HK1001139B - Increased sensitivity coriolis effect flowmeter using nodal-proximate sensors - Google Patents

Abstract

An increased sensitivity Coriolis flowmeter (310) having one or more drivers (D) oscillating a flow tube (130), or tubes (130, 130*), at pre-determined frequencies thereby producing nodes (N) including active nodes (AN) and/or static nodes (SN) at points along the flow tube(s) (130, 130*). The increased sensitivity of the present flowmeter (310) is provided by controllably locating sensors (S) in close proximity to the static nodes (SN) and /or active nodes (AN). In a first embodiment, the flow tubes (130, 130*) are oscillated in a manner generating a single active node (AN) with sensors (S) positioned in close proximity to, and on opposing sides of, the active node (AN).

Description

High sensitivity coriolis effect flow meter using nodal proximity type sensors

Background of the invention

The present invention relates to coriolis effect flow meters and, more particularly, to coriolis effect flow meters having pickoff sensors located in relatively close proximity to one or more oscillation nodes of the flow tube.

Problem solving

As is well known, coriolis effect flow meters can measure mass flow and other information as material flows through a conduit. Flow meters of this type have been disclosed in U.S. patent 4109524 at 29.8.1978, in U.S. patent 4491025 at 1.1.1985, and in U.S. reissue patent 31450 at 11.2.1982, all of which are issued to j.e. smith et al. These meters have one or more straight or curved flow tubes. Each flow tube configuration in a coriolis mass flowmeter has a set of natural vibration modes that may be simple bending, simple torsion, or a combination thereof. Each flow tube is driven to produce resonant vibration in one of these natural modes. Material flows into the meter from a conduit connected to the input side of the meter, is directed through one or more flow tubes, and is discharged out of the meter through the output side. The natural vibration modes that cause the fluid filling the flow tube to be excited are defined in part by the combined mass of the flow tube and the material within the flow tube.

When no fluid flows through the flowmeter, all points along the flow tube vibrate in the same phase under the applied driving force. When the material begins to flow in, coriolis accelerations cause each point along the flow tube to have a different phase. The phase at the input side of the flow tube will lag behind the driver, while the phase at the output side will lead the driver. If the sensor is placed on the flow tube, a sinusoidal signal corresponding to the flow tube motion is generated. The phase difference between the two sensor signals is proportional to the mass flow rate of the material flowing through the flow tube.

One complicating factor in such measurements is that the density of typical process fluids varies. This change in density results in a change in the frequency of the natural mode. Since the flowmeter control system is to remain resonant, its vibration frequency is also to change in response to changes in density. Under this condition, the mass-to-flow rate is proportional to the ratio of the phase difference to the vibration frequency.

In the aforementioned U.S. reissue patent 31450 to Smith, the coriolis flowmeter disclosed overcomes the need to measure both phase difference and vibration frequency simultaneously. It measures the phase difference by measuring the time delay between the horizontal zero crossings of the two sinusoidal signals of the flow meter. When this method is used, this change in vibration frequency is cancelled and the mass to flow is proportional to the measured time delay. This measurement method is referred to hereinafter as time delay or Δ t measurement.

Since the phase difference between the two sensor output signals is proportional to the mass specific flow rate of the material flowing through the one or more flow tubes, it is often possible to create a point where the phase difference is not detectable when the mass specific flow rate is reduced due to limitations in the sensitivity of the instrument and the effects of noise. If it is desired to measure the mass specific flow rate of a low density material, such as a gas or the like, at low pressure, a very high phase detection sensitivity is required to detect a corresponding relatively low phase difference in the flow meter output signal. Many conventional coriolis flowmeters do not have corresponding phase detection sensitivity and therefore they cannot be used to measure the flow of gas at low pressures or low flow rates.

Therefore, there is a need for a flowmeter that can measure the mass specific flow of materials, such as gases at low flow rates and low pressures, with improved sensitivity.

Technical solution

The above-identified problem of requiring coriolis flowmeters to have greater material mass to flow sensitivity is addressed by the flowmeter of the present invention. The flowmeter is configured to increase its sensitivity by controlling the position of the transducers as close as possible to the nodes of the flow tube. The nodes may be some static nodes or some vibrational nodes (hereinafter "dynamic nodes"). A node is a point along a vibrating flow tube where the amplitude of the vibration is zero. The stationary nodes are nodes at the flow tube support bar or other fixed flow tube ends where the flow tube vibrations are mechanically damped to form a zero amplitude point. Dynamic nodal points are one or more nodes formed freely along a vibrating flow tube other than at the location of the stationary nodal point, and the location of the dynamic nodal point is determined by the frequency of vibration, the drive location, and the resulting vibration of the flow tube when no material is flowing through the flow tube, etc.

