WO1999031464A1

WO1999031464A1 - Vibrating gyroscope

Info

Publication number: WO1999031464A1
Application number: PCT/FR1998/002720
Authority: WO
Inventors: Michel Dufour
Original assignee: Commissariat A L'energie Atomique
Priority date: 1997-12-15
Filing date: 1998-12-14
Publication date: 1999-06-24
Also published as: FR2772469B1; FR2772469A1

Abstract

The invention concerns a vibrating gyroscope comprising a mass (2) mobile relative to a substrate (1) and designed for measuring a rotation vector perpendicular to the substrate plane, the mobile mass (2) being connected to the substrate (1) by return means (3) having a constant degree of stiffness identical whatever the displacement direction of the mobile mass (2) in a plane parallel to the substrate (1) plane, the gyroscope comprising means for electrically exciting the mobile mass (2) to cause it to vibrate at a predetermined angular velocity, the gyroscope also comprising sensing means for supplying each time data concerning the position of the mobile mass (2) relative to the substrate (1).

Description

VIBRATING GYROSCOPE

Technical area

The present invention relates to a vibrating gyroscope. It relates in particular to a micromachined gyroscope on silicon.

Industries interested in guiding or controlling the trajectory of mobiles are today demanding miniaturized devices making it possible to locate, act or make the history of the movement of the object. Among the fields concerned, we can cite air navigation, the automobile, the control of measuring or taking cameras. To this end, several generations of linear accelerometers have been developed in the context of microtechnologies over the past decades. The demand is now directed towards micro-gyroscopes allowing to measure angular displacements. A complete history of the trajectory of the mobile is therefore possible.

For the envisaged applications, the aerospace industries are generally very demanding with regard to the accuracy of information, whereas on the contrary, in a field like the automobile, the level of electromechanical performance can be much less decisive than the cost price of the device when choosing a product.

In this perspective, microtechnologies are preferred because they allow to consider the production of precise gyroscopes and inexpensive gyroscopes, see both at the same time. Mass production favors the reduction of costs, while the mechanical properties of the basic materials of microelectronics, in particular those of silicon, are particularly suitable for the manufacture of high-performance sensors.

State of the art

Mechanical gyroscopes compatible with microtechnologies can be grouped into three categories: single oscillators, multiple oscillators (double, triple or more) or tuning forks and shell or Bryan effect resonators.

Multiple oscillators, such as tuning forks, have a better insensitivity to linear accelerations than the other two categories, but their resolution is often limited.

Shell resonators take advantage of the establishment of standing waves along a closed structure with symmetry of revolution. Document EP-A-0 619 471 discloses an apparatus of this type. In practice, the standing wave, generated on the periphery of the hull, is the result of a double excitation of a ring according to the mode of order 2, of type "cos 2θ". The Coriolis force, due to the speed of rotation to be measured, modifies the position of the standing wave. This displacement can be detected. These devices are characterized by the deformation and not the displacement of the mass formed by the shell.

The simple oscillators group together all the structures characterized by a displacement of the vibrating mass in resonance or not. The mass, which does not deform, generally put into vibration at its own pulsation, is subjected to a Coriolis force under the action of external rotation, which can be detected. In microtechnologies, it is simpler to carry out the linear movements which have been privileged. In the absence of rotation, the mass moves along a constant axis. The detection is carried out in the same plane, which may not be that of the substrate.

Statement of the invention

The gyroscope according to the invention belongs to the category of simple oscillators since it has a vibrating mass. It can be defined by analogy with the Foucault pendulum but adapted to a planar structure. The gyroscope according to the invention is in fact based on an axial movement of the vibrating mass. The axis of this movement is caused to rotate in the plane perpendicular to the rotation vector to be measured, under the effect of the Coriolis force. The movement of this axis is therefore a precession. The speed of rotation of the mass oscillation axis provides direct information on the speed of rotation to be measured. It is a frequency detection. The invention differs from the category of shell resonators insofar as it is precisely the property that the mass presents not to deform but on the contrary to move freely in the plane which is used. Like shell resonators, the structure can have radial symmetry.

