New Transducers Application

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TRANSDUCERS


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Classification of Transducer


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MEASURANDS

Displacement or Position

Velocity

Acceleration

Force and load

Strain Rotation or angel

Vibrations

Flow

Temperature

Pressure

Vacuum


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SELECTION OF TRANSDUCERS

Sensitivity

Range

Physical properties Loading effect and distortion

Frequency response

Electrical output format


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Classification of Transducers

Transducers

On The Basis of

principle UsedActive/Passive Primary/Secondary Analogue/Digital

Capacitive

Inductive

Resistive

Transducers/

Inverse Transducers

Transducers may be classified according to

their application, method of energy

conversion, nature of the output signal, andso on.


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Displacementand SpeedMeasurement


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Introduction

Displacement and speed are two importantparameters whose measurements are

important in many position and speed control

schemes. The displacement may be in the range of few

mto few cm.


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i i bl iff i l f


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Linear Variable Differential transformer

(LVDT)

works on the principle of variation of mutual

inductance

most popular types of displacement sensor.

has good linearity

displacement range of 0.25mm to 25mm


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Construction of LVDT


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Advantages of LVDT

Linearity

High Output

High Sensitivity Low Power Consumption


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Inductive type Sensors

In this case the inductance of a coil changes as a ferromagnetic

object moves close to the

magnetic former, thus change the reluctance of the magnetic path.

The measuring circuit is usually an a.c. bridge.


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Capacitance Sensors

It can also measure very small displacement in

micrometer range


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Construction


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Capacitance Proximity Sensor

two electrodes, one circular (B) and the other an annular

shaped one (A); separated by a small dielectrical spacing.

When the target comes in the closed vicinity of the sensor

head, the capacitance between the plates A and B would

change, which can be measured by comparing with a fixed

reference capacitor.


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Optical Sensors


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TemperatureMeasurement

Temperature transducers can be divided intofour main categories:

1.Resistance Temperature Detectors (RTD)2. Thermocouples3. Thermistor4. Ultrasonic transducers


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Detectors of wire resistance temperature common employ platinum,

nickel or resistance wire elements, whose resistance variation withtemperature has high accuracy. They are available in many

configurations and size and as shielded or open units for both

immersion and surface applications.

The relationship between temperature and resistance of conductors can

be calculated from the equation:

where

R = the resistance of the conductor at temperature t (0C)

R0 = the resistance at the reference temperature, usually

200C

= the temperature coefficient of resistance

T = the difference between the operating and the

reference temperature

1) Resistance Temperature Detector (RTD)

)1(0 TRR


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Resistance Temp Characteristic


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Advantagesof RTD

Linearity over wide operating range

Higher temperature operatiion

Better Stability at high Temperature High Accuracy


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Disadvantagesof RTD

Low Sensitivity

It can be affected by contact Resistance

Higher Cost compared to other TempTransducer

Requires 3 Wire or 4 Wire Compensation


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Thermistor

very high sensitivity but highly nonlinearcharacteristics.

Two types of Thermistors

(a) Negative temperature coefficient (NTC)thermistors and

(b) Positive temperature co-efficient (PTC)thermistors.

NTC thermistors made from oxides of iron,manganese, magnesium


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Useful range of themistors is normally -100 to

+300 C

Thermistors are available in different forms:

bead type, rod type disc type etc.


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Advantages of Thermistor

The small size of the sensing element makes it

suitable for measurement of temperature at a

point.

Small size and low cost

Fast response and narrow temp range.

High Sensitivity

Contact and Lead resistance problems notencountered.


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Disadvantages of Thermistor

Nonlinearity in resistance vs. temperature.

Unsuitable for wide temperature range.


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Thermocouple


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Vtg vs temp


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Cold Junction Compensation


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Magnetic Flow meter

Mechanical Flow Meter

Magnetic Flow meter

Works on principal of Faradays Lawof

Electromagnetic Induction.

Whenever a conductor moves through a magnetic

field of a given strength voltage is induced in the

conductor proportional to relative velocitybetween conductor and magnetic field .


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Equation to convert Velocity measurement to volumetric flow rate is

IMP


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Construction


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Advantages

It can handle corrosive fluids

It has very low pressure drop

It is totally obstruction less

It is available in large pipe size and capacity .

It can be used as a bidirectional meter.


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Disadvantages

Expensive

Relatively heavy in larger sizes


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Speed Measurement


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noncontact type measurements A LED source is aligned on one side of the disc in

such a way that its light can pass through thetransparent windows of the disc.

As the disc rotates the light will alternately passedthrough the transparent windows and blocked bythe opaque sections.

A photodetector fixed on the other side of the discdetects the variation of light and the output of thedetector after signal conditioning would be a squarewave (as shown) whose frequency is decided by thespeed and the number of holes (transparentwindows) on the disc.

Speed Measurement


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Unit 2 -Qualities of Measurement

Measuring is basically used to monitor aprocess or Operation or controlling application

The knowledge of any parameter depends on

the measurement. The major problem in measuring instruments

is the error.

So measuring instrument should not effect thequantity being measured.

Not practical. We need to minimize errors.


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Static Characteristic

Instrument-a device used to determine the present value of quantitybeing measured.

Measurement- the process of determining the amount ,degree, orcapacity by comparison (direct or indirect) with accepted standards.

Accuracy Degree of Closeness of measurement to the desired value.

Resolution The smallest change that measurement can measure.

Precision

A measure of repeatability.

Desired Value

Error- diff between true value and Desired Value.

Sensitivity Ratio of change in output to change in input .


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Errors In Measurement

Static error

Gross errors or human errors

Systematic errors

Random errors


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Gross errors

Human errors

Incorrect adjustments

Computational misteks.

Complete elimanation of error not possible we

can minimise it .

Reading 1.01 N/m2 as 1.10 N/m2


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Systematic Errors

These errors occur due to shortcoming of

Instrument.

Defective , worn parts ,or aging effect.

Instrument Error

Envirmental Errors

Observatiional Errors


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Instrument Error

None of the apparatus can be constructed tosatisfy all specifications completely. This is

the reason of giving guarantee within a

limit. therefore, a manufacturers always

mention the minimum possible errors in the

construction of the instruments.

Selection

Calibration

Correction Factor


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Envirmental Errors


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Observatiional Errors


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Random Errors

After corrections have been applied for all theparameters whose influences are known, there isleft a residue of deviation. These are random errorand their magnitudes are not constant.

Persons performing the experiment have no controlover the origin of these errors.

These errors are due to so many reasons such asnoise and fatigue in the working persons.

These errors may be either positive or negative. Tothese errors the law of probability may be applied.

Generally, these errors may be minimized by takingaverageof a large number of readings.

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New Transducers Application

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