CA1216360A - Pressure compensated differential pressure sensor and method - Google Patents
Pressure compensated differential pressure sensor and methodInfo
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- CA1216360A CA1216360A CA000451541A CA451541A CA1216360A CA 1216360 A CA1216360 A CA 1216360A CA 000451541 A CA000451541 A CA 000451541A CA 451541 A CA451541 A CA 451541A CA 1216360 A CA1216360 A CA 1216360A
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Abstract
PRESSURE COMPENSATED DIFFERENTIAL
PRESSURE SENSOR AND METHOD
ABSTRACT OF THE DISCLOSURE
A differential pressure sensor senses a difference in pressure between a first input (low) pressure or "reference pressure" and a second input (high) pressure and provides differential sensor signal representative of the difference in pressure.
The reference pressure is sensed by a reference pressure sensor which provides a reference signal representative of the reference pressure. The differential sensor signal and the reference signal are provided to a correction circuit. The correction circuit which is preferably a digital computer, adjusts the reference signal and provides an improved output signal as a function of the differential sensor signal and the adjusted reference signal.
PRESSURE SENSOR AND METHOD
ABSTRACT OF THE DISCLOSURE
A differential pressure sensor senses a difference in pressure between a first input (low) pressure or "reference pressure" and a second input (high) pressure and provides differential sensor signal representative of the difference in pressure.
The reference pressure is sensed by a reference pressure sensor which provides a reference signal representative of the reference pressure. The differential sensor signal and the reference signal are provided to a correction circuit. The correction circuit which is preferably a digital computer, adjusts the reference signal and provides an improved output signal as a function of the differential sensor signal and the adjusted reference signal.
Description
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.
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 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.
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.
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
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 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 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~
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~
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
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 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.
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.
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
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$se~ correction data ~uch a~
coefficlent~`~and con~tant9 represent~tive of the particular characteris~ics of differential pre~ure ~ensing means l0 with respect to nonlinearities, reference pres~ure de~endence and temperature dependence. ~emory s~orage means 42 can als~ be s integral ~o correcting means ~4 such as core memory, disk, tape, or other similar me~ory device. The correction data is al50 pre~erably representative of certain nonlineari~les and temperature dependence of reference presEure sensing means 26 and cer~n nonlinearities of temperature sensin~ means 34. When ~emory storage means 42 is addressed by correctin~
~eans 24 al~ng an address line 44, memory ~torage mean~
42 provides an enabling signal along an enable line dB
and provides the known data to correcting ~e~ns 2d through a data line 46.
~ he first and ~econd function~ ~i~ult~neously substantially correct th~ temperature ~ignal, reference siqnal and differential sensos sign~l to provlde the output signal. ~o~ever, ~t ls not ne~e~sary that corrections occur simultaneou~ly. For ~xample, in one preferred e~bodimen~, correct$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 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~
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~
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~
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$$ng ~:curacy w~th re~pect to ref~ren~ pressure dependency ~o about . 01 percent Eull ~cale or 0 00054 PSI ~.û037 ~ result~ng ~n e~bodlments of th~ pre3~nt 20 invention perfor~ing betteY t}Jan tbe ARI~C standard.
Thus the pr~sent inv~ntlon meets evels grea'c~r perfo~ance standard~ ~han s~e pre~ently r~uired., Sensor device~ buil'c in ~ccordance wlth ~h~
pre~en~ invention have been ~eseed to deter~sine t)Je 25 effectiven~s~ of tbe p~esent ~nv~ntion $n improving tb~
accuracy of di~ferent~ al pres~ure ~e~su~e~Rents. The se~sor devi~e~ easlly ex~ee~ed the ARI~C ~t~ndard ~hen the correc:tis:~n~ ~er~ taken lnto ~ccount. ~he sen~or devices utilised the second function. - 5uch ~ensor 30 device~ co~pri~ed a v~bratlng beam ~ype dlf~Eer~ntlal pressure BenSing Inean3 10 ~ith range of 0 - 16 PSI
(110 kPa~ O For such ~en~or devlce5" the ~or~t tot~l deviatlons ~rom actual dlfferential pressure varied froM .OO~l PSI ~.015 kPa) to .0039 PSI (.027 kP~) ~hile subjected to wide ranges of temperature, reference pres~ure and dif ~erential pressures exceeding tho~e 5 expec'ced to be encountered in subsoni c aerospace appl ications .
