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[DPRG] Borenstein squares
Subject: [DPRG] Borenstein squares
From: Dave Hylands
dhylands at gmail.com
Date: Wed May 21 12:28:22 CDT 2008
Hi Randy,
> The omniwheel is being used as a caster. It is just the rear
> wheel of the tripod. The front end used to be the backend of a
> Traxxas Emaxx. However it had two differentials up front, so the
> wheels could be driven differentially. We ran two swiss motors
> through the differentials to the two (now front) wheels.
When I converted Marauder (a mini-sumo) to do line following, one of
my attempts was to add a pair of front casters (since this is
relatively small, the casters done using Lego pieces). The casters
were setup like the traditional swiveling caster you find on
push-carts, where the axle of the wheel is located behind the vertical
swivel axis.
Everything seemed to move quite freely, however I noticed very odd
turning behaviors.
When Marauder tried to transition from going forwards to turning 90
degrees in place, I could observe noticeable wheel slippage of the
main drive wheels. Not having time to do anything about it (this was
just before a competition), I wound up putting a rounded scoop that
just gets dragged on the ground. The drive wheel slippage disappeared.
This was while operating on a relatively smooth surface, and Marauder
has polyurethane tires which provide fairly good traction.
I don't understand exactly why the casters were causing the drive
wheel slippage to occur, but since it seemed like it might be
relevant, I though I'd pass the information along.
Perhaps the whole notion of what's going one with a castered design
needs some investigation.
> Modifications done in the last two days have eliminated the
> differntials. So now we have the two swiss motors directly in
> line with the wheel drive shafts. Hopefully this will get our
> speed up 3x or 6x (never got a straight answer on the gear
> ration through the Emaxx transmissions). We had enough torque
> already. (I was told, anyway. We'll see when we encounter a curb
> with the new configuration.)
For the in-place turning, I would expect backlash to be a contributing
factor. The motors will turn some amount taking up the backlash before
the wheels will actually turn. If the encoders are on the wheels, then
backlash shouldn't be a contributing factor, but if the encoders are
on the motors, then it definitely will.
--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/
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