Patent Publication Number: US-2010122577-A1

Title: Evaluation electronics system for a rotation-rate sensor

Description:
BACKGROUND INFORMATION 
     Rotation-rate sensors for ascertaining rotation rates, about one or more measuring axes, are known from the related art. In known micromechanical rotation-rate sensors, two or more seismic masses are driven in such a way that they execute an antiparallel vibration. If a rotation rate occurs about a stipulated measuring axis, the seismic masses are deflected in an antiparallel manner by Coriolis forces, perpendicular to the drive direction. These deflections are detected using an evaluation electronics system, and they supply a measure for the rotation rate that is to be measured. 
     In rotation-rate sensors according to the related art, the sensitivity of the rotation-rate sensors to vibrations in the deflection direction of the seismic masses is observed in a frequency range of a few Hz up to a few kHz. An external vibration causes a motion of the seismic masses at the frequency of the external spurious response. The measurement of the rotation rate acting on the rotation-rate sensor may thereby be impaired. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved evaluation electronics system for a rotation-rate sensor. This objective is attained by an evaluation electronics system for a rotation-rate sensor according to the present invention. 
     An evaluation electronics system for a rotation-rate sensor according to the present invention, having a first and a second seismic mass, is developed for the purpose of ascertaining a rotation rate acting on the rotation-rate sensor, from a deflection of the first and the second seismic masses. The evaluation electronics system, in this instance, has a regulation member in order to minimize an undesired deflection of the first and second seismic masses, caused by interfering influences. The vibration sensitivity of the rotation-rate sensor may advantageously be reduced thereby. An additional advantage is that the evaluation electronics system according to the present invention is suitable for use with all types of rotation-rate sensors. 
     The rotation-rate sensor expediently has a drive mechanism that is developed to excite the first and second seismic masses to an antiparallel vibration along a drive direction. In this context, the first and second seismic masses are able to be deflected along a measuring direction which is oriented essentially perpendicular to the drive direction. Detection means are provided, furthermore, for detecting a deflection of the first and second seismic masses along the measuring direction. In addition, compensation means are provided for compensating for an undesired deflection of the first and second seismic masses. It is advantageous that an intermodulation of drive frequencies and interference frequencies is suppressed by this design. 
     The first and second seismic masses, the detection means, the regulation member and the compensation means preferably form a control loop. This advantageously permits a captive operation of the acceleration sensors formed by the first and second seismic masses. 
     The drive mechanism is expediently an electrostatic or piezoelectric drive mechanism. 
     In one specific embodiment, the detection means are developed to ascertain a deflection of the first and second seismic masses because of capacitance changes between the first and second seismic masses and first and second counter-electrodes situated on a substrate surface. This makes possible the use of an evaluation electronics system together with known rotation-rate sensors. 
     The compensation means in this specific embodiment are expediently developed to compensate for an undesired deflection of the first and second seismic masses by applying an electrical voltage between the first and second seismic masses and the first and second counter-electrodes situated on the substrate surface. This advantageously makes possible a compensation for undesired deflections without additional components being required. 
     In another specific embodiment, the detection means are developed to detect a deflection of the first and second seismic masses with the aid of a change in an electrical characteristics variable of at least one piezoelectric element. 
     The compensation means in this specific embodiment are expediently developed to compensate for an undesired deflection of the first and second seismic masses by the application of an electric voltage to the at least one piezoelectric element. In this case, too, advantageously no additional components will be required for compensating for undesired deflections. 
     In one refinement, the rotation-rate sensor has a mechanical low-pass filter, the evaluation electronics system being developed to minimize an undesired deflection of the first and second seismic masses in a frequency range of 0 Hz up to above a cutoff frequency of the mechanical low-pass filter. In this specific embodiment, the evaluation electronics system and the mechanical low-pass filter advantageously complement each other. 
