Draw Wire Encoder Overview
Rotary absolute encoders provide absolute position information to aid in improving machine performance in power-off and start-up modes of operation. However, there are times when this positional information is needed on a linear, not rotary basis. In some instances then, systems may employ a rotary encoder coupled with a spring-loaded reel to translate the linear motion. This package is known as a draw wire encoder.
Understanding Draw Wire Encoders
The basic operating principle of a draw wire encoder is simple; a rotary encoder is coupled to a spring loaded-reel through the use of a mechanical coupling device, internal to the housing of the reel. A cable is wound around the reel, usually a metal braid type. The braided cable is anchored to the drum of the reel and wrapped many times around it. The opposite end of the cable passes through an exit port of the draw wire encoder housing and is terminated with an anchoring device such as a clevis or loop to attach to the moving mechanism being measured. As the cable is pulled, the drum is rotated and the linear motion is translated into rotary motion, at which point the draw wire encoder encodes the information into an electric signal. In simplistic terms, the concept of a draw wire encoder can be likened to a fishing reel.
However, unlike the fishing reel analogy the drum is spring loaded to eliminate cable slack and therefore ensure accurate length measurement. Depending upon the linear distance being measured and the diameter of the draw wire encoder drum and cable, the absolute encoder must be of multi-turn type in order to provide a true absolute measurement over the entire range of the linear motion. Draw wire encoder measurement lengths can range from a few inches to tens of feet.
Why Use a Draw Wire Encoder?
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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