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Position Sensing: Mechanical, Optical, and Magnetic Sensors

This article provides an overview of mechanical, optical, and magnetic position sensors, including microswitches, LEDs and photodiodes, inductive proximity sensors, Hall sensors, capacitive sensors, ultrasonic sensors, variable reluctance sensors, incremental encoders, absolute encoders, potentiometers, and magnetostrictive sensors.

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Position Sensing: Mechanical, Optical, and Magnetic Sensors

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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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