     According to one additional refinement, the evaluation electronics system is developed for ascertaining an acceleration acting on the rotation-rate sensor in the measuring direction. It is advantageously made possible, thereby, to use the rotation-rate sensor as an acceleration sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a top view onto a rotation-rate sensor. 
         FIG. 2  shows a schematic block diagram of an evaluation electronics system for recording the Coriolis acceleration for a rotation-rate sensor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic top view onto a micromechanical rotation-rate sensor  100 , which is able to be used together with an evaluation electronics system  300 , shown in  FIG. 2 . However, rotation-rate sensor  100  shown in  FIG. 1  only represents an example. An evaluation electronics system  300  of  FIG. 2  is suitable for use with any type of vibrating rotation-rate sensors. In particular, an evaluation electronics system  300  may also be used in connection with rotation-rate sensors that are provided for the detection of rotation rates about other rotational axes than the one in  FIG. 1 . 
     Rotation-rate sensor  100  is situated above a surface of a substrate  110 . Substrate  110  may be a silicon substrate, for example. The surface of substrate  110  is situated in a plane that is generated by an x direction and a drive direction  205  perpendicular to it. A measuring direction  215  is oriented perpendicular to the substrate surface, that is, also perpendicular to the x direction and drive direction  205 . 
     Rotation-rate sensor  100  includes a first frame  120  and a second frame  220 . First frame  120  is connected to substrate  110  via first spring elements  130 . First spring elements  130  permit a motion of first frame  120  along drive direction  205 . Second frame  220  is connected to substrate  110  via second spring elements  230 . The second spring elements  230  permit a motion of second frame  220  along drive direction  205 . First spring elements  130  and second spring elements  230  may be developed as bar springs, for instance. 
     First frame  120  and second frame  220  are able to be set into antiparallel vibration along drive direction  205 , using a drive mechanism  200 . The antiparallel vibration may be developed in such a way that first frame  120  and second frame  220  move away from each other during a first vibration phase and move towards each other during a second vibration phase. In this context, the drive vibration may have a frequency of some 10 kHz, for instance, 15 kHz. Drive mechanism  200  may be an electrostatic or a piezoelectric drive mechanism. Such drive mechanisms are familiar from the related art. 
     Rotation-rate sensor  100  further includes a first seismic mass  140  which is situated above the surface of substrate  110 , in an area bordered by first frame  120 . First seismic mass  140  is connected to first frame  120  via third spring elements  150 . Rotation-rate sensor  100  also includes a second seismic mass  240  which is situated above the surface of substrate  110 , bordered by second frame  220 , and is connected to second frame  220  via fourth spring elements  250 . First seismic mass  140  and second seismic mass  240  are connected to each other via a fifth spring element  210 . In addition, first and second seismic masses  140 ,  240  are each connected to substrate  110  via sixth spring elements  211 . Third, fourth, fifth and sixth spring elements  150 ,  250 ,  210 ,  211  may be bar springs. Third spring elements  150  and fourth spring elements  250  are developed in the drive direction in such a stiff manner that first and second seismic masses  140 ,  240  follow drive motions of first and second frame  120 ,  220  in drive direction  205 . Besides that, third and fourth spring elements  150 ,  250  are designed so that first and second seismic masses  140 ,  240  are able to be deflected along measuring direction  215  against first and second frame  120 ,  220 . First and second seismic masses  140 ,  240  thus are able to move perpendicular to the substrate surface, away from substrate  110 , or towards substrate  110 . 
