Patent Publication Number: US-11378806-B1

Title: Multi-element electromechanical actuation mechanism for asymmetric optical applications

Description:
BACKGROUND 
     Some systems have the ability to move and reposition optical elements. For example, uniaxial lens translation mechanisms exist for various consumer products, scientific instruments, medical devices, and sensing systems. These mechanisms are generally employed in applications such as sensor imaging for zoom and autofocus functions, as well as in laser beam shaping and divergence control. Cameras, lasers, and sensor optical systems often use symmetrical cylindrically-profiled lens elements that are displaced using a single linear or rotary actuator to achieve a desired uniaxial motion. Another adjustment mechanism employs a “tube within a tube” design comprising a motor powered rotating collar having helical slots that displace an internal lens holder. The lens holder is displaced using cam followers that engage with the helical slots. 
     A problem arises in the case when the optical element is asymmetric (e.g., a non-circular lens) and/or requires unequal, precise, displacements at several points around the perimeter of the optical element. In addition, low size, weight, power, and cost are desirable for the optical path components, especially when the optical element is used in a wearable device such as head-mounted display (HMD). HMDs are a wearable form of near-eye display (NED) and are sometimes used for displaying content in an augmented reality (AR) or virtual reality (VR) system. In the case of an HMD, a user typically views displayed content through an optical aperture, which should be kept free of obstructions that might block the view of the user. 
     SUMMARY 
     Described herein is an actuator assembly comprising an electromechanical actuation mechanism for adjusting an optical element in an optical system. In some embodiments, the optical element is adjusted by translating the optical element along a linear axis of motion, moving the entire optical element. In other embodiments, the optical element is adjusted by applying force upon a surface of the optical element. For example, in some embodiments, force is applied upon a flexible membrane of a liquid lens to shape the liquid lens. Embodiments described herein are suitable for use with symmetric (e.g., circular) optical elements, but are especially advantageous for applications that involve asymmetric optical elements, as well as applications that require non-uniform (unequal) force or displacement around a perimeter of an optical element being adjusted. In particular, some embodiments may be used for applying unequal force around the perimeter of a non-circular lens in order to achieve a desired optical effect (e.g., a desired optical power), and to synchronously apply non-uniform force/displacement to multiple lenses. 
     In certain embodiments, an actuator assembly includes a plurality of lead screws and a mechanical linkage that intercouples the plurality of leads screws. The mechanical linkage is configured to simultaneously rotate the plurality of lead screws. The actuator assembly further includes at least one displacement element. Each displacement element is configured to act upon a respective optical element to which the displacement element is coupled, through translational motion of the displacement element in response to rotation of the plurality of lead screws. 
     In certain embodiments, a system includes a head-mounted device and an actuator assembly. The head-mounted device includes an optical system with at least one optical element, wherein at the least one of the optical element includes a lens. The actuator assembly is housed within the head-mounted device and includes a plurality of lead screws and a mechanical linkage that intercouples the plurality of leads screws. The mechanical linkage is configured to simultaneously rotate the plurality of lead screws. The actuator assembly further includes at least one displacement element. Each displacement element is configured to act upon a respective optical element of the at least one optical element, through translational motion of the displacement element in response to rotation of the plurality of lead screws. 
     In certain embodiments, a method includes determining, by one or more processors of a computer system, a desired optical characteristic of an optical system including at least one optical element. The method further includes determining, by the one or more processors, a target position for at least one displacement element in an actuator assembly based on the desired optical characteristic. The actuator assembly includes an actuator configured to produce rotational output, a plurality of lead screws, and a mechanical linkage that intercouples the plurality of leads screws. The mechanical linkage is configured to simultaneously rotate the plurality of lead screws based on the rotational output produced by the actuator. The actuator assembly further includes the at least one displacement element. Each displacement element is configured to act upon a respective optical element of the at least one optical element, through translational motion of the displacement element in response to rotation of the plurality of lead screws. The method further includes causing, by the one or more processors, power to be applied to the actuator to move the at least one displacement element toward the target position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments are described with reference to the following figures. 
         FIGS. 1 and 2  show examples of near-eye displays suitable for implementing one or more embodiments. 
         FIG. 3  shows a cross section of a near-eye display suitable for implementing one or more embodiments. 
         FIG. 4  is a perspective view of an actuator assembly, according to an embodiment. 
         FIG. 5  is a front view of the actuator assembly of  FIG. 4 . 
         FIG. 6  is an exploded view of the actuator assembly of  FIG. 4 . 
         FIG. 7  is a perspective view of a floating drive nut mechanism that can be used to implement an actuator assembly, according to an embodiment. 
         FIG. 8  is a front view of a belt slip prevention mechanism that can be used to implement an actuator assembly, according to an embodiment. 
         FIG. 9  shows cross-sectional views of an actuator assembly in different states of actuation, according to an embodiment. 
         FIG. 10  is a flowchart of a method for adjusting an optical system using an actuator assembly, according to an embodiment. 
         FIG. 11  is a block diagram of a system in which one or more embodiments may be implemented. 
     
    
    
     The figures depict embodiments of the present disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated may be employed without departing from the principles, or benefits touted, of this disclosure. 
     In the appended figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label by a dash and a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label. 
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, specific details are set forth in order to provide a thorough understanding of certain inventive embodiments. However, it will be apparent that various embodiments may be practiced without these specific details. The figures and description are not intended to be restrictive. 
