PLC control systems share many similarities with electrical control systems, but also have many differences. The main differences are as follows:
1) From a control method perspective, electrical control systems employ hardware wiring for control logic, utilizing series or parallel connections of relay mechanical contacts to form the control logic. This results in numerous and complex wiring connections, large size, and high power consumption. Once the system is configured, modifying or adding functions is difficult. Furthermore, the limited number of relay contacts significantly restricts the flexibility and scalability of electrical control systems. In contrast, PLCs utilize computer technology, storing their control logic as a program in memory. Changing the control logic only requires modifying the program, making it easy to modify or add system functions. The system has fewer wiring connections, smaller size, and lower power consumption. Moreover, the so-called "soft relays" in PLCs are essentially the states of memory units, so the number of contacts for "soft relays" is virtually unlimited. Therefore, PLC systems offer superior flexibility and scalability.
2) From the perspective of operation, in a relay control circuit, when the power is turned on, all relays in the circuit are in a constrained state. That is, relays that should be activated are activated simultaneously, while relays that should not be activated are restricted by certain conditions and cannot be activated. This operation mode is called parallel operation mode. However, the PLC user program is executed cyclically in a certain order, so each soft relay is in a periodic cyclic scanning and activation process. The action order of relays constrained by the same condition is determined by the program scanning order. This operation mode is called serial operation mode.
3) In terms of control speed, relay control systems rely on the movement of mechanical contacts to achieve control, resulting in low operating frequencies and the possibility of contact bounce. PLCs, on the other hand, control semiconductor circuits through program instructions, achieving high speed with program instruction execution times in the microsecond range, and eliminating contact bounce issues.
4) From the perspective of timing and counting control, electrical control systems use the delay action of time relays for time control. The delay time of time relays is easily affected by ambient temperature and temperature changes, resulting in low timing accuracy. In contrast, PLCs use semiconductor integrated circuits as timers, with clock pulses generated by crystal oscillators. This provides high accuracy, a wide timing range, and allows users to set timing values in the program as needed. Modification is convenient, and it is unaffected by the environment. Furthermore, PLCs have counting functions, while electrical control systems generally do not.
5) From the perspective of reliability and maintainability, electrical control systems use a large number of mechanical contacts, which are susceptible to mechanical wear, arc burns, and have short lifespans. The systems also have numerous wiring connections, resulting in poor reliability and maintainability. In contrast, PLCs utilize contactless semiconductor circuits for most switching actions, offering longer lifespans and higher reliability. Furthermore, PLCs have self-diagnostic capabilities, enabling them to identify their own faults and display them to operators in real time. They can also dynamically monitor the execution of the control program, facilitating on-site debugging and maintenance.
