Patent Publication Number: US-2007107526-A1

Title: Load sensor

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a load sensor, and specifically to a load sensor suitable for detection of a load upon a seat of an automobile or the like.  
      2. Description of the Related Art  
      Conventionally, the load sensor is used to various applications, such as the detection of a load upon a seat of an automobile, the detection of a force applied to a pointing device, and the like. For example, Japanese Unexamined Patent Application Publication No. 7-174646 discloses a load sensor wherein a load bearing portion serving as an operation portion is arranged in a substantially central portion on an elastic substrate and fixed portions are provided at end portions of the substrate, and wherein a strain detecting element is arranged between the load bearing portion and each of the fixed portions. Such a load sensor is configured to detect strains of the substrate in response to a load upon the load bearing portion by the strain detecting elements, and thereby to detect the magnitude and direction of the load upon the load bearing portion.  
      However, in the conventional load sensor as described above, undesirably, a stress applied to the substrate in response to a load is concentrated on positions on the substrate in the neighborhood of the load bearing portion and fixed portions. This causes a problem in that strains of the substrate in response to a load cannot be properly detected by the detecting elements.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an object of the present invention to provide a load sensor capable of dispersing the stress applied to the substrate in response to a load, and thereby properly detecting the magnitude and direction of the load.  
      A load sensor according to the present invention includes fixed portions, a load bearing portion subjected to a load, and strain detecting elements each detecting a strain of a substrate in response to the load, wherein slits each of which can disperse a stress applied to the substrate in response to the load, are formed in the substrate.  
      With this arrangement, since the stress applied to the substrate in response to the load is dispersed by the slits formed in the substrate, it is possible to avoid stress concentration on definite positions on the substrate, and to detect strains of the substrate in response to the appropriately dispersed stress by strain detecting elements. This allows the magnitude and direction of the load to be properly detected.  
      In the above-described load sensor, it is preferable that the strain detecting elements be disposed between each of the fixed portions and the load bearing portion. Thus disposing the strain detecting elements between each of the fixed portions and the load bearing portion, which are most susceptible to stress upon the substrate, allows the magnitude and direction of the load to be detected with even higher accuracy.  
      In the above-described load sensor, for example, the slits may be each formed in the neighborhood of the strain detecting elements. In this case, since the stress applied to the substrate in the neighborhood of the strain detecting elements is dispersed, directly detecting stresses dispersed by the strain detecting element enables the magnitude and direction of the load to be detected with even higher accuracy.  
      In particular, it is preferable that a pair of the slits be formed so as to sandwich the strain detecting elements. In this case, since the stress applied to the substrate is dispersed by the pair of slits sandwiching the strain detecting elements, the stress applied to the substrate can be uniformly dispersed.  
      In the above-described load sensor, it is preferable that the ends of the slit be each formed into a round shape. In this case, since the stress applied to the substrate can be borne by the round shape portions of the slits, it is possible to avoid stress concentration on the ends of the slits, and to prevent the occurrence of a failure of the substrate and a reduction of the product life that can be caused by the above-described stress concentration on the ends of the slits.  
      In the above-described load sensor, the slit may be formed into a substantially heart shape. In this case also, since the stress applied to the substrate can be borne by the round shape portions included in the substantially heart shape, it is possible to avoid stress concentration on the ends of the slits, and to prevent the occurrence of a failure of the substrate and a reduction of the product life that can be caused by the above-described stress concentration on the ends of the slits.  
      A load sensor according to the present invention includes fixed portions, a load bearing portion subjected to a load, and strain detecting elements each detecting a strain of a substrate in response to the load, wherein opening portions each of which can disperse a stress applied to the substrate in response to the load, are formed in the substrate.  
      With this arrangement, since the stress applied to the substrate in response to the load is dispersed by the opening portions formed in the substrate, it is possible to avoid stress concentration on definite positions and to detect strains of the substrate in response to the appropriately dispersed stress by strain detecting elements. This allows the magnitude and direction of the load to be properly detected.  
