Imagine an ordinary tape measure that provides feedback in electrical rather than visual form. That's the general idea behind draw wire sensors, more commonly known as cable extension transducers (CETs).

CETs are ideal for measuring linear displacement across long unsupported spans in places like elevators, boom cranes, presses, and wood processing machinery. They're relatively easy to install, maintenance free, and produce output signals recognized by most industrial controllers.

Q & A

What are the operating specs?

Measuring range: from a few inches to around 200 feet

Working speed: up to 65 ft/sec

Linearity: 0.01% to 0.35%

How tough are they?

CETs are protected against environmental assault, complying with IP67/68 standards. They also resist shock and vibration, and withstand temperatures to 85°C. They'll take even more abuse when equipped with cable wipers (to clean the draw wire during retractions), metal or plastic shields (to cover exposed cable runs), and high-temperature extension cable rated for 200°C.

Is it okay to use a pulley to guide or support the measuring cable?

If it's not possible to mount a CET in the plane of motion it's intended to measure, the cable can be rerouted through a low-friction pulley. This should be considered a last resort because it reduces cable life.

This month's handy tips provided by ASM Sensors Inc., Elmhurst, Ill. For more information on the use of CETs, call an ASM application engineer at (630) 832-3202 or e-mail info@asmsensors.com.