Stability and Insanity

For your next demo, use an electromagnet to lift a metal ball and hold it suspended. Sense the height with a light sensor. Use PID to achieve stability.

I saw an article that did this 30 or so years ago. They used a hollow steel ball with a map of the earth painted on. Can't remember the diameter of the ball, but maybe 1".

It was longer ago than that--iirc it was called "Li'l Atlas", and it could spin the ball as well as suspend it.

Cheers

Phil Hobbs

Found it. It was _fifty_ years ago:

Cheers

Phil "Old magazines rule" Hobbs

The light sensor is not strictly necessary, but then you need something mor e complex than a PID.

Bye Jack

Oh, thanks, Phil. That makes me 20 years older than I thought I was :P

Yeah, things have progressed over the last 50 years. Thanks for the link. I would have loved to see the current change in the electromagnet as additional weights were added.

I've done that. You need a honkin' big electromagnet to make it work with a plain steel load.

The executive desk-toys with the floating globes use big (30mm dia x

Try that again:

Oops! Sorry! I didn't know you were already doing it.

Cheers.

Well, the project is on long-term hold, but I did get as far as getting one of those desk toys and starting to take it apart.

Would it be inappropriate to demonstrate PID? Just curious.

I suspect that it would be a good way to demonstrate a number of principles in control systems, PID controllers included.

A lot of people seem to separate "PID" from other controllers -- somewhere in this thread someone made a comment about "oh, that wouldn't work with a PID controller". Yet most advanced controllers really boil down to a PID controller with:

• A different way of arriving at the gains; * various linear and nonlinear decorations; * and a fancy name.

But -- that's a rant for another day.

Usenet: bringing out the best in Humanity.

Well, there are various lead-lag type tricks that require basically lowpass filtering the D term. PLLs and diffusion-dominated temperature controls don't like terms with huge noise gain.

When I was a post-doc, in about 1988, I did a motion controller for a piezo bimorph used in a scanning force microscope. It had a notch filter for the lowest resonance, two integrators (with lead-lag to make the loop stable) and two more poles to get rid of wideband crap that would excite the higher resonances. Worked great--when tweaked for best settling time, the loop BW was about 30% of the resonant frequency, vs.

Would you include that in "decorated PID"?

I generally do it that way, i.e. use frequency compensation ideas, concentrate on phase margin, and then watch out for the transient response, because when you do something fancy like the above, the settling behaviour is liable to be a bit strange. Of course when one of my loops misbehaves a bit during testing, no machinery is ruined and nobody gets hurt.

Windup and nonlinear slewing are sometimes issues too, of course.

Overall it seems like the distinction between control guys and other EEs is a bit like the difference between civil and mechanical--a lot of the same concepts but completely different emphasis. Is that fair?

Cheers

Phil Hobbs

;)

There are better things in humanity than that. My wife's cooking, for one.

Cheers

Phil Hobbs

A levitated ball is a nearly undamped, nonlinear second order system, which seems like a good test case. Tim's pendulum with a Q of 80 is good too, and more linear for small deflections.

Cheers

Phil Hobbs

We seldom encounter chaotic behavior in electronic circuits... on purpose, I guess.

Superregenerative receivers are kind of chaotic, but mostly just noisy.

A laser driving some fiber with lots of reflections can be chaotic, but the only nonlinear element is the laser itself.

But I bet you learned loads!

Btw - did you ever discover the reason for the discrepancy between theoretical and actual coil resistance for hard drive voice coils?

Michael