Chip-scale inertial sensors and inertial measurement units

Abstract

An inertial sensor and an inertial measurement unit are provided. In one example, the chip-scale inertial sensor is for detecting a rotation rate of the inertial sensor about an axis. The inertial sensor comprises an optical vibratory gyroscope for detecting a first rotation rate of the inertial sensor about said axis. The optical vibratory gyroscope is configured to output a primary signal corresponding to the first rotation rate. The inertial sensor further comprises an optical Sagnac gyroscope for simultaneously detecting a second rotation rate of the inertial sensor about said axis. The optical Sagnac gyroscope is configured to output an auxiliary signal corresponding to the second rotation rate. The inertial sensor further comprises a microcontroller configured to receive one or more inputs based on the primary signal and the auxiliary signal. The microcontroller is further configured to determine a corrected first rotation rate of the inertial sensor about said axis based on the one or more inputs.

Description

[Technical field]

[0001] Technical Field The present disclosure relates generally to chip-scale inertial sensors and inertial measurement units. [Background technology]

[0002] background Rotation rate detection is increasingly being used to enhance device functionality, for example by accurately tracking the device's orientation. Vibration gyroscopes that detect rotation rate are used in many high-volume applications such as positioning, navigation, and rotation monitoring.

[0003] Vibration gyroscopes are very sensitive to rotation rate and therefore provide precise measurements. However, the long-term stability performance of these gyroscopes is limited by their sensitivity to vibration and linear acceleration, including gravity, also known as g-sensitivity. This increases the noise contribution in the gyroscope's output and reduces the accuracy of the output. Vibration gyroscopes can also suffer from a reduced sensing range.

[0004] It is an aim of embodiments of the present invention to at least alleviate one or more problems known in the art. Summary of the Invention [Means for solving the problem]

[0005] overview According to one aspect of the invention, a chip-scale inertial sensor is provided. The chip-scale inertial sensor is for detecting a rotation rate of the inertial sensor about an axis. The inertial sensor comprises an optical vibratory gyroscope for detecting a first rotation rate of the inertial sensor about said axis. The optical vibratory gyroscope is configured to output a primary signal corresponding to the first rotation rate. The inertial sensor further comprises an optical Sagnac gyroscope for simultaneously detecting a second rotation rate of the inertial sensor about said axis. The optical Sagnac gyroscope is configured to output an auxiliary signal corresponding to the second rotation rate. The inertial sensor further comprises a microcontroller configured to receive one or more inputs based on the primary signal and the auxiliary signal. The microcontroller is further configured to determine a corrected first rotation rate of the inertial sensor about said axis based on the one or more inputs.

[0006] Vibration gyroscopes are highly sensitive and mechanical gyroscopes, but are susceptible to undesirable motion such as vibration, acceleration, g-sensitivity, and cross-axis effects. Any acceleration will misalign the vibration gyroscope and distort the rotation rate measurements. Furthermore, the sensitivity to vibration and linear acceleration results in a large bias drift that increases the detected rotation error over time, resulting in inaccurate readings.

[0007] Conversely, current chip-scale optical Sagnac gyroscopes have low sensitivity due to their small size and limited by the sidewall roughness of the etch, which reduces the optical linewidth of the resonance. However, optical Sagnac gyroscopes are less susceptible to mechanical errors because they are stationary. This means that optical Sagnac gyroscopes are not sensitive to linear acceleration and do not suffer from long-term drift due to mechanical effects.

[0008] By providing an inertial sensor that includes both an optical vibratory gyroscope and an optical Sagnac gyroscope, the high sensitivity output of the optical vibratory gyroscope can be maintained and the stable output of the optical Sagnac gyroscope can be used to reduce errors in the output of the optical vibratory gyroscope. Thus, the stability of the optical Sagnac gyroscope can be combined with the sensitivity of the optical vibratory gyroscope to provide a highly sensitive hybrid gyroscope reading that remains accurate for a longer duration.

[0009] In particular, the output of the optical Sagnac gyroscope can be used to correct the output of the optical vibratory gyroscope to reduce errors in the output of the optical vibratory gyroscope. Such correction reduces the g-sensitivity and vibration effects present in the output of the optical vibratory gyroscope and improves the long-term stability performance of the optical vibratory gyroscope. Thus, the combination of the optical vibratory gyroscope with the optical Sagnac gyroscope allows for the provision of rotation rate measurements at the output of the inertial sensor that are highly sensitive, accurate and robust to errors due to effects in the optical vibratory gyroscope, such as vibration, shock, cross-axis effects and linear acceleration.

[0010] Another advantage of combining an optical vibratory gyroscope with an optical Sagnac gyroscope is that the optical vibratory gyroscope can have a low sensing range of about 300 degrees / second for closed-loop systems and 100 degrees / second for open-loop systems, while the optical Sagnac gyroscope can have a significantly larger sensing range of over 1000 degrees / second. The sensing bandwidth of the optical Sagnac gyroscope is limited by the round-trip time of the photons propagating in the optical cavity. Since this is very fast, the limit is set by the acquisition rate, which can be greater than kilohertz, or by the optimal averaging time (to reduce noise), which can set the bandwidth closer to 100 Hz. Thus, if the rotation rate of the inertial sensor exceeds the rotation rate detected by the optical vibratory gyroscope, the output of the optical Sagnac gyroscope can be used to detect the rotation rate instead. This increases the sensing range of the inertial sensor.

[0011] The use of an optical vibratory gyroscope rather than a purely electromechanical vibratory gyroscope allows for a significant reduction in noise and therefore an increase in the sensitivity of the sensor. Furthermore, implementing two optical gyroscopes allows both gyroscopes to share optical components. Both gyroscopes can also take advantage of the signal-to-noise amplification capabilities of optical resonance.

[0012] Although a purely electromechanical vibratory gyroscope can provide a greater sensing range than an optical vibratory gyroscope, for reasons discussed above, a combination of an optical vibratory gyroscope and an optical Sagnac gyroscope extends the sensing range of the inertial sensor beyond the sensing range achievable by an electromechanical vibratory gyroscope and counters any advantages offered by a purely electromechanical vibratory gyroscope.

[0013] Thus, the combination of features of the present invention provides a sensitive, accurate and compact inertial sensor with a large sensing range.

[0014] Both the optical vibration gyroscope and the optical Sagnac gyroscope detect the rotation rate of the inertial sensor about the same axis, therefore the gyroscopes will detect the same rotation rate of the inertial sensor and therefore any difference in the detected rotation rate will be due to limitations in one or both of the gyroscopes.

[0015] The microcontroller may receive the main signal and the auxiliary signal. The microcontroller may output an output signal from the inertial sensor corresponding to a corrected first rotational speed. The corrected first rotational speed may be, for example, a first rotational speed corrected to eliminate or at least reduce an error. Thus, the output signal may be a corrected main signal.

[0016] The optical vibratory gyroscope and the optical Sagnac gyroscope can receive light from the same light source.

[0017] Implementing two optical gyroscopes allows both gyroscopes to share a light source, which reduces the number of components required in the chip with the inertial sensor, and therefore the space and energy consumed. Furthermore, sharing the same light source allows the use of light with the same characteristics in both gyroscopes.

[0018] The light source may be a laser. The light received by the optical vibratory gyroscope and / or the optical Sagnac gyroscope may be broadband light. Alternatively, the light received by the optical vibratory gyroscope and / or the optical Sagnac gyroscope may be coherent single frequency light.

[0019] The chip-scale inertial sensor may further comprise a beam splitter or directional coupler for splitting the light from the light source for transmission to the optical vibratory gyroscope and the optical Sagnac gyroscope.

[0020] The chip-scale inertial sensor may further comprise a light source for transmitting light to the optical vibratory gyroscope and the optical Sagnac gyroscope.

[0021] An inertial sensor with a light source eliminates any coupling losses associated with connecting an external light source to the inertial sensor through a fiber. Furthermore, having the light source integrated within the sensor offers energy efficiency and energy proportionality.

[0022] The chip-scale inertial sensor may further include at least one detector for detecting light transmitted from the optical vibratory gyroscope and the optical Sagnac gyroscope, and the microcontroller receives one or more inputs from the at least one detector.

[0023] The inertial sensor may include a detector for detecting light transmitted from the optical vibratory gyroscope and a detector for detecting light transmitted from the optical Sagnac gyroscope. The primary signal may be the light transmitted from the optical vibratory gyroscope to the detector. The auxiliary signal may be the light transmitted from the optical Sagnac gyroscope to the detector.

[0024] Determining the corrected first rotational speed based on the one or more inputs may include comparing the first rotational speed to the second rotational speed and determining the corrected first rotational speed based on the comparison.

[0025] Comparing the first rotation rate to the second rotation rate allows for detection of any errors in the rotation being detected. A corrected first rotation rate can then be determined to reduce such errors. In particular, by correcting the first rotation rate rather than averaging the rotation, the sensitivity of the optical vibratory gyroscope is maintained while any errors in its output that are detected by the comparison are reduced.

[0026] Comparing the first rotation rate to the second rotation rate can include calculating a difference between the primary signal and the auxiliary signal to determine short-term and long-term errors of the optical vibratory gyroscope. Determining a corrected first rotation rate based on the comparison can include correcting the primary signal to remove the short-term and long-term errors.

[0027] Because the optical Sagnac gyroscope has low sensitivity to sources of short-term and long-term errors, such as linear acceleration, a comparison of the main signal output by the optical Sagnac gyroscope with the auxiliary signal output by the optical Sagnac gyroscope reveals the short-term and long-term errors of the optical vibratory gyroscope. These errors can then be removed from the main signal. Thus, such calibration using the optical Sagnac gyroscope improves the short-term and long-term stability performance of the optical vibratory gyroscope while maintaining its sensitivity.

[0028] The errors may be due to linear acceleration and / or long-term drift of the inertial sensor. Errors may be due to vibrations and / or other sources of mechanically induced drift.

[0029] Errors may be due to factors that affect the optical vibratory gyroscope but not the optical Sagnac gyroscope, and as a result, by comparing the outputs of the gyroscopes, such errors can be identified, thus allowing for their removal and improving the accuracy of the inertial sensor output.

[0030] Comparing the first rotation speed to the second rotation speed can include determining whether the primary signal is saturated. If the primary signal is saturated, the corrected first rotation speed can be determined using only the auxiliary signal.

[0031] The optical vibratory gyroscope may have a low sensing range of 300 degrees / second. Thus, the primary signal output from the optical vibratory gyroscope may become saturated as the rotation rate exceeds the sensing range of the optical vibratory gyroscope. However, the optical Sagnac gyroscope may have a significantly larger sensing range of over 1000 degrees / second, so the auxiliary signal is less likely to become saturated and is more likely to be able to accurately detect large rotation rates. Thus, if the primary signal is saturated, the auxiliary signal output by the optical Sagnac gyroscope may be used to provide a corrected rotation rate. In such a case, the primary signal is not used because it is saturated.

[0032] Thus, when the rotation rate of the inertial sensor is within the sensing range of the optical vibratory gyroscope, the corrected rotation rate may be a corrected version of the first rotation rate detected by the optical vibratory gyroscope, which is highly sensitive. When the rotation rate of the inertial sensor exceeds the sensing range of the optical vibratory gyroscope, the corrected rotation rate can be based on the second rotation rate being detected by the optical Sagnac gyroscope, so that it can still be detected by the inertial sensor. By allowing the corrected rotation rate to be based on the auxiliary signal output by the optical Sagnac gyroscope when the main signal is saturated, the sensing range of the sensor is increased. This provides an inertial sensor with a higher sensing range. Furthermore, this eliminates the common trade-off between sensitivity and sensing range, providing a highly sensitive inertial sensor with a high sensing range.

[0033] The optical vibratory gyroscope and the optical Sagnac gyroscope may be fabricated on the same chip. This provides a compact inertial sensor. Furthermore, this reduces the possibility of errors during calibration, since the relative motion between the gyroscopes is less likely, and therefore the gyroscopes are more likely to be rotated by the same amount. This allows errors in the output of the optical vibratory gyroscope due to long-term and short-term effects to be accurately determined and removed.

