MXPA00000281A - Drive circuit modal filter for a vibrating tube flowmeter - Google Patents

Abstract

A drive system (50, 104) for a vibrating tube-based measurement instrument (5) employing a spatial filter (500) to produce a drive signal having modal content only at a desired vibration mode. Multiple feedback sensors (105, 105') located at different locations along a vibrating tube (103A, 103B) produce multiple feedback sensors. Each feedback signal has applied to it a weighting or gain factor. All of the weighted feedback signals are then summed to produce a drive signal, or a signal proportional to a drive signal, having improved modal content as compared to any of the feedback signals by themselves. The weighting factors are selected by any of several means. One method is to build the eigenvector matrix for the vibrating flow tube by extracting the eigenvectors from a finite element model of the vibrating structure. The inverse or pseudo-inverse of the eigenvector matrix is calculated to obtain the modal filter vector. The appropriate set of weighting coefficients are selected from the modal filter vector.

Description

MODAL FILTER OF ACCESSORIES FOR A FTJ OMETRO OF VIBRATOR TUBE Field of the Invention The present invention relates to apparatus and methods for generating an excitation signal for a Coriolis mass flow actuator. More particularly, the present invention relates to generating an excitation signal, which excites only the desired vibration mode, within the vibration flow tube of a Coriolis flow meter. More particularly, the present invention relates to using modal filters .- to suppress the undesirable excitation signal components and to improve the excitation or drive signal components.

Statement of the Problem The use of flow meters is well known or mass flowmeters with Coriolis effect, to measure the flow of mass and other information tones, pa? materials that in Inven through a conduit. Coriolis flowmeters from REF .: 32288 example, are disclosed in the US Patents Nos. 4,109,524 of August 29, 1978, 4,491,025 of January 1, 1985, and Re. 31,450 of February 11, 1982, all assigned to J. E. Smi th et al. These flow meters have one or more flow tubes in a straight or curved configuration. Each flow tube configuration within a Coriolis mass flow meter has a set of natural-vibration modes, which can be of a simple bending, torsional or coupling type. - Each flow tube is excited to oscillate in a resonance within one of these natural modes. The material flows into the flowmeter from a duct connected to one side of the flowmeter inlet, and is directed through the flow tube or tubes, and exits the flow meter through the outlet side. The natural vibration modes of the system filled with material and vibrators are defined in part by the combined mass of the flow tubes and the material flowing within the flow tubes. When there is no flow through the flow meter, all the points along the flow tube oscillate due to an excitation force applied with an identical phase or a small initial fixed phase phase shift, which can be corrected. As the material begins to rise, the Corioli forces cause each point that is along the flow tube to have a different phase. The phase that is on the inlet side of the flow tube delays the exciter, while the phase that is on the output side precedes the exciter. Discharge sensors are placed in the flow tube to produce sinusoidal signals, which represent the movement of the flow tube. The output signals from the discharge sensors are processed to determine the phase difference between the discharge sensors. The phase difference that exists between two signals of the discharge sensor is proportional to the mass flow rate of the material passing through the flow tube. An essential component of any flowmeter Corioli, and also of any vibrating tube densitometer, is the excitation system. The excitation system operates to apply a periodic physical force to the flow tube, which causes the flow tube to oscillate. The excitation system includes an exciter mounted on the flow tube (s) of the flow meter. The excitation mechanism typically contains one of many known configurations, such as magnetically mounted to a conduit, and a wire spool mounted to the other conduit in an opposite relationship to the magnet.

