and Lisi PLC - the difference between PLC control system and relay control system

(1) From the point of view of control methods

The control logic of the electrical control system adopts hardware wiring, and the series connection or well connection of the electrical mechanical contact is combined into the control logic, and the connection is complex, large in size, and in large power consumption, and it is difficult to change or increase the function after the system is formed. In addition, the number of contacts for relays is limited, so the flexibility and scalability of the appliance control system is very limited. And PLC adopts computer technology, and its control logic is stored in the memory in the way of program, and only need to change the program to change the control logic, so it is easy to change or increase the system function. The system has few connections, small size, low power consumption, and the "soft relay" of the 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) From the perspective of working methods

in relay control circuits. When the power is turned on, all the relays in the circuit are in a constrained state, that is, the relays that should be engaged are engaged at the same time. Relays that should not be engaged are limited by certain conditions and cannot be engaged, and this mode of operation is called parallel working mode. The user program of PLC is executed in a certain order, so each soft relay is in the periodic cyclic scanning switch-on, and the action sequence of each relay restricted by the same condition is determined by the program scanning sequence, and this working mode is called serial working mode.

(3) From the point of view of control speed

The relay control system relies on the action of the mechanical contacts to achieve control, and the working child rate is low. Mechanical contacts can also have jitter issues. The PLC controls the semiconductor circuit through the program instruction to achieve control, which is fast, the program instruction execution time is at the microsecond level, and there will be no contact jitter problem.

(4) From the timing and counting control, the electrical control system adopts the delay action of the time relay to control the time division, and the delay time of the time relay is susceptible to the shape of the environmental silence and tear change, and the timing accuracy is not high. The PLC uses semiconductor integrated circuit as the timer, and the clock pulse is generated by the crystal oscillator, which has high precision and wide timing range. The user can set the timing value in the program according to the need, which is convenient to modify, and is not affected by the environmental sound, and the PLC has the function of counting, while the electrical control system generally does not have the counting function.

(5) From the perspective of reliability and maintainability

Because the electrical control system uses the mechanical contacts of the large basin, it has mechanical wear, arc burns, etc., the life is short, and the system has many connections, so the reliability and maintainability are poor. A large number of switching actions of PIZ are completed by non-contact semiconductor circuits, and its long life and high reliability PLC also has a self-diagnosis function, which can detect its own faults, display them to the operator at any time, and can dynamically monitor the implementation of the control program, which provides convenience for on-site debugging and maintenance.

and Lisi PLC - the difference between PLC control system and relay control system

and Lisi PLC - the difference between PLC control system and relay control system

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