tells about transducer

1. SENSORS
AND
TRANSDUCERS
By
Prof. M N Bhusavalwala
EED,SVNIT,SURAT
1

2. ARCHITECTURE OF INDUSTRIAL
AUTOMATION
2

3. Generalized instrumentation system
• It involves the precise measurement and recording of a
physical, chemical ,mechanical or an optical parameter.
• It plays a vital role in every branch of scientific research
and industrial processes interacting with control system
and process instrumentation.
• Instrumentation system provides a means of describing a
natural phenomena in quantitative terms.
3

4. Objective of Instrumentation System
1. Measurement of System parameters/Information.
2. Control of certain process or operation.
3. Simulation of system conditions
4. Testing of materials, maintenance of standards and
specifications of products.
5. Quality control in industry.
4

5. (1)Measurement of system parameters/information
• One of the important function of the instruments is to
determine the various parameters/information of the
system or a process.
• Instruments present the desired information of the
condition of the system in the form of visual indication /
registering / monitoring suitable transmission according
to needs and requirements of the system
5

6. (2) Control of a certain process or operation
• Important application of measurement / instrumentation is in
automatic control system.
• Measurement system is integral part of control system
• It provides guidance or manipulation to maintain various
parameters at or near set points or to change parameters
according to preset programme.
• Accurate measurement is required for accurate control of any
physical variable
• Application Areas: Oil refinery, chemical plant , textile mills for
controlling variables like temperature , pressure ,humidity ,
viscosity, flow rate etc..
• Other areas : Autopilots, Instrument landing of aircraft,
missile guidance, radar tracking.
6

7. 7
Measurement system
Control Elements
System/Process
Reference
signal (r)
Error
signal (e)
Input Controlled
output
Measurement system in automatic control

8. (3) Simulation of system conditions
• Actual condition of complex situation is simulated on a scale
model.
• Similarity of significant features between models and full
prototype are preserved
8

9. (4) Testing of materials , maintenance of standards, and
specifications of products:
1. Most countries have standards organizations that specify
material standards and product specifications based on
tests and measurements.
2. These organizations protect the interest of consumers. They
ensure the material/ products meet the specified
requirements so that they function properly, that is
enhancing reliability of the system.
3. An aircraft engine is subjected to extensive endurance tests
by the civil aviation authorities as per their specifications
before it is certified to be airworthy.
9

10. (5) Quality control in industries
1. Continuous quality control tests of mass produced industrial
products are carried out so that defective components can
be rejected outright at the early stage of production.
2. The final assembly of product or system is free from defects
if defective product/parts are detected at the early stage,
thereby improving reliability of the product.
3. A boiler plate has to undergo a number of quality control
tests before it is put to actual operation. The various tests
are: X ray examination of the plate for defects like blow
holes, cracks etc; metallographic examination for
metallurgical defects , period strength tests of samples etc.
10

11. 11
Input
Measurand
Digital display
Analog panel meter
Graphic recorder
Oscilloscope
Magnetic tape recorder
Signal conditioner
Power supply
Transducer
To control
system
Fig. A generalized instrumentation system

12. Generalized instrumentation system Comprises of the following
elements:
1. A transducer which converts measurand into a usable form.
2. The Signal conditioner which converts the transducer
output into an electrical quantity suitable for control ,
recording or display.
3. The display or readout devices ( also known as end devices
or output devices) which display the required information
about the measurand , generally in engineering units.
4. The electrical power supply which provides the required
excitation to the transducer and necessary electrical power
to the signal conditioners and display devices.
12

13. Definition of Transducer
• The transducer is defined as a device which , when actuated
by one form of energy, is capable of converting it to another
form of energy.
• The electrical transducer is a sensing device by which the
physical , mechanical or optical quantity is transformed into
an electrical voltage/current proportional to the input
measurand
13

14. Advantages of Electrical Transducers
1. Electrical amplification and attenuation can be
easily made.
2. Output can be indicated and recorded remotely at a
distance from the sensing area.
3. Output can be modified to meet the requirement of
output devices.
4. The signal can be conditioned or mixed to obtain
any combination with outputs of similar
transducers.
14

15. Classification of electrical transducers
Electrical transducers can be broadly classified into two major
classes:
(1) Active transducers
(2) Passive transducers
15

16. ACTIVE TRANSDUCERS
• Generates an electrical signal directly in response to the
measurand .
• Does not require an external power source for its operation.
• Self generating transducers
• Operate under energy conversion principle
• Examples are thermocouple, photovoltaic cells, piezo-electric
sensors, tachogenerators
16

17. Active transducers: Examples
1. Thermocouple Thermal energy is converted to
electrical energy.
2. Photovoltaic cells  Light energy is converted into
e.m.f..
3. Piezo -electric sensor Mechanical stress or
pressure is converted to electrical charge.
3. Tachogenerator Mechanical energy is
converted into e.m.f.
17

