Patent Description:
In a known manner, micromanufacturing techniques allow to manufacture microelectromechanical systems (MEMS) from semiconductor material layers, which have been deposited (for example a polycrystalline silicon layer) or grown (for example an epitaxial layer) above sacrificial layers, which are removed by etching. Inertial sensors, accelerometers and gyroscopes provided with this technology are having increasing success, for example in the automotive field, in the field of inertial navigation, or in the field of mobile or portable devices.

In particular, integrated gyroscopes of semiconductor material made using MEMS technology (hereinafter simply referred to as MEMS gyroscopes) are known, which operate on the basis of the relative acceleration theorem, exploiting Coriolis acceleration.

When a rotation at a certain angular velocity (the value whereof is to be detected) is applied to a mobile mass of the MEMS gyroscope, which is driven with a linear velocity, the mobile mass feels an apparent force, called Coriolis force, which determines a displacement thereof in a direction perpendicular to the direction of the linear driving velocity and to the axis about which the rotation occurs. The mobile mass is supported by elastic elements that allow a displacement thereof in the direction of the apparent force. According to Hooke's law, the displacement is proportional to the apparent force, such that the Coriolis force and the angular velocity value of the rotation that generated it may be detected from the displacement of the mobile mass.

The displacement of the mobile mass may, for example, be detected in a capacitive manner, determining, in a resonance condition, the capacitance variations caused by the movement of movable detection electrodes, integral with the mobile mass and coupled (for example in the so-called parallel-plate configuration, or in an interdigitated configuration) with fixed detection electrodes, so as to form a detection capacitor.

MEMS gyroscopes therefore generally comprise a detection structure, including the aforementioned mobile mass and the aforementioned detection electrodes; and also an ASIC (Application Specific Integrated Circuit) electronic circuit, electrically coupled to the detection structure, which receives at its input the capacitive variation produced by the detection capacitor and processes it for generating an electrical output signal, indicative of the angular velocity, which may be provided outside the MEMS gyroscope for subsequent processing.

The aforementioned ASIC electronic circuit and the detection structure are typically provided in respective dies of semiconductor material, which are arranged inside a package, which encloses and protects the same dies, also providing an electrical connection interface towards the outside environment.

The ASIC electronic circuit also implements a driving stage for driving of the mobile mass, maintaining it in oscillation at the resonance frequency with a desired amplitude (target amplitude), so that it may carry out the angular velocity detection by the aforementioned Coriolis effect.

In particular, the driving stage is configured, in an initial or start-up phase, to cause the oscillation to start and, subsequently, to maintain the oscillation at the desired amplitude, by a closed-loop (or feedback) control.

In this regard, <FIG> shows the trend over time of the driving displacement (or oscillation) Dosc of the mobile mass after receiving, by the ASIC circuit, a MEMS gyroscope start-up signal at an initial time t<NUM>.

In the example, this trend starts from an initial condition with null displacement and has a first phase (start-up phase) with a fast increase (a ramp increase) up to reaching a maintenance amplitude and a second phase (maintenance phase) of permanence at this maintenance amplitude for the entire duration of an angular velocity detection phase.

In particular, the aforementioned maintenance phase starts at a time t<NUM>, following time t<NUM>, at which the driving oscillation Dosc exceeds an amplitude threshold TH.

<FIG> shows the corresponding trend of a driving control signal SC (in particular a voltage signal) provided to the mobile mass by the aforementioned driving stage of the ASIC electronic circuit, which has, from reception of the aforementioned start-up signal, in the start-up phase, a high saturated value (that is, corresponding to a maximum control voltage), so as to cause fast increase of the mobile mass oscillation; and, in the maintenance phase, a suitable maintenance or steady state value, lower than the aforementioned high saturated value (this maintenance value being reached rapidly after time t<NUM>).

In the maintenance phase, the driving stage implements a closed-loop control of the oscillation amplitude, around the desired value, on the basis of a mobile mass oscillation amplitude detection signal (provided by suitable detection elements coupled to the same mobile mass).

In a known manner, the driving stage is required to provide, in the start-up phase, a maximum possible energy, to rapidly start the driving and make the start-up duration as short as possible; in fact, correct detection of the angular velocity by the gyroscope may be carried out only in the subsequent maintenance phase, when the driving oscillation of the mobile mass has reached the desired value.

In this first start-up phase, the driving control is an open-loop control, with the driving stage providing maximum energy (in the so-called full-steam mode); in this initial phase, the oscillation amplitude detection signal would be in any case too low to be distinguished from background noise.

