Micromechanical roatational rate sensor

Abstract

The present invention creates a micromechanical rotational rate sensor having a first Coriolis mass element (2a) and a second Coriolis mass element (2b) which are situated over a surface of a substrate (100); having an activating device by which the first Coriolis mass element (2a) and the second Coriolis mass element (2b) are able to have vibrations activated along a first axis (x); and having a detection device by which deflections of the first Coriolis mass elements (2a) and of the second Coriolis element (2b) are able to be detected along a second axis (y), which is perpendicular to the first axis (x), on the basis of a correspondingly acting Coriolis force; the first axis (x) and second axis (y) running parallel to the surface of the substrate (100);

Description

BACKGROUND INFORMATION

[0001] The present invention relates to a micromechanical rotational rate sensor.

[0002] A rotational rate sensors in which a first and a second Coriolis element are arranged on the surface of a substrate are already known from the U.S. Pat. No. 5,728,936. The Coriolis elements are induced to vibrate along a first axis. The deflections of the Coriolis elements due to a Coriolis force along a second axis, which is likewise parallel to the substrate, are verified.

[0003] DE 198 32 906 C1 describes a capacitive rotational rate sensor made up of a flexibly supported, mirror-symmetrically designed seismic mass, on which electrodes are fastened in a comb-like manner. At least two groups of comb-like counterelectrodes are provided, arranged in mirror symmetry, which are each fastened to a carrier and engage between the electrodes fastened to the seismic mass. The carriers of the counterelectrodes are fastened only in the vicinity of the axis of symmetry at the closest point on a ceramic carrier. A frame is also provided on which the seismic mass is fastened via two leaf springs. Two actuators are used for the excitation of vibrations of the frame, which has integrated vibratory springs and which is fastened on the ceramic carrier at at least two points of attachment.

[0004] EP 0 775 290 B1 describes a rotational rate sensor made up of at least two vibrating masses, which are connected to each other via a spring element to form a system capable of vibrating, which is supported on a substrate. Also provided are actuators for inducing vibrations, as well as at least one sensing element for the detection of the Coriolis force. The spring elements and the vibrating masses are positioned and designed in such a way that the system capable of vibrating is only able to execute vibrations in at least two vibration modes parallel to the plane of the substrate, one mode being used as excitation mode of the vibration excitation and the second mode, preferably orthogonal to it, is excited as the detection mode upon rotation about an axis perpendicular to the substrate to the Coriolis forces.

[0005] From M. Lutz, W. Golderer, J. Gerstenmeier, J. Marek, B. Maihöfer and D. Schubert, A Precision Yaw-Rate Sensor in Silicon Micromachining; SAE Technical Paper, 980267, and from K. Funk, A. Schilp, M. Offenberg, B. Elsner, and F. Lärmer, Surface-Micromachining of Resonant Silicon Structures, The 8th International Conference on Solid State Sensors and Actuators, Eurosensors IX, Stockholm, Sweden, Jun. 25-29, 1995, pp. 50-52, other rotational rate sensors are known.

[0006] One disadvantage of the known rotational rate sensors is the sensitivity of the structures with respect to interference accelerations, particularly with respect to angular accelerations about the sensitive axis, as well as with respect to the insufficient robustness of the structures.

[0007] One reason for the sensitivity with respect to interference accelerations is particularly founded in the low working frequency of these rotational rate sensors (1.5 kHz to 6 kHz), since in this frequency range interference accelerations may occur in the motor vehicle which have nonnegligible amplitudes.

[0008] A second reason is linked to the functioning principles of rotational rate sensors. In the case of a certain sensor type, besides a (desired) external rotational speed about the sensitive axis, a measuring signal is also triggered by a rotational acceleration about the same axis. Therefore, the known rotational rate sensors are particularly sensitive to this kind of interference acceleration.

[0009] The low working frequencies are also a reason for the inadequate robustness of the rotational rate sensors, particularly as regards falling protection. A further reason for the inadequate robustness is involved with a complicated process control, for example, a combination of bulk and surface micromechanics.

