Chapter 3 automation devices

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CHAPTER 3CHAPTER 3
AUTOMATION DEVICES

22
3.1 Introducton3.1 Introducton
• Automation includes sensors,
switches, actuators, transducers and
controllers

33
SwitchesSwitches
 Switches are frequently items in many
electrical actuation systems. Switches make
or break connections in an electrical circuit.
This may be to switch on electric motors,
switch on heating elements, actuate solenoid
valves controlling hydraulic or pneumatic
cylinders, control a set of sequential actions ,
etc.

55
Dual in-line Package (DIP) SwitchSlide Switch

77
Mechanically Operated SwitchesMechanically Operated Switches
 A mechanically operated switch is one that is
controlled automatically by factors such as
pressure, position and temperature. Examples
of mechanically operated switches are such as
limit switch, micro switch, temperature switch,
pressure switch and level switch

88
SensorsSensors
 Sensor is used for an element, which produces a
signal relating to the quality being measured.
Sensors are devices that are used to detect and
some time to measure. They are types of a
transducer.

99
Specification and Selection of a SensorSpecification and Selection of a Sensor
Range and span
Range and linearity error in a temperature sensor

1010
Repeatability
Sensor’s repeatability indicate the
range of output that the user can expect
when the sensor measures the same
input values several time On other
hand its mean the ability to give the
same output for repeated application of
the same input value.

1111
Linearity
Linearity often quoted on spec sheets
as a +/- value for the sensor’s output
signal.
Pressure

1212
 Accuracy
The extent which the value indicated by
a measurement system might be
wrong.
Sensitivity
The relationship indicating how much
output to get per unit input.
Stability
The ability to give the same output
when used to measure a constant input
over a period of time.

1313
Proximity SensorProximity Sensor
 Proximity sensors or switches are pilot
devices that detect the presence of object or
target without physical contact
Proximity sensors are used when:
 required, as in counting or ejection control
applications.
 The object being detected is too small, too
lightweight, or too soft to operate a
mechanical switch.

1414
 An object has to be sensed through non-
metallic barriers such as glass, plastic, and
paper cartons.
 Hostile environments demand improved
sealing properties, preventing proper
operation of mechanical switches.
 Long life and reliable service are required.
 A fast electronic control system requires a
bounce-free input signal.

1515
Type of Proximity sensorsType of Proximity sensors
Inductive proximity sensor
It is most widely used due to its small
size, robustness and low cost. An
inductive proximity sensor is a sensing
device that is actuated by a metal object.

1717
 When energy is supplied, the oscillator
operates to generate a high frequency field.
When a metal object enters the high
frequency field, eddy currents are induced in
the surface of the target. This results in a loss
of energy in the oscillator circuit;
consequently this causes a smaller amplitude
of oscillator. The detector circuit recognizes a
specific change in amplitude and generate a
signal that will turn the solid state output on or
off. When metal object leaves the sensing
area, the oscillator regenerates allowings the
sensor to return to its normal state.

1818
 Capacitive proximity sensor
It is a sensing device that is actuated
by conductive and non-conductive
materials.
Capacitive proximity sensor

1919
 The operation of capacitive sensor is also
based on principle of an oscillator. Inside
the sensor is a circuit that uses the supplied
DC power to generate AC, to measure the
current in the internal AC circuit, and to switch
the output circuit when the amount of AC
current changes. Capacitors can hold a
charge because, when one plate is charged
positively, negative charges are attracted into
the other plate, thus allowing even more
positive charges to be introduced into the first
plate. Unless both plates are present and
close to each other, it is very difficult to cause
either plate to take on very much charge.

2020
 Another type of a capacitive sensor is formed
by two metallic electrodes which are placed in
the feedback loop of a high frequency
oscillator that is inactive with no target
present. As the target approaches the face of
the sensor, it enters the electrostatic field that
is formed by the electrodes. This causes an
increase in the coupling capacitance and the
circuit begins to oscillate.

2222
Optical proximity sensor (LightOptical proximity sensor (Light
sensor/photoelectric sensor)sensor/photoelectric sensor)

2323
 They are widely used in automated systems.
These sensors are commonly known as light
beam sensors of the thru-beam and retro-
reflective types. Thru-beam type is usually
used to signal the presence of an object that
block light. The retro-reflective type has the
transmitter and receiver in the same package.
It detect targets that reflect light back to the
sensor.

2424
Magnetic SwitchesMagnetic Switches
A magnetic switch (also called reed
relay) contact is composed of two flat
contact tabs that are hermetically
sealed (airtight) in a glass tube filled
with protective gas. As a permanent
magnet approaches, the ends of the
overlapped contact tabs attract one
another come into contact and
demagnetizes and return to it original
position when the permanent magnet is
moved.

