Abstract:
An acceleration switch that improves detection sensitivity without being enlarged. The acceleration switch includes a switch body, a fixed electrode arranged in the switch body, and a movable weight arranged in the switch body. The movable weight is displaced when subjected to acceleration. The movable weight includes a movable electrode that contacts the fixed electrode when the movable weight is displaced, and a pair of beams connecting the movable weight and the switch body. The beams pivotally support the movable weight and extend into recesses formed in the movable weight.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an acceleration switch, and more particularly, to an acceleration switch including a movable electrode that contacts a fixed electrode when subjected to acceleration that exceeds a predetermined value. 
     Many automobiles are presently equipped with air bag systems. A typical air bag system includes an air bag, an ignitor, and an electronic control unit (ECU). The ECU includes an acceleration sensor, which detects a sudden change in acceleration upon collision of the vehicle. The employment of a mechanical acceleration switch (i.e., safing sensor) has been proposed as such acceleration sensor. 
     FIGS. 1A to  1 C schematically show the structure of a prior art acceleration switch  51 . The acceleration switch  51  includes a silicon chip  52  and a substrate  53 , which are connected to each other. As shown in FIG. 1B, the silicon chip  52  has a length of L 1  and a width of W 1 . Further, the silicon chip  52  has a hollow portion  52   a  in which a block-like inertia weight  54  is arranged. 
     A beam  55  is provided on each long side of the inertia weight  54  and extends from a position offset from the middle of the long side. The beams  55  connect the inertia weight  54  and the silicon chip  52 . The inertia weight  54  pivots about the beams  55  at a position offset from the center of gravity of the inertia weight  54 . Referring to FIG. 1B, the beams  55  each have a length of T 2 . Two movable electrodes  56 ,  57  are arranged on the lower side of the inertia weight  54 , as viewed in FIG.  1 A. The movable electrodes  56 ,  57  are located close to each other at the middle of the weight end that is farther from the beams  55 . 
     A hollow portion  53   a  is defined in the upper surface of the substrate  53 . A fixed electrode  58  is formed in the hollow portion  53   a  at a position corresponding to the movable electrodes  56 ,  57 . The movable electrodes  56 ,  57  are normally separated from the fixed electrode  58 . 
     When the acceleration switch  51  is subjected to acceleration, inertial force pivots the inertia weight  54  about the axis of the beams  55  in a downward direction (the direction indicated by arrow G in FIG.  1 A). When the acceleration becomes greater than or equal to a predetermined value, the inertia weight  54  pivots in a direction indicated by arrow F in FIG. 1A, and the movable electrodes  56 ,  57  contact the fixed electrode  58 . When the value of acceleration is small, the movable electrodes  56 ,  57  do not contact the fixed electrode  58 . Accordingly, the acceleration switch  51  is actuated only when the acceleration becomes greater than or equal to a predetermined value. 
     To actuate the acceleration switch  51  at a relatively small acceleration, the beams  55  may be thinned or elongated. However, the formation of thinner beams  55  has physical limitations and is thus not effective. Further, the formation of longer beams  55  limits miniaturization. For example, when the length of the beams  55  is changed to T 3  (T 2 &lt;T 3 ) as shown in FIG. 2, the width W 2  of the silicon chip  52  is increased by 2×T 3 −2×T 2  in comparison to when the width of the silicon chip  52  is W 1  (FIG.  1 B). This increases the area of the silicon chip  52  and enlarges the acceleration switch  51 . 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an acceleration switch having improved detection sensitivity while avoiding enlargement. 
     To achieve the above object, the present invention provides an acceleration switch including a switch body, a fixed electrode arranged in the switch body, and a movable weight arranged in the switch body. The movable weight is displaced when subjected to acceleration. The movable weight includes a movable electrode that contacts the fixed electrode when the movable weight is displaced, a recess formed in a side of the movable weight, and a beam connecting the movable weight and the switch body. The beam pivotally supports the movable weight and extends into the recess. 
     Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1A is a schematic cross-sectional view showing a prior art acceleration switch; 
     FIG. 1B is a schematic lower view showing a silicon chip showing the structure of the acceleration switch of FIG. 1A; 
     FIG. 1C is a cross-sectional view taken along line  1 C— 1 C in FIG. 1B; 
     FIG. 2 is a lower view showing the structure of another prior acceleration switch; 
     FIG. 3A is a schematic cross-sectional view showing an acceleration switch according to a first embodiment of the present invention; 
     FIG. 3B is a schematic lower view showing the structure of the acceleration switch of FIG. 3A; 
     FIG. 3C is a cross-sectional view taken along line  3 C— 3 C in FIG. 3B; 
     FIG. 4A is a schematic plan view illustrating the manufacturing procedure of the acceleration switch of FIG. 3A; 
     FIG. 4B is a cross-sectional view taken along line  4 B— 4 B in FIG. 4A; 
     FIG. 4C is a cross-sectional view taken along line  4 C— 4 C in FIG. 4A; 
     FIG. 5A is a schematic plan view illustrating the manufacturing procedure of the acceleration switch of FIG. 3A; 
     FIG. 5B is a cross-sectional view taken along line  5 B— 5 B in FIG. 5A; 
     FIG. 5C is a cross-sectional view taken along line  5 C— 5 C in FIG. 5A; 
     FIG. 6A is a schematic plan view illustrating the manufacturing procedure of the acceleration switch of FIG. 3A; 
     FIG. 6B is a cross-sectional view taken along line  6 B— 6 B in FIG. 6A; 
     FIG. 6C is a cross-sectional view taken along line  6 C— 6 C in FIG. 6A; 
     FIG. 7A is a schematic plan view illustrating the manufacturing procedure of the acceleration switch of FIG. 3A; 
     FIG. 7B is a cross-sectional view taken along line  7 B— 7 B in FIG. 7A; 
     FIG. 7C is a cross-sectional view taken along line  7 C— 7 C in FIG. 7A; 
     FIG. 8 is a schematic cross-sectional view showing a state in which acceleration is applied to the acceleration switch of FIG. 3A; 
     FIG. 9A is a schematic lower view showing a silicon chip of the acceleration switch of FIG. 3A; 
     FIG. 9B is a schematic cross-sectional view showing the acceleration switch of FIG. 3A; 
     FIG. 10 is a schematic lower view showing a silicon chip of an acceleration switch according to a further embodiment of the present invention; 
     FIG. 11A is a schematic lower view showing a silicon chip of an acceleration switch according to a further embodiment of the present invention; and 
     FIG. 11B is a schematic lower view showing a silicon chip of an acceleration switch according to a further embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawings, like numerals are used for like elements throughout. 
     FIG. 3A is a schematic cross-sectional view showing an acceleration switch  1  according to a first embodiment of the present invention. As shown in FIG. 3A, the acceleration switch  1  has a switch body  1 A, which is formed by connecting a silicon chip  2  to a substrate  3 . 
     The silicon chip  2  includes a chip body  4  and a double-layer structure of epitaxial growth layers  5 ,  6 . The chip body  4  is rectangular and formed from p-type monocrystal silicon of (110) orientation. The epitaxial growth layers  5 ,  6  are superimposed on one side of the chip body  4  and formed from n-type monocrystal silicon. The thickness of the chip body  4  is 500 to 600 μm. The thickness of each of the epitaxial growth layers  5 ,  6  is about 15 μm. Thus, the thickness of the two epitaxial growth layers  5 ,  6  is about 30 μm. To facilitate illustration, the thickness of the epitaxial growth layers  5 ,  6  is exaggerated. The silicon chip  2  has the same length L 1  and width W 1  as the silicon chip  52  of FIG.  1 . 
     A rectangular hollow portion  7  is formed in the lower side of the chip body  4 , as viewed in FIG.  3 A. The hollow portion  7  has a depth of about 100 μm and is thus deeper than the thickness of the two epitaxial growth layers  5 ,  6 . The hollow portion  7  accommodates a movable portion (movable weight) M 1 , which includes an inertia weight  8 , two beams  10 , and a plurality of flexible plates  11 . In the preferred embodiment, the beams  10  are formed integrally with the inertia weight  8 . 
     The inertia weight  8  is generally plate-like and has a thickness of about 20 μm. A pair of cutaway portions (recesses)  9  are formed on sides of the inertia weight  8  in a direction perpendicular to the longitudinal direction of the inertia weight  8 . The pairs of cutaway portions  9  are offset from the middle of the inertia weight  8 , which is indicated by a dot in FIG.  9 A. 
