I. Logical Stage
The so-called logic stage refers to the implementation of general logical designs in a relay system. Since it's a relay system, knowledge of electric drives is fundamental to this stage. My personal summary of learning relay systems boils down to one word: "grasp." The reason relay systems can achieve logical control lies in this word. A relay system mainly consists of three things: A) normally open; B) normally closed; and C) coils. These correspond to the basic elements in a PLC , only the reading methods are different.
So, can we simply copy the original relay system? No! Their operating methods are different. In a relay system, all hard elements compete simultaneously, while in a PLC , all soft elements are processed by the PLC's CPU through scanning calculations to arrive at the result for that specific time. This is the PLC's scan-cycle operating mode. (Any PLC book will explain this.)
Key point: This stage involves learning about electric drives, which correspond to normally open, normally closed, and coils in a PLC ladder diagram.
Simple system design can be completed.
II. Sequential Control Phase
Sequential control has a wide range of applications in industry; for example, a typical automatic machine tool is a sequential control process. There are two methods to achieve sequential control in PLC design: 1) sequential control instructions in the PLC , such as Mitsubishi STL; 2) start-stop control. Regardless of the control method, at the beginning of the design, we need to define the flow, which is the framework of the system and mainly consists of three aspects: 1) "steps," 2) "active steps," and 3) "transition conditions."
Key points: 1. Master the system framework and design the system process.
2. Master the "start-stop" control method and convert the flowchart into a ladder diagram.
It can complete general system design.
III. Assembly Stage
This stage is fundamentally different from relay control systems; it's something relay control systems cannot achieve, and it's the foundation for improving the functionality of PLC control systems! I call it the assembly stage because it's very similar to assembly language programming for microcontrollers. For example, the MOV instruction in a microcontroller has the same function in high-level instructions in a PLC. This stage is quite difficult. First, you need to learn computer fundamentals; second, you need to fully understand the internal functions and resources of the PLC; and third, you need to be familiar with the functions of all high-level instructions (without rote memorization). If you don't understand computer fundamentals, you simply won't be able to understand the high-level instructions and the internal resources of the PLC. The design thinking is very different from that of relay systems. For example, I0.0 and IB0: the first is a "bit," which is a "point" in logic design, and the second is a "byte," which is not involved in logic design.
Key points: 1. Computer basics
2. PLC Resources
3. Command Functions
4. Adapt to microcontroller programming thinking
Capable of performing complex system designs
IV. Special Phase
The special stage refers to systems with specific functions, such as motion control, PID temperature control, and network connectivity. Different PLCs can perform different functions; some functions are integrated within the PLC, while others require external expansion. Therefore, the appropriate PLC must be selected based on the specific controlled object. Mastering this stage can greatly improve PLC programming skills, but it also requires knowledge of other automation technologies besides PLCs, such as servo drives and frequency converters.
Key points: 1. Understanding the system's structural requirements
2. Select appropriate expansion units
3. Learn how to use the extended units.
Capable of performing special system designs
This stage of learning requires certain practical conditions to complete.
V. Network Phase
With the development of automation technology, the application of PLCs as lower-level controllers is becoming increasingly common. This stage involves learning the communication protocols and instructions of different PLCs, such as how a PLC controls a frequency converter through its programming port, using Siemens' USS protocol to exchange information with the frequency converter. Other technologies include industrial Ethernet and fieldbuses, such as Siemens' PROFIBUS and AS-i.
MODBUS is important in small-scale networks, for example, by establishing a MODBUS protocol between a PLC and a frequency converter to control the frequency converter.
In the network, some products sometimes use non-standard communication protocols, which is where free communication comes in. It is quite flexible, but requires a certain level of computer network knowledge.
