Abstract:
In order to provide a seat weight measuring apparatus in which the performance of load sensors is not spoiled due to dimensional error or deformation of a vehicle body or a seat and provides higher precision measurement, a seat weight measuring apparatus is provided for measuring a seat weight including the weight of a passenger sitting thereon and comprises load sensors for converting at least parts of the seat weight into electric signals and a dislocation/deflection absorbing mechanism (pin brackets and pins) arranged between a seat and the load sensors. Each load sensor comprises a cantilever-type sensor plate which is deformable when subjected to load, and a plurality of strain gauges attached to one surface (strain measuring surface) of the sensor plate. The load sensor is so structured that one of the strain gauges is subjected to tensile strain while the other one of the strain gauges is subjected to the compressive strain when the sensor plate is subjected to the applied load and is thus deformed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a seat weight measuring apparatus for measuring a load applied on a vehicle seat such as the weight of a passenger sitting thereon. Particularly, it relates to a seat weight measuring apparatus which has been improved not to spoil the performance of load sensors due to dimensional error or deformation of a vehicle body or a seat and also improved to provide higher precision measurement. 
     BACKGROUND OF THE INVENTION 
     Automobiles are equipped with seat belts and airbags to secure safety for passengers in the automobiles. In recent years, there is a trend for controlling the operation of such safety devices according to the weight (body weight) of a passenger for improved performance of seat belts and airbags. For example, the amount of gas to be introduced into the airbag, an airbag inflating speed, or a pre-tension of the seat belt may be adjusted according to the weight of a passenger. For that purpose, some means are needed for measuring the weight of the passenger sitting on the seat. An example of such means includes a proposal of an apparatus for measuring the weight of a passenger by the following steps of arranging load sensors (load cells) at four corners of the bottom of a seat, obtaining loads on the respective corners, and summing them to determine the seat weight including the weight of the passenger (Japanese Patent Application No. H09-156666, Japanese Patent Application No. H10-121627 filed by the applicant of this invention). 
     To make an accurate measurement by this seat weight measuring apparatus, it is necessary to eliminate loads besides the weight of the seat and the weight of the passenger (or an object) on the seat. One of loads needed to be eliminated is a load to be applied when the seat weight measuring apparatus is forced to be assembled despite the fact that the vehicle or the seat has dimensional error or deformation (in the specification, this load will be referred as to “assembly load”). 
     To obtain a seat weight measuring apparatus which has good durability and high precision but can be manufactured at low cost, it is necessary to establish a strain detecting mechanism which amplifies small strains. Further, it is necessary to reduce manufacturing errors of parts in the mechanism of the apparatus and measurement errors due to the frictional force. 
     The present invention is made under the above circumstances and the object of the present invention is to provide a seat weight measuring apparatus which has been improved not to spoil the performance of load sensors due to dimensional error or deformation of a vehicle body or a seat. 
     A further object of the present invention is to provide a seat weight measuring apparatus which has been improved to attain high precision measurement. 
     Yet another object of the present invention is to provide a seat weight measuring apparatus which has good durability and high precision but can be manufactured at low cost. 
     SUMMARY OF THE INVENTION 
     To solve these problems, the seat weight measuring apparatus of the present invention is a seat weight measuring apparatus for measuring a seat weight including the weight of a passenger sitting thereon, which comprises: load sensors arranged inside a seat or between the seat and a vehicle body for converting at least parts of the seat weight into electric signals; and an absorbing mechanism for absorbing dislocation and/or deflection between the seat and the vehicle body. 
     The aforementioned absorbing mechanism is arranged on the connecting and holding portions between the load sensors and the seat or between the load sensors and the vehicle body to absorb dimensional errors, not to transmit an assembly load exerted due to manufacturing error of parts and/or dimensional dislocation, and/or deformation caused by the installation to the load sensors. Therefore, only pure load to be measured (seat weight) is applied to the load sensors, thereby enabling secure measurement using the effective ranges of the load sensors enough widely. It can also prevent a load exceeding the measurable range of the load sensors from being exerted. 
     It should be noted that the intention of the seat weight measuring apparatus as described in this specification is basically to measure the weight of a passenger sitting on a vehicle seat. Accordingly, an apparatus for measuring only the weight of a passenger by canceling the weight of a vehicle seat itself is also contained in the range of the seat weight measuring apparatus disclosed by this specification. 
     According to the present invention, it is preferable that the absorbing mechanism works even after the seat weight measuring apparatus is installed in the vehicle body and the seat. 
     Even when an unexpected load is exerted on the seat and the seat thus deforms after the seat weight measuring apparatus is installed in the vehicle body or the vehicle body deforms while the vehicle runs, such deformation can be absorbed by connecting portions not to be transmitted to the load sensors. 
     In the seat weight measuring apparatus of the present invention, it is preferable that the absorbing mechanism includes a slide mechanism and a pivotal mechanism. 
     Positional, dimensional, and rotational dislocation can be absorbed. 
     The seat weight measuring apparatus of the present invention preferably further comprises a centering mechanism for regulating the position of the absorbing mechanism. 
     The centering mechanism is composed of, for example, a relatively weak spring and causes the slide mechanism or the pivotal mechanism to be positioned as close to the center in the slidable range or the pivotal angle as possible. By the function of the centering mechanism, the movement of the slide mechanism and the pivotal mechanism after the installation of the seat weight measuring apparatus can be secured in the both directions (right and left, up and down, forward and backward). 
