PATENT DOCUMENT

Publication Number: US-11385686-B2
Application Number: US-202016777239-A
Country: US
Kind Code: B2

Title: Electronic devices with flexible displays and hinges

Abstract:
An electronic device may have a flexible display that overlaps an axis. The display may be supported by a housing. The housing may have first and second portions that rotate relative to each other about the axis. The housing may be placed in an unfolded configuration to support the display in a planar state. The housing may also be placed in a folded configuration by rotating the first and second portions relative to each other. A hinge mechanism may be used to ensure adequate separation between the first and second portions when the housing is bent. Movable flaps may be retracted when the housing is bent to create room for a bent portion of the display.

Claims:
What is claimed is: 
     
       1. An electronic device operable in folded and unfolded positions, comprising:
 a housing that folds along a fold axis, wherein the housing has first and second housing portions on opposing sides of the fold axis and wherein the first and second housing portions respectively comprise first and second slots; 
 a flexible substrate that overlaps the first and second housing portions and that folds along the fold axis; 
 a pixel array on the flexible substrate that overlaps the first and second housing portions and that extends across the fold axis; and 
 a hinge coupled between the first and second housing portions, wherein the hinge comprises a first gear that rotates the first housing portion about the fold axis, a second gear that rotates the second housing portion about the fold axis, and a third gear coupled between the first and second gears, and wherein the first gear is coupled to a first pin that slides within the first slot during rotation and the second gear is coupled to a second pin that slides within the second slot during rotation. 
 
     
     
       2. The electronic device defined in  claim 1  wherein the third gear rotates about an axis that is perpendicular to the fold axis. 
     
     
       3. The electronic device defined in  claim 2  wherein the first and second gears rotate about respective first and second axes that are parallel to the fold axis. 
     
     
       4. The electronic device defined in  claim 1  wherein the third gear comprises a helical gear. 
     
     
       5. The electronic device defined in  claim 1  wherein the third gear causes the first and second gears to rotate by equal amounts when the electronic device moves between the folded and unfolded positions. 
     
     
       6. The electronic device defined in  claim 1  wherein the first housing portion comprises a first planar member and a first flap and the second housing portion comprises a second planar member and a second flap. 
     
     
       7. The electronic device defined in  claim 6  wherein the first flap rotates relative to the first planar member and the second flap rotates relative to the second planar member. 
     
     
       8. The electronic device defined in  claim 7  wherein the first and second gears respectively rotate the first and second flaps away from each other when the electronic device moves from the unfolded position to the folded position. 
     
     
       9. The electronic device defined in  claim 1  further comprising biasing structures that bias opposing edges of the flexible substrate outwardly from the hinge to hold the flexible substrate flat in the unfolded position. 
     
     
       10. The electronic device defined in  claim 9  wherein the biasing structures comprise springs. 
     
     
       11. The electronic device defined in  claim 10  further comprising a metal backplate behind the flexible substrate. 
     
     
       12. The electronic device defined in  claim 11  further comprising a metal stiffening plate attached to a backside of the metal backplate. 
     
     
       13. An electronic device that folds and unfolds along a fold axis, comprising:
 a flexible substrate that folds along the fold axis; 
 an array of organic light-emitting diode pixels on the flexible substrate; 
 a metal backplate that supports the flexible substrate; 
 a metal stiffening plate coupled behind the metal backplate, wherein the metal stiffening plate only partially overlaps the metal backplate; and 
 first and second housing portions coupled by a hinge having a first gear that rotates the first housing portion about the fold axis and a second gear that rotates the second housing portion about the fold axis. 
 
     
     
       14. The electronic device defined in  claim 13  further comprising a biasing member that holds the flexible substrate flat when the electronic device is unfolded. 
     
     
       15. The electronic device defined in  claim 13  wherein the biasing member comprises a spring. 
     
     
       16. The electronic device defined in  claim 13  wherein the first and second housing portions each comprise a planar member and a flap that pivots relative to the planar member. 
     
     
       17. The electronic device defined in  claim 13  further comprising a third gear coupled between the first and second gears that causes the first and second gears to rotate by equal amounts as the electronic device folds and unfolds. 
     
     
       18. An electronic device operable in folded and unfolded configurations, comprising:
 first and second housing portions that rotate about a fold axis; 
 a hinge coupled between the first and second housing portions, wherein the hinge comprises a gear system that rotates the first and second housing portions by equal amounts when the electronic device transitions between the folded and unfolded configurations; 
 a flexible display mounted to the first and second housing portions and overlapping the fold axis; 
 first and second overlapping metal plates attached to a rear side of the flexible display, wherein the first metal plate is interposed between the flexible display and the second metal plate and wherein the second metal plate spans only partially across a lower surface of the first metal plate; and 
 a spring-based biasing structure that holds the flexible display in a flat position when the electronic device is unfolded. 
 
     
     
       19. The electronic device defined in  claim 18  wherein the gear system comprises a first gear coupled to the first housing portion and having a first rotational axis, a second gear coupled to the second housing portion and having a second rotational axis, and a third gear coupled between the first and second gears and having a third rotational axis. 
     
     
       20. The electronic device defined in  claim 19  wherein the first and second rotational axes are parallel to the fold axis and the third rotational axis is perpendicular to the fold axis.

Description:
This application is a continuation of patent application Ser. No. 15/232,505, filed Aug. 9, 2016, which claims the benefit of provisional patent application No. 62/217,219, filed Sep. 11, 2015, and provisional patent application No. 62/338,827, filed May 19, 2016, all of which are hereby incorporated by reference herein in their entireties. 
    
