Abstract:
An electro-mechanical impact detecting device for a vehicle is provided which implements a multi-stage control for vehicle occupant protection systems and improves the operational delay characteristics of the vehicle occupant protection systems at an irregular collision based on the setting of multiple impact levels which are detected. When a main rotor rotates by a certain amount against the exertion force of a moving contact, the moving contact and fixed contact close the circuit, thereby detecting the first impact level. When the main rotor further rotates against the exertion force of the torsional coil spring, the remaining two pairs of fixed contacts and close the circuits sequentially, thereby detecting the second and third impact levels, respectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present invention is related to Japanese patent application No. Hei. 11-333125, filed Nov. 24, 1999; the contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to an electro-mechanical impact detecting device, and more particularily, to an acceleration detecting device or collision detecting device suitable for a vehicle occupant protection system, vehicle air-bag system or seat-belt pretensioner. 
     BACKGROUND OF THE INVENTION 
     A conventional electro-mechanical impact detecting device or collision detecting device is disclosed in JP-A No. Hei-9-306311. This device has a rotor with a weight positioned eccentric from the rotation shaft of the rotor. This device senses a vehicle collision when the rotor has rotated by a prescribed rotation value due to the eccentric mass of the weight. This means that the collision detecting device has its collision detection level corresponding to the prescribed rotation value of the rotor, and accordingly the device has a single detection level. This device produces a detection signal at a single impact level. However, when multi-stage control of the air-bag system is implemented, the dilating speed during an irregular collision rises. Without multi-stage control of the air-bag system, the system is incapable of timely air bag dilation, depending on the delay of collision detecting during irregular collision. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention overcomes the aforementioned drawbacks by providing an electro-mechanical impact detecting device for a vehicle which uses multiple impact levels to detect impact acting on the vehicle. As a result, multi-stage control is used for the occupant protection system and collision detection is based on improved operational delay characteristics of the occupant protection system during an irregular collision. 
     In one aspect of the invention, a displacement member deviates in position by a exerting means force responsive to an impact acting on a vehicle. The present invention includes switches which close circuits sequentially at displacement values of the displacement member corresponding to at least a first and second impact levels of impact. The device detects at least a first and second impact level in response to closing the switches. 
     The switches trip to detect the first and second impact levels. The present invention has at least two impact levels sensed and, by using the operational time difference between the first impact level and second impact level and splitting the control domain into an ON part if the time difference is within a prescribed time length and an OFF part if it exceeds the time length, the air bag is dilated without delay. Moreover, by setting two or more prescribed time lengths, the collision can be divided into more divisions, e.g., OFF/Lo/Hi or OFF/Lo/Mid/Hi. 
     In another aspect of the invention, a rotating member rotates about a center of rotation. The rotating member has a center of mass which is eccentric from the rotation center, and rotates about the rotation center against an exertion force of exerting means in response to an impact on the vehicle. Switches close circuits sequentially at rotation values of the rotating member corresponding to at least first and second impact levels, and the device detects at least the first and second impact levels in response to the closing switches. 
     In another aspect of the invention, a displacement member is supported to deviate in the axial direction against an exertion force of exerting means in response to impact. Also, switches are provided which close circuits sequentially at axial displacement values of the displacement member corresponding to at least a first and second impact levels of impact. The impact detecting device detects at least the first and second impact levels in response to the switches closing. 
     In another aspect of the invention, the exerting means includes a first spring which exerts a force on the rotating member toward an initial rotation position. At least one or more second springs exert forces simultaneously or sequentially on the rotating member against rotation after the rotating member has rotated by a certain amount against the force of the first spring. 
     In another aspect of the invention, the exerting means includes a first spring which exerts a force on the displacement member toward an initial axial displacement position. At least one second spring exerts forces simultaneously or sequentially on the displacement member against the displacement thereof after the displacement member has deviated by a certain displacement against the exertion force of the first spring. 
     In another aspect of the invention, the impact detecting device includes a cam provided on the rotating member concentrically with the rotation center and adapted to rotate integrally with the rotating member in response to impact. The switches include fixed contacts and flat-spring moving contacts which are in contact with the surface of the cam. The switches are pushed and bent as the cam rotates and contacts the fixed contacts to close the circuits. At the contact surface, the cam surface is shaped such that the cam does not increase the bending value of the moving contacts. 
     In another aspect of the present invention, at least the first or second springs works also as the flat-spring moving contact. This reduces component parts. In another aspect, an exerting means comprises flat exerting springs which extend from a root section toward the cam and align in the direction of push of the cam. The exerting flat springs have no spacing at their root section from each other. 
     This eliminates contact movement during contact between the flat exerting springs. Consequently, the creation of frictional force between the flat exerting springs is eliminated and the operational fluctuation of the impact detecting device can be reduced. 
     In another aspect, an auxiliary rotor is located concentrically with the rotation center of the rotating member, and the second spring comprises a plurality of springs. One of the second springs is a torsional spring located concentrically with the rotation center of the rotating member. The torsional spring has one end fixed to a stationary member and another end fixed to part of the auxiliary rotor. The torsional spring exerts a force on the rotating member through the auxiliary rotor against the rotation of the rotating member. When an exertion force of the torsional spring of the rotating member is imposed, the torsional spring has another end that twists and prevents increase of operational fluctuation of the impact detecting device. 
     In another aspect, the impact detecting device includes a cam provided on the rotating member concentric with the rotation center and rotates integral with the rotating member in response to impact. The switches include fixed contacts and flat-spring moving contacts in contact with the surface of the cam. The switches are pushed and bent as the cam rotates and contact the fixed contacts to close the circuits. The switches have equal spacing between the fixed contacts and moving contacts. The cam contact surface is formed to shift in position along the rotation direction of the cam. This prevents the flat-spring contacts from being damaged by chattering or exceeding the spring stress limit. 
     In another aspect, the present invention includes a detection signal generation means that generates detection signals in response to closing of the switches at stepped values which match with at least the first and second impact levels. In another aspect, the second impact level is set greater than the first level. The detection signal generation means includes first and second electrical load elements, and is an electrical closing circuit having a closed switch at the first impact level and the first electrical load element, and another closing circuit having a closed switch at the second impact level and the first and second electrical load elements. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a cross-sectional view along the line  1 — 1  of FIG. 3 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG. 1 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  of FIG. 1 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 4 is a plan view of the casing  30  of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 5 is a cross-sectional view taken along line  5 — 5  of FIG. 4 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 6 is a cross-sectional view taken along line  6 — 6  of FIG. 4 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 7 is a cross-sectional view taken along line  7 — 7  of FIG. 6 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 8 is a cross-sectional view taken along the line  8 — 8  of FIG. 6 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 9 is plan view of a bottom portion of a casing of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 10 is a plan view of the main rotor of the first embodiment of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 11 is a right side view of a main rotor of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 12 is a left side view of the main rotor of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 13 is a plan view of a sub rotor of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 14 is a right side view of the sub rotor of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 15 is a left side view of the sub rotor of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 16 is a front view of the contact mechanism of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 17 is a right side view of the contact mechanism of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 18 is a left side view of the contact mechanism of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 19 is a plan view of the contact mechanism of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 20 is a schematic circuit diagram of the moving contacts and fixed contacts (first through third switches) and the resistors of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 21 is a diagram showing the relation between the composite resistance values and the closing of the first through third switches of the circuit arrangement of FIG. 20 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 22 is a diagram showing the initial state of the impact detecting device of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 23 is a diagram showing the initial state of the impact detecting device of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 24 is a diagram showing the state when the weight rotates to come in contact with the sub rotor of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 25 is a diagram showing the state when the weight rotates to come in contact with the sub rotor of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 26 is a diagram showing the state when the cam  52  rotates, causing the moving contact  85  to become in contact with the cam surface  52   b  