Abstract: The HT-7U cryogenic system provides cooling for the longitudinal and poloidal superconducting magnets of the HT-7U tokamak advanced steady-state controllable nuclear fusion experimental device, and the screw compressor unit is one of the subsystems of the HT-7U cryogenic system. This paper discusses in detail the screw compressor unit monitoring system based on PLC and configuration software, and presents the hardware structure, control concept, software structure, and implemented functions of the monitoring system. Keywords: HT-7U; monitoring system; PLC; Controller Link network; configuration software Introduction The HT-7U superconducting tokamak nuclear fusion experimental device is a national-level major scientific engineering project undertaken by the Institute of Plasma Physics, Chinese Academy of Sciences. Its goal is to conduct exploratory experimental research on cutting-edge physics issues related to building a steady-state advanced tokamak nuclear fusion reactor on the device. The HT-7U cryogenic system, as one of the main subsystems of this project, provides cooling for the stable operation of the longitudinal and poloidal superconducting magnets of the HT-7U superconducting tokamak. Simultaneously, the system's 2KW/4K cryostat is also used to produce liquid helium to meet the needs of other experiments and users. The cryogenic system consists of three parts: a compressor station, a cryostat cold box section, and a tokamak magnet cooling section. An OMRON CS1G CPU44 PLC is used as the main equipment for data acquisition, control loops, automatic sequential operation, and calculation, supplemented by other measurement and monitoring equipment. This scheme achieves real-time monitoring, automatic control, and system operation diagnosis of the compressor station, meeting the requirements for system reliability, stability, and real-time performance. 1. System Introduction [align=center] Figure 1 Compressor Station System Flowchart[/align] The compressor station section of the HT-7U helium cryogenic system includes a two-stage screw compressor unit, an oil removal system, and other equipment. Its system flowchart is shown in Figure 1. The compressor section consists of two stages connected in series: a low-pressure stage and a high-pressure stage. The low-pressure stage comprises three LG25Ⅱ screw compressors connected in parallel, compressing helium from 0.104 MPa (P0) to 0.51 MPa (P1), with a total mass flow rate of 250 g/s. The high-pressure stage comprises two LG20Ⅱ screw compressors connected in parallel, compressing helium from 0.51 MPa (P1) to 2 MPa (P2), with a total mass flow rate exceeding 360 g/s. The compressor station's function is to provide the necessary stable pressure and high-purity helium flow rate for various operating modes of the refrigeration unit, including cooling, reheating, liquefaction, and refrigeration. It is crucial to the stability of the entire cryogenic system and the efficiency of the refrigeration unit. The number of compressors to be engaged or disengaged is selected based on the required helium flow rate of the refrigeration unit. By adjusting the energy slide valve of the screw compressor and five control valves, P0, P1, and P2 are stabilized within the required precision range. The main process parameters that need to be measured in the screw compressor station include: pressure, temperature, differential pressure, flow rate, liquid level, speed, vacuum, gas purity, valve opening, motor current, and compressor energy slide valve position. The system has 99 digital inputs, 59 analog inputs, 59 digital outputs, and 6 analog outputs. The main processes that need to be controlled include the screw compressor's start-up, shutdown, and safe operation; compressor oil temperature regulation; and handling of equipment malfunctions. 2. Monitoring System Hardware Structure The compressor unit monitoring system consists of a host computer and a slave computer. The system hardware structure diagram is shown in Figure 2. The host computer uses two Advantech industrial PCs. One acts as an operator station to monitor the entire system and detect data, while the other acts as an engineer station to design and develop the configuration software, develop the PLC program, and transmit the software to the PLC's CPU unit via serial port. The lower-level machine uses the powerful, reliable, easy-to-maintain, and anti-interference programmable controller OMRON CS1G-CPU44 PLC to acquire and control most parameters of the compressor station. Some parameters that do not participate in pressure control, such as oil adsorber pressure and buffer tank pressure, are acquired by Advantech's ADAM data acquisition module and transmitted to the upper-level machine in the form of serial data. Shielded cable is used as the medium for serial communication between the industrial control computer's serial port and the PLC and ADAM module. The ADAN 4520 RS485/232C converter is used to solve the problems of short communication distance and large interference of RS232C. [align=center] Figure 2 Hardware structure diagram of compressor station monitoring system[/align] The hardware configuration of the monitoring system is as follows: Advantech industrial computer, CPU is PIII 733, the operator station is also expanded with Controller Link support card 3G8F5 CLK01-E, the PLC is configured with the following modules: basic I/O units: 3 16-point input units ID212, 6 16-point output units OC225, high-density I/O units: 2 32-point input units ID216, special I/O units: 4 8-channel analog input units AD003, 1 8-channel analog output unit DA004, 3 temperature sensor units TS102, Controller Link communication unit CS1W CLK21, ADAN modules include 4 8-channel modules ADAN 4017 and 2 3-channel modules ADAN The system uses an ADAN 4520 RS485/232 converter and a 4013 PLC. An Omron Controller Link network is formed by connecting the host computer and slave devices via the CLK21 Controller Link cable communication unit on the field-level PLC and the 3G8F5-CLK01-E communication unit extended on the operator station's ISA slot. OMRON's Controller Link network is OMRON's primary FA (Factory Automation) level network, a token bus