SUMMARY OF THE INVENTION

[0010] The idea that the present invention is based on is that the centers of gravity of the first Coriolis mass element, the second Coriolis mass element, the first detecting mass device and the second detecting mass device, when at rest, coincide at one common mass center of gravity. If one operates a rotational rate sensor, constructed in this manner, using excitation in phase opposition which causes an opposite deflection of the first detecting mass device and the second detecting mass device under the influence of a Coriolis force, separate effects due to external linear accelerations or centrifugal accelerations may be removed, since these only bring with them a same-directed deflection of the first detecting mass device and the second detecting mass device. In addition, rotational accelerations about the sensing axis bring about no deflection, and consequently have no influence.

[0011] The micromechanical rotational rate sensor according to the present invention, having the features of Claim 1, thus have the special advantage compared to the known attempts to solve the problem, that sensitivity to interference and cross sensitivity, robustness and resolution range (driving dynamics range) are greatly improved. This is achieved by the possibility of selecting a high operating frequency and by the special symmetrical designs of the sensor mass elements.

[0012] The structure described is designed for manufacturing in straightforward micromechanical technology, but converting the functional principles to other technologies (bulk micromechanics, LIGA, etc.) is easily possible. The sensor element is designed so that, with respect to a silicon substrate, which is used at the same time as a reference coordinate system, movably suspended seismic masses are set into vibration parallel to the substrate plane. An external rotational rate acting about the substrate normal generates a Coriolis acceleration in the moved masses perpendicularly to the direction of motion and perpendicularly to the substrate normals, i.e. also parallel to the substrate plane. Thus, what is involved here in the system described, is an in-plane/in-plane linear vibrator system.

[0013] In the case of the structure described here, at the same time, the tuning fork principle and the inverse tuning fork principle play a role. The structure described is designed for operating frequencies >10 kHz. This leads to a further reduction of the sensitivity to interference of the sensor elements in the case of their use in the automobile field, since particularly interference accelerations in a vehicle are clearly reduced in this frequency range compared to the frequency range in use up to now, of typically 1.5 kHz to 6 kHz. The selection of the operating frequency furthermore leads to substantially more robust sensor structures having an increased falling protection. A suspension on numerous folded spring elements also contributes to robustness.

[0014] As the functional principle, both the tuning fork principle and the inverse tuning fork principle are implemented here. In that specific embodiment, the mass centers of gravity of the individual masses at rest coincide with the mass center of gravity of the entire vibrating structure at rest. The activation and detection preferably take place orthogonally to each other in the substrate plane, about the common mass center of gravity. The position of the mass center of gravity of the entire vibrating structure is time-invariant with respect to the substrate in normal operation. The advantage of this functional principle, as compared to the known principles, is that in the ideal case, both a rotational acceleration about the Z axis produces no measuring effect and, in response to a suitable design, centrifugal accelerations about the Z axis and linear accelerations in the sensing direction also produce no measuring effect, as a result of which sources of interference are able to be suppressed.

[0015] The dependent claims include advantageous further refinements of and improvements to the micromechanical rotational rate sensor indicated in Claim 1.

[0016] According to one preferred refinement, the first detecting mass device is connected via first springs, which are designed to be flexible along the first axis and stiff along the second axis, to the first Coriolis mass element, and, via second springs, which are designed to be stiff along the first axis and flexible along the second axis is connected to the substrate. At the same time, the second detecting mass device is connected to the second Coriolis mass element via third springs, which are designed to be flexible along the first axis and stiff along the second axis, and, via fourth springs, which are designed to be stiff along the first axis and flexible along the second axis, is connected to the substrate.

[0017] According to another preferred refinement, the activating device has a first activating mass device and a second activating mass device, the centers of gravity of the first activating mass device and the second activating mass device also coinciding at the common mass center of gravity in a state of rest.

[0018] According to a further preferred refinement, the first activating mass device has a first activating mass element and a second activating mass element, and the second activating mass device has a third activating mass element and a fourth activating mass element, which are able to be activated individually each via a respective comb actuator.

