Patent Publication Number: US-2023146777-A1

Title: Measurement individual difference correction system in ground voltage measurement

Description:
TECHNICAL FIELD 
     The present invention is related to a measurement individual difference correction system for measuring voltage to round. 
     BACKGROUND ART 
     Devices such as communication devices and precision devices are usually connected with cables. For this reason, electromagnetic noise occurring in a certain device may propagate through a cable, and in some situations, the electromagnetic noise may enter another device being connected and, for example, cause communication disruption or slow down the communication speed. For example, there are incidences where electromagnetic noise occurring in a rapid charger for electric vehicles enters an ADSL communication cable and disrupts communication. 
     Electromagnetic noise is a type of electric signal and is invisible. For this reason, when communication trouble suspected of being caused by electromagnetic noise has occurred, a maintenance person carries out maintenance work by measuring electromagnetic noise by using a measurement device such as an oscilloscope. On such an occasion, because electromagnetic noise often forms a loop while using the earth ground as a return path, the voltage to ground of the electromagnetic noise is measured. 
     The voltage to ground denotes voltage between a conductor such as a cable and the earth ground or another conductor continuous with the earth ground. Generally speaking, a measurement device (hereinafter, “voltage-to-ground measurement device”) such as an oscilloscope is grounded, and either a passive probe or a capacitive voltage probe (see Non-Patent Literature 1) of the voltage-to-ground measurement device is brought into contact with or clamped onto a cable subject to the measurement process, so as to measure the voltage to ground of the electromagnetic noise propagating through the cable. Alternatively, when it is difficult to have the voltage-to-ground measurement device grounded, the voltage to ground of the electromagnetic noise is measured without the grounding. Further, by using capacitance to ground measured by a capacitance-to-ground measurement device, a measurement error in the voltage to ground caused by the absence of the grounding ls corrected, so as to obtain an accurate value of the voltage to ground (see Non-Patent Literature 2). 
     CITATION LIST 
     Non-Patent Literature 
     Non-Patent Literature 1: Kobayashi, R. and five others, “A Novel Non-contact Capacitive Probe for Common-Mode Voltage Measurement”, IEICE TRANS. COMMUN., vol. E90-B, No. 6, June 2007, pp. 1329-1337 
     Non-Patent Literature 2: Arai and two others, “Sokuteiki no kan&#39;iteki setchi ni muketa taichi seiden youryou no mitsumori shuhou no teian (‘A proposal of a method for estimating electrostatic capacity to ground designed for conveniently grounding a measurement device’ in Japanese)”, the Institute of Electronics, Information and Communication Engineers (IEICE) General Conference 2019, Proceedings of Communications Conference 1, B-4-44, 2019, p. 264 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     To simplify the work of measuring the voltage to ground, for example, it may be possible to use a configuration in which a voltage-to-ground measurement device is installed in the sole of one of a pair or shoes, whereas a capacitance-to-ground measurement device is installed in the sole of the other shoe. In this configuration, by simply touching the cable subject to the measurement process, a maintenance person is able to measure the voltage to ground of electromagnetic noise propagating through the cable subject to the measurement process. 
     In this situation, although the work of measuring the voltage to ground is simplified, because the human body functions as a probe, the measurement result of the voltage to ground is impacted by data amounts caused by the intervention of the human body, such as electrostatic capacity (C h ) between the cable and the human body, impedance (Z h ) of the human body, and electrostatic capacity (C f ) between the human body and the voltage-to-ground measurement device. In that situation, normally, a single general standard value corresponding to the data amounts is estimated in advance, so as to further correct the voltage to ground by using the standard value, in addition to correcting the voltage to ground by using capacitance to ground. 
     In the standard value, however, an individual difference of the maintenance person regarding C h , Z h , and C f  are not taken into account. Further, Non-Patent Literature 2 merely discloses the method for estimating the capacitance to ground that is necessary in the correction of the voltage to ground. For this reason, there remains a problem that what can be measured is merely the voltage to ground of the electromagnetic noise for which the individual difference is ignored. 
     The present invention has been made in view of the circumstances described above, and it is an object of the present invention to provide a technique that makes it possible to correct a measurement error related to the individual difference of a human body that functions as a probe at the time of measuring the voltage to ground of electromagnetic noise. 
     Means for Solving the Problem 
     A measurement individual difference correction system for measuring voltage to ground according to an aspect of the present invention includes: a cable that is a conductor; a floor panel that is a conductor and is provided on a floor; an oscillation circuit that is connected between the cable and the floor panel and that outputs a signal; a voltage-to-ground measurement device that includes an upper electrode and a lower electrode positioned apart from each other and that measures voltage between the upper electrode and the lower electrode; and a computation device that communicates with the voltage-to-ground measurement device. When a user stands on the floor panel so that the lower electrode is directly connected to the floor panel and the user touches the cable while wearing a shoe of which a sole has the voltage-to-ground measurement device installed therein, the computation device calculates combined impedance of electrostatic capacity between the user and the cable, impedance of the user, and electrostatic capacity between the user and the upper electrode, by using voltage of the signal output from the oscillation circuit and the voltage between the upper electrode and the lower electrode measured by the voltage-to-ground measurement device. 