of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 27 is a diagram showing the state of the cam  53  when the cam  52  rotates, causing the moving contact  85  to come in contact with the cam surface  52   b  of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 28 is a diagram showing the state of contact of the moving contact  86   a  with the fixed contact  83  of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 29 is a diagram showing the contact of the moving contact  86   b  with the fixed contact  84  of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 30 is a diagram showing the state when the moving contact  86   b  becomes in contact with the cam surface  54   a  of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 31 is a diagram showing the state of the cam  52  and moving contact  85  when the moving contact  86   b  becomes in contact with the cam surface  54   a  of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 32 is a diagram showing the state when the moving contact  86   b  has come in contact with the cam surface  54   a  of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 33 is a graph showing the relation between the exertion force acting on the main rotor and the rotation value of the main rotor based on the first embodiment, with the closing of the first through third switches (first through third impact levels) being plotted as parameter of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 34 is a schematic circuit diagram of the conventional circuit arrangement used to explain the advantage of the circuit arrangement of FIG. 20 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 35 is a graph showing the variation in time of the impact detecting level for explaining the problem of the circuit arrangement of FIG. 34 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 36 is a graph showing the variation in time of the impact detecting level for explaining the advantage of the circuit arrangement of FIG. 20 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 37 is a graph used to explain the advantage of the inventive impact detecting device in contrast to the conventional impact detecting device of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 38 is a diagram taken along the line  38 — 38  of FIG. 40 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 39 is a diagram taken along the line  39 — 39  of FIG. 38 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 40 is a diagram taken along the line  40 — 40  of FIG. 38 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 41 is a diagram taken along the line  41 — 41  of FIG. 43 of the third embodiment of this invention of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 42 is a diagram taken along the line  42 — 42  of FIG. 41 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 43 is a diagram taken along the line  43 — 43  of FIG. 41 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 44 is a diagram taken along the line  44 — 44  of FIG. 42 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 45 is a diagram taken along the line  45 — 45  of FIG. 47 of the fourth embodiment of this invention of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 46 is a diagram taken along the line  46 — 46  of FIG. 45 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 47 is a diagram taken along the line  47 — 47  of FIG. 45 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 48 is a diagram taken along the line  48 — 48  of FIG. 46 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 49 is a diagram taken along the line  49 — 49  of FIG. 51 of the fifth embodiment of this invention of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 50 is a diagram taken along the line  50 — 50  of FIG. 49 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 51 is a diagram taken along the line  51 — 51  of FIG. 49 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 52 is a diagram taken along the line  52 — 52  of FIG. 50 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 53 is a diagram taken along the line  53 — 53  of FIG. 55 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 54 is a diagram taken along the line  54 — 54  of FIG. 53 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 55 A diagram taken along the line  55 — 55  of FIG. 53 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 56 is a diagram taken along the line  56 — 56  of FIG. 54 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 57 is a longitudinal cross-sectional diagram of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 58 is a diagram taken along the line  58 — 58  of FIG. 57 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 59 is a diagram taken along the line  59 — 59  of FIG. 57 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 60 is a longitudinal cross-sectional diagram showing the eighth embodiment of this invention of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 61 is a diagram taken along the line  61 — 61  of FIG. 60 of an electro-mechanical impact detecting device according to the present invention; 
     FIG. 62 is a diagram taken along the line  62 — 62  of FIG. 60 of an electro-mechanical impact detecting device according to the present invention; and 
     FIG. 63 is a diagram taken along the line  63 — 63  of FIG. 60 of an electro-mechanical impact detecting device according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG.  1  through FIG. 3 show a first embodiment of the electro-mechanical impact detecting device based on the present invention. This device is preferably for automobile air-bag systems. The device has an outer housing  10  and an inner housing  20 , of which the housing  10  is fixed to the vehicle body by a bracket  11  which is attached to the lower wall of the housing. 
     Inner housing  20  is fitted in the outer housing  10  as shown in FIG.  1  through FIG.  3 . The housing  20  has a connector  20   b  which extends from and is integral with a housing section  20   a , and the housing section  20   a  is located on the bottom of the housing  10 . The connector  20   b  is located in the opening section of the housing  10 . The connector  20   b  confronts the outside at its connecting section  21  through an opening  12  of the housing  10 . In FIG. 2, symbol  22  indicates terminals of the connector  20   b . In FIG.  1  and FIG. 2, symbol  10   a  indicates hermetic filling material. 
     This impact detecting device has a main body A, which is fitted in both housings  10  and  20  as shown in FIG.  1  through FIG.  3 . The device main body A includes a mechanical section Aa and an electrical circuit section Ab. The mechanical section Aa is fitted in the housing section  20   a  of the inner housing  20 , and the electrical circuit section Ab is fitted in the housing  10  on the lower wall of the housing section  20   a.    
     The mechanical section Aa includes a casing  30 , a rotation shaft  40 , a main rotor  50 , an auxiliary (sub) rotor  60 , a torsional coil spring  70 , and a contact mechanism  80 . The casing  30  is fitted in the housing section  20   a . The casing  30  is made of electrically insulating synthetic resin, which is shaped as shown in FIG.  4  through FIG.  9 . The casing  30  is seated by being coupled at its rectangular annular root section  31  (refer to FIG.  5  through FIG. 9) downward in FIG.  1  through FIG. 3 onto a base  81  of the contact mechanism  80  (will be explained later). 
     The rotation shaft  40  has both ends pivotally mounted in recess sections  32   a  at the top of support columns  32  (refer to FIG.  5  through FIG. 7) of the casing  30 . The main rotor  50  is coupled concentrically with the rotation shaft  40  together with the sub rotor  60  and torsional coil spring  70 . 
     The main rotor  50  has a plate weight  51  and plate cams  52  and  53 . The weight  51  is shaped so that the weight center is eccentric from the rotation center as shown in FIG.  1  through FIG.  10 . Specifically, the weight  51  is a stepped cylindrical boss  51   a  located at the rotation center. A weight section  51   b  is provided which causes the weight center to be eccentric from the boss  51   a . The weight  51  is coupled concentrically to the left-side section in FIG. 1 of the rotation shaft  40  by means of the boss  51   a , so that the weight section  51   b  is located below the rotation shaft  40 . 
     Accordingly, the weight  51  having the eccentric weight center from the rotation center locks initially at the shoulder section  15   c  of the weight section  51   b  upward against the upper-end stopper  33  of the casing  30  (refer to FIG.  2  and FIG.  5 ). The upper-end stopper  33  works for the initial stopper of the weight  51 . 
     The cam  52  is formed integral with the weight section  51   b  to extend along the left-side plane of the weight section  51   b  from a small-diameter section of the boss  51   a  in FIG.  10 . Also, the cam  52  is formed with a plate shape as shown in FIG.  10  and FIG.  11 . The cam  52  has two cam surfaces  52   a  and  52   b , and the cam surface  52   b  has an arcuate profile which is centered by the rotation center of the boss  51   a , i.e., the rotation shaft  40 . The cam surface  52   a  has a planar shape to crisscross the cam surface  52   b  right-upwardly from the right extreme of FIG.  11 . 
     The cam  53  is formed integral with the weight section  51   b  to extend along the right-side plane of the weight section  51   b  from a large-diameter section of the boss  51   a  in FIG.  10 . The cam  53  is formed as an L-shape plate as shown in FIG.  10  and FIG.  12 . The cam  53  has two cam sections  54  and  55 , with the cam section  54  being located on the right of the cam section  55  in FIG.  12 . 
     The cam section  54  extends a length longer than the cam section  55  to the right from the right-side plane of the weight section  51   b  as shown in FIG.  10 . The cam section  54  has an arcuate cam surface  54   a  centered by the rotation shaft of weight  51 . The cam section  55  has an arcuate cam surface  55   a  centered by the rotation shaft of the weight  51 . 
     The sub rotor  60  is coupled concentrically to the rotation shaft  40  in its section between the main rotor  50  and torsional coil spring  70  as shown in FIG.  1 . The sub rotor  60  has a plate rotor section  61 , a cylindrical boss section  62 , an arm section  63  and a trapezoidal coupling section  64 . These elements are formed as integral members as shown in FIG.  13  through FIG.  15 . 
     The boss section  62  is normal to the plane of the plate rotor section  61 , and the boss section  62  is coupled concentrically to the rotation shaft  40 . The arm section  63  extends from the left-side plane of the rotor section  61  in FIG. 13, parallel to the axis of the boss section  62  and in the direction opposite to the boss section  62 . The trapezoidal coupling section  64  is formed on the rotor section  61  on the same side as the boss section  62  and at the position with respect to the boss section  62  shown in FIG.  13  through FIG.  15 . 
     The torsional coil spring  70  is coupled concentrically to the rotation shaft  40  at its section between the sub rotor  60  and a side support section  32  of the casing  30  as shown in FIG.  1 . The torsional coil spring  70  has one end section  71  stopped in a stop hole section  63   a  of the arm section  63  of the sub rotor  60 . The torsional coil spring  70  has another end  72  stopped in a stop hole  34   a  which is formed in the wall section  34  of the casing  30  (refer to FIG. 1, FIG. 3, FIG.  4  and FIG.  7 ). As a consequence, the torsional coil spring  70  has a torsional force exertion on the sub rotor  60  in the rotation direction downwardly in FIG. 1 (clockwise direction of the main rotor  50  in FIG. 2) based on the stop hole  34   a.    
