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.  

Selecting linear position sensors requires an analysis of device categories and operating specifications. Suppliers typically divide products into the following categories: sensors, sensor elements, instruments, gauges, and recorders. Sensors, a category that includes transducers, include packaging and signal conditioning. By contrast, bare sensor elements or chips do not. Instruments or meters are self-contained linear position sensors that produce local outputs and include signal processing or analysis. Gauges or indicators are sensing devices with integrated dials or simple readouts. Recorders and totalizers measure values or accumulate totals. They may also provide statistical functions. Operating specifications for linear position sensors include temperature range, shock rating, and vibration rating. Linear range is usually measured in inches (in). Accuracy is a percentage of full-scale.   

Linear position sensors differ in terms of electrical outputs. Analog current levels such as 4 – 20 mA are suitable for sending signals over long distances. Analog voltages are simple, usually linear functions. Modulated analog output signals are encoded, but still analog in nature. Examples include sine wave, pulse wave, amplitude modulation (AM), and frequency modulation (FM) signals. Several types of digital outputs are available. RS232, RS422, and RS485 are common serial, digital protocols. Popular parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488. Other digital outputs for linear position sensors include transistor-transistor logic (TTL) signals. Outputs that change the state of a switch or alarm are also available.