Abstract: This paper discusses a novel hybrid process control system (HCS) that integrates the advantages of PLC and DCS. It demonstrates that this system possesses high reliability, high speed, flexible operation, and a high performance-price ratio in applications such as logic control, sequential control, and continuous control. Some perspectives on the development of this control system are also presented. Keywords: PLC, DCS, process control system. PLC, developed from the combination of early relay logic control systems and microcomputer technology, is an industrial control instrument based on a microprocessor. It integrates computer technology, control technology, and communication technology, combining sequential control, process control, and data processing. It boasts high reliability, powerful functions, flexible control, and simple operation and maintenance. In recent years, programmable logic controllers and their systems have been widely used in China's metallurgical, power plant, light industry, petrochemical, mining, and water treatment industries, achieving certain economic benefits. Since industrial production processes are distributed systems, process control is best performed in a decentralized manner, while monitoring, operation, and optimization management should be centralized. With the continuous expansion of industrial production scale, the requirements for control and management have been constantly increasing, process parameters have been growing, and control loops have become increasingly complex. Distributed control systems (DCS) emerged in the mid-1970s and were immediately favored by the industrial control community. A DCS is a system integrating computer technology, control technology, network communication technology, and graphic display technology. Compared with conventional centralized control systems, it has the following characteristics: 1. It achieves decentralized control. This makes system control risks dispersed, reliability high, investment reduced, and maintenance convenient. 2. It achieves centralized monitoring, operation, and management. This separates management from the field, making management more comprehensive and systematic. 3. It adopts network communication technology, which is a key technology of DCS. It enables real-time control and management and solves the problems of system expansion and upgrades. Currently, because PLCs have replaced dedicated data highways with universal networks and are gradually converging communication protocols between PLCs, they can be integrated with various other computer systems and equipment to form large-scale control systems. This gives PLC systems the form of DCS, and thus, PLC-based DCS systems are now widely used both domestically and internationally. Programmable Logic Controllers (PLCs) and Distributed Control Systems (DCS) are currently the two most widely used control technologies in the industrial control field. Each has its own distinct advantages and disadvantages. For example, PLCs dominate in high-speed sequential control, while DCSs excel in complex process control. PLCs are small, flexible, and relatively inexpensive, but they fall short of DCSs in communication and management capabilities. While DCSs offer stronger communication and management capabilities, they are larger and relatively more expensive. Therefore, users desire a control system that integrates the advantages of both PLCs and DCSs. This hybrid control system (HCS) should perfectly realize logic and sequential control, effectively perform process control, and also possess management functions, while being small, inexpensive, and highly reliable. Some automation equipment manufacturers have already launched products in this area; Rockwell's recently released Allen-Bradley (AB) ProcessLogix process control system is one such example. This article introduces a hybrid control system that combines the design essence of traditional DCS (centralized management, decentralized control) with the inherent flexibility and low cost of traditional PLCs. It is suitable for discrete, sequential, and continuous control applications, achieving high-tech automation at the lowest cost. I. Comparison of PLC and DCS If we categorize the performance of PLCs and DCS into 1 to 10 levels from weakest to strongest, their comparison can be listed in Table 1. Table 1: Comparison of PLC and DCS. Early PLCs primarily focused on sequential control of digital quantities. As PLC functions expanded, they added analog control, PID regulation, communication networking, and hierarchical control functions. Industries where DCS previously dominated, such as chemical and metallurgical industries, can now also be controlled by PLCs. However, PLCs are difficult to use to form large, complex, and integrated systems. If too many PCs attempt to communicate with too many PLCs via a network, it can lead to bottlenecks and timing difficulties. DCS evolved from analog instrument control systems, initially focusing on loop regulation, and later adding sequential control functionality. The design philosophy of DCS (Distributed Control System) is centralized operation and management with decentralized control to improve the reliability and management capabilities of the entire system. These advantages make DCS still mainstream in the high-end control system market. However, DCS is more expensive than PLC, which can be unaffordable for some small and medium-sized enterprises with limited funds. This paper discusses a hybrid control system that integrates the advantages of PLC and DCS, making it particularly suitable for applications requiring low-cost automation (LCA). II. Hybrid Control System Structure Figure 1 shows a hybrid control system. This system is mainly based on the design philosophy of DCS, combining the advantages of both PLC and DCS. It mainly consists of a system network, operator workstations, a central server, controllers, and input/output (I/O) modules. 1. System Network For DCS, the system network is the foundation and core of the entire system, playing a decisive role in the real-time performance, reliability, and scalability of the entire system. The same applies to the hybrid control system. As shown in Figure 1, HCS is a hierarchical control system, divided into two levels: the operation and management level and the process control level. The various devices in the operation and management level—operator workstations and the central server—are connected by an N1 network. N1 network is a local area network (LAN) requiring high-speed transmission of large amounts of data. Ethernet or ARCNET can be used. Ethernet uses a carrier sense/multiple access protocol, offering communication speeds of 10Mbps and 100Mbps, but it lacks real-time performance. ARCNET uses a token-passing protocol, offering a communication speed of 2.5Mbps and good real-time performance. N1 network topologies can include star, bus, and hybrid structures. Bus network technology is relatively mature, construction is simple, and node additions or removals do not require network interruption, making it a commonly used network structure in industrial control networks. N1 networks can use twisted-pair cable, coaxial cable, or fiber optic cable as the transmission medium. N2 network connects process control level controllers to controllers, controllers to remote I/O, and remote I/O to remote I/O, and connects them to a central server. N2 networks require real-time performance; once a node on the N2 network sends data, all connected nodes on the network should be able to receive the data simultaneously, achieving data sharing. This is crucial for process control with high real-time requirements. The N2 network also possesses other characteristics, such as the ability to add new devices without causing network congestion, completely eliminating system bottlenecks; its open architecture makes it compatible with other devices such as PLCs and intelligent motor drives. AB's ProcessLogix process control system uses a bus-structured Ethernet network for the N1 network and AB's own ControlNet network for the N2 network. With repeaters, the transmission distance can reach 30km. 2. Operator Workstations Operator workstations are devices that exchange information between the HCS and users. Their main function is to provide a human-machine interface for operators, enabling them to understand the system's operating status in a timely and comprehensive manner and to adjust and control the production process. With the continuous improvement of microcomputer performance, operator workstations can be handled by PCs. Since the operator workstations are connected to the central server via a local area network, adding an operator workstation is very easy; simply add a PC to the N1 network and have the central server provide the client software for the newly added operator workstation. 