WO2020234197A1

WO2020234197A1 - Force sensor

Info

Publication number: WO2020234197A1
Application number: PCT/EP2020/063737
Authority: WO
Inventors: Jason RILEY; Elena WATTS; Ning Xu
Original assignee: ROLI Limited; Imperial College Innovations Ltd
Priority date: 2019-05-17
Filing date: 2020-05-15
Publication date: 2020-11-26
Also published as: GB201907017D0; GB2584088A

Abstract

Disclosed herein is a force sensor, a method of measuring an input force using the force sensor, a method of fabricating a force sensor, a force sensor system, and a touch-sensitive user interface. The force sensor comprises: a substrate having a plurality of resilient protrusions on a first surface of the substrate; a first conductive layer disposed on the first surface at least partially covering the plurality of resilient protrusions; a first pair of electrodes; and an input mechanism arranged to receive an input force and communicate the input force to deform one or more resilient protrusions of the plurality of resilient protrusions to change an electrical impedance between the electrodes of the first pair of electrodes.

Description

FORCE SENSOR

FIELD OF INVENTION

This disclosure relates generally to force sensors and, in particular, electrical sensors for detecting pressure and tensional forces.

BACKGROUND

Force sensors are used across a wide range of technologies and at different scales in both non-commercial laboratories and in commercial products. One area in which electronic force sensors are especially relevant is electronic devices controlled by user touch. This includes mobile devices with touchscreens, user interfaces for controlling machinery or robotics, and electronic musical instruments.

Sensors in the field of touchscreens often use capacitive or resistive methods. By detecting a change in capacitance or resistivity at a surface of a user interface caused by the presence of a user input, e.g. a finger or stylus, a touch can be detected. These sensors can be used to receive user touch input at one or more locations on a screen. However, capacitive techniques typically have a low dynamic range of touch sensing; generally only the presence of a touch is detectable, whereas variations in the pressure or force of the touch input are not. Hence capacitive and resistive sensors are poor at detecting the magnitude of user input force.

An alternative to capacitive sensors is pressure sensors, which have been used in touch- based user interfaces. For touchscreen applications, the detection of user touch input is achieved by placing conventional pressure sensors, such as piezoelectric devices, around the edge of a screen. The pressure on the screen at touch locations is detected by measuring the force transferred to the edges of the screen. The signals at the pressure sensors can be analysed to work out the force of a touch input. However for two or more simultaneous touch inputs this technique is highly inaccurate, due in part to the distance between the location of the touch and the locations of the sensors which is even worse for larger screens. Further, the opacity of known pressure sensors prevents positioning them over the screen, since doing so would compromise the quality of the display. SUMMARY

A force sensor is provided comprising a substrate having a plurality of resilient protrusions on a first surface of the substrate. The term resilient is used herein to refer to materials having the property of returning, following deformation, to substantially the same shape as before the deformation.

The force sensor further comprises a first conductive layer disposed on the first surface at least partially covering the plurality of resilient protrusions. At least partially covering means that the first conductive layer is disposed on at least some of the resilient protrusions, e.g. so that the first conductive layer conforms to the shape of the resilient protrusions. As an example, the first conductive layer may be a thin layer, i.e. having a thickness much less that the thickness of the substrate or the height of the resilient protrusions. The first conductive layer may be a coating of the resilient protrusions. The first conductive layer may form a continuous area across the substrate with no gaps or holes. The first conductive layer may span the substrate.

The force sensor further comprises a first pair of electrodes. The force sensor further comprises an input mechanism arranged to receive an input force and communicate the input force to deform one or more resilient protrusions of the plurality of resilient protrusions to change an electrical impedance between the electrodes of the first pair of electrodes. The input mechanism may be a second surface of the substrate, e.g. the surface opposite the first surface on which the resilient protrusions are disposed, wherein the input force is communicated through the substrate from the second surface to the first surface. The resilient protrusions may be deformed against a deformation wall, which may be one or more electrodes, or a second substrate. The change in electrical impedance may be a change in resistance, capacitance, inductance, or a combination thereof.

One or more of the substrate, the first pair of electrodes, and first conductive layer may be substantially transparent. Any single item of this list, or any combination of two or three items of the list, may be substantially transparent. Substantially transparent may being 90% or greater transmittance in visible wavelengths of light, alternatively 70% or greater

transmittance, or 50% or greater transmittance depending on the implementation.

Accordingly, the force sensor can be incorporated into a screen without impeding the display.

The resilient protrusions may be positioned in a periodic array. For example, the resilient protrusions may be positioned in a two-dimensional array, i.e. a grid, on the first surface of the substrate. The resilient protrusions may be in perpendicular rows and columns, in a hexagonal array, or other base-shape array. The resilient protrusions may be periodic such that the resilient protrusions have equal spaces between adjacent resilient protrusions or may follow a larger repeat pattern across the substrate.

The resilient protrusions may be substantially domed. For example, each resilient protrusion may have a hemispherical shape, or have an ellipsoid shape, or have a cubic shape with rounded edges. Having domed resilient protrusions facilitates the deformation of the resilient protrusions, making the force sensor more sensitive to the input force.

The substrate may be resilient, e.g. made from a resilient material. This improves the communication between the input mechanism and the resilient protrusions. The substrate and the resilient protrusions may be integral, i.e. monolithic, and made of the same material.

One or both electrodes of the first pair of electrodes may oppose at least a respective portion of the first conductive layer. The change in electrical impedance between the electrodes of the first pair of electrodes may include change in electrical impedance between the one or both of the electrodes of the first pair of electrodes and the first conductive layer, wherein the change in electrical impedance is a result of deformation of the one or more resilient protrusions against the one or both of the electrodes of the first pair of electrodes. The one or both electrodes may therefore act as a deformation wall.

