_
`
`Unlted States Patent [19]
`Zolock
`
`USOO5231884A
`
`[11] Patent Number:
`[45] Date of Patent:
`
`5,231,884
`Aug. 3, 1993
`
`[54] TECHNIQUE FOR SUBSTANTIALLY
`ELIMINATING TEMPERATURE INDUCED
`MEASUREMENT ERRORS FROM A
`CORIOLIS METER
`
`[75]
`
`[73]
`
`[21]
`
`122]
`
`[51]
`[52]
`
`[53]
`
`[56]
`
`Inventor: Michael J. Zolock, Longmont, Colo_
`
`Assignee: Micro Motion, Inc., Boulder, Colo.
`
`Appl. No.: 728,546
`
`Filed:
`
`Jul. 11, 1991
`
`Int. Cl.5 .............................................. .. G01F 1/84
`US. Cl.
`.................... ,. 73/86L38; 324/7682;
`324/601
`Field of Search ...................... ,. 73/86l.37, 861.38;
`324/84 D, 84 Q, 601
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,585,841 6/1971 Brandau et al. . . . .
`
`. . . . . . . . . .. 73/4
`
`4,422,338 12/1983 Smith . . _ . . . . . . . . . . .
`
`. . . . .. 73/861.38
`
`4,491,025 1/1985 Smith et a1. . . . . . . . .
`
`. . . . .. 73/86l.38
`
`'
`
`4,817,448 4/1989 Hargarten et a1. . . . . .
`. . . . .. 73/861 38
`4,843,890 7/1989 Samson et a1. ............. .. 73/861 38
`4.852.409 8/1989 Herzl ............ ..
`4.872,35l 10/1989
`4,876,879 10/1989
`4,879,911 11/1989
`4,896,282 1/1990
`4,934,196 6/1990
`5,009,109 4/1991
`5,027,662 7/1991
`
`FOREIGN PATENT DOCUMENTS
`
`0275367 7/1988 European Pat. Off. ....... .. 73/861.38
`0151518 9/1983 Japan .............................. .. 73/86l.38
`Primary Examiner~Herbert ,Goldstein
`Attorney, Agent, or Finn-Peter L. Michaelson;
`Raymond R. Moser, Jr.
`[57]
`ABSTRACT
`Apparatus and accompanying methods for inclusion in
`a Coriolis meter that substantially eliminate temperature
`induced measurement errors which might otherwise be
`produced by performance differences existing between
`the separate input channels contained in the meter.
`Speci?cally, two pairs of input channels are used in the
`meter. In operation, the meter repetitively measures the
`internal phase delay of each of these pairs and then
`subtracts the delay associated with each pair from ac
`tual ?ow based measurement data subsequently ob
`tained therefrom. While one channel pair is measuring
`actual flow, the other channel pair is measuring its inter
`nal phase delay, with the channels being continuously
`cycled between these functions. Because both channel
`pairs are cycled at a suf?ciently rapid rate, the current
`value of the internal phase delay for each of the pairs
`accurately re?ects any temperature induced changes
`then occurring in the performance of that pair thereby
`eliminating substantially all temperature induced error
`components from the ?ow measurements subsequently
`obtained therefrom. In addition, the meter measures
`flow tube temperature in a manner that removes tem
`perature induced errors therefrom.
`
`26 Claims, 14 Drawing Sheets
`
`Micro Motion 1052
`
`1
`
`

`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 1 of 14
`
`5,231,884
`
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`

`

`US. Patent
`
`Aug. 3, 1993'
`
`Sheet 2 of 14
`
`5,231,884
`
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`US. Patent
`
`Aug. 3, 1993
`
`Sheet 3 of 14
`
`5,231,884
`
`FIG. 3
`
`FIG. 3A FIG. 38
`
`FIG. 34
`
`r_______________________________________________.__.__
`
`5 a
`
`A-C
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`if
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`
`I I l l In
`
`We REFERENCE J 39
`VOLTAGE
`GENERATOR
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`‘
`
`4
`
`

