Note: Descriptions are shown in the official language in which they were submitted.
P RES SU RE COMP ENS ATED D I FF~3RE~T I AI
P~E;SSURE 5~N50R ~ND METHOD
BAC~RO~D OF TEIE INVENTION
1. P`ield o~ the invention
The present ~nvention rel~tes to a
differentlal pre~ure sen~or which is compu'cer
compensated foa reference pre~su~e effects.
2, Pr ior Ar t
nigh accuracy i5 sequired ~or differential
10 pressure me~suremen~s in the aerospace industry. The
need for high accurasy dif!E~rential pres~uYe
measurement in aerospal:e applicatlons has forced th~
measure~en~c of two absolute pressures using two
~bsolute pre~ure sensors and then ~:omputing the
15 differerlce in pressureJ 'Phl~ type of mea~ure~ent" even
when pre~sure norll3.inearlty and temperatur2 dependent
errors werle dlgieally corrected resul ed in relati~rely
large errors as a perc~ent of required differen~cial
pressu~e mlea~ur~ent~ bec~u~e the dl~fererlce ~n
~o pressure is ~snall compared ~o the absolu~e pre~sures
~ea~ured .
Differentlal pressure sensor~ Jbose
diffesent$al pr~ssure nonlin~arity an~ t~laperature
dependent errors ~ere ad~u~ted by an~log circuitry and
who~e rePerence pressure dependen~ ~aY ~e~ani~ally
co~pen~ted al30 falled to ~eet the r~quir~d a~curacy
for differenti~l pse~sur~ ~ea~ure~ent~. Differential
pres~ure ~ensor3 u~ing already aY~ilabl~ reeren~e
pressure ~gnals for co~pen~ation by noninterchzng~abl~
~nalog circul~ry have ~lso f~1ed to ~eet su~
standar~s.
SUM.~ARY OF THE INVENTIO~
The present ~nvention comprises an i~proved
apparatu~ for providing an ad~usted ~lgnal
representative of a difference in pressure between a
first ~reference) pressure and a ~econd pressure. ~he
apparatus include~ differential pressure sensing means
fo~ providing a dlfferential sensor signal
represen~atlve of the difference in pres~ure. C~anging
reference pressures can cause mechanical variations o~
the differential pressure sen~ing means at e~uivalent
differential pressures. Such variations cause
undesirable effects of the reference pressure on the
dif ~eren~ial pres~ure sensing means to be reflected in
the differential pressure slgnal. The reference
pressure is also ~en~ed by a reference pressure sensing
~eans which proYid~s a re~erence signal representative
of the reflerence pr~sure. The reference signal ~ay
no~ vary linearly in resp~nse to certain ref~r~nce
pressure~. The referen~ signal and the differential
sen~or signal are trans~itt~d to a correcting mean~
where the referenc:e signal i~ ad~usted for ~rarianc~s
due ts~ reference pressore ~uch as certain nonlinear
respons~0 The differential ~ensor ~lgnal loay not v~ry
1inearly in re~ponse to c~rtain diferential pres~ures
and is adju~ted by t~e correcting ~eans for variance~
due to dif~erences ln pre~sure ~uch ~s cert~n
nonlinear respons~ and adver~e reference Rres3ure
ef~ects to provide an lmprov~d output signal more
representat~ve of the dlfference in pre~sure than ~h~
dlfferential sensor slgn~l~
In s:~ne preferred em~odiment of t)le invent~on,
a temper~ture ~ensing ~eans i5 included and ~en~e~ a
temperature repre~entative o~ the temperature~ of the
differential pressure sensing means and the reference
pressure senslng means and provid~s a temperature
signal to the correcting means. The temper~ture slynal
5 may not be linearly represen~ative of certain
temperatures sensed. The temperature c~n ad~ersely
aEfect the accuracies of the reference pressure ~enslng
means and the differential pres~ure sensing ~eans
because of their ther~al expansion characterlstlcs.
