JPH079164A - Instrument and method for measuring resistance between electrodes of resistance welding machine - Google Patents

Abstract

PURPOSE:To easily measure accurate resistance between electrodes of the resistance welding machine. CONSTITUTION:The resistance between electrodes measuring instrument of the resistance welding machine is provided with a voltage detecting means C1 which detects the voltage (detection voltage) between both electrodes A, a current differentiation value detecting means D1 which detects a current differentiation value being the value obtained by differentiating timewise a current flowing between both electrodes A, a current value calculating means E1 which integrates timewise the current differentiation value and calculates a current value flowing between both electrodes A, a voltage integrating means F11 which integrates timewise the detection voltage between current coincident timings which are between two timings when sizes of the current value coincide mutually, a current integrating means G11 which integrates timewise the current value between current coincident timings and a dividing means H11 which divides an integration value by the voltage integrating means F11 by an integration value by the current integrating means G11.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for measuring the resistance between two electrodes in a resistance welding machine in which a welding current is passed between two electrodes sandwiching a work to weld the work by Joule heat. It is about the method. And, in particular, the welding current flowing between both electrodes changes with time, and when an attempt is made to measure the voltage between both electrodes, an induced voltage is generated due to the temporal change of the welding current. In some cases, the present invention relates to an apparatus and method for measuring an accurate resistance that does not include a value related to the induced voltage component.

[0002]

2. Description of the Related Art As shown in FIG. 5, in a resistance welding machine 40, two works W, W are placed between two electrodes 44a, 44b.
, A large welding current i is caused to flow between the electrodes 44a and 44b, and the workpieces W and W are welded by the Joule heat generated by the current i. It is known that the resistance and voltage between the electrodes 44a and 44b change during the welding process by the resistance welding machine. Therefore, by measuring the resistance and voltage between the electrodes, it is possible to know to what stage welding has progressed at that time. Then, by appropriately applying pressure to the electrodes 44a and 44b according to the welding stage, good welding can be performed.

Therefore, for that purpose, both electrodes 44a,
It is necessary to accurately measure the resistance or voltage between 44b. However, in the case where the welding current i flowing through the arm 42b from the arm 42a to the electrodes 44a and 44b in FIG. 5 changes with time, the lead wires 72a and 72b connected to the electrodes 44a and 44b are connected.
To the respective arms 42a, 42b, and its detection end 74
When the voltage between a and 74b is detected, the detected voltage V is as follows. V = V ₀ + M · di / dt Here, V ₀ is an inter-electrode voltage, and is expressed as V ₀ = R · i (R is an inter-electrode resistance). M is a lead wire 72a,
The mutual inductance between 72b and the arms 42a and 42b. That is, the detection voltage V is the inter-electrode voltage V _0.
= R · i as well as a value obtained by adding the induced voltage M · di / dt induced in both lead wires 72a, 72b by the current i flowing through both arms 42a, 42b. Since this induced voltage component is included, the inter-electrode voltage V ₀ cannot be accurately determined, and the inter-electrode resistance cannot be accurately determined.

Under such a background, the inter-electrode voltage V₀Seeking
A device intended to be mounted is disclosed in JP-A-62-1138.
No. 6 publication. With this device, Rogowski
Depending on the coil, the differential value e (t) =-M 'of the welding current i
-Di / dt is calculated (in the publication, -M * di / dt
(Represented), it is amplified by the amplifier with the amplification factor α
α · e (t) = − α · M ′ · di / dt. That
This value is the above-mentioned detection voltage V = V₀+ M · di / dt
(V in the publication ₀+ K · di / dt)
Calculated as follows. V + α ・ e (t) = V₀+ M ・ di / dt-α ・ M '・
di / dt = V₀+ (M−α · M ′) · di / dt Then, the value of the amplification factor α is set to α = M / M ′.
The inter-electrode voltage V₀Is V + α · e (t) = V₀
= R · i.