The electrodes of the first pair of electrodes may be arranged on opposite sides of the conductive layer. In other words, the conductive layer is positioned between the first and second electrode of the pair of electrodes. The opposite sides of the conductive layer are opposite surfaces or faces of the conductive layer. The input force deforming first conductive layer against one of the electrodes therefore changes the impedance between the electrodes since the first conductive layer is located between the electrodes.

The electrodes of the first pair of electrodes may be arranged on the same side of the first conductive layer. For example, the first pair of electrodes may be positioned in the same plane adjacent to each other and separated by a gap. The electrodes may be arranged such that the input force brings the first conductive layer into contact with each of the electrodes and deforms the resilient protrusions, thereby changing the contact surface area between the electrodes and first conductive layer and changes the impedance between electrodes. The force sensor may further comprise one or more further pairs of electrodes, wherein the input mechanism communicates the input force received at respective locations to change an electrical impedance between the electrodes of a respective pair of electrodes.

The force sensor may further comprise a second substrate having a second plurality of resilient protrusions on a second surface of the second substrate and a second conductive layer disposed on the second surface at least partially covering the second plurality of resilient protrusions. The second substrate may have any of the features of the first substrate as described according to the present disclosure. The second conductive layer may have any of the features of the first conductive layer as described according to the present disclosure. The first and second conductive layers may function as a deformation wall for each other upon receiving the input force.

The first plurality of resilient protrusions may differ from the second plurality of resilient protrusions in one or more of: size, shape, height, width, material, hardness, elasticity, conductivity, and spacing between resilient protrusions. These parameters affect the deformation properties of the resilient protrusions and hence, if the different pluralities of resilient protrusions have different properties, the resulting force sensor will have a larger dynamic range of measurable forces. The first plurality of resilient protrusions may be arranged to deform in response to a lower input force than the input force in response to which the second plurality of resilient protrusions are arranged to deform. In this case, the force sensor may be sensitive to low forces, due to deformation of the first plurality of resilient protrusions, but still work for high forces, due to deformation of the second plurality of resilient protrusions.

The force sensor may be a pressure sensor, with the input mechanism being an input surface and the input force being an input pressure. For example, the input surface may be a surface of the substrate which an actuator, e.g. a finger, can exert a pressure which is communicated to deform the resilient protrusions. The input pressure may be input by depressing the input surface.

The force sensor may be a tension sensor, with the input force being an input tension and the input mechanism being arranged to communicate the input tension to stretch the resilient protrusions. For example, the input mechanism may be actuators on opposite sides of the first surface of the substrate which, when experiencing an input tension, pull in different directions to each other. This tensional force can stretch the resilient protrusions such that an electrical impedance between the electrodes of the first plurality of electrodes changes. The force sensor may also be arranged to detect input pressure and input tension.

A method is provided of measuring an input force using a force sensor as described above. The method comprises receiving the input force at the input mechanism and deforming, by the input force (i.e. the input force being communicated to deform), one or more of the resilient protrusions. The method further comprises detecting a change in the electrical impedance between the electrodes of the first pair of electrodes caused by the deforming of the one or more resilient protrusions. The method further comprises determining, based on the change in the detected electrical impedance, that an input force has been received at the input mechanism. The method of measuring may further comprise supplying a voltage across the electrodes of the first pair of electrodes and measuring an electrical current from one or other of the electrodes, wherein the detecting a change in the electrical impedance comprises detecting a change in the measured current caused by the deforming of the one or more resilient protrusions. The change in electrical impedance can be determined from the change in electrical current, or alternatively the change in electrical current can be used directly to determine that an input force has been received at the input mechanism and/or the magnitude of the input force.

The method of measuring may further comprise determining a magnitude of the input force based on the change in the detected electrical impedance. For example, a larger change in electrical impedance may correspond to a larger input force.

A method of fabricating a force sensor is provided. The method may be used to fabricate a force sensor as described above. The method comprises applying a first conductive layer into a patterned mould and applying a polymer onto the first conductive layer in the patterned mould. The method further comprises removing the polymer and first conductive layer from the patterned mould to produce a substrate having a plurality of resilient protrusions on a first surface of the substrate and the first conductive layer disposed on the first surface at least partially covering the plurality of resilient protrusions. The method further comprises arranging a pair of electrodes such that deformations of one or more of the resilient protrusions of the plurality of resilient protrusions changes an electrical impedance between the electrodes of the first pair of electrodes. The arranging of the pair of electrodes may comprise disposing the electrodes of the pair of electrodes on the same side of the first conductive layer. Alternatively, the arranging of the pair of electrodes may comprise disposing the electrodes of the pair of electrodes on opposite sides of the first conductive layer. Either way, the electrodes are arranged such that the first conductive layer is part of an electrical pathway between electrodes when the force sensor receives an input force.

The method of fabricating may further comprise providing the patterned mould by providing a metal-coated silicon wafer, transferring a plurality of monodisperse spheres onto the metal- coated silicon wafer, etching the plurality of monodisperse spheres according to a mould pattern, depositing a metal layer onto the metal-coated silicon wafer, and, after depositing the metal layer, removing the plurality of etched monodisperse spheres to produce the patterned mould.

A force sensing system is provided. The force sensing system comprises a force sensor as described above. The force sensing system further comprises a power supply arranged to supply a voltage across the electrodes of the first pair of electrodes. The force sensing system further comprises an electrical impedance detector arranged to measure an electrical impedance between electrodes of the first pair of electrodes. The force sensing system further comprises a processor configured to detect a change in measured electrical impedance from the electrical impedance detector and determining, based on the change in the measured electrical impedance, that an input force has been received at the input mechanism. The electrical impedance detector may be a current detector arranged to measure electrical current and may either determine the electrical impedance from the measured current or provide the processor with the electrical current which the processor can use to determine that the input force has been received. Hence the electrical current may be an indicator of, or an alternative to, the electrical impedance.

