Elements of Industrial Automation Week 03 Notes.pdf

Vidya Vikas Educational Trust (R),
Vidya Vikas Polytechnic
27-128, Mysore - Bannur Road Alanahally,Alanahally Post, Mysuru, Karnataka 570028
Prepared by: Mr Thanmay J.S, H.O.D Mechanical Engineering VVETP, Mysore Page | 26
Theory and Practical for week 03
A Programmable Logic
Controller (PLC) is a
microprocessor-based digital
electronic device, which uses a
programmable non-volatile
memory to store user-defined
instructions and implement
specific control functions like programming logic, and sequencing, timing, counting, and arithmetic
operations to control various electro-mechanical systems and industrial processes.
the working principle of a PLC system is not as simple as such. Because to control a process or machinery,
the PLC system must be equipped with sophisticated electronic Input and Output (I/O) modules. These I/O
modules interface the PLC CPU to the real world, or rather to the field input/output devices.
Digital inputs are the most common types of
inputs in PLC systems; due to the fact that PLCs
are digital electronic devices themselves, thus,
they’re able to easily process digital signals. A
digital PLC input is basically a binary signal that
is either ON or OFF, and which is applied to the
PLC processor from a digital field input device.
The concept of digital signals is based on the
binary number system, which consists of only two
possible digits, 1 or 0. Where 1 represents a HIGH
state and 0 indicates a LOW state. However, PLC normally operate on 24V DC.
Digital PLC outputs are control circuits that use only binary data (1 and 0) to give the PLC CPU control over
field output devices. A digital PLC output is thus a processed binary control output from the PLC to the field
devices. Digital PLC outputs are typically used to provide an ON or OFF (OPEN or CLOSED) control
scheme to any device or system being controlled by the connected PLC.
Input devices:
a) Strain Gauges
Strain is a dimensionless measurement that is a ratio of the change in length
to the original length of an object. Therefore, a positive strain is the result
of stretching a material and negative strain is the result of compression.
Stress is a measurement of the force applied divided by the initial cross-
sectional area of an object, or the internal resisting capacity of an object.
A strain gauge is a sensor whose measured electrical resistance varies
with changes in strain. Strain is the deformation or displacement of
material that results from an applied stress. Stress is the force applied to a
material, divided by the material’s cross-sectional area. Load cells are
designed to focus stress through beam elements where strain gauges are

located. Strain gauges convert the applied force, pressure, torque, ect., into an electrical signal which can be
measured.
The strain gauge must be connected to an
electrical circuit that is capable of accurately
responding to the minute changes in
resistance associated with strain. Multiple
strain gauges can be used in a divided bridge
circuit to measure small changes in electrical
resistance. This is called a Wheatstone bridge.
In a Wheatstone bridge configuration, an
excitation voltage is applied across the
circuit, and the output voltage is measured
across two points in the middle of the bridge.
When there is no load acting on the load cell,
the Wheatstone bridge is balanced and there is zero output voltage. Any small change in the material under
the strain gauge results in a change in the resistance of the strain gauge as it deforms with the material. This
causes the bridge to be thrown out of balance, resulting in a change in the output voltage. As stated earlier,
the resistance change is minute, which means that signal amplification is often needed to properly determine
changes. The amplification process strengthens the strain signal changes
b) Pressure Sensors
A pressure sensor is a device equipped with a pressure-sensitive element
that measures the pressure of a gas or a liquid against a diaphragm made
of stainless steel, silicon, etc., and converts the measured value into an
electrical signal as an output.
• A semiconductor piezo-resistance dispersion pressure sensor has a
semiconductor distortion gauge formed on the surface of the diaphragm,
and it converts changes in electrical resistance into an electrical signal
by means of the piezo-resistance effect that occurs when the diaphragm
is distorted due to an external force (pressure).
• A static capacitance pressure sensor has a capacitor that is formed
by a static glass electrode and an opposing movable silicon
electrode, and it converts changes in static capacitance that occur
when the movable electrode is distorted due to an external force
(pressure) into an electrical signal.
c) Liquid level detectors
The Liquid level sensor is a device that measures the liquid level in a fixed container that is too high or too
low. According to the method of measuring the liquid level, it can be divided into two types: contact type
and non-contact type. The input type water level transmitter we call is a contact measurement, which
converts the height of the liquid level into an electrical signal for output. It is currently a widely used water
level transmitter.
The different kinds of liquid level sensors include
1. Optical

