Startup and operational techniques for a digital flowmeter Number:7,146,280 from the United States Patent and Trademark Office (PTO) owispatent

Title: Startup and operational techniques for a digital flowmeter

Abstract: Startup and operational techniques for a digital flowmeter are described. The techniques select an optimal mode of operation for the digital flowmeter, depending on a current environment of the flowmeter. For example, during a startup operation of the flowmeter, the mode of operation might include a random sequence mode, in which filtered, random frequencies are applied as a drive signal to a flowtube associated with the digital flowmeter. Once the flowtube reaches a resonant mode of vibration, the digital flowmeter may transition to a positive feedback mode, in which a sensor signal representing a motion of the flowtube is fed back to the flowtube as a drive signal, as part of a feedback loop. Once an oscillation of the flowtube is achieved and analyzed, a digital synthesis mode of operation may be implemented, in which the analyzed sensor signals are used to synthesize the drive signal.

Patent Number: 7,146,280 Issued on 12/05/2006 to Henry,   et al.

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Inventors:	Henry; Manus P. (Oxford, GB), Zamora; Mayela E. (Oxford, GB)
Assignee:	Invensys Systems, Inc. (Foxboro, MA)
Appl. No.:	11/168,568
Filed:	June 29, 2005

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Related U.S. Patent Documents

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	Application Number	Filing Date	Patent Number	Issue Date
	10402131	Mar., 2003	6950760
	10400922	Mar., 2003
	60368153	Mar., 2002

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Current U.S. Class:	702/45 ; 73/861.12; 73/861.356
Current International Class:	G06F 19/00 (20060101); G06F 1/00 (20060101)
Field of Search:	702/45-47,50,54,100,104,106,137 73/861.02,861.04,861.12,861.355,861.356,861.357 318/640 386/34 361/740 340/606

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Primary Examiner: Nghiem; Michael
Assistant Examiner: Le; John
Attorney, Agent or Firm: Fish & Richardson P.C.

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Parent Case Text

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CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 10/402,131, filed Mar. 31, 2003, now U.S. Pat. No. 6,950,760 titled STARTUP AND OPERATIONAL TECHNIQUES FOR A DIGITAL FLOWMETER, which is a Continuation in Part of U.S. application Ser. No. 10/400,922, filed Mar. 28, 2003, now abandoned and titled DRIVE TECHNIQUES FOR A DIGITAL FLOWMETER, which claims priority from U.S. Provisional Application No. 60/368,153, filed Mar. 29, 2002, and titled ELLIPTICAL FILTER FOR DIGITAL FLOWMETER, all of which are incorporated by reference.

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Claims

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What is claimed is:

1. A method for operating a digital flowmeter comprising: operating the digital flowmeter in a random sequence mode, where the random sequence mode is characterized by a first drive signal that includes randomly-generated values and that causes a vibration of a flowtube; operating the digital flowmeter in a positive feedback mode, where the positive feedback mode is characterized by a sensor signal that corresponds to the vibration and that is used as a second drive signal for maintaining the vibration; and operating the digital flowmeter in a digital synthesis mode, where the digital synthesis mode is characterized by a synthesis of a third drive signal for maintaining the vibration.

2. The method of claim 1 further comprising transitioning the digital flowmeter between two or more of the random sequence mode, the positive feedback mode, and the digital synthesis mode.

3. The method of claim 1 wherein operating the digital flowmeter in the random sequence mode comprises filtering the randomly-generated values to remove frequency components above a pre-determined cut-off level.

4. The method of claim 3 wherein filtering the randomly-generated values comprises setting the pre-determined cut-off level based on a range of potential resonance frequencies associated with the flowtube.

5. The method of claim 1 wherein operating the digital flowmeter in the random sequence mode comprises filtering the randomly-generated values to remove frequency components above a first pre-determined cut-off level and below a second pre-determined cut-off level.

6. The method of claim 1 further comprising operating the digital flowmeter in a zero-output mode, the zero-output mode characterized by having a zero-value drive signal.

7. The method of claim 6 wherein operating the digital flowmeter in the random sequence mode comprises: detecting a pre-determined condition; and initiating operation of the zero-output mode, based on the predetermined condition.

