Patent Publication Number: US-11385454-B2

Title: Resonant frequency tuning of micromachined mirror assembly

Description:
TECHNICAL FIELD 
     The present disclosure relates to a micromachined mirror assembly, and more particularly to, a micromachined mirror assembly used in a scanner for light detection and ranging (LiDAR). 
     BACKGROUND 
     LiDAR systems have been widely used in autonomous driving and producing high-definition maps. For example, LiDAR systems measure distance to a target by illuminating the target with pulsed laser light and measuring the reflected pulses with a sensor. Differences in laser return times and wavelengths can then be used to make digital three-dimensional (3-D) representations of the target. The laser light used for LiDAR scan may be ultraviolet, visible, or near infrared. Because using a narrow laser beam as the incident light from the scanner can map physical features with very high resolution, a LiDAR system is particularly suitable for applications such as high-definition map surveys. 
     The scanner of a LiDAR system includes a mirror that can be moved (e.g., rotated) by actuators to reflect (and steer) incident laser beams from a laser source towards a pre-determined angle. The mirror can be a single, or an array of micromachined mirror assembly(s) made by semiconductor materials using microelectromechanical system (MEMS) technologies. In order to maximize the deflection angle of the micromachined mirror assemblies for a given voltage, they are operated in their resonant frequency. However, resonant frequency can shift due to thermal expansion when temperature changes. Also, there may be inherent process variations during fabrication of microstructures. Thus, achieving the target resonant frequency becomes especially important when multiple micro mirrors need to be synchronized to operate at the same resonant frequency. 
     Embodiments of the disclosure address the above problems by an improved micromachined mirror assembly in a scanner for LiDAR. 
     SUMMARY 
     Embodiments of the disclosure provide a micromachined mirror assembly. The micromachined mirror assembly includes a micro mirror, a first suspended beam, a second suspended beam, a first actuator, and a second actuator. The micro mirror is configured to tilt around an axis. The first suspended beam and second suspended beam each is mechanically coupled to the micro mirror along the axis. The first actuator is mechanically coupled to the first suspended beam and configured to apply a first torsional stress around the axis to the first suspended beam. The second actuator is mechanically coupled to the second suspended beam and configured to apply a second torsional stress around the axis to the second suspended beam. The first torsional stress and second torsional stress have a magnitude difference. 
     Embodiments of the disclosure also provide another micromachined mirror assembly. The micromachined mirror assembly includes a micro mirror, a first suspended beam, a second suspended beam, and at least one tensional actuator. The micro mirror is configured to tilt around an axis. The first suspended beam and second suspended beam each is mechanically coupled to the micro mirror along the axis. The at least one tensional actuator is mechanically coupled to an end of at least one of the first and second suspended beams and configured to apply a tensional stress along the axis to the at least one of the first and second suspended beams. 
     Embodiments of the disclosure also provide a method for driving a micromachined mirror assembly. A resonant frequency of the micromachined mirror assembly is set at an initial value. A tensional stress is applied along an axis of the micromachined mirror assembly to increase the resonant frequency to a first operational value greater than the initial value during operation of the micromachined mirror assembly. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a schematic diagram of an exemplary vehicle equipped with a LiDAR system, according to embodiments of the disclosure. 
         FIG. 2  illustrates a block diagram of an exemplary LiDAR system having a transmitter with a scanner, according to embodiments of the disclosure. 
         FIG. 3A  illustrates a schematic diagram of an exemplary micromachined mirror assembly, according to embodiments of the disclosure. 
         FIG. 3B  illustrates a top perspective view of the exemplary micromachined mirror assembly in  FIG. 3A , according to embodiments of the disclosure. 
         FIG. 4A  illustrates a waveform of an exemplary voltage signal applied to the actuators of a micromachined mirror assembly, according to embodiments of the disclosure. 
         FIG. 4B  illustrates a waveform of another exemplary voltage signal applied to the actuators of a micromachined mirror assembly, according to embodiments of the disclosure. 
         FIG. 5  illustrates a schematic diagram of another exemplary micromachined mirror assembly, according to embodiments of the disclosure. 