An excitation circuit continuously applies a periodic excitation voltage, in a typically sinusoidal or square fashion, to the exciter. Through an interaction in the continuous alternating magnetic field, produced by means of the coil, in response to the periodic excitation signal and the constant magnetic field produced by the magnet, both flow conduits are initially forced to vibrate within an opposite sinusoidal pattern, which is maintained from here on out. Those skilled in the art will recognize that any device capable of converting an electrical signal to a mechanical force is appropriate to be applied as an exciter. (See United States Patent 4,777,833, issued to Carpenter and assigned in its presence to Micro Moti on, Inc.). . A typical mode, although not the only one, in which the Cori olis flowmeters are excited to vibrate, is the first mode of bending out of phase. The first out-of-phase bending mode is a fundamental bending mode, in which two tubes of a dual-tube Coriolis flowmeter vibrate the one opposite the other. However, this is not the only mode of vibration present in the vibration structure of a Coriolis flowmeter, excited in the first bending mode out of phase. There are, of course, certain higher modes of vibration that can be excited. There is also, as a result of a fluid flowing through the flux tube in vibration and the consequent forces, a first off-phase torsional mode that is excited, as well as other modes. There are also lateral modes and within phase, of vibration. Recently, there are hundreds of vibration modes currently excited within a Cori olis flowmeter, which is excited to oscillate in the first bending mode out of phase. Even within a relatively narrow range of frequencies close to the first out-of-phase bending mode, there are at least several additional modes of vibration. In addition to multiple modes being excited by the excitation of the flow tubes, the modes can be excited due to vibrations external to the flow meter. For example, a pump located anywhere on a process line can generate a vibration along a pipe that excites a vibration mode within a Coriolis flow meter. Another reason why additional and undesirable modes are sometimes excited within a Coriolis flowmeter is when manufacturing tolerances are such that the excitation elements are not located symmetrically on the flow tubes. This results in a driver placing eccentric forces inside the flow tubes and therefore exciting multiple modes of vibration. Thus, a Coriolis flowmeter excited to oscillate or resonate in the first out-of-phase bending mode has, in fact, a duct (s) that oscillate in many other modes, in addition to the first bending mode on the outside of phase. Meters that are excited to oscillate in a different mode to the first out-of-phase bending mode experience the same phenomenon of multiple excitation modes in addition to the intended excitation mode. Existing excitation systems process a feedback signal, typically a signal from the signals of the discharge sensors, to produce an excitation signal. Unfortunately, the excitation feedback signal contains responses from other modes, in addition to the desired mode of excitation. Thus, the excitation feedback signal is filtered through a frequency domain filter, to remove the unwanted components y. thus, the filtered signal is then amplified and applied to the exciter. However, the frequency domain filter used to filter the excitation feedback signal is not effective in isolating the only desired excitation mode from responses in other modes, present in the excitation feedback signal. There may be responses outside resonance, coming from other modes that are close to the resonance frequency of the desired mode. There may also be resonance responses at frequencies that approach the desired resonance frequency. In any incident, the already filtered excitation feedback signal, i.e. the excitation signal, typically contains a modal content at different frequencies than only the mode desired for the excitation of the flow tube. An excitation signal composed of a resonant response from multiple modes supplies energy through the exciter to the flux tube which excites each mode for which the excitation signal contains the modal content. This multimode excitation signal causes operational problems within the Cori ol i s flow meters. Moreover, frequency domain filters introduce a phase delay in the filtered excitation signal. This may result in the requirement of a higher excitation power, to excite the flow tube to the desired amplitude. A problem caused by a multi-mode excitation signal is that external vibrations, such as vibrations of the pipe, are reinforced by the excitation signal. If the vibrations of the pipe, external to the Coriolis flowmeter, cause the flow meter to vibrate, the excitation feedback signal contains the response to tube vibration. The frequency domain filter fails to remove the undesirable response if the vibration of the pipe falls at least partly, within the filter frequency pass band. The filtered excitation feedback signal, including the undesirable response to tubing vibration, is amplified and applied to the exciter. The exciter then operates to reinforce the excitation mode of the pipe vibration. Another example problem, caused by a multimode excitation signal, occurs when the total amount of excitation power available to excite the flow tubes becomes a difficulty. In order to comply with the intrinsic safety requirements, established through several approval agencies, the total power available in the exciter of a Cori olis flowmeter is limited. This power limitation can be a problem for Coriolis flow meters, particularly with respect to larger flow meters and more particularly, with respect to larger flow meters that measure fluids with retained gas. A multi-mode excitation signal is inefficient, since it is placing energy in modes additional to the desired excitation mode. Accordingly, the intrinsic safety power limitation is reached more quickly than is necessary for a given set of operating conditions. A further problem is that, in the example of a meter energized in the first out-of-phase bending mode, the location of the driver is also the position of a maximum amplitude for the second out-of-phase bending mode. Accordingly, the second out-of-phase bending mode is solidly excited in a Corioli meter, energized to oscillate in the first out-of-phase bending mode. The excitation feedback signal, and subsequently the excitation signal, accordingly contain a response in the second out-of-phase flexing mode. A further problem of an excitation signal having a modal content at multiple frequencies occurs with respect to the density measurement effected by means of a Cori olis mass flowmeter. The measurement of the density in a Coriolis flowmeter, or in a vibration tube densitometer, rests in the measurement of the resonant frequency of the vibrating flow tube. A problem arises when the flow tube is excited, in response to an excitation signal, which contains a modal content in multiple modes. The superposition of the multiple modes in the excitation signal can result in a flow tube being excited out of resonance from the actual resonant frequency of the desired excitation mode. This can result in an error in density measurement. There is a need for an excitation circuit system for a Coriolis flow meter, that excites the vibrator tube (s) of the. flow meter, only at the desired excitation frequency. There is a further need for an excitation circuitry system that improves the desired excitation mode, on an excitation feedback signal, and suppresses undesirable vibration modes, to produce an excitation signal having a modal content, only at the desired excitation frequency.

Declaration of the Solution The problems identified above and other problems are solved and thus, a technical advance in the field is reached, by means of the excitation circuit system of the present invention. The present invention provides a method and an apparatus for using a modal filter to generate an excitation signal in a Cori olis flowmeter or a densitometer. The modal filter receives feedback signals from the vibrating flow tube and produces an excitation signal in which undesirable vibration modes are eliminated and desirable modes are improved. Thus, by using the excitation system of the present invention, an excitation signal is produced which contains only the desired excitation mode of the flow tube (s) of the Coriolis flow meter. The system of the present invention filters the feedback signals from the flow tube of a Coriolis flowmeter through a modal filter. A modal filter is a spatial filter that uses the summation of multiple feedback signals, measured at different points within a space and / or in different directions within the space, possibly including translation measurements and / or motion rotation measurements, of resistance, force (or a combination of these), as well as other quantities related to the movement of the flowmeter tube. The modal filter uses a summation of multiple feedback signals from different points that are along the length of a vibrating flow tube. The modal filter linearly combines calibrated feedback signals to produce a resultant, a filtered signal in which undesirable vibration modes are eliminated and desirable modes are improved. A feedback signal is a representative of the movement of a flow tube or the relative movement of multiple flow tubes, at a particular location on the flow tube (s). Typical Coriolis flow meters now have two available feedback signals, in the form of signals from the discharge sensors, which are used in the computation of the mass flow rate ratio of a Cori olis flowmeter. The signals generated by means of the discharge sensors on a Coriolis flow meter are used by the system of the present invention as feedback signals. A modal filter requires at least two feedback signals, as an input. The excitation system of the present invention is used in a manner to excite a Cori olis flowmeter having parallel and double flow tubes. Two discharge sensors provide two feedback signals. A third feedback signal is supplied by means of a sensor located in the position of the exciter. The three feedback signals are fed to a modal filter. The modal filter includes an amplifier for each feedback signal. A different calibration factor, that is, the gain of the amplifier, is applied to each feedback signal and the three feedback signals are combined linearly by means of an existing adder in the modal filter. The resulting output signal from the modal filter is amplified to produce the excitation signal and thus, the excitation signal is applied to the exciter. The amplifier gains of the modal filter amplifiers are selected in such a way that the modal filter operates to suppress the modal content that is in the excitation signal in the first torsion mode out of phase and in the second mode of bending out of phase. Moreover, the excitation signal has a substantial modal content and only in the first out-of-phase bending mode, which is the desired mode of excitation of the flow meter. The processing of the signal described above can, of course, be implemented in discrete analog components or be implemented digitally. The terms "amplifier" and "adder" as used herein, for example, are applicable to both analog and digital implementations.