    
     FIELD 
     This relates generally to electronic devices, and, more particularly, to electronic devices with displays. 
     BACKGROUND 
     Electronic devices often include displays for presenting image to a user. Displays are typically formed from rigid planar substrates. Although satisfactory in many situations, rigid displays such as these may be difficult to integrate into certain devices, such as devices with bendable housings. 
     SUMMARY 
     An electronic device may have a flexible display. The display may be mounted in a foldable housing that can bend about a bend axis. In a first configuration, the display may be supported by the housing and may be held in a planar state. In a second configuration, portions of the housing may be rotated about the bend axis with respect to each other, so the housing and display are bent. 
     A hinge mechanism may be used to ensure adequate separation between first and second portions of the housing when the housing is bent. This ensures that the flexible display can maintain a desired minimum bend radius in the vicinity of the bend axis. The hinge mechanism may be based on a rack-and-gear arrangement or other arrangement that maintains the first and second housing portions at a desired distance from each other. 
     With another arrangement, the housing may have movable flaps that extend parallel to the bend axis. The movable flaps may be placed in a planar configuration to support the display when the housing is in its unbent state. The movable flaps may be retracted when the housing is placed in its bent state. This creates room for a bent portion of the display along the bend axis. 
     Further features will be more apparent from the accompanying drawings and the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an illustrative electronic device having a display in accordance with an embodiment. 
         FIG. 2  is a schematic diagram of an illustrative electronic device having a display in accordance with an embodiment. 
         FIGS. 3 and 4  are cross-sectional side views of electronic devices with flexible displays in accordance with an embodiment. 
         FIG. 5  is a cross-sectional side view of an illustrative display layer with a bent portion in accordance with an embodiment. 
         FIG. 6  is a cross-sectional side view of a flexible display that has maintained a minimum bend radius while bending in accordance with an embodiment. 
         FIG. 7  is a cross-sectional side view of an illustrative flexible display in folded and unfolded configurations in accordance with an embodiment. 
         FIG. 8  is a cross-sectional side view of an illustrative electronic device having housing flaps adjacent to a flexible display bend axis in accordance with an embodiment. 
         FIG. 9  is a cross-sectional side view of the illustrative electronic device of  FIG. 8  in a configuration in which the housing has been folded and the flaps have retracted to accommodate bending of the flexible display in accordance with an embodiment. 
         FIG. 10  is a cross-sectional side view of the illustrative device of  FIG. 9  in a fully folded configuration in accordance with an embodiment. 
         FIG. 11  is a cross-sectional side view of an illustrative electronic device with flaps having bendable flap extensions in accordance with an embodiment. 
         FIG. 12  is a cross-sectional side view of an illustrative hinge structure for an electronic device that has a structure that helps position housing flaps during movement of the housing in accordance with an embodiment. 
         FIGS. 13, 14, 15, 16, 17, and 18  are cross-sectional side views of illustrative linkages for use in hinges for foldable electronic device housings in accordance with an embodiment. 
         FIG. 19  is an exploded perspective view of illustrative hinge structures for an electronic device in accordance with an embodiment. 
         FIG. 20  is a perspective view of structures of the type shown in  FIG. 19  assembled to form an electronic device hinge in accordance with an embodiment. 
         FIG. 21  is a cross-sectional side view of an illustrative electronic device having a hinge structure of the type shown in  FIG. 20  in accordance with an embodiment. 
         FIG. 22  is a cross-sectional side view of the electronic device of  FIG. 21  in a folded configuration in accordance with an embodiment. 
         FIG. 23  is a perspective view of illustrative electronic device hinge structures in accordance with an embodiment. 
         FIG. 24  is a cross-sectional side view of an illustrative electronic device with hinge structures of the type shown in  FIG. 23  in an unfolded configuration in accordance with an embodiment. 
         FIG. 25  is a cross-sectional side view of an illustrative electronic device with hinge structures of the type shown in  FIG. 24  in a folded configuration in accordance with an embodiment. 
         FIG. 26  is a cross-sectional side view of an illustrative electronic device with folded hinge structures and a bent flexible display in accordance with an embodiment. 
         FIG. 27  is an exploded perspective view of illustrative hinge structures for an electronic device with a foldable display in accordance with an embodiment. 
         FIG. 28  is a side view of an illustrative electronic device with hinge structures that allow the device to be bent in accordance with an embodiment. 
         FIG. 29  is a side view of the electronic device of  FIG. 28  in an unfolded configuration in accordance with an embodiment. 
         FIG. 30  is a front view of the device of  FIG. 29  in accordance with an embodiment. 
         FIG. 31  is a side view of the hinge structures of  FIG. 28  in accordance with an embodiment. 
         FIG. 32  is a cross-sectional side view of the hinge structures of  FIG. 29  in accordance with an embodiment. 
         FIG. 33  is an exploded perspective view of an illustrative electronic device housing having movable flaps in accordance with an embodiment. 
         FIG. 34  is perspective view of illustrative hinge structures for the device housing of  FIG. 33  in accordance with an embodiment. 
         FIG. 35  is a front view of the device housing of  FIG. 34  in accordance with an embodiment. 
         FIGS. 36, 37, and 38  are views of portions of a foldable electronic device having display biasing structures in accordance with an embodiment. 
         FIG. 39  is a front view of an illustrative device having a display biasing structure in accordance with an embodiment. 
         FIG. 40  is a perspective view of an illustrative protective rail structure to cover an edge portion of a flexible display in accordance with an embodiment. 
         FIG. 41  is a cross-sectional side view of an illustrative electronic device having biasing structures such as magnets to help maintain display flatness in accordance with an embodiment. 
         FIG. 42  is a cross-sectional side view of illustrative display layers to help prevent indentations in a flexible display in accordance with an embodiment. 
         FIG. 43  is a cross-sectional side view of an illustrative device having rounded hinge structures in accordance with an embodiment. 
         FIG. 44  is a cross-sectional side view of the illustrative device of  FIG. 43  showing how the hinge structures of  FIG. 43  allow the device to be folded outwardly to expose a flexible display on the outer surfaces of the device in accordance with an embodiment. 
         FIG. 45  is a perspective view of an illustrative foldable electronic device in accordance with an embodiment. 
         FIG. 46  is a cross-sectional side view of hinge portions of the foldable device of  FIG. 45  showing how signal paths formed in flexible printed circuits or other signal path structures may bend during folding and unfolding of the device in accordance with an embodiment. 
         FIG. 47  is a front view of an illustrative electronic device in accordance with an embodiment. 
         FIG. 48  is a side view of the device of  FIG. 47  showing how a display support structure may be moved outwardly when the device is folded in accordance with an embodiment. 
         FIG. 49  is a perspective view of the electronic device of  FIGS. 47 and 48  in accordance with an embodiment. 
         FIGS. 50 and 51  are perspective views of illustrative gear-based structures for coupling housing portions together in a foldable device in accordance with an embodiment. 
         FIG. 52  is a cross-sectional side view of an illustrative flexible display in accordance with an embodiment. 
