The daily output of Maoming No.2 Waterworks is 2×15 cubic meters, which provides more than 7% of daily water in Maoming city. In order to alleviate the shortage of water supply in the city, the municipal government increased investment and expanded the water plant. The water plant has a high degree of automation, and the whole automatic control system adopts the form of (PC+PLC). Filter control is a difficult design link in the automation of waterworks, which is mainly manifested in the complexity of opening and closing sequence and opening and closing conditions of valves in the backwashing process. This paper mainly expounds the main design process of the automatic control system of the extended filter in this factory.
2 control tasks of the filter system
2.1 process requirements
The newly expanded V-shaped filter in the Second Waterworks is equipped with six grids, each grid is equipped with a liquid level meter and a blocking instrument, and each grid has its own water inlet valve, clean water valve, air flushing valve, water flushing valve, drainage valve and exhaust valve. There are 3 blowers for air flushing (two for use and one for standby); 3 backwash pumps for water flushing (dual use and one standby); Two air compressors (one for use and one for standby); 1 dryer.
the water to be filtered enters each cell of the filter, and then enters the clear water tank after being filtered by quartz sand at a constant speed. The filtration process requires that the water level in the filter grid should be kept at 1.2 meters above the filter material, and the filtration effect is the best at this water level. In order to realize constant speed and constant water level filtration, the water output of the filter should be equal to the water input, and the opening degree of the water outlet valve should be adjusted according to the water level change of the filter to control the water output. When the operation of the filter meets the constraints of backwashing, backwashing is needed to clean the sand. Backwashing is realized by controlling the water inlet valve, clean water outlet valve, backwashing air valve, exhaust valve, backwashing water inlet valve, backwashing drainage valve and running backwashing water pump and fan.
Therefore, the task of the filter control system is mainly to control the liquid pulling during filtration and backwashing when cleaning the filtered sand, and the filtration and backwashing are continuously circulated and alternated.
2.2 The performance index requirements of the control system are as follows:
(1) Automatic constant water level filtration with an error of 1.5 ㎝;
(2) Automatic backwashing can be realized accurately according to one of the following constraints: if the filtration time reaches the backwashing set period (such as 48 hours), it has not been backwashed; The loss value of filtration head reaches the set value (15) and the delay time (15 minutes) has expired, but it has not been backwashed; The forced backwash button is triggered.
(3) The backwashing cycle and the time of each step in the backwashing process can be set by the program to meet the requirements of process and actual operation.
(4) It can directly display the filter water level, head loss and opening degree of outlet valve, and at the same time display the on-off status of backwashing equipment and local filter valves.
(5) The backwashing equipment, local filter valve and backwashing process can be controlled automatically or manually.
3 control principle and operation process of the filter
3.1 constant water level control principle
The constant water level control of the filter is shown in Figure 1.
each filter compares the detected value of filter water level with the set value of water level to obtain the water level deviation signal Δ e. After PID operation, the output signal is sent to the output additional processing program, and then is output to the servo motor of the water valve to control the opening of the water outlet valve. The increase of opening is determined by the speed of water level rise and the water level deviation in a certain cumulative time. If the inflow velocity is faster, the outlet valve opening will be larger, and vice versa. The goal of PID operation is to keep the water level at the set value, and the added value can be added to the output control as compensation. The output additional processing program is to output the operation result of PID to the servo motor of clean water valve according to certain rules.
fig. 1 constant water level control system diagram of filter
3.2 backwashing process
when the control system receives the backwashing instruction signal, it queues up for backwashing according to the principle of first-in first-out Backwashing is divided into three stages: gas washing, gas-water mixed washing and water washing. The process is as follows: firstly, close the water inlet valve of the water to be filtered, when the water level drops to the set backwashing level, close the clean water outlet valve and open the waste water drain valve. After the signal of the drain valve is in place, close the exhaust valve first, then open the backwashing valve, and start the first fan for air flushing, which takes 1-3min; After the completion, open the backwashing water inlet valve, then start the second fan and the first water pump for gas-water mixed washing for 5 minutes; Then close two fans, close the backwashing air valve, open the exhaust valve, start the second water pump, and carry out single water flushing, which takes 3-6min. After completion, close the backwashing water valve, stop two backwashing water pumps, close the wastewater discharge valve, open the water inlet valve to be filtered, and open the filtered water valve. When the water level rises to the constant filtration level, the system switches to the normal filtration program.
4 control system design
4.1 hardware structure and network structure
this system adopts the form of PC+PLC. The upper computer consists of a COMPAQ microcomputer and two printers, and the lower computer consists of an analog screen PLC8, a public flushing PLC7 and six unit filter PLCs 1-6 * * * eight Schneider PLCs, as shown in Figure 2.
each PLC adopts a bus-shaped topology communication network connected by twisted-pair cables, which is also called FIPWAY communication network with a transmission rate of 1Mbps. The PLCs communicate with each other to realize data sharing. Both the unit filter and the PLC for public flushing are equipped with an on-site XBT-B (artificial intelligence interface), which is connected with PLC through cables, and the filter can be manually controlled on the XBT operation panel. The filter PLC of each unit is connected with the public flushing PLC through the FIPWAY network, and the public flushing PLC enters the central control room of the water plant and is connected with the microcomputer through the network, so the system can remotely monitor the operation of the filter in the central control room, and realize the three-level control of centralized monitoring, PLC remote control and on-site XBT operation in the central control room, thus ensuring the safety and reliability of the filter production and operation.
the PLC configuration of this system is as follows:
PLC 8: one for CPU/COM, one for POWER and one for DI of TSX 47/415; DO is 9 yuan. The analog screen is equipped with a D/A converter.
