Electroadhesive grip device

Abstract

An electroadhesive device (1) configured for adhering to a surface of an object to be retained, said device (1) comprising a first insulating substrate (2) having a first relative electric permittivity value, a second insulating substrate (3) having a second relative electric permittivity value and having a surface (4) suitable for interfacing with the surface of the object (8) to be retained, at least one pair of electrodes (5) arranged between the first and second substrates (2,3), having a first resistivity value, and configured for generating a voltage difference in the substrate (3), wherein said voltage difference generates an electric field propagating to the surface (4) and interacting with the electric charges of the object (8), so as to generate, between the device (1) and the object (8), an electrostatic adhesion force capable of maintaining a current position of the device (1) relative to the object (8), a third substrate (6) arranged between the first and second insulating substrates (2,3) and in contact with the at least one pair of electrodes (5), so as to reduce the electric field concentration in the zones of electric field discontinuity on the interface surfaces between the at least one pair of electrodes (5) and the insulating substrates (2,3), said third substrate comprising a semi conductive material having a second resistivity value not smaller than the first resistivity value of the at least one pair of electrodes (5), or an insulating material having a third relative electric permittivity value.

Description

[0001] 26 November 2024

[0002] TRANSLATION (RULE 12.3) i

[0003] ELECTROADHESIVE GRIP DEVICE

[0004] DESCRIPTION

[0005] [FIELD OF THE INVENTION]

[0006] The present invention relates to devices capable of providing electrically controllable adhesion. More particularly, the present invention relates to an electroadhesive gripping device capable of using electric energy and electrostatic forces in order to exert attraction forces and obtain adhesion between two objects.

[0007] [PRIOR ART]

[0008] Electroadhesive gripping devices are gripping systems exploiting electrostatic attraction forces to adhere to the surface of an object to be retained made of any material (e.g. dielectric or conductive, rigid or soft, dense or porous, etc.) and having any shape (e.g. solids, films, powders, liquids, etc.) and any size.

[0009] From an architectural viewpoint, two macro categories of said devices are known in the art: monopolar devices and bipolar devices.

[0010] Monopolar devices are based on the concept of the capacitor with parallel flat plates, and generally consist of an electrode, to which a high electric voltage is applied, and a layer of insulating material, which separates the electrode of the device from the object to be retained. The gripping action is generated when at least a portion of the object to be retained is given an electric potential which is different from that of the electrode of the electroadhesive device.

[0011] On the other hand, bipolar devices exploit the marginal electric field, and generally consist of at least one pair of electrodes subjected to different electric potentials, completely buried in a layer of insulating material. An arrangement consisting of a plurality of pairs of electrodes with conductive elements arranged alternately and connected to two different electric potentials is referred to as “interdigitated”.

[0012] Fig. 1 shows a generic bipolar electroadhesive gripping device 1 according to the prior art, which comprises at least one pair of electrodes 5 buried in an insulating layer 23. Due to manufacturing requirements, the insulating layer 23 is actually divided into a first insulating substrate 2, the purpose of which is to electrically insulate the electrodes 5 from each other and from the surrounding environment, and a second substrate of insulating material 3, a surface 4 of which interfaces with the surface of an object 8 to be retained. Optionally, the device shown in Fig. 1 may also comprise a support layer 7 arranged on the outer surface of the device, opposite the interface surface 4. Said support layer 7 may have one or more functions including: structural functions (e.g. supporting, stiffening, increasing ductility under compression to obtain better pliability, etc.), secondary-actuation functions (e.g. vibration generator facilitating the detachment of the device 1 from the object 8, movement actuator facilitating the contact with the object 8 to be retained, etc.), connection functions (e.g. when installed, e.g. by threading, permanent glue, etc., in machinery configured to pick up, manipulate or transport objects, in brakes or clutches to generate a braking or engaging torque, or in climbing equipment to increase traction, etc.)

[0013] Fig. 1 also shows the interface zones 9a, 9b, where electric field concentration phenomena occur which may cause partial discharge, scintillation or failure, thus reducing the reliability and service life of the electroadhesive device 1.

[0014] Electric field concentrations occur in proximity to the electrodes 5 and are caused by an excessive electric potential variation in the discontinuity regions between the conductive material of the electrode 5 and the dielectric material of the insulator 2,3. It is therefore essential to be able to reduce electric field concentrations and peaks on the interface surfaces between the electrodes 5 and the insulator 2,3, and more specifically on the mutually facing sides of adjacent electrodes 5 in the interface region 9a where the electric field is highest, as well as at the triple points in the interface region of the metal-insulator-insulator junction 9b.

[0015] Said concentrations are a cause for partial discharges, scintillation and failures, which impair the reliability and life of the electroadhesive gripping device.

[0016] All other conditions being equal, a reduction in electric field concentrations makes it possible to obtain a longer-lasting device 1 as well as a better gripping action, because the potential difference across the electrodes 5 can be increased, resulting in more gripping force.

[0017] Controlled adhesion can be said to be successful when a technology is used which is controllable, reliable and sufficiently strong to be effective on a plurality of different surfaces in a real-world scenario, where environmental conditions are not always optimal (e.g. wet, dusty, slippery conditions).

[0018] Although good gripping control has already been achieved, reliability is still a requirement that has not yet been completely fulfilled, mostly because of the phenomenon of electric field concentration near the electrodes, particularly in the zones of electric field discontinuity on the interface surfaces between the electrodes and the insulating material.

[0019] The solutions currently available are focused on optimizing the geometry of the electroadhesive device 1 in order to find a proper balance between the width and gap of the electrodes 5 and the distance between the electrodes 5 and the object 8 to be retained.

[0020] Many techniques are known which are aimed at optimizing the geometry of the electroadhesive device in order to find the best trade-off between the width and gap of the electrodes and between the distance between the electrodes and the object on which a gripping force is to be exerted.

[0021] However, such techniques provide no appreciable results because of the abovedescribed problem.

[0022] [OBJECTS AND SUMMARY OF THE INVENTION]

[0023] It is one object of the present invention to provide an electroadhesive gripping device that solves, at least partly, those problems which impair the reliability and life of the electroadhesive device, and which cannot be overcome by the current solutions focused on optimizing the geometry of the device.

[0024] It is a further object of the present invention to provide an electroadhesive gripping device that reduces the electric field concentration on the interface surfaces between electrode and insulating material.

[0025] It is a further object of the present invention to provide an electroadhesive gripping device that mitigates the field emission phenomenon, i.e. migration of electrons from the conductive material of the electrodes to the insulating materials of the adjacent substrates.

[0026] It is a further object of the present invention to provide an electroadhesive gripping device that reduces the concentration of electric potential in proximity to the surfaces of the electrodes in favour of the regions above said electrodes, which affect the gripping force exerted on an object to be retained.

[0027] It is a further object of the present invention to provide an electroadhesive gripping device that promotes self-regeneration of the device’s materials following a localized dielectric breakdown event.

[0028] More particularly, in order to overcome the above-defined problems affecting the reliability and life of the device due to phenomena of partial discharge, scintillation and failure caused by electric field discontinuity on the electrode-insulator interface surfaces, the device of the present invention comprises: a first insulating substrate having a first relative electric permittivity value; a second insulating substrate having a second relative electric permittivity value and having a surface suitable for interfacing with the surface of the object to be retained; at least one pair of electrodes arranged between the first and second substrates, having a first resistivity value, and configured for generating a voltage difference in the substrate, wherein said voltage difference generates an electric field propagating to the surface and interacting with the electric charges of the object, so as to generate, between the device and the object, an electrostatic adhesion force capable of maintaining a current position of the device relative to the object; a third substrate arranged between the first and second insulating substrates and in contact with the at least one pair of electrodes, so as to reduce the electric field concentration in the zones of electric field discontinuity on the interface surfaces between the at least one pair of electrodes and the insulating substrates, said third substrate comprising a semi conductive material having a second resistivity value not smaller than the first resistivity value of the at least one pair of electrodes, or an insulating material having a third relative electric permittivity value.

