(12) United States Patent
`Henry et al.
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US006754594B2
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 6,754,594 B2
`*Jun.22,2004
`
`(54) DIGITAL FLOWMETER
`
`(75)
`
`Inventors: Manus P. Henry, Oxford (GB); David
`W. Clarke, Oxford (GB); James H.
`Vignos, Needham Heights, MA (US)
`
`(73) Assignee: Invensys Systems, Inc., Foxboro, MA
`(US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`This patent is subject to a terminal dis(cid:173)
`claimer.
`
`(21) Appl. No.: 09/931,057
`
`(22) Filed:
`
`Aug. 17, 2001
`
`(65)
`
`Prior Publication Data
`
`US 2002/0019710 Al Feb. 14, 2002
`
`(63)
`
`(60)
`
`(51)
`
`(52)
`(58)
`
`(56)
`
`Related U.S. Application Data
`
`Continuation of application No. 09/111,739, filed on Jul. 8,
`1998, now Pat. No. 6,311,136.
`Provisional application No. 60/066,554, filed on Nov. 26,
`1997.
`Int. Cl.7 . ... ... .. ... ... ... ... .. ... . GOlF 1/00; GOlF 23/00;
`GOlF 1/84
`U.S. Cl. ................ 702/45; 73/861.355; 73/861.356
`Field of Search ............................... 702/45-47, 50,
`702/100, 104-106; 73/861.355, 861.357
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`3,956,682 A
`RE29,383 E
`RE31,450 E
`
`5/1976 Van Dyck ................... 318/640
`9/1977 Gallatin et al. ............... 137 /14
`11/1983 Smith ... ... ... .. ... ... ... 73/861.356
`
`(List continued on next page.)
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`EP
`
`0 698 783 Al
`0 702 212 A2
`
`2/1996
`3/1996
`
`OTHER PUBLICATIONS
`
`Merriam-Webster's Collegiate Dictionary, Tenth Edition,
`1998, p. 747. *
`M. Henry et al., "The Implications of Digital Communica(cid:173)
`tions on Sensor Validation", Report No. QUEL 1912/92,
`University of Oxford, Department of Engineering Science,
`Apr. 1992.
`M.P. Henry et al., "Signal processing, Data Handling and
`Communications: The Case for Measurement Validation",
`Mar. 1992.
`M.P. Henry et al., "A New Approach to Sensor Validation",
`Improving Analyser Performance, IMC, Mar. 17, 1992.
`M.P. Henry, "Intelligent Behaviour For Self-Validating Sen(cid:173)
`sors", Advances in Measurement, pp 1-7 date unknown.
`J. Hemp et al.; "On the Theory and Performance of Coriolis
`Mass Flowmeters"; Proceedings of the International Con(cid:173)
`ference on Mass Flow Measurement Direct and Indirect;
`IBC Technical Services; 40 pages; Feb. 1989.
`Joseph DeCarlo; "True Mass-Flow Measurement"; Funda(cid:173)
`mentals of Flow Measurement, Unit 11-2; pp. 208-220;
`1984.
`David Spitzer; "Mass Flowmeters"; Industries Flow Mea(cid:173)
`surement, Chapter 12; pp. 197-210; 1990.
`
`Primary Examiner-Marc S. Hoff
`Assistant Examiner-Manuel L Barbee
`(74) Attorney, Agent, or Firm-Fish & Richardson P.C.
`
`(57)
`
`ABSTRACT
`
`A digital fiowmeter includes a vibratable conduit, a driver
`connected to the conduit and operable to impart motion to
`the conduit, and a sensor connected to the conduit and
`operable to sense the motion of the conduit. A control and
`measurement system is connected to the driver and the
`sensor. The control and measurement system includes cir(cid:173)
`cuitry to receive a sensor signal from the sensor, generate a
`drive signal based on the sensor signal using digital signal
`processing, supply the drive signal to the driver, and gen(cid:173)
`erate a measurement of a property of a two-phase material
`flowing through the conduit based on the signal from the
`sensor.
