JP2021004816A - Load calculation system - Google Patents

Abstract

To provide a load calculation system capable of highly accurately detecting a load of a rolling bearing with a simpler configuration.SOLUTION: A load calculation system 1 includes: a coil 20 in which a core material is fixed on the side surface orthogonal to the direction of a rotation shaft center Ax of a rolling bearing 10 and around which a conductor wire is wound; an exciting circuit 30 which applies an AC voltage of a predetermined frequency to the coil 20; and a current measuring circuit 40 which detects a current flowing through the conductor wire by applying an AC voltage; a voltage measuring circuit 50 for detecting the voltage generated in the conductor wire by applying the AC voltage; and an arithmetic device 60 for calculating a radial load R applied to the rolling bearing 10 based on the current and voltage.SELECTED DRAWING: Figure 1

Description

The present invention relates to a load calculation system.

In order to detect a radial load on a rolling bearing, a method of attaching a sensor to the outer peripheral side of the outer ring of the rolling bearing is known (for example, Patent Document 1).

JP-A-2017-026570

It is the inductance of the coil that functions as a sensor that changes with load. In Patent Document 1, the load is obtained by referring to the change in voltage caused by the change in inductance. In order to obtain the load with higher accuracy, it is desirable to obtain the inductance itself, but the method described in Patent Document 1 cannot obtain the inductance.

Further, in the method described in Patent Document 1, a space for mounting the sensor on the outer peripheral side of the bearing in the radial direction with respect to the outer ring is required. Further, in the method described in Patent Document 1, a reference sensor that is not affected by the load is required. As described above, the method described in Patent Document 1 has many design restrictions.

An object of the present invention is to provide a load calculation system capable of detecting the load of a rolling bearing with higher accuracy with a simpler configuration.

The load calculation system of the present invention for achieving the above object includes a rolling bearing, a coil in which a core material is fixed on a side surface orthogonal to the rotation axis direction of the rolling bearing and a lead wire is wound around the rolling bearing, and the coil. An excitation circuit that applies an AC voltage of a predetermined frequency, a current detection circuit that detects the current that flows through the lead wire by applying the AC voltage, and a voltage detection that detects the voltage generated by the lead wire by applying the AC voltage. It includes a circuit and a computing device that calculates a radial load applied to the rolling bearing based on the current and the voltage.

Therefore, the inductance of the coil corresponding to the load applied to the rolling bearing can be calculated based on the current and the voltage, and the load corresponding to the inductance can be derived. That is, since the inductance can be obtained from the current and the voltage, the inductance and the load corresponding to the inductance can be obtained with higher accuracy. Further, a space for providing a pressure detection sensor and a reference sensor on the outer peripheral side in the radial direction of the rolling bearing is not required.

In the load calculation system of the present invention, a plurality of the coils are provided, and the plurality of the coils are connected in series with the excitation circuit.

Therefore, it is possible to excite a plurality of coils with one excitation circuit. Further, it is not necessary to particularly control the timing of applying the alternating current to the plurality of coils, and the plurality of coils can be excited at the same time.

The load calculation system of the present invention includes a plurality of the coils and a plurality of the exciting circuits provided for each coil, and the application timings of the AC voltage by the plurality of the exciting circuits are synchronized.

Therefore, a plurality of coils can be individually excited. Therefore, it is possible to further reduce the possibility that an electrical change occurring in any of the plurality of coils affects the other coils. In addition, a plurality of exciting circuits synchronize and individually excite the coil. Therefore, the inductances of a plurality of coils corresponding to the loads at the same timing can be obtained, and the load can be obtained with higher accuracy.

According to the present invention, the load of the rolling bearing can be detected with higher accuracy with a simpler configuration.

FIG. 1 is a block diagram showing a main configuration of the load calculation system of the first embodiment. FIG. 2 is a cross-sectional view showing a main configuration of a rolling bearing with a sensor. FIG. 3 is a cross-sectional view showing another configuration example of the main configuration of the rolling bearing with a sensor. FIG. 4 is a diagram showing the relationship between the pitch angles of the two coils included in the load calculation system of the second embodiment and the predetermined pitch angles of a plurality of rolling elements included in the rolling bearing. FIG. 5 is a graph schematically showing the relationship between the inductance of each of the two coils and the combined inductance thereof and the radial load on the rolling bearing. FIG. 6 is a block diagram showing a main configuration of the load calculation system of the third embodiment. FIG. 7 is a block diagram showing a main configuration of the load calculation system of the fourth embodiment.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings, but the present invention is not limited thereto. The requirements of each embodiment described below can be combined as appropriate. In addition, some components may not be used.

