A precision analogue integrator system for heavy current measurement in MFDC resistance spot welding

Author： Xia Yu-Jun Zhang Zhong-Dian Xia Zhen-Xin Zhu Shi-Liang Zhang Rui

E-ISSN： 1361-6501|27|2|25104-25114

ISSN： 0957-0233

Source： Measurement Science and Technology, Vol.27, Iss.2, 2016-02, pp. : 25104-25114

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Abstract

In order to control and monitor the quality of middle frequency direct current (MFDC) resistance spot welding (RSW), precision measurement of the welding current up to 100 kA is required, for which Rogowski coils are the only viable current transducers at present. Thus, a highly accurate analogue integrator is the key to restoring the converted signals collected from the Rogowski coils. Previous studies emphasised that the integration drift is a major factor that influences the performance of analogue integrators, but capacitive leakage error also has a significant impact on the result, especially in long-time pulse integration. In this article, new methods of measuring and compensating capacitive leakage error are proposed to fabricate a precision analogue integrator system for MFDC RSW. A voltage holding test is carried out to measure the integration error caused by capacitive leakage, and an original integrator with a feedback adder is designed to compensate capacitive leakage error in real time. The experimental results and statistical analysis show that the new analogue integrator system could constrain both drift and capacitive leakage error, of which the effect is robust to different voltage levels of output signals. The total integration error is limited within ±0.09 mV s⁻¹ 0.005% s⁻¹ or full scale at a 95% confidence level, which makes it possible to achieve the precision measurement of the welding current of MFDC RSW with Rogowski coils of 0.1% accuracy class.