Patent Publication Number: US-2023152168-A1

Title: Load sensing apparatus

Description:
CLAIM OF PRIORITY 
     This application is a Continuation of International Application No. PCT/JP2021/029379 filed on Aug. 6, 2021, which claims benefit of Japanese Patent Application No. 2020-136350 filed on Aug. 12, 2020. The entire contents of each application noted above are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a load sensing apparatus for detecting a load. 
     2. Description of the Related Art 
     Load sensors for detecting a load have recently been widely used in electronic devices and so on. Japanese Unexamined Patent Application Publication No. 2002-124404 discloses a force detecting apparatus including a substrate including at least two force-detecting conductive lands and a common conductive land provided between them, an elastic plate made of conductive rubber or conductive elastomer opposed to the force-detecting conductive lands and the common conductive land, a contact-resistance generating surface including many microspikes formed on a surface of the elastic plate facing the force-detecting conductive lands, and a flat conductive elastic contact point formed on a surface of the elastic plate facing the common conductive land. 
     Japanese Unexamined Patent Application Publication No. 2014-142777 discloses a touch-panel input operation apparatus including a touch panel that is made of a transparent material, that includes an operating surface with which a subject who performs an input operation to come into contact, and that includes a touch sensor that detects a position where the subject comes into contact with the operating surface and outputs a signal, an upper holder holding the upper part of the touch panel and including an opening that opens the operating surface, a lower holder made of a light guiding member and disposed on the back of the touch panel and holding the lower part of the touch panel, a casing disposed on the back of the lower holder, a light source that introduces light to the lower holder made of a light guiding member, a pressure switch disposed between the lower holder and the casing and operated when the operating surface of the touch panel is pressed to output a pressure signal that confirms the input to the touch sensor, and a circuit board on which the pressure switch and the light source are mounted. 
     International Publication No. WO2019/167688 discloses a pressure-sensitive apparatus including a pressure sensor having a pressure sensitive surface, an elastic film member fixed to the periphery of the pressure sensitive surface so as to be opposed at a certain distance from the pressure sensitive surface, and an elastic member disposed between the pressure sensitive surface and the film member and transmitting a force applied to the film member to the pressure sensitive surface. 
     Load sensors for detecting a load require high detection accuracy and linearity of the detected value for the load and also tolerances necessary in housing a load sensing element in a housing for assembly. For example, if a member subjected to a load and the load sensing element need to be in contact with each other in assembly, the member subjected to the load and the load sensing element have to be accurately aligned, which makes the assembly difficult. 
     SUMMARY OF THE INVENTION 
     The present invention provides a load sensing apparatus configured to obtain high-accuracy detected values and sufficient tolerances in assembly. 
     In an aspect of the present invention, a load sensing apparatus includes a load sensing element including a pressure sensing portion, a housing that houses the load sensing element, and a pressing member supported by the housing, wherein the pressing member includes an elastic member that receives a load, a stiff pressing portion that is to come into contact with the pressure sensing portion; and an elastic supporting portion that supports the stiff pressing portion in the housing, and wherein, when no load is applied to the pressing member, a gap is formed between the stiff pressing portion and the pressure sensing portion. 
     With this configuration, the elasticity of the pressing member allows the load applied when the elastic member is pressed to be transmitted from the elastic member to the pressure sensing portion via the elastic supporting portion and the stiff pressing portion. In this case, the stiff pressing portion, which comes into contact with the pressure sensing portion, is made of, for example, a highly stiff material. This reduces or eliminates runout of the load, thereby enhancing the detection sensitivity. The presence of the gap between the stiff pressing portion and the pressure sensing portion allows providing tolerances in assembly. 
     In the load sensing apparatus, the elastic supporting portion may include a plate spring. The plate spring shape of the elastic supporting portion allows the load to be easily transmitted to the pressure sensing portion. In the load sensing apparatus, the elastic supporting portion may be part of the elastic member. This can simplify the apparatus configuration. 
     In the load sensing apparatus, a stiff plate may be provided between the elastic member and the stiff pressing portion. Providing the stiff plate allows reducing or eliminating runout of the load transmitted from the elastic member to the stiff pressing portion, thereby improving the measurement accuracy. 
     In the load sensing apparatus, the load sensing element preferably includes a displacement portion that is displaced by the load received by the pressure sensing portion and a plurality of piezoresistive elements that electrically detect an amount of displacement of the displacement portion. The use of such a load sensing element including the plurality of piezoresistive elements can enhance the linearity of load measurement. 
     In the load sensing apparatus, the stiff pressing portion may include metal or silicon. This enhances the durability at the contact between the stiff pressing portion and the pressure sensing portion of the load sensing element. 
     In the load sensing apparatus, the elastic member may include metal. This allows the load to be efficiently transmitted from the metal elastic member to the stiff pressing portion. 
     In the load sensing apparatus, the elastic member may include a first contact portion including a first contact point that receives the load, a second contact portion including a second contact point that is to come into contact with the stiff pressing portion, and a vertical portion provided between the first contact portion and the second contact portion and for forming a space between the first contact portion and the second contact portion, wherein, when a load is applied to the pressing member, the second contact portion may be elastically deformed toward the space by resistance from the stiff pressing portion. This allows the load to be efficiently transmitted to the pressure sensing portion while absorbing the resistance from the stiff pressing portion generated when the load is applied to the pressing member to be absorbed using the elastic deformation of the second contact portion. In this case, a structure in which the elastic deformation of the second contact portion occurs earlier than the elastic deformation of the vertical portion is preferable in the viewpoint of efficient transmission of the pressure received from the first contact point to the stiff pressing portion. 
     In the load sensing apparatus, the first contact portion, the second contact portion, and the vertical portion are preferably made of a metal plate into a single piece. This allows the elastic member to be made of one metal plate. In this case, when the second contact portion is formed of the opposite ends of the metal plate, joining the opposite ends together to form the second contact portion is preferable in the viewpoint of efficient transmission of the pressure received from the first contact point to the stiff pressing portion. 
     The load sensing apparatus may further include an integrated circuit housed in the housing and having the load sensing element thereon. This allows the load sensing apparatus to have the function of the integrated circuit. 
