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EXPERT SYSTEMS AND SOLUTIONS
Email: expertsyssol@gmail.com
expertsyssol@yahoo.com
Cell: 9952749533
www.researchprojects.info
PAIYANOOR, OMR, CHENNAI
Call For Research Projects Final
year students of B.E in EEE, ECE,
EI, M.E (Power Systems), M.E
(Applied Electronics), M.E (Power
Electronics)
Ph.D Electrical and Electronics.
Students can assemble their hardware in our
Research labs. Experts will be guiding the
projects.

1

BM 2203 - Sensors and
Measurment

G.Thiyagarajan
BM 17
REC/BME

2

Block diagram of a
generalized instrumentation
system
Process or
measurement Environmental
medium effects (noise,
Physical variable temperature etc)
to be measured
Observer
Feedback signal for control
Input
signal
Intermediate Modified
Controller
stage signal
Primary stage
Manipulation
Detector-transducer
Transduced & Indicator
Sensing & conversion
signal transmission
Calibration Recorder Quantity
signal presented
to observer
Calibration signal Final (output )
External
source representing stage
power
known value of
physical variable
3

Block diagram of the
pressure gauge based
Physical variable
on Bourdon tube
to be measured
Pressure

Pressure
(input)
Displacement Intermediate stage
Detector – transducer stage (transduced signal) Gearing arrangement that
Bourdon tube amplifies the displacement
Pressure to mechanical signal
displacement

Pressure Amplified displacement
(calibration) signal
Final stage
Calibration signal from Pointer and dial
a source with known arrangement
pressure values 5

A typical medical measurement
system
Outputs

Signal Signal Data
Sensor
Measurand conditioning processing displays

Feedback Data Data
Effector
storage communication

6

Feedback with and without
clinician

Patient
Instrument

Patient Clinician
Instrument

7

A patient monitors vital signs and
notify a clinician if abnormalities
occur

Abnormal
Clinician readings Patient Instrument

8

Detailed generalized medical
measurement system

9

Alternative operational modes
 Direct-indirect modes
 Sampling and continuous modes
 Generating and modulating sensors
 Analog and digital modes
 Real-time and delayed-time modes

10

Example to sampled data
Laboratory test Typical value
Hemoglobin 13.5 to 18 g/dL
Hematocrit 40 to 54%
Erythrocyte count 4.6 to 6.2 × 106/ µL
Leukocyte count 4500 to 11000/ µL

Neutrophil 35 to 71%
Band 0 to 6%
Lymphocyte 1 to 10%
Differential count
Monocyte 1 to 10%
Eosinophil 0 to 4%
Basophil 0 to 2%

Complete blood count for a male subject.
11

Analog and digital signals

Amplitude
Amplitude

Time Time

Analog signals can have any Digital signals have a limited
amplitude value number of amplitude values

12

Continuous and discrete-time
signals
Amplitude

Amplitude
Time Time

Continuous signals have values Discrete-time signals are sampled
at every instant of time periodically and do not provide
values between these sampling times

13

Origins of
common
biological
signal

14

Medical measurement
constraints Frequency,
Measurement Range Method
Hz
Blood flow 1 to 300 mL/s 0 to 20 Electromagnetic or ultrasonic
Blood pressure 0 to 400 mmHg 0 to 50 Cuff or strain gage
Cardiac output 4 to 25 L/min 0 to 20 Fick, dye dilution
Electrocardiography 0.5 to 4 mV 0.05 to 150 Skin electrodes
Electroencephalography 5 to 300 µ V 0.5 to 150 Scalp electrodes
Electromyography 0.1 to 5 mV 0 to 10000 Needle electrodes
Electroretinography 0 to 900 µ V 0 to 50 Contact lens electrodes
pH 3 to 13 pH units 0 to 1 pH electrode
pCO2 40 to 100 mmHg 0 to 2 pCO2 electrode
pO2 30 to 100 mmHg 0 to 2 pO2 electrode
Pneumotachography 0 to 600 L/min 0 to 40 Pneumotachometer
2 to 50
Respiratory rate 0.1 to 10 Impedance
breaths/min
Temperature 32 to 40 ° C 0 to 0.1 15
Thermistor

