as instrumentation

8/2/2019 As Instrumentation

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INSTR UMENTATION F OR PC BASED

VIRTUAL INSTRUMENTS

Lecture #1

Department of PhysicsA.Srinivasan

QIP-STC on VI (Nov., 1-7,2004), IITG

Basic modules of the inst rumentat ion


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S i g n a l T r a n s d u c e r s : F r o n tS i g n a l T r a n s d u c e r s : F r o n tS i g n a l T r a n s d u c e r s : F r o n tS i g n a l T r a n s d u c e r s : F r o n t ---- e n de n de n de n d

o f a n y me a s u r e me n t s y s t e mo f a n y me a s u r e me n t s y s t e mo f a n y me a s u r e me n t s y s t e mo f a n y me a s u r e me n t s y s t e m

THE FIRST MODULE




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!Most crucial component (module) of a measurement system.

! Accuracy, reliability and reproducibility of a measurement

depend on the characteristics of the transducer.

! Transducers with electrical output are preferred.

!Selecting an appropriate transducer depends on

1. Physical quantity to be measured2. Transducer principle best suited for this measurement

3. Accuracy required

a) Basic electrical an mechanical characteristics.- type & range, sensitivity, excitation, physical nature

b) Ambient and environmental conditions.

- intrinsic nature, temperature effects, vibration, c) Compatibility of the associated equipment.

- zero balance provision, impedence matching, .

Cl i f i t i f T dCl i f i t i f T dCl i f i t i f T dCl i f i t i f T d


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B a s e d o n e l e c t r i c a l p a r a me t e r me a s u r e dB a s e d o n e l e c t r i c a l p a r a me t e r me a s u r e dB a s e d o n e l e c t r i c a l p a r a me t e r me a s u r e dB a s e d o n e l e c t r i c a l p a r a me t e r me a s u r e dR pot ent iomet r ic (P, x), st r ain gage (F, , x), pir ani gage (P, f low),

r esist ance t her momet er (RTD) (T), t her mist or (T), .

C var . C pr essure gage (x, P), micr ophone (sound),

L LVDT (P, F, x), eddy cur r ent (x), magnet ost r ict ion (P, F, sound),

V, I t hermocouple (T, heat f l.), piezoelect r ic (sound, a, P),phot ovolt aic (light ), PMT (light ), Hall ef f ect (Mag. Flux),elect r ochemical sensors (ions, gases),

Cl a s s i f i c a t i o n o f T r a n s d u c e r sCl a s s i f i c a t i o n o f T r a n s d u c e r sCl a s s i f i c a t i o n o f T r a n s d u c e r sCl a s s i f i c a t i o n o f T r a n s d u c e r sBased on physical principle involved

Active & Passive

Primary & SecondaryAnalog & Digital

THE SECOND MODULE


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THE SECOND MODULE

R e a lR e a lR e a lRe a l ---- wo r l d t r a n s d u c e r s r e q u i r e s i g n a l c o n d i t i o n i n g .wo r l d t r a n s d u c e r s r e q u i r e s i g n a l c o n d i t i o n i n g .wo r l d t r a n s d u c e r s r e q u i r e s i g n a l c o n d i t i o n i n g .wo r l d t r a n s d u c e r s r e q u i r e s i g n a l c o n d i t i o n i n g .

I n c l u d e s s i g n a l a mp l i f i c a t i o n , a t t e n u a t i o n , f i l t e r i n g ,I n c l u d e s s i g n a l a mp l i f i c a t i o n , a t t e n u a t i o n , f i l t e r i n g ,I n c l u d e s s i g n a l a mp l i f i c a t i o n , a t t e n u a t i o n , f i l t e r i n g ,I n c l u d e s s i g n a l a mp l i f i c a t i o n , a t t e n u a t i o n , f i l t e r i n g ,

e l e c t r i c a l i s o l a t i o n , s i mu l t a n e o u s s a mp l i n g , a n de l e c t r i c a l i s o l a t i o n , s i mu l t a n e o u s s a mp l i n g , a n de l e c t r i c a l i s o l a t i o n , s i mu l t a n e o u s s a mp l i n g , a n de l e c t r i c a l i s o l a t i o n , s i mu l t a n e o u s s a mp l i n g , a n dmu l t i p l e x i n g .mu l t i p l e x i n g .mu l t i p l e x i n g .mu l t i p l e x i n g .

