Water is an indispensable and important part of human life and the development of the national economy, with the rapid development of science and technology level and the huge increase in the standard of living of mankind, the demand for clean and high-quality water also continues to be released dramatically. For the construction of reliable, stable, advanced, economic and scalable reasonable water treatment automation system has become the engineering and urban water industry operation and management department **** with the concern. Microelectronics, communications, computer technology development has greatly improved the water treatment control system of information technology and the degree of intelligence, and 3C technology combined with the PLC with its excellent reliability, anti-interference and flexible control mode to become the core controller of the water treatment automation system, with its open network communication system, *** with the development of water treatment automation system to promote the degree of intelligence.
The water treatment industry is mainly divided into two major parts of water purification and sewage treatment. Water purification plant control system is usually divided into water plant scheduling system, dosing room (chlorination room) PLC control station, filtering station PLC control station, water supply pump house PLC control station. Each control station works relatively independently, communicates through a wired network, and sends all data information to the water plant scheduling room for processing, or sends part of the data to the city's scheduling center through the scheduling system by means of wireless (or wired) communication. For sewage treatment, according to the status of the sewage water source to determine the sewage treatment process, due to the different sewage treatment processes and automatic control system application PLC requirements are different. Generally speaking, the entire sewage treatment plant has a general control room and a number of on-site control station, station and station connected through the controller layer network or information layer network, and then all connected to the general control room, the general control room of the multiple computers, workstations and graphic stations are connected with the information layer network, so that and on-site control station constitutes a centralized management, decentralized control, high-speed data exchange of the plant-level automation network [1]. PLC automatic control PLC automatic control system is the core part of the control of water treatment plant, its reasonable selection and design, whether the sewage plant can be efficient, automated operation is very important. However, the PLC network is one of the most important, the network of good and bad directly affect the normal operation of the sewage plant.
Two, the system composition
The sewage treatment plant automation system generally includes the sewage plant part and the pumping station outside the plant part. Monitoring system communication network and PLC is the core component of the sewage treatment automation system, their performance on the sewage treatment automation system will play a decisive role [2]. According to the characteristics of wastewater treatment automation itself and monitoring needs to choose the appropriate PLC and communication network is to ensure the performance of wastewater treatment automation system is an important factor.
Communication network:
In the structure of the sewage treatment automation system, the country in the management system is mainly used in three-tier management, namely, monitoring the main center, regional monitoring sub-center and monitoring station. Because the monitoring station does not directly control the outfield equipment of the sewage treatment plant, the engineering community in accordance with the division of the system structure of the monitoring system is divided into the information layer, control layer and equipment layer.
The first layer of the information layer, mainly responsible for a large amount of information and different manufacturers of different devices between the transmission of information, Industrial Ethernet Ethernet for the more commonly used type of information network, the world's major PLC manufacturers are supported by Industrial Ethernet, and they are in the original TCP / IP on the basis of the successive development of real-time higher Industrial Ethernet, such as Omron and Rockwell support Ethernet/IP. They have developed higher real-time industrial Ethernet based on the original TCP/IP, such as Ethernet/IP supported by Omron and Rockwell, Modbus-TCP/IP supported by Schneider, and ProfiNet supported by Siemens. Because of the large amount of information on Ethernet, Ethernet is mainly used in the automation system of wastewater treatment plant for the data transmission between each control substation and the monitoring center, including a large amount of historical data information such as various sensor data.
The second layer for the control layer, mainly using field buses to form a tunnel area controller network, which is characterized by the use of standard bus networking, both to meet the requirements of real-time communications, but also has a standard interface to open protocols, can be conveniently hooked up to a variety of out-of-area equipment on the bus, which is conducive to the expansion of the monitoring system. At present, there are more than 40 kinds of field bus, in the sewage treatment plant automation system in the application of field bus mainly ControllerLink, LonWorks, Inetrtbus, Profibus, Can and Modbus. their *** with the same characteristics of high speed, high reliability, suitable for PLC and computer, PLC and PLC and other equipment between the They are suitable for high speed and high reliability communication between PLC and computer, PLC and PLC and other devices with large amount of data. In order to make the system stable and reliable, the control layer of the network structure is composed of a ring network, including cable type and optical fiber as a transmission medium, the specific network will be made later to illustrate the example.
The third layer for the device layer, this layer is used for PLC and field devices, remote I/O terminals and field instrumentation communication between them, they have DeviceNet, Modbus and Profibus/DP, etc., of which DeviceNet has become the industry's standard bus and has been widely used, and Profibus/DP, although it has not become a standard, but it should also be a standard. become a standard, but it should also be quite extensive.
It is worth pointing out that in recent years, the wide application of Ethernet has made people cast their eyes on the fieldbus, and whether industrial Ethernet will eventually replace the fieldbus is still a topic of debate. However, whether it is Ethernet/IP or Modbus-TCP/IP, Ethernet still can not have the characteristics and advantages of fieldbus in some important performance indicators. In essence, Ethernet's carrier frame listening conflict monitoring CSMA/CD access, real-time is not as high as the token bus and token ring access used by the fieldbus, no matter what kind of methods people use, such as protocol encapsulation, time-sharing access control, etc., can only improve the real-time of Ethernet, and can't play an essential change. Before the current technology is fully mature, fieldbus applied to the control layer is a positive and sound choice. With the continuous development of Ethernet technology, it is very likely that it will be used in the control layer instead of fieldbus in the future.
