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Linear displacement transducer isn’t limited by stroke

Linear displacement transducer isn’t limited by stroke

One of the challenges with most linear displacement transducers (LDTs) for measuring hydraulic cylinder position is that the stroke must be known when the sensor is ordered. Furthermore, in-cylinder mounting generally requires gun-drilling the cylinder’s piston rod and mounting a magnet to the piston, both of which add to the final cost of the cylinder.

Illustration shows cutaway of cylinder and how LDT is installed as a replacement for a conventional LDT. For original application, piston rod would be solid and cable would be attached to piston or end of rod.

Plus, conventional in-cylinder mounting of LDTs cannot be used with double-rod-end or telescoping cylinders. In these cases, the LDT generally is mounted alongside, rather than inside, the cylinder. Finally, in-cylinder mounting can make LDTs susceptible to damage from radial shock and vibration for strokes longer than a few feet because the sensor inside the rod has a cantilevered mounting.

The unconventional approach
An alternative to conventional in-cylinder mounting is offered by Control Products Inc. (CPI), East Hanover, N. J. CPI introduced its unconventional in-cylinder mounted LDT ten years ago. Currently referred to as the SL720, the transducer consists of a cable and spring-loaded spool inside a housing.

One end of the cable attaches to a cylinder’s piston, and the other end wraps around the spool. As a cylinder’s piston retracts and extends, the wire wraps or unwraps around the spring-loaded spool. The inside diameter of the spool is threaded, and the OD of an LVDT has mating threads. Rotational motion of the spool, then,  causes linear motion of the LVDT, which produces a signal that is a function of piston position.

The SL720 does not require gun drilling of the piston rod, it can be used for any stroke up to 1 m, and it works in double rod and telescoping cylinders. However, the cylinder’s end cap must be machined to accommodate the LVDT housing.

Think outside the cylinder
CPI’s latest offering is a family of transducers that uses the same cable and LVDT technology, but the spool and LVDT housing gets mounted outside the cylinder, so no end cap machining is necessary. Instead, the cable attached to the piston is fed through a hydraulic hose that threads into the end cap of the cylinder. The other end of the hose is fitted to a steel enclosure that houses the spool and LVDT assembly.

This setup provides easy installation because all that’s needed is to attach the cable to the cylinder’s piston, thread one hose end into an 18 mm or #8 SAE port in the cylinder, then mount the housing to machine framework of the cylinder itself. The transducer is currently available in four models to accommodate a variety of applications with cylinder strokes to 120 in. All have a steel housing enclosing the LVDT sensor, and all can be used with any stroke up to its rated maximum.

The SL0390 and SL1200 have high shock and vibration resistance and measure strokes to 40 and 120 in., respectively. The SL0391 and SL1201 are intended as drop-in replacements for in-cylinder mounted magnetostrictive LDTs and measure strokes to 40 and 120 in., respectively. The sensor can be operated with any standard signal conditioner, with 0- to 5-Vdc, 0- to 10-Vdc, and 4- to 20-mA outputs and 10 to 30 Vdc input being most common. Output nonlinearity is less than ±0.01% FSO.

For more information, contact Control Products Inc. at (973) 887-9400 or visit


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