2. Capacitive
3. Conductive
4. Diaphragm
1. Optical Liquid Level Sensors
Optical sensors work by are solid state. They use an infra-red LED and
phototransistor which are optically coupled when the sensor is in air. When the
sensing tip is immersed in liquid, the infra-red-light escapes making the output
change state. These sensors can detect the presence or absence of almost any
liquid. They are not sensitive to ambient light and are not affected by foam when
in air or by small bubbles when in liquid. This makes them useful where the state
change must be quickly and dependably noted, and where they can function
reliably for extended periods without maintenance. The disadvantage of an optical liquid level sensor is that
it can only determine if liquid is present or not present. If variable levels are required, (25%, 50%, 100%,
etc.) each requires an additional sensor.
2. Capacitive Liquid Level Sensors
Capacitive liquid level switches use 2 conductive electrodes (usually made of metal)
in a circuit that are a short distance from each other. When the electrodes are
immersed in a liquid it completes a circuit.
The advantage of a capacitive liquid level switch is that it can be used to determine
the rising or falling of liquid in a container. By making the electrodes the same
height as the container, the capacitance between the electrodes can be measured. No
capacitance means no liquid. Full capacitance means a full container. Both “empty”
and “full” measurements must be recorded, then a meter calibrated with 0% and
100% to show the liquid level.
3. Conductive Liquid Level Sensors
Conductive liquid level switches are sensors with an electrical contact at a specific
liquid level. Two or more insulated electrodes with exposed tips are used inside a
pipe lowered into the liquid. A longer electrode carries a low voltage, while a shorter
one is used to complete the circuit when the liquid level rises to meet it.
Like capacitive liquid level switches, conductive liquid level switches depend on the
conductivity of the liquid. Therefore, they are only useful for measuring certain types
of liquids. In addition, these sensors tips must be cleaned at regular intervals to
reduce fouling.
4. Diaphragm Liquid Level Sensors
Diaphragm or pneumatic liquid level switches rely on air pressure
to push a diaphragm which engages a micro-switch inside the
body of the unit. As the liquid level rises, the internal pressure
inside a detecting pipe rises until the micro-switch or a pressure
sensor is activated. As the liquid level falls, the air pressure also
falls and the switch is disengaged.
The advantage of a diaphragm-based liquid level switch is that no
power source inside the tank is required, it can be used with many

types of liquids, and since the switch does not come in contact with the liquid. However, because it is a
mechanical device, over time it will require maintenance.
5. Float Liquid Level Sensors
Float switches are the original liquid level sensors. They are mechanical devices. A
hollow float is connected to an arm. As the float rises and falls in the liquid, the arm is
pushed up and down. The arm may be connected to a magnetic or mechanical switch to
determine on/off, or it may be connected to a gauge that rises from full to empty as the
liquid levels falls.
The ball float switch in a toilet tank is the most common type of float liquid level sensor
used. Sump pumps also use float switches (see right) as a cost-effective way to measure
the water level in a basement sump pit.
Float switches can measure any type of liquid, and can be designed to require no power to
operate. The disadvantage of float switches is that they are larger than other types of
switches and because they are mechanical, must be services more often than other liquid
level switches.
d) Fluid flow measurement
Measurement of mass flow rate or
volume flow rate of fluid flow is
defined as Flow Measurement. To
measure the fluid's mass flow rate or
volume flow rate, we need to use some
flow measurement devices
Working Principle of Electromagnetic
Flow Meter
Electromagnetic flowmeter works on
the principle of Faraday’s law of electromagnetic induction for flow measurement, according to which,
‘Whenever conductor moves through magnetic field of given field strength, a voltage (emf.) is induced in
the conductor, which is proportional to relative velocity between the conductor and magnetic field.” OR
“Whenever a conductor cuts magnetic flux lines (magnetic field), an emf is induced in the conductor, which
is directly proportional to rate of change of magnetic flux”.
In case of electromagnetic flow meter, fluid flow acts as a conductor.
Electromagnetic flow meter consists of electrically insulated or non-conducting pipe such as fiber glass. A
pair of electrodes is mounted opposite to each other and flushes with the inside wall of pipe carrying the
fluid, whose flow is to be measured. In the figure, it can be observed that, two electrodes are placed at right

angles to the plane of magnetic field, i.e. magnetic flux lines. The pipe is surrounded by an electromagnet,
which produces magnetic field. This magnetic field is generated by the current flowing through the coil
wounded on the electromagnet. The coil is powered by a steady D.C. supply.
A conductive fluid is passed through the pipe. As the fluid passes, its motion relative to magnetic feld
produces an e.m.f proportional to velocity of fluid. It is given by Faraday’s law as,
E = B. L. V in Volts
where,
B = Magnetic flux density in Weber / m2
L = Length of conductor (fluid) = diameter of pipe in m
V = Velocity of conductor (fluid) in m/sec.
This induced emf E is collected by the electrode and it is given to the external circuit. Since, this induced
emf E is assumed to be directly proportional to velocity of flowing fluid, therefore, the emf so induced or
produced becomes a measure of flow.
e) Smart Sensors
A sensor producing an electrical output when combined with interfacing electronic circuits is known as
“Smart Sensor", it is a combination of both sensor and actuator.
The smart sensor will have intelligent features and some electronics that can perform:
• Data conversion
• Bi-directional Communication
• Take a decision
• Perform Logical operations
Sensor + interfacing circuit = smart sensor
Smart sensors are devices that take
information from a physical
environment and use embedded
microprocessors and wireless
communication to monitor, examine,
and maintain various systems.
Smart sensors rely on built-in
microprocessors to help
them perform various functions, like
digital processing, code conversion
of analog to digital, interfacing functions, and calculations. They also determine when data needs to be
stored or deleted in accordance with the input they collect.
They also use Micro Electro Mechanical Systems (MEMS) and Very Large-Scale Integration technology
(VLSI) to help them function. MEMS allows the sensor to handle large amounts of data in a fraction of a
second. It also enhances and improves the sensor's self-calibration functions. VLSI is part of MEMS that
converts collected data to electrical signals that can be used for processing, display, recording, and
transmission.
Smart sensors have four main functions:
• Measurement
• Configuration
• Verification
• Communication

