United States Patent [191
`Kalotay
`
`mm mm mum ||||| {1131156111 [18111111 |||||||||||||| un
`5,469,748
`Nov. 28, 1995
`
`[11] Patent Number:
`[45] Date of Patent:
`
`[54] NOISE REDUCTION FILTER SYSTEM FOR
`A CORIOLIS FLOWMETER
`
`[75] Inventor: Paul Z. Kalotay, Lafayette, C010.
`
`[73] Assignee: Micro Motion, Inc., Boulder, C010.
`
`[21] Appl. No.: 278,547
`[22] Filed:
`Jul. 20, 1994
`
`[51] Int. Cl.6 ...................................................... .. G01F 1/68
`[52] US. Cl. ....................................... .. 731861.38; 324/601
`[58] Field of Search .......................... .. 73/861.37, 861.38,
`73/861; 324/601
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`Re. 31,450 11/1983 Smith ................................. .. 73/86138
`4,109,524 8/1978 Smith
`73/194 B
`4,491,025
`1/1985 Smith
`.., 737861.38
`4,879,911 11/1989 Zolock
`737861.38
`
`5,228,327 7/1993 Bruck . . . . . . . .
`
`. . . . . . . . . . .. 73/3
`
`73/86138
`8/1993 Zolock .... ..
`5,231,884
`.. 73/86138
`5,331,859 7/1994 Zolock .... ..
`5,343,761
`9/1994 Myers ..................................... .. 73/861
`
`Pn'mary Examiner—Hezron E. Williams
`Assistant Examiner—Harshad Patel
`Attorney, Agent, or F irm-Duft, Graziano & Forest
`[57]
`ABSTRACT
`
`A noise reduction system and method for measuring the
`phase difference between output signals of a Coriolis ?ow
`meter. The output signals are applied to signal processing
`circuitry having three measurement channels, each of which
`includes a multi-pole ?lter having a relatively large phase
`shift. A channel pair is alternately switched in successive
`time intervals between a calibration status and an active
`status. In the calibration status, the two channels are con
`nected during one time intervals to the same input signal and
`the output signals of the two channels are measured to
`determine the inherent phase delay between the two cali
`bration channels, The two channels are then switched during
`the next time interval to an active status in which they are
`connected separately to the two output signals from the
`?owmeter. The output signals of the channels are then
`measured and the resultant measured phase delay is alge
`braicly combined with the phase delay measured during the
`calibration status to determine the true phase delay between
`the two signals received from the Coriolis ?owmeter.
`
`11 Claims, 2 Drawing Sheets
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`Nov. 28, 1995
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`Nov. 28, 1995
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`

`

`1
`NOISE REDUCTION FILTER SYSTEM FOR
`A CORIOLIS FLOWMETER
`
`FIELD OF THE INVENTION
`
`This invention relates to a ?lter system and more particu
`larly to a noise reduction ?lter system for a Coriolis ?ow
`meter.
`
`PROBLEM
`
`It is known to use Coriolis effect mass ?owmeters to
`measure mass ?ow and other information for materials
`?owing through a conduit. Such ?owmeters are disclosed in
`U.S. Pat. Nos. 4,109,524 of Aug. 29, 1978, 4,491,025 of Jan.
`1, 1985, and Re. 31,450 of Feb. 11, 1982, all to J. E. Smith
`et a1. These ?owmeters have one or more ?ow tubes of
`straight or curved con?guration. Each ?ow tube con?gura
`tion in a Coriolis mass ?owmeter has a set of natural
`vibration modes, which may be of a simple bending, tor
`sional or coupled type. Each ?ow tube is driven to oscillate
`at resonance in one of these natural modes. Material ?ows
`into the ?owmeter ?om a connected conduit on the inlet side
`of the ?owmeter, is directed through the ?ow tube or tubes,
`and exits the ?owmeter through the outlet side. The natural
`vibration modes of the vibrating, ?uid ?lled system are
`de?ned in part by the combined mass of the ?ow tubes and
`the material within the ?ow tubes.
