1 BM 2203 - Sensors and Measurment G.Thiyagarajan BM 17 REC/BME.

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BM 2203 - Sensors and MeasurmentBM 2203 - Sensors and Measurment

G.ThiyagarajanG.ThiyagarajanBM 17BM 17REC/BMEREC/BME

BM 2203 - Sensors and MeasurmentBM 2203 - Sensors and Measurment

G.ThiyagarajanG.ThiyagarajanBM 17BM 17REC/BMEREC/BME

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Block diagram of a Block diagram of a generalized instrumentation generalized instrumentation

systemsystem Process or

measurement medium

Physical variable to be measured

Primary stage Detector-transducer

Sensing & conversion

Calibration signal source representing

known value of physical variable

Intermediate stage

Manipulation &

transmission

External power

Feedback signal for control Input signal

Calibration signal

Transduced signal

Modified signal Controller

Indicator

Recorder

Final (output ) stage

Environmental effects (noise, temperature etc)

Observer

Quantity presented

to observer

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The Bourdon The Bourdon GaugeGauge

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Block diagram of the Block diagram of the pressure gauge based pressure gauge based

on Bourdon tubeon Bourdon tube Physical variable to be measured Pressure

Detector – transducer stage Bourdon tube Pressure to mechanical displacement

Intermediate stage Gearing arrangement that amplifies the displacement signal

Pressure (input)

Pressure (calibration)

Displacement ( transduced signal)

Amplified displacement signal

Calibration signal from a source with known pressure values

Final stage Pointer and dial arrangement

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Sensor

Datacommunication

Datadisplays

Effector

Measurand

Signalconditioning

Signalprocessing

Datastorage

Feedback

Outputs

A typical medical measurement A typical medical measurement systemsystem

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InstrumentPatient

InstrumentPatient

Clinician

Feedback with and without Feedback with and without clinicianclinician

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InstrumentPatientClinician

Abnormalreadings

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

occuroccur

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Detailed generalized medical Detailed generalized medical measurement system measurement system

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Alternative operational modesAlternative operational modes

Direct-indirect modesDirect-indirect modes Sampling and continuous modesSampling and continuous modes Generating and modulating sensorsGenerating and modulating sensors Analog and digital modesAnalog and digital modes Real-time and delayed-time modesReal-time and delayed-time modes

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Laboratory testLaboratory test Typical valueTypical value

HemoglobinHemoglobin 13.5 to 18 g/dL13.5 to 18 g/dL

HematocritHematocrit 40 to 54%40 to 54%

Erythrocyte countErythrocyte count 4.6 to 6.2 4.6 to 6.2 10 1066/ / LL

Leukocyte countLeukocyte count 4500 to 11000/ 4500 to 11000/ LL

Differential countDifferential count

Neutrophil 35 to 71%Neutrophil 35 to 71%

Band 0 to 6%Band 0 to 6%

Lymphocyte 1 to 10%Lymphocyte 1 to 10%

Monocyte 1 to 10%Monocyte 1 to 10%

Eosinophil 0 to 4%Eosinophil 0 to 4%

Basophil 0 to 2%Basophil 0 to 2%

Complete blood count for a male subject.

Example to sampled dataExample to sampled data

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Analog and digital signalsAnalog and digital signals

Time

Am

plit

ude

Am

plit

ude

Time

Analog signals can have any amplitude value

Digital signals have a limited number of amplitude values

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Continuous and discrete-time Continuous and discrete-time signalssignals

Time

Am

plit

ude

Am

plit

ude

Time

Continuous signals have values at every instant of time

Discrete-time signals are sampled periodically and do not provide values between these sampling times

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Origins of Origins of common common biological biological

signalsignal

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MeasurementMeasurement RangeRange Frequency, HzFrequency, Hz MethodMethod

Blood flowBlood flow 1 to 300 mL/s1 to 300 mL/s 0 to 200 to 20 Electromagnetic or ultrasonicElectromagnetic or ultrasonic

Blood pressureBlood pressure 0 to 400 mmHg0 to 400 mmHg 0 to 500 to 50 Cuff or strain gageCuff or strain gage

Cardiac outputCardiac output 4 to 25 L/min4 to 25 L/min 0 to 200 to 20 Fick, dye dilution Fick, dye dilution

ElectrocardiographyElectrocardiography 0.5 to 4 mV0.5 to 4 mV 0.05 to 1500.05 to 150 Skin electrodesSkin electrodes

ElectroencephalographyElectroencephalography 5 to 300 5 to 300 VV 0.5 to 150 0.5 to 150 Scalp electrodesScalp electrodes

ElectromyographyElectromyography 0.1 to 5 mV0.1 to 5 mV 0 to 10000 0 to 10000 Needle electrodesNeedle electrodes