     The contact mechanism  80  has a base  81  as shown in FIG.  1  through FIG.  3 . This base  81  is coupled into the root section  31  of the casing  30  as mentioned above. The contact mechanism  80  has fixed contacts  82 ,  83  and  84  formed of elongate plates and moving contacts  85  and  86  formed of elongate plates as shown in FIG. 1, FIG.  2  and FIG.  16  through FIG.  19 . 
     The fixed contact  82  is fixed into the base  81  in its thickness direction on the left side of the main rotor  50  in FIG. 1, together with the moving contact  85 . The fixed contacts  83  and  84  are fixed into the base  81  in its thickness direction on the right side of the main rotor  50  in FIG. 1 together with the moving contact  86 . The fixed contacts  82 ,  83  and  84  are formed of a rigid, electro-conductive, metallic material, and the moving contacts  85  and  86  are formed of an electro-conductive spring material. 
     The moving contact  85  is located to confront the fixed contact  82 , and constitutes a normally-open switch (will be called a first switch hereinafter) in unison with the fixed contact  82 . The moving contact  86  has split moving contact sections  86   a  and  86   b , and these moving contact sections  86   a  and  86   b  are located to confront the fixed contacts  83  and  84  and constitute normally-open switches (will be called second and third switches hereinafter) in unison with the fixed contacts  83  and  84 , respectively. 
     The fixed contacts  82 ,  83  and  84  have their upper tip sections  82   a ,  83   a  and  84   a  bent in the counterclockwise rotation direction of the main rotor  50  in FIG.  2 . The moving contact  85  has its tip section  85   a  bent with an L-shape toward the tip section  82   a  of the fixed contact  82 , and the moving contact  86  has its moving contact sections  86   a  and  86   b  bent to have an L-shape as shown in FIG.  17 . 
     The fixed contacts  83  and  84  have their upper tip sections  83   a  and  84   a  formed with an L-shape configuration. The fixed contact  82  has its upper tip section  82   a  bent less than the upper tip sections  83   a  and  84   a . The fixed contacts  82 ,  83  and  84  and the moving contacts  85  and  86  are oriented in a thickness direction to the counterclockwise rotation direction of the main rotor  50  in FIG.  2 . The moving contact  86  locks upward in FIG. 3 against a stopper  35  of the casing  30  against the resilient force of the moving contact sections  86   a  and  86   b  to prevent chattering. 
     The electrical circuit section Ab is provided with a rid  90  having a U-shaped cross section, and a printed circuit board  100  mounted inside. Planted on the printed circuit board  100  are fixed contacts  82 ,  83  and  84  and moving contacts  85  and  86  of the contact mechanism  80 , which are connected electrically to the wiring section of the printed circuit board  100 . 
     The electrical circuit section Ab has resistors R 1  through R 3  as shown in FIG.  1  through FIG.  3  and FIG.  20 . Resistors R 1  through R 3  are connected in series. The resistor R 1  is connected between the lower end sections of the fixed contact  82  and moving contact  85  by way of the printed circuit board  100 . 
     The resistor R 2  is connected at one end to the moving contact section  86   a  of the moving contact  86  by resistor R 1 , the lower end section of the moving contact  86  and the printed circuit board  100 . The resistor R 2  has another end connected to the lower end section of the fixed contact  83  by printed circuit board  100 . The resistor R 3  is connected at one end to the moving contact section  86   b  of the moving contact  86  by resistors R 2  and R 1 , the lower end section of the moving contact  86  and the printed circuit board  100 . The resistor R 3  has another end connected to the lower end section of the fixed contact  84  by printed circuit board  100 . 
     Assuming the resistors R 1 , R 2  and R 3  have resistance values r 1 , r 2  and r 3 , respectively, when the first switch formed of the moving contact  85  and fixed contact  82 , the second switch formed of the moving contact section  86   a  of the moving contact  86  and fixed contact  83 , and the third switch formed of the moving contact section  86   b  and fixed contact  84  are all open, the electrical circuit section Ab has a composite resistance R which is equal to the sum of the r 1 , r 2  and r 3  (refer to FIG.  21 ). When only the first switch is closed, the composite resistance R is equal to the sum of r 2  and r 3  (refer to FIG.  21 ). When the second switch is closed, regardless of the state of the first switch, the composite resistance R is equal to the r 3  (refer to FIG.  21 ). When the third switch is closed, the composite resistance is zero (refer to FIG.  21 ). 
     In the first embodiment as described above, when the main rotor  50  has a position (initial position) shown in FIG. 2, FIG.  22  and FIG. 23, the weight  51  locks upward at its shoulder section  15   c  against the upper-end stopper  33  of the casing  30  (refer to FIG.  2  and FIG.  22 ). At this time, the first through third switches are all open, with the tip section  85   a  of the moving contact  85  having its root section being in right-to-left contact with the cam surface  52   a  of the cam  52  of the main rotor  50 . 
     In this state, if the vehicle in a running state comes to a sudden stop as in the case of a collision, the vehicle decelerates. When force caused by the deceleration acts on the main rotor  50  rightward in FIG. 2, the weight  51  has a moment of inertia at the weight center due to the eccentric weight center of the weight  51  from the axis of the rotation shaft  40  (which is also the rotation center of the weight  51 ). As a result, it begins to rotate counterclockwise in FIG. 24 about the rotation shaft  40  axis. 
     Accordingly, the tip section  85   a  of the moving contact  85  has its root section being in right-to-left contact with the cam surface  52   a , and the tip section  85   a  of the moving contact  85  is pushed rightward and deformed elastically by the cam surface  52   a  as the main rotor  50  rotates in the counterclockwise direction. Due to the displacement, the moving contact  85  contacts, at its tip section  85   a , with the tip section  82   a  of the fixed contact  82  (refer to FIG.  24 ). As the main rotor  50  further rotates in the same direction, the cam  52  increases the contact force at its cam surface  52  between the tip section  85   a  of the moving contact  85  and the tip section  82   a  of the fixed contact  82 . When the displacement of the moving contact  85  due to the increased contact force reaches a certain value, the main rotor  50  contacts, at its weight  51 , with the arm  63  of the sub rotor  60  (refer to FIG.  25 ). 
     When main rotor  50  further rotates in the same direction, it is subjected to torsional force by torsional coil spring  70  in the opposite rotation direction. If the main rotor  50  further rotates in the same direction against the torsional force of the torsional coil spring  70 , the tip section  85   a  of the moving contact  85 , in contact with the cam surface  52   a , begins to leave the cam surface  52   a  and contact cam surface  52   b  (refer to FIG.  26 ). Since the cam surface  52   b  has an arcuate profile, centered by the rotation center of the main rotor  50 , the moving contact  85  does not increase the bend any longer against the cam surface  52   b . Accordingly, out of forces acting on the main rotor  50  by the moving contact  85 , the exertion force becomes zero and there is only a frictional force between the moving contact  85  and the cam surface  52   b  of the cam  52 . 
     If it is assumed that moving contact  85  goes on deviating with the rotation of the main rotor  50  after contacting cam surface  52   b , the main rotor  50  would be subjected to the exertion force of the moving contact  85  and the frictional force of the moving contact  85  on the cam surface  52   b , in addition to the torsional force of the torsional coil spring  70 . In consideration of the fluctuation of the second and third impact levels (refer to FIG. 33) to be detected by the impact detecting device, it is advantageous to make the number of forces acting on the main rotor  50  as small as possible. Accordingly, in this embodiment, to reduce the number of forces acting on the main rotor  50 , the cam surface  52   b  to contacting the moving contact  85  is formed to have an arcuate profile centered by the rotation center of the main rotor  50  as described above. 
     When the main rotor  50  further rotates in the same direction, the cam section  55  abuts the moving contact section  86   a , causing the moving contact section  86   a  to deviate in position. When the main rotor  50  further rotates in the same direction, moving contact section  86   a  contacts the tip section  83   a  of the fixed contact  83  (refer to FIG.  27 ). When the main rotor  50  further rotates in the same direction, the moving contact section  86   a  increases the contact force to the tip section  83   a  of the fixed contact  83 . When the main rotor  50  further rotates by a certain amount, the moving contact section  86   a  of the moving contact  86  contacts the cam surface  52   a  of the cam section  55 ) (refer to FIG.  28 ). This is as effective as transitioning from contacting the moving contact  85  with the cam surface  52   a  to contacting the cam surface  52   b.    