         FIG. 6A  illustrates a schematic diagram of a design of the exemplary micromachined mirror assembly in  FIG. 5 , according to embodiments of the disclosure. 
         FIG. 6B  illustrates a schematic diagram of another design of the exemplary micromachined mirror assembly in  FIG. 5 , according to embodiments of the disclosure. 
         FIG. 7  illustrates a flow chart of an exemplary method for driving a micromachined mirror assembly, according to embodiments of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the exemplary embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 1  illustrates a schematic diagram of an exemplary vehicle  100  equipped with a LiDAR system  102 , according to embodiments of the disclosure. Consistent with some embodiments, vehicle  100  may be a survey vehicle configured for acquiring data for constructing a high-definition map or 3-D buildings and city modeling. 
     As illustrated in  FIG. 1 , vehicle  100  may be equipped with LiDAR system  102  mounted to a body  104  via a mounting structure  108 . Mounting structure  108  may be an electro-mechanical device installed or otherwise attached to body  104  of vehicle  100 . In some embodiments of the present disclosure, mounting structure  108  may use screws, adhesives, or another mounting mechanism. Vehicle  100  may be additionally equipped with a sensor  110  inside or outside body  104  using any suitable mounting mechanisms. Sensor  110  may 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 system  102  or sensor  110  can be equipped on vehicle  100  are not limited by the example shown in  FIG. 1  and may be modified depending on the types of LiDAR system  102  and sensor  110  and/or vehicle  100  to achieve desirable 3-D sensing performance. 
     Consistent with some embodiments, LiDAR system  102  and sensor  110  may be configured to capture data as vehicle  100  moves along a trajectory. For example, a transmitter of LiDAR system  102  is configured to scan the surrounding and acquire point clouds. LiDAR system  102  measures distance to a target by illuminating the target with pulsed laser beam and measuring the reflected pulses with a receiver. The laser beam used for LiDAR system  102  may be ultraviolet, visible, or near infrared. In some embodiments of the present disclosure, LiDAR system  102  may capture point clouds. As vehicle  100  moves along the trajectory, LiDAR system  102  may continuously capture data. Each set of scene data captured at a certain time range is known as a data frame. 
       FIG. 2  illustrates a block diagram of an exemplary LiDAR system  102  having a transmitter  202  with a scanner  210 , according to embodiments of the disclosure. LiDAR system  102  may include transmitter  202  and a receiver  204 . Transmitter  202  may emit laser beams within a scan angle. Transmitter  202  may include one or more laser sources  206  and a scanner  210 . As described below in detail, scanner  210  may include a micromachined mirror assembly (not shown) having a resonant frequency that can be tuned during the operation of the micromachined mirror assembly. 
     As part of LiDAR system  102 , transmitter  202  can sequentially emit a stream of pulsed laser beams in different directions within its scan angle, as illustrated in  FIG. 2 . Laser source  206  may be configured to provide a laser beam  207  (referred to herein as “native laser beam”) in a respective incident direction to scanner  210 . In some embodiments of the present disclosure, laser source  206  may generate a pulsed laser beam in the ultraviolet, visible, or near infrared wavelength range. 
     In some embodiments of the present disclosure, laser source  206  is a pulsed laser diode (PLD). A PLD may be a semiconductor device similar to a light-emitting diode (LED) in which the laser beam is created at the diode&#39;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 incident laser beam  207  provided by a PLD may be smaller than 1,100 nm, such as 405 nm, between 445 nm and 465 between 510 nm and 525 nm, 532 nm, 635 nm, between 650 nm 660 nm, 670 nm, 760 nm, 785 nm, 808 nm, or 848 nm. 
     Scanner  210  may be configured to emit a laser beam  209  to an object  212  in a first direction. Object  212  may 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 beam  209  may vary based on the composition of object  212 . At each time point during the scan, scanner  210  may emit laser beam  209  to object  212  in a direction within the scan angle by rotating the micromachined mirror assembly as the incident angle of incident laser beam  207  may be fixed. In some embodiments of the present disclosure, scanner  210  may also include optical components (e.g., lenses, mirrors) that can focus pulsed laser light into a narrow laser beam to increase the scan resolution and range of object  212 . 
