1 / 52

Sensors

Sensors. Efrain Teran Carol Young Brian O’Saben. Optical Encoders. Efrain Teran. What are Optical Encoders ?. An Optical R otary Encoder is an electro-mechanical device that converts the angular position of a shaft to a digital code. What are they used for?.

oren
Télécharger la présentation

Sensors

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Sensors Efrain Teran Carol Young Brian O’Saben

  2. Optical Encoders Efrain Teran

  3. What are Optical Encoders ? An Optical Rotary Encoder is an electro-mechanical device that converts the angular position of a shaft to a digital code. What are they used for? • Provide information on angular position, speed, and direction. • The information is used for system control (e.g. motor velocity feedback control). • It is the most popular type of encoder.

  4. How do they work? • Use light and photo detectors to produce a digital code • As the encoder shaft rotates, output signals are produced proportional to the angle of rotation. • The signal may be a square wave (for an incremental encoder) or an absolute measure of position (for an absolute encoder).

  5. Optical Encoder parts Light source: produces the light that will “trigger” the photodetectors during motion. Usually LEDs or IR LEDs Photodetector: electronic sensor that reacts to light. Usually a phototransistor or photodiode. Code disk: has one or more tracks with slits (windows) to allow light to pass through. Mask: collimates the beams of light

  6. Optical Encoder parts Shaft: mechanically attached to the system we want to measure; usually a motor. Housing: protection from the environment. Electronic board: filters signal into square wave used by microcontroller.

  7. Types of Optical Encoders Incremental Optical Encoders: • Single channel • Dual channel • Dual channel with Z index Absolute Optical Encoders

  8. Incremental Encoders • Generate a series of pulses as the shaft moves and provide relative position information. • They are typically simpler and cheaper than absolute encoders. • Need external processing of signals. TYPES

  9. Incremental Optical Encoder: Single channel • Has only one output channel for encoding information. • Used in unidirectional systems or where you don’t need to know direction. Voltage Lo Hi Lo Hi Lo Binary 0 1 0 1 0

  10. Incremental Optical Encoder: Dual channel • The output has two lines of pulses (“A” and “B” channel) • They are 90° offset in order to determine rotation direction. • This phasing between the two signals is called quadrature. Channel A Lo Hi Hi Lo Repetitive sequence Channel B Lo Lo Hi Hi

  11. Incremental Optical Encoder: Dual channel

  12. Incremental Optical Encoder: Dual channel with Z index • Some quadrature encoders include a third channel (Z or Index) • It supplies a single pulse per revolution used for precise determination of a reference position. • Need to do “homing” for it to work. Doesn’t hold after power down. Z

  13. Absolute Encoders • Provides a unique digital output for each shaft position • The code disk has many tracks. The number determines resolution. • Upon a loss of power it keeps the correct position value. • Uses binary or “grey” code.

  14. VIDEO: https://www.youtube.com/watch?v=cn83jR2mchw

  15. Absolute encoders: Binary vs. Gray code 010 001 011 000 100 111 101 110 Transition possible results: 011 - 010 - 001 - 011- 111 - 100

  16. Absolute encoders: Binary vs. Gray code 011 001 010 000 110 100 111 101 Transition possible results: 010 - 110

  17. Encoder Resolution Absolute Optical Encoder • Resolution can be given in number of bits or degrees • Depends on the number of tracks on the code disk. Each track requires an output signal, also known as an “encoder bit”. • Resolution = 360°/(2N) • N= number of encoder bits (number of tracks) Example: An absolute encoder has 8 tracks on the disc. What is its angular resolution in degrees? • Resolution = 360°/(2N) = 360°/(28) = 1.4°

  18. Encoder Resolution Incremental Optical Encoder • Resolution essentially depends on the number of windows on the code disk • Resolution = 360/N • N = number of windows on code disk Example: What number of windows are needed on the code disk of an incremental optical encoder to measure displacements of 1.5°? • Resolution=360° /N =1.5 °→ N = 240 windows • BUT, we can increase resolution by using channels A and B

  19. Encoder Resolution Incremental Optical Encoder • We may count rising and falling edges in both channel’s signals Today’s standard • X4 Resolution= 360/4N • N = number of windows (slits or lines) on the code disk

  20. (SabriCentinkunt, page 236) Example: Consider an incremental encoder that produces 2500-pulses/revolution. Assume that the photo detectors in the decoder circuit can handle signals up to 1 MHz frequency. Determine the maximum shaft speed (RPM) the encoder and decoder circuit can handle.

  21. Applications Incremental Single channel Incremental Dual channel Incremental with Z index Absolute Encoder

  22. REFERENCES: Mechatronics, SabriCetinkunt, Wiley, 2007. Section 6.4.3 http://en.wikipedia.org/wiki/Rotary_encoder http://www.ab.com/en/epub/catalogs/12772/6543185/12041221/12041235/Incremental-Versus-Absolute-Encoders.html http://www.ni.com/white-paper/7109/en/ http://www.digikey.com/PTM/IndividualPTM.page?site=us&lang=en&ptm=2420

  23. Laser Interferometer Carol Young

  24. What is a Laser Interferometer ? • Laser- single frequency light wave • Interferometry- Family of techniques where waves are super imposed in order to extract information about the waves • Uses the interference patterns from lasers to produce high precision measurements

  25. Physics BackgroundWaves • Light is an Electrometric wave and therefore has wave properties. http://en.wikipedia.org/wiki/File:Light-wave.svg

  26. Physics BackgroundDiffraction and Interference • Diffraction • Light spreads after passing a narrow point • Interference • superposition of two waves to form new wave with different amplitude • Constructive or Destructive http://en.wikipedia.org/wiki/File:Doubleslit3Dspectrum.gif

  27. Types of Laser Interferometers • Homodyne • Homo (same) + dyne (power) • Uses a single frequency to obtain measurements • Heterodyne • Hetero (different) + dyne (power) • Uses two different (but close) frequencies to obtain measurements.