     When the main rotor  50  further rotates in the same direction, the cam section  54  abuts against the moving contact section  86   b  of the moving contact  86 , causing the moving contact section  86   b  to deviate. When the main rotor  50  further rotates in the same direction, the moving contact  86  contacts, at its moving contact section  86   b , with the tip section  84   a  of the fixed contact  84  (refer to FIG.  29 ). 
     When the main rotor  50  further rotates in the same direction, the contact pressure of the moving contact section  86   b  against the tip section  84   a  of the fixed contact  84  increases. When the main rotor  50  further rotates in the same direction, the moving contact  86  contacts, at its moving contact section  86   b , the cam surface  54   a  of the cam section  54  (refer to FIG.  30  through FIG.  32 ). This is as effective as transitioning from contacting the moving contact  85  with the cam surface  52   a  to contacting the cam surface  52   b.    
     The foregoing is summarized in terms of relation A between the exertion force acting on the main rotor  50  and the rotation value of the main rotor  50  as shown in FIG.  33 . In the figure, gradient y 1 /x 1  indicates the spring constant of the moving contact  85 , and y 2 /x 2  indicates the spring constant of the torsional coil spring  70  which is greater than y 1 /x 1 . Symbol a indicates the closing position of the first switch (moving contact  85  contacting the fixed contact  82 ), symbol b indicates the closing position of the second switch (moving contact section  86   a  contacting the fixed contact  83 ), and symbol c indicates the closing position of the third switch (moving contact section  86   b  contacting the fixed contact  84 ). 
     FIG. 33 reveals that where the moving contact  85  exerts a force on the main rotor  50  out of the whole rotation range of the main rotor  50 , the exertion force of the moving contact  85  acting on the main rotor  50  increases along line Al proportional to the rotation value of the main rotor  50  at a rate of the spring constant y 1 /x 1 . At position a immediately before the torsional coil spring  70  begins to exert a force on the main rotor  50 , the flat-spring switch is closed. This closing position coincides with the first impact level detected by the impact detecting device. 
     The exertion force on the main rotor  50  increases sharply along line A 2  up to the initial exertion force of the torsional coil spring  70 . Thereafter, it increases along line A 3  at a rate of the spring constant y 2 /x 2  as the main rotor  50  rotates. At rotation positions b and c while increasing force along the line A 3 , the second and third switches are closed sequentially. Among these closing positions, the closing position b of the second switch coincides with the second impact level to be detected by the impact detecting device, and the closing position c of the third switch coincides with the third impact level to be detected by the impact detecting device. 
     Since the torsional coil spring  70  has one end section  71  inserted into the stop hole section  63   a  of the arm section  63  of the sub rotor  60  and another end inserted into the stop hole section  34   a  of the casing  30 , as described above, the arm  71  of the torsional coil spring  70  is untwisted at the rotation of the main rotor  50 , whereby the operational fluctuation of the impact detecting device is reduced. 
     Since the first through third switches are connected to the resistors R 1  through R 3  as described above and shown in FIG. 20, the composite resistance R decreases in steps as the impact level varies from the first through third levels as shown in FIG.  21 . Accordingly, by utilizing this change of composite resistance R, impact detection for the colliding vehicle can be done in three steps (or four steps inclusive of the off state). 
     For the first through third switches connected to the resistors R 1  through R 3  as shown in FIG. 34, if for example the first switch opens for some reason during operation of the impact detecting device at the second impact level, the signal at the resistance of the first impact level is released as shown by symbol P in FIG. 35, and it can be a cause of erroneous detection. 
     In contrast, based on the connection as shown in FIG. 20, even if the first switch opens by some reason during the operation of the impact detecting device at the second impact level, the signal is released at the resistance of the second impact level, and erroneous detection does not take place (refer to symbol Q in FIG.  36 ). 
     Next, a second embodiment of the present invention will be explained with reference to FIG.  38  through FIG.  40 . In the second embodiment, the sub rotor  60  described in the first embodiment is eliminated, and the material is changed and the wire diameter is increased for the torsional coil spring  70  so that the operational fluctuation of the impact detecting device caused by the twist at the end of the torsional coil spring  70  during the rotation of the main rotor  50  is reduced based on the rigidity of the torsional coil spring  70  itself. 
     The torsional coil spring  70  has one end  71  inserted into a long hole section  51   b  which is formed in the arcuate direction in the weight  51  of the main rotor  50 , in place of the sub rotor  60  described in the first embodiment, and the end  71  of the torsional coil spring  70  abuts at (in FIG. 40) at its rightward root section against the stopper  34   b  of the casing  30 . 
     Consequently, as main rotor  50  rotates by a certain amount, the end  71  of the torsional coil spring  70  abuts against one rotation end of the interior of the long hole section  51   b . The torsional coil spring  70  has another end  72  stopped by the stop hole section  34   a  of the casing  30  in the same manner as the first embodiment. The torsional coil spring  70  is stopped as mentioned above by having an exertion force produced by a certain twist angle. The remaining structure is virtually identical to the first embodiment. 
     In the second embodiment arranged as described above, when the vehicle undergoes a certain deceleration, the main rotor  50  rotates, causing the moving contact  85  to contact the fixed contact  82  and thus increases the contact force on the fixed contact  82  in the same manner as the first embodiment. When the main rotor  50  further rotates in the same direction by a certain amount, the long hole section  51   b  of the weight  51  contacts, at part of its interior surface, with the one end  71  of the torsional coil spring  70 . 
     When the main rotor  50  further rotates in the same direction, the main rotor  50  is subjected to an exertion force of the torsional coil spring  70 . During this time, the torsional coil spring  70 , which has a high rigidity due to an increased wire diameter or the like, does not have twisting in its end section  71  during elastic deformation from pushing by main rotor  50 . Accordingly, the post-operational fluctuation of the impact detecting device can be suppressed. Referring to FIG.  41  through FIG. 44, the impact detecting device of a third embodiment has an outer housing  100  and an inner housing  110  as shown in FIG.  41  through FIG.  44 . Housing  100  is fixed to the vehicle body at its proper location by a bracket  101  that is attached to the lower wall of the housing. 
     The inner housing  110  is fitted in the outer housing  100  as shown in FIG.  41  through FIG.  43 . The housing  110  has a connector  110   b  that extends from and is integral with housing section  110   a . The housing section  110   a  is located on the bottom of the housing  100  and the connector  110   b  is located in the opening section of the housing  100 . The connector  110   b  confronts the outside at its connecting section  111  through an opening  102  of the housing  100 . In FIG. 42, terminals  112  of the connector  110   b  are shown. 
     This impact detecting device has a device main body B, which is fitted in both housings  100  and  110  as shown in FIG.  41  through FIG.  43 . The device main body B includes a mechanical section Ba and an electrical circuit section Bb. The mechanical section Ba is fitted in the housing section  110   a  of the inner housing  110 , and the electrical circuit section Bb is fitted in the housing  100  on the lower wall of the housing section  110   a . The mechanical section Ba includes a casing  120 , a rotation shaft  130 , a rotor  140 , a contact mechanism  150 , and a flat-spring mechanism  160 . The casing  120  is fitted in the housing section  110   a . The casing  120  is made of a electrically insulating synthetic resin. This casing is shaped as shown in FIG.  41  through FIG. 44, and is seated by being coupled at its rectangular annular root section  121  downward in FIG.  41  and FIG. 42 onto a base  151  of the contact mechanism  150  (will be explained later). 
     The rotation shaft  130  has both ends pivotally mounted between the top sections of the support columns  122  and  123  of the casing  120 . The rotor  140  is coupled concentrically with rotation shaft  130 , and the rotor  140  has plate weight  141 , contact cams  142  through  144 , and exerting cams  145  and  146  integrally formed. 
     The weight  51  is plate shaped such that the weight center is eccentric from the rotation center (FIG.  42  and FIG.  43 . Specifically, the weight  141  has a cylindrical boss  141   a  located at the rotation center, and a weight section  141   b  which causes the weight center to be eccentric from the boss  141   a . The weight  141  is coupled concentrically to the rotation shaft  130  by means of the boss  141   a , so that the weight section  141   b  is located below the rotation shaft  130 . Accordingly, the weight  141  initially abuts obliquely from the top-left side at a protruding section  141   c  of the weight section  141   b  against the tip slant surface (refer to FIG. 43) of a stopper  151   a  of the base  151 . The stopper  151   a  works for the initial stopper of the weight  141 . 
