Piezo-resistive force sensor

Abstract

The present invention relates to a piezoresistive force sensor, particularly designed as a pressure sensor, capable of generating a sensor signal that depends on the amount of force applied to the force sensor in the force measurement direction. The force sensor comprises a first electrode, a second electrode, and an elastically deformable resistive layer electrically connecting the two electrodes. The resistance of the total resistance of the conductive path from the first electrode to the second electrode via the resistive layer varies depending on the amount of force applied. The sensor signal, which represents the amount of applied force, can be detected, for example, by measuring the voltage between the electrodes or the current flowing through the conductive path. The resistive layer comprises first conductive staple fibers and second non-conductive staple fibers. The proportion of the first staple fibers relative to the total amount of staple fibers can be varied to tailor the force-resistance characteristics of the force sensor.

Description

Technical Field

[0001] The present invention relates to a piezoresistive force sensor including a piezoresistive resistive layer of a fiber material.

Background Art

[0002] US 2007 / 0054577 A1 describes an electrically conductive fiber material in the form of a woven or non-woven fabric that can be used in the construction of a pressure sensor. This fiber material is pretreated for activation and for ease of application of the conductive layer. Next, the conductive coating is applied layer by layer onto the fiber material.

[0003] In such a method, it is difficult to control the conductivity of the electrical resistance of the conductive fiber material. During coating with a solution, the proportion of conductive particles in the solution frequently changes, and thus, during coating of parts of the fiber material, extensive volatilization occurs, and parts with a high proportion of unusable conductive fiber material can be formed.

[0004] Conductive fibers in fiber materials and their use are known. For example, US 2,845,962 describes antistatic fibers containing conductive carbon and having a relatively high resistance per unit area to enable the use of an antistatic fiber material regardless of the relative humidity in the environment.

[0005] EP 1 961 845 A2 discloses melt spinning, in which carbon black or graphite particles form aggregates along the longitudinal axis of the yarn to form a conductive path along the longitudinal axis of the yarn.

[0006] US 5,368,913 A describes an antistatic spunbond non-woven fabric and a method for manufacturing the same. Thereby, a plurality of conductive carbon filaments and metal filaments are arranged in a non-conductive non-woven fabric material during the manufacture of the non-woven fabric.

[0007] An antistatic laminate having a plurality of layers is known from US 4,540,624 A. Carbon fibers are arranged in the uppermost layer for heat dissipation.

[0008] US 6,346,491 B1 discloses a woven fabric, knitted fabric, or non-woven needle felt. In different layers, the needle felt includes different conductivities, which are adjusted by fiber density, fiber diameter, and fiber conductivity. The needle felt can remove or reduce electromagnetic wave interference in various applications. Fibers for similar purposes are known from, for example, US 4,684,762 A, US 4,943,477 A, and CN 101748599 B.

[0009] Antistatic clothing is described in US 3,699,590 A. A knit yarn structure with a conductive yarn of a metal material for use in RFID labels is described in WO 2008 / 098386 A1.

[0010] EP 1 716 085 B1 discloses a conductive coating composition for glass fibers and its production in a dipping bath.

[0011] Furthermore, it is known from a number of documents that a composite yarn in which a fabric carrier is combined with a conductive yarn is, for example, wound, twisted, spun, or drawn. Such composite yarns are described in, for example, the following documents: US 4,776,160A, US 4,813,219A, US 5,927,060A, EP 1 885 925 B1, EP 3 484 730 A1, EP 2 300 649 A1, US 7,135,227 B2, DE 20 2006 020 401 U1, WO 2004 / 027132 A1, DE 1 817 235 A, EP 0 250 260 B1, US 3,851,456 A, KR 100895092Bl, or EP 2 236 654 Bl.

[0012] In order to obtain particularly high conductivity, WO 2003 / 095724 A1 and EP 1 362 940 A1 propose to provide a thread of metal fibers for stainless steel fibers having a coating with high conductivity. US7845153B2 discloses, for example, a method for manufacturing conductive threads from metal filaments for use of the threads in information communication.

[0013] Conductors and simple switches can be realized by means of piercing threads having high conductivity, as described, for example, in JP 2011 137277 A, or KR 10 201 903 7721 A.

