Abstract:
The invention relates to a control device for a vehicular occupant restraint system comprising a housing with a metallic baseplate and an electronic module including an acceleration sensor and being mounted on a circuit board. The electronic module is insertable into the housing that is formed as a composite of an injection-molded plastic body and the metallic baseplate part of which is embedded in an injection molded wall of plastic.

Description:
FIELD OF THE INVENTION 
     The invention relates to a control device for a vehicular occupant restraint system. 
     BACKGROUND OF THE INVENTION 
     Safe and reliable functioning of such a control device is vital to the effectiveness of the restraint systems, such as belt tensioner and air bag connected thereto. The housing of such a control device has therefore to ensure safe protection against any interference that may affect the functioning of the electronic module. In addition, however, the housing rigidly bolted to the vehicle body is required to transfer the deceleration actions occurring an impact of the vehicle as directly as possible to the acceleration sensor of the electronic module. Therefore, solid metal housings, in particular heavy diecast housings made of aluminium, are used. Instead of metal housings, plastic housings have already been proposed provided at the attachment locations with metal fastening bushes embedded in an injection molded plastic material, since plastic materials fail to withstand the bolting pressure. However, the stiffness and rigidity of such a housing design is only just satisfactory. 
     BRIEF SUMMARY OF THE INVENTION 
     This is achieved in a control device for a vehicular occupant restraint system comprising a housing with a metallic baseplate and an electronic module including an acceleration sensor and being mounted on a circuit board. The electronic module is insertable into the housing that is formed as a composite of an injection-molded plastic body and the metallic baseplate part of which is embedded in an injection molded wall of plastic. 
     The housing of the control device combines the mechanical advantages of a metal design with the production engineering and cost advantages of a plastic design. According to the invention the metallic baseplate ensures a rigid coupling to the vehicle body. Since the circuit on which the acceleration sensor is mounted is arranged directly above the baseplate a direct momentum transfer from the baseplate to the acceleration sensor is assured. In addition, due to the baseplate being metallic the fastening points are precisely defined unlike a plastic design of the housing with fastening bushes embedded in the injection molded plastic material, the distances between which vary due to shrinkage of the plastic material. As compared to a design including fastening bushes embedded in the injection molded plastic material the solution in accordance with the invention has furthermore the advantage of facilitated fabrication since inserting the metallic baseplate in the plastic mold can be carried out much easier than correctly locating individual fastening bushes. 
     In the preferred embodiment the metallic baseplate is electrically connected to the ground of the circuit board. Serving this purpose is an electrically conducting link which may be provided in differing variants and which leads from the baseplate to a contact pin or contact tag on a plug socket and from there to the circuit board. 
     In one advantageous embodiment a mechanically solid and rigid connection between the metallic baseplate and the plastic body of the housing is achieved by providing a plurality of through-holes in the baseplate which are filled in a form-fitting manner by the plastic compound. As an alternative or in addition thereto the baseplate comprises a plurality of projected stamped tabs embedded in the plastic material. In the preferred embodiment the baseplate is furthermore flush with the outer bottom surface area of the housing, but is covered on the inside by a film of plastic so that the baseplate is ultimately inset in the bottom wall of the housing body. 
     According to a further embodiment of the invention the metallic baseplate takes up only part of the bottom surface area of the housing, more particularly half of the bottom surface area of the housing as defined by a diagonal line. It has been discovered that even in such a material and weight-saving design of the housing a sufficiently rigid connection and good mechanical coupling between the baseplate and acceleration sensor are assured. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages and features of the invention will become apparent from the following description of several embodiments as well as from the drawings to which reference is made and in which: 
     FIG. 1 is a slanting view in perspective of the control device as seen from above; 
     FIG. 2 is a plan view of the baseplate of the housing as seen from underneath; 
     FIG. 3 is a sectional view taken along the line III—III in FIG. 2; 
     FIG. 4 is a detail on a magnified scale as shown in FIG. 3; 
     FIGS. 5 to  8  show different embodiments of an electrical ground link between the baseplate of the housing and a contact pin connected to the circuit board of the electronic module; 
     FIG. 9 is a partly sectioned view in perspective of an advantageous further embodiment: and 
     FIG. 10 is a detail view of the bottom portion of the housing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The housing of the control device comprises a flat metal baseplate  10  and a hood-shaped plastic body  12  of a generally parallelepipedal form. The baseplate  10  has edge strips protruding beyond the plastic body  12  having arranged fastening holes  14  at the three corners. As evident from FIG. 2 the baseplate  10  takes up only part of the bottom surface area of the bottom wall of the housing. The portion of the bottom surface area of the bottom wall taken up by the baseplate  10  is defined by a diagonal line  16 , the baseplate  10  thus taking up only roughly half the bottom surface area of the bottom wall. 
