Pneumatic system in Mechatronics and Engineering

Pneumatic system
Pneumatic technology deals with the study of behavior and applications of
compressed air in our daily life in general and manufacturing automation in particular.
Pneumatic systems use air as the medium which is abundantly available and can be
exhausted into the atmosphere after completion of the assigned task.
1. Basic Components of Pneumatic System:
Fig. Components of a pneumatic system
Important components of a pneumatic system are shown in fig.
a)Air filters: These are used to filter out the contaminants from the air.
b)Compressor: Compressed air is generated by using air compressors. Air
compressors are either diesel or electrically operated. Based on the
requirement of compressed air, suitable capacity compressors may be used.
c)Air cooler: During compression operation, air temperature increases. Therefore
coolers are used to reduce the temperature of the compressed air.
d)Dryer: The water vapor or moisture in the air is separated from the air by using a
dryer.
e)Control Valves: Control valves are used to regulate, control and monitor for control
of direction flow, pressure etc.
f)Air Actuator: Air cylinders and motors are used to obtain the required movements
of mechanical elements of pneumatic system.
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g) Electric Motor: Transforms electrical energy into mechanical energy. It is
used to drive the compressor.
h) Receiver tank: The compressed air coming from the compressor is stored in
the air receiver.
These components of the pneumatic system are explained in detail on the next
pages.
2. Receiver tank
The air is compressed slowly in the compressor. But since the pneumatic
system needs continuous supply of air, this compressed air has to be stored.
The compressed air is stored in an air receiver as shown in Figure. The
air receiver smoothens the pulsating flow from the compressor. It also
helps the air to cool and condense the moisture present. The air receiver
should be large enough to hold all the air delivered by the compressor. The
pressure in the receiver is held higher than the system operating pressure
to compensate pressure loss in the pipes. Also the large surface area of
the receiver helps in dissipating the heat from the compressed air. Generally
the size of receiver depends on,
 Delivery volume of compressor.
 Air consumption.
 Pipeline network
 Type and nature of on-off regulation
 Permissible pressure difference in the pipelines
Fig. Air receiver
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3. Compressor:
It is a mechanical device which converts mechanical energy into fluid energy.
The compressor increases the air pressure by reducing its volume which also
increases the temperature of the compressed air. The compressor is selected based
on the pressure it needs to operate and the delivery volume.
The compressor can be classified into two main types
a. Positive displacement compressors and
b. Dynamic displacement compressor
Positive displacement compressors include piston type, vane type, diaphragm
type and screw type.
Piston compressors
Fig. Single acting piston compressor
Piston compressors are commonly used in pneumatic
systems. The simplest form is single cylinder
compressor. It produces one pulse of air per piston
stroke. As the piston moves down during the inlet stroke
the inlet valve opens and air is drawn into the cylinder.
As the piston moves up the inlet valve closes and
the exhaust valve opens which allows the air to be
expelled. The valves are spring loaded. The single
cylinder compressor gives significant amount of pressure
pulses at the outlet port. The pressure developed is about
3-40 bar.
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Double acting compressor
Fig. Double acting piston compressor
The pulsation of air can be reduced by using double acting compressor as shown in
Figure. It has two sets of valves and a crosshead. As the piston moves, the air
is compressed on one side whilst on the other side of the piston, the air is sucked in.
Due to the reciprocating action of the piston, the air is compressed and delivered twice
in one piston stroke. Pressure higher than 30bar can be produced.
Multistage compressor
Fig. Multi-stage compressor
As the pressure of the air increases, its temperature rises. It is essential to reduce the
air temperature to avoid damage of compressor and other mechanical elements. The
multistage compressor with intercooler in-between is shown in Figure. It is used
to reduce the temperature of compressed air during the compression stages. The inter-
cooling reduces the volume of air which used to increase due to heat. The compressed
air from the first stage enters the intercooler where it is cooled. This air is given as
input to the second stage where it is compressed again. The multistage
compressor can develop a pressure of around 50bar.
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Combined two stage
compressors
Fig. Combined to stage compressor
In this type, two-stage compression is carried out by using the same piston. Initially
when the piston moves down, air is sucked in through the inlet valve. During
the compression process, the air moves out of the exhaust valve into the intercooler.
As the piston moves further the stepped head provided on the piston moves
into the cavity thus causing the compression of air. Then, this is let out by the exhaust
port.
