As the demand for greater control and functionality increases, position-sensor-instrumented hydraulic cylinders are becoming more important in the heavy industry, mobile equipment, and subsea worlds. Position sensors for feedback in hydraulic or pneumatic cylinders have typically used one of three technologies: Linear Variable Inductance Transducers (LVITs), variable resistance potentiometers (Pots), or magnetostrictive transducers (MLDTs). While other sensor technologies have occasionally been used in these applications, the focus of this article is a comparison among these three popular technologies. Ultimately, a user or systems integrator must determine the exact requirements of the application and which technology best satisfies them on a total installed cost versus performance basis. The strengths and weaknesses of linear variable inductance, variable resistance pots, and magnetostrictive sensors are all examined below, together with a feature-by-feature comparison chart.
First, a point to be noted is that all three of these 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.
Linear Variable Inductance Transducer (LVIT) position sensors feature the level of performance and external port mounting usually associated with a magnetostrictive sensor and the ruggedness and lower price of an embedded resistance potentiometer. As a contactless technology, LVITs offer many significant advantages over resistance potentiometers regarding product life and long-term reliability, while competing favorably with the performance of magnetostrictive sensors in terms of non-linearity, resolution, and response, but at a lower cost. Equally important is the fact that these variable inductance transducers can withstand very great shock and vibration, such as found in heavy industrial and mobile equipment applications. The requirements for instrumented cylinders for various subsea applications have dramatically increased, so LVIT sensors are offered in high pressure sealed versions that allow installation of the sensor and cylinder in subsea environments to seawater depths of 10,000 feet (3000 m), with 3000 psig of internal pressure.
These sensors operate by using built-in electronics to measure the resonant frequency of an oscillator circuit with an inductive probe whose inductance is varied by the position of the conductive gundrilled rod over it. They feature an excellent stroke-to-length ratio and are typically offered in ranges from 4 inches (100 mm) to 36 inches (900 mm), in either port-mounted or embedded packages having either connector or cable terminations. LVIT sensors offer an analog voltage or current output, with some digital outputs available for OEMs. Remote field calibration is a standard feature offered on many variable inductance sensors, which permits a user to scale the output of the sensor after it is installed in the cylinder so it is no longer necessary to scale the unit in the associated control system.
Comparison of Analog Output Linear Position Sensors for Industrial Applications
An LVIT sensor presents a contactless solution that does not require a separate ring magnet as in a magnetostrictive sensor. In fact, if an LVIT sensor were used to replace an existing magnetostrictive sensor, the magnet can be left in the cylinder rod end without interfering with the LVIT’s operation.
Variable resistance potentiometertype sensors, commonly called “pots,” are typically 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. It uses an insulated round carrier attached to the internal end of the gun-drilled cylinder rod that supports an electrically conductive wiper running along the surface of a partially conductive plastic probe. As the wiper moves over the 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 voltage output, which is a high 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 wornout pot can be time-consuming and expensive, and could even result in the need for a completely new cylinder.
Magnetostrictive sensors have 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 counterbored 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 cylinder rod's gun-drilled bore. This technology uses the “time of flight” principle to determine the magnet's position over the probe 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 and the hole pattern. Magnetostrictive sensors can consume a large amount of power and are not the most mechanically rugged sensors. They offer electrical performance over mechanical robustness, because they often have issues around shock and vibration. But even with these potential drawbacks mechanically, a magnetostrictive position sensor is well suited for port-mounted or embedded service in cylinders.
So, to summarize, while there are many fluid power applications where resistance potentiometers and magnetostrictive sensors are a good solution, those applications often fall to either side of a bell curve. Recent smart electronic advancements and the flexibility of package design make the LVIT variable inductance sensor a very cost effective and technically sound solution for most mainstream in-cylinder applications, especially those that tend to be near the top of that applications bell curve.
This article was written by Edward E. Herceg, Chief Technology Officer, Alliance Sensors Group a div. of H. G. Schaevitz LLC (Moorestown, NJ). For more information, click here .