     The contact cams  142  through  144  extend along the left-side plane of the weight  141  downward to the circumferential plane of the left-side section in FIG. 41 of the boss  141   a  (refer to FIG.  42 ). These contact cams  142  through  144  are located by being more distant in this order from the left-side plane in FIG. 41 of the weight  141 . The contact cam  142  has two cam surfaces  142   a  and  142   b , the contact cam  143  has two cam surfaces  143   a  and  143   b , and the contact cam  144  has two cam surfaces  144   a  and  144   b . The cam surfaces  142   a ,  143   a  and  144   a  are located to shift sequentially to the left in FIG. 42, and the cam surfaces  142   b ,  143   b  and  144   b  have a same arcuate profile centered by the axis of the rotation shaft  130 . 
     The exerting cams  145  and  146  extend along the right-side plane of weight  141 , downward to the circumferential surface on the right in FIG. 41 of the boss  141   a  (refer to FIG.  43 ). These exerting cams  145  and  146  are more distant in this order from the right-side plane in FIG. 41 of the weight  141 . The exerting cams  145  and  146  have cam surfaces  145   a  and  146   a , respectively, which face to the left in FIG. 43, with the cam surface  145   a  shifting in position to the left more than the cam surface  146   a.    
     The contact mechanism  150  has a base  151  as shown in FIG.  41  through FIG.  43 . The base  151  is fixed into the rectangular annular root section  121  as mentioned previously. The contact mechanism  150  has fixed contacts  152 ,  153  and  154  formed of elongate plates and moving contacts  155 ,  156  and  157  formed of elongate plates as shown in FIG.  41  through FIG.  44 . 
     The fixed contact  152  in unison with the moving contact  155  constitutes the above-mentioned first switch, the fixed contact  153  in unison with the moving contact  156  constitutes the above-mentioned second switch, and the fixed contact  154  in unison with the moving contact  157  constitutes the above-mentioned third switch. The fixed contacts  152 ,  153  and  154  in parallel alignment are fed through the right-side wall  124  in FIG.  42  and FIG. 43 of the causing  120  and fixed into base  151 . These fixed contacts  152 ,  153  and  154  have their contact sections  152   a ,  153   a  and  154   a  extending in an L-shape fashion from the upper end of the right-side wall to the left. The moving contacts  155 ,  156  and  157  in parallel alignment are fed through the left-side wall  125  in FIG. 42 of the base  151  and fixed into the base  151 . These moving contacts  155 ,  156  and  157  have their contact sections  155   a ,  156   a  and  157   a  extending in an L-shape fashion from the upper end of the left-side wall, thereby confronting the fixed contacts  152 ,  153  and  154 . 
     The contact sections  155   a ,  156   a  and  157   a  extend along the upper face (refer to FIG. 42) of the contact sections  152   a ,  153   a  and  154   a  and have their tip section stopped in a preload stopper  124   a  which is located immediately above the right-side wall of the base  151 . Thus, they are subjected to a predetermined downward loading in advance. 
     The moving contacts  155 ,  156  and  157  have their contact sections  155   a ,  156   a  and  157   a  located immediately below the contact cams  142  through  144  of the rotor  140 . These contact sections  155   a ,  156   a  and  157   a  are pushed at their L-shaped protruding sections (refer to FIG. 42) by the cam surface of the contact cams  142  through  144  to contact the contact sections  152   a ,  153   a  and  154   a  of the fixed contacts  152 ,  153  and  154 . 
     The flat-spring mechanism  160  has exerting flat-springs  161  and  162  as shown in FIG.  41  through FIG.  44 . These springs  161  and  162  are planted at their root section on the right-side section of the base  151  as shown in FIG.  43 . The exerting flat-springs  161  and  162  coming from the right-side section of the base  151  run immediately below the exerting cams  145  and  146  of the rotor  140  and extend up-rightward obliquely. As a result, the exerting flat-spring  161  is subjected to an exertion force obliquely down-leftward in FIG. 43 by the cam surface  43  of the exerting cam  145 . The flat exerting spring  162  is subjected to an exertion force obliquely down-leftward in FIG. 43 by the cam surface  43  of the exerting cam  145 . 
     The electrical circuit section Bb is provided with a dish-shaped rid  170  as shown in FIG.  41  through FIG. 43. A printed circuit board  180  is mounted in the lower opening section of the inner housing  110  and located immediately above the rid  170 . Planted on the printed circuit board  180  are fixed contacts  152 ,  153  and  154  and moving contacts  155 ,  156  and  157 , which are connected electrically to the wiring section of the printed circuit board  180 . 
     The electrical circuit section Bb includes resistors  190   a  through  190   c . These resistors  190   a  through  190   c  are connected to the wiring section of the printed circuit board  180 . The resistor  190   a  mates with the fixed contact  152  and moving contact  155 . The resistor  190   b  mates with the fixed contact  153  and moving contact  156 . The resistor  190   c  mates with the fixed contact  154  and moving contact  157 . The resistors  190   a ,  190   b  and  190   c  are equivalent to the resistors R 1 , R 2  and R 3 , respectively, described in the first embodiment. The fixed contacts  152 ,  153  and  154  correspond to the fixed contacts  82 ,  83  and  84 , respectively, of the first embodiment. The moving contacts  155 ,  156  and  157  correspond to the contact sections  86   a  and  86   b  of the moving contacts  85  and  86 , respectively. To satisfy these relationships, the third embodiment has a wiring circuit arrangement as shown in FIG.  20 . In FIG. 42, symbol  100   a  denotes hermetic filling material. 
     In the third embodiment, when the vehicle decelerates, the rotor  140  rotates clockwise in FIG. 43 about the axis of rotation of shaft  130 . Since the exerting flat-spring  161  contacts, at its tip, the cam surface  145   a  of the exerting cam  145 , the tip of the exerting flat-spring  161  deviates to the left in FIG.  43 . 
     When the rotor  140  further rotates in the same direction, the surface of the contact cam  142  abuts cam surface  142   a  against the protruding section of the moving contact  155 . This causes the contact section  155   a  to bend downward as shown by the double-dash line in FIG.  42 . Consequently, the contact section  155   a  of the moving contact  155  gradually approaches the contact section  152   a  of the fixed contact  152 . It eventually contacts the contact section  152   a.    
     When the rotor  140  further rotates in the same direction, the force by the contact section  155   a  acting on the contact section  152   a  increases. After the rotor  140  has rotated by a certain amount, the surface of exerting cam  146  contacts the tip of flat exerting spring  162 . When the rotor  140  further rotates, it will be subjected to the exertion force of the flat exerting spring  162 . 
     When the rotor  140  further rotates by a certain amount in the same direction, the protruding section of moving contact  155  exits the cam surface  142   a  and contacts the cam surface  142   b . Since the cam surface  142   b  has an arcuate profile centered by the axis of rotation of shaft  130 , the downward bending displacement of the moving contact  155  following this contact transition becomes zero. Also, the moving contact  155  stays in contact with the fixed contact  152  at certain contact force. Accordingly, the force of the moving contact  155  acting on the rotor  140  resulting from the rotation of the rotor  140  is only a frictional force between the moving contact  155  and the cam surface  142   b.    
     If the moving contact  155  continues to bend downward at its contact section  155   a  contacting the protruding section of the cam surface  142 , the moving contact  155  would exert a force from friction between moving contact  155  and contact cam  142  and from flat springs  161  and  162  on the rotor  140 . 
     Because of fluctuations of the second and third impact levels detected by the impact detecting device of the third embodiment, it is desirable to reduce the number of forces acting on the rotor  140 . Accordingly, in this embodiment, the cam surface  142   b  contacting the moving contact  155  has an arcuate profile. When the rotor  140  further rotates in the same direction, the contact cam  143  abuts the protruding section of the moving contact  156 , causing the contact section  156   a  to bend and deviate downward. When the rotor  140  further rotates by a certain amount, the moving contact  156  eventually comes in contact, at its contact section  156   a , with the contact section  153   a  of the fixed contact  153 . 
