JP7699374B2 - Bending sensor and method for manufacturing bending sensor - Google Patents

Description

The present invention relates to a bending sensor that utilizes resistance changes caused by opening and closing cracks in a conductive film, and a method for manufacturing the bending sensor.

There are bending sensors and strain sensors for detecting changes in the amount of bending or curvature of components. In recent years, it has been proposed to integrate such sensors into prosthetics used by people to continuously measure the movements of the user and joints, and to link them to health management and IoT services. Among these sensors, sensors that detect the amount of bending and other parameters via changes in electrical resistance are known, in particular sensors that use resistance changes due to the opening and closing of cracks in a conductive film.

For example, Patent Document 1 discloses a bending sensor that detects bending and changes in a component by providing a conductive film with tiny cracks on a flexible substrate provided on the component, and opening and closing the cracks in the conductive film as the substrate is bent as the component deforms, thereby changing the electrical resistance (conductivity). Specifically, if a diagonal cut is made in a conductive elastomer in advance, the cut opens as the bend increases, and the electrical resistance of the conductive elastomer increases. On the other hand, when the conductive elastomer is straightened, the cut closes and the electrical resistance of the conductive elastomer decreases. The bending of the component to which the conductive elastomer is provided is detected from this change in electrical resistance.

In such sensors that use resistance changes caused by the opening and closing of cracks in a conductive film, if the cracks do not open and close stably, the sensitivity of the resistance change to bending will change, causing unstable operation.

For example, Patent Document 2 discloses a bending sensor in which a high-resistance conductive film (high-resistance layer) and a low-resistance conductive film (low-resistance layer) in which cracks have been introduced in advance are arranged with an insulating section between them, and the sensor is connected in parallel to multiple electrode parts to perform on/off operation. In the off state, the cracks in the low-resistance layer are closed, and the combined resistance of the electrical resistance of the high-resistance layer and the electrical resistance of the low-resistance layer is output as the off resistance from the multiple electrode parts. On the other hand, in the on state, the cracks in the low-resistance layer open, and at least the electrical resistance of the high-resistance layer is output as the on resistance from the multiple electrode parts. Since the electrical resistance of the low-resistance layer mainly depends on the state of the cracks (distribution, opening, shape, etc. of the cracks), the high-resistance layer is less likely to cause variations in electrical resistance and sensitivity than the low-resistance layer, and the sensor is said to be able to operate stably by outputting the electrical resistance of the high-resistance layer in the on state in which the cracks open.

U.S. Pat. No. 5,086,785 International Publication No. 2012/060427

As mentioned above, in a sensor that uses resistance changes caused by the opening and closing of cracks in a conductive film, it is necessary to have high sensitivity to resistance changes with bending and to be able to obtain stable operation. This requires that the cracks open and close stably. Furthermore, if there is unevenness in the introduction of cracks into the conductive film, even with the same amount of bending, differences in resistance changes will occur depending on the bending method, and stable operation will not be obtained.

The present invention was made in consideration of the above-mentioned circumstances, and its purpose is to provide a bending sensor that uses resistance changes caused by opening and closing cracks in a conductive film, and that has high sensitivity to resistance changes due to bending and can operate stably, and a method for manufacturing the same.

The bending sensor according to the present invention is a bending sensor that detects bending from a change in resistance caused by opening and closing of cracks introduced into a conductive film provided on the main surface of a substrate so as to form a strip-shaped conductive path, and is characterized in that the substrate is a woven fabric having irregularities at regular intervals on the main surface, the conductive path includes multiple cracks that penetrate the conductive film in the thickness and width directions, and the cracks are located at positions corresponding to the irregularities.

This feature makes it possible to obtain high sensitivity to resistance changes due to bending and stable operation in a bending sensor that uses resistance changes caused by the opening and closing of cracks in a conductive film.

