Download DRIVE SYSTEM ENT 271

HYDRAULICS & PNEUMATICS
Introduction to
Electro-pneumatics
Presented by: Dr. Abootorabi
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Introduction to electro-pneumatics
 Electro-pneumatic is widely used in many areas of industrial
automation, production, assembly, and packaging systems worldwide.
These systems are driven by electro-pneumatic control systems. The
following figures show different applications of electro-pneumatic
machines.
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different applications of electro-pneumatic machines
Milk filling machine
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different applications of electro-pneumatic machines
Yogurt filling machine
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Introduction to electro-pneumatics
 In electro-pneumatics, the pneumatic components are controlled by
using electrical and electronic circuits. Electronic and electromagnetic
sensors, electrical switches and industrial computers are used to
replace the manual control of a pneumatic system.
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Signal flow in electro-pneumatic control system
Command execution
Signal output
Signal processing
Signal Input
Next
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Signal flow in electro-pneumatic control system
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Advantages of electro-pneumatic systems:
1- Greater reliability:
Less moving parts subjected to wear compared to mechanical control
systems.
2- Reduced installation complexity:
Less components and hoses, leads to less effort in planning and
commissioning especially with large and complex systems.
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Advantages of electro-pneumatic systems:
3- The control system can be easily modified and adapted:
It is easier to change wiring and modify programs rather than changing
mechanical components and hose networks.
 Example: the AND gate is replaced with logic and through using
electrical switches.
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Advantages of electro-pneumatic systems:
4- Easy handling: Less complexity
5- Secure mounting: Fewer hoses
6- Environmentally-friendly coupling system: Less lubrication require
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Components of electro-pneumatic system
 The electro pneumatic system is normally consists of the following
items:
1.
DC power supply
2.
Switches
3.
Relays
4.
Solenoid valves
5.
Sensors
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DC power supply
 The power supply is used to reduce and convert the 230 V AC to a 24
V DC.
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DC power supply
 The power supply components which are shown below have the
following functions:
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The transformer
 The transformer reduces the main voltage (230 to 24 volt).
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The rectifier
 The rectifier converts the AC voltage to DC voltage.
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The stabilizer
 The stabilizer is used to smooth and maintain constant voltage at the
output.
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Switches
 Switches are installed in an electric circuit to connect or interrupt the
electric current.
 These switches are divided into:
1. Control switches: keep the selected position such as detent switches.
2. Push button switches: maintain the selected position as long as the
switch is activated.
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Switches
 In this module, three types of switches will be discussed:
Push button
switches
Detent
switches
Limit
switches
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Push button switches
 These switches are activated manually and used to connect or
disconnect the electric current in the control circuit. There are three
types of the push button switches:
Normally
opened contact
(make)
Normally closed
contact
(break)
Changeover
contact
(two-way)
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Normally opened contact (make)
 In the case of a normally opened switch, the circuit is open if the
switch is in its initial position.
 Pressing the pushbutton results in closing the circuit and then the
current will flow to load. When the plunger is released the spring will
returns the switch to its initial position.
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Normally closed contact (break)
 In the case of the normally closed switch, the circuit is closed when
the switch is in its initial position. The circuit is interrupted by pressing
the pushbutton. The figure shows the ISO symbol of the push button
N/C.
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Changeover contact (two-way)
The changeover contact, shown below combines the function of the
normally open and normally closed. Changeover contacts are used to
close one circuit and open another circuit in one switching operation.
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1
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Push button switches
 In many labs, these types of switches are combined in one switch
block as illustrated in the figure below:
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Detent switches
 These switches keep the selected position; the switch position
remains unchanged until a new switch position is selected. It is called
detent switch or a latching switch.
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Detent switches
 The figures below show the ISO symbol of the normally open detent
switch and normally closed detent switch respectively.
 Detent switches also designed to be as normally open, normally
closed or changeover switches.
