Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2000-03-14
2002-09-17
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S704000, C073S862590
Reexamination Certificate
active
06450032
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to vibrating beam force sensors in general and to the construction of producible accurate vibrating beam force sensors in particular.
2. Description of the Prior Art
Vibrating beam force sensors (also known as resonant sensors) are well known in the art. A vibrating beam comprised of a quartz crystal is supplied with an electrical drive signal which causes the beam to resonate at a frequency dependent upon axial force applied to the beam. If the force is compressive, it decreases the vibrating frequency and, if the force is tensile, it increases the vibration frequency. The vibrating frequency can be sensed and provides an indication of the force applied. The force applied can be created by any structure, but of particular interest is a force derived from a bellows providing an indication of pressure or a force derived from a proof mass providing an indication of acceleration.
In U.S. Pat. No. 4,406,966 a structure is shown which mounts a vibrating quartz beam in the form of a double ended tuning fork (DETF) design so that forces on the supporting structure change the vibrational frequency of the tuning fork. Difficulties of such a system are that, because of the different coefficients of expansion of the different materials (the DETF being quartz and the supporting structure being metal), inaccuracies in measurement arise.
In U.S. Pat. No. 5,596,145 issued to Albert et al on Jan. 21, 1997, a monolithic resonator for a vibrating beam sensor is disclosed (the entire subject matter of the '145 Albert patent is herein incorporated by reference). The benefit of such a monolithic resonator is that the entire structure, including the vibrating beam and associated vibrational isolator mechanism, can all be machined out of a single quartz structure. Benefits and details of such a monolithic structure are provided in the '145 patent and are also disclosed in FIGS.
1
(
a
) through
2
(
b
) in the present application.
In
FIG. 1
(
a
), a monolithic vibrating beam structure is disclosed having a mount structure
10
and a mounting hole
12
for securing the structure. In the prior art pressure sensor embodiment shown in FIGS.
1
(
a
) through
1
(
c
), two orthogonally arranged flexure beams
14
permit the lever arm portion
16
to pivot about pivot point
18
under the influence of bellows
20
. Bellows
20
may be attached to any corresponding structure which transduces changes in fluid pressure into changes in mechanical force and applies that mechanical force to the end of lever arm
16
.
In the event the pressure sensor is mounted as shown, it has been found helpful to provide a balance weight
22
which offsets the weight of the lever arm and the bellows. A vibrating beam
24
is located between the end of the lever arm portion
16
and the end of mount structure
10
. It is connected to these structures respectively by isolator beams
26
. In order to avoid transmitting vibrations to the lever arm and the mounting structure, isolator masses
28
are provided so as to avoid transmission of vibrating quartz beam root reactions through the isolator beams into the solid structure, thereby reducing the efficiency of resonant vibration or “Q.”
End view FIG.
1
(
c
) and cross-sectional view
1
(
b
) illustrate the different thicknesses of quartz material used including the mounting structure having thickness indicated at
30
, the isolator mass having thickness
32
and the vibrating beam with thickness
34
. The single vibrating beam vibrates in the plane of FIG.
1
(
a
), which also happens to be the plane in which flexing about pivot point
18
occurs. As discussed in U.S. Pat. No. 5,596,145, the above monolithic structure can be easily created. The multi-thickness integrated structure becomes practical due to the multiple thicknesses of the various portions of the structure. This allows for a very thin structure
34
for high vibrating beam sensitivity, a thicker structure
32
for desired vibration isolation mass and a substantially thicker structure
30
to provide strength of the overall design and its mount. The application of a suitable electrical drive voltage applied to electrodes
36
(whose pattern on the vibrating beam itself is not shown for clarity of illustration) causes the structure to operate.
FIG.
2
(a) also illustrates the acceleration sensor of U.S. Pat. No. 5,596,145. Similar structures to those identified in FIGS.
1
(
a
) through l(
c
) are indicated with similar terms in FIGS.
2
(
a
) and
2
(
b
). However, unlike the pressure sensor whose flexure beams
14
are orthogonally oriented (so as to provide rotation about pivot point
18
), the acceleration sensor flexure beams
38
are parallel and permit proof mass
40
to move in a direction orthogonal to the parallel flexure beams. With the mount structure rigidly mounted to a base whose acceleration is to be measured, movement of the base and consequently the mount structure
10
in the direction of arrows R will permit force to be applied to the vibrating beam where the force is proportional to the acceleration of the proof mass in an up or down direction (as shown in FIG.
2
(
a
)). The structure of
FIG. 2
would also be mounted in an evacuated and sealed housing (not shown).
As with the pressure sensor shown in FIGS.
1
(
a
) through
1
(
c
), the acceleration sensor shown in FIGS.
2
(
a
) and
2
(
b
) is a monolithic structure, in that the entire device is machined from a single piece of quartz crystal. The disadvantages of the prior art shown in FIGS.
1
(
a
) through
2
(
c
) are producibility and cost as a result of manufacturing limitations. The machining of the outer structure having the mounting structure thickness
30
is relatively economical, because tolerances are loose and “cookie cutter” methods of machining, such as ultrasonic machining, can be employed.
However, the inner structures comprising the isolator mass thickness
32
, and more particularly the vibrating beam thickness
34
, are much more difficult to machine. These features are not only delicate, but their tolerances must be kept relatively close. As a result, the more economical ultrasonic machining methods cannot be used and slower, more expensive methods are required. In addition, because even the thin inner structure features are still too thick, photo-etch processes such as those disclosed in U.S. Pat. No. 4,215,570 issued to EerNisse relating to the disclosed double-ended tuning fork (DETF) type vibrating beam assembly, cannot be used.
Thus, the methods used to easily and conveniently machine the thick outer structure compromises the ability to maintain the high degree of tolerance needed for proper machining of the vibrating beam structure thickness.
SUMMARY OF THE INVENTION
As a result of the above prior art difficulties, it is an object of the present invention to provide a construction for a force sensor in which an outer structure capable of being produced by conventional machining methods is combined with a second structure which is thin enough to be capable of machining by more accurate photo-etch processes.
It is an additional object of the present invention to provide an outer force carrying structure and a separate inner vibrating beam transducer structure in which each separate piece is machined in the most efficient manner.
It is a still further object of the present invention to provide a combination of outer structure and inner vibrating beam structures formed in the most economical manner and yet providing a high degree of accuracy.
The above and other objects are achieved by machining the outer structure in a conventional fashion for its thickness, i.e. ultrasonic machining, photo-etch machining or abrasive jet machining. The vibrating beam inner structure is created from a relatively thin quartz substrate with conventional “photo-etch” methods of machining. The need for intermediate thickness isolator beams is avoided by using a double-ended tuning fork design (DETF). In order to ensure equal force is applied to
Chapman John E.
Pressure Systems, Inc.
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