The method for manufacturing a bending sensor according to the present invention is a method for manufacturing a bending sensor that detects bending from a change in resistance due to the opening and closing of cracks introduced into a conductive film provided on the main surface of a substrate so as to form a strip-shaped conductive path, and is characterized in that the substrate is a woven fabric having irregularities at a constant interval on the main surface, a conductive material that provides the conductive film is applied onto the substrate, and the substrate is bent to provide the conductive path with multiple cracks that penetrate the conductive film in the thickness and width directions, and the cracks are provided at positions corresponding to the irregularities.

With these features, it is possible to easily manufacture a bending sensor that uses resistance changes caused by the opening and closing of cracks in a conductive film, and that has high sensitivity to resistance changes in response to bending and can operate stably.

1A is a top view of a bending sensor according to an embodiment of the present invention, and FIG. 1 is a side cross-sectional view of an embodiment of a bending sensor according to the present invention; 13A is a top view of a bending sensor according to another embodiment, FIG. 13B is an enlarged view of a main portion, and FIG. 13A and 13B are a top view and a side cross-sectional view, respectively, of a bending sensor according to still another embodiment. 10A to 10C are side cross-sectional views showing a manufacturing process of the bending sensor. 1 is an enlarged photograph of a main part of the manufactured bending sensor. 1A and 1B are photographs of a main part of the manufactured bending sensor, taken with reflected light and transmitted light, respectively. 1 is an enlarged photograph of a main part of a bending sensor manufactured using a woven fabric made of twisted yarn as a base material. FIG. 13 is a pattern diagram of a conductive film on a substrate in a production test. 1 is a diagram illustrating a method for measuring an electrical resistance value and a method for calculating a strain. 1 is a graph showing the relationship between strain and rate of resistance change. 1 is a table showing differences in gauge factor depending on the base material. 1A is a top view of a protective film formed on a conductive film in a manufacturing test, and FIG. 1B is a top view of a protective film formed with a gap therebetween. 11 is a table showing the relationship between the presence or absence of a gap between the protective film and the conductive film and the gauge factor. 13 is an enlarged photograph of a main part of a bending sensor manufactured by changing the diameter of the core material. 1 is a table showing the number of cracks and the gauge factor depending on the diameter of the core material.

Below, a bending sensor and its control method, which are one embodiment of the present invention, will be described with reference to Figures 1 to 5.

As shown in FIG. 1, the bending sensor 10 is formed by providing a conductive film 2 on the main surface of the substrate 1 so as to form a strip-shaped conductive path, and introducing multiple cracks 3 into the conductive film 2. Here, the conductive path is arranged in a U-shape. The substrate 1 is a woven fabric having irregularities at regular intervals on its main surface, and cracks are formed that penetrate the conductive film 2 in the thickness direction and width direction at positions corresponding to the irregularities. For example, the woven fabric is formed by weaving together warp threads 1a and weft threads 1b that are perpendicular to each other. In other words, the irregularities are formed on the surface of the woven fabric by weaving together the warp threads 1a and weft threads 1b of the woven fabric. If the woven fabric is made of plain weave warp threads 1a and weft threads 1b, it has high strength, is resistant to wear, and is less likely to stretch and deform, making it suitable for use as the substrate 1 of a bending sensor.

Here, the cracks 3 are formed so as to extend in the width direction of the bending sensor 10 and penetrate the conductive film 2 in the thickness direction and width direction. Furthermore, the cracks 3 are located at positions corresponding to the unevenness. For example, suppose that the warp threads 1a are arranged in the length direction of the bending sensor 10 and the weft threads 1b are arranged in the width direction. Then, the number of cracks counted in the length direction corresponds to the number of unevenness caused by the weft threads 1b, such as one crack 3 for every two weft threads 1b extending in the width direction. In other words, the number of cracks 3 corresponds to the number of meshes of the woven fabric, and the cracks 3 are also arranged periodically in accordance with the unevenness of a certain period caused by the mesh of the woven fabric, which is the base material 1. It has been observed that the number of cracks 3 is an integer divided by the number of weft threads 1b in this correspondence in the length direction. In other words, the number of cracks counted in the length direction is an integer divided by the number of threads counted in the same direction, and the period of the positions where the cracks are generated is an integer multiple of the period of the threads.

