1. Step

The basic idea of PLC Sequential control design method is to divide a working cycle of the system into several sequential connected stages. These stages are called steps, which can be represented by programming elements (such as auxiliary relay m and sequential control relay s). Step is divided according to the state change of the output. In any step, the on / off state of each output remains unchanged, but the total state of the output of the adjacent two steps is different. This division method of step makes there a very simple logical relationship between the state of the programming element representing each step and the state of each output.


The feeding trolley starts to stop at the left limit switch X2 (see Fig. 17), press the start button x0, X2 becomes on, open the gate of the storage hopper, start loading, and use the timer T0 to timing, close the gate of the storage hopper after 10s, Y0 becomes on, start to go right, stop to unload after touching the limit switch X1 (Y3 is on), and use the timer T1 to timing at the same time; After 5S, Y1 changes to on, starts to travel left, returns to the initial state after touching the limit switch X2, and stops operation.


According to the change of on / off state of Y0 ~ Y3, it is obvious that a working cycle can be divided into four steps: loading, right row, unloading and left row. In addition, the initial steps waiting for starting shall be set, which are represented by M0 ~ M4 respectively. The upper left part of Figure 17 is the spatial diagram of trolley movement, and the lower left part is the waveform diagram (sequence diagram) of programming elements, On the right is the sequence function diagram describing the system. In the diagram, a rectangular box is used to represent the step, and the number of the step can be represented by numbers in the box. Generally, the element number of the element representing the programming element of the step is used as the step number, such as M0, etc., so it is more convenient to design the ladder diagram according to the sequence function diagram.

2. Initial step

The step corresponding to the initial state of the system is called the initial step. The initial state is generally the relatively static state of the system waiting for the start command. The initial step is represented by a double line box, and each sequential function diagram should have at least one initial step.

3. Activity steps

When the system is in the stage where a step is located, the step is active, which is called “active step”. When the step is active, the corresponding action is executed: when it is inactive, the corresponding non storage action is stopped.

4. Action or command corresponding to step

A control system can be divided into controlled system and control system. For example, in the NC lathe system, the NC device is the control system, and the lathe is the controlled system. For the controlled system, some “actions” shall be completed in a certain step; For the control system, some “commands” shall be issued to the controlled system in a certain step. For the convenience of narration,

Next, commands or actions are collectively referred to as actions and are represented by words or symbols in a rectangular box, which should be connected with the corresponding symbols.

If there are several actions in a certain step, it can be represented by the two drawings in Fig. 18, but it does not imply any order between these actions. The statement describing the command should clearly indicate whether the command is stored or non stored. For example, the stored command “open No. 1 valve and hold” of a step means that it is opened when the step is active and continues to be opened when the step is inactive; The non storage command “open valve 1” means that it is opened when the step is active and closed when it is inactive.

In addition to the above basic structure, using action modifiers (see Table 1) can complete different actions in one step. Modifiers allow actions to be controlled without adding logic. For example, the modifier l can be used to limit the opening time of the batching valve.


In FIG. 17, the coil of timer T0 should be “powered on” when M1 is an active step and powered off when M1 is an inactive step. In this sense, the coil of t0 is equivalent to an action of step M1, so t0 is treated as an action of step M1. The conversion condition t0 under step M1 is provided by the normally open contact of t0 closed at the specified time. Therefore, t0 in the action box corresponds to the coil of T0, and the conversion condition t0 corresponds to the normally open contact of t0.

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