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.
Gyroscopes, or xe2x80x9cgyrosxe2x80x9d, 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 (xe2x80x2909) 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 xe2x80x2909 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 xe2x80x9ctunedxe2x80x9d speed. Gyroscope tuning permits the instrument to function as a xe2x80x9cfree rotor gyroscopexe2x80x9d. 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 in3, 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 xe2x80x9cgas fill pressure sensitivityxe2x80x9d. 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 the pressure gradients and turbulence to which the rotor subassembly is subjected. Furthermore, it would be advantageous to take advantages of gas bearings within such a device, without compromising the performance of the gyroscope.
The present invention is a hybrid wafer gyroscope comprised of micro-machined components and, potentially, conventional machined components, all of which are manufacturable by known low cost, high volume techniques. As a minimum, the hybrid wafer gyroscope includes a rotor subassembly micro-machined (e.g., etched) from a conductive wafer material, such as silicon. The rotor subassembly includes at least a micro-machined rotor and gimbal, and preferably a drive hub if two degrees of freedom are required. Each micro-machined component may be formed from one or more layers of wafer material. In the preferred form, the gyroscope includes an induction drive motor, including stators that may also be micro-machined from wafer material. The remainder of the gyroscope may include more traditional electro-mechanical components, such as a housing, a drive shaft, and other drive motor components. As an example, due to the relatively small micro-machined rotor subassembly, the overall size and weight of a DTG hybrid wafer gyroscope in accordance with the present invention, is about five a times smaller than typical machined DTGs. Additionally, a gyroscope in accordance with the present invention is capable of improved performance, having a stability of up to about 0.005 degree/hour. Of course, larger size and weight hybrid wafer gyroscopes may also be constructed, as dictated by the application for which the gyroscope is to be used. Because all of the gyroscope components are made from low cost, high volume techniques, the cost of a hybrid wafer gyroscope is about half that of similarly performing electromechanical gyroscopes.
The hybrid wafer gyroscope includes a housing substantially encasing and securing a pair of disk shaped stators in a parallel orientation, but offset within their respective planes from each other by about 90xc2x0. The stators are separated at a fixed distance by a ring shaped spacer disposed circumferentially therebetween. This combination defines a circular volume within which the rotor subassembly rotates about a fixed drive shaft, defining a spin reference Z-axis. Each stator is secured to the housing, which includes a top case and a bottom case. Within each case a circular arrangement of motor windings is housed, centered about the spin reference Z-axis. Rotation of the rotor subassembly between the stators and with respect to the drive shaft is facilitated using one of a variety of forms of bearing. In alternate forms, the drive shaft, or a portion thereof, may be rotational rather than stationary, potentially obviating the need for the use of bearings between the rotor subassembly and drive shaft. Also, with alternate motive forms, the windings may not be necessary.
Assuming a fixed drive shaft and bearing arrangement, in a first rotor subassembly embodiment, the drive hub includes a groove, as an inner bearing raceway, centered about the spin reference axis. In such a case, secured within (or as part of) each stator is a bearing plate which includes a corresponding outer bearing raceway. When assembled, the inner and outer bearing raceways nest a plurality bearing balls, thereby forming a ball bearing between the rotor subassembly and the drive shaft. In a second rotor assembly embodiment, a self-contained bearing mechanism, such as a cartridge bearing, may be coupled the rotor subassembly and thereby allow rotation with respect to the drive shaft. Other forms of bearing and rotation facilitating interfaces known in the art may be employed in alternate embodiments.
As part of the motive source which causes the rotation of the rotor subassembly, a magnet or series of magnets is coupled to or integral with the rotor subassembly and uniformly distributed about the spin reference axis. For example, in one form, the drive hub includes a plurality of magnet seats having magnets secured therein. In another embodiment, a magnet hub (i.e., a ring of magnets) is coupled to the rotor subassembly. The radius from the Z-axis to the center of the circular arrangement of windings is about equal to the radius from the Z-axis to the center of the circular magnet or arrangement of magnets. Together, the windings, a power source connected to the windings, and the magnet(s) form the motive source which rotates the rotor subassembly.