A known problem affecting MEMS gyroscopes is represented by the so-called hot start-up. In particular, a hot start-up is defined as a power-up of the gyroscope that occurs close to a previous power-down, when the mobile mass is still in its oscillation movement; the oscillation movement of the mobile mass, in the absence of the driving signal, in fact decays in rather long times, of the order of hundreds of ms.

In these cases, the previously described start-up phase, with the supply of maximum energy to the mobile mass by the driving stage, may not be optimal.

In fact, the mobile mass may undergo an overshoot phenomenon, i.e. assume an oscillation amplitude greater than the desired amplitude; in the worst case, the oscillation amplitude may be such as to cause the mobile mass to hit corresponding stopper elements (so-called stoppers).

In this regard, <FIG> shows the trend of the driving oscillation DOSC of the mobile mass, in different cases: a so-called "cold" start-up case, that is with a stationary mobile mass (desired condition), which corresponds to the trend shown previously in <FIG>; and four different "hot" start-up conditions, which differ for a different time interval between a previous power-down instant and the subsequent reception of the start-up signal (this time interval ranging from <NUM> to <NUM>).

As may be noted, in the worst hot start-up case, corresponding to the shortest time interval between the aforementioned power-down and power-up events, the mobile mass oscillation amplitude exceeds the target amplitude (before the driving control implemented by the driving stage brings the same oscillation amplitude back to the maintenance value).

The situation described may have a significant impact on the gyroscope performances, for a time that may also be quite long (tens of ms) from the beginning of the start-up phase, a time during which the gyroscope cannot provide a reliable detection.

In this regard, <FIG> shows the trend of the output signal Sout of the gyroscope (indicative of the detected angular velocity), in the same cases discussed for <FIG>.

In the presence of a hot start-up, in all cases, an evident oscillation of the output signal Sout is noted, which remains unstable for a rather long time; this oscillation has a greatest amplitude at the shortest time interval between the previous power-down instant and the subsequent reception of the start-up signal (case corresponding to the overshoot situation shown in <FIG>).

Such behavior of the gyroscope may be bad and not acceptable for various fields of use, for example for automotive or analogous applications, where high reliability and a prompt detection are required.

A known solution to overcome the problems associated with hot start-up events envisages preventing such events from occurring. In particular, this solution envisages that the application (the external electronic system), where the gyroscope is used, ensures a sufficient wait time interval after having commanded the power-down of the same gyroscope, such that it allows the mobile mass to stop its oscillation, so to ensure a cold start-up in case of the subsequent power-up.

The limits associated with this solution are, in an evident manner, the request for a wait time interval following each power-down of the gyroscope, this waiting being not feasible in various applications, as previously discussed.

Furthermore, a situation may occur wherein different applications (or external electronic systems) intervene to activate the power-down, respectively the start-up, of the gyroscope; in the event that these applications are independent and non-communicating with each other, implementing the aforementioned wait time interval may not be possible.

A further known solution envisages implementing detection of any residual mobile mass oscillation when the gyroscope is started and suitably adjusting the start-up phase, and in particular the driving energy provided to the mobile mass, as a function of this detection. In this solution, therefore, the start-up phase with full-steam approach (at maximum energy) is implemented only in the event that detection of the oscillation amplitude shows that the mobile mass is stationary before the beginning of the start-up phase.

However, also this solution has some drawbacks.

Firstly, this solution generally implies longer start-up times, due to the presence of the additional detection and evaluation phase of the mobile mass oscillation amplitude.

Furthermore, the possibility of error is high, due to the fact that the oscillation amplitude detection signal (on which the initial control is based) is very low and therefore susceptible to misinterpretations; for example, a cold start-up may be implemented instead of a hot start-up.

To overcome these errors, it is required to add further and sophisticated dedicated control electronics, thus increasing the size of the gyroscope device and the corresponding power consumption. <CIT> discloses a system for gyroscope dynamic motor amplitude compensation during startup, which comprises various program modules, including an a-priori motor amplitude module configured to generate an a-priori motor amplitude signal based on a model of gyroscope motor amplitude growth during startup; a steady state scale factor module configured to generate a steady state scale factor signal; and a dynamic motor amplitude compensation module configured to receive the a-priori motor amplitude signal, and the steady state scale factor signal. During startup, rate motion is sensed by the gyroscope and a sensed rate signal is output by the gyroscope. The dynamic motor amplitude compensation module receives a measured motor amplitude signal from the gyroscope, the a-priori motor amplitude signal, or a combination thereof, and outputs a time varying scale factor that is applied to the sensed rate signal to produce an accurate sensed rate from the gyroscope during the startup phase. <CIT> discloses a detection device including a driving circuit that drives a vibrator, and a detection circuit that receives a detection signal from the vibrator and performs a detection process of detecting a physical quantity signal corresponding to a physical quantity from the detection signal. The driving circuit performs intermittent driving in which the vibrator is driven in a driving period, and is not driven in a non-driving period, and the detection circuit performs the detection process of the physical quantity signal in the non-driving period of the intermittent driving.