The modal filter itself is comprised of a separate amplifier, associated with each feedback signal and by an adder, to add to the calibrated feedback signals. The magnitude of the gain of an amplifier within a modal filter is referred to as a calibration factor. The feedback signal is referred to as a calibrated feedback signal, after it has been amplified by means of its respective amplifier within the modal filter. The adder simply adds the calibrated feedback signal to produce the output signal of the filter. The output signal of the filter does not have an amplitude large enough to excite the flow tubes and thus, the output signal of the filter is amplified to produce the excitation signal. The excitation signal has the same modal content, but at a greater amplitude than the filter output signal. There are a large number of ways to determine the calibration factors applied by means of the modal filter to the feedback signals. All these practices are equivalent in their result, but certain practices are more efficient and repeatable than others. One practice is simply to select the calibration factors through trial and error, until an excitation signal is obtained, having a modal content only and substantially in the desired excitation mode. Several other practices include calculating the inverse or the pseudo-inverse of a matrix of eigenvectors of the flowmeter structure. Each row of this matrix comprises the appropriate calibration factors for a particular mode. The eigenvectors (or modal vectors) necessary to construct the eigenvector matrix can be obtained through various means, including but not limited to numerical means such as the finite element model of the flowmeter or experimental means such as the experimental modal analysis. Another practice for determining the calibration factors of the modal filter is to use a technique known as the modified reciprocal modal vector method. An additional practice is known as an adaptive modal filter. The means by which the calibration factors are determined are not critical and any method or combination of methods may be appropriate. The modal filter can be configured to filter a greater number of undesirable modes from the excitation signal, by using a greater number of feedback signals. At least two feedback signals must be supplied to the modal filter, in order to achieve the beneficial effects of the present invention. For example, two discharge signals from a Coriolis flowmeter can be used as the only feedback signals for the modal filter to produce an excitation signal that has two modes affected by the modal filter. In this case, the filter can effectively improve the first bending mode out of phase, that is, the desired excitation mode, and suppress the first torsion mode out of phase. To completely suppress all undesirable modes in a frequency range of interest, the same feedback signals are required as the total number of modes that are in the frequency range of interest. If fewer feedback signals are available than the number of modes, the amplitude of the desired mode is still improved, relative to the amplitudes of the undesirable modes, however, the response of the undesirable modes can not be totally eliminated. The Coriolis filter flow meter of the present invention can be used to add to the existing excitation signal systems, or it can be used in place of the existing excitation signal systems.

Brief Description of the Drawings Figure 1 illustrates a Coriolis meter and the associated electronic measuring devices. Figure 2 illustrates a block diagram of the electronic devices of a Cori olis flowmeter of the preceding art. Figure 3 illustrates a block diagram of an excitation system of the preceding art, for a Coriolis flowmeter. Figure 4 illustrates a block diagram of the electronic devices of a Coriolis flow meter, in accordance with the present invention. Figure 5 shows a block diagram of a Coriolis flowmeter excitation system, in accordance with the present invention. Figure 6 illustrates the frequency response function of a flux tube feedback signal and the frequency response function of the resultant excitation signal, in accordance with an existing excitation circuit. Figure 7 illustrates a frequency response function of a flow tube feedback signal of y to the additional frequency response functions, which represent the contribution of the constituent vibration modes to the feedback signal. Figure 8 is a flow chart illustrating the process steps for selecting the calibration coefficients of the modal filter, by trial and error. Figure 9 is a flow diagram illustrating the process for selecting the calibration coefficients of the modal filter, by calculating the inverse or the pseudo-inverse of a matrix of eigenvectors. Figure 10 is a flow chart illustrating the process for using a modal filter to develop a flowmeter drive signal.

Detailed description of the invention Coriolis Flowmeter in General - Figure 1 Figure 1 shows a Coriolis flow meter 5 comprising a Cori olis meter assembly 10 and electronic measuring devices 20. The electronic measuring devices 20 are connected to the measurement assembly 10, through the loads 100, for provide density, mass flow ratio, volume flow ratio and mass flow information totalizing on path 26. A structure of Corioli s flowmeter is described, although that apparent to those skilled in the art, that in the The present invention can be practiced in conjunction with a vibrating tube densitometer, without the need for the additional measuring capability, provided by a Cori olis mass flowmeter. The measurement assembly 100 includes a pair of flanges 101 and 101 ', the manifold 102 and the flow tubes 103A and 103B. Connected to the flow tubes 103A and 103B are the driver 104 and the discharge sensors 105 and 105 '. The creation bars 106 and 106 'serve to define the axes W and W', around which each flow tube oscillates. When the flow meter 10 is inserted into a pipe system (not shown) carrying the process material being measured, the material enters the measurement assembly 10 through the flange 101, passes through the manifold 102, in where the material is directed to enter the flow tubes 103A and 103B, it flows through the flow tubes 103A and 103B and back to the manifold 102, where it exits the measurement assembly 10, through the flange 101 '.

The flow tubes 103A and 103B are appropriately selected and mounted to the manifold 102, such that it has substantially the same mass distribution, moment of inertia and elasticity mode, around the bending axes WW and W'-W ' , respectively. The flow tubes extend outward from the manifold in an essentially parallel manner. The flow tubes 103A-103B are excited by means of the driver 104 in opposite directions about their respective bending axes W and W ', and what is known as the first out-of-phase bending mode of the flow meter. The exciter 104 may comprise any of the known configurations, such as a magnet mounted to the flow tube 103A and an opposite coil mounted to the tube. flow 103B and through which an alternating current is passed to vibrate both flow tubes. An appropriate excitation signal is applied by means of the electronic measuring devices 20, through the conductor 110, and towards the driver 104. The electronic measuring devices 30 receive the left and right speed signals that appear in the conductors 111 and 111 ', respectively. The electronic measuring devices 20 produce the excitation signal appearing on the conductor 110 and cause the exciter 104 to vibrate the tubes 103A and 103B. The electronic measuring devices 20 process the left and right speed signals, to compute the mass flow rate and the density of the material passing through the measurement assembly 10. This information is applied by means of the devices measuring devices 20, on the path 26, towards means of use (not shown). It is known to those skilled in the art that the Corioli 5 flowmeter is very similar in structure to a vibration tube densitometer. Vibrating tube densitometers also use a vibrating tube through which the fluids flow or, in the case of a sample densitometer, within which the fluid is maintained. Vibrating tube densitometers typically use a single feedback signal, since a density measurement requires only frequency measurement and thus, a phase measurement is not necessary. The descriptions of the present invention apply here equally, for vibrating tube densitometers. Those skilled in the art will recognize that where an existing Coriolis flowmeter already has two feedback signals available to enter the modal filter, an existing vibrator tube densitometer typically has a single feedback signal available. Thus, it is only necessary to provide additional feedback signals in a vibrator densitometer, in order to apply the present invention to a vibrating tube densitometer.

