Inertial measurement circuit, corresponding device and method

Abstract

Embodiments of the present disclosure relate to inertial measurement circuits, corresponding apparatuses, and methods. In one embodiment, a circuit comprises: an inertial measurement unit configured to oscillate via a drive signal provided by a drive circuit apparatus; a lock-in amplifier configured to receive a sense signal from the inertial measurement unit and a reference demodulation signal as a function of the drive signal, and to provide an inertial measurement signal based on the sense signal, wherein the reference demodulation signal is affected by a variable phase error; a phase meter circuit apparatus configured to receive the drive signal and the sense signal, and to provide as a function of a phase difference between the drive signal and the sense signal; a phase correction signal for the reference demodulation signal and a correction node configured to apply the phase correction signal to the reference demodulation signal, such that in response to applying the phase correction signal to the reference demodulation signal, the phase error is maintained near a reference value.

Description

[0001] Cross Reference to Related Applications

[0002] This application claims the benefit of Italian Application No. 102021000014621 filed June 4, 2021, which is incorporated herein by reference. TECHNICAL FIELD

[0003] The present description relates to inertial measurement units (IMUs). The present description also relates to micro-electro-mechanical system (MEMS) sensors (e.g., gyroscopes) as examples of IMUs. Moreover, the present description relates to automotive applications, in particular to navigation systems. BACKGROUND

[0004] Low bias drift performance is a desirable feature of inertial measurement units (IMUs) such as MEMS gyroscopes.

[0005] Today, MEMS gyroscopes are used in a wide range of products, such as consumer products, including augmented reality / virtual reality (AR / VR) applications, automotive and transportation (navigation), and other high-end applications where bias stability is a concern.

[0006] “Quadrature” bias leakage into the output channel of a gyroscope caused by phase error can adversely affect the stability performance.

[0007] This problem can be addressed by minimizing the quadrature value. This can be done electronically or electromechanically. Regardless of the approach taken, the compensation is imperfect and a residual drift is caused by the drift in the phase error.

[0008] As general background (and as known to those skilled in the art), gyroscopes are used to detect angular (rotational) rates with high precision (low bias drift) representing a desirable feature.

[0009] As Figure 1 As schematically shown in the

[0010] This can be minimized by coherent demodulation. However, the demodulation phase is affected by an error, err, which varies with temperature (T) and causes an output bias, q. err(T).

[0011] Acting directly on the system to compensate for the quadrature, q, has certain benefits. However, the compensation is imperfect and a residual quadrature, qr, remains, such that a residual error, qr err(T), remains and its drift adversely affects the gyroscope performance.

[0012] Various approaches have been proposed to address this problem.

[0013] For example, one can attempt to minimize the quadrature component using current injection at the analog front end (e.g., Ωqr.Φerr(T) - Ωcomp).

[0014] The drawback of this method is that it leaves a residual quadrature error, and thus a bias. Indeed, in the absence of a perfect match between the quadrature phase and its compensated phase, a residual error proportional to the phase error (Φerr) appears. In addition, the drift remains substantially uncompensated.

[0015] Another method can include minimizing the quadrature component by using a closed loop electro-mechanical compensation, e.g., (Ωq - Ωcomp)Φerr(T).

[0016] The drawback of this method is that it compensates the quadrature at its source, leaving a residual uncompensated term in any case, and thus a related drift.

[0017] Another method can include minimizing the phase error drift by relying on a linearized phase v to change phase. A temperature model, e.g., Ωq[Φerr(T) - Φmodel(T)].

[0018] The drawback of this method is that it relies on a linearized model, while the behavior of the real system is non-linear (it varies from component to component), and does not take into account other effects that contribute to the phase drift (sensing softening, driving hardening, deformation).

[0019] Therefore, the problem of enhancing the zero rate level (ZRL) stability during the system lifetime remains widely felt.

[0020] Therefore, there is a need for a solution that helps higher ZRL stability without limiting the noise and power consumption performances. SUMMARY

[0021] Embodiments provide an inertial measurement unit (IMU). Further embodiments provide a micro-electro-mechanical system (MEMS) sensor (e.g., a gyroscope), which is an example of an IMU. Other embodiments relate to automotive applications, and in particular to navigation systems. Embodiments can solve the above-mentioned problems.