However, the apparatus described in the above publication does not disclose a method of setting the value of the amplification factor α in the amplifier as α = M / M '. For this reason, the value of the amplification factor α is changed many times by trial and error, and the induction component (M−α ·
There is no choice but to guide it so that M ′) · di / dt becomes zero. However, in that case, the procedure is very complicated, and it is difficult to accurately determine the inter-electrode voltage V ₀ = R · i. Therefore, it is also difficult to measure the interelectrode resistance accurately.

Therefore, it is an object of the present invention to provide an apparatus and method capable of easily measuring the accurate interelectrode resistance of a resistance welding machine.

[0007]

In order to solve this problem, the invention according to claim 1 is, as schematically shown in FIG. 1, temporally between two electrodes A and A sandwiching a work W. A device B11 for measuring the resistance between both electrodes A and A in a resistance welding machine that welds a work W by applying a current that changes to a voltage, and is a voltage for detecting the voltage between both electrodes A and A. A detection means C1, a current differential value detection means D1 for detecting a current differential value which is a value obtained by temporally differentiating a current flowing between the electrodes A and A, and both electrodes by integrating the current differential value with time. Between the current value calculation means E1 for calculating the current value flowing between A and A and the current matching timing which is between two timings at which the magnitudes of the current values match each other
The voltage integration unit F11 that temporally integrates the voltage detected by the voltage detection unit C1, the current integration unit G11 that temporally integrates the current value between the current coincidence timings, and the integration value obtained by the voltage integration unit F11. And a dividing means H11 for dividing by the integrated value of the current integrating means G11.

The invention according to claim 2 is also the same as in FIG.
Between the two electrodes A and A in a resistance welding machine for welding the work W by Joule heat by passing a time-varying current between the two electrodes A and A sandwiching the work W. A device B12 for measuring the resistance of the device, a voltage detecting means C1 for detecting a voltage between the electrodes A and A, and a current differential value which is a value obtained by temporally differentiating a current flowing between the electrodes A and A. A current differential value detecting means D1 for detecting, a current value calculating means E1 for temporally integrating the current differential value to calculate a current value flowing between both electrodes A, A, and a current differential value for temporal integration. Voltage integration means F12 for temporally integrating the voltage detected by the voltage detection means C1 between the integration start timing from the start of integration and the timing at which the integration value becomes zero, and the integration value. Between zero timing A current integrating means G12 for integrating the serial current time, and having a dividing means H12 for dividing the integral value an integration value by the voltage integration means F12 by the current integrating means G12.

Further, in the invention according to claim 3, as schematically shown in FIG. 2, a time-varying current is caused to flow between the two electrodes A, A sandwiching the work W, and the work is heated by the Joule heat. Both electrodes A in the resistance welding machine for welding W,
A method B21 for measuring the resistance between the two electrodes A and B,
A voltage detection step C2 for detecting the voltage between A and both electrodes A,
A current differential value detection step D2 for detecting a current differential value which is a value obtained by temporally differentiating a current flowing between A, and a current value flowing between both electrodes A, A by temporally integrating the current differential value. Between the current value calculation step E2 to be calculated and the current coincidence timing between the two timings when the magnitudes of the current values are mutually equal, a voltage integration step of temporally integrating the voltage detected in the voltage detection step C2. F21, a current integration step G21 for temporally integrating the current value between the current coincidence timings, and a division step H2 for dividing the integrated value by the voltage integration step F21 by the integrated value by the current integration step G21.
1 and 1.

The invention according to claim 4 is the same as in FIG.
Between the two electrodes A and A in a resistance welding machine for welding the work W by Joule heat by passing a time-varying current between the two electrodes A and A sandwiching the work W. A method B22 for measuring the resistance of the electrode, the voltage detection step C2 for detecting the voltage between the electrodes A and A, and the current differential value that is a value obtained by temporally differentiating the current flowing between the electrodes A and A. A current differential value detection step D2 for detecting, a current value calculation step E2 for temporally integrating the current differential value to calculate a current value flowing between both electrodes A, A, and a current differential value for time integration A voltage integration step F22 for temporally integrating the voltage detected in the voltage detection step C2 between the integration start timing from the integration start timing to the timing when the integration value becomes zero; Between zero timing A current integrator stage G22 integrating the serial current time, and having a division step H22 dividing by the integral value integrated value by the voltage integration phase F22 by the current integration phase G22.