Excitation System of the Prior Art - Figures 2, 3 and 6 • Figure two illustrates a block diagram of electronic measurement devices 20. Electronic measurement devices 20 include a mass flow rate circuit 30- and an excitation circuit 40. The mass flow ratio circuit 30 is one of those circuits already known, for calculating the mass flow rate of a fluid, through a vibrating tube, based on the phase difference between: on the vibrating hub. The mass flow circuit 30 produces an output to means of use (not shown), on the line 26. The means of use can be, for example, a screen. The details of the mass flow ratio circuit 30 are already known to those skilled in the art and do not form part of the present invention. See US Patent RE 31,450, issued to Smi th on November 29, 1983 and assigned in its presence to Micro Moti on, Inc. or U.S. Patent 5,231,884 issued to Zolck on August 13, 1993 and assigned in its presence to Mi cro Moti on, Inc. for an exemplary information with respect to the mass flow ratio circuit 30. In existing excitation circuit systems, the excitation circuit 40 receives a feedback signal on the path 41 from the left discharge sensor 105. As shown in FIG. has described in more detail with respect to Figure 3, the existing excitation circuit systems produce an excitation signal on the path 110, towards the exciter 104. Those skilled in the art will recognize that existing excitation systems can alternatively use the sensor discharge right, as to the feedback for the excitation circuit 40. Also, some existing excitation systems use the Thorium sum of both discharge signals, such as to the feedback to the excitation circuit 40. Figure 3 illustrates a diagram of blocks of an existing excitation circuit 40. The excitation circuit 40 receives a signal from feedback in the form of one of the discharge signals, coming from the flowmeter and appropriately conditioning the magnitude of the discharge signal to produce a path 110 over the excitation signal. As denoted, some existing excitation systems add up to the two discharge signals and process the summed signal to produce an excitation signal. The excitation circuit 40 receives a signal from the discharge sensor 105, on the path 41. The discharge signal is fed to the rectifier 300 and then to the integrator 301. The signal output from the integrator 301 represents an average amplitude of the signal 105. The average amplitude signal is input to the amplitude control 302. The amplitude control 302 compares the average amplitude signal from the integrator 301 with a reference voltage Vrei. If the average amplitude falls below the reference voltage, then the discharge signal is amplified at the multiplier 303 and a conditioned discharge signal at its amplitude is outputted from the multiplier 303. The conditioned discharge signal at its amplitude is amplified by of the power amplifier 304, to produce the final excitation signal that is fed back to the exciter 104. An exciting circuit 40 thus operates to maintain a relatively constant amplitude. The details of the existing excitation control circuits 40 are already known to those skilled in the art of the electronics of a Coriolis flow meter and do not form part of the present invention. For a more detailed discussion of the multiple modalities of excitation circuit 40, see U.S. Patent Number 5, 009, 109. Figure 6 illustrates the modal content of the input going to, and the output from, the excitation circuit. Figure 6 illustrates two graphs of Frequency Response Function ("FRF") 600 and 602 that have vertical axes that represent the chronological relationship of the response amplitude of the flow tube, on the amplitude of input force and horizontal axes represent the frequency. The amplitude of input force includes the components due to the excitation signal, fluid flow turbulence, external vibration sources, etc. It is assumed, that for purposes of describing Figure 6, the input force has an equal amplitude at all frequencies. The scales of the vertical axes of graphs 600 and 602 are different and the scales of the horizontal axes are the same. Graphics 600 illustrates a frequency response function 601 that corresponds to a feedback signal. With reference to Figure 3, the frequency response function 601 characterizes a signal carried on the path 41 from the discharge sensor 105, to the existing excitation circuit 40, with respect to the input force applied to the flow tube. The frequency response function 601 has a modal content within the first bending mode out of phase (the peak of the amplitude being at point A), the first mode of twisting out of phase (the peak, of the amplitude being at point B) and the second mode of bending out of phase (the peak of the amplitude being at point C). Graphics 602 illustrates a frequency response function 603 that corresponds to an excitation signal that is produced in an existing excitation circuit, using a frequency domain filter. With reference to Figure 3, the frequency response function 603 characterizes a signal carried on a path 110, from the existing excitation circuit 40, to the exciter 104, with respect to the input force applied to the flow tube. The frequency response function 603 illustrates the effect of the frequency domain filtering of the excitation feedback signal 601. The frequency response function 603 has a modal content from the same three modes comprising signal 601 (indicated as a dotted line in the graph 602) but the high frequency components fall due to the frequency domain filtering of the existing charging circuit 40.

