Patent Publication Number: US-9841470-B2

Title: Triaxial coil sensor and magnetic field measuring device including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This U.S. non-provisional patent application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2015-0025290, filed on Feb. 23, 2015, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     The present disclosure herein relates to magnetic field measuring devices, and more particularly, to triaxial coil sensors for measuring a cumulative magnetic field in space and magnetic field measuring devices including the same. 
     Due to a magnet or an electric field changed according to a current or time, a space is produced in the surroundings in which a magnetic force acts. The space is referred to as a magnetic field. The magnetic field affects a moving charge, and the moving charge may generate a magnetic field. 
     Recently, in line with the provision of various services using radio technology in everyday life, an interest in the effect of electromagnetic waves on a user&#39;s health has been grown. Accordingly, a system for measuring the amount of electromagnetic waves, to which people are exposed in everyday life, has been studied. In particular, a measurement system for measuring the magnitude of a magnetic field in a low-frequency band (e.g., about 30 MHz or less) has been used. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides a triaxial coil sensor, which improves isolation characteristics of a sensor unit in each axial direction and reduces power consumption, and a magnetic field measuring device including the triaxial coil sensor. 
     An embodiment of the inventive concept provides a magnetic field measuring device including: a first sensor unit which includes a first coil sensor configured to output a first sensor signal and is connected to a first node and a second node; a second sensor unit which includes a second coil sensor configured to output a second sensor signal and disposed in a direction perpendicular to the first coil sensor, and is connected to the second node and a third node; a third sensor unit which includes a third coil sensor configured to output a third sensor signal and disposed in a direction perpendicular to the first and second coil sensors, and is connected to the third node and a fourth node; and a digital signal processor which is connected to the first and fourth nodes and outputs magnetic flux density based on a voltage difference between the first and fourth nodes, wherein the first to third sensor units respectively output first to third output signals in which specific voltages of the first to third sensor signals are maintained for a predetermined period of time. 
     In an embodiment of the inventive concept, a magnetic field measuring device includes: a first sensor unit which includes a first coil sensor configured to output a first sensor signal; a second sensor unit which includes a second coil sensor configured to output a second sensor signal and disposed in a direction perpendicular to the first coil sensor; a third sensor unit which includes a third coil sensor configured to output a third sensor signal and disposed in a direction perpendicular to the first and second coil sensors; and a digital signal processor configured to convert magnetic flux density based on first to third output signals from the first to third sensor units, wherein each of the first to third sensor units includes a band-pass filter that is independent from the other sensor units to select the first to third output signals having a desired frequency which are respectively included in the first to third sensor signals. 
     In an embodiment of the inventive concept, a triaxial coil sensor includes: a first coil sensor which includes a disc-shaped core having a first diameter and a first height and a first coil wound around the disc-shaped core; a second coil sensor which includes a first cylindrical core provided in the disc-shaped core to have a second diameter and a second height and a second coil wound on a cylindrical surface of the first cylindrical core; and a third coil sensor which includes a second cylindrical core provided in the disc-shaped core in a direction perpendicular to the first cylindrical core to have the second diameter and the second height and a third coil wound on a cylindrical surface of the second cylindrical core, wherein central axes of the first and second cylindrical cores are parallel to a disc surface of the disc-shaped core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the inventive concept, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the inventive concept and, together with the description, serve to explain principles of the inventive concept. In the drawings: 
         FIG. 1  exemplarily illustrates a magnetic field sensor for measuring the magnitude of a magnetic field; 
         FIG. 2  is a block diagram illustrating a magnetic field measuring device according to an embodiment of the inventive concept; 
         FIG. 3  is a block diagram exemplarily illustrating a digital signal processor of  FIG. 2 ; 
         FIG. 4  is a perspective view exemplarily illustrating first to third coil sensors of  FIG. 2 ; 
         FIG. 5  is a front view exemplarily illustrating the first to third coil sensors of  FIG. 4 ; 
         FIG. 6  is a right side view exemplarily illustrating the first to third coil sensors of  FIG. 4 ; 
         FIG. 7  exemplarily illustrates a magnetic flux density conversion table used in a magnetic flux density conversion circuit of  FIG. 3 ; 
         FIG. 8  is a block diagram illustrating a magnetic field measuring device according to another embodiment of the inventive concept; 
         FIG. 9  is a block diagram exemplarily illustrating a digital signal processor of  FIG. 8 ; 
         FIG. 10  is a timing diagram illustrating a voltage outputted from a band-pass filter of each sensor unit of  FIG. 8 ; 
         FIG. 11  is a timing diagram illustrating a voltage outputted from a unidirectional element of the each sensor unit of  FIG. 8 ; and 
         FIG. 12  is a timing diagram illustrating a voltage outputted from an electrical storage element of the each sensor unit of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     It should be construed that foregoing general illustrations and following detailed descriptions are exemplified and an additional explanation of claimed inventions is provided. Reference numerals are indicated in detail in preferred embodiments of the inventive concept, and their examples are represented in reference drawings. In every possible case, like reference numerals are used for referring to the same or similar elements in the description and drawings. 
     Hereinafter, a magnetic field measuring device will be used as one example of an electrical device for illustrating characteristics and functions of the inventive concept. However, those skilled in the art can easily understand other advantages and performances of the inventive concept according to the descriptions. Also, the inventive concept may be embodied or applied through other embodiments. Besides, the detailed description may be amended or modified according to viewpoints and applications, not being out of the scope, technical idea and other objects of the inventive concept. 
       FIG. 1  exemplarily illustrates a magnetic field sensor for measuring the magnitude of a magnetic field. Referring to  FIG. 1 , the magnetic field sensor may include three coil sensors  10 ,  20 , and  30  which are perpendicular to one another. Cores respectively included in the coil sensors  10 ,  20 , and  30  may be respectively disposed in X-axis, Y-axis, and Z-axis directions. The X-axis direction and Y-axis direction are perpendicular to each other and the Z-axis direction is a direction perpendicular to a plane formed by the X-axis and the Y-axis. 
     A magnetic field formed in space has a random direction. Thus, the magnetic field formed in space may be calculated by measuring a magnetic flux density in each of the mutually perpendicular X-axis, Y-axis, and Z-axis directions and then summing the magnetic flux densities in the three directions. When the magnetic flux densities corresponding to each direction (X-axis, Y-axis, and Z-axis) are denoted as B x , B y , and B z , a total magnetic flux density (B total) may be represented by Equation 1.
 
