General > General Technical Chat
Determining rotational accuracy of a drive unit.
Puffie40:
So we have a stepper motor/worm gear drive at work that does not provide very good positional accuracy at work, and I am trying to figure out the accuracy mathematically.
The Stepper motor has a 25,000 step/rev resolution, and it is coupled to a 20:1 gearbox. The supplier told us that means
(360/25,000)*(1/20) = 0.00072 degrees/step
I've determined the model of the gearbox used has a maximum backlash of 0.13 degrees, and I think this is a potential source of the problem (There is other issues, but this is the main one I can work with).
The problem I'm having is determining how that backlash factors into the original equation. So far, I think this would be inserted into the equation as an error tolerance (ie 0.00072 degrees/step,+/- 0.13 Degrees).
Any thoughts?
Benta:
Depends on whether is has to work bidirectionally or only in one direction, and if the mechanical load is reasonably constant.
In just one direction, backlash is not very important. But when you reverse direction, you'll have a 0.13 degree "dead zone" before the output shaft starts moving again.
HwAoRrDk:
Yeah, whether it's plus or minus 0.13 degrees will depend on the direction of rotation.
Seems silly having such a fine step resolution when it's massively outweighed by the mechanical backlash. :)
nctnico:
I'd use a toothed belt drive reduction instead of a gearbox combined with a servo motor. From there on an absolute position encoder along the worm drive to create a feedback system into the servo motor. One of the things I've learned is that stepper motors kinda suck as an open loop control system and if you can going for a closed loop, a servo motor gives you a much simpler control system.
Using gears together with stepper motors is a big no anyway because the stepper motor will wreck the gear teeth.
james_s:
CNC equipment typically has a backlash compensation setting, you measure the actual backlash under real world conditions and enter that constant and then whenever the drive changes direction it will account for that by turning the motor far enough to compensate. Can you do something like that with your system? You'll never completely eliminate backlash.
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