Capacitance sensing in a mirror assembly with a biased substrate

Embodiments of the disclosure provide a mirror assembly for controlling optical directions in an optical sensing system. The mirror assembly may include a substrate and a micro mirror suspended over the substrate by at least one beam. The at least one beam may be mechanically coupled to the substrate. The mirror assembly may also include an actuator configured to tilt the micro mirror with respect to the substrate. The mirror assembly may further include a position sensor configured to detect a position of the micro mirror. Moreover, the mirror assembly may include a bias voltage source electrically coupled to the substrate to bias the substrate with a bias voltage.

TECHNICAL FIELD

The present disclosure relates to optical sensing systems such as a light detection and ranging (LiDAR) system, and more particularly to, a mirror assembly for controlling optical directions in such an optical sensing system.

BACKGROUND

Optical sensing systems such as LiDAR systems have been widely used in autonomous driving and/or for producing high-definition maps. For example, a typical LiDAR system measures the distance to a target by illuminating the target with pulsed laser light beams and measuring the reflected pulses with a sensor such as a photodetector or a photodetector array. Differences in laser light return times, wavelengths, and/or phases can then be used to construct digital three-dimensional (3D) representations of the target. Because using a narrow laser beam as the incident light can map physical features with very high resolution, a LiDAR system is particularly suitable for applications such as sensing in autonomous driving and high-definition map surveys.

The pulsed laser light beams emitted by a LiDAR system are typically directed to multiple directions to cover a field of view (FOV). Various methods can be used to control the directions of the pulsed laser light beams. Existing LiDAR systems generally use electrostatic-, piezoelectric-, or magnetic-based actuators (e.g., electrostatic actuators, piezoelectric actuators, magnetic actuators, etc.) to drive an optical component, such as a mirror, in the LiDAR systems to direct the pulsed laser light beams to the surrounding environment when the mirror is oscillating back and forth. The oscillation of the mirror can be controlled based on sensing changes in capacitance caused by the movement of the mirror. However, the accuracy of capacitance sensing is hindered by parasitic capacitance.

Embodiments of the disclosure improve the accuracy of capacitance sensing by providing a mirror assembly with a biased substate to reduce the effect of parasitic capacitance.

SUMMARY

Embodiments of the disclosure provide a mirror assembly for controlling optical directions in an optical sensing system. The mirror assembly may include a substrate and a micro mirror suspended over the substrate by at least one beam. The at least one beam may be mechanically coupled to the substrate. The mirror assembly may also include an actuator configured to tilt the micro mirror with respect to the substrate. The mirror assembly may further include a position sensor configured to detect a position of the micro mirror. Moreover, the mirror assembly may include a bias voltage source electrically coupled to the substrate to bias the substrate with a bias voltage.

Embodiments of the disclosure also provide a method for controlling a mirror assembly in an optical sensing system. The method may include receiving, by a micro mirror of the mirror assembly, an optical beam emitted from an optical source. The micro mirror may be suspended over a substrate of the mirror assembly by at least one beam mechanically coupled to the substrate. The method may also include tilting, by an actuator, the micro mirror with respect to the substrate to change a direction of the optical beam. The method may also include detecting, by a position sensor, a position of the micro mirror. The method may further include controlling, by the actuator, the tilting of the micro mirror based on the detected position. In addition, the method may include biasing, by a bias voltage source electrically coupled to the substrate, the substrate with a bias voltage.

Embodiments of the disclosure further provide an optical sensing system. The optical sensing system may include an optical source configured to emit an optical beam to scan an environment around the optical sensing system. The optical sensing system may also include a mirror assembly configured to control a direction of the optical beam. The mirror assembly may include a substrate and a micro mirror suspended over the substrate by at least one beam. The at least one beam may be mechanically coupled to the substrate. The mirror assembly may also include an actuator configured to tilt the micro mirror with respect to the substrate. The mirror assembly may further include a position sensor configured to detect a position of the micro mirror. Moreover, the mirror assembly may include a bias voltage source electrically coupled to the substrate to bias the substrate with a bias voltage.

