Abstract:
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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     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  
       [0002]     1. Field of the Invention  
         [0003]     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.  
         [0004]     2. Description of the Related Art  
         [0005]     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.  
         [0006]     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.  
         [0007]     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  
       [0008]     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.  
         [0009]     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.  
         [0010]     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.  
         [0011]     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.  
         [0012]     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.  
         [0013]     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.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:  
         [0015]      FIG. 1  shows a configuration of the flux gate magnetometer  1  described as one embodiment of the present invention;  
         [0016]      FIG. 2  is a timing chart for describing the operation of the flux gate magnetometer  1  described as one embodiment of the present invention;  
         [0017]      FIG. 3  is shows an example of the SC integrator  80  described as one embodiment of the present invention;  
         [0018]      FIGS. 4A and 4B  show states of the switches SW 1  to SW 4  of the SC integrator  80  when generating signals in the step-up period of the drive signal;  
         [0019]      FIGS. 5A and 5B  show states of the switches SW 1  to SW 4  of the SC integrator  80  when generating signals in the step-down period of the drive signal;  
         [0020]      FIGS. 6A and 6B  show examples of the drive signal generated by the SC integrator  80 .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     An embodiment of the present invention will hereinafter be described in detail.  FIG. 1  shows a configuration of a flux gate magnetometer described as one embodiment of the present invention. A flux gate magnetometer shown in  FIG. 1  has three magnetic sensors  11 ,  12 ,  13  corresponding to the X-axis, Y-axis, and Z-axis, respectively. Each magnetic sensor  11 ,  12 ,  13  is constituted by winding an exciting coil  112 ,  122 ,  132  and a detecting coil  113 ,  123 ,  133  on a magnetic core  111 ,  121 ,  131  composed of a soft magnetic material such as a nanocrystal soft magnetic material. The exciting coil  112 ,  122 ,  132  is driven by an exciting coil drive circuit constituted by including an excitation switch circuit  21 , a noninverting amplifier  22 , an inverting amplifier  23 , a D/A converter  24 , and a control logic that controls the operation of the D/A converter  24  (hereinafter, DAC control logic  25 ). The output voltage of the detecting coil  113 ,  123 ,  133  is processed by a signal detecting circuit constituted by including a detection switch circuit  31 , a voltage adjustment circuit  32  that adjusts the output voltage to a predetermined voltage level, a differential amplifier  33  that amplifies the output voltage, a hysteresis comparator  34  that outputs a low level digital signal in a period between two spike voltages included in the output voltage, and a counter  35  that counts the number of pulses of a clock signal in a period when the digital signal output from the hysteresis comparator  34  is in the low level.  
         [0022]     A control circuit  41  controls the DAC control logic  25 . The control circuit  41  receives and stores a count value input from the counter  35  into an internal memory  411 . The control circuit  41  is connected to a control line  51  of the excitation switch circuit  21  and the detection switch circuit  31  and the control circuit  41  controls the opening/closing of the switch  21  and the switch  31  through the control line  51 . The control circuit  41  is communicably connected to a microcomputer  71  (external apparatus) via a bus line  61  and transmits the count value stored in the memory  411  to the microcomputer  71  as needed.  
         [0023]      FIG. 2  is a timing chart of the operation of the flux gate magnetometer  1  of the embodiment. The operation of the flux gate magnetometer  1  will be described with reference to the timing chart of  FIG. 2 . In the following description, it is assumed that all the contacts of the excitation switch circuit  21  and the detection switch circuit  31  are opened (turned off) in advance.  
         [0024]     As shown in  FIG. 2 , a measurement start signal is input from the microcomputer  71  to the control circuit  41  via the bus line  61  (t 1 ). When inputting the measurement start signal, the control circuit  41  outputs a signal (hereinafter, x-axis selection signal) for turning on the x-axis contacts of the excitation switch circuit  21  and the detection switch circuit  31  (t 2 ). When inputting the x-axis selection signal, the excitation switch circuit  21  and the detection switch circuit  31  turn on the contacts of the exciting coil  112  and the detecting coil  113  of the magnetic sensor  11  for measuring the magnetic field in the x-axis direction. In this way, the excitation switch circuit  21  selects the exciting coils  112 ,  122 ,  132  to which a drive signal P and a drive signal N are applied as described below.  
