If it is rotary voltage displacement then it sounds like the old 'selsyns' that used to be around and still are. No semiconductor devices are needed to accurately (well pretty accurate) read the displacement of the motor shaft. This motor shaft is connected to something else to give the motor shaft its movement. The motor is energized with 120 3 phase and any corresponding movement of the shaft is sensed by the receiving motor which moves the same amount as the motor which is sensing some process (liquid level, angular movements, ..). A mechanical display can hook to this receiving motor to indicate position or whatever of the process being monitored. I believe the non trade name of such equipment is synchro. No semiconductors needed. Hope this helps.