 B   total   =|B   x   |+|B   y   |+|B   z |  [Equation 1]
 
     In  FIG. 1 , the coil sensors  10 ,  20 , and  30  exemplarily illustrate magnetic field sensors corresponding to each direction (X-axis, Y-axis, and Z-axis). Each of the coil sensors  10 ,  20 , and  30  exemplarily illustrates an induction-type magnetic field sensor. Hereinafter, the coil sensor  10  will be described as an example. The coil sensors  20  and  30  may have the same structure and properties as the coil sensor  10 . In the induction-type coil sensor  10 , a coil  11  is a form of a conducting wire having high electrical conductivity, such as copper, to measure the magnitude of the magnetic field. A voltage V sx  is generated between both ends of the conducting wire of the coil  11  by the changing magnetic field according to Faraday&#39;s law. The coil sensor  10  may measure the magnetic field by using the voltage V sx  generated between the both ends of the conducting wire of the coil  11 .
 
 V   sx =0.5π 2   fnD   2   B   core   [Equation 2]
 
     The coil  11  having a circular cross section as well as a diameter D outputs the voltage V sx  as in Equation 2 by the magnetic field formed in the coil. Herein, f is a frequency of the magnetic field, n is the number of windings of the coil, and B core  represents magnetic flux density of the magnetic field formed in the coil. 
     When the magnetic flux density (B core ) is increased, sensing sensitivity of the coil sensor  10  may be improved. For this purpose, a magnetic material, such as ferrite, may be used in a core  12 . When the core  12  having a diameter D i  and a length l is disposed in the coil  11 , the magnetic flux density (B core ) is proportional to permeability of the core  12 . However, the magnetic flux density (B core ) is saturated by a physical size (i.e., D i  and l) of the core  12 . Thus, the voltage V sx  outputted from both ends of the coil  11  may be represented by Equation 3. 
     
