2013 Mechatronics Higher Finalised Marking Instructions

©
2013 Mechatronics
Higher
Finalised Marking Instructions
 Scottish Qualifications Authority 2013
The information in this publication may be reproduced to support SQA qualifications only on
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These Marking Instructions have been prepared by Examination Teams for use by SQA
Appointed Markers when marking External Course Assessments. This publication must not
be reproduced for commercial or trade purposes.
Part One: General Marking Principles for Mechatronics Higher
This information is provided to help you understand the general principles you must apply
when marking candidate responses to questions in this Paper. These principles must be
read in conjunction with the specific Marking Instructions for each question.
(a)
Marks for each candidate response must always be assigned in line with these
general marking principles and the specific Marking Instructions for the relevant
question. If a specific candidate response does not seem to be covered by either the
principles or detailed Marking Instructions, and you are uncertain how to assess it,
you must seek guidance from your Team Leader/Principal Assessor.
(b)
Marking should always be positive ie, marks should be awarded for what is correct
and not deducted for errors or omissions.
GENERAL MARKING ADVICE: Mechatronics Higher
The marking schemes are written to assist in determining the “minimal acceptable answer”
rather than listing every possible correct and incorrect answer. The following notes are
offered to support Markers in making judgements on candidates’ evidence, and apply to
marking both end of unit assessments and course assessments.
Page 2
Part Two: Marking Instructions for each Question
Section A
Question
1
1
Expected Answer/s
Figure Q1 illustrates the basic architecture of a
microcontroller. All parts have been identified
with the letters A, B, C, D, E, F and G.
a
Match each of the unit names to the letters in
Figure Q1.




b
D
E
B
C
Match each of the busses to the letters in Figure
Q1.