      It is preferable that the strain detecting elements be disposed between each of the fixed portions and the load bearing portion. Thus disposing the strain detecting elements between each of the fixed portions and the load bearing portion, which are most susceptible to stress upon the substrate, allows the magnitude and direction of the load to be detected with even higher accuracy.  
      In the above-described load sensor, for example, the opening portions may be each formed in the neighborhood of the strain detecting elements. In this case, since the stress applied to the substrate in the neighborhood of the strain detecting elements is dispersed, directly detecting the dispersed stress by the strain detecting element allows the magnitude and direction of the load to be detected with even higher accuracy.  
      In particular, it is preferable that a pair of opening portions be formed so as to sandwich the strain detecting elements. In this case, since the stress is dispersed by the pair of slits sandwiching the strain detecting elements, the stress applied to the substrate can be uniformly dispersed.  
      In the above-described load sensor, the opening portions may be formed into a substantially heart shape. In this case also, since the stress applied to the substrate can be borne by the round shape portions included in the substantially heart shape, it is possible to avoid stress concentration on the ends of the opening portions, and to prevent the occurrence of a failure of the substrate and a reduction of the product life caused by the above-described stress concentration on the ends of the opening portions.  
      In the above-described load sensor, the arrangement may be such that the fixed portions are each disposed at a corner portion of a substantially square substrate, and that the load bearing portion is disposed in a central portion of this substrate. In this case, since the main body of the sensor can be fixed on a mounting base at four fixed portions, it is possible to provide a load sensor that is resistant to torsion applied to the substrate in response to a load.  
      According to the present invention, it is possible to disperse stress applied to the substrate in response to a load to thereby properly detect the magnitude and direction of the load. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view showing the construction of a substrate of a load sensor according to an embodiment of the present invention;  
       FIG. 2  is a plan view showing wiring patterns formed on the substrate in  FIG. 1 ;  
       FIG. 3  is a plan view showing a distribution of stress applied to the substrate of the load sensor according to the embodiment;  
       FIG. 4  is a graph showing stresses in accordance with positions from the end of a load bearing portion to a fixed portion in the substrate in  FIG. 3 ;  
       FIG. 5  is a plan view showing a stress distribution in a load sensor having a substrate without any slit;  
       FIG. 6  is a graph showing stresses in accordance with positions from the end of a load bearing portion to a fixed portion in the substrate in  FIG. 5 ;  
       FIG. 7  is a plan view showing a stress distribution in a load sensor having a substrate without any round-shape portion at the ends of slits; and  
       FIG. 8  is a plan view showing a stress distribution in a load sensor having a substrate with a notch in the center of each of its sides. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.  
      First, the construction of a load sensor according to the embodiment of the present invention is described using  FIGS. 1 and 2 .  FIG. 1  is a perspective view showing the construction of a substrate of the load sensor according to the embodiment of the present invention.  FIG. 2  is a plan view showing wiring patterns formed on the substrate in  FIG. 1 .  
      The load sensor according to this embodiment has a substrate  100  made of a metal plate such as a stainless plate. As shown in  FIG. 1 , the substrate  100  is a flexible flat plate with a fixed thickness and formed into a substantially square shape. In a central portion of the substrate  100 , there is provided a load bearing portion  101  subjected to a load. The load bearing portion  101  protrudes from the substrate  100  in an arcuate shape in section, and has a hole  102  inside it. At corners of the substrate  100 , there are provided four fixed portions  103  for mounting this load sensor on the mounting base (not shown). As in the case of the load bearing portion  101 , each of the fixed portions  103  also protrudes from the substrate  100  in an arcuate shape in section, and has a mounting hole  104  inside it. Here, the height of the fixed portion  103  is arranged to be lower than that of the load bearing portion  101 . However, the height of the load bearing portion  101  may be either equal to or higher than that of the fixed portion  103 . Hereinafter, fixed portions  103  on the right side, lower side, left side, and upper side are referred to as fixed portions  103   a ,  103   b ,  103   c , and  103   d , respectively.  
