A Coriolis force type flow meter uses an optical interferometer
as the measuring device. When a tube that a fluid flows through
experiences a bending vibration caused by an external stimulating
source, the tube also has a twist vibration due to the action of
the Coriolis force. The optical interferometer is then employed
to measure the tiny angular change in the amplitude of the tube
vibration. From such a measurement, one can determine the flux of
the fluid in the tube.
What is claimed is:
1. A Coriolis force type flow meter utilizing a Fabry-Perot interferometer,
comprising: a substrate, which has a stimulating electrode for providing
an electrostatic force and two small holes symmetrically formed
on opposite sides of the stimulating electrode; a symmetric loop
tube, which is mounted on the substrate, the back end of which having
an opening for a fluid to enter and leave, the front end of which
being installed on the stimulating electrode and having a plurality
of through hole at the position corresponding to each of the small
holes; wherein the electrostatic force provided by the stimulating
electrode drives the loop tube into bending vibrations; a plurality
of reflective mirrors installed inside the small holes of the substrate
and the through holes of the loop tube; a plurality of light sources
installed above the through holes; and a plurality of photo probes
installed under the small holes of the substrate; wherein each of
the photo probes extracts interfered optical signals coming from
the associated light source and passing through the reflective mirrors
in the corresponding small hole and through hole, and the optical
signals are used to compute the flux of the fluid flowing through
the symmetric loop tube.
2. The Coriolis force type flow meter of claim 1 further comprising
a plurality of stimulating electrodes that are symmetrically distributed
about the central line of the loop tube.
3. The Coriolis force type flow meter of claim 1 wherein the loop
tube is made using chemical etching.
4. The Coriolis force type flow meter of claim 1 wherein the loop
tube is rectangular.
5. The Coriolis force type flow meter of claim 1 wherein the reflective
mirror in each of the through holes has a specific reflectivity.
6. The Coriolis force type flow meter of claim 1 wherein the distance
between the reflective mirrors in the associated small holes and
through holes is the resonance cavity length of the Fabry-Perot
7. The Coriolis force type flow meter of claim 1 wherein the reflective
mirror is made using the film coating technology.
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to a Coriolis force type flow meter and,
in particular, to a Coriolis force type flow meter that uses an
optical interferometer as its measuring device.
2. Related Art
In many manufacturing processes or applications that require flow
control, the first thing that one has to do is to precisely measure
and control the flux in order to properly make desired products.
For example, in the biochemical technology the formation of a chemical
compound requires two or more substances mixed in a specific ratio.
Another example is that in motor engines, the gasoline and air have
to be mixed in an appropriate ratio to achieve an optimal efficiency.
Currently, most flow meters utilize the changes in pressure, temperature
or acoustic wave propagation of the fluid flowing through a tube
to determine the flux. According to different measuring methods,
the flow meters can be categorized as the thermal, pressure-difference
and ultrasonic types. The physical quantity obtained from the above-mentioned
flow meters is normally the flow speed (meters per second) or volume
flux (cubic meters per second). Once the density of the fluid is
known, the mass flux of the fluid can be readily calculated.
However, the fluid flow measuring method of the above-mentioned
flow meters is indirect. The precision of the measurement is very
likely affected by changes of the fluid properties, such as its
temperature, pressure, density, viscosity, and homogeneity. Moreover,
the precision may also be affected by the change in the distribution
of the flow field.
To conquer the above drawbacks, Micro Motion, Inc first developed
a flow meter that utilizes the principle of Coriolis forces in 1997.
By directly or indirectly measuring the Coriolis force generated
by the fluid flowing inside a rotational tube, one is able to obtain
the mass flux of the fluid. This type of flow meter can directly
measure the fluid flux inside the tube. The best advantage is: a
high precision measurement can be achieved without being affected
by changes in the fluid properties. Nevertheless, such a flow meter
also has its shortcomings. In order to measure the tiny variation
in the flow field caused by the Coriolis force, the size of the
flow meter has to be large enough. A relatively complicated measuring
device has to be used in order to achieve the high precision requirement.
