Relay control relies on the mechanical action of contacts to achieve control. It operates at a low frequency, on the order of milliseconds, and the mechanical contacts are prone to chattering.
PLCs control semiconductors using program instructions, resulting in high speed (microsecond level), strict synchronization, and no jitter.
Relay control systems rely on the delayed action of time relays to achieve time-delay control. However, time relays have low timing accuracy, are greatly affected by the environment, and are difficult to adjust.
PLCs use semiconductor integrated circuits as timers. The clock pulses are generated by crystal oscillators, which are highly accurate, easy to adjust, and unaffected by the environment.
PLC ladder logic programs generally employ a sequential control design method, which is highly systematic and easy to master. For complex control systems, ladder logic design takes significantly less time than designing relay system circuit diagrams.
The PLC user program can be simulated and debugged in the laboratory. Input signals are simulated using small switches, and the status of the output signals can be observed through LEDs on the PLC. After the system installation and wiring are completed, problems found during on-site commissioning can generally be resolved by modifying the program. The system's debugging time is much shorter than that of a relay system.
PLCs have a low failure rate and can quickly identify the cause of a fault by providing addresses through LEDs or programmers. Faults can be quickly eliminated by replacing modules.
