Exciting coil drive circuit of magnetic sensor

An exciting coil drive circuit of a magnetic sensor comprises a D/A converter that receives input of digital data for detecting a magnetic field; a first amplifier that outputs a drive signal P applied to one end of an exciting coil of the magnetic sensor based on the output signal of the D/A converter; and a second amplifier that outputs a drive signal N applied to the other end of the exciting coil based on the output signal of the D/A converter, the drive signal N being an inversion signal of the drive signal P, the drive signal N intersecting with the drive signal P twice or more.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from Japanese Patent Application No. 2005-264284 filed on Sep. 12, 2005, which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an exciting coil drive circuit of a magnetic sensor, and more particularly, to technology for providing a highly-accurate and stable exciting coil drive circuit of a magnetic sensor.

2. Description of the Related Art

A so-called flux gate magnetometer is known which saturates a magnetic flux by inputting periodical drive signals to an exciting coil wound on a soft magnetic core to measure intensity of an external magnetic field from saturation time intervals changed depending on the size of the external magnetic field that is measured. The flux gate magnetometer has various excellent features for a magnetometer, such as (1) high sensitivity and magnetic field resolution, (2) capability of measuring a weak magnetic field, (3) a wide measurement range, (4) temperature stability better than magnetometers of other modes, and (5) high linearity to an input magnetic field.

Japanese Patent Application Laid-Open Publication No. 2005-147947 is an example of such a flux gate magnetometer and discloses a flux gate magnetometer with a magnetic sensor that excites a ring core composed of a magnetic detecting material up to a saturated magnetic field area by electrifying an exciting coil with an alternating signal to measure a magnetic flux density using symmetric property of a saturated magnetic flux density induced in the ring core. Japanese Patent Application Laid-Open Publication No. 1996-285929 discloses a magnetometer that supplies an excitation current from an oscillator to an exciting coil at a flux gate formed by winding the exciting coil and a detecting coil on a core to perform synchronous rectification of the output of the detecting coil with a synchronous rectification circuit. Recently, the flux gate magnetometer is expected to be applied to small devices such as a magnetic sensor for a portable compass. Japanese Patent Application Laid-Open Publication No. 2005-61969 discloses a flux gate magnetometer for accomplishing further improvement in accuracy in measurement of a magnetic force.

High accuracy and stability are required for a drive circuit driving an exciting coil of a magnetic sensor used in the flux gate magnetometer. For example, when the magnetic field measurement is performed in a plurality of spatial axis directions, a plurality of flux gate magnetometers is often used at the same time and, therefore, manufacturing variations must be reduced in each flux gate magnetometer in the case of mass production. When applying to small devices, a smaller component count is required and a chip area must not be occupied at the time of integration.

SUMMARY OF THE INVENTION

The present invention was conceived in consideration of such circumstances and it is therefore one object of the present invention to provide a highly-accurate and stable exciting coil drive circuit of a magnetic sensor having fewer manufacturing variations, which can be miniaturized.

In order to achieve the above and other objects, according to an aspect of the present invention there is provided an exciting coil drive circuit of a magnetic sensor, comprising a D/A converter that receives input of digital data for detecting a magnetic field; a first amplifier that outputs a drive signal P applied to one end of an exciting coil of the magnetic sensor based on the output signal of the D/A converter; and a second amplifier that outputs a drive signal N applied to the other end of the exciting coil based on the output signal of the D/A converter, the drive signal N being an inversion signal of the drive signal P, the drive signal N intersecting with the drive signal P twice or more.

Since the exciting coil drive circuit of the magnetic sensor is digitally constituted by using the D/A converter, the drive circuit can be achieved which is less affected by external factors such as temperature changes. Therefore, the highly-accurate and stable drive signal can be generated. The manufacturing variations can be constrained at the time of mass production.

In order to achieve the above and other objects, according to another aspect of the present invention there is provided an exciting coil drive circuit of a magnetic sensor, comprising an SC integrator that receives input of digital data for detecting a magnetic field; a first amplifier that outputs a drive signal P applied to one end of an exciting coil of the magnetic sensor based on the output signal of the SC integrator; and a second amplifier that outputs a drive signal N applied to the other end of the exciting coil based on the output signal of the SC integrator, the drive signal N being an inversion signal of the drive signal P, the drive signal N intersecting with the drive signal P twice or more.

