Nonlinear Current Control Method Research Articles

A Dead-Time Compensation Method for Parabolic Current Control With Improved Current Tracking and Enhanced Stability Range

Hysteresis current control is an attractive nonlinear current-control method for voltage source inverters when a fast system response is required. A well-known disadvantage of hysteresis current control is that the system has to operate over a wide switching frequency range. This causes an increase in the switching losses of the system and increases the difficulty in designing the output filter. The recently proposed parabolic current control solves this problem by employing a pair of parabolic carriers as the control band. Through the use of parabolic current control, constant switching frequency can be achieved. In the implementation of parabolic current control, dead time is employed to prevent shoot through of the inverter leg. The employment of dead time impacts the current-tracking precision and the stability range of the parabolic current-control method. Another side effect of using dead time is that the switching frequency deviates from the desired value. In this paper, the effects of dead time on parabolic current control are analyzed, and a compensation method is proposed for voltage source inverters that use parabolic current control. Using the output current direction of the voltage source inverter, a new pair of improved parabolic carriers is derived. As a result, the current error can be well controlled and the effects of dead time can be eliminated. The improvement in the current tracking of the system comes with an added benefit where the duty cycle range is extended. The effectiveness of the proposed dead-time compensation method is experimentally verified by the use of a full-bridge voltage source inverter.

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.