     In the load sensing apparatus, the housing may include a restricting portion that restricts motion of the stiff pressing portion in a direction perpendicular to a pushing direction. This allows reducing or eliminating the friction between the stiff pressing portion and the pressure sensing portion. 
     The load sensing apparatus may further include a cover that is disposed at an opposite side of the pressing member from the housing and that applies a load to the elastic member. This allows the load received with the cover to be transmitted from the pressing member to the load sensing element via the stiff pressing portion. 
     In the load sensing apparatus, the cover may include a protruding portion that is to come into contact with the elastic member. This allows the load to be efficiently transmitted from the protruding portion of the cover to the elastic member. 
     In the load sensing apparatus, the elastic member may be integrated with the protruding portion or connected to the protruding portion. This allows the load to be transmitted from the protruding portion of the cover to the elastic member without loss. 
     The load sensing apparatus may further include a stopper that restricts a decrease in relative distance between the cover and the housing. This stopper restricts a decrease in distance between the cover and the housing, thereby preventing an overload on the load sensing element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a perspective view of a load sensing apparatus according to an embodiment illustrating the configuration thereof; 
         FIG.  1 B  is a cross-sectional view of the load sensing apparatus illustrating the configuration thereof; 
         FIG.  2    is an exploded perspective view of the load sensing apparatus according to this embodiment; 
         FIG.  3    is a plan view of a displacement portion of a load sensing element; 
         FIG.  4 A  is a diagram illustrating the operation of the load sensing apparatus according to this embodiment; 
         FIG.  4 B  is a diagram illustrating the operation of the load sensing apparatus according to this embodiment; 
         FIG.  5 A  is a stress distribution chart when a load is applied to the load sensing apparatus according to this embodiment; 
         FIG.  5 B  is a stress distribution chart when a load is applied to the load sensing apparatus according to this embodiment; 
         FIG.  5 C  is a stress distribution chart when a load is applied to the load sensing apparatus according to this embodiment; 
         FIG.  6 A  is a diagram illustrating a pressing state in the case where a stiff pressing portion is used; 
         FIG.  6 B  is a diagram illustrating a pressing state in the case where a stiff pressing portion is used; 
         FIG.  7 A  is a diagram illustrating a pressing state in the case where an elastic pressing portion is used; 
         FIG.  7 B  is a diagram illustrating a pressing state in the case where an elastic pressing portion is used; 
         FIG.  8 A  is an enlarged stress distribution chart of a pressure sensing portion and a pressing portion; 
         FIG.  8 B  is an enlarged stress distribution chart of a pressure sensing portion and a pressing portion; 
         FIG.  9    is a graph showing the stress of the displacement portion against a load; 
         FIG.  10    is a graph showing the linear error of the output value of the load sensing element with respect to a load; 
         FIG.  11    is a cross-sectional view of another example of the load sensing apparatus according to this embodiment; 
         FIG.  12    is an exploded perspective view of the other example of the load sensing apparatus according to this embodiment; 
         FIG.  13 A  is a graph showing the output characteristics of a load sensing apparatus; 
         FIG.  13 B  is a graph showing the output characteristics of a load sensing apparatus; 
         FIG.  14 A  is a plan views of another example (1) of the elastic supporting portion; 
         FIG.  14 B  is a plan views of another example (2) of the elastic supporting portion; 
         FIG.  14 C  is a plan views of another example (3) of the elastic supporting portion; 
         FIG.  15 A  is a perspective view of another example (4) of the elastic supporting portion; 
         FIG.  15 B  is a perspective view of another example (5) of the elastic supporting portion; 
         FIG.  16 A  is a perspective view of another example (6) of the elastic supporting portion; 
         FIG.  16 B  is a perspective view of another example (7) of the elastic supporting portion; 
         FIG.  17    is a perspective view of another example (1) of the elastic member; 
         FIG.  18 A  is a perspective view of another example (2) of the elastic member; 
         FIG.  18 B  is a cross-sectional view of another example (2) of the elastic member; 
         FIG.  19    is a perspective view of another example (3) of the elastic member; 
         FIG.  20    is a perspective view of a load sensing apparatus including another example (4) of the elastic member; 
         FIG.  21    is an exploded perspective view of the load sensing apparatus including another example (4) of the elastic member; 
         FIG.  22    is a cross-sectional view of the load sensing apparatus including another example (4) of the elastic member; 
         FIG.  23    is a cross-sectional view of another example (4) of the elastic member; 
         FIG.  24 A  is a cross-sectional view of a load sensing apparatus including another example (4) of the elastic member illustrating the operation thereof; 
         FIG.  24 B  is a cross-sectional view of the load sensing apparatus including another example (4) of the elastic member illustrating the operation thereof; 
         FIG.  25 A  is a cross-sectional view of a load sensing apparatus including an elastic member of a comparable example illustrating the operation thereof; 
         FIG.  25 B  is a cross-sectional view of the load sensing apparatus including the elastic member of the comparable example illustrating the operation thereof; 
         FIG.  26 A  is a cross-sectional view of a load sensing apparatus including an integrated circuit; 
         FIG.  26 B  is a cross-sectional view of the load sensing apparatus including the integrated circuit; 
         FIG.  27    is a perspective view of an example of a load sensing apparatus including a cover; 
         FIG.  28    is an exploded perspective view of an example of the load sensing apparatus including the cover; 
         FIG.  29    is a cross-sectional view of an example of the load sensing apparatus including the cover; and 
         FIG.  30    is an enlarged cross-sectional view of an example of the load sensing apparatus including the cover. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described in detail hereinbelow with reference to the accompanying drawings. In the following description, like components are denoted by like reference signs, and descriptions of components once described are omitted as appropriate. 
     Configuration of Load Sensing Apparatus 
       FIGS.  1 A and  1 B  are diagrams illustrating the configuration of a load sensing apparatus according to an embodiment.  FIG.  1 A  is a perspective view of the load sensing apparatus.  FIG.  1 B  is a cross-sectional view of the load sensing apparatus.  FIG.  2    is an exploded perspective view of the load sensing apparatus according to this embodiment.  FIG.  3    is a plan view of a displacement portion of a load sensing element. 