The processor of the force sensing system may further be configured to determine a magnitude of the input force based on the change in the measured electrical impedance.

A touch-sensitive user interface is provided, comprising a force sensing system as described above and a display for providing visual feedback to a user of the touch-sensitive user interface. The display and the input mechanism may be located on opposite sides of the substrate and first conductive layer. This means the touch-sensitive user interface can be used by a user to interact with the display and use an input force to control a user device comprising the user interface, while the display provides visual feedback. Exemplary force sensors can be placed directly over the display without inhibiting the image since the arrangement of the force sensors described herein facilitate creating substantially

transparent force sensors. Certain exemplary force sensors, in accordance with the present disclosure, provide advantages such as:

improved sensitivity and/or force sensing dynamic range, e.g. by controlling the properties of the resilient protrusions of one or more substrate;

improved control for a user interface, since a user can have more options for expressing the input by varying touch location and force;

- versatile force sensors, e.g. by creating the force sensor using a mould pattern with customisable resilient protrusion parameters;

- transparent force sensors, e.g. because the structure of force sensors has a first conductive layer covering a plurality of resilient protrusions, and each layer of the force sensor can be made from substantially transparent materials. This allows advantageous integration with display screens, allowing for better optical

transmission through the display combined with the advantages listed above.

BRIEF DESCRIPTION OF FIGURES

Exemplary arrangements of the disclosure shall now be described with reference to the drawings in which:

Figure 1 is a cross-section view of a force sensor;

Figure 2 shows two cross-section views of a force sensor at rest and when receiving an input force; and

Figure 3 is an exploded perspective view of the force sensor of figure 2;

Figure 4 is a cross-section view of a force sensor;

Figure 5 is a cross-section view of a force sensor;

Figure 6 is a perspective view of the force sensor of figure 4 or 5;

Figure 7 is a cross-section view of a force sensor;

Figure 8 shows two perspective views of the force sensor of figure 7 at rest and when receiving an input force;

Figure 9 shows a method for measuring an input force using a force sensor;

Figure 10 shows a graph of resistance versus input pressure;

Figure 1 1 shows a method of fabricating a force sensor;

Figure 12 shows perspective views of stages of fabrication of a force sensor;

Figure 13 is a schematic view of a force-sensing system in a touch-sensitive user interface. DETAILED DESCRIPTION

In the present disclosure, an exemplary force sensor is provided wherein the force sensing technique is based on a patterned polymer substrate coated with a conductive polymer layer. A patterned surface of the polymer substrate is positioned in cooperation with another conductive surface such that when the patterned polymer substrate is deformed, along with the conductive polymer layer coating thereon, the resistance between the conductive surfaces changes. The change in resistance between the conductive surfaces can be measured using known techniques. Since the resistance is related to the deformation, which is in turn related to the amount of input force, the input force can be determined from the measured resistance.

The general principles of this technique are described below with reference to figure 1 , followed by further specific examples of forces sensors with reference to figures 2 to 8.

With reference to figure 1 , an exemplary force sensor 100 comprises a patterned polymer substrate 102. On a first surface of the substrate 102, the top surface of the substrate shown in figure 1 , there is a plurality of domes 104 protruding from the substrate 102. The substrate 102 is made from a resilient polymer such that the substrate can be compressed or stretched by external forces and will return to its original shape. The first surface is coated with a conductive polymer 106, which conforms to the shape of the plurality of domes 104 and is flexible. A first electrode 108 is connected to the conductive polymer 106. The substrate 102 is arranged against a second electrode 110 such that the domes 104 abut the second electrode 110, or are separated from the second electrode 110 by a small gap. A second surface of the substrate 102 is an input surface 112 which functions as an input mechanism for the force sensor 100. The first electrode 108 and second electrode 110 are a first pair of electrodes and are connected by a circuit (not shown) comprising measurement devices which allow, either directly or indirectly, the resistance between the first pair of electrodes 108, 110 to be determined. For example, the circuit may comprise a power supply of known or controllable voltage and an ammeter for measuring current between the first and second electrodes. The circuit is not considered part of the force sensor, but rather the circuit is introduced when the force sensor is put in use to measure the properties of the force sensor, such as the resistance between electrodes. Further, the circuit can be produced using standard components known to the person skilled in the art.

The input pressure at the input surface 112 of the force sensor 100 can be determined in the following way. An input pressure is received at the input surface 112 of the force sensor, which pushes the substrate 102 against the second electrode 110. The pressure forces one or more of the domes 104 to deform, e.g. by being compressed against the second electrode 110, which flattens the tops of the domes against the second electrode 110. This increases the contact surface area between the conductive polymer 106 and the second electrode 110, thereby reducing the resistance between the conductive layer 106 and the second electrode 110 and consequently also reducing the resistance between the first and second electrodes. The circuit measurement devices operating during the received input pressure will therefore detect a change in resistance (or alternatively current, or any other electrical property which varies corresponding to the deformation of the domes) which can be used to determine the input pressure. Once the pressure is released, the substrate 102 and domes 104 will return to the original shape and the resistance will return to the original value. Since different magnitudes of the pressure input will deform the domes 104 to different extents, creating a range of changes in resistance, the extent of resistance value change can be used to determine the magnitude of the input force. As described, a rate of pressure increase or a rate of pressure decrease can be accurately detected as the protrusions resiliently deform in response to an input pressure.

The general principles described above for the force sensor described with reference to figure 1 can be applied to force sensors having variations from the example given above.

