SENSOR UNIT FOR FLEXIBLE MATERIAL LAYERS

Description

[0001] The present invention relates to a sensor unit for detecting the elongation of a flexible, web- or strip-shaped material layer. The present invention also relates to a method for detecting the elongation of a flexible, web- or strip-shaped material layer.

[0002] Flexible material layers can be described by a variety of different properties. One of these properties is elongation. The elongation of a material is a relative change in length, i.e., extension (positive elongation) or shortening (negative elongation / compression), of a material under load. This load can be generated, for example, by external forces acting on the material or due to temperature changes. Measuring the elongation of a flexible material layer is important in many different applications or can be used for further evaluations of the material's properties.

[0003] In particular, the strain of a material layer can be used to determine the forces acting on it at a given moment. The forces thus measured can, in turn, be useful for analysis, for example, to evaluate the stress on a material.

[0004] Various methods for measuring the strain of a material are known from the prior art. Examples can be found in US 2017 / 322097 A1, DE 10 2014 111869 B3, DE 20 2007 018165 U1, DE 198 00 255 A1, US 2013 / 091962 A1, or US2016 / 317083A1. The strain of a material is often determined indirectly using displacement sensors. For example, a change in resistance, induction, or capacitance is detected in an electronic system. Electronic strain gauges, for example, detect a change in the resistance of a thin measuring grid made of resistance wire, which is applied to a thin plastic carrier. These strain gauges are glued directly to the material layer under test. Even with the smallest strain, the electrical resistance of the measuring strips changes, so that the strain can be measured immediately.

[0005] However, such electronic strain gauges have the disadvantage that they must be applied to each object to be tested. Furthermore, it must be ensured that the adhesive bond between the strain gauge and the material being tested is sufficiently strong to withstand the stresses during the strain test. If the strain gauge detaches from the material, the strain can no longer be measured accurately.

[0006] Alternatively, strain sensors are also known that optically detect the change in length of the material, for example, based on image correlation of surface patterns. Here, the movement of a pattern at specific measuring points is observed, thus determining the strain of a material. However, these measuring methods require precise alignment of the material to the measuring device. Furthermore, such measuring devices, which are often based on laser measurement methods or complex camera technology, are expensive and complex.

[0007] The problem underlying the invention is therefore to provide a sensor unit and a method for detecting the strain of a flexible material layer, with which the strain of a flexible material layer can be detected cost-effectively and reliably.

[0008] According to the invention, this problem is solved by a sensor unit of the type mentioned at the outset, wherein the sensor unit comprises a measuring magnet, wherein the measuring magnet has a fastening means with which the measuring magnet can be fastened to the material layer at a fixed position, and a sensor plate, wherein the sensor plate has a measuring device for detecting a parameter representing the magnetic flux density of a magnetic field of the measuring magnet, wherein the sensor plate further has a fastening means with which the sensor plate can be fastened to the material layer at a fixed position, and has an evaluation unit for determining the strain of the material layer on the basis of the parameter detected by the sensor plate, wherein the measuring device and the measuring magnet are arranged at a distance from one another,in which the magnetic flux density of the magnetic field of the measuring magnet can be detected by the measuring device, wherein the measuring device and the measuring magnet are movable relative to one another in a material layer plane by the stretching of the material layer in the direction of the web or strip, so that a distance of the measuring magnet from the measuring device depends on the stretching of the material layer in the direction of the web or strip, wherein the parameter detected by the measuring device depends on the distance between the measuring device and the measuring magnet and thus on the stretching of the material layer in the direction of the web or strip.

[0009] The sensor unit according to the invention is used to detect the expansion of a flexible material layer. A material layer, in the context of the present invention, is understood to be a flat object that has a significantly smaller expansion (preferably a difference of > a factor of 5) in one spatial direction than in the other two spatial directions.

[0010] A material layer is "flexible" within the meaning of the present invention if the material layer is reversibly deformable in more than one direction by a force acting on the material layer during intended use of a product consisting at least partially of the material layer. "Reversible" means that the material layer can be returned to its original shape prior to the application of the force by a force of the same magnitude, without being damaged by the force acting on it. A material layer is flexible within the meaning of the invention in particular if it is reversibly deformable by gravity and / or human muscle power.

