The Art Of Motion Control

Ribbon Dancer project scrapbook:

Early on, I had to quickly (and cheaply) come up with slip rings for the phi axis. This allowed testing of unlimited freedom to move in both axes-- a very nice addition.

(more on slip rings)

After receiving word that I was awarded the SCI project, I started serious thinking about the choice of hardware. Although I had demonstrated the feasibility of an open loop system, it was clear to me that a large robot, operating in a public space should be closed loop, to detect any "problems" (although none should ever occur :)

Once the decision to close the loop is made, the jump from steppers to servos is not a large one, in terms of expense. Having developed a certain fondness for Compumotor over the years (I have used steppers and drives from them that are over 20 years old, and NEVER seen them fail; plus they have a great web site and document their old products), I approached a local distributor for advice and pricing. The people at Braas. Co. were very helpful, and after some discussion about ways to approach my application (which I think surprised them), they lent me a 6K4 controller, with one Gemini drive and one size 23 servo with an 8:1 gearhead. I found a reasonably priced used TQ10 drive and motor at D&S, and quickly got both motors running by sending hi-level commands in 6K language over Ethernet:

Unfortunately, I thought my early ease getting things to work would mean that I would have no trouble streaming my dance paths to the motors. Wrong. Although the 6K is easy to use, it became clear after many hours of trying various methods, that it was not designed to produce multiaxis smooth contours in the way I desired-- arbitrary length, and the ability to repeat a contour without any "seam" (velocity = 0). I learned a whole lot, as this was my first experience using servos-- but mostly, I learned that the 6K controller is not what I'm looking for.

On the other hand, I did some testing of the servo motor's capabilities (there was some concern from the Parker engineering staff that I might have trouble tuning the servos in such a high compliance system). I was blown away by their performance:

Video clip of 4 revs/sec (wmv).

I fixtured a 92 oz-in motor with 8:1 gearhead, and attached a 6' graphite rod. I then tuned the motor without much difficulty. I then commanded 10 revs, v=1rps, a=10rpsps. No sweat. I increased the v to 2, then 4rps-- holy crap! I chickened out after 8 rps, which was truly frightening-- I later calculated the speed at the rod's tip: over 200 mph. I will not soon forget the sound it made slicing through the air. I'm not sure how much faster it could have gone before losing position.

I'm now trying to decide upon the fundamental question of how to "best" send my dance paths to the servo drives. Since I already have step/direction software written, that is an intriguing possibility...