System of Excitation of Conformity with the Present Invention - In General - Figures 4,5 and 7 Figure 4 illustrates a block diagram of an electronic measuring device 400 that includes a mass flow rate circuit 30 and or flow tube drive circuit 50. The electronic measuring device 400 is similar to the measuring device. electronic 20, described with respect to Figures 2-3, with the following exceptions. The drive circuit 50 differs from the drive circuit 40, as described in more detail with respect to FIG. 5. Also, the drive circuit 50 receives additional feedback signals as inputs, when compared to the drive circuit 40. The left and right discharge signals are received in the excitation circuit 50 on the paths 41-42, respectively. Additional feedback signals are received from the additional feedback sensor 401 on the path 403. The feedback sensor 401 represents any additional feedback defenders, fixed to the flow tube (s) of a Coriolis flowmeter. In one embodiment of the present invention to be discussed later, the feedback sensor 401 that sensing discharge located in the position of the exciter on a flow tube (s) of a Coriolis flow meter. Figure 5 illustrates a block diagram of an excitation circuit 50 including an existing excitation control circuit 40 and a modal filter 500. The existing excitation control circuit 40 is the same circuit described with respect to Figure 3. The place of receiving a discharge signal directly from one of the discharge sensors (feedback sensors 503), the existing excitation control circuit 40 receives the signal output by the modal filter 500. The modal filter 500 receives a feedback signals from the discharge sensors 105-105 'on the paths 111-111', respectively. Sensor 401 represents any additional feedback defenders numbers attached to the vibrator flow tube and producing a feedback signal for excitation circuit 50. Each additional feedback signal produced by each additional feedback sensor is communicated to excitation circuit 50. on a trajectory 402 separately. The discharge sensors 105-105 'and the additional sensor (s) 401 are collectively referred to herein as feedback sensors 503. Anyone skilled in the art recognizes that the discharge sensors 105-105' do not have to be used as one of the feedback sensors 503. However, these are preferably used, since they provide useful feedback signals, as will be described below, for modal filter 500 purposes and these are in any way necessary for the computation of the mass flow ratio. Each of the feedback signals 503 is input to one of the amplifiers 504-506. Again, the amplifier 506 represents any number of additional amplifiers to receive signals from any additional additional feedback defenders 401. The amplifier 504 has a gain of G, the amplifier 505 has a gain of G and the amplifier 506 has a gain of 506. a gain of Gn. The G: -Gr gains are referred to as calibration factors applied by the modal filter 500 for the feedback signals. The outputs of amplifiers 504-506 on wins 507-509 are referred to as calibrated feedback signals. The calibrated feedback signals are summed by the adder 510, to produce a filter output signal on the path 511. The gains G: -Gr. of the amplifiers 504-506 are selected in such a way that the filter output signal going over the path 511 has an improved modal content, when compared to any of the feedback signals from the feedback sensors 503. A filter output signal has improved modal content means, a filter output signal in which a desired modal response is amplified, and at least, an unwanted modal response is suppressed. Figure 7 illustrates the result of the modal filter described with respect to Figures 4-5. Figure 7 includes graph 600 of Figure 6, for comparison purposes. Like the graph 600, graphs 700-702 have vertical axes that represent a chronological relationship of a flow tube response amplitude over an input force amplitude. Graphs 700-702 illustrate individually the frequency response functions corresponding to the three vibration modes, in which, when added by means of superposition, they make up the Frequency Response Function 601.

The graph 700 illustrates the frequency response function 703, which corresponds to the component of the first out-of-phase defection mode of the frequency response function 601. The graph 701 illustrates the frequency response function 704, which corresponds to the component of the first off-phase torsion mode of the frequency response function 601. The graph 702 illustrates the signal component 705 corresponding to the component of the second out-of-phase defection mode of the frequency response function 601. With reference to Figure 5, the frequency response function 601 characterizes a signal from 1 of the feedback sensors 503, to the modal filter 500. The modal filter 500 operates, as described above with respect to FIGS. -5, to remove all vibration modes except the desired vibration mode from the frequency response function 601. Thus, the response function of frequency 703, corresponding to the component of the first out-of-phase defection mode of the frequency response function 601, represents the modal content of a signal going over the path 511 from modal filter 500 to the excitation control circuit 40. The The current excitation signal on the path 110 to the driver 104 has a different amplitude than the frequency response function 703, per modal content of the frequency response function 703 remains unchanged by means of a simple amplification. Accordingly, the excitation signal going over the path 110 to the driver 104 from the excitation circuit 50, only excites the first defection mode out of phase of the flow sensor 10. The difference between the signal, of excitation produced by the existing excitation circuits and the excitation signal produced by the excitation circuit of the present invention is illustrated graphically by means of comparing the frequency response function 603 of Figure 6 (modal content of the excitation signal from the existing excitation circuit) and the frequency response function 703 of Figure 7 (modal content of the excitation signal from the exciter circuit of the present invention).

Selection of Modal Filter Calibration Factors - Figures 8-9 The selection of calibration factors (gains G? -Gr. in Figure 5) for a modal filter of an excitation circuit of a Coriolis flowmeter, is discussed in more detail below, with respect to Figures 8-9. There are a variety of methods that anyone can use to select the calibration factors for the modal filter (s) applied to an excitation circuit of a Cori olis mass flowmeter. The means by which the calibration factors are determined are not critical and any method or combination of methods is appropriate and equivalent. A method for selecting the calibration factors for the modal filter of a Cori olis excitation circuit is through trial and error. As denoted with respect to Figures 5 and 7, the desired result of the modal filter 500 is to produce a filter output signal having an improved modal content, as compared to any of the input feedback signals to the modal filter . Figure 8 is a flow diagram illustrating the steps that a person uses to select the calibration coefficients of a modal filter, using trial and error practice. Steps 801-804 are repeated until a filter output signal (excitation signal) is obtained, having a desired modal content. Steps 801-804 are conducted using a current, instrumented 4-way flow meter. properly to provide the necessary feedback signals, together with an excitation circuit that allows changing the gains of the modal filter amplifiers. Alternatively, the feedback signals can be recorded, for example in a Digital Audio Cassette (DAT) format and applied again to the modal filter excitation circuit in each step, through steps 801-804. Alternatively, steps 801-804 are executed using a numerical model of. a Cori olis flowmeter and its associated excitation circuit. The process starts with step 800 and continues to step 801, where a first set of calibration coefficients are selected. During step 801, a person can select a new complete set of calibration coefficients (gains G? -Gn), each time step 801 is executed or a new calibration coefficient can be selected for only one feedback signal, each time that step 801 be executed. During step 802, the feedback signals are applied to the modal filter, wherein each modal filter amplifier has the set gain, as determined by step 801. During step 803, the filter output signal is measured and call as appropriate, to allow the necessary comparison with the input feedback signal towards the modal filter. Processing proceeds from step 803 to decision block 804. Decision block 804 operates to determine whether the filter output signal has an improved modal content, as compared to any of the input feedback signals to the modal filter. The user determines that the modal content within the filter output signal is satisfactory. So, the "improved modal content" can. means that a filter output signal, having only modal content from the desired excitation mode, is present. "Improved modal content" can also mean, depending on the user's specifications, that the amplitude of the desired excitation mode is at least 20 dB greater than the amplitude of the other modes present, for example. If it is determined, by means of the operation of the decision block 804, that the output signal of the filter does not have an improved modal content, then the process returns to step 801. A new set of calibration coefficients is selected during step 801 and steps 802-804 are processed again, to locate a set of calibration coefficients that produce a filter output signal having an improved modal content.