Address Bus
Data Bus
F
A or G
(Note A & G are interchangeable)
OR
A
B
C
D
E
F
2
Input Interface Unit
Output Interface Unit
Central Processing Unit
Memory Unit
Input Interface Unit
Output Interface Unit
Central Processing Unit
Memory Unit
1
Max
Mark
Data Bus (or G)
Central Processing Unit
Memory Unit
Input Interface Unit
Output Interface Unit
Address Bus
(boxes 0.5 each, buses 1 each)
Page 3
2
Additional Guidance
Question
1
c
Expected Answer/s
With reference to Figure Q1, state which one of
the following three statements correctly
describes the nature of the data flow on the
control bus in a microcontroller.
Statement 1:
Statement 2:
Statement 3:
Max
Mark
1
The data flow is
bi-directional on the control bus.
The data flow is
omni-directional on the control
bus.
The data flow is
uni-directional on the control bus.
Statement 1
2
Figure Q2 illustrates a liquid level control
system.
The header tank liquid level is controlled
between preset limits by a control system which
opens/closes the Input Control valve when
required. The manual valve is usually open and
it is only closed during system maintenance.
The header tank filling indicator illuminates
while the Input Control valve is open.
2
a
Sketch a flowchart that shows the operation of
the system. Start with an initial level maximum,
Input Control valve closed and manual valve
open.
NOTE: the inserted Flowchart Symbol sheet
Q2/Q11/Q12/Q13 gives a suitable selection of
Flowchart Symbols.
Page 4
5
Additional Guidance
Question
2
Expected Answer/s
Max
Mark
a
Start
Minimum
Level Sensor
Activated
N
Y
Open Input
Control Valve
Turn on Tank
Filling Indicator
Maximum
Level Sensor
Activated
N
Y
Close Input
Control Valve
Turn off Tank
Filling Indicator
End
1 for decision and 0.5 for each action (because a
quite linear action and some ordering flexibility is
allowed) = 2 marks decisions + 1 mark actions =
5 marks
Or other appropriate flow chart that works
Page 5
Additional Guidance
Question
2
b
Expected Answer/s
Max
Mark
List the various inputs and outputs required by
the system controller. Clearly identify which
signals are inputs and which are outputs.
2
Inputs
Outputs
Minimum level sensor
Input Control Valve
Maximum level sensor
Tank filling indicator
0.5 marks per box = 2 marks
3
a
The movement of each joint of a robotic system
can be described as either Rotary or Linear.
Figure Q3(a) shows a labelled diagram of a
Cartesian robot.
The movement for each joint of a Cartesian
robot is shown in Table Q3(a).
On Worksheet Q3(a), complete Table Q3(a) by
filling in the appropriate joint movement for a
Polar and a Revolute robot.
Robot
Joint 1
Joint 2
Joint 3
Cartesian
Linear
Linear
Linear
Polar
Rotary
Rotary
Linear
Revolute
Rotary
Rotary
Rotary
Table Q3(a)
0.5 marks per correct answer = 2 marks
Page 6
2
Additional Guidance
Question
3
b
Expected Answer/s
Max
Mark
In your workbook, briefly describe the basic
operating principles of either a vacuum end
effector or a magnetic end effector giving one
practical application of your chosen end
effector. You may wish to use a sketch to
clarify your answer.
2
Vacuum End Effector
A vacuum end effector uses the forces produced by
differing pressures to produce a grip with which to
lift/transfer an object. This method requires a
suitable leak free seal between the end effector
and the product. The vacuum (low pressure) is
switched on and off as required and the weight of
the product normally breaks the seal.
To vacuum
pump
Vacuum cup
suitably sized
for holding part
To vacuum
monitoring to
check part held
Part being
picked up
Note sketch is optional
An associated application using vacuum end
effector to transfer eggs as the product is fragile
and has a relatively smooth surface.
OR
Magnetic End Effector
This uses a coil wrapped round a soft iron core
which is often bar or horse-shoe shaped. When
this coil is energised by supplying an electric
current then a strong magnetic field is produced in
the iron core and it becomes like a magnet and it
can attract certain materials such as steel. When
the electric current is switched off then the
magnetic field collapes and the material (steel) is
no longer held but is released.
A typical application can be sorting scrap materials
by lifting only those that are magnetically attracted
to the electromagnetic end effector. Certain
materials such as steel are lifted whereas other
materials such as brass, copper or wood would not
be lifted.
Page 7
1
Additional Guidance
Question
4
Expected Answer/s
Max
Mark
Figure Q4 illustrates a set of electronic kitchen
scales.
Figure Q4
4
a
State a suitable sensor for this measurement
application.
1
Any suitable sensor for application. For example, a
load cell would be suitable for this application.
4
b
Describe the basic operation of your chosen
sensor in Q4(a) making reference to the output
signal of the sensor.
2
Any suitable description of the operation of the
chosen sensor in Q4(a).
For the load cell stated above then an example
answer might be: The system contains strain
gauges that detect force and the output from the
circuit is usually a small analogue electrical signal.
The scales may use one or more gauges to sense
the force/weight and if incorporated into a bridge
circuit the characteristic that changes is the
resistance of the individual gauges. This signal is
conditioned and the scales calibrated to display a
weight.
4
c
State two hazards that must be considered
when using your chosen sensor in this
environment.
Any appropriate answer for example, the scales
must operate using a low voltage power supply
(electric shock hazard). The scales must be
moisture proof as they will be used in an
environment that contains moisture (water or
steam hazard could damage scales).
Page 8
2
Additional Guidance
Question
5
a
i
Expected Answer/s
Identify the number of bits represented.
Max
Mark
1
The disc has a 3 bit code
5
a
ii
Complete the shading of the disk.
2
Figure Q5(a)
Note: there are quite a few variations possible but
the code MUST be pure/natural binary coded NOT
Gray Code.
5
b
Briefly explain one major problem with using
this disc code and state one solution to this
problem.
Any appropriate answer, for example, this disc
code has the problem that more than one bit
changes between some segments which makes
error detection challenging.
If Gray Code is used then only one bit changes
between segments and error detection.
Or other appropriate answer.
Page 9
2
Additional Guidance
Question
6
Expected Answer/s
A PLC based control system meets the
following specification.


