Ee16704 unit2 part2_electropneumatic

UNIT 2 PNEUMATICS & HYDRAULICS
Qualitative study of Pneumatics: Introduction to Fluid power –
classification, air actuators & its types, selection of actuators, free
air delivery, control valves for direction, pressure and flow, air
preparation. Compressor – types, air reservoir. Electro pneumatics
- components, relay ladder diagram. Vacuum - ejectors, suction
pad, selection.
Hydraulics: Pumps and motors, control valves for direction,
pressure and flow, servo and proportional valves. Power pack.
Selection of cylinders & valves.
Circuit design - symbols, schematic, travel step diagram, control
modes. Sequence control - Cascade method, KV mapping and Step
counter method.

ELECTRO – PNEUMATIC CONTROL
Electro-pneumatic control consists of electrical control
systems operating pneumatic power systems.
In this solenoid valves are used as interface between the
electrical and pneumatic systems.
Devices like limit switches and proximity sensors are used as
feedback elements.
In Electro Pneumatics, the signal medium is the electrical
signal, either AC or DC source is used. Operating voltages
from around 12V to 220V are often used.
Working medium is compressed air. The final control valve is
activated by solenoid actuation.

Control of Electro Pneumatic system is carried out either using
combination of Relays and Contactors or with the help of
Programmable Logic Controllers [PLC].
A Relay is often is used to convert signal input from sensors and
switches to number of output signals [either normally closed (NC) or
normally open (NO)].
Signal processing can be easily achieved using relay and contactor
combinations.
A Programmable Logic Controller can be conveniently used to obtain
the outputs as per the required logic, time delay and sequential
operation.
Finally the output signals are supplied to the solenoids activating the
final control valves which controls the movement of various air
cylinders.

The greatest advantage of electro pneumatics is the integration of
various types of proximity sensors [electrical] and PLC for very
effective control. As the signal speed with electrical signal, can be
much higher, cycle time can be reduced and signal can be conveyed
over long distances.
In Electro pneumatic controls, mainly three important steps are involved:
Signal input devices - Signal generation such as switches and contactor,
Various types of proximity sensors.
Signal Processing – Use of combination of Contactors of Relay or using
Programmable Logic Controllers.
Signal Outputs – Outputs obtained after processing are used for activation
of solenoids, indicators or audible alarms.

Signal flow and component of an Electro-Pneumatic
control system

BASIC ELECTRICAL DEVICES
Seven basic electrical devices commonly used in the control of fluid
power systems are:
1. Manually actuated push button switches
2. Limit switches
3. Pressure switches
4. Solenoids
5. Relays
6. Timers
7. Temperature switches
Other devices used in electro pneumatics are
1. Proximity sensors
2. Electric counters

1. Push button switches
A push button is a switch used to close or open an electric control
circuit. Push button switches are actuated by pushing the actuator
into the housing. This causes set of contacts to open or close.
Push buttons are of two types:
i) Momentary push button
ii) Maintained contact or detent push button
Momentary push buttons return to their unactuated position when
they are released. Maintained (or mechanically latched) push buttons
has a latching mechanism to hold it in the selected position.
The contact of the push buttons, distinguished according to their
functions,
i) Normally open (NO) type
ii) Normally closed (NC) type
iii) Change over (CO) type.
A changeover contact is a
combination of NO and
NC contacts.

Designation of the pushbuttons

2. Limit Switch
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.
There are two types of classification of Limit switches depending
upon method of actuations of contacts
a) Lever actuated contacts
b) Spring loaded contacts
In lever type limit switches, the contacts are operated slowly. In
spring type limit switches, the contacts are operated rapidly.

Limit switches (Mechanical Position sensor)
(Electro-Pneumatic)
Mechanical Position sensor
(Limit switch)

3. 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.
Figure shows a diaphragm type of pressure switch. When the
pressure is applied at the inlet (14) and when the pre-set pressure is
reached, the diaphragm expands and pushes the spring loaded
plunger to make/break contact.

4. Solenoids
Electrically actuated directional control valves (DCV) are switched
with the aid of solenoids.
They can be divided into two groups:
i) Spring return valves only remain in the actuated position as long
as current flows through the solenoid
ii) 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-energised and the solenoids are inactive. A double solenoid valve
has no clear initial position, as it does not have a return spring.
The possible voltage levels for solenoids are:
12V DC, 12V or 24V or 110/120V or 220/230V 50/60 Hz.

Cross sectional view of a 3/2 single solenoid valve

5. Relays
A relay is an electro magnetically actuated switch. It is a simple
electrical device used for signal processing.
When a voltage is applied to the solenoid coil, an electromagnet field
results. This causes the armature to be attracted to the coil core. The
armature actuates the relay contacts, either closing or opening them,
depending on the design. A return spring returns the armature to its
initial position when the current to the coil is interrupted.
Relay OFF Relay ON

More about Relays
 Relay has a few functions as a safety device:
1. The high voltage output (i.e. 240V) can be switched ON
through a contactor using relay with low voltage (i.e. 24V)
supplied to a coil.
2. The high current output can be switched ON through a
contactor using relay with low current supplied to a coil.
3. Functioned as Safety control circuit for emergency power
cut-off (EMERGENCY START and STOP button) to the whole
circuit.
Use in automation process  Switching more than one outputs
simultaneously using relay with a coil and multiple contactors.
To control ON and OFF of various outputs sequences using several
Relay.

