Researchers at the Fraunhofer Institute have developed a novel magnetic sensor that they claim for the first time detects tiny fluctuations in a small magnetic field – even when there is a strong magnet right beside it.
The sensor can therefore be utilised even in places where power cables generate an interference field – for instance, in a car’s side mirror.
In the car mirror application, if there is a change of driver it is normally required to adjust the position of the mirror and the seat.
To provide automatic adjustment for each driver, a tiny chip in the key or a corresponding button on the dashboard can be pressed, enabling all adjustment to be performed very easily.
There is a tiny magnet in the mirror and another in the seat, whose position is detected by a magnetic sensor and which enables the mirror to be correctly adjusted. The only problem with this system is that the cables supplying the power for heating the mirror and controlling the stepper motor also generate a magnetic field.
The sensor therefore sees not only the field generated by the magnet, but also that of the power cable – and errors can be made. Up to now, therefore, such magnetic field sensors have had to be screened. This is difficult and expensive.
A new type of integrated 3-D magnetic field sensor from the Fraunhofer Institute for Integrated Circuits (IIS) in Erlangen, Germany, can work without screening. The researchers have arranged several sensors in a pixel cell in such a way that they can measure all three components of the magnetic field in one place. If two of these pixel cells are placed on a chip, the sensor measures not only the magnetic field as such, but also how the position of the magnetic field changes. IIS team leader Dr Hans-Peter Hohe states: “This sensor enables us for the first time to identify magnetic interference fields as such and to separate them from the useful field. The sensor works perfectly even when the interference field is considerably larger than the useful field. There is therefore no need for shielding.”
The sensors have another advantage, too: they are suitable for high-temperature applications up to around 150 degrees C and can therefore be utilised in places such as the engine compartment. The sensors have already been tested and developed to a stage where they are suitable for industrial use. To facilitate serial production of the sensors, the researchers used low-cost standard CMOS techniques to manufacture them.
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