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PWM Control DC Motors

Discussion in 'Electronic Design' started by [email protected], Jun 11, 2007.

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  1. Guest

    This approach nearly always uses an H- bridge to drive the motor and
    needs a special reverse switch to reverse it. (see pic)

    So how do PWM servos work through zero degrees and into reverse? I
    thought it would be easier to have bipolar PWM which swings both
    positive and negative.

  2. This PWM H bridge does the direction reverse, as well as the
    speed control. No reversing switch is needed.Note the red
    and green arrows representing forward and reverse current
    that produces forward and reverse torque.
  3. Tim Wescott

    Tim Wescott Guest

    He's describing a situation where they go out of their way to avoid
    switching the high-side FETs. Once you get over that hurdle, it is
    easier to design an H-bridge PWM amplifier than a bipolar PWM amplifier.


    Tim Wescott
    Wescott Design Services

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at
  4. wrote in

    The control signals on the gates of the H-bridge determine the direction
  5. Guest

    I know that but when do you switch over? Is it smooth? With old-
    fashioned DC control and a bipolar supply it is smooth through zero
    and into negative voltage.
  6. If you pulse width modulate the average current to zero as
    you switch directions, the torque reversal is smooth.
  7. You can think of the motor as a giant low pass filter. What you do on any
    given pulse on the PWM would have very little impact on the motion profile.

    Bottom line, just change the direction, and don't worry too much about
    when. During direction changes, chances are the pulses will be small

    As a practical matter, motors have a pulse width at each frequency, below
    which they won't move at all. Be sure to add this dead width to all your
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