Force Sensing Device

Abstract

The force sensing device (201) includes a first electrode layer (202) including a material having a first resistivity and a second electrode layer (203) including a pressure sensitive material having a second resistivity. The second resistivity is relatively high compared to the first resistivity. The first electrode layer and the second electrode layer are spaced apart and configured to come together under an applied force. A first conductive material is applied to the first electrode layer to create a first deceleration layer (204), and a second conductive material is applied to the second electrode layer to create a second deceleration layer (205). The first conductive material and the second conductive material each include a material having a substantially low resistivity. The low resistivity is lower than the first resistivity and the second resistivity such that when the first conductive material and the second conductive material contact under an applied force, the flow of current between the first conductive material and the second conductive material depends on the contact area between the first deceleration layer and the second deceleration layer.

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims priority to UK patent application number 22 03 792.3, filed on March 18, 2022, the entire contents of which are incorporated herein by reference. [Background technology]

[0002] The present invention relates to a force sensing device and an electronic device including a force sensing device. Force sensing devices are used in many applications and may come in many different shapes, sizes and different technologies, each with their own limitations. A limitation of existing force sensing devices is that conventional force sensors often employ contact resistance (the electrical resistance that arises at the contact points when components are connected) as the key mechanism. As such, these types of force sensors are sensitive to changes in the mechanical interface materials, such as, but not limited to, hardness and roughness.

[0003] This can create stability issues for force sensors since the output from the sensor depends on how these mechanical properties change as a function of the surrounding environment. An example of such an environment would be applying a constant force while increasing the ambient temperature. This would result in a change in the measured sensor resistance, which in turn would cause the contact resistance to change due to the change in mechanical material properties. Other environments or scenarios may also produce similar results. Summary of the Invention [Problem to be solved by the invention]

[0004] There remains a need for alternative force sensing devices that can avoid these problems and that do not rely on contact resistance between components. [Means for solving the problem]

[0005] [Brief Description of the Invention] In accordance with an aspect of the present invention, there is provided a force sensing device comprising: a first electrode layer comprising a material having a first resistivity; a second electrode layer comprising a pressure sensitive material having a second resistivity, where the second resistivity is relatively high compared to the first resistivity, and where the first electrode layer and the second electrode layer are spaced apart and configured to come together under an applied force; and a first conductive material applied to the first electrode layer to form a first retardation layer, and a second conductive material applied to the second electrode layer to form a second retardation layer, where the first conductive material and the second conductive material each comprise a material having a substantially low resistivity, the low resistivity being lower than the first resistivity and the second resistivity, and where when the first conductive material and the second conductive material contact under an applied force, a current flowing between the first conductive material and the second conductive material is dependent on a contact area between the first retardation layer and the second retardation layer. [Brief description of the drawings]

[0006] [Figure 1] FIG. 1 is a diagram showing a contact resistance-dependent structure of an existing force sensing device; [Diagram 2] FIG. 2 is a diagram showing the configuration of a contact resistance free force detection device; [Diagram 3] FIG. 3 shows a top view of one implementation of a contact resistance free force sensing arrangement; [Figure 4] FIG. 4 further illustrates a plan view of the contact resistance free force sensing arrangement of FIG. 3; and [Diagram 5] FIG. 5 shows an example of an electronic device that uses the force detection device of the present invention.

[0007] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Embodiments of the present invention will now be described, by way of example only, with reference to the drawings. The detailed embodiments show the best mode known to the inventors and support the invention as claimed. However, they are merely illustrative and should not be used to interpret or limit the scope of the claims. Their purpose is to provide teachings to those skilled in the art.

[0008] [Detailed Description of the Invention] (Figure 1) The present application proposes a force sensing device that eliminates or minimizes the contact resistance component of such a force sensing device, thereby improving the stability of the sensor regardless of ambient fluctuations and changing environmental conditions. 1 shows a contact resistance dependent force sensing device 101 in line with conventional devices. In conventional force sensors, a low or medium resistance electrode 102 is combined with a high resistance base layer 103.