(Embodiment 1)
FIG. 1 is a block diagram showing a main configuration of the load calculation system 1 of the first embodiment. The load calculation system 1 includes a rolling bearing 10, a coil 20, an excitation circuit 30, a current measurement circuit 40, a voltage measurement circuit 50, and a calculation device 60.

The rolling bearing 10 rotatably supports the rotating shaft S. The coil 20 is attached to the rolling bearing 10. The impedance of the coil 20 changes according to a change in the radial load R applied to the rolling bearing 10. The rolling bearing 10 and the coil 20 constitute a rolling bearing 5 with a sensor.

FIG. 2 is a cross-sectional view showing a main configuration of the rolling bearing 5 with a sensor. FIG. 2 is a cross-sectional view of the rolling bearing 5 with a sensor cut in a plane passing through the center Ax of the rotating shaft of the rolling bearing 10 and the coil 20. The rolling bearing 10 has an outer ring 11, an inner ring 12, a rolling element 13, and a cage 14. The outer ring 11 and the inner ring 12 are provided so as to be relatively rotatable with the rolling element 13 interposed therebetween. A plurality of rolling elements 13 are provided, and are arranged between the outer ring 11 and the inner ring 12 along the circumferential direction centered on the rotation axis center Ax. The outer ring 11 is arranged on the outer peripheral side and the inner ring 12 is arranged on the inner peripheral side with the rolling element 13 interposed therebetween. The cage 14 maintains the distance between the plurality of rolling elements 13 arranged in the circumferential direction. In the embodiment, a ball bearing in which the shape of the rolling element 13 is a sphere is adopted, but this is an example of the form of the rolling bearing 10 and is not limited to this. The rolling bearing 10 may be a roller bearing or a needle bearing in which the shape of the rolling element 13 is columnar. Further, the rolling bearing 10 may be a tapered roller bearing or a spherical roller bearing.

In the embodiment, the outer ring 11 is fixed and the inner ring 12 and the rotating shaft S are rotatably supported, but the inner ring 12 and the rotating shaft S are fixed and the outer ring 11 is rotatably supported. There may be. The outer ring 11 of the embodiment is made of a conductive and magnetic material such as bearing steel.

FIG. 3 is a cross-sectional view showing another configuration example of the main configuration of the rolling bearing with a sensor. The coil 20 includes a core material 21 and a lead wire 22. The core material 21 is fixed so that the end portion of the core material 21 is in close contact with the side surface of the outer ring 11. Specifically, for example, as shown in FIG. 2, the outer ring 11 and the core material 21 are fixed with an adhesive. Further, as shown in FIG. 3, a fixing member 25 attached to one side surface of the outer ring 11 may be used. The fixing member 25 is, for example, a ring-shaped member having an outer diameter similar to that of the outer ring 11. The fixing member 25 is provided with a hole 25a into which the coil 20 can be inserted so as to be located on the side of the outer ring 11. The coil 20 is fixed to the hole 25a by mechanical fastening (for example, press fitting or the like) or adhesion. The fixing member 25 illustrated in FIG. 3 has a protruding portion 25b that is press-fitted into a notch 11a provided on one side surface side of the outer ring 11, and is fixed to the outer ring 11 by press-fitting, but may be adhered. The side surface of the outer ring 11 to which the coil 20 is fixed is the side surface of the outer ring 11 orthogonal to the center Ax of the rotation axis. The core material 21 is constructed by using a material capable of increasing the magnetic permeability inside the lead wire 22 as compared with a hollow coil, for example, iron or an alloy or compound containing iron as a main material. The lead wire 22 is wound around the core material 21. The conducting wire 22 is, for example, a copper wire, but may be a conducting wire made of another material having conductivity.