[0029] The presence of a third substrate of semiconductive material with a resistivity value not smaller than that of the at least one pair of electrodes and / or of insulating material with a third electric permittivity value makes is possible to generate a resistive and / or capacitive current, respectively, which can affect the spacing between the equipotential lines within the device.

[0030] The spacing between equipotential lines is closely related to electric field concentration; in particular, the greater the distance between the equipotential lines, the lower the electric field concentration.

[0031] Preferably, the third substrate can extend beyond the thickness of the at least one pair of electrodes in a direction perpendicular to the surface interfacing with an external object and towards said interface surface.

[0032] Preferably, the third substrate can extend beyond the thickness of the at least one pair of electrodes in a direction perpendicular to the surface interfacing with an external object and away from said interface surface. Preferably, the third substrate can extend beyond the thickness of the at least one pair of electrodes perpendicularly to the surface interfacing with an external object in both directions.

[0033] According to a first embodiment of the invention, the substrate of semiconductive material has linear, i.e. independent of the electric field, electric resistivity.

[0034] According to another embodiment of the invention, the substrate of semiconductive material has graded linear, i.e. independent of the electric field and varying in a continuous or discrete manner within the substrate, electric resistivity.

[0035] According to a further embodiment of the invention, the substrate of semiconductive material has non-linear, i.e. dependent on the electric field, electric resistivity.

[0036] According to a further embodiment of the invention, the third substrate is made of insulating material with a relative electric permittivity value greater than those of the first and second insulating substrates.

[0037] According to a further embodiment, the second insulating substrate has regions with higher permittivity than that of at least one of the other substrates, including the second substrate, in areas between each electrode of the at least one pair of electrodes and the surface suitable for interfacing with an external object.

[0038] Preferably, the first insulating substrate is divided into a first zone, adjacent to the third substrate, and a second zone, wherein the relative electric permittivity value of the first zone is greater than that of the second one.

[0039] Preferably, the relative electric permittivity of at least one of the first, second and third substrates can be obtained by using dielectric materials whose relative electric permittivity varies in a continuous or discrete manner within the substrates, so as to concentrate the relative electric permittivity in proximity to the electrodes of the at least one pair of electrodes, and between said electrodes and the surface suitable for interfacing with an external object.

[0040] According to a further embodiment of the invention, the device comprises a support substrate arranged on the outer surface of the device, opposite the surface suitable for interfacing with an external object, in order to provide said device with additional structural, secondary-actuation or connection properties.

[0041] Further advantageous features of the present invention are set out in the appended claims, which are an integral part of the present description.

[0042] [BRIEF DESCRIPTION OF THE DRAWINGS] These features as well as further advantages of the present invention will become more apparent in the light of the following description of a preferred embodiment thereof as shown in the annexed drawings, which are provided herein merely by way of nonlimiting example, wherein:

[0043] Fig. 1 shows a generic electroadhesive gripping device according to the prior art, wherein interface regions are highlighted where electric field concentration phenomena occur which cause partial discharge, scintillation and failure;

[0044] Fig. 2a shows an electroadhesive gripping device according to the present invention based on a resistive approach, wherein a semiconductive layer has been inserted between the insulating layers;

[0045] Figs. 2b-2d show some possible alternative embodiments of the electroadhesive gripping device of Fig. 2a, highlighting possible growths of the semiconductive layer beyond the thickness of the at least one pair of electrodes;

[0046] Figs. 3a-3d show some further alternative embodiments of the electroadhesive gripping devices of Figs. 2a-2d, wherein the semiconductive layer is a thin coating, the thickness of which is smaller than the thickness of the at least one pair of electrodes;

[0047] Figs. 4a-4d are diagrams showing the electric resistivity profiles of different types of semiconductive materials;

[0048] Fig. 5a shows a second embodiment of the electroadhesive gripping device of the present invention based on a capacitive approach, wherein an insulating substrate with a different relative electric permittivity value has been inserted between the insulating layers;

[0049] Figs. 5b-5d show some further alternative embodiments of the electroadhesive gripping device of Fig. 5a, highlighting possible growths of the third substrate of insulating material beyond the thickness of the at least one pair of electrodes;

[0050] Fig. 6a shows an alternative embodiment of the electroadhesive gripping device of Fig. 5a, wherein all the insulating layers are made of materials having the same relative electric permittivity value, and wherein in the second insulating substrate there are regions of insulating material having a relative electric permittivity value greater than that of the other layers;

[0051] Figs. 6b-6d show some further embodiments of the electroadhesive gripping device of Fig. 6a, wherein at least the third substrate has a relative electric permittivity value greater than that of the other insulating substrates; Figs. 7a-7c show a further embodiment according to the present invention, wherein the substrates of the electroadhesive gripping device are made of insulating material with graded relative electric permittivity, and the corresponding relative electric permittivity profiles;

[0052] Fig. 8 shows another embodiment of the electroadhesive gripping device according to the present invention based on a resistive-capacitive approach, wherein the third substrate is made of semiconductive material and the second substrate, which has a given relative electric permittivity value, comprises regions of insulating material having a relative electric permittivity value greater than the value of the second substrate.

[0053] The following description will illustrate several specific details to facilitate an in-depth understanding of one or more exemplary embodiments of the invention. The various embodiments may also be implemented without one or more of such specific details or by using other methods, components, materials, etc. In some cases, known structures, materials or operations will not be shown or described in detail to avoid shadowing some aspects of such embodiments. In this description, any reference to an “embodiment” will indicate that a particular configuration, structure or feature described herein in relation to an embodiment is comprised in at least one embodiment of the invention. Therefore, expressions such as “in one embodiment” and the like, which may be found in different parts of this description, will not necessarily refer to the same embodiment. Moreover, any particular configuration, structure or feature may be combined as deemed appropriate in one or more embodiments.

[0054] The references below are therefore used only for simplicity’s sake, and shall not limit the protection scope or extension of the various embodiments.

[0055] [DETAILED DESCRIPTION OF THE INVENTION]

[0056] The term “electroadhesion” refers to the mechanical coupling between two objects obtained by means of electrostatic forces. Electroadhesion as described hereinafter electrically controls such electrostatic forces to provide temporary and removable fastening of two objects. This electrostatic adhesion generates a gripping force that holds together two surfaces of said objects or increases traction or friction between two surfaces due to the electrostatic forces generated by an applied electric field.

[0057] As previously described and as shown in Fig. 1, in order to maximize the gripping forces as well as the life and reliability of electroadhesive gripping devices, it is necessary to ensure an appropriate distribution of the electric field generated by the electrodes 5 within the device 1, the object 8 to be retained, and the surrounding environment.

[0058] The present invention offers a solution that makes it possible to adjust the distribution of the electric field generated by the electrodes 5 within the device 1 by controlling the spacing between the equipotential lines of the electroadhesive device, which, as previously described, is closely related to electric field concentration.

[0059] Described below are three types of approach that allow controlling the spacing between equipotential lines: a resistive approach, a capacitive approach, and a combined resistive-capacitive approach.

[0060] The resistive approach provides a reduction in charge concentration, and in the resulting charge peaks, on the surfaces of the electrodes 5 as well as a reduction in field emissions, in addition to promoting the device’s self-healing capability.