[0009] The example of Figure 1 shows a simplified side view structure of a typical contact resistance dependent force sensing device 101. The contact resistance dependent sensing structure of Figure 1 includes two sides of an interface, a first side includes a low or medium resistance electrode 102, and further includes a positive voltage electrode and a ground electrode. The second side of the force sensing device 101 comprises a highly resistive material that has the necessary resistivity to make contact resistance a significant mechanism in this sensor design.

[0010] In use, a force is applied to the force sensing device 101 and a current flows when the electrode 102 contacts the base layer 103. The extent of current flow is determined by the interfacial pressure between the electrode 102 and the base layer 103 caused by the applied force. This conventional structure is highly sensitive to changes in the mechanical properties of the applied interface material. This is due to the fact that force sensing devices rely on contact resistance, which occurs at the interface contact points between the electrode 102 and the base layer 103 when the materials are pressed together. The applied force affects a very small number of contacts between the two surfaces of the electrode and the base layer.

[0011] The primary problem with this type of prior art configuration is that while material resistivity changes with applied force, contact resistance changes as the force sensing device experiences changing environments and different scenarios. For example, at higher temperatures, the component materials become softer, affecting contact resistance which depends on the hardness of the material. In one example, a thirty percent (30%) decrease in material hardness results in a decrease in contact resistance of approximately thirty percent (30%). This is a significant and undesirable change in performance that is difficult to overcome with firmware or software solutions.

[0012] Thus, FIG. 1 illustrates a force sensing device that relies on contact resistance having a low to medium resistivity layer that includes interdigitated fingers and a high resistivity layer, such as a high resistivity layer that includes a quantum tunnel composite material.

[0013] (Figure 2) To provide a force sensing device that has low temperature response, the invention described herein eliminates the resistance at the interface between materials that contact through an air gap that exists within the force sensing device. FIG. 2 is a simplified side view of a force sensing device without contact resistance. The force sensing device 201 includes a first electrode layer 202 and a second electrode layer 203, each electrode layer providing alternating sides of an interface configured to come into mating contact to provide an electrical output in response to an applied force.

[0014] The force sensing device 201 further includes a first deceleration layer 204 and a second deceleration layer 205. In an embodiment, the electrode layer 202 includes a material having a first resistivity and the electrode layer 203 includes a pressure sensitive material having a second resistivity. The first resistivity is medium or low, while the second resistivity is relatively high compared to the first resistivity. In an embodiment, the electrode layer 202 and the electrode layer 203 are spaced apart from each other and separated by a gap 206 and configured to move together when a force is applied to the force sensing device 201. The gap 206 is located between the first deceleration layer 204 and the second deceleration layer 205.

[0015] The retardation layer 204 comprises a first conductive material applied to the electrode layer 202 , while the retardation layer 205 comprises a second conductive material applied to the electrode layer 203 . In an embodiment, the deceleration layer 204 is in intimate contact with the electrode layer 202, which in this context is taken to indicate that there is no contact resistance between the electrode layer and the deceleration layer interface. Similarly, the deceleration layer 205 is in intimate contact with the electrode layer 203.

[0016] In an embodiment, an air gap 206 separates the two sides of the interface, with electrode layer 202 and electrode layer 203 being spaced a fixed distance apart. Thus, deceleration layer 204 and deceleration layer 205 are also spaced a fixed distance apart. Furthermore, electrode layer 202 and electrode layer 203 are provided on a first substrate 207 and a second substrate 208, respectively. The substrates are spaced apart by conventional spacer elements or the like, so that air gap 206 is maintained even when no force is applied. In an embodiment, the deceleration layer 204 and the deceleration layer 205 are appropriately aligned to ensure that only contact between the deceleration layer 204 and the deceleration layer 205 occurs in response to a force applied to the force sensing device, and the gap 206 is reduced and minimized.

