Abstract:
Rate sensor having a plurality of generally planar masses, means for driving the masses to oscillate about a drive axis in the planes of the masses, an input axis perpendicular to the drive axis, sense axes perpendicular to the drive axis and the input axis, means mounting the masses for torsional movement about the sense axes in response to Coriolis forces produced by rotation of the masses about the input axis, and means responsive to the torsional movement about the sense axis for monitoring rate of rotation about the input axis.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of Invention  
         [0002]     This invention pertains generally to angular rate sensors or gyroscopes and, more particularly, to a dual mass rate sensor in which momentum is balanced and the drive and sense modes are decoupled.  
         [0003]     2. Related Art  
         [0004]     In rate sensors or gyroscopes, the use of two coupled proof masses which oscillate in an out-of-phase manner in the drive mode is a common way of balancing the drive momentum in order to minimize the transfer of energy to the substrate. In addition, with the masses being driven in an anti-phase manner, the response of the masses to Coriolis forces will also be out of phase, which allows the effects of external vibrations to be cancelled.  
         [0005]     Out-of-phase oscillation in the drive mode is commonly achieved by coupling the two proof masses together with a spring, which has some significant disadvantages. The resulting dynamic system has two resonances—a parasitic in-phase mode and the desired out-of-phase mode, with the in-phase mode always being lower in frequency than the out-of-phase mode. In addition, imperfections in fabrication often result in the masses being of slightly different size, which prevents them from oscillating with equal amplitudes. The unequal oscillations produce an imbalance in momentum and result in undesired transfer of energy to the substrate.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0006]     It is in general an object of the invention to provide a new and improved angular rate sensor or gyroscope.  
         [0007]     Another object of the invention is to provide a rate sensor or gyroscope of the above character which overcomes the limitations and disadvantages of rate sensors of the prior art.  
         [0008]     These and other objects are achieved in accordance with the invention by providing a rate sensor having a plurality of generally planar masses, means for driving the masses to oscillate about a drive axis in the planes of the masses, an input axis perpendicularto the drive axis, sense axes perpendicular to the drive axis and the input axis, means mounting the masses for torsional movement about the sense axes in response to Coriolis forces produced by rotation of the masses about the input axis, and means responsive to the torsional movement about the sense axis for monitoring rate of rotation about the input axis. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a top plan view of one embodiment of a rate sensor incorporating the invention.  
         [0010]      FIG. 2  is a top plan view of another embodiment of a rate sensor incorporating the invention.  
         [0011]      FIGS. 2A and 2B  are enlarged fragmentary top plan views of portions of the rate sensor in the embodiment of  FIG. 2 .  
         [0012]      FIG. 3  is a fragmentary top plan view of another embodiment of a rate sensor incorporating the invention.  
         [0013]      FIGS. 4 and 5  are top plan views of additional embodiments of a rate sensor incorporating the invention.  
         [0014]      FIG. 6  is a fragmentary top plan view of another embodiment of a rate sensor incorporating the invention.  
         [0015]      FIGS. 7-17  are top plan views of additional embodiments of a rate sensor incorporating the invention.  
     
    
     DETAILED DESCRIPTION  
       [0016]     As illustrated in  FIG. 1 , the rate sensor has a pair of generally planar proof masses  11 ,  12  which lie in an x, y reference plane when the device is at rest. The input axis, i.e. the axis about which angular rate of rotation is measured, is the y-axis, and the proof masses are disposed side-by-side along that axis for torsional movement about a pair of drive axes  13 ,  14  which extend in a direction parallel to the x axis. The proof masses are driven in an anti-phase manner for out-of-phase movement through the reference plane as they pivot about the drive axes, with the inner edges  16 ,  17  of the two masses moving together in one direction and the outer edges  18 ,  19  moving together in the opposite direction.  
         [0017]     The proof masses are also mounted for torsional movement about a pair of sense axes  21 ,  22  which extend in the z-direction, perpendicular to the x, y plane and the other axes. Coriolis forces produced by rotation of the sensor about the y-axis cause the masses to rotate about the sense axes, and since the masses are driven in an out-of-phase manner, they rotate about the sense axes in opposite directions, with inner edges  16 ,  17  again moving together in one direction and the outer edges  18 ,  19  moving together in the other.  
         [0018]     The midpoints of the inner or adjacent edges of the two proof masses are connected together by a coupling link  24  which has a large out-of-plane stiffness. Thus, the proof masses are constrained to oscillate about the drive axes precisely out-of-phase and with exactly the same amplitude, thereby assuring that the angular momentum is perfectly balanced. In addition, the lowest mode of the overall system automatically becomes the anti-phase drive mode. This cannot be achieved in any previously known coupled mass system.  
