Compliant three-axis force sensor and its manufacturing method

Abstract

A compliant three-axis force sensor suitable for incorporation into soft robots or electronic skin is disclosed. The sensor comprises: a first electrode layer including column electrodes in a first orientation; a second electrode layer including column electrodes in a second orientation orthogonal to the first orientation; a force-dependent active layer disposed between the first and second electrode layers and configured to change at least one property, such as quantum tunneling, conductivity, resistivity, or charge, when subjected to a force; at least one three-dimensional bump disposed to transmit an externally applied force through the active layer; and at least one spacer disposed to maintain a spacing between two or more layers within the sensor until an external force is applied.

Description

[Technical field]

[0001] The present invention relates to a compliant three-axis force sensor and a method for manufacturing such a sensor. [Background technology]

[0002] For robots and machines to be able to interact with the world around them in the same way as humans, they also need a sense of touch as sensitive as humans. One of the most difficult areas to mimic in a robot is the palm, as it has one of the highest densities of nerve receptors in the human body. Furthermore, the skin has multiple mechanoreceptors to sense different forces / pressures as well as vibration frequencies. Static sensing of pressure is paramount for a robot's ability to handle delicate objects. For this purpose, multiple tactile sensors have been built to provide pressure data.

[0003] The majority of sensors currently on the market consist of individual force sensors or other sensors that only provide a scalar value of pressure that can be thought of as a relatively large surface area. Other sensors that provide more advanced tactile information include sensors that are robust but require replacing the entire robotic finger.

[0004] However, force is a vector with two components, its magnitude and direction. Directional information / data has been of great interest for the improvement of robotics for many years. Sensors that can determine both magnitude and direction are usually called 3-axis force sensors. These force-torque (F / T) sensors are primarily used as part of the end-effector arms of robots (see Reference 1 at the end of this specification) and more recently to measure the directional component of force on robot fingertips (see Reference 2 at the end of this specification). Historically there have been many attempts at such sensors, and while recent attempts have achieved relatively high density, they have been made using solid materials and / or micromachining techniques. Not only are the force readings sensitive to bending around certain surfaces, many of them are not bendable and can only be incorporated into hard robots. Recently, the need for compliant tactile systems has been recognized for both human and robot monitoring. Many of these compliant / conformable pressure sensing systems can be classified as a type of electronic skin (e-skin).

[0005] Some recent e-skins and data gloves have the capability to provide multi-point or high-density tactile information (see References 3 and 4 at the end of this specification), but most cannot provide directional information that is important for measuring many phenomena, which may enable a robot to grasp an object and detect its mass in a gravitational environment such as the Earth, but also, for example, to measure the ground reaction forces of an insole for other physical phenomena such as gait analysis.

[0006] Recently, there have been many attempts to make fully compliant three-axis force sensors. These have used a variety of tricky methods, including various designs of compliant strain gauges using liquid metals such as eGaIn (eutectic gallium indium) (see Reference 5 at the end of this specification) and several designs similar to rosette strain gauges, which are difficult to fabricate and can cause toxic leakage. Others include bulky pneumatic chamber-based sensors and self-aligning carbon nanotube (CNT) arrays.

[0007] If it is desired to integrate any of these sensing systems at high density, a matrix array is necessary to reduce wiring considerations and therefore the space they occupy on the e-skin. This has been done largely using capacitive sensing with metals, conductive liquids, or otherwise (see References 6, 7, and 8 at the end of this specification), but this type of sensing is sensitive to the substrate on which it is used (e.g., human skin, metallic robots) due to capacitive coupling that can occur between the surface and the sensor. Furthermore, capacitive sensing requires complex readout electronics and is susceptible to background electromagnetic (EM) noise, making it impractical for many applications. Other methods have also been used, but all have certain drawbacks: piezoelectric sensing (see Reference 9 at the end of this specification) has potential for actuation and detection (see Reference 10 at the end of this specification), but is susceptible to significant background electromagnetic compatibility (EMC) noise and cannot perform static sensing, and magnetic sensing (including Hall effect and inductive methods) (see References 11-14 at the end of this specification) is also subject to background EMC and ambient magnetic fields, some of which may be generated by actuators on the robot itself.

[0008] Furthermore, it is of utmost importance that the e-skin be as thin as possible so as not to interfere with the devices it will be retrofitted to, while at the same time having a compact electronic readout system, which is lacking in most solutions / systems. A very promising, but somewhat expensive, fabrication process has recently been developed in which self-aligning carbon nanotube arrays can be used to read out the 3-axis information of integrated arrays grown on the sidewall during the fabrication process with good cyclic repeatability (see Reference 15 at the end of this specification).

[0009] Quantum tunneling materials and piezoresistive composites have been used in the past, but none of the proposed systems were highly integrable or wrappable. In many cases, flexible wiring is not flexible or compliant enough, either because the material is too stiff or only compliant in one axis, or because the layers are not thin enough. The architecture of such devices typically involves the placement of a force-torque-converting “bump” that is placed directly above four orthogonally placed pressure sensors or pressure-sensing pixels (sensels). When a post experiences a force at its tip from a particular direction, the force is converted into a downward force and a lateral torque. As a result, the four pressure sensors experience different forces / pressures. The force can then be resolved into 3D Cartesian components (Fx, Fy, and Fz) by measuring the amount of resistance change of each of these sensors. A variety of electrode configurations can be used to read the signal from the piezoresistive material (see Reference 16 at the end of this specification).

[0010] To date, the closest approaches to fabricating piezoresistive three-axis sensor arrays are interlocking structures with sidewalls embedded in polydimethylsiloxane (PDMS) and micropyramid array structures (see Reference 17 at the end of this specification). The former have the advantage of being able to be deployed in relatively dense 3x3 arrays, but only function at very low loads of 0 to 225 Pa. The latter have the advantage of having a wide dynamic range, from 128 Pa to 44 kPa. However, they are difficult to deploy as a matrix, which requires a large amount of wiring. Also, the entire structure is several millimeters in size, so it cannot be wrapped around a surface, and its complexity makes mass production very difficult.

[0011] Quantum Tunneling Composites (QTC®) were discovered in 1996 by David Lussey. These materials are essentially composites consisting of metal particles and a non-conductive elastomeric binder or matrix. These materials have a very high resistance to electrical current and are essentially insulators when no pressure is applied. When pressure is applied, their resistance drops exponentially, which can be measured using the correct electrical circuit. The reason for this is that the metal particles are dispersed in the material at a certain concentration, but are not in physical contact with each other. When pressure is applied, the metal particles move closer to each other, they cannot touch each other, but they allow electrons to jump from one particle to another, via a phenomenon called quantum tunneling. The higher the pressure, the closer the particles are to each other, increasing this tunneling effect and decreasing the resistance of the material. For this to work well, there needs to be a good dispersion of the particles in the binder, and that they are small in size, since tunneling is only effective on a small scale. For this, the particles are usually micrometer or nanometer in size and composed of a conductive material, usually a metal such as silver, gold or magnetite. They are also often nanopatterned; for example, "spiky" nanoparticles show better tunneling behavior than round particles because the tiny spike tips enhance the tunneling current even when the particles are at a greater distance from each other, thereby improving sensitivity and overall performance.

[0012] QTC® pills provided by Peratech have been used to create 3-axis sensors, but these devices suffer from other issues such as miniaturization and sensor hysteresis. QTC® pills lack the ability to be integrated during mass production processes, are bulky in both thickness and minimum diameter, making them cumbersome to work with, and are not particularly sensitive.

[0013] In summary, commercially available flexible force sensors on the market only provide the scalar value of the force, but not its direction or shear force. They cannot withstand significant bending, nor can they change their value during bending. Furthermore, array technologies, including 3-axis technologies, are required to obtain the data required to enable multiple advanced applications. This is particularly important, for example, in human and robot gait analysis, where ground reaction forces need to be measured, in robots that manipulate objects in complex ways, in robots that handle delicate objects without damaging them, in robots that monitor physiological forces such as muscle activity and blood flow in clothed humans, in robots that detect flow phenomena such as air and water on tidal and wind turbines, and also to allow surgeons to "palpate" tissues, for example in surgical robots.

[0014] Compliant 3-axis sensors can be created (in the laboratory) using multiple transduction mechanisms, e.g., piezoresistive (e.g., strain gauges or nanocomposites), capacitive, piezoelectric, inductive, optical, or magnetic. Piezoresistive and capacitive sensors dominate due to their superior performance. However, capacitive sensors can be substrate sensitive due to capacitive coupling with the surface of the substrate on which they are placed, and require much more complicated readout electronics, which is an obstacle due to space requirements and implementation complexity. Drawbacks of other sensors include that piezoelectric sensors cannot sense static contact forces because the charge induced in the material dissipates very quickly, most of these sensors are temperature sensitive and generally not robust, lack a sensing range for dynamic external forces, are sensitive to EMC noise (especially magnetic and piezoelectric sensors), and some piezoelectric sensors are photosensitive. Furthermore, the best-known processes for creating these sensors are inherently low-yield, inconsistent, and not scalable. Measurements using such sensors are often not repeatable, making the sensors unreliable.

