Measuring and testing – Speed – velocity – or acceleration – Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
2000-09-12
2002-10-22
Moller, Richard A. (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Angular rate using gyroscopic or coriolis effect
Reexamination Certificate
active
06467349
ABSTRACT:
THIS INVENTION
Relates to an arrangement for measuring angular velocity. An arrangement in accordance with the invention may be used in a motor vehicle to detect a roll-over accident to control the deployment of a safety device such as an air-bag.
Vibrating angular velocity meters/gyros of the tuning fork type, or which use other vibrating members, have heretofore been constructed and tested both in metal and quartz and also in silicon. Both types may be constructed with an open loop and with a closed feed-back loop. The advantage of quartz and silicon based gyros over the prior proposed metal-fork gyro (Barnaby et. al., Aeronautical engineering review, p. 31, November 1953) is that they can be miniaturised and manufactured relatively cheaply in large production runs by batch manufacture.
Unlike rotating gyros in which the Coriolis force develops a constant torque during a turn, turning of a vibrating gyro results in an oscillating torque in which the amplitude is proportional to the turning speed and the phase indicates the turning direction. As a result, relatively extensive electronic circuitry is required for excitation and detection and interpretation of the gyro signal from a vibrating gyro as compared with a conventional rotating gyro.
Since silicon is a well-developed electronic material, there is considerable advantage in using a silicon-based gyro integrated together with associated excitation and sensor electronics. However, it has not been possible to produce and excite a traditional tuning fork in silicon easily.
The advantage of a tuning fork construction is that it can be made to swing in a dynamically balanced mode which makes the gyro insensitive to vibration and which gives a high Q-factor since little vibration energy is yielded to the surroundings. Even if it is possible, in principle, using moderm plasma etching techniques, to etch out, in silicon, a tuning fork (or any other horizontally swinging structure (Satchell, UK patent application, GB2198231A)), there still remains the excitation problem.
In silicon it is not possible to utilise the piezo-electric effect directly for excitation as in the case of quartz, since silicon is not a piezo-electric material. Of course a piezo-electric layer can be deposited on silicon, but if the structure has been etched out vertically down from the substrate surface, this means that a piezo-resistive layer of constant thickness has to be deposited on vertical walls, and this can be fairly problematic.
Instead, the hitherto most common solution to the excitation problem for silicon-based gyros has been to use electrostatic excitation at right angles to the substrate surface by making conductive plates on one or both sides of a bar or a swashplate (possibly with a weight thereon), or alternatively quite simply to “glue” practically the entire component on a piezo-electric plate.
Unfortunately, the micromechanical gyros previously proposed using electrostatic excitation at right angles to the substrate surface, which may be relatively easily produced, have suffered from a low gyroscopic scale factor of the order of magnitude of 0.01-0.2 (Boxhorn, U.S. Pat. No. 4,598,585). This can be compared with a tuning fork, which has a gyroscopic scale factor of 2 but which is much more difficult to produce, since it requires excitation parallel to the substrate surface, because the limbs must swing in the plane of the substrate in anti-phase with each other.
According to one aspect of this invention there is provided a sensor adapted to respond to a rotation, said sensor comprising a body formed from a substantially planar substrate, said body incorporating a beam, the beam having two opposed ends, each end being adapted to be fixed in position, the beam being provided with at least one inertia mass connected to the beam at a predetermined position, the body being associated with means for exciting a first oscillation of the inertia mass substantially about an axis perpendicular to the plane of the substrate, there being means for detecting a second oscillation of the mass, substantially around an axis coincident with the longitudinal axis of the beam, which second oscillation is caused by the coriolis force that arises when the body is subjected to rotation around an axis having at least a component in the said plane, but perpendicular to the said axis of the beam, the beam being configured so that the beam bends most easily in one predetermined direction, that direction making an acute angle with the plane, so that the direction is neither parallel with, nor perpendicular to the plane, so that the first oscillation may be initiated by an excitation force which is not parallel with said plane.
Preferably a sensor according to claim
1
wherein the beam is such that the first oscillation may be caused by an excitation force which is substantially perpendicular with said plane.
Conveniently the predetermined direction in which the beam bends most easily is at approximately 55° to the plane of the substrate.
Advantageously the or each inertia mass comprises two elements, located respectively on either side of the beam, within the plane of the substrate, the two elements being interconnected by a connecting bar, the central portion of the connecting bar being unitary with a portion of the beam.
Preferably the means for exciting the first oscillation of the or each inertia mass comprise at least one capacitative plate located adjacent a selected portion of the first inertia mass, and means to apply selected potentials to the capacitative plate, the inertia mass being provided with a conductive portion, there being means to apply a predetermined potential to the conductive portion, the arrangement being such that the potentials applied to the capacitative plate causes part of said inertia mass to tend to be deflected out of the plane of the substrate.
Conveniently there is a plurality of said capacitative plates.
Advantageously the or each capacitative plate is on a substrate of glass or semi-insulating silicon.
Preferably the means for applying potentials to said capacitative plate or plates are adapted to supply signals which generate a “see-saw” motion of the inertia mass about the connecting bar, the effect of the configuration of the beam being such that this oscillation generates the said first oscillation about the axis perpendicular to the plane of the substrate.
Conveniently the means for detecting the second oscillation comprise at least one further capacitative plate located adjacent on selected portions of the first inertia mass, and means to measure the capacitance between said further capacitative plate and the said body to detect said second oscillation.
Advantageously there is a plurality of said further capacitative plates.
In one embodiment there is only a single inertia mass, that inertia mass being located substantially centrally of the beam, half-way between the means provided at the opposed ends of the beam for mounting the beam in position.
Preferably the inertia mass is symmetrical about the axis of the beam.
In an alternative embodiment there are two inertia masses, the inertia masses being symmetrically disposed on the beam, each inertia mass being located at a position between the center part of the beam and a respective end of the beam.
Conveniently the two inertia masses are identical and symmetrical about the axis of the beam.
Preferably each inertia mass is provided with means for exciting a first oscillation of the mass around an axis perpendicular to the plane of the substrate, the arrangement being such that the masses oscillate in anti-phase.
Advantageously the body is formed from a mono-crystalline substrate, such as silicon.
The sensor may be adapted to provide an output signal representative of angular velocity.
The invention also relates to a sensor as described above mounted on a motor vehicle to control deployment of a safety device.
According to another aspect of this invention there is provided a sensor adapted to respond to a rotation, said sensor comprising a body formed from a substantially p
Andersson Gert
Hedenstierna Nils
Svensson Per
Autoliv Development AB
Kinberg Robert
Moller Richard A.
Venable
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