2626
Bar code technologyBar code technology
 It is widely implemented in industry and
supermarket. Bar code system consists of
three basic elements; bar code symbols,
scanner and decoder. Bar code symbol
composed of bars and spaces of varying with,
provides a means of expression for human
readable characters in a form (bars and
spaces) readable by machines. Bar code
symbols require a ‘quite zone’ of no mark in
front of the black marks. The code itself
contains coded data between start and stop
patterns. Both the width of dark and light
areas are significant.

2828
 The name bar code reader or scanner
designates the optical reading instrument
capable of emitting and collecting a coherent
(laser) or non coherent (LED) red,
intermediate or infrared beam. A bar code
reader is a computerized system and which
is build around retro-reflective optical
sensors. Bar code reader explore symbols
(bar codes) by shedding light across them
and collecting the reflected beam with an
optical transducer (photo detector) /
converting a invisible electromagnetic wave
into an analogical electric signal. Some bar
code readers can sweep a target area
hundred of times a second.

3030
 A decoder is an electronic circuit needed to
transform this signal into digital form
(succession of ones and zeros in the form of
pulses) corresponds 100 percent to the
programmed bar code .
Benefits of the bar code system
 The single item ID from primary production
sources to consumers, thus avoiding
alterations and errors.
 Precise information on production times and
cycles, inspection, storing, transportation and
sales.

3131
 General statistical data.
Minimum data error owing to auto check
system and control characters.
 Vertical oversize that allows a code to be
scanned even if, as a result of damage, only
a small part of its total height can be read.
 Faster, more efficient reception, sale and
collection, particularly at supermarket
checkout counters.
 No more price remarking or item by item
relabelling, especially in countries with high
inflation.
 Prompt information on stock, sales and
supplies.

3232
 Human error elimination in marking,
interpreting, invoice preparation and unknown
loss.
 Adaptable to most packing and printing
system and exciting packaging materials and
techniques.
 Easily adaptable and compatible with most
available computer brands and systems.
 Human error elimination in marking,
interpreting, invoice preparation and unknown
loss.
 Adaptable to most packing and printing
system and exciting packaging materials and
techniques.
 Easily adaptable and compatible with most

3333
Temperature sensorsTemperature sensors
 Thermocouple
A thermocouple consists essentially of a pair
of dissimilar conductors welded or fused
together at one end to form the "hot" or
measuring junction, with the free ends
available for connection to the "cold" or
reference junction. A temperature difference
between the measuring and reference
junctions must exist for this device to function
as a thermocouple. When this occurs, a small
dc voltage is generated. Thermocouples,
because of their rugged-ness and wide
temperature range, are used in industry to
monitor and control oven and furnace

3434
A reference junction is required for
measuring temperature with
thermocouples. The out-put voltage of a
thermocouple is approximately
proportional to the temperature
difference in voltage between the
measuring (hot) junction and the
reference (cold) junction.

3636
Resistance temperature detectors (RTDs)Resistance temperature detectors (RTDs)
The basic concept underlying the
measurement of temperature by resistance
temperature detectors (RTDs) is that the
electrical resistance of metals varies
proportionally with temperature. This
proportional variation is precise and
repeatable, therefore allowing the consistent
measurement of temperature through
electrical resistance detection. Platinum is the
material most often used in RTDs because of
its superiority regarding temperature limit,
linearity, stability, and reproducibility.

3838
ThermistorThermistor
A thermistor is a thermally sensitive resistor
that usually has a negative temperature
coefficient. As the temperature increases, the
thermistor's resistance decreases, and vice
versa. Thermistors are very sensitive (as
much as 5 percent resistance change per
°C); therefore, they are capable of detecting
minute changes in temperature. Their
sensing area is small, and their low mass
allows a fairly fast response time of
measurement. They are available in a variety
of sizes and styles. Because of the large
voltage output produced by a typical
thermistor bridge amplification is normally
unnecessary. The thermistor sensing unit is

4141
Encoder sensorsEncoder sensors
An encoder is used to convert linear or rotary
into a digital signal. There are two types of
encoder sensors:
i. Incremental encoder
The optical-type incremental is a rotary
encoder, creates a series of square waves as
its shaft is rotated. The encoder disk
interrupts the light as the encoder shaft is
rotated to produce the square wave output
waveform. It is very sensitive and accurate
(normally from 100 to 4000 pulses per
revolution).

4242
ii. The optical type absolute rotaryii. The optical type absolute rotary
encoderencoder
The optical-type absolute is also a rotary
encoder, operates in the same manner with
the exception that more traces have been
applied to the encoder disk. When scanned in
a parallel fashion, these traces provide angle
information in the form of code. The number
of traces corresponds to he number of steps
per rotation. The traces are arranged in Gray
code. The Gray coding has the advantage
over other coding in that only one bit (trace) is
changed per step. This helps avoid
misreading. The number of square waves
obtained from the output of the encoder can
be made to correspond to the mechanical
movement required.