     The inertia weight  8  has a joint  8   a,  a weight body  8   b,  and a balancer  8   c.  The joint  8   a  extends between and perpendicular to the beams  10 . The weight body  8  is connected to one end of the joint  8   a.  As shown in FIG. 3B, the width Wj of the joint  8   a  in the preferred embodiment is about the same as the width Wb of the beams  10 . 
     The two beams  10  are perpendicular to the longitudinal direction of the silicon chip  2  and toward the associated walls of the silicon chip  2  in the hollow portion  7 . The beams  10  are flexible and each have a thickness of about 7.5 μm, which is about three eighths of the thickness of the inertia weight  8 . Further, the beams  10  each have a length of T 1 . 
     In the preferred embodiment, the length T 1  of each beam  10  is about 4.4 times the beam length T 2  of the prior art acceleration switch  51  (FIG.  1 B). One end of each beam  10  is integrally connected with the side surface of the joint  8   a,  and the other end of each beam  10  is connected to the associated wall of the silicon chip  2 . The inertia weight  8  pivots about the beams  10 . 
     The two flexible plates  11  are formed integrally with the weight body  8   b  on the opposite side of where the cutaway portions  9  are located. That is, the flexible plates  11  are formed on the end of the weight body  8   b  that is opposite to the end facing the beams  10 . As shown in FIG. 3B, the flexible plates  11  are each trapezoidal when seen from above and become narrower toward the distal end. The thickness of each flexible plate  11  is about the same as the thickness of the beams  10  and is about 7.5 μm. The flexible plates  11  are formed at the middle of the end surface of the inertia weight  8 . The lower surfaces of the flexible plates  11  are flush with the lower surface of the weight body  8   b.    
     The flexible plates  11  are located near each other. More specifically, the gap between the flexible plates  11  is about 10 to 200 μm. In the preferred embodiment, the gap is about 40 μm. A movable electrode  12  is provided on the distal end of the lower side of each movable electrode  12 . The width of each movable electrode  12  is about the same as the width of the distal portion of each flexible plate  11 . The movable electrodes  12  are connected to external terminals (not shown) by wiring patterns  12   a  formed along the flexible plates  11 , the inertia weight  8 , and the beams  10 . 
     The substrate  3  is rectangular and has the same shape as the silicon chip  2 . In the preferred embodiment, a glass substrate is used as the insulative substrate  3 . Alternatively, substrates made of other materials such as silicon may be used as the substrate  3 . A rectangular hollow portion  13  is defined in the substrate  3 . The hollow portion may be formed by, for example, etching the portion of the substrate  3  corresponding to the hollow portion  7  of the silicon chip  2 . The substrate  3  and the silicon chip  2  are connected to each other using an anode connecting technique that is know in the art. The substrate  3  and the silicon chip  2  may also be connected with each other by using, for example, an adhesive. 
     A fixed electrode  14  is formed on the bottom surface of the hollow portion  13  at a position corresponding to the movable electrodes  12 . The movable electrodes  12  contact the fixed electrode  14  when the inertia weight  8  pivots and the flexible plates  11  incline toward the substrate  3 . This electrically connects the movable electrodes  12  via the fixed electrode  14 . 
     An example of a procedure for manufacturing the acceleration switch  1  of the preferred embodiment through a surface micro-machining technique will now be discussed with reference to FIGS. 4 to  7 . 
     The substrate  3  used in the acceleration switch  1  is manufactured by first etching a rectangular glass substrate (e.g., Pyrex glass) to form the hollow portion  13  having a predetermined shape. Then, after masking the glass substrate, conductive metal (e.g., aluminum Al) is sputtered to form the fixed electrode  14  on the bottom surface of the hollow portion  13 . In lieu of a dry film forming process such as sputtering, a wet film forming process such as electroless plating may be employed. 
     To manufacture the silicon chip  2 , a mask (not shown) is first applied to one side of a chip body  4 . The chip body  4  is then photoetched to form an opening in a predetermined portion of the mask. Then, the surface of the chip body  4  undergoes an ion implantation process to implant a predetermined concentration of p-type impurities such as boron. The p-type impurities are then thermally diffused. This forms a first high concentration p-type silicon layer (lower p +  silicon implantation layer)  21  (FIG. 4B) in the predetermined portion of the silicon chip  2 . The portion in which the lower p +  silicon implantation layer  21  is formed corresponds to where the hollow portion  7  will be subsequently formed. 
