PLC and wiring switch and analog - Simulation technology -

Everyone knows that PLC can handle input output consists of switch quantity, analog and digital communications, which switch amount and analog is we learn at an early stage in the process of PLC controller, the most used two types of input and output. What is that what is analog switch quantity? For some basic friend isn't the problem, but for new to PLC's white, this is a problem that must be made clear. Because it is the basis of the PLC to study in the foundation, and is also the basis in the field of electrical control. So for basic things or repetitive more necessary in a moment, the following content is suitable for the beginner of PLC, small white, if you are already could skip. First is the simplest on-off, as shown in figure 1. Figure 1

, in fact as long as seen in figure 1 I don't need to tell you can see what is the switch quantity, picture in picture is an electric contact pressure gauge, the pressure gauge also often see in many devices, it is a typical can switch output signal of the device. Principle is high pressure when the two contact closure through to C and B, high output pressure signal, the pressure is low C and two contact closure through to A low output pressure signal. With such signals will achieve the goal of in situ stress, far to the distance of the electrical control cabinet to participate in the automatic remote control, including C and B is A switch quantity, C and A, is also A switch quantity. So a switch contact is a switch, and the characteristics of it is the same time either on or off. Connect is 1, which represents have signal, disconnect is 0, representing no signal. This is the so-called on-off signal. Careful readers will find that at this moment, so of although the electric contact pressure gauge can put on the pressure signal is far to the distance, but it only have pressure and no pressure and no signal, that if we want to know on the real-time pressure value how much should I do? Then read the following introduction to analog you'll find a way.

at the picture, 2 in figure 2, the most on the left side of the device is called pressure transmitter, if we want to be in the distance on which all knew in situ pressure numerical value. How did he do it? 2 in the middle of the picture, actually it is a pressure transmitter circuit board, circuit board connected to a pressure sensor F. It is the working principle of pressure sensor F real-time detection of pipeline pressure, the detected pressure again to the circuit boards of C, detection signal into the circuit board, by the transformation of the circuit board and calculation, the pressure signal is converted into A current signal output from the two points A and B. Figure 2 most the right side is the diagram of the transition, we can put a 0 to see it 10 kpa pressure signal is transformed into a 4 - 20 ma current signal, the output from the two points A and B. When we say the two points A and B is an analog signal output. The analog signals can be 4 - 20 ma current signal, also can be 0 10 v voltage signal. The characteristics of the analog signal is its value is within the scope of a numerical continuous variable. This is known as analog signals. Look at the below analog signal is how to transmit the pressure signal far out.

figure 3, suppose we pipe is installed on a range of 0 - 10 kpa pressure transmitter, we make A power of the positive pressure transmitter point B, the cathode series A multimeter to point A pressure transmitter, and the multimeter to current file. Then let's look at the picture 3 left, when the pressure is 5 kpa pressure transmitter C point, multimeter 12 is the current reading of the ma. Just 4 - 20 ma current signal of intermediate value, and 5 kpa also happens to be 0 10 kpa pressure value of the intermediate value. Again at the picture 3 to the right, when the pressure transmitter C is 10 kpa pressure, a multimeter is precisely the current reading of the 20 ma. The 0 - Corresponding to the four to 10 kpa pressure value 20 ma current signal value, as long as we in the distance through the 4 - a receiving device 20 ma current signal value extracted, and then through a certain calculation, can know how much is the pressure value in situ. The receiving device can be a PLC can also be digital display instrument, etc.

here again, why the pressure signal into 4 - 20 ma current signal, instead of 0 Or 0 to 20 ma current signal 10 v voltage signal. Because 1, 0 10 v voltage signal is vulnerable to electromagnetic interference of the outside world, especially the long cable length is more obvious when the interference. 2, use a scale of 0 - 20 ma current signal, will not be able to determine when the current signal is 0 ma, what is the result of the cable bolt failure 0 ma, or is 0 kpa pressure and the output of the normal 0 ma. So now the market pressure transmitter use 4 - mostly Analog current signal of 20 ma.

figure 4, is a Siemens s7 - passing Read the pressure value of the pressure transmitter 200 PLC wiring diagram example, this is one of the most basic method of using switch quantity on the left, right is analog, different signal types to receive different input PLC. Tell me the pressure transmitter under section 4 Enter the Siemens s7-20 ma analog signal After 200 PLC, PLC is how this program to convert the analog current signal of the actual pressure value.

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