Abstract:
A microelectromechanical gyroscope includes: a substrate; a stator sensing structure fixed to the substrate; a first mass elastically constrained to the substrate and movable with respect to the substrate in a first direction; a second mass elastically constrained to the first mass and movable with respect to the first mass in a second direction; and a third mass elastically constrained to the second mass and to the substrate and capacitively coupled to the stator sensing structure, the third mass being movable with respect to the substrate in the second direction and with respect to the second mass in the first direction.

Description:
BACKGROUND 
     Technical Field 
     The present disclosure relates to a microelectromechanical gyroscope for sensing angular rate and to a method of sensing angular rate. 
     Description of the Related Art 
     As is known, use of microelectromechanical systems (MEMS) is increasingly widespread in various sectors of technology and has yielded encouraging results especially in the production of inertial sensors, microintegrated gyroscopes, and electromechanical oscillators for a wide range of applications. 
     In particular, there exist various types of MEMS gyroscopes, which are distinguished by their rather complex electromechanical structure and by the operating mode, but are in any case based upon detection of Coriolis accelerations. In MEMS gyroscopes of this type, a mass is elastically constrained to a substrate or stator to be able to translate in a driving direction and a sensing direction that are mutually perpendicular. By a control device, the mass is set in oscillation at a controlled frequency and amplitude in the driving direction. 
     When the gyroscope turns about an axis perpendicular to the driving direction and to the sensing direction at an angular rate, on account of the motion in the driving direction, the mass is subject to a Coriolis force and moves in the sensing direction. The displacements of the mass in the sensing direction are determined both by the angular rate and by the velocity in the driving direction and may be transduced into electrical signals. For instance, the mass and the substrate may be capacitively coupled so that the capacitance depends upon the position of the mass with respect to the substrate. The displacements of the mass in the sensing direction may thus be detected in the form of electrical signals modulated in amplitude in a way proportional to the angular rate, with carrier at the frequency of oscillation of the driving mass. Use of a demodulator makes it possible to obtain the modulating signal thus to derive the instantaneous angular rate. 
     In many cases, however, the acceleration signal that carries information regarding the instantaneous angular rate also contains spurious components that are not determined by the Coriolis acceleration and thus present in the form of disturbance. Not infrequently, for example, the spurious components may depend upon constructional imperfections of the micromechanical part, due to the limits of precision and to the production process spread. Typically, the effective oscillatory motion of the driving mass, as a result of a defect in the elastic constraints provided between the mass and the substrate, may be misaligned with respect to the direction expected theoretically. This type of defect commonly causes a quadrature signal component, which adds to the useful signal due to rotation of the microstructure. Like the Coriolis force, in fact, the misalignment causes the mass to displace also in the sensing direction, instead of just in the driving direction, and produces a variation of the capacitance between the mass and the substrate. 
     Obviously, the consequences are a degraded signal-to-noise ratio and an altered dynamic of the read interface, at the expense of the signal to be read, to an extent that depends upon the degree of the defects. 
     BRIEF SUMMARY 
     One or more embodiments of the present disclosure are directed to a microelectromechanical gyroscope and a method of sensing angular rates. 
