Abstract:
The invention relates to a gyroscope comprising at least one mass capable of vibrating along an x axis at a resonant excitation frequency F x  capable of vibrating along a y axis perpendicular to the x axis, at a resonant detection frequency F y , under the effect of a Coriolis force generated by a rotation about a z axis perpendicular to the x and y axes. It includes, connected to the mass or masses, a feedback control loop for controlling the resonant frequency F y  so that F y  is equal or practically equal to F x  throughout the duration of use of the gyroscope.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present Application is based on International Application No. PCT/EP2003/051053, filed on Dec. 18, 2003, which in turn corresponds to FR 02/16365 filed on Dec. 20, 2002, and priority is hereby claimed under 35 U.S.C §119 based on these applications. Each of these applications are hereby incorporated by reference in their entirety into the present application. 
   FIELD OF THE INVENTION 
   The invention relates to a vibrating gyroscope. 
   BACKGROUND OF THE INVENTION 
   The operating principle of a vibrating gyroscope is explained in relation to  FIG. 1 . 
   A mass M is suspended from a rigid frame C by means of two springs, of stiffness K x  and K y , It therefore possesses two degrees of freedom, along the x and y directions. 
   The system may be considered as an assembly of two resonators having eigenfrequencies or natural frequencies F x  along x and F y  along y. 
   The mass M is excited at its natural frequency F x  along the x axis. 
   When a speed of rotation Ω about the third, z axis is present, the Coriolis forces induce coupling between the two resonators, causing the mass to vibrate along the y axis. 
   The amplitude of the movement along y is then proportional to the speed of rotation Ω. 
   This amplitude is also a function of the difference in the natural frequencies F x  and F y —maximum sensitivity is achieved when the two natural frequencies are equal. 
   In particular, for high-performance gyroscopes, it is necessary to obtain maximum sensitivity of the displacement relative to the speed of rotation. It is therefore very desirable to make these frequencies equal. 
   However, when the frequency equality condition is met, the bandwidth of the gyroscope becomes very small. To increase it, the detection movement along y is feedback controlled, by applying an electrostatic or electromagnetic force along the y axis to the mass, which force counterbalances the force created by the Coriolis coupling. There is no longer any vibration of the mass along y and it is then the feedback force proportional to the speed of rotation Ω that is measured. 
   It is therefore desirable in vibrating gyroscopes of higher performance for the movement along the y axis to be feedback controlled and for the frequencies F x  and F y  to be made coincident. 
   However, the dispersion due to the method of production in manufacture does not allow a perfectly zero frequency difference to be obtained. It is therefore necessary to make an adjustment in order for the two frequencies to be equal. 
   A first method consists in making these frequencies equal by mechanical balancing. This therefore involves modifying the mass or stiffness characteristics of one or other of the resonators by removing material. This method may be used for carrying out a coarse initial adjustment of the frequencies. 
   Another method consists in carrying out electrical balancing. By means of electrodes, a variable electrostatic (or electromagnetic) stiffness is added to one of the two resonators so as to vary its natural frequency. This method allows a very fine initial adjustment of the frequencies to be made using an electrical voltage applied to the electrodes. 
   If a gyroscope whose frequencies have been initially adjusted by one of these methods is used, the initial adjustment of making the mechanical resonant frequencies F x  and F y  coincide cannot be maintained in the long term and under all environmental conditions. 
   This is because parasitic mechanical effects and the thermoelasticity effects are not strictly identical in both resonators and these effects may result in a frequency differentiation when the environmental, both mechanical and thermal, conditions vary. 
   One important object of the invention is therefore to propose a vibrating gyroscope that allows the initial adjustment of making the mechanical resonant frequencies F x  and F y  coincident able to be maintained in the long term and under all environmental conditions. 
   SUMMARY OF THE INVENTION 
   To achieve this object, the invention proposes a gyroscope comprising at least one mass M capable of vibrating along an x axis at a resonant excitation frequency F x  and capable of vibrating along a y axis perpendicular to the x axis, at a resonant detection frequency F y , under the effect of a Coriolis force generated by a rotation about a z axis perpendicular to the x and y axes, mainly characterized in that it comprises, connected to the mass or masses M, a feedback control loop for controlling the resonant frequency F y  so that F y  is equal or practically equal to F x  throughout the duration of use of the gyroscope. 
   This feedback control loop thus makes it possible for the stiffness K y  to be permanently feedback-controlled so as to make the natural frequencies F x  and F y  along the two directions equal. 
   According to one feature of the invention, the gyroscope includes a signal generator for generating a signal that disturbs the vibration of the mass M along y, said generator being connected to the mass M, and the feedback control loop comprises: means for modifying the resonant detection frequency F y , means for detecting the variation, induced by the disturbing signal, in the vibration of the mass M along y, an error signal representative of the difference between F x  and F y  being deduced from this variation, and control means for controlling the F y -modifying means, the control being established on the basis of the error signal. 
   According to a first embodiment of the invention, the disturbing-signal generator is connected to the mass M via the F y -modifying means. 
   According to another embodiment, when the gyroscope includes excitation means for exciting the mass M along y with the aim of counterbalancing the vibration along y generated by the Coriolis force, the disturbing-signal generator is connected to the mass M via these excitation means. 
   Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages of the invention will become apparent on reading the following detailed description, given by way of nonlimiting example and with reference to the appended drawings in which: 
       FIG. 1  illustrates schematically the operating principle of a vibrating gyroscope; 
       FIG. 2  shows schematically the necessary main components relating to a single mass of a gyroscope according to the prior art; 
       FIG. 3  shows schematically a curve representative of the variation of the amplitude (in dB) of the detection signal |U det,y |, corresponding to the movement of the mass along y, as a function of the frequency in Hz of the excitation signal U ex,y  according to the prior art; 
       FIGS. 4   a ) and  b ) show schematically the curves representative of the control signal (in this case a voltage) for controlling the frequency modulation ( FIG. 4   a ) and of the perturbing signal U ex,y  frequency-modulated about the central frequency F x  at the frequency F 0  ( FIG. 4   b ), expressed as a function of time; 
       FIGS. 5   a ),  5   b ) and  5   c ) show schematically, according to whether F y &gt;F x , F y =F x  or F y &lt;F x , the curves corresponding to those of  FIGS. 3 and 4   a ) and also the corresponding variation of the amplitude of the detection signal Δ|U det,y |; 
       FIG. 6   a ) shows schematically the detection signal U det,y , the envelope of which is given by Δ|U det,y | for the case in which F x ≠F y ; shown respectively in  FIGS. 6   b ) and  6   c ) are a reference demodulation signal of frequency F 0  and an error signal e; 
       FIG. 7  shows schematically the necessary main components relating to a signal mass in an example of a gyroscope according to the invention; and 
       FIG. 8  shows schematically the necessary main components relating to a signal mass of another example of a gyroscope according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   High-precision vibrating gyroscopes generally have two symmetrical vibrating masses operating in what is called tuning-fork mode. 
   In micromachined sensors, the excitation movement is generally provided by electrostatic forces along the x direction. These forces are often created by means of electrostatic combs. 
   The detection movement is picked up along a y direction perpendicular to x. In the case of micromachined sensors produced in a plane structure, this y direction may, depending on the case, lie in the plane of the plane structure or perpendicular to this plane. 
     FIG. 2  shows the necessary main components relating to a single mass, for the sake of simplicity. 
   Conventionally, means are provided:
         for applying excitation forces along the x direction and for detecting the movement of the masses along x so as to feedback control these excitation forces;   for detecting the movement of the masses along the y direction; and   for applying feedback forces to the masses along y, these forces being intended to counterbalance the forces created by the Coriolis coupling along y.       

