Difference between DCS control system and PLC control system

Differences from the basic structure of the DCS and PLC, DCS distributed control system of English (TOTALDISTRIBUTED CONTROL SYSTEM) for short. Refers to the control of dispersed risk, manage and display set. Late 60 programmable controller was developed as a logical operation (Programmable Logic Controller). Referred to as PLC. Mainly used in the automotive industry. Mid70 to complete analog of DCS to the market rather than by analog meter control of PID arithmetic.
DCS put forward the idea in the first place is the original manufacture of instrument manufacturers, mainly used in the chemical industry. Later engaged in the development of DCS of the computer industry. 70 microcomputer technology is not yet mature, computer technology is not developed enough. Action stations, controllers, I/O and network interface boards are developed by DCS suppliers, that is, all parts are dedicated. 70 early minicomputers such as PDP/1124 instead of centrally installed analog meter control. Lots of cable connected to the central control room. Such as small as controller, minicomputer as the CRT display devices connected simultaneously (that is, human-machine interface). A small machine needs receives thousands of transmitters or other sensor signals, completing hundreds of circuit operation. Obviously the risk a little bit concentration. As much as analog meter connection cable, and once the small machine is broken, controls and displays are gone. Digital control did not achieve the desired objective. After someone proposed a separate control and display. Complete control over computing tasks from a computer and another computer displays tasks completed. Addition, a process process as was control object may need displayed and control of points many, which some also need closed ring control or logic operation, process process as was control object of all part will has relative independence, can into several a independent of process, again put in computer control system in the independent of process Shang need displayed and control of entered, and output of points distribution to number Taiwan computer in the to, put original by a Taiwan minicomputer completed of operation task by several Taiwan or dozens of Taiwan computer (Controller) to completed. One machine is broken without affecting the global. The so-called Wolf Pack instead of the Tiger tactics, this is the meaning of risk dispersion. Operations, print, display, and other management functions together, above with network control and shows the two parts of a system. When someone call this system and distributed system.
Risk is spread what is right? This was related to the level of development of the computer technology. The mid70, dispersion is a thoroughly completed a circuit of the controller operation. At that time due to people not familiar with digital technology, accustomed to the analog meter, the late 70 ‘s and 80 ‘s once popular loop controllers, analog digital controller and the original almost exactly the same in appearance, does not change the habits and internal operation of digital PID. A meter (one computer) to complete a loop control tasks. Its price is more expensive, but the danger is distracting. And then with a communications network to the controller and CRT based human-machine interface of a system. Network structure is usually star-shaped structure. Loop controller of the production costs are too high, the price/performance ratio is not good. Later in order to reduce costs, there are two, fourcircuit controller circuit, its price/performance ratio is slightly better. For a medium to large systems, DCS system of price/performance ratio loop controller better. Some places still use some loop controller. If the loop too much to be done, such as a controller to collect thousands of points, completing hundreds of circuit operation, dangerous and too focused. In this case, risk must be dispersed. With the development of computer technology, computer processing power, storage capacity and reliability continues to improve, the task can also be accomplished by a single computer increases. Tasks can also be completed on one point. In addition, controllers, network redundancy also developed, some control operations can also be set.