One side of each stator, the side that faces the rotor subassembly, is generally divided into four quadrants and includes pick-offs and torquers. The pick-offs, torquers, and control electronics comprise the substantive portion of a control feedback loop that senses displacement of the rotor and adjusts for such displacements. Within each quadrant of each stator is a pair of arcuate capacitive pick-offs pads, concentric with the Z-axis, for sensing a displacement from a null position of the rotor as a function of the voltage between the rotor and pick-off pads. The pick-off pads are electrically isolated from each other and have separate leads which connect to separate pads at the outer circumference of their corresponding stator, for connection to the control electronics. The pick-off pads are disposed radially proximate to the outer half of the rotor. In a similar arrangement, within each quadrant of each stator is a pair of arcuate torquer pads concentric with the Z-axis for applying a nulling force to the rotor in response to voltage differences sensed by the pick-offs. The torquer pads are electrically isolated from each other and have separate leads which connect to pads at the outer circumference of their corresponding stator for connection to the control electronics. The torquer pads are disposed radially proximate to the inner half of the rotor. The stators also define several holes for securing together the components of the gyroscope and they define several cutouts at the outer circumference, some of which are metalized and form torquer and pick-off connection pads.
In the preferred embodiment the rotor subassembly includes a drive hub, gimbal, and rotor formed as a series of substantially coplanar concentric rings centered about the spin reference axis, preferably etched from a single silicon wafer. The gimbal is connected to the drive hub via two flexures formed between the two and along a gimbal-shaft pivot X-axis, which is orthogonal to the Z-axis. The rotor is attached to the gimbal via two flexures formed between the two and along a rotor-gimbal pivot Y-axis, which is orthogonal to the X and Z axes. Formed between the outer circumference of the drive hub and the inner circumference of the gimbal, and opposite to each other with respect to the Z-axis and along the Y-axis, are circular stop cutouts, within which stops are seated to limit or dampen the rotor deflection. Similarly, stop cutouts are formed between the gimbal and rotor along the X-axis and opposite each of other with respect to the Z-axis. Otherwise, the hub-gimbal and gimbal-rotor interfaces are formed by semicircular cuts on the wafer. Preferably, the stop cutouts and flexures are also etched into the silicon wafer during etching of the other rotor subassembly components.
In an alternate embodiment, the rotor subassembly is hermetically sealed within a gas-filled bearing cartridge having a gas pressure of up to about 1/10 atmospheres, wherein the cartridge is rotatable with respect to the drive shaft. In such an embodiment, the bearing cartridge includes two disk shaped couplers disposed in parallel planes on each side of the planar rotor subassembly, wherein the inner circumferences of the couplers are sealed together by a cylindrical inner spacer and the outer perimeters of the couplers are sealed together by a cylindrical outer spacer. Rotation of the cartridge with respect to the drive shaft is preferably accomplished using a bearing. In the preferred form, the bearing is a gas bearing. The bearing cartridge experiences a coincident rotation with the rotor subassembly. Consequently, there is a reduction of gas fill sensitivity of the gyroscope because the gas is rotating with the rotor, such that pressure gradients and turbulence within the cartridge are substantially nonexistent. The pressures within the bearing cartridge and outside the cartridge may be varied, but it is essential in hybrid wafer gyroscope embodiments including the bearing cartridge that the pressure within the cartridge is isolated and independent from the pressure outside the cartridge.
The couplers are micro-machined disks including pick-off couplers and torquer couplers disposed proximate to the rotor subassembly. The couplers are concentric with the rotor subassembly and have an inner perimeter substantially equal to that of the rotor subassembly. The outer perimeter of each coupler is slightly larger, radially, than the rotor subassembly and is coupled to a different edge of the outer spacer. Each coupler is divided into quadrants which include arcuate capacitive pick-off and torquer couplers oriented about the spin reference axis to be proximate to corresponding stator pick-offs and torquers. The pick-off couplers couple signals corresponding to displacements of the rotor to their corresponding stator pick-offs. Similarly, each torquer coupler couples signals from its corresponding stator to the rotor to correct rotor displacements. In an alternative approach, a circular band is used on the outside of the rotor to couple signals between the rotor and stators. In the preferred embodiment bearing cartridge embodiment, the bearing cartridge includes a circular arrangement of magnets which interact with the motor windings. Given that the rotor subassembly is coupled to the bearing cartridge, rotation of the cartridge causes coincident rotation of the rotor subassembly.
In an alternate approach, optical pick-offs are used in place of capacitive pick-offs. In such an embodiment, at least a portion of each stator and coupler is formed from an optically transparent material, wherein light is directed at an angle from a light emitting or laser diode through the glass and onto the rotor. The reflection of light from the rotor is measured by a quad cell to determine rotor displacement. Displacement signals are then communicated to the control electronics. For increased accuracy, an interferometer may be used in place of the quad cell.