Comparing position sensor technologies for hydraulic cylinder feedback: Page 3 of 4

September 18, 2018 // By Edward E. Herceg
As the demand for increased control and functionality has increased over the years, sensor- instrumented cylinders are becoming more important in the heavy industry, subsea, and mobile equipment worlds.  Position feedback sensors for hydraulic or pneumatic cylinders have most commonly used one of three technologies:  Magnetostrictive (MLDTs), Variable Resistance (Pots), and Variable Inductance (LVITs) sensors.

Linear Variable Inductance Transducer (LVIT) sensors cover the middle ground between the higher level of performance and external port mounting flexibility associated with a magnetostrictive sensor and the better ruggedness and lower price of an embedded resistance potentiometer. These sensors operate by measuring the resonant frequency of an oscillator circuit of an inductive probe whose inductance is varied by the position of the conductive gun-drilled rod over it, using built-in electronics. Typically offered in full scale ranges of 4 inches (100 mm) to 36 inches (900 mm), both port-mounted and embedded packages are available, with connector and cable terminations. LVIT sensors offer either an analog DC voltage or current output, with some types of digital output available for OEM applications. The LVIT sensor presents a contactless solution that does not require a ring magnet. In fact, if an LVIT sensor were installed to replace an existing magnetostrictive sensor, the magnet can be left in place in the cylinder rod end without interfering with the sensor’s basic operation.

Comparison of analog output linear position sensors for industrial applications.

In the past few years, the requirements for instrumented cylinders for subsea applications have dramatically increased. LVIT sensors are offered in a pressure-sealed version that allows a user to install the sensor and cylinder in subsea environments to depths of 10,000 feet (3000 m), with 3000 psig of internal pressure.

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