Abstract:
The deformation sensor does not require a supplementary sensor to check functionality. This deformation sensor includes an optical transmission medium, multiple transmission elements, which couple the light of various wavelengths into the transmission medium, and multiple reception elements, which selectively couple the transmitted light out of the transmission medium according to wavelength. An evaluation unit detects deviations between the output signals of the reception elements and signals a malfunction of the sensor if the deviations exceed a preset measure.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a deformation sensor, made of an optical transmission medium, means for coupling light into the transmission medium, and means for receiving the light transmitted via the transmission medium, with an evaluation unit being present which detects the change in intensity of the light received depending on the deformation. 
   BACKGROUND INFORMATION 
   A deformation sensor is known from German Patent Application No. 42 20 270 A1. Such a deformation sensor, which includes an optical transmission medium having transmission and reception elements coupled to it, is particularly to be used for detecting deformations of the vehicle body which a vehicle undergoes in the event of a crash. Such an optical deformation sensor can preferably be used for detecting side crashes in vehicles. An optical transmission medium, which includes an optical waveguide as in the specification cited, changes its light transmission properties due to the microbending effect upon curving such as that which occurs in the event of a deformation of the vehicle body. Specifically, the intensity of the light transmitted changes if the optical waveguide undergoes a curvature due to a deformation of the vehicle body part onto which the optical waveguide is affixed. 
   As described in U.S. Pat. No. 5,917,180, the optical transmission medium of a deformation sensor includes a deformable, light-transmitting body, made of, for example, polyurethane foam. The optical scattering properties of this deformable, light-transmitting body change if pressure is exerted on it, which occurs, for example, in the event of a deformation of the vehicle body part onto which the body is affixed. 
   In order to avoid spurious trippings of restraint devices (airbags, seat belt tightening system, roll cage, etc.) caused by the crash sensor in the vehicle, and also to avoid faulty non-tripping in the event of a crash, continuous function checking of the deformation sensor is necessary. The related art in this connection is the use of at least one further sensor of another type—this can be, for example, an acceleration sensor which is positioned in addition to the deformation sensor at any other location in the vehicle. As described in German Patent No. 42 20 270, the function checking can also be performed using a second identical deformation sensor. In each case, a further supplementary sensor must be located in the vehicle. 
   An object of the present invention therefore is to provide a deformation sensor of the type described above in which no further supplementary sensor is necessary for its function checking. 
   SUMMARY OF THE INVENTION 
   According to the present invention, multiple transmission elements are present which couple the light of various wavelengths into the transmission medium of the deformation sensor and multiple reception elements are present which selectively couple the transmitted light out of the transmission medium according to wavelength. An evaluation unit detects the deviations between the output signals of the reception elements and signals a malfunction of the sensor if the deviations exceed a preset measure. 
   According to the present invention, checking of the deformation sensors function for errors may be performed simultaneously using one single deformation sensor, in addition to its actual function of deformation sensing. A supplementary sensor which is responsible exclusively for function checking of the deformation sensor may be dispensed with. Since deformations may be sensed and function checking may be performed simultaneously with one single sensor, i.e. no additional querying of a further sensor is necessary, the time between the beginning of a crash with a vehicle and the provision of a trip signal for the restraint systems is reduced. 
   The transmission medium of the deformation sensor preferably comprises either one or more optical waveguides or a deformable, light-transmitting body. Optical waveguides may be inserted into the deformable, light-transmitting body, for example, to couple light waves into and out of the body. However, reception and transmission elements may also be directly implanted in the deformable, light-transmitting body. 
   To receive the light waves transmitted via the transmission medium, reception elements which react either uniformly or inversely to a change in intensity of the light transmitted via the transmission medium may be used. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a deformation sensor having an optical waveguide as a transmission medium. 
       FIG. 2  shows a deformation sensor having a deformable, light-transmitting body with optical waveguides for coupling out light as a transmission medium. 
       FIG. 3  shows a deformation sensor having a deformable, light-transmitting body, into which transmission and reception elements are directly integrated, as a transmission medium. 
       FIG. 4  shows various dependencies of the output signals of the reception elements depending on the intensity of light received. 
   

   DETAILED DESCRIPTION 
   In  FIG. 1 , a deformation sensor is shown whose optical transmission medium ÜM 1  includes one single optical fiber or a bundle of multiple optical fibers. If, for example, the deformation sensor is to be used as a side crash sensor for a vehicle, this optical fiber ÜM 1  is preferably installed in a side door of the vehicle. Thus, if a deformation of the side door occurs due to a side crash and thus a deformation, i.e. curvature, of optical waveguide ÜM 1  occurs, due to the known microbending effect, this curvature of the optical waveguide causes a change in intensity of the light transmitted via the optical waveguide. This change in intensity of the light transmitted via optical waveguide ÜM 1 , which is detected as described in the following, therefore provides direct information in the event of a deformation of the vehicle body caused by a crash. 
   At one end of optical waveguide ÜM 1 , an optical waveguide junction SK is connected, via which light of two different wavelengths λ 1  and λ 2  is coupled into waveguide ÜM 1  from two optical transmission elements S 1  and S 2 . Transmission elements S 1  and S 2  are electrooptical converters, e.g. light emitting diodes (LED). The light coupled into optical waveguide ÜM 1  from both transmission elements S 1  and S 2  should be clearly selectable. Therefore, transmission element S 1  transmits, for example, light in infrared wavelength range λ 1  and second transmission element S 2  transmits, for example, light in ultraviolet wavelength range λ 2 . 