[0034] The microcontroller may be further configured to output a feedback signal to actuate a proof mass of the optical vibratory gyroscope to recalibrate the optical vibratory gyroscope based on the comparison.

[0035] For example, if the same error is present in each output of the optical vibratory gyroscope, it can be determined that this is due to the proof mass of the optical vibratory gyroscope being in a different location than during the initial calibration. Recalibration then actuates the proof mass to return it to its position during the initial calibration in order to reduce the error present in the output of the optical vibratory gyroscope. The required recalibration can be determined from the error in the output of the optical vibratory gyroscope. Recalibration therefore reduces the error in the output of the optical vibratory gyroscope, thus reducing the correction of the output required by the microcontroller, increasing the speed of the inertial sensor and reducing the possibility of error in the output of the inertial sensor.

[0036] The feedback signal can be output to electrodes of an optical vibratory gyroscope to electrostatically actuate a proof mass.

[0037] An optical Sagnac gyroscope may comprise a coiled or spiral waveguide structure for propagating light in a clockwise and counterclockwise direction, and rotation of the sensor causes a change in the optical path length of the light propagating in the clockwise and counterclockwise directions.

[0038] The coiled or spiral waveguide structure thus provides a compact optical Sagnac gyroscope that can be integrated on the same chip as an optical vibratory gyroscope.

[0039] The rotation rate of the inertial sensor can be detected by an optical Sagnac gyroscope based on the change in optical path length of light propagating in clockwise and counterclockwise directions.

[0040] The rotation rate of the inertial sensor can be detected from the change in the optical path length of the clockwise and counterclockwise propagating light by detecting a change in the interference of the clockwise and counterclockwise propagating light, or by detecting a difference in the resonant frequencies of the clockwise and counterclockwise propagating light caused by different shifts in the resonant frequencies of the clockwise and counterclockwise propagating light. For example, rotation of the inertial sensor shifts the resonant frequency of the clockwise propagating light in a first direction and shifts the resonant frequency of the counterclockwise propagating light in a second direction opposite the first direction, resulting in a difference in the resonant frequencies of the clockwise and counterclockwise propagating light.

[0041] The coiled or helical waveguide structure may be in the form of one of a coiled optical fiber, a coiled waveguide, a helical waveguide, and a microdisk.

[0042] The diameter of the coiled or spiral waveguide structure may be on the order of millimeters or centimeters. For example, the diameter of the coiled or spiral waveguide may be about 2 cm. The small diameter thus provides a compact optical Sagnac gyroscope that can be integrated on the same chip as an optical vibratory gyroscope.

[0043] The microcontroller can control the frequency of the light propagating clockwise and counterclockwise based on the auxiliary signal output by the optical Sagnac gyroscope so as to reduce the frequency change of the light due to drift. Thus, feedback can be used to maintain the correct output of the optical Sagnac gyroscope. By maintaining the correct output of the optical Sagnac gyroscope, when comparing it with the output from the optical vibratory gyroscope, it is more likely to be in the output from the optical vibratory gyroscope. This ensures more accurate correction of the error and increases the accuracy of the corrected first rotation rate of the inertial sensor.

[0044] The optical Sagnac gyroscope may further comprise a phase shifter to shift the light to propagate 90 degrees in either a clockwise or counterclockwise direction before propagating within the coiled or spiral waveguide structure such that there is a 90 degree phase shift between the light propagating in a clockwise direction and the light propagating in a counterclockwise direction.

[0045] The optical Sagnac gyroscope and the optical vibratory gyroscope may be made from silicon or silicon nitride, which simplifies the fabrication process, especially if the gyroscopes are fabricated on the same chip, thus facilitating the integration of the gyroscopes on the same chip.

[0046] The optical vibratory gyroscope may comprise one or more microresonators, each microresonator supporting a corresponding optical resonance. The optical vibratory gyroscope may further comprise a microelectromechanical inertial proof mass suspended adjacent to and discontinuously from the one or more microresonators, the proof mass being deflectable under application of an inertial force. The optical vibratory gyroscope may further comprise one or more optical couplers for coupling light being transmitted to the optical vibratory gyroscope into and out of the corresponding microresonators. The optical vibratory gyroscope may further comprise one or more detectors for detecting light received from the one or more microresonators by the one or more optical couplers. A change in the spacing between the proof mass and at least one microresonator may cause a change in the optical resonance characteristics of that microresonator.

[0047] The use of optical systems to sense the rotation rate provides a highly sensitive measurement. The use of resonant light in an opto-mechanical readout mechanism provides even greater sensitivity because the motion of the proof mass shifts the resonance conditions, amplifying the signal rather than the noise.

[0048] The optical vibratory gyroscope may further comprise one or more electrodes for counteracting deflection of the proof mass with an electrostatic force. The microcontroller may be further configured to control the electrostatic force of the one or more electrodes to vibrate the proof mass in a first direction at a fixed frequency such that the optical vibratory gyroscope may detect a first rotational rate of the inertial sensor about said axis. The microcontroller may be further configured to receive electrical signals from the one or more detectors. The microcontroller may be further configured to detect a change in an optical resonance characteristic of the one or more microresonators in response to a change in spacing between the proof mass and the one or more microresonators in a second direction perpendicular to the first direction at the fixed frequency. The microcontroller may be further configured to determine a first rotational rate of the inertial sensor about said axis based on the change in the optical resonance characteristic of the one or more microresonators.

[0049] This configuration of the optical vibratory gyroscope offers the following advantages: The combination of a cavity-enhanced opto-mechanical readout mechanism with electrostatic actuation of the proof masses offers significant advantages over purely electromechanical gyroscopes (where both readout and actuation are performed using electrodes) and purely optical gyroscopes (where both readout and actuation are controlled by optical means). In particular, the use of voltages only for actuation of the proof masses, and not for sensing, allows for a significant reduction in noise and therefore an increase in the sensitivity of the gyroscope. The use of voltages for actuation also allows for the use of large proof masses without the need for high optical power, since voltages can be effectively and efficiently used for actuation, which is not possible when using optical means for actuation. Furthermore, the use of resonant light for the opto-mechanical readout mechanism provides even greater sensitivity, since the motion of the proof masses shifts the resonance conditions, amplifying the signal rather than the noise.

[0050] The combination of optical-mechanical sensing of the displacement of a proof mass (using one or more microresonators and one or more optical couplers) combined with electrostatic actuation of the proof mass (using one or more electrodes) in an optical vibratory gyroscope provides a hybrid optical-electro-mechanical gyroscope with improved sensitivity and improved signal-to-noise ratio compared to gyroscopes based on capacitive sensing, without compromising the size of the proof mass. Such a gyroscope retains the ability to actuate large proof masses for large mechanical response to inertial forces.

[0051] The hybrid nature of the optical vibratory gyroscope provides both improved sensitivity and improved response, which is not possible with all optical gyroscopes. Such improved response is important to effectively adjust or calibrate the gyroscope sensitivity. In fact, by providing sufficient actuation of the proof mass, a closed loop operation can be effectively implemented so that drift, e.g., thermally induced drift, or nonlinear response of the proof mass can be better controlled. This makes the gyroscope less susceptible to positioning errors. Furthermore, if a positioning error does occur, the inertial sensor described herein can correct such errors using the output of the optical Sagnac gyroscope. Thus, an inertial sensor comprising such an optical vibratory gyroscope reduces the likelihood of errors and corrects errors when they occur, providing a high accuracy sensor.

[0052] The combination of the optical readout mechanism and closed loop operation of the optical vibratory gyroscope with the optical Sagnac gyroscope allows for the best balance of sensitivity, control, and long term stability to provide a highly sensitive and stable inertial sensor that can be precisely controlled to achieve the required measurements.

[0053] The microcontroller can be further configured to control the electrostatic force of one or more electrodes to recalibrate the optical vibratory gyroscope based on the comparison. Thus, if the proof mass drifts over time, this can be detected in the output of the optical vibratory gyroscope using the output of the optical Sagnac gyroscope and can be corrected using the electrodes of the optical vibratory gyroscope so that the output of the optical vibratory gyroscope no longer outputs such errors or outputs smaller errors. Thus, the long-term and short-term errors of the optical vibratory gyroscope can be corrected not only in the output but also within the optical vibratory gyroscope using the output of the optical Sagnac gyroscope. Reducing the errors of the optical vibratory gyroscope by actively regulating its mechanical response reduces the possibility of errors that need to be computationally corrected while minimizing the increase in unregulated mechanical response. This improves the accuracy of the output of the inertial sensor and increases the sensing speed.

[0054] According to another aspect of the invention, an inertial measurement unit is provided. The inertial measurement unit comprises a plurality of chip-scale inertial sensors for detecting a rotation rate about a respective axis. Each inertial sensor comprises an optical vibratory gyroscope for detecting a first rotation rate of the inertial sensor about said axis. The optical vibratory gyroscope is configured to output a primary signal corresponding to the first rotation rate. Each inertial sensor further comprises an optical Sagnac gyroscope for simultaneously detecting a second rotation rate of the inertial sensor about said axis. The optical Sagnac gyroscope is configured to output an auxiliary signal corresponding to the second rotation rate. The inertial measurement unit further comprises a controller configured to receive, for each inertial sensor, one or more inputs based on the primary and auxiliary signals of the inertial sensor. The controller is further configured to determine, for each inertial sensor, a corrected first rotation rate of the inertial sensor about the respective axis based on the one or more inputs.

[0055] By providing a controller separate from the inertial sensors, the controller can be shared across all inertial sensors, reducing the circuitry required for each inertial sensor and the circuitry of the inertial measurement unit, resulting in reduced power and space for the inertial measurement unit.

[0056] The inertial measurement unit may comprise a first chip-scale inertial sensor for detecting a rotational rate of the first inertial sensor in a first axis, a second chip-scale inertial sensor for detecting a rotational rate of the second inertial sensor in a second axis perpendicular to the first axis, and a third chip-scale inertial sensor for detecting a rotational rate of the third inertial sensor in a third axis perpendicular to the first and second axes. The controller may be further configured to determine a total rotational rate of the inertial measurement unit based on the corrected first rotational rates determined for each inertial sensor.

[0057] By detecting the rotational speed of all three axes, it is possible to detect any rotation of the inertial measurement unit, which is useful for monitoring the precise position and movement of the inertial measurement unit.

[0058] According to another aspect of the invention there is provided a chip comprising a chip scale inertial sensor as described herein or an inertial measurement unit as described herein.

[0059] According to another aspect of the invention, there is provided a method implemented by a microcontroller for determining a rotation rate of an inertial sensor about an axis. The method includes receiving one or more inputs based on a primary signal output from an optical vibratory gyroscope for detecting a first rotation rate of the inertial sensor about the axis and an auxiliary signal output from an optical Sagnac gyroscope for simultaneously detecting a second rotation rate of the inertial sensor about the axis. The method further includes determining a corrected first rotation rate of the inertial sensor about the axis based on the one or more inputs.

[0060] According to another aspect of the present invention, there is provided a computer-readable medium storing instructions that, when executed by one or more processors, cause the one or more processors to perform a method as described herein.

[0061] The proof mass of the optical vibratory gyroscope can have an average thickness of greater than 1 micron.

[0062] The proof masses of optical vibratory gyroscopes can have an average thickness on the order of tens or hundreds of microns.

[0063] The test mass of the optical vibratory gyroscope can be significantly larger than the microresonator. The test mass can have an average thickness of 10 microns or more. The test mass can have an average thickness of less than 500 microns. The test mass can have an average thickness of 20 to 30 microns. The test mass can be significantly thicker than the microresonator, for example 100 times thicker. The test mass can have a larger surface area than the microresonator. The surface area of ​​the test mass can be less than 1 millimeter by 1 millimeter. The surface area of ​​the test mass can be about 250 microns by 250 microns. The diameter of the microresonator can be about 100 microns. Increasing the size of the test mass provides an improved response to rotational rate of the optical vibratory gyroscope.

[0064] The optical vibratory gyroscope can include at least two microresonators, at least two optical couplers, and at least two detectors, wherein a change in a first spacing between the proof mass and a first of the two microresonators and a change in a second spacing between the proof mass and a second of the two microresonators can cause a differential change in the optical resonance characteristics of the two microresonators.

