Hybrid wafer gyroscope

Details Hybrid wafer gyroscope Hybrid wafer gyroscope

2001-07-26

2003-09-09

Lorence, Richard M. (Department: 3681)

Machine element or mechanism

Gyroscopes

Gyroscope control

C074S00500R, C074S005700

Reexamination Certificate

active

06615681

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to gyroscopes, and more particularly to miniature gyroscopes having a high degree of bias stability, for example, having a stability of up to about 0.005 degrees/hour.
BACKGROUND OF THE INVENTION
Gyroscopes, or “gyros”, are used in many systems that require, as an example, an inertial guidance system. A significant feature of a gyroscope is that the momentum and the rotational axis of a gyroscope rotor generally preserve their direction in inertial space. Due to its ability to maintain the direction of its axis constant in space, the gyroscope can suitably be used for the stabilization of movements, that is, for maintaining an object in an orientation which is angularly fixed in inertial space. There are several classes of gyroscopes, for example, a floated single degree of freedom electro-mechanical gyroscope, an electrostatic gyroscope, a ring laser gyroscope, a tuning fork gyroscope, a fiber optic gyroscope, and a dry dynamically tuned gyroscope (DTG) having two degrees of freedom, but the basic functional characteristics of a gyroscope are common to all types.
A typical mechanical DTG is disclosed in U.S. Pat. No. 4,563,909 (′909) to Carroll et al. and shown herein in prior art
FIGS. 1A-1C
as an example of such devices. As shown in
FIG. 1A-1C
, a typical DTG
10
includes a drive shaft
14
which centers and rotates a gimbal
28
which in turn centers and rotates a rotor
20
. The drive shaft
14
is driven by a motor
16
under control of an electrical controller
42
and rotates about a longitudinal drive shaft spin axis (or Z-axis)
18
, also referred to as a spin reference axis. Gimbal
28
is attached to drive shaft
14
via a gimbal shaft that defines a gimbal-shaft pivot axis (or X-axis)
32
. Similarly, rotor
20
is attached to gimbal
28
via a rotor shaft that defines a gimbal-rotor pivot axis (or Y-axis)
30
. When tuned, gimbal
28
experiences a rotation about gimbal-shaft pivot X-axis
32
in response to a Y-axis component associated with the drive shaft
14
being displaced from a vertical Z-axis orientation. And, when tuned, rotor
20
experiences a rotation about gimbal-rotor Y-axis
30
in response to an X-axis component associated with drive shaft
14
being displaced from a vertical Z-axis orientation. The intersection of the X, Y, and Z axes is referred to as the pivot point
36
of the gyroscope. Rotor
20
spins about a rotor-spin axis
22
. When there is no displacement of drive shaft
14
in inertial space the rotor spins about and within a plane which is orthogonal to the spin reference Z-axis, but when a drive shaft displacement does occur, the rotor-spin axis shifts to remain orthogonal to the plane within which the rotor spins.
As shown in
FIGS. 1B and 1C
, gyroscope
10
also includes a case
12
which substantially encases the other gyroscope components. Motor
16
, which rotates the drive shaft of the ′909 gyroscope, is secured between case
12
and drive shaft
14
. A set of bearings
15
is disposed between case
12
and drive shaft
14
and maintains the orientation of the drive shaft relative to the case, while also facilitating rotation of drive shaft
14
, gimbal
28
, and rotor
20
. Drive shaft
14
is supported on a ball bearing (not shown) and spun by the electromagnetic drive motor
16
. Change in rotor position with respect to the case
12
results when an angular force input along the DTG's two mutually orthogonal axes
30
and
32
(i.e., X and Y-axes), which are normal to the spin reference axis
18
(Z-axis). Rotor position is sensed with a pick-off, and the rotor is re-balanced back to its null position using a torquer and control electronics
42
, in a closed loop operation.
A typical DTG is a two degree of freedom device, like that of
FIGS. 1A-1C
. The rotor
20
is attached to the drive shaft
14
through a universal joint hinge-gimbal assembly that provides the two rotational degrees of freedom of rotor
20
and gimbal
28
with respect to the drive shaft
14
, wherein drive shaft
14
is fixed in position relative to case
12
. The hinge stiffness of the flexures and gimbal inertias are sized to provide a dynamic decoupling of rotor
20
and shaft
14
when the assembly is spun at the gyroscope “tuned” speed. Gyroscope tuning permits the instrument to function as a “free rotor gyroscope”. A free rotor gyroscope is one in which the static torque of the flexures is canceled by the dynamic torque of the gimbal.
A common measure of the performance of a gyroscope is its stability, which may be measured in degrees/hour. The smaller the measure of degrees/hour, the more stable the gyroscope and the better its performance. The DTG 2000, a product of Litton Corporation, Woodland Hills, Calif., is a typical example of the state of the art in electro-mechanical DTGs. Such a device can cost several thousands of dollars and has a range of performance (or stability) of about 0.1 degree/hour. A considerable cost of the assembled instrument is in the fabrication and tuning of the rotor with thin flexures; thin flexures are less stiff, thus allow a lower tuned speed of the gyroscope. A lower tuned speed is generally desirable because it requires less energy input to the gyroscope to achieve the desired free rotor condition. Some commercially available electromechanical DTG instruments are capable of meeting a high level of performance, i.e., stability in the range of about 0.01 degree/hour. However, such devices consist of many hand assembled and costly conventionally machined parts and are relatively labor intensive to manufacture. Of course, such devices are more expensive than the 0.1 degree/hour devices to produce. Conventional electromechanical DTGs have high parts count and high labor input due to the many assembly and fine trimming operations required in their production. Their size and weight, about 2 in
3
, 100 gm, are attractive verses previous instruments, such as the floated gyroscope, but they are still relatively large for many applications.
An alternative to the typical mechanical gyroscope is a all micro-machined gyroscope, made from silicon, which tends to be smaller and less expensive than the typical electro-mechanical gyroscopes. An example of an all micro-machined gyroscope is the tuning fork gyroscope (TFG) by The Charles Stark Draper Laboratory, Inc., Cambridge, Mass. The all micro-machined TFG performs at only a moderate level of performance, about 10-100 degree/hour, and it is projected that it will take many years to improve performance to the better than 1 degree/hour level. So, with respect to performance, the all micro-machined gyroscope lags behind typical mechanical devices and, therefore, is primarily suited for applications requiring a small sized gyroscope with moderate performance.
Many gyroscopes are gas filled, having a gas generally occupying the volume within the case. Rotation of the rotor within the case causes pressure gradients and turbulence, which adversely effect the performance of the gyroscope. The gyroscope's vulnerability to such disturbances is referred to as “gas fill pressure sensitivity”. Despite such sensitivities, it is still typically considered advantageous to fill conventional DTG's, for example, with a gas at a fill pressure of, optimally, not more than 1/10 of atmosphere. At the same time, it is desirable, although not practical, to allow rotation of the rotor subassembly about the fixed drive shaft using a gas bearing, which has inherent low noise and long life characteristics. However, gas bearings typically require a fill pressures of 3 to 4 atmospheres, i.e., quite a bit higher than the fill pressure optimally required for the rotor subassembly.
It would be advantageous to have a gyroscope comprised of low cost, high volume components which performs at least as well as expensive traditional mechanical gyroscopes, but at significantly reduced size and weight. It would also be advantageous to substantially eliminate the gas fill pressure sensitivity of the gyroscope by substantially eliminating th

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