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Compressors
1. Diaphragm
compressor
Fig. Diaphragm compressor
These are small capacity compressors. In piston compressors the lubricating oil from
the pistons walls may contaminate the compressed air. The contamination
is undesirable in food, pharmaceutical and chemical industries. For such
applications diaphragm type compressor can be used. Figure shows the construction of
Diaphragm compressor. The piston reciprocates by a motor driven crankshaft. As
the piston moves down it pulls the hydraulic fluid down causing the diaphragm to
move along and the air is sucked in. When the piston moves up the fluid pushes the
diaphragm up causing the ejection of air from the outlet port. Since the flexible
diaphragm is placed in between the piston and the air no contamination takes place.
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2. Screw compressor
Piston compressors are used when high pressures and relatively low volume of air is
needed. The system is complex as it has many moving parts. For medium flow and
pressure applications, screw compressor can be used. It is simple in construction with
less number of moving parts. The air delivered is steady with no pressure pulsation. It
has two meshing screws. The air from the inlet is trapped between the meshing screws
and is compressed. The contact between the two meshing surface is minimum, hence
no cooling is required. These systems are quite in operation compared to piston type.
The screws are synchronized by using external timing gears.
Fig. Screw compressor
3. Rotary vane compressors
Fig. Rotary vane compressor
The principle of operation of vane compressor is similar to the hydraulic vane pump.
Figure shows the working principle of Rotary vane compressor. The unbalanced vane
compressor consists of spring loaded vanes seating in the slots of the rotor.
The pumping action occurs due to movement of the vanes along a cam ring. The rotor
is eccentric to the cam ring. As the rotor rotates, the vanes follow the inner surface of
the cam ring. The space between the vanes decreases near the outlet due to the
eccentricity. This causes compression of the air. These compressors are free
from pulsation. If the eccentricity is zero no flow takes place.
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Fig. Liquid ring compressor
Liquid ring vane compressor is a variation of vane compressors. Figure shows
the construction of Liquid ring compressor. The casing is filled with liquid up
to rotor center. The air enters the compressor through the distributor fixed to the
compressor. During the impeller rotation, the liquid will be centrifuged along the inner
ring of the casing to form the liquid ring. There are two suction and discharge ports
provided in the distributor. During the first quarter of cycle, the air is sucked in
both suction chambers of the casing and during the second quarter of the
cycle, the air is compressed and pushed out through the two discharge ports.
During the third and fourth quarters of the cycle, the process is repeated. This type of
compressor has no leakage and has minimal friction. For smooth operation, the
rotation speed should be about 3000 rpm. The delivery pressure is low (about 5 bar).
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4. Lobe compressor
Fig. Lobe compressor
The lobe compressor is used when high delivery volume but low pressure is needed. It
consists of two lobes with one being driven and the other driving. Figure shows the
construction and working of Lobe compressor. It is similar to the Lobe pump used in
hydraulic systems. The operating pressure is limited by leakage between rotors and
housing. As the wear increases during the operation, the efficiency falls rapidly.
5. Dynamic compressors
Fig. Blower (Centrifugal type)
When very large volume of compressed air is required in applications such as
ventilators, combustion system and pneumatic powder blower conveyors, the dynamic
compressor can be used. The pressure needed is very low in such applications. Figure
6.2.6 shows a typical Centrifugal type blower. The impeller rotates at a high speed.
Large volume of low pressure air can be provided by blowers. The blowers draw the
air in and the impeller flings it out due to centrifugal force. Positive
displacement
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compressors need oil to lubricate the moving parts, whereas the dynamic compressors
have no such need. The efficiency of these compressors is better than that of
reciprocating types.

Air Treatment and Pressure Regulation
1. Air treatment stages
For satisfactory operation of the pneumatic system the compressed
air needs to be cleaned and dried. Atmospheric air is contaminated
with dust, smoke and is humid. These particles can cause wear of the
system components and presence of moisture may cause corrosion.
Hence it is essential to treat the air to get rid of these impurities. The air
treatment can be divided into three stages as shown in Figure.
Fig. Stages of air treatment
In the first stage, the large sized particles are prevented from entering the compressor
by an intake filter. The air leaving the compressor may be humid and may be at high
temperature. The air from the compressor is treated in the second stage. In this stage
temperature of the compressed air is lowered using a cooler and the air is dried using a
dryer. Also an inline filter is provided to remove any contaminant particles present.