As shown in Figure 2, (a) the crack 3 closes when the substrate 1 of the bending sensor 10 is in a flat plate state, and (b) the crack 3 opens when the conductive film 2 of the bending sensor 10 is bent in the longitudinal direction so that it is on the outside of the substrate 1. The opening and closing of this crack changes the electrical resistance of the conductive film 2, and the bending of the bending sensor 10 and the member to which the bending sensor 10 is attached can be detected from this resistance change.

In this way, with the bending sensor 10, the cracks 3 penetrating in the thickness and width directions are arranged at positions corresponding to the periodic unevenness of the substrate 1 made of woven fabric, so that the reproducibility of the opening and closing of the cracks 3 for a certain bending can be increased. As a result, the resistance change due to the opening and closing of the cracks 3 is stable and high sensitivity can be obtained.

As shown in FIG. 3, a bending sensor 10a can be obtained in which a conductive film 2 is provided on the main surface of the substrate 1 via an insulating film 4. For example, the insulating film 4 is formed so as to cover the entire lower surface of the conductive film 2, and the conductive film 2 can be insulated from the substrate 1. This allows a conductive material to be used for the substrate 1. The crack 3 is formed so as to penetrate both the conductive film 2 and the insulating film 4 in the thickness direction and width direction.

As shown in FIG. 4, a bending sensor 10b having a protective film 5 can also be obtained. The protective film 5 faces the main surface of the substrate 1 and is formed to have a gap with the conductive film 2. In other words, the conductive film 2 is provided with a gap below the protective film 5, and this gap prevents the protective film 5 from affecting the opening and closing of the crack 3. Such an arrangement can be obtained, for example, by forming a spacer 6 between the substrate 1 and the conductive film 2 at a position that surrounds the conductive film 2 and is aligned with the outer periphery of the protective film 5. Note that a protective film 5 may be further provided on the bending sensor 10a using the insulating film 4 described above.

Next, we will explain the manufacturing method of bending sensors 10, 10a, and 10b.

As shown in FIG. 5(a), first, a substrate 1 is prepared. As described above, the substrate 1 is a woven fabric having irregularities at a regular interval on its main surface. The substrate 1 is of a size that allows multiple bending sensors to be arranged side by side, and multiple bending sensors may be manufactured simultaneously.

Next, as shown in FIG. 1B, a conductive material 2' that provides a conductive film 2 is applied to one main surface of the substrate 1. For example, a conductive carbon paste can be suitably used as the conductive material 2'. In applying the conductive material, a printing technique such as screen printing can be used to accurately obtain the pattern (shape) of the conductive film 2. After application, the conductive material 2' is dried, for example, to form the conductive film 2.

At this time, as shown in FIG. 1C, the warp threads 1a and weft threads 1b form unevenness on the substrate 1. The unevenness on the main surface of the substrate 1 makes the conductive film 2 thinner in parts, thereby varying the susceptibility of cracks to occur depending on the location.

Next, as shown in FIG. 1(d), the substrate 1 is bent with a constant curvature to cause cracks in the conductive film 2. The direction of bending is in the longitudinal direction of the strip-shaped conductive path formed by the conductive film 2. At this time, the conductive film 2 is bent so as to be positioned on the outside of the substrate 1 so as to apply a tensile strain to the conductive film 2. When bending, for example, the substrate 1 may be partially wrapped around a core material 11 such as a round bar having a constant curvature on its surface, with the conductive film 2 on the outside, and the entire substrate 1 may be bent by squeezing it.