DETAILED DESCRIPTION

Embodiments of the present disclosure provide systems and methods for controlling optical directions in an optical sensing system (e.g., a LiDAR system) using a mirror assembly. The mirror assembly may include a mirror configured to direct an optical beam into a plurality of directions to facilitate scanning of an environment around the optical sensing system. For example, the mirror can be driven by at least one actuator to tilt certain angles along an axis, thereby directing (e.g., guiding, reflecting, refracting, inflecting, deflecting, and/or diffracting) incident optical beams from an optical source (e.g., a laser source) toward multiple directions to, for example, scan the environment around the optical sensing system. The mirror can be implemented using a single micro mirror or an array of micro mirrors. In some embodiments, the mirror assembly can be made from semiconductor materials using microelectromechanical system (MEMS) technologies. Such a mirror assembly can also be referred to as a micromachined mirror assembly or a MEMS mirror assembly.

A mirror in a micromachined mirror assembly can be driven by one or more electrostatic actuators. For example, an electrostatic actuator may apply an electrical signal to generate a driving force (e.g., in the form of an electric force) pulling or attracting the mirror, often through a comb structure at an edge of the mirror, toward another comb structure on a stationary structure of the electrostatic actuator. By controlling the timing of the application of the electric force, the mirror can be driven to oscillate (e.g., tilting along an axis back and forth).

The timing of the application of the electric force can be controlled using a feedback-based closed-loop mechanism. For example, a position sensor may be used to detect the current position (e.g., a tilting angle) of the mirror relative to the initial position when the mirror is in a stationary state. Position sensing can be achieved through capacitance sensing because the capacitance between overlapping comb structures indicates the position of the mirror. Therefore, capacitance sensing is important in controlling the motion of the mirror as the result of capacitance sensing is used as feedback to regulate the application of the electric force. However, the accuracy of capacitance sensing is often hindered by parasitic capacitances, such as the parasitic capacitance of the substrate on which the mirror assembly is constructed. In some cases, the parasitic capacitance also varies dynamically with the electrical signal that drives the mirror, making accurate capacitance sensing even more challenging.

Embodiments of the present disclosure improve the accuracy of capacitance sensing by providing a mirror assembly having a biased substrate to reduce the effect of parasitic capacitance. For example, based on the capacitance-voltage (C-V) characteristic of the substrate, a positive or negative bias voltage may be applied to bias the substrate. By controlling the value of the bias voltage, the biased substrate may operate in an accumulation mode or a depletion mode. In either of these modes, the variation of the parasitic capacitance can be clamped into a relatively narrow region. In this way, the effect of the parasitic capacitance can be more accurately accounted for, which in turn improves the accuracy of the capacitance sensing. More accurate capacitance sensing leads to more accurate position sensing, which improves the overall controllability of the mirror assembly. The performance of an optical sensing system equipped with such an improved mirror assembly can be improved through a higher scanning accuracy, precision, range, and/or speed. Such an improved optical sensing system can be used in many applications, including, for example, autonomous driving and high-definition map survey, in which the optical sensing system can be equipped on a vehicle.

FIG.1illustrates a schematic diagram of an exemplary vehicle100equipped with an optical sensing system (e.g., a LiDAR system)102(hereinafter also referred to as LiDAR system102), according to embodiments of the disclosure. Consistent with some embodiments, vehicle100may be a survey vehicle configured for acquiring data for constructing a high-definition map, 3-D buildings, and/or, city modeling. Vehicle100may also be an autonomous driving vehicle.

As illustrated inFIG.1, vehicle100may be equipped with LiDAR system102. In some embodiments, LiDAR system102may be mounted to a body104via a mounting structure108. Mounting structure108may be an electro-mechanical device installed or otherwise attached to body104of vehicle100. Mounting structure108may use screws, adhesives, or another mounting mechanism. In some embodiments, LiDAR system102may be integrated with vehicle100without using mounting structure108. For example, LiDAR system102may be integrated as part of vehicle100on the top, side, front, and/or back of vehicle100. Vehicle100may be additionally equipped with a sensor110inside or outside body104using any suitable mounting mechanisms or integrated as part of vehicle100. Sensor110may include sensors used in a navigation unit, such as a Global Positioning System (GPS) receiver and one or more Inertial Measurement Unit (IMU) sensors. It is contemplated that the manners in which LiDAR system102or sensor110can be equipped on vehicle100are not limited by the example shown inFIG.1and may be modified depending on the types of LiDAR system102, sensor110, and/or vehicle100to achieve desirable 3D sensing performance.