Therefore, the manufacturing cost of the flow meter increases and
the product is not suitable for measurements in small flux fluid
The method disclosed in the U.S. Pat. No. 6412355 uses basically
the same idea as that of Micro Motion, Inc. However, the measurement
is made through electrical signals from two different points in
a tube. The flux inside the tube is obtained from its relation with
the phase difference and the vibration frequency. As in the previous
case, the size of this type of flow meter is larger and the device
has a rather complicated structure. Therefore, it is not suitable
for low fluid flow measurements either.
In the conference paper "A Coriolis Mass Flow Sensor Structure
in Silicon" presented in 1996 IEEE Meeting, Enoksson et. al.
proposed a new method of measuring the fluid flux by first projecting
a laser beam on a double-loop tube in motion and the computing a
rotation angle from the measurement of the position change of the
reflected light on a photon detection apparatus. However, both positioning
and calibration of the whole optical measuring system are not easy,
the fact of which in turn affects the sensitivities. Therefore,
one has to try to obtain compensations from other aspects, such
as increasing the input voltage of the stimulator.
SUMMARY OF THE INVENTION
In view of the difficulties of using the above-mentioned flow meters
to make measurements, an objective of the invention is to provide
a Coriolis force type flow meter that uses a Fabry-Perot interferometer
to measure the fluid flow inside a tube. Since the sensitivity of
this type of optical interferometers can reach the micrometer order,
it is ideal for measuring the minute flux changes inside a tube.
AS the setup and calibration of the Fabry-Perot interferometer is
not difficult at all, the manufacturing cost of the measuring device
can be lowered. In contrast, such advantages increase the competition
power of the disclosed Coriolis force type flow meter with others.
The Coriolis force type flow meter according to the invention has
a substrate with stimulating electrodes for providing an electrostatic
force and small holes symmetrically distributed on both sides of
the stimulating electrodes. The substrate is installed with a symmetric
rectangular loop tube, whose back end allows fluid to enter and/or
leave. Its front end is installed above the stimulating electrodes.
Driven by the electrostatic force provided by the stimulating electrodes,
the rectangular loop tube starts bending vibrations.
The front end of the rectangular loop tube has through holes that
are also symmetric, corresponding to the above-mentioned small holes.
Several reflective mirrors are installed in the small holes and
the through holes of the loop tube. A light source is provided above
the through holes of the loop tube. A photo probe is installed under
the small holes of the substrate.
The light emitted from the light source passes the reflective mirrors
in the through holes and small holes. The photo probe extracts the
interfered optical signals. After specific calculations, the fluid
flow inside the rectangular loop tube can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will become more fully understood from the detailed
description given hereinbelow illustration only, and thus are not
limitative of the present invention, and wherein:
FIG. 1 shows the symmetric rectangular loop tube of the invention;
FIG. 2 is a cross-sectional view of the static and vibrating front
end of the loop tube; and
FIG. 3 shows a schematic structure of the Coriolis force type flow
meter that utilizes a Fabry-Perot interferometer according to the
DETAILED DESCRIPTION OF THE INVENTION
To measure tiny variations caused by the Coriolis force inside
a flow meter, the invention provides a symmetric rectangular loop
tube 10 shown in FIG. 1. First, a fluid flows through the loop tube
10. A stimulator exerts a force on the loop tube 10 so that the
loop tube 10 undergoes bending vibrations. Due to the action of
the Coriolis force, the loop tube 10 also generates twist vibrations
at the same time.
