Modal excitation detection and related methods for vibratory flowmeters.

Abstract

A flow meter is provided that includes a sensor assembly (10) and meter electronics (20). The flow meter further includes one or more flow tubes (130, 130') and a drive mechanism (180) coupled to the flow tubes (130, 130') and oriented to induce a drive mode vibration therein. A pair of pick-off sensors (170L, 170R) are coupled to the flow tubes (130, 130') and configured to measure a vibration response induced by the drive mechanism (180). At least one strain gauge (200A, 200B) is coupled to the sensor assembly (10) and configured to detect strain in the sensor assembly (10). Meter electronics (20) is connected in series with the drive mechanism (180) and the strain gauges (200A, 200B). The meter electronics (20) is configured to detect a frequency at which the change in strain is occurring.

Description

[Technical field]

[0001] The embodiments described below relate to vibratory meters, and more particularly, to an improved vibratory flow meter that utilizes modal excitation detection. [Background technology]

[0002] Vibrating conduit sensors, such as Coriolis mass flowmeters and vibratory densitometers, typically operate by detecting the motion of a vibrating conduit containing a flowing material. Properties associated with the material in the conduit, such as mass flow rate, density, etc., can be determined by processing measurement signals received from motion transducers associated with the conduit. The vibration modes of a vibrating material filling system are generally influenced by a combination of the mass, stiffness, and damping characteristics of the conduit and the material contained therein.

[0003] It is well known to use vibratory flow meters to measure mass flow rates and other properties of materials flowing through pipelines. For example, vibratory Coriolis flow meters are disclosed in U.S. Patent No. 4,491,025, issued Jan. 1, 1985 to J.E. Smith et al., and Re. 31,450, issued Nov. 29, 1983, by J.E. Smith. These flow meters have one or more flow tubes. The configuration of each flow tube in a Coriolis mass flow meter has a set of natural vibration modes, which may be simple bending, torsional, radial, lateral, or coupled. Each flow tube is driven to vibrate sympathetically in one of these natural modes. Since the vibration modes are generally influenced by a combination of the mass, stiffness, and damping properties of the flow tubes and the materials contained therein, the mass, stiffness, and damping are typically determined during the initial calibration of the flow meter using well-known techniques. A common design vibrates two flow tubes in a single mode shape, described as an out-of-phase bending mode for those tubes. This mode is often referred to as the "drive" mode because it is the vibration mode that the meter's drive coil intentionally excites.

[0004] Material flows into the flow meter through a pipeline connected to the inlet side of the flow meter, and is then directed through one or more flow tubes before exiting the flow meter into a pipeline connected to the outlet side of the flow meter.

[0005] A driver, such as a voice coil type driver, applies a force to one or more flow tubes. This force causes the one or more flow tubes to vibrate. When no material is flowing through the flowmeter, all points along the flow tubes vibrate with the same phase. When material begins to flow through the flow tubes, Coriolis acceleration causes each point along the flow tube to have a different phase relative to other points along the flow tube. The phase on the inlet side of the flow tube lags the driver and the phase on the outlet side leads the driver. Sensors are placed at two different points on the flow tube and generate sinusoidal signals that represent the motion of the flow tube at the two points. The phase difference between the two signals received from the sensors is calculated in units of time.

[0006] The phase difference between the two sensor signals is proportional to the mass flow rate of the material through the flowtube or flowtubes. The mass flow rate of the material is determined by multiplying the phase difference by a flow calibration factor. The flow calibration factor depends on the material and cross-sectional properties of the flowtube. One of the main properties of the flowtube that affects the flow calibration factor is the stiffness of the flowtube. The flow calibration factor is determined by a calibration process prior to installation of the flowmeter in a pipeline. In the calibration process, a fluid is passed through the flowtube at a predetermined flow rate and the ratio between the phase difference and the flow rate is calculated. As is commonly known in the art, the stiffness and damping characteristics of the flowtube are also determined during the calibration process.

[0007] One advantage of Coriolis flowmeters is that because there are no moving parts inside the vibrating flowtube, the accuracy of the measured mass flow rate is not significantly affected by wear of moving parts inside the flowmeter. Flow rate is determined by multiplying the phase difference between two points on the flowtube by a flow calibration factor. The only input is a sinusoidal signal from a sensor that indicates the vibration of the two points on the flowtube. The phase difference is calculated from the sinusoidal signal. The flow calibration factor is proportional to the material and cross-sectional properties of the flowtube, therefore the phase difference measurement and the flow calibration factor are not affected by wear of moving parts inside the flowmeter.