The correcting Deans ad~u~ts the temperature signal ~or
nonlinearit~es, the reference signal for nonl~nearit~es
and temperature dependen~e, and the differential ~ensor
signal for nonllneari~ies, temperature dependence and
reference pressure dependence to provide an en~anced
output signal. ~he correcting means i~ o useul in
process~ng the signals to reduce the effect of other
predictabl~ undesir~d event~ ~uch as hydraullc sur~e~
and to ad~u~t the signals for other correctable
varlable~.
ln a urther preferred e~bodi~ent, the
correcting ~e~ns co~prl~es a digital co~p~ter whic~
imple~ents a function for ad~u~ting the signals. Su~h
function re~u~re~ cor~ect$on data such as ~ plurality
of coefflci~nt~ and constant~. A memory ~torage mesn~
provides such coeff~ient~ and constant~ based on the
characteris~ics of th~ te~p~ratu~e sen~ing ~eans,
reference pressure ~enslng mean6, and the differentl~l
pressure ~ensing me~ns. 8ince indlvidual sensing ~ans
may not be unifor~ly affected by the 2ff~cts of
nonllnearities, te~perature dependence and reference
pressure, the ~emory ~torage means comprise~ correction
data spec~f~cally represen~atlve of each indiv~dual
sensing rneans whose ch~racter~stlcs are ad~usted to
obtain even greater accuracy.
i~n advantaqe of the pre~ent invention is that
only one high accuracy differentlal pressure sensor i5
5 reauired. The reference pressure sensing means need
not be of h~gh accuracy because the reference signal
adjusted by the correc~$nq means thus allowing the use
of a low cost, low tJeigh'c, s~nall size reference
pressure sensing means.
Brlef De~crl~tion of the Drawinqs
FIG. 1 is a block di~gram repres~nt~t~on of a
preferred em~odiment of an adjusted dif ferential
pressure sensor made ss::s:ordinq to the present
invention,
FIG. 2 i~ a bloc~ dtagra~ repres~ntation of a
further preferred e~bodimen~ o the ~d~u~ted
differentii~l pressure sensor of FIG. 1,
Yl~. 3 is a block diagrzurl repres~nt~tion oiE
yet a further preferred embod~ment of the ad~usted
differential pressure sen~or of FIG. 1)
~ IG~ a sy~bDllc representation of a
vibrating beam type differ~nt~al pres~ure ~en~or used
with the pr~sent tn~ention,
FIG. 5 is ~ ~y~bolic representation of a
capacit~ve type dlff~rentlal pressure sen~or used wlth
the pres~nt inven~lon a~d
FIG. 6 1~ a block diagra~ repr~ent~tlon of a
preferred two wire embodiment of an adiu~ted
dif~erenti~l pressure sensor ~ade according to the
present inv~ntion~
Detailed DescrlE~tion of the Preferr~d ~51nbodl~ent~
In FlG. 1, a preferred em~odl~iE~nt oiE the
; 3!~ ~
in~ention comprises a differential pressure sen6in~
means lO such as a vibr~ting bea~ pressure sen~or~
Differential pressure sensing means ].0 ~enses a
dif ference in pressure between a first pressure or
reference pressure indlc~ted at 12 and a second
pressure indicated at l4~ Such diffe~ence in pres~ure
is useful in deter~ning air da~a characteristics such
a.s airspeed and ang~e of atta~k in aerospace
applications ~here the reference pressure correspond~
to a local sta~ic pressure and the ~econd pressure
corresponds to a local pitot ~ressure.
Differential pressure sensing means lO
provides a differential sensor signal through a
conducting means 18, such as an oscillating voltage
signal, who~e frequency is repre~entatlve of the
differ~nce in pressure. Th~ differential sensor signal
can also be other types of ~lectr~cal, pneu~atl~,
light, defined a~ includlnq electroma~netic radiation
in the infrar~d, vi~bl2, and ul;traviolet portton~ of
the spectru~ or oth~r for~ of slgnals representatlve
of the difference in pre~sure~ Conduceing means 18,
which can be any type of conductor co~patible with the
differential sensor siqnal, such a~ an el~ctrically
conductive wire, couples the uncorrected ~lgnal to a
first buffer ~eans 20. First bu~fer ~eans 20
preferably co~prlses known condi~ioning circui~ry such
a3 a comparator circuit comprislng an LM 193A
compar~tor sold by N~tlonal Se~iconductor Corp. ~lth a
differential line dr~Yer co~prising a 54LS40 dri~r
sold by Fairchild Industri~s to condition the
differentlal ~ensor ~ignal for carrying ~y a conducting
means 22 to a correcting means 2~ such that cor~ecting
--6--
means 24 is coupled ~o the diffe~ential sen~or signal.
Correcting ~eans 24 preferably is a digital co~puter
and compri3es known converting rneans such as counters
25, 2~ and analog to-digi~al converter 29 suitable for
con~-erting the dif~erential sensor signal and other
si~nals to a digital for~ compatible with correcting
means 240
The flrst pressure indlcated at 12, ~ al~o
sensed by a reference pressure sensing means 26.
