Abstract:
There is provided a load sensor including: a pressing shaft configured to transmit a load in an axial direction of the pressing shaft; a sensor element configured to measure the load being transmitted by the pressing shaft; and a load limiting means configured to limit the load applied to the sensor element and protect the sensor element from being applied with excessive load, the load limiting means being configured to be deformable by the load when the load transmitted by the pressing shaft exceeds an allowable measurement range set for the sensor element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a load sensor that measures a load acting in one axial direction. 
     2. Description of the Related Art 
     Conventionally, there have been known load sensors for measuring a load (a pressing force) acting in one axial direction (for example, a Z-axis direction in an orthogonal coordinate system). Examples of such load sensors are disclosed in JP-A-61-278719 and in JP-A-2000-214002. A load sensor disclosed in JP-A-61-278719 is configured as a top loading balance using a coil spring as a cushioning member. In this load sensor, a stopper is provided on the bottom of a shaft, and the diameter of a through-hole of a case is adjusted for the shaft, whereby a measure against cases where the load sensor receives an eccentric load or an impact in a direction other than a vertical direction is taken. A load sensor disclosed in JP-A- 2000-214002 is configured so as to detect a load in a wide range by two coil springs and two detection elements each made of a magneto resistance element (MR element). 
     In the configuration of the load sensor disclosed in JP-A-61-278719, a detector is arranged at a portion different from a portion that displaces in response to a load. Therefore, the load senor becomes large, and it is difficult to reduce the size of the entire load senor. Also, since a pan is configured to be large, a shaft should be formed to be long enough with respect to an eccentric load, this load sensor is not suitable for reducing the overall size. 
     Also, in the configuration of the load sensor disclosed in JP-A-2000-214002, since the coil springs are configured in two stages, the entire load sensor becomes large. Further, since the load sensor does not have a configuration for preventing an inclination of an axial direction in which a load to be measured acts, the load may not be measured accurately. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances, and one of objects of the present invention is to provide a load sensor capable of accurately measuring a load acting in one axial direction while protecting a sensor element. Another object of the present invention is to provide a load sensor capable of reducing the size of the entire configuration and capable of accurately measuring a load acting in one axial direction even if the load sensor is pressed from a direction deviating from the axial direction in which the load acts. 
     According to an illustrative embodiment of the present invention, there is provided a load sensor including: a pressing shaft configured to transmit a load in an axial direction of the pressing shaft; a sensor element configured to measure the load being transmitted by the pressing shaft; and a load limiting means configured to limit the load applied to the sensor element and protect the sensor element from being applied with excessive load, the load limiting means being configured to be deformable by the load when the load transmitted by the pressing shaft exceeds an allowable measurement range set for the sensor element. 
     According to the present invention, it is possible to provide a load sensor capable of accurately measuring a load acting in one axial direction while protecting a sensor element. Further, it is possible to provide a load sensor capable of reducing the size of the entire configuration and capable of accurately measuring a load (a pressing force) acting in one axial direction even if the load sensor is pressed from a direction deviating from the axial direction in which the load acts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a perspective view illustrating a load sensor according to an embodiment of the present invention; 
         FIG. 2  is a perspective view illustrating a cross section obtained by cutting the load sensor of  FIG. 1  along the longitudinal axis of the load sensor; 
         FIG. 3  is a cross-sectional view corresponding to  FIG. 2 ; 
         FIGS. 4A to 4D  are explanatory views for explaining the action of the load sensor shown in  FIG. 1 ; and 
         FIG. 5  is a characteristic diagram corresponding to  FIGS. 4A to 4D  for explaining the action of the load sensor shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a load sensor according to an embodiment of the present invention will be described with reference to the accompanying drawings.  FIG. 1  is a perspective view illustrating the load sensor according to the embodiment of the present invention.  FIG. 2  is a perspective view illustrating a cross section obtained by cutting the load sensor of  FIG. 1  along the longitudinal axis of the load sensor.  FIG. 3  is a cross-sectional view corresponding to  FIG. 2 . 
     A load sensor  1  according to the embodiment of the present invention is configured as a load sensor that measures a load by pressing a sensor element  10  with a pressing shaft  20 , and includes the pressing shaft  20  that transmits a load applied to the load sensor, in one axial direction, the sensor element  10  that measures the load through the pressing shaft  20 , a load limiting means that protects the sensor element  10 , and a support case (support body)  40  that supports the sensor element  10 , the load limiting means, and the pressing shaft  20  in a predetermined arrangement state. 