The outer ring 11 and the core material 21 have conductivity. Therefore, the outer ring 11 and the core material 21 are electrically continuous. Therefore, the outer ring 11 affects the electrical characteristics of the core material 21 of the coil 20. The rolling bearing 5 with a sensor including the outer ring 11 and the core material 21 has a characteristic that the impedance of the coil 20 fixed to the outer ring 11 changes according to the change of the radial load R with respect to the rolling bearing 10. When the impedance of the coil 20 changes according to the change of the radial load R, the inductance generated in the coil 20 changes due to the alternating current applied from the exciting circuit 30. That is, there is a correlation between the radial load R and the inductance of the coil 20. Therefore, the radial load R can be obtained from the inductance of the coil 20.

The excitation circuit 30 shown in FIG. 1 applies an AC voltage having a predetermined frequency to the lead wire 22. The exciting circuit 30 is connected to a power supply device (not shown), receives power from the power supply device, and applies an alternating current to the lead wire 22 of the coil 20. The current measuring circuit 40 detects the current flowing through the lead wire 22 due to the application of the AC voltage. The current measurement circuit 40 of the embodiment is a circuit that functions as an AC ammeter. The voltage measuring circuit 50 detects the voltage generated in the lead wire 22 by applying the AC voltage. The voltage measuring circuit 50 of the embodiment is a circuit that functions as an AC voltmeter. The lead wire 22 of the coil 20, the exciting circuit 30, the current measuring circuit 40, and the voltage measuring circuit 50 are electrically connected via the lead wire M to form an electric circuit. The arithmetic unit 60 calculates the radial load R based on the current flowing through the conducting wire 22 and the voltage generated in the conducting wire 22. That is, the arithmetic unit 60 derives the radial load R corresponding to the obtained inductance by calculating the inductance of the coil 20 based on the current flowing through the conducting wire 22 and the voltage generated in the conducting wire 22.

Based on the current value I flowing through the conducting wire 22 and the voltage value V generated in the conducting wire 22 by the alternating current applied to the coil 20 by the exciting circuit 30, the arithmetic unit 60 uses the equation (1) to determine the impedance Z of the coil 20. Is calculated. The voltage value V is the voltage between both ends of the lead wire 22 wound around the core material 21. The voltage measuring circuit 50 detects the voltage value V between both ends of the conducting wire 22 wound around the core material 21.

Since the coil 20 is present in the AC current transmission line from the excitation circuit 30, the phase of the current is delayed with respect to the phase of the voltage due to the self-induction of the current value I. Therefore, a phase difference θ is generated between the current value I and the voltage value V. The arithmetic unit 60 calculates the inductance L using the equation (2) based on the phase difference θ of the current value I and the voltage value V and the impedance Z. F in the formula (2) represents the frequency of the alternating current (predetermined frequency). For example, f = 128 [khz], but the present invention is not limited to this, and can be changed as appropriate. The unit of the phase difference θ is [degree (°)]. The phase difference θ is obtained based on the change in the current value I and the change in the voltage value V that occur within the same time according to the frequency (f). For example, the phase of the current value I lags the phase of the voltage value V by 90 degrees.

As shown in the equation (3), there is a correlation between the value of the inductance L and the value of the radial load R (Fr). A in the formula (3) is a coefficient set according to the sensitivity of the coil 20 with respect to the radial load R. b is an initial value of the inductance L (inductance measured when Fr = 0). a and b can be determined based on the results of prior experiments and measurements that change the radial load R applied to the rolling bearing 10 provided with the coil 20. The arithmetic unit 60 can obtain the radial load value Fr from the inductance L by using the equation (4) derived from the equation (3).

In equation (3), the correlation between the inductance L and the radial load R is shown by the correction equation by first-order approximation using a and b, but depending on the configuration conditions of the coil 20, the condition such as the second-order correction equation may be used. The correction formula can be adjusted accordingly.

Although the derivation of the radial load R by the arithmetic unit 60 has been described above, the arithmetic unit 60 may be a dedicated circuit for performing the above-mentioned processing, or executes a software program including the above-mentioned processing contents. It may be a computer.

The mechanism for the arithmetic unit 60 to obtain the current value I and the voltage value V from the current detected by the current measuring circuit 40 and the voltage detected by the voltage measuring circuit 50 is arbitrary. For example, the current measured by the current measuring circuit 40 and the voltage measured by the voltage measuring circuit 50 are converted into digital information by using an A / D converter (not shown) or a circuit having a similar function to the arithmetic unit 60. The arithmetic unit 60 includes an A / D conversion function to be input or related. Further, the current measurement circuit 40 and the voltage measurement circuit 50 may each have an A / D conversion function, and may be configured to output the digitized current value I and voltage value V to the arithmetic unit 60.