The present invention provides an improved method and apparatus for measuring the mass to flow rate of a material flowing through a conduit. The apparatus and method disclosed herein have a higher measurement sensitivity and can therefore be used to measure the mass specific flow of low density fluids such as low pressure gases. In operation, the flow tube using the present invention will vibrate and a measurement of the time difference (Δ t) can be obtained from the output signals of a pair of sensors controllably positioned near one or more nodes. This increased measurement sensitivity is ensured by placing the pair of sensors, where practicable, along the flow tube as close to the node as possible.

The flow meter of the present invention uses one or more drivers to vibrate one flow tube (or two flow tubes with a parallel configuration) at a frequency that produces the desired dynamic node. The drivers may be in contact with the one or more flow tubes at or near an antinode, or at any other location other than a node of the natural frequency of vibration of the one or more flow tubes.

In the two preferred embodiments of the invention disclosed herein, two parallel "modified U-shaped" flow tubes having substantially straight top portions are used which are connected to two downstream and inboard diagonal flow tube "legs". In one embodiment, the flow tube vibrates in a manner that creates a single dynamic node at the midpoint of the top portion that connects to the legs of the flow tube. Two sensors for detecting flow tube motion are located as close as possible to the dynamic node and on opposite sides of the dynamic node.

In further embodiments, the flow tube may be vibrated at a higher frequency (compared to the first embodiment) to create multiple dynamic nodes. In these further embodiments, two pickoff sensors may be provided on opposite legs of the pipe, i.e. one sensor is provided at a position above the dynamic node of one leg of the pipe and the other sensor is provided at a position below the corresponding dynamic node of the other leg of the pipe. Positioning the sensors relative to the flow tube legs allows the sensors to be positioned as close to the corresponding dynamic nodes as possible in a predetermined pattern without being limited by the physical dimensions of the sensors.

The pickoff sensors may be controlled to be positioned sufficiently close to the dynamic or static nodes in each vibration mode so that the signal-to-noise ratio generated by the sensor electronics is maximized.

The flow meter of the present invention may be used with substantially straight or curved tubes, as well as other types of tubes.

Brief description of the drawings

The above and other advantages and features of the present invention will be better understood from the following description taken in conjunction with the accompanying drawings.

Fig. 1 illustrates a prior art coriolis flowmeter.

Figure 2 shows graphically a graph of the relationship between the amplitude of the output signal, the phase and the flow tube position of the sensor, corresponding to the dynamic node and the noise level, for the instrument concerned.

Fig. 3 shows a preferred embodiment of the flow meter of the present invention using an improved "U" shaped flow tube.

Fig. 4 and 5 show schematic diagrams of pickup sensor and driver locations in general examples of bending and torsion modes, respectively.

FIG. 6 shows a schematic of sensor and driver positions when the flow meter is operating in bending mode.

Fig. 7 and 8 show schematic diagrams of various sensor and driver positions in first and second out-of-phase torsional modes, respectively.

Fig. 9 shows another alternative embodiment of the present invention using a straight flow tube.

Fig. 10 shows a graph of the displacement of various portions of the flow tube of fig. 9.

Detailed Description

Prior Art

Fig. 1 shows a mechanical assembly 10 and an electronic assembly 20 of a coriolis flowmeter. The electronics assembly 20 is connected to the mechanical assembly 10 by wires 100 and supplies density, mass specific flow, volumetric flow rate and summed mass flow information to the passageway 26.

The mechanical assembly 10 includes a pair of manifolds 110 and 110 ', tubular members 150 and 150 ', a pair of parallel flow tubes 130 and 130 ', a drive mechanism 180, and a pair of speed sensors 170L and 170R. The flow tubes 130 and 130 'have two substantially straight input side branch tubes 131 and 131' and output side branch tubes 134 and 134 'that converge forwardly toward each other at the manifold members 120 and 120'. Gussets 140 and 140 'define axes W and W' about which each flow tube vibrates.

The side branch tubes 131 and 134 of the flow tubes 130 and 130 ' are fixedly attached to the flow tube mounting members 120 and 120 ' and the latter are fixedly attached to the manifold members 150 and 150 '. This results in a continuous closed material path through coriolis flowmeter mechanical assembly 10.

When the mechanical assembly 10 with the flange 103 with the holes 102 is connected, via the input side end 104 'and the output side end 101', to a flow pipe system (not shown) for guiding the process material to be measured, the process material will enter the flow meter through the apertures 101 in the flange 103 of the end 104 on the input side manifold 110 and be guided to the manifold element 120 with the surface 121 by a passage route with a stepwise change of its cross section. Here, the material will flow separately through the branch tubes 131 and 131 ', the flow tubes 130 and 130 ' and the branch tubes 134 and 134 '. Upon reaching the output side branch pipes 134 and 134 ', the treatment material will again be combined in one piece in the manifold element 150 ' and then flow through the output manifold 110 '. In the output side manifold 110 ', material of another channel route through which the cross section changes stepwise will flow from the manifold 110 to the orifice 101 ' at the output side end 104 '. The outlet-side end 104 ' is connected to a pipe system (not shown) via a flange 103 ' having a threaded bore 102 '.