[0008] A typical Coriolis mass flow meter includes one or more transducers (or pickoff sensors), which are typically used to measure the vibrational response of one or more flow conduits, and are typically located upstream and downstream of a driver. The pickoff sensors are connected to electronic instrumentation. The instrumentation receives signals from the two pickoff sensors and processes the signals to derive a measurement, such as a mass flow rate measurement.

[0009] Any mechanical structure will always have vibration modes of various shapes and natural frequencies, and Coriolis meters are no exception. Under certain conditions, excitation of vibration modes other than those the meter is designed to excite may be undesirable. Such undesirable mode excitation may interfere with accurate measurement of fluid flow through the meter. Undesirable mode excitation may also adversely affect the reliability and lifespan of the meter.

[0010] The embodiments described below overcome these and other problems and achieve an advance in the art. The embodiments described below provide a flow meter that uses strain gauges to detect when undesired excitation of unintended vibration modes is occurring, and is used as a diagnostic tool to both troubleshoot flow measurement performance problems and protect the meter from damage. Summary of the Invention

[0011] According to one embodiment, a flow meter is provided that includes a sensor assembly and meter electronics. The flow meter includes one or more flow tubes and a drive mechanism coupled to the one or more flow tubes and oriented to induce a drive mode vibration in the one or more flow tubes. A pair of pick-off sensors are coupled to the one or more flow tubes and configured to measure a vibration response of the flow tube induced by the drive mechanism. At least one strain gauge is coupled to the sensor assembly, the at least one strain gauge configured to detect a strain in the sensor assembly. Meter electronics is connected to the drive mechanism and the at least one strain gauge, the drive mechanism and the at least one strain gauge being connected in series. The meter electronics is configured to detect a frequency at which a change in strain is occurring.

[0012] According to one embodiment, a method for detecting modal excitation in a flow meter having a sensor assembly and meter electronics is provided. The method includes vibrating at least one of one or more flow tubes in a driven mode vibration using a drive mechanism and measuring a vibration response of the flow tube induced by the drive mechanism using a pair of pick-off sensors. At least one strain gauge is provided coupled to the sensor assembly. The drive mechanism and the at least one strain gauge are connected to the meter electronics, and the drive mechanism and the at least one strain gauge are connected in series. A strain in the sensor assembly is detected using the at least one strain gauge. A frequency at which a change in strain is occurring is detected.

[0013] [Aspects] According to one aspect, a flow meter is provided having a sensor assembly and meter electronics, the flow meter comprising one or more flow tubes and a drive mechanism coupled to the one or more flow tubes and oriented to induce a drive mode vibration in the one or more flow tubes. A pair of pickoff sensors are coupled to the one or more flow tubes and configured to measure a vibration response of the flow tube induced by the drive mechanism. At least one strain gauge is coupled to the sensor assembly, the at least one strain gauge configured to detect a strain in the sensor assembly. The meter electronics is connected to the drive mechanism and the at least one strain gauge, the drive mechanism and the at least one strain gauge being connected in series. The meter electronics is configured to detect a frequency at which a change in strain occurs.

[0014] Preferably, the meter electronics is configured to detect vibrations at non-driven mode frequencies in the signal received from the at least one strain gauge.

[0015] Preferably, the meter electronics is configured to generate at least one of an alarm and a notification when the detected vibrations at the non-drive mode frequency are within a predetermined proximity to the drive mode frequency.

[0016] Preferably, the meter electronics is configured to output diagnostic information whether a separation between the non-driven mode frequencies and the driven mode frequencies remains stable or is changing when the detected vibrations at the non-driven mode frequencies are within a predetermined proximity to the driven mode frequencies, and if the separation between the non-driven mode frequencies and the driven mode frequencies remains stable, the diagnostic information includes instructions to calibrate the flow meter zero.

[0017] Preferably, the meter electronics is configured to output diagnostic information whether the separation between the non-driven mode frequencies remains stable or is changing if the detected vibrations at the non-driven mode frequencies are within a predetermined proximity to the driven mode frequencies, and if the separation between the non-driven mode frequencies and the driven mode frequencies is changing, the diagnostic information includes instructions for identifying and eliminating changes in installation and / or process conditions that may be causing the frequency separation to change.

[0018] Preferably, the meter electronics is configured to generate at least one of an alarm and a notification when a frequency of an undriven mode known to be associated with meter reliability issues is detected.

