Abstract: A control system for an HT-7 tokamak ion cyclotron resonance heating liquid mixer was designed based on virtual instrument technology. The system hardware consists of a PC, hydraulic sensors, data acquisition cards, analog output cards, interface circuits, and an oil circuit system composed of an oil pump and an electric ball valve. The system interface was designed using the virtual instrument development software LabVIEW. The system controls the silicone oil level in each section of the liquid mixer by comparing the measured silicone oil level with the matching height, using automatic or manual control methods, thereby achieving impedance matching of the ion cyclotron resonance heating system. Keywords: Virtual instrument; LabVIEW; Ion cyclotron resonance heating; Liquid mixer; Control system Introduction A virtual instrument is a computer testing system built on a computer basis, combining relevant hardware and software. Its functions are defined by the user and it has a visual interface. Using virtual instrument technology, users can make full use of the computer's software and hardware resources to realize software replacing hardware, customize the functions of the instrument, and build a flexible measurement and control system [1-3]. The liquid stub tuner is a novel impedance matching device used in the HT-7 tokamak ion cyclotron resonance heating system for matching the impedance of transmission lines and antennas. The tuner is made by filling the inner and outer conductors of a coaxial line with a liquid (silicone oil) with a low dielectric constant. By utilizing the difference in the propagation speed of electromagnetic waves in air and silicone oil, the characteristic impedance of each section of the tuner is changed, so that the antenna impedance and the characteristic impedance of the transmission line are matched[4]. Figure 1 is a schematic diagram of the liquid stub tuner section structure. [align=center] Figure 1 Schematic diagram of the liquid stub tuner section structure[/align] In practice, a three-stub liquid stub tuner is used, which consists of three structures as shown in Figure 1. According to the matching parameters of the liquid stub tuner at different transmission wave frequencies, the silicone oil level in each section is made to reach the corresponding matching height, so that the antenna impedance and the transmission line impedance are matched. This paper applies virtual instrument technology to establish a liquid mixer control system. Based on the collected data on the current silicone oil level in each section and its matching height, the system uses automatic or manual control methods to control the silicone oil level in each section of the liquid mixer, thereby achieving impedance matching between the antenna and the transmission line. 1. System Hardware Composition The system hardware consists of a personal computer, data acquisition card, analog output card, sensors, interface circuits, and an oil circuit system composed of an oil pump and an electric ball valve. A schematic diagram of the system structure is shown in Figure 2. [align=center]Figure 2 Schematic diagram of the liquid mixer control system[/align] Both the data acquisition card and the analog output card use ADLINK PCI NUDAQ cards: the acquisition card is PCI-91118HG, with 12-bit resolution, a maximum sampling rate of 333KS/s, 16-channel single-ended analog input (or 8-channel differential analog input), 2-channel analog output, 4-channel TTL digital input/output, and a maximum input range of 0-10V (single-ended) or 5V (differential); the analog output card is PCI-6208V, with 16-bit resolution, 8-channel voltage output, a voltage output range of [missing information], and 4-channel digital input/output. The hydraulic sensor uses the STD924 differential pressure transmitter from HONEYWELL Corporation (USA). Based on the pressure principle, it converts the silicone oil level in the liquid mixer section into a 4-20mA current signal. The silicone oil level in the mixing section can be determined from the output current of the differential pressure transmitter. The oil pump uses products from Grundfos, a Dutch company, known for its high reliability. Its LiqTec™ technology detects the presence of liquid in the pump, reducing the risk of dry running. Furthermore, Grundfos pumps offer extremely high sealing reliability. The electric ball valve uses products from OMAL, an Italian company, and is used in conjunction with an AM-type regulating electric actuator. The electric actuator receives a 4–20 mA current signal. When the input current signal is less than 4 mA, the electric actuator stops, and the electric ball valve returns to the closed position, fully closing the valve. When the input current signal is 20 mA, the electric ball