     When the rotor  140  further rotates in the same direction, the contact force of the contact section  156   a  acting on the contact section  153   a  of the fixed contact  153  increases. After the rotor  140  has rotated by a certain amount in the same direction, the protruding section of the moving contact  156  leaves the cam surface  143   a  and contacts the cam surface  143   b  of the contact cam  143 . To reduce the number of forces acting on the rotor  140 , the cam surface  143   b , for similar reasons as cam surface  142   b , has an arcuate profile. When the rotor  140  further rotates in the same direction, the contact cam  144  abuts the protruding section of the moving contact  157 , causing the contact section  157   a  to bend and deviate downward. When the rotor  140  further rotates by a certain amount, the moving contact  157 , at its contact section  157   a , eventually contacts the contact section  154   a  of the fixed contact  154 . 
     When the rotor  140  further rotates in the same direction, the contact force of contact section  157   a  acting on the contact section  154   a  increases. After the rotor  140  has rotated by a certain amount in the same direction, the protruding section of moving contact  157  leaves the cam surface  144   a  and contacts cam surface  144   b . To reduce forces, the cam surface  144   b  has an arcuate profile. 
     In the third embodiment, when the rotor  140  is within the rotation range where it is subjected to the exertion force by flat-spring  161 , the impact detecting device operates at the first impact level mentioned in the first embodiment. The device operates at the second and third impact levels mentioned in the first embodiment when the rotor  140  is within the rotation range in which it is subjected to the exertion force of the flat exerting spring  162 . The rotation range of the rotor  140 , the exertion force acting on the rotor  140 , and the closing positions of the moving contacts and fixed contacts (closing positions of the first through third switches) at the impact levels are then identical to the case shown in FIG.  33 . Also, by increasing the exertion force and spring constant of the flat exerting spring  162  relative to flat-spring  161 , the first through third impact levels can be altered. 
     FIG.  45  through FIG. 48 show the fourth embodiment of the impact detecting device according to the present invention. Here, the flat-spring mechanism  160 , has its flat exerting spring  162  extending from the base  151  to confront the exerting flat-spring  161  on the left-side plane thereof in FIG.  47 . The positions on the base  151  from which the exerting flat-springs  161  and  162  extend have a certain distance L as shown in FIG.  47 . The flat exerting spring  162  has a V-shaped protruding section  162   a  at its mid position which points to the tip section of the exerting flat-spring  161 . Because of this alteration, the contact cam  146  of the rotor  140  third embodiment is eliminated. The remaining arrangement is identical to the third embodiment. 
     When the rotor  140  rotates in response to a certain deceleration of the vehicle, the exerting flat-spring  161  is pushed at its tip section by the exerting cam  145  to deviate to the left in FIG.  47 . The rotor  140  further rotates in the same direction, causing the moving contact  155  to contact the fixed contact  152  the same as in the third embodiment. When the rotor  140  further rotates, the tip section of the exerting flat-spring  161  abuts the protruding section  162   a  of the flat exerting spring  162 . When the rotor  140  further rotates in the same direction, the rotor  140  is subjected to the exertion forces of the two exerting flat-springs  161  and  162 , to the right in FIG. 47, through the exerting cam  145 . The operation of the impact detecting device in the successive rotation of the rotor  140  in the same direction is identical to the third embodiment. 
     Placing the flat exerting spring  162  to confront the exerting flat-spring  161  on the left-side plane thereof in FIG. 47 makes the impact detecting device compact. Specifically, placing the flat exerting spring  162  parallel to the exerting flat-spring  161 , as described in the third embodiment, keeps the flat-springs  161  and  162  from contacting each other during operation. Although the impact detecting device is free from the operational fluctuation caused by the frictional force at the contact of the two springs  161  and  162 , the device must have a larger lateral dimension (axial direction of the rotation shaft  130 ). This makes it difficult to install in a small vehicle space. 
     In contrast, according to the fourth embodiment, in which the flat exerting spring  162  confronts the exerting flat-spring  161  on the left-side plane thereof in FIG. 47, the two springs  161  and  162  share lateral space. Accordingly, the impact detecting device has a smaller external lateral dimension. 
     However, during operation, the two exerting flat-springs  161  and  162  contact each other, with the contact point moving as the rotor  140  rotates. As a result, a frictional force is generated between the two exerting flat-springs  161  and  162 . This frictional force increases the operational fluctuation of the impact detecting device. Therefore, it is desirable to reduce this force to make the exertion forces and spring constants of the exerting flat-springs  161  and  162  as small as possible. 
     FIG.  49  through FIG. 52 show the fifth embodiment of the electro-mechanical impact detecting device based on this invention. The fifth embodiment employs a casing  200 , a rotor  210 , a contact mechanism  220  and a flat-spring mechanism  230 . Casing  200 , which replaces casing  120 , is fitted in the housing section  110   a  described in the fourth embodiment. The rotor  210 , which replaces the rotor  140 , is coupled concentrically to the rotation shaft  130 . The rotor  210  has a integral formation of a plate weight  211 , contact cams  212  through  214  and an exerting cam  215  which corresponds to the plate weight  141 , contact cams  142  through  144  and exerting cam  145  of the rotor  140 . The weight  211 , contact cams  212  through  214  and exerting cam  215  have virtually the same functions as of the weight  141 , contact cams  142  through  144  and exerting cam  145 . 
     The contact mechanism  220  has a base  221 , which is coupled into the rectangular annular root section  201  of the casing  200  as shown in FIG.  49  through FIG.  52 . The contact mechanism  220  has fixed contacts  222 ,  223  and  224  formed of elongate plates and moving contacts  225 ,  226  and  227  formed of elongate plates as shown in FIG.  51  and FIG.  52 . 
     The fixed contact  222  in unison with the moving contact  225  constitutes the first switch, the fixed contact  223  in unison with the moving contact  226  constitutes the second switch, and the fixed contact  224  in unison with the moving contact  227  constitutes the third switch. The fixed contacts  222 ,  223  and  224  in parallel alignment are fed through a supporting wall section  221 a and fixed to the base  221 . The moving contacts  225 ,  226  and  227  in parallel alignment are fed through the supporting wall section  221   a  and fixed into the base  221  to confront the fixed contacts  222 ,  223  and  224 , respectively, leftwardly in FIG.  52 . 
     The flat-spring mechanism  230  has flat exerting springs  231  and  232  which are planted at their root section at virtually the center and on both sides of the base  221  as shown in FIG.  51 . The flat exerting spring  231  extends upward from its root section, and is in resilient contact with the exerting cam  215  of the rotor  210  (rightwardly in FIG.  51 ). The flat exerting spring  232  extends up-leftward obliquely so as to confront the flat exerting spring  231  rightwardly in FIG. 51, and the flat exerting spring  232  has a protruding section  232   a  at its mid position, which points to the tip section of the flat exerting springs  231 . The flat exerting springs  231  and  232  have no spacing at their root section. At its tip, the flat exerting spring  232  abuts rightward against a stopper  202  provided on the wall section of the casing  200 . The remaining arrangement is virtually identical to the fourth embodiment. 
     In the fifth embodiment arranged as described above, in which the flat exerting springs  231  and  232  have no spacing at their root section, the flat exerting spring  231  is pushed by the exerting cam  215  of the rotor  210  during operation. The flat exerting spring  232  deviates together with the flat exerting spring  231  without transition of its contact point with the flat exerting spring  231  even after the tip section of the spring  231  contacts protruding section  232   a  of the spring  232 . As such, there is no frictional force between the flat exerting springs  231  and  232 . Accordingly, even if the flat exerting springs  231  and  232  undergo increased exertion force and spring constant, the impact detecting device can operate steadily without friction between the flat exerting springs  231  and  232 . The remaining operation and effectiveness are virtually identical to the fourth embodiment. 
     FIG.  53  through FIG. 56 show the sixth embodiment of the electro-mechanical impact detecting device based on this invention. The sixth embodiment uses the flat exerting spring  231  to move contact  225  (or use the exerting flat-spring  161  described in the fourth embodiment also for moving contact  155 ), and uses the flat exerting spring  232  described in the fifth embodiment for the fixed contact  222  (or uses the flat exerting spring  162  for the moving contact  152 ). This reduces the cost of the impact detecting device by reducing the number of component parts. 
     Therefore, the sixth embodiment removes the contact cam  212  from the rotor  210  of the fifth embodiment. In addition, the fixed contact  222  and moving contact  225  are removed from the contact mechanism  220 . 
     The flat exerting spring  232  described in the fifth embodiment has its root section planted leftward into the base  221  by being spaced out from the root section of the flat exerting spring  231  as shown in FIG.  55 . Due to the removal of the fixed contact  222  and moving contact  225 , the flat exerting springs  231  and  232  substitute these contacts  222  and  225  thereby constituting the first switch. The remaining arrangement is identical to the fifth embodiment. 