For example, the domes 104 on the first surface of the substrate 102 may be any shape of resilient protrusions, such as pyramids, cylinders, cubes, oblongs etc. Each of these shapes may have different deformation properties and respond differently to different types and magnitudes of input force. The substrate 102 may have resilient protrusions on more than one surface and/or be arranged to deform against more than one electrode.

The first pair of electrodes, first electrode 108 and second electrode 109, may be positioned in different arrangements to as shown in figure 1. For example, the conductive polymer layer 108 may be connected at a location other than at the edge of the force sensor. The force sensor may also comprise a plurality of pairs of electrodes for determining the input force at a plurality of locations on the input surface 112.

Instead of using a second surface of the substrate 102 to be the input mechanism, the input mechanism may instead an additional structure comprising moving parts. For example, a button could be used to communicate a press by a user to deform one or more of the resilient protrusions. With reference to figure 2a, an alternative arrangement of a force sensor 100 has the conductive polymer layer 106 not directly connected to an electrode. Instead, both the first electrode 108 and the second electrode 110 are disposed on the same side of the substrate 106 and the conductive polymer layer 106. The first and second electrodes are positioned side by side on a support 202 with a spacing between them. The substrate may be held in place with respect to the electrodes by a housing or supports (not shown). The circuit between the first and second electrodes produces a voltage across the electrodes but, in the inactivated state shown in figure 2a, there is no electrical connection through the conductive polymer layer 106 since this is not in contact with the electrodes. However, when an input force is received, e.g. by a human digit 204 as shown in figure 2b, the substrate 106 is pushed down onto the electrodes establishing an electrical pathway between the first and second electrodes. A current can flow from the first electrode 108, through the conductive layer 106 and into the second electrode 110 completing the measurement circuit. Further, the input force deforms one or more of the domes 104 thereby changing the resistance between the electrodes via the conductive polymer layer 106. Hence the input force can be measured. This arrangement means that the effect of changing contact surface area with received input pressure is increased, since there is a contact surface area increase at both the first electrode 108 and the second electrode 110.

When the input pressure is released the domes will revert to their original shape and position, breaking the electrical connection between the first and second electrodes. Hence, the force sensor 100 returns to the state described above with reference to figure 2a.

With reference to figure 3, the electrode arrangement described above with reference to figure 2 can be extended to provide an electrode matrix made up of a plurality of column electrodes and a plurality of row electrodes. The column electrodes extend in a direction perpendicular to the direction in which the row electrodes extend. At certain locations where the column and row electrodes would intersect there are spacings which will prevent the measurement circuit from being completed unless the domes 104 and conductive polymer layer 106 thereon are pushed into the spacings to complete the circuit. The column electrodes may be anodes and the row electrodes may be cathodes, or vice versa. The measuring circuit can determine the input pressure by sequentially interrogating each pair made up of one column electrode and one row electrode, e.g. using a multiplexer. The resistance measured between each pair of electrodes provides an indication of the input pressure received at the corresponding matrix point where that pair of electrodes intersects. By cycling through the pairs of electrodes, the input pressure across the input surface 112 can be determined, and therefore accurate touch location information is obtained. With reference to figure 4, an alternative arrangement of a force sensor 100 comprises, in addition to the first substrate 102 described above, a second substrate 402 which is patterned with a second plurality of domes 404. A second conductive polymer layer 406 coats the second substrate covering the second plurality of domes 404. The force sensor 100 is assembled so that the second conductive polymer layer 406 faces the first conductive polymer layer 106 of the first substrate 102. Hence the first and second pluralities of domes are directed towards each other. The substrates may be held in place by a housing or supports (not shown). The first electrode 108 is connected to the first conductive polymer layer 106 and the second electrode 110 is connected to the second conductive polymer layer 406.

An input force can be received via an input surface on one of the outside surfaces of the first and second substrate (i.e. a surface without domes), such as by a user digit 204. The input force pushes the first and second pluralities of domes together and deforms one or more dome from one or both of the first and second pluralities of domes. The deformation brings the respective conductive polymer layers 106, 406 together and increases the contact surface area between the conductive polymer layers. This changes the resistance between the first electrode 108 and the second electrode 110, which can be measured. Further, since the electrodes are positioned at edges of the conductive polymer layers, the lateral resistance of the conductive polymer layers, i.e. the resistance along each conductive polymer layer itself and not just the resistence at a point of contact between them, affect the measured resistance. Since the lateral resistance may also change in response to deformation of the domes, this may increase the sensitivity of the force sensor by producing a larger change in resistance.

The force sensor 100 optionally further comprises a second pair of electrodes 408, 410 which can also be used to measure the resistance between the first conductive polymer layer 106 and the second conductive polymer layer 406. Having two or more pairs of electrodes can increase the accuracy of the resistance measurement. The arrows in figure 4 indicate the direction of current flow from second electrodes 110, 410, through the conductive polymer layers 106, 406 pushed against each other, to the first electrodes 108, 408.

The first plurality of domes 104, on the first substrate 102, has a dome size of less than the second plurality of domes 404 on the second substrate 402. In particular, the dome radius of the first plurality of domes is smaller than the dome radius of the second plurality of domes. This means that it requires less force to deform a dome from the second plurality of domes than the force required to deform a dome from the first plurality of domes. Therefore, when an input force is received, domes from the second plurality of domes will deform at an earlier stage in the force application than the first plurality of domes, resulting in a more sensitive force sensor than if there were only the smaller sized domes. On the other hand, if the input force continues to increase, the larger domes of the second plurality of domes will reach maximum deformation at an earlier stage while the smaller domes of the first plurality of domes are still in the process of deforming. Hence, at higher forces, the smaller domes continue to deform and the change in resistance can still be measured. Hence the dynamic range of the force sensor 100 as a whole is higher than having only one size of dome.

