Microelectromechanical gyroscope with continuous self-test function

A microelectromechanical gyroscope includes a body and a sensing mass, which is movable with a degree of freedom in response to rotations of the body about an axis. A self-test actuator is capacitively coupled to the sensing mass for supplying a self-test signal. The capacitive coupling causes, in response to the self-test signal, electrostatic forces that are able to move the sensing mass in accordance with the degree of freedom at an actuation frequency. A sensing device detects transduction signals indicating displacements of the sensing mass in accordance with the degree of freedom. The sensing device is configured for discriminating, in the transduction signals, spectral components that are correlated to the actuation frequency and indicate the movement of the sensing mass as a result of the self-test signal.

BACKGROUND

1. Technical Field

The present invention relates to a microelectromechanical gyroscope with continuous self-test function, and to a method for controlling a microelectromechanical gyroscope.

2. Description of the Related Art

As is known, the use of microelectromechanical systems (MEMS) has increasingly spread in various technological sectors and has yielded encouraging results especially in providing inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.

MEMS systems of this type are usually based upon microelectromechanical structures comprising at least one mass, connected to a fixed body (stator) through springs and movable with respect to the stator according to pre-set degrees of freedom. The movable mass and the stator are capacitively coupled through a plurality of respective comb-fingered and mutually facing electrodes so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors, whence it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and consequently to the force applied. Instead, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass for setting it in motion. In addition, for providing electromechanical oscillators, the frequency response of MEMS inertial structures is exploited, which is typically of a second-order low-pass type, with one resonant frequency.

In particular, MEMS gyroscopes have a more complex electromechanical structure, which comprises two masses that are movable with respect to the stator and are coupled to one another so as to have a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is kept in oscillation at the resonant frequency. The other mass is driven in (translational or rotational) oscillatory motion and, in the event of rotation of the microstructure with respect to a pre-determined gyroscopic axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the driven mass, which is capacitively coupled to the fixed body through electrodes, as likewise the driving mass, operates as an accelerometer that enables detection of the Coriolis force and acceleration and hence of the angular velocity.

As practically any other device, MEMS gyroscopes are subject to production defects (which can regard the entire microstructure and the electronics) and wear, which can reduce the reliability thereof or jeopardize their operation completely.

For this reason, prior to being installed, gyroscopes undergo testing in the factory for proper operation, which enables identification and rejection of faulty examples.

In many cases, however, it is important to be able to carry out sampled checks at any stage of the life of the gyroscope, after its installation. In addition to the fact that, in general, it is advantageous be able to locate components affected by faults, to proceed to their replacement, MEMS gyroscopes are used also in critical applications, where any malfunctioning may have disastrous consequences. Just to provide an example, in the automotive sector the activation of many air-bag systems is based upon the response provided by gyroscopes. It is thus evident how important it is to be able to equip MEMS gyroscopes with devices that are able to carry out frequent tests on proper operation.

It should moreover be noted that the circuits for testing do not have to affect significantly the overall dimensions and the consumption levels, which assume increasing importance in a large number of applications.

In this connection, solutions have been developed that enable execution of in-field tests for proper operation in given conditions. According to known solutions, in particular, a self-test signal is generated starting from driving signals used for keeping the driving mass in oscillatory motion. The self-test signal is synchronous with the oscillations of the driving mass and is supplied to electrodes for self-testing of the sensing mass, which are configured so as to apply electrostatic forces to the sensing mass in the sensing direction, in response to the self-test signal. In practice, then, the self-test signal causes an effect that is altogether similar to that of a rotation of the microstructure about the gyroscopic axis. The amplitude of the self-test signal is known. In conditions of absence of any rotation of the microstructure, it is hence possible to detect the response of the gyroscope and, by comparing it with an expected response, determine whether the gyroscope functions properly or whether any malfunctioning has arisen.

BRIEF SUMMARY

It is noted that the known self-test process described above requires that the gyroscope be at rest, which is often impractical or even impossible, and that the self-test function cannot be exploited continuously or simultaneously with normal operation.

According to one embodiment, a device is provided that includes a microelectromechanical gyroscope having a body, a driving mass movably coupled to the body, and a sensing mass movably coupled to the driving mass. An actuator is provided, configured to apply a biasing force to the sensing mass to introduce a corresponding motion to the sensing mass relative to the driving mass. A sensing device is configured to detect motion of the sensing mass and to distinguish in the detected motion a component corresponding to motion of the body from a component corresponding to motion of the sensing mass introduced by the biasing force applied by the actuator. The sensing device is capacitively coupled to the sensing body, and is configured to detect differential changes in the capacitive coupling arising in response to motion of the sensing mass and to produce a corresponding first signal.

The sensing device can include demodulators configured to extract, from the first signal, a second signal corresponding to the motion of the body, and the second demodulator configured to extract, from the second signal, a third signal corresponding to motion of the sensing mass introduced by the biasing force applied by the actuator. Detection of the third signal at an appropriate amplitude is indicative of proper function of the gyroscope.