Since the exciting coil is selected by the switch for the application of the drive signals, a plurality of the exciting coils can be driven by the same exciting coil drive circuit. Therefore, the manufacturing variations can be constrained. Since the exciting coil drive circuit is used in common, the component count and chip area of the drive circuit can be reduced.

The present invention can thus provide a highly-accurate and stable exciting coil drive circuit of a magnetic sensor having fewer manufacturing variations, which can be miniaturized.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention will hereinafter be described in detail.FIG. 1shows a configuration of a flux gate magnetometer described as one embodiment of the present invention. A flux gate magnetometer shown inFIG. 1has three magnetic sensors11,12,13corresponding to the X-axis, Y-axis, and Z-axis, respectively. Each magnetic sensor11,12,13is constituted by winding an exciting coil112,122,132and a detecting coil113,123,133on a magnetic core111,121,131composed of a soft magnetic material such as a nanocrystal soft magnetic material. The exciting coil112,122,132is driven by an exciting coil drive circuit constituted by including an excitation switch circuit21, a noninverting amplifier22, an inverting amplifier23, a D/A converter24, and a control logic that controls the operation of the D/A converter24(hereinafter, DAC control logic25). The output voltage of the detecting coil113,123,133is processed by a signal detecting circuit constituted by including a detection switch circuit31, a voltage adjustment circuit32that adjusts the output voltage to a predetermined voltage level, a differential amplifier33that amplifies the output voltage, a hysteresis comparator34that outputs a low level digital signal in a period between two spike voltages included in the output voltage, and a counter35that counts the number of pulses of a clock signal in a period when the digital signal output from the hysteresis comparator34is in the low level.

A control circuit41controls the DAC control logic25. The control circuit41receives and stores a count value input from the counter35into an internal memory411. The control circuit41is connected to a control line51of the excitation switch circuit21and the detection switch circuit31and the control circuit41controls the opening/closing of the switch21and the switch31through the control line51. The control circuit41is communicably connected to a microcomputer71(external apparatus) via a bus line61and transmits the count value stored in the memory411to the microcomputer71as needed.

FIG. 2is a timing chart of the operation of the flux gate magnetometer1of the embodiment. The operation of the flux gate magnetometer1will be described with reference to the timing chart ofFIG. 2. In the following description, it is assumed that all the contacts of the excitation switch circuit21and the detection switch circuit31are opened (turned off) in advance.

As shown inFIG. 2, a measurement start signal is input from the microcomputer71to the control circuit41via the bus line61(t1). When inputting the measurement start signal, the control circuit41outputs a signal (hereinafter, x-axis selection signal) for turning on the x-axis contacts of the excitation switch circuit21and the detection switch circuit31(t2). When inputting the x-axis selection signal, the excitation switch circuit21and the detection switch circuit31turn on the contacts of the exciting coil112and the detecting coil113of the magnetic sensor11for measuring the magnetic field in the x-axis direction. In this way, the excitation switch circuit21selects the exciting coils112,122,132to which a drive signal P and a drive signal N are applied as described below.

The control circuit41then outputs a drive start enable signal to the DAC control logic25(t3). When inputting the drive start enable signal to the DAC control logic25, DAC data are input to the D/A converter24. Specifically, down-count data are input as the DAC data (t4to t5). Because of the down-count data, a signal is applied immediately before a step-up period to prevent the exciting coil112from generating a high-voltage back electromotive force causing damages of circuit elements such as the noninverting amplifier22and the inverting amplifier23. The DAC control logic25then outputs up-count data to the D/A converter24for the DAC data (t5). In this way, the D/A converter24outputs a signal for a step-up period of a triangular wave (t5to t8).

The DAC control logic25stops the output of the up-count data to the D/A converter24at t8and then outputs the down-count data. In this way, the D/A converter24outputs a signal for a step-down period of the triangular wave (t8to t11). The DAC control logic25stops the output of the down-count data to the D/A converter24at t11and then outputs the up-count data. Because of the down-count data, a signal is applied immediately after the step-down period to prevent the exciting coil from generating a high-voltage back electromotive force causing damages of circuit elements such as the noninverting amplifier22and the inverting amplifier23.

