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Sensors

Sensors. Bryson Cook James Wyler Hao Phan. Bryson Cook. Outline. Optical Encoders: Theory and applications –Types of encoders –Fundamental Components –Quadrature –Errors –Applications 

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Sensors

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  1. Sensors Bryson Cook James Wyler Hao Phan

  2. Bryson Cook Outline • Optical Encoders: Theory and applications –Types of encoders –Fundamental Components –Quadrature –Errors –Applications  • LVDT (Linear Variable Differential Transformer)–What is a LVDT –Types of LVDT –How do they work? –Applications

  3. Bryson Cook What are Encoders • For our class, an encoder is a device that senses position or orientation for use as a reference or active feedback to control position. • Most are either: • Rotary: converts rotary position to an analog or electronic signal. • Linear: converts linear position to an electronic signal. • Are also either absolute or incremental. • Absolute gives the absolute position and knowledge of the previous position is not needed. • Incremental encoders is more ambiguous and requires counting of cycles to determine absolute position.

  4. Bryson Cook Optical Encoders • Use light & photosensors to produce digital code • Most popular type of encoder. • Can be linear or rotary.

  5. Bryson Cook 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. Bryson Cook Optical Encoders: Theory Code Disk Photo-sensor LED

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

  8. Bryson Cook Binary and Gray Encoding • In some devices, Binary Encoding is used to keep track of the various positions. The areas of the disk are named counting in binary. • This can cause problems since multiple bits can change from one successive area to the next, such as in 011 to 100 all three bits change. • Gray Encoding is a binary system where the adjacent areas only differ in one bit.

  9. Bryson Cook Standard Binary Encoding *Note: Extremely simplified encoder

  10. Bryson Cook Gray Encoding Notice only 1 bit has to be changed for all transitions.

  11. James Wyler 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. Standard Encoder Track Gives velocity and position but not direction Quadrature Encoder Track Gives velocity, position AND direction James Wyler Quadrature

  13. James Wyler Quadrature – How It Works • Grey Encoding • Identical tracks • Phase offset of 90º • Two sensors • Current state vs. next state

  14. James Wyler Quadrature – Rotary Encoders • Examples of Quadrature Rotary Encoders 2 Bit Wheel 64 Bit Wheel

  15. Quantization Error – Dependent on resolution of sensor Assembly Error – Disk not positioned correctly with respect to sensor Manufacturing Error – Tolerances of sensor positioning and code printing lead to inaccurate signals James Wyler Optical Encoder Errors

  16. James Wyler Optical Encoder Errors – Cont. • Structural Limitations – Loading on shaft or disk deformation • Coupling Error – Gear backlash, belt slippage, etc… • Ambient Effects – Vibration, temperature, light noise, humidity, etc…

  17. James Wyler Optical Encoder Applications • Coordinate Measuring Machine (CMM) • Digital Calipers • CNC Machining • Electric Motors • Robotics

  18. James Wyler LVDT • What is a LVDT • Types of LVDT • How do they work? • Applications

  19. James Wyler What is a LVDT • Linear Variable Differential Transformer • Electrical transformer used to measure linear displacement

  20. James Wyler Construction of LVDT • One Primary coil • Two symmetric secondary coils • Ferromagnetic core

  21. Hao Phan Types of LVDT • Power supply : • DC • AC • Type of armature : • Free (Unguided) • Captive (Guided) • Spring-extended

  22. Hao Phan Power supply : DC LVDT • Easy to install • Signal conditioning easier • Can operate from dry cell batteries • High unit cost

  23. Hao Phan Power supply : AC LVDT • Small size • Very accurate –Excellent resolution (0.1 μm) • Can operate with a wide temperature range • Lower unit cost

  24. Hao Phan Armature : Free Core (Unguided) • Core is completely separable from the transducer body • Well-suited for short-range applications • high speed applications (high-frequency vibration)

  25. Hao Phan Captive Core (Guided) • Core is restrained and guided by a low-friction assembly • Both static and dynamic applications • Long range applications • Preferred when misalignment may occur

  26. Hao Phan 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 • Medium range applications

  27. Hao Phan How do they work? • An alternating current is driven through the primary, causing a voltage to be induced in each secondary proportional to its mutual inductance with the primary.

  28. Hao Phan How do they work? • The coils are connected in reverse series • The output voltage is the difference (differential) between the two secondary voltages

  29. Hao Phan Null Position • When the core is in its central position, it is placed equal distance between the two secondary coils. • Equal but opposite voltages are induced in these two coils, so the differential voltage output is zero.

  30. Hao Phan In Phase Voltage • Displacing the core to the left causes the first secondary to be more strongly coupled to the primary than the second secondary. • The higher voltage of the first secondary in relation to the second secondary causes an output voltage that is in phase with the primary voltage. • The phase of the voltage indicates the direction of the displacement.

  31. Hao Phan Out of Phase Voltage • Displacing the core to the right causes the second secondary to be more strongly coupled to the primary than the first secondary. • The greater voltage of the second secondary causes an output voltage to be out of phase with the primary voltage.

  32. Hao Phan How do they work? • The magnitude of the output voltage is proportional to the distance moved by the core, which is why the device is described as "linear". • Note that the output is not linear as the core travels near the boundaries of its range.

  33. Hao Phan LVDT Applications • Crankshaft Balancing • Testing Soil Strength • Automated Part Inspection • Automotive Damper Velocity

  34. References • http://www.macrosensors.com/lvdt_tutorial.html • http://zone.ni.com/devzone/cda/tut/p/id/3638#toc3 • http://en.wikipedia.org/wiki/Linear_variable_differential_transformer • http://prototalk.net/forums/showthread.php?t=78\ • http://www.transtekinc.com/support/applications/LVDT-applications.html • http://www.sensorsmag.com/sensors/position-presence-proximity/modern-lvdts-new-applications-air-ground-and-sea-7508 • http://www.macrosensors.com/lvdt_tutorial.html • http://zone.ni.com/devzone/cda/tut/p/id/3638#toc3 • http://en.wikipedia.org/wiki/Linear_variable_differential_transformer • Sensors Lecture: Fall ME6405 2009 • http://electricly.com/absolute-optical-encoders-rotary-encoders

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