[0017] In an embodiment, the resistivity of the deceleration layers 204 and 205 is very low. In one embodiment, this resistivity is typically 10 -7 When the deceleration layers 204 and 205 contact each other in a non-intimate manner, the contact resistance is extremely small, being less than ohm-meter (Ωm). This is because its absolute value depends on the sum of the resistivities of the contacting surfaces of the interface. If the resistance on one side of the force sensing device is high, as in some conventional systems, this dominates the sum and the contact resistance is large. Thus, in embodiments, the resistance on both sides of the interface is low, so the sum of the resistivities is also low. As a result, by introducing the low-resistance deceleration layers 204 and 205 to provide a contact interface between the two electrodes, the force sensing devices described herein have substantially no contact resistance.

[0018] By introducing the deceleration layers 204 and 205, the resulting force sensing device cannot rely solely on contact resistance. Otherwise, an electrical short occurs when the two opposite sides of the interface are in contact. Therefore, it is necessary for the electrode layer 203 to have a high resistivity on the opposite side of the interface to the deceleration layer 203. Typically, the deceleration layer 203 also includes relatively small features that are not limited by shape or distribution. In contrast, the deceleration layer 204 can include a variety of patterns. In one embodiment, such patterns can be substantially similar to the deceleration layer 203. Although alternative arrangements may be utilized, in any case, the deceleration layers 203 and 204 are configured to contact only at the interface between the two halves of the force sensing device.

[0019] In use, when a force is applied to the top surface 209 of the force sensing device 201, the electrode layer 202 and deceleration layer 204 move towards the deceleration layer 205 and electrode layer 203. The force sensing device 201 is connected in an electrical circuit such that when the deceleration layers 204 and 205 come into contact, an electric current flows. The extent to which the electric current flows is determined by the macroscopic contact area, not by the pressure at the contact interface. This ensures that the structure of the force sensing device 201 is not affected by changes in the mechanical properties of the materials used at the contact interface (the point where contact is made).

[0020] To ensure that a force sensing device exhibits a change in resistance with force, it must have a macroscopic contact area that changes as a function of force at the contact interface. This differs from the microscopic "true" contact area change observed in conventional force sensing devices that rely on contact resistance. The macroscopic contact area is defined as the area where the air gap is small enough that the deceleration layer 204 and the deceleration layer 205 are considered to be in contact.

[0021] Increasing the macroscopic contact area as a function of applied force increases the number of parallel current paths through the highly resistive material of the electrode layer 203. This increase in the number of parallel current paths reduces the overall resistance between the first and second electrode layers 203, 204. Therefore, the force sensing device 201 provides a solution to achieve these effects by removing the high resistance material of the electrode layer 203 in direct contact at the contact interface and replacing it with a corresponding deceleration layer 205 offering a lower resistance.

[0022] (Figure 3) FIG. 3 is a top view of the force sensing device 201 showing one embodiment and the arrangement of the electrode layer 202 and the moderator layer 204. In an embodiment, the first conductive material of the deceleration layer 204 is comprised of a silver-based material, such as a silver-based ink, and the material having the first resistivity of the electrode layer 202 is comprised of a carbon-based material.

[0023] In an embodiment, the first conductive material comprising the deceleration layer 204 is provided in the form of a printed pattern, as shown in Figure 3. In this particular embodiment, the printed pattern comprises a plurality of dots 301, although it will be appreciated that alternative patterns may be utilized. In an alternative embodiment, for example, the printed pattern comprises a plurality of interlocking fingers. In an embodiment, the electrode layer 202 includes a plurality of interdigitated fingers 302. In embodiments in which the printed pattern also includes a plurality of interdigitated fingers, it is understood that each of the plurality of interdigitated fingers are substantially similar and aligned with one another.

[0024] (Figure 4) 4 shows yet another plan view from below of the force sensing device 201. It shows the arrangement of the deceleration layer 205 and the electrode layer 203. In an embodiment, the second conductive material of the deceleration layer 205 comprises a silver-based material, such as a silver-based ink, which may be substantially similar to the conductive material of the deceleration layer 204. The pressure sensitive material of the electrode layer 203 comprises a quantum tunneling material, such as a quantum tunneling composite material available under the trademark QTC® from the applicant, Peratec Holdco Limited.

