EMC & Circuit Protection
Using magnetic sensor ICs in a strongly magnetic environment
Markus Zieserl, application engineer, magnetic position sensors for the automotive market, ams, talks about how to guarantee safe operations in a strongly magnetic environment
M
agnetic position sensors are one of the electronics industry’s technological success stories. In industrial and automotive 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. The industry’s most familiar device for measuring linear or angular displacement, the potentiometer, suffers in addition 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 industrial and automotive system designers. These engineers often design systems for use in an environment containing powerful sources of magnetism, such as motor drives and high-voltage power transmission lines. Surely the huge magnetic forces unleashed by these systems in, for instance, the enclosed space of a car body or a wind turbine will swamp the weak field generated by the target magnet with which a Hall effect sensor is paired? As this article will show, built-in immunity provides a complete protection against the magnetic power of
motor drives and power cables and guarantees safe and reliable operation without any need for expensive and bulky counter-measures.
The characteristics of stray magnetic fields
Stray magnetic fields may be generated by magnets, motors, transformers or any current-carrying conductors such as electric power lines. The number and the strength of the stray magnetic fields is greater in electric and hybrid-electric vehicles (EVs and HEVs) than in conventional cars powered by an internal combustion engine (ICE). In cars with an ICE, the sources of stray
fields are mainly located under the bonnet. In EVs and HEVs, stray fields are certainly present. But crucially, these vehicles also
20 September 2016
have large battery packs for powering the traction system. In passenger cars, for optimal weight distribution this heavy battery is commonly mounted centrally beneath the wheels, in a recess underneath the cabin. This means that cables are required to carry large amounts of current – as much as 400A at full load when the vehicle is accelerating – from the battery to one, two or four electric motors. In order to deliver power to the electric motor(s) driving the front wheels, these cables must pass close to safety-critical systems such as the gas and brake pedals. Car manufacturers are worried because a stray magnetic field of sufficient strength can cause conventional magnetic position sensors, if unprotected, to produce distorted or false output signals. A very strong stray field can in some cases even damage the sensor permanently. And this is of vital importance, because the brake pedal, gas pedal and steering systems present a
Figure 3: The principle of operation of a differential sensing architecture
reliability. System designers using magnetic position sensors must therefore know before they even start a new design how they are going to guarantee their system can withstand the powerful force of external magnetic stray fields.
Costs of mechanical counter-measures
When using a conventional magnetic position sensor, design engineers are forced to take counter-measures to protect it from stray magnetic fields. Since the stray fields are present most of the time, a popular counter-measure is to shield the sensor with special material: metal alloys with high magnetic permeability, such as permalloy or mu-metal, are commonly used. Unfortunately, the problem with these materials is that they do not block the magnetic field: instead, they absorb the field, providing a path for it to keep it at a known distance from the sensor.
Figure 2: A magnetic shield changes the shape of every magnetic field in its vicinity
severe risk to the life of the driver, passengers and other road users should they fail, and the position sensor plays a critical role in each one. The consequences of registering a small displacement when the driver has actually stamped the pedal to the floor in order to perform an emergency stop, could be catastrophic. Not surprisingly, therefore, the car industry has moved to tighten the specifications governing the safe operation of systems that might be exposed to stray magnetic fields. Standards such as ISO 26262 (functional safety) and ISO 11542-8 (immunity to magnetic fields) are meant to eliminate the risk of failure, including failure due to stray magnetic fields. Many industrial applications have similarly stringent requirements for safety and
Components in Electronics
But this absorption effect is not selective: it can equally affect the magnetic field of the sensor’s paired magnet (mounted on the object measured by the sensor, such as a motor’s rotor, or a shaft attached to a gas or brake pedal). This has the potential to distort the position measurements of the sensor. To avoid this unwanted effect on the paired magnet’s field, a minimum distance has to be maintained between it and the shielding material. This then increases the size of the complete sensing system, which must be big enough to accommodate an otherwise useless air gap between the sensor and the shielding material. Sensor manufacturer ams offers system designers built-in immunity through the application of a novel architecture: its differential sensing technology. An ams magnetic position sensor such as the AS5147 has four integrated Hall elements in a north/east/south/west configuration. These elements are only sensitive to changes in the magnetic field in the z
dimension, and do not measure magnetism in the x and y dimensions. (This means that they are inherently immune to stray magnetic field changes in the x and y dimensions.) The four Hall elements generate four sinusoidal waveforms when a diametrically magnetised (two-pole) magnet rotates over them. Each signal is phase-shifted by 90° from its neighbour. Differential amplification is applied to the two opposing pairs of Hall elements (that is, the pairs 180° from each other). This results in one sine and one cosine signal, both with doubled amplitude. These signals are digitised, and the digital signals are processed by an integrated CORDIC (COordinate Rotation DIgital Computer)
Figure 4: An ams position sensor’s operation is unaffected even by the very strong magnetic fields generated within a Helmholtz coil
digital block, which executes a mathematical arctangent function. Its output is an angle measurement, and a measurement of the magnitude of the sensed magnetic field.
As demand for vehicles with wholly or partly electric drivetrains starts to ramp up, encouraged by governments’ decarbonisation initiatives, the number of vehicles carrying powerful magnetic fields is only set to grow. This demands that users of magnetic position sensors should pay closer attention than ever before to the value of built-in immunity to magnetic interference.
www.ams.com www.cieonline.co.uk
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