A method for selecting the calibration coefficients for an excitation circuit of a Coriolis flowmeter is to calculate the inverse or the pseudo-inverse of an eigenvector matrix. As denoted above, a vibrating flow tube of a Coriolis flowmeter has a combination of vibration modes present. When analyzing the movement of the flow tube by means of physical coordinates, for example, the singular response in points and isolated lessons on the flow tube, requires the. analysis by means of simultaneous equations, which do not easily produce useful information about the movement of the flow tube. However, a modal transformation can be used to transform a vector of physical responses to the modal responses or modal coordinates of the system. The standard modal transformation is given by: (1) x = F? where: x is the vector of the physical response coordinates F is the eigenvector matrix, whose columns are the eigenvectors of the flow tube (also referred to as modal vectors) and interest, and? is the vector of the modal response coordinates.

The eigenvector array can, as will be described later, be developed by means of any Coriolis flowmeter flow tube. The physical vectors can be determined as the input to the modal filter, that is, the feedback signals. Therefore, equation (1) is solved by means of has, - the modal coordinate (s) response (s), as follows: (2) ? = ff} To put equation (1) in the form of the equation (2), requires taking the pseudo-inverse of the eigenvector matrix F. If the eigenvector matrix is square and not singular, then the inverse of the eigenvector matrix (F_i) is used in Equation (2) , instead of the pseudo-inverse. The eigenvector matrix is square and not singular, when the number of feedback signals from the flow tube, equal to the number of modes considered and the modal vectors are linearly independent.

The following example is used to illustrate the process by means of which the pseudo-inverse of a modal matrix is calculated to determine the calibration coefficients for a modal filter of the excitation circuit of a Coriolis flow meter. A physical or numerical model of the flowmeter can be used to construct the own vector matrix. The following example, a numerical model of the. flow meter. A finite element model of the tubes of a Cori ol i s mass flowmeter, model CMF100 (manufactured by Mi cro Moti on, Inc.) Is constructed. The model fixes to the ground the ends of the flow tube that, in a physical flow meter, connect to the manifold tube of the flow meter. Finite element modeling techniques are already known to those skilled in the art and do not form part of the present invention. The model of finite element eg emplificativos was constructed, using Ideas -SDRC and analyzed by means of an MSC / NASTRAN, a finite element code available with MacNeal -Sch wendl er. Those skilled in the art of finite element modeling will recognize that any finite element code can be used alternatively. The locations of the feedback sensors were moderate, to produce an output representative of the relative movement between the locations on the flow tube of the magnet and the coil corresponding to the right discharge sensor, the exciter and the left discharge sensor. These "scalar points" are a standard technique in advanced dynamic analysis. For more information about the finite element modeling of Coriolis flowmeters, see "A Finite The Epient for the V iewe on Analysis of a Fl ui-Conveying TimesShenko Beam." (AIAA Paper, 93-1552). The coefficients of eigenvalue of the model CMF100 are extracted from the finite element model, to construct the following own-vector matrix of three rows by 10 columns, for the CMF100 sensor: '0 25.08 0 0 0 40.3 0 0 0 36.78 F rotated! 0 35.39 0 0 0 0 0 0 0 36.55 0 25.08 0 0 0 40.3 0 0 0 36.78 Each row within the total own-vector matrix F-ctai of equation (3) corresponds to a physical location on the flow tube. The first row corresponds to the location of the left discharge, the second row corresponds to the location of the exciter and the third row corresponds to the location of the right discharge. Each column within the total own vector matrix F: ct: corresponds to a vibration mode. This matrix is used in a known way by means of the finite element model, to model the signals generated by means of the discharge sensors. The matrix is used, as will be described above, to develop the calibration coefficients for the excitation circuit modal filter. Columns (modes) with zeros in the total own vector matrix F o: -; they are "in-phase modes". This means that there is no relative movement between the tubes, since both tubes move with the same speed and direction. Thus, the sensors used to provide the feedback signals, speed sensors in this example, act on their own as a type of modal filter, by filtering all modes within phase. The total own reader matrix F - ..: -; a: is reduced by removing all the columns inside phase. 4) Reduced F Equation (4) is the reduced eigenvector matrix Freducida- Equation (1), the standard modal transformation, is rewritten using the reduced eigenvector matrix Fre3u;? A5, as follows: where? c is the modal coordinate response of the first bending mode out of phase, r). is the modal coordinate response of the first out-of-phase torsion mode and? c is the modal coordinate response of the second out-of-phase bending mode, and RPO is the physical response from the right discharge sensor, DRV is the The physical response from the feedback sensor that is at the location of the exciter and LPO is the physical response from the left discharge sensor. Equation (5) is solved by means of the unknown vector quantities, that is, the modal vectors: The reduced eigenvector matrix is inverted by importing into the matrix in a commercial, standard mathematical computing package, such as Ma thcad and using one of the standard investment or pseudo-investment functions, available in these packages of computing. The resulting equation is shown as equation (7): f RPO (7) DRV LPO the numerical coefficients in Equation (7) are the calibration factors for the modal filter amplifiers within the excitation circuit of a Cori olis flowmeter. For example, if it is desired to extract the first bending mode out of phase of the feedback signals, as in the case presented here, then the first row of the matrix of the previous modal filter vector is used as follows: ? p = 8 23S92 (RPO) + \ 6 5195 (DRJr) + 8.239 (LPO) the coefficients of the modal vector of the first bending mode for out of phase were multiplied by 103 to simplify Equation (8). With reference to Figure 5, the gain G: of the amplifier 504 is set to 8.2389 (the modal filter vector coefficients corresponding to the left discharge sensor), the gain G_- is set to 8.2389 (the modal filter vector coefficients corresponding to the right discharge sensor) and the gain Gn is set to 16.5795 (the modal filter vector coefficient corresponding to the location of the exciter). The calibration factors are linearly scaled in a group fashion, to provide a filter output signals on a path 511, having the appropriate amplitude for input to the drive control circuit 40. Figure 9 is a flow chart illustrating the process steps to determine the excitation circuit modal filter coefficients, by calculating the inverse or the pseudo-inverse of the eigenvector matrix. The calculation of the inverse or the pseudo-inverse of the matrix described above and with respect to Figure 9 is known to those skilled in the art of advanced dynamic analysis and is a useful tool for determining the excitation circuit modal filter coefficients. . The flow chart of Figure 9 starts with element 900 and proceeds to step 901. During step 901, the eigenvector array is constructed. As denoted above, a method for determining the eigenvectors of the eigenvector matrix is to construct a finite element model of the flowmeter from which the eigenvectors are extracted. Another practice is to use experimental modal analysis to directly determine the eigenvectors from a physical sample of the flowmeter. The experimental modal analysis is, already. known to those skilled in the art and their methods and used are not part of the present invention. Once the eigenvectors have been obtained by any suitable method, the eigenvector matrix is compiled. Equation (3) is an example of a complete eigenvector matrix for 10 vibration modes at three points on the flow tubes. Each column of its own vector matrix represents a different mode, while the number of rows of the eigenvector matrix represents the degrees of freedom. The eigenvector matrix is then reduced to the modes that are to be filtered. For the current example, this is done by eliminating columns that have zeros as coefficients. For the structure and the sensors, ie emplificativos, described here, the columns (modes) with coefficients at 0 are modes inside phase. The process proceeds from step 901 to step 902.