The domes of the first and second substrates 102, 402 may differ in parameters other than size. Further properties of resilient protrusions which affect the relationship between input force and change is resistance include shape, height, width, material, hardness, elasticity, conductivity, and spacing between resilient protrusions. For example, tall and thin resilient protrusions will change in contact surface area and/or resistance in a different way to short and wide resilient protrusions. Likewise, even when two sets of resilient protrusions are the same size as each other, having different hardness or elasticity affects how readily the protrusion deform, softer and/or more elastic protrusions are more sensitive to smaller input forces. The conductivity of the conductive polymer layers 106, 406 coating the resilient protrusions also exaggerate the effect of the deformation, if a protrusion is coated with a higher conductivity layer, a deformation causing a certain increase in contact surface area will produce a bigger change in resistance. Further, if resilient protrusions on a substrate are more spread out, i.e. the spacing between protrusions is larger, then a certain input force will have a lower net change in contact surface area since there are fewer protrusions to be deformed. In general, any one of these properties, or a combination of properties, may be chosen for the resilient protrusions on a substrate in order to provide the desired sensitivity for a particular application according to any of the force sensor arrangements described herein. Furthermore, in the arrangement described above in reference to figure 4, the choice of protrusion properties for each of the two substrates can produce an improved dynamic range.

Since the substrates 102, 402 and conductive polymer layers 106, 406 can each be made of substantially transparent material, the resulting device can be made as a transparent pressure sensor. As such, this form of sensor can be located on a display, such as mobile device touchscreen, to allow pressure sensing on a visual feedback user interface.

Accordingly, force sensors according to the present designs have a significant advantage over known pressure sensors, which are not transparent due to opaque components such as button structures or electrodes being required. Suitable transparent materials include

PEDOT:PSS for the conductive polymer layers 106, 406, and PET and/or EVA for the substrates 102, 402. Exemplary materials that the electrode(s) may be made from are ITO (Indium Tin Oxide), Aluminium Zinc Oxide, conductive polymers such as PEDOT:PSS, or silver nanowires. In arrangements where the transparency of the electrodes is not needed, any conductive material can be used.

With reference to figure 5, another arrangement of force sensor 100 has the first electrode 108 and the second electrode 110 positioned across the width of the force sensor 100. In this arrangement, the electrodes are not directly connected to the conductive polymer layers 106, 406 but instead the first and second substrates 102, 402 are conductive. Hence, when the force sensor 100 receives an input force, the electrical pathway from the second electrode 110 goes through the second substrate 402 to the second conductive layer 406, then between the conductive layers and from the first conductive layer 106 through the first substrate 102 to the first electrode 108. In this arrangement, at least one of the electrodes is flexible and resilient to act as an input surface to receive the input force from a user digit 204.

With reference to figure 6, the substrates 102, 104 comprise two-dimensional arrays of domes 104, 404. The two-dimensional arrays of domes are arranged in a square grid pattern, forming rows and columns of domes. The size, number and spacing of the first plurality domes 104 on the first substrate 102 are different to the second plurality of domes 404 on the second substrate 402. In alternative arrangements, the two-dimensional arrays may have any form of repeated pattern, protrusions of different forms instead of domes, and larger or smaller spacings between protrusions.

With reference to figure 7, an alternative arrangement of a force sensor 100 is designed to determine an input tension rather than an input pressure. The structure of the tension sensor is the same as described above with reference to figures 1 to 6, except that the input mechanism is designed to stretch the substrate 102 and domes 104 thereon. For example, the input mechanism may be fasteners attaching the sides of the first substrate 102 (and/or the second substrate 402) to an actuator for exerting a tension as shown by arrows in figure 7. In another example, the force sensor 100 may be disposed on or integrated into a piece of stretchable fabric such that stretching of the fabric with a tension force is communicated to the substrate 102 to deform the domes 104 with a stretching action. This approach could be applied to items of clothing having built-in flexible tensions sensors for measuring tension experienced by the fabric at the location of the tension sensor. With reference to figure 8, the input tension to the first substrate 102 can be applied in more than one direction. Figures 8a and 8b show the substrate 102 and plurality of domes 104 in the rest state and a tensioned state, respectively. The first and second electrodes are not shown in figure 8 for simplicity, but these can be included according to any of the

arrangements described above in order to create the force sensor 100. Further, a second substrate 402 can be included in the tension arrangements as described above with reference to figures 4 to 6.

With reference to figure 8a, tensions actuators 802, 804 are attached to the edges of the first substrate 102. This can be done using adhesives or physical fasteners. A first pair of tension actuators 802 are arranged on opposite sides of the first substrate, on either side of the first surface of the substrate 104 on which the domes 104 are located. The first actuators can be pulled to exert an input tension on the first substrate in the‘x’ coordinate direction. A second pair of tension actuators 804 are also arranged on another pair of opposite sides of the first substrate, on either side of the first surface of the substrate 104 on which the domes 104 are located. The second actuators can be pulled to exert an input tension on the first substrate in the y coordinate direction. While two pairs of tension actuators 802, 804 are shown in figure 8, alternatively, either a single pair of tension actuators can be used or a single tension actuator can be used with the opposite side being fixed in position.

With reference to figure 8b, when a tension is input along the first and second tension actuators, the first substrate will deform. The substrate 102 and plurality of domes 104 thereon will stretch in each direction corresponding to the tension applied along that respective direction. Analogously to the pressure sensor arrangements, the changing contact surface area between the conductive polymer layer 106 on the plurality of domes 104 and either an electrode or a second conductive polymer layer 406 changes. Hence the resistance between first and second electrodes changes according to the principles described above with reference to figures 1 to 6. The deformation of the plurality of domes 104 in general depends on both the tension received in the x and y directions, with the resulting resistance change being a result of both tensions. Hence the measurement of the change in resistance across the two electrodes will provide a measurement of the applied tension.