[0011]

In the first aspect of the invention, first, the voltage detecting means C1 detects the voltage (detection voltage) between the electrodes A and A. The detected voltage includes not only the voltage between the electrodes A and A, but also the induced voltage component due to the temporal change of the current flowing between the electrodes A and A. on the other hand,
The current differential value detecting means D1 detects a current differential value which is a value obtained by temporally differentiating the current flowing between the electrodes A and A, and the current differential value is temporally integrated by the current value calculating means E1. The current value flowing between the two electrodes A, A is calculated. The voltage integrator F11 temporally integrates the detected voltage between two timings (between current coincidence timings) in which the magnitudes of the current values match each other. At that time, the induced voltage component included in the detected voltage is proportional to the current differential value, and the integration range (between the current coincidence timings) by the voltage integrating means F11 is the both electrodes A, A as described above. Since the current values flowing between the current values (which are calculated by integrating the current differential value by the current value calculating means E1) match each other, the induced voltage components thereof are distributed over the current matching timings. The integrated value is zero.
That is, the induced voltage component included in the detected voltage is removed by being integrated by the voltage integrating means F11 over the current coincidence timing. On the other hand, the current integrator G11 temporally integrates the current value between the current coincidence timings. And
The dividing means H11 divides the integrated value by the voltage integrating means F11 by the integrated value by the current integrating means G11 to obtain the average resistance between the current coincidence timings. At that time, since the integrated value by the voltage integrating means F11 does not include the induced voltage component as described above, the resistance is a resistance between the electrodes A and A that does not include a value related to the induced voltage component. . In this way, the inter-electrode resistance measuring device B11 of the invention according to claim 1 can easily measure the accurate resistance between the electrodes A and A that does not include the value related to the induced voltage component.

In the invention according to claim 2, first, similarly to the invention according to claim 1, the voltage (detection voltage) between both electrodes A and A is detected by the voltage detection means C1 to detect the current differential value. A current differential value, which is a value obtained by temporally differentiating the current flowing between the electrodes A and A, is detected by the means D1.
The current value calculating means E1 calculates the value of the current flowing between the electrodes A and A. Then, during the period from the integration start timing when the voltage differential means F12 starts to integrate the current differential value temporally to the timing when the integrated value becomes zero (between the integrated value zero timing), the detected voltage is temporally changed. Integrated. At that time, the induced voltage component included in the detected voltage is proportional to the current differential value, and the range of integration by the voltage integrating means F12 (between the integration value zero timings) temporally changes the current differential value. Since the range is from the integration start timing when the integration starts to the timing when the integrated value becomes zero, the value of the induced voltage component integrated over the integrated value zero timing becomes zero. That is, the induced voltage component included in the detected voltage is removed by being integrated by the voltage integrating means F12 over the integrated value zero timing. On the other hand, the current integrator G12 temporally integrates the current value during the zero time of the integrated value. Then, the dividing means H12 divides the integrated value by the voltage integrating means F12 by the integrated value by the current integrating means G12 to obtain the average resistance between the integrated value zero timings. At that time, since the integrated value by the voltage integrating means F12 does not include the induced voltage component as described above, the resistance thereof is the resistance between the electrodes A and A which does not include the value related to the induced voltage component. . In this way, the interelectrode resistance measuring device B1 of the invention according to claim 2 is provided.
Even with 2, the accurate resistance between the electrodes A and A, which does not include the value related to the induced voltage component, can be easily measured.

In the invention according to claim 3, the processing by each means C1 to H11 of the invention according to claim 1 is
The resistance between both electrodes A and A is easily measured by performing the corresponding steps C2 to H21.

Further, in the invention according to claim 4, the processing by each means C1 to H12 of the invention according to claim 2 is
The resistance between both electrodes A and A is easily measured, which is carried out in corresponding steps C2 to H22.

