Preexcitation Signal When Driving a Motor With Ethercat Using Control Words

Issue Details

I am driving a third-party stepper motor (SW1A4185 from EVER Elettronica) using the 6040 control word. The following sequence is used to unlock the driver: 128, 0, 2, 6, 7, 15. Then, I use the control word 6 to set the motor's current to zero. On the first run of the VI, there is a preexcitation time of 2 seconds:

After rerunning the VI, the preexcitation disappears:

How can I fix this inconsistency?

Solution

The pre-excitation may be due to magnetic field alignment in permanent magnet stepper motors.

On the first run, the motor starts from a random rotor position, taking time to reach the optimal electrical angle.

On the subsequent runs, the motor retains alignment through detent torque, enabling immediate operation.

This happens because of an electrical angle mismatch between the position at command 7 and the optimized position at command 15. 

The recommended solution is to replace the current sequence with 128, 0, 2, 6, 14, 15, which should help avoid the problematic switch-on state.