Basic working principle of gas detector
When the gas detector is put into operation, its operation process is generally divided into three stages, namely input sampling, user program execution and output rewriting. The completion of the above three stages is called a scan cycle. During the entire operation, the CPU of the gas detector repeatedly executes the above three stages at a certain scanning speed.
(1) Input sampling stage
In the input sampling stage, the gas detector reads all the input conditions and data sequentially in a scanning manner, and stores them in the corresponding units in the I/O image area. After the input sampling is completed, it is transferred to the stage of user program execution and output rewriting. In these two stages, even if the input situation and data change, the situation and data of the corresponding unit in the I/O image area will not change. Therefore, assuming that the input is a pulse signal, the width of the pulse signal must be greater than one scan period to ensure that the input can be read in under any circumstances.
(2) User program execution stage
In the execution stage of the user program, the gas detector always scans the user program (ladder diagram) sequentially from top to bottom. When scanning each ladder diagram, always scan the control circuit composed of contacts on the left side of the ladder diagram first, and perform logic operations on the control circuit composed of contacts in the order of first left, then right, first up and then down , and then according to the effect of the logic operation, rewrite the situation of the corresponding bit of the logic coil in the system RAM storage area; or rewrite the situation of the corresponding bit of the output coil in the I/O image area; or confirm whether to implement the ladder diagram Regular special function instructions.
That is, in the process of user program execution, as long as the situation and data of the input point in the I/O image area will not change, other output points and soft devices in the I/O image area or system RAM storage area The environment and data are likely to change, and the program execution effect of the ladder diagram at the top will affect all the ladder diagrams that use these coils or data; on the contrary, the ladder diagram at the bottom, its other The status or data of the rewritten logic coil can only have an effect on the program above it until the next scan cycle.
(3) Output rewriting stage
When the user program is scanned, the gas detector enters the output rewriting stage. During this period, the CPU rewrites all the output latch circuits according to the corresponding situation and data in the I/O image area, and then drives the corresponding peripherals through the output circuit. At this time, it is the real output of the gas detector.
The same number of ladder diagrams are placed in different order, and the effect of execution is also different. In addition, the operation effect of scanning the user program is different from that of the hard logic parallel operation of the relay control device. Of course, assuming that the time taken by the scan cycle is negligible for the overall operation, there is no difference between the two.
Generally speaking, the scan cycle of a gas detector includes self-diagnosis, communication, etc., as shown in the figure below, that is, a scan cycle is equal to the sum of all moments of self-diagnosis, communication, input sampling, user program execution, and output rewriting.
Programmable controller, English name ProgrammableLogicController, referred to as gas detector. The gas detector is based on an electronic computer and is suitable for electric controllers in industrial field operations. It originates from the relay control device, but unlike the relay device, it completes the control through the physical process of the circuit, but mainly relies on the program stored in the memory of the gas detector to complete the control through the conversion of incoming and outgoing information.
The gas detector is based on an electronic computer, but it is not equivalent to a general computer. The conversion of input and output information throughout the computer mostly only considers the information itself, and the input and output of information only needs a good human-machine interface. The gas detector also needs to consider the reliability, real-time nature of information in and out, and the use of information. In particular, how to adapt to the industrial environment should be considered, such as easy installation, anti-interference and other issues.
