HK1057091B

HK1057091B - Low thermal stress balance bar for a coriolis flowmeter

Info

Publication number: HK1057091B
Application number: HK03109305.8A
Authority: HK
Inventors: Craig Brainerd Van Cleve; Gregory Treat Lanham
Original assignee: Micro Motion, Inc.
Priority date: 2000-05-02
Filing date: 2001-04-25
Publication date: 2011-06-03

Description

Low thermal stress balance bar for coriolis flowmeter

Technical Field

The present invention relates to a coriolis flowmeter having a balance bar capable of withstanding a wide range of thermal conditions without stressing a flow tube to which the balance bar is coupled.

Background

A single straight tube coriolis flowmeter typically has a concentric balance bar coaxial with the flow tube. The balance bar vibrates 180 degrees out of phase with respect to the flow tube to cancel the drive mode vibration of the flow tube. The balance bar and the material filled flow tube comprise a dynamically balanced structure that vibrates at its resonant frequency. The balance bar is rigidly secured at both ends to the flow tube by annular brace bars. Vibration-free zones, referred to as nodes, are located in the brace bars, which define the ends of the active portion of the flow tube.

The radial distance between the outer surface of the flow tube and the inner surface of the balance bar is typically kept small, both for compactness and to tune the resonant frequency of the balance bar. The small difference in diameter between the flow tube and the balance bar creates a very strong connection.

One problem with prior art designs of balance bars is that they apply a significant thermal stress to the flow tube. There are three distinct types of thermal stresses for coriolis flowtubes. The first is thermal shock. If a coriolis flowmeter in a cold climate suddenly receives hot material, the hot flow tube attempts to expand, but is restrained by the surrounding cold balance bar and flowmeter case. The prior art designs use a titanium flow tube with a very low modulus of elasticity. The low thermal expansion rate and high yield strength of titanium enable the flow tube to withstand the high stresses of thermal shock without damage.

The second type of thermal stress is caused by the elevated or reduced uniform temperature of the coriolis flowmeter. Such thermal stresses are common in chemical or food plants where the coriolis flowmeter housing is insulated or heated to maintain the entire meter at the temperature of the material. If the entire coriolis flowmeter is made of titanium, the uniform meter temperature does not create any stress, but titanium is too expensive to be used for the entire meter. Most prior art coriolis flowmeters use titanium flow tubes because the expansion and elastic modulus of titanium are low. For cost reasons, they use a stainless steel balance bar and housing, even though titanium is the best material. These coriolis flowmeters develop thermal stress at elevated, uniform temperatures because these different materials have different expansion moduli. A coriolis flowmeter with stress at 70 degrees has significant stress at a uniform 200 degrees because the balance bar and case of stainless steel have an expansion rate that is more than twice that of the titanium flow tube.

In a third type of thermal load, the stress placed on the flow tube is due to a steady state thermal condition in which the material and the environment have different temperatures. A coriolis flowmeter for measuring hot material in a cold climate eventually reaches a thermal equilibrium state where the titanium flow tube reaches the material temperature and the balance bar is only slightly cooler. However, the enclosure may be much cooler, depending on the ambient conditions. For example, if the enclosure is exposed to cold air, the temperature of the enclosure may be only a few degrees higher than the ambient temperature. Stresses are created when the cold case limits the attempted expansion of the balance bar and flow tube. Stresses also develop when the stainless steel balance bar attempts to expand at twice the expansion rate of the titanium flow tube.

Commercially available single straight tube coriolis flowmeters must be able to withstand all three types of thermal loads without encountering permanent damage and ideally without excessive error in material measurements. The balance bar ends are rigidly secured to the flow tube by brace bars. This effectively divides the flow tube into three sections. The central portion between the brace bars and inside the balance bar is the active portion of the flow tube. This portion vibrates out of phase with respect to the balance bar. The two portions of the flow tube extending from the axial ends of the balance bar to the ends of the case do not vibrate and are the inactive portions of the flow tube.

When the above-described prior art coriolis flowmeter is exposed to a first type of thermal load, i.e., thermal shock, both the active and inactive portions of the flow tube experience the same thermal stress. This is due to the fact that neither the balance bar, which confines the active portion of the flow tube, nor the case, which confines the inactive portion of the flow tube, has a temperature or length that does not change, while the three portions of the flow tube quickly reach elevated temperatures like the material and have the same thermal stresses. When the prior art coriolis flowmeter is exposed to a second type of thermal load having a uniformly elevated temperature, the three portions of the flow tube are again subjected to the same thermal stress. Both the balance bar and the outer shell are stainless steel and expand at the same expansion rate. Titanium flow tubes attempt to expand at different rates of expansion but are constrained by the balance bar and case.

In the third thermal state of the thermal load, the flow tube and balance bar almost reach the material temperature, while the case is still cold. The hot balance bar expands its length while the cold outer shell does not. The inactive portion of the flow tube is located between the ends of the case and the elongated balance bar. The cross-sectional areas of both the balance bar and the case are much larger than the flow tube, forcing the inactive portion of the flow tube to shorten in length. Because the non-working portion of the flow tube is hot, it will increase in length if not restrained, and the forced shortening of the length will create stresses that may even exceed the yield strength of the titanium flow tube. At the same time, the active portion of the flow tube is restrained at both ends by attachment to a heated stainless steel balance bar. Stainless steel has a much higher expansion coefficient than titanium flow tubes. Depending on the temperature difference between the balance bar and the flow tube, the active portion of the flow tube may be under tension because the balance bar temperature is approximately equal to the flow tube temperature. The balance bar may also be compressed when its temperature is lower than the flow tube temperature.

The non-working portion of the flow tube is highly stressed by temperature gradients, a condition that is a problem with prior art coriolis flowmeters. This problem is typically addressed in prior art coriolis flowmeters by limiting the temperature range over which the coriolis flowmeter may operate. This is undesirable because many customers desire to measure the flow rate of the material at temperatures that exceed the limits specified by thermal stress.

