Design and Applied Technology (Secondary 4

Design and Applied Technology (Secondary 4
Design and Applied Technology (Secondary 4 - 6)
Design and Applied Technology (Secondary 4 - 6)
Design and Applied Technology
(Secondary 4 – 6)
Elective Module 1
Automation
[Learning Resource Materials]
Resource Materials Series
In Support of the Design and Applied Technology Curriculum
(S4 –6)
Technology Education Section
Developed by
Curriculum Development Institute
Education Bureau
The Government of the HKSAR
Institute of Professional Education
And Knowledge (PEAK)
Vocational Training Council
Design and Applied Technology (Secondary 4 - 6)
Technology Education Section
Curriculum Development Institute
Education Bureau
The Government of the Hong Kong Special Administrative Region
Room W101, 1/F, West Block, Kowloon Tong Education Service Centre,
19 Suffolk Road, Kowloon Tong, Hong Kong
Reprinted with minor amendments 2010
Project Advisor:
Mr. Eric Liu
(Head, Department of Multimedia and Internet Technology, IVE/Tsing Yi)
Author:
Mr. Li Yu Wai
(Design and Technology Teacher)
Project Coordinators:
Mr. Li Yat Chuen
Mr. Tsang Siu Wah
(Senior Training Consultant, PEAK/VTC)
(Training Consultant, PEAK/VTC)
The copyright of the materials in this package, other than those listed in the Acknowledgments section and the
photographs mentioned there, belongs to the Education Bureau of
the Government of the Hong Kong Special Administrative Region.
© Copyright 2009
Duplication of materials in this package other than those listed in the Acknowledgements section may be used
freely for non-profit making educational purposes only. In all cases, proper acknowledgements should be made.
Otherwise, all rights are reserved, and no part of these materials may be reproduced, stored in a retrieval system
or transmitted in any form or by any means without the prior permission of the Education Bureau of
the Government of the Hong Kong Special Administrative Region.
Design and Applied Technology (Secondary 4 - 6)
PREFACE
A set of curriculum resource materials is developed by the Technology Education Section of
Curriculum Development Institute, Education Bureau for the implementation of the Design and
Applied Technology (Secondary 4-6) curriculum in schools.
The aim of the resource materials is to provide information on the Compulsory and Elective
Part of the DAT (Secondary 4-6) to support the implementation of the curriculum. The
resource materials consist of teacher’s guides and student’s learning resource materials of each
Strand and Module of the DAT (Secondary 4-6) arranged in eight folders.
All comments and suggestions related to the resource materials may be sent to:
Chief Curriculum Development Officer (Technology Education)
Technology Education Section
Curriculum Development Institute
Education Bureau
Room W101, West Block, 19 Suffolk Road
Kowloon Tong
Hong Kong
Design and Applied Technology (Secondary 4 - 6)
CONTENTS
Introduction
Chapter 1 – Basics of Control Systems
1
1.1
Open-Loop, Closed-Loop and Sequential Control Systems
2
1.2
System and Sub-systems
7
1.3
Operation of a Washing Machine
9
1.4
Operation of Traffic Lights
13
1.5
Control of Fluid Level in a Tank
17
1.6
Application of Control Systems in a Buggy
20
1.7
Application of Control Systems in Air Conditioner
23
Chapter 2 – Pneumatics
25
2.1
Pressure
26
2.2
Pneumatics Components and Symbols
28
2.3
Understanding Pneumatic Components
31
2.4
Pneumatic Circuitry
38
2.5
Electro-pneumatic Systems
59
Chapter 3 – Programmable Control Systems
64
3.1
What is Programmable Logic Controller ( PLC)?
65
3.2
Programming the PLC
71
3.3
Application of Ladder Logic Diagram
76
3.4
Programmable Interface Controller
83
3.5
Stepper Motor and Servomotor
89
Chapter 4 – Robotics
95
4.1
Definition of Robots
96
4.2
Mechanical Structure of Industrial Robotic Arms
98
4.3
Robot Anatomy
106
4.4
Robot Control Systems
116
4.5
Applications of Robots
122
Theme-based Learning Tasks
126
•
Practical Design Appreciation - Case Study of Intelligent Fire Alarm System
126
•
Hands-on Activity – Controlling an Automated Traffic Lights using Programmable
130
Logic Controller
Design and Applied Technology (Secondary 4 - 6)
•
Design and Make Project – Pipe Cleaning/Inspection Robot
133
•
Mars Exploration – Design an Innovative End-effector for Mars Lander
136
Assessment Tasks
138
Useful Web Sites
155
References
156
Glossary of Terms
157
Acknowledgements
161
INTRODUCTION
Automation is happening around us in our daily life - from early morning “snooze” of an
alarm clock to boarding the MTR through an automatic door. Both of them are examples of
industrial automation application and involve different levels of automation. An alarm clock is
basically a timer. The “snooze” function is in fact a sequential control with preset time
variables. An automatic door is a typical application of Pneumatic/Electro-pneumatic systems.
The duration of door opening is another example of a sequential control with delay function.
Sometimes the MTR driver performs the “Manual Override” function for emergency, which is,
to re-open the doors when a passenger is clamped between the closing doors.
Students will be introduced to the basic control systems, pneumatics/electro-pneumatics
systems, programmable control systems and robotics. After completing this module, students
will have an integrated knowledge in understanding, interpreting and appraising basic
automation systems, such as washing machine, traffic lights, buggy, air conditioner,
production line, automatic door and fire alarm system etc.
New topics like electro-pneumatics, micro-controller, PLCs and robotics, will also be
introduced in this module. For better understanding, four thematic learning tasks will be
assigned to students. With the experience after the tasks, students will be technologically
competent to solve practical design problems in different scenarios. Our students will gain
some hands-on experience from these tasks.
From the application point of view, this module will be very challenging to the students who
have desires in pursuing innovative design and advanced technology. This module will
provide students who have interest in logics, electronics, computer control and system
engineering with a lot of opportunities to explore and unlimited space for their innovation.
Design and Applied Technology (Secondary 4 - 6)
CHAPTER 1 – BASICS OF CONTROL SYSTEMS
This chapter covers topics on:
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Open-Loop, Closed-Loop and Sequential Control Systems
System and Sub-systems
Operation of a Washing Machine
Operation of Traffic Lights
Control of Fluid Level in a Tank
Application of Control Systems in a Buggy
Application of Control Systems in Air Conditioner
These topics include learning materials that facilitate you to:
Identify Open-Loop, Closed-Loop and Sequential control system
Identify input, process, output, state output diagram and time-phase diagram
Understand some control basics in washing machine, traffic lights, water level,
buggy and air-conditioner
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Design and Applied Technology (Secondary 4 - 6)
1.1
OPEN-LOOP AND CLOSED-LOOP CONTROL SYSTEM
(I) Basic terminology of control system
1. Controlled elements are defined as those which are involved in realizing automatic
controlling tasks. Commonly they are referring to electrical motors, pneumatic
cylinders, and hydraulic pumps in machines, equipment and industrial production
processes.
2. Controllers are defined as those used to make the controlled elements to perform
their controlling tasks.
3. Controlled values are referring to those physical properties that are to be controlled
according to the desired value by an automatic control system. Commonly they are
the temperature, speed and displacement.
4. References (Set points) are referring to the values of input signals required by an
automatic control system to control the controlled elements.
5. Disturbances are referring to those which make the controlled values deviated from
the desired value in an automatic control system. For those disturbances that are
come from the control system itself are called the Internal Disturbance. Those come
from the surrounding environment are called the External Disturbance.
6. Automatic Control System is composed of controlled elements and controllers which
work together according to a preset program so that the controlled elements can
perform the controlling tasks.
(II) Open-Loop Control System
Open-Loop control is the most basic automatic control system and is composed of controller
and controlled element. It involves the forward function of the control system but without the
“feedback control”. The block diagram shows their relationship.
Disturbance, U
R
Reference
C
Controller
Figure 1.1
Controlled Element
Controlled Value
Block diagram of Open-Loop control system
Taking a furnace temperature control as an example for the Open-Loop control system:
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Design and Applied Technology (Secondary 4 - 6)
SW
Heating Coil
Furnance
Figure 1.2
Open-Loop control system of Furnace Temperature (Controlled value)
In this case, the controlled element is the furnace, or more specifically referring to the heating
coil. U is the control value that determines the power (voltage, current) to the heating element.
The furnace temperature, c, is the controlled value. The SW is the switch that is controlled by
an electromechanical relay. The switch will be turned ON or OFF in a pattern according to the
timed sequence. The timed sequence is used to maintain the temperature inside the furnace
within a controllable range.
The Disturbance in this case is the frequency at which the furnace door is opened. If the door
is opened so often, the furnace temperature will drop and the controlled value c becomes
deviated (lower) from the desired value. However, the SW in an Open-Loop control system
will not be closed to give power to the heating coil to restore the temperature. The ON/OFF
time of this switch has been preset and will not be changed with the furnace temperature in
this case.
In an Open-Loop control system, the controlled value will not influence the value of input
signal. Therefore, there is no need to measure the furnace temperature (controlled value). The
control system becomes simple but the accuracy and stability is usually not high. The only
way to increase the stability is to make sure that every component used in this system is of
high quality and accuracy but it is usually unpredictable. This system cannot deal with the
disturbance, such as frequent opening of the furnace door.
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Design and Applied Technology (Secondary 4 - 6)
Thermometer
Manual
SW override
Heating Coil
Furnance
Figure 1.3
Human interrupt control of the Open-Loop control system
Disturbance, U
Reference, R
C Controlled Value
Controller
Controlled Element
Human control
Figure 1.4
Block diagram of Human override in Open-Loop control system
The above problem can be solved by the use of manual override to the electromechanical
switch. A thermometer is used to measure the actual furnace temperature for visual
monitoring. If the controlled value is deviated from (usually lower than) the desired value, the
operator will close the switch in order to give power to the heating coil. The furnace
temperature can hence be more closely stabilized within the controlled range. It is an
immediate solution that the controlled value can be used ‘back” to control the system,
however, it is by no means an automatic control system. If we use some sort of sensor and
electrical circuit to measure the temperature and provide the “feedback” of controlled value to
the system, the system becomes a Closed-Loop Control system.
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Design and Applied Technology (Secondary 4 - 6)
(III) Closed-Loop Control System
Thermometer
Mercury Switch
Sensor circuit
High
Low
SW
Heating Coil
Furnance
Figure 1.5
Closed-Loop control system of furnace
Disturbance
e, Error signal
Reference, R
C Controlled Value
Controller
-
Feedback signal
Figure 1.6
Controlled Element
Measuring Device
Block diagram of Closed-Loop control system
The main difference between Closed-Loop system and Open-Loop system is the Feedback
function. The controlled value is measured and used to compare with the input signal
(Reference). An error signal is then generated by this comparison. The error signal is
amplified and converted to be used to change the controlled value in an opposing way so as to
delete or at least minimize the deviation from the desired value. It is called the Feedback
control.
If the feedback signal is used to enhance the controlled value and make it go beyond the
desired value, it is known as Positive Feedback. If the feedback signal is used to reduce the
deviation of the controlled value from the desired value, it is known as Negative Feedback.
Most of these control systems are negative feedback, so the word “negative” is usually
omitted for simplicity.
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Design and Applied Technology (Secondary 4 - 6)
The advantage of Closed-Loop control system is that the reference is changed in accordance
with the level of disturbance. The error signal generated can compensate and oppose the effect
of disturbance. Therefore, the controlled value can go back to the level before the disturbance
is interfered.
(IV) Sequential Control System
Sequential control means one operation or process must be completed before the next one is
initiated. The execution of next operation or process depends on the execution of the
preceding one. It will continue in a stepwise order until a termination command is met.
Sequential control is usually referring to automatic and mostly used in engineering and
industry, as most of the processes in production line are sequential in nature. Take a lift as an
example. The lift can only move up or down after the door has closed properly and the door
will only open when the lift is level with the building floor. It implies that automatic
sequential control usually work with sensory feedback mechanism.
We will explore the application of sequential control system by exploring the operation of a
washing machine, traffic lights, and the control systems in a buggy in topics 1.3, 1.4 and 1.6
respectively.
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Design and Applied Technology (Secondary 4 - 6)
1.2 System and Sub-systems
Systems are actually everywhere in our daily life, from the universe solar system to home
entertainment systems, from Automatic Teller Machine (ATM) to missile defense systems. In
laymen terms, people say everything goes smoothly when “the system works” but disaster
comes when “the system fails”.
The definition in dictionary for system is that a system is as a whole but with inter-related
parts and emphasizing on internal structure. The system approach means, in engineering sense,
integrating analysis and synthesis. Let’s take automobile as an example.
When driving a car in a normal situation, it works as a whole. However, when it is tugged to a
garage, it is obviously a system composed of many inter-related subsystems, such as
transmission, ignition, steering, braking, lubricating, suspension, and more. Each of these
subsystems is in turn made up of many parts, such as the clutch, stick shift, and gear box for
transmission. The gear box again in turn consists of many components, such as gear, shaft and
a lot of nuts and bolts.
In engineering system approach, when engineers want to design a car, they do not start with a
bunch of bolts and nuts. They start with a conception of the intended car as a whole with a set
of functional requirements. Then, they decompose the concept car into a number of functional
sub-systems, most importantly, with proper interface design for assembly in later stages. For
example, there are sub-systems for Power and the subsystem for Transmission. Interfacing is
the connection of two sub-systems to work together. Then, engineers further analyze the
sub-systems into their inter-related components. Finally, they come to the specifications of
manufacturing a single part in details under the component level.
After thousands of individual parts are made, they will be tested, brought together and
assembled into a number of sub-systems. The sub-systems will be brought together again and
assembled into larger sub-systems. Finally, they are assembled together into a car (a system)
for test drive.
The sub-systems approach at different intermediate levels is crucial for engineers and any
personnel involved in managing a large complex system. Through division of sub-systems
according to their functions, engineers can make a detailed design (less complex) and physical
assembly (in a manageable scale) at a time. The sub-system approach is also related to the
concept of Modularity which is widely adopted in today’s civil construction and
manufacturing. The system approach can be visualized by the Vee model.
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Design and Applied Technology (Secondary 4 - 6)
Performance Test
Funtional
Requirement
Finished
Product
Implementation
Detail
design
l In
t eg
r at
ion
Integration
verification
Subsystem
design
Figure 1.7
Integration
validation
Ph
ys
ica
ion
sit
po
om
ec
lD
na
ti o
nc
Fu
Feedback
System
design
Vee Model of System and Sub-system approach
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Design and Applied Technology (Secondary 4 - 6)
1.3 Operation of a Washing Machine
Figure 1.8
Typical structure of a washing machine
A washing machine has a rotating drum for holding laundry and water. The amount of fabrics
and the amount of water in the drum may vary. Power is supplied from an inverter to an A.C.
motor which drives the rotating drum. A speed control regulates the drum’s rotating speed
within a selected range.
During the tumble-wash phase, the motor works at low speed and high torque. Although
power consumption during the tumble-wash phase is comparatively low, this phase lasts much
longer than the spin-dry phase. Therefore, driving the drum motor efficiently during the
tumble-wash phase can reduce the total power consumption. Balancing the load before fast
spinning is very important because it reduces power consumption during acceleration. In
order to achieve a rapid, power-saving spin-dry phase, maximum drum speed is required.
S T O P
A N D
T H I N K
Why A.C. motor, not D.C. motor, is used to drive the washing machine? State your reasons.
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Design and Applied Technology (Secondary 4 - 6)
(I) Timing diagram for the Washing - Drying operation
Drum Speed
[RPM]
400
40
Time
[min.]
-40
Tumble - wash
Figure 1.9
Spin - dry
Timing diagram for washing processes
During the spin-dry phase, the washer drum rotates at high speed, typically 400rpm for brief
intervals as shown in the Time-phase diagram in Figure 1.9, Between these high-speed
intervals, the drum rotates at low speed for longer intervals. Regarding the diagram, the drum
rotates for 1 min at 400 rpm, and then rotates at low speed for about 3 minutes in both
directions. Then, there is a second 400-rpm spin (1-min long) followed by another low-speed
spin for 3 minutes in both directions. Finally, there is a high-speed spin for 1 minute,
followed by a low-speed spin for another 1 minute in a single direction.
(II) Timing diagram for Tumble-wash phase
Drum Speed
[RPM]
40
Clockwise
50
95
100
45
Time
[min.]
Counter
Clockwise
-40
Figure 1.10
The timing phase diagram for tumble-wash phase
The tumble-wash phase is typically three to four times longer than the spin-dry phase. When
the scale of the time-phase diagram is enlarged as in Figure 1.9, It can be found that during
the tumble-wash phase, the drum rotates slowly, typically at 40 rpm, first turns clockwise
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Design and Applied Technology (Secondary 4 - 6)
(CW), and stops, then turns counter clockwise (CCW), and stops, and so on. Rotation time
interval varies and depends on the nature of laundry, such as delicate, light, heavy or mixed
according to the manufacturer’s specifications. The diagram shows that there is a 90-percent
rotation time in a 100-sec interval.
S T O P
A N D
T H I N K
Why the tumble-wash cycle is not 100 percent continuous rotating and needs to be stopped
for a while (10 percent time) before reversing in direction?
(III) Flow diagram analysis of washing machine operation
Flow diagrams are widely used to explain decision making processes in a logical manner.
They are particularly useful for converting sequential events into a series of ‘Yes’ or ‘No’
operations to each decision to be made. Flow diagram is also used for understanding a process
to be controlled before producing a ladder diagram in PLC programming. Here are some
major symbols used in flow diagrams.
Symbol
Function
Elongated circle/Terminator
Represent the start or end of a process
Connector
Rectangles
Represent process and action to be taken
Input/Output
Diamond/Decision
Represent what decisions that must be made
Table 1
Main symbols for Flow diagram
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Design and Applied Technology (Secondary 4 - 6)
Steps of constructing a simple flow diagram
1. Start the flow chart by drawing an elongated circle/terminator, and label it with
"Start".
