Tactile sensor

Abstract

Embodiments of the present techniques provide a tactile sensor, a method for making a tactile sensor, and a system for using a tactile sensor to obtain information about a contact on the tactile sensor. The tactile sensor comprises a multilayer structure which is flexible and stretchable and may be termed a sensor skin. Electrodes are attached physically and electrically to the edges of the multilayer structure and electrical impedance tomography EIT techniques are used to obtain information from the electrodes to determine a location and other information about a contact on the multilayer structure.

Description

Tactile SensorField

[0001] The present techniques generally relate to a tactile sensor, a method for making a tactile sensor, and a system for using a tactile sensor to obtain information about a contact on the tactile sensor using electrical impedance tomography (EIT). The sensor can be used as a tactile skin for an arbitrary shaped sensing surface.Background

[0002] EIT is a non-invasive, radiation-free technique which has been used for reconstructing the resistivity distribution within a biological tissue. EIT was originally used in the medical field primarily because researchers discovered that each biological tissue exhibits some type of resistance to electrical currents, known as impedance. By reconstructing the impedance distribution within a biological tissue, it was possible to gain insight into its current physiological state. EIT technology has been extended to potential applications in the field of artificial skin as described for example in “Electrical impedance tomography for artificial sensitive robotic skin: A review” by Tawil et al published in IEEE Sensors Journal in 2015. Since artificial skin has properties similar to those of biological tissues, when stimulated by external stimuli, artificial skin exhibits changes in impedance corresponding to its current state.

[0003] EIT works by measuring electrical properties across the body using surface electrode measurements and then reconstructing a tomographic image of that part, like ultrasound imaging. By replacing the human body with a soft conductive material, a sensorised robotic skin can be developed, for example as described in EP4016028.

[0004] Typically, such robotic skins using EIT present advantages over other designs. First, all electronic components and electrodes can be located away from the sensing area. Second, the number of measurements scale to the power of the number of electrodes. In other words, an electronic skin with 128 electrodes can provide a similar number of sensory channels as the human hand (e.g., 128*128= 16384 independent channels). The sensing principle for EIT is to use two electrodes for injecting current and two for measuring the resultant voltages and then alternate between different combinations of electrodes. When the skin comes into contact with an external object, these conductive pathways are affected, and the cause of the deviation can be inferred(e.g., the location and area of the contact). EIT provides a non-invasive, continuum sensing method that does not interfere with the physical interactions between objects and offers relatively low cost and high temporal resolution.

[0005] There are limitations with EIT. For example, as the current flows through the path of least resistance, the sensing surface of the electronic skin is limited in morphology if the electrodes are placed away from the sensing surface. Accordingly, most EIT skins are flat with sensing electrodes placed around the surface. Typically, the electrodes are placed directly on the sensing surface. However, this makes the skin non-stretchable and prone to damage.

[0006] The present applicant has recognised there are still challenges facing the widespread use of EIT in electronic skin architectures, including the morphology of the sensing surface, placement of the electrodes and accuracy and / or sensitivity of the sensing surface.Summary

[0007] In a first approach of the present techniques, there is provided a tactile sensor for sensing a contact, the tactile sensor comprising: a flexible multilayer structure, a plurality of electrodes and an electronic unit for obtaining electrical impedance tomography (EIT) information to analyse a contact on the flexible multilayer structure. The flexible multilayer structure comprises a first layer for receiving a contact; a second layer; and a central layer which is between and is attached to the first and second conductive layers; wherein the central layer is made from a material having a lower conductivity than the material used for the first and second layers. The plurality of electrodes comprise a first set of electrodes which are attached to at least one edge of the first conductive layer, and a second set of electrodes which are attached to at least one edge of the second conductive layer. The electronic unit comprises a current drive circuit for injecting current between a first electrode in the first set of electrodes and a second electrode in the second set of electrodes; a voltage measurement circuit for measuring multiple voltage measurements between multiple pairs of electrodes in the plurality of electrodes while current is being injected between the first and second electrodes, and a control circuit for controlling the current drive circuit and the voltage measurement circuit. When measuring the voltage, each pair of electrodes may comprise an electrode from the first set of electrodes and an electrode from the second set of electrodes. Alternatively, each pair of electrodes may be attached to the samelayer, i.e. a pair of electrodes may comprise two electrodes from the first set of electrodes or two electrodes from the second set of electrodes.

[0008] By attaching the electrodes to the edges of the first and second conductive layers, the electrodes are placed away from the sensing location, namely the location of the contact on the first layer. There are no electronic parts on the multilayer structure and in particular, on the first layer. This allows for the multilayer structure to be flexible, deformable, soft and tactile. As is known in the art, EIT involves passing high frequency low amplitude currents between pairs of electrodes and simultaneously measuring the potentials resulting between all other pairs of electrodes to generate a tomographic image of the multilayer structure. The principles of EIT are described, for example, in “Tactile perception in hydrogel-based robotic skins using data-driven electrical impedance tomography” by Hardman et al published in Materials Today Electronics in 2023. However, unlike the known art, the present technique applies EIT to a multilayer structure.

[0009] The central layer may have a conductivity which is ten times lower, more preferably one hundred times lower, than the conductivity of the first and second layers. This helps to improve the sensitivity of the sensor in detecting information about the contact. For example, each of the first and second layers are made from a material comprising conductive carbon. The central layer may be made from a hydrogel material, for example a gelatin-based hydrogel. The layers may be fixed together using any suitable technique, including insulating transparent tape or using the central layer itself.

[0010] In addition to being flexible, the multilayer structure is preferably stretchable so that it may be used in a variety of applications, e.g. a glove or other wearable for a human or a robot. The flexibility and stretchability may be achieved by appropriate selection of the materials within the multilayer structure. Each of the first layer, the second layer and the central layer have a Youngs Modulus of less than 10MPa. More specifically, the central layer may have a Youngs Modulus of less than 1 MPa. The multilayer structure may have an overall value of Youngs Modulus of less than 10MPa and a maximum strain range between 30-60%.