With reference to figure 9, a force sensor 100 described above according to any of the described arrangements or variations thereof can be used to measure an input force according to the following method 900. An input force is received 902 at an input mechanism. The input force may be a pressure force and the input mechanism may be an input surface of a substrate 102, 104 or electrode 108, 110, of the force sensor 100. Alternatively, the input force may be a tensional force and the input mechanism may be tension actuators 802, 804 attached to sides of a substrate 102, 104 or may be a stretchable material which the substrate is positioned in or on.

One or more resilient protrusions, e.g. a plurality of domes, of the substrate are deformed 904 by the input force, thereby changing the contact surface area of the conductive layer coating the resilient protrusions and either an electrode or a second conductive layer. The deforming 904 may be a compression or a stretch of one or more resilient protrusions. As described above with reference to figures 1 to 8, this changes the electrical impedance, e.g. resistance, between a first pair of electrodes of the force sensor 100. The change in electrical impedance between the electrodes caused by the deforming 904 of the resilient protrusions is detected 906. This can be done using a measuring circuit connected between the two electrodes, such as by measuring a current flow between the two electrodes at a known voltage or by any other method for determining electrical impedance across two points.

The input force can be determined 908 based on the change in electrical impedance detected 906 across the electrodes. In general, any change in electrical impedance will determine that an input force has been received at the input mechanism. Hence a change in resistance will indicate that an input pressure or tension is present. For some

implementations, this level of determination is all that is required. In other implementations, the determination may also need to provide a relative or absolute value for the input force. A relative input force value can be determined by detecting a change in electrical impedance, wherein the electrical impedance decreasing indicates that a larger input force has been received. This value can be tracked and normalised to provide a variable value for relative input force across a period of time. To provide an absolute value for input force, calibration data such as a‘resistance across electrodes’ versus‘input force’ graph can be used which relates the measurable quantity of electrical impedance to a value for input force. The calibration data may be provided in advance for a particular force sensor, or calibration with known input forces can be carried out by measuring the electrical impedance for each known input force. An example of a calibration table is shown in figure 10, demonstrating an inverse relationship between input pressure and resistance. On the y axis is resistance in Ohms, on a logarithmic scale. On the x axis is pressure in kPa, on a linear scale. The data points indicate the measured resistance for an input force of each pressure.

With reference to figure 11 , a force sensing system 1100 comprises a force sensor 100 according to any of the arrangements as described above with reference to figures 1 to 8, or variants thereof. The force sensing system 1100 also comprises a power supply 1110 arranged to supply a voltage across the electrodes 108, 110 of the first pair of electrodes. This may be a direct current or alternating current power supply. The system also includes an electrical impedance detector 1120 arranged to measure electrical impedance between the electrodes 108, 110. The electrical impedance detector 1120 may be a combination of circuitry and components which produces a signal indicating the electrical impedance. The signal may directly correspond to the electrical impedance, e.g. resistance in direct current implementations, or may require further calculations and/or measurements in order to determine the electrical impedance. For example, the electrical impedance detector could include an ammeter to measure current and a component which calculates the electrical impedance from the current, such as by using the voltage supplied by the power supply 1110.

The system also comprises a processor 1130 configured to detect a change in the measured electrical impedance from the electrical impedance detector and determining, based on the change in the measured electrical impedance, that an input force has been received at the input mechanism. The processor 1130 determines if an input force has been received using the signal from the electrical impedance detector 1120. The processor 1130 may run from a computer-readable medium stored on memory in the force sensing system or externally. The processor 1130 may also perform calculations for the electrical impedance detector 1120 in order to determine the electrical impedance. The processor 1130 may also control the operation of the power supply 1110, electrical impedance detector 1120 and any controllable features of the force sensor 100. The processor 1130 may output the result of the input force determination to another entity or store the result to a memory. The processor 1130 may perform one or more actions based on the determined input force.

The force sensing system 1100 may be included in a touch-sensitive user interface 1150, such as a mobile electronic device, smartphone or electronic instrument. The touch-sensitive user interface includes a display 1140 for providing visual feedback to a user of the touch- sensitive user interface. Unlike other touchscreen sensors, the force sensor 100 as described above can be positioned over the top of the display without compromising the display picture. This is because a force sensor 100 according to any of the arrangements described herein can be made from transparent materials which do not substantially block visible light. This produces a touchscreen with both touch and pressure sensing, improving the range of inputs the user can provide to control the device. The processor 1130 may, in response to detecting an input force, control a user device to perform an action such as presenting images on the display 1140 or provide haptic feedback to the user. The processor may perform operations in response to the received input via the user interface into order to carry out a command from the user. The results of the operations may be presented on the display 1140.

With reference to figure 12, a force sensor 100 described above according to any of the described arrangements or variations thereof can be fabricated using the following method 1200. The method comprises applying 1220 a first conductive layer into a patterned mould. The conductive layer may be PEDOT:PSS and may coat the inside of the patterned mould or be deposited according to a particular pattern in the patterned mould. For example, the conductive layer may be deposited into the mould in lines or a grid pattern. The patterned mould may comprise a number of recesses, which will be the shape and size of the plurality of resilient protrusions of the resulting force sensor 100 and the pattern of the patterned mould determines the positioning of the resilient protrusions. For example, the recesses may be domed.

