Abstract:
The invention relates to a pressure sensor comprising a housing ( 12 ), a membrane ( 26 ) arranged in said housing and which can be deflected by the pressure to be measured, a light source (LED) having an optical axis, a light detector having an optical axis and a light blocker ( 36 ) that is coupled to the membrane and that can be deflected thereby, the blocker being arranged in the beam path of the light source. The light source and the light detector are arranged inside the housing ( 12 ), wherein their optical axes lie parallel to one another. A first prism ( 40 ) is assigned to the light source and a second prism ( 42 ) is assigned to the light detector in such a way that an uninterrupted beam path from the light source through both prisms to the light detector is obtained. To this end, the light blocker ( 36 ) is arranged between the two prisms.

Description:
CROSSREFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of copending international patent application PCT/EP02/03145, filed on Mar. 21, 2002 and designating the U.S., which claims priority of German patent application DE 101 14 751.1 filed on Mar. 22, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a pressure sensor with features defined in the preamble of claim  1 . 
     A pressure sensor of the afore-mentioned kind is, for example, known from U.S. Pat. No. 3,100,997 A. 
     Further pressure sensors are, for example, known from DE 79 06 132 U1, EP 0 580 458 A1, U.S. Pat. No. 4,122,337 or U.S. Pat. No. 5,005,584. DE 74 05 439 U1 also shows a pressure sensor. 
     All the afore-mentioned pressure sensors have the disadvantage that they are costly designed with respect to their structure so that their use in mass production is not possible due to high costs. Furthermore, the respective design is sensitive to vibrations so that this characteristic prevents their use on a large scale. 
     SUMMARY OF THE INVENTION 
     In view of the above, it is an object of the present invention to provide a pressure sensor of the afore-mentioned kind which may be produced cost effectively on the one hand, and which is small on the other hand, however, without deteriorating its sensibility. 
     This object is solved by a pressure sensor of the afore-mentioned kind such that a holding device is provided which may be inserted into the housing and which comprises a receiving opening for the light means and the light detection means, the holding device supporting the first and the second reflection surface, preferably the first and the second prism in the area of the respective receiving opening and being made of a transparent material, preferably a plastic material. 
     This kind of arrangement results in a very compact assembly of the pressure sensor and allows a very cost effective production. Particularly, off-the-shelf components may be used because of the employment of prisms for deflecting the light beams, without enlarging the design. Furthermore, the use of prisms allows to employ the normal geometry for example of standard LEDs and to use the whole beam width of the LEDs, so that the detectible path of the deflectable membrane increases. Therewith, the measuring range of the pressure sensor or its sensitivity may be increased. 
     At this point, it is to be noted that the expression “optical axis” means the axis of the light means and the light detection means, respectively, which defines the main beam direction. A further advantage may be seen in that the orientation of the light means and the light detection means, respectively, to the respective prism is already defined by the holding device so that no errors will occur during assembly. The light means and the light detection means, respectively only have to be inserted into the receiving opening of the holding device and will then have the desired orientation with respect to the respective prism. Moreover, the integration of the prisms in the holding device results in a very cost effective design. A further cost reduction is achieved in that the holding device is made of a transparent material, preferably a plastic material. That means in other words that the whole holding device with the prisms is made of one material. 
     In a preferred embodiment of the inventive pressure sensor the light blocker comprises a truncated portion lying in the beam path. 
     This measure allows a very simple and hence cost effective design of the light blocker, wherein there is a further advantage that due to the rotational symmetry of this portion no orientation or adjusting errors of the light blockers may occur during assembly. 
     In a preferred embodiment the holding device comprises at least two snap-in pins mounted on the exterior which engage with respective recesses in the housing. 
     This measure has the advantage that the assembly of the pressure sensor is simple and fast and may be carried out without tools. 
     In a preferred embodiment the housing comprises an interior shoulder extending along the whole circumference, a respective supporting surface of the holding device lying on the shoulder, wherein a edge region of the membrane is clamped between the supporting surface and the shoulder. 
     This measure has the advantage that a separate mounting element for the membrane in the interior of the housing is not necessary. Rather, fixing the membrane within the housing is achieved by putting on the holding device, which clamps the membrane to the shoulder of the housing. The advantage is that a further simplification of the production may be achieved. 
     In a preferred embodiment, the light means comprises a solid-state light source such as a LED and the light detection means comprises a solid-state light detector such as a phototransistor. 
     This measure has the advantage that off-the-shelf components may be used so that the use of expensive special components may be avoided. 
