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Tacho or encoder?

Discussion in 'Electronic Design' started by Pubo, Jan 13, 2006.

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  1. Pubo

    Pubo Guest

    Would like to know how a tacho is used for feedback n control the speed

    which is better to use a tachogenerator or an encoder?

    what is the diff for cost n size?
     
  2. Guest

    Encoders are more reliable and cheaper, as long as your drive supports
    them thats the way to go.
     
  3. Guest

    Probably the cheapest and smallest solution today (when building in
    quanitity from the ground up) is to use a motor withough a commutator,
    with a driver circuit that switches the phases at the desired speed,
    possibly with some feedback detection from the same coils. I think
    this is what you will find in modern disk drives, for example.

    For a one-off, use whatever you can get your hands on surplus - tacho,
    surplus encoder which could be quite coarse, even one pulse per
    revolution if the speed is high, etc.
     
  4. Naebad

    Naebad Guest

    You have to be careful about the number of pulses per rev - I think you will
    find if there are not enough pulses then your phase amrgin will suffer. Also
    what's teh point in an encoder if you are using analogue control? You would
    need to convert back to analogue.

    Naebad
     
  5. Depends on the application.
     
  6. Pubo

    Pubo Guest

    How a tacho is used??

    Now in my application am giving it to an opamp(error amplifier).

    the application is to drive a DC servo motor(90Vmax,3A )

    Thanks
     
  7. Yes, but do you also need to stop and hold the shaft at a fixed
    position, do you need to know the exact position of the
    shaft, etc.

    Encoders can be used to generate a tacho-like signal, but at
    very low RPM including stopped position, the signal isn't
    as smooth as a true DC-tacho, and you may get small oscillations
    in the shaft position and audible noise. But if you need to
    know the exact position you *need* to use an encoder. I had
    a system once that used both a DC-tacho and an encoder with
    sine output. Tacho for velocity feedback and encoder to keep
    track of position during run, and sine output was used to lock
    the shaft in stopped position.

    So, again, it depends on the application. You can't just
    say that a tacho is better than an encoder or vice verca.

    When in doubt, try both ;)
     
  8. Joseph2k

    Joseph2k Guest

    I vote for the encoder. If the control system is analog you only need to
    feed the encoder output (1 bit for unidirectional systems, 2 to include
    direction) to a little logic and a "F to V" converter. benefits: better
    linearity (over almost 4 decades of speed), and retains analog control
    system (if that is a requirement). for digital control systems it is
    obviously no contest in favor of the encoder; you could also get as many
    bits absolute position as you can pay for. speed would be taken from the
    top three bits to include measure of acceleration.
     
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