     The load sensing apparatus  1  according to this embodiment is an apparatus that receives an external load and outputs a signal responsive to the load. The load sensing apparatus  1  includes a load sensing element  10 , a housing  20  that houses the load sensing element  10 , and a pressing member  30  supported by the housing  20 . In the description of the embodiments, the direction of normal to the load sensing element  10  mount surface of the housing  20  is a Z-direction, one of the directions perpendicular to the normal direction (Z-direction) is an X-direction, and another of the directions perpendicular to the normal direction is a Y-direction. 
     The load sensing element  10  includes a pressure sensing portion  11  and a sensor substrate  12 . The pressure sensing portion  11  is a portion protruding, for example, in a columnar shape, from the top of the sensor substrate  12  to receive an external load. The pressure sensing portion  11  is made of a silicon compound or silicon (the same material as that of the sensor substrate  12 ). 
     The sensor substrate  12  includes a displacement portion  121  that is displaced by the load received by the pressure sensing portion  11  and a plurality of piezoresistive elements  122  that electrically detects the amount of displacement of the displacement portion  121 . The sensor substrate  12  is joined to the top of a base substrate  13  and is connected to the housing  20  via the base substrate  13 . The displacement portion  121  is a portion that is displaced by the load received by the pressure sensing portion  11  and is disposed on the opposite surface of the sensor substrate  12  from the pressure sensing portion  11 . 
     The piezoresistive elements  122  are elements that electrically detect the amount of displacement of the displacement portion  121 . The plurality of piezoresistive elements  122  is disposed on the displacement portion  121 . The plurality of piezoresistive elements  122  is disposed along the periphery of the displacement portion  121  in such a manner that the adjacent elements are 90° out of phase with each other (mutually orthogonal positional relationship). When the displacement portion  121  is displaced by the load received by the pressure sensing portion  11 , the electrical resistance of the plurality of piezoresistive elements  122  changes according to the amount of displacement, and the midpoint electrical potential of a bridge circuit constituted by the plurality of piezoresistive elements  122  changes. This midpoint electrical potential is the sensor output. 
     The housing  20  has, for example, a box shape and includes an edge  21  and a housing space  22  which is a central recess. The edge  21  is the uppermost surface of the housing  20  and serves as a stopper when receiving an external load. 
     The housing space  22  houses the load sensing element  10 . The housing space  22  is provided with pads. The housed load sensing element  10  and the pads are electrically connected by bonding wires  15 . The housing space  22  may have resin (not shown) embedded therein for the purpose of protection of the bonding wires  15  and so on. 
     A level-difference portion  23  is provided inside the edge  21  so as to enclose the housing space  22 . A stiff pressing member  32  of the pressing member  30 , described later, is placed on the level-difference portion  23 . 
     The pressing member  30  includes an elastic member  31  that receives an external load, a stiff pressing portion  32  that comes into contact with the pressure sensing portion  11 , and an elastic supporting portion  33  that receives the stiff pressing portion  32  in the housing  20 . The elastic member  31  includes a protruding portion  311  and a flange  312 . The elastic member  31  is made of rubber, for example. The protruding portion  311  is columnar, for example. The flange  312  has a surface for placing the protruding portion  311  on the stiff pressing portion  32 . 
     The stiff pressing portion  32  is a plate-like member made of a material harder than the elastic member  31 . The stiff pressing portion  32  is made of, for example, stainless steel with a thickness of about 0.2 mm. Examples material for the stiff pressing portion  32  include silicon, ceramic, glass, and aluminum. The elastic modulus of the stiff pressing portion  32  is higher than the elastic modulus of the elastic member  31 , and preferably 60 GPa or more. 
     The elastic supporting portion  33  includes a frame  331  placed on the level-difference portion  23  of the housing  20  and an arm  332  that connects the frame  331  and the stiff pressing portion  32  together. Since the stiff pressing portion  32  is supported by the elastic supporting portion  33 , the stiff pressing portion  32  is positioned above the housing space  22  with the elastic supporting portion  33  therebetween. 
     The arm  332  is a plate spring portion that acts as a plate spring. The stiff pressing portion  32  is supported using the elastic deformation of the arm  332  with a predetermined spring constant. This spring constant is adjusted by means of the material, width, thickness, length, and shape of the arm  332 . Because the arm  332  has a plate spring shape and is disposed symmetrically about the stiff pressing portion  32 , the load from the elastic member  31  can easily be transmitted to the pressure sensing portion  11  directly below. 
     The elasticity of the pressing member  30  allows the load when the elastic member  31  is pressed to be transmitted from the elastic member  31  to the pressure sensing portion  11  of the load sensing element  10  via the elastic supporting portion  33  and the stiff pressing portion  32 . In this case, the stiff pressing portion  32 , which comes into contact with the pressure sensing portion  11 , is made of, for example, a highly stiff material (metal or silicon). This reduces or eliminates runout of the load, thereby enhancing the detection sensitivity. 
     The configuration of the load sensing element  10  that produces an output using a bridge circuit with the plurality of piezoresistive elements  122  requires to receive the load with the protruding pressure sensing portion  11  to deform the displacement portion  121  efficiently. For this reason, if the stiffness of a member that comes into contact with the pressure sensing portion  11  is low, the load to be transmitted from the pressing member  30  to the pressure sensing portion  11  cannot be efficiently transmitted to the pressure sensing portion  11 . In this embodiment, the pressure sensing portion  11  is pressed using the stiff pressing portion  32 . This allows reducing or eliminating the runout of an external load, thereby transmitting the load to the pressure sensing portion  11  efficiently. 
     Assembly of Load Sensing Apparatus 
     In the above configuration, the load sensing element  10  is housed in the housing space  22  of the housing  20 , and the load sensing element  10  and the pads in the housing space  22  are connected together using the bonding wires  15 . The elastic supporting portion  33  of the pressing member  30  is placed on the level-difference portion  23  of the housing space  22 , and the elastic member  31  is placed on the stiff pressing portion  32 . 
     In this state, the housing  20  is covered with the frame  40 . The frame  40  is fixed by engaging with hooks  25  provided on the sides of the housing  20 . The frame  40  has a hole 40h at the center. When the frame  40  is placed over the housing  20 , the protruding portion  311  protrudes upward through the hole 40h. The elastic member  31  is held at the flange  312  by the frame  40 . Thus, the pressing member  30  is fixed to the housing  20 . 