6
Max
Mark
a
The system has two inputs, X1 and X2 and
one output, Y1.
Output Y1 is only energised if either or both
input switches, X1 and X2, are closed.
Construct a table showing the output for each
of the four possible input combinations.
1
Appropriate table such as the one shown below.
6
b
X1
X2
Y1
0
0
0
0
1
1
1
0
1
1
1
1
Construct a ladder diagram which would allow
the control to be realized using a PLC
(Programmable Logic Controller).
Note: the inserted PLC Datasheet Q6/Q8/Q12
gives the PLC instruction set.
X1
Y1
X2
Ladder diagram only required. No description
necessary.
Page 10
2
Additional Guidance
Question
6
c
Expected Answer/s
Max
Mark
Redraw your ladder diagram in Q6(b) to change
the operation such that the output Y1 can be
latched and also enable the output Y1 to be deenergised at any time using an additional input
X3.
2
X1
Y1
X3
X2
Y1
Ladder diagram only required. No description
necessary.
1 mark for each of 2 added contacts = 2 marks
7
Figure Q7 illustrates a cut away view of an
optical incremental encoder.
Figure Q7
Page 11
Additional Guidance
Question
7
a
Expected Answer/s
Max
Mark
Explain the basic operation of this type of
encoder and describe the type of signal
generated by this device. Illustrate your answer
with a labelled sketch of a signal.
3
A beam is transmitted through the slotted area of
the disc and passes to a receiver on the other side
of the disc. As the disc rotates the beam is blocked
by the solid sections but passes through the slots.
This produces a pulsed beam and a waveform
similar to that shown. The frequency of the pulsed
beam can be calibrated into rotational speed/
distance. Or similar answer which includes a
labelled sketch.
2 marks explanation
1 mark labelled sketch
Output
Time
7
b
An encoder disk has 180 equally spaced slots
and is rotated through 3 turns. State the count
that would be accumulated in a counter if the
initial count was zero.
1
Count = 180 × 3 = 540
7
c
It takes 1·5 seconds to accumulate the count in
Q7(b), what is the rotational speed of the
encoder in rpm?
Speed =
Distance
Time

3
= 2 rps × 60 = 120 rpm
1 5
OR
It takes 1·5 seconds to do 3 rotations
0·5 seconds to do 1 rotation
60 seconds to do 120 rotations
(1 minute)
per minute (rpm)
Page 12
1
Additional Guidance
Question
Expected Answer/s
8
Max
Mark
5
Figure Q8
Figure Q8 illustrates a simplified feed system
for a recycling system controlled by a PLC.



When the feed system is in operation, the
conveyor runs continuously and feeds
material to the recycling system.
When the feed hopper sensor detects an
empty hopper, Normally Closed contact X1
opens, the conveyor motor (Y1) will stop
and the “EMPTY” indicator (Y2) will be
illuminated.
Ladder Diagram Q8 shows the ladder
diagram program. The first two rungs have
been numbered for convenience.
Describe the operation of the program in
relation to the feed system, assuming initially:
 the feed hopper is full;
 the conveyor is running and;
 the EMPTY indicator is not illuminated.
Note: the inserted PLC Datasheet Q/6/Q8/Q12
gives the PLC instruction set.
Page 13
Additional Guidance
Question
Expected Answer/s
8
Max
Mark
(cont)
Normally Closed (NC) Contact X1 is closed if the
hopper is NOT empty. This energises Y1 which
runs the conveyor (1). In Rung 2, while Y1 is
energised, NC contact Y1 is open (1), hence Y2
is not energised and the EMPTY indicator is not
illuminated (1). When the hopper becomes empty,
X1 opens and Y1 is de-energised making the
conveyor stop (1). In Rung 2 when Y1 is
de-energised, NC contact Y1 will be closed and
Y2 will be energised turning the EMPTY indicator
on (1).
9
a
Sketch and label a basic block diagram of a
closed loop control system.
2
Any suitable diagram such as the one below or
similar that contains the key elements.
Measured
Error
Controller
Reference
+
-
Measured Output
9
9
b
c
System
Input
System
System
Output
Sensor
State how an open loop contol system
generally differs from a closed loop control
system when applied to the same process in
terms of:
i
Accuracy;
1
ii
Complexity.
1
i
&
ii
Open loop control systems are generally less
accurate and less complex than closed loop control
systems
State the main advantage of incorporating an
integral element into a proportional control
system.
An integral element introduced into a proportional
control system has the effect of removing/reducing
the steady state error/offset within a control system
and thus the system output will settle closer to the
desired value.
Page 14
1
Additional Guidance
Question
10
10
Expected Answer/s
This question consists of a series of multiple
choice questions and answers for a number of
mechatronic related themes. On Worksheet
Q10, answer the multiple choice question by
putting a tick  in the correct box.
a
A pneumatic based Mechatronic System
A
B
C
D
E
10
b
c
d
e
1
is an open loop system
has no feedback
provides closed loop control
runs with a large offset
uses ON/OFF control.