[0019] According to one preferred refinement, the first and second activating mass elements are connected to the first Coriolis mass element via fifth springs, which are designed to be stiff along the first axis and flexible along the second axis, and, via sixth springs, which are designed to be flexible along the first axis and stiff along the second axis are connected to the substrate. At the same time, the third and fourth activating mass elements are connected to the second Coriolis mass element via seventh springs, which are designed to be stiff along the first axis and flexible along the second axis, and, via eighth springs, which are designed to be flexible along the first axis and stiff along the second axis are connected to the substrate.

[0020] According to a further preferred refinement, the first Coriolis mass element has the shape of a closed polygonal frame, preferably of an essentially square frame.

[0021] According to still another preferred refinement, the second Coriolis mass element is situated within the first Coriolis mass element, and has a polygonal shape, preferably an essentially square shape.

[0022] According to an additional preferred refinement, the first Coriolis mass element and the second Coriolis mass element are able to be activated by the activating device to vibrations in phase opposition along a first axis, and the first detecting mass device and the second detecting mass device are able to be deflected in various directions along the second axis, on account of the acting Coriolis force.

[0023] In conformance with yet another preferred further development, the first activating mass device has a first detecting mass element and a second detecting mass element, and the second detecting mass device has a third detecting mass element and a fourth detecting mass element, which each have a plurality of fingers situated along the second axis, and movable electrodes are provided at the which cooperate with electrodes firmly anchored to the substrate in detecting the deflections.

[0024] According to another preferred refinement, the first activating mass element and the third activating mass element, as well as the second activating mass element and the fourth activating mass element are coupled to one another pairwise by a connecting spring in each case, which is designed to be flexible along the first axis and preferably stiff along the second axis.

[0025] According to another preferred refinement, the first activating mass element and the third activating mass element, as well as the second activating mass element and the fourth activating mass element are coupled to one another pairwise by a connecting spring in each case, which is designed preferably to be stiff along the first axis and flexible along the second axis.

[0026] According to one more preferred further development, a mechanical coupling is provided along the x axis and along the y axis by a coupling spring device between the Coriolis mass elements, the coupling spring device being designed flexible along the x axis and along the y axis.

BRIEF DESCRIPTION OF THE DRAWING

[0027] An exemplary embodiment of the present invention is represented in the drawing and explained in detail in the following description.

[0028] FIG. 1 shows a schematic top view onto a specific embodiment of the micromechanical rotational rate sensor according to the present invention.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0029] FIG. 1 shows a schematic top view onto a specific embodiment of the micromechanical rotational rate sensor according to the present invention.

[0030] It should be mentioned at this point that, for reasons of clarity, not all appropriate elements in FIG. 1 have been provided with reference numerals.

[0031] In FIG. 1, 1a denotes a first activating mass element, 1a′ a second activating mass element, 1b a third activating mass element and 1b′ a fourth activating mass element. 2a is a first Coriolis mass element and 2b is a second Coriolis mass element. 3a denotes a first detecting mass element, 3a′ a second detecting mass element, 3b a third detecting mass element and 3b′ a fourth detecting mass element.

[0032] As may be seen clearly in FIG. 1, all the functional mass elements 1a, 1a′, 1b, 1b′, 2a, 2b, 3a, 3a′, 3b, 3b′ are situated symmetrically in such a way that their center of gravity coincides at a common mass center of gravity SP, which lies at the center of the vibrating structure. All individual masses are suspended movably over substrate 100. Besides setting the common mass center of gravity, the selected symmetrical construction also assures nonsensitivity to process tolerances.

[0033] In the following, the activation of first and second Coriolis mass elements 2a, 2b is described, of which the first, 2a, has a closed frame structure and the second, 2b, has an essentially square shape having right-angled angle continuations hanging from it.

[0034] Activating mass elements 1a, 1b and 1a′, 1b′ are coupled to one another by a respective connecting spring 9 and 9′. Connecting spring 9 and 9′ are designed to be flexible along a first axis x and preferably stiff along a second axis y perpendicular to it. Axes x, y form a plane which runs parallel to the plane of a substrate 100 in question, over which the vibrating structure is suspended. Perpendicularly out of the plane of the drawing, that is, as the normal to the substrate surface, the z axis is oriented, about which the rotational rate according to the Coriolis principle is to be detected.