     Hereinafter, the individual components configuring the load sensor  1  will be described in detail. The sensor element  10  is provided with an elastic body  110  and strain gages  120 . The elastic body  110  is made of a metal in a thin plate shape, and the tip of the pressing shaft  20  is pressed against one surface (the upper surface in  FIGS. 2 and 3 ) of the elastic body  110  while the pressing shaft  20  is configured to be able to be spaced apart from the upper surface of the elastic body  110 , and on the other surface the lower surface in  FIGS. 2 and 3 ) of the elastic body  110 , the strain gages  120  are being attached. The outer area (edge portion  111 ) of the elastic body  110  having the strain gages  120  attached thereon abuts on one end portion (the upper end portion in  FIG. 2 )  31  of a coil spring  30  that is configured to serve as the load limiting means. 
     On the other surface of the elastic body  110 , an insulating film is formed, and on the insulating film, the strain gages  120  are attached. The strain gages  120  are electrically connected to each other so as to form a Wheatstone bridge circuit. The output of the strain gages  120  is taken out of the load sensor  1  through a flexible printed circuit (FPC) board, which will not be described in detail here. 
     The coil spring  30  serving as the load limiting means is assembled in a state where the coil spring  30  urges toward the support case  40  (to be described below), and has a spring constant such that in a. case where a load is within the allowable measurement range of the sensor element  10 , the load is measured by the sensor element  10 , and in a case where a load exceeding the allowable measurement range is applied to the sensor element  10 , the coil spring  30  is deformed in the load range corresponding to the excess exceeding the allowable measurement range of the sensor element  10 . 
     The pressing shaft  20  is made of for example, a metal, and includes a shaft body  210  which has a solid rod shape, and a pressure receiving portion  220  which is provided at one end portion of the shaft body  210  and receives a load. Also, at the other end portion of the shaft body  210  of the pressing shaft  20 , a gentle tapered portion is formed over the whole of the radial direction such that the center portion of the shaft body becomes the peak having a large curvature radius, and a pressing portion  230  protruding at the center of the tapered portion abuts on the center portion of the one surface (the opposite surface to the surface on which the strain gages  120  have been attached) of the elastic body  110  of the sensor element  10 . 
     On the shaft body  210  of the pressing shaft  20 , in the vicinity of the pressure receiving portion  220 , a mechanical stopper  250  is formed over the entire circumference of the shaft body  210  so as to be a diameter larger than that of the shaft body  210 . The outer diameter of the mechanical stopper  250  is set to be larger than the inner diameter of a shaft side opening portion  41  which belongs to the support case  40  (to be described below) and is for the pressing shaft  20 , such that in a case where an excessive load or an impact force is applied to the pressure receiving portion  220 , the mechanical stopper  250  bumps into the shaft side opening portion  41 , thereby preventing the pressing shaft  20  from further moving toward the sensor element  10 . This function may also be implemented by the pressure receiving portion  220  without forming the mechanical stopper. Also, in the present embodiment, between the mechanical stopper  250  and the pressing portion  230  in the longitudinal direction of the shaft body  210 , only two leaf spring engagement grooves  211  and  212  (see  FIG. 3 ) are formed over the entire circumference of the shaft body  210  with a predetermined interval. 
     The support case  40  is made of, for example, a metal, in a cylindrical shape, and a portion of the internal space of the support case  40  from one end portion to a substantially center portion in the longitudinal direction is formed as a shaft insertion space  420  for the shaft body  210  of the pressing shaft  20 , and a portion of the internal space from the other end portion to the center portion in the longitudinal direction is formed as a spring accommodating space  430  which accommodates the sensor element  10  and the coil spring  30 . The inner circumferential surface of the shaft insertion space  420  and the outer circumferential surface of the shaft body  210  which is inserted into the shaft insertion space  420  are distant from each other to a certain extent, and in the shaft insertion space  420 , only two leaf spring engagement grooves  411  and  412  (see  FIG. 3 ) are formed over the entire inner circumferential surface, and between the two leaf spring engagement grooves  411  and  412  and leaf spring engagement grooves  211  and  212  formed over the entire outer circumferential surface of the shaft body  210  and corresponding to the leaf spring engagement grooves  411  and  412  of the shaft insertion space  420 , ring-shaped leaf springs  51  and  52  ( 50 ) are interposed so as to be engaged with the corresponding grooves. Also, the leaf springs and the engagement grooves do not necessarily need to be provided at two positions. 
