Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Mechanical measurement system
Reexamination Certificate
1997-09-30
2001-05-08
Wachsman, Hal (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Mechanical measurement system
C702S045000, C702S064000, C073S861354, C073S861356, C073S861357
Reexamination Certificate
active
06230104
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the field of oscillatory vibrational drivers that are used to convert electrical power into mechanical actuation and, particularly, oscillatory drivers of the type that vibrate the flow tubes of a Coriolis flowmeter in obtaining Coriolis-based flow measurements. Still more specifically, the oscillatory driver according to the present invention incorporates circuitry that permits the oscillatory driver to be used as a signal pickoff device which measures vibrational modes of the Coriolis flow tubes.
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. No. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 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 that is driven in the first out-of-phase bending mode. 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. 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, which is typically one of the pick-off sensor signals, in order 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. The frequency domain filter that is 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. 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.
Problems that derive from the drive signal having modal content at multiple frequencies affect the density measurement made by a Coriolis mass flowmeter. The density measurement in a Coriolis flowmeter or vibrating tube densitometer relies on the measurement of the resonant frequency of the vibrating flow tube. A problem arises when the flow tube is driven in response to a drive signal containing modal content at multiple modes. Superposition of the multiple modes in the drive signal can result in a flow tube that is driven off-resonance from the true resonant frequency of the desired drive mode. An error in the density measurement can result.
Modal filtering techniques can be used to isolate and identify the vibrational modes that are of interest to Coriolis mass flow rate and density calculations. Modal filtering requires additional signal pickoffs to be attached to the vibrating tubes of a Coriolis flow meter, e.g., as in copending U.S. patent application Ser. No. 08/890,785 filed Jul. 11, 1997, to applicant Timothy J. Cunningham, which is hereby incorporated by reference to the same extent as though fully disclosed herein. The use of an additional signal pickoff is associated with additional cost.
Yet another problem arises whenever a driver or pickoff device is connected to the vibrating tube of a Coriolis flowmeter or densitometer. The connection of additional apparatus changes the mass of the total vibrating system and, consequently, alters the natural system harmonics to vibration at different frequencies. The precision of measurement that is obtainable from the meter system having increased mass declines because the increased mass causes the meter to become less sensitive to small vibration variances. Calibration to correct for these differences is complicated by the fact that variations in system mass also affect the placement of the driver and pickoffs for maximum performance, meter power consumption, bend modes, and other problems that are discussed above.
There is a need for a drive circuit system for a Coriolis flowmeter that doubles as a signal pickoff to reduce the amount of mass that is connected to the Coriolis flowtubes.
STATEMENT OF THE SOLUTION
The above identified problems, and others, are solved and a technical advance achiev
Cunningham Timothy J.
Shelley Stuart J.
Chrisman Bynum & Johnson, P.C.
Micro Motion Inc.
Wachsman Hal
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