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Pneumatic system
• Introduction to pneumatic system.
• Basic component in pneumatic system.
– Compressor.
– valve.
– Cylinders and pneumatic motor(actuator).
• Elements of control chain.
• Pneumatic control circuit.

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Introduction of pneumatic system
• A system that uses air(pressurize gases) to do
mechanical motion(work).
• Pneumatics uses an easily compressible gas such
as air or a suitable pure gas.
• The accomplishment of work requires the
application of kinetic energy to a resisting object
resulting in the object moving through a distance.
• The energy in the pneumatic system is stored in a
potential state under the form of compressed air.

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Advantages of pneumatics
• Cleanliness
– Air is used by a machine and is then exhausted to the atmosphere -
no return line is necessary.
– Any leaks will be of air
• Availability
– Air is freely available in the pneumatics.
– very easy to set up a manufacturing process.
• Simplicity of Design And Control
– Control is as easy as it is simple ON - OFF type control.
• Reliability
– Pneumatic systems tend to have long operating lives and require
very little maintenance.
– The equipment is less likely to be damaged by shock

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• Storage
– Compressed Gas can be stored, allowing the use of machines
when electrical power is lost.
• Safety
– Very small fire hazards (compared to hydraulic oil).
– Machines can be designed to be overload safe.

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Disadvantages of pneumatic system are:
• Low accuracy and control limitation because
of compressibility.
• Noise pollution.
• Leakage of air can be of concern.
• Additional drying and filtering may be
required.
• Difficult speed control.

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Basic component in the
pneumatic system
• Air Compressor unit.
• Air Compressors compress air from atmosphere to
higher pressures. There are two (2) basic types of
compressors in use today:
– Positive displacement (Intermittent Flow)
– Dynamic (Continuous-flow)
• Air is pumped through a non-return valve into a
strong metal tank called a receiver.

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• The positive displacement type of compressor uses a
squeezing effect to force gas molecules into a
smaller space.
• The dynamic compressor applies mechanical force
to gas to increase its velocity, which is the
converted to pressure.
• Several types of positive displacement compressor
are:
– Reciprocating Single Acting
– Reciprocating Double Acting
– Rotary Helical Screw
– Liquid-Ring Rotary compressor
– Rotary Lobe type

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Reciprocating Single Acting
Rotary Lobe type
Rotary Helical Screw
Liquid-Ring Rotary
compressor
Reciprocating Double Acting

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• Types of dynamic compressor are:
– Dynamic (Centrifugal)
– Ejector

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• Valve
– Several types of valve consist in pneumatic
system:
• Safety relief valve.
• Flow control valve.
• Shuttle valve.
• Check valve.
• Directional control valve.

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• Safety relief valve
– A safety relief valve is a normally closed valve.
– Consist a poppet of the safety relief valve
seated on the valve inlet, a spring holds the
poppet firmly on its seat, which makes air
cannot pass through the valve until the force of
the spring biasing the poppet is overcome.

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• Flow control valve.
– Also called speed control valve.
– Allows free flow in one direction but restricts flow in
the other direction.

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• Shuttle valve
– Consists three-port valve with two inlets and one
outlet.
– Ports 1 and 2 are inlets, port 3 the outlet.
– If pressure is applied to port 1, it will appear out port 3,
but will not back-flow out port 2.
– If pressure is applied to port 2, it will appear out port 3
but will not back-flow out port 1.
– When the pressures are removed from ports 1 and 2,
port 3 will exhaust back out one of the two inlet ports,
but it may exhaust out either of them.
1 2
3

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• Check valve
– Allows flow in one direction, but blocks flow in the
other direction.