18. Passive transducers
1. Operate under energy controlling principles.
2. External excitation source is necessary.
3. Depend upon change in an electrical parameter: Resistance
Inductance or Capacitance in general.
Examples:
1 Strain gauge Resistance changes in response to pressure
or stress
2 RTD Resistance changes in response to change in
temperature
18

19. A transducer….
…..converts a non electrical quantity into an analog
electrical signal may be considered of two parts:
Sensing element and the transduction element .
1. Sensing Element is that part which responds to a
physical phenomenon.
2. Transduction element transforms the output of a
sensing element to an electrical output. This acts
as a secondary transducer.
19

20. A TRANSDUCER
20
Primary
sensing element
Transduction
Element
Output
Input
Transducers may be
classified into different
categories depending upon
the transduction principle
employed as follows:

21. Classification on the basis of Transduction principle
1. Thermoelectric Active
2. Piezoelectric Active
3. Photovoltaic Active
4. Electrodynamic Active
5. Resistive Passive
6. Inductive Passive
7. Capacitive Passive
8. Photoconductive Passive
9. Piezoresistive Passive
21

22. General Characteristics of Transducers
1. Performance characteristics
2. Measurand characteristics
3. Electrical design characteristics
4. Mechanical characteristics
5. Environmental characteristics
6. Reliability characteristics
22

23. Performance Characteristics
Static performance parameters are as follows:
1. Accuracy
2. Precision
3. Resolution
4. Threshold
5. Static sensitivity
6. Linearity
23

24. 1 Accuracy
• Accuracy of a transducer is defined as the degree of closeness
of the output to the true value of the measurand.
• In actual practice , it is specified as the % deviation from the
true value.
measured value - true value
Accuracy= ----------------------------------- X 100 %
True value
OR
measured value - true value
Accuracy= ----------------------------------- X 100 %
Full Scale value
24

25. 2 Precision
• Precision is defined as the ability of the transducer to
reproduce a certain set of output readings with in a given
accuracy.
• For example if a particular transducer is subjected to an
accurately known input and if the repeated outputs of the
transducer lie within say ± 1 %,then the precision would be
stated as ± 1 %.
• Highly precise transducer gives same output for a given
input when the observation is repeated a large number of
times.
• Precision depends upon repeatability.
• Inconsistencies in the measured values are due to random
errors
25

26. Precision versus Accuracy
• Accuracy represents the degree of correctness of the
measurand with respect to true value whereas precision
represents the degree of repeatability of several
independent measurements of the desired input at the
same reference conditions.
• Accuracy depends upon the systematic errors whereas
precision depends upon the random errors.
• Accuracy is determined by calibration whereas precision is
determined by probability.
• A precise output measurement may not necessarily be
accurate and vice versa.
26

27. A B
C D

28. 3 Resolution or Discrimination
• It is defined as the smallest increment in the measurand
that can be detected with certainty by the transducer.
• It is the degree of fineness with which a measurement can
be made.
• Resolution may be specified in absolute value or as a
percentage of full scale value.
• Resolution has to be at least as good as its precision
28

29. 4. THRESHOLD
• If the transducer input is increased very gradually from zero
there will be some minimum value below which ,no output
change can be detected.
• This minimum value is defined as the threshold of that
transducer.
• Threshold may be specified in absolute value or as a
percentage of full scale value.
• The total range of input over which the output remains zero
is the DEADZONE
29

30. 5. STATIC SENSITIVITY
• Sensitivity is the ratio of the change in magnitude of the
output to the corresponding change in the measurand.
• It is also termed as gain or scale factor.
• Reciprocal of sensitivity is termed as Deflection Factor or
inverse sensitivity.
• Sensitivity is determined from results of static calibration.
• Sensitivity remains constant at all inputs for linear calibration
curve.( Fig. a)
• Sensitivity varies with input for nonlinear calibration curve.
(Fig. b)
30

31. STATIC SENSITIVITY ….
31
Input
O
U
T
P
U
T
Input
O
U
T
P
U
T
qi
qi
qo qo
∆qo
∆qi
Sensitivity=

32. 7. LINEARITY
• One of the most desirable feature of transducer.
• Manufacturers always attempt to design their transducers so
that output is a linear function of the measurand.
• Linearity is never completely achieved and its deviations from
the ideal are termed as linearity tolerances.
• Maximum departure from linearity is often specified in one of
the following ways:
(1) Absolute maximum deviation.
(2) As a % of full scale value.
32

33. Measurand Characteristics
• Transducer is normally designed to sense a specific
measurand and to respond to only that measurand.
• RANGE: It is the upper and lower limits of measurand values .
Unidirectional range: e.g. 0 to 2.5 m
Bidirectional range: e.g. ±45 mm symmetrical
-2 to 45 mm asymmetrical
70 to 120 rad /sec zero suppressed
33

34. Measurand Characteristics……
• Span: Algebraic difference between two range limits is known
as span.
The span of a ±30° angular displacement transducer is 60 ° .
• Over range or Overload or Maximum Measurand
Maximum measurand that can be applied to a transducer
without causing a change in performance beyond specified
tolerances. A specified amount of recovery time ,after
removal of overload , may have to be allowed to elapse before
the transducer again performs within specified tolerances.
34