[0019] Preferably, the at least one strain gauge is coupled to at least one of the one or more flow tubes.

[0020] Preferably, the at least one strain gauge is coupled to the brace bar.

[0021] According to one aspect, a method is provided for detecting modal excitation in a flowmeter having a sensor assembly and meter electronics. The method includes vibrating at least one of one or more flow tubes in a driven mode vibration with a drive mechanism and measuring a vibration response of the flow tube induced by the drive mechanism with a pair of pickoff sensors. At least one strain gauge is provided coupled to the sensor assembly. The drive mechanism and the at least one strain gauge are connected to the meter electronics, with the drive mechanism and the at least one strain gauge being connected in series. Strain in the sensor assembly is detected using the at least one strain gauge. A frequency at which a change in strain occurs is detected.

[0022] Preferably, the meter electronics is configured to detect vibrations at non-driven mode frequencies in the signal received from the at least one strain gauge.

[0023] Preferably, the meter electronics is configured to generate at least one of an alarm and a notification when the detected vibrations at the non-drive mode frequency are within a predetermined proximity to the drive mode frequency.

[0024] Preferably, the method further includes outputting diagnostic information by the meter electronics whether the separation between the non-driven mode frequencies and the driven mode frequencies remains stable or is changing if the vibration at the non-driven mode frequencies is within a predetermined proximity to the driven mode frequencies, and if the separation between the non-driven mode frequencies and the driven mode frequencies remains stable, the diagnostic information includes instructions to calibrate the flow meter zero.

[0025] Preferably, the method further includes outputting diagnostic information by the meter electronics whether the separation between the non-driven mode frequencies and the driven mode frequencies remains stable or is changing if the vibration at the non-driven mode frequencies is within a predetermined proximity to the driven mode frequencies, and if the separation between the non-driven mode frequencies and the driven mode frequencies is changing, the diagnostic information includes instructions for identifying and eliminating changes in installation and / or process conditions that may be causing the frequency separation to change.

[0026] Preferably, the method further includes generating at least one of an alarm and a notification when a frequency of an undriven mode known to be associated with a meter reliability issue is detected by the meter electronics.

[0027] Preferably, the method further comprises coupling at least one strain gauge to at least one of the one or more flow tubes.

[0028] Preferably, the method further comprises the step of coupling at least one strain gauge to the brace bar. [Brief description of the drawings]

[0029] In all drawings, the same reference numbers represent the same elements. [Figure 1] FIG. 1 shows a prior art flow meter. [Diagram 2] FIG. 2 illustrates a flow meter according to one embodiment. [Diagram 3] FIG. 3 is a diagram of the meter electronics. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] 1-3 and the following description depict specific examples to teach those skilled in the art how to make and use the best modes of embodiments of the flowmeter and related methods. Some conventional aspects have been simplified or omitted for the purpose of teaching the principles of the invention. Those skilled in the art will recognize variations of these examples that fall within the scope of the invention. Those skilled in the art will appreciate that the features described below can be combined in various ways to form multiple variations of the invention. As a result, the invention is not limited to the specific examples, but only by the claims and their equivalents.

[0031] FIG. 1 illustrates a prior art flow meter 5, which may be any vibratory meter, such as a Coriolis flow meter. The flow meter 5 includes a sensor assembly 10 and meter electronics 20. The sensor assembly 10 is responsive to the mass flow rate and density of a process material. The meter electronics 20 is connected to the sensor assembly 10 via leads 100 and provides density, mass flow rate, and temperature information, as well as other information not related to the present invention, to a path 26. The sensor assembly 10 includes a pair of manifolds 150 and 150', flanges 103 and 103' having flange necks 110 and 110', a pair of parallel flow tubes 130 (first flow tube) and 130' (second flow tube), a driver mechanism 180, a temperature sensor 190, such as a resistance temperature detector (RTD), and a pair of pickoffs 170L and 170R, such as magnet / coil pickoffs, strain gauges, optical sensors, or any other pickoff sensor known in the art. Flow tubes 130 and 130' have inlet legs 131 and 131' and outlet legs 134 and 134' that converge toward flow tube mounting blocks 120 and 120', respectively. Flow tubes 130 and 130' are bent at at least one symmetrical location along their length and are essentially parallel throughout their length. Brace bars 140 and 140' serve to define axes W and W' about which each flow tube oscillates.

