Abstract: The vacuum tuning capacitor used to adjust the ICRH antenna impedance in the EAST device must be remotely controlled. Based on serial communication technology, a master-slave architecture was adopted, using an industrial computer as the master and a Siemens PLC as the slave to achieve remote control of the vacuum tuning capacitor. The master control interface was implemented using LabVIEW virtual instrument development software, which not only achieved remote control of the capacitor but also provided a highly interactive human-machine interface. Keywords: EAST; virtual instrument; impedance matching; programmable logic controller Introduction EAST (Experimental Advanced Superconducting Tokamak), as China's first fully superconducting tokamak nuclear fusion experimental research device designed and developed independently, is about to be built and put into trial operation at our institute. Compared with the original HT-7 device, this device has made significant improvements in many aspects. Among them, the antenna impedance matching system used for radio frequency wave heating plasma will be improved from the original liquid matching system to vacuum tuning capacitor matching. During plasma discharge, experimental personnel cannot approach the fusion device, and the tuning capacitor is more than 100 meters away from the control room. Therefore, the matching adjustment of the antenna impedance must be remotely controlled. 1. Remote Control System Block Diagram The control system uses an industrial computer as the host computer and a Siemens S7-224 PLC as the slave computer. Long-distance transmission of monitoring signals is achieved between the computer's serial port and the PLC's communication port. Since the computer uses an RS-232 serial port, but the RS-232 standard cannot meet the required communication distance, and the PLC's serial communication port is an RS-485 interface, which fully meets the 100-meter communication distance requirement, only an RS232/485 physical protocol converter needs to be installed on the host computer's serial port. Here, a JARA2102 converter is used. The actuator is a Yaskawa high-precision servo motor, model SGMAH-08ADA, controlled by a driver. This motor drives the external screw of the tuning capacitor to rotate, thus changing the capacitance. The two are connected by a coupling, and the driver operates in position control mode. The entire control system hardware connection block diagram is shown in Figure 1. [align=center]Figure 1. Block Diagram of Remote Control System[/align] 2. Communication Protocol Between Upper and Lower Computers The RS-485 communication port of the Siemens S7-224 PLC can operate in free port communication mode, meaning the communication protocol can be user-defined. Therefore, we stipulate that communication between the upper and lower computers is initiated by the upper computer, and the upper computer requires the lower computer to provide a data frame feedback for each command frame sent. This allows the upper computer to monitor command execution and process the feedback data. The format of the upper computer command frame is shown in the table below, where the left 33 bytes represent the upper computer command frame format, and the right 13 bytes represent the lower computer feedback frame format. The start character is designated as the letter "g," which the lower computer uses to indicate the start of a data frame. The end character is designated as the letter "G," which the lower computer uses to determine the end of a data frame. The function character is represented by 05H, indicating the write to a register on the lower computer. In Siemens S7-224 PLCs, the address of a register can be represented by an 8-bit hexadecimal number: 0000 (H): I register; 0100 (H): Q register; 0200 (H): M register; 0800 (H): V register. For example, 0000000 (H) represents IB000; 08000064 represents VB100. During data transmission, instructions may be interfered with and erroneous. To detect errors during transmission, the receiver must further verify the received instructions to prevent erroneous instructions from being executed. One method is to use a checksum, which in this paper is defined as the XOR sum of the ASCII codes of strings Byte 1 to 29, byte by byte. According to the above instruction format, if the host computer writes 2000 pulses to VB000 of the slave computer, the command frame to be sent should be "g602080000C808000007D0000000003CG", where "3C" is the checksum. 3. Host Computer Program Design and Implementation 3.1 Host Computer Program Flow Design After the program runs, it first enters the While loop, within which the program executes cyclically according to the flowchart shown in Figure 2. First, the serial port is opened according to the set serial communication parameters. It is important to note that the serial port parameters must be consistent with those of the slave computer to ensure normal communication between the two computers. In this paper, the baud rate is set to 9.6kbps, 1 start bit, 1 stop bit, 8 data bits, and no parity. Then, the program enters the judgment of the rotation command, using three nested CASE statements to determine whether the motor is rotating forward, backward, or stopped. If the forward or reverse button is pressed, the program first sends the number of pulses required for the motor's rotation angle. Then, it delays and waits for the PLC's feedback frame. Based on this feedback frame, a CASE statement is used to determine if the pulse data frame was sent correctly. If an error occurs, the indicator light turns red, prompting a retransmission. If the pulse data frame has been sent correctly, a rotation command frame is sent, and then the program delays and waits for the PLC's feedback frame. If an error occurs, the indicator light also turns red, prompting a retransmission. If the stop button is pressed, a command frame indicating a stop is sent directly. Finally, the program updates the capacitor value and closes the serial port. [align=center]Figure 2 Upper Computer Program Flow Design[/align] 3.2 Sub-VI Program Development To facilitate the implementation of the main program's functions, several sub-programs were developed, namely: Rotation Command Frame Packaging.VI, Pulse Data Frame Packaging.VI, and Rotation Command Packaging.VI. The development of these sub-programs is similar; only Pulse Data Frame Packaging.VI (excluding the start character) is listed below. When pulse data needs to be written to the lower computer VD200, its program's front and back panels are shown in Figure 3: [align=center]Figure 3 Pulse Data Frame Front and Back Panels[/align] 3.3 LabVIEW Implementation of Upper Computer Control Interface 3.3.1 LabVIEW Serial Communication Module The LabVIEW platform for virtual instruments includes a serial communication control module. In the serial communication process controlled by the LabVIEW platform, data is composed of strings. Each character in the string actually corresponds to a familiar ASCII character. Data must be converted to an ASCII string before it can be sent and received. The LabVIEW platform's serial communication module provides functions such as VISA Open.vi, VISA Configure Serial Port.vi, VISA Read.vi, VISA Write.vi, VISA Bytes at serial port.vi, and VISA Close.vi. When actually using serial port read/write functions, to ensure the correctness of the read/write data, there should generally be a delay between two consecutive read/write operations. The length of this delay depends on the amount of data sent; in this paper, the delay is set to 80ms. 3.3.2 Control Interface Design According to actual requirements, the upper-level control interface needs to be able to display capacitor values, set motor forward/reverse speeds, send rotation commands, and display various error messages. The upper-level control interface implemented using LabVIEW is shown in Figure 4. [align=center] Figure 4 Control System Operation Interface[/align] 4 System Test Results As a position servo control system, its quality can be evaluated from three aspects: high positioning accuracy, positioning process without overshoot, and fast dynamic response. For this system, no overshoot positioning process was observed during multiple tests; therefore, the main focus was on testing control accuracy and system response time. After multiple runs and tests, the system's characteristics in these two aspects are as follows: In this paper, the minimum angle of motor rotation is set to 0.72 degrees, and the capacitor value changes by 3.2 pF per revolution. Therefore, the control accuracy of the capacitor value can reach 0.0064 pF, fully meeting the requirements. During the host computer's serial port read/write process, multiple read/write operations are required to send speed, pulse count, rotation commands, etc. Each read/write operation has an 80ms delay to ensure normal communication. Considering the PLC CPU self-check and program execution delays, a single motor rotation operation takes approximately 300ms. The entire system achieved good results using EAST ICRH true harmonic capacitor remote control. The author's innovation: This article is the first to demonstrate the method of implementing the Siemens series PLC free port communication protocol using LabVIEW, and elaborates on the details of the protocol implementation. It also has certain reference value for the development of serial communication application programs. References: [1] Lei Zhenshan, "LabVIEW 7 Express Practical Technology Tutorial", China Railway Publishing House, 2005 [2] Deng Yan, Wang Lei, "LabVIEW 7.1 Test Technology and Instrument Application", Machinery Industry Press, 2004 [3] Siemens (China) Co., Ltd. Automation and Drive Group, "S7-200 System Manual", 2005 [4] Jin Yuanyu, Pang Zhonghua, Real-time Monitoring System Based on VC++ and AI Regulator Serial Communication, Microcomputer Information, No. 4, 2005 [5] Li Guangming, Li Yan, Li Qian, Using VB to Implement Ordinary Serial Communication between S7-300 PLC and PC, Microcomputer Information, No. 07S, 2005