      When such a substrate  100  is installed on a seat of an automobile or the like, mounting screws (not shown) are inserted through the mounting holes  104  of the fixed portions  103  and fastened to an automobile body, as well as a lever (not shown) abutted against a portion of the seat is engaged with the hole  102  of the load bearing portion  101 . The substrate  100  is bent as appropriate by a load applied to the above-described lever, and consequently strain stresses are applied to strain detecting elements that will be described later, whereby the load is detected.  
      Slits  105  each having a substantially C-shape that opens toward the outside of the substrate, i.e., in the direction of a side of the substrate, are each formed between the fixed portions  103 . For example, between the fixed portion  103   a  and fixed portion  103   b , there is provided a slit  105   a , which has a shape that opens in the direction of the side between the fixed portions  103   a  and  103   b . Here, the ends of the slit  105  each have a round shape directed toward the inside of the slit  105 .  
      In the load sensor according to this embodiment, by forming such slits  105  at predetermined positions on the substrate, it is possible to disperse the stress applied to the substrate  100  while maintaining spaces for forming wiring patterns on the substrate  100 . In particular, forming slits with substantially C-shape each opening toward the outside of the substrate  100 , between the fixed portions  103 , ensures the flexibility of the substrate  100  as in the case where the substrate would be formed into a cruciform.  
      Between the load bearing portion  101  and each of the fixed portions  103 , there are provided a pair of slits  106  having the same shape. Each pair of slits  106  have symmetrical shapes with respect to a line connecting the center of the load bearing portion  101  and the center of a respective one of the fixed portions  103 . Each pair of slits  106  that are mutually opposed are spaced apart by a predetermined distance. The pair of slits  106  has each rectilinear portions  106   a  that substantially orthogonally intersect each other, and round shape portions  106   b  each being directed from the ends of the rectilinear portions  106   a  toward the inside of the slit, thereby forming a substantially heart shape. The pair of slits  106  are configured so that the ends  106   c  in the slits  106   a  that substantially orthogonally intersect each other (hereinafter, these ends are referred to as “orthogonal ends” as appropriate) are opposed to each other.  
      In the load sensor according to this embodiment, by forming such pairs of slits at predetermined positions on the substrate, it is possible to disperse the stress applied to the substrate  100  in response to a load upon the load bearing portion  101 .  
      As shown in  FIG. 2 , between the load bearing portion  101  and each of fixed portion  103 , and in the neighborhood of the pair of slits  106 , there are provided a pair of strain detecting elements  201  ( 201   a  and  201   b ). Specifically, the pair of strain detecting elements  201  are provided on the line connecting the center of the load bearing portion  101  and the center of each the fixed portions  103 . Also, with respect to a line connecting the orthogonal ends  106   c  of the slit  106 , the strain detecting element  201   a  as one of the pair of strain detecting elements  201  is disposed on the side of the load bearing portion  101 , and the strain detecting element  201   b  as the other of the pair of strain detecting elements  201  is disposed on the side of the fixed portion  103 .  
      As shown in  FIG. 2 , the strain detecting elements  201  are provided in twos between the load bearing portion  101  and each of the fixed portions  103 , and in total, eight strain detecting elements  201  are arranged on the substrate  100 . The strain detecting elements  201  detect strains of the substrate  100  in response to a load upon the load bearing portion  101 . The strain detecting element is made of, for example, a material formed by dispersing a metal or metal oxide constituted of an electrically conductive material into a binder constituted of a low-melting glass material or the like. The strain detecting element is arranged so that the electrically conductive material that is dispersed in the binder varies in density under a stress such as a compressive stress or tensile stress to thereby vary in resistance value. Here, the resistance values of all strain detecting elements  201  are set to an identical value.  
