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[DPRG] PWM vs. voltage motor control

Subject: [DPRG] PWM vs. voltage motor control
From: Dan Creagan dcreagan at bellevue.edu
Date: Thu Jun 14 11:38:36 CDT 2001

Robert Jordon's suggestions are pretty much along the lines that I take
for PWM.  Choose a gearbox combination that will get you the maximum
speed you know you want to go but no more.  Usually, our hobby robots
don't work well at high speed because of sensor lag and inertia so the
max speed should be limited to something pretty mild.  For those of you
creating a sentient bullet, skip this message.

If you use PWM to tame a really fast motor and gearbox combination, you
will get some strange duty cycle behaviors and a really difficult tuning
problem. 

Generally, I would recommend *not* using PWM as a fix for an incorrect
gearbox choice.

Dan

-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org] On Behalf
Of Robert L. Jordan
Sent: Thursday, June 14, 2001 11:18 AM
To: dprglist at dprg.org
Subject: Re: [DPRG] PWM vs. voltage motor control

I have worked with H-Bridges off and on for a long time.
My approach is more do and observe the results.

I have found that different motors make a big difference in the 
behavior of slow speed PWM. Also, properly matching the motors torgue
and optimal RPM to the ground speed and startup requirements makes a 
large difference in performance.

Since most robots don't have a multi spee gear box, they don't 
perform well in all speed ranges. Trying to use PWM for slow speeds 
on a motor and gear train optomized for higher ground speed is not 
reasonable. If you want great slow speeds with or without PWM, design 
the drivetrain for that goal.

The GARP wheel chair has a two speed motor to address this very issue.
Commercial robots seem to take a similar approach.

My $.02 worth.

Regards,
Robert L. Jordan

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