displacement measurement 2

8/6/2019 Displacement Measurement 2

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BY- Anil Pathak, Manoj Kumar,Mayank Tripathi,ME( Ist Year), I & C Dept.,

NITTTR ,Chandigarh



� In an industry it is not possible to measure the

unknown parameters like pressure,

temperature, force, etc directly.

� So we first convert these unknown parameter

in to the displacement and then convert it into electrical form.



� Now the displacement is can be converted in

to electrical in nature with the help of Linear

variable differential transformer(LVDT),which works as a secondary transducers

�

Where as the bourdon tube is working as aprimary transducers(because it is convertingfirst the pressure in to displacement)



� The displacement can be dived into two

parts.

� a) Relative Displacement

� b) Absolute Displacement



� The relative displacement is measured with

respect to a fixed reference.

� In such device one terminal of the instrument

is attached to a point fixed in space and theother terminal is attached( mechanically or

electrically) to the point whose motion is tobe measured.



� The devices used for the measurement of

relative displacement are :-

i) Inductive Type

ii) Capacitive typeiii) Piezo-electric Type

iv) Digital Displacement measuring typev) Resistive Typevi) Ultrasonic Type



� The most widely used inductive transducers

to translate the linear displacement into

electrical signal are � Linear Variable Differential Transformer

(LVDT)� And for the translation of rotational

displacement into electrical signals are-� Rotary Variable Differential Transformer

(RVDT)



� The transformer consists of a single primary

winding(P) and two secondary windings S1 and S2wound on a cylindrical former.

� The secondary windings have equal numbers of turns and are identically placed on either side of the

primary windings.� The primary winding is connected to an alternating

current source.� A movable soft iron core is placed inside the former.



� The displacement to be measured is applied

to the arm attached to the soft iron core.

� The assembly is placed in a stainless steelhousing.



� Since the primary winding is excited by an alternatingcurrent source, it produce an alternating magnetic field

which in turn induces alternating voltages in the twowindings.� The output voltage of secondary S1 and S2 are Es1 and

Es2 respectively.



� To convert the output from S1 and S2 intosingle voltage signal, the two secondaries S1

and S2 are connected in series-opposition.

� Thus the output voltage of the transducers isthe difference of the two voltages.

� The differential voltage, E0 = Es1 ~ Es2



� At NULL position, the flux linking with bothsecondary windings is equal and hence equal emfsare induced in them.(i.e., E

s1= E

s2)and then the

output differential voltage will be zero, E0 = 0.� Now if the core is moved to the left of the NULL

position ,more flux links with S1 and less with S2 , soemf induced in S1 , Es1 is more then emfs induced in

S2 , Es2 (i.e.,Es1 > Es2). So the output differentialvoltage will be, E0 = Es1 - Es2

� And this voltage is in phase with primary voltage.



� Now if the core is moved to the right of the NULLposition ,more flux links with S2 and less with S1 , so

emf induced in S2 , Es2 is more then emf induced inS1 , Es1 (i.e.,Es2 > Es1). So the output differentialvoltage will be, E0 = Es2 - Es1

� And this voltage is 180° out of phase with primary

voltage.



� Therefore , The two voltages(Es1 & Es2 ) are 180° out

of phase with each other .



� So the amount of voltage change neither

secondary winding is proportional to the

amount of movement of the core.

� By noting which output voltage is increasingor decreasing ,we can determine the

direction of motion.



� By comparing the magnitude and phase of theoutput(differential)voltage with that of the source, the

amount and direction of the movement of the coreand hence of displacement may be determined.� The output voltage of an LVDT is

a linear function of Core

displacement with a limited rangeof motion ,say about 5 mm fromnull position.



� High range:- LVDT can be used used for measurementof displacement of 1.25 mm to 250 mm.

� Frication less:- There is no physical contact betweenthe movable core and the coil structure which meansthe LVDT is a frictionless device

� Low Power Consumption: Most of LVDTs consume

power which is less then 1 watt.



� Many a times, the transducer performance is

affected by vibrations.

� Large displacement are required fordifferential output.

� Temperature affects the performance of theLVDT.



� LVDT can be use as a secondary transducers so wecan convert pressure, force and weight intodisplacement and then into electrical output.



� Clockwise rotation produces an Increasing voltageof a secondary winding of one phase.

� while counterclock-wise rotation produces anincreasing voltage of opposite phase.



� The principal of operation of capacitive

transducer is based upon equation of a

parallel plate capacitor.� Which is

� Where, A=Overlapping area of plates(m2)d=Distance between the plates(m)

=Permittivity of the medium (F/m)



� A parallel plate capacitor is shown in figure-

� The capacitive transducer works on the principle of change of capacitance which may be caused by:-

i) Change in overlapping area Aii) Change in the distance d between

the platesiii) Change in dielectric constant



� These changes are caused by physical variable likedisplacement, force and pressure in most of the cases.

� The change in change in dielectric constant as is thecase in measurement of liquid or gas levels.

�

The capacitance may be measured with bridge circuit.



� The capacitive transducer are commonly

used for measurement of linear

displacement.� These transducers use the following effect-

i) Change in capacitance due to change inoverlapping area of plates.

ii) Change in capacitance due to change indistance between the two plates.