`When there is no ?ow through the ?owmeter, all points
`along the ?ow tube oscillate with identical phase due to an
`applied driver force. As material begins to ?ow, Coriolis
`accelerations cause each point along the ?ow tube to have a
`different phase. The phase on the inlet side of the ?ow tube
`lags the driver, while the phase on the outlet side leads the
`driver. Sensors are placed on the ?ow tube to produce
`sinusoidal signals representative of the motion of the ?ow
`tube. The phase di?’erence between two sensor signals is
`proportional to the mass ?ow rate of material through the
`?ow tube.
`A complicating factor in this measurement is that the
`density of typical process ?uids varies. Changes in density
`cause the frequencies of the natural modes to vary. Since the
`?owmeter’s control system maintains resonance, the oscil
`lation frequency varies in response to changes in density.
`Mass ?ow rate in this situation is proportional to the ratio of
`phase difference and oscillation frequency.
`The above~mentioned US. Pat. No. Re. 31,450 to Smith
`discloses a Coriolis ?owmeter that avoids the need for
`measuring both phase difference and oscillation frequency.
`Phase difference is determined by measuring the time delay
`between level crossings of the two sinusoidal signals of the
`?owmeter. When this method is used, the variations in the
`oscillation frequency cancel, and mass ?ow rate is propor
`tional to the measured time delay. This measurement method
`is hereinafter referred to as a time delay or At measurement.
`Information regarding the characteristics of material
`?owing in a Coriolis mass ?owmeter is typically derived by
`instrumentation which measures the phase or time delay
`between two output signals of the sensors of the ?owmeter.
`These measurements must be made with great accuracy
`since this is often a requirement that the derived ?ow rate
`information have an accuracy of at least 0.15% of reading.
`These ?owmeter output signals are sinusoidal and are dis—
`placed in time or phase by an amount determined by the
`Coriolis forces generated by the meter through which the
`material ?ows. The signal processing circuitry which
`receives these sensor output signals measures this phase
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`difference with precision and generates the desired charac
`teristics of the ?owing process material to the required
`accuracy of at least 0.15% of reading.
`In order to achieve these accuracies, it is necessary that
`the signal processing circuitry operate with precision in
`measuring the phase shift of the two signals it receives from
`the ?owmeter. Since the phase shift between the two output
`signals of the meter is the information used by the process
`ing circuitry to derive the material characteristics, it is
`necessary that the processing circuitry not introduce any
`phase shift which would mask the phase shift information
`provided by the meter output signals. In practice, it is
`necessary that this processing circuitry have an extremely
`low inherent phase shift so that the phase of each input
`signal is shifted by less than 0.001° and, in some cases, less
`than a few parts per million. Phase accuracy of this magni
`tude is required if the derived information regarding the
`process material is to have an accuracy of less than 0.15%.
`The frequencies of the Coriolis ?owmeter output signals
`fall in the frequency range of many industrially generated
`noises. Also, the amplitude of the meter output signals is
`often small and, in many cases, is not signi?cantly above the
`amplitude of the noise signals. This linrits the sensitivity of
`the ?owmeter and makes the extraction of the useful infor
`mation quite di?icult.
`There is not much a designer can do either to move the
`meter output signal frequency out of the noise band or to
`increase the amplitude of the output signal. Practical Corio
`lis sensor and ?owmeter design requires compromises that
`result in the generation of an output signal having a less than
`optimum signal to noise ratio and dynamic range. This
`limitation determines the ?owmeter characteristics and
`speci?cations including the minimum and maximum ?ow
`rates which may be reliably derived from the ?owmeter’s
`output signals.
`The magnitude of the minimum time delay that can be
`measured between the two Coriolis ?owmeter output signals
`at a given drive frequency is limited by various factors
`including the signal to noise ratio, the complexity of asso
`ciated circuitry and hardware, and economic considerations
`that limit the cost and complexity of the associated circuitry
`and hardware. Also, in order to achieve a ?owmeter that is
`economically attractive, the low limit of time delay mea
`surement must be as low as possible. The processing cir
`cuitry that receives the two output signals must be able to
`reliably measure the time delay between the two signals in
`order to provide a meter having the high sensitivity needed
`to measure the ?owing characteristics of materials having a
`low density and mass such as, for example, gases.