         [0019]     The coupling link also ensures that the sense mode responses of the proof masses are precisely out-of-phase and equal in amplitude, independent of the operating frequency.  
         [0020]     In the embodiment of  FIG. 1 , each of the proof masses is mounted on a pair of gimbals or frames  26  for the torsional movement about the drive and sense axes. The gimbals are pivotally mounted to anchors  28  by narrow flexures  27  which are aligned with drive axes  13 ,  14  and permit torsional movement of the gimbals about the drive axes. The proof masses are suspended from the gimbals by flexures  29  which permit torsional movement of the masses about the sense axes. In the embodiment illustrated, flexures  29  are located at the corners of the masses and extend in a diagonal direction, i.e. at an angle of 45 degrees to the x and y axes.  
         [0021]     A sensing frame  31  surrounds the proof masses in the x, y plane and is suspended from anchors  28  by folded flexures  32  for linear movement in the x-direction. Flexures  32  are flexible in the x-direction but relatively stiff in the y and z directions so as to constrain the frame for movement in the x-direction only.  
         [0022]     Links  33  are connected between the midpoints of the outer edges of the proof masses and the sensing frame for transforming torsional movement of the proof masses about the sense axes into linear movement of the sensing frame along the x-axis. Those links are connected to the frame by folded flexures  34  which are flexible in the y-direction and relatively stiff in the x-direction. They allow out-of-plane deflections of the proof masses in the drive mode while transferring sense mode responses to the sensing frame.  
         [0023]     The position of the sensing frame and hence the torsional response of the proof masses is monitored by capacitors  36 ,  37  with electrode plates  38  which are affixed to the frame and interleaved with stationary plates  39  which are affixed to anchors  41 . The capacitors are positioned on opposite sides of the frame and therefore react differentially to movement of the frame.  
         [0024]     In this embodiment, the gimbals  26  deflect only in the out-of-plane torsional drive direction, whereas the linear springs or flexures  29  that suspend the proof masses from the gimbals allow the proof masses to oscillate only in the in-plane torsional sense mode relative to the gimbals. Thus, the suspension members utilized in the drive and sense modes are independent of each other, which minimizes undesired dynamic coupling between the two modes and suppresses the quadrature error and bias that result from such coupling.  
         [0025]     Furthermore, the folded springs or flexures  32  which suspend the sensing frame  31  maintain it in perfect alignment with the sense direction.  
         [0026]     Since the drive and sense mode responses of the proof masses are torsional, any external linear acceleration results in zero displacement of the proof masses. However since linear sense mode deflection of sensing frame  31  is coupled to the torsional sense mode response of the proof masses, the sensing frame deflects with linear acceleration in the x-direction.  
         [0027]     In the sense mode, however, coupling link  24  and frame  31  deflect in opposite directions, with one being connected to the inner edges and the other being connected to the outer edges of the rotating proof masses. Hence, the net effect of the linear external acceleration can be expressed as 
 
 k   x   x=a   x ( m   F   −m   c ) 
 
         [0028]     By making the mass of the frame (including links  33  and flexures  34 ) equal to the mass of coupling link  24 , the effect of any linear or angular acceleration can be nulled out, and susceptibility of the device to vibration is minimized.  
         [0029]     Similarly, the device can be adapted for simultaneous detection of angular rate and linear acceleration by making the mass of the frame much greater than the mass of the coupling link. In that case, any linear acceleration along the deflection axis of the sensing frame can be detected by low-pass filtering of the output signals before demodulating them to extract the Coriolis signals.  
         [0030]     The drive and sense modes can also be decoupled by suspending the proof masses from a structure that deflects only in the in-plane torsional sense mode, with the proof masses deflecting only in the torsional out-of-plane drive mode with respect to the supporting structure and the in-plane deflections of the supporting structure being coupled to the sensing frame. Such an embodiment is shown in  FIG. 2 .  
         [0031]     In this embodiment, proof masses  42 ,  43  are pivotally mounted on gimbals or frames  44  by flexures  46  which permit out-of-plane torsional movement of the masses about drive axes  13 ,  14 . The gimbals are suspended from anchors  47 ,  48  by flexures  49  for in-plane torsional movement about sense axes  21 ,  22 . As best seen in  FIG. 2A , the midpoints of the inner or adjacent edges of the proof masses are connected together by a coupling link  51  consisting of thin flexures  52  which extend in the x-direction. Thus, the inner portions of the masses are tied together for movement in concert both in the out-of-plane drive mode and in the in-plane sense mode, as in the embodiment of  FIG. 1 . The gimbals which support the two masses are connected together by flexures  53  in the region between the adjacent edges of the masses.  
