Using 12bit DAC, I would like to have 32 microsteps.
I have mounted a laser on the shaft so I can see the laser beam on my wall, where I have 32 grid lines.
I've experimented, and it goes pretty well, all 32 steps are more or less on grid - when I go right.
But when I turn back left, the laser dot moves fast, then slow, then fast again - not on grid.
I haven't found anything on the net about this. Maybe I could have two DAC tables, one for moving right and for moving left, but I would like to read something about this - if anyone can guide me further...?
I'm not sure I understand your use of this figure in what follows. The motor has a fixed (*whole*) step size. E.g., 1.8 degrees (200 steps per shaft revolution). Microstepping allows you to drive the motor to "fractional step positions". So, for the
1.8 degree step size, 32 microsteps PER STEP would translate to
6400 microsteps per shaft revolution.
[In which case, I can't understand how you are "seeing this" in a light projection on the wall]
Here I don't quite follow. Is the laser's light *parallel* to the motor shaft (so the laser traces out a circle when the shaft rotates)? In which case, the "grid lines" are really *rays* emanating from the point at which the center of the shaft intersects the wall (?)
"Right" being clockwise when viewing the laser light on the wall?
Sounds like mechanical slop, backlash or stiction in the motor (is there a gearhead on the motor?). OTOH, if it is that "regular", it could be that your excitation isn't as it should be!
The first thing to verify is the voltage from your controller at each microstep point. I.e., the waveform should have the same relative shape (and instantaneous values) just "reflected" as the direction changes (looking at each individual drive channel in isolation).
Remember, microstepping "balances" the armature's position based on the relative magnetic strengths of the motor's coils. So, if one waveform isn't in the proper phase relationship with the other, this "sweet spot" can move off of "ideal".
[In practice, this doesn't hold up as the motor is accelerated or decelerated]
Finer grained microstepping (32 is a rather high target) suffers from repeatability issues. Especially if the motor's manufacturing tolerances didn't take this form of operation into account.
"Lots of microsteps" are often of primary value in delivering
*smooth* operation -- not "near infinite angular shaft resolution". I suspect you may be looking at signal amid noise.
Does the motor "run as smooth" clockwise vs. counterclockwise? I.e., the *sound* of the motor should have roughly the same frequency distributions regardless of direction -- unless it truly *is* cogging in one direction, only (or, "more than the other") WHEN OPERATED AT SOME NOMINAL SPEED.
But, still, to resolve a 32nd of a step you'd need either *big* steps or to be pretty far from the wall (and doing your math carefully!) 0-th order approximation: sine = tan = angle for angle close to 0. Smallest angle would be O(2pi/6400) which my shirtsleeve says is "about 0.001". So, the spacing between lines would approach D/1000 (where D is distance to wall). At 5 *ft* you're looking at 1/16" -- double that at 10'. How good is the laser? One of those handheld "pointers"? :>
[Assuming my shirtsleeve isn't out of calibration]
It would seem simpler to mount a mirror on the shaft and aim the laser at the "center" of the mirror -- less mass to support and accelerate (maybe even "borrow" a mirror from a barcode scanner). *If* you want to go that way...
Seems like I would be looking at waveforms before messing with clumsy measurement jigs.
Has the OP decided that mechanical reduction can't provide the resolution/holding torque desired?
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