LECTURE 1
Introduction to
Measurement &
Instrumentation System
LESSON
OUTCOME
AT THE END OF THE LESSON,
STUDENT SHOULD BE ABLE TO:
1. Explain the application/purpose of
measurement system.
2. Elaborate, discuss and explain the
structure of measurement system in term
of:
i.
ii.
Basic functional elements
Auxiliary functional elements
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INTRODUCTION
Body
temperature
Body
weight
Measurement
in everyday
life
Temperature
Distance
travel
Strain
Measurement
in
mechanical
engineering
Pressure
Vibration
Room
temperature
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WHY DO
MEASUREMENT?
TO DETERMINE VALUE OF
VARIABLE
TO UNDERSTAND HOW
VARIABLES CHANGE
WITH TIME & LOCATION
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is an act of assigning a
specific value to a physical
variable
WHAT IS
MEASUREMENT?
is a tool used that provide
quantitative information on
the actual state of physical
variables and processes.
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•Control of certain process or
operations
•Simulation of system conditions
•Experimental design studies
•Testing of materials, maintenance
of standards and specifications of
products
•Verification of physical
phenomena/scientific theories
APPLICATIONS OF
MEASUREMENT
SYSTEM
•Quality control in industry
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Basic functional
element
STRUCTURE OF
MEASUREMENT
SYSTEM
• Sensing element (sensor
element)
• Signal conditioning element
• Signal processing element
• Data presentation element
Auxiliary functional
element
•
•
•
•
Calibration element
External power element
Feedback element
Microprocessor element
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STRUCTURE OF MEASUREMENT SYSTEM
Input
True value
of variable
Input
True
value
Sensing
Element
Transmission
path
Signal
Conditioning
Element
Measurement
system
Output
user
Measured value
of variable
Signal
Processing
Element
Data
Presentation
Element
Output
Measured
value
Data Acquisition System
(DAQ)
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SENSING ELEMENT
(SENSOR)
A sensor is a device that
measures physical input
from its environment and
converts it into data that can
be interpreted by either a
human or a machine.
Sensors with a
mechanical output
are commonly used
as the primary
sensing element in
measurement systems
for mechanical
variables.
E.g.: Force and flow
rate
Most sensors are electronic (the
data is converted into electronic
data), but some are simpler, such
as a glass thermometer, which
presents visual data
Sensors convert
physical parameters
to electrical signals.
In order to obtain an
electrical signal, this
primary element is
followed by a
secondary sensing
element with an
electrical output
signal
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SENSING ELEMENT
(SENSOR)
In contact/contactless
with the monitored
structure, detect or
sense the measurand.
Gives an output
which depends on the
variable to be
measured. Example:
Thermocouple –
milivolt depends on
temperature
A flow meter is a device that
measure the amount of liquid,
gas, or vapor moving through
a pipe (m3/s)
strain gauge –
resistance depends
on mechanical strain
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DESIRABLE
CHARACTERISTIC
OF TRANSDUCER
ELEMENT
It should recognize and sense the desired
input signals present simultaneously in the
measurand. E.g. a speed sensor should sense
the instantaneous velocity and should be
insensitive to the local pressure or
temperature
It should not alter the event to be measured
The output should preferably be electrical
to obtain the advantages of modern
computing and display devices
It should have good accuracy
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DESIRABLE
CHARACTERISTIC
OF TRANSDUCER
ELEMENT
It should have good reproducibility (i.e.
precision)
It should have amplitude linearity
It should have adequate frequency
response (i.e. good dynamic response)
It should not induce phase distortions (i.e.
should not induce time lag between the input
and output transducer signal)
It should be able to withstand hostile
environments without damage and should
maintain the accuracy within acceptable limit
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It should have high signal level and low
impedance
DESIRABLE
CHARACTERISTIC
OF TRANSDUCER
ELEMENT
It should be easily available, reasonably
priced and compact in shape and size
(preferably portable)
It should have good reliability and
ruggedness. In other words, if a transducer
gets dropped by chance, it should still be
operative
Leads of the transducer should be sturdy
and not easily pulled off
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SIGNAL
CONDITIONING
ELEMENT
Definition
How it
works
Signal conditioning is the
technique of making a
signal from a sensor
suitable for processing by
data acquisition
equipment.
Takes the output of the sensing
element and converts it into a form
more suitable for further processing,
usually in the form of a DC voltage,
current, variable frequency AC
voltage.
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SIGNAL CONDITIONING ELEMENT
Example of
problems
How to solve
The output of the sensing element is usually too small to
operate an indicator or a recorder. Therefore, it is
suitably processed and modified in the signal
conditioning element so as to obtain the output in the
desired form.
• Using amplifier-increasing the amplitude of the signal
without affecting it waveform.
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SIGNAL CONDITIONING ELEMENT
Example of
problems
How to solve
Signal attenuation is reducing the amplitude of the
signal that is travelling from the sensor to the DAQ
via cable.
• Using amplifier-increasing the amplitude of the signal
without affecting it waveform.
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Deflection/
Wheatstone
bridge –
converts an
impedance
change into a
voltage change
SIGNAL CONDITIONING
ELEMENT
(EXAMPLE-ELECTRICAL
CIRCUIT)
Oscillator – converts
an impedance
change into a
variable frequency
voltage
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SIGNAL
PROCESSING
ELEMENT
Takes the output of the conditioning element
and converts it into a form more suitable for
presentation
Example
 Analogue-to-digital converter – converts a voltage
into a digital form for input to a computer
 Computer – calculates the measured value of the
variable from the incoming digital data. E.g.
calculation of temperature from a thermocouple emf
signal.
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DATA PRESENTATION
ELEMENT
Presents the measured value in a form
which can be easily recognized by the
observer
Example




simple pointer-scale indicator
chart recorder
alphanumeric display
visual display unit (VDU)
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DATA PRESENTATION ELEMENT
This element should:



Have a fast response as possible
Impose a little drag on the system as possible
Have very small inertia & frictions
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Signal
Processing
Data
presentation
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EXAMPLE: BLOCK DIAGRAM OF MEASUREMENT SYSTEM STAGES
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TUTORIAL 1
1. Choose one sensor (related with mechanical engineering & make
sure not to repeat the same sensor with your other friends)
2. Briefly explain the applications of the chosen sensor
3. Describe the measurement structure regarding the chosen sensor.
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