     Further advantages and embodiments of the invention can be taken from the following description and the enclosed drawings. It is to be understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation without leaving the scope of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to an embodiment and the drawings in detail. In the drawings: 
     FIG. 1 is a schematic sectional view of the pressure sensor according to the present invention; 
     FIG. 2 shows two respective views of a holding device; 
     FIG. 3 shows a perspective view of the housing; 
     FIG. 4 shows a perspective view of a membrane; 
     FIG. 5 shows a perspective view of a spring cup; and 
     FIG. 6 shows a perspective view of a light blocker. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1, a pressure sensor is shown schematically and in a sectional view and is indicated with reference numeral  10 . The pressure sensor  10  comprises a cup-shaped housing  12  which is closed by a printed circuit board  14  on which the pressure sensor is soldered. The housing  12  and the printed circuit board  14  enclose an interior  16 . Inside the interior  16  a holding device  18  is provided which receives a LED  20  and a phototransistor  22  in receiving openings  24  provided for that. 
     In the housing  12  being made of a transparent material further a membrane  26  is arranged which divides a part of the interior  16  in a first partial volume  28  and in a second partial volume  30  together with a holding device  18 . Both partial volumes are sealed against each other by the membrane  26 . Dependant on the pressure difference between both partial volumes  28 ,  30  the membrane  26  is deflected upwards or downwards. 
     A spring cup  32  lies extensively on the membrane  26  and supports at its opposite end an end of a spring  34 . The other end of the spring  34  is supported by the holding device  18 . 
     The spring cup  32  supports at its end facing the spring  34  an aperture or light blocker  36  which is put on the spring cup  32  and which is held by friction contact. 
     The holding device  18  comprises a first reflection surface  40  and a second reflection surface  42 . The first reflection surface  40  is assigned to the LED  20  and the second reflection surface  42  is assigned to the phototransistor  22 . The reflection at the reflection surfaces  40 ,  42  is caused by a so-called total reflection which requires that the inclination of the reflection surfaces  40 ,  42  is selected correspondingly. Both reflection surfaces are formed as conical surfaces as can be seen in FIG.  2 . The conical surfaces are dimensioned such that the light beams of the LED  20  are line-focused on an axis L. 
     In the present embodiment, the reflection surface  40  and  42 , respectively, is arranged with an inclination angle of 45° with respect to a vertical axis L. The receiving openings  24  are provided in the holding device  18  such that the inserted LED  20  lies with its optical axis  44  parallel to the vertical axis. The same applies also for the phototransistor  22  which optical axis  44  also lies parallel to the vertical axis. Therewith, the LED  20  emits a beam in an angle of 45° onto the reflection surface  40  which deflects this beam by 90° and reflects the beam in a horizontal direction to the opposite reflection surface  42  which in turn reflects the beam to the phototransistor  22 . Two rays lying in the marginal region are drawn in FIG. 1 for illustrative purposes and are indicated with reference numeral  46 . 
     In FIGS. 2 through 6 the elements of the pressure sensor shown in FIG. 1 are shown in detail and in a perspective view. The holding device  18  as shown in FIG. 2 comprises a cylindrical portion  50  and a following receiving portion  52 . Inside the cylindrical portion  50  both conical designed reflection surfaces  40 ,  42  are clearly shown, each being part of a prism  54 . Both prisms  54  are, as already shown in FIG. 1, arranged opposite to each other. The perspective view further shows a centric opening  56  which is in connection with a tubular portion  58 . The tubular portion  58  extends perpendicularly to the longitudinal axis of the cylindrical portion  50  and is supported by the receiving portion  52 . The tubular portion  58  and the opening  56  serve to apply the first pressure to the partial volume  30 . 
     FIG. 2 further shows that in the receiving portion  52  both receiving openings  24  are provided in which the LED  20  and the phototransistor  22 , respectively, are inserted, wherein by providing a step-in the inner wall of the receiving opening  24  a stop means  59  may be provided which engages with a respective flange surface of the LED  20  and the phototransistor  22 , respectively. By that the assembly and positioning and adjusting, respectively, of the LED  20  relative to the reflection surface  40  and the phototransistor  22  relative to the reflection surface  42 , respectively, may be achieved easily. 
     On the cylindrical portion  50  four snap-in pins  57  in total are provided which serves to engageably connect them with the housing  12 . Further, two further snap-in pins  55  are provided which engage with the printed circuit board  14  before soldering for achieving a pre-fixation of the pressure sensor on the printed circuit board  14 , what may be clearly seen in FIG.  1 . There, the snap-in pins  55  extend through the printed circuit board  14 . 
     The prisms  54  shown in FIG. 2 are made of a transparent material, so that the beams emitted by the LED  20  may enter the prisms and may be reflected by the respective reflection surface  40  and  42 , respectively, wherein concurrently a line-focusing onto the axis L is carried out. Preferably, the whole holding device  18  is made of this material so that it may be cast in a simple manner. 
     In FIG. 3 the housing  12  is shown in a perspective view. The housing  12  is designed in a cup shape and comprises a bottom  61  as well as a cylindrical side wall  62 . In the side wall  62  recesses  63  are provided in which the snap-in pins  57  may engage. On the inner surface of the side wall  62  a step or shoulder  64  extending circumferentially is arranged which can also be clearly seen in FIG.  1 . 
