Getting loose of absolute position with incremental encoders

My question is what happens, if the incremental position encoder e.g. optical looses the absolute position?

Say the motor is running in velocity mode and the load exceeds the moment the motor is capable with this volatge/current at the moment. In that case the electrical angle gets an offset because the motor slips through say by one pole-pair. Is this detected in any case? If so the SW should stop the motor and do a realign.

Thanks for any hints.

Are you thinking on an open-loop or closed loop set-up?

If you’re thinking closed loop and the sensor is very securely fixed to the motor’s rotor you will never loose the electrical angle offset, even if the torque of the motor is not enough to move the load (considering sensor alignment was done without any load).

To further clarify, perhaps @wimalopaan refers to the case where the external force exceeds the torque and the motor stops and even could be moved backwards (in his understanding it “slips”) and his confusion is how the algorithm knows what to do?

My take on his question is, if that’s the case, the algorithm will “stop” or “count backwards” the alignment, synchronizing the counter with the apparent angular position of the coils and apply the maximum current to the coils corresponding to the position, while constantly updating the position. In other words, the motor will start “fighting you” until you release it and then it will spin forward always trying to reach the velocity you set. In a closed loop the electrical angle never gets “offset” w.r.t. the sensor output, unless there is some severe mechanical problem such as sensor slippage or electrical problem in the circuit. That’s why it’s called “closed loop”.

Did I get that right?

Yes, thank you for your explanation.
The electrical-angle is fixed to the mechanical-angle, so it is incremented properly when the mechanical-encoder steps forward.

You’re totally correct :slight_smile: