Patent Application
20240191994
The present invention relates to inertial sensors, particularly to angular displacement sensors that can be used to detect angular position, speed or acceleration, and more particularly to single axis pitch/roll miniaturized high performance gyroscopes.
Rotational movement detection is important in the automotive and aviation sectors as well as satellite navigation systems. Gyroscopes based on microelectromechanical systems can be miniaturized so as to be integrated in smartphones and smart devices.
Gyroscopes are typically referred as pitch, roll and/or yaw gyroscopes according to along which axis they are measuring the angular velocity. Pitch and roll gyroscopes measure the angular velocity around in-plane directions, while yaw gyroscopes measure the angular velocity around a direction perpendicular to the device plane. A so-called 3-axis gyroscope simultaneously measures the angular velocity around three perpendicular directions.
It is well known that pitch/roll microelectromechanical systems (MEMS) gyroscopes are one of the main limitations towards a high-performance 3-axis MEMS inertial measurements units (MEMS IMU) due to inherent constraints given by the classical MEMS processes for what concern out-of-plane devices, e.g. constant out-of-plane thickness, fixed gap between suspended structure and the substrate.
Capacitive solutions proposed in the literature cannot reach the target performances of several high-end applications. A possible solution to obtain a high-performance 3-axis MEMS IMU can be to exploit yaw devices only. In this case, three yaw devices are carefully aligned during the assembly process to create a system sensitive to angular velocities in all the three space directions. However, this approach requires a higher cost in the mounting process and a larger package with respect to 3-axis monolithic devices. Besides, alignment is limited by the assembly process and is subject to errors.
Micro and nanoelectromechanical systems (M&NEMS) technology based on piezoresistive NEMS sensing can potentially achieve high-end performance. However, as a matter of fact, previous literature disclosures of pitch/roll gyroscopes showed many drawbacks in such devices:
The best solution for a 3-axis system would be to solve at least some of the issues above for pitch (or roll) devices, so that they can be fabricated on the same substrate as yaw devices, giving intrinsically perfect alignment between them and allowing smaller packages. Some pitch/roll inertial sensors based on piezoresistive sensing are disclosed in the following publications:
However, none of them discloses an inertial sensor that satisfactorily solve the above-stated problems.
Therefore, a general objective of the present invention is to provide a pitch/roll inertial sensor which features a better sensitivity and that is more immune to quadrature effects than already existing devices based on piezoresistive sensing.
To at least one of these ends, it is provided an inertial sensor comprising:
In this inertial device, the first sense-to-drive elastic means, the first sense-to-lever elastic means, the second sense-to-drive elastic means and the second sense-to-lever elastic means are all configured so that upon the inertial sensor being subjected to a rotational movement about the rotation axis, in a transversal plane extending along the drive excitation direction and the detection direction, the ratio of the first tilt to the lever tilt and the ratio of the second tilt to the lever tilt are both lower than 0.1, preferably lower than 0.01.
Some of the advantages of the ratio of the first tilt to the lever tilt and the ratio of the second tilt to the lever tilt being lower than 0.1 are:
The foregoing and other aspects of the embodiments of this invention are made more evident in the following Detailed Description, when read in conjunction with the attached Drawing Figures, wherein:
The figures are given as example and are not restrictive to the invention. They are guideline schematic representations designed to facilitate the understanding of the invention and they are not necessarily at the scale of practical implementations.
The present invention also relates to a detection system comprising a plurality of inertial sensors according to the present invention. According to an advantageous embodiment, the detection system features at least two inertial sensors having perpendicular rotation directions.
Before performing a detailed review of the different embodiments of the present invention, here are stated the optional characteristics that can eventually be implemented, in association or alternatively.
According to a particularly advantageous embodiment:
According to an example, the first sense-to-drive elastic means also comprise at least one first additional sense-to-drive spring, the at least one first sense-to-drive spring connecting the first sense frame to a first portion of the first drive frame extending mainly along the rotation direction and the at least one first additional sense-to-drive spring connecting the first sense frame to a second portion of the first drive frame extending mainly along the drive excitation direction.