     A measurement individual difference correction system for measuring voltage to ground according to an aspect of the present invention includes: a cable that is a conductor; a counter earth ground surface electrode provided on a floor; an oscillation circuit that is connected between the cable and the counter earth ground surface electrode and that outputs a signal; a voltage-to-ground measurement device that includes an upper electrode and a lower electrode positioned apart from each other and that measures voltage between the upper electrode and the lower electrode; a capacitance-to-ground measurement device that measures capacitance to ground; and a computation device that communicates with the voltage-to-ground measurement device and the capacitance-to-ground measurement device. When a user touches the cable while wearing shoes of which a sole of one shoe has the voltage-to-ground measurement device installed therein and of which a sole of the other shoe has the capacitance-to-ground measurement device installed therein, the computation device calculates combined impedance of electrostatic capacity between the user and the cable, impedance of the user, and electrostatic capacity between the user and the upper electrode, by using voltage of the signal output from the oscillation circuit, the voltage between the upper electrode and the lower electrode measured by the voltage-to-ground measurement device, and the capacitance to ground measured by the capacitance-to-ground measurement device. 
     Effects of the Invention 
     According to the present invention, it is possible to provide the technique that makes it possible to correct the measurement error related to the individual difference of the human body that functions as a probe at the time of measuring the voltage to ground of electromagnetic noise. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a drawing showing a configuration of a voltage-to-ground measurement system and a measurement concept assumed by the present invention. 
         FIG.  2    is a drawing showing a configuration of a voltage-to-ground measurement device. 
         FIG.  3    is a drawing showing an equivalent circuit of  FIG.  1   . 
         FIG.  4    is a drawing showing a configuration of a capacitance-to-ground measurement device. 
         FIG.  5    is a drawing showing a configuration of a measurement individual difference correction system and a measurement concept according to a first embodiment. 
         FIG.  6    is a drawing showing a configuration of an oscillation circuit. 
         FIG.  7    is a drawing showing an equivalent circuit of  FIG.  5   . 
         FIG.  8    is a drawing showing a processing flow of the measurement individual difference correction system according to the first embodiment. 
         FIG.  9    is a drawing showing (a modification example of) the configuration of the measurement individual difference correction system and the measurement concept according to the first embodiment. 
         FIG.  10    is a drawing showing a configuration of a measurement individual difference correction system and a measurement concept according to a second embodiment. 
         FIG.  11    is a drawing showing an equivalent circuit of  FIG.  10   . 
         FIG.  12    is a drawing showing a processing flow of the measurement individual difference correction system according to the second embodiment. 
         FIG.  13    is a drawing showing a hardware configuration of a computation device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be explained below, with reference to the drawings. In the drawings, some of the elements that are the same as one another will be referred to by using mutually the same reference characters, and the explanations thereof will be omitted. 
     The present invention is an invention related to a technique for measuring the voltage to ground of electromagnetic noise propagating through a cable, without a maintenance person being concerned about the work of grounding a voltage-to-ground measurement device. In particular, the invention is related to a technique for correcting a measurement error related to an individual difference of a human body, with regard to a technique for measuring voltage to ground while using the human body as a probe. 
     &lt;A Configuration of a Voltage-to-Ground Measurement System&gt; 
       FIG.  1    is a drawing showing a configuration of a voltage-to-ground measurement system and a measurement concept assumed by the present invention. Two devices A 1  and A 2  are connected together by a coated cable W. It is assumed that electromagnetic noise is propagating from one of the devices A 1  to the other device A 2 . In this situation, when a maintenance person U touches the cable W, it is possible to measure the voltage to ground of the electromagnetic noise, by using a voltage-to-ground measurement device  10  and a capacitance-to-ground measurement device  20  that are respectively installed in the two soles of a pair of shoes. 
     More specifically, when the maintenance person U touches the cable W or grips the cable W, an electric circuit is formed via the maintenance person U and the earth ground (the earth surface or the floor surface). As shown in the example in  FIG.  2   , the voltage-to-ground measurement device  10  has a four-layer structure including an upper electrode  11 , a spacer, a lower electrode  12 , and another spacer. The device has a mechanism in which a voltage measurement circuit  13  measures the voltage occurring between the upper electrode  11  and the lower electrode  12  (more accurately, the voltage occurring in a resistance element connected between the upper electrode  11  and the lower electrode  12 ). As a result of the voltage-to-ground measurement device  10  measuring the voltage between the upper electrode  11  and the lower electrode  12 , it is possible to measure the voltage to ground of the electromagnetic noise propagating through the cable W. 
     An equivalent circuit of  FIG.  1    is shown in  FIG.  3   . The symbol C denotes electrostatic capacity (=capacitance to ground) between the lower electrode  12  of the voltage-to-ground measurement device  10  and the earth ground or a conductor continuous with the earth ground. The symbol C e  denotes electrostatic capacity between the upper electrode  11  and the lower electrode  12  of the voltage-to-ground measurement device  10 . The symbol C f  denotes electrostatic capacity between the sole of the foot of the maintenance person U and the upper electrode  11  of the voltage-to-ground measurement device  10 . The symbol Z h  denotes impedance of the maintenance person U. The symbol C h  denotes electrostatic capacity between a hand of the maintenance person U and the cable W gripped by the hand of the maintenance person U. The symbol Z n  denotes impedance in the electric circuit path through which the electromagnetic noise is flowing. The symbol V n  denotes the voltage to ground of the electromagnetic noise to be measured. The symbol V m  denotes the voltage measured by the voltage measurement circuit  13  of the voltage-to-ground measurement device  10 . 
     The voltage-to-ground measurement device  10  is able to obtain the voltage to ground (V n ) of the electromagnetic noise to be measured, through a calculation where the electrostatic capacity values (C, C e , C f , and C h ), the impedance values (Z h  and Z n ), and the voltage (V m ) described above are assigned to Expression (1) presented below. It should be noted that Z n  is ignored because the value thereof is generally small. Further, the symbol Z m  denotes combined impedance of C h , Z h , and C f . 
     