[0035] Each of the activating mass elements 1a, 1b, 1a′, 1b′ has an assigned comb actuator, using which a linear motion along the x axis may be induced. Each respective comb actuator includes fixed electrodes 12a, 12b, 12a′, 12b′ that are anchored to substrate 100, as well as movable electrodes 13a, 13b, 13a′, 13b′ which are mounted on the respective activating mass elements 1a, 1b, 1a′, 1b′.

[0036] The activating mass elements 1a, 1b, 1a′, 1b′ are anchored to substrate 100 on the side facing away from connecting spring 9 and 9′, via anchoring springs 5a, 5b, 5a′, 5b′. Reference numeral 18 denotes anchorings for springs 5a, 5b, 5a′, 5b′. These anchoring springs 5a, 5b, 5a′, 5b′ are designed to be flexible along the x axis and preferably stiff along the y axis, in order to avoid a deflection of activating mass elements 1a, 1a′, 1b, 1b′ along the y axis and to make possible only an approximately one-dimensional motion along the x axis.

[0037] Using connecting springs 8a, 8a′, the first and second activating mass element 1a and 1a′, respectively, are each connected to the external frame-shaped Coriolis mass element 2a at their longitudinal ends. These springs 8a, 8a′ are designed in such a way that they are stiff along the x axis and flexible along the y axis. Accordingly, Coriolis mass element 2a follows the x motion of activating mass elements 1a and 1a′.

[0038] In an analogous fashion, the longitudinal ends of activating mass elements 1b, 1b′ are connected to the angle continuations of second Coriolis mass element 2b, via connecting springs 8b, 8b′. These connecting springs 8b, 8b′ are also designed to be stiff along the x axis and flexible along the y axis. Accordingly, Coriolis mass element 2b follows the x motion of activating mass elements 1b and 1b′.

[0039] Consequently, a symmetrical activation of Coriolis mass elements 2a, 2b may be accomplished which, as will be explained below, is expediently designed in such a way that it brings about vibrations of the Coriolis mass elements 2a, 2b that are in phase opposition.

[0040] First of all, let us explain detecting mass elements 3a, 3a′, 3b, 3b′ in greater detail. Detecting mass elements 3a, 3a′, 3b, 3b′ each have a plurality of fingers F which intermesh and which are directed counter to one another with respect to detecting mass elements 3a, 3b and 3a′, 3b′. Detecting mass elements 3a, 3b and 3a′, 3b′ are connected to one another at their middle, in each case via a connecting spring 10 and 10′. Connecting springs 10 and 10′ are designed to be flexible along the y axis and preferably stiff along the x axis. The individual fingers F are oriented along the y axis and have movable electrodes 16a, 16b, 16a′, 16b′ which cooperate with electrodes 14, 14′, that are firmly anchored on substrate 100, in order to detect the deflections along the y axis according to the principle of the differential capacitor.

[0041] The first and second detecting mass element 3a, 3a′ are connected at their longitudinal ends to first Coriolis mass element 2a, via respective connecting springs 7a, 7a′. These springs 7a, 7a′ are designed to be flexible along the x axis and stiff along the y axis.

[0042] In an analogous manner, third detecting mass element 3b and fourth detecting mass element 3b′ are connected to the angle continuations of second Coriolis mass element 2b via connecting springs 7b, 7b′. These connecting springs 7b, 7b′ are also designed to be flexible along the x axis and stiff along the y axis. These springs 7a, 7a′, 7b, 7b′ make it possible for the Coriolis force acting along the y axis on Coriolis mass elements 2a, 2b to be transmitted to detecting mass elements 3a, 3a′, 3b, 3b′. Anchoring springs 6a, 6b, 6a′, 6b′, by which detecting mass elements 3a, 3a′, 3b, 3b′ are connected to substrate 100, prevent, on the other hand, the activating motion from being transmitted along the x axis to detecting mass elements 3a, 3a′, 3b, 3b′. Reference numeral 18 denotes anchorings for springs 6a, 6b, 6a′, 6b′.

[0043] After all that, the present structure has a double decoupling of Coriolis mass elements 2a, 2b, on the one hand, from the activation and, on the other hand from the detection.

