FIELD

The present invention relates to microelectromechanical devices and specifically to an inertial sensor with continuous self-test capability and a continuous self-test method for an inertial sensor.

BACKGROUND

Micro-Electro-Mechanical Systems or MEMS can be defined as micro-scale mechanical and electro-mechanical systems where at least some elements have a mechanical functionality. MEMS structures can be applied to quickly and accurately detect very small changes in physical properties.

Motion can be considered to have six degrees of freedom: translations in three orthogonal directions and rotations around three orthogonal axes. The latter three may be measured by an angular rate sensor, also known as a gyroscope. MEMS gyroscopes use the Coriolis Effect to measure the angular rate. When a mass is moving in one direction and rotational angular velocity is applied, the mass experiences a force in orthogonal direction as a result of the Coriolis force. The resulting physical displacement caused by the Coriolis force may then be read from, for example, a capacitively, piezoelectrically or piezoresistively sensing structure.

In MEMS gyroscopes the primary motion is typically not continuous rotation as in conventional ones due to lack of adequate bearings. Instead, mechanical oscillation may be used as the primary motion. When an oscillating gyroscope is subjected to an angular motion, an undulating Coriolis force results. This creates a secondary oscillation orthogonal to the primary motion and to the axis of the angular motion, and at the frequency of the primary oscillation. The amplitude of this coupled oscillation can be used as the measure of the angular motion. The secondary oscillation may be called a secondary motion, a detection motion or a sense motion.

MEMS sensors have become part of many consumer devices and they are used also in a variety of safety critical applications, such as electronic stability control (ESC). Especially in the safety related applications, it is important to identify potential failures in mechanical or electrical signal paths of the sensor. MEMS gyroscopes are becoming continuously more important components for example in automated driving applications in order to allow inertial navigation during absence of external navigation data. When angular rate signal is used for navigation purposes in vehicles, it becomes highly important that the sensor is capable of fast and reliable self-diagnostics. This has become moderately pronounced also in ESC systems but is becoming more demanding in future for example with respect to detection time requirements. Ideally, the component should be capable of identifying its own erroneous operation very rapidly. On the other hand, false alarms that cause unnecessary inconvenience to system users should be effectively avoided. Thus, at the same time the sensor is required to tell whether it is malfunctioning, it should also be nearly fully insensitive toward triggering of the self-test features due to external disturbances.

A type of advanced vibratory gyroscopes applies a closed-loop system where the vibrational sense motion is feedback controlled for enhanced performance. The closed-loop configuration, however, creates also aspects that cannot be met with conventional self-test methods.

DESCRIPTION OF THE RELATED ART

U.S. Pat. No. 9,109,901 discloses a system and method for monitoring operation of a MEMS a gyroscope, where a test signal generator generates and applies a test signal to the angular rate feedback loop of the gyroscope. The test signal detector is coupled to a quadrature feedback loop. The sense loop includes demodulations and modulations in both angular rate phase and quadrature phase, and therefore an additional filtering is required within the closed-loop secondary for removing primary frequency and higher than primary frequency signal content from the signal before modulation. Thus, the speed of the feedback loop and the bandwidth of the angular rate sensor becomes limited.

U.S. Pat. No. 7,127,932 discloses a method for monitoring a rotational speed sensor with a vibrational gyroscope. Also, this solution the closed sense loop includes both modulations and demodulations both in angular rate and quadrature phase, which requires an additional filter for removing primary frequency signal content from the signal before modulation, which limits the speed of the feedback loop and the bandwidth of the angular rate sensor.

U.S. Pat. No. 9,846,037 a method and device for continuous self-test of a gyroscope, that requires high test signal magnitudes to avoid false triggering and requires selecting between robustness of the self-test and achievable error detection time.

Thus, an improved continuous self-test method and system is needed that avoids the trade-offs of the prior art.

SUMMARY

An object is to provide a method and apparatus so as to solve the problem of providing a continuous self-test that is both robust and fast, while avoiding necessity for high test signal levels.

The objects of the present invention are achieved with an apparatus according to the claim1. The objects of the present invention are further achieved with a method according to the claim8.

The preferred embodiments of the invention are disclosed in the dependent claims.

According to a first aspect, a microelectromechanical (MEMS) gyroscope is provided. The MEMS gyroscope comprises a drive loop comprising a drive element configured to be excited into a vibrational primary motion in a first direction, and drive loop circuitry configured to receive from a detection transducer associated with the drive element a first electrical signal for producing a quadrature-phase detection signal that corresponds to position of the drive element. The drive loop circuitry comprises a clock generating circuitry for generating from the quadrature-phase detection signal at least one test clock signal, an angular rate phase demodulation signal and a quadrature phase demodulation signal. The MEMS gyroscope further comprises a sense loop comprising a sense element configured to be driven into a vibrational sense motion in a direction that is perpendicular to the first direction, and sense loop circuitry configured to receive from a detection transducer associated with the sense element a second electrical signal corresponding to position or speed of the sense element and to generate a force-feedback signal on basis of the second electrical signal. The MEMS gyroscope also comprises a test signal generator configured to receive from the drive loop circuitry the quadrature-phase detection signal to be used as a quadrature-phase carrier signal and the at least one test clock signal configured to carry at least two test frequency signals with distinctive fundamental test frequencies, and to generate a test signal in quadrature phase by modulating the quadrature-phase carrier signal with the at least two test frequency signals.

