Optical force sensing system and related method

Abstract

The objective of the invention is to provide a reliable and flexible optical force sensing system that accurately detects forces applied to a touch element in some distance to an optical force sensor while isolating sensitive components from direct mechanical stress and enabling diverse design configurations. This objective is met by an optical force sensing system (100) comprising: · a deformable light guide (12, 14) configured to transmit light, the light guide having a first end (24) and a second end (26), · a touch element (16, 18) mechanically attached to the first end (24) of the light guide (12, 14), · an optical force sensor (8, 10) positioned on a printed circuit board (2) or flex board at a distance from the touch element (16, 18), wherein the second end (26) of the light guide (12, 14) is optically coupled to the optical force sensor (8, 10), and wherein the optical force sensor (8, 10) is configured to detect a magnitude or level of force applied on the touch element (16, 18).

Description

[0001] 2024PF01810 1

[0002] OPTICAL FORCE SENSING SYSTEM AND RELATED METHOD

[0003] DESCRIPTION

[0004] TECHNICAL FIELD

[0005] The present invention relates to optical force sensing systems and related methods . This invention has applications in user interfaces across a wide range of industries, including home appliances, consumer devices, automotive interiors, industrial machinery, and medical equipment .

[0006] BACKGROUND

[0007] Optical force sensing (OFS) has become an essential component in modern user interfaces . These systems are often used in devices such as smartphones, home appliances, and automotive interiors, where tactile feedback and precise force measurement are necessary. Other typical requirements may include low cost, small size, design freedom, no mechanical or electrical coupling, and / or shy tech (hidden buttons) . Traditional optical force sensing systems, however, face limitations in terms of the distance between the optical sensor and the touch surface, as well as the inability to accommodate deeper button presses or angled configurations .

[0008] In many conventional systems, the optical sensor must be positioned close to the touch surface in order to accurately detect force . This requirement limits design flexibility and increases the complexity of the system.

[0009] The need for a more flexible, robust, and scalable optical force sensing solution is apparent . In particular, this invention aims at a solution that allows for force sensing at greater distances from the touch surface while maintaining accuracy, providing deeper button press detection, and improving tactile feedback.

[0010] SUMMARY OF THE INVENTION

[0011] The present invention proposes an optical force sensing system that overcomes the limitations of prior art by incorpo-2024PF01810 2

[0012] rating a deformable light guide, which enables force sensing at greater distances and from non-planar surfaces . This also enables different angles between the surface and the sensor, so that one can have, e . g. , a tilted plate compared to the sensor location. However, planar touch surfaces are possible as well .

[0013] The system includes a touch element that is mechanically attached to the first end of the deformable light guide, with the second end of the light guide being optically coupled to an optical force sensor positioned on a printed circuit board (PCB) at a distance from the touch element . This arrangement allows for the detection of force applied to the touch element based on changes in the optical properties of the light guide .

[0014] A key feature of the invention is the ability to measure force at a distance, even when the touch element is angled or displaced relative to the sensor . This flexibility enables designers to create more ergonomic and aesthetically pleasing user interfaces . Additionally, the deformable light guide allows for deeper button presses, improving tactile feedback and user experience .

[0015] DETAILED DISCUSSION OF CLAIMS

[0016] According to the invention, the system comprises a deformable light guide that is configured to transmit light and has two ends . The first end is mechanically attached to a touch element with a touch surface, and the second end is optically coupled to an optical force sensor that is positioned on a printed circuit board (PCB) or a flex board, a distance away from the touch element . The sensor detects the magnitude of force applied on the touch element by measuring changes in the optical properties of the light guide . This arrangement allows for force sensing to occur at a distance from the touch surface, enabling design flexibility and more complex device configurations . The use of the deformable light guide allows for precise detection of force without the need for2024PF01810 3

[0017] direct contact between the touch surface and the optical sensor .

[0018] In a preferred embodiment, the system further includes a mechanical holder, also called force relief holder, that is attached to the PCB . The holder ' s primary function is to prevent direct application of force onto the optical force sensor . This protective feature ensures that the sensor remains undisturbed by any external forces or pressures that could potentially interfere with its accurate operation. By isolating the sensor from direct mechanical impact, the holder enhances the system' s durability and reliability, especially in high-usage or harsh environments . This feature also ensures the light guide does not flip away (detaches) when pressing or when the device is shaken. A further advantage, depending on the circumstances, may be easier manufacturability by having a place or base or socket to "plug" the lightguide in.

