Measuring and testing – Speed – velocity – or acceleration – Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
1999-06-07
2002-02-26
Chapman, John E. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
active
06349597
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to movement sensors and in particular to micromechanical rotational rate gyroscopes using the Coriolis force.
BACKGROUND ART
Micromechanical Coriolis-force rotational rate gyroscopes have many fields of application, for example the determination of the position of a motor vehicle or aircraft. Such devices or sensors in general have a movable mechanical structure that is excited to perform a periodic oscillation. This periodic oscillation induced by excitation is referred to as primary oscillation. When the sensor experiences a rotation about an axis perpendicular to the primary oscillation or primary movement, the movement of the primary oscillation results in a Coriolis force proportional to the measurement quantity, i.e. the angular velocity. The Coriolis force induces a second oscillation orthogonal to the primary oscillation. This second oscillation orthogonal to the primary oscillation is referred to as secondary oscillation. The secondary oscillation, which is also termed detection oscillation, can be detected by several measuring methods, with the quantity detected serving as a measure for the rotational rate acting on the rotational rate gyroscope.
To generate the primary oscillation, thermal, piezoelectric, electrostatic and inductive methods are used among others, which are known in the art. For detecting the secondary oscillation, piezoelectric, piezoresistive and capacitive principles are state of the art.
DESCRIPTION OF PRIOR ART
Known micromechanical rotational rate gyroscopes are described in K. Funk, A. Shilp, M. Offenberg, B. Elsner and F. Lärmer, “Surface Micromachining Resonant Silicon Structures”, The 8th International Conference on Solid-State Sensors and Actuators, Eurosensors IX, NEWS, pages 50 to 52. In particular, a known, quasi-rotating gyroscope described in that publication comprises a circular oscillator supported on a base so as to be rotatable in two directions. The oscillator of the known gyroscope is of disc-shaped configuration with respect to an x-y plane, with comb electrode configurations be attached on two opposite sides of the disc. A comb electrode configuration is used for driving the oscillating body and is composed of fixed comb electrodes and the comb electrodes of the oscillator engaging with the fixed comb electrodes. A similar comb electrode detection assembly consists of fixed comb electrodes engaging with corresponding comb electrodes attached to the primary oscillator. The comb electrode configuration on the input side serving for driving the oscillator and being also referred to as comb drive, is suitably connected to an excitation voltage, such that a first comb electrode configuration is fed with an a.c. voltage, whereas a second comb electrode configuration of the comb drive is fed with a second voltage phase-shifted by 180° with respect to the first voltage. Due to the applied a.c. voltage, the oscillator is excited to perform a rotational oscillation about the z axis normal to the x-y plane. The oscillation of the oscillator in the x-y plane is the afore-mentioned primary oscillation.
When the known gyroscope is rotated about an y axis with a specific angular velocity, a Coriolis force acts on the oscillator that is proportional to the applied angular velocity about the y axis. This Coriolis force generates a rotational oscillation of the oscillator about the x axis. This rotational oscillation or periodic “tilting” of the oscillator about the x axis can be measured capacitively by means of the two electrodes located underneath the gyroscope or sensor.
A disadvantage of this known structure consists in that the primary oscillation and the secondary oscillation, which is the oscillation of the oscillating body due the Coriolis force acting thereon, are carried out by one single oscillator supported by means of a two-axis joint in order to be able to perform the two mutually orthogonal oscillations. The two oscillation modes, i.e. the primary oscillation and the secondary oscillation, thus are not decoupled from each other, and this is why the intrinsic frequencies of primary and secondary oscillations cannot be balanced in exact manner independently of each other in order to obtain an as high as possible sensing accuracy of the rotational rate gyroscope. Furthermore, in case of the known gyroscope, the secondary oscillation has the effect that the comb electrode assembly for driving the oscillator is tilted, thereby affecting the primary oscillation by the secondary oscillation. This influence results in a primary oscillation that is not controlled in fully harmonic manner, which is a reaction to the retroactive effect of the secondary oscillation on the primary oscillation, i.e. a reaction to tilting of the comb drive for generating the primary oscillation.
Another known rotational rate gyroscope described in that publication comprises two mutually separate oscillatory masses which may be brought into opposite-phase oscillation by respective comb drives connected to one mass each by spring beams. The two masses are connected to each other by a spring beam arrangement and, due to suspension of the assembly of the two masses and the connecting webs of the masses, carry out a rotational oscillation in the x-y plane when the gyroscope is subjected to rotation about the z axis. Displacement of the assembly of the two masses and the spring beams mutually connecting the masses, in the direction of the y axis as a reaction to rotation of said assembly is detected capacitively by means of four comb electrode configurations.
Just as the first known gyroscope described, the second known gyroscope also comprises merely one single oscillator both for the primary oscillation and for the secondary oscillation, so that the two orthogonal oscillation modes are coupled with each other and the secondary oscillation generated by the Coriolis force may have a retroactive effect on the primary oscillation. This structure, too, thus permits no exact selective balancing of the intrinsic frequencies of the primary and secondary oscillations.
A further known oscillatory gyroscope is described in the article by P. Greiff et al., entitled “Silicon Monolithic Micromechanical Gyroscope” in the conference band of Transducers 1991, pages 966 to 968. This gyroscope is a double gimbal structure in the x-y plane, which is supported by torsion springs. A frame-shaped first oscillator structure surrounds a plate-shaped second oscillator structure. The second oscillator structure comprises an inertia element projecting in z-direction from the plane thereof. In operation, rotary excitation about the y axis of the first oscillator structure is transferred by torsion springs rigid in the direction of the first oscillation to the second oscillator structure. In the presence of an angular velocity about the z axis, a Coriolis force is generated in the y direction, which engages the projecting inertia element or gyro element in order to deflect the second oscillator structure about the x axis, whereby the second oscillator structure performs a Coriolis oscillation about the x axis orthogonal to the excitation oscillation, which is rendered possible by the torsion springs suspending the second oscillator structure on the first oscillator structure. The Coriolis force present with this gyroscope only in y direction does not result in movement of the remaining structure since the latter is fixedly held in the y direction. Only the gyro element projecting in y direction offers a point of application for the Coriolis force, so that this force can effect a measurable movement proportional to the forced rotation.
Although the first and second oscillations are decoupled from each other in this structure and no retroactive effect of the second oscillation on the excitation of the first oscillation takes place, a disadvantage resides in that the second oscillator structure cannot be made in planar manner due to the projecting gyro element. Upon manufacture of the gyroscope structure, the gyro element is formed by gold ele
Folkmer Bernd
Geiger Wolfram
Lang Walter
Sobe Udo
Beyer Weaver & Thomas LLP
Chapman John E.
Hahn-Schickard-Gesellschaft für angewandte Forschung e.V.
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