A drive signal of the D/A converter24is supplied to the noninverting input terminal of the noninverting amplifier22. A Vref signal of the D/A converter24is supplied to the noninverting input terminal of the inverting amplifier23. The output of the noninverting amplifier22is fed back negatively to the inverting input terminal of the noninverting amplifier22. The output of the noninverting amplifier22is input to the inverting input terminal of the inverting amplifier23. In this way, the noninverting amplifier22outputs a signal shown by a solid line ofFIG. 2(hereinafter, drive signal P), which is acquired by amplifying the output signal of the D/A converter24, and the inverting amplifier23outputs a signal shown by a dotted line ofFIG. 2(hereinafter, drive signal N), which is acquired by inverting the oscillation of the drive signal P.

The drive signal P output from the noninverting amplifier22is applied to one of two terminals of the exciting coil112. The drive signal N output from the inverting amplifier23is applied to the other of two terminals of the exciting coil112. Therefore, a difference voltage between the drive signal P and the drive signal N is applied to the exciting coil112(hereinafter, this voltage is referred to as an exciting voltage).

As shown inFIG. 2, spike voltages (t7, t10) generated between the terminals of the detecting coil113are caused by the electromotive force generated in a non-saturated section of a B-H curve (B: magnetic flux density, H: magnetic field) of the magnetic sensor11. A time interval (Tx) of two spike voltages at t7and t10is changed depending on an external magnetic field ΔH applied to the magnetic sensor11. That is, information about intensity, etc. of the external magnetic field AH can be acquired by measuring the time interval (Tx) of the output of the two spike voltages.

The spike voltages generated in the detecting coil113are converted to predetermined voltage levels by the voltage adjustment circuit32and are input to the differential amplifier33for amplification. The output voltage amplified by the differential amplifier33is input to the hysteresis comparator34.

The hysteresis comparator34outputs a digital signal becoming low level in a period defined by adjacent spike voltages included in the output voltage and becoming high level in other periods. In an initial state, the hysteresis comparator34outputs the high level. The hysteresis comparator34starts the output of the low level at the timing of the input of the spike voltage generated due to the polarity inversion of the exciting voltage at t6(t7). The hysteresis comparator34switches the output to the high level at the timing of the input of the spike voltage generated due to the polarity inversion of the exciting voltage at t9(t10).

The digital signal output from the hysteresis comparator34is input to the counter35. The clock signal is input to the counter35, and the counter35counts the number of pulses of the clock signal in a period when the digital signal output from the hysteresis comparator34is in the low level. When the digital signal becomes high level and the counting of the number of the pulses is terminated, the counter35outputs the count value to the control circuit41. The control circuit41stores the input count value to the memory411.

The control circuit41then turns off the drive start enable signal that is input to the DAC control logic25(t13). The control circuit41stops the input of the X-axis selection signal to the excitation switch circuit21and the detection switch circuit31(t14). In this way, the contacts are turned off in the exciting coil112and the detecting coil113of the magnetic sensor11for measuring the magnetic field in the x-axis direction.

The control circuit41then transmits a signal (hereinafter, Y-axis selection signal) for turning on the Y-axis contacts of the excitation switch circuit21and the detection switch circuit31(t15). In this way, the process for the Y-axis is started. The process for the Y-axis is performed in a period from t15to t16as is the case with the X-axis. The process for the Z-axis is also performed in a period from t17to t18as is the case with the X-axis.

When the count value is stored in the memory411for each of the X-axis, Y-axis, and Z-axis, the control circuit41transmits to the microcomputer71an interrupt signal notifying that the writing of the count values is completed (t19). When receiving the interrupt signal, the microcomputer71transmits a read request to the control circuit41. In this way, the microcomputer71reads the count value stored in the memory411of the control circuit41for each of the X-axis, Y-axis, and Z-axis (t20). The count values read by the microcomputer71are utilized for measuring the intensity, etc. of the external magnetic field ΔH.

In the flux gate magnetometer1of the embodiment with the configuration described above, the signals for driving the exciting coils112,122,132are generated with the digital circuits such as the DAC control logic25and the D/A converter24. Therefore, the highly-accurate and stable drive signals are generated which are less affected by the temperature, etc., as compared to the case of using analog circuits. The manufacturing variations are also constrained by using the digital circuits.

In the flux gate magnetometer1of the embodiment, a plurality of the exciting coils112,122,132is driven by the same D/A converter24. Therefore, the uniform exciting voltage can be applied to each of the exciting coils112,122,132and variations of the output can be constrained. Since the circuit is used in common, the component count and the chip area can be reduced at the time of integration.