[0015]

【Example】

<First Embodiment> Next, an embodiment in which the inventions of claims 1 and 3 are embodied will be described with reference to FIGS. Figure 3
As shown in FIG. 4 and FIG.
0 (FIG. 3) and various circuits 12, 14,
16 etc. (Fig. 4). Resistance welding machine 4 of them
0 (no reference numeral in FIGS. 3 and 4) is as shown in FIG.
It has a pair of arms 42a and 42b, and electrodes 44a and 44b (corresponding to electrodes A and A) are provided at the tips thereof (see also FIGS. 3 and 4). Then, by the power supply circuit 30 and the switching circuit 32 in FIG.
A welding current i is applied to the arm 42b from a through the electrodes 44a and 44b, and the workpieces W and W are welded by the Joule heat generated in the workpieces W and W sandwiched between the electrodes 44a and 44b. is there.

First, the power supply circuit 30 and the switching circuit 32 in FIG. 3 will be described with reference to FIG. The current from the AC power supply 50 is rectified by the rectifier circuit 52 into a pulsating current, smoothed by the capacitor 54, and flows through the transistor circuit 56. The switching circuit 32 that outputs a pulsed switching signal for controlling the inverter is connected to the transistors 58a and 58b of the transistor circuit 56. In the switching circuit 32, ON signals are alternately output from the signal output lines 60a and 60b with a time interval in which the OFF signals are output from both the signal output lines 60a and 60b. As a result, the transistor 58a is turned on, the transistors 58a and 58b are turned off, the transistor 58b is turned on, and the transistors 58a and 58b are turned off (hereinafter, the transistor 58a indicates that one of the transistors 58a and 58b is on. It is said that the transistors 58a and 58b are in the on state, and the fact that both transistors 58a and 58b are in the off state means that the transistor 58 is in the off state.) On the primary coil 64 side of the transformer 62, the time during which no current flows in either direction An electric current flows alternately in each direction sandwiching it. Then, through the transformer 62 and the diode 66, the electrodes 44a, 4
As shown in Fig. 7 (1), a welding current i that has been transformed, rectified, and considerably smoothed flows between 4b. That is, when the transistor 58 is turned on, the current i gradually increases, and when the transistor 58 is turned off, the current i gradually decreases, and the value of the current i is triangular as shown in the figure. It becomes. In this way, the welding current i changes with time. The graph of FIG. 7 (1) is calculated from the graph of FIG. 7 (2) as follows.

As shown in FIG. 3, one electrode 44a is provided with a toroidal coil 70. Based on the toroidal coil 70, the current differential value detecting circuit 12 (corresponding to the current differential value detecting means D) causes the electrodes 44a, A value (current differential value) di / d obtained by temporally differentiating the welding current i flowing between 44b.
t is detected. The detection result is shown in Fig. 7 (2). Then, while the welding current i is being conducted between the electrodes 44a and 44b by the energization detection circuit 17, the current differential value di / dt in FIG. The value of the welding current i shown in FIG. 7 (1) is calculated by the integration by the calculation means E1).

Further, as shown in FIGS. 3 to 6, each electrode 4
Lead wires 72a and 72b are connected to the wires 4a and 44b, and the lead wires 72a and 72b are connected to the arms 42a and 4b.
2b, and its detection ends 74a and 74b are connected to the voltage detection circuit 14 (corresponding to the voltage detection means C). The detection terminal voltage V is expressed by the following equation, and the detection result is as shown in FIG. 7 (3). V = V ₀ + M · di / dt Here, V ₀ is an inter-electrode voltage, and is expressed as V ₀ = R · i (R is an inter-electrode resistance). Also, M is a lead wire 7.
It is the mutual inductance between 2a, 72b and the arms 42a, 42b. That is, the lead wires 72a, 72b
, The welding current i flowing through the arms 42a and 42b
As a result, an induced voltage M · di / dt is generated, so that a voltage obtained by adding the induced component to the inter-electrode voltage V ₀ is detected between the detection ends 74a and 74b of the lead wires 72a and 72b.