[0032] The side legs 131, 131' and 134, 134' of flow tubes 130 and 130' are fixedly attached to flow tube mounting blocks 120 and 120' which are further fixedly attached to manifolds 150 and 150'. This provides a continuous, closed material path through sensor assembly 10.

[0033] When flanges 103 and 103', having bolt holes 102 and 102', are connected via inlet end 104 and outlet end 104' to a process line (not shown) carrying the process material to be measured, the material enters the meter end 104 through orifice 101 in flange 103 and is directed through manifold 150 to flow tube mounting block 120 having surface 121. The material is split within manifold 150 and directed through flow tubes 130 and 130'. Upon exiting flow tubes 130 and 130', the process material is recombined within manifold 150' into a single stream and then directed to outlet end 104', which is connected to the process line (not shown) by flange 103', having bolt holes 102'.

[0034] Flow tubes 130 and 130' are selected and appropriately mounted in flow tube mounting blocks 120 and 120' so that they have substantially the same mass distribution, moment of inertia, and Young's modulus about bending axes W--W and W'--W', respectively. These bending axes pass through brace bars 140 and 140'. Because the Young's modulus of the flow tube changes with temperature and this change affects flow rate and density calculations, a temperature sensor 190 is attached to flow tube 130' to continuously measure the temperature of the flow tube. The temperature of the flow tube, and therefore the voltage appearing at temperature sensor 190 for a given current passing therethrough, is determined by the temperature of the material passing through the flow tube. The temperature dependent voltage appearing at temperature sensor 190 is used by meter electronics 20 in a well known manner to compensate for changes in the elastic modulus of flow tubes 130 and 130' due to changes in flow tube temperature. Temperature sensor 190 is connected to meter electronics 20 by leads 195.

[0035] Both flow tubes 130 and 130' are driven in opposite directions about their respective bending axes W and W' by a driver 180 in what is referred to as the first out-of-phase bending mode of the flowmeter. This driver 180 may comprise any one of a number of well known configurations, such as a magnet attached to flow tube 130' and an opposing coil attached to flow tube 130 through which an alternating current is passed to vibrate both flow tubes. An appropriate drive signal is applied to driver 180 by meter electronics 20 via leads 185.

[0036] Meter electronics 20 receives the temperature signal on lead 195 and the left and right velocity signals appearing on leads 165L and 165R, respectively. Meter electronics 20 generates a drive signal appearing on lead 185 to driver 180, causing tubes 130 and 130' to vibrate. Meter electronics 20 processes the left and right velocity signals and the temperature signals to calculate the mass flow rate and density of the material passing through sensor assembly 10. This information, along with other information, is sent by meter electronics 20 over path 26 to a utilization means.

[0037] Typically, the Coriolis meter is driven in a first out-of-phase bending mode with flow-induced phasing between the inlet and inlet legs being sensed using coil / magnet pickoffs attached to the inlet and inlet legs of the flow meter. In one embodiment, a combined signal from one or more strain gauges attached to the internal vibrating structure of the meter is input to the meter electronics. A Wheatstone bridge circuit can be used to amplify the signal. In one embodiment, a strain signal from the internal vibrating structure of the flow meter is input to the meter electronics and processed to detect the natural frequencies of the various multiple vibration mode shapes excited within the structure. The detected modal frequencies are analyzed to reveal diagnostic information for optimizing the installation and operation of the meter. In one embodiment, the signal from one or more strain gauges is transmitted superimposed on other signals transmitted by existing signal conductors. By transmitting the strain gauge signal through signal conductors already present in the existing flow meter design, this embodiment can be easily implemented and retrofitted to existing flow meter designs.

[0038] For clarity, the number of conductors shown has been minimized. Although only a single line is drawn for 26, 165L, 165R, 185, and 195, this single line may represent one or more conductors. The driver circuit using lead 185 is shown in more detail than the other circuits to visually convey its series nature. The other circuits, whether or not specifically shown, may be in series, parallel, or a combination thereof.

[0039] FIG. 2 illustrates a flowmeter 5 according to one embodiment. Although the structure of a Coriolis flowmeter is described, it will be apparent to one skilled in the art that the present invention can be implemented as a vibrating tube densitometer without the additional measurement capabilities provided by a Coriolis mass flowmeter. Elements in common with the prior art device of FIG. 1 are given the same reference numerals. Flowtubes 130 and 130' are driven in opposite directions about their respective bending axes W and W' in what is referred to as the first out-of-phase bending mode of the flowmeter by driver 180. This driver 180 can comprise any one of many well known configurations, such as a magnet attached to flowtube 130' and an opposing coil attached to flowtube 130 through which an alternating current is passed to vibrate both flowtubes. It should be noted that flowtubes 130, 130' are substantially rigid, for example formed from metal, and therefore only limited movement, e.g., oscillatory motion induced by the driver, is possible. An appropriate drive signal is applied to driver 180 by meter electronics 20 via leads 185. A pair of pickoffs 170L and 170R are provided, such as magnet / coil pickoffs, strain gauges, optical sensors, or other pickoff sensors known in the art.

