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[DPRG] Pittman GM87XX Homebrew

Subject: [DPRG] Pittman GM87XX Homebrew
From: David P. Anderson dpa at io.isem.smu.edu
Date: Mon Aug 27 19:16:10 CDT 2001


I took the Pittman GM8712 motors from Tanner's out to visit Michael Hamilton,
designer extraordinare for Traxxus et al, and we took one apart and played
with it.  He pronounced it "first rate stuff."  Not bad for $11.00!!

Here are some pictures of it disassembled:


It is a seven pole motor, which accounts for the high torque at low rpm
and for it's extremely quiet operation, always a plus for hobby robots.
Notice in the upper right-hand image that the seven poles are wound
with lots of extremely fine wire (we measured 12 ohms).  The poles are
also "skewed" helically.  Michael tells me this provides for smoother
operation, as the poles "overlap" each other.

The gearhead has 4 stages.  We counted the teeth and the last three
stages have 32 tooth spur gears driven by 10 tooth pinions, 3.2:1
per stage.  The first stage has slightly smaller teeth and we were
not able to count them.  However, they must provide about 1.8:1 ratio
for a final ratio of 60:1.  All gears are "high quality" metal gears
with steel axles riding in bronze bushings.  

He is of the opinion that we should probably NOT mount wheels directly
on the output shaft because of the small diameter of the rear pin and
bushing.  I told him we were thinking of a 5 to 10 pound platform.

I have gathered together the parts for a simple home-brew encoder for
these motors as discussed previously.  This consists of a paper encoder
wheel, Hamammatsu emitter/detector, black plastic film canister, and
a cable tie.  

Here is a picture of the pieces:


I'm using a 28 division encoder wheel as a starting place.  The wheel is
super-glued to the rear of the motor shaft.  The Hamammatsu is mounted
on a small piece of perfboard with a couple of other components.  This
will probably change.  The perfboard is glued into the back of the film
canister with the leads exiting through a slot in the side.  The canister
was cut down with an xacto knife and slotted to clear the motor wires.

Here is a picture of the assembled encoder.  The cable tie allows the
film canister to slide in and out to adjust the exact distance for the
Hammamatsu sensor.  Then it is tightened to hold it in place.  


I haven't had a chance to test it yet, so more improvements will no doubt
come along the way.  This home-brew encoder requires no machining and
can be assembled with simple hand tools.  It represents an improvement
over our previous encoders in that it is adjustable and also has a
light shield and protective cover over the encoder wheel.

More to come!


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