percent full ~cale for re~erence pre~sures equivalent to full scile dlfferential pressures, which is well above the ~otal . 09 percent full scale accuracy required to meet the ARI~C standardO If the tot~l 5 accuracy of reference pre~sure sensing ~eans 26 i~ 2 percent, a 50:1 correction of reference pressur~
dependencie~ is achievable. ~he reerence pre~sure for subsonic aircraft varie~ bet~?een 2.5~ PSI (17.6 kPa) at an altltude o approximstely 12,600 ~eter~ and 15D6 PSI
~107.6 kPal at 500 meters belo~ sea level or a total chanqe of 13 PSI (90 ItPa). Such change i~ about twice full scale of differe~ntial pressure ~en~ing ~eans 10 wh1 ch double~ ~he total ref~rence pres~ure dependency to . 5 percent of ~ull scale . Thus the 50 s 1 correction 15 reduc~ the referenc~ pr2 sure depend~ncy of di~ferent~al pres~ure ~en~lng means 10 thereby increa$$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 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 .
Claims (38)
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:
1. Apparatus for sensing a difference in pressure between a first pressure and a second pressure and pro-viding an output signal representative of the difference in pressure, the apparatus comprising:
differential pressure sensing means for providing a differential sensor signal representative of the difference in pressure between the first and second pressures;
reference pressure sensing means for providing a reference sensor signal representative of the first pressure;
memory storage means for providing correction data for known errors in the reference sensor signal and the differential sensor signal; and correcting means for providing the output signal as a function of the reference sensor signal, the differential sensor signal and the correction data, the out-put signal being more representative of the difference in pressure than is the differential sensor signal as a result of correction for known errors in both the reference sensor signal and the differential sensor signal.
differential pressure sensing means for providing a differential sensor signal representative of the difference in pressure between the first and second pressures;
reference pressure sensing means for providing a reference sensor signal representative of the first pressure;
memory storage means for providing correction data for known errors in the reference sensor signal and the differential sensor signal; and correcting means for providing the output signal as a function of the reference sensor signal, the differential sensor signal and the correction data, the out-put signal being more representative of the difference in pressure than is the differential sensor signal as a result of correction for known errors in both the reference sensor signal and the differential sensor signal.
2. The apparatus of Claim 1 wherein the differ-ential sensor signal does not vary linearly in response to variations in the difference in pressure between the first and second pressures and wherein the correction data com-prises data representative of the differential sensor signal nonlinearities.
3. The apparatus of Claim 1 wherein the reference sensor signal does not vary linearly in response to varia-tions in the first pressure and wherein the correction data comprises data representative of the reference sensor signal nonlinearities.
4. The apparatus of Claim 3 wherein the differ-ential sensor signal does not vary linearly in response to variations in the difference in pressure between the first and second pressures and wherein the correction data further comprises data representative of the differential sensor signal nonlinearities.
5. The apparatus of Claim 1 wherein accuracy of the differential sensor signal is affected by variations in the first pressure and wherein the correction data comprises data representative of effects of variations in the first pressure on the differential sensor signal.
6. The apparatus of Claim 5 wherein the differ-ential sensor signal does not vary linearly in response to variations in the difference in pressure between the first and second pressures and wherein the correction data further comprises data representative of nonlinearities in the differential sensor signal and wherein the reference sensor signal does not vary linearly in response to variations in the first pressure and wherein the correction data further comprises data representative of nonlinearities in the reference sensor signal.