     The seat weight measuring apparatus according to another aspect of the present invention is a seat weight measuring apparatus for measuring a seat weight including the weight of a passenger sitting thereon, comprising load sensors arranged inside a vehicle seat or between the seat and a vehicle body for receiving load (including moment, hereinafter referred as to “applied load”) relating to the seat weight and for converting the applied load into electric signals, characterized in that each of the load sensors has a cantilever-type sensor member which is deformable when subjected to the applied load, and a plurality of strain gauges attached to one surface (strain measuring surface) of the sensor member; and that the load sensor is so structured that one of the strain gauges is subjected to tensile strain while other one of the strain gauges is subjected to the compressive strain when the sensor plate is subjected to the applied load and thus deformed. 
     The strain gauge to be subjected to tensile strain and the strain gauge to be subjected to compressive strain are wired on a bridge circuit in such a manner as to form an opposite phase, thereby increasing the outputs of the strain gauges. This attains highly precise measurement with small strains on the sensor, thereby increasing the life of the sensor. 
     Because the surfaces on which the strain gauges are attached are on one side of the sensor member (e.g. one side of a plate), printing process should be made only on one side when the strain gauges and wirings are printed by, for example, screen printing. This allows sensors to be manufactured at lower cost. 
     The seat weight measuring apparatus according to another aspect of the present invention is a seat weight measuring apparatus for measuring a seat weight including the weight of a passenger sitting thereon, comprising load sensors arranged inside a vehicle seat or between the seat and a vehicle body for receiving load relating to the seat weight and for converting the load into electric signals, characterized in that each of said load sensors has a sensor member of which thickness is constant and width is partially different and which is deformable when subjected to the applied load, and a plurality of strain gauges attached to one surface (strain measuring surface) of the sensor member; that the strength in deformation of an elastic deforming portion of the sensor member is adjusted so as to form a region providing substantially constant surface strain on a portion of the strain measuring surface of the sensor member when the sensor member is subjected to the applied load; and that the strain gauges are attached to that region. 
     Since the strain gauges are attached (printed) on the region providing substantially constant surface strain, the strains may not vary even though the positions of the strain gauges are slightly shifted, thereby preventing measurement error. Therefore, this can ensure high precision of measurement and can reduce the quality requirement on manufacturing. 
     The seat weight measuring apparatus according to another aspect of the present invention is a seat weight measuring apparatus for measuring a seat weight including the weight of a passenger sitting thereon, comprising load sensors arranged inside a vehicle seat or between the seat and a vehicle body for receiving load relating to the seat weight and converting the load into electric signals, characterized in that each of the load sensors has a sensor member of which thickness is constant and width is partially different and which is deformable when subjected to the applied load, and a plurality of strain gauges attached to one surface (strain measuring surface) of the sensor member; that the sensor member is a cantilever; that one end is a fixed portion, the other end is a subjected portion to be subjected to the applied load, and a portion between the ends is a strain gauge fixing portion; and that both the fixed portion and the subjected portion are patched with reinforcing members to concentrate strain in the strain gauge fixing portion. 
     Because strains are concentrated in the portion on which the strain gauges are attached, highly precise measurement is performed and measurement error can be prevented even with manufacturing error and/or assembling error of other parts. 
     According to this aspect, it is preferable that the subjected portion of said sensor member is provided with a half arm, wherein the half arm has a body having relatively high rigidity to be in contact with the subjected portion and wings projecting the body, wherein said wings have action portions to be subjected to simple load (normal load not moment), wherein said half arm has such a structure (reversing structure) that the simple load is transmitted mainly as bending moment to the subjected portion of said sensor member via the body of said half arm, and wherein according to the reversing structure of said half arm, a wave-like strain is applied to the strain measuring surface of said sensor member. 
     Further, it is preferable that said half arm is provided with a releasing mechanism at the action points thereof, wherein the releasing mechanism releases load besides vertical load by sliding or rotation, and that the strain gauge fixing portion of said sensor member has a compression-side region and a tension-side region which are arranged symmetrically about a neck portion with a reduced width in its plane view. 
     When load is applied in the longitudinal direction and/or rotational moment is applied, the balance of strain between the tension side and the compression side is changed in such a manner as not to change the total sensitivity of the sensors relative to the vertical load. Even with dislocation in a horizontal direction (e.g. the longitudinal direction of the vehicle) or axial force acting on the sensor plate, error can be cancelled by means of the compression-side strain gauge and the tension-side strain gauge and the total sensitivity obtained by summing the outputs of both the strain gauges may not include such error. 
     In the seat weight measuring apparatus of the present invention, it is preferable that the action points of said half arm and the center line in the thickness of the sensor member are set to be in substantially the same level or have height difference of ±5 mm or less. That is, when frictional force (axial force) acts on the aforementioned action points, the moment arm for deforming the sensor plate due to the frictional force is short. This means that the deformation of the sensor plate due to the frictional force is little, thereby reducing the measurement error. 
    
