Patent Publication Number: US-2023152169-A1

Title: Biometric sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application No. PCT/JP2021/023375, filed on Jun. 21, 2021, which claims priority to Japanese Patent Application No. 2020-124741 filed in Japan on Jul. 21, 2020. The entire disclosures of International Application No. PCT/JP2021/023375 and Japanese Patent Application No. 2020-124741 are hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a sensor, particularly a biometric sensor. 
     Background Information 
     Various efforts have been undertaken to detect movement of measurement targets such as people and/or animals and convert the movement into numerical data. 
     A strain sensor which employs a strain sensor element having a resistance value that can vary in accordance with stretching and recovering is well-known as a device for detecting the movement of such measurement targets (see, for example, PCT Publication No. 2019/031381). Adopting the strain sensor element to detect the movement enables configuring the sensor to be noninvasive and superior in terms of wearing sensation. 
     SUMMARY 
     The strain sensor element is string-shaped or strip-shaped, and is capable of detecting the movement of the measurement target in accordance with a change in the resistance value resulting from the stretching and/or recovering in a lengthwise direction of the strain sensor element. In a case in which this strain sensor element is attached to the measurement target at an equilibrium length, when the measurement target moves in a direction so as to elongate the strain sensor element (hereinafter, may be also referred to as merely “elongating direction”), the resistance value varies in accordance with the elongation of the strain sensor element, whereby the movement of the measurement target can be detected. In contrast, when the measurement target moves in a direction so as to shorten the strain sensor element (hereinafter, may be also referred to as merely “shortening direction”), since the strain sensor element is attached at the equilibrium length, the strain sensor merely induces slack, and the strain sensor element does not shorten. In other words, since no change in the resistance value is generated in the strain sensor, the movement of the measurement target cannot be detected. 
     In the case of the conventional strain sensor, tension is applied to the strain sensor element beforehand (pretension) to enable detecting the movement of the measurement target, even if the measurement target moves in the shortening direction. In other words, the strain sensor element is attached to the measurement target in a state of being elongated to a predetermined length. In this case, since the strain sensor element shortens in a direction involving returning to the equilibrium length when the measurement target moves in the shortening direction, a change in the resistance value is generated in the strain sensor element. Thus, the movement of the measurement target can be detected. 
     If the pretension is insufficient, the strain sensor element slackens, whereby the movement in the shortening direction cannot be detected. On the other hand, if the pretension is excessive, an error can occur when the measurement target moves in the elongating direction, due to, e.g., deviation from a measurable range on a measurement circuit side, whereby the measurement may fail. Thus, since it is necessary to apply the pretension appropriately, putting on the strain sensor element requires labor. 
     The present disclosure was made in view of the foregoing circumstances, and an object of the present disclosure is to provide a biometric sensor which is noninvasive, superior in terms of wearing sensation, and can be put on easily, and enables detecting the movement of a measurement target even in the shortening direction. 
     The biometric sensor according to one aspect of the present disclosure includes: a fixed member having a frame which is ring-shaped; and a first strain sensor element and a second strain sensor element, each of the first strain sensor element and the second strain sensor element being string-shaped or strip-shaped, and the first strain sensor element and the second strain sensor element being stretchable and recoverable in a lengthwise direction of the first strain sensor element and a lengthwise direction of the second strain sensor element, wherein the first strain sensor element and the second strain sensor element extend across the frame and are disposed such that the first strain sensor element and the second strain sensor element cross each other, and wherein the frame is configured to be deformable at least in the lengthwise direction of the first strain sensor element and the lengthwise direction of the second strain sensor element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic perspective view illustrating a biometric sensor according to one embodiment of the present disclosure. 
         FIG.  2    is a schematic lateral view of the biometric sensor illustrated in  FIG.  1   . 
         FIG.  3    is a partially enlarged schematic plan view illustrating a frame area of a biometric sensor according to an embodiment which differs from that of  FIG.  1   . 