Referenc~ pressure ~ensing ~eans 2~ preferably is an
inexpensive uncorrected absolute pressure sensor ~uch
as a batch ~abricated silicon piezvresistive sen~or or
a soli~ state pressure sensor a Reference prQsxure
sensing means 26 sense~ ehe reference pr~ssure and
provide~ a reference ~ignal, 3uch as an analog voltage
or an o~cillating volta~e, ~hose frequency i~
representative of the reference pressur~ or other form
of signal whicb i~ carried by ~ condu~ting ~ans 28, to
a ~econd buffer means 36 ~hlcb ~erve~ to isolate
2D referenc~ pre~ure sen~ing aean~ 26 fro~ correcting
~eans 2A. Second bufer ~ean~ 30 prefer~bly co~pris~
conditlon~nq cir~ultry ~u~h a~ an LM 193A co~parator
sold by N~tional Se~icond~ctor Corp. with a
differ~nt~al line driver ~ompri~ing a 5~LS~0 driver
sold by ~airchild Industries to condition the ~feren~e
sign~l. The reference signal i8 then carried to
correct~ ng ~eans 24 by a conducting ~ean~ 320 When
using a piezoresisttve sen~or having ~n analog output
as reference pressure sensing me~n~ 26~ s~ond bu~fer
~eans 30 co~prises an op~atlonal amplifier and
converting mean~ 25 comprises an analog-to-dlg~t31
converter.
- 7 -
A temperature sensing means 34 such as a
model 7006 5600 ohm ? 2 percent temperature sensor with
a positive temperature coefficient available from AMF
Incorporated/Electro Components Division, 195 McGregor
Street, Manchester, New Hampshire 03102 is positioned
in or proximate to the differential pressure sensing
means 10 preferably in intimate thermal contact
therewith to sense the temperature of differential
pressure sensing means 10 which is preferably at the
same temperature as reference pressure sensing means
26. Temperature sensing means 34 provides a
temperature signal such as an analog voltage or other
form of signal representative of the temperature of
differential pressure sensing means 10. The
temperature signal is carried by a conducting means 36,
to a third buffer means 38 for conditioning the
temperature signal for carrying by a conducting means
40 to correcting means 24. Third buffer means 38 can
be a Model LM158 operational amplifier available from
National Semiconductor Corp.
Correcting means 24 preferably performs
routines implemented by software or firmware. Such
routines comprise functions such as a suitable lookup
table or polynomial function for adjusting the
reference signal as a function of the reference signal
and temperature signal and adjusts the differential
sensor signal as a function of the reference signal,
temperature signal and differential sensor signal to
provide a substantially corrected output signal on line
41, preferably a digital signal which can be converted
to a 4 to 20 milliampere signal or other form of signal
representative of the difference in pressure for direct
readout or control purposes.
--8--
Differen~ial ~re3sure sens1ng means lO may
not respond exactly linearly to certain dlfferences ln
pressure~ The resultant d{ferential sensor ~lgnal
from differential pre~ure sen~ing means 10 co~prise~
certain nonlinearities. The reference pre~sure al~o
causes effects on the ~lfferen~ial pressure sen~ing
means lO not repre~entati~e of differences ~n pressure.
~t dif~erent magnitudes of reference pressure,
~lfferential pressure ~ens~ng means 10 provide~
slightly different different~al ~ensor signals in
response t~ the same differ~noe in pressure, re~ulting
in a reference pressure dependence of the differential
sensor signal. Differenti~l pressure sens~ng ~eans lO
iS al50 sffected by te~per~ture becau~e of v~rying
therm~l e~pansion coeffic~ents causing response~ not
representat~ve of the difference in pr~ssure, re~ulting
ln a te~perature depen~ence of the differential ~ensor
signal. At least to the extent t~at he above
nonlinearltie~, reference pre~su~ dependence and
temperature dependenc~ of differential pres6ure senslng
mean~ lO are repeat~ble, the different~al sensor ~lgn~l
is ad~usted and subseantially corrected by ~orrecting
means 24. Other predic'cable dep~ndenciefi can al30 be
substantially correct~d by corr0cting T~ean~ 24. Gther
25 repeatable nonlinearitles and temperature dep~nder1ce s~f
reference pre~sure ~ensing means 26 reflected in the
reference slgnal ar~ ad~u~ted and ~ubstanti~lly
correc~ed by corr~c~lng means 24 for a ~re accurate
ad~ustment of the differ~nt~al sensor sign~l. The
te~perature 3i~nal is ~l~o linearized by correcting
~ean~ 24. A ir~t ~unctlon ~ple~ented by corr~cting
means 24 for ad~usting the ~ignals co~pri~es t~e
follo~ing polyno~ial ~erie~:
~ - a + bx ~ cx2
where:
Q ~ output signal
x Z0 2
80 ~ ~ constant
z ~ dif~erential sen~or ~lgnsl
and: a o a~ + a~LY + a;~Y 2 ~ .. .