The subject of the invention is therefore a vibrating gyroscope comprising a mobile mass relative to a substrate and intended to measure a rotation vector perpendicular to the plane of the substrate, the mobile mass being attached to the substrate by return means having an identical stiffness whatever either the direction of movement of the moving mass in a plane parallel to the plane of the substrate and allowing movement of the mass in all directions of said plane, the gyroscope comprising means of electrical excitation of the mobile mass to make it vibrate at a determined pulsation in the plane of the substrate, the gyroscope also comprising detection means making it possible to supply information on the position of the mobile mass relative to the plane of the substrate.

The mobile mass and the return means can be located in a recess produced from one face of a substrate. The mobile mass can also be located above one face of the substrate, the return means connecting the mobile mass to anchoring means integral with the substrate. In this case, if the mobile mass has an annular shape, or any other shape of the annular type such as a hollowed out polygon, the anchoring means are advantageously constituted by an anchoring point located in the center of the ring.

Depending on the case, the return means may consist of flexible arms or by a bellows. Advantageously, the means for returning the mobile mass are arranged so as to provide the mobile mass with an isotropic return force in the plane of the substrate.

The electrical excitation means may be capacitive excitation means constituted by means forming electrodes, some linked to the moving mass facing the other, integral with the substrate.

The detection means can be capacitive detection means constituted by means forming electrodes, some linked to the moving mass facing the others secured to the substrate.

Means for controlling the moving mass can also be provided. These control means can be capacitive control means constituted by means forming electrodes, one linked to the moving mass facing the other, integral with the substrate.

The means forming electrodes linked to the moving mass can be conductive parts of the moving mass itself.

The detection and / or excitation means can also include strain gauges located on the return means.

Brief description of the drawings

The invention will be better understood by means of the description which follows, given by way of nonlimiting example, accompanied by the appended drawings among which:

FIG. 1 represents a first variant of a vibrating gyroscope according to the present invention,

FIG. 2 represents a second variant of a vibrating gyroscope according to the present invention,

FIG. 3 represents a third variant of a vibrating gyroscope according to the present invention, FIG. 4 represents a fourth variant of a vibrating gyroscope according to the present invention,

- Figure 5 shows a detail of the gyroscope shown in Figure 1, - Figure 6 shows a gyroscope according to the first embodiment, equipped with means for exciting the moving mass and means for detecting the position of the mass mobile, FIGS. 7 and 8 represent gyroscopes according to the third variant embodiment, equipped with means for exciting the moving mass,

- Figure 9 shows a gyroscope according to the third embodiment, equipped with means of excitation, detection and control of the moving mass.

Detailed description of embodiments of the invention

The inertial force, known as Coriolis, acts on a mobile having a relative speed with respect to a reference point itself in rotation with respect to a fixed reference point. In practice, this relative movement is a periodic movement of pulsation Ω ₀ . The intensity of the inertial force is proportional to the sine of the angle between the relative speed vector and the axis of the rotation vector. It is therefore advantageous, for optimization questions, to design devices in which the relative speed is perpendicular to the rotation vector.

In the case of microtechnology gyroscopes, it is convenient to reason by attaching a Cartesian coordinate system (x, y, z) to the substrate, the x and y axes being in the plane of the substrate and the z axis being perpendicular to it. The rotation vector then designates the optimally detected vector. Indeed, its component parallel to the relative speed vector does not generate a Coriolis force. One can thus call in a generic way "rotation vector" the component of the rotation which remains perpendicular both to the relative speed imposed on the mobile and to the direction according to which the Coriolis force is measured. If we call ω the pulsation of rotation we show that, in the plane (x, y), the coordinates

(x ₂ , y ₂ ) as a function of time t of a mass with an oscillating movement relative to its rest position, are described by the following system of equation:

_^ ω _^ - x ₂ = X [cos (Ω _0. t) .cos (ω. t) + - sin (Ω _0. t). sin (ω. t)]

x ₂ = X [cos (Ω _0. t). sin (ω. t) - - sin (Ω _0. t). cos (ω. t)]

The value of the amplitude X is determined by the initial pulse, the stiffness of the return device and the friction losses. In practice, it may be advantageous to increase this amplitude by working at the resonant frequency of the device.