[0065] The proof mass of the optical vibratory gyroscope may be suspended between the first microresonator and the second microresonator.

[0066] The proof mass of the optical vibratory gyroscope can include a protrusion, the protrusion being located between the first microresonator and the second microresonator. The protrusion can act as an optical channel to guide photons that leak from the first microresonator and / or the second microresonator, thereby enhancing the sensitivity of the gyroscope as the photons are absorbed or transmitted by the protrusion.

[0067] The proof mass of the optical vibratory gyroscope can further include one or more additional protrusions, each of the one or more additional protrusions being located between the two microresonators.

[0068] At least two of the electrodes of the optical vibratory gyroscope may each include fingers that are stationary relative to the gyroscope, and the proof mass may include fingers that are movable relative to the gyroscope, the movable fingers of the proof mass being positioned between the stationary fingers of the at least two electrodes such that the fingers of the proof mass and the at least two electrodes interdigitate with each other.

[0069] The motion of the fingers is strongly coupled to the motion of the proof mass such that the motion of the fingers results in the motion of the proof mass as a whole. Thus, the stationary fingers of the electrodes are used to control the motion of the fingers of the proof mass and, consequently, the motion of the proof mass. The fingers may be integral to the proof mass or rigidly fixed to the proof mass so as to be stationary relative to the proof mass. Alternatively, the fingers of the proof mass may have their own mechanical degree(s) of freedom, e.g., in a cantilever mode. However, the fingers of the proof mass may be arranged to be used only to move or maintain the position of the entire proof mass.

[0070] One or more microresonators of the optical vibratory gyroscope may be radially separated from the proof mass.

[0071] One or more electrodes and / or one or more microresonators of the optical vibratory gyroscope may be fixed relative to the gyroscope.

[0072] One or more microresonators of the optical vibratory gyroscope may be whispering gallery mode resonators.

[0073] The one or more microresonators of the optical vibratory gyroscope can each have an evanescent field that extends beyond the edge of the microresonator when in use. The amount that the evanescent field extends beyond the edge of the microresonator can be based on the size of the wavelength of the light coupled into the optical coupler. The evanescent field can extend at least 1 micron beyond the edge of the microresonator when in use. This provides a particularly effective gyroscope when the wavelength of the light coupled into the optical coupler is 1550 nm.

[0074] The proof mass of the optical vibratory gyroscope can be larger than each of the one or more microresonators.

[0075] The change in the optical resonance characteristics of the optical vibratory gyroscope can be a shift in the optical resonance and / or a broadening of the optical resonance.

[0076] The one or more microresonators of the optical vibratory gyroscope can each have a different optical resonance.

[0077] In light of the teachings presented herein, many modifications and other embodiments of the inventions described herein will occur to those skilled in the art to which these inventions pertain. It will therefore be understood that the disclosure herein is not limited to the specific embodiments disclosed herein. Moreover, while the descriptions provided herein provide example embodiments in the context of specific combinations of elements, steps and / or functions may be provided by alternative embodiments without departing from the scope of the invention.

[0078] BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0079] [Figure 1] 1 is a schematic diagram of an exemplary inertial sensor; [Diagram 2] 1 is a schematic diagram of an exemplary inertial sensor; [Diagram 3] 1 illustrates the operation of an optical Sagnac gyroscope and an example output of the optical Sagnac gyroscope. [Figure 4] FIG. 1 illustrates an exemplary optical Sagnac gyroscope. [Diagram 5] FIG. 1 is a diagram of an example optical vibratory gyroscope. [Figure 6] 6 is a diagram of an opto-mechanical element of the optical vibratory gyroscope of FIG. 5 and an example output of the optical vibratory gyroscope. [Figure 7] 1 is a diagram of an example optical vibratory gyroscope and an example output of the optical vibratory gyroscope before and during rotation. [Figure 8] FIG. 1 is a schematic diagram of an example inertial sensor implementing a closed-loop feedback system. [Figure 9] FIG. 2 is a diagram of a gyroscope of an inertial sensor according to an example. [Figure 10] FIG. 1 illustrates an exemplary closed loop feedback system. [Figure 11] FIG. 2 shows examples of errors in the output of an optical vibratory gyroscope and an optical Sagnac gyroscope. [Figure 12] 1 illustrates exemplary outputs of an optical vibratory gyroscope, an optical Sagnac gyroscope, and an inertial sensor. [Figure 13] FIG. 1 is a block diagram of an example inertial measurement unit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0080] Like reference characters refer to like parts throughout the specification and drawings. Detailed Description Various embodiments are described below, but the present invention is not limited to these embodiments, and variations of these embodiments may be well within the scope of the present invention, which should be limited only by the claims.

[0081] Below, a chip-scale inertial sensor for detecting rotational rate is described. Although exemplary optical Sagnac gyroscopes and optical vibratory gyroscopes are provided, the inertial sensors described herein may be implemented with other optical Sagnac gyroscopes and optical vibratory gyroscopes as would be understood by one of ordinary skill in the art.

[0082] As will be understood from reading the detailed description, a chip-scale inertial sensor is an inertial sensor that is small enough to fit on a chip. The size of the chip can vary, but its area is typically less than 1 mm 2 ~600mm 2 Thus, in some instances, the chip scale is 600 mm 2 It may be smaller. A chip is also known as an integrated circuit (IC). Throughout this specification, inertial sensor and chip-scale inertial sensor are used interchangeably to mean chip-scale inertial sensor.

[0083] As will be understood from reading the detailed description, the structure within the optical Sagnac gyroscope that enables detection of rotation rate may be any Sagnac structure. A coiled or spiral waveguide is one example of a Sagnac structure.

[0084] As will be understood from reading the detailed description, a signal being saturated means that it has reached and optionally exceeded its upper detection limit. For example, for measuring a rotation rate, a main signal (as output by an optical vibratory gyroscope) being saturated means that the rotation rate of the inertial sensor has reached or exceeded the detection limit of the optical vibratory gyroscope. Saturation can occur in the main signal as the maximum possible signal that the optical vibratory gyroscope can output. Saturation results in a main signal that no longer corresponds to the actual rotation rate of the inertial sensor. This is because any rotation rate above the detection limit will provide the same main signal. An exemplary detection limit of an optical vibratory gyroscope is 300 degrees / second.

[0085] As will be understood from reading the detailed description, a microresonator is a closed circuit object that supports optical resonance. By a microresonator that supports optical resonance, it is meant that light entering the closed circuit of the microresonator is amplified at at least one resonant frequency by constructive interference and total internal reflection within the microresonator. Exemplary materials for microresonators include silicon, silica, silicon nitride, and crystalline fluorides. Exemplary diameters for microresonators range from microns to hundreds of millimeters.

[0086] As will be understood from reading the detailed description, proof mass refers to a mechanical structure for use in a MEMS sensor. Exemplary materials for the proof mass are silicon and quartz.

[0087] As will be understood from the detailed description, an optical coupler is a means for coupling light into and out of a microresonator. An optical coupler may be a waveguide. An optical coupler may be attached to a waveguide. An optical coupler may guide light from a light source into the microresonator. An optical coupler may guide light from the microresonator to a detector. An optical coupler may be a buried waveguide, a ridge waveguide, or a rib waveguide. An optical coupler may be fabricated simultaneously with the microresonator.

[0088] As will be understood from reading the detailed description, the change in the optical resonance characteristic is any change in the characteristics of the optical resonance. The change in the optical resonance characteristic may be a change in the line shape of the optical resonance characteristic, e.g., a decrease in peak amplitude. The change in the optical resonance characteristic may be a change in the location of the optical resonance.

[0089] FIG. 1 shows a schematic diagram of an exemplary inertial sensor 100. The inertial sensor 100 is for detecting a rotation rate of the inertial sensor about an axis. The inertial sensor 100 comprises an optical vibration gyroscope 104 for detecting a first rotation rate of the inertial sensor 100 about said axis. The optical vibration gyroscope 104 is configured to output a primary signal corresponding to the first rotation rate. The inertial sensor 100 further comprises an optical Sagnac gyroscope 106 for simultaneously detecting a second rotation rate of the inertial sensor 100 about said axis. The optical Sagnac gyroscope 106 is configured to output an auxiliary signal corresponding to the second rotation rate. The inertial sensor 100 further comprises a microcontroller 108 configured to receive one or more inputs based on the primary signal and the auxiliary signal. The microcontroller 108 is further configured to determine a compensated first rotation rate of the inertial sensor about said axis based on the one or more inputs. The microcontroller 108 may output the corrected first rotational rate from the inertial sensor 100 (not shown).

[0090] The optical vibration gyroscope 104 and the optical Sagnac gyroscope 106 can be fabricated on the same chip to create a hybrid chip. In fact, the optical vibration gyroscope 104 and the optical Sagnac gyroscope 106 may be fabricated on a common substrate. The optical vibration gyroscope 104 and the optical Sagnac gyroscope 106 are fixed in the inertial sensor 100 such that when the inertial sensor 100 is rotated, the inertial sensor 100, the optical vibration gyroscope 104 and the optical Sagnac gyroscope 106 rotate at the same rotational speed. Thus, detecting the rotational speed of one of the gyroscopes is the same as detecting the rotational speed of the inertial sensor 100.

[0091] The optical vibration gyroscope 104 and the optical Sagnac gyroscope 106 can independently detect the rotation rate of the inertial sensor 100, as described in more detail below. The microcontroller 108 can receive one or more inputs based on the primary signal output by the optical vibration gyroscope and the auxiliary signal output by the optical Sagnac gyroscope. For example, the microcontroller 108 can receive the primary signal and the auxiliary signal. The microcontroller 108 can then compare one or more inputs based on the output of the optical vibration gyroscope 104 and the optical Sagnac gyroscope 106 to determine the true rotation rate of the inertial sensor 100. This is because the auxiliary signal output by the optical Sagnac gyroscope 106 does not include false readings from vibrations and linear acceleration.

[0092] For example, when comparing the primary and auxiliary signals, if the microcontroller finds that the primary signal output from the optical vibratory gyroscope 104 corresponds to a greater rotation rate than the auxiliary signal output from the optical Sagnac gyroscope 106, the microcontroller can determine that the difference in rotation rate is due to an error in the optical vibratory gyroscope 104 and can therefore remove the error in the primary signal to provide a corrected primary signal at the output of the inertial sensor 100. The microcontroller 108 may calculate the error as the difference between the primary and auxiliary signals or the difference between the first and second rotation rates being detected. The microcontroller 108 may calculate the error based on multiple primary and auxiliary signals. For example, the microcontroller 108 can average the difference between the first and second rotation rates being detected to determine the corrected first rotation rate.

[0093] In another example, if, when comparing the signals, the microcontroller finds that the primary signal output from the optical vibratory gyroscope 104 corresponds to a much smaller rotation rate than the auxiliary signal output from the optical Sagnac gyroscope 106 and the first rotation rate being detected by the optical vibratory gyroscope 104 is the maximum rotation rate detectable by the gyroscope 104, the microcontroller can determine that the optical vibratory gyroscope 104 is saturated and therefore can output an auxiliary signal from the inertial sensor 100.

[0094] In addition to correcting errors in the output of the optical vibration gyroscope 104 to provide a corrected first rotation rate after comparing the outputs from the gyroscopes, the microcontroller 108 can provide a feedback signal to the optical vibration gyroscope 104 to reduce drift in the optical vibration gyroscope 104 and, therefore, reduce errors in the output of the optical vibration gyroscope 104. The microcontroller 108 can also implement a feedback mechanism to reduce drift in the optical vibration gyroscope 104 based on information that does not originate from the Sagnac gyroscope.

[0095] FIG. 2 shows a schematic diagram of an exemplary inertial sensor 200. The inertial sensor 200 is an example of the inertial sensor 100 of FIG. 1. The inertial sensor 200 comprises an optical vibration gyroscope 204, which is an example of the optical vibration gyroscope 104 of FIG. 1, an optical Sagnac gyroscope 206, which is an example of the optical Sagnac gyroscope 106 of FIG. 1, and a microcontroller 208, which is an example of the microcontroller 108 of FIG. 1. The inertial sensor 200 further comprises a light source 202 and two light detectors 210, 212. Exemplary internal components of the optical vibration gyroscope 204 and the optical Sagnac gyroscope 206 are shown in FIG. 2 to illustrate example connections between the gyroscopes, the light source 202, and the detectors 210, 212. These internal components are described in more detail below.