pressure
flow
force
movement
temperature.
1
a type of robot geometry
a type of pneumatic motor
a control strategy
a mechatronic actuator
a coding system.

A thermocouple is a device used to sense
A
B
C
D
E
1

BCD is
A
B
C
D
E
10

An absolute linear encoder system directly
measures
A
B
C
D
E
10
is maintenance free
uses compressible fluid
always provides linear motion
uses incompressible fluid
always provides rotary motion.
1
A PID control system
A
B
C
D
E
10
Max
Mark
time
heat
force
speed
light level.
1

Page 15
Additional Guidance
Section B
Question
11
Expected Answer/s
Figure Q11 illustrated the main parts of a fanassisted shortbread oven with circulating air
flow as shown.
A microcontroller is used to control the oven.
The oven door has an independent safety
interlock that provides a signal to the
microcontoller to indicate the door status,
“door open” or “door closed”.
The operator opens the oven door, places the
shortbread tray on the shelf, selects the
required baking time and required temperature
and then closes the oven door.
During oven operation, the temperature sensor
informs the microcontroller of the actual oven
temperature. The microcontroller uses ON/OFF
control of the heating elements to control the
oven temperature.
The circulating fan runs constantly to ensure a
continual flow of heated air.
The filter condition sensing system detects the
filter condition and provides a filter condition
signal. The audible alarm is activated when the
filter needs to be cleaned or replaced.
Page 16
Max
Mark
Additional Guidance
Question
11
a
i
Expected Answer/s
Max
Mark
Construct a table that clearly identifies all of the
Input signals to the microcontroller.
4
Inputs
Door status
Temperature sensor
Filter condition sensing system
Required temperature
Required baking time
11
a
ii
Construct a table that clearly identifies all of the
Output signals from the microcontroller.
Outputs
Audible alarm
Heating elements
Circulating fan
0·5 marks each
11
b
Assuming the oven door is closed, sketch a
flowchart which shows how the ON/OFF
decision is made for the control of the heating
elements during one cycle of operation of the
“temperature controlling process”.
The flowchart should start with “read actRual
oven temperature” and should use the signals:


required temperature and;
actual oven temperature.
Note: the inserted Flowchart Symbol sheet
Q2/Q11/Q12/Q13 gives a suitable selection of
Flowchart Symbols.
Page 17
2
Additional Guidance
Question
11
b
Expected Answer/s
Max
Mark
(cont)
Read actual
oven
temperature
Actual oven
temperature less
than required
temperature
Yes
Heater ON
No
Heater OFF
Or any other suitable flow chart
1 mark for decision,
0·5 for each heater ON/OFF box.
11
c
Sketch a flowchart which describes the overall
operation of the oven which includes:




