linear encoders include built-in limit switches; either optical or magnetic. Two limit switches are frequently included such that on power-up the controller can determine if the encoder is at an end-of-travel and in which direction to drive the axis. Physical arrangement and protection Linear encoders may be either enclosed or open. Enclosed linear encoders are employed in dirty, hostile environments such as machine-tools. They typically comprise an aluminium extrusion enclosing a glass or metal scale. Flexible lip seals allow an internal, guided readhead to read the scale. Accuracy is limited due to the friction and hysteresis imposed by this mechanical arrangement. For the highest accuracy, lowest measurement hysteresis and lowest friction applications, open linear encoders are used. Linear encoders may use transmissive (glass) or reflective scales, employing Ronchi or phase gratings. Scale materials include chrome on glass, metal (stainless steel, gold plated steel, Invar), ceramics (Zerodur) and plastics. The scale may be self-supporting, thermally mastered to the substrate (via adhesive or adhesive tape) or track mounted. Track mounting may allow the scale to maintain its own coefficient of thermal expansion and allows large equipment to be broken down for shipment. Encoder terms Resolution Repeatability Hysteresis Signal-to-noise ratio/noise/jitter Lissajous figure Quadrature Index/reference mark/datum/fiducial Distance coded reference marks (DCRM) See also Rotary encoder References Further reading Electromechanical engineering Position sensors