Consistent with some embodiments, LiDAR system102and sensor110may be configured to capture data as vehicle100moves along a trajectory. For example, a transmitter of LiDAR system102may be configured to scan the surrounding environment. LiDAR system102may measure the distance to a target by illuminating the target with optical signals such as pulsed laser beams and measuring the reflected pulses with a receiver. The laser beams used by Li DAR system102may be in the ultraviolet, visible, or near infrared frequency range. In some embodiments of the present disclosure, LiDAR system102may capture point clouds including depth information of the objects in the surrounding environment. As vehicle100moves along the trajectory, LiDAR system102may continuously capture data. Each set of scene data captured at a certain time point or range is known as a data frame.

FIG.2illustrates a block diagram of an exemplary implementation of LiDAR system102, according to embodiments of the disclosure. LiDAR system102may include a transmitter202and a receiver204. Transmitter202may emit laser beams along multiple directions. Transmitter202may include one or more optical sources206(e.g., one or more laser sources) and one or more scanners210. As will be described below in greater detail, scanner210may include a micromachined mirror assembly having a mirror driven by one or more actuators.

Transmitter202can sequentially emit a stream of pulsed laser beams in different directions within a scan range (e.g., a range in angular degrees), as illustrated inFIG.2. Optical source206may be configured to provide a laser beam207(also referred to as a “native laser beam”) to scanner210. In some embodiments of the present disclosure, optical source206may generate a pulsed laser beam in the ultraviolet, visible, or near infrared wavelength range.

In some embodiments of the present disclosure, optical source206may include a pulsed laser diode (PLD), a vertical-cavity surface-emitting laser (VCSEL), a fiber laser, etc. For example, a PLD may be a semiconductor device similar to a light-emitting diode (LED) in which the laser beam is created at the diode's junction. In some embodiments of the present disclosure, a PLD includes a PIN diode in which the active region is in the intrinsic (I) region, and the carriers (electrons and holes) are pumped into the active region from the N and P regions, respectively. Depending on the semiconductor materials, the wavelength of laser beam207provided by a PLD may be smaller than 1,100 nm, such as 405 nm, between 445 nm and 465 nm, between 510 nm and 525 nm, 532 nm, 635 nm, between 650 nm and 660 nm, 670 nm, 760 nm, 785 nm, 808 nm, or 848 nm. It is understood that any suitable laser source may be used as optical source206for emitting laser beam207.

Scanner210may be configured to emit a laser beam209to an object212in a first direction. Object212may be made of a wide range of materials including, for example, non-metallic objects, rocks, rain, chemical compounds, aerosols, clouds and even single molecules. The wavelength of laser beam209may vary based on the composition of object212. In some embodiments, at different time points during the scan, scanner210may emit laser beam209to object212in different directions within a range of scanning angles by tilting the mirror of the micromachined mirror assembly. In some embodiments of the present disclosure, scanner210may also include optical components (e.g., lenses, other mirrors) that can focus pulsed laser light into a narrow laser beam to increase the scan resolution and/or the range to scan object212.

In some embodiments, receiver204may be configured to detect a returned laser beam211returned from object212. The returned laser beam211may be in a different direction from laser beam209. Receiver204can collect laser beams returned from object212and output electrical signals reflecting the intensity of the returned laser beams. Upon contact, laser light can be reflected by object212via backscattering, such as Rayleigh scattering, Mie scattering, Raman scattering, and fluorescence. As illustrated inFIG.2, receiver204may include one or more lenses214and one or more photodetectors216. Lens214may be configured to collect light from a respective direction in its field of view (FOV). At different time points during the scan, returned laser beam211from different directions may be collected by lens214. Returned laser beam211may be returned from object212and may have the same wavelength as laser beam209.

Photodetector216may be configured to detect returned laser beam211returned from object212. In some embodiments, photodetector216may convert the laser light (e.g., returned laser beam211) collected by lens214into an electrical signal218(e.g., a current or a voltage signal). Electrical signal218may be generated when photons are absorbed in a photodiode included in photodetector216. In some embodiments of the present disclosure, photodetector216may include a PIN detector, a PIN detector array, an avalanche photodiode (APD) detector, a APD detector array, a single photon avalanche diode (SPAD) detector, a SPAD detector array, a silicon photo multiplier (SiPM/MPCC) detector, a SiPM/MPCC detector array, or the like.

LiDAR system102may also include one or more processor220. Processor220may receive electrical signal218generated by photodetector216. Processor220may process electrical signal218to determine, for example, distance information carried by electrical signal218. Processor220may construct a point cloud based on the processed information. Processor218may include a microprocessor, a microcontroller, a central processing unit (CPU), a graphical processing unit (GPU), a digital signal processor (DSP), or other suitable data processing devices. Processor220may control the operation of transmitter202and/or receiver204. For example, processor220may control scanner210based on feedback signals from capacitance sensing, which will be described in greater detail below.

While scanner210is described as part of transmitter202, it is understood that in some embodiments, receiver204may also include a scanner, e.g., before photodetector216in the light path. The scanner included in receiver204may be the same as or similar to scanner210, and may operate in synchronization or in tandem with scanner210. The inclusion of such a scanner in receiver204can improve the signal-to-noise ratio (SNR) and sensitivity of receiver204. For example, photodetector216can capture light, e.g., returned laser beam211from desired directions, thereby avoiding interferences from other light sources, such as the sun and/or other LiDAR systems.

FIG.3illustrates a schematic diagram of an exemplary mirror assembly300(top view), according to embodiments of the disclosure.FIG.4illustrates a section view of mirror assembly300along line A-A′ when mirror assembly300is in a static state.FIG.5illustrates a section view of mirror assembly300along line A-A′ when mirror assembly300is in a dynamic state. In the following passages,FIGS.3-5are discussed together.

As shown inFIG.3, mirror assembly300may include a substrate302. Substrate302may be used as a base on which other components of mirror assembly300can be formed. In some embodiments, substrate302may include a single layer, such as a silicon (Si) layer. In other embodiments, substrate302may include multiple layers. For example, as shown inFIGS.4and5, substrate302may include a semiconductor layer304and an insulator layer306. Semiconductor layer304may be a silicon (Si) layer, although other semiconductor materials may be used. Insulator layer306may be formed on top of semiconductor layer304. In some embodiments, insulator layer306may be a silicon dioxide (SiO2) layer. Multiple other components of mirror assembly300may be formed on top of insulator layer306.

Returning toFIG.3, mirror assembly300may include a mirror310suspended over substrate302by beams322and332. Mirror310may be implemented by a single micro mirror and an array of micro mirrors. For simplicity, mirror310is also referred to as micro mirror310. Beam322may be mechanically coupled to substrate302through an anchor320. Similarly, beam332may be mechanically coupled to substrate302through an anchor330.FIGS.4and5schematically show that mirror310is suspended over substrate302. In some embodiments, substrate302, anchors320and330, beams322and332, and mirror310may couple to one another to form a single mechanical structure. Anchors320and330may be rigidly coupled to substrate302. As used herein, rigid coupling refers to a fixed mechanical coupling such that relative motion or displacement between the two coupling components is not allowed. Beams322and332may also be referred to as springs, which may allow limited flexibility such that the suspended mirror310may tilt along an axis defined by beams322and332. For example, referring toFIG.5, mirror310may tilt with respect to substrate302back and forth in a clockwise and counterclockwise manner. The tilting motion of mirror310may be utilized to change the direction of an incident optical beam into multiple scanning directions.

