Abstract:
The invention relates to an optical pressure sensor based on light intensity measurements and comprises at least one membrane and two parallel optical fibers. At least one first fiber has a fiber end and a light emission surface for emitting light in the direction of the membrane. At least one second fiber has a fiber end having a light admission surface for receiving the light reflected from the membrane and transmitting that reflected light. The light emission surface and the light admission surface of the two fibers are disposed facing away from each other. This changes the optical path of the light during use such that the light portion received by the at least one second fiber is very sensitive to the position of the membrane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to International Application Ser. No. PCT/CH2007/000589 filed Nov. 26, 2007, which claims priority to Swiss Application No. CH/01914/06 filed Nov. 27, 2006. 
     TECHNICAL FIELD 
     The invention relates to an optical pressure sensor based on light intensity measurements comprising at least one membrane as well as at least a first optical fiber and a light emission surface and at least a second optical fiber arranged in parallel to the first optical fiber and a light admission surface wherein a light beam is guided from the first fiber via the light emission surface to the membrane where it can be reflected and wherein the reflected light beam can enter via the light admission surface into the second fiber in which it can be further transmitted. 
     BACKGROUND 
     Optical sensors of this type are, for example, employed for engine pressure measurements and are e.g. built into standard spark plugs for this purpose. Other types are used in miniaturized nozzle pressure sensors, for example. In such sensors, light is emitted from a first fiber to a membrane. This membrane is located at a variable position, i.e. closer to or farther away from the emitting fiber, depending on the amount of pressure that acts thereonto from the other side. Then, the light is reflected at the membrane. A portion of the reflected light impinges onto the second fiber that guides the light to a measuring device in which this light intensity of the light is measured. Eventually, the position of the membrane with respect to the optical fibers and, thus, the pressure prevailing at the membrane at that time of measurement can be deduced from the light intensity measured. 
     It is a disadvantage of such systems that a small signal is superposed on a huge offset. Therefore, the smallest disturbances of this offset result in dramatic errors in the pressure signal measured. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to suggest an optical pressure sensor of the type described in the beginning which is insensitive to load change drift, thermal shock and drift. 
     To be able to incorporate sensors for example in standard spark plugs requires a small diameter. Thus, the required miniaturization of the total sensor diameter of &lt;2 mm and before long of &lt;1.5 mm or even &lt;1 mm, poses a permanent challenge. 
     The object has been achieved by the characterizing parts of the independent claim. 
     The idea underlying the present invention is that the light emission surface and the light admission surface are disposed facing away from each other. The optical path of the light is altered so that in use the portion of light that is received by the receiving fiber  4  greatly depends on the membrane position. 
     Furthermore, due to the favorable optical path the membrane can be disposed close to the fiber ends so that a major proportion of the light intensity can be utilized. In this way, the dynamics with respect to disturbances is enhanced. In addition, the variance in light intensity is proportional to the pressure applied. 
     The easiest way to accomplish the invention is by means of a roof-like edge of a ferrule that incorporates these two fibers wherein the light emission surface and the light admission surface each are arranged on one side of the roof-like edge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following the invention will be explained in more detail with respect of the drawings which show 
         FIG. 1   a  a schematic representation in cross-section of an optical sensor according to the prior art in the region of the sensor head; 
         FIG. 1   b  a schematic perspective representation of an optical sensor according to the prior art in the region of the fiber ends of the light guide; 
         FIG. 2   a  a schematic cut-open view of an optical sensor according to the invention in the region of the sensor head; 
         FIG. 2   b  a schematic perspective representation of an optical sensor according to the invention in the region of the fiber ends of the light guides; 
         FIG. 3  a schematic representation in cross-section of a fiber; 
         FIG. 4  a perspective view of an alternative embodiment of a sensor according to the invention in the region of the fiber ends; 
         FIG. 5  a plan view of an alternative embodiment of a sensor according to the invention in the region of the fiber ends including a plurality of fibers; 
         FIG. 6   a - d  perspective views of alternative embodiments of light emission and light admission surfaces having different shapes; 
         FIG. 7  a schematic cut-open representation of an alternative embodiment of an optical sensor according to the invention in the region of the sensor head; 
         FIG. 8  a schematic time-dependent sensor signal obtained using a) a sensor according to the prior art, and b) a sensor according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The reference numerals were kept the same in all drawings. 