     As part of LiDAR system  102 , receiver  204  may be configured to detect a returned laser beam  211  returned from object  212  in a different direction. Receiver  204  can collect laser beams returned from object  212  and output electrical signal reflecting the intensity of the returned laser beams. Upon contact, laser light can be reflected by object  212  via backscattering, such as Rayleigh scattering, Mie scattering, Raman scattering, and fluorescence. As illustrated in  FIG. 2 , receiver  204  may include a lens  214  and a photodetector  216 . Lens  214  may be configured to collect light from a respective direction in its field of view (FOV). At each time point during the scan, returned laser beam  211  may be collected by lens  214 . Returned laser beam  211  may be returned from object  212  and have the same wavelength as laser beam  209 . 
     Photodetector  216  may be configured to detect returned laser beam  211  returned from object  212 . Photodetector  216  may convert the laser light (e.g., returned laser beam  211 ) collected by lens  214  into an electrical signal  218  (e.g., a current or a voltage signal). The current is generated when photons are absorbed in the photodiode. In some embodiments of the present disclosure, photodetector  216  may include an avalanche photodiode (APD), such as a single photon avalanche diode (SPAD), a SPAD array, or a silicon photo multiplier (SiPM). 
     Although scanner  210  is described as part of transmitter  202 , it is understood that in some embodiments, scanner  210  can be part of receiver  204 , e.g., before photodetector  216  in the light path. The inclusion of scanner  210  in receiver can ensure that photodetector  216  only captures light, e.g., returned laser beam  211  from desired directions, thereby avoiding interferences from other light sources, such as the sun and/or other LiDAR systems. By increasing the aperture of mirror assembly in scanner  210  in receiver  204 , the sensitivity of photodetector  216  can be increased as well. 
     As described above, the incident angle of incident laser beam  207  may be fixed relative to scanner  210 , and the scanning of laser beam  209  may be achieved by rotating a single micro mirror or an array of micromachined mirror assembly in scanner  210 .  FIG. 3A  illustrates a schematic diagram of an exemplary micromachined mirror assembly  300 , according to embodiments of the disclosure.  FIG. 3B  illustrates a top perspective view of micromachined mirror assembly  300  in  FIG. 3A , according to embodiments of the disclosure. Different from some micromachined mirror assemblies having fixed resonant frequencies that cannot be adjusted during the operation, the operational resonant frequency of micromachined mirror assembly  300  can be adjusted online during the operation, thereby allowing resonant frequency compensation due to temperature variation and/or enabling resonant frequency match among multiple micromachined mirror assemblies (e.g., in an array) due to fabrication process variation. 
     As illustrated in  FIGS. 3A-3B , micromachined mirror assembly  300  may include a micro mirror  302  and a pair of first and second suspended beams  304  and  306  mechanically connected to micro mirror  302  along an axis  303  of micro mirror  302 . Micro mirror  302  may be configured to tilt around axis  303  as suspended beams  304  and  306  rotate due to the rigid joint between micro mirror  302  and suspended beams  304  and  306 . In some embodiments, micro mirror  302  and suspended beams  304  and  306  are formed using MEMS microfabrication techniques from a same rigid semiconductor structure, such as a silicon wafer. Micro mirror  302  may be covered by a reflective layer disposed on its top surface (facing incident laser beam). The reflective layer may be reflective to an incident laser beam, which is reflected by micromachined mirror assembly  300  to form a reflected laser beam. By tilting micro mirror  302 , the incident laser beam may be reflected to a different direction, i.e., to form another reflected laser beam. It is understood that although micro mirror  302  is in a rectangle shape as shown in  FIGS. 3A-3B , it is understood that the shape of micro mirror  302  is not limited to a rectangle shape and may vary in other examples, such as a square, round, or eclipse shape. 
     Micromachined mirror assembly  300  may further include a pair of first and second anchors  308  and  310  each mechanically coupled to a respective end of suspended beam  304  or  306  that is farther away from micro mirror  302  and along axis  303 . The other end of suspended beam  304  or  306  is mechanically coupled to micro mirror  302 . Each one of anchors  308  and  310  is affixed on a base (not shown) of micromachined mirror assembly  300 , according to some embodiments. Anchors  308  and  310  may be affixed to the base as both are formed using MEMS microfabrication techniques from a same rigid semiconductor structure, such as a silicon wafer or may be joined together using thermal bonding, adhesive bonding, or soldering. Each one of suspended beams  304  and  306  is suspended from the base, i.e., leaving a space therebetween, to allow certain movement (e.g., rotation and/or displacement) of suspended beams  304  and  306  with respect to the base and anchors  308  and  310 . In some embodiments, each one of suspended beams  304  and  306  is configured to tilt around axis  303 , thereby driving the rotation of micro mirror  302 . In some embodiments, each one of suspended beams  304  and  306  is made of a rigid material, such as silicon, with substantially zero displacement in a direction along axis  303  (i.e., the axial direction). 
     As illustrated in  FIGS. 3A-3B , micromachined mirror assembly  300  may further include a pair of first and second actuators  312  and  314  mechanically coupled to pair of suspended beams  304  and  306 , respectively. A first actuator  312  may be configured to apply a first torsional stress around axis  303  to a first suspended beam  304 , and a second actuator  314  may be configured to apply a second torsional stress around axis  303  to a second suspended beam  306 . Consistent with some embodiments of the present disclosure, the first torsional stress and second torsional stress have different magnitudes, resulting in a magnitude difference between the stresses. In other words, first and second actuators  312  and  314  can create unbalanced torsional stresses on suspended beams  304  and  306  on different sides of micro mirror  302  along axis  303 . The unbalanced torsional stresses (i.e., a torsion) can be translated into a tensional stress in the axial direction, which can increase the resonant frequency of micro mirror  302 . In some embodiments, the tensional stress caused by the torsion is maintained during the operation of micromachined mirror assembly  300 , thereby tuning the operational resonant frequency of micro mirror  302  from its initial resonant frequency. It is understood that the direction of torsion may not affect the increase of the operational resonant frequency of micro mirror  302 . That is, the first torsional stress applied by first actuator  312  may be greater than the second torsional stress applied by second actuator  314 , or vice versa. 
     As illustrated in  FIGS. 3A-3B , first and second actuators  312  and  314  may be electrostatic actuators, such as a set of comb drives. Electrostatic actuators rely on the force between two conducting electrodes when a voltage is applied between them. Depending on the arrangement of the electrodes, various types of electrostatic actuators are possible, such as comb drive electrostatic actuators, parallel plate electrostatic actuators, rotational electrostatic actuators cantilever electrostatic actuators, to name a few. For example, as shown in  FIG. 3B , first actuator  312  may be an electrostatic comb drive actuator that includes a moveable comb  320  fixed to first suspended beam  304  and a pair of fixed combs  322  and  324  fixed to the base on different sides of first suspended beam  304 . First suspended beam  304  and moveable comb  320  may be arranged in a plane above fixed combs  322  and  324 . By alternatingly applying a voltage signal to the pair of fixed combs  322  and  324 , first suspended beam  304  and moveable comb  320  can tilt around axis  303 . Similarly, second actuator  314  may be an electrostatic comb drive actuator that includes moveable comb  326  fixed to second suspended beam  306  and a pair of fixed combs  328  and  330  fixed to the base on different sides of second suspended beam  306 . Second suspended beam  306  and moveable comb  326  may be arranged in a plane above fixed combs  328  and  330 . By alternatingly applying a voltage signal to the pair of fixed combs  328  and  330 , second suspended beam  306  and moveable comb  326  can tilt around axis  303  as well. 
     In some embodiments, the unbalanced torsional stresses created by first and second actuators  312  and  314  are achieved by applying two different AC voltages V 1  and V 2  to first and second actuators  312  and  314 , respectively. The difference between AC voltages V 1  and V 2  may be converted into the magnitude difference of the first and second torsional stresses by a pair of electrostatic actuators, such as two sets of comb drives, as shown in  FIGS. 3A-3B . The difference between AC voltages V 1  and V 2  may be created in any suitable ways, such as by introducing a DC offset (DC bias) or a phase offset (phase shift). For example, as shown in  FIG. 4A , a DC offset Av may be applied to one of the two AC voltages, such as V 1 , to cause the difference of voltage magnitude between V 1  and V 2  at each time point. In another example as shown in  FIG. 4B , a phase offset may be applied to one of the two AC voltages, such as V 2 , to cause a phase shift in V 2  relative to V 1 . The phase shift in turn can cause the difference of voltage magnitude between V 1  and V 2  at each time point. In some embodiments, the difference of voltage magnitude between V 1  and V 2  is maintained to be at substantially the same level at each time point to maintain a constant operational frequency increase during the entire operation cycle. The control of voltage signals V 1  and V 2  may be achieved by a controller (not shown) operatively coupled to first and second actuators  312  and  314  to create and maintain the operational frequency increase as the desired level. 
     It is understood that the type of electrostatic actuators for creating unbalanced torsional stresses is not limited to comb drive actuators and can include any other suitable electrostatic actuators, such as parallel plate electrostatic actuators, rotational electrostatic actuators, or cantilever electrostatic actuators, to name a few. It is also understood that the type of actuators for creating unbalanced torsional stresses is not limited to electrostatic actuators and can include any other suitable actuators, such as piezoelectric actuators, electromagnetic actuators, thermal actuators, etc. 
     As described above, the torsion induced by first and second actuators  312  and  314  can increase the resonant frequency of micro mirror  302  from its initial resonant frequency. In some embodiments, to allow tuning of the operational resonant frequency of micro mirror  302  in both ways, i.e., increase and decrease, heating elements  316  and  318  may be included as part of micromachined mirror assembly  300 . Heating elements  316  and  318  may be disposed under and thermally coupled to both first and second suspended beams  304  and  306 , respectively (as shown in  FIG. 3A ), or may be disposed under and thermally coupled to just one of first and second suspended beams  304  and  306 . During the operation of micromachined mirror assembly  300 , heating elements  316  and  318  can heat first suspended beam  304  and/or second suspended beam  306  to increase the temperature thereof. The resulting thermal expansion of first suspended beam  304  and/or second suspended beam  306  can cause the decrease of the operational resonant frequency of micro mirror  302 . In some embodiments, a controller (not shown) is configured to dynamically tune the operational resonant frequency of micro mirror  302  by adjusting the voltage signals V 1  and V 2  applied to first and second actuators  312  and  314  and/or adjusting the temperature of heating elements  316  and  318 . 
     In addition to indirectly translating a torsion into the tensional stress, another way to introducing tensional stress for increasing resonant frequency of a micro mirror is to directly apply a tensional stress through one or two suspended beams along the axis of the micro mirror. For example,  FIG. 5  illustrates a schematic diagram of another exemplary micromachined mirror assembly  500 , according to embodiments of the disclosure. Similar to micromachined mirror assembly  300  described above in  FIGS. 3A-3B , micromachined mirror assembly  500  also includes a micro mirror  502 , a first suspended beam  504  and a second suspended beam  506  each mechanically coupled to micro mirror  502  along an axis  503  of micro mirror  502 , and a first anchor  508  and a second anchor  510  each fixed on the base of micromachined mirror assembly  500  and mechanically coupled to a respective end of first or second suspended beam  504  or  506 . In some embodiments, micromachined mirror assembly  500  further includes a first torsional actuator  512  mechanically coupled to first suspended beam  504  and configured to apply a first torsional stress around axis  503  to first suspended beam  504 , and a second torsional actuator  514  mechanically coupled to second suspended beam  506  and configured to apply a second torsional stress around axis  503  to second suspended beam  506 . The first torsional stress and second torsional stress may be different to create a torsion that can be translated into a tensional stress in the axial direction, as described above with respect to micromachined mirror assembly  300 . The details of micro mirror  502 , first and second suspended beams  504  and  506 , first and second anchors  508  and  510 , and first and second torsional actuators  512  and  514  of micromachined mirror assembly  500  have been described above with respect to their counterparts of micromachined mirror assembly  300  in  FIGS. 3A-3B  and thus, are not repeated. 