Ma n y t r a n s d u c e r s r e q u i r e e x c i t a t i o n c u r r e n t s o rMa n y t r a n s d u c e r s r e q u i r e e x c i t a t i o n c u r r e n t s o rMa n y t r a n s d u c e r s r e q u i r e e x c i t a t i o n c u r r e n t s o rMa n y t r a n s d u c e r s r e q u i r e e x c i t a t i o n c u r r e n t s o r

v o l t a g e s , b r i d g e c o mp l e t i o n o rv o l t a g e s , b r i d g e c o mp l e t i o n o rv o l t a g e s , b r i d g e c o mp l e t i o n o rv o l t a g e s , b r i d g e c o mp l e t i o n o r l i n e a r i z a t i o nl i n e a r i z a t i o nl i n e a r i z a t i o nl i n e a r i z a t i o n ....

S i g n a l c o n d i t i o n i n g r e q u i r e me n t s a r e s p e c i f i c t o e a cS i g n a l c o n d i t i o n i n g r e q u i r e me n t s a r e s p e c i f i c t o e a cS i g n a l c o n d i t i o n i n g r e q u i r e me n t s a r e s p e c i f i c t o e a cS i g n a l c o n d i t i o n i n g r e q u i r e me n t s a r e s p e c i f i c t o e a c

t y p e o f t r a n s d u c e r .t y p e o f t r a n s d u c e r .t y p e o f t r a n s d u c e r .t y p e o f t r a n s d u c e r .

L e t u s c o n s i d e r t h e s p e c i f i c c a s e s o f F S C r e q u i r e dL e t u s c o n s i d e r t h e s p e c i f i c c a s e s o f F S C r e q u i r e dL e t u s c o n s i d e r t h e s p e c i f i c c a s e s o f F S C r e q u i r e dL e t u s c o n s i d e r t h e s p e c i f i c c a s e s o f F S C r e q u i r e df o r a t h e r mo c o u p l e , RT D,f o r a t h e r mo c o u p l e , RT D,f o r a t h e r mo c o u p l e , RT D,f o r a t h e r mo c o u p l e , RT D, t h e r mi s t o rt h e r mi s t o rt h e r mi s t o rt h e r mi s t o r a n d s t r a i n g a g e .a n d s t r a i n g a g e .a n d s t r a i n g a g e .a n d s t r a i n g a g e .

F r o n tF r o n tF r o n tF r o n t ---- E n d S i g n a l Co n d i t i o n e r ( F S C)E n d S i g n a l Co n d i t i o n e r ( F S C)E n d S i g n a l Co n d i t i o n e r ( F S C)E n d S i g n a l Co n d i t i o n e r ( F S C)

S t i f f t &S t i f f t &S t i f f t &S t i f f t &


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S t r a i n g a g e s f o r f o r c e , t o r q u e &S t r a i n g a g e s f o r f o r c e , t o r q u e &S t r a i n g a g e s f o r f o r c e , t o r q u e &S t r a i n g a g e s f o r f o r c e , t o r q u e &d i s p l a c e me n t me a s u r e me n t sd i s p l a c e me n t me a s u r e me n t sd i s p l a c e me n t me a s u r e me n t sd i s p l a c e me n t me a s u r e me n t s

* V or I excitation

* High amplification

* Bridge Completion* Linearization

* Shunt calibration

* Low resistance

* Low sensitivity

* Nonlinear output

FSC RequirementCharacteristics

* Strain (): Amount of deformation of abody due to an applied force.