[0096] The light source 202 is for transmitting light to the optical vibratory gyroscope 204 and the optical Sagnac gyroscope 206. The same light source 202 is used to transmit light to both gyroscopes. The light source is split into two parts by a beam splitter 214 or directional coupler, with the first part being transmitted to the optical vibratory gyroscope 204 and the second part being transmitted to the optical Sagnac gyroscope 206. Since the change in resonance can be used to detect the rotation rate by both gyroscopes, the light output from the light source can be scanned across the optical resonance to enable detection of the rotation rate by both gyroscopes. Alternatively, a fixed wavelength of light can be used and the change in the intensity of the light in each gyroscope due to rotation can be detected.

[0097] The microcontroller 108 can control the light source 202 to stabilize the light source, for example. The light source 202 can be a laser. The microcontroller 108 can use laser stabilization techniques to stabilize the laser. Such laser stabilization techniques improve the stability of the output of the inertial sensor 200 by providing low noise and low drift of the output intensity and frequency. In other examples, the light source 102 can be external to the inertial sensor 200.

[0098] Photodetector 210 receives light from optical vibration gyroscope 204, and photodetector 212 receives light from optical Sagnac gyroscope 206. The photodetectors then output signals to microcontroller 208. Microcontroller 208 then determines a corrected first rotation rate of inertial sensor 200 about the axis based on these signals. Microcontroller 208 can then provide the corrected first rotation rate as an output from inertial sensor 200. The microcontroller can also control optical vibration gyroscope 204 and optical Sagnac gyroscope 206 to ensure they are functioning properly and to reduce errors, as described further below.

[0099] In one example, the light received by photodetector 210 from optical vibration gyroscope 204 is the primary signal, and the light received by photodetector 212 from optical Sagnac gyroscope 206 is the auxiliary signal. In this example, the signals output from photodetectors 210, 212 to microcontroller 208 are one or more inputs. In another example, photodetector 210 may be incorporated into optical vibration gyroscope 204, and photodetector 212 may be incorporated into optical Sagnac gyroscope 206. In this example, the primary signal output by optical vibration gyroscope 204 may be the signal output by photodetector 210 incorporated into optical vibration gyroscope 204, and the auxiliary signal output by optical Sagnac gyroscope 206 may be the signal output by photodetector 212 incorporated into optical Sagnac gyroscope 206. In this example, the one or more inputs are the primary signal and the auxiliary signal.

[0100] FIG. 3 illustrates the operation of an optical Sagnac gyroscope and an example output of the optical Sagnac gyroscope. The optical Sagnac gyroscope described in connection with FIG. 3 is an example of the optical Sagnac gyroscope 206 of FIG. 2 and the optical Sagnac gyroscope 106 of FIG. 1. The optical Sagnac gyroscope may be a fiber optic gyroscope. For ease of understanding, the X, Y, and Z axes are shown in schematic diagram A of FIG. 3 and apply to both schematic diagrams A and B. However, the directions X, Y, and Z are provided for understanding, and although a description of a gyroscope with rotation about the Z axis is provided below, it should be understood that the optical Sagnac gyroscope is not limited to being utilized only to measure rotation rates about the Z axis. Additionally, although a description of the gyroscope when the rotation is clockwise is provided below, the optical Sagnac gyroscope can measure counterclockwise rotation as well.

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[0102] Diagram A300 shows the optical Sagnac gyroscope when stationary. The axes of diagram A300 show that the optical Sagnac gyroscope is in the XY plane, with the Z axis passing substantially through the center of the optical Sagnac gyroscope, so that rotation about the Z axis is in the direction that light is shown propagating clockwise (CW) 302 or counterclockwise (CCW) 304 in both diagrams A and B. As shown in diagram A300, light propagating clockwise (CW) 302 and counterclockwise (CCW) 304 have the same optical path length when the optical Sagnac gyroscope is stationary.

[0103] Diagram B 310 illustrates an optical Sagnac gyroscope as it rotates about the Z-axis in a clockwise direction. As shown in diagram B, the optical path length of light propagating in a clockwise direction 302 has been increased from when the gyroscope was stationary, and the optical path length of light propagating in a counterclockwise direction 304 has been decreased from when the gyroscope was stationary. Thus, in this example, because the optical Sagnac gyroscope is rotating in a clockwise direction about the Z-axis, the light propagating in a clockwise direction 302 has a greater path length than the light propagating in a counterclockwise direction 304. The size of the difference between the path lengths is based on the rate of rotation. Alternatively, if the gyroscope is rotating in a counterclockwise direction about the Z-axis, the light propagating in a clockwise direction 302 has a smaller path length than the light propagating in a counterclockwise direction 304.

[0104] One way to detect the rotation rate about the Z-axis from the change in the optical path length of the clockwise and counterclockwise propagating light is to detect the phase difference between the clockwise and counterclockwise propagating light. This difference arises due to the different shifts in the resonant frequencies of the clockwise and counterclockwise propagating light. This can be detected by scanning a laser transmitting light to an optical Sagnac gyroscope across the resonant frequencies and detecting the resulting frequency difference in the resonant frequencies with one or more photodetectors. Graph C320 shows the shift in the resonant frequencies of the clockwise and counterclockwise propagating light. The clockwise and counterclockwise propagating light inherently have a resonant curve 326. When the gyroscope is rotated clockwise about the Z-axis, the resonant frequency of the clockwise propagating light shifts to a higher wavelength as shown by its new resonant curve 322. Additionally, the resonant frequency of the counterclockwise propagating light shifts to a lower wavelength as shown by its new resonant curve 324. Alternatively, if the gyroscope is rotating in a counterclockwise direction about the Z-axis, the resonant frequency of the light propagating clockwise will shift to a lower wavelength and the resonant frequency of the light propagating counterclockwise will shift to a higher wavelength. Thus, the amount and direction of the shift in the resonant frequency of the light propagating both clockwise and counterclockwise corresponds to the rotation rate about the Z-axis. Thus, the resonant frequency of the light propagating both clockwise and counterclockwise can be detected to determine the rotation rate of the optical Sagnac gyroscope.

[0105] Another way to detect the rotation rate about the Z-axis from the change in the optical path length of the clockwise and counterclockwise propagating light is to detect the change in the interference of light. Graph D330 shows an interference pattern of counter-propagating light fields, known as an interference reading, that can be obtained by a photodetector that simultaneously receives both the counterclockwise and counterclockwise propagating light. The optical modes of the clockwise and counterclockwise propagating light interfere with each other and generate a cosine-shaped response 334 around a rotation rate of 0 degrees. To help measure such interference patterns, the optical Sagnac gyroscope can further comprise a phase shifter in front of the Sagnac structure to shift either the clockwise or counterclockwise propagating light by 90 degrees. By shifting either the clockwise or counterclockwise propagating light by 90 degrees, the interference pattern has a sine-shaped response around 0 degrees / sec instead of a cosine-shaped response.

[0106] Graph D 330 shows an interference read pattern with no phase shift 334 providing a cosine shaped response, and with a 90 degree phase shift 332 providing a sine shaped response. As shown in the graph, when there is a 90 degree phase shift, there is a linear readout just before and just after a rotation speed of 0 degrees. This linear readout at about a rotation speed of 0 degrees facilitates detection of motion of the interference pattern, allowing motion of the interference pattern in either direction corresponding to the rotation speed about the Z axis. Furthermore, the sine shaped response is asymmetric, meaning that the response for rotation speeds above 0 is different from the response for rotation speeds below 0, resulting in the ability to distinguish between negative and positive sign rotations when detected.

[0107] The detector can detect the difference in the light, e.g., interference pattern or resonance, from the optical Sagnac gyroscope and send a signal to the microcontroller based on the detection. Based on the signal, the microcontroller can provide feedback to the optical Sagnac gyroscope and / or the light source. In particular, the microcontroller can control the frequency of the light propagating clockwise and counterclockwise based on the auxiliary signal output by the optical Sagnac gyroscope to reduce frequency changes of the light due to drift. Thus, the output of the optical Sagnac gyroscope can also be corrected.

[0108] The optical Sagnac gyroscope may comprise any Sagnac structure capable of propagating light in a clockwise and counterclockwise direction. The optical Sagnac gyroscope may comprise a coiled or spiral waveguide structure for propagating light in a clockwise and counterclockwise direction. For example, the optical Sagnac gyroscope may be a coiled piece of optical fiber, a spiral or coil made directly on the chip from a rectangular waveguide, or an on-chip bulk microdisk structure. In the case of a microdisk structure, the Sagnac gyroscope may further comprise a waveguide that provides evanescent coupling to the microdisk. These Sagnac structures are chip-scale and therefore can be integrated on the same chip as the optical vibration gyroscope. In particular, the diameter of the coiled or spiral waveguide structure may be on the order of millimeters or centimeters. FIG. 4 shows an exemplary optical Sagnac gyroscope. These optical Sagnac gyroscopes are examples of the optical Sagnac gyroscope 106 of FIG. 1 or the optical Sagnac gyroscope 206 of FIG. 2 and may operate as described in connection with FIG. 3. All these gyroscopes have different Sagnac structures. Image A is a planar spiral gyroscope 400 with a waveguide structure etched from a single layer. Image B is a microresonator gyroscope 402 in the shape of a toroid, although this type of gyroscope can also be disk-shaped. Image C is a coiled fiber gyroscope 404, which is an optical fiber wound in multiple layers.

[0109] The optical Sagnac gyroscope has no moving parts and therefore has high stability and a wide sensing range. Over time, the drift in accuracy is very small, so the optical Sagnac gyroscope can be used for long periods of time without significant long-term drift and can therefore be used to reset the errors of the vibrating gyroscope.

[0110] FIG. 5 shows a schematic diagram of an exemplary optical vibration gyroscope 500. The optical vibration gyroscope 500 is also known as a micro-optical electromechanical system (MOEMS) vibration gyroscope. Although FIG. 5 shows XY axes for understanding, the optical vibration gyroscope 500 is not limited to motion in these axes. The optical vibration gyroscope 500 is an example of the optical vibration gyroscope 204 of FIG. 2 and the optical vibration gyroscope 104 of FIG. 1.

[0111] The optical vibratory gyroscope 500 comprises a microresonator 502 supporting optical resonance. The optical vibratory gyroscope 500 further comprises a microelectromechanical inertial proof mass 504 adjacent to and discontinuously suspended from the microresonator 502. That is, the proof mass 504 is arranged within the optical vibratory gyroscope 500 so as to be close to the microresonator 502 but spaced a distance / spacing "d" (labeled as 512 in FIG. 5) from the microresonator 502. The proof mass 504 is deflectable under application of an inertial force. That is, under the influence of an inertial force, the spacing 512 between the proof masses 504 and 502 may change temporarily. Rotation of the gyroscope 500 causes application of an inertial force to the gyroscope 500. The optical vibratory gyroscope 500 further comprises an electrode 506 for counteracting the deflection of the proof mass 504 with an electrostatic force. The optical vibratory gyroscope 500 further comprises an optical coupler 508 for coupling light into and out of the microresonator 502. The optical vibratory gyroscope 500 further comprises a detector 510 for detecting light received from the microresonator 502 by the optical coupler 508. A change in the spacing "d" 512 between the proof mass 504 and the microresonator 502 causes a change in the optical resonance properties of the microresonator 502.