3. Central Server All system information, reports, and the overall database are centrally managed by the central server to achieve centralized information management. The central server uses Windows NT as its operating system, along with system application software, and can also connect to enterprise management systems. The configuration functions of the DCS are also provided by the central server in the HCS. Engineers can use the system application software on the central server to modify or add control configurations and download them to the controllers. 4. Controller The controller is the control center in an automatic control system. Like a PLC, the HCS controller adopts a typical computer architecture, mainly including a processor, memory, I/O interfaces, and a communication interface. The HCS controller's frame follows the traditional PLC's frame size structure, thus its size is much smaller than a DCS. Although the HCS controller is extremely similar to a PLC in size and appearance, it is by no means a simple replica of a PLC. The HCS controller performs closed-loop and sequential control in process control and can handle the process control tasks undertaken by the DCS. The HCS controller adopts a modular structure; the processor module, local I/O module, and communication interface module are all inserted into the same frame and connected via a data bus to achieve "soft wiring." In addition, remote I/O modules can be expanded via the N2 network. Each HCS can support multiple controllers, and each controller can support up to hundreds of control loops. Thus, HCSs can be used to form large-scale control systems with distributed control. For example, the ProcessLogix control system can support up to 16 controllers, each supporting 125-150 control loops. 5. Input/Output (I/O) Modules: HCS provides various specifications of I/O modules that can be directly connected to industrial field I/O signals, such as analog/digital, DC/AC, voltage/current, and I/O modules of different voltage levels. These I/O modules can be directly connected to industrial field devices such as buttons, transmitters, sensors, solenoid valves, and motor controllers, offering flexibility and convenience. III. Characteristics of Hybrid Control Systems: 1. Hierarchical Distributed Control, Centralized Management: HCS retains the advantages of centralized information and distributed control found in DCS. The system is vertically divided into two levels based on function: operation management level and process control level. Each level has its own division of labor and is interconnected, operating under the coordination of the system. Simultaneously, it is horizontally decomposed according to the production process to meet the need for control throughout the entire plant area. The distributed control structure is adopted, and multiple controllers and I/O frameworks are distributed and networked. On the one hand, all information of the production process can be transmitted to the central server through the network to achieve information centralization. On the other hand , it avoids the danger caused by the failure of individual equipment affecting the entire system and improves reliability. 2. High flexibility and strong scalability HCS adopts a modular and building block structure. Users can choose different numbers and specifications of unit equipment to form hardware systems with different requirements and scales. For example, the controller and its remote I/O adopt a PLC-style modular structure. Users can choose different specifications of modules according to different application scenarios. The whole system adopts a hierarchical distributed network structure, so that adding or removing certain units will not affect the performance of the whole system. This flexible assembly method makes system expansion easy and is conducive to the factory to configure the system according to the current scale and improve the utilization efficiency of equipment. 3. High reliability (1) Redundancy technology HCS allows users to expand redundant components in any critical parts as needed to avoid affecting the operation of the system due to the failure of a certain component. For example, adding redundant processors, redundant central servers and redundant transmission media (such as coaxial cables) to the controller. (2) Self-diagnostic function: The HCS system software can monitor the hardware and software status of the entire system online. Once an abnormality is detected, effective measures can be taken immediately to prevent the fault from escalating. (3) Power failure protection function: The processor module of the HCS controller is equipped with a lithium battery to prevent data loss due to power failure. IV. Prospects for Hybrid Control Systems 1. Smaller size, lower price, and stronger function: With the development of ultra-large-scale integrated circuit technology, the HCS controller will use a higher-performance microprocessor as the processor, which will greatly enhance the control function. With the development of installation and wiring technology, HCS can adopt surface mounting and flat enclosure technology, making the HCS smaller and cheaper. 2. More diverse I/O modules: Since the I/O modules are directly connected to industrial field equipment, the strength of the I/O module function directly affects the system control capability. For this reason, various manufacturers are competing to develop a variety of I/O modules, such as positioning control modules, CRT modules, numerical control modules, computing modules, voice processing modules, fuzzy modules, etc. Most of them are intelligent I/O modules with built-in processors, which can meet the complex control needs of different occasions. If HCS adopts these intelligent I/O modules, its control function will be greatly enhanced. 3. Introduce artificial intelligence technology (1) Adopt intelligent control algorithms in the system application software, such as fuzzy logic, expert system, genetic algorithm, neural network and other advanced control algorithms, to form control modules and network systems with artificial intelligence, and improve the control level of the system. (2) Use artificial intelligence technology for self-diagnosis and early prediction of faults. Since artificial neural networks have the ability to self-organize, self-learn and adapt, as well as parallel processing capabilities, after training, they can monitor and detect faults in HCS, make early predictions of faults, optimize process control, and improve the reliability of the system.