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Clutch Drag detector.

Discussion in 'Electronic Design' started by Jamie, Feb 4, 2005.

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  1. Jamie

    Jamie Guest

    i have made a design in the pass using motors as tension drag and was
    able to detect the tension so that one could detect the absence of no
    tension and thus turn off the machine.

    now we have a case using magnetic drag (not friction) clutches and
    must be able to detect using only the 2 DC lines used to energies the
    field. i haven't disassembled one of these units yet but it appears they
    may have a PM (permanent magnet) in the rotor and Fixed coil in the back
    plate, from what i can gather from the feel if it there must be poles
    because it feels like what a stepping motor would feel like if you were
    to have low current in the stepping more and manually turned the shaft.
    in any case, this brake unit is part of the mounting plate where mylar
    tape is loaded, cable is past through the center of a rotating shaft.
    when the tap has tension (not broke or run out), the clutch is also
    slipping with a drag in it. this unit is powered by 2 slip rings.
    we can not put any more slip rings and because of how the surroundings
    are set up, we can not use a strobe tach of any kind and can mount any
    wireless devices on the plates. the solution must be of no devices
    hanging out in the open in the way.. other attempts of such devices has
    proven to not work well or just get broke by the machine operators.

    I have already tested for current and voltage changes when the
    unit is being dragged, there is no noticeable effects that can be used.
    what i purpose is to modulate the DC current slightly at a freq where
    the moving PM's over the poles will cause change effects in the wave
    that can be detected as a phase shift offset. when the rotor is spinning
    and not slipping in the field the phase should not be showing any phase
    timing shifts.
    of course the modulation level is only going to be no more than abt
    5 % of the DC current being applied to the brake field.
    what do you think about this idea ?
  2. Ken Smith

    Ken Smith Guest

    Does this include there not being an AC waveform imposed on the DC when
    you rotate it? At low speeds it would be small.

    I expect that this clutch works like an induction motor in reverse. The
    magnetic field is non-rotating and thuse makes a torque when the shaft is
    rotated. If that is the case and there is no evidence of AC, you will
    have to do something a bit trickier. If there is some AC it could be as
    simple a putting a transformer in one wire.

    I think that if you measure the impedance vs frequency of this critter, it
    will be slightly different when it is rotating. I suggest you jig up
    something that will let you measure it while it is contantly being turned.
    This will geive you an idea of what you have to work with,
  3. Jamie

    Jamie Guest

    Thanks for replying.
    i have done some more research and it appears the initial method of
    testing could have been flawed. i was using an incandescent lamp as the
    shunt (because it was handy at the time). it was suggested that maybe
    the fact that the filament changes R, this could be suppressing the effect.
    i can say that i did notice at the start and end of rotor movement,
    there seem to be an effect, but it balances out as it runs. in other
    words it only seems to be showing effects on the start and stop.
    The brake field is static and rotor has a PM in it. i am next going
    to try using a xformer (current config) transformer. by passing the
    DC current through one field, i may be able to see small levels of
    AC/Pulses appear on the other field. this would be good to also isolate
    the dc.
    if that does not work, then i am thinking of modulating DC line also
    via the Xformer with around 5K hz or better monitoring phase shift and
    load effects to see if i can detect the PM in the pole line.
    I think that a PM should make some inductive change effects when the
    PM is in the pole lines.
    with this, i can convert the stream of pulses to form a Logic train
    signal using a Capacitor Decoupler to generate Pulses which would
    indicate the rotor is moving.
    we will see how that goes.
    Thanks, for replying.
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