         [0032]     Sensing beams or frames  56  extend in a direction parallel to the input or y-axis on opposite sides of the masses and are suspended from anchors  48  by flexures  57  for linear movement along the drive axes. The ends of the sensing frames are connected to the outer edges of the proof masses by links  58 . Linear movement of the sensing frames and, hence, the in-plane sense mode torsional movement of the proof masses is monitored by capacitors  59  consisting of plates  61  connected to the frames and stationary plates  62  connected to anchors  63 .  
         [0033]     As in the embodiment of  FIG. 1 , the proof masses are driven in an anti-phase manner, and Coriolis forces produced by rotation about the input axis cause the masses to oscillate in opposite directions about the sense axes, with the inner edges of the masses moving in one direction and the outer edges moving in the other. This torsional movement is converted to linear movement of the sensing frames and monitored by capacitors  59 .  
         [0034]     The embodiment of  FIG. 3  is similar to the embodiment of  FIG. 2 , with the addition of diagonally extending flexures  66  at the corners of the gimbals. The additional flexures provide increased stiffness in the x and y directions and significantly reduce the susceptibility of the sensor to vibration. In this embodiment, the portions of the gimbals  44   a  between the proof masses serve as counterbalances, and the susceptibility to vibration can be reduced to zero by increasing the mass of these counterbalances.  
         [0035]     The embodiments of  FIGS. 4 and 5  are similar to the embodiment of  FIG. 3 , and like reference numerals designate corresponding elements in all three of them. In the embodiment of  FIG. 4 , however, the arms  44   a  of the gimbals which serve as a counterbalance are made equal in mass to sensing frames  56  so the inertial force on the frame due to linear acceleration is nulled out by the inertial force on the counterbalance. In addition, each of the sensing frames is suspended from anchors  48  by a two pairs of flexures  57  instead of just one, which further stabilizes the device.  
         [0036]     The embodiment of  FIG. 5  is designed for simultaneous detection of angular rate and linear acceleration. In this embodiment, the mass of sensing frames  56  is made substantially larger than the mass of the counterbalance arms  44   a , and the counterbalance mass is thus eliminated. The deflection due to linear acceleration along the deflection axis of the sensing frames is maximized and detected by low pass filtering of the output signals from capacitors  59  before demodulating those signals to extract the Coriolis signal.  
         [0037]     In the embodiment of  FIG. 6 , gimbals or frames  68  are suspended by flexures  69  from anchors  71  which are located inboard or toward the centers of proof masses  72 ,  73 . The flexures extend diagonally from the four corners of each of the proof masses and permit the torsional in-plane oscillation of the proof masses and gimbal in the sense mode. Having the anchors located close to the centers of the masses minimizes the effects of stress on the packaging of the sensor. If desired, the four flexures can be suspended from a single, centrally located anchor instead of the four separate anchors shown in this embodiment.  
         [0038]     As in the previous embodiments, the proof masses are connected to the gimbals by flexures  74  for out-of-plane oscillation about the drive axes in the drive mode. The midpoints of the adjacent edges of the proof masses are connected together by a coupling  76 , and the arms of the gimbals between the masses are connected together by flexures  77 .  
         [0039]     Sensing beams or frames  79  are suspended from anchors  81  by folded flexures  82  which constrain the frames for linear movement in the x-direction. The midpoints of the outer arms of the gimbals are connected to the frames by links  83  to transform sense mode oscillations of the proof masses and gimbals into linear motion of the frames. Those oscillations are monitored by capacitors  84  with interleaved plates  86  connected to the sensing frames and to anchors  87 .  
         [0040]     Finite element analysis of the embodiment shown in  FIG. 6  shows that the first resonance mode of the structure is the anti-phase drive mode and that the second mode is the sense mode. The lowest unwanted resonance mode was also observed to be over 5 KHz higher than the sense mode, providing the desired separation of the unwanted modes from the operational modes. This mode separation cannot be achieved with any other known mechanically coupled system with two degrees of freedom.  
         [0041]     The embodiment of  FIG. 7  is generally similar to the embodiment of  FIG. 6 , with gimbals or frames  68  suspended by flexures  69  from inboard anchors  71  for torsional in-plane oscillation in the sense mode, proof masses  72 ,  73  connected to the gimbals by flexures  74  for out-of-plane oscillation about the drive axes in the drive mode, and the midpoints of the adjacent edges of the proof masses connected together by a coupling  76  for movement in concert.  
