Accelerometer

Details Accelerometer Accelerometer

2002-07-03

2003-10-14

Williams, Hezron (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Acceleration determination utilizing inertial element

C073S514220

Reexamination Certificate

active

06631643

ABSTRACT:

This invention relates to an accelerometer and particularly to an accelerometer at which is made at least partially from silicon.
The conventional quartz and silicon accelerometers as shown in
FIG. 1
of the accompanying drawings have a bulk machined (wet etched) quartz or silicon structure which is thinned along a line to create a hinge
1
, with a proof mass
2
, on one side thereof in the form of a pendulum. The structure is normally implemented in the form of a sandwich with two fixed capacitor plates
3
, located on either side of the moveable proof mass
2
which is hinged at
1
from a support
4
on which the capacitor plates
3
are mounted. With an acceleration in the direction A in
FIG. 1
which is perpendicular to the plane of the silicon or quartz wafer and proof mass
2
, the proof mass is deflected arcuately about the hinge
1
by an amount proportional to acceleration. This movement is usually sensed electrostatically and a restoring force is applied to return the proof mass
2
to a null position conveniently by use of electromagnetic feedback using a wire wound coil on the proof mass. Electrostatic force may also be used for the feedback. Feedback improves the scale factor linearity at high forces as the proof mass does not move. High accuracy accelerometers are usually closed loop.
Such a conventional accelerometer can provide high accuracy over a high gravity range but is generally expensive to manufacture and of relatively large size. Additionally as the structure is a pendulous structure the motion of the proof mass
2
is arcuate which gives rise to vibro pendulosity which introduces a cross axis sensitivity under a vibration. An additional problem which is common to this and other types of conventional accelerometers is vibration rectification. This means that in the presence of a vibration but no static gravity load such a conventional accelerometer can provide an erroneous output signal which is due to an imbalance between the two capacitor plates
3
which sense the deflection of the proof mass
2
and act to give an electrostatic restoring force.
The conventional accelerometer as shown in
FIG. 1
requires the two capacitor plates
3
to be operated in anti-phase with the difference in capacitance value then being linearly proportional to the offset position and acceleration. An electrostatic force is supplied for feedback. This gives rise to three particular disadvantages. Firstly the electrostatic forces are quadratic in voltage so it is necessary to linearise the force which is proportional to acceleration. This can be difficult to do for precision accelerometers. Secondly, the conventional silicon accelerometer of
FIG. 1
is pendulous which means that the proof mass
2
moves in a curved arc as a function of increasing acceleration. This arcuate motion means that when the proof mass
2
is away from the null position there is a sensitivity at right angles to the main sensing axis. This effect which is generally called vibro-pendulosity is an error which is particularly apparent for vibration when it is applied to excite both axis. At high frequency of movement the proof mass is not correctly restored to the null position. Thirdly, any offset between the values of the two capacitor plates
3
which are used differentially to detect movement of the proof mass
2
away from the null position can cause the vibration rectification effect as well as bias (zero offset). Hence it is necessary accurately to match the values of the two capacitor plates
3
which is difficult to do with the sandwich structure of FIG.
1
. Accordingly electronic offsets are typically used to null out any imbalance which is not desirable as any drift in this nulling signal will cause a drift in the accelerometer bias, which is a key parameter to keep stable.
A second type of conventional accelerometer is shown in
FIG. 2
of the accompanying drawings which uses vibrating beams
5
, the frequency of vibration of which varies with strain and acceleration. The vibrating beams
5
are attached to a proof mass
6
so that the acceleratative force on the proof mass
6
changes the strain on the vibrating beams
5
, either in compression or tension, to give a frequency output which varies with the gravitational force. The vibrating beams
5
generally operate differentially so that one side is in compression and the other is in tension with a frequency increase and decrease respectively of the beams. The difference frequency is then a good measure of acceleration. Movement of the proof mass is in a sensing direction B as shown in
FIG. 2
, with the vibrating beams
5
in effect suspending the proof mass
6
between two mounting supports
7
.
The conventional accelerometer of the type shown in
FIG. 2
can use quartz for the vibrating beams
5
and proof mass
6
. Such accelerometers can be made smaller and slightly cheaper than the pendulum type accelerometer of
FIG. 1
but they are still considerably expensive to manufacture, and are open loop accelerometers which do not usually have force feedback and which may be subject to linearity errors at high input accelerations.
There is thus a need for an improved accelerometer which utilises silicon and which at least minimises the foregoing difficulties inherent in the conventional accelerometers illustrated in
FIGS. 1 and 2
.
According to one aspect of the present invention there is provided an accelerometer having a substantially planar plate-like proof mass, four or more flexible mounting legs each co-planar with the proof mass, a substantially planar, ring-like support, in which the proof mass is movably mounted, which support is fixedly mounted relative to the proof mass and co-planar therewith, with each mounting leg being connected at one end to the proof mass and connected at another end to the support so that the proof mass is mounted for linear movement in a sensing direction in the plane containing the proof mass, mounting legs and support, in response to acceleration change applied to the accelerometer, and with the mounting legs extending substantially perpendicularly to the sensing direction, at least two spaced apart substantially planar capacitor plates, mounted in the ring-like support substantially transverse to the sensing direction with the proof mass located between the capacitor plates and with each capacitor plate being coplanar with the proof mass, mounting legs and support, for sensing linear movement of the proof mass in the sensing direction, a plurality of interdigitated fingers in air, comprising first arrays of laterally spaced fingers extending substantially perpendicularly to the sensing direction from the support towards the proof mass and second arrays of laterally spaced fingers extending substantially perpendicularly to the sensing direction from the proof mass towards the support, with the first arrays of fingers being interdigitated with the adjacent second arrays of fingers to provide air squeeze damping for movement of the proof mass in the sensing direction relative to the support, with the proof mass, mounting legs, support capacitor plates and interdigitated fingers are formed from a single plate of silicon, and restoring means for returning the proof mass in the sensing direction towards a null position.
Preferably the proof mass, mounting legs, support capacitor plates and interdigitated fingers are formed by dry etching from a plate of silicon which is orientated in a [111] or [100] crystal plane.
Conveniently the support has a substantially rectangular ring-like shape surrounding an inner open area in which is located the proof mass which has a substantially rectangular shape, and wherein the mounting legs extend substantially perpendicularly to the sensing direction in spaced array, with at least two between a first inner wall of the support defining the inner open area and a facing first outer wall of the proof mass and with at least two between the opposing second inner wall of the support defining the inner open area and the facing second outer wall of the pro

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