      Each of the strain detecting elements  201   a  and a respective one of the strain detecting elements  201   b  are connected via wiring  202 , and to these, an input electrode  203 , output electrode  204 , and ground electrode (not shown) are connected via the wiring  202 , thereby forming a bridge circuit. Each of the input electrodes  203  and a respective one of the output electrodes  204  are disposed in a region formed between a corresponding slit  105  and a corresponding side portion facing the opening portion side of the pertinent slit  105  (hereinafter, this region is referred to as a “slit external region”). Here, the strain detecting elements  201  and wiring patterns with respect thereto are formed on the substrate  100  by, for example, printing them at desired positions with an electrically conductive ink. However, this method for forming the strain detecting elements  201  and wiring patterns is not restrictive.  
      The input electrode  203  is connected to the end of a strain detecting element  201   a  provided in the neighborhood of the pertinent side portion which end is adjacent to the load bearing portion  101 , and the end of a strain detecting element  201   b , adjacent to the fixed portion  103   a  across the pertinent side portion, via the wiring  202 . On the other hand, the output electrode  204  is connected to the end of the strain detecting element  201   b  provided in the neighborhood of the pertinent side portion via the wiring  202 , the end being adjacent to the load bearing portion  101 . Specifically, the input electrode  203  provided in the slit external region of the slit  105   a  is connected to the end of the strain detecting element  201   b  provided in the neighborhood of the fixed portion  103   a , the end being adjacent to the  103   a ; while it is connected to the end of the strain detecting element  201   a  provided in the neighborhood of the fixed portion  103   b , the end being adjacent to the load bearing portion  101 . On the other hand, the input electrode  204  provided in the slit external region of the slit  105   a  is connected to the end of the strain detecting element  201   b  provided in the neighborhood of the fixed portion  103   b , the end being adjacent to the  103   a.    
      In a state where no load is applied to the strain detecting elements  201 , the bridge circuit is in balance. If a load is applied to the load bearing portion  101 , the substrate  100  bends, and the strain detecting elements  201   a  and  210   b  distort in proportion to the load so as to be compressed or expanded, so that resistance values of the strain detecting elements  201  change. A voltage applied to the bridge circuit by the input electrode  203  is divided by the strain detecting elements  201 , and appears as a differential voltage of the output electrode  204 . This differential voltage is converted into, e.g., a weight value by a circuit (not shown).  
      Next, the distribution of stress applied to the substrate  100  of the load sensor according to this embodiment will be described with reference to  FIGS. 3 and 4 .  FIG. 3  is a plan view showing a distribution of stress applied to the substrate  100 .  FIG. 4  is a graph showing stresses in accordance with positions from the end of the load bearing portion  101  to the fixed portion  103   a  in the substrate  100  in  FIG. 3 . In  FIG. 3 , the compressive stress serving as a stress applied to the substrate  100  is indicated by oblique lines, and the tensile stress serving as a stress applied to the substrate  100  is indicated by horizontal lines. Here, the spacings between solid lines in the oblique lines and horizontal lines are made smaller as the magnitude of the compressive or tensile stresses increases. In other words, the larger the compressive or tensile stress, the smaller are the spacings between solid lines. For convenience of description, in  FIG. 3 , the compressive and tensile stresses are each classified into two levels.  
      In this load sensor, when a load is applied to the load bearing portion  101 , as shown in  FIG. 3 , a stress is mainly applied to the substrate  100  along the direction of a straight line connecting the centers of the fixed portion  103   a , load bearing portion  101 , and fixed portion  103   c . Specifically, a compressive stress is applied to the portions from the slits  106  up to the fixed portions  103  while a tensile stress is applied to the portions from the slits  106  up to the fixed portions  103 .  
      More specifically, the compressive stress is applied to the portions from the neighborhood of the peripheral surface of the load bearing portion  101  up to the neighborhood of the orthogonal ends  106   c  along one of the straight line portions  106   a  of the slit  106 , while the tensile stress is applied to the portion from the neighborhood of the peripheral surface of the fixed portion  103   a  up to the neighborhood of the orthogonal ends  106   c  along the other of the straight line portions  106   a  of the slit  106 .  