         [0042]     In this embodiment, however, the detection frame is divided into four sections  91 - 94  which are suspended from anchors  96 ,  97  by folded flexures or springs  98  for movement independently of each other in the sensing (y) direction.  
         [0043]     The midpoints of the side arms of the gimbals are connected to the frames by links  99  to transform torsional sense mode oscillations of the proof masses and gimbals about the z-axes into linear displacement of the frames. Those displacements are monitored by capacitors  101 A- 101 D having interleaved plates  102 ,  103  connected to the sensing frames and to anchors  104 .  
         [0044]     As in the other embodiments, masses  72 ,  73  are driven to oscillate in see-saw fashion about the x-axes, and rotation about the y-axis causes the masses to oscillate about the z-axes, with the rotation about the z-axes being converted to linear movement of sensing frames  91 - 94 . In this embodiment, however, the capacitor electrodes or plates extend in the x-direction, and the sensing frames move in the y-direction.  
         [0045]     The flexures are orders of magnitude stiffer in all directions other than the sensing direction, which minimizes the motion of the sensing electrodes due to the drive motion and to any other spurious motion of the structure. It also provides better decoupling of the drive mode from the sense mode and minimizes quadrature error.  
         [0046]     A significant advantage of the split frames is the ability to reject external accelerations as common mode by employing double differential detection, e.g. (C A +C C )-(C B +C D ), where C A , C B , C C  and C D  are the capacitances of capacitors  101 A- 101 D. The split frames also minimize the total additional inertia of the detection frame.  
         [0047]     In the embodiment of  FIG. 8 , inner sensing frames  106  are suspended from anchors  107  by diagonally extending flexures  108  and thereby constrained for torsional movement only in the x, y plane. These frames are positioned within windows  109   a ,  111   a  in proof masses  109 ,  111 , with the proof masses being suspended from the frames by torsional hinges  112  which permit the masses to pivot in see-saw fashion about x-axes in the drive mode but transmit sense mode motion about the z-axes to the frames.  
         [0048]     The adjacent edges of the proof masses are connected together by a coupling  113  for movement in concert both in the z-direction in the drive mode and in the x-direction in the sense mode.  
         [0049]     This embodiment also has four outer sensing frame sections  114  which are suspended from anchors  116  by a combination of flexures  117  and folded flexures  118  for rotational movement about z-axes. The outer frame sections are connected to inner sensing frames  106  by links  119  so that the outer sections rotate in concert with the inner frames. Rotation of the sensing frames is monitored by capacitors  121  which have interleaved plates  122 ,  123  connected to the outer sensing frames and to anchors  124 .  
         [0050]     In the drive mode, masses  109 ,  111  are driven out-of-phase and pivot in see-saw fashion about the x-axes. Rotation about the y-axis produces Coriolis forces which cause the masses to rotate in opposite directions about the z-axes. The rotation of the masses is transferred to inner sensing frames  106  by hinges  112  and then to outer frames  114  by links  119 . The sensing capacitors at the ends of the outer frames are driven differentially, with the plates moving closer together at one end of each frame and farther apart at the other.  
         [0051]     Having substantial portions of the sensing frames inside the proof masses minimizes the inertia of the sense mode, and the modes are effectively decoupled since the inner sense mode frames are free to oscillate only in the torsional sense mode and are strictly constrained in all other modes of motion. The torsional hinges which connect the proof masses to the inner frames allow the masses to move relative to the frames in the drive mode while transmitting the sense mode responses of the masses to the sensing frames. Transmitting the sense mode oscillation of the inner frames to outer frames which carry the sensing electrodes provides differential torsional detection which enhances immunity to both linear vibration and angular vibration.  
         [0052]     In the embodiment of  FIG. 9 , the sensing frames  126  are located entirely within windows  127   a ,  128   a  in proof masses  127 ,  128 . As in the embodiment of  FIG. 8 , the sensing frames are suspended from central anchors  129  by flexures  131 , and the proof masses are suspended from the sensing frames by torsion springs  132  which permit the masses to pivot in see-saw fashion about the x-axes, but transfer sense mode rotation about the z-axes from the masses to the frames.  
         [0053]     The adjacent edges of the proof masses are connected together by a coupling  133  for movement in concert both in the z-direction in the drive mode and in the x-direction in the sense mode.  
         [0054]     Rotation of the sensing frames is monitored by capacitors  134  which are also located entirely within the proof masses. These capacitors have electrodes or plates  136  which are attached directly to the sensing frames and interleaved with stationary electrodes or plates  137  affixed to anchors  138 .  