[0014] In the field of conductive fibers, a further group of documents refers to polymer fibers containing conductive fillers and methods for manufacturing such polymer fibers. Examples include the following documents: US 2005 / 0170177 A1, US 4,457,973 A, US 4,216,264 A, DE 2 329 484 B2, US 5,952,099 A, EP 0 343 496 B1, US 4,704,311 A, US 5,260,013 A, US 5,277,855 A, US 4,545,835 A, US 4,129,677 A, US 4,997,712 A, US 5,916,506 A, DE 2 251 071 B2, US 4,045,949 A, US 4,756,969 A, US 5,262,234 A, and US 4,267,233 A.

[0015] US 3,206,923 A proposes to guide a composite thread with viscose into a dipping bath, which contains carbon black particles. This viscose absorbs the carbon black particles.

Summary of the Invention

Problems to be Solved by the Invention

[0016] Starting from the prior art, the problem of the present invention is to create a force sensor. The force sensor has a configuration in which there is almost no scattering of the force sensor effect and can be manufactured simply and inexpensively, and can be configured to respond not only to particularly large forces but also to small forces.

Means for Solving the Problem

[0017] This problem is solved by a force sensor having the features of claim 1. Claim 15 shows a method for manufacturing a force sensor.

[0018] The piezoresistive force sensor of the present invention has an elastically deformable resistive layer. This resistive layer has a first outer surface and a second outer surface on the opposite side of the first outer surface. The two outer surfaces are arranged at a distance in a direction in which the force sensor can detect a force applied from the outside between the two outer surfaces, particularly a compressive force. The resistive layer has a mixture in which conductive first staple fibers and non-conductive second staple fibers are spatially dispersed. Preferably, the resistive layer consists only of a mixture of the first and second staple fibers. Other components can be excluded.

[0019] In the resistor layer, at least some or a large number of the first staple fibers are in contact with one or more further first staple fibers respectively, and conductive contacts are formed between these first staple fibers in contact with each other. As a result, one or more parallel conductive connections are generated between the first outer surface and the second outer surface, or at least while an external force is applied. The number and / or arrangement and / or conductivity of the conductive connections generated by the first staple fibers coming into contact with each other can vary according to the amount and direction of the applied force.

[0020] Preferably, in the observed volume portion of the mixture, the density of the first staple fiber and / or the density of the second staple fiber is substantially constant or within a predetermined tolerance range, so that the mixture of the first and second staple fibers is substantially homogeneous. The deviation in the density of the first staple fiber and / or the second staple fiber between volume portions of the same size observed within the entire mixture preferably deviates by 5% or less, or 10% or less. When the deviation in density is within a predetermined tolerance range, in this application, the mixture is regarded as a homogeneous mixture. The observed volume portion can have a size of, for example, at least 1 mm 3 and / or at most 1 cm 3 . Each volume portion of the resistive layer observed individually from the perspective of uniformity can be defined by a region with a size of at least 1 mm 2 and / or at most 1 cm 2 , and can extend orthogonally to this region between the first outer surface and the second outer surface through the resistive layer.

[0021] The mixture of the first and second staple fibers can form a resistive layer in the form of a non-woven fabric. Alternatively or additionally, one or more staple fiber yarns can be manufactured from the first staple fiber and the second staple fiber and may be present in the resistive layer as a scrim, and / or a knitted fabric, and / or a warp knitted fabric, and / or a woven fabric. Alternatively or additionally, one or more staple fiber twisted yarns can be manufactured from the above-mentioned staple fiber yarns and can be present in the resistive layer as a scrim, and / or a knitted fabric, and / or a warp knitted fabric.

[0022] The staple fiber yarn can be a ring yarn, a rotor yarn, a friction yarn or an air jet yarn. The resistive layer can include any combination of the above-mentioned yarns. From any combination of the above-mentioned staple fiber yarns, a twisted yarn or a wound yarn can be formed, and the resistive layer can include at least one twisted yarn or wound yarn. The portion of the twisted yarn or wound yarn, and / or the composition, and / or the component can be non-conductive. For example, the twisted yarn can include one or more non-conductive yarns.

