1. Project Introduction
As environmental issues are increasingly valued by all sectors of society, sewage treatment has become the most urgent need to be resolved in environmental protection and most closely related to social development
Related topics. The Xi’an Municipal Government implements national policies and vigorously builds water treatment projects. Xi’an ’s fourth sewage treatment plant is the most
One of the representative projects.
The Xi'an Fourth Sewage Treatment Plant project is an ODA loan project of the Japan International Cooperation Bank. The plant accepts sewage services including some areas in the urban area of ​​Xi'an, part of the northern suburbs and east suburbs. The planned drainage area of ​​the sewage is about 45km2. The project design scale is 500,000L / day, the design scale of the first phase is 250,000T / day, the sewage treatment adopts anoxic / anaerobic / aerobic (inverted A2 / 0) two-stage biological treatment process, and the effluent is discharged to the Weihe River through the open channel of the transportation. Mud treatment adopts two-stage medium temperature digestion and mechanical dehydration after gravity concentration, and the mud cake is transported to landfill after dehydration. The digested biogas is used for sludge heating to achieve resource reuse. The structures that mainly produce odor in the sewage plant are equipped with an odor collection system, which is purified by the biological deodorization system. The specific process flow is shown in Figure 1.
Figure 1 Process flow chart
According to the process flow and layout of the wastewater treatment plant, the "central monitoring, decentralized control" automatic control system is adopted. The system includes a central monitoring system, a field control station, a TV monitoring system, a communication network, and intelligent instruments. The central monitoring system is located in the central control room of the comprehensive office building in the factory, including monitoring workstations, management workstations, and engineer stations. The central monitoring station communicates with five field control stations via fiber optic Ethernet. The five field control stations are divided according to the process area, of which PLC1 control station is located in the coarse grid and sewage lifting pump room, PLC2 control station is located in the power distribution room, PLC3 control station is located in the dosing and chlorination room, and PLC4 control station In the sludge dewatering room, the PLC5 control station is located in the boiler room and bathroom. Use flow, pressure, temperature, liquid level and water quality analysis instruments that support PROFIBUS protocol at each monitoring point to achieve online measurement and fault diagnosis, and send the signal to the nearest field control station for monitoring.
According to the project situation, Siemens s7-400 series PLC is used as the main control system. The specific hardware configuration of the control station is as follows:
Second, the control system structure
In this project, the automatic control system adopts three levels of control methods: management, control and field level. The control network is shown in Figure 2. For the lower-level control system, each station performs hardware selection and configuration according to the difference in the number of control points and the actual situation on site as described below.
Figure 2 Control network
PLC1 control station-coarse grid and on-site control station of sewage lifting pump room
This station PLC selects Siemens PLCS7-400 series products, CPU is CPU414-3DP. The field PLC control station communicates with the instrument and special control equipment through the standard industrial field bus (PROFIBUS-DP / PA). The field control station is connected to the lOOMbit / s redundant optical fiber Ethernet ring network through the Siemens SCALANCEX408-2 switch. Communication with the monitoring computer in the central control room.
The I / O points of the coarse grid and the on-site control station of the sewage lifting pump room are: DI = 181 points, DO = 79 points, AI = 0 points, AO = 0 points. The hardware configuration is shown in Figure 3.
Figure 3 PLC1 control station hardware configuration
PLC2 control station-on-site control station in transformer room
This station PLC selects Siemens PLCs7-400 series products, the CPU is CPU414-3DP. The field PLC control station communicates with the instrument and special control equipment through the standard industrial field bus (Profibus-DP / PA). The field control station is connected to the lOOMbit / s redundant optical fiber Ethernet ring network through the Siemens SCALANCEX408-2 switch. Communication with the monitoring computer in the central control room.
The number of I / O points in the on-site control station of the transformer room is: DI = 208 points, DO = 72 points, AI = 0 points, AO = 0 points. The hardware configuration is shown in Figure 4.
Figure 4 PLC2 control station hardware configuration
PLC3 control station-on-site control station for dosing and chlorination
This station PLC selects Siemens PLCS7-400 series products, CPU is CPU414-3DP. The field PLC control station communicates with the instrument and special control equipment through standard industrial field bus (PROFIBUS-DP / PA). The field control station is connected to the 100Mbit / s redundant optical fiber Ethernet ring network through the Siemens SCALANCEX408-2 switch. Communication with the monitoring computer in the central control room.
PLC3 control station and instrument and special equipment all carry on data communication through PROFIBUS-DP interface. The hardware configuration is shown in Figure 5.