`
`EP
`
`0696726 A
`
`2/1996
`
`26 Claims, 71 Drawing Sheets
`
`Mas sf low
`Measurement
`
`100
`;V
`
`Temp.
`Sensor
`
`I
`I
`I
`
`'--D-riv_e.,..r _. ___ r:::-i ___ ,__ _ _,......
`115~, 120
`
`Micro Motion 1001
`
`1
`
`

`

`US 6,754,594 B2
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`4,419,898 A
`4,422,338 A
`4,491,025 A
`4,688,418 A
`4,727,746 A
`4,773,257 A
`4,782,711 A
`4,801,897 A
`4,817,448 A
`4,823,614 A *
`4,852,410 A
`4,879,911 A
`4,891,991 A
`4,895,030 A
`4,911,006 A
`4,911,020 A
`4,934,195 A
`4,934,196 A
`4,996,871 A
`5,027,662 A
`5,029,482 A
`5,050,439 A
`5,052,231 A
`5,054,313 A
`5,054,326 A
`5,218,869 A
`
`12/1983
`12/1983
`1/1985
`8/1987
`3/1988
`9/1988
`* 11/1988
`1/1989
`4/1989
`4/1989
`8/1989
`11/1989
`1/1990
`1/1990
`3/1990
`3/1990
`6/1990
`6/1990
`3/1991
`7/1991
`7/1991
`9/1991
`10/1991
`* 10/1991
`10/1991
`6/1993
`
`........... 73/861.02
`Zanker et al.
`Smith ... ... ... .. ... ... ... 73/861.356
`Smith et al. .. ... .. ... . 73/861.356
`Cheung et al.
`............ 73/29.01
`Mikasa et al. ............. 73/23.31
`Aslesen et al.
`............ 73/61.44
`Pratt ........................... 702/45
`Flecken . . . . . . . . . . . . . . . . . . . . . . . 331/65
`Hargarten et al.
`. ... . 73/861.356
`Dahlin ........................ 73/198
`Corwon et al. . ... ... . 73/861.356
`Zolock . ... ... .. ... ... ... 73/861.356
`Mattar et al. . .. ... ... . 73/861.357
`Bergamini et al. . ... . 73/861.355
`Hargarten et al.
`............ 73/198
`Thompson .. .. ... ... ... 73/861.356
`Hussain ... .. ... ... ... ... 73/861.355
`Romano .. ... .. ... ... ... 73/861.356
`Romano ..................... 73/32 A
`Titlow et al. . .. ... ... . 73/861.356
`Liu et al. ................. 73/861.04
`Thompson .. .. ... ... ... 73/861.356
`Christ et al. .. .. ... ... . 73/861.356
`Fitzgerald et al. ......... 73/54.27
`Mattar .. ... ... .. ... ... ... 73/861.355
`Pummer .. ... .. ... ... ... ... .. . 73/629
`
`7/1993 Bruck ........................ 73/1.34
`5,228,327 A
`11/1993 Kolpak .................. 73/861.355
`5,259,250 A
`12/1993 Mattar et al. .......... 73/861.355
`5,271,281 A
`9/1994 Matter et al. .......... 73/861.355
`5,343,764 A
`9/1994 Kalotay et al.
`........ 73/861.357
`5,347,874 A
`3/1995 Kalotay ................. 73/861.355
`5,400,653 A
`7/1995 Colman ................. 73/861.356
`5,429,002 A
`11/1995 Kalotay ................. 73/861.356
`5,469,748 A
`3/1996 Cage et al.
`............ 73/861.356
`5,497,665 A
`3/1996 Patten et al. ........... 73/861.355
`5,497,666 A
`7/1996 Kolpak .................... 73/861.04
`5,535,632 A
`9/1996 Derby et al. .................. 702/45
`5,555,190 A
`10/1996 Henry et al. .................. 702/45
`5,570,300 A
`11/1996 Yokoi et al. ........... 73/861.356
`5,578,764 A
`1/1997 Carpenter et al.
