Linear position sensors use contact or non-contact methods to measure the speed and/or position of an object. They use many different sensing technologies. Capacitance sensors are used with both conductive and nonconductive materials, but are very sensitive to environmental variables. Eddy current sensors contain two coils: an active coil that indicates the presence of a conducting target, and a secondary coil that completes a bridge circuit. Fiber optic sensors use a pair of adjacent fibers to carry light to a target and receive reflected light from the object. Inductive sensors are non-contact devices that determine an object’s coordinates with respect to a reference point. Linear encoders sense and digitize position changes for control systems. Linear potentiometers produce a resistance output proportional to an object’s displacement or position. Linear variable differential transformers (LVDT) produce electrical outputs proportional to the position of a solid, cylindrical core. With magnetoresistive sensors, the resistance of a conducting strip is a function of the direction and magnitude of an applied magnetic field. With magnetostrictive sensors, displacement is deduced from the change in state of a magnetic field generated by a ferromagnetic material. Photoelectric sensors use reflected beams of light to measure distance or displacement. Ultrasonic sensors reflect acoustical signals and calculate distance based on the signal’s return time and the measurement medium’s propagation velocity. Variable reluctance sensors are also available.