ME 106 Spring 1997BJ Furman Sensor Broad Outline March 1, 19971Sensors: Applications• DisplacementLinear and AngularPotentiometer (linear and rotary (through gears or belts))EncoderLVDT and RVDT (Linear Variable Differential Transformer)Synchros and ResolversInductive SensorsCapacitiveEddy currentMagneticHall effectMagnetostrictiveMagnetic encoderSwitches and proximity sensorsMicroOptointerrupterReed switchInductive, capacitive, and ultrasonicOptical leverInterferometer•StrainStrain gages• Force/TorqueLoad cellsTorque sensors• Pressure• TemperatureThermocoupleThermistorRTDIC temperature sensor•MotionAccelerometersTachometers and velocity pick-ups• Acoustics/SoundMicrophones•OthersNear-infrared (880 nm) proximity sensors (VCR remote control)ME 106 Spring 1997BJ Furman Sensor Broad Outline March 1, 19972Pyroelectric (8-10 micron)Sonar (Polaroid)HumidityME 106 Spring 1997BJ Furman Sensor Broad Outline March 1, 19973Sources for more information on sensors1. Beckwith, T. G., Marangoni, R. D., Lienhard, J. H., Mechanical Measurements, 5th ed.,Addison-Wesley, Reading, Mass., 1993.2. Dally, J. W., Riley, W. F., McConnell, K. G., Instrumentation for EngineeringMeasurements, 2nd ed., J. Wiley, New York, 1993.3. Trade journals such as NASA Tech Briefs, Mechanical Engineering, Design News, PowerConversion and Intelligent Motion (PCIM).4. The Internet!!• Thomas Register (www.thomcat.com)ME 106 Spring 1997BJ Furman Sensor Broad Outline March 1, 19974Sensors: Applications• DisplacementLinear and AngularPotentiometer (linear and rotary (through gears or belts))A resistive element with a movable contact (wiper), hence a resistor whose resistance varies as functionof displacement.Translational strokes from 0.1 in to about 20 in.Rotational motion from about 10• to about 60 turns (or single turn no-limit versions)Resistance element: wire-wound, carbon film, or conducting plasticIdeally linear input output relationshipAccuracy (linearity) 0.01% to 0.5% FSOBe careful with “loading” the potentiometer. Keep Rp/Rld << 1 (Use amplification stage withsufficiently high input stage, or voltage follower)RpRldVoVex+-+-AwiperVo/Vex = [1/(xi/xt) + Rp/Rld(1 - xi/xt)]-1Where: xi/xt = the fraction of Rp measured between the wiper and point A.Max error is about 12% FSO if Rp/Rld=1For Rp/Rld < 0.1, max error will occur near xi/xt=0.67, and is about 15(Rp/Rld)% FSO.Repeatability: typically 10 times the accuracySensitivityDepends on Vex, but will be limited by power dissipationResolutionDepends on construction of resistance element. Lower limits: wire wound, (limited by number ofturns of wire, 0.001 in. linear, 0.12/D degrees rotational) carbon-film, conductive plastic (5E-6 in.)Frequency Response: Linear, 1 - 25 m/s; Rotary, 100 rpm max.Life: 50E6 to 200E6 cyclesOther ConcernsFrictionNoiseInertiaEnvironmental effectsPros:Relatively inexpensive, $10 - $150Easy to useVarious resistance functions available: sinusoidal, log, etc.Cons:WearFrictionLimited speedVendors: Systron-Donner, NovotechnicME 106 Spring 1997BJ Furman Sensor Broad Outline March 1, 19975EncoderEx. - HP printer carriage, linear encoderWidely used in machine tools, photolithography tools, etc.Incremental vs. absolute opticalpulse train vs. coded numberPhototranmissive or photoreflective (Hamamatsu P306201)Resolutionabsolute limited by number of bits (20 about max, 0.0003•)incremental limited by diffraction. (with interpolation, 0.01 micron, 3.6E-5• possible) 0.5 micron,0.1• typicalRepeatability: linear 0.5 µm to 0.05 µm; rotary, 0.05• - 0.0005•Accuracy: half (or worse) of the repeatabilityFrequency Response: linear, 0.5 - 1 m/s; rotary, 4000 - 10,000 rpm.Pro’s:Non-contactDigital outputRelatively inexpensive modular units are availableCon’s:High resolution devices are expensive $30 - $10kAlignmentNeed to keep contamination awayOther varieties:magnetic, magnetoresitive: rugged, flexible, long lengthsVendors: HP, Oak-Grigsby, DRCLVDT and RVDT (Linear Variable Differential Transformer)Consists of a movable core, primary coil, and two secondary coils symmetrically spaced on a cylindricalform.Core couples excitation voltage in primary to the two secondaries.Phase and amplitude of the secondary outputs varies with the position of the core.Range and ResolutionFull scale ranges from 0.05 to 10 in.Linear range in RVDT’s is about ±40•Resolution is infinite. Practical limits are about 0.1 micron (linear), 0.001• (rotary)Accuracy (linearity):Typically 0.25%, best is 0.01%Repeatability: typically better than 0.1 micronFrequency Response: 400 Hz - 10 kHzPro’s:Non-contact displacement measuring device! No friction, no wear, etc.Infinite resolution, readability limited only by external electronicsCan isolate core and coil by simply a static seal for high pressure or corrosive environments.Complete isolation of excitation and output.Relatively insensitive to temperature changes.High reliability.IC driver and demodulator available, compact unit. (Signetics NE/SE5520, Analog Devices)Con’s:Limited frequency response, about 10% of driving frequency.Mass of coreCost: min $400Vendors: Omega, Lucas SchaevitzSynchros and ResolversAre rotary variable transformers, but instead of a movable magnetic core, there is primary coil woundon a rotor. Magnitude and phase of induced secondary voltages depends on position and voltage applied(through slip-rings) to rotor.ME 106 Spring 1997BJ Furman Sensor Broad Outline March 1, 19976Synchros have 3 secondary stator coils separated by 120•Resolvers have 2 secondary stator coils separated by 90•Accuracy (linearity): 0.1•Repeatability: 2 to 5 times the accuracyResolution: 0.0003• to 0.03•Frequency response: 0 to 20 HzVendors: LittonInductive SensorsLinearRelated to LVDT’s, but only two inductive coils, connected in a bridge circuit.RotaryHave a specially shaped rotating coreRange and Resolution±10 mm to ±75 mm range0.3 µm to 2.5 µm resolutionAccuracy (linearity):±0.2% to ±0.15%, best 0.05%Frequency Response:Flat, 0 to 6 kHzPro’s:Smaller, lower temperature coefficient, higher frequency response, better shock and vibrationresistance, and CHEAPER than LVDT’s.Vendors: OmegaCapacitiveProbe and conductive target form parallel plates of a capacitor. Change in spacing and resulting changein capacitance modulates the ouput of an oscillator.Range: on the order of ±0.13 mmAccuracy: 0.1% - 0.2% FS rangeRepeatability: two to five time resolutionResolution: 2.5 nm typically, but