`....... 73/86.356
`5,594,180 A
`7/1997 Keel
`..................... 73/861.356
`5,648,616 A
`8/1997 Dutton .................... 73/152.18
`5,654,502 A
`5,687,100 A * 11/1997 Buttler et al. ............... 702/100
`5,732,193 A * 3/1998 Aberson ..................... 700/174
`5,774,378 A
`6/1998 Yang .......................... 702/104
`5,804,741 A
`9/1998 Freeman ................ 73/861.356
`7/1999 Mattar et al. ............... 390/606
`5,926,096 A
`6,073,495 A
`6/2000 Stadler . ... ... ... ... .. ... 73/861.356
`7/2000 Cunningham et al. . 73/861.356
`6,092,429 A
`6,311,136 Bl * 10/2001 Henry et al. .................. 702/45
`
`* cited by examiner-
`
`2
`
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`
`

`

`U.S. Patent
`
`Jun. 22, 2004
`
`Sheet 1 of 71
`
`US 6,754,594 B2
`
`-
`
`Mas sf low
`Measurement
`
`105
`J
`
`Digital
`Controller
`
`100
`;ii
`
`125
`Temp. _/
`Sensor
`
`Driver
`\ ...
`
`,._. ____
`
`115
`
`I
`I
`I
`I
`I
`
`Flow Tube
`
`.
`
`120
`FIG. 1
`
`----- Sensor -
`" 110
`
`3
`
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`

`

`U.S. Patent
`
`Jun.22,2004
`
`Sheet 7 of 71
`
`US 6,754,594 B2
`
`600
`JIL/
`
`6 05
`'-
`
`' '
`Collect
`Data
`,
`610
`'- Determine
`Frequency
`
`,
`
`615
`"- Determine/Eliminate
`Offset
`
`,
`
`I
`
`620
`'- Determine
`Amplitude
`
`6 25
`, '
`'- Determine
`Phase
`
`,,
`630
`' . ...__ Generate Drive
`Signal
`,
`
`635
`\.._
`
`Generate
`Measurements
`
`FIG. 6
`
`9
`
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`

`U.S. Patent
`
`.Jun. 22, 2004
`
`Sheet 11 of 71
`
`US 6,754,594 B2
`
`1000
`~
`
`1005
`'- Find f h
`'2, m, & m2
`tor d1, d2
`1'
`1010
`'- Determine
`
`f2m1, f1m2
`
`1 •
`
`101 5
`'- Determine Start and
`End of d1m2, d2m1
`,,
`1020
`'- Determine Phase,
`Amplitude for d, and
`d2m1, and Phase
`Difference
`
`1 025
`'- Repeat for
`
`1 '
`
`d2, d1m2
`
`' '
`103 0
`'- Generate Averages
`of Amplitude &
`Phase Differences
`
`FIG .10
`
`13
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`

`

`U.S. Patent
`
`Jun.22,2004
`
`Sheet 15 of 71
`
`US 6,754,594 B2
`
`1405
`'- Estimate Nominal
`Operating Frequency
`
`14 10
`'--
`
`, r
`
`Synthesize
`Nominal Signals
`
`1 •
`
`141 5
`'- Multiply by Original
`Sensor Signals
`
`142 0
`'-
`
`I
`
`Eliminate
`High f Components
`
`.
`
`I
`
`14 25
`'- Combine Signals
`to Produce u[k]
`,,
`143 0
`'- Calculate Frequency
`Deviation
`
`143 5
`'-
`
`' I
`
`Add to
`Nominal Frequency
`
`14 40
`'-....
