Abstract:
A micromechanical tuning fork gyroscope has an input axis out of the plane of the structure. In one embodiment, capacitor plates are provided in parallel strips beneath two apertured, planar proof masses suspended from a substrate by a support structure. The proof masses are paired and set in opposed vibrational motion by an electrostatic comb drive. In response to an input angular rate about the out-of-plane input axis, the proof masses translate with respect to the striped capacitors, thereby varying the capacitance between the capacitor strips and the proof masses as a function of the input rate. In another embodiment, proof mass combs of a comb drive are meshed between fixed drive combs which are electrically excited in pairs 180° out of phase. As the proof masses translate in response to an angular input, the distance between the proof mass combs and the fixed combs varies, thereby varying the capacitance between the combs resulting in an unbalanced voltage on the proof masses that is detected as an indication of input rate. The out-of-plane tuning fork gyroscope can be combined with two in-plane tuning fork gyroscopes to provide a complete three-axis inertial measurement unit from a single wafer or on a single chip.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to microfabricated tuning fork gyroscopes. 
     Microfabrication enables mechanical gyroscopes and other devices to be constructed using batch fabrication techniques known for fabricating solid state electronics. These techniques result in instruments of smaller size, lower cost, and greater reliability than those constructed by conventional techniques. 
     Micromechanical tuning fork structures are known for use as inertial rate sensors. Known tuning fork structures typically sense angular rate along an axis in-plane with a major planar surface of a substrate on or in which the device is constructed. One such device is an in-plane tuning fork gyroscope, which employs meshing drive and driven finger electrodes or combs associated with two vibrating tuning fork elements or proof masses. 
     The fabrication of such known devices is fairly straightforward, involving photolithographic and other semiconductor fabrication techniques. For damping and cross-coupling reasons, the plates of such known devices may be made with holes or apertures. Some fabrication sequences such as polysilicon and bulk silicon require the holes to enhance under cut etching. However, such devices are known and configured to sense only angular rates imposed in the plane of the major planar surface of the proof mass(es), and not for sensing angular rate about an axis perpendicular to the major plane of the substrate. Mechanical fixturing and wire bonding to sense angular rate about an axis perpendicular to the major plane of the substrate, and/or to realize a three axis system from known in-plane tuning fork gyroscope configurations is expensive and cumbersome. 
     Other relatively sophisticated micromechanical configurations are known for sensing out-of-plane angular rates. U.S. Pat. No. 5,016,072 to Greiff describes a double gimbal gyroscope structure which senses out-of-plane angular rates. However, the processing required to achieve such a double gimbal structure is not compatible with the processing required to achieve the referenced in-plane structures. Thus, realization of a three axis inertial measurement unit on a single chip would be difficult and perhaps commercially impracticable. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention defines the structure for a microfabricated out-of-plane tuning fork gyroscope which senses angular rate about an axis perpendicular to a major plane of the substrate upon which the device is constructed. The out-of-plane tuning fork gyroscope is fabricated by processing similar to and compatible with that of the referenced in-plane tuning fork gyroscope, making construction of a three axis angular rate sensor on a single chip viable. 
     In a first embodiment, the out-of-plane tuning fork gyroscope incorporates a striped capacitor readout having two apertured proof masses and electrodes in the form of first and second sets of strips deposited on a substrate below (or above) the apertures in the proof masses. A comb drive causes each proof mass to vibrate in the major plane, typically in opposition. The vibrating proof masses are compliant in an axis parallel to the major plane of the substrate but different from the vibration axis, and translate along the axis in response to an angular rate or acceleration input about an axis normal to the substrate. As the proof masses translate, the apertures cover the electrode strips to varying relative degrees, so that the capacitance between the proof masses and each set of electrodes increases and decreases in proportion to the differential axial position of the proof masses and hence to the input angular rate. 
