1 / 39

Optical Encoders, LVDT

Optical Encoders, 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.

studs
Télécharger la présentation

Optical Encoders, LVDT

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. Optical Encoders, 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. Linear Variable Differential Transformer（LVDT） Lei Yang

  22. 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

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

  24. 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

  25. 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

  26. 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.

  27. 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).

  28. 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).

  29. Lei Yang How LVDT works

  30. 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

  31. 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

  32. 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

  33. 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)

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

  35. Lei Yang Applications of LVDT • For power generation • Conditioning valves for large and medium steam turbines. • Reheat and stop valves for large and medium steam turbines. • Feed water boiler pump valve positioning. • Natural gas fuel valve position for gas turbines for throttle control. • Monitoring hydraulic fluid level in reservoir of feed water pumps in nuclear reactor core.

  36. Lei Yang Applications of LVDT • For manufacturing • Measuring final height placement for automotive wheel trim • Measuring injector height for diesel engines Feed water boiler pump valve positioning. • Thickness measuring in multiple locations of fly-wheel to insure balance. • Controlling depth of hole during machining operations in a rotary transfer machine. • Providing indication and feedback position of rocket engine nozzle actuators during testing.

  37. Lei Yang Other Applications • Automation Machinery • Civil / Structural Engineering • Metal Stamping / Forming • OEM • Pulp and Paper • Industrial Valves • R&D and Test • Automotive Racing

  38. Lei Yang References • http://www.macrosensors.com/lvdt_macro_sensors/lvdt_tutorial/index.html#automation • http://en.wikipedia.org/wiki/Linear_variable_differential_transformer • http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm • http://www.directindustry.com/industrial-manufacturer/lvdt-73930.html • http://www.macrosensors.com/lvdt_macro_sensors/lvdt_products/lvdt_position_sensors/dc_lvdt/free_core_dc/se_750_single_ended.html • Alexandre Lenoble’s lecture

  39. Lei Yang Thank you!

More Related