2. Draw a rectangle or diamond to represent the first process or action. Write the key
word of action or question down, and then draw an arrow (connector) from the
“Start” to this shape.
3. Represent the actions and decisions of the whole process and in the order of their
occurrence. Using arrows (connectors) to indicate the flow of the process.
4. Draw a diamond to indicate where a decision needs to be made, using arrows leaving
out of the diamond to indicate the possible outcomes.
5. Use an elongated circle labeled with "Finish" to indicate the end of the process.
6. Review the flow chart from step to step to see if the flow diagram represents the
sequence of actions and decisions involved in the process and in a good order.
START
Close Door
Start tumble
No
Read sensor
Drain Water
No
Read timer
Door close?
Read timer
Time up?
Yes
No
Time up?
Yes
Yes
Open Door
Open Valve
Start Spin
END
No
Read sensor
Water full?
No
Read timer
Time up?
Yes
Yes
Figure 1.11
Flow diagram for a classical washing machine
The operation of washing machine may vary according to type, price and country of origin.
Some washing machine controls may be highly sophisticated and use artificial intelligence to
improve washing efficiency, water consumption, acoustic noise, electromagnetic interference
(EMI) and the power consumption. The example above is the flow diagram for a classical
washing machine (human operation is included in this case).
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Design and Applied Technology (Secondary 4 - 6)
1.4 Operation of Traffic Lights
Traffic lights become an integral part of a modernized city. The proper operation of traffic
lights can smooth the flowing traffic and boost the economic growth. Proper operation means
precise timing, cycling through the states correctly, and responding to outside inputs like
Walk signals. Traffic lights are at least come in pairs and need to be synchronized for proper
operation. Below is a typical 3-lamp, 4-state traffic light signals.
Figure 1.12
A typical 3 lamps traffic light sequence
When considering the design and implementation of traffic light control at an intersection, it
is typically between a busy (Main) road and a less busy (Side) road. Both streets have the
ordinary (Red, Amber, Green) signal lights. The intersection is sometimes fitted with a sensor
to detect the vehicle for the side- road traffic condition and with a walk request button
controlled by pedestrians.
A single 3-lamp traffic light has three states, Red, Yellow, and Green, which are also the
outputs. A single input for the traffic light is values 0 for no change and 1 for state change.
This input is connected to the output of a countdown timer, which outputs a value of 1 when it
counts down to zero. Thus for a single light, we can draw the state transition diagram as
below.
Figure 1.13
A State diagram of traffic signal sequence (a single light)
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Design and Applied Technology (Secondary 4 - 6)
(I) Three important parts of Traffic Light Control System
Figure 1.14
A Block Diagram of Typical Traffic Control system
The first part is the controller, which represents the brain of a traffic system. It consists of a
computer or a programmable logic controller (PLC) that controls the selection and timing of
traffic lights in accordance to the varying demands of traffic loads.
The second part is the traffic light unit or “signal face”. Signal faces are used to provide
controlling signals to the traffic in a single direction. It consists of 3 signal states and operates
in synchronization with the sequence of other traffic lights. The lamps are conventionally
comprised of red, amber and green lights.
The third part is the detector or sensor to indicate the presence of vehicles. The detector
consists of wire loops placed in the pavement at intersections. They are activated by the
change of electrical inductance caused by a vehicle passing over or standing over the wire
loop. For a more recent technology, it uses the video detection system for the indication of
vehicles on the road. A small camera is fixed in the traffic light pole to “see” if any vehicle is
present.
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Design and Applied Technology (Secondary 4 - 6)
Figure 1.15 A diagram of Induction loop traffic sensor.
(II) A timing diagram to traffic control
Figure 1.16
A diagram of a pair of traffic signal unit
The sequence of traffic light operation and timing duration can be analyzed or represented by
a timing diagram. The code O represents an output signal. The codes are O:2/00 and O:2/04
for Red; O:2/01 and O:2/05 for Amber and O:2/01 and O:2/05 for Red. A 1 second delayed
period for RED lights on both directions to be illuminated is for road safety reason. 1 second
delayed period lets drivers from both directions have a brief for readiness and let the vehicles
on the junction be cleared off.
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Design and Applied Technology (Secondary 4 - 6)
Red = O:2/00
Green = O:2/06
Green = O:2/02
Amber = O:2/05
8 Sec.
4 Sec.
Figure 1.17
Amber = O:2/01
Red = O:2/04
1
8 Sec.
A diagram of a pair of traffic signal unit
16
4 Sec.
Design and Applied Technology (Secondary 4 - 6)
1.5 Control of Fluid Level in a Tank
u,Desired Fluid Level
C Actual Fluid level
Controller
Tank
Float Switch
Figure 1.18
Block diagram of fluid level control system
A cold water tank is supplied with water via a float operated control valve ‘F’. A manual
globe valve ‘V’ is on the outlet pipe. Both valves are assumed to have same size in term of
flow capacity (mass flow rate) and flow characteristic (pressure drop) along the pipe. The
desired water level in the tank is set at the point B. This is equivalent to the set (desired) point
of a Closed-Loop control system.
Here, the tank is the controlled element. The float switch is the measuring device which
provides the feedback to the controller. The controlled value is the fluid level and the control
value is the mass flow rate.
(I) When Loading is 50% (Half-open)
Figure 1.19
Valve at 50% open
It can be assumed that, with valve ‘V’ half open (50% load), the flow rate of water entering
via the float operated valve is equal to that leaving the discharge pipe via globe valve “V”,
Water level can be maintained in the tank at point B.
The system can be said to be in equilibrium, under control and in a stable condition. It means
that the flow rate of water entering and leaving the tank is the same and, therefore, the level is
not varying. The water level is precisely at the desired water level (B) and giving the required
outflow.
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Design and Applied Technology (Secondary 4 - 6)
a. When the loading is 0% (Fully closed)
Figure 1.20
Valve fully closed
With the globe valve ’V’ is fully closed, the level of water in the tank rises to point A. The
float operated valve cuts off the water supply.
The system is still under control and stable but the water level is above level B. The difference
between level B (set point) and the actual level A is the proportional band of the control
system.
If globe valve ‘V’ is half open to give 50% load, the water level in the tank will return to the
desired level, point B.
b. When the loading is 100% (Fully open)
Figure 1.21
Valve fully open
The globe valve ‘V’ is fully opened (100% load). The float operated valve will be dropped to
widely open the inlet valve. This allows a higher flow rate of water to meet the increased
demand from the discharge pipe. When it reaches level C, enough water will be entering to
meet the discharged needs. The water level will be maintained at point C.
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Design and Applied Technology (Secondary 4 - 6)
Figure 1.22
Proportional band
The system in this case is still regarded as under control and stable, but there is an offset that
is the deviation in level between points B and C. The difference in levels between points A
and C is known as the Proportional Band, It is the change in level for the float operated
control valve to move from fully close to fully open.
c. Level Control Basics
The above water level control case illustrates several basic and important concepts, such as
feedback and proportional control:
1. The control valve is triggered in proportion to the error (offset) in the water level
from the set point. (level A and C)
2. The set point can only be achieved at certain load level (50% in this case).
3. A stable control state will be achieved between points A and C (proportional band).
Any load (disturbance) causing a difference in level other than that of B will be the
offset.
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Design and Applied Technology (Secondary 4 - 6)
1.6 Application of Control Systems in a Buggy
The buggy is an obstacle-avoiding robot comprised solely of two motors, two wheels, two
contact (bump) switches, and a few discrete electronic components. This obstacle-avoiding
robot is designed to illustrate the basic knowledge of a sequential (Open-Loop) control
system.
This buggy is entirely an analog circuitry without any integrated circuit. The block diagram
illustrates how the bump sensor is connected to the motor–driven wheel (actuators). Signal
will be created when the bump sensor comes into contact with obstacles. This signal is sent to
the motor-driver circuitry – the signal amplifier for each wheel, signaling the robot to back up.
An adjustable timer is the RC circuit that is associated with each motor driver to determine
how long each wheel should reverse.
Left Wheel
Adjustable
Timer
Motor
Driver
Bevel Gears
Bump Sensor
Left Motor
Right Motor
Adjustable
Timer
Bevel Gears
Motor
Driver
Right Wheel
Figure 1.23 The block diagram shows two motors, two wheels, a bump sensor and two
potentiometers for programming the time events.
The diagram illustrates the sequence of actions when the obstacle-avoiding buggy strikes an
obstacle. The robot is initially moving forward. When it strikes the obstacle, both motors are
switched to reverse and the buggy moves straight backwards.
How the buggy makes a turn? The right motor reverses for a longer time period than the left
motor, causing the robot to turn to right. After a certain period, the right motor stops reversing
and both motors go forward, leading the robot off in a new direction. If the robot bumps into
obstacle again, the process repeats the sequence of actions until the obstacle clears off from its
way.
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Design and Applied Technology (Secondary 4 - 6)
Obstacle
Obstacle
Obstacle-avoiding
robot moving forward
Figure 1.24
Obstacle
Robot moving
backward
after colision
Obstacle
Robot turns right
left wheed moves forward
right wheel moves backward
Robot moves forwards again
Clear of obstacle
The basic operation of the obstacle-avoiding buggy
Colision detected
Signal from
bump sensor
Time
forward
Command to
right motor
Time
backward
forward
Robot moving
forward
Robot
Robot
backing up turning
right
Command to
left motor
Robot moving
forward
Time
backward
Figure 1.25
Timing sequence illustrating the obstacle-avoiding buggy’s backing up
behavior.
The sequence of actions can be illustrated in timing diagrams. Figure1.25 depicts the signal
generated by the front bumper sensor. Figure 1.26 illustrates the signals sent to the right and
left drive motors.
Initially, both motors receive signals to go forward. If a collision occurs, the bumper sends a
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Design and Applied Technology (Secondary 4 - 6)
binary signal to the adjustable timers, low (0) for no contact, high (1) when an obstacle is
struck. The timers, in turn, provide a binary signal to the motor drivers – high (1) for reverse
rotation, low (0) for forward rotation.
Assume that the timers are set for delays of TR seconds and TL seconds for the right and left
motors respectively (TR > TL) for reversing. After encountering an obstacle, the buggy will
backup for a time TL. The left motor turns forward after TL, the right motor stays in reverse
for a time TR - TL. It will then resume moving forward. Therefore, the buggy will move in a
different direction and avoid the obstacle. The buggy will repeat the sequence until it can
totally avoid the obstacle.
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Design and Applied Technology (Secondary 4 - 6)
1.7 Application of Control Systems in Air Conditioner
(I) Temperature Control
Air conditioning systems are essential in most of our daily lives though we are facing a
critical issue of global warming. With a view of that, an effective control that can improve the
cooling efficiency will save the power consumption. A simple air conditioning system is
shown in Fig 1.26. The only control task in this system is temperature. There are two
adjustable valves to regulate the temperature.
Figure 1.26
Block diagram of temperature feedback control system
The temperature sensor provides the feedback to the Closed-Loop control system.
If temperature measured Tm is lower than the set point Ts, open heating valve Hv fully and
close the cooling valve Cv fully to restore the temperature.
If temperature measured Tm is higher than the set point Ts, open cooling valve Cv fully and
closes the heating valve Hv fully to restore the temperature.
State
Action
Tm < Ts
Cv Low (0), Hv High (1)
Tm > Ts
Cv High (1), Hv Low (0)
Table 1.1
Relationship of control and controlled valve
(II) Humidity control
In the real world, however, it is usually not enough to manage an air conditioning system with
temperature control only. We need to control humidity as well. A modified air conditioning
system is shown in Fig 1.27. There are two sensors in this system: one is to monitor
temperature and another is to monitor humidity. There are three control elements: cooling
valve, heating valve, and humidifying valve (water spray nozzle), to adjust temperature and
humidity of the air supply.
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Design and Applied Technology (Secondary 4 - 6)
Figure 1.27
Block diagram of temperature and humidity control system
The humidity sensor provides the feedback to the Closed-Loop control system. If humidity
measured HUm is lower than the set point HUs, open humidifying valve HUv fully to restore
the humidity. If humidity measured is HUm higher than the set point HUs, close humidifying
valve HUv fully to reduce the humidity.
State
Action
HUm < HUs
HUv, High (1)
HUm > HUs
HUv, Low (0)
Table 1.2
Relationship of control and controlled valve
S T O P
A N D
T H I N K
1.
Suggest any disadvantage of this type of temperature and humidity control.
2.
Suggest any method which can improve this control system.
QUIZZES (CHAPTER 1)
1. Why automated control systems are so important in industry?
2. State the differences between Open-Loop and Closed-Loop control system.
3. Give one daily example in Open-Loop control system and Closed-Loop control
system. Draw the block diagrams for the examples.
4. Suggest and name the types of sensors used in a washing machine.
5. What is the meaning of set point?
6. What is the meaning of proportional band?
7. What is the meaning of error signal?
8. What is the meaning of offset?
9. Describe how an RC circuit functions as a timer.
10. What sensors can be used as a bump switch for an obstacle avoiding buggy?
24
Design and Applied Technology (Secondary 4 - 6)
CHAPTER 2 – PNEUMATICS
This chapter contains topics on:
2.1
2.2
2.3
2.4
2.5
Pressure
Pneumatics Components and Symbols
Understanding Pneumatic Components
Pneumatic Circuitry
Electro-pneumatic Systems
These topics include learning materials that facilitate you to:
Understand the characteristics of pressure
Identify pneumatics and electro-pneumatics components
Understand design of pneumatics and electro-pneumatics circuits
Understand the industrial application of pneumatics.
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Design and Applied Technology (Secondary 4 - 6)
2.1 Pressure
(I) Units
The atmosphere exerts a pressure on earth surface. As a standard, pressure is measured at sea
level. This pressure can be described as a force per unit area. The metric unit of pressure is
Pascal (Pa).
1 Pa = 1 N/m2 (Newton per square meter)
The force exerted by the atmosphere at sea level is 100,000 Pa. 1 standard atmosphere is
approximately 14.696 psi or 1.01325 bar or 1.03323 kgf/cm2. In English, pressure is
expressed in psi, or pounds per square inch. It is also defined as a ratio of force to area.
1 MPa =10 bar or 145 psig
Figure 2.1
The various systems of pressure notation
In the application of pneumatics, pressure is usually referring to gauge pressure (GA or psig).
As the pressure is always above atmospheric pressure, it is also regarded as over-pressure. For
gauge pressure in pneumatics application, the atmospheric pressure is referenced as zero
pressure.
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Design and Applied Technology (Secondary 4 - 6)
Figure 2.2
Gauge Pressure dial
In a full vacuum scenario, pressure can be expressed as absolute pressure (Pa ABS or psia). In
vacuum technology, a pressure is always below atmospheric and in a state of under pressure.
The actual atmospheric pressure is 1.013 bars. Disregarding the decimals, the standard
atmospheric temperature is referenced to 1 bar.
(II) Pressure and Flow
The most important relationship in pneumatics is pressure and flow. If there is no flow,
pressure over the entire system will be the same at every point. However, once there is a flow
from one point to another, pressure will be decreasing along the path. This difference in
pressure is called pressure drop. Pressure drop depends on three factors:
1. Initial pressure
2. Volume of flow
3. Flow resistance
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Design and Applied Technology (Secondary 4 - 6)
2.2 Pneumatics Components and Symbols
(I) Air Handling Units (AHUs)
a. Why needs Air Handling Unit?
All atmospheric air carries dust and moisture. After compression, moisture will condense in
the air. Dust and moisture combine with other contaminants, such as pipe scales and worn
sealing materials, will cause detrimental effect to the pneumatic system. Therefore, AHU is
the primary element in a pneumatic system to provide clean, regulated, and/or lubricated
compressed air in all industrial applications.
The function of an AHU is to prolong the life of pneumatic tools, devices and valves, and
hence reduce the maintenance and downtime costs. AHU is also known as Air Preparation
Unit (APU). It is usually composed of Filter, Regulator and/or Lubricator.
b. Filter
A standard filter consists of two units: water separator and filter. Water separator will collect
a considerable quantity of water and the filter will prevent contaminants, such as dust and rust
particles from entering into the pneumatic system. Water collected can be drained off through
a manual drain cock or an automatic drain. The time required to replace a filter can be alerted
by excessive drop of pressure across the filter.
Figure 2.3
Typical structure and symbol of a Filter with Drain
c. Regulator
Pressure regulator has a diaphragm to balance the output pressure against an adjustable spring
force.
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Design and Applied Technology (Secondary 4 - 6)
Figure 2.4
Regulator sectional diagram
If the consumption required by the pneumatic system increases, the output pressure will
decrease accordingly. This decreases the force acting on the diaphragm and against the spring
poppet force. The diaphragm and the poppet will lower until the spring force to be balanced
again. This will increase the air flow through the orifice until it meets the increased
consumption of compressed air.
If the consumption rate drops, output pressure slightly increases. This increases the force
acting on the diaphragm and valve will then lift until the spring force is equaled again. The air
flow through the valve will then be reduced until it matches with the reduced consumptions
rate. The output pressure can be maintained.
Figure 2.5
Structure and symbol of a Pressure Regulator
S T O P
A N D
T H I N K
Why the output pressure will increase when the consumption flow rate drops and vice versa?
Can you explain this phenomenon?
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Design and Applied Technology (Secondary 4 - 6)
d. Lubricator
It is designed to dispense a certain amount of lubrication oil continually to the compressed air.
Some pneumatic devices need lubrication to run at peak efficiency. However, some
pneumatic applications will omit the lubricator for the following reasons:
1. Clean and hygienic environment for food and pharmaceutical application
2. Oil free, healthier and safer working environment
3. Reduce cost of additional lubrication equipment, lubricating oil and maintenance.
Structure and symbol of a Lubricator
In most applications, especially for those used in school and vocational training, a combined
unit or modular design of filter, pressure regular and lubricator is commonly used.