Flow tubes 130 and 130 'may be suitably selected and mounted on members 120 and 120' to provide the same mass distribution, moment of inertia, and modulus of elasticity with respect to bending axes W-W and W '-W', respectively. These bending axes are static nodes and are located at the respective flow gussets 140 and 140 'and members 120 and 120'. The flow tubes extend outwardly from the mounting member in a substantially parallel manner and have substantially the same mass distribution, moment of inertia, and modulus of elasticity relative to the respective bending axes.

The two flow tubes 130 are driven by the driver 180 in opposite directions relative to the respective bending axes W and W' at the first out of phase natural frequency of the meter. This mode of vibration is also referred to as out-of-phase bending mode. The two flow tubes 130 and 130' vibrate out of phase like the fingers of a tuning fork. The drive mechanism 180 may be provided in any known manner including, for example, a magnet mounted to the flow tube 130' and an opposing coil mounted to the flow tube 130 and alternating current is passed through the coil to vibrate the two flow tubes, etc. The meter electronics 20 can be used to send an appropriate drive signal to the drive mechanism 180 via lead 185.

The drive mechanism 180 and the resulting coriolis force will cause the flow tube 130 to vibrate in a periodic manner with respect to the axes W and W'. During the first oscillation half-cycle of the flow tube 130, the adjacent side branches 131 and 131 'are forced closer than their cooperating side branches 134 and 134' and reach the end of travel earlier than their cooperating parts, where their velocity crosses zero. During the second half of the Coriolis vibration, the flow tube 130 will undergo opposite relative motion, i.e., adjacent side branches 134 and 134 'are forced closer than their associated side branches 131 and 131' and reach the end of travel earlier than their associated components, where their velocity crosses zero. The time interval (hereinafter referred to as the frequency-specific phase difference, or time difference, abbreviated as the "Δ t" value), at which the pair of adjacent side branches reach the end of travel earlier than the associated components (i.e., the forced apart components) reach the end of travel, is substantially proportional to the mass-to-flow rate of the treatment material through meter assembly 10.

To measure this time interval Δ t, a pair of sensors 170L and 170R may be attached near the upper ends of the flow tubes 130 and 130'. The sensor may be of any known type. The signals generated by the sensors 170L and 170R give the velocity profile of the entire travel of the flow tube, which can be processed by meter electronics 20 in any known manner to calculate the time interval Δ t and hence the mass specific flow rate of the material through the meter.

Sensors 170L and 170R provide left and right velocity signals to leads 165L and 165R, respectively. The time difference, or delta t measurement, provides a representation of the phase difference that results between the left and right velocity sensor signals. Note, however, that the two sensors 170L and 170R are located at considerable distances from the static nodes located at the gussets 140 and 140', respectively. As described below, this larger distance between the static node and the sensor will reduce the resolution of the material flow measurement.

Meter electronics 20 receives left and right speed signals over conductors 165L and 165R, respectively. The electronics assembly 20 also generates a drive signal that is sent through a lead 185 to a drive mechanism 180, which drives the flow tubes 130 and 130' into vibration. The electronics assembly 20 processes the received left and right velocity signals to calculate the mass specific flow, volumetric flow rate and material density of the material flowing through the meter assembly 10.

Description of the drawings FIG. 2

Fig. 2 schematically illustrates the relationship between various parameters of coriolis flowmeter 310 of fig. 3, using the position of sensor S on flow tubes 130 and 130' as an example. The parameters illustrated by FIG. 2 include the phase and displacement amplitude of the vibrating flow tube for various possible sensor locations, the sensor output signals obtainable from the sensor locations corresponding to different flow tube locations of the sensor, the resulting phase shift between the two sensor output signals corresponding to different sensor locations, and the noise level in the sensor output signals. Fig. 2 illustrates bending and torsion modes of operation that are applicable to, but not limited to, coriolis flowmeters of various forms, including those shown in fig. 1, 3 and 9.

The term "output signal amplitude" refers to the amplitude of the output signal from the pick-off sensors SL and SR in fig. 3. The output signal amplitude is proportional to the displacement of the flow tube from its center position. The Y-axis is called the tangent and represents the phase shift of the two sensor output signals. The X-axis represents the distance between a single dynamic node AN and various sensor locations. The sensors may be controlled to be positioned at different locations on either side of the dynamic node AN at the locations represented by the vertical lines in the middle of fig. 2. The left vertical line BL represents the position of a left strut, such as strut BL in fig. 3. The rightmost vertical line BR represents the right side gusset BR in fig. 3 with respect to the position of the dynamic node AN. Vertical lines DL and DR to the left and right of the AN dynamic node represent the locations of the drivers DL and DR in fig. 3.