At this time, the conductive film 2 is thin on the surface of the weft thread 1b, making it prone to cracking (small arrow in the figure). When the core material 11 is bent with a large curvature on the outside, cracks 3 extending in the direction along the weft thread 1b are generated, absorbing the tensile strain. Then, the strain is reduced in the vicinity of the part where the crack 3 has occurred, making it difficult for further cracks to occur. In this way, multiple cracks 3 can be formed at positions corresponding to the unevenness caused by the weft thread 1b. The correspondence between the number of weft threads 1b on a straight line extending in the longitudinal direction (bending direction) and the number of cracks 3 can be controlled by selecting the curvature at which the cracks 3 are formed. In other words, the number of cracks 3 can be controlled by appropriately selecting the diameter of the core material 11.

As a result, as shown in FIG. 1(e), a conductive film 2 is obtained in which cracks 3 are formed at positions corresponding to the unevenness of the substrate 1. This allows the bending sensor 10 to be obtained.

Furthermore, as shown in FIG. 1(f), a bending sensor 10b can be obtained by forming a spacer 6 surrounding the conductive film 2 on the substrate 1 and then overlaying a protective film 5 on top of the spacer 6.

When forming the insulating film 4, the insulating film 4 is formed on the main surface of the substrate 1 before applying the conductive material 2', and then the conductive material 2' is applied on top of the insulating film 4. This allows the bending sensor 10a to be obtained.

[Production testing]
A bending sensor was manufactured and the results of investigating the details thereof will be described with reference to FIGS.

[Test 1]
Bending sensors were manufactured using a monofilament woven fabric and a twisted yarn woven fabric as the substrate, and cracks formed in the conductive film were observed.

As shown in Figure 6, a bending sensor was manufactured using a plain weave fabric with monofilament warp and weft as the substrate. (a) When a 100-mesh woven fabric with a monofilament wire diameter of 0.12 mm was used as the substrate, and (b) when a 200-mesh woven fabric with a monofilament wire diameter of 0.09 mm was used as the substrate, the correspondence between the unevenness of the woven fabric and the cracks was similar in both cases. That is, the number of cracks intersecting with the straight lines extending in the longitudinal direction of the bending sensor (horizontal direction of the paper) was 1/2 of the number of threads (threads extending up and down on the paper) intersecting with the straight lines extending in the longitudinal direction. Although the cracks may branch along the way, it can be seen that the number of cracks is constant in response to the number of threads, i.e., in response to the unevenness of the substrate 1. In addition, in the manufacturing process, an iron bar with a diameter of 2.0 mm was used as the core material, and the bending was imparted by repeatedly folding it over 20 times.

As shown in Figure 7, when comparing (a) a photograph observed with reflected light and (b) a photograph observed with transmitted light, a bright area observed with transmitted light is observed in the same position as the crack observed with reflected light. In other words, it can be seen that the crack penetrates at least in the thickness direction.

As shown in Figure 8, a bending sensor was manufactured using a plain weave fabric with twisted warp and weft threads as the base material. The twisted threads have a wire diameter of 0.18 mm, and the mesh number of the fabric is 100. When taking the photographs, the amount of light required to observe the number of threads was different from the amount of light required to observe the number of cracks, so two photographs were arranged side by side: (a) a photograph in which the number of threads can be observed, and (b) a photograph in which the number of cracks can be observed. In this case, the number of threads and the number of cracks are the same. As in the case of monofilament, it can be seen that even if the cracks branch out along the way, the number of cracks in a straight line crossing the longitudinal direction is constant.

[Test 2]
Bending sensors were manufactured using plain weave fabrics with different mesh counts and smooth PET (polyethylene terephthalate) films as substrates, and the electrical resistance was measured when bending was applied.

The woven fabrics used as the substrates were all plain weave woven fabrics made of polyethylene monofilaments, with mesh numbers of 100, 150, and 200, respectively. The monofilament wire diameters were 0.12 mm, 0.10 mm, and 0.08 mm, respectively. The thickness of the substrate was the same as the wire diameter. The thickness of the substrate, made of PET film, was 0.1 mm.