* Expressed in 10-6 in./in. or 10-6mm/mm.

* Fine wire / metallic foil in a grid form.

* Gage Factor (GF) = (R / R) / (L / L)

= (R / R) /

# S t r a i n g a g e me a s u r e me n t s r e q u i r e a c c u r a t eS t r a i n g a g e me a s u r e me n t s r e q u i r e a c c u r a t eS t r a i n g a g e me a s u r e me n t s r e q u i r e a c c u r a t eS t r a i n g a g e me a s u r e me n t s r e q u i r e a c c u r a t e


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# S t r a i n g a g e me a s u r e me n t s r e q u i r e a c c u r a t eS t r a i n g a g e me a s u r e me n t s r e q u i r e a c c u r a t eS t r a i n g a g e me a s u r e me n t s r e q u i r e a c c u r a t eS t r a i n g a g e me a s u r e me n t s r e q u i r e a c c u r a t eme a s u r e me n t o f v e r y s ma l l r e s i s t a n c e v a l u e s .me a s u r e me n t o f v e r y s ma l l r e s i s t a n c e v a l u e s .me a s u r e me n t o f v e r y s ma l l r e s i s t a n c e v a l u e s .me a s u r e me n t o f v e r y s ma l l r e s i s t a n c e v a l u e s .

Co n s i d e r a s p e c i me n wi t h a s t r a i n o f 5 0 0Co n s i d e r a s p e c i me n wi t h a s t r a i n o f 5 0 0Co n s i d e r a s p e c i me n wi t h a s t r a i n o f 5 0 0Co n s i d e r a s p e c i me n wi t h a s t r a i n o f 5 0 0 ....

T h e c o r r e s p o n d i n gT h e c o r r e s p o n d i n gT h e c o r r e s p o n d i n gT h e c o r r e s p o n d i n g R = G FR = G FR = G FR = G F ( 5 0 0( 5 0 0( 5 0 0( 5 0 0 1 01 01 01 0 ---- 6666 ) .) .) .) .No w, i f GF = 2 , t h e nNo w, i f GF = 2 , t h e nNo w, i f GF = 2 , t h e nNo w, i f GF = 2 , t h e n R = 0 . 1 %.R = 0 . 1 %.R = 0 . 1 %.R = 0 . 1 %.F o r a 1 2 0F o r a 1 2 0F o r a 1 2 0F o r a 1 2 0 g a g e , t h i sg a g e , t h i sg a g e , t h i sg a g e , t h i s R = 0 . 1 2R = 0 . 1 2R = 0 . 1 2R = 0 . 1 2 ! ! ! !! ! ! !! ! ! !! ! ! !

He n c e s t r a i n g a g e s a r e u s e d i n aHe n c e s t r a i n g a g e s a r e u s e d i n aHe n c e s t r a i n g a g e s a r e u s e d i n aHe n c e s t r a i n g a g e s a r e u s e d i n a b r i d g eb r i d g eb r i d g eb r i d g e

c o n f i g u r a t i o nc o n f i g u r a t i o nc o n f i g u r a t i o nc o n f i g u r a t i o n wi t h a v o l t a g e o r c u r r e n twi t h a v o l t a g e o r c u r r e n twi t h a v o l t a g e o r c u r r e n twi t h a v o l t a g e o r c u r r e n te x c i t a t i o n s o u r c ee x c i t a t i o n s o u r c ee x c i t a t i o n s o u r c ee x c i t a t i o n s o u r c e ....

For the Wheatstone bridge shown in figure,the output voltage,

When R1/R2=R3/R4, Vo = 0 = BALANCED!

