I. Project analysis
1. Project Analysis
Engineering and technical personnel must first analyze the project, that is, the control process of the project and the type of control of each process, and prejudge the possible problems of the entire project.
(1) Analyze the control process. When analyzing the control process, it is recommended to draw the relevant control flow chart to clearly mark the content of each step and the conditions to the next step.
(2) Analyze the type of control and predict the parameters required for PLC selection. General PLC is suitable for four control types, namely sequential control, process control, motion (or position) control and network communication. After analyzing the control requirements, the engineering and technical personnel classify the control types of each control flow according to the drawn control flow chart, and then combine the control types according to the complexity of the project. Therefore, the control type of each step is accurately analyzed in the early stage. Will contribute to the accuracy of the selection and the estimation of the problem.
While analyzing the type of project control, engineers and technicians must also estimate the important parameter values ​​required for PLC selection. For example, the number of I/O points in the sequence control; if an encoder is used, the frequency value of the output pulse is calculated according to the parameters of the encoder, and then converted into the pulse frequency of the PLC high-speed counting. For example, the analog quantity and analog precision in process control, the response speed of PLC to servo drive feedback signal and the number of PLC high-speed pulse output in motion control, and whether the selected PLC supports the corresponding network type during network communication, etc. parameter.
2. Estimate possible problems
Estimates of possible problems are more difficult aspects of engineering analysis. This requires not only the engineering and technical personnel to grasp the working environment of the site and the difficulties of the entire project control, but also the preliminary prediction of the possible emergencies and dangers.
(1) Grasping the working environment of the equipment. Engineers need a more comprehensive understanding of the production environment. If the working environment of the textile machinery is high in humidity and vibration, it is necessary to do earthquake-proof treatment when designing the PLC system. Another example is that the building material processing plant has a relatively high ambient temperature and has a large amount of dust and strong static electricity. Therefore, engineers and technicians should further protect the electric control cabinet from dust and static electricity.
The grasp of the working environment of the equipment is not limited to the physical environment. With the deepening of the PLC application, it is necessary to consider the personnel factor. If the quality of equipment operators is low, it is necessary to develop a more concise equipment operation interface.
(2) Expectations of difficult projects. The expectation of the project's difficulties is actually the grasp of the core issues of the project. For example, the control core of the air jet loom equipment is how to control the solenoid valve quickly and orderly, and the frictional force of the compressed air is used to pull the weft yarn through the textile shuttle to complete the weft insertion work of the textile, which requires the PLC to have a very fast response speed. After determining the difficulty of the project, the engineering and technical personnel can select the PLC according to the difficulty. From the perspective of the whole project, the difficulty of the project is the characteristics of the system design, and also the direction of PLC selection.
(3) Pre-project estimates of project hazards. In the early stage of project design, engineers and technicians need to prejudge the dangerous situation that may occur in this project. For the protection of malfunctions when debugging equipment in sequence control or motion control; in process control, whether there is high pressure, high temperature or toxic and hazardous substances and related protective measures during testing. Estimating hazards in the early stages of project design helps to enhance the safety awareness of engineers and technicians.
Second, PLC hardware selection
The selection of PLC is based on the previous project analysis and the difficulty of the project to select the appropriate PLC, mainly based on the following principles.
1. First special general principle
According to engineering experience, the factors that restrict PLC selection in most engineering projects are mainly concentrated on several key points, so PLC should be selected according to the principle of special and general.
The so-called special, that is, the project has special control requirements, and different control types have different primary constraints. For example, in the sequence control, the program capacity of the CPU and the expansion capability of the I/O point are the main factors for PLC selection. In the process control, the selection is made by controlling the number of analog quantities and the accuracy of the analog quantity as a starting point. In the simpler motion control, the PLC needs to receive the position signal sent back from the field encoder and correspondingly emit a pulse of a certain frequency to control the servo motor. Therefore, the speed at which the PLC processes the data, the ability of the input terminal to receive the high-speed pulse, and the output end The ability to transmit high-speed pulses will be the primary factor in PLC selection. In large-scale composite projects, different PLC networking is required, so the network type supported by PLC becomes the primary factor for PLC selection.
Engineers and technicians need to arrange different control requirements in order from special to general according to the core requirements of the project. This type of selection will do more with less, and reduce the overall difficulty of the project.
2. Bottom-up principle
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