On the other hand, the detection level setting circuit 20 outputs a reference current is as shown by a broken line in FIG. 7 (1). As shown in the figure, the reference current is is set to be an intermediate value between a plurality of maximum values and a plurality of minimum values of the welding current i. Then, by the level determination circuit 21,
The values of the welding current i and the reference current is are compared, and the timings t ₁ , t ₂ , t ₃ , ...
_2n-1 , t _2n , ... _Are detected. Then, between the adjacent timings (the timings t ₁ and t _{2, the} times t ₃ and t ₄ , ..., T) where the welding current i has a larger value than the reference current is
_{Between 2n-1} and _t2n , ...) is T1, T during the current coincidence timing.
, ..., Tn, ..., At the beginning of each current coincidence timing (timing t ₁ , t ₃ , ..., T _2n-1 , ...), the integration start signal from the timing generation circuit 22 is given to each integration circuit 24 ,.
25, and the integration end signal is output at the end of each current coincidence timing (timing t ₂ , t ₄ , ..., T _2n , ...). Based on each of these signals, the integrating circuit 24 (corresponding to the voltage integrating means F11) causes a voltage V between the detection terminals.
Is integrated (constant integration) for each current matching timing,
The voltage integrated values SV1, SV2, ..., SVn, ...
Is output. The voltage integrated values SV1, SV2, ..., S
The detection result of Vn, ... Is as shown in FIG. 7 (6). In addition,
Hereinafter, as for the value related to the definite integral, the one having the subscript n is appropriately expressed as a representative value.

[Equation 1] Here, since the welding current i at the start (timing t _2n-1 ) and the end (timing t _2n ) of the current matching timing Tn is the same value at i = is as described above, it is the differential value di. The value obtained by definite integration of / dt during the current coincidence timing becomes zero, and Equation 2 holds for SVn of Equation 1.

[Equation 2] That is, the voltage V between the detection ends is Tn during the current coincidence timing.
The value relating to the inductive component M · di / dt is removed from the inter-detection-end voltage V by the definite integration at.

Similarly, the integrator circuit 25 (corresponding to the current integrator G11) integrates the welding current i at each current coincidence timing Tn, and the integrated current values SI1, SI2, ... SIn, ... Are output. The detection result of the integrated current values SI1, SI2, ..., SIn, ... Is as shown in FIG. 7 (7).

[Equation 3] Then, the values of the voltage integrated value SVn and the current integrated value SIn are temporarily held in the sample hold circuits 26 and 27, and the voltage integrated value SVn is divided by the current integrated value SIn by the division circuit 28 (corresponding to the dividing means H11). , Divided values D1, D2, ..., Dn, ... Shown in Equation 4 are calculated.

[Equation 4] That is, the divided value Dn is obtained by dividing the value SVn obtained by integrating the inter-electrode voltage V ₀ at the current matching timing Tn by the value obtained by integrating the welding current i at the current matching timing Tn. The average inter-electrode resistance R in the interval Tn is obtained. As described above, since the value related to the inductive component is removed from the voltage integrated value SVn, the resistance R is an accurate interelectrode resistance that does not include the value related to the inductive component.

As described above, according to this apparatus and method, it is possible to easily measure the interelectrode resistance that does not include the value related to the inductive component.

During the current coincidence timings in the inventions according to claims 1 and 3, between the adjacent timings (timing t ₁ where the welding current i is larger than the reference current is as in the first embodiment). between · t _2, t ₃ · t
₄₎ , ..., Between t _2n-1 and t _2n , ...), but also between timings t ₁ and t _3, between timings t ₁ and t ₄ , and timing t
Every time the welding current i matches the reference current is, such as ₂ · t _3, it corresponds to the current matching timing. Further, even if the concept of the reference current is not used, it corresponds to the current coincidence timing as long as the current values coincide with each other.

<Second Embodiment> Next, an embodiment in which the inventions of claims 2 and 4 are embodied will be described with reference to FIG. 3 and FIGS. I will explain mainly. In this embodiment, there are the following differences in the method of obtaining the timing between the definite integration in the first embodiment. Therefore, in the interelectrode resistance detection circuit 102 in FIG. 3, an integration circuit 90 and a timing detection circuit 91 are provided as shown in FIG. 8 instead of the detection level setting circuit 20 and the level determination circuit 21 in FIG. Has been.