    
     BRIEF EXPLANATION OF THE DRAWINGS 
     FIGS.  1 (A),  1 (B) are views showing a dislocation/deflection absorbing mechanism for a seat weight measuring apparatus according to a first embodiment, in which FIG.  1 (A) is an exploded perspective view thereof and FIG.  1 (B) is a front sectional view of a pin bracket. 
     FIGS.  2 (A) through  2 (D) are views illustrating the entire construction of a seat weight measuring apparatus according to the first embodiment of the present invention, in which FIG.  2 (A) is a plan view thereof, FIG.  2 (B) is a side sectional view thereof, and FIGS.  2 (C) and  2 (D) are front sectional views thereof. 
     FIG. 3 is a partially broken perspective view showing detailed structure around the sensor plate. 
     FIG.  4 (A) is a plan view showing the detailed structure of the sensor plate, FIG.  4 (B) is a side sectional view of the sensor plate taken along the line X—X of FIG.  4 (A), and FIG.  4 (C) is a circuit diagram of the sensor. 
     FIGS.  5 (A) through  5 (C) are views showing the relation between the sensor plate and half arms, in which FIG.  5 (A) is a plan view, FIG.  5 (B) is a side view illustrating the non-loaded state, FIG.  5 (C) is a side view schematically illustrating the loaded state. 
     FIG.  6 (A) is a front sectional view schematically showing a structural example for fixing a seat to a vehicle body and FIG.  6 (B) is a side view thereof. 
     FIGS.  7 (A) and  7 (B) are views for explaining the way of examining the slidable dimension of the absorbing mechanism relative to the vertical direction of the vehicle body. 
     FIGS.  8 (A) and  8 (B) are views for explaining the way of examining the slidable dimension of the absorbing mechanism relative to the width direction of the vehicle body, in which FIG.  8 (A) shows a state before a load is applied and FIG.  8 (B) shows a state after the load is applied. 
     FIG.  9 (A) is a side sectional view showing the construction of a seat weight measuring apparatus according to a second embodiment of the present invention. 
     FIG.  9 (B) is a perspective view showing the detailed structure of a load cell, hollow suspension, and XY slider according to the second embodiment. 
     FIGS.  10 (A) through  10 (C) are views for explaining the actions of the structure around a sensor plate of a seat weight measuring apparatus according to the first embodiment of the present invention. FIG.  10 (A) is a side view thereof, FIG.  10 (B) is a plan view of the sensor plate, and FIG.  10 (C) is a graph schematically illustrating strain distribution on the surface of the sensor plate. 
     FIG. 11 is a side view for explaining the action when axial force (force in the longitudinal direction) is applied to supports of half arms according to the first embodiment of the present invention. 
     FIG. 12 is a graph showing data indicating the influences of the axial force applied to the supports of the half arms on the measurement data according to the first embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the drawings. First, the structure around a vehicle seat will be described with reference to FIGS.  6 (A),  6 (B). 
     FIG.  6 (A) is a front sectional view schematically showing a structural example for fixing a seat to a vehicle body and FIG.  6 (B) is a side view thereof. It should be understood that arrows in the drawings indicate as follows. UP: the opposite direction of the gravitational direction when the vehicle is placed horizontally, DOWN: the gravitational direction, FORWARD: the forward direction of the vehicle, BACKWARD: the reverse direction of the vehicle, LEFT: the left side as facing the forward direction of the vehicle, RIGHT: the right side as facing the same. 
     A seat  3  is shown in FIGS.  6 (A),  6 (B). A passenger  1  sits on a seat squab  3   a  of the seat  3 . The seat squab  3   a  is supported at its lower surface by a seat frame  5  made of a steel plate. The seat frame  5  comprises components including a bottom plate  5   a , lateral plates  5   c , vertical plates  5   e , and slide plates  5   g . The bottom plate  5   a  extends to cover the lower surface of the seat squab  3   a . The lateral plates  5   c  extend along the left and right sides of the lower surface of the bottom plate  5   a . The vertical plates  5   e  are hung from the center portions of the lower surfaces of the lateral plates  5   c , respectively. The slide plates  5   g  project right and left of the respective vertical plates  5   e  like wings and the end portions of each slide plate  5   g  are bent upward. 
     Two seat rails  7  are arranged beneath right and left portions of the seat  3  so as to extend in the forward and backward direction (the lengthwise direction) and parallel to each other. The cross section of each seat rail  7  is formed in a U-like shape and has a concavity  7   c  therein and a groove  7   a  formed in the upper portion of the concavity  7   c . Inserted in the groove  7   a  is the vertical plate  5   e  of the seat frame  5 . 
     The slide plate  5   g  of the seat frame  5  is housed in the concavity  7   c  of the seat rail  7 . The slide plate  5   g  is slidable in the lengthwise direction in the seat rail  7 . 
     Connected to the lower surface of each seat rail  7  is a seat weight measuring apparatus  9 . The seat weight measuring apparatus  9  has an elongated box-like profile extending in the lengthwise direction. The seat weight measuring apparatus  9  will be detailed later. 
     Attached to front and rear ends of the lower surface of the seat weight measuring apparatus  9  are seat brackets  11 . The seat brackets  11  are fixed to seat fixing portions  13  of the vehicle body by means of bolts. 
     FIGS.  2 (A)- 2 (D) are views illustrating the entire construction of a seat weight measuring apparatus according to a first embodiment of the present invention. FIG.  2 (A) is a plan view thereof, FIG.  2 (B) is a side sectional view thereof, and FIGS.  2 (C) and  2 (D) are front sectional views thereof. In FIGS.  2 (A) and  2 (B), the illustration of the seat weight measuring apparatus on the rear side is omitted. 
     FIG. 3 is a partially broken perspective view showing the details of the construction around a sensor plate. 
     FIGS.  4 (A) through  4 (C) show a structural example of the sensor plate of the seat weight measuring apparatus according to the first embodiment of the present invention. FIG.  4 (A) is a plan view showing the details of the construction of the sensor plate, FIG.  4 (B) is a side sectional view of the sensor plate taken along the line X—X of FIG.  4 (A), and FIG.  4 (C) is a circuit diagram of the sensor. 
     FIGS.  5 (A)- 5 (C) are views showing the relation between the sensor plate and half arms. FIG.  5 (A) is a plan view thereof, FIG.  5 (B) is a side view thereof when no load is applied, and FIG.  5 (C) is a side view thereof when a load is applied. 
     The seat weight measuring apparatus  9  comprises an elongate base  21  as a substrate. The base  21  extends lengthwise in the forward and backward direction when mounted to the vehicle body and is a product made by the press-working of a steel plate having a U-shaped cross section, as shown in FIGS.  2 (C),  2 (D). The bottom of the base  21  is referred as to a bottom plate  21   c  and portions which stand from the left and right edges of the bottom plate  21   c  to form corners of 90° there between are referred to as side plates  21   a ,  21   a′.    
