When linear measurement is required, sometimes it is not feasible to utilize a linear encoder and a draw wire encoder is preferred. A linear encoder typically requires a fixed length scale and with longer measuring lengths this can become a challenge. As an example, mobile crane booms of telescopic type have a varying linear distance of considerable length. For this reason, draw wire encoders are a suitable choice.
Historically, multiple-turn potentiometers have been coupled with draw wires due to their simple nature and inherent absolute output (known as ¡°string pots¡±). However, the contacting point, or wiper in the potentiometer is a wear point and is a life-limiting factor. It is also an analog device and, therefore, more susceptible to electrical noise. With encoders, the sensing element (whether optical or magnetic) is non-contact and therefore not a wear consideration. Also coupled with the fact that many absolute encoders are serial bus networked devices (i.e. CAN Open, DeviceNet, ProfiBus, etc), they make wiring and communication with controls simpler. It is for these reasons that draw wire encoders are many times preferable over ¡°string pots¡±.
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