6. Timers or Time Delay relays
Timers are required in control systems to effect time delay between
work operations. Most of the timers we use are Electronic timers.
There are two types of time relay
i) Pull-in delay ( on-delay timer)
ii) Drop-out delay (off-delay timer)
Pull-in delay ( on-delay timer) Drop-out delay ( off-delay timer)

Timing diagram for on and off delay timer

7. 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.

1. Proximity sensors
a) Reed Proximity switches
Reed switches are magnetically actuated proximity switches. Reed
switches are similar to relays, except a permanent magnet is used
instead of a wire coil. The reed switches comprise two ferromagnetic
reeds placed with a gap in between and hermetically sealed in a glass
tube. The glass tube is filled with inert gas to prevent the activation of
the contacts. The surfaces of the reed contacts are plated with
rhodium or iridium. Whole unit is encapsulated in epoxy resin to
prevent mechanical damage to the switch. They are also provided
with LED indicator to show its switching status.

Advantages of reed switches:
1. Reed switches are cheap.
2. They have long service life
3. They have shorter switching time (in the order of 0.2 to 0.3
millis seconds)
4. They are compact and maintenance free
Disadvantages of reed switches:
1. They cannot be used in environments subjected to
magnetic fields (like resistance welding machine)
2. Closing of contacts in reed switch is not bounce free
When the magnet is away, the switch is open. But when the magnet is
brought near, the switch is closed. The reed switch is operated by the
magnetic field of an energized coil or a permanent magnet which
induces north (N) and south (S) poles on the reeds. The reed contacts
are closed by this magnetic attractive force.
When the magnetic field is removed, the reed elasticity causes the
contacts to open the circuit.

b) Electronic sensors
Inductive, Optical and capacitive proximity switches are
electronic sensors. They normally have three electrical contacts. One
contact for supply voltage , other for ground and third for output
signal.
In these sensors, no movable contact is switched. Instead, the
output is either electrically connected to supply voltage or to ground.
There are two types of electronic sensors with regard to the polarity
of output voltage.
Positive switching sensors: In this output voltage is zero if no part is
detected in the proximity. The approach of a work piece or machine
part leads to switch over of the output, applying the supply voltage.
Negative switching sensors: In this the supply voltage are applied to
the output if no part is detected in the proximity. The approach of a
work piece or machine part leads to switch over of the output,
switching the output voltage to 0 volts.

a) Inductive sensors
Inductive sensor use currents induced by magnetic field to detect the
nearby metal objects. The inductive sensor uses a coil or inductor to
generate a high frequency magnetic field as shown in Figure. If there
is a metal object near the changing magnetic field, current will flow in
the object. This resulting current flow sets up a new magnetic field
that opposes the original magnetic field. The net effect is that it
changes the inductance of the coil in the inductive sensor. By
measuring the inductance the sensor can determine when a metal
have been brought nearby.
Shields will often be
available for inductive
sensors to improve
their directionality
and accuracy.

Advantages of inductive proximity sensors are
1. They are self contained, rugged and extremely reliable
2. They have long service life
3. They have shorter switching time
4. They are compact and maintenance free
Disadvantages of inductive proximity sensors are
1. Like reed switches , they cannot be used in environments subjected
to magnetic fields (like resistance welding machine)
Applications of inductive proximity sensors
Sensing of end position of linear actuators like cylinders and semi
rotary actuators
They are used to detect metallic pieces on conveyor. That is
presence or absence of work piece on conveyor
They are used in press to detect the end position
They are used to monitor drill breakage while drilling.
They are also used as feed back devices in speed measuring devices

Factors influences the sensing distance
The sensing distance of inductive sensors depends on the
conductivity and permeability of the metal part whose presence or
absence is to be detected. This distance varies with the material
composition of the target object, with mild steel taken as the material
for standard reference (Table). This is described by the reduction
factor. The reduction factor is the factor by which the sensing range
of the inductive sensor is reduced based on material composition of
the objected to be sensed, compared to steel [FE 360] as the standard
reference. Table
Another factor which affects the sensing range of inductive sensors is the
diameter of sensing coil. A small sensor with a coil diameter of 18mm has
a typical range of 1mm, while a large sensor with core diameter of 75mm
has a sensing range up to 50mm or even more

b) Capacitive sensors
Capacitive sensors are able to detect most materials at distances upto
a few centimetres. We know that
In the sensor the area of the plates and distance between them is
fixed. But, the dielectric constant of the space around them will vary
as different material is brought near the sensor. An illustration of a
capacitive sensor is shown Figure. An oscillating field is used to
determine the capacitance of the plates. When this changes beyond
selected sensitivity the sensor output is activated.