[0011] The sensor unit according to the invention is used to detect the elongation of web- or strip-shaped material layers. In this case, the expansion of the material layer in a spatial direction of the web or strip is significantly greater than the expansion in a spatial direction perpendicular to it (preferably > a factor of 10). In this case, the expansion of the material layer in the direction of the web or strip is detected in particular.

[0012] In certain embodiments, the material layer is a layer made of a material selected from among stretchable plastic sheets, films, textile fabrics, and combinations thereof. In particular embodiments, the material layer is a stretchable fitness band, a fabric section of a sportswear item, a layer of an orthopedic bandage or medical dressing, or a cover of a sports equipment.

[0013] In embodiments where the sensor unit detects the stretch of a fitness band, the resulting stretch data can be used to provide the user of the fitness band with feedback, preferably in real time, on how much force is being exerted during training. This allows for more targeted exercise and thus increased training success.

[0014] However, the present invention can be used to determine not only the stretch of fitness bands, but also the stretch of any other stretchable material layer, such as the stretch of a fabric. In certain embodiments of the invention, at least one sensor unit according to the invention is attached to a fabric section of a piece of sportswear in order to measure the movements of a joint during the practice of a sport. If multiple sensor units are attached to multiple fabric sections of a piece of sportswear, each joint on the human body could be measured in its range of motion, thus determining a movement profile of the wearer of the sportswear.

[0015] Alternatively, the sensor unit according to the invention can also provide information about the remaining elasticity of a fabric. Particularly with plastics, material fatigue over time leads to a material layer increasingly failing to return to its original state after exposure to stress. The elasticity of the material layer decreases accordingly. If the elasticity of such a material layer is monitored with the sensor unit according to the invention, it may be possible, based on empirical values, to determine a point in time at which a fatigue fracture of the material is to be expected. For example, accidents involving sports equipment or extended downtimes for machinery can be avoided if the material layer is replaced before fatigue fracture occurs.

[0016] To detect the expansion of the flexible material layer, the invention provides that the sensor unit essentially comprises two parts. One part of the sensor unit contains the measuring magnet, and another part of the sensor unit contains a sensor plate with a measuring device for detecting the measuring magnet. These two parts are each separately attached to the flexible material layer at a fixed position. This means that once the parts are attached, they can no longer move relative to the material layer.

[0017] It goes without saying that the measuring device is also firmly attached to the sensor plate. However, by attaching the parts separately using appropriate fasteners, movement of the parts relative to one another is enabled when the flexible material layer stretches, i.e., when a load acts on the material layer. In other words, the measuring magnet only moves relative to the measuring device when the flexible material layer is stretched. The movement of the measuring magnet relative to the measuring device therefore always occurs in the direction of the stretching and in a material layer plane or in a plane parallel to the material layer plane.

[0018] In one embodiment, the measuring magnet has axial or radial magnetization. This offers the advantage that the sensor unit operates reliably even when the material layer is rotated relative to the direction of stretching. The measuring magnet is particularly preferably a neodymium disc magnet with axial magnetization.

[0019] To ensure proper strain measurement, it is particularly advantageous if the sensor unit exerts as little influence as possible on the strain of the material layer. In one embodiment, the fastening means of the sensor plate and the measuring magnet are designed to ensure a selective attachment of the measuring magnet and the sensor plate to the material layer at fastening points, allowing the material layer to slide freely outside the fastening points during strain, with the strain of the material layer preferably not being influenced by the fastening means.

[0020] For example, if the material layer is a fabric, the fastening means can be designed in the form of needles or centering pins, each of which penetrates only one fabric honeycomb or several adjacent fabric honeycombs, but does not affect or damage the fibers of the fabric. At the same time, however, the fastening means must also be suitable for attaching the sensor plate and the measuring magnet to the specified location on the material layer. In certain embodiments, needles or centering pins are attached to the sensor plate and the measuring magnet for this purpose and, on a side of the material layer facing away from the sensor unit, are engaged with plugs that prevent spontaneous loosening of the connection between the sensor unit and the material layer.

[0021] In order to detect the relative distance between the two parts of the sensor unit and thus the expansion of the material layer, a measuring device is provided on the sensor plate according to the invention, which measures a parameter of the magnetic field of the measuring magnet. This parameter represents, in particular, the magnetic flux density of the magnetic field. However, it is understood that any other parameter of a magnetic field is also suitable, provided that it can be used to determine the distance between the measuring magnet and the measuring device.