This treatment is called primary air treatment. In the third stage which is the
secondary air treatment process, further filtering is carried out. A lubricator introduces
a fine mist of oil into the compressed air. This will help in lubrication of the moving
components of the system to which the compressed air will be applied.
Filters
To prevent any damage to the compressor, the contaminants present in the air need to
be filtered out. This is done by using inlet filters. These can be dry or wet filters. Dry
filters use disposable cartridges. In the wet filter, the incoming air is passed through an
oil bath and then through a fine wire mesh filter. Dirt particles cling to the oil drops
during bubbling and are removed by wire mesh as they pass through it. In the
dry filter the cartridges are replaced during servicing. The wet filters are cleaned
using detergent solution.
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Cooler
As the air is compressed, the temperature of the air increases. Therefore the air
needs to be cooled. This is done by using a cooler. It is a type of heat exchanger.
There are two types of coolers commonly employed viz. air cooled and water
cooled. In the air cooled type, ambient air is used to cool the high temperature
compressed air, whereas in the water cooled type, water is used as cooling
medium. These are counter flow type coolers where the cooling medium flows
in the direction opposite to the compressed air. During cooling, the water vapor
present will condense which can be drained away later.
2.Main line filter
These filters are used to remove the water vapors or solid contaminants present in
the pneumatic systems main lines. These filters are discussed in detail as follows.
Air filter and water trap
Air filter and water trap is used to
 prevent any solid contaminants from entering in the system.
 condense and remove water vapor that is present in the compressed air.
Fig. Air filter and water trap
The filter cartridge is made of sintered brass. The schematic
of the filter is shown in Fig. The thickness of sintered
cartridge provides random zigzag passage for the air to
flow-in which helps in arresting the solid particles. The air
entering the filter swirls around due to the deflector cone.
The centrifugal action causes the large contaminants and
water vapor to be flung out, which hit the glass bowl
and get collected at the bottom. A baffle plate is provided
to prevent the turbulent air from splashing the water into the
filter cartridge. At the bottom of the filter bowl there is a
drain plug which can be opened manually to drain off the
settled water and solid particles. Page
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Refrigerated dryers
Fig. Refrigerated dryers
It consists of two heat exchangers, refrigerant compressor and a separator. The system
circuitry is shown in Figure The dryer chills the air just above 0 °C which condenses
the water vapor. The condensate is collected by the separator. However such
low temperature air may not be needed at the application. Therefore this chilled air is
used to cool the high temperature air coming out from the compressor at heat
exchanger 2. The moderate temperature dry air coming out from the heat exchanger 2
is then used for actual application; whilst the reduced temperature air from compressor
will further be cooled at heat exchanger 1. Thus, the efficiency of the system is
increased by employing a second heat exchanger.
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Chemical dryers
When absolute dry air is needed chemical dryers are used. These dryers are of two
types viz. adsorption dryer and absorption dryer.
Adsorption dryers
Fig. Adsorption dryer
In Adsorption dryers, the moisture collects on the sharp edges of the granular
material. The adsorbing materials can be silicon dioxide (silica gel) or other materials
which exist in hydrated and dehydrated state (copper sulphate, activated
alumina). Moisture from the adsorbing material can be released by heating in
the column as shown in Fig. At a given time, one column will dry the air while the
other column will regenerate the adsorption material by heating and passing low purge
air. The column B dries the air and column C regenerates. The rotary valves are
opened using time clock at regular interval to reverse the process. These
dryers are also called regenerative dryers.
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Absorption dryers
These are also called as deliquescent dryers. Figure shows a schematic of the same.
It uses chemical agents like phosphoric pentoxide or calcium chloride as drying
agents. The moisture in the compressed air chemically reacts with the drying agent.
The agent dissolves to form a liquid compound which collects at the bottom of the
dryer where it can be drained out. The deliquescent agent has to be replenished
regularly as it gets consumed during the drying process.
Fig. Absorption dryer
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3. Lubricators
Fig. Air lubricator
The compressed air is first filtered and then passed through a lubricator in order to
form a mist of oil and air to provide lubrication to the mating components. Figure
shows the schematic of a typical lubricator. The principle of working of venturimeter
is followed in the operation of lubricator. The compressed air from the dryer enters in
the lubricator. Its velocity increases due to a pressure differential between the upper
and lower changer (oil reservoir). Due to the low pressure in the upper chamber the oil
is pushed into the upper chamber from the oil reservoir through a siphon tube with
check valve. The main function of the valve is to control the amount of oil passing
through it. The oil drops inside the throttled zone where the velocity of air is much
higher and this high velocity air breaks the oil drops into tiny particles. Thus a mist of
air and oil is generated. The pressure differential across chambers is adjusted
by a needle valve. It is difficult to hold an oil mixed air in the air receiver as oil may
settle down. Thus air is lubricated during secondary air treatment process. Low
viscosity oil forms better mist than high viscosity oil and hence ensures that oil is
always present in the air.