         FIG.  4    is a partially enlarged schematic plan view illustrating a frame area of the biometric sensor illustrated in  FIG.  1    in a case in which a measurement target does not move. 
         FIG.  5    is a partially enlarged schematic plan view illustrating the frame area of the biometric sensor illustrated in  FIG.  1    in a case in which the measurement target moves in an elongating direction of a first strain sensor element. 
         FIG.  6    is a partially enlarged schematic plan view illustrating the frame area of the biometric sensor illustrated in  FIG.  1    in a case in which the measurement target moves in a shortening direction of the first strain sensor element. 
         FIG.  7    is a partially enlarged schematic plan view illustrating a frame area of a biometric sensor according to an embodiment which differs from those of  FIG.  1    and  FIG.  3   . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The biometric sensor according to one embodiment of the present disclosure includes: a fixed member having a frame which is ring-shaped; and a first strain sensor element and a second strain sensor element, each of the first strain sensor element and the second strain sensor element being string-shaped or strip-shaped, and the first strain sensor element and the second strain sensor element being stretchable and recoverable in a lengthwise direction of the first strain sensor element and a lengthwise direction of the second strain sensor element, wherein the first strain sensor element and the second strain sensor element extend across the frame and are disposed such that the first strain sensor element and the second strain sensor element cross each other, and wherein the frame is configured to be deformable at least in the lengthwise direction of the first strain sensor element and the lengthwise direction of the second strain sensor element. 
     The biometric sensor according to the one embodiment of the present disclosure is noninvasive and superior in terms of the wearing sensation due to being able to be used by being affixed onto a surface of the measurement target, e.g., a human body. Furthermore, in a case in which the measurement target moves in the elongating direction of the first strain sensor element, the biometric sensor can detect the movement of the measurement target by means of the first strain sensor element. On the other hand, in a case in which the measurement target moves in the shortening direction of the first strain sensor element, since the second strain sensor element, which crosses the first strain sensor element, elongates due to a deformation of the frame, the movement of the measurement target can be detected by means of the second strain sensor element. In other words, in addition to the movement of the measurement target in the elongating direction of the first strain sensor element, the biometric sensor can also detect the movement in the shortening direction thereof. Furthermore, since, at a time of putting on the biometric sensor, it is not necessary to apply pretension, the biometric sensor can be put on easily. 
     The first strain sensor element and the second strain sensor element preferably cross each other orthogonally. 
     A crossing position where the first strain sensor element and the second strain sensor element cross each other preferably corresponds to a central position of the frame. 
     The frame is preferably circular ring-shaped or polygonal ring-shaped. 
     The fixed member preferably has a first reinforcing part and a second reinforcing part, each of the first reinforcing part and the second reinforcing part being rod-shaped or plate-shaped, and the first reinforcing part and the second reinforcing part extending in mutually opposite directions outwards from outer edges of the frame and along the lengthwise direction of the first strain sensor element. 
     The biometric sensor preferably includes: a substrate being strip-shaped, which is flexible and secures the fixed member; and a first holding part and a second holding part, each of the first holding part and the second holding part being rod-shaped or plate-shaped, and the first holding part and the second holding part extending, on a top face of the substrate, in the lengthwise direction of the first strain sensor element, wherein the first holding part and the second holding part are disposed along the lengthwise direction of the first strain sensor element and on outer sides of the frame such that the frame is disposed between the first holding part and the second holding part. 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawing or drawings as appropriate. 
     The biometric sensor  1  illustrated in  FIG.  1    and  FIG.  2    includes: a substrate  10 ; a fixed member  20 ; a first strain sensor element  31  and a second strain sensor element  32 ; and holding parts  40 . Since all of these components can be made thin, the biometric sensor  1  as a whole can also be made thin. Hereinafter, the present disclosure is described taking the biometric sensor as an example, but the present disclosure can also be used as a sensor which detects movements other than those of organisms. 
