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 analysis of the previous project is not in place, it will result in inaccurate hardware selection and delay in the project.
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 estimate the parameters required for PLC selection. The 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 help in 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 materials 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 electrical 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.
Engineering and technical personnel shall arrange different control requirements in order from special to general according to the core requirements of the project. Such selection will be more effective and reduce the overall difficulty of the project.
2, the principle of bottom-up
The purpose of the bottom-up principle is to maximize the cost-effectiveness of PLC selection. At present, most manufacturers' PLC products are divided into multiple series. When selecting the engineering and technical personnel, follow the first step from the special to the general selection sequence, starting from the lowest PLC and comparing the performance parameters one by one. Consider a higher product when it is found that the requirements are not met. And so on, until all the PLC models that meet the requirements are selected. If you choose from top to bottom, it will waste the PLC function, resulting in a big horse-drawn car.
3. Selection of PLC switch input/output unit
The PLC's digital input point is used to accept the level signal input by the field sensor. The output point of the switch acts to drive the external load according to the internal control signal.
(1) Selection of the digital input terminal. Now the PLC input points on the market are transistor inputs, and the user only needs to select according to the number of input points estimated in the previous period. However, it should be noted here that there are two input modes, NPN and PNP, depending on the type of PLC terminal wiring. The meaning is whether the input terminal is active at low level or high level. Once the input terminal type is determined, it needs to be selected. Sensors of the same type input, ie NPN and PNP type sensors, cannot share the input terminals of one PLC.
Now the PLC input terminal on the market is mostly DC 24V input voltage. If you need to connect his voltage specification sensor to the PLC, you need to use the relay to do the corresponding isolation to ensure that the signal connected to the PLC input is DC 24V.
(2) Selection of the switching output terminal. The types of PLC switching output points are mainly relay type output and transistor output type.
1) Relay output type. The relay has good output load capacity, can withstand high overvoltage and overcurrent for a short time, and has strong isolation. However, since the inside of the relay is a mechanical contact and has a limited operating life, it can only be used in applications where the connection frequency is low and high-speed pulse output is not required.
2) Transistor output type. The transistor type output is achieved by controlling the conduction of its internal transistor to achieve the purpose of controlling the output terminal to be turned on and off, and there is no mechanical contact structure inside, so the transistor output contact has a long life compared to the relay output contact. 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 the switch output terminal
1 Similar to the input terminal, the transistor output terminals are also divided into NPN type and PNP type. Once the model is determined, the load can only be connected in the same way.
2 In practical applications, it is recommended that engineers and technicians choose transistor output type PLCs and use relays to connect external loads at the output to form electrical isolation for downstream load devices. This combination combines long transistor life and relay load capacity. advantage. If an electrical fault occurs 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.
4, first built-in post-expansion principle
With the continuous upgrading of PLCs, especially the continuous enhancement of the functions of minicomputers, PLC single machines have built in many functions of expansion modules, such as analog functions and communication functions. Therefore, when selecting the model, the PLC with built-in functions should be used as much as possible, which not only reduces the cost, but also saves the space of the control cabinet, and simplifies the setting and programming workload.
5, the grasp of PLC selection redundancy
Due to the preliminary estimation, on-site construction changes and the need for post-maintenance upgrades, PLC selection needs to consider a certain amount of redundancy. The main consideration is the number of I/O points, the smaller engineering control is in the 20% redundancy range; the larger engineering control is in the 5% to 10%. Other redundancy problems such as analog, communication and bus functions require engineers and technicians to flexibly grasp the hardware configuration according to the field. If the control functions are built into the PLC, they need to replace the high-level single-machine PLC; if the control function is through the expansion module If it is implemented, only the corresponding module needs to be updated when considering redundancy.
Third, PLC programming points
(1) Allocating blocks according to the control flow chart
According to the previous control flow chart, the control program is decomposed into different program segments, which can make the overall structure of the program clear and facilitate the later debugging process. If the project is more complicated, the program is segmented and easily distributed to several programmers for simultaneous programming and debugging, which improves the programming efficiency as a whole.
