Application of PLC and MDS Wireless Data Radio SCADA System in Water Plant

[Key words]
Programmable Controller (PLC) Monitoring Digital Radio Communication Interface Industrial Control Computer MCGS Configuration Software I. Overview A water plant control system consists of 5 deep well water pumping stations, storage tanks, and user pipe networks distributed over a dozen kilometers. The entire water supply system has a height drop of about 150 meters. Due to the uniqueness of the water supply system and the topographical structure of the water supply system, past manual monitoring has brought inconvenience to production management and water supply dispatch.
After the implementation of the microcomputer monitoring, it can monitor the main process parameters (such as pressure, flow, water level, voltage, current, etc.) of the water supply system in real time, control the operation status of the deep well pump, monitor the pump, and provide the reports required for production management. Curve, data query and other functions. Its operation has important significance for the safe production and scientific dispatch of the water supply system.
Second, system composition The system mainly by the monitoring center, the wireless communication system, the scene monitoring terminal, the sensor and the instrument four parts.
Monitoring Center: It is composed of microcomputer, MCGS configuration software, wireless data transmission radio, omnidirectional antenna, analog screen and UPS. It mainly completes the real-time collection, monitoring, control, data storage, print report, data query and other functions of the field terminal data. .
Wireless communication system: Wireless communication between the monitoring center and the terminals of each pump station. The monitoring center is an active station, and the other terminal substations are passive slave stations. The system uses the data frequency given by the radio management committee to communicate with the slave station in a point-to-multipoint manner. The monitoring center is an omnidirectional antenna, and each substation is Directional antenna.
Field monitoring terminal: The core is PLC, which is a smart device. It has its own CPU and control software. It mainly completes on-site data acquisition, conversion, storage, alarm, control and other functions, and communicates with the monitoring center microcomputer through wireless channel. . According to the commands of the monitoring center, tasks such as system self-test, data transmission, and control output are completed.
Sensors and Meters: It is the “eye” of PLC to monitor the field signals. All the signals on the site need to be converted by sensors and meters to output the standard signals and accepted by the PLC terminal. The system mainly measures parameters such as voltage, current, level, pressure, flow, and power consumption.

Water source well pump control l manual control. The operator starts and stops the water pump according to the level of the clear water tank.
l Automatic control. The PLC starts and stops the automatic water pump in accordance with the clear water level, the starting water level and the stop water level of each water pump.
3. On-site PLC terminal The on-site PLC monitoring terminal is the bridge link between the industrial site and the monitoring center. On the one hand, it collects signals such as on-site meters, transmitters, and equipment operation status. On the other hand, it communicates with the monitoring center and performs related command. Field terminals are generally unattended. Therefore, the performance and quality of the terminal have a great influence on the reliability of the system. After full demonstration, the Siemens S7-200 series PLC is chosen as the field terminal with high performance-cost ratio. It has the characteristics of small size, easy expansion, and excellent performance, which is very suitable for small-scale on-site monitoring.
1, PLC hardware design
A terminal at the scene needs to measure and control the switch input signal, switch output signal path, and analog input signal. Therefore, we use S7-212 basic unit, analog input expansion unit (EM231), analog output expansion unit (EM232). Meet site requirements.
2, communication interface
PLC and radio communication from the station: The S7-214PLC basic unit provides an RS-485 interface. In order to connect with the data transmission machine (power supply, data transmission radio) of the wireless channel, we specially designed a dedicated Modem with RS-485 interface, and The use of optical isolation technology, so that the two completely independent in the electrical, to avoid mutual interference, due to the need for RTS signals when the data transmission device, and RS-485 interface does not provide RTS signal, there are two ways to solve this problem. First, the RTS signal is generated by the wireless Modem according to the PLC's transmission information. This requires that the Modem must be intelligent. At the same time, the PLC must communicate with the Modem before sending the information to let it output the RTS signal and send back the RTS generated information. The PLC then sends the field information. Second, using an I/O output point of the PLC, an RTS signal is generated. The PLC turns on the point before sending the information, controls the data transmission, and delays for a period of time (the radio station establishes the carrier time). information. The latter method is simple and practical, and it solves the problem of wireless communication interface.

Communication between PLC and IPC of the main control room: Because the PLC of the main control room must communicate with the data transmission station and communicate with the upper computer (IPC), the PLC of the main control room selects the basic unit of the S7-216 with two ports. One of the ports communicates with the data radio station. The other port communicates directly with the host computer using the PPI cable from Siemens. The host computer uses the MCGS configuration software provided by Beijing Kunlun Technologies to configure and program the water level and other signals in real time. Monitoring and processing.
3, anti-jamming design
In order to improve the reliability of the system, on-site terminals, data transfer machines, PLCs, DC temperature power supplies and some transmitters are installed in a control cabinet. Each part is relatively independent and easy to maintain. PLC switch input, output and field isolation between the relay, the analog signal isolation using the signal isolator and distribution, power supply using isolation transformer to reduce the power "noise", while the system is set up a good grounding.
Fourth, PLC software design PLC terminal software is written in ladder diagram language, in order to improve the terminal's anti-jamming capability, the software design uses digital filtering, fault self-test, control passwords and other measures to ensure the correctness and reliability of the control operation. The program design adopts a modular and functional structure for easy maintenance and expansion. The terminal software consists mainly of the following modules.
1, the initialization program: set the initial value of each register, counter, PLC operating mode, communication mode and other parameters.
2, data acquisition subroutine: The analog data acquisition, filtering, averaging and other processing.
3. Accumulative running time subroutine: accumulate the operating time of the pump and other equipment.
4, remote signaling subroutine: detection of motor, valves, alarm switches and other equipment operating status.
5, set the initial value subroutine: the monitoring center on the time, power consumption, flow and other cumulative parameters according to the user's requirements set initial value.
6, fault self-test subroutine: Test PLC fault information, verification information, and sent to the monitoring center.
7, control subroutine: According to the monitoring center's command, or on-site automatic control conditions output corresponding operation.
8. Communication subroutines; Complete various communication functions with the monitoring center.
In the communication program, the receive command and the send command use interrupt processing, and the ATCH instruction causes the interrupt event 8 to execute different programs under the command of receiving different features. The timeout limit for serial communication is controlled by setting an internal timer interrupt with an event number of 10 and the timing is determined by the value of SMB34. In order to reduce the error of communication, double check and XOR double check measures are adopted.
V. Concluding remarks The system has taken many measures in terms of software and hardware. In particular, S7-200 PLC has been selected at the field terminal to improve the reliability of the system. The automatic control system of the water plant has achieved good results. The PLC-based SCADA system can fully meet the requirements of the water plant control system, and plays a crucial role in the safe operation of the water plant, improving the quality of water supply, saving energy, reducing consumption, and optimizing management. The system integrates wireless communication with S7-200 PLC organically to solve the problem of system monitoring where the field distribution is relatively scattered, distant, and has a large range. The water supply, power supply, gas supply, oil field, meteorology, hydrology and water conservancy departments have Better application prospects.

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