`There are limitations regarding the extent to which con
`ventional circuit design can, by itself, permit accurate time
`delay measurements under all possible operating conditions
`of a Coriolis ?owmeter. These limitations are due to the
`inherent noise present in any electronic equipment including
`the imperfections of semi-conductor devices and noise gen
`erated by other circuit elements. These limitations are also
`due to ambient noise which similarly limits the measurement
`can be reduced to some extent by techniques such as
`shielding, guarding, grounding, etc. Another limitation is the
`signal to noise ratio of the sensor output signals themselves.
`Good circuit design can overcome some of the problems
`regarding noise in the electronic equipment as well as the
`ambient noise in the environment. However, an improve
`ment in the signal to noise ratio of the output signals cannot
`be achieved without the use of ?lters. But ?lters alter the
`amplitude and phase of the signals to be processed. This is
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`5,469,748
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`3
`undesirable, since the time delay between the two signals is
`the base information used to derive characteristics of the
`process ?uid. The use of ?lters having unknown or varying
`amplitude and/or phase characteristics can unacceptably
`alter the phase difference between the two sensor output
`signals and preclude the derivation of accurate information
`of the ?owing material.
`The ?owmeter’s drive signal is typically derived from one
`of the pick-off output signals after it is conditioned, phase
`shifted and used to produce the sinusoidal drive voltage for
`the drive coil of the meter. This has the disadvantage that
`harmonics and noise components present in the pick-off
`signal are ampli?ed and applied to the drive coil to vibrate
`the ?ow tubes at their resonant frequency. However, an
`undesirable drive signal can be generated by unwanted
`mechanical vibrations and electrical interferences that are
`fed back to the meter drive circuit and reinforced in a closed
`loop so that they create relatively high amplitude self
`perpetuating disturbing signals which further degrade the
`precision and accuracy of the time delay measurement.
`A successful technique to reduce some of the above
`problems is described in U.S. Pat. No. 5,231,884 to M.
`Zolock and US. Pat. No. 5,228,327 to Bruck. These patents
`describe Coriolis ?owmeter signal processing circuitry that
`uses three identical channels having precision integrators as
`?lters. A ?rst one of these channels is permanently con
`nected to one pick-off signal, say, for example, the left. The
`other two channels (second and third) are alternately con
`nected in successive time intervals, one at a time, to the right
`pick-01f signal. When one of these, say the second channel,
`is connected to the right pick-off signal, the third channel is
`connected, along with the ?rst channel, to the left pick-off
`signal. The inherent phase delay between the ?rst and third
`channel is measured during a ?rst time interval by compar
`ing the time diiference between the outputs of the two
`channels now both connected to the left signal. Once this
`characteristic delay is determined, the role of this third
`channel and the second channel connected to the right
`pick-off signal is switched during a second time interval. In
`this new configuration, the second channel undergoes a
`calibration of its delay characteristics while the third cali
`brated channel is connected to the right pick-o?.D signal. The
`roles of second and third channels are alternately switched
`by a control circuit approximately once every minute. Dur
`ing this one-minute interval (about 30 to 60 seconds), aging,
`temperature, and other effects have no meaningful in?uence
`on the phase-shift of the ?lters and therefore their phase
`relationship is known and considered de?ned.
`The precisely calibrated integrators used by Zolock pro
`vide a signal to noise ratio improvement amounting to about
`6 db/octave roll-off in the amplitude transfer function of the
`integrator. Unfortunately, this 6 db/octave improvement is
`not enough under many circumstances in which Coriolis
`?owmeters are operated. The reason for this is that a
`single-pole ?lter, such as the Zolock integrator, has a rela
`tively wide band width. As a result, noise signals generated
`by unwanted ?ow tube vibration modes, noisy environment,
`material ?ow noise, electromagnetic or radio frequency
`interference and disturbances are not removed to the extent
`necessary for high meter sensitivity required for precision.
