Abstract:
A device and a method for recognizing the occupancy of a seat has two sensor mats which independently of one another detect the pressure exerted on them.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a device and a method for recognizing the occupancy of a seat, in particular a motor vehicle seat, in which for detecting a force acting on the seat surface a sensor installation is provided so that the force can be classified for recognition. By classification here is meant that the device should contribute to making a distinction as to who is in fact located on the seat so that components of the vehicle safety system, such as an air bag or belt tightener for example, can be adaptively controlled.  
       BACKGROUND OF THE INVENTION  
       [0002]     Devices of the type identified at the outset are generally well known. In these the sensor installation can be constructed as a sensor mat filled with a fluid, wherein recognition of the seat occupancy ensues by determining the increase in the fluid pressure in the sensor mat when the seat is acted upon by a certain weight.  
         [0003]     Since the signals output from the sensor installation do not suffice for classification, in known devices signals from belt tension sensors and acceleration sensors are additionally recorded.  
       SUMMARY OF THE INVENTION  
       [0004]     It is an object of the invention to refine a device and a method of the type identified at the outset in such a way that signal evaluation is made easier.  
         [0005]     This task is solved by the characteristics of the independent claims and in particular in that the sensor installation in the device comprises two sensor mats which each independently of one another detect the pressure exerted on them.  
         [0006]     By increasing the signal sources from the seat, that is to say the number of sensor mats, it is possible when the mats are of suitable shape reliably to detect and classify the seat occupancy. The need of additionally measuring the belt tension or acceleration is then eliminated.  
         [0007]     Advantageous embodiments of the invention are described in the description, the drawing and the subsidiary claims.  
         [0008]     According to an advantageous embodiment one of the sensor mats can be shaped so that it is suitable for detecting a child&#39;s seat provided with two runners. For this purpose it can also be advantageous for the two sensor mats to be interlocked in one another since by this means selective evaluation of the pressure exerted on the sensor mats is possible.  
         [0009]     According to another advantageous embodiment viewed in plan view one sensor mat is located inside the other sensor mat. This allows ready discrimination as to whether, for example, a child&#39;s seat or an adult is located on the seat or whether, for example, an adult of small size or low weight or alternatively an adult of average or high weight is sitting on the vehicle seat.  
         [0010]     It is furthermore advantageous for one of the sensor mats to be of such a shape that it is suitable for emitting signals which on their own can serve to detect the state that a person belonging either to the 5% lightest or 5% heaviest of their gender is sitting on the seat. Accordingly, the shaping of the mats takes into account differing weight distributions in differently shaped buttocks in humans.  
         [0011]     The recognition of the occupancy of the seat can be optimised by detailed specification of the shape of the sensor mats. Accordingly, it is advantageous for an inner mat having a substantially closed surface area to be surrounded by an outer mat in semicircular or U-shaped manner. Relatively heavy adult occupants are also larger and by acting on the outer mat with their weight are distinguishable in this way.  
         [0012]     Advantageously, the inner mat is substantially rectangular and possesses two arms as extensions of two opposite sides of the rectangle which project into the interior of the outer mat, ie they are oriented towards the base of the U.  
         [0013]     Particularly high sensitivity and differentiability is obtained when a projection on the outer mat juts out in between the two arms. This can be triangular, trapezoidal or semicircular.  
         [0014]     Furthermore, trials have shown that the legs of the U-shape of the outer mat need not be of the same width everywhere, but rather that particularly good selectivity is achieved by the legs being wider at those points where they are adjacent to the arms of the inner mat than at those points where they are adjacent to the rectangular shape of the inner mat. It is also particularly advantageous if the legs of the outer mat have recesses on their inner sides adjacent to the rectangular section of the inner mat since by this means different hipbone shapes can be differentiated particularly well.  
         [0015]     It is advantageous for the outer sensor mat to have approximately 1.5 to 2.5 times the area, preferably slightly more than twice the area, of the inner sensor mat, that is to say approximately 2 to 2.25 times the area of the inner mat. At its widest point the U-shape can be up 2.5 times the width of the rectangle and also at the end of the legs is preferably more than twice the width of the rectangle. In length it can slightly exceed twice the length of the rectangle (without the arms being taken into account).  