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4. Pressure regulation
In pneumatic systems, during high velocity compressed air flow, there is flow-
dependent pressure drop between the receiver and load (application). Therefore
the pressure in the receiver is always kept higher than the system pressure. At
the application site, the pressure is regulated to keep it constant. There are three ways
to control the local pressure, these are shown in Figure.
Fig. Types of pressure regulation
In the first method, load X vents the air into
atmosphere continuously. The pressure regulator
restricts the air flow to the load, thus controlling
the air pressure. In this type of pressure regulation,
some minimum flow is required to operate the regulator.
If the load is a dead end type which draws no air, the
pressure in the receiver will rise to the manifold
pressure. These type of regulators are called as ‘non-
relieving regulators’, since the air must pass through
the load.
In the second type, load Y is a dead end load. However
the regulator vents the air into atmosphere to reduce the
pressure. This type of regulator is called as ‘relieving
regulator’.
The third type of regulator has a very large load Z.
Therefore its requirement of air volume is very high and
can’t be fulfilled by using a simple regulator. In such
cases, a control loop comprising of pressure
transducer, controller and vent valve is used. Due to
large load the system pressure may rise above its critical
value. It is detected by a transducer. Then the signal will
be processed by the controller which will direct the valve
to be opened to vent out the air. This technique can be
also be used when it is difficult to mount the pressure
regulating valve close to the point where pressure
regulation is needed.
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5. Relief valve
Fig. Relief valve
Relief valve is the simplest type of pressure regulating device. The schematic of its
construction and working is shown in the Figure. It is used as a backup device if the
main pressure control fails. It consists of ball type valve held on to the valve seat by a
spring in tension. The spring tension can be adjusted by using the adjusting cap. When
the air pressure exceeds the spring tension pressure the ball is displaced from its seat,
thus releasing the air and reducing the pressure. A relief is specified by its span of
pressure between the cracking and full flow, pressure range and flow rate. Once the
valve opens (cracking pressure), flow rate depends on the excess pressure. Once the
pressure falls below the cracking pressure, the valve seals itself.
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6. Non-relieving pressure regulator
In a non-relieving pressure regulator the outlet pressure is sensed by a
diaphragm which is preloaded by a pressure setting spring. If outlet pressure is
too low, the spring forces the diaphragm and poppet to move down thus
opening the valve to admit more air and raise outlet pressure. If the outlet
pressure is too high the air pressure forces the diaphragm up hence reduces the air
flow and causing a reduction in air pressure. The air vents away through the
load. At steady state condition the valve will balance the force on the diaphragm
from the outlet pressure with the preset force on the spring.
Fig. Non-relieving type pressure
regulator
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7. Service units
During the preparation of compressed air, various processes such as filtration,
regulation and lubrication are carried out by individual components. The
individual components are: separator/filter, pressure regulator and lubricator.
Preparatory functions can be combined into one unit which is called as ‘service unit’.
Figure shows symbolic representation of various processes involved in air preparation
and the service unit.
(a) (b)
Fig. (a) Service unit components (b) Service unit
symbol
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Actuators
Actuators are output devices which convert energy from pressurized hydraulic oil or
compressed air into the required type of action or motion. In general,
hydraulic or pneumatic systems are used for gripping and/or moving operations in
industry. These operations are carried out by using actuators.
Actuators can be classified into three types.
1.Linear actuators: These devices convert hydraulic/pneumatic energy into linear
motion.
2.Rotary actuators: These devices convert hydraulic/pneumatic energy into
rotary motion.
3.Actuators to operate flow control valves: these are used to control the flow and
pressure of fluids such as gases, steam or liquid.
The construction of hydraulic and pneumatic linear actuators is similar. However they
differ at their operating pressure ranges. Typical pressure of hydraulic cylinders
is about 100 bar and of pneumatic system is around 10 bar.
1. Single acting cylinder
Fig. Single acting cylinder
These cylinders produce work in one direction of motion hence they are named
as single acting cylinders. Figure shows the construction of a single acting cylinder.
