Chapter 3: Sensor and Application
By Tesfaye M.
Electrical Measurements and Instrumentation
(Introduction to Instrumentation Engineering)
20/12/19
Debre Markos University
ደብረ ማርቆስ ዩኒቨርስቲ ማርቆስ ዩኒቨርስቲ ዩኒቨርስ ዩኒቨርስቲቲ
Debre Markos Institute of Technology
ደብረ ማርቆስ ዩኒቨርስቲ ማርቆስ ዩኒቨርስቲ ቴክኖሎጂ ኢንስቲትዩት ኢንስ ዩኒቨርስቲቲትዩት
Department of Electrical and Computer Engineering
Goal of the chapter
•
classification of Transducers
•
Mechanical system Measurement
•
Thermal measurement
•
etc….
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Sensor and Application
• Measurement of both electrical and non-electrical quantities are required
industrial systems and research.
• The electrical quantities includes: voltage, current, impedance parameters, and
phase, frequency and time.
• The non-electrical measurand may be force, velocity, accelerations, mass,
pressure, strain, etc.
• Measurement of non-electrical quantities requires application of transducers, a
device that converts non-electrical quantities or signals into electrical signal that
can be conditioned.
Question
•What is the difference between sensor and transducer ?
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Classification of Transducer
•“A transducer is a device that converts a signal from one physical form to a
corresponding signal having a different physical form. Therefore, it is an energy
converter. ”
•Input signals may be electrical, mechanical, thermal, magnetic, chemical, and
radiation (corpuscular and electromagnetic)
•Sensors are a device which can quantitatively measure a certain quantity and
ideally no energy is drawn from a system being measured.
•Based on their output signal, the transducers are categorized into two:
• Active; generate voltage or current signal
• Passive; generate a change in measurable parameter like impedance variation
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Classification of Transducer
•The input-output relation of transducers can be represented as:
Qo = f (Qi )
• The general classification of transducer based on input signal:
Mechanical
• Force, pressure, and displacement ⇒ impedance variation: resistance,
capacitance, and
• inductance
• Speed, acceleration ⇒ electromotive force (EMF) due to electrodynamic and
piezoelectric effect
(a) Dynamometer
Piezoresistive pressure
(d) Tachometer
(b) Gyroscope
(e) Anemometer
(c)
(f)
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Classification of Transducer
Thermal
• Temperature change ⇒ resistance variation e.g. thermometer, thermistor with
positive and negative coefficient
• Seeback effect ⇒ generate emf and thermo-couple effect
• Thermal radiation intensity ⇒ radiation pyrometer
(g) NTC-thermistor
pyrometer
(h) PTC-thermistor
(j)Thermal camera
thermal camera
Question
•
(i) Optical
(k) Image captured by
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Classification of Transducer
Seeback effect
•When the two terminal of electrically connected conducting wire is set at different
temperature, the electrons at the hot junction at higher thermal velocity diffuse to
cold junction.
•Such electron diffusion causes unbalanced charge concentration between cold and
hot junction as the result, EMF is induced between them.
•Seeback has thought that he invented way of thermal to electromagnetic energy
conversion method. Later it is discovered that electron diffusion produce a magnetic
field (a change in emf).
seeback effect
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Classification of Transducer
•The produced emf between two terminals depends on magnitude of temperature
difference ∆T1,2 and material properties through linear relationship defining seeback
coefficient S for material.
Esb = S∆T 1 ,2
•The Seeback coefficient (S) can be experimentally determined using following
circuit.
(a) Thermoelectric seeback module
(b) Seeback generator
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Classification of Transducer
Optical
• Luminous flux variation = ⇒ resistance variation eg. photo-resistors,
photo-diodes (transistors) and photo-multiplier
• Photo-electric effect =⇒ EMF: photo-voltaic cell
Infrared Reflective sensor
Module
Optical sensor
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Classification of Transducer
Magnetic
• Magnetic flux variation ⇒ Hall effect, magnetic resonance, magnetic
recording Geometric change of magnetic circuit ⇒ Impedance change:
inductance and flow measurement
• Permeability change ⇒ variable voltage transformation
Molecular e.g. PH meter, electrolytic sensor
Miscellaneous transducers
• Radioactive and nuclear reaction detector (Scintillation counter)
Humidity sensor ⇒ Politer effect (reverse of seeback effect)
• Viscosity variation alters torque in rotating disk which is coupled
with electric motor.
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Mechanical system Measurement
Displacement Measurement
Potentiometer
Linear and rotary potentiometer
Schematic diagram of rectilinear and angular displacement
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Mechanical system Measurement
• For the given potentiometer in schematic diagram, the open circuit voltage E t h is
given by:
•For the linear(rectilinear) potentiometer the open circuit output voltage is given by:
•The open circuit or no-load output voltage of angular or rotary potentiometer with
constant per unit resistance can be expressed as:
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Mechanical system Measurement
• The maximum displacement (θT & dT ) depends on geometric design of
potentiometer.
• If the potentiometer is required to select potentiometer for specific application,
the power rating W m a x should be greater than actual rating W m a x = V 2 /R p .
•The potentiometer can be made of wire wound or conductive plastic.