[0112] The optical vibratory gyroscope 500 detects inertial forces caused by rotation based on the displacement of the proof mass 504 as it moves in response to the inertial force. More specifically, the optical vibratory gyroscope 500 of FIG. 5 operates as follows. When the optical vibratory gyroscope 500 rotates, for example as part of an inertial sensor, the optical vibratory gyroscope 500 rotates to a certain angle. The proof mass is suspended by a suspension means 514, which in this example is a spring, connected to an anchor 516. The proof mass 504 responds to the inertial force caused by the rotation rate and is displaced based on the size and direction of the inertial force. The displacement of the proof mass 504 may be proportional to the inertial force caused by the rotation rate. This displacement changes the gap "d" 512 between the proof mass 504 and the microresonator 502, which causes a change in the optical resonance properties of the microresonator 502. An optical coupler 508 receives light from and couples light to the microresonator 504. Based on the change in the optical resonant properties of the microresonator 504, one or more properties of the light coupled out of the microresonator 504 change, and such change is detected at the detector 510. This change in the transmitted power at the detector 510 is related to the displacement of the proof mass, and therefore can be used to calculate the inertial force experienced by the optical vibratory gyroscope 500 due to rotation. Thus, the microresonator 504, optical coupler 508, detector 510, proof mass 504, spring 514, and anchor 516 together form the optical-mechanical readout mechanism of the gyroscope, allowing for the determination of the inertial force, and therefore the rotation rate, based on the detected change in the optical transmission at the detector 510. The gyroscope 500 may be operated and packaged in a vacuum to increase the mechanical quality factor, and therefore the sensitivity, of the gyroscope 500.

[0113] Electrodes 506 are provided and a voltage can be applied to the electrodes 506 such that the movement of the proof mass 504 can be controlled. The electrodes 506 are configured to output an electrostatic force resulting from the applied voltage to actuate the proof mass 504. The magnitude of the electrostatic force is based on the magnitude of the applied voltage. Because the size of the actuation is based on the size of the electrostatic force, the size of the actuation is also based on the applied voltage and can be controlled by varying the voltage applied to the electrodes 506. Thus, the electrodes 506 can change the position of the proof mass 504 relative to the microresonator by moving the proof mass 504 using the electrostatic force from the applied voltage. Such actuation is sometimes referred to as capacitive actuation. The electrodes 506 may be fixed relative to the gyroscope 500. The electrodes can move the proof mass in an X-direction and / or a Y-direction, where the X- and Y-directions are indicated in FIG. 5. The electrodes can also move the proof mass in a Z-direction (not shown).

[0114] Proof mass 504 has an initial position, which is its initial position relative to other components of gyroscope 500 when no inertial forces are acting on it. The use of the initial position in calculating rotation rate is described below in connection with FIG. 6. However, when proof mass 504 is displaced multiple times by inertial forces, the proof mass may begin to respond nonlinearly and drift over time. Drift is when a proof mass does not return to its initial position after being displaced. Thus, drift causes a change in the location of the proof mass when no inertial forces are acting on it. The size of the drift is the distance between the proof mass and its initial position when no inertial forces are acting on it. Over time, the drift increases, and therefore the distance between the proof mass position and its initial position increases. Because rotation rate calculations are based on proof mass position changes from its initial position, the drift of the proof mass causes long-term errors in the output of the optical vibration gyroscope.

[0115] Drift can be due to the sensitivity of the proof mass 504 to vibrations and linear acceleration. Drift can be due to thermal changes to the proof mass, or can be thermally induced, i.e., due to temperature changes of the gyroscope. Drift can be due to asymmetric fabrication or damping.

[0116] To eliminate errors caused by drift, the drift can be determined using the output of the optical vibratory gyroscope 500 and the output of the optical Sagnac gyroscope. The determined drift can then be utilized in the calculation to correct the output of the optical vibratory gyroscope 500. Additionally or alternatively, the drift can be reduced by actuating the proof masses, as described below, to approach an initial position. Thus, the use of both gyroscope outputs helps reduce errors in the output of the optical vibratory gyroscope.

[0117] The optical vibratory gyroscope 500 implements a closed loop system using a feedback mechanism of electrodes 506 that actuate the proof mass. The actuation force of the electrodes 506 can be used to actively restore the proof mass to its initial position, extending linearity and sensing range, and reducing or counteracting drift. The actuation force from the electrodes 506 can be adjusted using feedback to ensure that the proof mass 504 is in a position that provides optimal sensor measurements. Thus, the electrodes 506 improve the long-term stability of the gyroscope. The electrodes 506 may also be used to damp any vibrations of the proof mass 504. The optical-mechanical readout combined with electromechanical control provides a highly sensitive optical vibratory gyroscope 500. The combination of the optical Sagnac gyroscope to determine drift and the electrodes 506 to actuate the proof mass based on the drift further improves the long-term stability of the gyroscope.

[0118] Although only one of the components of the optical vibratory gyroscope 500 is shown in Figure 5, the optical vibratory gyroscope 500 may include multiple components. The optical vibratory gyroscope 500 may include multiple microresonators 502, electrodes 504, springs 514, anchors 516, and optical couplers 508. The microresonators may be fixed relative to the gyroscope 500. In Figure 5, the proof mass 504 is positioned radially from the microresonators 502, but the proof mass 504 may be located at other positions relative to the microresonators 502.

[0119] Displacement of the proof mass 504 changes the optical resonance characteristics. The change in the optical resonance characteristics can be a shift in the resonance wavelength and / or a broadening or deepening of the optical resonance curve in response to the proof mass moving relative to the microresonator. The shift can occur in either direction to progressively either red-detune or blue-detune. The broadening or deepening is due to a change in optical losses, such as dissipative or scattering opto-mechanical coupling, as described below in connection with FIG. 6.

[0120] As shown in Figure 5, the proof mass may be suspended using a suspension means 514 in the form of a spring. One end of the spring is fixed relative to the gyroscope by being connected to an anchor 516, and the other end of the spring is connected to and suspends the proof mass. The spring 514 allows for the displacement of the proof mass 504. Although the spring 514 and anchor 516 are shown in this figure, it will be apparent to one skilled in the art that other types of suspension means may be used in the optical vibration gyroscope 500 in place of the spring 514 and anchor 516.

[0121] A microresonator 502 is positioned near a proof mass 504 such that deflection of the proof mass 504 causes a change in the optical resonant properties of the microresonator 502 .

[0122] The electrostatic actuation provided by the electrodes 506 can be controlled by a controller (not shown in FIG. 5), as described further below. The optical vibratory gyroscope 500 may include the controller, or the controller may be external to the optical vibratory gyroscope 500. For example, the microcontroller 108 of FIG. 1 or the microcontroller 208 of FIG. 2 may control the electrostatic actuation provided by the electrodes 506.

[0123] A voltage is applied to electrode 506, as shown by the voltage source in Figure 5. The anchor may be grounded such that the applied voltage is the potential difference between electrode 506 and anchor 516. The applied voltage may vary based on the electrostatic force required to counter the deflection of proof mass 504. Thus, although a voltage source is shown in Figure 5, the voltage may instead be applied by a controller (not shown in Figure 5) as described above.

[0124] The detector 510 may be any device that converts an optical signal into an electrical signal. The detector 510 may be an optical detector. The detector 510 may include a processor for processing the optical signal to calculate the inertial force sensed by the gyroscope. The detector 510 may receive the optical signal from the optical coupler 508 and the microresonator 502 as its input and output the inertial force measurement. Alternatively, the controller (not shown in FIG. 5) described above may be used to calculate the inertial force based on the optical signal being detected by the detector 510. Although the optical vibration gyroscope 500 of FIG. 5 includes a detector 510, the detector may instead be outside the gyroscope, as illustrated by the detector 210 in the inertial sensor 200 of FIG. 2.

[0125] The optical coupler 508 can be coupled to the waveguide. The optical coupler 508 can be a waveguide arranged in close proximity to the microresonator 502 such that light is coupled from the waveguide to the microresonator 502 and from the microresonator 502 to the waveguide. Light from a light source (not shown in FIG. 5) can be input to the optical coupler 508 and transmitted to and from the microresonator 502 to the detector 510. The optical vibratory gyroscope can include a light source. Alternatively, the light source can be external to the optical vibratory gyroscope and transmit light to both the optical vibratory gyroscope and the optical Sagnac gyroscope as shown in the inertial sensor 200 of FIG. 2. The light input to the optical coupler 508 can be broadband light or coherent single frequency light.

[0126] The proof mass 504 is displaced toward or outward from the microresonator, for example along the Y axis. The proof mass 504 may additionally or alternatively be displaced laterally relative to the microresonator, for example along the X axis. The proof mass may be made of silicon.

[0127] The microresonator 502 shown in FIG. 5 is a ring resonator, but this microresonator can be replaced with another type of microresonator 502, such as a disk microresonator or a racetrack microresonator.

[0128] 5 has at least a portion of an evanescent field that extends beyond the edge of the microresonator 502 when the gyroscope is in use. The evanescent field may extend only 1 micron from the edge of the microresonator 502. An evanescent field that extends substantially from the edge of the microresonator 502 enhances the sensitivity of the gyroscope as the interaction between the evanescent field and the proof mass 504 increases.

[0129] The microresonator 502 may be a whispering gallery mode microresonator. When the microresonator is a whispering gallery mode microresonator, it traps light as a whispering gallery mode optical resonance and has an evanescent field that extends beyond its edges. When the proof mass 504 is displaced, it interacts with the evanescent field and alters the properties of the microresonator as it interacts with the evanescent field, changing the effective refractive index of the microresonator. This change shifts the whispering gallery mode optical resonance of the microresonator. Thus, when the proof mass 504 moves within the evanescent field of the whispering gallery mode microresonator, the optical resonance of the microresonator is perturbed and the properties of the optical resonance change.

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[0135] The resulting measurements of transmitted intensity at detector 510 can be compared to a previous or reference value to determine a change in transmission. This allows a change in optical resonance to be detected. The change in optical resonance is indicative of a displacement of the proof mass. Thus, from the detection of the change in optical resonance, an inertial force on the proof mass can be calculated. The rotational rate of the gyroscope can then be determined from the inertial force.

[0136] In summary, inertial forces applied during rotation of the gyroscope can cause a displacement of the proof mass, with the displacement being based on the size and direction of the inertial force. The displacement of the proof mass 504, and the resulting change in distance between the proof mass 504 and the microresonator 502, causes a change in the coupling velocity and the refractive index of the microresonator 502, causing a change in detuning. The change in detuning changes the intracavity electromagnetic field and the optical intensity at the output of the waveguide coupler. As a result, a change in the optical resonant frequency of the microresonator 502 is detectable at the detector 510. Graph 650 of FIG. 6 illustrates such a change.

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[0138] To detect inertial forces from the displacement of the test mass 504 using the transmitted power detected at the optical coupler 508, a scale factor, for example, a change in transmission per nanometer of displacement, can be defined. The relationship between transmission and displacement is provided above in connection with FIG. 6, showing that the transmission, and therefore the scale factor, is based on the coupling velocity and detuning.

[0139] To measure the inertial forces, an equilibrium position can be determined, which can be or correspond to the separation distance between the proof mass 504 and the microresonator 502 when the proof mass 504 is in its initial position. Small movements about the equilibrium position can then be detected. Because such small movements are detected about the equilibrium position, the response is substantially linear to these movements.

[0140] However, the initial position of the proof mass 504 may change over time, which causes the equilibrium position to change. At different equilibrium positions, the optical-mechanical coupling velocity is different due to the exponential strength distribution of the evanescent field around the resonator. Thus, when the initial position of the proof mass changes, the equilibrium position changes, which results in a change in the scale factor of the gyroscope 500. The closed-loop operation attempts to maintain the initial position, and therefore the scale factor. Additionally, a comparison of the output of the optical vibration gyroscope to the output of the optical Sagnac gyroscope can be used to attempt to maintain the initial position, since the comparison can detect any errors due to changes in the initial position. The proof mass can then be actuated based on the detected errors to return it to its initial position.

[0141] 7 shows a schematic diagram of an example optical vibration gyroscope 700 and an example output of the optical vibration gyroscope 700 before and during rotation. The optical vibration gyroscope 700 is an example of the optical vibration gyroscope 500 of FIG. 5. The optical vibration gyroscope 700 comprises the components of the optical vibration gyroscope 500 of FIG. 5. The optical vibration gyroscope 700 further comprises another spring 514, anchor 516, and electrode 506 that operates perpendicular to the original spring 514, anchor 516, and electrode 506 of FIG.