     In the load sensing apparatus  1  assembled in this manner, a gap d is formed between the stiff pressing portion  32  and the pressure sensing portion  11  in a state in which no load is applied to the pressing member  30 . In other words, a surface of the stiff pressing portion  32  adjacent to the pressure sensing portion  11  is not in contact with the pressure sensing portion  11 . The presence of the gap d between the stiff pressing portion  32  and the pressure sensing portion  11  allows providing tolerances in assembly. 
     In other words, if the stiff pressing portion  32  and the pressure sensing portion  11  are in contact or close enough to come into contact, the stiff pressing portion  32  and the pressure sensing portion  11  may collide with each other because of the dimensional errors of the components or misalignment in assembly. Collision of a high-stiffness member, like the stiff pressing portion  32 , with the pressure sensing portion  11 , may exert adverse influences on the load sensing element  10 . Providing the gap d between the stiff pressing portion  32  and the pressure sensing portion  11  as in this embodiment allows collision at assembly to be positively prevented, thereby protecting the load sensing element  10 . 
     Operation of Load Sensing Apparatus 
       FIGS.  4 A and  4 B  are diagrams illustrating the operation of the load sensing apparatus according to this embodiment.  FIG.  4 A  illustrates a state in which the load sensing apparatus  1  is subjected to a load.  FIG.  4 B  illustrate an example of the output of the load sensing element. The horizontal axis of  FIG.  4 B  represents the stroke of a plate  50  in the Z-direction, and the vertical axis represents the output values (relative values). 
     As shown in  FIG.  4 A , the elastic member  31  of the pressing member  30  of the load sensing apparatus  1  is subjected to a load via the plate  50 . When a load is applied in the Z-direction from the plate  50  to the elastic member  31 , the stiff pressing portion  32  supported using the spring action of the elastic supporting portion  33  is pushed in the Z-direction. 
     Since the gap d is provided between the stiff pressing portion  32  and the pressure sensing portion  11  of the load sensing element  10 , the pressure sensing portion  11  is subjected to no load until the stiff pressing portion  32  comes into contact with the pressure sensing portion  11 . 
     Accordingly, no output is produced during the period after the load sensing apparatus  1  is subjected to a load to a predetermined stroke S 1 , as shown in  FIG.  4 B . This region is referred to as a prestroke region R 1 . In the prestroke region R 1 , the pressing member  30  makes a stroke, but the load is not transmitted to the pressure sensing portion  11 , so that the output value does not increase during the period after the elastic supporting portion  33  is elastically deformed under the load until the stiff pressing portion  32  comes into contact with the pressure sensing portion  11 . The length of the prestroke region R 1  can be set using the gap d. 
     Next, when the load is applied across the prestroke region R 1 , the output value increases according to the stroke. This region is referred to as a force receiving region R 2 . In the force receiving region R 2 , the stiff pressing portion  32  is in contact with the pressure sensing portion  11 , so that the load is transmitted from the elastic member  31  to the pressure sensing portion  11  via the stiff pressing portion  32 . The output value from the load sensing element  10  increases substantially in proportion to the stroke (load) because of the stiffness of the stiff pressing portion  32  that is in contact with the pressure sensing portion  11 . The output value increases to V 1  according to the stroke. 
     The force receiving region R 2  continues until the edge  21  of the housing  20  functions as a stopper. In other words, when the pressing member  30  is pushed to bring the plate  50  into contact with the edge  21  of the housing  20 , the pushing stops. Thus, the stroke of the pressing member  30  stops at S 2 , so that the output value does not increase, and therefore an overload on the load sensing element  10  is prevented. 
       FIGS.  5 A to  5 C  are stress distribution charts when a load is applied to the load sensing apparatus according to this embodiment.  FIG.  5 A  shows a state in which no load is applied (the state at stroke 0 in  FIG.  4 B ). In this state, since the arm  332  of the elastic supporting portion  33 , which supports the stiff pressing portion  32  between the elastic member  31  and the pressure sensing portion  11 , is not deformed, the stiff pressing portion  32  is hidden by the frame  331 . 
       FIG.  5 B  shows a stress distribution in a state in which the pressing member  30  is pushed in the Z-direction by application of a load to bring the stiff pressing portion  32  into contact with the pressure sensing portion  11  (the state at stroke S 1  in  FIG.  4 B ). The load from the plate  50  is transmitted to the stiff pressing portion  32  via the elastic member  31 . This causes the stiff pressing portion  32  to move toward the pressure sensing portion  11  in the Z-direction into contact therewith while being supported by the arm  332  of the elastic supporting portion  33 . 
       FIG.  5 C  shows a stress distribution in a state in which the plate  50  is in contact with the edge  21  of the housing  20  (the state at stroke S 2  in  FIG.  4 B ). The change in stress distribution shows that the load applied from the plate  50  is effectively transmitted from the stiff pressing portion  32  to the load sensing element  10  via the pressure sensing portion  11 . 
     Even if the plate  50  is diagonally proximate to (in partial-contact with) the load sensing apparatus  1  for any reason (assembly variation), deformation of the elastic member  31  that is in direct contact with the plate  50  allows stabilizing the state of contact between the plate  50  and the elastic member  31 . The elastic member  31  subjected to a load displaces the stiff pressing portion  32  in the Z-direction toward the pressure sensing portion  11  while deforming the arm  332 . Thus, the elastic member  31  is for stabilizing the contact between the pressing member  30  and the plate  50 , and the stiff pressing portion  32  is for achieving proper contact between the pressing member  30  and the load sensing element  10 . 
     EXAMPLES 
     Comparison of the hardness of pressing portions that come into contact with the pressure sensing portion  11  will be described. 
       FIGS.  6 A and  6 B  are diagrams illustrating a pressing state in the case where the stiff pressing portion  32  comes into contact with the pressure sensing portion  11  in an example of the present invention.  FIG.  6 A  is a schematic cross-sectional view of the stiff pressing portion  32  and the pressure sensing portion  11  in contact with each other.  FIG.  6 B  is a stress distribution chart in the case where a load is applied from the stiff pressing portion  32  to the pressure sensing portion  11 . 