If R is replaced with a strain gage then


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If R4 is replaced with a strain gage, thenR= RG GF . If R1=R2 and R3=RG,

then for the quarter

-bridge circuit,

Sensitivity is doubled

the half-bridge circuit(R3 and R4 are replace

by two active gages).V

o/V

EX= - GF / 2/ 2/ 2/ 2

In the full-bridge circuit, R1, R2, R3 and R4of the general Wheatstone bridge is made

of four active strain gages (two in tensionand two in compression mode). For this

circuit, Vo/VEX = - GF

T h e F S C f o r a s t r a i n g a g e s h o u l d t a k e c a r eT h e F S C f o r a s t r a i n g a g e s h o u l d t a k e c a r eT h e F S C f o r a s t r a i n g a g e s h o u l d t a k e c a r eT h e F S C f o r a s t r a i n g a g e s h o u l d t a k e c a r e


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T h e F S C f o r a s t r a i n g a g e s h o u l d t a k e c a r eT h e F S C f o r a s t r a i n g a g e s h o u l d t a k e c a r eT h e F S C f o r a s t r a i n g a g e s h o u l d t a k e c a r eT h e F S C f o r a s t r a i n g a g e s h o u l d t a k e c a r eo f t h e f o l l o wi n go f t h e f o l l o wi n go f t h e f o l l o wi n go f t h e f o l l o wi n g :Bridge completion: If full-bridge circuit is not used, the bridge

has to be completed with two or more high-precision (matched) reference resistors.

Bridge excitation: Normally a constant voltage source is used.

There is no standard voltage level (3V and 10Vare commonly used).

Signal amplification: Typical strain gage outputs are < 10 mV/V. So

if VEX

= 10V, V0

=100 mV. A good voltage

amplifier with gain up to 2000 is required.

Shunt calibration: A calibrated shunt resistance is used across

an arm of the bridge to get a known R.Vogenerated is then compared with expected value

Others: Lead wire R compensation & bridge balancing.

T h e r mo c o u p l eT h e r mo c o u p l eT h e r mo c o u p l eT h e r mo c o u p l e t h e r mi s t o rt h e r mi s t o rt h e r mi s t o rt h e r mi s t o r a n d RT Da n d R T Da n d R T Da n d R T D


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T h e r mo c o u p l e ,T h e r mo c o u p l e ,T h e r mo c o u p l e ,T h e r mo c o u p l e , t h e r mi s t o rt h e r mi s t o rt h e r mi s t o rt h e r mi s t o r a n d RT Da n d R T Da n d R T Da n d R T Df o r t e mp e r a t u r e me a s u r e me n t sf o r t e mp e r a t u r e me a s u r e me n t sf o r t e mp e r a t u r e me a s u r e me n t sf o r t e mp e r a t u r e me a s u r e me n t s

T h e r mo c o u p l e :T h e r mo c o u p l e :T h e r mo c o u p l e :T h e r mo c o u p l e : T wo d i s s i mi l a r me t a l s j o i n e dT wo d i s s i mi l a r me t a l s j o i n e dT wo d i s s i mi l a r me t a l s j o i n e dT wo d i s s i mi l a r me t a l s j o i n e dt o g e t h e r .t o g e t h e r .t o g e t h e r .t o g e t h e r . S e e b e c kS e e b e c kS e e b e c kS e e b e c k c o e f f i c i e n t , S =c o e f f i c i e n t , S =c o e f f i c i e n t , S =c o e f f i c i e n t , S = V /V /V /V / T .T .T .T .

J1, J2 and J3 are three metal-metal junctions. To get the temperature o

J1, contribution from J2 and J3 should be subtracted. For this, J2 & J3temperatures and their V-to-T relationship should be known. Cold

Junction and isothermal J2 and J3 eliminates parasitic effects.

Types of thermocouple


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Types of thermocouple

Type Composition Useful T (0C) Abs. A

B Pt30%Rh (+) , Pt6%Rh (-) 1370 1700 ---

C W5%Re (+) , W26%Re (-), 1650 2315 ---

E Chromel (+) , Constantan (-) 95 900 1.

