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 technological 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-scale automation system projects. So how can the concept of distributed control be implemented in small- and medium-scale 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-scale chemical projects. This concept has been successfully applied in the liquid ammonia storage and transportation section of the technological upgrading project of the 800,000-ton-per-year heavy calcium carbonate plant to ammonium phosphate plant at Guizhou Hongfu Industrial Development Co., Ltd.
1. Basic principles of distributed control system (DCS) using PC and PLC
The basic idea of distributed control is centralized management and decentralized control. That is, 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 and relatively independently by each control station, 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, thus limiting various faults to a localized area and greatly improving the overall safety and reliability of the automatic control system; on the other hand, they also communicate and exchange data in real time, enabling operators in the central control room to manage and adjust the entire automatic control process.
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 functionality, with no special requirements for computer hardware or operating systems; a standard PC with monitoring software is sufficient.
When a distributed control system is composed of a PC and a PLC, the PLC performs the functions of the field control station, while the PC performs the functions of the operator station and engineer station. The PLC's control application software (generally called a ladder diagram) 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. The various functions of the operator station can be implemented through a combination of real-time monitoring software and a PC. A PC with real-time monitoring software installed can conveniently monitor the production process.
2. An example of implementing distributed control system (DCS) using PC and PLC
2.1. Brief Introduction to the Process:
The liquid ammonia storage and transportation section is a crucial component of the technical upgrade project of Guizhou Hongfu Industrial Development Co., Ltd., which converts its 800,000-ton-per-year heavy calcium carbonate plant into an ammonium phosphate plant. It is designed to unload ammonia at a capacity of 250 tons per hour, with a tank area 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 company's high-risk areas, making safe production a key consideration for this automatic control system.
2.2. Overview of the Control System:
To improve production safety, this control system adopts a "3-out-of-2 voting" strategy for critical process parameters and designs 21 automatic interlocking loops to provide interlocking protection for the production process. To ensure stable operation and energy conservation, the system includes 6 regulating loops. For ease of monitoring and operation, a general flow chart is designed at the operator station, centrally displaying a set of process parameters closely related to safe production, equipment operating status, and alarm information. Local flow charts are designed for other chemical unit operations, comprehensively displaying various detailed production information. Based on operator habits, four sets of screens are designed at the operator station, centrally displaying temperature, pressure, flow rate, and liquid level signals. Regulating screens are designed for each of the 6 regulating loops, enabling PID parameter tuning, switching between manual and automatic modes, and manual operation of regulating valves. Pop-up switch screens are designed for each of the 21 main valves, enabling automatic or remote control of the production process. Historical trend charts are designed for key process parameters, providing data for fault diagnosis and optimized control. To ensure safe production and enable emergency response to accidents, external automatic tracking controllers were installed on six controller loops. In the event of a control system failure, the system will automatically switch to the tracking controller to control the control valves independently of the DCS. Emergency handling buttons were also installed on 21 main valves to enable 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 control modules (PID03); the operator station uses a DELLOPTIPLEX 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 is configured using OMRON's SSS system software 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: A PLC's I/O address is a unique, one-to-one register address for data communication between the PLC and field monitoring devices and actuators. I/O address allocation is the basis for further PLC configuration. For the 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 I/O module's installation location and its pin number. For field devices connected to analog or PID modules, the I/O address depends on the I/O module's unit number (different modules should be identified through the module's hardware). The switches are configured with different unit numbers and point numbers on the module; for example, in this system configuration, the 4~20mA analog signal output by the radar level transmitter LT-101 in the field 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 operator station and control station is accomplished by reading and writing PLC internal registers. In order to achieve real-time communication between operator station and control station, sufficient internal register addresses must be configured for PLC to store this data. For example, if DM0232 is defined as the internal register for exchanging LT-101 data between operator station and control station, then PLC will store the acquired LT-101 level signal in DM0232 after preprocessing, while operator station will read LT-101 data from DM0232 of PLC to build its own database. In this system, a total of 184 such addresses are defined.
Intermediate address allocation: During the operation of the PLC, a large number of intermediate registers are needed to store temporary data during the operation. In order to improve the readability of the application, these registers must be defined and commented as necessary.
2.4.2. Write the ladder diagram for the control strategy
Automatic adjustment: This system uses three PID03 modules to form six adjustment loops to complete the automatic control of the production process. In order to facilitate the operator's management of the control process at the operator station, SW2 of PID03 should be set to ON, and a corresponding ladder diagram should be written to realize the data exchange between PLC and PID03. For example, the adjustment of PIC111 is completed by the second loop of PID03 with unit number 5. When the program shown in Figure 2 is executed, the data in address DM0060 in PLC is defined as the setpoint of adjustment loop PIC111.
Three-out-of-two voting: To ensure safe production, the pressure of the atmospheric pressure tank must be controlled within a specified range. Whenever the pressure rises (falls) to a certain range, the corresponding equipment must be started (stopped). Therefore, three pressure gauges are used on-site to measure the pressure. The PLC compares the three pressures, and the interlock only activates when two of the three gauges simultaneously meet the required conditions. This control strategy can be implemented by using comparison instructions, along with AND, OR, and NOT logic instructions, when writing the ladder diagram. (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 a total of 21 interlocking circuits. (Ladder diagrams omitted)
2.4.3. I/O Module Setup and Calibration:
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 large library of I/O interface device drivers. Since this system uses an OMRON PLC as the control station, the I/O driver OMR.drv must be installed to enable communication 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 functional block. FIX provides various functional blocks to meet different needs. These functional blocks either read and write data from interface devices or perform calculations and alarm processing on the data. Establishing a data point in the database is equivalent to defining a functional block, whose contents include: functional 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, if you add an AI module to the database and define the following in its properties dialog box: "Tag Number" as "LT-101", "Description" as "Buffer Tank F0101A Level", "Interface Device" as "OMR", "I/O Address" as "D:DM:232", "Data Format" as "12AL", "Zero Point" as "0", "Range" as "17", and "Unit" as "M", then a data point LT-101 will be created in the database. It reads the data (0-4095) from the register with address DM0232 in the PLC and converts it into 0-17M data for use by other function blocks and flowcharts in FIX.
Flowchart creation: A flowchart is essentially a human-machine interface. Operators use the flowchart to understand and control the production process. Therefore, the flowchart needs to be comprehensive yet concise. The FIX system provides Windows-style drawing tools and related controls, making it easy to create various dynamic flowcharts to meet operator requirements. For example, in the process flow diagram, to visually display the liquid level of buffer tank F0101A, simply select the dynamic fill attribute in the dynamic properties dialog box of its graphic, 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, connected to the data point with "tag number" "LT-101"; to intuitively display the working status of various valves, select dynamic color change in the dynamic properties dialog box of its graphic, displaying a static red when the valve is closed, a static green when the valve is open, a flashing red when the valve is closed and a flashing green when the valve is open; to quickly control the valve, connect its pop-up switch screen to its graphic, and simply click on its graphic to pop up the switch screen, achieving WYSIWYG for the objects on the process flow diagram.
Report Definition: Considering that the head office has implemented electronic office, all reports are set to be saved to files on a schedule. Operators can access them at any time as needed. With the promotion of office automation, they are connected to the company's internal management network and can be viewed and accessed through a web page.
3. Conclusion
Since its commissioning more than two years ago, this control system has demonstrated 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 DCS composed of PC and PLC offers flexible system configuration, simple software configuration, ease of self-design and debugging, superior 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.
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 and preprocessing methods corresponding to the field equipment, and the 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.
2.5. Configuration of the operator station software:
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.
