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What is the difference between PLC control and relay contactor hardware control?
The traditional relay contactor control system is composed of three parts: input equipment (buttons, switches, etc.), control lines (connected by various types of relays, contactors, and wires to perform certain logic functions) and output equipment (contactor coils, indicator lights, etc.). This is a control system composed of physical devices. Although the ladder diagram of PLC is similar to the control circuit of relay contactor, its control components and working methods are different. The main differences are as follows. The control circuit of relay contactor with different components is composed of various hardware low-voltage electrical appliances, while the soft relays such as input relay, output relay, auxiliary relay, timer and counter in PLC ladder diagram are completed by software, not real hardware relays.
When the relay contactor control circuit works in different working modes, the hardware relays in the circuit are in a controlled state. All hardware relays that meet the conditions are in a closed state at the same time, and hardware relays that should not be closed by various constraints are in a closed state at the same time. The software relays in the PLC ladder diagram are all in the periodic cyclic scanning working state, and the action order of each soft relay limited by the same condition depends on the program scanning sequence. The number of electric shock of different hardware relays with the number of component contacts is limited. Generally, only 4-8 pairs are required. The number of contacts of software relays in PLC ladder diagram can be used indefinitely during programming, and can be normally open and normally closed.
The control circuit is implemented in different ways. The relay contactor control circuit is implemented through wiring between various hardware relays. The control function is fixed. When the control function is to be corrected, it must be rewired. PLC control circuit is implemented by software programming, which can change sensitively and correct online.
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