Micromachined devices with apertures

A micromachined device has a body suspended over a substrate and movable in a plane relative to the substrate. The body has a perimeter portion, a first cross-piece portion extending from one part of the perimeter portion to another part of the perimeter portion to define at least first and second apertures, a first plurality of fingers extending along parallel axes from the perimeter portion into the first aperture, and a second plurality of fingers extending along parallel axes from the perimeter portion into the second aperture.

BACKGROUND OF THE INVENTION

This invention relates to micromachined gyros.

A surface micromachined gyro has a planar body (or a number of bodies) suspended with anchors and flexures over and parallel to an underlying substrate. The body is dithered along a dither axis in a plane parallel to the substrate and perpendicular to a sensitive axis that can be in the plane of the body or perpendicular to the body and to the substrate. As is generally known, rotation by the body about the sensitive axis causes the body move along a Coriolis axis, which is mutually orthogonal to the dither axis and the sensitive axis. This motion can be sensed to derive a signal that indicates the angular velocity of the rotation.

Because of mechanical imperfections in the body and in the flexures, a suspended mass will typically not be perfectly parallel to the substrate, and the dither and sensitive will typically not be perfectly orthogonal. Consequently, when the body is dithered, an interference signal, referred to as the quadrature signal, is induced by the dithering motion itself. This quadrature signal, which is unrelated to the rotation to be sensed, interferes with the desired signal relating to the rotation. The quadrature signal (a) is proportional to the acceleration in the dither direction with a constant of proportionality indicative of the mechanical misalignment; (b) has the same frequency as the dither frequency; and (c) is 90° out of phase with the dither velocity, unlike the Coriolis signal which is in phase with the velocity. Because of this 90° phase difference, the quadrature signal can be partially rejected with a phase-sensitive detector. The effectiveness of such rejection, however, depends on how precise the phase relationships are maintained in the electronics.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a micromachined gyro in which there is minimal interference in the output signal caused by the dither signal. The gyro has a first body, suspended over a substrate and dithered along a dither axis, and a second body coupled to the first body and also suspended over the substrate. The first and second bodies are coupled together and anchored to the substrate such that the first body can move along the dither axis but is substantially inhibited from moving along a Coriolis axis (perpendicular to the dither axis) relative to the second body, and the second body is movable with the first body along the Coriolis axis but is substantially inhibited from moving along the dither axis. The coupling between the first body and the second body substantially decouples the dithering movement from the movement along the Coriolis axis in response to rotation about the sensitive axis, thus minimizing the unwanted quadrature signal. One of the first and second bodies preferably surrounds the other; the dithered first body is preferably on the inside and surrounded by the second body, although the first body can surround the second body.

In another aspect, a micromachined device has a first body with fingers interdigitating with fixed drive fingers that cause the first body to dither along a dither axis. At least one conductive member is formed under some, but not all, of the fixed dither drive fingers and is electrically coupled to the drive fingers to keep the first body in the desired vertical plane and to prevent the first body from levitating due to fringe effects.

In yet another aspect, a micromachined device has a movable body suspended over a substrate and at least one stop member positioned near the movable body. The stop member includes a hook portion extending over the movable body such that the stop member limits both lateral movement and vertical movement by the body.

In still another aspect, a micromachined device has a suspended movable body with an outer perimeter portion and at least one cross-piece that defines a number of apertures enclosed by the perimeter portion. The body has fingers extending into the apertures. These fingers can be used either to dither the body or to sense motion of the body.

In another aspect, the micromachined device has an inner body surrounded by an outer body, the outer and inner bodies being inhibited from movement together along one axis by flexures oriented along that one axis. These flexures are connected between the body and an elongated stationary member anchored at a midpoint and with the flexures extending from each end to the body. The elongated member is preferably between the inner and outer bodies

In still another aspect of the invention, a first micromachined structure is positioned near a second micromachined structure, and the first micromachined structure is dithered relative to the second micromachined structure. These first and second structures are connected together with coupling structures designed to minimize stress and to encourage opposite ends of the structure to move together in the direction of dithering toward and away from the second structure. While there are a number of variations of coupling structures that can be used, these include structures that have elongated members extending from ends of the first structure and extending toward the center of the structure along a direction perpendicular to the dithering direction. These elongated members are connected by a short connecting beam that encourages the elongated members to move together in the same direction at the same time, rather than moving in opposite directions. These elongated members are connected to the first structure with perpendicular members that define a pivot point.

