Hi Pascal,
You may be reaching the limit of the motor. The back EMF may be preventing the desired current level. As you observed, raising the voltage is the best approach to reach the desired current.
There are a few items to try:
1) Change to slow decay on increasing, and mixed or automixed decay on decreasing.
2) Lower the step frequency. This will allow more time for the current to reach the desired level.
3) Talk to your motor manufacturer about alternate motors.
You may be reaching the limit of the motor. The back EMF may be preventing the desired current level. As you observed, raising the voltage is the best approach to reach the desired current.
There are a few items to try:
1) Change to slow decay on increasing, and mixed or automixed decay on decreasing.
2) Lower the step frequency. This will allow more time for the current to reach the desired level.
3) Talk to your motor manufacturer about alternate motors.