~ ~ bo ~ b~ b2Y
~ ~ Co + Cl~ + ..o
.
O
w~ere:
Y . Ro R
R~ on~tang
8 ~er~nce ~ign~l
e ~0O ~ a~ T ~ ~02 T2 + ...
al ~ a lQ~ al1 T + al2 T + 0~-
a2 ~ ~ 20 ~ ~21 T . .
~nd s bo ~ bo~ ~ b~ T ~ b o2 T
bl ~ bto ~ blt T
J~nd cO ~ cO0 +
.~P ~3~:i,V
-10-
where ~
T ~ te~perature ~;ignal,
aOO,aOl,aO2,~0.,boo,bol;..~,
0O, c 01' ~ a 10~ a 11~ . ~ . . are
5 const~nts.
The o~ders of t~e f ir~3t function ar~ of a
magn:Ltude sufflcient to ~eflect meaningful correlat~on
of Z, P~, and T ~i~h respect to the sensed differ~nce in
pressure 0
q'h~ fl~t functlon can ~e redu~ed to c:omprise
a preferred second furlction:
Q ~ (~ + aO ~ alR 2 a2P~2)3
~1 ~ c~ ~ ~1 R + c2 R ~ ~ 3R ) ~ do
0' ~ cO~ cl~ C2~ C3 ana do are polyno~ial
15 function3 o~ tll~ te~peratuEe signal o~ t~i~d order o~
le~ havlng corresponain~ly dl~er~nt constants t2~an
those of th~ first functlon~. Other func:tlon~ or me3~n~
oP co~pens~tiTIg the diff~rential ~en~or ~gnal for
nonllnearity, refer~nce pr~ure dependen~:e ~nd
210 te~operatuge depend;~nce ~e ~ritbirt the ~op~ oiE tb~
present ~nvent~on. ~o~ in~tance, t~le dl~fer~rltlal
5en50r slgnal, re~r~n~ lgnal and temE>eratur~ ~ignal
.; can be ad~u7ted by correctlng ~ean~ 24 oo~?~lslr~g a
n~ r~ )
otorolal 6800 series mlc:roprocessor based co~?u~er or
25 an air v~ cl~ on boar~ allr d~ta co7r~pu'cer to l~plem2nt
one or Dlore o~ the a~ve furlGt~on~.
:For certaln dif~erences ~n pres~u~/ the
diEferes~tlal sensor ~lgnal does vary line~rly. The
f lr~t an~ set:ond function~ acco2~0date such lineaziti~6
30 wi~h prop6!r ~el~ctlon of con~tzlnt~. C~rta~r~ linear
re8pon9~3 o the referenc:e slgnal ~nd th-3 e~mperature
signal are si~llarly accommodated.
The f ir~t and second functions requ1re a
plurality of con.stants co~pri~ing correction values or
data. The correct10n data is pre~erably selected to be
representative of the characterlstic~ of differential
~ressure sen~ing means lO such that certain
nonlinearities~ temperature dependence and reference
pressure dependence reflec~ed in the differen~ial
sensor signal are accurately co~pensated by ~orrec~ing
means 24. The differential pressure sensing means lO,
refer~nce pressure sen~ing mean~ 26 and te~perature
sensing means 34 are sub~ected to different known
reference pressures, te~per~tures and dlfferences ln
pressure and the differential sen~or signal, refer~nce
si~nal and ~e~perature signal ar~ recorded at the kno~n
reference pres~ures, te~p~r~tures ~nd ditference~ in
pressure. Such recorded ~ignals are then fit to the
second function using for example a lea~t squar~ fit
method to determine correction v~lues su~h ~h~t the
reerence slgnal, te~per~tur~ ~ignal and differenti~l
sensor ~ignal ar~ adju~ted to provide an improved
output ~gnal. The correction value~ ~re then stored
in a me~ory storage mean~ ~2. 5~3ch v~lu~ can also be
theoretically determ~ned ~ro~ known des~gn
characteristics and con~truction mAterials of
diff~ren~ial pr~a~ur~ ~enaing ~e~n5 lO or an avera~s of
the data can be used ~or all dlfferential pre~ure
senslng ~eans lO.
Ma~ory ~torage ~ean~ ~2 is prefer~bly ~ read
only ~e~ory dev1ce ~u h ~s a 5454~2 programmable read
only me~ory avallable from National Se~conductor
Corp. which compr$se~ correction data ~uch a~
coefficlent~`~and con~tant9 represent~tive of the
-12-
particular characteris~ics of differential pre~ure
~ensing means l0 with respect to nonlinearities,
reference pres~ure de~endence and temperature
dependence. ~emory s~orage means 42 can als~ be
s integral ~o correcting means ~4 such as core memory,
disk, tape, or other similar me~ory device. The
correction data is al50 pre~erably representative of
certain nonlineari~les and temperature dependence of
reference presEure sensing means 26 and cer~n
nonlinearities of temperature sensin~ means 34. When
~emory storage means 42 is addressed by correctin~
~eans 24 al~ng an address line 44, memory ~torage mean~
42 provides an enabling signal along an enable line dB
and provides the known data to correcting ~e~ns 2d
through a data line 46.
~ he first and ~econd function~ ~i~ult~neously
substantially correct th~ temperature ~ignal, reference
siqnal and differential sensos sign~l to provlde the
output signal. ~o~ever, ~t ls not ne~e~sary that
corrections occur simultaneou~ly. For ~xample, in one
preferred e~bodimen~, correct$ng mean~ 24 compen~te~
the ~emperature ~ignal for nonlinearity ~d then
com~ensate8 t~e re~erence ~ignal for nonlinearity and
temperature aepsndence. Pinally, tbe differentl31
sen~or signal i~ ~o~pens~ted for nonline~rity~
temperature dependencQ ænd reference pre~sure
dependence to provide the output signal. ~ach
compensation is a function of the prevlou~ly
compensated siqnals and correction value~ aft~r the
te~pera~ure 3ignal is co~pen~ated. Other compensation
routines are within the ~cope of the present invention.
In a ~urther preferred embodiment a~ 3een in
3r
-13-
FIG. 2 wherein the numbering is con~i6tent ~ith FIG.
l, reference pressure sensing ~eans 26 i8 physically
coupled to the hou~ing of differential pressure sens~ng
~ean~ lO. A circuit com~on ls shown at 50. Memory
S storage means 42 is also physically coupled to the
housing of differential pres~ure sen~ing ~eans lO.
Temperature sensing ~eans 34 ls phy~ically coupled ~n
intimate ~hermal co~tact wi~h differential pre~sure
sensing means 10 and reference pressure sensing means
26 such tbat ~e sensed temperature is representative
of the ~e~perature of different~al pre~sure sensing
means lO and reference pressure senfiing mean~ 26. ~hen
physically coupled ~ogether, different~al pre~ure
sensing means lO, referencg pres~ure sensing means 26,
temperatur~ sen~ing ~eans 34 and m0mory ~tor~ge mean~
42 co~prise ~ sensor m~dule 60. Correction d~ta 18
then generated from tests or theoretical analy~l~
completed and the correction dat~ ~epr~sentati~e of the
character~st~cs of eac~ s~nsor ~dule 60 comprislng
nonlinearltie3, te~perature dependencies and referenc~
pre~sure de~endenc~ and other correctable variables ar~
stored in ~emory ~torage ~eans 42. Sensor m0dule~ 60
are then ~nterchangeably coupled with correcting ~e~ns
24 without altering the r~utine ~plemented by
correcting ~eans 2~. Correcting mean~ ~4 then ad~usts
the te~perature ~ignal, reference slgnal and
differential sen~or ~i~nal a~ ~ function of the
dlfferentlal ~ensor ~lgn~l, r~ference signal,
temperature slgnal and the correctlon data to prov~de
the l~proYed output ~ign~l.