According to an embodiment, the sensing mass is one of a plurality of sensing masses, each movably coupled to the driving mass according to a respective axis of freedom, and the actuator is configured to apply a biasing force to each of the plurality of sensing masses to introduce a corresponding motion to each sensing mass according to the respective axis of freedom. The sensing device is configured to detect motion of each of the sensing masses according to the respective axis of freedom and to distinguish in the detected motion a component corresponding to motion of the body in a corresponding one of a plurality of axes of detection from a component corresponding to motion of the respective sensing mass introduced by the biasing force applied by the actuator.

According to respective embodiments, the device is a palm-top computer, a laptop computer, a cell phone, a messaging device, a digital music player, and a digital camera, each of which incorporates the gyroscope to detect movement of the device.

According to an embodiment, a method for testing the operation of a microelectromechanical gyroscope is provided, including introducing a first signal to a sensing mass that is movably coupled to a body of the gyroscope, detecting a second signal corresponding to movement of the sensing mass according to a degree of freedom, and separating from the second signal a third signal corresponding to movement of the body according to a sensing axis of the gyroscope and a fourth signal corresponding to movement of the sensing mass in response to introduction of the first signal. The first signal can be introduced by modulating a fifth signal having a frequency equal to a drive frequency of the sensing mass with a sixth signal having a frequency lower than the drive frequency, to produce the first signal, and introducing the resulting first signal to the sensing mass.

Separating the third and fourth signals from the second signal can include demodulating the second signal with a seventh signal having a frequency equal to the drive frequency, to produce a first demodulated signal, and filtering the first demodulated signal to derive the third signal; and demodulating the first demodulated signal with an eighth signal having a frequency equal to the frequency of the sixth signal, to produce a second demodulated signal, and filtering the second demodulated signal to derive the fourth signal.

Proper function of the gyroscope can be determined on the basis of an amplitude of the fourth signal, with respect to a reference value.

DETAILED DESCRIPTION

FIG. 1is a schematic illustration of a microelectromechanical gyroscope1, which comprises a microstructure2, made of semiconductor material, a driving device3, a self-test actuator4, and a sensing device5.

The microstructure2is made of semiconductor material and comprises a fixed structure6, a driving mass7, and at least one sensing mass8. For reasons of simplicity, in the embodiment illustrated herein reference will be made to the case of a uniaxial gyroscope, in which only one sensing mass8is present. The ensuing description applies, however, also to the case of multiaxial gyroscopes, which comprise two or more sensing masses for detecting rotations according to respective independent axes.

The driving mass7is elastically connected by suspensions (not shown) to the fixed structure6so as to be oscillatable about a rest position according to a translational or rotational degree of freedom.

The sensing mass8is mechanically coupled to the driving mass7so as to be driven in motion according to the degree of freedom of the driving mass7itself. In addition, the sensing mass8is elastically connected to the driving mass7so as to oscillate in turn with respect to the driving mass7itself, with a respective further translational or rotational degree of freedom. In the embodiment described herein, in particular, the driving mass7is linearly movable along a driving axis X, whilst the sensing mass8is movable with respect to the driving mass7according to a sensing axis Y perpendicular to the driving axis X. It is understood, however, that the type of movement (whether translational or rotational) allowed by the degrees of freedom and the arrangement of the driving and sensing axes can vary according to the type of gyroscope. With reference to the movements of the driving mass7and of the sensing mass8, moreover, either of the expressions “according to an axis” and “in accordance with an axis” can be understood as indicating movements along an axis or about an axis, according to whether the movements allowed to the masses by the respective degrees of freedom of a particular device are translational or else rotational, respectively. Likewise, either of the expressions “according to a degree of freedom” and “in accordance with a degree of freedom” can be understood as indicating either translational or rotational movements, as allowed by the degree of freedom itself.

In addition, the driving mass7(with the sensing mass8) is connected to the fixed structure6so as to define a resonant mechanical system with a resonant frequency ωR(according to the driving axis X).

The driving mass7(FIG. 2) is capacitively coupled to the fixed structure6by driving units10and feedback sensing units12. The capacitive coupling is preferably of a differential type.

In greater detail, the driving units10comprise first and second fixed driving electrodes10a,10b, anchored to the fixed structure6and extending substantially perpendicular to the driving direction X, and movable driving electrodes10c, anchored to the driving mass7and arranged also substantially perpendicular to the driving direction X. The movable driving electrodes10care comb-fingered and capacitively coupled to respective first fixed driving electrodes10aand second fixed driving electrodes10b. In addition, the first and second fixed driving electrodes10a,10bof the driving units10are electrically connected, respectively, to a first driving terminal13aand to a second driving terminal13bof the microstructure2. As already mentioned, moreover, the coupling is of a differential type. In other words, in each driving unit10a movement of the driving mass7along the driving axis X produces an increase in the capacitance between each movable driving electrode10cand one of the corresponding fixed driving electrodes10a,10b, while the capacitance between the respective movable driving electrode10cand the other of the corresponding fixed driving electrodes10a,10bdecreases, accordingly.

