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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 09:59:43 CDT 2001

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.


Perhaps I should demo this experiment at the next meeting.
We could have a meeting focused on motor controllers.

-Clay Timmons-



Kipton Moravec wrote:

> I was thinking around the same thing, but for a slightly different reason.
>
> At the lower PWM percentages you need to get the current flowing.
>
> I have been wondering if some of the problem is the battery.  The chemical
> reaction for producing electricity is not instantaneous.  Perhaps if he had
> a large capacitor to help provide the current for the short "on" times, it
> might fix the problem.
>
> This is another reason to have very low resistance devices for your
> H-bridge.
>
> Kip
>
> ----- Original Message -----
> From: "Dean Hall" <dwhall256 at yahoo.com>
> To: <dprglist at dprg.org>
> Sent: Wednesday, June 13, 2001 1:51 PM
> Subject: RE: [DPRG] PWM vs. voltage motor control
>
> > Here's a little explanation into what Clay was seeing:
> >
> > Motor torque is proportional to the input current, not
> > the voltage.  A PWM signal connected to a motor-driver
> > chip will drive current for the "high-time" of the PWM
> > signal.  When the PWM signal has a short "high-time"
> > the motor experiences very little current to drive it.
> >
> > In order for a motor driver chip to drive a motor at
> > low RPMs, it needs to be able to push a significant
> > amount of current in a small amount of time.
> > "significant amount of current" is purely a matter of
> > scale.  A LM293 or SN754410 can drive a motor that's
> > the size of a soupcan (but not very well at the low
> > RPMs).  However, those same drivers can drive at low
> > RPM a motor that's the size of an egg just fine.
> >
> > Another major reason that motors stall at low RPMs is
> > due to friction.  So don't try to solve everything
> > electrically; make sure your mechanical system spins
> > freely before plopping in a 24V/5A motor driver.
> > (I'm saying this in general to all aspiring robot
> > builders, not directly to Clay;  I'm pretty sure he
> > knows all this <smiley>)
> >
> > !!Dean
> >
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