Setting sensor specifications
Specification Value

–

Pressure range

Sensor specifications for a blood pressure sensor are
determined by a committee composed of individuals from
academia, industry, hospitals, and government.
16

Specifications for ECG
Specification Value
Input signal dynamic range ±5 mV
Dc offset voltage ±300 mV
Slew rate 320 mV/s
Frequency response 0.05 to 150 Hz
Input impedance at 10 Hz 2.5 MΩ
Dc lead current 0.1 µΑ
Return time after lead switch 1s
Overload voltage without damage 5000 V
Risk current at 120 V 10 µΑ
Specification values for an electrocardiograph are agreed
upon by a committee.
17

Classification of biomedical
instruments
 Quantity sensed: pressure, flow,
temperature etc.
 Principle of transduction: resistive,
inductive, capacitive, ultrasonic or
electrochemical
 Organ system studied: cardiovascular,
pulmonary, nervous, and endocrine
systems.
 Clinical medical specialties: pediatrics,
18

Interfering and modifying inputs

Original waveform An interfering
input may shift A modifying
the baseline input may
change the gain

19

Simplified
Electrocardiographic
recording system
20

Compensation Techniques
 Inherent insensitivity
 Negative feedback
 Signal filtering
 Opposing inputs

21

Negative feedback

( xd − H f y )Gd = y
y
xd
+ ∑ Gd xd Gd = y (1 + H f Gd )
- Gd
Hf y= xd
1 + H f Gd

22

Signal filtering

Signals without noise are Interference superimposed on
uncorrupted signals causes error.
Frequency filters can be used
to reduce noise and
interference

23

Opposing inputs
 Differential amplifier: v0 = Gd(vA- vB)
 DC cancellation (bucking)

A m p lit u d e

D c o ffs e t
T im e

An input signal with dc offset
An input signal without dc
offset
24

Generalized Static
Characteristics
 Accuracy  Zero drift
 Precision and  Sensitivity drift
reproducibility  Linearity
 Resolution  Input ranges
 Statistical control  Input impedance
 Static sensitivity

25

Data points with
Accuracy
Accuracy: closeness with which an
instrument reading approaches the
true or accepted value of the variable
(quantity) being measured. It is
considered to be an indicator of the
total error in the measurement without low accuracy
looking into the sources of errors.

true value − measured value
accuracy =
true value

Accuracy is often expressed in percentage

26 accuracy
high

Precision Data points with

1. A measure of the reproducibility
of the measurements; i.e., given a
fixed value of a variable,
precision is a measure of the
degree to which successive low precision
measurements differ from one
another.

2. Number of distinguishable
alternatives. 2.434 V is more
precise than 2.43 V.

high precision
27

Resolution
 The smallest change in measured value to
which the instrument will respond.

Statistical control: random variations
in measured quantities are tolerable,
Coulter counter example

28

Tolerance
 Maximum deviation allowed from the conventional
true value.
 It is not possible to built a perfect system or make
an exact measurement. All devices deviate from
their ideal (design) characteristics and all
measurements include uncertainties (doubts).
 Hence, all devices include tolerances in their
specifications. If the instrument is used for high-
precision applications, the design tolerances must
be small.
 However, if a low degree of accuracy is acceptable,
it is not economical to use expensive sensors and
precise sensing components
29

Static sensitivity

Sensor
Sensor
signal
signal

Measurand
Measurand

A low-sensitivity sensor has low A high sensitivity sensor has
gain high gain

30

Static sensitivity constant over a limited
range

n∑ xd y − (∑ xd )(∑ y ) (∑ y )(∑ xd ) − (∑ xd y )(∑ xd )
2
m= b=
n ∑ x − ( ∑ xd )
2
d
2
n ∑ xd − ( ∑ xd ) 2
2
31

Zero and sensitivity drifts

32

Linearity

Output Output

Input Input
A linear system fits the A nonlinear system does not fit
equation y = mx + b. a straight line 33

Calibration for linearity
Output Output

Input Input

The one-point calibration may The two-point calibration may
miss nonlinearity also miss nonlinearity

Measuring instruments should be calibrated against a
standard that has an accuracy 3 to 10 times better than the
desired calibration accuracy
34

Hysteresis

Sensor
s ig n a l

M e a s u ra n d

A hysteresis loop. The output curve obtained when increasing the
measurand is different from the output obtained when decreasing
the measurand.

