[BornaE]

You need to use and encoder to "measure" the Back EMF of the motor.

BackEMF = EncoderRPM * 12V/(12V Free RMP)


Motor output = Control Output + BackEMF

This essentially linearizes the motor to act as a torque source.

Now you can set up a standard PID with RPM as input. and Plug in the output to the equation : Motor output = Control Output + BackEMF.