Electromagnetic and mechanical sensors

BY
P. MARIA SHEEBA
II ME COMMUNICATION SYSTEMS
NANO-SENSORS

A sensor is a device that measures a physical quantity and converts
it into a 'signal' which can be read by an observer or by an
instrument.
It is a type of transducer, (ie) they change one form of energy into
another.
Sensors are mostly electrical or electronic.
SENSORS ??

 Automobiles
 Machines
 Robotics
 Industry
 Medicine
 Aerospace etc.
APPLICATIONS

Types of Sensors (few)
Electromagnetic Mechanical
4. Power
1. Resistance
3. Voltage
2. Current 2. Position
3. Gas
1. Liquid
flow
Ohmmeter
Galvanome-
ter
Multimeter
Ammeter
Leaf
Electroscope
Voltmeter
Watt-hour
meters
Orifice
plate
Turbine
meter
Potentiom
-etric
MOS Type
Gas Sensor
4. Pressure
1. Absolute
2.Differential
3.Gauge

 The term “electromagnetic” refers to any form of energy that
is propagated as a wave.
 An electronic device used to measure a physical quantity
such as pressure or loudness and convert it into an electronic
signal of some kind (e.g. a voltage).
ELECTROMAGNETIC SENSORS

 Electrical resistance and electrical conductance sensors
measure the resistance or conductance of an electrical
component or system.
 Electrical resistance and conductance sensors are used in a
wide variety of industries, across a very broad spectrum of
applications.
1. ELECTRICAL RESISTANCE
SENSOR

 It is an instrument which measures resistance of a quantity.
 Every device has a resistance, it may be large or small and it
increases with temperature for conductors, however for
semiconducting devices the reverse is true.
 There are many types of ohmmeters available such as
1. Series ohmmeter.
2. Shunt ohmmeter.
3. Multi range ohmmeter.
OHMMETER

 The instrument is connected with a battery, a series adjustable resistor and an
instrument which gives the reading. The resistance to be measured is connected
at terminal ab. When the circuit is completed by connecting output resistance,
the circuit current flows and so the deflection is measured.
 When the resistance to be measured is very high then current in the circuit
will be very small and the reading of that instrument is assumed to be maximum
resistance to be measured. When resistance to be measured is zero then the
instrument reading is set to zero position which gives zero resistance.
CONTD..

 Almost all multimeters can
measure voltage, current,
and resistance.
 Some multimeters also
have a continuity check and
diode check.
MULTIMETER

Continuity Check
This is a setting used to check if
two things are electrically
connected. The multimeter will
beep if there is a conductive path
between the two probe tips, and will
not make any noise if there is no
conductive path.
Diode Check
This function is used to test a diode,
which is like a one-way valve for
electricity; it only lets current flow
in one direction. The exact function
of the diode check can be different
on different multimeters.

A current sensor is a device that detects electric current (AC or
DC) in a wire, and generates a signal proportional to it.
 The generated signal could be analog voltage or current or
even digital output.
 It can be then utilized to display the measured current in an
ammeter or can be stored for further analysis in a data
acquisition system or can be utilized for control purpose.
2. ELECTRICAL CURRENT SENSOR

 The sensed current and the output signal can be:
1. Alternating current input,
 analog output, which duplicates the wave shape of the sensed current.
 bipolar output, which duplicates the wave shape of the sensed current.
 unipolar output, which is proportional to the average or RMS value of the sensed
current.
2. Direct current input,
 unipolar, with a unipolar output, which duplicates the wave shape of the sensed
current
 digital output, which switches when the sensed current exceeds a certain threshold

A galvanometer is a type of ammeter. It is an instrument for
detecting and measuring electric current. It is an analog
electromechanical transducer that produces a rotary deflection,
through a limited arc, in response to electric current flowing
through its coil
GALVANOMETER

Ammeter means Ampere-
meter which measures ampere
value. Ampere is the unit of
current so an ammeter is a
meter or an instrument which
measures current.
AMMETER

 The main principle of ammeter is that it must have a very
low resistance and also inductive reactance.
It has very low impedance because it must have very low amount
of voltage drop across it and must be connected in series connection because
current is same in the series circuit.
Also due to very low impedence the power loss will be low and if it is
connected in parallel it becomes almost a short circuited path and all the current
will flow through ammeter as a result of high current the instrument may burn.
WORKING PRINCIPLE

 A voltage sensor can in fact determine, monitor and can measure the
supply of voltage.
 It can measure AC level or/and DC voltage level.
 The input to the voltage sensor is the voltage itself and the output can be
analog voltage signals, switches, audible signals, analog current level,
frequency or even frequency modulated outputs.
Mainly two types are of voltage sensors are available- Capacitive type
voltage and Resistive type voltage sensor.
3. ELECTRICAL VOLTAGE SENSOR

 They have been used for many
years for the accurate measurement
of charge.
Their operation is based on the
principle of like sign charge
repulsion.
LEAF ELECTROSCOPE

