Sensor magnet quality control using a magnetic field camera

Inhalt:
A method is presented for magnetically characterizing 2-pole rotary encoder magnets, which are widely used in angular encoders in e.g. electric motors and rotary positioning systems. The method uses a proprietary ‘Distance Filter’ algorithm, which allows to extrapolate a magnetic field distribution recorded at one certain (close) distance above a magnet or magnet assembly to another (larger) distance in a very fast way and with a strong suppression of measurement noise. This allows very fast and at the same time highly accurate measurements, making the method suitable for production quality control. The ‘Distance Filter’ extrapolation algorithm is used in combination with Magcam’s magnetic field camera systems. When the extrapolation is in the direction away from the magnet, there is a strong suppression of noise, resulting in muTesla magnetic field resolutions. This makes the Distance Filter algorithm very powerful for determining magnetic field distributions far away from a magnet, since a direct measurement at the farther distance would suffer from a poorer signal-to-noise ratio. In typical end applications, a Bx,By sensor is positioned in the center of the rotary encoder magnet at a certain distance from the magnet surface, which is typically several mm. At such distance the Bxy magnetic field is typically of the order of 50mT. The Bx,By sensor then measures Bx and By and from those calculates the in-plane angle of the magnetic field using atan2(By,Bx). Inhomogeneities of the magnetic field distribution can cause errors on this measured angle value. During quality control on such magnets, this angle error needs to be determined with high accuracy, typically in the order of 0.1deg or better. By using a magnetic field camera in com bination with the Distance Filter this can be achieved in a superior way.