Since the digital circuits are used in the flux gate magnetometer1of the embodiment, the lengths of the step-up period (t5to t8) and the step-down period (t8to t11) of the drive signal can be matched highly accurately to improve the measurement accuracy. Since some circuits necessary in the case of the analog circuit are not needed which are, for example, circuits measuring the overall lengths of the drive signals for correcting the effects of the measurement intervals of errors included in the time intervals (Tx, Ty, Tz), the small-sized flux gate magnetometer1with lower electric power consumption can be achieved.

Since the flux gate magnetometer1of the embodiment counts the number of the pulses of the clock signal with the counter35to measure the time intervals (Tx, Ty, Tz) of two spike voltages, the measurement can be performed with accuracy higher than the case of using the analog circuits. Although an A/D converter occupying large chip area is generally needed at the time of integration if the analog circuits measure the time intervals (Tx, Ty, Tz), since the counter35is a counter35occupying small chip area, the small-sized flux gate magnetometer1can be achieved.

Although an integrator is needed for improving accuracy of a measurement value and it is difficult to reduce the measurement time in a conventional method of measuring the time intervals (Tx, Ty, Tz) with the use of the combination of phase detection and filters, the measurement can be performed in a short time since the counter35is used. This constrains consumption currents as well.

The flux gate magnetometer1of the embodiment is less affected by temperature and noises since the output voltage of the detecting coil113,123,133is digitalized by the hysteresis comparator33at an early stage.

In the flux gate magnetometer1of the embodiment, since the same differential amplifier33and the same hysteresis comparator34perform processes for the output voltages of a plurality of the detecting coils113,123,133, variations are reduced in the measurement values of the detecting coils113,123,133. Since the same circuits are used in the processes for the output voltages of the detecting coils113,123,133in this way, the component count and the chip area can be reduced at the time of integration.

In the flux gate magnetometer1of the embodiment, since the differential amplifier33is used for amplifying the output voltages, less common-mode noise is mixed. Since the detecting coils113,123, and133are not grounded, this also prevents the common-mode noise from being mixed.

Although one embodiment of the present invention has been described in detail as above, the description of the embodiment is for the purpose of facilitating the understanding of the present invention and is not intended to limit the present invention. The present invention may be changed and altered without departing from the spirit thereof and the present invention encompasses the equivalents thereof. For example, a low-pass filter may be inserted at the stage after the D/A converter24to smooth the drive signal output from the D/A converter24.

The drive signal may be generated by an SC (switched capacitor) integrator with a configuration shown inFIG. 3instead of the D/A converter24. The SC integrator80shown inFIG. 3is constituted by four switches SW1to SW4, a capacitor C1, and an integration circuit81using an operational amplifier. The switch SW1, the capacitor C1, and SW4are serially connected in this order; SW1is connected to a direct-current power source Vin; and the output of SW4is input to the noninverting input terminal of the operational amplifier constituting the integration circuit81. The switch SW2is connected between the switch SW1and the capacitor C1and one end of the switch SW2is grounded. The switch SW3is connected between the capacitor C1and the switch SW4and one end of the switch SW3is grounded.

When the SC integrator80ofFIG. 3generates signals in the step-up period of the drive signal constituted by a triangular wave, the switches SW1to SW4are switched at constant intervals Δt1such that states shown inFIGS. 4A and 4Bare achieved alternately (crawl type driving mode). In this way, as shown inFIG. 6A, a drive signal can be acquired which is pressured up stepwise at constant inclination. When generating signals in the step-down period of the drive signal, the switches SW1to SW4are switched at constant intervals Δt2such that states shown inFIGS. 5A and 5Bare achieved alternately (butterfly type driving mode). In this way, as shown inFIG. 6B, a drive signal can be acquired which is pressured down stepwise at constant inclination. A linear drive signal can be acquired by smoothing the drive signal output from the SC integrator80through the low-pass filter.

The SC integrator80can match Δt1and Δt2accurately with the use of known digital circuits and can generate the exact triangular wave with the step-up period and the step-down period having inclinations matched highly accurately. Therefore, when the SC integrator80is used, the flux gate magnetometer1can also be achieved which can measure a magnetic field highly accurately as is the case with the D/A converter24.