The design of PLC in electrical equipment consists of three parts:
First, it involves the process design of the control system;
Second, the design of the I/O address;
Third, is the design of the control system.
PLC control system process design
The entire design process begins by clarifying the control objectives that the PLC needs to achieve in the electrical equipment. Then, based on this, the control range of the PLC in the electrical equipment is determined. The corresponding host is determined according to the selected control system circuit. Finally, the corresponding matching modules are selected based on the selected host.
PLCI/O interface address design
I/O interface address design is a crucial part of PLC system design. From a software perspective, only after determining the I/O interface addresses can related software programming begin. Furthermore, for hardware and PLC peripherals, only after determining the I/O interface addresses can tasks such as drawing, wiring, and assembly be performed. To facilitate viewing and processing, the various technical specifications and codes involved in determining input/output addresses are typically displayed clearly and concisely, such as using an Excel spreadsheet.
Design of PLC Control System
The complete design of a PLC control system is divided into two main parts: software system design and hardware system design. Software system design generally refers to writing the PLC control system program, which can be divided into main programs, subroutines, and interrupt programs. It is used to control the hardware and software of electrical equipment. Hardware system design generally includes the design of anti-interference measures, the selection of electrical equipment control components, and the design of the electrical equipment control system.
(1) Software design of PLC control system
There is no fixed method for designing PLC control system software; there are no shortcuts to writing an optimized PLC program. It relies entirely on the programmer's skill and experience. Therefore, the programmer's individual ability and experience are crucial to completing this task successfully. Of course, there are basic design methods for PLC software, including flowcharts, Boolean algebra, and function charts. PLC program design generally involves five steps: First, determine the start-up and shutdown conditions of the control system; second, determine if there are start-up or shutdown constraints on the output objects in the control program; third, program the output objects according to standard equations. For no constraints, use equation [equation missing]; for constraints, use equation [equation missing]; then substitute the known conditions to design the ladder diagram of the program; finally, check and modify the written program. Another point to note is that ladder diagrams are generally the best programming method for control system design because they are more intuitive and visual than statement programming.
(2) Hardware design of PLC control system
In the hardware design of a PLC control system for electrical equipment, the most crucial element is the design of the control system itself, which controls the operation of the entire electrical equipment. The quality of the control system design significantly impacts the normal operation of the equipment. Anti-interference measures in electrical equipment are primarily designed to improve the stability of the control system's hardware and software, as well as its adaptability to external environmental influences. Typical PLC anti-interference design includes three parts: power supply anti-interference design, primarily to control interference from the power grid; input/output anti-interference design, primarily to control current interference; and external wiring anti-interference design, primarily to prevent interference between external wiring components. Component selection also plays a vital role in hardware design; inappropriate component selection can significantly impact the overall hardware design of the control system.
PLC control relies heavily on the design of electrical circuit diagrams. Proper use of electrical graphic symbols and text symbols requires cultivating the habit of drawing according to national standards. Older designers, accustomed to older symbols, often mix old and new ones when using the new national standards. Those who graduated in the 1990s, however, only need to master the new standards.
In the mid-1980s, the former State Bureau of Standards issued its first batch of national standards for electrical drawing and electrical graphic symbols, including 13 items in GB4728 "Graphical Symbols for Electrical Drawings", 7 items in GB6988 "Electrical Drawings", 2 items in GB5465 "Graphical Symbols for Electrical Equipment", and GB5094 "Item Codes in Electrical Technology", etc. These standards were compiled based on the corresponding IEC standards and related documents. In 1987, the former State Bureau of Standards issued Document No. 079, "Notice on the Comprehensive Implementation of National Standards for Electrical Drawing and Graphic Symbols in the National Electrical Field", stipulating that "from January 1, 1990, all electrical technical documents and drawings shall use the new national standards, and the old national standards shall no longer be used."
In PLC electrical control, the number of electrical symbols used is relatively small. Commonly used electrical symbols such as circuit breakers, contactors, relays, normally open and normally closed pushbuttons, solenoid valves, three-phase and single-phase AC motors, DC motors, servo motors, transformers, reactors, instrument transformers, indicator lights, and buzzers are very common. See the table below:
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