35. ELECTRICAL DESIGN CHARACTERISTICS
• A transducer is viewed as a black box that is without regard to
its internal working and just as a device with which other
equipment must electrically interface.
35
MEASURAND
ZS
ZL
SENSING
ELEMENT
TRANSDUCTION
ELEMENT
Zin
Zout
Excitation
source
Load

36. ELECTRICAL DESIGN CHARACTERISTICS ……
• Impedance of the excitation supply, presented to the
transducer is the Source impedance ZS
• Impedance presented to the excitation supply by the
transducer is the Input impedance Zin
• Impedance of the excitation cabling is considered part of the
source impedance.
• The impedance across the output terminals of the transducer
is termed as Output impedance Zo
• Impedance presented to the output terminals of the
transducer by the associated circuitry is known as load
impedance ZL( Output conditioning circuitry , display devices
etc)
36

37. ELECTRICAL DESIGN CHARACTERISTICS ……
Breakdown voltage rating:
• When two or more portions of a transducer are electrically
insulated from each other, the resistance between them as
measured while a specified dc voltage is applied ,is the
insulation resistance.
• The degree of insulation can also be expressed in terms of the
breakdown voltage rating.
• The breakdown voltage is the magnitude of voltage at which
arcing or excessive conduction occurs.
• The breakdown voltage can be determined by a test.
37

38. ELECTRICAL DESIGN CHARACTERISTICS ……
OUTPUT:
• Output is the electrical quantity, produced by a
transducer, which is a function of the applied measurand.
• Output may be analog voltage amplitude , voltage ratio,
current or sometimes just as changes in C,L,R or so on.
• Output may be in terms of frequency, where the number
of cycles or pulses per second are the function of input
measurand.
• Output may be digital which represents measurand in
terms of discrete quantities coded in some system of
notation (e.g. binary code
38

39. ELECTRICAL DESIGN CHARACTERISTICS ……
Loading Error
• It is important for transducers to be approximately matched
to and interfaced with the associated measurement
system.
• Output impedance of transducer must be matched to load
impedance.
• A mismatch in these impedances can cause loading effect.
• Attention must be paid to manufacturer’s
recommendations for excitation source requirement.
39

40. MECHANICAL DESIGN CHARACTERISTICS
Specified primarily for three purposes:
1. to facilitate handling and installation.
2. to prevent malfunction or performance degradation.
3. to interface the transducer properly with the system as part
of which it is expected to operate.
40

41. MECHANICAL DESIGN CHARACTERISTICS……
• Dimensions, mounting provisions ,type, size and locations of
all external electrical and mechanical connections are
always specified.
• Provisions for external zero and gain adjustments may be
indicated.
• Case material and case sealing may be specified.
• Transducer may be explosion proof or waterproof
• It may operate in a high temperature or nuclear radiation
environment.
• Name plate information: Range, Excitation, output ,sr. no.,
manufacturer's name, external electrical connections, Pin
labeling
41

42. ENVIRONMENTAL CHARACTERISTICS
• When a transducer is expected to operate under conditions
other than those under which it was calibrated (operating
environmental conditions) the environmental effects must be
known and resulting deviations from the static performance
must be limited by tolerances.
• Such additional tests ( temperature tests, vibration tests etc.)
may have to be performed on each transducer, more
commonly they are performed on a sampling basis.
42

43. ENVIRONMENTAL CHARACTERISTICS……
• However, the transducer is expected to perform within
specified tolerances after exposure to such non-operating
environmental conditions.
• When non-operating environmental conditions, including
those encountered during storage, shipping and handling are
known or suspected to alter the behavior of a transducer,
they should be included in a specification and the absence of
out of balance non-operating environmental effects should be
verified by testing.
43

44. ENVIRONMENTAL CHARACTERISTICS……
• Temperature effects must be known, and accounted for, for
essentially all type of transducers
• The operating temperature range is the range of ambient
temperature , given by their lower and upper extremes (e.g. -
50º C to 250º C ) within which the transducer is intended to
operate and within which all specifications related to
temperature effects apply.
• Some manufacturers of transducers incorporate the
compensating elements. They call this the compensated
temperature range.
44

45. RELIABILITY CHARACTERISTICS
LIFE OF THE TRANSDUCER
• Operating life is the minimum length of time over which the
transducer will operate either continuously or over a number
of on/off cycles whose duration is specified, without changing
its performance characteristics beyond specified tolerance
• Cycling life: The minimum number of full range excursions
over which transducer will operate without changing its
performance beyond specified tolerance .
45

46. RELIABILITY CHARACTERISTICS…..
Storage life
In some cases , it may also be necessary to specify or be
concerned about transducer’s storage life, length of time over
which it can be exposed to specified storage conditions
without changing its performance beyond specified
tolerances.
46