8. The method of claim 7 wherein operating the digital flowmeter in the zero-output mode comprises: detecting a pre-determined amount of time has passed; and transitioning operation of the digital flowmeter into the positive feedback mode.

9. The method of claim 7 wherein detecting the pre-determined condition comprises detecting an end of a pre-determined time period.

10. The method of claim 1 wherein operating the digital flowmeter in the positive feedback mode comprises: detecting a first pre-determined condition; and initiating operation of the digital synthesis mode, based on the first pre-determined condition.

11. The method of claim 10 wherein detecting the first pre-determined condition comprises: detecting a signal waveform within the sensor signal; determining that the signal waveform has a pre-determined characteristic; and transitioning operation of the digital flowmeter from the positive feedback mode into the digital synthesis mode.

12. The method of claim 11 wherein determining that the signal waveform has a pre-determined characteristic comprises determining that the signal waveform has sinewave characteristics.

13. The method of claim 11 further comprising: detecting an instability associated with the operation of the digital flowmeter in the digital synthesis mode; and transitioning operation of the digital flowmeter from the digital synthesis mode into the positive feedback mode, in response to detecting the instability.

14. The method of claim 11 further comprising: detecting an instability associated with the operation of the digital flowmeter in the digital synthesis mode; and transitioning operation of the digital flowmeter from the digital synthesis mode into the random sequence mode, in response to detecting the instability.

15. The method of claim 10 wherein operating the digital flowmeter in the positive feedback mode comprises: detecting a second predetermined condition; and transitioning operation of the flowmeter from the positive feedback mode into the random sequence mode, based on the second pre-determined condition.

16. The method of claim 15 wherein detecting the second predetermined condition comprises detecting an instability in an operation of the flowmeter.

17. The method of claim 1 wherein operating the digital flowmeter in the positive feedback mode comprises filtering an initial sensor signal to obtain the sensor signal.

18. The method of claim 1 wherein the sensor signal is a digital signal, and operating the digital flowmeter in the positive feedback mode comprises converting an analog sensor signal into the sensor signal.

19. The method of claim 1 wherein operating the digital flowmeter in the positive feedback mode comprises: buffering the sensor signal to obtain a buffered sensor signal; and selecting values from the buffered sensor signal to use as the second drive signal so as to compensate for a delay associated with the second drive signal, where the delay is associated with a digital element associated with the flowmeter.

20. The method of claim 1 wherein operating the digital flowmeter in the digital synthesis mode comprises filtering an initial sensor signal to obtain the sensor signal.

21. The method of claim 1 wherein operating the digital flowmeter in the digital synthesis mode comprises basing the synthesis of the third drive signal on an analysis of the sensor signal.

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Description

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TECHNICAL FIELD

The invention relates to flowmeters.

BACKGROUND

Flowmeters provide information about materials being transferred through a conduit. For example, mass flowmeters provide a direct measurement of the mass of material being transferred through a conduit. Similarly, density flowmeters, or densitometers, provide a measurement of the density of material flowing through a conduit. Mass flowmeters also may provide a measurement of the density of the material.

Coriolis-type mass flowmeters are based on the Coriolis effect, in which material flowing through a rotating conduit becomes a radially travelling mass that is affected by a Coriolis force and therefore experiences an acceleration. Many Coriolis-type mass flowmeters induce a Coriolis force by sinusoidally oscillating a conduit about a pivot axis orthogonal to the length of the conduit. In such mass flowmeters, the Coriolis reaction force experienced by the traveling fluid mass is transferred to the conduit itself and is manifested as a deflection or offset of the conduit in the direction of the Coriolis force vector in the plane of rotation.

Energy is supplied to the conduit by a driving mechanism that applies a periodic force to oscillate the conduit. One type of driving mechanism is an electromechanical driver that imparts a force proportional to an applied current. In an oscillating flowmeter, the applied current is periodic, and is generally sinusoidal. The period of the input current may be chosen so that the motion of the conduit matches a resonant mode of vibration of the conduit, which generally reduces the energy needed to sustain oscillation. An oscillating flowmeter may use a feedback loop in which a sensor signal that carries instantaneous frequency and phase information related to oscillation of the conduit is amplified and fed back to the conduit using the electromechanical driver. Other types of driving mechanisms, such as an electromechanical driver that imparts a force proportional to an applied voltage, also may be used.

Many conventional flowmeters are essentially analog devices in which a sensor signal frequency and phase information are amplified, for example by an analog op-amp, before being fed back into the electromechanical driver. In such flowmeters, there may be little or no phase delay between the signal(s) being sensed at the conduit and the driving signal being applied to the conduit at the other end of the feedback loop. Such analog flowmeters may be prone to the introduction of high harmonics of a desired oscillation frequency, particularly during start-up operations when an estimated drive signal is applied to the conduit to begin the feedback loop described above. Moreover, analog flowmeters may be prone to gain saturation of the op amp, which may occur during "two-phase flow" through the conduit (e.g., an air pocket or entrained air in a flow of liquid) and which can lead to a damping effect on the conduit, or a stalling of the entire oscillation process. Finally, analog flowmeters may be prone to typical shortcomings of analog circuitry, e.g., relatively low precision and high noise measurements.

In contrast to analog flowmeters, digital flowmeters also exist. For example, U.S. Pat. No. 6,311,136 and U.S. Pat. No. 6,507,791, which are hereby incorporated by reference, disclose the use of a digital flowmeter and related technology. Such digital flowmeters may have various advantages over analog flowmeters; for example, they may be more precise in their measurements, with less noise, and may be capable of enabling a wide range of positive and negative gains at the driver circuitry. Such digital flowmeters are thus advantageous in a variety of settings. For example, U.S. Pat. No. 6,505,519 discloses the use of a wide gain range, and/or the use of negative gain, to prevent stalling and to more accurately exercise control of the flowtube, even during difficult conditions such as two-phase flow.

SUMMARY

According to one general aspect, a digital flowmeter is operating in a random sequence mode, where the random sequence mode is characterized by a first drive signal that includes randomly-generated values and that causes a vibration of a flowtube. The digital flowmeter also is operated in a positive feedback mode, where the positive feedback mode is characterized by a sensor signal that corresponds to the vibration and that is used as a second drive signal for maintaining the vibration. The digital flowmeter also is operated in a digital synthesis mode, where the digital synthesis mode is characterized by a synthesis of a third drive signal for maintaining the vibration.

Implementations may include one or more of the following features. For example, the digital flowmeter may be transitioned between two or more of the random sequence mode, the positive feedback mode, and the digital synthesis mode.

In operating the digital flowmeter in the random sequence mode, the randomly-generated values may be filtered to remove frequency components above a pre-determined cut-off level, where the pre-determined cut-off level may be selected based on a range of potential resonance frequencies associated with the flowtube. Alternatively in operating the digital flowmeter in the random sequence mode, the randomly-generated values may be filtered to remove frequency components above a first pre-determined cut-off level and below a second pre-determined cut-off level.

The digital flowmeter also may be operated in a zero-output mode, the zero-output mode characterized by having a zero-value drive signal. In this case, operating the digital flowmeter in the random sequence mode may include detecting a pre-determined condition, and initiating operation of the zero-output mode, based on the predetermined condition.

In operating the digital flowmeter in the zero-output mode, the fact that a pre-determined amount of time has passed may be detected, and operation of the digital flowmeter may be transitioned into the positive feedback mode. Further, detecting the pre-determined condition may include detecting an end of a pre-determined time period.

In operating the digital flowmeter in the positive feedback mode, a first pre-determined condition may be detected, and operation of the digital synthesis mode may be initiated, based on the first pre-determined condition. In this case, in detecting the first pre-determined condition, a signal waveform may be detected within the sensor signal, it may be determined that the signal waveform has a pre-determined characteristic, such as sinewave characteristics, and operation of the digital flowmeter may be transitioned from the positive feedback mode into the digital synthesis mode.

Further, an instability associated with the operation of the digital flowmeter may be detected in the digital synthesis mode, and operation of the digital flowmeter may be transitioned from the digital synthesis mode into the positive feedback mode, in response to detecting the instability. Alternatively, an instability associated with the operation of the digital flowmeter in the digital synthesis mode may be detected, and operation of the digital flowmeter may be transitioned from the digital synthesis mode into the random sequence mode, in response to detecting the instability.