     Each of the base side plates  21   a ,  21   a ′ is provided with two pairs of pin holes  21   e  and  21   g  in front and rear portions, respectively. The pin holes  21   e ,  21   g  are formed to face the pin holes  21   e ,  21   g  of the opposite side plates  21   a ,  21   a′.    
     The holes  21   e  near the front and rear ends of the base  21  are formed in portions away at a distance from the front and rear ends, respectively, wherein the distance corresponds to approximately ⅛ of the overall length of the base  21 . The holes  21   e  are vertically elongated holes, as shown in FIG.  2 (B). Inserted through the elongated holes  21   e  are ends of bracket pins  27 . 
     There are clearance between the bracket pin  27  and the elongated hole  21   e  all around the bracket pin  27  so that the bracket pin  27  is normally prevented from being in contact with the inner periphery of the elongated hole  21   e . However, when excessive load is applied to the seat weight measuring apparatus  9  (concretely, to pin brackets  25 ), the bracket pins  27  are lowered to come in contact with lower portions of the inner peripheries of the elongated holes  21   e  whereby excess load is not transmitted to load sensors (the sensor plates  51 , detailed later). That is, the pins  27  and the elongated holes  21   e  compose a part of a mechanism for defining the upper limit of load to be exerted on the sensor plates  51 . The main function of each bracket pin  27  is transmitting the seat weight exerted on the pin bracket  25  to a Z arm  23 . 
     The pin holes  21   g  are formed in positions closer to the center than the positions of the elongated holes  21   e  (at a distance corresponding to approximately {fraction (1/10)} of the overall length of the base  21  from the elongated hole  21   e ). Inserted into the holes  21   g  are base pins  31 . Each of the base pins  31  extends to bridge the left and right side plates  21   a ,  21   a ′. Fitted on the left and right ends of the pin  31  are retainers  33 , thereby fixing the base pin  31  to the base  21 . The base pin  31  is the pivot shaft of the Z arm  23 . 
     The Z arms  23  are arranged inside of the base  21 . Each of the Z arms  23  has a center portion, when seen in plan view, which is forked (into two branches  23   h ) and has a rectangular portion near the end. The Z arm  23  has side plates  23   a ,  23   a ′ formed by upwardly folding left and right edge portions thereof by 90°. The side plates  23   a , 23   a ′ extend from the end to the middle portion. The branches  23   h  are only flat plates. The side plates  23   a , 23   a ′ extend along the inner surfaces of the side plates  21   a , 21   a ′ of the base  21 . There are clearances between the side plates  23   a , 23   a ′ and the side plates  21   a , 21   a′.    
     Each of the Z arm side plates  23   a ,  23   a ′ is provided with two pin holes  23   c ,  23   e  formed therein. Inserted into the pin holes  23   c  formed near the end is the bracket pin  27 . The bracket pin  27  and the pin holes  23   c  slide little relative to each other. Inserted into the pin holes  23   e  at the center side is the base pin  31 . The base pin  31  is the pivot of the Z arm  23 , so the base pin  31  and the pin holes  23   e  slide relative to each other by pivotal movement of the Z arm  23 . Sandwiched between the base side plates  21   a  and the Z arm side plates  23   a  about the base pin  31  are disk-like spacers  35  having holes. 
     The length of the branches  23   h  of the Z arm  23  corresponds substantially to a half of the overall length of the Z arm  23 . The branches  23   h  are separated from each other to the right and left and extend toward the center of the base  21 . Each of the branches  23   h  has a reduced width near the center. The action portions  23   j  at the ends of the branches  23   h  are clamped between wings  41   a ,  42   a  of upper and lower half arms  41 ,  42  as shown in FIGS.  3  and  5 (A) through  5 (C). 
     As a load is exerted on the pin bracket  25 , the Z arm  23  slightly pivots (the maximum pivotal angle being approximately 5°) whereby the action portions  23   j  transmit the load to the sensor plate  51  through the half arms  41 ,  42 . 
     The pin bracket  25  is formed to have an inverted U-shaped cross section as shown in FIG.  2 (C). The length of the pin bracket  25  in the forward and backward direction substantially corresponds to {fraction (1/20)} of that of the base  21 . The pin bracket  25  has a flat upper surface  25   a  on which the seat rail  7  shown in FIGS.  6 (A),  6 (B) is mounted. The seat rail  7  is strongly fixed to the pin brackets  25  by bolts or other fastening means. 
     The pin bracket  25  has left and right side plates  25   b  downwardly projecting, of which lower ends are bent inwardly. The pin bracket  25  is disposed inside the Z arm  23  in such a manner as to have clearances between the side plates  25   b  and the Z arm side plates  23   a ,  23   a ′. The side plates  25   b  are provided with pin holes  25   c  formed therein. Inserted into the pin holes  25   c  is a bracket pin  27 . The inner diameter of each pin hole  25   c  is larger than the diameter of the bracket pin  27 . The clearance between them absorbs dimensional tolerance of the seat and the vehicle body and/or unexpected deformation. 
     Disposed between the side plates  25   b  of the pin bracket  25  and the side plates  23   a ,  23   a ′ of the Z arm  23  is a spring plate  29  having spring washer portions with holes. The bracket pin  27  is loosely inserted into the holes of the spring washer portions. The spring plate  29  composes a centering mechanism for biasing the pin bracket  25  toward the center. 
     The centering mechanism as mentioned above causes the pin bracket  25  to be positioned as close to the center in the slidable range as possible. By the function of the centering mechanism, the movement of the slide mechanism and the pivotal mechanism after the installation of the seat weight measuring apparatus can be secured in the both directions (right and left, up and down, forward and backward). 
     Hereinafter, the structure around the sensor plate  51  will be described. 
     First, the structure of the sensor plate  51  itself will be described. 
     FIGS.  4 (A) through  4 (C) show a structural example of the sensor plate of the seat weight measuring apparatus according to the first embodiment of the present invention. FIG.  4 (A) is a plan view of the sensor plate, FIG.  4 (B) is a side sectional view of the sensor plate taken along the line X—X of FIG.  4 (A), and FIG.  4 (C) is a circuit diagram of the sensor. 
     Applied on the sensor plate (spring member)  51  as a base of the sensor  50  is an insulating layer (lower insulating layer)  52  for electrical insulation. Selectively formed on the insulating layer  52  is a wiring layer  53 . Further selectively formed on the wiring layer  53  is a resistant layer  54  which composes the strain gauge. In addition, an insulating layer (upper insulating layer)  55  is applied over these layers as a protective layer. In this manner, the electrical circuit including resistors is directly laminated on the spring steel plate  51 , thereby reducing the working cost and the assembly cost and further improving the heat resistance and the corrosion resistance. 