[0061] The capacitive approach provides a reduction in charge concentration, and in the resulting charge peaks, on the surfaces of the electrodes 5, while concentrating the electric potential in the regions above the electrodes.

[0062] Lastly, the combined resistive-capacitive approach provides all the advantages described above for the resistive approach and for the capacitive approach.

[0063] According to the resistive approach, it is possible to increase the spacing between the equipotential lines in proximity of the pair of electrodes 5 of an electroadhesive device 1 by using a semi conductive material positioned in the areas adjacent to the pair of electrodes 5 and having a resistivity value not smaller than the resistivity value of the pair of electrodes 5.

[0064] Fig. 2a shows an embodiment of an electroadhesive gripping device 1 of the present invention based on the resistive approach, which comprises: a first substrate of insulating material 2 having a first relative electric permittivity value; a second substrate of insulating material 3 having a second relative electric permittivity value and having a surface 4 suitable for interfacing with the surface of the object 8 to be retained; a pair of electrodes 5 arranged between the first and second substrates 2,3 and configured for generating a voltage difference within the second substrate 3, wherein said voltage difference generates an electric field which, as it propagates to the surface 4, interacts with the electric charges of the object 8, thereby generating, between the device 1 and the object 8, an electrostatic adhesion force capable of maintaining a current position of the device 1 relative to the object 8; a substrate 6 of semi conductive material arranged at the same level as the pair of electrodes 5 and having a resistivity value not smaller than the resistivity value of the pair of electrodes 5.

[0065] The relative electric permittivity values of the first and second substrates 2,3 may be either equal to or different from each other.

[0066] The resistivity value of the semi conductive layer 6 must be sufficient to ensure a less sharp electric potential variation in the zones of discontinuity between the electrodes 5 and the insulating 2,3 and semiconductive 6 substrates.

[0067] Suitable semiconductive materials that can be used as the third substrate 6 are those having volume resistivity values ranging between 104and 1012ohm m, more preferably ranging between 106and IO10ohm m.

[0068] Semiconductive materials with higher resistivity values are not generally capable of reducing electric field concentrations.

[0069] The use of semiconductive materials with lower resistivity values would imply excessive dissipations that would reduce the energetic efficiency of the electroadhesive device 1, potentially leading to damage caused by overheating.

[0070] Depending on the geometry of the device 1 and the insulating and semiconductive materials employed for manufacturing it, the substrate 6 can extend: beyond the thickness of the pair of electrodes 5 in a direction perpendicular to the surface 4 suitable for interfacing with the external object 8 and towards said interface surface 4, as shown in Fig. 2b; beyond the thickness of the pair of electrodes 5 in a direction perpendicular to the surface 4 suitable for interfacing with the external object 8 and away from said interface surface 4, as shown in Fig. 2c; beyond the thickness of the pair of electrodes 5 in a direction perpendicular to the surface 4 suitable for interfacing with the external object 8 and in both directions, as shown in Fig. 2d.

[0071] The extension of the substrate 6 of semiconductive material makes it possible to reduce the interface areas between the electrodes 5 and the insulating substrates 2,3, and hence the zones of discontinuity in which electric field concentrations occur. According to further embodiments of the present invention, the connection between the electrodes 5 and the substrate 6 of semi conductive material may consist of a thin coating of semiconductive material, the thickness of which is smaller than the thickness of the electrodes 5.

[0072] Fig. 3 shows some possible embodiments of said connection on the surface that separates the first substrate 2 of insulating material from the second substrate 3 of insulating material.

[0073] In particular:

[0074] Fig. 3a shows the substrate 6 of semiconductive material which separates the first and second insulating substrates 2,3, and which is in contact with the pair of electrodes 5;

[0075] Fig. 3b shows, in addition to what is shown in Fig. 3a, that the thin coating of semiconductive material used as substrate 6 is also deposited on the surfaces of the pair of electrodes 5 substantially perpendicular to the interface surface 4;

[0076] Fig. 3c shows, in addition to what is shown in Fig. 3b, that the thin coating of semiconductive material used as substrate 6 is also deposited on the surfaces of the electrodes 5 that are exposed to the first substrate 2 of insulating material;

[0077] Fig. 3d shows, in addition to what is shown in Fig. 3c, that the thin coating of semiconductive material used as substrate 6 is also deposited on the surfaces of the electrodes 5 that are exposed to the second substrate 3 of insulating material.

[0078] Just like the embodiments shown in Figs. 2a-2d, also the embodiments shown in Figs. 3a-3d aim at increasing the spacing between the equipotential lines in proximity to the electrodes, thereby reducing electric field discontinuity, but in this case a manufacturing process is employed which is more cost-effective in that it requires less semiconductive material.

[0079] The semiconductive material used in the possible embodiments described so far may be a uniform electrically linear one, a graded electrically linear one, or an electrically non-linear one.

[0080] A uniform electrically linear material is a material whose resistivity value is independent of the electric field and constant within the substrate.

[0081] A graded electrically linear material is a material whose resistivity value is independent of the electric field and variable in a continuous or discrete manner within the substrate. The use of a semiconductive material of the uniform electrically linear type in the semi conductive substrate 6, with a resistivity value significantly greater than the resistivity value of the pair of electrodes 5, makes it possible to obtain, in the layer comprising the substrate 6 and the pair of electrodes 5, along a direction parallel to the interface surface 4, an electric resistivity profile with discontinuous horizontal segments, resulting in the equipotential lines being equally spaced within the substrate 6.

[0082] Fig. 4a shows the above-described resistivity profile, where one can see the segmented profile with almost null resistivity values 12 in the areas of the pair of electrodes 5 and high resistivity values 11 in the substrate 6 of semiconductive material.

[0083] Prototypes of the electroadhesive device 1 characterized by the presence of a semiconductive substrate 6 of the uniform electrically linear type were manufactured by using: a commercial Polyimide film (hereafter referred to as PI), having a thickness of 24 pm, as the second insulating substrate 3; a pair of Ag electrodes 5 deposited by ink-jet printing; an insulating substrate having a thickness of 143 pm, made of silicone-based elastomer by blade casting, as substrate 2; a semiconductive third substrate 6 made from a mixture of silicone-based elastomer and carbon black (with a volume resistivity value of approx. 7.1 • 107ohm m), deposited by ink-jet printing.

[0084] The results obtained from the prototypes indicated an increase in electric breakdown voltage exceeding 48%, with a similar increase in the maximum attainable gripping force.

[0085] The use of a semiconductive material of the graded electrically linear type with a resistivity value significantly greater than that of the pair of electrodes 5 makes it possible to obtain, in the layer comprising the substrate 6 and the pair of electrodes 5, an electric resistivity profile independent of the electric field and variable along the direction parallel to the interface surface 4 either in a continuous manner, as shown in Fig. 4b, or in a discrete manner, as shown in Fig. 4c.

[0086] Unlike the profile of Fig. 4a, where the resistivity value is constant within the substrate 6 and the equipotential lines are equally spaced, the use of a semiconductive material of the graded electrically linear type provides a variation in the resistivity value, according to a continuous 13 or discrete 16 profile, from a highest value in regions of the substrate 6 far from the electrodes 5 to a lowest value in regions proximal to the electrodes 5. This resistivity variation, which is related to the slope of the curve, results in a potential variation which is greater in those regions of the substrate 6 which are farther from the electrodes 5 than that which occurs in those regions of the substrate 6 which are closer to said electrodes 5.