     If a rotation rate acts on rotation-rate sensor  100 , about an axis of rotation that is parallel to the x direction, while first and second frames  120 ,  220  are carrying out an antiparallel vibration, the rotation rate brings about, in measuring direction  215 , Coriolis forces that have an effect on first and second seismic masses  140 ,  240 . The direction of the Coriolis forces acting on first and second seismic masses  140 ,  240 , in this instance, is a function of the direction of rotation and the directions of motion of first and second seismic frames  120 ,  220 . In one vibrational phase, during which the first and the second frame  120 ,  220  move away from each other, a force, for example, may act on first seismic mass  140 , which points away from substrate  110 , while a force in a direction towards the substrate acts on second seismic mass  240 . In a vibrational phase during which first and second frame  120 ,  220  move towards each other, a Coriolis force then acts on first seismic mass  140  in the direction of substrate  110 , while a Coriolis force pointing away from substrate  110  acts on second seismic mass  240 . The Coriolis forces acting on the first and second seismic masses  140 ,  240  cause periodic deflections of first and second seismic masses  140 ,  240  along measuring direction  215  at the frequency of the drive motion effected by drive mechanism  200 . The amplitudes of these deflections, in measuring direction  215 , represent a measure for the magnitude of the rotation rate acting on rotation-rate sensor  100 . 
     Rotation-rate sensor  100  includes detection means  260  for detecting a deflection of first and second seismic masses  140 ,  240  along measuring direction  215 . Detection means  260  may be electrostatic detection means, for example. A first counter-electrode  265  may be situated on substrate  110 , for instance, below first seismic mass  140 , and a second counter-electrode  266  may be situated on substrate  110  below second seismic mass  240 . In this case, first seismic mass  140  and first counter-electrode  265  form a first capacitor, whose capacitance is a function of the distance of first seismic mass  140  from first counter-electrode  265 , that is connected to substrate  110 . Second seismic mass  240  and second counter-electrode  266  form a second capacitor, whose capacitance changes in response to the deflection of second seismic mass  240  in measuring direction  215 . By a measurement and evaluation of the capacitances of the first and second capacitors, one is able to draw a conclusion on the deflections of the first and second seismic masses  140 ,  240  effected by the Coriolis forces, and because of that, on a rotation rate acting on rotation-rate sensor  100 . By the application of electrical voltages to the first and the second counter-electrodes  265 ,  266 , deflections of first and second seismic masses  140 ,  240  in measuring direction  215  may also be specifically influenced. This being the case, first and second counter-electrodes  265 ,  266  also represent a compensation means  270 . In another specific embodiment of the present invention, detection means  260  and compensation means  270  may also be formed by piezoelectric elements which may, for example, be situated at suspension springs  150 ,  250 . In this specific embodiment, during a deflection of first and second seismic masses  140 ,  240  along the measuring direction  215 , there is a change in an electrical characteristics variable, such as a voltage, a load or a resistance. By applying electric voltages to the piezoelectric elements, deflections of first and second seismic masses  140 ,  240  may also be specifically influenced. 
     Accelerations acting on rotation-rate sensor  100  in measuring direction  215  are also able to lead to a deflection of first and second seismic masses  140 ,  240  in measuring direction  215 . Such accelerations may, for instance, originate with vibrations in measuring direction  215 . For damping such vibrations, rotation-rate sensor  100  may be situated on a mechanical low-pass filter. Such mechanical low-pass filters are known from the related art, but they work only above a certain minimum frequency, for example, above a few kHz. Vibrations having a low frequency were not sufficiently damped. 
     Coriolis forces effected by a rotation rate lead to deflections of first and second seismic masses  140 ,  240  that are in phase opposition or antiparallel. On the other hand, a vibration acting on rotation-rate sensor  100  causes parallel or in-phase deflections of seismic masses  140 ,  240  to the frequency of vibration, which may be in a range of up to 4 kHz, for example. An evaluation electronics system connected to rotation-rate sensor  100  does not react to such direct component signals. However, the mechanical structure of rotation-rate sensor  100  also vibrates without interference vibrations, not only with the antiparallel drive frequency but proportionally also with other adjacent forms of vibration, for instance, of a parallel phase resonance. Under the influence of interference vibrations, there is an intermodulation of the partaking frequencies at the nonlinear characteristics curve of detection means  260 . Mixed products created thereby may hit the operating frequency of rotation-rate sensor  100  by convolution. In this case, there is a vibration sensitivity of rotation-rate sensor  100  since, because of the influence of the interference vibrations, a corruption of the vibration of first and second seismic masses  140 ,  240  is created at the drive frequency of rotation-rate sensor  100 . 