`Depending on their frequency, their amplitude is reduced
`somewhat, but they can still interfere with the precision time
`delay measurement between the two pick-01f output signals
`when measuring low mass materials such as gases.
`There is another source for errors in the Zolock and Bruck
`system. The integrator time delay measurements are made at
`three (3) certain well de?ned points of the sinusoidal pick
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`off signals. The two pick-off signals are ideal only when
`their shape is the same and is symmetrical around their peak
`values. However, when the two magnetic circuits (sensors)
`that generate the pick-off signals are not identical, the
`resulting non-ideal wave forms contain di?‘erent amounts of
`harmonics with possibly unde?ned phase conditions which
`can alter their shape and potentially change their symmetri
`cal character. The result of such variations is such that when,
`during normal operations, a Zolock integrator is calibrated
`with one wave form and is subsequently used to measure
`another wave form, the difference in wave forms may
`produce an unde?ned and unknown amount of error due to
`its harmonic content and its unde?ned and varying phase of
`its harmonics.
`There are techniques currently available, such as digital
`signal processing and ?ltering, to overcome the above
`discussed problems and simultaneously improve the signal
`to noise ratio of the signals being processed. However, these
`alternatives are complicated, expensive, and in most cases
`require design compromises that render their use less than
`ideal. It can therefore be seen that it is a problem to process
`the output signals of Coriolis ?owmeters by circuitry that
`maintains the original phase shift between the two output
`signals and that by itself does not generate any unknown and
`unwanted phase shifts or other signal alterations which
`could degrade the accuracy of the output information gen‘
`erated by the processing circuitry regarding the character
`istics of the material ?owing through the ?owmeter.
`
`SOLUTION
`
`The above problems are solved and an advance in the art
`is achieved by the present invention which provides addi
`tional and improved ?ltering apparatus and methods for
`Coriolis ?owmeter output signal processing circuitry that
`uses the techniques of Zolock and Bruck. The integrating
`ampli?ers of Zolock are relatively broad band 6 db/octave
`type ?lters that are primarily effective to ?lter out frequen
`cies substantially higher than that of the meter output signals
`while maintaining a precise amount of phase shift in?uenced
`only by minor component variations. Since they are not of
`the sharp cut-off type, they are not effective in reducing and
`eliminating noise signals having a frequency close to that of
`the meter output signals. Thus, even though the Zolock
`self-calibrating feature eliminates errors due to long-term
`phase shift, the undesirable noise signals immediately adja
`cent the frequency of the process signals remain in the
`output of the Zolock circuit.
`In accordance with my invention, I provide a multi-pole
`?lter, such as a ?lter having eight or more poles, ahead of the
`precision integrators of the Zolock type. Since my ?lters are
`of the multi-pole type, they have a sharp roll-o?’ character
`istic and provide a narrow pass band that e?ectively elimi
`nates all noise signals having a frequency near that of the
`processed ?owmeter pick-01f signals. The use of my multi
`pole ?lters with the Zolock circuitry permits better ?ltering
`of undesirable signals so that the processed Coriolis pick-01f
`signals have signi?cantly reduced undesirable noise com
`ponents. These signals can therefore be processed to gener
`ate information of improved precision regarding various
`characteristics of the process material.
`Even with the use of my multi-pole ?lter, it is still
`necessary to meet the requirement of less than 0.001° of
`phase shift stability so that the time delay between the two
`pick-off signals can be measured with required precision.
`My multi~pole ?lters may have an unde?ned and unknown
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`phase shift that can amount to several hundred degrees or
`more. Normally, it might present a problem to insert a ?lter
`having a large, and unknown phase shift of several hundred
`degrees or more into a circuit that measures the time
`difference between two signals to an accuracy of 0.001° or
`better. However, the phase shift of my multi-pole ?lters,
`although large and unde?ned, is relatively constant over a
`period of a few minutes. The Zolock system performs its
`customary channel switching and calibration so that any
`errors due to phase shifts between channels are effectively
`canceled.