[0015] Of all the available sensors, none are robust enough to function in a desired manner in a real-world setting. The industry urgently needs robust sensors with durable materials that can withstand thousands to millions of cycles, are insensitive to temperature and harsh environmental conditions, and perform three-axis force sensing. Sensors also need to be very thin so that they can be added without compromising the functionality of existing devices. The operation of the sensors also needs to be insensitive to bending so that they can function on soft surfaces such as soft robots and human skin.

[0016] It is therefore an object of the present invention to provide a compliant three-axis force sensor and a method of manufacturing such a sensor that addresses at least some of the above problems discussed with the current state of the art. Summary of the Invention

[0017] According to a first aspect of the present invention there is provided a compliant three-axis force sensor comprising: a first electrode layer having an array of column electrodes in a first orientation; a second electrode layer having an array of column electrodes in a second orientation orthogonal to the first orientation; a force-dependent active layer configured to change at least one property when subjected to a force and disposed between the first electrode layer and the second electrode layer; at least one three-dimensional bump arranged to transmit an externally applied force through the active layer; At least one spacer is provided that is positioned to maintain a spacing between two or more layers in the sensor until an external force is applied.

[0018] Thus, embodiments of the present invention constitute a compliant three-axis force sensor that can be made of thin layers of material so that it is fully compliant and flexible, but includes one or more spacers to maintain a specific interlayer spacing, so that bending of the sensor itself does not cause erroneous force readings, and therefore the sensor is robust, reliable, and can be used repeatedly. Thus, the sensor can withstand thousands to millions of cycles without failure, and thus can operate effectively in industrial environments. The consistent output also eliminates the significant time that may be required to individually calibrate each sensing element. This is particularly useful in some embodiments that use four sensels (sensing elements) per tactel (tactile element), i.e., bump. Furthermore, the sensor is less susceptible to background noise and can be easily configured (i.e., by increasing or decreasing the thickness of the active layer) for different force sensing ranges depending on the application.

[0019] Advantageously, the sensor can be configured as a thin, stretchable, conformable, nanocomposite-based tactile device capable of measuring forces in three different axes (x, y, and z). Embodiments of the present invention are compliant enough to be wrapped around very small bending radii or soft robots, yet still allowing accurate measurement of force magnitude and direction with minimal impact on signal integrity or response. Thus, the performance of the sensor is insensitive to bending. Flexible connections and electrical wiring are integrated into the sensor through layers and electrode structures, with a "bump" layer on top that converts force into torque. Furthermore, the sensor is suitable for mass production using any of the existing straightforward mass production processes, such as screen printing or roll-to-roll printing with customizable materials, while maintaining high yields. Thus, it can be deployed in large-scale arrays such as those required for e-skin applications. Currently, no general-purpose e-skin of this type exists. The sensor is highly integratable into a variety of devices, products, or garments, and can be retrofitted to hard, soft, and stretchable surfaces. Furthermore, environmentally friendly materials can be used.

[0020] Each layer can be printed (e.g., by screen printing or inkjet printing) and can be made very small (at least to the ideal 1 mm x 1 mm 3-axis pixel density desired for a robotic end effector). In some embodiments, the first and / or second electrode layers can be formed from a conductive ink deposited on top of the layer below.

[0021] All of the above advantages may make embodiments of the present invention the provision of commercially viable sensors suitable for "real world" industrial applications outside of controlled laboratory environments or academia.

[0022] The active layer may include at least one of a quantum tunneling material, a piezoresistive material, or a piezoelectric material. Thus, at least one property that may change when the active layer is subjected to a force is quantum tunneling, conductivity, resistivity, or the amount of charge. Thus, embodiments of the present invention may use composite materials that utilize quantum tunneling conduction mechanisms or piezoresistive conduction mechanisms to convert an applied force into a sense signal. The active layer is advantageously anisotropic, so that when deposited in a thin layer, it responds more selectively to normal forces due to its material properties.

[0023] At least one three-dimensional bump may have a footprint that extends at least partially over at least two column electrodes of the first electrode layer and over at least two column electrodes of the second electrode layer such that a force applied to the bump may be transmitted through the active layer in at least four distinct regions. In this manner, each tactel (tactile element) is connected to four sensels (sensing elements) or pixels, and the strength of the force at each sensel may be used to determine both the magnitude and direction of the applied force (i.e., resulting in a combination of shear and compression forces).

[0024] In other embodiments, at least one three-dimensional bump has a footprint that is contained within the space between two column electrodes of the first electrode layer and two column electrodes of the second electrode layer, in which case one or more spacers can be used to redistribute the force applied to the bump to each of the four sensels (sensing elements).

[0025] The active layer includes a first active layer and a second active layer having a gap therebetween.

[0026] The sensor can include one or more of a base layer below the first electrode layer, a top layer above the second electrode layer, a first carbon layer between the first electrode layer and the active layer, a second carbon layer between the second electrode layer and the active layer, and a cover layer on the at least one three-dimensional bump.

[0027] The first and / or second carbon layers may extend continuously across a plurality of column electrodes of the first and / or second electrode layers.

[0028] At least one spacer may be provided within the active layer, between the base layer and the top layer, or on or through the first or second electrode layer.

[0029] The at least one spacer may be in the shape of a dot, a column, a post, a cylinder, a tube, a pyramid, or a mesh.

[0030] The at least one spacer may have a solid or hollow cross-section in the shape of a circle, square, rectangle, diamond, pentagon, hexagon, or honeycomb lattice.

[0031] The sensor may include an array of the spacers. The spacers in the array may have a number of different dimensions.

[0032] The active layer may form a continuous layer across column electrodes of the first and / or second electrode layers.

[0033] The active layer may form a discontinuous layer across column electrodes of the first and / or second electrode layers.

[0034] The sensor may comprise an insulator between adjacent electrodes of the first and / or second electrode layers.

[0035] The at least one spacer may take the form of a binder provided between the active layer and the first and / or second electrode layers, the binder being provided in the gaps between adjacent column electrodes and extending between the active layer and respective edges of the adjacent column electrodes.

[0036] The sensor may include an adhesive layer disposed between the one or more adjacent layers.

[0037] The at least one three-dimensional bump may include a polymer having a different elasticity than the remainder of the sensor.

[0038] The at least one three-dimensional bump may be a mesa, dome, hemisphere, hemispherical section, cone, conical section, cube, cylinder, hemicylinder, pyramid, pyramidal section, tetrahedron, tetrahedral section, hexahedron, triangular array, polyhedron, or other shape.

[0039] The at least one three-dimensional bump may have a height at least twice that of the rest of the sensor. In some embodiments, the bump may be significantly taller than the rest of the sensor (e.g., up to 3 mm in height) to convert the shear force into a torque high enough to compress the active layer material underneath at different pressures. In some embodiments, the active layer has a width of 1 mm between the electrodes, and each tactel has a sensing area of ​​3 mm x 3 mm = 9 mm. 2 A tactel may consist of 4 pixels in a matrix array, each spaced 2 mm apart, such that: tactel = 1; tactel = 1; tactel = 2 ...1; tactel = 2; tactel = 1; tactel = 1; tactel = 2; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel = 1; tactel

[0040] The sensor may include an array of the three-dimensional bumps.

[0041] According to a second aspect of the present invention there is provided a method of manufacturing a sensor according to the first aspect above, the method comprising the steps of: providing a first electrode layer having an array of column electrodes in a first orientation; providing a second electrode layer having an array of column electrodes in a second orientation orthogonal to the first orientation; providing a force dependent active layer configured to change at least one property when subjected to a force, the force dependent active layer being disposed between the first electrode layer and the second electrode layer; providing at least one three-dimensional bump arranged to transmit an externally applied force through said active layer; providing at least one spacer arranged to maintain a spacing between two or more layers in the sensor until an external force is applied; Equipped with.

[0042] Thus, embodiments of this aspect of the invention provide a method of sensor manufacturing that can be easily mass-produced using a variety of existing technologies.

[0043] Each layer, bump, or spacer can be applied by one of screen printing, roll-to-roll printing, inkjet printing, 3D printing, electrospinning, deposition, droplet dispensing, casting, coating, molding, spinning, or weaving.

[0044] The method may include forming at least a first electrode layer and a second electrode layer on a single substrate, separating the substrate into two portions, and stacking the two portions.

[0045] The method may comprise the step of forming said at least one spacer on at least one portion prior to said separating and / or stacking steps.