[0087] This difference in potential variation makes it possible to obtain equipotential lines that are more widely spaced apart, resulting in lower electric field peaks near the electrodes 5. The resistivity difference AR at the interface between the substrate 6 and the electrodes 5 can be optimized for specific applications.

[0088] The grading of the electric resistivity of the semiconductive substrate 6 can be obtained by using materials having constant volume resistivity (e.g. ZnO, CuO, SeCh, AI2O3, C, SiC, a-Si:H, a-Ge:H, a-C:H; polymers like, for example, silicone-based elastomers, epoxy resins, polyurethanes charged with particles of the above materials, even combined together, or carbon black, carbon nanotubes, mica, BaTiO3, TiO2, SiO2, Fe3O4, graphene oxide, etc.), and by adjusting the thickness of said substrate, which thus will not be even along the direction parallel to the interface surface 4, through the use of different fabrication techniques (e.g. masked chemical vapor deposition, variable-pitch ink-jet printing, selective laser ablation of previously deposited layers, etc.), or by using materials with uneven volume resistivity values (e.g. unevenly oxidized metals, such as an Al layer selectively oxidized to AI2O3; polymers evenly charged with conductive nanoparticles (e.g. Ag, Cu) and unevenly sintered; polymers like silicone-based elastomers, epoxy resins, polyurethanes unevenly charged with carbon black, carbon nanotubes, graphene oxide, etc.), which can be deposited by means of known additive manufacturing techniques (e.g. multi -material aerosol -jet printing, selective sintering of previously deposited materials whose resistivity is dependent on the exposure energy level, etc.).

[0089] Finite-element simulations of an electroadhesive device 1 characterized by the presence of a semiconductive substrate 6 of the graded electrically linear type were conducted by using: a PI insulating substrate having a thickness of 25 pm, as the second insulating substrate 3; a pair of Ag electrodes 5 having a height of 5 pm; an insulating substrate made of silicone-based elastomer, having a thickness of 150 pm, as the first substrate 2; a third substrate 6 of semi conductive material, said substrate 6 not extending beyond the thickness of the pair of electrodes 5 and having a volume resistivity value ranging between 106and 107ohm m according to the profile described and shown in Fig. 4b.

[0090] The results of these simulations indicated that the electric field peak could be reduced by more than 60%. As a consequence, the stress applied to the insulating layers 2,3 being equal, an increase by at least 2.5 times could be obtained in the potential difference applied to the electrodes 5 of the electroadhesive device 1, with a similar increase in the maximum attainable gripping force.

[0091] The use of a non-linear semi conductive material, such that its electric resistivity is dependent on the electric field, can ensure high conductivity only with high electric field values exceeding that which is defined as switching electric field.

[0092] The switching electric field is a threshold value belonging to a narrow field region where conductivity increases from a low value to a high value; beyond such threshold value, the material becomes electrically conductive.

[0093] The latter must be calibrated according to the dielectric rigidity of the insulating materials of the adjacent layers, i.e. the first and second substrates 2,3. The advantage over electrically linear materials lies in the fact that losses by electric conduction occur only when the electric field is higher than the switching electric field, thus preventing the device from overheating and avoiding any damage that may result. In addition, the effectiveness of the spacing between the equipotential lines is independent of the frequency of activation of the electroadhesive device 1, and this extends the operating range of the latter.

[0094] Fig. 4d shows the profile 15 resulting from the use of electrically non-linear semi conductive material, highlighting the trend of resistivity values (on the y-axis) in relation to electric field variations (on the x-axis) for a silicone-ZnO compound.

[0095] Finite-element simulations of said electroadhesive device 1 characterized by the presence of a semi conductive substrate 6 of the electrically non-linear type were conducted by using: a layer of PI insulating material having a thickness of 25 pm, as substrate 3; a pair of Ag electrodes having a thickness of 5 pm; an insulating layer of silicone-based elastomer having a thickness of 150 pm, as substrate 2; a third substrate 6 of semi conductive material, said substrate 6 not extending beyond the thickness of the pair of electrodes 5 and having a non-linear volume resistivity value according to the profile described and shown in Fig. 4d.

[0096] These simulations provided numerical results indicating a reduction in the electric field peak in excess of 70%.

[0097] This means that, the stress applied to the insulating layers being equal, the potential difference applied to the electrodes 5 of the electroadhesive device 1 can be tripled, with a similar increase in the maximum attainable gripping force.

[0098] The solutions using a resistive substrate 6 with the pair of electrodes 5 completely surrounded by semi conductive material also offer further benefits, including in particular: if the semi conductive material is also refractory, such as AI2O3, then the semi conductive substrate 6 can accelerate the self-healing process of the device 1 by protecting the adjacent insulating layers from the heat generated by vaporization of the material of the electrodes 5, thus reducing vaporization and the tendency of the insulating layers towards carbonization. This eliminates, or substantially reduces, the risk of failure of the electroadhesive device 1, while also decreasing the energy consumption; a reduction in the frequency of dielectric breakdowns within the electroadhesive device 1, thanks to a reduction in field emissions, i.e. migration of electrons from the conductive material of the electrodes to the insulating material of the adjacent layers. A further advantage provided by the above-described embodiments following the resistive approach, which is due to the insertion of a semiconductive layer 6, lies in the fact that said substrate 6 forms a resistor in parallel with the capacity of the electroadhesive device 1, which eliminates the need for an external discharge resistor in order to remove the residual electric voltages and ensure safety in operation. According to the capacitive approach, it is possible to increase the spacing between equipotential lines in proximity to the pair of electrodes 5 of an electroadhesive device 1 by using insulating materials having different relative electric permittivity for the first substrate 2, the second substrate 3 and the third substrate 6, and by positioning the insulating material having the highest relative electric permittivity value in the areas adjacent to the pair of electrodes 5. In this way, the spacing between equipotential lines is determined by the capacitive current generated by electric polarization or induction, not by the resistive current generated by electric conduction. Compared with the previously described resistive approach, the capacitive approach reduces the current flowing across the electrodes, thereby minimizing the overheating due to the Joule effect and the damage that may result. It follows that the energetic efficiency and service life of the device 1 are both improved.

[0099] Nevertheless, the resistive approach is generally simpler to implement and takes up less space because thinner material layers are employed.

[0100] Depending on the geometry and insulating materials in use, the material with the highest relative electric permittivity value may be used only for the substrate 6 at the level of the pair of electrodes 5, as shown in Fig. 5a.

[0101] Therefore, according to another embodiment of the present invention, shown in Fig. 5a, the electroadhesive gripping device 1 based on the capacitive approach comprises, much like the previously described resistive case: a first substrate made of insulating material 2 having a first relative electric permittivity value; a second substrate of insulating material 3 having a second relative electric permittivity value and having a surface 4 suitable for interfacing with the surface of the object 8 to be retained; a pair of electrodes 5 arranged between the first and second substrates 2,3 and configured for generating a voltage difference within the substrates 1 of the device 1, wherein said voltage difference generates an electric field which, as it propagates to the surface 4, interacts with the electric charges of the object 8, thereby generating, between the device 1 and the object 8, an electrostatic adhesion force capable of maintaining a current position of the device 1 relative to the object 8; a third substrate 6 of an insulating material having a higher relative electric permittivity value than at least either the first or the second insulating substrate 2,3. The relative electric permittivity values of the first and second substrates 2,3 may be either equal or different, and may range between 1 and 2.8- 105.

[0102] The ratio between the relative electric permittivity of the substrate 6 and that of at least one of the substrates 2,3 must be greater than 1.1.

