1. Transducer & Sensor 1

2. Contents Objectives Sensor and Transducer Sensing Process
Definition of Transducer
Function of Transducer
Classification of Transducer
Selecting of Transducer
Temperature Transducer 2

3. Contents…contd Resistive Position Transducers Capacitive Transducers
Inductive Transducers
Strain Gauge
Photoelectric Transducer 3

4. Contents Objectives Sensor and Transducer Sensing Process
Definition of Transducer
Function of Transducer
Classification of Transducer
Selecting of Transducer
Temperature Transducer 4

5. Objectives
Ability to understanding the definition, functions & categories of transducers.
List the classes and types and examples of transducers.
Operations and applications for each transducers

6. Contents Objectives Sensor and Transducer Sensing Process
Definition of Transducer
Function of Transducer
Classification of Transducer
Selecting of Transducer
Temperature Transducer 6

7. Sensor and Transducer As a comparison……
Sensor is a device that detects a change in a physical stimulus and turns it into a signal which can be measured or recorded.
E.g. : Thermistor
Transducer is a device that transfers power from one system to another in the same or in the different form'.
E.g. Thermistor with it associate circuit convert heat to electricity.
As a comparison……
‘Sensor' for the sensing element itself and 'transducer' for the sensing element plus any associated circuitry. All transducers would thus contain a sensor and most (not all) sensors would also be transducers.

8. Contents Objectives Sensor and Transducer Sensing Process
Definition of Transducer
Function of Transducer
Classification of Transducer
Selecting of Transducer
Temperature Transducer 8

9. Sensing Process

10. Contents Objectives Sensor and Transducer Sensing Process
Definition of Transducer
Function of Transducer
Classification of Transducer
Selecting of Transducer
Temperature Transducer 10

11. Definition of a Transducer
Transducer is any device that converts energy in one form to another energy. The majority either convert electrical energy to mechanical displacement or convert some non-electrical physical quantity, such as temperature, sound or light to an electrical signal.

12. Type of Transducers Electrical Transducers Mechanical Transducers
Converts the input measurand into an electrical voltage/current
Non electrical
physical quantity
Mechanical Transducers
Converts the input measurand into a mechanical energy
Electrical signal
Transducer

13. Contents Objectives Sensor and Transducer Sensing Process
Definition of Transducer
Function of Transducer
Classification of Transducer
Selecting of Transducer
Temperature Transducer 13

14. Functions of Transducer
1. To sense the presence, magnitude, change in, and frequency of some measurand.
2. To provide an electrical output that, when appropriately
processed and applied to readout device, gives accurate
quantitative data about the measurand
Transducer
Electrical output
Measurand
Excitation
Measurand – refers to the quantity, property or condition which the transducer translates to an electrical signal.

15. Contents Objectives Sensor and Transducer Sensing Process
Definition of Transducer
Function of Transducer
Classification of Transducer
Selecting of Transducer
Temperature Transducer 15

16. Classification of Transducers
Transducer can be classified according to
their application, based primarily on the physical
quantity, property, or condition that is measured.
A transducer can be classified
(i)  On the basis of transduction form used,
(ii)  As primary and secondary transducers
(iii) As passive and active transducers
(iv) As analog and digital transducers and
(v)  As transducers and inverse transducers

17. Fig. Measurement of pressure using Bourdon tube and LVDT
Classification based upon principle of transduction
The transducers can be classified on the basis of principle of transduction as resistive, inductive, capacitive etc. depending upon how they convert the input quantity into resistance, inductance or capacitance respectively.
2. Primary and Secondary Transducers
Primary transducers are transducers which convert a physical quantity into another form. But the secondary transducers convert the output signal from the primary transducer into a usable output (i.e. an electrical signal).
For eg., the Bourdon tube converts the input pressure into a displacement, as primary transducer and the
LVDT converts this displacement into an analogous voltage, as secondary transducer.
Fig. Measurement of pressure using Bourdon tube and LVDT

18. 3. Passive and Active Transducers
Generates an electrical signal directly in response to the physical parameter and does not require an external power source for its operation. Thus active transducers are self-generating devices.
Typical examples of active transducers are piezo electric sensors (for generation of charge corresponding to pressure) and photo voltaic cells (for generation of voltage in response to illumination).
(ii) Passive transducers
Derive the power required for transduction from an auxiliary power source. Thus passive transducers require an external electrical source to convert the physical parameters into an electrical signal. They are also known as externally powered transducers. They depend upon the change in an electrical parameter (R, L and C).
Typical examples are strain gauges (for resistance change in response to pressure) and thermistors (for resistance change corresponding to temperature variations).