`
`14 45
`'-
`
`1 •
`
`Determine
`Amplitude
`
`r
`
`Determine
`Phase Difference
`
`,v1400
`
`FIG.14
`
`17
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`U.S. Patent
`
`Jun. 22, 2004
`
`Sheet 27 of 71
`
`US 6,754,594 B2
`
`2600
`~
`
`2605
`"- Produce Frequency
`Estimate
`, '
`261 0
`"- Estimate Amplitude
`& Phase
`
`1 I
`
`261 5
`'- Calculate Phase
`Difference
`, '
`2620
`'- Determine Amplitude
`, ,
`2 625
`'- Correct
`Frequency
`
`Rate of Change
`
`u
`
`26 30
`'- Compensate
`Phase
`
`FIG. 26
`
`29
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`US 6,754,594 B2
`
`1
`DIGITAL FLOWMETER
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`This application is a continuation of U.S. application Ser.
`No. 09/111,739, filed Jul. 8, 1998, and entitled DIGITAL
`FLOWMETER, now U.S. Pat. No. 6,311,136, which claims
`priority from U.S. Provisional Application No. 60/066,554,
`filed Nov. 26, 1997, and entitled DIGITAL FLOWMETER,
`which is incorporated by reference.
`
`TECHNICAL FIELD
`
`The invention relates to flowmeters.
`
`BACKGROUND
`
`Flowmeters provide information about materials being
`transferred through a conduit. For example, mass flowmeters
`provide a direct indication of the mass of material being
`transferred through a conduit. Similarly, density flowmeters,
`or densitometers, provide an indication of the density of
`material flowing through a conduit. Mass flowmeters also
`may provide an indication of the density of the material.
`Coriolis-type mass flowmeters are based on the well(cid:173)
`known Coriolis effect, in which material flowing through a
`rotating conduit becomes a radially traveling mass that is
`affected by a Coriolis force and therefore experiences an
`acceleration. Many Coriolis-type mass flowmeters induce a
`Coriolis force by sinusoidally oscillating a conduit about a
`pivot axis orthogonal to the length of the conduit. In such
`mass flowmeters, the Coriolis reaction force experienced by
`the traveling fluid mass is transferred to the conduit itself
`and is manifested as a deflection or offset of the conduit in
`the direction of the Coriolis force vector in the plane of
`rotation.
`Energy is supplied to the conduit by a driving mechanism
`that applies a periodic force to oscillate the conduit. One
`type of driving mechanism is an electromechanical driver
`that imparts a force proportional to an applied voltage. In an
`oscillating flowmeter, the applied voltage is periodic, and is
`generally sinusoidal. The period of the input voltage is
`chosen so that the motion of the conduit matches a resonant
`mode of vibration of the conduit. This reduces the energy
`needed to sustain oscillation. An oscillating flowmeter may
`use a feedback loop in which a sensor signal that carries
`instantaneous frequency and phase information related to
`oscillation of the conduit is amplified and fed back to the
`conduit using the electromechanical driver.
`
`SUMMARY
`
`The invention provides a digital flowmeter, such as a
`digital mass flowmeter, that uses a control and measurement
`system to control oscillation of the conduit and to generate
`mass flow and density measurements. Sensors connected to
`the conduit supply signals to the control and measurement
`system. The control and measurement system processes the
`signals to produce a measurement of mass flow and uses
`digital signal processing to generate a signal for driving the
`conduit. The drive signal then is converted to a force that
`induces oscillation of the conduit.
`The digital mass flowmeter provides a number of advan(cid:173)
`tages over traditional, analog approaches. From a control
`perspective, use of digital processing techniques permits the
`application of precise, sophisticated control algorithms that,
`relative to traditional analog approaches, provide greater
`responsiveness, accuracy and adaptability.
`
`10
`
`2
`The digital control system also permits the use of negative
`gain in controlling oscillation of the conduit. Thus, drive
`signals that are 180° out of phase with conduit oscillation
`may be used to reduce the amplitude of oscillation. The
`5 practical implications of this are important, particularly in
`high and variable damping situations where a sudden drop in
`damping can cause an undesirable increase in the amplitude
`of oscillation. One example of a variable damping situation
`is when aeration occurs in the material flowing through the
`conduit.