         [0055]     In operation, proof masses  127 ,  128  are driven to oscillate in an anti-phase manner about the x-axes, and the Coriolis forces produced by rotation about the y-axis cause the masses to rotate about the z-axes. This rotation is transferred to sensing frames  126  to move plates  136  and produce a change in the capacitance of sensing capacitors  134 .  
         [0056]     Having the capacitor plates attached directly to the sensing frame, with both the plates and the frame being positioned entirely within the masses permits the device to be constructed in compact form with essentially perfect symmetry, which makes this embodiment even more immune to linear and angular vibration than the embodiment of  FIG. 8 .  
         [0057]     In the embodiment of  FIG. 10 , proof masses  139 ,  141  are mounted on frames  142  for rotation about the y-axis in the drive mode and the z-axis in the sense mode. The frames are mounted to the substrate by torsion springs  143  which extend in the y-direction and permit the frames to pivot in see-saw fashion about the y-axis, with the masses being connected to the frames by flexures or springs  144  which extend in the x-direction. The drive and sense modes of the masses are thus decoupled, with the drive mode frequency (rotation about the y-axis) being dominated by torsion springs  143  and the detection mode frequency (rotation about the z-axis) being determined primarily by the bending of springs  144 .  
         [0058]     Masses  139 ,  141  are identical, and frames  142  are positioned centrally of the masses, with the y- and z-axes of rotation passing through the physical centers of the masses.  
         [0059]     Each of the masses is driven by an electrode (not shown) positioned beneath it for rotation about the y-axis in an anti-phase manner. The two masses are coupled together at the midpoints of their adjacent edges by a coupling spring  146  which permits the masses to be driven either in phase or out of phase. The spring separates the anti-phase drive mode from the in-phase mode, with the anti-phase frequency increasing with the stiffness of the spring, while the in-phase mode frequency remains the same. The coupling spring also constrains the two masses for rotation together about the z-axis so that when one mass rotates in one sense, the other must rotate in the opposite sense, thereby ensuring anti-phase proof mass motion in the detection mode.  
         [0060]     Detection, or sensing, frames  148  are mounted to the substrate by flexures  149 ,  151  for rotation about the z-axis. These frames are coupled to the proof masses by link springs  152 , which transfer rotational movement about the z-axes from the masses to the frames. Such rotation is monitored by capacitors  153  which have plates  154  attached to the frames and interleaved with stationary plates  156  affixed to anchors  157  on the substrate. These capacitors also provide feedback for force rebalancing.  
         [0061]     Link springs  152  extend in the y-direction and are preferably made torsionally weak in order to minimize coupling between the drive mode oscillation of the masses and the detection frames. Such coupling could result in a slight vertical vibration of the capacitor plates carried by the frames which could detract from the performance of the sensor. At the same time, however, the springs should not be made so weak that the transfer of z-axis rotation is degraded.  
         [0062]     To enhance immunity to angular vibration noise, coupling springs  159  are connected between the confronting arms of the detection frames. These springs are relatively soft in the y-direction and stiff in the x-direction in order to enhance coupling and ensure that the frames oscillate in a true anti-phase manner.  
         [0063]     In operation, proof masses  139 , 141  are driven to oscillate out-of-phase with each other in see-saw fashion about the y-axis, and rotation of the sensor about the x-axis results in out-of-phase Coriolis induced rotation of the masses about the z-axes. That rotation is transferred to detection frames  148  by link springs  152 , with the rotation of each of the frames being opposite to that of the adjacent proof mass. Thus, for example, when mass  139  rotates in a clockwise direction about the z-axis, the upper link spring  152  forces the upper detection arm to rotate in a counter-clockwise direction about its z-axis. At the same time, mass  141  rotates in a counter-clockwise direction, and the lower detection frame rotates in a clockwise direction.  
         [0064]     Having the two detection frames rotate in opposite directions provides a significant advantage in reducing the sensitivity of the device to vibration noise and angular acceleration about the z-axis.  
         [0065]     The embodiment of  FIG. 11  is similar to that of  FIG. 10  in that proof masses  161 ,  162  are mounted on frames  163  for rotation about the y-axis in the drive mode and the z-axis in the sense mode. The frames are mounted to anchors  164  on the substrate by flexures or torsion springs  166  which extend in the x-direction, and the masses are connected to the frames by flexures or torsion springs  167  which extend in the y-direction.  
         [0066]     As in the previous embodiments, the two masses are coupled together at the midpoints of their adjacent edges by a coupling spring  168 . This spring is in the form of a small rectangular frame, with relatively long, flexible flexures extending in the x-direction and relatively short, stiff ones in the y-direction. This design decouples the torsional stiffness of the spring about the y-axis for drive from the linear stiffness along the x-axis for detection while permitting the two masses to be driven either in an in-phase manner or in an anti-phase manner.  