4545
A tachometer is used to detect the speed
of a motor. There are two types of
tachometer: frequency and magnitude.
A tachometer normally refer to a small
permanent magnet dc generator. When the
generator is rotated, it produces a dc voltage
directly proportional to speed. Tachometers
coupled to motors are commonly used in
motor speed control to provide a feedback
voltage to the controller that is proportional to
motor speed. The rotating speed of a shaft
often measured using a magnetic (inductive)
pickup sensor. A magnet is attached to the
shaft.
RPM sensorsRPM sensors

4646
A small coil of wire held near the
magnet receives a pulse each time the
magnet passes. By measuring the
frequency of pulses, the shaft speed
can be determined. The voltage output
of a typical pick up coil is quite small
and requires amplification.

4848
TransducerTransducer
A transducer is any device that converts
energy from one form to another.
Transducer can be divided into to: i/p
transducer and o/p transducer. For e.g.
electric i/p transducer convert non-electric
energy such as sound, light into electric
energy. Electric o/p transducer works in the
reverse order-convert electric energy to forms
of non-electric energy

4949
Displacement transducersDisplacement transducers
 Potentiometers, also called variable
resistors, are making a comeback as position
sensors. It can be used as linear or as rotary
position sensors. The output is a voltage
proportional to the position of a wiper along a
variable resistor. They have a reputation for
being too inaccurate because of the effects of
temperature changes, wear, and resistance
changes due to pollutants between the wiper
and the variable resistor. Its making a
comeback because developments in
materials, especially in conductive plastics,
are improving their performance.

5151
Linear variable differential transformerLinear variable differential transformer
(LVDT)(LVDT)
It is basically a transformer with a
movable core and two secondary
windings. LVDT sensor can be used to
control the water level in a tank .
especially in conductive plastics, are
improving their performance. The LVDT
is provided with AC at its central (input)
coil.

5353
CapacitiveCapacitive Position SensorsPosition Sensors
Have been used as dial position sensors in
radios for years. (Their variable capacitance
is used in the radio frequency selection
circuitry, so perhaps calling them "sensors" in
this application isn't quite right.) A capacitor
increases in capacitance as the surface area
of the plates facing each other increases. If
one 180 degree set of plates is attached to a
rotating shaft, and another 180 degree set of
plates is held stationary, then capacitance
increases linearly with shaft rotation through
180 degrees.

5454
Capacitive sensors are used in a
slightly different way to detect the height
of fluids that have higher dielectric
constraints than air. If the fluid rises in
the capacitive height sensor, then more
surface area of the capacitor plates are
coupled by the fluid, and capacitance
rises proportionally with fluid height.

5656
Force and pressure transducerForce and pressure transducer
 Strain gauge
It is a sensor that measure deformation due
to pressure. When the strain gauge is
stretched, the conductor reduces its cross-
sectional area and thus can carry less
current. The piezoelectric strain gauge sensor
includes a crystalline material that develops a
voltage across the crystal when the crystal is
deformed. A strain wire gauge transducer
converts a mechanical strain into an electric
signal. The force applied to the gauge causes
the gauge to bend. This bending action also
distorts the physical size of the gauge, which,
in turn, changes its resistance

5858
Flow transducers/SensorFlow transducers/Sensor
 Pitot (pea-toe) type
Pressure upstream from a restricted orifice in
a pipe is always higher than pressure
downstream from that restricted. The greater
the flow rate, the greater the pressure
difference. If a differential pressure sensor
compares pressures before and after
restriction, then flow rate can be determined

5959
Turbine typeTurbine type
The blades of turbine flow meter are
often magnetized, so that a magnetic
sensor outside the pipe can detect the
rotation speed of the turbine. Although
the output rotation speed of this
flowmeter is theoretically proportional to
the flow rate, several factors such
leaking, viscosity of fluid, etc can effect
the precision.

6060
Vortex sheddingVortex shedding
A Vortex shedder obstruction
creates swirling areas of low pressure
behind and on side of the obstruction as
fluid flows past. The faster the fluid flow,
the faster this vortex is created and
destroyed. A vibration sensor on the
obstruction measures the frequency at
which the obstruction vibrates as the
vortex location alternates from one side
to the other.

6262
ActuatorActuator
Actuation systems are the elements of
control systems which are responsible
for transforming the output of a
microprocessor or control system into a
controlling action on a machine or
device. E.g. we might have an electrical
output of the controller which has to be
transformed into a linear motion to move
a load.