     The elasticity of the leaf springs  50  in the longitudinal direction of the pressing shaft is set so as not to interfere with movement of the pressing shaft  20  in the axial direction of the pressing shaft  20 . Also, since the two leaf springs  51  and  52  are formed in a ring shape, the movement direction of the pressing shaft  20  is the same as the axial direction thereof. Further, elastic deformation of the leaf springs  50  causes the shaft body  210  of the pressing shaft  20  inside the shaft insertion space  420  of the support case  40  to move in the central axis direction in a state where the central axis of the shaft body  210  has been aligned with the central axis of the shaft insertion space  420 , without receiving resistance from the leaf springs  50 . Also, in a case of assembling the load sensor  1 , for example, the leaf springs  51  and  52  are engaged with the leaf spring engagement grooves  211  and  212  formed in the shaft body  210  of the pressing shaft  20 , respectively, and in this state, the shaft body  210  is inserted from the shaft side opening. portion  41  of the support case  40 , whereby the outer circumferential portions of the leaf springs  51  and  52  are engaged with the leaf spring engagement grooves  411  and  412  formed in the shaft insertion space  420  of the support case  40 . 
     The inner diameter of the spring accommodating space  430  of the support case  40  is set to be larger than the inner diameter of the shaft insertion space  420 , and be larger than the inner diameter of a spring side opening portion  42  of the spring accommodating space ( 430 ) side. Further, between the spring accommodating space  430  and the shaft insertion space  420 , a first step portion  431  is formed, and on the side where the spring side opening portion  42  is formed, a second step portion  432  is formed. 
     The first step portion  431  of the spring accommodating space  430  of the support case  40  abuts on the edge portion  111  of the one surface of the elastic body  110 , and an edge portion  112  of the other surface of the elastic body  110  abuts on the one end. portion  31  of the coil spring  30 . Also, on the second step portion  432  of the spring accommodating space  430  of the support case  40 , the other end portion  32  of the coil spring  30  is set. Also, in a case of assembling the load sensor  1 , for example, in work for storing the sensor element  10  and the coil spring  30  in the spring accommodating space  430  of the support case  40 , each of the sensor element  10  and the coil spring  30  is deformed with fingers, thereby being reduced in external size, and is inserted into the spring accommodating space  430  through the spring side opening portion  42  of the support case  40 . 
     The pressing shaft  20  is connected to the support case  40  by the leaf springs  50  so as to move in one specific axial direction in response to a load acting on the shaft as described above. The load sensor  1  includes the support case  40  that supports the sensor element  10  and the pressing shaft  20  in the predetermined arrangement state as described above, and the pressing shaft  20  is connected to the support case  40  by the leaf springs  50  so as to move in one specific axial direction in response to a load acting on the shaft. Therefore, it is possible to measure a load by pressing the sensor element  10  with the pressing shaft  20 . 
     Subsequently, function of the load sensor  1  will be described.  FIGS. 4A to 4D  are explanatory views for explaining the function of the load sensor shown in  FIG. 1 .  FIG. 5  is a characteristic diagram corresponding to  FIGS. 4A to 4D  for explaining the function of the load sensor  1  shown in  FIG. 1 . In order to facilitate understanding of the function of the load sensor  1 .  FIGS. 4A to 4D  hyperbolically show the degree of deformation of each component as compared to the degree of actual deformation. In  FIG. 5 , K 1  represents the spring constant of the sensor element  10 , and K 2  represents the spring constant of the coil spring  30 , and b represents the initial load of the coil spring, and δ represents the maximum displacement amount of the pressing shaft  20 , that is, the displacement amount of the pressing shaft  20  from when the pressing portion  230  of the pressing shaft  20  comes into contact with the sensor element  10  to when the mechanical stopper  250  bumps into the shaft side opening portion  41  of the support case  40 , thereby being stopped. 
     Hereinafter, changes in the state of the load sensor according to load changes from  FIG. 4A  to  FIG. 4D  will be described.  FIG. 4A  shows a state where no pressing force is being applied on the load sensor  1 . This state corresponds to the position of F 0  in the horizontal axis of FIG,  5 . In this state, the pressing portion  230  which is the tip portion of the pressing shaft  20  is in contact with the elastic body  110  of the sensor element  10 . 
