Optical Encoders, LVDT

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!