[0022] According to one or more embodiments, a circuit can have the features set forth below.

[0023] A MEMS (micro-electro-mechanical system) gyroscope based circuit can be an example of such a circuit.

[0024] One or more embodiments relate to corresponding devices.

[0025] Various types of consumer products, such as augmented reality / virtual reality (AR / VR) viewer viewing, including navigation devices of the automotive industry that utilize output from IMUs can be examples of such devices.

[0026] One or more embodiments relate to corresponding methods.

[0027] The claims are an integral part of the technical teaching provided herein with respect to the embodiments.

[0028] Examples as discussed herein can provide one or more of the following advantages:

[0029] Due to the potential feedback mechanism, the phase error can be kept close to zero at any time; thus, its component does not leak into the signal channel regardless of the quadrature value;

[0030] In certain embodiments, the solution as discussed herein can be combined with other compensation mechanisms and provide further improved overall compensation;

[0031] Using a closed-loop approach, which on the one hand keeps the phase error close to zero and on the other hand does not take into account the variability of e.g. gyroscope parameters, which makes the temperature (“T”) dependent part different from the part;

[0032] In certain embodiments, e.g. applied to gyroscopes, no compensation at the gyroscope level is needed; this has significant advantages compared to conventional techniques.

[0033] Examples as discussed herein propose a closed-loop compensation approach to improve the zero rate output (ZRO) stability performance of an amplitude modulated (AM) capacitive MEMS gyroscope.

[0034] Examples as discussed herein rely on a direct measurement (and closed-loop compensation) of the relative phase variation between the quadrature signal modulated by the driving carrier frequency and the demodulation reference.

[0035] In examples as discussed herein, the closed-loop arrangement comprises a phase meter acting on the phase of a phase locked amplifier in readout mode, an open loop, a demodulation chain.

[0036] Examples as discussed herein can be applied in MEMS gyroscopes, for automotive or augmented reality / virtual reality (AR / VR) applications.

[0037] Examples as discussed herein can be applied to other conventional gyroscopes and inertial measurement units (IMUs), such as 6x IMUs, which have enhanced capabilities representing “mainstream” inertial MEMS products.

[0038] Examples as discussed herein do not involve a substantial change in the electromechanical design of such conventional products.

[0039] The examples discussed in this article may include additional (analog) stages that take signals from the drive / sensor chain and square them. For example, these may include a two-path implementation of a bandpass filter (BPF) stage (with associated passive components) operating at the gyroscope's operating frequency.

[0040] Other additional (analog) stages and related signals can appear along with the phase meter in the path toward the lock-in amplifier (LIA) reference.

[0041] In practice, the examples discussed in this paper can produce output tones associated with the periodic activation of the compensation features.

[0042] Furthermore, in some instances, periodic changes in the sign of the voltage applied to the MEMS quadrature compensation electrode can be detected. Attached Figure Description

[0043] One or more embodiments will now be described by way of example only with reference to the accompanying drawings, in which:

[0044] Figure 1 The output of the gyroscope is shown.

[0045] Figure 2 A block diagram of a circuit based on the examples discussed in this specification is shown;

[0046] Figure 3 A block diagram illustrating a circuit according to an embodiment is shown; and

[0047] Figure 4 A time diagram illustrating the possible temporal behavior of the signal that may occur in the embodiments is shown.

[0048] Unless otherwise specified, corresponding numbers and symbols in the different figures generally refer to corresponding parts. The figures are drawn to clearly illustrate relevant aspects of the embodiments and are not necessarily drawn to scale. The edges of features drawn in the figures do not necessarily indicate the termination of the feature range. Detailed Implementation

[0049] In the following description, various specific details are shown to provide a thorough understanding of various examples of embodiments according to the description. Embodiments may be obtained without one or more specific details, or by utilizing other methods, components, materials, etc. In other instances, known structures, materials, or operations are not shown or described in detail so as not to obscure the various aspects of the embodiments.