[0046] Non-limiting embodiments of the present invention will now be described, by way of example only, with reference to the following drawings: [Brief description of the drawings]

[0047] [Figure 1A] FIG. 1 illustrates a top perspective view of a compliant three-axis force sensor including a dome-shaped bump according to a first embodiment of the present invention. [Figure 1B] FIG. 1B shows a side view of the sensor of FIG. 1A. [Figure 1C] FIG. 1B shows a plan view of the sensor of FIG. 1A. [Figure 2A]FIG. 1 shows a top perspective view of a compliant three-axis force sensor including cube-shaped bumps according to a second embodiment of the present invention. [Figure 2B] FIG. 2B shows a side view of the sensor of FIG. 2A. [Figure 2C] FIG. 2B shows a plan view of the sensor of FIG. 2A. [Figure 3A] FIG. 13 shows a top perspective view of a compliant three-axis force sensor including cylindrical bumps according to a third embodiment of the present invention. [Figure 3B] FIG. 3B shows a side view of the sensor of FIG. 3A. [Figure 3C] FIG. 3B shows a plan view of the sensor of FIG. 3A. [Figure 4] 4 is a flow chart of a method for manufacturing a sensor according to an embodiment of the present invention. [Diagram 5] 5A-5I show steps of a method for manufacturing a sensor according to another embodiment of the invention. [Figure 6] 6A-6I show steps of a method for manufacturing a sensor according to a further embodiment of the invention. [Figure 7] 7A-7G show steps of a method for manufacturing a sensor according to yet another embodiment of the invention. [Figure 8] 8A and 8B show plan views of a portion of a sensor including different cylindrical post arrangements according to embodiments of the present invention. [Figure 9] 9A and 9B show plan views of a portion of a sensor including different diamond-shaped post arrangements according to embodiments of the present invention. [Figure 10] 10A and 10B show plan views of a portion of a sensor including different square post arrangements according to an embodiment of the present invention. [Figure 11] 11A, 11B, and 11C show plan views of a portion of a sensor including different hexagonal post arrangements according to an embodiment of the present invention. [Figure 12]FIG. 12A shows a top perspective view of a single tactile element (tactel) of a sensor according to an embodiment of the present invention, FIG. 12B shows a side perspective view of the tactile element of FIG. 12A before and after an oblique force is applied along with a corresponding force versus time graph, and FIG. 12C shows a side view of the tactile element of FIG. 12B when an oblique force is applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0048] 1A, B and C, a compliant three-axis force sensor 100 including four three-dimensional dome-shaped bumps 102 according to a first embodiment of the present invention is shown. The sensor 100 comprises a base layer 104 in the form of a bottom membrane encapsulation layer, a first (bottom) electrode layer 106 having an array of column electrodes 106a in a first orientation, a second (top) electrode layer 108 having an array of column electrodes 108a in a second orientation orthogonal to the first orientation, a force-dependent active layer 110 in the form of a quantum tunneling material layer provided between the first electrode layer 106 and the second electrode layer 104 and configured to change at least one property when subjected to a force, and a top layer 112 in the form of a top membrane encapsulation layer. The dome-shaped bumps 102 are provided on the top layer and are arranged to transmit an externally applied force through the active layer 110. Although not shown in Figures 1A, B and C, at least one spacer is provided that is positioned to maintain spacing between two or more layers in sensor 100 until an external force is applied. These spacers are described in more detail below in connection with Figures 4 through 11C.

[0049] 1A and 1B, an optional insulating array 114 is provided between the column electrodes 106a of the first electrode layer 106. A similar insulating array 116 is provided between the column electrodes 108a of the second electrode layer 108. Each bump 102 has a circular bottom 102a and curved sidewalls 102b that extend to a flat top 102c.

[0050] In this embodiment, each of the base layer 104, the first electrode layer 106, the active layer 110, the second electrode layer 108, and the top layer 112 are 0.10 mm thick, forming a sensor body that is 0.50 mm thick. Additionally, each column electrode 106a, 108a is 1 mm wide, and each bump 102 is 1 mm high with a 3 mm maximum diameter footprint. There is a 1 mm gap between each adjacent bump 102. In other embodiments, other dimensions can be used to suit a particular application. However, it is generally envisioned that the sensor body will be formed in a relatively thin layer to allow the sensor to be compliant and to be given curved or other shaped surfaces. As shown here, in some embodiments, the bumps 102 can be sufficiently taller than the sensor body to convert shear forces into sufficiently high torques to press the underlying active layer 110 with different pressures.

[0051] As shown in FIG. 1C, each three-dimensional dome-shaped bump 102 has a footprint that extends at least partially over at least two column electrodes of the first electrode layer 106 and at least partially over at least two column electrodes of the second electrode layer 108, such that a force applied to the bump 102 may be transmitted through the active layer 110 in at least four distinct regions. In this manner, each bump 102 may be considered a tactile element (tactel) connected to four sensing elements (sensels) or pixels, and the force strength at each sensor may be used to determine the location, magnitude and direction of the applied force (i.e., resulting in a combination of shear and compression forces), as will be further described below with reference to FIGS. 12A, B and C. Note that in FIG. 1C, the column electrodes 106a, 108a are shown in dashed lines as if the layers of the sensor 100 were transparent.

[0052] Although the sensor 100 is shown with an array of four bumps 102, the sensor body can be extended to include any number of bumps 102 as required for any given application.

[0053] In any of the described embodiments, the sensor may be formed from any of the materials listed below.

[0054] The base layer 104 and the top layer 106 may be formed from any suitable substrate material, including one or more of poly(paraxylylene) polymers (parylene), 2,8-dichlorotricyclo[8.2.2.2 4,7] hexadeca-1(12), 4,6,10,13,15-hexaene (parylene-C), polydimethylsiloxane (PDMS), polyimide (PI), polybutylene adipate terephthalate (polybutyrate), poly(methyl methacrylate) (PMMA / acrylic), prop-2-enoate (acrylate), polyethylene (PE), high density polyethylene (HDPE), low density polyethylene (LDPE), polyethylene terephthalate (PET), thermoplastic polyurethane (TPU), polyurethane (PU), polysiloxane (silica), polyvinyl chloride (PVC), polyethyleneimine (PEI), polyethylene naphthalate (PEN), propylene (PP), polystyrene (PS), aliphatic or semi-aromatic polyamide (PI), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), non-woven fabrics or blends thereof. A polymer with a low Young's modulus may help achieve a higher compatibility, such a polymer having a Young's modulus of at least 500 kPa, such as in the range of 700 kPa to 1 MPa, for example 700-800 kPa, up to 1.5 MPa (megapascals), more preferably up to 1.2 MPa, even more preferably up to 1 MPa. This is especially true for PDMS, where the drawback of these low Young's modulus polymers is that they are often fragile or not heat resistant enough for some applications. For this reason, blends such as TPU coated silicone can be used. The more rigid polymers in the list (such as PEN) are very flexible, such as at a thickness of 25 microns. Examples of different substrate thicknesses that may be considered are Parylene-C, 500-10000 nm; PEN, 25-100 microns; PET, 36-125 microns; silicone, 5-500 microns; and TPU, 50-75 microns.

[0055] The column electrodes 106a, 108a of both the first electrode layer 106 and the second electrode layer 108 can be formed of conductive composite materials such as inks containing metal filler particles (micro- or nanoscale). Carbon, silver, gold, and copper are most commonly used. Metal thin films (such as gold / copper) can be used if the substrate is not stretchable or is particularly thin, or if the metal has flexibility and bends while the substrate is being stretched instead of stretching itself. These all need to be highly conductive, and the best metal to use is gold, which is the most malleable and ductile of all metals. Some exemplary materials that can be used for the column electrodes 106a, 108a are Engineered Materials Solutions, Inc. (EMS) Conductive Inks (CI) (e.g., CI-1031, a silver conductive ink with a sheet resistance of less than 0.010 Ω / square at a thickness of 25.4 microns; or CI-2051, a carbon-based conductive ink with a sheet resistance of less than 50 Ω / square at a thickness of 25.4 microns). Other suitable materials include Asahi's Conductive Ink (CI) (e.g., CI-2051, a carbon-based conductive ink with a sheet resistance of less than 50 Ω / square at a thickness of 25.4 microns). TM Approximately 2.44x10 in ink and at 20 degrees Celsius -8 A thin film of gold is included having a resistivity of less than Ωm.

[0056] The active layer 110 may include a quantum tunneling material, a piezoresistive material, or a piezoelectric material. A quantum tunneling material has an exponential decrease in resistance due to quantum tunneling, and exhibits a large dynamic force range of about 10 12This results in a large resistance change of up to Ω. In some embodiments, the range of sensible forces can be as low as 0.003N to 200N, and the range can be extended higher if stiffer materials are used in the sensor. An example of a suitable quantum tunneling material is Quantum Tunneling Composite (QTC®), which includes nickel or silicon (semiconductor) filler particles with high aspect ratio tips (e.g., round, spiked, or needle-shaped) in addition to a binder material. Particles composed of dielectric materials such as titanium dioxide or fumed silica particles can also be used in the material. Metals or metal alloys with void-bearing structures can also be used with a variety of different binders. Another suitable quantum tunneling material is QTSS®, which includes spiked or needle-shaped magnetite and / or nickel particles and a polyurethane binder. The particles do not contact each other, but current can pass through them by the mechanism of quantum tunneling. Zero-dimensional, one-dimensional, or two-dimensional nanoparticle materials are desirable because of their enhanced dynamic range and sensitivity. Part of the reason for this may be due to the local concentration of the electric field at the quantum tip enhancing quantum tunneling, and the low percolation threshold (and therefore quantum) of the material required. Advantageously, quantum tunneling materials are most sensitive to compressive pressure between the electrodes whose resistance they measure, which allows for significant selectivity not offered by, for example, strain gauges. Quantum tunneling materials are also less sensitive to temperature and environmental electromagnetic compatibility noise than many other sensors and materials.