Openings can be cut out of the second structure to reduce the combined mass of the first and second structures, while still maintaining stiffness in the structure. In addition to the coupling structures between the first and second structures, the second structure is also anchored to the substrate through plates that are relatively wide compared to the width of the second structure itself. These plates are connected together by perpendicular members that define a pivot point.

The gyro of the present invention minimizes the quadrature signal, and thus is very accurate compared to prior gyro designs. The accuracy due to the structure of the gyro obviates the need for complex electronics, and also allows the device to be packaged under ambient conditions. The stop members, the conductive members on the substrate, use of multiple apertures with inwardly extending fingers, and use of a centrally anchored stationary member for supporting flexures along an axis of inhibited movement all improve performance and reliability of a micromachined device in general and a gyro in particular. Other features and advantages will become apparent from the following detailed description, the drawings, and the claims.

DETAILED DESCRIPTION

FIG. 1illustrates a simplified surface micromachined gyro10that is structurally and operationally similar to known gyros. Gyro10has an essentially planar body12suspended over and parallel to an underlying substrate14. Body12is supported with four flexures16, each of which extends from a respective support anchor18to a different corner of body12. Fingers22extend from body12and interdigitate with fixed drive fingers24that are coupled to an AC voltage source (not shown) to dither the body at its resonant frequency along a dither axis26. When body12rotates about a sensitive axis36, body12moves along a Coriolis axis28that is mutually orthogonal to sensitive axis36and dither axis26. This movement is sensed with a differential capacitor that includes fingers30that extend away from body12along axes parallel to dither axis26, and two sets of inwardly extending fixed sensing fingers32and33. The differential capacitor is formed from many individual cells, each cell having two fixed fingers32,33and one finger30serving as a movable finger and interdigitation with fixed fingers32,33.

As is generally known, if the dither motion is x=X sin(wt), the dither velocity is x′=wX cos(wt), where w is the angular frequency and is directly proportional to the resonant frequency of the body by a factor of 2π. In response to an angular rate of motion R about sensitive axis36, a Coriolis acceleration y″=2Rx′ is induced along Coriolis axis28. The signal of the acceleration thus has the same angular frequency w as dithering velocity x′. By sensing acceleration along Coriolis axis28, rotational velocity R can thus be determined.

Due to mechanical imperfections, e.g., if one flexure is more compliant than the others due to overetching, the center of suspension of the body may not coincide with its center of mass and thus the mass can wobble during the dithering motion. Such wobbling causes a component of the dither motion to appear along the sensitive axis. This component creates the interfering quadrature signal. This signal can be very large compared with the desired rotational signal being measured; e.g., it can be as much as 10% of the dither motion, creating a signal 10,000 times greater than the Coriolis signal. The need to eliminate this quadrature signal places a great burden on the signal processing electronics.

FIG. 2is a plan view of a surface micromachined gyro50illustrating a simplified first embodiment of the present invention. Gyro50has a suspended body with an inner frame52and an outer frame54surrounding inner frame52. Frames52,54are coplanar and are suspended over and parallel to an underlying substrate55. Outer frame54is suspended with flexures56that extend along axes parallel to a dither axis58and are anchored to substrate55with anchors53. This orientation of flexures56allows outer frame54to move along a Coriolis axis60, but substantially prevents outer frame54from moving along dither axis58.

Inner frame52is coupled to and suspended from outer frame54with flexures62that extend along axes parallel to Coriolis axis60. The orientation of flexures62allows inner frame52to move along dither axis58relative to outer frame54, substantially inhibits relative motion of the frames along Coriolis axis60, but allows inner frame52and outer frame54to move together along Coriolis axis60. Accordingly, for both inner frame52and the outer frame54, control over allowable and inhibited directions of movement is achieved by orienting the axes of the flexures along the inhibited axis.

To summarize these allowed and inhibited movements, inner frame52:

(a) can move relative to outer frame54along the dither axis independent of the movement of outer frame54; and

(b) cannot move along the Coriolis axis relative to outer frame54, but can move along the Coriolis axis with outer frame54;

while outer frame54:

(a) cannot move along the dither axis; and

(b) can move along the Coriolis axis but only by moving inner frame52along with it.

Anchors53for flexures56are preferably located within the space between inner frame52and outer frame54. Referring toFIG. 9, this structure is useful because a suspended structure, such as outer frame54, can tend to have a bowed shape with a high point in the center and low points at the ends. It is desirable for the anchors to be located at the vertical position of the center of gravity57so that the extremes of bowing do not unmesh the fingers and so that any wobbling induced by the center of gravity being vertically displaced from the center of suspension is minimized.