It is from the position of the mass in the plane (x, y) that we can calculate the angle of rotation of the entire device with respect to a fixed reference. As the system of equations of movement shows, position involves a product of temporal functions depending on the pulsation of vibration and that of rotation.

By signal processing, it is possible to dissociate the effects relating to the two pulses. In practice, this means that a single electrical device can perform the two functions of excitation of the movement of the mass and detection of that of precession. It may however be advantageous, in certain applications, to provide two separate systems, one for excitation, the other for detection. FIG. 1 represents a first variant of a vibrating gyroscope according to the invention, obtained for example by micromachining of a substrate 1. From a plane face of the substrate 1, defining the plane (x, y), a mobile mass 2 has been obtained housed in a recess 4 of the substrate and attached to the substrate by three return arms 3 regularly distributed and of curved shape. The moving mass can have an indifferent shape. In Figure 1, the moving mass is circular.

The return means of the mobile mass must provide an isotropic return force in the plane (x, y) of the substrate. It is preferable that the return elements of the movable mass have an angular periodicity in their arrangement and that they are at least three in number. The return elements can be arms or beams, straight or not, in one or in several segments.

FIG. 2 represents a second variant of a vibrating gyroscope according to the invention. We find a flat substrate 5 provided with a recess 6 in which is housed the movable mass 7. The movable mass is, in this example, of square shape. The return elements 8 are this time four in number and each formed from several segments arranged end to end. They are regularly distributed to provide an overall isotropic stiffness in the plane (x, y) of the substrate.

FIG. 3 represents a third variant of a vibrating gyroscope according to the invention. The mobile mass 11 is in a plane parallel to the plane of the substrate 10 but, unlike the first two variants, the mobile mass, which has a ring shape, is located above the substrate. The mobile mass 11 is attached to the substrate 10 by a central anchoring point 12 to which it is connected by flexible arms 13. As before, the flexible arms 13 are evenly distributed to provide an overall isotropic stiffness.

FIG. 4 represents a fourth variant of a vibrating gyroscope according to the invention. The circular mobile mass 16 is this time in the plane of the substrate 15 but the return means are constituted by a continuous bellows type medium 17.

These gyroscopes according to the present invention can be manufactured by micromachining process steps well known to those skilled in the art. Since the mobile mass must be produced on the front face of a substrate, it is possible to start by etching the rear face of the substrate in order to give the mobile mass the desired thickness. Then, an etching step via the front face of the substrate makes it possible to delimit the mobile mass and the return means.

The steps described above are supplemented or modified according to the nature of the means provided for obtaining the excitation of the mobile mass and the detection of its movement. These means will put the seismic mass in motion relative to the support and will detect its position in the plane (x, y). These two functions can possibly be dissociated. The position of the moving mass can be detected by a family of displacement or stress detectors. These detectors can be capacitors with variable capacity, by varying the size of the air gap or the surface of the electrodes actually facing each other. They can also be devices with piezoresistive or piezoelectric strain gauges, or devices using, for example, Laplace force according to a principle already listed in the prior art. Detectors may or may not be associated with the suspension device.

An example of detection by a strain gauge is shown in Figure 5. This figure is a detail view of Figure 1. It shows the connection of a return arm 3 on the substrate 1. At this location, a gauge constraint 20 has been formed. Each of its ends is connected to a conductor 21, 22 intended to transmit the information collected. The strain gauge 20 can be formed by doping the substrate at the very start of the process for producing the gyroscope. The conductors 21 and 22 can be produced, at the end of the process, by depositing a metal layer and etching this layer. To vibrate the moving mass at the pulsation Ω ₀ , a single electrode may suffice. The conventional techniques of excitation of microtechnologies are suitable. We can cite the capacitive type excitation. Figures 6 and 7 illustrate two possible embodiments. Figure 6 illustrates a gyroscope of the type of Figure 1, seen from above. We recognize the substrate 25, the moving mass 26 produced in a recess of the substrate and attached elastically to this substrate by return arms 27 symbolically represented. The mobile mass 26, completely or partially conductive (for example by doping the silicon substrate) constitutes one of the electrodes of an excitation capacitor. A second electrode 28, fixed relative to the substrate, was provided during the process for producing the gyroscope. It is located opposite the mobile mass 26 and forms, with this mobile mass, a capacitor. If an excitation signal is applied between the electrode 28 and the moving mass 26, the moving mass is alternately attracted and released (or repelled) so as to vibrate at its own pulsation. This minimum configuration can advantageously be supplemented by the addition of additional excitation electrodes intended, for example, to obtain better control of the vibration or of the position of the mass, or any other function.