During step 902, the inverse or pseudo-inverse of the eigenvector array is computed. Each row of the inverse or the pseudo-inverse of the eigenvector matrix contains the modal filter coefficients, associated with a particular mode. This is expressed in general by means of Equation (2) and is shown by means of the previous example by Equation (7). The process now proceeds to step 903. During step 903, the appropriate modal filter calibration coefficients are selected. In the previous example, the flowmeter is excited in the first bending mode out of phase and therefore, the modal filter coefficients for the first out-of-phase bending mode are selected. However, you can build different modal filters for different applications, by selecting the modal filter coefficients for different modes. For example, it could be that a flow meter having multiple exciters placed to excite the flow tube in the first torsion mode is constructed out of phase, instead of the first mode of out-of-phase bending. In this case, the modal filter coefficients for the first off-phase torsion mode, ie the second line of in Equation (7), are used as the calibration factors for modal filter. A further example is where it is chosen to simultaneouand precisely excite multiple modes within a Cori olis flow tube. If one wants to excite a flow tube in both first bending and torsion modes out of phase, then two modal filters are used. A modal filter uses the calibration coefficients for first mode out of phase and produces a filter output signal having a modal content only in the first out-of-phase bending mode. The second modal filter uses the calibration coefficients for the first off-phase torsion mode and produces a filter output signal having a modal content only in the first out-of-phase torsion mode. The two filter output signals are summed, to be developed to an excitation signal having a modal content only in the first out-of-phase bending mode and the first out-of-phase torsion mode. After selecting the appropriate modal filter calibration coefficients, the process concludes with element 904.

Flow Diagram of the Flow Modal Filter of the Flowmeter Excitation Circuit - Figure 10 Figure 10 is a flow chart illustrating the process steps for using a modal filter to produce an excitation signal from a Coriolis flow meter. Processing begins with step 1000 and proceeds to step 1001. During step 1001, the modal filter excitation circuit receives two or more feedback signals from the vibrating flow tube. The processing then proceeds to step 1002 where each feedback signal is amplified by its corresponding calibration factor. The calibrated power signals are, during step 1003, summed to produce a filter output signal. During step 1004, the filter output signal is amplified to produce an excitation signal that is fed back to the excitation element of the flow tube. Step 1004 may also include a gain control function. This process continues until element 1005, during which, the operation of the flow meter is concluded. Although specific modalities have been discovered here, it is expected that persons skilled in the art may design Coriolis flowmeter excitation systems, employing modal filters that are within the scope of the following claims, either literally or under the Doctrine of Equivalents. It is noted that, with regard to this date, the best method known by the requested, to carry out the present invention, is that which is clear from the present, discovering the invention. Having described the invention as above, the content of the following is claimed as property.

Claims (5)

1. CLAIMS An apparatus for measuring a property of a material having a flow tube through which the material flows, a first sensor means fixed to the first location on the flow tube to produce a first movement signal indicative of tube movement flow in the first location, where the The first movement signal has a modal content, a second sensor means fixed to a second location on the flow tube to produce a second movement signal indicative of the 15 movement of the flow tube at the second location where the second movement signal has a modal content, and an excitation system for vibrating the flow tube, the apparatus characterized 20 because it includes: drive or drive means positioned adjacent to the flow tube and susceptible to an excitation signal to vibrate to the flow tube; Y space filtering means for receiving the first and second motion signals and generating the excitation signal with a modal content, in less than the plurality of vibration modes, the spatial filtering means including a: 10 first calibration means to apply a first calibration factor to the first movement signal, to develop a first signal 15 calibrated, a second calibration means to apply a second calibration factor to the second motion signal, to 20 develop a second calibrated signal, and add the means to combine the first calibrated signal and the second calibrated signal, to produce the excitation signal.
2. The apparatus according to claim 1, characterized in that the flow tube is part of a Coriolis mass flow meter.
3. The apparatus according to claim 1, characterized in that the flow tube is part of a densitometer 10 vibrator tube.
4. The apparatus according to claim 1, characterized in that the excitation means are fixed to the flow tube.
5. 5. The apparatus according to claim 1, characterized in that the means of addition include: a sum means to combine the first calibrated signal and the second 20 calibrated signal to produce a modally filtered signal; and amplification means for amplifying the modally filtered signal, to produce the excitation signal. The apparatus according to claim 5, characterized in that it also comprises: a third sensor means fixed to a third location on the flow tube, to produce a third signal of 10 movement, indicative of the movement of the flow tube at the third location. The apparatus according to claim 6, characterized in that the third location is close to one 15 position in which, - the excitation means interact with the flow tube. The apparatus according to claim 6, characterized in that 20 comprises, in addition: third calibration means for applying a third calibration factor to the third movement signal, to develop a third calibrated signal; Y summation means for combining the first, second and third calibrated signals to produce the excitation signal.