         [0067]     Detection, or sensing, frames  169  are coupled to the proof masses by link springs  170 . These frames are similar to frames  148  in the embodiment of  FIG. 10 , although they are perforated to reduce the mass of the detectors. The frames are mounted to the substrate by flexures  171 ,  172  for rotation about z-axes, and the rotation of the frames, and hence the masses, is monitored by capacitors  173  which are similar to capacitors  153 .  
         [0068]     Coupling springs  174  are connected between the confronting arms of the detection frames. These springs are similar to coupling springs  159  in the embodiment of  FIG. 10  in that they are relatively soft in the y-direction and stiff in the x-direction in order to enhance coupling and ensure that the frames oscillate in a true anti-phase manner. However, they are simpler than springs  159  in that they have only two leaves instead of four.  
         [0069]     Operation and use of the embodiment of  FIG. 11  is similar to that of the embodiment of  FIG. 10 , with coupling springs  168 , and link springs  170  providing both good coupling of z-axis rotation between the proof masses and the detection frames in the sense mode and good isolation between the masses and the frames for y-axis rotation in the drive mode.  
         [0070]     In the embodiment of  FIG. 12 , detection frames  176  and sensing capacitors  177  are located entirely within proof masses  178 ,  179 , and the detection frames and the masses are symmetrical about the x- and y-axes and have the same centers of rotation in the sense mode. This effectively prevents rotational responses about the z-axis from being induced by linear acceleration along the x-axis because the detectors are inertia lly balanced along the x-axis with regard to their centers of rotation. Also, since the masses and the detectors have the same centers of rotation, there is no tendency to produce rotation about the z-axis as can happen when the rotation centers are separated and the linear displacements produced by linear acceleration along the x-axis are different.  
         [0071]     Frames  176  are mounted to anchors  181  by flexures or springs  182 ,  183  which extend along the x- and y-axes that intersect at the centers of the masses and the frames. Each of the masses is connected to one of the frames by a pair of torsion springs  184  which extend along the y-axis, and the two masses are coupled together at the midpoints of their adjacent edges by a coupling spring  186  which is similar to spring  168  in the embodiment of  FIG. 10 . Since torsion springs  184  determine the in-phase drive mode frequency, they are normally made much stiffer than link springs  152  in the embodiment of  FIG. 10 .  
         [0072]     Capacitors  177  have plates  187  which extend from frames  176  and are interleaved with stationary plates  188  attached to anchors  189 ,  191 .  
         [0073]     In this embodiment, proof masses  178 ,  179  are once again driven to oscillate in an anti-phase see-saw fashion about the y-axis, and rotation about the x-axis is sensed. The Coriolis forces produced by that rotation cause the masses to rotate in opposite directions about the z-axes which pass through the centers of the masses. The z-axis rotation is transferred to detection frames  176  by torsion springs  184 , and the frames rotate in the same direction as the masses and about the same centers of rotation as the masses. As noted above, since the proof masses and the detectors are symmetrical about the x- and y-axes and have the same centers of rotation, this embodiment is substantially immune to linear acceleration along the x-axis.  
         [0074]     In the embodiment of  FIG. 13 , the rate sensor has two proof masses  193 ,  194  in the x-y plane, with mass  193  being positioned inside mass  194 . Each mass is mounted to the substrate through a pair of torsion springs  196  attached to anchors  197  for rotation about the y-axis. Thus, instead of having spaced apart drive axes as in some of the other embodiments, these masses have coincident drive axes.  
         [0075]     The ends of the masses are connected to a pair of coupling frames  198  by torsion springs  199 , and each of the coupling frames is mounted on a detection frame  201  by additional torsion springs  202  which allow the coupling frames to rotate about the y-axis. The two masses are thus coupled together and constrained for rotation with equal amplitude in opposite directions. By proper design of the masses, the linear and angular momentum of the drive motion can be balanced.  
         [0076]     Detection frames  201  are mounted on the substrate by flexures or springs  203  which are attached to anchors  204  at the centers of the frames for rotation about the z-axis.  
         [0077]     In the presence of input rotation about the x-axis, the anti-phase drive motion generates Coriolis forces that cause masses  193 ,  194  to rotate about the z-axis in an anti-phase manner. This motion is transferred to detection frames  201  through coupling frames  198 , resulting in in-phase rotational motion of the detection frames about the z-axes, with the coupling frames constraining the detection frames to rotate at the same amplitude and in the same direction about the z-axes, independent of frequency. The response to the rotation about the x-axis is monitored by capacitors  206  with plates  207  attached to the detection frames and static plates  208  on the substrate.  