The aim of the present solution is to overcome the previously highlighted problems, and in particular to provide a hot start-up solution for a MEMS gyroscope device which is more efficient and has limited energy or resource consumption.

According to the present solution, a microelectromechanical gyroscope device and an associated method are therefore provided, as defined in the attached claims.

For a better understanding of the present invention, a preferred embodiment thereof is now described, purely by way of non-limiting example and with reference to the attached drawings, wherein:.

As will be described in detail below, an aspect of the present solution stems from the observation that the mobile mass oscillation of the gyroscope decays in a decay time that is known or in any case determinable as a function of the characteristics and configuration of the same mobile mass (in particular, being proportional to its quality factor Q) and envisages suitably adjusting the start-up phase as a function of the time interval elapsed from a previous power-down command.

This elapsed time interval, which may be measured by a suitable counter (using, for example, a low-power oscillator), compared with the aforementioned decay time, allows to establish whether the mobile mass is still moving and in particular to establish whether the subsequent start-up corresponds to a cold start-up or, instead, to a hot start-up, in this case allowing to suitably adjust the way of implementing the same hot start-up.

<FIG> shows a gyroscope device <NUM>, of a microelectromechanical, MEMS, type, which comprises: a detection structure <NUM>, of a known type (not described in detail herein), provided with a mobile mass for detecting angular accelerations on the basis of Coriolis effect; and an integrated electronic circuit <NUM>, ASIC (Application Specific Integrated Circuit), coupled to the detection structure <NUM>.

The electronic circuit <NUM> is configured to provide bias signals Sb to the same detection structure <NUM> to cause it to oscillate at the resonance frequency and furthermore to acquire detection signals Sd from the same detection structure <NUM>, indicative of the detected angular velocities.

As previously discussed, the detection structure <NUM> and the electronic circuit <NUM> may be integrated in a respective die of semiconductor material and the dies arranged, with suitable mutual electrical connections, in a same package of the gyroscope device <NUM>.

In greater detail, the electronic circuit <NUM> comprises: a control stage <NUM>, for example provided with a microcontroller, a processor resident in the ASIC or with a similar processing unit; and a biasing stage <NUM>.

The control stage <NUM> is configured to receive, externally from the gyroscope device <NUM>, for example from an external electronic system <NUM>, shown schematically in <FIG>, management signals based on which to control the operation of the same gyroscope device <NUM> and to provide to the same external electronic system <NUM> output signals Sout indicative of detected angular velocities.

In particular, the aforementioned management signals may include a power-up or start-up signal, denoted with PUP, for causing the power-up or start-up of the gyroscope device <NUM>, for example from a full power-down or an idle (inactivity) condition, and a stop or power-down signal, denoted with PDOWN, for causing the power-down (turn-off) of the gyroscope device <NUM> and the return to the aforementioned condition, of full power-down or idle.

This external electronic system <NUM> may for example be an electronic unit of an electronic apparatus, for example of mobile or portable type (such as a smartphone, a tablet, a phablet, a smartwatch or other wearable device, a smart pen or the like), which uses the aforementioned gyroscope device <NUM> for detecting angular velocity.

The biasing stage <NUM> of the electronic circuit <NUM> is controlled by the control stage <NUM> for generating the aforementioned bias signals Sb for the detection structure <NUM>, in particular during the start-up condition (following the reception of the aforementioned start-up signal PUP), i.e., as previously discussed with reference to <FIG>, to cause the mobile mass to rapidly oscillate at the resonance frequency and at the desired maintenance amplitude.

According to a particular aspect of the present solution, the electronic circuit <NUM> further comprises a time counter stage <NUM>, controlled by the control stage <NUM> and configured to measure a wait time interval from reception of the aforementioned power-down signal PDOWN. In particular, the value of this wait time interval corresponds to the oscillation decay time of the mobile mass of the detection structure <NUM>.

The electronic circuit <NUM> may further comprise a non-volatile memory <NUM>, coupled to the control stage <NUM> and storing a suitable stored value TMEM, associated with the aforementioned wait time interval.