   These means generally consist of sets of electrodes. The x and y resonators therefore have various types of electrodes:
         excitation electrodes  1 , for applying an excitation force along x proportional to a control voltage U ex,x , and detection electrodes  2  that deliver a detection voltage U det,x  proportional to the movement along x;   detection electrodes  3  that deliver a detection voltage U det,y  proportional to the movement along y; and   feedback electrodes  4  which are in fact excitation electrodes for applying a feedback force to the y resonator proportional to a control voltage U ex,y .       

   The means  2  for detecting the movement of the mass along x are connected to the means  1  for applying excitation forces along the x direction via an oscillator  5  and an amplitude regulation device  6  placed in parallel with the oscillator  5 . 
   An excitation or feedback loop for excitation along y comprises the following elements. The means  3  for detecting the movement of the mass along y are connected to the means  4  for applying feedback forces along the y direction by a shaping device  7 , in series with a synchronous demodulator  8 , a corrector  9  and then a modulator  10 . The output signal from the gyroscope comes from the corrector  9 . 
   The object of the invention is to provide permanent feedback control of F y , for example by controlling the stiffness K y , so as to make the natural frequencies F y  and F x  equal. To do this, a feedback control loop is proposed, which includes F y -modifying means  11  (shown in  FIGS. 7 and 8 ) such as, for example, electrodes for controlling the stiffness K y , which are controlled on the basis of an error signal representative of the difference between F x  and F y . The error signal is determined as follows. 
     FIG. 3  shows schematically a curve representative of the variation of the amplitude (in dB) of the signal |U det,y | coming from the electrodes for detecting the movement of the mass along y, as a function of the frequency in Hz of the excitation signal U ex,y  applied to the excitation electrodes. This curve shows a maximum when F x =F y  and decreases otherwise. 
   By disturbing the frequency of the excitation signal U ex,y , that is to say by applying a disturbing force along O y  to the mass, a disturbance of the detection signal, corresponding to the movement of the mass along y, is obtained, this disturbance being representative of the error signal. 
   The disturbing force is generated by applying, to the y excitation electrode  4 , a disturbing voltage U ex,y  frequency-modulated about the central frequency F x  at the frequency F 0  of the following form:
 
 U   ex,y   =U   ex,0  sin(2π( F   x   +ΔF  sin(2 πF   0   t ) t ),
 
U ex,0  being a constant.
 