[0057] Suitable piezoresistive materials include nanocomposites and electrospun micro- or nanofibers. Piezoresistive materials modulate the flow of current when a potential difference is applied across them and pressure is applied to the material. They contain active fillers (micro / nanoparticles) that can be carbon, carbon nanotubes (CNT), multi-walled carbon nanotubes (MWCNT), graphene, borophene, silver, quantum dots, or other conductive or semiconducting micro / nanoparticles. They may also contain surfactants, which are usually functionalized on the surface of the nanoparticles to increase dispersion and reduce aggregation, for example, due to van der Waals forces. Binders such as PDMS, silicone, tetrafluoroethylene (TFE), propylene, PVDF (not piezoelectric in all states), and other polymers found in substrates and concrete are also used. The polymers may be thermosetting (i.e., undergo permanent crosslinking reactions by heat, moisture, light, or air) or non-thermosetting (i.e., melt). They may also be fluoropolymers. Piezoresistive materials may also contain a solvent to control viscosity (e.g., during printing), but it usually mostly evaporates in air or during curing. The nanoparticles are ideally aligned vertically (and well-dispersed) to enhance the anisotropic behavior of the material to sense normal pressure while minimizing pressure readings due to bending and stretching of the material. Some particles in a piezoresistive material can be electrically or magnetically aligned (by applying static or changing electric and / or magnetic fields across the material before and / or during curing and / or deposition). For example, this can be done with dielectrophoretic alignment by changing an AC (alternating current) electric field through the material before curing, or DC (direct current) alignment by applying a DC current through the material, or purely by a DC electric field (in which case a static field exists in the material without a current passing through the conductive particles). This alignment can be done to make the piezoresistive material more anisotropic or optically transparent.If the conductive particles of the piezoresistive material themselves do not respond to an applied electric field, they can be functionalized with other nanoparticles to align them (e.g., iron oxide nanoparticles, which are ferromagnetic). Other potentially suitable piezoresistive materials include thin films of any of the fillers used in the nanocomposites described above.

[0058] Suitable piezoelectric materials also include nanocomposites and electrospun micro- or nanofibers, which contain active fillers (micro / nanoparticles) that exhibit charge separation upon external stress, and include zinc oxide (ZnO), polyvinylidene fluoride (PVDF), poly[(vinylidene fluoride-cotrifluoroethylene)][P(VDF-TrFE)], lead zirconate titanate (PZT), barium titanate (BaTiO 3 ), lead(II) titanate (PbTiO 3 ), lithium tantalite (LiTaO 3 Other potentially suitable piezoelectric materials include thin films of any of the fillers used in the nanocomposites described above, in addition to a binder as described above.

[0059] The bumps 102 may include a polymer, such as a dielectric polymer, polysiloxane, rubber, silicone, or an amorphous crystalline material, such as glass.

[0060] 2A, B, and C show a compliant three-axis force sensor 200 including four three-dimensional rectangular bumps 202 according to a second embodiment of the present invention. The sensor 200 comprises an identical sensor body as described above for the sensor 100, but with the four dome-shaped bumps 102 replaced by four rectangular bumps 202. Thus, like reference numbers are used for like parts. As described above, the bumps 202 each have a footprint that extends over two column electrodes 106a of the first electrode layer 106 and over two column electrodes of the second electrode layer 108, such that a force applied to the bumps 202 is transferable through the active layer 110 in at least four distinct regions.

[0061] 3A, B, and C show a compliant triaxial force sensor 300 including four three-dimensional cylindrical bumps 202 according to a third embodiment of the present invention. The sensor 300 comprises an identical sensor body as described above for the sensor 100, but with the four dome-shaped bumps 102 replaced by four cylindrical bumps 302. Thus, like reference numbers are used for like parts. As described above, the bumps 302 each have a footprint that extends over two column electrodes 106a of the first electrode layer 106 and over two column electrodes of the second electrode layer 108, such that a force applied to the bumps 302 is transmissible through the active layer 110 in at least four distinct regions.

[0062] The above three-dimensional bumps are merely examples, and in other embodiments, each bump can be a mesa, dome, hemisphere, hemispherical portion, cone, cone portion, cuboid, cylinder, semicylinder, pyramid, pyramidal portion, tetrahedron, tetrahedral portion, hexahedron, triangular prism, polyhedron, or other shape. Additionally, the sensor can include bumps of different shapes and / or materials to suit any given application.

[0063] 4 is a flow chart of a general method 400 of manufacturing such a sensor according to an embodiment of the present invention. The method 400 comprises steps 402 of providing a first electrode layer 106 having an array of column electrodes 106a in a first orientation, 404 of providing a second electrode layer 108 having an array of column electrodes 108a in a second orientation orthogonal to the first orientation, 406 of providing a force-dependent active layer 110 configured to change at least one property when subjected to a force and provided between the first electrode layer 106 and the second electrode layer 108, 408 of providing at least one three-dimensional bump 102, 202, 302 arranged to transmit an externally applied force through the active layer 110, and 410 of providing at least one spacer arranged to maintain a spacing between two or more layers in the sensor until an external force is applied. As mentioned above, the spacer is illustrated and will be described in more detail below.

[0064] Each layer, bump, or spacer can be provided by any of printing, screen printing, roll-to-roll printing, inkjet printing, 3D printing, electrospinning, deposition, droplet dispensing, casting, coating, molding, spinning, or weaving. The method 400 may include forming at least a first electrode layer 106 and a second electrode layer 108 on a single substrate 104, separating the substrate into two portions, and stacking the two portions, as described in more detail below. The method 400 may also include forming at least one spacer on at least one portion prior to the separating and / or stacking steps.

[0065] 5A-5I show steps of a particular method 500 of fabricating a sensor according to an embodiment of the invention. In FIG. 5A, a base layer 504 is spin-coated, deposited, cast, or spray-coated into a thin film that forms a bottom polymer encapsulation layer. Ideally, the base layer 504 is made of an insulating polymer with desired elastic properties, such as PDMS or Parylene-C, which may also form part of the binder material of the active layer. In the case of spin-coating, this can be done on a silanized silicon wafer so that the polymer does not adhere and the device can be peeled off after fabrication. In FIG. 5B, a first electrode layer 506 is screen-printed, cast, sputter-coated, or spin-coated (e.g., using a photolithographic pattern) onto a matrix array of bottom column electrodes. The first electrode layer 506 can be composed of a conductive adhesive, such as a carbon-impregnated elastomer, an intrinsically conductive polymer, such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), or a metal, such as gold or silver. FIG. 5C illustrates an optional step of adding an insulating layer 514 between each column electrode of the first electrode layer 506 using screen printing or photolithographic patterning for additional stiffness or robustness. In this case, the insulating layer 514 may be of the same material as the base layer 504. FIG. 5D illustrates screen printing, casting, spin coating, spray coating, or attachment of a thin active layer 510 of quantum tunneling material, for example, as described above. The thickness of the active layer 510 can determine the sensitivity of the sensor and can vary between tens of microns to millimeters. The type of metal and binder matrix material can vary depending on the desired application, and the composition of the material can be in liquid or solid form. For example, the active layer 510 can include gold nanoparticles and PDMS, or it can include magnetite and a water-based polymer ink. It may be a known material or a custom formulated material.FIG. 5E shows screen printing, casting, sputter coating, spin coating or attachment (e.g., using photolithographic patterns) to create a thin layer (~10-100 μm thick) of binder material and form spacers only in the areas between where the electrodes are to be deposited, with the edges of the electrodes over the edges of the spacers. These spacers form small air gaps between the electrodes. Standard microfabrication or chemical techniques such as patterning a partially dissolvable layer, e.g., using a photocurable polymer, can be used to create the air gaps. FIG. 5F shows screen printing, casting, sputter coating, spin coating or attachment (e.g., using photolithographic patterns) to form a matrix array of top electrodes of a second electrode layer 508 orthogonal to the first electrode layer 506, on top of the spacers 520. The second electrode layer 508 can be composed of a conductive adhesive such as a carbon-impregnated elastomer, an intrinsically conductive polymer such as PEDOT:PSS, or a metal such as gold or silver. Optionally, a support layer is provided between the spacers 520 on which the top electrodes are formed. FIG. 5G illustrates an optional step of adding an insulating layer 514 between the top electrodes of the second electrode layer 508 for additional stiffness or robustness using screen printing or photolithographic patterning techniques. As mentioned above, the insulating layer 514 can be the same material as the base layer 504. In FIG. 5H, a top layer 512 is spin-coated, deposited, cast, or spray-coated into a thin film to form a top polymer encapsulation layer. The top layer 512 can be made of an insulating polymer with desired elastic properties, such as PDMS or Parylene-C, and ideally can be formed of the same material as the binder material of the active layer 510. In FIG. 5I, four three-dimensional surface bumps 502 are added to the top layer 512 by casting a polymer layer to form the bumps 502 over every set of four overlapping electrodes or sense cells. In particular, the bumps 502 can have a different elasticity and stiffness than the rest of the sensor (e.g., the bumps 502 can be made stiffer than the sensor body).The three-dimensional shape of each bump 502 can be, for example, a cube, cone, or mesa / hemisphere, and its corners should overlap to some extent with the four underlying electrodes. This allows each group of four electrodes (i.e., sensors) in the array to become a single three-dimensional or three-axis force-sensing tactile element (tactel). After all steps described in method 500, an optional oxygen plasma layer, thiol layer, or other chemical adhesive layer can be added to increase the robustness of the sensor. If the base layer 502 is spin-coated, a further step may be required to detach the entire assembly from the wafer by peeling it off.

