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[DPRG] Balancing robot difficulties

Subject: [DPRG] Balancing robot difficulties
From: Kenneth Maxon kmaxon at qwest.net
Date: Tue Jul 3 14:21:12 CDT 2007

Hello David,

Noted the comment on Harmonic drives.  I've used them in a few projects and
encourage everyone to at least put their hands on a set, disassemble,
reassemble and fully understand how they work.  They are one of the most
significant developments in mechanical technology in the last 50yrs..
Benefits: multiple gear teeth concurrently engaged engagement at opposite
ends of a force ring, extremely low backlash, back drivable and very high
reduction ratios in a small space...



Give the four blue graphics, middle of the following page, a check and then
watch the animation...


The one in the photo is somewhat small, good for 152 in-lb from my forth
coming long term Sojourner robot project.  I've also got a set rated at 5000
ft-lbs that are only 8 in across that came from eBay for barely more than
the price of shipping.  Eventually, the later set will build a horizontal
rotary table.


-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On
Behalf Of dpa
Sent: Tuesday, July 03, 2007 2:08 PM
To: dprglist at dprg.org
Subject: [DPRG] Balancing robot difficulties

Hi Luke,

First of all, congratulations!  You've done the hard
part: getting the robot to balance.

Looks to me like you are right about backlash in the
gears.  That would explain the twitchy behavior.  I've
seen the same thing on some of my balancer experiments.

Two-wheel balance, particularly stationary, puts very
tight requirements on the acceptable backlash of the
gear train, because the motors are constantly and
rapidly changing direction, unlike most robot
applications.  Even when driving at a steady
velocity, the balancing algorithm is continuously
loading and unloading the gears to keep the robot
in balance.

Turns out that this is not a trivial problem to solve.

Planetary gears have very little backlash, so that's one
possible solution.  The Segway uses "harmonic drives"
to get rid of the backlash problem with 2 wheel balance,
not quite sure how that works but it looks like a variation
on planetary gears, makes for nice flat "pancake" motors,
and probably costs arm+leg.

I think you may have to go to some more expensive motors
and gearheads, unfortunately.

High friction in the drive train can also cause the kind of
overshoot visible in the video.  But its more likely a
problem with slop in the gear train, as you surmised.

Keep up the great work!

best regards,

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