`The ability to provide negative feedback also is important
`when the amplitude of oscillation is controlled to a fixed
`setpoint that can be changed under user control. With
`negative feedback, reductions in the oscillation setpoint can
`be implemented as quickly as increases in the setpoint. By
`15 contrast, an analog meter that relies solely on positive
`feedback must set the gain to zero and wait for system
`damping to reduce the amplitude to the reduced setpoint.
`From a measurement perspective, the digital mass flow(cid:173)
`meter can provide high information bandwidth. For
`20 example, a digital measurement system may use analog-to(cid:173)
`digital converters operating at eighteen bits of precision and
`sampling rates of 55 kHz. The digital measurement system
`also may use sophisticated algorithms to filter and process
`the data, and may do so starting with the raw data from the
`25 sensors and continuing to the final measurement data. This
`permits extremely high precision, such as, for example,
`phase precision to five nanoseconds per cycle. Digital pro(cid:173)
`cessing starting with the raw sensor data also allows for
`extensions to existing measurement techniques to improve
`30 performance in non-ideal situations, such as by detecting
`and compensating for time-varying amplitude, frequency,
`and zero offset.
`The control and measurement improvements interact to
`provide further improvements. For example, control of
`oscillation amplitude is dependent upon the quality of ampli(cid:173)
`tude measurement. Under normal conditions, the digital
`mass flowmeter may maintain oscillation to within twenty
`parts per million of the desired setpoint. Similarly, improved
`control has a positive benefit on measurement. Increasing
`40 the stability of oscillation will improve measurement quality
`even for meters that do not require a fixed amplitude of
`oscillation (i.e., a fixed setpoint). For example, with
`improved stability, assumptions used for the measurement
`calculations are likely to be valid over a wider range of
`45 conditions.
`The digital mass flowmeter also permits the integration of
`entirely new functionality (e.g., diagnostics) with the mea(cid:173)
`surement and control processes. For example, algorithms for
`detecting the presence of process aeration can be imple-
`50 mented with compensatory action occurring for both mea(cid:173)
`surement and control if aeration is detected.
`Other advantages of the digital mass flowmeter result
`from the limited amount of hardware employed, which
`makes the meter simple to construct, debug, and repair in
`55 production and in the field. Quick repairs in the field for
`improved performance and to compensate for wear of the
`mechanical components (e.g., loops, flanges, sensors and
`drivers) are possible because the meter uses standardized
`hardware components that may be replaced with little
`60 difficulty, and because software modifications may be made
`with relative ease. In addition, integration of diagnostics,
`measurement, and control is simplified by the simplicity of
`the hardware and the level of functionality implemented in
`software. New functionality, such as low power components
`65 or components with improved performance, can be inte(cid:173)
`grated without a major redesign of the overall control
`system.
`
`35
`
`74
`
`Invensys Ex. 2023
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`
`

`

`US 6,754,594 B2
`
`3
`In one general aspect, a digital flowmeter includes a
`vibratable conduit, a driver connected to the conduit and
`operable to impart motion to the conduit, and a sensor
`connected to the conduit and operable to sense the motion of
`the conduit. A control and measurement system connected to
`the driver and the sensor includes circuitry that receives a
`sensor signal from the sensor, generates a drive signal based
`on the sensor signal using digital signal processing, supplies
`the drive signal to the driver, and generates a measurement
`of a property of material flowing through the conduit based
`on the signal from the sensor.
`Embodiments may include one or more of the following
`features. The meter may include a second sensor connected
`to the conduit and operable to sense the motion of the
`conduit. In this case, the control and measurement system is
`connected to the second sensor and receives a second sensor
`signal from the second sensor, generates the drive signal
`based on the first and second sensor signals, and generates
`the measurement of the property of material flowing through
`the conduit based on the first and second sensor signals. The
`control and measurement system may digitally combine the
`first and second sensor signals and generate the drive signal
`based on the combination of the sensor signals.
`The control and measurement system may generate dif(cid:173)
`ferent drive signals for the two drivers. The drive signals
`may have, for example, different frequencies or amplitudes.
`The digital flowmeter also may include circuitry for
`measuring current supplied to the driver. The circuitry may
`include a resistor in series with the driver and an analog(cid:173)
`to-digital converter in parallel with the resistor and config(cid:173)
`ured to measure a voltage across the resistor, to convert the
`measured voltage to a digital value, and to supply the digital
`value to the control and measurement system.
`The digital flowmeter also may include a first pressure
`sensor connected to measure a first pressure at an inlet to the
`conduit and a second pressure sensor connected to measure
`a second pressure at an outlet of the conduit. Analog-to(cid:173)
`digital converters may be connected and configured to
`convert signals produced by the first pressure sensor and the 40
`second pressure sensor to digital values and to supply the
`digital values to the control and measurement system. Tem(cid:173)
`perature sensors may be connected to measure temperatures
`at the inlet and outlet of the conduit.
`The control and measurement system may generate the
`measurement of the property by estimating a frequency of
`the first sensor signal, calculating a phase difference using
`the first sensor signal, and generating the measurement using
`the calculated phase difference. The control and measure(cid:173)
`ment system may compensate for amplitude differences in
`the sensor signals by adjusting the amplitude of one of the
`sensor signals. For example, the control and measurement
`system may multiply the amplitude of one of the sensor
`signals by a ratio of the amplitudes of the sensor signals.
`When the sensor signal is generally periodic, the control
`and measurement system may process the sensor signal in
`sets. Each set may include data for a complete cycle of the
`periodic sensor signal, and consecutive sets may include
`data for overlapping cycles of the periodic sensor signal. The
`control and measurement system may estimate an end point
`of a cycle using a frequency of a previous cycle.
`The control and measurement system may analyze data
`for a cycle to determine whether the cycle merits further
`processing. For example, the system may determine that a
`cycle does not merit further processing when data for the
`cycle does not conform to expected behavior for the data,
`where the expected behavior may be based on one or more
`
`4
`parameters of a previous cycle. In one implementation, the
`system determines that a cycle does not merit further pro(cid:173)
`cessing when a frequency for the cycle differs from a
`frequency for the previous cycle by more than a threshold
`5 amount. The system may determine whether the frequencies
`differ by comparing values at certain points in the cycle to
`values that would occur if the frequency for the cycle
`equaled the frequency for the previous cycle.
`The control and measurement system may determine a
`10 frequency of the sensor signal by detecting zero-crossings of
`the sensor signal and counting samples between zero cross(cid:173)
`ings. The system also may determine a frequency of the
`sensor signal using an iterative curve fitting technique.
`The control and measurement system may determine an
`15 amplitude of the sensor signal using Fourier analysis, and
`may use the determined amplitude in generating the drive
`signal.
`The control and measurement system may determine a
`phase offset for each sensor signal and may determine the
`20 phase difference by comparing the phase offsets. The system
`also may determine the phase difference using Fourier
`analysis. The control and measurement system may deter(cid:173)
`mine a frequency, amplitude and phase offset for each sensor
`signal, and may scale the phase offsets to an average of the
`25 frequencies of the sensor signals. The control and measure(cid:173)
`ment system may calculate the phase difference using mul(cid:173)
`tiple approaches and may select a result of one of the
`approaches as the calculated phase difference.
`The control and measurement system may combine the
`sensor signals to produce a combined signal and may
`generate the drive signal based on the combined signal. For
`example, the control and measurement system may sum the
`sensor signals to produce the combined signal and may
`35 generate the drive signal by applying a gain to the combined
`signal.