[0040] A first strain gauge 200A and a second strain gauge 200B are provided. As shown, the first strain gauge 200A is located on the inlet leg 131 of the first flow tube 130 and the second strain gauge 200B is located on the outlet leg 134 of the first flow tube 130. In some embodiments, strain gauges may be located on both flow tubes 130, 130'. The maximum strain amplitude is near the brace bars 140, 140' of the flow tubes 130, 130', where the strain gauges 200A, 200B are located in some embodiments. However, other locations on the flow tubes are contemplated. Additionally, placement on a support structure such as the brace bars 140, 140' is also contemplated. Generally, the strain elements are attached to the flow tubes and / or other portions of the meter structure that experience strain when the meter is vibrating in one or more undesirable mode shapes.

[0041] As shown, the strain gauges 200A, 200B are connected in series to the driver 180 circuit. This provides the advantage of being able to transmit signals from these strain gauge elements to the Coriolis transmitter without requiring any changes to the existing meter feed-through design or the number of conductors in the transmitter connection, as the elements are connected in series with each other and with the existing drive coil circuit. By using the drive coil circuit, the PO coil signal, which is important for the meter to measure flow and density, remains intact. In the illustrated series connection, the driver is placed between the two strain gauges 200A, 200B. It is also envisioned that the driver will be the first element in the circuit, and also the last element in the circuit, in terms of current flow.

[0042] In one embodiment, each strain gauge is oriented to detect strain induced by drive mode operation of the flow tubes 130, 130. In one embodiment, the strain gauges 200A, 200B are oriented substantially parallel to the longitudinal axis of the flow tube to which they are coupled. Perpendicular and non-orthogonal orientations are also contemplated.

[0043] The change in resistance of the strain gauges 200A, 200B is caused by strain on the underlying surface to which they are attached. The magnitude of the change in resistance does not necessarily need to be measured precisely for the embodiments to function as intended. Of particular relevance is the frequency at which the change in strain occurs, which can be obtained without necessarily measuring precisely the resistance or the magnitude of the strain.

[0044] The most likely adverse effect on flow measurements from undesired mode excitation will manifest as a zero shift or instability in the meter. The phase relationship between the signals from pickoffs 170L and 170R is indicative of the flow rate through the meter. A measurement of this phase difference, which corresponds to a zero fluid flow condition in the meter, is obtained when calibrating the meter's zero offset value. This zero offset value is subtracted from future flow measurements made by the meter, making the accuracy of these flow measurements more accurate.

[0045] However, the actual meter's zero offset phase difference may drift or become unstable from the original calibrated value if mounting conditions change from when the meter was calibrated. Additionally, different fluid properties may move undesirable mode shape frequencies closer to the drive mode frequency. If the natural frequencies of different mode shapes get too close to the drive mode frequency, this may cause the drive coil to excite vibrations in these other modes and interference between these modes and the drive mode may destabilize the meter's zero offset value or shift it away from the previously calibrated value.

[0046] When the meter zero is affected in this way, resolving the problem may require different actions depending on whether the relative separation of these frequencies is stable or fluctuating. If the amount of separation remains stable, simply recalibrating the zero value to the new conditions may resolve any flow measurement issues. If the frequency separation is not stable because one or more frequencies are constantly changing relative to the drive mode frequency in response to changing conditions, the solution will likely require a change in installation and / or operating conditions.

[0047] Detecting and trending the amount of separation between the drive modal frequency and any nearby modal frequencies can inform either calibrating the meter zero in-place to resolve the problem if the modal separation is held constant, or alternatively, investigating further to find out which installation and / or process conditions are causing the instability if the modal separation is fluctuating. In the latter case, the meter's ability to detect and trend the amount of modal separation would be useful to further diagnose, through trial and error, whether changes in specific installation and / or process conditions are resulting in changes in the modal separation and therefore zero instability.

