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[DPRG] PWM vs. voltage motor control

Subject: [DPRG] PWM vs. voltage motor control
From: Clay Timmons ctimmons at cadence.com
Date: Thu Jun 14 10:41:21 CDT 2001

I didn't have a full H-bridge for my PWM testing.  I only had one N-channel
MOSFET to turn the motor on/off in one direction only.  No braking when
the single MOSFET is off.

Using an H-bridge you may or may not brake the motor between pulses
depending on how you have it setup. Most H-bridges have both direction
and enable pins.  If you PWM on the enable pin then there is no braking
between pulses.   Ideally you want 3 inputs per H-bridge,  one for the
left and one for the right side of the H and one enable.    Then you can
brake, coast, go forward, and go reverse.   The MIT handboard only
has enable and direction control and there is no way to brake.  If your
gearmotor has really high gear ratio then braking is not really that
important.


Good point about the braking.  I've done it the wrong way before and
had braking during the off pulse.  Motor buzzed very loudly and things
got hot.

Later,

-Clay Timmons-


Rodent wrote:

> Just out of curriosity, how were you implementing PWM with the discrete
> H-bridge? The H-bridge Rick designed has a 5th MOSFET that actually turns
> the motor on and off -- the other MOSFETS only select the direction. As he
> explained it, if you try to do PWM with just the H-bridge, you are actually
> braking the motor between pulses.
>
> ----- Original Message -----
>
> > In my testing I used a bench power supply, not a battery.
> > Also I used a big beefy TO-220 case mosfet, not a whimpy H-bridge driver
> chip.
> > I don't have the specs handy but I suspect extremly low on resistance and
> > several amps of current capability.
> >
> > I'm still convinced that PWM at low speeds is nearly worthless.
> > It was suggested that my test was missing flyback diodes.  I can't
> > remember but I think I tried adding diodes also.
> >
> > I suspect the only way to get decent torque at low speeds is to
> > increase the drive voltage to get more current in the short amount of
> > time the PWM is on.   This is the way high performance stepper
> > drivers work.  Take a 5v stepper and drive it at 30, 40, 50, even 80volts
> > using current limiting chopper type drive.  The drive switches the full
> > 50volts on then cuts it back off when it reaches a preset current limit.
> > This works great but is not very practical for a robot since it is
> > hard to get those high voltages (with high current too) from batteries.

Rodent wrote:

> Just out of curriosity, how were you implementing PWM with the discrete
> H-bridge? The H-bridge Rick designed has a 5th MOSFET that actually turns
> the motor on and off -- the other MOSFETS only select the direction. As he
> explained it, if you try to do PWM with just the H-bridge, you are actually
> braking the motor between pulses.
>
> ----- Original Message -----
>
> > In my testing I used a bench power supply, not a battery.
> > Also I used a big beefy TO-220 case mosfet, not a whimpy H-bridge driver
> chip.
> > I don't have the specs handy but I suspect extremly low on resistance and
> > several amps of current capability.
> >
> > I'm still convinced that PWM at low speeds is nearly worthless.
> > It was suggested that my test was missing flyback diodes.  I can't
> > remember but I think I tried adding diodes also.
> >
> > I suspect the only way to get decent torque at low speeds is to
> > increase the drive voltage to get more current in the short amount of
> > time the PWM is on.   This is the way high performance stepper
> > drivers work.  Take a 5v stepper and drive it at 30, 40, 50, even 80volts
> > using current limiting chopper type drive.  The drive switches the full
> > 50volts on then cuts it back off when it reaches a preset current limit.
> > This works great but is not very practical for a robot since it is
> > hard to get those high voltages (with high current too) from batteries.



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