[0066] If the binder material in the above method is a stretchable elastomer, it is also possible to make the entire sensor array stretchable, further increasing its potential applications.

[0067] The substrate or base layer 504 used to create the sensor depends somewhat on the application. Traditional flexible electronics substrates that are compliant in only one axis can be used, such as PET (polyethylene terephthalate) with thicknesses ranging from 25 to 125 microns, or PEN (polyethylene naphthalate) with thicknesses varying from 10 to 100 microns (usually 25 microns). These materials may be necessary if high resolution is required, or if the device must withstand harsh external environments such as underwater. For less harsh conditions, substrates that are compliant in multiple axes can be used, such as TPU (thermoplastic polyurethane) or silicone. Also, there is TPU coated silicone to reduce shrinkage induced in silicone during thermal curing of the nanocomposite ink after deposition. These materials may also be used in a thickness range between 25 and 125 microns. Nonwoven fabrics may also be used as substrates, as may paper, various rubbers and latexes. Various other polymers may also be used as substrates. Parylene-C can be used in the range of 10 nm to 1 micron, for example, for biocompatible applications and applications requiring extreme compliance and / or complete environmental sealing. Additionally, if desired, oil can be provided on top of the substrate to facilitate the spreading of the ink for the electrodes during the printing process.

[0068] A first layer of conductive ink can be printed, which acts as the conductive tracks of the first electrode layer 506 and is highly conductive. To define the matrix pattern of this first electrode layer 506, a stencil film or mask or screen printing mesh with appropriate openings cut out or washed away with a solvent can be used, through which the ink can be spread onto the substrate using a squeegee, as is common in screen printing or roll-to-roll printing. 2This electrode layer, with a sheet resistivity in the range of 100-2000 nm, is made of nanocomposite materials and can be either flexible or stretchable. Stretchable inks work with a substrate that allows a certain percentage of stretchability, such as TPU or silicone. Such inks contain conductive particles, often with one dimension on the order of nanometers and an overall size often on the order of microns, embedded in a polymer matrix that is usually mixed with solvent chemicals and surfactants to ensure that they are uniformly distributed among the particles of the ink. The particles are usually composed of silver or carbon, but can be composed of gold, graphene, copper, tungsten, zinc, etc. as well. With screen printing, the thickness of the deposited ink can be on the order of 10-50 microns (sometimes 100), but can be less when inkjet printing is used. The width of some electrodes can be as low as 20 microns to 1 millimeter.

[0069] After the electrode ink is deposited, it needs to be cured, typically at temperatures between 80 and 120 °C. During the curing process, some of the solvent evaporates while the polymer composite hardens.

[0070] The configuration of the tracks that may be needed to connect the sensor to readout electronics and the electrodes themselves may be the same or different. Optionally, a coating of ink of different composition can be printed for the electrodes over the conductive tracks that extend into the area under each electrode.

[0071] One or more sensitive (potentially anisotropic, ie only sensitive in the vertical direction, depending on the thickness of the material or its intrinsic properties) active layers are provided in each sensor.

[0072] Additionally, spacers made of a dielectric polymer can be printed or deposited onto the sensor using a droplet dispensing method. These spacers are approximately 10-50 microns in height and 1-200 microns in diameter or width and function to keep the grid layers of the sensor spaced apart from each other until a certain force is applied. They also function to redistribute forces when bent and prevent creases or wrinkles in the sensor skin, which can create localized pressure points or "bubbles" that can cause pressure readings on the sensor.

[0073] Once the sensor is printed, for highly compliant and stretchable materials, or for very thin materials such as 25 microns or less, reinforcements can be added to the ends of the connected electrode traces or tracks and regular solder tabs or cables can be crimped to them, connecting the sensor electronics to a rigid electronic processing circuit.

[0074] 6A-6I show steps of a method 600 for fabricating another sensor according to a further embodiment of the invention. At each step, the sensor is shown in both three-dimensional and two-dimensional views. In FIG. 6A, a suitable substrate material 604 (e.g., PEN, PET, TPU, PDMS, TPU-coated silicone, latex, etc.) is selected and fabricated to the desired thickness. In this case, the substrate 604 forms both the base and top layers of the sensor, as described in more detail below. In this embodiment, the substrate 604 is PET with a thickness of 36 microns. FIG. 6B shows the steps of printing (e.g., screen printing or inkjet printing) both the first electrode layer 606 and the second electrode layer 608 in a matrix array with row and column electrodes oriented at right angles to each other. The electrodes are typically formed from inks containing silver or low resistivity carbon. In this embodiment, the electrodes are made from CI-1036 stretchable silver-based conductive ink with a low sheet resistivity of less than 0.010 Ω / square at a thickness of 25.4 microns. In this example, the electrodes are screen printed at the thickness of the emulsion, so that the thickness before curing is 14-25 microns. Three electrodes are provided in each of the first and second electrode layers 606, 608, such that each electrode has a width of 2 mm, forming a sensor that is 1 cm wide by 1 cm long when assembled. A 2 mm gap is provided between the first and second electrode layers 606, 608 on the substrate 604 to allow for easy separation of each layer, as described below. Figure 6C shows the optional step of screen printing carbon layers 607, 609 on each electrode of the first and second electrode layers 606, 608. Figure 6D shows the step of depositing (e.g., screen printing, inkjet printing, electrospinning) an active layer 610 on each electrode of the first and second electrode layers 606, 608. The active layers can include piezoresistive and / or quantum tunneling and / or piezoelectric material layers, and can be deposited as a single layer or multiple layers on top of each other to control the thickness of the layers. The active layer may be a continuous sheet or may include discrete portions, for example lines, squares, dots or other shapes.In this embodiment, the active layer is formed of Quantum Technology Super Sensor (QTSS®) C75 piezoresistive ink printed over the electrode columns and rows in the grid area only, with a similar thickness as the previous layer. In FIG. 6E, the spacer layer 620 is deposited using a dielectric, insulating, or conductive material. The spacer layer 620 can be deposited, for example, by screen printing, inkjet printing, or by using a droplet dispenser. In this embodiment, the spacer layer 620 is formed on only one side of the substrate 604, in the gaps between the electrodes that form the second set of electrodes 608. In this example, the spacer layer 620 constitutes an array of cylindrical posts, each with a diameter of 50 microns and a height of 100 microns, spaced 1 mm apart across the substrate 604. The material used for the spacer layer 620 in this example is a dielectric such as EMS DI-7548, which is stretchable and flexible at the same time. The spacer layer 620 can be printed as a single thick layer on one side of the sensor, as multiple stacked layers, or if the layers are thinner, printed on both sides of the sensor (in which case precise alignment is required).

[0075] In FIG. 6F, the substrate 604 is cut, for example with scissors, a laser, a knife plotter or a scalpel, to separate the first electrode layer 606 and the second electrode layer 608 into two parts. The two parts are then folded inward such that the second electrode layer 608 is stacked on top of the first electrode layer 606 to form a conductive matrix grid, as shown in FIG. 6G. In this configuration, the substrate 604 attached to the second electrode layer 608 forms the top layer of the sensor. Furthermore, the posts of the spacer layer 620 extend between the substrate 604 forming the base layer and the substrate 604 forming the top layer, such that an air gap 622 exists between the portion of the active layer 610 provided on the first electrode layer 606 and the portion of the active layer 610 provided on the second electrode layer 608. When the sensors are stacked as in FIG. 6G, the spacer layer 620 acts to distribute the force when the sensor is bent, in this case to maintain an air gap between the active layers 610, ensuring an open circuit until a predetermined offset force is applied. In some embodiments, the sides of the sensor can be bonded with an adhesive paste (e.g., silicone PDMS) applied along the edges of the device and around the active layer area to seal the sensor from the external environment in a durable manner and laminate the sides of the sensor. Additionally, a vapor deposit of APTES (3-aminopropyltriethoxysilane) linker can be applied to the top layer for 1 hour. In some embodiments, the sensor can be completely coated with a compliant polymer such as latex at this stage. For very thin sensors, a dip coating technique can also be used.

[0076] Next, bumps 602 are provided on the top surface of the sensor to convert the force into torque, as shown in FIG. 6H. To sense both the direction and magnitude of the force, a bump must be placed in the center of each set of four orthogonally arranged electrodes (sensors) and, when pressed, converts the applied force into a torque. That torque then compresses some sensors more than others, which can be read through the connected electronic circuitry. The bumps can be created by printing multiple dielectric layers using various printing techniques (such as 3D printing or silicon etching) or by molding the bumps (in this case, the sensor needs to be placed upside down in the mold while the polymer used is in a liquid or partially cured state, and then the whole assembly needs to be cured). In this embodiment, a master mold is created from PLA for four cubic bumps 602, 2 mm high and 6 mm long and wide. Liquid PDMS (or silicone / latex / TPU / rubber) is then poured into the mold (10:1 weight ratio of elastomer to curing agent) and cured in an oven at 80° C. for 1 hour. The PDMS is then treated with UV-ozone or oxygen plasma to further increase adhesion before it is bonded to the PET top layer of the substrate 604. The assembly can be left overnight to form strong bonds between the PDMS bumps and the PET. The bumps 602 can themselves be rigid or compliant, as long as they allow the device to bend at a given thickness. Thinner bumps 602, as well as those made from highly compliant materials, will distort the measurements less when bent, and therefore give better anisotropic selectivity to normal forces while the device is in the bent state.