The MEMS gyroscope also comprises a summing element configured to sum the test signal with the force-feedback signal to form a sense feedback signal to be fed back towards the sense element. The MEMS gyroscope further comprises a rate phase demodulator configured to produce a rate signal by demodulating a sense signal received from the sense loop with the angular rate phase demodulation signal, wherein the rate signal comprises angular rate information, and a quadrature-phase demodulator configured to produce a quadrature-phase output signal by demodulating the sense signal received from the sense loop with the quadrature-phase demodulation signal. The quadrature-phase output signal comprises self-test information.

According to a second aspect, the at least one test clock signal is in quadrature phase.

According to a third aspect, the sense loop circuitry comprises a frontend circuitry configured to receive the second electrical signal and to produce a sense loop output signal and a backend circuitry comprising at least one of a damping circuitry for damping the force-feedback signal and an amplifier for amplifying the sense loop output signal to produce the sense signal.

According to a fourth aspect, the at least two distinctive fundamental test frequencies are selected such that the modulated test frequency signals are within the signal band of the sense loop.

According to a fifth aspect, the MEMS gyroscope further comprises a quadrature correction circuitry configured to generate a quadrature feedback signal from the quadrature-phase output signal and to feed the quadrature feedback signal towards the sense element. The frequency of the at least two modulated test frequency signals are outside a signal band of the quadrature correction circuitry and/or one or more quadrature correction electrodes are configured to input the quadrature correction feedback signal towards the sense element.

According to a sixth aspect, the MEMS gyroscope comprises capacitive or piezoresistive electrodes for drive, detection and feedback of the drive element and for detection and force-feedback of the sense element.

According to a seventh aspect, the quadrature-phase detection signal is configured to be obtained at an output of a charge sensitive amplifier of the drive loop circuitry.

According to an eighth aspect, a wanted test clock signal frequency is configured to be obtained by dividing the quadrature-phase carrier signal frequency with an integer value and wherein the test clock signal switches its state at a zero crossing of the quadrature-phase carrier signal.

According to a first method aspect, a method for continuous self-testing of a microelectromechanical (MEMS) gyroscope is provided. The method comprises receiving a quadrature-phase detection signal from a detection transducer of a drive loop circuitry of the gyroscope, wherein the quadrature-phase detection signal corresponds to position of a drive element of the gyroscope, generating from the quadrature-phase detection signal at least one test clock signal carrying at least two test frequency signals with distinctive fundamental test frequencies, a rate phase demodulation signal and a quadrature-phase demodulation signal to be used as a quadrature-phase carrier signal, and generating a test signal in quadrature phase by modulating the quadrature-phase carrier signal with the at least two test frequency signals. The method also comprises receiving a second electrical signal corresponding to position or speed of a sense element of the gyroscope, generating a force-feedback signal in a sense loop of the gyroscope on basis of the second electrical signal, summing the test signal into the force-feedback signal to produce a sense feedback signal, and feeding the sense feedback signal towards the sense element of the gyroscope. The method further comprises producing a rate signal by demodulating a sense signal received from the sense loop with the rate phase demodulation signal, wherein the rate signal comprises angular rate information, and producing a quadrature-phase output signal by demodulating the sense signal received from the sense loop with the quadrature-phase demodulation signal, wherein the quadrature-phase output signal comprises self-test information.

According to a second method aspect, the at least one test clock signal is in quadrature phase.

According to a third method aspect, the method further comprises producing a sense loop output signal on basis of the second electrical signal, and at least one of damping the force-feedback signal before said summing and amplifying the sense loop output signal to produce the sense signal.

According to a fourth method aspect, the at least two distinctive fundamental test frequencies are selected such that the modulated test frequency signals are within the signal band of the sense loop.

According to a fifth method aspect, the method further comprises generating a quadrature feedback signal from the quadrature-phase output signal and feeding the quadrature feedback signal towards the sense element. The frequency of the at least two modulated test frequency signals are outside a signal band of the quadrature correction circuitry and/or one or more quadrature correction electrodes are configured to input the quadrature correction feedback signal towards the sense element.

According to a sixth method aspect, the method further comprises obtaining wanted test clock signal frequency by dividing the quadrature-phase carrier signal frequency with an integer value and wherein the test clock signal switches its state at a zero crossing of the quadrature-phase carrier signal.

The present invention is based on the idea of taking a carrier signal for self-test signal generation from primary loop charge-sensitive amplifier output, which carrier signal is in phase with position of the primary element of the MEMS gyroscope, and summing a self-test signal in quadrature phase into a force-feedback signal fed back towards the sense element. By selecting suitable test frequencies and designing response functions of the various signal loops of the MEMS gyroscope appropriately, the self-test signal for detecting proper functionality of major part of gyroscope signal path may be fed continuously to the MEMS gyroscope without affecting its normal, active operation.

The present invention has the advantage that it provides a robust continuous self-test with efficient immunity towards any external inertial signals, as long as they are not strong enough to inflict unwanted mechanical contact within the MEMS gyroscope, in other words exceeding the MEMS gyroscope's defined operation range. The present invention also allows fast error detection time, typically below 5 ms. Further, the present invention requires only modest test signal level despite robust operation while providing good detection coverage.

