Measuring and testing – Instrument proving or calibrating – Volume of flow – speed of flow – volume rate of flow – or mass...
Reexamination Certificate
2000-07-21
2002-04-30
Raevis, Robert (Department: 2856)
Measuring and testing
Instrument proving or calibrating
Volume of flow, speed of flow, volume rate of flow, or mass...
Reexamination Certificate
active
06378354
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an apparatus for measuring properties of a material flow through apparatus such as a conduit of a Coriolis mass flowmeter. More particularly, this invention relates to calibrating a driver affixed to the conduit to excite the conduits in only a desired mode of vibration. Still more particularly, this invention relates to determining a drive signal that causes the driver to vibrate the conduit in a desired mode of vibration.
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 in the flowmeter. 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 U.S. Pat. No. Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more conduits of a straight or a curved configuration. Each conduit configuration in a Coriolis mass flowmeter has a set of natural modes of vibration, which may be of a simple bending, torsional or coupled type. Each conduit is driven to oscillate at a resonance in one of these natural modes of vibration. Material flows into the flowmeter from a connected pipeline on the inlet side of the flowmeter, is directed through the conduit or conduits, and exits the flowmeter through the outlet side of the flowmeter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flowmeter, all points along the conduit oscillate, due to an applied driver force, with identical phase or a small initial fixed phase offset which can be corrected. As material begins to flow, Coriolis forces cause each point along the vibrating conduit to have a different phase. The phase on the inlet side of the conduit lags the driver, while the phase on the outlet side of the conduit leads the driver. Pick-offs are placed on the conduit(s) to produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-offs are processed to determine the phase difference between the signals. The phase difference between two pick-offs signals is proportional to the mass flow rate of material through the conduit(s).
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 conduit which causes the conduit to oscillate. The drive system includes a driver mechanism mounted to the conduit(s) and a drive circuit for generating a drive signal to operate the drive mechanism. 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.
The drive circuit continuously applies a periodic drive signal to the drive mechanism. The drive signal is typically sinusoidally or square shaped. In a typical magnetic-coil drive mechanism, the periodic drive signal causes the coil to produce an alternating magnetic field. The alternating magnetic field of the coil and the constant magnetic field produced by the magnet force causes the flow conduit(s) to vibrate in a sinusoidal pattern. 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. 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.).
For a dual tube flowmeter, a typical mode, although not the only mode, in which Coriolis flowmeters are typically driven to vibrate is a first out-of-phase bending mode. The first out-of-phase bending mode is the fundamental resonant bending mode at which the two conduits of a dual tube Coriolis flowmeter vibrate in phase opposition. However, this is not the only mode of vibration present in the vibrating structure of the Coriolis. Higher modes of vibration may be also be excited in the conduits. For example, a first out-of-phase twist mode may be excited as a result of material flowing through the vibrating conduit and the consequent Coriolis forces caused by the flowing material. Other higher modes of vibration that may be excited include in-phase bending and lateral modes of vibration.
Hundreds of vibration modes may be excited in a Coriolis flowmeter that is driven in the first out-of-phase bending mode. Even within a relatively narrow range of frequencies near the first out-of-phase bending mode, there are at least several additional modes of vibration that may be excited by the drive system. In addition to multiple modes being excited by the driver, undesired modes of vibration can also be excited due to vibrations external to the flowmeter. For example, nearby machinery in a process line might generate a vibration that excites an unwanted mode of vibration in a Coriolis flowmeter.
The drive system can excite additional and undesirable modes of vibrations because the driver mechanism is not ideal. The drive system is comprised of the drive circuitry generating a command signal and a driver mechanism that converts the received command signal into a force. An ideal driver mechanism would be linear, and the mechanism would generate a force that is linearly related to the command signal. However, the relationship between the command signal applied to the driver and the force generated by the driver is non-linear due to various reasons. Manufacturing tolerances require that driver elements be located symmetrically on the conduits. Any resulting non-linearity causes a distortion in the drive force, which can appear as forces applied to the structure at harmonics of the original drive signal. The drive system is configured to apply a drive signal to the driver that applies a sufficient force to the conduit(s) to vibrate them in the desired mode of vibration. However, when the driver is not ideal, the force applied to the conduit(s) is not ideal, and forces at higher frequencies are generated. These higher frequency forces may excite other unwanted structural modes.
The application of eccentric forces excites multiple modes of vibration in the conduits. Thus, a Coriolis flowmeter driven to oscillate or resonate in a desired mode of vibration, such as the first out-of-phase bending mode, may actually have a conduit(s) oscillating in many other modes in addition to the desired mode. Meters driven to oscillate in a different mode other than the first out-of-phase bending mode experience the same phenomenon of having multiple excited modes of vibration in addition to the intended drive mode.
The application of eccentric forces by a conduit on a driver can be a particular problem if an apparatus, such as a Coriolis flowmeter, is unbalanced. An apparatus is balanced when vibrations within the apparatus cancel one another out to create a zero sum vibration of the apparatus. Apparatus is unbalanced when vibration are not canceled out. This causes a force to be added to a system. A typical dual conduit apparatus, such as a dual tube Coriolis flowmeter, is balanced because the two conduits vibrate in phase opposition to one another and cancel opposing vibration. However, unbalanced apparatus does not have a conduits vibrating in opposite directions to cancel the vibrational forces from the conduit.
Unbalance can cause significant coupling between the ambient environment and the conduit(s). This coupling increases the impact of structural dynamics of the ambient environment, and may cause an undesired mode of vibration to be excited by a harmonic of the force applied by the driver to the conduit. Therefore, it is desirable in an unbalance apparatus to have a driver applying a force that only excit
Faegre & Benson LLP
Micro Motion Inc.
Raevis Robert
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