       
         
           
             
               
                 
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     In Expression (1), the symbol C e  denotes a known Parameter value determined by a designer of the voltage-to-ground measurement device  10 . Further, it is possible to measure C by using the capacitance-to-ground measurement device  20  that measures capacitance to ground (see Non-Patent Literature 2). 
     Next, the capacitance-to-ground measurement device  20  will be explained. As shown in  FIG.  4   , the capacitance-to-ground measurement device  20  is structured with an upper electrode  21 , a first lower electrode  22  and a second lower electrode  23  provided as a set of two electrodes, spacers inserted between the electrodes, an oscillation circuit  24 , a resistance element  25 , and a voltage measurement circuit  26 . The first lower electrode  22  and the second lower electrode  23  are both installed so as to face the earth ground serving as a reference potential. 
     In this situation, output voltage V of the oscillation circuit  24 , output resistance R out  of the oscillation circuit  24 , resistance R of the resistance element  25 , electrostatic capacity C 4  between the upper electrode  21  and the first lower electrode  22 , and electrostatic capacity C 5  between the upper electrode  21  and the second lower electrode  23  are known parameter values determined by a designer of the capacitance-to-ground measurement device  20 . Further, it is possible to design electrostatic capacity C 1  between the first lower electrode  22  and the earth ground, electrostatic capacity C 2  between the upper electrode  21  and the earth ground, and electrostatic capacity C 3  between the second lower electrode  23  and the earth ground, as values that can be expressed by using one variable. Further, when the capacitance-to-ground measurement device  20  approaches the earth ground, the values of C 1  to C 3  increase, and the voltage occurring in the resistance element  25  when a signal is output from the oscillation circuit  24  increases. 
     Thus, by using the notion that the values of C 1  to C 3  can be expressed by the one variable, it is possible to estimate the values of C 1  to C 3  from the voltage occurring in the resistance element  25  on the basis of Kirchhoff&#39;s laws. Accordingly, the capacitance-to-ground measurement device  20  is able to measure the capacitance to ground. Alternatively, the capacitance-to-ground measurement device  20  may be installed as a stationary device, instead of being installed in the sole of the other shoe. Further, as long as the capacitance-to-ground measurement device  20  is capable of measuring the capacitance to ground, configurations other than the configuration shown in  FIG.  4    may be used. 
     Returning to the explanation of Expression (1), the symbols C e  and C in Expression (1) represent values that can be defined. In contrast, the symbol Z m  denotes the combined impedance of C h , Z h , and C f  and represents a value that varies among individuals. For this reason, conventional techniques have only been able to use a predetermined single general standard value, which are unable to take the individual difference of the maintenance person U into account. 
     To cope with this situation, the present invention makes it possible to correct a measurement error related to the individual difference of the maintenance person U, by measuring, with respect to each user U representing the maintenance person U, a value of the combined impedance (Z m ) of C h , Z h , and C f  in advance, so as to calculate V n  by assigning the value of Z m  of the maintenance person U who performs the measurement process to Expression (1), at the time of measuring the voltage to ground. 
     Next, two embodiments of the present invention will be explained. In a first embodiment, a measurement individual difference correction system of a stationary type will be explained. In a second embodiment, a measurement individual difference correction system of a portable type will be explained. 
     First Embodiment 
     A Configuration of the Measurement Individual Difference Correction System According to the First Embodiment 
       FIG.  5    is a drawing showing a configuration of the measurement individual difference correction system and a measurement concept according to the first embodiment. The measurement individual difference correction system is structured with a grip cable  30 , a floor panel  40 , a height adjustment table  50 , an oscillation circuit  60 , the voltage-to-ground measurement device  10 , and a computation device  70 . In the first embodiment, because there is no need to use capacitance to ground since the lower electrode  12  of the voltage-to-ground measurement device  10  is directly connected to the floor panel  40 , the capacitance-to-ground measurement device  20  will not be used. 
     The grip cable  30  is a conductor cable to be gripped by the user U. 
     The floor panel  40  is a conductor panel provided on the floor. 
     The height adjustment table  50  is a conductor pedestal which is provided on the floor, is taller than the floor panel  40 , is electrically connected to the floor panel  40 , and is used for adjusting the heights of the grip cable  30  and the oscillation circuit  60 . 
     The oscillation circuit  60  is a circuit that is connected between the grip cable  30  and the height adjustment table  50  and that outputs a signal having a prescribed frequency. As shown in  FIG.  6   , the oscillation circuit  60  has a structure in which a resistance element  61  and an oscillator  62  are connected in series. 
     As shown in  FIG.  2   , the voltage-to-ground measurement device  10  is a device that includes the upper electrode  11  and the lower electrode  12  positioned apart from each other, measures the voltage between the upper electrode  11  and the lower electrode  12  by using the voltage measurement circuit  13 , and transmits the measured voltage resulting from the measurement process to the computation device  70  in a wired or wireless manner. The voltage-to-ground measurement device  10  is installed in the sole of one of the shoes of the user U. 
     Further, the voltage-to ground measurement device  10  has stored therein the combined impedance (Z m ) of each user U calculated by the computation device  70 . Further, when the maintenance person (=the prescribed user) U measures the voltage to ground of the electromagnetic noise propagating through the cable W subject to the measurement process, the voltage-to-ground measurement device  10  calculates the voltage to ground (V n ) of the electromagnetic noise, by changing the voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured at the time of the measurement process, with the use of the value of Z m  of the maintenance person U. 
     The computation device  70  is a device that communicates with the voltage-to-ground measurement device  10 . Further, when the user U stands on the floor panel  40  so that the lower electrode  12  is directly connected to the floor panel  40  and the user U touches the grip cable  30  while wearing the shoe of which the sole has the voltage-to-ground measurement device  10  installed therein, the computation device  70  calculates the combined impedance (Z m ) in the following manner: The computation device  70  calculates the combined impedance (Z m ) of the electrostatic capacity (C h ) between the hand of the user U and the grip cable  30 , the impedance (Z h ) of the user U, and the electrostatic capacity (C f ) between the sole of the foot of the user U and the upper electrode  11 , by using the voltage (V) of the signal output from the oscillation circuit  60  and the voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured by the voltage-to-ground measurement device  10 . 
     A Method for Calculating the Combined Impedance Implemented in the First Embodiment 
     While wearing the voltage-to-ground measurement device  10  implemented in the form of the shoe, the user U stands on the floor panel  40  and grips the grip cable  30 . In this situation, the floor panel  40  is electrically connected to the height adjustment table  50 . One end of the grip cable  30  is connected to an output side of the oscillation circuit  60 , whereas the other end of the grip cable  30  is not electrically connected anywhere and is in an open state. A ground side of the oscillation circuit  60  is connected to the height adjustment table  50 . The lower electrode  12  of the voltage-to-ground measurement device  10  worn by the user U is directly connected to the floor and  40  physically. The floor panel  40  and the height adjustment table  50  are each made of a conductive material (e.g., aluminum). 
     An equivalent circuit expressing the state described above is shown in  FIG.  7   . It is possible to express the voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured by the voltage-to-ground measurement device  10 , by using Expression (2) presented below. 
     