The structure of the feedback sensing units12is similar to that of the driving units10. In particular, the feedback sensing units12comprise first and second fixed sensing electrodes12a,12b, anchored to the fixed structure6, and first movable sensing electrodes12c, anchored to the driving mass7and comb-fingered and capacitively coupled to respective first fixed sensing electrodes12aand second fixed sensing electrodes12b. In addition, the first and second fixed sensing electrodes12a,12bof the feedback sensing units12are electrically connected, respectively, to a first feedback sensing terminal14aand to a second feedback sensing terminal14bof the microstructure2.

The sensing mass8is capacitively coupled to the fixed structure6through signal sensing units15(FIG. 3) and self-test actuation units17. More precisely, the signal sensing units15comprise third and fourth fixed sensing electrodes15a,15b, anchored to the fixed structure6, and second movable sensing electrodes15c, anchored to the sensing mass8and set between respective third fixed sensing electrodes15aand fourth fixed sensing electrodes15b. In addition, the third and fourth fixed sensing electrodes15a,15bof the signal sensing units15are electrically connected, respectively, to a first signal sensing terminal16aand to a second signal sensing terminal16bof the microstructure2.

The self-test actuation units17comprise first and second fixed self-test electrodes17a,17b, anchored to the fixed structure6, and movable self-test electrodes17c, anchored to the sensing mass8and set between respective first fixed sensing electrodes17aand second fixed sensing electrodes17b. In addition, the first and second fixed self-test electrodes17a,17bof the self-test sensing units17are electrically connected, respectively, to a first self-test terminal18aand to a second self-test terminal18bof the microstructure2.

Also for the electrodes of the signal sensing units15and of the self-test actuation units17the capacitive coupling is of a differential type, but is obtained by parallel-plate electrodes, perpendicular to the sensing direction Y.

As illustrated schematically inFIG. 1, in practice the driving mass7is coupled to the driving terminals13a,13bthrough driving differential capacitances CD1, CD2and to the sensing terminals14a,14bthrough feedback sensing differential capacitances CFBS1, CFBS2. The sensing mass8is instead coupled to the signal sensing terminals17a,17bthrough signal sensing differential capacitances CSS1, CSS2, and to the self-test terminals18a,18bthrough self-test differential capacitances CST1, CST2. In particular, the self-test actuation electrodes17a,17bare shaped in such a way that the sensing mass8is subject to an electrostatic actuation force FAparallel to the sensing direction Y in the presence of a non-zero voltage between the sensing mass8itself and the self-test terminals18a,18b. In addition, the sensing mass8is connected to an excitation terminal8afor receiving a square-wave excitation signal VE, supplied for example by the driving device3.

With reference once again toFIG. 1, the driving device3is connected to the driving terminals13a,13band to the feedback sensing terminals14a,14bof the microstructure2so as to form, with the driving mass7, a microelectromechanical loop19. The driving device3is configured so as to maintain the microelectromechanical loop19in oscillation at a driving frequency ωDclose to the resonant frequency ωRof the mechanical system defined by the driving mass7(with the sensing mass8) connected to the fixed structure6. In addition, the driving device3supplies a carrier signal VCof a frequency equal to the driving frequency ωDand in phase with the oscillations of the microelectromechanical loop19.

The self-test actuator4comprises a frequency-divider module20and a modulator21, having outputs connected to the self-test terminals18a,18bof the microstructure2. The self-test actuator4is thus capacitively coupled to the sensing mass8.

The frequency-divider module20is coupled to the driving device3for receiving the carrier signal VCand generates a base-band self-test signal VSTB, for example a sinusoidal or square-wave signal, with a frequency equal to or lower than the driving frequency ωD. In particular, the base-band self-test signal VSTBhas a self-test frequency ωSTequal to ωD/A (with A being an integer) and higher than the frequency band of the signals indicating the Coriolis acceleration to which the sensing mass8is subject as a result of a rotation about the sensing axis Y. Said frequency band will be hereinafter referred to as Coriolis band BC. Preferably, the self-test frequency ωSTis equal to one half of the driving frequency ωD(A=2) and is of the order of kilohertz (whilst the upper margin of the Coriolis band BC is of the order of tens of hertz).

The modulator21has inputs21a,21b, respectively connected to the frequency-divider module20and to the driving device3for receiving the carrier signal VCand the base-band self-test signal VSTB. In addition, the modulator21generates a modulated self-test signal VSTM, which is in practice obtained by modulating the carrier signal VCwith the base-band self-test signal VSTB. The modulation is preferably of the DSB-SC (Double Side Band—Suppressed Carrier) type.