the door status signal;
filter condition check;
the “temperature controlling process” in
Q11(b); (This should be shown as a single
labelled box)
the duration of operation (required baking
time) at the required temperature.
Assume the circulating fan runs constantly and
need not be shown on your flowchart.
Page 18
5
Additional Guidance
Question
11
c
Expected Answer/s
(cont)
START
Door status?
Open
Closed
Filter
condition
OK?
No
Sound alarm
Yes
Temperature
controlling process
Required
baking time
complete?
No
Yes
Heater OFF
END
Or alternative flowchart that provides a suitable
functionality, eg Sound Alarm could be followed by
"Stop system".
1 mark each decision, 0·5 mark for each action
box, 1 mark correct positioning of "temperature
controlling process" box = Total 5 marks
Page 19
Max
Mark
Additional Guidance
Question
11
d
Expected Answer/s
Max
Mark
Sketch and briefly describe a suitable
differential pressure sensor that could be used
in the filter condition sensing system.
3
Any suitable differential pressure sensor sketch
and associated brief description.
Flexible
diaphragm
Air
connection A
Air
connection B
Microswitch
1 mark
In this differential pressure sensor, horizontal
movement is produced in the flexible diaphragm by
the differences in pressure between Air connection
A and Air connection B. This movement is
transmitted to a microswitch which can be
calibrated to give a digital output when the different
pressures across the filter element reached a
preset value. This signal could be used to indicate
that the filters required changing/cleaning.
2 marks
Answer must include sketch AND brief description
for 3 marks.
Page 20
Additional Guidance
Question
11
e
Expected Answer/s
Max
Mark
The Microcontroller uses On/Off control of the
heating elements to control the oven
temperature. Describe with the aid of a sketch
how the oven ON/OFF control operates. Your
description and sketch should make reference
to:
 heater ON time
 heater OFF time
 required temperature
 actual oven temperature.
A system using ON/OFF control of heating
elements means that the power to the heaters is
either fully on or fully off. When the actual oven
temperature is less than the required temperature
the power is supplied to the heating elements and
they are fully on (Heater ON time). The heat
produced by the elements causes the temperature
to rise. When the actual oven temperature is more
than the required temperature then power is NOT
supplied to the heating elements and they are fully
off (Heater OFF time). No heat produced by the
elements which causes the temperature to fall.
The sketch may vary considerably owing to the
complex issues discussed below in the topic note.
Examples include:
on
heater
off
0.6
0.5
required
temperature
0.4
0.3
0.2
actual
temperature
0.1
0
Time
Page 21
4
Additional Guidance
Question
11
e
Expected Answer/s
Max
Mark
(cont)
Alternatively
Heater ON
time
Temperature
Heater OFF
time
Required
temperature
Actual oven
temperature
Time
Topic note – candidates may include some of this
information but it is NOT mandatory as the various
interactions are complex and candidates at this
level would not be expected to understand or
provide these discussions.
11
f
11
f
The manufacturer wishes to improve the energy
efficiency of the oven when there is no tray in
the oven. This is to be achieved by ensuring
that the heating elements’ energy input is
reduced to 20% of full power and the fan speed
reduced to 10% of full speed.
i
Briefly describe one method of reliably
detecting when there is no tray in the oven.
Any suitable method of detecting when there is no
tray in the oven that takes account of the likely
fluctuating temperature and impaired visibility.
A method such as using ultrasonic beams to detect
the absence of the tray.
Alternative method to weigh the load on the shelf
where the tray would be placed.
Note, a solution that failed to take account of the
temperature variations or potentially impaired
visibility (such as an infrared detector or optical
system) would not earn full marks.
Page 22
2
Additional Guidance
Question
11
f
ii
Expected Answer/s
Describe one method of reducing either the
energy input to the heating elements or the
speed of the circulating fan.
Max
Mark
3
Any suitable method such as:
Power: switch off 4 out of 5 elements (assumes
each element has the same power rating) or switch
off elements for 80% of the time.
Fan Speed: reduce fan voltage (for information, it
is NOT likely to reduce voltage to 10% of normal as
response often non-linear and some fans will not
work with a very low voltage) or use PWM (Pulse
Width Modulation) on fan.
11
g
State one hazard in this system and briefly
describe how this hazard could be overcome.
Any suitable hazard and way of overcoming the
hazard such as the following:
It may be possible to touch the heating elements –
provide cover or grill to stop this.
No indication when oven hot – provide indicator to
show heaters on.
No indication when oven hot – provide indicator to
show oven temperature.
No overall ON/OFF button – provide overall
ON/OFF button.
Heat still on when door opens – link heaters and
fan to door opening sensing.
1 mark for the hazard, 1 mark for the solution.
Page 23
2
Additional Guidance
Question
12
Expected Answer/s
Max
Mark
Figure Q12 shows an inspection system
controlled by a PLC.
The system operates as follows.