       
         
           
             
               
                 
                   
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                       10 
                       
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                             ( 
                             
                               
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                   [ 
                   
                     Equation 
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                     3 
                   
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     Where H core  represents magnetic field strength formed in the coil and d represents a diameter of the conducting wire used in the coil. Since the magnetic field strength (H core ) is independent of a magnetic material, the magnetic field strength (H core ) is the same as magnetic field strength of a magnetic field formed in a free space. Thus, the coil sensor  10  including the core  12  may measure the magnetic field strength (H core ) of a magnetic field which is formed in a longitudinal direction (i.e., X-axis direction) of the core  12 . Thus, each of the coil sensors  10 ,  20 , and  30  may measure a magnetic field in each direction (X-axis, Y-axis, and Z-axis). Eventually, the magnetic field in space may be obtained by summing the magnetic fields in each of the mutually perpendicular directions (X-axis, Y-axis, and Z-axis). 
       FIG. 2  is a block diagram illustrating a magnetic field measuring device according to an embodiment of the inventive concept. Referring to  FIG. 2 , a magnetic field measuring device  100  may include first to third sensor units  110 ,  120 , and  130  and a digital signal processor  140 . Each sensor unit may include independent coil sensor, band-pass filter, and amplifier. Each sensor unit may measure a magnetic field in each direction (X-axis, Y-axis, and Z-axis). 
     The first sensor unit  110  may include a coil sensor  111 , a band-pass filter  112 , and an amplifier  113 . The first sensor unit  110  may measure the magnetic field in the X-axis direction. For example, the coil sensor  111  may output a sensor signal V sx  according to the magnetic field in the X-axis direction. The coil sensor  111 , as illustrated in  FIG. 1 , may include a core formed of a magnetic material and a coil in which the core is wound with a conducting wire. 
     The band-pass filter  112  may remove a noise component by receiving the sensor signal V sx . The band-pass filter  112 , for example, may be realized by using passive components such as a resistor, an inductor, and a capacitor. Also, the band-pass filter  112  may be realized by using active components such as an operational (OP) amplifier. The band-pass filter  112  may output a filter signal V fx  from which the noise component is removed. The band-pass filter  112  may be set to output the filter signal V fx  corresponding to a desired magnetic field frequency from the sensor signal V sx . 
     In order to increase sensing sensitivity of the first sensor unit  110 , there is a need to amplify the filter signal V fx . The amplifier  113  may receive and amplify the filter signal V fx . The amplifier  113  may output an amplified signal V ax . The amplified signal V ax  is proportional to the magnetic flux density of the magnetic field to be detected by the coil sensor  111 . Thus, the magnetic field measuring device  100  may obtain the magnetic flux density B x  in the X-axis direction based on the amplified signal V ax . 
     The second and third sensor units  120  and  130  may be configured in the same manner as the first sensor unit  110 . The second sensor unit  120  may include a coil sensor  121 , a band-pass filter  122 , and an amplifier  123 . The third sensor unit  130  may include a coil sensor  131 , a band-pass filter  132 , and an amplifier  133 . The second sensor unit  120  may measure the magnetic field in the Y-axis direction. The third sensor unit  130  may measure the magnetic field in the Z-axis direction. For example, the second sensor unit  120  may output an amplified signal V ay . The third sensor unit  130  may output an amplified signal V az . Since generation processes of the amplified signals V ay  and V az  are the same as a generation process of the amplified signal V ax , the generation processes of the amplified signals V ay  and V az  will not be provided. 
     The digital signal processor  140  may receive the amplified signals V ax , V ay , and V az . The digital signal processor  140  may convert the amplified signals V ax , V ay , and V az  into digital codes. The digital signal processor  140  may convert the converted digital codes into the magnetic flux densities corresponding to each direction (X-axis, Y-axis, and Z-axis). 