         FIG. 53  is cross-sectional side view of an illustrative flexible display with flexibility enhancement structure such as parallel grooves in a backplate layer in accordance with an embodiment. 
         FIG. 54  is a top view of the illustrative flexible display backplate layer of  FIG. 53  in accordance with an embodiment. 
         FIG. 55  is a top view of an illustrative flexible display layer such as a backplate layer having flexibility enhancement recesses of various patterns in accordance with an embodiment. 
         FIGS. 56 and 57  are cross-sectional side views of illustrative hinge structures with a dual axis linkage in accordance with an embodiment. 
         FIG. 58  is a perspective view of an illustrative dual axis linkage hinge in a bent (folded device) configuration in accordance with an embodiment. 
         FIG. 59  is a perspective view of the illustrative hinge of  FIG. 58  in a straight (unfolded device configuration in accordance with an embodiment). 
         FIGS. 60 and 61  are cross-sectional side views of a foldable device showing how lateral movement in flexible layers such as flexible display layers may be dynamically adjusted to prevent bulging in accordance with an embodiment. 
         FIGS. 62 and 63  are cross-sectional side views of a foldable device in partly folded and open configurations in accordance with an embodiment. 
         FIGS. 64 and 65  are cross-sectional side views of a foldable device showing how biasing structures such as magnets may help flatten a flexible display in accordance with an embodiment. 
         FIGS. 66 and 67  are cross-sectional side views of a foldable device showing how biasing structures such as springs may help flatten a flexible display in accordance with an embodiment. 
         FIGS. 68A and 68B  are cross-sectional side views of a portion of an illustrative electronic device showing how pressure sensitive adhesive that stretches preferentially along a dimension that is perpendicular to a surface normal for a flexible display may prevent bulging in accordance with an embodiment. 
         FIG. 69  is a cross-sectional side view of a portion of an illustrative electronic device showing how flaps or other structures in a foldable device may have unidirectional teeth to help remove slack from the bending region of a flexible display and thereby tighten the display in accordance with an embodiment. 
         FIG. 70  is a cross-sectional side view of a portion of an illustrative electronic device showing how a foam biasing member may be used to pull an edge of a flexible display outwards to help flatten the display in accordance with an embodiment. 
         FIG. 71  is a cross-sectional side view of an illustrative electronic device showing how a biasing structure such as a torsion spring may be used to apply rotational force to flaps or other structures in an electronic device to flatten a display in accordance with an embodiment. 
         FIG. 72  is a cross-sectional side view of an illustrative electronic device showing how a biasing structure such as a clip spring may be used to retain the end of a display in a housing while allowing the display to slide relative to the housing in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with a flexible display is shown in  FIG. 1 . Electronic device  10  may be a computing device such as a laptop computer, a computer monitor containing an embedded computer, a tablet computer, an electronic book, a cellular telephone, a media player, or other handheld or portable electronic device, a smaller device such as a wrist-watch device, a pendant device, a wearable or miniature device of other types, a computer display that does not contain an embedded computer, a computer display that includes an embedded computer, a gaming device, a navigation device, an embedded system such as a system in which electronic equipment with a display is mounted in a kiosk or automobile, equipment that implements the functionality of two or more of these devices, or other electronic equipment. In the illustrative configuration of  FIG. 1 , device  10  is a portable device such as a cellular telephone, media player, tablet computer, electronic book, watch or other wrist device, or other portable computing device. Other configurations may be used for device  10  if desired. The example of  FIG. 1  is merely illustrative. 
     In the example of  FIG. 1 , device  10  includes a display such as display  14  mounted in housing  12 . Housing  12 , which may sometimes be referred to as an enclosure or case, may be formed of plastic, glass, ceramics, fiber composites, metal (e.g., stainless steel, aluminum, etc.), other suitable materials, or a combination of any two or more of these materials. Housing  12  may be formed using a unibody configuration in which some or all of housing  12  is machined or molded as a single structure or may be formed using multiple structures (e.g., an internal frame structure, one or more structures that form exterior housing surfaces, etc.). Housing  12  may have hinge structures such as hinge  20  to allow device  10  to fold or otherwise bend about bend axis  22  (sometimes referred to as a fold axis, hinge axis, etc.). 
     Display  14  may be a touch screen display that incorporates a layer of conductive capacitive touch sensor electrodes or other touch sensor components (e.g., resistive touch sensor components, acoustic touch sensor components, force-based touch sensor components, light-based touch sensor components, etc.) or may be a display that is not touch-sensitive. Capacitive touch screen electrodes may be formed from an array of indium tin oxide pads or other transparent conductive structures. A touch sensor may be formed using electrodes or other structures on a display layer that contains a pixel array or on a separate touch panel layer that is attached to the pixel array (e.g., using adhesive). 
     Display  14  may include pixels formed from liquid crystal display (LCD) components, electrophoretic pixels, microelectromechanical (MEMs) shutter pixels, electrowetting pixels, micro-light-emitting diodes (small crystalline semiconductor die), organic light-emitting diodes (e.g., a thin-film organic light-emitting diode display), or pixels based on other display technologies. Display  14  may be formed from a single display panel (e.g., a single organic light-emitting diode layer) or may be formed form two panels (e.g., two organic light-emitting diode layers, two liquid crystal display modules, etc.). 
     In a configuration with a single display panel, the entire display panel or at least the center of the display panel may be formed using flexible structures that allow the display to be bent along the bend axis of device  10 . A display cover layer or other layer may form the outermost surface of the display. Display layers such these (e.g., display cover layers) may be formed from glass, plastic, and/or other transparent display cover layer structures. The outermost display layer (e.g., a display cover layer) and underlying display layers (e.g., a polymer substrate, metal traces, and other conducting and dielectric layers in an organic light-emitting diode panel) may be flexible or may at least have a flexible center portion aligned with the bend axis of device  10 . As shown in  FIG. 1 , for example, display  14  may have three areas such as areas  14 A,  14 B, and  14 C. In areas  14 A and  14 C, display  14  may be flexible or may be rigid. In area  14 B, which forms a strip that lies between areas  14 A and  14 C, display  14  is flexible to accommodate bending of display  14 , housing  12 , and device  10  about bend axis  22 . 
     A schematic diagram of an illustrative electronic device such as device  10  of  FIG. 1  is shown in  FIG. 2 . As shown in  FIG. 2 , electronic device  10  may have control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  18  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  18  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, etc. A user can control the operation of device  10  by supplying commands through input-output devices  18  and may receive status information and other output from device  10  using the output resources of input-output devices  18 . Input-output devices  18  may include a display such as display  14  of  FIG. 1 . 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may display images on display  14  (e.g., video, still images such as text, alphanumeric labels, photographs, icons, other graphics, etc.) using an array of pixels in display  14 . 