35

Independent
nonlinearity

36

Objectives
At the end of this chapter, the students
should be able to:

 describe the principle of operation of
various sensors and transducers; namely..
Resistive Position Transducers.
Capacitive Transducers
Inductive Transducers

37

Introduction
 Sensors and transducers are classified
according to;
 the physical property that they use
(piezoelectric, photovoltaic, etc.)
 the function that they perform
(measurement of length, temperature, etc.).
 Since energy conversion is an essential
characteristic of the sensing process, the various
forms of energy should be considered.

38

Introduction
 There are 3 basic types of transducers namely
self-generating, modulating, and modifying
transducers.

The self-generating type (thermocouples,
piezoelectric, photovoltaic) does not require the
application of external energy.

39

Introduction
 Modulating transducers (photoconductive
cells, thermistors, resistive displacement devices) do
require a source of energy.
For example, a thermocouple is self-generating,
producing a change in resistance in response to a
temperature difference, whereas a
photoconductive cell is modulating because it
requires energy.
 The modifying transducer (elastic beams,
diaphragms) is characterized by the same form of
energy at the input and output. The energy form on
both sides of a modifier is electrical.
40

Definition
 The words 'sensor' and 'transducer' are
both widely used in the description of
measurement systems.
 The former is popular in the USA
whereas the latter has been used in Europe
for many years. The word 'sensor' is derived
from entire meaning 'to perceive' and
'transducer' is from transducer meaning 'to
lead across'.

41

Definition
 A dictionary definition of 'sensor' is
`a device that detects a change in a physical
stimulus and turns it into a signal which can
be measured or recorded;
 The corresponding definition of
'transducer' is 'a device that transfers energy
from one system to another in the same or in
the different form'.

42

Features of Sensors
The desirable features of sensors are:

1. accuracy - closeness to "true" value of variable;
accuracy = actual value - sensed value;
2. precision - little or no random variability in
measured variable
3. operating range - wide operating range; accurate
and precise over entire sensing range
4. calibration - easy to calibrate; no "drift" - tendency
for sensor to lose accuracy over time.
5. reliability - no failures
6. cost and ease of operation - purchase price,
cost of installation and operation
43

Sensors Types
A list of physical properties, and sensors to
measure them is given below:

44

Common Sensors
Listed below are some examples of common
transducers and sensors that we may encounter:

 Ammeter - meter to indicate electrical current.
 Potentiometer - instrument used to measure
voltage.
 Strain Gage - used to indicate torque, force,
pressure, and other variables. Output is change in
resistance due to strain, which can be converted into
voltage.
 Thermistor - Also called a resistance thermometer;
an instrument used to measure temperature. The
operation is based on change in resistance as a
function of temperature. 46

Sensors Types
• There are several transducers that will be
examined further in terms of their
principles of operations.
• Those include :

1. Resistive Position Transducers
2. Strain Gauges
3. Capacitive Transducers
4. Inductive Transducers
5. And a lot more…

47

Resistive Position Transducers
• The principle of the resistive position transducer
is that the measured quantity 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 the distance it has
moved.
• One type of displacement transducer uses a
resistance element with a sliding contact linked to the
object being monitored.
• Thus the resistance between the slider and one
end of the resistance element depends on the
position of the object.
48

• The output voltage depends on the wiper position and
therefore is a function of the shaft position.
• In figure below, the output voltage Eout is a fraction
of ET, depending on the position of the wiper.
• The element is considered perfectly linear if
the resistance of the transducer is distributed
uniformly along the length of travel of wiper.