     Example embodiments relate to an electromechanical actuation mechanism for adjusting an optical element in an optical system, for example, by displacing all or a portion of the optical element. In some embodiments, the optical element is adjusted by translating the optical element along a linear axis of motion, moving the entire optical element. In other embodiments, the optical element is adjusted by applying force upon a surface of the optical element. For example, in some embodiments, force is applied upon a flexible membrane of a liquid lens to shape the liquid lens. Embodiments described herein are suitable for use with symmetric (e.g., circular) optical elements, but are especially advantageous for applications that involve asymmetric optical elements, as well as applications that require non-uniform (unequal) force or displacement around a perimeter of an optical element being adjusted. In particular, some embodiments may be used for applying unequal force around the perimeter of a non-circular lens in order to achieve a desired optical effect (e.g., a desired optical power), and to synchronously apply non-uniform force/displacement to multiple lenses. 
     Example applications for an embodiment of the present disclosure include moving or distorting non-circular ophthalmic lenses for presbyopia correction or visual accommodation correction in a stand-alone HMD (e.g., an AR or VR device with an integrated controller) or in a system employing an HMD (e.g., an AR/VR device and a remote console controlling the AR/VR device). Accommodation refers to the change of optical power within the human eye as distance to the viewed object changes. Presbyopia is an age related condition characterized by lack of range of focus of the eye and inability to focus on close objects. An embodiment of a system according to the present disclosure could perform accommodation correction to provide a better viewing experience to the user by, for example, changing the relationship of the focal distance of a display image to a real-world image to correct for the natural focal shift of the user&#39;s eye when looking at near field objects, thereby keeping the display image in focus regardless of the eye&#39;s natural focal plane. The system could also correct for presbyopia by, for example, adaptively changing a focal length of a lens to provide the necessary corrective optical power to the user&#39;s eye at different focal distances (e.g., continuously adjusting the focal length to cover all near field focal distances). 
     Other potential applications include manipulation of non-circular lenses that are not used for viewing by a user, such as lenses in a sensor system. Embodiments can also be applied for asymmetric laser beam shaping or any other application where displacement or distortion of an optical element is desired. 
     In some embodiments, the optical elements include one or more liquid lenses. However, it is understood that the embodiments can be applied for displacement/distortion of other types of optical elements, such as solid lenses. Liquid lenses comprise a sealed cavity filled with fluid, e.g., a fluid having one or more desired properties such as a particular index of refraction, a particular viscosity, and/or a particular degree of light transmissivity. According to some embodiments, a liquid lens is shaped by expanding or compressing a flexible membrane of the lens. Expansion or compression of the membrane causes the fluid to be displaced to create an optical surface on one side of the lens. For example, optical power can be changed by applying pressure to the membrane to mechanically displace fluid from the perimeter of the lens toward the optical center of the lens, causing the membrane to bulge, thereby increasing refraction of light and thus the optical power of the lens. 
     Example embodiments relate to an electromechanical actuation mechanism that is operable to effect displacement of an optical element at multiple points around a perimeter of the optical element, using components located outside of an optical aperture. In this manner, the actuation mechanism can precisely control the displacement without impeding light transmission or image quality through the optical element, making the actuation mechanism especially suited for use with HMDs and other wearable devices where the optical aperture is a viewing aperture. 
     Example embodiments relate to an actuator assembly comprising an electromechanical actuation mechanism for synchronized displacement of a plurality of optical elements that are intercoupled to permit the optical elements to be driven by a single actuator. The ability to precisely control displacement of multiple optical elements using a single actuator facilitates flexible and rapid configuration of the optical system while minimizing size, weight, power consumption, and cost. 
     In some embodiments, an actuator assembly includes an actuator operable to produce rotational output (e.g., a motor), a plurality of lead screws including a first lead screw driven by the rotational output of the actuator, and a mechanical linkage (e.g., a belt or cable) configured to simultaneously rotate the plurality of lead screws (e.g., by distributing torque from the first lead screw to a remainder of the plurality of lead screws). The actuator assembly further includes at least one displacement element (e.g., a lens holder or displacement ring). Each displacement element is configured to act upon a respective optical element to which the displacement element is coupled, through translational motion of the displacement element in response to rotation of the plurality of lead screws. 
     In some embodiments, a system includes an HMD that houses an actuator assembly. The HMD includes an optical system with a plurality of optical elements, at least one of which is a lens that the actuator assembly acts upon. 
     In some embodiments, a method performed by one or more processors of a computer system includes determining, by the one or more processors, a desired optical characteristic of an optical system including at least one optical element. The method further includes determining (e.g., mapping or calculating), by the one or more processors, a target position for at least one displacement element in an actuator assembly based on the desired optical characteristic. The method further includes causing, by the one or more processors, power to be applied to an actuator of the actuator assembly to move the at least one displacement element toward the target position (e.g., using a belt or cable to simultaneously drive a plurality of lead screws). 
     In some embodiments, a displacement element is moved by different amounts (i.e., non-uniform displacement) at various points along the displacement element. In some embodiments, different displacement elements are non-uniformly displaced with respect to each other. The movements for a single displacement element or for multiple displacement elements can be synchronized using a mechanical linkage that causes a plurality of lead screws to simultaneously rotate. Depending on how the lead screws are threaded, different amounts and/or directions of displacement can be produced. 
     Embodiments of the present disclosure may include or be implemented in conjunction with an artificial reality system. Artificial reality is a form of reality that has been adjusted in some manner before presentation to a user, which may include, e.g., a VR, an AR, a mixed reality (MR), a hybrid reality, or some combination and/or derivatives thereof. Artificial reality content may include completely generated content or generated content combined with captured (e.g., real-world) content. The artificial reality content may include video, audio, haptic feedback, or some combination thereof, and any of which may be presented in a single channel or in multiple channels (such as stereo video that produces a three-dimensional effect to the viewer). Additionally, in some embodiments, artificial reality may also be associated with applications, products, accessories, services, or some combination thereof, that are used to, e.g., create content in an artificial reality and/or are otherwise used in (e.g., perform activities in) an artificial reality. The artificial reality system that provides the artificial reality content may be implemented on various platforms, including an NED connected to a host computer system, a standalone NED, a mobile device or computing system, or any other hardware platform capable of providing artificial reality content to one or more viewers. 
