How can a novice independently design a complete PLC application system?

“Project analysis requires a comprehensive analysis of the project’s production process, working environment, hardware requirements and control requirements. This work is the basis of the entire system design. If the preliminary project analysis is not in place, it will cause inaccurate hardware selection later, leading to project delays.

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Project Analysis

Project analysis requires a comprehensive analysis of the project’s production process, working environment, hardware requirements and control requirements. This work is the basis of the entire system design. If the preliminary project analysis is not in place, it will cause inaccurate hardware selection later, leading to project delays.

1. Project analysis

Engineering and technical personnel must first analyze the project, that is, the control process of the project and the control type of each process, and make predictions about possible problems in the entire project.

(1) Analyze the control process. When analyzing the control process, it is recommended to draw the relevant control flow chart, clearly marking the content of each step of the work and the conditions to the next step.

(2) Analyze the control type and estimate the parameters required for PLC selection. Generally PLC is suitable for four control types, namely sequence control, process control, motion (or position) control and network communication. After analyzing the control requirements, engineers and technicians classify the control types of each control process according to the drawn control flowchart, and then combine the control types according to the complexity of the project, so the early stage accurately analyzes the control types of each step, Will help 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. Such as the number of I/O points in sequence control; if an encoder is used, the frequency value of its output pulse should be calculated according to the parameters of the encoder, and then converted into the pulse frequency of PLC high-speed counting. For another example, the analog quantity and accuracy in process control, the response speed of PLC to servo drive feedback signal and the quantity 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. Estimated possible problems

The estimation of possible problems is a difficult part of engineering analysis. This not only requires engineering and technical personnel to have a good grasp of the on-site working environment and the difficulties of the entire project control, but also to have a preliminary judgment of possible emergencies and dangers.

(1) Grasp the working environment of the equipment. Engineers need to have a more comprehensive understanding of the production environment. For example, the air humidity and vibration in the working environment of textile machinery are high, so shock-proof treatment should be done when designing the PLC system. Another example is the relatively high ambient temperature in the building materials processing plant, and there is a lot of dust and strong static electricity. Therefore, engineers and technicians must take further measures to prevent dust and eliminate static electricity on the premise of ensuring that the electrical control cabinet is well ventilated.

The grasp of the equipment working environment is not limited to the physical environment. With the deepening of PLC applications, human factors must be considered. For example, when the quality of equipment operators is low, it is necessary to develop a more concise equipment operation interface.

(2) Anticipation of project difficulties. The anticipation of project 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 quickly and orderly control the solenoid valve, and use the friction of the compressed air to pull the weft through the textile shuttle to complete the weft insertion of the textile. This 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 entire project, the difficulty of the project is the characteristics of the system design and the direction of PLC selection.

(3) Preliminary estimation of project risks. In the early stage of project design, engineering and technical personnel need to predict the possible dangerous situations of the project. Such as sequence control or motion control for the protection of misoperation when debugging equipment; in the process control, whether there are high pressure, high temperature or toxic and harmful substances and related protective measures during the test. Estimating the hazards in the early stage of project design helps to strengthen the safety awareness of engineering and technical personnel.

PLC hardware selection

The selection of PLC is that the engineering and technical personnel choose the appropriate PLC according to the preliminary project analysis and the expectation of the difficult points of the project, mainly based on the following principles.

1. Special first and general principle

According to engineering experience, the factors that restrict PLC selection in most engineering projects are mainly concentrated on a few key points, so PLC should be selected according to the principle of first special and general.

The so-called special refers to the special control requirements of the project, and different control types have different primary constraints. For example, in sequence control, the program capacity of the CPU and the expansion capability of I/O points are the main factors for PLC selection. In process control, the number of analog quantities and the accuracy of analog quantities are taken as the starting point for selection. In relatively simple motion control, the PLC needs to receive the position signal sent back from the field encoder and correspondingly send out pulses of a certain frequency to control the servo motor. Therefore, the speed of the PLC processing data, the ability of the input to receive high-speed pulses, and the output The ability to send high-speed pulses will become the primary factor in PLC selection. In large-scale complex projects, different PLC networking is required, so the network type supported by PLC becomes the primary factor in PLC selection.