     The fixed member  20  has a frame  21  which is ring-shaped, wherein the first strain sensor element  31  and the second strain sensor element  32  extend across the frame  21  and are disposed such that the first strain sensor element  31  and the second strain sensor element  32  cross each other. Furthermore, the substrate  10  secures the fixed member  20 . The biometric sensor  1  is suitably used as a device for measuring behaviors of a human body such as respiration. 
     Substrate 
     The substrate  10  is strip-shaped and flexible. 
     The substrate  10  is preferably a substrate which is flexible in such a way that the elongation and shortening of the fixed member  20  are not impeded, and is exemplified by a knitted fabric, a woven fabric, a nonwoven fabric, a rubber, leather, or the like. Of these, the knitted fabric, which is superior in terms of a property of being stretchable and recoverable, is suitably used. 
     In the biometric sensor  1 , as illustrated in  FIG.  2   , the fixed member  20 , the holding part  40 , and the wiring  33 , described later, are secured on a top face of one strip of the substrate  10 . It is to be noted that in  FIG.  2   , a configuration in which the fixed member  20  is secured on the one strip of the substrate  10  is illustrated, but a configuration in which one further substrate  10  is laminated on a top face side of this substrate  10  to secure the fixed member  20  by sandwiching between the two strips of the substrate  10  is preferred. Thus sandwiching the fixed member  20  between the two strips of the substrate  10  can improve the wearing sensation. 
     The substrate  10  is preferably carved out along the frame  21  of the fixed member  20 , described later. In other words, the substrate  10  preferably has a hole  11  which overlaps with the frame  21  of the fixed member  20  in a planar view. When the hole  11  is thus provided in the substrate  10 , there is an absence of hindrance to the movement of the fixed member  20  in an inner side of the ring of the fixed member  20 . Thus, concentration of the movement of the measurement target to the first strain sensor element  31  and the second strain sensor element  32  is facilitated, whereby sensitivity of the biometric sensor  1  can be increased. 
     A size of the substrate  10  is appropriately decided based on, e.g., a size of the fixed member  20  and the like, and can be, for example, 5 cm or more and 15 cm or less in a lengthwise direction, and 2 cm or more and 5 cm or less in a crosswise direction. A size of the biometric sensor  1  in the planar view is decided based on the size of the substrate  10 , and can thus be made to be comparatively small. 
     It is to be noted that a tacky layer may be provided on a bottom face (a face on which the fixed member  20  and the like are not provided) of the substrate  10 . When the tacky layer is thus provided on the bottom face of the substrate  10 , for example, the substrate  10  can be easily affixed to and peeled off of the measurement target. 
     Fixed Member 
     As illustrated in  FIG.  1   , the fixed member  20  has, in addition to the frame  21 , two reinforcing parts  22  (a first reinforcing part  22   a  and a second reinforcing part  22   b ) each being rod-shaped or plate-shaped, which extend in mutually opposite directions outwards from outer edges of the frame  21  along the lengthwise direction of the first strain sensor element  31 . As described above, the fixed member  20  is secured to the substrate  10 . A method for securing the fixed member  20  is not particularly limited, and for example, a method involving adhering to the substrate  10 , an entire face of the fixed member  20  which comes in contact with the substrate  10  can be exemplified. 
     Frame 
     The frame  21  is configured to be deformable at least in the lengthwise direction of the first strain sensor element  31  and the lengthwise direction of the second strain sensor element  32 . In the biometric sensor  1  illustrated in  FIG.  1   , the frame  21  is circular ring-shaped, and the frame  21  is deformable in an arbitrary direction. Due to the frame  21  thus being deformable in the arbitrary direction, for example, even if the movement of the measurement target does not run along the lengthwise direction of the first strain sensor element  31  or the second strain sensor element  32 , the movement can be detected more easily. However, it is not necessary for the frame  21  to be deformed in a direction other than the lengthwise direction of the first strain sensor element  31  and the lengthwise direction of the second strain sensor element  32 . 