Referring again toFIG. 2, inner frame52is shaped generally as a rectangular annulus with a central rectangular aperture64. Drive fingers66and sensing fingers67extend inwardly from inner frame52into aperture64along parallel axes that are parallel to dither axis58. Positioning drive fingers66in the aperture as shown helps to maximize the outer perimeter and area of inner frame52, thus allowing for a larger numbers of drive fingers, thereby improving the response to the dither signal. Drive fingers66interdigitate with fixed dither drive fingers68, while sensing fingers67interdigitate with fixed sensing fingers69. Fixed fingers68and69are anchors to and fixed relative to underlying substrate55, while fingers66and67move with inner frame52and thus are movable relative to substrate55.

A drive signal is provided from a dither drive mechanism (not shown) that includes an AC voltage source coupled to fixed dither drive fingers68to cause inner frame52to be dithered relative to outer frame54along dither axis58at a velocity such as x′=wX cos(wt) as noted above; more preferably, the dither is caused by a square wave. The dithering motion is sensed by the change in capacitance between movable sensing fingers67and fixed sensing fingers69. This sensed motion is amplified and fed back to the dither drive mechanism to sustain the dithering motion at the resonant frequency of the inner frame.

When there is no rotational velocity R about sensitive axis64, outer frame54does not move relative to substrate55. When there is a rotational velocity R, inner frame52will tend to move along Coriolis axis60with an acceleration y″=2Rx′, which is 2(R/w)x″(cos(wt)/sin(wt)), because x″=w2x sin(wt). Flexures56allow the inner frame52to move with outer frame54along the Coriolis axis. Note that the ratio of y″ to x″ is 2R/w, which is actually modified by m/M, where m is the mass of the inner frame, and M is the total mass of both frames. Assuming m/M=1/2, R=1 rad/sec, and w=2π×104rad/sec, the ratio of the magnitudes of y″ to x″ is about 16 ppm.

To sense the movement along Coriolis axis60, outer frame54has fingers70that extend along axes parallel to the dither axis and interdigitate with fixed fingers72,74on either side of fingers70(fingers72,74are only shown on one side). Fingers72,74are fixed with anchors73to substrate55. Fingers72are electrically connected together to a first fixed DC voltage source with a voltage V1, and fingers74are connected together to a second fixed DC voltage source with a voltage V2. As fingers70of outer frame54move toward one or the other of fingers72or74, the voltage on outer frame54changes. By sensing the voltage on outer frame54, the size and direction of movement can therefore be determined.

If desired, a carrier signal with a frequency much larger than the dither frequency can be applied to fixed fingers72,74, and the resulting output is then amplified and demodulated. Such sensing techniques are known in the field of linear accelerometers. A carrier signal is not necessary with the structure of the present invention, however, because this structure substantially eliminates the interfering quadrature signal, and thus the added complexity in the circuitry is undesirable if avoidable.

Referring toFIG. 3, in another embodiment of the present invention, one large gyro150includes four substantially identical gyros152a-152darranged in a rectangular configuration and shown here in a simplified form. Gyros152a-152dhave respective inner frames154a-154d, outer frames156a-156d, dither drive structures158a-158dand159a-159don opposite sides of the inner frames, dither sensing structures160a-160dand161a-161d, and fixed fingers162a-162dand164a-164dfor sensing motion along the Coriolis axes. These gyros are connected in a “cross-quad” manner as shown. With this interconnection, fixed fingers162aand162care electrically connected together and to fixed fingers164band164d; fixed fingers162band162dare electrically coupled together and to fixed fingers164aand164c; dither drive structures158a,158c,159b, and159dare electrically connected together; dither drive structures158b,158d,159a, and159care electrically connected together; dither sensing structures160a,160c,161b, and161dare electrically connected together; and dither sensing structures160b,160d,161a, and161care electrically connected together.

Such a cross-quad connection eliminates errors due to manufacturing and temperative gradients and also eliminates sensitivity to external linear acceleration. Such a connection is also described in Patent Publication No. WO 96/39615, which is expressly incorporated by reference for all purposes.

