department of electrical and computer engineering introduction to instrumentation engineering...
TRANSCRIPT
Department of Electrical and Computer Engineering
Introduction to Instrumentation Engineering
Chapter 3: Sensors and Transducers
By Sintayehu Challa
Goals of the Chapter
Define classification of sensors and some terminologies Introduce various types of sensors for measurement
purpose and their applications Example: Displacement, motion, level, pressure, temperature, …
2Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Overview
Introduction Classification of sensors Passive sensors Active sensors
3Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Introduction
Sensors Elements which generate variation of electrical quantities (EQ) in
response to variation of non-electrical quantities (NEQ)
Examples of NEQ Temperature, displacement, humidity, fluid flow, speed, pressure,…
Sensor are sometimes called transducers
4
Sensing element
Signal conditioning
element
Signal conversion/processing
element
Output presentation
Non-electrical quantity
Electricalsignal
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Introduction …
Advantages of using sensors include 1. Mechanical effects such as friction is reduced to the minimum
possibility
2. Very small power is required for controlling the electrical system
3. The electrical output can be amplified to any desired level
4. The electrical output can be detected and recorded remotely at a distance from the sensing medium and use modern digital computers
5. etc …
5Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Introduction - Use of Sensors
1. Information gathering: Provide data for display purpose This gives an understanding of the current status of the system
parameters Example: Car speed sensor and speedometer, which records the
speed of a car against time
2. System control: Signal from the sensor is an input to a controller
6
Controller
System under control Output signalDesires signal
Sensor
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Introduction – Sensor Requirements
The main function of a sensor is to respond only for the measurement under specified limits for which it is designed
Sensors should meet the following basic requirements 1. Ruggedness: Capable of withstanding overload
Some safety arrangements should be provided for overload protection
2. Linearity: Its input-output characteristics must be linear
3. Repeatability: It should reproduce the same output signal when the same input is applied again and again
4. High output signal quality
5. High reliability and stability
6. Good dynamic response
7. No hysteresis, …
7Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Overview
Introduction Classification of sensors Passive sensors Active sensors
8Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Classification of Sensors
Sensors can be divided on the basis of Method of applications Method of energy conversion used Nature of output signals Electrical principle
In general, the classification of sensors is given by Primary and secondary sensors Active and passive sensors Analog and Digital sensors
9Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Primary and Secondary Sensors
Classification is based on the method of application Primary sensor
The input NEQ is directly sensed by the sensor The physical phenomenon is converted into another NEQ
Secondary sensor The output of the primary sensor is fed to another (secondary)
sensor that converts the NEQ to EQ
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Load cell
Strain- gauge
NEQ
Secondary sensor
Weight(Force F)
Primary sensor
NEQ EQ
Displacementd
Resistance R
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Active and Passive Sensor Classification based on the basis of energy conversion
Active sensor Generates voltage/current in response to NEQ variation Are also called self-generating sensors Normally, the output of active sensors is in V or mV
Examples Thermocouples: A change in temperature produces output voltage Photovoltaic cell: Change solar energy into voltage Hall-effect sensors, …
11
Active sensors
NEQ
Ex. Temperature
EQ
Voltage or current
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Active and Passive ….