[0142] The rotation rate is sensed using the Coriolis effect on the vibrating gyroscope structure. Thus, the optical vibratory gyroscope 700 may also be known as a Coriolis gyroscope. To measure the rotation rate, the proof mass 504 is driven in a first direction, for example along the X-axis, so that it oscillates at a frequency Ω. Due to the Coriolis effect, rotation about the Z-axis produces a displacement along the Y-axis at a frequency Ω. The rotation rate can then be determined by determining the change in amplitude of the oscillation of the proof mass 504 in the Y-axis at frequency Ω.

[0143] Image A 710 shows the opto-mechanical elements when the gyroscope 700 is not rotating, e.g., when the gyroscope 700 is stationary, and image B 720 shows the opto-mechanical elements when the gyroscope 700 is rotating about the Z axis. It should be understood that the directions X, Y, and Z are provided for understanding and that the gyroscope is not limited to being utilized only to measure rotational rate about the Z axis.

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[0148]

number

[0149] A change in the distance between the microresonator and the proof mass causes a change in the resonant frequency characteristic of the microresonator which changes the transmission output from the optical coupler at the detector. As described in connection with FIG. 6, the change in resonant frequency can be detected from the change in transmission at the detector. Because the change in resonant frequency is based on the displacement of the proof mass, the change in amplitude of the signal at frequency Ω can be determined, and thus the Coriolis force and, consequently, the rotation rate can be measured.

[0150] The optical vibratory gyroscope 700 has a sense mode and a drive mode that are simultaneously implemented in the vertical direction. In the optical vibratory gyroscope 700, the sense mode is implemented along the Y axis and the drive mode is implemented along the X axis. Thus, conceptually, the optical vibratory gyroscope 700 can be equivalent to two optical vibratory gyroscopes 500 of FIG. 5 that are perpendicular to each other and have a common proof mass, with the main purpose of the optical vibratory gyroscope extending along the X axis being drive, and the main purpose of the other optical vibratory gyroscope extending along the Y axis being sense. For the optical vibratory gyroscope extending along the X axis, since its purpose is drive and not sense, the microresonator 502 and the optical coupler 508 are optional and are not shown in images A710 and B720.

[0151] A first one of each of the anchors 516, springs 514, and electrodes 506 is used for the drive mode, and a second one of each of the anchors 516, springs 504, and electrodes 506 is used for the sense mode, in addition to the microresonator 502 and optical coupler 508. Since the drive mode is along the X-axis, the electrodes 514 on the right side of the proof mass are for driving the proof mass to oscillate at a frequency Ω, and the springs 514 and anchors 516 on the right side of the proof mass in the figure allow the proof mass 504 to move freely in the X-direction to enable such oscillation. As mentioned above, the optical vibratory gyroscope can further include an optical coupler 508 and a microresonator 502 on the left side of the proof mass to sense that the proof mass is being driven at the correct frequency. The electrodes 506 on the right side of the proof mass may also be for reducing noise and drift, as described in connection with FIG. 5.

[0152]

number

[0153] Displacement of the proof mass 504 along the Y-axis changes the spacing between the proof mass 504 and the microresonator 502 below the proof mass 504. This changes the optical resonance characteristics of the microresonator 502 below the proof mass 504, which in turn changes the transmitted power detected from an optical coupler 508 below the microresonator by a detector (not shown).

[0154] As mentioned above, the electrodes 506 on the top of the proof mass and / or to the right of the proof mass are controlled to reduce noise and drift that cause errors in the output of the gyroscope 700. Comparing the outputs of the optical Sagnac gyroscope and the optical vibration gyroscope makes it possible to determine the size of the drift as it can be reduced by the electrodes 506. Thus, the Sagnac reading minimizes detrimental errors caused by the Coriolis mechanism.

[0155] FIG. 8 shows a schematic diagram of an exemplary inertial sensor 800 implementing a closed-loop feedback system. The inertial sensor 800 is an example of the inertial sensor 100 of FIG. 1 and the inertial sensor 200 of FIG. 2. The closed-loop feedback system comprises a closed-loop feedback 808 for the optical vibration gyroscope 804 and a closed-loop feedback 810 for the optical Sagnac gyroscope 806. The components of the closed-loop feedback 808 may be within the optical vibration gyroscope 804 or may be external to the optical vibration gyroscope 804 but within the sensor 800. The components of the closed-loop feedback 810 may be within the optical Sagnac gyroscope 806 or may be external to the optical Sagnac gyroscope 806 but within the sensor 800. The optical vibration gyroscope 804 is shown in the figure to be the optical vibration gyroscope 700 of FIG. 7 with the optional microresonator 502 and optical coupler 508. Optical Sagnac gyroscope 806 is shown in the figure as having a coiled waveguide structure, however, it should be apparent that optical vibratory gyroscope 804 may be any optical vibratory gyroscope and optical Sagnac gyroscope 806 may be any optical Sagnac gyroscope.

[0156] The inertial sensor 800 includes two photodetectors 812, 826 for receiving light from an optical vibratory gyroscope 804. Light from the light source 802 is split to be input to an optical coupler 508 to the left and below the proof mass 504.

[0157] A first photodetector 812 is for receiving light from the microresonator 502 via an optical coupler 508 to the left of the proof mass to sense that the proof mass is being driven at the correct frequency. A second photodetector 826 is for receiving light from the microresonator 502 via an optical coupler 508 below the proof mass to sense changes in the optical resonance properties of the microresonator 502 caused by displacement of the proof mass 504 along the Y-axis, and thus measure the rotation rate.

[0158] Thus, the output of the first photodetector 812 is converted to a digital output by an analog-to-digital converter 814. The digital signal output from the analog-to-digital converter 814 is then compared to the signal that would be received if the proof mass was being driven at the correct frequency by a drive mode controller 816. Based on the comparison, the drive mode controller 816 outputs a digital feedback signal to correct for any errors in the drive signal to ensure that the proof mass is being driven at the correct frequency. This digital feedback signal is converted to an analog signal by a digital-to-analog converter 818. The digital-to-analog converter 818 then provides the converted analog signal to the right electrode 506 of the proof mass 504 to control the voltage of the electrode and therefore the drive signal.

[0159] The output of the second photodetector 826 is converted to a digital output by an analog-to-digital converter 824. The digital signal output from the analog-to-digital converter 824 is then input to the sense mode controller 822 to determine any errors in the measurements due to, for example, drift. Based on this determination, the sense mode controller 822 outputs a digital feedback signal to reduce the errors in the measurements by actuating the proof masses, as described above in connection with FIG. 5. This digital feedback signal is converted to an analog signal by a digital-to-analog converter 820. The digital-to-analog converter 820 then provides the converted analog signal to the electrodes 506 above the proof mass 504 to control the voltage of the electrodes and thus actuate the proof mass accordingly.

[0160] The sense mode controller 822 also outputs a signal based on the detected transmission power from the optical coupler 508 corresponding to the rotational speed of the sensor 800 to a digital signal processor 828 to determine the rotational speed of the sensor 800 .

[0161] The inertial sensor 800 comprises a photodetector 832 for receiving light from the optical Sagnac gyroscope 806. The light from the light source 802 is split by a directional coupler 844 such that both clockwise and counterclockwise propagating light are input to the Sagnac structure. For example, the coupler 844 can take a portion of the input laser light and send it to the clockwise optical resonance, while the remainder of the input laser light is the counterclockwise optical resonance. The light propagating either clockwise or counterclockwise is phase shifted by a phase shifter 846 before entering the Sagnac structure, as described in connection with FIG. 3. The light output from the Sagnac structure then propagates back through the directional coupler 844 to the photodetector 832. Alternatively, the clockwise and counterclockwise propagating light may interfere in the directional coupler, and this interference signal may be sent to the photodetector 832.

[0162] The output of the photodetector 832 is converted to a digital output by an analog-to-digital converter 834. The digital signal output from the analog-to-digital converter 834 is then input to a controller 836 to determine any error in the frequency or phase of the input light. Based on this determination, the controller 836 outputs a digital feedback signal to reduce the error in the input light and maintain the sine-shaped response described in connection with FIG. 3 using a phase shifter. This digital feedback signal is then converted to an analog signal by a digital-to-analog converter 838. The digital-to-analog converter 838 then provides a converted analog signal that is combined with a bias signal 842. The bias signal 842 provides the voltage required by the phase shifter 846 to initially set the phase shift to 90 degrees. The converted analog signal is added to or subtracted from this voltage and the resulting signal is provided to the phase shifter 846 to maintain the 90 degree phase shift to reduce the error in the input light.

[0163] The controller 836 also outputs a signal to the digital signal processor 828 that corresponds to the rotational rate of the sensor 800 and is based on the detected transmission output from the optical Sagnac gyroscope.

[0164] Thus, the digital signal processor 828 receives signals corresponding to the rotation rate of the sensor 800 from both the optical vibration gyroscope 804 and the optical Sagnac gyroscope 806. The digital signal processor 828 then determines an error in the signal output from the optical vibration gyroscope based on the signals and corrects the rotation rate obtained by the optical vibration gyroscope 804 based on the two signals. The digital signal processor 828 may determine the error using cross-correlation, averaging, and / or weighting. The corrected rotation rate is then output 830 by the inertial sensor 800. The determination of the correction was described above in relation to FIG. 1.

[0165] The microcontroller 108 of FIG. 1 or the microcontroller 208 of FIG. 2 may serve as the drive mode controller 816 , the sense mode controller 822 , the controller 836 and the digital signal processing 828 .

[0166] The light source 802 may also be connected in a closed loop feedback. For example, if the light source 802 is a laser, a laser stabilization control path (not shown) may be used to maintain the stability of the light source 802. Laser stabilization techniques are known and therefore will not be described further.

[0167] Figure 9 shows a schematic diagram of an example optical vibratory gyroscope 900 and an example optical Sagnac gyroscope 950 of an inertial sensor. The optical vibratory gyroscope 900 is an example of the optical vibratory gyroscope 104 of Figure 1, the optical vibratory gyroscope 204 of Figure 2, and the optical vibratory gyroscope 700 of Figure 7. The optical Sagnac gyroscope 950 is an example of the optical Sagnac gyroscope 106 of Figure 1 and the optical Sagnac gyroscope 206 of Figure 2. In this example, both gyroscopes can be fabricated on the same substrate and / or using the same fabrication process.

[0168] The optical vibration gyroscope 900 comprises an outer microelectromechanical inertial proof mass acting as a frame 904 and an inner microelectromechanical inertial proof mass 954. The outer proof mass frame 904 is suspended by four suspension means 934, which in this example are flexible parts of the outer proof mass frame 904 connected to respective anchors 916. The anchors 916 are fixed with respect to the optical vibration gyroscope 900. These flexible parts of the outer proof mass frame 904 can behave as springs. The suspension means 934 are stiff in the X direction but can bend in the Y direction. Thus, the outer proof mass frame 904 is constrained to move in the Y direction. The inner proof mass 954 is suspended by two suspension means 944, which in this example are flexible parts of the inner proof mass 954, connected from the outside of the inner proof mass 954 to the inside of the outer proof mass frame 904. These flexible parts of the inner proof mass 954 can behave as springs. The suspension 944 is stiff in the Y direction but is flexible in the X direction. Thus, the inner proof mass 954 is constrained from moving in the Y direction relative to the outer proof mass frame 904 and therefore moves in the Y direction with the outer proof mass frame 904. The inner proof mass 954 is allowed to move in the X direction relative to the outer proof mass frame 904. Thus, due to the stiffness of the suspension 934 of the outer proof mass frame 904 in the X direction, only the inner proof mass 954 experiences the X-axis Coriolis force that is generated when rotation is applied.

[0169] The optical vibratory gyroscope 900 also includes four microresonators 902. An inner proof mass 954 is suspended adjacent to and discontinuous with two of the microresonators 902, and an outer proof mass frame 904 is suspended adjacent to and discontinuous with the other two microresonators 902. The optical vibratory gyroscope 900 further includes four optical couplers 908 for coupling light into and out of the corresponding microresonators 902. Light enters the optical couplers as indicated by the arrows shown in FIG. 9, is input and output to the corresponding microresonators, and then output to a detector.

