United States Patent
`
`[19]
`
`[11] Patent Number:
`
`5,379,649
`
`Jan. 10, 1995
`Kalotay
`[45] Date of Patent:
`
`US005379649A
`
`[54] CORIOLIS EFFECT METER USING
`OPTICAL FIBER SENSORS
`
`[75]
`
`Inventor:
`
`Paul Z. Kalotay, Lafayette, C010.
`
`[73] Assignee:
`
`Micro Motion, Inc., Boulder, Colo.
`
`[21] Appl. No.: 809,146
`
`[22] Filed:
`
`Dec. 23, 1991
`
`Int. 01.6 ................................................ 0011? 1/84
`[51]
`[52] US. Cl. .................................... 73/861.38; 73/657
`[58] Field of Search ............. 73/861.37, 861.38, 32 A,
`73/653, 655, 657
`
`[56]
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`Re. 31,450 11/1983 Smith ............................... 73/861.38
`4,275,296 6/1981 Adelfsson .
`.
`4,281,245 7/1981 Brogardh et a1.
`4,358,678 11/1982 Lawrence ........................... 250/227
`
`4,407,561 10/1983 Wysocki ..........
`350/963
`4,418,984 12/1983 Wysocki et a1.
`350/9633
`.
`
`4,724,316 2/ 1988 Morton ................... 250/227
`
`4,843,890 7/1989 Samson et a1.
`.......
`73/861.38
`5,038,620
`8/1991 Rogers, Jr. et a1.
`............. 73/861.38
`
`FOREIGN PATENT DOCUMENTS
`
`2071321
`
`9/1981 United Kingdom .
`
`OTHER PUBLICATIONS
`
`Bruce Johnson, Dave Brodeur, Tom Lindsay and
`Randy Morten; “Macrobend fiber optic transducer for
`aerospace applications”; SPIE vol. 989 Fiber Optic
`Systems for Mobile Platforms II (1988); pp. 68—77.
`Y. Ohtsuka, M. Kamaishi and Y. Imai; “Fibre—coil
`deformation-sensor immune from temperature disturb-
`ances”; International Journal of Optoelectronics, 1988,
`vol. 3, No. 5; pp. 371-380.
`C. A. Wade and A. Dandridge; “An optical fiber flow-
`meter based on the Coriolis effect”; SPIE Voo. 985
`Fiber Optic and Laser Sensors VI (1988); pp. 299—304.
`
`Stewart D. Personick; “Fiber Optics Technology and
`Applications”; Aug. 1985; pp. 226-227.
`D. A. Krohn; “Fiber Optic Sensors Fundamentals and
`Applications”; 1988; pp. 3, 32—35, 95—102.
`Technical Staff of Newport Corporation; “Projects in
`Fiber Optics”; 1986; p. 77.
`The 5th International Conference on Solid—State Sen-
`sors and Actuators & Eurosensors III; “Abstracts”; Jun.
`25—30, 1989, Montreux, Switzerland; pp. 165 and 329.
`
`Primary Examiner—Herbert Goldstein
`Attorney, Agent, or Firm—Duft, Graziano & Forest
`
`[57]
`
`ABSTRACT
`
`A Coriolis mass flow rate meter for measuring the mass
`flow rate of material flowing through a conduit. The
`flow meter includes at least one flow tube through
`which the material to be measured passes. The flow
`tube is vibrated at its natural frequency so that the con-
`current flow of material through the vibrating tube
`produces a displacement of the tube with the magnitude
`of the displacement being dependent upon the magni-
`tude of the generated Coriolis forces and the mass flow
`rate of the measured material. The phase of the dis—
`placement of the flow tube is measured using optical
`fiber sensors comprising at least one loop of optical fiber
`which is flexed by the displacement of the flow tube.
