Rotary position sensors provide electrical outputs proportional to shaft rotation. They measure multiturn motion and angular displacement of less than one turn. Rotary position sensors use many different sensing technologies. Potentiometers detect variable resistance and measure the angle of rotation as the shaft turns. Hall effect sensors develop an electrical potential across an axis perpendicular to an applied current flow. Permanent magnets mounted to the rotating sensor shaft supply a magnetic field. Variable inductance sensors include a component on the rotating sensor shaft that induces magnetic field effects on multiple stationary coils. Resolvers consist of a rotating coil and two stationary coils that are 90° apart. They measure the currents generated in the stationary coils and compare them, providing precise measurements of the rotor position. Synchros are variable transformers that are similar to resolvers, but that contain three stationary coils 120° apart. The coil on the rotating shaft induces a current in the other two coils that can be used to provide angular position outputs. Rotary variable differential transformer (RVDT) signal conditioners translate the rotary movement of a ferromagnetic armature into an AC voltage that is proportional to the armature position. Capacitive displacement transducers are non-contact devices that use differential capacitance for high resolution angular measurement. Other unlisted, specialized, and proprietary sensing technologies are also available.