Newsgroups: comp.robotics
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From: jab@hpuerca.atl.hp.com (Alan Barrow)
Subject: Re: A sense of balance
Message-ID: <C1uIpL.1KL@hpuerca.atl.hp.com>
Date: Wed, 3 Feb 1993 00:00:56 GMT
References: <1993Jan31.072524.14704@adobe.com> <1993Feb1.025125.26895@muddcs.claremont.edu>
Organization: Hewlett-Packard NARC Atlanta
Lines: 43

In <1993Feb1.025125.26895@muddcs.claremont.edu> bgribble@jarthur.claremont.edu (Bill Gribble) writes:

>The big problem with swinging LEDs and things is damping them. What happens 
>if  you shake the thing, or (much more important in a robot) you accelerate
>the thing linearly and then stop it?  does the angle still read properly? 
>The gyro solution is cheap and practical and required a few hours of lathe
>and mill work to make.  

The pendulum is succeptable to acceleration. Gyro's precess (sp?).

So you add accelerometers to measure and factor out these issues.

But if you have accelerometers, you could build an INS instead.

Forget about the sphere's and such. You can do this with rotary shaft 
encoders and pendulums. You only need two. (one for each axis.)

Any way you do it, you end up with a trig problem. The Cartesian coord
approach has simpler trig than the spherical approach. You can use
lookup tables to make it simple. (Given the limited resolution of your
shaft encoders.) Do you really want to do trig with your 6511/8051?

You could even combine the shaft encoder with the pendulum. I have made
shaft encoder disks for tachometers. Just add a lead weight, and a low
friction shaft for the disk, and instant stable pendulum. You can use
magnetic damping as well. (Like on chemistry balances)

I like commercial shaft encoders, as I can use quadrature to determine
direction of motion as well as the position. I can also use off the
shelf chips to read/manage the position info. I use the "low cost"
optical potentiometer replacements.

Use averaging or calculus to factor out the "spikes" caused by motion, as
well.

Hope this helps!


 Alan Barrow  km4ba | I've seen things you people wouldn't believe. Attack
 jab@atl.hp.com     | ships on fire off the shoulder of Orion. I watched
                    | C-beams glitter in the dark near the Tannhauser gate.
 ..!gatech!kd4nc!   | All those moments will be lost in time -
         km4ba!alan | like tears in rain. Time to die.          Roy Batty
