Patent ID: 12235129

DETAILED DESCRIPTION

InFIG.1, an assembly1comprises a micromachined inertial angle sensor2and a calibration system4configured to calibrate the angle sensor2. Hereafter, the micromachined inertial angle sensor2is referred to as the angle sensor2.

The angle sensor2is for example a gyrometer configured to measure an angular velocity. Alternatively or additionally, the angle sensor2is a gyroscope for measuring an angular position.

The angle sensor2is a micromachined sensor, and thus forms a microelectromechanical system, also defined by its acronym MEMS (Microelectromechanical Systems).

In particular, the angle sensor2is a sensor intended to be carried in a vehicle, not shown, for example in an aircraft, a drone or a ship.

The angle sensor2is for example intended for use in a vehicle navigation, steering, or guidance system.

The angle sensor2is in particular a tuning fork gyro, particularly a tuning fork gyro with two vibrating masses.

“Vibrating mass” means that the mass is capable of oscillating, for example driven by means described below, and by the Coriolis effect when the angle sensor2is rotated.

With reference to the examples inFIGS.2and3, the angle sensor2comprises a support6extending in a plane of the support6along a first axis X and a second axis Y perpendicular to the first axis X.

The angle sensor2comprises at least one vibrating mass8,10, preferably two vibrating masses8,10arranged around each other, to form a so-called inner mass8and an outer mass10.

The vibrating masses8,10are movable relative to the support6, and movable relative to each other. In particular, the centres of gravity0of the vibrating masses8,10coincide when at rest.

The angle sensor2comprises suspension springs12, for example four for each vibrating mass8,10, suspending each vibrating mass8,10from a respective anchor point14which is fixed relative to the support6.

The angle sensor2further comprises coupling springs16, for example four when the angle sensor2comprises two masses8,10, coupling the vibrating masses8,10to each other to allow vibration of the masses8,10in phase opposition.

The example of the angle sensor2inFIG.3differs from the example inFIG.2in that, inFIG.3, the suspension springs12of the outer mass10are oriented towards the inner mass8and fixed to the same anchor points14as the corresponding suspension springs12of the inner mass8.

The angle sensor2further comprises, particularly visible inFIG.3, at least one excitation transducer Ex, Ey of a vibrating motion of the vibrating mass8,10, at least one detection transducer Dx, Dy of a vibration of the vibrating mass8,10, and at least one electrostatic transducer Tx, Ty adapted to apply an adjustable electrostatic stiffness to the vibrating mass8,10

The fact that the static stiffness is “adjustable” indicates that the electrostatic transducer Tx, Ty is capable of changing the electrostatic stiffness.

The angle sensor2preferably further comprises at least one electrostatic quadrature bias compensation transducer Q+, Q− configured to compensate for a quadrature bias. The quadrature bias corresponds to a coupling of a stiffness of the suspension springs12along the first axis X and the second axis Y.

For the sake of visibility, the transducers Ex, Ey, Dx, Dy, Tx, Ty, Q+ and Q− are not shown inFIG.2. Preferably, the angle sensor2according to the example inFIG.2comprises the same transducers as in the example inFIG.3.

An example of the arrangement of the transducers Ex, Ey, Dx, Dy, Tx, Ty, Q+ and Q− is described in the following with reference toFIG.3.

The angle sensor2comprises for example an excitation transducer Ex configured to excite the inner mass8along the first axis X, and an excitation transducer Ey configured to excite the inner mass8along the second axis Y in particular during an operation of the angle sensor2for example to obtain an angular velocity.

The angle sensor2comprises for example two detection transducers Dx configured to detect the vibration of the inner mass8along the first axis X, and two detection transducers Dy for such detection along the second axis Y.

The angle sensor2further comprises an electrostatic transducer Tx capable of applying the adjustable electrostatic stiffness to the inner mass8along the first axis X, and a corresponding electrostatic transducer Ty for the second axis Y, also called the additional electrostatic transducer.

The angle sensor2comprises, for example, two Q+ compensating electrostatic transducers configured to compensate for a positive quadrature bias, and two Q− compensating electrostatic transducers configured to compensate for a negative quadrature bias.

InFIG.3, the transducers Ex, Ey, Dx, Dy, Tx, Ty, Q+ and Q− are shown only for the inner mass8. Preferably, the angle sensor2further comprises at least one and preferably each transducer Ex, Ey, Dx, Dy, Tx, Ty, Q+ and Q− for the outer mass10as well. For example, each transducer Ex, Ey, Dx, Dy, Tx, Ty, Q+ and Q− is as described in EP 2 960 625 A1.

