At present, the most frequently used PLC programming languages are structured text and ladder diagram. For technicians with no basic knowledge, it is the fastest way to start learning PLC programming from ladder diagram. No matter what brand of PLC, the structure of its ladder diagram is consistent with the actual electrical The control loops are similar. Below, we recommend several of the most commonly used control circuits.
1. Start, hold and stop circuits
The four ladder diagrams to realize the start, hold and stop of Y10 are shown in the figure. These ladder diagrams can realize the functions of start, hold and stop. x0 is the start signal and X1 is the stop signal. Figures a and c use Y10 normally open contacts to achieve self-locking retention, while Figures b and d use SET and RST instructions to achieve self-locking retention.
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2. Multi-ground control circuit
The picture below is a program for controlling a relay coil in two places. Among them, X0 and X1 are the start and stop control buttons in one place, and X2 and x3 are the start and stop control buttons in another place.
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3. Interlock control circuit
The picture below is the interlock circuit of three output coils. Among them, X0, X1 and X2 are the start buttons, and X3 is the stop button. Since only one of Y0, Y1, and Y2 can be turned on at a time, the normally closed contacts of Y0, Y1, and Y2 are connected in series to the control circuits of the other two coils.
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4. Sequential start control circuit
As shown in the figure, the normally open contact of Y0 is connected in series in the control loop of Y1, and the turning on of Y1 is conditional on the turning on of Y0. In this way, Y1 is allowed to be turned on only when Y0 is turned on. After Y0 is turned off, Y1 is also turned off and stopped, and when Y0 is turned on, Y1 can turn on and stop by itself. X0 and X2 are start buttons, X1 and X3 are stop buttons.
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5. Motor forward and reverse circuit
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6. Centralized and decentralized control circuits
In an automatic line composed of multiple single machines, there is centralized control on the main operating table and interlocking of decentralized control on the single machine operating table. The ladder diagram of centralized and decentralized control is shown in the figure. x2 is a selector switch, and its contacts are interlocking contacts for centralized control and decentralized control. When X2 is ON, it is single-machine distributed starting control; when x2 is OFF, it is centralized overall starting control. In both cases, the stand-alone machine and the main operating console can issue stop commands.
