Monolithic miniature accelerometer

Details Monolithic miniature accelerometer Monolithic miniature accelerometer

2001-04-10

2002-05-14

Kwok, Helen (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Acceleration determination utilizing inertial element

C073S514010

Reexamination Certificate

active

06386035

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a miniature accelerometer that can be used in aircraft, helicopter or automobile navigation, for example, or for improved power assistance of braking or active suspension of terrestrial vehicles.
The invention relates more particularly to a monolithic accelerometer including a fixed part, two mobile mass parts referred to as test weights, and two resonators, each of which has one end fastened to one of the two mobile mass parts.
2. Description of the Prior Art
The resonators constituting the responsive members of the accelerometer according to the invention are preferably flexional or torsional vibratory blades of piezoelectric material. The vibration frequencies of each of the blades are highly sensitive to the tensile or compression force that is exerted longitudinally on the blade when the inertial mass fastened to it subjected to acceleration. The extension of the blades and the compression of the other blade are converted into electrical signals that are picked up by electrodes supported by the vibratory blades and connected to two oscillator circuits, for example. A signal at a differential frequency whose variations are representative of those of the acceleration is produced at the output of the oscillator circuits. The benefit of using the difference between the two frequencies is that this reduces the effect on the two blades of spurious common mode inputs, for example temperature.
Another important aspect is the monolithic nature which enables miniature accelerometers to be fabricated at relatively low cost by chemical machining and promotes good performance, since the process of assembling together component parts generally constitutes a major limitation of non-monolithic accelerometers. The materials most frequently used to make monolithic accelerometers are quartz and silicon, which are appreciated for the excellent stability of their mechanical characteristics.
FIG. 1
shows an accelerometer of the above type disclosed in U.S. Pat. No. 4,945,765. The body of this accelerometer
14
is monolithic and is obtained by chemically machining a silicon plate. The body includes a fixed part
18
, two inertial masses
20
and
22
, two resonators
28
and
30
and two hinges
24
and
26
. The resonators
28
and
30
vibrate in torsion and are excited electrostatically by means of a device (not shown) at whose output their resonant frequencies are delivered. The direction of sensitivity of the accelerometer is close to perpendicular to the faces of the plate. Acceleration applied in this direction causes a tension force to one resonator and a compression force to the other resonator, and the output signal of the accelerometer is the difference between the frequencies of the two resonators. The mechanical design of the accelerometer
14
nevertheless has a drawback associated with the vibration of the two resonators
28
and
30
. The alternating mechanical forces generated by the vibrations of the two resonators where they are “built into” the fixed part
18
lead to dissipation of vibratory mechanical energy in the fixed part. This reduces the Q quality factor of the vibration of each of the resonators
28
and
30
. This affects the precision of the measurement of the differential frequency and therefore the value of the acceleration deduced therefrom.
FIG. 2
shows another accelerometer disclosed in our U.S. Pat. No. 5,962,786. The body of the accelerometer AD′ is monolithic and is obtained by chemically machining a quartz plate. This body includes a fixed part
1
′ with an I-shaped face contour, four U-shaped mobile mass parts comprising two inertial masses
2
1
and
2
2
and two resonators
3
1
and
3
2
, four parallelepiped-shaped articulation blades
81
1
,
82
1
,
81
2
,
82
2
and two flexible frames
5
1
and
5
2
. The resonators
3
1
and
3
2
vibrate in flexion and are excited piezoelectrically by means of a device (not shown) at whose output their resonant frequencies are delivered. The direction of sensitivity of this accelerometer is close to perpendicular to the faces of the plate. Acceleration applied in this direction causes a tension force to one resonator and a compression force to the other resonator, the output signal of the accelerometer being the difference between the frequencies of the two resonators. This accelerometer does not have the drawback of dissipation of vibratory mechanical energy in the fixed part because the flexibility of the frames
5
1
and
5
2
provides a mechanical filtering effect between the resonators and the fixed part. Also, the accelerometer eliminates coupling between the two resonators (see U.S. Pat. No. 5,962,786, col. 4, lines 13-15). This accelerometer is therefore very suitable for industrial applications that require excellent precision and moderate cost. On the other hand, it has drawbacks in applications which require very low fabrication costs, in particular the field of automotive engineering. The relative complexity of the structure shown in
FIG. 2
impacts on the yield of fabrication by chemical machining and limits the possibilities of miniaturization, which limits the number of structures that can be made on a quartz wafer of given dimensions. These drawbacks make it impossible to obtain a very low fabrication cost.
OBJECT OF THE INVENTION
The present invention proposes a geometrical shape which prevents leakage of vibratory mechanical energy from the resonators to the fixed part and is more suitable for miniaturization. This reduces the fabrication cost and satisfies industrial requirements for very cheap accelerometers offering high performance.
SUMMARY OF THE INVENTION
According to the invention, this monolithic miniature accelerometer comprising a fixed part, two first mobile mass parts referred to as inertial masses, two hinge blades each having one end fastened to one of the two mobile mass parts, and two resonators each having one end fastened to one of the two mobile mass parts, is characterized in that it comprises a third mobile mass part fastened to the other end of each of the two resonators and of each of the two hinge blades, and a flexible stem situated between the first two mobile mass parts and connecting the third mobile mass part to the fixed part.
Locating the stem between the two inertial masses helps to maximize the total mass of the mobile parts. The flexibility of the stem combined with the total mass of the three mobile parts provides a mechanical filter between the resonators and the fixed part of the accelerometer. The Q quality factors of the resonators is therefore not degraded much and the precision of the measurement is excellent. The simplicity and compactness of the structure achieved by locating the stem between the first two mobile mass parts is also beneficial for miniaturization and achieving a good fabrication yield. On the other hand, the presence of a mobile mass part common to the two resonators rules out eliminating mechanical coupling between them and a different solution must be found to resolve this problem thereby maintaining the precision of the accelerometer.
According to a preferred embodiment, the flexible stem is a beam extending substantially parallel to the resonators and whose height is significantly greater than the dimensions of its cross section.
To maximize the efficiency of the mechanical filter, the longitudinal axis of symmetry of the flexible stem is substantially an axis of symmetry of the body of the accelerometer.
To exploit its performance optimally, the accelerometer is preferably fixed to a base whose larger faces are not parallel, which enables the axis of sensitivity of the accelerometer to be strictly perpendicular to the plane of the support.


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patent: 4939935 (1990-07-01), Amand
patent: 4945765 (1990-08-01), Roszhart
patent: 5005413 (1991-04-01), Novack et al.
patent: 5456110 (1995-10-01), Hulsing, II
patent: 5962786 (1999-10-01), Le Traon et al.
patent: 2685964 (1996-06-01), None
patent: WO 93/20413 (1993-10-

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