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Stepper Fundamentals Slide 39

Another anomaly that can affect microstepping is a result of the decay mode that is selected. These two different decay modes, fast decay and slow decay, were discussed earlier. In the slow decay mode, during the off time, the current is re-circulating around a path that is one on transistor and one diode (or one MOS conducting in the reverse direction). During this time, the current decays rather slowly since the voltage across the coil is very small. However, the slow decay rate may limit the ability to follow a fast changing waveform that is on the downward slope of the sine curve. The green and orange traces represent the current waveform when the decay rate is limited by the slow decay and the inductance of the motor. Since the entire voltage is applied across the motor when energizing the inductance, the di/dt can be relatively fast. In that case the current can follow the rising slope of the sine wave but the fall time is limited by di/dt.

PTM Published on: 2011-12-14