     Vapor phase epitaxy is performed to form the first epitaxial growth layer  5 , which is made of n-type monocrystal silicon, on the entire surface of the chip body  4  on which the p +  silicon implantation layer  21  has been applied. As a result, the p +  silicon implantation layer  21  is implanted in the first epitaxial growth layer  5  (FIG.  4 B). A mask (not shown) is then applied to the first epitaxial growth layer  5  and photoetched to form an opening at a predetermined portion of the mask. In this state, for example, ion implantation is performed to implant p-type impurities. The implanted p-type impurities are then thermally diffused. This forms a second high concentration p-type silicon layer (upper p +  silicon implantation layer)  22  in the first epitaxial growth layer  5 . The upper p +  silicon implantation layer  22  extends to the lower p +  silicon implantation layer  21 . The portion in which the upper p +  silicon implantation layer  22  is formed also corresponds to the hollow portion  7 . The portion that is masked when forming the upper p +  silicon implantation layer  22  corresponds to the inertia weight  8 . 
     Subsequently, vapor phase epitaxy is performed to form the second epitaxial growth layer  6 , which is made of n-type monocrystal silicon, on the entire surface of the first epitaxial growth layer  5 . As a result, the upper p +  silicon implantation layer  22  is implanted in the second epitaxial growth layer  6  (FIGS. 4B,  4 C). Then, a mask (not shown) is applied to the second epitaxial growth layer  6  and photoetched to form openings at predetermined portions. P-type impurities are then implanted and thermally diffused. This forms a third high concentration p-type silicon layer (p +  silicon diffusion layer)  23  in the first and second epitaxial growth layers  5 ,  6  (FIGS.  5 B and  5 C). The p +  silicon diffusion layer  23  extends to the upper p +  silicon implantation layer  22 . The portions in which the third high concentration p-type silicon layer (p +  silicon diffusion layer)  23  is formed correspond to the hollow portion  7 . The portions that are masked when forming the p +  silicon diffusion layer  23  correspond to the inertia weight  8 , the beams  10 , and the flexible plates  11 . In other words, the p +  silicon diffusion layer  23  is formed to leave space for forming the inertia weight  8 , the beams  10 , and the flexible plates  11 . 
     After the high concentration p-type silicon layer forming process is completed, the silicon chip is heated in the presence of oxygen or in an atmospheric environment to form an oxidation film (not shown) on the upper and lower surfaces of the silicon chip  2 . In this state, aluminum (Al) is sputtered or vapor deposited on the oxidation film. Then, photolithography is performed on the silicon chip  2 . This forms the movable electrodes  12  and the wiring pattern  12   a  on the surface of the silicon chip  2  in correspondence with the locations where the inertia weight  8  and the flexible plates  11  will be formed. 
     Subsequently, sputtering or vapor deposition of, for example, tungsten (W) or molybdenum (Mo) is performed on the silicon chip  2 . The silicon chip  2  then undergoes lithography. This forms a metal protection film (not shown) having openings. The oxidation film is then removed through the openings of the metal protection film to expose the upper surface of the p +  silicon diffusion layer  23 . Tungsten and molybdenum are sputtered or vapor deposited on the silicon chip  2  since these metals resist hydrofluoric acid. 
     After the masking process is completed, the silicon chip  2  performs an anode conversion process. 
     A high concentration hydrofluoric acid (HF) solution, which is an anode conversion acid solution, is filled in an anode conversion treatment tank. Counter electrodes, which are formed from, for example, platinum, and the silicon chip  2 , which is faced toward the counter electrodes, are immersed in the hydrofluoric acid solution. A cathode of a direct current power supply is connected to the lower side of the silicon chip  2 . An anode of the direct current power supply is connected to the upper side of the silicon chip  2 . Thus, direct current flows from the lower surface to the upper surface of the silicon chip  2 . This results in the portions formed from high concentration p-type silicon in the silicon chip  2  (i.e., p +  silicon implantation layer  21 ,  22  and the p +  silicon diffusion layer  23 ) becoming porous. Therefore, the first to third high concentration p-type silicon layers  21 ,  22 ,  23  are reformed into porous silicon layers. 
