PLC programming example: contactor interlock forward and reverse circuit

PLC programming learning requires not only the enthusiasm for independent learning, but also the skills of imitation. When I have nothing to do, I always practice and explore. Here is a real example in PLC programming. Through this example, I hope it will be helpful to everyone in the process of learning PLC.

First of all, this is a simple contactor interlock forward and reverse circuit. Let's take it as an example to see how it is programmed and briefly understand this circuit diagram:

FR thermal relay, SB1 stop button, the left side is the forward control circuit, the right side is the reverse control circuit, SB2 forward start button, SB3 reverse start button, KM1 forward AC contactor, KM2 reverse AC contactor, this pair of points is KM1's normally open point and the forward start button in parallel, which is its self-locking point. Similarly, this pair of normally open points is the self-locking point of the reverse control circuit. Let's take a look at the interlocking part, we can see KM2, its normally closed point and KM1's coil are connected in series, KM1, its long closed point and KM2, its coil are connected in series, when KM1 is actuated, this pair of normally closed points are disconnected, ensuring that the reverse circuit cannot be connected. Similarly, when KM2 is connected, this pair of points are disconnected. This is the contactor interlocking circuit.

Let's see how to do address coding. We code the SB1 stop button address as x1, SB2 address code as x2, SB3 address code as x3 thermal machine electrical appliance, address code x0 output KM1 AC contactor address code Y1, KM2 address code Y2. Let's see how to draw a ladder diagram. The ladder diagram corresponds to the electrical control schematic diagram.

First, you can see SB2. After x2, there is the self-locking key, and it is connected to a pair of normally open points. In case the normally open point of the AC contactor is followed by the x1 stop button and then the heat meter appliance. After x0, there is the AC contactor. The normally closed point serves as an interlocking key. You can see Y2. No slash refers to the normally open point, and a slash refers to the normally closed point. When using it, you must clearly distinguish the output of the positive transmission. In case of KM1 AC contactor, this is the forward control circuit. Let's take a look at the reverse control circuit. They are the same. Then there is the self-locking point, the self-locking point of Y2-KM2, which is all normally open points, followed by the SB1 stop button, the normally closed point, the logical closed point, and x0. After the search point of Y1, there is a slash, which refers to the logical closed point, and no slash refers to the logical open point, and finally the output Y2.

Next, how to convert it into programming language, first of all, the normally open point of the symptom control loop connected to the bus can be recorded as ldx2 with the ld instruction, and then the parallel connection, the normally open instruction we use the o2 instruction, can be recorded as o2y1, the series long close instruction, we can record as ANIx1, ANIx0, ANIY2, and then the output OUTY1, we have marked it here, what does it represent the address code mean? Corresponding to the motor control schematic diagram, this is the forward rotation circuit.

Let's take a look at the reverse loop and the busbar connected to the open point. We can use the ld instruction to record it as LDx3. Similarly, the parallel, normally open instruction ORY2. We use the anI instruction, which can be recorded as anIx1, x0, y1, and then the output coil OUTY2. In the logic programming language, when we are actually programming, we can use ladder diagrams for programming, or we can use logic programming languages ​​to program directly. Both methods are possible.

Next, let's look at the PLC wiring diagram. Let's look at the input part. You can see that there are 4 inputs corresponding to thermal relays, SB stop button, SB2, SB3 start buttons, and two outputs corresponding to Y1, Y2, connected to the AC contactor. After you can see its coil, friends will ask why the thermal relay is a normally closed point in the electrical control schematic, but a normally open point is used in the PLC wiring diagram. The same is true for the stop button. Here, because we programmed it into a logical closed point when programming, we can see in the ladder diagram that the thermal relay x0 uses a logical closed point. The stop button SB1 also uses a logical closed point, so we can use a normally open point on the outside. Of course, we can also use a normally closed point. We only need to change the logical closed point here to a logical open point when programming. In this case, we can use a normally closed point on the outside. When using it, we must distinguish it clearly and use it flexibly. In fact, both are possible. Simple basics, electrical schematics, address coding ladder diagrams, simple understanding of logic programming languages ​​and PLC wiring diagrams.

Learning PLC requires four steps: self-study, imitation, error correction and innovation. Many people stagnate in the self-study step. A big reason is that they don't have perseverance and don't insist on reading all the book knowledge. They always think that the theoretical knowledge is too boring. In fact, this step is very critical. It can help you build a complete PLC concept framework in your mind. Secondly, you need an object to imitate and help you correct errors, so that you can grow slowly in practice.