As shown inFIG.4, mirror310may include a reflective layer316and a supporting layer318. Support layer318may be made of the same material as beams322/332and anchors320/330. For example, support layer318may be formed together with beams322/332and anchors320/330. Reflective layer316may include a high-reflection material to reflect optical signals.

In some embodiments, mirror assembly300may include one or more actuators configured to tilt mirror310with respect to substrate302.FIG.3shows an electrostatic actuator that includes an anchor340, a first plurality of comb fingers342extending from anchor340toward mirror310, a second plurality of comb fingers312extending from mirror310toward anchor340and interleaving with comb fingers342, and a driving voltage source380configured to apply a driving voltage across comb fingers342and comb fingers312. Anchor340may be similar in composition to that of anchors320and330, and may be formed in a similar manner. Anchor340may be mechanically coupled to substrate302. In some embodiments, anchor340may be rigidly coupled to substrate302. Comb fingers342may be mechanically or even rigidly coupled to anchor340and maintain a static state during operation of mirror assembly300. Comb fingers312may be mechanically or even rigidly coupled to mirror310(e.g., at an edge of mirror310). During operation of mirror assembly300, comb fingers312may be set in motion by the attraction force resulting from the application of the driving voltage to comb finger342, thereby tilting mirror310.

In some embodiments, two actuators may be disposed in a symmetrical manner to drive mirror310on both sides. As shown inFIG.3, a second actuator on the left side may include an anchor350, a first plurality of comb fingers354, a second plurality of comb fingers314, and a driving voltage source382, similar to its counterpart on the right side.

Referring toFIG.4, when mirror310is in a static state, mirror310is parallel to substrate302. In other words, the relative angle between mirror310and substrate302is zero degree. Turning toFIG.5, when mirror310is in a dynamic state, in which mirror310is titling back and forth (clockwise and counterclockwise), the position of mirror310can be indicated by the relative angle between mirror310and substrate302, denoted as angle β. Information of the position of mirror310(e.g., β) is important to control the tilting of mirror310because it is more efficient to apply the attraction force when comb fingers312are moving toward comb fingers342. In this way, the attraction force can accelerate the tilting motion of mirror310. On the other hand, if the attraction force is applied when comb fingers312are moving away from comb fingers342, the tilting motion would be decelerated.

Mirror assembly300may include a position sensor390configured to detect a position (e.g., β) of mirror310. In some embodiments, position sensor390may be implemented by a capacitance sensor (also referred to as390) configured to measure the capacitance or a change in capacitance between mirror310and anchor340. Referring toFIG.4, the capacitance between comb fingers312and342(denoted as Cf) is proportional to the overlapping areas (denoted as A) between these two sets of interleaving comb fingers. Now referring toFIG.5, when mirror310is in motion, the change in capacitance Cf(denoted as ΔCf) is proportional to the change in areas A (denoted as ΔA), which is also proportional to angle β. Therefore, angel β can be determined or derived based on ΔCf. Therefore, it is desirable to accurately measure Cfor ΔCfusing capacitance sensor390.

However, the capacitance value directly measured by capacitance sensor390disposed between mirror310and anchor340is not the same as Cf. The presence of the parasitic capacitance of substrate302makes accurate capacitance sensing challenging. As shown inFIG.6, the capacitance value directly measured by capacitance sensor390is the total capacitance CT, which is the sum of the capacitance between the two sets of comb fingers (Cf) and the parasitic capacitance of substrate302(Cs) because Cfis in parallel to Cs. Csvaries when the driving voltage is applied (e.g., by driving voltage source380) due to the transition from charge depletion to charge accumulation, or vice versa.FIG.7shows an exemplary capacitance-voltage (C-V) characteristic of silicon, the material of semiconductor layer304. As shown inFIG.7, a capacitance curve702indicates that the parasitic capacitance changes from about 93 pF to about 110 pF when the applied voltage changes from −30 V to 30 V. To drive mirror310to tilt back and forth, the driving voltage provided by driving voltage source380is typically an alternate current (AC) voltage, which changes from a negative peak to a positive peak. Therefore, when such an AC driving voltage is applied, Csconstantly varies, making it difficult to accurately determine Cffrom the measured total capacitance CT.