       FIG. 1  shows an NED  100  suitable for implementing one or more embodiments. The NED  100  presents media to a user. Examples of media presented by the NED  100  include one or more images, video, and/or audio. In some embodiments, audio is presented via an external device (e.g., speakers and/or headphones) that receives audio information from the NED  100 , a console, or both, and presents audio output based on the audio information. The NED  100  can be configured to operate as a VR display. In some embodiments, the NED  100  is modified to operate as an AR display and/or an MR display. 
     The NED  100  includes a frame  105  and a display device  110 . The frame  105  is shaped to enable the NED  100  to be worn in the manner of a pair of eyeglasses. Thus, the NED  100  is an example of an HMD. The frame  105  is coupled to one or more optical elements (e.g., lenses integral with the display device  110 ). The display device  110  is configured for the user to see content presented by NED  100 . In some embodiments, the display device  110  comprises a waveguide display assembly for directing light from one or more images to an eye of the user. 
     The NED  100  may include one or more optical sensors (not shown) that capture optical data about the user and/or the external environment. For example, the optical sensors may include at least one pixel cell array comprising an plurality of pixel cells (e.g., a two-dimensional (2D) pixel cell array) configured to generate image data representing a particular field of view along a particular direction toward the user or toward the external environment. 
     In some embodiments, the NED  100  may include one or more active illuminators configured to project light toward the user and/or toward the external environment. Active illuminators are activated using electrical signals that cause the illuminators to project light. The projected light may form one or more light patterns, can be associated with different frequency spectrums (e.g., visible light, infrared (IR) light, near infrared (NIR) light, ultra-violet (UV) light, etc.), and can serve various purposes, including illuminating the user&#39;s face in connection with eye tracking or facial recognition and illuminating the external environment in connection with tracking of the location or head movement of the user. 
       FIG. 2  shows an NED  200  suitable for implementing one or more embodiments. Like the NED  100 , the NED  200  is an HMD designed to be worn by a user. The NED  200  includes a housing  220  and a display device (not shown) inside the housing. The display device may be positioned near a front side  225  of the NED  200 . The housing  220  forms an enclosed viewing environment for the user and includes a pair of eye cups  230 -A and  230 -B that are attached to a top side  223  of the NED and surround the eyes of a user when the NED is being worn. The NED  200  further includes a strap  240  configured to secure the NED against a back of the user&#39;s head and various electronics (e.g., an antenna unit  250  for wireless communication with a remote computing device) located on a right side  227  of the NED  200 . Both the NED  100  and the NED  200  may include optical elements that are integral with a display device or located along an optical path between the display device and an eye of the user. 
       FIG. 3  shows a cross section of an NED  300  suitable for implementing one or more embodiments. The NED  300  may correspond to the NED  100  or the NED  200 . The NED  300  includes a frame or housing  305 , a display device  310 , and an optical system  340 . The display device  310  is configured to present image content to the user. The optical system  340  is configured to direct image light from the display device  310  to an eye  320  of the user. The optical system  340  may include a waveguide, lenses, and/or other optical elements that guide or adjust the image light  350 . In some embodiments, additional optical elements are embedded within the display device. When placed into an operative position with respect to the user, e.g., when the user wears the NED  300 , the NED  300  forms an exit pupil  330  at a location where the eye  320  is positioned in an eyebox region. For purposes of illustration,  FIG. 3  shows the cross section associated with a single eye  320  and optical system  340 , but a second optical system  340  can be used for a second eye of the user. 
     The optical system  340  is configured to direct the image light to the eye  320  through the exit pupil  330 . The optical system  340  may include optical elements composed of one or more materials (e.g., plastic, glass, etc.) with one or more refractive indices. For example, the optical system  340  may include a waveguide composed of one or more materials with one or more refractive indices that effectively minimize the weight and widen a field of view (FOV) of the NED  300 . The optical system may include other types of optical elements that adjust or guide the image light  350 , e.g., to correct aberrations in the image light or magnify the image light. Example optical elements include an aperture stop, a Fresnel lens, a convex lens, a concave lens, a filter, a reflector, or any other suitable optical element that affects image light. In some embodiments, the optical system is part of or attached to the display device  310 . In other embodiments, the optical system is separate from the display device  310  and located along an optical path between the eye  320  and the display device  310 . The optical system may be driven by a controller of the NED  300  to, for example, adjust one or more optical characteristics of the optical system  340  using an electromechanical actuation mechanism in accordance with an embodiment described herein. 
       FIG. 4  shows a perspective view of an actuator assembly  400  according to an embodiment. The actuator assembly  400  can be incorporated into a wearable device that includes an optical system, e.g., an HMD. The actuator assembly  400  includes an aperture  405 , an actuator  410 , lead screws  420 -A to  420 -D, tension idlers  425 -A to  425 -C, displacement rings  430 -A and  430 -B, a belt  440 , a housing  450 , an end plate  455 , and lenses  460 -A and  460 -B. For simplicity, other types of optical elements have been omitted. However, it is understood that the actuator assembly  400  can include optical elements besides lenses. For example, the lenses  460  may be stacked together with a removable prescription lens or a Fresnel lens. Further, although described with respect to synchronized displacement of a pair of optical elements (the lenses  460 -A and  460 -B), the actuator assembly  400  can be adapted for displacement of any number of optical elements. For example, an additional displacement ring could be introduced for each optical element to be displaced. The embodiment of  FIG. 4  can be used for translation of lenses or other optical elements by moving the displacement rings  430 . 