7. The apparatus of Claim 6 further comprising temperature sensing means for providing a temperature signal representative of the temperature of the apparatus.
8. The apparatus of Claim 7 wherein accuracy of the reference sensor signal is affected by variations in the temperature of the apparatus and wherein the correction data comprises data representative of effects of variation in the temperature on the reference sensor signal.
9. The apparatus of Claim 8 wherein accuracy of the differential sensor signal is affected by variations in the temperature of the apparatus and wherein the correction data comprises data representative of effects of variation in the temperature on the differential sensor signal.
10. The apparatus of Claim 9 wherein the tempera-ture signal does not vary linearly in response to variation in temperature of the apparatus and wherein the correction data comprises data representative of nonlinearities in the temperature signal.
11. The apparatus of Claim 1 further comprising temperature sensing means for providing a temperature signal representative of the temperature of the apparatus.
12. The apparatus of Claim 11 wherein the correct-ing means comprises a digital computer which provides the output signal in accordance with the formula a + bx + cx2 + ...
where: x = Z0 - Z
Z = the differential sensor signal Z0= correction data and: a = a0 + a1Y + a2Y2 + ...
b = b0 + b1Y + b2y2 + ...
c = c0 + C1Y + c2Y2 + ...
where: Y = R0 - R
R = the reference sensor signal R0= correction data and: a0= a00 + a-01 T + a02 T2 +...
a1 a10 + a11 T + a12 T2 + ...
a2= a20 + a21 T + ...
and: b0= b00 + b01 T + b02 T2 + ...
b1= b10 + b11 T + ...
and: c0= c00 + c01 T + ...
where: T = the temperature signal and: a00, a01..., b00, b01,..., c00, c01, ... , a10,a11, .... are correction data
where: x = Z0 - Z
Z = the differential sensor signal Z0= correction data and: a = a0 + a1Y + a2Y2 + ...
b = b0 + b1Y + b2y2 + ...
c = c0 + C1Y + c2Y2 + ...
where: Y = R0 - R
R = the reference sensor signal R0= correction data and: a0= a00 + a-01 T + a02 T2 +...
a1 a10 + a11 T + a12 T2 + ...
a2= a20 + a21 T + ...
and: b0= b00 + b01 T + b02 T2 + ...
b1= b10 + b11 T + ...
and: c0= c00 + c01 T + ...
where: T = the temperature signal and: a00, a01..., b00, b01,..., c00, c01, ... , a10,a11, .... are correction data
13. The apparatus of Claim 12 wherein selected correction data are chosen as a zero value and the formula a + bx + cx2 + ...
comprises the formula (Z + a0 + a1R + a2R2) (1 + c0 + c1R + c2R2 + c3R3) + d0 wherein: a0 a1 a2 c0 c1 c2 c3 and d0 are functions of the temperature signal of third order or less having correspondingly different correction data.
comprises the formula (Z + a0 + a1R + a2R2) (1 + c0 + c1R + c2R2 + c3R3) + d0 wherein: a0 a1 a2 c0 c1 c2 c3 and d0 are functions of the temperature signal of third order or less having correspondingly different correction data.
14. The apparatus of Claim 11 wherein the refer-ence pressure sensing means is physically coupled to the differential pressure sensing means and the correcting means adjusts the reference sensor signal simultaneously with the adjustment of the differential sensor signal.
15. The apparatus of Claim 14 wherein the tempera-ture sensing means is physically coupled in intimate thermal contact with the differential pressure sensing means and the correcting means adjusts for nonlinearities in the temperature signal simultaneously with the adjust-ment of the reference sensor signal and the output signal.
16. The apparatus of Claim 15 wherein the memory storage means is physically coupled to the differential pressure sensing means such that the differential pressure sensing means, reference pressure sensing means, temperature sensing means and memory storage means comprise a sensor module which is interchangeably connectable to the correct-ing means.
