Stability or insanity
I've just spent over two weeks getting ready to do my next video. It was a combination of one of those vast underestimations one occasionally makes, combined with falling into a bit of an obsession.
I am, at this point, not only wondering if it was worth it, but questioning my sanity in carrying on even when the going went beyond tough to just plain crazy.
At any rate, a good video needs a visual aid, and I decided that my video needed to demonstrate stability with a pendulum. Moreover, it needed a pendulum that could be worked electronically. So, I've
- Disassembled a hard drive for it's head positioner. This took a day or two.
- Decided that wasn't good enough and wound my own custom coil (220 feet of #40 wire, woo hoo!). This took a false start (18 feet of #34 wire) and several days.
- Mounted the coil into a custom pendulum, running on Real Ball Bearings. Several more days, and if you touch it wrong the Q goes down from about 80 to about 10, then you have to fiddle with it for several minutes so the moving parts don't rub.
- Built an oscillator that uses the pendulum as its resonator (this is where stability comes in -- is an oscillator stable? How is it stable? What if it's showing chaotic behavior?). This was astonishingly frustrating, and didn't finally work until I carefully modeled the pendulum as a resonator AND took the coil inductance into account in the circuit. This part too about a week.
And for all that, I now have the time base for an exceptionally inaccurate electro-mechanical clock! Check out the picture. That's one cycle of the pendulum, running off of a "tick-toc" circuit that (A) minimizes the load on the pendulum (to give a high loaded Q, essential for wringing as much accuracy as possible out of a pendulum, never mind that it's made of wood, masking tape, and car parts that I picked up off the floor), and (B) has to be started by hand (I wanted to demonstrate a hard limit cycle).
More on all of this when I post the video.
Previous post by Tim Wescott:
PID Without a PhD
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