[0023] The piezoresistive force sensor has a conductive first electrode and a conductive second electrode. Each electrode is disposed indirectly or directly on the first outer surface or the second outer surface. Thereby, a conductive path is provided from the first electrode to the second electrode through the resistive layer, or vice versa. The resistance value of the ohmic resistance of this conductive path depends on the external force applied to the piezoresistive force sensor. This force may be, for example, a compressive force or a tensile force caused by elastic deformation of the resistive layer such that the distance between the first outer surface and the second outer surface changes. This external force can include a force component (e.g., a vector component of a spatial vector) that promotes moving the two outer surfaces apart or bringing them closer together. For example, the detected force is at least one force component applied orthogonally or parallel to a plane on the piezoresistive force sensor, and the plane is oriented substantially parallel to the two outer surfaces.

[0024] In this specification, when resistance is mentioned, unless otherwise specified, it always means ohmic resistance.

[0025] With the configuration of a force sensor comprising an elastically deformable resistive layer from conductive first staple fibers and non-conductive second staple fibers, the desired resistance value of the conductive path can be adjusted easily and inexpensively. By changing the ratio of the conductive first staple fibers to the whole of the first and second staple fibers, adjustment of the resistance value according to the range of the force to be measured can be performed. The smaller the amount of the external force applied, the higher the ratio of the first staple fibers to the total amount of the first and second staple fibers.

[0026] Staple fibers are commercially available and can be purchased simply and inexpensively. The staple fibers can be processed to form non-woven fabrics, yarns, spun yarns, etc., whereby the resistive layer can be formed from a non-woven fabric and / or at least one staple fiber yarn. For example, one or more staple fiber yarns can be connected to each other by weaving, knitting, warp knitting, etc., and can be used as components of the resistive layer or can form the resistive layer.

[0027] Preferably, the proportion of the first staple fiber in the total amount of the first and second staple fibers is at least 5%. More preferably, the proportion of the first staple fiber in the total amount of the first and second staple fibers exceeds 10% or 20%.

[0028] In a preferred embodiment, the first staple fiber and / or the second staple fiber has a fiber length of at least 20 mm. Alternatively or additionally, the first staple fiber and / or the second staple fiber can have a maximum fiber length of 100 mm.

[0029] In an embodiment, the average fiber length of the first staple fiber and / or the average fiber length of the second staple fiber can reach at least 20 mm and / or a maximum of 100 mm.

[0030] The resistance layer has a volume resistance between the first outer surface and the second outer surface. The contact resistance in the resistance layer is formed by the contact resistances connected in parallel and / or in series with each other obtained by the first staple fibers in contact with each other.

[0031] It is more advantageous if the total resistance of the conductive path is formed from a series connection of a plurality of resistances. The series connection preferably includes, or consists of, the total resistance of the first contact resistance between the first electrode and the resistance layer, the second contact resistance between the second electrode and the resistance layer, and the volume resistance of the conductive path in the elastically deformable resistance layer. When an external force is applied, at least the volume resistance of the conductive path in the resistance layer changes, and optionally or preferably, the first contact resistance and / or the second contact resistance also change. Thereby, the sensitivity of the force sensor can be improved.

[0032] In a preferred embodiment, when no external force is applied to the force sensor, the total resistance of the conductive path reaches a maximum of 1000 MΩ. Additionally or alternatively, when a force is applied to the force sensor, the value of the total resistance of the conductive path can reach a minimum of 1 kΩ. It is advantageous if the maximum value of the total resistance when no external force is applied to the force sensor can be adjusted between 1 kΩ and 1000 MΩ by the portion of the first staple fiber relative to the entire portion of the first and second staple fibers.

[0033] It is advantageous if the value of the total resistance of the conductive path changes according to a defined correlation depending on the amount of external force applied to the force sensor. This correlation is also defined, at least, by the ratio of the first staple fiber to the total amount of the first and second staple fibers. Preferably, this correlation is non-linear, for example, exponential.

[0034] This correlation is also defined, at least, by the fiber type and / or the conductivity of the first staple fiber. For example, the operating range of the force sensor can be adjusted by the fiber type and / or conductivity of the first staple fiber, particularly by the maximum resistance value and / or the minimum resistance value and / or the slope of the non-linear correlation between the resistance value of the total resistance and the external force.