[0011] The multilayer structure may be any shape to suit the application of the tactile sensor. Each of the first layer, the second layer and the central layer may be coterminous. In other words, each layer may have the same shape and may be aligned with each other. The multilayer structure may be generally circular, rectangular or may have another shape in which there are two opposed sides which are shorter than the other opposed sides. The sides may be straight or curved to form a complex shape. The multilayer structure may have a symmetric or non-symmetric design.

[0012] The arrangement of the electrodes may be selected based on the shape of the multilayer structure and the application for the tactile sensor. For example, when the multilayer structure is circular, the electrodes may be equally spaced around a circumference. In such an arrangement, the electrodes are thus uniformly distributed around the outer edge of the multilayer structure. Alternatively, when the multilayer structure is rectangular or otherwise has two shorter sides, the electrodes may be spaced along the shorter sides, more specifically equally spaced. The resulting arrangement may be considered to form a sensor with non-uniformly distributed electrodes. In other words, the electrodes may be uniformly or non-uniformly distributed and thus the arrangement of the electrodes like the shape of the multilayer structure may be symmetric or non-symmetric.

[0013] Electrode mounting pads may be used to attach the electrodes to the multilayer structure. This may provide a more secure connection to the electronic unit. There may be a first set of electrode mounting pads attaching the first set of electrodes to the first conductive layer and a second set of electrode mounting pads attaching the second set of electrodes to the second conductive layer. The electrode mounting pads may be in the form of strips (e.g. rectangular strips) which extend around the multilayer structure.

[0014] Any suitable arrangement of electrodes may be used. For example, the first set of electrodes may alternate with the second set of electrodes. In other words, any electrode on the first conductive layer is between two electrodes on the second conductive layer and vice versa. Such an arrangement may be easier to manufacture.

[0015] Each of the first layer, the second layer and the central layer may have a constant thickness. The layers may also be the same or different thicknesses to each other. Alternatively, it may be possible to improve sensitivity further by tailoring the thickness of each layer, particularly the central layer. Each of the first layer and the second layer may have a constant thickness and the central layer may have a thickness which decreases towards its centre.

[0016] The fundamental approach of EIT is adapted in the present techniques to connect any number of electrodes to the multilayer structure. By applying a controlled and safe driving alternating current (AC) using the current drive circuit through the layers between pairs of electrodes in the first and second layers, the resulting voltage response of the other electrodes is measured using the voltage measurement circuit. In other words, the electrical current can flow from each electrode attached to the first layer to each electrode attached to the second layer and vice versa. This information is then used to reconstruct the resistivity distribution within the multilayer structure.

[0017] The tactile sensor may provide the measurements to a computing device for analysis. Thus, according to another aspect there is provided a system comprising the tactile sensor described above and a computing device comprising a processor for analysing the multiple voltage measurements to determine information about a contact on the flexible multilayer structure. The processor may use a machine learning model to determine information about the contact. The information may comprise the location of at least one contact on the multilayer structure and / or level of force of the at least one contact.

[0018] According to another aspect, there is provided a method of making a tactile sensor as described above. The method comprises fitting an inner conductive layer into a mould; filling the mould with material; curing the material to form the central layer over the inner conductive layer; removing the inner conductive layer and central layer from the mould; fitting an outer conductive layer over the central layer; and connecting a plurality of electrodes so that a first set of electrodes are connected to the outer conductive layer and a second set of electrodes are connected to the inner conductive layer. The outer conductive layer is the layer on which a contact is made.

[0019] In a related approach of the present techniques, there is provided a non- transitory data carrier carrying processor control code to implement any of the processes and techniques described herein.

[0020] As will be appreciated by one skilled in the art, the present techniques may be embodied as a system, method or computer program product. Accordingly, present techniques may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects.

[0021] Furthermore, the present techniques may take the form of a computer program product embodied in a computer readable medium having computer readable program code embodied thereon. The computer readable medium may be a computer readable signal medium or a computer readable storage medium. A computer readable medium may be, for example, but is not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing.

[0022] Computer program code for carrying out operations of the present techniques may be written in any combination of one or more programming languages, including object-oriented programming languages and conventional procedural programming languages. Code components may be embodied as procedures, methods or the like, and may comprise sub-components which may take the form of instructions or sequences of instructions at any of the levels of abstraction, from the direct machineinstructions of a native instruction set to high-level compiled or interpreted language constructs.

[0023] Embodiments of the present techniques also provide a non-transitory data carrier carrying code which, when implemented on a processor, causes the processor to carry out any of the methods described herein.

[0024] The techniques further provide processor control code to implement the abovedescribed methods, for example on a general purpose computer system or on a digital signal processor (DSP). The techniques also provide a carrier carrying processor control code to, when running, implement any of the above methods, in particular on a non-transitory data carrier. The code may be provided on a carrier such as a disk, a microprocessor, CD- or DVD-ROM, programmed memory such as non-volatile memory (e.g. Flash) or read-only memory (firmware), or on a data carrier such as an optical or electrical signal carrier. Code (and / or data) to implement embodiments of the techniques described herein may comprise source, object or executable code in a conventional programming language (interpreted or compiled) such as C, or assembly code, code for setting up or controlling an ASIC (Application Specific Integrated Circuit) or FPGA (Field Programmable Gate Array), or code for a hardware description language such as Verilog (RTM) or VHDL (Very high speed integrated circuit Hardware Description Language). As the skilled person will appreciate, such code and / or data may be distributed between a plurality of coupled components in communication with one another. The techniques may comprise a controller which includes a microprocessor, working memory and program memory coupled to one or more of the components of the system.

[0025] It will also be clear to one of skill in the art that all or part of a logical method according to embodiments of the present techniques may suitably be embodied in a logic apparatus comprising logic elements to perform the steps of the above-described methods, and that such logic elements may comprise components such as logic gates in, for example a programmable logic array or application-specific integrated circuit. Such a logic arrangement may further be embodied in enabling elements for temporarily or permanently establishing logic structures in such an array or circuit using, for example, a virtual hardware descriptor language, which may be stored and transmitted using fixed or transmittable carrier media.