[0103] As previously described with reference to the resistive approach, also when using the capacitive approach, depending on the geometry and the insulating materials employed for manufacturing the device 1, the substrate 6 of a material having a higher relative electric permittivity value can extend: beyond the thickness of the pair of electrodes 5 in a direction perpendicular to the surface 4 suitable for interfacing with the external object 8 and towards said interface surface 4, as shown in Fig. 5b; beyond the thickness of the pair of electrodes 5 in a direction perpendicular to the surface 4 suitable for interfacing with the external object 8 and away from said interface surface 4, as shown in Fig. 5c; beyond the thickness of the pair of electrodes 5 perpendicularly to the surface 4 suitable for interfacing with the external object 8 and in both directions, as shown in Fig. 5d.

[0104] Finite-element simulations of an electroadhesive device 1 characterized by the presence of a substrate 6 of insulating material having a greater relative electric permittivity value than the first and second insulating substrates 2,3 were conducted by using: a first insulating substrate 2 having a thickness of 150 pm and a relative electric permittivity value of 3.5 (e.g. PET); a second insulating substrate 3 having a thickness of 25 pm and a relative electric permittivity value of 3.5 (e.g. PET); a pair of Ag electrodes having a height of 5 pm; a third substrate 6 of a material having higher relative electric permittivity, said substrate 6 not extending beyond the thickness of the pair of electrodes 5, with a relative electric permittivity value of 40 (e.g. the Piezotech® RT P(VDF-TrFE-CTFE / CFE terpolymer by Arkema) and a thickness of 25 pm.

[0105] The results of these simulations indicated a reduction in the electric field peak by approx. 23% in comparison with the case wherein also the third substate 6 is made of a material having a relative electric permittivity value of 3.5 (e.g. PET).

[0106] This means that, the stress applied to the insulating layers being equal, the potential difference applied to the electrodes 5 of the electroadhesive device 1 can be increased by approx. 1.3 times, with a similar increase in the maximum attainable gripping force. For the same device, simulations carried out using, for the third substrate 6, materials having a relative electric permittivity value of 3.9 (e.g. alkyd resin or shellac), 300 (e.g. Ba(Zro.2Tio.s)03 or BaSrTiCh), 960 (e.g. Pbo.92Lao o8Zro.52Tio.48O3) and 2.8- 105(e.g. CaCuaTuO 12) show that, the stress applied to the insulating layers being equal, the potential difference applied across the electrodes, and hence also the attainable gripping force, can be increased by, respectively, 1.02, 1.345, 1.349 and 1.352 times in comparison with the case wherein also the third substrate 6 is made of a material having a relative electric permittivity value of 3.5 (e.g. PET).

[0107] For the same device, simulations carried out using a material having a relative electric permittivity value of 1.04 (e.g. Eccostock SH by Laird Technologies) for the first and second substrates 2,3 and materials having a relative electric permittivity value of 3 (e.g. SU-8 or PI), 40 (e.g. Piezotech® RT P(VDF-TrFE-CTFE / CFE terpolymer by Arkema), 300 (e.g. Ba(Zro.2Tio.s)03 or BaSrTiCh), 960 (e.g. Pbo.92Lao o8Zro.52Tio.48O3) and 2.8 - 105(e.g. CaCu3Ti40i2) for the third substrate 6 show that, the stress applied to the insulating layers being equal, the potential difference applied across the electrodes, and hence also the attainable gripping force, can be increased by, respectively, 1.496, 1.985, 2.05, 2.057 and 2.059 times in comparison with the case wherein also the third substrate 6 is made of a material having a relative electric permittivity value of 1.04 (e.g. Eccostock SH by Laird Technologies).

[0108] For the same device, simulations carried out using a material having a relative electric permittivity value of 10 (e.g. alumina) for the first and second substrates 2,3 and materials having a relative electric permittivity value of 40 (e.g. Piezotech® RT P(VDF-TrFE-CTFE / CFE terpolymer by Arkema), 300 (e.g. Ba(Zro.2Tio.8)03 or BaSrTiCh), 960 (e.g. Pbo.92Lao o8Zro.52Tio.48O3) and 2.8- 105(e.g. CaCu3Ti40i2) for the third substrate 6 show that, the stress applied to the insulating layers being equal, the potential difference applied across the electrodes, and hence also the attainable gripping force, can be increased by, respectively, 1.021, 1.303, 1.345 and 1.352 times in comparison with the case wherein also the third substrate 6 is made of a material having a relative electric permittivity value of 10 (e.g. alumina).

[0109] According to a further embodiment, materials having different relative electric permittivity values can be used for manufacturing the different substrates in order to concentrate the electric potential in regions 10 of the second insulating substrate 3 above each electrode of the at least one pair of electrodes 5, thereby increasing the depth of penetration of the electric field within the object to be retained.

[0110] An example of an architecture providing electric field concentration is shown in Fig. 6a, wherein the first substrate 2, the second substrate 3 and the third substrate 6 of the device 1 are made of insulating materials having the same relative electric permittivity value, ranging between 1 and 2.8 105. The second substrate 3 comprises also, in the areas between each electrode of the pair of electrodes 5 and the interface surface 4, regions 10 made of insulating material having a relative electric permittivity value which is greater than at least the relative electric permittivity value of the second substrate 3.

[0111] The ratio between the relative electric permittivity value of the regions 10 and those of the substrates 2,3,6 must be greater than 1.1.

[0112] Finite-element simulations of said electroadhesive device 1 having an architecture like the one described above were conducted by using: a substrate 2 of insulating material having a thickness of 150 pm and a relative electric permittivity value of 3.5 (e.g. PET); a second substrate 3 having a thickness of 25 pm and a relative electric permittivity value of 3.5 (e.g. PET); a pair of Ag electrodes 5 having a thickness of 5 pm; regions 10 of the second substrate 3 including a material having a higher relative electric permittivity value of 40 (e.g. Piezotech® RT P(VDF-TrFE-CTFE / CFE) terpolymer) in the areas of said second substrate 3 between each electrode of the pair of electrodes 5 and the interface surface 4; a third substrate 6 of insulating material having a relative electric permittivity value of 3.5 (e.g. PET).

[0113] The results of these simulations indicated an increase in gripping forces by approx. 56%, along with a reduction in the electric field peak by approx. 23%, compared with the case wherein also the regions 10 are made of a material having a relative electric permittivity value of 3.5 (e.g. PET). Therefore, this architecture provides, the stress applied to the insulating layers being equal, an increase in the maximum attainable gripping force by about 2.03 times in comparison with the case wherein also the regions 10 are made of a material having a relative electric permittivity value of 3.5 (e.g. PET). For the same device, simulations carried out using, for the regions 10, materials having a relative electric permittivity value of 3.9 (e.g. alkyd resin or shellac), 300 (e.g. Ba(Zro.2Tio.s)03 or BaSrTiCh), 960 (e.g. Pbo.92Lao o8Zro.52Tio.48O3) and 2.8- 105(e.g. CaCu3Ti4O 12) show that, the stress applied to the insulating layers being equal, the attainable gripping force can be increased by, respectively, 1.04, 2.36, 2.40 and 2.42 times in comparison with the case wherein also the regions 10 are made of a material having a relative electric permittivity value of 3.5 (e.g. PET).