The present lnv~ntion i~ les~ expensive tban
uslng two absolute pre~sure ~ensors ~5 only one h~gh
accur~cy dl~ferent~al pre~sure senqing means 10 ~t
about ~he same co~t as one accurate absolute pre~6ure
sensor is requiredO By de~ermlning data for sen~or
module 60 ~o be ~eored in ~emory s~orage ~eans 42 by
actual testing of ~en~or ~odule~ 60, the
nonl$nearitie~, refer~n~e pressure depen~ence and
temperature dependence o dlfferential pre~sure ~ensinq
means 10, nonl$nearl~ies and te~perature dependenc~ of
reference pre-~sure se~in~ ~ean~ 26 and nonlinearities
of temperature ~en~n9 mean~ 34 are represented in th~
data. Therefore inexpens{ve pre~sure senso~s such as
batch fabricated silicon piezoreslstive sensor~ or
~olid ~ta~ pressure sensors can be used a3 reference
pressure sen~ors 260 Althougb such ~ensor~ may have
high tem~ra~ure dep~nd~nc~ and other d2pendencle on
t~e order of t~n (10) ~ t~en~y ~20) percent, ~u~h
errors can ~* corr~c~ed to with~n two (2) p~rcent of
full ~cale or b~tker by correctlng ~ean~ 24~ ~be
inherent ~811 31~ ~nd low ~e$gb~ of ~uch sen~or~
~o per~its aerospace pack~g~ng advantages o~r tbe u~e of
t~o high accur~cy ab~olute pre3s~1re s~n~or8 ~blc~ ~igh
more ana requir~ mor~ 9pace than sen~or ~odule 60.
Such pac~aging advantage~ ar~ further enhanced a3 ~ho~n
in FI~. 3, ~herein t~ nu~berlng is con~tent ~ith
25 FIGo 2 uith tbe add~tion o~ an ~A~ follo~ing ea~h
nu~ber, by including ~ r~f~r~noe pre~sura sensing ~eans
26A within the housing o~ ~ diff~entlal press~re
sens~ng ~n~ lOA. Th~ pack~ging ndv~nt~g~s a
fur~her ~nh~nced ~y includinq a ~e~oEy ~eoragQ ~n~
~2h and a ts~peratur~ ~en~ing ~ean~ 34~ hin the
hou3ing of di~ferenti~l pre~8ure sen~in~ ~ans lOA.
t~
-15-
As stated above, differential pressure
sensing means 10 in FIG. 1 is preferably a vibrating
beam pressure sensor which is the subject of U.S.
Patent No. 4,311,053 which is assigned to the same
05 assignee as this application. In FIG. 4, a vibrating
beam section 42V such as shown in U.S. Patent No.
4,311,055 is positioned between a first ancl second
isolator sections 43V and 44V which support the
vibrating beam section 42V under a tension or
compression stress or load representative o~ a
differential pressure. A capacitor plate 61V forms a
capacitor with beam section 42V to pick off the
differential pressure representative vibration
frequency of beam section 42V. The vibrating beam
pressure sensor is useful in aerospace applications.
The linearity, temperature and reference pressure
adjustments described above greatly improve
performance at certain reference pressures.
In a further preferred em~odiment,
differential pressure sensing means 10 of FIG. 1
comprises a capacitive type sensor such as the sensor
of U.S. Patent No. 3,646,538 which is assigned to the
same assignee as this application, or the capacitive
sensor of U.S. Patent No. 4,389,895 assigned to the
same assignee as this application coupled with known
circuitry or the capacitive transducer of U.S.
Patent 4,370,890 which is assigned to the same
assignee as this application coupled with
known circuitry~ The difference in pre~ure ~herl
sens~d by the capacitlve~ type sen~or o ~J.S. Pat~nt
No. 3,646,53a 1 s useful for deter~inlng veloc~ty o~
flow of a fluid tbrough an orifice in a conduit where
5 the referenc~ pres~3ure and the second pres~ur~
corre~pond to pressures the fluid in the pipe e~sert~ on
oppo~lte side~ of the orifice., The capacitive ~ensors
are represented in PIG. 5 and comprise a senPiing
dlaphragDI 21 such as sbown in U.5, Patent No. 3,646,538
10 whose posltion varies in response ~o the difference in
pr~sure which is applied 21CrO88 it. On oppo~lte sides
of sensing diaphr~gm 21 are capacitor plate~ 39C and
40C. The change~ in capacitance b~S~een sensing
d~aphr2~ 21 and diaphrag~n plates 39C and 40C ~re
15 represen~a~lve of ths difference tn p~es~u~e.
