Presentation on theme: "A brief overview of sensors (and why you should love them)"— Presentation transcript:
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 devicesEncodersVisionMine detectors (detector vs. sensor)
5 Some types of Sensors: Ladar (laser distance and ranging) Sonar Radar Time of flightPhase shiftSonarRadarInfra-redLight sensingHeat sensingTouch 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 meatNow 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.
11 Resistive Sensors Bend Sensors Resistance = 10k to 35k Force to produce 90deg = 5 grams= 10$PotentiometersFixed Rotation SensorsEasy to find, easy to mountLight SensorGood for detecting direction/presence of lightNon-linear resistanceSlow responseResistive Bend SensorPotentiometerCadmium Sulfide Cell
12 Applications Measure bend of a joint SensorMeasure bend of a jointWall Following/Collision DetectionWeight SensorSensorsSensor
13 Inputs for Resistive Sensors Voltage divider:You have two resisters, oneis fixed and the other varies,as well as a constant voltageV1 – V2 * (R2/R1+R2) = VR1VAnalog to Digital(pull down)R2V2microKnown unknownmeasuremicro+BinaryThresholdSingle PinResistanceMeasurement-Comparator: ifvoltage at + is greaterthan at -, high value out
14 Intensity Based Infrared Increase in ambient lightraises DC biasvoltageEasy to implement (few components)Works very well in controlled environmentsSensitive to ambient lighttimevoltagetime
15 Modulated Infrared Insensitive to ambient light amplifierbandpass filterintegratorlimiterdemodulatorcomparatorInputOutput600us600usInsensitive to ambient lightBuilt in modulation decoder (typically 38-40kHz)Used in most IR remote control units ( good for communications)Mounted in a metal faraday cageCannot detect long on-pulsesRequires modulated IR signal
16 Digital Infrared Ranging Modulated IR beamOptical lenses+5voutputinput1k1kgndposition sensitive device (array of photodiodes)Optics to covert horizontal distance to vertical distanceInsensitive to ambient light and surface typeMinimum range ~ 10cmBeam width ~ 5degDesigned to run on 3v -> need to protect inputUses Shift register to exchange data (clk in = data out)Moderately reliable for ranging
17 Polaroid Ultrasonic Sensor ElectricMeasuring TapeMobile RobotFocus for Camera
18 Digital Init Chirp 16 high to low -200 to 200 V Internal Blanking Theory of OperationDigital InitChirp16 high to low-200 to 200 VInternal BlankingChirp reaches object343.2 m/sTemp, pressureEchoesShapeMaterialReturns to XducerMeasure the time
19 Problems Azimuth Uncertainty Specular Reflections Multipass Highly sensitive to temperature and pressure changesMinimum Range
27 VendorsMicromintWirzGleason Research (Handyboard)Polaroid-oem
28 Metal Detector Oscillator signal coupled via transformer When T2 is turned off, T3 is turned on112kHzLC OscillatorLED will drop about 2voltsDiode converts ACsignal to DC rippleand applies as bias to T3
29 9v Signal to 5v logic + - +9V +5V Rpullup PIC LM311comparatorA 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
30 Accelerometers adxl202 2-axis accelerometer Mems technology provides precision mechanical electrical devicesADXL202 outputs convenient PWM output whose duty cycle is proportional to accelerationCost 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 ADCAverages signalFilter cutoff frequency should be < 0.1 bandwidth+5V0.1mFRLM324PICViADXL202EBBandwidth = 100 Hz+ADC0C-VoViVot
33 Digital Tilt Measurements Send PWM signal directly to CCPSet to measure pulse widthUses a valuable microcontroller resource+5V0.1mFPicADXL202EBBandwidth = 100 HzViCCPton50% duty cycle = 0 acceltperiod
34 Analog Velocity Measurements Pass acceleration signal through integrator, then to ADCNeed to compensate for 2.5V offsetNeed to choose RC such that Vo does not saturateNeed to periodically reset integrator to prevent overflowRR+5VPICLM324+RADC0ADXL202EBBandwidth= 1000 Hz-RbVo+-ViCViVot
35 Threshold Measurements Pass acceleration signal through comparator, then to input captureNeed high signal bandwidth to see pulseRLM311 comparatorViADXL202EBBandwidth= 1000 Hz+VoRbC-PICRpullup+5VVthVoViVtht