Passive sensors Sensors that does not generate voltage or current, but produce element variation
in R, L, or C Need an additional circuit to produce voltage or current variation
Examples Thermistor: Change in temperature leads to change in resistance Photo resistor: Change in light leads to change in resistance Straingauge: Change in length or position into change in resistance) LVDT, Mic
12
Passive sensors
NEQ R, L, C
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Analog and Digital Sensors
Classification based on the nature of the output signal Analog sensor
Gives an output that varies continuously as the input changes Output can have infinite number of values within the sensor’s range
Digital sensor Has an output that varies in discrete steps or pulses or sampled form
and so can have a finite number of values E.g., Revolution counter: A cam, attached to a revolving body whose
motion is being measured, opens and closes a switch The switching operations are counted by an electronic counter
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Sensor Classification
Sensors can also be classified according to the application Example
Measurement of displacement, motion, temperature, intensity, sensors
14Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Overview
Introduction Classification of sensors Passive sensors
Resistive sensors Potentiometers, temperature dependent resistors, strain gauge,
photoconductors (photoresistors), Piezoresistive
Capacitive sensors Inductive sensors
Active sensors
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Resistive Sensors - Potentiometer
Examples: Displacement, liquid level (in petrol-tank level indicator) using potentiometer or rheostat Convert s linear (translatory) or angular (rotary) displacement
into a change of resistance in the resistive element provided with a movable contact
16
Petrol-tank level indicator Change in petrol level moves a
potentiometer arm Output signal is proportion to the
external voltage source applied across the potentiometer
The energy in the output signal comes from the external power source
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 17
Resistive Sensors – Potentiometer …
A linear or rotary movement of a moving contact on a slide wire indicates the magnitude of the variable as a change in resistance which can easily be converted by a proper electrical circuit into measurements of volt or current
18Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Resistive Sensors – Temperature Dependent Resistors
Two classes of thermal resistors are Metallic element Semiconductor
For most metals, the resistance increases with increase in temperature
Where is the temperature coefficient of resistance and given as
Example: Platinum Has a linear temperature-resistance characteristics Reproducible over a wide range of temperature Platinum Thermometers are used for temperature measurement
19
]1[...]1[)(R 02
210 TRTTRT
0
1
R
R
T
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 20
Resistive Sensors – Temperature Dependent…
Semiconductor based resistance thermometers elements The resistance of such elements decreases with increasing
temperature Example: Thermistor
The resistance-temperature relationship is non-linear and governed by
Where R0 is the resistance at absolute temp (in Kelvin) and is material constant expressed in degree Kelvin
Most semiconductor materials used for thermometry possess high resistivity and high negative temperature coefficients
21
KTeRT TT 00
)11
(
0 300;)(R 0
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Resistive Sensors – Temperature Dependent…
The temperature coefficient of resistance is
is typically 4000 k and for T = 300k,
22
20
1
TR
R
T
044.0300
400022
T
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 23
ECEg535:-Instrumentation Eng'g By Sintayehu Challa
strain gauge
If a strip of conductive metal is stretched, it will become skinnier and longer, both changes resulting in an increase of electrical resistance end-to-end.
Conversely, if a strip of conductive metal is placed under compressive force (without buckling), it will broaden and shorten. Such a device is called a strain gauge.
ECEg535:-Instrumentation Eng'g By Sintayehu Challa
Contd.
Most strain gauges are smaller than a postage stamp, and they look something like this
Resistive Sensors – Strain Gauges
Is a secondary transducer that senses tensile or compressive strain in a particular direction at a point on the surface of a body or structure
Used to measure force, pressure, displacement
Where e=l/l is the strain The resistance of an unstrained conductor is given as
Under strained condition, resistance of conductor changes by R because of l, A, and/or
26
)(R eR
A
lR
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Resistive Sensors – Strain Gauges …
To find the change in resistance R,
Dividing both sides by R, we get the fractional change as
Let us define eL = l/l as the longitudinal stain and eT as the transversal strain
Also assume that eT = -eL ,where is the Poisson’s Ratio
27
A
lA
A
ll
A
RA
A
Rl
l
R
2
R
A
A
l
l
R
R
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Resistive Sensors – Strain Gauges …
Then, Gauge Factor, G is defined as
G is also known as Strain-Sensitivity factor; rearranging terms, we get
Where is the Piezoresistive term
For most metals, the Piezoresistive term is about 0.4 and 0.2 < < 0.5
Thus, Gauge factor for metallic stain gauges is in the range 2.0–2.5 (not sensitive)
28
LeG
/)21(
Le
/
LellG
R/R
/
R/R
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Resistive Sensors – Strain Gauges …
Sensitive measurements require very high Gauge factors in the range of 100-300
Such factor can be obtained from semiconductor strain gauges
Due to the significant contribution from the Piezoresistive term
29Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Piezoresistive Pressure Sensor
Piezoresistivity is a strain dependent resistivity in a single crystal semiconductor
When pressure is applied to the diaphragm, it causes a strain in the resistor
Resistance change is proportional to this strain, and hence change in pressure
30Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Resistive Sensors – Photoconductor
Are light sensitive resistors with non-linear negative temperature coefficient
Are resistive optical radiation transducers
Photoconductors have resistance variation that depends on illumination
The resistance illumination characteristics is given by
Where RD is Dark Resistance and E is illumination level in Lux
Photoconductors are used in Cameras, light sensors in spectrophotometer Counting systems where an object interrupts a light beam hitting
the photoconductor, etc.