Curve 201 represents the phase shift that may occur when the left sensor SL moves in the middle of the range of possible positions from the left gusset BL to the right dynamic node AN. As can be seen, the phase shift of the sensor output signal is moderate in magnitude near the vertical line BL and thus starts to decrease until the region of the vertical line 206. The reduced noise level is then maintained until the region bordering vertical line 207. From here to the right, the phase shift becomes considerably larger at sensor positions close to the dynamic node AN. The phase shift of the right sensor DR is negative and, as shown in the lower right quadrant of fig. 2, it changes from a moderate level around the position of the vertical line BR. This begins to decrease, always in the region of the vertical lines 214 and 213. At the sensor locations close to the dynamic node AN, it becomes considerably larger in the negative direction.

The displacement over different parts of the flow tube is shown by curve 203. The curves 203 also show the relative amplitude of the sensor output signal at the location with respect to each curve 203. As can be seen, sensor output signal 203 is below the noise level near both the right and left vertical lines BR and BL, and near the dynamic node AN represented by the position between vertical lines 209 and 211. The flow tube position between vertical lines 206 and 207 is not the optimal position for left sensor SL because the available phase shift 201 is quite small. A similar state can also be obtained if the right sensor SR is disposed at a position between the vertical lines 213 and 214. Since the output signal output from the left sensor SL is made large in both amplitude and phase shift, the position between the vertical lines 207 and 209 is the optimum position for the arrangement of the left sensor SL. Similarly, the position between vertical lines 211 and 213 is the optimum position for the right sensor SR for its placement, since the signal amplitude and phase shift of the available sensor output are both large and the noise level is minimized.

In accordance with the teachings of the present invention, to eliminate the noise problem and obtain an output signal with the appropriate amplitude and phase shift, the left sensor SL may be position controlled between the flowtube positions corresponding to lines 207 and 209. Similarly, to obtain an output signal with the appropriate amplitude and phase shift, minimizing the noise potential, the right sensor SR may be controllably positioned between vertical lines 211 and 213.

The principle that the sensor S should be positioned as close to a node N as possible within a practically feasible range is the technical teaching of the present invention, regardless of which flow pipe is used, and regardless of whether the node N is a dynamic node AN or a static node SN, a node or a plurality of nodes. If the node is not located at a spreader or other support point, the sensor S may bridge the dynamic node AN, or for a static node SN located at a spreader B, the sensor S may be located as close as practical to the static node SN. The sensor S may also be connected across two dynamic nodes AN, as shown later in fig. 5. The closer the sensor S is located to the node N, the greater the value of Δ t and hence the greater the sensitivity of the measurement to mass flow. However, the amplitude of the output signal of the flow tube is inversely proportional to the value of Δ t. The present invention is directed to positioning the sensor S control as close to the node N as possible, but still at a sufficient distance from the node N to produce an output signal amplitude having a usable signal-to-noise ratio.

Description of the drawings 3

FIG. 3 illustrates an exemplary embodiment of a flow meter 310 of the present invention that utilizes a modified "U" shaped flow tube. The term "modified 'U' shaped flow tube" refers to a flow tube that includes a substantially "D" shaped flow tube having a substantially straight portion and a substantially "D" shaped flow tube having a non-linear or curved portion. The structure and function of the embodiment shown in fig. three is basically the same as the example shown in fig. one, with the only differences being the locations of the drivers DL and DR, and the locations of the pickup sensors SL and SR. Although the drivers DL and DR are also located at different positions of flowtubes 130 and 130 'than the instrument shown in figure one, the description of the present embodiment is primarily used to discuss various arrangements when locating the sensors as close as possible to a dynamic node located at a midpoint in the top region of flowtubes 130 and 130'. It will be understood that this embodiment is described for illustrative purposes only and is not meant to limit the scope of the present invention. Other embodiments are also contemplated as falling within the scope of the present invention.

The flowmeter shown in fig. 3 is operated in AN out-of-phase torsional mode, which produces a dynamic node AN at the intersection of axis NP and the center of the plane defined by the centers of flow tubes 130 and 130'. The drivers DL and DR are positioned at opposite ends of a straight region of the flow tubes 130 and 130', referred to below as the "top" region of the flow meter 310. Drivers DL and DR are driven out of phase by drive signals 322 and 324 to twist the top portions of flow tubes 130 and 130' relative to axis NP. Coriolis flowmeter 310 has pick-off sensors SL and SR located as close as possible to dynamic node AN to maximize the value of at over the signal-to-noise ratio rejection range of the flowmeter apparatus. Mass flow instrument 320 is connected to sensors SL and SR through paths 326 and 328, respectively, and to drivers DL and DR through paths 322 and 324. Mass flow instrument 320 may effectively perform the same function as electronic assembly 20 shown and described in connection with fig. one.