J Iron (+) , Constantan (-) 95 760 1.

K Chromel (+) , Alumel (-) 95 1260 1.N Nicrosil (+) , Nisil (-) 650 1260 1.

R Pt13%Rh (+) , Pt (-) 870 1450 1.

S Pt10%Rh (+) , Pt (-) 980 1450 1.T Cu (+) , Constantan (-) -200 - 350 0.

Size: Larger diameter wire means longer life at higher temperatures.

But thinner wire gives higher sensitivity.Length: Minimum immersed length should be four times the diameter

the outer (protective) tubing.

FSC R i tCh t i ti


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* TREF sensor

* High amplification* Linearization

* Low voltage OP

* Low sensitivity

* Nonlinear output


TREF sensor and cold junction compensation:

H/W method: A variable voltage source (voltage source with apotentiometer) with a pre-calibrated setting for variations in J2 (and J3)

temperature is used. The reference emf TREF sensor.

S/W method: Reference junction temperature TREF is measured and

converted in to voltage. This value is added to measured voltage of J1.

High amplification: Thermo emf changes only 7 to 50V for 1oC.

Low-noise mV amplifier is required.

Linearization: Thermo emf is non-linear. S can vary by a factor of

V vs. T curves are fit to T = a0 + a1V + a2V2 +


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T h e r mi s t o rT h e r mi s t o rT h e r mi s t o rT h e r mi s t o r ::::


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T h e r mi s t o rT h e r mi s t o rT h e r mi s t o rT h e r mi s t o r ::::

T h i s i s a s e mi c o n d u c t o r c o mp o s e d o f me t a l l i cT h i s i s a s e mi c o n d u c t o r c o mp o s e d o f me t a l l i cT h i s i s a s e mi c o n d u c t o r c o mp o s e d o f me t a l l i cT h i s i s a s e mi c o n d u c t o r c o mp o s e d o f me t a l l i co x i d e s s u c h a s n i c k e l , ma n g a n e s e , c o b a l t ,o x i d e s s u c h a s n i c k e l , ma n g a n e s e , c o b a l t ,o x i d e s s u c h a s n i c k e l , ma n g a n e s e , c o b a l t ,o x i d e s s u c h a s n i c k e l , ma n g a n e s e , c o b a l t ,

c o p p e r , i r o n , a n d t i t a n i u m.c o p p e r , i r o n , a n d t i t a n i u m.c o p p e r , i r o n , a n d t i t a n i u m.c o p p e r , i r o n , a n d t i t a n i u m. T h e r mi s t o r sT h e r mi s t o r sT h e r mi s t o r sT h e r mi s t o r s i n wa f e r , d i s k , b e a d , a n d o t h e ri n wa f e r , d i s k , b e a d , a n d o t h e ri n wa f e r , d i s k , b e a d , a n d o t h e ri n wa f e r , d i s k , b e a d , a n d o t h e r

s h a p e s .s h a p e s .s h a p e s .s h a p e s .

T wo b a s i c t y p e s o fT wo b a s i c t y p e s o fT wo b a s i c t y p e s o fT wo b a s i c t y p e s o f t h e r mi s t o r st h e r mi s t o r st h e r mi s t o r st h e r mi s t o r s ---- n e g a t i v en e g a t i v en e g a t i v en e g a t i v et e mp e r a t u r e c o e f f i c i e n t ( NT C) a n d p o s i t i v et e mp e r a t u r e c o e f f i c i e n t ( NT C) a n d p o s i t i v et e mp e r a t u r e c o e f f i c i e n t ( NT C) a n d p o s i t i v et e mp e r a t u r e c o e f f i c i e n t ( NT C) a n d p o s i t i v et e mp e r a t u r e c o e f f i c i e n t ( P T C) .t e mp e r a t u r e c o e f f i c i e n t ( P T C) .t e mp e r a t u r e c o e f f i c i e n t ( P T C) .t e mp e r a t u r e c o e f f i c i e n t ( P T C) .