     The aperture  405  forms a viewing window through which the user can see through the lenses  460 -A and  460 -B. The aperture  405  can include a layer of clear or light transmissive material that forms a protective layer for the lenses. In some embodiments, protective layers may be attached to both the housing  450  and the end plate  455  to seal the actuator assembly  400  against intrusion of dust, water, or other contaminants. Alternatively, the aperture  405  can be an opening defined by the housing  450  and the end plate  455 . 
     The actuator  410  can be an electrically activated motor that generates rotational motion. Various types of motors are suitable for implementing the actuator  410 , including stepper type, servo, direct current (DC), alternating current (AC), brushed, or brushless motors. In  FIG. 4 , the actuator  410  is mounted on a user facing side of the housing  450 . Other locations for the actuator  410  are equally feasible including, for example, on the end plate  455  or along a side of the housing  450 . The actuator  410  is coupled to the lead screw  420 -A through a gear set including a gear  412  attached to the output end of the actuator  410  and a gear  414  (shown in  FIG. 6 ) attached to the lead screw  420 -A. The gear set can be implemented using spur gears (as shown in the figures) if the actuator  410  is mounted with its axis of rotation parallel to the longitudinal axes of the lead screws  420 . Alternatively, the gear set can be implemented using bevel gears if the actuator  410  is mounted with the axis of rotation perpendicular to the longitudinal axes of the lead screws  420 . The ratio of the gears can be set to achieve a desired travel speed for the displacement rings  430 . For example, a speed reduction and torque amplification can be achieved using a smaller drive gear (e.g., the gear  412 ) than driven gear (e.g., the gear  414 ). 
     The lead screws  420  are located around the periphery of the optical elements, e.g., the lenses  460 . The lead screws are placed outside of the aperture  405  in order not to obstruct the view of the user. The lead screws  420  operate to mechanically support the displacement rings  430  and to move the displacement rings  430  linearly in response to rotational output of the actuator  410 . Each lead screw includes a threaded shaft in contact with the displacement rings  430  and a toothed head in contact with the belt  440 . The shaft and head are shown more clearly in  FIGS. 6 and 7 . The threading of the shaft converts the rotary motion of the actuator  410  to linear motion (in  FIG. 4 , motion along the z direction). The lead screws  420  can be threaded in various ways. If uniform displacement is desired, the threading can be made uniform. In one embodiment, each lead screw has an individual, unique thread pitch so as to provide non-uniform displacement. For example, the distance between threads of the lead screw  420 -A could be different from the distance between threads of the lead screw  420 -B. For simplicity, the threading has been omitted from the figures, which depict the shaft sections of the lead screws as being smooth. 
     In some embodiments, one or more of the lead screws  420  are shaped as twin lead screws with a first threaded section at one end and a second threaded section at the opposite end, where the first threaded section is coupled to the displacement ring  430 -A and the second threaded section is coupled to the displacement ring  430 -B. The first threaded section and the second threaded section could be threaded in the same direction (e.g., both right-hand threaded) or in opposite directions (e.g., one section is right-hand threaded and the other section is left-hand threaded). Opposite threading of the sections would enable the lead screws to provide motion in opposite directions (e.g., so that the displacement rings  430 -A and  430 -B move away from each other). If the sections are threaded in the same direction, then the displacement rings  430 -A and  430 -B would move in the same direction. 
     The lead screws  420  can be supported on low friction sintered sleeve bearings or jeweled bearings (not shown). The bearings can be coupled to one or more ends of the lead screws to reduce friction and thus wearing as the lead screws are rotated. For example, bearings may be included at points where the lead screws meet the end plate  455 . 
     The tension idlers  425  can be implemented as pulleys that rotate and prevent the belt  440  from contacting the end plate  455 , thereby avoiding friction between the belt  440  and the end plate  455 . The tension idlers  425  are positioned to provide sufficient tensioning of the belt  440  such that there is little or no slack that could otherwise cause the belt  440  to come into contact with the end plate  455 . 
     The displacement rings  430  are relatively thin (in the z direction) and narrow in width (in the x direction). The thickness may be approximately of the same order of magnitude as the thickness of the lenses  460 . The displacement rings  430  may operate as lens holders for the lenses. In this respect, the width of the displacement rings can be minimized so as not to obstruct the aperture  405 , while being wide enough to securely hold the lenses. 
     In some embodiments, the travel of the displacement rings  430  may be limited using one or more sensors, for example using end-of-travel micro-switches positioned in the housing  450 , such that the displacement rings  430  activate the micro-switches at a desired end of travel, thereby signaling a controller to shut off power supplied to the actuator  410 . Alternatively, travel may be limited using a combination of an electronic shaft encoder (e.g., an absolute position type or relative counter type encoder) coupled with a micro-switch that is positioned on the actuator  410 . The encoder could track the number of revolutions and/or angular position of an output shaft of the actuator  410  (e.g., the shaft to which the gear  412  is attached) to generate a signal indicating to the controller the actual position of the displacement rings  430 , for example through a lookup table. Yet another way to limit travel would be to use a current sensor that senses a current of the actuator  410 , together with a “bumper stop” element at the end of travel (e.g., a linear spring or elastomer that compresses with an increasing reaction force toward the end of travel). This resistance to motion would cause the actuator current to ramp up, allowing a current sensing controller to shut current off at a certain threshold current level corresponding to the end of travel position. 