[0022] In one embodiment, the measuring device is a Hall sensor, where the measured parameter is a voltage or a current. A Hall sensor is typically a semiconductor element through which a current flows. If this semiconductor element is placed in a magnetic field, the moving charge carriers in the semiconductor element are influenced by the Lorentz force acting in the magnetic field, resulting in charge carrier separation. This charge carrier separation, in turn, creates an electric field that counteracts the Lorentz force. A voltage created in the semiconductor element due to the electric field can then be measured. This voltage directly represents the magnetic flux strength of the magnetic field.

[0023] In a further embodiment, the fastening means of the sensor plate comprise two position magnets arranged on the sensor plate and two corresponding counter magnets, wherein the measuring device is arranged such that the measuring device has the same distance from each of the position magnets, wherein the parameter detected by the measuring device depends on a distance of the measuring magnet perpendicular to a connecting line of the position magnets.

[0024] The position magnets, which are fixedly arranged on the sensor plate, together with the corresponding counter magnets, serve to attach the sensor plate to the flexible material layer by clamping the material layer between the position magnets and the counter magnets. Furthermore, a centering pin can be provided, which is arranged on the position magnet or the counter magnet, penetrates the material layer at the specified position, and engages an opening in the respective opposite counter magnet or position magnet to attach the position magnets and thus the sensor plate and the measuring device to the material layer.

[0025] On the other hand, the position magnets enable the definition of a reference line to which the distance of the measuring magnet is determined. The position magnets are arranged on the flexible material layer in such a way that the direction of extension of the material layer, and thus the distance of the measuring magnet from the measuring device, is always recorded in a direction perpendicular to the connecting line of the position magnets.

[0026] The position magnets thus form fixed reference points that are defined independently of the material layer in the sensor unit. Fixing the sensor plate to the flexible material layer via the two position magnets allows the measurement data to be transferred to different material layers with the same manufacturing pattern, without the need for individual calibration for each material.

[0027] In order to enable repeatable measurements of the strain of the material layer, it is essential that the measuring points do not shift relative to the material layer.

[0028] The movement of the parts of the sensor unit occurs solely through the stretching of the material layer.

[0029] In a further embodiment of the invention, at least the measuring magnet and preferably also the position magnets have a circular or annular cross-section, wherein particularly preferably the fastening means are arranged in the center of the circular or annular cross-section.

[0030] This offers the advantage that the measuring magnet and the position magnets can be attached to a fixed position on the material layer, and that the position magnets can be arranged symmetrically relative to the measuring device. This is important to ensure that the position magnets do not influence the measuring device when measuring the magnetic field of the measuring magnet. The position magnets must therefore be arranged so that their magnetic forces cancel each other out. The precise positioning of the sensor plate and the measuring magnet by the position magnets increases the measurement accuracy of the sensor unit according to the invention.

[0031] In addition, a circular measuring magnet offers the advantage that even if the magnet is rotated relative to the measuring device, the measuring accuracy is not affected.

[0032] In a further embodiment, the sensor plate has a recess on a side facing the measuring magnet, wherein the measuring magnet can be arranged at least partially in the recess. The measuring magnet is therefore always located in the recess of the sensor plate when no or only little positive expansion is being exerted on the flexible material layer. With increasing positive expansion of the flexible material layer, the measuring magnet moves out of the recess of the sensor plate and the distance between the measuring device and the measuring magnet increases. The recess offers the advantage that the measuring magnet can be arranged more easily in the correct position relative to the measuring device. This is particularly advantageous in applications in which the user himself positions the sensor unit. This could, for example,This can be the case when using a fitness band if the user wants to independently select and change the area in which the stretch is to be recorded.

[0033] In another embodiment, the sensor plate is made of a flexible, non-stretchable material, so that the sensor plate adapts to the bending but not to the stretching of the material layer. As with the fastening means, it is important that the sensor plate also influences the stretching of the material layer as little as possible. At the same time, however, a flexible sensor plate increases the stability of the connection between the sensor plate and the flexible material layer. Furthermore, wearing comfort is increased when the sensor unit according to the invention is attached to sportswear, for example.

[0034] With regard to the flexibility and adaptability of the sensor unit to the shape of the material layer, in one embodiment the thickness of the sensor unit perpendicular to the material layer is at most 1 cm (without an intermediate material layer), preferably at most 5 mm (without an intermediate material layer). To ensure sufficient stability of the sensor unit, the thickness of the sensor unit without a material layer is at least 2 mm, preferably 3 mm.