valve fully opens. The OMAL electric ball valve also features a valve position transmitter, providing feedback on the current valve opening. The hydraulic circuit system of the liquid distributor, consisting of the oil pump and electric ball valve, is shown in Figure 3. The rising and falling circuits are interlocked independent circuits, and can be switched between them using a selector switch. [align=center]Figure 3 Schematic diagram of the hydraulic circuit system of the liquid mixer[/align] The output signal of the hydraulic sensor and the input signal of the valve electric actuator are both 4-20mA DC current signals, while the input and output signals of the data acquisition card PCI-9118HG and the analog output card PCI-6208V are both DC voltage signals. Therefore, an interface circuit needs to be designed to perform the corresponding current-to-voltage or voltage-to-current conversion. Since the total height of the liquid mixer section is 6 meters, in order to correspond the 4-20mA current signal output by the hydraulic sensor with the actual liquid level, the 4-20mA current signal can be converted into a 0-6V voltage signal. Figure 4 shows the corresponding current-to-voltage interface circuit diagram. [align=center]Figure 4 4-20mA current to 0-6V voltage interface circuit diagram[/align] Similarly, in order to convert the voltage signal output by the analog output card PCI-6208 into the drive current signal of the electric actuator, an interface circuit for converting 1-5V voltage to 4-20mA current, as shown in Figure 5, was designed. [align=center]Figure 5 Circuit diagram of 1-5V voltage to 4-20mA current interface[/align] 2 System Software Design The system software was designed using the graphical virtual instrument development platform NI LabVIEW 7.0. The software flowchart is shown in Figure 6. [align=center]Figure 6 Flowchart of Liquid Mixer Control Program[/align] Since a three-section liquid mixer is used, each section should have a corresponding operation panel for controlling the silicone oil level. The system software test results are shown in Figure 7. [align=center]Figure 7 Liquid Mixer Control System Software[/align] By selecting the manual/automatic switch, the program can realize manual and automatic control of the liquid mixer. In manual control mode, the operator can select the rise/fall switch according to the current liquid level and matching height of each section, freely set the valve speed, and flexibly control the rise or fall of the liquid level in each section. When the matching height is reached, pressing the stop button will shut off the oil circuit system of that section, stabilizing the liquid level at the matching height. In automatic control mode, the operator first sets the matching height of the liquid level in each segment, and after selecting the start button, the liquid level in each segment automatically reaches the matching height. Simultaneously, the program panel displays the rise/fall and valve speed status of each segment's liquid level, and the upper and lower limit indicators will also provide alarm displays based on whether the current liquid level in each segment has reached the upper or lower limit, prompting the operator to stop control to prevent silicone oil leakage or dry running of the oil pump. In this control program, the operator can flexibly and in real-time control the valve opening of the electric ball valve. The "4~20" on the panel corresponds to the 4~20 input current of the electric actuator; at 4mA, the electric ball valve is closed; at 20mA, the electric ball valve is fully open; the higher the current, the larger the valve opening, and the faster the liquid level rises/falls in the segment. 3. Conclusion This system operates stably and reliably in practical applications, is easy to operate, and effectively controls the liquid mixer, meeting the actual requirements. The innovation of this article is that various interface circuits were designed in terms of system hardware, and the virtual instrument software LabVIEW was applied in terms of system software, so that the whole system has the characteristics of user-friendly interface, convenient operation and easy expansion. References [1] Zheng Ting, Wang Yong. Application of virtual instrument in automatic test system [J]. China Test Technology, Vol. 32, No. 1, 2006 [2] Deng Yan, Wang Lei et al. LabVIEW test technology and instrument application [M]. Beijing: Machinery Industry Press, 2004 [3] Zhu Yuqing, Wu Weibin, Hong Tiansheng. Fuzzy control system of diesel engine fuel injection quantity based on virtual instrument [J]. Microcomputer Information, 2006, 3: 24-26 [4] Pan Yaping, Wang Lei, Zhao Yan ping, et al. Design and Realization of Liquid Stub Tuner Contron System. Plasma Science & Technology, 2004: Vol6, No.6