`This flexing of the fiber causes a corresponding change
`in its Optical conductivity and a corresponding change
`in the intensity of the light transmitted through the fiber
`from a light signal source to a signal detector. The
`modulated light signal received by the optical signal
`detector is converted to an electrical signal which is
`processed to generate the mass flow rate and other
`information for the flowing material. A feedback circuit
`for the light signal source monitors the average inten-
`sity of the light received by the signal detector and
`controls the drive signal applied to the light source to
`maintain the average intensity of the light signal re-
`ceived by the detector at a predetermined initial inten-
`sity.
`
`23 Claims, 5 Drawing Sheets
`
`UflLEfiflON
`MEANS
`
`
`
`Micro Motion 101 5
`
`1
`
`Micro Motion 1015
`
`

`

`US. Patent
`
`Jan. 10, 1995
`
`Sheet 1 of 5
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`5,379,649
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`US. Patent
`
`Jan. 10, 1995
`
`Sheet 2 of 5
`
`5,379,649
`
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`UTILIZATION
`MEANS
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`

`

`US. Patent
`
`Jan. 10, 1995
`
`Sheet 3 of 5
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`5,379,649
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`» US. Patent
`
`Jan. 10,1995
`
`Sheet 4 of 5
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`5,379,649
`
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`UTILIZATION
`
`5
`
`

`

`US. Patent
`
`Jan. 10, 1995
`
`Sheet 5 of 5
`
`5,379,649
`
`164L
`
`164R
`
`
`
`170L
`
`170R
`
`F l G. 5
`
`6
`
`

`

`1
`
`5,379,649
`
`CORIOLIS EFFECT METER USING OPTICAL
`FIBER SENSORS
`
`FIELD OF THE INVENTION
`
`This invention relates to structure comprising and a
`method of operating a vibrating tube Coriolis effect
`meter and, more particularly, to a Coriolis effect mass
`flow rate meter using optical fibers as displacement
`sensors.
`
`10
`
`PROBLEM—BACKGROUND OF THE
`INVENTION
`
`Coriolis effect mass flow rate meters are well-known.
`One such meter that has gained widespread commercial 15
`acceptance is shown in U.S. Reissue Patent 31,450 to
`James E. Smith of Nov. 29, 1983. As taught by Smith,
`the flow of a material through an oscillating conduit
`produces Coriolis forces which are perpendicular to
`both the velocity of the mass moving through the con- 20
`duit and the angular velocity vector of the oscillation of
`the conduit. The magnitude of the generated Coriolis
`forces is related to the material mass flow rate as a
`function of the angular velocity of the mass flowing
`through the conduit.
`Coriolis effect flow meters typically use one or two
`flow tubes to direct the measured material flow from a
`pipe, through the meter tubes and then back to the pipe.
`These tubes may be either straight or curved, or irregu-
`lar shaped, and they may be mounted in the flow line or 30
`attached to a substantially rigid support. The tubes are
`normally vibrated by an electromagnetic drive at the
`natural frequency of the tube structure including the
`measured material. The Coriolis forces resulting from
`the mass of the material flow through the vibrating 35
`tubes causes a displacement of portions of the tubes.
`The displacement is measured at points on the tubes by
`position or velocity sensors. The time differential At
`between the movements of the tube elements at spaced
`apart locations is used for a determination of informa- 40
`tion including the mass flow rate of the measured mate-
`rial.
`
`25
`
`One step in measuring the generated Coriolis forces is
`to track the relative movement of different portions of
`the legs of meters having U-shaped tubes. This is typi- 45
`cally done by attaching two electromagnetic velocity
`sensors each comprising a magnet and a pickup coil in
`opposing relative positions on the side legs of the flow
`conduit or conduits as described in U.S. Pat. No.
`4,422,338 entitled, “Method and Apparatus for Mass 50
`Flow Measurements” and issued Dec. 27, 1983,
`to
`James E. Smith. This is also shown in U.S. Pat. No.
`4,491,025 of Jan. 1, 1985, to James E. Smith and Donald
`R. Cage. In a parallel dual tube design as disclosed in
`the patent to Smith and Cage, a sensing coil is attached 55
`to one of the two flow conduits. A cooperating magnet
`mounted to the other flow conduit is positioned coaxi-
`ally within the sensing coil. As the tube is vibrated by
`the drive coil, the sensing coil produces a signal which
`is representative of the movement of the conduit leg. By 60
`this means, a complete velocity profile is generated for
`each leg. The signals generated by the two sensors are
`applied to signal processing circuitry which produces
`an output representing the desired information for the
`flowing material (such as, for example, the mass flow 65
`rate, the density, etc).
`Although the currently available Coriolis effect me«
`ters (including those disclosed in the above-identified .
`
`2
`patents) operate satisfactorily and produce excellent
`results under most conditions, there are certain circum-
`stances in which their performance is not wholly satis-
`factory. For example, since they use electromagnetic
`devices as sensors, these devices can be affected by
`external magnetic fields. Under such circumstances
`their output data may be subject to error.
`Electromagnetic sensors are also complex to manu-
`facture due to the small gauge of the wire used in the
`sensing coils and due to the required resin coating and
`curing process for protecting the coils. Despite the resin
`coating, the sometimes harsh operating environment for
`these meters can cause the sensor coils to fail.
`Another disadvantage is that the coils are inductive
`devices which store energy that can generate arcing.
`This is a problem if the meter is used in an explosive
`atmosphere.
`Also, meters using these sensors are often used to
`measure the flow of material at high temperatures.
`These temperatures are often at such a level that the
`magnetic coils of these sensors have a high failure rate
`or else become unstable and generate output data that is
`unreliable.
`In view of the above, it can be seen that there cur-
`rently exists a need for Coriolis effect meters having
`sensors which are immune or resistant to harsh environ-
`mental conditions (such as, for example, strong electro-
`magnetic fields or high temperatures). It can further be
`seen that there exists a need for sensors which are more
`economical to manufacture, are more reliable, are im-
`mune to explosion hazards, and are both mechanically
`and thermally rugged.
`
`SUMMARY OF THE INVENTION
`
`The present invention overcomes the foregoing dis-
`advantages and achieves an advance in the art by pro-
`viding sensing apparatus for a Coriolis effect meter that
`is rugged, that is highly reliable, that will not degrade
`over time, and that is better suited for use in harsh envi~
`ronmental conditions (such as high temperatures) and in
`strong electromagnetic fields or explosive atmospheres.
`The invention comprises a Coriolis effect meter and,
`in particular, a Coriolis effect mass flow rate meter
`which uses one or more metal-clad optical fibers as flow
`tube displacement sensors. Preferably, the metal with
`which the optical fiber is coated is aluminum or gold. A
`specific application of the invention uses a pair of metal-
`clad optical fiber for a pair of sensors in which the
`microbend characteristic of the fibers is used to produce
`modulated optical signals corresponding to the dis-
`placement of the flow tubes. These modulated optical
`signals are produced when the displacement of the flow
`tube flexes the optical fiber sensor and attenuates its
`optical output signal by an amount that corresponds to
`the displacement of the flow tubes. An optical detector
`converts the modulated optical signals of the sensors
`into electrical signals which are processed to derive the
`mass flow rate of material through the flow tubes. The
`use of an outer metal coating on the fiber sensors pro-
`tects the fibers from harsh environmental conditions
`and atmospheres.
`The present
`invention provides a Coriolis effect
`meter having reliable displacement sensors which are
`unaffected by the presence of strong external electro-
`magnetic fields and high temperatures. Optical fiber
`sensors are well-suited for use in explosive atmospheres
`since no sparks or arcing is generated by the fiber.