     Subsequent to the anode conversion process and prior to the removal of the metal protection film, the silicon chip  2  undergoes alkali etching. A substance such as tetramethylammonium hydroxide (TMAH) is used as the etching agent. The etching dissolves the porous silicon layers. The porous silicon layer, which define a reformed portion, is easily dissolved by alkali in comparison to the non-porous silicon layers, which define a non-reformed portion. Thus, the porous silicon layers are easily hollowed out to form the hollow portion  7 . This also forms the movable portion M 1  in the hollow portion  7  (FIGS.  7 A- 7 C). The silicon chip  2  is then reversed and attached to the substrate  3 . This completes the acceleration switch  1  of FIG.  3 . 
     The operation of the acceleration switch  1  will now be discussed. Referring to FIG. 8, when the acceleration switch  1   a  is subjected to acceleration, an inertial force is applied to the movable portion M 1  in the direction of arrow G. When the acceleration applied to the acceleration switch  1  becomes greater than or equal to a predetermined value, the inertia weight  8  pivots downward about the beams  10  as shown by arrow F in FIG.  8 . The weight body  8   b  is heavier than the balancer  8   c.  Thus, although inertial force is also applied to the balancer  8   c,  the weight body  8   b  is pivoted downward. 
     The joint  8   a  is thicker than the beams  10  and thus more rigid than the beams  10 . Hence, even if the joint  8   a  is subjected to acceleration that elastically deforms the beams  10 , the joint  8   a  does not flex. This integrally pivots the weight body  8   b  and the balancer  8   c.  The weight body  8   b  is pivoted downward and the balancer  8   c  is pivoted upward. As a result, the movable electrodes  12  contact the fixed electrode  14 . This conducts electricity between the movable electrodes  12  through the fixed electrode  14  and actuates the acceleration switch  1 . 
     The balancer  8   c  causes a time delay from when inertial force, which is produced by acceleration, is applied to the inertia weight  8  to when the inertia weight  8  starts to pivot. Response to acceleration applied to the inertia weight  8  is delayed by the balancer  8   c.  Therefore, the movable electrodes  12  do not contact the fixed electrode  14  unless acceleration is applied to the inertia weight  8  for a predetermined time. Accordingly, the acceleration switch  1  is not actuated when momentary acceleration is applied to the inertia weight  8 . In other words, the acceleration switch  1  is not actuated when, for example, noise, which is produced by vibrations and which acts in the same manner as acceleration, is applied to the inertia weight  8 . 
     When the acceleration switch  1  is subjected to acceleration that is smaller than the predetermined value, the inertia weight  8  does not pivot about the beams  10 . Thus, even if the beams  10  flex for a certain amount, the inertia weight  8  is not displaced to a predetermined position and electricity is not conducted between the movable electrodes  12 . In other words, the acceleration switch  1  is actuated only when the acceleration switch  1  is subjected to acceleration that is greater than or equal to the predetermined value. 
     The equations listed below are satisfied by the acceleration switch  1 . In the equations, the distance between the movable electrodes  12  and the fixed electrode  14  is represented by δ (mm), the acceleration required for the movable electrodes  12  to contact the fixed electrode  14  (i.e., the acceleration produced by the inertial force applied to the inertia weight  8 ) is represented by G (m/s 2 ), and the coefficient of spring of the beams  10  is represented by k. 
     
       
         δ=sin (2 ·m*·G/k )×( Lm/ 2 +R ) 
       
     
     
       
           k =(β· Wb·hb   3   ·Gs· 2)/ Lb   
       
     
     Referring to FIGS. 9A and 9B, the length of the weight body  8   b  is represented by Lm, the mass (kg·m) of the weight body  8   b  is represented by m*, the distance (mm) between the center of gravity of the inertia weight to the middle of the joint  8   a  is represented by R. The length (mm) of each beam  10  is represented by Lb, the width (mm) of each beam  10  is represented by Wb, and the thickness (mm) of each beam  10  is represented by hb. The coefficient of torsion of each beam  10  is represented by β, and the transverse elasticity (N/m 2 ) of each beam  10  is represented by Gs. 