The compressed air pushes the piston located in the cylindrical barrel causing the
desired motion. The return stroke takes place by the action of a spring. Generally the
spring is provided on the rod side of the cylinder.
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2. Double acting cylinder
Fig. Double acting cylinder
The main parts of a hydraulic double acting cylinder are: piston, piston rod, cylinder
tube, and end caps. These are shown in Figure The piston rod is connected to piston
head and the other end extends out of the cylinder. The piston divides the
cylinder into two chambers namely the rod end side and piston end side. The seals
prevent the leakage of oil between these two chambers. The cylindrical tube is
fitted with end caps. The pressurized oil, air enters the cylinder chamber through the
ports provided. In the rod end cover plate, a wiper seal is provided to prevent the
leakage of oil and entry of the contaminants into the cylinder. The combination of
wiper seal, bearing and sealing ring is called as cartridge assembly. The end caps may
be attached to the tube by threaded connection, welded connection or tie rod
connection. The piston seal prevents metal to metal contact and wear of piston head
and the tube. These seals are replaceable. End cushioning is also provided to prevent
the impact with end caps.
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3. Cylinder end cushions
Fig. Cylinder end cushioning
Double acting cylinders generally contain cylinder cushions at the end of the cylinder
to slow down the movement of the piston near the end of the stroke. Figure shows the
construction of actuating cylinder with end cushions. Cushioning arrangement avoids
the damage due to the impact occurred when a fast moving piston is stopped by the
end caps. Deceleration of the piston starts when the tapered plunger enters the
opening in the cap and closes the main fluid exit. This restricts the exhaust flow from
the barrel to the port. This throttling causes the initial speed reduction. During the last
portion of the stroke the oil has to exhaust through an adjustable opening since main
fluid exit closes. Thus the remaining fluid exists through the cushioning valve.
Amount of cushioning can be adjusted by means of cushion screw. A check valve is
provided to achieve fast break away from the end position during retraction motion. A
bleed screw is built into the check valve to remove the air bubbles present in
a hydraulic type system.
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4. Gear motor: a rotary actuator
Rotary actuators convert energy of pressurized fluid into rotary motion. Rotary
actuators are similar to electric motors but are run on hydraulic or pneumatic power.
Fig. Gear motor
It consists of two inter meshing gears inside a housing with one gear attached to the
drive shaft. Figure shows a schematic diagram of Gear motor. The air enters from the
inlet, causes the rotation of the meshing gear due to difference in the pressure
and produces the torque. The air exists from the exhaust port. Gear motors tend to leak
at low speed, hence are generally used for medium speed applications.
5. Vane motor: a rotary actuator
A rotary vane motor consists of a rotor with sliding vanes in the slots provided on the
rotor. The rotor is placed eccentrically with the housing. Air enters from the inlet port,
rotates the rotor and thus torque is produced. Air is then released from the
exhaust port (outlet).
Fig. Vane motor
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6. Limited rotation actuators
It consists of a single rotating vane connected to output shaft as shown in Figure. It is
used for double acting operation and has a maximum angle of rotation of about 270°.
These are generally used to actuate dampers in robotics and material handling
applications. Other type of limited rotation actuator is a rack and pinion type actuator.
Fig. Semi rotary vane type
actuator
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7. Speed control
For an actuator, the operational speed is determined by the fluid flow rate and
the cylinder actuator area or the motor displacement. The speed can only be controlled
by adjusting the fluid flow to the actuator, because the physical dimension of the
actuator is fixed. Since the air is compressible, flow control is difficult as
compared to the hydraulic system. There are various ways of controlling the
fluid flow. One of the methods is discussed as below-
Fig. Speed control by pump
volume
Figure shows the circuit diagram of hydraulic system developed to control the speed
of motion of a piston. Consider a pump which delivers a fluid volume of ‘V’
per minute. The pump has a fixed displacement. The volume of fluid goes either to the
pump or to the actuator. When the direction control valve moves from its
center position the actuator of area ‘A’, the piston moves with a velocity,
v =
𝑉
𝐴
(6.4.1)
If the pump delivery volume ‘V’ can be adjusted by altering swash plate angle of a
piston pump or by using a variable displacement vane pump, no further speed control
will be needed.
