A position sensor is any device that permits position measurement. It can either be an absolute position sensor or a relative one (displacement sensor). Position sensors can be linear, angular, or multi-axis.
Some position sensors available today:
Capacitive transducer
Capacitive displacement sensor
Eddy-current sensor
Ultrasonic sensor
Grating sensor
Hall effect sensor
Inductive non-contact position sensors
Laser Doppler Vibrometer (optical)
Linear variable differential transformer (LVDT)
Multi-axis displacement transducer
Photodiode array
Piezo-electric transducer (piezo-electric)
Potentiometer
Proximity sensor (optical)
Rotary encoder (angular)
Seismic displacement pick-up
String potentiometer (also known as string pot., string encoder, cable position transducer)