[0019] In this embodiment, the light guide is configured to rest against the holder, which stabilizes the system and helps maintain proper alignment between the light guide, the touch element, and the optical force sensor . This ensures that the deformation of the light guide caused by applied force results in a predictable optical change, enabling accurate force measurements . The resting position of the light guide against the holder provides additional mechanical support, contributing to the robustness and longevity of the system.

[0020] In a preferred embodiment, a gap is introduced between the second end of the light guide and the optical force sensor . This gap allows for mechanical tolerance during assembly, ensuring that slight misalignments do not negatively affect the system' s performance . It also ensures that the optical sensor responds only to the changes in light transmitted through the light guide, not to any unintended contact or interference . The gap provides an additional layer of flexibility in the design process and facilitates integration into various applications .2024PF01810 4

[0021] In a preferred embodiment, the light guide is positioned obliquely or slanted relative to the surface normal of the PCB . This allows the system to accommodate non-planar configurations, where the touch element or the surface is at an angle rather than being flat . Many modern devices, such as automotive interfaces, have curved surfaces and thus require angled buttons or touch surfaces . By enabling the light guide to be aligned in a slanted manner, the system allows for greater flexibility in design, making it adaptable to a wide range of user interfaces and devices that incorporate such angled configurations . However, in other use cases the arrangement of the light guide with respect to the circuit board may be perpendicular .

[0022] In a preferred embodiment, the system allows for the distance between the touch element and the optical force sensor to be significantly increased, with this distance being at least in the range of millimeters, or even centimeters or more . This increased distance enables the optical force sensing system to be used in devices where the sensor needs to be placed further away from the touch surface, such as in larger or more complex designs . This feature enhances the versatility of the system, allowing it to be integrated into various types of devices while maintaining effective force detection at greater distances .

[0023] In a preferred embodiment, the light guide is designed to enable a force-induced displacement of the touch element, which can range from micrometers to millimeters or even greater . This feature is essential for applications that require deeper button presses or the ability to detect varying levels of force . The deformability of the light guide allows for the touch element to move when force is applied, and the resulting displacement is detected optically. This capability is beneficial for user interfaces where tactile feedback is critical, such as in industrial machinery, medical devices, or consumer electronics .2024PF01810 5

[0024] In a preferred embodiment, the light guide is constructed from a compressible material that allows it to deform when force is applied. Suitable materials include hollow tubes, solid transparent elastomers, or materials coated with reflective properties . These materials are chosen for their ability to maintain optical transparency while still providing the necessary compressibility and bendability. The compressibility of the light guide ensures that the system can accurately detect force-induced changes, even when the touch element undergoes significant displacement . This characteristic is key to the system' s ability to handle deeper presses and provide enhanced tactile feedback.

[0025] In a preferred embodiment, the optical force sensor includes an emitter and a detector . The emitter shall preferably emit a constant stream of light, but light pulses work fine too, as long as they are synchronized with the detector . The sensor preferably works by counting photons or measuring the intensity of photons that are emitted by the emitter and reflected by the first end of the light guide or the touch element or in between (e . g. within a coating layer) . As the light guide deforms in response to force, the amount of light reflected changes, which is detected by the optical force sensor . This working principle enables precise and sensitive force detection based on changes in optical properties, without the need for physical contact between the sensor and the touch element .

[0026] In a preferred embodiment, the optical force sensing system is integrated into user interface devices across a wide range of industries . These applications include home appliances, consumer electronics (including accessories and wearables, smart phones and smart speakers) , automotive interiors and exteriors, industrial machinery, and medical equipment . The system' s ability to handle greater distances between the touch element and the sensor, as well as its adaptability to non-planar configurations, makes it ideal for a variety of user interfaces that require high levels of interactivity and tactile feedback. The versatility of the system enables it to2024PF01810 6

[0027] enhance the user experience in diverse fields, where precision and ease of use are essential .

[0028] An alternative to the use of printed circuit boards (PCBs) could involve other substrates or technologies suitable for integrating optical and mechanical components . For instance, the optical force sensing system could be implemented on flexible or flex-rigid circuit boards, flexible films, or even hybrid substrates or In-Mold Electronics ( IME) , which offer greater adaptability to various form factors and application scenarios . In the claims the term "flex board" is intended to cover all these variations .