Figure 5 PLC3 control station hardware configuration
PLC4 control station-on-site control station of sludge dewatering room
This station PLC selects Siemens PLCS7-400 series products, CPU is CPU414-3DP. The field PLC control station communicates with the instrument and special control equipment through the standard industrial field bus (PROFIBUS-DP / PA). The field control station is connected to the lOOMbit / s redundant optical fiber Ethernet ring network through the Siemens SCALANCEX408-2 switch Communication with the monitoring computer in the central control room.
The I / O points of the on-site control station of the dehydration machine room are: DI = 128 points, DO = 42 points, AI = 20 points, AO = 0 points. The hardware configuration is shown in Figure 6.
Figure 6 PLC4 control station hardware configuration
3. The functions completed by the control system
The fourth sewage treatment plant project in Xi'an adopts three levels of control methods: management, control and field level. Hierarchical control makes the user's authority clear, the overall network structure is clear, local faults do not affect the operation of other parts of the system, and the system is easy to expand and maintain.
1. Brief description of the three-level control method
(1) Management
The management is responsible for monitoring the operation of the five PLC control stations in the plant and its subordinate slave stations. The management is composed of four SIE-MENS industrial control computers. The installation of WinCC configuration software can realize the monitoring of the upper system and the whole plant. Generate reports and data trends. The management layer communicates with the five control layer PLCs through the optical fiber Ethernet ring network, and performs remote control of the field layer equipment through the control layer. The management uses Siemens SCALANCEX408-2 photoelectric switches for network connection and sets a fixed IP address for communication, as shown in Figure 7.
Figure 7 Set a fixed IP address for communication
(2) Control layer
The main function of the control layer is to receive data and instructions from the management layer. The PLC operates the equipment according to a pre-programmed program, and periodically scans the status of each slave station to transmit the operation of the field device to the management computer of the management layer. Command transmission and information transmission between the central control and the scene. The control layer is composed of five PLC main control stations. Each main control station and the central control computer are connected by optical fiber Ethernet ring network. Each main control PLC station is configured according to the difference of process and control equipment. Its core is Siemens s7- In the 400 series PLC, the CPU module is CPU414-3DP, and it is connected to the optical fiber ring network through the CP443-1 Ethernet module and SCALANCEX408-2 photoelectric switch. The master control PLC communicates with the field layer S7-200 slave station through the CP443-5Profibus bus module cascaded field bus to achieve master-slave distributed control, as shown in Figure 8.
Figure 8 Master-slave distributed control
(3) Field layer
The field layer communicates with the control layer through the PROFIBUS-DP bus. The field general equipment performs related operations through the MCC cabinet. The MCC and the main control PLC are hard-wired, and the equipment I / O point is expanded into a remote I / O bus station by using the Siemens IM153_2 module. Inverter, grid machine, mud scraper, sludge discharge valve and other integrated Siemens S7-200PLC are used as on-site control slave stations. S7-200PLC is connected to the PROFIBUS-DP bus through the Siemens EM277 communication module, and transmits the collected data to the control layer master PLC (S7-400).
The field-level process detection and online analysis instruments are smart instruments supported by Siemens, HACH, and E + H companies that support the PROFIBUS-DP bus; the sludge treatment area is an explosion-proof area, so the PROFIBUS-PA bus with intrinsic safety is used. Configure IM153H2 module and FDC157-0DP / PA coupler on the main control PLC, couple the PA bus to the DP bus to realize the data transmission between the PA protocol instrument and the main control PLC, as shown in Figure 9.
Figure 9 Data transmission between the PA protocol instrument and the PLC
2. Realization method of master-slave distributed control system and instrument integration based on PROFIBUS
Carry out hardware configuration and program design for each system and equipment according to the above configuration scheme. The programming and configuration software is STEP7v5.4 simplified Chinese version. The system uses a modular structure to facilitate maintenance and expansion. The implementation methods of the control system and the instrument system are described below.
(1) Realization method of master-slave distributed control system
For the bus remote I / O mode, set the bus address of the IM153_2 module, add termination resistors at both ends of the bus, and configure the hardware to implement the monitoring function. For the S7-200 PLC, set the bus address of the EM277 module, and the "DXMODE" light on the module lights, indicating that the physical connection between the S7-200 and the bus is normal. Map the variables that need to be transferred in S7-200 to the V-Memory memory swap area, so that the CPUs of S7-200 and s7-400 can realize data exchange through the one-to-one correspondence of this address, as shown in Figure 10.