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• Directional control valve (DCV)
– 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.
– It change the direction of airflow to and from the
cylinder.
– The valve mechanism can be moved by direct
mechanical action, a spring, an electrical solenoid or by
pneumatic air pressure (signal operated).
Supply
Actuator
exhaust

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• Directional control valve symbol
– Symbols are used to simplify the drawing and
explanation of pneumatic systems.
– Standardised symbols exist for most pneumatic
components.
– The symbol below 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(supply) is the 'air in' Port 3 is the
'exhaust' .
Supply
Actuator
exhaust

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– The shape on the end of the symbols show how the
internal mechanism is operated.
Plunger
Solenoid
Pedal
Level
Push button
Roller trip
Spring
Plunger

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• Classifying a directional control valve.
– One method of classifying a directional control valve is
by the flow paths that are set up in its various operating
conditions.
– Information about the type of valve can be established
from the following features:
• Number of ports
• Number of switching positions
• Port numbering
– The following applies to the numbering of the ports:
• Air supply port 1
• Exhaust ports 3,5
• Working or outlet ports(actuators) 2,4

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• Two-Way Directional Valve
– A two-way directional valve consists of two ports
connected to each other with passages, which are
connected and disconnected
Flow Path Open Flow Path Closed
2/2 – way valve, normally open
position

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• Three-Way Directional Valve
– A three-way directional valve consists of three ports
connected through passages within a valve body that
are shown here as port A, port P and port Ex.
– The function of this valve is to pressurize and exhaust
one actuator port.
3/2 – way valve, normally closed position.

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• Four-Way Directional Valve
– The most common directional valve in simple
pneumatic systems.
– A common application of four-ported four-way
directional valve is to cause reversible motion of a
cylinder or motor.
4/2 – way valve flow from 1 to 2 & from 4 to 3
Supply
Actuator
exhaust
Actuator

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• Five-Port / Four-Way Directional Valve
5/2 – way valve flow from 1 to 2 & from 4 to 5
5/3 – way valve mid position closed

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• Pneumatic cylinder
– Is one of the actuator(working element) in
pneumatic system.
– Cylinders are linear actuators which convert
fluid power into mechanical power.
– Also known as JACKS or RAMS.
– Types of cylinder in pneumatic system:
• Single acting cylinder with or without spring return.
• Double acting cylinder.

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Single acting cylinder(SAC)
– The cylinder is only powered in one direction
and needs another force to return it such as an
external load or a spring.

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3/2 ways DCVs
Single acting cylinder
Push button
Supply
Exhaust

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Double acting cylinder(DAC)
• Uses the force of air to move in both extend and
retract strokes.
• They have two ports to allow air in, one for
outstroke and one for instroke.
• A four-way directional control valve is
normally used to control the operation of this type
of cylinder.

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Example;
Double acting cylinder with 5/2 wa
y DCV
Double acting cylinder with 3/2 way
DCV
Several types of double
acting cylinder

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Pneumatic circuit
Controlling one double
acting cylinder.
• List of the component:
– 1 Air supply.
– 2 Service unit.
– 3 Valve(with roller in left and
with lever in right).
– 4 Directional Control Valve,
3/2 ways.
– One way flow control valve.
– Double acting cylinder.
http://www.mekanizmalar.com/v
alf5.shtml

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Pneumatic controller
• The basic pneumatic control mechanism is
the flapper-nozzle unit.
• Using with amplifying relay and feedback
bellow for rapid control mechanism.
• Convert a small motion(position) or force
into an equivalent(proportional) pneumatic
signal.
• Control pressure is;
– 20 to 100 kPa (3 to 15 psi)
– 0 % signal pressure is equivalent to 20 kPa and
100 % signal is equivalent to 100 kPa.

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The Flapper-Nozzle unit
• Working principle:
– Input air(regulated at 20 psi
or 138kPa ) is fed to the
nozzle through a reducing
tube(restriction).
– The pressure of the nozzle
drop when the flapper is
moved away from the
nozzle.
– The pressure will constantly
build up when the flapper is
moved closer to the nozzle.
‾ The flapper movement is
very small and rapidly,
produce a proportional
pneumatic signal.

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• The limitation of the simple flapper-nozzle:
– Output pressure is slow because of the constriction
when directly used to operate a control valve. Causing
the sluggish action on the system.
– The flapper must big and strong enough to overcome
the blast of air leaving the nozzle.
– Because of the flapper moving rapidly the system is
susceptible to vibration and instability.
• The limitation can be reduce by adding:
– Relay or amplifier.
– Feedback system (repositions the flapper or
nozzle.)

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PNEUMATIC CONTROL SYSTEM IN CONTROL ENGINEERING

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