Abstract: To ensure the safe and reliable operation of water diversion equipment, a field monitoring system consisting of an S7-200 PLC, a touch screen, and an industrial computer was designed. It adopts the widely applicable Modbus communication protocol and seamlessly integrates hardware from different manufacturers. Field operation results demonstrate that this typical industrial field control scheme has advantages such as high reliability and comprehensive functions, meeting the control requirements of field equipment, reducing operating costs, and providing a reference scheme for the control of water diversion systems in other areas of Xinjiang.
Keywords : Water diversion project, S7-200 PLC, Touch screen, Modbus
0 Introduction
Water resources have always been the biggest constraint on the construction and development of Xinjiang. The rational utilization of existing water resources under the conditions of sustainable development has been a hot topic of research for scholars and experts. The Korla Economic and Technological Development Zone, established in 2007 with a planned area of 140 square kilometers, is intended to be the backbone of industrial development in Korla City and even Bayingolin Mongol Autonomous Prefecture, driving the development of the southern slope of the Tianshan Mountains economic belt. However, Korla's arid climate and water scarcity have become a bottleneck for the development zone. A water diversion project in Xinjiang, undertaken with the dual considerations of economic development and ecological balance, began construction in early 2009.
The key point of the water diversion project is the pump house near the water source. Its function is to pump water from the water source into a 6.02-kilometer-long concrete pipeline to ensure that the water reaches the development zone smoothly. The height difference between the water intake point and the development zone is about 40 meters, which means that the power of the water pump used for pumping is 1250KW to meet the power requirements. Moreover, the return water pressure when the pump house is shut down is huge. Without a scientific control and management system, it will cause pipeline damage and threaten the personal safety of management personnel [1][2]. This paper solves the above problems through the current advanced industrial site control scheme and realizes the harmonious development of economy and environment.
1. System Composition of the Pump Station
1.1 System Equipment Introduction
The basic structure of the pump house is shown in Figure 1. To ensure the efficiency and stability of the system, two parallel water supply lines are used, operating selectively. Simultaneously, before the water from the source enters the pumping system, it passes through a filtration tank to remove various impurities, ensuring the safety of the system during operation.
The water pump uses a 1250KW motor with a rated voltage of 10KV, and the motor is started in stages using a soft starter cabinet; the five regulating valves are SK series valves from Shanghai Stock Company, with a drive voltage of 380V or 220V, and the control signal and the feedback signal of the remote valve position are both standard 4-20mA current; each outlet valve has a protective check valve at the front end, and the combined pipeline has a drain pipe and a drain check valve at the front end; a Chongqing Chuanyi PDS423 pressure transmitter is also installed between the water pump and the outlet valve to monitor the pressure changes in the pipeline.
Figure 1 System composition of the pump station
1.2 System Operating Requirements
For the first working circuit, when motor #1 starts, first adjust the #1 inlet valve to fully open and the #1 outlet valve to fully close. Then, motor #1 starts, and the water pump begins to work, water begins to enter the pipeline, and the water pressure increases. When the pressure transmitter detects that the water pressure reaches 0.65MPa, the #1 outlet valve begins to open until it is fully open. When motor #1 stops, motor #1 stops working first, and after a 20-second delay, the #1 outlet valve begins to close until it is fully closed. After the #1 outlet valve is fully closed, the #1 inlet valve begins to close until it is fully closed. The operating conditions and procedures for motor #2 are the same as for motor #1. However, it should be noted that the time interval between starting motor #1 and motor #2 is 30 minutes, and the time interval between stopping motor #1 and motor #2 is 20 minutes. If one of the motors is working independently, it can be started or stopped at any time.
2. Construction of the pump room control system
The safety status of the pump room is affected by the working condition of the equipment. The key to selecting the controller is its working stability. After comparison from multiple aspects, it was decided to use Siemens' S7-200PLC as the control core. S7-200PLC is a small programmable controller developed by Siemens. It can be controlled by a single machine or expanded by modules. It belongs to the integrated structure. It is inexpensive, compact, reliable, fast, has a rich instruction set, powerful multi-integration functions and real-time characteristics, and has a high cost performance. The CPU selected is 224XPCN, which integrates 14 inputs/10 outputs for a total of 24 digital I/O points; the digital expansion module selected is EM223, which has 8 inputs/8 outputs for a total of 16 digital I/O points; the analog module selected is EM235, which has 4 analog inputs and 1 analog output [3]. The upper-level human-machine interface is selected in two ways: one is an industrial control computer, and the other is a touch screen.
The structure of the entire control system is shown in the figure below.
Figure 2. Structure of the control system
The PLC controls the high-voltage soft starter cabinet to start or stop the motor, and transmits the motor status information back to the host computer and touch screen. The signals from the pressure sensor installed on the pipeline and the valve position signals from the regulating valve enter the EM235 module through the anti-interference signal isolator, and the information is transmitted to the CPU through the inter-module cable.