     The digital signal processor  140  may output a total magnetic flux density B total  by summing the magnetic flux densities B x , B y , and B z  corresponding to each direction (X-axis, Y-axis, and Z-axis). However, a method of obtaining the total magnetic flux density B total  is not limited thereto. For example, the converted digital codes may be first summed and the digital signal processor  140  may then convert it into the total magnetic flux density B total . 
     The digital signal processor  140  may convert the digital codes into the magnetic flux densities by various methods. For example, the digital signal processor  140  may convert the digital codes into the magnetic flux densities using Equation 3. Also, the digital signal processor  140  may store a magnetic flux density conversion table. The magnetic field measuring device  100  may store a digital code measured in a specific magnetic field. Thus, the digital signal processor  140  may calculate current magnetic flux density using the magnetic flux density conversion table stored in advance. However, a method of calculating the magnetic flux density is not limited thereto. 
     The magnetic field measuring device  100  according to the inventive concept may use the independent amplifiers  113 ,  123 , and  133  in the first to third sensor units  110 ,  120 , and  130 . Thus, the magnetic field measuring device  100  may obtain improved isolation characteristics between each sensor unit. Also, since a multiplexer or switch for selecting output signals of the coil sensors  111 ,  121 , and  131  is not used, the magnetic field measuring device  100  may reduce power consumption. 
       FIG. 3  is a block diagram exemplarily illustrating the digital signal processor of  FIG. 2 . Referring to  FIG. 3 , the digital signal processor  140  may include analog-digital converters  141 ,  142 , and  143  and a magnetic flux density conversion circuit  144 . The digital signal processor  140  may receive the amplified signals V ax , V ay , and V az  corresponding to each direction (X-axis, Y-axis, and Z-axis). The digital signal processor  140  may output the total magnetic flux density B total  based on the amplified signals V ax , V ay , and V az . 
     For example, the analog-digital converter  141  may receive the amplified signal V ax . The analog-digital converter  141  may convert the amplified signal V ax  into a voltage code S dx . Also, the analog-digital converter  142  may receive the amplified signal V ay . The analog-digital converter  142  may convert the amplified signal V ay  into a voltage code S dy . Furthermore, the analog-digital converter  143  may receive the amplified signal V az . The analog-digital converter  143  may convert the amplified signal V az  into a voltage code S dz . 
     The magnetic flux density conversion circuit  144  may output the total magnetic flux density B total  by receiving the voltage codes S dx , S dy , and S dz . For example, the magnetic flux density conversion circuit  144  may respectively convert the voltage codes S dx , S dy , and S dz  into the magnetic flux densities B x , B y , and B z  corresponding to each direction (X-axis, Y-axis, and Z-axis). The magnetic flux density conversion circuit  144  may calculate the total magnetic flux density B total  by summing the converted magnetic flux densities B x , B y , and B z . 
     Also, the magnetic flux density conversion circuit  144  may calculate a total voltage code S total  by summing the voltage codes S dx , S dy , and S dz . The magnetic flux density conversion circuit  144  may calculate the total magnetic flux density B total  by converting the total voltage code S total . The magnetic flux density conversion circuit  144  may output the total magnetic flux density B total . 
     The magnetic flux density conversion circuit  144  may convert the voltage codes S dx , S dy , and S dz  into the magnetic flux densities by various methods. For example, the magnetic flux density conversion circuit  144  may convert the voltage codes S dx , S dy , and S dz  into the magnetic flux densities using Equation 3. Also, the magnetic flux density conversion circuit  144  may store a magnetic flux density conversion table. The magnetic field measuring device  100  may store the voltage codes S dx , S dy , and S dz  measured in a specific magnetic field or the total voltage code S total . Thus, the magnetic flux density conversion circuit  144  may calculate current magnetic flux density using the magnetic flux density conversion table stored in advance. 
       FIG. 4  is a perspective view exemplarily illustrating the first to third coil sensors of  FIG. 2 .  FIG. 5  is a front view exemplarily illustrating the first to third coil sensors of  FIG. 4 .  FIG. 6  is a right side view exemplarily illustrating the first to third coil sensors of  FIG. 4 . Referring to  FIGS. 4 to 6 , a thickness of the magnetic field measuring device  100  may be reduced. 
