Abstract:
A rotation-rate sensor includes a substrate having a surface, a movable element situated above the surface, which is deflectable based on a Coriolis force along a first axis that runs perpendicular to the surface, a driving device which is prepared to activate the movable element along a second axis that runs parallel to the surface, a compensation device, in order to generate an electrostatic force along the first axis, including electrodes corresponding to one another, developed on the substrate and on the movable element; a relative degree of covering of the electrodes in the direction of the first axis being a function of the deflection of the movable element along the second axis; and the electrode developed on the movable element runs around an insulating region of the movable element.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rotation-rate sensor, especially for use in motor vehicles. 
     BACKGROUND INFORMATION 
     A rotation-rate sensor described in German Patent No. DE 10 2006 058 746 A1 has a movable element that is situated above the surface of a substrate and is able to be driven to vibrations by a driving device along a first axis running along the surface, and which is deflectable along a second axis that runs perpendicular to the surface of the substrate (out of plane) by the action of a Coriolis force, and has a compensation device that is equipped to compensate for undesired vibrations of the movable element along the second axis, which are caused by the driving device. 
     The undesired vibrations along the second axis are recorded as so-called quadrature signals, and they falsify the measurement results. The cause of the quadrature signals, in this instance, typically is asymmetries in the sensor structure, which are given rise to by production tolerances. The undesired vibrations along the second axis have the same frequency as the vibrations along the first axis. Their direction is determined by the type/shape of the asymmetry. 
     The present invention is based on the object of creating an improved compensation device for quadrature signals in a rotation-rate sensor that is equipped to record a Coriolis force directed perpendicular to the substrate. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is attained by a rotation-rate sensor according to the present invention. 
     The compensation device described may advantageously be implemented in a particularly simple manner, and is reliable and cost-effective. Furthermore, a detection mass element is given which is compact and has only few fissures on its outer edges, whereby there is an increase in the robustness of the sensor to a fracture of a spring that is fastened to the detection mass element. The structures of the proposed compensation device are advantageously smaller than those known up to now, and therefore have an increased natural frequency, whereby the inclination of an undesired interaction with the measuring process is decreased. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a view of a rotation-rate sensor having a compensation device. 
         FIGS. 2   a  to  2   c  show views of an electrode constellation of the compensation device in  FIG. 1  in different deflections of the detection mass element. 
         FIGS. 3   a  to  3   b  show views corresponding to  2   a  to  2   c  of an alternative electrode constellation of the compensation device in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a view of a rotation-rate sensor. The rotation-rate sensor includes two identical structures  1 ,  2  having a constant thickness which are situated above a substrate running in the plane of the paper. Structures  1 ,  2  are produced, for example, by depositing an electrically conductive polysilicon layer onto an oxide layer which is, in turn, provided on a silicon substrate. Recesses are formed in the oxide layer, so that interconnections are created from the polysilicon layer to the silicon substrate in these recesses. The patterns are then specified and the oxide layer is removed in an etching process. In so doing, the polysilicon layer remains joined to the silicon substrate. 
     Each of structures  1 ,  2  has two drive mass elements  3 . Drive mass elements  3  are connected to the substrate lying below them via four drive mass springs  4  at ends  5 . In each case two drive mass springs  4 , which connect the same drive mass element  3  to the substrate lying below it, lie opposite in the y direction, in this context, which runs along the surface of the substrate. The deflections of drive mass elements  3  are thus limited in the y direction by opposite ends  5  relative to the substrate lying below them. Drive mass springs  4  are each situated in a rectangular recess  6  in one of drive mass elements  3 . Because of the alignment of their folds, springs  4  are above all extensible in the y direction, while oscillations of drive mass elements  3  in the x direction are suppressed. Because of the application of drive mass springs  4  in recesses  6 , in this context, there is still sufficient space on the sides of drive mass elements  3  to position comb drives  9 , by which drive mass elements  3  may be put into oscillation in the y direction. 
     The two drive mass elements  3  of each structure  1 ,  2  are connected via eight detection mass springs  7  to an essentially rectangular detection mass element  8  (two springs  7  on each side). Detection element  8  may be provided with holes going all the way through (e.g. perforations). The two drive mass elements  3  almost completely surround detection mass element  8 , but leave room for connecting a coupling spring  10  and a substrate spring  11  to detection mass element  8 . In each case, two of detection mass springs  7  lying opposite to each other are mounted on two sides of detection mass element  8 . Because of the development and this mounting of detection mass springs  7 , oscillations of detection mass element  8  to drive elements  3  in the y direction and in the x direction are suppressed, while a relative motion of detection mass element  8  in a z direction, perpendicular to the surface of the substrate, is easily possible. 
     Detection mass elements  8  are coupled to each other via coupling spring  10 . For the purpose of stabilization, detection mass elements  8  are connected to the underlying substrate via substrate springs  11 , at the ends  12  of substrate springs  11 . 
     At the opposite sides of detection mass elements  8 , along the y direction, rectangular recesses  13  are provided, between which rectangular projections  14  are developed. On the substrate, under rectangular recesses  13 , electrode pairs  15 ,  16  are developed, which are electrically insulated from the substrate. Electrodes  15  are each electrically connected to current supply V 1 , and electrodes  16  are each electrically connected to a current supply V 2 , so that electrodes  15  are able to be supplied with a different voltage from that of electrodes  16 . Structures  1 ,  2 , and thus projections  14 , are electrically connected to current supply V 3 . 