6363
Comparison of actuating systemsComparison of actuating systems
Hydraulic
• Advantages
- Good for large robots and heavy payload
- Highest power/weight ratio
- Stiff system, high accuracy, better response
- No reduction gear needed
- Can work in wide range of speeds without
difficulty
- Can be left in position with out any damage

6464
•DisadvantagesDisadvantages
- May leak. Not fit for clean room applications
- Requires pump, reservoir, motor, hoses, etc.
- Can be expensive and noisy.
- Requires maintenance
- Viscosity of oil changes with temperature
- Very susceptible to dirt and other foreign
material in oil
- High torque, high pressure, large inertia on
the actuator

6565
ElectricalElectrical
• Advantages
- Good for all sizes of robots
- Better control, good for high precision
robots
- Higher compliance than hydraulics
- Reduction gears used re duce inertia
on the
motor
- Does not leak, good for clean room
- Reliable, low maintenance

6666
- Can be spark, explosive environments
- Needs reduction gears,Low stiffness
- increased backlash, cost, weight, etc.
- Motor needs braking device when not
powered.

6767
PneumaticPneumatic
• Advantages
- Many components are usually off-the-
shelf
- Reliable components
- No leaks or sparks
- Inexpensive and simple
- Low pressure compared hydraulics
- Good for on-off application and for
pick
and place Compliant systems

6868
- Noisy systems
- Require air pressure, filter. etc.
- Difficult to control their linear position
- Deform under load constantly
- Very low stiffness. Inaccurate response
- Lowest power to weight ratio

7070

Pneumatic power supply

7171
Directional Control ValvesDirectional Control Valves
Pneumatic and hydraulic systems use
directional control valve to direct the flow of
fluid through a system. It is used to
open/close , or on/off devices. These valves
are used to develop sequences control
systems. They can be activated to switch the
fluid flow direction of mechanical, electrical or
fluid pressure signals. Figure 4.3(a) shows a
typical directional control valve (spool valve).
In (a) the air supply is connected to port 1
and port 3 is closed. This will make the
device connected to port2 is pressurized. In
(b) the air supply is cut off and port 2 is
connected to port 3.

7272
A typical directional
control valve (spool
valve). In (a) the air supply
is connected to port 1 and
port 3 is closed. This will
make the device connected
to port2 is pressurized. In
(b) the air supply is cut off
and port 2 is connected to
port 3.

7373
Another type of
directional control
valve, the poppet valve.
There is no connection
between port 1 to which
the system is connected.
When the button is
released the spring
forces the ball back up
against its seat and so
closes off the flow.

7474
(a) Flow path, (b) flow shut off, (c) initial
connections

7777
CylinderCylinder
a) A Double acting cylinder
b) Single acting cylinder

7878
• The hydraulic or pneumatic cylinder is an
example of a linear actuator. The principle
and form are the same for both hydraulic and
pneumatic with the difference of size and
pressures(hydraulic requires more pressure).
The cylinder consist of a cylindrical tube
along which a piston/ram can slide. Single
acting cylinder is used when the control
pressure is applied to just one side of the
piston, a spring often being used to provide
the opposition to the movement of the piston.
When a current passes through the solenoid,
the valve switches position and pressure is
applied to move the piston along the cylinder.
As a consequence the spring returns the
piston back to initial position.

7979
Control of Single acting cylinder

8080
• Double acting cylinder is used when the
control pressures are applied to each
side of the piston. A difference in
pressure between the two sides then
results in motion of the piston, the
piston being able to move in either
direction along the cylinder as result of
high pressure signals.

8181
Control of Double acting cylinder

8282
Figure below shows how a 4/3 way
directional control valve controls a double
acting cylinder

8383
 Relay
• is an electrically operated device that
mechanically switches electric circuits. It is an
important part of many control system
because it is useful for remote control and for
controlling high voltage and current devices
with a low voltage and current control signal.
When current flows through the electromagnet
in an electromechanical control relay, a
magnetic field that attracts the iron arm of the
armature to the core of the magnet is set up.
As a result, the contacts on the armature and
relay frame are switched. Relay may have NO
contacts or NC contacts or combinations of

8484
An electromechanical control relay

8585
 Solenoid
• is a device used to convert an electrical
signal or an electrical current into linear
mechanical motion. The solenoid is
made up of a coil with a movable iron
core. When the coil is energized, the
core, or armature as it is sometimes
called, is pulled inside the coil. The
amount of pulling or pushing force
produced by the solenoid is determined
by the number of turns of copper wire
and the amount of current flowing
through the coil.

8686
 A solenoid valve
is a combination of two basic functional units.
A solenoid (electromagnet) with its core or
plunger.
• A valve body containing an orifice in which a
disc or plug is positioned to restrict or allow
flow.
• Flow through an orifice is off or allowed by the
movement of the core and depends on
whether the solenoid is energized or de-
energized. When the coil is energized, the
core is drawn into the solenoid coil to open
the valve. The spring returns the valve to its
original closed position when the current
ceases. Solenoid valves are available to
control hydraulics (oil fluid), pneumatics (air),

Chapter 3 automation devices

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