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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: Earl Bollinger earlwbollinger at comcast.net
Date: Fri Dec 26 08:21:00 CST 2003

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.





-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org] On Behalf
Of David P. Anderson
Sent: Monday, December 22, 2003 11:47 PM
To: dprglist at dprg.org
Subject: [DPRG] Driving an H-Bridge with a switching power supply

Howdy

On Using a Switching Power Supply to Drive an H_Bridge.

One of the problems I've struggled with on the two-wheel
balancing robot is the change in behavior as the battery
drains.  I'm running the robot on 18 Nmh AA cells, at 1600
mAh.  Fully charged that's about 25 volts, and fully discharged
(at 1 volt per cell) is 18 volts.

The robot balancing behavior is optimal at about 23 volts.
Higher than that, it is quite "jerky", and below about 22 volts
it is quite "loopy."   I can adjust it so that it is optimal
in any of these ranges, but then it works poorly in the others.

On my other non-balancing robots, this problem is solved with
shaft encoders and a PID speed controller.  The speed controller
increases the pulse widths to compensate for the sagging battery
voltage, and the behaviors remain constant across the range of
the battery.

For the balancing robot, this has proved more problematic.
I originally thought that I could deal with this by monitoring
the battery voltage, which I do, and adjusting the gain or gains
as the voltage drooped, but in practice this has proven very
difficult to do.

About a month ago at one of the RBNOs at the warehouse, I was
demonstrating this behavior and the Most Excellent Brian Merritt
suggested that this problem might better be solved in hardware,
and thereafter gave me a pointer to a Dallas Semiconductor
Maxim MXL1074CT  --- here's a handy reference:

http://www.chipdocs.com/pndecoder/datasheets/MAXIM/MXL1074CT.html

So I ordered a couple of samples and after some consultation with Brian
and Ron Grant, made a trip to Tanners for the appropriate inductor
and a handful of capacitors, and built a little 23 volt, 5 amp
switching power supply.  I added three more AA cells to nBot's
battery pack and mounted the switcher between the power switch
and the H-Bridge.

I've been running it for a couple of weeks now, and it works just
dandy!  The output remains at a steady 23 volts as the pack drains
>from about 29 volts to about 25v.  I hung a 3amp load on the supply
and watched the output on the scope, it appears to regulate down to
within about 2.5 volts of the requested output voltage (which I can
adjust with a 10-turn pot).  There is a little bit of ringing on
the output waveform, but basically it's pretty clean.

I'm now in the process of re-calibrating the robot, which is about
1/2 done, but so far the performance is markedly improved.   Further,
during a two hour session of testing and calibrating, the behavior
remains consistent throughout.  As a result, it is easier to tune
the system, and the robot already seems more stable.

This has made me wonder if this sort of thing might be a useful
addition to a non-balancing robot, as well.  And if so, if the
little switcher should just be part of the H-Bridge board to
begin with?  (It's only something like 5 components)

Anybody have any similar experience?  How about with industrial
controllers?  

merry christmas,
dpa


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