Perforu~3nce of th2 cap~ci~lva ~ensor~ reatly
enhanced by the linearity, t~mpe~atur~ and r~~rence
pre~ure adl~ust~ent~ of the pres~nt $nven'cion. l~n Many
velocl~y o flo~r appli~:at~on~ the reference pre~sureR
~an be many order~ of ~agnitude higher than tbe
differential pres~re de~lr~d to be sen~edO Th~ two
absolute pre~3ure ~en30r approach f~ils b2cause ~e~
~ull ~cale ~rror~ can bæ large oomp~red ~o th~
d~ferential pxes~ur~. Prio~ art dlferential pr~s~ure
sensor~ ref~renc~ pre~sur2 errors ~re al~o large
compared to ~he d~fferen~ial pre~sur~ at high re~erence
pressures. The present invention comp~nsate~ or
reference pre~sure th~s permitting accur~te senslng of
the different~al pres~ure~
The present invent~on ~lso permi~ de~ign
cons~er~ti~n~ o~her t~n reference pre~ure
dependence, nonl1ne~rity and te~p~rat~re dependenc~ ~o
be evaluated. Repeatabillty of performance become~
primar~ly iMportant. Materials and de~ign~ ~r~
selected for repeat~billty o performance wlth respect
~o t~me, temper2ture, pressure and linearity. l~7~th
s repeatable errors co~pensated for by the pr~ent
inven~lon, longer ~erm performan~:e is permltted to be
improved by the selection of appropeiate material6 and
mechanical de~ignsO The inv~n~on as descr$bed ~bove
an~ furehe~ descr~bed below i~ po~ered in a
10 conventional manner.
I n F~G . 6, ~ typlcal ar range~ent for use of a
compensated dif ferent~al pressure BenSOr 1~0 per t
present invention in combination ~ith a t~o ~ire
process control sys~em i8 shown in bl~k d$agrams. ïn
15 ~his inst,ance, compen~t~d s~nqor 100 co~pri~e~ ~
current control ~ne~ns 10~. Co~npen~t~d ~enso~ 100
provid~ ,a s:ontrol 919n~1 to the current ~:on?crol ~an~
102 for controlllng a slgnal ~uch a a ~C c:urren'c It
~ut:h t~at It ~3 rep~sent2~ ive of a dlffer~nc~ ln
20 pre~ure ~ The s:llrrent I ,~ also provldes po~r ~n a
conventlonal two ~lre laann~r to coEnpens~ted ~ensor 100.
The two trir~ 2~y~!3teDD of the present iLnvention
include~ a direct eurrent supply 110 having a line for
carrylng the current It ~onnected in serles throu~h a
~irst ~erm~n21 112 through a line 113 to ~ fir~t input
terD~inal 114 to coE~pensated sensor 130 and current
control ~ans 102. $he current It is then c~rri~d
tl~rough ~ line to a ~econd lnput ter~inal 116 lthrough a
line 117 to ~ ~econd t~rlainal 118. A load ~n~an3 120 ~
30 coopled to ~econd ter~nlnal 118 and in ~curn i8 conne~ted
in BerieS ~dlth the ~u~ply 110 thus co~nplet~ng the t~o
wire current path. Load ~eans 120 m~y c~npris~ an
actuator, a controller, a recorder or simply a current
ind1ca~ing in~tru~en~0 The connec:tlons of ~upply 110
and co~pen~ated s~n~or 100 can al~o be mad~ ~o that
they are siloilar to t~at shown In United States Patent
No. 3, 764, 880 which shows a DC to DC conv~rt~r .
Current control ~eans 102 controls the total
DC current I~ which 18 preferably an industry star~dard
4-20 milliamp signal or other forz~ of signal such that
the current It is repres~ntatlv~ of the d1f~erence in
lû pressure and provides power to compensated sensor lOOo
When ~ is a 4 20 milliasllp signal, components of
compen~ated ~ensor 100 n~ust be ~elected eo operat~
50Iely on 4 ~illi~lp~i of current or les~ wh~n ~ 3 4
mill ia~p~ .
A~ prevlo~ly stat~d, the pr~sent invention
can be u~ed in det:~rD~ining alrspeed o~ an aircra~t.