       FIGS.  7 A and  7 B  are diagrams illustrating a pressing state in the case where an elastic pressing portion  32 B with a low elastic modulus, in place of the stiff pressing portion  32 , comes into contact with the pressure sensing portion  11  in a comparable example.  FIG.  7 A  is a schematic cross-sectional view of the stiff pressing portion  32 B and the pressure sensing portion  11  in contact with each other.  FIG.  7 B  is a stress distribution chart in the case where a load is applied from the stiff pressing portion  32 B to the pressure sensing portion  11 . 
       FIGS.  8 A and  8 B  are enlarged stress distribution charts of the pressure sensing portion and the pressing portions.  FIG.  8 A  is an enlarged stress distribution chart of the contact between the stiff pressing portion  32  and the pressure sensing portion  11  shown in  FIG.  6 B .  FIG.  8 B  is an enlarged stress distribution chart of the contact between the elastic pressing portion  32 B and the pressure sensing portion  11  shown in  FIG.  7 B . 
     The stiff pressing portion  32  shown in  FIGS.  6 A and  6 B  and  FIG.  8 A  was made of stainless steel (SUS304). The elastic pressing portion  32 B shown in  FIGS.  7 A and  7 B  and  FIG.  8 B  was made of rubber (a hardness of  70 , measured using a durometer Type A). Both of them applied a load of 6 N to the pressure sensing portion  11 . 
     The example showed that using the stiff pressing portion  32  caused the load to be intensively transmitted toward the sensor substrate around the point of contact with the pressure sensing portion  11 . In contrast, using the elastic pressing portion  32 B caused the pressure sensing portion  11  to bite into the elastic pressing portion  32 B, causing the load to disperse. Accordingly, the stiff pressing portion  32  should have stiffness to the extent that the surface of contact with the pressure sensing portion  11  is within the pressure sensing surface of the pressure sensing portion  11 . The area of the contact surface can be obtained using Hertzian contact theory. 
       FIG.  9    is a graph showing the stress of the displacement portion against a load. When the elastic pressing portion  32 B was used, the load dispersed and therefore could not be efficiently transmitted from the pressure sensing portion  11  to the sensor substrate  12 , and as a result, the stress against the load, transmitted to the displacement portion  121  of the sensor substrate  12 , was not increased. In contrast, when the stiff pressing portion  32  was used, the load could be efficiently transmitted to the sensor substrate, and as a result, the stress against the load, transmitted to the displacement portion  121  of the sensor substrate  12 , could be increased. The change in the stress of the displacement portion  121  against the load shown in  FIG.  9    is equivalent to sensitivity. Accordingly, using the stiff pressing portion  32  provides more sufficient sensitivity than using the elastic pressing portion  32 B. 
       FIG.  10    is a graph showing the linear error of the output value of the load sensing element with respect to a load. When the elastic pressing portion  32 B was used, the pressure sensing portion 11 bit into the elastic pressing portion  32 B to 4 N, which caused the load to disperse, causing a large linear error. In contrast, when the stiff pressing portion  32  was used, the pressure sensing portion  11  did not bite into the stiff pressing portion  32 , which caused almost no runout of the load. This showed that the linear error was very small. 
     Another Example of Load Sensing Apparatus 
       FIG.  11    is a cross-sectional view of another example of the load sensing apparatus according to this embodiment.  FIG.  12    is an exploded perspective view of the other example of the load sensing apparatus according to this embodiment. 
     As shown in  FIGS.  11  and  12   , a load sensing apparatus  1 B of another example includes a stiff plate  60  between the elastic member  31  and the stiff pressing portion  32 . 
     The load sensing apparatus  1  described above directly presses the stiff pressing portion  32  using the elastic member  31 . However, since part (flange  312 ) of the elastic member  31  is in contact with the frame and the frame  331  of the elastic supporting portion  33 , part of the received load disperses to cause attenuation of the force to be transmitted to the load sensing element  10 . 
     In contrast, the load sensing apparatus  1 B transmits the load received by the elastic member  31  to the stiff pressing portion  32  via the stiff plate  60 . The stiff plate  60  can transmit the force to the center of the stiff pressing portion  32  without interfering with another member. This facilitates transmitting the load received by the elastic member  31  to the load sensing element  10 , thereby increasing the sensitivity. 
       FIGS.  13 A and  13 B  are graphs showing the output characteristics of load sensing apparatuses. In  FIGS.  13 A and  13 B , the horizontal axis represents the load, and the vertical axis represents the output value of the load sensing element  10 .  FIG.  13 A  shows the output characteristics of the load sensing apparatus  1 .  FIG.  13 B  shows the output characteristics of the load sensing apparatus  1 B. Under the same load, the load sensing apparatus  1 B obtains an output value about 1.6 times that of the load sensing apparatus  1 . 
     Even with the load sensing apparatus  1 B including the stiff plate  60 , the prestroke region R 1  (see  FIG.  4 B ) was set using the gap d provided between the stiff pressing portion  32  and the pressure sensing portion  11  of the load sensing element  10 , in the load sensing apparatus  1 . In this case, the length from the start of application of the load to the predetermined stroke S 1 , that is, the prestroke region R 1 , can be set using the thickness of the stiff plate  60 . In other words, the thinner the stiff plate  60 , the larger the gap d, and the larger the prestroke region R 1 . In contrast, the thicker the stiff plate  60 , the smaller the gap d, and the smaller the prestroke region R 1 . 
     Other Examples of Elastic Supporting Portion 
       FIGS.  14 A to  14 C  are plan views of other examples of the elastic supporting portion.  FIG.  14 A  shows another example (1) of the elastic supporting portion  33 .  FIG.  14 B  shows another example (2) of the elastic supporting portion  33 .  FIG.  14 C  shows another example (3) of the elastic supporting portion  33 . 
     As shown in  FIGS.  14 A to  14 C , the arm  332  of the elastic supporting portion  33  may have various shapes. In  FIG.  14 A , the arm  332  is disposed axisymmetrically about the stiff pressing portion  32  disposed at the center. In  FIGS.  14 B and  14 C , the arm  332  is disposed symmetrically about a point with respect to the stiff pressing portion  32  disposed at the center. In  FIG.  14 B , the arm  332  has a turned over shape. In  FIG.  14 C , the arm  332  has a spiral shape. 