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Detent switches
 In many labs, the detent switches are included in the same switch
block with pushbutton switches, as shown in the figure below:
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Limit switches
 The limit switch (shown below) is actuated when a machine part or a
work-piece is in a certain position. Normally, actuation is affected by a
cam or cylinder piston.
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Limit switches
 Limit switches are normally changeover contacts and can be
connected according to the required control circuit. The limit switch can
be used in circuit according to one of the following:
 Normally open switch
 Normally closed switch
 Changeover switch
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Relays
 A relay is defined as an electromagnetically actuated switch.
 When the voltage is applied to a solenoid coil terminals (A1, A2) in
Figure below, it will become an electromagnet which in turn attracts the
contacts of the relay either closing or opening them.
 The spring returns the contacts to the initial position immediately after
disconnecting the voltage at the coil terminals.
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Construction of the Relay
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Relays
 An ISO symbol of the relay and a lab relay block is also illustrated in
the below figure:
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Advantages of Relays
 Some advantages of a relay:
1.
It can be used to switch one or more contacts.
2.
To switch a high current circuit with a low current circuit.
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Safety and operation
 The following points should be observed while working with electro-
pneumatic systems:
1. Pressurized air lines that become
detached
can
cause
accidents.
Switch off pressure immediately.
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Safety and operation
2. First connect all tubing and secure before switching on the
compressed air.
3.
Cylinders
may
advance
or
retract as soon as the compressed
air is switched on.
4. Do not operate the electrical limit switch manually during fault
finding (use a tool only).
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Safety and operation
5. Limit switches should be fixed in such a way that they contact the
trip cam of the cylinder only in the determined direction.
6. Do not exceed the permissible working pressure.
7. Use only low voltages of ≤ 24 V.
8. Switch off the air and voltage supply before disconnecting the circuit.
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Introduction
 Two forms of energies are used to operate any electro-pneumatic
control system:
 Electrical energy
 Compressed air energy
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Introduction
 Electrically actuated directional control valves (DCVs) are switched
with the aid of an electric coil that is called a solenoid. The electric coil
attracts or repels the valve spool as shown below:
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Introduction
 When the pushbutton switch is pressed (activated), an electric
current flows through the solenoid coil, the solenoid is energized
causing the valve spool to move, which in turn will switch the valve to
the second position where the air flows to move the cylinder piston
forward as illustrated in the following figure:
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Introduction
 Releasing the pushbutton terminates the current flow, which in turn
de-energizes the solenoid and the DCV moves back to its normal
position.
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Types of solenoid valves
 There are two types of solenoid valves used in electro-pneumatic
systems:
Single solenoid valve
with a spring return
(reset)
Double solenoid valves.
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Single solenoid valve with a spring return (reset)
 The valve remains in the actuated position as long as the current
flows through the solenoid.
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Single solenoid valve with a spring return (reset)
 Example: 5/2 DCV with single solenoid and spring return:
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Double solenoid valves
 The valve will keep and maintain the last switched position even
when no current flows through the solenoid. It is sometimes called
memory valve.
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Double solenoid valves
 Example: 5/2 DCV, with double solenoid:
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Control in electro-pneumatics
 There are two general ways to control the electro-pneumatic systems:
Direct Control
Indirect
Control
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Direct control in electro-pneumatics
 Direct control is the control of an
electro-pneumatic
valve
without
using
intermediate components such as a relay,
a contactor or an industrial computer
(PLC). The valve is connected directly to
electric switch as shown in the figure:
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Advantages of direct control
 Simple and easy
 Less wiring
 Cheap
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Disadvantages of direct control
 Remote control is not possible
 Switching more than one valve at a time is not possible
 Latching is not possible
 Design improvement is not flexible
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Indirect control in electro pneumatics
 Indirect control is the control of an electro-pneumatic valve using
intermediate components such as relays, contactors or programmable
logic controllers (PLC).
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Advantages of indirect control systems
 Remote control is possible
 Switching more than one valve at a time is possible
 Latching is possible
 Flexible design improvement and development
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Disadvantages of Indirect control
 Complicated
 More wiring
 More cost involved
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The end.
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