17. The apparatus of Claim 16 wherein the memory storage means comprises a read only memory device.
18. The apparatus of Claim 1 having two terminals for connection to two wires from a series connected source of DC voltage and a load, through which a direct current signal flows, the apparatus further comprising:
current control means coupled to the two terminals for controlling the direct current signal as a function of the output signal such that the direct current signal is representa-tive of the difference in pressure and pro-vides sole energization of the apparatus.
current control means coupled to the two terminals for controlling the direct current signal as a function of the output signal such that the direct current signal is representa-tive of the difference in pressure and pro-vides sole energization of the apparatus.
19. The apparatus of Claim 18 further comprising temperature sensing means for providing a temperature signal representative of the temperature of the apparatus.
20. The apparatus of Claim 18 wherein the direct current signal comprises a signal varying between 4 milli-amperes and 20 milliamperes.
21. A method of determining a difference in pressure between a first pressure and a second pressure and providing an output signal representative of the difference comprising the steps of:
sensing the difference in pressure between the first and second pressures;
providing a differential signal representative of the sensed difference in pressure;
sensing the first pressure;
providing a reference signal representative of the sensed first pressure;
calculating a corrected differential pressure value as a function of the differential signal, the reference signal and previously stored correc-tion values which provide corrections for known correctable errors in the differential signal and the reference signal; and providing the output signal as a function of the corrected differential pressure value.
sensing the difference in pressure between the first and second pressures;
providing a differential signal representative of the sensed difference in pressure;
sensing the first pressure;
providing a reference signal representative of the sensed first pressure;
calculating a corrected differential pressure value as a function of the differential signal, the reference signal and previously stored correc-tion values which provide corrections for known correctable errors in the differential signal and the reference signal; and providing the output signal as a function of the corrected differential pressure value.
22. The method of Claim 21 wherein:
the difference in pressure is sensed by a differential pressure sensor;
the first pressure is sensed by a reference pressure sensor;
the correction values are stored in a memory device; and a digital computer calculates the corrected differ-ential pressure value and provides the output signal.
the difference in pressure is sensed by a differential pressure sensor;
the first pressure is sensed by a reference pressure sensor;
the correction values are stored in a memory device; and a digital computer calculates the corrected differ-ential pressure value and provides the output signal.
23. The method of Claim 22 wherein the differ-ential pressure sensor comprises a capacitive type differential pressure sensor.
24. The method of Claim 22 wherein the differ-ential pressure sensor comprises a vibrating beam type differential pressure sensor.
25. The method of Claim 22 wherein the reference pressure sensor comprises an absolute pressure sensor which can be adjusted to two percent accuracy or less.
26. The method of Claim 22 wherein the memory device comprises a read only memory.
27. The method of Claim 22 wherein accuracy of the reference signal is affected by variations in the first pressure and wherein accuracy of the differential signal is affected by variations in the difference in pressure and by variations in the first pressure.
28. The method of Claim 27 wherein the correction values comprise data representative of nonlinearities of the reference signal with variations in the first pressure, of nonlinearities of the differential signal with variations in the difference in pressure, and variations in the differ-ential signal as a function of variations in the first pressure.
29. A method of determining a difference in pressure at variable temperatures between a first pressure and a second pressure and providing an output signal repre-sentative of the difference in pressure comprising the steps of:
sensing the difference in pressure;
providing a differential signal representative of the sensed difference in pressure;
sensing the first pressure;
providing a reference signal representative of the sensed first pressure;
sensing the temperature;
providing a temperature signal representative of the sensed temperature;
adjusting the temperature signal as a function of the temperature signal and a first set of previously stored correction values;
adjusting the reference signal as a function of the adjusted temperature signal, the reference signal and a second set of previously stored correction values; and providing the output signal as a function of the adjusted temperature signal, the adjusted reference signal, the differential signal and a third set of previously stored correction values.
sensing the difference in pressure;
providing a differential signal representative of the sensed difference in pressure;
sensing the first pressure;
providing a reference signal representative of the sensed first pressure;
sensing the temperature;
providing a temperature signal representative of the sensed temperature;
adjusting the temperature signal as a function of the temperature signal and a first set of previously stored correction values;
adjusting the reference signal as a function of the adjusted temperature signal, the reference signal and a second set of previously stored correction values; and providing the output signal as a function of the adjusted temperature signal, the adjusted reference signal, the differential signal and a third set of previously stored correction values.
