Rotary encoders are widely used to provide feedback for linear measurement. As the encoder generates pulses in response to linear displacement of the material or object to be measured, it is a matter of simple math for the receiving device to convert those pulses into distance.
Mechanically, a primary consideration is how to effectively obtain feedback from the item to be measured. One common method is to apply a measuring wheel to the encoder. The wheel either contacts the surface to be measured (direct feedback) or a shaft that moves in conjunction with the material (indirect feedback). Measuring wheel applications need a means of mounting the encoder and maintaining adequate wheel pressure at all times. Separate brackets, pivot arms and springs can be used, but they can be cumbersome. EPC's Model TR1 TruTrac™ is an all-in-one solution that makes for fast and easily installation.
Another means of using a rotary encoder for linear measurement is to attach a thru-bore or hollow bore encoder to a roller shaft, such as a conveyor head-roll. In such cases, a clear understanding of the relationship between shaft rotation and corresponding linear displacement, as well as system mechanical tolerances, is essential.
For reciprocating linear measurement, a measuring wheel may not be the best choice; there is always the potential for slippage during start/stop cycles. In such cases, a solution that eliminates or limits backlash, such as a pinion gear and rack (as with the TR2 TruTracTM), a belt and pulley, or a chain and sprocket may be preferred. Another solution is the Linear Cable Encoder (LCE), where a spring-driven retractable cable spool is attached to a shaft encoder.
Electrically, incremental encoders are most often specified for linear measurement, with some specialized applications requiring absolute positioning. For applications with frequent start/stop or bi-directional travel, quadrature output is required.
Environmentally, proximity of the encoder to potential exposure to liquids, fine particulates and extreme temperatures will dictate specification. In applications with washdown requirements, an IP66 or IP67 seal can offer protection against moisture ingress.