The cross sections of the static and vibrating front end of the
loop tube 10 are shown in FIG. 2. The dashed lines in the drawing
outline the front-end cross section of the static loop tube 10
whereas the solid lines outline the front-end cross section of the
vibrating loop tube 10. By measuring the displacement of 1.sub.1
and 1.sub.2 one can compute the maximum vibration angle .theta..sub.b
of the bending vibrations and the maximum amplitude angle .theta..sub.t
of the twist vibrations from the following Eqs. (1) and (2): ##EQU1##
The obtained angles .theta..sub.b and .theta..sub.t can be further
used to determine the flux .phi. by putting into the following Eq.
(3) that relates the flux .phi. with the angles .phi..sub.b and
.phi..sub.t : ##EQU2##
where a is the length of the loop tube 10 b is its width, c is
the vibration frequency, and k is the torsional spring stiffness.
The structure of the disclosed Coriolis force type flow meter is
shown in FIG. 3. A symmetric rectangular loop tube 10 is mounted
on a substrate 20. The fluid enters the loop tube 10 via the inlet
11 and leaves via the outlet 12 on the back. The flowing path is
a symmetric rectangle.
The cross section of the loop tube 10 can be of any geometric shape
that is adjusted taking into account the size and manufacturing
process of the flow meter. Taking a micro tube as an example, the
cross section of the loop tube 10 can be any symmetric shape, such
as rectangles and hexagons. The upper and lower symmetric surfaces
of the loop tube 10 are made using the etching technique in the
microelectromechanics (MEMS). Afterwards, the upper and lower surfaces
are combined using the bonding technology.
Furthermore, stimulating electrodes 30 are provided on the substrate
20 under the front end of the loop tube 10. The purpose of these
stimulating electrodes 30 is to provide an electrostatic force as
the stimulating source of the bending vibrations. If an even number
of stimulating electrodes 30 are provided, the positions have to
be distributed symmetrically about the central line of the loop
tube 10. A small hole 21 is formed on the substrate 20 on each side
of the stimulating electrodes. The positions of the small holes
21 are also symmetric about the central line of the loop tube 10.
A through hole is formed on the front end of the loop tube at the
position of each of the small holes 21. A reflective mirror 41 is
provided for each of the small holes 21 and the through holes 13.
A light source 42 is provided on top of each of the through holes
13. A corresponding photo probe 43 is provided under the small hole
21 of the substrate 20. The distance between the reflective mirrors
of the associated small hole 21 and through hole 13 is the resonance
cavity length of the Fabry-Perot interferometer.
Light emitted from each of the light sources 42 passes the reflective
mirrors 41 in the corresponding through hole 13 and the small hole
21. The photo probe 43 receives the optical signals from the interference
due to the two reflective mirrors 41. The received optical signals
can be computed to obtain the displacement values 1.sub.1 and 1.sub.2
shown in FIG. 2. One is then able to compute the fluid flux through
the loop tube 10 using Eq. (3). The precision of the Coriolis force
type flow meter is determined by the optical properties of the Fabry-Perot
interferometer, such as the wavelength of the light from the light
source 42 the reflectivity of the reflective mirrors 41 and the
length of the resonance cavity.
EFFECTS OF THE INVENTION
Using the Coriolis force type flow meter making use of the Fabry-Perot
interferometer according to the invention, the precision of its
measurements is higher than the Coriolis force type flow meter developed
by Micro Motion, Inc. The positioning and calibration of the interferometer
are much easier than the laser measuring system employed in the
flow sensor disclosed in the paper "A Coriolis Mass Flow Sensor
Structure in Silicon" by Enoksson et. al. Due to the increase
in the precision of measurements, the structure of the invention
can be further simplified and the voltage required by the stimulator
can be lowered. The manufacturing cost of the whole system is greatly
While the invention has been described by way of example and in
terms of the preferred embodiment, it is to be understood that the
invention is not limited to the disclosed embodiments. To the contrary,
it is intended to cover various modifications and similar arrangements
as would be apparent to those skilled in the art. Therefore, the
scope of the appended claims should be accorded the broadest interpretation
so as to encompass all such modifications and similar arrangements.