First, the value of the current differential value di / dt shown in FIG. 7 (2) is integrated by the integrating circuit 16 while the welding current i is being supplied, and the welding current i shown in FIG. 7 (1) is obtained. Desired. This point is similar to the first embodiment. On the other hand, as shown in FIG. 7 (3), the timing when the welding current i is increasing (time _{t 1, t 3, ... t} 2n-1, ...) from the current by the integrating circuit 90 differential values di The value of / dt is integrated to calculate the current differential integration value In shown in Equation 5 (this value only differs from the value of the welding current i by a predetermined constant value).

[Equation 5]Then, the first tie when this current calculus value I becomes zero
Ming (timing t ₂, T_Four, ... t_2n,…) Is Taimi
Detected by the ringing detection circuit 91, and the integration start timing is detected.
Timing until the current calculus I becomes zero
Interval (timing t₁・ T₂Between, t₃・ T_FourBetween, ..., t
_2n-1・ T_2n,) Is the integration value zero timing T1, T
2, ..., Tn ,. And this integrated value zero
During the imming, the current matching timing of the first embodiment is mathematically
Coincides with the interval between the integration circuits 24 and 25, and sample and hold times.
The processing in the paths 26 and 27 and the division circuit 28 is also the first embodiment.
Therefore, the same symbols as those in the first embodiment are used as appropriate.
Explain.

That is, the timing generation circuit 92 outputs an integration start signal at the beginning of the integration value zero timing Tn (timing t ₁ , t ₃ , ..., T _2n-1 , ...), and during the integration value zero timing. An integration end signal is output at the end of Tn (timing t ₂ , t ₄ , ..., T _2n , ...). Based on each of these signals, the integrator circuit 24 (voltage integrator F12) causes the inter-detector voltage V to be detected, as in the first embodiment.
Are integrated for each integration value zero timing Tn, and the voltage integration value SVn shown in Expression 6 is calculated. The detection result is as shown in FIG. 7 (6) as in the first embodiment.

[Equation 6] Here, since the current differential integration value In in the integrated value zero timing Tn is zero as described above, the expression 7 holds for the SVn of the expression 6 as in the first embodiment, and the detection end voltage V The induction component M · di / dt is removed.

[Equation 7]

Further, the integrating circuit 25 (current integrating means G1
By 2), the welding current i is integrated for each integrated value zero timing Tn, and the voltage integrated value SIn shown in Expression 8 is calculated. The detection result is also shown in FIG.
It becomes like (7).

[Equation 8]

Then, similarly to the first embodiment, the division circuit 2
The voltage integration value SVn is divided by the current integration value SIn by 8 (dividing means H12), and the interelectrode resistance R that does not include the value related to the inductive component is obtained as R = SVn / SIn.

During the integrated value zero timing in the inventions according to claims 2 and 4, the current differential value di / dt changes from the timing when the welding current i is increasing as in the second embodiment. current calculus values definite integral is not limited to between the timing until the first timing becomes zero, the timing of integrated current calculus value becomes zero at the second time or more from the integration start timing (t _3, t ₄ , ..., t _2n-1 , t _2n , ...) may be the end of the integration value zero timing, and the timing (t ₂ , t ₄ , ..., t _2n , ...) during which the welding current i is decreasing. May be integrated from

[0029]

According to the present invention, it is possible to easily measure an accurate resistance between the electrodes A and A which does not include a value related to the induction component. For this reason, it is possible to know to what stage welding has progressed due to the resistance, and the electrode A,
The work W can be satisfactorily welded by appropriately applying the pressure of A or the like.

[Brief description of drawings]

FIG. 1 is a block diagram schematically showing the contents of the inventions of claims 1 and 2.

FIG. 2 is a block diagram schematically showing the contents of the inventions of claims 3 and 4.

FIG. 3 is a block diagram showing an entire apparatus according to first and second embodiments of the present invention.

FIG. 4 is a block diagram showing in detail the inter-electrode detection circuit of FIG. 3 in the first embodiment.