     According to one embodiment of the present disclosure, a microelectromechanical gyroscope includes a substrate and a stator sensing structure fixed to the substrate. The gyroscope further includes a first mass elastically coupled to the substrate and movable with respect to the substrate in a first direction and a second mass elastically coupled to the first mass and movable with respect to the first mass in a second direction. The gyroscope includes a third mass elastically coupled to the second mass to enable movement in the first direction and elastically coupled to the substrate to enable movement in the second direction, the third mass being capacitively coupled to the stator sensing structure. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the disclosure, some embodiments thereof will now be described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  is a simplified block diagram of a microelectromechanical gyroscope according to an embodiment of the present disclosure; 
         FIG. 2  is a simplified top plan view of a portion of the microelectromechanical gyroscope of  FIG. 1 ; 
         FIG. 3  is a simplified top plan view of a portion of a microelectromechanical gyroscope according to a different embodiment of the present disclosure; and 
         FIG. 4  is a simplified block diagram of an electronic system incorporating a microelectromechanical gyroscope according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a microelectromechanical gyroscope according to an embodiment of the present disclosure is designated as a whole by the number  1  and comprises a substrate  2 , a microstructure  3 , a control device  4 , and a read device  5 . As explained in detail hereinafter, the microstructure  3  comprises movable parts and parts that are fixed with respect to the substrate  2 . The control device  4  forms a control loop with the microstructure  3  and is configured to keep movable parts of the microstructure  3  in oscillation with respect to the substrate with controlled frequency and amplitude. For this purpose, the control device  4  receives position signals S P  from the microstructure  3  and supplies driving signals S D  to the microstructure  3 . The read device  5  supplies output signals S OUT  as a function of the movement of the movable parts of the microstructure  3 . The output signals S OUT  indicate an angular rate of the substrate  2  with respect to a gyroscopic axis of rotation. 
     Illustrated in  FIG. 2  are the substrate  2  and, in greater detail, the microstructure  3  according to one embodiment. In particular, the microstructure  3  comprises a driving mass  7 , a transduction mass  8 , and a movable sensing structure  10 . 
     The driving mass  7  is elastically constrained to the substrate  2  and is movable with respect to the substrate  2  in a driving direction D 1 . In use, the control device  4  keeps the driving mass  7  in oscillation in the driving direction D 1  about a resting position. For this purpose, the control device  4  uses movable driving electrodes  12   a , fixed to the driving mass  7 , and stator driving electrodes  12   b , fixed to the substrate  2 . The movable driving electrodes  12   a  and the stator driving electrodes  12   b  are capacitively coupled in comb-fingered configuration and are substantially parallel to the driving direction D 1 . The stator driving electrodes  12   b  receive the driving signals S D  from the control device  4  through electrical-connection lines (not illustrated for reasons of simplicity). The oscillations of the driving mass  7  define a signal carrier for the transduction chain of the gyroscope  1 . 
     The elastic connection of the driving mass  7  to the substrate  2  is obtained by elastic suspension elements  11  or “flexures”, which are configured to enable oscillations of the driving mass  7  with respect to the substrate  2  in the driving direction D 1  and to prevent other movements of the driving mass  7 , in particular in a transduction direction D 2  perpendicular to the driving direction D 1 . Here and in what follows, the expression “prevent movements in a direction” and similar expressions, both in reference to the driving direction D 1  and in reference to the transduction direction D 2  or in any other direction, are to be understood in the sense of substantially limiting the movements in said direction, compatibly with what is allowed by the technological and geometrical limits in definition of the constraints. It is thus not to be understood that the expressions referred to are in contradiction with the presence of possible spurious movements in the forbidden directions that may be at the origin of signals of disturbance with respect to the carrier defined by the oscillations of the driving mass, but these movements are only ideally prevented by the specific configuration of the flexible elements and constraints that are practically rigid in these directions. 
     The transduction mass  8  is elastically constrained to the driving mass  7  and is movable with respect to the driving mass in the transduction direction D 2 . 
     The elastic connection of the transduction mass  8  to the driving mass  7  is obtained by elastic suspension elements  13 , which are configured to enable oscillations of the transduction mass  8  with respect to the driving mass  7  in the transduction direction D 2  and prevent other relative movements of the transduction mass  8  with respect to the driving mass  7 , in particular in the driving direction D 1 . With respect to the substrate  2 , instead, the transduction mass  8  is movable both in the transduction direction D 2  and also in the driving direction D 1  as a result of the drawing action of the driving mass  7  and of the constraint imposed by the elastic suspension elements  13 . 
     The movable sensing structure  10  comprises a frame  15  and a set of movable sensing electrodes  16   a , which are supported by the frame  15  and extend parallel to the driving direction D 1 . The frame  15  is elastically constrained to the transduction mass  8  and to the substrate  2 . With respect to the substrate  2 , the frame  15  is movable in the transduction direction D 2 . With respect to the transduction mass  8 , the frame  15  is movable in the driving direction D 1 . 