The conveyor is initially stopped with no
box present.
When a toffee box is loaded, Sensor S1
sends a “Box loaded” signal to the PLC.
The PLC then starts the conveyor which
should carry the box to the Inspection area.
When the box is in the Inspection area
Sensor S2 sends a “Box Present” signal to
the PLC.
The PLC stops the conveyor when it
receives the “Box Present” signal.
If the weight of the box is unacceptable,
Sensor S3 sends a “Weight Error” signal to
the PLC.
If the barcode is incorrect, Sensor S4 sends
a “Barcode Error” signal to the PLC.
If either the “Weight Error” or the “Barcode
Error” signal is received then the “REJECT”
indicator is illuminated by the PLC.
A separate handling system loads each box.
12
a
Sketch a flowchart which shows the operation
of the system. Start with the initial conditions:


Conveyor stopped;
Conveyor empty.
Note: the inserted Flowchart Symbol sheet
Q2/Q11/Q12/Q13 gives a suitable selection of
Flowchart Symbols.
Page 24
6
Additional Guidance
Question
12
a
Expected Answer/s
(cont)
START
S1 Detect
NO
YES
Start Conveyor
S2 Detect
YES
Stop Conveyor
S3 Weight
Error?
NO
1 mark each
decision, 0·5
mark for action
and 1 mark for
OR loop
OR any other
suitable
flowchart.
YES
NO
S4 Barcode
Error?
YES
REJECT
Indicator ON
NO
STOP
Page 25
Max
Mark
Additional Guidance
Question
12
b
Expected Answer/s
With reference to the initial Ladder Diagram
Q12(b), briefly explain why a box loaded at
Sensor S1 will NOT reach the inspection area
and Sensor S2.
Max
Mark
2
Note: the inserted PLC Datasheet Q6/Q8/Q12
gives the PLC instruction set.
Sensor S1 (X1) will activate Y1 (1) starting the
conveyor and moving the box.
Sensor S1 (X1) will deactivate (1) as soon as the
box moves away towards S2 and before reaching
the Inspection area.
12
c
Produce a table which shows the PLC I/O
allocation for the inspection system shown in
Figure Q12.
Inputs
Outputs
X1 = S1 box loaded
Y1 = Conveyor
X2 = S2 box present
in Inspection
area
Y2 = Reject indicator
X3 = S3 weight error
X4 = S4 barcode
error
6 × 0·5 marks each = total 3 marks
Note: X1 and Y1 defined by part (b) above is the
logical allocation for the remaining contacts but
alternatives are possible and if chosen may change
the answers to parts (d), (e) and/or (f).
Page 26
3
Additional Guidance
Question
12
d
Expected Answer/s
Redraw the initial Ladder Diagram Q12(b)
showing how it could be modified to ensure
that the conveyor motor continues to run until
the box is detected in the inspection area.
X1 (S1)
Max
Mark
2
X2 (S2)
Y1
Y1
No explanation needed, only a correct ladder
diagram needed.
12
e
Add another rung to your ladder diagram which
reads the “Weight Error” and “Barcode Error”
signals to produce the required output for the
“REJECT” indicator.
3
The additional rung is shown below.
X3 (S3)
Y2
X4 (S4)
No explanation needed, only a correct ladder
diagram rung needed.
12
f
It normally takes a box 30 seconds to travel
from Sensor S1 to Sensor S2. As an upgrade to
the existing system, an audible alarm, Y3, is to
be activated if a box has not arrived at S2 after
40 seconds.
Sketch a ladder diagram that includes only this
upgrade and briefly describe its operation –
there is no need to redraw other parts of the
ladder diagram that remain unchanged.
Page 27
4
Additional Guidance
Question
12
f
Expected Answer/s
Max
Mark
(cont)
Y1
T1
Timeout = 40s
Y3
T1
Alarm
Or other suitable ladder diagram with correct
functionality.
Note: Y1 here could NOT be S1 unless there are
additional elements because of the problem
identified in Q12(b).
Appropriate description to match the ladder
diagram supplied.
Exemplar brief description – when S1 is activated
