Resonant element

Details Resonant element Resonant element

2000-11-30

2002-08-13

Moller, Richard A. (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Angular rate using gyroscopic or coriolis effect

C073S514180

Reexamination Certificate

active

06430998

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resonant element used as an angular velocity sensor, filter, or the like.
2. Description of the Related Art
FIG. 7A
is a perspective view showing a previous resonant element
16
. The resonant element
16
is a microelement produced utilizing a conventional silicon micromachining technique and the like. More specifically, the resonant element
16
is produced by forming a nitride film
7
on a silicon substrate
1
, then forming a polysilicon film
5
thereover, and forming the films
7
and
5
into a predetermined pattern by dry etching or the like.
The substrate
1
functions as a fixed substrate of which the substrate plane direction is an X-Z two-dimensional plane direction. A weight portion
2
is disposed above the substrate in a state isolated from the substrate
1
. In the resonant element
16
shown in
FIG. 7A
, the weight portion functions as a planar vibrating body
10
. The planar vibrating body
10
is supported via support beams
3
so as to be vibratable in the X-direction. One end side of each of the support beams
3
is fixed to the substrate I via a fixing portion
35
.
Comb electrodes
6
B are formed on both sides of the planar vibrating body
10
outwardly in the transverse direction (X-direction), and comb electrodes
6
A are each disposed inwardly in the transverse direction at positions opposed to and interdigitated with the comb electrodes
6
B. Conductive layers for driving
11
A and
11
B are connected to the comb electrodes
6
A and
6
B, respectively, and are connected with outside electrode pads (not shown) via conductor patterns (not shown), and thus form an exciter
4
.
Once an AC voltage is applied to these conductive layers for driving
11
A and
11
B of the exciter
4
, an electrostatic force is generated between the comb electrodes
6
A and
6
B, and the planar vibrating body
10
is vibrated in the arrow F direction (X-direction) by this electrostatic force.
When the resonant element
16
is rotated around the Z-axis while the planar vibrating body
10
is vibrated in the X-direction by driving the comb electrodes
6
A and
6
B, a Coriolis force occurs in the Y direction orthogonal to the above-described X-Z two-dimensional plane direction. The Coriolis force is applied to the planar vibrating body
10
constituted of the weight portion
2
, and the planar vibrating body
10
vibrates in the direction of the Coriolis force. By measuring an electric signal corresponding to the magnitude of the vibration amplitude of the planar vibrating body
10
due to the Coriolis force, for example, the magnitude of the rotational angular velocity can be detected.
In the case where the resonant element
16
is used as an angular velocity sensor, there is provided a detecting portion for measuring the electric signal corresponding to the magnitude of the vibration amplitude of the planar vibrating body
10
due to the Coriolis force.
When the resonant element
16
is produced, the resonance frequency of the planar vibrating body
10
in the direction of the Coriolis force (Y-direction) is previously set at the design stage to the resonance frequency in the X-direction, and the shape, dimension, weight, etc. of the planar vibrating body
10
are designed and produced so that the above-mentioned resonance frequency is obtained. In many cases, however, the shape, dimension, weight, etc. of the planar vibrating body
10
are not achieved as designed, because of the machining accuracy of silicon micromachining technique. Accordingly, deviation of the resonance frequency of the planar vibrating body
10
from the designed frequency often occurs. If the vibration of the planar vibrating body
10
is in a resonant state, the amplitude is greatly amplified by virtue of the value of the Q (quality factor) related to the structure, but if the frequency deviates, a problem arises in that the amplitude is not nearly amplified as much, resulting in the sensitivity of the resonant element begin significantly reduced. It is, therefore, necessary to perform trimming with respect to the weight portion
2
and/or the support beams
3
by, for example, a complicated machining process, to thereby adjust the resonance frequency of the planar vibrating body
10
to the design frequency.
Since the resonant element
16
is, however, a minute resonant element
16
, it is practically impossible because of the accuracy of conventional mechanical trimming techniques to perform trimming of the minute weight portion
2
and/or support beams
3
so as to have the desired dimensions, shape, and weight, etc. It is, therefore, very difficult to adjust the resonance frequency of the planar vibrating body
10
to a set value.
Therefore, in the resonant element
16
, as shown in
FIG. 7B
, a conductive layer
12
for providing an electrostatic attractive force
15
is located at a position opposed to the weight portion
2
in the Y-direction with a gap therebetween. As shown in
FIG. 7A
, the conductive layer
12
is connected to a conductive pad
14
via a conductive pattern
13
. By controlling the voltage to be applied to the conductive layer
12
via the conductive pattern
13
and conductive pad
14
, the resonance frequency of the resonant element
16
is adjustable to a set value.
Once a DC voltage is applied to the conductive layer
12
, an electrostatic force acts on the planar vibrating body
10
as an electrostatic spring. Specifically, when the planar vibrating body
10
vibrates in the direction such that the planar vibrating body
10
approaches the substrate
1
, an electrostatic force acts in the direction such that the amplitude is increased, and hence the application of the DC voltage to the conductive layer
12
has an effect of generating a force in the opposite direction as if a mechanical spring were being compressed. This results in a reduction in the resonance frequency in the Y-direction. Since this reduced amount of the resonance frequency varies in accordance with the electrostatic attractive force
15
, a fine-adjustment of the resonance frequency of the planar vibrating body
10
from the natural frequency thereof to the lower frequency side can be performed by adjusting the magnitude of the DC voltage applied to the conductive layer
12
.
Utilizing this effect, by designing the natural resonance frequency of the planar vibrating body
10
in the Y-direction to be slightly higher than the most sensitive resonance frequency (the resonance frequency in the X-direction), i.e., by designing the resonance frequency of the planar vibrating body
10
in the detection direction to be higher than the resonance frequency thereof by the exciter
4
in the vibrational direction, the sensitivity of the resonant element
16
can be increased by adjusting the DC voltage applied to the conductive layer
12
.
In the resonant element
16
, it is important to adjust the resonance frequency thereof to a set value and to keep the vibrating state of the planar vibrating body
10
on-target.
FIGS. 6A and 6B
illustrate examples of movements of a planar vibrating body
10
in the X-Y plane without angular velocity around the Z-axis, when the planar vibrating body
10
is vibrated in the X-direction. In the resonant element
16
shown in
FIGS. 7A and 7B
, if the vibration of the planar vibrating body
10
is deflecting in the Y-direction, which is the detection direction, that is, if the planar vibrating body
10
tilts with respect to the substrate plane, a Coriolis force cannot be accurately measured if this tilt is substantial, and the gyro characteristics of the angular velocity sensor or the like deteriorates.
It is therefore desirable that the vibratory state of the planar vibrating body
10
hardly exhibits any deflection in the Y-direction, as shown in FIG.
6
B.
Generally, the less the difference (&Dgr;f) in the resonance frequency of the planar vibrating body
10
between the vibrational direction and detection direction, the larger the mechanical coupling between the two directions (the

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