Disclosure of Invention

The present invention overcomes the above and other problems with a balance bar that minimizes the stresses in the active and inactive portions of the flow tube at any thermal condition. The mid-section of the balance bar is axially variable so that the change in length of the balance bar ends does not exert a significant axial force on the flow tube. This ensures that the thermal stresses on the active and inactive portions of the flow tube are always equal. This stress equalized state is the state of minimum possible stress of the flow tube. Residual stresses in the flow tube due to the axially displaceable balance bar are a function only of differential expansion between the flow tube and the case. The expansion and contraction of the balance bar is eliminated without stressing the flow tube.

Yet another advantage of the stabilizer bar of the present invention is cost. Most prior art coriolis flowmeters require a stainless steel balance bar to keep the cost reasonable. To extend the temperature range of a coriolis flowmeter, the prior art balance bar is required to have a coefficient of expansion as close as possible to the flow tube material (titanium). The best prior art balance bar is made entirely of titanium. However, the cost of a titanium balance bar in a large coriolis flowmeter may be as much as six times that of a stainless steel balance bar. The balance bar of the present invention has increased axial compliance without exerting axial forces on the flow tube. The thermal expansion of the balance bar is insignificant and can therefore be manufactured from less expensive materials and have a wide temperature range.

There are several possible exemplary embodiments of the invention. The first embodiment is a stabilizer bar having two separate end portions and a hollow center portion. Each end portion is secured to a respective brace bar and is secured to the ends of the active portion of the flow tube by the brace bars. The two independent balance bar end portions behave like cantilever beams designed to have the resonant frequency of the material filled flow tube. The central void section is capable of reducing the balance bar drive mode frequency to that of the flow tube without the need for an additional balance bar mass of prior art meters. It does this by removing the stiffness from the balance bar. This allows the coriolis flowmeter to reach dynamic equilibrium. The driver includes a drive coil secured to the housing due to the hollow section of the center portion of the balance bar, and a magnet secured to the flow tube. The two independent balance bar end portions are passively driven by the brace bar motion in response to the drive mode vibrations of the flow tube. The two independent balance bar end portions vibrate in response to the drive mode of the flow tube and apply a torque to the brace bar regions that counteracts the torque applied to the brace bars by the ends of the active portion of the flow tube. The twisting of the end portions of the balance bar also counteracts the momentum of the vibrating flow tube.

The balance bar design has an added advantage in addition to cost savings and an extended temperature range without stressing the flow tube. The balance bar in a prior art single tube flowmeter is capable of canceling drive mode vibrations of the flow tube, but it does not balance flowmeter vibrations generated by coriolis forces exerted on the flow tube during material flow conditions. Coriolis forces and deflections are applied to a vibrating flow tube with material flow. The two axial halves of the flow tube have coriolis forces applied in opposite directions. The resulting coriolis deflections of the two axial halves of the flow tube are also opposite. These forces and deflections are proportional to the material flow rate and the vibrations they produce cannot be countered with the fixed weight on a conventional balance bar.

The balance bar of the present invention is able to counteract these coriolis forces due to the independence of the two end portions of the balance bar. The void section in the center of the balance bar lowers the resonant frequencies of the ends of the balance bar in vibration modes that are out of phase with each other. This mode is called a coriolis-like mode due to its shape. The central void region reduces the resonant frequency of this mode below the drive frequency. Each balance bar end portion is resonant out of phase with respect to the flow tube at the drive mode vibrational frequency. Because coriolis deflections of the flow tube occur at the drive mode frequency, the two independent balance bar end portions respond to these coriolis deflections as easily as they do. The driving force for both responses is the same. It is the movement of two struts. The left balance bar end portion has the same response to coriolis excitations as it does to drive mode excitations. The difference between these two excitation modes is that the drive mode excitation is a constant amplitude excitation, with the ends of the active portion of the flow tube being in phase with each other. The amplitude of the coriolis excitation is proportional to the flow rate, and the two ends of the flow working portion are 180 degrees out of phase with each other. The separate balance tube end portions have coriolis-like deflections that effectively cancel the coriolis forces of the flow tube. As the flow rate (and thus coriolis force) increases, the out of phase coriolis-like deflections increase the amplitude of the balance bar out of phase with the coriolis vibrations of the flow tube.

The cancellation of the coriolis force vibrations by the end portions of the balance bar results in a more accurate coriolis flowmeter. The unbalanced coriolis forces of the prior art coriolis flowmeter produce a kind of rocking of the coriolis flowmeter at the drive mode frequency. This flow rate proportional shaking changes the coriolis acceleration of the material flow and the output signal produced by the sensor. This error can be compensated for except that it depends on the stiffness of the coriolis flowmeter mounting. A coriolis flowmeter having a rigid mounting has a slight error while a coriolis flowmeter having a flexible mounting has a large error. Because the mounting conditions of commercial coriolis flowmeters are unknown, it is often not possible to compensate for them.

Another embodiment of the invention has the balance bar end portions weakly coupled by the drive coil bobbin. The brackets enable the driver to be mounted in the axial center portion so that the coil and magnet of the driver can drive the balance bar end portions and the flow tube in opposite phases. These brackets are made of a sufficiently thin metal and have a geometry that allows the balance bar to expand and contract axially with little resistance at its ends. These bendable brackets also facilitate out of phase movement of the two balance bar end portions that counteract the applied coriolis force.

Another alternative embodiment facilitates the expansion and contraction of the two balance bar end portions, but does not facilitate out of phase movement of the two end portions. This allows the use of an inexpensive balance bar material together and provides a large temperature range. This embodiment does not facilitate out of phase motion of the ends of the balance bar counteracting the coriolis forces.

Yet another embodiment provides a stabilizer bar having separate end portions coupled to a central section by flexible side straps. The cutouts in the central section and the balance bar halves increase the axial compliance.