[0026] In an embodiment, the present techniques may be implemented using multiple processors or control circuits. The present techniques may be adapted to run on, or integrated into, the operating system of an apparatus.

[0027] In an embodiment, the present techniques may be realised in the form of a data carrier having functional data thereon, said functional data comprising functional computer data structures to, when loaded into a computer system or network and operated upon thereby, enable said computer system to perform all the steps of the above-described method.Brief description of the drawings

[0028] Implementations of the present techniques will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0029] Figures 1 a and 1 b are side views of a tactile sensor without and with an external stimulus, respectively;

[0030] Figures 1c and 1d are top views of the tactile sensors of Figures 1a and 1b;

[0031] Figure 2a is a schematic view of the tactile sensor of Figure 1a showing a possible arrangement of electrodes;

[0032] Figures 2b and 2c show current flow through the tactile sensor of Figure 2a when current is injected through two different pairs of electrodes;

[0033] Figures 3a, 4a and 5a illustrate three alternative tactile sensors each having a multi-layer arrangement such as that shown in Figure 1a;

[0034] Figures 3b, 4b and 5b illustrate the location of electrodes on the sensors of Figures 3a, 4a and 5a, respectively;

[0035] Figure 6 is a block diagram of a system incorporating a sensor such as those shown in Figures 1a, 3a, 4a and 5a;

[0036] Figure 7 is a flowchart for using the system of Figure 6;

[0037] Figure 8 is an illustration of an experimental set-up for testing sensors such as those shown in Figures 1a, 3a, 4a and 5a;

[0038] Figures 9a and 9b are graphs plotting voltage difference against electrode combination pattern indices for a single-layer sensor as a test and a multi-layer sensor such as the one shown in Figure 1a;

[0039] Figure 9c shows the location of the three different contact points on the sensors tested using the set-up of Figure 8a;

[0040] Figures 10a and 10c plot the predicted X location against the true X location for the single-layer sensor and the multi-layer sensor respectively;

[0041] Figures 10b and 10d plot the predicted Y location against the true Y location for the single-layer sensor and the multi-layer sensor respectively;

[0042] Figure 11 shows the mean absolute error for the single layer sensor and multilayer sensors as shown in Figures 3a, 4a and 5a;

[0043] Figures 12a to 12d plot the error distribution for the single layer sensor and multi-layer sensors as shown in Figures 3a, 4a and 5a, respectively;

[0044] Figure 13 illustrates a method of fabricating a sensor having the form of a human finger;

[0045] Figure 14a and 14b show the inner part and external parts of the mould which is used to form the sensor using the method of Figure 13;

[0046] Figure 14c shows one conductive sheet for the sensor made using the method of Figure 13; and

[0047] Figures 14d to 14g illustrate steps in the method of fabricating a sensor of Figure 13.Detailed description of the drawings

[0048] Broadly speaking, embodiments of the present techniques provide a tactile sensor, a method for making a tactile sensor, and a system for using a tactile sensor to obtain information about a contact on the tactile sensor. The tactile sensor comprises a multilayer structure which is flexible and stretchable and may be termed a sensor skin. Electrodes are attached physically and electrically to the edges of the multilayer structure and electrical impedance tomography EIT techniques are used to obtain information from the electrodes to determine a location and other information about a contact on the multilayer structure.

[0049] Figures 1a to 1d illustrate a tactile sensor 10 which comprises a multilayer structure comprising a central layer 12 sandwiched between two first and second layers 14a, 14b. The first and second layers thus form a top and bottom layer for the multilayer structure and may also be termed outer layers. The first and second layers 14a, 14b are made from a material which has significantly higher conductivity than the material which is used for the central layer 12. The central layer 12 may thus be considered to be a low conductive layer and the first and second layers 14a, 14b may be considered to be highly conductive layers. Any suitable material for the first and second layers may be used provided it exhibits good electrical conductivity, typically ranging from 100 to 5K Ohms, depending on measurement distance. Similarly, any suitable material for the central layer can be used, provided it exhibits a much lower conductivity (e.g. at least 10 times and ideally 100 times lower) than the outer layers.

[0050] A plurality of electrodes is physically and electrically connected to the multilayer structure. A first set of first electrodes (16a, 16b, ..., 16m, ... 16n) are located on the first conductive layer 14a and a second set of second electrodes (18a, 18b, ..., 18m, ...18n)are located on the second conductive layer 14b. There are typically the same number of electrodes on each conductive layer, and as examples the total number of electrodes may be 8, 16, 32, 64 or even 128 electrodes. As explained in more detail below, the electrodes are used to provide electrical impedance tomography (EIT) across and through the multilayer structure. EIT involves passing high frequency low amplitude currents between pairs of electrodes and simultaneously measuring the potentials resulting between all other pairs of electrodes.

[0051] As an example, the central layer may be made from a hydrogel material (e.g. a gelatin-based hydrogel) and each outer layer may be made from the same material, e.g. carbon black sheets or a fabric coated with conductive carbon such as EconTex ™ Conductive Fabric. In addition to providing the desired conductivity properties, these materials are flexible and stretchable. The desired flexibility may be achieved by using materials which together provide a multi-layer structure having a Youngs Modulus of less than 10MPa. The desired stretchability may be defined as having a maximum strain range between 30 to 60%, for example to mimic the performance of human skin. Together the three layers create a multi-layer soft structure which has some of the properties of human skin. Thus, the sensors may be known as sensor skins and the terms may be used interchangeably.

[0052] To prevent undesired separation of the multiple layers of skin during use, e.g. during experiments, the layers may be fixed together using any suitable technique, including insulating transparent tape or using the central layer itself. For example, for the gelatin-based central layer, heating this layer will cause it to adhere to the adjacent layers at contacting surfaces. The three layers may be attached to or supported on a substrate (not shown). For example, if the three layers are used as a wearable skin for a user, the substrate is human skin. Alternatively, if the three layers are used for robotic applications, the substrate may be the body of a robot which is typically more rigid than human skin.

[0053] As an example for the central layer, the hydrogel layer may be created by mixing gelatin (from pork), glycerol, water, and table salt (NaCI) in a 1 :1.5:2.5:0.1 wt% ratio, following the method described by Hardman et al.