Figure 2.6a Structure and symbol of a combined APU
S T O P
A N D
What are the advantages of a combined APU?
30
T H I N K
Design and Applied Technology (Secondary 4 - 6)
2.3 Understanding Pneumatic Components
2
3
Figure 2.6b
4
1
5
Position or box details
When looking at the box in details, the number of ports is determined by the number of end
points in a given box and only counted in one box per symbol. In this example, there is a total
of 5 ports. In practice, the exhaust port sometimes goes directly to atmosphere with no
physical port exists. It can be noted that the actual ports line will extend beyond the box,
while the exhaust port is blocked with a symbol T.
(I) Directional Control Valves
The function of a directional valve is used to determine the direction of compressed air flow
through its ports by changing its internal connections. The descriptions of directional valves
are described by the following parameters:
1. The number of ports
2. The number of switching positions
3. The normal (non-operated, initial) position
4. The method of operation or actuation.
Directional Valves symbols
Switching function
2/2 ON/OFF without
A
Major application
Pneumatic tools
exhaust
P
3/2 Normally closed NC
A
Single acting cylinders
(push type)
P
R
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Design and Applied Technology (Secondary 4 - 6)
Directional Valves symbols
Switching function
3/2 Normally open NO
A
Major application
Single acting cylinders
(pull type)
R
P
A
B
4/2 Switching between
Double acting cylinders
output A and B ports with
common exhaust R
P
R
B
A
5/2 Switching between
Double acting cylinders
output A and B ports with
separate exhaust R
R2 P R1
B
A
5/3 Switching between
Double acting cylinders,
output A and B ports with
with neutral position for
mid-position fully sealed
stopping all cylinders
R2 P R1
Table 2
action
Symbols (ISO) of common direction valves
Remarks: P is inlet port of working air line; A is the outlet port of working air line; R is the
exhaust port.
(II) Valve actuator
The directional valve can be controlled directly by manual control or automatically by
mechanical, electrical and air actuation. Below are some common actuator symbols:
Symbols (ISO)
Actuator type
Spring return
Roller type (2 directional)
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Design and Applied Technology (Secondary 4 - 6)
Manual control
Lever type manual control
Push button manual control
Direct Acting Solenoid
Air Pilot
Pilot Assist Solenoid
Table 2.1
Symbols for common valve actuator
Take a typical 5/2 directional valve as an example. It shows the method of actuation, the
number of positions, the flow paths and the number of ports. Here is a brief illustration of
how to read a symbol:
Figure 2.7 Directional Valve symbol
(2 position, lever actuated and spring return)
The left actuator is manually controlled by a lever. It is used to shift the valve from right to
left when actuated. This directional valve has at two positions (boxes) and each position has
three flow paths.
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Design and Applied Technology (Secondary 4 - 6)
When the lever is not activated, the spring return actuator in the right side will take control of
the valve and the right box will be in operation. When the lever is actuated, the box next to the
lever is then in control of the valve. A valve can only be in one “Position” at a time. It should
be reminded that there is no external movement occur and only switching of internal
connections taking place within the valve housing.
The number of boxes in a valve symbol indicates the number of positions the valve has. Flow
direction is indicated by the arrows in each box. The number of arrows represents the number
of flow paths the valve has when it is in that position. The position is depending upon which
actuator is taking control of the valve at that time.
In practice, the exhaust port goes directly to atmosphere and no physical port exists. The
exhaust port is indicated with symbol T.
(III) Adjustable speed control of air flow
A speed control valves consists of a check valve and a variable throttle valve in one housing
to restrict the air flow in one direction.
The air can flow freely from left to right direction to the cylinder but the air has restricted
flow for speed regulation in reverse direction to the cylinder.
Figure 2.8
Structure and symbol of flow control Valve
S T O P
A N D
T H I N K
Would you describe briefly the following two valves to see how much you understand at
this stage?
(a)
(b)
34
Design and Applied Technology (Secondary 4 - 6)
H
I
G
H
L
I
G
H
T
1. Cylinder Sizing Calculation
The air cylinder size calculation steps are as follow:
1. Calculate the area of the cylinder piston
Area = Pi x r2
2. Multiply the piston area by the air pressure to be used
Area x Pressure = Force Output
The real force output of a cylinder will be less than the theoretical output because of
internal friction and external loading. It is best to use a cylinder that will generate, from
25% to 50% (safety factor). more force than theoretically needed.
2. Cylinder Bore Size Selection
Figure 2.9
Cylinder bore Size
Four easy steps:
1. Determine the force needed to move the load. Add 25% (safety factor) for
friction and to provide enough power for the cylinder rod to move at a
reasonable rate of speed
2. Find out how much air pressure will be used and maintained in the system.
3. Calculate the power factor by the formula (Air pressure x Power factor =
Cylinder force required).
4. Once the power factor is found, the bore diameter can be checked from the table
below. For safety sake, the higher approximate value in the table will be chosen
for the determination of the bore size.
Power Factor Table
Bore Diameter
Power Factor
3
/4
1
0.4 0.8
11/8 11/2
1.0
1.8
Table 2.2
2
3.1
21/4 21/2
4.0
4.9
3
31/4
7.1
8.3
Power Factor Table
35
4
6
12.6 28.3
Design and Applied Technology (Secondary 4 - 6)
Example:
Estimated force needed is 900 N (25% safety factor included). Air pressure to be used is 80
N:
80 (N) x Power Factor = 900 (N).
Power Factor = 900 N / 80 (N) = 11.25
The power factor just above 11.25 is 12.6. Therefore, a bore diameter of 4cm cylinder
should be used.
(IV) Factors affecting the performance of cylinder
There are many factors that affect the performance of a cylinder.
Some of these factors are:
1. Quantity and type of fittings leading to the cylinder
2. Hose tube length and capacity
3. Cylinder operating load
4. Air pressure.
(V) Single Acting Cylinder
Figure 2.10
Structure and symbol of Single acting cylinder
A single-acting cylinder has one air inlet to provide power to the “extend” stroke. The piston
rod is returned by an internal spring. Single-acting cylinders use theoretically one-half as
much air as double-acting cylinders. It is operated by a 3-way valve.
(VI) Double Acting Cylinder
Double-acting cylinders have two inlets to provide power to both the “extend” and “retract”
stroke. It requires four ways or five ways directional control valves to control.
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Design and Applied Technology (Secondary 4 - 6)
Figure 2.11
Structure and symbol of Double acting cylinder
(VII) Single and Double Rod
Figure 2.12
Single acting (above) and Double acting cylinder (below)
Single-rod cylinder has a piston rod protruding from only one end of the cylinder. Double-rod
cylinder has a common rod, driven by a single piston, protruding from both cylinder ends.
When one end retracts, the other end extends.
Double-rod cylinders are excellent for providing an adjustable stroke and additional rigidity.
Also, a double-rod with attached cam may be used to trip a limit switch.
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Design and Applied Technology (Secondary 4 - 6)
2.4 Pneumatic Circuitry
When constructing a pneumatic circuit, it is drawn from bottom to top and from left to right.
The circuit layout basically consists of 4 layers. The bottom layer is Air Supply Unit; the
second layer is Signal Level; the third level is Control or Logic Level; and the top level is
Power level.
Pneumatic circuits are the assemblies of valves and a collection of elementary sub-circuits to
perform control functions. These functions are usually consist of the followings:
1.
2.
3.
Control output actuators, such as single-acting, double-acting cylinders, rotary
actuator and slide unit
Operate another valves, such as remote control and safety interlocks
Perform logic control functions, such as AND, OR and NOT.
(I) Number notation in pneumatic circuits
1.3
1.0
1.2
1.01
2.2
2.02
2.01
1.02
2.1
1.1
1.2
2.0
2.3
2.2
1.3
Start
0.1
Figure 2.13
Pneumatic circuit with number notation
38
2.3
Design and Applied Technology (Secondary 4 - 6)
Pneumatics
Components
Numbering Notation
Remarks
Top level:
Working
components
1.0, 2.0,3.0, ……
Mark the actuators.
1.01,1.02,2.01,2.02,
3.01,3.02,…….
Mark the auxiliary components, i.e. flow
restriction valve.
Third level:
Control
components
1.1, 2.1, 3.1,…..
Mark the components connected to working
components, i.e. 3/2 directional valve to the
single acting cylinder; 5/2 directional valve to
the double acting cylinder.
Second level:
Signal
Components
1.2,1.4,1.6,
2.2,2.4,2.6, ….
1.3,1.5,1.7, ……
2.3,2.5,2.7, ……
Mark the signal components responsible for
the retraction of cylinder, i.e. 3/2 NC valve
Bottom level:
Air Supply
Components
0.1, 0.2, 0.3, ……..
Mark the elementary air supply unit to
pneumatic circuit. i.e. AHUs
Table 2.3
….. Mark the signal components responsible for
the outstroke of cylinder, i.e. 3/2 NC valve
Numbering notation in pneumatic circuit
(II) Elementary functions
a. Flow amplification and remote control
A large power rating cylinder is needed for most industrial application. Thus a large
directional valve with sufficient air capacity is required. However, it is dangerous to operate a
large valve directly or in close proximity by an operator. A small manually operated valve is
used to control a large pilot-operated valve at a remote distance. The large capacity valve can
be installed close to the cylinder.
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Design and Applied Technology (Secondary 4 - 6)
2
1
Figure 2.14
Flow amplification and remote control of cylinder
b. Signal Inversion
A 3/2 normally closed valve (NC) is connected to the 3/2 normally open valve (NO). When
the NC valve is pressed, the NC changes position and pressure releases.
2
1
Figure 2.15
Circuit diagram of Signal inversion
c. Mono-stable circuit
A 3/2 manual operated NC valve is used to control a 5/2 valve which actuates a double acting
cylinder. This circuit has two functions: (1) one is for flow amplification, (2) Another is the
control of double acting cylinder with one 3/2 NC valve.
40
Design and Applied Technology (Secondary 4 - 6)
2
1
Figure 2.16
Switching of two circuits with one manual operated valve
d. Bi-stable Circuit
Two normally closed 3/2 valves are used to control a 5/2 directional valve which is used for
actuating a double acting cylinder. When the left valve is pressed momentarily, the cylinder
will retract and hold its position until the right 3/2 valve is pressed. When the right 3/2 valve
is pressed momentarily, the cylinder will extend. This circuit has two functions: (1) flow
amplification (2) memory function.
3
1
Figure 2.17
2
Switching of two circuits with one manual operated valve
e. Timing Circuits
A 3/2 NC valve is used to control another 3/2 NC valve. A flow restriction valve is connected
in the pilot line and in a direction that the ON signal is delayed. When the valve 1 is pressed,
the air flow is restricted to the directional valve. The cylinder will extend after a certain time
delay. When the valve 1 is released, the exhaust air flows freely. The cylinder retracts
immediately.
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Design and Applied Technology (Secondary 4 - 6)
2
1
Figure 2.18
Delayed switching ON circuit
A 3/2 NC valve is used to control another 3/2 NC valve. A flow restriction valve is connected
in the pilot line and is in a direction that the OFF signal is delayed. When the valve 1 is
pressed, the air flow freely to the directional valve. However, when the valve 1 is released, the
exhaust flow is restricted. Therefore, the cylinder retracts after a delay.
2
1
Figure 2.19
S T O P
Delayed switching OFF circuit
A N D
T H I N K
Why the air pressure fluctuates within the plants? Please list any 4 points.
(1) _______________________________________________________
(2) _______________________________________________________
(3) _______________________________________________________
(4) _______________________________________________________
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Design and Applied Technology (Secondary 4 - 6)
f. Direct operation and speed control of single acting cylinder
A manual operated 3/2 NC valve is used to directly control a single-acting cylinder. A flow
restriction valve is connected in the pilot line. The circuit can control the speed of the cylinder
outstroke. The cylinder is returned by a spring force once the valve is released. No flow
restriction in the return stroke.
Figure 2.20
(III) Logic Circuits
Direct control of a single acting cylinder
a. Logic OR function for a single acting cylinder
With a shuttle (OR) valve connected for the operation of a single acting cylinder, the
outstroke of the cylinder can be actuated by either of the two 3/2 valve. If the shuttle valve is
absent in practicality, the air from one valve will escape directly through the exhaust port of
another valve. A flow restriction valve is added to regulate the speed of the outstroke.
3
2
1
Figure 2.21
“OR” Operation of a single-acting cylinder
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Design and Applied Technology (Secondary 4 - 6)
b. Logic AND function for a single acting cylinder
This function is commonly known as an interlock function for safety purpose. Two 3/2 NC
valves are connected in series. Both valves need to be pressed together for the outstroke
operation of a single acting cylinder. The first valve can be a mechanical plunger operated
valve to make sure that the safety door is in place for common machine operation. Then, the
second valve may be the manual switch pressed by the operator. The outstroke of single
acting cylinder can be made only when?? both valves are actuated.
2
1
Figure 2.22
Interlock and “AND” function of a single-acting cylinder
c. Inverse Operation NOT function
A 3/2 manual actuated NC valve is used to control a 3/2 NO valve for the inverse operation of
single acting cylinder. The cylinder is in an outstroke condition as an initial position. When
the NC valve is pressed momentarily, the cylinder will retract at a regulated speed, and a flow
restriction valve is connected in the exhaust line. It may be used for an unlocking function of
some mechanical devices.
44
Design and Applied Technology (Secondary 4 - 6)
2
1
Figure 2.23
Signal inversion of a cylinder
d. Direct control of a double acting cylinder
A 5/2 directional valve is used for a basic operation of a double acting cylinder. The cylinder
is in a retracted position when the spring takes the control of that valve. Once the valve is
pressed momentarily, the cylinders will outstroke. The speed of both directions can be
regulated independently by two flow restriction valve.
1
Figure 2.24
Direct control of a double acting cylinder
e. Bi-stable control of a double acting cylinder
Two 3/2 NC valves are used for the control of 5/2 valve and in turn control the double acting
cylinder. When valve 1 is pressed momentarily, the cylinder will extend and hold its position
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Design and Applied Technology (Secondary 4 - 6)
even the valve is released. The cylinder will retract until the valve 2 is pressed momentarily.
The speed of both directions can be regulated by two flow restriction valves. This circuit is
also known as a “memory” function.
3
2
1
Figure 2.25
Memory function of double acting cylinder control
f. Automatic return of double acting cylinder
One of the two manual operated 3/2 NC valves is replaced by a roller actuated valve and is
put to the position at the end of the cylinder outstroke. When the valve 1 is pressed
momentarily, the cylinder extends. When the cylinder rod trips the valve 2 at the end of
outstroke, the cylinder will retract automatically.
Valve 2
3
2
1
Figure 2.26
Automatic control of a double acting cylinder
46
Design and Applied Technology (Secondary 4 - 6)
S T O P
1.
A N D
T H I N K
What will happen in the above circuit if valve 1 is still pressed even when valve 2 is
tripped at the end of the outstroke?
2.
Would you suggest any modification to the circuit to improve this situation?
g. Reciprocating strokes
Repeating strokes of a double acting cylinder can be designed by two roller-actuated 3/2 NC
valves. They are at both ends of stroke. A 3/2 manual operated valve is connected in series
with one of the roller-operated valve to perform an interlock (AND) function as a “manual”
switch to start the automatic reciprocating stroke.
Valve 4
Valve 2
3
2
4
1
Figure 2.27
H
I
G
Reciprocating stroke of a double acting cylinder
H
L
Working principle of electromagnetic relay
47
I
G
H
T
Design and Applied Technology (Secondary 4 - 6)
Figure 2.28
Schematic diagram of electro-mechanical relay principle.
An electromagnetic relay is an electrical switch that can be actuated indirectly by another
switch. When the coil is energized, current flows through the coil and generates an
attractive electromagnetic force. The armature is pulled down, causing the lever arm to
close the contacts. When the coil is de-energized, the spring pulls the lever arm down and
opens the contacts. Electromagnetic relay can be designed with normally open contacts or
with normally closed contacts. It is a commonly used device in electro-pneumatics and PLC
applications.
1.
Functions of Electromagnetic relay
It is an electrical switch with a high current rating that is indirectly operated by
a low control current
It acts as an interface between the low signal levels (5-12V) from controllers to
high current rating devices
It can provide contact points for one to many
It can provide contact points from normal close to normal open or vise versa
2.
Limitations of Electromagnetic relay
It contains moving parts and electrical contacts; it has limited operating speed,
reliability and lifespan
It is big in size and requires large mounting racks in application
Each relay can only provide a small number of contacts
It is very difficult to change the control function of a relay system once it is
connected up. A complete re-wiring may be necessary for changing control tasks
Due to limited lifespan, the replacement and maintenance involves high cost
3. Latching function of Electromechanical relay
It can be connected to an electro-pneumatic circuit for the control of solenoid valves and the
control function of Programmable Logic Controller (PLC), an automatic pneumatic system
can be realized.
The latching function of electromagnetic relay can be done by connecting the power line to
one of the contact pins. Once the coil is energized, the contact closes and allows power to
48
Design and Applied Technology (Secondary 4 - 6)
the coil. When the ON switch is turned off, the coil remains energized by the power line. An
OFF switch is connected to “reset” the relay.
1
2
ON
R1
OFF
OFF
R1
+
ON
R1
Figure 2.29
+
2
(left) Electrical circuit diagram; (right) Electro-pneumatic circuit of
“latch” relay
(IV) Electro-pneumatic
a. Solenoid valve
For electro-pneumatic applications, there are two types of solenoid valve: double solenoid and
single solenoid.
For double solenoid valve, the cylinder will extend if one of the solenoid is energized. The
extended cylinder will hold its position by either leaving the solenoid in the energized state or
un-energized state. When energizing the opposite solenoid, the cylinder will retract.