ElectroretinographyElectroretinography 0 to 900 0 to 900 V V 0 to 500 to 50 Contact lens electrodesContact lens electrodes

pHpH 3 to 13 pH units3 to 13 pH units 0 to 10 to 1 pH electrodepH electrode

ppCOCO22 40 to 100 mmHg40 to 100 mmHg 0 to 20 to 2 ppCOCO22 electrode electrode

ppOO22 30 to 100 mmHg30 to 100 mmHg 0 to 20 to 2 ppOO22 electrode electrode

PneumotachographyPneumotachography 0 to 600 L/min0 to 600 L/min 0 to 400 to 40 PneumotachometerPneumotachometer

Respiratory rateRespiratory rate 2 to 50 2 to 50 breaths/minbreaths/min 0.1 to 100.1 to 10 ImpedanceImpedance

TemperatureTemperature 32 to 40 °C32 to 40 °C 0 to 0.10 to 0.1 ThermistorThermistor

Medical measurement Medical measurement constraintsconstraints

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Specification Value

Pressure range –30 to +300 mmHg

Overpressure without damage –400 to +4000 mmHg

Maximum unbalance ±75 mmHg

Linearity and hysteresis ± 2% of reading or ± 1 mmHg

Risk current at 120 V 10 A

Defibrillator withstand 360 J into 50

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

Setting sensor specificationsSetting sensor specifications

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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 M

Dc lead current 0.1

Return time after lead switch 1 s

Overload voltage without damage 5000 V

Risk current at 120 V 10

Specification values for an electrocardiograph are agreed upon by a committee.

Specifications for ECGSpecifications for ECG

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Classification of biomedical Classification of biomedical instrumentsinstruments

Quantity sensed: pressure, flow, Quantity sensed: pressure, flow, temperature etc.temperature etc.

Principle of transduction: resistive, Principle of transduction: resistive, inductive, capacitive, ultrasonic or inductive, capacitive, ultrasonic or electrochemicalelectrochemical

Organ system studied: cardiovascular, Organ system studied: cardiovascular, pulmonary, nervous, and endocrine pulmonary, nervous, and endocrine systems.systems.

Clinical medical specialties: pediatrics, Clinical medical specialties: pediatrics, obstetrics, cardiology, or radiology.obstetrics, cardiology, or radiology.

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Interfering and modifying inputsInterfering and modifying inputs

Original waveform An interfering input may shift the baseline

A modifying input may change the gain

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Simplified Simplified Electrocardiographic Electrocardiographic

recording systemrecording system

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Compensation TechniquesCompensation Techniques

Inherent insensitivityInherent insensitivity Negative feedbackNegative feedback Signal filteringSignal filtering Opposing inputsOpposing inputs

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Negative feedbackNegative feedback

ddf

d

dfdd

dfd

xGH

Gy

GHyGx

yGyHx

1

)1(

)(

Gd

Hf

xd

y+

-

22

Interference superimposed on signals causes error.

Frequency filters can be used to reduce noise and

interference

Signal filteringSignal filtering

Signals without noise are uncorrupted

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Opposing inputsOpposing inputs

Differential amplifier: vDifferential amplifier: v00 = G = Gdd(v(vAA- v- vBB)) DC cancellation (bucking)DC cancellation (bucking)

T im e

Am plitude

D c o ffse t

An input signal without dc offset

An input signal with dc offset

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Generalized Static Generalized Static CharacteristicsCharacteristics

AccuracyAccuracy Precision and Precision and

reproducibilityreproducibility ResolutionResolution Statistical controlStatistical control Static sensitivityStatic sensitivity

Zero driftZero drift Sensitivity driftSensitivity drift LinearityLinearity Input rangesInput ranges Input impedanceInput impedance

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Accuracy Accuracy Data points with

low accuracy

high accuracy

valuetrue

valuemeasuredvaluetrueaccuracy

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 looking into the sources of errors.

Accuracy is often expressed in percentage

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Precision Precision Data points with

low precision

high precision

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 measurements differ from one another.

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

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Resolution Resolution

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

Statistical control: random variations in measured quantities are tolerable,

Coulter counter example

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ToleranceTolerance Maximum deviation allowed from the Maximum deviation allowed from the

conventional true value. conventional true value. It is not possible to built a perfect system or make It is not possible to built a perfect system or make

an exact measurement. All devices deviate from an exact measurement. All devices deviate from their ideal (design) characteristics and all their ideal (design) characteristics and all measurements include uncertainties (doubts).measurements include uncertainties (doubts).

Hence, all devices include tolerances in their Hence, all devices include tolerances in their specifications. If the instrument is used for high-specifications. If the instrument is used for high-precision applications, the design tolerances must precision applications, the design tolerances must be small. be small.

However, if a low degree of accuracy is However, if a low degree of accuracy is acceptable, it is not economical to use expensive acceptable, it is not economical to use expensive sensors and precise sensing components sensors and precise sensing components

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Static sensitivityStatic sensitivity

Sensorsignal

Measurand

Sensorsignal

Measurand

A low-sensitivity sensor has low gain

A high sensitivity sensor has high gain

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Static sensitivity constant over a limited Static sensitivity constant over a limited rangerange

22 )(

))((

dd

dd

xxn

yxyxnm

22

2

)(

))(())((

dd

ddd

xxn

xyxxyb

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Zero and sensitivity driftsZero and sensitivity drifts

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Linearity Linearity

Output

Input

Output

Input

A linear system fits the equation y = mx + b.