[0170] The optical vibration gyroscope 900 further comprises four electrodes 906 for reacting with an electrostatic force to deflections of the outer proof mass frame 904 in the Y direction. There are two electrodes 906 above the outer proof mass frame 904 and two electrodes below the outer proof mass frame 904, so that electrostatic forces can be applied to opposite sides of the outer proof mass frame 904 to precisely control the movement or maintaining the position of the outer proof mass frame 904. The optical vibration gyroscope 900 further comprises two electrodes 946 for reacting with an electrostatic force to deflections of the inner proof mass 954 in the X direction. Each of the six electrodes 906 includes two fingers that are interdigitated with each other.

[0171] The optical vibratory gyroscope 900 has a sense mode and a drive mode implemented simultaneously, as described above in connection with Figure 7. As shown in Figure 9, the drive mode is along the Y axis and the sense mode is along the X axis. The orientation of the suspensions 944, 934 constrains the direction of travel of the inner proof mass 954 and the outer proof mass frame 904, as described above, so that the drive mode does not cross-couple to the sense mode and the Coriolis effect does not couple to the drive mode.

[0172] With regard to the drive mode, the four electrodes 906 on the top and bottom of the outer proof mass frame 904 are for driving the frame 904 to vibrate at the frequency Ω. The suspension means 944 connecting the outer proof mass frame 904 and the inner proof mass 954 is stiff in the Y direction and therefore passes vibrations from the outer proof mass frame 904 to the inner proof mass 954. Thus, the vibration energy is transferred from the outer proof mass frame 904 to the inner proof mass 954 such that the inner proof mass experiences the Coriolis effect. The microresonators 902 on either side of the protrusions on the top and bottom of the outer proof mass frame 904 and their corresponding optical couplers 908 are for sensing that the proof mass frame 904 is being driven at the correct frequency.

[0173]

number

[0174] When the outer proof mass frame 904 is driven in the Y direction at a frequency Ω, vibration energy is transferred to the inner proof mass 954, which is constrained to move in the X axis due to the suspension geometry. As the gyroscope 900 rotates, the inner proof mass 954 deflects along the X axis at a frequency Ω due to the Coriolis effect. The left and right microresonators 902 on the central post of the inner proof mass can then be used to detect the rate of rotation in a manner similar to that described in connection with FIG.

[0175] Although FIG. 9 shows electrostatic actuation performed by interdigitated electrodes arranged at specific locations around the proof mass, the electrostatic actuation can be arranged in any manner that generates X and Y forces on the proof mass.

[0176] The arrangement shown in FIG. 9 offers the advantage of reducing cross-axis coupling of vibrations in the sense and drive directions because it uses two frames whose springs can be adjusted to be very stiff in either the drive or sense directions.

[0177] Although FIG. 9 shows an optical vibratory gyroscope 900 having an inner proof mass 954 and an outer proof mass frame 904, the gyroscope can have a single proof mass and can be designed in other ways, such as a disk-type hemisphere system or a tuning fork structure.

[0178] In the case of the optical vibratory gyroscope 900, due to the arrangement of the microresonator 902, movement of the proof mass 904 differentially changes the spacing between the proof mass 904 and the microresonator 902. By comparing the different changes in the light output from each optical coupler 908, a differential output can be determined. Such a differential output, being differentially focused, eliminates errors that arise due to changes in the proof mass, for example due to thermal expansion. The arrangement of the optical vibratory gyroscope 900 of FIG. 9 thus allows for differential operation that significantly improves readings by reducing drift and temperature offsets.

[0179] A pair of detectors can perform a differential measurement on the detection signals to determine a differential output. The differential measurement is performed by comparing a signal received from a first detector of the pair of detectors with a signal received from a second detector of the pair of detectors. To obtain a differential output, the inertial sensor can include a first detector pair for receiving light from an output of the optical coupler 908 of the optical vibration gyroscope 900 labeled A1 962 and an output of the optical coupler 908 of the optical vibration gyroscope 900 labeled A2 964. The first detector pair can obtain a differential output for motion along the X direction. The inertial sensor can include a second detector pair for receiving light from an output of the optical coupler 908 of the optical vibration gyroscope 900 labeled B1 966 and an output of the optical coupler 908 of the optical vibration gyroscope 900 labeled B2 968. The second detector pair can obtain a differential output for motion along the Y direction.

[0180] The optical Sagnac gyroscope 950 comprises a Sagnac structure and two optical couplers for coupling light into and out of the Sagnac structure. As described above in connection with FIG. 3, one optical coupler couples light into the Sagnac structure in a clockwise direction and the other optical coupler couples light into the Sagnac structure in a counterclockwise direction to detect the rotation rate. One of the optical couplers is connected to a phase modulator 952 to shift the phase of the light propagating through that optical coupler and the Sagnac structure by 90 degrees as previously described. The light coupled into the optical coupler from the Sagnac structure has a different optical path length due to the rotation of the gyroscope. The inertial sensor may comprise a third detector pair for receiving light from the output of the optical coupler of the optical Sagnac gyroscope 950, labeled C1 970, and the output of the optical coupler 908 of the optical Sagnac gyroscope 950, labeled C2 972. This detector pair may perform a differential measurement on the detection signals to determine a differential output. For example, if a detector is detecting a shift in the resonance of the light propagating in each optical coupler, a differential measurement will indicate the frequency difference between the resonances and eliminate any common errors, e.g., due to changes in ambient temperature, that may move both resonances in the same direction.

[0181] FIG. 10 illustrates an exemplary control loop feedback system for use with an inertial sensor. The control loop system comprises closed loop control for the sense and drive modes of the optical vibratory gyroscope of the inertial sensor, and closed loop control for the optical Sagnac gyroscope of the inertial sensor. The closed loop control is described below as being external to the gyroscope, but it may be implemented by a controller within the gyroscope. The closed loop control may be implemented by the sensor's microcontroller, for example microcontroller 108 of FIG. 1 or microcontroller 208 of FIG. 2.

[0182] Closed loop controls for the sense and drive modes of the optical vibratory gyroscope are used to regulate the mechanical response. When the optical vibratory gyroscope is rotated about its Z-axis 1020, the gyroscope is in both sense and drive modes. A controller implementing closed loop feedback has separate closed loop controls for the drive and sense modes. The closed loop control for the drive mode is to keep the amplitude stable using closed loop feedback to adjust the electrostatic force 1022 by regulating the drive mode vibration amplitude compared to a given reference set point. The closed loop control for the sense mode is to monitor the gyroscope output, find changes in amplitude at frequency Ω, and counteract such changes using electrostatic actuation 1046.

[0183] In the drive mode, the proof mass is driven in the X-direction 1084 at a frequency Ω using electrostatic actuation 1022. A drive force is exerted on the optical vibration gyroscope in the X-direction, causing the gyroscope to output an output voltage V. The output voltage of the optical vibration gyroscope is input to a controller for closed-loop control of the drive mode. The output voltage is input to an analog-to-digital converter (ADC) 1028 in the controller to convert the voltage to a digital signal and filtered using a digital signal processor (DSP) 1030. The signal is then demodulated 1032 to obtain a drive-mode vibration amplitude that can be compared to a given reference point. In particular, the demodulated signal is used to extract information about the amplitude of the vibration and offsets relative to the frequency of the vibration, which can then be corrected by feedback from the closed-loop control. The closed-loop control 1034 compares the drive-mode vibration amplitude to a given reference set point and determines the required actuation of the gyroscope to maintain stability. A closed loop control 1034 sends drive signals to the associated electrodes of the optical vibratory gyroscope for electrostatic actuation 1022 in the X-axis via a modulator 1026 and a digital-to-analog converter (DAC) 1024 to maintain the correct vibration 1084. There is also a phase-locked loop (PLL) 1006 connected to a digitally controlled oscillator 1008 that keeps the frequency locked and generates the frequency reference used by other parts of the system. It is important that the drive force is very accurate and produces vibrations with stable peak amplitude and frequency to increase the sensitivity of the gyroscope to rotation rate.

[0184] In the sense mode, rotation of the optical vibratory gyroscope about the Z-axis 1020 exerts a force on the proof mass along the Y-axis. The gyroscope outputs an output voltage V. The gyroscope output voltage is input to the controller for closed-loop control for the sense mode. The output voltage is input to the ADC 1036 for conversion to a digital signal and filtered using the DSP 1038. The signal is then demodulated 1040 to obtain an in-phase rate, which is proportional to the rotation rate and represents the amplitude of the Coriolis waves produced in response to the rotation rate. The demodulator 1040 also outputs a quadrature signal, which represents the error associated with the frequency mismatch. The closed-loop control 1042 then uses the in-phase rate to determine a signal corresponding to the rotation rate and outputs 1052 to the digital signal processor 1080. The closed-loop control 1042 also outputs a correction signal to suppress the quadrature signal and the in-phase rate. The correction signal is modulated 1050 and converted through a DAC 1048 into a voltage signal to the associated electrodes of the optical vibratory gyroscope for electrostatic actuation in the Y-axis 1046 .

[0185] The closed loop control of the inertial sensor's optical Sagnac gyroscope is used to adjust the phase shifter to keep the input counter-propagating light field shifted by 90 degrees and keep the laser locked to the resonance without drifting. Rotation of the optical Sagnac gyroscope about the Z-axis 1060 causes a change in the optical path length of the clockwise and counterclockwise propagating light, which can be detected by detecting the phase difference between the resonances of the clockwise and counterclockwise propagating light or a change in the output intensity from an optical interference readout. The phase shifter 1062 shifts the phase of the light propagating either clockwise or counterclockwise through the Sagnac structure by 90 degrees using a bias signal 1064. The output of the optical Sagnac gyroscope is then detected by a detector 1066 to detect the frequency difference or change in optical interference of the clockwise and counterclockwise propagating light. The detector outputs an analog signal corresponding to the rotation rate, which is converted to a digital signal by an ADC 1068 and demodulated 1070 to obtain a signal corresponding to the difference in the interference pattern or resonance, as well as an error signal related to frequency and / or phase. These are sent to a proportional-integral (PI) controller 1072, which outputs 1078 a signal corresponding to the rotation rate to a digital signal processor 1080. The PI controller 1072 also corrects for any drift in the input light by applying a feedback signal to the Sagnac gyroscope or input light source using a DAC 1076, or by modifying the bias signal 1064.

[0186] A digital signal processor 1080 receives the outputs from the optical vibration gyroscope and the optical Sagnac gyroscope. The digital signal processor 1080 compares these outputs to determine any error in the output of the optical vibration gyroscope. The digital signal processor 1080 then corrects the output of the optical vibration gyroscope to reduce the error and outputs a corrected output 1082 of the optical vibration gyroscope from the inertial sensor.

[0187] FIG. 11 shows an example of the error of the output of the optical vibratory gyroscope and the optical Sagnac gyroscope. Graph A1100 shows the relationship between the Allan deviation and the sampling time. The Allan deviation provides an indication of the noise contributions and when they affect the sensor output, for example, in the short or long term. The dashed line shows the noise of the optical vibratory gyroscope, and the dotted line shows the noise of the optical Sagnac gyroscope. As shown in graph A1100, as the sampling time increases, the optical vibratory gyroscope has a much larger error caused by the drift of the proof mass. Conversely, the optical Sagnac gyroscope has a lower long-term drift. Thus, the optical Sagnac gyroscope provides a stable low-drift signal that can compensate for the optical vibratory gyroscope. Furthermore, the difference in the Allan deviation of the two gyroscopes at sampling time 1 indicates that the optical vibratory gyroscope has a lower noise density, which means that the optical vibratory gyroscope may have a lower noise level in the short term. The curves also show that the optical Sagnac gyroscope may have lower bias instability because the curve for the optical Sagnac gyroscope has a lower minimum.

[0188] Graph B1150 shows the rotation error accumulated when the gyroscope is stationary in time. The dashed line shows the rotation error of the optical Sagnac gyroscope, and the dotted line shows the rotation error of the optical vibratory gyroscope. As shown in graph B1150, as time increases, the error of the optical vibratory gyroscope increases at a much higher rate than the error of the optical Sagnac gyroscope. Thus, the use of an optical Sagnac gyroscope can benefit long-term stability by reducing the accumulation of rotation error of the optical vibratory gyroscope. Graph B1150 also shows that the optical Sagnac gyroscope may start with a higher amount of noise (short-term error), but has a lower long-term error.