[0050] Reference within the specification to "one embodiment" or "an embodiment" is intended to mean that a particular configuration, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. The appearances of the phrase "in one embodiment" or "in an embodiment" in various places in the specification are not necessarily all referring to the same embodiment. Furthermore, the particular configurations, structures, or characteristics can be combined in any suitable manner in one or more embodiments.

[0051] The headings / reference herein are provided merely for convenience and do not limit the scope of protection or the embodiments.

[0052] For brevity and simplicity, the same reference designators can be used in the various drawings and the detailed description to designate the same elements or components that are shown in the figures and described in the detailed description.

[0053] By way of overview of the instant detailed description, reference can be made to document US2020 / 400434A1. This document discloses a MEMS gyroscope having a moving mass carried by a support structure to move in a drive direction and a first sense direction perpendicular to each other.

[0054] The structure of such a gyroscope will be assumed to be known to the skilled person. For brevity, the detailed description will not be repeated herein.

[0055] For the purposes of this document, the principle of operation of such a gyroscope can be briefly recalled: even if the support of the vibrating body is rotated, the vibrating body tends to continue vibrating in the same plane. Due to the Coriolis effect, the vibrating body exerts a significant force on the support. The rate of rotation can be determined by measuring the displacement produced by this force.

[0056] Reliable and inexpensive vibrating structure gyroscopes can be manufactured with MEMS technology. These applications are in mobile communication devices, electronic games, cameras and various other applications.

[0057] An overview of MEMS gyroscope technology is provided in "Overview of MEMS Gyroscopes: History, Principles of Operations, Types of Measurements" by A. A. Trusov, Microsystems Laboratory, Mechanical and Aerospace engineering university of California, Irvine, CA, 92697, USA, May 10, 2011 (see uci.edu).

[0058] And, in S. Facchinetti, L. Guerinoni, L.G. Falorni, A. Donadel, and C. Valzasina, "Development of a complete model to evaluate the zero rate level drift over temperature in MEMS coriolis vibrating gyroscope," 2017 IEEE International Symposium on Inertial Sensors SanD Systems (INERTIAL), 2017, pp. 125-128, doi: 10.1109 / ISISS.2017.7935673, a comprehensive model is presented to analytically estimate the zero rate level (ZRL) over temperature in a micro-machined Coriolis vibrating gyroscope (CVG) with associated electronics, with the aim of providing solid guidance for the development of high-stability MEMS inertial measurement units (IMUs).

[0059] The examples given herein rely on a compensation method based on a direct measurement of the phase error Φerr.

[0060] In particular, the exemplified method exploits the quadrature channel to recover the phase information.

[0061] Note that the quadrature channel signal is (far) larger than the rate full-scale signal.

[0062] This is particularly true if the embedded quadrature compensation electrodes are used to purposefully de-compensate the phase. It is found that these are susceptible to increase "chopping" or modulation in order to increase resolution in the phase measurement.

[0063] Various implementation options can be considered.

[0064] In some examples, no compensation electrodes are provided and the measurement relies on the quadrature signal present (inevitably) in the system output.

[0065] In other examples, by providing quadrature compensation electrodes, the quadrature can be intentionally increased (or modulated) to facilitate the phase measurement.

[0066] In either case, the process can be applied in continuous-time mode, or only at certain times (each time it occurs) the temperature drift transients involved are considered.

[0067] Hence, the basic principle aims to continuously keep the term Φerrzero (zero) via a closed loop feedback, so that the bias and its associated drift are zero.

[0068] In such Figure 2 In the traditional, uncompensated configuration shown (as in Figure 3 In the case of (considering, as an example, installation on vehicle V), the circuit 10 considered here includes (actually built around) a MEMS gyroscope 12 (any known type suitable for the purposes herein).

[0069] During the drive mode, the gyroscope 12—represented herein in a deliberately simplified manner—is kept oscillating along the drive direction (which may be assumed to be horizontal in the figure) via a first electrode 14A, which is driven by drive stages 16A(D+) and 16B(D-), and the drive stages 16A(D+) and 16B(D-) are coupled to a variable gain amplifier (VGA) 18.

[0070] Second electrodes 20A and 20B, sensitive to oscillations in the driving direction, generate signals applied to differential stage 22, which provides phase adjusters / gain stages 24 and 26. The output from the gain stage is closed into the loop of MEMS 12 via variable gain amplifier 18.