`The use of my multi-pole sharp cutoff ?lters into the
`Zolock system permits the resultant system to have a very
`high db/octave roll-off characteristics so as to constitute a
`sharp cut-off ?lter that eliminates substantially all unwanted
`noise from the pick~oif signals being processed. The phase
`shift of my multi-pole ?lters do not present a problem since
`they change slowly only over a relatively long period of
`time, as active and passive components age. Any phase shift
`is detected once per minute by the Zolock calibration
`circuitry as the status of the switchable charmels is changed
`during each successive interval from active to stand-by to
`calibrate and vice-versa.
`Another advantage of my invention is that the drive
`circuit can utilize a ?ltered pick-01f signal from which
`undesirable components have been removed by the ?ltering
`provided in accordance with my invention. The resulting
`improved sinusoidal drive signal does not in itself excite and
`generate further undesirable modes that can contribute to
`noisy pick-off signals.
`
`DESCRIPTION OF THE DRAWINGS
`
`These and other advantages and features of the invention
`can be better understood from a reading of the following
`description thereof taken in conjunction with the drawing in
`which;
`FIG. 1 discloses the invention embodied in a Coriolis
`material flow measurement system.
`FIG. 2 discloses further details of the meter electronics
`element of FIG. 1.
`
`DETAILED DESCRIPTION
`
`FIG. 1 shows a Coriolis meter assembly 10 and meter
`electronics 20. Meter electronics 20 are connected to meter
`assembly 10 via leads 100 to provide density, mass ?ow rate,
`volume ?ow rate and totalized mass ?ow information to path
`26.
`Meter assembly 10 includes a pair of tubular members
`110 and 110', manifolds elements 150 and 150', a pair of
`parallel ?ow tubes 130 and 130', drive mechanism 180, and
`a pair of velocity sensors 170L and 170R. Flow tubes 130
`and 130' have two essentially straight inlet legs 131 and 131'
`and outlet legs 134 and 134' which converge towards each
`other at elements 120 and 120' having surfaces 121 and 121'.
`Brace bars 140 and 140' de?ne the axis W and W‘ about
`which each ?ow tube oscillates.
`The side legs 131 and 134 of ?ow tubes 130 and 130' are
`?xedly attached to surfaces 121 and 121' of elements 120
`and 120' which, in turn, are attached to manifold members
`150 and 150'. This provides a continuous closed material
`path through Coriolis meter assembly 10. When meter
`assembly 10 having ?ange 103 and 103' and ?uid openings
`101 and 101' are connected, via outlet end 104' and outlet
`end 104' to a ?ow tube system (not shown) which carries the
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`process material to be measured, material enters the meter
`through ?uid opening 101 in ?ange 103 of inlet end 104 is
`extended through inlet manifold 110 and a passageway
`therein having a gradually changing cross-section to element
`120 having a surface 121. There, the material is divided and
`routed through legs 131 and 131' and ?ow tube elements 130
`and 130'. Upon exiting ?ow tubes 130 and 130' and legs 134
`and 134', the process material is recombined in a single
`stream within element 120' having a surface 121 and is
`thereafter routed to exit manifold 150 . Within exit manifold
`150, the material ?ows through a passageway having a
`similar gradually changing cross~section to that of inlet
`manifold 150 to ?uid opening 101' in outlet end 104‘. Outlet
`end 104‘ is connected by ?ange 103' having bolt holes 102'
`to a conduit system (not shown).
`Flow tubes 130 and 130' are selected and appropriately
`mounted to elements 120 and 120' so as to have substantially
`the same mass distribution, moments of inertia and elastic
`modulus about bending axes W-W and W'—W', respec
`tively. These bending axes are located near respective ?ow
`tube brace bars 140 and 140' and elements 120 and 120'.
