Pneumatics

In this topic you need to be able to identify and describe pneumatic circuits and also, as for the NAB, draw these circuits from scratch.

It is therefore very important that you understand how to draw the symbols that you will need to use.

Cylinders:

You can see that the single acting cylinder has one input port which, when air flows in, will outstroke the cylinder.  The spring will make the cylinder instroke as soon as this input air stops.

The double acting cylinder has two input ports, one at each end. This means that air operates the cylinder  for both instroke and outstroke.

Valve Symbols:

The symbols for valves show where external connections (i.e. for main air or exhaust) are connected, as well as the internal connections for each state.

3/2 Valves:

3/2 valves are so called because they have 3 ports and two different states.  The symbol for a 3/2 valve is in two parts to show the two different states it can be in.  The bottom box typically shows the state a valve would be in when just out of the box, or unactuated.   This means that main air is not flowing into the valve, and any excess air in the system can escape out of the exhaust:

The top box is the "action" box.  This is when actuated the valve will allow main air to flow through the valve:

(note, that although I have added main air and exhaust here to help explain, all connections should be drawn on the bottom box.)

To finish your symbol you need to add the actuators.  The actuator make the box they are attached to "happen".  So in this case the push button actuates the valve and allows main air to flow from port 1 to 2.  The spring pushes the top back up again to block off main air when the button is released.

5/2 Valves:

5/2 valves are so called because they have 5 ports and 2 states.  They are primarily used to control a double acting cylinder as main air is always flowing through the 5/2 valve to hold the cylinder outstroke or instroke.  The change of state in a 5/2 valve simply means that main air flows from port 1 > 2 or port 1 > 4, unlike the 3/2 valve which either allowed main air to flow, or blocked it off.

State 1 > 2:

State 1 > 4:

5/2, lever lever valve controlling a double acting cylinder:

Note how all connections are drawn on the left hand box.

Unidirectional Flow Control Restrictors:

A unidirectional FCR combines a bi-directional FCR and a one way valve.  In the diagram below, if the air flows from right to left, it pushes the ball out of the one way valve and air can freely flow along the top of the Unidirectional FCR.  If the air flows from left to right the air pushes the ball into the one way valve which blocks it off and therefore the air is forced to go through the restrictor.

When using a FCR to control the speed of a cylinder it is important to remember to restrict the EXHAUST of the cylinder (the air coming out of the cylinder) so that the cylinder has full pressure air pushing it outstroke and the motion will be smooth with full force.  This is shown below so that the cylinder is being slowed down on outstroke, but will instroke full speed:

Note: just remember that the arrows for the one way valve point away from the cylinder.

Time Delay:


A reservoir can be added after an FCR to cause a time delay.  The air is restricted and therefore takes some time to fill up the reservoir causing the delay. The time can be adjusted by tightening or loosening the FCR or the size of the resistors.

For more information on AND and OR control, and an excellent interactive diagram please look at the BBC Bitesize website here.