     In the sixth embodiment as described above, when the rotor  210  rotates in response to vehicle deceleration, the flat exerting spring  231  is pushed at its tip section by the exerting cam  215  to move left in FIG.  55 . When the rotor  210  further rotates in the same direction, the tip section of flat exerting spring  231  eventually contacts protruding section  232   a  of the flat exerting spring  232 . This closes the first switch. Accordingly, the exerting action between the flat exerting spring  231  and exerting cam  215  is implemented together with the switch closing. This reduces the number of component parts. 
     When the rotor  210  further rotates in the same direction, it is subjected to the exertion forces of the flat exerting springs  231  and  232 . Further rotation of the rotor  210  in the same direction causes moving contact  226  to contact fixed contact  223  and moving contact  227  to contact fixed contact  224  in the same manner as the fifth embodiment. The remaining operation and effectiveness of the impact detecting device is identical to the fifth embodiment. 
     FIG.  57  through FIG. 59 show the seventh embodiment of the electro-mechanical impact detecting device based on this invention. This impact detecting device is adopted in place of the impact detecting device of the first embodiment. This impact detecting device has an outer housing  300  and an inner housing  310 , of which the housing  300  is fixed to the vehicle body at its location by a bracket  301  which is attached to the lower wall of the housing. 
     The inner housing  310  is fitted in the outer housing  300  as shown in FIG.  57 . The housing  310  has a connector  310   b  which extends from and is integral with a housing section  310   a . The housing section  310   a  is located deep within the housing  300  and the connector  310   b  is located in the opening section of the housing  300 . The connector  310   b  confronts the outside at its connecting section  311  through an opening  302  of the housing  300 . In FIG. 57, symbol  312  indicates terminals of the connector  310   b.    
     This impact detecting device has a device main body C, which is fitted in the housing  310  as shown in FIG.  57 . The device main body C includes a mechanical section Ca and an electrical circuit section Cb. The mechanical section Ca is fitted on the interior bottom of the housing section  310   a , and the electrical circuit section Cb is fitted in the inner opening section of the housing section  310   a.    
     The mechanical section Ca includes a casing  330 , a rotation shaft  340 , a main rotor  350 , a sub rotor  360 , two torsional coil springs  370  and  380 , and a contact mechanism  390 . The casing  330  is fitted to the interior bottom of the housing section  310   a . The rotation shaft  340  is supported concentrically between the lower wall of the housing section  310   a  and the base  391  of contact mechanism  390  which is coupled into the opening  331  of the casing  330 . 
     The main rotor  350  pivots concentrically with the rotation shaft  340  on the right in FIG. 57 of the rotation shaft  340  inside the casing  330 . The main rotor  350  is formed of a plate weight, and has arcuate plate shape (a disc with a V-shaped section being cut away as shown in FIG. 59) to position its weight center eccentric from the rotation center. The main rotor  350 , at its cut-off edge  351 , initially abuts stopper  332  under the exertion force of the torsional coil spring  370 , formed on the interior wall of casing  330  to protrude toward the axis of the casing  330  as shown in FIG.  59 . 
     The sub rotor  360  pivots concentrically with rotation shaft  340 , and the protrusion  361  of sub rotor  360  initially abuts rightward. Protrusion  361  is formed outwardly in the radial direction on the circumferential section of sub rotor  360 . This abutment is under the exertion force of the torsional coil spring  378  against a protruding bar  332  (refer to FIGS. 57 and 59) which extends from part of the lower wall of the casing  330  as shown in FIG.  59 . 
     The sub rotor  360  has a solid-cylindrical protrusion  362  as shown in FIGS. 57 and 59, and this protrusion  362  extends axially from the left-side plane in FIG. 57 of the sub rotor  360 . Protrusion  362  is positioned where it is hit by another cut-off edge  352  of the main rotor  350  following a predetermined rotation in the clockwise direction in FIG. 59 (explained later). The sub rotor  360  has a smaller diameter as compared with the main rotor  350 . Also, the sub rotor  360  is formed of a material having a small specific gravity, such as resin, and is lighter in weight than the main rotor  350 . 
     Torsional coil springs  370  and  380  are coupled concentrically to the rotation shaft  340  inside casing  330 , with springs  370  and  380  being held between the lower wall of the casing  330  and the main rotor  350  between the lower wall of the casing  330  and the sub rotor  360 , respectively. 
     The torsional coil spring  370  has one end  371  caught by protrusion  333  which protrudes axially inward from the lower exterior wall section of the casing  330 , and has another end section  372  caught by a protrusion  353  which protrudes axially from the circumferential section of the main rotor  350  to the lower wall section of the casing  330 . Based on this attachment of torsional coil spring  370 , it produces an exertion force in the counterclockwise direction in FIG.  59 . 
     The torsional coil spring  380  has one end section  381  caught by a protrusion  334  which protrudes axially inward from the lower central wall of the casing  330 . The coil has another end  382  caught by a protrusion  363  which protrudes axially from the circumferential section of the main rotor  350  to the lower wall section of the casing  330 . Because of this attachment, coil spring  380  produces an exertion force in the counterclockwise direction in FIG.  59 . 
     The contact mechanism  390  includes base  391 , fixed contacts  392  through  394  and moving contacts  395  through  397 . Fixed contacts  392  through  394  have concentric semicircular shapes centered by the rotation axis of shaft  340  on the left-side plane of the base  391  (side of the main rotor  350 ). The fixed contacts  392  through  394  have increasing radius in this order. 
     The moving contacts  395  through  397  are disposed along circles having the same radii as fixed contacts  392  through  394  on the right-side plane of the main rotor  350 . As such, the moving contacts  395  through  397  can confront the fixed contacts  392  through  394 , respectively. The moving contacts  395  through  397  are disposed on the right-side plane of the main rotor  350  so that the distance in circumferential direction from the left extreme section in FIG. 58 of the moving contact  395  and fixed contact  392 , the distance in circumferential direction from the left extreme section in FIG. 58 of the moving contact  396  and fixed contact  393 , and the distance in circumferential direction from the left extreme section in FIG. 58 of the moving contact  397  and fixed contact  394  have ascending values in this order when the fixed contacts  392  through  394  are located above the axis of the rotation shaft  340  in FIG.  57 . 
     The moving contact  395  has split contact sections  395   a . These contact sections  395   a  are fixed at their root section to the right-side plane of the main rotor  350 . The contact sections  395   a  extend from the root section to the tip section toward the fixed contact  392 . Accordingly, the moving contact  395  in unison with the fixed contact  392  constitutes the above-mentioned first switch. The moving contact  396  has split contact sections  396   a . These contact sections  396   a  are fixed at their root section to the right-side plane of the main rotor  350 . The contact sections  396   a  extend from the root section to the tip section toward the fixed contact  393 . Accordingly, the moving contact  396  in unison with the fixed contact  393  constitutes the above-mentioned second switch. The moving contact  397  has split contact sections  397   a . These contact sections  397   a  are fixed at their root section to the right-side plane of the main rotor  350 . The contact sections  397   a  extend from the root section to the tip section toward the fixed contact  394 . Accordingly, the moving contact  397 , in unison with the fixed contact  394 , constitutes the above-mentioned third switch. 
     The electrical circuit section Cb has a printed circuit board  300   a . The fixed contacts  392  through  394  and moving contacts  395  through  397  of the contact mechanism  390  are fed through the base  391  and printed circuit board  300   a  and connected to resistors  398   a  through  398   c . The resistors  398   a  through  398   c  are equivalent to the resistors R 1  through R 3 , respectively, described in the first embodiment. The moving contact  395  and fixed contacts  392  are equivalent to the moving contact  85  and fixed contacts  82  described in the first embodiment. The moving contact  396  and fixed contacts  393  are equivalent to the contact section  86   a  of the moving contact  86  and the fixed contacts  83  described in the first embodiment. The moving contact  397  and fixed contacts  394  are equivalent to the contact section  86   b  of the moving contact  86  and the fixed contacts  84  described in the first embodiment. Accordingly, the electrical circuit section Cb has a circuit arrangement identical to that of the first embodiment shown in FIG.  20 . When the vehicle decelerates, the main rotor  350  works as a weight to rotate clockwise in FIG. 59 about the axis of the rotation shaft  340  in the same manner as the first embodiment. At this time, the sub rotor  360 , which is lighter in weight than the main rotor  350  and is subjected to a large exertion force in the counterclockwise direction, does not rotate. 