Figure 10 Address mapping of each variable
Through the above setting and programming methods, the devices and systems controlled by S7-200PLC are integrated into the overall automatic control system through the bus connection.
(2) Implementation method of intelligent instrument system integration
When configuring the instrument, it should be noted that the PA bus connected by the DP / PA coupler is an extension of the DP bus in the topology, so the coupler is not configured. The address of the instrument on the same DP bus cannot be repeated, no matter which PA bus it is distributed on. Use the "PI direct read" command to read the value from the meter address. Different read addresses correspond to different meter information. The content depends on the configuration and settings of the meter itself. When using online analytical instruments in water treatment, a transmitter may be connected to multiple sensors, and related values ​​should be read according to the probe address.
3. Typical program function description of automatic control system
The process control of the bus electric valve in this project can reflect the characteristics of bus control, which is now described as follows. There are a total of 24 electric valves on the bus. Because the control technology of each group of valves is the same, but the actual variable address is different, a custom function is used to implement the relevant functions. Formal parameters are used as intermediate variables in the custom function. Each time the same function is called, different actual parameters are assigned to the function interface to realize linkage control. The control flow is shown in Figure 11.
Figure 11 Control flow
4. Problems encountered in actual engineering and solutions
The fourth sewage treatment plant project in Xi'an is the first large-scale automatic control instrument system integration project using Profibus fieldbus technology in the northwest region. After project practice, the following issues that need to be noted when using the bus technology are summarized as follows:
1) Reliability of bus connection. In the actual debugging process, the shielding layer of the twisted pair may be damaged and lead to grounding, which makes the line grounding potentials unequal and causes bus communication failure. The single-ended grounding method should be adopted, which can effectively remove the influence of grounding unequal potential; adding the "OB80-OB87" and "OB120-122" system organization blocks during configuration can make a slave station drop without affecting bus communication.
2) The problem of instruction address mapping when reading and writing to S7-200. When communicating with the S7-200 via the bus, the setting of the "V-Memory" address mapping area is the key to successful communication. According to Siemens bus communication regulations, write operations in the V area are at the front-end address, and read operations are at the back-end address. If it is not read according to this rule, the slave and master cannot communicate.
3) The problem of reading intelligent instrument parameters and values. The value of the bus instrument is a digital signal processed by the transmitter. Therefore, when reading the meter, the data type and data length of the signal must be considered first. For example, the flow signal transmitted by the HACHSC1000 transmitter is a floating-point 32-bit data. If the "PIW" command is used in the PLC, data overflow will occur. Similarly, if the MD register is used to store the value in the program, the meter The data type set on the above is "INT" integer type, the value cannot be read correctly. Therefore, we must pay attention to the problem of data type matching in communication.
4) The influence of the strong electromagnetic environment on the bus communication. In this project 2 # PLC main control station, there are 26 instruments connected to the bus, and a large submersible sewage pump inverter is next to the PLC main control cabinet. During debugging, it was found that the bus instrument of the 2 # PLC main control station was disconnected from time to time. The analysis found that the bus was subjected to strong electromagnetic interference when the inverter was running. The solution is that the bus cable is laid in a single layer on the bridge, keeping the straight distance between the bus cable and the power cable above 20cm. The non-parallel laying of the bus cable and the power cable can reduce the phase-to-phase electromagnetic interference.
4. Project operation and application experience
Xi'an No. 4 Sewage Treatment Plant has now been successfully delivered to the owner, and the entire system is operating normally, meeting the design requirements. As a mature bus technology, PROFI-BUS has significant advantages compared with traditional hard wiring, such as high transmission rate, all-digital communication, strong anti-interference ability, saving wiring costs, and reducing construction volume; while the distributed automatic control system based on bus technology It has the advantages of wide control range, high system stability, easy post-maintenance and system expansion; intelligent instrument integration based on bus technology can greatly improve the real-time and stability of data transmission. The control system based on this can effectively improve production efficiency and shorten the development cycle, and will play an important role in various fields of industrial process control.
references
[1] Cui Jian. Siemens S7 Programmable Controller-STEly7 Programming Guide [M]. Beijing: Machinery Industry Press, 2007.
[2] Cui Jian, Li Jia. Siemens Industrial Network Communication Guide [M]. Beijing: Machinery Industry Press, 2004.
[3] Tang Jiyang. Fieldbus (PROFIBUS) Technology Application Guide [M]. China Fieldbus Professional Committee, 1998.
[4] Siemens. STEP7v5.4 programming manual.
[5] Siemens. PROFIBUS-DP application manual.
[6] Siemens. PROFIBUS-PA application manual.
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