     As illustrated in  FIG. 1 , a triaxial coil sensor generally requires cores corresponding to each direction (X-axis, Y-axis, and Z-axis). The cores are generally realized in the form of cylinders having the same diameter and length. Thus, the magnetic field measuring device  100  may have the thickness as much as at least the length of the core. 
     A triaxial coil sensor  3 CS according to the inventive concept may reduce the length of the core in the Z-axis direction. For example, the triaxial coil sensor  3 CS may include a disc-shaped core  131   a  for measuring a magnetic field in the Z-axis direction. Column-shaped cores  111   a  and  121   a  for measuring a magnetic field in the X-axis or Y-axis direction may be included in the disc-shaped core  131   a . The column-shaped cores  111   a  and  121   a  may have a length equal to or less than a diameter of the disc-shaped core  131   a . The column-shaped cores  111   a  and  121   a  may have a diameter equal to or less than a thickness of the disc-shaped core  131   a . Thus, the triaxial coil sensor  3 CS according to the inventive concept may have various thicknesses. Coils wound on the column-shaped cores  111   a  and  121   a  and the disc-shaped core  131   a  may be the same. 
     Although shapes of the column-shaped cores  111   a  and  121   a  and the disc-shaped core  131   a  are different, the column-shaped cores  111   a  and  121   a  and the disc-shaped core  131   a  may be used together to measure a magnetic field. For example, the diameters and lengths (or thicknesses) of the column-shaped cores  111   a  and  121   a  and the disc-shaped core  131   a  may be adjusted to output the same voltage in the same magnetic field using Equation 3. Also, the magnetic field measuring device  100  may use different magnetic flux density conversion tables to convert voltages measured by the column-shaped cores  111   a  and  121   a  and the disc-shaped core  131   a  into magnetic flux densities. 
     As described above, the triaxial coil sensor  3 CS according to the inventive concept may reduce the length of the core in the Z-axis direction by using the disc-shaped core. Thus, the magnetic field measuring device  100  may reduce its thickness by using the triaxial coil sensor  3 CS. 
       FIG. 7  exemplarily illustrates a magnetic flux density conversion table used in the magnetic flux density conversion circuit of  FIG. 3 . Referring to  FIGS. 3 and 7 , the magnetic flux density conversion circuit  144  may convert voltage codes into magnetic flux densities using the magnetic flux density conversion table. 
     First, the magnetic field measuring device  100  may store a voltage code measured in a specific magnetic field. For example, when the magnetic flux density is 1, the measured total voltage code S total  may be V 1 . When the magnetic flux density is 2, the measured total voltage code S total  may be V 2 . The magnetic field measuring device  100  may generate and store a magnetic flux density conversion table in the above manner. The magnetic flux density conversion table may be generated with respect to the total voltage code S total  measured in a specific magnetic field. Also, the magnetic flux density conversion table may be generated with respect to the voltage codes S dx , S dy , and S dz  corresponding to each direction (X-axis, Y-axis, and Z-axis) measured in a specific magnetic field. 
     When measuring a magnetic field, the magnetic flux density conversion circuit  144  may convert the voltage codes S dx , S dy , and S dz  or the total voltage code S total  into magnetic flux densities using the stored magnetic flux density conversion table. For example, when the measured total voltage code S total  is V 1 , the magnetic flux density conversion circuit  144  may output the total magnetic flux density B total  having a value of 1. With respect to the total voltage code S total  and the voltage codes S ax , S dy , and S dz  which are not included in the magnetic flux density conversion table, the total voltage code S total  and the voltage codes S ax , S dy , and S dz  may be converted into magnetic flux densities corresponding thereto by an interpolation method. 
       FIG. 8  is a block diagram illustrating a magnetic field measuring device according to another embodiment of the inventive concept. A magnetic field measuring device  200  may include first to third sensor units  210 ,  220 , and  230 , a digital signal processor  240 , and an amplifier  250 . Each sensor unit may include a coil sensor, a band-pass filter, a unidirectional element, and an electrical storage element. Each sensor unit may measure a magnetic field in each direction (X-axis, Y-axis, and Z-axis). 