The wait time interval value may be determined in the design stage of the gyroscope device <NUM>, based on constructive and operating characteristics of the detection structure <NUM> and in particular, as previously indicated, on the basis of the quality factor Q associated with the corresponding mobile mass.

This value may also advantageously be adjusted (with so-called trimming operations) during manufacturing of the gyroscope device <NUM> and consequently stored in the non-volatile memory <NUM> as the aforementioned stored value TMEM, so as to adapt to the actual real characteristics of the gyroscope device <NUM>.

The operation of the gyroscope device <NUM> relative to management of a corresponding start-up mode, is now discussed in greater detail, also with reference to <FIG>.

In an initial step, denoted with <NUM>, which is supposed to correspond to a normal operating phase of the gyroscope device <NUM> (i.e. to a maintenance phase, see the previous discussion, with the mobile mass which oscillates at the desired amplitude and is operative for the angular velocity detection), the control stage <NUM> receives the power-down signal PDOWN.

According to an aspect of the present solution, instead of directly controlling power-down of the gyroscope device <NUM> (as in traditional solutions), the control stage <NUM> determines, step <NUM>, the start of a pre-power down, pre-PD, phase and in particular starts the time count by the time counter stage <NUM> (which is supposed to have been previously reset, so to start from an initial null count value).

In this pre-power down phase, the control stage <NUM> controls power-down of a large part of the gyroscope device <NUM> (in particular by interrupting biasing of the mobile mass of the detection structure <NUM>, the oscillation whereof thus begins to decay), except the aforementioned time counter stage <NUM>.

As indicated at step <NUM>, the control stage <NUM> then monitors, for the entire duration of the aforementioned pre-power down phase, the time count value (denoted with TC) and assesses whether the wait time interval, corresponding to the decay time (here denoted with TDEC) of the oscillation of the mobile mass of the detection structure <NUM>, has been reached.

For instance, the control stage <NUM> compares the time count value TC with the stored value TMEM present in the non-volatile memory <NUM>.

If the wait time interval corresponding to the decay time of the mobile mass oscillation has elapsed from the reception of the power-down signal PDOWN (that is, if the time count value TC reaches the design value or the stored value TMEM), the control stage <NUM> determines the end of the aforementioned pre-power down phase and, step <NUM>, activates the actual power-down, PD, phase of the gyroscope device <NUM> (in a per-se known manner, not discussed in detail herein).

In particular, the control stage <NUM> also determines the power-down of the time counter stage <NUM> (the count whereof is thus reset).

Subsequently, step <NUM>, the reception of the start-up signal PUP awakens the control stage <NUM> (acting as an interrupt) and the same control stage <NUM> determines the start of the start-up phase for the fast increase of the oscillation of the mobile mass of the detection structure <NUM> up to reaching the maintenance amplitude.

In particular, the control stage <NUM> implements in this case, step <NUM>, a cold start-up, with the supply of full energy (with full-steam mode) to the detection structure <NUM> by the biasing stage <NUM>, since the same control stage <NUM> has previously verified that the mobile mass was stationary, a time interval greater than the decay time of the oscillation of the same mobile mass having elapsed from the previous power-down signal PDOWN.

Reversely, as shown at step <NUM>, if the control stage <NUM> receives the start-up signal PUP before the wait time interval corresponding to the decay time of the mobile mass oscillation has elapsed (i.e. before the time count value TC has reached the design or stored value TMEM), the same control stage <NUM> determines the occurrence of a hot start-up, with a new power-up request when the mobile mass of the detection structure <NUM> is still oscillating.

As a result, according to an aspect of the present solution, the control stage <NUM> is configured to suitably adjust the start-up phase, to avoid possible drawbacks such as overshoot of the mobile mass. In other words, the control stage <NUM> implements in this case a "soft" start-up, as shown at step <NUM>.

In particular, the control stage <NUM> adjusts, through the biasing stage <NUM>, the amount of bias energy and the mode of supplying the same bias energy to the mobile mass, as a function of the current time count value TC, that is as a function of the time interval elapsed from the previous power-down.

In general, the shorter the time elapsed (and therefore the lower the time count value TC), the greater a limitation implemented by the control stage <NUM> to the bias energy provided by the biasing stage <NUM> to the mobile mass. Reversely, the higher the time count value TC, the lower the limitation implemented by the control stage <NUM> to the energy provided by the biasing stage <NUM> to the mobile mass, thus approaching the full-steam mode envisaged for a cold start-up.

In any case, both from step <NUM> and from the aforementioned step <NUM>, at the end of the start-up phase, the control stage <NUM> controls the biasing stage <NUM> to implement the maintenance phase for sustaining the oscillation of the mobile mass of the detection structure <NUM>, in a per se known manner, so that the procedure returns to the aforementioned initial step <NUM>, to await the reception of a new power-down signal.