[0044] In the present sensor structure, the detection takes place at a structure at rest, which means that the part of the masses, that is, the detecting mass elements, which forms an electrode of the plate capacitor system, essentially carries out no activating motion. The rotational rate sensor described here is a linearly vibrating system in which both the activation and the detection take place in the substrate plane.

[0045] As was indicated above, the activation of the structure takes place preferably in the antiparallel activating mode, which means that activating mass elements 1a and 1b and 1a′ and 1b′, and thus also Coriolis mass elements 2a, 2b move in phase opposition. The Coriolis accelerations appearing at an external rotation about the z axis are then also in phase opposition, and if there is an appropriate design of the structures, this leads to an activation of an antiparallel detection mode. In other words, at a certain rotational direction, detecting mass elements 3a and 3a′ are deflected in the positive y direction and 3b and 3b′ into the negative y direction.

[0046] The desired measuring effect generated thereby may then, by a suitable evaluation, be directly distinguished from an undesired interference effect, brought on by an external linear acceleration in the y direction or a centrifugal acceleration, which would respectively act in phase on the detection mass elements of both partial structures.

[0047] Add to this, that a rotational acceleration about the sensing axis does not lead to any deflection of the detection elements in the sensing direction.

[0048] Although the present invention is described above on the basis of preferred exemplary embodiments, it is not limited to them, and may be modified in numerous ways.

[0049] The Coriolis mass elements may be, but do not have to be suspended via additional springs at the substrate for the further stabilization.

[0050] The activating mass elements may also be connected indirectly via springs between the detection mass elements in such a way that a mechanical coupling of both partial structures in the activating direction is present, and the formation of parallel and antiparallel vibration modes in the x direction comes about.

[0051] The activating mass elements may, on the other hand, also be connected indirectly via springs between the activating mass elements in such a way that a mechanical coupling of both partial structures in the detecting direction is present, and the formation of parallel and antiparallel vibration modes comes about.

[0052] The design of the plate capacitor structures connected to the movable structure may be made with or without a cross bar. A cross bar is used to avoid comb finger vibrations which may lead to undesired signal fluctuations in the electrical evaluation.

[0053] A mechanical coupling in the activating direction and in the detecting direction may also be achieved by suitable coupling spring constructions between the Coriolis mass elements, the coupling spring constructions to be designed flexible in the activating direction and the detecting direction.

[0054] The individual masses are preferably designed as closed frame structures, which increases stability and takes care that the frequency of the undesired out-of-plane vibration modes lies in a more favorable range.

[0055] In one embodiment of the masses as open frame structures, preferably a torque adjustment is achievable by the suitable choice of the spring linking points and also by a suitable design of the frames.

[0056] The mass elements may be designed to be perforated (like lattice-work) or not perforated.

Claims

1. A micromechanical rotational rate sensor having: a first Coriolis mass element (2a) and a second Coriolis mass element (2b) which are situated over a surface of a substrate (100); an activating device by which the first Coriolis mass element (2a) and the second Coriolis mass element (2b) is able to have vibrations activated along a first axis (x); and a detecting device by which deflections of the first Coriolis mass elements (2a) and of the second Coriolis element (2b) are able to be detected along a second axis (y), which is perpendicular to the first axis (x), on the basis of a correspondingly acting Coriolis force; the first axis (x) and second axis (y) running parallel to the surface of the substrate (100); the detecting device having a first detection mass device (3a, 3a′) and a second detection mass device (3b, 3b′); and the centers of gravity of the first Coriolis mass element (2a), the second Coriolis mass element (2b), the first detection mass device (3a, 3a′) and the second detection mass device (3b, 3b′) coinciding at a common mass center of gravity (SP) when they are at rest.

2. The micromechanical rotational rate sensor as recited in claim 1, wherein the first detection mass device (3a, 3a′) is connected to the first Coriolis mass element (2a) via first springs (7a, 7a′) which are designed to be flexible along the first axis (x) and stiff along the second axis (y), and is connected to the substrate (100) via second springs (6a, 6a′), which are designed to be stiff along the first axis (x) and flexible along the second axis (y); and the second detection mass device (3b, 3b′) is connected to the second Coriolis mass element (2b) via third springs (7b, 7b′) which are designed to be flexible along the first axis (x) and stiff along the second axis (y), and is connected to the substrate (100) via fourth springs (6b, 6b′), which are designed to be stiff along the first axis (x) and flexible along the second axis (y).

