Software design is the core of PLC control system design. To design effective PLC application software, a thorough understanding of the controlled object's production process, technical characteristics, and control requirements is essential. The PLC application software then performs the system's various control functions.
1. Contents of PLC application software design
PLC application software design refers to the process of creating user control programs and corresponding documentation using appropriate programming languages, based on the hardware structure and technological requirements of the control system. The main contents include: determining the program structure; defining parameter tables such as input/output, intermediate flags, timers, counters, and data areas; programming; and writing program specifications. PLC application software design also includes configuring human-machine interface (HMI) devices such as text displays or touchscreens, as well as other special function modules.
2. Familiarize yourself with the controlled object and formulate equipment operation plans.
Based on the system hardware design, and according to the requirements of the production process, the logical relationships between various inputs/outputs and operations are analyzed to determine the detection quantities and control methods. The operation content and sequence of each device in the system are then designed. For more complex systems, the system can be divided into zones for control based on physical location or control function. More complex systems generally also require a system control flowchart to clearly illustrate the sequence and conditions of actions; this is generally not necessary for simple systems.
3. Familiar with programming languages and programming software
The best way to become familiar with a programming language is to practice on a computer and write some experimental programs to run on a simulation platform. This will allow you to understand the functions and uses of the instructions in detail, laying a good foundation for subsequent program design and avoiding detours.
4. Define parameter table
Before programming begins, an input/output signal table must first be defined. This is primarily based on the PLC input/output electrical schematic. Each PLC has its own specific rules for input and output point numbering. After determining the PLC model and configuration, the input/output signals must be assigned PLC input/output numbers (addresses) and compiled into a table.
Generally, input/output signal tables should clearly indicate the location of the template, input/output address numbers, signal names, and signal types. In particular, the comments in the input/output definition tables should be as detailed as possible. Addresses should be arranged in ascending order, and undefined or spare points should not be omitted. This facilitates finding and using these points during programming, debugging, and program modification.
Intermediate flags, timers, counters, and data areas may not have been easy to define in the past. They are usually defined as they are used during the programming process and then organized together with the input/output signal table during or after the program is written.
5. Programming
If the operating system supports it, try to use a high-level programming language, such as ladder logic. During the programming process, define the intermediate flag signal table and memory unit table one by one according to actual needs, and be sure to reserve enough common temporary storage area to save memory usage.
Many small PLCs use simple programmers that can only input instruction codes. After designing the ladder diagram, it is necessary to write a program based on the instruction statements and list the program. After becoming familiar with the selected PLC instruction system, it is easy to write the statement list program based on the ladder diagram.
6. Program Testing
During testing, begin with each functional unit, setting input signals and observing the effect of changes in these signals on the system. Instruments can be used if necessary. After testing each functional unit, connect the entire program and test the interfaces of each part until satisfactory results are achieved.
Program testing can be performed in a laboratory or on-site. If on-site testing is performed, the PLC must be isolated from the field signals to prevent accidents.
7. Writing the program specification
A program specification typically includes the basis for program design, the basic structure of the program, analysis of each functional unit, the formulas and principles used, the source and calculation process of each parameter, and the program's testing status.
Each step in the above process is an indispensable part of application design. To design a good application, each step must be done well. However, the core of application design is program writing; all other steps serve it. (Forwarding code: Yiwei focuses on core control)
8. Commonly Used Programming Methods
PLC programming methods mainly include empirical design and logical design. Logical design is based on Boolean algebra, involving writing logical expressions for inputs and outputs and then converting them into ladder diagrams. Because logical design is generally complex and time-consuming, empirical design is often used. If the control system is complex, flowcharts can be used. Empirical design, on the other hand, is based on typical applications. It involves selecting basic components, appropriately combining, modifying, and refining them according to the specific requirements of the controlled object, to create a program that meets the control requirements. There are generally no set rules for empirical design; it requires continuous accumulation and refinement through numerous program designs, gradually forming one's own design style. The quality of a program design and the time taken are often closely related to the programmer's experience.