**** L a r g e c h a n g e i n r e s i s t a n c e f o r 1L a r g e c h a n g e i n r e s i s t a n c e f o r 1L a r g e c h a n g e i n r e s i s t a n c e f o r 1L a r g e c h a n g e i n r e s i s t a n c e f o r 1 OOOOC c h a n g e .C c h a n g e .C c h a n g e .C c h a n g e .

Co mp a c t s h a p e a n d s i z e e n a b l e s t e mp e r a t u r eCo mp a c t s h a p e a n d s i z e e n a b l e s t e mp e r a t u r eCo mp a c t s h a p e a n d s i z e e n a b l e s t e mp e r a t u r eCo mp a c t s h a p e a n d s i z e e n a b l e s t e mp e r a t u r eme a s u r e me n t o f s ma l l a r e a s .me a s u r e me n t o f s ma l l a r e a s .me a s u r e me n t o f s ma l l a r e a s .me a s u r e me n t o f s ma l l a r e a s .

H i g h r e s i s t a n c e mi n i mi z e s c o n t r i b u t i o n f r o mHi g h r e s i s t a n c e mi n i mi z e s c o n t r i b u t i o n f r o mHi g h r e s i s t a n c e mi n i mi z e s c o n t r i b u t i o n f r o mHi g h r e s i s t a n c e mi n i mi z e s c o n t r i b u t i o n f r o ml e a d wi r e s .l e a d wi r e s .l e a d wi r e s .l e a d wi r e s .


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L i n e a r i z a t i o nL i n e a r i z a t i o nL i n e a r i z a t i o nL i n e a r i z a t i o n : T h e S t a i n h a r t - Ha r t e q u a t i o ne x p r e s s e s t h e r e l a t i o n s h i p b e t we e n r e s i s t a n c eo f a t h e r mi s t o r a n d t e mp e r a t u r e , n a me l y ,

1 / T = a + b ( l n R) + c ( l n R) 3

wh e r e T i s t e mp e r a t u r e , R i s r e s i s t a n c e a n da , b , c a r e c o e f f i c i e n t s d e r i v e d f r o m

me a s u r e me n t s .

* I or V excitation with

reference resistor

* Linearization

* High resistance

* High sensitivity* Highly NL output



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Re s i s t a n c e T e mp e r a t u r e De t e c t o r ( RT D)Re s i s t a n c e T e mp e r a t u r e De t e c t o r ( RT D)Re s i s t a n c e T e mp e r a t u r e De t e c t o r ( RT D)Re s i s t a n c e T e mp e r a t u r e De t e c t o r ( RT D)

* Co n s i s t s o f a w i r e c o i l o r d e p o s i t e d f i l mo f p u r e me t a l wh o s e r e s i s t a n c e i n c r e a s e s

wi t h t e mp e r a t u r e .

* Us u a l l y ma d e o f p l a t i n u m a n d h a s a n o mi n a l

r e s i s t a n c e o f 1 0 0 a t 0 C [ P T 1 0 0 ] . Ot h e r

v a r i a n t s a r e P T 5 0 0 , P T 1 0 0 0 , e t c .

* H i g h s t a b i l i t y a n d a c c u r a c y o v e r a w i d e T .

* RT Ds a r e p a s s i v e r e s i s t i v e d e v i c e s .

In the 2-wire RTD (RT) measurement, the


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* I or V excitation with

reference resistor

* Linearization

* High resistance

* High sensitivity* Highly NL output


w ( ) a ,voltage drops due to IEXC, passing through

the lead resistance, RL, add to the measured

voltage, VO.In the 4-wire RTD, one pair of wires carries

the excitation current through the RTD; the

other pair senses the voltage across the RTD.Since negligible current flows through the

sensing wires, the lead resistance error is

very small.


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