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The accuracy of a linear potentiometer is limited to its resolution, which is determined by how tight the wires for the resistor can be wound. Other linear position sensors have been developed that provide a greater degree of accuracy. The magnetostrictive-type position sensor uses a magnetic field that is distorted as a waveguide is moved through the field to determine position. Fig. 1 shows an example of these sensors. One type has a magnetostrictive sensor with a movable rod. For this tvpe of sensor the body is mounted on the fixed part of a machine, and the rod is mounted to the movable part of the machine. When the movable part of the machine moves back and forth, the rod is extended and retracted in the sensor body.
Above: Fig. 1 (a) Examples of two different types of magnetostrictive
positioning sensor. Three sensors are shown with movable rods, and the
fourth sensor is shown with a fixed rod and movable magnet. (b) An example
of a magnetostrictive type positioning sensors mounted on an injection
molding machine.
The second type of magnetostrictive positioning sensor has the rod fixed to the body. The body is mounted to the stationary part of the machine and a set of four magnets shaped like a doughnut are mounted to the movable part of the machine. When this type of sensor is mounted on the machine, the rod is inserted through the center of the magnetic doughnut. When the movable part of the machine moves back and forth, the magnet will move back and forth over the rod. This is the same effect as the sensor in the first example where the rod moves back and forth through the doughnut.
The Benefits of Magnetostrictive Sensing
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The two image shown above use the time-based magnetostrictive position sensing principle developed. Within the sensing element, a sonic strain pulse is induced in a specially-designed magnetostrictive waveguide by the momentary interaction of two magnetic fields. One field comes from a movable permanent magnet that passes along the outside of the sensor. The other field comes from an "interrogation" current pulse applied along the waveguide. The resulting strain pulse travels at sonic speed along the wave-guide and is detected at the head of the sensing element. The position of the magnet is determined with high precision and speed by accurately measuring the elapsed time between the application of the interrogation pulse and the arrival of the resulting strain pulse with a high speed counter. Using the elapsed time to determine position of the permanent magnet provides an absolute position reading that never needs recalibration or re-homing after a power loss. Noncontact sensing eliminates wear, and guarantees the best durability and output repeatability.
