Kang Zhou

A Nonlinear Current Control Method for Resistance Spot Welding

Resistance spot welding (RSW) is one of the most commonly used methods in the manufacturing process for joining sheet metal together. This paper deals with the constant current control for RSW, which is the most common control strategy in actual applications. Since the process of RSW is nonlinear and time-varying, conventional control schemes do not yield satisfactory performance. To cope with this problem, a new control method is proposed in this paper. By solving the governing equation of the equivalent circuit of the RSW electrical system at each control cycle at its local coordinate frame, a nonlinear relationship between input and output variables is obtained. The relationship can be used to estimate an approximate value of the trigger time of the silicon-controlled rectifier for the next control cycle based on the information of the previous control cycle. In order to compensate the estimation error and improve the performance of the closed-loop system, a proportional-derivative (PD) controller is employed. Because the proposed controller regulates trigger time error instead of output current error, the parameters of the PD controller are easy to determine. The effectiveness of the proposed controller was verified through experiments under different conditions. Experimental results showed that the performance of the proposed controller was much better than that of a well-tuned proportional-integral-derivative controller.

On the development of nugget growth model for resistance spot welding

In this paper, we developed a general mathematical model to estimate the nugget growth process based on the heat energy delivered into the welds by the resistance spot welding. According to the principles of thermodynamics and heat transfer, and the effect of electrode force during the welding process, the shape of the nugget can be estimated. Then, a mathematical model between heat energy absorbed and nugget diameter can be obtained theoretically. It is shown in this paper that the nugget diameter can be precisely described by piecewise fractal polynomial functions. Experiments were conducted with different welding operation conditions, such as welding currents, workpiece thickness, and widths, to validate the model and the theoretical analysis. All the experiments confirmed that the proposed model can predict the nugget diameters with high accuracy based on the input heat energy to the welds.

Online Measuring Power Factor in AC Resistance Spot Welding

The power factor is a very important process variable in ac resistance spot welding (RSW) operation. Because of the variation of welding load during the welding process, the power factor is a time-varying variable. The power factor of RSW cannot be obtained online because the value of power factor angle cannot be directly measured in real time. After analyzing the electrical structure of RSW, a new algorithm for measuring the power factor angle in real time is developed. Then, an integrated algorithm for calculating the power factor on a practical discrete system in real time is proposed. Experiments were conducted to validate the effectiveness of the proposed method. The experimental results showed that the proposed algorithm can be used to obtain more accurate values of the power factor angle than using other methods over a very large operation range. The final results of power factors obtained from the proposed algorithm are very close to the results from the numerical simulation.

Study on effect of electrode force on resistance spot welding process

This paper deals with the effect of electrode force on resistance spot welding process. As one of the most important parameters during the process, the electrode force can influence the process in different aspects. Dynamic resistance, which can reflect the internal physical variation of the workpiece, is employed to monitor the characteristic variation of the workpieces. According to theoretical analysis, large electrode force may easily induce surface expulsion. Also, it can decrease the overall dynamic resistance values and postpone the time when first melting point appears, as well as enlarge the duration between times when first melting point and peak value appear. Larger electrode force induces the initial smaller nugget diameter, and then the nugget diameter has a higher growth speed. However, the overall nugget diameter may be smaller and its growth terminates earlier than when smaller electrode force is applied. Final experiments validated all the theoretical analysis.

Improvement in control system for the medium frequency direct current resistance spot welding system

The paper deals with a medium frequency direct current resistance spot welding system. The system consists of special electromagnetic structure and works in a particular pattern. After studying the structure, working principle and possible operating modes of the system, a new mathematic model is developed to precisely describe the dynamic behavior of the whole system. A phenomenon that the operation of the system can be in dead zone is observed. And the performance of the system would be deteriorated if the phenomenon is not handled properly. And then a control algorithm is proposed to improve the performance of the system. The effectiveness of the proposed algorithm is verified through a numerical simulation.

Sample Entropy-Based Approach to Evaluate the Stability of Double-Wire Pulsed MIG Welding

According to the sample entropy, this paper deals with a quantitative method to evaluate the current stability in double-wire pulsed MIG welding. Firstly, the sample entropy of current signals with different stability but the same parameters is calculated. The results show that the more stable the current, the smaller the value and the standard deviation of sample entropy. Secondly, four parameters, which are pulse width, peak current, base current, and frequency, are selected for four-level three-factor orthogonal experiment. The calculation and analysis of desired signals indicate that sample entropy values are affected by welding current parameters. Then, a quantitative method based on sample entropy is proposed. The experiment results show that the method can preferably quantify the welding current stability.

On current control method for single-phase AC resistance spot welding

The paper deals with the constant current control for single-phase AC resistance spot welding (RSW), which is the most commonly used control strategy in modern welding industry. The process of RSW is nonlinear and time-varying. After analyzing the working principle and nonlinear properties of RSW, a mathematical relation between input and output variables for constant current control is developed. A PI controller, which is used to regulate the desired current used in the mathematical relation, is employed to improve the performance. The final experimental results showed the performance obtained by new controller was better than that by a fine-tuned PID controller.

Mathematical description of AC resistance spot welding control problem and limits of its controller design

Resistance spot welding (RSW) is widely used in sheet metal fabrication industries even though the medium frequency type has appeared. Since there are lots of demands to significantly improve its performances depending on better control schemes, it is valuable to answer the question: how much of the performance it might improve if more advanced control strategies could be used. In addition, various modeling work has been done in the past twenty years. However, there are few new papers showing controller design based on these models and improving its performances effectively. This paper presented the complete mathematical description of the control problem firstly and revealed the critical points in the model of the whole process which significantly affect the control strategy design. Then it explained why most feedback strategies ordinarily used lose their theoretical foundations and few existing control algorithms can be directly used for this control problem with RMS valued current as the output and input constraints. Moreover, a practical control strategy with solid theoretical support was proposed, which seems to be simple but actually with lots of complicated theoretical issues concerned and is one of the few reasonable control algorithms considering the unique properties of RSWs “single phase AC model, input constraints and nonlinearity of the output”. In the end, this paper verified the proposed scheme against the experimental results, and introduced a new interesting general control problem for later researches.