Each transducer Ex, Ey, Dx, Dy, Tx, Ty, Q+ and Q− is in particular a transducer comprising interdigitated combs. These interdigitated combs comprise a mobile comb rigidly connected to the inner mass8, in particular fixed with respect to the inner mass8, and a comb rigidly connected to the support6, in particular fixed with respect to the support6.

Each transducer Ex, Ey, Dx, Dy, Tx, Ty, Q+ and Q− is configured either to apply a force to the inner mass8, based on a received voltage, or to detect movement of the inner mass8by measuring changes in load between the fixed and moving combs.

The calibration system4comprises an excitation device18configured to emit a predetermined vibrational excitation to the angle sensor2, and a computing device20.

The excitation device18comprises for example a loudspeaker, a piezoelectric element and/or a vibration generator.

The excitation device18is for example configured to emit to the angle sensor2a vibrational excitation of a frequency range centred around a resonant frequency of the excitation device18, for example of a frequency range equal to +/−2 k Hz relative to the resonant frequency.

The resonant frequency of the excitation device18is a predetermined frequency.

For example, the excitation device18is integrated into a calibration bench, not shown, allowing the angle sensor2to be positioned at a predetermined position for calibration by receiving the vibrational excitation.

According to another example, the excitation device18is attached to the angle sensor2, for example by being arranged together with the angle sensor2in a single housing. This allows, for example, a self-calibration of the angle sensor2.

In some examples, the excitation device18is arranged outside or inside the angle sensor2. In an example not shown, the excitation device18is integrated into the angle sensor2.

The computing device20comprises a reception module22, a transformation module24, a determination module25, a feedback loop26and a transmission module28.

The reception module22, the transformation module24, the determination module25, the feedback loop26and the transmission module28are each integrated in at least one computer.

In this case, each of the reception module22, the transformation module24, the determination module25, the feedback loop26and the transmission module28is at least partly in the form of software that can be executed by a processor and stored in a memory of the computer.

In a variant or in addition, each of the reception module22, the transformation module24, the determination module25, the feedback loop26, and the transmission module28is at least partially integrated into a physical device, such as a programmable logical component, such as a FPGA (“Field-Programmable Gate Array”), or as a dedicated integrated circuit, such as an ASIC (“Application-Specific Integrated Circuit”).

The reception module22is configured to receive a measurement signal Smobtained from a measurement of the vibration of the vibrating mass8,10by each detection transducer Dx, Dy.

The transformation module24is configured to transform the measurement signal Smover a predetermined time window of that measurement signal Sminto a power spectral density.

The determination module25configured to determine at least one value, called noise value V, as a function of a portion of said power spectral density comprising a frequency less than or equal to a predetermined frequency. This portion in particular has a frequency lower than or equal to the predetermined frequency.

The predetermined frequency is, for example, 0.1 Hz.

In other examples, the predetermined frequency has a value greater than or less than 0.1 Hz.

The feedback loop26is configured to adjust the electrostatic stiffness to be applied by each electrostatic transducer Tx, Ty to the vibrating mass8,10.

The feedback loop26is configured to receive as input the noise value V and to output a control signal Scto the electrostatic transducer Tx, Ty comprising the electrostatic stiffness to be applied to the vibrating mass8,10to minimise the noise value V. Preferably, the feedback loop26is configured to output the control signal Scthat minimises the amplitude of the noise value V.

The transmission module28is configured to transmit the control signal Scto the angle sensor2to each electrostatic transducer Tx, Ty to apply the electrostatic stiffness in accordance with the control signal Sc.

A calibration method100for the angle sensor2is now described, with reference toFIG.4showing a flowchart of this method.

The calibration method100comprises a receiving step110, a measuring step120, a transforming step130, a determining step135, an adjusting step140and an applying step150.

In the receiving step110, the angle sensor2receives the predetermined vibrational excitation from the excitation device18which is separate from the excitation transducer Ex, Ey.

Each vibrating mass8,10is subjected to different forces in the receiving step110. An example is described below.

Each vibrating mass8,10is subjected to return forces by the suspension springs12connecting each vibrating mass8,10to the support6. The suspension springs12generate a sum of stiffnesses Ki applied to the inner mass8and a sum of stiffnesses Ke applied to the outer mass10.