In summary, the present invention solves three problems of the stabilizer bar by breaking the two end portions of the stabilizer bar. It allows the balance bar to be made of inexpensive materials. It allows a wider temperature range resulting in less axial stress on the flow tube and it provides a more accurate coriolis flowmeter by counteracting the coriolis forces exerted on the flow tube.

One aspect of the invention is a coriolis flowmeter adapted to receive a flow of material at an inlet and to extend said flow of material through flow tube means to an outlet of the coriolis flowmeter; the coriolis flowmeter further comprises:

a balance bar positioned parallel to said flow tube means;

brace bars for coupling said balance bar first and second ends to said flow tube means;

a driver for vibrating said flow tube means and balance bar in opposite phases;

sensor means coupled to said balance bar and said flow tube means for generating signals representative of the coriolis response of said vibrating flow tube means with material flow;

an axially central portion including a drive coil carrier mechanism and a spring mechanism;

said balance bar including a first portion extending axially inwardly from said first one of said struts toward said axial center portion;

said balance bar including a second portion extending axially inwardly from said second one of said struts toward said axially central portion; and

the axial center portion couples the first balance lever portion to the second balance lever portion,

wherein said spring mechanism is oriented perpendicular to the axis of said flow tube mechanism and couples said drive coil carrier mechanism to the axially inner ends of said first and second portions of said balance bar, and said spring mechanism has an axial compliance that causes said balance bar to change length in response to thermal changes in the coriolis flowmeter without imparting any additional axial stresses to said flow tube mechanism.

In another aspect, the coriolis flowmeter comprises a case and wherein said flow tube means comprises a straight flow tube.

In another aspect, the coriolis flowmeter is characterized in that the balance bar is coaxial with the flow tube.

In another aspect, the coriolis flowmeter is characterized in that said sensor means comprises a pair of velocity sensors, a first of said sensors being coupled to said first end portion of said balance bar and said flow tube and a second of said sensors being coupled to said second end portion of said balance bar and said flow tube.

Another aspect is the coriolis flowmeter, wherein the case encloses the straight flow tube, the brace bar, and the balance bar.

In another aspect, the coriolis flowmeter wherein said flow tube mechanism is a single flow tube, said sensor mechanism comprising a pair of velocity transducers, a first of said velocity transducers coupled to said first balance bar portion and said single flow tube, and a second of said velocity transducers coupled to said second balance bar portion and said single flow tube.

In another aspect, the coriolis flowmeter, wherein the driver comprises:

said drive coil carrier means being coaxial with said flow tube means and having an axial length less than the distance between said first and second balance bar portions;

an axially extending support rod coupling said first and second balance bar portions to said drive coil carrier mechanism;

said extended support bar lying in a vibration neutral plane of said balance bar and oriented parallel to the longitudinal axis of said flow tube means;

the drive coil carrier mechanism defining first slots in a wall of the drive coil carrier mechanism, the first slots being parallel to and proximate an axially outer end of the drive coil carrier mechanism;

a set of first set of springs defined by walls of said drive coil carrier mechanism and positioned between said first slot and an axially outer end of said drive coil carrier mechanism, said first set of springs being curved in response to changes in axial length of the balance bar.

In another aspect, the coriolis flowmeter is characterized in that,

the first end of the spring mechanism is connected to the driving coil frame mechanism;

a second end of the spring mechanism is connected to the first and second balance bar portions;

the spring mechanism is bent according to the change in the axial length of the first and second balance bar portions.

In another aspect, the coriolis flowmeter is characterized in that the drive coil holder mechanism includes:

a first drive coil carrier having a flat surface parallel to the longitudinal axis of said flow tube mechanism;

a second drive coil carrier having a flat surface parallel to the longitudinal axis of the flow tube mechanism;

the spring mechanism includes a first set of springs connecting the first drive bobbin to the first and second balance bar portions;

the flat surface of the first drive coil former being adapted to receive a coil of the driver;

a drive magnet coupled to the flow tube mechanism and magnetically coupled to the drive coil;

the spring mechanism further includes a second set of springs connecting the second drive bobbin to the first and second balance bar portions; and

a block secured to the flat surface of the second drive coil former.

In another aspect, the coriolis flowmeter wherein said first and second sets of springs have ends connected to said first and second balance bar portions.

a first drive coil bobbin secured to the top of said balance bar;

a second drive bobbin secured to the bottom of said balance bar;

the spring mechanism couples the first drive bobbin to the second drive bobbin; and is

The spring mechanism is adapted to bend about its ends in accordance with changes in the axial length of the balance bar.

In another aspect, the coriolis flowmeter, wherein the spring mechanism comprises:

a first end of said spring mechanism coupled to said first drive bobbin;

a second end of said spring mechanism coupled to said second drive bobbin.

In another aspect, the coriolis flowmeter further comprises:

a first block fixed to a lower portion of an axially inner end of said balance bar;

a second block is fixed to an upper portion of said axially inner end of said balance bar.

In another aspect, the coriolis flowmeter, wherein the driver comprises:

a first drive coil secured to one surface of said first drive coil bobbin;

a first magnet magnetically coupled to said first drive coil and fixed to said flow tube mechanism;

a second drive coil secured to one surface of said second drive coil bobbin;

a second magnet magnetically coupled to said second drive coil and fixed to said flow tube mechanism;

the first and second drive coils and the first and second magnets act upon receipt of a drive signal to cause the flow tube mechanism and the balance bar to vibrate in opposite phases.

In another aspect, the coriolis flowmeter is characterized by:

the walls of the first and second balance bar portions defining a plurality of circumferentially oriented second slots;

a second set of springs is positioned in the wall between the second slot and the axially elongated support bar, the second set of springs bending axially in response to changes in the axial length of the balance bar.

In another aspect, the coriolis flowmeter is characterized by:

the support bar and the first and second sets of springs define springs that flex in response to changes in the axial length of the balance bar without imparting any additional axial stress to the flow tube mechanism.