     A material and a ring thickness (width and diameter) of the frame  21  are decided such that the frame  21  has deformable elastic force in response to the movement of the measurement target. For example, a urethane rubber having a width of 0.5 mm or more and 1 mm or less, a spring wire having a diameter of 0.1 mm or more and 1.0 mm or less, or the like can be adopted. Examples of the spring wire include a hard steel wire, a piano wire, a stainless steel wire, phosphor bronze for a spring, and the like. It is to be noted that the thickness of the ring of the frame  21  is preferably uniform. When the thickness of the ring of the frame  21  is uniform, the first strain sensor element  31  and the second strain sensor element  32  can be elongated in proportion to the movement of the measurement target, thereby increasing the measurement accuracy. 
     The size of the frame  21  (an outer diameter, as of the frame  21  being circular ring-shaped in the biometric sensor  1  illustrated in  FIG.  1   ) is appropriately decided in accordance with the sensitivity and the like of the first strain sensor element  31  and the second strain sensor element  32  which extends across the frame  21 , and can be, for example, 1 cm or more and 3 cm or less. 
     Reinforcing Parts 
     The reinforcing parts  22  accurately grasp the movement of the measurement target in the lengthwise direction of the first strain sensor element  31 , and are members for guiding the deformation of the frame  21 . For example, in a case in which the measurement target is a human body and the movement thereof results from respiration, the movement of a surface of the human body due to the respiration is not uniform. If, by providing the reinforcing parts  22 , the movement of the surface of the human body can be grasped at any of the sites where the reinforcing parts  22  are located, the movement can be transmitted to the frame  21 . 
     A first reinforcing part  22   a  and a second reinforcing part  22   b,  being two reinforcing parts  22 , are provided symmetrically with the frame  21  being interposed therebetween. By enhancing the symmetry with respect to the lengthwise direction of the first strain sensor element  31 , the deformation of the frame  21  due to the movement of the measurement target can be prevented from becoming uneven, thereby increasing the measurement accuracy. Furthermore, in light of the measurement accuracy, the first reinforcing part  22   a  and the second reinforcing part  22   b  are preferably disposed so as to align with the first strain sensor element  31  in a straight line. 
     As a material of the reinforcing parts  22 , the same material as that of the frame  21  can be used. 
     A width and length of the reinforcing parts  22  are appropriately optimized in accordance with the measurement target. When the width and length of the reinforcing parts  22  are too short, the movement of the measurement target may not be accurately grasped. Conversely, when the width and length of the reinforcing parts  22  are too long, displacing the reinforcing parts  22  in accordance with the movement of the measurement target may become more difficult, leading to a decrease in the amount of deformation of the frame  21 , whereby measurement sensitivity may decrease. 
     It is to be noted that in light of the measurement sensitivity, the reinforcing parts  22  are preferably disposed to be parallel to the lengthwise direction of the first strain sensor element  31 , but this does not exclude disposing the reinforcing parts  22  at an angle with respect to the lengthwise direction of the first strain sensor element  31 . Even if the reinforcing parts  22  are disposed at an angle with respect to the lengthwise direction of the first strain sensor element  31 , the biometric sensor  1  achieves similar effects. It is to be noted that the angle formed between each of the reinforcing parts  22  and the lengthwise direction of the first strain sensor element  31  is, for example, 30° or less, and the angle being smaller is more preferable. 
     Strain Sensor 
     The first strain sensor element  31  and the second strain sensor element  32  (hereinafter, may be also collectively referred to as “strain sensor element”) are each a string-shaped or strip-shaped component which is stretchable and recoverable in a lengthwise direction thereof. The strain sensor element can directly detect the movement of the measurement target. For example, in the case of measuring the respiration of a human body, since changes in a state of the respiration can be grasped in real time, taking control of data indicative of various diseases based on respiration, such as respiratory failure, is enabled without delay. 
     The strain sensor element is acceptable as long as it has the property of being stretchable and recoverable, and the electrical characteristics thereof vary in accordance with elongation and shortening. A strain resistance element in which the electrical resistance varies in accordance with the elongation and shortening is suitably used. In particular, a carbon nanotube (hereinafter, may be also referred to as “CNT”) strain sensor which employs CNT(s) is particularly suitably used. 