`
`7
`
`

`

`5,379,649
`
`10
`
`4
`Flow tubes 12 and 14 are cantilever mounted by their
`side legs 131 and 134 to blocks 120 and 120' of manifold
`body 30 which is formed of castings 32 and 32’. Flow
`meter 10 is adapted to be attached to a supply conduit
`such as a pipeline (not shown) by flanges 36 and 36’.
`Manifold body 30 diverts the flow from the supply
`conduit into flow tubes 12 and 14 and then back to the
`supply conduit.
`When meter 10 having flange 103 having holes 102 is
`connected via its inlet end 104 to an upstream conduit
`(not shown) carrying the material that is being mea-
`sured, the material enters through an orifice 101 of inlet
`flange 103, flows through manifold elements 150 and
`block 120 of casting 32, to side legs 131 and 131' of flow
`5 tubes 14 and 12. From there, the material flows through
`the upper portion of flow tubes 12 and 14 and back
`down through side legs 134 and 134’ to block 120’ and
`elements 150’ of manifold body 30. From there, the
`material flows through outlet end 104’ which is con-
`nected by flange 103’ to the downstream conduit (not
`shown).
`The optical fiber sensors 170R and 170L are affixed to
`the right and left ends of flow tubes 12 and 14 so that the
`fiber element of each sensor bends or flexes as ends of
`tubes 12 and 14 are displaced with respect to each other.
`The optical fiber sensors 170R and 170L are preferably
`made of glass. The flexing of the fiber sensors 170L and
`170R causes a change in the light attenuation character-
`istics of each fiber sensor. Each fiber sensor is energized
`by optical source 160, which may comprise a laser or a
`light emitting diode (LED). Optical source 160 applies
`its output to one end of fiber 161 that extends to optical
`coupler 162. The single fiber path -161 splits into three
`parts within coupler 162 with path 163 returning di-
`rectly to detector 191 of meter electronics 20. The other
`two fiber paths extending from optical coupler 162
`comprise the fiber path sections 164L and 164R. Fiber
`path section 164L extends to one end of fiber sensor
`170L the output of which extends over fiber path 165L
`to detector 192L of meter electronics 20. Fiber path
`164R extends to fiber sensor 170R the output of which
`extends over fiber path 165R to detector 192R of meter
`electronics 20.
`
`3
`The use of optical fiber permits the sensors to be
`connected to light sources and cooperating optical de-
`tectors by means of closed optical fiber signal paths
`rather than copper conductors. This eliminates the
`problems associated with the transmission of small sig-
`nals over copper conductors (such as, for example,
`ground loops interference, etc).
`The light sources and the optical detectors are advan-
`tageously remotely situated, such as four feet, from the
`vibrating tube apparatus. This protects the light source
`and the optical detectors from the harsh environmental
`conditions, such as high temperatures or corrosive at-
`mospheres, to which the vibrating tube apparatus may
`be subjected.
`BRIEF DESCRIPTION OF THE DRAWING
`
`The above and other advantages and features of the
`invention may be better understood from a reading of
`the following detailed description thereof taken in con-
`junction with the drawing in which:
`FIG. 1 discloses one possible exemplary embodiment
`of the invention;
`FIG. 2 discloses further details of the invention;
`FIG. 3 discloses further details of the flow meter
`electronics;
`FIG. 4 discloses an alternative embodiment to that of
`FIG. 2;
`FIG. 5 is a top view of the embodiment shown in
`FIG. 1 on a smaller scale and with parts omitted for
`clarity.
`
`DETAILED DESCRIPTION
`
`Description of FIGS. 1 and 5
`FIGS. 1 and 5 disclose a Coriolis effect mass flow
`meter 10 having flow tubes 12 and 14. Flow tubes 12
`and 14 are selected and mounted to manifold body 30 so
`that they have substantially identical spring constants
`and moments of inertia about their respective bending
`axes W and W'. Flow tubes 12 and 14 have left side legs
`131 and 131' and right side legs 134 and 134'. The side
`legs converge toward each other at manifold block
`elements 120 and 120’. Brace bars 140 and 140’ serve to
`define the axes W and W’ about which flow tubes 12
`and 14 vibrate when a drive coil 180 is energized over
`path 185. Drive coil 180 is mounted at a midpoint region
`between the ends of flow tubes 12 and 14 to vibrate the
`flow tube structure about axes W and W’ parallel to the
`direction of fluid flow in the upper portion of flow tubes
`12 and 14.