For a single solenoid valve, the valve will shift back to its original position and the cylinder
will retract when power is turned off.
Figure 2.30
(left) single solenoid valve;
(right) double solenoid valve
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Design and Applied Technology (Secondary 4 - 6)
b. Major symbols for Electro-pneumatic circuit
Operation
ISO Symbol
N.O. spring return ,2 points contact
N.C. spring return ,2 points contact
N.O. roller-operated, spring-return contact
N.C. roller-operated, spring-return contact
N.O. Solenoid controlled proximity contact
Control relay
Solenoid
Table 2.4
ISO symbols of Electro-pneumatic circuit
c. Electro-pneumatic circuit
(i)
Manual control
Manual control (push button) of a double-acting cylinder can be done by using solenoid
valves and a 5/2 valve respectively. When pressing the pushbutton, it energizes the solenoid to
extend the cylinder. Once the pushbutton is released, the cylinder retracts by the spring force.
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Design and Applied Technology (Secondary 4 - 6)
Pb1
A
1
A
Figure 2.31
Manual operation of a double-acting cylinders
(ii) “OR” control
For electro-pneumatic system, two or more pushbuttons can be used to control the actuation
of cylinders through an “OR” logic control without the problem of air leakage through
switches as in pneumatic system.
Pb1
A
1
Pb2
2
A
Figure 2.32
“OR” operation of double-acting cylinders
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Design and Applied Technology (Secondary 4 - 6)
(iii) Multiple contacts
When pressing either the pushbutton of pb1 or pb2, the solenoid will be energized and the
lamp will be turned on as shown in Figure 2.33
In Figure 2.34, when pb1 is closed, it energizes control relay R1 which will then turn on the
lamp and solenoid valve (A) to extend the cylinder. When pressing pb2, only the solenoid (A)
is energized to extend the cylinder but it does not turn on the lamp.
Pb1
1
R1
Pb2
2
A
Figure 2.33
Control of cylinder and lamp by either pb1 or pb2
Pb1
R1
1
R1
2
R1
A
3
A
Pb2
4
Figure 2.34
Control of cylinder and lamp by pb1; pb2 for controlling cylinder only.
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Design and Applied Technology (Secondary 4 - 6)
(iv) Control of Slide Unit
Double solenoid valve has a bi-stable characteristic. When Pb1 is pressed, the slide moves
from left to right and is held in that position until Pb2 is pressed.
Pb1
S+
1
Pb2
S-
2
S+
Figure 2.35
S-
Control of Slide unit movement.
Figure 2.36
Slide Unit
(v) Control of rotary actuator
The rotary actuator has a built-in positional switch at the limit. When the START (pb1) button
is pressed, it energizes solenoid S+, the rotary actuator turns from left to right and is held in
that position. Once the START button is released, the “triggered” positional switch a1
energizes the solenoid S- immediately and the rotary actuator returns to the left automatically.
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Design and Applied Technology (Secondary 4 - 6)
Pb1
S+
1
a1
S-
2
S+
Figure 2.37
S-
Automatic return of rotary actuator
Figure 2.38
Rotary Actuator
(vi) Reciprocating movement of linear drive
Slide actuator usually has positional switches at the ends of movement limit. When the
START button is pressed and held in this position, the side will move back and forth because
of the alternate switching of positional switches ao and a1. ao and a1 are used for energizing
the S+ and S- solenoids for controlling the forward and backward movements respectively.
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Design and Applied Technology (Secondary 4 - 6)
Start
a0
S+
1
a1
S2
S+
Figure 2.39
S-
Reciprocating movement of Linear drive
(vii) Time delay circuit
A double acting cylinder can be reciprocating. The time of action can be adjusted by delay on
timer. In case of power failure, the cylinder will keep on moving to the end and held in this
position until the power resumes.
The double acting cylinder has two built-in positional switches, a0 and a1, at both ends of its
stroke. In Figure 2.40, the normal open a0 is closed in the initial position. When the START
button is pressed, the timer T1 is energized. The timer will in turn energize the solenoid S+ to
extend the cylinder when the preset time is over.
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Design and Applied Technology (Secondary 4 - 6)
a0
a1
a0
Start
T1
1
T1
2
S+
2
a1
S-
S+
3
Figure 2.40
S-
Double acting cylinder with delay on timer in extend stroke
In Figure 2.41 a0 is closed in the initial position as it is triggered in the retract stroke. When
START button is pressed, T1 is energized and cylinder will extend after a certain time of delay.
When the cylinder rod is fully extended, a1, as a positional switch, will be triggered. It will
energize another delay on timer. After a preset period of time, the solenoid S- will be
energized. The directional valve will move to the right and the cylinder rod will retract.
a1
a0
Start
a0
1
T1
2
T1
S+
2
a1
T2
3
4
S+
T2
S-
S+
4
Figure 2.41
Double acting cylinder with delay-on timer in both extend and retract stroke
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Design and Applied Technology (Secondary 4 - 6)
(viii) Sequential control of electro-pneumatic system
For simplicity, cylinder is omitted in the illustration. R1 is a normal close control relay. When
START button is pressed, T1 is energized and the LED is on. After the preset period, R1 is
energized by T1 in line 3, the normal close R1 become open. The LED will then be turned off.
At the same time, control relay R1 in line 4 is closed and T2 is energized. After the preset
duration of T2, T2 opens and de-energize R1 in line 1, R1 in line 1 backs to its NC state and
the LED turns on again. This sequential operation repeats itself.
Start
R1
T1
1
2
T1
R1
3
R1
T2
4
T2
Figure 2.42
E
X
Control of bi-stable LED flashing
A
M
P
L
E
Take a game kiosk in an amusement park as an example, a pneumatic 2-fingers gripper is
installed in a double acting slide. The actions are as follow:
1.
2.
3.
4.
5.
6.
7.
A 2-fingers gripper is open in initial position
The gripper is attached to the double acting slide
The gripper and the slide are driven by pneumatic but controlled by PLC
The slide moves to the right from left when the START button is pressed
When it moves to the preset position, the gripper will close and try to pick up a toy
The slide will move to the initial position with the gripped closed
At the end of the return stroke, the gripper finger will open and drop the toy.
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Design and Applied Technology (Secondary 4 - 6)
For simplicity, the downward movement of gripper for picking up a toy is omitted. a0, a1, b0
and b1 are positional switches for slide and gripper respectively. bo is closed in its initial
state and the gripper is in open position. When the START button is pressed, A+ is energized
and the slide extends to the left. At the end of extend stroke, a1 is triggered and energizes the
solenoid B+. With the energizing of B+, the 2-fingers gripper closes. When b1 is triggered
by closing the gripper, it in turn energizes A-. The slide is then retracted. After it is fully
retracted, a0 is closed and in turn energizes B- to open the 2-fingers gripper.
Start
b1
A+
1
a1
B+
2
b1
A3
a0
B4
a1
A+
a0
b1
A-
B+
b0
B-
Figure 2.43 Sequential controls of pneumatic slide and gripper
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Design and Applied Technology (Secondary 4 - 6)
2.5 Electro-pneumatic Systems
This section introduces the use of electro-pneumatics parts that can be controlled by
Programmable Logic Controller (PLCs). This will also be discussed in Chapter 3.
When the electro-pneumatic solenoid receives an electrical signal from the electronic
controller, which can be either PC or PLC. Practically, the input signal can be accepted in
form of 0 - 20mA DC or 0-10V DC. With this input, the unit regulates the high pressure air
supply to the final control element, such as control valve, to give a corresponding
displacement.
(I) Applications of pneumatic/electro-pneumatic systems
Pneumatic and Electro-pneumatic control systems are widely adopted in process control and
the production line automation in manufacturing industry.
a. Materials Transfer
Figure 2.44
Examples of material transfer application
In Figure 2.44 (left) a tailor-made container is attached to the end of the cylinder rod to collect
the pre-cut work pieces that are coming progressively from the conveyor belt. As the work
pieces fill up the container, the cylinder has to move down (extend) progressively to collect
the new coming work pieces at an appropriate height. The timing of the cylinder displacement
must synchronize with the conveyor speed. This is achieved after careful calculation and a
number of trial runs in the production line. The cylinder is fixed to an electro-pneumatic slide
unit that moves back and forth to serve other conveyor belts.
For figure 2.44 (right), it is a CD production line. There are two cylinders in this transfer
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Design and Applied Technology (Secondary 4 - 6)
mechanism. One cylinder steadily pushes the CD from the conveyor belt to the material
magazine. Another cylinder that connects to the magazine needs to vertically move down
(retract) progressively to allow each CD feed to the empty slot every time.
S T O P
A N D
T H I N K
For the two cases above, please suggest with reasons for (a) what types of cylinder to be
used? (b) Should air-piloted or solenoid control valves be used in this application?
b. Testing of limit switch
Figure 2.45
Testing of limit switches
The pneumatic systems above are used for the automatic testing and transfer mechanism. The
linear drive on the left is used to trip the limit switch for functional or failure test. Limit
switches are moved along the conveyor belt. This action is tedious and requires constant
magnitude of force for testing. Only automation can do this job at a standard and efficient
way.
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Design and Applied Technology (Secondary 4 - 6)
c. Work piece ejection in punching machine
Figure 2.60
Punching Machine Accessory Function
For a conventional metalworking punching machine, a continuous supply of compressed air is
needed to “blow” the finished work pieces off the way. It will cause wastage of compressed
air if it is supplied continuously during the punching action. To improve this situation, a
2-way solenoid valve and an opt-electrical switch will work together. The control valve will
actuate only when the dies are open, the compressed air will then “blow” out of the nozzle to
push the finished parts out of the way for collection. That will save 50% volume of
compressed air.
S T O P
A N D
T H I N K
One of the advantages of pneumatic system is theoretically of no cost at all as the supply of
air is free in the atmosphere. Therefore, in the above case, why engineers need to design an
automatic system to reduce the wastage of compressed air?
(I) Safety Consideration of Pneumatic systems
1.
2.
3.
4.
5.
6.
7.
Contaminants of solid particles may come from damaged pump and valve sealing
Liquid contamination may come from oil, water, or cleaning solvents
A loose fitting or damaged hose allowing contaminants into the system by-passing
the filter
Worn out, misused, or incorrectly routed hoses
Use the right voltage for solenoid valve
Calculate the correct air pressure to activate your device
Use pressure relief valves
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Design and Applied Technology (Secondary 4 - 6)
8. Repair air leaks immediately
9. Wear safety glasses when working with pneumatic cylinders
10. Protect equipment and patrons with a good safety envelope.
(II) Advantages of Pneumatics
1.
2.
3.
4.
5.
6.
7.
High efficiency. A relatively small compressor can fill a large storage tank to meet
intermittent high demands for compressed air
Unlike hydraulic systems, no return lines are required. The used compressed air can
be released through the exhausted ports to atmosphere without harm
High reliability, because of fewer moving parts
Low cost, easy installation and maintenance
Availability of components of wide range of standard sizes and ratings
Air devices create no sparks in explosive atmospheres. They can also be used under
wet conditions with no electrical shock hazard
The design problems involved are usually not too difficult to solve, and equipment
selection procedures are relatively simple and straightforward. Installation is
relatively simple because of relatively low power and light duty.
(III) Limitations of Pneumatics
1.
2.
3.
4.
5.
6.
Difficult to perform speed control in pneumatics cylinders. Load line and return
(exhaust) line will be required in this case as flow restriction devices are installed.
Usually no return line is needed in pneumatics circuits as compressed exhaust air can
release into working environment with no harm and damage
Additional drying and filtering systems are needed as dust and moisture needed to
be removed before entering into the pneumatic systems. Dust and moisture will
accelerate wearing of pneumatic devices
Compressed air will inevitably cause noise pollution. It will be a concern if it is used
in laboratory or clinical applications
Leakage of air is unavoidable in compressed air system. Routine maintenance and
troubleshooting are required for pneumatic systems
The compressed air is compressible. The accuracy, say, the stroke of cylinder, will be
deteriorating and varied over times. Adjustment needs to be done after certain period
of servicing
The power rating is limited as the compressible nature of air. Some critical situation,
the pneumatics systems will not be applied, such as the life-saving ladder in a fire
engine. The power rating and the speed cannot reach the requirement.
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Design and Applied Technology (Secondary 4 - 6)
Quizzes (chapter 2)
1.
2.
3.
4.
5.
6.
7.
8.
9.
What are the limitations of using Pneumatics?
Why elevator in fire engine cannot be driven by Pneumatics?
What are the limitations of using a single acting cylinder?
Suggest methods to detect the leakage of compressed air in a manufacturing plant.
What are the major reasons of using Electro-pneumatics?
Describe the differences between Solenoid and Electromechanical relay.
Why some application prefer to use a non-lubricated pneumatic system?
Why two 3/2 NC and spring return valve cannot be used to directly control a double
acting cylinder?
Why a shuttle valve must be present to perform a OR logic function?
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Design and Applied Technology (Secondary 4 - 6)
CHAPTER 3 - PROGRAMMABLE CONTROL SYSTEMS
This chapter contains topics on:
3.1
3.2
3.3
3.4
3.5
What is Programmable Logic Controller (PLC)?
Programming the PLC
Application of Ladder Logic Diagram
Programmable Interface Controller
Stepper Motor and Servomotor
These topics include learning materials that facilitate you to:
Understand the operation of PLCs
Understand the Ladder Logic Function
Understand the programming of PLCs
Understand the use of PLCs in control.
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Design and Applied Technology (Secondary 4 - 6)
3.1 What is Programmable Logic Controller (PLC)?
Figure 3.1
PLCs of different brands
A programmable logic controller is defined by the National Electrical Manufacturers
Association (NEMA) as:
A digitally operating electronic apparatus which uses programmable memory for the internal
storage of instructions for implementing specific functions such as logic, sequencing, timing,
counting, and arithmetic to control, through digital or analogue input/output modules,
various types of machines or processes.
(I) Why Programmable Logic Controller (PLC) is needed?
Basically system automation and process control require at least an "on/off" control in modern
commerce and industry application. These control systems are no longer built from
electromechanical relays, switches, timers, counters and other discrete logic gates to perform
sequential control. It is because they are all hard-wired for specific purpose or controlling
specific machines. These types of dedicated systems are of no flexibility. Therefore, digital
devices, which can be programmed to do a variety of logical functions, play an important part
in our daily industrial automation.
The Programmable Logic Controller (PLC) was invented in 1960s to replace the sequential
relay circuits that were commonly used in machine control. PLC is a solid-state, electronic
device that controls the operation of a machine. It uses logic functions, which can be
programmed into its built-in memory via software. This program can be changed or modified
when necessary.
(II) Basic component of the PLC
A typical Programmable Logic Controller contains the following major components:
Input module
Output module
Processor
Memory
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Design and Applied Technology (Secondary 4 - 6)
Power supply
Programming device
Interface
A schematic diagram is shown below:
Memory
Power
Supply
Processor
Input
module
Output
module
Handheld
Programmer
Figure 3.2
Schematic diagram of the basic Programmable Logic Controller
a. Inputs devices
The inputs to the controller are signals from limit switches, pushbuttons, proximity sensors,
and any other digital (binary) or analog (continuous) devices.
b. Output devices
The outputs from the controller are on/off signals to the operating motors, cylinders, relays,
solenoid valves and any other actuators.
c. Input and output module
Input and output modules are the I/O connections to the industrial process that is to be
controlled.
d. Processor
The processor is the Central Processing Unit (CPU) of the programmable controller. It
executes various logic and sequencing functions.
e. Memory
It consists of input, output and flag memory. Its storage capacity may ranges from 1K to 48K.
It contains the program of logic, sequencing and other input/output operations.
f. Power supply
A power supply of 115Vac or 220Vac is typically used to drive the PLC. The components of
industrial process that are controlled by PLC can have a higher voltage and power rating than
the PLC.
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Design and Applied Technology (Secondary 4 - 6)
g. Programming device
The programming device is usually detachable from the PLC cabinet. It may be either a
hand-held programmer, similar to those used in robotics, or a computer-based programming
package, using PC to input program into the PLC memory.
Figure 3.3
Functionality of PLC components
(III) How the PLC operate
Programmable Logic Controllers (PLCs) work by continually scanning a program. This scan
cycle consists of 3 important steps: (1) checking input status, (2) executing the program, and
(3) updating output status.
Step 1
Step 2
Scan Time
Step 3
Figure 3.4
Scan cycle of PLC
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Design and Applied Technology (Secondary 4 - 6)
Step 1—Check Input Status
Firstly, the input signals to the PLC, through the input module, are sampled by the processor.
The contents are stored in the input memory.
Step 2—Execute Program
The control program is executed by the processor. The input values stored in the input
memory are used in the control logic operation to determine the values of outputs.
Step 3—Update Output Status
Finally, the PLC updates the status of the outputs which is based on the input values and the
results of executing the control program.
Scan and Scan time
A “scan” is referred to the cycle of reading the inputs, executing the control program and
updating the outputs.
A “scan time” is referred to the time taken to complete one scan. The time vary from 1 to
100ms. It depends on the number and complexity of control functions to be performed for
each scan cycle. This means it depends on the number of rungs in the ladder diagram and the
complexity of the logic operation to be carried out on each rung.
S T O P
A N D
T H I N K
What will happen if the value of input changes immediately after it has been sampled during
a scan cycle?
(IV) Advantages of Programmable Logic Controller
Programming the PLC is easier than wiring the relay control panel.
The PLC can be reprogrammed. Conventional controls must be rewired.
PLCs take less floor space than relay control panel.
Maintenance of PLC is easier and reliability is greater.
The PLC can be connected to digital systems more easily than relays.
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Design and Applied Technology (Secondary 4 - 6)
Below is the comparison between Relays logic control and PLC.
Relays
PLCs.
Large complicated systems that take up
One PLC can control a large system. Takes up less
a lot of space.
floor space than a relay-based system.
Hard-wired devices are used to
Only the input and output devices are hard-wired.
configure relay ladder.