`In general, the control and measurement system may
`initiate motion of the conduit by using a first mode of signal
`generation to generate the drive signal, and may sustain
`motion of the conduit using a second mode of signal
`generation to generate the drive signal. The first mode of
`signal generation may be synthesis of a periodic signal
`having a desired property, such as a desired initial frequency
`of conduit vibration, and the second mode of signal genera-
`45 tion may use a feedback loop including the sensor signal.
`In other instances, the first mode of signal generation may
`include use of a feedback loop including the sensor signal
`and the second mode of signal generation may include
`synthesis of a periodic signal having a desired property. For
`50 example, the control and measurement system may generate
`the drive signal by applying a large gain to the combined
`signal to initiate motion of the conduit and generating a
`periodic signal having a phase and frequency based on a
`phase and frequency of a sensor signal as the drive signal
`55 after motion has been initiated. The desired property of the
`synthesized signal may be a frequency and a phase corre(cid:173)
`sponding to a frequency and a phase of the sensor signal.
`The control and measurement system generates an
`adaptable, periodic drive signal. For example, the meter may
`60 include positive and negative direct current sources con(cid:173)
`nected between the control and measurement system and the
`driver, and the control and measurement system may gen(cid:173)
`erate the drive signal by switching the current sources on and
`off at intervals having a phase and frequency based on the
`65 sensor signal. The control and measurement system may
`generate the drive signal by synthesizing a sine wave having
`a property corresponding to a property of the sensor signal,
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`such as a phase and a frequency corresponding to a phase
`and a frequency of the sensor signal.
`The control and measurement system may digitally gen(cid:173)
`erate a gain for use in generating the drive signal based on
`one or more properties of the sensor signal. For example, the 5
`control and measurement system may digitally generate the
`gain based on an amplitude of the sensor signal.
`The driver may be operable to impart an oscillating
`motion to the conduit. The control and measurement system
`also may digitally implement a PI control algorithm to
`regulate the amplitude of conduit oscillation. The control
`and measurement system also may digitally generate the
`drive signal based on the sensor signal so as to maintain an
`amplitude of oscillation of the conduit at a user-controlled
`value. In support of this, the control and measurement
`system may generate a negative drive signal that causes the
`driver to resist motion of the conduit when the amplitude of
`oscillation exceeds the user-controlled value and a positive
`drive signal that causes the driver to impart motion to the
`conduit when the amplitude of oscillation is less than the 20
`user-controlled value.
`The control and measurement system may include a
`controller that generates a gain signal based on the sensor
`signal and a multiplying digital-to-analog converter con(cid:173)
`nected to the controller to receive the gain signal and 25
`generate the drive signal based on the gain signal.
`When the digital flowmeter includes a second sensor
`connected to the conduit and operable to sense the motion of
`the conduit, the control and measurement system may
`include a controller that generates the measurement, a first
`analog-to-digital converter connected between the first sen(cid:173)
`sor and the controller to provide a first digital sensor signal
`to the controller, and a second analog-to-digital converter
`connected between the second sensor and the controller to 35
`provide a second digital sensor signal to the controller. The
`controller may combine the digital sensor signals to produce
`a combined signal and to generate a gain signal based on the
`first and second digital sensor signals. The control and
`measurement system also may include a multiplying digital(cid:173)
`to-analog converter connected to receive the combined
`signal and the gain signal from the controller to generate the
`drive signal as a product of the combined signal and the gain
`signal.
`The control and measurement system may selectively
`apply a negative gain to the sensor signal to reduce motion
`of the conduit.
`The control and measurement system also may compen(cid:173)
`sate for zero offset in the sensor signal. The zero offset may
`include a component attributable to gain variation and a
`component attributable to gain nonlinearity, and the control
`and measurement system may separately compensate for the
`two components. The control and measurement system may
`compensate for zero offset by generating one or more
`correction factors and modifying the sensor signal using the
`correction factors.
`The control and measurement system may calculate phase
`offsets for the first and second sensor signals. The phase
`offset may be defined as a difference between a zero(cid:173)
`crossing point of a sensor signal and a point of zero phase
`for a component of the sensor signal corresponding to a
`fundamental frequency of the sensor signal. The control and
`measurement system may combine the calculated phase
`offsets to produce a phase difference.
`The control and measurement system may generate the
`measurement of the property by estimating a frequency of
`the first sensor signal, estimating a frequency of the second
`
`6
`sensor signal, with the frequency of the second sensor signal
`being different from the frequency of the first sensor signal,
`and calculating a phase difference between the sensor sig-
`nals using the estimated frequencies.
`When the sensor is a velocity sensor, the control and
`measurement system may estimate a frequency, amplitude,
`and phase of the sensor signal, and may correct the estimated
`frequency, amplitude, and phase to account for performance
`differences between a velocity sensor and an absolute posi-
`10 tion sensor. Instead of controlling the apparent amplitude of
`oscillation (i.e., the velocity of oscillation when the sensor
`is a velocity sensor), the system may control the true
`amplitude by dividing the sensor signal by the estimated
`frequency. This correction should provide improved ampli-
`15 tude control and noise reduction.
`The control and measurement system may estimate a first
`parameter of the sensor signal, determine a rate of change of
`a second parameter, and correct the estimated first parameter
`based on the determined rate of change. For example, the
`system may correct an estimated frequency, amplitude, or
`phase of the sensor signal based on a determined rate of
`change of the frequency or amplitude of oscillation of the
`conduit. The system may perform separate corrections for
`each sensor signal.
`The digital flowmeter may be a mass flowmeter and the
`property of material flowing through the conduit may be a
`mass flow rate. The digital flowmeter also may be a densi(cid:173)
`tometer and the property of material flowing through the
`conduit may be a density of the material.
`The control and measurement system may account for
`effects of aeration in the conduit by determining an initial
`mass flow rate, determining an apparent density of material
`flowing through the conduit, comparing the apparent density
`to a known density of the material to determine a density
`difference, and adjusting the initial mass flow rate based on
`the density difference to produce an adjusted mass flow rate.
`The system may further account for effects of aeration in the
`conduit by adjusting the adjusted mass flow rate to account
`40 for effects of damping. To further account for effects of
`aeration in the conduit, the system may adjust the adjusted
`mass flow rate based on differences between amplitudes of
`the first and second sensor signals.
`The vibratable conduit may include two parallel planar
`45 loops. The sensor and driver may be connected between the
`loops.
`The meter may include a power circuit that receives
`power on only a single pair of wires. The power circuit
`provides power to the digital control and measurement
`50 system and to the driver, and the digital control and mea(cid:173)
`surement system is operable to transmit the measurement of
`the property of material flowing through the conduit on the
`single pair of wires. The power circuit may include a
`constant output circuit that provides power to the digital
`55 control and measurement system and drive capacitor that is
`charged by excess power from the two wires. The digital
`control and measurement system may discharge the drive
`capacitor to power the driver, and may monitor a charge
`level of the drive capacitor and discharge the drive capacitor
`60 after a charge level of the capacitor reaches a threshold level.
`The digital control and measurement system also may
`discharge the drive capacitor periodically and to perform
`bidirectional communications on the pair of wires.
`The control and measurement system may collect a first
`65 data set for a period of the periodic signal and process the
`first data set to generate the drive signal and the measure(cid:173)
`ment. The system may collect a second data set for a
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`subsequent period of the sensor signal simultaneously with
`processing the first data set. The period corresponding to the
`first data set may overlap the period corresponding to the
`second data set.
`The control and measurement system may control the
`drive signal to maintain an amplitude of the sensor signal at
`a fixed setpoint, reduce the fixed setpoint when the drive
`signal exceeds a first threshold level, and increase the fixed
`setpoint when the drive signal is less than a second threshold
`level and the fixed setpoint is less than a maximum permitted 10
`value for the setpoint. The first threshold level may be 95%
`or