[0035] In one embodiment, the length of the sensor unit parallel to the direction of stretching is at least 20 mm, preferably at least 45 mm and at most 60 mm. The width of the sensor unit is preferably adapted to the width of the material layer and, in one embodiment, is 30 mm, which corresponds, for example, to the standard width of a narrow fitness band.

[0036] In a further embodiment, a counterplate for the sensor plate and a counterpart to the measuring magnet are also provided, wherein the material layer can be arranged between the sensor plate and the counterplate and the measuring magnet and the counterpart, wherein the counterplate and the counterpart have fastening means corresponding to the fastening means of the sensor plate and the measuring magnet, so that the counterplate can be connected to the sensor plate and the counterpart to the measuring magnet, in particular by means of a plug connection, for example by means of the centering pins described above, wherein the fastening means of the counterplate and the counterpart are preferably designed to be magnetic, so that the counterplate and the counterpart are additionally held to the sensor plate and the measuring magnet by a magnetic connection.

[0037] The counterplate and counterpart allow the sensor unit to be attached to the material layer particularly effectively. A plug connection as thin as possible between the respective components ensures that the expansion of the material layer is influenced as little as possible by the fastening elements. Furthermore, the strength of the magnetic connection between the components is selected so that expansion of the material is still unhindered.

[0038] Particularly preferably, the position magnets, the measuring magnet, and the respective counter magnets are designed as ring-shaped magnets. Using ring-shaped magnets on both sides further improves the positioning accuracy of the sensor unit, since two ring-shaped magnets interacting with each other tend to center each other, preventing the magnets from slipping, even if the material layer is exposed to vibrations, for example.

[0039] In a further embodiment, in addition to the fastening means, holding magnets are provided on the sensor plate, wherein the holding magnets interact with opposing holding magnets arranged on the counterplate. These holding magnets provide additional fastening of the sensor plate to the counterplate. This is particularly advantageous if, in addition to the measuring device and the position magnets, the sensor plate contains further electronic components that require space in addition to the position magnets and the measuring device and therefore cannot be held close to the material layer solely by the position magnets on which the fastening means are arranged.

[0040] In a further embodiment, the sensor unit further comprises a transmission unit configured to transmit, preferably wirelessly, a signal corresponding to the parameter detected by the measuring device to an evaluation unit during intended use of the sensor unit. This offers the advantage that data detected by the measuring device can be transmitted directly, for example, to a user's terminal device. Thus, the user of a fitness band receives immediate feedback on the force exerted during a training session. Exercises currently being performed can be adapted in real time to the optimal force requirements.

[0041] In a further embodiment, the evaluation unit is therefore configured in such a way that the force applied for the extension can be determined from the transmitted signal.

[0042] In a further embodiment, the sensor unit further comprises a position, temperature, and / or acceleration sensor. Thus, in addition to the applied force, the position or acceleration during a workout can also be recorded. This also offers the advantage that, in the case of a fitness band, for example, it is possible to distinguish what type of movement is currently being performed. If, for example, the band is only being transported, different accelerations occur than in training mode.

[0043] In a further embodiment, the sensor unit further comprises control elements for switching the measuring device on and off. This offers the advantage that the measuring device can be switched off when not in use, thus increasing the battery life of the sensor unit. However, it is also conceivable that in an alternative embodiment, the measuring device is automatically switched off after a certain period of time, for example, when no more strain is detected in the material layer.

[0044] Furthermore, the problem underlying the invention is also solved by a method for detecting the elongation of a flexible, web- or band-shaped material layer, the method comprising the following steps: A) Attaching a sensor plate with a measuring device to the material layer, B) Attaching a measuring magnet to the material layer, wherein the measuring device and the measuring magnet are movable relative to one another in a material plane in the direction of the stretching of the material layer in the direction of the web or strip, so that a distance of the measuring magnet to the measuring device depends on the stretching of the material layer in the direction of the web or strip, C) Detecting a parameter representing the magnetic flux density of a magnetic field of the measuring magnet as a function of the distance of the measuring magnet to the measuring device, D) Determining the stretching of the material layer in the direction of the web or strip on the basis of the parameter detected in step C).

