The invention relates to a vibrating gyroscope.
The operating principle of a vibrating gyroscope is explained in relation to
A mass M is suspended from a rigid frame C by means of two springs, of stiffness Kx and Ky, 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 Fx along x and Fy along y.
The mass M is excited at its natural frequency Fx 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 Fx and Fy—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 Fx and Fy 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 Fx and Fy 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 Fx and Fy coincident able to be maintained in the long term and under all environmental conditions.
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 Fx and capable of vibrating along a y axis perpendicular to the x axis, at a resonant detection frequency Fy, 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 Fy so that Fy is equal or practically equal to Fx throughout the duration of use of the gyroscope.
This feedback control loop thus makes it possible for the stiffness Ky to be permanently feedback-controlled so as to make the natural frequencies Fx and Fy 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 Fy, 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 Fx and Fy being deduced from this variation, and control means for controlling the Fy-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 Fy-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.
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:
a) and b) show schematically the curves representative of the control signal (in this case a voltage) for controlling the frequency modulation (
a), 5b) and 5c) show schematically, according to whether Fy>Fx, Fy=Fx or Fy<Fx, the curves corresponding to those of
a) shows schematically the detection signal Udet,y, the envelope of which is given by Δ|Udet,y| for the case in which Fx≠Fy; shown respectively in
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.
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 Uex,x, and detection electrodes 2 that deliver a detection voltage Udet,x proportional to the movement along x;
detection electrodes 3 that deliver a detection voltage Udet,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 Uex,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 Fy, for example by controlling the stiffness Ky, so as to make the natural frequencies Fy and Fx equal. To do this, a feedback control loop is proposed, which includes Fy-modifying means 11 (shown in
By disturbing the frequency of the excitation signal Uex,y, that is to say by applying a disturbing force along Oy 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 Uex,y frequency-modulated about the central frequency Fx at the frequency F0 of the following form:
Uex,y=Uex,0 sin(2π(Fx+ΔF sin(2πF0t)t),
Uex,0 being a constant.
Uex,y is shown in
b) indicates certain frequencies of Uex,y.
In practice, the frequency modulation is not necessarily sinusoidal, but triangular. F0 is chosen to be above the bandwidth of the gyroscope, but very much below Fx. For example, ΔF is about 10% of Fx.
Depending on whether the resonant frequency Fy is below, equal to or above the excitation frequency Fx, the variations in the amplitude of the detection signal |Udet,y| will be different:
These variations in the amplitude of the detection signal |Udet,y| are thus representative of the difference in Fx and Fy: 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 F0 in phase with the control signal (sector 1) or in phase opposition (sector 3) or a signal of frequency 2F0 (sector 2).
These three situations are illustrated in
In the case of
In the case of
In the case of
a) shows the detection signal Udet,y, the envelope of which is shown as Δ|Udet,y| in the case of which Fx≠Fy. A demodulation reference signal of frequency F0 and the error signal e coming from the synchronous demodulation device 15 are shown in
A gyroscope according to the invention will now be described. It comprises, as shown in
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 Uex,y frequency-modulated about the central frequency Fx at the frequency F0. The control signal from the oscillator is that shown in
The feedback control loop comprises the following elements.
The amplitude of the signal Udet,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 |Udet,y| and, after the signal |Udet,y| has passed through an F0-centered narrow band-pass filter 14 and then through an F0 reference frequency demodulator 15, an error signal e is produced, which becomes zero when the frequency Fy becomes equal to Fx.
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 Ky and therefore the frequency Fy.
The natural frequency Fy of the mass M along y is therefore properly slaved to the natural frequency Fx 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+v0 sin(2πF0t) 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.
The various elements described in relation to
The vibrating gyroscope according to the invention may have a plane or three-dimensional structure. It may or may not be micromachined.
Number | Date | Country | Kind |
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02/16365 | Dec 2002 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP03/51053 | 12/18/2003 | WO | 6/20/2005 |