1. A brief overview of sensors (and why you should love them)
I’m Mr. Sensitivity. . .
copyright 2002: Sean Pieper, Bob Grabowski, Howie Choset

2. Why Sensors Are Important:
The world is NOT static.
It can also be incredibly complex.

3. 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. . .

4. 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)

5. Some types of Sensors: Ladar (laser distance and ranging) Sonar Radar
Time of flight
Phase shift
Sonar
Radar
Infra-red
Light sensing
Heat sensing
Touch sensing

6. How to Choose a Sensor:
There are four main factors to consider in choosing a sensor.
Cost: sensors can be expensive, especially in bulk.
Environment: there are many sensors that work well and predictably inside, but that choke and die outdoors.
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.
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.

7. 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.

8. Example of sharp IR mounted to sweep for a wider field of view.
Shadow cast indicates obstacle:
one way to navigate with photo resistors.

9. 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.

10. Resistive Light Sensor
Gas Sensor
Accelerometer
Gyro
Metal Detector
Pendulum Resistive
Tilt Sensors
Piezo Bend Sensor
Gieger-Muller
Radiation Sensor
Pyroelectric Detector
UV Detector
Resistive Bend Sensors
CDS Cell
Resistive Light Sensor
Digital Infrared Ranging
Pressure Switch
Miniature Polaroid Sensor
Limit Switch
Touch Switch
Mechanical Tilt Sensors
IR Pin
Diode
IR Sensor w/lens
Thyristor
Magnetic Sensor
Polaroid Sensor Board
Hall Effect
Magnetic Field
Sensors
IR Reflection
Sensor
Magnetic Reed Switch
IR Amplifier Sensor
IRDA Transceiver
IR Modulator
Receiver
Lite-On IR
Remote Receiver
Radio Shack
Remote Receiver
Solar Cell
Compass
Compass
Piezo Ultrasonic Transducers

11. Resistive Sensors Bend Sensors Resistance = 10k to 35k
Force to produce 90deg = 5 grams
= 10$
Potentiometers
Fixed Rotation Sensors
Easy to find, easy to mount
Light Sensor
Good for detecting direction/presence of light
Non-linear resistance
Slow response
Resistive Bend Sensor
Potentiometer
Cadmium Sulfide Cell

12. Applications Measure bend of a joint
Sensor
Measure bend of a joint
Wall Following/Collision Detection
Weight Sensor
Sensors
Sensor

13. Inputs for Resistive Sensors
Voltage divider:
You have two resisters, one
is fixed and the other varies,
as well as a constant voltage
V1 – V2 * (R2/R1+R2) = V R1 V
Analog to Digital
(pull down) R2 V2
micro
Known unknown
measure
micro +
Binary
Threshold
Single Pin
Resistance
Measurement -
Comparator: if
voltage at + is greater
than at -, high value out

14. Intensity Based Infrared
Increase in ambient light
raises DC bias
voltage
Easy to implement (few components)
Works very well in controlled environments
Sensitive to ambient light
time
voltage
time

15. Modulated Infrared Insensitive to ambient light
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

16. 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

17. Polaroid Ultrasonic Sensor
Electric
Measuring Tape
Mobile Robot
Focus for Camera

18. Digital Init Chirp 16 high to low -200 to 200 V Internal Blanking
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

19. Problems Azimuth Uncertainty Specular Reflections Multipass
Highly sensitive to temperature and pressure changes
Minimum Range

20. Beam Pattern
Not Gaussian!!

21. (Naïve) Sensor Model

22. Problem with Naïve Model

23. Reducing Azimuth Uncertainty
Pixel-Based Methods (Most Popular)
Region of Constant Depth
Arc Transversal Method
Focusing Multiple Sensor

24. Certainty Grid Approach
Combine info with
Bayes Rule
(Morevac and Elfes)

25. Arc Transversal Method
Uniform Distribution on Arc
Consider Transversal Intersections
Take the Median

26. Mapping Example

27. Vendors
Micromint
Wirz
Gleason Research (Handyboard)
Polaroid-oem

28. 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

29. 9v Signal to 5v logic + - +9V +5V Rpullup 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

30. 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

31. Accelerometer Uses Measure tilt of arm Measure Weight ADXL202EB

32. Analog Tilt Measurements
Pass signal through low-pass filter, then to ADC
Averages signal
Filter cutoff frequency should be < 0.1 bandwidth
+5V
0.1mF R
LM324
PIC Vi
ADXL202EB
Bandwidth = 100 Hz +
ADC0 C - Vo Vi Vo t

33. Digital Tilt Measurements
Send PWM signal directly to CCP
Set to measure pulse width
Uses a valuable microcontroller resource
+5V
0.1mF
Pic
ADXL202EB
Bandwidth = 100 Hz Vi
CCP
ton
50% duty cycle = 0 accel
tperiod

34. 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
+5V
PIC
LM324 + R
ADC0
ADXL202EB
Bandwidth
= 1000 Hz - Rb Vo + - Vi C Vi Vo t

35. Threshold Measurements
Pass acceleration signal through comparator, then to input capture
Need high signal bandwidth to see pulse R
LM311 comparator Vi
ADXL202EB
Bandwidth
= 1000 Hz + Vo Rb C -
PIC
Rpullup
+5V
Vth Vo Vi
Vth t