FIG. 4is a detailed view of a little more than one-half of one gyro80; the other half of gyro80is substantially the same as the half that is shown. As in the embodiment ofFIG. 2, each gyro has an inner frame82and an outer frame88. Flexures90extend along axes parallel to a Coriolis axis86from inner frame82to outer frame88. With these flexures, inner frame82can move along a dither axis84relative to an outer frame88, but is substantially inhibited from moving along Coriolis axis86relative to outer frame88. Outer frame88is movable along Coriolis axis86along with inner frame82.

The structures have a number of larger openings146in the inner and outer frames resulting from the removal of pedestals made of photoresist and later etched away as part of the manufacturing process. Smaller holes148are formed in the structures so that a solvent can be introduced to etch out a sacrificial oxide layer. Such processing techniques for surface micromachined accelerometers are generally known and are described, for example, in U.S. Pat. No. 5,326,726, which is expressly incorporated by reference for all purposes.

Inner frame82is roughly shaped as a rectangular ring with two relatively long sides92(one of which is shown) and two relatively short sides94. Extending along the interior aperture surrounded by frame82are two elongated cross-pieces96(one of which is shown) integrally formed with the outer ring of inner frame82and extending parallel to relatively long sides92from one relatively short side to the other. Inner frame82thus has three elongated apertures98(one and a half of which are shown), rather than the one shown in the embodiment of FIG.2. With these multiple apertures, there can be additional rows (six in this case) of movable dither fingers and fixed dither fingers instead of two, thus increasing response and consistency.

Extending into apertures98from both long sides92and from elongated cross-pieces96are drive fingers100and sensing fingers102extending in parallel and along axes parallel to dither axis84. Drive fingers100and sensing fingers102interdigitate with fixed drive fingers104and with fixed dither sensing fingers106, respectively. Fixed drive fingers104are driven with an AC signal to cause drive fingers100, and hence inner frame82, to move along dither axis84. If the AC signal is sinusoidal, the inner frame moves with a displacement x=X sin(wt), and therefore with a velocity of x′=wX cos(wt), with angular frequency w=2πfres(resonant frequency frescan be different for different types of structures, but is typically in the 10-25 KHz range).

Fixed dither sensing fingers106interdigitate with fingers102, and the change in capacitance between these fingers is sensed to monitor the dither motion and to provide a feedback signal to the dither drive to maintain the dither motion at the desired angular frequency w. Fixed sensing fingers106are anchored to substrate98with anchors130and are electrically coupled together with conductive lines132formed on substrate98. Fixed dither drive fingers104are anchored to substrate98with anchors134and are electrically coupled away from the gyro with conductive lines136.

Along relatively short sides92between inner frame82and outer frame88are two stationary members110anchored to and fixed relative to substrate98. Flexures114extend from outer frame88to stationary members110along axes parallel to dither axis84, and therefore substantially prevent outer frame88from moving along dither axis84. Stationary members110are anchored to substrate98with anchors109that are located at the midpoint of stationary members110. This location minimizes stress because any shrinkage that occurs in stationary members110and flexures114during manufacturing is similar to that in outer frame88. Therefore, there is no residual stress in the dither direction in flexures114.

Stationary members110are very useful because they provide for flexures114to have the correct length, provide attachment points for flexures114that are far from a center line of the device, thereby stabilizing outer frame86against tilting, and provide freedom from shrinkage along the lengthwise direction of flexures114.

Extending outwardly away from outer frame88along axes parallel to dither axis84is a large number of fingers111, each of which is disposed between two fixed sensing fingers112,113to form a capacitive cell. The large number of cells together form a differential capacitor. Fixed sensing fingers112,113are anchored at their ends and are electrically connected to other respective fingers112,113and to a different DC voltage as noted in connection with FIG.2.

This assembly of sensing fingers on a suspended frame essentially forms a sensitive accelerometer of the type disclosed in the incorporated patent publication, with its function being to sense the Coriolis acceleration. The accelerometer is also sensitive to externally applied accelerations, but two of the gyros in the cross-quad arrangement are sensitive in the opposite sense to the other two, thereby canceling such external interference.

In one exemplary embodiment, inner frame82and outer frame88are each at 12 volts DC, while fixed fingers112,113are all at 0 volts DC. As outer body88and its fingers111move, a change in voltage is induced on fingers112,113. A high frequency carrier signal can be provided to the fixed sensing fingers, but with the accuracy of the gyro according to the present invention, the carrier is not needed, and thus the required circuitry is minimized by avoiding the need for a high frequency demodulator.