3. The micromechanical rotational rate sensor as recited in claim 1 or 2, wherein the activating device has a first activating mass device (1a, 1a′) and a second activating mass device (1b, 1b′) and the centers of gravity of the first activating mass device (1a, 1a′) and the second activating mass device (1b, 1b′) also coincide at the common mass center of gravity (SP) when they are at rest.

4. The micromechanical rotational rate sensor as recited in claim 3, p1 wherein the first activating mass device (1a, 1a′) has a first activating mass element (1a) and a second activating mass element (1a′), and the second activating mass device (1b, 1b′) has a third activating mass element (1b) and a fourth activating mass element (1b′), which are able to be individually activated via a respective comb actuator (12a, 12b, 13a, 13b, 12a′, 12b′, 13a′, 13b′).

5. The micromechanical rotational rate sensor as recited in claim 4, wherein the first and the second activating mass device (1a, 1a′) are connected to the first Coriolis mass element (2a) via the fifth springs (8a, 8a′) which are designed to be stiff along the first axis (x) and flexible along the second axis (y), and are connected to the substrate (100) via the sixth springs (5a, 5a′), which are designed to be flexible along the first axis (x) and stiff along the second axis (y); and the third and the fourth activating mass element (1b, 1b′) are connected to the second Coriolis mass element (2b) via the seventh springs (8b, 8b′) which are designed to be stiff along the first axis (x) and flexible along the second axis (y), and are connected to the substrate (100) via the eighth springs (5b, 5b′), which are designed to be flexible along the first axis (x) and stiff along the second axis (y).

6. The rotational rate sensor as recited in one of the foregoing claims, wherein the first Coriolis mass element (2a) has the shape of a closed polygonal frame, preferably of an essentially square frame.

7. The rotational rate sensor as recited in claim 6, wherein the second Coriolis mass element (2b) is situated within the first Coriolis mass element (2a), and has a polygonal shape, preferably an essentially square shape.

8. The rotational rate sensor as recited in claim 2, wherein the first Coriolis mass element (2a) and the second Coriolis mass element (2b) are able to have vibrations that are in phase opposition activated along a first axis (x) by the activating device; and the first detection mass device (3a, 3a′) and the second detection mass device (3b, 3b′) are able to be deflected in various directions along the second axis y, based on the acting Coriolis force.

9. The rotational rate sensor as recited in one of the foregoing claims, wherein the first detection mass device (3a, 3a′) has a first detection mass element (3a) and a second detection mass element (3a′), and the second detection mass device (3b, 3b′) has a third detection mass element (3b) and a fourth detection mass element (3b′), which each have a plurality of fingers (F), which are situated along the second axis (y); and at the fingers (F), movable electrodes (16a, 16b, 16a′, 16b′) are provided, which cooperate with the electrodes (14, 14′), that are firmly anchored to the substrate (100), to detect the deflections.

10. The rotational rate sensor as recited in one of the preceding claims 4 through 9, wherein the first activating mass element (1a) and the third activating mass element (1b), as well as the second activating mass element (1a′) and the fourth activating mass element (1b′) are coupled to one another pairwise by a connecting spring (9, 9′) in each case, which is designed to be flexible along the first axis (x) and preferably stiff along the second axis (y).

11. The rotational rate sensor as recited in one of the preceding claims 9 or 10, wherein the first detection mass element (3a) and the third detection mass element (3b), as well as the second detection mass element (3a′) and the fourth detection mass element (3b′) are coupled pairwise to one another by a respective connecting spring (10, 10′) in each case, which is preferably designed to be stiff along the first axis (x) and flexible along the second axis (y).

12. The rotational rate sensor as recited in one of the foregoing claims, wherein a mechanical coupling is provided along the x axis and along the y axis by a coupling spring device between the Coriolis mass elements (2a, 2b), the coupling spring device being designed to be flexible along the x axis and along the y axis.