     In accordance with the two equations, the acceleration G changes as the length Lb of each beam  19  changes. More specifically, the acceleration G required for the movable electrodes  12  to contact the fixed electrode  14  decreases as the length Lb of the beam  10  increases. 
     The inventors of the present invention have confirmed that when, for example, the length Lm of the weight body  8   b  is 7 mm, the width of the weight body  8   b  is 1.5 mm, the thickness of the main weight  8   b  is 0.015 mm, the distance δ between the movable electrodes  12  and the fixed electrode  14  is 0.02 mm, and the length T 1  of each beam  10  is about 0.88 mm (Lb=T 1 =about 0.88 mm), the acceleration G required for contact between the electrodes  12 ,  14  is about 2.5 g (m/s 2 ) in which g represents gravitational acceleration. 
     In comparison, the inventors of the present invention have confirmed that when the beam length T 2  of the conventional acceleration switch  51  is 1/4.4 of the beam length T 1  and the beam length T 2  (Lb) is about 0.2 mm, the acceleration G required for contact between the electrodes  12 ,  14  increases to about  11   g  (m/s 2 ). This is about four times the acceleration G required for contact between the electrodes  12 ,  14  when the beam length is T 1 . 
     Accordingly, the acceleration G required for contact between the movable and fixed electrodes  12 ,  14  is inversely proportional to the beam length Lb. The desired threshold value of acceleration is easily obtained by varying the beam length Lb. 
     The acceleration switch  1  of the preferred embodiment has the advantages described below. 
     (1) A pair of cutaway portions  9 , or recesses, extending perpendicular to the longitudinal axis of the inertia weight  8  are formed on sides of the weight  8 . Each of the beams  10  is formed in one of the cutaway portions  9 . This increases the beam length T 1  by the length h 1  of the cutaway portions  9  without increasing the silicon chip area. Thus, the beams  10  are elastically deformed easily and enable the inertia weight  8  to be pivoted by a small acceleration. Accordingly, the sensitivity of the acceleration switch  1  to acceleration is improved without enlarging the switch  1 . 
     (2) The beams  10  support the inertia weight  8  at the joint  8   a.  Thus, the inertia weight  8  is pivoted smoothly. 
     (3) The inertia weight  8  is provided with the balancer  8   c.  This delays the response of the inertia weight  8  when the acceleration switch  1  is subjected to acceleration. Thus, the inertia weight  8  is not pivoted by momentary acceleration. This prevents inadvertent actuation of the acceleration switch  1  when subjected to noise-like acceleration (e.g., sudden vibration). 
     (4) When the acceleration switch  1  is subjected to acceleration, the beams  10  are elastically deformed but the joint  8   a  is not. Thus, the joint  8   a,  the weight body  8   b,  and the balancer  8   c  are pivoted integrally, and the balancer  8   c  is not displaced in an undesirable manner. 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms. 
     As shown in FIG. 10, the flexible plates  11  may be eliminated, and the movable electrodes  12  may be formed on the weight body  8   b.    
     As shown in FIG. 11A, a cutaway portion  9  may be formed in one end  8   d  of the inertia weight  8 , and a single beam  10  extending between the cutaway portion  9  and connected to the inertia weight  8  may be formed. In this case, since there is only one beam, the sensitivity of the acceleration switch  1  is further improved. 
     As shown in FIG. 11B, a pair of cutaway portions  9  extending in the longitudinal direction of the inertia weight  8  and spaced from each other by a predetermined distance may be formed in the end  8   d  of the inertia weight, and a beam  10  may be formed between each cutaway portion  9  and the hollow portion  7 . This further stabilizes the pivoting of the inertia weight  8 . 
     The beams  10  may be formed in any part of the corresponding cutaway portion  9 . 
     The width Wj of the joint  8   a  may be equal to the width Wb of each beam  10 , and the thickness of the joint  8   a  may be equal to the thickness hb of each beam  10 . In this case, the joint  8   a  functions in the same manner as the beams  10  and enables the weight body  8   b  to be pivoted by a smaller acceleration. 
     The balancer  8   c  may be eliminated. This would make the acceleration switch  1  more compact. 
     The width Wj of the joint  8   a  is not restricted to any value. The width Wj may be such that the joint  8   a  is more rigid than the beams  10 . 
     The present examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.