[0015] , After a two-day drying process, the mixture completely solidifies and reaches equilibrium with the humidity of the surrounding environment. The resulting hydrogel layer is bendable, stretchable, and resembles a slightly yellowish, transparent gelatin layer. The normal resistance typically ranges from 100k ohms to 800k ohms, depending on the humidity of the hydrogel.

[0054] The conductivity of the outer layers may be denoted as 01 and the conductivity of the central layer may be denoted as 02. A rough estimate of the material properties used in each layer is shown in the table below.

[0055] The total resistance of our multilayered sensor may be approximately 160k ohms, with the exact measurement depending on the specific electrodes used. The overall resistance Rtbetween two electrodes can be written as:Rt=Ri + R2 + R3R2=^2 / a2R3=3 / o±A where Rtis the resistance in each of the layers and Ltis the thickness of each layer, A is the area of each layer.

[0056] Assuming the area of the three layers and the width of each layer is comparable, we can see that R2» Ri=Rs- When the skin is pressed, L1;L2, and L3decreased and the magnitude of the change will depend on the stiffness of the layers and the stiffness of any base substrate to which the three layers are attached. Nonetheless, now this contact location has a smaller value of each of the widths of the layers, i.e. each of L1;L2, and L3and as this change in length is amplified by the inverse of the conductivity to determine the overall resistance, the change in overall resistance ARtis mostly determined by the middle layer. This may be expressed as:ARf = A / ?x+ AR2+ AR3R2» RI=R3SOARt=AR2

[0057] This decrease in resistance at the location of the applied stimulus changes the pathways of the currents between pairs of electrodes as shown in Figures 1a to 1d. Figure 1a shows a cross-sectional view of the tactile sensor in which four current paths 20, 22, 24, 26 through the tactile sensor are illustrated. The current paths may also be termed the injection paths. Figure 1c schematically shows the same when a section is viewed from above the tactile sensor. Although the paths are passing through allthree layers and not just through the upper layer 14a, the complete path is shown for simplicity. Similarly, for simplicity, only a segment of the upper layer 14a is shown. The principle of EIT is to alternate between different combinations of electrodes and thus it will be appreciated that there will be more pathways than the four shown for illustrative purposes. Similarly, it will also be appreciated that there are multiple paths for measuring the voltage between two electrodes, which may be in the same layer or in different layers.

[0058] There are two current paths 20, 26 between adjacent or neighbouring electrodes on the first and second layers. It is noted that although the adjacent electrodes appear to be aligned vertically in Figure 1a, they are offset horizontally as shown in Figure 1c. One current path 20 is from a first electrode 16a on the first layer to the adjacent first electrode 18a on the second layer and the other current path 26 is between the mth electrode 16m on the first layer and the adjacent mth electrode 18m on the second layer. Both of these current paths are relatively short curves. Figure 1a also shows two current paths 22, 24 between opposite electrodes on the first and second layers. One current path 24 is from the first electrode 16a on the first layer to the opposite electrode 18m on the second layer and the other path 22 is from the first electrode 18a on the second layerto the opposite electrode 16m on the first layer. In this example, the design is symmetric and thus each path 22, 24 between opposed electrodes is generally S shaped with the centre of the S aligned centrally with the width and height of the multilayer structure. All the paths 20, 22, 24, 26 pass through all three layers of the multilayer structure.

[0059] In both Figures 1a and 1c, there is no contact on the multilayer structure. Figures 1 b and 1 d show the changes to the current paths in Figures 1 a and 1 c when an external stimulation in the form of a contact 28 is applied to an outer surface of the first layer 14a. The outer surface is the opposite surface to that which is adjacent the central layer and may also be termed an upper layer. In Figures 1 b and 1 d, the four paths are labelled 20a, 22a, 24a and 26a. Paths 20a and 26a are the paths between adjacent electrodes and paths 22a and 24a are the paths between opposite electrodes.

[0060] Figures 1a to 1d show that conductive pathways are directed to the region in which the stimulus is applied depending on their vicinity to the electrodes. Thus, the relatively short paths 20, 26 shown in Figures 1a and 1c are much longer paths 20a, 26a in Figures 1 b and 1d. For the S-shaped paths 22, 24, the centre of the S-shape is drawn towards the location of the stimulus as shown by the changed paths 22a, 24a in Figures 1 b and 1d. The current flows between pairs of electrodes and follows the path of least resistance.

[0061] By tuning the shape and thickness of the conductive layers, we can tune the sensitivity over different regions for tactile cues. This allows us to place the electrodes distally and have any sensing shape. In other words, by changing the morphology of the conducting material along each layer, complex current paths can be obtained, enabling various sensor shapes and electrode placements. A learning-based approach may be used to model and / or calibrate the sensor.

[0062] The fundamental approach of EIT is adapted in the present techniques to connect any number of electrodes to the multilayer structure. As shown in Figure 2, the multilayer structure may have a circular shape and the first set of electrodes 16a, 16b, ..., 16m, ...16n on the first layer may be arranged in an alternating pattern with the second set of electrodes 18a, 18b, ..., 18m, ...18n on the second layer. In other words, any electrode (e.g. 16b) on the first conductive layer is between two electrodes (18a, 18b) on the second conductive layer. In other words, the electrodes are alternately placed in the first and second layers. Other patterns of electrodes may be used. The dotted line illustrates the section which is shown in Figures 1c and 1d.

[0063] By applying a controlled and safe driving alternating current (AC) through the layers between pairs of electrodes in the first and second layers, the resulting voltage response of the other electrodes is measured. In other words, the electrical current can flow from each electrode attached to the first layer to each electrode attached to the second layer and vice versa. This information is then used to reconstruct the resistivity distribution within the multilayer structure. Although the enhancement of the EIT resolution can be achieved by increasing the number of electrodes, Figure 2a shows a common configuration of 16 electrodes due to the limitations of the measurement space. Other numbers of electrodes may be used, for example 8, 16, 32, 64 or even 128 electrodes.

