The invention relates to a control device for a vehicular occupant restraint system.
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.
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.