3. Control Program and Monitoring Screen Design
3.1 Control Flowchart
Based on the pump station's workflow, a control flowchart was created, as shown in Figure 3.
Figure 3 Pump Room Control Flowchart
3.2 Ladder Diagram and HMI Screen Programming
The S7-200 PLC uses STEP7-Micro/WINV4.0 software for programming, and the programming language chosen is LAD, which is familiar to technicians. The PLC CPU needs to communicate with both the touch screen and the industrial computer at the same time. The 224XPCN model CPU has two 485 serial ports. Port1 is connected to the touch screen, and Port0 is connected to the industrial computer using the Modbus communication protocol [4]. In the human-machine interface of the industrial computer, there is no separate configuration software to monitor the water supply equipment. This information, together with the information of the high and low voltage substation equipment in the pump room, is integrated into the monitoring software independently developed by Sichuan Jintaige Company.
The Modbus initialization program of the ladder diagram is shown in Figure 4. In the MBUS_INIT instruction, Mode is used to select the communication protocol. Setting its input value to "1" indicates that the CPU defines Port0 as the Modbus communication protocol and enables the protocol. Addr is used to set the station address value. Baud is used to set the baud rate of communication. It can be set to 1200, 2400, 4800, 9600, 19200, 38400, 57600 or 115200, a total of 8 values. Here, 9600 is selected. Parity is used to set the parity so that the slave station of Modbus matches the master station. Delay is used to set the delay parameter. Since this system is a wired network, the parameter is set to the typical value of 0. MaxIQ is used to specify the number of I/O points that the master device can use. MaxAI specifies the number of analog input words that the master device can use. MaxHold specifies the maximum number of holding registers that the master device can access. HoldSt~ is used to set the starting address of the holding register [5]. When the MBUS_SLAVE instruction responds to a Modbus request, the "Complete" output is turned on, and when there is no request requiring service, the "Complete" output is turned off. The "Error" output contains the result of executing the instruction, and this output is only valid when "Complete" is turned on. If "Complete" is turned off, the error parameter will not change [6].
Furthermore, since the two water supply lines in the pumping station operate in the same sequence, subroutines were written for each, which are then called separately in the main program with different parameters. This advantage shortens the programming time and increases CPU efficiency.
Figure 4 Modbus initialization
Figure 5 MT500HMI Interface
The HMI equipment selected is the MT500 touch screen produced by Weintek Technology Co., Ltd., which has a high cost performance and can meet the on-site work requirements. The software for editing the touch screen screen is EasyBuilder, which has created five windows: status screen, No. 1 motor operation screen, No. 2 motor operation screen, drain valve control, and alarm screen [7]. Figure 5 shows the No. 1 motor operation screen, which has manual buttons for stopping and starting the motor, motor status indicators, pipeline working pressure values, and control buttons and status indicators for the inlet and outlet valves. The screen structure is simple and can be easily understood by the staff.
4. Conclusion
The control system adopts a typical industrial control scheme, and the selection and matching of hardware takes into account both operational reliability and economic applicability. Since the completion of the water diversion project, the operation results have proved that the pump house control system operates stably, with no safety accidents occurring during start-up and shutdown. It has saved manpower, reduced water transportation costs, and achieved functions such as real-time status monitoring and data management of the pump house, playing a crucial role in the automated management of the entire water diversion project.
References:
[1] Deng Xuerang. Several issues to be noted in the trial operation design and implementation of long-distance pumping station water diversion and supply projects [J]. Water Resources Planning and Design, 2010(3):69-70
[2] Xiao Shunli. Application of PLC in multiple pumps in sewage treatment plants [J]. Electrical Age, 2007(7):92-93
[3] Wu Zhimin, Yang Shengfeng. Comprehensive Application Tutorial of Siemens PLC, Inverter and Touch Screen [M]. China Electric Power Press, 2009: 1-8
[4] Liao Changchu. S7-200 PLC Programming and Application [M]. Machinery Industry Press, 2007: 88-89, 149-156
[5] Chen Ming. Research on Communication between Devices and PLC Based on MODBUS Protocol [J]. Journal of Hunan University of Science and Technology, 2009, 30(4): 60-62
[6] SIMATIC S7-200 PLC Manual [M]. Siemens, 2003.
[7] MT500 User Manual [M]. Weintek Technology Co., Ltd., 2008.
Author Basic Information
Name: Li Xiaohai
Institution and Position: Postgraduate student, School of Electrical Engineering, Xinjiang University
Research Interests: Fieldbus and Industrial Field Control
Email: [email protected]
Mobile phone: 15999118761
Contact Address: 2009 Graduate Students, School of Electrical Engineering, South Campus, Xinjiang University, Tianshan District, Urumqi, Xinjiang
Postal code: 830047
