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Stepper Fundamentals Slide 37

If the transfer function that was derived in the discussion on minimum current is applied to the desired sine wave current for microstepping, the effect of minimum current shown here in the orange and green occurs. If the output of the DAC sets current near zero, the circuit will respond with the minimum current. To get zero current, the bridge can be disabled and all of the transistors can be turned off. Without any drive, the current will go to zero in order to get the zero points on the sine wave. Since zero current is possible, look only at what affect the minimum current will have on the sine wave just before and just after the zero value. Luckily the first micro step after zero and the last one before zero is the largest step that is needed. So if the minimum current is less than the sine of the first microstep, there should be no distortion in the sine wave. In the example shown here, the minimum current starts to effect the waveform at about 11°. So if the first microstep is more than 11°, there should be no distortion on the sine caused by the minimum current. In this case, each quarter cycle could be divided into eight microsteps of 11.25° each.

PTM Published on: 2011-12-14