       FIG. 1   a  shows a schematic representation in cross-section of an optical sensor according to the prior art in the region of the sensor head. Within a ferrule  11  are represented a first light-conducting fiber  1  as well as a second light-conducting fiber  4  arranged in parallel to the first fiber  1  and having a fiber end  5 . In operation, light  10  is emitted through the first fiber  1  at a light emitting surface  3  towards a membrane  8  where it is reflected. A portion of this light beam  10  eventually enters into a light admission surface  6  of the second fiber  4  and is transmitted for evaluation of the light intensity. The membrane  8  as well as the ferrule  11  enclosing the two light-conducting fibers  1 ,  4  are kept in a predetermined position by a housing  9 . Depending on the amount of pressure acting from outside of the housing  9  onto the membrane  8 , the membrane  8  will be displaced closer to the fiber ends  2 ,  5  of the fibers  1 ,  4 . This changes the proportion of light  10  which was originally emitted through the first fiber  1  and which enters into the second fiber  4 . The pressure prevailing at this time can be deduced from the light intensity transmitted through fiber  4  since the light intensity impinging onto the first fiber  1  is known. 
       FIG. 1   b  shows the end of the ferrule  11  with the two fiber ends  2 ,  5 , the light emitting surface  3  of the first fiber  1  as well as the light admission surface  6  of the second fiber  4  according to the prior art in a perspective view. The end of the ferrule as a whole has a planar edge so that the light emission surface  3  and the light admission surface  6  both are disposed in one plane extending parallel to the membrane  8 . 
       FIG. 2   a  shows the same arrangement as in  FIG. 1   a  with the exception that the light admission surface  3  and the light emission surface  6  are arranged facing away from each other. In contrast to  FIG. 1   a  they are not arranged in one plane that extends parallel to the membrane but in one which is inclined with respect to the membrane in an angle α. The emerging light beam  10  is refracted at the light emission surface  3  of fiber  1  towards the center of the ferrule  11  and is reflected at the membrane  8  towards the light admission surface  6 . Because of the favorable entrance angle a light beam  10  reaching the light admission surface  6  is transmitted within the second fiber  4 . It is crucial, however, that the quantity of light of the impinging light beam  10  strongly depends on the membrane position and changes in a manner proportional thereto. 
     The two surfaces  3  and  6  are facing away from each other if their inner surfaces are facing each other. Specifically, parallel surfaces are neither facing each other nor facing away from each other.  FIGS. 2 ,  4 , and  6  show various examples illustrating the expression “facing away from each other”. 
     Due to the arrangement of the light emission surface  3  and the light admission surface  6  of the fiber ends  2 ,  5  the wanted signal is amplified with respect to the offset and the quality of the measurement is enhanced. The distance of the membrane  8  to the fiber ends  2 ,  5  as well as the angle α are optimized under several aspects. On the one hand, the refractive indices on both sides of the light emission surface  3  as well as the light admission surface  6  define the angle of total reflection limiting the angle of incidence and the angle of emergence. On the other hand, the difference in the light impinging onto the light admission surface that is caused by the variable membrane position should be as dynamic as possible. That means, that the intensity of the light  10  entering into the second fiber  4  varies as much as possible due to a change in the position of membrane  8 . 
       FIG. 2   b  shows the end of the ferrule  11  with the two fiber ends  2 ,  5 , the light emission surface  3  of the first fiber  1  as well as the light admission surface  6  of the second fiber  4  in a perspective view in an embodiment of the invention. In this embodiment, the end of the ferrule  11  has a root-like edge where each of the fiber ends  2 ,  5  terminates in a different roof plane. The fiber ends  2 ,  5  are arranged symmetrically with respect to a central plane  14  of the sensor. In this embodiment, this central plane  14  is represented by the ridge of the roof-like edge. Preferably, the fiber ends  2 ,  5  are disposed close to each other, if possible touching each other. 
     In another preferred embodiment the light emission surface  3  and the light admission surface  6  are disposed in two planes  12 ,  13 . These planes  12 ,  13  define the two roof planes of the roof-like edge in  FIG. 2   b.    
     The angle α between the two planes of the roof-like edge and a plane which extends parallel to the membrane  8  should be as steep as possible, however, without leading to total reflection at the light emission surface  3  or the light admission surface  6 . Angles of between 20 and 40°, in particular between 25 and 35°, have been found to be particularly suitable. 
       FIG. 8  schematically shows a time-dependent sensor signal, in the first portion  18  without any load and in the second portion  19  with full load wherein in a) a prior art sensor according to  FIG. 1  and in b) a sensor according to the invention, for example according to  FIG. 2 , was used. The first portion  18  shows an offset signal  20 , the second portion  19  a wanted signal  21  that is superposed on the offset signal. 
     It can be seen that in the arrangement according to the invention the ratio of wanted signal to offset signal was improved by multiple orders of magnitude compared to the arrangement according to the prior art. In this way, the sensor according to the invention has been strongly improved with respect to load change drift, thermal shock and drift. 
       FIG. 3  represents a light-conducting fiber in cross-section. The fiber is composed of a light-conducting core  15  surrounded by a cladding  16 . This cladding  16  is itself enclosed by a protective layer  17 . In the embodiment of the invention a fiber with a core  15  that encompasses at least 40% of the total area or 60% of the total diameter of the fiber should be used. 