FIG. 7 illustrates a gyroscope of the type of FIG. 3, seen from above. We recognize the substrate 30 and the movable mass 31 attached elastically to the anchor point 32 by return arms 33 symbolically represented. As before, the seismic mass is completely or partially conductive and constitutes one of the electrodes of an excitation capacitor. In this case, the second electrode 34 of the excitation capacitor, which is attached to the substrate 30, can be located outside of the moving mass 31. It can also be located inside the moving mass 31. shows such an electrode, referenced 35, in broken lines.

The detection must provide the position information of the moving mass at all times. The evolution of this position over time results in a movement dependent on the vibration Ω ₀ and the precession at the frequency ω. Frequency analysis therefore makes it possible to calculate the angular speed sought.

The detectors are such that the variation of the electrical quantity measured, for example an impedance, is a strictly monotonic function of the distance "d" between a fixed point which is attached to them and the center of gravity of the whole of the mobile structure . We can then show that there are such detectors. A capacitor such as those envisaged previously for excitation, that is to say a fixed electrode and the other constituted by the moving mass, is suitable. If it is supposed that, under the effect of the displacement of the mass, the surface of the opposite electrodes does not vary, the value of the capacity ε. S is then written C = which is indeed a strictly monotonic function e (d) of "d". At least two such detectors must be used to determine by

"triangulation" and unequivocally the position of the mobile mass whose displacement of the center of gravity is inscribed in the circle whose center is its position at rest, and the radius its distance from a detector.

The minimum configuration necessary for determining the position of the mass requires two electrodes not aligned with the center of gravity. One of these electrodes can optionally also be used alternately for the excitation of the moving mass. The conventional means for detecting microtechnologies can be used and among them capacitive detection. Thus, in the case of FIG. 6, the addition of an additional electrode 29, not aligned with the electrode 28, is sufficient if the movable mass 26 constitutes another electrode opposite the electrode 29. The moving mass 26 constitutes, as for the excitation, a reference. The electrodes 28 and 29 make it possible to determine the distance separating them from the moving mass.

By geometric construction we deduce the relative position of the mass at a given instant. The number of electrodes can advantageously be increased. It is also advantageously possible to arrange the electrodes equidistant angularly from one another as shown in FIG. 8 which illustrates a gyroscope of the type of FIG. 3, seen from above. We recognize the substrate 40 and the movable mass 41 attached elastically to the anchor point 42 by return arms 43 symbolically represented. The mobile mass being conductive, n capacitive detection capacitors are formed by regularly placing n electrodes 44 facing the mobile mass 41, inside the mobile mass. Instead of placing them inside, these detection electrodes can be placed outside. This is the role of the electrodes 45 shown in dashed lines and angularly equidistant.

The configurations in FIGS. 6 and 8 are not limiting and, for each structure, two electrodes (not aligned with the center of gravity) with n electrodes can be arranged relative to the mobile mass. The increase in the number of electrodes can advantageously be envisaged to increase the resolution of the sensor or improve the signal / noise ratio. It may be advantageous for one or more excitation devices to contribute to both the excitation and the detection. Indeed, insofar as the excitation signals and the precession characteristic of the rotation are at different frequencies, it is possible, for example, to use a multiplexing effect.

The structure can advantageously be slaved in position using electrodes used for excitation and detection. The servo-control can advantageously be carried out using additional electrodes which can also be of the capacitive type and arranged around the moving mass. Enslavement consists in the creation of a force which opposes the detected Coriolis force and maintains the moving mass along its trajectory initial resulting from its vibration at its pulsation Ω ₀ along an axis defined by the excitation electrode (s).