     In the case in which the strain sensor element is string-shaped, the strain sensor element can be constituted to include a CNT bundle. The CNT bundle is a fiber bundle in which a plurality of the CNTs (single fibers) are roughly orientated in the lengthwise direction of a CNT element, and is coated with a resin. The string-shaped strain sensor element has, from a center to an outer side in a radial direction: an electrically conductive part consisting of the CNT bundle; an electrically conductive layer being a composite of CNT fibers and the resin; and a coating film made of the resin, in this order. The strain sensor element can undergo a change in resistance by rupture of the CNT bundle in the center, followed by alteration of gaps generated by the rupture. 
     On the other hand, in the case in which the strain sensor element is strip-shaped, the strain sensor element can be constituted from a resin composition containing a plurality of CNT fibers. Specifically, the strip-shaped strain sensor element has: a sheet of a plurality of fiber bundles obtained by orientating a plurality of CNTs (single fibers) roughly in the lengthwise direction of the CNT element; and a resin which coats the sheet of these fiber bundles. In a case in which an extension strain is applied, the strain sensor element undergoes a change in the resistance value by, for example, breaking of the internal CNT fibers causing splitting of the CNTs at ends, and/or relaxation of the extension strain resulting in contact of the ends of the CNTs again. 
     As the CNT(s), either of a monolayer single-wall nanotube (SWNT) and a multilayer multi-wall nanotube (MWNT) can be used. Of these, in view of, e.g., the electrical conductivity and thermal capacity, the MWNT is preferred, and the MWNT having a diameter of 1.5 nm or more and 100 nm or less is more preferred. 
     The CNTs may be produced by a well-known method, and may be produced by, for example, a CVD method, an arc method, a laser ablation method, a DIPS method, a CoMoCAT method, or the like. Of these, in light of enabling efficiently obtaining a CNT (MWNT) having a desired size, the CNTs are preferably produced by the CVD method involving adopting iron as a catalyst, and using ethylene gas. In this case, CNT crystals of the desired length grown with a vertical orientation can be obtained after forming a thin film of iron or nickel serving as the catalyst on a quartz glass substrate or a silicon substrate with an oxide film attached. 
     Both ends of each of the first strain sensor element  31  and the second strain sensor element  32  are connected to a measuring part (not shown in the figure) which measures the change in resistance via the wiring  33 . It is to be noted that the wiring  33  is connected to enable independently measuring the change in resistance of each of the first strain sensor element  31  and the second strain sensor element  32 . 
     It is preferred that the first strain sensor element  31  and the second strain sensor element  32  cross each other orthogonally, that is to say, that the lengthwise direction of the first strain sensor element  31  and the lengthwise direction of the second strain sensor element  32  cross each other orthogonally. Even if the lengthwise direction of the first strain sensor element  31  and the lengthwise direction of the second strain sensor element  32  do not cross each other orthogonally, the biometric sensor  1  achieves similar effects; however, when these cross each other orthogonally, particularly the measurement sensitivity in the direction in which the first strain sensor element  31  shortens can be increased. 
     On the other hand, it is not necessary for the lengthwise direction of the first strain sensor element  31  and the lengthwise direction of the second strain sensor element  32  to cross each other. A biometric sensor  2  illustrated in  FIG.  3    shows a configuration in the case in which the lengthwise direction of the first strain sensor element  31  and the lengthwise direction of the second strain sensor element  32  do not cross each other. In this case, the configuration is such that an extension line of the lengthwise direction of the first strain sensor element  31  and an extension line of the lengthwise direction of the second strain sensor element  32  cross each other. It is to be noted that the biometric sensor  2  illustrated in  FIG.  3    is provided with two sets of the first strain sensor element  31  and the second strain sensor element  32 . It is also possible to adopt a configuration in which the biometric sensor  2  is thus provided with a plurality of sets of the first strain sensor element  31  and the second strain sensor element  32 . 