Outputs of the modulator21are coupled to the self-test terminals18a,18bof the microstructure2so that the modulated self-test signal VSTMis applied to the self-test actuation units17and produces a net electrostatic actuation force FAthat tends to shift the sensing mass7according to the sensing axis Y (for example, in the direction indicated inFIG. 2).

The sensing device5comprises a read interface23, a signal branch24, and a self-test branch25.

The read interface23comprises a charge-to-voltage converter, for example a fully differential charge amplifier, having inputs connected to the signal sensing terminals16a,16bof the microstructure2. The read interface23receives from the sensing terminals16a,16bof the microstructure2electrical sensing signals indicating the displacements of the sensing mass8according to the sensing axis Y. The electrical sensing signals can be charge packets QS1, QS2having a value modulated by the angular velocity ΩC, as in the embodiment described herein, or else currents or voltages.

The read interface23converts the electrical sensing signals (in the example, the charge packets QS1, QS2) into a transduction signal VT, in turn indicating the displacements of the sensing mass8according to the sensing axis Y.

The signal branch24comprises a signal demodulator27and a first low-pass filter28, which are cascade-connected to the read interface23. In greater detail, the signal demodulator27is coupled to outputs of the read interface23for receiving the transduction signal VT. In addition, the signal demodulator27has a demodulation input27aconnected to the driving device3for receiving the carrier signal VC. The transduction signal VTis demodulated using the carrier signal VC. A first demodulated signal VD1is thus present on the output of the signal demodulator27.

The first low-pass filter28is connected downstream of the signal demodulator27and has a cut-off frequency comprised between approximately the upper margin of the Coriolis band BC and the self-test frequency ωST. Preferably, the cut-off frequency is close to the upper margin of the Coriolis band BC so as not to jeopardize the signal components produced by the rotation about the sensing axis Y.

The output of the first low-pass filter28defines a signal output1aof the gyroscope1and supplies an angular-velocity signal VΩindicating the angular velocity Ω about the sensing axis Y.

The self-test branch25comprises a self-test demodulator30, a second low-pass filter31, a comparator32, and a status register33, cascade-connected together.

In detail, the self-test demodulator30is coupled to the signal demodulator27, for receiving the first demodulated signal VD1, and has a demodulation input30aconnected to the frequency-divider module20of the self-test actuator4, for receiving the base-band self-test signal VSTB. The self-test demodulator30, in practice, carries out a further demodulation of the first demodulated signal VD1, using the base-band self-test signal VSTBas carrier, and thus generates a second demodulated signal VD2.

The second low-pass filter31is connected downstream of the self-test demodulator30and has a cut-off frequency such as to attenuate the harmonic components of the second demodulated signal VD2equal to or higher than the self-test frequency ωST. In one embodiment, the first low-pass filter28and the second low-pass filter31have the same cut-off frequency. However, the second low-pass filter31has the basic purpose of preserving the DC component of the second demodulated signal VD2and attenuating or eliminating all the higher components. Thus, the cut-off frequency of the second low-pass filter31may even be lower than that of the first low-pass filter28. A DC self-test signal VSTCis present on the output of the second low-pass filter31.

The comparator32is coupled to the low-pass filter31for receiving the DC self-test signal VSTC,and to a programmable reference generator35, which provides a threshold value VTH. The comparator32supplies a self-test logic signal FST having a first value when the DC self-test signal VSTCis greater than the threshold value VTH, and a second logic value otherwise.

The status register33stores the self-test logic signal FST and makes it available as status flag. In particular, the first logic value of the self-test logic signal FST indicates proper operation of the gyroscope1, whilst the second logic value indicates malfunctioning.

Operation of the gyroscope1is described in what follows.

The driving device3acts so as to maintain the microelectromechanical loop19in oscillation at the driving frequency ωD. Consequently, the driving mass7and the sensing mass8vibrate along the driving axis X about a resting position. When the gyroscope1turns about a gyroscopic axis perpendicular to the driving axis X and to the sensing axis Y, the sensing mass8oscillates along the sensing axis Y as a result of the Coriolis force. For reasons of simplicity, we shall assume that the gyroscope turns with a constant angular velocity ΩCin the Coriolis band BC (without any loss of generality to the ensuing treatment, which applies also in the case of angular velocity variable within the Coriolis band BC). The amplitude of the displacement, which is proportional to the rotation rate about the gyroscopic axis and to the velocity along the driving axis X, is transduced by the read interface23and converted into a spectral component of the transduction signal VT. More precisely, from the standpoint of the harmonic content of the transduction signal VTthe effect of the rotation about the gyroscopic axis is that of a carrier signal with driving frequency ωDamplitude-modulated by a signal having a frequency equal to the angular velocity ΩC. As illustrated inFIG. 4a, then, the component of the transduction signal VTdue to rotation about the gyroscopic axis comprises the frequencies ωD±ΩC(more in general, the band ωD±BC in the case of rotations at variable angular velocity, illustrated here with a dashed line).