[0048] Alternatively, there may be some mode shapes that do not occur close enough to the drive frequency to pose a risk to the integrity of the meter zero, but which nevertheless could pose a risk to the mechanical reliability of the meter structure if excited to extreme levels by external forces. If the undesired excitation of these modes could also be detected, potential future failures of the meter could be avoided by preventative maintenance or adjustments to the mounting or process conditions that are causing the undesired mode excitation.

[0049] Coriolis meters with many pick-off coils may not be able to detect excitation of some or all of the undesired modes because they are typically designed and positioned to optimize the measurement of the meter tube vibration in the primary drive mode to achieve optimal flow and density measurement performance of the meter. Thus, vibrations in other modes may not produce a significant detectable contribution to the pick-off coil signal. In mode shapes where the flow tubes vibrate directionally in phase with each other, there is little or no relative motion between the PO coil and magnet, and therefore nothing correspondingly added to the signal at the frequency of the mode. In mode shapes where the primary motion of the flow tube is transverse or perpendicular to the direction the tube moves in the drive mode, there may also be little or no motion between the pick-off coil and magnet that is in the correct orientation to generate a signal at the frequency of the mode. The same is true for the drive coils and magnets as they are typically positioned.

[0050] The strain gauges 200A, 200B are therefore used to detect structural vibrations at the frequencies of the undesired modes of interest (i.e., non-driven mode vibrations). The meter electronics 20 can do this by applying well-established digital signal processing (DSP) techniques to convert either the circuit's dynamic resistance measurements and / or the dynamic changes in the drive current into the frequency domain. Once the signals are processed to identify all frequencies superimposed on top of the drive coil circuit signal, the frequencies of mode shapes other than the driven mode become apparent. The detected resistance and / or current changes that occur at these other frequencies are the result of strains that change the resistance of the element when these other modes are excited.

[0051] For any particular flowmeter model, the full range of frequencies that may occur in response to changes in fluid properties and mounting conditions can be predicted in advance by computer modeling for each mode known to potentially affect the meter's accuracy or reliability. Any modal vibrations detected using this method can be identified as being active by checking the observed frequencies against the known range of possible frequencies for that mode.

[0052] If the possible frequency range of a particular mode is known to be close enough or overlapping in frequency with the drive mode, then this mode may affect meter zero stability because it is more likely to change frequency as mounting conditions change compared to the drive mode. If the observed frequencies for these modes are too close to the drive frequency, this is an indication that the undesired modes are being excited by the same energy provided by the drive coil to excite the drive mode. This phenomenon can very well cause zero stability issues for the meter as the cumulative motion of both combined modes will constantly change as mounting conditions change, affecting meter zero stability. In one embodiment, the meter electronics 20 generates an alarm and / or notification when the observed frequencies for these modes are within a predefined proximity to the drive frequency.

[0053] By observing and trending the difference between the live frequency of the driven mode and the live frequencies of nearby modes, it is possible to detect when the modal frequencies are too close or may cross periodically. This observation provides a clear indication that modal interference is the root cause of the observed meter zero stability and accuracy problems. It will be appreciated that a threshold can be set by the meter electronics that indicates that the modal frequencies are too close to each other.

[0054] Similarly, if a particular mode when excited is known to be associated with meter reliability problems, detection of a frequency within the range of known frequencies for that mode would be an indication that this mode is being excited by external vibrations or by energy entering the meter structure from some other source. Since this mode is not intended to be active, an indication of the occurrence of frequencies in the known frequency range of this mode would be a valuable diagnostic indicator that could guide measures to make adjustments to the installation to eliminate excitation of this mode.

[0055] In one embodiment, diagnostic information may be output by the meter electronics to direct the operator to the most appropriate course of action for a particular undesirable mode excitation. This is based on measurements of the frequency of the detected signal disturbance, the mode shape that it represents, and the likely outcome of exciting that mode shape. Potentially problematic modes fall into two main categories: 1) This mode is too close in frequency to the drive frequency (common mode preventing zero stability) due to fluid density, mounting conditions, etc. 2) This mode may be harmful to the meter and is excited externally (e.g., a transverse mode that may damage the meter if excited sufficiently strongly).

[0056] If a mode shape is at risk for zero stability due to its frequency being close to the drive frequency, the separation between this mode and the drive mode frequency can be monitored and a diagnostic command is sent based on whether the frequency separation remains stable or is changing. If the frequency separation is stable, the command is to calibrate the zero. In one embodiment, the meter electronics 20 can automatically calibrate the zero when flow conditions are suitable for zero calibration. In another embodiment, the meter electronics 20 can prompt the user to calibrate the zero. If the frequency separation is fluctuating, the command provided by the meter electronics 20 is to identify and eliminate changes in installation and / or process conditions that are causing the frequency separation to fluctuate.