Judging from the present DCS, a controller to complete dozens of loop operation and hundreds of points for the collection, and then add just the right amount of logical operations, field use, the effect is good. This controller upgrade problems. Sometimes the distance between controller and sensor is quite far away, this would promote the development of fieldbus. Such as CAN, LOONWORKS, FF fieldbus and HART protocol receiver boards are used in the DCS system. DCS is divided into three parts, I/O controllers, communications network and human-machine interface (HMI). From the I/O Board by Terminal board directly connected with the production process, read sensor signals. I/O boards are of several different types, each I/O Board has the appropriate Terminal Board. Simulation volume entered, 4-20 Ma of standard signal board and to read thermocouple of MV signal board; 4-16 a channel ranging; simulation volume output, usually are is 4-20 Ma of standard signal, General it of channel compared less, 4-8 all channel; switch volume entered; 16-32 a channel: switch volume output, switch volume entered and output also points different voltage grade of Board, as DC 24 v, and 125 v; Exchange 220 v or 115 v,; 8-16 a channel ranging; pulse volume entered, For the sampling rate of a signal; 4-8 channels ranging from fast interrupt input, HART protocol input Board fieldbus I/O Board on each I/O boards are connected to the I/O bus. For the security and integrity of the signal, signal in the signal into the I/O Board before the renovation, such as upper and lower bounds checking, temperature compensation, filtering, these jobs can be performed in the Terminal Board, also can be done separately, complete refurbishment of some people have called them signal processing Board. I/O bus and the controller. 80 the DCS because the controller operation is not strong, in order to increase I/O points, to separate the controller‘s task, in fact there are three types of controllers. That is: a complete closedloop operation of controllers, analog data acquisition devices, and logical operators. They each have their own I/O bus various different DCS I/O bus. If you require a quick, the best use of parallel bus. Serial bus is usually used more. More particularly the RS485 bus, analog data acquisition and I/O points can be more of a logical operator.
Closedloop controllers, analog data acquisition device and logical operations can interface directly connected to the communications network, in each of the different controllers on the network as a separate node on the network. Each node perform a variety of functions. They should have a network interface. All DCS in order to save network interface, process control for all equipment that is used in closedloop controllers, analog data acquisition device and logical operations prior and continuous on the control bus, known as the control station. This process can increase the control station can receive I/O points and saving interface. And then through the interface connected to network, connected with the human-machine interface. With the development of meter, controller of the growing computing power, such as a PC controller are strong, can receive both analog operation, also received digital logic operations. A controller into a node on the network. Through a network connected to the interface.
Controller is a core part of DCS, it is equivalent to a PC machine. DCS is PC machine controller itself. It includes CPU, RAM, E2PROM and ROM chips, there are two interface, and a receive I/O bus down to signal another interface is up and send signals to the network connected to the interface. ROM for storing all kinds of operation functions of control algorithms (DCS function block library). In library in the Save function block, as control algorithm PID, and with died district PID, points separation PID, arithmetic operation added, and reduction, and took, and except, and square, and prescribing, and function operation once filter, and sine, and cosine, and X-Y function generator, and ahead-lag; compared advanced of algorithm has Smith estimates, c language interface, and matrix added, and matrix took; logic operation has logic and, and logic or, and logic non-, and logic and non-,. Usually use station function block switches combine analog and not only, also linked up with people. Function blocks the more userwritten applications (configuration) more convenient. Configuration according to technological requirements, linked in a control function blocks. E2PROM control programmes exist. Because E2PROM can erase, change configuration changing with processes, E2PROM configuration exists. Different users have different configurations. Configuration from which users you wish to function block function block library, fill in the parameters, to connect the function blocks. Form a control scheme to save the E2PROM. When the controller configuration mode, running in operation mode the controller after you have installed the operating system, function block configuration software and communications software. For safe operation of the system, closedloop controller must be redundant, with a ready and hot spare. In order to succeed the redundancy, the following points should be noted: both the hardware, the software version of the controller must be consistent; check the sendingreceiving chip is intact; redundant chip is intact. Set of two modules, are asked to check for manual stations and so on. Communications networks and HMI in a system process. Communication network structure of several different line. Such as bus, ring, and star-shaped (see picture). Total linear logic is circular. Star shape is only suitable for small systems. Neither the ring nor the bus, is used for broadcasting. Some other agreement has been used less. Communications network rate around 10M and 100M.
Human-machine interface there are 4 different types of nodes, they are operating station, engineering workstations, historical trends and dynamic data server.
Install operating system operation, monitoring software and controller driver software. Display a label system, dynamic flowcharts, and alarm information.