`Brace Bars 140 and 140' together with axes W and W‘
`comprise the out-of-phase bending axes for ?ow tubes 130
`and 130' as they are driven out of phase by driver 180.
`Manifold elements 120 and 120' comprise the in-phase
`bending axis for ?ow tubes 130 and 130'. The ?ow tubes
`may be vibrated in-phase by disturbances from the environ
`ment such as earthquakes, human activity, movements
`caused by adjacent machinery, etc. The ?ow tubes extend
`outwardly from the mounting blocks in an essentially par
`allel fashion and have substantially equal mass distributions,
`moments of inertia and elastic modules about their respec~
`tive bending axes.
`Both ?ow tubes 130 are driven by driver 180 in opposite
`directions about their respective bending axes W and W' at
`what is termed the ?rst out-of-phase natural frequency of the
`?owmeter. This mode of vibration is also referred to as an
`out-of-phase bending mode. Both ?ow tubes 130 and 130' ,
`vibrate out of phase as the tines of a tuning fork. This drive
`mechanism 180 may comprise any one of many well known
`arrangements, such as a magnet mounted to ?ow tube 130'
`and an opposing coil mounted to ?ow tube 130 and through
`which an alternating current is passed for vibrating both ?ow
`tubes. A suitable drive signal is applied by meter electronics
`20, via lead 185, to drive mechanism 180.
`Drive element 180 and the generated Coriolis forces cause
`a Coriolis oscillation of ?ow tubes 130 in a periodic manner.
`During the ?rst half of the Coriolis oscillation period of the
`?ow tubes 130, the adjacent side legs 131 and 131', are
`forced closer together than their counterpart side legs 134
`and 134', and reach the end point of their travel where their
`velocity reaches zero before their counterparts do so. In the
`second half of the Coriolis oscillation period, the opposite
`relative motion of the ?ow tubes 130 occurs, i.e., adjacent
`side legs 134 and 134', are forced closer together than their
`counterpart side legs 131 and 131'. Therefore, legs 134 reach
`the end point of their travel where their velocity crosses zero
`before legs 131 do so. This time interval (also referred to
`herein as the phase difference at a particular frequency, or
`time delay di?ference, or simply "At” value) which elapses
`from the instant one pair of adjacent side legs reaches their
`end point of travel to the instant the counterpart pair of side
`legs (i.e., those forced further apart), reach their respective
`end point, is substantially proportional to the mass ?ow rate
`of the material ?owing through meter assembly 10. The end
`points of travel of the respective ?ow tubes is a convenient
`point to make the At measurement. The same At relationship
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`5,469,748
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`between the two ?ow tubes exists throughout the entire
`range of motion of the ?ow tubes.
`To measure the time delay interval, At, sensors 1701. and
`170R are attached to ?ow tubes 130 and 130' near their free
`ends. The sensors may be of any well-known type. The
`signals generated by sensors 170L and 170R provide a
`velocity (or displacement or acceleration) pro?le of the
`complete travel of the ?ow tubes and can be processed by
`any one of a number of Well known methods to compute the
`time interval and, in turn, the mass ?ow rate of the material
`passing through the meter.
`Sensors 170L and 170R produce the left and right velocity
`signals that appear on leads 165L and 165R, respectively.
`The time delay di?erence, or At, measurement provides a
`manifestation of the phase difference that occurs between
`the left and right velocity sensor signals.
`Meter electronics 20 receives the left and right velocity
`output signals appearing on leads 165L and 165R, respec
`tively. Meter electronics 20 produces the drive signal
`appearing on lead 185 to drive element 180 which vibrates
`tubes 130 and 130‘. Meter electronics 20 processes the left
`and right velocity signals to compute the mass ?ow rate,
`volume ?ow rate and the density of the material passing
`through meter assembly 10.
`The characteristics of the process material ?owing in the
`?owmeter 10 is derived by meter electronics 20 which
`measures the phase or time delay At between the two sensor
`output signals 165L and 165R. These time delay measure
`ments must be made with great accuracy. It is a requirement
`that the derived ?ow output information for the process
`material have an accuracy of at least 0.15% of rate when
`applied to output conductor 26 of meter electronics 20.