[0057] FIG. 3 illustrates meter electronics 20 of a flow meter 5 in accordance with one embodiment of the present invention. Meter electronics 20 may include an interface 201 and a processing system 203. Meter electronics 20 receives first and second sensor signals, such as, for example, strain gauge 200A, 200B signals, from sensor assembly 10. Meter electronics 20 processes the first and second sensor signals to obtain flow characteristics of the fluid material flowing through sensor assembly 10. For example, meter electronics 20 may determine one or more of phase difference, frequency, time difference (Δt), density, mass flow rate, strain, and volumetric flow rate from the sensor signals. Additionally, other flow characteristics may be determined in accordance with the present invention.

[0058] The interface 201 receives the strain gage signal via the leads utilized for the drive signal. The strain gages 200A, 200B and the driver 180 are connected in series. The interface 201 may perform any necessary or desired signal conditioning, such as formatting, amplifying, buffering in any manner. Alternatively, some or all of the signal conditioning may be performed in the processing system 203. As previously mentioned, the meter electronics may apply well-established digital signal processing (DSP) techniques to convert the dynamic resistance measurements of the circuit and / or dynamic changes in the drive current into the frequency domain.

[0059] Additionally, the interface 201 may enable communication between the meter electronics 20 and external devices, such as over the communications path 26. The interface 201 may enable any form of electronic, optical, or wireless communication.

[0060] In one embodiment, the interface 201 includes a digitizer 202, and the sensor signal includes an analog sensor signal. The digitizer samples and digitizes the analog sensor signal to generate a digital sensor signal. The interface / digitizer may also perform any necessary decimation, where the digital sensor signal is thinned out to reduce the amount of signal processing required and thus the processing time.

[0061] The processing system 203 performs the operations of the meter electronics 20 and processes the flow measurements from the sensor assembly 10. The processing system 203 executes one or more processing routines, thereby processing the flow measurements to generate one or more flow characteristics.

[0062] Processing system 203 may comprise a general purpose computer, a microprocessor system, logic circuitry, or other general purpose or customized processing device. Processing system 203 may be distributed across multiple processing devices. Processing system 203 may include any form of integrated or stand-alone electronic storage medium, such as storage system 204.

[0063] In the illustrated embodiment, the processing system 203 determines vibration mode frequency characteristics from the two or more vibration / strain responses 220, 226. The processing system 203 can determine at least the magnitude, phase difference, time difference, and frequency of the two or more responses 220, 226.

[0064] The storage system 204 can store flow meter parameters and data, software routines, constant values, and variable values. In one embodiment, the storage system 204 includes routines executed by the processing system 203. In one embodiment, the storage system 204 stores a phase shift routine 212, a notification routine 213, a phase difference routine 215, a frequency routine 216, a time difference (Δt) routine 217, and a distortion detection routine 218. In some embodiments, the storage system 204 stores one or more flow characteristics derived from the flow measurements.

[0065] In these embodiments, a bridge circuit may be used to amplify the distortion signal, however, in other embodiments, the distortion signal is utilized without a bridge circuit.

[0066] The detailed description of the above embodiments is not an exhaustive description of all embodiments that the inventors consider to be within the scope of the present invention. Indeed, those skilled in the art will recognize that certain elements of the above embodiments can be combined or deleted in various ways to create additional embodiments, and that such additional embodiments are within the scope and teachings of the present invention. It will also be apparent to those skilled in the art that the above-described embodiments can be combined in whole or in part to create additional embodiments within the scope and teachings of the present invention.

[0067] Thus, while specific embodiments and examples of the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the art will recognize. The teachings provided herein may be applied to other devices and methods in addition to the embodiments described above and illustrated in the accompanying figures. The scope of the invention should therefore be determined from the claims that follow.

