4.pneumatic and electropneumatic systems

Prepared by
David Blessley S
AP/Mechanical Engg.,
Kamaraj College of Engineering and Technology,
Virudhunagar
Hydraulics and Pneumatics
Pneumatic and Electropneumatic systems
Kamaraj College of Engineering and Technology, Virudhunagar

Boyle’s law
The volume of the gas will change
inversely with the absolute pressure of the
gas, if the temperature of a given amount
of gas is held constant.
𝑉1
𝑉2
=
𝑝2
𝑝1
Charles’ law
The volume of the gas will change
directly proportional to the absolute
temperature if the pressure on a given
amount of gas is held constant.
𝑉1
𝑉2
=
𝑇1
𝑇2
Constant Temperature Process
Constant Pressure Process

Gay Lussac’s law
The pressure exerted by the gas is
directly proportional to its absolute
temperature if the volume of a given gas is
held constant
𝑝1
𝑝2
=
𝑇1
𝑇2Constant Volume Process
General Gas law
Boyle’s, Charles’ and gay lussac’s laws can be combined into a single
general gas law as defined by
𝑝1 𝑉1
𝑇1
=
𝑝2 𝑉2
𝑇2

Compressor
• A compressor is a machine that compresses air or gas from low inlet pressure (atm)
to high desired pressure.
• This can be accomplished by reducing the volume of gas
• Generally positive displacement, reciprocating piston type, rotary screw type and
rotary vane type are its classification
• Piston type reciprocating compressors
• Most commonly used type and operating principle is similar to IC engines
• It consists of cylinder, cylinder head, piston, piston rings, inlet and exhaust valves,
crank, crankshaft, connecting rod etc.,
• A prime mover generally motor is used to run the compressor. The rotary motion to
the crank shaft is converted to reciprocating motion of piston through crank and
connecting rod
• During downward stroke, pressure inside the cylinder falls below atmospheric
pressure due to pressure difference and the inlet valve is opened
• As the piston moves upwards, the inlet valve gets closed and the pressure increase
makes the exhaust valve to open and delivers the air to the outlet

Multi stage compressors
• If the compression of air from initial to final pressure is carried out in more than one cylinder, then the
compressor is known as multi stage compressor
• If a single stage compressor can give compressed air of about 5 bar, the compressed air temperature can
increase to 200OC which in turn increases the power required to drive the compressor. Thus effective
cooling is needed
• When multi stage compressors are used, it reduces the input power requirements and increases
efficiency of the compressor
Staging: Staging is dividing the total pressure among two or more cylinders by allowing the outlet from one cylinder into the
inlet of the next cylinder and so on.

Screw Compressor
• Similar to screw pump
• Compression is accomplished by rolling the trapped
air into a progressively smaller volume as the screws
rotate
• The rotors turn freely with controlled clearance
between both rotors and the housing
• A precisely measured amount of filtered and cooled
air is injected into the compression chamber, mixing
with the air as it is compressed
• Single stage compressors are available with capacities
1450 cfm and pressures of 120psi

Rotary Vane Compressor
• In this, a cylindrical slotted rotor turns inside
of a stationary outer casing
• Each rotor slot contains a rectangular vane,
which slides in and out of the slot due to
centrifugal force
• As the rotor turns, air is trapped and
compressed between the vanes and then
discharged through a port to the receiver
• Vane compressor can operate up to approx.
50psi in single stage and 150 psi in two stage
design

• Air filter
• The function of air filter is to remove contaminants
from the air before it reaches the pneumatic
components such as valves and actuators
• It can filter particles in the range of 5μm to 50μm
• The air entering the filter swirls around due to the
deflector cone
• The centrifugal action causes the large contaminants
and water vapour to be flung out, which gets collected
at the bottom
• A baffle late 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

• Lubricator
• A lubricator ensures proper lubrication of internal
moving parts
• The compressed air enters the lubricator, its
velocity increases due to pressure difference
between upper and lower chamber
• 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 high and this high velocity air
breaks the oil drops into tiny particles. Thus a mist
of air and oil is generated.
• 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.Kamaraj College of Engineering and Technology, Virudhunagar

• Air pressure regulator
• The function of the air pressure regulator is to regulate the
pressure of the incoming compressed air
• It consists of diaphragm, valve, main and dampening springs
• The diaphragm allows the proper amount of movement for
opening and closing at the valve seat
• When the adjusting screw is in the fully retracted position,
the valve is closed. When the adjusting screw is turned to
compress the adjusting and dampening springs, the valve is
opened and allows air from inlet to outlet port
• The pressure of the outlet air depends upon the size of the
valve opening that is maintained. Which can be determined
by the compression of the adjustable spring
• Vent hole is provided to let out the undesirable excessive
outlet pressure
• Dampening spring is provided to act as a dampening device
needed to stabilize the pressure.