The inner configuration of PLC is solid-state.
Difficult to modify or update a program
By the programming software, it becomes simple to
write a new program (or modify an existing one),
and download it into the PLC.
Limited service of life for mechanical
The PLC is a solid-state device which has the
devices.
characteristic of
long service life and little
maintenance
Require separate hard-wired timers and
Counters and timers are internal solid-state devices
counters
Table 3
H
I
Comparison between Relays and PLCs
G
H
L
Coil
I
G
H
T
Core
Spring
Direction
of force
when coil
is energized
Switch
+
Figure 3.5
Working principle of solenoid
A solenoid is an electromagnetic actuator that can be used to open and close a valve,
electrical contact, or other mechanical devices. The solenoid operates by means of an
electrical current flowing through a wire coil to produce magnetic field inside the coil. A
mechanical spring causes the core to be retracted out of the coil when the electrical
current is turned off.
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Design and Applied Technology (Secondary 4 - 6)
S T O P
A N D
T H I N K
What will happen to the above solenoid design in case of power failure during the course
of control? Would you suggest any modification to alleviate this problem?
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Design and Applied Technology (Secondary 4 - 6)
3.2
Programming the PLC
PLCs is relatively easy to program. The programming language is designed to resemble
ladder logic diagram. It is designed for an industrial electrician or electrical engineer who is
accustomed to reading ladder logic schematics in electromechanical control relay. The
learning curve becomes short for programming a PLC to perform the same logic control
functions.
(I) Constructing a Ladder Logic Diagram
The power (e.g.110V AC) to the components is provided by two vertical rails. The left rail is
the power rail and the right rail is the ground bus. The horizontal line is the “rung”. It is
common to locate inputs and outputs to the left and right of each rung respectively. Power
flows through a series of normally open or normally closed contacts. It powers a coil from left
to right and top to bottom over a ladder diagram.
(II) Symbols of components in ladder diagram
Normally open contacts are symbolized by two vertical lines along a horizontal rung.
Normally closed contacts are shown by a diagonal line across a normally open contact. Both
types of contacts represent ON/OFF inputs to the ladder logic. These inputs can be limit
switches, relays, photo-detectors and any binary contact devices.
The output loads can be represented by circles in the rung. The output loads are motors, lights,
alarms, solenoids and any other electrical components.
The timers and counters are symbolized by squares on the right of a rung. When the input
signal is received, the timer waits the specified delay time before switching on the output
signal. The timer is reset by turning off the input signal.
Counters require two inputs. The first is a pulse train that is counted by the counter, The
second is a signal to reset the counter and restart the counting procedure.
Resetting the counter means zeroing the count for a count-up device, and resetting the starting
value for a count-down device. The accumulated count can be stored in the flag memory for
use if required by the application.
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Design and Applied Technology (Secondary 4 - 6)
Ladder Symbol
Functions
Normally open contacts
(switch, relay, other
devices)
ON/OFF
Normally closed contacts
(switch, relay, other
devices)
ON/OFF
Output loads
(motor, lamp,. Solenoid, alarm, etc)
Timer
TMR
s
Counter
CTR
Table 3.1
Symbols of common components in ladder diagram
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Design and Applied Technology (Secondary 4 - 6)
(III) Ladder control logic
Input contacts can be arranged to conduct logic functions in the ladder diagram.
Function
Ladder
Truth Table
AND
X1
X2
Y
OR
X1
X2
Time Chart
X1
X2
Y
0
0
0
0
1
0
1
0
0
1
1
1
X1
X2
Y
X1 AND X2
X1
0
0
0
0
1
1
1
0
1
1
1
1
73
(X1.X2)
X2
Y
X1 OR X2
X1
Y
Boolean
X2
Y
(X1+X2)
Design and Applied Technology (Secondary 4 - 6)
NOT
Y
C1
X1
C1
Y
0
0
1
1
1
0
NOT X1
X1
C1
Y
X1
NAND
X1
X2
Y
X1
X2
Y
0
0
1
0
1
1
1
0
1
1
1
0
X1 AND X2
X1
74
X2
Y
(X1.X2)
Design and Applied Technology (Secondary 4 - 6)
NOR
X1
X2
Y
X1
X2
Y
0
0
1
0
1
0
1
0
0
1
1
0
X1 OR X2
X1
Figure 3.6
X2
Y
Ladder logic diagram
75
(X1+X2)
Design and Applied Technology (Secondary 4 - 6)
3.3 Application of Ladder Logic Diagram
(I) Starting and stopping an electric motor
Stop
Start
K1
K1
K1
Figure 3.7
M1
Motor ON/OFF control (Industrial 3 phase motor)
Pushbutton is used for starting and stopping an electric motor - one for START and the other
for STOP. When the Start button is pressed momentarily by a human operator, power is
supplied and maintained for the motor until the STOP button is pressed.
The operation logic is as follow:
START
STOP
MOTOR
0
0
0
0
1
0
1
0
1
1
1
0
Table 3.3
Truth table of Motor ON/OFF control
X1 and X2 are input contacts for START and STOP respectively and K1 is the output load to
represent MOTOR. Y serves as a latch function to maintain the power to motor when the
START button is released.
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Design and Applied Technology (Secondary 4 - 6)
Start
Stop
K1
K1
Figure 3.8
Ladder diagram of motor ON/OFF control
For a better understanding on the differences between hard-wired logic control and
programmable control, a logic circuit design for the same motor ON/OFF control application
is provided below for comparison.
Stop
Motor
control relay
Power to
Motor
Start
K1
Figure 3.9
Hard-wired logic circuit for the same motor ON/OFF control
(II) Use of control relay for alternate switching of motors
A relay can be used to control ON/OFF actuation of a powered device at remote location. A
control relay in different rungs of a ladder diagram is used to serve multiple logic function.
The output load (control relay, C) on one rung in a ladder diagram can be inputs for other
rungs.
When the normally opened contact X is open, the relay is not energized. The output motor Y1
in the second rung is connected to the power line. The motor Y1 turns on. When the contact X
is closed, the normally closed control relay in second rung will open and the normally opened
contact in third rung will close. The motor Y1 and Y2 will turn off and on respectively.
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Design and Applied Technology (Secondary 4 - 6)
Figure 3.10
X
C
C
Y1
C
Y2
Use of control relay for multiple logic function
(III) Level control in sewage treatment tank
X1
Control Relay
Manual
START
C1
Float
Switcch
FS
S1
150s
Timer T1
Control
Valve
C2
Control Relay
60s
Timer T2
S2
Control
Valve
Figure 3.11
Schematic diagram of fluid level control
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Design and Applied Technology (Secondary 4 - 6)
When the START button X1 is pressed, it energizes the control relay C1. C1 in turn energizes
the solenoid S1 which is used to actuate a motorized valve, allowing sewage flows into the
storage tank. When the level rises to a certain level, the float switch FS will close. This will
open C1 and in turn de-energize S1 to stop the sewage from flowing in.
FS also energizes other control relay C2 which in turn energizes a timer T1 to provide a 150s
time delay for the chemical reaction to take place. At the end of time delay, C2 powers the
solenoid S2 which actuates another motorized valve to drain the tank. At the same time, C2
also initiates another timer T2 to allow a delay of 90s for the drainage to complete. At the end
of 60s, T2 opens and de-energizes C2 and thus de-energizes solenoid S2 and stops the out
flow.
X1
FS
C1
C1
C1
FS
S1
T2
C2
C2
C2
T1
TMR
150s
T1
S2
T1
T2
TMR
60s
Figure 3.12
Ladder logic diagram for fluid level control
Exercise 1 : Sequential control of drill automation
The upper limit switch L1 (normally opened contact) is closed at the beginning of the drilling
cycle. The START (normally open contact) button is pressed momentarily to start the drilling
cycle. At the same time, the output load motor M1 starts rotating the drill and the other motor
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Design and Applied Technology (Secondary 4 - 6)
M2 starts to descend the drill. The drill (M1) will stop at the lower limit switch L2. At this
time, the motor M2 starts to reverse and ascend the drill.
Upper
Limit Switch
L1
M2
M1 Drill Motor
Vertical
Motor
for Up and Down
motion
Workpiece
L2
Lower
Limit Switch
Figure 3.13
Schematic diagram of automatic drilling system
Limit1
START
Motor1
Limit2
Motor2
Down
Motor2
Up
Figure 3.14
Timing diagram for sequential control of automatic drilling system
Exercise 2: Control of traffic light
Pushbutton S1 is pressed momentarily to start a traffic light cycle. The Red signal will be ON
for 5s. Then, followed by the Yellow signal for 2s. Final, the Green will be ON for 8s. The
cycle will be reset and wait until the start button is pressed again.
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Time T1 = 5s
Time T2 = 2s
Time T3 = 8s
Figure 3.15
Traffic light control
(IV) Entry of ladder logic diagram into the PLC
The ladder logic diagram is directly entered into the PLC memory. It requires PC-based
application software or a handheld programmer with limited graphics user interface to display
the symbols in the ladder diagram. The ladder diagram is inputted to the PLC memory rung
by rung. Below is an example of ladder logic program entry by a handheld programmer to a
typical PLC.
Figure 3.16
Handheld Programmer for PLCs
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Boolean
Ladder Diagram
Address
X0
X1
Instruction
Handheld
Programmer
0
ST X0
ST.ST.0.WRT
1
ST X1
ST.ST.1.WRT
2
AN X2
AN.ST.2.WRT
3
OR
OR.STK.WRT
4
OT Y1
OT.AN.1.WRT
Y1
X2
Table 3.4
Example of inputting ladder diagram into PLC
Remarks: Command button ST is START, AN is AND, OR is OR,OT is Output, STK is Stack,
WRT is Write.
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3.4 Programmable Interface Controller (PIC)
Programmable Logic Controllers are usually used for industrial and electrical application.
They are rather bulky and expensive for use in school. An alternative solution is to introduce a
controller based PIC for controlling tasks in student projects.
(I) Introduction of PIC
A Programmable Interface Controller or PIC, contains a microprocessor and EEPROM. PIC
can be provided with 8, 18 and 28 pin configurations which provide a variety of outputs and
digital/analogue inputs.
The chips use reprogrammable 'flash memory' which can be written and rewritten.
Constructing a working controller involves connecting the chip to power, interfacing input,
output components and adding a capacitor, resonator and a reset switch.
'Flash memory’ is EEPROM (Electrically Erasable Programmable Read Only Memory). It
means that the PIC is capable of being re-programmed over 10,000 times.
The most commonly used PIC is the 16F84 shown below. This is a 18-pin device which has 8
outputs and 5 inputs.
Figure 3.17
S T O P
Pin layout diagram of PIC 16F84
A N D
T H I N K
Why EEPROM memory is used for PIC?
(II) Configuration of PIC 16F84
The 16F84 requires a 6V DC supply. This can simply be provided by 4 x AA cells. A 4MHz
ceramic resonator must also be connected as shown below. The 16F84 provides an internal
clock pulse. The resonator is used to regulate the speed of the clock pulse (4MHz).Pin 4 (reset)
and must be connected via a 4k7 resistor to +V.
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(III) Interfacing Input and Output Devices
a. Digital input sensors
Common input devices are Micro-Switches, Reed Switches, Tilt Switches and Push Switches.
They can all be directly connected to any input pin of PIC 16F84. Care should be taken to add
10k resistor to prevent short circuit and the 1k resistor to protect the input pin. The digital
input will trigger from logic 0 to logic 1 when the switch is pressed.
Limit switch
Reed Switch
Figure 3.18
Figure 3.19
Tilt Switches
Toggle and Push Switches
Common digital input device to PIC
Circuit diagram for connecting input device
b. Analogue input sensors
Although the 16F84 does not have analogue input pins, an analogue sensor can also be used
by connecting them via a potential divider and a transistor interfacing circuit. A
phototransistor can be used to switch the input directly.
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Design and Applied Technology (Secondary 4 - 6)
Figure 3.20
Interfacing circuit for analogue photo-sensors
c. Output devices
Buzzer
Light bulb
Figure 3.21
Solenoid
Common output devices
Common output devices are LED, 7 segment display, Piezo sounder, speaker, light bulb and
solenoid.
An LED can be driven directly from any output. The 330k resistor serves the functions of
protecting the input pin from the risk of short circuit in the event of LED blows and prevents
the LED from blowing out by the over-current.
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Design and Applied Technology (Secondary 4 - 6)
Figure 3.22
Output circuitry for LED
7 segment displays can display the number from 0 to 9 by outputting logic 1 or high signal to
the display segment in correct sequence.
Figure 3.23
Output circuitry for 7 segment display
A Piezo speaker can be directly connected to the output pin to produce a range of sounds
because of its high internal resistance. The sounds are produced by the output signals that are
in form of pulses and at a variety of frequencies. The frequencies can be generated by the
programmed instructions.
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Design and Applied Technology (Secondary 4 - 6)
Figure 3.24
Output circuitry for Piezo speaker
Higher current devices cannot be driven directly by the output signal from PIC which is
usually as low as 6V DC. It requires a simple transistor-to- transistor pair switching circuit
(the Darlington driver) to amplify the current rating. It is significant when the PIC is required
to control electrical actuators in industrial application, the power rating is usually higher, say
24V DC or more. A relay is also a common device used to do this type of switching to drive
the output load of different or higher voltage and current rating.
Figure 3.25
Output circuitry for buzzer
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Design and Applied Technology (Secondary 4 - 6)
Figure 3.26
Figure 3.27
Output circuitry for Light bulb
Output circuitry for Solenoid
(IV) Programming PIC
The PIC is programmed using a “compiler” or “assembler” which is a high-level
programming language. However, they are still not easily to be understood by senior
secondary students. It can be made easier to program this PIC by a program editor which uses
flowchart approach. The editing software has graphical user interface and adopts “drag and
drop” operation for programming. The software will convert the flowchart into BASIC or
Assembly language which will be downloaded to the PIC.
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3.5
Stepper Motor and Servomotor
Figure 3.28
Photo of a typical stepping motor
Stepping motor is an electric motor without commutators. All the windings in the motor are
the stator and the rotor is either a permanent magnet or a toothed block of magnetically soft
material. All commutations are handled externally by a motor controller.
The motor is designed so that it can be held in any fixed position or being rotated
bidirectionally. Most stepping motors can also be spinned quickly at audio frequencies.
Both stepping motor and servomotor can be used for precise positioning in industrial
application but they are different in working principle:
Servomotors require analog feedback control systems. This usually involves a
potentiometer to provide feedback about the rotor position. External circuits are
needed to drive a current through the motor proportional to the difference between
the desired position (set point) and the actual position.
Stepping motors can be used in Open-Loop control system. This is generally
adequate for systems that operate at low acceleration with static load.
Worktable
Pulse
train
Stepping Motor
Gear
Leadscrew
(a)
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Design and Applied Technology (Secondary 4 - 6)
Worktable
Input
+
Comparator
DAC
DC Servomotor
Gear
Leadscrew
Sensor
Feedback Signal
(b)
Figure 3.29
(a) Closed loop control of servomotor (b) Open loop control of stepping motor
(I) Stepping Motor Principle
Figure 3.30
Exploded view of a typical stepping motor
Stepping motor divides a revolution into discrete steps. It can be held standstill in a motor
position when not rotating and without the need for a positional feedback sensor. The steps
are created by sequentially energizing the stator electromagnets causing the rotor to line up
each time with the resultant magnetic field. The shaded area in the diagram indicates the
position of the energized magnets and the wedge indicates the angle rotated according to the
resultant magnetic field.
Figure 3.31 Alternate energizing stator for the rotation of rotor.
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Design and Applied Technology (Secondary 4 - 6)
Stepping Motors can achieve a short and precise movement with quick acceleration. No brush
maintenance is required. This greatly reduces its cost compared with any other brushed
motors. However, the rapid energizing of electromagnets is inefficient at high speed condition.
Any excessive loads can cause the motor to skip its sequence and stall in the worst case.
H
I
G
H
L
I
G
H
T
Control of stepping motor by PIC
Unipolar stepping motors have four coils which must be switched on and off in the correct
sequence to make the motor turn as shown in previous section. The table below shows the
correct sequence.
Step
Coil 1
Coil 2
Coil 3
Coil 4
1
1
0
1
0
2
1
0
0
1
3
0
1
0
1
4
0
1
1
0
5
1
0
1
0
Table 3.5
The ULN 2003A is a Darlington driver IC used to drive the stepping motor. The PIC (6V) and
stepping motor (12V) have different supply voltages but they must be common to ground 0V.
Figure 3.32
Control circuitry for stepping motor by PIC 16F84
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Design and Applied Technology (Secondary 4 - 6)
(II) Advantages of Servomotors
Figure 3.33
Servomotor with integrated feedback device
Servomotor has the following advantages when compared to other motor technologies:
fast positioning
high peak torques
wide speed ranges
high controllability
In general, servomotor is common in positioning applications. It provides a wide range of
power output that will be suitable for a variety of industrial applications, such as packaging,
material handling, laser trimming and automation where high throughput is of concern.
a. High peak torque and High productivity
Servomotor of same power rating offers peak torques of 200-400 percent over the continuous
duty torque. Higher peak torques, in practical terms, mean that the motor can accelerate and
position the load faster.
b. Light weight and Energy savings
Servomotors can fit in a tight and compact location. Smaller motor size means less weight. It
is significant in some applications where a load includes the motor itself, such as the joint in
robotic arm.
c. High controllability
Servomotors can offer excellent controllability. Servomotors are used with closed loop motion
controllers and can position accurately. Servos can position up easily to a diameter of one hair.
d. Fast Response and Reliability
A measure of controllability is bandwidth. This is an indication of response time. The higher
bandwidth, the faster the response. Comparing to other motor technologies, servos have the
highest bandwidth. This means that if there is a disturbance in loading, servos can make
corrections faster.