19. Passive Transducers -Require external power source to operate
Active Transducers - Do not requires external power produce an analog voltage
Passive Transducers -Require external power source to operate
measurand
electrical output
Active Transducer
measurand
electrical output
Passive Transducer
external power

20. Contents Objectives Sensor and Transducer Sensing Process
Definition of Transducer
Function of Transducer
Classification of Transducer
Selecting of Transducer
Temperature Transducer 20

21. Selecting a Transducers
Operating range
=> The transducer should maintain range
requirements and good resolution.
2. Sensitivity
=> the transducer must be sensitive enough to allow
sufficient output.
Environmental compatibility
=> ability to make applicable and interactions
Minimum sensitivity measurand. => The transducer must be minimally sensitivity
Accuracy => Subject to repeatability and calibration error
Physical condition => Depend on its usage
Electrical parameters => Length and type of cable required, signal to noise ratio (SNR) when combined with amplifiers and frequency response limitations.

22. Transducers Resistive Transducer => Potentiometer
Resistive Position Transducer
Capacitive Transducer
Inductive Transducer
Temperature transducers
Strain Gauge
Photoelectric

23. Potentiometer
Electromechanical device containing a resistance element that is contacted by a movable slider
The motion of the movable slider may be translatory or rotational.
Helipots Type
Rotational Type
Translatory type

24. Resistive Position Transducer

25. APPLICATION
• Potentiometer senses displacement by means of sensing shaft, which is mechanically connected to the point or objects whose displacement, is to be measured.
• Example:
Petrol-tank level indicator.
In this case, potentiometer is used to indicate/sense the petrol
level in a tank as shown in Figure below. The output signal
(voltage) is proportional to the petrol level.

26. Resistive Position Transducer
The principle of the resistance transducer is that the physical variable under measurement causes a resistance change in the sensing element.
A common requirement in industrial measurement and control work is to be able to sense the position of an object or distance it has moved. .
Potentiometer
R: resistance change
: density
L: Length
A: area

27. Cont’d…
FIG 1 (a)
FIG 1 (b)
Figure shows the construction of a displacement transducer uses a resistance element with a sliding contact or wiper linked to the object being monitored.
The resistance between the slider and one end of the resistance element depends on the position of the object. The output voltage depends on the wiper position and therefore is a function of the shaft position.

28. Consider Fig 1 (b), if the circuit is unloaded, the output voltage V0 is a certain fraction of VT, depending on the position of the wiper:
This equation shows that the output voltage is directly proportional to the position of the wiper (R2), if the resistance of the transducer is distributed uniformly along the length of travel of the wiper.
EXAMPLE 1
A displacement transducer with a shaft stroke of 4 in. is used in the circuit of figure 1 (b). R1 +R2 is 1000 Ω and VT = 4 V.
The wiper is 1.5 in from B. Find V0?

29. Capacitive Transducer
• Capacitive transducers are nothing but the capacitors with the variable capacitance. These are mainly used for the measurement of displacement, pressure etc.
• A capacitor consists of two parallel plates separated by an air space or by a dieletric (insulating material).
• The capacitance of the pair of plates is a measure of the amount of charge that can be transferred before a certain voltage is reached.
• If the capacitance is large, more charge is needed to establish a given voltage difference.

30. Capacitive Transducer
The capacitance of a parallel plate capacitor is given by
where
k = dielectric constant
A = the area of the plate, in m2
εo = x F/m
d = the plate placing in m

31. Cont’d Forms of Capacitance Transducers Rotary plate capacitor
Thin diaphragm
Rectilinear Capacitance Transducer

32. Cont’d Rotary plate capacitor:
The capacitance of this unit proportional to the amount of the fixed plate that is covered, that shaded by moving plate. This type of transducer will give sign proportional to curvilinear displacement or angular velocity.

33. Cont’d Rectilinear capacitance transducer:
It consists of a fixed cylinder and a moving cylinder. These pieces are configured so the moving piece fits inside the fixed piece but insulated from it.

34. Cont’d Thin diaphragm:
A transducer that varies the spacing between surfaces. The dielectric is either air or vacuum.
Often used as Capacitance microphones.

35. Cont’d Advantages: Has excellent frequency response
Can measure both static and dynamic phenomena.
Disadvantages:
Sensitivity to temperature variations
the possibility of erratic or distortion signals owing to long lead length
Applications:
As frequency modulator in RF oscillator
In capacitance microphone
Use the capacitance transducer in an ac bridge circuit

36. Example : A capacitive transducer is used for the measurement of linear displacement, X, as shown in below. The parallel plate has a dimension of 5.0cm X 5.0cm and is separated by a distance of 1.0cm. The space between the plates is filled with a dielectric material of 1.0cm thick, which has a dielectric constant of 4.0. If the dielectric constant for air is 1.0cm, determine the value of the capacitance when x is equal to: (i) 0.0cm (ii) 2.0cm

38. Inductive Transducer
Inductive transducers may be either of the self generating or passive type. The self generating type utilises the basic electrical generator principle, i.e, a motion between a conductor and magnetic field induces a voltage in the conductor (generator action). This relative motion between the field and the conductor is supplied by changes in the measurand.
An inductive electromechanical transducer is a device that converts physical motion (position change) into a change in inductance. Transducers of variable inductance type work upon one of the following principles:
Variation of self inductance
Variation of mutual inductance

39. Cont..
Inductive transducers are mainly used for the measurement of displacement. The displacement to be measured is arranged to cause variation in any of three variables:
Number of turns
Geometric configuration
Permeability of the magnetic material or magnetic circuits

40. Temperature Transducers
Temperature transducers can be divided into four main categories:
1. Resistance Temperature Detectors (RTD)
2. Thermocouples
3. Thermistor

41. 1) Resistance Temperature Detector (RTD)
Detectors of wire resistance temperature common employ platinum, nickel or resistance wire elements, whose resistance variation with temperature has high intrinsic accuracy. They are available in many configurations and size and as shielded or open units for both immersion and surface applications.
The relationship between temperature and resistance of conductors can be calculated from the equation:
where
R = the resistance of the conductor at temperature t (0C)
R0 = the resistance at the reference temperature, usually C
α = the temperature coefficient of resistance
ΔT = the difference between the operating and the reference temperature

42. RTD (resistance temperature detector)
=> A temperature sensor that operates on the measurement principle that a material’s electrical resistance changes with temperature.
Exercise:

43. 2) Thermocouple
It consists of two wires of different metals are joined together at one end, a temperature difference between this end and the other end of wires produces a voltage between the wires. The magnitude of this voltage depends on the materials used for the wires and the amount of temperature difference between the joined ends and the other ends.

44. How does a thermocouple look like ?
please note the two wires (of two different metals) joined in the junction.

45. How does a thermocouple work ?

46. In normal operation, cold junction is placed in an ice bath

47. Cont’d The emf of the thermocouple : E = c(T1 – T2) + k(T12 – T22)
Where
c and k = constant of the thermocouple
materials
T1 = The temperature of the “hot”
junction
T2 = The temperature of the “cold” or “reference” junction

48. Cont’d Advantages of thermocouples: 1. It has rugged construction
2. Temperature range -2700C to 27000C
3. Using extension leads and compensating cables, long transmission distances for temperature measurement are possible
4. Bridge circuits are not needed for temperature measurement
5. Comparatively cheaper in cost
6. Calibration checks can be easily performed
7. Thermocouples offer good reproducibility
8. Speed of response is high compared to the filled system thermometer
9. Measurement accuracy is quite good

49. Cont’d
Thermocouples and thermocouple assemblies: (a) Uninsulated thermocouple (b) insulated thermocouple (c) probe assembly
(d) thermocouple well

51. Thermistor is a type of resistor used to measure
temperature changes, relying on the change in its resistance with changing temperature.
(ii)Thermistor (THERmally sensitive resISTOR)
=> non-metallic resistors (semiconductor material), made by sintering mixtures of metallic oxides such as manganese, nickel, cobalt, copper and uranium.’