Engineering workstations to the controller configuration (CAD) or to an operation station configuration (dynamic flow chart). If monitoring software mapping ability is very strong, graphic work can be done by independent monitoring software. Engineer another function is to read the controller configuration for controller upgrades, you find the problem. We call it reverse engineering station. History trend historical data will be stored, generally with the disk array (RAID technology). Dynamic data servers are DCS and MIS system interfaces, and isolation of DCS and Web devices.
DCS and PLC design difference is large, PLC is developed by touches like the original relay control, 70 only switch logic control PLC, first apply the automobile manufacturing industry. Its stores perform logical operations, sequence control, clock, counting and arithmetic operation instructions and through digital input and output operations to control various types of machinery or production processes. User prepared controls expression of the technological requirements of the production process, in the user program memory and advance into the PLC. Article-by-article implementation of the stored procedure returns the running in order to complete the process the requested operation. PLC program indicates that the program step to store addresses in the CPU counters, while the program is running in the process every step of the implementation of the counter automatically plus 1, from the initial step in the process (step ordinal of zero) are performed to the final step (usually the END directive), and return to the startingstep cycle operation. PLC each the time required to complete a cycle operation called a scanning cycle. Different types of PLC, circular scanning cycle between 1 microsecond to dozens of microseconds. Program counter, this cycle of operations, this is no DCS. That‘s why redundant PLC than DCS. DCS is operational amplifier based on development. All the functions, the relationships between process variables are function blocks (DCS system, known as expanded blocks). Mid 70 ‘s DCS analog control. TDC2000 system, a controller for a second to complete within 8 PID loop operation. First of all is the application of chemical industry. DCS and PLC for showing the main differences in the volume switch logic solution and analog operation, even though the two some penetration, but there are still differences. 80 ‘s, PLC in addition to logical operations, also have some algorithm for control loop, but is more difficult to perform complex operations, PLC ladder diagram programming, analog computing less intuitive programming, programming is too much trouble. But in terms of solving logic, showed rapid advantage of microsecond magnitude less than 1 millisecond resolution 1K logic programs. Switch to handle all inputs as 16-bit (32-bit) for an analog. While the DCS analogue input as if all, 1 is the switch. A logic solution is of the order of hundreds of microseconds to milliseconds. Counted as PID algorithm in PLC solutions to dozens of MS, and the operation of DCS as the same period. Use another CPU to complete large PLC analog operation. Controller for PLC results. Different types of DCS and time needed to calculate PID, but dozens of millisecond order of magnitude. Early TDC2000 system within 1 second to complete 8 loop control operations. With the development of microarray technology, an algorithm to calculate Jane shorten. An algorithm to calculate the required time and the arrangement and configuration of function block mode.
In terms of grounding resistance, to the PLC may demand is not high, but DCS must be below the few ohms (usually below 4 ohms). Analog isolation is also important. In explosion hazard areas, you should configure the intrinsic safety barrier. Same number I/O systems with PLC than with DCS, its cost is lower (around 40%, about). PLC do not have dedicated stations, which is common with software and hardware, so maintenance costs are much lower than DCS. A PLC controller, you can receive thousands of I/O points (up to a maximum of more than 8,000 more I/O). DCS controllers, only hundreds of I/O points (no more than 500 I/O). If control is rarely equipment chain, loop, PLC is appropriate. If is analog control, and function a lot, the best use of DCS. DCS controllers, I/O boards, communication network redundancy, advanced computing, industry specific requirements much better terms than PLC. PLC as a result of monitoring software, in terms of enterprise management information system design, easier. Special note is, dedicated DCS operation station, not the norm. It is formed by historical reasons. DCS manufacturers such as don’t open operator station, and the plant‘s management information system network, some DCS are in danger of disappearing from the market.
With the emergence of new technologies, negative effects follow. Opening of the new station, viruses and hackers easily penetrated to the system. At design time, set at the operating station password more isolation and firewall systems. Minimizing negative impacts.
DCS and PLC is not the industry developed two computer controlled systems, DCS process control instrumentation evolved, and that of PLC logic control in process industry evolved, DCS to emphasize is centralized management,