     Such shapes of the arm  332  make the stiff pressing portion  32  likely to be deformed elastically in the direction of proximity to the pressure sensing portion  11  (Z-direction) but make the stiff pressing portion  32  difficult to be elastically deformed in another direction (for example, the in-plane direction of the frame  331 , the X-Y direction). In other words, the arm  332  has an anisotropic nature in ease of elastic deformation. For this reason, the load applied to the plate  50  is efficiently transmitted to the pressure sensing portion  11 . Even if the direction of the load applied to the plate  50  varies, the elastic member  31  can receive the load properly by being elastically deformed, and the anisotropic arm  332  can efficiently transmit the load received by the elastic member  31  toward the pressure sensing portion  11  (Z-direction). In particular, the shape of the elastic supporting portion  33  shown in  FIG.  14 A  has a high anisotropic effect of elastic deformation, increasing the reliability of contact with the pressure sensing portion  11 . With the shapes of the elastic supporting portion  33  shown in  FIGS.  14 B and  14 C , the arm  332  is long and easily elastically deformed, easily providing a soft prestroke feeling. 
     The spring structure of the elastic supporting portion  33  allows the stiff pressing portion  32  disposed at the center to be supported at a predetermined spring constant. In this spring structure, the longer and the narrower the arm  332 , the smaller the spring constant (easily deformed under a small load). Likewise, the thinner the arm  332 , the smaller the spring constant. In the case of stainless steel, the arm  332  preferably has a thickness of about 0.2 mm in consideration of the strength of the stiff pressing portion  32 . 
     Reducing the deformation of the stiff pressing portion  32  in the in-plane direction supported by the arm  332  allows reducing the risk of wear due to the contact between the stiff pressing portion  32  and the pressure sensing portion  11  of the load sensing element  10 , allowing providing more reliable products. 
       FIG.  15 A  is a perspective view of another example (4) of the elastic supporting portion.  FIG.  15 B  is a perspective view of another example (5) of the elastic supporting portion.  FIG.  16 A  is a perspective view of another example (6) of the elastic supporting portion.  FIG.  16 B  is a perspective view of another example (7) of the elastic supporting portion. Any of the above diagrams shows a state in which the elastic member  31  is removed for the convenience of description. 
     In other examples (4) to (7) of the elastic supporting portion  33 , the frame  331  has positioning holes  331   h . The level-difference portion  23  of the housing  20  on which the frame  331  is to be placed has protrusions  23   a  for positioning. When the frame  331  is placed on the level-difference portion  23 , the protrusions  23   a  are fitted in the positioning holes  331   h  to locate the elastic supporting portion  33 . 
     The elastic supporting portions  33  shown in  FIG.  15 A  to  FIG.  16 B  differ in the width (Y-direction) of joining portions  332   a  of the respective arms  332  joining to the stiff pressing portion  32 . In other words, the width of the j oining portion  332   a  of the arm  332  of the elastic supporting portion  33  shown in  FIG.  15 A  is the smallest, and the widths of the j oining portions  332   a  of the arms  332  shown in  FIGS.  15 B,  16 A, and  16 B  increase in this order. The spring constant decreases to facilitate obtaining a soft prestroke feeling as the width of the joining portion  332   a  of the arm  332  of the elastic supporting portion  33  decreases. In contrast, the spring constant increases to facilitate obtaining a pushing feeling as the width of the joining portion  332   a  of the arm  332  increases. 
     Other Examples of Elastic Member 
       FIG.  17    is a perspective view of another example (1) of the elastic member. An elastic member  31 B shown in  FIG.  17    doubles as the elastic supporting portion  33 . In other words, the elastic supporting portion  33  is part of the elastic member  31 B. Specifically, the stiff pressing portion  32  is fitted in a surface of the elastic member  31 B adjacent to the load sensing element  10 , and the flange  312  of the elastic member  31  also has the function of the elastic supporting portion  33  that supports the stiff pressing portion  32 . The use of the elastic member  31 B eliminates the need for the plate-like elastic supporting portion  33  shown in  FIG.  2   , simplifying the configuration of the pressing member  30 . 
       FIG.  18 A  is a perspective view of another example (2) of the elastic member.  FIG.  18 A  shows a state in which the frame  40  is removed for the convenience of description.  FIG.  18 B  is a cross-sectional view of another example (2) of the elastic member. 
     In another example (2) of the elastic member  31 , the flange  312  of the elastic member  31  extends in the X-direction to the edge  21  of the housing  20 . The edge  21  of the housing  20  includes steps  21   a . The flange  312  of the elastic member  31  is placed on the steps  21   a  of the edge  21 . Sides  31   a  of the elastic member  31  opposing the edge  21  include recessed portions  31   b . The recessed portions  31   b  engage with protruding portions  20   a  of the housing  20  to position the elastic member  31 . By placing the frame  40  over the housing  20 , the elastic member  31  is sandwiched between the frame  40  and the housing  20 . Providing the elastic member  31  with the flange  312  allows the flange  312  to be sandwiched between the frame  40  and the housing  20 , thereby effectively preventing the elastic member  31  from coming off when a lateral force is applied to the elastic member  31 . 
       FIG.  19    is a perspective view of another example (3) of the elastic member. In another example (3) of the elastic member  31 , the flange  312  of the elastic member  31  extends in the X-direction to the edge  21  of the housing  20 , and the flange  312  is placed on the steps  21   a  of the edge  21 . Although another example (3) of the elastic member  31  does not include the recessed portions  31   b , the elastic member  31  is positioned by the contact between the sides  31   a  of the elastic member  31  and the inner peripheral surface of the housing  20  and the engagement of inward protruding portions  21   b  provided at the four corners of the edge  21  with notches 312b of the flange  312 . In another example (3) of the elastic member  31  also, the elastic member  31  is sandwiched between the frame  40  and the housing  20  by placing the frame  40  over the housing  20 . Providing the elastic member  31  with the flange  312  allows the flange  312  to be sandwiched between the frame  40  and the housing  20 , thereby effectively preventing the elastic member  31  from coming off when a lateral force is applied to the elastic member  31 . 
