Sequence control

Sequence control in pneumatics

Associated with pneumatic sequence control, the following terms are used:

Cascade control (Spanish cascada Δ stepped waterfall). Conferred to automated systems, this means that the work process and thus the structure of the control itself consist of several steps. Control blocks with logical AND valve as a set condition for the next step are also available in the market. Here an example of a well-known manufacturer from its online catalog:

pneumatic control block

Pneumatic control block

This control block aside consists of four stages (5/2-way pulse valve) and an AND gate as a switching condition and an OR gate as a reset condition.

Source: Festo

 

 

For a process-controlled sequence, an end position detection of the cylinders is always necessary!

Description of the work process by path-step diagram

Since very often purely pneumatic controls use binary way valves as actuators, the path-step diagram is well used to describe the work flow of sequential control. In principle, this diagram shows graphically the cylinder positions over each work steps.


Path-step diagram in pneumatics

Path-step diagram in pneumatics

The steps are numbered horizontally. In the last step, the cylinders have the same position as in the first step.

A cylinder is either retracted ('0') or extended ('1'). When a cylinder moves, you can recognize this by its oblique line.

To get to the next step, one or more signals must be present. These signals are marked as narrow lines and have an arrow that indicates the effective direction.

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Symbols of signal elements:

Switch elements within the path-step diagram

Switch elements within the path-step diagram

The different symbols represent the multiplicity of signal elements. Here you see some of the most important ones:

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Logical operations within the path-step diagram

Logical operations within the path-step diagram

Often, you get to the next step by several input signals. The basic logical operations hereby are 'AND', 'OR', NOT '.


Time function within the path-step diagram

Time function within the path-step diagram

Time function: Here you can distinguish between turn-on delay and off delay.


Signals and signal lines within the path step diagram:

Path-step diagram - signal lines

Path-step diagram - signal lines

Example for a path-step diagram with signals – now a bit more complex:

Path-step diagram - example

Path-step diagram - example