Mode of operation

In the known flow meters, there are two interesting modes of operation vibration. They are "bending" and "torsion" modes. The flow tube can be driven in several modes including "bending mode" and various "out of phase torsional modes". Bending modes can be achieved by driving the flow tube out of phase with respect to axes W and W' at relatively low resonant frequencies, as shown in the flow meter of figure one. Thus, static nodes will be formed at the gussets 140 and 140'. Gussets 140 and 140' are also pivot points for the flow tubes to vibrate out of phase. Out of phase torsional modes can be achieved when the flow tube is driven in a torsional mode at the end at frequencies typically higher than those used for bending modes. The flow tube vibrates in a typical torsional mode that is possible, creating a single dynamic node AN at the top (at the midpoint) of the flow tube. This is shown in figure three.

In the prior art described above and shown in fig. one, driver 180 is positioned at the top region of flow tubes 130 and 130 ' connected to flow tube branches 131/131 ' and 134/134 '. In this arrangement, the flow tubes will operate in a first out of phase "bending" mode, creating static nodes at the gussets 140 and 140'. Conventional coriolis flow measurement instruments also have their sensors properly positioned to produce a sufficiently large output signal amplitude. However, these prior coriolis flow meters do not place the sensors close to the nodes in order to maximize the phase difference of the output signals.

When flow tube legs 131/131 ' and 134/134 ' are driven in a first out of phase "twist" mode as shown in fig. 3, static node SN is located at or near the gussets BR and BL, respectively, and dynamic node AN will be created at the top center region of flow tubes 130 and 130 '. However, conventional systems do not utilize either the dynamic nodes AN or the static nodes SN as the "focus" of the positioning sensor.

The present invention is not limited to positioning the sensor S close to a single top area central dynamic node AN to increase the measurement accuracy. The present invention can also be used to increase the accuracy of the measurement of a conventional coriolis flowmeter using other "torsional" modes. The present invention can also utilize higher drive frequencies in torsional mode operation to create two or more dynamic nodes AN. The number and location of dynamic nodes AN can be determined by the location and frequency of drivers DL and DR along flow tubes 130 and 130'.

In the torsional mode of operation, for example as shown in fig. 3, drivers DL and DR may be positioned at opposite ends of the legs of flow tubes 130 and 130' at non-nodal locations. In either mode of operation (bending or torsion), the sensors SL and SR can be controlled to be positioned near (or relative to) the dynamic node AN to allow operation at AN acceptable signal-to-noise ratio and to maximize the value of at.

Description of figures 4 and 5

Fig. 4 and 5 show the positioning of the transducers and drivers relative to the nodal locations for a "normal" flow meter, which may be straight, U-shaped, or irregularly shaped. The amplitude of flow tube displacement is shown by curve a in fig. 4 as a function of the position of sensors S1 and S2, and drivers DL and DR, disposed on flow tube FT, relative to a dynamic node AN. Although the preferred embodiment of the present invention uses a pair of parallel flow tubes FT, only one flow tube is shown in fig. 3 and 4 for ease of understanding. When the meter conduit is operated in the torsional mode according to the embodiment shown in fig. 3, the sensors S1 and S2 can be placed at a location close to the dynamic node AN in a practically operable condition within the range of signal-to-noise ratio rejection shown in fig. 2, as shown in fig. 4. In some cases, the embodiment shown in fig. 5 presents another solution to this particular problem, since the physical size of the sensors S may prevent them from being placed close to the dynamic node AN. The static nodes SN are located at or near the gussets BL and BR.

As will be described in further detail below, the flow tube shown in fig. 5 is operated in a second out of phase twist mode. In this torsional mode, there are two dynamic nodes AN1 and AN2, and two drives DL and DR. For ease of illustration, only one flow tube is shown, but two flow tubes may be used. The presence of two dynamic nodes AN1 and AN2 allows pairs of sensors S1-S2 and S3-S4 to be located at any one of four possible available locations. Thus, the sensor pairs may be positioned at positions S1 and S2, S3 and S4, S1 and S4, or S2 and S3. Since there are two dynamic nodes, AN1 and AN2, the sensors can be placed on opposite sides of the flow tube so that the sensors can be placed as close as possible to the intended node under practical operational conditions, eliminating the limitation in body proximity when two sensors need to be placed on opposite sides of a given node. The embodiment of fig. 5 may also use a central DC driver, and drivers DL and DR, if desired. The static nodes SN are located at or near the gussets BL and BR.