communication network where each node can act as a master station for sending and receiving data. Data can be automatically exchanged between preset areas by configuring data link nodes. The node controlling communication in this network is called the token issuing unit; it controls tokens, checks the network, and performs related tasks. This bus topology offers maximum flexibility, ease of expansion and maintenance, and meets system scalability requirements. The use of distributed control technology ensures that the Controller Link network will not collapse due to the failure of a single site, improving system stability. This system uses shielded twisted-pair cable as the communication medium for the Controller Link network. Since the distance between nodes is less than 500m, the transmission rate reaches 2Mbps, which meets the system's real-time requirements. After the PLC network is physically connected, necessary parameter settings and path tables must be established; this is the most crucial part of the entire network configuration process. The parameters set include the communication unit number, the node number in the network, the I/O table, and data links. Only after completing these necessary tasks can the PLC network interconnection be achieved. The parameter settings for the CLK21 and 3G8F5 CLK01-E modules in this system are shown in Table 1. Table 1 Controller Link Communication Unit Parameter Settings Table 3. Monitoring Software Structure Design Industrial control configuration software is a software package that can collect data in real time from devices such as programmable controllers and various data acquisition cards, issue control commands, and monitor whether the system is operating normally. Configuration software can fully utilize the powerful graphical editing functions of Windows to display the operating status of monitored devices in an animated manner, easily construct monitoring screens and implement control functions, and generate reports, historical databases, etc., providing a convenient software development platform for industrial monitoring software development and improving the overall quality of industrial control software. KingView 5.1, developed by Beijing Yacon Company, is a configuration software that runs on Windows 98/NT, consisting of two parts: the project browser TouchMAK and the screen running system TouchVEW. TouchMAK is the core part of the KingView software and the management and development system; its function is to create animated display windows. Through its toolbox, it is easy to create real-time curves, historical trend charts, and alarm record displays. TouchVEW is the running environment for displaying the graphical windows created in TouchMAK. In the screw compressor monitoring system, the engineer station can run TouchMAK and TouchVEW, while the operator station is only allowed to run TouchVEW. Figure 3 shows the structure of the monitoring software. The KingView 5.1 driver communicates with the PLC via the Controller Link network and with the ADAN module via the serial port, accessing the corresponding registers to obtain the actual values ​​of various process parameters at the compressor site or to control the opening degree of on-site switch and analog quantities such as various control valves. In this system, DM0 to DM199 of the PLC are set as read-write areas, meaning the host computer can read and write to this area on the slave computer; DM200 to DM399 are set as read-only areas, meaning the host computer can only read the values ​​in this area on the slave computer and cannot change them. [align=center] Figure 3 Monitoring Software Structure Diagram[/align] The configuration software of the host computer of the screw compressor station measurement and control system basically realizes the measurement and control requirements of the screw compressor station. It concisely and vividly simulates the process flow of the compressor station, and the operator can understand the entire operating status of the compressor station, including various alarms, on the computer screen in the central control room. Authorized operators can control any compressor individually or online from the central control room. All switching between automatic and semi-automatic modes is seamless. Each control button and each automatic/semi-automatic switch button has further confirmation or cancellation to prevent accidental operation. 4. Control System Design Philosophy Large screw compressor systems employing a two-stage series connection of low-pressure and high-pressure stages have been successfully implemented in Germany, Japan, and France, but are not yet widely used in China. There are two main pressure control methods for this system: pre-stage control and post-stage control. Pre-stage control uses the compressor intake pressure as the control basis, which reduces system coupling but results in larger fluctuations in compressor discharge pressure. Post-stage control uses the compressor discharge pressure as the control basis, offering high precision in compressor discharge pressure control, but with greater system coupling and more complex implementation. The screw compressor regulates its volumetric flow rate by increasing or decreasing the load on a structural element called an energy slide valve (achieved through the on/off switching of an electromagnetic directional valve). The gas supply valve and gas collection valve regulate the gas flow rate in the system. The compressor control system is required to provide the refrigeration unit with different gas flow rates under various operating conditions while ensuring stable pressure. Therefore, the compressor control system must first maintain the flow balance of the helium circulation system and ensure the stability of the supply pressure (secondary pressure). According to the structure of the compressor system, P0, P1, and P2 in Figure 1 are controlled separately and then form a cascade control. This control scheme can reduce the coupling degree within the system and reduce the complexity of control. Since the compressor control system is a large-time-delay, strongly coupled, multivariable process control system, the mathematical model of the