 Voltmeters provide a way to safely
measure the voltage, or the difference
in electric potential, between two
points in a circuit while not changing
the voltage in that circuit.
 The capability to measure voltage
is critical to designing and
maintaining advanced technology, but
it also has more common and
practical applications.
VOLTMETER

 Electrical power sensors are used to measure characteristics of
electric power such as voltage, current, power.
 For AC circuits, an electric power sensor can also be used to
measure power quality.
 These measurements are then converted into corresponding analog
or digital signal to be read by another device or displated.
4. ELECTRICAL POWER SENSORS

Hall Effect
Inductive sensors
Direct measurement
Voltage response measurements
TECHNOLOGIES

Energy meter or watt-
hour meter or is an
electrical instrument that
measures the amount of
electrical energy used by
the consumers.
WATT-HOUR METER

 An electrical energy meter, that is, an electricity meter that measures and
registers the integral, with respect to time, of the
power in the circuit in which it is connected.
 This instrument can be considered as having two parts: a transducer, which
converts the power into a mechanical or electrical signal, and a counter, which
integrates and displays the value of the total energy that has passed through
the meter. Either or both of these parts can be based on mechanical or
electronic principles.
WORKING

 Class of sensors to measure mechanical phenomena.
Includes
 Pressure sensor
 Force and torque sensor
 Inertial sensor
 Flow sensor
MECHANICAL SENSOR

 Gas sensor is a subclass of chemical sensors.
 Gas sensor measures the concentration of gas in its vicinity. Gas
sensor interacts with a gas to measure its concentration. Each gas has
a unique breakdown voltage i.e. the electric field at which it is
ionized. Sensor identifies gases by measuring these voltages. The
concentration of the gas can be determined by measuring the current
discharge in the device.
1. GAS SENSOR

 Process control industries
 Environmental monitoring
 Boiler control
 Fire detection
 Alcohol breath tests
 Detection of harmful gases in mines
 Home safety
 Grading of agro-products like coffee and spices
APPLICATIONS OF GAS SENSOR

Operating parameters:
 Operating temperature
 Operating humidity
Disadvantages:
 Bulky
 Consume lots of power
 Require “risky” high voltage to operate.

 Metal Oxide Based Gas Sensors
 Capacitance Based Gas Sensors
 Acoustic Wave Based Gas Sensors
 Calorimetric Gas Sensors
 Optical gas sensors
 Electrochemical gas sensors
GAS SENSING TECHNOLOGIES

 Metal oxide sensors are also known as chemiresistors.
 The detection principle of resistive sensors is based on change of the
resistance of a thin film upon adsorption of the gas molecules on the
surface of a semiconductor.
 The gas-solid interactions affect the resistance of the film because of
the density of electronic species in the film.
METAL OXIDE BASED GAS SENSORS

MOS TYPE GAS SENSOR (CARBON
MONOXIDE)

In clean air, donor electrons in tin
dioxide are attracted toward oxygen
which is adsorbed on the surface of
the sensing material, preventing
electric current flow.
In the presence of reducing gases,
the surface density of adsorbed
oxygen decreases as it reacts with
the reducing gases. Electrons are
then released into the tin dioxide,
allowing current to flow freely
through the sensor.
STEP1 STEP2

In the most extreme case where oxygen
concentration is 0%, when metal oxide sensor
material (typically tin dioxide [SnO2-x]) is
heated at high temperature such as 400˚C, free
electrons flow through the conjoined parts (grain
boundary) of tin dioxide crystals. In clean air
(approx.. 21% O2), oxygen is adsorbed on the
metal oxide surface. With its high electron
affinity, adsorbed oxygen attracts free electrons
inside the metal oxide, forming a potential
barrier (eVs in air) at the grain boundaries. This
potential barrier prevents electron flow, causing
high sensor resistance in clean air.
OPERATING PRINCIPLE

When the sensor is exposed to
combustible gas or reducing gas
(such as carbon monoxide), the
oxidation reaction of such gas with
adsorbed oxygen occurs at the
surface of tin dioxide.

As a result, the density of adsorbed
oxygen on the tin dioxide surface
decreases, and the height of the potential
barrier is reduced. Electrons easily flow
through the potential barrier of reduced
height, and the sensor resistance
decreases. Gas concentration in air can
be detected by measuring the resistance
change of MOS-type gas sensors. The
chemical reaction of gases and adsorbed
oxygen on the tin dioxide surface varies
depending on the reactivity of sensing
materials and working temperature of
the sensor.

