Compared with relay control system, PLC control system has many similarities and differences. The differences are mainly in the following aspects:
(1) From the perspective of control method, the control logic of the electrical control system adopts hardware wiring, and the control logic is composed of series or well connection of relay mechanical contacts. Its wiring is complex, bulky, and high power consumption. After the system is constituted, it is difficult to change or add functions. In addition, the number of relay contacts is limited, so the flexibility and scalability of the electrical control system are greatly limited. PLC adopts computer technology, and its control logic is stored in memory in the form of program. To change the control logic, only the program needs to be changed, so it is easy to change or add system functions. The system has few connections, small volume and low power consumption, and the "soft relay" of PLC is essentially the state of the memory unit, so the number of contacts of the "soft relay" is the most unlimited, and the flexibility and scalability of the PLC system are good.
(2) In terms of working mode, it is in the relay control circuit When the power supply is turned on, all relays in the circuit are in a restricted state, that is, the pulled in relays are pulled in at the same time The relay that should not be pulled in cannot be pulled in due to certain conditions. This working mode is called parallel working mode. The user program of PLC is executed cyclically in a certain order, so each soft relay is in the process of periodic cyclic scanning, and the action sequence of each relay subject to the same condition is determined by the program scanning sequence, which is called serial working mode.
(3) In terms of control speed, the relay control system relies on the action of mechanical contacts to achieve control, and the working rate is low Mechanical contacts also exhibit jitter problems. The PLC controls the semiconductor circuit through the program command, which is fast. The program command execution time is microseconds, and there is no contact jitter problem.
(4) From the perspective of timing and counting control, the electrical control system uses the time delay action of the time relay for time control. The time delay of the time relay is easily affected by the change of ambient noise and voltage, and the timing accuracy is not high. PLC uses semiconductor integrated circuit as timer, and clock pulse is generated by crystal oscillator, with high precision and wide timing range. The user can set the timing value in the program as required, which is easy to modify and is not affected by the environment, and the PLC has the counting function, while the electrical control system generally does not have the counting function.
(5) From the perspective of reliability and maintainability, the electrical control system uses large basin mechanical contacts, which have mechanical wear, arc burns, short life, and many system connections, so the reliability and maintainability are poor. While a large number of switch actions of PIZ are completed by contactless semiconductor circuit, which has a long life and high reliability. PLC also has a self diagnosis function, which can find out its own fault, display it to the operator at any time, and dynamically monitor the implementation of the control program, providing convenience for on-site debugging and maintenance.