Simultaneously, the self-test actuator4applies the modulated self-test signal VSTMto the self-test electrodes18a,18bof the microstructure2. Thanks to the capacitive coupling with the sensing mass8, the modulated self-test signal VSTMproduces an electrostatic actuation force FAand consequently causes displacements of the sensing mass8along the axis Y with actuation frequencies ωD±ωST. In normal operating conditions, said displacements are proportional to the amplitude of the modulated self-test signal VSTM; in effect, they add to the displacements caused by the rotation about the gyroscopic axis and, like these, are transduced by the read interface23. Consequently, the transduction signal VTalso comprises a component indicating the displacements caused by the self-test actuator4, even though in a different frequency band with respect to the component due to rotation about the gyroscopic axis. In particular, given that the modulated self-test signal VSTMis obtained by DSB-SC modulation of the carrier signal VCwith the base-band self-test signal VSTB, its spectrum contains the actuation frequencies ωD±ωST, which are again encountered also in the spectrum of the transduction signal VT. To sum up, the sensing mass8operates as adder of the effects due to the rotation about the gyroscopic axis (which it is desired to measure through the gyroscope1) and to the modulated self-test signal VSTMsupplied by the self-test actuator4, and the transduction signal VTgenerated by the read interface23contains (but for disturbance) the spectral components ωD±ωST(actuation frequencies) and ωD±ΩC(ωD±BC).

The demodulation performed by the signal demodulator27brings the frequency components ωSTand ΩCback into band base, in addition, of course, to introducing frequency components 2ωD±ωSTand 2ωD±ωC(FIG. 4b; in practice, the spectrum of the first demodulated signal VD1contains the frequencies ωD±(ωD±ωST) and ωD±ΩC, or ωD±(ωD±BC) in the case of rotations at variable angular velocity).

The first low-pass filter28(the transfer function FLP1of which is illustrated inFIG. 4b) eliminates or in any case attenuates all the spectral components having a frequency higher than the Coriolis band BC, and hence the angular-velocity signal VΩindicates just the angular velocity ΩCabout the gyroscopic axis.

The self-test demodulator30carries out a further demodulation of the first demodulated signal VD1using the base-band self-test signal VSTBas carrier. The component of the first demodulated signal VD1at the self-test frequency ωSToriginates a DC component DCST(FIG. 4c), which is due exclusively to the effect of the modulated self-test signal VSTMon the movement of the sensing mass8and corresponds (is for example proportional) to the maximum amplitude of the modulated self-test signal VSTMitself The other components of the first demodulated signal VD1introduce higher frequency components into the spectrum of the second demodulated signal VD2, among which frequency components ωST±ΩC(further spectral components are not illustrated for reasons of simplicity).

The second low-pass filter31(the transfer function FLP2of which is illustrated inFIG. 4c) eliminates or attenuates all the components with frequency higher than the DC component. The DC self-test signal VSTCis hence indicative only of the contribution of the modulated self-test signal VSTMand can thus be compared with a threshold value VTH, which can possibly be calibrated. As already mentioned, in practice, the effect of the base-band self-test signal VSTBis equivalent, as regards the displacements of the sensing mass8, to a rotation about the gyroscopic axis, but falls within a different frequency band, which the self-test branch25makes it possible to isolate and discriminate in order to determine whether the gyroscope1is operating properly. If in fact any malfunctioning arises, whether in the micromechanical part or in the associated electronics, the amplitude of the DC self-test signal VSTCis reduced with respect to the expected value. The comparator32then switches, and the self-test logic signal FST passes to the value indicating a fault, which is immediately identified.

In practice, then, the gyroscope1uses a self-test signal (the modulated self-test signal VSTM) with harmonic content basically in a band distinct from the Coriolis band BC. The effect of the self-test signal can be added to the effect of rotations about the gyroscopic axis through the sensing mass8. Hence, the signals fetched and transduced by the read interface23have spectral components in distinct bands that can be put down either to the rotations about the gyroscopic axis or to the self-test actuation. The reading branch24and the self-test branch25of the reading device5separate the spectral components, in particular isolating the contribution of the self-test actuation. The latter can hence be examined for determining the compatibility with normal operating conditions. In the embodiment described, the separation of the spectral components is obtained effectively by a double demodulation, using first the carrier signal VCand then the base-band self-test signal VSTas demodulating signals.

The gyroscope1described above advantageously enables continuous exploitation of the self-test functions, irrespective of the operating conditions. In particular, the self-test can be conducted even when the gyroscope1is not at rest, and it is not necessary to suspend the operations of measurement of the angular velocity, because the self-test actuator4and the self-test branch25of the sensing device5enable discrimination, in the spectrum of the signal transduced by the read interface23, of the effect of the self-test signal applied to the sensing mass8. In addition, using a very selective low-pass filter for isolating the DC component, it is possible to apply self-test signals of reduced amplitude, which practically have no effect as regards sensing of the angular velocity (in other words, the dynamics of the read interface23is almost entirely available for detection of the angular-velocity signal, and the self-test signals have an altogether negligible effect on the signal-to-noise ratio).