`Left optical sensor 170L and right optical sensor
`170R are mounted near the respective ends of flow
`tubes 12 and 14 to sense the displacement of the end
`portion of flow tubes 12 and 14 caused by the combined
`action of the vibrations caused by drive coil 180 and the
`Coriolis motion caused by the flow of material through
`tubes 12 and 14. The mass of the left and right optical
`sensors 170L and 170R is preferably small relative to
`the mass of the corresponding flow tubes 12, 14 so as to
`minimize the impact of the optical sensors 170L and
`170K on the vibration of the corresponding flow tube
`12, 14. Temperature detector 190 is mounted onto one
`of the legs 131 or 131’ of flow tubes 12 and 14 to mea-
`sure the flow tube temperature caused by the tempera—
`ture of the material therein. Temperature information is
`used to determine changes in the spring constant as well
`as other relevant information for the flow tubes and the
`measured material.
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`65
`
`The fiber sensors 170L and 170R may each comprise
`one or more loops of optical fiber which are affixed to
`each of tubes 12 and 14 so that the displacement of the
`tubes causes a corresponding displacement or bending
`of the fiber loop comprising each fiber sensor 170L and
`170R.
`
`It is well known that optical fibers possess what is
`known as “microbending” characteristics wherein their
`light transmission capability is greatest when the fiber is
`straight and devoid of bends. Conversely, it is also well
`known that a bending of an optical fiber reduces its light
`transmission capability by an amount dependent upon
`the degree of bending and that the amount of the reduc-
`tion is repeatable to a very high degree of accuracy for
`small bends (termed “microbends”). Since the fiber
`comprising each fiber sensor 170L and 170R is bent or
`flexed by the displacement of flow tubes 12 and 14 with
`respect to each other, the optical transmission capability
`of each fiber sensor 170L and 170R is varied by the
`instantaneous displacement of flow tubes 12 and 14
`caused by the generated Coriolis forces in combination
`with the driven vibration. This instantaneous flexing of
`the sensors causes a corresponding instantaneous varia-
`tion in the light transmission capability of each sensor
`170L and 170R. This results in an optical output signal
`
`8
`
`

`

`5,379,649
`
`5
`being applied to each of fiber paths 165L and 165R that
`is modulated by the Coriolis displacement of the ends of
`flow tubes 12 and 14. The output of fiber paths 165L
`and 165R is applied to optical detectors 192L and 192R
`of meter electronics 20.
`
`With the above arrangement, optical detectors 192L
`and 192R receive a modulated light signal whose instan-
`taneous intensity is determined by the tube vibration
`amplitude and the instantaneous Coriolis displacement
`of the portion of tubes 12 and 14 to which optical sen-
`sors 170L and 170R are affixed.
`
`The electrical output of optical detectors 192L and
`192R is applied, as subsequently described, to the ele-
`ments of meter electronics 20 which generate mass flow
`rate and other information pertaining to the material
`flowing through flow tubes 12 and 14. The output of
`temperature detector 190 is extended over metallic path
`195 to meter electronics 20, which use this output infor-
`mation to provide the highest possible accuracy of gen-
`erated material mass flow rate and other information.
`
`The output of meter electronics 20, representing the
`measured mass flow rate is applied over path 26 to
`utilization means 29—which may comprise either a
`display device, such as an indicator, or a plurality of
`indicators. Alternatively, utilization means 29 may com-
`prise an industrial system whose process is controlled in
`whole or in part by the data applied over path 26.
