Indirect rotor position sensing for variable reluctance motors

Rotor position sensing is essential in providing stable operation for closed-loop control of variable reluctance (VR) motors over a wide speed range. Position sensing can be carried out by two different methods; either by using direct position sensors or by using an indirect sensing method. There are obvious disadvantages in using direct position sensors. In this paper further developments in indirect rotor position detection using the waveform detection (WD) technique for VR motors are described. In principle, rise and fall times of an active or inactive phase can be used for position detection. However, position detection by monitoring the rise or fall times of the active phase current has been found suitable only in a narrow operating range. In order to overcome this problem, the rise or fall times of a small magnitude diagnostic current injected into an inactive phase can be monitored for position detection. Theoretical analysis and simulation results reported elsewhere have revealed that the use of the inactive (diagnostic) phase chop current rise time leads to instability in the drive system and hence is not recommended. In this paper, the inactive (diagnostic) phase chop current fall time is used instead for position detection. A hard-wired digital closed-loop controller is designed and implemented to drive a 3-phase VR stepping motor. Experimental results obtained conclude that rotor position detection using the WD technique with diagnostic phase chop current fall time provides reliable position detection and stable operation for the drive system over a wide operating range