WO1996026445A1

WO1996026445A1 - Coupled resonator vibratory rate sensor

Info

Publication number: WO1996026445A1
Application number: PCT/US1996/001716
Authority: WO
Inventors: Yishay Netzer
Original assignee: Netzer, Yohay
Priority date: 1995-02-23
Filing date: 1996-02-08
Publication date: 1996-08-29
Also published as: IL112770A0; AU5169796A; EP0805984A1; EP0805984A4

Abstract

An inertial angular rate sensor (IARS) which includes a symmetrical mechanical resonator. The rate sensor includes one or more pair of vibrating masses (20, 21). The sensor further includes a mechanical coupler (24) which forces the masses to move in a symmetrical manner despite mechanical tolerances.

Description

COUPLED RESONATOR VIBRATORY RATE SENSOR

FIELD OF THE INVENTION

This invention relates to symmetrical mechanical resonators and in

particular to vibratory or Coriolis type rate sensors employing the same.

BACKGROUND OF THE INVENTION

The terms "tuning forks" and "symmetrical mechanical resonators"

are used herein as synonyms and both can be defined as signifying

mechanical structures that include two masses that are counter-oscillating

in such a manner that their common center-of-mass is ideally stationary

and their total linear and angular momentums are zero at any time. The

classical tuning fork includes two tines connected to a common stem,

which in turn is connected to a stationary base. The purpose of the stem

is to decouple the tines as much as possible from the base and to couple

them to each other, so that they have a common resonance frequency and

a minimum energy is dissipated to the stationary base.

Tuning forks have been used as frequency standards by employing

piezoelectric crystalline quartz. They have also been used as angular

inertial rate sensors, wherein the excited vibrational counter-motion of the

tines combines with the inertial rotation of the tuning-fork base to induce

in the tines so-called Coriolis accelerations which are perpendicular to the

plane of the excited vibrations and of opposite sense in each of the two tines. These accelerations induce vibratory motions in the tines

perpendicular to the excited vibrations, the difference of which is

indicative of the input angular inertial rate. Although each of the tines

responds to said inertial rotation, it also responds to vibratory motion of

the mounting base that would lead to an output error that is

indistinguishable from the rate signal; however, by processing the

differential induced vibrations, the error is ideally eliminated. The earliest

application of a tuning fork resonant structure for angular inertial rate

sensing is described in "New space rate sensing instrument, " by J. Lyman

in Aeronautical engineering review, Vol. 12, pp. 24-30, 1953. A

modified rate sensor that utilizes a double tuning fork is described in

"Reduction of errors in vibratory Gyroscopes by Double Modulation" by

R.W. Bush and G.C. Newton, Jr. in IEEE Transactions on automatic

control, October 1964 pp. 525-535. Numerous other rate sensors are

based on vibrating structures that are essentially constituted by two

counter-oscillating masses of various structures. In order to decrease the

cost of vibratory rate sensors, they are often manufactured as monolithic

structures by employing photolithographic microfabrication techniques.

All monolithic tuning fork geometries utilized in the prior art belong to

either of the following three families:

Single tuning-fork, as in US Patent 5.343.749. H-shaped structures that are essentially two tuning forks with a

common base, as in US Patents 4.524.619 and 5.056.366.

Vibrating frame constructions that can be regarded as two

tuning-forks with the ends of the ends of their corresponding tines

connected, as in US Patents 4.654.663 and 5.349.855.

It is obvious to those skilled in the art that there are four main

vibration modes in the conventional tuning fork, these being:

1. An in-plane, symmetrical, vibration mode depicted with short

arrows in Fig. 1.

2. An asymmetrical, in-plane vibration mode depicted with long

arrows in Fig. 1.

3. A symmetrical vibration mode perpendicular to the plane of the

tuning fork.

4. An asymmetrical vibration mode perpendicular to the plane of

the tuning fork.