If there is rotation about a sensitive axis130(which is mutually orthogonal to both dither axis84and Coriolis axis86), outer frame86and inner frame82move together along Coriolis axis86in response to the rotation. If there is no such rotation about sensitive axis130, the dither motion of inner frame82causes substantially no motion by outer frame88along Coriolis axis86.

The decoupling of motion along the dither axis and sensitive axis has significant beneficial effects. Imbalances in the flexures produce very little dither motion along the sensitive axis. In this case, the interfering quadrature signal can be reduced to as low as 0.5 parts per million (ppm) or 0.00005%; this small quadrature signal results from the same types of mechanical imbalances that otherwise could produce a 10% interference signal in a gyro of the type generally shown in FIG.1. Moreover, the rotationally induced acceleration that the gyro is designed to sense is inhibited very little. Because of this accuracy, the circuitry need not be particularly complex.

Another benefit from this structure arises in the packaging. The increase in the signal from the large number of fingers due to the apertures, and from the four gyros in the cross-quad arrangement, eliminates the need to enhance the signal by reducing air damping, and thus makes ambient packaging possible, rather than more costly vacuum packaging.

Referring also toFIGS. 5 and 6, another aspect of the present invention is illustrated. Along much of the row of fixed driveFIGS. 104are conductive members126at voltage V, preferably the same DC voltage as inner frame82and outer frame88(i.e., 12 volts). Meanwhile, the drive fingers are preferably driven with a square wave with an amplitude of 12 volts. At several other locations along the row of fixed dither drive fingers104, conductive members120are formed on substrate98under groups of fingers and are electrically coupled to drive fingers104. As shown inFIG. 4, conductive members120have a length coextensive with the length of fingers104and a width that extends across five fingers104, while conductive members126extend across the inner frame with a width that extends along fourteen fingers104.

Referring toFIG. 5, where movable drive fingers100and fixed drive fingers104are formed over conductive members126, there will be a net upward force on movable drive fingers100due to fringe effects from adjacent fingers104, and thus fingers100will have a tendency to levitate. Conductive members126are used and kept at 12 volts to prevent static collapse and makes the stray capacitance well-defined.

As shown inFIG. 6, however, where conductive members120are formed under drive fingers100and fixed dither fingers104, an attraction by movable drive fingers100toward substrate98causes a net downward force that should counteract the net upward force shown in FIG.5. The downward force due conductive members120is greater per finger100than the net upward force shown inFIG. 5per finger100because conductive members120are formed under fewer fingers. By positioning anti-levitating conductive members120periodically along the length, levitation is prevented.

Referring toFIGS. 4 and 7, another aspect of the present invention is illustrated. Gyro80inFIG. 4has four stop members140(two of which are shown) positioned relative to substrate98and to inner frame82to prevent excessive movement in any direction. Stop members140have a first portion144that is substantially coplanar with inner frame82, and a hook portion146that extends over frame82. Stop member140is connected with an anchor142to substrate98. Frame82is substantially inhibited from movement both into the stop member in the plane of frame82due to coplanar portion144, and also is inhibited from moving too far upwardly due to hook portion146.

As described in incorporated U.S. Pat. No. 5,326,726, to produce suspended inner frame82, a layer of polysilicon is formed over a sacrificial oxide. When the oxide is removed (etched), a suspended polysilicon structure is left behind. To form stop members140, a further oxide layer is formed over inner frame82, and then a material for forming stop members140is formed over that further oxide at locations147. Etching out this further oxide leaves behind stop members140. The material used for stop members140preferably is one that minimizes the risk of inner frame82contacting and sticking to stop member140(a problem referred to as “stiction”). The preferred material is titanium tungsten (TiW) because this material has low stiction, compatibility with electronics processing, good conductivity, and high mechanical strength. An appropriately coated silicon could also be used.

Referring toFIG. 8, a circuit is shown for use with gyros such as those shown inFIGS. 2 and 4. InFIG. 8, a gyro body200includes both a first body and a second body interconnected to decouple the dither motion from the Coriolis motion. Body200is maintained at an elevated voltage relative to sensing plates204, and is driven with a signal from dither drive plates202to create a dither motion that is sensed by dither sensing plates204. The motion along a Coriolis axis is sensed by Coriolis plates206. This circuitry would be considered rather simple in that it has a relaxed phase specification, and is made possible by the design of the body that substantially eliminates the quadrature signal.