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Design and Applied Technology (Secondary 4 - 6)
e. Motor speed, voltage and current
Speed is directly proportional to the amount of voltage applied; the more voltage, the faster
the motor will operate. The torque delivered is directly proportional to the current. Just like a
conventional DC motor, the more the current, the more the torque to be delivered. Both
characteristics are very predicable, thus servos become easy to apply in any application.
S T O P
A N D
Can you suggest one application of
(I) servomotor and
(II) stepping motor with an appropriate reason?
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Design and Applied Technology (Secondary 4 - 6)
Quizzes (Chapter 3)
1.
2.
3.
4.
5.
6.
7.
8.
Name the 6 major components of a typical PLC.
What are the three main steps of each scan cycle performed by a PLC?
What are the drawbacks of using electromechanical relays in automated control?
Give a brief description of what a PLC is.
Can PLC be regarded as a computer? If not, why?
What is the history of PLCs?
Why PLCs are more commonly used for industrial applications than computers?
Convert the following logic diagram into a ladder diagram and write a Boolean
expression for it?
A
B
Z
C
D
9. What are the usage of Tilt switch and Reed switch?
10. List three advantages for stepping motors and servomotors?
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CHAPTER 4 – ROBOTICS
This chapter contains topics on:
4.1
4.2
4.3
4.4
4.5
Definition of Robots
Mechanical Structure of Industrial Robotic Arms
Robot Anatomy
Robot Control Systems
Applications of Robots
These topics include learning materials that facilitate you to:
Identify different mechanical structure of industrial robots
Identify different robot anatomy
Understand different robot control methods
Understand various applications of robots.
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4.1 Definition of Robots
Robot is officially defined by the Robot Institute of America as "A re-programmable,
multi-functional manipulator designed to move materials, parts, tools, or special devices
through variable programmed motions for the performance of a variety of tasks".
Figure 4.1
Typical industrial robotic arm.
Variations of robots are available for use in industrial applications. They are used to carry out
repeated actions with high accuracy and without any variation. Robots require a control
program to govern its velocity, direction, acceleration, deceleration and the distance of
movement at any time.
An Industrial Robot (IR), which is usually referring to a robotic arm, consists of several links
connected in series by linear, revolute or prismatic joints. At one end the robot is fixed to a
supporting base while the other end is equipped with a tool and manipulated into position to
perform tasks.
However, the definition of robots by Robot Institute of America seems to be focused on
industrial application, robots nowadays are quite popular for leisure and some of them are for
innovative purposes, such as humanoid and military purpose.
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Design and Applied Technology (Secondary 4 - 6)
Figure 4.2
Examples of different types of robotic applications
Therefore, robot can also be defined as "Human made semi or fully autonomous
(self-controlled) object or cooperating objects (with common objectives) with
intelligence which is programmable".
S T O P
A N D
Please state your reasons for the following questions.
1. Are movable machines, such as cars, "Robot"?
2. Are computers “Robot”?
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Design and Applied Technology (Secondary 4 - 6)
4.2 Mechanical Structure of Industrial Robotic Arms
Since 80’s of last century, there was a rapid growth in the application of robotic arms in
industry, especially in the car manufacturing and welding process. Identifying different types
of mechanical robotic structures seem to be an effective and visual way of classifying robots
and are a good starting point for learning robotics.
(I) Cartesian Coordinate Robot
Figure 4.3
Typical Cartesian Robot
Cartesian robot is formed by 3 prismatic joints, which axes are coincident with the X, Y and
Z planes. A Cartesian Coordinate robot with the horizontal member supported at both ends is
sometimes called Gantry robot. It can be quite large in size. Cartesian robot uses 3
perpendicular translational slides along the x, y , z axes, and it is also called the xyz robot or
rectilinear robot. It has a rectangular work volume. The joint types are obviously LLL.
Figure 4.4
Schematics diagram of Cartesian robots
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Design and Applied Technology (Secondary 4 - 6)
Advantages:
The rigid structure can support large robots and heavy payloads:
Easily controlled/programmed movements
High accuracy
Accuracy, speed and payload capacity are constant over entire working range
Control system simplicity
Familiar X, Y, Z coordinates can be easily understood
Inherently stiff structure
Large area coverage
Structural simplicity, offering good reliability
Easy to expand in modular fashion
Drawbacks:
The workspace is limited within the robot size. A big robot will require a very large
area if it works for a large work piece, such as a car
Work piece underneath is out of reach by this type of robot
Prismatic joints are easily contaminated with dust, especially in folds around the
flexible bellows
(II) Cylindrical Robot
Figure 4.5
Typical Cylindrical Robot
Cylindrical robot is able to rotate along its main axis forming a cylindrical work space. It has
two linear axes and one rotary axis around its base.
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Advantages:
Base rotation gives high speeds
Can reach under objects (compared with Cartesian robots)
Easily controlled/programmed movements
Control system simplicity
Good accuracy
Fast operation
Good access to front and sides
Structural simplicity, offering good reliability
Drawbacks:
Dust contamination around the flexible bellows is hard to avoid for prismatic joint
Relatively small workspace (compared with Cartesian robot)
Figure 4.6
Schematics diagram of Cylindrical Robot
Cylindrical robot can move up and down and around the column, and the arm can be
telescopic. It has a cylindrical work volume. The joint types are usually LTL or TLL.
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(III) Spherical / Polar Robot
Figure 4.7
Typical Polar Robot
A Polar Robot has one linear axis and two rotary axes. It is able to rotate in two different
directions along its main axes and the third joint moves in translation forming a hemisphere or
polar coordinate system. The joint type is usually TRL
Advantages:
Has a large workspace, and can reach below its base
Easily controlled/programmed movements
Familiar polar coordinates easily understood
Large payload capacity
Fast operation
Accuracy and repeatability at long reaches
Drawbacks:
Resolution is relatively low, and is variable over the workspace
The resolution is lower when the end effector is around the base - small change in
angle produce a large movement
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Figure 4.8 Schematics diagram of Polar robot
(IV) SCARA Robot
Figure 4.9
Typical SCARA Robot
SCARA (Selective Compliance Assembly Robot Arm) is a particular robot design developed
in the late 1970's in Japan. It is a version of articulated robot, where shoulder and elbow joints
rotate about vertical axis, and there is a prismatic joint at the shoulder for elevation.
The basic configuration of a SCARA is a four d.of.f robot with horizontal positioning, much
like a shoulder and elbow held perfectly parallel to the ground.
Advantages:
Fast cycle times and fast operation
Excellent repeatability and high accuracy
Relatively high payload capacity due to stiff structure in the vertical direction
Extremely good maneuverability and access within its programmable area
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Drawbacks:
Difficult to program offline
Highly complex arm in mechanical structure
Figure 4.10
Work space of polar robot
(V) Parallel Robots
Figure 4.11
Delta type (left); triceps type (mid) Hexapod type (right) Parallel Robots
Parallel robot uses three parallelograms to build a robot with three translational and one
rotational degree of freedom. The parallelograms ensure consistent orientation of one end of a
link with respect to the other. The rotational axis can only be provided by the end effector. As
the arms are parallel with each other, the weight of load is distributed over all three links.
Good examples of application are flying simulator and 4-D cinema.
Advantages:
Increased stability and arm rigidity
Faster cycle times than serial linked robots
End-of-arm errors are averaged over parallel link structure
Drawbacks:
Relatively large footprint-to-work space ratio
Small range of motion
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Figure 4.12
Schematics diagram of Parallel Robot
(VI) Articulated/ Revolute Robots
Articulated or Joint-Arm Robots are the most versatile robots available. It closely simulates
the natural form of human arm. Articulated robots are mechanical manipulator that looks
like an arm with at least three rotary joints. One joint is around the base (A1) and the others
are on the joints A2 and A3. In terms of a human arm, these can be compared to the shoulder,
bicep and forearm. A six-axis jointed robot includes the axis of the wrist (A4, A5 & A6),
known as pitch, roll and yaw respectively. With these extra axes added, this robot can move
the end effector to any point at any orientation in the workspace. The joint types used are
TRR.
Figure 4.13
Typical Articulated Robots for material handling application
Advantages:
The wrist can reach any position and orientation within the work envelope
It can reach areas that are difficult to be reached by other robots
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It is compact and provides the largest work envelope relative to their size
Extremely good working ability
Ability to reach over obstructions
Easy access to front, sides, rear and overhead
Large reach for small floor area
Slim design allowing easy integration into restricted workplace layouts
Fast operation due to rotary joints
Ability to traverse complex continuous paths
Drawbacks
Not easy to control
The motion of robot from one point to another can be difficult to visualize, as the
robot will move each joint through the minimum angle required
Accuracy is decreased due to the accumulation of joint errors
Control of motion is difficult due to the gravitational loading
Resolution control varies and has less increment at full reach
Pitch
Roll
Yaw
Figure 4.14
Schematic diagram of Articulated Robots
S T O P
A N D
T H I N K
1. Why Articulated robots are commonly used in welding process in industrial application?
Please state your reasons.
2. What are the limitations of Cartesian robots used in this application?
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4.3 Robot Anatomy
Robot Anatomy is to examine matters including types, sizes of joints, links and any other
aspects of a robot.
(I) Degree of Mobility, Degree of Freedom
A joint of an industrial robotic arm, especially the articulated robot, is quite similar to a
human arm. It provides relative motion or links between two parts of the robot.
Each link-joint pair is known as Degrees of Mobility (DOM).
Degrees of Freedom (DOF) is the number of independent movements the arm can make,
referring to the point of view of the end effector (e.g. a grasping hand).
Figure 4.15
Total 12 Degree of Freedom of an object
Each joint enables the robot with a certain number of Degree of Freedom (DOF) of motion.
Each joint can provide more than one DOF. For an industrial robot, there are commonly less
than or equal to 3 DOF in the body-arm assembly and up to 3 in the wrist assembly but it is
possible have any number of DOM. Another DOF may come from the end effector, such as
open and close of gripper.
DOF of an arm can be a combination of vertical, radial and translational movements
Vertical - ability to move up and down (z-axis motion)
Radial - extension and retraction (in-and-out or y-axis motion)
Rotational - rotation about the vertical axis (x-axis motion or swivel about the
vertical axis at the base)
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Design and Applied Technology (Secondary 4 - 6)
Figure 4.16
Combination of vertical, radial and translational movements
It is important to identify the number of DOF of a robot because the most important
specifications and criteria of selecting robots is to state the number of degree of freedom that
the robot possesses.
To establish the orientation of the object, the wrist assembly may have the following 3 typical
DOF configurations:
Pitch – type R joint for up-and-down motion of the object
Yaw – type R joint for right-to-left rotation of the object
Roll – type T joint to rotate the object about the arm axis
Figure 4.17
A typical wrist would have 3 DOF
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Design and Applied Technology (Secondary 4 - 6)
S T O P
A N D
T H I N K
1. What will be the number of mobility (DOM) and number of freedom (DOF) for a
telescopic arm which has 4 steps of extension?
DOM = _____
DOF = _____
(II) Joints and links
Links are considered to be rigid components of robots. There will be two links connected to
each joint: the input link and output link. The purpose of each joint is to provide controlled
relative movement between the input and output link.
There are five types of mechanical joints in industrial robots. Two of them provide the linear
motion whilst the other three types provide the rotary motion. They are listed below:
a. Linear (prismatic) joint - type L joint
The relative movement between the input link and the output link is a linear sliding motion,
with the axes of the two links parallel to each other.
Figure 4.18
Radial, sliding, or translational movement – Type L
b. Orthogonal Joint – type O joint
The relative movement between the input link and the output link is a linear sliding motion,
with the axes of the two links are perpendicular to each other.
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Design and Applied Technology (Secondary 4 - 6)
Output link
Input link
Output link
Figure 4.19
Input link
Two types of orthogonal joint are also known as Type O
c. Rotational Joint – type R joint
The relative movement between the input link and the output link is rotational, with the axes
of rotation perpendicular to the axes of the input and output links.
Figure 4.12
Axis of rotation is perpendicular to axis of the 2 connecting links – Type R
d.Twisting joint – type T joint
The relative movement between the input link and the output link is a rotary motion, with the
axes of rotation parallel to the axes of the input and output links.
Figure 4.21
Axis of rotation is parallel to the links – Type T
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Design and Applied Technology (Secondary 4 - 6)
(III) Revolving joint – type V joint
The axis of output link is perpendicular to the axis of rotation, with the axis of rotation of the
joint parallel to the axis of input link.
Figure 4.22
Output link is perpendicular to the input joint, but parallel to the axis of
rotation – Type V
(IV) Joint notation scheme
Figure 4.23
Joint and link nomination in an Articulated robot
For a typical robotic arm, it can be divided into 2 main sections:
a body-and–arm assembly
a wrist assembly.
Joint notation scheme is that a robotic arm that can be described in terms of the type of joints
(L O R T V) it has, which is listed in the order from the base to the end effector.
5 joints types can be labeled with L.O.R.T and V for the linear, orthogonal, rotational,
twisting and revolving joints respectively. They are commonly used in joint notation system
to describe a robotic arm.
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Design and Applied Technology (Secondary 4 - 6)
E
X
A
M
P
L
E
We can use TRR as an example to explain the joint notation scheme of TRL.
A TRR represents a 6 DOF robot where (1) the Body-and-Arm assembly is made up of 3
joints: a twisting joint (Type T) at the Base, rotational joint (Type R) at Joint 1 and linear
joint (Type R) at Joint 2; (2) the Wrist assembly consists of 3 joints: a twisting joint (Type T)
for Rolling, rotational joint (Type R) for Pitching and rotational joint (Type R) for yawing. A
colon is used to denote the Body-and-Arm assembly from the Wrist assembly.
Adopting this joint notation system to identify the 6 configurations of robots is listed below:
Robot Type
Joint Notation Scheme
Cartesian Robot
LOO
Cylindrical Robot
TLO
Polar Robot
TRL
SCARA Robot
VRO
Parallel Robot
TRL
Articulated Robot
TRR
Table 4.1
Joint types for different robot configurations
(V) End Effector
A device which is attached to the wrist of the robotic arm to perform specific tasks, such as
grippers for material transfer, welding torches for joining, and spray guns for surface finishing,
etc. They are mainly divided into two categories by functions:
Grippers - To hold and move objects
Tools - To perform work on a part. A tool can be held by collets, making the tools
changeable and more flexible
a. Grippers
Vacuum or Suction cups
Vacuum or suction caps are appropriate when the objects to be handled have a flat, smooth,
clean surface, e.g. for lifting glass.
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Design and Applied Technology (Secondary 4 - 6)
Figure 4.24
Vacuum gripper
Advantages:
simple in mechanically design
gentle force can be applied to lift objects without causing
reliable
light weight
can be used on a wide range of materials
damage
(VI) Magnetic
Useful only for ferrous materials (containing iron). Objects of different shapes can be picked
up quickly. It can pick up parts with holes and irregular surface. It is inevitable to picks up dirt,
rubbish or unwanted objects during operation. In addition, slippage may occur and handling
will not very precise.
Figure 4.25
Articulated robot per permanent magnetic gripper
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It can be powered electromagnetically or use a permanent magnet. Dropping an object is easy
for an electromagnetic one by simply powering off the gripper. However, it may be dangerous
in the event of power failure. For a permanent magnetic one, it needs specific mechanism to
unload the object from the gripper.
Other gripper designs may use adhesives, hooks, scoops, etc, They are specially designed for
picking up parts of particular shapes.
(VII) Mechanical gripper
Figure 4.26
Mechanical gripper of fingers type
It uses fingers / jaws - usually 2 fingers, with 2 positions - open and closed, making control
relatively simple. Detachable fingers allow worn fingers to be replaced, and is
interchangeable between different types of fingers.
It can have hard fingers which allow precise handling but may damage delicate materials.
The finger may not be versatile or effective for picking up objects of different shapes. The
other can be compliant fingers which have some "allowance” for dealing with some
unpredictable shapes.
Sensors on grippers can provide information to the robot control system that an object has
been picked up and allow fingers to apply appropriate force to grasp the object. Sensors can
be force sensors, pressure sensors, strain gauges and touch sensors
S T O P
A N D
T H I N K
Can you use the Joint Notation Scheme to describe the following robots - (1) 6 DOF
Articulated robot (2) 6 SCARA?
(i) Body and Arm assembly : (1) _______ ; (2) ________
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Design and Applied Technology (Secondary 4 - 6)
(ii) Wrist assembly : (1) _______ ; (2) ________
(iii) Joint notation scheme : (1) ____:____ ; (2) ____:_____
Figure 4.27
(a) Articulated robot
(b) SCARA robot
(VIII) Work Space
Work space is also called Work volume or work envelope. It is defined as the space within
which the robot can manipulate by the end of its wrist. It is determined by the following
factors:
1. The number and types of joints in the manipulator (body-and-arm
and wrist
assembly)
2. The physical size of each joints and links
3. The ranges of each joint
Cartesian robot has a rectangular work space. Polar robot has a partial spherical work
envelope and a cylindrical robot has a cylindrical work volume.
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Figure 4.28
Diagrams show the work envelopes of an articulated robot
Maximum envelope is the envelope that encompasses the maximum designed
movements of all robot parts, including the end effector, work piece and attachments.
Restricted envelope is a portion of the maximum envelope which a robot is
restricted by some limiting devices.
Operating envelope is the restricted envelope that is used by the robot while
performing its programmed motions.
S T O P
A N D
T H I N K
What is/are the use(s) of knowing the work envelope for a robot?
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4.4 Robot Control Systems
(I) Drive Systems
Usually there are three main classifications of power drive for robots - electric, pneumatic and
hydraulic. They are used to drive robots through the use of actuators
a. Electric
For electrical driven robots, four major types of electric drive can be used:
(i)
Stepping Motors: These are used mainly for simple pick and
place mechanisms, especially when low-cost is the more
important consideration than power or controllability.
