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Comparison of PLC control system and relay control system
2023-12-08
1.from the control method, relay control system control logic using hardware wiring, the use of relays mechanical contacts in series or parallel and other combinations into the control logic, its connection is more and more complex, large volume, power consumption, the system is composed, want to change or increase the function is more difficult. In addition, the number of contacts of relays is limited, so the flexibility and scalability of the electrical control system is greatly restricted. PLC uses computer technology, its control logic is stored in the memory in the form of a program, to change the control logic only to change the program, so it is easy to change or increase the system functions. System connection less, small size, power consumption, and PLC so-called "soft relay" is essentially the state of the memory unit, so "soft relay" the number of contacts is unlimited, the flexibility and scalability of the PLC system.
2. from the working mode, in the relay control circuit, when the power is on, all the relays in the circuit are in a constrained state, that is, the suction of the relays are simultaneously suction, should not be suction of the relays subject to certain conditions and can not be suction, this mode of work is called parallel mode of work. PLC user program is a certain order of cyclic implementation, so the soft relays are in a cyclic cycle of scanning on, subject to the same conditions of the action sequence of the various relays determined by the scanning sequence of the program, this mode of operation is called serial mode of work.
3.from the control speed, the relay control system relies on the action of the mechanical contacts to achieve control, low operating frequency, mechanical contacts will also have jitter problems. PLC control semiconductor circuit through the program instructions to achieve control, fast, program instruction execution time in microseconds, and will not have contact jitter problems.
4.from the timing and counting control, the relay control system uses time relays for time control of the delay action, time relays are vulnerable to ambient temperature and temperature changes in the delay time, timing accuracy is not high. The PLC uses semiconductor integrated circuits for timer, clock pulse generated by the crystal oscillator, high precision, wide timing range, the user can set the timing value in the program as needed, easy to modify, not affected by the environment, and PLC has a counting function, while the electrical control system generally does not have a counting function.
5. from the reliability and maintainability, because the relay control system uses a large number of mechanical contacts, its existence of mechanical wear, arc burns, etc., short life, the system's connection more, so the reliability and maintainability is poor. And PLC a large number of switching action by the semiconductor circuit without contacts to complete, its long life, high reliability, PLC also has a self-diagnostic function, can find out their own fault, at any time to show the operator, and can dynamically monitor the implementation of the control program, for on-site debugging and maintenance provides a convenient.