     The lenses  460  are asymmetrically shaped, with lens  460 -A being mounted on the displacement ring  430 -A and lens  460 -B being mounted on the displacement ring  430 -B. The lenses  460  may be solid lenses. Alternatively, as explained below in connection with  FIG. 9 , lenses can be liquid lenses that include a flexible membrane. The lenses can be mounted to the displacement rings in various ways. For example, the lenses  460  may be mounted using an adhesive or other fixing agent. In some embodiments, the displacement rings  430  may include a grooved inner surface into which the lenses are friction fit in a similar manner to lenses in an eyeglass frame. Other mounting techniques are also possible. 
     The belt  440  operates as a mechanical linkage that converts rotational motion of the lead screw  420 -A into rotational motion of the lead screws  420 -B,  420 -C, and  420 -D. Like the lead screws  420 , the belt  440  is located outside of the aperture  405 . The actuator  410  applies torque via the gears  412  and  414  to the lead screw  420 -A, causing the lead screw  420 -A to displace the displacement ring  430 -A (e.g., by pushing against the displacement ring using a drive nut that is threaded onto the lead screw, as shown in  FIG. 7 ). The belt  440  distributes the torque applied to the lead screw  420 -A to the other lead screws  420 -B,  420 -C, and  420 -D. The belt  440  can be formed of a flexible material (e.g., a polymer or elastomer). The belt  440  can be spring loaded to increase belt tension and therefore increase the angle of wrap against the lead screws  420 , thereby preventing slippage as the belt relaxes over the life of the actuator assembly  400 . In some embodiments, the belt  440  may include teeth that engage the teeth of the lead screws  420 . Other linkage mechanisms are also possible. For example, the belt  440  can be replaced with a beaded cable, a roller chain, or a gear train. 
     The use of a flexible belt for torque distribution helps prevent noise and vibration as compared with other torque distribution methods using more rigid components, due to the energy damping characteristics of the flexible material. The prevention of noise and vibration is desirable for an HMD as the proximity of the user&#39;s ear and direct wave propagation path from the device to the ear will make even slight vibrations and noise easily detectable to the user. In some embodiments, vibration is further reduced by using a soft motor mount between the housing  450  and the actuator  410  to damp any motor imbalance, acceleration or deceleration forces, or other sources of rotating mass-based vibration. 
     The housing  450  provides mechanical support for the actuator  410 , the lead screws  420 , the optical elements, and any other components which may reside within the housing  450 . The housing can be formed of a rigid material such as a metal, a metal alloy, or a polymer metal (i.e., a polymer-metal composite). The housing  450  is mated to the end plate  455  and held in place against the end plate  455  by the lead screws  420 -A to  420 -D. In some embodiments, the housing  450  and the end plate  455  are attached to each other using fixed screws, adhesive, a snap fit or friction fit mechanism, or some other attachment mechanism. The housing  450  may include space for mounting additional optical elements such as fixed-location lenses. If the actuator assembly  400  is incorporated into an HMD or other wearable device, the housing  450  and the end plate  455  can be integrated into the wearable device such that the actuator assembly  400  is housed within the wearable device. 
     The end plate  455  provides thrust support for the lead screws  420  and, if bearings are included, may also provide support for the bearings. The end plate  455  includes a groove that receives the belt  440 , the tension idlers  425 , and the geared portions of the lead screws  420 .  FIG. 5  shows in further detail the arrangement of the lead screws  420 , the belt  440 , and the tension idlers  425  relative to the end plate  455 . 
       FIG. 5  is a front view of the actuator assembly  400 . As shown in the figure, the end plate  455  includes an outer wall  550  and an inner wall  560  that together form a groove in which the lead screws  420 , the belt  440 , and the tension idlers  425  are received. The tension idlers  425  apply enough tension to the belt  440  that the belt maintains continuous contact with the lead screws and does not come into contact with the walls  550  and  560  as the belt  440  moves along the lead screws. The walls  550  and  560  can be continuous or broken into segments. For example,  FIG. 5  shows the inner wall  560  being formed of a first segment  562  that spans a majority of the perimeter of the inner wall and a shorter segment  564  that extends along the bottom of the inner wall, with spaces between the segments. 
     The actuator assembly  400  can include a floating drive nut mechanism (shown in  FIG. 7 ) that allows for a small amount of transverse movement of the displacement rings  430 . The transverse movement accommodates rotation of the displacement rings (e.g., rotation caused by non-uniform displacement at different points along the displacement rings) by preventing side loading of the lead screws  420 . The floating drive nut mechanism also prevents binding of the mechanical linkage (e.g., the belt  440 ) that might otherwise occur as result of structural deflections of the housing  450 . 
       FIG. 6  is an exploded view of the actuator assembly  400 .  FIG. 6  shows in more detail the structure of the lead screws  420 , each of which comprises a shaft section and a head section. For example, the lead screw  420 -D includes a shaft  610  and a head  620 . As mentioned earlier, the shaft may be threaded while the head is toothed. The head  620  operates as a sheave that rotates against the belt  440 . Teeth of the head grip the belt  440  so that the belt  440  moves in response to rotation of the lead screw  420 -A to drive the other lead screws  420 -B,  420 -C, and  420 -D. Other head shapes are also possible, such as a toothless groove with bumps, ridges, or some other surface texture to facilitate gripping of the belt  440 . 
