Accelerometer strain isolator

Details Accelerometer strain isolator Accelerometer strain isolator

2002-04-05

2003-10-21

Moller, Richard A. (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Acceleration determination utilizing inertial element

Reexamination Certificate

active

06634231

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to suspension devices and methods, and in particular to structures for mounting force-versus-displacement sensors, whereby external stress sources are isolated from active sensor components.
BACKGROUND OF THE INVENTION
Accelerometers generally measure acceleration forces applied to a body by being mounted directly onto a surface of the accelerated body. One common type of accelerometer employs one or more force-versus-displacement or “force/displacement” sensors for measurement of acceleration. Accelerometers employing two force/displacement sensors instead of the necessary minimum one sensor gain considerable advantage. If the two sensors operate in a push-pull mode, then many error sources such as thermally driven effects or drift may be rejected as common mode, while the difference signal represents the desired acceleration measurement. Occasionally, designs using two force/displacement sensors include two completely separate proof masses, which results in essentially two accelerometers, each having its own sensor, but operating in opposite directions. For numerous reasons, however, a two proof mass solution is not preferred. Rather, it is generally advantageous to have only one proof mass in an accelerometer.
A typical example of a prior art micromachined two-sensor/single proof mass accelerometer, commonly referred to as a Rectangle design, is illustrated in
FIGS. 1A and 1B
. The accelerometer
10
illustrated in
FIGS. 1A and 1B
is a miniature structure fabricated from a substrate
12
of semiconductor material by conventional micromachining techniques. The substrate
12
is formed of a monocrystalline silicon material in a substantially planar structure, i.e., having substantially planar and parallel opposing offset upper and lower surfaces. The silicon substrate
12
often includes an upper silicon or active layer
14
that is electrically isolated from an underlying substrate
16
by an insulating layer
18
, or an insulating layer is applied to active layer
14
, as shown and described in U.S. Pat. No. 5,948,981, entitled Vibrating Beam Accelerometer, issued on Sep. 7, 1999, and assigned to the Assignee of the present application, the entirety of which is incorporated herein by reference. The insulating layer
18
is may be a thin layer, e.g., about 0.1 to 10.0 micrometers, of an oxide, such as silicon oxide. The silicon substrate
12
is usually formed by oxidizing active layer
14
and underlying substrate
16
, and adhering the two layers together. A portion of active layer
14
may be removed to bring the layer
14
to the desired thickness. The silicon oxide layer
18
retains its insulating properties over a wide temperature range to ensure effective mechanical resonator performance at high operating temperatures on the order of 100 degrees Celsius. In addition, the insulating layer
18
inhibits undesirable etching of the active layer
14
during manufacturing.
The micromachined accelerometer
10
includes an acceleration sensor mechanism
20
having one or more flexures
22
pliantly suspending a proof mass
24
from an inner sensor frame or plate
26
for movement of the proof mass
24
along an input axis I normal to the proof mass
24
. The flexures
22
are preferably etched near or at the center of the underlying substrate
16
, i.e., substantially centered between the opposing upper and lower surfaces of the underlying substrate
16
. Optionally, the flexures
22
are formed by anistropically etching in a suitable etchant, such as potassium hydroxide (KOH). The flexures
22
define a hinge axis H about which the proof mass
24
moves in response to an applied force, such as the acceleration of the accelerated body, for example, a vehicle, aircraft or other moving body having the accelerometer
10
mounted thereon. The sensor mechanism
20
includes a pair of force/displacement sensors
28
coupled between the proof mass
24
and the sensor frame
26
for measuring forces applied to the proof mass
24
. The force/displacement sensors
28
are, for example, mechanical resonators formed from the active silicon layer
14
as double-ended tuning fork (DETF) force sensors.
A known oscillator circuit, shown in FIG.
1
C and described in above-incorporated U.S. Pat. No. 5,948,981, drives the mechanical resonators
28
at their resonance frequency.
FIG. 1C
illustrates a representative oscillation circuit
50
in which vibrating beams of the transducers
28
function as a resonator. A transimpedance amplifier
52
converts a sense current received from vibrating beams to a voltage. This voltage is filtered by a bandpass filter
54
, which reduces noise, and the voltage amplitude is controlled by an amplitude limiter
56
. The resulting signal is combined with the output or DC bias voltage from a DC source
58
in a summing junction
60
. The DC bias voltage generates a force between electrodes and the beams of the force/displacement sensors
28
. The signal from amplitude limiter
56
modulates this force causing the beams of the transducers
28
to vibrate laterally at their resonant frequency. This lateral beam motion, in turn, generates the sense current. An output buffer
62
isolates the oscillator from external circuitry connected to an output
64
of oscillation circuit
50
. The gain in oscillation circuit
50
sustains oscillation of the beams of the force/displacement sensors
28
.
In response to an applied force, the proof mass
24
rotates about the hinge axis H, causing axial forces, either compressive or tensile, to be applied to the mechanical resonators
28
. The axial forces change the frequency of vibration of the mechanical resonators
28
, and the magnitude of this change serves as a measure of the applied force or acceleration. In other words, the force/displacement sensors
28
measure the applied acceleration force as a function of the displacement of the proof mass
24
.
Undesirable external stresses and strains may be induced in the sensitive acceleration sensor mechanism
20
by, for example, mechanical coupling of the accelerometer sensor frame
26
to a silicon cover plate
30
which in turn is typically connected to a ceramic or metal mounting plate
32
. Any strains occurring in the sensor frame
26
are transmitted not only to the proof mass
24
, but through the proof mass
24
to the two DETF resonators
28
. Since the only significant compliance in the system is the sensing DETF resonators
28
themselves, almost the entire strain appears as an error output from the DETF resonators
28
. Thus, undesirable errors are generated in the DETF resonators
28
from inputs having nothing to do with the acceleration being measured. These errors can be quite large since the compliance through the DETF resonators
28
must be low to detect acceleration with sufficient accuracy to be useful in practical systems.
Strain isolation within the micro-machined accelerometers is thus of paramount importance for good performance, i.e., accuracy. Strain isolation separates the mechanism from stresses mechanically induced during fabrication and assembly, and thereby reduces variations in resonance within the beams of the two vibrating-beam force sensing portion of the accelerometer mechanism. Strain isolation also separates the mechanism from stresses externally induced by shock, vibration and temperature variation within the operating environment.
Many methods are known for isolating the sensitive acceleration sensor mechanism
20
from such undesirable stresses and strains. Typically, cantilever-style isolation is provided wherein the sensor frame
26
is suspended from a second outer or external frame portion
34
by flexures
36
formed by overlapping slots
38
and
40
through the substrate
12
. The sensor frame
26
is thus able to move relative to the outer frame
34
, as shown and described in U.S. Pat. No. 5,948,981, which is incorporated herein. Such isolation minimizes the distortion of the sensor frame
26
, and thereby decreases the effects of external stresses and strains on the

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