31
EDeR
R
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Photoconductive Transducers
A voltage is impressed on the semiconductor material When light strikes the semiconductor material, there is a
decrease in the resistance resulting in an increase in the current indicated by the meter
They enjoy a wide range of applications and are useful for measurement of radiation at all levels
The schematic diagram of this device is shown below
32Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Photovoltaic Cells
When light strikes the barrier between the transparent metal layer and the semiconductor material, a voltage is generated
The output of the device is strongly dependent on the load resistance R
The most widely used applications is the light exposure meter in photographic work
Schematic of a photovoltaic cell.
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Overview
Introduction Classification of sensors Passive sensors
Resistive sensors Capacitive sensors Inductive sensors
Active sensors
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Capacitive Transducers
The parallel plate capacitance is given by
d= distance between plates A=overlapping area 0 = 8.85x10-12 F/m is the absolute permittivity, r =dielectric
constant (r =1 for air and r =3 for plastics)
d
AC r 0
Displacement measurement can be achieved by varying d, overlapping area A and the dielectric constant
Schematic of a capacitive transducer.
35Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Capacitive Transducers – Liquid Level Measurement
A simple application of such a transducer is for liquid measurement
The dielectric constant changes between the electrodes as long as there is a change in the level of the liquid
Capacitive transducer for liquid level measurement.
36Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Capacitive Transducers – Pressure Sensor
Use electrical property of a capacitor to measure the displacement
Diaphragm: elastic pressure sensor displaced in proportion to change in pressure
Acts as a plate of a capacitor
37Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Capacitive Transducers – Pressure Sensor
Components: Fixed plate, diaphragm, displaying device, dielectric material (air)
When the diaphragm deflects, there is change in the gap between the two plates which in turn deflects the meter
dC
1
38
Capacitance C of the capacitor is inversely proportional to distance d between the plates, i.e.,
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Capacitive Transducers - Linear Displacement
Variable area capacitance displacement transducer
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Overview
Introduction Classification of sensors Passive sensors
Resistive sensors Capacitive sensors Inductive sensors
Active sensors
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Inductive Sensors
For a coil of n turns, the inductance L is given by
Where N: Number: of turns of the coil l: Mean length of the magnetic path A: Area of the magnetic path : Permeability of the magnetic material R: Magnetic reluctance of the circuit
Application of inductive sensors Force, displacement, pressure, …
Inductance variation can be in the form of Self inductance or Mutual inductance: e.g., differential transformer
41
R
N
l
NAL
22
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Input voltage (alternating current): One primary coil There will be a magnetic coupling between the core and the coils
Output voltage: Two secondary coils connected in series Operates using the principle of variation of mutual
inductance
Linear Variable Differential Transformer (LVDT)
Schematic diagram of a differential transformer
The output voltage is a function of the core’s displacement
Widely used for translating linear motion into an electrical signal
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LVDT - Output Characteristics
Output characteristics of an LVDT
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LVDT – Applications
Measure linear mechanical displacement Provides resolution about 0.05mm, operating range from 0.1mm
to 300 mm, accuracy of 0.5% of full-scale reading The input ac excitation of LVDT can range in frequency from 50 Hz
to 20kHz
Used to measure position in control systems and precision manufacturing
Can also be used to measure force, pressure, acceleration, etc..
