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Optical Encoders, Laser Interferometer, LVDT

Optical Encoders, Laser Interferometer, LVDT. Rushi Vyas Xiaoyu Ding Lei Yang. Rushi Vyas. Outline. Optical Encoders: Theory and applications Fundamental Components Theory Types of optical encoders Quadrature Errors Applications. Rushi Vyas. What are Encoders.

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Optical Encoders, Laser Interferometer, LVDT

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  1. Optical Encoders, Laser Interferometer, LVDT RushiVyas Xiaoyu Ding Lei Yang

  2. Rushi Vyas Outline • Optical Encoders: Theory and applications • Fundamental Components • Theory • Types of optical encoders • Quadrature • Errors • Applications

  3. Rushi Vyas What are Encoders • An accessory to a mechanical device that translates mechanical motion into a measurable electrical signal Digital or Analog (preferably digital). • Optical Encoders • Use light & photosensors to produce digital code • Most popular type of encoder. • Can be linear or rotary.

  4. Rushi Vyas Optical Encoders: Components • Code Disk: Used to produce different light patterns on a photo detector assembly from a stationary light source. • Code Disk: Determines the Optical Encoder type.

  5. Rushi Vyas Optical Encoders: Components • Light source(s) • LEDs or IR LEDs provide light source. • Light is collimated using a lens to make the beams parallel. • Photodetector(s) • Either Photodiodes or Phototransistors. • Opaque disk (Code Disk) • One or more “tracks” with slits to allow light to pass through.

  6. Rushi Vyas Optical Encoders: Theory Code Disk Photo-sensor LED

  7. Rushi Vyas Optical Encoder Types Incremental Encoder code Disk Absolute Encoder code Disk • Incremental Encoders: Mechanical motion computed by measuring consecutive “on” states. • Absolute Encoders: Digital data produced by code disk, which carries position information. Lab 3

  8. Rushi Vyas Standard Binary Encoding

  9. Rushi Vyas Problem with Binary Code • One angle shift results in multiple bit changes. • Example: 1 => 2 • 001 (start at 1) • 000 (turn off bit 0) • 010 (turn on bit 1)

  10. Rushi Vyas Gray Encoding Notice only 1 bit has to be changed for all transitions.

  11. Rushi Vyas Quadrature • ❖ Quadrature describes two signals 90° out of phase • ❖ Used to determine direction of measurement • ❖ Only two directions possible, A leads B or B leads A

  12. Rushi Vyas Quadrature An incremental rotary encoder, also known as a quadrature encoder or a relative rotary encoder, has two outputs called quadrature outputs that are 90 deg out of phase. Direction of rotation can be determined from output sequence.

  13. Rushi Vyas Encoder Resolution: • Absolute Optical Encoder • Resolution = 360º/(2n) • n = number of encoder bits • Measures the rotational displacement that can be measured per bit change. • Incremental Optical Encoder • Resolution = 360/n • N = number of windows on code disk • Resolution can be increased by reading both rising and falling edges ( ) and by using quadrature ( ).

  14. Rushi Vyas Examples Number of bits on encoder code disk n = 3 Resolution = 360º/23 = 45º Number of bits on encoder code disk n = 4 Resolution = 360º/24 = 22.5º

  15. Rushi Vyas Example: • What resolution absolute optical encoder is needed to be able to measure rotational displacements of 1.5 degrees? • N = ? • Resolution = 1.5 degrees For absolute optical encoder: Resolution=360/2N =1.5 → N = 7.91 ≈ 8 bits

  16. Rushi Vyas Example: • What number of slits (windows) are needed on the code disk of an incremental optical encoder to be able to measure rotational displacements of 1.5 degrees? • N = ? • Resolution = 1.5 degrees For incremental optical encoder Resolution=360/N =1.5 → N = 240 windows

  17. Rushi Vyas Optical Encoders: Reliability • Encoder errors • Quantization Error – Dependent on digital word size. • Assembly Error – Due to instability in rotational motion of code disk • Manufacturing tolerances – Code printing accuracy, sensor position, and irregularities in signal generation.

  18. Rushi Vyas Optical Encoders: Reliability • Structural Limitations – Disk Deformation, physical loads on shaft. • Coupling Error – Gear backlash, belt slippage, etc… • Ambient Effects – Vibration, temperature, light noise, humidity, etc… • Diffraction of light: occurs due to edge of codes disk windows. Fixed in newer encoders by using mask and minimizing distance to photodetector.

  19. Rushi Vyas Applications • Primarily used in motors for monitoring velocity and position. • Robotics • Conveyor belts • Locomotives: Automobiles, planes.. • Tachometers

  20. Rushi Vyas References • Kawasaki Industries Optical Encoders: www.khi.co.jp • Compumotors: www.compumotor.com • ME class notes: Dr. Kurfess, Georgia Tech • www.motioncontrol-info.com • Sensors: Fall 08. ME6405 • Wikipedia • Computer Optical Products: http://www.opticalencoder.com/

  21. Laser interferometer Xiaoyu Ding

  22. Xiaoyu Ding Laser Interferometer • What’s laser interferometer? • The principle of standard interferometer • Types of interferometers • Applications

  23. Xiaoyu Ding What’s a Laser Interferometer • Laser Interferometer: • the instrument used for high precision measurements (distance, angles…. etc.) • it uses interferometry as the basis for measurement. • it uses the very small, stable and accurately defined wavelength of laser as a unit of measure.