and Y1 (the conveyor is started and latched) then a
timer T1 with a 40 second timeout is started. This
timer will continue to increase while the conveyor is
running. If S2 is reached then X2 will de-activate,
the rung will be broken and Y1 will de-activate
which resets the timer. If however T1 counter
reaches 40 seconds then contact T1 in the next
rung operates Y3 the alarm.
Ladder diagram = 2 marks. Associated brief
description = 2 marks.
12
g
State a suitable type of sensor for use as
Sensor S1.
Any suitable sensor such as an optical beam which
is interrupted by the box.
Page 28
1
Additional Guidance
Question
12
h
Expected Answer/s
Identify two safety hazards inherent in the
entire inspection system and state how each
safety hazard could be reduced.
Any two appropriate inherent safety hazards with
their associated ways of reducing the hazards.
Exemplar answers Auto start & stop is a potential hazard – this can be
reduced by machine starting announcements.
Auto start & stop is a potential hazard – can be
reduced by personnel isolation system.
There is no "PLC independent" way of stopping the
conveyor – hazard can be reduced by fitting an
emergency stop button.
No control on the number of simultaneous boxes
on the conveyor – hazard can be reduced by
additional sensor and PLC program enhancement.
2 marks for hazards and 2 for reduction methods.
Page 29
Max
Mark
4
Additional Guidance
Question
13
Expected Answer/s
Figure Q13 illustrates a powder coating system
in which domestic radiators are manually
loaded onto the overhead linear conveyor. The
radiators come in three sizes (small, medium
and large). The radiators move through the
various work sections and are then manually
uploaded as a coated product.
The overhead linear conveyor runs
continuously at a constant speed. Closed loop
control ensures the overhead conveyor speed
remains constant regardless of how many
radiators are loaded on or unloaded from the
conveyor.
 In Section 1 (load area), each radiator is
manually loaded onto the overhead linear
conveyor.
 In Section 2 (clean and dry), each radiator is
preheated to 35°C and is cleaned by
blasting it with compressed air.
 In Section 3 (powder coat), Sensor A detects
the arrival of a radiator. Sensor system B
detects the radiator size and this
information is used to decide which one of
three coating programs is to be carried out
by the robot. the radiator is powder coated
by the automated robot.
 In Section 4 (dry and cure), the coated
radiator spends 5 minutes at 85°C where the
coating hardens and dries.
 In Section 5 (unload area), the radiators are
manually unloaded and are collected in
batches for transportation to the final
baking ovens (not shown).
Page 30
Max
Mark
Additional Guidance
Question
13
a
Expected Answer/s
State and briefly describe one type of sensor
which would be appropriate for sensing the
speed of the overhead linear conveyor.
Max
Mark
3
For example: a linear encoder installed on the
conveyor belt system with a suitable optical sensor
and signal conditioning system could be used. The
linear conveyor would be constantly moving
producing a pulse waveform from the encoder
which could be used to sense the linear belt speed.
Or any other suitable sensor capable of sensing
speed and associated brief description.
1 mark for stating sensor, 2 marks for brief
description of stated sensor.
13
b
Briefly explain two safety hazards associated
with this system and describe how they could
be minimised or resolved at the design stage.
Any suitable two safety hazards and associated
minimisation/resolution measures.
Exemplar answers include:



