How does PLC replace traditional relay control and contactor control systems?
PLC is now very popular, almost universally used in the development of industrial production and science and technology, gradually replacing the traditional contactor control and relay control system, many people can not help but ask: PLC in the end is how to replace the traditional relay control and contactor control system?
PLC has a variety of functional components.
Many internal components of the PLC have different functions. These components are composed of electronic circuits and memories. For example, the input relay (X) is composed of an input circuit and an input shadow register, and the output relay (Y) is composed of an output circuit and an output map register. The timer (T), counter (C), auxiliary relay (M), State relays (S), data registers (D), index registers (V, Z), etc. are all composed of memory.
In order to distinguish it from the usual hardware components, these components are often referred to as soft components. They are equivalent abstract simulated components and are not actual physical components. From the point of view of the work process, only the functions of the components are emphasized, and the names are given according to the functions of the components, and each component has a definite address number.
1. Input relay (X); It is connected with the PLC's input terminal, which is the gate for the PLC to receive the external switch signal. PLC reads the external signal status through the input terminal and stores it in the input image register. The input relay connected to the input terminal is an opto-isolated electronic relay. In the PLC program, there are a lot of moving contacts and moving contacts.
For example, the input relay of FX2N PLC adopts the octal address number, such as Xo~X7, X10~X17, X20~X27 and so on. The wiring diagram of PLC input signal refers to the following figure:
If the start button SB1 signal and the stop button SB2 signal are sent to the PLC, and one end is connected to Xo for X1 and the other end is connected to the CoM field of the input end, the corresponding button switch signal can be sent to the PLC. If the SB1 button is in the 0FF state, all contacts of the Xo in the PLC program remain in their original state, ie, the movable contact remains open, and the movable contact remains closed. If the SB1 button is in the 0N state, all contacts of the X0 in the pLC program are to be actuated, that is, the open contact is opened and the movable contact is closed.
2. Output relay (Y); The output relay is connected with the output terminal of PLC, it is the window that PLC sends signal to external load. The output relay is used to transmit the output signal of the PLC to the output unit, and then the output unit drives the external load. There are output relay coils and their contacts in the PLC program. In the program its coils typically only appear once, and their contacts can be unlimited. Its address number is similar to that of the input relay, and it uses octal address numbers, such as Y0~Y7, Y10~Y17, Y20~Y27.
When the state of the Y0 coil in the PLC program is turned ON to 0N, all the contacts of Y0 in the program will be activated, and the moving and closing contacts will be closed, and the moving and breaking contacts will be opened. And in the peripheral circuit (as shown in the figure above), Y0 terminal and C0M1 terminal will be connected, then the load HL1 controlled by Y0 will turn on its working power and light up. When the coil of Y0 is disconnected to 0FF in the program, all the contacts of Y0 in the program remain in the original state, and the movable contact is opened, and the movable contact is closed. And in the peripheral circuit, the Y0 terminal and the C0M1 terminal will be disconnected, then the load HL1 controlled by Y0 is not turned on, and the lamp whose operating power is controlled is not on.