     As shown in  FIG. 5 , micromachined mirror assembly  500  further includes a pair of tensional actuators  516  and  518  mechanically coupled to an end of respective first and second suspended beams  504  and  506  and configured to apply a tensional stress along axis  503  to first and second suspended beams  504  and  506 . The tensional stress can be applied directly by tensional actuators  516  and  518  in the axial direction away from micro mirror  502  to increase the operational frequency of micro mirror  502 . It is understood that in some embodiments, instead of having tensional actuators  516  and  518  on both sides of micro mirror  502  as shown in  FIG. 5 , only one of tensional actuators  516  and  518  is kept on one side of micro mirror  502 , i.e., mechanically coupled to one of suspended beams  504  and  506 , which still can apply a tensional stress along axis  503 . Tensional actuators  516  and  518  can be any suitable actuators that can apply a tensional stress to first and second suspended beams  504  and  506  in the axial direction away from micro mirror  502 , i.e., pulling first and second suspended beams  504  and  506  away from micro mirror  502  to increase the tension axially within micromachined mirror assembly  500 , and thus, increase the resonant frequency of micro mirror  502  during its operation. Tensional actuators  516  and  518  can be, for example, electrostatic actuators, piezoelectric actuators, electromagnetic actuators, thermal actuators, etc. 
     For example,  FIG. 6A  illustrates a schematic diagram of a design of micromachined mirror assembly  500  in  FIG. 5 , according to embodiments of the disclosure.  FIG. 6B  illustrates a schematic diagram of another design of micromachined mirror assembly  500  in  FIG. 5 , according to embodiments of the disclosure. In both examples, a tensional actuator, such as one or more sets of comb drive electrostatic actuators, is disposed on one side of micro mirror  502  to apply a tensional stress along axis  503  of micro mirror  502  to increase the operational resonant frequency of micro mirror  502 . It is understood that in other embodiments, another tensional actuator may be similarly disposed on the other side of micro mirror  502  as well. 
     In one design as shown in  FIG. 6A , a set of comb drives  602  is mechanically coupled to an end of second suspended beam  506  and configured to apply a tensional stress along axis  503  to second suspended beam  506 . In some embodiments, set of comb drives  602  include a fixed comb  604  fixed to second anchor  510  that does not move relative to the base of micromachined mirror assembly  500 , and a movable comb  606  fixed to second suspended beam  506  that is movable along the axial direction. By applying a voltage to set of comb drives  602 , movable comb  606  can be attracted by fixed comb  604  toward second anchor  510  and thus, create a tensional stress to second suspended beam  506  in the axial direction away from micro mirror  502 . It is understood that fixed comb  604  may not be fixed to anchor  510  in some embodiments. In some embodiments, fixed comb  604  is electrically separated from the rest of components (e.g., micro mirror  502 , first and second suspended beams  504  and  506 , first and second anchors  508  and  510 , and movable comb  606 ) in micromachined mirror assembly  500 . 
     In another design as shown in  FIG. 6B , two sets of comb drives  610  and  616  each is mechanically coupled to a respective end of a first sub-suspended beam  608  and a second sub-suspended beam  614 . First and second sub-suspended beams  608  and  614  may be mechanically coupled to a connection point of second suspended beam  506  in a certain angle to form rigid joints, such that tensional stresses applied to first and second sub-suspended beams  608  and  614  can be transferred to second suspended beam  506 , which can be in turned transferred to micro mirror  502  along axis  503  of micro mirror  502  to increase the operational resonant frequency of micro mirror  502 . In some embodiments, the connection point is close to anchor  510  to minimize the rotation of first and second sub-suspended beams  608  and  614  around axis  503 . Similar to set of comb drive  602  described above with respect to  FIG. 6B , each set of comb drives  610  or  616  may include a movable comb drive fixed to respective first sub-suspended beam  608  or second sub-suspended beam  614 , and a fixed comb drive fixed to a respective anchor  612  or  618 . By applying a voltage to each set of comb drives  610  or  616 , each movable comb can be attracted by the respective fixed comb toward anchor  612  or  618  and thus, create a tensional stress to respective first sub-suspended beam  608  or second sub-suspended beam  614 , which can be in turned transferred to second suspended beam  506  along axis  503 . 