Description of the drawings FIG. 6

In accordance with the principles of the present invention, FIG. 6 illustrates the position of the transducer and driver in a flow meter operating in a first out-of-phase bending mode. Fig. 6 shows a zero flow condition of the flow meter element. The zero offset state is shown in dashed lines. The offset state is shown as a solid line. In fig. 6, driver D1 is positioned near the midpoint of the tops of flow tubes 130 and 130' similar to the prior art shown in fig. 1, so that the static nodes SN of the flow tubes when vibrating appear at gussets BL and BR, respectively. However, in the embodiment shown in fig. 6, the sensors SL and SR are moved to the lower side than the sensors 170L and 170R in the prior art so as to position them closer to the respective static nodes SN at the gussets BL and BR, respectively. The increased flow measurement sensitivity is obtained by the node-proximal positioning of the sensor.

Description of figures 7 and 8

Fig. 7 and 8 show the position of the sensor and driver corresponding to the first out of phase torsional mode and the second out of phase torsional mode, respectively.

As shown in fig. 7, drivers DL and DR drive flow tubes 130 and 130' in a first out of phase twist mode. Dashed lines FTO and FTO' show the zero offset condition. Lines FT1 and FT 1' show the normal flow regime. In this particular torsional mode, the sensors SL and SR are located close to the dynamic node AN. The proximity of the sensors SL and SR to the dynamic node AN may be determined according to the criteria discussed above in fig. 2.

FIG. 8 illustrates a second out-of-phase torsional mode constructed in accordance with the present invention. Comparing fig. 8 with fig. 5, it can be seen that the former will result in two dynamic nodes AN1 and AN2, which allows the sensor to be located in a larger area. The dashed lines FTO and FTO' represent no-flow conditions. Solid lines FT1 and FT2 represent normal flow conditions. In this particular mode, three drivers DL, DC and DR are used. The drive system may create dynamic nodes AN1 and AN2 on the top areas of the flow tubes 130 and 130'. It is also possible to create the same node with two drivers, but in this case one driver must be placed at the top region of the flow tube and the other driver at one of the two sides. Sensors SL1 and SL2 are disposed at locations near dynamic node AN1, while sensors SR1 and SR2 are disposed at locations near dynamic node AN 2.

Description of the drawings 9

Another preferred embodiment of the invention which can be implemented is shown in fig. 9, in which the tubular portions are supported by members 912, 914. The distance between these two elements determines the frequency of vibration of tube 910, since this distance is at least one wavelength long of the drive frequency. If the length of pipe 910 is relatively long in order to enable the meter to function in practice, then supports attached to elements 912 and 914 and located between elements 912 and 914 may be mounted on pipe 910. The flow meter element of the present invention can be clamped to the pipe 910 without requiring any major modifications or changes to the pipe and can measure the mass specific flow of material flowing through the pipe. As shown, tube 910 is substantially straight and constant in cross-section. It will be understood that the flow meter of the present invention may be used with tubes of various shapes and configurations.

The embodiment of fig. 9 includes a driver 920 clamped directly to tube 910 at or near an antinode of the second harmonic of the natural frequency, or anywhere other than at a node of the second harmonic of the natural frequency. An auxiliary drive, similar to drive 920, may also be clamped to tube 910 to increase symmetry and balance the load applied to the tube. However, as shown, the system of the present invention may also operate with only one drive. The driver 920 may be connected to a feedback loop that includes a motion sensor 930, and the motion sensor 930 may be mounted directly opposite the driver 920, near the driver 920, or attached to the driver 920.

The flow meter element may also include motion transducers 932L and 932R mounted on pipe 910 as close as practical to the dynamic node, as indicated by dashed line 931. Counterweight 940 may be mounted at a predetermined location of tube 910, such as at an antinode of a harmonic of a natural vibration frequency of tube 910, or the like, to balance the load generated by driver 920. A second actuator may also be installed at this location if desired, and the counterweight 940 or second actuator may be removed.

Description of the operation of the system of FIG. 10

Figure 10 pictorially illustrates the displacement of various parts of flow tube 910 during operation. The amplitude curve 1000 in fig. 10 graphically illustrates the zero flow condition of tube 910 when it vibrates at its second harmonic frequency.

Curve 1000 is of zero amplitude at each end of the tube held by supports 912, 914 and at the dynamic node position 1002 in the non-flow state. The peak amplitudes of curve 1000 occur at antinodes 1004 and 1006. Driver 920 applies a lateral force to tube 910 and then releases the force, thereby vibrating tube 910. The amplitude plot 1000 shows the force cycle portion, while the amplitude plot 1000' shows the non-force cycle portion. Antinodes 1004, 1006 are of opposite amplitude in each cycle and they are located at 1004 ', 1006' during the part of the cycle that is not under load.

