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[DPRG] Driving an H-Bridge with a switching power supply

Subject: [DPRG] Driving an H-Bridge with a switching power supply
From: Rick J. Bickle rbickle at swbell.net
Date: Fri Dec 26 11:28:01 CST 2003

I think a good solution to this voltage problem would be to use one of
the buck regulators from either National Semiconductor or Linear Tech.
These regulators essentially perform the same function as a standard 3
terminal linear regulator, but with much more efficiency. Thus bettery
drain is minimized.


-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org] On Behalf
Of Earl Bollinger
Sent: Friday, December 26, 2003 8:18 AM
To: dprglist at dprg.org
Subject: RE: [DPRG] Driving an H-Bridge with a switching power supply

I had observed problems like this before. The most obvious is when a
robot did a 90 or 180 degree turn on a freshly charged versus a low
battery. You would get a difference. You can readily see this in the
small robots. 
So it does make sense to use a regulated supply to furnish voltage to a
drive motor system. It would keep the voltage and current more constant
during the charge life of the battery pack.

So it looks like a good thing to do, if you can get a motor power
regulator into your robot, is to do so.

For a small robot I think we need to come up with a reliable sensor for
determining whether we have done a 90 or 180degree turn. GPS looks
promising, but the military have the accurate system. We'd have to at
the least, use a fixed and mobile GPS receiver to determine positions
more accurately. I have tried wheel encoders, but the slippage is
cumulative and leads to error. A digital compass doesn't work well
inside as all the nearby metal interferes with the compass, sometimes
rendering it useless. I have been thinking that maybe a dual roller
encoder with both encoders set at 90 degrees to each other. When the
robot starts a turn, one roller works for a while then stops as it's
skidding, then the other roller starts turning. Adding the combined
total tick counts from the two rollers looks promising. This would use
two wheels like those multi-roller wheels they sell at Acroname.com.
Then of course maybe a simple castor wheel encoder would work.

Outdoors, a digital compass looks much better, but the larger electric
PM motors cause the compass to point at them. If you use a soft iron
shield around the motors, it works for a while until the iron starts to
get magnetized, then the compass starts pointing at the motors again.
GPS seems to be the only practical way at the moment here. But maybe a
castor wheel type encoder would work Ok enough. But grass, sidewalks,
dirt, bumps, ditches, etc. all cause turn problems too.

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