0. Introduction With the continuous development of chemical automation technology, the concept of distributed control (DCS) is increasingly favored by automation engineers and is gradually being applied to new construction, expansion, and technical upgrading projects. However, traditional DCS systems are generally manufactured by specialized companies and have a certain degree of proprietary nature; furthermore, traditional DCS systems generally have a large control scale and high cost, thus limiting their application in small and medium-sized automation system projects. So how can the concept of DCS be implemented in small and medium-sized control systems? With this question in mind, based on extensive reading of technical materials, the author summarized and compared existing DCS and PLC control systems, proposing the idea of ​​using PC + PLC to construct a DCS in small and medium-sized chemical projects. This idea was successfully applied in the liquid ammonia storage and transportation section of the technical upgrading project of converting an 800,000-ton-per-year heavy calcium carbonate plant to ammonium phosphate at Guizhou Hongfu Industrial Development Co., Ltd. 1. Basic Principles of Distributed Control (DCS) Using PC and PLC The basic idea of ​​distributed control is centralized management and decentralized control. In other words, the automatic control process in the process industry is relatively separated from the management process of the automatic control process by operators. The automatic control process in the process industry is completed automatically by each control station relatively independently, while the management of the automatic control process by operators is completed by the operator station in the central control room. On the one hand, the central operator station and each field control station operate relatively independently, thereby limiting various faults to a local scope and greatly improving the overall safety and reliability of the automatic control system; on the other hand, they also conduct real-time data communication and information exchange with each other, enabling operators to manage and adjust the entire automatic control process from the operator station in the central control room. The primary task of the field control station is to achieve automatic control of the production process. Therefore, it must be able to automatically collect various process parameters from the entire plant (such as temperature, pressure, flow rate, viscosity, composition, and level of various process media) and equipment operating status (such as valve opening, pump start/stop, equipment vibration, and mechanical displacement). Then, it performs extensive numerical calculations according to a pre-programmed control program, and finally outputs a 4-20mA standard analog signal (or ON/OFF digital signal) to drive various valves, motors, and other actuators, adjusting various process parameters to achieve automatic control of the production process. In addition, it must communicate with the operator station in real time, transmitting the collected production information to the operator station for use by the operators, and simultaneously receiving various commands issued by the operators through the operator station to adjust the automatic control scheme and optimize the production process in real time. Therefore, it also needs a standardized communication interface. Current PLCs all possess these functions, and they offer high capacity flexibility, convenient expansion, simple and easy-to-learn control scheme configuration, and excellent performance-price ratio, making them ideal choices for operator stations in small and medium-sized DCS systems. The operator station in the central control room is essentially a human-machine interface. On one hand, it processes various production information collected by the control station and provides it to operators in familiar formats such as flowcharts, production reports, historical trends, and audible and visual alarms. On the other hand, it encodes operator commands and transmits them to the operator station for adjustments to the control scheme, optimizing the production process or handling emergencies. For small and medium-sized DCS systems, most popular monitoring software on the market can achieve this function without special requirements for computer hardware and operating systems; a regular PC with monitoring software is sufficient. When using a PC and PLC to form a distributed control system, the PLC performs the work of the field control station, while the PC performs the work of the operator station and engineer station. The PLC's control application software (generally called ladder diagrams) can be edited offline (or online) on a PC with the PLC system software installed. After the control application software is downloaded to the PLC, the PLC independently completes field data acquisition, logic control, and analog control. All functions of the operator station can be implemented through "real-time monitoring software" + "PC". The production process can be easily monitored on the PC with the real-time monitoring software installed. 2. An example of distributed control (DCS) using PC and PLC 2.1. Process overview: The liquid ammonia storage and transportation section is an important part of the technical renovation project of Guizhou Hongfu Industrial Development Co., Ltd., converting its 800,000-ton/year heavy calcium carbonate plant to ammonium phosphate. It is designed to unload ammonia at a capacity of 250 tons/hour, with a tank buffer capacity of 9,000 tons. Ammonia is a gas at normal temperature and pressure, and is flammable, explosive, toxic, and harmful. The liquid ammonia storage and transportation section is one of the high-risk areas of the company, and safe production is a key consideration for this automatic control system. 