Im Mr. Sensitivity... A brief overview of sensors (and why you should love them) copyright 2002:...
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Transcript of Im Mr. Sensitivity... A brief overview of sensors (and why you should love them) copyright 2002:...
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