     As the crossing position where the first strain sensor element  31  and the second strain sensor element  32  cross each other, an arbitrary site within the frame  21  can be adopted, and in particular, a central position of the frame  21  is preferred. More specifically, in the case of the frame  21  being circular ring-shaped, the crossing position where the first strain sensor element  31  and the second strain sensor element  32  cross each other preferably corresponds to the center of the circle. When the crossing position thus corresponds to the central position of the frame  21 , the measurement sensitivity can be increased. It is to be noted that the first strain sensor element  31  and the second strain sensor element  32  are not joined at the crossing position. In other words, the first strain sensor element  31  and the second strain sensor element  32  are arranged so as to act independently. 
     Holding Parts 
     The holding parts  40  consist of two members, being a first holding part  40   a  and a second holding part  40   b.  Each of the two holding parts  40  (the first holding part  40   a  and the second holding part  40   b ) is rod-shaped or plate-shaped, and extends, on the top face of the substrate  10 , in the lengthwise direction of the first strain sensor element  31 . The two holding parts  40  are disposed so as to be perpendicular to the lengthwise direction of the second strain sensor element  32  and on outer sides of the frame  21 , with the frame  21  being interposed therebetween. 
     The holding parts  40  prevent a phenomenon in which the movement of the measurement target is not sufficiently transmitted to the frame  21  because the substrate  10  moves in the lengthwise direction of the first strain sensor element  31  and bends, for example, when the measurement target moves. 
     As a material of the holding parts  40 , the same material as that of the frame  21  can be used. Furthermore, a width and length of the holding parts  40  and a distance from the frame  21  are appropriately decided such that the movement of the measurement target is effectively transmitted to the frame  21 . 
     Principles of Operation 
     The biometric sensor  1  is capable of detecting the movement of the measurement target with respect to the elongating direction and the shortening direction of the first strain sensor element  31  without applying the pretension to the first strain sensor element  31  and the second strain sensor element  32 . Hereinafter, the principles of operation of the biometric sensor  1  are explained with reference to  FIG.  4    to  FIG.  6   . 
       FIG.  4    is a drawing illustrating the area of the frame  21  of the biometric sensor  1  in a case in which the measurement target does not move. In the case in which the measurement target does not move, due to maintaining the original shape, the frame  21  is circular ring-shaped. 
     A case in which the measurement target moves in the elongating direction of the first strain sensor element  31  is illustrated in  FIG.  5   . When the measurement target moves in the elongating direction of the first strain sensor element  31 , the frame  21  deforms to be elliptical ring-shaped, having a major axis in the lengthwise direction of the first strain sensor element  31 . In this case, since the first strain sensor element  31  is pulled and elongated, a change in resistance is generated in the first strain sensor element  31 . On the other hand, since the pretension has not been applied, slack is generated in the second strain sensor element  32  due to the deformation of the frame  21 , whereby the resistance does not change. Accordingly, in the case in which the measurement target moves in the elongating direction of the first strain sensor element  31 , the movement can be detected based on the change in resistance of the first strain sensor element  31 . 
     In contrast, a case in which the measurement target moves in the shortening direction of the first strain sensor element  31  is illustrated in  FIG.  6   . When the measurement target moves in the shortening direction of the first strain sensor element  31 , the frame  21  deforms to be elliptical ring-shaped, having a major axis in the lengthwise direction of the second strain sensor element  32 . In this case, since the pretension has not been applied, slack is generated in the first strain sensor element  31  due to the deformation of the frame  21 , whereby the resistance does not change. Conversely, since the second strain sensor element  32  is pulled and elongated, a change in resistance is generated in the second strain sensor element  32 . Accordingly, in the case in which the measurement target moves in the shortening direction of the first strain sensor element  31 , the movement can be detected based on the change in resistance of the second strain sensor element  32 . 
     Thus, in the case of the biometric sensor  1 , whether the measurement target moves in the elongating direction or the shortening direction of the first strain sensor element  31 , the movement can be detected by the change in resistance of the first strain sensor element  31  or the change in resistance of the second strain sensor element  32 . 