These sensors work well for insulators (such as plastics) that tend to
have high dielectric coefficients, thus increasing the capacitance.
But, they also work well for metals because the conductive materials
in the target appear as larger electrodes, thus increasing the
capacitance as shown in Figure.
In total the capacitance changes are normally in the order of pF.

Advantages of capacitive proximity sensors:
1. They are widely used because of their ability to react with wide
range of materials
2. They are suitable for detecting non metallic objects
3. They can be used to sense and monitor level in storage containers
Disadvantages of capacitive proximity sensors:
1. They are sensitive especially in humid environment
2. Without the compensator ring, the sensor would be very
sensitive to dirt, oil and other contaminants that might stick to the
sensor.

c) Optical proximity sensors
Optical sensors require both a light source (emitter) and detector.
Emitters will produce light beams in the visible and invisible spectrums
using LEDs and laser diodes. Detectors are typically built with
photodiodes or phototransistors. The emitter and detector are
positioned so that an object will block or reflect a beam when present. A
basic optical sensor is shown in Figure.
This arrangement works well with opaque and reflective objects with the emitter
and detector separated by distances of up to hundreds of feet

Opposed mode optical sensor
Emitter and detector in one unit
An emitter can be set up to point directly at a detector, this is known as opposed mode. When
the beam is broken the part will be detected.
The reflector is constructed with polarizing screens oriented at 90 deg. angles. If the light is
reflected back directly the light does not pass through the screen in front of the detector.

Polarized light in retroreflective sensors
Diffuse sensor
For retroreflectors the reflectors are quite easy to align, but this
method still requires two mounted components. A diffuse sensors is
a single unit that does not use a reflector, but uses focused light as
shown in Figure

The relationship between beam width and object size
The emitter beam is focused at a distance so that the light intensity
is greatest at the focal distance.
Point detection using focused optics
Multiple point detection using optics

2. Electric counters
An electric counter consist of a coil, associated circuits and contacts, a
reset coil, manual reset, release button and a display window.
Pressure the release button of the counter and entering the desired
count valve sets the pre-determined count value. The pre-determined
count valve is displayed in the window.
There are two types of counters
1. Up counter
2. Down counter
Up counter: An up counter counts electrical
signal upwards from zero. For each electrical
counting pulse input to an up-counter coil, the
counter value is incremented by 1. When the
predetermined valve has been reached, the
relay picks up and the contact set is actuated.
Down counter: A down counter
counts electrical signal downwards
from preset valve. If the count valves
of zero is reached the relay picks up
and the contact set is actuated.
The pre-determined value is
maintained when the counter is reset.

Direct control of single acting cylinder
Electro pneumatic circuits for single acting actuators

Indirect Control of single acting cylinder
Electro pneumatic circuits for single acting actuators

Direct Control of double acting cylinder
a) Position when cylinder is extended b) Position when cylinder is retracted
Electro pneumatic circuits for double acting actuator

Indirect Control of double acting cylinder (using 5/2 way, single solenoid)

a) Position when cylinder is extended b) Position when cylinder is retracted
Indirect control of double acting cylinder (using 5/2 way double solenoid)

Control of double acting cylinder OR logic (Parallel circuit)
a) Cylinder is extended using PB1 OR PB2 b) Cylinder is retracted (both PB released)

a) Cylinder is extended using PB1 AND PB2 b) Cylinder is retracted (any PB released)
Control of double acting cylinder AND logic

Q: Double acting cylinder is to be controlled using 5/2 directional
control valve, single solenoid, spring return. When push button PB1 is
pressed, cylinder should extend and remains in that position though
PB1 is released. The cylinder is to retract completely when PB2 is
pressed. In addition, the cylinder is to remain in the retracted position
though PB2 is released. Develop a Electro-pneumatic control circuit
with an electrical latching with a) dominant Off and b) dominant On.
*Dominant position refers to status of relay coil (circuit) when both
the start and stop signals are applied simultaneously.
When Start button (PB1) and Stop button (PB2) are pressed
simultaneously, if the circuit goes to OFF position, then such a circuit
is called Dominant OFF latching circuit.
When Start button (PB1) and Stop button (PB2) are pressed
simultaneously, if the circuit goes to ON position, then such a circuit is
called Dominant ON latching circuit.
Latching circuits

Automatic return of a double acting cylinder (spring return) using Limit Switch

Direct control of automatic return of a double acting cylinder (double solenoid)
using Limit Switch

Indirect control of automatic return of a double acting cylinder (double solenoid)

Oscillating motion of a double acting cylinder (Forward)

Oscillating motion of a double acting cylinder (Return)

Control of double acting cylinder with time delay (Double solenoid)

Control of double acting cylinder using electric counter with two end sensors
Q: Design a electro pneumatic circuit for a double acting cylinder to perform a continuous to
and fro motion. The cylinder has to stop automatically after performing 50 cycles operations

Ee16704 unit2 part2_electropneumatic

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