[0077] As shown in Figure 6I, a final optional step may include laminating or insulating the top of the sensor or the edges of the sensor or the entire sensor structure to create an impermeable cover layer 624 to external elements. Lamination can be done, for example, with a fully conformal Parylene coating (more specifically, Parylene-C, 1 nanometer to 1 micron thick), and insulation can be achieved with silicone, rubber, polyurethane, or other polymer blends.

[0078] It will be appreciated that any suitable printer may be used for any layer in the sensor, although one example of a suitable screen printer is the Horizon Dek i3 TM It is.

[0079] Readout of the sensors can be done using the known zero potential method. This method minimizes crosstalk between adjacent sensors in a large array. In resistive film array circuits, large interference currents can occur when the resistance of one sensing element approaches that of other surrounding sensing elements. In negative piezoresistive coefficient sensors such as those described herein, the sensing elements start out with a high resistance and the resistance decreases as pressure is applied. This means that in the described sensors, lower pressures will generate higher interference because the material has a high resistance at rest. More details of the operation of these sensors are provided below in conjunction with Figures 12A-C.

[0080] 7A-7G show steps in a method of manufacturing a sensor according to yet another embodiment of the present invention. This sensor can be considered a four-wire single point sensor that differs in operation from the other sensors described above. In particular, this sensor differs in that it measures the "center of gravity" of pressure being applied to a resistive area between the four wires of an electrode grid. The bumps of this sensor function in a similar manner to a joystick in that the location of the center of gravity can indicate the direction of the force.

[0081] As shown in FIG. 7A, the appropriate material for the substrate 704 is selected along with its thickness. In this embodiment, the substrate 704 is formed of PTE with a thickness of 36 microns. FIG. 7B shows the screen printing of both the first and second electrode layers 706, 708 on the same substrate 704, with the electrodes of each layer oriented perpendicular to each other. In this embodiment, the electrodes are made of a stretchable silver-based conductive ink EMSCI-1036™ with a low sheet resistivity of less than 0.010 Ω / square at a thickness of 25.4 microns. In this embodiment, the electrodes are screen printed with an emulsion thickness of 14 to 25 microns for an ink thickness before curing. The electrodes are 2 mm wide and only two electrodes are provided along the edges of each of the first and second electrode layers 706, 708, forming a footprint sensor of 1 cm by 1 cm.

[0082] 7C shows the screen printing of a first highly resistive carbon layer 707 deposited on and extending between the two electrodes of a first electrode layer 706, and a second highly resistive carbon layer 709 deposited on and extending between the two electrodes of a second electrode layer 708. In this embodiment, both highly resistive carbon layers are made from two Henkel inks called ECI 7004HR and NCI 7002 inks. TM It is mixed from an ink (80 / 20% mix ratio by weight) to obtain a sheet resistivity of about 10,100 Ω / sq at a thickness of 10 microns after curing. The deposition is performed through a mesh with an emulsion thickness of about 20-40 microns. In other embodiments, the high resistance carbon layer may be formed using EMSMT CI-2050 LR (1,500-2500 Ω / sq / mil at 25.4 microns) and / or HR (less than 10 MΩ / sq / mil at 25.4 microns). In some embodiments, the first and second high resistance carbon layers 707, 709 may be different.

[0083] For this type of sensor, it is important that the highly resistive carbon layers 707, 709 have a resistance much higher than the sum of the lead resistance within the device and the contact resistance with the outside world. The reason for this is that when a voltage is applied between the top electrode layer 708 or bottom electrode layer 706 of the device, it is desirable for the majority of the voltage drop to occur within the resistive carbon region. This allows a reliable measurement of the location of the center of pressure of the torque applied to the bump layer. When pressure is applied to the device, both active layers 710 contact each other around a given point. When a potential difference is applied between the electrodes on one side of the device (top or bottom) and a voltage is measured from one of the electrodes on the other side of the device, this forms a voltage divider circuit. The voltage read from one of the electrodes on the opposite side of the carbon face indicates the location of the pressure (one axis) between the two electrodes that have a potential difference applied across them and where pressure is being applied. This electrical configuration can be reversed to read the location of the other axis with the opposite electrode / carbon layer of the device.

[0084] 7D shows screen printing, inkjet printing or electrospinning of an active layer 710 on each of the first and second highly resistive carbon layers 707 and 709. In this embodiment, the active layer 710 is formed from Quantum Technology SuperSensors (QTSS®) C75 piezoresistive water-based ink.

[0085] In Figure 7E, a spacer layer 720 consisting of a square array of short cylindrical spacer dots is provided on one side (first side) of the substrate over the active layer 710. Each spacer dot is 200 microns high and spaced 2 mm from adjacent dots. In this embodiment, the spacer layer 720 is made of Stratasys' Tango TM (i.e. TangoPlus TM The 3D printed parts were printed using PolyJet FLX930 material, which simulates a rubber-like thermoplastic elastomer. TMThe spacer is a printable material of 1000 nm in diameter. Typical properties of this material are a Shore hardness of 26-28 and an elongation at break of 170-220%. Thus, the spacer can withstand the difference between tensile and compressive stresses at the top and bottom of the device while it is curved, even if the device itself is used in a flat state. The substrate 704 is then cut, for example with scissors, a laser, a knife plotter or a scalpel, to separate the first electrode layer 706 and the second electrode layer 708 into two parts. The two parts are then folded inward such that the second electrode layer 708 is stacked on top of the first electrode layer 706 to form a conductive matrix grid, as shown in FIG. 7E. In this configuration, the substrate 704 attached to the second electrode layer 708 forms the top layer of the sensor. Additionally, as shown in Figure 7F, the posts of the spacer layer 720 extend between the active layer 710 on the first electrode layer 706 and the active layer 710 on the second electrode layer 708 such that there is an air gap in the center of the active layer 710. Once the device is cut and folded, the sides are glued together with a silicone PDMS adhesive applied around the edges of the device. A vapor deposit of APTES (3-aminopropyltriethoxysilane) linker is then applied to the top surface of the device for 1 hour to promote PDMS adhesion as described below.

[0086] In FIG. 7G, bumps 702 are provided in the center of the sensor, away from the electrode grid. The bumps 702 can be cast or printed using many polymers, such as the more compliant PDMS, or the less compliant SU8. The bumps 702 can have a variety of shapes, sizes, and heights, depending on the desired application. In this embodiment, a master mold is made from PLA for a rectangular bump 702 that is 2 mm high and 6 mm long and wide. Liquid PDMS is poured into the mold (10:1 elastomer to curing agent weight ratio) and cured in an oven at 80° C. for 1 hour. After this, the PDMS is treated with UV-ozone or oxygen plasma to further increase adhesion before being glued to the top layer of the substrate 704. The assembly is left overnight to form a strong bond between the PDMS bumps and the substrate 704.

[0087] For this type of sensor, the reading consists of three separate readings per bump 702. One reading is required for the X position and one for the Y position, as explained below. Each time, a voltage (and current) is applied across the electrodes of one plane (i.e., the first or second electrode layer 706, 708) and one electrode on the opposite plane (i.e., the other of the first or second electrode layer 706, 708) is used to read the voltage. This forms a voltage divider circuit and the voltage read corresponds to the position (x or y) where a force is applied and both planes come into contact. It is important that the highly resistive carbon layer has a much higher resistance than the lead and contact resistances connecting the device to the outside world, and that their resistance is essentially negligible. Otherwise, there would be a significant voltage drop in the leads, limiting the position reading.

[0088] To read the force itself, a current must flow from one plane to the other, with the active layer 710 (i.e., the piezoresistive material) in between. This can be done by applying a potential difference between one electrode on the top surface (i.e., the second electrode layer 708) and another electrode on the bottom surface (i.e., the first electrode layer 706). In this case, the applied pressure affects the resistance of the active layer, changing the current through the device. As a result of this, the overall voltage drop across the active layer 710 also changes. By using opposing electrodes on both planes (not used to carry current), the voltage at either location can be measured. This corresponds to what is known in the electronic literature as a four-point probe measurement, because it does not measure the contact resistance, in this case the voltage drop of the electrodes and their corresponding leads to the device. However, it is not perfect, since it measures the voltage drop. Depending on the location of the applied pressure, the current must pass different distances on the highly resistive surface, which changes the resistance and therefore the voltage drop in the resistive layer depending on the location of the pressure. Knowing the resistance and uniformity of the high resistivity layer tells us how much we need to adjust our resistance measurements by calibrating the device based on location. Thus, when the bump 702 on the top of the sensor experiences a force, the shear component of it creates a torque on the bump 702, concentrating the center of pressure on a particular area of ​​the device. By measuring the location and magnitude of this, we can measure the average direction and magnitude of the force.