[0064] There are several methods for injecting current in EIT. The electrodes in Figure 2a may be used to inject current using pairs of adjacent electrodes (e.g. 16a, 18a) or pairs of opposite electrodes (e.g. 16a, 16m) or using another method (e.g. the trigonometric method). The adjacent injection method means that two electrodes which inject the driving current are adjacent to each other, and then measure the relative voltages between the other electrodes. Similarly, the opposite injection methods means that two electrodes which inject the driving current are generally opposite to each other, and then measure the relative voltages between the other electrodes. The adjacent method is the most commonly used method for EIT and is illustrated in Figures 2b and 2c.

[0065] Figure 2b shows a pair of injection electrodes comprising a first electrode in the first layer (numbered 1) and a first electrode in the second layer (numbered 2). Current flows through the multilayer structure to each of the other fourteen electrodes as illustrated by the lines connecting the electrodes. The equipotential lines are also shown with arrows. Voltage measurements between multiple pairs of electrodes can then be taken, e.g. between a second electrode in the first layer (numbered 3) and a second electrode in the second layer (numbered 4) or between the second electrode in the second layer (numbered 4) and a third electrode in the first layer (numbered 5) and so on. The voltage measurements are used to build a resistivity distribution within the multilayer structure. The voltage measurements may also be taken between electrodes in the same layer, e.g., between the second electrode in the first layer (numbered 3) and the third electrode in the first layer (numbered 5).

[0066] Figure 2c shows a similar arrangement to that of Figure 2b except that the pair of injection electrodes comprise the second electrode in the first layer (numbered 3) and a first electrode in the second layer (numbered 2). As in Figure 2b, current flows through the multilayer structure to each of the other fourteen electrodes as illustrated by the lines connecting the electrodes and the equipotential lines are shown. Voltage measurements between multiple pairs of electrodes can then be taken, e.g. between a second electrode in the second layer (numbered 4) and a third electrode in the first layer (numbered 5) and so on. The voltage measurements may be taken between a first electrode in the first layer and a second electrode in the second layer. Alternatively, or additionally, the voltage measurements may be taken between a pair of electrodes in the first layer. Alternatively, or additionally, the voltage measurements may be taken between a pair of electrodes in the second layer.

[0067] The performance of the EIT is also affected by the frequency of the injected current, which depends on the conductivity and dielectric constant of the analyzed medium. As with standard EIT applications, we use the standard measurement frequency which is 50 kHz, as well as frequencies ranging from 10 kHz to 100 kHz depending on the medium’s physical properties.

[0068] Figure 3a illustrates an alternatively shaped tactile sensor 30. In this arrangement, the multilayer structure forms a central, circular area 32 from which electrode mounting pads 34a, ... ,34n in the form of rectangular strips extend radially and are equally spaced around the central area. Merely as example, the overall diameter of the central area may be 110 mm. As in the embodiment of Figures 1a to 1 d, there are three layers and the material used to form each layer in this arrangement is the same as described above. Each electrode is mounted to an electrode mountingpad and as in the arrangement of Figures 1a to 1d, the electrodes are alternately connected to the first and second conductive layers. As shown in Figure 3b, there are thus 16 electrodes which are equally spaced in a circular arrangement. The electrodes are thus uniformly distributed around the outer edge of the sensor 30.

[0069] Figure 4a illustrates an alternatively shaped tactile sensor 40 with a symmetrical design. In this arrangement, there is a central, rectangular area 42 from which electrode mounting pads 44a, ... ,44n in the form of rectangular strips extend from opposed edges. There are an equal number of mounting pads on each opposed edge and they are equally spaced along the edge. The resulting arrangement may be considered to form a rectangular sensor with non-uniformly distributed electrodes. Merely, as an illustrative example, the rectangular central area measures 100 x 80 mm. The dotted line illustrates the section which is shown in Figures 1c and 1d. As shown in Figure 4b, the electrodes (1 , 3, 5...) which are connected to the mounting pads on one side of the central portion are connected to the first conductive layer and the electrodes (2, 4, 6, ... ) which are connected to the mounting pads on the opposed side of the central portion are connected to the second conductive layer.

[0070] Figure 5a illustrates an alternatively shaped tactile sensor 50. In this arrangement, there is a central, curved shaped area 52 from which electrode mounting pads 52a, ... ,52n in the form of rectangular strips extend from opposed curved edges. There are an equal number of mounting pads on each opposed edge, but in this arrangement they are not equally spaced along the edge. The resulting arrangement may be considered to form irregularly shaped sensor with non-uniformly distributed electrodes. Merely, as an illustrative example, the irregularly shaped central portion has a transverse length of 130 mm, a maximum longitudinal height of 75 mm, and a minimum longitudinal width of 45 mm. As shown in Figure 5b, the electrodes (1 , 3, 5...) which are connected to the mounting pads on one side of the central portion are connected to the first conductive layer and the electrodes (2, 4, 6, ...) which are connected to the mounting pads on the opposed side of the central portion are connected to the second conductive layer.

[0071] In each of the arrangements of Figures 3a to 5a, the use of the rectangular strips, which may also be termed legs, ensures better electrode placement on the respective conductive layer. This helps to provide a more secure connection to the EIT board. Additionally, sectors may be used around each electrode to isolate the 16 electrodes on the conductive sheet. The connection and isolation of the electrodes may be carried out before assembling the three layers to prevent short circuits resulting from different combinations due to the high conductivity of the sheets.

[0072] As shown in Figure 6, the sensor 60 forms part of a system for determining a location of a contact point on the multilayer structure. The sensor 60 comprises the multilayer structure 64 and each of the electrodes 62a, 62b, ... , 62n connected to the multilayer structure 64. Each of the electrodes 62a, 62b, ... , 62n are also connected to an electronic unit 70 (which may also be termed an EIT motherboard). Any suitable electronic unit may be used such as the one described in “EIT-kit: An Electrical Impedance Tomography Toolkit for Health and Motion Sensing” by Zhu et al published in HCI engineering in IISIT 21.

