Abstract: This paper addresses the challenges of multi-unit coordination and the complex, nonlinear, and highly interference-prone nature of plasma spraying processes. A Siemens S7-300 PLC is used as the core of the field equipment control system, while a PC is used as the host computer to perform jet image acquisition and processing, process monitoring, process optimization, and processing strategy adjustment. Finally, data communication between the PC and PLC is established via the OPC protocol. Experimental results show that the control system operates stably, ensuring the plasma spraying formability and final film quality. Keywords: Plasma spray; PC+PLC; OPC; Control system; Design Abstract: To solve the problem that the process of plasma spray is complex, nonlinear, uncertain and adaptive distributed control required by the whole working unit, Siemens S7-300 PLC is chosen as the central control unit of the local devices, and PC is also selected for image gathering and processing of plasma jet, process monitoring, running of optimization algorithm and regulating of spraying strategies. Finally, OPC protocol is applied for the data communication between PC and PLC. The system is proved to be robust with respect to significant variations in the operating conditions by experimental results, hence it can improve the formability and finally coatings quality of plasma spray. Keywords: Plasma spray; PC+PLC; OPC; Control system; Design 1 Introduction Plasma spraying is widely used in surface modification, functional thin film preparation and materials processing engineering due to its high temperature and concentrated energy, which can spray metals, ceramics or composite materials [1-2]. In order to ensure the formability and quality of the melt-sprayed film, it is necessary to carry out system integration control and process optimization based on digital image processing, process control, artificial intelligence and other methods. At present, several major international thermal spraying equipment and material manufacturers, such as Metallisation in the UK, Sulzer-Metco in Switzerland and Praxair in the US, have launched plasma melt-spraying systems based on PC+PLC+on-site detection+process control. However, due to the high price of related melt-spraying equipment internationally, it is not possible to introduce it to every processing workshop or research institute in China. Therefore, it is necessary to independently develop a melt-spraying process detection and control system suitable for specific processes. At present, there are relevant studies and reports on the development of melt-spraying control systems based on single-chip microcomputers, microcomputers, PLCs and other technologies in China [3]. However, how to integrate the advantages of PC and design a plasma melt-spraying control system based on PC+PLC still needs more in-depth research. This paper develops a plasma melt-spraying control system based on PC+PLC, uses the open OPC protocol to realize communication between the two, and performs robot path planning, online monitoring and melt-spraying process data management in the PC. Combining the stability of PLC field control with the computational and data storage capabilities of the computer, the stability and real-time control of the plasma spraying process are ensured, thereby guaranteeing high-quality applications of plasma spraying in surface modification and rapid mold manufacturing. 2. Structure of the Automatic Control System for Plasma Spraying 2.1 System Composition and Working Principle The PC+PLC-based plasma spraying control system composition principle is shown in Figure 1. The PC mainly performs 3D modeling and slicing of the solid object, finally generating robot path codes that the robot can recognize, and adjusting the path based on field feedback information. It also monitors the entire spraying process, samples key process parameters, and saves them to the processing document. The PLC implements real-time control of the CNC rotary table and the entire plasma arc generation subsystem, and transmits the preliminary processed field sampled data to the host PC. 2.2 Functional Allocation of PC and PLC During plasma spraying, the environment is harsh, with severe noise pollution, strong interference, and a long system operating cycle. Therefore, the core of the field equipment control adopts a Siemens S7-300 PLC, fully utilizing its reliability and good anti-interference capabilities to ensure system reliability. It is equipped with A/D, D/A modules and CP5611 communication card, which can realize analog sampling and output and communication with the host computer. At the same time, the PLC system is also equipped with Siemens dedicated voltage regulator to ensure the stability of system operation and avoid interference caused by sharing power with the whole system. Since the PLC cannot perform monitoring chart display, image processing and complex algorithm design, the operator cannot intuitively understand the on-site situation [4]. In order to make up for the above deficiencies, the system adds a PC for on-site monitoring and data calculation. Its main tasks are to obtain robot status information and film temperature sampling information, and adjust the melting path in real time according to the execution result of the set process optimization algorithm; process the plasma jet detection image and feed back the adjustment information to the PLC to realize the control of the plasma jet generator; at the same time, it can make timely alarms for system faults and take corresponding emergency handling measures and processing site breakpoint protection, etc. [align=center] Figure 1 Composition principle diagram of PC+PLC plasma melting control system [/align] 3 Control system software design 3.1 Control software design The main functions of the control software system include: parameter setting, process monitoring, process optimization, fault information processing and report system, etc. These components work together to achieve real-time monitoring of the entire plasma spraying process status and field data, system fault alarms and corresponding handling, recording of key process parameters and report printing functions. 