     The vibration sensitivity of rotation-rate sensor  100  is reduced by an evaluation electronics system  300  shown in  FIG. 2 , by reducing or suppressing the direct component motions of seismic masses  140 ,  240 , effected by interference vibrations, by a regulated or captive operation of an evaluation electronics system  300 . Direct component motions in the frequency range between 0 Hz to barely above the cutoff frequency of a possibly present low-pass filter should expediently be suppressed. 
     In an evaluation electronics system  300 , detection means  260  detects a first capacitance  316  of the first capacitor formed by first seismic mass  140  and first counter-electrode  265  and a second capacitance  317  of the second capacitor formed by second seismic mass  240  and second counter-electrode  266 . If detection means  260  is a piezoelectric detection means, detection means  260  instead detects electrical characteristics variables of the piezoelectric detection means. First capacitance  316  and second capacitance  317  are converted to first and second voltages  326 ,  327 , that are proportional to capacitances  326 ,  327 , by a capacitance/voltage converter  320 . A differential element  330  gathers an analog differential signal  335  from the difference between first voltage  326  and second voltage  327 . An analog/digital converter  340  converts the analog differential signal  335  to a digital differential signal  345 . Via a controller  350 , digital differential signal  345  is fed back via a feedback  360  and compensation means  270  as a force to seismic masses  140 ,  240 , and at the same time is available as Coriolis acceleration  355  for a subsequent synchronous demodulation and low-pass filtering for determining the rotation rate. 
     In addition, an evaluation electronics system  300  includes a regulation member  310  which generates a compensation signal  315  from first voltage  326  and second voltage  327 . Regulation member  310  has a sufficiently high amplification at low frequencies so as, because of the control function, to hold the motions of seismic masses  140 ,  240  sufficiently small, based on interference accelerations, at sufficiently small phase rotation, so that the stability of the control loop is ensured. Furthermore, at the resonance points of the mechanical sensor structure of rotation-rate sensor  100 , particularly at the parallel resonance of seismic masses  140 ,  240 , regulation member  310  has a sufficiently high damping so that the loop gain of the open control loop does not exceed the factor 1 in this frequency range, and the control loop remains stable at these points. Compensation signal  315  is supplied to compensation means  270  in order to achieve the compensation for undesired interference deflections of the first and second seismic masses  140 ,  240 . If compensation means  270  is formed by capacitors made up of seismic masses  140 ,  240  and counter-electrodes  265 ,  266 , compensation means  270  is able to compensate for undesired interference deflections of seismic masses  140 ,  240 , for instance, by applying suitable voltages to the capacitors. 
     The compensations for undesired deflections of seismic masses  140 ,  240 , undertaken by compensation means  270 , puts active forces onto seismic masses  140 ,  240  which superpose themselves to form an overall force  306 , together with Coriolis forces  305  effected by a rotation rate acting on rotation-rate sensor  100 . This overall force  306  determines the effective deflections of first and second seismic masses  140 ,  240  which, in turn, are detected by detection means  260 . First and second seismic masses  140 ,  240 , detection means  260 , regulation member  310  and the compensation means  270  thereby form a control loop. 
     Besides the described use for the detection of rotation rates, rotation-rate sensor  100  explained with the aid of  FIG. 1 , and an evaluation electronics system  300  explained with the aid of  FIG. 2 , additionally offer the possibility of using first and second seismic masses  140 ,  240  for the detection of accelerations, acting on rotation-rate sensor  100  in measuring direction  215 , in the frequency range between 0 Hz and the cutoff frequency of the mechanical low-pass filter.