Electricity: measuring and testing – Magnetic – Calibration
Reexamination Certificate
1999-11-10
2001-10-16
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Calibration
C324S207210, C324S207120, C324S207250
Reexamination Certificate
active
06304074
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for the offset calibration of a magnetoresistive angle sensor for the determination of the directions of magnetic fields, which sensor comprises at least a Wheatstone bridge with at least four magnetoresistive resistors, which Wheatstone bridge is capable of receiving an input signal at its input side, in particular an input voltage, and which Wheatstone bridge is designed for supplying an angle signal at its output side, in particular an angle voltage, in dependence on the direction of a magnetic field which acts on the Wheatstone bridge, said Wheatstone bridge comprising a first and a second pair of mutually opposed, substantially parallel magnetoresistive resistors, the first and the second pair being arranged substantially at right angles to one another, while the direction of the magnetic field can be determined from the angle signal supplied by the Wheatstone bridge (by means of an evaluation circuit), and the invention also relates to a sensor device for measuring the angle of a magnetic field, comprising a magnetoresistive angle sensor which comprises at least a Wheatstone bridge with at least four magnetoresistive resistors, which Wheatstone bridge is capable of receiving an input signal at its input, in particular an input voltage, and which Wheatstone bridge is designed for supplying an angle signal at its output side in dependence on the direction of a magnetic field which acts on the Wheatstone bridge, said Wheatstone bridge comprising a first and a second pair of mutually opposed, substantially parallel magnetoresistive resistors, the first and the second pair being substantially orthogonal to one another, while the direction of the magnetic field can be determined from the angle signal supplied by the Wheatstone bridge by means of an evaluation circuit.
Such a method and such a sensor device are known, for example, from data book SC 17 “Semiconductor Sensors” of the Philips company. This known sensor device comprises a magnetoresistive angle sensor KMZ41 which consists of two Wheatstone bridges with four magnetoresistive resistors each. The two Wheatstone bridges are arranged so as to enclose an angle of 45° with one another. An input voltage is applied to the input of each of the Wheatstone bridges. The output voltage of the first Wheatstone bridge is dependent on the angle of the magnetic field acting on the first Wheatstone bridge in a cosine function. The output voltage of the second Wheatstone bridge is dependent on the angle of the magnetic field acting on the second Wheatstone bridge in a sine function. The known sensor device is combined with an evaluation circuit which calculates the angle of the magnetic field from the output voltages of the first and the second Wheatstone bridges by means of the so-called Cordic algorithm. Irreproducibilities in the manufacturing process will in general render the magnetoresistive resistors of any Wheatstone bridge unequal, so that an unknown offset voltage will be added to each output voltage. An offset calibration is accordingly necessary so as to render possible an error-free angle measurement. The calibration in the known method takes place by means of a magnet which rotates in a very exact manner in front of the sensor. The offset voltages are determined iteratively through a comparison of the sensor output signal with a pulse width modulated signal. Such a calibration method is time-consuming owing to the iterative process, requires an expensive equipment on account of the rotating magnet during the calibration process, and requires additional inputs for the evaluation circuit, which are used for trimming only.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a calibration method which can be carried out more simply and which requires a less expensive evaluation circuit.
According to the invention, this object is achieved by means of a method which is characterized in that in a first step the Wheatstone bridge is acted upon by a direct magnetic field which is so aligned that it is neither substantially perpendicular to the magnetoresistive resistors of the first pair nor substantially perpendicular to the magnetoresistive resistors of the second pair of the Wheatstone bridge, in that in a second step the direct magnetic field is removed, and in that in a third step an evaluation circuit is provided for determining an offset calibration signal, the angle signal supplied by the Wheatstone bridge without direct magnetic field being a measure for the offset of the Wheatstone bridge.
In the first step, a direct magnetic field is applied to the Wheatstone bridge. The output voltage of the Wheatstone bridge depends on the angle at which the direct magnetic field acts on the Wheatstone bridge. The resistance values of the individual magnetoresistive resistors of the Wheatstone bridge are at their lowest when the direct magnetic field is directed perpendicularly to the magnetoresistive resistors, i.e. perpendicularly to the longitudinal axis and perpendicularly to the current flow direction of the magnetoresistive resistors. If the direct magnetic field is perpendicular to the magnetoresistive resistors of the first pair or perpendicular to the magnetoresistive resistors of the second pair of the Wheatstone bridge, the total value of the output voltage of the Wheatstone bridge will be at a maximum. According to the invention, the direct magnetic field acting on the Wheatstone bridge in the first step should be so directed that it is neither substantially perpendicular to the magnetoresistive resistors of the first pair nor substantially perpendicular to the magnetoresistive resistors of the second pair of the Wheatstone bridge. This means that the maximum ranges of the output voltage of the Wheatstone bridge should be avoided. Now if the direct magnetic field acting on the Wheatstone bridge is removed in the second step, the output voltages of the Wheatstone bridge depend on the angle at which the direct magnetic field has acted on the Wheatstone bridge. The invention is based on the recognition that the output voltage in the second step, i.e. after the removal of the direct magnetic field, deviates only very little from the offset voltage of the Wheatstone bridge over comparatively wide angle ranges of the angle of the direct magnetic field which was active during the first step. This renders it possible to determine the offset voltage of the Wheatstone bridge from the output voltage with the direct magnetic field being removed. Larger deviations in the measured output voltage from the offset voltage are measured upon the removal of the direct magnetic field only if the direct magnetic field acting on the Wheatstone bridge in the first step is directed perpendicularly to the magnetoresistive resistors of the first pair or perpendicularly to the magnetoresistive resistors of the second pair.
The output voltage supplied by the Wheatstone bridge after the removal of the direct magnetic field thus is a measure for the offset voltage of the Wheatstone bridge. This output signal supplied by the Wheatstone bridge without direct magnetic field is accordingly used in the third step by the evaluation circuit for offset calibration.
This method for the offset calibration does not require any expensive measuring devices and can accordingly be realized in a simple and inexpensive manner. The requirements imposed on the angular accuracy of the direct magnetic field acting on the Wheatstone bridge are very low, so that an expensive and accurate mounting of a magnet is not necessary. Deviations or drift of the angle of the direct magnetic field in the order of ±10° are definitely acceptable and have hardly any influence on the accuracy of the offset calibration. It is advantageous when the direct magnetic field acting on the Wheatstone bridge is directed at an angle of approximately 45° to the magnetoresistive resistors of the first pair and the second pair of the Wheatstone bridge. The offset calibration is particularly accurate in the case of su
Patidar Jay
U.S. Philips Corporation
Waxler Aaron
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