   9. The apparatus according to claim 1, characterized in that the first and second means of sensitivity 10 are speed sensors.

   10. The apparatus according to claim 1, characterized in that the first and second sensitivity means are position sensors.

   11. The apparatus according to claim 1, characterized in that the first and second sensitivity means are acceleration sensors.

   12. The apparatus in accordance with 20 claim 1, characterized in that the first and second means of sensitivity, are strain gauges.

   13. The apparatus according to claim 1, characterized in that the first and second calibration means are analog amplifiers.

   14. The compliance apparatus or claim 1, characterized in that the spatial filtering means include, in addition: 'an analog to digital converter 10 for converting the first and second motion signals to digital signals; and the first and second calibration means are amplifiers 15 digital

   15. The apparatus according to claim 1, characterized in that the spatial filtering means include, in addition: 20 and amplitude control means, susceptible to the excitation signal and to a reference voltage, to maintain a maximum vibration amplitude of the flow tube, at a substantially constant level.

   16. A method for vibrating a flow tube, for measuring a property of a material flowing through the flow tube, the method characterized in that it comprises the steps of: receive a first movement signal indicative of the movement of the 10 flow tube, to a first location on said flow tube, the first movement signal having a modal content to a plurality of vibration modes; receive a second signal 15 movement indicative of the movement of the flow tube, to a second location on said flow tube, the second movement signal having a modal content to a plurality of modes of movement. 20 vibration; apply a first calibration factor to the first movement signal, to develop a first calibrated signal; apply a second calibration factor to the second movement signal, to develop a first calibrated signal; add the first calibrated signal and the second calibrated signal, to produce the excitation signal; Y 10 apply the excitation signal to an exciter with such an operation, to cause the flow tube to vibrate in response to the excitation signal. 17 The method of compliance with 15 claim 16, characterized in that the application step includes: apply the excitation signal to an exciter with such an operation as to cause the flow tube to vibrate in 20 answer The method according to claim 16, characterized in that the sum step includes: adding the first calibrated signal and the second calibrated signal, to produce a modally filtered signal; Y amplify said signal filtered modally, to produce the excitation signal.

   19. The method according to claim 18, characterized in that it further comprises: receiving a third movement signal indicative of the movement of the flow tube at the third location on the flow tube, said third movement signal having a modal content 15 to a plurality of vibration modes.

   20. The method according to claim 19, characterized in that it also comprises: apply a third factor of 20 calibration to the third movement signal, to develop a third calibrated signal; and add to the first, second and third calibrated signals, to produce the excitation signal.

   21. The method according to claim 16, characterized in that it also comprises: control a maximum vibration amplitude in said flow tube, susceptible to the excitation signal and to a voltage of 10, to maintain the maximum vibration amplitude of the flow tube, at a substantially constant level.

   22. The method according to claim 18, characterized in that 15 further comprises the steps of: construct an own vector matrix for the movement of the vibrating tube, at N locations on the flow tube; To solve the inverse and the pseudo-inverse of the eigenvector matrix, to have a modal filter vector for said flow tube, the modal filter vector containing N sets of coefficients, wherein each of said sets N of coefficients refers to one of the vibration modes present in said vibrator tube; Y select one of the N sets of coefficients, such as the first and second calibration factors of the filter, to be applied to the signals of 10 feedback from the sensor (s) located at the N locations on said vibrator tube.

   23. The method of compliance with 15 claim 22, characterized in that the construction step includes: perform an experimental modal analysis on said vibrating tube, to generate eigenvectors for the 20 matrix of eigenvectors. The method according to claim 22, characterized in that the construction step includes: developing a finite element model of said flow tube; Y extract the eigenvectors from the finite element model, for the eigenvector matrix.

   25. The method according to claim 22, characterized in that the resolving step includes: solve the equation x = F? for ?, 10 where: x is the vector of physical response coordinates F is the eigenvector matrix; Y ? is the modal filter vector that 15 contains the N sets of coefficients.

   26. The method according to claim 25, characterized in that the selection step includes: determining which of the vibration modes present in the vibrating tube, which will be extracted as the excitation signal, to cause the vibrating tube to vibrate; Y select, susceptible to the determination step, a desired set of coefficients from the N sets of coefficients, such as the modal filter calibration factors.

   27. The method according to claim 18, characterized in that 10 further comprises the steps of: choose a first temporary calibration factor and a second temporary calibration factor; apply the first factor of 15 temporary calibration 'as the first calibration factor at the first movement signal, to produce the first calibrated signal; apply the second factor of 20 temporary calibration as the second calibration factor to the second movement signal, to produce the second calibrated signal; determining whether the excitation signal has improved the modal content, when compared to the first and second motion signals; Y select the first temporary calibration factor as a first operational calibration factor and the second temporary calibration factor as a second operational calibration factor, 10 in response to determine if the excitation signal has improved to the modal content. Summary of the Invention An excitation system 50, 104 for a vibrator tube-based measuring instrument 5, which employs a spatial filter 500, to produce an excitation signal having a modal content only in the desired vibration mode. Multiple feedback sensors 105, 105 'located in different locations along the vibrator tube 103A, 103B produce multiple feedback sensors. Each feedback signal has a calibration or gain factor applied to it. All calibrated feedback signals are then summed to produce an excitation signal, a signal proportional to the excitation signal, having an improved modal content, compared to any of the feedback signals by themselves. The calibration factors are selected by any means. One method is to construct a matrix of eigenvectors for the vibrating flow tube, by extracting the eigenvectors from a finite element model of vibration structure. The inverse or the pseudo-inverse of the eigenvector matrix is calculated to have a modal filter vector. The appropriate set of calibrated coefficients is selected from the modal filter vector.