3.2 OPC Client Program Design The OPC specification defines an industry standard interface, which makes COM technology applicable to process control and manufacturing automation applications. OPC uses the OLE/COM mechanism as the communication standard for applications. OLE/COM is a client/server model with advantages such as language independence, code reusability, and ease of integration. OPC specifies interface functions, and regardless of the form of the field device, the client accesses it in a unified way, thereby ensuring the transparency of the software to the client and freeing the user from low-level development [5-6]. The OPC client software design process is shown in Figure 2. The purpose of its client program development is to realize communication between the computer and the PLC based on the OPC protocol, to directly read and write variables in the PLC through the PC, improve data access speed, ensure that the calculation results of the plasma spraying process optimization algorithm are transmitted to the PLC field control equipment in a timely manner, realize real-time control of the entire system, and thus make full use of the computer's data processing capabilities and rich software resources. 3.3 PLC Program Design The plasma spraying system uses a Siemens S7-300 PLC as the core for field device control, enabling control of field devices, motion control throughout the process, and field data sampling. The PLC's internal program is divided into manual control and automatic operation sections, responding to button actions on the control panel and control commands from the host computer, respectively. The PLC program is designed using Step7, and the main process includes: First, a new project, SprayControl, is created in Step7, and then a SIMATIC 300 Station is inserted. Hardware configuration is performed according to the PLC hardware configuration and the physical installation location of the template. Next, a Simatic PC Station is inserted, and an OPC Server and CP5611 are inserted within it. In the Connections section of the OPC Server, a network connection is established between the PC Station and the Simatic 300 Station based on the MPI network. After the MPI network is successfully established, all digital, analog, and data block variables designed within the PLC's CPU unit can be seen in the Symbols list in the OPC Server. The network connection diagram after networking based on the MPI method is shown in Figure 3. Finally, an OPC server named Spray was established based on SimaticNet software, allowing access to PLC variables via an OPC client program. [align=center] Figure 2: OPC Client Program Design Flowchart Figure 3: Network Connection Diagram Based on MPI Networking[/align] The PLC's running programs are concentrated in the Blocks section of S7 Program. The main modules include the system main control module OB1, responsible for calling other function blocks. Then, function blocks for powder feeder control, turntable control, robot fault handling, and system fault handling were designed for the main control block to call. To ensure the safe execution of the PLC program, object blocks OB80, OB82, and OB85 were added to handle template diagnostic errors and timeout errors respectively, while OB121 and OB122 responded to synchronization errors. During the design process, variables can be categorized or dedicated data blocks can be designed for specific function blocks to manage variables in the control system in a unified group. 4 Conclusion This paper developed a PC+PLC-based automatic control system for plasma spraying. Experimental verification has shown that the system possesses excellent anti-interference capabilities and can adapt to the requirements of plasma spraying processes, laying a foundation for the transformation of this technology into productivity. Simultaneously, the PC, acting as the host computer, provides a user-friendly interface and effective system monitoring and management, while the PLC, acting as the slave computer, performs reliable on-site control, ensuring system stability. This control system can be easily integrated with robots, other actuators, or production lines to form a plasma spraying system. The author's innovation lies in designing a plasma spraying control system combining PC and PLC. This integrates the stable performance of the PLC in harsh spraying environments with the PC's advantages in image processing and complex algorithm calculations. Communication between the PC and PLC is achieved based on the OPC protocol, ensuring the real-time acquisition, transmission, and processing of multi-variable information in process control. This automatic control system lays the foundation for improving the formability and quality of plasma sprayed films. References [1] Zhang Haiou, Han Guangchao, Wang Guilan. Rapid mold manufacturing technology [J], China Mechanical Engineering, 2002, 13 (22): 1903-1906. [2] Sofiane B, Ghislain M, Patrick G, et al. Designing expert system using neural computation in view of the control [J], Materials and Design, 2003, 24: 497-502. [3] Wang Wei, Xia Weisheng, Zhang Haiou, Wang Guilan. Design and research of robot plasma spraying control system [J]. Journal of Huazhong University of Science and Technology (Natural Science Edition), 2005, 33 (7): 74-76. [4] Guo Shigang. Human-machine interface and programming of PLC [J]. Microcomputer Information, 2006, 19: 42-44. 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