[0089] The spacer in any of the embodiments of the present invention helps to distribute the force and prevent wrinkles from forming in the sensor that would be erroneously recorded as pressure, while ensuring that an open circuit exists when no offset force is applied. Figures 8A-11C show various exemplary spacer arrangements for use in embodiments of the present invention. For purposes of illustration, only a portion of the sensor in each case is shown.

[0090] 8A and 8B show plan views of a portion of a sensor including different cylindrical spacer arrangements according to embodiments of the invention. In each case, a substrate 804 includes three horizontal electrodes in a first electrode layer 806 and three orthogonal electrodes in a second electrode layer 808. An active material 810 is provided between the electrodes in the first electrode layer 806 and the electrodes in the second electrode layer 808. In each case, the spacers are formed as cylindrical posts, pillars, or dots.

[0091] In FIG. 8A, spacers 820a are printed on both sides of substrate 804 and are positioned between overlapping portions of the electrode matrix to extend from and through each electrode layer to substrate 804.

[0092] In FIG. 8B, spacers 820b are printed on only one side of substrate 804 and are positioned to extend between the bottom and top of substrate 804, between overlapping portions of the electrode matrix.

[0093] The spacers in Figures 8A and 8B can be formed of polymers such as dielectric polymers, polysiloxanes, rubbers, silicones, or amorphous crystalline materials such as glass. The spacers can be cylindrical, conical or cone fragments, pyramids or pyramid fragments, rectangular, hemispheres or hemisphere fragments, hemicylinders, tetrahedrons or tetrahedron fragments, hexahedrons, triangular prisms, polyhedrons, and other shapes. In some embodiments, the spacers can be tapered to change height, shape, size, or spacing across the sensor. In one example, the spacers can be cone-shaped with a width of 50 microns, a height of 25 microns, and a spacing of 1 mm.

[0094] 9A and 9B show plan views of a portion of a sensor including different diamond-shaped spacer arrangements, according to embodiments of the invention. In each case, a substrate 904 includes three horizontal electrodes in a first electrode layer 906 and three orthogonal electrodes in a second electrode layer 908. An active material 910 is provided between the electrodes in the first electrode layer 906 and the second electrode layer 908. In each case, the spacers are formed of a diamond or square mesh.

[0095] In FIG. 9A, spacers 920a are printed on only one side of substrate 904 and are positioned between overlapping portions of the electrode matrix to extend from and through each electrode layer.

[0096] In FIG. 9B, spacers 920b are printed on both sides of substrate 904 and are positioned to extend over and / or between the bottom and top portions of substrate 904, over and / or between overlapping portions of the electrode matrix.

[0097] The spacers in Figures 9A and 9B can be made, for example, of a woven mesh or a flexible / rubberish polymer mesh, 3D printed using any of a variety of soft polymers.

[0098] 10A and 10B show plan views of a portion of a sensor including different square spacer arrangements, according to an embodiment of the invention. In both cases, a substrate 1004 is provided with three horizontal electrodes in a first electrode layer 1006 and three orthogonal electrodes in a second electrode layer 1008. An active material 1010 is provided between the electrodes in the first electrode layer 1006 and the second electrode layer 1008. In both cases, the spacers are formed of hollow square posts.

[0099] In FIG. 10A, spacers 1020a are printed on only one side of substrate 1004 and are arranged to span a large sensor area, for example including an overlap of nine electrodes per square.

[0100] In FIG. 10B, spacers 1020b are printed on only one side of substrate 1004 and are positioned, for example, to extend around the small sensor areas between the overlapping portions of each electrode matrix.

[0101] The spacers of Figures 10A and 10B may be formed from materials similar to those described above for the cylindrical or diamond shaped spacers.

[0102] 11A, 11B, and 11C show plan views of a portion of a sensor including different hexagonal spacer arrangements, according to an embodiment of the invention. In each case, a substrate 1104 includes three horizontal electrodes in a first electrode layer 1106 and three orthogonal electrodes in a second electrode layer 1108. An active material 1110 is provided between the electrodes in the first electrode layer 1106 and the second electrode layer 1108. In each case, the spacers are formed of hollow hexagonal (i.e., honeycomb) meshes of different sizes.

[0103] In FIG. 11A, spacers 1120a are printed on only one side of the substrate 1104 and are positioned to extend between overlapping portions of the electrode matrix.

[0104] In FIG. 11B, spacers 1120b are printed on only one side of the substrate 1104 and are positioned over and / or between the overlapping portions of each electrode matrix, extending around the small sensor area.

[0105] In FIG. 11C, spacers 1120c are printed on only one side of the substrate 1104 and are positioned to extend between and join overlapping portions of the electrode matrices.

[0106] The spacers in Figures 11A, 11B, and 11C can be formed of extruded or printed mesh. They can be honeycomb, triangular, quadrilateral, diamond, diamond with ferrules, square, or plain shapes. Different weaves can be chosen depending on the application, and should allow the sensor active layers to contact each other sufficiently. The woven mesh has the same purpose as the other spacer layers. Plain tatami weave, twill weave, plain tatami weave, twill tatami weave, lock crimp, intercrimp, double or strand weave of twill tatami weave can be used. The mesh can be made of non-conductive fiber yarns, or conductive yarns (e.g., silver or titanium) can be used to increase sensitivity while distributing pressure within the sensor.

[0107] There are multiple challenges in making a fully compliant flat sensor. The sensor needs to be thin, the electrodes need to be compliant, and the sensor must not react to forces other than the desired force. The quantum tunneling active layer material has a very high resistance (~10 9 This is advantageous because the resistance starts at 0.05 Ω. Contacts to this material can be made of several different materials, but the electrodes need to be flexible and slightly elastic so that the material can be used to wrap around objects. One way to do this is to create stretchable silver electrodes or alternate conductive adhesives, even if they change slightly with pressure.

[0108] As explained above, a grid / matrix of these sensors can be created by sandwiching an active layer between row and column electrodes running at right angles on either side. This has been done before with non-compliant electrodes and electronics, and also with "flexible" electronics that have limited bending radii but are not fully compliant on macroscopically observable surfaces. Reading the matrix requires a multiplexer that reads every sensor (sensing pixel) in the array one by one and connects it to a microcontroller or other signal processing hardware and software. This has been done before with non-compliant electrodes and electronics, and also with "flexible" electronics that have limited bending radii but are not fully compliant on macroscopically observable surfaces. Reading the matrix requires a multiplexer that reads every sensor (sensing pixel) in the array one by one and connects it to a microcontroller or other signal processing hardware and software.

[0109] In an embodiment of the present invention, a simple device is proposed where not only is the entire device compliant, but every cluster of four sensels in the grid can be converted into a 3D force-sensing "tactel" (tactile pixel) by including bumps or shapes in the top layer of the device that act as force-transmitting structures.

[0110] The proposed device is tailored for mass production using printing, such as roll-to-roll printing, screen printing, or similar microfabrication techniques.

[0111] The main working mechanism of the above sensor is as follows: the resistance changes of each of the four sensors and their normal distribution are used to estimate the forces acting on the sensor in three directions. Each bump acts as a force transmission structure. Stress is generated by deforming the "bump" by applying a directional force to each bump. The normal component of the force compresses each sensor below it equally (resulting in a symmetric resistance change), while the shear component generates a torque on the active layer, compressing some sensor cells more than others (resulting in an asymmetric resistance change). This tuning of sensitivity can be done by varying both the thickness and binder material of the active layer, and the height and stiffness of the "bump" and the substrate material underneath it.

[0112] Individual grid lines can be as small as 10um and the spacing between them can be as small as necessary, provided tunneling is not established between layers which would cause crosstalk or shorting.

[0113] To read the signal, each column and row electrode must be connected to an appropriate electrical read circuit. When reading the signal, each electrode is spatially separated from each other, so that resistive crosstalk noise is greatly reduced. However, to achieve a further reduction in crosstalk or an improvement in the signal-to-noise ratio (SNR) of each sensor, one of the following methods can be adopted: i) Publick's voltage mirror method (in this case the drive lines not considered at the measurement point are set to a potential equal to the output voltage), or ii) the simpler zero potential method (in this case the voltage of the scanning electrodes is set to zero). These form an equipotential zone that eliminates the effects of crosstalk. There are several ways to actually construct the above circuit.

[0114] Calibration of the device is required, but machine learning or artificial intelligence algorithms can also be used to "learn" how the sensor operates given the appropriate training data. The range of forces that can be measured with such sensors ranges from 0.01 N to theoretically infinite amounts, depending on the formulation of the active layer, its binder, and the overall thickness of the sensor.

[0115] There is no preferred thickness for any layer, but for the entire device to be compliant, it is expected that they will be less than 1 mm thick and more than 10 nm thin. The surface bumps can be of different shapes than those listed, but in most cases the constraint of having one bump for every four sensors applies.

[0116] 12A shows a top perspective view of a single tactile element (tactel) 1200 of a sensor according to an embodiment of the present invention. The tactel has dome-shaped bumps 1202 provided on four sensor cells C1, C2, C3 and C4 that represent pixels to be read. Table 1 below shows the change in electrical resistance experienced by each of the four sensor cells when subjected to different forces. Thus, electrical readings of such sensor cells can be converted into measurements of the position, magnitude and direction of the applied force. [Table 1]

[0117] 12B shows a side perspective view of the haptic element 1200 of FIG. 12A before and after an oblique force is applied to the bump 1202, along with a corresponding force versus time graph, illustrating the difference in forces experienced in the x, y, and z axes when an oblique force is applied.