5 is a diagram showing a main part of the resistance welding machine 40 in FIG. It also serves as an explanatory view of a conventional general resistance welding machine 40.

6 is a wiring diagram showing specific contents of a power supply circuit 30 and a switching circuit 32 in FIG.

FIG. 7 is a graph showing detected values in each circuit in FIG.

FIG. 8 is a block diagram showing in detail the inter-electrode detection circuit of FIG. 3 in the second embodiment.

[Explanation of symbols]

 12 current differential value detection circuit (current differential value detection means D1) 14 voltage detection circuit (voltage detection means C1) 16 integration circuit (current value calculation means E1) 24 integration circuit (voltage integration means F11, F12) 25 integration circuit (current Integrating means G11, G12) 28 Dividing circuit (dividing means H11, H12) 44a, 44b Electrode (A, A) W Work

Claims (4)

[Claims] 1. A device for measuring a resistance between both electrodes of a resistance welding machine, wherein a current that changes with time is passed between two electrodes sandwiching a work, and the work is welded by its Joule heat. A voltage detection means for detecting a voltage between the electrodes, a current differential value detection means for detecting a current differential value which is a value obtained by temporally differentiating a current flowing between the electrodes, and the current differential value with time. Current value calculating means for calculating the value of the current flowing between the electrodes by integrating the voltage and the voltage detecting means between the two current matching timings at which the magnitudes of the current values are mutually matched. A voltage integrating unit that temporally integrates the detected voltage, a current integrating unit that temporally integrates the current value between the current coincidence timings, and an integrated value obtained by the voltage integrating unit by the current integrating unit. And an inter-electrode resistance measuring device of a resistance welding machine. 2. A device for measuring the resistance between both electrodes in a resistance welding machine for welding the work by means of Joule heat by passing a time-varying current between two electrodes sandwiching the work. A voltage detection means for detecting a voltage between the electrodes, a current differential value detection means for detecting a current differential value which is a value obtained by temporally differentiating a current flowing between the electrodes, and the current differential value with time. Current value calculating means for calculating a current value flowing between the both electrodes by integrating the current, and an integration start timing at which the current differential value starts to be integrated temporally to a timing at which the integrated value becomes zero. Between integration timing zero timing, voltage integration means for temporally integrating the voltage detected by the voltage detection means, and current integration means for temporally integrating the current value between the integration value zero timing, An inter-electrode resistance measuring device for a resistance welding machine, comprising: a dividing unit that divides an integrated value by the voltage integrating unit by an integrated value by the current integrating unit. 3. A method for measuring the resistance between both electrodes in a resistance welding machine in which a time-varying electric current is passed between two electrodes sandwiching a work to weld the work by its Joule heat. A voltage detection step of detecting a voltage between the electrodes, a current differential value detection step of detecting a current differential value that is a value obtained by temporally differentiating a current flowing between the electrodes, and the current differential value with time. Between the current value calculation step of calculating the current value flowing between the two electrodes by performing the integral integration and the current matching timing between the two timings at which the magnitudes of the current values match each other in the voltage detection step. A voltage integration step of temporally integrating the detected voltage, a current integration step of temporally integrating the current value between the current coincidence timings, and an integration value of the voltage integration step by the current integration step. And a division step of dividing by an integral value. 4. A method for measuring the resistance between both electrodes of a resistance welding machine, wherein a current that changes with time is passed between two electrodes sandwiching a work, and the work is welded by its Joule heat. A voltage detection step of detecting a voltage between the electrodes, a current differential value detection step of detecting a current differential value that is a value obtained by temporally differentiating a current flowing between the electrodes, and the current differential value with time. Between a current value calculation step of calculating a current value flowing between the two electrodes by integrating the current, and an integration start timing when the current differential value starts to be integrated temporally to a timing when the integration value becomes zero. Between the integration value zero timing, a voltage integration step of temporally integrating the voltage detected in the voltage detection step, a current integration step of temporally integrating the current value between the integration value zero timing, And a division step of dividing the integrated value of the voltage integration step by the integrated value of the current integration step.