     The sensor plate  51  is a rectangular plate having two necks as a whole. The sensor plate  51  is provided with a central hole  51   a  formed in the center thereof and bolt holes  51   b  formed in end portions thereof. The sensor  50  is formed around the central hole  51   a  and between the central hole  51   a  and the bolt holes  51   b . V-like concavities are provided in both side edges of regions  51   c  between the central hole  51   a  and the bolt holes  51   b . These concavities ensure positions to be deformed of the sensor plate  51 , thereby preventing positional variation of dislocation and stabilizing the sensitivity of the sensor  50 . 
     The sensor  50  is substantially symmetrical about the center of the central hole  51   a . The sensor  50  is composed of four strain gauges  54   a ,  54   b ,  54   c , and  54   d . Two of them  54   a ,  54   b  to be applied with tensile strain are arranged near the bolt holes  51   b  (near the ends), while the other two strain gauges  54   c ,  54   d  to be applied with compressive strain are arranged near the central hole  51   a  (central side). The strain gauges  54   a ,  54   b ,  54   c , and  54   d  are connected to each other by wirings  53   a ,  53   b ,  53   c , and  53   d  to form a bridge circuit shown in FIG.  4 (C). Squares marked by numerals  1 ,  2 ,  3 ,  4  in FIGS.  4 (A),  4 (C) are terminals. 
     Arranged between the strain gauges  54   a ,  54   c  and the strain gauges  54   b ,  54   d  is a sensitivity control resistor  54   e.    
     It should be noted that the load may be obtained by conversion from deflection of the sensor plate  51  detected by electrical capacitance pressure sensors or Hall elements, instead of the detection of dislocation of the sensor plate  51  being detected by the strain gauges  54   a ,  54   b ,  54   c , and  54   d.    
     Hereinafter, the structure around the sensor plate  51  will be described with reference to FIGS.  3  and  5 (A)- 5 (C). 
     The sensor plate  51  is strongly fixed to the top of a column  63  at the center of the base bottom plate  21   c  by means of a washer  67  and a nut  68 . 
     As shown in FIGS.  3  and  5 (A)- 5 (C), the half arms  41 ,  42  are provided in the form of two pairs to be arranged above and below of the front and rear portions of the sensor plate  51  to clamp the sensor plate  51 . Since the half arms  41 ,  42  have same configuration, a description will be made as to only an upper half arm  41 . 
     The half arm  41  comprises a half arm body  41   c  which is a rectangular plate with an attachment hole  41   e  (FIGS.  5 (B),  5 (C)) formed in the center thereof. The half arm  41  further comprises wings  41   a  extending in the right and left directions from edges of the body  41   c  near the sensor plate center, and levee-like supports  41   b  formed on the backs of the wings  41   a  and extending in the right and left directions. The top of each support  41   b  is slightly edged. 
     Hereinafter, a description will be made as regard to the assembly structure of the upper and lower half arms  41 ,  42 , the sensor plate  51 , and the Z arm action portion  23   j.    
     Since the lower surface of the body  41   c  of the upper half arm  41  and the upper surface of the body  42   c  of the lower half arm  42  are flat, these are fixed to the surfaces of the sensor plate  51  without backlash by means of screw  43 . The wings  41   a ,  42   a  of the upper and lower half arms  41 ,  42  are arranged in such a manner that the supports  41   b ,  42   b  confront each other. Sandwiched between the supports  41   b ,  42   b  are the action portions  23   j  of the Z arm  23 . The supports are positioned at the middle (the neck (the region  51   c ) of the sensor plate  51 ) of a region between the two strain gauges  54   a  and  54   c  or  54   d  and  54   b.    
     As a load is exerted on the pin brackets  25  of the seat weight measuring apparatus  9 , the Z arms  23  slightly pivot to raise the action portions  23   j . FIG.  5 (C) schematically and exaggeratedly shows the state of the sensor plate and the half arms. 
     As the action portions  23   j  of the Z arms are raised, the supports  41   b  of the upper half arms  41  are pushed up. Therefore, moments M are applied to the front or rear end portion of the sensor plate  51 . By these moments M, the strain gauge  54   a ,  54   b  on the front and rear end portions are tensioned, while the strain gauges  54   c ,  54   d  near the center are compressed. The variations in the resistances of the respective strain gauges are obtained as electrical signals, thereby measuring the strain on the sensor plate and thus the load applied on the pin brackets  25 . 
     Hereinafter, the entire structure of a dislocation/deflection absorbing mechanism for the seat weight measuring apparatus of this embodiment will be described. 
     FIGS.  1 (A),  1 (B) are views showing the dislocation/deflection absorbing mechanism for the seat weight measuring apparatus of this embodiment. FIG.  1 (A) is an exploded perspective view thereof and FIG.  1 (B) is a front sectional view of a pin bracket. The pin bracket  25  is securely fixed to the seat rail  7  by a bolt or the like. The constructions of the respective components and their assembling relation of the seat weight measuring apparatus  9  have been described above with reference to FIG.  3 . 
     In the vertical direction of the vehicle body, dislocation is absorbed by a space between the pin hole  25   c  of the pin bracket  25  and the bracket pin  27 . The quantitative constructions thereof will be described later. 
     In the lengthwise direction of the vehicle body, dislocation is absorbed by forming the pin hole  25   c  of the pin bracket  25  into an elongated hole. 
     In the width direction of the vehicle body, dislocation is absorbed by spaces between the side plates  25   b  of the pin bracket  25  and the side plates  23   a  of the Z arm. It should be noted that this portion is provided with the centering mechanism by the spring plate  29 . 
     As for the rotation about the axis in the vertical direction of the vehicle body, dislocation is absorbed mainly by spaces between the side plates  25   b  of the pin bracket  25  and the side plates  23   a  of the Z arm. 
     As for the rotation about the axis in the lengthwise direction of the vehicle body, dislocation is absorbed mainly by spaces between the side plates  25   b  of the pin bracket  25  and the side plates  23   a  of the Z arm, as well as the vertical direction of the vehicle body. 
     As for the rotation about the axis in the width direction of the vehicle body, dislocation is absorbed mainly by rotation of the pin bracket  25  about the bracket pin  27 . 
     The quantitative constructions of the absorbing mechanism will now be described. 
     The absorbing mechanism of the seat weight measuring apparatus of this embodiment can slide the following L UD  [mm] or more relative to the vertical direction of the vehicle body. 
     