[0029] In terms of method, the invention provides a novel approach to optical force sensing. The method involves using a deformable light guide to detect the force applied to a touch element . The first step of the method includes mechanically attaching the touch element to the first end of the light guide . The second end of the light guide is optically coupled to an optical force sensor located on a printed circuit board (PCB) , a distance away from the touch element . When force is applied to the touch element, the light guide deforms, causing changes in its optical properties . These changes are detected by the optical force sensor, allowing for the measurement of the magnitude or level of the applied force .

[0030] All features and advantages of the device can be directly applied to the method and vice versa, as both are based on the same fundamental principles and are closely interconnected in their operation.

[0031] In summary, the system according to the invention offers significant improvements in terms of design flexibility, sensor performance, and tactile feedback, making it suitable for a wide range of applications and industries . By using a deformable light guide, the invention extends the operational domain of optical force sensing and solves several challenges faced by conventional systems .2024PF01810 7

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The single figure illustrates in a purely schematic manner a cross-sectional view of an optical force sensing system according to an embodiment of the invention.

[0034] DETAILED DESCRIPTION

[0035] Referring to the figure, the optical force sensing system 100 comprises a printed circuit board (PCB) 2, which serves as the base for mounting other components of the system. In this particular embodiment, the PCB 2 is mounted on a carrier structure 4, which also supports the dome-shaped structure 6. This carrier structure 4 provides mechanical stability for the entire system and ensures that the various components are securely positioned.

[0036] The PCB 2 supports two optical force sensors (OFS) 8 and 10, which are responsible for detecting the force applied to the system. These sensors are located on the surface of the PCB 2 and are optically coupled to light guides 12 and 14, respectively.

[0037] Each light guide 12 and 14 has a first end 24 and a second end 26. The first ends 24 of the light guides are mechanical ly coupled to the touch elements 6 and 18, while the second ends 26 are optically coupled to the optical force sensors 8 and 10.

[0038] Each optical force sensor (OFS) 8 and 10 consists of two main components : an emitter and a detector (not explicitly shown in the figure) . The emitter sends light through the light guide 12 or 14, while the detector measures the amount of light reflected back at the first end 24 of the light guide or the surface of the touch elements 16 or 18. In the absence of force, the light passing through the light guide 12 or 14 remains at a certain intensity. When force is applied to the touch elements 16 or 18, the light guide 12 or 14 deforms, changing the path or the intensity of the reflected light .2024PF01810 8

[0039] This change in light intensity is detected by the detector, and the optical force sensor 8 or 10 interprets the variation to determine the magnitude of the applied force .

[0040] In the example embodiment, the touch elements 16 and 18 are mounted on a dome-shaped structure 6, which is positioned above the PCB 2. The dome-shaped structure 6 is designed to allow for a more ergonomic and intuitive user interface, providing a tactile response when the user presses the touch elements 16 and 18. The dome shape helps facilitate a deeper press and improved tactile feedback.

[0041] One of the light guides 12 is oriented perpendicularly to the PCB 2, which means it extends straight upward from the surface of the PCB 2. This orientation allows the light guide 12 to directly transmit light from the optical force sensor 8 to the touch element 16, which is located directly above it .

[0042] The second light guide 14 is oriented at an oblique angle relative to the surface normal of the PCB 2, meaning it is tilted sideways . This allows the system to accommodate force detection in configurations where the touch element 18 is not directly above the sensor 10, providing greater flexibility in design and enabling force sensing from various angles .

[0043] To support the light guides 12 and 14 and prevent them from being directly exposed to external forces, the system incorporates force relief holders 20 and 22. These holders 20, 22 are attached to the PCB 2 (or alternatively the carrier structure 4 ) and act as supports for the light guides 12 and 14. The holders ensure that the light guides 12 and 14 are positioned correctly and that they do not suffer from any undue deformation or force that might interfere with the accurate measurement of force by the optical force sensors 8 and 10. The holders may typically also prevent direct contact between the light guides 12 and 14 and the force sensors 8 and 10, ensuring that only the deformation of the light guides 12 and 14, and not any external pressure applied directly to the sensors, is measured. However, a no-gap arrangement with re-2024PF01810 9

[0044] spect to the light guides 12 and 14 and the force sensors 8 and 10 may be expedient in other cases .

