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Position feedback sensors for hydraulic or pneumatic cylinders have most commonly used one of three technologies: magneto-strictive (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 comparing 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-vs.-performance basis. The strengths and weaknesses of magneto-strictive, variable resistance, and variable inductance sensors are examined below, along with a chart for feature-by-feature comparisons.
Linear variable inductance transducer (LVIT) sensors cover the middle ground between the higher level of performance and external port mounting flexibility associated with a magneto-strictive sensor.
Magneto-Strictive Technology
First, a point to be noted is that all three of these 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.
Magneto-strictive 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 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 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....