Acceleration detector

An acceleration detector, the spring is a substrate plate, e.g., of Al.sub.2 O.sub.3, which is fixed at one end in a housing. The resistors and the evaluating circuit are arranged on the spiral spring and can be applied to the substrate plate in a single work step using thick-film technology.

BACKGROUND OF THE INVENTION 
The invention relates to an acceleration detector. It is known in 
acceleration detectors to fix a flat spring at one end and to determine 
the deflection of the spiral spring, for example, by means of 
piezoelectric elements. An evaluation circuit is arranged so as to be 
separate from the elements in the housing of the acceleration detector. 
The construction is accordingly made more complicated. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an acceleration detector which 
has the advantage that acceleration and deceleration, particularly during 
an accident of the motor vehicle, can be reliably detected as soon as 
determined boundary values are exceeded. In so doing, a particularly 
simple construction, which is therefore also free from disturbances, is 
ensured. Since the resistors and the evaluating circuit are arranged on a 
substrate plate which serves, at the same time, as a bending flat spring, 
the acceleration detector can be constructed inexpensively and simply and 
in a manner which benefits assembly. Transmission losses in the lines 
between the resistors and the evaluating circuit can be avoided to a great 
extent. The sensitivity to interference is low.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The plastic housing of an acceleration detector 11, which is shaped as a 
hollow body, is designated by 10 in FIG. 1, the flat spring 12 of the 
acceleration detector 11 being fixed at one end. The flat spring 12 is a 
ceramic substrate plate formed of aluminum oxide (Al.sub.2 O.sub.3), for 
example. A metal or ceramic block 13 is arranged at the free end 12a of 
the spring as a seismic mass. In the area of the metal block 13, two 
mechanical stops 14, 15, which define the maximum deflection of the spring 
12, are formed at the plastic housing 10. In addition, two retaining 
devices 16, 17, which almost contact the spring 12 at upper and lower 
intermediate regions thereof to delimit a non-bending area of the spring, 
are located in the housing 10. Thus, the spring 12 should only bend 
between the metal block 13 and the retaining device 16. A plurality of 
expansion-sensitive sensor resistors 18 are also arranged in this area of 
the spring 12. The sensor resistors 18 are applied to the spring 12 with 
the use of so-called thick-film technology. In addition, an evaluating 
circuit 20 is located between the retaining devices 16, 17 and between the 
retaining device 17 and the housing 10 on the spring 12. The evaluating 
circuit 20 is also applied using thick-film technology. It has active 
components, e.g. amplifying members (semiconductors), and passive 
components, e.g. R-, C-, L-components. 
Electric lines 21 lead from the evaluating circuit 20 to a control device, 
not shown, which can trigger the passenger protection devices of the motor 
vehicle, for example, belt tighteners, air bag, roll bar, warning flasher 
system, central locking mechanism or the antilocking system for the 
brakes. Fastening pins 22 are located at the lower side of the housing 10, 
the acceleration detector 11 can be mounted on a printed circuit board by 
means of them. 
The function of an acceleration detector is sufficiently known and is 
therefore not described here in more detail. If the metal block 13 is 
deflected from its rest position, the flat spring 12 is bent in the area 
of the resistors 18 and a proportional change in resistance is produced in 
the latter. This change in resistance is evaluated in the evaluating 
circuit 20 and conveyed to the control device. 
Since the spring 12 is a substrate plate, the resistors 18 and the 
evaluating circuit 20 can be applied in one work step using thick-film 
technology. The packing cost is minimal.