Control speed: The relay control logic is controlled by the mechanical action of the contact, the working frequency is low, the millisecond level, and the mechanical contact has jitter. PLC is controlled by program instructions to control the semiconductor circuit, fast, microsecond level, strict synchronization, no jitter.
Delay control: The relay control system realizes the delay control by the hysteresis action of the time relay, while the timing relay precision is not high, which is greatly affected by the environment and the adjustment time is difficult. The PLC uses a semiconductor integrated circuit as a timer, and the clock pulse is generated by a crystal oscillator with high precision, convenient adjustment time, and no environmental influence.
In general, the relay control system is designed for a certain production machinery and fixed production process. It is assembled by hard wiring and can only perform the functions of logic control, timing and counting. Once the production process changes, the control cabinet must be redesigned and rewired. It is an inevitable trend that the traditional relay control system has been replaced by PLC. Since PLC uses microelectronics technology and computer technology, various control functions are realized by software. As long as the program is changed and a small number of terminals are changed, it can adapt to the change of production process. From the aspects of adaptability, reliability and installation and maintenance, PLC has significant advantages. Therefore, the PLC control system will replace most traditional relay control systems.