`In order to attain these output signal accuracies, it is
`necessary that the signal processing circuitry within meter
`electronics 20 determine with precision the phase difference
`between the sensor output signals. Since the phase difference
`between the sensor output signals comprises the input infor
`mation upon which the processing circuitry operates to
`derive the process material information, it is necessary that
`the signal processing circuitry within meter electronics 20
`not introduce any phase shift which would mask or alter the
`phase shift between the sensor output signals from the
`?owmeter. It is necessary that this signal processing cir
`cuitry have a constant or an extremely stable phase shift so
`that the phase of the sensor signals 165R and 165L is shifted
`by less than 0.001 degree and, in some cases, less than a few
`parts per million. Phase accuracy of this magnitude is‘
`required if the meter electronics 20 is to attain an accuracy
`of less than 0.15% for the signals it applies to its output
`conductor 26.
`Description of FIG. 2
`FIG. 2 discloses the circuitry comprising the meter elec
`tronics 20 of FIG. 1. As shown, meter electronics 20
`comprises a drive circuit 27 which is connected to drive coil
`180 by path 185. Drive circuit 27 applies the appropriate
`signals to path 185 to energize drive coil 180 at the proper
`amplitude and frequency to produce the desired out-of-phase
`bending vibrations to the: ?ow tubes 130 and 130'. Circuit
`27 is well known in the art and since its speci?c implemen
`tation does not form any part of the present invention, it is
`not further discussed in detail herein. However, the reader is
`referred to U.S. Pat. No. 5,009,109 issued to P. Kalotay, et
`al. on Apr. 23, 1991; U.S. Pat. No. 4,934,196 issued to P.
`Romano on Jun. 19, 1990; and U.S. Pat. No. 4,876,879
`issued to J. Ruesch on Oct. 31, 1989. All of these are owned
`by the present assignee of the present invention and describe
`different embodiments for a ?ow tube drive circuit.
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`Meter electronics 20 also comprises ?ow measurement
`circuit 23 which on its left is connected to sensors 170L and
`170R over paths 165L and 165R to receive the sensor output
`signals. It is the phase di?erence between these two sensor
`output signals that comprises the input information mea
`surement circuit 23 receives and processes to derive accurate
`information regarding the process material. Input signals
`165L and 165R are applied to multiplexer 31 whose output
`is connected over paths 45, 55 and 65 to inputs of channels
`44, 54 and 64, respectively (channels A, C and B). The
`channels operate in the manner subsequently described to
`process the sensor signals and control the operation of
`counters 74 and 76 whose outputs represent the phase
`difference between the sensor output signals.
`The outputs of counters 74 and 76 are applied over path
`87 to microprocessor 80 which functions in the manner
`subsequently described to derive the desired information for
`the process material. The derived information is applied over
`path 91 to output circuitry 90 which applies the desired
`process material output information to its conductor 26 via
`conductors 263 and 262. The operation of the counters 74
`and 76, as well as multiplexer 31, is controlled as subse
`quently described by control logic 72.
`Each of channels A, B, and C contains a multi-pole ?lter,
`an integrating ampli?er, and at least one level detector. The
`output of the multi-pole ?lter in each channel is applied to
`the integrating ampli?er of the channel whose output, in
`turn, is applied to a level detector whose outputs are applied
`to counters 74 and 76 which measure the timing interval in
`terms of clock pulses that occur between corresponding
`changes in the detector outputs. The counter output is the
`well-known At value and varies with the mass ?ow rate of
`the processing ?uid. The resulting At value and counts are
`applied to microprocessor 80 over path 87 which responds
`to this information, computes mass and volumetric ?ow
`rates and density, and generates the desired output informa
`tion concerning the process material.
`The circuitry contained within channels 44, 54 and 64 can
`inject phase error into the mass ?ow information generated
`by processor 80. The circuitry of each channel not only
`possesses a different amount of internal phase delay as
`measur