• FRL Unit
• The compressed air is first filtered and then
regulated to a specific pressure and made
to pass through a lubricator for lubricating
the oil
• Usually, filter, regulator, lubricator are
placed in the inlet line to air circuit. But,
more often they are used in the form of a
combined unit
• This combination of filter, regulator and
lubricator is called FRL Unit
Simplified symbol for FRL Unit

• Mufflers or pneumatic silencers
• The function of muffler is to reduce the
noise caused by a rapidly exhausting air
stream flowing into the atmosphere
• Noise in air system may cause nervous
tension, mental fatigue, lack of
concentration and dissatisfaction among
the operators
• The exhaust air stream enters one end and
passes out at another end passing through
a series of baffles
• This baffles are perforated with a large
number of small holes. The outer shell acts
as a barrier and helps guide the stream
toward the exit to the atmosphere.

Air control valves
• Air control valves are used to control the pressure, flow rate and direction of air in
pneumatic circuits
Pressure regulator
• The purpose of pressure regulator is to set the
working pressure of the particular circuit
• Sometimes it is installed within a circuit to
provide two or more different pressure levels
for separation portions of the unit
• The desired pressure is established by a T
handle which exerts a compressive force on
the spring. The spring transmits a force to the
diaphragm which regulates the opening an
closing of the control valve
• This regulates the airflow rate to establish the
desired downstream pressure

Check valve
• The purpose of check valve is to allow free flow of compressed air in one
direction only and to prevent flow in opposite direction
• When flow is in forward direction, the compressed air pushes the disk seal
and allows free flow
• If flow is attempted in the opposite direction, the seal is in closed condition
and thus no flow is permitted

• Shuttle valve
• Also known as double check valve
• Shuttle valve consists of two inlet ports and a outlet port
• If the pressure in the right inlet port is greater than the left, the spool closes the
left port
• When the pressure at the left inlet port becomes greater than the right, the spool
moves and closes the right port and allows flow through the left port

• Direction control valves
• Its purpose is to control the direction of flow of pneumatic fluids
• DCVs are used to start, stop and regulate the direction of air flow
• DCVs are classified
• Based on the construction as
• Poppet valves
• Sliding spool valves
• Rotary spool valves
• Based on the number of ports present
• Two way valves
• Three way valves
• Four way valves
Sliding Spool type
Rotary type
poppetKamaraj College of Engineering and Technology, Virudhunagar

• Two way valves
• Basically On-off type valve (similar to water
faucet which allows flow or stops flow
manually)
• A normally open two way valve allows flow
in its normal position and restricts when it
is actuated
• A normally closed valve restricts flow in its
normal position and allows flow when
actuated

• Three way valves
• A three way valve has three ports i.e., inlet, exhaust and a cylinder port
• One flow port is connected to the any of the other two ports. This valve may be used
to pressurize one port and exhaust the other port
• These valves can be used as a pilot relay to operate the other valves

• Four way valves
• In four way valve, inlet port is connected to cylinder port A and cylinder port B is
connected to exhaust port
• When the valve is actuated, they become unseated and inlet port is connected to
cylinder port B and cylinder port A is connected to exhaust port.

• Quick exhaust valve
• The purpose of this valve is to exhaust the cylinder to
the atmosphere quickly
• Basically spring return single acting pneumatic cylinder to
increase the speed
• The principle behind this valve is by reducing the
resistance to flow of air, the air can be exhausted to
atmosphere quickly
• It consists of a movable disc and three ports namely inlet
port (P), exhaust port ® and a cylinder port (A)
• When the air flowing to the cylinder from the DCV is
applied at port P, then the movable disc covers the
exhaust port R. thus the compressed air passes from port
P to port A
• When the DCV is switched to its next position, air from the cylinder port A pushes the
movable disc to mask the inlet port P whereby the air is exhausted through the
exhaust port R immediately
• Thus the resistance to piston movement is reduced considerably and the speed of the
piston is accelerated proportionately.Kamaraj College of Engineering and Technology, Virudhunagar

• Flow control valve
• The purpose of flow control valve is to regulate the volumetric flow of the
compressed air to different parts of a pneumatic system
• The spring loaded disk allows free flow in one direction and an adjustable or
controlled flow in the opposite direction
• A tapered brass stem controls the flow adjusted by controlling the flow through the
cross hole disc in the disk.