  24. Xiaoyu Ding Physics Review • Diffraction Diffraction is a sure sign that whatever is passing through the hole is a wave. Diffraction of Water Waves

  25. Xiaoyu Ding Physics Review • Diffraction of Light Light, just like a water wave, does spread out behind a hole is the hole is sufficiently small. Light is a electromagnetic wave. Diffraction of light Wave

  26. Xiaoyu Ding Physics Review • A Double-Slit Interference Experiment Interference of Light

  27. Xiaoyu Ding Principle of Michelson Interferometer • Albert Michelson (1852~1931) • the first American scientist to receive a Nobel prize, invented the optical interferometer. • The Michelson interferometer has been widely used for over a century to make precise measurements of wavelengths and distances. Albert Michelson

  28. Xiaoyu Ding Principle of Michelson Interferometer • Michelson Interferometer • Separation • Recombination • Interference A Michelson Interferometer for use on an optical table

  29. Xiaoyu Ding Principle of Michelson Interferometer • Analyzing Michelson Interferometer • The central spot in the fringe pattern alternates between bright and dark when Mirror M2 moves. If we can know the spacing distance of M2 betweentwo sequent central bright spots and the number of central bright spots appeared, then we can calculate how long M2 moved. Photograph of the interference fringes produced by a Michelson interferometer.

  30. Xiaoyu Ding Principle of Michelson Interferometer • Analyzing Michelson Interferometer • Spacing distance of M2 is . laser has very small, stable and accurately defined wavelength which can help us get high precision measurement.

  31. Xiaoyu Ding Types of Laser Interferometers • Homodyne Laser Interferometer (Standard) • It is based on interference of laser waves (Michelson interferometer) • Heterodyne Laser interferometer • It is based on Doppler Effect.

  32. Xiaoyu Ding Principle of Heterodyne Laser interferometer • Doppler Effect Doppler Effect: The change of frequency when a source moves relative to an observer. We can get the velocity of an object by measure the frequency change between incident laser wave and reflected laser wave.

  33. Xiaoyu Ding Applications • Measurement of Distance • 1) frequency stabilized He-Ne laser tube • 2) combination of beam-splitter and retroreflector • 3) a moving retroreflector • 4) detection electronics Aerotech’s LZR3000 Series Laser Interferometer System

  34. Xiaoyu Ding Applications • Other Applications • Measure angles, flatness, straightness, velocity and vibrations, etc. Rearrangements of the light paths

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

  36. Xiaoyu Ding 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 • PHYSICS FOR SCIENTISTS AND ENGINEERS, Randall D. Knight, 2003.

  37. Linear Variable Differential Transformer（LVDT） Lei Yang

  38. Lei Yang LVDT • What is LVDT? • Construction of LVDT • How LVDT works • Support electronics of LVDT • Properties of LVDT • Types of LVDT • Applications of LVDT

  39. Lei Yang What is a LVDT • Linear variable differential transformer • Electrical transformer measuring linear displacement

  40. Lei Yang Construction of LVDT • One Primary coil • Two symmetric secondary coils • Ferromagnetic core Primary coil • The primary coil is energized with a A.C. • The two secondary coils are identical, symmetrically distributed. • The two secondary coils are connected in opposition Ferromagnetic core Secondary coils

  41. Lei Yang Recall of conventional transformer • Mutual induction • the secondary voltage proportional to the primary voltage • The transformer core is fixed • Energy transferred is high

  42. Lei Yang How LVDT works • If the core is located midway between S1 and S2 • Equal flux is coupled to each secondary. • Voltage E1 and E2 are equal. • The differential voltage output, (E1 - E2 ), is zero. • This core position is called null point.

  43. Lei Yang How LVDT works • If the core is moved closer to S1 than to S2 • More flux is coupled to S1 than S2 . • The induced voltage E1 is increased while E2 is decreased. • The differential voltage is (E1 - E2).

  44. Lei Yang How LVDT works • If the core is moved closer to S2 than to S1 • More flux is coupled to S2 than to S1 . • The induced E2 is increased as E1 is decreased. • The differential voltage is (E2 - E1).

  45. Lei Yang How LVDT works

  46. Lei Yang Support electronics of LVDT • LVDT signal conditioning equipment • Supplying excitation power for an LVDT • typically 3 V rms at 3 kHz • Converting AC output into DC signals with directional information from the 180 degree output phase shift • External electronics • Self-contained electronics e.g. DC-LVDT

  47. Lei Yang Properties of LVDT • Friction-Free Operation • Infinite Resolution • Unlimited Mechanical Life • Single Axis Sensitivity • Environmentally Robust • Null Point Repeatability • Fast Dynamic Response • Absolute Output

  48. Lei Yang Types of LVDT • DC LVDT • Signal conditioning easier • Can operate from dry cell batteries • High unit cost • AC LVDT • Small size • Very accurate – Excellent resolution (0.1 µm) • Can operate with a wide temperature range • Lower unit cost

  49. Lei Yang Types of LVDT • Free 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-extended core • 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. Lei Yang Example of commercial LVDT • SE-750 Series General Purpose Free Core Single-Ended DC-LVDT Position Sensors

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