FIG. 9 shows an example of a gyroscope according to the invention, of the type of FIG. 3, comprising detection, servo-control and excitation electrodes. We recognize the substrate 50 and the moving mass 51 attached elastically to the anchor point 52 by return arms 53 symbolically represented. As before, the moving mass 52 constitutes as many capacitor electrodes for the electrodes which are placed opposite it.

The mobile mass 41 is excited by applying an appropriate voltage, at the resonant frequency, between the mobile mass 41 and, alternately, between the electrodes 54 and 55. An oscillating movement of the mobile mass is obtained. the pulse Ω ₀ .

Control can be obtained by electrodes 56 and 57 or 58 and 59 in association with the conductive moving mass. Under the effect of the Coriolis force, the mobile mass, oscillating in the direction of an axis passing through the electrodes 54 and 55, tends to rotate around the anchor point 52. An excitation force is then created offset which maintains the excitation in the direction defined by the electrodes 54 and 55.

The same electrodes (54 and 55 and possibly 56 and 57, 58 and 59) in association with the moving mass can be used to detect the position of the moving mass from which the angular speed orthogonal to the plane can be calculated.

The structure can also be slaved in frequency by requesting all of the excitation to produce a rotating force field so as to maintain the overall movement of the mobile mass, that is to say its oscillation with precession. In this case, the frequency of rotation of the rotating field is controlled by the precession frequency of the mobile.

Claims

1. Vibrating gyroscope comprising a mass (2, 7, 11, 16, 26, 31, 41, 51) movable relative to a substrate (1, 5, 10, 15, 25, 30, 40, 50) and intended for measure a rotation vector perpendicular to the plane of the substrate, the mobile mass being attached to the substrate by return means (3, 8, 13, 17, 27, 33, 43, 53) having an identical stiffness whatever the direction of movement of the moving mass in a plane parallel to the plane of the substrate and allowing a displacement of the mass in all directions of said plane, the gyroscope comprising means of electrical excitation of the moving mass to make it vibrate at a determined pulsation in the plane of the substrate, the gyroscope also comprising detection means making it possible to supply information at all times on the position of the moving mass relative to the plane of the substrate.

2. vibrating gyroscope according to claim 1, characterized in that the movable mass (2, 7) and the return means (3, 8) are located in a recess

(4, 6) produced from one face of a substrate (1, 5).

3. Vibrating gyroscope according to claim 1, characterized in that the mobile mass (11) is located above one face of the substrate (10), the return means (13) connecting the mobile mass (11) to anchoring means (12) integral with the substrate.

4. Vibrating gyroscope according to claim 3, characterized in that the mobile mass (11) having an annular shape, the anchoring means consist of an anchoring point (12) located in the center of the ring.

5. Vibrating gyroscope according to claim 1, characterized in that the return means consist of flexible arms (3, 8, 13).

6. Vibrating gyroscope according to claim 1, characterized in that the return means consist of a bellows (17).

7. Vibrating gyroscope according to any one of claims 1 to 6, characterized in that the means for returning the moving mass are arranged so as to provide the moving mass with an isotropic restoring force in the plane of the substrate.

8. Vibrating gyroscope according to any one of claims 1 to 7, characterized in that the electrical excitation means are capacitive excitation means constituted by means forming electrodes, some linked to the moving mass (26, 31 ) facing the others (28, 34) integral with the substrate (25, 30).

9. vibrating gyroscope according to any one of claims 1 to 7, characterized in that the detection means are capacitive detection means constituted by means forming electrodes, some linked to the movable mass facing the others secured to the substrate .

10. Vibrating gyroscope according to any one of claims 1 to 7, characterized in that it further comprises means for controlling the mobile mass.

11. Vibrating gyroscope according to claim 10, characterized in that the servo means are capacitive servo means constituted by means forming electrodes, some linked to the moving mass facing the others secured to the substrate.

12. Vibrating gyroscope according to any one of claims 8, 9 and 11, characterized in that the means forming electrodes linked to the moving mass are conductive parts of the moving mass itself.

13. Vibrating gyroscope according to any one of claims 1 to 7, characterized in that the detection and / or excitation means comprise strain gauges (20) located on the return means (3).

14. Vibrating gyroscope according to any one of the preceding claims, characterized in that it is formed from a silicon substrate.