Gyroscopic mass flowmeter

Details Gyroscopic mass flowmeter Gyroscopic mass flowmeter

2000-11-28

2003-02-18

Fuller, Benjamin R. (Department: 2855)

Measuring and testing

Volume or rate of flow

Mass flow by imparting angular or transverse momentum to the...

Reexamination Certificate

active

06520028

ABSTRACT:

FIELD OF THE INVENTION
This Invention relates to a flowmeter and, in particular, to a flowmeter that imparts a rotation to a material flow within a vibrating flow tube and measures generated gyroscopic forces to determine information regarding the material flow.
Problem
Mass flowmeters measure the mass flow rate rather than the volumetric flow rate of material. They are desirable because mass measurement is often needed for chemical reactions, recipes, custody transfer, and many other applications. Furthermore, the accuracy of mass flowmeters is not impaired by changing material density, temperature, or viscosity. Coriolis effect mass flowmeters have been on the market for at least twenty years. They are well liked because of their accuracy and their ability to measure density as well as mass. However, the high cost of Coriolis flowmeters has limited their acceptance in the market.
In prior art single straight tube Coriolis mass flowmeters, the flow tube is connected at both ends to a parallel balance bar. The flow tube is vibrated out of phase in a drive plane with respect to the balance bar at a resonant frequency. An electromagnetic driver maintains the desired amplitude of the vibration. The flow tube and balance bar act as counterbalances to each other to create a dynamically balanced structure. Velocity sensors are located at two locations along the flow tube to measure the relative velocities between the flow tube and balance bar. The velocity sensors are usually located equal distances upstream and downstream from the flow tube's midpoint.
The vibrating flow tube imposes rotations on the upstream and downstream halves of the flow tube. The rotations stop and change direction along with the flow tube's vibration direction. The fixed ends of the flow tube are the pivot points for the rotation and the flow tube's longitudinal center is the point of maximum amplitude. The material moving through the rotating segments of the flow tube creates a Coriolis force that deforms the flow tube and produces a phase delay between the signals output by the velocity sensors. The phase delay between the velocity sensor output signals is proportional to the mass flow rate of the material.
The prior art single straight flow tube Coriolis mass flowmeters have a short straight flow tube that is very stiff in bending. The high stiffness results in high frequencies for the higher modes of flow tube vibration. The driven mode of vibration is usually the mode with the lowest frequency, the first bending mode. In this mode, both the flow tube and the balance bar vibrate out of phase with each other in the drive plane. The shape of this vibration mode is the same as the shape of a vibrating guitar string. The maximum vibration amplitude is in the center and the nodes (fixed points) are on the ends. The driver keeps the flow tube and balance bar vibrating and is located at the center of the flow tube and balance bar.
With straight flow tube geometry, the Coriolis force bends the flow tube in the shape of the second bending mode. The second bending mode is shaped like a stretched S and has three nodes. Two nodes are on the flow tube ends and the third node is in the center. When a flow tube vibrates in the second bending mode, the two halves of the flow tube (located on either side of the central driver) appear to be vibrating out of phase with each other. The second bending mode has a resonant frequency that is almost three times that of the first bending mode. It has a high resonant frequency because the flow tube is very stiff and it is very difficult to bend the flow tube in the shape of the second bending mode.
Coriolis forces are applied to the flow tube at the drive frequency (the resonant frequency of the first bending mode). Assuming the flow tube is horizontal and is vibrated in a vertical drive plane in the first bending mode, the Coriolis deformation of the flow tube is also in the drive plane and has the shape of the second bending mode. If material is flowing from left to right and the flow tube center is passing through the zero displacement point while traveling downward, the Coriolis force on the left half of the flow tube is in the upward direction while the Coriolis force on the right half of the flow tube is in the downward direction. When the flow tube is passing through zero displacement while traveling upward, the direction of the Coriolis force is reversed. The Coriolis force is applied to the flow tube in a sinusoidal manner (with respect to time) at the drive frequency. The Coriolis force is at its peak when the flow tube velocity is at its peak and the Coriolis force is zero when the flow tube velocity is zero as it changes direction.
The Coriolis force deflects the flow tube in the second bending mode shape but at the first mode (drive) frequency. The drive frequency is so far below the resonant frequency of the second bending mode that the maximum deflection of the flow tube due to the Coriolis force is very low. The Coriolis deflection is comparable in amplitude to the static deflection that would result from a static application of the Coriolis force. The Coriolis force due to material flow thus has to deform the stiff flow tube in a very stiff mode shape (the second bending) at a frequency (first bending) that is far removed from the second bending resonant frequency. The result is an extremely small Coriolis deflection of the flow tube and a very small phase difference between the signals produced by the two velocity sensors. A typical time delay (phase difference divided by frequency) between the two signals resulting from a maximum flow rate through a typical meter is ten microseconds. If the meter is to have no more than 0.15% error at ten percent of maximum flow, then the time delay measurement accuracy has to be better than 1.5 nanoseconds (1.5×10
−9
seconds). Accurately measuring such small time increments requires extremely sophisticated and expensive electronics.
Solution
The present invention overcomes the problems of prior Coriolis mass flowmeters by using gyroscopic force rather than Coriolis force in the material flow measurement. In accordance with one possible exemplary embodiment of the invention, a single straight tube gyroscopic flowmeter is provided that looks like the Coriolis flowmeter described above except that its flow tube has an internal helical baffle which causes the material to rotate about the longitudinal axis of the flow tube as the material flows through the tube. The rotating material causes the flow tube to act as a gyroscope. The gyroscopic meter is also different from Coriolis flowmeters in that it has its velocity sensors at the longitudinal center of the flow tube rather than upstream and downstream from the center as do Coriolis flowmeters.
In order to understand how the gyroscopic force of the rotating material can be used to measure flow, the nature of gyroscopic behavior and force will first be examined in two examples. The first example illustrates the motion (precession) that results from a torque applied to a gyroscope axle when the axle is unconstrained. The second example allows the calculation of the torque that the gyroscope axle applies to its mounting when the gyroscopic motion (precession) of the first example is prevented by constraints. It is this resultant torque that deforms the flow tube of the present invention and enables measurement of the mass flow rate.
Gyroscopes are devices having mass that rotates about an axis (called the spin axis) giving them angular momentum. Typical gyroscopes have a circular disk-like mass that is mounted on a thin axle. Conservation of the angular momentum of the rotating disc gives gyroscopes their unique properties. In understanding the present invention it is only necessary to understand how gyroscopes behave, not why they behave the way they do. Therefore, the following is limited to a description of gyroscopic behavior as pertains to the gyroscopic flowmeter of the present invention.
Consider a typical toy gyroscope having a flywheel rotating on

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