Figure 4.29
Stepping motor
(ii) DC Servos: For the early electric robots, the DC servo drive was used extensively. It
provides good power output with a high degree of speed and position control.
(iii) AC Servos: In recent years the AC servo has
taken over from the DC servo as the standard
drive. These modern motors give higher
power output and are almost silent in
operation. They have no brushes in structure.
They are very reliable and require almost no
maintenance in operation.
Figure 4.30
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AC Servos with controller
Design and Applied Technology (Secondary 4 - 6)
(iv) Solenoids Actuator: A major advantage of solenoid actuator is their quick operation.
Also they are much easier to install than pneumatic or hydraulic actuators. However,
solenoid can only have fully open (extend) or closed (retract) function. They cannot
bear much loading.
Figure 4.31
Operation of solenoid actuator
b. Pneumatic
Robots, that use compressed air, may come in a wide
variety of sizes. Most of the simple pick and place arms are
driven by pneumatics. This makes the system low in cost.
However, it has the disadvantage of being difficult to
control with high accuracy.
Figure 4.32
c. Hydraulic
Robots, that use hydraulic, are generally
performing heavy duty jobs. This power type
is noisy, large and heavier than the other
power drive sources. Hydraulic drives were
used on a large number of early robots as it
was more rigid and controllable than
pneumatics. It could provide more power
than the electric drives. The problems with
hydraulics are that it tends to be fairly slow
in operation, but the leaks, that due to the
high-pressured oil, can be very messy.
Figure 4.33 Hydraulic actuator
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Pneumatic cylinder
Design and Applied Technology (Secondary 4 - 6)
S T O P
A N D
T H I N K
1. Why solenoid actuator can only perform duty of light loading?
2. Why pneumatic drive system has difficulty of controlling with high accuracy?
(II) Types of robot control
a. Limited-sequence robot
It is the most elementary control type and can only be used for simple motion cycle, such as
pick and place application. It is designed by arranging the limit switches or mechanical stops
at each joints and sequencing the actuation of joints to accomplish the cycle. Simple
pneumatic driven robot without an electronic controller is the type of limited sequenced robot.
b. Point to Point control
The controller has a memory for storing the sequence of motion and the location of each joint
in a given work cycle. Feedback control is used to assure that an individual joint has achieved
the desired location defined in the program. However, in this PTP control, only the final
location of an individual joint is controlled. The path taken for the joint to move from the
initial location to the final location is not of concern.
c. Continuous Path control
The movement of the arm and wrist is controlled during the motion. Servo control is used to
maintain the continuous path control over the position and speed of the robotic arm. Its
advantage is to provide a smooth continuous path for the robot.
d. Intelligent Robot
With the advancement in microprocessor and Artificial Intelligence (AI) technology, robots
can be equipped with advanced sensory systems, such as vision sensors and face recognition
technology, that process information and function like a human brain. AI allows a robot to
perceive conditions and make decision based on the perceived condition. An intelligent robot
can make decisions when things go wrong during the work cycle, communicate with human
beings and make computation and correction during the motion cycle.
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Figure 4.34
Intelligent robot with machine vision and face recognition technology
(III) Programming a robot
a. Teach Pendant Method
A teach pendant (handheld programmer) is used to control and program the robot. Operator
can teach the robot by driving individual robotic joints independently. The operator can use
either the world coordination system (WCS) which is located at the robot base or Tool
coordinate system (TCS) which is originated at the robot wrist. This system is especially
useful when the tool is near to the work piece.
This method of programming is very simple to use where simple movements are required.
However, when teaching, the robot cannot be run in production, this reduces the machine
utilization rate.
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Figure 4.35
Teach pedant for teach method of robot programming
b. Leadthrough Programming
This programming method is mainly used by spray painting robots. The robot is programmed
by being physically moved through the task by an operator. This is exceedingly difficult
where large robots are being used. Therefore, a smaller version of robot is sometimes used for
this purpose.
Any hesitations or inaccuracies that are introduced into the program cannot be edited easily
without reprogramming the whole task. The robot controller simply records the joint positions
at a fixed time interval during the leadthrough and then plays back the sequences.
c. Off-line Programming
It is used to program robots from the CAD models of the robots, fixtures and accessories. The
program structure is built up in much the same way as for teaching programming but
intelligent programming tools are available which allow the CAD data to be used to generate
sequences of location and process information. The benefits of this form of programming
are: Reduced down time for programming.
Programming tools make programming off-line, thus can reduce product lead time.
Work cell design and allows process optimization.
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However, because off-line programming is not accurate, it requires adjustment interactively
on the factory floor until all positions and orientations are correct before the robot can run in
the production.
Figure 4.36
Teach pedant for teach method of robot programming
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4.5
Applications of Robots
Robots offer some obvious advantages in terms of speed, accuracy and productivity as they
can run 24/7 repeating jobs, which would be tedious to operators, and without human error.
Below are some common applications which can provide a robotic solution to increase
productivity. It may also be possible to integrate more than one application of robots into a
system to suit variety of needs.
(I) Medical Robots
Robots are used in medical fields because they are
highly precision machines. By tooling the end-effector
with surgical instruments, they used to perform
delicate surgery.
These machines still require a human surgeon to
operate and input instructions. Remote control though
Internet and voice activation are going to be used to
control these surgical robots.
Figure 4.37
Surgery Robot
(II) Robots in Automobile Industries
In the automobile industry, robotic arms
are used in diverse manufacturing
processes including assembly, spot
welding, arc welding, part transfer, laser
processing, cutting, grinding, polishing,
deburring, testing, and painting. Robots
have been proved to help automakers to be
more agile, flexible and to reduce
production lead times.
Figure 4.38
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Industrial robot in manufacturing plant
Design and Applied Technology (Secondary 4 - 6)
(III) Electronics/Semi-Conductor
Application of clean room Robots in
semiconductor manufacturing results in
the reduction of scraps from broken chips.
The avoidance of contamination and the
savings in scraps from dropped wafers in
machine loading and unloading can be a
major savings. Typically, clean Room
robots are used in machine loading,
unloading, and parts transfer in the
semiconductor
industry.
Assembly,
packaging, and testing processes are other
Figure 4.39
application areas for clean room robots.
Industrial robot in manufacturing plant
(IV) Food & Beverage
Food and beverage applications represent a small
fraction of industrial robotics application. However,
it is widely recognized as one of the fastest growing
segments, like automotive industry. The vast
majority of robots in the Food & Beverage industry
are found in the packaging area. High-speed
material handling robotic arms and vision-guided
systems are beginning to work in food factories.
Figure 4.40
Industrial robot in manufacturing plant
(V) Construction
Construction robots aim to improve the efficiency of work at construction sites. Robots are
used in the applications like inner pipe crawling, excavation, load transport, mining,
bricklaying, earth work, foundation, prefabrication of reinforcement and pavement work.
Generally, construction robots will replace human workers in dangerous conditions or
problem of limited accessibility.
Figure 4.41
Construction Robot for pavement work
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(VI) Space Robots
Space robotics is generally divided into two main areas: 1. serving the functions of robotic
manipulators, such as the mechanical arm installed in US space shuttle for maintenance of
space station or as a crane for material transfer or construction work. 2. Robotic explorer for
inspection of hostile environment in planetary surfaces and collection of soil samples for
analysis.
Figure 4.42
MARS lander for US MARS Exploration Program
(VII) Military/Security Robots
They are usually deployed as unmanned remote-control vehicles, typically of terrace-type.
They are capable of taking surveillance photographs and serve as mine sweepers or bomb
disposal to safeguard people from endangering into a hostile environment.
Figure 4.43
Bomb disposal robot checking suspicious
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Design and Applied Technology (Secondary 4 - 6)
Quizzes (Chapter 4)
1.
2.
3.
4.
5.
6.
7.
What is the limitation of Gantry Robot?
What are the differences of DOM and DOF?
List the five types of joints used in robots.
Suggest any specific applications of electric, pneumatics and hydraulic drive systems of
robots? State your reasons.
What are regarded as Intelligent Robot?
What will the possible limitations of using surgery robot through Internet be?
Which of the followings are regarded as robots? State your reasons.
a
b
c
d
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Design and Applied Technology (Secondary 4 - 6)
THEME-BASED LEARNING TASK 1
Practical Design Appreciation - Case Study of Intelligent Fire Alarm System
Figure 1.1
High rise commercial building on fire
(I) Background
Most of the Hong Kong people may remember the tragedy of Grade 5 fire in Garley
Commercial Building in 1996. The fire had caused a great causality and alerted people to the
safety of public premises. After this tragedy, there is a law stating that all the commercial
buildings, which were built before 1987, are enforced to install Intelligent Fire Alarm system
to safeguard the people from severe causality by early warning and to extinguish the flame
automatically and intelligently.
(II) What is Automatic Fire Alarm (AFA) or Intelligent Fire Alarm system (IFA)?
1. Conventional system
Figure 1.2
Sprinkler head (left); Alarm gong (mid); Break glass (right)
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Design and Applied Technology (Secondary 4 - 6)
Traditionally, the fire alarm system is triggered by the “break glass” manually to alert people
inside the building in the event of fire. The red alarm gong will be triggered to alert the
residents inside the building and the pedestrians outside the building. The fire is extinguished
by the sprinkler system that is triggered locally by the heat of fire. The pre-pressurized water
inside the pipes of sprinkle system will be sprayed through the nozzle of each sprinkler head
until the pressure head along the pipeline drops to zero.
2. Automatic Fire Alarm (AFA) or Intelligent Fire Alarm (IFA) system
Figure 1.3
Block diagram of Intelligent Fire Alarm system
For the Intelligent Fire Alarm System, the fire is detected by intelligent sensors, which verify
the flame by the heat and smoke of the location. The signal will be transmitted to the Control
Panel that is usually housed in the security room and monitored by the on-duty operator. The
control panel is basically comprised of a CPU and a number of interfacing cards and
networking cards. Once the flame is “verified” by the intelligent system, the signal will be
sent to the pump room to turn on the duty pumps to charge the sprinkle system. The fire alarm
signal will also be relayed to the Regional Fire Office automatically through the dedicated
signaling line. The fire engines will then be sent to the fire location through this AFA or IFA.
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Design and Applied Technology (Secondary 4 - 6)
Figure 1.4
User Interface of Flame and Smoke detection system
Figure 1.5
A cutaway diagram of intelligent smoke sensor
The owner or responsible personnel of this property will also be notified by the messaging
function of the control panel through mobile phone. The user interface has imported the
building floor plan in the initial configuration. The floor plan is divided into number of zones
according to the arrangement of smoke and heat sensors. Once the fire happens, the zone
concerned will be flashing and alerting sound on the screen to alert the responding personnel
to take any pre-planned emergency procedure.
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Design and Applied Technology (Secondary 4 - 6)
This intelligent fire alarm system will be integrated with the surveillance system to have a real
time monitoring of the fire location as shown in Figure 1.4. Once the flame is notified in one a
(zone), the surveillance system will alert the on-duty personnel and indicate the exact location
of the building by the pre-loaded site drawing or the building floor plan.
All the intelligent sensors are connected by the twisted-pair cables and are communicated
through an Ethernet network. Each sensor has its own IP address and their distribution over
the building are stipulated by the fire regulation.
(III) Learning Tasks
1. Investigative Questions:
Students are expected to form groups, by conducting information search through Internet and
small group discussion, to answer the following questions:
A. Explain why the Intelligent Fire Alarm system does not use the pre-pressurized
sprinkle system but use the “dry” pipe.
B. Explain why each sensor has its own IP address.
C.
Investigate how the intelligent sensors ‘confirm” the fire is happening.
D.
Investigate what other fire extinguish methods are used besides the sprinkle
system.
2. Follow-up Activities
A.
Form groups and carry out a field study of fire safety measures taken in the school
premises. Appraise the level of automation for this fire safety measure.
B. Also, other groups of students can take a look at the shopping mall nearby. Study
what fire safety devices and systems are being adopted. A comparison can be made
with those used in the school campus. Prepare a brief verbal report to state the
differences and give appropriate reasons.
C. Suggest any improvement plan to upgrade the fire safety system for your school.
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Design and Applied Technology (Secondary 4 - 6)
Hands-on Activity – Controlling an Automated Traffic Lights using
Programmable Logic Controller (PLC)
Figure 2.1
Traffic light control between Oi Kwan Road and Heard Road
(I) Background
A pair of traffic lights is set at the intersection of Oi Kwan Road and Heard Road. Oi Kwan
Road is the busy highway, Heard Road is the little-used road. One traffic light is used for
highway and the other is used for the little-used road.
Three timers are used for the traffic light control:
1. First timer is used to provide a short delay of 2 seconds, TS_2
2. Second timer is used to provide a medium delay of 5 seconds, TM_5
3. Third timer is used to provide a long delay of 10 seconds, TL_10
The state variables are as follows:
1.
2.
3.
4.
5.
OG for Oi Kwan Road Green
OY for Oi Kwan Road Yellow
AR for both roads Red
HG for Heard Road Green
HY for Heard Road Yellow
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Design and Applied Technology (Secondary 4 - 6)
Converting the traffic lights situations into a light sequence condition for analysis.
State
Oi Kwan Road Light
Heard Road Light
OG
Green
Red
OY
Yellow
Red
AR
Red
Red
HG
Red
Green
HY
Red
Yellow
AR
Red
Red
Table 2.1
Light and state conditions
The light sequence can be represented by the state diagram and timing chart for better
understanding.
Figure 2.2
State diagram for the traffic lights intersection
(II) Learning tasks
Part A
1.
Equipment
A PLC with a handheld programmer or a computer
LEDs (Red, Yellow and Green for 2 sets)
x 1k resistors
Connecting wires
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Design and Applied Technology (Secondary 4 - 6)
2. Tasks
A.
B.
C.
Construct the timing diagram
Construct the ladder diagram
Setup the PLC control and demonstrate to teacher
Part B
Figure 2.3
Traffic light on a motorway for pedestrian
Students can conduct a simple field study at the zebra crossing point and observe the traffic
light operation in a real scenario. They can record the sequence of light cycle. The traffic light
should have a button (interrupt) for the pedestrian to adjust the traffic light to cross the road.
1. Equipment
A PLC with a handheld programmer or a computer
LEDs (Red, Yellow and Green for 1 set)
A push button
x 1k resistors
Connecting wires
2. Tasks
A.
B.
C.
D.
Construct the timing diagram
Construct the ladder diagram
Setup the PLC control and demonstrate to teacher
Give any suggestions to improve the road safety for this zebra crossing condition.
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Design and Applied Technology (Secondary 4 - 6)
Design and Make Project – Pipe Cleaning/Inspection Robot
(I) Introduction
In Hong Kong, most of the drivers complain that the maintenance and construction work of
underground pipe works are so frequent which are blamed for the major cause of traffic jams.
The repair of underground pipes is mainly due to the leakage caused by prolonged clogging in
the old metal pipe and accumulation of rubbish in PVC pipe.
These pipes cannot be cleaned by routine maintenance. Once the leakage happens, the only
way is to replace them with new ones. Therefore, it is unavoidable to cause a lot of
disturbances to the traffic condition. If these pipes can be cleaned regularly to reduce the
clogging of dirt, they can “live” longer and, thus, reduce the tremendous maintenance works.
Figure 3.1
A typical pipe inspection robot
Students are required to apply their knowledge to design and make a Pipe Cleaning/Inspection
Robot.
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Design and Applied Technology (Secondary 4 - 6)
(II) Project Brief
(a) Consumables
No.
01
Descriptions
Qty.
Aluminum (of wide range of forms and size, such numerous
as angle, flat, rod, channel and tube, etc.)
02
Acrylics (of wide range of forms, size and color, numerous
such as triangle, flat, rod and tube, etc.)
03
Fasteners (i.e. machine screws, washers, nuts, numerous
rivets, pop rivets, lock clips, studs, self-locking
nuts, strap fasteners and spanners, etc.)
04
Conventional workshop hand tools (marking-out Sufficient
tools, cutting tools and finishing tools) and numbers
machinery (such as drilling machine, sander and
buffing machine, etc.)
Table 3.1
Consumables needed for making the robots
(b) Test Rig
No.
01
Descriptions
Qty.
A 300mm internal diameter opaque PVC or
1 pc.
transparent acrylic pipe with length of not less than
4 feet.
02
A tailor-made wooden container with nylon
1pc.
covering for collection of leakage water (placed
under the test pipe)
03
A pair of tailor made wooden stands to support the
test pipe and with appropriate fasteners for security
and safety.
Table 3.2
Part list for the test rig
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1pc.
Design and Applied Technology (Secondary 4 - 6)
Figure 3.2
Schematic diagram of Test rig
(c) Tasks
Students are required to design and make a pipe cleaning robot prototype that can
demonstrate how to clean a pipe in the test rig.
The robot can be autonomous with suitable sensory feedback, execution of preset
program or real-time remote control.
Students have to design an appropriate locomotion that can move effectively
along the inner wall of the test pipe.
An end-effector should be a pipe cleaning device.
It may need to install a camera or appropriate sensors to collect image and
information from the pipe being inspected.
Robots can also be controlled by wireless through radio frequency or bys wired
real-time remote control according to the competency of students.
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Design and Applied Technology (Secondary 4 - 6)
Mars Exploration – Design an Innovative End-effector for Mars Lander
(I) Background
Figure 4.1
MARS Lander used in sample extraction on Mars surface
Though Mars exploration seems to be a far-reaching issue to a hassle and financial city like
Hong Kong, the patented “Space Plier” that was designed and developed by a group of
engineers and scientists from Hong Kong Polytechnics University had been used by Europe
NASA in the Mars Exploration Program in 1995 and 2003 respectively. The “Space Plier” is
used to pick up soil samples of MARS surface. It has a small drilling bit to dig a hole on the
surface and has a little “pliers”, like a chopstick, to collect the soil sample out of the drilled
hole.