     When the main rotor  350  further rotates, the distance in circumferential direction between the moving contact  395  and fixed contact  392  decreases. Eventually, the moving contact  395  contacts the fixed contact  392 . When the main rotor  350  further rotates in the same direction, it abuts at another cut-off edge  351  against the stopper  362  of the sub rotor  360 . 
     When the main rotor  350  further rotates in the same direction, it is subjected to the exertion forces of the two torsional coil springs  370  and  380  in the counterclockwise direction. With further rotation, the moving contact  396  contacts the fixed contact  393 , and thereafter the moving contact  397  contacts the fixed contact  394 . 
     During operation, with the main rotor  350  rotating only against the exertion force of the torsional coil spring  370 , the impact detecting device operates based on the first impact level. Otherwise, when main rotor  350  rotates against the exertion forces of the two torsional coil springs  370  and  380 , the impact detecting device operates based on the second and third impact levels. The relation among the rotation value of the main rotor  350 , the exertion force acting on the main rotor  350  and the closing positions of the first through third switches (which correspond to the first through third impact levels) is identical to the case shown in FIG.  33 . 
     In this embodiment, the moving contacts  395  through  397 , which contact the fixed contacts  392  through  394 , respectively, slide on the surface of the respective fixed contacts. In this case, only frictional forces exists, and there is no exertion force acting on the main rotor  350 . Also, the exertion force and spring constant of the torsional coil spring  370  can be changed to alter the first through third impact levels. 
     FIGS. 60 through 63 show the eighth embodiment of the electro-mechanical impact detecting device of the invention. This impact detecting device is adopted in place of the impact detecting device of the first embodiment. This impact detecting device has an outer housing  400  and an inner housing  410 , of which the housing  400  is fixed to the vehicle body at its proper location by a bracket  401  which is attached to the lower wall of the housing. 
     The inner housing  410  is fitted in the outer housing  400  as shown in FIG.  61 . The housing  410  has a connector  410   b  which extends from and is integral with a housing section  410   a , and the housing section  410   a  is located in the deep section of the housing  400  and the connector  410   b  is located in the opening section of the housing  400 . The connector  410   b  confronts the outside at its connecting section  411  through an opening  402  of the housing  400 . In FIG. 60, symbol  412  indicates terminals of the connector  410   b.    
     This impact detecting device has a device main body D, which is fitted in the housing  410  as shown in FIG.  60  and FIG.  61 . The device main body D is constituted of a mechanical section Da and an electrical circuit section Db. The mechanical section Da is fitted on the interior bottom of the housing section  410   a , and the electrical circuit section Db is fitted in the inner opening section of the housing section  410   a.    
     The mechanical section Da includes a cylindrical casing  430 , a shaft  420 , a weight  440 , compression springs  450  and  460 , and a contact mechanism  390 . The casing  420  is fitted on the interior bottom of the housing section  410   a . The shaft  430  is supported concentrically between the lower wall of the casing  420  and the base  471  of contact mechanism  470  which is coupled into the opening  421  of the peripheral wall  421  of the casing  420 . 
     The weight  440  has a shape of rectangular parallelepiped, and it is supported slidably and concentrically with the shaft  430  on the interior bottom of the casing  420  as shown in FIG.  60  through FIG.  62 . 
     The compression spring  450  is coupled concentrically onto the shaft  430  in its section between the base  471  and the weight  440  inside the casing  420 . The compression spring  450  exerts a force rightward in FIG. 60 to the weight  440  so that it is seated on the lower wall  422  of the casing  420 . The compression spring  460  is coupled concentrically onto the compression spring  450  in its section between the base  471  and an exerting plate  460   a  inside the casing  420 . The compression spring  460  exerts a force rightward in FIG. 60 to the exerting plate  460   a  so that it is seated on two protrusions  423  of the casing  420 . The two protrusions  423  protrude from the interior surface of the circumferential wall  421  of the casing  420  toward the axis to confront each other. The distance between the left-side plane in FIG. 60 of the weight  440  which is seated on the lower wall  422  of the casing  420  and the right-side plane in FIG. 60 of the exerting plate  460   a  which is seated on the protrusions  423  is set to have a certain value. 
     The contact mechanism  470  includes two fixed contacts  472 , two fixed contacts  473 , two moving contacts  474  and two moving contacts  475 . The two fixed contacts  472  are embedded by being spaced out from each other in one protruding wall section  421   b . They extend along the inner surface of the wall  421  along the axial direction so that the two fixed contacts  472  are exposed to the interior of the casing  420  as shown in FIG.  61  through FIG.  63 . The two fixed contacts  473  are embedded by being spaced out from each other in another protruding wall section  421   c  of the circumferential wall  421  of the casing  420  and extend axially along the inner surface of the wall  421  so that the two fixed contacts  473  are exposed to the interior of the casing  420  as shown in FIG.  61  through FIG.  63 . 
     The protruding wall sections  421   b  and  421   c  protrude from the inner surface of the circumferential wall  421  of the casing  420  toward the axis to confront each other and axially extend on the inner surface of the circumferential wall  421 . These protruding wall sections  421   b  and  421   c  are formed from the open end of the circumferential wall  421  of the casing  420  toward the lower wall  422 , with the protruding wall section  421   c  being axially shorter than the protruding wall section  421   b . The two fixed contacts  472  are axially shorter than the two fixed contacts  473  to match with the different lengths of the wall sections  421   b  and  421   c  (refer to FIG.  61 ). 
     The two moving contacts  474  are fixed on the weight  440  at positions on the sides  442  of the two fixed contacts  472 . The moving contacts  474  extend outward from the sides  442  to contact contacts  472 . When the weight  440  is seated on the lower wall  422 , the tip sections of these moving contacts  474  are located on the right in FIG. 61 of the protruding wall section  421   b  do not-contact fixed contacts  472 . 
     The two moving contacts  475  are fixed on the weight  440  on sides  443  of contacts  473 . The moving contacts  475  confront the two moving contacts  474  on the opposite side of the weight  440  and extend outward from sides  443  to contact contacts  473 . When the weight  440  is seated on the lower wall  422 , the tip sections of these moving contacts  475  are located on the right in FIG. 61 of the protruding wall section  421   c  and do not contact the fixed contacts  473 . 
     The electrical circuit section Db has a printed circuit board  480 , and the fixed contacts  472  and moving contacts  474  and  475  of the contact mechanism  470  are fed through the printed circuit board  480  and connected to resistors  490   a  and  490   b  which are connected to the wiring section of the printed circuit board. The resistor  290   a  mates with the two fixed contacts  472  and two moving contact  474 , and the resistor  290   b  mates with the two fixed contacts  473  and two moving contacts  475 . 
     In the eighth embodiment, when the vehicle decelerates, the weight  440  slides along the shaft  430  against the exertion force of the compression spring  450 . When the weight  440  further slide in the same direction, the distance between the two moving contacts  474  and the two fixed contacts  472  decreases, and eventually the moving contacts  474  contact fixed contacts  472 . 
     When the weight  440  further slides, the weight  440  contacts (at its left-side plane  441 ) exerting plate  460   a . With further sliding, the weight  440  undergoes additional exertion forces by the two compression springs  450  and  460 . With further sliding against the exertion force of the two compression springs  450  and  460 , the distance between the two moving contacts  475  and the two fixed contacts  473  decreases. Eventually the moving contacts  475  contact fixed contacts  473 . 
     During operation, with the weight  440  sliding only against the exertion force of the compression spring  450 , the impact detecting device operates based on the first impact level. Otherwise, when sliding against the exertion forces of the compression springs  450  and  460 , the impact detecting device operates based on the second impact level. By greatly increasing the exertion force and spring constant of the compression spring  460  relative to compression spring  450 , the first and second impact levels are altered. The amount of slide of weight  440 , exertion force acting on the weight  440  and closing positions of the first and second switches is identical to the first embodiment. 
     The present invention is not confined in practice to automobiles, but it may be applied to electro-mechanical impact detecting devices equipped on other vehicles including buses and trucks. Also, the present invention is not confined in practice to air-bag systems, but it may be applied to electro-mechanical impact detecting devices for the vehicle occupant protection systems such as the seat-belt pretensioner for automobiles. The moving contact of the contact mechanism is not required for the exertion force acting on the rotor, a spring which produces the exertion force may be employed separately. 
     While the above-described embodiments refer to examples of usage of the present invention, it is understood that the present invention may be applied to other usage, modifications and variations of the same, and is not limited to the disclosure provided herein.