Monolithic vibrating rate gyro structure

Details Monolithic vibrating rate gyro structure Monolithic vibrating rate gyro structure

2001-07-26

2002-07-02

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S329000, C310S367000

Reexamination Certificate

active

06414416

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a vibrating rate gyro that can be used to stabilize or guide vehicles, for example, or for automobile navigation.
The invention relates more particularly to a monolithic vibrating rate gyro structure comprising a fixed part, a resonator and a mechanical system linking the resonator to the fixed part and preventing leaks of vibratory mechanical energy from the resonator to the fixed part.
2. Description of the Prior Art
The resonator is intended to operate in two different vibration modes, the vibration directions of the two modes being mutually perpendicular. Accordingly, when the rate gyro is rotating at an angular speed {right arrow over (&OHgr;)} about an axis perpendicular to these two vibration directions, this rotation {right arrow over (&OHgr;)} being referred to a so-called “Galilean” inertial frame of reference, coupling occurs between the two vibration modes of the resonator. The coupling is due to Coriolis accelerations {right arrow over (&ggr;)}
c
that apply at all material points of the resonator and whose analytical expression can be written in the form of the following vector product:
{right arrow over (&ggr;)}
c
=2
{right arrow over (&OHgr;)}&Lgr;{right arrow over (v)}
,
{right arrow over (v)} being the speed of the material
point concerned, expressed in the frame of reference tied to the resonator.
There are several ways to exploit this coupling to measure the angular speed {right arrow over (&OHgr;)}.
For example, one of the two vibration modes of the resonator can be maintained in vibration. The rotation at speed {right arrow over (&OHgr;)} then causes vibration in the other mode, with an amplitude proportional to &OHgr;. The value of &OHgr; can be determined from the measured amplitude, generally converted to the form of an electrical signal.
A second way to exploit the coupling between the two vibration modes of the resonator requires the resonant frequencies of the two modes to be identical, and consists of maintaining them in vibration simultaneously so that one of the modes is in phase quadrature relative to the other mode. Accordingly, if the vibration amplitudes are equal for the two modes, each material point of the resonator traces out a circle in each period of the vibration. The angular frequency &ohgr;
0
of that vibration, observed in an inertial frame of reference, is not affected by the rotation at angular speed {right arrow over (&OHgr;)}. On the other hand, the angular frequency &ohgr; of the same vibration, as observed by measuring means tied to the resonator, depends on &OHgr;, as expressed by the equation &ohgr;=&ohgr;
0
±&OHgr;. Thus an oscillator whose frequency variations are representative of rotation speed variations can be made.
A third way to exploit the coupling between the two vibration modes of the resonator requires the resonant frequencies of the two modes to be identical, as before, and consists of maintaining them in vibration simultaneously so that the two modes are in phase relative to each other. Accordingly, the vibrations of each material point of the resonator are contained in a plane. If the system for maintaining the vibrations is designed not to give preference to any particular orientation in that plane relative to the resonator, then the initial orientation, as observed in an inertial frame of reference, is not affected by the rotation at the angular speed {right arrow over (&OHgr;)}. On the contrary, the same vibration plane, as observed using means linked to the resonator, undergoes the opposite rotation to that imposed on the resonator. Unlike the first two ways of exploiting the coupling, this third way does not enable the rotation speed &OHgr; to be measured directly because the sensor provides the orientation &phgr; of the vibration plane. This operation in gyroscope mode is beneficial in some applications, and also enables the speed to be determined by means of a differentiator circuit (&OHgr;=d&phgr;/dt).
For each of the above three ways of exploiting the coupling between the two vibration modes of the resonator, the accuracy of the measurement supplied by the rate gyro depends on the quality of the vibrations of the two modes. To be more precise, for each of the modes, it is preferable for the quality factor Q to be high, enabling mechanical phenomena resulting from Coriolis accelerations to be amplified. For one resonator the Q-factor is defined as the quotient of the energy stored in the resonator divided by the energy loss during one period of the vibration. The Q-factor depends on a plurality of parameters. Firstly, the nature of the material used to fabricate the resonator conditions the mechanical energy losses by internal friction. For this reason the material is advantageously quartz or silicon, for which these losses are small, and which further have the benefit of a low acquisition cost. A second parameter influencing the Q-factor is the nature of the vibration of the resonator. For example, if the resonator is a beam vibrating flexionally, the Q-factor depends on exchanges of heat between the fibers that are alternately stretched and compressed, and this value is therefore proportional to the square of the thickness of the beam. These first two parameters determine what might be called the “intrinsic” quality factor Q of the resonator. In practice, the real Q-factor of the resonator is at most equal to this intrinsic factor, because it is generally affected by other influencing parameters, in particular the means of fixing the resonator to a support. It is desirable for the fixing means to avoid mechanical energy leakages from the resonator to the support. To be more precise, it is desirable to limit these energy leakages to a level related to the intrinsic quality factor of the resonator. For example, to obtain the greatest benefit from a resonator whose intrinsic quality factor is 3×10
5
, it is necessary for the fixing means to limit energy leakages in each period to a value significantly lower than (3××10
5
)
−1
times the energy stored in the resonator. To this end, the fixing means can consist of a fixed part and a mechanical system connecting the resonator to the fixed part and reducing sufficiently leakages of mechanical energy from the resonator to the fixed part. The device therefore performs a function of mechanically filtering vibrations of the resonator. The fixed part is therefore little affected by the vibrations of the resonator. Accordingly, when the fixed part is fixed to a support, for example glued to the support, the mechanical energy leakages to the support can be sufficiently small. The resonator, the fixed part and the mechanical filtering system constitute a rate gyro structure.
The rate gyro structure is preferably monolithic, which avoids the drawbacks inherent to assemblies of several components, such as behavior that is unstable in time or in the event of temperature variations.
The monolithic structure of the rate gyro is preferably machined from a plate of material of uniform thickness, which enables it to be fabricated at low cost by chemical machining.
The resonator is preferably a tuning fork formed of two identical and parallel branches facing each other and each fixed at one end to a common part. One of the vibration modes is a flexional mode, in which the two branches vibrate flexionally in phase opposition, the vibratory displacements of the two branches being parallel to the plane of the plate of material. This vibration mode is therefore similar to that of a musical tuning fork. The other vibration mode is a torsional mode, in which the two branches vibrate flexionally in phase opposition, the vibratory displacements of the two branches occurring perpendicularly to the plane of the plate. The vibration directions of the two modes are therefore mutually perpendicular, and coupling occurs between the two modes if the tuning fork is subjected to rotation about an axis parallel to the two branches. That axis

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