     In a further embodiment, the meshing finger electrodes of the comb drive are used for both drive and angular rate sensing. Fixed combs are arranged in electrically isolated pairs 180° out of phase. As the proof masses translate in response to an out-of-plane angular rate input, the distance between the combs on the proof masses and the fixed combs varies, varying the capacitance. The combs can be driven with a voltage at the drive axis resonance frequency to provide both drive and sense operation. 
     The sensitivity of the out-of-plane tuning fork gyroscope approaches that of the in-plane tuning fork gyroscope for a given proof mass size and separation of resonant frequencies. 
     In a further embodiment, the out-of-plane tuning fork gyroscope incorporates a center motor that is split into two halves for common mode rejection of electrical coupling, which can cause gyroscope errors. 
     The microfabrication process of the out-of-plane tuning fork gyroscope is compatible with that of the in-plane tuning fork gyroscope, so that both types of devices can be made on the same silicon wafer or even the same chip. Thus, a complete inertial measurement unit, having three axes of rate and three axes of acceleration, can be built on a single silicon substrate. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The above and other features of the present invention are more fully set forth below in the detailed description of illustrative embodiments, and the accompanying drawing of which: 
     FIG. 1 is a schematic diagram of a prior art in-plane tuning fork gyroscope; 
     FIG. 2 is a schematic diagram of an out-of-plane tuning fork gyroscope according to the present invention; 
     FIG. 3 is a schematic diagram of part of a striped capacitor readout in the tuning fork gyroscope of FIG. 2; 
     FIGS. 4-6 are schematic diagrams of alternative capacitive readouts for an out-of-plane tuning fork gyroscope according to the present invention; 
     FIGS. 7-10 are schematic diagrams of further embodiments of an out-of-plane tuning fork gyroscope according to the present invention incorporating alternative capacitive readouts of FIGS. 4-6; 
     FIGS. 11-14 are schematic diagrams of further embodiments of an out-of-plane tuning fork gyroscope according to the present invention incorporating alternative suspension configurations; and 
     FIG. 15 is a schematic view of a three-axis inertial measurement unit incorporating two in-plane tuning fork gyroscopes and an out-of-plane tuning fork gyroscope according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As illustrated in FIG. 1, a prior-art in-plane tuning fork gyroscope includes vibrating elements in the form of proof masses  10  and combs  12 ,  14 . The proof masses  10  may have holes or apertures  40  therein, only some of which are shown in FIG. 1 for clarity. The proof masses  10  and combs  12 ,  14  are supported on an assembly including supporting members or beams  16 , flexures  18  connecting the proof masses  10  to the supporting members  16 , and flexures  20  that connect the supporting members  16  to a substrate  22  at anchor points  24 . The proof masses  10 , supporting members  16 , and flexures  18  and  20  are indicated as Suspended Material  2  on FIG. 1, and may be formed from metal, doped silicon, silicon, or polysilicon in the illustrated embodiment. 
     The outer combs  14  are excited with a DC bias and an AC drive signals via contact pads  26  to cause the proof masses  10  to vibrate in opposition along a drive axis  30 . Optionally, the outer combs  14  are driven at half the mechanical resonant frequency with no bias, or at two different frequencies. 
     The inner combs  12  are used to sense the vibration motion of the proof masses for use in a self-drive oscillator circuit, not shown in FIG.  1 . 
     For the in-plane tuning fork gyroscope of FIG. 1, an angular rate in the plane of the substrate  22  along an input axis  32  causes Coriolis forces which move one proof mass  10  up and the other down along an output motion axis that is normal to the substrate  22 . The motion of each proof mass  10  causes a change in the capacitance between the proof mass  10  and a corresponding aligned electrode plate  34 L,  34 R on the substrate  22 . The plates  34 L,  34 R are driven by AC sense signals used to detect the changing capacitance. For example, the right plate  34 R may be excited with 100 Khz, phase angle 0°, while the left plate  34 L is excited with 100 kHz, phase angle 180°. Other frequencies and DC can be used also. The differential AC current from the proof masses  10  at the output node  36  is proportional to the input angular rate. 
