Step 1: Analyze the controlled object and propose control requirements
System planning is the first step in design, which includes two parts: determining the control scheme and overall design. When determining the control system scheme, the actions and functions that the controlled object needs to achieve should be clearly defined first. Then, the technological process and working characteristics of the controlled object should be analyzed in detail, the coordination between the mechanical, electrical and hydraulic systems of the controlled object should be understood, the control requirements of the controlled object for the PLC control system should be proposed, and the control scheme should be determined.
Step 2: Identify input/output devices and allocate I/O points
Based on the system's control requirements, determine all the necessary input devices (such as buttons, position switches, selector switches, and various sensors) and output devices.
(e.g., contactors, solenoid valves, indicator lights, and other actuators), thereby determining the input/output devices related to the PLC, determining the number of I/O points of the PLC, allocating I/O, and drawing a connection diagram or correspondence table between the PLC's I/O points and input/output devices.
Step 3: Hardware Configuration
Determine the configuration requirements based on the requirements, select the PLC model and specifications, determine the number and specifications of I/O modules, and determine whether to select special function modules, human-machine interface, servo, frequency converter, etc.
Step 4: Design the PLC peripheral hardware circuitry
Based on the overall plan, complete the electrical control schematic diagram and draw the electrical circuit diagrams for other parts of the system, including the main circuit and control circuits not connected to the programmable controller. The PLC I/O connection diagram and the PLC peripheral electrical circuit diagram constitute the system's electrical schematic diagram.
Step 5: Program Design
1. Programming
The program design should be based on the determined overall scheme and the completed electrical schematic diagram. Using the allocated I/O addresses, the PLC user program should be written to implement the control requirements and functions. It is important to use appropriate design methods to design the PLC program. The program should focus on meeting the system control requirements, writing the program for each control function or subtask one by one, gradually improving the system's specified functions. In addition, the program should typically include the following:
1) Initialization Program. After the PLC is powered on, some initialization operations are generally performed to prepare for startup and prevent system malfunctions. The main contents of the initialization program include: clearing certain data areas and counters, restoring required data in certain data areas, setting or resetting certain relays, and displaying certain initial states, etc.
2) Detection, fault diagnosis, and display programs. These programs are relatively independent and are generally added after the basic program design is completed.
3) Protection and interlocking procedures. Protection and interlocking are indispensable parts of the procedure and must be carefully considered. They can prevent control logic chaos caused by illegal operations.
PLCs are primarily used in automation control technology. This section will introduce the general methods for constructing a programmable controller automatic control system by comprehensively applying the knowledge points learned previously and based on the specific engineering requirements of the effective capacity automatic control system.
Main process of PLC programmable controller control system design
1. Specific content of system development
(1) Formulate process standards for control system design. Technical standards are generally defined by the type of design specification, and they serve as the basis for all designs;
(2) Select the process control system type and actuators such as motors and vacuum solenoid valves;
(3) Select the PLC model and specifications;
(4) Write the PLC's input/export dispatch table or create the input/export terminal wiring method;
(5) Edit the software specification according to the regulations of the control system, and then carry out programming design using the corresponding computer language (commonly PLC ladder diagram);
(6) Understand and follow customer personality psychology, attach great importance to the design of industrial touch screens, and improve the friendly relationship between people and equipment;
(7) Design of workbench, electrical control cabinet and non-standard household appliance components;
(8) Write design specifications and user manuals;
Depending on the specific content, the above details may be adjusted accordingly.
2. Main process of system development
The first step in the design and adjustment of a PLC programmable controller software system is...
(1) Thoroughly understand and analyze the technical standards and control regulations of the target under test.
a. The target being measured is controllable mechanical equipment, electrical equipment, production lines, or production processes.
b. The key operating procedures refer to the main methods of operation, the actions to be performed, the repetitive structure of fully automatic operation, necessary maintenance, and interlocks. For more complex automatic control systems, the operating tasks can be divided into several individual parts, which simplifies the process and facilitates programming and adjustment.