[0045] The force relief holders 20, 22 rest against the light guides 12 and 14, providing them with mechanical stability and ensuring that the system is able to accurately detect force, even when the touch elements 16 and 18 are pressed at various angles and distances .

[0046] Overall, this configuration of the system enables force sensing with a large degree of flexibility. The system can accommodate touch elements 16 and 18 that are located at varying distances from the PCB 2, and it can detect force applied at different angles, even if the touch elements 16 and 18 are not aligned directly above the optical force sensors 8 and 10. This configuration extends the operational range of optical force sensing systems, allowing for deeper button presses and non-planar button orientations while still providing accurate force measurement .

[0047] By using deformable light guides 12 and 14, this system ensures reliable and accurate detection of applied forces, while the design of the holders and the light guides 12 and 14 allows for improved flexibility and ease of integration into a variety of products, including home appliances, consumer devices, automotive interiors, and other applications requiring force-sensitive user interfaces .

[0048] It should be noted that the description provided herein is merely exemplary and not intended to limit the scope of the invention. Numerous variations and modifications are possible within the framework of the claims . Specifically, the domeshaped structure 6, while advantageous for providing an ergonomic and tactile interface, can be replaced by alternative configurations tailored to specific applications . For example, the dome-shaped structure could be substituted with a flat panel for compact designs, a concave structure for improved grip in handheld devices, or a stepped surface for multi-level interactions . Other potential alternatives in-2024PF01810 10

[0049] elude spherical, pyramidal, or asymmetrical forms, depending on the functional and aesthetic requirements of the end product . These variations, and others, are fully encompassed within the scope of the claimed invention.

[0050] In addition to the previously mentioned variations, the touch element can also be extended, spanning over larger portions of the dome-shaped structure 6. This design allows for a broader interaction area, providing users with more flexibility and control when applying force . The extended press buttons 16 or 18 could cover significant areas of the dome, ensuring that force can be applied over a wider surface, which may be beneficial for applications requiring larger or more distributed touch zones . Furthermore, the dome-shaped structure 6 itself could function as the touch element, eliminating the need for a separate button element . In this case, the deformation of the dome structure in response to user input would directly trigger the optical force sensing mechanism, streamlining the design and reducing the number of components .

[0051] In one preferred embodiment the respective touch element 16 or 18 can be a press button. In another preferred embodiment the touch elements 16 and 18 may be realized as ID sliders or 2D sliders, preferably based on a working principle which involves comparing two sensor inputs and judging a force and position of displacement by their ratio of amplitudes .

[0052] The term "deformable" used in the preceding description to characterize the light guides 12, 14 refers to the ability of a material or component to change its shape, size, or structure when subj ected to an external force, without causing permanent damage or breakage . Within the anticipated lifetime of the product, this deformation is preferably reversible, such as in elastic materials that return to their original shape, size, and / or structure (or at least close to it) after the force is removed. It can include various types of deformation such as bending, compressing, stretching, or flexing. Importantly, "deformable" covers a wide range of suitable ma-2024PF01810 11

[0053] terials and mechanisms, including elastic, plastic, and viscoelastic behaviors, as well as compressible or stretchable materials . The essential characteristic is that the deformation leads to a detectable change in the optical properties of the light guide, which can be measured by the optical force sensor 8 or 10.

[0054] The system is not limited to a single or two sensor units, as illustrated, but may comprise multiple sensor units to enable advanced functionalities . With the inclusion of three or more optical force sensing units strategically positioned within the system, it becomes possible to determine not only the magnitude but also the direction of the force applied by the user . By employing geometrical triangulation or similar computational methods, the relative deformation patterns of the light guides connected to the different sensors can be analyzed to calculate the vector of the applied force . This capability is particularly advantageous for applications requiring precise input recognition, such as gaming controllers, touch-sensitive interfaces with directional gestures, or medical devices for detailed force mapping. The ability to determine force direction significantly enhances the versatility and potential use cases of the system, while maintaining the flexibility and design freedom provided by the deformable light guides .2024PF01810 12

[0055] LIST OF REFERENCE SIGNS

[0056] printed circuit board (RGB) 2 carrier structure 4 dome-shaped structure 6 optical force sensor (OFS) 8 optical force sensor (OFS) 10 light guide 12 light guide 14 touch element 16 touch element 18 force relief holder 20 force relief holder 22 first end of light guide 24 second end of light guide 26 gap 30 optical force sensing system 100