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Pneumatic controllers
In automated industrial processes, it is always essential to
keep the process variables such as temperature, flow rate,
system pressure, fluid level, etc. at the desired value for
safety and economical operation. Consider an example
where the flow of water through a pipe has to be kept
constant at some predetermined value (Fig. 6.5.1). Let the
value of flow to be measured is ‘V’ (process variable
PV). This flow rate is compared with the required
flow value say ‘V1’ (set point SP). The difference
between these two values is the error which is sent
to the controller. If any error exists, the controller
adjusts the drive signal to the actuator, informing it to
move the valve to give the required flow (zero error).
This type of control system is called closed loop
control system. It mainly includes a controller, actuator
and a measuring device.
Fig. Closed loop control system
The control can be achieved by using control electronics or by pneumatic
process control. The pneumatic systems are quite popular because they are safe. In the
process industries like refinery and chemical plants, the atmosphere is explosive.
Application of electronics based systems may be dangerous in such cases. Since
the pneumatic systems use air, there are very scant chances of any fire hazards.
Even though electrical actuators are available, but most of the valves employed
are driven by pneumatic signals.
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1. Components of a pneumatic controller
 Flapper nozzle amplifier
 Air relay
 Bellows
 Springs
 Feedback arrangements
Flapper nozzle amplifier
A pneumatic control system operates with air. The signal is transmitted in the form of
variable air pressure (often in the range of 0.2 to 1.0 bar (3-15 psi)) that initiates
the control action. One of the basic building blocks of a pneumatic control system
is the flapper nozzle amplifier. It converts very small displacement signal (in
order of microns) to variation of air pressure. The basic construction of a
flapper nozzle amplifier is shown in Figure 6.5.2
Fig Nozzle flapper amplifier
Constant air pressure is supplied to one end of the pipeline. There is an orifice at this
end. At the other end of the pipe, there is a nozzle and a flapper. The gap between the
nozzle and the flapper is set by the input signal. As the flapper moves closer to the
nozzle, there will be less airflow through the nozzle and the air pressure inside
the pipe will increase. On the other hand, if the flapper moves further away
from the nozzle, the air pressure decreases. At the extreme, if the nozzle is open
(flapper is far off), the output pressure will be equal to the atmospheric pressure.
If the nozzle is blocked, the output pressure will be equal to the supply pressure.
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Air Relay
The major limitation of a flapper nozzle amplifier is its limited air handling
capacity. The variation of air pressure obtained cannot be used for any useful
application, unless the air handling capacity is increased. It is used after the
flapper nozzle amplifier to enhance the volume of air to be handled. The principle
of operation of an air relay can be explained using the schematic diagram shown in
Figure 6.5.3. It can be seen that the air relay is directly connected to the
supply line (no orifice in between). The output pressure of the flapper nozzle
amplifier (p2) is connected to the lower chamber of the air relay with a
diaphragm on its top. The variation of the pressure p2 causes the movement (y)
of the diaphragm. There is a double-seated valve fixed on the top of the
diaphragm. When the nozzle pressure p2 increases due to decrees in xi, the
diaphragm moves up, blocking the air vent line and forming a nozzle between the
output pressure line and the supply air pressure line. More air goes to the output line
and the air pressure increases. When p2 decreases, the diaphragm moves
downwards, thus blocking the air supply line and connecting the output port to
the vent. The air pressure will decrease.
Fig Air relay
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2. Types of pneumatic controllers
Following is the list of variants of pneumatic controllers.
 Proportional only (P) controller
 Proportional-Derivative (PD) controller
 Proportional-Integral (PI) controller
 Proportional-Integral-Derivative (PID) controller
Proportional only (P) controller
The simplest form of pneumatic controller is proportional only controller. Figure 6.5.4
shows the pneumatic circuit of ‘proportional only’ controller. The output signal is
the product of error signal multiplied by a gain (K).
Output = (Error * gain) (6.5.1)
Fig. Proportional only
controller
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Consider the pneumatic system consisting of several pneumatic components, viz.
flapper nozzle amplifier, air relay, bellows and springs, feedback arrangement.
The overall arrangement is known as a pneumatic proportional controller as
shown in Figure.
Proportional only (P) controller elements
It acts as a controller in a pneumatic system generating
output pressure proportional to the displacement at one
end of the beam. The action of this particular controller
is direct, since an increase in process variable signal
(pressure) results in an increase in output signal (pressure).