[0114] For the same device, simulations carried out using a material having a relative electric permittivity value of 1.04 (e.g. Eccostock SH by Laird Technologies) for the first, second and third substrates 2,3,6 and materials having a relative electric permittivity value of 3 (e.g. SU-8 or PI), 40 (e.g. Piezotech® RT P(VDF-TrFE-CTFE / CFE) terpolymer by Arkema), 300 (e.g. Ba(Zro.2Tio.s)03 or BaSrTiCh), 960 (e.g. Pbo.92Lao.osZro.52Tio.48O3) and 2.8 - 105(e.g. CaCu3Ti40i2) for the regions 10 show that, the stress applied to the insulating layers being equal, the attainable gripping force can be increased by, respectively, 2.01, 5.44, 6.30, 6.40 and 6.44 times in comparison with the case wherein also the regions 10 are made of a material having a relative electric permittivity value of 1.04 (e.g. Eccostock SH by Laird Technologies).

[0115] For the same device, simulations carried out using a material having a relative electric permittivity value of 10 (e.g. alumina) for the first, second and third substrates 2,3,6 and materials having a relative electric permittivity value of 40 (e.g. Piezotech® RT P(VDF-TrFE-CTFE / CFE) terpolymer by Arkema), 300 (e.g. Ba(Zro.2Tio.8)03 or BaSrTiCh), 960 (e.g. Pbo.92Lao o8Zro.52Tio.48O3) and 2.8- 105(e.g. CaCu3Ti40i2) for the regions 10 show that, the stress applied to the insulating layers being equal, the attainable gripping force can be increased by, respectively, 1.313, 1.541, 1.574 and 1.588 times in comparison with the case wherein also the regions 10 are made of a material having a relative electric permittivity value of 10 (e.g. alumina).

[0116] For the same device, simulations carried out using a material having a relative electric permittivity value of 28 (e.g. 3YSZ) for the first, second and third substrates 2,3,6 and materials having a relative electric permittivity value of 40 (e.g. Piezotech® RT P(VDF-TrFE-CTFE / CFE) terpolymer by Arkema), 300 (e.g. Ba(Zro.2Tio.8)03 or BaSrTiCh), 960 (e.g. Pbo.92Lao o8Zro.52Tio.48O3) and 2.8- 105(e.g. CaCu3Ti40i2) for the regions 10 show that, the stress applied to the insulating layers being equal, the attainable gripping force can be increased by, respectively, 1.059, 1.289, 1.335 and 1.357 times in comparison with the case wherein also the regions 10 are made of a material having a relative electric permittivity value of 28 (e.g. 3 YSZ).

[0117] Other embodiments of the present invention, which can reduce the electric field peaks even further, and which include different combinations of areas having lower relative electric permittivity values and areas having higher relative electric permittivity values, are shown in Figs. 6b-6d. In such illustrative embodiments, unlike the previous example, according to which only the regions 10 of the second substrate 3 are made of insulating material having a relative electric permittivity value which is higher than that of the other substrates 2,3,6, also the third substrate 6 is made of an insulating material having a relative electric permittivity value which is higher than that of the other insulating substrates 2,3.

[0118] It should be noted that, in the solutions shown in Figs. 6c and 6d, the first insulating substrate 2 is also divided into an upper first portion 2a, facing the pair of electrodes 5 and having a relative electric permittivity value which is higher than that of a lower second portion 2b.

[0119] Each one of such embodiments may be preferred over the others depending on the geometry of the device, the characteristics of the materials employed, and the manufacturing process adopted.

[0120] According to other embodiments (not shown) of the solutions of Figs. 6c and 6d, the substrate 2 is wholly made of an insulating material having a high permittivity value. Compared with the solutions shown in Figs. 6c and 6d, these further embodiments make the process of manufacturing the substrate 2 simpler, although they use larger amounts of high-permittivity materials, which are generally more expensive than low- permittivity materials, and increase the capacity of the device 1, resulting in more accumulated energy and longer response times.

[0121] According to a further embodiment of the invention, the variation in relative electric permittivity within the insulating layers 2,3,6 can be achieved by using graded dielectric materials.

[0122] A graded dielectric material is a material whose relative electric permittivity value varies in a continuous or discrete manner within the substrate.

[0123] Such a solution, as shown in Fig. 7a, makes it possible to concentrate the high relative electric permittivity in proximity to the electrodes 5 and between the electrodes 5 and the interface surface 4, thus reducing the electric field concentrations at the electrodes 5 while at the same time increasing the gripping force.

[0124] The insulating material with graded relative electric permittivity employed for this type of architecture has a value ranging between 1 and 2.8- 105.

[0125] Figs. 7b-c illustrate, respectively, a relative electric permittivity profile associated with the architecture solution of Fig. 7a, which is variable in a continuous or discrete manner along the direction parallel to the surface 4, showing the highest relative electric permittivity value in regions 18,19 near the electrodes 5, with values gradually increasing or decreasing in the other regions of the device 1.

[0126] Insulating materials with graded relative electric permittivity suitable for manufacturing the substrates 2,3,6 of this embodiment, in accordance with the relative electric permittivity profile shown in Figs. 7b-c, must have a ratio greater than 1.1 between the regions 18,19 with maximum relative electric permittivity and the other regions of the device 1.

[0127] The profiles shown in Figs. 7b-c can be obtained, for example, through the use of an epoxy resin charged with TiCh in a variable volumetric fraction within the device 1.

[0128] Finite-element simulations of an electroadhesive device 1 characterized by graded relative electric permittivity within the insulating layers 2,3,6 were conducted by using: a first substrate 2 of insulating material having a thickness of 150 pm, further divided into an upper first portion 2a, facing the pair of electrodes 5, made of a material with graded relative electric permittivity of 3.5 to 7 according to the profile of Fig. 7b, and a lower second portion 2b with a relative electric permittivity value of 5; a second substrate 3 having a thickness of 25 pm, made of an insulating material with graded relative electric permittivity of 3.5 to 7 according to the profile of Fig. 7b; a pair of Ag electrodes 5 having a height of 5 pm; a third substrate 6, between the pair of electrodes 5, made of an insulating material with graded relative electric permittivity of 3.5 to 7 according to the profile of Fig. 7b.

[0129] The results of these simulations indicated that the electric field peak is reduced by more than 30%. Therefore, the stress applied to the insulating layers being equal, this architecture permits increasing by 1.4 times the potential difference applied across the electrodes 5 of the electroadhesive device 1, resulting in a similar increase in the maximum attainable gripping force.

[0130] Fig. 7a shows, by way of example, an embodiment of the device 1 wherein the upper first portion 2a of the first substrate 2, the second substrate 3, and the third substrate 6 are made of the same insulating material with graded relative electric permitivity. According to other embodiments (not shown), only a subset of the substrates 2a, 3, 6 are made of an insulating material with graded relative electric permittivity.

[0131] According to a further embodiment of the device 1, also the lower portion 2b of the first substrate 2 is made of an insulating material with graded relative electric permittivity. According to further embodiments of the device 1, the substrates of insulating material 2a, 2b, 3, 6, or a subset thereof, are made of materials with graded relative electric permittivity having different relative electric permittivity values.

[0132] Lastly, Fig. 8 shows an architecture solution that comprises a combination of the abovedescribed embodiments of the electroadhesive device 1. In particular, this solution follows both the resistive approach and the capacitive approach. In this solution, the electroadhesive device 1 comprises a third substrate 6 of semiconductive material and a second substrate 3 of insulating material having a given relative electric permittivity value and comprising regions 10, between each electrode of the pair of electrodes 5 and the interface surface 4, made of an insulating material having a relative electric permittivity value which is greater than that of the other insulating substrates 2,3.

[0133] Relative electric permittivity values of insulating materials suitable for manufacturing the substrates 2,3 and the regions 10 range between 1 and 2.8- 105, wherein the ratio between the relative electric permittivity of the regions 10 and that of the substrates 2,3 must be greater than 1.1.