       
           L   UD =β{( H   1   +H   2 )/4α+ F}   
       
     
     wherein 
     β: deflection amount per unit load [mm/kgf] of the absorbing mechanism in the vertical direction, 
     H 1 : dimensional error [mm] of the legs of the seat in the vertical direction, 
     H 2 : dimensional error [mm] of the seat fixing portions of the vehicle body in the vertical direction, 
     α: deflection amount per unit load [mm/kgf] when three of four legs of the seat are fixed and the residual one is deformed in the vertical direction, and 
     F: the lower limit [kgf] of the weight measurable range of one leg of the seat. 
     FIGS.  7 (A),  7 (B) are views for explaining the way of examining the slidable dimension of the absorbing mechanism relative to the vertical direction of the vehicle body. 
     FIG.  7 (A) shows the dimensional error of the seat. When the leg portion of the seat is put on a flat surface  10 , one of the legs (brackets  11 ) is away from the flat surface  10  by an error H 1 . The seat means a unit comprising a seat frame (designated by a numeral  5  in FIG. 6) and a seat rail (designated by a numeral  7  in FIG.  6 ). 
     FIG.  7 (B) shows the dimensional error of the seat mounting portions  13  of the vehicle body. There is an error H 2  on one of the seat mounting portions  13  when a flat surface  10  formed by the other three seat mounting portions  13  is assumed as a reference surface. 
     In the state shown in FIGS.  7 (A) and  7 (B), the leg  11  having the error H 1  is forced to be pressed to the seat mounting portion  13  having the error H 2  for assembly. Force F A  (assembly load) exerted on the leg  11  due to the deformation is given by the following expression, assuming the worst case that the other three legs are not deformed: 
     
       
           F   A =( H   1   +H   2 )/α  (1) 
       
     
     In case where the four mounting portions have the respective absorbing mechanisms, the absorption of deformation can be shared by the four mounting portions so that the force F A  exerted on the leg is given by the following expression: 
     
       
           F   A =( H   1   +H   2 )/4α  (1) 
       
     
     On the other hand, the slidable dimension L UD  in the vertical direction to be absorbed by the absorbing mechanism is given by the following expression: 
     
       
           L   UD =β( F   A   +F)   (2) 
       
     
     This expression means that the load sensor mechanism may deform a value obtained by multiplying the sum of F A  (assembly load) and F by a deformation amount β per unit load in the vertical direction of the load sensor mechanism wherein F is the sum of F S  (the seat itself) and F M  (a person or an object on the seat) and the sum of F A  and F is the total weight to be applied to the seat weight measuring apparatus. The absorbing mechanism should absorb the deforming value. 
     The following expression is given according to the above expressions (1′) and (2): 
     
       
           L   UD =β{( H   1   +H   2 )/4α +F}   (3) 
       
     
     Followings are values according to one example of design specification: 
     dimensional tolerance (H 1 +H 2 )=±5 mm 
     4α=0.25 mm/kgf 
     F=60 kgf 
     β=0.02 mm/kgf 
     L UD =0.02×80 kgf=1.6 mm 
     Description will now be made with regard to the absorbency of the absorbing mechanism in the width direction of the vehicle body. 
     When a load is applied to the seat after the seat weight measuring apparatus is installed to the vehicle body, loads are applied due to the deformation of the seat and the deformation of the vehicle body while the vehicle runs. These loads are absorbed by the connecting portion not to transmit the loads to the load sensors. The range of the absorbable deformation is given by the following expression. 
     The absorbing mechanism of this embodiment can slide the following L LR  or more relative to the width direction of the vehicle body. 
     