In a similar manner, according to an example, the second sense-to-drive elastic means also comprise at least one second additional sense-to-drive spring, the at least one second sense-to-drive spring connecting the second sense frame to a first portion of the second drive frame extending mainly along the rotation direction and the at least one second additional sense-to-drive spring connecting the second sense frame to a second portion of the second drive frame extending mainly along the drive excitation direction. Increasing the number of springs connecting the sense frames to the drive frames and distributing the additional springs in different portions of the drive and sense frames like perpendicular portions of the drive frames can help better adjust the elastic means between the frames and therefore help achieve a very low tilting of the sense frames when the inertial device is subjected to a rotational movement.
According to an advantageous embodiment, the sense-to-substrate elastic means is accommodated within the sense lever.
According to an example, the first drive frame and the second drive frame are connected through at least one connection spring. This ensures a single coupled anti-phase drive mode.
According to an advantageous embodiment, the inertial sensors further comprises at least one compensation electrode mainly extending into a plane parallel to the device plane. Compensation electrode help reduce instability and noise levels in the inertial device. Combined with a translational motion of the sense frames, compensation electrodes are particularly advantageous as they counterbalance a simple force and not a torque as in previous device that feature a larger tilting of the sense frames.
According to an example, the sensing system is accommodated within the sense lever. This can help mechanically protect the sensing system and improve the robustness of the inertial device.
According to an advantageous embodiment, the sense lever comprises a central beam which extends along the rotation axis and to which the sensing system is connected.
According to an advantageous embodiment, the sensing system comprises at least one connection zone configured to electronically connect the at least one strain gauge.
Unless otherwise indicated in the context of the present application, the terms “thickness” and “depth” refer to dimensions oriented along the thickness of the substrate.
Terms like “on”, “underlying”, “positioned on”, or “above” or the equivalents thereof do not necessarily mean a direct contact, but, a priori, any position where staking is effective. This includes the presence of interlayers between the superimposed parts.
The expression “being tilted” is used as a synonym for “forming an angle”.
The following detailed description of the invention refers to the accompanying drawings. While the description includes exemplary embodiments, other embodiments are possible, and changes may be made to the embodiments described without departing from the spirit and scope of the invention.
Some advantageous features will be described below. Then some exemplary embodiments and use cases will be further detailed in regard with the drawings.
The inertial sensor 1 will now be described referring to
The inertial sensor 1 comprises a substrate 2 extending along a drive excitation direction x and a rotation direction y. The drive excitation direction x and the rotation direction y are normal to each other. They are both also normal to a third direction called detection direction z. The substrate 2 defines a plane called device plane also extending along the drive excitation direction x and the rotation direction y.
The inertial sensor 1 includes an excitation device 3 required for setting into motion the device thereby creating a Coriolis effect. The excitation device 3 typically includes push-pull comb finger electrodes 8 in an anti-phase motion. Comb finger electrodes actuators are common knowledge in microelectronics. It makes use of tangential electrostatic forces for driving. Such actuators are consisting of pairs of comb-fingers stators facing each other and driving comb-fingers rotors attached to the device to be actuated.
In this case, the excitation device is configured to put into motion along the drive excitation direction x a first drive frame 110 and a second drive frame 120, as depicted in
Preferably, the first drive frame 110 and the second drive frame 120 are symmetrical to each other with respect to a rotation axis 7 extending along the rotation direction y. Advantageously, the first drive frame 110 and the second drive frame 120 are connected to each other through connection springs 1500. The presence of such connections springs 1500 ensures a single coupled anti-phase drive mode. The connection springs 1500 are preferably tuning fork springs.
Advantageously, the first drive frame 110 and the second drive frame 120 are both connected to anchor points 4 of the substrate 2 through at least one additional drive spring 1600.