[0035] It is advantageous if the resistance value of the conductive path decreases as the amount of external force increases starting from the maximum resistance value. Preferably, the resistance value can asymptotically approach the minimum resistance value as the force increases in the range up to at least the maximum allowable external force. The operating range of the force sensor is preferably in the range starting from the minimum resistance value of the total resistance up to the resistance value of the total resistance where the amount of the slope of the non-linear correlation between the resistance value of the total resistance and the external force exceeds a threshold value.

[0036] In an advantageous embodiment, the first staple fiber comprises a plurality of components, at least one of which is conductive. For example, the first staple fiber can contain a polymer containing a conductive filler. The first staple fiber can be configured, for example, as a bicomponent fiber. In an embodiment, the bicomponent fiber can include a core and an outer layer. The outer layer can completely or partially cover the core. The outer layer contains a conductive material, in particular a conductive filler, preferably conductive carbon (for example, so-called carbon black). The core of the bicomponent fiber may be elastically deformable, not plastic with respect to the generated force, and not elastically deformable. The core is preferably non-conductive. The bicomponent fiber can contain other components. Instead of the bicomponent fiber, the first staple fiber can also have a different configuration, for example, consisting of a polymer containing a conductive filler and including a core with a conductive coating applied thereto later.

[0037] Regarding the manufacture of a force sensor, in particular a force sensor according to one of the embodiments described above, it can proceed as follows.

[0038] First, a conductive first staple fiber and a non-conductive second staple fiber are prepared. Next, the first and second staple fibers are mixed with each other to form a mixture, preferably such that a uniform dispersion of the first and second staple fibers is present in the mixture. A method such as carding can be used to mix the first and second staple fibers.

[0039] Next, using this mixture, an elastically deformable resistive layer is formed. For this purpose, the mixture can be felted, or a single staple fiber yarn, or a twisted yarn consisting of a plurality of fiber yarns can be made from the mixture. The resistive layer can include a non-woven fabric and / or at least one staple fiber yarn and / or at least one twisted yarn consisting of a plurality of staple fiber yarns. The staple fiber yarns, or the twisted yarns or wound yarns can be connected to each other by methods such as weaving and / or knitting and / or warp knitting. The non-woven fabric can be manufactured, for example, by solidifying the first and second staple fibers using felting needles and / or water jets. The solidification can also be carried out, additionally or alternatively, by applying heat with the addition of a chemical substance.

[0040] The resistive layer has a first outer surface and a second outer surface. The conductive first electrode and the conductive second electrode are respectively attached to one of the outer surfaces. Thereby, a conductive path is formed from the first electrode, through the resistive layer, to the second electrode, or vice versa. The resistance value of this conductive path of the manufactured force sensor depends on the external force applied in a tensile or compressive manner between the two outer surfaces.

Brief Description of the Drawings

[0041] Advantageous improvements of the present invention are derived from the dependent claims, the description, and the drawings. Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The drawings show the following.

[0042]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0043] FIG. 1 shows a block diagram of the connection to the force sensor 15 and the circuit 16. The force sensor 15 includes an elastically deformable resistive layer 17. The resistive layer 17 has a first outer surface 18 and a second outer surface 19 on the opposite side facing away from the first outer surface 18. The two outer surfaces 18, 19 are arranged at opposite distances from each other in the force measurement direction M. In this example, the force measurement direction M is oriented such that it is substantially orthogonal to the plane oriented parallel to the outer surfaces 18, 19. Alternatively or additionally, the force sensor 10 can detect the force of the force component oriented parallel to this plane. The force sensor 10 can be a pressure sensor and / or a tension sensor and / or a strain sensor. Basically, the resistive layer 17 can be elastically deformed, and in particular, the force or force component that changes the distance between the outer surfaces 18, 19 can be measured.

[0044] The force sensor 15 further has a first electrode 20 and a second electrode 21. The two electrodes 20, 21 are indirectly mechanically and electrically connected to each other via the resistive layer 17 of the force sensor. There is no direct electrical connection between the two electrodes 20, 21. Therefore, current can flow between the electrodes 20, 21 only through the resistive layer 17.

[0045] In the embodiment of FIG. 1, the first electrode 20 is attached to the first outer surface 18, and the second electrode 21 is attached to the second outer surface 19. The sizes of the electrodes 20, 21 in the plane perpendicular to the force measurement direction M can be adapted to the sizes of the outer surfaces 18 or 19 of the resistive layer 17. The electrodes 20, 21 can be made smaller than the outer surfaces 18 or 19 of the resistive layer 17 to which they are respectively attached in at least one direction perpendicular to the force measurement direction M (FIG. 2).