 A flow sensor is a device for sensing the rate of flow of material.
 Material may be solid liquid or gas.
 Flow rate is measured by weighting the amount of material on a platform
of length L
WR
Q
L

flow (kg/min or lb/min)
weight of material on section of length L (kg or lb)
conveyor speed (m/min, or ft/min)
length of weighting platform
Q
W
R
L





 It is simply a disc, with a central hole, which is placed in the tube
through which the fluid is flowing.
 The pressure difference is measured between a point equal to the
diameter of the tube upstream and a point equal to half the diameter
downstream.
 It is simple, cheap, with no moving parts and is widely used.
 The accuracy is typically about +/- 1.5% of full range, it is non-linear,
and it does produce quite an appreciable pressure loss in the system to
which it is connected.

 The turbine flow meter consists of a multi-bladed rotor that is supported
centrally in the pipe along which the flow occurs.
 The fluid flow results in rotation of the rotor, the angular velocity being
approximately proportional to the flow rate.
 The rate of revolution of the rotor can be determined using a magnetic
pick-up.
The pulses are counted and so the number of revolutions of the rotor can
be determined.
 The meter is expensive with an accuracy of typically about +/- 0.3%.

Position sensors are basically sensors for measuring the distance travelled
by the body starting from its reference position. How far the body has
moved from its reference or initial position is sensed by the position sensors
and often the output is given as a fed back to the control system which takes
the appropriate action. Motion of the body can be rectilinear or curvilinear;
accordingly, position sensors are called linear position sensors or angular
position sensors.
3. POSITION SENSOR

Position sensors use different sensing principles to sense the displacement
of a body. Depending upon the different sensing principles used for
position sensors, they can be classified as follows:
1. Resistance-based or Potentiometric Position sensors
2. Capacitive position sensors
3. Linear Voltage Differential Transformers
4. Magnetostrictive Linear Position Sensor
5. Eddy Current based position Sensor
6. Hall Effect based Magnetic Position Sensors
7. Fiber-Optic Position Sensor
8. Optical Position Sensors
TYPES OF POSITION SENSOR

Potentiometric position sensor use resistive effect as the sensing principle.
The sensing element is simply a resistive (or conductive) track. A wiper is
attached to the body or part of the body whose displacement is to be
measured. The wiper is in contact with the track. As the wiper(with the
body or its part) moves, the resistance between one end of the track and the
wiper changes. Thus, the resistance becomes a function of the wiper
position. The change in resistance per unit change in wiper position is
linear.
POTENTIOMETRIC POSITION SENSORS

 Resistance, proportional to wiper position, is measured using voltage
divider arrangement. A constant voltage is applied across the ends of the
track and the voltage across the resistance between the wiper and one end of
the track is measured. Thus, voltage output across the wiper and one end of
the track is proportional to the wiper position.
 The conductive track can be made linear or angular depending upon the
requirements. The tracks are made from carbon , resistance wire or piezo
resistive material.

A pressure sensor is a device which senses
pressure and converts it into an analog
electric signal whose magnitude depends
upon the pressure applied. Since they convert
pressure into an electrical signal, they are
also termed as pressure transducers.
4. PRESSURE SENSOR

1. Touch screen devices
2. Automotive Industry
3. Biomedical Instrumentation
4. Industrial uses
5. Aviation
6. Marine Industry
NEED FOR PRESSURE SENSOR

1. Absolute Pressure Measurement
2. Differential Pressure Measurement
3. Gauge Pressure Measurement
TYPES OF PRESSURE
MEASUREMENTS

Pressure measured relative to perfect vacuum is termed as absolute
pressure. Perfect vacuum is a condition where there is no matter present
in the atmosphere and hence, nil air pressure exists in that region.
Absolute pressure sensors have limited usage because it is impossible to
attain a state of perfect vacuum. Hence, sensors based on absolute
pressure measurement require strict specifications for precise outputs.
Sensors based on this type of measurement are used in barometric or
altitude related pressure measurements.
ABSOLUTE PRESSURE
MEASUREMENT

 In differential pressure measurement, pressures of two distinct
positions are compared.
 For example, pressure difference calculated by measuring it at
different floors of a tall building will give us differential pressure.
 Differential pressure measurements, typically taken in pound per
square inch differential (psid), are applied when high amount of
pressure is to be measured.
 These types of measurements are used for feed pressure monitoring
purposes where the pressure with which the fluid is flowing in a
medium is monitored, so that homogeneity in the flow can be
maintained.
DIFFERENTIAL PRESSURE
MEASUREMENT

 Differential pressure measurements find an important application in
monitoring filters in various types of purification systems.
They take the reference of the normal pressure with which the filters
clean the fluid.
Whenever the filters face the problem of clogging due to
contaminants, these pressure sensors give a reading relative to the
normal pressure.
 This helps in keeping the filter clean and operational.
CONTD….

 It can be defined as a subtype of differential pressure measurement
where we compare pressure at any point to the current atmospheric
pressure.
Gauge pressure measurement is used in applications like tire pressure
or blood pressure measurement.
There is no consistency in gauge pressure measurements because
atmospheric pressure does vary with altitude and hence its applications
are limited to non-critical measurements.
GAUGE PRESSURE
MEASUREMENT

Electromagnetic and mechanical sensors

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