In another aspect, the coriolis flowmeter is characterized by:

a top portion of said drive coil carrier mechanism having a flat surface with an aperture for receiving a coil of said actuator;

a magnet of the driver is electromagnetically coupled to the driver coil and is coupled to the flow tube mechanism.

In another aspect, the coriolis flowmeter is characterized by:

the drive coil carrier mechanism is cylindrical and has a diameter equal to the diameter of the balance bar.

Drawings

The invention will be better understood from a reading of the following detailed description taken in conjunction with the drawings in which:

fig. 1 discloses a prior art straight tube coriolis flowmeter.

Fig. 2 discloses a straight tube coriolis flowmeter according to a first exemplary embodiment of the invention.

Fig. 3, 4 and 5 disclose the vibrational mode shapes of the flow tube and balance bar according to the present invention.

Fig. 6 and 7 disclose a straight tube coriolis flowmeter according to a second exemplary embodiment of the invention.

Fig. 8 and 9 disclose a straight tube coriolis flowmeter according to a third exemplary embodiment of the invention.

Fig. 10, 11 and 12 disclose a straight tube coriolis flowmeter according to a fourth exemplary embodiment of the invention.

Detailed Description

Description of FIG. 1

Fig. 1 discloses a straight tube coriolis flowmeter 100 having a straight flow tube 101 surrounded by a balance bar 102, with flow tube 101 and balance bar 102 being surrounded by a case 104. Brace bars 110 and 111 couple the ends of balance bar 102 to the outer wall of flow tube 101. Flow tube 101 also includes flow tube extension members 101A and 101B. Part 101 is the active portion of the flow tube between brace bars 110 and 111. The extension members 101A and 101B are the inactive portions of the flow tube and connect the brace bars 110 and 111 to the case ends 108 and 109. Parts 113 and 114 can be considered part of the flow tube because they extend through the top portions 105 and 115 to the flanges 112 and 112A. Component 106 is the material input port of the flow meter. Component 107 is the material outlet of the flow meter. Meter electronics 121 applies a signal to driver D via path 123 to vibrate balance bar 102 and flow tube 101 in opposite phases. Sensors (velocity transducers) LPO and RPO detect vibrations of the flow tube 101 with material flow and generate output signals indicative of the phase of the Coriolis response. The sensor and output signals are applied via paths 122 and 124 to meter electronics 121, which produces an output on path 125 containing information about material flow.

Balance bar 102 is rigidly coupled to flow tube 101 by brace bars 110 and 111. Flow tube 101 is sealingly coupled to case ends 108 and 109 by flow tube sections 101A and 101B. The sealed coupling of the flow tube to the balance bar and the case creates thermal stresses on the flow tube during conditions of sudden elevation of the flow tube temperature relative to the balance bar 102 and case 104 and steady state conditions where the flow tube temperature is different than the temperature of the balance bar 102 and/or case 104.

There are three possible types of thermal stresses within a coriolis flowmeter. The first type is thermal shock. In this type, the flow tube 101 may suddenly receive a hot (or cold) material. The hot flow tube 101 attempts to expand but is restrained by the surrounding cold balance bar 102 and case 104. This stress created under such conditions causes the active portion 101 of the flow tube to attempt to expand axially more than the cold balance bar. The inactive portions 101A and 101B of the flow tube experience this stress and attempt to expand axially more than the case 104. If the flow tube is made of titanium, the problems of thermal shock are minimized due to the small elastic modulus of titanium. While the use of titanium flow tubes minimizes stress problems, stress on the flow tubes can change the stiffness of the flow tubes. This can degrade the accuracy of the output information produced by the vibrating flow tube and, in turn, the accuracy of the coriolis flowmeter.

A second type of thermal stress occurs when the entire flow meter is subjected to a uniform temperature increase or decrease. Even with titanium flow tubes, the flow tubes are subject to thermal stresses because the rate of expansion of stainless steel balance bar 102 and case 104 is greater than twice the rate of expansion of titanium flow tube 101. Even if the titanium flow tube is able to withstand such stresses without permanent mechanical deformation, its varying stiffness can degrade the accuracy of the output information produced.

A third type of thermal stress is characterized by a steady state thermal state in which the flowing material and the environment have different temperatures. A coriolis flowmeter for measuring hot material in cold climates eventually reaches a thermal equilibrium state where the titanium flow tube reaches the temperature of the material and the balance bar is only slightly cooler. The housing may be much cooler depending on environmental conditions, such as use in arctic regions. Stresses are created when the cold case limits the expansion requirements created by the balance bar and flow tube. Stresses also develop when the stainless steel balance bar attempts to expand at twice the expansion rate of the titanium flow tube. Under these conditions, the hot balance bar attempts to expand in length, while the cold outer shell does not. The inactive portions 101A and 101B of the flow tube are connected between the ends of the case and the expanded balance bar. The balance bar and case are both of much larger cross-section than the flow tube and they force the inactive portions 101A and 101B of the flow tube to shorten in length. Because the two non-working portions of the flow tube attempt to elongate in length, the force exerted by the larger balance bar applies stress to the flow tube portions 101A and 101B. This stress level may exceed the yield strength of the titanium flow tube. At the same time, the ends of the active portion 101 of the flow tube are bounded by balance bars and brace bars. The stainless steel balance bar has a much greater coefficient of expansion than the titanium flow tube. Thus, depending on the temperature differential between the stainless steel balance bar and the titanium flow tube, the active portion of flow tube 101 may be under tension. The balance bar may also be subjected to compressive forces when its temperature is lower than the flow tube temperature.

It can thus be seen that the prior art straight tube coriolis flowmeter shown in fig. 1 is subject to thermal stresses on the flow tube which degrade the output information produced by the flowmeter and, in extreme cases, can also permanently damage the flow tube.

Description of FIG. 2

Fig. 2 discloses a first possible exemplary embodiment of the present invention, including a straight tube coriolis flowmeter 200 that is similar in many respects to the prior art coriolis flowmeter 100 of fig. 1. The difference is that fig. 2 and the center section of the balance bar have been removed.