     For clarity, the other Figures aside from  FIG. 3  each only show the core  15  of a fiber  1 ,  4  without cladding and protective layer. In the representations, the fibers  1 ,  4  that touch each other therefore have always a distance of twice the cladding thickness including the protective layer. 
     The fibers  1 ,  4  are led in parallel whereby their handling and processing is simplified and miniaturization of the sensor is enabled. In a preferable embodiment as for example represented in  FIG. 2   b  the fibers  1 ,  4  are conducted within a ferrule  11  which, however, is not obligatory for carrying out the invention. In addition, also the symmetrical arrangement of the light emission surface  3  and the light admission surface  6  within the sensor is not mandatory but simplifies mounting and evaluation. 
     An alternative embodiment with regard to  FIG. 2   b  is shown in  FIG. 4 . In this embodiment the ferrule  11  has a cone-shaped tip similar to a pencil with two leads arranged side by side representing the fibers  1 ,  4 . 
     Another alternative embodiment is shown in  FIG. 5  as a plan view onto a ferrule  11  containing the fiber ends  2 ,  5 . In this embodiment several or a plurality of first and second fibers  1 ,  4  are represented wherein in operation the first fibers  1  are the emitting fibers and the second fibers  4  are the receiving fibers. These fibers  1 ,  4  are arranged on both sides of the central plane  14 . All advantageous embodiments as described for  FIG. 2  apply analogously also to this arrangement with several first and second fibers  1 ,  4 . Specifically, all light emitting surfaces  3  and all light admitting surfaces  6  each can be arranged in planes wherein preferably all light emitting surfaces  3  lie in a first plane  12  and all light admitting surfaces  6  lie in a second plane  13 . Each of these first  1  and second fibers  4  can be arranged in an array, as depicted, on both sides of and close to the central plane  14 , preferably touching each other. They can also be arranged in several arrays or in a random order on both sides of the central plane. 
       FIG. 6  represents further embodiments in a perspective view. The Figures illustrate different cut shapes wherein in each case—as shown—there can be arranged only one fiber per non-planar surface  12 ′,  13 ′ or, in a manner analogous to the representation in  FIG. 5 , several fibers per non-planar surface  12 ′,  13 ′. The preferred arrangements and embodiments described above also apply here in an analogous manner. 
     In these representations,  FIG. 6   a  shows a concave cut and  FIG. 6   b  an essentially convex cut in which the surfaces  3  and  6  are located. In  FIGS. 6   c  and  6   d  the surfaces  3  and  6  are formed to represent concave ( FIG. 6   c ) or convex ( 6   d ) segments of cylinders the axes of which intersect the central plane  14 , or concave ( FIG. 6   c ) or convex ( 6   d ) spherical segments, respectively. 
     All cuts described herein can be easily prepared if the fibers ( 1 ,  4 ) are held by the ferrule. Without an appropriate hold a sensor according to the invention would be difficult to fabricate, especially in the required miniaturized embodiment as described. Another advantage of the ferrule ( 11 ) is protection of the fiber ends in the case of strong vibrations as they occur in engines. 
       FIG. 7  shows a schematic representation in a cut-open view of an alternative embodiment of an optical sensor according to the invention in the region of the sensor head. In contrast to  FIG. 2   a , in this embodiment the ends of the first fiber  1  and the second fiber  4  are located in the same plane parallel to the membrane  8 . A light-conducting insert body  7  is arranged adjacent to these fiber ends  2 ′,  5 ′. This insert body  7  has the same function as the fiber ends  2 ,  5  of the arrangement shown in  FIG. 2   a  which are integrally connected to the two fibers  1 ,  4 . In particular, the light emission surface  3 ′ and the light admission surface  6 ′ of the insert body are arranged facing away from each identically to the other arrangements described. Thus, the optical path in this alternative embodiment is essentially the same as in the arrangement depicted above and has the same advantages as described. The light path in the region of the insert body just is slightly conical because the reflecting lateral walls of a light guide are missing in the region of the insert body  7 . All of the embodiments described herein above and in particular those depicted with regard to  FIGS. 4-6  can be achieved accordingly by using an insert body  7  and the same advantages as illustrated above can be achieved. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  first fiber 
           2   2 ′ fiber end of a first fiber 
           3   3 ′ light emission surface 
           4  second fiber 
           5   5 ′ fiber end of a second fiber 
           6   6 ′ light admission surface 
           7  light-conducting insert body 
           8  membrane 
           9  housing 
           10  light, light beam 
           11  ferrule 
           12   12 ′ first plane 
           13   13 ′ second plane 
           14  central plane 
           15  core 
           16  cladding 
           17  protective layer 
           18  first portion, without load 
           19  second portion, with full load 
           20  offset signal 
           21  wanted signal