Output Devices:
a) Relays: A Relay is a type of Switch which can switched ON or OFF with the help of
a signal or a pulse of electricity. These are the essential component
for protection and switching of a number of the control circuits and other electrical
components. All the Relays react to voltage or current with the end goal that they
open or close the contacts or circuits. This article briefly discusses the relay basics
and different types of relays that are utilized for a variety of applications.
A switch is a component that opens (turn off) and close (turn on) an electrical circuit. whereas, a relay is an
electrical switch that control (switch on and off) a high voltage circuit using a low voltage source. A relay
completely isolates the low voltage circuit from the high voltage circuit. There are different types of relays like:
• Electromagnetic Relays
• Latching Relays
• Electronic Relays
• Non-Latching Relays
• Reed Relays
• High-Voltage Relays
• Small Signal Relays
• Time Delay Relays
• Multi-Dimensional Relays
• Thermal Relays
• Distance Relays
• Automotive Relays
• Frequency Relays
• Polarized Relays
• Rotary Relays
• Sequence Relays
• Moving Coil Relays
• Buchholz Relays
• Safety Relays
• Differential Relays
b) Directional control Valve: The directional control valves can be used to start, stop, and to change the
fluid flow in a hydraulic system. The major function of a directional control valve is to control the
direction of flow in hydraulic systems. They are capable to determine the path through which the fluid
should flow in a circuit. One can use the directional control valve to direct the inlet flow to a specific outlet
port. Directional control valves are classified according to certain factors like inlet control element
structure, number of ports or ways, number of positions, method of actuation, and center position flow
pattern. In a directional control valve, the internal control element would be a sliding spool, rotary spool or
ball. The construction and design of the directional control valves make it suitable for different
applications.
Classification of Directional Control Valves
The below are the types of directional control valves as follows.
1. Based on the type of construction.
2. Based on the number of ports.
3. Based on the number of switching positions.
4. Based on actuating mechanism.
i. Based on the Type of Construction: The most basic configuration of the directional control valve is a spool inside a
cylinder where the movement of the spool allows and stops the flow of the fluids through it. Other types include a ball,
spool (sliding spool or rotating spool), or poppet.

ii. Based on Number of Ports: Two-way, three-way, four-way valves.
Denoted in capital letters P (pump port), T (Tank port), A, B (supply
ports), or numerals 1, 2, 3, 4 are shown on the boxes that indicate
connections.
iii. Based on a number of switching positions: The square box represents
position. The symbol of the directional control valve is made of a
number of square boxes which are adjacent to each other depending on
the number of positions. One, two, or three positions.
iv. Based on Actuating Mechanism: The directional control valves are actuated by a variety of
methods includes manual, mechanical, electrical, pneumatic, and hydraulic and as shown in
the table below. When the actuator has pressed the spool inside the directional control valve
changes its position and controls the fluid flow.
Various Types of Directional Control Valves
2/2 way valves are for ON/OFF function for fluid supply. It can be
normally open or closed.
3/2 way valves are generally used to control single-acting actuators.
It can be normally open or closed.
4/2, 4/3, 5/2, 5/3 way valves are normally used for double-acting
actuators.
Summary
Quality Being Measured The input device (Sensors) Output device (Actuators)
Light
Light Dependent Resistor (LDR)
Photo Diode
Phototransistor
Solar cell
Lights and Lamps
LEDs and Displays
Fibre Optics
Temperature
Thermocouple
Thermistor
Thermostat
Resistive Temperature Detectors (RTD)
Heater
Fan
Force or Pressure
Strain Gauge
Pressure switch
Load cells
Lifts and Jacks
Electromagnetic vibrations
Position
Potentiometer
Encoders
Reflective/slotted opto-switch
LVDT
Motor
Solenoid
Panel Meters
Speed
Tacho – generator
Reflective/slotted optocoupler
Doppler effect sensors
AC and DC Motors
Stepper Motor
Brake
Sound
Carbon microphone
Piezoelectric crystal
Bell
Buzzer
Loudspeaker

Elements of Industrial Automation Week 03 Notes.pdf

More Related Content

What's hot (20)

Similar to Elements of Industrial Automation Week 03 Notes.pdf (20)

More from THANMAY JS (20)

Recently uploaded (20)

Elements of Industrial Automation Week 03 Notes.pdf