     It is to be noted that in the above description, the case in which the pretension has not been applied to the first strain sensor element  31  and the second strain sensor element  32  is described; however, the biometric sensor  1  functions similarly even if the pretension is applied. Thus, applying the pretension to the first strain sensor element  31  and the second strain sensor element  32  is not to be excluded. However, it is preferred that the pretension is not applied in such a manner that the frame  21  is deformed. 
     The biometric sensor  1  can be affixed on the surface of the measurement target, e.g., a human body, for use; thus, the biometric sensor  1  is noninvasive and superior in terms of the wearing sensation. Furthermore, in the case in which the measurement target moves in the elongating direction of the first strain sensor element  31 , the biometric sensor  1  can detect the movement of the measurement target by means of the first strain sensor element  31 . On the other hand, in the case in which the measurement target moves in the shortening direction of the first strain sensor element  31 , due to the elongation of the second strain sensor element  32 , which crosses the first strain sensor element  31  due to the deformation of the frame  21 , the movement of the measurement target can be detected by the second strain sensor element  32 . In other words, in addition to the movement of the measurement target in the elongating direction of the first strain sensor element  31 , the biometric sensor  1  can also detect the movement in the shortening direction thereof. Furthermore, since, at a time of putting on the biometric sensor  1 , it is not necessary to apply the pretension, the biometric  1  sensor can be put on easily. 
     Other Embodiments 
     The embodiments described above do not restrict the constituent features of the present disclosure. Therefore, constituent elements of each part of the above-described embodiment may be omitted, replaced, or added based on the description in the present specification and common technical knowledge, and such omission, replacement, and addition should be construed as falling within the scope of the present disclosure. 
     In the above-described embodiment, the case in which the fixed member has two reinforcing parts is described; however, there may be one, or three or more reinforcing part(s). Furthermore, the reinforcing part(s) is/are not a necessary component. In a case in which the movement of the measurement target can be grasped by the frame alone, the reinforcing part(s) may be omitted. 
     In the above-described embodiment, the case in which the biometric sensor has two holding parts is described; however, there may be one, or three or more holding part(s). Furthermore, the holding part(s) is/are not a necessary constituent feature, and may be omitted. In the case of the biometric sensor not having the holding part(s), the substrate may also be omitted. Even in the case of the configuration not involving the holding part(s) or the substrate, the biometric sensor of the present disclosure achieves similar effects. 
     In the above-described embodiment, the case in which the frame is circular ring-shaped is described, but as long as the frame is configured to be deformable at least in the lengthwise direction of the first strain sensor element and the lengthwise direction of the second strain sensor element, another shape may be adopted. Such a shape of the frame may be exemplified by a polygonal ring shape. In a biometric sensor  3  illustrated in  FIG.  7   , a case of the frame  23  having a diamond-shaped ring, being polygonal ring-shaped, is illustrated. 
     In the case of the frame  23  thus being polygonal ring-shaped, a configuration in which the lengthwise direction of the first strain sensor element  31  and the lengthwise direction of the second strain sensor element  32  are diagonals, in part, of the frame  23  is preferred. In the case of the frame  23  having the diamond shape, the lengthwise direction of the first strain sensor element  31  and the lengthwise direction of the second strain sensor element  32  form two diagonals of the diamond shape. When such a configuration is carried out, a deformation also occurs in the lengthwise direction of the second strain sensor element  32  due to the elongation and/or shortening in the lengthwise direction of the first strain sensor element  31 . 
     Furthermore, when the frame  23  has the diamond shape, the first strain sensor element  31  and the second strain sensor element  32  cross each other orthogonally, and the crossing position corresponds to the central position of the frame  23 . Making such a configuration enables an increase in the measurement accuracy and the measurement sensitivity of the biometric sensor  3 . 
     Industrial Applicability 
     As described above, the biometric sensor according to the present disclosure is noninvasive, superior in terms of wearing sensation, and can be put on easily, and enables detecting the movement of a measurement target even in the shortening direction.