control system is difficult to identify. The parameters of the PID controller that adjusts the opening of each valve are tuned by trial and error. Based on the operator's manual control experience, a scheme of fuzzy control + expert control + PID control is adopted to achieve the system control requirements. The control system structure diagram is shown in Figure 4. [align=center] Figure 4 Control System Structure Diagram[/align] Two fuzzy controllers, Fuzzy Controller 1 and Fuzzy Controller 2, are constructed. The design of the fuzzy controller is explained using Fuzzy Controller 1 as an example. Based on the accumulated manual operation experience in screw compressor operation, the fuzzy subsets of the controlled variable and the control variable are determined as follows: E1, EC1, U1: {large negative deviation, medium negative deviation, small negative deviation, no deviation, small positive deviation, medium positive deviation, large positive deviation}, abbreviated as {NB, NM, NS, ZO, PS, PM, PB}, where E1 is the deviation of the low-pressure stage compressor outlet pressure P1, EC1 is the rate of change of P1 deviation, and U is the energizing time of the solenoid directional valve. The basic universes of discourse for the controlled variable and the control variable are selected as e1:[-1,1]; ec1:[-0.3,0.3]; u1:[-10,10], where a negative u1 value indicates that the energy slide valve is unloaded, and a positive value indicates that it is overloaded. The membership functions of E1, EC1, and U1 are determined based on expert experience. The fuzzy inference adopts the Mamdani synthesis algorithm, and the anti-fuzzy operation adopts the Centriod method. 1) During compressor startup and operation, helium is continuously drawn in. The helium pressure P0 drawn in by the compressor is maintained within the required precision through PID control of the replenishment valve. 2) Fuzzy Control + Expert Control: After the compressor starts and builds pressure, P0, P1, and P2 change with the refrigeration unit's gas demand. When the pressure deviation is large, two designed fuzzy controllers are used to adjust the positions of the energy valves of the low-pressure and high-pressure stages of the compressor, fundamentally balancing the amount of gas discharged by the compressor with the refrigeration unit's gas demand. To avoid excessive pressure fluctuations due to the lag in the energy valve's action, two bypass valves are given corresponding openings based on expert experience. Gas that the energy valve cannot transfer in time is transferred through the bypass. As the energy valve moves, the opening of the bypass valve gradually decreases. 3) PID Control: When the pressure deviation is small (a threshold is set), due to the mechanical dead zone of the energy valve, fine adjustment of the bypass regulating valve is needed to compensate for this dead zone. At this time, the energy valve no longer moves, and the PID control of the bypass regulating valve keeps P1 and P2 within the required precision. 4) In case of abnormal situations, such as a quench in the nuclear fusion device, the system's return gas volume is very large. If the energy slide valve and bypass valve operate simultaneously but P2 still cannot decrease, the return helium should be collected into the buffer tank using the gas recovery valve. 5. System Software Configuration Operator station and engineer station: Windows 2000 operating system and KingView 5.1. In addition, the operator station requires: OMRON FinsGateway software for setting up the data link table of the Controller Link network and monitoring network operation, and ADAN software for monitoring the ADAN module's operating status. The engineer station also requires: Omron CX-Programmer 2.0 ladder logic programming software. 6. Main Functions of the Monitoring System : 1) Display Function: Displays process flow, measured values, equipment operating status, operating modes, alarms, etc., and allows for screen recall; 2) Alarm Processing and Report Generation Function: Records alarm occurrence time, fault content, and other information, and manages alarm information. The system outputs daily, monthly, and other reports; 3) Historical Trend Function: Displays on-site helium pressure, liquid helium level, helium temperature, valve opening, etc., as curve graphs. Each trend curve display screen mainly includes screen name, time, trend, and description; 4) Database Storage and Access: Automatically creates and records Access historical databases on a minute-by-minute basis during each system run, and stores on-site data; 5) Screen Function: The system can modify and store system parameters and controller parameters, and can achieve seamless switching between automatic/semi-automatic/manual operation modes; 6) Management Permissions: Implements different levels of system management permissions. System operators can select operating modes, view trend curves and reports, etc.; system engineers can modify the monitoring software and lower-level software according to actual needs. 7. Conclusion This paper studies a screw compressor unit monitoring system based on PLC and configuration software. It leverages the strong anti-interference capability, convenient networking, and suitability for industrial sites of PLCs, while utilizing the powerful data processing and graphical representation capabilities of configuration software. The system integrates advanced automation, computer, communication, fault diagnosis, and software technologies, resulting in high reliability, simple networking, and easy maintenance. Currently, this system has been successfully applied in the HT-7 tokamak nuclear fusion experiment and superconducting magnet experiment with good results. It is of great significance to the operating efficiency of the chiller and even the smooth conduct of the nuclear fusion experiment, while greatly improving the level of automation and reducing the labor intensity of workers. References [1] Xu Shixu. Programmable Controller Principles and Applications Networks [M]. University of Science and Technology of China Press, 2000. [2] KingSCADA version 5.1 User Manual [Z]. 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