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Measurement SystemsAsst. Prof. Ahmed A. Al-Rajihy

University of KarabalCollege of engineering

Department of Mechanical Engineering

Text: Measurement Systems, Applicationand Design by Ernest O. Doebelin.

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:

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This recently developed instrument is

illustrated in the figure shown. In thisparticular design, a dual-frequency

heliumneon (HeNe) laser is used thatgives an output pair of light waves at anominal fre uenc of Hz. The two14

waves differ in frequency by Hz andhave opposite polarization. This dual-

frequency output waveform is split into ameasurement beam and a reference

beam by the first beam splitter.

6102

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The reference beam is sensed by

the polarizer and photodetector, A,which converts both waves in the

light to the same polarization. Thetwo waves interfere constructively

and destructively alternately,producing lightdark flicker at a

frequency of Hz. This excitesa 2MHz electrical signal in the

photodetector.

6

102

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The measurement beam is separated

into the two component frequencies bya polarizing beam splitter. Light of the

first frequency, f1, is reflected by a fixedreflecting cube into a photodetector and

, .frequency, f2, is reflected by a movablereflecting cube and also enters B. Thedisplacement to be measured is appliedto the movable cube.

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,

, .

.

. . .

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The outputs from A and B are

amplified and subtracted. Theresultant signal is fed to a counter

whose output indicates the magnitudeof the displacement in the movable

cu e an w ose ra e o c angeindicates the velocity of motion.

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:

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The linear inductosyn is an extremely

accurate instrument that is widely usedfor axis measurement and control within

machine tools. Typical measurementresolution is 2.5 microns. The instrumentconsists of two ma neticall cou led

parts that are separated by an air gap,typically 0.125mm wide, as shown in

Figure. One part, the track, is attached tothe axis along which displacements are to

be measured.This would generally be

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.

, ,

. .

&Vs= Vsin t V1= 0; V2= Vsin t

V1= Vsin

t sin (2

x/s) ; V2= Vsin

t cos (2

x/s)

where xis the displacement of the

slider away from the null position.

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:

Vs = V in

, ,

(()

Va = Ka sin t&

Vb = Kb sin(t-)

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, ,

, .

. . . , :

(

)()

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(.. ) ,

,

:()()()( +)

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+ () ,

.

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eo