Electropneumatic system
• An electropneumatic system may consist of a pneumatic driving element
and an electromechanical control part
• The electromechanical parts used may be a solenoid, push button switches,
relays, contactors, limit switches etc.,
• A pneumatic control system with solenoid operated valves may be called
electropneumatic system. Solenoid operated direction control valves, limit
switches, pressure switches are other examples

• Basic electrical devices
• Manually actuated push button switches
• Limit switches
• Pressure switches
• Solenoids
• Relays
• Timers
• Temperature switches

• Push button switches
• A push button is a switch used to close or open an electric control circuit.
• They are primarily used for starting and stopping of operation of machinery. They
also provide manual override when the emergency arises.
• Push button switches are actuated by pushing the actuator into the housing. This
causes set of contacts to open or close.
• Fig. shows four common types of push button switches
• Single pole single throw NO
• Single pole single throw NC
• Double pole single throw
• Double pole double throw

• Limit switches
• Any switch that is actuated due to the position of a fluid power component (usually a
piston rod or hydraulic motor shaft or the position of load is termed as limit switch.
• The actuation of a limit switch provides an electrical signal that causes an
appropriate system response.
• Limit switches perform the same function as push button switches. Push buttons are
manually actuated whereas limit switches are mechanically actuated.

• Pressure switches
• A pressure switch is a pneumatic-electric
signal converter. Pressure switches are
used to sense a change in pressure, and
opens or closes an electrical switch when
a predetermined pressure is reached.
• Bellow or diaphragm is used to sense the
change of pressure. Bellows or
Diaphragm is used to expand or contract
in response to increase or decrease of
pressure.
• Figure shows a diaphragm type of
pressure switch. When the pressure is
applied at the inlet and when the pre-set
pressure is reached, the diaphragm
expands and pushes the spring loaded
plunger to make/break contact.

• Solenoids
• Solenoids are electromagnets that provide a push or pull force to operate fluid
power valves remotely. Electrically actuated directional control valves are switched
with the aid of solenoids.
• They can be divided into two groups:
• Spring return valves only remain in the actuated position as long as current flows through the
solenoid
• Double solenoid valves retain the last switched position even when no current flows through the
solenoid.
• In the initial position, all solenoids of an electrically actuated DCVs are de-energized
and the solenoids are inactive. A double valve has no clear initial position, as it does
not have a return spring.
• The possible voltage levels for solenoids are 12 V DC, 12V AC, 12 V 50/60 Hz, 24V
50/60 Hz, 110/120V 50/60 Hz, 220/230V 50/60 Hz.

• Relays
• Relays are switches whose contacts open or close when their corresponding coils are
energized
• The use of relays also provides interlock capability, which prevents the accidental
energizing of two solenoids at the opposite ends of a valve spool.
• Relays are commonly used to open or close the contacts and thereby energize or
deenergize solenoids which operate at a high current level

• Timer Or Time delay relays
• Timers are required in control systems to effect time delay between work operations.
• This is possible by delaying the operation of the associated control element through a
timer.
• There are two types of time relay
• Pull in delay ( on –delay timer)
• Drop –out delay (off delay timer)
• In the on-delay timer, when push button PB is pressed (ON), capacitor is charged
through potentiometer as diode D is reverse –biased.
• The time taken to charge the capacitor, depends on the resistance of the potentiometer
and the capacitance of the capacitor.
• By adjusting the resistance of the potentiometer, the required time delay can be set.
When the capacitor is charged sufficiently, coil K is energized, and its contacts are
operated after the set time delay.
• When the push button is released (OFF), the capacitor discharges quickly through a
small resistance as the diode by passes resistor, and the contacts of relay return to their
normal position without any delay.
• In the off-delay timer, the contacts are operated without any delay when the push
button is pressed (ON). The contacts return to the normal position after the set delay
when the push button is releasedKamaraj College of Engineering and Technology, Virudhunagar

• Temperature Switch
• Temperature switches automatically senses a change in temperature and opens
or closes an electrical switch when a predetermined temperature is reached.
• This switch can be wired either normally open or normally closed.
• Temperature switches can be used to protect a fluid power system from serious
damage when a component such as a pump or strainer or cooler begins to
malfunction.