As shown in Figure 9, a conductive material made of conductive carbon paste (CH-8, manufactured by Jujo Chemical Co., Ltd.) was patterned by screen printing on each substrate 1, and then dried in an oven at 100°C for 30 minutes. The thickness of the obtained conductive film was 0.03 mm, and the resistance value of the conductive film over its entire length was 5 kΩ. The substrate 1 was A4 size, and the conductive film 2 was patterned so that a total of 16 bending sensors could be arranged on one substrate, four vertically and four horizontally.

Next, the core material 11, made of an iron bar with a diameter of 2.0 mm, was used to bend the conductive film 2 back and forth 20 times to form a crack 3 in the conductive film 2 (see FIG. 5(d)). After the cracks were formed, the substrate was cut away to produce 16 bending sensors each under the same manufacturing conditions.

As shown in FIG. 10(a), the obtained bending sensor 10 was fixed to a semi-cylindrical jig 21 having a predetermined radius of curvature R, and the electrical resistance value was measured. Here, the bending sensor 10 was oriented such that the longitudinal direction was along the outer periphery of the jig 21, and the conductive film 2 was facing outward. The curvature radius R was set to six types: 10 mm, 20 mm, 30 mm, 40 mm, 50 mm, and infinity (flat). Then, as shown in FIG. 10(b), the electrical resistance of the entire length of the conductive film was measured with strain ε=t/2R, where t is the thickness of the bending sensor, and the relationship between the resistance change rate and strain was obtained. The resistance change rate was expressed as a ratio value, with the electrical resistance value at strain 0 (when using a flat jig) being 1.

As shown in Figure 11, it was found that the rate of resistance change changes almost linearly with strain. It was also found that the gauge factor, calculated as the rate of resistance change/strain, changes depending on the number of meshes in the substrate. Furthermore, it was also found that the gauge factor becomes higher, i.e., the sensitivity to bending becomes higher, by using a woven fabric with regular irregularities as the substrate compared to using a PET film with no irregularities as the substrate.

Furthermore, as shown in Figure 12, when the variation in gauge factor was calculated, it was found that when a woven fabric (100 to 200 mesh) was used as the substrate, the variation in gauge factor could be reduced compared to when a PET film with no irregularities was used as the substrate. In other words, it was found that the bending sensor can operate stably by using a woven fabric with irregularities at a constant interval as the substrate.

[Test 3]
Bending sensors with different overcoats were fabricated and their performance compared alongside that of an unovercoated bending sensor.

As shown in FIG. 13(a), a bending sensor was manufactured by applying a resin that would become a protective film 5' so that it would adhere closely to the conductive film 2 without any gaps. Two types of resin were used to make the protective film 5': an epoxy resin that uses PGMEA (propylene glycol monomethyl ether acetate) as a solvent, and a PVA (polyvinyl alcohol) resin that uses water as a solvent.

As shown in FIG. 2B, an adhesive resin that will become the spacer 6 is applied by printing to surround the conductive film 2 except for the electrode portion, and a PET film that will become the protective film 5 is attached on top of the adhesive resin. Two types of adhesive resin were used: epoxy resin and PVA resin.

As shown in Figure 14, of the "with protective film" cases, the two without the "hollow" designation had the protective film adhered to the conductive film, and the "hollow" case had the protective film 5 placed via a spacer 6. When the protective film was adhered to the conductive film, the gauge factor was reduced to less than half that of the "without protective film" case. On the other hand, when the protective film 5 was placed with a gap, there was almost no reduction in the gauge factor compared to the "without protective film" case. In other words, by providing the protective film with a gap from the conductive film, a high gauge factor could be maintained. No significant difference was observed between the epoxy resin and the PVA resin, whether the protective film was adhered to the conductive film or the spacer was provided.

[Test 4]
In the manufacture of bending sensors, the effect of changing the curvature of the core material for forming cracks was investigated.