     The first sensor unit  210  may include a coil sensor  211 , a band-pass filter  212 , a unidirectional element  213 , and an electrical storage element  214 . The first sensor unit  210  may measure the magnetic field in the X-axis direction. For example, the coil sensor  211  may output a sensor signal V sx  according to the magnetic field in the X-axis direction. The band-pass filter  212  may remove a noise component by receiving the sensor signal V sx . The band-pass filter  212 , for example, may be realized by using passive components such as a resistor, an inductor, and a capacitor. Also, the band-pass filter  212  may be realized by using active components such as an OP amplifier. The band-pass filter  212  may output a filter signal from which the noise component is removed. The band-pass filter  212  may be set to output the filter signal corresponding to a desired magnetic field frequency from the sensor signal V sx . For example, the filter signal may be a sinusoidal signal. 
     The unidirectional element  213  may remove a negative component from the filter signal. For example, the unidirectional element  213  may be realized as a diode. The unidirectional element  213  may convert the filter signal into a half-wave rectified signal. The electrical storage element  214  may convert the half-wave rectified signal into a peak signal. For example, the electrical storage element  214  may be realized as a capacitor. The electrical storage element  214  may maintain a voltage signal having a peak value of the half-wave rectified signal for a predetermined period of time to output the voltage signal. 
     The second and third sensor units  220  and  230  may be configured in the same manner as the first sensor unit  210 . The second sensor unit  220  may include a coil sensor  221 , a band-pass filter  222 , a unidirectional element  223 , and an electrical storage element  224 . The third sensor unit  230  may include a coil sensor  231 , a band-pass filter  232 , a unidirectional element  233 , and an electrical storage element  234 . The second sensor unit  220  may measure the magnetic field in the Y-axis direction. The third sensor unit  230  may measure the magnetic field in the Z-axis direction. For example, the electrical storage element  224  of the second sensor unit  220  may output a peak signal corresponding to a sensor signal V sy . The electrical storage element  234  of the third sensor unit  230  may output a peak signal corresponding to a sensor signal V sz . Since a generation process of the peak signal of each sensor unit is the same, the generation process of the peak signal will not be provided. 
     In this case, the electrical storage elements  214 ,  224 , and  234  of the first to third sensor units  210 ,  220 , and  230  and the amplifier  250  may be connected in series. For example, a first input terminal of the amplifier  250  and one end of the electrical storage element  214  may be connected to a node N 1 . The other end of the electrical storage element  214  and one end of the electrical storage element  224  may be connected to a node N 2 . The other end of the electrical storage element  224  and one end of the electrical storage element  234  may be connected to a node N 3 . The other end of the electrical storage element  234  and a second input terminal of the amplifier  250  may be connected to a node N 4 . Accordingly, a total peak signal summing the peak signals of the electrical storage elements  214 ,  224 , and  234  of the first to third sensor units  210 ,  220 , and  230  may be input to the amplifier  250 . Thus, the amplifier  250  may output a total voltage signal V total  by amplifying the total peak signal. 
     The digital signal processor  240  may receive the total voltage signal V total . The digital signal processor  240  may convert the total voltage signal V total  into a digital code. The digital signal processor  240  may convert the converted digital code into a total magnetic flux density B total . For example, the digital signal processor  240  may convert the digital code into the total magnetic flux density B total  using a magnetic flux density conversion table stored in advance. 
     The magnetic field measuring device  200  according to the inventive concept does not use a multiplexer or switch for selecting output signals of the coil sensors  211 ,  221 , and  231 . Thus, the magnetic field measuring device  200  may reduce power consumption. 
       FIG. 9  is a block diagram exemplarily illustrating the digital signal processor of  FIG. 8 . Referring to  FIG. 9 , the digital signal processor  240  may include an analog-digital converter  241  and a magnetic flux density conversion circuit  242 . The digital signal processor  240  may receive the total voltage signal V total . The digital signal processor  240  may output the total magnetic flux density B total  based on the total voltage signal V total . 
