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Minimal inductive proximity sensor?

Discussion in 'Electronic Design' started by Dave, Aug 31, 2005.

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  1. Dave

    Dave Guest

    I have a situation where an 8-pin PIC would monitor an inductive
    proximity sensor through a 3 ft wire and control a relay. The proximity
    sensor would detect the spokes of a cast iron wheel at maybe 5mm
    distance. Since the PIC is available is there any cheaper alternative
    to the inductive proximity sensor? Permanent magnets can't be used in
    this application.

  2. John Fields

    John Fields Guest

  3. Guest

    Reflection sensor? Farnell has about half a page of "reflective optical
    sesnsors", most of them costing a bit under $10 each in small

    If the cast iron wheel had any significant residual magnetism, you
    might be able to do something with a linear Hall effect sensor and
    comparator, but it would be more expensive,
  4. Dave

    Dave Guest

    Gritty, dirty, oily environment...
  5. Dave

    Dave Guest

    What would be the basic setup for this approach? An oscillator on the
    remote board that the PIC would monitor the frequency of?
  6. Ken Smith

    Ken Smith Guest

    The low cost way to do it is to make the oscillator at the PIC side of the
    capacitor and have the object provide a ground that increases the

    The PIC is wired to the plate at two points. One is an output from the
    PIC that drives the plate through a resistor and the other is an input.
    The PIC asserts a high for a long time and then a low. It then measures
    how long it takes for the input to go low too. It compares this value to
    a value from longish ago to remove drifts. If the time gets longer, the
    object has arrived.
  7. quietguy

    quietguy Guest

    Maybe the old fashioned approach would work for you - microswitch with
    wand protruding into wheel arch - reliable, dust and water etc proof units
    available - very reliable - use the PIC to count and de bounce etc

  8. Guest

    Right - that wipes out optical sensors, unless you can get the system
    to squirt filtered oil at your sensor all the time.
  9. Dave

    Dave Guest

    I considered that. The wheel speed is slow, but it can be at 100RPM for
    long periods so I thought the resulting reliability would be
  10. John Larkin

    John Larkin Guest

    Sounds like you need a nonmagnetized variable reluctance pickup, like
    the ones used for aircraft fuel flow sensors. It looks like a spark
    plug, sort of; just a ferrous metal tube with a center post barely
    peeking out, and a coil inside. Inductance increases as a gear
    tooth/spoke/turbine blade comes close.

    You can signal condition this with an oscillator that uses the coil in
    its tank (observe frequency shift) or you can put it in an AC bridge.

    Google "magnetic proximity sensor." You can buy these with the
    necessary electronics inside if you don't want to do it yourself.
    You'd have to make a lot of these to justify homebrew.

  11. Dave

    Dave Guest

    I can't imagine this working when the probable capacitance change would
    only be a few dozen picofarads on the sensor board. I'm more inclined
    to believe a seperate monitored oscillator might work.
  12. John Fields

    John Fields Guest

    No, I think that method would be too "drifty", and the frequency too
    high because of the size of the plate.

    What I'd do is something like this:

    +HV +V +V +V
    | | | |
    | | | [R7]
    [R1] [R2] | |
    || | | | +--[R6]--+
    || | | | | | |
    || |--+---[C2]---+------|----+--|+\ |
    || | | | | >--+
    || C1 [R3] [POT[<----|-/
    ___||___ | |
    |___ ___|--+--------+------+
    || |
    || GND

    The fixed plate of C1 (the capacitor formed by the plate and the
    wheel spoke) is charged up to some relatively high voltage through
    R1, a high-valued resistor, and connected to C2, which is used to
    block the DC bias, but to let through the change in voltage which
    occurs because of the change in capacitance as the spoke passes by
    the plate. R2 and R3 are also high-valued resistors which comprise
    a voltage divider which serves to bias the comparator's
    non-inverting input and set it at a particular voltage. The pot is
    used to set the trigger threshold of the comparator, and when the
    voltage on the non-invertiing input becomes more positive than the
    voltage on the inveting input, the output of the comparator will go
    high, then when the voltage on the + falls to below the reference
    voltage on the - input, the output will go low. Properly adjusted,
    there should be a single pulse out of the comparator every time a
    spoke passes the plate. R6 is used to set the hysteresis in the
    circuit, which will keep the comparator from chattering around the
    switching point and generating multiple outputs for a single spoke

    The nice thing about this circuit is that since a _change_ in
    capacitance is all that's being detected, instead of some phenomenon
    which depends on the absolute value of the capacitance, the drift in
    capacitance (from whatever source) becomes unimportant. Also, the
    amplitude of the output from the capacitor can easily be adjusted by
    changing the value of the high voltage bias on the cap, which can
    probably be used to advantage in dirty environments. Also, the cap
    ought to be a snap to clean; just blow it off with an air hose when
    it gets dirty.
  13. Dave

    Dave Guest

    Thanks, that looks like a pretty reasonable approach. I'll do some
  14. Ken Smith

    Ken Smith Guest

    10pF times a 1Meg resistor gives a 10uS change in the time constant so
    whats the problem?
  15. Dave

    Dave Guest

    Ok, now I understand what you're suggesting. Somehow I was thinking you
    meant a split plate.
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