         [0016]     The invention also relates to a method for recognizing the occupancy of a seat in which two sensor mats located in the seat each emit a pressure signal and the pressure signals are individually evaluated or sums and differences are also calculated and likewise used for evaluation. When the sensor mat has a suitable shape the signal from an inner mat and the sum of its signal with that from a surrounding U-shaped outer mat is sufficient.  
     
    
     BRIEF DESCRIPTION OF THE INVENTION  
       [0017]     The invention is described below purely by way of example on the basis of an advantageous embodiment and with reference to the drawing. The latter shows:  
         [0018]      FIG. 1  is a plan view on to two sensor mats according to the invention; and  
         [0019]      FIG. 2  is a graph in which pressure signals from the sensor mats are illustrated. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]      FIG. 1  shows a plan view on to a device for recognizing the occupancy of a seat comprising an inner sensor mat  10  of closed surface area surrounded by an outer sensor mat  12 . In plan view the outer sensor mat  12  exhibits substantially the shape of a U while the inner sensor mat  10  possesses a rectangular, approximately square section  14  amounting to approximately 85% of its total area. From two opposite sides of the section  14  project two arms  16  in the direction of the base of the outer mat  12 , wherein the two arms extend away from two opposite, shorter sides of the rectangular section  14  and as a result point towards the base of the U of the outer mat  12 .  
         [0021]     In the region located between the two arms  16  the outer mat  12  possesses a projection  18  of approximately trapezoidal construction projecting in between the arms  16 .  
         [0022]     The side legs  20  of the U-shaped outer mat  12  begin starting from the base of the U at a slightly tapering section  22  and are widened in a section  24  located adjacent to the arms  16  of the inner mat  10 . In an adjoining section  26  the two legs  20  of the outer mat  12  possess recesses both on the inside of the legs and on the outside of the legs which recesses are located at the level of the rectangular section  14  of the inner mat  10 . This gives rise in turn to a slightly widened end section  28 .  
         [0023]     In  FIG. 1  the length of the legs is designated by the reference symbol l 2  and the width of the base by the reference symbol d 2 , whereas the length of the rectangular section  14  of the inner mat  10  is designated by l 1  and the width of this section  14  extending parallel to the base is designated by d 1 . As can be seen the U-shaped outer mat  12  has a width d 2  which is approximately 2.5 times the width d 1  of the rectangular section  14 . The length  1   2  is approximately 2.5 times the length l 1  of the rectangular section  14 . The length of the arms  16  is approximately 0.6 times the length l 1  of the rectangular section  14 . The area of the outer mat  12  is approximately 2.25 times the area of the inner mat  10 .  
         [0024]      FIG. 2  illustrates the evaluation of signals from both mats  10  and  12 .  
         [0025]     The signal from the inner sensor mat  10  is plotted on the X axis and the sum of the signals from the inner sensor mat  10  and the outer mat  12  is plotted on the Y axis. By constructing regions  30 ,  32 ,  34 ,  36  the types of occupancy of the seat can be clearly distinguished, ie classified.  
         [0026]     The diamonds in region  30  represent a child (approximately 12 months old) in a child&#39;s seat. In this case a good part of the signal comes from the outer sensor mat  12  because the child&#39;s seat is standing on runners which exert pressure on sections  24 ,  26 ,  28 . On the other hand, due to its narrow hipbones an approximately six year-old child presses down almost exclusively on to the inner sensor mat  10  and, accordingly, is to be differentiated from the value range of the child&#39;s seat (square values in region  32 ). In a central region  34  is found the group of particularly light adults belonging to the so-called 5 percent, ie to the 5% lightest of their gender. The circles in region  34  represent values for living persons and the triangles those of associated test dummies. The group of adults belonging to the 50th percentile whose weight is therefore the statistical median for their gender yields values in an upper region  36  of the graph in  FIG. 2  (crossed circles: test dummies, crosses: living persons). If the sum of the signals from the two mats  10  and  12  yields a value of more than 300 (arbitrary units) a normal person of at least average weight is sitting on the seat.  
         [0027]     Accordingly, the device according to the invention with two sensor mats and the method according to the invention for evaluating the signals from these two mats allows clear electronic recognition of different seat occupancy situations as required for controlling the deployment of air bags. The belt tension and acceleration sensors from the state of the art are no longer required. The entire system is simplified.