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LVDT – Bourdon Tube Pressure Gauge
LVDT can be combined with a Bourdon tube
LVDT converts displacement into an electrical signal
The signal can be displayed on an electrical device calibrated in terms of pressure
45Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
When pressure is applied via the hole, the bellow expands a distance d
This displacement can be calibrated in terms of pressure
Where: d: distance moved by the bellows in m, A is cross sectional area of the bellow in m2
is the stiffness of the below in N.m-1
LVDT – Bellows
The bellow is held inside a protective casing Differential pressure sensor
Used to measure low pressure
46
A
dPunknown
Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
LVDT and Bellow Combination
Bellows produce small displacement Amplified by LVDT and potentiometer
47Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Overview
Introduction Classification of sensors Passive sensors Active sensors
Thermoelectric transducers Photoelectric transducers Piezoelectric transducers Hall-effect transudes Tachometric generators
48Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Active Sensors - Thermocouple
Thermoelectric transducers provide electrical signal in response to change in temperature
Example: Thermocouple
Thermocouple: Converts thermal energy into electrical energy
Application: To measure temperature
Contains a pair of dissimilar metal wires joined together at one end (sensing or hot junction) and terminated at the other end (reference or cold junction)
When a temperature difference exists b/n the sensing junction and the reference, an emf is produced
49
)(....)()( 212
22
121 TTTTTTEemfInduced
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers 50
Active Sensors – Thermocouple …
Typical material combinations used as thermocouples
To get higher output emf Connect two or more Thermocouples in series
For measurement of average temperature Connect Thermocouples in parallel
51
Type Materials Temp. Range Output voltage (mV)
T Copper-Constantan -2000C to 3500C -5.6 to 17.82
J Iron-Constantan 0 to 7500C 0 to 42.28
E Chromel-Constantan -200 to 9000C -8.82 to 68.78
K Chromel-Alumel -200 to 12500C -5.97 to 50.63
R Platinum = 13%Rhodium = 87%
0 to 14500C 0 to 16.74
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 52
Active Sensors – Thermocouple …
Applications Temperature measurement Voltage measurement
Rectifier based rms indications are waveform dependent They are normally designed for sinusoidal signals
Hence, error for non-sinusoidal signals
Use thermocouple based voltmeters Here, temperature of a hot junction is proportional to the true rms
value of the current
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Active Sensors – Thermocouple Meter
The measured a.c. voltage signal is applied to a heater element
A thermocouple senses the temperature of the heater due to heat generated ( )
The d.c. voltage generated in the thermocouple is applied to a moving-coil meter
The thermocouple will be calibrated to read current (Irms)
AC with frequencies up to 100 MHz may be measured with thermocouple meters
One may also measure high frequency current by first rectifying the signal to DC and then measuring the DC
Schematic of a thermocouple meter.