       FIG.  20    is a perspective view of a load sensing apparatus including another example (4) of the elastic member.  FIG.  21    is an exploded perspective view of the load sensing apparatus including another example (4) of the elastic member.  FIG.  22    is a cross-sectional view of the load sensing apparatus including another example (4) of the elastic member.  FIG.  23    is a cross-sectional view of another example (4) of the elastic member. 
     An elastic member  31 C according to another example (4) is made of metal. For example, the elastic member  31  is formed by bending a metal plate. The elastic member  31 C includes a first contact portion  3101  including a first contact point CP 1  that receives a load, second contact portions  3102  including a second contact point CP 2  that is in contact with the stiff pressing portion  32 , and vertical portions  3103  provided between the first contact portion  3101  and the second contact portions  3102  to provide a space S between the first contact portion  3101  and the second contact portions  3102 . 
     In this elastic member  31 C, the first contact portion  3101  and the vertical portions  3103  form a convex shape. This convex shape serves as the protruding portion  311 . The second contact portions  3102  face the first contact portion  3101  and have an interval from the first contact portion  3101  according to the height of the vertical portions  3103 . This interval forms the space S between the first contact portion  3101  and the second contact portions  3102 . 
     The elastic member  31 C is formed by bending, for example, a metal plate, to integrally form the first contact portion  3101 , the second contact portions  3102 , and the vertical portions  3103 . The opposite ends of the metal plate are joined (for example, welded) to form the elastic member  31 C having an annular portion. The inside of the annular portion is the space S. 
     Specifically, the first contact portion  3101  including the first contact point CP 1  and extending in the lateral direction (the direction along the X-Y plane) is formed of a metal plate, and the two vertical portions  3103  are formed by bending the plates at the opposite ends of the first contact portion  3101  downward at substantially right angles. The second contact portions  3102  are formed by bending the plate from the vertical portions  3103  to the opposite side from the lateral first contact portion  3101  and folding back the plate 180 degrees at predetermined positions. 
     The opposite ends of the metal plate of the second contact portions  3102  may be joined together in overlapped state or with the end faces butted. The overlapped joining is preferable from the viewpoint of joining strength. Since the overlapped portion functions as the stiff plate  60 , the sensitivity can be enhanced. The flange  312  of the elastic member  31 C is formed by folding the metal plate from the vertical portions  3103  to the second contact portions  3102  so as to extend laterally. 
       FIGS.  24 A and  24 B  are cross-sectional views of a load sensing apparatus including another example (4) of the elastic member illustrating the operation thereof.  FIGS.  25 A and  25 B  are cross-sectional views of a load sensing apparatus including an elastic member of a comparable example illustrating the operation thereof. 
       FIG.  24 A  shows a state before the load sensing apparatus  1  including the elastic member  31 C is subjected to a load.  FIG.  24 B  shows a state in which the load sensing apparatus  1  including the elastic member  31 C is subjected to a load. When a load is applied to the elastic member  31 C on which the plate  50  is placed, the load received at the first contact point CP 1  of the first contact portion  3101  is applied to the stiff pressing portion  32  that is in contact with the second contact point CP 2  of the second contact portions  3102 . 
     The application of the load to the stiff pressing portion  32  causes the elastic supporting portion  33  to bend to bring the stiff pressing portion  32  close to the pressure sensing portion  11  of the load sensing element  10  into contact therewith. When the stiff pressing portion  32  comes into contact with the pressure sensing portion  11 , the second contact portions  3102  are elastically deformed toward the space S because of the resistance from the stiff pressing portion  32 . The load from the plate  50  is transmitted to the stiff pressing portion  32  via the elastic member  31 C and is transmitted from the stiff pressing portion  32  to the load sensing element  10  via the pressure sensing portion  11 . 
       FIG.  25 A  shows a state before the load sensing apparatus  1  including the elastic member  31 D of the comparative example is subjected to a load.  FIG.  25 B  shows a state in which the load sensing apparatus  1  including the elastic member  31 D of the comparative example is subjected to a load. Although the elastic member  31 D of the comparable example is formed by bending a metal plate, as is the elastic member  31 C, the ends of the second contact portions  3102  (the ends of the metal plate) are not joined together. 
     When a load is applied to the elastic member  31 D on which the plate  50  is placed, the load received by the first contact portion  3101  is applied to the stiff pressing portion  32  that is in contact with the second contact portions  3102 . The application of the load to the stiff pressing portion  32  causes the elastic supporting portion  33  to bend to bring the stiff pressing portion  32  close to the pressure sensing portion  11  of the load sensing element  10  into contact therewith. When the stiff pressing portion  32  comes into contact with the pressure sensing portion  11 , the second contact portions  3102  are elastically deformed toward the space S because of the resistance from the stiff pressing portion  32 . 
     Since the ends of the second contact portions  3102  are not joined together, the two second contact portions  3102  move in the expanding direction with application of the load. The expansion of the two second contact portions  3102  causes the protruding portion  311  to collapse such that portions of the vertical portions  3103  adjacent to the second contact portions  3102  (lower parts) expand. Thus, in the elastic member  31 D, the protruding portion  311  is more prone to collapse than that of the elastic member  31 C. 
     In a simulation, when the same pressure was applied to the first contact portion  3101  of the elastic member  31 C and the first contact portion  3101  of the elastic member  31 D, the force transmitted to the stiff pressing portion  32  via the second contact portions  3102  of the elastic member  31 D was one fifth of the force transmitted to the stiff pressing portion  32  via the second contact portions  3102  of the elastic member  31 C. This simulation showed that the load sensing apparatus  1  including the elastic member  31 C with a structure in which the elastic deformation of the second contact portions  3102  occurs earlier than the elastic deformation of the vertical portions  3103  could transmit the load from the plate  50  to the load sensing element  10  more efficiently than the load sensing apparatus  1  including the elastic member  31 D with a structure in which the elastic deformation of the vertical portions  3103  occurs earlier than the elastic deformation of the second contact portions  3102 . In the case where the elastic member including the first contact portion  3101 , the second contact portions  3102 , and the vertical portions  3103  is formed by bending a metal plate, and the second contact portions  3102  are formed of the opposite ends of the metal plat, the pressure that the first contact portion  3101  receives can be transmitted to the stiff pressing portion  32  efficiently by forming the second contact portions  3102  by joining the opposite ends of the metal plate together, as is the elastic member  31 C. 