The flow tube of the straight tube coriolis flowmeter 200 disclosed in fig. 2 has a working portion 201 and non-working portions 201A and 201B. Coriolis flowmeter 200 further includes balance bar end sections 202, 203 and an empty center section 202V, a housing 204 and end flanges 212, 212A. The housing 204 has end portions 208 and 209 connected to end flanges 212 and 212A by necks 205 and 215. The inlet of the flow meter is the left part 206 and the outlet is the right part 207. Tapered connecting segments 213 and 214 couple the inner walls of necks 205 and 215 to the outer surfaces of flow tube sections 201A and 201B. Brace bars 210 and 211 couple axially outer end portions of balance bar sections 202 and 203 to flow tube 201. The transducers LPO and RPO each comprise a coil C and a magnet M. The driver D includes a magnet 217 secured to the flow tube 201 and a coil 216 attached to the flat surface of a driver bobbin 221, the leg portion of the bracket 221 being attached to the inner wall 220 of the housing 204. The component 222 is the axially inner end of the balance bar segment 202; the component 223 is the axially inner end of the balance bar segment 203.

In the same manner as described in fig. 1, driver D causes flow tube 201 and balance bar sections 202, 203 to vibrate in opposite phases. The vibration of the flow tube 201 extends the vibrational forces through the brace bars 210 and 211 to the ends of the balance bar end sections 202 and 203 causing them to vibrate in opposite phase to the flow tube 201 with respect to the drive mode vibration of the flow tube. The pickoffs (speed pickoffs) LPO and RPO sense the coriolis response of the vibrating flow tube 201 with material flow and generate an output signal indicative of the material flow. These output signals are transmitted via paths 122 and 124 to meter electronics 121, which processes the signals and generates an output signal indicative of the material flow.

Because the balance bar center portion of the coriolis flowmeter 200 of fig. 2 has an empty section 202V, two separate balance bar end sections 202 and 203 are secured to respective brace bars 210 and 211 and, by way of these brace bars, to the active portion 201 of the flow tube. The balance bar end sections 202 and 203 behave like cantilever beams, each having a resonant frequency like a vibrating material filled flow tube. Because the flow tube and balance bar end sections 202, 203 are vibrating in opposite phases and because they have the same resonant frequency, they form a dynamically balanced vibrating structure that does not transmit vibration from the outside to the flow meter.

Another advantage of the coriolis flowmeter 200 of fig. 2 is that the balance bar's hollow central section 202V enables the balance bar end sections 202 and 203 to expand or contract in length in response to changing thermal conditions without transferring a stress to the active portion 201 of the flow tube. As a result, the material used for the balance bar need not be expensive stainless steel, but can be made of a less expensive material. The only resulting thermal stress on the flow tube of fig. 2 is that transferred by the relative thermal expansion or contraction of the case.

Summarizing the embodiment of fig. 2, the void section 202V located halfway between the two end sections 202 and 203 of the balance bar lowers the drive mode resonant frequency of the sections 202 and 203 to the resonant frequency of the flow tube 201. The void section 202V also lowers the coriolis-like mode resonant frequency of the balance bar below the drive frequency. This void section thereby enhances the coriolis like out of phase response of the balance bar end sections 202 and 203 with respect to the flow tube 201. This enhances the material flow sensitivity of the coriolis flowmeter. Flowmeter 200 requires that driver D be mounted between the housing and the flow tube. The embodiment of FIG. 2 also facilitates the dead zone section 202V to protect the flow tube 601 from any stresses due to the axial length variation of the balance bar end sections 202 and 203.

Description of FIGS. 3, 4, and 5

Fig. 3 illustrates how the individual balance bar sections 202 and 203 of fig. 2 respond to drive mode vibrations of flow tube 201. The drive mode vibrations generate a torque that is applied by the flow tube to the brace bars 210 and 211. This torque extends to the end sections 202 and 203 of the balance bar causing them to vibrate in opposite phase to the corresponding portions of flow tube 201. This deflection of the balance bar end section opposes the deflection of the vibrating flow tube such that the flow tube and balance bar end section together cancel each other's vibrations and torques and create a dynamically balanced vibrating structure. The balance bar also has the advantage that it reduces the cost of the material used for the balance bar and produces less stress on the flow tube over an extended temperature range.

The balance bar of the coriolis flowmeter of fig. 1 counteracts drive mode vibrations of the flow tube, but it never balances the vibrations of the flowmeter generated by coriolis forces exerted on the flow tube during material flow. Fig. 4 illustrates coriolis forces and deflections occurring in a vibrating flow tube 201 with material flow. The arrows indicate that the coriolis forces exerted on the two halves of the working flow tube 201 are in opposite directions. In FIG. 4, the Coriolis force arrows on the left half of the flow tube are in the upward direction; the arrow on the back half is in a downward direction. As a result, the coriolis deflections produced on the two halves of the flow tube are in opposite directions. These forces and deflections are proportional to the magnitude of the material flow rate and cannot be offset by the additional weight on the balance bar. Moreover, the force applied to the flow tube varies in magnitude and direction sinusoidally continuously in the manner of the drive. For the condition shown in fig. 4, it can be seen that the flow tube 201 attempts to rotate in a clockwise direction about its center C because an upward force is applied to its left half 303 and a downward force is applied to its back half 304. At a later time in the oscillation cycle, these forces change direction and the flow tube then attempts to rotate in a counterclockwise direction about its center C. This oscillatory change in rotational force on the flow tube produces undesirable vibrations that can adversely affect the accuracy of the output of material flow information produced by the flowmeter.