Eout R2
=
ET R1 + R2

49

Example 1
An RPT with a shaft stroke of 5.5 inches is applied in
the circuit as below. The total resistance of the
potentiometer is 4.7kΩ. The applied voltage is
ET= 3V.
When the wiper is 0.9 in. from B, what is Eout?

50

Strain Gauges
• The Strain Gauge is an example of a passive
transducer that uses electrical resistance variation
in wires to sense the strain produced by a force on
the wire.
• It is a very versatile detector and transducer for
measuring weight, pressure, mechanical force or
displacement.

51

Strain Gauges
The construction of a bonded strain gauge shows a
fine wire looped back and forth on a mounting plate,
which is usually cemented to the element that
undergoing stress.

52

Strain Gauges
• For many common materials, there is a constant
ratio between stress and strain.
• Stress is defined as the internal force per unit
area.
•
F
S= S – Stress (kg/m2)
F – Force (kg)
A A - Area (m2)

• The constant of proportionality between stress
and strain for the curve is known as the modulus of
elasticity of the materials, E or Young’s Modulus.

53

Capacitive Transducers
• The capacitance of a parallel plate is given
by:

kAε k= dielectric constant

C= o A= area of the plate
εo=8.854x10-12 F/m
d d= plate spacing

• Since the capacitance in inversely
proportional to the spacing of the parallel
plates, any variations in d will cause a variation
in capacitance.
54

• Some examples of capacitive transducers

55

Example 2:
An electrode-diaphragm pressure transducer has
plates whose area is 5x10-3 m2 and distance
between plates is 1x10-3.
Calculate its capacitance if it measures air
pressure with k=1.

56

Inductive Transducers
• Inductive Transducers may be either the self-
generating or the passive type transducers.

• In the Self-Generating IT, it utilises the basic
electrical generator principle that when there is
relative motion between conductor and magnetic
field, a voltage is induced in the conductor.

• An example of this is Tachometer that directly
converts speeds or velocity into an electrical
signal.

57

Tachometers
• Examples of a Common Tachometer

58

Linear Variable
Differential Transformer (LVDT)
• Passive inductive transducers require an external
source of power.
• The Differential transformer is a passive inductive
transformer, well known as Linear Variable
Differential Transformer (LVDT).
• It consists basically of a primary winding and two
secondly windings, wound over a hollow tube and
positioned so that the primary is between two of
its secondaries.

59

Linear Variable
• Some examples of LVDTs.

60

Linear Variable
• An example of LVDT electrical wiring.

61

Linear Variable
• An iron core slides within the tube and therefore
affects the magnetic coupling between the primary
and two secondaries.
• When the core is in the centre , the voltage
induced in the two secondaries is equal.
• When the core is moved in one direction of centre,
the voltage induced in one winding is increased and
that in the other is decreased. Movement in the
opposite direction reverse this effects.

62

Linear Variable
•In next figure, the winding
is connected ‘series opposing’
-that is the polarities of V1
and V2 oppose each other
as we trace through the circuit
from terminal A to B.
•Consequently, when the core
is in the center so that V1=V2,
there is no voltage output,
Vo = 0V.

63

Linear Variable
• When the core is away from S1, V1 is greater than
V2 and the output voltage will have the polarity of V1.
• When the core is away from S2, V2 is greater than
V1 and the output voltage will have the polarity of V2.
• That is the output of ac voltage inverts as the core
passes the center position.
• The farther the core moves from the centre, the
greater the difference in value between V1 and V2,
and consequently the greater the value of Vo.

64

Linear Variable
• Thus, the amplitude of Vo is a function of distance
the core has moved. If the core is attached to a
moving object, the LVDT output voltage can be a
measure of the position of the object.
• The farther the core moves from the centre, the
greater the difference in value between V1 and V2,
and consequently the greater the value of Vo.

