lect9 - Lecture 9 Lecture 9 Sensors, A/D, sampling noise...

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Unformatted text preview: Lecture 9 Lecture 9 Sensors, A/D, sampling noise and jitter Sensors, A/D, sampling noise and jitter Forrest Brewer Light Sensors - Photoresistor Light Sensors - Photoresistor voltage divider V signal = (+5V) R R /(R + R R ) Choose R=R R at median of intended measured range Cadmium Sulfide (CdS) Cheap, relatively slow (low current) t RC = C l *(R+R R ) Typically R~50-200k C~20pF so t RC ~20-80uS => 10-50kHz Light Sensors - Phototransistor Light Sensors - Phototransistor- Much higher sensitivity- Relatively slow response (~1-5uS due to collector capacitance) Light Sensors - Pyroelectric Sensors Light Sensors - Pyroelectric 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 Sensors Other 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 Dielectric Incidence -- photoreflectors Incidence -- photoreflectors Rotational Position Sensors Rotational Position Sensors Optical Encoders Relative position Absolute position Other Sensors Resolver Potentiometer Jizhong Xiao Optical Encoders Relative position mask/diffuser grating light emitter light sensor decode circuitry Jizhong Xiao Optical Encoders Relative position - direction- resolution Ronchi grating light emitter light sensor decode circuitry A B A leads B Phase lag between A and B is 90 degrees (Quadrature Encoder) Jizhong Xiao Optical Encoders Detecting absolute position Typically 4k-8k/2 Higher Resolution Available Laser/Hologram (0.1-0.3 resolution) Jizhong Xiao 00 00 1 00 1 1 00 1 1 10 1 11 1 01 1 00 1 100 .. 1001 Jizhong Xiao Gray Code Gray 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 2 n-1 segment in reverse order as next bit is added Preserves unambiguous absolute position and direction Other Motor Sensors Resolver Selsyn pairs (1930-1960) High speed Potentiometer High resolution Monotone but poor linearity Noise!...
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This note was uploaded on 12/29/2011 for the course ECE 253 taught by Professor Brewer,f during the Fall '08 term at UCSB.

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lect9 - Lecture 9 Lecture 9 Sensors, A/D, sampling noise...

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