(2) Prepare I/O tables and memory tables
Programming an I/O table is to assign an address to each input/output point and make comments to avoid I/O point confusion during programming. The memory table is compiled by assigning the PLC memory address to the intermediate variables of the program and making comments for easy reference during programming.
(3) simplify programming
Based on the familiarity with the PLC instruction system, the programmer is proficient in using advanced instruction programming, which can greatly reduce the programming workload, save the PLC memory space, and help to better play the PLC function.
(4) Clear comments
In order to facilitate the debugging of the program, the annotation of each relevant point should be clearly marked in the program during programming, including the purpose of the special instruction used. The program is readable and lays the foundation for later project maintenance and upgrade.
Fourth, PLC program debugging method
The debugging work of the PLC application can be divided into two steps: analog debugging and online debugging.
1, analog debugging
The analog debugging refers to the debugging according to the display state of the corresponding LEDs on the switch I/O unit without the output device.
After designing the control program, it is generally used for analog debugging. Some PLC manufacturers provide simulation software that can be used to replace the PLC hardware to debug the program, such as the CX-Simulator simulation software that Omron and 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 is rewritten, and whether the function of the monitoring system can be correctly implemented.
If you connect the PLC hardware to debug the program, you can use the small switches and buttons connected to the input terminals to simulate the actual input signals of the PLC, such as using them to issue operational commands, or use them to simulate actual feedback signals, such as travel switch contacts. Turn the point on and off, etc. Through the LEDs corresponding to the output points on the switching output unit, observe whether the output signal meets the design requirements.
The main task of the debug sequence control program is to check whether the running of the program conforms to the specification of the sequence diagram, that is, whether a correct change of the active step state occurs when a certain conversion is implemented, and whether all the previous steps of the conversion become inactive steps Whether all subsequent steps become active steps and whether the load driven by each step changes accordingly.
All kinds of possible situations should be fully considered during commissioning. Each different working mode of the system, each branch in the sequence diagram, and various possible progress routes should be checked one by one and cannot be omitted. After the problem is discovered, the program is modified in time until the relationship between the input signal and the output signal is fully met under various possible conditions. If the settings of some timers or counters in the program are too large, in order to shorten the debugging time, you can reduce them during debugging, and write the actual set values ​​after the simulation is completed.
In short, analog debugging is an important part of the overall programming work, it can initially check the actual effect of the program. Analog debugging and programming are inseparable, and many of the functions of the program are constantly modified and gradually improved during debugging. Simulation commissioning can be done either in the laboratory or on site. If the simulation 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 loss.
2, online debugging
Online debugging refers to the process of installing the PLC into the control cabinet, connecting the input components and the output load, and running the control program for overall debugging.
While the program is being simulated and debugged, the control cabinet can be designed and manufactured. The installation and wiring work of other hardware besides the PLC can also be performed simultaneously. After completing the internal wiring of the control cabinet, the wiring should be tested. The switch input signal outside the PLC can be simulated on the terminal block of the control cabinet, or the button and command switch on the panel of the control cabinet can be operated to observe whether the state change of the corresponding PLC input point is correct. Use the programmer or programming software to forcibly set or reset the output point of the PLC, observe whether the load of the corresponding PLC (such as external relay, contactor, etc.) is normal, or whether the status of the output signal on the terminal of the control cabinet is correct. .
For systems with analog inputs, the transmitter can be supplied with a standard input signal. By adjusting the potentiometer on the unit or the parameters in the program, the relationship between the analog input signal and the converted digital quantity is met.
After the control cabinet is installed on site and the wiring test in the cabinet is completed, the external input components and actuators are connected to the PLC, the PLC is placed in the operation mode, and the control program is run to check whether the control system can meet the requirements.
During the debugging process, the hardware problems that may exist in the PLC system and the problems in the ladder diagram design will be exposed. After the problem is found, it will be solved on the spot until it meets the requirements. After all the commissioning is completed, it will take a period of trial operation to verify the reliability of the system.
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