   U ex,y  is shown in  FIG. 4   b ) and obtained by applying, to an oscillator, a signal (in this case a voltage) for controlling the frequency modulation shown in  FIG. 4   a ). 
     FIG. 4   b ) indicates certain frequencies of U ex,y . 
   In practice, the frequency modulation is not necessarily sinusoidal, but triangular. F 0  is chosen to be above the bandwidth of the gyroscope, but very much below F x . For example, ΔF is about 10% of F x . 
   Depending on whether the resonant frequency F y  is below, equal to or above the excitation frequency F x , the variations in the amplitude of the detection signal |U det,y | will be different:
         if F y &gt;F x , Δ|U det,y |=u sin(2πF 0 t) (sector  1 , shown in  FIG. 5   a )   if F y =F x , Δ|U det,y |=u sin(4πF 0 t) (sector  2 , shown in  FIG. 5   b )   if F y &lt;F x , Δ|U det,y |=−u sin(2πF 0 t) (sector  3 , shown in  FIG. 5   c ).       

   These variations in the amplitude of the detection signal |U det,y | are thus representative of the difference in F x  and F y : the error signal e is deduced from this difference. 
   Depending on the sector in question, the amplitude of the error signal is a signal of frequency F 0  in phase with the control signal (sector  1 ) or in phase opposition (sector  3 ) or a signal of frequency 2F 0  (sector  2 ). 
   These three situations are illustrated in  FIGS. 5   a ),  5   b ) and  5   c ), respectively. Each case shows the same curve as that in  FIG. 3  and the variation in the signal for controlling the frequency modulation of U ex,y  as shown in  FIG. 4   a ), and the corresponding variation in the amplitude of the detection signal Δ|U det,y | from which the error signal e is deduced. 
   In the case of  FIG. 5   a ) where F x &lt;F y , Δ|U det,y | is a signal of frequency F 0  in phase with the control signal. 
   In the case of  FIG. 5   b ) where F x =F y , Δ|U det,y | is a signal of frequency 2F 0 . 
   In the case of  FIG. 5   c ) where F x &gt;F y , Δ|U det,y | is a signal of frequency F 0  in phase opposition with the control signal. 
     FIG. 6   a ) shows the detection signal U det,y , the envelope of which is shown as Δ|U det,y | in the case of which F x ≠F y . A demodulation reference signal of frequency F 0  and the error signal e coming from the synchronous demodulation device  15  are shown in  FIGS. 6   b ) and  6   c ) respectively. 
   A gyroscope according to the invention will now be described. It comprises, as shown in  FIG. 7 , in addition to the elements described in relation to  FIG. 2  and identified by the same references, a signal generator  12  for generating a signal that disturbs the vibration of the mass along y, connected to the mass M, and a feedback control loop for slaving the resonant frequency F y  to the frequency F x . 
   The disturbing force is generated by applying, to the y excitation electrode  4 , by means of the generator  12  such as a VCO (voltage-controlled oscillator) connected to the y excitation loop, a disturbing voltage U ex,y  frequency-modulated about the central frequency F x  at the frequency F 0 . The control signal from the oscillator is that shown in  FIG. 4   a ). 
   The feedback control loop comprises the following elements. 
   The amplitude of the signal U det,y  is recovered by means of an amplitude detector  13  after a shaping device  7  has shaped the signal coming from the detection electrodes  3 . This detector  13  delivers |U det,y | and, after the signal |U det,y | has passed through an F 0 -centered narrow band-pass filter  14  and then through an F 0  reference frequency demodulator  15 , an error signal e is produced, which becomes zero when the frequency F y  becomes equal to F x . 
   After integration by means of an integrator/corrector  16 , this error signal may control a voltage V on the stiffness electrode  11  that modifies the stiffness K y  and therefore the frequency F y . 
   The natural frequency F y  of the mass M along y is therefore properly slaved to the natural frequency F x  along x. 
   In the case described above, a disturbing force was applied to the mass along y by modulating the frequency of the excitation signal. 
   Rather than modulating the excitation frequency, it is possible, according to a variant of the invention, to modulate the amplitude of the electrostatic stiffness. 
   In this case, a voltage V+v 0  sin(2πF 0 t) is applied to the stiffness electrode  11 . The effect on the detection signal is then equivalent to that obtained by modulating the frequency of the excitation signal. 
     FIG. 8  shows the gyroscope corresponding to this variant. The disturbing force is then generated by applying, to the y stiffness electrode  11 , the disturbing voltage v 0  sin(2πF 0 t) generated by an oscillator ( 12 ′) centered on the frequency F 0 , connected to the feedback control loop for slaving F y  to F x . The feedback control loop is the same as that described in relation to  FIG. 7 . 
   The various elements described in relation to  FIGS. 2 ,  7  and  8  may of course be produced in analogue or digital technology. 
   The vibrating gyroscope according to the invention may have a plane or three-dimensional structure. It may or may not be micromachined. 
   It will be readily seen by one of ordinary skill in the art that the present invention fulfills all of the objects set forth above. After reading the foregoing specification, one of ordinary skill will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof.