L9942 stall detection methods

AN3327

12/22

DocID18343 Rev 3

Figure 8. Back EMF waveforms on an active stepper motor

In an unloaded motor winding at the points where the current is at or near zero the BEMF is

the strongest.

The concern comes when you consider the effects of motor loading on the phasing of the

BEMF. Since this algorithm only looks for BEMF when the phase is not being driven there is

a very short window to "look". As the motor is loaded the BEMF shifts such that it is more in

line with the driving voltage/current. As a result, motor loading adds some variation to the

BEMF detection. A fully loaded motor just on the edge of stall looks to be the same as a fully

stalled motor. Fortunately a stepper motor is not intended to be driven with that much load.