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A brief overview of sensors (and why you should love them)

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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

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