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Position accuracy & torque of stepper before slip & slip amount
Interesting posts on the Geckodrive Yahoo group last night:
(from http://finance.groups.yahoo.com/group/geckodrive/message/19229) ************************************* Hi, Rather than doing the research myself , does anyone know if a stepper will hold its position at rated torque, or will there be some error? If so, is it up to a full step? Also my guess is that at a full step position the accuracy is greater than at a microstep position. Cheers, Peter. ************************************************** * Yes, standing still, at rated holding torque, the error is almost exactly two full steps. If the error ever exceeds 2 full steps, the motor will jump to the next detent, which is every 4 full steps. Note that when running, the motor cannot deliver the holding torque, but something quite a bit less, and any load that approaches the 2 full steps lag is dangerously close to stalling the motor. Jon ************************************************** ********* Hi, not exactly, I think the correct formula is torque = sin (pi/2 * dp) * rated torque with dp = position deviation in full steps. So the maximum torque is delivered at one full step from the unloaded position. It decreases to zero between one and two full steps and becomes negative between 2 and 4 full steps and that's why the motor jumps 4 full steps when overloaded. Most manufacturers specify 20% tolerance for the max. torque and up to 5% distortion from the ideal sine waveform. Microstep or not does not matter, only the absolute value of the current vector counts. Actually, microstepping can result in higher torque when running because less resonances means less risk for stalling. One tricky thing is that the max holding torque is reached only if both windings are energized with the full rated current what results in a vector of 1.41 times the rated current. You cannot run the motor continously with that current level at any speed considerably higher than zero or the motor will overheat. If you run the motor with only one winding energized (or a current vector of 1.0 times rated current when microstepping) you will only get 0.71 times the rated torque. So the rated max. holding torque is a "marketing trick". Best Regards Nicolas Benezan |
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