The inertial sensor 1 also features a first sense frame 210 and a second sense frame 220 that have two degrees of freedom, along the drive excitation direction x and the detection direction z. The first sense frame 210 is connected to the first drive frame 110 through first sense-to-drive elastic means that comprise at least one first sense-to-drive springs 1100. The first sense-to-drive elastic means can also comprise at least one first additional sense-to-drive spring 1200. As depicted in
The second sense frame 220 is connected to the second drive frame 120 through second sense-to-drive elastic means that comprise second sense-to-drive springs 2100. The second sense-to-drive elastic means can also comprise second additional sense-to-drive springs 2200. The features of the first sense-to-drive springs 1100 and the first additional sense-to-drive springs 1200 can be applied mutatis mutandis to the second sense-to-drive springs 2100 and the second additional sense-to-drive springs 2200, respectively.
The inertial sensor 1 further comprises a sense lever 300 pivotably mounted to the substrate 2 around a rotation axis 7 directed along the rotation direction y. The sense lever 300 is mounted to the substrate 2 through sense-to-substrate elastic means, comprising at least one sense-to-substrate spring 1400. The sense-to substrate springs 1400 are preferably serpentine springs or folded springs. The sense lever 300 preferably features at least one aperture 303 that accommodates the sense-to-substrate springs 1400 and the anchor points 4 they are connected to.
The sense lever 300 is furthermore connected to the first sense frame 210 and to the second sense frame 220 through first sense-to-lever elastic means comprising at least one first sense-to-lever spring 1300 and through second sense-to-lever elastic means comprising at least one second sense-to-lever spring 2300, respectively.
Upon being excited along the drive excitation direction x and under the effect of the Coriolis acceleration, the first and second sense frames 210, 220 are driven along the detection direction z in an anti-phase motion. More specifically, as depicted in
A pure translation motion of the sense frames 210, 220 along the detection axis z, without any rotational component, can be reached through an accurate balance of the torques resulting from the sense-to-drive elastic means, the sense-to-lever elastic means and the distribution of the centres of gravity of the different frames. A pure translation motion results in the ratio of the first tilt to the lever tilt and the ratio of the second tilt to the lever tilt being equal to zero, which is the best-case scenario and makes it possible to achieve excellent device performances.
Within the structure described above, the tilting of the sense frames 210, 220 can be adjusted, in combination with or independently of adjusting the ratio of the first tilt to the lever tilt and the ratio of the second tilt to the lever tilt. The tilting of the sense frames 210, 220 relatively to the device plane is as low as possible. Advantageously, the first tilt θ1 and second tilt θ2 are both lower than 5°, preferably lower than 3º. Preferably, the sense frames are constantly in a plane parallel to the device plane, i.e. the first tilt θ1 and second tilt θ2 are equal to 0, like in
Some of the advantages of the sense frames moving in a translational motion or having a very low inclination relatively to the device plane are:
All these advantages derive from the fact that making the sense frames move in a translational or almost translational manner nulls or at least reduces the parasitic rotation of the sense lever.
How such low ratios θ1/θs and θ2/θs can be achieved and/or how such a pure translational or approaching motion can be achieved will now be described.
Each of the elastic means involved in the inertial sensor 1 can be characterized by a translational stiffness and/or a torsional stiffness. The table below synthesizes the different stiffnesses of each of the elastic means and along/about which axis they are measured.
Other parameters are important when designing the inertial device 1:
It has been found that, so as to achieve a pure translational motion of the sense frames 210, 220 and therefore in order to have θ1/θs=0 and θ2/θs=0, the parameters characterizing the inertial device 1 should follow the following formula:
with
COS is called total sense-to-substrate torsional stiffness. It is the overall torsional stiffness around the rotation direction y resulting between the sense lever 300 and the anchor points 4, thus taking into account the contribution of the strain gauges 3000. ZG is called gauge center height and is set by process thicknesses hs and hG.
The fact that the sense frames 210, 220 move in a pure translational motion simplifies the model development thanks to a rigid body assumption. This results in simplifying the prediction of the sensitivity of the device, thus making the rigid body assumption more reliable than in former pitch devices from the literature based on NEMS piezoresistive sensing.
