Pneumatic Systems

In pneumatic systems, force is produced by air pressure acting on the surface of a piston or valve.

Air is a colourless, odourless and tasteless gas consisting of approximately 78% Nitrogen and 20% Oxygen. The remaining 2% consists of about 1% Argon and a mixture of other trace elements such as helium, hydrogen and neon.

Air

Compressed air is produced in a compressor and stored in a receiver. From here it is routed to valves which control the direction of fluid flow, flow control valves which control the amount of power produced by the cylinders which convert the potential energy of the compressed air into kinetic energy at the output.

The physical behaviour of a fluid was first discovered by Blaise Pascal. Pascal found that when a force is applied to the end of a container of liquid, the force is transmitted

Blaise Pascal also discovered that pressure is equal to force per unit area (or the force divided by the area on which it acts)

Force = Pressure x Area

A force of 100 kPa (100 000 pascal's) is known as 1 bar. Normal atmospheric pressure ( the force exherted on a square metre of land at sea level by a square metre column of air rising from sea level to the outer atmosphere is approx 101.3 kPa (1.013 bar)

Compressors come in many shapes and sizes, but they all work on the same principle.

Air is pumped through a non-return valve into a strong metal tank called a receiver.

When the air inside the receiver reaches, the required pressure (usually set on a regulator) the pump switches off. In case the regulator fails a safety valve is fitted to the receiver so that if a dangerous pressure is reached the safety valve opens to allow air to escape, reducing the pressure in the receiver.

Directional Control Valves

Valves receive external commands from a mechanical, pneumatic or electrical source and release, stop or re-direct the air that flows through them.

Directional Control Valves can be used to provide a number of different functions. They can:

Control the direction of cylinder movement;

Select the path air takes through the system;

Perform logic control functions;

Stop and start air flow (on-off valves);

Sense cylinder positions (limit valves)

Directional Control valves are classified according to a number of design characteristics:

The internal valve mechanism (i.e. 'poppet' or 'sliding spool')

The number of switching positions (usually 2 or 3)

The number of connecting ports (i.e. 3 or 5 port)

The method of valve actuation ( i.e. lever, roller, plunger)

Valve Symbols

Symbols are used to simplify the drawing and explanation of pneumatic systems in much the same way as electronic systems.

Standardised symbols exist for most pneumatic components. The symbol on the right is for a three port two position poppet valve (or 3/2 poppet valve).

The top box shows the normal position of the valve. the bottom box shows the actuated position after the valve is pressed. Port 1 is the 'air in' Port 3 is the 'exhaust' .

Move cursor over the valve symbol to see how it operates

Two port valves are used to simply turn airflow on or off.

Three port valves are used as selector valves to route air to create a pilot signal, to control single acting cylinders or to make logic functions.

Five port valves are used to control double acting cylinders

Valve mechanisms

The valve mechanism directs the compressed air supply, through the valve body to the selected output ports or stops the air from passing through the valve.

The valve mechanism can be moved by direct mechanical action, a spring, an electrical solenoid or by pneumatic air pressure (signal operated).

The shape on the end of the symbols show how the internal mechanism is operated.

Plunger

Spring

Roller trip

Push Button

Lever

Pedal

Solenoid

Pilot operated

Low pressure pilot operated

Cylinder Control

Cylinders are either single acting (spring return) or double acting (air return) the diagram below shows a 3 port push button spool valve used to control a single acting cylinder.

You can see how the spring is used to expel or exhaust the air when the cylinder retracts. In contrast the diagram below shows a double acting cylinder operated by a 5 port push button spool valve. The used air in this case is exhausted as the incoming air pushes the cylinder forwards and backwards.

Controlling Cylinder Speed
Controlling the speed at which the cylinder operates can in some situations be very important. If for example, the cylinder were closing the guard on a machine or a door at the exit of a supermarket, you would not want the cylinder to close too suddenly for reasons of safety.

Connecting a cylinder directly to a 3 or 5 port valve does not allow you any control. The cylinder operates very quickly and the speed is determined by the rate of air flow (please remember that the simulations shown are deliberately slowed for clarity). The rate of air flow will be determined to a large extent by the diameter of the pipes and ports as well as the air pressure.

Restricting the flow of air into a cylinder will slow down its speed of operation. If we want to control air flow we use a special valve called a flow control valve.

The symbol for a flow control valve and a simulation of its operation are shown on the right.

Simple Control Circuits

The simplest control circuits are circuits which produce the logic functions AND / OR / NOT

Logic Functions
Connecting the air supply to the exhaust port (port 3) instead of the air inlet port (port 1) means that the cylinder will normally go positive when the button valve is NOT pressed

Connecting two valves in series, means that both valve A AND valve B have to be pressed before anything happens. This type of valve control is often used as a safety device to ensure that operators have to remove both hands to press the valves before a guard is closed and a machine is operated

Connecting two valves in parallel means that pressing valve A OR valve B will operate the output device. This circuit can be used to operate a machine from a number of different positions. The OR function requires the use of a shuttle valve. A simple explanation of how this works is shown right.