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Resistance welding current measurement solution

This article mainly talks about how HANGZHI high-precision current transducer helps customers solve the problem of current measurement for resistance welding.


Resistance welding, as a welding method in which two metals are fused and combined by resistance heat, has the characteristics of low costs, high safety, low consumption, high welding strength, high degree of automation, and easy operation. It is especially suitable for welding requirements of automobile body sheet covering parts. It is widely used in the automotive industry. However, due to factors such as power grid voltage, welding electrode tip, material, and surface cleanliness of welding workpieces, de-soldering and false welding of components often occur in automobile production, which affects the quality of automobiles. Therefore, it is necessary to analyze the possible factors that affect the quality of resistance welding and provide solutions to improve the quality of automobile welding and automobiles eventually. The welding process of resistance welding is controlled by 4 basic steps: pressure, welding, maintenance, and rest. The principle is to use the Joule heating effect for welding, and the current is the most important factor affecting resistance welding. The main interference factors that affect current accuracy are network voltage fluctuations, shunting, changes in welding circuit impedance, electrode wear, and changes in sheet thickness and number of welded layers. Therefore, during welding, adjusting the parameters of welding current according to the properties and thickness of the welded metal is the key to welding quality control. The generally adopted method is to monitor the current regularly and its changes during the working process, and use constant current to ensure stable welding quality.


At present, for a customer's resistance welding machine usage and current monitoring analysis, HANGZHI uses high-precision current sensors and electrical measuring instruments to provide corresponding solutions.


1. Current situation faced by the customer 

1.1 The controller model of the resistance welding machine used by the customer is ST21C, see below for details.

1.1.1 Device parameters


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1.1.2 Equipment technical requirements


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1.1.3 Precision

The power supply voltage variation rate is within ±10%

Resistance load changes within ±10%

Setting accuracy ±3% (full scale)

Repeatable accuracy ±3% (full scale)


1.1.4 Current use status: The equipment manufacturer has given the equipment accuracy ±3% (full scale), which needs to be calibrated regularly. There is a calibration knob on the equipment, which can calibrate the accuracy of the welding power supply through the current data measured by the standard test. At present, the customer used the welding tester MM-315B to test and found inaccurate current, but the welding power source was not calibrated. Desoldering and false welding of components often occur in the production process, which affects the quality of vehicles. It is urgent to make adjustments of the welding equipment parameters to improve product quality.


1.2  The welding test machine currently used by the customer:


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1.2.1 The specifications are as follows:

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1.2.2 Current state of use: The welding tester MM-315B has not been tested, and it is uncertain whether it is within the accuracy range. According to the standard requirements of industrial measurement, the accuracy of the measurement equipment needs to be 2 class higher than the tested equipment. The accuracy of MM-315B is ±1%, so it cannot be used as a tool for measuring and calibrating the welding power source. It can only be used as a verification device. It also needs to be submitted for inspection. It is urgent to verify whether the welding tester MM-315B is within the accuracy range.


1.2.3 Through the analysis of the above requirements of the equipment, the customer's demands are as follows:

l  The welding power source has been tested and calibrated to ensure accuracy within ±3% (full scale)

l  Welding tester MM-315B has passed inspection and calibration to ensure accuracy within ±1%


2. HANGZHI solution


The most important quality control method for regular monitoring of welding current. According to the characteristics of welding current, HANGZHI proposes to adopt a high-precision tester effective value detection resistance welding current adjustment technology. After analyzing its working principle, it is proved that this solution can solve the accurate measurement of the current under the conditions of resistance welding welding current waveform distortion and frequency change.


2.1 Characteristics of resistance welding current

During current stability monitoring of resistance welding, the correct collection of current feedback signals is very important. The primary coil of the resistance welding power transformer has a voltage of 500 V. The current can vary from 150 A to 10 kA; the secondary side coil outputs a low voltage below 30 V, the current waveform is pulsating direct current, and the current value is very large, generally from a few kA to tens of kA, and some are above 100kA. And the power-on time is short (generally 0.1-0.5s). Moreover, the current waveform is irregular, and there are many distortions. According to the characteristics of resistance welding current, the current detection of resistance welding requires fast and accurate response to achieve a good control effect.


Because the resistance welding process is quite complicated, the welding current can be changed by various factors such as the power supply fluctuation of the power grid, the change of resistance during welding (during welding, the contact state of weldment-weld-piece and electrode-electrode, welding temperature field and the constant change of electric field causes constant changes of welding resistance).


Due to various factors, the output current of resistance welding is a pulsating direct current. The waveform is complicated, and there are many distortions, which increases the difficulty of detecting the welding current.

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The detection of resistance welding current signal can be embodied in the form of effective value, average value and peak value. Among the three methods, the peak detection method is the fastest and the easiest to implement, but the error is the largest; the average detection method is between the effective value detection and the peak detection in terms of speed and accuracy. Because the effective value detection is the most accurate, it can best reflect the true situation of the current and is more suitable for the detection of waveform distortion. This solution uses a high-precision tester to accurately measure complex current waveforms.


2.2 The composition of the high-precision tester system provided by HANGZHI

This solution uses a high-precision fluxgate current sensor as the acquisition current sensor. The current sensor is directly placed on the electrode arm of the welding machine, and the welding current signal is directly extracted from the high-precision current sensor side, and the test current is directly displayed by the integration circuit of the tester. The fluxgate high-precision current sensor can measure current from several amperes to 15K amperes, and there is no magnetic saturation. There is no direct circuit connection between the current. It has strong instantaneous tracking ability and can be used to measure current with irregular shape which are particularly suitable for resistance welding current measurement sensors.


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2.2.1 AIT15000-SG high precision current sensor

The multi-point zero magnetic flux technology system is applied to the existing high-precision DC sensor technology. The combination of excitation magnetic flux closed-loop control technology, self-excited fluxgate technology and multi-closed-loop control technology realizes the control of excitation magnetic flux, DC magnetic flux. The zero-flux closed-loop control of AC magnetic flux, and the detection of high-frequency ripple by constructing a high-frequency ripple sensing channel, enables the sensor to have relatively high gain and measurement accuracy within the full bandwidth.


2.2.2 Portable AC/DC standard meter

As a new generation of high-precision AC/DC meter produced by HANGZHI, the product adopts a brand-new hardware and software design. It can measure single-phase AC and DC voltage, current, frequency, phase, active power, etc., and can be widely used in AC and DC in the fields of measurement research, electric power, metering, military industry, manufacturing, scientific research, etc. Measurement, government inspection agencies and other metrology fields. It can measure single-phase AC and DC voltage, current, frequency, phase, and active power. Ripple test can be performed to detect AC ripple below 1kHz. Equipped with RS232 and RS485 communication interfaces, it can directly communicate with PC online, and can be equipped with CAN communication interface. The voltage and current multi-ranges are automatically switched, and the measurement limit can be 110%. Configure 5.6 inch LCD. With the online upgrade function of the product program.






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