In operating the digital flowmeter in the positive feedback mode, a second predetermined condition may be detected, and operation of the flowmeter may be transitioned from the positive feedback mode into the random sequence mode, based on the second pre-determined condition. In this case, detecting the second predetermined condition may include detecting an instability in an operation of the flowmeter.

In operating the digital flowmeter in the positive feedback mode, an initial sensor signal may filtered to obtain the sensor signal. The sensor signal may be a digital signal, and operating the digital flowmeter in the positive feedback mode may include converting an analog sensor signal into the sensor signal.

In operating the digital flowmeter in the positive feedback mode, the sensor signal may be buffered to obtain a buffered sensor signal, and values may be selected from the buffered sensor signal to use as the second drive signal so as to compensate for a delay associated with the second drive signal, where the delay is associated with a digital element associated with the flowmeter.

In operating the digital flowmeter in the digital synthesis mode, an initial sensor signal may be filtered to obtain the sensor signal. Also, the synthesis of the third drive signal may be based on an analysis of the sensor signal.

According to another general aspect, a digital flowmeter includes a vibratable flowtube, a sensor connected to the flowtube and operable to sense information about a motion of the flowtube, a driver connected to the flowtube and operable to impart energy to the flowtube, a filter system, a random-value generator operable to generate random-frequency signals, and a control and measurement system operable to apply the filter system to the random-frequency signals and supply filtered, random-frequency signals to the driver for application to the flowtube of a first drive signal during a first mode, and further operable to transition the digital flowmeter into a second mode characterized by a second drive signal.

Implementations may include one or more of the following features. For example, the control and measurement system may be operable to transition the digital flowmeter from the second mode to the first mode, in response to detecting a system disturbance associated with the digital flowmeter.

The second mode may be a zero-output mode in which the second drive signal has a magnitude of substantially zero, and the control and measurement system may transition the digital flowmeter from the first mode to the second mode after a pre-determined amount of time. In this case, the control and measurement system may be further operable to transition the digital flowmeter from the second mode into a third mode, the third mode being a positive feedback mode characterized by a third drive signal that includes components of a sensor signal detected by the sensor and fed back to the driver.

Further, the control and measurement system may be further operable to transition the digital flowmeter from the third mode back into the first mode, in response to detecting a system disturbance associated with the digital flowmeter. Also, the control and measurement system may be further operable to transition the digital flowmeter from the third mode into a fourth mode, the fourth mode being characterized by a drive signal that is synthesized by the control and measurement system based on an analysis of the sensor signal. In this case, the filter system may be operable to filter an initial sensor signal to obtain the sensor signal, which may be a digital signal.

The control and measurement system may initiate transition from the third mode to the fourth mode in response to a pre-determined event, which may include detection of sinewave components within the sensor signal.

The control and measurement system may be further operable to transition the digital flowmeter from the fourth mode to the third mode, upon detecting a system disturbance associated with the digital flowmeter. The control and measurement system may be further operable to transition the digital flowmeter from the fourth mode to the first mode, upon detecting a system disturbance associated with the digital flowmeter.

The second mode may be a positive feedback mode in which the second drive signal includes components of a sensor signal detected by the sensor and fed back to the driver. Also, the second mode may be a digital synthesis mode in which the second drive signal is synthesized based on an analysis of a sensor signal detected by the sensor. The filter system is operable to filter the random-frequency signals to include a pre-determined range of frequencies within the filtered, random-frequency signals.

According to another general aspect, a flowmeter may include a vibratable flowtube, a sensor connected to the flowtube and operable to sense information about a motion of the flowtube by way of a sensor signal, a driver connected to the flowtube and operable to impart energy to the flowtube by way of a drive signal, and a digital transmitter operable to select and implement, from among a plurality of operational modes, an operational mode determined to be best-suited for a current operational environment of the digital flowmeter.

Implementations may include one or more of the following features. For example, the plurality of operational modes may include a random-sequence mode to generate the drive signal, in which the digital transmitter generates random frequencies, filters the random frequencies to obtain filtered random frequencies and delivers the filtered random frequencies to the driver. In this case, the digital transmitter may be filter the random frequencies to retain frequencies within a pre-determined frequency range. The random-sequence mode may be selected and implemented during a startup operation of the flowmeter.

The plurality of operational modes includes a zero-output mode, in which a magnitude of the drive signal is substantially zero. The plurality of operational modes may include a positive feedback mode, in which a sensor signal is processed and supplied to the driver. The plurality of operational modes may include a digital synthesis mode, in which the drive signal is digitally synthesized, based on an analysis of the sensor signal.

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is an illustration of a digital flowmeter using a bent flowtube.

FIG. 1B is an illustration of a digital flowmeter using a straight flowtube.

FIG. 2 is a block diagram of an operation of a digital flowmeter.

FIG. 3 is a block diagram of the digital transmitter of FIG. 2.

FIG. 4 is a block diagram of a digital flowmeter.

FIG. 5 is a flowchart illustrating a positive feedback mode of operation of the system of FIG. 4.

FIG. 6 is a timing diagram illustrating a buffering process implemented during the positive feedback operation of FIG. 5.

FIG. 7 is a flowchart illustrating a digital synthesis mode of operation of the system of FIG. 4.

FIG. 8 is a first timing diagram illustrating a timing of a synthesis of a sine wave to be used as part of a drive signal.

FIG. 9 is a second timing diagram illustrating a timing of a synthesis of a sine wave to be used as part of a drive signal.

FIGS. 10A and 10B are flowcharts illustrating operations of a digital flowmeter.

FIG. 11 is a flowchart illustrating further operations of a digital flowmeter.

FIG. 12 is an illustration of a zero-crossing of a signal output by a filter used in an operation of a digital flowmeter.

FIG. 13 is an illustration of a sine wave illustrating synthesis parameters for synthesizing a drive signal.

FIGS. 14A, 14B, and 14C are graphs showing filter characteristics of a filter used in an operation of a digital flowmeter.

FIG. 15 is a block diagram of a closed-loop system for compensating digital delay in a flowmeter.

FIG. 16 is a flow diagram illustrating start-up and operational techniques for a digital flowmeter.

FIG. 17 is a flowchart illustrating the random sequence mode and the zero-output mode of FIG. 16 in more detail.

FIG. 18 is a flowchart illustrating the positive feedback mode 1608 of FIG. 16 in more detail.

FIGS. 19A 19F describe a start sequence for one implementation of a digital flowmeter as applied to a bent flowtube.

FIGS. 20A 20F illustrate the start sequence of FIGS. 19A 19F in more detail.

DETAILED DESCRIPTION

FIG. 1A is an illustration of a digital flowmeter using a bent flowtube 102. Specifically, the bent flowtube 102 may be used to measure one or more physical characteristics of, for example, a (traveling) fluid, as referred to above. A detailed description of a structure and operation(s) of the bent flowtube 102 is provided in, for example, commonly-assigned U.S. Pat. No. 6,311,136. Flowtubes which are similar in concept to the bent flowtube 102 are also discussed in, for example, U.S. Pat. No. 6,327,914 B1, which is incorporated by reference in its entirety.

In FIG. 1A, a digital transmitter 104 exchanges sensor and drive signals with the bent flowtube 102, so as to both sense an oscillation of the bent flowtube 102, and to drive the oscillation of the bent flowtube 102 accordingly. By quickly and accurately determining the sensor and drive signals, the digital transmitter 104, as referred to above, provides for fast and accurate operation of the bent flowtube 102.

More specifically, the digital transmitter 104 serves to select signal characteristics such as a frequency, phase, and amplitude of the drive signals, in order to obtain a desired oscillation of the bent flowtube 102, e.g., an oscillation which aligns these signal characteristics of the sensor and drive signals with one another, at a natural resonant frequency of the bent flowtube 102. For example, the digital transmitter 104 may generate the drive signals by applying the sensor signals (perhaps with suitable adjustments) as the drive signals, as part of a feedback loop. As another example, the digital transmitter 104 may generate the drive signals by synthesizing a new signal having the desired characteristics.

FIG. 1B is an illustration of a digital flowmeter using a straight flowtube 106. More specifically, in FIG. 1B, the straight flowtube 106 interacts with the digital transmitter 104 to produce, for example, density and/or mass flow measurements. Such a straight flowtube operates similarly to the bent flowtube 102 on a conceptual level, and has various advantages/disadvantages relative to the bent flowtube 102. For example, the straight flowtube 106 may be easier to (completely) fill and empty than the bent flowtube 102, simply due to the geometry of its construction. In operation, the bent flowtube 102 may operate at a frequency of, for example, 50 110 Hz, while the straight flowtube 106 may operate at a frequency of, for example, 300 1,000 Hz.

Various relevant details of the structure and operation(s) of the bent flowtube 102 and/or the straight flowtube 104 are discussed below. However, such description is not intended to be an exhaustive description of such flowtubes, of which many conventional examples exist. Rather, the flowtube description(s) provided herein are intended to provide context for the below description of a structure and operation of the digital transmitter 104.

More specifically, the digital transmitter 104, as described in more detail below, is capable of initiating and maintaining a flowtube oscillation, such that the drive signals output by the digital transmitter 104 are compensated for phase delays caused by analog and/or digital elements within or associated with the digital transmitter 104. In this way, the drive signals have a desired phase relationship with the oscillation of the particular flowtube being used (i.e., with the sensor signals detected at the flowtube). As shown in the examples below, such phase compensation may be implemented in a variety of ways, for a number of uses in different settings. Specifically, for example, digital drive techniques described herein may be applied to a variety of flowtubes and flowtube geometries.

Referring to FIG. 2, a digital mass flowmeter 200 includes the digital transmitter 104, one or more motion sensors 210, one or more drivers 215, a flowtube 220 (which also may be referred to as a conduit, and which may represent either the bent flowtube 102, the straight flowtube 106, or some other type of flowtube), and a temperature sensor 220. The digital transmitter 104 may be implemented using one or more of, for example, a processor (including a Digital Signal Processor (DSP)), a field-programmable gate array (FPGA), an ASIC, other programmable logic or gate arrays, or programmable logic with a processor core. Examples of implementations of the digital transmitter 104 are discussed in more detail below.

The digital transmitter 104 generates a measurement of mass flow through the flowtube 215, based at least on signals received from the motion sensors 205. The digital transmitter 104 also controls the drivers 210 to induce and sustain motion in the flowtube 215. This motion is sensed by the motion sensors 205.

Mass flow through the flowtube 215 is related to the motion induced in the flowtube in response to a driving force supplied by the drivers 210. In particular, mass flow is related to the phase and frequency of the motion, as well as to the temperature of the flowtube (as this temperature reflects the temperature of the flowing material, which also may be directly measured). The digital mass flowmeter 200 also may provide a measurement of the density of material flowing through the flowtube. The density is related to the frequency of the motion and the temperature of the flowtube. Many of the described techniques are applicable to a densitometer that provides a measure of density rather than a measure of mass flow.

The temperature in the flowtube 215, which is measured using the temperature sensor 220, affects certain properties of the flowtube, such as its stiffness and dimensions. The digital transmitter 104 compensates for these temperature effects. The temperature of the digital transmitter 104 affects, for example, an operating frequency of the digital transmitter 104, a sampling rate of an analog-to-digital converter, and/or a crystal frequency associated with a reference clock used by the transmitter 104. In general, the effects of transmitter temperature are sufficiently small to be considered negligible. However, in some instances, the digital transmitter may measure the transmitter temperature using a solid state device, and may compensate for effects of the transmitter temperature. Although not shown in FIG. 2, similar comments and considerations may be applied with respect to a pressure sensor that is operable to sense a pressure of a fluid flowing through the flowtube 215.

As referred to above, control of the drive signal sent to driver 210 for inducing vibration of the flowtube 215 is a factor in starting and operating a digital flowmeter. More specifically, it is desirable to maintain precise control over the frequency, phase and amplitude of the drive signal.

With respect to frequency, it should first be understood that flowtube vibration is generally determined by physical characteristics of the flowtube 215 itself. In particular, the flowtube 215 will generally have a high Q factor, thereby ensuring that only a very narrow band of frequencies around the natural frequency of the flowtube generate a low-damped response in the desired mode of vibration and thereby permit the type of system operation desired. Therefore, it is desirable to match the natural frequency of the flowtube as closely as possible in the drive waveform.

