Sensing Linear Position in Telescoping Hydraulic Cylinders

Telescoping hydraulic cylinders

Telescoping Hydraulic Cylinders find uses in everything from raising the bed of a dump truck, to the massive riser tensioner cylinders used on offshore oil rigs. Telescoping cylinders were first designed to provide long stroke lengths where the total length required would not fit in into the machine. Instead these cylinders are multi-stage and their construction presents challenges for accurate and reliable position sensing, especially in harsh environments.

Two Stage Telescopic cylinder

Two Stage Telescopic Cylinder

Some of these challenges are inherent to the nature of telescoping cylinders, in that the stroke must typically be known before the sensor can be made. Furthermore, in-cylinder installation generally requires gun-drilling the piston rod to mount a magnet to the piston, which is difficult and adds to the final cost of the cylinder. Not to mention that in-cylinder mounting can make these LDT’s susceptible to damage from excessive vibration, radial shock, and heat.

Telescoping Hydraulics Require a Different Sensor Approach

Enter the SL Series Linear Position Sensors from Control Products Inc. This sensor technology combines some well-known components of linear position sensors built in a whole new way. The sensor design utilizes well known draw-wire technology coupled with an LVDT to deliver an absolute positioning system that is easily mounted internal to the cylinder and highly resistant to shock, vibration and EMI. Most importantly it is one of the most robust and versatile overall solutions to linear position sensing for a telescoping hydraulic cylinder available today.

The SL1300 is an example of this new kind of sensor, appropriate to the needs of telescoping cylinder designers with a maximum stroke length of 23 feet. The transducer for the 1300 consists of a cable and spring-loaded spool inside a housing. One end of the cable attaches to the moving piston, and the other end wraps around the spool. As a cylinder’s piston changes position, the draw-wire wraps or unwraps around the spring-loaded spool. The inside diameter of the spool is threaded, and the outside diameter of an LVDT has mating threads. Rotational motion of the spool is thus translated into the linear motion of the LVDT, which produces a signal that is a function of absolute piston position.

A significant benefit to the SL Series Sensor is that it works for any stroke length over its sensing range with almost no modification to an existing design, most notably without core drilling of the rod. This makes position sensing much more practical in long stroke applications, especially telescoping cylinders where internal piston position sensing was not previously possible.

About Linear Position Sensors Made By Control Products Inc.

Linear Position Sensor by CPICPI introduced its first generation of SL series LVDT based linear position sensors in 2001 and CPI’s latest offering is a family of sensors that uses the same cable and LVDT technology, but the spool and LVDT housing can be 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 comes in 4 models to accommodate cylinder strokes up to 120 in.  The SL0390 and SL1200 have high shock and vibration resistance and measure strokes to 40 and 120 inches, 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.

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