         [0078]     In this embodiment, the symmetry of both the drive elements and the detection elements provides for momentum balance, immunity to linear vibration excitation, and reduced susceptibility of these properties to variations in the fabrication process. The use of mechanical constraint rather than coupling springs enables the drive and detection modes to be the lowest frequency modes of the system. Also, since the proof masses are mounted to the substrate through their own anchors and connect to the detection frames through torsion springs, these springs can be made much weaker than the springs which mount the masses, thereby minimizing the leakage of driving motion into the detection frames, quadrature error, and other spurious error signals. The weaker coupling springs also minimize the inertia forces induced on the detection frames by the masses due to externally applied accelerations.  
         [0079]     The embodiment of  FIG. 14  has a proof mass  211  and a pair of coupling frames  212  which also serve as proof masses. Mass  211  is mounted on a frame  213  which is mounted to anchors  214  by springs or flexures  216 ,  217  for rotation about the z-axis at the center of the mass, and it is connected to the frame by torsion springs  218  for rotation about the y-axis. Mass  211  is connected to coupling frames  212  by torsion springs  219  which are similar to torsion springs  199  in the embodiment of  FIG. 13 .  
         [0080]     Torsion springs  220  connect coupling frames  212  to detection frames  221  which are constrained for rotation about z-axes by flexures  222 ,  223  connected to anchors  224 ,  226 . The coupling frames are generally U-shaped and extend along three sides of the detection frames. Rotation of the detection frames is monitored by capacitors  227  which have electrodes or plates  228  attached to the frames and stationary electrodes or plates  229  affixed to anchors  231 . The open sides of the detection frames simplify fabrication of the device in that they permit the electrodes to be routed out from the side without the need for backside vias through the substrate.  
         [0081]     In operation, proof mass  211  and coupling frames  212  are driven to oscillate in see-saw fashion about y-axes, with the coupling frames being out of phase with the proof mass and in phase with each other. Coriolis forces produced by rotation about the x-axis cause the proof mass and the coupling frames to rotate about z-axes, with the frames once again in phase with each other and out of phase with the proof mass. The z-axis rotation of the coupling frames is transferred to detection frames  221  by torsion springs  220 .  
         [0082]     Since the rotation of coupling frames  212  is opposite to that of proof mass  211  in the drive mode, the drive momentum can be balanced relatively easily by proper design of these masses, as can inertia balance in the detection mode for the rejection of angular vibration. In addition, pivotally mounted frame  213  helps to decouple the out-of-plane drive motion of the proof mass from the rotational sense motion of that mass and detection frames  212 .  
         [0083]     The embodiment of  FIG. 15  has a central proof mass  233  which is mounted on a frame  234  for rotation about the y- and z-axes and a pair of outer proof masses  236  on opposite sides of the central mass. The frame is mounted to anchors  237 ,  238  by flexures  239 ,  241  for rotation about the z-axis at the center of mass  233 , and the mass is connected to the frame by torsion springs  242  for rotation about the y-axis.  
         [0084]     Each of the outer proof masses  236  is rotatively mounted on a detection frame  244  by torsion springs  246  for rotation about the y-axis which passes through the center of the mass, and the detection frames are mounted to anchors  247 ,  248  by flexures or springs  249 ,  251  for rotation about the z-axes at the centers of the masses. Flexures  251  extend in the y-direction and are aligned along common axes with torsion springs  246 .  
         [0085]     The outer edges of central proof mass  233  are connected to the adjacent edges of outer proof masses  236  by couplings  252  whereby the outer masses are constrained for out-of-phase movement relative to the central mass and in-phase movement relative to each other about both the y- and z-axes.  
         [0086]     Detection frames  244  are generally U-shaped and extend along three sides of outer proof masses  236 . Rotational movement of the proof masses about the z-axes is monitored by capacitors  254 ,  256 . Capacitors  254  have electrodes or plates  257  on the detection frames and interleaved with stationary electrodes or plates  258  affixed to anchors  259 , with the plates extending in the y-direction and being spaced apart in the x-direction. Capacitors  256  have electrodes or plates  261  on the detection frames and interleaved with stationary electrodes or plates  262  affixed to anchors  263 , with the plates extending in the x-direction and being spaced apart in the y-direction.  
         [0087]     In the drive mode, central proof mass  233  is driven out of phase with outer proof masses  236  for oscillation about the respective y-axes. Rotation about the x-axis produces Coriolis forces which cause the masses to rotate about their z-axes, with the outer masses being out of phase with the central mass and in phase with each other.  