[0046] It is clear from the embodiment illustrated in FIG. 2 that the electrodes 20, 21 do not necessarily have to be arranged on opposite sides of the resistive layer 17. Rather, the electrodes 20, 21 can also be attached together to the first outer surface 18 of the resistive layer 17 or, as described, to the second outer surface 19.

[0047] The resistance layer 17 is conductive and has a conductivity lower than that of the two electrodes 20, 21. The electrodes 20, 21 can be made of, for example, a metal-containing material. For attachment, the electrodes 20, 21 can be attached to the resistance layer 17, for example, by means of a conductive adhesive bond.

[0048] Referring to FIGS. 3 to 5, the configuration of the resistance layer 17 will be described. The resistance layer 17 includes a mixture of conductive first staple fibers 25 and non-conductive second staple fibers 26. In FIGS. 3 to 5, the conductive first staple fibers 25 are shown in black and the non-conductive second staple fibers 26 are shown in white. The first staple fibers 25 and the second staple fibers 26 are substantially uniformly dispersed within the resistance layer 17. According to an embodiment, this means that, for example, in each observation volume portion having a specific minimum size that extends in the force measurement direction M from the first outer surface 18 to the second outer surface 19 and has a cross-section perpendicular to the force measurement direction M that is smaller than the first outer surface 18 and smaller than the second outer surface 19, the ratio of the number of first staple fibers 25 to the number of second staple fibers 26 is substantially equal. The deviation in the number of the first staple fibers 25 and / or the second staple fibers 26, or the ratio between two observation volume portions, preferably reaches a maximum of 5% or a maximum of 10%. The volume portion has a constant cross-section in the force measurement direction M. The cross-sectional area can be, in particular, on the order of a factor 5 or 7 or 10 smaller than the first outer surface 18 and the second outer surface 19.

[0049] In the resistive layer 17, a large number of first staple fibers 25 always abut against one or more additional first staple fibers 25, and conductive contacts are formed between these abutting first staple fibers 25. The arrangement or density of the first staple fibers 25 is such that one or more parallel conductive connection sites are formed between the first outer surface 18 and the second outer surface 18, or can be formed at least while an external force is applied. Accordingly, there is a conductive connection between the two outer surfaces 18, 19 via the first staple fibers 25 having a variable volume resistance 37. The volume resistance 37 of the resistive layer 17 is defined in the equivalent circuit (Figs. 3 - 5) by the individual contact resistances formed by the first staple fibers 25 being in contact with each other and connected in parallel and / or in series with each other. Figs. 3 - 5 are only schematic diagrams for explaining the principle of different parts of the first staple fibers 25 and do not explicitly illustrate the conductive connection between the two outer surfaces 18, 19.

[0050] For the purpose of achieving a uniform dispersion within the resistive layer 17 by mixing the first staple fibers 25 and the second staple fibers 26, the staple fibers 25, 26 can be dispersed and oriented, for example, by carding or carding. According to an embodiment, the main elongation directions of the staple fibers 25, 26 are, for example, perpendicular to the force measurement direction M, preferably not parallel thereto. This means that more than 50% or more than 75% or more than 90% of the first and second staple fibers have an elongation component parallel to the force measurement direction M that is smaller than the elongation component perpendicular to the force measurement direction M.

[0051] The resistive layer 17 is shown very schematically in the drawing. It can be configured as a non - woven fabric and / or a woven fabric and / or a knitted fabric and / or a warp - knitted fabric, etc. For example, the resistive layer 17 can include woven, knitted, or warp - knitted staple fiber yarns and / or staple fiber twisted yarns that are connected to each other or form a laid fabric. The staple fiber yarns can be realized as ring yarns, rotor yarns, friction yarns, wrap yarns, or air - jet yarns.

[0052] In an embodiment, the second staple fiber 26 is made of plastic, preferably a polymer.

