While other sensor technologies have occasionally been used successfully in this application, the focus of this article is the comparison among these three most popularly used technologies.
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.
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 discussed below, along with a chart for feature-by-feature comparisons.
Initially, a point to be noted is that all of these common sensing technologies utilize a long probe that extends into a deep, small diameter blind hole which has been gun-drilled into the internal end of the cylinder rod.
Magnetostrictive technology has been the preferred technology for high accuracy applications. The sensor, often called an LDT or MLDT, incorporates a stainless steel tubular probe and a short toroidal permanent magnet assembly around it that is installed in a counterbore in the 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 inserted into the rod's bore.
It uses the “time of flight" principle to determine the magnet's position with high accuracy and moderate response time. The magnet is used to reflect a torsional mechanical pulse which is transmitted along a special wire inside of the probe called a waveguide. Typically, each of the magnetostrictive sensor manufacturers has its 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 are subject to shock and vibration issues. Yet, while there are some potential drawbacks mechanically, 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 where purchase cost is a driver and high accuracy is not paramount. A resistance pot is usually embedded into the cylinder's rear 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.
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 big 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.
Variable inductance position sensors have been used in the cylinder industry but have not had the widespread 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 sensors can withstand much greater shocks and vibration, such as those commonly found in heavy industrial and mobile equipment applications.