     Fabricating the housing is done by injection-molding it in a mold in which the baseplate  10  has been inserted prior. The baseplate  10  has a plurality of through-holes  18  flared conically towards the outer surface of the bottom wall. Referring now to FIGS. 3 and 4 there is illustrated how the baseplate  10  is brought into composite with the plastic body of the housing in injection-molding. The baseplate  10  coincides on the outside with the bottom surface area of the housing. On the inside the baseplate  10  is coated with a film of plastic having a thickness that corresponds roughly to the thickness of the baseplate. The through-holes  18  in the baseplate  10  are positively filled by the plastic material. As an alternative or in addition thereto the baseplate comprises a plurality of stamped-out and inwardly bent tabs  19  (FIG. 10) embedded in the plastic material. Molded to the inside of the plastic body  12  are latching keys  20  or latching notches, whose functioning is explained next. 
     The housing has an open narrow side into which a frame-type plug socket  22  of plastic is inserted to thus close off this open side of the housing. The plug socket  22  is connected by a plurality of contact pins and by latching means to a circuit board  23  (FIG. 9) onto which the electronic module of the control device including an acceleration sensor is mounted. The complete electronic module comprising plug socket  22  and circuit board  23  is inserted into the housing, the circuit board being received by guide slots incorporated on the inside of the housing. Due to the latching keys  20  molded to the inside of the housing and engaging the plug socket  22  the electronic module is reliably locked in place in the housing. The housing is sealed by a silicone rubber sealing ring at the transition to the plug socket  22 . As an alternative latching keys are molded on the plug socket  22  and the housing comprises corresponding latching notches. 
     To produce an electrical connection between the baseplate and ground on the circuit board of the electronic module an electrically conducting link is provided, four variants of which are apparent from FIGS. 5 to  8 . 
     In the embodiment as shown in FIG. 5 a tab  24  is stamped out and bent upwards at right angles from the baseplate  10  alongside one of the fastening holes  14 . The free end of this tab  24  is connected to the end of a contact pin, for example, by welding or soldering, the contact pin passing through the plug socket  22  and is soldered on the inside to the circuit board. 
     In the embodiment as shown in FIG. 6 a contact pin  25  soldered to the circuit board passes through the plug socket  22  and is bent as a contact strap  26  towards the baseplate where it is secured in full contact, for example, by welding or soldering. 
     In the embodiment as shown in FIG. 7 the end of the contact strap  26  is configured as a contact lug  28  made to coincide with a fastening hole  14  fully or partly and is clamped in place between screw head and baseplate  10  in being screw-fastened. 
     In the embodiment as shown in FIG. 8 a contact pin  30  is secured by its free end to the baseplate  10  by welding or soldering, the other end of the contact pin  30  being connected to a contact pin jutting out from the plug socket  22 . The contact pin  30  forms at the same time a mechanical lock making it difficult for the plug socket  22  to become loose from the housing. As an alternative, as shown in FIG. 8, another arrangement of the contact pin is shown at  30 a. Several such contact pins may also be provided. 
     In a further embodiment of the control device as shown in FIG. 9 guide and supporting rails  32  are molded on the inside of the plastic body  12  of the housing, these rails extending in the plug-in direction of the electronic module comprising the circuit board  23  with the plug socket  22 . Insertable between these guide and supporting rails  32  is a component  34  such as a cylindrical capacitor. This component  34  is mounted freely supported on the plug socket  22  or on the circuit board  23  as well as being electrically in contact therewith and first receives support by the guide and supporting rails  32  load-bearingly in the housing and locked in place.