Since the coriolis deflections of fig. 3 occur at the drive mode frequency, it can be seen that the balance bar end section is readily responsive to these coriolis deflections of the flow tube as it is responsive to the drive mode deflections of the flow tube. The driving force for both responses is the same. Which is a brace bar 210. And 211. This is illustrated in fig. 5. It can be seen that the left balance bar segment 202 has the same response to the same excitation as the left half of the balance bar of fig. 3. The difference between these two excitation modes is that the drive excitation is of constant amplitude and the two ends of the active portion of flow tube 201 are in phase with each other. The amplitude of the coriolis excitation mode is proportional to the material flow rate, while the vibrations of the two end sections 202 and 203 of the balance bar are 180 degrees out of phase with each other. The balance bar end sections 202 and 203 effectively cancel the coriolis force on the flow tube because they increase their amplitude as the flow rate and coriolis force increase. It can be seen in fig. 5 that the deflections of balance bar end sections 202 and 203 are out of phase with the coriolis deflections of their respective portions of flow tube 201. As a result, coriolis forces exerted on the vibrating flow tubes with material flow are effectively cancelled out by the offset vibrational deflections of their respective portions of the balance bar end sections 202 and 203. This coriolis force cancellation results in a more accurate coriolis flowmeter because the unbalanced coriolis forces of prior art coriolis flowmeters that would cause the coriolis flowmeter to oscillate at the drive frequency are eliminated in the coriolis flowmeter of the present invention.

Description of FIGS. 6 and 7

Fig. 6 and 7 disclose another embodiment of a coriolis flowmeter 600 embodying the invention. This embodiment differs from the embodiment of fig. 2 primarily in that the two balance bar end sections 602 and 603 are coupled by a central section that includes a flexible drive coil former 640. The bracket 640 allows the coils of a drive D to be mounted in conventional positions as part of a balance bar. The driver coil and an attached magnet on the flow tube can directly drive the balance bar end section in phase opposition to the flow tube 601. The structure of the drive coil bobbin 640 includes a leaf spring 638 that can flex, allowing the balance bar end section to expand and contract axially without creating more stress on the flow tube than those stresses attendant to the forces required to flex the leaf spring 638. Leaf spring 638 also allows balance bar end sections 602 and 603 to adopt a coriolis like response that is out of phase with the coriolis response of the flow tube and counteracts the coriolis deflections of the vibrating flow tube.

The drive bobbin structure 640 includes a flat surface 646 on which the driver coil 644 is mounted. The structure 640 comprises four leaf springs 638 having a right angle bend at their lower ends and being secured to support rods 642, the support rods 642 forming an extension of the inner ends 636, 637 of the equalizer bar end sections 602 and 603. Part 640A is a bracket having an aperture 641 which receives block 643. The bracket 640A is coupled to the support rods 642 with a set of lower leaf springs 638A. The mass dynamically balances the mass of the drive coil 644.

The remainder of the flow meter structure of the embodiment of fig. 6 and 7 is similar to the remainder of the embodiment of fig. 2 and includes the following components: housing 604, housing ends 608 and 609, inlet 606, necks 605 and 615, tapered connecting members 613 and 614, flow tube 601 including its non-working end portions 601A and 601B, housing connecting segments 631 and 632 with out-of-plane bending member 634, struts 610 and 611 including side wall extensions 610A main 611A, inner wall 620 of housing 604, sensors LPO and RPO and driver D, magnet carrier 639, magnet M mounted on carrier 639, coil 644, inner walls 602A and 603A of balance bar end sections 602 and 603, and outlet 607. These components are all similar to their counterparts in the embodiment of fig. 2 and perform the same function.

The leaf spring 638 of fig. 6 and 7 has thermal expansion properties such that the leaf spring 638 does not stress the flow tube 601 as the balance bar end sections 602 and 603 change length. The elongation or contraction of the end section of the balancing rod bends the leaf spring leg. Since the leaf spring is thin, this bending creates only a small stress in the leaf spring legs. The only stress on the flow tube is the stress with little force required to bend the leaf spring 638. This embodiment reduces the drive mode resonant frequency of the balance bar end sections 602 and 603 to the resonant frequency of the flow tube 601. It also lowers the coriolis-like mode resonant frequency of the balance bar below the drive frequency. The reduced resonant frequency of the balance bar end sections 602 and 603 enables them to have a coriolis like response with a phase opposite to the coriolis deflection of the flow tube 601. Balancing this coriolis-like response of the tube end sections increases the material flow sensitivity of the coriolis flowmeter of the embodiment of fig. 6 and 7 and balances the coriolis forces on the flow tube.

In the same manner as described in the embodiment of FIG. 1, meter electronics 121 sends a signal via path 123 to driver D causing balance bar 602 and flow tube 601 to vibrate in opposite phases. The pickoffs LPO and RPO sense the vibration of the flow tube 601 with the material flow and generate output signals indicative of the magnitude and phase of the coriolis response. The output signal of the sensor is transmitted via paths 122 and 124 to meter electronics 121, which produces an output on path 125 containing information about the material flow.

Summarizing the embodiments of fig. 6 and 7, a bendable drive coil former 640 is located halfway between the balance bar end sections 602 and 603. The resonant frequency of the drive mode of sections 602 and 603 is reduced to the resonant frequency of flow tube 601. The flexible drive coil former 640 also reduces the resonant frequency of the coriolis-like deflection of the balance bar end segments 602 and 603 below the drive frequency. This enhances the coriolis like out of phase response of balance bar end sections 602 and 603 with respect to flow tube 601. This increases the material flow sensitivity of the coriolis flowmeter. However, care must be taken to design the drive coil former 640 to prevent unwanted vibrations that may adversely affect the accuracy or output data of the coriolis flowmeter. An advantage of this embodiment is that leg leaf spring 638 easily bends and protects flow tube 601 from axial stress in response to changes in the axial length of balance bar end sections 602 and 603.