     When the sensor is rotated about the x axis, drive mass elements  3  for all specific embodiments are excited by comb drives  9  to oscillations along the y axis. The Coriolis force is then directed in the z direction, perpendicular to the surface of the substrate. In the process, the frequency of comb drives  9  is preferably selected in such a way that detection mass elements  8  are excited to oscillations in phase opposition, based on the coupling. Below detection mass elements  8 , there is developed in each case an electrode as a detection device in the substrate. If detection mass elements  8  are set into oscillations in the z direction by the Coriolis force, there is a change in the capacitances with respect to the superjacent detection mass elements. By subtraction of the signals of the electrodes, interference accelerations, for instance, by translation of the sensor, may simply be compensated for. In addition, because of the suitable dimensioning of drive mass elements  3  and detection mass elements  8 , it is assured that their common center of mass is invariant in time. 
     Between drive mass elements  3  and detection mass elements  8 , additional oscillating mass elements may also be provided that are coupled to one another. Thus, it is possible to transfer only the oscillation in the z direction, based on the Coriolis force, to detection mass elements  8 . 
       FIG. 2   a  shows an electrode pair of the compensation device which are situated under detection mass element  8  that is not deflected. In position Y 0  that is not deflected, a covering region between electrode  15  and projection  14  and a covering region between electrode  16  and projection  14  are of the same magnitude. Based on voltage V 3  applied to detection mass elements  8 , voltage V 2  applied to electrode  15  and voltage V 1  applied to electrode  16 , a force acts between electrodes  15 ,  16  and projection  14  which slightly shifts detection elements  8  in a decreasing z direction. 
     The electrostatic compensation force acting on detection mass elements  8  along the z axis is determined by forces F static  and F dynamic  acting along the z axis. F static  is a proportion of the force which acts independently of the deflection. F dynamic  is dependent on a deflection ΔY of the detection mass element along the y axis from resting position Y 0 . In  FIG. 2   a , the deflection is zero and F dynamic  is zero; the resulting force is F static . 
       FIG. 2   b  shows a view of an electrode pair  15 ,  16  of the compensation device having a detection mass element  8  deflected in one direction. In deflected position Y 0 +ΔY, a covering region between electrode  15  and projection  14  is smaller than a covering region between electrode  16  and projection  14 . F dynamic  acts in the direction of F static , since now electrode  16  has a greater influence on detection mass elements  8  than it had in the non-deflected position of  FIG. 2   a . F is greater than in the representation of  FIG. 2   a.    
       FIG. 2   c  shows a view of an electrode pair  15 ,  16  of the compensation device having a detection mass elements  8  deflected in the opposite direction. In deflected position Y 0 +ΔY, a covering region between electrode  15  and projection  14  is larger than a covering region between electrode  16  and projection  14 . F dynamic  acts in the direction opposite to F static , since now electrode  15  has a greater influence on detection mass element  8  than it had in the non-deflected position of  FIG. 2   a . F is smaller than in the representation of  FIG. 2   a.    
     In response to a suitable setting of voltages V 1  and V 2 , those forces may be compensated for, by the force action of electrodes  15  and  16  on electrode  14 , which lead to the undesired oscillations of detection mass element  8  in the z direction, and thus to quadrature signals. Based on the separate current supply for electrodes  15  and  14 , the quadrature signals may be corrected individually in two opposite directions, in this context. Alternatively to this, a correction may also be implemented in one direction using only one voltage. 
       FIGS. 3   a  to  3   c  show an alternative design of the electrodes of the compensation device of  FIGS. 2   a  to  2   c . Detection mass element  8  is not shown here; it extends from electrode  14  towards the x direction in the x-y plane. However, electrode  14  of detection mass element  8  is not finger-shaped as in  FIGS. 2   a  to  2   c , but runs around an insulating region  17  in detection mass element  8 , so that the edge of electrode facing the observer, running in the y direction, closes in a straight manner. The insulating region  17  may be a recess in detection mass element  8 . Alternatively, the insulating region  17  may be made up of an electrically insulating material. In another specific embodiment, an electrode  14  extending into this region may also be screened from electrostatic forces, for instance, using an electrically conductive covering (not shown). 
     The electrode shape shown in  FIGS. 3   a  to  3   c  may be mounted along two opposite edges of detection mass element  8 , for example (cf. the position of electrodes  14  in  FIG. 1 ). Detection mass element  8  having the electrodes as in  FIGS. 3   a  to  3   c  is clearly more compact than is shown in  FIG. 1 , and has four straight outer edges. Because of this minimization of the fissures in the outer edge, the danger is significantly reduced that filigree spring structures bordering on, or striking detection mass element  8  (such as detection mass springs  7 , coupling spring  10 , substrate spring  11 ) might break when there is a great deflection of detection mass element  8 . Such great deflections might come about in the motor vehicle environment based on unevenness in the ground, strong braking or rapid cornering. 
     A further advantage of the electrode shapes shown in  FIGS. 3   a  to  3   c  is that the lengths of the individual structures of the compensation device are able to be kept short, conditioned on the design. The natural frequency of the structures of electrode  14  are therefore above a range in which the influencing of the functioning of the sensor element is able to take place, for instance, by exciting such a natural frequency (eigenmode) by the drive motion of detection mass element  8 . In addition, the straight outer edge of electrode  14  stiffens detection mass element  8  in such a way that natural oscillations that might occur are effectively damped.