30. The method of Claim 29 wherein;
the difference in pressure is sensed by a differ-ential pressure sensor;
the first pressure is sensed by a reference pressure sensor;
the temperature is sensed by a temperature sensor;
the first, second and third sets of correction values are stored in a memory device; and a digital computer adjusts the temperature signal, the reference signal, and provides the output signal.
the difference in pressure is sensed by a differ-ential pressure sensor;
the first pressure is sensed by a reference pressure sensor;
the temperature is sensed by a temperature sensor;
the first, second and third sets of correction values are stored in a memory device; and a digital computer adjusts the temperature signal, the reference signal, and provides the output signal.
31. The method of Claim 30 wherein the digital computer provides the output signal in accordance with the formula a + bx + cx2 + ...
where: x = Z0 - Z
Z = the differential signal Z0= a correction value of the third set and: a = a0 + a1Y + a2Y2 + ...
b = b0 + b1Y + b2Y2 + ...
c = c0 + c1Y + c2Y2 + ...
where: Y = R0 - R
R = the reference signal R0= a correction value of the second set and: a0= a00 + a01 T + a02 T2 + ...
a1= a10 + a11 T + a12 T2 + ...
a2= a20 + a21 T + ...
and: b0= b00 + b01 T + b02 T2 + ...
b1= b10 + b11 T + ...
and: c0= c00 + c01 T + ...
where: T = the temperature signal and: a00 a01 ..., b00 b01 ..., c c01 ...., a10 a11 .... are correction values
where: x = Z0 - Z
Z = the differential signal Z0= a correction value of the third set and: a = a0 + a1Y + a2Y2 + ...
b = b0 + b1Y + b2Y2 + ...
c = c0 + c1Y + c2Y2 + ...
where: Y = R0 - R
R = the reference signal R0= a correction value of the second set and: a0= a00 + a01 T + a02 T2 + ...
a1= a10 + a11 T + a12 T2 + ...
a2= a20 + a21 T + ...
and: b0= b00 + b01 T + b02 T2 + ...
b1= b10 + b11 T + ...
and: c0= c00 + c01 T + ...
where: T = the temperature signal and: a00 a01 ..., b00 b01 ..., c c01 ...., a10 a11 .... are correction values
32. The method of Claim 31 wherein selected correc-tion values from among the first; second and third sets are chosen to have a zero value and the formula a + bx +cx2 + ...
comprises the formula (Z + a0 + a1R + a2R2) (1 + c0 + c1R + c2R2 + c3R3) + d0 wherein: a0 a1 a2 c0 c1 c2 c3 and d0 are functions of the temperature signal of third order or less having correspondingly different correction values.
comprises the formula (Z + a0 + a1R + a2R2) (1 + c0 + c1R + c2R2 + c3R3) + d0 wherein: a0 a1 a2 c0 c1 c2 c3 and d0 are functions of the temperature signal of third order or less having correspondingly different correction values.
33. A two wire differential pressure sensing appara-tus having two terminals for connection to two wires from a series connected source of DC voltage and a load through which a DC current of variable magnitude flows, the DC
current providing sole energization for the apparatus, the apparatus comprising:
differential pressure sensor means for providing a differential sensor signal which is a function of a difference in pressure between first and second pressures;
reference pressure sensor means for providing a reference sensor signal which is a function of the first pressure;
temperature sensor means for providing a temperature sensor signal which is representative of tempera-ture of the apparatus;
memory means for storing correction data uniquely associated with the differential pressure sensor means, the reference pressure sensor means and the temperature sensor means for cor-recting known correctable errors in the differ-ential sensor signal, the reference sensor signal and the temperature sensor signal;
digital computer means for computing a corrected differential pressure value based upon the differential sensor signal, the reference sen-sor signal, the temperature sensor signal and the correction data and for producing a control signal as a function of the corrected differ-ential pressure value; and current control means coupled to the two terminals for controlling the magnitude of the DC current as a function of the control signal so that the magnitude of the DC current is representative of the corrected differential pressure value.