       
         
           
             
               
                 
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     The symbol Z m  denotes the combined impedance of the electrostatic capacity (C h ) between the hand of the user U and the grip cable  30 , the impedance (Z h ) of the user U, and the electrostatic capacity (C f ) between the sole of the foot of the user U and the upper electrode  11 . The symbol V denotes the output voltage of the oscillator  62  included in the oscillation circuit  60 . The symbol R denotes an output resistance value of the resistance element  61  included in the oscillation circuit  60 . The symbol C e  denotes the electrostatic capacity between the upper electrode  11  and the lower electrode  12  of the voltage-to-ground measurement device  10 . By using the relationship among the variables expressed in Expression (2), it is possible to obtain Z m  by using Expression (3) presented below. 
     
       
         
           
             
               
                 
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     The computation device  70  is used for performing the calculation in Expression (3). The computation device  70  is communicably connected to the oscillation circuit  60  and the voltage-to-ground measurement device  10  in a wired or wireless manner. The computation device  70  performs the calculation in Expression (3) by using the output voltage (V) of the signal output from the oscillation circuit  60  and the measured voltage (V m ) measured by the voltage-to-ground measurement device  10 . Further, by sweeping an oscillation frequency of the oscillation circuit  60 , the computation device  70  measures not only the combined impedance (Z m ) at a single frequency, but also frequency characteristics of the combined impedance. 
     Subsequently, the computation device  70  transmits frequency characteristic data of the combined impedance (Z m ) of each user obtained as measurement results, to the voltage-to-ground measurement device  10 . The voltage-to-ground measurement device  10  has stored therein the value of Z m  of each user U so as to be kept in association with a user ID. Further, in the voltage-to-ground measurement system shown in  FIG.  1   , when the maintenance person U measures the voltage to ground of the electromagnetic noise propagating through the cable W, the voltage-to-ground measurement device  10  obtains the voltage to ground (V m ) of the electromagnetic noise by assigning the value of Z m  of the maintenance person U to Expression (1) and thereby makes it possible to correct the measurement error related to the individual difference of the human body. 
     Next, a flow in the operation to measure the combined impedance (Z m ) will be explained.  FIG.  8    is a drawing showing a processing flow of the measurement individual difference correction system according to the first embodiment. The processes in the processing flow are performed with respect to each user. 
     Step S 101 : 
     First, the oscillation circuit  60  outputs a signal of the first one of frequencies. 
     Step S 102 : 
     After that, the computation device  70  assigns the output voltage (V) of the signal output from the oscillation circuit  60  and the measured voltage (V m ) measured by the voltage-to-ground measurement device  10  to Expression (3), and also assigns the electrostatic capacity (C e ) between the upper electrode  11  and the lower electrode  12  of the voltage-to-ground measurement device  10  and the output resistance value (R) of the resistance element  61  of the oscillation circuit  60 , which are default values, to Expression (3). In this manner, the computation device  70  calculates the combined impedance (Z m ) of the electrostatic capacity (C h ) between the hand of the user U and the grip cable  30 , the impedance (Z h ) of the user U, and the electrostatic capacity (C f ) between the sole of the foot of the user U and the upper electrode  11 . 
     Step S 103 : 
     Subsequently, the computation device  70  determines whether or not there is an unmeasured frequency among the plurality of frequencies that are predetermined. When there is at least one unmeasured frequency, the process proceeds to step S 104 . On the contrary, when there is no unmeasured frequency, the process proceeds to step S 106 . 
     Step S 104 : 
     When there is at least one unmeasured frequency, the computation device  70  instructs the oscillation circuit  60  to output a signal of the next frequency. 
     Step S 105 : 
     After that, on the basis of the instruction from the computation device  70  to output the signal, the oscillation circuit  60  outputs the signal of the next frequency, and the process returns to step S 102 . 
     Step S 106 : 
     When there is no unmeasured frequency, the computation device  70  transmits the measured combined impedance (Z m ) of each of the frequencies to the voltage-to-ground measurement device  10  and thus ends the process. 
