Wednesday, January 29, 2025
Direct wire actuator and complementary table test
I made a quick hack of a job on making the direct wire actuator and connected it up to the new complimentary flexure table, not so much as to see if it worked, but how badly it failed. Interesting data emerged. Here's the test rig:
I can't say how well the table works, but I can define a few characteristics of the stepper itself and I don't like them.
When microstepped, the motor would move a little bit and then leap by approximately 20-25 microns. Doing the maths for a 5mm diameter shaft (roughly 16mm circumference with the wire) it works out as roughly 800 usable steps per revolution - half-stepping in fact. I was hoping for much better than that, though with a better driver chip I might achieve more accuracy.
For now though, direct wire drive is out by a factor of 5 (I consider 5 microns just good enough). It is immune to the vagarities of thread and nut precision, alignment, coupling offsets and so forth so a hybrid approach might be possible. Needing a 5:1 mechanical advantage would mean spooling up 50mm of wire, 3-4 wraps on the shaft, which could present wire management issues.
Plugging the numbers back into the screw-driven axis with a 0.5mm thread pitch and 800 steps gives about 0.6 microns per usable step (plus the 3:1 advantage of the flexure arm) so that at least is comforting. Plus I finally have something I can fit under a real microscope!