According to the first embodiment, the inductance L of the coil 20 corresponding to the radial load R applied to the rolling bearing 10 is calculated based on the current value I and the voltage value V, and the radial load R corresponding to the inductance L is derived. Can be done. That is, since the inductance L can be obtained from the current value I and the voltage value V, the radial load R corresponding to the inductance L and the inductance L can be obtained with higher accuracy. Further, it is possible to provide the coil 20 in the direction in which the center Ax of the rotation axis of the outer ring 11 extends without requiring a space for providing the pressure detection sensor and the reference sensor on the outer peripheral side in the radial direction of the outer ring 11. If there is, the radial load R can be obtained. Further, the radial load R can be obtained with a simpler configuration without the need for an expensive system using a reference sensor, a load cell, a strain gauge amplifier, or the like.

(Embodiment 2)
FIG. 4 is a diagram showing the relationship between the pitch angles P2 of the two coils 20 included in the load calculation system 1A of the second embodiment and the predetermined pitch angles P1 of the plurality of rolling elements 13 included in the rolling bearing 10. The pitch angle centered on the rotation axis center Ax of the two rolling elements 13 adjacent to each other in the circumferential direction among the plurality of rolling elements 13 included in the rolling bearing 10 is a predetermined pitch angle P1 defined by the cage 14. .. In the example shown in FIG. 4, the rolling elements 13a and the rolling elements 13b are adjacent to each other in the circumferential direction, and the pitch angle centered on the rotation axis center Ax is a predetermined pitch angle P1. The pitch angle between the two rolling elements 13 adjacent to each other in the circumferential direction, including the rolling elements 13b and the rolling elements 13c, is a predetermined pitch angle P1, not limited to the rolling elements 13a and the rolling elements 13b. For example, when the number of rolling elements 13 included in one rolling bearing 10 is 8, the predetermined pitch angle P1 is 45 degrees (°).

In the second embodiment, one rolling bearing 10 is provided with two coils 20. In FIG. 4, for the purpose of distinguishing two coils 20 having different arrangements in the circumferential direction centered on the center Ax of the rotation axis, a is added to the end of one code and b is added to the end of the other code. ing. The pitch angle P2 of the coil 20a and the coil 20b adjacent to each other about the center Ax of the rotation axis is half the angle of the predetermined pitch angle P1. The coil 20a and the coil 20b have the same configuration as the coil 20 described above, except that the arrangement in the circumferential direction about the center Ax of the rotation axis is different.

FIG. 5 is a graph schematically showing the relationship between the respective inductances La and Lb of the two coils 20a and 20b and their combined inductance Lc and the radial load R on the rolling bearing 10.

The rolling element 13 revolves in the circumferential direction around the center Ax of the rotation axis as the outer ring 11 and the inner ring 12 rotate relative to each other. Therefore, the positional relationship between the coil 20 and the rolling element 13 changes with the relative rotation of the outer ring 11 and the inner ring 12.

The rolling element 13 of the embodiment is a magnetic material. This is because a magnetic material such as bearing steel is used as the material of the rolling element 13. When the positional relationship between the coil 20 and the rolling element 13 changes with the relative rotation of the outer ring 11 and the inner ring 12, the degree of magnetic influence of the rolling element 13 on the coil 20 changes. Due to such a change in the degree of magnetic influence, the inductance L obtained from the current value I and the voltage value V of one coil 20 has a waveform like a sine curve as shown in the inductances La and Lb in FIG. While drawing, it increases (or decreases) as a whole as the radial load R increases (or decreases). That is, although the inductance L of one coil 20 should ideally increase (or decrease) linearly with an increase (or decrease) of the radial load R, the rolling element 13 is a coil. It increases (or decreases) while drawing a waveform like a sine curve depending on the degree of magnetic influence on 20. As described above, the change in the positional relationship between the rolling element 13 and the coil 20 causes an error in the relationship between the inductance L and the radial load R.