current providing sole energization for the apparatus, the apparatus comprising:
differential pressure sensor means for providing a differential sensor signal which is a function of a difference in pressure between first and second pressures;
reference pressure sensor means for providing a reference sensor signal which is a function of the first pressure;
temperature sensor means for providing a temperature sensor signal which is representative of tempera-ture of the apparatus;
memory means for storing correction data uniquely associated with the differential pressure sensor means, the reference pressure sensor means and the temperature sensor means for cor-recting known correctable errors in the differ-ential sensor signal, the reference sensor signal and the temperature sensor signal;
digital computer means for computing a corrected differential pressure value based upon the differential sensor signal, the reference sen-sor signal, the temperature sensor signal and the correction data and for producing a control signal as a function of the corrected differ-ential pressure value; and current control means coupled to the two terminals for controlling the magnitude of the DC current as a function of the control signal so that the magnitude of the DC current is representative of the corrected differential pressure value.
34. Apparatus for sensing a difference in pressure between a first pressure and a second pressure and providing an output signal representative of the difference in pres-sure, the apparatus comprising:
differential pressure sensor means for providing a differential sensor signal which is a function of the difference in pressure between the first and second pressures;
reference pressure sensor means for providing a reference sensor signal which is a function of the first pressure;
memory storage means for providing correction data for reference sensor signal nonlinearities, differential sensor signal nonlinearities, and effects of variations of the reference sensor signal on the differential sensor signal; and correcting means for providing the output signal as a function of the reference sensor signal, the differential sensor signal and the correction data, the output signal being more representa-tive of the difference in pressure than is the differential sensor signal as a result of correction for the reference sensor signal nonlinearities, the differential sensor signal nonlinearities, and the effects of variation of the reference sensor signal on the differential sensor signal.
differential pressure sensor means for providing a differential sensor signal which is a function of the difference in pressure between the first and second pressures;
reference pressure sensor means for providing a reference sensor signal which is a function of the first pressure;
memory storage means for providing correction data for reference sensor signal nonlinearities, differential sensor signal nonlinearities, and effects of variations of the reference sensor signal on the differential sensor signal; and correcting means for providing the output signal as a function of the reference sensor signal, the differential sensor signal and the correction data, the output signal being more representa-tive of the difference in pressure than is the differential sensor signal as a result of correction for the reference sensor signal nonlinearities, the differential sensor signal nonlinearities, and the effects of variation of the reference sensor signal on the differential sensor signal.
35. Apparatus for sensing a difference in pressure between a first pressure and a second pressure and pro-vidiny a corrected differential pressure value representa-tive of the difference in pressure, the apparatus comprising:
differential pressure sensor means for providing a differential sensor signal which is a function of the difference in pressure between the first and second pressures;
reference pressure sensor means for providing a reference sensor signal which is a function of the first pressure;
memory storage means for providing correction data for reference sensor signal nonlinearities, differential sensor signal nonlinearities, and effects of variations of the reference sensor signal on the differential sensor signal; and means for calculating the corrected differential pressure value based upon a predetermined formula which includes the reference sensor signal, the differential sensor signal and the correction data, the formula providing correc-tion for the reference sensor signal non-linearities, the differential sensor signal nonlinearities, and the effects of variation of the reference sensor signal on the differ-ential sensor signal.
differential pressure sensor means for providing a differential sensor signal which is a function of the difference in pressure between the first and second pressures;
reference pressure sensor means for providing a reference sensor signal which is a function of the first pressure;
memory storage means for providing correction data for reference sensor signal nonlinearities, differential sensor signal nonlinearities, and effects of variations of the reference sensor signal on the differential sensor signal; and means for calculating the corrected differential pressure value based upon a predetermined formula which includes the reference sensor signal, the differential sensor signal and the correction data, the formula providing correc-tion for the reference sensor signal non-linearities, the differential sensor signal nonlinearities, and the effects of variation of the reference sensor signal on the differ-ential sensor signal.