[0073] The electronic unit 70 comprises a current drive circuit 74 which is used to inject AC current into pairs of electrodes. The current drive circuit 74 comprises the standard components such as a signal generator, an adjustable amplifier and a voltage- controlled current source so that a small, constant-amplitude, differential sine waveform at the desired injection frequency can be generated. The electronic unit 70 also comprises a voltage measurement circuit 76 for measuring the voltage between pairs of electrodes through which current is not being injected. The voltage measurement circuit 76 comprises the standard components such as two input buffers, an adjustable instrumentation amplifier, and an analog-to-digital converter (ADC). The electronic unit 70 also comprises a control circuit 72 which controls both the current drive circuit 74 and the voltage measurement circuit 76.

[0074] The electronic unit 70 comprises a multiplexer board 78 with several multiplexers. For example, there may be four multiplexer each of which is a 32:1 analog multiplexer. Each source pin (e.g. one of the 32 in the example of a 32:1 multiplexer) is connected to an electrode. Each drain pin (e.g. the one pin for the 32:1 multiplexer) is attached to either the current drive circuit 74 or the voltage measurement circuit 76. More specifically when there are four multiplexers, the four drain pins may be connected to the current drive positive, the current drive negative, the voltage measurement positive and the voltage measurement negative respectively. By using the multiplexer, each of the electrodes may be connected to any one of the drain pins. Thus, electrodes can be switched between injection electrodes and measurement electrodes. Each pair of injection electrodes has an electrode in each layer. Each pair of measurement electrodes may have an electrode in each layer or may have two electrodes from the same layer.

[0075] The electronic unit 70 may comprise other standard components which are not shown, e.g. memory and / or an input / output interface for transmitting data. For each data collection cycle, the electronic unit 70 may collect multiple sets (e.g. 256) of voltage measurement data, including the electrode used as the current source.However, the measurements of the electrodes that serve as the current source are set to 0. These measurements may be stored on the electronic unit 70 and may be processed on the electronic unit. However, the processing is normally done in a separate computer 80 which may be connected to the electronic unit in any suitable way, e.g. via a wired or wireless connection or via the Internet.

[0076] The computer 80 may be any suitable computing device. Figure 6 shows some of the components of the computer 80 and it will be appreciated that they may additionally be other standard components which are not shown. The computer 80 comprises at least one processor 82 coupled to memory 84. The at least one processor 82 may comprise one or more of: a microprocessor, a microcontroller, and an integrated circuit. The memory 84 may comprise volatile memory, such as random access memory (RAM), for use as temporary memory, and / or non-volatile memory such as Flash, read only memory (ROM), or electrically erasable programmable ROM (EEPROM), for storing data, programs, or instructions, for example. The computer 80 typically comprises at least an input / output interface 86 for a user to input instructions and / or receive information from the electronic unit 70. The at least one input / output interface 86 may take any appropriate form, e.g. a keyboard, a mouse, a touchpad or other input device for inputting instructions from the user and / or a display or other output device for providing the results and / or data generated during the method described below. The computer 80 also comprises a machine learning, ML, model 88 which is used to process the received information to determine the nature and position of a contact on the multilayer structure.

[0077] Figure 7 shows a flowchart for using the system of Figure 6. Initially there is a calibration phase for each sensor and as shown a learning-based approach is used. The model to be trained may be a simple neural network which maps the impedance measurements to the contact location. The hidden layer size may be set to any suitable value, e.g. to 400 neurons. In a first step S700, training data which is used for training, validating and testing the model is collected. Such training data may be collected using any suitable technique, for example the experimental set-up of Figure 8. The collected data is divided into training, validation, and test datasets in a ratio of 80: 10: 10. When there are 16 electrodes in the sensor, the input data for the neural network has a dimension of 416. It is generated by taking the difference between the impedance measurements before and after probing, eliminating any instances with an intermediate value of 0.

[0078] At step S702, the model is trained using any suitable technique to generate the output. For example, MATLAB deep learning toolbox may be used for training. Oneoutput of the model is a two-dimensional variable representing the coordinates of each contact location (e.g. a probe in the experimental set-up or multiple probes) in the x and y directions. Other outputs may be a level of force which is detected at each contact location. For example, the probe(s) may be used to apply known forces and measurements of voltage changes collected as training data. In this case, a regression model or other training technique may be used to map voltage changes to the applied forces. Similarly any tactile cues that affect local conductivity can be inferred. These include, but are not limited to temperature changes, shear and normal forces, multicontact detection, human touch detection, etc. Once the model is trained to output all the desired outputs, there is then a validation step S704 to validate the model and there may also be a testing step (not shown).

[0079] Once the model is calibrated and trained, it can be used to determine the location of a contact on the sensor and / or other information about the contact. During use, or inference, the new input data from the electrodes will be collected as indicated at step S706. The collected input data is then input to the trained model and a location of any contact on the sensor can be determined at step S708. If a contact is detected by analysing the collected input data, the determined location is output at step S710. The determined location can then be used for any suitable output, e.g. if the sensor is incorporated in a glove, the glove may be controlled to close when a contact is detected, or if the sensor is incorporated in a robotic gripper, the contact location(s) can be used to infer the object shape and / or pose to control the closing of the gripper to apply an appropriate amount and type of grasp to hold the object.

[0080] Figure 8 is an arrangement of an experimental set-up for testing a sensor 82 which comprises a plurality of electrodes 84 as described above. During the entire experiment, we used the Universal Robots UR5 robotic arm 86 to calibrate and characterize the three different shapes of the multi-layer soft tactile sensor (e.g. as shown in Figures 3a to 5a). This involved attaching a 3D printed polylactic acid (PLA) probing tip 88 to the robotic arm 86. In this example, the probing tip 88 has a diameter of 5 mm and a depth of 15 mm. We programmed the robotic arm to randomly select a target point within the skin region for each stimulation. For each probe, we captured the EIT output signals from the multi-layer sensor before and after the probe, simultaneously recording the corresponding location for each skin stimulation.