[0045] Further advantages, features, and possible applications of the present invention will become clear from the following description of an embodiment and the accompanying figures. In the figures, identical elements are designated by identical reference numerals. Figure 1 shows a schematic representation of an embodiment of the sensor unit according to the invention on a flexible material layer with the cover open. Figure 2a shows a plan view of an embodiment of the sensor unit according to the invention while the material layer is not stretched. Figure 2b shows the Figure 2a shown embodiment during stretching of the material layer. Figure 3a shows a three-dimensional representation of the sensor plate and the fastening means of an embodiment of the sensor unit according to the invention while the material layer is not stretched. Figure 3b shows the Figure 3ashown embodiment during a stretching of the material layer. Figure 4 shows a schematic representation of an embodiment of the sensor unit according to the invention with a sensor plate, a measuring magnet and corresponding opposites. Figure 5a shows the Figure 4 The embodiment shown is arranged on a material layer while the material layer is not stretched. Figure 5b shows the Figure 4 shown embodiment on a material layer, during stretching of the material layer.

[0046] In Figure 1 The sensor unit 1 according to the invention is shown with a sensor plate 20 and a measuring magnet 10 on a material layer 2. The sensor unit 1 can be covered with a cover 5 so that the electronics of the sensor unit 1 are protected from external influences.

[0047] Both the measuring magnet 10 and the sensor plate 20 are attached to the material layer 2 by means of appropriate fastening means. For this purpose, the measuring magnet 10 has an opening 11 in which a centering pin 31 of the counterpart (see Figure 4 ) engages. Likewise, the position magnets 23, 23' of the sensor plate 20 have openings 22, 22' into which centering pins 41, 41' of the counter-plate 40 engage in order to fasten the sensor unit 1 to the material layer 2.

[0048] The centering pins 31, 41, 41', which are used particularly in Figure 4 , but also in the Figures 3a and 3bare shown, result in both the sensor plate 20 and the measuring magnet 10 being attached to a fixed position on the material layer 2. The attachment of the components of the sensor unit 1 to the material layer 2 is additionally supported by the magnetic properties of the measuring magnet 10 or the position magnets 23, 23' and corresponding counter magnets 24, 24', 30.

[0049] Furthermore, a measuring device 21 is arranged on the sensor plate 20, which is at the same distance from both position magnets 23, 23'. The measuring device 21 is configured such that a distance to the measuring magnet 10 can be detected with the measuring device 21. In particular, a distance of the measuring magnet 10 perpendicular to a connecting line 50 of the position magnets 23, 23' can be detected, whereby the sensor unit 1 according to the invention can be applied to different material layers 2 of the same tissue type without the need for recalibration.

[0050] The distance between the measuring magnet 10 and the measuring device 21 changes when the material layer 2 undergoes stretching. In this case, the measuring magnet 10 moves out of the recess 25 of the sensor plate 20 in the direction of the stretching. If the material layer 2 is, for example, a fitness band, the band stretches depending on the force applied by the user. Consequently, by detecting the stretching of the material layer 2 using the sensor unit 1 according to the invention, the user can be given feedback on how much the fitness band was stretched during a workout, i.e., how much force was applied.

[0051] The measuring device 21 is a Hall sensor that detects a change in a voltage or current value when the magnetic field of the measuring magnet 10 is moved, thus exerting a changed Lorentz force on the Hall sensor 21. From these values measured by the measuring device 21, a distance between the measuring device 21 and the measuring magnet 10 can be determined, thus determining the strain of the material layer 2. The strain of the material layer 2, in turn, provides information about the forces acting on the material layer 2.

[0052] In the embodiment shown, the data recorded by the measuring device 21 is transmitted to a transmission device 3, which in turn transmits the data via a wireless connection to an evaluation unit, for example, a smartphone. The user can then view the force exerted during training in real time on the smartphone, for example, and thus optimize the training.

[0053] The sensor unit 1 according to the invention also has a position, temperature, and acceleration sensor 4, with which further information can be made available to the user in addition to the applied force. In particular, the acceleration sensor 4 can also be used to draw conclusions about the type of movement the sensor unit 1 according to the invention is exposed to at a specific point in time. For example, the acceleration forces acting while transporting the fitness band in a bag differ from those during training. Data acquisition using the measuring device 21 therefore particularly preferably only takes place during those movement states that also require data acquisition. This extends the battery life of the sensor unit 1 according to the invention. The battery of the sensor unit 1 according to the invention can be charged, for example, inductively or via a detachable cable connection.

