Comparing position sensor technologies for hydraulic cylinder feedback

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.

While other sensor technologies have occasionally been used in this application, the focus of this article is the comparison among these three most popularly used technologies. Ultimately, a user or systems integrator must determine the requirements of the application and which technology best satisfies it on a total installed cost versus performance basis. The strengths and weaknesses of magnetostrictive, variable resistance, and variable inductance sensors are examined below, along with a chart for feature-by-feature comparisons.

First, a point to be noted is that all of these three common sensing technologies utilize a long probe that extends into a deep, small diameter blind hole gun-drilled into the internal end of the cylinder rod. 

 Magnetostrictive technology has traditionally been the preferred technology for use in high accuracy applications. These sensors, often called LDTs or MLDTs, incorporate a stainless steel tubular probe and a short toroidal permanent magnet assembly around it that is installed in a counter-bored recess in the piston. The most common package threads the sensors' electronics housing into an O-ring port in the back of a cylinder, with the long slender probe inserted into the rod's bore. This technology uses the “time of flight" principle to determine the magnet's position with very high accuracy and moderate response time.

In operation, the magnet is used to reflect a torsional mechanical pulse transmitted along a special wire called a waveguide inside of the probe. Typically each magnetostrictive sensor manufacturer has their own style of magnet with unique mounting features like the number of holes, the hole pattern, etc.  Magnetostrictive sensors can consume a fair amount of power and are not the most mechanically rugged sensors. They offer electrical performance over mechanical robustness, because they often encounter shock and vibration issues.  Yet, even with these potential drawbacks mechanically, a magnetostrictive position sensor's package is tailor-made for port-mounted in-cylinder use.

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