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Are magnetic position sensors truly reliable?

Posted: 28 Jul 2015     Print Version  Bookmark and Share

Keywords:Magnetic position sensors  EMI  magnetism  stray fields  optical encoder 

In automotive and industrial applications specifically, the magnetic sensor's capacity to withstand the dust, dirt, grease, vibration and humidity that commonly disable optical encoders—the best known alternative contactless type of position sensor—is highly valued. Like optical encoders, the potentiometer—the industry's most familiar device for measuring linear or angular displacement, also suffers from the effects of mechanical wear, a common source of premature failure.

By contrast with both the optical encoder and the potentiometer, a magnetic position sensing system is far more durable, and operates far more reliably no matter how dirty, damp or unstable the operating conditions.

And yet, a stubborn question about the reliability of the magnetic position sensor lingers in the minds of some automotive and industrial system designers.

Since they often design systems for use in an environment containing powerful sources of magnetism, such as motor drives and high-voltage power transmission lines, they are wary of magnetic position sensors.

They fear the huge magnetic forces unleashed by these systems—for instance, in the enclosed space of a car body or wind turbine—will swamp the weak field generated by the target magnet with which a Hall Effect sensor is paired.

That's partly true.

Magnetic position sensors can be sensitive to the paired target magnet field, and also to unintended magnetic stray fields, which can impair the accuracy of the magnetic position sensor's output by reducing the signal-to-noise ratio (SNR) to unacceptable levels within the device.

If the issue of unintended stray fields is not addressed, a sensor sub-system could yield inaccurate results, potentially leading to reduced system performance and safety issues.

Stray fields are of particular concern in industrial and automotive applications where high levels of electro-magnetic interference (EMI) is commonly found. The EMI issue has become an even greater concern with the increased electrification of automobiles, and particularly so with electric cars where large high-current carrying wires run between the front and back of the vehicle.

Traditional solutions
Historically, design engineers working in high EMI environments and implementing magnetic sensor solutions use a number of countermeasures to prevent the degradation in performance of the magnetic sensor IC.

Magnetic shielding
One common technique involves adding magnetic shielding around the magnetic sensor solution, thus preventing any stray magnetic fields from interfering with the sensor's performance. Incorporating magnetic shielding, however, is expensive and requires space, adding to the cost and size of the sensor sub-system.

Another potential downside with magnetic shielding is that besides shunting the stray fields away from the magnetic sensor IC, the shielding also might shunt away the target magnetic field the sensor is supposed to be measuring. In addition, the shielding itself may become magnetized over time and its performance can vary with temperature.

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