[0118] 12C shows a side view of the tactile element 1200 of FIG. 12B when an oblique force is applied to bump 1202. It shows a simplified sensor layer structure including an active layer 1210 sandwiched between top and bottom substrates 1204 for ease of illustration. In this example, sensor C3 experiences a small deformation upon application of force, while sensor C4 experiences a large deformation. This translates to a weaker change in C3 than in C4, helping to pinpoint the direction of the applied force.

[0119] There are many applications for the described sensor. These stem from the large industrial, domestic and medical demand for compliant 3-axis force sensors with reliable operation. Examples include robotics (hard and soft robots, slip detection), electronic skin, prosthetics, human-robot interaction, tactile systems and surfaces such as walls and floors, virtual reality, gaming, structural integrity monitoring (airplane wings, buildings), smart clothing (gloves etc.), gait analysis insoles (ground reaction force direction, total pressure, diabetic foot ulcer monitoring etc., they can replace the force sensing plate), sports impact monitoring (can detect impact direction and magnitude), implantable devices, wearables (e.g. for use in smart homes), biomedical devices, drug delivery patches / systems, stretchable electronic circuits, adult and sex toy industry (e.g. toys or dolls with real-time performance / feedback and / or adaptive devices), motion sensing pillows, bed sores, hospital beds monitoring and detection of blood pressure (e.g. blood pressure and blood flow in clothing or wearables etc.) for coronary heart disease (CVD) diagnosis by monitoring force, muscle strength / exercise, fatigue monitoring and detection, monitoring flow characteristics around tidal or wind turbine blades to improve lifespan or energy generating capacity, autonomous robots could be envisioned and in all areas requiring high levels of dexterity and tactile information (e.g. warehouse picking and sorting, space exploration, search and rescue, elderly care, cooking, electronics assembly, fishing, social robots, etc.), airplane wings (load / force monitoring to help design better wings along with weight and cost reduction), force monitoring in construction work environments, saddle pressure monitoring on horses, muscle strength mapping / monitoring, tissue stiffness / density monitoring (e.g. breast cancer diagnosis / prediction of childbearing time in women), prosthetics (e.g. in the case of amputees but also for tremor compensation in certain diseases such as Parkinson's, ALS, among others).

[0120] In some embodiments, the piezoresistive and piezoelectric layers can be stacked before attaching the bump layer to better mimic the high and low frequency mechanoreceptors of the skin.

[0121] Additionally, the device can be used upside down (i.e., with the bumps in contact with the user's skin and exerting a force on the base layer), which is particularly beneficial if the aerodynamic or hydrodynamic properties of the device's surface are maintained.

[0122] While the foregoing description describes exemplary embodiments, those skilled in the art will understand that many variations can be made to these embodiments within the scope of the invention as defined by the claims. Furthermore, the features of one or more embodiments can be mixed and combined with the features of one or more other embodiments.

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Claims

1. A compliant three-axis force sensor, a first electrode layer having an array of column electrodes in a first orientation; a second electrode layer having an array of column electrodes in a second orientation orthogonal to the first orientation; a force-dependent active layer disposed between the first electrode layer and the second electrode layer, the force-dependent active layer configured to change at least one property when subjected to a force; at least one three-dimensional bump arranged to transmit an externally applied force through the force-dependent active layer; at least one spacer arranged to maintain a spacing between two or more layers in the compliant three-axis force sensor until an external force is applied; the at least one spacer is in the form of a binder material disposed between the force-dependent active layer and the first and / or second electrode layer, the binder material being disposed in gaps between adjacent column electrodes and extending between the force-dependent active layer and respective edges of the adjacent column electrodes; or the force-dependent active layer comprises a first active layer and a second active layer having a gap between each other, and the at least one spacer is disposed within the force-dependent active layer. Compliant 3-axis force sensor.

2. The compliant three-axis force sensor of claim 1 , wherein the force-dependent active layer comprises at least one of a quantum tunneling material, a piezoresistive material, or a piezoelectric material.

3. The compliant three-axis force sensor of claim 1 , wherein the at least one property is quantum tunneling, conductivity, resistivity, or charge.

4. 2. The compliant three-axis force sensor of claim 1, wherein the at least one three-dimensional bump has a footprint that extends at least partially over at least two column electrodes of the first electrode layer and over at least two column electrodes of the second electrode layer such that a force applied to the at least one three-dimensional bump can be transferred through the force-dependent active layer in at least four distinct regions.

5. 2. The compliant three-axis force sensor of claim 1 , wherein the at least one three-dimensional bump has a footprint contained within a space between two adjacent column electrodes of the first electrode layer and two adjacent column electrodes of the second electrode layer.

6. 6. The compliant three-axis force sensor of claim 1, further comprising one or more of a base layer below the first electrode layer, a top layer above the second electrode layer, a first carbon layer between the first electrode layer and the force-dependent active layer, a second carbon layer between the second electrode layer and the force-dependent active layer, and a cover layer on the at least one three-dimensional bump.

7. 7. A compliant three-axis force sensor as claimed in claim 6, wherein the first and / or second carbon layers continuously cross a plurality of column electrodes of the first and / or second electrode layers.

8. 6. A compliant three-axis force sensor as claimed in claim 1, wherein the at least one spacer is provided between a base layer and a top layer, or on or through the first or second electrode layer.

9. The compliant three-axis force sensor according to any one of claims 1 to 5, wherein the at least one spacer is in the shape of a dot, a column, a pillar, a cylinder, a tube, a pyramid or a mesh.

10. A compliant three-axis force sensor according to any of claims 1 to 5, wherein the at least one spacer has a solid or hollow cross-section in the shape of a circle, a square, a rectangle, a rhombus, a pentagon, a hexagon, or a honeycomb lattice.

11. A compliant three-axis force sensor as described in any one of claims 1 to 5, wherein the at least one spacer comprises an array of spacers.

12. The compliant three-axis force sensor of claim 11 , wherein the spacers in the array have a plurality of different dimensions.

13. A compliant three-axis force sensor according to any preceding claim, wherein the force-dependent active layer forms a continuous layer across a plurality of column electrodes of the first and / or second electrode layers.

14. A compliant three-axis force sensor according to any preceding claim, wherein the force-dependent active layer forms a discontinuous layer across column electrodes of the first and / or second electrode layers.

15. The compliant three-axis force sensor according to any one of claims 1 to 5, wherein an insulator is provided between adjacent electrodes of the first and / or second electrode layers.

16. A compliant three-axis force sensor according to any preceding claim, wherein an adhesive layer is provided between the one or more adjacent layers.

17. The compliant three-axis force sensor of any of claims 1 to 5, wherein the at least one three-dimensional bump is made of a polymer having a different elasticity than the remainder of the compliant three-axis force sensor.

18. 6. The compliant three-axis force sensor of claim 1, wherein the at least one three-dimensional bump is in the shape of a mesa, a dome, a hemisphere, a hemispherical portion, a cone, a conical portion, a cube, a cylinder, a hemispherical portion, a pyramid, a pyramidal portion, a tetrahedron, a tetrahedral portion, a hexahedron, a triangular prism, or other polyhedron.

19. A compliant three-axis force sensor according to any preceding claim, wherein the at least one three-dimensional bump has a height at least twice the height of the remainder of the compliant three-axis force sensor.

20. A compliant three-axis force sensor as described in any one of claims 1 to 5, wherein the at least one three-dimensional bump comprises an array of the three-dimensional bumps.

21. A method for manufacturing a compliant three-axis force sensor according to any one of claims 1 to 5, comprising: providing a first electrode layer having an array of column electrodes in a first orientation; providing a second electrode layer having an array of column electrodes in a second orientation orthogonal to the first orientation; providing a force dependent active layer disposed between the first electrode layer and the second electrode layer, the force dependent active layer configured to change at least one property when subjected to a force; providing at least one three-dimensional bump positioned to transmit an externally applied force through the force-dependent active layer; providing at least one spacer positioned to maintain spacing between two or more layers in the compliant three-axis force sensor until an external force is applied; providing the at least one spacer in the form of a binder material provided between the force-dependent active layer and the first and / or second electrode layer, the binder material being provided in gaps between adjacent column electrodes and extending between the force-dependent active layer and each edge of the adjacent column electrodes, or providing the force-dependent active layer as first and second active layers having a gap between each other and providing the at least one spacer within the force-dependent active layer; A method comprising:

22. 22. The method of claim 21, wherein each of the layers, at least one three-dimensional bump, or spacer is provided by one of printing, screen printing, roll-to-roll printing, inkjet printing, 3D printing, electrospinning, deposition, droplet dispensing, casting, coating, molding, spinning, or weaving.

23. 23. The method according to claim 21 or 22, comprising forming at least the first and second electrode layers on a single substrate, dividing the substrate into two parts, and stacking the two parts.

24. 24. The method of claim 23, comprising forming said at least one spacer on at least one portion prior to said dividing and / or stacking steps.

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