Increasing process variable (PV) pressure attempts to push
the right-hand end of the beam up, causing the baffle
to approach the nozzle. This blockage of the nozzle causes
the nozzle’s pneumatic backpressure to increase, thus
increasing the amount of force applied by the output
feedback bellows on the left- hand end of the beam and
returning the flapper (very nearly) to its original position. If
we wish to reverse the controller’s action, we need to
swap the pneumatic signal connections between the input
bellows, so that the PV pressure will be applied to the upper
bellows and the SP pressure to the lower bellows. The ratio
of input pressure(s) to output pressure is termed as a gain
(proportional band) adjustment in this mechanism.
Changing bellows area (either both the PV and SP bellows
equally, and the output bellows by itself) influences this
ratio. Gain also affects by the change in output bellows
position. Moving the fulcrum left or right can be used to
control the gain, and in fact is usually the most convenient to
engineer.
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Proportional-Derivative (PD) controller
A proportional-derivative (PD) controller is shown in Figure. To add
derivative control action to a P-only controller, all we need to place a restrictor
valve between the nozzle tube and the output feedback bellows, causing the bellows
to delay filling or emptying its air pressure over time.
Proportional-Derivative (PD) controller
If any sudden change occurs in PV or SP, the output
pressure will saturate before the output bellows has the
opportunity to equalize in pressure with the output signal
tube. Thus, the output pressure “spikes” with any sudden
“step change” in input: exactly what we would expect
with derivative control action. If either the PV or the SP
ramps over time, the output signal will ramp in direct
proportion (proportional action). But there will be an
added offset of pressure at the output signal in order to
keep air flowing either in or out of the output bellows
at a constant rate to generate the necessary force to
balance the changing input signal. Thus, derivative action
causes the output pressure to shift either up or down
(depending on the direction of input change) more than
it would with just proportional action alone in response
to a ramping input.
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Proportional-Integral (PI) controller
In some systems, if the gain is too large the system may become unstable. In these
circumstances the basic controller can be modified by adding the time integral of
the error to control the operation Thus the output can be given by an equation,
Block diagram of P-I controller
The Ti is a constant called integral time. As long as there is an error the output of the
controller steps up or down as per the rate determined by Ti. If there is no error then
the output of the controller remains constant. The integral term in the above equation
removes any offset error.
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Figure shows the configuration of pneumatic proportional plus integral
controller. Integral action requires the addition of a second bellows (a “reset” bellows,
positioned opposite the output feedback bellows) and another restrictor valve to the
mechanism.
Fig. Proportional-Integral (P-I) controller
As the reset bellows fills with pressurized air, it begins to
push down the left-hand end of the force beam. This
forces the baffle closer to the nozzle, causing the
output pressure to rise. The regular output bellows has
no restrictor valve to impede its filling, and so it
immediately applies more upward force on the beam with
the rising output pressure. With this greater output
pressure, the reset bellows has an even greater “final”
pressure to achieve, and so its rate of filling continues.
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Proportional-Integral-Derivative (PID) controller
Three term pneumatic control can be achieved using a P-I-D controller. Here the
action of the feedback bellows is delayed. The output is given by,
The terms gain K, derivative time Td, integral time Ti which can be set by beam
pivot point and two bleed valves. This is a combination of all the three controllers
described above. Hence it combines the advantages of all three. A derivative
control valve is added to delay the response at feedback bellow. Addition of
derivative term makes the control system to change the control output quickly when
SP and PV are changing quickly. This makes the system more stable.
Proportional-Integral-Derivative (P-I-D)
controller
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Advantages of pneumatic controllers
Simplicity of the components and no complex structure
Easy maintainability
Safe and can be used in hazardous atmospheres
Low cost of installation
Good reliability and reproducibility
Speed of response is relatively slow but steady
Limited power capacity for large mass transfer
Limitations of pneumatic controllers
Slow response
Difficult to operate in sub-normal temperatures
Pipe-couplings can give rise to leaks in certain ambient conditions
Moving parts - more maintenance

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Applications of pneumatic systems
In this section we will study the application of various
pneumatic components in designing a pneumatic circuit. The
graphical symbols of pneumatic components and equipments have
already been discussed in lecture 7 of module 5.
1.Case study A
Consider a simple operation where a double-acting cylinder is used
to transfer parts from a magazine. The cylinder is to be advanced
either by operating a push button or by a foot pedal. Once the
cylinder is fully advanced, it is to be retracted to its initial position.
A 3/2-way roller lever valve is to be used to detect the full
extension of the cylinder. Design a pneumatic circuit for the above-
mentioned application.