     Subsequently, the voltage-to-ground measurement device  10  has stored therein the frequency characteristics of the combined impedance (Z m ) of each user transmitted thereto from the computation device  70 , so as to be kept in association with the ID of the user U. When the maintenance person U measures the voltage to ground of the electromagnetic noise, the voltage-to-ground measurement device  10  obtains the voltage to ground (V n ) of the electromagnetic noise for which the measurement error related to the individual difference of the human body has been corrected, by assigning the combined impedance (Z m ) of the maintenance person U to Expression (1) and changing the measured voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured at the time of the measurement process. 
     A Modification Example of the First Embodiment 
     The example was explained above in which the height adjustment table  50  is used; however, when there is no need to adjust the heights of the grip cable  30  and the oscillation circuit  60 , it is also acceptable to directly connect the oscillation circuit  60  to the floor panel  40 , as shown in  FIG.  9   . 
     Advantageous Effects of the First Embodiment 
     According to the first embodiment, provided are: the grip cable  30  that is a conductor; the floor panel  40  that is a conductor and is provided on the floor; the height adjustment table  50  that is a conductor, is provided on the floor, is taller than the floor panel  40 , and is electrically connected to the floor panel  40 ; the oscillation circuit  60  that is connected between the grip cable  30  and the height adjustment table  50  and that outputs the signal having the prescribed frequency; the voltage-to-ground measurement device  10  that includes the upper electrode  11  and the lower electrode  12  positioned apart from each other and that measures the voltage between the upper electrode  11  and the lower electrode  12 ; and the computation device  70  that communicates with the voltage-to-ground measurement device  10 . When the user U stands on the floor panel  40  so that the lower electrode  12  is directly connected to the floor panel  40  physically and the user U touches the grip cable  30  while wearing the shoe of which the sole has the voltage-to-ground measurement device  10  installed therein, the computation device  70  calculates the combined impedance (Z m ) of the electrostatic capacity (C h ) between the hand of the user U and the grip cable  30 , the impedance (Z h ) of the user U, and the electrostatic capacity (C f ) between the sole of the foot of the user U and the upper electrode  11 , by using the voltage (V) of the signal output from the oscillation circuit  60  and the voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured by the voltage-to-ground measurement device  10 . It is therefore possible to provide the technique that makes it possible to correct the measurement error related to the individual difference of the human body that functions as a probe at the time of measuring the voltage to ground of the electromagnetic noise. As a result, it is possible to measure the voltage to ground of the electromagnetic noise with a higher level of precision. 
     Second Embodiment 
     In the second embodiment, a measurement individual difference correction system is configured as a portable type, in contrast to the system of the stationary type in the first embodiment. The biggest problem of the portable type is that, because the floor panel  40  cannot be used, it is impossible to have the lower electrode  12  of the voltage-to-ground measurement device  10  directly connected to the floor panel  40  physically. Accordingly, in the second embodiment, the problem is solved by using the capacitance-to-ground measurement device  20  that measures the capacitance to ground. 
     A Configuration of the Measurement Individual Difference Correction System According to the Second Embodiment 
       FIG.  10    is a drawing showing a configuration of the measurement individual difference correction system and a measurement concept according to the second embodiment. The measurement individual difference correction system is structured with the grip cable  30 , the oscillation circuit  60 , a pedestal  80 , a desk  90 , a counter earth ground surface electrode  100 , the voltage-to-ground measurement device  10 , the capacitance-to-ground measurement device  20 , and the computation device  70 . 
     The grip cable  30  is a conductor cable gripped by the user U. 
     The oscillation circuit  60  is a circuit that is connected between the grip cable  30  and the counter earth ground surface electrode  100  and that outputs a signal having a prescribed frequency. As shown in  FIG.  6   , the oscillation circuit  60  has a structure in which the resistance element  61  and the oscillator  62  are connected in series. 
     The pedestal  80  is a pedestal disposed on the desk  90  and used for adjusting the heights of the grip cable  30  and the oscillation circuit  60 . The pedestal  80  may be a conductor or may be a non-conductor. 