65

Linear Variable
Among the advantages of LVDT are as follows:

• It produces a higher output voltages for small
changes in core position.
• Low cost
• Solid and robust -capable of working in a wide
variety of environments.
• No permanent damage to the LVDT if
measurements exceed the designed range.

66

Linear Variable
Example 3:
An ac LVDT has the following data; input 6.3V,
output 5.2V, range ±0.50 cm. Determine:
a) Plot of output voltage versus core position for a
core movement going from +0.45cm to -0.03cm?
b) The output voltage when the core is -0.35cm from
the center?
c) The core movement from center when the output
voltage is -3V?
d) The plot of core position versus output voltages
varying from +4V to -2.5V.

67

Piezoelectric Transducers
• When a mechanical pressure is applied to a
crystal of a Rochelle salt, quartz, or tourmaline type, a
displacement of the crystals that will produce a
potential difference will occur.

• This property is used in piezo-
electric transducers; where a
crystal is placed between a solid
base and force-summing element,
as shown below:

68

Piezoelectric Transducers
• When externally force is applied to the plates, a
stress will be produced in the upper part of the crystal.

• This deformation will produce a potential
difference at the surface of the crystal. This produces
an electromotive force across the crystal proportional
to the magnitude of the applied pressure. This effect is
called piezoelectric effects.
•
• The induced charge on the crystal is proportional
to the impressed force and given by:
Q = dF; where d = piezoelectric constant.

69

Temperature Transducers
• The temperature transducers can be divided
into four main categories:

o Resistance Temperature Detectors (RTD)
o Thermocouples
o Thermistors
o Ultrasonic transducers

70

Resistance Temperature
Detectors (RTDs)
• Detectors of resistance temperatures
commonly employ platinum, nickel, or
resistance wire elements, whose resistance
variation with temperature has a high intrinsic
accuracy.

• They available in many configurations and
sizes and as shielded and open units for both
immersion and surface applications.

71

Detectors (RTDs)
• Some examples of RTDs are as follows:

72

Detectors (RTDs)
• The relationship between temperature and
resistance of conductors can be calculated from
this equation:

R = Ro (1 + α∆T )
where;
R= resistance of the conductor at temp t (oC)
Ro=resistance at the reference temp.
α= temperature coefficient of resistance
∆= difference between operating and reference
temp.
73

Detectors (RTDs)
Example:
A platinum resistance thermometer has a
resistance of 220Ω at 20oC. Calculate the
resistance at 50oC?
Given that α20oC=0.00392.

74

Thermocouples
• A thermocouple is a sensor for measuring
temperature. It consists of two dissimilar / different
metals, joined together at one end, which produce a
small unique voltage at a given temperature. This
voltage is measured and interpreted by the
thermocouple.

•The magnitude of this voltage depends on the
materials used for the wires and the amount of
temperatures difference between the joined end and
the other ends.

75

Thermocouples
• Some examples of the thermocouples are as
follows:

76

Thermocouples
• Common commercially available
thermocouples are specified by ISA
(Instrument Society of America) types.

• Type E, J, K, and T are base-metal
thermocouples and can be used up to about
1000°C (1832°F).

• Type S, R, and B are noble-metal
thermocouples and can be used up to about
2000°C (3632°F).
77

Thermocouples
• The following table provides a summary of basic
thermocouple properties.

78

Thermocouples
•Calibration curves for several commercially
available thermocouples is as below:

79

Thermocouples
• The magnitude of thermal emf depends on the
wire materials used and on the temperature difference
between the junctions.
• The effective emf of the thermocouple is given as:

E = c(T1 − T2 ) + k (T − T ) 1
2
2
2

•Where;
c and k – constant of the thermocouple materials
T1 - temperature of the ‘hot’ junction.
T2 - temperature of the ‘cold’ or
‘reference’ junction.
80

Thermocouples
Example

During experiment with a copper- costantan
thermocouple, it was found that
c= 3.75x10-2 mV/oC and k = 4.50x10-5 mV/oC. If
T1= 100oC and the cold junction T2 is kept
in the ice, compute the resultant electromotive
force, emf?

81

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