     As shown in  FIG. 3 , device  10  may be folded (bent outwardly by 180° or other suitable amount) about bend axis  22  so that display  14  is visible from the outside of device  10  in its folded state. In this configuration, a first portion of display  14  faces outwardly from one half of device  10  and a second portion of display  14  faces outwardly from another half of device  10  (and faces away from the first portion).  FIG. 4  shows how device  10  may be folded inwardly by 180° or other suitable amount about bend axis  22  so that display  14  is protected within the interior of device  10 . Device  10  may have hinges that allow outward bending (folding) of the type shown in  FIG. 3 , that allow inward bending (folding) of the type shown in  FIG. 4 , or that allow bending of both the type shown in  FIG. 3  and the type shown in  FIG. 4 . Configurations in which device  10  is flexed by different amounts (e.g., more than 180° or less than 180°) may also be used. 
     Display  14  may have an outermost layer formed from clear glass, transparent plastic, sapphire, or other transparent materials that serve as a protective layer for thin-film transistor circuitry and other display structures. The outer display layer may sometimes be referred to as a display cover layer. In some configurations for display  14 , the outermost layer of the display may serve both as a protective layer (display cover layer) and as a substrate for display structures (touch sensors electrodes, color filter elements, thin-film transistors, etc.). In other configurations, the display cover layer is free of circuitry and serves solely as a protective layer for underlying display structures (e.g., one or more underlying display panels). 
     As shown in  FIG. 5 , the display cover layer and other display layers forming display  14  (i.e., display layers  24 ) may have outer portions  24 A and  24 C that are formed on opposing sides of center portion  24 B. Outer portions  24 A and  24 C may be rigid planar layers or may be planar flexible layers. Configurations in which portions  24 A and  24 C have non-planar shapes may also be used. Between portions  24 A and  24 C of display layers  24 , display layers  24  may have a flexible portion such as portion  24 B. Portion  24 B may bend about bend axis  22  to allow display  14  to bend as housing  12  is bent about hinge  20 . 
     To prevent metal lines and other thin-film structures in display  14  from becoming damaged (e.g., due to cracking), a safe minimum bend radius R may be maintained for display  14  when bending display  14  about bend axis  22 , as shown in  FIG. 6 . The value of R may be 0.5 mm to 5 mm, 0.3 mm to 3 mm, more than 1 mm, more than 5 mm, more than 10 mm, less than 7 mm, less than 4 mm, less than 1 mm, or other suitable value. 
     Housing  12  preferably has structures that help maintain a suitable minimum bend radius R in display  14  during bending. With one suitable arrangement, which may sometimes be described herein as an example, housing  12  may be provided with movable structures such as retractable flaps that run along axis  22 . When device  10  is in its planar state, the flaps may rest in a planar orientation to support display  14  and ensure that display  14  is planar where overlapping axis  22 . When device  10  is in its folded state, the flaps may retract, thereby providing display  14  with room to accommodate a desired bend radius R in display  14 . 
       FIG. 7  is a cross-sectional side view of an illustrative flexible display in folded and unfolded configurations. Display  14  may be a flexible display that can bend about bend axis  22 , as described in connection with  FIG. 3 . Display  14  may, as an example, be placed in unfolded position  14 - 1  in which the surface of display  14  is planar or in folded position  14 - 2  in which portions of the active surface of display  14  are folded inwardly in directions  50  towards each other (and are separated by distance D). The value of distance D may be 0-2 mm, less than 1 mm, less than 0.2 mm, etc. To ensure that display  14  is not damaged, the portion of display  14  that lies near the bend axis for display  14  may be bent while maintaining a minimum bend radius of R. In some configurations, this may cause the separation ( 2 R) between opposing portions of display  14  to be larger in the vicinity of the bend in display  14  than at other locations such as the edges of display  14  (i.e., the value of  2 R may be more than D). 
     To accommodate the relatively wide distance between the opposing portions of display  14  in the vicinity of the bend in display  14  (i.e., near axis  22 ), housing  12  may be provided with retractable flaps. As shown in  FIG. 8 , for example, housing  12  may have planar portions  12 P (sometimes referred to as housing plates, planar support members, planar support structures, or display support plates) and flaps  12 F. Flaps  12 F may run along the center of display  14  in the portion of display  14  that overlaps bend axis  22 . 
     Planar portions  12 P may be planar support members (e.g., rectangular support plates). Flaps  12 F may be mounted to flap hinges  54  or other structures that allow flaps  12 F to pivot relative to housing plates  12 P when housing  12  and display  14  are folded about bend axis  22 . As shown in  FIG. 9 , for example, flaps  12 F may pivot outwardly in directions  56  (away from and out of the respective planes in which planar members  12 P lie) when device  10  is folded about axis  22 . As shown in the fully folded configuration of  FIG. 10 , the pivoting motion of flaps  12 F creates additional room for display  14  in the portion of display  14  that overlaps axis  22  (e.g., to allow a desired minimum bend radius R to be maintained in display  14 , as described in connection with  FIG. 8 ). 
     If desired, the pivoting flaps in housing  12  may have multiple parts. As shown in  FIG. 11 , for example, left flap  12 F-L may be a two-part flap that includes main left flap  12 F-LM and left flap extension  12 F-LE. Right flap  12 F-R may be a two-part flap that includes main right flap  12 F-RM and right flap extension  12 F-RE. Main left flap  12 F-LM may pivot about hinge  54 L. When device  10  is folded, flap  12 F-LM may pivot out of the plane in which left plate  12 P-L lies. Flap extension  12 F-LE may pivot relative to flap  12 F-LM about hinge axis  58 L. When device  10  is folded, for example, flap  12 F-LE may pivot out of the plane in which main left flap  12 F-LM lies. Main right flap  12 F-RM may pivot about hinge  54 R. When device  10  is folded, flap  12 F-RM may pivot out of the plane in which right plate  12 P-R lies. Flap extension  12 F-RE may pivot relative to flap  12 F-RM about hinge axis  58 R. When device  10  is folded, for example, flap  12 F-RE may pivot out of the plane in which main right flap  12 F-RM lies. As shown in the partly folded configuration of  FIG. 11 , the movements of the two-part flaps during device folding may help create additional room for display bending while maintaining a desired minimum bend radius in display  14 . 
     An illustrative arrangement that may be used to help pivot flaps in housing  12  during device folding is shown in  FIG. 12 .  FIG. 12  is a cross-sectional side view of an illustrative hinge structure for an electronic device that has a flap positioning structure that helps position housing flaps during movement of the housing. As shown in  FIG. 12 , housing  12  may include foldable plates  12 P and flaps  12 F that pivot about hinge axes  54 . In the unfolded state for device  10 , plates  12 P and flaps  12 F lie in a common plane. When device  10  is bend about bend axis  22 , plates  12 P move in directions  50  to positions  12 P′ and flaps  12 F pivot about hinges  54  to positions  12 F′. Flap positioning structure  66 , which may sometimes be referred to as a rear housing cover or hinge cover, other structure in device  10  may be coupled to other structures in housing  12 . When device  10  and housing  12  are folded, flap tips  60  may contact inner surface  64  of structure  66 , thereby causing tips  60  to more towards each other in directions  62 . The presence of flap positioning structures  66  therefore helps create appropriate pivoting movement of flaps  12 F when device  10  is folded. 
     The sliding movement of flap tips  60  relative to structure  66  and the rotational motion of plates  12 P and flaps  12 F relative to each other can be accommodated using any suitable linkages (e.g., gears, rotating shafts, pin-and-slot structures, sliding members, hinges, or other linkages).  FIGS. 13, 14, 15, 16, 17, and 18  are cross-sectional side views of illustrative linkages for use in hinges for foldable electronic device housings. In the examples of  FIGS. 13, 14, 15, 16, 17, and 18 , two different housing structures  12 - 1  and  12 - 2  are being coupled to each other (while being allowed to rotate, slide, and otherwise move with respect to each other). Structures  12 - 1  and  12 - 2  may be portions of flaps, portions of flap extensions (e.g., in a two-part flap configuration), may be portions of plates  12 P, may be internal housing structures, may be external housing walls or portions of external housing walls, may be attached to display  14  or portions of display  14 , or may otherwise be associated with housing  12  and device  10 . 
     In the example of  FIG. 13 , housing structures  12 - 1  and  12 - 2  have mating grooves or other structures that allow structures  12 - 1  and  12 - 2  to slide with respect to each other. Structure  12 - 1  may, for example, have a portion such as portion  12 - 1 ′ with an opening that receives structure  12 - 2  and thereby allows structure  12 - 2  to slide in direction  68  with respect to structure  12 - 1 . 
       FIG. 14  shows how structures  12 - 1  and  12 - 2  may include gears with teeth (e.g., teeth that engage corresponding teeth in structure  12 - 3 ). In this type of structure, structures  12 - 1  and  12 - 2  may rotate relative to each other and relative to structure  12 - 3 . Structure  12 - 3  may be, for example, a helical gear having teeth than engage both the teeth of structure  12 - 1  and the teeth of structure  12 - 2  (as an example). This type of linkage may be used to couple movement of structure  12 - 1  to movement of structure  12 - 2 . 
       FIG. 15  shows how structure  12 - 1  may have an elongated shape with teeth and structure  12 - 2  may be a gear with mating teeth. This type of arrangement may allow structure  12 - 1  to rotate in directions  70  while walking around structure  12 - 2 . 
     With the illustrative linkage of  FIG. 16 , structure  12 - 1  has pin  72  that is received within slot  74  of structure  12 - 2 . This allows structures  12 - 1  and  12 - 2  to rotate and slide with respect to each other. 
     In the illustrative configuration of  FIG. 17 , structures  12 - 1  and  12 - 2  bear against each other, which allows structure  12 - 1  to walk around the surface of structure  12 - 2  in direction  70 . Because structures  12 - 1  and  12 - 2  of  FIG. 17  do not have gears, there may be slippage between the mating surfaces of structures  12 - 1  and  12 - 2 . 
       FIG. 18  shows how structures  12 - 1  and  12 - 2  may be linked using multiple hinge structures (e.g., hinges  76  and  78  on linking member  12 - 4 ). If desired, hinges  76  and/or  78  may use pin and slot structures to accommodate sliding movement. 
     If desired, biasing structures (coil springs, leaf springs, elastomeric materials, compressible foam, magnets, ferromagnetic material, etc.) may be used in biasing structures  12 - 1  and  12 - 2  relative to each other and/or relative to other structures. Moreover, different types of linkages may be used in coupling structures  12 - 1  and  12 - 2 . The configurations of  FIGS. 13, 14, 15, 16, 17, and 18  are merely illustrative. 
     An exploded perspective view of illustrative hinge structures for electronic device  10  is shown in  FIG. 19 . As shown in  FIG. 19 , at each end of axis  22 , housing  12  may include gears  80 L and  80 R. Gear  80 L may be mounted to a first half of housing  12  (e. g., left housing portion  12 L) and gear  80 R may be mounted to a right half of housing  12  (e.g., right housing portion  12 R). Housings  12 R and  12 L (and therefore respective gears  80 L and  80 R) may rotate about respective axes that run parallel to axis  22 . 
     Helical gear  82  may have teeth that engage the respective teeth of gears  80 L and  80 R, thereby coupling movement of gear  80 L (and therefore housing portion  12 L) to movement of gear  80 R (and therefore housing portion  12 R). Helical gear  82  may rotate about helical gear axis  88  (i.e., an axis that runs perpendicular to axis  22 ). Bracket  84  has openings that receive the shaft of helical gear  82  and shafts associated with gears  80 L and  80 R. Bracket  84  and the gears coupled to bracket  84  may be mounted within gear housing  86 . 
     During movement of left housing  12 L (e.g., the main left plate of housing  12 ) about bend axis  22 , the presence of gears  80 L,  82 , and  80 R ensures that right housing  12 R will rotate about bend axis  22  by an equal amount (i.e., gears  80 L,  82 , and  80 R form a linkage that causes rotation of one half of housing  12  to be mirrored by rotation of the other half of housing  12 ). During rotation of the main left and right portions of housing  12  (i.e., the left and right plates  12 P), pin  90 L of bracket  84  will slide within a mating slot in the left flap  12 F of housing  12  and pin  90 R will slide within a mating slot in the right flap  12 F of housing  12 , thereby causing flaps  12 F to retract as described in connection with  FIGS. 8, 9, and 10 . 
     An assembled version of device  10  in which gear housing  86  has been removed so as not to obscure gears  80 L and  80 R is shown in  FIG. 20 .  FIG. 21  shows how flaps  12 F may have slots such as left slot  92 L to receive pin  90 L on bracket  82  and right slot  92 R to receive pin  90 R on bracket  82 . When housing  12  is folded along axis  22 , pin  90 P will slide within slot  92 R and the associated flap  12 F in housing  12  will move from planar position  12 F′ to retracted (pivoted) position  12 F″. The position of flap  12 F when retracted is shown in  FIG. 22 , in which housing  12  is in its folded configuration. Gears  80 R and  80 L, which are coupled using helical gear  82 , are coupled to the left and right portions of housing  12 , respectively, and ensure that while the left flap is retracting by a given amount, the right flap is retracting by the same amount. 
     If desired, the left and right halves of housing  12  can be coupled using a gear-and-rack mechanism. An illustrative mechanism of this type is shown in  FIGS. 23, 24, 25, and 26 . 
     As shown in  FIG. 23 , right-hand housing portion  12 R may have a pin such as pin  100 R that extends into slot  102 R of sliding bar member  104  and may have teeth  116 R that engage teeth  114 R on right-hand gear member  112 R. Housing  12  may also have a left housing portion  12 L with a pin  100 L that extends into left slot  102 L and teeth  116 L that engage teeth  114 L on left gear member  112 L. 
     Clamp  118  prevents gear members  112 L and  112 R from separating during folding and unfolding operations in which housing portions  12 L and  12 R are rotated about axis  22 . Bar member  104  maintains a desired separation distance between gear members  112 L and  112 R during folding and unfolding. Bar member  104  has opening  106  that allows bar member  104  to slide up and down in directions  108  along vertical post  110  (e.g., a post coupled to clamp  118 ). As bar  104  moves down post  110 , teeth  116 L move around teeth  114 L and teeth  116 R move around teeth  114 R (i.e., left flap and pin  100 L walk around teeth  114 L and the right flap and pin  100 R walk around teeth  114 R), thereby pivoting flaps  12 F on the left and right of housing  12  away from each other. If desired, the teeth of  FIG. 23  may be omitted (i.e., the mating surfaces may be smooth and free of teeth). The use of teeth is merely illustrative. 
     A cross-sectional side view of housing  12  prior to folding of housing portions  12 R and  12 L relative to each other is shown in  FIG. 24 . As housing  12  is folded about axis  22 , portions  12 L and  12 R will walk around gears  112 L and  112 R, respectively and will maintain a desired separation from each other ( FIG. 25 ). This allows housing  12  to accommodate a folded display  14  with a desired minimum bend radius R, as shown in  FIG. 26 . 
     Another illustrative configuration for housing  12  that is based on a gear-and-rack arrangement is shown in the exploded perspective view of  FIG. 27 . As shown in  FIG. 27 , member  120 L may have teeth  116 L and may be attached to a planar housing structure to form left housing portion  12 L. Member  120 R may have teeth  116 R and may be attached to a planar housing structure to form right housing portion  12 R. Pin  100 L may be received within slot  102 L of slider member  128 . Pin  100 R may be received within slot  102 R of slider member  128 . Slider member  128  may have an opening such as opening  134  that receives steel pin  132  of base member  126 . Base member  126  may be formed from a material such as metal (e.g., aluminum). Screw  130 L may be received within an opening in clip  124 L and may be used to mount gear  122 L to base member  126 . Screw  130 R may be received within an opening in clip  124 R and may be used to mount gear  122 R to base member  126 . 
     When in an unfolded state, housing portions  12 L and  12 R lie in a common plane and mate with each other along their edges. When in a folded configuration, the separation between gears  122 L and  122 R helps maintain a desired horizontal separation between housing portions  12 L and  12 R, thereby accommodating a bent flexible display such as display  14  of  FIG. 27  without bending display  14  more tightly than desired. 
       FIG. 28  is a cross-sectional side view of device  10  in an arrangement that uses a rack-and-gear mechanism of the type shown in  FIG. 27 . In the configuration of  FIG. 28 , housing  12  is folded. In the configuration of  FIG. 29 , housing  12  is unfolded.  FIG. 30  is a front view of housing  12  of  FIG. 29  in the unfolded configuration.  FIGS. 31 and 32  are detailed views of housing  12  in the vicinity of the bend axis in respective folded and unfolded configurations. 
       FIG. 33  is an exploded perspective view of an illustrative electronic device housing having movable flaps. The arrangement of  FIG. 33  illustrates how a top down assembly technique may be used to construct device  10 . As shown in  FIG. 33 , housing  12  may include top module  140  and lower module  142 . Top module  140  may include left upper planar housing member  12 LT (e.g., a left support plate), right upper planar housing member  12 RT, and flaps  12 F. Hinge module  130  may include gears and other linkages (i.e., structures  133 ) for coupling motion of the left and right halves of housing  12  and may include attachment members  132 . Lower housing module  142  may include outer left housing member  12 LB and outer right housing member  12 RB. Mounting rails  136  may be coupled to lower housing module  142 . During assembly operations, screws  134  may be used to mount attachment members  132  of hinge module  130  to mounting rails  136 , as shown in  FIG. 34 . The resulting partly assembled housing for device  10  (housing  12 ) is shown in  FIG. 35 . Module  140  and display  14  may be mounted to the assembly of  FIG. 35  to form device  10 . 
     Flexible displays may contain materials that stretch over time, so it may be desirable to provide device  10  with biasing structures that help maintain display  14  in a planar state whenever housing  12  is unfolded. With one illustrative configuration, a spring or other biasing structure  160  of  FIG. 36  may be used to stretch display  14  outward and thereby help ensure that display  14  is taut and flat during use. If desired, biasing structure  160  may be used to pull the edge of display  14  downward to and therefore outward to ensure that display  14  is taut ( FIG. 37 ) or the biasing structure may be incorporated into a roller such as biasing roller  160  of  FIG. 38  to pull display  14  outward to ensure that display  14  is taut. 
     In the illustrative configuration of  FIG. 39 , biasing structure  160  is being used to push flap  12 F away from housing plate  12 P so that display  14  (which is attached to housing plate  12 P) is stretched taut. Biasing structures may be formed from spring metal, foam, elastomeric material, magnets, or other biasing structures. Other arrangements for biasing display  14  outward may be used if desired. Moreover, flattening layers such as a resilient backplate, biasing strips, a thin and highly elastic backing layer that helps counteract any plastic deformation in the layers of display  14 , a frame with a spring mechanism, a mesh-shaped backing mechanism, bistable plates or strips to hold display  14  flat, and/or other structures may be used to enhance display flatness. The illustrative biasing mechanisms of  FIGS. 36, 37, 38, and 39  are merely illustrative. 
     If desired, the edge of housing  12  may be provided with bezel structures such as bezel structure  12 BE or other lip-shaped edge portion along the edge of housing  12  that helps hold down the edges of display  14  and helps reduce dust intrusion under display  14 . 
       FIG. 41  is a cross-sectional side view of display  14 . Display  14  may have active layers  14 A for displaying images. The side view of display  14  of  FIG. 41  shows how the rear surface  14 R of display  14  may be provided with a ferromagnetic material that is pulled by magnets  170  in housing  12  (e.g., to ensure that display  14  follows the retraction of flaps in housing  12 , to hold display  14  in a planar configuration, etc.). This type of arrangement may help maintain display  14  in a desired configuration (e.g., flat when device  10  is unfolded, retracted when device  10  is bent, etc.). 
     In the example of  FIG. 42 , display  14  has been provided with intermediate layer  121  between rear layer  14 R and active layer(s)  14 A. Rear layer  14 R of  FIG. 42  may be a support layer that helps hold display  14  flat (e.g., a thin layer of metal, a polymer layer that resists deformation, etc.). Intermediate layer  141  may be used to help prevent deformation of display  14  when display  14  is exposed to an undesired impact from an object. Layer  141  may include, for example, one or more layers of protective material such as foam, elastomeric polymer, a thin steel layer or other metal layer, etc. 
     In arrangements in which the outer surfaces of housing portions  12 L and  12 R have curved shapes of the type shown in  FIG. 43 , it is possible to fold housing  12  outwardly while preserving a desired separation between the housing portions (i.e., a separation that does not cause display  14  to stretch or buckle), as shown in  FIG. 44 . Linkage  180  may be a pin and slot linkage, a biasing structure such as a spring, a rack-and-gear structure, or other suitable linkage for coupling housing portions  12 L and  12 R together while allowing housing portions  12 L and  12 R to be moved between the open (unfolded) configuration of  FIG. 43  and the outwardly folded configuration of  FIG. 44 . If desired, device  10  of  FIGS. 43 and 44  may also be folded inwardly. 
       FIG. 45  is a perspective view of an illustrative electronic device with a foldable housing. Display  14  is not shown in  FIG. 45 . Left and right housing portions  12 L and  12 R may have curved mating surfaces of the type shown in  FIGS. 43 and 44  (as an example). As shown in  FIG. 45 , circuitry in the left and right portions of housing  12  may be coupled using flexible printed circuits  182 . Magnets  183  may be used to help hold left and right housing portions  12 L and  12 R together. Magnets  183  may, for example, include pairs of magnets, each pair having a left-hand magnet on portion  12 L and a mating right-hand magnet at an opposing location on portion  12 R. 
       FIG. 46  is a cross-sectional side view of the hinge region of device  10  of  FIG. 45  showing how some of the flexible printed circuits such as flexible printed circuit  182 - 1  may extend from the upper surface of left housing portion  12 L to the lower surface of adjacent portions of right housing portion  12 R and how other flexible printed circuits such as flexible printed circuit  182 - 2  may extend from the lower surface of left housing portion  12 L to the upper surface of right housing portion  12 R. Printed circuits  182  may contain signal lines for routing signals between the circuitry of the left and right housings and/or may serve as a mechanical hinge structure for device  10 . The layout of printed circuits  182 - 1  and  182 - 2  allows left and right portions  12 L and  12 R respectively to be placed in either planar (unfolded) position P or bent (folded) position B without stretching the printed circuits. 
       FIG. 47  is a front view of an illustrative electronic device having a hinge structure that protrudes and retracts as a function of the folding state of housing  12 . When housing  12  is in the unfolded configuration of  FIG. 47 , retractable hinge portion  200  is retracted and lies flat with the planar front surface of housing  12 , as shown by retracted position RP of portion  200  in  FIG. 48 . As left portion  12 L and right portion  12 R of housing  12  are bent in directions  202  from planar positions P to bent positions B, portion  200  advances outwardly in direction  204  to support display  14 , which lies on the exposed outer surface of device  10 . During bending, portions  12 L and  12 R compress spring plate  206 . When unbent, the spring plate may help flatten display  14 .  FIG. 49  is a perspective view of a portion of device  10  in an illustrative configuration in which housing  12  has a retractable hinge portion such as portion  200  of  FIGS. 47 and 48 . 
     In general, any suitable coupling mechanism may be used to couple motion of the first and second portions of the housing of foldable device  10  when device  10  is bent. In the example of  FIG. 19 , gear  80 L is coupled to left housing portion  12 L and gear  80 R is coupled to right housing portion  12 R and helical gear  82  is used to couple gears  80 R and  80 L together. The teeth on gears  80 R and  80 L may be helical, as shown by illustrative helical gears  250  and  254  of  FIG. 50  (i.e., gears  250 ,  252 , and  254  of  FIG. 50  may serve respectively as gears  80 L,  82 , and  80 R of  FIG. 19 ). As shown in  FIG. 50  this type of coupling mechanism ensures that clockwise motion of gear  250  (i.e., inward bending of the left housing portion) will result in associated counterclockwise motion of gear  254  (i.e., inward bending of the right housing portion). If desired, other configurations may be used for the mechanism that couples the left and right housing portions. An illustrative four gear arrangement is shown in  FIG. 51 . The system of  FIG. 51  includes spur gears  256 ,  258 ,  260 , and  262 . The teeth on gear  256  engage the teeth on gear  258 . The teeth on gear  258 , in turn, engage the teeth on gear  260 , whose teeth engage the teeth on gear  262 . Gear  256  may be connected to left housing portion  12 L and gear  262  may likewise be coupled to right housing portion  12 R, so that clockwise motion of gear  256  (i.e., inward bending of the left housing) will result in counterclockwise motion of gear  262  (i.e., inward bending of the right housing). The examples of  FIGS. 50 and 51  are merely illustrative. Any suitable mechanism may be used to couple motion of the left and right housing portions of housing  12 . 
       FIG. 52  is a cross-sectional side view of an illustrative flexible display of the type that may be used in device  10 . As shown in  FIG. 52 , display  14  may include a supporting layer such as backplate  14 - 1 . Backplate  14 - 1  may be formed from a thin stiff material to help display  14  resist indentations and other damage during use in device  10  (e.g., damage from impact events) while allowing display  14  and device  10  to fold and unfold about bend axis  22 . As an example, backplate  14 - 1  may be formed from one or more layers of plastic, carbon-fiber composite material or other fiber composites, metal, or other materials. Backplate  14 - 1  may have a thickness that is thinner in bend region  300  than in edge regions  302  or that contains recesses or other structures to enhance flexibility of backplate  14 - 1  in region  300 . Bend region  300  may overlap and run along bend axis  22  for device  10 . Backplate  14 - 1  may be formed from a material that helps spread heat generated by light-emitting diode pixels in display  14  (e.g., backplate  14 - 1  may be formed from a material with a high thermal conductivity such as metal). 
     Adhesive layer  14 - 2  (e.g., pressure sensitive adhesive) may be used to attach flexible organic light-emitting diode display  14 - 3  to backplate  14 - 1 . Adhesive layer  14 - 4  (e.g., an optically clear adhesive) may be used to attach flexible touch sensor  14 - 5  to display  14 - 3 . Touch sensor  14 - 5  may be a capacitive touch sensor having an array of capacitive touch sensor electrodes formed on a substrate such as a flexible polymer sheet. Adhesive layer  14 - 6  may be used to attach polarizer and functional layer  14 - 7  to touch sensor layer  14 - 5 . Layer  14 - 7  may include a polymer polarizer film and optional coating layers (e.g., a scratch resistant inorganic coating, which may sometimes referred to as a hard coat), a smudge resistant coating, antireflection coating material, etc. 
       FIG. 53  shows how recesses such as elongated grooves  304  that run parallel to axis  22  may be formed in bend region  300  of backplate  14 - 1  to locally reduce the stiffness of backplate  14 - 1 . If desired, elastomeric polymers or other flexible materials such as material  306  and optional additional material  308  may be formed in and/or on grooves  304  or other recesses in backplate  14 - 1  to help provide structural support while allowing backplate  14 - 1  to maintain enhanced flexibility in region  300  relative to regions such as regions  302  that do not contain flexibility-enhancing recesses in backplate  14 - 1 .  FIG. 54  is a top view of grooves  304  of  FIG. 53 .  FIG. 55  shows how recesses  304  in bend region  300  may have the shapes of spots, slots that run perpendicular to axis  22  and bend region  300 , serpentine grooves, etc. In general, recesses in backplate  14 - 1  and/or any other layers in display  14  may have these shapes, other shapes, or combinations of shapes. 
     If desired, the hinge for folding device  10  may be formed using a linkage with multiple axes. An illustrative hinge mechanism with two rotational axes (i.e., two rotational axis for each half of the hinge to accommodate left and right housing portions in housing  12 ) is shown in  FIGS. 56, 57, 58, and 59 . As  FIGS. 56 and 57  are cross-sectional side views of a hinge linkage (e.g., a right-hand portion of a hinge) with two axes.  FIG. 56  shows the hinge mechanism in the closed state and  FIG. 57  shows the hinge mechanism in the open state. As shown in  FIG. 56 , the linkages of the hinge may have hinge members such as hinge block  310  that are coupled to hinge members such as hinge plates  312  by linkage members  320  and  322 . Linkage members  320  and  322  have holes that receive rods  316 . The ends of rods  316  move within linkage grooves  324  in hinge blocks  310 . This allows plate  312  of  FIG. 56  (e.g., the right portion of housing  12  in this example) to move outwardly in direction  314  of  FIG. 56  (e.g., to place the hinge in the open hinge state of  FIG. 57  so that device  10  is unfolded) and inwardly in direction  322  of  FIG. 57  (e.g., to place the hinge in the closed hinge state of  FIG. 56  so that device  10  is in a folded configuration). In bending device  10  open and closed in this way, display  14  experiences relatively small amounts of stress (i.e., the distance along the surface of display  14  between points A and B in  FIGS. 56 and 567  does not change appreciably between the open and closed states due to the use of the dual axis linkages formed using pairs of rods  316  and associated grooves  324 .  FIGS. 58 and 59  are perspective views of the dual-axis (two rod) hinge linkage mechanism of  FIGS. 56 and 57  in closed and open states, respectively. 
       FIGS. 60 and 61  are cross-sectional side views of a foldable device showing how lateral movement in flexible layers such as flexible display layers may be dynamically adjusted. Flexible layer  330  may be a flexible structure in device  10  such as a flexible display layer (e.g., backplate  14 - 1  of  FIG. 52  or other display layer or support layer). Strain sensors such as strain sensors  324  may be used to detect changes in the strain in layer  330  (e.g., bending stress remaining in layer  330  after device  10  has been folded and unfolded, etc.). Strain sensors  324  may be thin-film strain gauges (e.g., strain gauges embedded in organic light-emitting diode layer  14 - 3  of  FIG. 52 , strain gauges embedded in touch sensor layer  14 - 5  of  FIG. 52 , or formed on other layers in display  14  or device  10 ), piezoelectric strain gauges attached to a backplate or other display layer, or other suitable strain gauges. When stress is detected, control circuitry in device  10  can take appropriate action. For example, an embedded actuator such as piezoelectric actuator  322  may be directed to move laterally outwards in directions  326 , thereby flattening out layer  330  as shown in  FIG. 61 . If desired, electromagnetic actuators (e.g., solenoids), piezoelectric actuators, motors, or other actuators may be located at one or both ends of layer  330  (see, e.g., actuators  328  of  FIG. 61 ) and may be electronically controlled to pull on layer  330  in directions  326  to flatten layer  330  and thereby prevent bulging in layer  330 . 