`It should be noted that the optical source 160 and
`detectors 192L and 192R are remotely situated, such as
`four feet, from the vibrating tube apparatus so as to
`isolate them from the harsh conditions, such as high
`temperatures and corrosive atmospheres, to which the
`vibrating tube apparatus may be subjected.
`
`Description of FIG. 2
`
`FIG. 2 discloses further details of the circuitry and
`apparatus comprising the presently preferred embodi-
`ment of the invention. Elements on FIG. 2 which are
`also shown on FIG. 1 are designated in the same man-
`ner as on FIG. 1. Elements on FIG. 2 which are not
`shown on FIG. 1 have a reference number in the 200
`series.
`
`Optical signal source 160 contains an LED or laser
`light source 203 which is energized by the potential on
`path 202 from the output of drive amplifier 201. The
`intensity of the optical output of source 203 is deter-
`mined by the amplitude of the current on path 202.
`The optical output of source 203 is received by the
`left end (FIG. 2) of fiber section 161 and is transmitted
`to optical coupler 162, where the single optical path 161
`is split into the three optical paths 163, 164L and 164R.
`Fiber path 163 extends directly from coupler 162 to
`diode D1 of optical detector 191, which generates an
`electrical signal that is applied to the input of reference
`amplifier 190. The output of reference amplifier 190 is
`extended over path 204 to the upper input of the differ-
`ential amplifier 201. The lower input of amplifier 201
`receives a gain control signal over path 207 from poten-
`tiometer R1. Potentiometer R1 and amplifier 201 cause
`the potential applied over conductor 202 and resistor
`R3 to light source 203 to be constant in response to
`changes in the amplitude of the signal applied to path
`204. The intensity of light generated by either an LED
`or a laser may decrease with time. Also the light con-
`ductivity of a fiber may decrease with time. If steps
`were not taken to compensate for this effect, it could
`cause the intensity of the steady state optical signals
`received by decoder D2 and D3 to decrease. This
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`6
`would cause erroneous signals to be generated by the
`remainder of the circuitry shown on FIG. 2.
`Potentiometer R1 is adjusted so that potential 202 is
`set with a voltmeter to a level that causes source 203 to
`generate an optical signal whose intensity, when applied
`to optical detectors 191, 192L and 192R, causes a de-
`sired steady state voltage to be applied to paths 204,
`206L and 206R. Subsequently, if the output from source
`203 changes with age or temperature or from any other
`effect, output voltage 204 changes in a corresponding
`manner. This change is detected by differential ampli-
`fier 201, which alters its gain as required to cause the
`drive signal 202 to change by the amount required so
`that the intensity of light source 203 changes and the
`potential on conductor 204 returns to the level which
`was initially set by potentiometer R1. This feedback
`arrangement maintains the outputs 206R and 206L of
`the optical detectors 192R and 192L to be held constant
`in spite of any aging of source 203 or change in the
`optical conductivity of the fibers. This arrangement
`ensures that the accuracy of the output data generated
`by meter electronics 20 is not degraded. Potentiometers
`R20 and R21 control the gain of amplifiers 193L and
`193R.
`
`The output 206R of optical detector 192R is applied
`to the input of the right channel element 218R, and the
`output 206L of optical detector 192L is applied to the
`input of the left channel element 218L. Element 218L is
`identical to element 218R, whose details are fully dis-
`closed. The signal 206R comprises an AC signal super-
`imposed on a DC signal from the output of optical
`detector 192R. The amplitude of the DC component of
`the signal is substantially larger than that of the AC
`component. It is therefore necessary that the DC com—
`ponent be removed so that the AC component carrying '
`the intelligence representing the flexing of fiber loop
`170R can be accurately detected and processed.