The sum of stiffnesses Ki applied to the inner mass8by the suspension springs12of the inner mass8is for example defined as follows:

K⁢i=[K⁢x⁢iK⁢x⁢y⁢iK⁢x⁢y⁢iK⁢y⁢i],
whereKxi is the stiffness along the first axis X generated by the suspension springs12connecting the inner mass8to the support6;Kyi is the stiffness along the second axis Y generated by the suspension springs12connecting the inner mass8to the support6;Kxyi represents the quadrature stiffness forming a coupling between the first axis X and second axis Y.

The sum of stiffnesses Ke applied to the outer mass10is defined in an equivalent way, with the index e instead of i:

K⁢e=[K⁢x⁢eK⁢x⁢y⁢eK⁢x⁢y⁢eK⁢y⁢e].

In addition, we define:

X⁢i=[x⁢iy⁢i]
as the displacement of the inner mass8, and

Xe=[xeye]
as that of the outer mass10.

Each vibrating mass8,10is also subjected to forces generated by the electrostatic transducers Tx, Ty. The electrostatic transducers Tx, Ty generate a sum of stiffnesses Kti applied to the inner mass8and a sum of stiffnesses Kte applied to the outer mass10.

The sum of stiffnesses Kti applied to the inner mass8is for example defined as follow:

Kti=[KtxiKtxyiKtxyiKtyi],Ktxi is the stiffness along the first axis X generated by the or each electrostatic transducer Tx along the X axis;Ktyi is the stiffness along the first axis X generated by the or each electrostatic transducer Ty along the Y axis.

The sum of stiffnesses Kte applied to the outer mass10is defined in an equivalent way, with the index e instead of i.

For example, the inner mass8is subjected to an oscillating force Foscillantwhich is defined as follows:
Mi·{umlaut over (X)}ι+Ki·Xi=Foscillant, whereMi is the mass of the inner mass8;{umlaut over (X)}ι is the acceleration of the inner mass8.

Similarly, the outer mass10has, for example, an oscillating force Foscillantwhich is defined as follows:
Me·{umlaut over (X)}e+Ke·Xe=Foscillant, whereMe is the mass of the outer mass10;{umlaut over (X)}e is the acceleration of the outer mass10.

The skilled person will understand that these definitions of the oscillating forces are simplified definitions which disregard certain forces, for example coupling forces between the internal and outer mass8,10, Coriolis forces, and differences in the excitation force applied to each mass.

From the first terms Mi·{umlaut over (X)}ι and Me·{umlaut over (X)}e of the oscillating force equations Foscillantfor each mass8,10, we have:

Mi.X¨⁢i-Me.Xe¨=(M⁢i-M⁢e)⁢(X¨⁢i+Xe¨)2+M⁢i+M⁢e2⁢(X¨⁢i-Xe¨).

In particular, this equation has a phase acceleration

(X¨⁢i+Xe¨)2,
the average acceleration of the two masses8,10, associated with the mass difference (Mi−Me) and an opposite phase acceleration ({umlaut over (X)}i−{umlaut over (X)}e), associated with the average mass

M⁢i+M⁢e2.

We now consider the second terms Ki·Xi and Ke·Xe of the oscillating force equations Foscillantfor each mass8,10. These terms relate to the forces generated by the suspension springs12connecting the inner8and outer 10 masses to the support6.

The movement

(X⁢i+X⁢e)2
described below is driven by the excitation device18.

We have:

[KxiKxyiKxyiKyi][x⁢iy⁢i]-[KxeKxyeKxyeKye][xeye]=K⁢i.X⁢i-K⁢e.Xe=(Ki-K⁢e)⁢(X⁢i+X⁢e)2+K⁢i+K⁢e2⁢(X⁢i-X⁢e)

That is:

M⁢i+M⁢e2⁢(X¨⁢i-Xe¨)+K⁢i+K⁢e2⁢(X⁢i-X⁢e)=-(Mi-Me)⁢(X¨⁢i+Xe¨)2-(K⁢i-K⁢e)⁢(X⁢i+X⁢e)2.

In the reception step110, the movement of the inner and outer mass8,10has in particular two disturbing forces

(Mi-Me)⁢(X¨⁢i+Xe¨)2⁢and⁢(Ki-K⁢e)⁢(X⁢i+X⁢e)2,
related to a difference of the masses8,10and a difference of the sums of the stiffnesses of the suspension springs12. In particular, these disturbing forces are reduced, preferably cancelled, by the feedback loop26in the adjusting step140, described later.