Claims

1. A flow meter (5) including a sensor assembly (10) and meter electronics (20), one or more flow tubes (130, 130'); a drive mechanism (180) coupled to the one or more flow tubes (130, 130') and oriented to induce a drive mode vibration within the one or more flow tubes (130, 130'); a pair of pick-off sensors (170L, 170R) coupled to the one or more flow tubes (130, 130') and configured to measure a vibrational response of the flow tubes (130, 130') induced by the drive mechanism (180); and At least one strain gauge (200A, 200B) coupled to the sensor assembly (10), the at least one strain gauge (200A, 200B) configured to detect strain in the sensor assembly (10). Equipped with the meter electronics (20) is connected to the drive mechanism (180) and the at least one strain gauge (200A, 200B), the drive mechanism (180) and the at least one strain gauge (200A, 200B) are connected in series; The flow meter (5), wherein the meter electronics (20) is configured to detect a frequency at which a change in distortion occurs.

2. The flow meter (5) of claim 1, wherein the meter electronics (20) is configured to detect vibrations at a non-driven mode frequency in a signal received from the at least one strain gage (200A, 200B).

3. 3. The flow meter of claim 2, wherein the meter electronics is configured to generate at least one of an alarm and a notification when the detected vibration at a non-driven mode frequency is within a predetermined proximity to the driven mode frequency.

4. 3. The flowmeter of claim 2, wherein the meter electronics is configured to output diagnostic information whether a separation between the non-driven mode frequency and the driven mode frequency remains stable or is changing if the vibration at the detected non-driven mode frequency is within a predetermined proximity to the driven mode frequency, and if the separation between the non-driven mode frequency and the driven mode frequency remains stable, the diagnostic information includes instructions to calibrate a flow meter zero.

5. 3. The flowmeter of claim 2, wherein the meter electronics is configured to output diagnostic information whether a separation between the non-driven mode frequencies and the driven mode frequencies is stable or varying if the vibration at the detected non-driven mode frequencies is within a predetermined proximity to the driven mode frequencies, and if the separation between the non-driven mode frequencies and the driven mode frequencies is varying, the diagnostic information includes instructions for identifying and eliminating changes in installation and / or process conditions that may be causing a variation in frequency separation.

6. 3. The flow meter of claim 2, wherein the meter electronics is configured to generate at least one of an alert and a notification when a frequency of an undriven mode known to be associated with meter reliability issues is detected.

7. The flow meter (5) of claim 1, wherein the at least one strain gauge (200A, 200B) is coupled to at least one of the one or more flow tubes (130, 130').

8. The flow meter (5) of claim 1, wherein the at least one strain gauge (200A, 200B) is coupled to a brace bar (140, 140').

9. 1. A method for detecting modal excitation in a flow meter having a sensor assembly and meter electronics, comprising: vibrating at least one of the one or more flow tubes in a driven mode vibration using a drive mechanism; measuring a vibration response of the flow tube induced by the drive mechanism with a pair of pickoff sensors; providing at least one strain gauge coupled to a sensor assembly; connecting the drive mechanism and the at least one strain gauge to the meter electronics, the drive mechanism and the at least one strain gauge being connected in series; detecting strain on the sensor assembly using the at least one strain gauge; Detecting the frequency at which the distortion change occurs A method comprising:

10. The method of claim 9 , wherein the meter electronics is configured to detect vibrations at non-driven mode frequencies in the signal received from the at least one strain gauge.

11. 11. The method of claim 10, wherein the meter electronics is configured to generate at least one of an alarm and a notification when the vibration at a detected non-drive mode frequency is within a predetermined proximity to the drive mode frequency.

12. 11. The method of claim 10, further comprising outputting, by the meter electronics, diagnostic information whether a separation between the non-driven mode frequencies and the driven mode frequencies remains stable or is changing if the vibration at the detected non-driven mode frequencies is within a predetermined proximity to the driven mode frequencies, and if the separation between the non-driven mode frequencies and the driven mode frequencies remains stable, the diagnostic information includes instructions to calibrate a flow meter zero.

13. 11. The method of claim 10, further comprising outputting, by the meter electronics, diagnostic information whether a separation between the non-driven mode frequencies and the driven mode frequencies remains stable or is changing if the vibration at the detected non-driven mode frequencies is within a predetermined proximity to the driven mode frequencies, and if the separation between the non-driven mode frequencies and the driven mode frequencies is changing, the diagnostic information includes instructions for identifying and eliminating changes in installation and / or process conditions that may be causing a change in frequency separation.

14. 11. The method of claim 10, further comprising generating at least one of an alarm and a notification when a frequency of an undriven mode known to be associated with a meter reliability issue is detected by the meter electronics.

15. The method of claim 9 , comprising coupling the at least one strain gauge to at least one of the one or more flow tubes.

16. The method of claim 9 , comprising coupling the at least one strain gauge to a brace bar.

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