There are fire hazards associated with heat in
the various sections – the system could be
fitted with a fire detection system to sound an
alarm and automatically shut down the system.
There are hazards to people in the robot
section due to the automatic movement of the
robot – the robotic section should be fitted with
a key-lock entry system to ensure no
unauthorised entry during operation.
Radiators are not monitored in most sections –
add sensors to ensure radiators have not fallen
off the conveyor or got stuck somewhere.
2 marks for hazards, 2 marks for associated
minimisation/resolution measures.
Page 31
4
Additional Guidance
Question
13
c
13
c
Expected Answer/s
The rotary position of one axis of the robot
used in Section 3 is sensed using an optical
rotary encoder which uses an 8 bit Gray code.
i
Calculate the resolution in degrees for this Gray
code if the 8 bit code is distributed evenly over
the full 360 degrees of the encoder disc.
28 = 256,
13
c
Max
Mark
ii
360
256
2
 1 41 degrees or ± 0·7 degrees
State one suitable robotic drive system and
give two reasons, other then cost, that justify
your choice.
3
Any suitable example of a robot drive sysyem with
justified reasons.
Exemplar answers An electrical drive system could be used as the
power supply is readily available and they require
less maintenance than pneumatic/hydraulic
systems.
1 mark for drive system, 2 marks for appropriate
reasons.
13
d
The robot programs were created using a “leadto-nose” technique. Describe this method of
programming robots and justify its suitability
for this application.
"Lead by nose" is where the robot is manually
taught the task and this is recorded on the robotic
system and this recorded program can be retrieved
and played back when required.
"Lead by nose" is an appropriate method as the
actions of a skilled worker are passed to the robot
using minimal programming.
Or any other suitable description of "lead by nose"
programming and suitable justification of the
method.
2 marks for description of "lead by nose"
programming, 1 mark for justification of this
application (powder coating spraying).
Page 32
3
Additional Guidance
Question
13
e
Expected Answer/s
Max
Mark
Sensor system B in Section 3 (powder coat) is
used to detect the size of each radiator. Briefly
describe the operation of a suitable sensing
system stating the sensors being used.
3
A variety of solutions are possible.
Exemplar answer:
Sensor A detects radiator presence. It would then
be possible with only two height sensors (light
beams) and then rely on simple logic to detect the
radiator size. The light beam sensors would need
to be able to work in the spray environment but as
the powder coat process is quite clean and the
robot only powder coats after the radiator has been
identified, this type of sensor would be appropriate.
OR
Three light sensors set at the correct height could
be used to detect the radiator sizes. If only the
higher sensor beam is broken then it is a small
radiator requiring powder coating. If only the upper
two sensor beams are broken the radiator is
medium sized and if all three are broken then it is a
large radiator.
Other alternatives – a barcode system could be
used. The weight of the radiator detected.
Electronic labels could be used.
3 marks for solution that takes account of the
environment and radiator size and nature.
13
f
Describe the operation and process decisions
carried out as a radiator passes through
Section 3 (powder coat). Start your description
with the radiator entering Section 3 and make
reference to your sensing system chosen in
Q13(e). Either a written descriptive answer or
Flowchart can be used.
Note: the inserted Flowchart Symbol sheet
Q2/Q11/Q12/Q13 gives a suitable selection of
Flowchart Symbols.
Page 33
3
Additional Guidance
Question
13
f
Expected Answer/s
(cont)
Either a written solution or a flowchart would be
acceptable. Logic must be correct and the solution
may be sensitive to the answer given in Q13(e).
START
Radiator at
Sensor A
N
Y
Radiator
large
Y
Paint Program
Large
N
Radiator
medium
Y
Paint Program
Medium
N
Paint Program
Small
END
Page 34
Max
Mark
Additional Guidance
Question
13
g
Expected Answer/s
Max
Mark
A sensor is required to monitor the flow rate of
the powder being fed to the robot during the
powder coating process in Section 3.
4
State and describe with the aid of a sketch a
suitable sensor.
Any suitable flow sensor with associated
description and sketch.
Example solution shows a paddle wheel sensor.
The flow turns the paddle wheel which in turn
outputs a signal proportional to the flow rate.
Or similar suitable description.
1 mark for the type, 3 for description and diagram.
[END OF MARKING INSTRUCTIONS]
Page 35
Additional Guidance