     If a pressing force is applied to the pressure receiving portion  220  of the pressing shaft  20 , as shown in  FIG. 4B , in a state where the coil spring  30  is not deformed, the center portion of the elastic body  110  of the sensor element  10  is pressed by the pressing portion  230  of the pressing shaft  20 , whereby only the elastic body  110  is deformed, and the pressing force applied to the sensor element  10  is accurately measured by use of a potential difference occurring in the Wheatstone bridge circuit due to a change in the resistance value of the strain gages  120 . This state corresponds to a section from the position of F 0  to the position of F S  in the horizontal axis of  FIG. 5  representing the pressing force F. In  FIG. 5 , in order to improve the accuracy of measurement, a section from F 0  to F 1  (F 1 &lt;F S ) in the allowable measurement range of the sensor element  10  from F 0 to F s  has been set as an actual measurement range. 
     If a pressing force exceeding the allowable measurement range of the sensor element  10  is further applied, as shown in  FIG. 4C , the coil spring  30  is urged, thereby being deformed in a load range corresponding to the excess. This state corresponds to a section from F S  to F L  in the horizontal axis of  FIG. 5  representing the pressing force F. 
     If a pressing force or an impact force for further urging the coil spring  30  is applied, as shown in  FIG. 4D , the mechanical stopper  250  provided on the shaft body  210  of the pressing shaft  20  bumps into the circumference of the shaft side opening portion  41  of the support case  40 , thereby preventing the pressing shaft  20  from further moving toward the sensor element  10 . Therefore, in some cases such as a case where an excessive pressing force considerably exceeding the allowable measurement range of the sensor element  10 . or an impact force is applied to the load sensor  1 , it is possible to protect the sensor element  10  from the excessive pressing force or the impact force. This state corresponds to a section (F≧F L ) on the right side from F L  in the horizontal axis of FIGS representing the pressing force F. 
     As described above, in the load sensor  1 , the pressing shaft  20 , the sensor element  10 , and the coil spring  30  are arranged in series, and the shaft body  210  of the pressing shaft  20 , the sensor element  10 , and the coil spring  30  are accommodated in the support case  40 . Further, the sensor element  10  is provided with the thin elastic body  110 , and the strain gages  120  attached on the elastic body  110 , and the width of each of the above described components in its arrangement direction is very small. Therefore, it is possible to reduce the full length of the load sensor  1 . Since the load sensor  1  has the configuration as described above, it is possible to considerably reduce the size and weight of the load sensor as compared to load sensors according to the related art. As a result, it is possible to reduce the size of a target product incorporating the load sensor  1 . 
     Also, since the pressing shaft  20  is supported inside the support case  40  through the two leaf springs  50 , the pressing shaft  20  is moved in the axial direction in accordance with a load applied to the pressure receiving portion  220  of the pressing shaft  20 , such that the pressing portion  230  of the tip of the pressing shaft is pressed perpendicularly against the one surface of the elastic body  110  of the sensor element  10 . As a result, even if the load sensor is pressed from a direction deviating from the axial direction in which a load acts, it is possible to accurately measure the load acting in one axial direction, and excessive stress is prevented from being generated between the strain gages  120  and the elastic body  110  and thus the life of the sensor element is extended, and it is possible to perform accurate load measurement over a long period of time. 
     Particularly, in a case of reducing the size of the load sensor  1 , in order to maintain the output characteristic of the sensor element  10  at a high degree of accuracy, according to the reduction in the size of the load sensor  1 , the elastic body  110  of the sensor element  10  needs to be formed so as to have a small diameter and a considerably small thickness. In this case. if the pressing portion  230  of the tip of the pressing shaft presses the elastic body  110  while moving, in an unexpected direction, the elastic body  110  may be damaged. However, in the present embodiment, this problem is prevented due to the above described configuration in which the pressing portion  230  of the tip of the pressing shaft is pressed perpendicularly against the one surface of the elastic body  110  of the sensor element  10 . 
