Fiber optic linear position sensors use fiber optic technology to sense position and displacement. They use an adjacent pair of fiber optic elements, flexible strands of glass that transmit light along their length by maintaining the near-total internal reflection of light accepted at the input. One fiber optic element carries light from a remote source to the object or target whose displacement is to be measured. The other fiber optic element receives the light reflected from the object and carries it to a remote photosensitive detector. Output types include analog current levels, analog voltages, modulated analog outputs, and digital signals. 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 outputs such as sine waves, pulse waves, amplitude modulation (AM), and frequency modulation (FM) signals are encoded, but still analog in nature. 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 inductive linear position sensors and switches include transistor-transistor logic (TTL) signals.
