PWM Control DC Motors

[[email protected]]

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

 

[John Popelish]

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.

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.

 

[Tim Wescott]

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

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
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Scott Seidman]

[email protected] wrote in @g37g2000prf.googlegroups.com:

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

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

 

[[email protected]]

[email protected] wrote in @g37g2000prf.googlegroups.com:



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

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.

 

[John Popelish]

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.

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

 

[Scott Seidman]

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

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.