Capacitive sensors for position acquisition of linear drives with passive vehicles

Long stator permanent magnet synchronous linear drives with passive, lightweight movers (vehicles) are proposed for material handling systems. For high quality speed and position control of the vehicles, precise and fast position measurement is necessary. These passive vehicles provoke challenges for the position acquisition system as neither energy nor information must be transmitted to the moving parts. The paper analyses the principle of operation of a capacitive sensor and presents two methods of extracting the position information, one uses instantaneous demodulation and the other is based on phase measurement.