      In this case, as shown in  FIG. 4 , the stress continues to maintain a high compressive stress on the portion from the load bearing portion  101  up to the neighborhood of the orthogonal ends  106   c . The stress changes from the compressive stress into the tensile stress with the orthogonal ends  106   c  of the slit  106  as a boundary, and it continues to maintain a high tensile stress on the portion from the neighborhood of the orthogonal ends  106   c  up to the fixed portion  103   a . For example, at the nearest position to the load bearing portion  101 , a compressive stress of −3e+8 [N/m 2 ] to −5e+8 [N/m 2 ] is applied, and up to the neighborhood of the orthogonal ends  106   c , the same level of the compressive stress is applied although it slightly decreases. On the other hand, at the nearest position to the fixed portion  103 , a tensile stress of +3e+8 [N/m 2 ] to +5e+8 [N/m 2 ] is applied, and up to the neighborhood of the orthogonal ends  106   c , the same level of the tensile stress is applied although it slightly decreases.  
      In the load sensor according to this embodiment, since the slits  106  are provided in the neighborhood of the strain detecting elements  201  so as to disperse stresses (compressive stress and tensile stress) applied to the substrate  100 , it is possible to avoid the concentration of stresses applied to the substrate  100  on definite positions on the substrate  100 , and to properly detect strains of the substrate  100  by strain detecting elements  201 .  
      When a load is applied to the load bearing portion  101 , as shown in  FIG. 3 , a compressive stress is applied to the portion from the peripheral surface of the load bearing portion  101  up to the neighborhood of one of the round shape portions  106   b  of the slit  106 , while a tensile stress is applied to the portion from the peripheral surface of the fixed portion  103   a  up to the neighborhood of the other of the round shape portion  106   b  of the slit  106 . At this time, in the neighborhood of the round shape portions  106   b  of the slit  106 , stresses (compressive stress and tensile stress) are applied along the shape of the pertinent round shape portion  106   b.    
      In the load sensor according to this embodiment, since it is arranged that the ends of the slits  106  is each formed into a round shape and that the stress applied to the substrate  100  is borne by the pertinent round shape portions, it is possible to avoid stress concentration on the ends of the slits  106 , and to prevent the occurrence of a failure of the substrate and a reduction of the product life caused by the above-described stress concentration on the ends of the slits  106 .  
      When a load is further applied to the load bearing portion  101 , as shown in  FIG. 3 , a stress that twists the substrate  100  is applied to the substrate  100  along the direction of a straight line connecting the centers of the fixed portion  103   b , load bearing portion  101 , and fixed portion  103   d . However, since this load sensor is fixed by the four fixed portions  103   a  to  103   d , an adverse effect of the pertinent stress on the detection of load is reduced.  
      Here, regarding a load sensor adopting a substrate  100  different from that of the present embodiment, distributions of stresses applied to the substrate  100  will be described with reference to FIGS.  5  to  8 .  
       FIG. 5  is a plan view showing a stress distribution in a substrate  100  without any slit.  FIG. 6  is a graph showing stresses in accordance with positions from the end of a load bearing portion  101  up to a fixed portion  103   a  in the substrate  100  in  FIG. 5 .  FIG. 7  is a plan view showing a stress distribution in the substrate  100  without any round-shape portion at the ends of the slits.  FIG. 8  is a plan view showing a stress distribution in a substrate that has a notch in the center of each of its sides. In FIGS.  5  to  8 , the components same as or equivalent to those in the above-described embodiment are designated by the same reference numerals, and description thereof is omitted to avoid redundancy. Here, it is assumed that each of these substrates  100  has strain detecting elements  201  at the same positions as those in the above-described embodiment. In  FIGS. 5, 7 , and  8 , stresses (compressive stress and tensile stress) are represented as in the case of  FIG. 3 .  
