A Strain gauge bridge

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Technical Report, 1943-09-01

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A Strain gauge bridge

Korenberg, S.; McKinley, D. W. R.; NRC Radio Branch

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-. SECRET

ＮＭＺﾷ ｾ ｾＭＹＵ＠

1

COPY NO·--··-·

NATIONAL

RESEARCH COUNCIL OF

CANADA

RADIO BRANCH

A

STRAIN GAUGE BRIDGE

OTTAWA

•

_SECRET

PRA-95 Page 1

A STRAIN GAUGE BRIDGE

Summary:

., .. .

.. , · Thip ﾷ ｳＩ［ｾ｡ｩｮ＠ gauge bridge was developed E:Ja rly in 1942 by the Radio

Branch a-t the , r e que st of th e Aeronautical Engineering Department of the

National ｒｾｳ ･｡ ｲ｣ｨ＠ Cotmcil. It was neither int ende d as a prototype, nor

os a compet itor -in the commer ci a l field0

With a .stora ge batt ery for filmnent a nd dry batt e ri es for plate volta ge, it is compact enough and sufficiontly rugged ·to permit of

operation in· an a ircraft in flight. Its usa ble s ensitivity is roughly

six times that of the Bal.dwi.n Southwark Strain Gauge Brid ge, . #SR-4, and abou:t fifteen times· that of a standa rd d-e galvanometer typo bridge.

Tho rea ding is ·dire ct and lin G'cr with strain0

General ·

ｾｨｇ＠

specific at ions .for th o

ｾｯｲｦｯｲＮｭ｡ｮ｣ｯ＠

ot t h is.bridge wero

'tent-a tively outlined 'tent-as

foltows:-1. I t should be ca pable of de t ecting · stra ins of th o order

of ono part in a million, and h ave a linea r · scale ､ｩｲ ･ ｣ｴ Ｎｾｲ ｯ ｣ ｾ ､ｩｮ ｧ＠ indica

tor-' . ·It should: ·b.o suffici ently compact

to

usc in aircraft

from stor.a go ba tt eri es or the i::d.rcrnft' s gene r a tor.

.

..

. 3.

...

·when. o·p<:i:rE:'ting o.t requir ed I t .should. be shock-proof to typic a l ·eircrnft · vibl'ations ｳ･ｮｳｩｴｩｶｩｴｩ･ｳｾ＠

Di-r eqt .current bri.i:lgo. methods were out· of th o que stion for the s ensitivity ond ruggednoss roquir<:-'Cl, so o.n n-c bridge wcs d es ignqCJ. ; · which incorporc t os an oscilliitor-amplifior to supply voltage to the. bridge prop er, togethe r with a tuned amplifior-dotoctor c::i.rcuit to .·

indica te unbalance. Th o out p ut of tho tunod a-c amp lifier is rectifi e d

2nd fed to two 0 - 100 mi croormcto rs, tho r o.::td incss of which ｾ ｲ･＠ . .

dire ctly proportionq.l .t9 t ho streins,

' . . . . . . . :

: · Rubb e r shock ｭｯ ｵｮｾｩ ｰ｟ｧｳ＠ oro ' :provid ed for tho brid ge . oquipHont q:p.d .

for th o indic a ting mot ors. (Sou·- Photo.gra:phs, Fi gs.

3

o:ud

4.)

In .. _th.ese

photographs a rubber pad is visibl e under th o n Gtor, but the.Lord mmmt·ing

framework . ho.s boe.n . dyt nched .fr an the ｢ｾｩ､ｇ［ｯ＠ i tsolf o

Gonorntor . ' .

A ;Colpitts o scilla tor circuit (Fig. 1) ie us od with o high-Q, .- t .a·nk

SECRET

PRA-95

Page 2 .. ..

This ensures that the freq_uency will be .. · stable under conditions of

varyin g voltace s, Half of a 6F8G tube is used for the oscillator and .

th e other half coapris es a buffer anplifier, The buffer amplifier

drives a . 6v6 .Power tJ.?.bO capable of

ｰｲｯｶｩ､ｩｾｧ＠

ＱＯｌｾ＠

to 1/2 watt output.

A step-dovm transfornor is ·used · in tho output of the 6v6 with

ｾｮ＠

output

impedance of 125 ohnls to ' nntch the .bridge approximat e ly. .'J;'ho o;onorator

c1raws · approxir:tately

9

r'lils ot 18d volts {or the 6v6 stage, a.nd ·about .

2 mils at 90 volts for tho oscillator buffer st age.

.Anplif i er-Do1t _o.ctor

A

ｴｲ､ｮＤｾｯｲｭ

･ ｾ＠

is

ｊＡｬｾ｣ｯ､＠

across the bridge with an input .impodan<'o·

of 200 obr.ili wi t'h a voltage

ｰｴﾢｰＭｾｊ＿＠

ratio of 20:1. Tho primary is

con-trc-ts:ppod to ground. A I

ｬＩ ｏｴ･ｾ

ｴｩｯｲ｡･ｴｯｲ＠

₁

R6

1 with .3 db. steps,. follows

this transformer to udj ust tho sensi ti vi ty of tho instrui:wnt to tho

oporatinc; conditions. The first

ｳｴｯｾ･＠

of the anplifier is a high gnin ·

pontode, type 6SJ7, with n tuned circuit in its Grid. This is followed

by o high mu tri ode, 6SF5, having tuned circuits in both its grid· and

ｾ ｬ ｡ ｴｯ＠

circuits, Tho output of tho . 6sF5 is fed througp _an isolating

ｶｯｬｴｯ

ｲ ｾ･＠

divider to tho :pentod e section of a 6B8, The output of the

pentodc is r ectifi e d by .the diode section of tho 6B8

ｯ ｾ､＠

passed turough

two 0 ·- 100 d-e nicrospnetors in series, These noters ore locntod in

soparnto boxes and connected to tho bridBG through lone lengths of

shielded ca ble. One of tho notors is 'use d for line-up purpos0s and for

viB.ual observation, whilo tho othor notor is plscocl in a dark box .ond

photogrophoc'l from time t.o tine when a . record of' the rondings is roq_uired.

Tho a mplifier has £1 band pass which is l)onked at tho oscillnt;or

f r oqucmcy nnd · is half down in

ｶＮｯｬｴ｡ｾｯ＠

nt 3900 c,p.s, and 48'50 c.p.s.

'l'ho r esponse at low froquencios such as encountered und er normal

vibr-ation conditions up to about 1000 cycles is negligible. The. ga in of tho omplifi or is r a ther nore thon 1:90 db, and 150 microvolts a-c. input will give full scale outiJut on tho Iil.Gt cr. There is no evidence of ·noise or r.Ucrophonics.

Bridge Circuit

Tho 12.5-ohm output of tho · genorntor passes throush two 2000-ohr.l isoloting resistors to o .special centerinG potentionotor, R1, having

discrete stops. A v e rnier control, R₂, ·is proyided for fin G control of

c ent ering. · · ·

Two 8.rms of th o bridge ₁ of course, ore

ｦｯｲｲｾｴｯ､＠

by th e two 100-ohr1

strain gauges, Th es e gouges ore usually nountod on the opparntus

ｾ､Ｙｲ＠

..

exominotion so th at thoy 01)erote in opposite phase. Alterna tely, mie gouge may be inoperoti ve, in whieh .case -its function is to provid·e a r e sistance with the same ter.rpe rnture characteristics as tho active gauge•

SEC:RET

ｾｒａ ＭＹＵ＠ ｐｾｧｾ Ｌ ｊ＠

The third a r m of the ｢ｲｾ､＠ .

iome ter

R

t ge lS made up of th

res. t .,

3'

he

Ｌｓｬ ｬｾｯｯｾｨ＠

adjustment pot ent. (3 rough adj ustment

uotent-ls ance. Th e fourtb lonet er R4 and

r· .. .

