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Displacement Step Diagram Pneumatic Controls

Principle of a displacement step diagram

A displacement step diagram illustrates how the sequence of movements of an electro-pneumatic control system functions in a clear and easily understood fashion. The individual movements of all the working components are plotted along an axis that shows the individual steps. Although such functional diagrams are no longer included in the standards, they are still used frequently in practice. They are particularly useful for simple circuits without too many control sequences.

 

Example: drilling machine

The following animation shows the example of a drilling machine with pneumatic cylinders (M1 and M2), each equipped with two limit switches (S1.1/S1.2 and S2.1/S2.2).

drilling-machine-pneumatic-cylinders-animation.gif

To start with, both cylinders are in their rest positions, i.e. retracted. The drilling machine requires four steps:

(1) Extend cylinder M1 (clamp the workpiece)
 
(2) Extend cylinder M2 (drill the workpiece)
 
(3) Retract cylinder M2 (raise the drill)
 
(4) Retract cylinder M1 (release the workpiece)
 

The displacement step diagram shows the movement of the cylinders or the pistons plotted along the step axis. The example above produces the following diagram.

cylinders-pistons-plotted-along-step-axis.jpg

The x-axis has the number of each step (labelled i in this instance) while the y-axis has the position of each cylinder entered, M1 (top, red curve) and M2 (bottom green curve). A value of 0 implies that the cylinder is fully retracted and a value of 1 means it is fully extended. Thus, in the first step cylinder M1 extends at constant speed, i.e. its position traverses linearly from 0 to 1 where it remains until step 4. Cylinder M2 extends during the second step and retracts again during the third. The displacement step diagram thus includes all aspects of the sequence of movements.

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