[0081] For preliminary investigation of the sensor, a probe test at three different locations is done and the results are shown in Figures 9a and 9b which plot voltage difference against electrode combination pattern indices. Figure 9c shows the locationof the three different locations. These tests involved collecting the raw change in voltage data before and after probing using the multi-layer sensor described above in Figure 3a and as a comparison a sensor having the same shape and same non- conductive material as a single layer. The results for testing the comparison sensor are shown in Figure 9a and the results for the multi-layer sensor described above are shown in Figure 9b.

[0082] In the case of single-layer sensor tests shown in Figure 9a, the minimum to maximum voltage differences ranged from -0.03 to 0.06 V. However, in the multi-layer sensor tests shown in Figure 9a, the minimum to maximum voltage differences ranged from -0.28 to 0.45 V, showing higher sensitivity. When considering the multilayer sensor tests, we observed that the amplitude generated at point C was smaller than at points A and B. Nevertheless, point C activated in multiple directions. On the other hand, the peak amplitudes produced at points A and B, located at the edges, were greater than at point C, but they activated fewer indices. This property enables our multi-layered sensor to respond uniquely to external stimuli at different locations, leading to better accuracy and sensitivity.

[0083] We also conducted a comparative analysis of prediction errors in the X and Y directions for single-layer and multi-layer sensor with the same shape using the learned model. For different skin shapes, we determined the appropriate EIT parameters, including the frequency of the injected alternating current (AC), the number of signal periods (NUM PERIODS), and the number of analog-to-digital converter (ADC) samples to average for each analog reading (ADC AVG). These parameters were determined to achieve optimal EIT performance, and the corresponding values are listed in the table below. The table also illustrates the experimental data collected for each tested skin.

[0084] Figures 10a and 10c plot the predicted X location against the true X location for the single-layer sensor and the multi-layer sensor respectively. Figures 10b and 10d plot the predicted Y location against the true Y location for the single-layer sensor and the multi-layer sensor respectively. A clear linear relationship between the ground truth and predicted values is evident in both X and Y directions for the multi-layer sensor(Figures 10b and 10d). This indicates a significantly stronger correlation compared to the case of the single-layer skin, where the data distribution is more scattered. The reason for this difference is the low sensitivity of the single-layer sensor when probed, attributed to minimal changes in impedance. In contrast, the multi-layer sensors of the present disclosure exhibit higher sensitivity due to substantial changes in impedance, mainly driven by the cross-sectional thickness of the non-conductive layer, which is significantly affected when probed.

[0085] Figure 11 shows the mean absolute error for all the four different tested sensors (single layer sensor and multi-layer sensors as shown in Figures 3a, 4a and 5a). All three different shapes of multi-layered sensor exhibit better accuracy compared to the single-layered sensor. Among them, the circular multi-layer soft sensor of Figure 3a performs the best, followed by the irregularly shaped skin of Figure 5a and the rectangular sensor of Figure 4a. Notably, the circular multi-layer soft sensor of Figure 3a reduces the mean absolute error from 15.8 mm to 4.4 mm, which significantly improves accuracy in predicting external stimuli. While the accuracy of rectangular and irregularly shaped sensors is not as high as that of the circular-shaped sensor, they still exhibit better accuracy compared to the single-layer comparison EIT sensor.

[0086] The lower accuracy of rectangular and irregularly shaped sensors, in contrast to the circular skin, can be mainly attributed to differences in electrode placement and the suboptimal shape of the sensing surface. In the circular sensor, electrodes are evenly distributed, while in the rectangular and irregularly shaped sensors, they have a non-uniform distribution. This is also why the localization error is higher in the Y direction than the X direction for both cases. Nonetheless, because of the multilayer architecture, we are still able to obtain good localization accuracy even for these complex structures.

[0087] Figures 12a to 12d plot the error distribution for the four tested sensor. Figure 12a shows the results for the single-layer circular shape. Figure 12b shows the results for the multi-layer circular shape of Figure 3a. Figure 12c shows the results for the multilayer rectangular shape of Figure 4a. Figure 12d shows the results for the multi-layer irregular shape of Figure 5a. Each of Figures 12a to 12d provides a more detailed visualization of the error distribution for each tested sensor.

[0088] The results show that a multi-layer EIT-based soft sensor may be used successfully for distributed high-density touch localization. Three distinct shapes for this multi-layer structured sensor have been proposed and experimentally compared to a single-layer architecture. The experiments demonstrated that our multi-layer soft sensor outperforms a single-layer hydrogel sensor in sensing external stimuli, evenwhen the electrodes are non-uniformly distributed. This provides us with greater flexibility in the design and deployment of our soft tactile skins without compromising localization accuracy.

[0089] The next extension of this work is to expand the multi-layer design for more complex sensing surfaces with even more restrictive electrode placement. One such example would be a tactile glove that can be used to track human interactions with the environment. In this scenario, electrodes have to be placed near the wrist to avoid interfering with user motions while still achieving high localization accuracy across the sensing surface. The insights gained from this exploration could have practical applications in the development of bionic hands, offering an approach to replicate human-like tactile perception in robotic systems.

[0090] Figure 13 illustrates a method of fabricating a sensor having the form of a human finger but it will be appreciated that any shape can be made using the appropriate moulds and shapes of the non-conductive layers. In a first step S1300, the mould(s) are obtained and for the example of a finger shaped sensor, moulds such as those shown in Figures 14a and 14b can be used. Figure 14a shows the inner part of the mould which is in the form of a spindle and Figure 14b, one half of the external part of the mould which is curved to form the finger shape.

[0091] The first step S1300 of Figure 13 also involves obtaining the desired shape for the conductive layers. As before, each conductive layer may be made from carbon or a similar material. The conductive material is typically available as a flat sheet and this flat sheet can be cut to the desired shape. In this example, as shown in Figure 14c is in the form of a star with eight points on which electrodes can be attached.

[0092] At step S1302, the first conductive layer (i.e. the inner conductive layer) is fit into the mould. In other words, the first conductive layer is attached to (e.g. wrapped around) the inner part of the mould as shown in Figure 14d. The external parts of the mould are then attached as shown in Figure 14e. Once the mould is assembled, at step S1304, the mould can be filled with the non-conductive material so that the first conductive layer is covered with the non-conductive material. As before the non-conductive material may be a hydrogel. Once the mould is filled, the non-conductive material is cured at step S1306, and the first conductive layer and the non-conductive layer can be removed from the mould at step S1308.

