There are two types of piston movements while considering programming, cyclic movement and sequential movement.
Cyclic movement of a piston:
Consider the task of obtaining the cyclic movement of a piston in a cylinder. This could be to periodically push the workpieces to their position on a machine tool with another similar, but outdated, arrangement, which is used to remove the finished workpieces.
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Consider both timers set for 10 s. Wlien the start contacts X400 are closed, timer T450 starts
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The output Y431 is one of the solenoids used to actuate the valve. When it is energised it causes tlie pressure supply P to be applied to the right-hand end of the cylinder and the left-hand side to be connected to the vent to the atmosphere.
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The piston thus moves to the left. After 10 s, the normally open T450 contacts close and the normally closed T450 contacts open.
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This stops the output Y431, starts the timer T451 and energises the output Y430.
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Then the pressure supply P is applied to the left-hand side of the piston and the right-hand side connected to the vent to the atmosphere.
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The piston now moves to the right. After 10 s, the T451 normally closed contacts are opened. This causes the normally closed contacts of T450 to close and so Y431 is energised. Thus the sequence repeats itself.
Sequential movement of pistons:
Consider another task involving three pistons A, B and C that have to be actuated in the sequence:
A to the right, A to the left, B to the right, B to the left, C to the right, C to the left (such a sequence is often written A-i-, A-, B-I-, B-, C+, C-).
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X400 is the start switch. When it is closed there is an output from Y430 and the timer T450 starts.
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The start switch is latched by the output. Piston A moves to the right. After the set time, K = 4, the normally closed timer T450 contacts open and the normally open timer T450 contacts close.
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This switches off Y430 and energises Y431 and starts timer T451.
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Piston A moves to the left. In rung 2, the T450 contacts are latched and so the output remains on until the set time has been reached.
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When this occurs the normally closed timer T451 contacts open and the normally open T451 contacts close.
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This switches ofif Y431 and energises Y432 and starts timer T452. Piston B moves to the right. Each succeeding rung activates the next solenoid. Thus in sequence, each of the outputs is energised.