`Conductor 206R is applied to the lower input of
`buffer amplifier 208, whose output is applied over path
`209 to the upper input of differential amplifier 216. Path
`209 also extends to the junction of capacitor C1 and
`resistor R2. Capacitor Cl, switched capacitor low pass
`filter 211 (SCF-LPF), resistor R2 and clock generator
`212 operate so that, as clock generator 212 drives the
`low pass filter 211 over path 213, the output of the low
`pass filter on path 214 comprises a DC only signal that
`is applied to the lower input of differential amplifier
`216. Other types of low pass filters could be used in
`place of the switched capacitor filter for element 211.
`The upper input of differential amplifier 216 on path
`209 comprises an AC signal superimposed on a DC
`signal having an amplitude equal to the DC signal on
`path 214. The equal DC signals on the upper and lower
`inputs of differential amplifier 216 cancel one another so
`that the differential amplifier 216 responds only to the
`received AC component of signal 209 on its upper in-
`put. This AC signal is amplified by differential amplifier
`216 and applied over path 217R to the circuitry of FIG.
`3. The phase of signal 217R is determined by the instan-
`taneous flexing of fiber sensor 170R due to the displace—
`ment of the portion of flow tubes 12 and 14 associated
`with sensor 170R.
`
`65
`
`The left channel element 218L is identical to right
`channel element, and it receives signal 206L from the
`output of left optical detector 192L. It operates in the
`same manner to apply an output signal over path 217L
`to the comparator circuitry of FIG. 3. The phase of
`signal 217L represents the instantaneous displacement
`
`9
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`

`7
`of the fiber loop sensor 170L determined by the instan-
`taneous displacement of the portion of the flow tubes 12
`and 14 associated with fiber sensor 170L.
`
`5,379,649
`
`Description of FIG. 3
`
`FIG. 3 discloses the details of the circuitry 20 that
`receives the conditioned output of fiber detectors 192L
`and 192R on paths 217L and 217R and,
`in response
`thereto, generates output information, such as the mass
`flow rate, for the material flowing through the flow
`tubes 12 and 14. FIG. 3 also includes the circuitry that
`drives drive coil 180 to vibrate flow tubes 12 and 14 at
`their natural frequency.
`The drive circuitry receives an input signal from the
`output 217R of the right channel amplifier 218R. This
`input is applied to the input of a low pass filter 351
`whose output extends over path 354 to the upper input
`of the drive amplifier 356. The lower input of the drive
`amplifier 356 receives a control signal over path 353
`from potentiometer 352. Potentiometer 352 is adjusted
`to cause the amplitude of the signal on path 185 to be of
`the level required to energize drive coil 180 so that it
`vibrates the flow tubes 12 and 14 at a desired amplitude
`to generate a useable signal in sensors 170L and 170R.
`This amplitude adjustment is made at the factory using
`precision adjustment techniques.
`The signals 217L and 217R from the left and right
`channel amplifiers 218L and 218R, respectively, are
`applied to the inputs of the flow measurement circuitry
`23 of FIG. 3. Flow measurement circuitry 23 includes
`processing circuitry 335 which processes the left and
`right position signals on leads 217L and 217R, respec-
`tively, along with the temperature signal on lead 195, in
`a well known manner (disclosed, for instance, in US.
`Pat. No. 4,843,890 of Jul. 4, 1989, to Allan L. Sampson
`and Michael J. Zolock) to calculate information includ-
`ing the mass flow rate of the material passing through
`Coriolis effect flow meter assembly 10. Output informa-
`tion is applied over path 26 to utilization means 29
`which may be either a display or a process control
`system.
`Inasmuch as the method by which flow measurement
`circuitry 23 generates information (including the mass
`flow rate) is well known to those skilled in the art, only
`that portion of electronics 20 that is germane to the
`present invention is discussed below.