     Example of Load Sensing Apparatus Including Integrated Circuit 
       FIGS.  26 A and  26 B  are cross-sectional views of a load sensing apparatus including an integrated circuit. An integrated circuit  70  used in the load sensing apparatus  1 C is an application specific integrated circuit (ASIC) that converts an analog signal, for example, an output from the load sensing element  10 , to a digital signal. The integrated circuit  70  may be a circuit other than that for signal conversion. 
     The integrated circuit  70  has the load sensing element  10  thereon. In other words, the housing space  22  of the housing 20 houses the integrated circuit  70  and the load sensing element  10  layered therein. This allows providing a package configuration of one load sensing apparatus  1 C including the integrated circuit  70 . 
       FIG.  26 A  illustrates an example in which the housing  20  of the load sensing apparatus  1 C is made of a mold resin.  FIG.  26 B  illustrates an example in which the housing  20  of the load sensing apparatus  1 C is made of ceramic. Using a mold resin for the housing  20  allows providing a low-price light weight load sensing apparatus  1 C. Using ceramic for the housing  20  allows forming a fine metalizing pattern on the housing  20 , thereby thinning the electrical connection to the integrated circuit  70 . 
     Example of Load Sensing Apparatus Including Cover 
       FIG.  27    is a perspective view of an example of a load sensing apparatus including a cover.  FIG.  28    is an exploded perspective view of an example of the load sensing apparatus including the cover.  FIG.  29    is a cross-sectional view of an example of the load sensing apparatus including the cover.  FIG.  30    is an enlarged cross-sectional view of an example of the load sensing apparatus including the cover. 
     A load sensing apparatus  1 D includes a cover  80  disposed at the opposite side from the housing  20  of the pressing member  30 . The housing  20  of the load sensing apparatus  1 D includes a restricting portion  27  that restricts the motion of the stiff pressing portion  32  in the directions (X-direction and Y-direction) perpendicular to the pushing direction (Z-direction). Specifically, the restricting portion  27  is a hole provided in the housing  20 . The hole houses the load sensing element  10  and the pressing member  30  (the elastic member  31 , the stiff pressing portion  32 , and the elastic supporting portion  33 ). The load sensing apparatus  1 D includes the stiff plate  60  between the elastic member  31  and the stiff pressing portion  32 . The stiff plate  60  may be provided as need arises. 
     The lengths of the hole of the restricting portion  27  in the X-direction and the Y-direction are fixed in the Z-direction. The pressing member  30  is slidable in the Z-direction in the hole of the restricting portion  27  but is restricted in movement in the X-direction and the Y-direction. 
     The cover  80  includes a protruding portion  81  that comes into contact with the elastic member  31 . This allows a load, when applied from the cover  80 , to be transmitted through the protruding portion  81  to the elastic member  31  and to the pressure sensing portion  11  of the load sensing element  10  via the stiff pressing portion  32 . At that time, the movement of the pressing member  30  in the X-direction and the Y-direction is restricted by the restricting portion  27  of the housing  20 . For this reason, the movement of the stiff pressing portion  32  in the X-direction and the Y-direction is also restricted, and as a consequence, the friction between the stiff pressing portion  32  and the pressure sensing portion  11  in the X-direction and the Y-direction is reduced or eliminated. Accordingly, even if the pressure sensing portion  11  is made of a high-hardness low-toughness material, damage to the pressure sensing portion  11  due to the friction between the pressure sensing portion  11  and the stiff pressing portion  32  can be reduced or eliminated. 
     The protruding portion  81  of the cover  80  may be formed integrally with the elastic member  31 , or the protruding portion  81  and the elastic member  31  may be connected together. This allows the load to be transmitted to the elastic member  31  through the protruding portion  81  of the cover  80  without loss. 
     The load sensing apparatus  1 D preferably includes stoppers  85  for restricting a decrease in relative distance between the cover  80  and the housing  20 . For example, the stoppers  85  are protrusions provided at portions of the cover  80  facing the housing  20  in the vicinity of the protruding portion  81 . Pushing the cover  80  causes the stoppers  85  and the housing  20  to come into contact with each other, preventing further pushing of the cover  80 . Thus, even if a load more than necessary is applied to the cover  80 , an overload on the load sensing element  10  can be prevented. The stoppers  85  are preferably disposed in the vicinity of the protruding portion  81  of the cover  80 . For example, when the cover  80  is pushed in, low stiffness of the cover  80  tends to cause deflection of the cover  80 . Disposing the stoppers  85  in the vicinity of the protruding portion  81  provides a sufficient stopping effect when the load sensing element  10  is pushed by the protruding portion  81  even if the cover  80  is bent. The stoppers  85  may be preliminarily disposed at a position distant from the protruding portion  81 . This provides the effect of reducing or eliminating the bending of the cover  80 . The stoppers  85  may be provided at the cover  80  or at the housing  20 . In other words, the stoppers  85  need only be disposed at positions where the cover  80  and the housing  20  face each other or may be provided at both the cover  80  and the housing  20 . 
     Since the load sensing apparatus  1 D includes the cover  80 , the load sensing apparatus  1 D may be used as a component disposed at a position of a product visible to the user. A specific example of such a component is a panel switch disposed indoors or in vehicles. An image display may be provided at the position of the cover  80  visible to the user. Adjusting a material (elastic modulus) for the elastic member  31  allows changing the pushing feeling of the cover  80  (the responsive touch of the panel switch). Adjusting the thickness of the elastic member  31  allows adjusting the surface position of the cover  80  (the touch position of the panel switch). 
     Thus, the embodiments allow providing the load sensing apparatuses  1 ,  1 B,  1 C, and  1 D that provide high detection accuracy and linearity of the detected value against the load, and having sufficient assembly tolerances. 
     Having described the embodiments, it is to be understood that the present invention is not limited to the examples. For example, the shape of the arm  332  of the elastic supporting portion  33  is not limited to the above examples. It is to be understood that addition, deletion, and design change of components, as well as combinations of the features of the embodiments as appropriate, will occur to those skilled in the art without departing from the scope of the present invention.