This cross-sectional view shows a hydraulic cylinder fitted with an internal position sensor. Optional signal-conditioning electronics could be mounted outside the cap end at left.
Populating these cylinders are position feedback sensors, which employ one of three traditional technologies: magnetostrictive, variable resistance, and variable inductance sensors. Other sensor technologies have seen some success in this application, but the focus of this article is to compare these three most popular technologies.
Incorporating these sensors into a cylinder typically involves the use of a long probe that extends into a deep, small-diameter blind hole drilled into the internal end of the cylinder rod. This process is often called gun drilling, due to its resemblance to a gun barrel.
A Glance at the Big Three
Magnetostrictive technology has been the preferred technology for high-accuracy applications. The sensor incorporates a stainless-steel tubular probe and a short toroidal permanent magnet, which is installed in a counterbore in the cylinder’s piston. The most common package is designed to thread the sensors’ electronics housing into an O-ring port in the back of a cylinder, with the long, slender probe fitting inside into the rod’s bore.
Magnetostrictive technology uses the “time of flight" principle to determine the magnet's position with high accuracy and moderate response time. The magnet reflects a torsional mechanical pulse, which is transmitted along a wire inside the probe, called a waveguide. Typically, each magnetostrictive-sensor manufacturer has its own style of magnet with proprietary mounting features, such the number of holes, the hole pattern, etc.
Magnetostrictive sensors can consume a fair amount of power and may not be as mechanically rugged as the other two main sensor types. They offer high electrical performance, and the magnetostrictive sensor's package design is tailor-made for port-mounted in-cylinder use.
Variable-resistance (potentiometer-type) sensors, commonly called pots, are selected when purchase cost becomes more of a driving factor than high accuracy. A resistance pot is usually embedded into the cylinder's cap end plate, as opposed to the port mounting of magnetostrictive sensors. It uses an insulated round carrier, which is attached to the internal end of the gun-drilled cylinder rod, and supports 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.