In the measurement step120, the or each detection transducer Dx, Dy measures the vibration of the vibrating mass8,10, to obtain the measurement signal Smfrom the measurement by the or each detection transducer Dx, Dy.

The measurement signal Smis in particular a signal at the output of the angle sensor2, for example an angular velocity or an angular position.

For example, the angle sensor2determines an angular velocity corresponding to the measurement signal Sm, depending on the amplitude of the vibration of the vibrating mass8,10.

For example, when the angle sensor2is a gyroscope, the measurement signal Smis a measurement of the direction of vibration of the vibrating mass8,10, in the plane of the support6, measured by the detection transducers Dx, Dy. For example, each detection transducer Dx, Dy measures the amplitude of the vibration along the corresponding X and Y axis, and the angle sensor2obtains the measurement signal Smfrom these measurements.

When the angle sensor2is a gyrometer, the measurement signal Smis, for example, proportional to excitation forces applied by the or each excitation transducer Ex, Ey. The angle sensor2determines these excitation forces in particular on the basis of the measurement of the vibration of the vibrating mass8,10by the or each detection transducer Dx, Dy.

For example, the measurement step120comprises the measurement by the detection transducer Dx of the vibration of the inner mass8along the first axis X, and the measurement by the detection transducer Dy, also called additional detection transducer, of the vibration of the inner mass8along the second axis Y.

According to one example, the measurement step120comprises measurements along any direction in the plane of the support6, in particular different from the X or Y axis.

In particular, this vibration corresponds to the displacement referred to as Xi above and possibly its first and/or second order derivatives.

Preferably, the measurement step120further comprises measuring the vibration of the outer mass10along the first axis X and the second axis Y by the corresponding detection transducers, not shown.

In particular, this vibration corresponds to the displacement referred to as Xe above and possibly its first and/or second order derivatives.

In the case of two masses, namely the inner mass8and the outer mass10, the measuring step120comprises in particular measuring a movement of each mass8,10along the first axis X, and along the second axis Y. For example, the angle sensor2determines the measurement signal Smfrom the difference in movement, in particular in phase opposition, between the inner mass8and the outer mass10along the first axis X, and the difference in movement between these masses8,10along the second axis Y.

According to one example, the angle sensor2determines a resulting direction of vibration of each vibrating mass8,10, in the plane of the support6, from the measurements of the detection transducers Dx and Dy, to obtain the measurement signal Sm.

In the transformation step130, the transformation module24transforms the measurement signal Smover a predetermined time window of this measurement signal Sminto a power spectral density.

The power spectral density is in particular the power spectral density of the measurement signal Sm.

The transformation module24obtains the power spectral density, for example by applying the periodogram method to the measurement signal Sm.

The power spectral density shows the frequency distribution of the power of the measurement signal Smaccording to the frequencies of this signal. For example, the power spectral density has the unit:

∘/⁢sHz,
where°/s is the rotational velocity obtained by measurement by the angle sensor2in degrees per second; andHz is the frequency in Hertz.

In the determination step135, the determination module25determines the noise value V, based on a portion of the power spectral density comprising a frequency less than or equal to the predetermined frequency, for example 0.1 Hz.

This portion in particular has a frequency lower than or equal to the predetermined frequency.

For example, the determination module25determines the arithmetic mean of each value of the power spectral density less than or equal to the predetermined frequency to obtain the noise value V.

In another example, the determination module25determines an Allan variance to obtain the noise value V.

According to one example, the determination module25determines a plurality of noise values V, corresponding for example to different measurement directions in the plane of the support6.

In the adjusting step140, the feedback loop26adjusts the electrostatic stiffness to be applied to each ground8,10.

“Electrostatic stiffness” is understood to mean a stiffness which is constant over a predetermined period of time and which is applied to the vibrating mass8,10, in particular constant over a period of time which is several orders of magnitude longer than an oscillation period of the predetermined vibrational excitation generated by the excitation device18.

According to one example, the electrostatic stiffness depends on a direction in the plane of the support6formed by the X and Y axes and/or on a temperature of the angle sensor2.

The feedback loop26receives as input the noise value V and provides as output the control signal Scto each electrostatic transducer Tx, Ty comprising the electrostatic stiffness to be applied to the respective vibrating mass8,10, that minimises the noise value V.

