The difference between PLC Control and Relay Control.
According to our many items as well as the long time study to sum up the experience, the following shallowly discusses the PLC control system and the relay control system difference.
There are many similarities and differences between PLC control system and relay control system. The main differences are as follows:
(1) from the point of view of control method, the control logic of electric control system adopts hardware wiring, and uses the combination control logic such as series or parallel connection of relay mechanical contacts, which has many connections and is complex, large in volume and large in power consumption. After the system is constituted, It is difficult to change or add features. In addition, the number of relay contacts is limited, so the flexibility and scalability of the electrical control system is greatly limited. PLC uses computer technology, its control logic is stored in the memory in the form of a program, to change the control logic only needs to change the program, so it is easy to change or increase the system functions. The "soft relay" of PLC is essentially the state of the memory cell, so the number of contacts of the "soft relay" is the most unlimited, and the system has good flexibility and expansibility.
(2) in the way of work, in the relay control circuit. When the power is switched on, all the relays in the circuit are in a restricted state. That is to say, all the relays should be sucked together at the same time. The relay that should not be sucked in is limited by certain conditions and cannot be sucked in. This mode of operation is called parallel mode of operation. And the user program of PLC is executed in a certain order, so the soft relays are all in the periodic cycle scanning connection, and the action order of each relay restricted by the same condition is decided by the scanning sequence of the program. This mode of operation is called serial mode.
(3) from the view of control speed, the relay control system depends on the action of mechanical contacts to realize the control, and the rate of working children is low. The mechanical contact will also appear to shake the problem. But PLC realizes the control through the program instruction control semiconductor circuit, the speed is quick, the procedure instruction execution time is in the microsecond level, and does not appear the contact jitter between problems.
(4) from the view of timing and counting control, the electric control system adopts the time-delay action of the time relay to control the time. The delay time of the time relay is susceptible to the change of the environment quiet degree and the Ju degree, and the timing precision is not high. PLC uses semiconductor integrated circuit as timer, clock pulse is generated by 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 environment, and the PLC has the function of counting, but the electric control system generally does not have the function of counting.
(5) from the reliability and maintainability, because the electrical control system uses the mechanical contact of the big basin, its mechanical wear, electric arc burn and so on, the life is short, the system connection is many, so the reliability and maintainability are poor. PIZ has a long life and high reliability. It also has the function of self-diagnosis. It can detect the fault of itself, show to the operator at any time, and monitor the execution of the control program dynamically. It provides convenience for field debugging and maintenance.