A polymer is applied 1240 onto the first conductive layer in the patterned mould. The polymer may be PET and/or EVA, or any other polymer which can be used to create a substrate having resilient protrusions. The polymer may fill the mould such that each recess in the patterned mould is filled with the polymer and the polymer covers the space between recesses. The polymer may be left to solidify or can be treated in order for the polymer to cure, according to whatever process is required for the polymer to take the form of the patterned mould. During this process, the polymer binds to the first conductive layer applied to the inside of the patterned mould. The polymer and first conductive layer thereon are removed 1260 from the patterned mould. The polymer is a substrate 102, 402 as described above with reference to figures 1 to 8, having a plurality of resilient protrusions on a first surface and the first conductive layer disposed on the first surface at least partially covering the resilient protrusions. Instead of applying the first conductive layer into a patterned mould, the method may include applying the first conductive layer onto a patterned polymer after it has been removed from the patterned mould.

A pair of electrodes 108, 110, 408, 410 are arranged 1280 such that deformation of the resilient protrusions on the substrate 102 change an electrical impedance between the electrodes of the pair of electrodes. This may be done by depositing electrodes directly onto the substrate 102 or by bringing together the substrate and one or more electrodes on an electrode support 202 and securing the two with, for example, a housing or attachment means. Arranging the pair of electrodes may include coupling an electrical connection to the first conductive layer 106 of the substrate 102. Arranging the electrodes can be performed in such a way to produce any of the force sensor electrode arrangements described above with reference to figures 1 to 8 or variants thereof. In particular, the electrodes may be arranged on the same side of the substrate 102 according to the arrangement described with reference to figures 2 or 3. Alternatively, in arrangements as described with reference to figure 4, the electrodes may be arranged in electrical connection to conductive layers 106, 406 of first and second substrates 102, 402. In any of these examples, the electrodes are positioned so that deformations due to an input force result in a change to the electrical impedance between the two electrodes and any alternative electrode arrangement which achieves this is a suitable alternative.

An exemplary method 1300 of fabricating a force sensor includes a number of stages a) to h) described below with reference to figure 13. The method begins at stage a) by providing a support layer 1310 of titanium on a silicon wafer and depositing a gold layer 1320 onto the support layer 1310. Any suitable material for support layer may be used, since this functions just as a mechanical base on which the subsequent stages take place. Likewise, materials other than gold may be suitable for depositing onto the support layer 1310. In stage b) a plurality of polystyrene nanospheres 1330 is transferred onto the gold layer 1320. Alternative particles can any be made from materials other than polystyrene and have other sized dimensions, e.g. on the order of 1 micron. Further, shapes other than spheres can be used in order to produce a force sensor 100 having resilient protrusions which are not domed but instead cuboid or cylindrical. The nanospheres 1330 can be deposited using nanosphere lithography. The plurality of nanospheres 1330 are etched with oxygen plasma in stage c) to reduce the nanospheres 1330 to a size to produce the desired shape and size of the resilient protrusions of a resulting force sensor 100. As an example, the plasma etch may reduce 1 micron nanospheres to a size of approximately 400 nm. Stage c) can also be used to control the shape of the resulting resilient protrusions, e.g. by directional etching.

In stage d) a further gold layer 1340 is sputtered onto the etched nanosphere-coated surface. The sputtered gold layer 1340 partially surrounds the etched nanospheres. Optionally, the sputtered gold layer 1340 has a thickness of the radius of the etched nanospheres. In stage e) the etched nanospheres 1330 are removed by mechanical exfoliation, leaving a recess pattern 1350 of a plurality of recesses 1355 in the gold layer 1340. The recesses 1355 have the same shapes and sizes as the lower halves of the etched nanospheres 1330. For example, the further sputtered gold layer 1340 may be approximately 200nm thick for 400nm sized nanospheres.

In stage f) a conductive polymer layer 1360, e.g. of PEDOT:PSS, is electrodeposited into the mould. This may completely fill the recesses 1355 left by the removed nanospheres, or may partially fill the recesses. In stage g) a substrate polymer 1370 is applied on top of the conductive polymer layer 1360 and cured or left to solidify into a substrate 102 as described for a force sensor 100 according to the present disclosure. The substrate polymer 1370 may comprise two layers, e.g. approximately 150 micron thick layer of EVA followed by a 150 micron thick layer of PET which may not be flexible or elastomeric.

In stage h), the substrate polymer layer 1370 and conductive polymer layer 1360 are removed from the patterned mould, i.e. the recess pattern 1355. This may be done using a hot plate to heat the layers before lifting off with laminate plastic. The resulting sensor structure 1390 is a substrate 102, i.e. the polymer layer 1370, having a first surface pattern 1380 comprising a plurality of resilient protrusions 1385. The resilient protrusions 1385 have a shape and design determined by the recesses 1355 in the recess pattern 1350, which in turn is produced by the etched nanospheres 1330. In alternative methods, the patterned mould produced by the end of stage e) is directly provided, e.g. having been used to previously make another sensor structure 1390 comprising a substrate 102 with resilient protrusions 1385.

The sensor structure 1390 produced by stage h) can then be made into a force sensor 100 as described above with reference to figures 1 to 8 by arranging 1280 a pair of electrodes on the sensor structure 1390 as using the method described above with reference to figure 12.

Below are some examples of the material properties of a force sensor 100 as described herein, which can be used to perform the described methods of measuring and can be produced by the above method of fabrication.

Exemplary sizes for the plurality of resilient protrusions 104 on the first substrate 102 are between 200nm to 3000nm in width or diameter and between 100nm to 1500nm in height or radius. The spacing between resilient protrusions 104 may be approximately the diameter of each resilient protrusion, e.g. a spacing of 1 micron for resilient protrusions having a radius or height of 500 nm.

Exemplary hardness values of the substrate polymers used for resilient components are Shore Hardness values A:40 to 85, having good elastomeric properties. In implementations requiring substantially transparent components, the transparency across visible wavelengths may be 60% transmission or higher. Any of the arrangements described herein may be produced from substantially transparent materials. The properties and functions of each component in any of the described arrangements will be understood by the skilled person to be applicable to each of the other described arrangements unless otherwise stated or impractical. For example, the material properties, shape or size of any substrate 102, 402; conductive layer 106, 406; and plurality of resilient protrusions 104, 404, 1385 as described for arrangements with reference to any particular figure is also applicable to the arrangements described with any of the other figures.