FIG. 5illustrates a triaxial gyroscope100in accordance with a different embodiment of the invention. The gyroscope100comprises a microstructure102, a driving device103, a self-test actuator104, a sensing device105, and a control unit150.

The microstructure102is, for example, of the type described in detail in the published European patent application No. EP-A-1 832 841 and in the corresponding U.S. patent publication No. 2007/0214883, now U.S. Pat. No. 7,694,563, and comprises a fixed structure106, a driving mass107, and three sensing-mass systems108-X,108-Y,108-Z. InFIG. 5, however, the microstructure102is represented only schematically, for reasons of simplicity.

The driving mass107is elastically connected through suspensions (not shown) to the fixed structure106so as to oscillate in a plane XY about a resting position according to a degree of freedom, in this case rotational. The sensing masses108-X,108-Y,108-Z are mechanically coupled to the driving mass107so as to be driven in motion according to the rotational degree of freedom of the driving mass107itself. In addition, the sensing masses108-X,108-Y,108-Z are elastically connected to the driving mass107so as to oscillate in turn with respect to the driving mass107itself, with respective further translational or rotational degrees of freedom, in response to rotations of the microstructure102about respective mutually perpendicular sensing axes X, Y, Z.

The sensing masses108-X,108-Y,108-Z are capacitively differentially coupled to the fixed structure106through respective sets of sensing electrodes115-X,115-Y,115-Z (here not shown individually and illustrated schematically as capacitors), which are connected to respective pairs of signal sensing terminals116-X,116-Y,116-Z. The sets of sensing electrodes115-X,115-Y,115-Z are shaped in such a way that, in the presence of an electrical signal on the signal sensing terminals116-X,116-Y,116-Z, the corresponding sensing masses108-X,108-Y,108-Z are subject to electrostatic forces according to the respective degrees of freedom.

The driving device103is connected to the microstructure102so as to form, with the driving mass107, a microelectromechanical loop119. The driving device103is configured so as to maintain the microelectromechanical loop119in oscillation at a driving frequency ωDclose to the resonant frequency ωRof the mechanical system defined by the driving mass107(with the sensing mass108) connected to the fixed structure106. In addition, the driving device103supplies a carrier signal VCwith a frequency equal to the driving frequency ωDand in phase with the oscillations of the microelectromechanical loop119.

The self-test actuator104comprises a frequency divider120and a modulator121, substantially as already described with reference toFIG. 1, and, moreover, a first demultiplexer122, which is controlled by a first selection signal SEL1generated by a control unit150. In particular, the frequency-divider module120generates a base-band self-test signal VSTB, for example a sinusoidal or square-wave signal, with a frequency equal to or lower than the driving frequency ωD, starting from the carrier signal VC. The modulator121generates a modulated self-test signal VSTMthat is obtained by modulating the carrier signal VCwith the band-base self-test signal VSTB. In this case, the modulated self-test signal VSTMis supplied on outputs of the modulator121that are selectively connectable, through the first demultiplexer122, to the signal sensing terminals116-X,116-Y,116-Z of the microstructure102. The first demultiplexer122is moreover configured so as to decouple in a controlled way the modulator121from the signal sensing terminals116-X,116-Y,116-Z during sensing steps of each read cycle of the gyroscope100while the sensing circuit105is connected to the signal sensing terminals116-X,116-Y,116-Z. For example, the modulator121may be provided with a floating output or else an enable output that enables setting of the outputs in a high-impedance state.

The sensing device105comprises a multiplexer126, a read interface123, a signal demodulator127, a self-test demodulator130, a low-pass filter131, a comparator132, a second demultiplexer134, and a status register133.

The read interface123is selectively connectable in succession to the signal sensing terminals116-X,116-Y,116-Z through the multiplexer126, which is controlled by a second selection signal SEL2. In addition, the multiplexer126is configured so as to decouple the read interface123from the signal sensing terminals116-X,116-Y,116-Z through the multiplexer126during self-test steps of each read cycle of the gyroscope100, where the signal sensing terminals116-X,116-Y,116-Z receive the modulated self-test signal VSTM. When connected, the read interface23receives from the signal sensing terminals116-X,116-Y,116-Z electrical sensing signals (charge packets in the embodiment described) and converts them into respective transduction signals VTX, VTY, VTZ.

