Lecture 9 Sensors, A/D, sampling noise and jitterLight Sensors - PhotoresistorLight Sensors - PhototransistorLight Sensors - Pyroelectric SensorsOther Common SensorsIncidence -- photoreflectorsRotational Position SensorsPowerPoint PresentationSlide 9Slide 10Gray CodeSlide 12Draper Tuning Fork GyroImprovement in MEMS GyrosPiezoelectric GyroscopesPiezoelectric GyroscopeAbsolute Angle MeasurementDesign considerationMeasurement Accuracy vs. PrecisionDeviationsRandom ErrorsModel based CalibrationNoise Reduction: FilteringFilter Noise ExampleAveraging (filter analysis)‘Normal’ Gaussian StatisticsIssues with Normal statisticsCharacteristic of ADC and DACMonotonic and missing codeOffset and Gain ErrorD/A nonlinearity (D/A)D/A nonlinearity (A/D)Glitch (D/A)Sampling TheoremAliasing (Sinusoids)Aliased ReconstructionAlaisingQuantization EffectsQuantization ErrorQuantization Noise (A/D)Quantization NoiseSampling Jitter (Timing Error)Sampling-Time UncertaintyJitter Noise AnalysisSampling Jitter BoundsDAC Timing JitterSlide 47DAC Jitter BoundsDecoder-Based D/A convertersSlide 50Slide 51Slide 52Binary-Scaled D/A ConvertersSlide 54Thermometer-Code ConverterFlash (Parallel) ConvertersFeedback in Sensing/ConversionNyquist-Rate A/D convertersIntegrating convertersSuccessive-Approximation ConvertersSlide 61Sigma Delta A/D ConverterModulator CharacteristicsLecture 9Lecture 9Sensors, A/D, sampling noise and Sensors, A/D, sampling noise and jitterjitterForrest BrewerLight Sensors - PhotoresistorLight Sensors - Photoresistorvoltage divider Vsignal = (+5V) RR/(R + RR)–Choose R=RR at median of intended measured range–Cadmium Sulfide (CdS)–Cheap, relatively slow (low current)•tRC = Cl*(R+RR)Typically R~50-200k C~20pF so tRC~20-80uS => 10-50kHzLight Sensors - PhototransistorLight Sensors - Phototransistor-Much higher sensitivity-Relatively slow response (~1-5uS due to collector capacitance)Light Sensors - Pyroelectric Light Sensors - Pyroelectric Sensors Sensors lithium tantalate crystal is heated by thermal radiationtuned to 8-10 m radiation – maximize response to human IR signaturemotion detecting burglar alarmE.g. Eltec 442-3 sensor - two elements, Fresnel optics, output proportional to the difference between the charge on the left crystal and the charge on the right crystal.Other Common SensorsOther Common SensorsForce–strain gauges - foil, conductive ink–conductive rubber–rheostatic fluids•Piezorestive (needs bridge)–piezoelectric films–capacitive force•Charge sourceSound–Microphones•Both current and charge versions–Sonar•Usually PiezoelectricPosition–microswitches–shaft encoders–gyrosAcceleration–MEMS–PendulumMonitoring– Battery-level•voltage– Motor current•Stall/velocity– Temperature•Voltage/Current SourceField–Antenna–Magnetic•Hall effect•Flux GateLocation–Permittivity–DielectricIncidence -- photoreflectorsIncidence -- photoreflectorsRotational Position SensorsRotational Position SensorsOptical Encoders–Relative position–Absolute positionOther Sensors–Resolver–PotentiometerJizhong XiaoOptical Encoders• Relative position mask/diffuser gratinglight emitterlight sensordecode circuitryJizhong XiaoOptical Encoders• Relative position - direction - resolution Ronchi gratinglight emitterlight sensordecode circuitryABA leads BPhase lag between A and B is 90 degrees (Quadrature Encoder)Jizhong XiaoOptical Encoders• Detecting absolute position•Typically 4k-8k/2•Higher Resolution Available – Laser/Hologram (0.1-0.3” resolution) Jizhong Xiao000000010011001001100111010101001100..1001Jizhong XiaoGray CodeGray CodeAlmost universally used encodingOne transition per adjacent number–Eliminates alignment issue of multiple bits–Simplified Logic–Eliminates position jitter issuesRecursive Generalization of 2-bit quadrature code–Each 2n-1 segment in reverse order as next bit is added–Preserves unambiguous absolute position and directionOther Motor Sensors• Resolver•Selsyn pairs (1930-1960)•High speed• Potentiometer •High resolution•Monotone but poor linearity•Noise!•Deadzone!Jizhong XiaoDraper Tuning Fork GyroDraper Tuning Fork GyroThe rotation of tines causes the Coriolis ForceForces detected through either electrostatic, electromagnetic or piezoelectric.Displacements are measured in the Comb driveImprovement in MEMS GyrosImprovement in MEMS GyrosImprovement of drift–Little drift improvement in last decade–Controls/Fabrication issueImprovement of resolution00.20.40.60.811.2drift '93 drift '98 drift '08Deg / hr0.0010.010.1110Resolution '93 Resolution '94Resolution '97degree/secPiezoelectric GyroscopesPiezoelectric GyroscopesBasic Principles–Piezoelectric plate with vibrating thickness–Coriolis effect causes a voltage form the material–Very simple design and geometryPiezoelectric GyroscopePiezoelectric GyroscopeAdvantages–Lower input voltage than vibrating mass–Measures rotation in two directions with a single device–More RobustDisadvantages–(much) Less sensitive–Output is large when Ω = 0•Drift compensationAbsolute Angle MeasurementAbsolute Angle MeasurementBias errors cause a drift while integratingAngle is measured with respect to the casing–The mass is rotated with an initial θ–When the gyroscopes rotates the mass continues to rotate in the same directionAngular rate is measured by adding a driving frequency ωdDesign considerationDesign considerationDamping needs to be compensated Irregularities in manufacturingAngular rate measurementFor angular rate measurementCompensation forceMeasurement Accuracy vs. Measurement Accuracy vs. PrecisionPrecisionExpectation of deviation of a given measurement from a known standard–Often written as a percentage of the possible values for an instrumentPrecision is the expectation of deviation of a set of measurements–“standard deviation” in the case of normally distributed measurements–Few instruments have normally distributed errorsDeviationsDeviationsSystematic errors–Portion of errors that is constant over data gathering experiment–Beware timescales and conditions of experiment– if one can identify a measurable input parameter which correlates to an error – the error is systematic–Calibration is the process of reducing systematic errors–Both means and medians provide estimates of the systematic portion of a