Moreover, such a translational pitch/roll architecture results in a better coupling between the sense frames 210, 220 and the sense lever 300. This results in a lower displacement along the detection direction z of the sense frames for a desired stress target applied on the strain gauges 3000. For example, it was shown that, for the same amount of stress of 200 MPa applied to the strain gauges 3000, the herein described solution requires a lower out-of-plane displacement, in the order of 50 nm, with respect to the maximum z-displacement of the previous design depicted in
The balanced architecture of the inertial sensor and balanced motion of the sense frames are extremely beneficial as it ensures a lower native quadrature in the device.
As it will be described below, a pure translational motion of the sense frames is also beneficial when it comes to quadrature compensation.
It should be understood that a significant improvement of the coupling between the sense frames and the sense lever, of the minimum lever of native quadrature, and in quadrature compensation efficiency can be achieved even though the previous formula isn't perfectly matched. It is considered that significant improvements over already existing technologies
The expression of the axial stress that is applied on the strain gauges 300 when a pure translational motion of the sense frames 210, 220 is achieved will now be detailed. When pure translational motion is achieved, the rotation θs of the sense lever 300 is given by the following formula:
where wc is the translational displacement of the sense frames 210, 220, as depicted in
The sense lever factor Γs, indicative of the coupling between the sense frames and the sense lever, is given by:
where ΔL is the elongation of the strain gauge and IG is the distance between the center of the strain gauges 3000 (characterized by the gauge center height ZG) and the rotational axis 7 in the transversal plane xz.
The axial stress σ applied to the strain gauges 3000 can therefore be expressed as follows:
where LG is the length of the strain gauges 3000 along the drive excitation direction x when at rest and E is the Young modulus of the strain gauges 3000.
The above-stated formulas highlight the fact that in the present device design, the stiffnesses and position of the elastic means between the sense lever 300 and the sense frames 210, 220 are the most important design parameters for setting the stress applied to the strain gauges 3000 to a target value. Then, the translational motion of the sense frames 210, 220 is obtained through the correct sizing and positioning of the elastic means that connect them to the drive frames 110, 120.
The inertial sensor 1 further includes a sensing system connected to the sense lever 300. The sensing system is actuated by the sense lever 300 when it rotates around the rotation axis 7, allowing Coriolis force sensing.
The sensing system comprises at least one strain gauge 3000 that gets subjected to axial stress when the sense lever 300 rotates around the rotation axis 7 as depicted in
As depicted in
The central portion 350—comprising the central frame 351 and the central beam 352—and the rest of the sense lever 300 are configured in one piece. More specifically, they can be formed during the same fabrication steps and be made of the same material like monocrystalline silicon or polysilicon. As a result of these elements being in a single bloc, when the inertial device 1 is subjected to Coriolis acceleration, the central beam 352 tilts around the rotational axis 7 at the same angle as the sense lever 300 as a whole. Monitoring the tilting of the central beam 352 is therefore equivalent to monitoring the tilting of the sense lever 300. This central beam 352 features a first side 353 and a second side 354, opposite each other. Preferably, the first and second strain gauges 3001, 3002 are connected to the first and second sides 353, 354 of the central beam 351. As depicted in
Advantageously, the central portion 350 of the lever 300 is designed to have a stiffness that allow the strain gauges 3000 to be properly stressed, that is to say that the stress on the gauges 3000 are the largest allowed by the technology when the incoming angular rate is the largest to measure.
In a preferred embodiment, the inertial sensor 1 further includes drive springs 1700, 2700 that respectively connect the first and the second drive frames 110, 120 to drive sense levers 1710, 2710. Advantageously, the drive springs 1700, 2700 are 4-fold springs. Each of the drive sense levers 1710, 2710 is connected at one end to a drive spring 1700, 2700 and at the other end to an anchor point 4 of the substrate 2. The drive springs 1700, 2700 are mainly extending along the drive direction x. They allow but attenuate mechanical drive in the drive excitation direction x between the drive frames 110, 120 and the drive sense levers 1710, 2710. As a result, when the drive frames 110, 120 are moving along the drive excitation direction x, the drive sense levers 1710, 2710 get deformed and deliver stress into drive sense gauges 3500. The drive sense gauges 3500 can be piezoresistive gauges. This system, depicted in
In an advantageous embodiment, the inertial sensor 1 comprises compensation electrodes 4000.
Such electrodes typically help reduce the zero-rate-output (ZRO) instability in inertial devices. Indeed, among the various sources of zero-rate-output (ZRO) instability, leakage of quadrature Ωq into the sense channel after demodulation errors ϕer is a huge contribution due to possible drifts of both these terms. Referred quadrature Ωq originates from electromechanical imperfections, in particular related to non-orthogonality between drive motion and the sense axis—here between the drive excitation direction x and the detection direction y. Suitable compensating electrodes 4000 in closed-loop circuits can compensate quadrature caused by such imperfections.
Here especially, the implemented quadrature compensation is based on strain sensing, typically NEMS (nanoelectromechanical systems) resistive sensing, as opposed to capacitance detection. Capacitance couplings are therefore minimized, yielding an optimal compensation because of the reduced error in phase der resulting from this minimized capacitance couplings.
Furthermore, the herein described design allows a more efficient quadrature compensation. Indeed, the quadrature electrodes 4000 are preferably extending into a plane parallel to the device plane. In the case of the present architecture, the quadrature compensation forces applied by the electrodes 4000 underneath the sense frames 210 are independent on the position of the electrode 4000 itself along the drive excitation direction x, since they counterbalance directly a force and not a torque as in torsional pitch/roll devices like the one depicted in
The presence of quadrature compensation electrodes 4000, together with appropriate electronic control of said compensation, makes it possible to reach better specifications in terms of noise and stability.
The inertial device 1 can be realized starting from a SOI (Silicon On Insulator) wafer. During a first production step, monocrystalline silicon NEMS layer can be patterned to define the strain gauges 3001, 3002, 3500. Following production steps, during which other elements of the inertial sensors 1 (sense lever 300—comprising the central portion 350 and the central beam 352—, drive frames 110, 120, sense frames 210, 220 . . . ) are made—typically including epitaxial growth, ion etching and oxide release—, are common to conventional MEMS processes. The MEMS wafer can then be bonded with eutectic Al—Ge alloy or with other bonding materials like Au—Si, glassfrit . . . to a cap wafer, where cavities and getters yield the final pressure, typically ranging from of a few ten to a few hundred μbar. Advantageously, the strain gauges 3001, 3002, 3500 are of the same material as the sense lever 300. Preferably, at least some of the strain gauges 3001, 3002, 3500 fabrication steps and of sense lever 300 fabrication steps are carried out concurrently.
Multiple inertial sensors of the same type can be merged to further improve the minimum achievable stability and noise levels. This technique is commonly known as sensor fusion. This has the advantage of improving sensing properties of the inertial device without increasing its thickness if sensors are merged in the device plane. Large packages for specialized applications could fit several tens of inertial sensors 1, potentially exploiting redundancy and cross-correlation techniques for additional performance improvements.
The design herein disclosed is well suited also for high frequency gyroscopes, which could improve the rejection to external vibrations and shocks.
The presented gyroscope could be also properly designed to be a bi-axial device sensitive to out-of-plane and in-plane angular rates. As a matter of fact, finite element method (FEM) simulations show the presence of a mode that is sensitive to out-of-plane rotations that, with a careful mechanical design, could be optimized for the desired behaviour and frequency.
The present inertial sensor can be implemented in a 3-axis MEMS IMU. Thanks to the improvements made with regards to quadrature compensation and sensing coupling, pitch/roll IMU based on the present technology can meet very tight specifications like navigation-grade specifications. High performance 3-axis MEMS IMU could therefore be designed by relying on herein described innovation.
It was shown that, thanks to a balanced design, the inertial sensor 1 can feature very low quadrature effects and an optimized coupling between sense frames and sense lever.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22315317.2 | Dec 2022 | EP | regional |