36. The apparatus of Claim 35 and further comprising:
temperature sensor means for providing a tempera-ture sensor signal which is representative of temperature of the apparatus;
wherein the memory storage means also provides correction data for effects of variation in temperature on the differential sensor signal and the reference sensor signal and tempera-ture sensor signal nonlinearities; and wherein the formula used by means for calculating also includes the temperature signal and the correction data for effects of variation in temperature and temperature sensor signal nonlinearities.
temperature sensor means for providing a tempera-ture sensor signal which is representative of temperature of the apparatus;
wherein the memory storage means also provides correction data for effects of variation in temperature on the differential sensor signal and the reference sensor signal and tempera-ture sensor signal nonlinearities; and wherein the formula used by means for calculating also includes the temperature signal and the correction data for effects of variation in temperature and temperature sensor signal nonlinearities.
37. A method of determining a difference in pressure between a first pressure and a second pressure comprising the steps of:
sensing the difference in pressure between the first and second pressures;
providing a differential signal value representa-tive of the sensed difference in pressure;
sensing the first pressure;
providing a reference signal value representative of the sensed first pressure;
calculating a corrected differential pressure value based upon a formula which includes the differential signal value, the reference sig-nal value and previously stored correction values representing corrections for known correctable errors in the differential signal value and the reference signal value; and providing the output signal as a function of the corrected differential pressure value.
sensing the difference in pressure between the first and second pressures;
providing a differential signal value representa-tive of the sensed difference in pressure;
sensing the first pressure;
providing a reference signal value representative of the sensed first pressure;
calculating a corrected differential pressure value based upon a formula which includes the differential signal value, the reference sig-nal value and previously stored correction values representing corrections for known correctable errors in the differential signal value and the reference signal value; and providing the output signal as a function of the corrected differential pressure value.
38. A method of determining a difference in pressure at variable temperatures between a first pressure and a second pressure and providing an output signal repre-sentative of the difference in pressure comprising the steps of:
sensing the difference in pressure;
providing a differential signal value representa-tive of the sensed difference in pressure;
sensing the first pressure;
providing a reference signal value representative of the sensed first pressure;
sensing the temperature;
providing a temperature signal value representative of the sensed temperature;
calculating a corrected differential pressure value using a formula which simultaneously adjusts the differential signal value, the reference signal value, and the temperature signal value based upon a set of previously stored correc-tion values; and providing the output signal as a function of the corrected differential pressure value.
sensing the difference in pressure;
providing a differential signal value representa-tive of the sensed difference in pressure;
sensing the first pressure;
providing a reference signal value representative of the sensed first pressure;
sensing the temperature;
providing a temperature signal value representative of the sensed temperature;
calculating a corrected differential pressure value using a formula which simultaneously adjusts the differential signal value, the reference signal value, and the temperature signal value based upon a set of previously stored correc-tion values; and providing the output signal as a function of the corrected differential pressure value.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000451541A CA1216360A (en) | 1984-04-09 | 1984-04-09 | Pressure compensated differential pressure sensor and method |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000451541A CA1216360A (en) | 1984-04-09 | 1984-04-09 | Pressure compensated differential pressure sensor and method |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1216360A true CA1216360A (en) | 1987-01-06 |
Family
ID=4127605
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000451541A Expired CA1216360A (en) | 1984-04-09 | 1984-04-09 | Pressure compensated differential pressure sensor and method |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1216360A (en) |
-
1984
- 1984-04-09 CA CA000451541A patent/CA1216360A/en not_active Expired
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