Subject:
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Re: Walkers
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 14 Feb 2001 05:12:54 GMT
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Original-From:
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Steve Baker <sjbaker1@airmail.%ihatespam%net>
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Reply-To:
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SJBAKER1@AIRMAIL.NETnospam
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Viewed:
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617 times
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"Boorman, Mathew" wrote:
>
> A problem with any 'pendulum' based system will have is that they are
> effected by the movement of the robot.
> > From a robot which attempted to balance using 2 wheels I found...
>
> From stationary start falling forward...
> detect pendulum swinging forward
> accelerate to bring upright again
> pedulum is pushed back by acceleration
> detect 'upright', stop
> fall over really, really hard!
Well, I think your software has to be able to predict (mathematically)
what will happen to the robot and the pendulum as it accellerates the
motor.
You can't rely on the simple "if pendulum in middle all is well" thing.
You need to know the accelleration you applied, the torque that'll apply
to the robot, the consequent rate of rotation, etc. In short, your software
has to do the physics to predict what it's actions should produce and
only use the pendulum to confirm that the error in your calculations
are not adding up too far in either direction.
I've seen a device demonstrated that consisted of a lever with a free-
swinging pendulum connected one end - with a motor at the other end
of the lever. There were rotation sensors at the motor and the fulcrum
of the pendulum. (This wasn't a Lego device) The software was able to
jiggle the motor about until both lever and pendulum were
pointed straight up - and to balance them there indefinitely.
You could even push the pendulum with quite a bit of force to
try to knock it over and the system would catch it and rebalance
the system within a small fraction of a second. The entire thing
was completely hypnotic because it worked with more speed and
precision than you'd think possible. It was the robotic equivelent
of balancing a pencil point-down on the tip of your finger without
moving your arm...and then letting someone try to knock it off your
finger without losing it!
This was done by emulating the physics of the system in software
and applying the precise amount of force theoretically needed to
exactly halt the pendulum in the vertical position. By recomputing
this solution about 500 times a second and adjusting the speed of
the motor accordingly, the system could have the pendulum appear
to stop dead in the correct position.
I wish I could recollect where I'd seen that - I think it was some
college student's term-paper project shown at a local science fair.
> A gyro would eliminate that, although I suspect building a compact lego one
> would be tricky.
Whilst a gyro helps to prevent the robot from falling, it also makes it
harder to correct any tilt you do accumulate. However, it may just give
you more time to react which could be a net win.
> As an alternative I am thinking of using an IR distance sensor to measure
> the lean by measuring the distance to the ground.
It would have to be pretty sensitive. A few degrees of tilt won't produce
a measurable difference in the distance to the ground - so it'll be a
very insensitive detector of small amounts of tilt.
--
Steve Baker HomeEmail: <sjbaker1@airmail.net>
WorkEmail: <sjbaker@link.com>
HomePage : http://web2.airmail.net/sjbaker1
Projects : http://plib.sourceforge.net
http://tuxaqfh.sourceforge.net
http://tuxkart.sourceforge.net
http://prettypoly.sourceforge.net
http://freeglut.sourceforge.net
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Message has 2 Replies:
 | | Re: Walkers
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| Hey good points! What if one took the motor driven pendulum idea and applied it to a gyro to produce torue counteracting the tilt from stepping forward? Good point about the distance to ground being nil for detectability, expecially if you want to (...) (24 years ago, 15-Feb-01, to lugnet.robotics)
| | | Re: Walkers
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| (...) Try rebuilding you robot with the wheels tilted. Instead of the having the axel parallel with the ground (0 deg), tilt the wheel so the axel is is at 60+ deg. This moves the effective center of rotation upwards. Once you get it above the CG (...) (24 years ago, 15-Feb-01, to lugnet.robotics)
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