Engineering and technical personnel need to arrange the different control requirements in order from special to general according to the core requirements of the project. This selection will double the result with half the effort and reduce the overall difficulty of the project.

2. Bottom-up principle

The purpose of the bottom-up principle is to maximize the cost-effectiveness of PLC selection. At present, PLC products of most manufacturers are divided into multiple series. When engineering technicians select models, follow the first step from special to general selection order, starting with the lowest PLC, and compare the performance parameters one by one. When it is found that the requirements are not met, consider a higher product. And so on, until all PLC models that meet the requirements are selected. If the model is selected from top to bottom, it will waste PLC functions and cause a large horse-drawn cart.

3. Selection of PLC switch input/output unit

The switch value input point of PLC is used to receive the level signal input by the field sensor, and the function of the output point when switching is used to drive the external load according to the internal control signal.

(1) Selection of digital input terminals. Now the PLC input points on the market are all transistor inputs, and the user only needs to choose according to the number of input points estimated in the previous period. However, it should be noted here that due to the different wiring types of the PLC terminal, there are two input methods, NPN and PNP, respectively. The meaning is whether the input terminal is active at low level or active at high level. Once the wiring type of the input terminal is determined, you need to choose Sensors with the same type of input, namely NPN and PNP type sensors, cannot share one PLC input terminal.

At present, PLC input terminals on the market are mostly DC 24V input voltage. If it is necessary to connect sensors of other voltage specifications to the PLC, a relay must be used for corresponding isolation to ensure that the signal connected to the PLC input terminal is a DC 24V voltage.

(2) Selection of switch output terminals. The main types of PLC switch output points are relay output and transistor output.

1) Relay output type. The relay has a good output load capacity, can withstand higher overvoltage and overcurrent in a short time, and has a strong isolation effect. However, because the internal relay is a mechanical contact, the action life is limited, so it can only be used when the connection action frequency is low and high-speed pulse output is not required.

2) Transistor output type. Transistor output is to control the on and off of the output terminal by controlling the conduction of its internal triode, and there is no internal mechanical contact structure, so compared to the relay output contact, the transistor output contact has a long life. , The action frequency is high, and it is not easy to be damaged. The disadvantage is that the load capacity is poor.

(3) Precautions for the selection of switch output terminals

Similar to the input terminal, the transistor output terminal is also divided into two types, NPN type and PNP type. Once the model is determined, the load can only be connected according to the same wiring method.

In practical applications, it is recommended that engineers and technicians use transistor output PLCs and use relays to connect external loads at the output end to form electrical isolation for downstream load equipment. This combination combines the advantages of long transistor life and strong relay load capacity. . If there is an electrical failure on site, the PLC output terminal will be protected from damage by the isolation relay, and only the damaged relay needs to be replaced. Once the relay output type PLC terminal is damaged, the damaged terminal cannot be repaired.

Fourth, the principle of built-in first and then extended

With the continuous updating of PLCs, especially the continuous enhancement of minicomputer functions, a single PLC has built-in many expansion module functions, such as analog functions, communication functions, and so on. Therefore, the PLC with many built-in functions should be used as much as possible when selecting the model, which not only reduces the cost, but also saves the space of the control cabinet, and can simplify the setup and programming workload.

Fifth, grasp the redundancy of PLC selection

Due to the needs of early estimation, on-site construction changes, and later maintenance and upgrades, PLC selection needs to consider a certain amount of redundancy. Mainly consider the number of I/O points, the smaller project is controlled at 20% redundancy range; the larger project is controlled at 5%-10%. Other redundancy problems such as analog quantity, communication and bus functions need to be flexibly grasped by engineers and technicians according to the on-site hardware configuration. If the control functions are all built-in PLCs, the higher-level stand-alone PLC needs to be replaced; if the control functions are through expansion modules To achieve this, only the corresponding modules need to be updated when redundancy is considered.

Main points of PLC programming

(1) Allocate program segments according to the control flow chart

According to the early control flow chart, the control program is divided into different program segments, which can make the overall structure of the program clear and facilitate the later debugging of the program. If the project is more complex, it is convenient to assign the program to several programmers to program and debug at the same time after segmenting the program, which improves the programming efficiency as a whole.