� The area changes linearly with displacement

and also the capacitance.

� As mentioned earlier the

response is linear as shown,the initial non-linearity is

due to edge effect.



� Figure shows the basic form of a capacitivetransducer utilizing the effect of change in distancebetween the two plates.

� One is a fixed plate and displacement to bemeasured is applied to the other plate, which ismovable.



� Since the capacitance , C, varies inversely as thedistance d; between the plates the response of thistransducer is not linear, as shown in the figure-

Thus this transducer is useful only formeasurement of extremely smalldisplacements.



� They require extremely small forces to

operate them and hence are very useful for

use in small systems.

� They are extremely sensitive.

� They require small power to operate them.



� Capacitive transducers are used for the

measurement of both linear and angular

motion.� Capacitive transducers are used for the

measurement of pressure and force, thisforce and pressure is first will converted into

displacement which causes change incapacitance.



Piezo-electric Transducers mainly used for measurementof displacement. They can be used for measurement of force, pressure, or acceleration. these quantities when

measured with piezo- electric Transducers are firstconverted in to displacement is subsequently applied tothese Transducers to produce an out put voltage.



� A piezo-electric material is one in which electricpotential appears across certain surface of a crystalif the dimensions of the crystal are changed by theapplication of a mechanical force, this potential isproduced by the displacement of charges. Thiseffect is also reversible, i.e if a varying potential isapplied to the Piezo-electric material.



Digital displacementTransducers thatcommunicate ith digital computer and hich have a digital out put. uch transducer have to use ( /D)

converters to realize the digital data.DigitalTransducers are alled ncoders.They are normally in the f orm of linear or otarydisplacement transducers. ther name isDigitizers

Digital encoders linear digitizers.



� Classification of Encoders-

� Digital Displacement transducers can be

classified in to 3 categories.� 1- Tachometer transducers

� 2- incremental Transducers

� 3- absolute Transducers



� Two tracks mechanically shifted by ¼ cyclerelative to each other. this is allows detection of motion which signal rises first. Thus up down

pulse counter can be used to subtract pulses .Whenever the motion reverses. Thus a third output , which produces one pulse per revolution ata distinct point. Or zero reference track.

� Advantage that any false pulse resulting fromelectric noise will cause errors. Incrementalencoder used.



� Absolute transducers-

� Generally limited to measurement of single

revolution. They use multiple tracks andoutputs, which are read out in parallel to

produce a binary representation of angularshaft input position. Since, there is a one- to-

one correspondence between binaryoutput.



Three type of encoders are given below.

� 1- Tachometer encoders� 2- incremental encoders� 3- absolute encodersThese are Constructed as

(1)Contacting type OR Brush type (Resistiveencoders)

(2) Non Contacting type

The shaded areas are made of conductingmaterial

& unshaded areas are made of non conductingmaterials. As shown in fig.



1. It is relatively inexpensive.

2. It can be to any degree of desired accuracyprovided that the sector is made largeenough to accommodate the large no. of rows for binary nos. and are quite adequate

for slowly moving system.



�It consists of a resistance with amovable contact�The motion of contact can betranslational, rotational or helical



� SINGLE SLIDE WIRE: Step less variation of resistance as wiper travels over it Length of the wire is limited by the desired stroke in

translational device and by the diameter in rotationaldevice

Limited to small value of resistances Resistance per unit length can be increased by

decreasing the area but then it suffers in the point of view of strength

� WIRE WOUND: Resistive wire is wound on a straight or circular card

depending on the type of device



Power loss: if heat dissipated is limited to P

watts the maximum excitation voltage isgiven by

eex = (P Rp)1/2

for high sensitivity output voltage should be

high for this eex should be high it means for

high sensitivity Rp should also highThis situation contradicts linearity thus the

choice of Rp must be influenced by tradeoff between linearity and sensitivity condition

FACTORS TO BE CONSIDER



ADVANTAGES:1. Inexpensive and simple to use

2. Rather large displacement can be measured3. Sufficient output to drive the control circuitry

DISADVANTAGES:1. Mechanical loading owing to wiper friction2. Electrical noise from wiper contacts3. Quick manipulation generates heat



Un-bonded metal wire gauge: At theinitial preload the strain and

resistance of four wires are normallyequal which gives a balanced bridge.Application of motion increasestension in two wires and decreases intwo others causing corresponding

change in resistance so bridgebecome unbalanced and output isproportional to input motion

TYPES OF STRAIN GAUGE



Semiconductor type: The resistivity of dopedsilicon and germanium undergoes a changewhen stressed. This property is called piezo-resistivity is utilized to construct strain gaugewith these metals.

ADVANTAGES:1. No moving parts

2. These are small and inexpensiveDISADVANTAGES:1. Needs to be calibrated2. Usually they are nonlinear

TYPES OF STRAIN GAUGE



Sound Generator: The face of transducer must be kept

clean and damage free to prevent losses Transmitter should have low impedance

at resonant frequency to obtain highmechanical efficiency

Receiver should have max impedance atresonant frequency to obtain highelectrical efficiency

Surface to Beam angle: Transducers are carefully designed to

produce beam angle as small as possible Sometimes beam angle is small, but due

to tilted object echo is deflected away

displacement measurement 2

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