When material flows through tube 910, the vibration of tube 910 will generate coriolis forces on each element of the tube. Amplitude curves 1010, 1020 for pipes with fluid flow have been shown in fig. 2. The magnitude of the deflection of tube 910 has been greatly exaggerated in fig. 2 to illustrate the manner in which the system operates. The effect of the coriolis force acting on tube 910 will shift the amplitude curve 1010 (corresponding to the first portion of the drive cycle) to the left as compared to the amplitude curve 1000 for the zero flow condition. The material flowing through the tube 910 will resist this effect transmitted by the vibrating tube. The initial portion of curve 1010 is smaller in amplitude than curve 1000 due to the influence of the Coriolis force effect of the material reacting against the wall of tube 910. This will shift the dynamic node (zero amplitude point) of the amplitude curve 1010 towards the position 1020. Similarly, the effect of Coriolis forces on tube 910 during the second portion of the cycle will produce amplitude curve 1020 for the effect of Coriolis forces on tube 910. Node 1022 (zero amplitude point) of curve 1020 precedes node 1002 of curve 1000.

The cyclic longitudinal offset of nodes 1012 and 1022 will produce a cyclic lateral amplitude displacement at location 1002 of tube 910. As shown in FIG. 10, this lateral displacement will occur between point 1018 on curve 1010 representing curve 1010 offset from dynamic node position 1002, and point 1028 on curve 1020 representing curve 1020 offset from dynamic node position 1002. This cyclic lateral displacement at the dynamic node location is due to the effect of coriolis forces generated by the fluid flow through vibrating tube 910. Since this Coriolis force effect is due to the mass of the material flowing through tube 910, the measured lateral acceleration and displacement resulting therefrom is directly representative of the mass specific flow rate of the material.

It will be understood that the invention is not limited to these details of the preferred embodiment, but encompasses other modifications and variations within the spirit and scope of the present invention.

Claims (18)

1. A coriolis flowmeter (310) for measuring a property of a process material flowing through said flowmeter, said flowmeter (310) comprising: a flow tube assembly (130, 130'; 910) for flowing the treatment material therethrough, the flow tube assembly being secured at two locations spaced along its length; a drive assembly (DL, DR; DL, DC, DR; 920) arranged to vibrate said flow tube assembly (130) to create at least one dynamic node (AN; AN 1; AN 2; 931) at a location on said flow tube assembly between said two fixed locations; sensor means including a pair of sensors (SL, SR; S1, S2; S1-S4; 932L, 932R) responsive to said vibration of said flow tube assembly and said process material flowing through said flow meter for generating sensor output signals indicative of vibration of said flow tube assembly due to Coriolis forces generated by said process material flowing through said flow tube assembly; and a signal processing assembly (320) responsive to the sensor output signals for generating information indicative of the process material flowing in the flow meter (310), characterized by: the sensor pairs (SL, SR; S1, S2; S1-S4; 932L, 932R) are secured to the flow tube assembly (130, 130'; 910) on opposite sides of the at least one dynamic node (AN; AN 1; AN 2; 931) as close as possible to the dynamic node to maximize the phase difference between the sensor output signals while maintaining sufficient separation from the dynamic node to provide a usable signal-to-noise ratio for the amplitude of the output signal.

2. The flowmeter of claim 1, wherein said flow tube assembly comprises a pair of substantially parallel flow tubes (130, 130').

3. The flowmeter of claim 2, wherein said flow tube (130, 130 ') has a top portion and a pair of side branches (134, 134 ', 131, 131 ') having lower portions connected to said brace plates (BL, BR) forming two fixed positions of said flow tube.

4. The flowmeter of claim 3, wherein said drive assembly (DL, DR) vibrates said flow tube (130, 130 ') to create a single dynamic node (AN) located intermediate a top portion of said flow tube (130, 130').

5. The flowmeter of claim 3, wherein said drive assembly (DL, DR; DL, DC, DR) vibrates said flow tube (130, 130 ') to create a pair of dynamic nodes (AN1, AN2) on said flow tube (130, 130').

6. The flowmeter of claim 5, wherein said sensor assembly comprises two pairs of sensors (S1, S2, S3, S4; SL1, SL2, SR1, SR2), one pair (S1, S2; SL1, SL2) being located on opposite sides of one (AN1) of said pair of dynamic nodes, and the other pair (S3, S4; SL1, SL2) being located on opposite sides of the other (AN2) of said pair of dynamic nodes.

7. The flowmeter of claim 5, wherein the sensor pairs are positioned as follows: one sensor (S1) is located on one side of the first node (AN1) in the pair of dynamic nodes, and the other sensor (S4) is located on the opposite side of the other node (AN2) in the pair of dynamic nodes.

