Drive Question

We use Siemens S-120 drive for motor carriage and the control sensorless vector control. The carriage have to stop three position. Start position , middle position and end position . The position is measured by laser encoder and PLC check measured position and reference position and position controller is in PLC. PLC send to drive speed reference . The carriage stop at start and end position without overshoot and when stop motor current is almost 40%. When it tries to stop at middle position it overshoots and stopping time motor current increase to 70%.

The same control used for stopping at start, middle and end position. Could you please tell me why current is increasing to 70% stopping time at middle and also please what i need to do to stop overshoot.

If you are using a zero speed reference or motion control, it is not unusual for a motor to draw a lot of current trying to maintain position. If the motor is rated for VFD control and does not overheat, it may not be a problem. If you want the motor to draw no current when stopped, and the cycle of operations permits it (what is the duration of the stopped parts of the sequence?) then perhaps you should stop in position and disable the VFD, perhaps add a mechanical brake if necessary to hold position.

As for the overshoot, tell us more about how the motor control works. Also the mechanical system info. Is it belt chain or screw driven?

I am not a Siemens user nor a motion expert but have worked on a few problems similar to yours. Sometimes a fine tuning of the motor control is all that is needed and other times I have fixed this sort of problem with logic changes. In one case we had to change the gear ratio and another case we added an external brake.