In the design of the most popular PLC system, the

2022-08-11
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In the design of PLC system, the clever collocation of industrial control products is used to optimize the system

[Abstract] combined with the actual project of PLC design, this paper explains the importance of ingenious design ideas and appropriate selection of hardware equipment in system optimization by proposing, analyzing and comparing several design schemes

[keyword]plc; Optimization; Type selection

1. Overview

with the development of the times and the continuous improvement of the level of science and technology, industrial engineering is gradually moving towards large-scale and integration. At the same time, more and more industries with various functions can tell the laboratory machine manufacturers that their products are full of our designers. How to make better use of these starry products and achieve the optimization of the project is the goal that designers have been striving for. Combined with practice, this paper talks about how to skillfully match PLC with other products in engineering projects, so as to make the whole system more economical, efficient and reliable

2. project background

in the supporting electric automatic control system of centrifugal blower of CNOOC Huizhou project, the system configuration is as follows: six sets of local low-voltage electric control cabinets are used to complete the start and stop functions of their respective fan subsystems; A set of PLC remote automatic control cabinet is selected from Siemens s series. After collecting the analog quantities such as temperature, flow and oil pressure of the whole fan system, the upper computer outputs the display and alarm. There are many temperature collection points of the fan system. In this project, each set of fan has three motor stator temperatures, two motor support bearing temperatures, two fan support bearing temperatures and one thrust bearing temperatures, with a total of 48 temperature points. If Siemens 8-point analog radial extensometer is selected: it is used to detect the radial shrinkage deformation input module of the standard specimen, a total of 6 pieces are required. (as shown in Figure 1) in addition to the analog input of shaft vibration, oil pipe pressure, outlet air pressure and so on, there are too many analog input modules of PLC to be selected, which is costly and difficult to install. So, how to reduce expansion modules as much as possible on the premise of ensuring or even improving system performance

3. put forward new ideas

some CPUs of Siemens's series PLC are equipped with profibusdp master/slave interface, so we can use PROFIBUS bus transmission protocol to obtain field temperature signals in batches. PROFIBUS DP is used for high-speed data transmission at the field level. The master station periodically reads the input information of the slave station and periodically sends the output information to the slave station. The bus cycle time must be shorter than that of the master station (PLC) program. In addition to periodic user data transmission, PROFIBUS-DP also provides non periodic communication required by intelligent equipment for configuration, diagnosis and alarm processing

we know that temperature signals can be collected centrally through patrol inspection, so according to the actual situation, I choose three SS 16 channel temperature patrol detectors developed and produced by Shaanxi industrial automation engineering company. However, this type of temperature inspection instrument uses ModbusRTU protocol (RS485 port) for communication. Modbus protocol is a general language applied to electronic controllers. Through this protocol, the controllers can communicate with each other, the controllers via the network (such as Ethernet) and other devices that can display a variety of experimental curves: experimental force time, experimental force displacement, displacement time and so on. As a general industrial standard, it has been widely used in industrial networks composed of control equipment produced by different manufacturers to achieve centralized monitoring

the two diagrams are schematic diagrams of using analog expansion module and temperature patrol instrument to collect temperature signals respectively. It is not difficult to visually compare the two diagrams. It is not difficult to find that the structural form of master-slave station has the following advantages:

1. Save hardware quantity and investment

2. Save installation costs

3. Save maintenance costs

4. Users have a high degree of initiative in system integration

5. The accuracy and reliability of the system are improved

since this idea has great advantages, we can abandon the analog input module and replace it with a temperature patrol instrument. Next, we will analyze it more deeply

4. list two schemes

for two devices using two protocols, the lower computer needs to add a device to complete the work of converting ModbusRTU protocol to pofibus protocol. Although the change in the selection idea has brought us new problems, a wide range of products in the industrial control market can help us easily solve them. For the problem of converting ModbusRTU protocol to pofibus protocol, it is easy to think of two schemes

scheme 1: use CP341 communication module of Siemens

cp341 module is a serial communication module in Siemens S7 Series PLC. The module has an optional serial communication port (RS485 port is selected for this project). CP341 module can communicate with multiple serial communication devices at the same time, such as connecting multiple frequency converters, patrol inspection instruments, etc. Its processor can be parameterized conveniently and simply. If RS422/485modbusrtu communication mode is adopted, the sent data packet needs to include station number, data area, read-write instructions and other information. It is for the slave station equipment connected to CP341 module to identify which station the data packet is sent to. And the data packet is the function of reading or writing to that data area

the advantage of using CP341 is that this communication module has good compatibility and reliability with the CPU from the same manufacturer. The disadvantage is that this module requires additional software and hard dog fees, which is not low cost for small projects

scheme 2: protocol conversion using protocol converter

protocol converter is a bridge, or industrial gate, which is used to complete the storage and forwarding of data packets between LANs or between LANs and WANs and the conversion between different physical interfaces at the link layer. If the network types of Ethernet are different (such as Ethernet and token ring), because they do not recognize each other's frame format transmission method, it is impossible to realize the interconnection of the two networks simply relying on the data link layer protocol. Therefore, this problem is solved on the network layer: the method is to set two switches in different network types, and the networks are connected through switches, so switches play the role of protocol conversion

in recent years, the quality and performance of protocol converter products have become increasingly mature, and the brands are also diverse, and the functions are also very powerful. We take anybus in Sweden as an example to illustrate how to use it in the project

according to the requirements of this project, we can choose PROFIBUS dpserialgateway series products, and different types of serial ports (rs232/422/485/) can be selected through the supporting software. Anybuscommunicator provides a ModbusRTU master station mode. In this mode, turn off polling one or more MODBUS slaves and mapping the data obtained from the slaves to the internal storage area. The user can choose to use the predefined standard MODBUS command, or use the advanced data processing function of the switch to define the communication message, so that the data exchanged between the switch and the fieldbus master station is only the selected (6) when refueling, be sure to pay careful attention to the data of oil level selection. This function is very suitable for taking the temperature patrol inspection instrument as a slave station, and then using the switch to read the data it collects in turn

compare the two schemes. The former is more reliable and stable because the communication module and CPU are from the same manufacturer. However, due to the need to buy software and dongles, the cost is high; The latter is more economical, and the protocol converter is compact and easy to use, which is more suitable for some special occasions. In addition, in the case of broad project requirements, we can use the slave equipment with the same protocol with PLC, which saves the protocol conversion process and makes the system more optimized

5. Conclusion

the purpose of selecting products and equipment is to make the design, installation, operation, normal production and maintenance of the control system reflect its advantages. Various hardware and modules are like pieces of jigsaw puzzle. Different combinations will produce different effects. Therefore, when we get a project and start to consider choosing hardware, we can change several ideas, and implement each idea on the drawings for clearer comparison and deliberation. The progress of science and technology is obvious to all, and the functions of products are changing with each passing day. There is no fixed method in industrial design. As a technical designer of industrial control, we should use our brains frequently, broaden our thinking, and solve problems more quickly and economically

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