      In the load sensor having the substrate  100  without any slit, when a load is applied to the load bearing portion  101 , as shown in  FIG. 5 , stresses concentrate on the neighborhood of the peripheral surfaces of the load bearing portion  101  and fixed portion  103   a  ( 103   c ). Specifically, a compressive stress concentrate on the neighborhood of the peripheral surface of the load bearing portion  101  while a tensile stress concentrate on the neighborhood of the peripheral surface of the fixed portion  103   a  ( 103   c ). In particular, a high compressive stress and tensile stresses, respectively, concentrates on positions adjacent to the load bearing portion  101  and fixed portions  103 , and they decrease as the position on the substrate is spaced apart from the load bearing portion  101  and fixed portions  103 , respectively.  
      In this case, as shown in  FIG. 6 , the stress exhibits the maximum compressive stress in the neighborhood of the load bearing portion  101 , and gradually declines up to an intermediate position between the load bearing portion  101  and the fixed portion  103 . The stress changes from the compressive stress into the tensile stress with the pertinent intermediate position as a boundary, and it gradually increases from the intermediate position toward each of the fixed portions  103 , until it exhibits the maximum tensile stress in the neighborhood of the fixed portion  103 . For example, at the nearest position to the load bearing portion  101 , a compressive stress of −3e+8 [N/m 2 ] to −5e+8 [N/m 2 ] is applied, and decreases in accordance with a distance from the load bearing portion  101 . On the other hand, at the nearest position to the fixed portion  103 , a tensile stress of +1e+8 [N/m 2 ] to +3e+8 [N/m 2 ] is applied, and decreases in accordance with a distance from the fixed portion  103 .  
      As shown in  FIG. 5 , in a load sensor having the substrate  100  without any slit, as compared with the load sensor according to this embodiment shown in  FIG. 3 , the stresses (compressive stress and tensile stress) undesirably concentrate on the neighborhood of the load bearing portion  101  and fixed portions  103 . This makes it difficult to detect strains of the substrate  100  by the strain detecting elements  201   a , and thereby to detect the magnitude and direction of a load, with high accuracy.  
      On the other hand, regarding a load sensor having the substrate  100  without any round shape portion at the ends of the slits, when a load is applied to the load bearing portion  101 , as shown in  FIG. 7 , it exhibits a stress distribution similar to that in the present load sensor shown in  FIG. 3 , but is subjected to a concentration of compressive stress on the neighborhood of the ends of the slits. Specifically, compressive stresses concentrate on the ends of the slits  106  adjacent to the load bearing portion  101 , while tensile stresses concentrate on the ends of the slits  106  adjacent to the fixed portion  103   a  ( 103   c ). Also, a tensile stress concentrates on one end of each of the slits  105 .  
      As shown in  FIG. 7 , in the load sensor having the substrate  100  without any round shape portion at the ends of the slits, as compared with the load sensor according to this embodiment shown in  FIG. 3 , stresses (compressive stress and tensile stress) concentrate on the neighborhood of the ends of the slits  106 . This can cause a failure of the substrate  100  and a reduction of the product life.  
      Regarding the load sensor having the substrate  100  with a notch in the center of each of its sides, when a load is applied to the load bearing portion  101 , as shown in  FIG. 8 , it exhibits a stress distribution similar to that in the present load sensor shown in  FIG. 3 . This indicates that the load sensor according to the present embodiment is capable of detecting the magnitude and direction of a load, with an accuracy similar to that of the substrate  100  with a notch in the center of each of its sides.  
      However, as shown in  FIG. 8 , as compared with the present load sensor shown in  FIG. 3 , the load sensor having the substrate  100  with a notch in the center of each of its sides has a small space for forming wiring patterns on the substrate  100 . This necessitates taking countermeasures such as separately providing a space for forming wiring patterns or reducing the wiring patterns.  
