Comparing position sensor technologies for hydraulic cylinder feedback: Page 2 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.

Variable Resistance potentiometer-type sensors, commonly called pots, are usually selected where purchase cost is a driver and high accuracy is not paramount. A resistance pot is usually embedded into the cylinder's rear end cap, as opposed to the port mounting of a magnetostrictive sensor. It uses an insulated round carrier attached to the internal end of the gun-drilled cylinder rod supporting an electrically conductive wiper that contacts the surface of a partially conductive plastic probe. As the wiper moves along this plastic element, its resistance changes in a linear fashion, making it fairly easy to determine the carrier's position and, thus, the rod's position.  Pots have been seen as a good position measurement solution for use in cylinders because of their ruggedness, favorable stroke-to-length ratio, and their large analog DC voltage output, which is a large percentage of the input voltage.  The major drawback to resistance pots is wearout, especially if the cylinder is actuated at a high frequency, or even more importantly, dithered over a short range to improve a system's dynamic characteristics. Since a resistance pot is embedded into the cylinder, replacement of a worn out pot can be very time consuming and expensive, and could even result in the need for a completely new cylinder.

Linear Variable Inductance Transducer (LVIT) position sensors have been used in the cylinder industry but have not had the industry-wide recognition of magnetostrictive sensors or resistance potentiometers.  This non-contacting technology has many significant advantages over resistance potentiometers regarding product life and long-term reliability, and usually can compete favorably with the performance of magnetostrictive sensors in terms of linearity, resolution, and frequency response, but at a significantly lower cost. Equally important is the fact that variable inductance transducers can withstand much greater shocks and vibration, such as those commonly found in heavy industrial fluid power and mobile equipment applications.

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