Claims

2024PF01810 13CLAIMS1 . An optical force sensing system ( 100 ) comprising :• a deformable light guide ( 12 , 14 ) configured to transmit light , the light guide having a first end ( 24 ) and a second end ( 26 ) ,• a touch element ( 16 , 18 ) mechanically attached to the first end ( 24 ) of the light guide ( 12 , 14 ) ,• an optical force sensor ( 8 , 10 ) positioned on a printed circuit board ( 2 ) or flex board at a distance from the touch element ( 16 , 18 ) ,wherein the second end ( 26 ) of the light guide ( 12 , 14 ) is optically coupled to the optical force sensor ( 8 , 10 ) , and wherein the optical force sensor ( 8 , 10 ) is configured to detect a magnitude or level of force applied on the touch element ( 16 , 18 ) .2 . The optical force sensing system ( 100 ) of claim 1 , further comprising a force relief holder ( 20 , 22 ) attached to the printed circuit board ( 2 ) , wherein the holder ( 20 , 22 ) is configured to prevent direct application of force applied to the touch element ( 16 , 18 ) onto the optical force sensor ( 8 , 10 ) .3 . The optical force sensing system ( 100 ) of claim 2 , wherein the second end ( 26 ) of the light guide ( 12 , 14 ) rests against the holder ( 20 , 22 ) .4 . The optical force sensing system ( 100 ) of any one of the preceding claims , wherein there is a gap ( 30 ) between the second end ( 26 ) of the light guide ( 12 , 14 ) and the optical force sensor ( 8 , 10 ) .

5. The optical force sensing system ( 100 ) of any one of the preceding claims , wherein the light guide ( 12 , 14 ) is aligned in an obliqued or slanted manner with respect to a surface normal on the printed circuit board ( 2 ) .2024PF01810 146. The optical force sensing system ( 100) of any one of the preceding claims, wherein the distance between the touch element ( 16, 18 ) and the optical force sensor ( 8, 10) is at least in the range of millimeters, preferably even centimeters or above .

7. The optical force sensing system ( 100) of any one of the preceding claims, wherein the light guide ( 12, 14 ) allows for a force-induced displacement of the touch element ( 16, 18 ) in the range of micrometers, preferably even millimeters or above .

8. The optical force sensing system ( 100) of any one of the preceding claims, wherein the light guide ( 12, 14 ) is made of a compressible material selected from one or more of the following :• hollow tubes,• solid transparent materials, for example based on elastomers, or• coated materials, preferably with reflective properties .

9. The optical force sensing system ( 100) of any one of the preceding claims, wherein the optical force sensor ( 8, 10) comprises an emitter and a detector and has a working principle which is based on counting photons or measuring an intensity of photons emitted by the emitter and reflected at the first end (24 ) of the light guide ( 12, 14 ) or at the touch element ( 16, 18 ) or in between.

10. A user interface device comprising the optical force sensing system ( 100) of any one of the preceding claims, wherein the user interface device is implemented in at least one of :• home appliances,• consumer devices, including accessories and wearables, smart phones and smart speakers,• automotive interiors or exteriors,• industrial machinery, or• medical equipment .2024PF01810 1511 . A method for sensing force using an optical force sensing system ( 100 ) , the system comprising a deformable light guide ( 12 , 14 ) having a first end ( 24 ) and a second end ( 26 ) , a touch element ( 16 , 18 ) mechanically attached to the first end ( 24 ) of the light guide ( 12 , 14 ) , and an optical force sensor ( 8 , 10 ) positioned on a printed circuit board ( 2 ) at a distance from the touch element ( 16 , 18 ) , wherein the second end ( 26 ) of the light guide ( 12 , 14 ) is optically coupled to the optical force sensor ( 8 , 10 ) , the method comprising :• applying a force to the touch element ( 16 , 18 ) ,• deforming the light guide ( 12 , 14 ) in response to the applied force ,• transmitting light through the light guide ( 12 , 14 ) , • detecting, by the optical force sensor ( 8 , 10 ) , changes in optical properties of the light guide ( 12 , 14 ) caused by the deformation, and• determining the magnitude or level of the applied force based on the detected changes in optical properties .

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