[0134] Finite-element simulations of an electroadhesive device 1 characterized by the presence of a substrate 6 of semiconductive material and regions 10, within the second substrate 3, having a relative electric permittivity value greater than those of the insulating substrates 2 and 3 were conducted by using: a first substrate 2 having a thickness of 150 pm, made of a material having a relative electric permittivity value of 3.5 (e.g. PET); a second substrate 3 having a thickness of 25 pm, made of a material having a relative electric permitivity value of 3.5 (e.g. PET); a pair of Ag electrodes 5 having a height of 5 pm; a third substrate 6 of semiconductive material, said substrate 6 not extending beyond the thickness of the pair of electrodes 5 and having a volume resistivity value of 107ohm-m. regions 10 of the second substrate 3 including a material with higher relative electric permitivity, having a relative electric permittivity value of 40 (e.g. Piezotech® RT P(VDF-TrFE-CTFE / CFE) terpolymer) in the areas of said second substrate 3 between each electrode of the pair of electrodes 5 and the interface surface 4.

[0135] The results of these simulations indicated a reduction in the electric field peak in excess of 70%.

[0136] This means that, the stress applied to the insulating layers being equal, the potential difference applied across the electrodes 5 of the electroadhesive device 1 can be tripled, with a similar increase in the maximum attainable gripping force.

[0137] The reductions in charge concentration and charge peaks are obtained, in the abovedescribed embodiments, at the interfaces between the electrode 5 and the insulating substrates 2,3 and at the interfaces between the electrode 5 and the substrate 6.

[0138] As previously described with reference to the resistive approach, also the architecture solution of Fig. 8 can be implemented through further embodiments wherein the manufacturing processes uses less semiconductive material.

[0139] The various embodiments described herein comprise, for simplicity, just one pair of electrodes, but they can nevertheless be extended to architectures including multiple pairs of electrodes as used, for example, in interdigitated solutions.

[0140] Regardless of the architecture, the materials employed for the electrodes 5 and for the insulating layers may be rigid, flexible or deformable, with rheological properties that may be either constant or controllable by physical or chemical stimulation (e.g. by means of electric, magnetic or temperature fields, or by irradiation). In particular, the electrodes and the insulating layers may be made as a rigid, flexible or deformable envelope containing liquid, gas, powder, or a mixture thereof, to improve the flexibility or deformability properties of the insulating layers and of the electrodes 5 and allow the interface surface 4 of the electroadhesive device 1 to adapt and adhere to the shape and surface finish of the object 8 to be retained.

[0141] Examples of insulating materials that can be used, depending on the operating environment and the dielectric and resistive properties of the object to be retained, are those materials which have relative electric permittivity values in the range of 1 to 2.8- 105, such as: rigid polymeric materials, which can, if required, become flexible when shaped as thin films, such as PI, PET, PEN, PE, PP, PC, PS, PPS, PTFE, PVC, epoxy resins, SU-8, PEEK, PEI, PVDF (in its P(VDF-TrFE) copolymers and P(VDF-TrFE- CTFE / CFE) terpolymers), Parylene (C, F, HT types); soft polymeric materials, such as PDMS silicone-based elastomers, acrylic elastomers, polyurethanes, natural rubber and styrene rubber, optionally charged with SiCE, TiCE, AI2O3, BaTiCh particles, carbon black, quaternary salts, plasticizers like DOP and DIOP, or mixtures thereof, Beyolex thermosetting stretchable material by Panasonic, FR4; ceramic materials, which can, if required, become flexible when shaped as thin films, such as AI2O3, SiO2, AIN, SiaN4, 3YSZ, alkali-free glass like borosilicate glass and borosilicate-alumina glass; materials based on ferroic perovskites (such as relaxing ferroelectric materials, antiferroelectric materials), ferroelectric and antiferroelectric materials based on hafnium oxide and zirconium oxide.

[0142] As concerns the other electric properties of the insulating materials suitable for use in all of the above-described embodiments: materials with volume resistivity greater than 1013Ohm m are used in order to exploit Coulomb’s forces due to the electrostatic attraction between the charges generated in the materials, whether by polarization or by induction, by the potential difference applied across the electrodes; materials with volume resistivity values between 107Ohm m and 1013Ohm m, preferably between 109Ohm m and 1012Ohm m, are used in order to exploit Johnsen- Rahbek forces due to the electrostatic attraction between the charges that accumulate by conduction at the interface between materials having different relative electric permittivity and resistivity upon application of a potential difference across the electrodes.

[0143] As concerns the electric properties of the semiconductive materials suitable for use in all of the above-described embodiments, they include materials having volume resistivity values ranging between 104and 1012ohm m, more preferably between 106and IO10ohm m.

[0144] Some examples of semiconductive materials that can be used for implementing the above-described architecture are: ZnO; CuO; SeO2; A12O3; C; SiC; a-Si:H; a-Ge:H; a- C:H; polymers like, for example, silicone-based elastomers, epoxy resins, polyurethanes charged with particles of the above-mentioned materials, even combined together, as well as carbon black, carbon nanotubes, mica, and oxides such as BaTiO3, TiO2, SiO2, Fe3O4 and graphene oxide.

[0145] As concerns the electric properties of the conductive materials suitable for making the electrodes 5, they include highly conductive materials like, for example, Al, Cu, Sn, Al-Zn alloy, Ag, and Au, which reduce the energy consumption and activation time of the device 1, and also semi conductive materials like, for example, silicone-based elastomers, polyurethanes or other polymers charged with carbon black particles with surface resistivity ranging between 105Ohm / sq and 109Ohm / sq, which make for reduced short-circuit currents in case of failure of the device 1, thus improving safety. The thickness of the second insulating substrate 3 is in the range of 0.1 pm to 10 mm.

[0146] The thickness of the first insulating substrate 2 is typically similar to the above, but this substrate may also be as thick as a few centimetres.

[0147] The thickness of the electrodes 5 is in the range of 100 Angstroms to 200 pm.

[0148] Small thicknesses of the second substrate 3 of insulating material are often desirable to reduce the supply voltage of the electroadhesive device 1.

[0149] In general, small thicknesses of the electroadhesive device 1 as a whole are desirable when said device 1 needs to be flexible or easily deformable to better comply with the object to be retained.

[0150] Small thicknesses of the electrodes 5 are often desirable to ensure good self-healing properties of the electroadhesive device 1, i.e. when the electric arc generated during a failure, resulting from a local dielectric breakdown caused by impurities in the insulating layers, overvoltage, etc., can vaporize the electrode of the surrounding area, thus insulating it from the rest.

[0151] The present invention makes it possible to overcome the problems suffered by the prior art, which cannot be solved by means of known solutions focused, as previously described herein, on the geometry of the device, by providing a reduction in electric field concentration which extends the service life of the electroadhesive device 1.

[0152] An additional advantage of the present invention, which also contributes to extending the life of the electroadhesive device 1, lies in the fact that it provides reduced field emissions, i.e. migration of electrons from the conductive material of the electrodes 5 to the insulating material 2,3 of the adjacent layers.

[0153] A second additional advantage of the present invention, which also contributes to extending the life of the electroadhesive device 1, lies in the fact that it promotes the self-healing process, i.e. self-regeneration of the materials of the device following a localized dielectric breakdown event.

[0154] A third additional advantage of the present invention lies in the fact that the electric potential is concentrated in the regions above the electrodes, resulting in increased depth of penetration within the object 8 to be retained, and hence increased gripping force of the electroadhesive device 1.