     The configuration of the suspension in the tuning fork gyroscope of FIG. 1, that is, the thickness, length, and width of the suspension members  16 ,  18 , and  20 , can be selected to achieve a desired in-plane sensitivity appropriate for intended uses of the device. 
     In FIG. 2, the elements of an out-of-plane tuning fork gyroscope that correspond to similar elements of the in-plane gyroscope of FIG. 1 are referred to using the same reference numbers. The gyroscope of FIG. 2 employs a striped capacitor readout in place of the plates  34 L,  34 R. The striped capacitor readout includes electrodes formed in paired strips  42 ,  43  on the substrate  22 . The strips  42 ,  43  are formed parallel to the drive axis  30  below the proof masses  10 . The pitch, or distance between corresponding points, of the apertures  40  along a proof mass motion axis  44  (discussed below) is substantially the same as that of the pairs of strips  42 ,  43 . To maximize the sensitivity, the edges of the apertures  40  lie over the conductive strips  42 ,  43 , as is shown more particularly in FIG.  3 . The strips  42 ,  43  may be formed by metallization on the substrate surface or by diffusion regions in the substrate. 
     The structure shown in FIG. 2 is compliant along a Z axis  44  parallel to the substrate  22 . As in the prior-art tuning fork gyroscope of FIG. 1, the thickness, length, and width of the suspension members  16 ,  18  and  20  can be selected to achieve a desired out-of-plane sensitivity. An angular rate about an input axis  38  orthogonal to the substrate causes one proof mass to translate along +Z and the other along −Z. This axial motion causes changes in the capacitance between the capacitor strips  42 ,  43  and the proof masses  10  as the apertures  40  cover the strips  42 ,  43  to varying relative degrees. One set of capacitor plates  42  is excited with, for example, a DC voltage and frequency of 50 to 500 kHz at 0° phase angle, and the other set of capacitor strips  43  at 180° phase angle. Other frequencies can be used also. Also, the set of capacitor strips  42 ,  43  beneath one proof mass  10  are oppositely excited from the set of capacitor plates  42 ,  43  beneath the other proof mass  10 . Accordingly, the current sensed from the output node  36  is proportional to the differential axial position of the two proof masses  10  and, hence, to the input angular rate. The greater the number of capacitor strips  42 ,  43 , the greater the sensitivity to the input angular rate. 
     A portion of the strips  42 ,  43  can be dedicated to torque rebalancing if desired. The torque rebalancing may be accomplished as taught in the prior art. 
     The sensitivity of the striped capacitor out-of-plane tuning fork gyroscope can range from 30 to 100 percent of the sensitivity of the in-plane tuning fork gyroscope for a given proof mass size and separation of resonant frequencies. Neglecting fringing fields, the capacitance between parallel, rectangular plates is described by:              C   =         ɛ                 Lw     h        1             (   1   )                                
     where C=capacitance; 
     ε=dielectric constant; 
     L=length of plates; 
     w=width of plates; and 
     h=gap between plates. 
     When the plates are moved apart, capacitance varies as:                  ∂   C       ∂   y       =       -       ɛ                 L     h            (     w   h     )        2             (   2   )                                
     When the gap is held constant and the motion is parallel to edge w, the change in capacitance with displacement y is:                  ∂   C       ∂   x       =         ɛ                 L     h        3             (   3   )                                
     With the striped pattern, the sensitivity is multiplied by the number of active edges and a factor of {fraction (1/2+L )} to account for the fact that the holes do not cross the proof mass completely and for fringing fields. The number of active edges is 2w/L p  from FIG.  3 . Therefore:                  ∂   C       ∂   x       =         ɛ                 L     h          (     w     L   p       )        4             (   4   )                                
     where L p =center-to-center spacing of holes along w. 
     In gyroscope operation, readout sensitivity is proportional to the change in capacitance and the excitation voltage. The excitation voltage is proportional to the snap down voltage. The snap down voltage is a DC voltage which brings the proof mass into the sense electrodes and is given by:                V   snap     =           8        h   3          k   t         27      ɛ                 A            5             (   5   )                                
     where k t =spring stiffness-translation normal to plane; and 
     A=area of opposing capacitor plates. 
