What about magnetic position sensor reliability?

What about magnetic position sensor reliability?

Feature articles |
By eeNews Europe

Magnetic position sensors are an electronics industry’s technological success.
In automotive and industrial applications in particular, the magnetic sensor’s ability 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 electromagnetic interference (EMI) are commonly found.

Next: Traditional solutions

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 subsystem.

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.

As a result, a great deal of care, often addressed by multiple magnetic shielding design iterations, is required to find the effective shielding solution. This takes time and effort, as well as adds to the development cost of the sensor sub-system.

Stronger or repositioned magnets

Two other solutions often used for combatting the effects of stray fields on magnetic position sensors involve using a stronger magnet (or, more specifically, a magnet with a higher eminence) and/or positioning the magnet closer to the magnetic sensor. Both of these solutions drive costs higher.

Magnets with higher remanence, or a stronger field, drive up the sensor cost to unacceptable levels. And the solution involving narrowing the gap between the magnet and sensor tends to drive up costs as well, since this fix requires tighter mechanical tolerances.

Next; Integration

Integrating circuitry

The most effective solution for addressing the stray field issue is integrating IC circuitry into the magnetic position sensor, which makes the device immune to any magnetic stray fields.

At AMS, for example, magnetic position sensors are based around two unique and fundamental architectural design features that prevent magnetic stray fields from interfering with their performance. First, the magnetic position sensors measure only the Z field component of a target magnetic field (the field perpendicular to the face of the sensor IC). The results is all magnetic fields in the X and Y direction, fields planar to the surface of the IC, are completely rejected when making a measurement of the target magnetic field.

Second, and even more significantly, the ams magnetic position sensors implement a differential mathematical operation that cancels out any homogenous magnetic stray field in the Z direction.

These two important design features result in magnetic position sensors that are completely immune to any stray magnetic field, in any direction.
It takes two

By using the two architectural design features described above, magnetic position sensors will provide high accuracy measurements even in the noisiest of EMI environments, such as those commonly found in automotive and industrial applications.

Mark J. Donovan is a senior product marketing manager at AMS AG, with marketing responsibility for magnetic position sensor ICs. He has more than 30 years of industry experience in magnetic sensing, telecom, and radar signal processing and holds 6 patents, with two additional patents pending.

This article first appeared on EE Times’ website.

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