The first and third modes are referred to as the excitation mode and

the output or Coriolis mode, and are the only modes relevant to rate

sensing. The second and fourth modes are parasitic and lead to

sensitivities of the rate sensor to linear accelerations. It is well known,

however, to those skilled in the art that, regardless of the specific

geometry of the tuning fork, there are additional, higher order, vibration modes, that are, however, of little consequence to its applications as a rate

sensor.

A shortcoming of all prior art implementations of the tuning fork

resonators is that excitation of the first mode may be accompanied by an

undesirable excitation of the second mode due to asymmetry in the

excitation forces applied to the two tines or due to asymmetry in the mass

or stiffness of the tines as a result of manufacturing tolerances. This

results in vibrational energy that is transferred to the mounting base

through the stem and may lead to output errors in rate sensors of this type.

It may also combine with linear oscillations to further deteriorate the

fidelity of the output signal.

It is an object of the present invention to provide an improved

tuning fork and other symmetrical resonator topologies that is relatively

insensitive to mechanical asymmetries, suitable for inertial rate sensors.

It is another object of the invention to provide other improved

symmetrical resonator structures that are relatively insensitive to

mechanical asymmetries.

It is a further object of the invention to provide such resonator

structures that are more complex and include more than two moving

masses.

It is a still further object of the invention to provide a symmetrical

resonator wherein the second excitation mode is essentially eliminated by mechanically constraining the two oscillating masses to move in opposite

directions.

It is a still further object of the invention to provide tuning fork

mechanisms wherein the excitation of a pair of tines is effected indirectly

by applying force to a single point.

It is a still further object of the invention to provide an improved

single-axis rate sensor.

It is a still further object of the invention is to provide a 2-axis rate

sensor.

Further objects and advantages of the invention will become

apparent as the description proceeds.

SUMMARY OF THE INVENTION

According to the invention, a symmetrical mechanical resonator is

provided, which comprises at least one pair of vibrating masses and

includes one or more mechanical coupling means that force the masses to

move in a symmetrical manner despite mechanical tolerances.

The said mechanical resonator may be, in particular, a tuning fork

resonator, a frame-type resonator, or any planar resonator.

According to a preferred embodiment of the invention, the said

symmetrical mechamcal resonator, comprising the said mechamcal

coupling means, also comprises means for indirectly exciting said movable masses by applying at least one force that acts on said mechanical coupling

means.

In a further embodiment, a planar symmetrical mechanical resonator

is provided wherein said force is applied by parametrically modulating the

effective length of a mechanical member.

According to a further embodiment of the invention, a single axis

inertial angular rare sensor is provided, comprises a mechanical resonator

having one or more of the aforesaid characteristics.

A still further embodiment of the invention is a planar mechanical

resonator comprising four arms assembled in a cruciform shape, each pair

of adjacent arms being connected by mechanical coupling means that force

the masses to move in a symmetrical manner, whereby said arms are

vibrated in a scissors mode. Preferably said arms are indirectly excited

by forces applied in the plane of the resonator along at least one of its axes

of symmetry, which forces are more preferably generated by

parametrically modulating the effective length of a mechanical member.

A resonator according to this embodiment may be comprised in a dual axis

inertial angular rate sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates a prior art tuning fork.

Fig. 2 illustrates an H-type conventional tuning fork. Fig. 3 illustrates a conventional frame-type symmetrical resonator.

Fig. 4 illustrates a tuning fork according to the present invention.

Fig. 5 illustrates an idealized equivalent of a tuning fork according

to the present invention.

Fig. 6 illustrates an H-type tuning fork according to the present

invention.

Fig. 7 illustrates a frame-type double tuning fork resonator

according to the present invention.

Fig. 8 illustrates a frame-type symmetrical resonator according to

the present invention.

Fig. 9 illustrates another frame-type symmetrical resonator

according to the present invention.

Fig. 10 shows a modified flexible element employed in the gyro

geometry of Fig. 9.

Fig. 11 shows a flexible element as in Fig. 10 but including two

flexible elements.

Fig. 12 illustrates a centrally excited tuning fork according to the

present invention.

Fig. 13 illustrates a centrally excited H-type tuning fork according

to the present invention.

