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Practical application of small and medium-sized PLC in large Waterworks

I. Introduction

this paper introduces the application of the control system composed of small and medium-sized PLC in the automatic control project of the second Waterworks in Nanhai City, Guangdong Province. The total design scale of Nanhai No.2 water plant is 1million m3 of daily water supply. The first phase of the project is 250000 m3 of daily water supply. It was completed and put into operation in April 1997. The second phase of the 250000 m3/d project is about to be completed and put into operation. The automatic control project mainly includes the transformation of the first phase of the automatic control project and the construction of the second phase of the automatic control project, and reserves interfaces for subsequent expansion

II. Production process

the production process of the water plant used in this system is shown in Figure 1:

it is mainly divided into the following process:

(1) taking water and pumping river surface water into water purification plants such as wool and acrylic fiber plants through multiple large-scale centrifugal pumps

(2) preparation and dosing of agents prepare appropriate coagulants according to the process requirements, and input coagulants and chlorine to achieve the purpose of coagulation and disinfection

(3) coagulation includes mixing and flocculation, that is, the surface water reacts after adding coagulants, and the sludge precipitated after the reaction is discharged

(4) the pig iron market continues to be weak. The water after the reaction between advection sedimentation and coagulant flows through the advection sedimentation tank at a low speed, so that the suspended particles can settle and the settled sludge can be discharged

(5) filter the precipitated water through the particle medium (quartz sand) to remove the suspended impurities in it, make the water clear, and backwash the quartz sand regularly

(6) water delivery: tap water is delivered to the city pipe at a certain pressure and flow rate through multiple large centrifugal pumps

III. control scheme

because the tap water production process mainly has the following characteristics: (1) each production process section is relatively independent, and there are many single equipment. (2) The amount of data collected is large. The whole system has more than 3000 digital inputs and outputs, and more than 1000 analog inputs and outputs. There are many kinds of process parameters, including pressure, flow, temperature, differential pressure, liquid level, current, voltage, power, etc., but there are few upstream and downstream related production parameters. (3) The production of tap water is continuous, irreplaceable and uninterrupted. (4) Each process section is far away, the equipment is scattered, and the group is relatively complex. According to the above characteristics, the system selects the small and medium-sized PLC of OMRON to control the production equipment of each process section in a decentralized manner, and uses omronprotocol and controllerlink to form a network. The upper computer is set in the control room and central control room of each process section to build a man-machine interface for production management and subsequent processing of production data. The control network of the whole plant is shown in Figure 2

in the water intake and delivery process section, the main equipment is composed of multiple large-scale centrifugal pumps and 10kv High-voltage direct distribution motors, each of which is controlled by the corresponding high-voltage distribution cabinet. Therefore, a sepam2000 (Small PLC produced by Schneider and dedicated to the control of distribution cabinet) is selected for each high-voltage distribution cabinet for data acquisition and control, and an omroncpm2a is selected for each pump valve on site for data acquisition and control, Connected through RS422 interface, omronc200hg medium-sized PLC in the control room communicates with them using omronprotocol protocol to read and write data and conduct unified scheduling, which can save a lot of data acquisition cables, and when a PLC fails, it can be easily disconnected for maintenance without affecting the normal production of other equipment. For the control of the sludge car of the sedimentation tank, because the sludge car moves back and forth on the sedimentation tank up to nearly 100 meters, the small PLC used for its control uses c200hg between the radio and the control room to communicate 1:n through the RS232 interface. The radio model is mds-scada-24810, which is a direct digital modulation and demodulation radio with a working frequency range of 2.4g~2.4835ghz. It supports the standard asynchronous communication protocol and works stably and reliably. The protocol also adopts omronprotocol, The software is compiled with Omron CX protocol. Multiple small PLC (omroncqm1h) are used for decentralized control of phase II filter, which can better solve the problem of affecting the safe water supply due to the shutdown of all filters caused by the failure of control equipment. The whole controllerlink network is divided into two sections by the repeater, which is mainly to meet the requirements of controllerlink for communication distance and meet the needs of future expansion. All the parameters required by the production process in the system are collected and controlled by PLC. The upper computer is only a man-machine interface and subsequent processing of production data, which greatly improves the reliability of the system by tightening the oil box to a torque of 8.5Kg when measuring the sintering load. This control scheme adopts small and medium-sized PLC to control the main production equipment dispersedly, and uses the network to connect them closely, so as to realize centralized management, reduce the failure risk and improve the reliability. It is an economic and feasible scheme

IV. introduction to related small and medium-sized PLC

1. Omronc200hg has the characteristics of fast speed, strong function, convenient programming and reliable operation. The maximum i/o points are 1184, the program capacity is 15.2k, and the instruction execution time is 0.15 μ S~0.6 μ S. It can support various communication units

2. Omroncqm1h is applicable to compact PLC with decentralized control, with 512 i/o points and 7.2k program capacity. It supports various built-in boards and controllerlink units

3. Omroncpm2a is designed to meet the system control operation of 10~60 point i/o, which can meet the requirements of efficient control of single equipment and effectively replace relay controller and sensor controller

4. Controllerlink is a factory automation network provided by OMRON. It can conveniently and flexibly send and receive large capacity data packets between suitable PLC and various microcomputers, support data links that can share data and information services that can send and receive data when needed. The network adopts shielded twisted pair cable or optical fiber connection, and the maximum transmission distance changes with the baud rate. In the case of two-layer repeater, when the baud rate is 500kbit/s, the transmission distance can reach 3km, It supports a maximum of 62 nodes. Controllerlink network is a network that uses token bus communication. This bus topology has the greatest flexibility, is easy to expand and maintain, and meets the needs of system scalability. Due to the distributed control technology, it can ensure that the controllerlink network will not collapse due to the failure of a site, and improve the stability of the system

5. Omronprotocol uses the communication board of OMRON to send and receive data with various general components connected to RS232 or rs422/485 (such as PLC of various brands and models, field instruments, etc.). Through the communication protocol support software (Omron CX protocol), users can freely compile the original communication protocol that can be implemented with pmcr instructions

v. program structure

the control of all equipment in this system is completed by PLC. The program is compiled by OMRON CX programmer software and transmitted to the CPU unit of PLC through controllerlink network or serial port on the upper computer. In each process section and single equipment, its control program is also relatively independent, and some of the same processes adopt subroutine mode. For the control of some process parameters requiring high precision, the closed-loop control using the PID command of PLC can meet the production requirements. Therefore, the program structure is relatively simple and convenient for debugging and maintenance. The following describes the automatic operation of the tensile strength testing pool of the test pieces or test blocks of gray iron castings of different brands with the start and stop of the water pump and the tensile strength testing machine of filter gray iron castings

1. The start and stop control process of the water pump. The start and stop of the water pump are commanded by the operator on the upper computer, and the operation is completed by PLC step by step. The program structure is shown in Figure 3

2. The automatic control process of the filter is completed by multi grid filter. For each grid of filter, the process is basically the same, including the normal filtration and backwashing states. Therefore, the control mode of subroutine is adopted, and the program structure diagram is shown in Figure 4

VI. conclusion

this project is a distributed control system composed of industrial computer and small and medium-sized PLC, which uses the PLC's strong anti-interference ability, convenient organization, and suitable for the holding point of the industrial site, and realizes the control of the whole plant's production equipment and the setting, adjustment and monitoring of process parameters in the upper function, so as to meet the requirements of automatic control of large-scale waterworks. The whole scheme is safe, reliable, economical and practical, easy to program, operate and maintain, and has been well applied in Guangdong Nanhai No. 2 water plant