     The desk  90  is a desk provided on the floor and is used for adjusting the heights of the grip cable  30  the oscillation circuit  60 . The desk  90  may be a conductor or may be a non-conductor. 
     The counter earth around surface electrode  100  is an electrode provided on the floor. The counter earth ground surface electrode  100  is connected to the oscillation circuit  60  by a conductor cable. When the pedestal  80  and the desk  90  are each a conductor, the counter earth ground surface electrode  100  may be connected to the desk  90 . 
     As shown in  FIG.  2   , the voltage-to-ground measurement device  10  is a device that includes the upper electrode  11  and the lower electrode  12  positioned apart from each other, measures the voltage between the upper electrode  11  and the lower electrode  12  by using the voltage measurement circuit  13 , and transmits the measured voltage resulting from the measurement process to the computation device  70  in a wired or wireless manner. The voltage-to-ground measurement device  10  is installed in the sole of one of the shoes of the user U. 
     Further, the voltage-to-ground measurement device  10  has stored therein the combined impedance (Z m ) of each user U calculated by the computation device  70 . Further, when the maintenance person (=the prescribed user) U measures the voltage to ground of the electromagnetic noise propagating through the cable W subject to the measurement process, the voltage-to-ground measurement device  10  calculates the voltage to ground (V n ) of the electromagnetic noise, by changing the voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured at the time of the measurement process, with the use of the value of Z m  of the maintenance person U. 
     As shown in  FIG.  4   , the capacitance-to-ground measurement device  20  includes the upper electrode  21 , the first lower electrode  22  and the second lower electrode  23  provided as a set of two electrodes, the spacers inserted between the electrodes, the oscillation circuit  24 , the resistance element  25 , and the voltage measurement circuit  26 . Further, the capacitance-to-ground measurement device  20  measures one value of capacitance to ground corresponding to total electrostatic capacity made up of the electrostatic capacity C 1  between the first lower electrode  22  and the earth ground, the electrostatic capacity C 2  between the upper electrode  21  and the earth around, and the electrostatic capacity C 3  between the second lower electrode  23  and the earth ground, and further transmits the measured capacitance to ground to the computation device  70  in a wired or wireless manner. The voltage-to-ground measurement device  10  is installed in the sole of the other shoe of the user U. 
     The computation device  70  is a device that communicates with the voltage-to-ground measurement device  10  and the capacitance-to-ground measurement device  20 . Further, when the user U touches the grip cable  30  while wearing the shoes of which the sole of one shoe has the voltage-to-ground measurement device  10  installed herein and of which the sole of the other shoe has the capacitance-to-ground measurement device  20  installed therein, the computation device  70  calculates the combined impedance (Z m ) in the following manner: The computation device  70  calculates the combined impedance (Z m ) of the electrostatic capacity (C h ) between the hand of the user U and the grip cable  30 , the impedance (Z h ) of the user U, and the electrostatic capacity (C f ) between the sole of the foot of the user U and the upper electrode  11 , by using the voltage (V) of the signal output from the oscillation circuit  60 , the voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured by the voltage-to-ground measurement device  10 , and the capacitance to ground measured by the capacitance-to-ground measurement device  20 . 
     A Method for Calculating the Combined Impedance Implemented in the Second Embodiment 
     The user U grips the grip cable  30  while wearing the voltage-to-ground measurement device  10  implemented in the form of the shoe. In this situation, the grip cable  30  and the oscillation circuit  60  are provided on the pedestal  80  and the desk  90  having appropriate sizes so as to be easily gripped by the user U. In the second embodiment, the grip cable  30  and the oscillation circuit  60  are used like tn the first embodiment, but the floor panel  40  and the height adjustment table  50  are not used. Further, the counter earth ground surface electrode  100 , which is not used in the first embodiment, is connected to the ground side of the oscillation circuit  60 . Further, the pedestal  80  is provided to facilitate placement on the desk or the like. The counter earth ground surface electrode  100  is installed so as to be horizontal on the earth ground surface. 
     An equivalent circuit expressing the state described above is shown in  FIG.  11   . It is possible to express the measured voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured by the voltage-to-ground measurement device  10  by using Expression (4) presented below. 
     