     Referring back to  FIG. 5 , in some embodiments, to allow tuning of the operational resonant frequency of micro mirror  502  in both ways, i.e., increase and decrease, heating elements  520  and  522  may be included as part of micromachined mirror assembly  500 . Heating elements  520  and  522  may be disposed under and thermally coupled to both first and second suspended beams  504  and  506 , respectively (as shown in  FIG. 5 ), or may be disposed under and thermally coupled to just one of first and second suspended beams  504  and  506 . During the operation of micromachined mirror assembly  500 , heating elements  520  and  522  can heat first suspended beam  504  and/or second suspended beam  506  to increase the temperature thereof. The resulting thermal expansion of first suspended beam  504  and/or second suspended beam  506  can cause the decrease of the operational resonant frequency of micro mirror  502 . In some embodiments, a controller (not shown) is configured to dynamically tune the operational resonant frequency of micro mirror  502  by adjusting the voltage signals applied to first and second torsional actuators  512  and  514 , the voltage signals applied to first and second tensional actuators  516  and  518 , and/or adjust the temperature of heating elements  520  and  522 . 
       FIG. 7  illustrates a flow chart of an exemplary method  700  for driving a micromachined mirror assembly, according to embodiments of the disclosure. For example, method  700  may be implemented by micromachined mirror assemblies  300  and  500  described above. However, method  700  is not limited to that exemplary embodiment. Method  700  may include steps S 702 -S 706  as described below. It is to be appreciated that some of the steps may be optional to perform the disclosure provided herein. Further, some of the steps may be performed simultaneously, or in a different order than shown in  FIG. 7 . 
     In step S 702 , a resonant frequency of a micromachined mirror assembly is set at an initial value. The initial value may be pre-set by the design and fabrication process of the micromachined mirror assembly. In some embodiments, the initial value is pre-set as the minimum value, which can be increased to a higher value during the operation by increasing the resonant frequency during the operation of the micromachined mirror assembly. In some embodiments, the initial value is pre-set as the maximum value, which can be decreased to a lower value during the operation by decreasing the resonant frequency during the operation of the micromachined mirror assembly. 
     In step S 704 , a tensional stress is applied along an axis of the micromachined mirror assembly to increase the resonant frequency to a first operational value greater than the initial value during the operation of the micromachined mirror assembly. In some embodiments, the tensional stress is directly applied to one or two suspended beams of the micromachined mirror assembly by one or more tensional actuators. In some embodiments, the tensional stress is indirectly transferred from unbalanced torsional stresses applied to two suspended beams of the micromachined mirror assembly by a plurality of torsional actuators. In some embodiments, two different voltages are applied to two electrostatic actuators on different sides of the micro mirror of the micromachined mirror assembly, respectively, to create two torsional stresses with different magnitudes. In some embodiments, the initial value of the resonant frequency is pre-set as the minimum value, such that the operational resonant frequency is increased to the desired first operational value by applying a suitable level of tensional stress along the axis of the micromachined mirror assembly. 
     In step S 706 , heat is applied to the micromachined mirror assembly to decrease the resonant frequency to a second operational value smaller than the initial value during the operation of the micromachined mirror assembly. In some embodiments, heating elements are thermally coupled to one or two suspended beams of the micromachined mirror assembly to heat the suspended beam(s), thereby decreasing the second operational value. In some embodiments, the initial value of the resonant frequency is pre-set as the maximum value, such that the operational resonant frequency is increased to the desired second operational value by applying a suitable level of heat to the micromachined mirror assembly. 
     It is understood that in some embodiments, the initial value of the resonant frequency of the micromachined mirror assembly is pre-set at neither the maximum value nor the minimum value, and step S 704  and step S 706  can be both performed in any suitable times and order to dynamically tune the initial value to a desired operational value in both ways. 
     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.