[0053] The first staple fiber 25 contains a conductive material, particularly a conductive carbon such as so-called "carbon black". The first staple fiber 25 can further contain one or more non-conductive components, and for example, can be configured as a bicomponent fiber 27 (Fig. 6). According to this embodiment, the bicomponent fiber 27 has a core 28 with an outer layer 29 entirely or partially coated thereon. The core 28 can be made of a non-conductive and elastically deformable material, or a material that does not elastically deform with respect to the generated force F, for example, a plastic such as a polymer. The outer layer 29 contains a conductive component, for example, conductive carbon. Also, other configurations of the first staple fiber 25 can include a plurality of components, at least one of which is a conductive component. For example, the first staple fiber can contain or consist of a polymer containing a conductive filler.

[0054] In an embodiment, the first staple fiber 25 and the second staple fiber 26 have a fiber length in the range of 20 to 100 mm.

[0055] In Fig. 7, the force sensor 15 is illustrated in an initial position or a stationary position where it does not receive an external force F. Due to the conductive first staple fibers 25 in the resistance layer 17 that are in contact with each other, a conductive path is formed between the first electrode 20 and the second electrode 21 or vice versa through the resistance layer 17, and current can flow. When a force F is applied to the force sensor 15 (Fig. 8), the resistance value R of this conductive path changes. If the force F is a force that compresses the force sensor 15 such that the first outer surface 18 and the second outer surface 19 approach each other compared to the initial state, the resistance value R of the conductive path decreases. Conversely, if a tensile force is applied between the two outer surfaces 18 and 19 instead of the compressive force, the resistance value R of the conductive path will increase.

[0056] To measure an external force F, for example a compressive force, a voltage or current source 33 can be electrically connected to the electrodes 20, 21 via a series resistor 34, as shown by the circuit 16 in FIGS. 1 and 2. At this time, a measurable voltage U that varies in proportion to the resistance value of the total resistance 35 of the conductive path is applied between the first electrode 20 and the second electrode 21. The total resistance 35 of the conductive path is configured in an embodiment of a series connection of a plurality of resistances: a first contact resistance 36 between the first electrode 20 and the resistive layer 17, a volume resistance 37 inside the resistive layer 17 along the formed conductive path, and a second contact resistance 38 between the resistive layer 17 and the second electrode 21. Therefore, the resistance value R of the total resistance 35 corresponds to the sum of the individual resistance values of this series connection. As schematically shown in the equivalent circuits of FIGS. 1 and 2, all of the series-connected resistances 36, 37, 38 can be made variable according to the amount of the applied force F. At least the thickness or height of the elastically deformable resistive layer 17 changes in the force measurement direction M while the force is being applied, and as a result, its volume resistance 37 changes.

[0057] The resistance value R of the total resistance 35 of the conductive path is, in the initial state of the force sensor 15, for example, at most 1000 MΩ when no external force F is applied (FIG. 7). When the maximum external force F is applied to the force sensor 15 that is configured or approved (FIG. 8), the resistance value R of the total resistance 35 of the conductive path is preferably at least 1 kΩ.

[0058] It can be seen schematically from FIGS. 3 to 5 and FIG. 9 that the ratio A of the first staple fiber 25 among the total amount of the first staple fiber 25 and the second staple fiber 26 can vary. The higher the ratio A of the first staple fiber 25 among the total amount of the staple fibers 25 and 26, the higher the conductivity σ of the resistance layer 17 in the force measurement direction M, that is, it is preferably orthogonal to the outer surfaces 18 and 19. By changing the ratio A of the first staple fiber 25, the desired conductivity σ of the resistance layer 17 can be achieved. Thereby, the force sensor 15 can be adapted to different applications, for example, to adjust sufficient sensitivity according to the amount of the external force F to be measured within a possible range. When the range of the amount of the applied force F is very small, it may be reasonable to increase the conductivity σ of the resistance layer 17 in the initial state of the sensor by increasing the ratio A of the first staple fiber 25, which improves the sensitivity to changes in the case of a small force F. When a large force F is measured, the ratio A of the first staple fiber 25 can be selected to be smaller.

[0059] In FIGS. 3 to 5, resistance layers 17 having different ratios A of the first staple fiber 25 are schematically shown. The ratio A is highest in FIG. 3 and lowest in FIG. 5. The higher the ratio A of the first staple fiber 25, the more individual resistors are connected in parallel in the equivalent circuit forming the volume resistance 37 in the resistance layer 17 along the conduction path. This means that in the initial state of the force sensor 15 to which no external force F is applied, the volume resistance 37 in the resistance layer 17 is low and the ratio A of the first staple fiber 25 is high. This correlation is only schematically shown by exemplifying the equivalent circuit of the volume resistance 37 in the resistance layer 17.