     If desired, device  10  may have biasing structures such as compression springs that help stretch display  14  into a flat configuration. This type of arrangement is illustrated in the cross-sectional side views of device  10  of  FIGS. 62 and 63 . 
       FIG. 62  shows how flexible display  14  may be attached to support structures such as edge support plates  334  using pressure sensitive adhesive layers  338  (as an example). Inner support members  332  may have curved mating surfaces (i.e., rounded edges) that allow these structures to press against each other and to rotate relative to each other as device  10  is being folded and unfolded about bend axis  22 . Compression springs  336  or other biasing structures may push members  332  against each other and away from outer support structures such as edge support plates  334 . This ensures that display  14  will be held taught and will be free of bulges. As shown in  FIG. 63 , for example, springs  336  will push members  332  inwardly against each other in directions  340  while pushing edge support plates  334  outwardly away from members  332  in directions  342 . This pulls the ends of display  14  in directions  342  and helps flatten display  14 . 
       FIGS. 64 and 65  are cross-sectional side views of foldable device  10  in an illustrative configuration in which biasing structures such as magnets  350  and  352  are used to bias display  14  outwardly in directions  356  and thereby tighten and flatten display  14 . Magnets  350  may be attached to the underside of display layer  330  (e.g., backplate  14 - 1 , a stiffener, or other support structure, etc.). Corresponding magnets  352  may be attached to support structure  354  (e.g. part of housing  12 , an internal support structure that is coupled to housing  12 , etc.). Magnets  352  pull magnets  350  outwardly in directions  356 , so that display layer  330  is flat when device  10  is in an unfolded configuration of the type shown in  FIG. 65 . 
       FIGS. 66 and 67  are cross-sectional side views of foldable device  10  in an illustrative configuration in which springs  360  are used to help bias display  14  outwardly in directions  366  and thereby tighten and flatten display  14 . Display  14  may include, for example, one or more display layers such as display layer  330  (e.g., backplate  14 - 1 ). Flexible display stiffening plates  364  may be attached to the lower surfaces of layer  330  (e.g., adhesive may be used to attach plates  364  to the outer edges of layer  330  at opposing ends of device  10 ). Spring support structures  358  may be used to attach an inner ends of springs  360  to stiffening plates  364 . An opposing outer end of each of springs  360  may be attached to a housing structures or other support structure (see, e.g., structures  362 ). When springs  360  are stretched due to a bulge in layer  330 , springs  360  will tend to contract, thereby pulling stiffening plates  364  in outward directions  366 . As shown in  FIG. 67 , this will tend to flatten layer  330  and therefore flatten display  14 . 
       FIGS. 68A and 68B  are cross-sectional side views of a portion of electronic device  10  in an illustrative configuration in which flexible display layer (flexible display)  370  has been attached to device structures  378  (e.g., a flap such as flap  12 F of  FIG. 9 , a support plate that may or may not be associated with a flap, a fixed or stationary portion of housing  12 , or other supporting structure) using pressure sensitive adhesive  372 . Display layer  370  may a layer such as one of the layers of display  14  of  FIG. 52  (as an example). Device structures  378  may be another layer in display  14  or may be a supporting structure such as flap  12 F). 
     Pressure sensitive adhesive  372  may stretch preferentially in direction  376  of  FIG. 68A  (i.e., adhesive  372  may not stretch in direction  374 , which is parallel to the surface normal n of layer  370 , but may stretch in directions  376 , which is perpendicular to surface normal n of layer  370  and which may run perpendicular to bend axis  22  when device  10  is folded). As a result, the edge of layer  370  may protrude over the edge of layer  378  after device  10  is folded around bend axis  22 , thereby stretching adhesive  372  in directions  376  ( FIG. 68B ). This type of arrangement may help maintain display  14  attached to flat structures in device  10  (e.g., flaps  12 F, etc.) and may help to relieve interlayer stresses in display  14  and stresses between a flexible display layer and flaps or other rigid support structures when display  14  is stretched by bending display  14  about axis  22 . 
       FIG. 69  shows how a unidirectional sliding mechanism may be used to help prevent bulges in display  14 . Initially, display  14  may be in a flat configuration (i.e., an unfolded device configuration). Display  14  may be supported by elastomeric layer  380 . One-way teeth  384  on flaps or other support structures in the vicinity of bend axis  22  may selectively engage elastomeric layer  380 . During folding operations, teeth  384  or other one-way sliding structures tend to engage elastomeric layer  380  and therefore pull display  14  outwardly to tighten display  14  and remove bulges from display  14 . During unfolding operations, teeth  384  or other one-way sliding structures may slip past the surface of elastomeric layer  380 , thereby allowing display  14  to lie flat. Biasing structures may pull the ends of display  14  outwardly to further flatten display  14 . Unidirectional sliding mechanisms such as the illustrative structures of  FIG. 69  may be used in conjunction with displays having layers attached to each other and/or to support structures using pressure sensitive adhesive of the type described in connection with  FIGS. 68A and 68B . 
       FIG. 70  is a cross-sectional side view of a portion of electronic device  10  in a configuration in which foam biasing member (foam)  386  serves as an outwardly biasing structure that pulls display  14  outwardly in direction  388  relative to housing  12  and thereby helps maintain display  14  in a flat configuration. 
       FIG. 71  is a cross-sectional side view of electronic device  10  in an illustrative configuration in which flaps  12 F have been biased for rotational motion in directions  392  by biasing structures such as coil springs  390  or other springs. This motion (i.e., the rotational force that biases flaps  12 F away from display  14 ) tends to remove bulges from display  14  and thereby flatten display  14 . 
       FIG. 72  shows how a sliding clip structure such as clip spring  396  may be used to retain display  14  within housing  12 . Clip spring  396  may help hold display  14  in place against housing  12  so that dust and other foreign matter does not enter the interior of device  10 . At the same time, spring  396  may allow display  14  to slide past housing  12  (see, e.g., dashed line  400 ) when device  10  is unfolded. By allowing the end of display  14  to slip outwardly relative to housing  12 , stress that might otherwise cause display  14  to bulge can be avoided. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20200130
Publication Date: 20220712
Grant Date: 20220712
Priority Date: 20150911
Inventors: AI, JIANG
HESCHKE, MITCHELL A.
KIM, SOYOUNG
MCCLURE, STEPHEN R.
Assignee: APPLE INC
CPC Classifications: [{"code": "H04M1/0268", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/1616", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/0216", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F1/1652", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04M1/022", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/1641", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/1618", "inventive": false, "first": false, "tree": "[]"}, {"code": "G06F1/1681", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/1681", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F1/1652", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/1681", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F1/1652", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/1618", "inventive": false, "first": false, "tree": "[]"}, {"code": "H04M1/0268", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 69230072