Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2001-01-26
2002-08-13
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S207210
Reexamination Certificate
active
06433545
ABSTRACT:
CROSS REFERENCE TO RELATED APPLICATIONS
Applicants claim priority under 35 U.S.C. §119 of GERMAN Application No. 198 34 153.9 filed Jul. 29, 1998. Applicants also claim priority under 35 U.S.C. §120 of PCT/EPT99/05233 filed on Jul. 22, 1999. The international application under PCT article 21 (2) was not published in English.
The invention relates to a method and a number of devices for carrying out said method for evaluating the signals of magnetoresistive sensors, in connection with which for the purpose of eliminating an interference signal that is proportional to the zero offset, the magnetization of the magnetoresistive resistor strips is periodically adjusted or only modulated by flip current pulses alternating in the positive and negative directions by way of an integrated flip line or by an external flip coil. The magnetoresistive sensor can be employed in this connection for high-resolution measurements of a magnetic field or of a magnetic field gradient, or of values based thereon, for example for potential-free current measurements.
Measuring devices for measuring magnetic fields that employ such methods with the objective of separating the offset voltage caused in magnetoresistive sensor bridges by unevenness of the resistances and their temperature dependency, from the sensor signal that is proportional to the magnetic field, are already known. For example, laid-open patent specification DE-OS 34 42 278 describes a magnetic field measuring device that contains a magnetic field sensor with four sensor elements consisting of magnetoresistive layer strips with Barber pole structures. Said sensor elements are wired as a Wheatstone bridge and are fed by a direct current source. Owing to tolerances in the manufacture of the magnetoresistive layer strips, not all four bridge resistances are completely the same, and without the presence of a magnetic field to be measured, the offset voltage ensuing from the dc voltage value appears on the output of the sensor bridge. All of the magnetoresistive layer strips of the sensors each have in each case at the same time the same magnetically preferred direction ensuing from the direction of their respective magnetization. A magnetizing coil is arranged within the proximity of the sensor elements, which is supplied by a current pulse generator with short, alternating positive and negative current pulses. Said current pulses generate in the magnetizing coil an alternating magnetic field that reverses the magnetism of the magnetoresistive layer strips at the cycle of the flip current pulses. When a magnetic field to be measured is applied, an ac voltage appears on the zero branch of the sensor bridge in addition to the aforementioned dc offset voltage. Said ac voltage can be indicated by a phase-sensitive rectifier that is controlled by the current pulse generator. On the output, said rectifier has an integrator for eliminating the offset voltage contained as the ac voltage component. The output voltage of said integrator is proportional to the magnetic field associated with the magnetic field sensor as long as it lies within the linearity region of the magnetic field sensor.
An important drawback of the described measuring method consists in that magnetic fields can still be measured only with a maximal bandwidth amounting to about one hundredth up to about one tenth of the frequency of the flip current pulses because the offset voltage of the sensor bridge present in the signal as the alternating component downstream of the phase-sensitive rectifier is otherwise present in an excessive proportionate amount. A second drawback of the measuring method is conditioned by the limited linearity range of the characteristic of the sensor bridge, as well as by the temperature dependency of the gradient of the characteristic as well as its dependency on the component of the magnetic field acting in the longitudinal direction of the magnetoresistive layer strips. This leads to the fact that the proportionality of the output voltage of the described measuring circuit to the measured component of the magnetic field is available only within a highly limited range of the field intensity and only at a constant temperature and constant magnetic field component in the longitudinal direction of the strips.
Said second drawback is already no longer present in connection with an improved measuring method according to U.S. Pat. No. 5,351,005. In the circuit specified in said patent specification, a current is generated by phase-sensitive rectification that is proportional to the magnetic field intensity to be measured. Said current is fed into a compensating coil in which the magnetoresistive sensor bridge is located, and generates there a magnetic field. The circuit controls said current to a value that generates in the coil a magnetic field that just about cancels the magnetic field applied from the outside. The magnetoresistive sensor bridge still acts here only as a zero instrument. The non-linearities of the characteristic and their dependency on the temperature and the corresponding magnetic field component no longer play any role here. The output signal of the circuit is obtained, for example from a voltage drop that is generated by the current through the compensating coil on a fixed resistor. Two “sample and hold” amplifiers are alternately employed, controlled at the cycle of the flip current generator, for measuring the respective voltage drop for the two directions of magnetization in the magnetoresistive sensor. The difference between the two voltages of the “sample and hold” amplifiers, which is formed by a low-pass amplifier, still contains only the component of the signal that is proportional to the magnetic field, whereas the component that is proportional to the offset of the sensor bridge drops out due to the formation of the difference. The bandwidth with which magnetic fields can be measured based on the circuit described herein is substantially determined by the bandwidth of the low-pass amplifier. In the most favorable case it is possible to reach a bandwidth of half of the frequency of the flip current pulses because for obtaining the correct value of the magnetic field it is necessary that both “sample and hold” amplifiers each have picked up at least one measured value. Another circuit for eliminating the bridge offset resulting from the sensor signal is specified in the “Data Handbook SC 17” [1997] of Philips Semiconductors, page 36. In the present case, the bridge signal is first supplied to a differential amplifier. The dc voltage component of the signal downstream of the differential amplifier, which contains the offset voltage of the sensor bridge and the differential amplifier, is fed back negative to the input of the differential amplifier via a low-pass filter and is thus controlled to zero. The ac voltage component of the signal that is proportional to the magnetic field to be measured is rectified via a controlled amplifier. The rectified signal is fed as current into a compensating coil and cancels there the magnetic field to be measured. A filter is connected downstream because the jump-like change of the input voltage of the controlled amplifier by the normal deviation leads to voltage peaks at its output. This means that in the present case, too, the magnetic field can be measured only with a bandwidth that is far below the flip frequency.
The known methods proposed heretofore for eliminating the zero offset of the magnetoresistive bridge circuits all relate to those bridges whose resistor strips are provided with Barber pole structures, and whose magnetization can be adjusted in the positive or negative longitudinal direction by means of a dc or pulse field. The direction of the magnetic fields used in this connection substantially correspond with the light direction of the magnetoresistive resistor strips. The direction of the field to be measured extends perpendicular thereto in the heavy direction of the strips. However, patent specification U.S. Pat. No. 4,596,950 describes a magnetoresistive bridge
Dettmann Fritz
Kunze Jürgen
Loreit Uwe
Schepp Gunter
Collard & Roe P.C.
Lefkowitz Edward
Lust Antriebstechnik GmbH
Zaveri Subhash
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