PLC 7: 1 CPU/COM, 1 POWER, 1 DO and 1 AI(TSXAEM811) of TSX 67/455; DI is 2 yuan.
PLC 1-6: one CPU/COM, one POWER, one DI, one DO and one AI(TSXAEM411) for TSX 47/415.
in the communication between PLC and PC, the special FIPWAY communication network card of TE company should be installed on the PC first, and then data communication should be carried out through RS422 communication interface.
4.2 control function of PLC
The PLC of unit filter mainly completes the constant water level filtration control of this filter and the automatic control of inlet valve, outlet valve, blowdown valve, backwash valve, exhaust valve and backwash valve of each filter, and collects data, and exchanges data information with the public flushing PLC. When the blocking instrument under the filter plate transmits the blocking degree signal of the filter bed to the filter unit PLC, after receiving the signal, the PLC compares it with the set value of the water head and displays it to decide whether the filter should be backwashed or not, and transmits it to the public flushing PLC. The opening number of the filter is determined by the inflow. The water level and head loss of each filter are measured by the liquid level gauge and the blocking instrument, and sent to the unit PLC together with the opening of the filtered water valve. After the built-in PID operation of the PLC, if the water level deviation exceeds 1.5cm, the PLC will immediately start the control unit to automatically adjust the opening of the butterfly valve of the filter outlet to maintain the water level of the filter basically constant, thus realizing constant water level filtration.
The public flushing PLC is responsible for the backwashing queue coordination of the six filters, and the monitoring of backwashing equipment (backwashing pumps, blowers, etc.) and its import and export valves. When the unit PLC sends a backwash request to the public flushing PLC, the public flushing PLC starts the backwash program to control the backwash of the filter. When a filter is backwashing, if one or more filters send out backwashing request signals, the signals will be stored in the public washing PLC memory, and then washed according to the storage sequence, and the filters waiting in line for backwashing will maintain normal production.
the function of the analog screen PLC is to drive the analog screen to work and realize the communication with the radio system and microcomputer of the water company. The simulation screen can dynamically display the process flow, equipment operation status and main process parameters of the whole water plant, and realize acousto-optic alarm, which is convenient for production scheduling management.
4.3 programming
backwashing is carried out when the filter meets one of the constraints of backwashing control. This system uses a backwashing PLC to realize the queuing backwashing of six filters, and collects the backwashing information of the whole group of filters and the specific water level of the filters through the reading and writing commands of the public program and issues commands. The main contents of the public program include: the control program of the recoil pump fan, the reading and writing program of the public PLC and other units' PLC information, and the filter queuing program.
the process of each filter is basically the same, and its PLC program structure is also the same, which can be in the form of subroutine, as shown in figure 3. Each filter program includes initialization command, automatic state, manual state, field state and other programs. The automatic state program of filter includes three subprograms: backwashing state, finishing state and normal filtering state. The manual status program of filter contains the manual operation commands of each valve. The main contents of the on-site status program of the filter include: (1) All the commands issued when the filter is transferred from the automatic status to the site must be reset. (2) Some variables in the automatic state, such as time variables and counter variables, must be reset. (3) For recoil, an end recoil command must be issued in this state.
4.4 system monitoring software
The upper computer of this system adopts Windows NT operating system, and the real-time monitoring software adopts InTouch7. industrial configuration software of Wonderware company, which mainly includes two programs: WindowMaker and WindowViewer. The upper computer is equipped with a communication network card following FIPWAY communication protocol to collect production data in real time. The monitoring computer can clearly display the dynamic process simulation picture during the filter's filtration, waiting and backwashing, and can remotely operate and control all the equipment in the system. It also has the functions of displaying process layout, real-time dynamic parameters, equipment's working status and real-time/historical alarm signals, real-time/historical trend curve of online instruments, motor running time, etc. At the same time, it can carry out offline/online programming and modification of set parameters, and compile and print production and management reports.
5 networking between old and new systems
Because the newly-built filter system and the original water plant system are two independent systems developed by different companies' PLC, the communication protocols of the two systems are different, and there is no data communication between them, which brings certain troubles to production and management. InTouch is adopted in the two phases of monitoring configuration software, but the versions used are different. Considering the cost of technical transformation and the technical strength of the company, it is decided to use the network function of InTouch based on Ethernet and compatible with TCP/IP communication protocol to realize the networking control of two independent systems. The specific methods are as follows:
First, an Ethernet is set up with a switch, and the system schematic diagram is shown in Figure 4. TCP/IP communication protocol and NetDDE program are installed on the original system monitoring microcomputer PC1
and the newly-built system monitoring microcomputer PC2 respectively.
Then design the software of InTouch monitoring system. Run windowmaker, the development environment of InTouch, and use the "import" function to integrate the data of the old and new programs into a complete application program, which is installed on PC1 and PC2 respectively, so that production can be monitored on any PC. B. Set the DDE Access of InTouch by adding \\PC2\viewer (on PC1) and \\PC1\viewer (on PC2) in the column DDE Application/Server Name in the Modify DDE Access Name dialog box. Through this setting, PC1 and PC2 can communicate with each other in real time via Ethernet. C. Initialize NetDDE, run InTouch windowviewer, and PC1 and PC2 can communicate in real time.
6 Conclusion
After a period of operation, the filter shows that the application effect of the control system is good, and all the control performance indexes of the system can meet the design requirements. Under normal circumstances, the water level fluctuation of this filter is controlled within ±1.5cm of the set value, and automatic filtration, automatic queuing and backwashing of six filters are realized, and the network control with the original system of the water plant is indirectly realized. The design of the whole control system basically meets the production requirements and achieves the expected effect.