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Position Sensing

Position Sensing. Mechanical Optical Magnetic. Mechanical Sensing - Microswitch. Microswitch Operation. Switch exhibits mechanical hysteresis. Microswitch Actuators. Optical Sensing. LED’s and Photodiodes Transmissive/Reflective Modulated/Unmodulated Light-on/Dark-on Fiber optic.

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Position Sensing

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  1. Position Sensing • Mechanical • Optical • Magnetic

  2. Mechanical Sensing - Microswitch

  3. Microswitch Operation Switch exhibits mechanical hysteresis.

  4. Microswitch Actuators

  5. Optical Sensing • LED’s and Photodiodes • Transmissive/Reflective • Modulated/Unmodulated • Light-on/Dark-on • Fiber optic

  6. LED and Photodiode Characteristics

  7. Transmissive & Reflective Sensors

  8. Beam Pattern and Reflectance

  9. Specular Reflection

  10. Modulation • “Chop” LED on and off at many kHz rate • Bandpass filter after photodiode at the same frequency as chopping • Threshold circuit after BPF generates on/off output

  11. Magnetic Position Sensors • Reed switches (sense permanent magnet) • Inductive proximity sensors (eddy current) • Hall Sensors (sense permanent magnet)

  12. Inductive Proximity Sensor

  13. Inductive Proximity Sensors

  14. Hall Sensors • Hall effect: • constant voltage forces a constant current in semiconductor sheet • magnetic field flux lines perpendicular to current cause proportional voltage across sheet. • discovered by E.F.Hall in 1879 • Linear sensor needs voltage regulator and amplifier • Switch also needs threshold circuit, with hysteresis

  15. Hall Switch • Magnet motion • head-on • bypass or slide-by • Total effective air gap (TEAG) • Sensitivity, Hysteresis, & Temperature

  16. Other Discrete Position Sensors • capacitive • ultrasonic • variable reluctance (coil around magnet, senses moving ferrous material)

  17. Incremental Encoders

  18. Incremental Encoders • Encoders typically run on +5V, not +24V • Outputs are typ. not 24V compatible either

  19. Absolute Encoders • doubling resolution requires adding another photodiode/LED pair • cost is much higher than incremental • does not require seeking to establish reference location

  20. hybrid incr/absolute encoders • add more information to index channel to reduce amount of seeking required to find reference position. • interface requires lots of wires (parallel) or a special comm. protocol

  21. Potentiometer • A potentiometer (or pot) is a variable resistor wired to obtain a variable DC voltage proportional to position

  22. Magnetostrictive Pos. Sensor • Pulse sent down magnetostrictive material • Pulse reflects off position magnet’s field • Position is proportional to trcvd - tsent • Pulse propagates at ~2800 m/s • Resolution is ~.001” with tupdate ~1msec/in.

  23. Magnetostrictive Sensor

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