       
         
           
             
               
                 
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     The symbol Z m  denotes the combined impedance of the electrostatic capacity (C h ) between the hand of the user U and the grip cable  30 , the impedance (Z h ) of the user U, and the electrostatic capacity (C f ) between the sole of the foot of the user U and the upper electrode  11 . The symbol V denotes the output voltage of the oscillator  62  included in the oscillation circuit  60 . The symbol R denotes an output resistance value of the resistance element  61  included in the oscillation circuit  60 . The symbol C e  denotes the electrostatic capacity between the upper electrode  11  and the lower electrode  12  of the voltage-to-ground measurement device  10 . The symbol C denotes electrostatic capacity (=capacitance to ground) between the earth ground or a conductor continuous with the earth ground and the lower electrode  12  of the voltage-to-ground measurement device  10 . The symbol C a  denotes electrostatic capacity (=capacitance to ground) between the earth ground or a conductor continuous with the earth ground and the counter earth ground surface electrode  100 . It is possible to measure C and C a  by using the capacitance-to-ground measurement device  20 . By using the relationship among the variables expressed in Expression (4), it is possible to obtain Z m  by using Expression (5) presented below. 
     
       
         
           
             
               
                 
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     The computation device  70  is used for performing the calculation in Expression (5). The computation device  70  is communicably connected to the oscillation circuit  60 , the voltage-to-ground measurement device  10 , and the capacitance-to-ground measurement device  20 , in a wired or wireless manner. The computation device  70  performs the calculation in Expression (5) by using the output voltage (V) of the signal output from the oscillation circuit, the measured voltage (V m ) measured by the voltage-to-ground measurement device  10 , and the capacitance to ground (=the electrostatic capacity values C and C a ) measured by the capacitance-to-ground measurement device  20 . Further, by sweeping an oscillation frequency of the oscillation circuit  60 , the computation device  70  measures not only the combined impedance at a single frequency, but also frequency characteristics of the combined impedance. 
     Subsequently, the computation device  70  transmits frequency characteristic data of the combined impedance (Z m ) of each user obtained as measurement results, to the voltage-to-ground measurement device  10 . The voltage-to-ground measurement device  10  has stored therein the value of Z m  of each user U so as to be kept in association with a user ID. Further, in the voltage-to-ground measurement system shown in  FIG.  1   , when the maintenance person U measures the voltage to ground of the electromagnetic noise propagating through the cable W, the voltage-to-ground measurement device  10  obtains the voltage to ground (V m ) of the electromagnetic noise by assigning the value of Z m  of the maintenance person U to Expression (1) and thereby makes it possible to correct the measurement error related to the individual difference of the human body. 
     Next, a flow in the operation to measure the combined impedance (Z m ) will be explained.  FIG.  12    is a drawing showing a processing flow of the measurement individual difference correction system according to the second embodiment. The processes in the processing flow are performed with respect to each user. 
     Step S 201 : 
     First, the computation device  70  receives the capacitance to ground measured by the capacitance-to-ground measurement device  20  and regards the received capacitance to ground as the electrostatic capacity (C) between the earth ground or a conductor continuous with the earth ground and the lower electrode  12  of the voltage-to-ground measurement device  10  and as the electrostatic capacity (C a ) between the earth ground or a conductor continuous with the earth ground and the counter earth ground surface electrode  100 . In this situation, the computation device  70  may regard a value adjusted, for example, by multiplying the capacitance to ground measured by the capacitance-to-ground measurement device  20  by a prescribed value as the electrostatic capacity values (C and C a ). 
     Step S 202 : 
     Subsequently, the oscillation circuit  60  outputs a signal of the first one of frequencies. 
     Step S 203 : 
     After that, the computation device  70  assigns the output voltage (V) of the signal output from the oscillation circuit  60 , the measured voltage (V m ) measured by the voltage-to-ground measurement device  10 , and the electrostatic capacity values (C and C a ) obtained at step S 201  to Expression (5), and also assigns the electrostatic capacity (C e ) between the upper electrode  11  and the lower electrode  12  of the voltage-to-ground measurement device  10  and the output resistance value (R) of the resistance element  61  of the oscillation circuit  60 , which are default values, to Expression (5). In this manner, the computation device  70  calculates the combined impedance (Z m ) of the electrostatic capacity (C h ) between the hand of the user U and the grip cable  30 , the impedance (Z h ) of the user U, and the electrostatic capacity (C f ) between the sole of the foot of the user U and the upper electrode  11 . 
     Step S 204 : 
     Subsequently, the computation device  70  determines whether or not there is as unmeasured frequency among the plurality of frequencies that are predetermined. When there is at least one unmeasured frequency, the process proceeds to step S 205 . On the contrary, when there is no unmeasured frequency, the process proceeds to step S 207 . 