Fig. 14 illustrates a centrally excited frame-type symmetrical

resonator according to the present invention. Fig. 15 illustrates a 2-axis rate-sensor according to the present

invention.

Fig. 16 illustrates a modified 2-axis rate-sensor according to the

present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates a conventional, prior art, tuning fork mechanism

including tines 10 and 10', stem 12 and mounting base 13. The first

symmetrical vibrational mode is indicated by short arrows, the second

asymmetrical mode is indicated by long arrows. Stem 12 is optional and

the tines may be directly connected to the mounting base.

Fig. 2 illustrates a two-ended conventional tuning fork, as described

in US Patent 4.524.619, which includes tines 14 and 14' at one end and

tines 15 and 15' at the other end and is provided with mounting bases 60,

60'. A mounting structure 16 is common to all the tines.

Fig. 3 illustrates a frame type, double-tuning fork resonator,

including counter moving masses 17 and 17', that are supported by a

flexible beam structure that is anchored by mounting bases 18.

All of the above resonator based rate sensors have a left-right

symmetry, and are typically excited by applying electrostatic or

piezoelectrically generated symmetrical opposing forces on the tines or

masses, in order to ideally obtain symmetrical motion. This symmetry is important in order to minimize coupling of vibrational energy to the

mounting base by unbalanced reaction forces. Also, the tines static

deflection in the presence of external accelerations may combine with the

asymmetrical vibrational mode to cause sensitivity of the Coriolis output

to acceleration or gravity. In practice, the motion of the tines is never

symmetrical, because, due to mechanical tolerances, the tines have

different natural resonance frequencies, but since they are excited at a

common natural frequency, they do not react symmetrically, i.e., they

react with different amplitudes and different phases. The stiffer the stem,

the more sensitive is the differential motion to the mechamcal tolerances;

however, decreasing the stiffness of the stem lowers the frequency of the

asymmetrical mode and increases the sensitivity of the structure to external

accelerations. The above phenomena may be aggravated by asymmetrical

excitation forces on the tines due, for example, to tolerances in the

excitation electrodes deposited on the tines that also tends to excite the

asymmetrical mode.

A description of the excitation of the asymmetric mode due to

asymmetries in a frame-type resonator rate sensor is described in the

article: "A Micro machined Comb-Drive Tuning fork Rate Gyroscope" by

M. Weinberg et al. which appeared in the Proceedings of the 49th Annual

Meeting of the Institute of Navigation, June 21-23, 1993 page 599. The purpose of the present invention is to provide symmetrical

resonators for inertial angular rate sensors where the excited motion is

essentially symmetrical in spite of asymmetries in the structure and the

excitation forces. Fig. 4 illustrates a tuning fork according to the present

invention. The tuning fork includes tines 20 and 21, a mounting base 61,

flexible links 22 and 23, and a flexible mechanical coupling 24 between

the two tines. The purpose of the coupling is to force the two tines to

move as a mirror image of each other and thus to essentially eliminate the

asymmetrical mode. The bases of the two tines have portions 20' and 21'

which have apertures for the purpose of reducing weight. Similar

apertures portions will be found in the following embodiments as well.

The operation of the coupling in Fig. 4 is easier to understand with

the aid of Fig. 5, where an idealized equivalent of the tuning fork of the

present invention is illustrated. The two tines 20 and 21 are stiff and are

mounted on pivots 25 and 26 that represent the flexible links 22 and 23.

They are also coupled by means of a third pivot 27 that forces their

motion to be symmetrically equal. The spring 28 represent the equivalent

spring rate of all the flexible 22, 23, 24 links in Fig 4. It is obvious that,

regardless of the symmetry of the forces applied to the tines or their

dimensional symmetry, their motion will be essentially symmetrical.

Fig. 6 illustrates an H-type tuning fork according to the present

invention. The respective tines are coupled as before and stems 29 and 30 allow torsional motion of the tuning fork around its line of symmetry. 62

and 63 indicate two mounting bases.