Components used
The pneumatic components which can be used to implement the
mentioned task are as follows:
 double acting cylinder
 3/2 push button valve
 3/2 roller valve
 shuttle valve
 3/2 foot pedal actuated valve
 5/3 pneumatic actuated direction control valve
 compressed air source and connecting piping

Working
Pneumatic circuit for shuttle valve operation
Figure shows the proposed circuit diagram. As the problem
stated, upon actuation of either the push button of valve (S1)
or the foot pedal valve (S2), a signal is generated at 1 or 1(3)
side of the shuttle valve. The OR condition is met and the
signal is passed to the control port 14 of the direction control
valve (V2). Due to this signal, the left position of V2 is
actuated and the flow of air starts. Pressure is applied on the
piston side of the cylinder (A) and the cylinder extends. If the
push button or pedal valve is released, the signal at the
direction control valve (V2) port is reset. Since DCV (V2) is a
double pilot valve, it has a memory function which
doesn’t allow switching of positions. As the piston reaches
the rod end position, the roller valve (S3) is actuated and a
signal is applied to port 12 of the DCV (V2). This causes
actuation of right side of DCV (V2). Due to this actuation,
the flow enters at the rod- end side of the cylinder, which
pushes the piston towards left and thus the cylinder retracts.
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2. Case study B
A plastic component is to be embossed by using a die which is powered by a double
acting cylinder. The return of the die is to be effected when the cylinder rod has
fully extended to the embossing position and the preset pressure is reached. A
roller lever valve is to be used to confirm full extension. The signal for retracting
must only be generated when the piston rod has reached the embossing position.
The pressure in the piston chamber is indicated by a pressure gauge.
Components used
The pneumatic components to be used to implement this task are:
 double acting cylinder
 3/2 push button valve
 3/2 roller valve
 shuttle valve
 5/3 pneumatic actuated direction control valve
 pressure sequence valve
 compressed air source
 pressure gauge and connecting piping
Working
Pneumatic circuit for embossing
application
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The proposed pneumatic circuit diagram for embossing application is shown in fig. When the push
button valve (S1) is pressed, the flow takes place through the valve and a signal is sent to
the control port 14 of the direction control valve (V2). The left position of the direction control
valve (V2) is switched-on and the flow enters at the piston-end of the cylinder (A). It
causes the extension of the cylinder. Even if the push button is released the position of the DCV
(V2) will not change due to its memory function. As the piston nears the end position, the roller
valve (S2) is actuated and pressure line is connected to the pressure sequence valve (V1). During
the embossing process the pressure at the piston-side of the cylinder (A) increases. This increase in
pressure is indicated by the pressure regulator (Z1). When the pressure reaches to its pre-set
value in the pressure sequence valve (V1), the 3/2 valve of pressure sequence valve switches and
the signal is applied to port 12 of the DCV (V2). The right position of the DCV is
actuated and the piston retracts. During the retracting movement, the roller valve (S2) is
released and the signal at the control port 12 of the DCV (V2) is reset and the pressure sequence
valve is also reset.
3. Case study C
Sequencing application
In process control applications such as sequencing, the Pneumatics systems are generally
employed. Electrical components such as relays, programmable logic controllers are used to
control the operations of Pneumatic systems. A simple example of a pneumatic sequencing is
shown in Figure
Cylinder sequencing- Oscillating
cylinder
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Components used
The components used in the circuit are: double acting cylinder, 3/2 roller lever
valve, 5/3 pilot operated direction control valve and a 3/2 push button valve. By
using this circuit, a continuous to and fro motion of the actuator is obtained.
Working
When the 3/2 push button is actuated, the air flows from the source through the
push button valve to the 3/2 roller valve (S1). The roller valve is already actuated by
the cylinder when the piston rod hits the lever of S1. Therefore, there is continuous
flow to the 5/3 pilot operated direction control valve (DCV). The flow given to the
pilot line 14 actuates the first position of DCV. The air flows from port 1-4
pushes the piston head which causes the extension of the cylinder. As the cylinder
fully extends it actuates the 3/2 roller lever valve (S2). The roller valve is actuated
and air flows through the valve to the 5/3 DCV. The air enters the DCV
through pilot port 12 actuating the second position. Hence the air flows from port
1-2 to the actuator rod end, causing its retraction. The cylinder reciprocates till the
supply is stopped. In this way, we can achieve the sequencing operation by
controlled actuation of various valves in a pneumatic system.

Pneumatic system in Mechatronics and Engineering

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