     Step S 204 : 
     When there is at least one unmeasured frequency, the computation device  70  instructs the oscillation circuit  60  to output a signal of the next frequency. 
     Step S 206 : 
     After that, on the basis of the instruction from the computation device  70  to output the signal, the oscillation circuit  60  outputs the signal of the next frequency, and the process returns to step S 203 . 
     Step S 207 : 
     When there is no unmeasured frequency, the computation device  70  transmits the measured combined impedance (Z m ) of each of the frequencies to the voltage-to-ground measurement device  10  and thus ends the process. 
     Subsequently, the voltage-to-ground measurement device  10  has stored therein the frequency characteristics of the combined impedance (Z m ) of each user transmitted thereto from the computation device  70 , so as to be kept in association with the ID of the user U. When the maintenance person U measures the voltage to ground of the electromagnetic noise, the voltage-to-ground measurement device  10  obtains the voltage to ground (V n ) of the electromagnetic noise for which the measurement error related to the individual difference of the human body has been corrected, by assigning the combined impedance (Z m ) of the maintenance person U to Expression (1) and changing the measured voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured at the time of the measurement process. 
     Advantageous Effects of the Second Embodiment 
     According to the second embodiment, provided are: the grip cable  30  that is a conductor; the counter earth ground surface electrode  100  provided on the floor; the oscillation circuit  60  that is connected between the grip cable  30  and the counter earth ground surface electrode  100  and that outputs the signal; the voltage-to-ground measurement device  10  that includes the upper electrode  11  and the lower electrode  12  positioned apart from each other and that measures the voltage between the upper electrode  11  and the lower electrode  12 ; the capacitance-to-ground measurement device  20  that measures the capacitance to ground; and the computation device  70  that communicates with the voltage-to-ground measurement device  10  and the capacitance-to-ground measurement device  20 . When the user U touches the grip cable  30  while wearing the shoes of which the sole of one shoe has the voltage-to-ground measurement device  10  installed therein and of which the sole of the other shoe has the capacitance-to-ground measurement device  20  installed therein, the computation device  70  calculates the combined impedance (Z m ) of the electrostatic capacity (C h ) between the hand of the user U and the grip cable  30 , the impedance (Z h ) of the user U, and the electrostatic capacity (C f ) between the sole of the foot of the user U and the upper electrode  11 , by using the voltage (V) of the signal output from the oscillation circuit  60 , the voltage (V m ) between the upper electrode  11  and the lower electrode  12  measured by the voltage-to-ground measurement device  10 , and the capacitance to ground values (=C and C a ) measured by the capacitance-to-ground measurement device  20 . It is therefore possible to provide the technique that makes it possible to correct the measurement error related to the individual difference of the human body that functions as a probe at the time of measuring the voltage to ground of the electromagnetic noise. As a result, it is possible to measure the voltage to ground of the electromagnetic noise with a higher level of precision. 
     &lt;Others&gt; 
     The present invention is not limited to the embodiments described above and may be modified in various manners within the scope of the gist thereof. 
     It is possible to realize the computation device  70  according to the first embodiment and the second embodiment by using, for example, a generic computer system including a central processing unit (CPU)  901 , a memory  902 , a storage  903  (a hard disk drive [HDD] and/or a solid state drive [SSD]), a communication device  904 , an input device  905 , and an output device  906 , as shown in  FIG.  13   . The memory  902  and the storage  903  are each a storage device. In the computer system, as a result of the CPU  901  executing a prescribed program loaded into the memory  902 , various functions of the computation device  70  are realized. 
     Further, the computation device  70  may be implemented by a single computer or may be implemented by two or more computers. Further, the computation device  70  may be a virtual machine installed in a computer. The program used by the computation device  70  may be stored in a computer-readable recording medium such as an HDD, an SSD, a Universal Serial Bus (USB) memory, a compact disc (CD), or a digital versatile disc (DVD) and may be distributed via a network. 
     REFERENCE SIGNS LIST 
       10  Voltage-to-ground measurement device 
       11  Upper electrode 
       12  Lower electrode 
       13  Voltage measurement circuit 
       20  Capacitance-to-ground measurement device 
       21  Upper electrode 
       22  First lower electrode 
       23  Second lower electrode 
       24  Oscillation circuit 
       25  Resistance element 
       26  Voltage measurement circuit 
       30  Grip cable 
       40  Floor gavel 
       50  Height adjustment table 
       60  Oscillation circuit 
       61  Resistance element 
       62  Oscillator 
       70  Computation device 
       80  Pedestal 
       90  Desk 
       100  Counter earth ground surface electrode 
       901  CPU 
       902  Memory 
       903  Storage 
       904  Communication device 
       905  Input device 
       906  Output device