First, the basic concept of distributed control system
Distributed control system is a centralized and decentralized control system based on microprocessor, and its main characteristics are centralized management and decentralized control.
Basic idea: (Honeywell)
1, the computer centralized control system is decomposed into decentralized control systems, and there is a special process decentralized control device to complete part of the control and operation of the process control level.
2. On the basis of simulating the operating habits of electrical instruments, a good man-machine interface is developed for the monitoring operation of operators.
3. In order to establish data connection between the operation station and the process control equipment, a data communication system is established to transmit data between the operator and the production process.
Second, the basic structure of distributed control system
1, distributed process control device 2, operation management device 3, communication system
Third, the basic composition of DCS
1. Field I/O control station for controlled site.
2. Operator's console.
3.DCS monitoring and management engineer station.
The main function of DCS operator station is to provide man-machine interface for the operation of the system, so that the operator can know the on-site operation status, various operation parameters and whether there is any abnormality in time through the operator station.
Fourth, the characteristics of DCS
1, hierarchical structure 2, decentralized control 3, autonomy and coordination
Function design of verb (verb abbreviation) DCS
1, field data acquisition function
2. Monitoring and alarm function
3. Log management server function
4, accident memory function
5. Time series recording function (SOE)
6. Secondary advanced computing function
7. Man-machine interface of 7.DCS
Brief introduction of distributed control system (DCS)
DCS, the so-called decentralized control system, or decentralized control system in some data, is a new computer control system relative to centralized control system, which is developed and evolved on the basis of centralized control system. In terms of system functions, there is little difference between DCS and centralized control system, but the way to realize system functions is completely different.
The first is the framework of DCS-system network, which is the foundation and core of DCS. Because the network plays a decisive role in the real-time, reliability and expansibility of the whole DCS system, all manufacturers have carefully designed this aspect. For the system network of DCS, it must meet the requirements of real-time, that is, complete the transmission of information within a certain time limit. The "definite" time limit mentioned here means that the information transmission can be completed within this time limit under any circumstances, which is determined according to the real-time requirements of the controlled process. Therefore, the index to measure the network performance of the system is not the speed of the network, that is, bits per second (bps), but the real-time performance of the system network, that is, how long it takes to ensure the transmission of the required information. The system network must also be very reliable, and the network communication can't be interrupted under any circumstances, so most manufacturers' DCS adopt double bus, ring or double star network topology. In order to meet the requirements of system scalability, the maximum number of nodes accessible on the system network should be several times larger than the number of nodes actually used. In this way, on the one hand, new nodes can be added at any time, on the other hand, the system network can run under light communication load, ensuring the real-time and reliability of the system. During the actual operation of the system, all nodes can go online and offline at any time, especially the operator station. In this way, network reconfiguration will be carried out frequently, and this operation must not affect the normal operation of the system. Therefore, the system network should have powerful online network reconfiguration function.
Secondly, it is a network node that completely handles field I/O and realizes direct digital control (DOS) function. In general, a set of DCS should be equipped with on-site I/O control station to share the I/O and control functions of the whole system. This can not only avoid the failure of one station leading to the failure of the whole system, improve the reliability of the system, but also enable all stations to share data acquisition and control functions, which is conducive to improving the performance of the whole system. The DCS operator station is a network node that handles all HMI- man-machine interface or operator interface functions related to operation.
System network is the engineer station of DCS, and it is the network node of off-line configuration, configuration work and on-line system monitoring, control and maintenance of DCS. Its main function is to provide tool software (configuration software) for DCS configuration and configuration work, and monitor the operation of each node on DCS network in real time when DCS is running online, so that system engineers can adjust the system configuration and some system parameter settings in time through the engineer station, so that DCS can be in the best working state at any time. Different from centralized control system, all DCS need system configuration function. It can be said that a system without system configuration function cannot be called DCS.
DCS has developed for more than 20 years since 1975 came out. In the past twenty years, although the system architecture of DCS has not changed obviously, its function and performance have been greatly improved through continuous development and improvement. Generally speaking, DCS is developing towards more openness, standardization and productization.
As a computer control system in the field of production process automation, the traditional DCS is only a narrow concept. If we think that DCS is only the automation system of production process, we will come to a wrong conclusion, because the meaning of computer control system has been greatly expanded, including not only all kinds of contents contained in DCS in the past, but also every measuring equipment and actuator on site, up to all aspects of production management and enterprise management. In the traditional sense, DCS only refers to the automation of production process control, while the concept of industrial automation system should be positioned at the level of overall solution. Only by putting forward and solving problems from this angle can computer automation really play its due role.
Since 1990s, computer technology has advanced by leaps and bounds, and more new technologies have been applied to DCS. PLC is an electronic device developed for sequential logic control, which is mainly used to replace inflexible and bulky relay logic. Since the mid-1990s, fieldbus technology has developed rapidly. It is predicted that FCS based on fieldbus will replace DCS as the protagonist of control system.