8. The flow meter of claim 5, wherein the pair of sensors (S1, S2) are located on opposite sides of one node (AN1) of the pair of dynamic nodes.

9. The flowmeter of any of claims 3-8, wherein said drive assembly comprises a pair of spaced drivers (DL, DR) located on said flow tubes (130, 130').

10. The flow meter of claim 1 wherein the flow tube assembly is a substantially straight tube (910).

11. The flowmeter of claim 1, wherein the flow tube assembly comprises a pair of flow tubes having different configurations.

12. A coriolis flowmeter (310) for measuring a property of a process material flowing through said flowmeter, said flowmeter (310) comprising: a flow tube assembly (130, 130') for flowing said process material therethrough, said flow tube assembly being secured at two locations (BL, BR) spaced along its length; a drive assembly (D1) for vibrating said flow tube assembly to create a Static Node (SN) at each of said fixed locations (BL, BR); sensor means including a pair of sensors (SL, SR) responsive to said vibration of said flow tube assembly and said process material flowing through said flow meter (310) for generating sensor output signals indicative of motion of said vibrating flow tube assembly due to coriolis forces generated by said process material flowing through said flow tube assembly; and a signal processing assembly (320) responsive to generating said sensor output signal to generate information indicative of said process material flowing in said flow meter (310), characterized by: the sensor pairs (SL, SR) are secured to the flow tube assembly (130, 130') proximate the securing locations (BL, BR) such that each sensor (SL, SR) is located as close as possible to an associated one of the Static Nodes (SN) to maximize the phase difference between the sensor (SL, SR) output signals while remaining sufficiently spaced from the static nodes to provide a usable signal-to-noise ratio of the amplitude of the sensor output signals.

13. The flowmeter of claim 12, wherein said flow tube assembly comprises a pair of substantially parallel flow tubes (130, 130').

14. The flowmeter of claim 13, wherein said flow tube (130, 130 ') has a top portion and a pair of side branches (134, 134 ', 131, 131 ') having lower portions defining a fixed portion connected to a brace plate (BL, BR), and wherein said drive assembly (D1) is fixed to the top of said flow tube and vibrates said flow tube (130, 130 ') in an out-of-phase bending mode such that said flow tube (130, 130 ') pivots out-of-phase with respect to each other about said brace plate (BL, BR).

15. A method for operating a coriolis flowmeter (310), said flowmeter (310) having a flow tube assembly (130, 130'; 910) for flowing said process material therethrough, said method comprising: vibrating the flow tube assembly with a drive assembly that produces at least one dynamic node (AN, AN1, AN 2; 931) on the flow tube assembly; receiving output signals from a sensor (SL, SR; S1, S2; 932L, 932R) secured to said flow tube assembly and responsive to vibration of said flow tube assembly to generate a signal indicative of vibration of said flow tube assembly due to Coriolis forces generated by said process material flowing through said flow tube assembly; and operating a signal processing assembly (320) in response to receiving said output signal to generate information indicative of said process material flowing in said flow meter (310); the method is characterized in that: the sensor pair (SL, SR; S1, S2; 932L, 932R) is secured to the flow tube assembly (130, 130'; 910) with opposite sides of the at least one dynamic node (AN; AN 1; AN 2; 931) as close as possible to the dynamic node to maximize the phase difference between the sensor output signals while maintaining sufficient separation from the dynamic node to provide a usable signal-to-noise ratio in the amplitude of the output signals.

16. The method of claim 15 wherein said flow tube assembly comprises a pair of flow tubes (130, 130 ') of modified U-shaped configuration having a top portion and a pair of side branches (134, 134', 131, 131 '), and said drive assembly (D1) is fixed at said top portion and vibrates said flow tubes (130, 130') in out-of-phase bending mode relative to each other about said strut assembly (BL, BR) as a Static Node (SN).

17. The method of claim 15 wherein said flow tube assembly comprises a pair of flow tubes (130, 130 ') of modified U-shaped configuration having a top portion and a pair of side branches (134, 134 ', 131, 131 '), and a drive assembly (DL, DR) secured at said side branches vibrates said flow tubes (130, 130 ') in out-of-phase torsional mode relative to each other such that a single dynamic node (AN) is created at the top portion of said pair of flow tubes (130, 130 ').

18. The flow tube of claim 15, wherein said flow tube comprises a pair of flow tubes (130, 130 ') of modified U-shaped configuration having a top portion and a pair of side branches (134, 134 ', 131, 131 '), and said drive assembly includes a drive assembly (DL, DR) secured at said side branches to vibrate said flow tubes (130, 130 ') in out-of-phase torsional modes relative to each other to create two dynamic nodes (AN1, AN2) at said top portion of said flow tubes (130, 130 ').