[0189] Thus, these graphs show that the error of the optical vibratory gyroscope over time can be much larger than the error of the optical Sagnac gyroscope, and therefore, using the output of the optical Sagnac gyroscope to correct the output of the optical vibratory gyroscope reduces the error in the sensor's output signal and provides a more accurate determination of rotation rate.

[0190] FIG. 12 shows exemplary outputs of an optical vibration gyroscope, an optical Sagnac gyroscope, and an inertial sensor. Graph A1200 shows the output of the optical vibration gyroscope as a line and the output of the optical Sagnac gyroscope as a box, indicating that this output has more noise. The output of the optical vibration gyroscope is not linear and includes minimum and maximum values ​​where errors occur due to the undesirable effects of vibration or linear acceleration on the optical vibration gyroscope. Since the optical vibration gyroscope relies on measuring motion, any vibration or linear acceleration will generate erroneous readings, as shown in this graph. As shown in the graph, the optical Sagnac gyroscope is not subject to such undesirable effects and responds linearly to changes in rotation rate. Therefore, the optical Sagnac gyroscope does not provide erroneous readings and can be utilized to remove erroneous readings generated by the optical vibration gyroscope. Graph B shows the output of the inertial sensor and indicates that the error in the output of the optical vibration gyroscope was successfully removed by calibrating the output of the optical vibration gyroscope with the output of the optical Sagnac gyroscope.

[0191] FIG. 13 illustrates a block diagram of an exemplary inertial measurement unit (IMU) 1300. The IMU 1300 includes multiple chip-scale inertial sensors 1302 and a controller 1304. FIG. 13 illustrates three inertial sensors 1302, with one of the inertial sensors 1302 shown in dashed lines to indicate that it is optional. Although three sensors 1302 are shown in the IMU 1300, the IMU 1300 may include more or fewer inertial sensors.

[0192] Each inertial sensor 1302 comprises an optical vibratory gyroscope for detecting a first rate of rotation of the inertial sensor about said axis. The optical vibratory gyroscope is configured to output a primary signal corresponding to the first rate of rotation. Each inertial sensor 1302 further comprises an optical Sagnac gyroscope for simultaneously detecting a second rate of rotation of the inertial sensor about said axis. The optical Sagnac gyroscope is configured to output an auxiliary signal corresponding to the second rate of rotation.

[0193] The inertial measurement unit 1300 further comprises a controller 1304 configured to receive, for each inertial sensor 1302, one or more inputs based on the primary and auxiliary signals of the inertial sensor 1302. The controller 1304 is further configured to determine, for each inertial sensor 1302, a corrected first rotational rate of the inertial sensor 1302 about a respective axis based on the one or more inputs.

[0194] The controller 1304 may perform the operations of any of the microcontrollers described herein. Each inertial sensor 1302 may be the inertial sensor 100 of Figure 1, the inertial sensor 200 of Figure 2, or the inertial sensor 800 of Figure 8. The controller 1304 of the IMU 1300 may be shared between all sensors 1302, so that each inertial sensor 1302 may include elements of the inertial sensor 100 of Figure 1, the inertial sensor 200 of Figure 2, or the inertial sensor 800 of Figure 8 without a microcontroller.

[0195] The inertial measurement unit 1300 may include a first chip-scale inertial sensor 1302 for detecting a rotational rate of the first inertial sensor in a first axis, a second chip-scale inertial sensor 1302 for detecting a rotational rate of the second inertial sensor in a second axis perpendicular to the first axis, and a third chip-scale inertial sensor 1302 for detecting a rotational rate of the third inertial sensor in a third axis perpendicular to the first and second axes. The controller 1304 may be further configured to determine a total rotational rate of the inertial measurement unit 1300 based on the corrected first rotational rates determined for each inertial sensor 1302.

[0196] For example, the three inertial sensors 1302 may include a sensor 1302 for detecting a rotation rate about an X axis, a sensor 1302 for detecting a rotation rate about a Y axis, and a sensor 1302 for detecting a rotation rate about a Z axis, thereby providing three degrees of freedom for 3D orientation tracking.

[0197] Many variations of the methods described herein will be apparent to those of skill in the art. Each feature disclosed in this specification (including any accompanying claims, abstract, and drawings), unless expressly stated otherwise, may be replaced by alternative features serving the same, equivalent, or similar purpose. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features.

[0198] The invention is not limited to the details of any of the preceding embodiments. The invention extends to any novel or any novel combination of features disclosed in this specification (including the accompanying claims, abstract, and drawings), or any novel or any novel combination of steps of any method or process so disclosed. The claims should be construed to encompass not only the preceding embodiments, but also any embodiment that falls within the scope of the claims.

Claims

1. A chip-scale inertial sensor for detecting the rotational speed of the inertial sensor about an axis, the inertial sensor comprising: An optical vibrating gyroscope for detecting a first rotational speed of the inertial sensor about the axis, the optical vibrating gyroscope being configured to output a main signal corresponding to the first rotational speed; An optical Sagnac gyroscope for simultaneously detecting a second rotational speed of the inertial sensor about the axis, the optical Sagnac gyroscope being configured to output an auxiliary signal corresponding to the second rotational speed; Receiving one or more inputs based on the main signal and the auxiliary signal; A microcontroller configured to determine a corrected first rotational speed of the inertial sensor about the axis based on the one or more inputs. A chip-scale inertial sensor comprising the above.

2. The chip-scale inertial sensor according to claim 1, wherein the optical vibrating gyroscope and the optical Sagnac gyroscope receive light from the same light source.

3. The chip-scale inertial sensor according to claim 2, further comprising a beam splitter for splitting the light from the light source for transmission to the optical vibrating gyroscope and the optical Sagnac gyroscope.

4. The chip-scale inertial sensor according to claim 1, further comprising a light source for transmitting light to the optical vibrating gyroscope and the optical Sagnac gyroscope.

5. The chip-scale inertial sensor according to claim 1, further comprising at least one detector for detecting the light transmitted from the optical vibrating gyroscope and the optical Sagnac gyroscope, wherein the microcontroller receives the one or more inputs from the at least one detector.

6. Determining a corrected first rotational speed based on the one or more inputs includes comparing the first rotational speed with the second rotational speed and determining the corrected first rotational speed based on the comparison. The chip-scale inertial sensor according to claim 1.

7. Comparing the first rotational speed with the second rotational speed includes calculating the difference between the main signal and the auxiliary signal to determine the short-term error and long-term error of the optical vibration gyroscope. Based on the comparison, determining the corrected first rotational speed includes correcting the main signal to remove the short-term error and long-term error, according to the chip-scale inertial sensor of claim 6. **Claim 8** The error is due to the linear acceleration and / or long-term drift of the inertial sensor, according to the chip-scale inertial sensor of claim 7. **Claim 9** Comparing the first rotational speed with the second rotational speed includes determining whether the main signal is saturated. When the main signal is saturated, the corrected first rotational speed is determined using only the auxiliary signal, according to the chip-scale inertial sensor of claim 6. **Claim 10** The optical vibration gyroscope and the optical Sagnac gyroscope are fabricated on the same chip, according to the chip-scale inertial sensor of claim 1. **Claim 11** The microcontroller is further configured to output a feedback signal for actuating the test mass of the optical vibration gyroscope to re-calibrate the optical vibration gyroscope based on the comparison, according to the chip-scale inertial sensor of claim 6. **Claim 12** The feedback signal is output to the electrodes of the optical vibration gyroscope to electrostatically actuate the test mass, according to the chip-scale inertial sensor of claim 11. **Claim 13** The optical Sagnac gyroscope includes a coiled or spiral waveguide structure for propagating light clockwise and counterclockwise, and the rotation of the sensor causes a change in the optical path length of the light propagating clockwise and counterclockwise, according to the chip-scale inertial sensor of claim 1. **Claim 14** The coiled or spiral waveguide structure is in the form of one of a coiled optical fiber, a coiled waveguide, a spiral waveguide, and a microdisk, according to the chip-scale inertial sensor of claim 13. **Claim 15** The diameter of the coiled or spiral waveguide structure is on the order of millimeters or centimeters, according to the chip-scale inertial sensor of claim 13. **Claim 16** The microcontroller controls the frequencies of the light propagating clockwise and counterclockwise based on the auxiliary signal output by the optical Sagnac gyroscope so as to reduce the frequency change of the light due to drift, for the chip-scale inertial sensor according to claim 13.

17. The optical Sagnac gyroscope and the optical vibrating gyroscope are made of silicon or silicon nitride, for the chip-scale inertial sensor according to claim 1.

18. The optical vibrating gyroscope comprises one or more micro-resonators, each micro-resonator being a micro-resonator that supports a corresponding optical resonance, a micro-electromechanical inertial test mass that is adjacent to and discontinuously suspended from the one or more micro-resonators, the test mass being deflectable under the application of an inertial force, one or more optical couplers for coupling the light transmitted to the optical vibrating gyroscope to the inside and outside of the corresponding micro-resonators, and one or more detectors for detecting the light received from the one or more micro-resonators by the one or more optical couplers and is provided with, a change in the distance between the test mass and at least one micro-resonator causes a change in the optical resonance characteristics of the micro-resonator, for the chip-scale inertial sensor according to claim 1.

19. The optical vibrating gyroscope further comprises one or more electrodes for reacting with an electrostatic force against the deflection of the test mass, The microcontroller controls the electrostatic force of the one or more electrodes so as to vibrate the test mass at a fixed frequency in a first direction perpendicular to the axis so that the optical vibrating gyroscope can detect the first rotational speed of the inertial sensor about the axis, receives an electrical signal from the one or more detectors, detects a change in the optical resonance characteristics of the one or more micro-resonators in response to a change in the distance between the test mass and the one or more micro-resonators in a second direction perpendicular to the first direction at the fixed frequency, and determines the first rotational speed of the inertial sensor about the axis based on the change in the optical resonance characteristics of the one or more micro-resonators and is further configured as such, for the chip-scale inertial sensor according to claim 18.

20. The chip-scale inertial sensor according to claim 19, wherein the microcontroller is further configured to control the electrostatic force of the one or more electrodes to recalibrate the optical vibration gyroscope based on the comparison.

21. An inertial measurement unit, A plurality of chip-scale inertial sensors for detecting a rotational speed about each axis, each inertial sensor comprising: An optical vibration gyroscope for detecting a first rotational speed of the inertial sensor about the axis, the optical vibration gyroscope being configured to output a main signal corresponding to the first rotational speed; and An optical Sagnac gyroscope for simultaneously detecting a second rotational speed of the inertial sensor about the axis, the optical Sagnac gyroscope being configured to output an auxiliary signal corresponding to the second rotational speed. A plurality of chip-scale inertial sensors comprising: A controller, for each inertial sensor, Receiving one or more inputs based on the main signal and the auxiliary signal of the inertial sensor; Determining a corrected first rotational speed of the inertial sensor about each respective axis based on the one or more inputs. A controller configured as such, An inertial measurement unit comprising.

22. The inertial measurement unit includes a first chip-scale inertial sensor for detecting a rotational speed of a first inertial sensor about a first axis, a second chip-scale inertial sensor for detecting a rotational speed of a second inertial sensor about a second axis perpendicular to the first axis, and a third chip-scale inertial sensor for detecting a rotational speed of a third inertial sensor about a third axis perpendicular to the first axis and the second axis. The inertial measurement unit according to claim 21, wherein the controller is further configured to determine a total rotational speed of the inertial measurement unit based on the corrected first rotational speed determined for each inertial sensor.

23. A chip comprising the chip-scale inertial sensor according to any one of claims 1 to 20, or the inertial measurement unit according to claim 21 or 22.

24. A method implemented by a microcontroller for determining the rotational speed of an inertial sensor about an axis, comprising: receiving one or more inputs based on a main signal output from an optical vibrating gyroscope for detecting a first rotational speed of the inertial sensor about the axis and an auxiliary signal output from an optical Sagnac gyroscope for simultaneously detecting a second rotational speed of the inertial sensor about the axis; determining a corrected first rotational speed of the inertial sensor about the axis based on the one or more inputs; A method comprising the steps of: **Claim 25** A computer-readable medium storing instructions that, when executed by one or more processors, cause the one or more processors to perform the method according to claim 24.

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