[0060] The correlation between the applied force F, for example the amount of compressive force, and the resistance value R of the total resistance 35 of the conductive path is illustrated in FIG. 10 based on the first curve K1 and the second curve K2. The correlation is non-linear. When the applied force F is equal to zero, the force sensor 15 has, in its initial condition, a maximum value Rmax that likewise depends on the proportion A of the first staple fibers 25. The gradients of the curves K1, K2 are higher in the range of smaller forces than in the range of larger forces. The curves K1, K2 can have an exponential progression and can asymptotically approach the minimum value Rmin of the resistance value R as the compressive force increases.

[0061] The first curve K1 in FIG. 10 shows the correlation between the applied compressive force (force F) and the resistance value R for the case of the first proportion A1 of the first staple fibers 25, and the curve K2 shows the correlation for the case of the second proportion A2 of the first staple fibers 25. The second proportion A2 is higher than the first proportion A1. When no external force F is applied, the resistance value R has a maximum value Rmax, the amount of which depends on the proportion A of the first staple fibers 25. Thus, suitable force-resistance characteristics of the force sensor 15 adapted to the application can be achieved by varying the proportion A of the first staple fibers 25.

[0062] Generally, the correlation between the amount of the applied force F and the resistance value R of the total resistance 35 of the conductive path can be specifically influenced or adjusted by the following parameters: The proportion (A) of the first staple fibers (25) and / or among the total amount of the first and first staple staple fibers (26) The fiber type of the first staple fibers (25) and / or The conductivity of the first staple fibers (25). By means of one or more of the indicated parameters, it is possible to influence or adjust, for example, the measurement range of the force F, and / or the minimum resistance value Rmin and / or the maximum resistance value Rmax and / or the difference component between the minimum resistance value Rmin and the maximum resistance value Rmax.

[0063] The present invention relates to a piezoresistive force sensor 15 configured in particular as a pressure sensor and capable of generating a sensor signal that depends on the amount of force F applied to the force sensor 15 in the force measurement direction M. The force sensor 15 comprises a first electrode 20, a second electrode 21, and an elastically deformable resistive layer 17 that electrically connects the two electrodes 20, 21. The resistance value R of the total resistance 35 of the conductive path from the first electrode 20 to the second electrode 21 via the resistive layer 17 changes depending on the amount of force F applied. Therefore, for example, by measuring the voltage U between the electrodes 20, 21 or the current flowing along the conductive path, a sensor signal characterizing the amount of force F applied can be detected. The resistive layer 17 includes conductive first staple fibers 25 and non-conductive second staple fibers 26. In order to adapt the force and resistance characteristics of the force sensor 15 to their respective applications, the ratio A of the first staple fibers 25 to the total amount of the staple fibers 25, 26 can be varied.

Explanation of symbols

[0064] 15 Force sensor 16 Circuit 17 Resistive layer 18 First outer surface 19 Second outer surface 20 First electrode 21 Second electrode 25 First staple fiber 26 Second staple fiber 27 Bicomponent fiber 28 Core 29 Outer layer 33 Voltage or current source 34 Series resistance 35 Total resistance of the conductive path 36 First contact resistance 37 Volume resistance of the conductive path in the resistive layer 38 Second contact resistance A Ratio F Force Kl First curve K2 Second curve M Force measurement direction R Resistance value of the total resistance Rmax Maximum resistance value of the total resistance Rmin The minimum resistance value among the total resistances U Voltage

Claims

1. An elastic deformable resistive layer (17) having a first outer surface (18) and a second outer surface (19) opposite to the first outer surface (18), and containing a mixture of dispersed conductive first staple fibers (25) and non-conductive second staple fibers (26); a conductive first electrode (20) and a conductive second electrode (21); each electrode (20, 21) is disposed on the first outer surface (18) or the second outer surface (19) such that a conductive path is formed from the first electrode (20), through the resistive layer (17), to the second electrode (21); a piezoresistive force sensor (15) in which a resistance value (R) of the resistance of the conductive path depends on an amount of an external force (F) applied between the first outer surface (18) and the second outer surface (19).