Ladder diagram
• A ladder diagram is a representation of hardware connections between switches,
relays, solenoids etc., which constitute the basic components of an electrical
control system
• A ladder diagram is nothing but an electrical diagram showing the hardware
connections between the various electrical control devices
• Ladder diagram provide a circuit designer with a
practical means to examine input process and out
functions to quickly plan the circuit layout design
for a particular hydraulic or pneumatic application
Legs
Rungs

• Reciprocation of a cylinder using limit switches
• When the start button is pressed, relay coil 1-CR gets energized and closes both the contacts of 1-CR
• The upper 1-CR serves to keep coil 1-CR energized even though the start button is released
• The lower 1-CR closes to energize solenoid A to extend the cylinder
• When the piston end actuates limit switch 1-LS, the coil 1-CR gets deenergized and the valve returns to spring offset
mode and cylinder retracts
• The operation can be repeated by pressing START button switch manually.
• The STOP button is an emergency button to stop the whole processKamaraj College of Engineering and Technology, Virudhunagar

• Reciprocation of a double acting cylinder using pressure switches
• When SOL A is energized, the valve is shifted to
left mode and forces air to the blind end of the
cylinder
• When cylinder extension is maximum, the
pressure buildup actuates limit switch 1-PS and
thus energizes SOL B
• When SOL B is energized, the valve is shifted to
right mode and forces air to the rod end and the
cylinder retracts
• When cylinder retraction is maximum, the
pressure buildup actuates limit switch 2-PS and
thus energizes SOL A and the reciprocation
continues

• Reciprocation of a cylinder using limit switches
• When SOL A is energized, the valve is shifted to
left mode and forces air to the blind end of the
cylinder
• When cylinder extension is maximum, limit
switch 2-LS gets actuated and energizes SOL B
• When SOL B is energized, the valve is shifted to
right mode and forces air to the rod end and the
cylinder retracts
• When cylinder retraction is maximum, the limit
switch 2-PS gets actuated and energizes SOL A
and the reciprocation continues

• Dual cylinder sequencing circuit
• When STSRT button I pressed, SOL A gets energized
• When SOL A energizes, the valve is shifted to left mode and air enters
through the blind end of cylinder 1 and activates limit switch 1-LS
• Actuation of 1-LS energizes SOL C and the valve 2 shifts to its left mode and
supplies air to the blind end of cylinder 2. The cylinder extension actuates
limit switch 2-LS
• When limit switch 2-LS is activated, this energizes SOL B and the valve 1
shifted to its right mode and causes retraction of cylinder 1
• As cylinder 1 retraction continues it deactuates limit switch 1-LS which
deenergizes limit SOL C and the valve 2 is shifted right by spring

• Regenerative circuit of a Double acting Pneumatic cylinder
• When switch SW-1 is manually shifted to EXTEND
position, the solenoid SOL A gets energized and the valve
is shifted to it left mode
• This allows air to enter through the blind end of the
cylinder and the cylinder extension starts
• Air from the rod end passes through the check valve V3
and join with the incoming air. Thus it facilitates rapid
extension
• When the cylinder reaches its maximum position,
pressure buildup arises which is sensed by the pressure
switch 1-PS
• When the pressure switch 1-PS gets activated, this
energizes coil 1-CR and SOL C
• When SOL C is energized, the V2 valve is shifted to its left
mode and exposes the air to atmosphere
• Relay contacts 1-CR hold SOL C energized during the slow
extension of the cylinder to prevent any fluttering of the
pressure switch
• When the solenoid is set to UNLOAD position, all the
solenoids and coil gets deenergized

Fluidics
• The term fluidics is derived from the words fluid and logic
• The fluidic devices perform control function by a stream of gas or
liquid directing itself through precision channels
• Thus, fluidics is the technology that utilizes fluid flow phenomena in
components and circuits to perform a wide variety of control functions
such as sensing, logic, memory, timing and interfacing to other control
media

Coanda effect
When a stream of fluid meets other stream, the effect is to change its
direction of flow and effect is the fluid sticks to the wall
• When there is greater availability of this entraining air from one side than from the other ,
then a small vertex area ( having a low pressure) is created near the nozzle exit. This low
pressure area tends to attract the free jet, distorting it and pulling it toward the angled
wall
• Once this free jet is pulled tangent to the exit wall, the continued existence of this
vortex tends to hold the jet attached to that wall. Hence this phenomenon is termed as
wall attachment phenomenon
• When a free fluid jet (i.e., a stream
of air/liquid) passing through an
orifice will continue in a given
direction, pulling in with it any
available fluid that surrounds the
jet as it leaves the orifice.

