Method for offset compensation of a magnetoresistive position or angular position measuring system

#The invention relates to a method for offset compensation of a magnetoresistive position or angular position measuring system, which comprises at least one Wheatstone bridge with four locally offset magnetoresistive resistors to which a DC voltage is applied and past which a permanent magnet is moved, wherein the respective bridge voltage is measured.

#It is currently common to use magnetoresistive sensors for position and angular position measurement. These consist of magnetoresistive resistors which are arranged in the manner of a Wheatstone bridge on a generally disk-shaped support in front of which a permanent magnet rotates. To obtain a linear output signal, two bridges are used whose resistors are arranged at a mechanical angle to one another of 45°, such that one bridge generates a sinusoidal signal and the other a cosinusoidal signal. The ARCTAN function then allows a linear function over 180° as the output signal. The signals are evaluated, for example, in a mixed-signal IC which together with the sensor constitutes a measuring system.

#Due to the production process, the magnetoresistive resistors are non-identical, which, however, may be compensated in the IC when the measuring system is started up. This compensation thus takes place individually for each individual measuring system. During a final measuring process, the sensor is magnetically excited in such a way prior to delivery that the individual offset, i.e. the respective offsets of the two bridges, may be determined from the signals. These values are written into an EEPROM in the IC and subsequently taken into account by the IC.

#The invention will be further described with reference to an embodiment. In the appended drawings:

#FIG. 1 shows the gradient of the sensor output signals for clarification of the method, and

#FIG. 2 is a block diagram of the measuring system.

#FIG. 1 shows the gradient of the voltage signals from two Wheatstone bridges arranged at 45° to one another as a function of the angle of a permanent magnet rotating past the magnetoresistive resistors of the bridges. This angular alignment corresponds to an electrical angle of 90°. The reference system shows a sinusoidal profile I of a sensor A and a negatively cosinusoidal signal profile II of a sensor B. An offset would reveal itself in a constant displacement of the voltage signal obtained along the angle (W).

#An offset may therefore be determined by measurement of the maxima (Ma) and minima (Mi). In a calibration cycle, the four extreme values are determined, in that the measuring cycle is performed once by rotating the permanent magnet, and at the same time four extreme-value registers are activated, i.e. each sensor A and B has two registers, in one of which the minimum is determined and in the other the maximum. This may occur in the manner of a comparison, for example, in that a time-discrete signal is stored and compared with the next sampled value. The following applies for the maximum register: if the next value is higher than the last, accept the value, otherwise not. The reverse is true of the minimum register. At the end of this measuring process, an acknowledgement may optionally be provided by the user. The sensor IC then calculates the mean value (offset) and stores a suitable compensation value in a register, which from then on is used for offset compensation.

#FIG. 2 is a block diagram of the angular position measuring system for a sensor. A permanent magnet 2 is positioned rotatably in front of a magnetic field sensor 1, which comprises magnetoresistive resistors arranged in bridge circuits. The magnetic field sensor 1 presents the angular position of the permanent magnet 2 as an electrical variable, as was shown in FIG. 1. The sensor signal passes via an amplifier 3 and an A/D converter 4 to a digitally operating control unit (“digital control”) 5. During the calibration cycle, the maxima and minima of the sensor signals are sought, the offset voltage is determined, and a compensation value is established, as described above.