         [0025]     The control circuit  41  then outputs a drive start enable signal to the DAC control logic  25  (t 3 ). When inputting the drive start enable signal to the DAC control logic  25 , DAC data are input to the D/A converter  24 . Specifically, down-count data are input as the DAC data (t 4  to t 5 ). Because of the down-count data, a signal is applied immediately before a step-up period to prevent the exciting coil  112  from generating a high-voltage back electromotive force causing damages of circuit elements such as the noninverting amplifier  22  and the inverting amplifier  23 . The DAC control logic  25  then outputs up-count data to the D/A converter  24  for the DAC data (t 5 ). In this way, the D/A converter  24  outputs a signal for a step-up period of a triangular wave (t 5  to t 8 ).  
         [0026]     The DAC control logic  25  stops the output of the up-count data to the D/A converter  24  at t 8  and then outputs the down-count data. In this way, the D/A converter  24  outputs a signal for a step-down period of the triangular wave (t 8  to t 11 ). The DAC control logic  25  stops the output of the down-count data to the D/A converter  24  at t 11  and 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 amplifier  22  and the inverting amplifier  23 .  
         [0027]     A drive signal of the D/A converter  24  is supplied to the noninverting input terminal of the noninverting amplifier  22 . A Vref signal of the D/A converter  24  is supplied to the noninverting input terminal of the inverting amplifier  23 . The output of the noninverting amplifier  22  is fed back negatively to the inverting input terminal of the noninverting amplifier  22 . The output of the noninverting amplifier  22  is input to the inverting input terminal of the inverting amplifier  23 . In this way, the noninverting amplifier  22  outputs a signal shown by a solid line of  FIG. 2  (hereinafter, drive signal P), which is acquired by amplifying the output signal of the D/A converter  24 , and the inverting amplifier  23  outputs a signal shown by a dotted line of  FIG. 2  (hereinafter, drive signal N), which is acquired by inverting the oscillation of the drive signal P.  
         [0028]     The drive signal P output from the noninverting amplifier  22  is applied to one of two terminals of the exciting coil  112 . The drive signal N output from the inverting amplifier  23  is applied to the other of two terminals of the exciting coil  112 . Therefore, a difference voltage between the drive signal P and the drive signal N is applied to the exciting coil  112  (hereinafter, this voltage is referred to as an exciting voltage).  
         [0029]     As shown in  FIG. 2 , spike voltages (t 7 , t 10 ) generated between the terminals of the detecting coil  113  are 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 sensor  11 . A time interval (Tx) of two spike voltages at t 7  and t 10  is changed depending on an external magnetic field ΔH applied to the magnetic sensor  11 . 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.  
         [0030]     The spike voltages generated in the detecting coil  113  are converted to predetermined voltage levels by the voltage adjustment circuit  32  and are input to the differential amplifier  33  for amplification. The output voltage amplified by the differential amplifier  33  is input to the hysteresis comparator  34 .  
         [0031]     The hysteresis comparator  34  outputs 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 comparator  34  outputs the high level. The hysteresis comparator  34  starts 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 t 6  (t 7 ). The hysteresis comparator  34  switches 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 t 9  (t 10 ).  
         [0032]     The digital signal output from the hysteresis comparator  34  is input to the counter  35 . The clock signal is input to the counter  35 , and the counter  35  counts the number of pulses of the clock signal in a period when the digital signal output from the hysteresis comparator  34  is in the low level. When the digital signal becomes high level and the counting of the number of the pulses is terminated, the counter  35  outputs the count value to the control circuit  41 . The control circuit  41  stores the input count value to the memory  411 .  
         [0033]     The control circuit  41  then turns off the drive start enable signal that is input to the DAC control logic  25  (t 13 ). The control circuit  41  stops the input of the X-axis selection signal to the excitation switch circuit  21  and the detection switch circuit  31  (t 14 ). In this way, the contacts are turned off in the exciting coil  112  and the detecting coil  113  of the magnetic sensor  11  for measuring the magnetic field in the x-axis direction.  
         [0034]     The control circuit  41  then transmits a signal (hereinafter, Y-axis selection signal) for turning on the Y-axis contacts of the excitation switch circuit  21  and the detection switch circuit  31  (t 15 ). In this way, the process for the Y-axis is started. The process for the Y-axis is performed in a period from t 15  to t 16  as is the case with the X-axis. The process for the Z-axis is also performed in a period from t 17  to t 18  as is the case with the X-axis.  