Fig. 7 illustrates a frame-type resonator according to the present

invention. The structure is essentially two tuning forks with their

5 respective tines (20-21 and 20a-21a) connected. The notches 31 are

provided in order to eliminate excessive tension in the tines when

deflected. 64 and 65 indicate two mounting bases.

Fig. 8 illustrates another frame-type resonator according to the

present invention, each mechanism comprising bars 34, 35, 36, 37. 66,

10 66', 67, 67', and 68, 68' indicate mounting bases. The two masses 32

and 33 are supported on either of its sides by parallel-motion bar

mechanisms (four altogether) well known to those skilled in the art, and

is coupled on either side by mechamcal couplings 37 (two altogether), as

in Fig. 4, that forces their motion to be the mirror image of each other,

15 as desired.

Fig. 9 illustrates a modified frame-type resonator of Fig. 8 where

the two masses are mounted on a simpler bar mechanism comprising bars

38, 39 and tension relief elements 40, 41. 69 and 70 indicate two

mounting bases.

20 The flexible link 24 in Fig. 4 can be modified so that it will be

stressed in compression and tension rather than in flexure, and thus further

increase the stiffness of the tines to non symmetrical motion, this is achieved by employing a flexible element perpendicular to its original

orientation.

In Fig. 10 the modified flexible element is employed in the gyro

geometry in Fig. 9 - wherein the flexible element comprises a single

element. In Fig. 11 the flexible element comprises two flexible elements

to restore the symmetrical construction.

In prior art tuning forks and symmetrical resonators the two masses

are excited by two opposing forces individually applied to them, the forces

are typically electrostatic magnetic or piezoelectric. Fig. 12 illustrates an

excitation method according to the present invention applied to the tuning

fork in Fig. 4, wherein the tines are deflected by applying a single force

on the mechanical linkage 24 through member 42. 71 indicates a

mounting base. A preferred method for the excitation as in Fig. 12 is by

piezoelectrically modulating the length of element 42 at the resonant

frequency of the tuning fork. An alternative method is by modulating the

apparent length of member 42 by applying a force on its center which is

perpendicular to the plane of the paper. In this method, referred to as

parametric excitation, the excitation frequency should be one half the

resonant frequency of the tuning fork. A possible implementation of this

method, applicable to micromechanical devices, is by means of

electrostatic attraction forces applied to element 42. This may be effected

by constituting a capacitor that comprises a first stationary planar electrode set and a second electrode set deposited on a planar surface of element 42,

and applying an alternating excitation voltage on the resulting capacitor.

Alternatively, the electrode set could be of the comb type described in "A

Micro machined Comb-Drive Tuning fork Rate Gyroscope" by M.

Weinberg et al. which appeared in the Proceedings of the 49th Annual

Meeting of the Institute of Navigation, June 21-23, 1993 page 599,

wherein the force is applied in the plan of the sensor.

Another preferable parametric excitation method applicable to

piezoelectric crystalline sensor is by deflecting element 42 by means of

shear forces generated with a set of electrodes deposited on it.

Fig. 13 illustrates the application of the above excitation concept to

the double-ended H-type tuning fork of Fig. 6, where the excitation is

applied differentially between the two individual tuning forks by means of

member 43'. 72 and 72' indicate two mounting bases.

Fig. 14 illustrates the application of the above excitation method to

the frame-type resonator of Fig. 7, where the excitation is applied

differentially between the two individual tuning forks by means of member

43. 73 and 73' indicate two mounting bases. In a similar manner the

excitation can be applied to the configurations in Figs. 8 and 9.

Until now the discussion was limited to single-axis rate sensors

where Coriolis forces are the result of inertial rotation around a single

sensitive axis which is parallel to the axis of symmetry of each of the structures. A typical application of single-axis rate-sensors is automotive

vehicle yaw-sensors for skid sensing.