(2) Define the I/O equipment
Based on the basic functional requirements of the PLC control system software for the target being tested, the user input and external devices required by the system software are clearly defined. Common output devices include buttons, toggle switches, limit switches, and sensors, while common external devices include solenoid valves, AC contactors, indicator lights, and vacuum solenoid valves.
(3) Select the appropriate type of PLC
Based on the confirmed customer I/O equipment, statistical analysis is conducted on the required input and output information levels to select the appropriate PLC type, including model selection, capacity selection, I/O control module selection, and power supply module selection.
(4) Assign I/O points
Assign PLC output points, write input/output assignment tables, or draw the wiring methods for input/output terminals. Then, PLC programming design can be carried out, and at the same time, the design of control boxes or workbenches and on-site construction can be carried out.
(5) Design scheme software system PLC ladder diagram program flow
Based on the data charts or process flow tables used in the work, the PLC ladder diagram, i.e., the program, is created. This step is the most crucial and also the most difficult part of the entire software system design. To design a good PLC ladder diagram, one must first have a thorough understanding of the operating requirements, and at the same time, have some experience in power distribution design.
(6) Input the program into the PLC
When inputting a program into a PLC using a simple microcontroller programmer, the PLC ladder diagram must first be converted into command mnemonics for easy input. When using PLC-assisted CNC programming software to write programs on a computer, the system software can be downloaded to the PLC via the connection cable between the upper and lower computers.
(7) Conduct software performance testing
After the program flow is input into the PLC, testing should be carried out first. Since oversights are inevitable during the programming design process, software performance testing is essential before connecting the PLC to the field equipment. This eliminates errors in the program flow and lays a solid foundation for overall adjustment, reducing the overall calibration cycle time.
(8) Overall adjustment of software system
After the PLC hardware and software design, control box, and on-site operation are completed, the entire system can be networked and adjusted. If the automatic control system consists of multiple parts, partial adjustments should be made first, followed by overall adjustments. If the management program has many steps, segment adjustments can be made first, followed by interconnection and overall adjustments. Problems discovered during adjustment should be eliminated one by one until the adjustment is successful.
1. What are the characteristics of a PLC control system?
PLC control systems are characterized by strong computing power, high control precision, complete functions, strong programmability, wide control range, high reliability, and convenient maintenance, and can meet various types of industrial control requirements. They can achieve more refined control and perform complex control functions, thus satisfying different types of industrial control needs.
2. What are the application areas of PLC control systems?
PLC control systems are mainly used in industrial production automation, such as automation control in industries like machining, electronics manufacturing, chemicals, metallurgy, petroleum, and textiles. They can also be used for control in industries like metallurgy, gas, water treatment, food processing, beverage production, tobacco processing, paper, ceramics, construction, agriculture, ports and docks, public transportation, and power plants.
3. What are the maintenance requirements for a PLC control system?
The maintenance of a PLC control system mainly includes the following points: 1. Regularly check the hardware for any abnormalities; 2. Regularly check the software for any program abnormalities; 3. Regularly update the software program to ensure the reliability of the control system; 4. Regularly check the operating status of the control system to ensure its reliability; 5. Regularly maintain the equipment in the control system to ensure its normal operation.
4. What is the operating principle of a PLC control system?
The operating principle of a PLC control system is as follows: The processor in the PLC control system converts the signals received from the input devices into digital signals, then processes the signals using programs and arithmetic modules, and finally converts the processed signals into control signals through the output devices, thereby controlling the operation of the machine equipment.
5. What are the advantages of a PLC control system?
The advantages of PLC control systems are: 1. Powerful functions, capable of realizing many complex control functions; 2. High control precision, enabling more refined control; 3. Simple operation, enabling unattended operation; 4. Low cost, effectively reducing production costs; 5. High flexibility, allowing adjustment of control programs according to actual needs.