       FIG. 7  is a perspective view of a floating drive nut mechanism  700  that can be used to implement an actuator assembly (e.g., the actuator assembly  400 ). The housing, end plate, lenses, and various other components have been omitted for clarity. The floating drive nut mechanism allows for some structural distortion of the housing as a result of natural forces arising during operation of the actuator assembly, without applying significant side loads to the lead screws  420 . Otherwise, the use of rigid, close tolerance components in the presence of side loads could cause binding of the lead screws  420  or binding of the belt  440 . 
     The floating drive nut mechanism  700  includes a set of drive nuts  710 -A and  710 -B with respective anti-rotation flats  720 -A and  720 -B. The drive nuts  710 -A and  710 -B can be threaded onto a lead screw  420 . Only one lead screw  420  is shown, but corresponding drive nut mechanisms can be used for each of the lead screws. The drive nut  710 -A contacts the displacement ring  430 -A and the drive nut  710 -B contacts the displacement ring  430 -B. The anti-rotation flats  720  contact an inner surface of the housing  450  (not shown) to prevent rotation of the drive nuts  710  as the lead screw  420  is rotated. 
     The inset image shows the structure of the drive nut  710 -B in more detail. The drive nut  710 -A can be similarly shaped. As shown in the inset, the drive nut  710 -B includes a flanged section  712  and a cylindrical section  714 . The flanged section  712  contacts a back side of the displacement ring  430 -B while the cylindrical section  714  is inserted through an opening of the displacement ring  430 -B. The lead screw  420  passes through the cylindrical section  714 , which can be inner-threaded to match the threads of the lead screw. The flanged section  712  of the drive nut  710 -A is separated from the displacement ring  430 -A by a gap  730 . The flanged section  712  of the drive nut  710 -B is likewise separated from the displacement ring  430 -B. Further, the cylindrical section  714  of drive nut  710 -B is separated from the displacement ring  430 -B by a gap  732 . The cylindrical section  714  of drive nut  710 -A is likewise separated from the displacement ring  430 -A. The gaps  730  and  732  provide diametrical clearance between the drive nuts and the displacement rings, allowing for a limited degree of angular movement (e.g., rotation) of the displacement rings before side loading occurs. 
       FIG. 8  is a front view of a belt slip prevention mechanism  800  that can be used to implement an actuator assembly (e.g., the actuator assembly  400 ). Belt slippage is a concern in the presence of external loads and housing distortion, and could result in loss of synchronization between the lead screws. The mechanism  800  comprises a small gap  805  between the end plate  455  and the belt  440  at a location of a lead screw  420 . A similar gap  805  may be provided at the locations of each lead screw  420  in the actuator assembly. The gap  805  is sized to be smaller than a thickness  810  of the belt  440 , preventing the belt  440  from slipping off the lead screw  420  by limiting radial displacement of the belt with respect to the lead screw. In particular, the gap  805  may prevent the belt  440  from radially displacing enough to separate from contact with the teeth  820  of the head of the lead screw  420 . 
       FIG. 9  shows cross-sectional views of an actuator assembly  900  in different states of actuation.  FIG. 9  illustrates the adjustment of a liquid lens using the actuator assembly  900 , which includes components similar to those described above with respect to the actuator assembly  400 . For example, as shown, the actuator assembly  900  includes a pair of displacement rings  930 -A and  930 -B, a pair of lenses  960 -A and  960 -B, a plurality of lead screws  920 -A and  920 -B, and a belt  940 . However, unlike in the actuator assembly  400 , the displacement rings  930  do not operate as lens holders, but are instead used to apply a force around a perimeter of the lenses  960 , which are held stationary against a housing  950  and an end plate  955  of the actuator assembly  900 . The force need not be applied directly along a boundary of the lenses (e.g., because the displacement rings may not be exactly aligned with the edges of the lenses). Therefore, applying a force around a perimeter of a lens or other optical element can include applying force near the edge of the optical element. Each of the lenses  960  includes a flexible membrane  910  that is displaced by the force applied around the perimeter of the lens. Opposite the membrane is a lens surface that contacts the housing  950  or the end plate  955  and which may be rigid or semi-rigid. Alternatively, the lenses  960  can be mounted on stationary holders. Torque distribution can be performed in the same manner as described with respect to the actuator assembly  400 , e.g., using the belt  940  as a mechanical linkage that intercouples the lead screws  920 , with the lead screw  920 -A being driven by the rotational output of an actuator (not shown). 
     The left side of  FIG. 9  shows the displacement rings  930  in a first configuration, with displacement ring  930 -A resting against a membrane  910 -A of the lens  960 -A and displacement ring  930 -B resting against a membrane  910 -B of the lens  960 -B. In this configuration, the lenses  960  are in a relaxed state, with the internal liquid being uniformly distributed across the lens area. The uniform distribution of the liquid produces a relatively flat membrane shape, and therefore low optical power. The membranes  910  may be configured to be in a state of tension in the relaxed state, thereby providing a natural inward spring pressure against the displacement rings  930 . 
     When the lead screws  920  are rotated, the displacement rings  930  move away from each other in the directions shown by the arrows in the right side of the figure (e.g., if the lead screws  920  are twin lead screws with opposite direction threading for the shaft sections that couple to the displacement rings  930 ). The movement of the displacement rings  930  applies pressure to the lens membranes  910 , causing liquid to be pushed from a periphery of the lenses  960  toward the centers of the lenses, so that the membranes  910  bulge toward the interior of the actuator assembly  900  to form a more spherical shape that increases the refraction of light through the lenses. Thus, the configuration shown on the right side of  FIG. 9  has a higher optical power than the first configuration on the left side. 
