What is a DCS control system?
DCS stands for Distributed Control System, also known as Distributed Control System in the domestic automation industry. The so-called distributed control system, or in some materials referred to as a distributed system, is a new type of computer control system relative to centralized control systems. It is developed and evolved on the basis of centralized control systems. It is a multi-level computer system composed of process control level and process monitoring level, connected by a communication network. It integrates 4C technologies such as computer, communication, display, and control. Its basic idea is decentralized control, centralized operation, hierarchical management, flexible configuration, and convenient configuration.
Firstly, the backbone of DCS – the system network, is the foundation and core of DCS. Due to the decisive role that networks play in the real-time, reliability, and scalability of the entire DCS system, various manufacturers have carefully designed in this regard. For the system network of DCS, it must meet the real-time requirements, that is, complete the transmission of information within a determined time limit. The “definite” time limit referred to here refers to the time limit within which information transmission can be completed under any circumstances, and this time limit is determined based on the real-time requirements of the controlled process. Therefore, the indicator for measuring system network performance is not the network rate DCS, commonly known as bits per second (bps), but the real-time performance of the system network, that is, how long it can ensure the completion of the required information transmission. The system network must also be very reliable, and network communication cannot be interrupted under any circumstances. Therefore, most manufacturers use a dual bus, ring, or dual star network topology for their DCS. In order to meet the requirements of system scalability, the maximum number of nodes that can be accessed on the system network should be several times larger than the actual number of nodes used. In this way, on the one hand, new nodes can be added at any time, and on the other hand, the system network can operate under a lighter communication load state to ensure the real-time and reliability of the system. During the actual operation of the system, the connection and disconnection of various nodes can occur at any time, especially at the operator station. As a result, network reconstruction will occur frequently, and this operation must not affect the normal operation of the system. Therefore, the system network should have a strong online network reconstruction function.
Secondly, this is a network node that fully handles on-site I/O processing and implements direct digital control (DDC) functionality. Generally, a field I/O control station should be set up in a DCS to share the I/O and control functions of the entire system. This can not only avoid the failure of the entire system due to the failure of one site, improve system reliability, but also enable each site to share data collection and control functions, which is conducive to improving the performance of the entire system. The operator station of DCS is a network node that handles all HMI Human Machine Interface or operator interface functions related to operational operations.
The engineer station is a network node that performs offline configuration, configuration work, and online system supervision, control, and maintenance on DCS. Its main function is to provide tool software (i.e. configuration software) for DCS configuration and configuration work, and to monitor the operation of various nodes on the DCS network in real time during DCS online operation. This allows system engineers to adjust system configuration and some system parameter settings in a timely manner through the engineer station, Keep DCS in optimal working condition at all times. Unlike centralized control systems, all DCS require system configuration functions. It can be said that a system without system configuration functions cannot be called DCS. DCS has gone through more than thirty years of development since its inception in 1975. Over the past thirty years, although DCS has not undergone significant changes in its system architecture, its functionality and performance have been greatly improved through continuous development and improvement. Overall, DCS is developing towards greater openness, standardization, and productization.
As a computer control system in the field of production process automation, traditional DCS is only a narrow concept. If one thinks that DCS is just an automation system in the production process, it will lead to incorrect conclusions, because the meaning of computer control systems has been greatly expanded. It not only includes various contents contained in DCS in the past, but also delves into every measuring device and executing mechanism on site, and develops to all aspects of production management and enterprise operation. In the traditional sense, DCS now only refers to the automation of production process control, while the concept of industrial automation systems should be positioned at the level of comprehensive solutions for enterprises, namely total solution. Only by posing and solving problems from this perspective can computer automation truly play its due role.
Functional composition of DCS control system
The typical DCS architecture is divided into three layers, with the first layer being the decentralized process control level; The second layer is the centralized operation monitoring level; The third layer is the comprehensive information management level. The communication lines between layers are connected by high-speed data path HW and local area network LAN, and the communication connections between devices within the layer are carried out by the communication network of the same level.
First layer: Decentralized process control level
The decentralized process control level is the foundation layer of DCS, which directly faces industrial objects downwards. Its input signals come from sensors (such as thermocouples, thermal resistors, etc.), transmitters (such as temperature, pressure, liquid level, flow rate, etc.), and electrical switches (input contacts) in the production process site. Its output drives actuators (such as regulating valves, solenoid valves, motors, etc.) to complete functions such as data acquisition, closed-loop regulation control, and sequential control in the production process; It communicates with the centralized operation monitoring level for data, receives parameters and operation commands downloaded from the operation station, and organizes on-site work information to report to the operation station.
The main devices that make up this level include: on-site control stations, programmable controllers, intelligent regulators, and other measurement and control devices.
1. On site control station
The on-site control station has multiple functions – integrating continuous control, sequential control, batch control, and data acquisition functions. (1) The hardware composition of the on-site control station is generally a standard cabinet type structure, which is composed of power supply, bus, I/O modules, processor modules, communication modules, and other parts inside the cabinet. Generally, a fan component is installed at the top of the cabinet to remove the heat emitted by the electronic components inside the cabinet; The cabinet is equipped with a dry layer module installation unit, with processor modules and communication modules installed on the upper layer, I/O modules installed in the middle, and power components installed at the bottom. There are also various buses inside the cabinet, such as power bus, grounding bus, data bus, address bus, control bus, and so on. The power supply of the on-site control station should not only provide power for the cabinet, but also provide external power supply for the on-site detection devices. These two types of power supplies must be isolated from each other and cannot be grounded together to prevent interference signals from coupling into the I/O channel through the power circuit.
The system structure in a field control station, which includes one or more basic control units. The basic control unit is composed of a processor module that completes control or data processing tasks and several input/output modules connected to it (similar to IPC). The basic control units are connected together through the control network Cnet, and the uploaded information on the Cnet network is sent to the monitoring network Snet through the communication module. Similarly, the downlink information of Snet is also transmitted to various basic control units through the communication module and Cnet. In each basic control unit, the information exchange between processor modules and I/O modules is completed by the internal bus. The internal bus may be a parallel bus or a serial bus. In recent years, serial bus has been widely used.
(2) Software functions of on-site control stations
There are six main functions of the on-site control station, namely data acquisition function, DDC control function, sequence control function, signal alarm function, report printing function, and data communication function:
Data collection function: Collect, transform, process, display, store, trend curve display, accident alarm, etc. for process parameters, mainly analog signals from various sensing transmitters.
DDC control function: including receiving on-site measurement signals, calculating the deviation between the set value and the measured value, performing PID control operation on the deviation, and finally obtaining a new control quantity. This control quantity is converted into the corresponding current and sent to the actuator to drive the controlled object.
Sequential control function: By using input and output signals from process status and feedback control functions, etc., sequentially control each stage of the control according to pre-set sequence and conditions.
Signal alarm function: Set upper and lower limit values for process parameters, and if they exceed the upper or lower limit, perform limit exceeding alarms separately; Alarm for illegal switch status; Report any accidents that occur to the police. Signal alarms are indicated by changes in sound, light, or CRT screen display color.