I’m Mr. Sensitivity. . .

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

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.

Shadow cast indicates obstacle: one way to navigate with photo resistors.

Example of sharp IR mounted to sweep for a wider field of view.

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.

Solar Cell

Digital Infrared Ranging

Compass

Touch Switch

Pressure Switch

Limit Switch

Magnetic Reed Switch

Magnetic Sensor

Miniature Polaroid Sensor

Polaroid Sensor Board

Piezo Ultrasonic Transducers

Pyroelectric Detector

Thyristor

Gas Sensor

Gieger-MullerRadiation Sensor

Piezo Bend Sensor

Resistive Bend Sensors

Mechanical Tilt Sensors

Pendulum Resistive Tilt Sensors

CDS Cell Resistive Light Sensor

Hall EffectMagnetic Field

Sensors

Compass

IRDA Transceiver

IR Amplifier Sensor

IR ModulatorReceiverLite-On IR

Remote Receiver

Radio ShackRemote Receiver

IR Sensor w/lens

GyroAccelerometer

IR Reflection Sensor

IR Pin Diode

UV Detector

Metal Detector

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

Light Sensor• Good for detecting direction/presence of light • Non-linear resistance• Slow response

Resistive Sensors

Resistive Bend Sensor

Cadmium Sulfide Cell

Potentiometer

Sensor

• Measure bend of a joint

• Wall Following/Collision Detection

• Weight Sensor

Sensors

Sensor

Applications

micro+-

Single Pin Resistance

Measurement

Binary Threshold

micro

Analog to Digital(pull down)

Inputs for Resistive Sensors

Voltage divider: You have two resisters, oneis fixed and the other varies,as well as a constant voltage

V1 – V2 * (R2/R1+R2) = V

R1

R2

measureKnown unknown

V

V1

V2

Comparator: ifvoltage at + is greaterthan at -, high value out

Intensity Based Infrared

• Easy to implement (few components)• Works very well in controlled environments• Sensitive to ambient light

time

volt

age

timevo

ltag

e

Increase in ambient light raises DC bias

Modulated Infrared

http://www.hvwtechnologies.comhttp://www.digikey.com

• 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

limiter demodulatorbandpass filteramplifier

comparatorintegrator

600us 600us

Input

Output

Digital Infrared Ranging

position sensitive device (array of photodiodes)

Optical lenses

Modulated IR beam

• 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

+5voutputinput

gnd

1k 1k

Polaroid Ultrasonic Sensor

•

Mobile Robot Electric Measuring Tape

Focus for Camerahttp://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 withBayes Rule (Morevac and Elfes)

Arc Transversal Method

• Uniform Distribution on Arc

• Consider Transversal Intersections

• Take the Median

Mapping Example

Vendors

• Micromint

• Wirz

• Gleason Research (Handyboard)

• Polaroid-oem

Metal Detector

112kHz LC Oscillator

Oscillator signal coupled via transformer

Diode converts AC signal to DC rippleand applies as bias to T3

When T2 is turned off, T3 is turned on

LED will drop about 2volts

9v Signal to 5v logic

+-

LM311 comparator

PIC

+5V

Rpullup

+9V

9v signal

A comparator can be used to convert a two-state signal to digital logicWhen the + voltage is above the voltage on the - pin, the output is highWhen the + voltage is below the - voltage, the output is lowThe LM311 has an open collector (you need to provide pullup resistor)This allows conversion from 9volt logic to 5volt logic

Accelerometers

adxl2022-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

• Measure Weight

ADXL202EB

ADXL202EB

• Pass signal through low-pass filter, then to ADC– Averages signal

– Filter cutoff frequency should be < 0.1 bandwidth

C

R

+

-

ADXL202EBBandwidth = 100 Hz

LM324PIC

ADC0

+5V0.1F

Hz102

1 RC

t

ViVo

Vo

Vi

Analog Tilt Measurements

• Send PWM signal directly to CCP – Set to measure pulse width

– Uses a valuable microcontroller resource

ADXL202EBBandwidth = 100 Hz

Pic

CCP

+5V0.1F

Vi

ton

0.5 on

period

taccel

t

50% duty cycle = 0 accel

tperiod

Digital Tilt 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

C

R +-ADXL202EB

Bandwidth = 1000 Hz

LM324

PIC

ADC0

+5V

t

Vi

Vo

VoVi

R R

+ -

Rb

Analog Velocity Measurements

• Pass acceleration signal through comparator, then to input capture– Need high signal bandwidth to see pulse

C

R

+-

ADXL202EBBandwidth = 1000 Hz

LM311 comparator

PIC

Rb

Hz1002

1 RC

t

Vi

Vo

Vo

Vi

+5V

Vth

RpullupVth

Threshold Measurements