2.2. Control system overview: To improve production safety, this control system adopts a "3-out-of-2 voting" strategy for important process parameters and designs 21 automatic interlocking loops to provide interlocking protection for the production process. To ensure stable operation and energy saving, the system is designed with 6 regulating loops. To facilitate monitoring and operation, a general flow chart was designed at the control station, centrally displaying a set of process parameters, equipment operating status, and alarm information closely related to safe production. Local flow charts were designed for other chemical unit operations, comprehensively displaying various detailed production information related to them. Based on operator habits, four sets of screens were designed at the control station, centrally displaying temperature, pressure, flow rate, and liquid level signals. Control screens were designed for each of the six control loops, enabling PID parameter tuning, switching between manual and automatic modes, and manual operation of control valves. Pop-up switch screens were designed for each of the 21 main valves, enabling automatic or remote control of the production process. Historical trend charts were designed for the main process parameters, providing data for fault diagnosis and optimized control. To ensure safe production and enable emergency handling, external automatic tracking controllers were installed in the six controller loops. In the event of a control system failure, the system automatically switches to the tracking controller to control the control valves independently of the DCS. Emergency handling buttons were installed on the 21 main valves, enabling forced opening and closing of the valves independently of the DCS. 2.3 Hardware Configuration: The control station uses an OMRON C200 PLC, configured with 9 digital modules (OD211/ID212), 4 analog modules (AD003), and 3 regulating modules (PID03); the operator station uses a DELL OPTIPLEX GX150 computer; and the engineer station uses a COMPA PC. The configuration diagram is as follows: 2.4. Control Station Software Configuration: The control station configuration uses OMRON's system software SSS as the technical platform and ladder diagrams as the programming tool. Its configuration content mainly includes: 2.4.1. PLC Internal Address Allocation: I/O Address Allocation: The PLC's I/O address is a unique, one-to-one corresponding register address for data communication between the PLC and field detection devices and actuators. I/O address allocation is the basis for further PLC configuration. For OMRON-C200, the I/O address is related to the connected I/O module. For field devices connected to digital modules, the I/O address depends on the installation location of the I/O module and the point number on that module. For field devices connected to analog modules or PID modules, the I/O address depends on the unit number of the I/O module (different modules should have different unit numbers set through the module's hard switch) and the point number on that module. For example, in this system configuration, the field radar level transmitter LT-101 outputs 4~20mA... The analog signal is connected to the second point of the analog input module AD003 with unit number 3, so its configured address in the PLC is IR: 132; while the closed status signal (closed) of valve HV120 is connected to the tenth point of the digital input module ID212 installed in the second slot of the expansion rack, so its configured address in the PLC is IR: 01210; in this system, a total of 142 I/O addresses are defined. Data exchange address allocation between operator station and control station: Data communication between the operator station and control station is accomplished by reading and writing the PLC's internal registers. To achieve real-time communication between the operator station and control station, sufficient internal register addresses must be configured for the PLC to store this data. For example, if DM0232 is defined as the internal register for exchanging data from LT-101 between the operator station and control station, the PLC will preprocess the acquired LT-101 level signal and store it in DM0232, while the operator station will read the LT-101 data from the PLC's DM0232 to build its own database. In this system, a total of 184 such addresses are defined. Intermediate address allocation: During operation, the PLC also requires a large number of intermediate registers to store temporary data during calculations. To improve the readability of the application program, these registers must also be defined and commented as necessary. 2.4.2. Ladder Diagram Writing for Automatic Adjustment: This system uses three PID03 modules to form six adjustment loops to complete the automatic control of the production process. To facilitate operator management of the control process at the workstation, SW2 of PID03 should be set to ON, and a corresponding ladder diagram should be written to realize data exchange between the PLC and PID03. For example, the adjustment of the PIC111 in the furnace is completed by the second loop of PID03 with unit number 5. After executing the program shown in Figure 2, the data in address DM0060 in the PLC is defined as the setpoint of the adjustment loop PIC111. (Complete ladder diagram omitted) Two-out-of-three voting: To ensure safe production, the pressure of the atmospheric pressure tank must be controlled within the specified range. Whenever it rises (falls) to a certain range, the corresponding equipment must be started (stopped). For this purpose, three pressure gauges are used on site to measure the pressure. The PLC compares the three pressures, and the interlock only operates when two of the three pressure gauges meet the conditions simultaneously. When writing the ladder diagram, comparison instructions, plus AND, OR, NOT and other logical instructions, can be used to implement this control strategy. (Ladder diagram omitted) Interlocking