   At the other end of optical waveguide ÜM 1 , an optical waveguide junction EK is connected which distributes the light transmitted via optical waveguide ÜM 1  to a first reception element E 1  and a second reception element E 2 . Reception elements E 1  and E 2  may be conventional photodiodes or photo transistors. To separate two wavelength ranges λ 1  and λ 2 , reception element E 1  has a sensitivity for wavelength range λ 1  (e.g. infrared) and reception element E 2  has a sensitivity for wavelength range λ 2  (e.g. ultraviolet). To improve the separation between two wavelength ranges λ 1  and ë 2  even further, optical filters, which are tuned to desired wavelength ranges λ 1  and λ 2 , may be inserted between the ends of junction EK and reception elements E 1  and E 2 . 
   Instead of optical waveguide junction EK on the reception side, a wavelength-selective coupler may also be provided, which supplies light separated according to wavelengths λ 1  and ë 2  to reception elements E 1  and E 2  at its two outputs. 
   Electrical output signals a 1  and a 2  of two optoelectronic reception elements E 1  and E 2  are supplied to an evaluation circuit AW. During the transmission via optical waveguide ÜM 1  of light of both wavelength ranges λ 1  and  2  coupled out of transmission elements S 1  and S 2 , a curvature of optical waveguide ÜM 1  affects the light of both wavelength ranges λ 1  and λ 2 . Both output signals a 1  and a 2  of reception elements E 1  and E 2  thus show a reaction in the form of a change in signal level in the event of a deformation of optical waveguide ÜM 1  on the basis of the change in intensity of the transmitted light. Thus, if one of transmission elements S 1 , S 2  or reception elements E 1 . E 2  is defective, a clear mutual deviation will show in both output signals a 1  and a 2  of reception elements E 1 , E 2 . Evaluation circuit AW therefore compares both output signals a 1  and a 2  of reception elements E 1 , E 2  with one another and signals a malfunction of the sensor if a level deviation which exceeds a preset measure exists between the two output signals a 1 , a 2 . In this way, reliable function checking of the sensor may be performed. If a malfunction of the sensor is established, the information that this sensor may not be considered when tripping decisions are made is relayed to a control device for restraint systems present in the vehicle.
         If a break of optical waveguide ÜM 1  occurs, evaluation circuit AW recognizes this defect in that both output signals a 1  and a 2  have no signal level.       

   A further exemplary embodiment for a transmission medium of a deformation sensor is shown in FIG.  2 . The same means are used for the coupling in and out of light of various wavelength ranges λ 1  and λ 2  as was already described in connection with the exemplary embodiment of FIG.  1 . Therefore, these means will not be described in more detail here. 
   Transmission medium ÜM 2  of the deformation sensor shown in  FIG. 2  includes a deformable, light-transmitting body K, which is surrounded by a jacket M which is opaque to light. The deformable, light-transmitting body is made of, for example, polyurethane foam. This body K has the property that it changes its optical scattering properties upon a pressure acting on it from outside or a deformation. Thus, if light is coupled into body K, the intensity of the light coupled out of body K will change in the event of a deformation of body K. 
   As shown in  FIG. 2 , light of both wavelength ranges λ 1  and λ 2  from transmission elements S 1  and S 2  is coupled into body K 1  via optical waveguide junction SK and a transmission optical waveguide LS. This transmission optical waveguide LS penetrates into body K for this purpose. A reception optical waveguide LE also penetrates body K and feeds the light thus coupled out of the body to two reception elements E 1  and E 2  via the optical waveguide junction and/or via wavelength-selective coupler EK. In the exemplary embodiment shown in  FIG. 2 , transmission optical waveguide LS and reception optical waveguide LE are introduced into body K at the same location. Notwithstanding this, two optical waveguides LS and LE may penetrate into body K from different sides. 
   Transmission medium ÜM 2  shown in FIG.  2  and its action in the event of a deformation is described in U.S. Pat. No. 5,917,180, cited above. 
   The coupling of light into and out of deformable, light-transmitting body K of transmission medium ÜM 2  may also occur, as shown in  FIG. 3 , in that transmission elements S 1 , S 2  and reception elements E 1 , E 2  are integrated directly into body K, allowing the optical waveguides LS, LE, which couple the light in and out, to be dispensed with. There is any desired number of configurations for the insertion locations of transmission elements S 1 , S 2  and reception elements E 1  and E 2 , notwithstanding the exemplary embodiment shown in FIG.  3 . 
   There is the possibility that both reception elements E 1  and E 2  react uniformly to changes in intensity of the light received, i.e. both output signals a 1 , a 2  of reception elements E 1 , E 2  experience an increase or decrease in level with increasing intensity of the light received. Reception elements E 1 , E 2  may also, however, be implemented in such a way that they react inversely to changes in intensity of the light received. As shown in FIG.  4 . for example, the level of output signal a 1  of first reception element E 1  would then increase with increasing light intensity and the level of output signal a 2  of second reception element E 2  would drop with increasing light intensity. The evaluation of signals a 1  and a 2  must then be appropriately adjusted in evaluation circuit AW. Inversely operating receivers have the advantage that interference signals (e.g. those caused by electromagnetic irradiation) and actual measurement signals may be differentiated using them. Output signal levels a 1  and a 2  of two receivers E 1  and E 2  are changed in the same direction by malfunctions. In contrast, the actual fractions of light to be measured, which are coupled out of optical transmission medium ÜM 1  or ÜM 2 , respectively, change output signal levels a 1  and a 2  of receivers E 1  and E 2  in opposite directions. Malfunctions and measurement signals may thus be differentiated from one another.