Monitoring sub-center and upper monitoring software:
Monitoring sub-center will generally set up more than one SCADA workstation (industrial computer). Separately for the water plant scheduling system, dosing room (chlorination room), filtering station, water pumping station and other monitoring, complete the status of various equipment in the sewage plant display, automatic control, semi-automatic control, print alarms, analysis reports and other work. At the same time, the monitoring sub-center will also set up a number of servers to provide support for other computers and communication with the monitoring center.
PLC selection:
Schneider (Schneider), Siemens (Siemens), Omron (Omron), Rockwell (Rockwell), General Electric (GE) is the world's top five PLC manufacturers and providers of total solutions, their products for their different fields, which is in the automation of sewage treatment The application of the system, and Rockwell, Omron and Schneider's most widely used.
The automatic control system for sewage treatment puts forward higher requirements for the performance of PLC, as the core controller of the automatic control system for sewage treatment, it must have the following major functional characteristics: first of all, it must be stable and reliable, and has the ability to pre-process data and centralized data transmission, with a high degree of fault protection; secondly, the control substation local controllers can independently undertake control Secondly, the control substation local controller can independently undertake the control of the basic control tasks in the sub-area, even if the monitoring station or monitoring center stops running due to faults, the controllers in the adjacent areas can also exchange data information; again, when a change in the control volume of a control station, according to the predetermined program and procedures to take the appropriate algorithm, the control of the relevant areas of the object, such as pumps or dosage systems to make the appropriate adjustments. Therefore, it must have at least the following functional modules, data acquisition, storage and processing functions (to achieve centralized and independent mode of operation, especially in the independent control can be achieved with the adjacent controller data exchange); communication functions, fault tolerance, automatic diagnostic functions and local operation (that is, with a touch screen).
Must take into account the performance requirements of the entire monitoring system and the natural conditions and operating cycle of the equipment requirements for the choice, especially in extreme weather and harsh environmental conditions conditions or larger-scale wastewater treatment plants, the need to choose a better performance of the dual-machine hot standby redundant PLC, such as Schneider's 2Quantom series, Rockwell's 2ControlLogix, Omron's CS1D series, Siemens' S7-417 series; the difference is that Omron's dual system is realized on a baseboard, while Siemens and other two baseboards are connected by fiber optics, which will take up the space of the control cabinet to a certain extent, but their configurations are very flexible, and they can arbitrarily realize dual-CPU dual power supply, dual-CPU single power supply, single-CPU single power supply, and dual-CPU single power supply. CPU single power supply, single CPU single power supply redundancy structure.
In the general environmental conditions or smaller scale sewage treatment plant, the standard models are used as field controllers, such as Schneider's Quantom140 series, Rockwell's ControlLogix, Omron's CS1 series, Siemens's S7-400 series, etc.; they all support industrial Ethernet and a variety of field buses. Ethernet and a variety of field buses, the control method adopts the intelligent distributed structure with CPU remotely, the system is open and compatible, rich I/O and high-function modules, which fully meets the requirements of the automatic control system of sewage treatment for signal processing.
Three, application cases
The following Tianjin Xianyang Road sewage treatment plant as an example [3], specify the composition of the automatic control system of the sewage treatment plant, the control system topology as shown in Figure 1:
Information layer: Xianyang Road sewage treatment system because of its distribution area is large, the plant **** there are five PLC sub-stations: pre-treatment system subcontrol master station PLC1, Biological treatment system sub-control master PLC2, sludge treatment system sub-control master PLC3, effluent and rainwater system sub-control master PLC4 and sludge digestion system PLC5, using the CPU are OMRON's CS1H-CPU66H. The functional layer to realize the sewage treatment plant unit process all process parameters, equipment operating status and electrical parameters of the data acquisition, the unit process and the control of the equipment, and through OMRON, the unit process and the equipment. This functional layer realizes data acquisition of all process parameters and electrical parameters of each unit process of the sewage treatment plant, control of the unit process and equipment, and transmission of data to the monitoring layer and acceptance of control instructions from the monitoring layer through the OMRON network module CS1W-ETN21, and the central control room through the Hirschmann Ethernet switches, which form a 100M fiber-optic Ethernet ring network. In the central control room, as an industrial Ethernet node, the system data server, two engineer/operator station computers, printers, UPS power supply and monitoring screen and other equipment, its main function is to carry out the information exchange and information display and control in the system. This layer realizes the control and scheduling of the main process equipment through the upper monitoring software, data acquisition, monitoring, optimization and adjustment of process parameters in the whole process of sewage treatment, dynamic simulation and trend analysis, real-time data processing and real-time control of the main process, and realizes a variety of conventional and complex optimization control, expert control, fuzzy control and other advanced intelligent control in the control configuration. At the same time, the powerful and stable real-time and historical databases also become the information channel between the upper and lower levels through Ethernet. The control station in the central control room of the sewage plant also communicates remotely through RIAMBView with the information center, portable computers and pumping stations outside the plant (Xianyang Road pumping station, Miyun Road pumping station), etc. RIAMBView is equipped with a remote data service (best suited for SCADA) function, which receives or sends the relevant data via broadband and achieves the access to part of real-time screen and process database from the remote end.
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