     In a typical tuning fork gyroscope, L p ˜10 μm and h˜3 μm. The opposing area for the striped geometry is 50% that of the normal capacitors of the in-plane tuning fork gyroscope. From Equations (2), (4), and (5), the sensitivity of the striped capacitor readout should be 45% that of the present in-plane tuning fork gyroscope. 
     The widths and lengths of the beams and flexures can be chosen to optimize the axial compliance, so that the resonance of the sense motion approximates that of the tuning fork drive. The resonances of both the sense and drive are independent of the thickness of the proof mass and beams (if all are the same thickness). This independence could make matching of sense and drive axis resonant frequencies easier in the out-of-plane tuning fork gyroscope than in the in-plane tuning fork gyroscope. As device processing has improved, thicker beams and flexures have become possible, so that the sensitivity of the out-of-plane gyroscope can approach or even exceed that of the in-plane gyroscope. 
     In a further embodiment of the present invention, the out-of-plane tuning fork gyroscope incorporates a dual function drive which uses meshing finger electrodes or combs  50 ,  52  for both drive and angular rate sensing, obviating the need for capacitor plates located below the proof masses  10 . This dual-function drive is illustrated schematically in FIG.  4 . Fixed combs  50 ,  52  are arranged in pairs, the combs in each pair being electrically isolated from each other. One comb  50  of each pair is excited with a carrier, for example, at 100 kHz, at 0° phase angle. The other comb  52  of each pair is excited with a carrier at 180° phase. Other frequencies and DC can be used also. As shown, the outer, or 180°, combs  52  are each attached to a long footing  54 . The inner, or 0°, combs  50  are each attached to the substrate  22  at anchors  51  in a space  56  defined between the long footing  54  and two 180° combs  52 . The phase angle of the combs could be reversed if desired, such that the 180° combs form the inner combs. Electrical connection to the inner combs  50  may be accomplished through a conductive lead  58  underneath the outer combs  52 . The lead out of the inner combs may also be parallel to the combs and cross under the long footing  54 . 
     The combs  60  extending from the proof mass  10  lie between each tooth of a pair of the fixed combs  50 ,  52 . An angular rate about the out-of-plane or input axis  38  causes the proof mass  10  to move axially along axis  44  as described above. This axial motion varies the distance and thus the capacitance between the proof mass comb  60  and the fixed combs  50 ,  52 , so that the current flowing through the proof mass output node  36  (shown in FIG. 2) is proportional to the input angular rate. 
     The combs  50 ,  52  can be driven with bias and voltage at the drive axis resonance to realize the dual functions of drive and sense operation. Similarly, by applying a DC bias and 100 kHz, the combs can excite the drive axis motion and detect sense axis displacement. The combs may also be segmented so that some are used only for rate sensing while others are used only for drive or drive sensing. 
     The dual function combs  50 ,  52  of FIG. 4 can also be used in a “differential” mode in which both combs are used to sense displacement along the motion axis  44 . This read out is used with polysilicon depositions with sacrificial etch, which offer sound, small-area anchors. 
     FIGS. 5 and 6 show alternative structures for the sense combs in a gyroscope like that of FIG.  4 . The embodiment of FIG. 5 has gaps  73  and  75  of unequal size between each tooth of a fixed comb  53  affixed to the substrate  22  and the two adjacent teeth of a proof mass comb  60 . Likewise, the embodiment of FIG. 6 has gaps  73 ′ and  75 ′ of unequal size between each fixed tooth  55  and the two adjacent strips of a ladder-like proof mass comb  60 ′. The teeth  55  are connected by a conductive lead  58 . Typically the fixed comb  53  or the fixed teeth  55  are driven by a DC or AC voltage signal. 