A nonlinear system does not fit a straight line

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Calibration for linearityCalibration for linearity

Output

Input

Output

Input

The one-point calibration may miss nonlinearity

The two-point calibration may 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

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Hysteresis Hysteresis

Sensors ignal

M easurand

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

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Independent Independent nonlinearity nonlinearity

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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 TransducersInductive Transducers

ObjectivesObjectivesObjectivesObjectives

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IntroductionIntroductionIntroductionIntroduction 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.

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IntroductionIntroductionIntroductionIntroduction 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.

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IntroductionIntroductionIntroductionIntroduction 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.

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DefinitionDefinitionDefinitionDefinition 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'.

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DefinitionDefinitionDefinitionDefinition 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'.

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Features of SensorsFeatures of SensorsFeatures of SensorsFeatures of SensorsThe 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

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

Sensors TypesSensors TypesSensors TypesSensors Types

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Sensors TypesSensors TypesSensors TypesSensors Types

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Common SensorsCommon SensorsCommon SensorsCommon SensorsListed 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.

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Sensors TypesSensors TypesSensors TypesSensors Types• There are several transducers that will be

examined further in terms of their principles of operations.

• Those include :

1. Resistive Position Transducers2. Strain Gauges3. Capacitive Transducers4. Inductive Transducers5. And a lot more…

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• 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.

Resistive Position TransducersResistive Position TransducersResistive Position TransducersResistive Position Transducers

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• 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.

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2

RR

R

E

E

T

out


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Example 1An 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?


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• 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.

Strain GaugesStrain GaugesStrain GaugesStrain Gauges

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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.


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• For many common materials, there is a constant ratio between stress and strain.

• Stress is defined as the internal force per unit area. •

S – Stress (kg/m2)F – Force (kg)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.


A

FS

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• The capacitance of a parallel plate is given by:

k= dielectric constantA= area of the plateo=8.854x10-12 F/md= 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.

Capacitive TransducersCapacitive TransducersCapacitive TransducersCapacitive Transducers

d

kAC o

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• Some examples of capacitive transducers


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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.


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• 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.

Inductive TransducersInductive TransducersInductive TransducersInductive Transducers

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• Examples of a Common Tachometer

TachometersTachometersTachometersTachometers

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Linear Variable Linear Variable Differential Transformer (LVDT)Differential Transformer (LVDT)

Linear Variable Linear Variable Differential Transformer (LVDT)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.

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Linear Variable Linear Variable Differential Transformer (LVDT)Differential Transformer (LVDT)• Some examples of LVDTs.

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Linear Variable Linear Variable Differential Transformer (LVDT)Differential Transformer (LVDT)• An example of LVDT electrical wiring.

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Linear Variable Linear Variable Differential Transformer (LVDT)Differential Transformer (LVDT)• 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.

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Linear Variable Linear Variable Differential Transformer (LVDT)Differential Transformer (LVDT)•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.

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Linear Variable Linear Variable Differential Transformer (LVDT)Differential Transformer (LVDT)• 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.

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Linear Variable Linear Variable Differential Transformer (LVDT)Differential Transformer (LVDT)• 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.

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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.

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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.

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Piezoelectric TransducersPiezoelectric TransducersPiezoelectric TransducersPiezoelectric 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:

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Piezoelectric TransducersPiezoelectric TransducersPiezoelectric TransducersPiezoelectric 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.

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Temperature TransducersTemperature TransducersTemperature TransducersTemperature Transducers• The temperature transducers can be divided

into four main categories:

o Resistance Temperature Detectors (RTD)o Thermocoupleso Thermistorso Ultrasonic transducers

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Resistance Temperature Resistance Temperature Detectors (RTDs)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.

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• Some examples of RTDs are as follows:

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

where;R= resistance of the conductor at temp t (oC)Ro=resistance at the reference temp.= temperature coefficient of resistance= difference between operating and reference

temp.

)1( TRR o

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

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ThermocouplesThermocouplesThermocouplesThermocouples• 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.

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ThermocouplesThermocouplesThermocouplesThermocouples• Some examples of the thermocouples are as follows:

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ThermocouplesThermocouplesThermocouplesThermocouples• 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).

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

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

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• 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:

•Where; c and k – constant of the thermocouple materialsT1 - temperature of the ‘hot’ junction.T2 - temperature of the ‘cold’ or

‘reference’ junction.

)()( 22

2121 TTkTTcE

ThermocouplesThermocouplesThermocouplesThermocouples

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ThermocouplesThermocouplesThermocouplesThermocouplesExample

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?

1 BM 2203 - Sensors and Measurment G.Thiyagarajan BM 17 REC/BME.

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Transcript of 1 BM 2203 - Sensors and Measurment G.Thiyagarajan BM 17 REC/BME.