[0054] In the Figures 2a and 2b A top view of the cover 5 of the sensor unit 1 according to the invention is shown. The cover 5 of the sensor unit 1 according to the invention essentially represents the individual components of the sensor plate 20. If the material layer 2 is not stretched, as in Figure 2a As shown, the measuring magnet 10 is completely in the recess 25 of the sensor plate 20. As soon as the material layer 2 experiences an expansion, the measuring magnet 10 moves out of the recess 25 of the sensor plate 20, as shown in Figure 2b The effect of the stretching of the material layer 2 on the movement of the measuring magnet 10 can also be seen in the Figures 3a and 3b as well as 5a and 5b.

[0055] In the Figures 3a and 3bAlso shown are the centering pins 31, 41, 41' of the counterplate 40 and the counterpart 30, which engage in the openings 11, 22, 22' of the measuring magnet and the sensor plate to secure the sensor plate 20 and the measuring magnet 10 to the material layer 2 at a fixed position. The centering pins 31, 41, 41' are dimensioned such that they do not affect the fabric of the material layer 2, but rather engage in a fabric honeycomb of the material layer 2. Thus, the extensibility of the material layer 2 is not affected by the centering pins 31, 41, 41'.

[0056] In Figure 4 It is shown how the centering pins 31, 41, 41' engage in the openings 11, 22, 22'. The material layer 2 is arranged between the sensor plate 20, the measuring magnet 10, the counter plate 40 and the counterpart 30, as shown in the Figures 5a and 5b shown.

[0057] To securely hold the sensor plate 20 to the material layer 2 at its full extension, additional holding magnets 26 on the sensor plate and holding magnets 42 on the counterplate are provided, which interact with each other. The sensor plate 20 is also made of a flexible material, allowing the sensor plate 20 to follow any bend in the material layer 2. The same applies to the counterplate 40.

[0058] In the embodiment shown, the sensor unit according to the invention has a total dimension of 48.5 mm x 30 mm x 4.75 mm (length x width x height). The sensor unit 1 according to the invention is thus compact enough not to impair the movement of the material layer 2, e.g., the use of a fitness band or other sports equipment. The weight of the sensor unit 1 is correspondingly low. List of reference symbols

[0059] 1Sensor unit 2Material layer 3Transmission unit 4Position, temperature, and acceleration sensor 5Cover 10Measuring magnet 11Opening of the measuring magnet 20Sensor plate 21Measuring device 22, 22'Openings of the sensor plate 23, 23'Position magnets 24, 24'Counter magnets to position magnets 25Recess 26Holding magnets on sensor plate 30Counterpiece 31Centering pin for counterpart 40Counterplate 41, 41'Centering pins for counterplate 42Holding magnets on counterplate 50Connecting line

Claims

1. Sensor unit (1) for detecting the elongation of a flexible, web- or strip-shaped material layer (2), wherein the sensor unit (1) comprises - a measuring magnet (10), wherein the measuring magnet (10) has a fastening means (11) with which the measuring magnet (10) can be fastened to the material layer (2) at a predetermined position, and - a sensor plate (20), wherein the sensor plate (20) has a measuring device (21) for detecting a parameter representing the magnetic flux density of a magnetic field of the measuring magnet (10), wherein the sensor plate (20) further has a fastening means (22) with which the sensor plate (20) can be fastened to the material layer (2) at a predetermined position, and - an evaluation unit for determining the elongation of the material layer (2) on the basis of the parameter detected by the sensor plate (20), wherein the measuring device (21) and the measuring magnet (10) are arranged at a distance from each other at which the magnetic flux density of the magnetic field of the measuring magnet (10) can be detected by the measuring device (21), wherein the measuring device (21) and the measuring magnet (10) are movable relative to each other in a material layer plane due to the elongation of the material layer (2) in the direction of the web or strip, so that a distance between the measuring magnet (10) and the measuring device (21) depends on the elongation of the material layer (2) in the direction of the web or strip, wherein the parameter detected by the measuring device depends on the distance between the measuring device (21) and the measuring magnet (10) and thus on the elongation of the material layer (2) in the direction of the web or strip.

2. Sensor unit (1) according to one of the preceding claims, wherein the material layer (2) is a layer consisting of a material which is selected from stretchable plastic films, foils, textile fabrics, and combinations thereof, wherein the material layer (2) is preferably an expandable fitness band, a section of fabric of sportswear, a layer of an orthopedic bandage or a medical dressing, or a cover of a sports device.

