1. Introduction to Instrumentation Engineering
Chapter 3: Sensors and Transducers
By Sintayehu Challa

2. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
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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, …
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

3. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
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Overview
Introduction
Classification of sensors
Passive sensors
Active sensors
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

4. Introduction Sensors Examples of NEQ
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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
Sensing element
Signal conditioning element
Signal conversion/processing element
Output presentation
Non-electrical quantity
Electrical
signal
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

5. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
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Introduction …
Advantages of using sensors include
Mechanical effects such as friction is reduced to the minimum possibility
Very small power is required for controlling the electrical system
The electrical output can be amplified to any desired level
The electrical output can be detected and recorded remotely at a distance from the sensing medium and use modern digital computers
etc …
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

6. Introduction - Use of Sensors
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Introduction - Use of Sensors
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
System control: Signal from the sensor is an input to a controller
Controller
System under control
Output signal
Desires signal
Sensor
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

7. Introduction – Sensor Requirements
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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
Ruggedness: Capable of withstanding overload
Some safety arrangements should be provided for overload protection
Linearity: Its input-output characteristics must be linear
Repeatability: It should reproduce the same output signal when the same input is applied again and again
High output signal quality
High reliability and stability
Good dynamic response
No hysteresis, …
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

8. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
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Overview
Introduction
Classification of sensors
Passive sensors
Active sensors
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

9. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

10. Primary and Secondary Sensors
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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 EQ
NEQ
NEQ
Load
cell
Strain- gauge
Secondary sensor
Primary sensor
Weight
(Force F)
Displacement d
Resistance R
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

11. 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, …
NEQ EQ
Active sensors
Ex. Temperature
Voltage or current
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

12. 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
NEQ
R, L, C
Passive sensors
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

13. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

14. Sensor Classification
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Sensor Classification
Sensors can also be classified according to the application
Example
Measurement of displacement, motion, temperature, intensity, sensors
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

15. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
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Overview
Introduction
Classification of sensors
Passive sensors
Resistive sensors
Potentiometers, temperature dependent resistors, strain gauge, photoconductors (photoresistors), Piezoresistive
Capacitive sensors
Inductive sensors
Active sensors
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

16. Resistive Sensors - Potentiometer
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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
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

17. Instrum. & Control Eng. for Energy Systems - Ch
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers

18. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

19. Resistive Sensors – Temperature Dependent Resistors
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

20. Instrum. & Control Eng. for Energy Systems - Ch
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers

21. Resistive Sensors – Temperature Dependent…
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

22. Resistive Sensors – Temperature Dependent…
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Resistive Sensors – Temperature Dependent…
The temperature coefficient of resistance is
 is typically 4000 k and for T = 300k,
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

23. Instrum. & Control Eng. for Energy Systems - Ch
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers

24. 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

25. ECEg535:-Instrumentation Eng'g By Sintayehu Challa
Contd.
Most strain gauges are smaller than a postage stamp, and they look something like this
ECEg535:-Instrumentation Eng'g By Sintayehu Challa

26. Resistive Sensors – Strain Gauges
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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 
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

27. Resistive Sensors – Strain Gauges …
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

28. Resistive Sensors – Strain Gauges …
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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)
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

29. Resistive Sensors – Strain Gauges …
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Resistive Sensors – Strain Gauges …
Sensitive measurements require very high Gauge factors in the range of
Such factor can be obtained from semiconductor strain gauges
Due to the significant contribution from the Piezoresistive term
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

30. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

31. Resistive Sensors – Photoconductor
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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.
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

32. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

33. Introduction 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.
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

34. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
4/23/2017
Overview
Introduction
Classification of sensors
Passive sensors
Resistive sensors
Capacitive sensors
Inductive sensors
Active sensors
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

35. 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)
Displacement measurement can be achieved by varying d, overlapping area A and the dielectric constant
Schematic of a capacitive transducer.
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

36. 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.
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

37. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

38. 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
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

39. Capacitive Transducers - Linear Displacement
Variable area capacitance displacement transducer
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

40. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
4/23/2017
Overview
Introduction
Classification of sensors
Passive sensors
Resistive sensors
Capacitive sensors
Inductive sensors
Active sensors
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

41. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

42. Linear Variable Differential Transformer (LVDT)
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
The output voltage is a function of the core’s displacement
Widely used for translating linear motion into an electrical signal
Schematic diagram of a differential transformer
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

43. LVDT - Output Characteristics
Output characteristics of an LVDT
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

44. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
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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..
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

45. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

46. Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers
LVDT – Bellows
The bellow is held inside a protective casing
Differential pressure sensor
Used to measure low pressure
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
Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers

47. LVDT and Bellow Combination
Bellows produce small displacement
Amplified by LVDT and potentiometer
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

48. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
4/23/2017
Overview
Introduction
Classification of sensors
Passive sensors
Active sensors
Thermoelectric transducers
Photoelectric transducers
Piezoelectric transducers
Hall-effect transudes
Tachometric generators
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

49. Active Sensors - Thermocouple
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

50. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

51. Active Sensors – Thermocouple …
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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
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

52. Instrum. & Control Eng. for Energy Systems - Ch
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers

53. Active Sensors – Thermocouple …
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

54. 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.
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

55. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
4/23/2017
Overview
Introduction
Classification of sensors
Passive sensors
Active sensors
Thermoelectric transducers
Photoelectric transducers
Piezoelectric transducers
Hall-effect transudes
Tachometric generators
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

56. Photoelectric Transducers
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Photoelectric Transducers
Versatile tools for detecting radiant energy or light
Are extensively used in instrumentation
Most known photosensitive devices include
Photovoltaic cells
Semiconductor junction devices used to convert radiation energy into electrical energy
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

57. Photoelectric Transducers …
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Photoelectric Transducers …
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
Photo transistor
Also operate in reverse-biased
Responds to light intensity on its lens instead of base current
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

58. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
4/23/2017
Overview
Introduction
Classification of sensors
Passive sensors
Active sensors
Thermoelectric transducers
Photoelectric transducers
Piezoelectric transducers
Hall-effect transudes
Tachometric generators
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

59. Piezoelelectric Transducers
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

60. 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
With certain crystals, this deformation will produce a potential difference at the surface of the crystal
This effect is called piezoelectric effect
The piezoelectric effect
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

61. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

62. Piezoelelectric Transducers ….
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Piezoelelectric Transducers ….
Piezoelelectric sensors have good frequency response
Example: Accelerometer
Piezoelectric accelerometer
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

63. 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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

64. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
4/23/2017
Overview
Introduction
Classification of sensors
Passive sensors
Active sensors
Thermoelectric transducers
Photoelectric transducers
Piezoelectric transducers
Hall-effect transudes
Tachometric generators
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

65. Hall-effect Transducers
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

66. Hall-effect Transducers …
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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
Introduction to Instrumntaion Eng‘g – Ch.3 Sensors and Transducers

67. Hall-effect Transducers …
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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
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

68. Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers
4/23/2017
Overview
Introduction
Classification of sensors
Passive sensors
Active sensors
Thermoelectric transducers
Photoelectric transducers
Piezoelectric transducers
Hall-effect transudes
Tachometric generators
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

69. Tachometric Generators
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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.
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

70. Tachometric Generators
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Tachometric Generators
Voltage generated is proportional to rotation of the shaft
D.C. tachometric generator
A.C. tachometric generator
Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers

71. Notice Mid Semester Exam On December 1,2012@8:30 A
Notice Mid Semester Exam On December A.M(Saturday) In Room 109 and 113
Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers