Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
1997-07-11
2001-03-06
Wachsman, Hal (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S045000, C702S194000, C073S861356, C073S861355, C073S861354
Reexamination Certificate
active
06199022
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatus and methods of generating a drive signal for a Coriolis mass flowmeter driver. More particularly the present invention relates to generating a drive signal which excites only the desired vibration mode in the vibrating flow tube of the Coriolis flowmeter. More particularly the present invention relates to using modal filters to suppress the undesirable drive signal components and enhance the desirable drive signal components.
STATEMENT OF THE PROBLEM
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information for materials flowing through a conduit. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. Nos. 4,109,524 of Aug. 29, 1978, 4,491,025 of Jan. 1, 1985, and Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more flow tubes of straight or curved configuration. Each flow tube configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional or coupled type. Each flow tube is driven to oscillate at resonance in one of these natural modes. Material flows into the flowmeter from a connected conduit on the inlet side of the flowmeter, is directed through the flow tube or tubes, and exits the flowmeter through the outlet side. The natural vibration modes of the vibrating, material filled system 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 flowmeter, all points along the flow tube oscillate due to an applied driver force with identical phase or small initial fixed phase offset which can be corrected. As material begins to flow, Coriolis forces cause each point along the flow tube to have a different phase. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Pick-off sensors are placed on the flow tube to produce sinusoidal signals representative of the motion of the flow tube. Signals output from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between two pick-off sensor signals is proportional to the mass flow rate of material through the flow tube.
An essential component of every Coriolis flowmeter, and of every vibrating tube densitometer, is the drive or excitation system. The drive system operates to apply a periodic physical force to the flow tube which causes the flow tube to oscillate. The drive system includes a driver mounted to the flow tube(s) of the flowmeter. The driver mechanism typically contains one of many well known arrangements, such as a magnet mounted to one conduit and a wire coil mounted to the other conduit in an opposing relationship to the magnet. A drive circuit continuously applies a periodic, typically sinusoidally or square shaped, drive voltage to the driver. Through interaction of the continuous alternating magnetic field produced by the coil in response to the periodic drive signal and the constant magnetic field produced by the magnet, both flow conduits are initially forced to vibrate in an opposing sinusoidal pattern which is thereafter maintained. Those skilled in the art recognize that any device capable of converting an electrical signal to mechanical force is suitable for application as a driver. (See U.S. Pat. No. 4,777,833 issued to Carpenter and assigned on its face to Micro Motion, Inc.) Also, one need not use a sinusoidal signal but rather any periodic signal may be appropriate as the driver signal (see U.S. Pat. No. 5,009,109 issued to Kalotay et. al. and assigned on its face to Micro Motion, Inc.).
A typical mode, although not the only mode, in which Coriolis flowmeters are driven to vibrate is the first out-of-phase bending mode. The first out-of-phase bending mode is the fundamental bending mode at which the two tubes of a dual tube Coriolis flowmeter vibrate in opposition to one another. However, this is not the only mode of vibration present in the vibrating structure of a Coriolis flowmeter driven in the first out-of-phase bending mode. There are, of course, higher modes of vibration which may be excited. There is also, as a result of fluid flowing through the vibrating flow tube and the consequent Coriolis forces, a first out-of-phase twist mode that is excited as well as other modes. There are also in-phase and lateral modes of vibration. Ultimately, there are hundreds of vibration modes actually excited in a Coriolis flowmeter that is driven to oscillate in the first out-of-phase bending mode. Even within relatively narrow range of frequencies near 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 driven excitation of the flow tubes, modes can be excited due to vibrations external to the flowmeter. For example, a pump located elsewhere in a process line might generate a vibration along a pipeline that excites a mode of vibration in a Coriolis flowmeter. Another reason that additional and undesirable modes are sometimes excited in a Coriolis flowmeter is when manufacturing tolerances are such that the driver elements are not located symmetrically on the flow tubes. This results in the driver putting eccentric forces into the flow tubes hence exciting multiple modes of vibration. Thus a Coriolis flowmeter driven to oscillate or resonate at the first out-of-phase bending mode actually has a conduit(s) oscillating in many other modes in addition to the first out-of-phase bending mode. Meters driven to oscillate in a different mode than the first out-of-phase bending mode experience the same phenomenon of multiple excited modes in addition to the intended drive mode.
Existing drive systems process a feedback signal, typically one of the pick-off sensor signals, to produce the drive signal. Unfortunately, the drive feedback signal contains responses from other modes in addition to the desired mode of excitation. Thus, the drive feedback signal is filtered through a frequency domain filter to remove unwanted components and the filtered signal is then amplified and applied to the driver. However, the frequency domain filter used to filter the drive feedback signal is not effective at isolating the single desired drive mode from other mode responses present in the drive feedback signal. There can be off-resonance responses from other modes which are near the desired mode resonance frequency. There might also be resonant responses at frequencies approaching the desired resonance frequency. In any event, the filtered drive feedback signal, i.e., the drive signal, typically contains modal content at frequencies other than just the desired mode for excitation of the flow tube. A drive signal composed of resonant response from multiple modes inputs, through the driver, energy to the flow tube that excites each mode for which the drive signal contains modal content. Such a multi-mode drive signal causes operational problems in Coriolis flowmeters. Further, frequency domain filters introduce phase lag in the filtered drive signal. This can result in a requirement for higher drive power to drive the flow tube at the desired amplitude.
One problem caused by a multi-mode drive signal is that external vibrations such as pipeline vibrations are reinforced by the drive signal. If pipeline vibrations external to the Coriolis flowmeter cause the flowmeter to vibrate, the drive feedback signal contains the response to the pipeline vibration. The frequency domain filter fails to remove the undesired response if the pipeline vibration falls at least in part within the frequency pass band of the filter. The filtered drive feedback signal, including the undesired response to the pipeline vibration, is amplified and applied to the driver. The driver then operates to reinforce the excitation mode of the pipeline vibration.
Another exemplary problem caused by a multi-mode drive signal occurs when the total amount of drive power available for
Duft, Graziano & Forest, P.C.
Micro Motion Inc.
Wachsman Hal
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