At present, there are three main control systems in continuous process production automatic control (PA) or industrial process control, namely PLC, DCS and FCS. Their basic characteristics are as follows:
1 PLC
(1) The development from on-off control to sequential control and conveying treatment is from bottom to top.
(2) Multi-functions such as continuous PID control, and the interrupt station is equipped with PID.
(3) One PC can be used as the master station, and multiple PLCs of the same type can be used as the slave stations.
(4) A PLC can also be used as a master station, and multiple PLCs of the same type can be used as slave stations to form a PLC network. This is much more convenient than using PC as the main station: when there is user programming, you don't need to know the communication protocol, just write it according to the instruction format.
(5)PLC power grid can be used as an independent DCS/TDCS or as a subsystem of DCS/TDCS.
(6) Large-scale systems are the same as DCS/TDCS, such as TDC3000, CENTUMCS, WDPFI and MOD300.
(7)PLC networks such as SINEC-L 1 of Siemens, SINEC-H 1, S4, S5, S6, S7, GENET of GE, MELSEC-NET of Mitsubishi and MELSEC-NET/MINI.
(8) It is mainly used for sequential control in industrial process, and the new PLC also has closed-loop control function.
(9) Manufacturers: Gould (USA), AB (USA), GE (USA), Omron (Japan), Mitsubishi (Japan), Siemens (Germany), etc.
2 developing countries or TDCS
(1) DCS and TDCS are monitoring technologies integrating 4C (communication, computer, control and CRT) technologies.
(2) Top-down tree topology large-scale system, in which communication is the key.
(3)PID In the interrupt station, the interrupt station is connected with computer, field instrument and control device.
(4) It is a tree topology and a parallel and continuous link structure. There are also a large number of cables running in parallel from the relay station to the field instrument.
(5) Analog signal, A/D-D/A, mixed with microprocessor.
(6) A pair of wires of the instrument are connected to I/O, and the instrument is connected to the local area network LAN from the control station.
(7)DCS is a three-level structure of control (engineer station), operation (operator station) and field instrument (field measurement and control station).
(8) The disadvantage is that the cost is high, and the products of various companies cannot interoperate with each other, and the large DCS system is different from company to company.
(9) Used for large-scale continuous process control, such as petrochemical industry.
(10) Manufacturers: Bailey (USA), westinghouse (USA), HITACH (Japan), Leeds &; Northrop (USA), Siemens (Germany), Foxboro (USA), ABB (Switzerland), Hartman &; Braun (Germany), Yokogawa (Japan), Honewell (America), Taylor (America), etc.
Three functional constituencies
The basic tasks of (1) are: intrinsic safety, dangerous area, changeable process and difficult environment.
(2) Fully digital, intelligent and multifunctional, replacing instruments, meters and control devices that simulate a single function.
(3) Use two wires to connect scattered field instruments, control devices, PID and control centers, instead of using two wires for each instrument.
(4) PID is equal to the instruments, meters and control devices on the bus.
(5) Multivariable, multi-node, serial and digital communication systems have replaced univariate, single point, parallel and analog systems.
(6) It is interconnected, two-way and open, not one-way and closed.
(7) Replace centralized control station with decentralized virtual control station.
(8) Controlled by the on-site computer, or connected to the upper computer and the upper computer on the same bus.
(9) Local area network, which can be connected with the Internet.
(10) Transform the traditional signal standard, communication standard and system standard into enterprise management network.
(1 1) Manufacturers: Honeywell, Smar, Fisher— Rosemount, AB/Rockwell, Elsag— Bailey, Foxboro, Yamatake, Yokogawa, Siemens Europe, GEC—-Alstom, Schneider, Proceedings-Data, ABB, etc.
Typical (12) Class 3 Fieldbus
1) continuous process automatic control, such as petrochemical industry, in which "intrinsically safe and explosion-proof" technology is absolutely important. Typical products are FF, FIP and PROFIBUS-PA；;
2) Discrete automatic control of process actions, such as automobile manufacturing robots and automobiles. Typical products are PROFIBUS-DP and CANbus；;
3) Multi-point control such as building automation, typical products are LON Work and Profibus—FMS.
From the description of the above basic points, do we notice that the three systems used for process control were not developed for power plants, or that in the early stage of their development, power plants were not the first choice for system control? In the operating instructions of these systems, power stations are never regarded as the preferred scope of application, and some do not mention power stations at all. Strangely, these three control systems, especially DCS and PLC, have been widely used in power stations and have achieved good results.