     Elastic connection of the frame  15  to the substrate  2  is obtained by elastic suspension elements  18 , which are configured to enable oscillations of the frame  15  with respect to the substrate  2  in the transduction direction D 2  and prevent other movements of the frame  15  with respect to the substrate  2 , in particular in the driving direction D 1 . 
     The frame  15  is coupled to the transduction mass  8  by elastic connection elements  20 , which are configured to prevent relative movements between the transduction mass  8  and the frame  15  in the transduction direction D 2 . The elastic connection elements  20  enable, instead, other relative movements between the transduction mass  8  and the frame  15 . In particular, translatory oscillations in the driving direction D 1  and rotary oscillations are allowed. Consequently, the movements of the transduction mass  8  in the transduction direction D 2  are transmitted substantially in a rigid way, whereas the translatory movements in the driving direction and the rotary movements of the transduction mass are at least in part compensated by the elastic connection elements  20 . Due to the elastic connection elements  20 , which enable displacements between the frame  15  and the transduction mass  8  in the driving direction D 1 , the frame  15  may be constrained to the substrate  2  as already described without cancelling out the useful displacement components due to the Coriolis force that acts on the transduction mass  8 . This would not be possible with a simple rigid connection between the transduction mass  8  and the movable sensing structure  10 . 
     The movable sensing structure  10  is capacitively coupled to a stator sensing structure  30 , which comprises stator sensing electrodes  16   b  fixed to the substrate  2  and extending in the driving direction D 1 . In particular, the movable sensing electrodes  16   a  and the stator sensing electrodes  16   b  are coupled according to a “parallel plate” scheme and define a capacitor with a capacitance variable as a function of the position of the movable sensing structure  10  with respect to the substrate  2  in the transduction direction D 2 . 
     As mentioned, in use, the control device  4  keeps the driving mass  7  in oscillation in the driving direction D 1  with controlled frequency and amplitude. The transduction mass  8  is drawn by the driving mass  7  in the motion in the driving direction D 1  as a result of the connection by the elastic suspension elements  13 , which enable relative motion between the driving mass  7  and the transduction mass  8  only in the transduction direction D 2 . When the substrate  2  turns about a gyroscopic axis G perpendicular to the driving direction D 1  and to the transduction direction D 2 , the transduction mass  8  is subjected to a Coriolis force in the transduction direction D 2 . The transduction mass  8  thus oscillates in the transduction direction D 2  with an amplitude that depends upon the linear drawing velocity in the driving direction D 1  and by the angular rate of the substrate  2  about the gyroscopic axis G. A spurious displacement, caused by imperfections of the elastic suspension elements  11 , may be added to the displacement due to the Coriolis force. The component due to the spurious displacement varies at the same frequency as that of the carrier, but is phase-shifted by 90° with respect to the Coriolis forcing term because it depends upon the position and not upon the velocity in the driving direction Dl. The overall displacement of the transduction mass  8  in the transduction direction D 2  is transmitted to the movable sensing structure  10  as a result of the elastic connection elements  20 , which allow relative translatory motion only in the driving direction Dl. 
     The effect of the imperfections of the constraints, in particular of the elastic suspension elements  11  that connect the driving mass  7  to the substrate  2  is, however, transferred to the movable sensing structure  10  to an extent much smaller than the contribution due to the Coriolis force. The contribution due to the defects of driving in the transduction direction D 2  is thus attenuated for the transduction mass  8  and the sensing mass  10  both by the elastic suspension elements  13  between the driving mass  7  and the transduction mass  8  and by the elastic suspension elements  18  between the movable sensing structure  10  and the substrate  2 , as well as by the elastic connection elements  20  between the transduction mass  8  and the frame  15 . In particular, the elastic connection elements  20  are able to attenuate also spurious rotary movements, which are transmitted to the transduction mass  8  by the driving mass  7  and are not completely compensated for by the elastic suspension elements  13 . Instead, the contribution due to the Coriolis force in the transduction direction D 2  arises directly from the transduction mass  8  and is transmitted without appreciable attenuation by the elastic connection elements  20 , which enable a substantially rigid coupling in the direction D 2 , and this contribution is affected the action of the elastic suspension elements  13  and of the elastic suspension elements  18  and is consequently transmitted in a non-attenuated way on the sensing mass  10 . The Coriolis force on the driving mass  7  is, instead, completely balanced by the elastic suspension elements  11 . 