      As described above, the load sensor according to this embodiment includes the load bearing portion  101 , and strain detecting elements  201  each detecting a strain of the substrate  100  in response to a load, wherein slits  105  and  106  each of which can disperse a stress applied to the substrate in response to the load, are formed in the substrate. According to the present load sensor, a stress applied to the substrate in response to a load are dispersed by the slits  105  and  106  formed in the substrate  100 , whereby it is possible to avoid stress concentration on definite positions on the substrate  100 , and to detect strains of the substrate  100  in response to the appropriately dispersed stress by the strain detecting elements  201 . This enables the magnitude and direction of the load to be properly detected.  
      In particular, in the load sensor according to this embodiment, the strain detecting elements  201  are disposed between the load bearing portion  101  and each of the fixed portions  103 . Thus, by disposing the strain detecting elements  201  between and the load bearing portion  101  and each of the fixed portions  103 , which are most susceptible to stress upon the substrate  100 , it is possible to detect the magnitude and direction of the load with even higher accuracy.  
      In this embodiment, the slits  106  are formed in the neighborhood of the strain detecting elements  201 . Thereby, the stress applied to the substrate  100  in the neighborhood of the strain detecting elements  201  is dispersed, so that directly detecting the dispersed stress by the strain detecting element  201  allows the magnitude and direction of the load to be detected with even higher accuracy.  
      In particular, in this embodiment, the pair of slits  106  is formed so as to sandwich the strain detecting elements  201 . Thereby, since the stress is dispersed by the pair of slits  106  sandwiching the strain detecting elements, the stress applied to the substrate can be uniformly dispersed.  
      In this embodiment, the ends of the slit  106  are each formed into a round shape. Thus, by forming each of the ends of slits  106  into a round shape, the stress applied to the substrate can be borne by the round shape portions  106   b  of the slits  106 . This makes it possible to avoid stress concentration on the ends of the slits  106 , and to prevent a failure of the substrate and a reduction of the product life caused by the above-described stress concentration on the ends of the slits  106 .  
      Especially, in this embodiment, the slits  106  are formed into a substantially heart shape. Thereby, since the stress applied to the substrate can be borne by the round shape portions included in the substantially heart shape portions, it is possible to avoid stress concentration on the ends of the slits  106 , and to prevent a failure of the substrate and a reduction of the product life caused by the above-described stress concentration on the ends of the slits  106 .  
      Also, in this embodiment, the fixed portions  103  are each disposed at the corner portion of the substantially square substrate, and the load bearing portion  101  is disposed in the central portion of this substrate  100 . Thereby, since the main body of the sensor can be fixed on the mounting base at the four fixed portions, it is possible to provide a load sensor that is resistant to torsion applied to the substrate in response to a load.  
      The present invention is not limited to the above-described embodiment, but various changes and modifications can be made therein. In the above-described embodiment, the sizes and shapes of the parts illustrated in the accompanying drawings are not limited, but can be changed as appropriate within the scope in which the effect of the present invention is exerted. In other respects, changes can also be made as appropriate without departing the spirit and scope of the present invention.  
      For example, in the load sensor according to the above-described embodiment, the case was described in which the slits  105  and  106  are provided at predetermined positions on the substrate  100  in order to ensure the dispersion of stress, but the method for ensuring the dispersion of stress is not limited to this. Instead of the slits  105  and  106 , opening parts corresponding to these slits may be provided. The opening parts may be formed by punching processing. At that time, as in the case of the above-described embodiment, the opening parts may be arranged pairwise so as to sandwich the strain detecting elements  201 . Also, the ends of the opening parts may be each formed into round shape, and the opening parts themselves may be each formed into a heart shape. The use of such opening parts allows an effect similar to that in the above-described embodiment to be produced.  
      Also, in the above-described embodiment, the case was described in which the substrate  100  is formed into a square shape and its four corners are fixed on the mounting base, but the shape of the substrate  100  can be changed as appropriate. For example, the substrate  100  may be formed into a long shape and its both ends may be fixed on the mounting base. In this case also, by arranging slits  106  as in the above-described embodiment between the load bearing portion and each of fixed portions, it is possible to disperse the stress applied to the substrate in response to a load to thereby properly detect the magnitude and direction of the load.