3. Sensor unit (1) according to one of the preceding claims, wherein the fastening means (11, 22) cause the measuring magnet (10) and the sensor plate (20) to be fastened at specific points on the material layer (2) at fastening points so that the material layer (2) can slide freely outside the fastening points during elongation, wherein preferably the elongation of the material layer (2) is not influenced by the fastening means (11, 22).

4. Sensor unit (1) according to one of the preceding claims, wherein the measuring device (21) is a Hall sensor and wherein the parameter detected is a voltage or a current.

5. Sensor unit (1) according to one of the preceding claims, wherein the fastening means (22) of the sensor plate (20) have two position magnets (23, 23') arranged on the sensor plate (20) and two corresponding counter magnets (24, 24'), wherein the measuring device (21) is arranged such that the measuring device (21) has the same distance from each of the position magnets (23, 23'), wherein the parameter detected by the measuring device (21) depends on a distance of the measuring magnet (10) perpendicular to a connecting line (30) of the position magnets (23, 23').

6. Sensor unit (1) according to one of the preceding claims, wherein at least the measuring magnet (10) and preferably also the position magnets (23, 23') have a circular or ring-shaped cross-section, wherein it is particularly preferred that the fastening means (11, 22) are arranged in the center of the circular or ring-shaped cross-section.

7. Sensor unit (1) according to one of the preceding claims, wherein the sensor plate (20) has a recess (25) on a side facing the measuring magnet (10), wherein the measuring magnet (10) can be arranged at least partially in the recess (25).

8. Sensor unit (1) according to one of the preceding claims, wherein the sensor plate (20) is made of a flexible, non-stretchable material so that the sensor plate (20) adapts to the bending but not to the stretching of the material layer (2).

9. Sensor unit (1) according to one of the preceding claims, wherein a counterplate (40) is provided for the sensor plate (20) and a counterpart (30) is provided for the measuring magnet (10), wherein the material layer (2) can be arranged between the sensor plate (20) and the counterplate (40) and the measuring magnet (10) and the counterpart (30), wherein the counterplate (40) and the counterpart (30) have fastening means (31, 41) corresponding to the fastening means (11, 22) of the sensor plate (20) and the measuring magnet (10), so that the counter plate (40) can be connected to the sensor plate (20) and the counterpart (30) to the measuring magnet (10), in particular by means of a plug connection, wherein preferably the fastening means (31, 41) of the counterplate (40) and the counterpart (30) are magnetically designed so that the counterplate (40) and the counterpart (30) are additionally held on the sensor plate (20) and the measuring magnet (10) by a magnetic connection.

10. Sensor unit (1) according to the preceding claim, wherein, in addition to the fastening means (11, 22, 31, 41), holding magnets (26) are provided on the sensor plate (20), wherein the holding magnets (26) interact with opposing holding magnets (42) arranged on the counterplate (40).

11. Sensor unit (1) according to one of the preceding claims, wherein the sensor unit (1) further comprises a transmission unit (3) which is designed such that, during the intended use of the sensor unit (1), it is suitable for the preferably wireless transmission of a signal corresponding to the parameter detected by the measuring device (21) to an evaluation unit.

12. Sensor unit (1) according to the preceding claim, wherein the evaluation unit is designed to determine the required force for the elongation from the transmitted signal.

13. Sensor unit (1) according to one of the preceding claims, wherein the sensor unit (1) further comprises a position, temperature and / or an acceleration sensor (4).

14. Sensor unit (1) according to one of the preceding claims, wherein the sensor unit (1) further comprises operating elements for switching the measuring device (21) on and off.

15. Method for detecting the elongation of a flexible, web- or strip-shaped material layer (2), comprising the steps: a. attaching a sensor plate (20) with a measuring device (21) to the material layer (2), b. attaching a measuring magnet (10) to the material layer (2), wherein the measuring device (21) and the measuring magnet (10) are movable relative to each other in a material layer plane in the direction of the elongation of the material layer (2) in the direction of the web or strip, so that a distance between the measuring magnet (10) and the measuring device (21) depends on the elongation of the material layer (2) in the direction of the web or strip, c. detecting a parameter representing the magnetic flux density of a magnetic field of the measuring magnet (10) as a function of the distance between the measuring magnet (10) and the measuring device (21), d. determining the elongation of the material layer (2) in the direction of the web or strip on the basis of the parameter detected in step c.