2rmsI
54Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Overview
Introduction Classification of sensors Passive sensors Active sensors
Thermoelectric transducers Photoelectric transducers Piezoelectric transducers Hall-effect transudes Tachometric generators
55Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Photoelectric Transducers
Versatile tools for detecting radiant energy or light Are extensively used in instrumentation
Most known photosensitive devices include
1.Photovoltaic cells Semiconductor junction devices used to convert radiation energy
into electrical energy
56Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Photoelectric Transducers …
2. Photo diode A diode that is normally reverse-biased=> Current is very low When a photon is absorbed, electrons are freed so current starts
to flow, i.e., the diode is forward biased Has an opening in its case containing a lens which focuses
incident light on the PN junction
3. Photo transistor Also operate in reverse-biased Responds to light intensity on its lens instead of base current
57Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Overview
Introduction Classification of sensors Passive sensors Active sensors
Thermoelectric transducers Photoelectric transducers Piezoelectric transducers Hall-effect transudes Tachometric generators
58Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Piezoelelectric Transducers
Convert mechanical energy into electrical energy If any crystal is subject to an external force F, there will be
an atomic displacement, x The displacement is related to the applied force in exactly the
same way as elastic sensor such as spring
Asymmetric crystalline material such as Quartz, Rochelle Salt and Barium Tantalite produce an emf when they are placed under stress
An externally force, entering the sensor through its pressure port, applies pressure to the top of a crystal
This produces an emf across the crystal proportional to the magnitude of the applied pressure
59Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Piezoelelectric Transducers
A piezoelectric crystal is placed between two plate electrodes
Application of force on such a plate will develop a stress and a corresponding deformation
The piezoelectric effect
With certain crystals, this deformation will produce a potential difference at the surface of the crystal
This effect is called piezoelectric effect
60Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Piezoelelectric Transducers …
Induced charge is proportional to the impressed force
Q = d F d= charge sensitivity (C/m2)/(N/m2) = proportionality constant
Output voltage E= g t P t= crystal thickness P = impressed pressure g=voltage sensitivity (V/m)/(N/m2)
Shear stress can also produce piezoelectric effect Widely used as inexpensive pressure transducers for
dynamic measurements
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Piezoelelectric Transducers ….
Piezoelelectric sensors have good frequency response Example: Accelerometer
62
Piezoelectric accelerometer
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Piezoelelectric Transducers …
Example: Pressure Sensors Detect pressure changes by
the displacement of a thin metal or semiconductor diaphragm
A pressure applied on the diaphragm causes a strain on the piezoelectric crystal
The crystal generates voltage at the output
This voltage is proportional to the applied pressure
63Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Overview
Introduction Classification of sensors Passive sensors Active sensors
Thermoelectric transducers Photoelectric transducers Piezoelectric transducers Hall-effect transudes Tachometric generators
64Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Hall-effect Transducers
Hall voltage is produced when a material is Kept perpendicular to the magnetic field and A direct current is passed through it
The Hall-voltage is expressed as
Where Ic: Control current flowing through the Hall-effect sensor, in Amps
B: Flux density of the magnetic field applied, in Wb/m2
t: Thickness of the Hall-effect sensor, in meters KH : Hall-effect coefficient
Hall-effect sensors are used to measure flux density Can detect very week magnetic fields or small change in magnetic
flux density
65
t
BIKV CHH
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Hall-effect Transducers …
Like active sensors, it generates voltage VH
It also need an external control current IC like passive sensors
The sensor can be used for measurement of
Magnetic quantities (B, ) Mobility of carriers Very small amount of power
66Introduction to Instrumntaion Eng‘g – Ch.3 Sensors and Transducers
Hall-effect Transducers …
Magnetic field forces electrons to concentrate on one side of the conductor (mainly uses semiconductor)
This accumulation creates emf, which is proportional to the magnetic field strength
Used in proximity sensors
67Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Overview
Introduction Classification of sensors Passive sensors Active sensors
Thermoelectric transducers Photoelectric transducers Piezoelectric transducers Hall-effect transudes Tachometric generators
68Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
Tachometric Generators
Tachometer – any device used to measure shaft’s rotation
Tachometric generator A machine, when driven by a rotating mechanical force, produces
an electric output proportional to the speed of rotation Essentially a small generators
Tachometric generators connect to the rotating shaft, whose displacement is to be measured, by, e.g.,
Direct coupling or Means of belts or gears
They produce an output which primarily relates to speed Displacement can be obtained by integrating speed
Types of Tachometric generators: Generally a.c. or d.c.
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Tachometric Generators
Voltage generated is proportional to rotation of the shaft
70
D.C. tachometric generator A.C. tachometric generator
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
NoticeMid Semester Exam On December
1,2012@8:30 A.M(Saturday) In Room 109 and 113
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 71