`Measurement circuit 23 contains two separate input
`channels: a left channel and a right channel. Each chan-
`nel contains a zero crossing detector 308 or 318. The left
`and right displacement signals 217L and 217R are ap-
`plied to respective zero. crossing detectors (effectively
`comparators) 308 and 318, which generate level change
`signals whenever the corresponding position signal
`exceeds a voltage window defined by a small predefined
`positive and negative voltage level, e.g. :2.5 V. The
`outputs 309 and 310 of zero crossing detectors 308 and
`318 are fed as control signals to counter 320 to measure
`a timing interval, in terms of clock pulse counts, that
`occurs between corresponding changes in outputs 309
`and 310. This interval is the At value of sensors 170L
`and 170R and it varies with the mass flow rate of the
`material through tubes 12 and 14. This At value,
`in
`counts, is applied in parallel as input data to processing
`circuitry 335.
`Temperature detector element 190 is connected by
`path 195 to circuit 324. This circuit supplies a constant
`drive current to temperature detector element 190, lin-
`earizes the voltage that appears across the temperature
`
`10
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`15
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`20
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`25
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`30
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`35
`
`45
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`50
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`55
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`65
`
`10
`
`8
`detector element 190 and converts this voltage using
`voltage/frequency (V/F) converter 326 into a stream of
`pulses that has a scaled frequency which varies propor-
`tionally with any temperature changes detected by tem-
`perature sensor 190. The resulting pulse stream pro—
`duced by circuit 324 is applied as an input to counter
`328, which periodically counts the stream and produces
`a signal, in counts, that is proportional to the measured
`temperature. The output of counter 328 is applied as
`input data to processing circuit 335.
`Processing circuit 335, which is advantageously a
`microprocessor based system, determines the mass flow
`rate from the digitized At information and temperature
`values applied thereto. The digitized temperature value
`is used to modify a meter factor value based upon the
`temperature of the flow tubes. This compensates for
`changes in flow tube elasticity with temperature. The
`temperature compensated meter factor is then used to -
`calculate the mass flow rate and volume flow rate from
`the measured At value and calculated density value.
`Having determined the mass flow rate and the volume
`flow rate, circuitry 335 then updates the output signals
`applied over leads 26 to utilization means 29.
`Processing circuitry 335 on FIG. 3 includes micro-
`processor 336 and memory elements including a ROM
`memory 337 and a RAM memory 338. The ROM 337
`stores permanent information that is used by micro-
`processor 336 in performing its functions, while RAM
`memory 338 stores temporary information used by mi-
`croprocessor 336. Microprocessor 336 together with its
`ROM 337 and RAM 338 memories and bus system 339
`control the overall functions of the processing circuitry
`335 so that it can receive signals from counters 320 and
`328 and process them in the manner required to calcu-
`late and apply over path 26 to utilization means 29 the
`various items of data the Coriolis effect meter of the
`present invention generates. This information includes
`the mass flow rate and volume flow rate of the mea-
`sured material.
`
`The following describes in further detail how the
`processing circuitry 335 on FIG. 3 operates to compute
`the mass flow rate of the material flowing in flow tubes
`12 through 14. The output of counter 320 represents the
`term At which is the time difference between the time at
`which sensor 170R crosses a predetermined reference
`point as it is vibrated and twisted by forces and the time
`at which sensor 170L crosses a corresponding reference
`point. Counter 320 is started by one of the zero crossing
`detector outputs 309 or 310 and is stopped by the other
`output. This At factor is multiplied by a calibration
`factor K which is dependent upon the material and
`geometry of the meter structure. K is empirically deter-
`mined in a flow calibration facility and is input to micro—
`processor circuitry 335 at the factory in which the
`meter is constructed. This K is corrected for tempera-
`ture during the operation of the flow meter under con-
`trol of the output 312 of counter 328. The units of K are
`grams/seconds/microsecond. This means that for every
`microsecond of phase shift there will be a certain num-
`ber of grams per second of mass flow rate. The mass
`flow rate is calculated by the processing circuitry 335
`according to the formula MFR=AtK, where MFR is
`the computed mass flow rate. To determine K for a
`given meter, fluid is run through the flow meter for