       
         
           L 
           LR 
           =ax+by+c−L 
           S 
         
       
     
     wherein 
     x: distance [mm] from the center of the seat in the width direction to the center of the seat leg, 
     a: rate of change of the aforementioned x per unit load (within the measurable range), a=Δx/x, 
     b: sinusoidal quantity (sin θ) per unit load (within the measurable range) of a deflection angle θ in the horizontal direction between the center of the bottom of seat leg and the center of the seat leg, 
     y: the height of the seat leg, 
     c: dimensional tolerance [mm] from the center of the seat and the center of the seat leg, and 
     L S : deflection amount [mm] allowed by the load sensors. 
     FIGS.  8 (A),  8 (B) are views for explaining the way of examining the slidable dimension of the absorbing mechanism relative to the width direction of the vehicle body. FIG.  8 (A) shows a state before a load is applied and FIG.  8 (B) shows a state after the load is applied. 
     FIGS.  8 (A),  8 (B) show the seat  3 , the seat frame  5 , and the seat brackets  11 . It should be noted that the seat frame  5  shown in the drawings includes the seat rails  7 . 
     In FIG.  8 (A), “x” is a distance from the center of the seat in the width direction to the center Z 1 , Z 1 ′ of the right or left seat leg. The height of each seat bracket  11  is “y”. 
     In FIG.  8 (B), when the load is applied to the seat  3 , “x” is changed to “x′” and the vertical center line of the seat rail  11  is inclined by θ. Therefore, deflection between the center of the seat leg and the center of the seat bracket is the sum of “x′−x=Δx” and “y sin θ”. 
     Here, expressions “Δx=ax” and “y sin θ=by” are established. 
     The absorbing mechanism should further absorb the original dimensional tolerance c. The slidable dimension L LR  of the absorbing mechanism in the width direction is given by the following expression in which the deflection amount L S  is subtracted from the sum of the deflection between the center of the seat leg and the center of the seat bracket and the dimensional tolerance. 
     
       
         
           L 
           LR 
           =ax+by+c−L 
           S 
         
       
     
     The following are values according to one example of a design specification when x=250 mm and y=20 mm: 
     a=0.002-0.005 
     b=0.01-0.07 
     c=0.1-1 mm 
     L S =1.3-2.5 mm 
     L LR =1.3-2.0 mm 
     Because of the clearance between the pin hole  25   c  of the pin bracket  25  and bracket pin  27 , the absorbing mechanism of this embodiment can rotate by θ LR  or more about an axis extending in a widthwise direction of the vehicle body: 
     
       
         θ LR =tan −1 ( H/W ) 
       