     For example, the analog-digital converter  241  may receive the total voltage signal V total . The analog-digital converter  241  may convert the total voltage signal V total  into a total voltage code S total . The magnetic flux density conversion circuit  242  may output the total magnetic flux density B total  by receiving the total voltage code S total . For example, the magnetic flux density conversion circuit  242  may include a magnetic flux density conversion table. A voltage corresponding to a specific magnetic flux density may be measured and stored in the magnetic flux density conversion table in advance. The magnetic flux density conversion circuit  242  may convert the total voltage code S total  into the total magnetic flux density B total  with reference to the magnetic flux density conversion table. 
       FIG. 10  is a timing diagram illustrating a voltage outputted from the band-pass filter of the each sensor unit of  FIG. 8 .  FIG. 11  is a timing diagram illustrating a voltage outputted from the unidirectional element of the each sensor unit of  FIG. 8 .  FIG. 12  is a timing diagram illustrating a voltage outputted from the electrical storage element of the each sensor unit of  FIG. 8 . Referring to  FIGS. 8 to 12 , the coil sensors  211 ,  221 , and  231  of the each sensor units  210 ,  220 , and  230  may output the sensor signals V sx , V sy , and V sz  correspond to the magnetic field in each direction (X-axis, Y-axis, and Z-axis). 
     In  FIG. 10 , each of the band-pass filters  212 ,  222 ,  232  of the sensor units  210 ,  220 , and  230  may output a filter signal V bpf  in which noise is removed from the sensor signals V sx , V sy , and V sz . For example, the band-pass filter  212  of the first sensor unit  210  may output the filter signal V bpf  having a peak value of V x . The band-pass filter  222  of the second sensor unit  220  may output the filter signal V bpf  having a peak value of V y . The band-pass filter  232  of the third sensor unit  230  may output the filter signal V bpf  having a peak value of V z . 
     The filter signal V bpf  of the each sensor units  210 ,  220 , and  230  may be a sinusoidal signal. The filter signals V bpf  of the each sensor units  210 ,  220 , and  230  may have different phases and peak values from one another. For example, the filter signal V bpf  of the first sensor unit  210  may have the peak value V x  at a first time t 1 . The filter signal V bpf  of the second sensor unit  220  may have the peak value V y  at a second time t 2 . The filter signal V bpf  of the third sensor unit  230  may have the peak value V z  at a fourth time t 4 . 
     In  FIG. 11 , each of the unidirectional elements  213 ,  223 , and  233  of the sensor units  210 ,  220 , and  230  may output a half-wave rectified signal V diode  by removing a negative portion from the filter signal V bpf . The half-wave rectified signal V diode  of the each sensor units  210 ,  220 , and  230  may have the same phase and peak value V x , V y , or V z  as the filter signal V bpf . 
     In  FIG. 12 , each of the electrical storage elements  214 ,  224 , and  234  of the sensor units  210 ,  220 , and  230  may output a peak signal V cap  decreasing at a predetermined slope from the peak values V x , V y , and V z  of the half-wave rectified signal V diode . Electric storage capacities of the electrical storage elements  214 ,  224 , and  234  may be controlled so that the peak signals V cap  may respectively maintain the peak values V x , V y , and V z  for a predetermined period of time. That is, the peak signals V cap  may respectively maintain the peak values V x , V y , and V z  for a predetermined period of time by adjusting time constants of the electrical storage elements  214 ,  224 , and  234 . 
     The magnetic field measuring device  200  according to the inventive concept may sum the peak signals V cap  of the sensor units  210 ,  220 , and  230  by connecting the amplifier  250  and the sensor units  210 ,  220 , and  230  in series. Thus, the magnetic field measuring device  200  does not use a multiplexer or switch for selecting output signals of the coil sensors  211 ,  221 , and  231 . As a result, the magnetic field measuring device  200  may reduce power consumption. 
     According to an embodiment of the inventive concept, provided is a magnetic field measuring device including a triaxial sensor which may improve isolation characteristics of a sensor unit in each axial direction and may reduce power consumption because a multiplexer or switch for selecting output signals of coil sensors is not used. 
     Hitherto, the best mode was disclosed in the drawings and specification. While specific terms were used, they were not used to limit the meaning or the scope of the inventive concept described in claims, but merely used to explain the inventive concept. Accordingly, a person having ordinary skill in the art will understand from the above that various modifications and other equivalent embodiments are also possible. Hence, the real protective scope of the inventive concept shall be determined by the technical scope of the accompanying claims.