受工作环境和时间的影响射频负载阻抗在工作时是不断变化的, 影响射频源的输出功率和工作效率。因此, 需要在射频源和负载之间构造匹配网络, 并且能够实时跟踪负载阻抗的变化, 对匹配网络元件的参数做出相应调整, 使射频源能够实现最大功率输出。根据射频源负载变化特点选择Γ型匹配网络, 利用Smith圆图的变化规律控制匹配网络可调元件变化的约束规律。实现方式是由罗氏线圈和电容分压器检测传输线上的电流和电压信号, 输入AD8302幅相检测电路得到信号幅度比和相位差; 经过模数转换输入STM32; 根据Smith圆图的阻抗和导纳的变化特点, 采用PID算法控制步进电机的转角和转向, 由此形成全新的闭环控制系统。由实验和仿真结果可知, 此种匹配方法能够使电路在5 s内实现快速匹配, 并且使误差控制在3°以内。

Influenced by the operating environment and working time, the radio frequency impedance load is constantly changing, which may affect the output power and the efficiency of the RF source. Therefore, a matching network is needed between the RF source and the load, which can track the changes of load impedance dynamically and make corresponding adjustments to matching network elements, thus to maximize the output power of the radio frequency source. According to the changing characteristics of the radio frequency source load, model matching network is chosen in this paper. The control law of the adjustable elements of the matching network is controlled by the change rule of Smith chart. The current and voltage signals are detected respectively by Roche coil and the capacitive divider, which are then input to AD8302 amplitude/phase detection circuit for obtaining signal amplitude ratio and phase difference. After analog-to-digital conversion, the signals are input to STM32. According to the characteristics of impedance and admittance in Smith chart, PID algorithm is used to control the stepper motor and the steering angle, thus a new closed-loop control system is realized. The experiment and simulation results show that the circuit can be quickly matched in 5 s, and the error is controlled within 3 degrees.