     Also, even under a measurement environment in Which although a load to be measured is small and the allowable measurement range is narrow, the load relative to the pressure receiving portion  220  of the pressing shaft  20  is not constant, and excessive loads considerably exceeding the range of the magnitude of the load to be measured, and impact forces act frequently, the load sensor  1  according to the present invention sufficiently exerts its action. That is, as described above, in a case of reducing the size of the load sensor  1 , according to the reduction in the size of the load sensor  1 , the elastic body  110  of the sensor element  10  is formed so as to have a small diameter and a considerably small thickness. In this case, if an excessive load or an impact force is applied, at the first stage, the coil spring  30  is deformed in a load range due to the impact or a load corresponding to the excess exceeding the measurement range, and at the second stage, the mechanical stopper  250  bumps into the shaft side opening portion  41  of the support case  40 , thereby preventing the pressing shaft  20  from moving. Therefore, it is possible to protect the sensor element  10 , and it is possible to perform accurate load measurement over a long period of time. 
     Since the coil spring  30 , which serves as the load limiting means, is deformed in the load range in a case where an impact load or a load exceeding the measurement range is applied, it is possible to perform accurate measurement, and in a case where a load exceeding the measurement range is applied, the coil spring  30  is deformed in the load range, whereby a load corresponding to the excess does not exert adverse influence on the sensor element  10 , and thus accurate load measurement is possible over a long period of time. 
     The load sensor  1  as described above is only shown as an embodiment according to the invention. The shape, material and size of the load sensor  1  and its components may be arbitrary modified without departing from the scope of the present invention. 
     For example, in the above described embodiment, in order to support the pressing shaft  20  on the support case  40  such that the pressing shaft  20  is movable in the axial direction, the ring-shaped leaf springs  50  are used. However, in place of such ring-shaped leaf springs  50 , cantilever-shaped leaf springs having flexibility may be arranged to extend from the inner circumferential surface of the support case at even intervals in the circumferential direction, and the pressing shaft  20  may be fixed to the leading end portions of the cantilever-shaped leaf springs, such that the pressing shaft  20  is supported on the support case  40  so as to be movable in the axial direction. 
     Also, for example, a linear bearing may be interposed between the pressing shaft  20  and the support case  40 , thereby preventing resistance from occurring when the pressing shaft  20  moves in the axial direction. 
     In the above described embodiment, the elastic body  110  of the sensor element  10  is made of a metal. As long as the sensor element  10  can exert its performance, the kind of the metal is not especially limited and may be an aluminum alloy, stainless steel (SUS), or the like. Alternatively, the elastic body  110  may be formed of a resin exerting elastic force. 
     In the above described embodiment, plural strain gages  120  are electrically connected to each other so as to form a Wheatstone bridge circuit. However, the load sensor  1  may be configured to have only one strain gage being attached on the center portion of the elastic body  110 , and a pressing force may be detected from the distortion amount of the single strain gage. 
     In the above described embodiment, the sensor element  10  is configured as a distortion sensor which is provided with the strain gages  120  and the elastic body  110 . However, the sensor element  10  may be configured by other types of sensors that measures a load by detecting pressing force, sensors such as magnetostrictive sensor, a piezo type pressure sensor, or an electrical capacitance pressure sensor formed in a semiconductor chip form. 
     Also, in the above described embodiment, the coil spring  30  is used for a member serving as the load limiting means. However, other types of members, such as rubber having appropriate elasticity, bellows, or a sponge having sufficient degree of hardness that is able to serve as the load limiting means, may also be used as a member serving as the load limiting means. 
     In the above described embodiment, the spring side opening portion  42  and the spring accommodating space  430  are defined by a single component, which is the support case  40 , and the inner diameter of the spring side opening portion  42  is set to be smaller than the inner diameter of the spring accommodating space  430 . However, the spring side opening portion  42  and the spring accommodating space  430  may be configured to be defined by plural members that form the support case  40  when combined together. By such modification, the sensor element  10  and the coil spring  30  may be inserted into the spring accommodating space  430 , and thereafter, another member that configures a part of the support case  40  may be attached to cover the spring accommodating space  430 . According to this configuration, assembling of the sensor element  10  and the coil spring  30  into the spring accommodating space  430  may be performed easily without heavily deforming the sensor element  10  and the coil spring  30  when these components are inserted into the spring accommodating space  430   
     The present invention is not limited to the illustrative embodiment described above but can be embodied by modifying the components without departing from the gist of the invention. Further, various inventions can be made by appropriately combining a plurality of components described in the above illustrative embodiment. For example, some of all components described in the above illustrative embodiment may be removed. Furthermore, the components according to another illustrative embodiment may be appropriately combined.