lecture 10lecture 1 lecture 1 lecture 1 lecture 1 lecture 1 lecture 1 lecture 1 lecture 1 lecture 1...
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GENERALIZED PERFORMANCE
CHARACTERISTICS OFINSTRUMENTS
RESISTIVE TRANSDUCERS
Lecture 10Instructor : Dr Alivelu M Parimi
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Problem
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Solution
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Problem contd,,
What is the actual temperature at 3000 m
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Problems
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Solution
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y(t) = AK (1e- t/)
Error = em= inputoutput = // )1(/)()( tt AeeAAKtytx
em = M- Me-t/= M( 1- e
-t/)
Steady state error = em,ss( emas t ) = M, Transient error = Me-t/
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Solution
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)(1
1
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Kx
y
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Example
A first order measurement system with a time constant of =
0.1 seconds is used to measure a harmonically varying voltage.
The voltage has a peak-to-peak amplitude of 6 Volts and a
frequency of 5 Hz. Calculate the following:
Frequency of the observed signal Mean-to-peak amplitude of the observed signal
Phase (in degrees) of the observed signal
Time delay (lag) of the observed signal.
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Solution
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Solution
(i) The input and observed signals both have the same frequency. Therefore, observed
frequency = 5 Hz.
(ii)Mean-to-peak amplitude of input = (peak-to-peak)/2 = 6/2 = 3 V
= 2 f =31.42 rad/s
Observed value of mean to peak = V910.0)(1
1
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(appreciable attenuation!)
(iii) Phase angle = tan-1
(- ) = - 72.30
(iii) One cycle takes 1/5 = 0.2 seconds which corresponds to 3600.
Therefore 72.30correspond to
0.2*72.30/360
0=0.0402sec
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Outline RESISTIVE TRANSDUCERS
Introduction
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Introduction
Instrument Transducers are devices which for the purpose of
measurement convert physical input quantities into an
electrical output signal.
Measurand is converted into electrical signal becauseelectrical signals are easy to manipulate, process, transmit,
store and display.
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Radiant to Electrical Photodiode/phototransistor/LDR
Mechanical to Mechanical Gear Box
Mechanical to Electrical Piezoresistance
Thermal to Thermal Heat exchanger
Thermal to Electrical Seebeck Efefct
Electrical to Chemical Electroplating
Electrical to Radiant LED/LCD
Electrical to Mechanical Piezoelectricity
Electrical to Thermal Peltier effect
Electrical to Electrical Transistor
Electrical to Magnetic Coil
Magnetic to Magnetic Magnetic circuit
Chemical to Electrical pH
Chemical to Chemical Chemical reaction
Some examples of energy transformation
devices/ effects are as follows:
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Transducers examples
Any energy conversion device is not a transducer. Only those
which convert for the purpose of measurement are called
Transducers.
LED/LCD are not transducers since the energy conversionprocess in them is utilized for display purpose, while
photodiode which is made to measure light intensity is a
transducer.
Thermocouple utilizing Seebeck effect is a transducer for
temperature measurement.
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Transducers are also classified
on the basis of: Transduction principle
This classification is based on the transduction principle used for converting non-electrical signal to electrical signal. For example inductive transducer will change itsinductance upon change in measurand.
Examples : Resistive, Capacitive, Inductive transducers
Primary Input quantity
This classification depends on the measurand for which they are used, like motiontransducer are the ones used to measure motion.
Example : Flow, Temperature, Motion, Pressure transducers
Material and technology
This classification depends on technology used in making transducers, for examplefibre optic transducers use principle and components of fiber optics to sense, SMTsuse IC technology to make sensors.
Example : SMT (Silicon Micro Technology), MEMS ( Microelectomechanicsystems), Fiber Optic transducers
Application
This classification is based on the broad area of application in which transducers areused. For example biomedical transducers cover all types of transducer used infield of biomedical.
Example : Environmental, Biomedical , Industrial etc.
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RESISTIVE transducers
The major types of resistive transducers are Potentiometers,
Strain Gages Resistance Temperature Detector (RTD),
Thermistors, Light Dependent Resistor (LDR) etc.
Resistive transducers have been used to measuredisplacement, load, pressure, temperature, flow, light
intensity, humidity etc.
This chapter mainly deals with Strain Gages and
Potentiometers.
RTDs and Thermistors would be covered under temperature
measurement and LDR would be covered under Photoelectric
Transducers. 15
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RESISTIVE transducers
Three main parameters of an electrical system are
Conductance (Resistance), Inductance and Capacitance.
All these parameters depend on (i) Geometrical
configuration and (ii) Material properties.
Resistance R is given by where L(length) & A (area)
are geometrical properties while (resistivity) is a
material property. 16
,A
L
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RESISTIVE
POTENTIOMETERS Resistive potentiometer consists of a resistance element
provided with a movable contact.
The contact motion can be translational, rotational, or a
combination of the two such as helical thus allowingmeasurement of rotatory, translatory and helical
displacements.
This displacement could be caused by load, force, vibration,sound pressure etc. The resistance element is excited with
either dc or ac voltage, and the output voltage is a linear
function of the input displacement. 17
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RESISTIVE
POTENTIOMETERS Resistance elements in common use may be classified as wire-
wound, slide wire, conductive plastic, carbon film, thin film
metal resistor, conducting plastic.
Figure shows different types of potentiometer configuration
like slide type, wire wound, linear, rotary, helical; and graphicalrelation of open circuit output voltage vs displacement (linear
or angular).
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