[0025] In an embodiment, the deceleration layer 205 is provided in the form of a printed pattern, and as shown, the printed pattern includes a plurality of dots 401 that correspond to the plurality of dots 301 of the deceleration layer 204 described above with respect to Figure 3. The dot pattern avoids shorting the force sensing device when the deceleration layer 204 and the deceleration layer 205 come into contact, thereby forcing current to flow along a path in the higher resistivity material of the electrode layer 203.

[0026] In an embodiment, a dot pattern of silver-based ink is printed onto the quantum tunneling material according to the dot arrangement shown in Figure 3. In an embodiment, this provides silver-to-silver contact and creates parallel paths between the dots, eliminating contact resistance and allowing current to flow through the more resistive material. This effectively has the effect of increasing the number of parallel resistors in the electrical circuit, which reduces the resistance and therefore the force sensing device can reduce the resistance.

[0027] (Figure 5) FIG. 5 illustrates an exemplary electronic device 501 with a keyboard 502 in accordance with an embodiment of the present invention. In an embodiment, electronic device 501 may be a desktop computer, a notebook computer, or any other suitable type of electronic device capable of using a keyboard. Keyboard 502 includes a number of keys 503 that a user can use to provide input to electronic device 501 that can then be output to a display 504 of electronic device 501.

[0028] In an embodiment, each of the plurality of keys 503 includes a force sensing device in accordance with the present invention, as described herein. The force sensing device may be utilized as part of a keyboard membrane to provide a force sensitive response when a force is applied to any of the plurality of keys 503. As a result, the force sensing device described herein may provide a more appropriate and stable response even when environmental conditions change.

Claims

1. a first electrode layer comprising a material having a first resistivity; a second electrode layer including a pressure sensitive material having a second resistivity, where the second resistivity is relatively high compared to the first resistivity, and where the first electrode layer and the second electrode layer are spaced apart and configured to converge under an applied force; and a first conductive material applied to the first electrode layer to form a first rate-down layer, and a second conductive material applied to the second electrode layer to form a second rate-down layer, The first conductive material and the second conductive material each comprise a material having a substantially low resistivity, the low resistivity being lower than the first resistivity and the second resistivity, and when the first conductive material and the second conductive material are in contact under an applied force, a current flowing between the first conductive material and the second conductive material depends on a contact area between the first retardation layer and the second retardation layer: Force sensing device.

2. The force sensing device of claim 1 , further comprising an air gap between the first and second deceleration layers.

3. The force sensing device of claim 1 or claim 2, wherein the first conductive material comprises a silver-based material.

4. 4. The force sensing device of claim 1, wherein the second conductive material comprises a silver-based material.

5. 5. The force sensing device of claim 1, wherein the current flow includes multiple parallel paths configured to reduce an overall resistance between the first electrode layer and the second electrode layer.

6. 6. A force sensing device according to claim 1, wherein the material having the first resistivity comprises a carbon-based material.

7. A force sensing device according to claim 1 , wherein the pressure sensitive material comprises a quantum tunneling material.

8. 8. The sensing device according to claim 1, wherein the first electrode layer and the second electrode layer are provided on a first substrate and a second substrate, respectively.

9. 9. A force sensing device according to claim 1, wherein the first conductive material is provided in the form of a printed pattern.

10. The force sensing device of claim 9 , wherein the printed pattern includes a plurality of interlocking fingers.

11. The force sensing device of claim 9 , wherein the printed pattern comprises a plurality of dots.

12. 12. A force sensing device according to claim 1, wherein the first electrode layer comprises a plurality of interdigitated fingers.

13. 13. An electronic device comprising a force sensing device according to any one of claims 1 to 12.

14. The electronic device of claim 13 , wherein the electronic device comprises an electronic keyboard.

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