[0155] A fourth additional advantage of the present invention, as far as the resistive approach is concerned, lies in the fact that the resistor, which is arranged in parallel with the capacity of the electroadhesive device 1 due to the presence of a semiconductive substrate, eliminates the need for using an external discharge resistor in order to remove the residual electric voltage and ensure safety in operation.

[0156] Therefore, according to the previously described embodiments, the present invention makes it possible to improve reliability and service life while maintaining the same gripping force; - improve gripping force while maintaining the same reliability and service life; improve gripping force, reliability and service life at the same time.

[0157] The present invention is not limited to above-described embodiments, but may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the inventive idea, as clearly specified in the following claims.

Claims

CLAIMS1. An electroadhesive device (1) configured for adhering to a surface of an object (8) to be retained, said device (1) comprising:- a first insulating substrate (2) having a first relative electric permittivity value;- a second insulating substrate (3) having a second relative electric permittivity value and having a surface (4) suitable for interfacing with the surface of the object (8) to be retained;- at least one pair of electrodes (5) arranged between the first and second substrates (2,3), having a first resistivity value, and configured for generating a voltage difference in the substrate (3), wherein said voltage difference generates an electric field propagating to the surface (4) and interacting with the electric charges of the object (8), so as to generate, between the device (1) and the object (8), an electrostatic adhesion force capable of maintaining a current position of the device (1) relative to the object (8);- a third substrate (6) arranged between the first and second insulating substrates (2,3) and in contact with the at least one pair of electrodes (5), so as to reduce the electric field concentration in the zones of electric field discontinuity on the interface surfaces between the at least one pair of electrodes (5) and the insulating substrates (2,3), said third substrate comprising a semi conductive material having a second resistivity value not smaller than the first resistivity value of the at least one pair of electrodes (5), or an insulating material having a third relative electric permittivity value.

2. The electroadhesive device (1) according to claim 1, wherein a thickness of the third substrate (6) can extend beyond a thickness of the at least one pair of electrodes (5) in a direction perpendicular to the surface (4) and towards said surface (4) or away from said surface (4) or in both directions, for the purpose of reducing the interface areas between the at least one pair of electrodes (5) and the insulating substrates (2,3) and lessening the zones of discontinuity.

3. The electroadhesive device (1) according to claim 1, wherein the third substrate (6) of semi conductive material has a thickness smaller than the thickness of the at least one pair of electrodes (5) and is capable of separating the first substrate (2) of insulating material from the second substrate (3) of insulating material.

4. The electroadhesive device (1) according to claim 3, wherein the thirdsubstrate (6) of semi conductive material is further arranged on at least one of the surfaces of the at least one pair of electrodes (5) exposed to the first or second substrate (2, 3) of insulating material, in order to reduce the interface areas between the at least one pair of electrodes (5) and the insulating substrates (2,3) and lessen the zones of discontinuity.

5. The electroadhesive device (1) according to one or more of claims 1 to 4, wherein the third substrate (6) of semi conductive material has uniform linear electric resistivity independent of the electric field, in order to obtain, within said substrate (6), an electric resistivity profile of the type formed of discontinuous horizontal segments following a direction parallel to the surface (4).

6. The electroadhesive device (1) according to one or more of claims 1 to 4, wherein the third substrate (6) of semiconductive material has graded linear electric resistivity independent of the electric field, in order to obtain an electric resistivity profile that varies in a continuous or discrete manner within said substrate (6).

7. The electroadhesive device (1) according to one or more of claims 1 to 4, wherein the third substrate (6) of semiconductive material has non-linear electric resistivity dependent on the electric field, so that electric conduction losses occur for electric field values greater than the switching electric field.

8. The electroadhesive device (1) according to claim 1 or 2, wherein the third substrate (6) of insulating material has a relative electric permittivity value greater than the first and second relative electric permittivity values of the insulating substrates (2,3).

9. The electroadhesive device (1) according to at least one of claims 1 or 2, wherein the third substrate (6) is made of insulating material and the first, second and third substrates (2,3,6) are made of insulating materials having the same relative electric permittivity values, and wherein the second substrate (3) of insulating material having a second relative electric permittivity value comprises regions (10) of insulating material with a relative electric permittivity value greater than at least the second relative electric permittivity value of the substrate (3), said regions (10) being comprised between each electrode of the at least one pair of electrodes (5) and the interface surface (4).

10. The electroadhesive device (1) according to claim 1 or 2, wherein the third substrate (6) is made of insulating material having a relative electric permittivityvalue greater than at least one of the first and second relative electric permittivity values of the insulating substrates (2,3), and wherein the second substrate (3) of insulating material having a second relative electric permittivity value comprises regions (10) of insulating material with a relative electric permittivity value greater than at least the second relative electric permittivity value of the substrate (3), said regions (10) being comprised between each electrode of the at least one pair of electrodes (5) and the interface surface (4).

11. The electroadhesive device (1) according to claim 1 or 2, wherein the relative electric permittivity of at least one of said first, second and third substrates (2,3,6) of insulating material is obtained from insulating materials having graded relative electric permittivity, with a relative electric permittivity profile that varies in a continuous or discrete manner within the substrate, in order to concentrate the relative electric permittivity in proximity to each electrode of the at least one pair of electrodes (5).

12. The electroadhesive device (1) according to one or more of claims 8 to 11, wherein the first substrate (2) of insulating material is further divided into a first zone (2a) and a second zone (2b) having different relative electric permittivity values.

13. The electroadhesive device (1) according to one or more of claims 1 to 4, wherein the third substrate (6) is made of semiconductive material, and wherein the second substrate (3) of insulating material having a second relative electric permittivity value comprises regions (10) of insulating material with a relative electric permittivity value greater than at least the second relative electric permittivity value of the substrate (3), said regions (10) being comprised between each electrode of the at least one pair of electrodes (5) and the interface surface (4).

14. The electroadhesive device (1) according to one or more of the preceding claims, wherein the reduction in electric field concentration is obtained in the zones of electric field discontinuity on the interface surfaces between the at least one pair of electrodes (5) and the third substrate (6).

15. The electroadhesive device ( 1 ) according to one or more of the preceding claims, wherein said device (1) further comprises a support substrate (7) arranged on the outer surface opposite the surface (4) to provide further structural, secondaryactuation or connection properties.

16. The electroadhesive device (1) according to one or more of the precedingclaims, wherein the thickness of the first substrate (2) of insulating material is greater than 0.1 m, the thickness of the second substrate (3) of insulating material ranges between 0.1 pm and 10 mm, and the thickness of the at least one pair of electrodes (5) ranges between 100 Angstroms and 200 pm.

17. The electroadhesive device (1) according to one or more of the preceding claims, wherein the voltage difference generated by the at least one pair of electrodes (5) propagates in all the substrates (2,3,6) of the device (1) in contact with said pair of electrodes (5).

18. The electroadhesive device ( 1 ) according to one or more of the preceding claims, wherein the semiconductive materials suitable for use as the third substrate (6) have volume resistivity values ranging between 104and 1012ohm m, more preferably ranging between 106and 1010ohm m.

19. The electroadhesive device (1) according to one or more of the preceding claims, wherein the insulating materials have relative electric permittivity values ranging between 1 and 2.8- 105.

20. The electroadhesive device (1) according to claim 19, wherein the insulating materials with graded relative electric permittivity have a profile in which the ratio between the maximum relative electric permittivity in the regions (18,19) adjacent to the electrodes of the at least one pair of electrodes (5) and the relative electric permittivity in the other regions of the device (1) is greater than 1.1.

21. The electroadhesive device (1) according to claim 19, wherein the ratio between the values of relative electric permittivity of the insulating materials used in the different substrates within the device (1) is greater than 1.1.

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