In many applications, e.g. optical line-of-sight stabilization,

two-axis inertial rate measurement is necessary. Therefore, if the

advantages of vibratory rate sensors are desired, two single axis sensors

would be used, alternatively, a single two-axis rate sensor would be

advantageous in such applications. Fig. 15 illustrates a two-axis vibratory

rate-sensor according to the present invention, including four vibrating

tines 44, 45, 46, and 47 and a mounting base 74. The arrows indicate the

polarity of the motion at a specific instant. As long as the four excitation

amplitudes are equal, the total net angular momentum of the sensor is

zero, and the structure is similar to two center-mounted beams that move

in a scissors-like motion. The zero total angular momentum ensures no

angular vibratory interaction with the mounting base 48 around an axis

perpendicular to the plane of the paper.

When the two-axes sensor in Fig. 15 experiences inertial rate

around the x-axis, i.e. around tines 46, 47 - or around the y-axis parallel

to tines 44 and 45, Coriolis accelerations will induce the tines to vibrate

perpendicular to the plane of the paper and in opposite phase. The

amplitude of the Coriolis accelerations will be equal if the excitation

amplitude of the tines were equal. In that case, subtracting Coriolis

induced deflections will provide an inertial angular rate signal that is insensitive to extraneous linear vibrations of the mounting base along the

axis perpendicular to the plane of the paper, that might otherwise interfere

with the output signal. This is so, since the extraneous accelerations act

similarly on the two tines and their effect will thus be nullified by the

subtraction. It is thus obvious that the amplitude of the excitation should

be equal in the four tines. In order to ensure that the amplitude of the

four tines is equal in spite of practical imperfections, as described above,

the four tines are linked with mechamcal couplings 49, 50, 51, and 52,

that evenly distribute the excitation forces and equalize the arms'

deflection.

Fig. 16 illustrates a modified two-axis angular rate sensor,

according to the present invention, provided with a mounting base 75,

wherein the tines are indirectly excited by means of elements 53 and 54,

in a manner similar to that illustrated in Fig. 12. The operation of this

embodiment is self-explanatory, but it should be emphasized that,

depending on the specific design, four such excitation elements may be

used.

While preferred embodiments of the invention have been described,

it should be understood that the invention may be carried out with many

modifications, variations and adaptations by persons skilled in the art,

without departing from its spirit or exceeding the scope of its claims.

Claims

WHAT IS CLAIMED IS:

1. An inertial angular rate sensor (IARS), including a

symmetrical mechanical resonator, comprising at least one pair of

vibrating masses and including mechanical coupling means that force the

masses to move in a symmetrical manner despite mechanical tolerances.

2. An IARS including a tuning fork resonator with a mechanical

coupling as in claim 1.

3. An IARS including a frame-type resonator with a mechanical

coupling as in claim 1.

4. An IARS including a planar resonator as in claims 1, 2 and

3.

5. An IARS including an H-type tuning fork, comprising two

pairs of tines, the tines of each pair being coupled as in claim 1 , and stems

29 and 30 allowing torsional motion of the tuning fork around its line of

symmetry.

6. An IARS including a frame-type resonator, comprising two

tuning forks according to claim 1 , with their respective tines connected.

7. An IARS including a frame-type resonator, comprising two

masses supported by parallel-motion bar mechamsms and coupled on either

side by mechanical couplings, as in claim 1.

8. An IARS including a symmetrical mechanical resonator,

according to any one of claims 1 to 7, comprising means for exciting the

vibrating masses by applying at least one force to the mechanical coupling

means.

9. An IARS including a planar symmetrical mechamcal

resonator as in claim 8, wherein the force is applied by parametrically

modulating the effective length of a mechamcal member.

10. A two-axis IARS including a planar mechanical resonator,

comprising four arms assembled in a cruciform shape and means for

vibrating said arms in a scissors mode, wherein each pair of adjacent arms

are coupled as in claim 1.

11. A two-axis IARS including a mechanical resonator as in

claim 10, wherein the arms are indirectly excited by means of forces

applied in the plane of the resonator along at least one of its axes of

symmetry.

12. A two-axis IARS including a mechanical resonator as in

claim 11 , wherein said force is generated by parametrically modulating the

effective length of a mechanical member.

13. A IARS according to claim 1 , substantially as described and

illustrated.