Robot Sensors Robot Sensors Sensor Characteristics Switch Sensor

Robot Sensors Robot Sensors Sensor Characteristics Switch Sensor
Robot Sensors
• Allow a robot to interact with its environment in a
flexible, and intelligent manner
Robot Sensors
Dr. Rohan Munasinghe
BSc, MSc, PhD, MIEEE
Department of Electronic and Telecommunication Engineering
Faculty of Engineering
University of Moratuwa 10400
• A robot that can see and feel is much easier to
train and deploy to perform complex tasks
• Provide position and velocity of wheels as a
continuous stream of feedback signals that
possess an integral part of the control loop
• Provide information about the environment and
the objects therein
Sensor Characteristics
• Dynamic range
– Minimum and maximum values of the input signal for which
the sensor responds
• Response
Switch Sensor
• Indicates whether or not a contact has been made, while
disregarding the magnitude of the contact force
– Micro switches, limit switches
47kΩ
Ω
– Sensor should respond to the stimuli almost instantaneously
+5V
signal
0V
• Sensitivity
GND
– The change in sensor output for a unit change in input
• Linearity
– Whether the sensor maintains same sensitivity within the
entire dynamic range
5V:not pressed (“false”)
0V:pressed (“true”)
• Other considerations
– Sensor should not disturb the physical quantity it measures
– Sensor should be suitable for the environment it is exposed
– Sensor should be isolated from noise, and protected from
physical damages
– Size, cost, and ease of operation
47kΩ
Ω
47kΩ
Ω
signal
5V
+5V
GND
Applications of Switch Sensors
• Collision detection and ground detection
Optosensors
• Passive Sensors
– Photocell (LDR): street light trigger
– Slow response
– 10kΩ
Ω in the dark (open cct), and 10Ω
Ω(short cct) in bright light
• Used in potential divider configuration
• Light (photons) transfer energy to bound electrons
• Responsiveness to a wide range of frequencies
– IR, visible, UV
Optosensors cntd..
• Active sensors
– Emitter : Infra red LED
– Detector : Photodiode (faster), or Phototransistor (sensitive)
– Performance
Interfacing a Photocell
Potential divider
Robot
Control
board
• fast response, low light levels, wavelength match
– Two kinds of active Optosensors
• Reflective: light is reflected off a surface onto the detector
– Used for proximity sensing
• Break beam: light is shined directly onto the detector, and is
obstructed by an opaque object
– Used for shaft encoding (with a slotted wheel)
5V
Light Baffling
Gnd 7 Analog inputs
Interfacing Active Optosensors
Properties of Optosensors
• Emitter testing
– IR is invisible
– use IR detector card, camcoder, or camera phone to check
whether IR emitter works
OFF 5V
ON 0V
• IR filter
– Semiconductor junction is most efficient in infra-red
wavelengths. Detector is usually attached with a filter, which
allows infra-red to pass through while blocking ambient light
interference
• Adjust pull-up resister
Light falls on ⇒ detector current ↑ ⇒ voltage across pull-up
resister ↑⇒ analog reading ↓
– Use different pull-up resisters to improve response
• Correction for ambient light
– Switch emitter ON and OFF and get the difference of the two
readings
– Reduce power consumption too
• Emitter brightness control
– 5V through 500Ω
Ω ⇒ 10mA. Check whether it is bright enough.
Break-Beam Sensor
• Digital IR Sensors
– Whether you see through the slot
Reflective Optosensor
• Digital IR Sensors
– Whether you are in very close proximity of an obstacle
5V (logic 1) on black
0V (logic 0) on white
Absolute Encoder
Qudrature Shaft Encoder (two tracks)
Resolution = 360 / 2 n
n = number of bits
1800
Shaft
position
This track and
sensor Is missing
In figure
inner
outer
wheel motion
Eg. 110010
= 3×360/26
= 6.875°
time
time
Phase ⇒Direction of motion
Pulse count (and derivative) ⇒ position (and velocity)
Qudrature Shaft Encoder (one track)
Encoder A
1
clockwise
-1
• Measures distance to obstacle
Encoder B
0
+1
counter
clockwise
Counter clockwise motion
A
B
clockwise
IR Distance Measuring Sensors
counter
clockwise
IR Distance Measuring Sensor
IR Distance Measuring Sensor
• Use reception intensity as a measure of distance
• SHARP GP2D12 : Distance output type (10cm~80cm)
• SHARP GP2D12 : Distance judgment type (24cm)
Timing
Diagram
Reflectivity
Sensing High Speeds
Illumination
Sensing Low Speeds
ω≈
3
T
or
ω≈
4
T + δT
Ultrasonic Range Sensor
• 3cm ~4m, 40~60kHz
• Emits ultrasonic ping and listen to the echo
time of flight × 0.5 × sound speed = distance to obstacle
• Problems
– Wide field of reception: where is the obstacle exactly??
– False echoes: ground reflection, etc..
– Not very accurate: sound speed changes on temperature and humidity
Sonar Sensor Interface
Internal resonances takes
2.38ms to decay out.
Echoes can be detected
after this interval (min
distance limit)
Pin
Configuration
At a distance, echo
becomes too weak to be
detected (max distance)
Timing Diagram
Mode 1 (two pin)
Sonar Sensor Beam width
Robots with Ultrasonic Sensors
• Real-time Collision Avoidance
As frequency increase attenuation increases and beam width reduces
Beam width shouldn’t be too wide to hear false echoes. It shouldn’t be too
narrow to miss a legitimate obstacle
Narrow beam width sensors are used atop a scanning (servo) mechanisms
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