Capacitive sensing plates204are coupled to inputs of an amplifier210that provides two outputs211, each of which is coupled to inputs of amplifier210through a feedback impedance network Z1, Z2that is primarily resistive. The output of amplified210is provided to a second amplifier212that provides two outputs along two paths. The first paths214,216provide the feedback signal to dither drive plates202to help keep the body200dithering at the resonant frequency. The other two paths218,220from amplifier212are provided to a two pole, double throw analog switch222that serves as a synchronous rectifier. Switch222also receives two inputs from the output of an amplifier224that receives inputs from Coriolis sensing plates206. Amplifier224has feedback networks Z3and Z4that are primarily capacitive. The signals from amplifier212alternate the polarity of the Coriolis signals from amplifier224, thereby phase demodulating the Coriolis signals. The output from switch222is provided to a buffering low pass filter230.

FIG. 10illustrates a simplified plan view of a gyro250according to another embodiment of the present invention. Gyro250has an outer frame252and an inner frame254. A dither drive mechanism256can be positioned to apply a dithering motion to outer frame252through fingers260extending from outer frame252parallel to a dither axis262. Inner frame254is coupled to outer frame252through flexures270oriented in parallel to a Coriolis axis272that is perpendicular to dither axis262. Elongated stationary members268extend along dither axis262and are centrally anchored to the underlying substrate264through anchors269. Flexures266extend from each end of each anchored stationary member268in a direction parallel to dither axis262. Flexures266thus prevent outer frame252from moving along dither axis262, while flexures270allow outer frame252and inner frame254to move together along Coriolis axis272. As noted above, stationary members268control stress and tilt and help keep the flexures at their appropriate length.

In response to rotation about a sensitive axis276(which is mutually orthogonal to axes262and272), outer frame252and inner frame254move along Coriolis axis272. Inner frame has sensing fingers278extending inwardly into an aperture280, each located between two fixed fingers282such that fingers278and fingers282form a differential capacitor with a number of individual cells. The voltage on inner frame254can be sensed to determine the change in motion, which, as noted above, indicates the rotational velocity about axis276. As inFIG. 3, four gyros of the type shown inFIG. 10can be connected together in a cross-quad manner. Moreover, other features discussed above, such as the stop members, positioning of anchors, and conductive members on the substrate can be employed with this embodiment of FIG.10.

Referring toFIG. 11, another improvement is illustrated. In the situation in which a number of movable fingers are between two sets of fixed fingers to make up capacitive cells, one or both of the fixed fingers can be arranged to extend across two gyros or two sets of fingers to reduce space and reduce processing. As shown in simplifiedFIG. 11, movable masses280and282are each movable along the direction of arrows284and286. Each of these masses has respective fingers288and290that move with the respective mass. Fingers288and290are between two stationary fingers, including first fingers292and second fingers294. As shown here, fixed fingers292are formed in substantially straight lines to form one electrode of the differential capacitor with movable finger288, and also to form one electrode of a differential capacitor with a movable finger290. Fixed fingers292are formed with a dog-leg configuration so that they extend from one side of each movable finger288to another side of each movable finger290(with the sides being in reference to the direction indicated by arrows284and286). With this arrangement, fewer separate fingers need to be manufactured, and fewer connections need to be made to the stationary fingers. Electrical contact points296,298to fingers292and294, respectively, are offset along a direction perpendicular to the direction of arrows284and286so that contacts can be made in a straight line with conductors on the surface of the substrate and anchors at the contact points to the conductors on the substrate.

The arrangement shown inFIG. 11can be used when there are multiple adjacent gyros, such as in the situation illustrated byFIG. 3, and as the connections would be made inFIG. 4with multiple gyros. Indeed, inFIG. 4, the connections to fixed fingers112and113are arranged in such a staggered fashion, but there is no gyro shown to the side of the gyro in FIG.4. The arrangement of fixed fingers as shown inFIG. 11could be used in the aperture region of the inner frame in FIG.10. By arranging the fixed fingers in this manner, processing is reduced as the number of fingers to be formed is reduced, and also space can be made for additional cells, thereby increasing the signal that is received and improving accuracy.

Referring toFIG. 12, in another embodiment of the present invention, a portion of a gyro300is shown. As shown inFIGS. 4 and 10, gyro300has an inner frame302, an outer frame304, anchored stationary beams306between the inner and outer frames and flexures308for preventing the outer frame from moving along a dither axis310, which is parallel to the elongated direction of flexures308. Inner frame302is dithered along dither axis310relative to outer frame304, which is inhibited from moving along dither axis310.