     The weight of the spurious contributions is thus attenuated with respect to that of the contributions useful for detection of the angular rate, and the signal-to-noise ratio is accordingly improved. 
       FIG. 3  illustrates a different embodiment of the disclosure. In this case, a microelectromechanical gyroscope  100  comprises a substrate  102  and a microstructure  103 , in addition to a control device and to a read device (not illustrated). 
     The microstructure  103  comprises two actuation masses  106 , two driving masses  107 , two transduction masses, and four movable sensing structures  110 , all arranged symmetrically about a central anchorage  109 . 
     In detail, the actuation masses  106  are arranged symmetrically with respect to the central anchorage  109  and are aligned in an actuation direction. The actuation masses  106  are elastically coupled to the substrate  102  for oscillating in a fixed actuation direction. Connection to the substrate  102  is obtained by elastic elements  111  for connection to respective outer ends. The actuation masses  106  are further coupled together through elastic connection elements  112  and a bridge  113 , which is in turn connected to the central anchorage  109 . The bridge  113  is defined by a frame surrounding the central anchorage  109  and connected thereto to be able to oscillate out of plane with respect to two perpendicular axes. 
     The actuation masses  106  are provided with respective sets of movable actuation electrodes  115   a , which are capacitively coupled in comb-fingered configuration to stator actuation electrodes  115   b  fixed to the substrate  2 . The control device (not illustrated) uses the movable actuation electrodes  115   a  and the stator actuation electrodes  115   b  for keeping the actuation masses  106  in oscillation with respect to the actuation direction with controlled frequency and amplitude and, for example, with a mutual phase shift. 
     The driving masses  107  are arranged symmetrically with respect to the central anchorage  109  and are aligned in a driving direction D 1 ′ perpendicular to the actuation direction. The driving masses  107  are elastically coupled to the substrate  102  and to the actuation masses  106  for oscillating in the driving direction D 1 ′. In particular, each driving mass  107  is coupled to both of the actuation masses  106  by respective elastic suspension elements  117 , which are configured to convert the motion of the actuation masses  106  in the actuation direction into motion of the driving masses  107  in the driving direction D 1 ′. The mutually phase-shifted oscillatory motion of the actuation masses  106  in the actuation direction causes a corresponding mutually phase-shifted oscillatory motion of the driving masses  107  in the driving direction D 1 ′. 
     The driving masses  107  are further coupled to the substrate  102  by elastic suspension elements  118  and to the bridge  113  by elastic connection elements  120 . The elastic suspension elements  118  and the elastic connection elements  120  are configured to prevent movements of the driving masses  107  transverse to the driving direction D 1 ′. 
     Each transduction mass  108  is elastically coupled to a respective one of the driving masses  107  by elastic connection elements  121 . The transduction masses  108  are arranged symmetrically with respect to the central anchorage  109 . The elastic connection elements  121  are configured to enable relative movements of the transduction masses  108  with respect to the driving masses  107  in a transduction direction D 2 ′ perpendicular to the driving direction D 1 ′ and for preventing relative movements of the transduction masses  108  with respect to the driving masses  107  in the driving direction D 1 ′ (in one embodiment, the transduction direction D 2 ′ is parallel to the actuation direction). 
     Coupled to each transduction mass  108  are two respective movable sensing structures  110  on opposite sides with respect to the transduction direction D 2 ′. 