A bending sensor was fabricated in the same manner as in Test 2, using a polyethylene mesh with a mesh count of 100 as the base material, and the gauge factor was measured. However, the diameter of the core material used to form the crack was 2.0 mm, 4.0 mm, and 6.0 mm.

Figure 15 shows photographs of the cracks. The symbols a, b, and c indicate core diameters of 2.0 mm, 4.0 mm, and 6.0 mm, respectively, with the number 1 indicating that the photograph was taken with reflected light and the number 2 indicating that the photograph was taken with transmitted light. This shows that in (a1) and (a2), where the core diameter was 2.0 mm, the number of cracks crossed by a straight line in the longitudinal direction (horizontal direction on the page) was 1/2 the number of threads. Similarly, in (b1) and (b2), where the core diameter was 4.0 mm, the number was 1/3, and in (c1) and (c2), where the core diameter was 6.0 mm, the number was 1/6.

As shown in Figure 16, the measurement results of the gauge factor are also shown. In the test range this time, a higher gauge factor was obtained by making the diameter of the core material smaller. In other words, it was found that the diameter of the core material can control the period of the cracks relative to the period of the unevenness of the base material, and the gauge factor of the bending sensor can be controlled.

Although the above describes an embodiment of the present invention and variations based thereon, the present invention is not necessarily limited thereto, and a person skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the appended claims.

REFERENCE SIGNS LIST 1 Substrate 2 Conductive film 3 Crack 10 Bending sensor

Claims (10)

A bending sensor that detects bending from a resistance change caused by opening and closing of a crack introduced in a conductive film provided on a main surface of a substrate so as to form a strip-shaped conductive path,
A bending sensor characterized in that the substrate is a woven fabric having irregularities on the main surface at a regular interval, the conductive path includes a plurality of cracks that penetrate the conductive film in the thickness and width directions, and the cracks are located at positions corresponding to the irregularities. The bending sensor according to claim 1, characterized in that the woven fabric is made by weaving together warp threads and weft threads that are perpendicular to each other, and the warp threads or the weft threads are arranged in the width direction of the conductive path. The bending sensor according to claim 2, characterized in that the woven fabric is formed by plain weaving the warp and weft threads. The bending sensor according to any one of claims 1 to 3, characterized in that the conductive film is provided on the main surface of the woven fabric with an insulating film interposed therebetween. The bending sensor according to any one of claims 1 to 4, characterized in that the conductive film is provided below a protective film provided opposite the main surface of the substrate with a gap therebetween. A method for manufacturing a bending sensor that detects bending from a resistance change caused by opening and closing of a crack introduced in a conductive film provided on a main surface of a substrate so as to form a strip-shaped conductive path, comprising:
The method for manufacturing a bending sensor is characterized in that the substrate is a woven fabric having irregularities on its main surface at a regular interval, a conductive material that provides the conductive film is applied onto the substrate, and the substrate is bent to provide a plurality of cracks in the conductive path that penetrate the conductive film in the thickness and width directions, and to provide the cracks at positions corresponding to the irregularities. The method for manufacturing a bending sensor according to claim 6, characterized in that the woven fabric is made by weaving together warp and weft threads that are perpendicular to each other, and the conductive material is applied onto the substrate so that the warp threads or the weft threads are arranged in the width direction of the conductive path. The method for manufacturing a bending sensor according to claim 7, characterized in that the woven fabric is made by plain weaving the warp and weft threads. The method for manufacturing a bending sensor according to any one of claims 6 to 8, characterized in that an insulating film is provided on the main surface of the woven fabric, and the conductive material is applied onto the insulating film. The method for manufacturing a bending sensor according to any one of claims 6 to 9, characterized in that a spacer is provided on the substrate surrounding the conductive film so that the conductive film is provided below the protective film provided facing the main surface of the substrate with a gap therebetween, and the protective film is formed on top of the spacer.

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