•In wire wound potentiometer resistive track there is n discrete turns as slider move
(dT or θT ). As the result, the resolution error becomes dT /n or θT /n
•The conductive plastic potentiometer has smooth and continuous track where no
resolution error occur as slider moves, but it has high temperature coefficient.
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Mechanical system Measurement
Capacitive displacement sensors
• The simplest capacitor consists of two parallel plate separated by dielectric
material and its capacitance is proportional to area of plate (A), inter-plate gap
(d), and permittivity of dielectric ( ɛ ) material.
•
The parallel plate capacitor can be used as: variable separation (d ± ∆d),
variable area (A ± ∆A), variable dielectric (∆ɛ).
Parallel plate capacitive sensor
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Mechanical system Measurement
•
•
•
•
•
Variable separation displacement sensor
Variable area displacement sensor
Variable dielectric displacement sensor
capacitive pressure sensor
Capacitive level sensor
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Mechanical system Measurement
Inductive Displacement Sensor
• Inductive displacement sensors can be variable reluctance, mutual inductance, or
eddy current type.
Variable reluctance
• The self-inductance of magnetic circuit, depends on number of turns and
reluctance of magnetic circuit.
Where: N is number of turns and R is reluctance.
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Mechanical system Measurement
•
For the medium with l mean flux path, A cross-sectional area,
and µr relative permeability, the reluctance is calculated by;
•
Thus variable reluctance is achieved by altering relative
permeability (µr ) (i.e. change in medium), or mean flux path to
cross-sectional area ratio (l/A).
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Mechanical system Measurement
• Different configuration of variable reluctance displacement sensor
• small change in air gap cause
significant change in inductance
of the variable reluctance
inductive sensor.
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Mechanical system Measurement
Linear variable differential transformer (LVDT)
• Electromechanical device designed to produce an AC voltage output proportional
to relative displacement of transformer and iron core.
• This sensor is a transformer with a single primary winding and two identical
secondary windings wound on a tubular ferromagnetic former
• It consists a primary coil between two secondary coils that can be connected
either in series additive or subtractive fashion.
• The LVDT primary coil is excited by 1 − 15 volt amplitude at 50 Hz − 20 kHz
frequency range.
• Most commonly, the 1.0 V olt amplitude at 10 kHz ac signal is used for
excitation.
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Thermal Measurement
Thermoelectric or thermocouple sensing elements
• They are commonly used for measuring temperature.
• If two different metals are joined together, there is a difference in
electric potential across junction called junction potential.
commercially available thermocouple
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Thermal Measurement
• Junction potential depends on the metals A and B and the temperature T °C of the
junction, and is given by a power series of the form:
• The values of constants a1, a2, etc., depend on the metals A and B.
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Thermal Measurement
Law of Thermocouple Behavior
I) Junction emf of thermocouple depends on temperature of
junction and independent of temperature of wire connecting
the junctions.
II) If third metal whose junction temperature is introduced
between thermocouple metals, the junction temperature
remain unchanged.
Thus the voltmeter can be introduced to measure output voltage
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Thermal Measurement
III) If third metal is inserted between to thermocouple-metal at either junction
provided that the two new junctions are at the same temperature ( T1 or T2), the emf
is unchanged.
• Thus the thermocouple junctions can be soldered, brazed with
other metal without affecting its characteristics.
IV) Law of intermediate metal is used to determine
thermocouple effect of unknown metal, provided that the two
other metals are known.
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Thermal Measurement
Thermistor
•It is the resistive temperature sensor made from semiconductor material.
NTC Thermistors
• The most commonly used type is prepared from oxides of the iron group of
transition metal elements such as chromium, manganese, iron, cobalt and nickel.
• The resistance of such element decrease with increase in temperature are known as
Negative Temperature Coefficient.
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Thermal Measurement
NTC Thermistors
Where K, and β are constants and θ is temperature in Kelvin.
• The most commonly used alternative equation is:
Where R θ 1 is reference temperature θ 1 K which is commonly 25 oC
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Strain Measurement
•Stress is defined as the ratio of force over area experienced by body. It can be either
tensile stress (+F/A) which tends to increase length of material or compressive stress
(−F/A) which tends to reduce the length of body.
•Strain is the effect of applied stress in the body.
Where ∆l is change in length, and l0 original unstressed length.
•Longitudinal strain (e) is positive for tensile stress (e = +∆l/l 0 )
and negative for compressive stress (e = −∆l/l0 ).
•Over a certain range of values, relation between stress and strain
is linear. The ratio of stress and strain is known as elastic
modulus.
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Strain Measurement
•Elastic modulus can be:
• Young’s modulus (E): the linear tensile and compressive
stress
• Shear modulus (S): elastic modulus of shear stress.
•Longitudinal tensile strain is accompanied by increase in length
and decrease in cross-sectional area. Therefore, longitudinal tensile
strain(el ) is accompanied by transversal compressive strain (eT ).
et = −vel where v is Poisson’s ratio
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Strain Measurement
The resistance of strain gauge of area (A) and length (l) is given by:
The strain gauge is a semiconductor or metal material whose resistance changes
under strain.
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• The longitudinal tensile strain which is accompanied by
compressive transversal strain can be shown as:
• Gauge factor of strain gauge,
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THANK YOU…..
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