52. 3) Thermistor …cont
Termistors have negative temperature coefficient (NTC). That is, their resistance decreases as their temperature rises.
Types of thermistor Resistance
Disc to 1MΩ
Washer to 50kΩ
Rod high resistance

53. Advantages of Thermistor
Small size and low cost
Fast response over narrow temperature range
Good sensitivity in Negative Temperature Coefficient
(NTC) region
Cold junction compensation not required due to dependence of resistance on absolute temperature.
Contact and lead resistance problems not encountered due to large resistance

54. Limitations of Thermistor
Non linearity in resistance vs temperature characteristics
Unsuitable for wide temperature range
Very low excitation current to avoids self heating
Need of shielded power lines, filters, etc due to high resistance
Electrical symbol of a thermistor

55. Figure 3: Various configurations of thermistor
Cont’d
Figure 3: Various configurations of thermistor

56. Strain Gauge
The strain gauge is an example of a passive transducer that uses electric resistance variation in wires to sense the strain produced by a force on wires. It is a very versatile detector and transducer for measuring weight, pressure, mechanical force, or displacement.
The construction of a bonded strain gauge (see figure) shows a fine wire element looped back and forth on a mounting plate, which is usually cemented to the member undergoing stress. A tensile stress tends to elongate the wire and thereby increase its length and decrease its cross-sectional area.

57. The combined effect is an increase in resistance:
Where,
ρ: the specific resistance of the conductor material in ohm meters
L : length of conductor (meters)
A : area of conductor (m2)
As consequence of strain, 2 physical qualities are particular interest:
The change in gauge resistance
The change in length
The relationship between these two variables called gauge factor,
K, is expressed mathematically as

58. Where
K= the gauge factor
R=the initial resistance in ohms (without strain)
∆R= the change in initial resistance in ohms
L= the initial length in meters (without strain)
∆L=the change in initial length in meters
∆L/L same unit with G, therefore

59. From Hooke theory, stress, S, is defined as internal force/area.
and (circle area)
Where
S= the stress in kilograms per square meter
F= the force in kilograms
A= area in square meters
j = radius in meters
D= diameter in meters
Then the modulus of elasticity of material E or called Young’s modulus (Hooke’s Law) is written as:
Where,
E= Young modules in kg per square meter
S= the stress in kilograms per square meter
G= the strain (no units)
∆L = the change in initial length in meters
L = the initial length in meters

60. Metallic strain gauge – formed from thin resistance wire or etched from thin sheets of metal foil.
Wire gauge (small) – to minimum leakage – for high T applications
Semiconductor strain gauge – high output transducers as load cells
Strain gauge is generally used as one arm of bridge

61. What’s Photoelectric Effect?
-is the emission of electrons from matter upon the absorption of electromagnetic radiation, such as ultraviolet radiation or x-rays.
-refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light.

62. Photoelectric Transducer
Can be categorized as: photoemissive, photoconductive, or photovoltaic.
No.
Types
Characteristics 1.
Photoemmisive
radiation falling into a cathode causes electrons to be emitted from cathode surface. 2.
Photoconductive
the resistance of a material is change when it’s illuminated. 3.
Photovoltaic
Generate an output voltage proportional to radiation intensity

63. Examples of Photoelectric Transducer
(i) The Photomultiplier Tube
(ii) Photoconductive Cells or Photocells
the electrical resistance of the materials
varies with the amount of light striking.
(iii) The Photovoltaic Cell or solar cell
- produce an electrical current when
connected to the load.

64. …….The end …. 