`

`U.S. Patent
`
`Aug. 3, 1993
`
`Sheet 4 of 14
`
`5,231,884
`
`PUTICwbnIC
`YTimUT.
`
`INPUT
`CIHCUITRY
`
`USER SMITCHES
`
`FIG. 3B
`
`(INITIAL VALUES.
`E.G. TIMING
`INTERVALS IN
`TUBE coums. AND
`STATE INFORMATION)
`
`RTD/REF. VOLTAGE
`MEASUREMENT
`DATA
`
`1IIIIIIIIIIIlIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIlIIIIIIIIIIIIIIIIIIIIIIIIIIIIla
`
`coumen
`
`rIIIIIIIIIIIIIIIIIlIIIIIIIIIIIIIIIIIIIIIIIII|IIIIIIIIIIIIIIIIIIIIIIIIIIIn
`
`FLOM MEASUREMENT CIRCUIT
`
`5
`
`
`
`
`
`

`

`US. Patent
`
`Aug. 3, 1993~
`
`Sheet 5 of 14
`
`5,231,884
`
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`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 6 of 14
`
`5,231,884
`
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`

`

`U.S. Patent
`
`Aug. 3, 1993
`
`Sheet 7 of 14
`
`5,231,884
`
`
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`

`

`US. Patent
`
`Aug. 3, 1993‘
`
`Sheet 8 of 14
`
`5,231,884
`
`FIG ' 6
`
`ENTER
`
`FLOH
`MEASUREMENT .
`BASIC
`MAIN LOOP
`59D
`
`f 610
`
`'
`
`READ CURRENT RAM PHASE DIFFERENCE MEASLBEMENT DATA.
`I.E. RAM_RATE__A AND RAM_RATE_D, AND STATE
`INFORMATION FROM CIRCUIT 7O
`
`1
`
`520
`2
`
`EXECUTE ZERO DETERMINATION ROUTINE 700 USING CURRENT
`RAM_RATE_A AND RAM_RATE_B VALUES AND STATE
`INFORMATION TO: PROCESS FLOM BASED At MEASUREMENTS FOR A
`CHANNEL PAIR OPERATING IN ITS MEASUREMENT MODE AND DETERMINE
`ELECTRONIC ZERO FOR THE OTHER CHANNELv PAIR OPERATING IN ITS
`ZERO MODE FROM ITS INTERNAL PHASE DELAY MEASUREMENTS.
`AND DETERMINE MECHANICAL ZERO FOR METER, MHEN DESIRED; AND TO
`APPROPRIATELY CORRECT THE CURRENT MEASURED At VALUE USING THE
`MECHANICAL ZERO VALUE AND THE APPLICABLE ELECTRONIC ZERO VALUE
`
`T
`
`FILTER CORRECTED At VALUE f 53°
`THROUGH THO-POLE SOFTHARE FILTER
`TO YIELD FILTEHED At VALUE
`
`1
`
`640
`CALCULATE VOLUMETRIC AND MASS FLOM RATES /
`USING FILTERED At VALUE AND
`TEMPERATURE CORRECTED RATE FACTOR
`
`1
`
`cuIoFF; m0
`IF voLunEmIc AND ms FLOR
`RATE VALUES ARE LESS mm LOH
`FLOR RATE CUTOFF. SET voLunLzmlc AND
`MASS FLOH mus VALUES I0 ZERO
`
`sso
`/
`
`1
`
`STORE VOLUMETRIC m0 mss FLOH f 56°
`RATE VALUES FOR SUBSEOUENT use
`(as. PERIODIC UPDATING OF TOTALIZEO FLOM)
`
`9
`
`

