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[DPRG] grounding motor cases

Subject: [DPRG] grounding motor cases
From: Ted Larson ted at larsonland.com
Date: Wed Jul 6 13:18:27 CDT 2005


We have used both the Devantech compass, and the PNI Vx2E (www.pnicorp.com).
The PNI does have soft, hard iron calibration, and is a very sensitive, and
accurate device.  The problem we ran into with it, was we needed better
motor shielding, based upon where the compass is mounted on our robot.  Our
motors are right angle, and the compass sits right in-between the mag-field
bubbles of each.  Use a gauss-meter to look at where you plan to mount it,
especially with the robot driving around on the ground under load, so you
can see what the ideal mounting will be.  The Devantech works good in that
position, and we get +/- 5 degrees out of it.  The PNI is good to 1 degree
or better, but hops all over the place as soon as we get the motors moving.
I have done testing with it mounted up on a pole above the robot which
eliminates the field problems.  HOWEVER, you really want your compass as low
to the ground as possible.  These compasses are not tilt-compensated.  The
larger the moment-arm, the more tilt from bumps in the road, and the more
the compass is swinging around from tilt issues.  So, this is why we are
currently using the Devantech, and it just barely works well enough.  We
hope to go back to the PNI once we shield our motors better...more on this
in a sec.

The ultimate is to either own a tilt compensated compass ($$$).  Or here I
am again, going on the cheap for more robot fun.....build your own Z-axis
magnetometer and tilt-compensate the compass yourself.  I think it might be
possible to tilt-compensate a cheap compass compass using a Z-axis
magnetometer, and a 2-axis accelerometer together with a Kalman filter,
pretty easy.  I have the math for doing it using just the Z-axis iron, and
it is a nasty piece of calcs.  More on this later...all it takes is time I
don't have. 

Make sure whatever enclosure you build for your compass has NO FERROUS
materials in it.  Ours is enclosed in a PVC box held together will all brass
fasteners, and aluminum stand offs.  One steel screw somewhere is enough to
mess things up.

I have been looking into building Mumetal shields for our motors.  That
would be the ideal setup for keeping the motors from messing with your
compass.  Here is a company that sells Mumetal foil that might work.  I am
not sure what thickness we will need to block the field strength.  Probably
need to spend some time with a gauss meter and doing some calcs to figure
out what would help or not.


One more thing on the subject of Sonar units to look at.  The main reason we
went to a separate battery for the sonar was that the 400v trigger, causes
all kinds of power supply spikes on your logic.  It even bleeds back over
the trigger and echo lines.  We have actually thought about just putting a
PIC in there that does nothing but read Sonar, and then optically isolating
the whole thing from the rest of the robot.  You might want to make sure
that isn't something that is going on for you.

GPS....It has taken us MONTHS to get our GPS working correctly.  Issues with
noise, and bad data, and math round-off errors, and more....I think I have
spent as many hours getting our GPS to work on our robot as I did trying to
get Bender to balance the first time.   I finally have it working REALLY
well.  If I can get 5 satellites on a position fix, I can drive to within 3
feet of the cone on GPS alone.  We did a recent Magellan challenge at our
club last week, there were 4 cones set out on a course, and we touched 3 out
of 4.  The only reason we didn't get the first cone correctly was because I
forgot to take the lens cap off the camera (not the robots fault).  We still
need improvements on obstacle avoidance, and handling areas where GPS
reception is poor using our inertial unit, however, we are working better
than ever at this point.  The only bummer is, that Seattle is the same
weekend as Robonexus, so I am not sure if we will be able to compete it in
Seattle this year.

So...the answer is....GPS is possible to work well, but getting it to work
perfectly (as one would expect it SHOULD work), is up there in difficulty to
building a balancing robot.  I figure you of all people will understand that
comparison...but, I also know you are up for the challenge! :-)


-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On Behalf
Of David P. Anderson
Sent: Wednesday, July 06, 2005 8:44 AM
To: dprglist at dprg.org
Subject: [DPRG] grounding motor cases


Ted wrote:

> David,
> Did you notice the problem before you added the .1uf caps to your motor
> cases?  I usually avoid doing the motor case caps unless the chassis is
> out of a conductive material (i.e. not plastic).  I think it would be
> interesting to look at your ground with the caps on, and with them off,
> also with your ground clips on, and off.

I will try this.

> Are you putting GPS on your robot?  Ground-plane noise issues will get
> more particularly interesting (sticky), once you have a GPS antenna in the
> mix too.  We ended up with a huge aluminum shield on the top of our robot
> get our GPS to work right.  Steel would have been even better, but that
> chunk of ferrous material really messed with our compass.

What sort of compass are you using?  Does it allow for hard/soft iron

> We eventually went to three battery backs.  One for Motors, one for logic,
> and one for Sonar, with the grounds tied together at one point only, back
> near the batteries.
> -Ted

I have a single tie point for all the grounds, but I'm, using a single 24
battery pack, with a 5v switching regulator for the logic supply, and a
separate 7v
regulator for the sensors and IMU.  The GPS is a 3 volt device and at this
point has it's own pair of AA cells, (which is sort of a pain because I
forget to turn it off.)  I don't think the noise problems are due to the
power, as I get the same results when powering the sonar from a separate
bench supply.

Ted, are you getting any useful information from the GPS?  I ask because I
don't really think that I am...


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