The relationship between the inductance L and the radial load R indicated by each of the inductances La and Lb becomes an error in obtaining the correct radial load R for the amplitude of the waveform. The current value I and the voltage value V are detected at a plurality of timings, and the moving average lines L1 and L2 of the inductance L calculated based on the current value I and the voltage value V at each timing are obtained, and the intermediate values thereof are used. It is also possible to reduce the influence of error by deriving the combined inductance Lc. However, in this method, it is difficult to improve the calculation accuracy of the radial load R until the current value I and the voltage value V are detected a plurality of times, which causes a time lag. That is, it is difficult to improve the response speed related to the sensing of the radial load R only by the moving average.

On the other hand, according to the second embodiment, the pitch angle P2 of the coil 20a and the coil 20b adjacent to each other about the center Ax of the rotation axis is half the angle of the predetermined pitch angle P1. Therefore, considering the case where the center of the rolling element 13 is within the range of the pitch angle P2, when the rolling element 13 moves away from one of the coil 20a and the coil 20b, the other and the rolling element 13 come close to each other. When one of the coil 20a and the coil 20b has the shortest distance to the rolling element 13, the other has the longest distance to the rolling element 13. When the relationship between the inductance L of one of the coils 20a and the coil 20b and the radial load R becomes like the inductance La due to the positional relationship between the coils 20a and 20b and the rolling element 13, the other inductance L and the radial The relationship with the load R is like the inductance Lb. Therefore, the combined inductance Lc, which is the sum of the inductances La and Lb of the coils 20a and 20b and divided by 2, draws a substantially ideal linear line in the graph showing the relationship between the inductance L and the radial load R.

As described above, the load calculation system 1A of the second embodiment is the same as the load calculation system 1 of the first embodiment, except for the matters specially mentioned. According to the second embodiment, by setting the pitch angle P2 of the coil 20a and the coil 20b adjacent to each other about the rotation axis center Ax to half the predetermined pitch angle P1, the presence or absence of rotation and the rotation angle of the rolling bearing 10 It is possible to derive the radial load R with higher accuracy by suppressing the influence of the above. That is, according to the rolling bearing 5A with a sensor of the embodiment, it is possible to obtain information for stably obtaining the radial load R regardless of the position of the rolling element 13.

(Embodiment 3)
FIG. 6 is a block diagram showing a main configuration of the load calculation system 1B of the third embodiment. In the third embodiment, the plurality of coils 20 are connected in series with the arithmetic unit 60. The current measurement circuit 40 and the voltage measurement circuit 50 of the third embodiment are provided for each coil 20. The first current measuring circuit 40a detects the current flowing through the lead wire 22 of the coil 20a. The second current measuring circuit 40b detects the current flowing through the lead wire 22 of the coil 20b. The first voltage measuring circuit 50a detects the voltage generated in the conducting wire 22 of the coil 20a. The second voltage measuring circuit 50b detects the voltage generated in the lead wire 22 of the coil 20b. Except for these special matters, the first current measuring circuit 40a and the second current measuring circuit 40b are the same as the current measuring circuit 40 of the first embodiment, and the first voltage measuring circuit 50a and the second voltage measuring circuit 50b are This is the same as the voltage measuring circuit 50 of the first embodiment.

The excitation circuit 30 of the third embodiment applies an alternating current to two coils 20 connected in series via a lead wire M. In FIG. 6, the coil 20a and the coil 20b are connected in series with the excitation circuit 30. The exciting circuit 30 applies an alternating current to the coils 20a and 20b. As described above, the load calculation system 1B of the third embodiment is the same as the load calculation system 1 of the first embodiment, except for the matters specially mentioned.

According to the third embodiment, one excitation circuit 30 can excite a plurality of coils 20. Further, it is not necessary to particularly control the timing of applying the alternating current to the plurality of coils 20, and the plurality of coils 20 can be excited at the same time.

(Embodiment 4)
FIG. 7 is a block diagram showing a main configuration of the load calculation system 1C of the fourth embodiment. The load calculation system 1C of the fourth embodiment includes a plurality of coils 20 and a plurality of excitation circuits 30 provided on the coils 20. Further, in the fourth embodiment, the application timings of the AC voltage by the plurality of exciting circuits 30 are synchronized. The first current measurement circuit 40a, the second current measurement circuit 40b, the first voltage measurement circuit 50a, and the second voltage measurement circuit 50b of the fourth embodiment are the same as those of the third embodiment.

