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Quadrature decoder

Discussion in 'Electronic Design' started by GregS, Jun 1, 2010.

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  1. GregS

    GregS Guest

    Been searching all morning for one, and did find one by LSI/ LS7184.

    I have been looking at a drawing of one from the mid 70's, drawn up by
    a coworker who is long gone.
    using 74121;'s and 7400.

    I got a sample request, but I think i can do this myself quickly. I got stepper motors
    and controllers requiring step and direction input.
    Good thing I have this diagram because I can't find others.
    Must be some other stuff around ??

    I don't need indexing or counters.
    I also see optical encoders that have the outputs I need for the controllers.
    But I need something fast.

  2. GregS

    GregS Guest

  3. GregS

    GregS Guest

    Right, pulse and direction.

  4. GregS

    GregS Guest

    I also found this. All I would need is a separate toggle
    switch to choose direction, but not so cool......

    ay be sufficient.

  5. GregS

    GregS Guest

    Its been a long project. i tried to get an existing controller to work
    with my steppers. For a Z axis motion. I thought i needed more power
    so I used two audio amps to act as buffers from the stepper
    output. That did not help much. i didn't figure out why the motors
    did not like the drive output. So now I am wiring a separate Z axis
    system, which is a lot simpler than using the existing system.
    The movement is by knobs, and the speed will not
    be over 500 Hz. I also did not want to drive two steppers
    in sync, but thats what I was given. I could have also
    ordered an opto knob with step and direction outputs but its
    a time thing right now.

  6. GregS

    GregS Guest

    Missed the diagrams, I was looking at earlier today.
    My first thought, simple, and perhaps even better, was
    to use just a toggle switch. I thought about using a VCO
    that would speed up the longer you hold the toggle
    up or down. You go up, and you go down. Not as precise though
    as in focusing a microscope.

  7. Jamie

    Jamie Guest

    That can be done with a 7474 (dual Data flip flp).

    I'm not going to get into details but, you do need to
    invert the CLK signal for each FF in the package..

    Channel A drives FF #2's Data and Clr pin while it
    also drives the inverted circuit for the CLK of FF #1

    Channel B drives FF #1's data and clr pin while it
    also drives the inverted circuit for the CLK for FF #2

    Net results are:

    FF #1 outputs CW pulses while FF #2 outputs CCW pulses..

    Using a basic NPN common emitter type inverting circuit for the
    CLK with a pull up R, should work just fine.. I guess one could also
    use a pre biased transistor..

    Most encoders use a open collector scheme. You'll need to supply a
    pull-up or use a PNP inverting circuit which would remove the need of the
    pull up of the encoder line.. You would still need a pull down R on the
    inputs of the CLK I guess..
  8. Jamie

    Jamie Guest

    Really, I've never seen that. I use a dual D-ff, the other Channel holds
    the Data and CLr pin to one state. Even if lets say channel A is
    chattering, channel B is in the middle of the phase supplying the DATA
    pin for the state which isn't moving.

    Oh well, maybe we have different circuits..
  9. Guest

    You only get the direction once per "cycle". The four flip-flop (two two-bit
    shift registers) and XOR solution updates every quarter cycle and is more
    stable around edges.
  10. GregS

    GregS Guest

    Well, I liked the idea of having current control, and I allready
    had one channel wired up. I DO have a driver chip that will do what you said.
    I am going to consider it for simplicity.
    Darn stepper is only 3.3 ohoms.

  11. Ian Bell

    Ian Bell Guest

    The problem with all clocked or sampled quadrature decoded such as those
    referenced above is transition jitter. I know this from personal
    experience where such a detector on a mechanical system was subject to
    vibrations that could occasional cause a rapid set of transition
    forward&backwards some of which would be missed by a clocked system.
    This leads to false counts.


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