(II) Task
Students are provided with a remote-controlled locomotive, either wheel-driven or
terrace-type (common mechanical toys can be considered). The locomotive is a platform on
which students are expected to design and make a special end-effector to simulate the tasks in
MARS. Teacher can provide a Mars-like field to promote the students’ motivation.
The end-effector can be driven by whatever the means students can think of, maybe powered
by water or air syringes, elastic bands, DC motors or any other means. The design of this
end-effector is encouraged to be more innovative, not only to the use of 2 or 3-fingers
mechanical gripper.
Figure 4.2
Toys chassis used as a locomotive platform
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Design and Applied Technology (Secondary 4 - 6)
Figure 4.3
Typical 2-fingers gripper design based on linkage and driven by electric motor
(III) Follow-up Activities
A.
B.
Describe any trade-off when designing an “innovative” end-effector.
If the answer to the last question is “absolute”, why do we still need to pursue
innovation?
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Design and Applied Technology (Secondary 4 - 6)
ASSESSMENT TASKS
QUESTION 1: (RELEVANT TOPIC: BASICS OF CONTROL
SYSTEMS)
Students are required to study the sequential operations of a washing machine at home.
Figure A.1
A cutaway view of a washing machine
A. Complete the Record Form and study the sequential operations of the washing
cycle.
B. Draw a flow chart/block diagram to present the sequence of the washing cycle.
C. Draw a timing diagram to represent the washing cycle.
D. List any state variables for the washing cycle.
Figure A.2
Control panels of washing machines
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Design and Applied Technology (Secondary 4 - 6)
Design and Applied Technology
Washing Machine Analysis and Record Form
Student Name:
Class:
Date:
Machine Model:
Brand:
Capacity: (if any)
Country:
Tasks:
Record the sequential actions of the washing cycle?
Steps No
Descriptions
Periods
001
e.g. Close the door
N/A
002
e.g. Turn on the power
N/A
003
e.g. Turning the control dial to start position
N/A
004
e.g. Inlet of water automatically
005
e.g. Water inlet stops.
006
e.g. Drum turn at low speed (CW)
3mins.
N/A
007
008
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1min.
Design and Applied Technology (Secondary 4 - 6)
QUESTION 2 (RELEVANT TOPIC: PNEUMATICS)
(I) Situation
Packs of cut foods are accumulating at the end of conveyor. These parts need to be transferred
to other conveyor for quality inspection. An operator needs to activate a transfer device to
remove these accumulated packs. This transfer device is powered by a pneumatic cylinder.
Figure A.3
A conveyor belt
(II) Control Task
To design and assemble a circuit that can extend and retract a single acting, spring-return
cylinder by an operator.
(III) Circuit Problem
Using the given components and layout, design a schematic circuit which will operate a
spring return cylinder with a two-position, spring return, three-way valve.
Design and draw the schematic circuit diagram
1.
2.
3.
Connect components according to the schematic diagram
Operate and explain to teacher.
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Design and Applied Technology (Secondary 4 - 6)
(IV) Hints
3/2 Normally Close NC x 1
single acting cylinder x 1
Note: If there is no pneumatic learning/training kit, magnetic symbols and white board can be
used for demonstration.
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Design and Applied Technology (Secondary 4 - 6)
QUESTION 3 (RELEVANT TOPIC: PNEUMATICS)
(I) Situation
A part needs to be clamped for a drilling operation. An operator needs to activate and
deactivate a pneumatic clamp that holds the part in a fixture on a drilling table. The clamp
must be activated before the drilling cycle begins and deactivated at the end of the drilling
cycle.
Figure A.4
A drilling machine
(II) Control Task
To design and assemble a circuit that extends and retracts a double acting cylinder.
(III) Circuit Problem
Using the given components and layout, design a schematic circuit which will operate a
double acting cylinder with a two position five -way valve.
1. Design and draw the schematic circuit diagram
2. Connect components in according to the schematic diagram
3. Operate and explain to teacher
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Design and Applied Technology (Secondary 4 - 6)
(IV) Hints
3/2 Normally Close NC x 2
5/2 directional valve, air pilot x 1
double acting cylinder x 1
Note: If there is no pneumatic learning/training kit, magnetic symbols and white board can be
used for demonstration.
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Design and Applied Technology (Secondary 4 - 6)
QUESTION 4 (RELEVANT TOPIC: PNEUMATICS)
(I) Situation
A large stamping press must have the work piece in place, clamps engaged and safety guard in
position before the “Start” button can activate. This interlock design is needed for minimizing
potential risks.
Figure A.5
A Stamping machine
(II) Control Task
To design and assemble an “AND” logic circuit to control a single acting, spring-return
cylinder.
(III) Circuit Problem
Using the given components and layout, design a schematic circuit which will only operate
the cylinder when the three valves are all simultaneously operated, implying the safety
precautions are all in effect.
1. Design and draw the schematic circuit diagram
2. Connect components in according to the schematic diagram
3. Operate and explain to teacher
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Design and Applied Technology (Secondary 4 - 6)
(IV) Hints
3/2 Normally Close NC x 3
AND valve, air pilot x 2
Single acting cylinder x 1
Note: If there is no pneumatic learning/training kit, magnetic symbols and white board can be
used for demonstration.
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Design and Applied Technology (Secondary 4 - 6)
QUESTION 5 (RELEVANT TOPIC: PNEUMATICS)
(I) Situation
Semi-finished parts are accumulating on a conveyor belt and waiting to be released and
transferred to the next stage for wrapping. Operators at different points can turn on the gate
release mechanism to let the trays moving into the packaging unit.
Figure A.6
A conveyor for food industry
(II) Control Task
To design and assemble an “OR” logic circuit to actuate a single-acting, spring return
cylinder.
(III) Circuit Problem
Using the given components and layout, design a schematic circuit which will operate a
spring return cylinder from any one of three identical valves.
1. Design and draw the schematic circuit diagram
2. Connect components in according to the schematic diagram
3. Operate and explain to teacher
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Design and Applied Technology (Secondary 4 - 6)
(IV) Hints
3/2 Normally Close NC x 3
OR Shuttle valve, air pilot x 1
single acting cylinder x 1
Note: If there is no pneumatic learning/training kit, magnetic symbols and white board can be
used for demonstration.
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Design and Applied Technology (Secondary 4 - 6)
QUESTION 6 (RELEVANT TOPIC: PNEUMATICS)
(I) Situation
A plastic thermo-forming machine is capable of heating and forming parts of various
thicknesses. The parts must be held in their molded positions for curing to its final form. The
length of time needed to cure the plastic will vary depending on its thickness. This requires
a forming operation to have a variable time delay function so that the part can be held in place
before the part is automatically released.
Figure A.7
A Forming Machine
(II) Control Task
To design and assemble a “Time delay off” circuit to actuate a single acting spring return
cylinder.
(III) Circuit Problem
Using the given components and layout, design a schematic circuit which will extend a
cylinder for an adjustable period of time, then automatically retract the cylinder.
1. Design and draw the schematic circuit diagram
2. Connect components in according to the schematic diagram
3. Operate and explain to teacher
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Design and Applied Technology (Secondary 4 - 6)
(IV) Hints
3/2 Normally closed NC, air-pilot x 1
3/2 Normally closed NC, manual x 1
Flow speed control valve x 1
Single-acting cylinder x 1
Note: If there is no pneumatic learning/training kit, magnetic symbols and white board can be
used for demonstration.
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Design and Applied Technology (Secondary 4 - 6)
QUESTION 7 (RELEVANT TOPIC: PNEUMATICS)
(I) Situation
A furnace outlet has a single conveyor that can transport iron bars to two different loading
docks. In order to shift the out feed of the conveyor to the alternate loading dock, the operator
must push a button. As a safety precaution, the conveyor will always be held in the last shifted
position until the reception of next signal.
Figure A.8
Furnace and a conveyor belt
(II) Control Task
To be able to design and assemble a “Memory” circuit to actuate a double acting cylinder.
(III) Circuit Problem
Using the given components and layout, design a schematic circuit that requires the operator
to push one of two buttons that in turn shifts a retented, two position, four-way valve. The
valve is air-piloted in both directions to operate a double acting cylinder.
1. Design and draw schematic diagram.
2. Connect components in according to the schematic diagram.
3. Operate and explain to teacher.
150
Design and Applied Technology (Secondary 4 - 6)
(IV) Hints
3/2 Normally Close NC, manual x 2
5/2 directional valve, air pilot x 1
Double acting cylinder x 1
Note: If there is no pneumatic learning/training kit, magnetic symbols and white board can be
used for demonstration.
151
Design and Applied Technology (Secondary 4 - 6)
QUESTION 8 (RELEVANT TOPIC:
ELECTRO-PNEUMATICS/PROGRAMMABLE LOGIC CONTROLLER)
When the push button is pressed, two motor M1 and M2 (outputs) must run. After 4 minutes
Motor 1 stops. Motor 2 keeps running for another 2 minutes and stops. At this moment, a
lamp is switched on. After a further 90 seconds, the lamp will go off and the cycle restarts. If
the stop switch is pressed at any time, the motor will continue until the completion of this
cycle, then stop.
Figure A.9
1.
2.
Timing diagram for the operation sequence
Construct a ladder diagram and program it to PLC to make it work.
Draw an electro-pneumatic circuit diagram.
152
Design and Applied Technology (Secondary 4 - 6)
QUESTION 9 (RELEVANT TOPIC: PROGRAMMABLE LOGIC
CONTROLLER)
Components pass along a pair of photo-sensors along the conveyor belt. When the
components pass the sensors, the light beam is interrupted and the signal will go low (Off).
After 6 components have been counted by the sensors, an eject operation will be actuated and
used to remove the batch out of the conveyor. Then, the cycle starts again.
The ejecting device is pushed by a single-acting cylinder that is controlled by a solenoid
valve.
Figure A.10
1.
Schematic diagram of the system
Construct a ladder diagram and program it to PLC to make it work.
153
Design and Applied Technology (Secondary 4 - 6)
QUESTION 10 (RELEVANT TOPIC: PROGRAMMABLE LOGIC
CONTROLLER)
A machine is switched on by pressing either A or B push button. A safety guard D is in place
with a limit switch D triggered in the closed position. In addition, there is a proximity switch
C to detect if anyone is standing inside the dangerous zone. All the switches are normally
open contacts and the machine will not start when they remain open.
A
B
C
D
Figure A.11
1.
2.
3.
Schematic diagram of the system
Draw a ladder diagram for the system
Write a Boolean expression for the system
Construct a truth table for all possibilities
154
Design and Applied Technology (Secondary 4 - 6)
USEFUL WEB SITES
TITLE
URL
Explanatory Note
Traffic light control
circuit
http://home.cogeco.ca/~rpaisley4/20step.ht
ml#Traffic
20 sequential circuits
Information and
Computer Science
http://www.ics.uci.edu/~mghodrat/ics151/h
w5/prob8/final.html
Traffic light Lab Sheet
Sequential Circuit
Design
http://www.cs.swarthmore.edu/~mstone/sch
ool/cs/cs24/web/lab3/
Dynamic Traffic Controller
Electrical Training
Series
http://www.tpub.com/content/neets/14187/css/
Open loop control
O’Reilly Network
http://www.oreillynet.com/pub/a/network/sy
nd/2003/08/05/closed_loop.html
Closed-Loop control
Delphion Integrated
View
http://www.delphion.com/details?pn10=US
03264544
Washing machine patent article
LG Washing Machine
User Manual
http://www.lgwasherdryer.com/pdf/3431_m
anual.pdf
LG Washing Machine User Manual
Festo Pneumatics
Learning System
http://www.festo-didactic.com/int-en/learni
ng-systems/equipment-sets/562/564/semi-ro
tary-drive,size-16,184.htm
Fuzzynet Online
Application Note
http://www.aptronix.com/fuzzynet/applnote/
air.htm
Air Conditioning Temperature
Control
Control Weekly
Review
http://controlsweekly.com/pneumatics.htm#
Tool
Pneumatics
KINEQUIP INC.
http://www.kinequip.com/basic_advantages.
asp
Basic advantage of Pneumatics
Fluid Power
Education Foundation
http://www.clippard.com/downloads/genera
l/PDF_Documents/Intro_to_Pneumatics.pdf
Introduction to pneumatics and
Pneumatics Circuit problem for
FPEF trainer
101 Basic Series Electrical
http://www.eatonelectrical.com/html/101bas
ics/Modules/Module24.pdf
Module 24 Programmable Logic
Controller
Introduction to PLC
programming and
implementation –
from Relay logic to
PLC logic.
http://www.idc-online.com/technical_refere
nces/pdfs/instrumentation/Intro_to_PLC_20
Pro.pdf
Pneumatic
Application and
Reference Handbook
http://www.allair.com/pdf/mead_pneumatic
_handbook.pdf
Loop Technology, UK
http://www.looptechnology.com/index.asp
14187_92.htm
155
Industrial Automation Technology,
Machine Vision System
Design and Applied Technology (Secondary 4 - 6)
REFERENCES
于長官主編。
(2007)
。
《自動控制技術及應用高等學校”十一五”》
。哈爾濱工業大學出版
社。
Groover, M. (2000). Automation, Production Systems, and Computer Integrated
manufacturing. NJ: Prentice-Hall.
Jones, J.L., Flynn, A.M. (1998). Mobile robots: inspiration to implementation. AK Peters,
Ltd.
Mitchell, F.H. (1991). CIM Systems, An Introduction to Computer-Integrated Manufacturing,
Englewood Cliffs : Prentice Hall.
156
Design and Applied Technology (Secondary 4 - 6)
GLOSSARY OF TERMS
Term
Algorithms
Definition
an algorithm is a sequence of instructions, often used for
calculation and data processing. It is formally a type of
effective method in which a list of well-defined instructions for
completing a task will, when given an initial state, proceed
through a well-defined series of successive states, eventually
terminating in an end-state. The transition from one state to the
next is not necessarily deterministic; some algorithms, known
as probabilistic algorithms, incorporate randomness.
Analog signal
Any type of input or output that has more than two states (on
and off). An analog signal can vary in magnitude from “off” to
a high-end value or between two non-zero values. An example
of an analog device would be a level sensor that returns a
voltage somewhere between 0 and 10 V that can vary over
time.
Bit
A single digit that only has two possible values either 0 or 1.
Boolean expression
A general term used to describe logic functions. It includes
AND, OR, XOR, etc.
Central Processing Unit
The main processor of information in a computer. This single
(CPU)
chip performs all of the logic and math operations of the PLC.
Digital signal
Any type of input or output signal that has exactly two states,
on and off. An example of a digital device would be a
pushbutton, which can either be pressed (ON) or released
(OFF).
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Design and Applied Technology (Secondary 4 - 6)
Term
Definition
DSP (Digital Signal
is concerned with the representation of the signals by a
Processing)
sequence of numbers and the processing of these signals. DSP
measures or filters continuous real-world analog signals,
usually to convert the signal from an analog to a digital form
Globe Valve
is a type of valve used for regulating flow in a pipeline,
consisting of a movable disk-type element and a stationary ring
seat in a generally spherical body.
I/O (Inputs and Outputs)
refers to the communication between an information processing
system (such as a computer). Inputs are the signals or data
received by the system, and outputs are the signals or data sent
from it.
Ladder Diagram
The logic of ladder programming used to program and control
a PLC. The fundamental theories of ladder diagram are
consistent among all manufacturers but each PLC manufacturer
generally has a proprietary ladder software package.
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Design and Applied Technology (Secondary 4 - 6)
Term
Logic
Definition
A series of directives or boundaries created to allow a process
to be controlled. Logic can be programmed via hard wiring (as
is the case with relay logic) or via a PLC.
Multiple bits
can be combined to form bytes or words.
Network
Several devices connected together, through electrical means,
for data acquisition and/or control.
Non-retentive
All values are resent to zero after powering down the unit.
Off-Delay Timer
Will turn an output OFF after X amount of seconds has passed.
On-Delay Timer
Will turn an output ON after X amount of seconds has passed.
Operator Interface (O/ I)
A device that allows the operator of a machine to monitor and
control devices attached to a PLC.
Register
A storage area, within the PLC, for information.
Relays
An electromechanical switch that can control on/off of AC or
DC loads.
Relay Circuits.
Devices often used in control. Can be opened and closed
electronically to perform logic circuits.
Retentive
Will store data in memory so that it remains intact after
powering down the unit.
Sensor
The basic element that usually changes some physical
parameter to an electrical signal.
Solenoid
A type of output device and a specific type of coil. Both coils
and solenoids utilize voltage to convert electrical energy to
mechanical energy via magnetic fields.
Starter
A control device usually consisting of a contact and overload. It
will also contain a communication module used for starting and
stopping loads.
Transistors
A solid-state, electronic switch. It is fast, switches a small
current, has a long lifetime, and works with DC only.
Triacs
Or Silicon Controlled Rectifiers (SRCs) act as a mediator
159
Design and Applied Technology (Secondary 4 - 6)
Term
Definition
between the PLC and the AC output device. The triac or SCR
functions as a switch that responds to the commands of the
PLC logic.
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Design and Applied Technology (Secondary 4 - 6)
ACKNOWLEDGEMENTS
The authors wish to thank the following persons/organizations for permission to use their
photographs and images:
Under the GNU Free Documentation License:
Figure 4.2, 4.34, P.120
Every effort has been made to trace the copyright for the photographs and images as needed.
We apologize for any accidental infringement and shall be pleased to come to a suitable
arrangement with the rightful owner if such accidental infringement occurs.
161
Design and Applied Technology (Secondary 4 - 6)
Technology Education Section
Curriculum Development
Development Institute
Institute
Curriculum
Education Bureau
Education Bureau
The Government of the HKSAR
The Government of the HKSAR
162
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Developedby
by
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Instituteof
ofProfessional
ProfessionalEducation
Education
and Knowledge (PEAK),
And Knowledge (PEAK)
Vocational Training Council
Vocational Training Council
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