Design principles of PLC control system

The essential difference between a programmable controller and a traditional relay control system is the use of the scanning mode and the concept of a relay. PLC system design includes two parts: hardware design and software design. The design method of parallel development of hardware and software can be used in the design, which can speed up the development of the entire system. The content and principles of system design are as follows:

  1. The hardware design includes selection of PLC model, selection of input / output equipment, and drawing of various patterns (such as wiring diagrams, etc.). Principles for hardware design:

  (1) Economical. Under the premise of ensuring the control function, the selected components and equipment should be fully considered for their cost-effectiveness to reduce the design, use and maintenance costs.

  (2) Reliability. The failure rate of the control equipment during operation should be the lowest.

  (3) Advancedness and scalability. On the premise of satisfying the first two conditions, the system should be advanced within a certain period of time, and there should be room for expansion of functions according to the requirements of the production process, so as not to redesign the entire system.

  2. Software design software is to write a PLC user program that meets the requirements of production control, that is, draw a ladder diagram or write a statement table. Software design principles:

  (1) The logical relationship should be simple and clear, and easy to program. For example, the contacts of the relay can be used countless times. As long as it is required to implement a certain logic function, it can be used at any time to make the program readable, but avoid using unnecessary contacts.

  (2) When programming, on the premise of ensuring the function of the program, reduce instructions as much as possible and use various techniques to reduce the running time of the program.