The signal demodulator127receives in cyclic succession the transduction signals VTX, VTY, VTZfrom the read interface123. In addition, a demodulation input127aof the signal demodulator127is connected to the driving device3for receiving the carrier signal VC. The transduction signals VTX, VTY, VTZare demodulated using the carrier signal VC. On the output of the signal demodulator27there are hence cyclically present first demodulated signals VD1X, VD1Y, VD1Z(in other words, the signal present on the output of the signal demodulator127cyclically represents the movements of the sensing masses108-X,108-Y,108, which are all due to rotations about the axes X, Y, Z and to self-test actuation).

The self-test demodulator130is coupled to the signal demodulator127, for receiving the first demodulated signals VD1X, VD1Y, VD1Zcyclically and has a demodulation input130aconnected to the frequency-divider module120for receiving the base-band self-test signal VSTB. The self-test demodulator130carries out a further demodulation of the first demodulated signals VD1X, VD1Y, VD1Zusing the base-band self-test signal VSTBas carrier and thus generates cyclically second demodulated signals VD2X, VD2Y, VD2Z.

The second demultiplexer134, which is controlled by a third selection signal SEL3, receives the first demodulated signals VD1X, VD1Y, VD1Zfrom the signal demodulator127and forwards them on respective signal output lines134-X,134-Y,134-Z. In addition, the second demultiplexer134receives the second demodulated signals VD2X, VD2Y, VD2Zfrom the self-test demodulator130and forwards them on a self-test output line134-ST.

The low-pass filter131has a plurality of filtering lines, connected to a respective one between the signal output lines134-X,134-Y,134-Z and the self-test output line134-ST. The cut-off frequency of the low-pass filter131is comprised between approximately the upper margin of the Coriolis band BC and the self-test frequency ωST. Signal outputs of the low-pass filter131(in particular, the outputs corresponding to the signal output lines134-X,134-Y,134-Z of the second demultiplexer134) define respective signal outputs100-X,100-Y,100-Z of the gyroscope100and supply angular-velocity signals VΩX, VΩY, VΩZthat indicate angular velocities ΩX, ΩY, ΩZabout the sensing axes X, Y, X, respectively.

A self-test output131-ST of the low-pass filter131, corresponding to the self-test output line134-ST of the second demultiplexer134, cyclically supplies DC self-test signals VSTCX, VSTCY, VSTCZ.

The comparator131is coupled to the self-test output131-ST of the low-pass filter131, for receiving the DC self-test signals VSTCX, VSTCY, VSTCZ, and to a programmable reference generator135, which yields a threshold value VTH. The comparator131supplies self-test logic signals FSTX, FSTY, FSTZ, having a first value, when the corresponding DC self-test signal VSTCX, VSTCY, VSTCZis higher than the threshold value VTH, and a second logic value otherwise. The self-test logic signals FSTX, FSTY, FSTZare indicative of proper operation of the gyroscope100each as regards a respective one of the sensing axes X, Y, Z. In one embodiment (not illustrated), distinct threshold values are used for the different sensing axes.

The self-test logic signals FSTX, FSTY, FSTZmay be made available as status flags in a register (not shown).

The working principle of the gyroscope100is similar to what has already been described for the gyroscope1, with the difference that, in the gyroscope100, the self-testing function is provided on each sensing axis X, Y, Z using the time-division signal sensing terminals.

In practice, each read cycle TR(FIG. 6) of the gyroscope100is divided into three sensing intervals TX, TY, TZ, each dedicated to reading according to a respective one of the sensing axes X, Y, Z. The sensing intervals TX, TY, TZeach comprise a sensing step TSX, TSY, TSZand a self-test step TSTX, TSTY, TSTZ. In the sensing steps TSX, TSY, TSZdedicated to reading of the respective sensing axis X, Y, Z, the signal sensing terminals116-X,116-Y,116-Z of the sensing-mass systems108-X,108-Y,108-Z are selectively coupled to the sensing device105through the multiplexer126(not shown inFIG. 6). In each self-test step TSTZ, TSTY, TSTZ, instead, the modulated self-test signal VSTMis applied to the signal sensing terminals116-X,116-Y,116-Z involved through the first demultiplexer122(not shown inFIG. 6).

As has been mentioned previously, the modulated self-test signal VSTMis generated by the modulation of the carrier signal VCwith the base-band self-test signal VST. Each sensing-mass system108-X,108-Y,108-Z sums up the contribution due to displacements caused by the self-test device104and the contribution due to rotation of the gyroscope100about the respective sensing axis X, Y, Z. The latter present as carrier signals with frequency ωDmodulated by angular velocities ΩX, ΩY, ΩZ, respectively, about the sensing axes X, Y, Z. The transduction signals VTX, VTY, VTZsupplied in sequence by the read interface123hence contain the spectral components ΩD±ωST(actuation frequencies, due to the movements of the sensing masses108-X,108-Y,108-Z as a result of the modulated self-test signal VSTM) and ωD±BC (due to rotation of the gyroscope100). The signal demodulator127, as already described, introduces into baseband the frequency components ωSTand ΩC, whereas the self-test demodulator130introduces a DC component that is due exclusively to the effect of the self-test actuation on the movement of the sensing-mass systems108-X,108-Y,108-Z and corresponds to the maximum amplitude of the modulated self-test signal VSTM.