AE~O~aAUTICAL ~ADI0, Itn:~ (ARINC~ of 2551 Rlva Road,
P.nnapoli~, Maryland ~1401 is a co~pgny that form~l~tes
standards ~o~ electrols1c ~ p~erlt and Sy5teJ115 for
2~ airlInes. The standards ~re finE~ ed ~fter
$nvestig~tlon, coordination and general a~r2~men'c ~lth
the a1rlIne~, w~tb otheY a~rcraft opera~tor~, ~lth the
military services having similar requir~aent~ and ~ith
equi pa~ent ~sanufacturer~. For de~e~ining ind1catsd
75 airspeed, the gre2~t~t di~f~ntial pra~ re fo~
sub~onic aircr~f t ~ e~u~ed by the pre~ent
inven~ion i8 approxl~ately 5.4 P~I (pound~ p~r 8quare
inch) (37.~ ~Pa lkI~opa~c2~1~ 3 . R~ferenc~ pr~s~ure can
be as high as approxl~ately 15.6 P~;I (1~7.7 IcPa~ at
30 about 500 s~e'c~r~ belo~ sels level uh~ le ~eoond pr~ssure
can be a~ high as approx~at~ly 21 PSI ~144.9 kPa) at
speeds approaching t~e ~p~ed of sound and at lo~
-19-
altitudes. To meet a 1978 standard set by ARINC, for
indicated airspeed determination using two separate
absolute pressure sensors, the implied accuracy of the
sensors respectively is .013 percent full scale and
.014 percent full scale. The accuracies are based on a
maximum error for the differential pressure of
approximately .005 PSI (.034 kPa). Presently available
absolute pressure sensors are accurate to about .02
percent full scale which would result in a possible
error of .0002 x 21 = .0042 PSI (.0002 x 144.9 = .0290
kPa) considering just one of the absolute pressure
sensors. The additional error of the second absolute
pressure sensor makes it improbable that the ARINC
standard can presently be met by the two absolute
pressure sensor approach as the error of one absolute
sensor is about the same as the allowed maximum
dif~erential error of .005 PSI (.034 kPa). However,
~f~erc.~ IQC I P~IJ;o Inc. )
-- the present invention meets the ARINCJstàndard. In
FIG. 1 to determine airspeed using differential
pressure sensing means 10 with reference pressure
sensing means Z6, differential pressure sensing means
10 is required to have a range of at least 0 to 5.4 PSI
(37.2 kPa) c~rresponding to the greatest difference in
pressure to be measured and reference pressure sensing
means 26 is required to have a range of from 0 to lg.6
PSI (107.7 kPa). The standard then implies from the
worst case error based on the indicated airspeed that
the total accuracy of differential pressure sensing
means 10 must be at least .09 percent full scale to
remain within .005 PSI (.034 kPa) o the differential
pressure. Deviations in the differential sensor signal
caused by reference pressure dependencies can be .25
3~
-20-
percent full ~cale for re~erence pre~sures equivalent
to full scile dlfferential pressures, which is well
above the ~otal . 09 percent full scale accuracy
required to meet the ARI~C standardO If the tot~l
5 accuracy of reference pre~sure sensing ~eans 26 i~ 2
percent, a 50:1 correction of reference pressur~
dependencie~ is achievable. ~he reerence pre~sure for
subsonic aircraft varie~ bet~?een 2.5~ PSI (17.6 kPa) at
an altltude o approximstely 12,600 ~eter~ and 15D6 PSI
~107.6 kPal at 500 meters belo~ sea level or a total
chanqe of 13 PSI (90 ItPa). Such change i~ about twice
full scale of differe~ntial pressure ~en~ing ~eans 10
wh1 ch double~ ~he total ref~rence pres~ure dependency
to . 5 percent of ~ull scale . Thus the 50 s 1 correction
15 reduc~ the referenc~ pr2 sure depend~ncy of
di~ferent~al pres~ure ~en~lng means 10 thereby
increa$$ng ~:curacy w~th re~pect to ref~ren~ pressure
dependency ~o about . 01 percent Eull ~cale or 0 00054
PSI ~.û037 ~ result~ng ~n e~bodlments of th~ pre3~nt
20 invention perfor~ing betteY t}Jan tbe ARI~C standard.
Thus the pr~sent inv~ntlon meets evels grea'c~r
perfo~ance standard~ ~han s~e pre~ently r~uired.,
Sensor device~ buil'c in ~ccordance wlth ~h~
pre~en~ invention have been ~eseed to deter~sine t)Je
25 effectiven~s~ of tbe p~esent ~nv~ntion $n improving tb~
accuracy of di~ferent~ al pres~ure ~e~su~e~Rents. The
se~sor devi~e~ easlly ex~ee~ed the ARI~C ~t~ndard ~hen
the correc:tis:~n~ ~er~ taken lnto ~ccount. ~he sen~or
devices utilised the second function. - 5uch ~ensor
30 device~ co~pri~ed a v~bratlng beam ~ype dlf~Eer~ntlal
pressure BenSing Inean3 10 ~ith range of 0 - 16 PSI
(110 kPa~ O For such ~en~or devlce5" the ~or~t tot~l
-21-
deviatlons ~rom actual dlfferential pressure varied
froM .OO~l PSI ~.015 kPa) to .0039 PSI (.027 kP~) ~hile
subjected to wide ranges of temperature, reference
pres~ure and dif ~erential pressures exceeding tho~e
5 expec'ced to be encountered in subsoni c aerospace
appl ications .