  28. Homodyne Interferometer(Michelson) Mirror Reference Laser Mirror Moveable (Sample) Beam Splitter Screen

  29. Homodyne InterferometerAnalysis • λ is the wavelength of the light • Lref is the distance to the reference mirror • L is the distance to the moveable mirror • n is the number of fringes Photograph of the interference fringes produced by a Michelson interferometer.

  30. Homodyne InterferometerUses • Absolute distance • Optical testing • Refractive index • Angles • Flatness • Straightness • Speed • Vibrations

  31. Physics BackgroundDoppler Effect • Point creating a wave and movement • Wave ahead of point has higher frequency • Wave behind point has lower frequency • Frequency change corresponds to velocity http://en.wikipedia.org/wiki/File:Dopplereffectsourcemovingrightatmach0.7.gif

  32. Physics BackgroundBeat Frequency • Rate of constructive and destructive interference

  33. Heterodyne Interferometer • Produces two close but not equal frequencies (Creating a Beat Frequency) • Doppler effect from moving reflector shifts the frequency proportional to the velocity

  34. Heterodyne / HomodyneInterferometerComparison • Comparing with a Homodyne Interferometer • Can determine movement direction (but limited range) • More useful when direction of movement is important

  35. Heterodyne / HomodyneInterferometer Comparison • Homodyne • Smooth surfaces only • Heterodyne • Can be used for • Distance to rough surfaces • Surface roughness measurements

  36. Xiaoyu Ding Resolution • XL-80 Laser Measurement System

  37. References • http://www.aerotech.com/products/engref/intexe.html • http://www.renishaw.com/en/interferometry-explained--7854 • http://en.wikipedia.org/wiki/Michelson_interferometer • http://en.wikipedia.org/wiki/Interferometry • http://en.wikipedia.org/wiki/Doppler_effect • www.ljmu.ac.uk/GERI/GERI_Docs/interferometry_presentation(1).ppt • http://www.olympus-controls.com/documents/GEN-NEW-0117.pdf • http://www.lambdasys.com/product/LEOI-20.htm • http://www.intechopen.com/books/advances-in-solid-state-lasers-development-and-applications/precision-dimensional-metrology-based-on-a-femtosecond-pulse-laser • http://en.wikipedia.org/wiki/Fringe_shift • http://www.gitam.edu/eresource/Engg_Phys/semester_1/optics/intro_polari.htm • A. F. Fercher, H. Z. Hu, and U. Vry, “Rough surface interferometry with a two-wavelength heterodyne speckle interferometer”, Applied Optics

  38. Linear Variable Differential Transformer (LVDT) Brian O’Saben

  39. Outline • What is a LVDT? • How LVDTs Works • LVDT Properties • LVDT Support Electronics • Types of LVDTs • LVDT Applications

  40. What is a LVDT? • Linear variable differential transformer • Electromechanical transducer measuring linear displacement

  41. What is a LVDT? • Primary coil • Energized with constant A/C • Two identical secondary coils • Symmetrically distributed • Connected in opposition • Ferromagnetic core

  42. How LVDT works • If core is centered between S1 and S2 • Equal flux from each secondary coil • Voltage E1 = E2

  43. How LVDT works • If core is closer to S1 • Greater flux at S1 • Voltage E1 increases, Voltage E2 decreases • Eout=E1 – E2

  44. How LVDT works • If core is closer to S2 • Greater flux at S2 • Voltage E2 increases, Voltage E1 decreases • Eout=E2 – E1

  45. How LVDT works

  46. LVDT properties • Friction-free operation • Unlimited mechanical life • Infinite resolution • Separable coil and core • Environmentally robust • Fast dynamic response • Absolute output

  47. LVDT support electronics • LVDT signal conditioning equipment • Supply excitation power for the LVDT • Typically 3 Vrms at 3 kHz • Convert low level A/C output to high level DC signals • Gives directional information based on phase shift

  48. Types of LVDTs • DC LVDT • Signal conditioning equipment built in • Pre-calibrated analog and/or digital output • Lower overall system cost • AC LVDT • Wide operating environments • Shock and vibration • Temperature • Smaller package size

  49. Types of LVDTs • Separate core • Core is completely separable from the transducer body • Well-suited for short-range (1 to 50mm), high speed applications (high-frequency vibration) • Guided core • Core is restrained and guided by a low-friction assembly • Both static and dynamic applications • working range (up to 500mm) • Spring-loaded • Core is restrained and guided by a low-friction assembly • Internal spring to continuously push the core to its fullest possible extension • Best suited for static or slow-moving applications • Lower range than guided core(10 to 70mm)

  50. LVDT applications • Industrial gaging systems • Electronic dial indicators • Weighing systems • Crankshaft balancer • Final product inspection (checking dimensions) • Octane analyzer (provides displacement feedback for Waukesha engine) • Valve position sensing

More Related