In the embodiment ofFIG. 4, flexures90were oriented perpendicular to the dither axis for allowing movement along the dither axis. With such a structure, these flexures are under a high tensile force and have a tendency to stretch. If significant enough, such stresses could start to buckle the frame and/or could change the resonant frequency of the system.

Referring toFIG. 12, the connection between inner frame302and outer frame304is made through a connection structure that includes pivoting beams312and314, that are connected to inner frame302through flexures316and318, and to outer frame304with flexures320and322. Pivoting beams312and314are connected together with a small cross-piece324.

Referring toFIG. 13, a close-up and simplified view of the connection structure is shown. As inner frame302is dithered along dither axis310, inner frame302moves as indicated by arrows330, causing perpendicular stresses along flexures316and318in the direction indicated by arrows332. Because pivoting beams312and314are connected to flexures320and322, each of which is oriented along a direction parallel to arrows332, the intersection of beam312and flexure320and the intersection of beam314and flexure322form pivot points336and338, respectively, causing beams312and314to move along the direction indicated by arrows340and342, respectively. This movement of the pivoting beams causes a small movement by cross-piece324along the direction indicated by arrow344, which is parallel to dither axis310.

This structure encourages movement of beams312and314in an opposite rotational direction while discouraging simultaneous rotation in the same direction; i.e., the structure allows anti-phase movement, and substantially inhibits in-phase movement. If the pivoting beams were to try to rotate in the same direction at the same time, the cross-piece would need to lengthen and would undergo a complex twisting motion. Consequently, this structure helps to prevent such movement. The pivoting mechanism thereby prevents the unwanted motion of dither frame302perpendicularly to the preferred dither axis, i.e., from producing a motion which interferes with the Coriolis signals. By alleviating the tensile forces in flexures316,318, frame302can move more freely along dither axis310and produce a larger signal. The alleviation of these tensile forces also prevents distortions of the accelerometer frame by the dither motion, while such distortions could otherwise produce interfering signals if unchecked.

In the embodiment ofFIGS. 12 and 13, outer frame308is shown with a linear inner edge350that faces the connection structure and inner frame302. As an alternative, a portion of outer frame308may be recessed relative to edge350for connection to flexures320and322. Regardless of the recess, it is desirable for cross-piece324to be in a line with flexures320and322.

FIGS. 12 and 13each show flexures316and318extending to a corner of beams312and314, effectively forming a linear and continuous edge with beams312and314. To create more space between beams312,314and inner frame302when beams312,314pivot, it can be desirable to shave off portions of the edges of beams312and314facing inner frame302, particularly at the corner most remote from respective pivot point336and338.

Referring again toFIG. 12, there is a difference in the arrangement of the apertures and fingers relative to the embodiment of FIG.4. As shown inFIG. 4, there are three elongated apertures with fingers, and each of the apertures has some drive fingers and some pickoff fingers.FIG. 12, by contrast, shows one aperture out of five, and that aperture has only drive fingers connected together. In this embodiment ofFIG. 12, there are five apertures, the middle of which is used only for pickoff fingers and not driving fingers, while the other four apertures have only driving fingers and not pickoff fingers.

Another difference with respect to the embodiment ofFIG. 4is that in the embodiment ofFIG. 12, the connectors that are used to drive and pick off combs are made from polysilicon formed on the surface of the substrate, rather than diffused n+ connectors. With the polysilicon on the surface, the fingers can be made more accurately, thus allowing more fingers in the same space and therefore more force per unit area.

Another embodiment of the present invention is illustrated inFIGS. 14 and 15. A gyro400has an inner frame402surrounded by an outer frame404. Inner frame402is dithered along a dither axis410through the use of a dither drive mechanism406. As described in the embodiments above, dither drive mechanism406is preferably formed with combs of drive fingers that interdigitate with fingers on inner frame402and are driven with voltage signals to produce the sinusoidal motion. In the embodiment ofFIG. 14, inner frame402has four elongated and parallel apertures that include the drive fingers.

In the four corners of inner frame402are apertures408that have dither pick-off fingers for sensing the dithering motion. As discussed in embodiments above, this sensed dithering motion is fed back to the dither drive mechanism that drives inner frame402along dither axis410.

