A new development in stepper control is to incorporate a rotor position feedback (eg. an encoderEncoder
An encoder is a device, circuit, transducer, software program, algorithm or person that converts information from one format or code to another for the purposes of standardization, speed, secrecy, security, or saving space by shrinking size....
or resolverResolver
Resolver may refer to:* Resolver , a type of rotary electrical transformer used for measuring degrees of rotation* Resolver , an album by the band Veruca Salt...
), so that the commutation can be made optimal for torque generation according to actual rotor position. This turns the stepper motor into a high pole count brushless servo motor, with exceptional low speed torque and position resolution. An advance on this technique is to normally run the motor in open loop mode, and only enter closed loop mode if the rotor position error becomes too large -- this will allow the system to avoid hunting or oscillating, a common servo problem.

can you provide any details , driver schematics theory etc. I am very interested in building a drive that morphs between open loop and closed loop
Amplexus