[0071] The amplitude of the oscillation maintained by the main phase loop just discussed is precisely controlled by an automatic gain control (AGC) negative feedback loop, which includes a rectifier circuit 28 coupled to the output from the differential amplifier stage 22.

[0072] The output from rectifier circuit 28 is applied to the first input (e.g., inverted) of comparator 32 via low-pass filter 30, and the second input (e.g., non-inverted) of comparator 32 is coupled to reference voltage Vref.

[0073] The output of comparator 32 controls the gain of variable gain amplifier 18.

[0074] As shown in the figure, sensing is performed via an open-loop chain including third electrodes 34A and 34B (assumed to be perpendicular in the figure).

[0075] The signals generated by the third electrodes 34A and 34B are applied to the full differential stage 36, which provides the differential output signals S+ and S- to the lock-in amplifier (LIA) 38, which generates the desired output signal Vout.

[0076] A LIA (Low-Input Amplifier) ​​is an amplifier capable of extracting signals with a known carrier from a (quite) noisy environment. It can be implemented as a zero-difference detector with cascaded adjustable low-pass filters.

[0077] like Figure 2 As shown, LIA38 uses the demodulation reference signal Φopt obtained from the drive loop (e.g., from the output of the variable gain amplifier 18).

[0078] In Figure 2 , the phase of the signal is designated at T0 as Φopt, in order to highlight the fact that its phase value is the one reached at a certain temperature T0, and therefore can be exposed to variations depending on the temperature.

[0079] The skilled person can understand, Figure 2 that the conventional implementation shown is merely exemplary.

[0080] For both the drive chain and the sensing chain associated with the gyroscope 12, a plurality of alternative implementations are possible: in fact, the examples provided herein are substantially "transparent" to the specific implementation of the drive chain and / or of the sensing chain.

[0081] It is noted that, in principle, the "optimal" demodulation phase (Φopt found in reference conditions) can be calibrated. Due to both mechanical elements (mainly the MEMS gyroscope 12) and electronic elements in the drive and sensing chains, a drift in Φopt would negatively affect the output signal Vout in operation.

[0082] Figure 3 The basic principles of the embodiments are exemplified.

[0083] As mentioned above, corresponding numbers and symbols in the different drawings generally refer to corresponding parts, unless explicitly specified otherwise. Thus, like parts or elements in several Figure 2 discussed components or elements are indicated by like numbers and symbols in Figure 3 , and the respective description will not be repeated for the sake of brevity.

[0084] It should be understood that specific parts or elements denoted by the same numbers or symbols in Figure 2 and Figure 3 do not imply that such parts or elements must be implemented in the same way in Figure 2 and Figure 3 .

[0085] Furthermore, a user circuitry is shown in Figure 3 , denoted as UC.

[0086] This can be any type of circuitry configured to exploit the inertial measurement (gyroscopic) output signal Vout in, for example, an augmented reality / virtual reality (AR / VR) viewer (such as a product of a navigation device of the automotive industry).

[0087] As Figure 3 shown, a phase meter 40 is provided to measure the phase difference Φer between the signal Dsq indicative of the "drive phase" and the signal Ssq indicative of the "sense phase".

[0088] As shown in Figure 3 , the signal Dsq is sensed across the first electrodes 14A, 14B (D+, D-), e.g. at the output of the drivers 16A, 16B, and is conveyed to the phase meter 40 via an instrumentation amplifier (INA) 44 and a high gain (hiG) stage 46 (after possible band-pass filtering at 42A, 42B).

[0089] Also as shown in Figure 3 , the signal Ssq is sensed across the third electrodes 34A, 34B (S+, S-), and is conveyed to the phase meter 40 via an instrumentation amplifier (INA) 50 and a high gain (hiG) stage 52 (after possible band-pass filtering at 48A, 48B).

[0090] The output from the phase meter 40 is a measure of the phase difference between the signals Dsq and Ssq (see Figure 4 , denoted as Figure 3 .

[0091] The reference phase delay

[0092] The LIA phase regulator thus closes the feedback on the phase of the reference demodulation wave entering the lock-in amplifier 38, so that

[0093] In short, Figure 3 The exemplary circuit 10 comprises an inertial measurement unit 12 (e.g. a MEMS gyroscope) configured to oscillate via a drive signal, e.g. the signal D+, D- generated by a drive circuit arrangement comprising elements 14A, 14B, 16, 18, 20A, 20B, 22, 24, 26, 28, 30 and 32.

[0094] The lock-in amplifier 38 receives a sense signal S+, S- from the inertial measurement unit 12 and a reference demodulation signal as a function of the drive signal D+, D-, which ultimately gives rise to the signal Dsq.

[0095] The lock-in amplifier 38 is configured to generate an inertial measurement signal Vout on the basis of the sense signal S+, S- from the inertial measurement unit 12 and the reference demodulation signal, which is affected by a variable phase error

[0096] The phase meter circuit arrangement 40 is configured to receive the drive signals (D+, D->>Dsq) and the sense signals (S+, S->>Ssq), and to generate a phase correction signal (via nodes 54, 56) for the reference demodulation signal of the lock-in amplifier 38 in dependence on a phase difference ΔΦds between the drive signals Dsq and the sense signals Ssq (see Figure 4 ).

[0097] A correction node such as 56 is provided, which is configured to apply such a phase correction signal to the reference demodulation signal of the lock-in amplifier 38.

[0098] In response to the phase correction signal being applied to the reference demodulation signal of the lock-in amplifier 38, the associated phase error Φer is kept near a (constant) reference value (i.e. Φer0).

[0099] A trim node 54 can be provided between the phase meter circuit arrangement 40 and the correction node 56, the trim node 54 being configured to trim the phase correction signal at T0 to the reference value Φer0.

[0100] As shown in Figure 3 , the circuit 10 includes sensing circuitry intermediate the inertial measurement unit 12 and the lock-in amplifier 38, the sensing circuitry including electrodes 34A, 34B configured to generate the sense signals S+, S->>Ssq.

[0101] As discussed below, such orthogonal electrodes (and pads) are not mandatory, and can be dispensed with, e.g. in an On / Act / On option in combination with electronic compensation, as discussed below, although they are advantageous for reliability.

[0102] As shown in Figure 3 , the sensing circuitry 34A, 34B, 36 can be configured to provide both sense signals S+, S->>Ssq to the lock-in amplifier 38 and the phase meter 40.

[0103] Advantageously, the phase meter 40 can be implemented in the digital domain (see Figure 4 ).

[0104] As shown in Figure 3 , the phase meter circuit arrangement 40 is coupled to input signal paths for the drive signals D+, D-, Dsq and the sense signals S+, S-, Ssq.

[0105] As shown, these input signal paths include:

[0106] bandpass filter circuitry (e.g. filters 42A, 42B; 48A, 48B), and / or

[0107] The saturation circuit arrangement, optionally comprising a instrumentation amplifier such as 44 or 50 and a high gain stage such as 46 or 52, is arranged in cascade.

[0108] The LIA 38 with phase adjuster can be implemented in the analog or digital domain.

[0109] The pre-filter (BPF) stages 42A, 42B, 48A, 48B, the INA amplifiers 44, 50 and the hiG stages 46 and 52 are analog stages.

[0110] As shown, the pre-filtering (BPF) and saturation (INA + hiG) are applied to the sense and reference signals before (upstream of) the phase meter. This was found to improve the resolution of the phase measurement, reducing noise folding.

[0111] Figure 3 The architecture exemplified is suitable for use in various ways.

[0112] In the On / Act / Off method, in combination with electronic compensation, the quadrature can be intentionally increased (the "on" step) and measured and corrected for the best phase (the "act" step). In the last step, the intentional increase of the quadrature is removed and the quadrature itself is compensated for again (the "off" step).

[0113] This option does not in principle require dedicated quadrature electrodes and pads.

[0114] In the independent method of continuously applying quadrature modulation, the quadrature can be intentionally increased again to apply "chopping" at a frequency higher than the sensing bandwidth of the Coriolis channel. The modulated quadrature can be used to detect the phase and apply (continuous) correction.

[0115] This option does of course involve quadrature electrodes and pads.

[0116] Another On / Act / Off method in combination with electromechanical compensation can be similar to the first method discussed previously, in which the quadrature is intentionally increased and compensated for again with an electromechanical method.

[0117] This (more reliable) option again involves quadrature electrodes and pads.

[0118] The examples discussed herein thus improve the zero rate output (ZRO) stability of an inertial measurement unit (IMU) such as a MEMS gyroscope by essentially intervening at the hardware level.

[0119] The examples discussed herein are however suitable for cooperation with appropriate software acting on the phase meter and phase adjuster (as shown in blocks 38 and 40 in Figure 3

[0120] ​It will be appreciated that the apparatus exemplified herein can be applied to multi-axis, multi-parameter IMUs, and the present specification is intentionally simplified for the sake of explanation and understanding.

[0121] Details and embodiments can vary significantly in relation to what is described by way of example only, without departing from the basic principles, without leaving the scope of protection. The scope of protection is determined by the attached claims.

Claims

1. An inertial measurement circuit, comprising: an inertial measurement unit configured to oscillate via a drive signal provided by drive circuitry; a phase locked amplifier configured to: receive a sense signal from the inertial measurement unit and a reference demodulation signal dependent on the drive signal; provide an inertial measurement signal based on the sense signal, wherein the reference demodulation signal is subject to a variable phase error; phase meter circuitry configured to: receive the drive signal and the sense signal; and provide a phase correction signal for the reference demodulation signal as a function of a phase difference between the drive signal and the sense signal; a correction node configured to apply the phase correction signal to the reference demodulation signal such that the phase error is maintained around a reference value in response to the phase correction signal being applied to the reference demodulation signal; and a trim node arranged between the phase meter circuitry and the correction node, wherein the trim node is configured to trim the phase correction signal to a reference phase correction value. sense circuitry arranged between the inertial measurement unit and the phase locked amplifier, wherein the sense circuitry is configured to provide the sense signal.

3. The inertial measurement circuit of claim 2, wherein the sense circuitry is configured to provide the sense signal to both the phase locked amplifier and the phase meter circuitry.

2. The inertial measurement circuit of claim 1, further comprising:

4. The inertial measurement circuit of claim 1, wherein the phase meter circuitry is coupled to input signal paths for the drive signal and the sense signal, respectively, the input signal paths comprising: band pass filter circuitry; and / or saturation circuitry.

5. The inertial measurement circuit of claim 4, wherein the saturation circuitry comprises a cascade arrangement of an instrumentation amplifier and a high gain stage.

6. The inertial measurement circuit of claim 1, wherein the phase meter circuitry comprises a digital phase meter.

7. The inertial measurement circuit of claim 1, wherein the inertial measurement unit comprises a MEMS gyroscope.

8. An inertial measurement device, comprising: the inertial measurement circuit of claim 1; and user circuitry coupled to the phase locked amplifier in the circuit, wherein the user circuitry is configured to utilize the inertial measurement signal produced by the phase locked amplifier.

9. A method for closed loop compensation, comprising: oscillating an inertial measurement unit via a drive signal provided by drive circuitry; receiving, by a phase locked amplifier, a sense signal from the inertial measurement unit and a reference demodulation signal as a function of the drive signal; providing, by the phase locked amplifier, an inertial measurement signal based on the sense signal and the reference demodulation signal, wherein the reference demodulation signal is subject to a variable phase error; receiving, by phase meter circuitry, the drive signal and the sense signal, and providing a phase correction signal for the reference demodulation signal as a function of a phase difference between the drive signal and the sense signal; trimming, by a trim node, the phase correction signal to a reference phase correction value; and ​ ​ ​ ​ ​ applying the phase correction signal to the reference demodulation signal of the phase locked amplifier by a correction node, such that the phase error is maintained around a reference value in response to the phase correction signal being applied to the reference demodulation signal of the phase locked amplifier.

10. The method of claim 9, further comprising: providing the sense signal by a sense circuit arrangement arranged between the inertial measurement unit and the phase locked amplifier.

11. The method according to claim 10, wherein the sense circuit arrangement provides the sense signal to both the phase locked amplifier and the phase meter circuit arrangement.

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