     FIG. 7 shows a tuning fork gyroscope using the sense combs of FIG.  4 . Dual combs  50 L,  52 L and  50 R,  52 R are formed within openings  80  and  82  in the proof masses  10 . The combs  60  are formed in the proof masses  10  between the openings  80  and  82 . Combs  50 L and  50 R are attached to the substrate at anchors  51 , and combs  52 L and  52 R are attached to the substrate at anchors  54 . Combs  50 L and  52 L are biased at positive voltages, and combs  50 R and  52 R are biased at negative voltages, or vice versa. 
     As also shown in FIG. 7, the center or inner motor can be split into two parts  90 L and  90 R. The split can be either vertical, as shown, or horizontal (i.e., creating vertically separated motor sections, with each section driving both proof masses  10 ). The combs  92 L,  92 R of the inner motor, which mesh with the combs  12  of the proof masses, sense the motion of the driven proof mass  10 . The inner combs  92 L and  92 R are biased with DC voltages of opposite sign and the same magnitude. A differential, integrating amplifier  93  senses the signal across the split motor parts  90 L and  90 R. As the proof masses  10  are driven parallel to the combs  92 L and  92 R, electrical current flows into and out of the inner combs  92 L and  92 R. The integrating differential amplifier  93  senses the low-impedance voltage signal proportional to the proof masses&#39; positions generated by the electrical current flow. The split, rather than solid, inner combs  92 L,  92 R enable the use of separate biases of opposite polarity that results in an electrical anti-symmetry between motor parts  90 L and  90 R. This anti-symmetry causes any common mode signal which can cause gyroscope errors to be rejected by the differential action of integrating amplifier  93 . 
     FIGS. 8-10 show additional alternative ways of configuring the sense combs. FIGS. 8 and 9 employ the unequal-gap structure of FIG.  5 . In FIG. 8, the sense combs are located entirely within the boundaries of the proof masses  10 , whereas in FIG. 9 additional sense combs are also placed outside the boundaries of the proof masses  10 . FIG. 10 shows a variation of the internal comb-tooth structure of FIG.  6 . 
     An alternative suspension configuration for both the striped capacitor readout gyroscope of FIG.  2  and the dual function comb gyroscope of FIG. 7 is illustrated schematically in FIG.  11 . In the illustrated configuration, two cross beams  70  are fixed to the substrate  22  at anchors  72 . The proof masses  10  are suspended from the beams  70  by flexures  76 . Other forms of suspension are also possible. In addition, the number of leads and bonding pads from the sense combs can vary. For example, although four bonding pads are shown in FIG. 7, a separate bonding pad for each row of sense combs, that is, eight bonding pads, could be used. In another option, all positive voltage combs can be connected to one sense pad; two, three, or four pads could be used for the negative combs. These options provide compensation against quadrature and allow for a continuous guard plane beneath the proof mass. 
     FIGS. 12-14 show alternative suspension configurations for out-of-plane gyroscopes. FIGS. 12 and 13 show one quarter of a complete structure including proof-mass and springs, and FIG. 14 shows a complete structure. In the gyroscope of FIG. 14, the flexures attached to the proof mass  10  are disposed in elongated cutouts, and attached to the proof masses  10  at the innermost region of the cutout. This configuration makes for an area-efficient design. 
     An out-of-plane tuning fork gyroscope can be used as a single sensor or in combination with two in-plane tuning fork gyroscopes as a three-axis inertial measurement unit for automotive, military, medical, and computer game applications. The out-of-plane tuning fork gyroscopes illustrated herein can be made by the same process used for the prior-art in-plane tuning fork gyroscope of FIG.  1 . Thus, a three-axis inertial measurement unit can be constructed from a single wafer or on a single chip, as is shown schematically in FIG.  15 . The devices can be fabricated, for example, according to a dissolved wafer process, various silicon-on-insulator (SOI) processes, or by a surface-micromachining polysilicon process. Fabrication via etching of bulk silicon is also possible. 
     It will be apparent to those skilled in the art that modification to and variation of the above-described methods and apparatus are possible without departing from the inventive concepts disclosed herein. Accordingly, the invention should be viewed as limited solely by the scope and spirit of the appended claims.