Protection: Ladder diagrams are very similar to electrical interlocking logic diagrams. Once the I/O addresses are determined, writing ladder diagrams for interlocking protection is both simple to operate and highly readable. To protect equipment and production safety, this system has written a total of 21 interlocking circuits. (Ladder diagram omitted) 2.4.3. I/O Module Settings and Calibration: After the ladder diagram is completed, the I/O modules must be set and calibrated as necessary for the PLC to work properly. The analog module should be set with the input signal types corresponding to the field equipment and the preprocessing method for the input signals. Zero point and range should also be calibrated. In addition to setting the input signal types and preprocessing methods, the PID module should also set the contents of the PID module storage area and its modification method, the modification method of the control loop setpoint, the control action of the PID and its control method, etc. 2.5. Operator Station Software Configuration: The operator station configuration uses INTELLUTION's system software FIX32 as the technical platform. Its main contents include: system configuration, database establishment, flowchart drawing, definition of historical trends and reports, etc. System configuration in this system essentially involves installing the FIX system on a PC. Its main components include defining the FIX system's installation directory, installing interface device drivers, configuring the SCADA system, configuring the alarm system, and configuring the network. FIX provides a vast library of I/O interface device drivers. This system uses an OMRON PLC as the control station; therefore, the I/O driver OMR.drv must be installed to communicate with the OMRON PLC. Database establishment: The database is the foundation upon which the SCADA system operates. It consists of a series of data points, each of which is essentially a function block. FIX provides various function blocks to meet different needs. These function blocks either read and write data from interface devices or perform calculations and alarm processing. Creating a data point in the database defines a function block, which includes: function block type, data point tag number, comments, zero point, range, interface device, I/O address, data format, alarm upper and lower limits, etc. For example, add an AI module to the database and define it in its properties dialog box as follows: "Tag Number" is "LT-101", "Description" is "Buffer Tank F0101A Level", "Interface Device" is "OMR", "I/O Address" is "D:DM:232", "Data Format" is "12AL", "Zero Point" is "0", "Range" is "17", and "Unit" is "M". Then, a data point LT-101 is created in the database. It reads data (0-4095) from the register at address DM0232 in the PLC and converts it into 0-17M data for use by other FIX function blocks and flowcharts. Drawing the flowchart: The flowchart screen is essentially a human-machine interface. Operators understand and control the production process through the flowchart screen, so the flowchart screen must be comprehensive yet concise. The FIX system provides Windows-style drawing tools and related controls, which can easily draw various dynamic screens to meet the requirements of operators. For example, in the process flow diagram, to visually display the liquid level of buffer tank F0101A, simply select the dynamic fill attribute in its dynamic properties dialog box and define that the height of its fill color changes according to the size of the data point with "tag number" "LT-101". To accurately display the actual height of the liquid level, a dynamic data connection can be defined next to the tank graphic, connecting to the data point with "tag number" "LT-101". To intuitively display the working status of various valves, select dynamic color change in its dynamic properties dialog box; a closed valve displays a static red, an open valve displays a static green, a closed valve displays a flashing red when malfunctioning, and an open valve displays a flashing green when malfunctioning. To quickly control valves, connect their pop-up switch screens to their graphics; simply click on the graphic to pop up the switch screen, achieving a WYSIWYG (What You See Is What You Get) experience for objects on the process flow diagram. Report Definition: Considering that the head office has implemented electronic office procedures, all reports are set to be saved to files on a schedule. Operators can access them at any time as needed. With the implementation of office automation, these reports are connected to the company's internal management network and can be viewed and accessed via a web page. 3. Conclusion This control system has been in use for over two years, demonstrating stable performance, reliable operation, a user-friendly interface, simple operation, and minimal maintenance, making it popular with operators and maintenance personnel. After its initial deployment, it underwent two expansions to meet the needs of technical upgrades without affecting normal production. Practice has proven that a PC+PLC DCS configuration offers flexible system configuration, simple software configuration, ease of self-design and debugging, excellent performance-price ratio, easy system expansion, and minimal maintenance, making it the preferred automation system for enterprises undertaking technical upgrades and small-to-medium-sized production processes. References: 1) C200H programmable Controllers Operation Manual, OMRON, 1995. 2) C200H-PIDOX PID Control Unit Operation Manual, OMRON, Revised March 1995. 3) C200H-AD003 Analog I/O Unit Operation Manual, OMRON, Revised June 1998. 4) FIX-OMR Driver Manual, Intellution, 2000. 5) Intellution-FIX Electronic Book, Intellution, 2000. About the author: Li Wancheng, male, born in 1963, senior engineer in instrumentation, research direction: data acquisition and monitoring of chemical processes.