     In some embodiments, one or more displacement rings  930  are attached to a corresponding membrane, e.g., using an adhesive, so that when the displacement ring is moved away from the lens, the membrane becomes stretched, e.g., to form a concave optical surface that provides negative optical power. Additionally, in some embodiments, the displacement rings  930  may flex in response to rotation of the lead screws  920 . Further, each displacement ring  930  may have a varying cross-sectional thickness, with some sections being thicker and some sections being thinner, in order to facilitate non-uniform deflection of the displacement ring. For example, smaller cross-sections can be used in areas where less deflection is desired, and larger cross-sections can be used in areas where more deflection is desired. 
     Example embodiments of actuator assemblies have been described which use an electrically controlled actuator to produce rotational output that causes a plurality of intercoupled lead screws to rotate. In particular, the actuator drives a first lead screw to cause the remaining lead screws to rotate via a mechanical linkage. In some embodiments, an actuator may be omitted so that rotation of a lead screw is performed manually. Some embodiments may feature an actuator that drives a non-lead screw element (e.g., an elongated element with a non-threaded shaft and a toothed head) that is coupled to the lead screws via the mechanical linkage to provide a rotational motion that causes rotation of the lead screws. Other modifications of the disclosed embodiments are also possible. 
       FIG. 10  is a flowchart of a method  1000  for adjusting an optical system using an embodiment of an actuator assembly. The method  1000  can be applied to adjust an optical system in an HMD, for example, to move or distort (e.g., shape in a defined manner) lenses in the HMD in order to provide presbyopia correction or visual accommodation correction. The method  1000  can also be applied to other types of optical systems including, for example, optical systems that include optical elements for producing an image captured by an image sensor and optical systems that include optical elements for generating focused light output (e.g., for asymmetric laser beam shaping). The method  1000  can be performed by a controller that is implemented in hardware, software, or a combination thereof. For example, the controller may be a control unit of an HMD, or one or more processors executing computer-readable instructions on a computer system. 
     At step  1010 , the controller determines one or more desired optical characteristics of the optical system. The optical characteristics may pertain to any number of parameters that define the performance of the optical system, e.g., optical power, focal length, zoom factor, radius of curvature, etc. The desired optical characteristics can be user selected or determined by the controller, e.g., based on a measurement of the refractive error of an eye of an HMD user. 
     At step  1020 , the controller maps or calculates target positions of a plurality of displacement elements to which optical elements of the optical system are coupled, based on the desired optical characteristics. The displacement elements can be holders on which the optical elements are mounted (e.g., displacement rings that act as lens holders) or elements that apply force to the optical elements (e.g., the displacement rings of  FIG. 9 ). The controller may perform mapping by referencing a stored lookup table or other data structure containing information indicating which target positions achieve the desired optical characteristics. Alternatively, the controller may calculate the target positions using the desired optical characteristics as input. 
     At step  1030 , the controller applies power (e.g., a drive current or voltage) to an actuator of the actuator assembly, causing the actuator assembly to synchronously drive the displacement elements toward their respective target positions using a set of lead screws that are intercoupled via a mechanical linkage. The mechanical linkage can, for example, include a belt or a cable. In some instances, a single displacement element may have multiple target positions. For example, it may sometimes be desirable to use a displacement ring to apply different amounts of force along a perimeter of a lens. This could be achieved by varying the displacement at different areas of the displacement ring (e.g., each lead screw to which the displacement ring is coupled could drive the displacement ring for a different distance). As explained earlier, the actuator assembly can accommodate such asymmetric displacement through appropriate design of the lead screws (e.g., using varying thread pitch, thread direction, and/or thread length). Thus, it may even be possible for one lead screw to drive a displacement element in one direction and another lead screw to drive the same displacement element in the opposite direction. 
     In step  1040 , the controller shuts off power to the actuator upon detecting that the displacement elements have reached their target positions. For example, the controller may receive a signal from a sensor (e.g., a shaft encoder) that enables the controller to determine the positions of the displacement elements. Further, as explained earlier, the actuator assembly may include end-of-travel micro-switches or some other mechanism that limits the travel of the displacement elements (e.g., to prevent the displacement elements from being driven beyond a specific range of positions). Thus, the controller can also shut off power upon detecting that an end-of-travel position has been reached for one or more displacement elements (e.g., to shut off power as soon as an end-of-travel position is detected for any displacement element). 
       FIG. 11  is a block diagram of a system  1100  in which one or more embodiments may be implemented. The system  1100  includes an HMD  1110 , a control unit  1130 , and an input/output (I/O) interface  1140 . The HMD  1110  includes a display device  1112 , an optical system  1113 , an actuator assembly  1114 , at least one proximity sensor  1116 , at least one illuminator  1118 , at least one optical sensor  1120 , at least one position sensor  1122 , and an inertial measurement unit  1124 . 
     The display device  1112  includes a display screen for presenting visual media, such as images and/or video, to a user. In addition to visual media, the HMD  1110  may include an audio output device (not shown) for presenting audio media to the user, e.g., in conjunction with the presentation of the visual media. 
     The optical system  1113  includes at least one optical element (e.g., a lens, a waveguide, a reflector, etc.) that affects image light. Multiple optical elements may be stacked together to, for example, direct and guide light from the display device  1112  toward an eye of the user. 
     The actuator assembly  1114  includes an actuator operable to produce rotational output (e.g., a motor), a plurality of lead screws, and a mechanical linkage (e.g., a belt or cable) configured to simultaneously rotate the plurality of lead screws. The actuator assembly  1114  further includes a plurality of displacement elements (e.g., displacement rings). Each displacement element is configured to act upon a respective optical element to which the displacement element is coupled, through translational motion of the displacement element in response to rotation of the plurality of lead screws. 