The excitation circuit 30 of the fourth embodiment is provided for each coil 20. The first excitation circuit 30a applies an AC voltage having a predetermined frequency to the lead wire 22 of the coil 20a. The second excitation circuit 30b applies an AC voltage of a predetermined frequency to the lead wire 22 of the coil 20b. Except for these special matters, the first exciting circuit 30a and the second exciting circuit 30b are the same as the exciting circuit 30 of the first embodiment.

In the synchronous circuit 70, the first exciting circuit 30a and the second exciting circuit 30b have the same timing so that the exciting timing of the coil 20a by the first exciting circuit 30a and the exciting timing of the coil 20b by the second exciting circuit 30b are the same. Performs synchronous processing to control the operation. As an example of a specific form, the synchronization circuit 70 includes a clock circuit that outputs a clock pulse used for controlling the excitation timing to the first excitation circuit 30a and the second excitation circuit 30b. The first excitation circuit 30a and the second excitation circuit 30b each output an alternating current under timing control by a clock pulse. As described above, the load calculation system 1C of the fourth embodiment is the same as the load calculation system 1 of the first embodiment, except for the matters specially mentioned.

According to the fourth embodiment, the plurality of coils 20 can be individually excited. Therefore, it is possible to further reduce the possibility that an electrical change generated in any of the plurality of coils 20 affects the other coils 20. Further, the plurality of exciting circuits 30 synchronize and individually excite the coil 20. Therefore, the inductances L of the plurality of coils 20 corresponding to the radial load R at the same timing can be obtained, and the radial load R can be obtained with higher accuracy.

The excitation of the coils 20a and 20b in the second embodiment may be performed by one exciting circuit 30 as in the third embodiment, or by the individual first exciting circuit 30a and the second exciting circuit 30b as in the fourth embodiment. You may go.

The two coils 20 in the third and fourth embodiments may be the two coils 20a and 20b arranged as in the second embodiment, or the two coils 20 having other arrangement relationships. Good. Further, in the third embodiment and the fourth embodiment, three or more coils 20 may be provided. In that case, in the third embodiment, three or more coils 20 are connected in series with the excitation circuit 30. In the fourth embodiment, the excitation circuits 30 are individually provided for each of the three or more coils 20, and the three or more excitation circuits 30 are synchronized under the control of the synchronization circuit 70.

In the configuration including the plurality of coils 20 as in the second embodiment, the third embodiment and the fourth embodiment, it is possible to measure a load other than the radial load R applied to the rolling bearing 10. Specifically, the radial load applied to the rolling bearing 10 at the timing when the inductance L of the plurality of coils 20 arranged at different positions with respect to the outer ring 11 and the inductance L of the plurality of coils 20 are obtained. The correlation with R, axial load, presence / absence of moment load, and magnitude is measured in advance. By storing and holding the data indicating the measurement result in a referenceable manner from the arithmetic unit 60, the radial load applied to the rolling bearing 10 by collating the inductance L of the plurality of coils 20 calculated by the arithmetic unit 60 with the data. R, axial load and moment load can be derived.

1,1B, 1C Load calculation system 10 Rolling bearing 11 Outer ring 12 Inner ring 13 Rolling element 14 Cage 20 Coil 21 Core material 22 Lead wire 30 Excitation circuit 30a First excitation circuit 30b Second excitation circuit 40 Current measurement circuit 40a First current measurement Circuit 40b Second current measurement circuit 50 Voltage measurement circuit 50a First voltage measurement circuit 50b Second voltage measurement circuit 60 Computing device 70 Synchronous circuit

Claims (3)

Rolling bearings and
A coil in which a core material is fixed and a lead wire is wound on a side surface orthogonal to the direction of rotation of the rolling bearing.
An exciting circuit that applies an AC voltage of a predetermined frequency to the coil,
A current detection circuit that detects the current flowing through the lead wire by applying the AC voltage, and
A voltage detection circuit that detects the voltage generated in the lead wire by applying the AC voltage, and
A load calculation system including an arithmetic unit that calculates a radial load applied to the rolling bearing based on the current and the voltage. A plurality of the coils are provided,
The load calculation system according to claim 1, wherein the plurality of the coils are connected in series with the excitation circuit. With the plurality of the coils
It is equipped with a plurality of the excitation circuits provided for each coil.
The load calculation system according to claim 1, wherein the timing of applying the AC voltage by the plurality of excitation circuits is synchronized.

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