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, be 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.
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