Magneto-restriction Principle In Linear Position Sensors

By Author: Control Products
Total Articles: 33
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Magnetic position sensing is done in linear position sensors by attaching a sensor to the moving component, which corresponds with the stationary sensors. Measurement accuracy can be greatly improved by using multiple sensors.

Linear position sensors operate on the principle of magneto-restriction, a principle that helps determine the position, distance and speed accurately. Based on elapsed time measurement, it helps measure parameters with pinpoint precision. Manufacturers opt for sensors in a pressure-resistant stainless steel housing that have been made using a non-contact and wear-free measuring technique. The sensors are ready to be fully integrated into any hydraulic cylinder.

Accuracy measurement is done in terms of resolution, non-linearity, hysteresis, repeatability and temperature coefficient. Ambient conditions include ambient temperature range, storage temperature range, relative humidity, vibration resistance, shock resistance, interference transmission and interference resistance. Other data you need to factor in is the electrical connection, supply voltage and current consumption without load.

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... implementing a non-contact location of motional objects, magnetic position sensing using anisotropic magneto-resistive (AMR) sensors has become very popular. These sensors operate by connecting a sensor to a linear or angular moving object that corresponds with the stationary sensors. Quantification of the relative direction of the resulting magnetic field is done electronically. The capability of measurements can be enhanced using multiple sensors. It is often used in real-life circuit applications to solve engineering problems.

Anisotropic magneto-resistance can be applied as a thin strip to become a resistive element. When creating a sensor from the AMR elements, the ends are connected together by metallization. The top and bottom connections are given a Direct Current (DC) stimulus. With no magnetic field, the side contacts should be at the same voltage. However, there is a small offset voltage thanks to manufacturing tolerances. The side contacts produce a differential voltage as a function of the MR ratio, angle theta and supply voltage. The applied field must saturate the permalloy material to have the correct element magnetization direction. Position sensing is much like a saturation mode function as opposed to other AMR sensor elements.

One common error to be accounted for in sensor products is the drift in the material constant versus temperature. The error affects the offset and the bridge sensitivity. If the application has a large operating temperature range, the temperature monitoring circuitry must also be factored in. This helps produce error corrections to offset the temperature drift.

When using multiple bridge sensors, the part-to-part tolerance in material constant is the most common error. This creates a change in sensitivity, resulting in varying peak-to-peak voltage swings. Digitized value corrections and analog signal processing can counter this sensitivity tolerance.

Bridge sensitivity is a specification that has to be factored in. Sub micro-volt noise floors and a nominal output voltage allows angular resolution to be measured at a tenth of a degree. By choosing the right amount of effective bits of analog-to-digital converter resolution and the right signal conditioning bandwidth, the user can play into the strengths of the sensors. Bandwidth of the sensor bridge is another key specification that needs to be factored in while procuring linear position sensors.

Most customers require custom components designed to fit their specific requirements. Be sure to choose an experienced manufacturer who has expert technicians who can design and build the exact product you are looking for. They will provide quality components and timely delivery on whatever products you may need.

The author of the article is associated with Control Products Inc. that deals in magnetostrictive transducers, long stroke sensors, piston accumulators and other such components.