Failure to perform this frequency matching will thus generally result in energy being wastefully imparted to the system at other frequencies. Moreover, frequencies which are close (but not equal to) the resonant frequency of the flowtube 215 may result in an operation of the flowtube 215 at those frequencies. In such situations, errors may result in calculations of, for example, a massflow and/or density of the fluid traversing the flowtube 215.

One way that improper frequencies are generated during flowmeter start-up is by generating harmonics of the natural frequency within the drive signal. Generally speaking, flowtubes have several modes of vibration. Usually only two vibration modes are of interest: the "driven mode" which is the main mode of oscillation, and the "coriolis" mode at which the coriolis forces are manifest, leading to phase offset between the sensors observed in the driven mode. In "one-drive" flowtubes, the driven mode is usually the lowest mode of vibration of the flowtube, and the coriolis mode is the next highest mode of vibration. In a "two-driver" design for the bent flowtube 102, the driven mode may be, for example, the second mode of vibration (e.g., 70 90 Hz), and the coriolis mode may be at the fundamental, lower frequency (e.g., 50 60 Hz). In any case, any other harmonic frequencies generated beyond those desired for a particular mode of vibration may result in inefficient and/or inaccurate operation of the system.

Another way that improper frequencies may be generated is through the presence of a phase offset between the sensor signal and the drive signal. That is, as this phase offset increases, the vibration changes from being at the desired natural frequency to a forced oscillation. As this phase offset is typically a function of frequency (e.g., a constant time delay of 5 ms may correspond to a phase offset of 180 degrees at 100 Hz, but may correspond to 360 degrees (i.e., 0 degrees) at 200 Hz), a phase offset close to 180 degrees for a particular frequency will typically result in stalling of the oscillation at that frequency. Conversely, a phase offset close to a multiple of 360 degrees for a given frequency will often result in oscillation of the flowtube 215 at that frequency (even if it is not the desired natural frequency).

Also with respect to phase, it should be understood that, as is known, the drive waveform should be at 90 degrees out of phase with the motion of the flowtube in order to affect vibration at the natural resonant frequency. However, if a flowtube design uses velocity sensors, which automatically provide a signal at 90 degrees to the motion of the flowtube, then the optimal drive waveform is one which is in phase with the sensor voltage signals. Thus, it should be understood that the phrases such as "in phase," "having the proper phase," "having the proper phase offset" and similar phrases refer to the condition in which the drive signal has a relationship with the sensor signal such that it affects proper vibration of the flowtube.

Once vibration at the proper frequency has been initiated and maintained, the amplitude of the drive signal must be set so that a proper amount of energy (and thus, amplitude of vibration at the chosen frequency) is actually imparted to the flowtube. Ideally, the amplitude should be frequently updated so as to ensure a constant amplitude of the oscillation of the flowtube.

As referred to above, implementations may use different techniques for obtaining drive signals which have the desired frequency, phase, and amplitude characteristics. One such technique is referred to herein as "positive feedback," in which a sensor signal (having the desired frequency and phase characteristics) is multiplied by a drive gain factor (either by analog or digital control loop techniques) and fed back to the flowtube via the drive signal. A non-linear amplitude control algorithm may be used to provide stable oscillation and selection of a sustainable set-point for an amplitude of oscillation, even during situations such as a highly-damped operation of the flowtube 215 (e.g. two-phase flow), or beginning/ending operation of the flowtube 215 in an empty state.

While this approach, by itself, may be adequate in some conventional flowmeter applications, it may not be effective when dealing with difficult situations such as batching to/from empty and two-phase flow. In these cases, flowtube stalling, where oscillation ceases and measurements cannot be calculated, may occur.

A second technique for generating desired drive signals is digital synthesis of the drive signals. That is, rather than simply feeding back the sensor signals as the drive signals, the digital transmitter 104 may actually synthesize pure sine waves having the desired drive signal characteristics. As in the case of positive feedback above, a non-linear amplitude control algorithm may be used to provide stable oscillation and amplitude set-point selection.

Moreover, the use of digital synthesis of drive signals results in a high precision in producing the