TechArchive

Original article date: November 1997

Roy Moffatt, Managing Director of Variohm Components, explains the basics of the cable extension transducer and how the CET originally came about.

It is possible to deploy one or more methods to measure position on machines or equipment. Cable extension transducers (CETs) are just one of these. They are used to measure the movement and displacement of objects. The body of the transducer is mounted to a fixed surface and the stainless steel cable attached to the moveable object. As the object moves, the transducer produces an electrical signal proportional to the cable’s linear extension or velocity for display, recording or feedback to a controller.

Inside the Celesco industrial grade design, a stainless steel cable is wound on a precisely machined cylindrical spool of constant diameter. This turns as the cable reels and unreels. To maintain tension, a spring is coupled to the spool. The spool is coupled to the shaft of a rotational sensor (an encoder or potentiometer). As the transducer cable extends along the movable object, it causes the spool and sensor shafts to rotate. The rotating shaft creates an electrical signal proportional to the linear extension of the cable or its velocity. The signal is in pulses per unit of distance of cable displacement for an encoder and volts per unit of distance of cable displacement for a potentiometer.

Cables made of materials other than stainless steel, including synthetics, do exist, but these are not so reliable, because they are more elastic and have higher coefficients of expansion. The cable guide (or exit point from the CET) is often via a hole bored into a self-lubricating plastic unit. These are least expensive to buy, but paradoxically, seem to perform the best. They are simple to install and, with a reading accuracy of better than 0.01%, they offer better precision than any other sensor. Cable length is usually up to 2.5m, but for special applications can be increased up to six times that length.

The CET was brought about by a specific need. In the early days of aircraft engineering, before computers were powerful enough to undertake structural calculations, a method was required to measure the movement of the wings along their length when they were jacked up hydraulically to the extreme in order to pinpoint the critical positions on the airframe.

With the aircraft body held in a fixed position, a number of CETs were fixed to the hangar roof in several places along the wing length, their cables were extended downwards and secured to the top of the wings. When jacked from below, the amount of wing flexing can be measured accurately, hopefully with potential weak points spotted.

Although originally developed for the aircraft industry, they are now found in a multitude of engineering applications.

A short brochure is available free of charge to aid the designer and engineer in selecting the right cable extension transducer for the job.

• Variohm Components
• Roy Moffatt
• Tel: 01296 770341

November 1997