2. The piezoresistive force sensor according to claim 1, wherein the first staple fibers (25) and the second staple fibers (26) are substantially uniformly dispersed in the resistive layer (17).

3. The piezoresistive force sensor according to claim 1 or claim 2, wherein a ratio (A) of the first staple fibers (25) in a total amount of the first staple fibers (25) and the second staple fibers (26) reaches at least 5% or at least 10%.

4. The piezoresistive force sensor according to claim 3, wherein a ratio (A) of the first staple fibers (25) in a total amount of the first staple fibers (25) and the second staple fibers (26) is higher than 20%.

5. The piezoresistive force sensor according to any one of claims 1 to 4, wherein the first staple fibers (25) and / or the second staple fibers (26) include a fiber length of 20 to 100 mm.

6. The piezoresistive force sensor according to any one of claims 1 to 5, wherein the resistive layer (17) is formed at each contact site between two first staple fibers (25) in contact with each other, and includes a volume resistance (37) depending on an external force (F), which is caused by individual contact resistances connected in parallel and / or in series with each other.

7. The piezoresistive force sensor according to claim 6, characterized in that the total resistance (35) of the conductive path is formed by a series connection of a first contact resistance (36) between the first electrode (20) and the resistance layer (17), a volume resistance (37) of the conductive path in the resistance layer (17), and a second contact resistance (38) between the resistance layer (17) and the second electrode (21).

8. The piezoresistive force sensor according to any one of claims 1 to 7, characterized in that when no external force (F) is applied to the force sensor (15), the resistance value (R) of the total resistance (35) of the conductive path reaches a maximum of 1000 MΩ.

9. The piezoresistive force sensor according to any one of claims 1 to 8, characterized in that when an external force (F) is applied to the force sensor (15), the resistance value (R) of the total resistance (35) of the conductive path reaches a minimum of 1 kΩ.

10. The correlation relationship (K1, K2) is defined by the ratio (A) of the first staple fiber (25) in the total amount of the first and second staple fibers (26), and / or the fiber type of the first staple fiber (25), and / or the conductivity of the first staple fiber (25), The piezoresistive force sensor according to any one of claims 1 to 9, characterized in that the correlation relationship characterizes a change in the resistance value (R) of the total resistance (35) of the conductive path according to the amount of the external force (F) applied to the force sensor (15).

11. The piezoresistive force sensor according to claim 10, characterized in that the correlation relationship (K1, K2) is non-linear.

12. The piezoresistive force sensor according to claim 10 or claim 11, characterized in that starting from the maximum resistance value (Rmax), the resistance value (R) decreases and asymptotically approaches the minimum resistance value (Rmin) as the amount of the external force (F) increases.

13. The piezoresistive force sensor according to any one of claims 1 to 12, characterized in that the first staple fiber (25) is a bicomponent fiber (27).

14. The piezoresistive force sensor according to any one of claims 1 to 13, characterized in that the resistance layer (17) includes at least one staple fiber yarn including the first staple fiber (25) and the second staple fiber (26).

15. The piezoresistive force sensor according to claim 14, wherein the at least one staple fiber yarn forms a scrim, and / or a woven fabric, and / or a warp knitted fabric.

16. The piezoresistive force sensor according to any one of claims 1 to 15, characterized in that the resistive layer (17) includes a non-woven fabric including the first staple fiber (25) and the second staple fiber (26).

17. A method for manufacturing a force sensor, comprising the following steps: Preparing a conductive first staple fiber (25) and a non-conductive second staple fiber (26); Mixing the first staple fiber (25) and the second staple fiber (26) to form a mixture in which the first staple fiber (25) and the second staple fiber (26) are dispersed; Forming an elastically deformable resistive layer (17) having a first outer surface (18) and a second outer surface (19) on the opposite side of the first outer surface (18) from the mixture; and Attaching the conductive first electrode (20) to the first outer surface (18) or the second outer surface (19), and attaching the conductive second electrode (21) to the first outer surface (18) or the second outer surface (19) such that a conductive path is formed from the conductive first electrode (20) through the resistive layer (17) to the conductive second electrode (20), wherein a resistance value (R) of a total resistance (35) of the conductive path depends on an external force (F) applied between the first outer surface (18) and the second outer surface (19).

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