Embodiments of the present disclosure address this problem by applying a bias voltage to substrate302. Referring toFIGS.3-5, mirror assembly300may include a bias voltage source370electrically coupled to substrate302to bias substrate302with a bias voltage. In some embodiments, bias voltage source370may be electrically coupled to semiconductor layer304to bias semiconductor layer304, as shown inFIGS.4and5. Biasing substrate302may force substrate302(or its semiconductor layer304) to operate in either an accumulation mode or a depletion mode. Referring toFIG.7, capacitance curve702in general has three regions corresponding to three operation modes. When voltage is lower than about −10 V, capacitance curve702enters depletion region710. When voltage is higher than about −3 V, capacitance curve702enters accumulation region720. Between depletion region710and accumulation region720is a transition region730, in which the parasitic capacitance changes abruptly. On the other hand, in either depletion region710or accumulation region720, the parasitic capacitance is clamped to a relatively stable value.

The accuracy of capacitance sensing can be improved when substrate302operates in either the depletion mode or the accumulation mode during the application of the driving voltage. To that end, the bias voltage can be a direct current (DC) voltage, the absolute value of which can be set higher than the peak-to-peak value of the driving voltage (or the peak-to-peak value plus any DC offset contained in the driving voltage, hereinafter collectively referred to as the peak-to-peak value of the driving voltage) such that substrate302would operate in the depletion mode even when the driving voltage reaches its positive peak, or that substrate302would operate in the accumulation mode even when the driving voltage reaches its negative peak. In other words, the voltage offset of substrate302created by the bias voltage can be greater than the peak-to-peak value of the driving voltage. In this way, the substrate parasitic capacitance Cscan be kept relatively constant, leading to more accurate Cf.

For example, when the driving voltage is an AC voltage having a peak-to-peak value of 110 V, the bias voltage can be a negative DC voltage lower than −120 V (e.g., offsetting substrate302by a negative offset of more than 120 V), such that when the driving voltage reaches its positive peak (e.g., +110 V relative to the negative offset), the total voltage applied to substrate302is still lower than −10 V, within the depletion region shown inFIG.7. Alternatively, the bias voltage can be a positive DC voltage higher than 110 V (e.g., offsetting substrate302by a positive offset of more than 110 V), such that when the driving voltage reaches its negative peak (e.g., −110 V relative to the positive offset), the total voltage applied to substrate302is still higher than 0 V, within the accumulation region shown inFIG.7. It is noted that the value of the bias voltage depends on the driving voltage as well as the C-V characteristic of the substrate, and can be adjusted accordingly.

Referring toFIGS.3-5, mirror assembly300may include a voltage regulator360electrically coupled to bias voltage source370to regulate the bias voltage based on the driving voltage. For example, voltage regulator360may include a feedback network to sense the driving voltage of driving voltage source380, and change the bias voltage accordingly.

FIG.8illustrates another exemplary mirror assembly in which bias voltage source370can be electrically coupled to semiconductor layer304from a different side than the example shown inFIG.5. While other components or configurations of the mirror assembly shown inFIG.8can be the same as or similar to those shown inFIG.5, bias voltage source370can be electrically coupled to semiconductor layer304from the top side of substrate302instead of from the bottom side. As used herein, the top side of substrate302refers to the side on which mirror310is suspended, and the bottom side of substrate302refers to the opposite side to the top side. As shown inFIG.8, bias voltage source370may be electrically coupled to semiconductor layer302via a conductor layer308(e.g., a metal pad, an electrode, etc.) that is formed on the top side of semiconductor layer304. WhileFIG.8shows that conductor layer308is separate from insulator layer306, in some embodiments, conductor layer308may be surrounded by insulator layer306. For example, conductor layer308may form a conductive path through insulator layer306to reach semiconductor layer304. Coupling bias voltage source370from the top side of substrate302may be compatible with the fabrication process flow, in which conductor layer308may be formed as part of the fabrication process and bias voltage source370may be formed along with other components on top of substrate302.