Description of FIGS. 8 and 9

Fig. 8 and 9 disclose yet another exemplary embodiment of a coriolis flowmeter 800 incorporating the invention. This embodiment is similar in many respects to the embodiment of fig. 2, 6 and 7, except for the drive bobbin structure in the mid-balance bar between the balance bar end sections 802 and 803. The embodiment of fig. 2 has a hollow section 202V as the central section of the balance bar; the embodiment of fig. 6 and 7 has a bendable drive coil former 640 as the central section of the balance bar. The flow meter 800 of fig. 8 and 9 has a central drive coil former 841 which connects the axially inner ends 836 and 837 of the balance bar end sections 802 and 803.

The drive coil former 841 has an outer circumferential surface 843 on top of which a flat surface 838 can receive the coils 844 of the driver D. The drive coil former 841 also has a slot 842. The drive coil former 841 is connected to the axially inner ends 836 and 837 of the balance bar end sections 802 and 803 by support rods 835. The equalizer bar end section 802 has a slot 833 near its right end; the balance bar end section 803 has a slot 833 near its left end. The slots 833 of the two balance bar end sections and the corresponding slots 842 of the drive coil former 841 form leg springs 846 which provide an axial compliance that accommodates thermal expansion and contraction of the balance bar end sections 802 and 803. The rear side of the equalizer bar end section and the rear side of the drive coil former 841 have similar slots which are not visible in this figure. The compliance provided by the leg spring 846 is not as great as in the two embodiments described above. However, this compliance can significantly reduce the stresses in the flow tube caused by expansion and contraction of the balance bar. The slots 832 and 833 also reduce the resonant frequency of the balance bar end sections 802 and 803 thereby also facilitating the springs determining the balance of the components, providing the balance bar end sections 802 and 803 with a lower resonant frequency that allows the components to have a coriolis-like response in phase opposition to the coriolis deflection of the flow tube 801. The remaining components of the embodiment of fig. 8 and 9 are similar to those already described for the embodiment of fig. 2 and 6 and 7. These components include housing 804, housing ends 808 and 809, necks 805 and 815, inlet 806, outlet 807, tapered connecting components 813 and 814, flow tube sections 801A and 801B, housing connecting segments 831 and 832 with out-of-plane bends 834 and 834A, struts 810 and 811 along with strut inner wall extensions 810A and 811A, sensors LPO and RPO, driver D, inner wall 820 of housing 804.

In the same manner as described for the embodiment of FIG. 1, meter electronics 121 applies a signal to driver D via path 123 to vibrate balance bar 802 and flow tube 801 in opposite phases. The pickoffs LPO and RPO sense the vibration of the flow tube 801 with the material flow and generate output signals indicative of the magnitude and phase of the coriolis response. The sensor output signals are transmitted via paths 122 and 124 to meter electronics 121, which produces an output on path 125 containing information about the material flow.

Summarizing the embodiments of fig. 8 and 9, a bendable drive coil former 841 halfway between balance bar end region ends 802 and 803 lowers the drive mode resonant frequency of sections 802 and 803 to the resonant frequency of the flow tube. It also reduces the resonant frequency of the coriolis-like deflection mode below the drive frequency. This enhances the coriolis like out of phase response of balance bar end sections 802 and 803 with respect to flow tube 801 and increases the material sensitivity of the coriolis flowmeter. However, the drive coil former 841 must be carefully designed to prevent unwanted vibrations that may adversely affect the accuracy or output data of the coriolis flowmeter. An advantage of this embodiment is that leg springs 846 formed by slots 833 and 842 act as springs that flex and protect flow tube 801 from axial stresses in response to changes in the axial length of balance bar end sections 802 and 803.

Description of FIGS. 10, 11 and 12

Fig. 10, 11 and 12 disclose a coriolis flowmeter 1000 implementing yet another exemplary embodiment of the invention. This embodiment differs from the previously described embodiment only in the details of the central drive bobbin 1040 which makes up the central portion of the balance bar, the other two sections of the balance bar being the left end portion 1002 and the right end portion 1003. The drive bobbin 1040 includes a pair of drivers D1 and D2, a block member 1041 on the right end of the balance bar end section 1002, a block 1035 on the left end of the balance bar end section 1003, bobbins 1042 and 1043, a leaf spring 1045 connecting the drive bobbins 1042 and 1043, driver coils 1044 and 1045 and associated magnets 1202 and 1204, a flow tube bracket 1042 having a flat surface 1046 capable of mounting coils 1044 and 1044A. As shown in detail in fig. 12, the top surface 1046 of the drive bobbin 1042 has an arcuate cutout 1208 for receiving the magnet 1202. The top ends of the leaf springs 1045 are fixed to the right vertical surface 1209 of the drive bobbin 1042.

To minimize the complexity of the pattern, the block 1035 and the drive bobbin 1043 of fig. 10, which are fixed to the balance bar segment 1003, are not shown in fig. 12. However, as will be apparent to those skilled in the art, the coil 1044 of the driver D2 of fig. 12 will be secured to the driver bobbin 1043, while the lower ends of the leaf springs 1045 will be secured to the left vertical surface of the driver bobbin 1043.

A leaf spring 1045 movably couples the central end portions of the balance bar sections 1002 and 1003 to enable them to change length in response to changing thermal conditions. This change in length of the balance bar sections 1002 and 1003 causes the leaf spring 1045 to bend without creating stress on the flow tube. In other words, the change in length of the balance bar segments 1002 and 1003 only causes the leaf spring 1045 to bend, without creating any stress on the flow tube 1001 other than the stress associated with the small force required to bend the spring 1045.

Fig. 12 discloses details of brace bar 1010 and its lateral projections 1001A, projections 1001A tightly coupling the sides of flow tube 1001 to the sides of the inner wall 1002A of balance bar sections 1002, 1003. This coupling increases the frequency of the undesired lateral vibrations of the flow tube so they do not interfere with the drive frequency signal from the velocity sensor.

