PWM Control DC Motors

This approach nearly always uses an H- bridge to drive the motor and needs a special reverse switch to reverse it. (see pic)
http://www.4qdtec.com/bridge.html
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.
W.King
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snipped-for-privacy@gmail.com wrote:

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.
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snipped-for-privacy@gmail.com wrote:

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.
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Tim Wescott
Wescott Design Services
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snipped-for-privacy@gmail.com wrote in

The control signals on the gates of the H-bridge determine the direction
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Scott
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wrote:

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.
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snipped-for-privacy@gmail.com wrote:

If you pulse width modulate the average current to zero as you switch directions, the torque reversal is smooth.
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wrote:

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 anyway.
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 widths.
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Scott
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