The low-pass filter131eliminates the components with frequency higher than the self-test frequency ωDso that at output the angular-velocity signals VΩX, VΩY, VΩZare defined only by the components in the Coriolis band BC, and the DC self-test signals VSTCX, VSTCY, VSTCZhave substantially only DC components.

Finally, the DC self-test signals VSTCX, VSTCY, VSTCZare compared with respective threshold values by the comparator131to generate the self-test logic signals FSTX, FSTY, FSTZ.

In addition to the advantages already referred to, regarding the possibility of performing self-testing continuously and in any condition of operation of the gyroscope, both the microstructure and the sensing device are simplified. In fact, the microstructure102does not require dedicated self-test terminals and is hence simpler to design and build and is less subject to possible failures. The sensing device105is instead provided with just one low-pass filter.

According to the embodiment ofFIG. 6, the operation of the gyroscope100is divided into three sensing intervals TX, TY, TZ, each dedicated to operation of a respective one of the sensing masses108-X,108-Y,108-Z, and each divided into a respective sensing step and self-test step. According to an alternative embodiment, the sensing step TScorresponding to each of the sensing masses overlaps, at least partially, with the self-test step TSTof another of the sensing masses. Illustrated inFIG. 7is a portion of an electronic system200in accordance with one embodiment of the present invention. The system200incorporates the gyroscope1and may be used in devices, such as, for example, a palm-top computer (personal digital assistant, PDA), a laptop or portable computer, possibly with wireless capacity, a cell phone, a messaging device, a digital music player, a digital camera, or other devices designed to process, store, transmit or receive information. For example, the gyroscope1may be used in a digital camera for detecting movements and performing an image stabilization. In other embodiments, the gyroscope1is included in a portable computer, a PDA, or a cell phone for detecting a free-fall condition and activating a safety configuration. In a further embodiment, the gyroscope1is included in a motion-activated user interface for computers or video-game consoles. In a further embodiment, the gyroscope1is incorporated in a satellite-navigation device and is used for temporary position tracking in the event of loss of the satellite-positioning signal.

The electronic system200can comprise a controller210, an input/output (I/O) device220(for example, a keyboard or a display), the gyroscope1, a wireless interface240, and a memory260, of a volatile or nonvolatile type, coupled to one another through a bus250. In one embodiment, a battery280may be used for supplying the system200. It should be noted that the scope of the present invention is not limited to embodiments having necessarily one or all of the devices listed.

The controller210can comprise, for example, one or more microprocessors, microcontrollers and the like.

The I/O device220may be used for generating a message. The system200can use the wireless interface240for transmitting and receiving messages to and from a wireless communications network with a radiofrequency signal (RF). Examples of wireless interface can comprise an antenna, a wireless transceiver, such as a dipole antenna, even though the scope of the present invention is not limited from this standpoint. In addition, the I/O device220can supply a voltage representing what is stored either in the form of digital output (if digital information has been stored) or in the form of analog information (if analog information has been stored).

Finally, it is evident that modifications and variations may be made to the method and to the device described, without departing from the scope of the present invention, as defined in the annexed claims.

In the first place, the discrimination in frequency of the self-test signals may be used for testing proper functionality of gyroscopes with any microelectromechanical structure that are based upon detection of Coriolis forces.

The discrimination of the spectral component due to the self-test actuation may be conducted directly by the signals coming from the signal demodulator or even directly by the read interface. In this case, a selective pass-band filter may be used, centered, for example, around the frequency 2ωD−ωSTfor the signals coming from the signal demodulator, or else around the frequency ωD−ωSTfor the signals coming from the read interface.

Furthermore, the embodiments described above can be combined to provide further embodiments. For example, according to an embodiment, in a microelectromechanical device configured to detect rotation about a single sensing axis, similar to the gyroscope1described with reference toFIGS. 1-4, a modulator and a read interface can be cyclically coupled to a sensing mass via a multiplexer and a single capacitive coupling, as described with reference to the gyroscope100ofFIGS. 5 and 6. According to another embodiment, in a multi-axial gyroscope, such as, e.g., the device100, each sensing mass can be provided with a respective self test actuator and a respective sensing device, substantially as described with reference to the device1.

According to an embodiment, the electronic system200described with reference toFIG. 7can be provided with a multi-axial device in place of the gyroscope1. As used herein, the symbol “±” means “plus and minus.” Thus, for example, the phrase “the frequencies ωD±ωST” can be interpreted as referring inclusively to the frequency ωD+ωSTand the frequency ωD−ωST.

All of the U.S. patents, U.S. patent application publications, U.S. patent application, foreign patents, foreign patent application and non-patent publications referred to in this specification are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, application and publications to provide yet further embodiments.