Preferably, the feedback loop26minimises the amplitude of the noise value V by providing the control signal Sc.

In particular, the feedback loop26implements a negative feedback that tends to reduce the noise value V.

For example, the control signal Sccomprises a different electrostatic stiffness for each vibrating mass8,10to account for an imbalance of the vibrating masses8,10, preferably a different stiffness along the X-axis with respect to the Y-axis.

In the example of an angle sensor2with the two vibrating masses8,10, the imbalance is the difference in mass between the vibrating masses8and10.

The control signal Sccomprises, for example, the electrostatic stiffness to be applied in the X-axis by the electrostatic transducer Tx, and the electrostatic stiffness to be applied in the Y-axis by the additional electrostatic transducer Ty, for each vibrating mass8,10.

For example, the control signal Scfurther comprises an electrostatic stiffness to be applied by each electrostatic quadrature bias compensation transducer Q+, Q−.

For example, the feedback loop26provides the control signal Sccomprising electrostatic stiffeners generating a force determined as follows:

(Kti-Kte)⁢(X⁢i+X⁢e)2+Kti+Kte2⁢(X⁢i-Xe).

In particular, this determined force minimises, and preferably cancels, the following disturbing forces:

(Mi-Me)⁢(X¨⁢i+Xe¨)2⁢and⁢(Ki-Ke)⁢(X⁢i+X⁢e)2.

In the application step150, the electrostatic transducer Tx, Ty applies the electrostatic stiffness according to the control signal Sc.

In particular, the electrostatic transducer Tx applies the respective electrostatic stiffness along the X-axis, and the electrostatic transducer Ty along the Y-axis, according to the control signal Se.

The electrostatic transducers Tx, Ty cancel out the diagonal terms of the disturbing forces in particular, by applying the following stiffnesses:

(Kti-Kte)⁢(Xi+Xe)2=[Ktxi-Ktxe00Ktyi-Ktye]⁢(Xi+Xe)2.

For example, each electrostatic compensation transducer Q+, Q− applies the respective electrostatic stiffness according to the control signal Sc.

The electrostatic compensation transducers Q+, Q− cancel out the non-diagonal terms of the disturbing forces.

For example, the calibration method100is repeated several times, as illustrated in particular by arrow R inFIG.4.

For example, when repeating the calibration method100, the excitation device18continuously emits a predetermined vibrational excitation to the angle sensor2.

After a first execution of steps110,120,130,135,140and150and the application of the electrostatic stiffness in accordance with the control signal Sc, the noise value V, and in particular its amplitude, is reduced in a second execution compared to the first execution.

Preferably, the calibration method100is repeated until a predetermined value of the noise value V is obtained.

According to one example, the calibration method100comprises at least one repetition of the steps of receiving110, measuring120, transforming130, determining135, adjusting140, and applying150.

For example, the control signal Sccomprises an electrostatic stiffness to be applied in a direction, in the plane of the support6, which is distinct from the first axis X and the second axis Y.

For example, the calibration method100comprises the implementation of several repetition of the steps of receiving110, measuring120, transforming130, determining135, adjusting140, and applying150. In this case, in a given repetition, the control signal Sccomprises an electrostatic stiffness to be applied in a first direction that is distinct from a second direction. The second direction corresponds in particular to the direction of the electrostatic stiffness to be applied in a previous repetition. The first and second directions are in the plane of the support6, and have for example a predetermined angle between them, such as 10°.

Of course, other embodiments than those described above can be envisaged.

For example, the angle sensor2is a different sensor from the one described above. For example, the angle sensor2comprises only one or more than two vibrating masses.

The calibration system4and the calibration method100according to the invention have a large number of advantages.

In particular, the calibration system4and the calibration method100allow a calibration of the angle sensor2which is simple to implement due to the excitation by the excitation device18which is separate from the excitation transducer Ex, Ey, and due to the adjustment from the output measurement of the angle sensor2.

Also, calibration by the method according to the invention makes it possible to obtain a good measurement accuracy of the angle sensor2, because the low frequency noise of the measurement of the angle sensor2is reduced by the feedback implemented by the feedback loop26.

In the example where the sensor receives a predetermined vibrational excitation from an excitation device which is separate from the excitation transducer, the implementation of the calibration method is particularly simple, as it is sufficient to position the angle sensor for example in a predefined area to receive this excitation for calibration, especially in the absence of added compensation elements inside the sensor.