     Each movable sensing structure  110  comprises a frame  115  and a set of movable sensing electrodes  126   a , which are supported by the respective frame  115 . The frames  115  are elastically constrained to the respective transduction masses  108  and to the substrate  102  and are movable with respect to the substrate  102  in the transduction direction D 2 ′ and with respect to the respective transduction masses  108  in the driving direction D 1 ′. 
     Elastic connection of the frames  115  to the substrate  102  is obtained by elastic suspension elements  128 , which are configured to enable oscillations of the frames  115  with respect to the substrate  102  in the transduction direction D 2 ′ and prevent movements of the frames  115  with respect to the substrate  102  in the driving direction D 1 ′. 
     Elastic connection of the frames  115  to the respective transduction masses  108  is obtained by elastic suspension elements  129 , which are configured to enable oscillations of the transduction masses  108  with respect to the respective frames  115  in the driving direction D 1 ′ and prevent relative movements between the frames  115  and the respective transduction masses  108  in the transduction direction D 2 ′. 
     The movable sensing structures  110  are capacitively coupled to respective stator sensing structures  130 , which comprise respective sets of stator sensing electrodes  126   b  fixed to the substrate  102 . In particular, the movable sensing electrodes  126   a  and the stator sensing electrodes  126   b  are coupled according to a parallel-plate scheme and define a capacitor with capacitance variable as a function of the position of the movable sensing structures  110  with respect to the substrate  102  in the transduction direction D 2 ′. 
     In the embodiment described, the actuation masses  106  and the driving masses  107  may be constrained to the substrate  102  so that respective out-of-plane rotary movements are allowed. In practice, the elastic connection elements of the actuation masses  106  and of the driving masses  107  may be configured to enable rotations about respective axes parallel to the driving direction D 1 ′ (for the actuation masses  106 ) or to the transduction direction D 2 ′ (for the driving masses  107 ). In this case, the actuation masses  106  and driving masses  107  may be capacitively coupled to electrodes (not illustrated) arranged on respective portions of the substrate  102 . This makes it possible to provide multiaxial gyroscopes, which may detect rotations of the substrate  102  also with respect to axes parallel to the driving direction D 1 ′ or to the transduction direction D 2 ′ (in practice, parallel to the surface of the substrate  102 ). 
     Also in this case, the transduction masses  108  and the movable sensing structures  110  are separated from the driving masses  107  and coupled for penalizing transfer of the spurious movements (due to defects of the constraints) to the sensing structures. In particular, the result is favored by the elastic suspension elements  128  between the frames  115  and the substrate  102  and by the elastic suspension elements  129  between the transduction masses  108  and the respective frames  115 . 
     Illustrated in  FIG. 4  is a portion of an electronic system  200  according to an embodiment of the present disclosure. The system  200  incorporates the electromechanical transducer  1  and may be used in devices such as, for example, a laptop computer or tablet, possibly with wireless-connection capacity, a cellphone, a smartphone, a messaging device, a digital music player, a digital camera, or other devices designed to process, store, transmit, or receive information. In particular, the electroacoustic transducer  1  may be used for performing functions of voice control, for example, in a motion-activated user interface for computers or consoles for video games or in a satellite-navigation device. 
     The electronic system  200  may comprise a control unit  210 , an input/output (I/O) device  220  (for example, a keyboard or a screen), the gyroscope  100 , a wireless interface  240 , and a memory  260 , of a volatile or nonvolatile type, coupled together through a bus  250 . In one embodiment, a battery  280  may be used for supplying the system  200 . It should be noted that the scope of the present disclosure is not limited to embodiments necessarily having one or all of the devices listed. 
     The control unit  210  may comprise, for example, one or more microprocessors, microcontrollers and the like. 
     The I/O device  220  may be used for generating a message. The system  200  may use the wireless interface  240  for transmitting and receiving messages to and from a wireless-communication network with a radiofrequency (RF) signal. Examples of wireless interface may comprise an antenna, a wireless transceiver, such as a dipole antenna, even though the scope of the present disclosure is not limited from this point of view. Furthermore, the I/O device  220  may 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 microelectromechanical gyroscope and to the method described, without thereby departing from the scope of the present disclosure. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.