     
     wherein 
     H: a difference in height between the front leg and the rear leg of the seat (connecting points) and 
     W: a space between the front leg and the rear leg of the seat (connecting points). 
     The clearance between pin hole  25   c  of the pin bracket  25  and bracket pin  27  allows the absorbing mechanism to rotate about an axis extending in a vertical direction of the vehicle body. The degree of pivoting allowed is more than a difference from parallel of the seat rails. 
     FIG.  9 (A) is a side sectional view showing the construction of a seat weight measuring apparatus according to a second embodiment of the present invention. FIG.  9 (B) is a perspective view showing the detailed structure of a load cell, hollow suspension, and XY slider according to the second embodiment. 
     A seat weight measuring apparatus  100  is arranged between the seat rail  7  and the seat mounting portion  11  of the vehicle body or between the seat frame  5  and the seat rail  7 . 
     The seat weight measuring apparatus  100  comprises load cells  105 , hollow suspensions  103 , and XY sliders  102 . 
     Each load cell  105  includes a strain gauge therein to measure the seat weight. Each load cell  105  is fixed to the seat mounting portion  11  by a fixing plate  107 . 
     Each hollow suspension  103  is a member transmitting a load to each load cell  105 . Each hollow suspension  103  is a hollow member having a relatively small thickness. Therefore, the hollow suspension  103  can elastically deform in the vertical, lengthwise, and width directions. 
     Each XY slider  102  is a disk-like member and is engaged with a disk-like concavity  101   a  of a base  101  with a space therebetween. The XY slider  102  can slide in the lengthwise and width directions (XY directions) within the concavity  101   a.    
     The hollow suspensions  103  and the XY sliders  102  compose an absorbing mechanism of this seat weight measuring apparatus  100 . 
     Hereinafter, the actions of the structure around the sensor plate of the seat weight measuring apparatus according to the first embodiment of the present invention will be described with reference to FIG.  10 (A) through FIG.  12 . 
     FIGS.  10 (A) through  10 (C) are views for explaining the actions of the structure around the sensor plate of the seat weight measuring apparatus according to the first embodiment of the present invention. FIG.  10 (A) is a side view thereof, FIG.  10 (B) is a plan view of the sensor plate, and FIG.  10 (C) is a graph schematically illustrating strain distribution on the surface of the sensor plate. It should be noted that these figures show only the front half and the central portion of the sensor plate. 
     FIG. 11 is a side view for explaining the action when axial force (force in the longitudinal direction) is applied to the supports of the half arms. 
     FIG. 12 is a graph showing data indicating the influences of the axial force applied to the supports of the half arms on the measurement data. 
     As shown in FIG.  10 (A), the Z arm  23  slightly pivots upwardly (see FIG.  2 (B)) according to the seat weight to raise up the supports  41   b  of the wings  41   a  of the half arm  41 . According to the force W raising up the supports  41   b , moment M is transmitted from the half arm body  41   c  to the sensor plate  51 . By the moment M, the sensor plate  51  is deformed in wave-like profile so as to create a region  51   y  of which strain is tensile (+) and a region  51   z  of which strain is compressional (−). 
     Since the bodies  41   c  and  42   c  of the half arms  41  and  42  clamping the ends of the sensor plate  51  are thick and have high rigidity as described above, a load applied portion  51   x  of the sensor plate  51  clamped by the bodies  41   c  and  42   c  is little deformed. In addition, the central portion (fixed portion  51   w ) of the sensor plate  51  is also little deformed because the portion  51   w  is vertically clamped by a washer  67  and a column  63  which both have high rigidity. 
     On the other hand, a portion between the load applied portion  51   x  and the central fixed portion  51   w  has such a plane configuration that two triangular portions are disposed to oppose to each other about the neck  51   c  as shown in FIG.  1 (B). Formed on this portion are the tension-side constant surface strain region  51   y  and the compression-side constant surface strain region  51   z . Attached to the regions  51   y  and  51   z  are a tension-side strain gauge  54   a  and a compression-side strain gauge  54   c , respectively. 
     Therefore, the tension-side strain gauge and the compression-side strain gauge are wired on the bridge circuit in such a manner as to form an opposite phase, thereby increasing the outputs of the strain gauges. This attains high precise measurement with small strains on the sensor, thereby increasing the life of the sensor. 
     Since the surfaces on which the strain gauges are attached are on one side of the sensor plate  51 , printing process should be made only on one side when the strain gauges and wirings are printed by, for example, screen printing. This allows sensors to be manufactured at lower cost. 
     The strain gauges are attached (printed) on regions providing constant surface strain, so strain may not vary even through the positions of strain gauges are slightly shifted, thereby preventing measurement error. Therefore, this can ensure high precision of measurement and can reduce quality requirement on manufacturing. 
     Because strains are concentrated in portions on which the strain gauges are attached, highly precise measurement is performed and measurement error can be prevented even with manufacturing error and/or assembling error of other parts. 
     Hereinafter, axial force applied from the Z arm  23  to the half arm  41  will be described with reference to FIG.  2 . As described above, force W is applied from the action portions  23   j  of the Z arm  23  to the supports  41   b  of the wings  41   a  of the half arm  41 . The force W is mainly composed of vertical component W v  because of the structure of the half arm  23 . However, the force W may partially composed of horizontal component (axial force) W h . Due to elongation or dislocation of the Z arm  23 , frictional force may act on the supports  41   b  in the lateral direction. 
     Force due to the friction and deformation can be released not to apply axial force to the half arm  41  because the action portions  23   j  of the Z arm  23  and the supports  41   b  are not restrained in the axial direction (the longitudinal direction) so that they are allowed to slip relative to each other. 
     When the axial force W h  is applied, axial stress acting on the sensor plate  51  can be cancelled by offsetting the outputs of the tension-side strain gauge  54   a  and the compression-side strain gauge  54   c.    
     This state is shown in FIG.  12 . In the graph of FIG. 12, the abscissa indicates the load (kgf) applied on the seat and the ordinate indicates the output (mV) of the strain gauges. 
     Hysteresis in the outputs of the compression-side strain gauge and the tension-side strain gauge is clearly shown in this graph, the hysteresis being developed by rise and fall in load. This is because axial force is produced between the Z arms and the half arms during rise or fall in load. However, in the total output obtained by subtracting the output of the compression-side strain gauge from the output of the tension-side strain gauge, nearly non hysteresis is shown so that the data of the total output is almost linear. This is because the axial force is cancelled as described in the above. 
     When load is applied in the longitudinal direction and/or rotational moment is applied, the balance of strain between the tension side and the compression side is changed in such a manner as not to change the total sensitivity of the sensors relative to the vertical load. That is, even with dislocation in a horizontal direction (e.g. the longitudinal direction of the vehicle) or axial force acting on the sensor plate, error can be cancelled by means of the compression-side strain gauge and the tension-side strain gauge and the total sensitivity obtained by summing the outputs of both the strain gauges may not include such error. 
     As shown in FIG. 11, with the axial force W h , a space S between the center line of the sensor plate  51  and the supports  41   b  of the half arm  41  acts as a moment arm so as to apply W h ×S moment on the sensor plate  51 , creating an error factor. In the seat weight measuring apparatus of the present invention, therefore, the load applied points  41   b  of the half arm  41  and the center line in the thickness of the sensor plate  51  are set to be in substantially the same level or have height difference of ±5 mm or less. According to this structure, when frictional force (axial force W h ) acts on the action points  41   b , the moment arm for deforming the sensor plate due to the frictional force is short. This means that the deformation of the sensor plate due to the frictional force is little, thereby reducing the measurement error. 
     As can be seen from the foregoing description, the present invention can provide a seat weight measuring apparatus in which the performance of load sensors is not spoiled due to dimensional error or deformation of a vehicle body or a seat. Further, the present invention can provide a seat weight measuring apparatus which has good durability and high precision but can be manufactured at low cost. Further still, the present invention can provide a seat weight measuring apparatus with reduced measurement error due to manufacturing error of parts of a mechanism and/or frictional force. 
     The priority documents here Japanese Application No. H11-061340, filed Mar. 9, 1999, and Japanese Application No. H11-061341, filed Mar. 9, 1999 (including their specifications, drawings, and claims), are hereby incorporated by reference into this application.