`

`US. Patent
`
`Aug. a, 1993
`
`Sheet 9 of 14
`
`5,231,884
`
`FIG ['76- 7
`M
`FIG
`78
`
`F I 6. 7A
`
`ZERO
`DETERHINATIDN
`noglgnue
`m
`
`I. ___________________________________ __
`ELECTRONIC ZERO
`DETERMINATION ROUTINE
`m
`
`SET:
`TE_"‘’__5T2AETRE0ING
`CHANNEL]
`
`_
`INCREMENT COUNTER.
`coumen~
`coumsn + 1
`
`718 \
`TOTAL RATE
`\ ELECT_ZEHD_A —-<
`'
`)
`COUNTER
`
`721
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`\ TEMP_STATE--NOT_ZEROING_
`CHANNEL_A
`
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`coumen+o
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`
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`10
`
`

`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 10 of 14
`
`5,231,884
`
`T
`i
`i
`i
`i
`i
`i
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`739 /
`INCREMENT COLNTER;
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`COUNTER + 1
`
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`7
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`CHANNEL__B
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`ELECTRONIC ZERO
`COMPENSATION ROUTINE
`ZOO
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`763
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`MECHANICAL ZERO
`DETERMINATION
`ROUTINE
`ZOO
`
`7B1
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`YES »
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`MECHANICAL
`ZERO?
`
`EXECUTE MECHANICAL ZERO
`ROUTINE 800 TO
`DETERMINE CURRENT
`MECHANICAL ZERO VALUE.
`I.E. MECH__ZERO
`
`MECHANICAL ZERO
`DETERMINATION
`ROUTINE
`ZQQ
`
`At<-At - MECH ZERO fm
`
`(ébENO
`
`FIG. 7B
`
`11
`
`

`

`US. Patent
`
`Aug. 3, 1993-
`
`Sheet 11 of 14
`
`5,231,884
`
`FIG F I 6. 8
`8A
`
`F15
`as
`
`f B06
`
`ZEHOJOTAL
`—— zanojom
`+ At
`
`/ B09
`
`INCREMENT:
`ZERO_COUNT *
`magnum + 1
`
`F I 6. 8A
`
`MECHANICAL
`ZERO
`ROUTINE
`m
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`SET:
`zeno_sme ~ ACTIVE
`ZEHO_TOTAL--0
`zsno_coum ‘ o
`MIN_STD_DEV ~ BIG
`
`B16
`\
`RESET ALL MECHANICAL
`ZERO ennon FLAGS
`
`N0
`
`822
`2
`
`s23
`,
`f
`CALCULATE sunomn DEVIATIUN, smpev:
`smpav ~ 0 (At)
`
`a29\\
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`new ZERO TEMP
`zsnuouml
`-
`-
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`
`12
`
`

`

`US. Patent
`
`Aug. 3, 1993
`
`Sheet 12 of 14
`
`5,231,884
`
`FIG. 8B
`
`SET ‘MECHANICAL
`ZERO T00 LOH‘
`ERROR FLAG
`TRUE
`
`SZEQRO
`ERROR FLAG
`TRUE
`_
`
`SET ‘MECHANICAL
`ZERO 10o
`NOISY' ERROR
`me mm:
`
`f ass
`
`MECHJEHD
`__ MECHJEROJEHP
`
`e70
`
`\ SET:
`ZEHO_STATE~—INACTIVE
`
`55 gm
`
`13
`
`

`

`U.S. Patent
`
`Aug. 3, 1993
`
`Sheet 13 of 14
`
`5,231,884
`
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`

`US. Patent
`
`Aug. a, 1993 '
`
`Sheet 14 of 14
`
`5,231,884
`
`F I G .
`
`1 1
`
`QFEUTER
`
`RTU TEMPERATURE
`PROCESSING ROUTINE
`
`“Q0
`
`INSTRUCT CIRCUIT 70 TO APPLY SUITABLE SELECT SIGNALS TO
`1110 \ LEADS 34 TD SELECT RTD VOLTAGE AS INPUT TD V/F 42; AND
`AFTER APPROPRIATE COUNTING INTERVAL, READ CONTENTS OF
`COUNTER 78 IN CIRCUIT 7O
`
`FILTER CONTENTS OF COUNTER 78 THROUGH
`1120 \- A TMU POLE soETMARE FILTER; AND
`STORE RESULTING FILTERED VALUE IN
`VARIABLE V-TO-F
`
`CALCULATE:
`1 130
`\ CURRENT_FREO-— (FREG_AT_OV) - (V__TO_F)
`MRERE: FREO_AT_0V IS THE LATEST FILTERED
`V/F EREUUEUUY UUTPUT VALUE IN
`COUNTS, EUR 0v INPUT
`
`1140 \_ CALCULATE:
`
`“5° \ CALCULATE cURREUT. RTU TEMPERATURE, TEMP;
`TEMP - (CURRENLFREO) / (FREU_PER_c)
`
`“wk CALCULATE TEMPERATURE coMPEMsATEU METER FACTOR (RF):
`RF <- (PETER FACTOR) * l II-TEMPTETCT I
`NHERE TC 15 A TEMPERATURE COEFFICIENT
`
`é) EXIT
`
`15
`
`

`

`1
`
`5,231,884
`
`TECHNIQUE FOR SUBSTANTIALLY
`ELIMINATING TEMPERATURE INDUCED
`MEASUREMENT ERRORS FROM A CORIOLIS
`METER
`
`CROSS-REFERENCE TO RELATED
`APPLICATION
`This application describes and claims subject matter
`10
`that is also described in a co-pending United States
`patent application entitled “A TECHNIQUE FOR
`DETERMINING A MECHANICAL ZERO
`VALUE FOR A CORIOLIS METER” from applicant
`R. Bruck, Ser. No. 728,547, ?led Jul. 11, 1991 simulta
`neously herewith and owned by the present assignee
`hereof.
`
`2
`a respective mid-plane of oscillation, until the instant a
`corresponding point situated on the opposite side leg of
`the same ?ow conduit, crosses its corresponding loca
`tion, e.g. its respective mid-plane of oscillation. For
`parallel dual conduit Coriolis mass ?ow rate meters, this
`interval is generally equal to the phase difference be
`tween the velocity signals generated for both ?ow con
`duits at the fundamental (resonant) frequency at which
`these conduits are driven. In addition, the resonant
`frequency at which each ?ow conduit oscillates de
`pends upon the total mass of that conduit, i.e. the mass '
`of the conduit itself, when empty, plus the mass of any
`?uid ?owing therethrough. Inasmuch as the total mass
`varies as the density of the ?uid ?owing through the
`conduit varies, the resonant frequency likewise varies
`with any changes in ?uid density and, as such, can be
`used to track changes in ?uid density.
`For some time, the art has taught that both velocity
`signals are processed through at least some analog cir
`cuitry in an effort to generate output signals that are
`proportional to the mass ?ow rate of the process ?uid.
`In particular, the output signal associated with each
`velocity sensor is ordinarily applied through analog
`circuitry, e.g. an integrator followed by a zero crossing
`detector (comparator), contained within a separate cor
`responding input channel. In this regard, see illustra
`tively U.S. Pat. No. 4,879,911 (issued to M. J. Zolock on
`Nov. 14, 1989), U.S. Pat. No. 4,872,351 (issued to J. R.
`Ruesch on Oct. 10, 1989), U.S. Pat. No. 4,843,890 (is
`sued to A. L. Samson et al on Jul. 4, 1989) and U.S. Pat.
`No. 4,422,338 (issued to J. E. Smith on Dec. 27,
`l983)—all of which are also owned by the present as
`signee hereof. While the various approaches taught in
`these patents provide sufficiently accurate results in a
`wide array of applications, the meters disclosed in these
`references, as well as similar Coriolis meters known in
`the art, nevertheless suffer from a common drawback
`which complicates their use.
`Speci?cally, Coriolis mass ?ow meters operate by
`detecting what amounts to be a very small inter-channel
`phase difference between the signals produced by both
`velocity sensors, i.e. the At value, and transforming this
`difference into a signal proportional to mass ?ow rate.
`While, at its face, a At value is obtained through a time
`difference measurement, this value, in actuality, is also a
`phase measurement. Using such a time difference mea
`surement conveniently provides a way to accurately
`measure a manifestation of a phase difference appearing
`between the velocity sensor signals. In Coriolis meters
`currently manufactured by the present assignee, this
`difference tends to amount to approximately 130 usec at
`maximum ?ow. Each input channel in a Coriolis meter
`imparts some internal phase delay to its input signal.
`While the amount of this delay is generally quite small,
`it is often signi?cant when compared to the small inter
`channel phase difference, i.e. I30 usec or less, that is
`being detected. Currently available Coriolis meters
`have relied on assuming that each input channel imparts
`a ?nite and ?xed amount of phase delay to its corre
`sponding velocity signal. As such, these Coriolis meters
`generally rely on ?rst measuring, at a true zero ?ow
`condition occurring during meter calibration, either the
`inter-channel phase difference (At) or the indicated
`mass ?ow rate. Subsequently, while metering actual
`?ow, these meters will then subtract the resulting value,
`in some fashion, from either the measured At or mass
`?ow rate value, as appropriate, in order to generate an
`
`BACKGROUND OF THE INVENTION
`1. Field of the Invention
`The present invention relates to apparatus and meth
`ods for inclusion in, illustratively, a Coriolis mass ?ow
`rate meter that substantially eliminate temperature in
`duced measurement errors which might otherwise be
`produced by performance differences existing between
`two separate input channel circuits contained in the
`meter.
`2. Description of the Prior Art
`Currently, Coriolis meters are ?nding increasing use
`in a wide variety of commercial applications as an accu
`rate way to measure the mass ?ow rate of various pro
`cess ?uids.
`Generally speaking, a Coriolis mass ?ow rate meter,
`such as that described in U.S. Pat. No. 4,491,025 (issued
`to J. E. Smith et al on Jan. 1, 1985 and owned by the
`present assignee hereof--hereinafter referred to as the
`‘025 Smith patent), contains one or two parallel con
`duits, each typically being a U-shaped ?ow conduit or
`tube. As stated in the ’025 Smith patent, each ?ow con
`duit is driven to oscillate about an axis to create a rota
`tional frame of reference. For a U-shaped ?ow conduit,
`this axis can be termed the bending axis. As process
`?uid ?ows through each oscillating ?ow conduit,
`movement of the ?uid produces reactionary Coriolis
`forces that are orthogonal to both the velocity of the
`?uid and the angular velocity of the conduit. These
`reactionary Coriolis forces, though quite small when
`compared to a force at which the conduits are driven,
`nevertheless cause each conduit to twist about a tor
`sional axis that, for a U-shaped ?ow conduit, is normal
`to its bending axis. The amount of twist imparted to
`each conduit is related to the mass ?ow rate of the
`process ?uid ?owing therethrough. This twist is fre
`quently measured using velocity signals obtained from
`magnetic velocity sensors that are mounted to one or
`both of the ?ow conduits in order to provide a complete
`velocity pro?le of the movement of each ?ow conduit
`with respect to either the other conduit or a ?xed refer
`ence. In dual conduit Coriolis meters, both ?ow con
`duits are oppositely driven such that each conduit oscil
`lates (vibrates) as a separate tine of a tuning fork. This
`“tuning fork” operation advantageously cancels sub
`stantially all undesirable vibrations that might otherwise
`mask the Coriolis force.
`In such a Coriolis meter, the mass flow rate of a ?uid
`that moves through the meter is generally proportional
`to the time interval (the so-called “At” value) that elap
`ses between the instant one point situated on a side leg
`of a flow conduit crosses a predetermined location, e. g.
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`3
`ostensibly accurate mass flow rate value for the process
`?uid then flowing therethrough.
`Unfortunately, in practice, this assumption has
`proven to be inaccurate. First, not only does each input
`channel often produce a different amount of internal
`phase delay with respect to the other, but also the phase
`delay that is produced by each channel is temperature
`dependent and varies differently from one channel to
`the other with corresponding changes in temperature.
`This temperature variability results in a temperature
`induced inter-channel phase difference. Because the
`measured phase difference (At) that results from actual
`flow through the meter is relatively small, then an error
`in the measured phase difference between the velocity
`signals and attributable to the temperature induced in
`ter-channel phase difference can, in certain instances, be
`signi?cant. This error is generally not taken into ac
`count in currently available Coriolis mass ?ow rate
`meters. In certain situations, this error can impart a
`noticeable temperature dependent error into mass ?ow
`20
`rate measurements, thereby corrupting the measure
`ments somewhat.
`In an effort to avoid this error, one well known solu
`tion in the art is to shroud an installed piped Coriolis
`meter, including its electronics, with a temperature
`controlled enclosure. This approach, which prevents
`the meter from being exposed to external temperature
`variations and maintains the meter at a relatively con
`stant temperature while it is in operation, greatly in
`creases the installed cost of the meter and is thus not
`suited for every application. Hence, in those applica
`tions where installed cost is a concern, this approach is
`generally not taken. Speci?cally, in those applications
`and particularly‘ where the meter is to be sited indoors
`and not exposed to wide temperature variations, then
`the measurement error which results from the tempera