     The actuator assembly  1114  may include a housing for the optical elements of the optical system  1113 . The housing of the actuator assembly can be integrated into a housing of the HMD  1110 . For example, the HMD  1110  may include a housing similar to the housing  220  in  FIG. 2 . The actuator assembly  1114  and the optical system  1113  may be positioned along an optical path between the eye of the user and the display device  1112 , with the optical path passing through the optical elements of optical system  1113 . A separate actuator assembly and optical system may be provided for each eye of the user. 
     The proximity sensor  1116  can be a sensor configured to detect that the user is wearing the HMD  1110 . For example, the proximity sensor  1116  can be a simple mechanical switch that is activated when the user&#39;s head is pressed against a frame of the HMD  1110 . Alternatively, the proximity sensor  1016  can be a resistive or capacitive touch sensor configured to detect contact with the user&#39;s head based on electrical measurements. In some embodiments, the proximity sensor  1116  is an optical sensor. 
     The illuminator  1118  is an electrically triggered light source that generates light for use in connection with presentation of image content on the display device. For example, the generated light could be used in combination with one or more optical sensors  1120  to perform eye tracking or tracking of head movements in order to update image content from one or more applications executed by the control unit  1130 . In some embodiments, the generated light may be used to perform a measurement that determines a degree to which a vision of the user needs to be corrected, e.g., a degree of presbyopia or visual accommodation error. The illuminator  1118  can be placed in a frame of the HMD  1110  or integrated into an optical component such as the display device  1112 . 
     The optical sensor  1120  can be an image sensor configured to capture 2D and/or 3D image data, for example, a 2D image of the user&#39;s eye or the external environment around the user. 
     The position sensor  1122  can be a gyroscope, an accelerometer, a global positioning system device, or any other device that detects changes in the location and/or orientation of the HMD  1110 . 
     The inertial measurement unit  1124  is an electronic device that generates data indicating an estimated position of the HMD  1110 , e.g., based on measurement signals received from the position sensor  1122 . The measurement signals can include, for example, signals indicative of roll, pitch, yaw, or acceleration. 
     The I/O interface  1140  is a device that allows the user to send action requests to the control unit  1130 . An action request is a request to perform a particular action. For example, an action request may be to start or end an application or to perform a particular action within the application. 
     The control unit  1130  is configured to direct the operation of the HMD  1110  including, for example, selecting image content for presentation on the display device  1112 , activating the illuminator  1118 , and adjusting the optical system  1113  (e.g., moving or shaping one or more optical elements of the optical system  1113  using an embodiment of an actuator assembly). The control unit  1130  includes an optical system adjustment module  1132 , a tracking module  1134 , one or more processors  1136 , a control information data store  1137 , and an application store  1139 . The control unit  1130  can include components that are integrated into the HMD  1110 . In some embodiments, one or more components of the control unit  1130  are remotely located. For example, the control information data store  1137  can be located on a remote server or distributed between a memory of the control unit  1130  and a remote server. 
     The control unit  1130  outputs signals that control the actuator of the actuator assembly  1114 . The control signals can be sent directly to the actuator (e.g., the control unit  1130  may output voltage or current signals that drive the actuator) or sent to a power source that produces power for the actuator (e.g., a voltage or current generator). 
     The control information data store  1137  stores control information that the control unit  1130  uses for controlling the actuator assembly  1114 . For example, the control information may include a lookup table mapping one or more optical characteristics of the optical system  1113  to target positions of displacement elements in the actuator assembly  1114 . 
     The application store  1139  stores one or more applications for execution by the control unit  1130 . An application is a set of instructions executable by a processor, for example instructions that cause the processor to generate content for presentation to the user on the display device  1112 . Examples of applications include: gaming applications, conferencing applications, video playback application, or other suitable applications. 
     The optical system adjustment module  1132  can be implemented in hardware and/or software and is configured to adjust one or more optical characteristics of the optical system  1113  using the control information stored in the control information data store  1137 . In some embodiments, the optical system adjustment module  1132  comprises instructions stored on a non-transitory computer-readable medium, the instructions being executable by the processor  1136  to control the actuator assembly  1114 . 
     The tracking module  1134  can be implemented in hardware and/or software and is configured to track changes in the position of the HMD  1110  and/or the position of the user&#39;s facial features (e.g., eye tracking). In some embodiments, the tracking module  1134  may track the movements of the HMD  1110  and correlate the HMD movements to movement of the user&#39;s head. 
     The processor  1136  executes instructions from applications stored in the application store  1139  and/or instructions provided to the processor  1136  by the optical system adjustment module  1132  or the tracking module  1134 . The processor  1136  can receive various items of information used in the applications. This includes, for example, position information, acceleration information, velocity information, and captured images. Information received by processor  1136  may be processed to produce instructions that determine content presented to the user on the display device  1112 . 
     The foregoing description of the embodiments of the disclosure has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. 
     Some portions of this description may describe the embodiments of the disclosure in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules, without loss of generality. The described operations and their associated modules may be embodied in software and/or hardware. 
     Steps, operations, or processes described may be performed or implemented with one or more hardware or software modules, alone or in combination with other devices. Although the steps, operations, or processes are described in sequence, it will be understood that in some embodiments the sequence order may differ from that which has been described, for example with certain steps, operations, or processes being omitted or performed in parallel or concurrently. In some embodiments, a software module is implemented with a computer program product comprising a computer-readable medium containing computer program code, which can be executed by a computer processor for performing any or all of the steps, operations, or processes described. 
     Embodiments of the disclosure may also relate to an apparatus for performing the operations described. The apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a non-transitory, tangible computer readable storage medium, or any type of media suitable for storing electronic instructions, which may be coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability. 
     The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the disclosure, which is set forth in the following claims.