In a possible implementation, which is now described purely by way of a non-limiting example, the start-up phase, when the gyroscope device <NUM> is powered-up, may be divided into a plurality of sub-phases, in the example:.

Afterwards, the control stage <NUM> implements the maintenance phase for sustaining the oscillation, with the closed-loop control, during which the reading of the oscillation amplitude is enabled and the feedback control around the maintenance amplitude is implemented.

According to an aspect of the present solution, the adjustment of the start-up phase by the control stage <NUM> may envisage, in this case, eliminating one or more of the sub-phases into which the start-up phase is normally divided (i.e., in case of a cold start-up) and/or reducing the duration or the associated bias energy of one or more of the same sub-phases, again as a function of the current time count value TC, that is of the time interval elapsed from the previous power-down.

Purely by way of example, assuming that the decay time of the mobile mass (corresponding to the stored value TMEM in the non-volatile memory <NUM>) is of the order of <NUM>, the control stage <NUM> may be configured to:.

The advantages of the solution described are clear from the preceding description.

In any case, it is again highlighted that this solution allows the problems associated with the hot start-up of the gyroscope device to be effectively avoided, in particular by avoiding possible overshoots of the mobile mass and considerably reducing the oscillation of the output signal Sout of the gyroscope device <NUM> in the period immediately following the start-up phase.

In this regard, <FIG> shows the trend of the aforementioned output signal Sout of the gyroscope device <NUM>, considering the same cases shown in <FIG> for a traditional gyroscope, highlighting the significant reduction in the oscillation amplitude, regardless of the time elapsed between the power-down and the subsequent start-up.

The solution described requires a substantially negligible increase in energy consumption, this increase being substantially associated with the power-up of the time counter stage <NUM> in the pre-power down phase, having very low consumption, in particular in its implementation by a low-power oscillator.

In this regard, it is highlighted that the present solution does not require a high accuracy in the time count, so that using resources that are not excessively expensive in terms of energy and area occupation for the implementation of this time counter is actually possible.

Advantageously, the solution described, being integrated in the gyroscope device (in particular in the ASIC electronic circuit coupled to the detection structure within the package of the same gyroscope device), does not require the intervention of the external electronic system (or of the user), neither to know the time elapsed from the power-down nor to wait or verify that the mobile mass is stationary before activating a start-up.

The solution described has no impact on cold start-up times and allows for shorter hot start-up times (since, as previously discussed, the adjustment by the control stage <NUM> may envisage eliminating or reducing the duration of one or more of the sub-phases into which the start-up phase is normally divided).

Furthermore, this solution is not affected by the errors induced by the use of the oscillation detection signal, which has a reduced value during the start-up phase, since the mobile mass oscillation is deduced in an indirect, but in any case sufficiently accurate manner, from the measurement of the time elapsed from the power-down (related to the decay time of the oscillation, which is also known or in any case can be obtained).

Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present invention, as defined in the attached claims.

In particular, it is highlighted that the solution described may find advantageous application in any (monoaxial, biaxial or triaxial) gyroscope device or in other combined devices which envisage gyroscopic sensors and possible further sensors (for example accelerometer sensors), such as for example in Inertial Measuring Units (IMU).

Claim 1:
A microelectromechanical gyroscope device (<NUM>) comprising:
a detection structure (<NUM>), provided with a mobile mass; and
an integrated electronic circuit (<NUM>), coupled to the detection structure (<NUM>) and configured to provide a bias signal (Sb) to the detection structure (<NUM>) to cause its oscillation at a resonance frequency and to acquire a detection signal (Sd) from the detection structure (<NUM>) indicative of a detected angular velocity,
wherein, upon powering of the gyroscope device (<NUM>), the integrated electronic circuit (<NUM>) is configured to implement a start-up phase, following a previous power-down, during which the mobile mass is biased to have an increase in the oscillation up to a target oscillation amplitude, followed by a maintenance phase at the target oscillation amplitude,
and wherein the integrated electronic circuit (<NUM>) comprises a time counter stage (<NUM>) designed to measure a duration of a time interval elapsed from a previous power-down command and is configured to adjust biasing of the mobile mass during said start-up phase as a function of the measured duration of said time interval;
characterized in that the integrated electronic circuit (<NUM>) is configured to: carry out a comparison between the measured duration of said time interval and a wait interval associated with a decay time of the mobile mass oscillation; and adjust biasing of said mobile mass during said start-up phase on the basis of said comparison.