`ture induced inter-channel phase difference, while gen
`erally expected, tends to remain quite small and rela
`tively constant. As such, this error is usually tolerated
`by a user. Unfortunately, in other applications where
`40
`the meter is not housed in a temperature controlled
`enclosure, such as outdoor installations where the meter
`is expected to experience wide ?uctuations in operating
`temperature, the error generally varies and can become
`signi?cant, and thus needs to be taken into account.
`45
`Apart from errors arising from temperature induced
`inter-channel phase differences, many currently avail
`able Coriolis vmass flow rate meters also disadvanta
`geously exhibit an additional source of measurement
`inaccuracy related to temperature. In particular, Corio
`50
`lis meters generally measure the temperature of the ?ow
`conduit and, owing to changes in flow conduit elasticity
`with temperature, accordingly'modify a meter factor
`value based upon the current temperature of the con
`duit. This meter factor, as modi?ed, is then subse
`quently used to proportionally relate the inter-channel
`phase difference (At) value to mass flow rate. Flow
`conduit temperature is measured by digitizing an output
`of a suitable analog temperature sensor, such as a plati
`num RTD (resistive temperature device), that is
`mounted to an external surface of a flow conduit. The
`digitized output usually takes the form of a frequency
`signal, oftentimes produced by a voltage-to-frequency
`(V /F) converter, that is totalized (counted) over a
`given timing interval to yield an accumulated digital
`value that is proportional to flow conduit temperature.
`Unfortunately, in practice, V/F converters usually ex
`hibit some temperature drift which, based upon the
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`4
`magnitude of a change in ambient temperature, could
`lead to an error, amounting to as much as several de
`grees, in the measurement of ?ow conduit temperature.
`This error will, in turn, corrupt the mass flow rate.
`A solution proposed in the art to ostensibly deal with
`temperature dependent variations in the performance of
`the input channels of Coriolis meters is taught in US.
`Pat. No. 4,817,448 (issued to J. W. Hargarten et al on
`.Apr. 4, 1989 and also owned by the present assignee
`hereof-hereinafter referred to as the ’448 Hargarten et
`al patent). This patent discloses a two channel switching
`input circuit for use in a Coriolis meter. In particular,
`this circuit includes a two-pole two-throw FET (?eld
`effect transistor) switch located between the outputs of
`the velocity sensors and the inputs to both of the chan
`nels. In one position, the FET switch connects the out
`puts of the left and right velocity sensors to correspond
`ing inputs of the left and right channels, respectively;
`while in the opposite position, these connections are
`reversed. The switch is operated to change its position
`at every successive cycle of flow conduit movement. In
`this manner, the output of each velocity sensor is alter
`nately applied to both channels in succession. Over a
`two cycle interval, appropriate time intervals are mea
`sured with respect to the velocity waveforms applied to
`both channels and then averaged together to yield a
`single time interval value from which errors attributable
`to each individual channel have been canceled. This
`resulting time interval value is then used in determining
`mass flow rate through the meter.
`While this solution does indeed substantially elimi
`nate temperature induced inter-channel phase differ
`ences, it possesses a drawback which limits its utility
`somewhat. Speci?cally, this input circuits'in the appara
`tus taught in ’448 Hargarten et a1 patent do not include
`integrators. Owing to the lack of any low pass ?ltering
`that would have been provided by integrators, these
`input circuits are therefore susceptible to noise. Unfor
`tunately, the switching scheme taught in this patent
`does not permit integrators to be included in the
`switched portion of the input circuitry, hence requiring
`that, to provide noise immunity, an integrator must be
`located after the FET switch. Unfortunately, in this
`location, the phase delay inherent in the integrator can
`not be readily compensated, if at all. Inasmuch as the
`integrator disadvantageously tends to provide the larg
`est source of phase delay in the input circuitry, inclusion
`of such an integrator would add an error component,
`i.e. an uncompensated phase delay, to the measured At
`values. Moreover, this phase delay would also vary
`with temperature changes. Consequently, the resulting
`measured flow rate values would contain an error com
`ponent. Thus, it became apparent that the solution
`posed in the ‘448 Hargarten et al patent has limited
`applicability to relatively noise-free environments.
`Therefore, a need exists in the art for a Coriolis meter
`that provides accurate ?ow and ?ow rate output values
`that are substantially insensitive to ambient temperature
`variations and hence does not appreciably exhibit ad
`verse temperature affects an could provide appreciable
`noise immunity. Such a meter should possess negligible,
`if any, temperature induced measurement inaccuracies
`over relatively wide variations in ambient temperature
`thereby permitting the meter to be used to provide
`highly accurate ?ow measurements in a wide variety of
`applications and particularly without a need to house
`the meter in a temperature controlled enclosure. Ad
`vantageously, the increased measurement accuracy
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`5,231,884
`5
`provided by such a meter and the attendant installed
`cost savings associated therewith would likely broaden
`the range of applications over which such a meter could
`be used.
`
`6
`surement modes involve measuring the inter-channel
`phase difference in a pair of channels, the principal
`distinction between the modes is that in the zero mode,
`the same velocity sensor signal is applied to both chan
`nels in that pair so that the resulting inter-channel phase
`difference measurement provides a measurement of the
`internal phase delay for that pair; while, in the measure
`ment mode, the left and right velocity signals are ap
`plied to different corresponding channels in that pair so
`as to provide a measurement, though uncorrected, of
`the current flow based At value for subsequent use in
`determining current mass flow and flow rate values.
`Though inter-channel phase difference (At) measure
`ments are taken during both modes, to simplify matters
`and avoid confusion, I will distinguish between these
`values in terms of their occurrence. I will henceforth
`refer to those phase measurements which occur during
`the zero mode as being inter-channel phase difference
`measurements and those which occur during the mea
`surement mode as being At values.
`Speci?cally, for any channel pair operating in the
`zero mode, such as pair A-C, the same, i.e. left, velocity
`sensor signal is applied to the inputs of both channels in
`that pair. Inter-channel phase difference measurements
`are then successively and repetitively taken during a
`so-called “zeroing" interval with the results being aver
`aged during this interval. Ideally, if both of the channels
`in this pair exhibit the same internal phase delay, i.e. the
`phase delay through channel A equals that of reference
`channel C, then the resulting inter-channel phase differ
`ence measurements will all equal zero. However, in
`actuality, at any instant, all three channels usually pos
`sess different internal phase delays. Nevertheless, since
`the phase delay for each pair is measured with respect
`to the same reference channel, i.e. channel C, any differ
`ences in the phase delay between the two pairs is caused
`by differences in the internal phase delay occurring
`between channels A and B. Once the “zeroing” interval
`has terminated, the input to the non-reference channel
`in that pair is switched to the other velocity sensor
`signal, i.e. the right velocity sensor signal. A finite per
`iod of time, i.e. including a so-called “switching” inter
`val, is then allowed to expire before that channel pair is
`operated in the “measurement” mode during which
`flow based At values are measured. The switching inter
`val is sufficiently long to enable all resulting switching
`transients to settle out.
`While one pair of channels, e.g. A-C, is operating in
`its zero mode, the other pair, e.g. B-C, is operating in its
`measurement mode in order to provide continuous flow
`metering. For any channel pair, each successive current
`flow based At value obtained during its measurement
`mode is compensated by, typically subtracting, the most
`recent value of the internal phase delay that has been
`measured for this channel pair during its preceding zero
`mode.
`The time during which one channel pair operates in
`the measurement mode, i.e. the measuring interval,
`equals the entire time that the other pair operates in the
`zero mode. This latter time includes the time during
`which the latter channel switches its non-reference
`channel input from the right to the left velocity sensor
`signal, then performs zeroing, and ?nally switches its
`non-reference channel input from the left back to the
`right velocity sensor signal.
`At the conclusion of the measurement interval, the
`channel pairs simply switch modes, with illustratively
`channel pair B-C initially switching its non-reference
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`SUMMARY OF THE INVENTION
`An object of the present invention is to provide a
`Coriolis meter that provides accurate output measure
`ments that are substantially insensitive to variations in
`ambient temperature.
`A speci?c object is to provide such a meter that sub
`stantially, if not totally, eliminates the need for a tem
`perature controlled enclosure.
`Another speci?c object is to provide a Coriolis meter _
`in which the measured flow and ?ow rate values do not
`contain appreciable error, if any at all, that would other
`wise result from switching transients appearing in the
`input channels.
`These and other objects are accomplished in accor
`dance with the teachings of my invention by cycling the
`operation of each channel, particularly using a rela
`tively short period, between: (a) measuring the internal
`phase delay of that channel and (b) measuring raw flow
`based At value(s). The raw value(s) are then compen
`sated, typically by subtracting, the measured phase
`delay value therefrom in order to yield a corrected At
`value. A current value mass flow rate is then deter
`mined using the corrected rather than, as occurs in the
`art, the raw At value(s).
`‘
`Speci?cally, the two identical input channels (i.e. left
`and right), as commonly used in prior art Coriolis flow
`meters, are replaced with two pairs of input channels
`(i.e. pairs A-C and B-C) that permit the current internal
`phase delay exhibited by each channel pair to be mea
`sured. Each of the channel pairs is operated to cycle
`between measuring its own internal phase delay, i.e. a
`“zeroing” mode, and measuring At values for actual
`flow conditions, i.e. a “measurement” mode. Given the
`short cycle time, the current phase delay value accu
`rately re?ects any temperature induced changes then
`occurring in the performance of each channel pair.
`Once the current internal phase delay value is known
`for each pair, that value is then used to correct flow
`based At values subsequently produced by that pair
`during its next measurement mode. Because the At ?ow
`45
`based measurements provided by each channel pair are
`corrected for the current internal phase delay associ
`ated with that particular pair, these At values do not
`contain any appreciable temperature induced error
`components regardless of the ambient temperature of
`the meter and its variation. As such, a Coriolis meter
`constructed in accordance with my invention, can ad
`vantageously be used in environirients with widely
`varying temperatures with essentially no diminution in
`accuracy owing to temperature changes.
`In accordance with the teachings of a preferred em
`bodiment of my invention, my inventive ?ow measure
`ment circuit utilizes three separate similar input chan
`nels (i.e. channels A, B and C) through which inter
`channel phase difference measurements are successively
`and alternately taken for each of two pairs, i.e. pairs
`A-C and B-C, of the three channels. Channel C serves
`as a reference channel and is continuously supplied with
`one of the two velocity waveform sensor signals, and
`speci?cally for purposes of the preferred embodiment
`65
`the left velocity sensor signal, as its input signal. The
`input to channels A and B is either the left or right
`velocity sensor signals. While both the zero and mea
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`5,231,884
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`channel input from the right to the left velocity sensor
`FIGS. 3A and 3B collectively depict a high level
`signal, and channel pair A-C commencing ?ow based
`block diagram of a preferred embodiment of ?ow mea
`At measurements. Once this input