In response to an angular velocity about a central sensitive axis412, outer frame404is caused to move along a Coriolis axis414. As described above, inner frame402can be dithered relative to outer frame404, while inner frame402is coupled to outer frame404so that inner frame402and outer frame404move together along Coriolis axis414. In this embodiment, the coupling between inner frame402and outer frame404, and the anchoring of outer frame404to the substrate are designed to improve performance and to reduce the interfering quadrature signal to produce a very high performance gyro.

The couplings are shown in more detail inFIG. 15, which shows one-quarter of gyro400. The other three quarters of the gyro are substantially identical to the portion shown. A dither flexure mechanism430is coupled between inner frame402and outer frame404to allow inner frame402to move along dither axis410, but to prevent inner frame402from moving along Coriolis axis414relative to outer frame404, but rather to move along Coriolis axis414only with outer frame404.

FIG. 15shows half of one dither flexure mechanism430, which has a dither lever arm432connected to outer frame404through a dither main flexure434, and connected to inner frame402through pivot flexures436and438. Identical components would be on the other side of dashed line442connected through a small central beam440to lever arm432. Similar to the embodiment ofFIG. 12, central beam440encourages lever arm432and the corresponding lever arm connected on the other side of beam440to move in the same direction along dither axis410. At the other end of lever arm432, flexures436and438extend toward inner frame402at right angles to each other to create a pivot point near the junction of flexures436and438.

This coupling and connection mechanism has a number of advantages over other structures recited herein. Because the length of the lever arm from the pivot point to the small central flexure is long relative to the total length of the inner frame, the ratio of stiffness of the mechanism for perpendicular motion of the dither mass and for relief of tension in the main dither flexure is increased. For a given residual tension, there is good resistance to perpendicular motion, or for a given resistance, the perpendicular motion creates less distortion in the accelerometer frame compared to the embodiments of FIG.12. Flexures436and438can be made long, thereby reducing tension for a given dither displacement. The flexures436and438are connected to inner frame402at points nearer to the center of the inner frame in the length and width directions, the distortion for a given amount of tension is reduced relative to other embodiments. Because the two pivoting flexures are perpendicular to each other, the pivot point is better stabilized than in other structures. By moving the effective attachment point of the dither mechanism to the inner frame toward the outside of the inner frame, there is better stability in the inner frame against tilting. To keep lever arm432stiff compared to central beam440, lever arm432is made wide; while this greater width does improve stiffness, it has the drawback in requiring additional space.

Compared to the embodiment ofFIG. 12, outer frame404is made stiffer by increasing its width. As indicated above, however, the ratio of signals is modified by m/M, where M is the total mass of both frames, and m is the mass of the inner frame. Consequently, it is desirable to reduce the mass of the outer frame, so that M is as small as possible relative to m. Consequently, a number of holes444are cut out of outer frame404. While the existence of holes444reduces the mass, they do not have any substantial effect on the stiffness because they create, in effect, a number of connected I-beams. This increase in stiffness and performance does come at the price of increased size of the device, however, thereby lowering yield on the wafer level.

To further improve the performance of device400compared to prior embodiments, outer frame404is coupled and anchored to the substrate through a connection mechanism450and a pair of anchors452that are connected together. Connection mechanism450includes plates453and454connected together with short flexures456and458, which are perpendicular to each other.

The structural polysilicon used to make the masses and flexures should be somewhat tensile in comparison with the substrate so that the structures fabricated from the polysilicon have a well-defined and singular form. A consequence of this is that the accelerometer flexures are slightly bent as manufactured. If these flexures are imbalanced in terms of stiffness, a slight static tilt can be introduced into the overall structure with consequences similar to a tilt of the normal mode. The forces from the dither motion can differentially straighten the flexures, thereby providing an additional dynamic tilt around the gyro axis. Moreover, the frame bows in response to the tension in the dither flexures, thereby giving a similar effect as does differential stretching of the flexures from the reaction forces which tilt the structure in the plane of the substrate.

In the embodiment ofFIG. 15, the pivot points are defined by flexures456and458so that outer frame404can easily move perpendicular to the dither motion by pivoting plate453relative to plate454thereby giving a single bending action to flexures456and458at the ends and in the center. To accomplish this, center beam440should be co-linear with the pivot points.

Having described embodiments of the present invention, it should be apparent that modifications can be made without departing from the scope of the invention as defined by the appended claims. While a cross-quad arrangement with four gyros has a number of benefits described above, with highly accurate processing, one may only use two to eliminate common mode external accelerations. In yet another alternative, a larger array with more than four gyros could be arranged and coupled together.