[0093] In step S1310, the second conductive layer which is the same shape as the first conductive layer is then attached over the non-conductive layer to form the sensor having three layers as described above. In this arrangement, as shown in Figure 14f, the first and second conductive layers are offset from each other so that there are 16projections to which electrodes can be connected. In a final step S1312, the electrodes are connected to the conductive layers and the electrodes are also connected to the electronic unit as shown in Figure 14g. As described above, half are attached to the first conductive layer and half are attached to the second conductive layer in an alternating arrangement. The electrodes are also attached to the electronic unit.

[0094] At least some of the example embodiments described herein may be constructed, partially or wholly, using dedicated special-purpose hardware. Terms such as ‘component’, ‘module’ or ‘unit’ used herein may include, but are not limited to, a hardware device, such as circuitry in the form of discrete or integrated components, a Field Programmable Gate Array (FPGA) or Application Specific Integrated Circuit (ASIC), which performs certain tasks or provides the associated functionality. In some embodiments, the described elements may be configured to reside on a tangible, persistent, addressable storage medium and may be configured to execute on one or more processors. These functional elements may in some embodiments include, by way of example, components, such as software components, object-oriented software components, class components and task components, processes, functions, attributes, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuitry, data, databases, data structures, tables, arrays, and variables. Although the example embodiments have been described with reference to the components, modules and units discussed herein, such functional elements may be combined into fewer elements or separated into additional elements. Various combinations of optional features have been described herein, and it will be appreciated that described features may be combined in any suitable combination. In particular, the features of any one example embodiment may be combined with features of any other embodiment, as appropriate, except where such combinations are mutually exclusive. Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of others.

[0095] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[0096] Although a few preferred embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all ofthe steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

Claims

CLAIMS1. A tactile sensor for sensing a contact, the tactile sensor comprising: a flexible multilayer structure comprising: a first layer for receiving a contact; a second layer; and a central layer which is between and is attached to the first and second conductive layers; wherein the central layer is made from a material having a lower conductivity than the material used for the first and second layers; a plurality of electrodes comprising: a first set of electrodes which are attached to at least one edge of the first conductive layer, and a second set of electrodes which are attached to at least one edge of the second conductive layer, and an electronic unit for obtaining electrical impedance tomography information to analyse a contact on the flexible multilayer structure, the electronic unit comprising a current drive circuit for injecting current between a first electrode in the first set of electrodes and a second electrode in the second set of electrodes; a voltage measurement circuit for measuring multiple voltage measurements between multiple pairs of electrodes in the plurality of electrodes while current is being injected between the first and second electrodes, and a control circuit for controlling the current drive circuit and the voltage measurement circuit.

2. The tactile sensor of claim 1 , wherein the central layer has a conductivity which is ten times lower, more preferably one hundred times lower, than the conductivity of the first and second layers.

3. The tactile sensor of claim 1 or claim 2, wherein each of the first and second layers are made from a material comprising conductive carbon.

4. The tactile sensor of any one of the preceding claims, wherein the central layer is made from a hydrogel material.

5. The tactile sensor of any one of the preceding claims, wherein each of the first layer, the second layer and the central layer have a Youngs Modulus of less than 10MPa.

6. The tactile sensor of claim 5, wherein the central layer has a Youngs Modulus of less than 1 MPa.

7. The tactile sensor of any one of the preceding claims, wherein each of the first layer, the second layer and the central layer are coterminous.

8. The tactile sensor of any one of the preceding claims, wherein the multilayer structure is generally circular.

9. The tactile sensor of claim 8, wherein the plurality of electrodes are equally spaced around a circumference of the multilayer structure.

10. The tactile sensor of any one of the preceding claims, wherein the multilayer structure has two opposed sides which are shorter than the other opposed sides.

11. The tactile sensor of claim 10, wherein the plurality of electrodes are equally spaced along the shorter sides of the multilayer structure.

12. The tactile sensor of claim 10 or claim 11 , wherein the sides are curved.

13. The tactile sensor of any one of the preceding claims, further comprising a first set of electrode mounting pads attaching the first set of electrodes to the first conductive layer and a second set of electrode mounting pads attaching the second set of electrodes to the second conductive layer.

14. The tactile sensor of claim 13, wherein the first and second sets of electrode mounting pads are in the form of strips which extend around the multilayer structure.

15. The tactile sensor of any one of the preceding claims, wherein the plurality of electrodes are arranged with the first set of electrodes alternating with the second set of electrodes.

16. The tactile sensor of any one of the preceding claims, wherein each of the first layer, the second layer and the central layer have a constant thickness.

17. The tactile sensor of any one of claims 1 to 15, wherein each of the first layer and the second layer have a constant thickness and the central layer has a thickness which decreases towards its centre.

18. The tactile sensor of any one of the preceding claims, wherein each of the multiple pairs of electrodes for measuring voltage measurements comprises an electrode from the first set of electrodes and an electrode from the second set of electrodes or two electrodes from the first set of electrodes or two electrodes from the second set of electrodes.

19. A system comprising a tactile sensor according to any one of claims 1 to 18, and a computing device comprising a processor for processing the multiple voltage measurements to determine information about a contact on the flexible multilayer structure.

20. The system of claim 19, wherein the processor uses a machine learning model to determine information about the contact.21 . The system of claim 20, wherein the information comprises the location of at least one contact on the multilayer structure and / or level of force of the at least one contact.

22. A method of making a tactile sensor of any one of claims 1 to 18, the method comprising fitting an inner conductive layer into a mould; filling the mould with material; curing the material to form the central layer over the inner conductive layer; removing the inner conductive layer and central layer from the mould; fitting an outer conductive layer over the central layer; and connecting a plurality of electrodes so that a first set of electrodes are connected to the outer conductive layer and a second set of electrodes are connected to the inner conductive layer.

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