The embodiment of fig. 10, 11 and 12 has good thermal response because the bendable springs 1045 allow the balance bar sections 1002 and 1003 to freely change length without transferring the resulting stresses to the flow tube 1001. The central drive bobbin 1040 has minimal spurious modes of vibration. Leaf spring 1045 couples inner ends 1036 and 1037 of balance bar segments 1002 and 1003, preventing them from having significant movement out of phase with each other. As a result, coriolis-like deflections are not induced in the balance bar segments 1002 and 1003. Thus, the embodiments of fig. 10, 11, and 12 do not have the material flow sensitivity of the previously described embodiments.

Blocks 1035 and 1041 improve accuracy by making the mass distribution symmetric about a plane perpendicular to the drive plane and containing the flow tube axis. For example, the weight of the block 1041 is equal to the weight of the drive coil 1044-driver drive bobbin 1042. Without these added masses, the vibrations transmitted axially to the meter produce an erroneous flow signal because it transmits a coriolis-like deflection to the balance bar.

The remainder of the coriolis flowmeter shown in fig. 10, 11 and 12 is similar to that already described in the previous embodiment. These components include housing 1004, housing ends 1008 and 1009, housing necks 1005 and 1015, flow tube inlet 1006 and flow tube outlet 1007, tapered connection members 1013 and 1014, inactive portions 1001A and 1001B of flow tube 1001, housing connection segments 1031 and 1032 having lateral ends 1033 connected to inner wall 1020 of housing 1004, transducers LPO and RPO, a pair of drivers D1 and D2, drive bobbins 1042 and 1043, blocks 1041 and 1035, out-of-plane bends 1034 in housing connection segments 1031 and 1032.

In the same manner as described for the embodiment of fig. 1, meter electronics 121 applies a signal to driver D via path 123 to vibrate balance bar 1002 and flow tube 1001 in opposite phases. The sensors LPO and RP0 detect the vibration of the flow tube 1001 with the material flow and generate output signals indicative of the magnitude and phase of the coriolis response. The sensor output signals are transmitted via paths 122 and 124 to meter electronics 121, which generates a signal containing information about material flow on path 125.

Summarizing the embodiments of fig. 10, 11 and 12, the advantage of the bendable drive coil former 1040 halfway between the balance bar end sections 1002 and 1003 is that the leaf springs 1045 are easily bendable and protect the flow tube 1001 from axial stresses, depending on the axial length variation of the balance bar end sections 1002 and 1003. Unlike the previous embodiment, the leaf springs of this embodiment do not sufficiently reduce the frequency of the coriolis-like deflections and therefore do not increase the sensitivity of the flow meter.

It is to be clearly understood that the invention as claimed is not limited to the description of the possible preferred exemplary embodiments but includes other modifications and variations which are within the scope and spirit of the inventive concept. For example, while the present invention is disclosed as comprising a portion of a single straight tube coriolis flowmeter, it is to be understood that the present invention is not so limited and can be used with other types of coriolis flowmeters including single tube flowmeters in irregular or curved configurations as well as coriolis flowmeters having multiple flowtubes.

Claims

1. A coriolis flowmeter adapted to receive a flow of material at an inlet and pass said flow of material through a length of flow tube means to an outlet of said coriolis flowmeter, said coriolis flowmeter further comprising:

a balance bar positioned parallel to said flow tube means;

a driver for vibrating said flow tube means and balance bar in opposite phases;

2. The coriolis flowmeter of claim 1 including a case and wherein said flow tube means comprises a straight flow tube.

3. The coriolis flowmeter of claim 2 characterized in that said balance bar is coaxial with said flow tube.

4. The coriolis flowmeter of claim 2 characterized in that said pick-off means comprises a pair of velocity pick-offs, a first of said pick-offs being coupled to said first portion of said balance bar and said straight flow tube and a second of said pick-offs being coupled to said second portion of said balance bar and said straight flow tube.

5. The coriolis flowmeter of claim 2 said case enclosing said straight flow tube, said brace bar and said balance bar.

6. The coriolis flowmeter of claim 1 characterized in that said flow tube mechanism is a single flow tube and said sensor mechanism comprises a pair of velocity transducers, a first of said velocity transducers coupled to said first balance bar portion and said single flow tube and a second of said velocity transducers coupled to said second balance bar portion and said single flow tube.

7. The coriolis flowmeter of claim 1 characterized in that said driver comprises:

the spring mechanism includes a set of first set of springs defined by walls of the drive coil carrier mechanism and positioned between the first slot and an axially outer end of the drive coil carrier mechanism, the first set of springs being curved in response to changes in axial length of the balance bar.

8. The coriolis flowmeter of claim 1 characterized in that,

9. The coriolis flowmeter of claim 8 characterized in that said drive coil carrier means comprises:

a block secured to the flat surface of the second drive coil former.

10. The coriolis flowmeter of claim 9 characterized in that said first and second sets of springs have ends connected to said first and second balance bar portions.

11. The coriolis flowmeter of claim 1 characterized in that said drive coil carrier means comprises:

a first drive coil bobbin secured to the top of said balance bar;

a second drive bobbin secured to the bottom of said balance bar;

12. The coriolis flowmeter of claim 11 characterized in that said spring mechanism comprises:

a first end of said spring mechanism coupled to said first drive bobbin;

a second end of said spring mechanism coupled to said second drive bobbin.

13. The coriolis flowmeter of claim 12 further comprising:

a first block fixed to a lower portion of the axially inner end of the first portion of the balance bar;

a second block fixed to an upper portion of said axially inner end of said balance bar second portion.

14. The coriolis flowmeter of claim 13 characterized in that said driver comprises:

a first drive coil secured to one surface of said first drive coil bobbin;

a second drive coil secured to one surface of said second drive coil bobbin;

15. The coriolis flowmeter of claim 7 characterized in that:

the spring mechanism includes a second set of springs positioned in the wall between the second slot and the axially extending support bar, the second set of springs being axially curved in accordance with the change in axial length of the balance bar.

16. The coriolis flowmeter of claim 15 characterized in that:

17. The coriolis flowmeter of claim 16 characterized in that:

18. The coriolis flowmeter of claim 16 characterized in that: