A brief overview of sensors
(and why you should love them)
I’m Mr. Sensitivity. . .
copyright 2002: Sean Pieper, Bob Grabowski, Howie Choset
Why Sensors Are Important:
The world is NOT static.
It can also be incredibly complex.
Oh no. I thought you said the
40’th canal on sector 15. .
Sometimes, the environment where a robot will be deployed is simply unknowable. . .
What Is a Sensor?
• Anything that detects the state of the environment.
• Yep. You’ve already used sensors before in the
Braitenburg lab.
• Are the following sensors?
–
–
–
–
positioning devices
Encoders
Vision
Mine detectors (detector vs. sensor)
Some types of Sensors:
• Ladar (laser distance and ranging)
– Time of flight
– Phase shift
•
•
•
•
•
•
Sonar
Radar
Infra-red
Light sensing
Heat sensing
Touch sensing
How to Choose a Sensor:
There are four main factors to consider in choosing a sensor.
1)
Cost: sensors can be expensive, especially in bulk.
2)
Environment: there are many sensors that work well and
predictably inside, but that choke and die outdoors.
3)
Range: Most sensors work best over a certain range of distances.
If something comes too close, they bottom out, and if something
is too far, they cannot detect it. Choose a sensor that will detect
obstacles in the range you need.
4)
Field of View: depending upon what you are doing, you may want
sensors that have a wider cone of detection. A wider “field of
view” will cause more objects to be detected per sensor, but it also
will give less information about where exactly an object is when
one is detected.
Creative Uses:
• Sharp IR sensors are very accurate and operate well over a large range
of distances proportional to the size of a Lego robot. However, they
have almost no spread. This can cause a robot to miss an obstacle
because of a narrow gap. One solution is to make the sensor pan.
• One could also use a light sensor to detect obstacles indoors. Inside,
there tend to be lights at many angles and locations. Thus, around the
edges of most obstacles, a slight shadow will be cast. A light sensor
could detect this shadow and thus the associated object. Warning: this
could be a very fickle design.
• Touch sensors can have their spread increased with large bumpers, and
can be used for wall following to implement bug2. They are also dirt
cheap.
Example of sharp IR mounted to
sweep for a wider field of view.
Shadow cast indicates obstacle:
one way to navigate with photo resistors.
Time for some meat
• Now that you understand the vital
importance of sensors to constructing robust
and interesting robots, you’re probably
curious about what’s available.
• Here’s a small sampling of commonly used
sensors and their applications. . .
• Hang on tight.
Gas Sensor
Gyro
Accelerometer
Pendulum Resistive
Tilt Sensors
Metal Detector
Piezo Bend Sensor
Gieger-Muller
Radiation Sensor
Pyroelectric Detector
UV Detector
Resistive Bend Sensors
Digital Infrared Ranging
CDS Cell
Resistive Light Sensor
Pressure Switch
Miniature Polaroid Sensor
Limit Switch
Touch Switch
Mechanical Tilt Sensors
IR Pin
Diode
IR Sensor w/lens
Thyristor
Magnetic Sensor
IR Reflection
Sensor
Magnetic Reed Switch
IR Amplifier Sensor
Hall Effect
Magnetic Field
Sensors
Polaroid Sensor Board
IRDA Transceiver
Lite-On IR
Remote Receiver
Radio Shack
Remote Receiver
IR Modulator
Receiver
Solar Cell
Compass
Compass
Piezo Ultrasonic Transducers
Resistive Sensors
Bend Sensors
• Resistance = 10k to 35k
• Force to produce 90deg = 5 grams
• www.jameco.com = 10$
Potentiometers
• Fixed Rotation Sensors
• Easy to find, easy to mount
Resistive Bend Sensor
Potentiometer
Light Sensor
• Good for detecting direction/presence of light
• Non-linear resistance
• Slow response
Cadmium Sulfide Cell
Applications
Sensor
• Measure bend of a joint
Sensors
• Wall Following/Collision
Detection
Sensor
• Weight Sensor
Inputs for Resistive Sensors
V1
Voltage divider:
You have two resisters, one
is fixed and the other varies,
as well as a constant voltage
R1
V
Analog to Digital
(pull down)
R2
V2
V1 – V2 * (R2/R1+R2) = V
Known unknown
micro
measure
micro
Single Pin
Resistance
Measurement
+
-
Binary
Threshold
Comparator: if
voltage at + is greater
than at -, high value out
Intensity Based Infrared
voltage
Increase in ambient light
raises DC bias
time
voltage
• Easy to implement (few components)
• Works very well in controlled environments
• Sensitive to ambient light
time
Modulated Infrared
amplifier
bandpass filter
integrator
limiter
demodulator
comparator
Input
Output
600us
600us
• Insensitive to ambient light
• Built in modulation decoder (typically 38-40kHz)
• Used in most IR remote control units ( good for communications)
• Mounted in a metal faraday cage
• Cannot detect long on-pulses
• Requires modulated IR signal
http://www.hvwtechnologies.com
http://www.digikey.com
Digital Infrared Ranging
Modulated IR beam
Optical lenses
+5v
output
input
1k
1k
gnd
position sensitive device
(array of photodiodes)
• Optics to covert horizontal distance to vertical distance
• Insensitive to ambient light and surface type
• Minimum range ~ 10cm
• Beam width ~ 5deg
• Designed to run on 3v -> need to protect input
• Uses Shift register to exchange data (clk in = data out)
• Moderately reliable for ranging
Polaroid Ultrasonic Sensor
•
Mobile Robot
Electric
Measuring Tape
Focus for Camera
http://www.robotprojects.com/sonar/scd.htm
Theory of Operation
• Digital Init
• Chirp
– 16 high to low
– -200 to 200 V
• Internal Blanking
• Chirp reaches object
– 343.2 m/s
– Temp, pressure
• Echoes
– Shape
– Material
• Returns to Xducer
• Measure the time
Problems
•
•
•
•
•
Azimuth Uncertainty
Specular Reflections
Multipass
Highly sensitive to temperature and pressure changes
Minimum Range
Beam Pattern
Not Gaussian!!
(Naïve) Sensor Model
Problem with Naïve Model
Reducing Azimuth Uncertainty
•
•
•
•
Pixel-Based Methods (Most Popular)
Region of Constant Depth
Arc Transversal Method
Focusing Multiple Sensor
Certainty Grid Approach
Combine info with
Bayes Rule
(Morevac and Elfes)
Arc Transversal Method
•
•
•
Uniform Distribution on Arc
Consider Transversal Intersections
Take the Median
Mapping Example
Vendors
•
•
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Micromint
Wirz
Gleason Research (Handyboard)
Polaroid-oem
Metal Detector
Oscillator signal
coupled via transformer
When T2 is turned off, T3 is turned on
112kHz
LC Oscillator
LED will drop about 2volts
Diode converts AC
signal to DC ripple
and applies as bias to T3
9v Signal to 5v logic
+9V
+5V
Rpullup
9v signal
+
-
PIC
LM311
comparator
A comparator can be used to convert a two-state signal to digital logic
When the + voltage is above the voltage on the - pin, the output is high
When the + voltage is below the - voltage, the output is low
The LM311 has an open collector (you need to provide pullup resistor)
This allows conversion from 9volt logic to 5volt logic
Accelerometers
adxl202
2-axis
accelerometer
• Mems technology provides precision mechanical electrical devices
• ADXL202 outputs convenient PWM output whose duty cycle is proportional to acceleration
• Cost about 30$ - easy to interface to PIC
Accelerometer Uses
• Measure tilt of arm
ADXL202EB
ADXL202EB
• Measure Weight
Analog Tilt Measurements
• Pass signal through low-pass filter, then to ADC
– Averages signal
– Filter cutoff frequency should be < 0.1 bandwidth
+5V
ADXL202EB
Bandwidth = 100 Hz
Vi
LM324
R
PIC
+
-
C
1
 10 Hz
2RC
0.1mF
ADC0
Vo
Vi
Vo
t
Digital Tilt Measurements
• Send PWM signal directly to CCP
– Set to measure pulse width
– Uses a valuable microcontroller resource
+5V
0.1mF
ADXL202EB
Bandwidth = 100
Hz
Vi
CCP
ton
accel  0.5 
ton
t period
Pic
tperiod
50% duty cycle = 0 accel
Analog Velocity Measurements
• Pass acceleration signal through integrator, then to ADC
– Need to compensate for 2.5V offset
– Need to choose RC such that Vo does not saturate
– Need to periodically reset integrator to prevent overflow
R
R
+
-
R
ADXL202EB
Bandwidth
= 1000 Hz
+
Vi
-
Vo
t
PIC
LM324
C
Vi
+5V
ADC0
Vo
Rb
Threshold Measurements
• Pass acceleration signal through comparator, then to
input capture
– Need high signal bandwidth to see pulse
ADXL202EB
Bandwidth
= 1000 Hz
R
Vi
LM311 comparator
+
-
C
Vo
Rb
PIC
Rpullup
+5V
Vth
Vo
1
 100 Hz
2RC
Vi
Vth
t