. .

with a 1- h . · • a r n ha s a fixed 100- h ' . ' a · 1xed series

the 1-ohm orr:

ｾ ･ ｳｬｳｴ｡ｮｾ･Ｎ＠

A 200-ohr.l rheostat o n

ｲ･ｾｾｳｴ

｡ｮ｣･＠

in <?eries ·

f , . . " . Slstance ln series with th " '

ｒｾＬ＠

1"' connected across

OIJl"'er 01 the anplifier-d t . o Sense Key". The o t, t

usual 'f:iarmor _{. •} e ector ls co.m octod across tl _{le rldge ln the} b

ｾ＠

pu .

trans-Operation ·

. The batteri es are ·connect ed .

m,nutes before usin T _ ,

ｾ Ｎ＠ ｾｮｾ＠

t ho s ot turn.od on to •

tho preliminary

｢｡ｬＺｾ｣ｩｮｾ

･＠ ｳｾｾＺ｣［ｬ＠

ｾＱ＠

Vl ty control, R6 i s

ｴｵｲＺｾｬ､＠

up a few

burning out the . . . o • ·wJ..ll b o no t ed that

t ' ., . -

own for

s t a ge of tho

ｾｭｰ ｾｾｾｾｾｾｴｬｮｾ＠

a ot ers uncl or any

｣ｩｲ｣ｭｬｳｾｾｾｾ･＠

ls

ｾｯ＠

danger of'

r Gached tl,o u"',.., . t .

ｬｾｵｬ｣ｬ｣ｬｹ＠

ｾ｡ｴｵｲ｡ｴｯｳ＠

boforo tllo d s Sln?e the output

' - o-u 1 ls ln qar u t nn gor polnt .

st r a in gau ges ha vo . . . p . . o full scoJ.o of tho

ｮｾ＠

, I lS

n rounh

｢ｾｬ＠

reslstancos that do n ot d' f·" . . utcr , If tho two

a u anco nay bo ob t a · . . 1 .tGr by nor o than., .

ne c essary to adjust th . l nod by a dJustr.tc nt of R.3 It - a fow ohms,

br' d . o step pot o·1t i on t • lJtay a lso be

｡ ､ｾ＠

go 1 ,s

ｮｭｾ＠

incr o<) sed and th e

ｦｾｮ

Ｍ ｢ ﾷｯ ｬｾｲ＠

Rl•

Ｇｾ ｨｯ＠

r3Gnsi ti vi ty of tho

JUstmo.lt 01 t ho fino t . 0 a 3nce aclnovcd by "'· 1. ·

with i·he usunl b . .PO Olltl or.toter R2 £111d of R . ol!.1U tanoous

th o o;orotor ....

ｳ ｨｯ ｾｾｾ ｧ ｾ＠ Ｎ｢ｾｾｾＡｾ｣

ｩ ｮｧ＠

.

Ｎｴｯ｣ｨｮｩｱｾ･Ｇ＠

-r:md wi

ｴｬｾ＠

a

ｔｾｾｾｊＮ＠

ｾ＠

s in

ｯ｣ｾｯｲ､｡ｮ｣ｯ＠

ina tic,ns noc o ssory to o' cllo o t o pr;;rfc,rr.t tho sori os of sue t... e. expe rl ence

oa 8IlCO th o brid o· . ｾＮ＠ cessl v o approx-oo Ouo lly and quickly.

Tho b ·· d _{_rl ge should be ba l anc ed .}

:po sl.·ti on ln wh ich it WJ..'ll b - - Wlth tho s cmsitivity

ｰ ｮ ｾｬ＠

ti . e us ed b oce u . control in tho

Ｇｾ ＢＧ＠ ｾｭ＠ 111 t ho circuit tl I , • se Wl th th o control . .

nn gl'rt - · b ' w ro 1s o sli 1 t J..n 1 ts present

. , - _poss l ly b e obvi n t od b' . ·, g..l reacti on on th o br'd '"'- Thl' s

the 6SF5.. In fact

Ｎｾ＠

h Y puth.lg tho control b

ｾ ｴＬ＠

1 oe .