Similar toFIGS.3-5, voltage regulator360may be electrically coupled to bias voltage source370to regulate the bias voltage based on the driving voltage. As shown inFIG.8, voltage regulator360may be coupled to bias voltage source370from the top side of substrate302. For example, bias voltage source370may be electrically coupled to voltage regulator360, which in turn may be electrically coupled to conductor layer308such that both bias voltage source370and voltage regulator360are electrically coupled to semiconductor layer304.

FIG.9illustrates a flow chart of an exemplary method900for controlling a mirror assembly (e.g., mirror assembly300) in an optical sensing system (e.g., LiDAR system102), according to embodiments of the disclosure. Method900may include multiple steps. It is to be appreciated that some of the steps may be omitted to perform method900. Further, some of the steps may be performed simultaneously, or in a different order than shown inFIG.9.

In step910, a micro mirror (e.g., mirror310) may receive an optical beam (e.g., laser beam207) emitted from an optical source (e.g., optical source206). For example, optical source206may be controlled by processor220to emit laser beam207(e.g., a pulsed laser beam), which may travel toward mirror310of mirror assembly300, which may be part of scanner210. Mirror310may receive laser beam207and change the direction of laser beam207to scan an environment around optical sensing system102.

In step920, an actuator (e.g., the actuator that includes anchor340, comb fingers342, comb fingers312, and driving voltage source380) may tilt mirror310with respect to substrate302to change the direction of laser beam207(e.g., becoming laser beam209) to scan the environment. For example, driving voltage source380may apply a driving voltage across the interleaving comb fingers312and342to attract comb fingers312toward comb fingers342, thereby tilting mirror310along an axis (e.g., the axis defined by beams322and332). The driving voltage may be an AC voltage with alternating polarities (e.g., changing between positive and negative voltages). The frequency of the AC voltage may be set to be or close to the resonant frequency of mirror310to achieve the maximum tilting range.

In step930, a position sensor (e.g., position sensor390) may detect a position of mirror310(e.g., tilting angle (3). For example, position sensing may be implemented by capacitance sensing, in which position sensor390may measure the capacitance or a change in capacitance between mirror310and anchor340. As discussed above, the capacitance measured by position sensor390is the total capacitance (CT) including the capacitance between comb fingers312and342(Cf) and the parasitic capacitance of substrate302(Cs). With the applicant of the AC driving voltage, the parasitic capacitance Csvaries, as shown inFIG.7. To maintain a relative constant parasitic capacitance Cs, in step940, a bias voltage is applied to substrate302by a bias voltage source (e.g., bias voltage source370) to force substrate302to operate in either an accumulation mode or a depletion mode. The bias voltage (e.g., the absolute value thereof) can be set to be higher than the peak-to-peak value of the AC driving voltage such that substrate302would maintain its operation mode (e.g., accumulation or depletion) even under the peaks of the AC driving voltage. In this way, the parasitic capacitance can be clamped to a relatively stable range, improving the accuracy of capacitance sensing. In some embodiments, when substrate302has multiple layers, bias voltage source370may be electrically coupled to a semiconductor layer (e.g., semiconductor layer304) to apply the bias voltage to the semiconductor layer.

In step950, the actuator may control the tilting of mirror310based on the detected position. For example, a closed-loop control may be used to control the timing of the application of the driving voltage based on the position of mirror310fed back from position sensor390.

In step960, a voltage regulator (e.g., voltage regulator360) electrically coupled to bias voltage source370may regulate the bias voltage based on the driving voltage. For example, voltage regulator360may include passive components (e.g., resistors, capacitors, etc.) and/or active components (e.g., amplifier, sensors, transistors, etc.) to dynamically adjust the bias voltage such that the driving voltage would not change the operation mode of substrate302during the operation of mirror assembly300.

It is noted that step940may be performed before step910,920, or930, or be performed in parallel or simultaneously with any of the other steps in method900.

It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed system and related methods. Other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the disclosed system and related methods.

It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.