By Josh Cosford, Contributing Editor
A draw-wire encoder (also known as a cable extension transducer or string potentiometer) is ideal for measuring linear displacement (distance) in a variety of applications where precise position is required. A draw-wire encoder uses a wire or cable wound around a spool, connected to a rotary encoder or potentiometer housed in a protective casing. A spring mechanism ensures the cable stays in constant tension, even during rapid retraction.
The spool’s rotation converts into an electrical signal by the rotary encoder, which outputs position data in either analog or digital format. The housing protects the mechanism and has some type of bracket to help secure the encoder to the machine. With such a simple and effective design, there are many reasons you should use a draw-wire encoder.
- Draw-wire encoders are excellent for measuring long distances, often ranging from just inches to many yards. The encoder itself has no limitations, so the measuring length is only limited by the reel mechanism.
- They are compact and require minimal mounting space, making them suitable for applications where space is restricted, especially at the attachment end where a simple eyelet is used.
- The draw-wire encoder isn’t relegated to a single axis of motion since the wire can follow curved or nonlinear paths; they are useful when the measurement path is not straight or direct. The cable can bend around idler pullies as well, so even the mounting point of the reel does not need to be along the same axis as the measured end.
- They can be mounted in various orientations and are easy to integrate into existing systems. If you find out later that you want to accurately measure the cylinder stroke length, the draw-wire encoder can be added to the machine with some simple brackets rather than a complex installation inside the cylinder itself.
- They are suitable for measuring the movement of objects like platforms, doors, or machinery components, where both position and velocity must be relayed back to the PLC or machine controller.
- Compared to other types of linear measurement devices, they are often more economical for long-range applications. LDTs installed into cylinders, for example, can be quite expensive even before the provisions are made in the cylinder to accommodate the sensor. The rod must be gun-drilled and the cap machined to accept the electronics and connection point.
- Many models are rugged enough for industrial settings, with resistance to dust, dirt, and moisture. If you have a particular level of weather or washdown protection you’re looking for; chances are a manufacturer will have a solution for you.
You can find draw-wire encoders on many machines and applications. They’re perfect for hydraulic cylinders to monitor the position and velocity of both extension and retraction travel. They’re popular on construction equipment, such as booms and platforms with very long extension lengths.
And it’s no surprise these are used on various components of automation to track linear mechanical movement in machinery. Material handling and positioning applications are perfect for draw-wire encoders, as are test machines requiring position measurements, such as for tensile strength. If precision, robustness, and compact installation for long linear measurements are key factors, a draw-wire encoder is a fantastic choice.