• I G oro arc so . 1 u .l!eon the 6SJ7 8 d

constructl on of tl . . v era srmll points . - . . n

model .ns lnstrumcnt wh ich r.ti r91t we ll 1

Ｑｾ＠

tho desl gn a nd

• )0 Hlprovod in a futur•e

The. "s ens o n switch is us e d t . . .

or ｾ･ｾ｡ｴｊＮＮ＠ vo. Boca us e a-c . .:o , o lndlC n to whothor tlw strni . . .

pos l tl vo dor"l oct . f' lu us e d t hrour,-11out tJl o I t . n lS pOsl tl ve

· · 1 on o · t ｾｾ＠ , • :1.o or wi 11 · · ·

boen obtainod i . r OJ. hor strnin . However of - ' . . . gl

v?

a

certa in th e-. 11 t. ls only nec e ssory t o d opross tl , t or d dof l oct um has

__ ._. po a r1 ty of th no sense sw · t h

pol a rity a nd doer " o stra in since tho r cnd in ; . . . 1 c t o

as-unbalo n . . ea se I or th o op·oo si t, 'l'l .. . g Wlll l ncr ense for one

nee ls adJusted by t ho 1 - ｾＮ＠ .Jo I. ne;ln tude of Ulis ·nso

and c n,

1v en · -• · r 1oo:::;tnt R t ) . t J

• nse

L _ - l anc e , since it wi ll dopond 4 ｾ＠ SUl . cho ope r a t or's t aste

0 11 _{th J sens1 tivity l ov ol .}

Usi 111 use

ng stcmd. ord 100-120 hri . •

e l ement so tl'>..nt th · - o . !l strmn t:;;nu gos c onnect ·d t ..., . .

defl e t· . _ey operate ln nush 11 . c o

ｾ＠

strolnod

d ·· c l on ls ohtoj_nc d on this b .. .

d

-.-pu ' 1 t is f ound tha t full seal

:

･ ｾ＠

tho samo c onditi ons tha t

ｴｾｾ＠

t,e f o r. £1 strain of 24 'lrbitrnry

ｵｮｾｴ＠

qulres a strain of 115 . - Bo l dv.'ln Suuthwork SR-4 "'t ,., . . . s

. r _{Ulll ts for full scnlo dofl t.} 0 r c. ln lnch en tor

S E C R .E T

PRA-95

ｐ ｯ ﾣｾ ｃ＠

4

gnugo equi:pmcnt · was not available r or this t e st, so that it is not

possible to express ,the,strains i n absolute units, i.e .• in

ｮ ｩ｣ｲｯｾｩｮｇｨ･ｳ＠

per inch .- _{, .}

_.

_.

The l i near

ｩｮ､ｩ｡

｡ ｴｾｲ＠

ｳｾ ｡ ｬ ･ﾷ ﾷｮ｡ｹ＠

be ca libra t ed for a giv en sen si

t-ivity set t ing in the laboratory by using standa rd s tra in gauge s and by

produc i n g lcnown s tra ins in t ho not a l under t est. The gen er a l cali bra-

..

.

tion curve for thi s instrunent is gi von in Fi gur e 2·. The brid ge will

than hold it s c a libration a s long a s tho b at teri e s r ema in in good

condition. A shift of t h e z ero bet ween exporir.J.Gnts or betwee n

r:i.easure-monts i s of no consoquenc e sinc e tho bridge may b e r e-b alance d to tho

new z e ro with confidei;l.c e t ha t the uet or calibrati on r mn.a ins unchanged.

s .

Ko:ronb.erg D. W. R. HcKinley HI? Sept emb er

1943

... ·. oOo

..

..

' :

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lo

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ｾ＠ r-.. ｾ＠ \9

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ｾ＠ ﾷ ｲ ｾ ｯｯｧ＠ !<I _ｒｾ＠ ｾＭｾｾ _{Ｂｬｪ ｾ＠}ＡＱＮＺ＠ _{ｾ＠ ｾ＠}

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