• When a finger is held near the stream of water flowing at from the water tap, then one
can see that the water attaches itself to the figure as shown. This is due to Coanda effect

Basic fluidic devices
• Bistable flip flop
• A basic bistable flip-flop provides controlled assurance as to which of the
two output ports will deliver the power stream.
• In this device, the air flow will be from
input power stream (Ps) to either the
O1 output port or the O2 output port,
depending on the existence of a signal
from either C1 channel or C2 channel.
• This device operates on the Principle
of the low pressure vortex being
created on one side or the other of
the element, creating the wall-
attachment effect toward either the
O1 or O2 output ports. Kamaraj College of Engineering and Technology, Virudhunagar

Flipflop with startup preference
• flip-flop with start-up preference is used in applications where a specific
output is required when the power supply is first turned ON and all controls
are OFF.
• (+) sign indicates that the output O1 is preferred over O2. this can be
accomplished by building the splitter slightly off center. That’s why when the
flipflop first receives its power supply, the wall attachment is preferred to
O1 output

SRT flip flop (SET-RESET)
• In addition to all the capabilities of a basic flip flop, a SRT flip flop can
also switch by applying a signal to the trigger port
• In fig. The letters S and R stand for SET and RESET respectively. They
perform as regular control signals similar to C1 and C2
• The letter T stands for Trigger, whenever trigger signal is applied, it
switches the output

OR/NOR gate
• Here, O1 represents OR out put and O2 represents NOR output
• OR – pressure at one or any combination of the control parts
• NOR – pressure at none of the control parts
• A control signal at any one or any combination of these ports will switch the device
to the O1 output
• With all control signals OFF, the device will switched to O2 output

AND/NAND gate
• The operation of AND gate is similar to the NOR gate, except that the NOR gate is
used to determine when none of the controls is present, whereas the AND gate
is used to determine when all the control signals are present
• Both C1 and C3 control signals must be present to get an output at the O1 port.
The absence of either or both will result in the stable output at the O@ port

Exclusive OR gate
• An exclusive OR gate provides output only
when C! or C3 o C5 or C7 is ON
• The exclusive OR gate is obtained by
feeding the output of a het interaction
cavity into the control port of an OR gate
• When control signal C1 or C3 is ON, the jet
interaction cavity produces an output at
O2 port.
• When both C1 and C3 control signals are
ON, then their interaction causes them to
vent to the atmosphere and hence there is
no output at port O2

Programmable Logic controller (PLC)
• A programmable logic controller can be defined as a digital electronic device
that uses a programmable memory to store instructions and to implement
functions such as logic, sequencing, timing, counting and arithmetic in order
to control machines and processes
• PLC vs computers
• PLCs are rugged and designed to withstand vibrations, temperature, humidity and
noise whereas computers are not
• The interfacing for inputs and outputs is inside the controller
• In PLCs, programming can be done by using simple logic and switching functions
whereas in computers, it’s not the case

• Elements of PLC
• Central processing unit (CPU)
• Programmer/Monitor
• Input/Output (I/O)

• Central processing unit
• The CPU controls and processes all the operations within the PLC
• The CPU receives input data from various sensing devices such as switches, executes the
stored program and delivers corresponding output signals to various load control devices such
as relay coils and solenoids
• It consists a microprocessor with a fixed memory and a variable memory i.e., ROM and RAM
• Programmer/Monitor
• The programmer/monitor unit allows the user to enter the desired programme into the RAM
• The programme which is entered in relay logic determines the sequence operation of the
symbol to be controlled
• Input/Output module
• This module interfaces between the fluid power system input sensing and output load devices
and the CPU
• The purpose of the I/O module is to transform the various signals received from or sent to the
fluid power interface devices such as push button switches, pressure switches, limit switches,
solenoid coils, motor relay coils and indicator lights

• PLC Ladder programs for logic functions
AND Logic
NAND LogicNOR Logic
OR Logic

4.pneumatic and electropneumatic systems

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