         [0088]     Outer masses  236  are symmetrical in shape, which provides more robustness against process variations in maintaining momentum balance, and detection efficiency is increased by having detection frames  244  outside those masses. In the sense mode, the rotation of the outer masses in phase with each other and in phase opposition to the central mass produces a net cancellation of linear and angular momentum. The torsional springs and flexures provide a high level of decoupling between the drive and sense motions, and the symmetry of the sensor makes it insensitive to externally applied accelerations.  
         [0089]     The embodiment of  FIG. 16  is similar to the embodiment of  FIG. 12 , and like reference numerals designate corresponding elements in the two. In the embodiment of  FIG. 16 , however, proof masses  178 ,  179  are suspended from anchors  266  by drive springs consisting of torsion springs  267 ,  268  and a rigid connecting bar  269 . The drive springs have a generally U-shaped configuration, with the two torsion springs being spaced apart and extending parallel to the y-axis and the bar extending between them. At the ends opposite the bars, springs  267  are connected to the anchors, and springs  268  are connected to the masses. Each of the masses is suspended by four of the drive springs which are disposed symmetrically of the y-axis and the centers of the masses. These springs constrain the masses for see-saw movement about the y-axis and rotational movement about the z-axes which pass through the centers of the masses.  
         [0090]     The masses are connected to sensing frames  176  by link springs  271  which are substantially weaker than springs  267 ,  268  in torsional stiffness.  
         [0091]     In operation, proof masses  178 ,  179  are driven to oscillate in an anti-phase see-saw fashion about the y-axis, and rotation about the x-axis is sensed. The Coriolis forces produced by that rotation cause the masses to rotate in opposite directions about the z-axes which pass through the centers of the masses. The z-axis rotation is transferred to detection frames  176  by link springs  271 , and the frames rotate in the same direction as the masses and about the same centers of rotation as the masses. Since the proof masses and the detectors are symmetrical about the x- and y-axes and have the same centers of rotation, this embodiment is substantially immune to linear acceleration along the x-axis.  
         [0092]     Moreover, with drive springs  267 ,  268  being much stiffer torsionally than link springs  271 , the drive springs dominate the in-phase drive mode frequency, and the transfer of angular momentum from the drive mode to the sensing frame is greatly reduced, resulting in significantly less vertical motion of the sensing frame.  
         [0093]     The embodiment of  FIG. 17  is similar to the embodiment of  FIG. 7  in that gimbals  272  are suspended by diagonally extending flexures  273  from inboard anchors  274  for torsional in-plane oscillation in the sense mode. Proof masses  276 ,  277  are connected to the gimbals by torsion springs  278  for out-of-plane oscillation about the drive axes in the drive mode, and the midpoints of the adjacent edges of the proof masses connected together for movement in concert by a coupling  279 .  
         [0094]     In this embodiment, however, the proof masses and gimbals are generally circular, and the movable electrodes or plates  281 ,  282  of feedback capacitors  283  and sensing capacitors  284  are attached directly to the gimbals rather than to separate sensing frames. The fixed electrodes or plates  286 ,  287  of the capacitors are mounted in a stationary position on frames  288 ,  289  attached to anchors  291 ,  292 . The inner end portions of the gimbals are connected together by couplings  293 .  
         [0095]     As in the embodiment of  FIG. 7 , the sensing capacitors are divided into four separate sections, and a differential torsional detection scheme can be employed. That provides a significant improvement in immunity to linear and angular vibration. Moreover, with the capacitor plates attached directly to the gimbals, the structure of this embodiment is much simpler than those which separate sensing frames.  
         [0096]     In operation, masses  276 ,  277  are driven to oscillate in see-saw fashion about the x-axes which pass through their centers, and rotation about the y-axis causes the masses to oscillate about the z-axes which pass through their centers. The rotation about the z-axes is transferred to gimbals  272  by torsion springs  278  and monitored by capacitors  284 . With their inner edge portions coupled together, the two masses are constrained for rotation in opposite directions, as are the two gimbals.  
         [0097]     As in the embodiment of  FIG. 7 , the sensing capacitors are divided into four separate sections, and a differential torsional detection scheme can be employed. That provides a significant improvement in immunity to linear and angular vibration. Moreover, with the capacitor plates attached directly to the gimbals, the structure of this embodiment is much simpler than those which separate sensing frames.  
         [0098]     It is apparent from the foregoing that a new and improved rate sensor has been provided. While only certain presently preferred embodiments have been described, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.