         [0035]     When the count value is stored in the memory  411  for each of the X-axis, Y-axis, and Z-axis, the control circuit  41  transmits to the microcomputer  71  an interrupt signal notifying that the writing of the count values is completed (t 19 ). When receiving the interrupt signal, the microcomputer  71  transmits a read request to the control circuit  41 . In this way, the microcomputer  71  reads the count value stored in the memory  411  of the control circuit  41  for each of the X-axis, Y-axis, and Z-axis (t  20 ). The count values read by the microcomputer  71  are utilized for measuring the intensity, etc. of the external magnetic field ΔH.  
         [0036]     In the flux gate magnetometer  1  of the embodiment with the configuration described above, the signals for driving the exciting coils  112 ,  122 ,  132  are generated with the digital circuits such as the DAC control logic  25  and the D/A converter  24 . 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.  
         [0037]     In the flux gate magnetometer  1  of the embodiment, a plurality of the exciting coils  112 ,  122 ,  132  is driven by the same D/A converter  24 . Therefore, the uniform exciting voltage can be applied to each of the exciting coils  112 ,  122 ,  132  and 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.  
         [0038]     Since the digital circuits are used in the flux gate magnetometer  1  of the embodiment, the lengths of the step-up period (t 5  to t 8 ) and the step-down period (t 8  to t 11 ) 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 magnetometer  1  with lower electric power consumption can be achieved.  
         [0039]     Since the flux gate magnetometer  1  of the embodiment counts the number of the pulses of the clock signal with the counter  35  to 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 counter  35  is a counter  35  occupying small chip area, the small-sized flux gate magnetometer  1  can be achieved.  
         [0040]     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 counter  35  is used. This constrains consumption currents as well.  
         [0041]     The flux gate magnetometer  1  of the embodiment is less affected by temperature and noises since the output voltage of the detecting coil  113 ,  123 ,  133  is digitalized by the hysteresis comparator  33  at an early stage.  
         [0042]     In the flux gate magnetometer  1  of the embodiment, since the same differential amplifier  33  and the same hysteresis comparator  34  perform processes for the output voltages of a plurality of the detecting coils  113 ,  123 ,  133 , variations are reduced in the measurement values of the detecting coils  113 ,  123 ,  133 . Since the same circuits are used in the processes for the output voltages of the detecting coils  113 ,  123 ,  133  in this way, the component count and the chip area can be reduced at the time of integration.  
         [0043]     In the flux gate magnetometer  1  of the embodiment, since the differential amplifier  33  is used for amplifying the output voltages, less common-mode noise is mixed. Since the detecting coils  113 ,  123 , and  133  are not grounded, this also prevents the common-mode noise from being mixed.  
         [0044]     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 converter  24  to smooth the drive signal output from the D/A converter  24 .  
         [0045]     The drive signal may be generated by an SC (switched capacitor) integrator with a configuration shown in  FIG. 3  instead of the D/A converter  24 . The SC integrator  80  shown in  FIG. 3  is constituted by four switches SW 1  to SW 4 , a capacitor C 1 , and an integration circuit  81  using an operational amplifier. The switch SW 1 , the capacitor C 1 , and SW 4  are serially connected in this order; SW 1  is connected to a direct-current power source Vin; and the output of SW 4  is input to the noninverting input terminal of the operational amplifier constituting the integration circuit  81 . The switch SW 2  is connected between the switch SW 1  and the capacitor C 1  and one end of the switch SW 2  is grounded. The switch SW 3  is connected between the capacitor C 1  and the switch SW 4  and one end of the switch SW 3  is grounded.  
         [0046]     When the SC integrator  80  of  FIG. 3  generates signals in the step-up period of the drive signal constituted by a triangular wave, the switches SW 1  to SW 4  are switched at constant intervals Δt 1  such that states shown in  FIGS. 4A and 4B  are achieved alternately (crawl type driving mode). In this way, as shown in  FIG. 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 SW 1  to SW 4  are switched at constant intervals Δt 2  such that states shown in  FIGS. 5A and 5B  are achieved alternately (butterfly type driving mode). In this way, as shown in  FIG. 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 integrator  80  through the low-pass filter.  
         [0047]     The SC integrator  80  can match Δt 1  and Δt 2  accurately 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 integrator  80  is used, the flux gate magnetometer  1  can also be achieved which can measure a magnetic field highly accurately as is the case with the D/A converter  24 .