(2) Compile I/O tables and memory tables

The preparation of the I/O table is to assign addresses and make notes to each input/output point to avoid confusion of I/O points during programming. To compile a memory table is to allocate PLC memory addresses to the intermediate variables of the program and make comments for easy reference during programming.

(3) Simplify programming

On the basis of being familiar with PLC instruction system, programmers are proficient in using advanced instruction programming, which can greatly reduce the programming workload, save PLC memory space, and help better exert PLC functions.

(4) Clear notes

In order to facilitate the later debugging of the program, the comments of each relevant point should be clearly marked in the program during programming, including the purpose of the special instructions used. The program has good readability and lays the foundation for later project maintenance and upgrades.

PLC program debugging method

The debugging of PLC application program can be divided into two steps: simulation debugging and online debugging.

1. Simulation debugging

Analog debugging refers to debugging based on the Display status of the LEDs corresponding to each bit on the switch I/O unit without output devices.

After the control program is designed, it is usually simulated and debugged first. Some PLC manufacturers provide simulation software that runs on a computer and can be used to replace PLC hardware to debug programs, such as the CX-Simulator simulation software provided by Omron and the CX-Programmer programming software. In the simulation, according to the requirements of the system function, some input component bits are forced to ON or OFF, or the data in some components are rewritten to monitor whether the system functions can be implemented correctly.

If the PLC hardware is connected to debug the program, the small switches and buttons connected to the input terminals can be used to simulate the actual input signals of the PLC. Point on and off, etc. Observe whether the output signal meets the design requirements through the light-emitting diodes corresponding to each output point on the switch output unit.

The main task of debugging the sequence control program is to check whether the operation of the program meets the requirements of the sequence diagram, that is, whether the correct change of the active step status occurs when a certain transition is implemented, and whether all the previous steps of the transition become inactive steps. , Whether all subsequent steps become active steps, and whether the load driven by each step changes accordingly. When debugging, various possible situations should be fully considered, and the various working methods of the system, each branch in the sequence control diagram, and various possible progress routes should be checked one by one, and no omissions should be made. After discovering the problem, modify the program in time until the relationship between the input signal and the output signal fully meets the requirements under various possible conditions. If the set value of some timers or counters in the program is too large, in order to shorten the debugging time, you can reduce them during debugging, and then write their actual set values ​​after the simulation debugging is over.

In short, simulation debugging is a very important content in the entire program design work, it can initially check the actual effect of the program. Simulation debugging and program writing are inseparable, and many functions of the program are constantly modified and gradually improved during debugging. Simulation debugging can be carried out either in the laboratory or on-site. If the simulation debugging is performed on site, the PLC system should be isolated from the on-site signal, and the external power supply of the I/O unit should be cut off to avoid unnecessary losses.

2. Online debugging

Online debugging refers to the process of installing the PLC into the control cabinet, connecting the input components and output loads, and running the control program for overall debugging.

While simulating and debugging the program, the control cabinet can be designed and manufactured, and the installation and wiring of other hardware besides PLC can also be carried out at the same time. After completing the internal wiring of the control cabinet, the wiring should be tested. You can simulate the PLC external switch input signal on the wiring terminal of the control cabinet, or operate the buttons and command switches on the control cabinet panel to observe whether the status change of the corresponding PLC input point is correct. Use a programmer or programming software to forcibly set or reset the PLC output point, and observe whether the corresponding PLC load (such as external relays, contactors, etc.) is operating normally, or whether the status change of the output signal on the corresponding control cabinet wiring terminal is correct .

For systems with analog input, standard input signals can be provided to the transmitter, and the relationship between the analog input signal and the converted digital quantity can meet the requirements by adjusting the potentiometer on the unit or the parameters in the program.

After installing the control cabinet on site and completing the wiring test inside the cabinet, connect the external input components and actuators to the PLC, put the PLC in operation mode, run the control program, and check whether the control system can meet the requirements.

During the debugging process, the possible hardware problems of the PLC system and the problems in the ladder diagram design will be exposed. After the problems are found, they will be solved on the spot until they fully meet the requirements. After all commissioning is completed, a period of trial operation is required to verify the reliability of the system.