ECT418 - MECHATRONICS

Temperature, flow, pressure sensors, Resolvers

and synchros. Piezoelectric sensors. Acoustic
Emission sensors. vibration sensors. Force and
tactile sensors. Range finders: ultrasonic and light
based range finders
 Temperature Sensors
• Temperature- is the degree of hotness or
coldness
• By the law of thermodynamics heat flows from
higher temperature to lower temperature in
the absence of other effects.
• Pressure, Volume, Electrical resistance,
Expansion coefficient, etc. are related to
temperature. ie. they change with temperature
 Instruments used for temperature
measure are:
• Bimetallic strip
• Resistance temperature detectors (RTDs)
• Thermocouple
• Thermopile
• Thermistor
• Thermo-Diodes and Thermo- Transistors
• Pyrometers
– Total radiation
– Optical Pyrometer
 Bimetallic Strip
• A bimetallic strip is used to convert a
temperature change into mechanical
displacement.
• The method is based on Principle of change in
dimension of the metal, that is expand or contract
when there is a change in temperature.
• Expansion or contraction depends on the thermal
expansion coefficient.
• The strip consists of two strips of different metals
which expand at different rates as they are heated,
usually steel and copper, or in some cases steel
and brass.
Bimetallic strips
Bimetallic thermostat
-
• The strips are joined together throughout their
length by riveting, brazing or welding.
• The different expansions force the flat strip to bend
one way if heated, and in the opposite direction if
cooled below its initial temperature.
• The metal with the higher coefficient of thermal
expansion is on the outer side of the curve when
the strip is heated and on the inner side when
cooled.
• The bimetallic strip material should have
high coefficient of expansion, high ductility,
high modulus of elasticity and high electrical
conductivity.
Resistance temperature
detectors (RTDs)
• The material used for RTD are nickel, iron,
platinum, copper, lead, tungsten, mercury,
Manganin, silver etc.
• Platinum is the commonly used RTD, as it having
linear relation over the temperature range between
-2000c to 400c
Resistance temperature detectors (RTDs)
Pharmaceutical Industry
VIRUS INACTIVATION UNIT
 Thermistor
• Thermistors are temperature sensitive variable
resistors made of a ceramic – like semiconducting
material
• Made of metal oxide, cobalt, nickel etc.
• Thermistor respond negatively to temperature
(negative temperature coefficient of resistance-NTC).
• For each 10C rise in temperature, The resistance of a
thermistor decreases by 5%.
• High sensitivity to temperature changes make the
thermistor useful in precision measurements.
• Resistance varies from 0.5ohms to 0.75M ohms
Thermistor - types
 Thermocouple
• Most frequently used method to measure
temperatures with an electrical output signal.
• Basic Principle: When two dissimilar metals
are joined together an emf will exist between
the two points A and B, which is primarily a
function of junction temperature.
• This principle is called Seebeck effect
• The potential difference depends on the
metals used and the temperature of the
junction.
• If both the junctions are at the same
temperature there is no emf
Commonly used thermocouple and
its range
Thermocouple Material Vs EMF
Types T, J, and K are most commonly used thermocouples
(see Table 16.8 of the “Handbook”).
Thermocouple
 Thermocouple Laws
• 1. Law of intermediate metals: Insertion of
an intermediate metal into a thermocouple
circuit will not affect the emf voltage output
so long as the two junctions are at the same
temperature and the material is
homogeneous.
 Law of Intermediate Temperatures
If a thermocouple circuit develops a
net emf1-2 for measuring junction
T2 temperatures T1 and T2, and a net
T1
emf2-3 for temperatures T2 and T3,
then it will develop a net voltage
of emf1-3 = emf1-2 + emf2-3 when
T3 T2 the junctions are at temperatures
T1 and T3.

T3 T1 emf1-2+ emf2-3= emf1-3

 Thermopile
• A thermopile is an
device that converts
Thermal energy into electrical
energy.
• It is composed of several
thermocouples connected
usually in series or, less
commonly, in parallel
Automobile
Water related equipment
Air Conditioning Equipment
Fire Prevention and Security
Equipment
Food Processing Machines and
Cooking Appliances
Office Automation Equipment
Thermodiodes and transistors
Thermodiodes and transistors
Fluid Pressure Sensors
Wheatstone bridge
Fluid Pressure Sensors
Fluid Pressure Sensors
Fluid Pressure Sensors
Fluid Pressure Sensors
 Piezo electric sensor
• A piezoelectric transducer (also known as a
piezoelectric sensor) is a device that uses the
piezoelectric effect to measure changes in acceleration,
pressure, strain, temperature or force by converting
this energy into an electrical charge.
• A transducer can be anything which converts one form
of energy to another.
• Piezoelectric material is one kind of transducers.
When we squeeze this piezoelectric material or apply
any force or pressure, the transducer converts this
energy into voltage.
• This voltage is a function of the force or pressure
applied to it.
• The electric voltage produced by a
piezoelectric transducer can be easily
measured by voltage measuring instruments.
• Since this voltage will be a function of the
force or pressure applied to it, we can infer
what the force/pressure was by the voltage
reading.
• In this way, physical quantities like
mechanical stress or force can be measured
directly by using a piezoelectric transducer.
Piezoelectric Actuator
• A piezoelectric actuator behaves in the
reverse manner of the piezoelectric sensor.
• It is the one in which the electric effect will
cause the material to deform i.e. stretch or
bend.
• That means in piezoelectric sensor, when
force is applied to stretch or bend it, an
electric potential is generated and in opposite
when on a piezoelectric actuator, an electric
potential is applied it is deformed i.e.
stretched or bend.
• A piezoelectric transducer consists of quartz crystal
which is made from silicon and oxygen arranged in
crystalline structure (SiO2).
• Generally, unit cell (basic repeating unit) of all crystal
is symmetrical but in piezoelectric quartz crystal it is
not.
• Piezoelectric crystals are electrically neutral.
• The atoms inside them may not be symmetrically
arranged but their electrical charges are balanced
means positive charges cancel out negative charge.
• The quartz crystal has the unique property of
generating electrical polarity when mechanical stress
applied on it along a certain plane.
• There are two types of stress. One is compressive
stress and other is tensile stress.
• When there is unstressed quartz no charges
induce on it.
• In the case of compressive stress, positive
charges are induced in one side and negative
charges are induced in the opposite side.
• The crystal size gets thinner and longer due
to compressive stress.
Piezoelectric sensors
Piezoelectric sensors
 Piezoelectric sensors

Piezoelectric sensors are used for the measurement of

pressure, force and acceleration.
Flow sensors
Flow sensors
Flow sensors
Flow sensors
Flow sensors
Flow sensors
Flow sensors
Flow sensors