Discussion on the selection and maintenance of PLC

2022-08-23
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Talking about the selection and maintenance of PLC in Waterworks

1 introduction

with the popularization of PLC, the types and quantities of PLC products are more and more, and the functions are becoming more and more perfect. It is more and more widely used in waterworks, which can not only improve the automation level of waterworks, speed up production, reduce production costs, but also improve the quality of water supply

2 selection of PLC in Waterworks

2.1 it is advocated to choose modular PLC

according to the structural form, PLC can be divided into integral type and modular type. The integrated PLC centralizes the power supply, CPU and i/o components in one chassis. Modular PLC structure is to divide each part of PLC into several separate modules, such as CPU module fast, i/o module fast, etc. Considering that 95% of the reconstruction (especially energy conservation, replacement of old equipment), expansion and PLC failure rate of the waterworks are caused by the damage of i/o components; At the same time, the configuration of modular PLC is flexible and convenient for assembly and maintenance. As long as the corresponding i/o module is quickly replaced or expanded and then programmed, automation can be easily realized for the change of water plant equipment and process. Therefore, in the long run, modular PLC is advocated

2.2 unified selection of models

when selecting PLC, attention should be paid to whether the after-sales service is guaranteed, while taking into account the convenience of future maintenance of the water plant, the inventory of spare parts and software programming. The common steps of making tap water are mainly divided into: coagulation, sedimentation, filtration, disinfection and storage. On the premise that the functions meet the requirements, it is best to choose the products of the same company for the selected models

2.3 according to the input and output, the main equipment in the waterworks are: reaction tank, clarifier, filter, clean water tank, chlorinator, chlorine absorption device, air compressor, blower, dosing equipment, valves, pumps, mixing equipment, metering equipment. According to the requirements of the control system and the control method adopted in an automatic system, for each controlled object, the i/o points used will not change easily. The i/o module selected according to the required i/o points can be flexibly combined with the host, but considering the future changes of technology and equipment, or the damage and failure of i/o points, generally, 1/8 margin should be reserved

in addition to the number of i/o points, attention should also be paid to the properties, parameters and characteristic requirements of input and output signals. For example, whether the valve in the water plant is controlled by analog quantity or switching quantity; Whether the signal source of pH meter, flowmeter, turbidity meter, residual chlorine meter, liquid level meter and other water quality monitoring instruments is voltage output type or current output type, active output or passive output, and whether the relay output is NPN output type or PNP output type. In addition, pay attention to the load characteristics of the output terminal (type of load voltage and current), quantity level and requirements for response speed

based on this, i/o modules suitable for the characteristics and requirements of input and output signals are selected and configured

2.4 select

according to the memory capacity. Generally, the memory capacity of PLC is in words, such as 64K words. The memory capacity required by the application program can be estimated in advance. The first method of selection and calculation is to accurately calculate the actual capacity of memory according to the number of nodes used in programming. The second method is the estimation method. Users can estimate the mechanical performance testing equipment such as electronic universal testing machine, hydraulic universal testing machine, horizontal tensile testing machine, fatigue testing machine according to the formula in the attached table according to the control scale and application purpose

when using, you can choose the appropriate model according to the storage needs of programs and data, and when necessary, you can also specially design the expansion of memory. In order to be convenient for use, and taking into account the changes of the process and equipment of the water plant and the needs of programming, a margin of 25% - 30% should generally be reserved

2.5 choose

at present, PLC adopts various industrial standards, such as IEC61131, IEEE802.3 Ethernet, tcp/ip, udp/ip, etc. various de facto industrial standards, such as WindowsNT, OPC, etc., integrate IT technology, and can be connected with intelligent MCC motor control center, other operation control systems, electronic control equipment, frequency converter, soft starter, etc

at present, the control system composed of industrial computer and PLC is the most widely used in the automation of waterworks. Generally, PLC in the system is divided into water intake pump station, dosing station, filter station and water supply pump station. The monitored parameters, such as residual chlorine, flow, turbidity, etc., should be transmitted between stations, and they are centrally controlled by the computer in the central control room. The basic requirements of communication are real-time, stable, reliable and economical. According to its own equipment, invested funds, response speed and future development, the water plant should choose field buses and networks that are easy to expand, connect and develop mature, such as Ethernet, PROFIBUS, MODBUS, Fipio, ASI, etc., so that a more long-term goal is to select PLC communication modules

3 problems to pay attention to during maintenance

(1) the location of PLC installation should avoid direct sunlight, ensure sufficient heat dissipation space and ventilation conditions, and avoid installation in the environment with serious interference, high temperature and high humidity, such as dust, unclean and corrosive gas in the modified plastics of various nano inorganic powder materials. In addition, vibration reduction measures should be taken when PLC is installed in a place with vibration source

(2) do not use the same cable for input and output lines in order to save investment. At the same time, the power cable and control cable should be laid separately to avoid interference

(3) after installation, it is necessary to check clearly, and clean the thin and short wires, copper filings, iron filings and screws before power on. After putting into use, regularly check whether the installation is firm and whether the connection and wiring of terminals and modules are reliable, and regularly clean the dust to ensure safety

(4) in order to suppress the interference added to the power supply, input and output terminals, the PLC should be connected to a special ground

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