The claims are not limited to particular embodiments set out in the description, and the skilled person will appreciate that multiple modifications or additions may be made to the specific embodiments described herein, without departing from the scope of the claims. Those skilled in the art will recognize that a wide variety of modifications, alterations, and combinations can be made with respect to the above described embodiments without departing from the scope of the disclosed concepts, and that such modifications, alterations, and combinations are to be viewed as being within the ambit of the disclosed concepts.

Claims

1. A force sensor comprising:

a substrate having a plurality of resilient protrusions on a first surface of the substrate;

a first conductive layer disposed on the first surface at least partially covering the plurality of resilient protrusions;

a first pair of electrodes;

an input mechanism arranged to receive an input force and communicate the input force to deform one or more resilient protrusions of the plurality of resilient protrusions to change an electrical impedance between the electrodes of the first pair of electrodes.

2. The force sensor of claim 1 , wherein one or more of the substrate, the first pair of electrodes and first conductive layer is substantially transparent.

3. The force sensor of claim 1 or 2, wherein the resilient protrusions are positioned in a periodic array.

4. The force sensor of any preceding claim, wherein the resilient protrusions are substantially domed.

5. The force sensor of any preceding claims, wherein the substrate is resilient.

6. The force sensor of any preceding claim, wherein

one or both electrodes of the first pair of electrodes oppose at least a respective portion of the first conductive layer, and

the change in electrical impedance between the electrodes of the first pair of electrodes includes change in electrical impedance between the one or both of the electrodes of the first pair of electrodes and the first conductive layer, wherein the change in electrical impedance is a result of deformation of the one or more resilient protrusions against the one or both of the electrodes of the first pair of electrodes.

7. The force sensor of any preceding claim, wherein the electrodes of the first pair of electrodes are arranged on opposite sides of the first conductive layer.

8. The force sensor of any of claims 1 to 6, wherein the electrodes of the first pair of electrodes are arranged on the same side of the first conductive layer.

9. The force sensor of any preceding claim, wherein the force sensor further

comprises one or more further pairs of electrodes, wherein the input mechanism communicates the input force received at a respective location to change an electrical impedance between the electrodes of a respective pair of electrodes.

10. The force sensor of any preceding claim, herein the force sensor further

comprises:

a second substrate having a second plurality of resilient protrusions on a second surface of the second substrate; and

a second conductive layer disposed on the second surface at least partially covering the second plurality of resilient protrusions.

11. The force sensor of claim 10, wherein the first plurality of resilient protrusions differs from the second plurality of resilient protrusions in one or more of: size, shape, height, width, material, hardness, elasticity, conductivity, and spacing between resilient protrusions.

12. The force sensor of claim 10 or 11 , wherein the first plurality of resilient

protrusions is arranged to deform in response to a lower input force than the input force in response to which the second plurality of resilient protrusions are arranged to deform.

13. The force sensor of any preceding claim, herein the force sensor is a pressure sensor and the input mechanism is an input surface and the input force is an input pressure.

14. The force sensor of any of claims 1 to 12, wherein the force sensor is a tension sensor, the input force is an input tension, and the input mechanism is arranged to communicate the tension to stretch the resilient protrusions.

15. A method of measuring an input force using a force sensor according to any preceding claim, wherein the method comprises:

receiving the input force at the input mechanism;

deforming, by the input force, one or more of the resilient protrusions; detecting a change in the electrical impedance between the electrodes of the first pair of electrodes caused by the deforming of the one or more resilient protrusions;

determining, based on the change in the detected electrical impedance, that an input force has been received at the input mechanism.

16. The method of claim 15, wherein the method further comprises:

determining a magnitude of the input force based on the change in the detected electrical impedance.

17. A method of fabricating a force sensor, comprising:

applying a first conductive layer into a patterned mould;

applying a polymer onto the first conductive layer in the patterned mould;

removing the polymer and first conductive layer from the patterned mould to produce a substrate having a plurality of resilient protrusions on a first surface of the substrate and the first conductive layer disposed on the first surface at least partially covering the plurality of resilient protrusions; and

arranging a pair of electrodes such that deformations of one or more of the resilient protrusions of the plurality of resilient protrusions changes an electrical impedance between the electrodes of the first pair of electrodes.

18. The method of claim 17, wherein method further comprises:

providing the patterned mould by:

providing a metal-coated silicon wafer; transferring a plurality of monodisperse spheres onto the metal- coated silicon wafer;

etching the plurality of monodisperse spheres according to a mould pattern;

depositing a metal layer onto the metal-coated silicon wafer; and after depositing the metal layer, removing the plurality of etched monodisperse spheres to produce the patterned mould.

19. A force sensing system comprising:

a force sensor according to any of claims 1 to 14;

a power supply arranged to supply a voltage across the electrodes of the first pair of electrodes;

an electrical impedance detector arranged to measure an electrical impedance between electrodes of the first pair of electrodes;

a processor configured to detect a change in measured electrical impedance from the electrical impedance detector and determining, based on the change in the measured electrical impedance, that an input force has been received at the input mechanism.

20. The force sensing system of claim 19, wherein the processor is further

configured to determine a magnitude of the input force based on the change in the measured electrical impedance.

21. A touch-sensitive user interface comprising:

the force sensing system according to claim 19 or 20; and a display for providing visual feedback to a user of the touch-sensitive user interface.

22. A touch-sensitive user interface according to claim 21 , wherein the display and the input mechanism are located on opposite sides of the substrate and first conductive layer.