How to realize the idea of ​​distributed control in small and medium-sized control systems? The author puts forward the idea of ​​using PC + PLC to form DCS in small and medium-sized chemical projects, and has been successfully applied.
1. Using PC and PLC to realize the basic principle of distributed control (DCS)
The basic idea of ​​distributed control is centralized management and decentralized control. That is: the process industry's automatic control process is relatively separated from the operation management personnel's management process of the automatic control process; the process industry's automatic control process is relatively automatically completed by each control station, and the operator's management of the automatic control process is The operation station of the central control room is completed. On the one hand, the central operation station and each field control station operate relatively independently, thereby limiting various faults to a local scope, greatly improving the overall safety and reliability of the automatic control system; on the other hand, real-time data are exchanged with each other. Communication and information exchange enable operators to manage and adjust the entire automatic control process at the operation station in the central control room.
The main task of the on-site control station is to realize automatic control of the production process, so it must be able to automatically collect various process parameters of the whole plant (such as the temperature, pressure, flow, viscosity, composition and level of various process media) Etc.) and the operating status of the equipment (such as the opening of the valve, the start and stop of the pump, the vibration of the equipment, the mechanical displacement) and other production information, and then perform a large number of numerical calculations according to the pre-programmed control program, and finally output the 4 ~ 20mA standard Analog signals (or ON / OFF digital signals) to drive various valves, motors and other actuators, adjust various process parameters, and realize automatic control of the production process; in addition, real-time communication with the operation station is needed to collect various The production information is transmitted to the operation station for the operator to use, and at the same time receives various instructions issued by the operator through the operation station to adjust the automatic control scheme in real time and optimize the production process. Therefore, it also needs to have a standardized communication interface. At present, all kinds of PLCs have such a function, and its capacity is flexible, it is easy to expand, the configuration of the control scheme is simple and easy to learn, and the performance and price are superior, so it is an ideal choice for small and medium-sized DCS operation stations.
The operation station in the central control room is actually a human-machine interface. On the one hand, it processes various production information collected by the control station, and then uses various process screens, production reports, historical trends and sounds that are familiar and familiar to the operators. Provide the operator with light alarm and other forms. On the other hand, the operator's various instructions are encoded and transmitted to the operating station to adjust the control scheme to optimize the production process or emergency handling of special situations. For small and medium-sized DCS, all the popular monitoring software on the market can achieve such a function, and there is no special requirement on the hardware and operating system of the computer. It can be realized by adding a set of monitoring software with an ordinary PC.
When a PC + PLC is used to form a distributed control system, the PLC undertakes the work of the site control station, and the PC undertakes the work of the operation station and the engineer station. PLC control application software (commonly called ladder diagram) can be edited offline (or online) on a PC with PLC system software installed. After the control application software is downloaded to the PLC, the PLC independently completes field data collection, logic control, analog control, etc . The various functions of the operation station can be realized by "real-time monitoring software" + "PC". It is convenient to monitor the production process on the PC with real-time monitoring software installed.
2. An example of using PC and PLC to implement distributed control (DCS)
2.1. Process introduction:
The liquid ammonia storage and transportation section is an important part of the technological transformation project of Guizhou Hongfu Industrial Development Co., Ltd. with an annual output of 800,000 tons of heavy calcium equipment to ammonium phosphate. The designed ammonia unloading capacity is 250 tons / hour, and the tank area buffer capacity is 9000. Ton. Ammonia is a gas at normal temperature and pressure, which is flammable, explosive, toxic, and harmful; the liquid ammonia storage and transportation section is one of the high-risk areas of the head office, and safe production is the focus of this automatic control system.
2.2. Overview of the control system:
In order to improve the safety of production, in this control system, the important process parameter points were adopted "3 takes 2 votes" strategy, and 21 automatic chain loops were designed to protect the production process; in order to ensure the production process For smooth operation and energy saving, the system has designed 6 adjustment loops. In order to facilitate monitoring and operation, an overview picture of the process screen is designed at the operation station, which collectively displays a batch of process parameters, equipment operating status and alarm information that are closely related to safety production; it is designed for other chemical unit operation processes. The local process screen fully displays various detailed production information related to it; according to the habits of the operator, 4 groups of screens are designed at the operation station, which display the temperature, pressure, flow and liquid level signals respectively; for 6 Each adjustment circuit has separately designed adjustment screens to realize the PID parameter setting, manual automatic mode switching and manual operation of the adjustment valve; pop-up switch screens are respectively designed for 21 main valves to realize automatic control or remote control of the production process The historical trend chart is designed for the main process parameters, which provides data for fault diagnosis and optimized control. In order to ensure safe production and achieve emergency treatment for emergency accidents, an external automatic tracking regulator is added to the 6 regulator circuits. Once a control system failure occurs, it automatically switches to the tracking regulator to control the control valve independently of the DCS; for 21 The main valve is equipped with an emergency treatment button to realize the forced switching of the valve independently of the DCS.
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