     At the junction between bottom  61  and side wall  62  a tubular portion  65  is mounted which provides a connection into the interior of the housing  12  in the bottom region. In the region of the side wall  62  opposing the bottom  61  an opening  66  having an opened edge is provided which serves to receive the tubular portion  58  of the holding device  18 —as will be described below—. The opening  66  lies in line with the tubular portion  65 . However, this opening  66  may also be provided at another location along the circumference of the side wall  62 . 
     In FIG. 4, the circular membrane  26  is shown in a perspective view. The membrane  26  comprises a circular edge  71  and a membrane surface  72  enclosed by the edge. A circular raised region  74  is provided centrical to this circular membrane surface  72 , which region  74  may also be clearly seen in FIG.  1 . The membrane surface  72  is made of silicone as to provide the necessary flexibility. 
     In FIG. 5 a spring cup  32  is shown in a perspective view. This rotationally symmetrical component comprises a large supporting surface  78  at its one end which lies on the membrane surface  72 . As to center the spring cup  32  on the membrane surface  72  and to avoid any slipping, the raised region  74  of the membrane surface  72  engages into a tubular portion  79 . This can also be clearly seen in FIG. 1 in a sectional view. At the other end of the tubular region  79  a step  18  is provided, which supports one end of the spring  34 . Furthermore, the tubular portion  79  supports the light blocker  36 . To avoid any light reflections, the spring cup  32  has a black surface. 
     This light blocker  36  is shown in FIG. 6 in a perspective view. It is also arranged as a rotationally symmetrical component and comprises a first truncated portion  82  and a cylindrical portion  84 . The cylindrical portion  84  is arranged such that it may be inserted into the tubular portion  79  of the spring cup  32  and may be held there for example by self-locking frictional engagement. Then the truncated portion  82  lies on the tubular portion  79 . This may also be clearly seen in FIG.  1 . As to avoid any reflection at the light blocker, it has a black surface. 
     Preferably, the light blocker may be provided with openings in the area of the portion  82  serving as windows, wherein respective two openings lie opposite to each other. Via two opposite openings light may travel in a radial direction through the portion  82 . 
     The assembly of the pressure sensor  10  shown in FIG. 1 is carried out as follows: 
     The light blocker  36  is inserted into the spring cup  32  and then the spring  34  is mounted on the tubular portion  79 . After that, this assembly is laid on the membrane surface  72  and is centered via the raised region  74 . 
     Then, the holding device  18  with the centric region  50  is mounted on this assembly, wherein the edge  71  of the membrane  26  is put over the cylindrical portion  50  of the holding device  18  and comes there into a snap-in or quasi snap-in engagement. This assembly may then be inserted easily into the housing  12 , wherein the fixation is achieved by the snap-in pins  57  which engage in the recess  63 . By that, the holding device  18  is totally pushed against the edge  71  and the edge  71  itself to the housing  12  in the area of the step  64  of the housing  12  so that a good sealing is achieved. 
     In the next step, the LED  20  and the phototransistor  22  are then inserted into the respective receiving opening  24  of the holding device  18 , wherein the end position is defined by the stop  59 . 
     Afterwards, the pressure sensor  10  is mounted on the printed circuit board  14 , wherein first the snap-in pins  55  engage with the printed circuit board and a pre-fixation is achieved. Then, the terminals of the LED  20  and the phototransistor  22  are soldered with the printed circuit board  14 . 
     The pressure sensor shown in FIG. 1 now operates as follows: 
     The first partial volume  28  will be applied with a first pressure via the tubular portion  65  of the housing  12 . Via the tubular portion  58  of the holding device  18  the second partial volume  30  is applied with a second pressure, wherein one of both pressures may be the ambient pressure. If both pressures are different, a pressure difference is caused which results in a deflection of the membrane  26  against the force of the spring  34 . This deflection of the membrane  26  is transferred to the light blocker  36  via the spring cup  32 , which light blocker  36  moves in a longitudinal direction L either upwards or downwards. 
     The light blocker  36  and its truncated portion  82 , respectively, lies in the beam path or beam passage of the LED  20 , which is indicated by the two single rays  46 . Depending on the position of the light blockers  36  in a longitudinal direction L a more or less large region of the beam  46  reflected by the reflection surface  40  is blocked and, hence, does not achieve the phototransistor  22  via the reflection surface  42 . Due to the focusing effect of the reflection surface it is not important whether the rotational axis of the light blocker is offset to the axis L. 
     If openings are provided in the light blocker  36  the light beam may pass these openings. The released surface of the openings determines the amount of light which is blocked or passed. 
     Hence, it is apparent that the amount of light which reaches the phototransistor  22  depends on the position of the light blocker  36  and, hence, the pressure difference between both partial volumes  28 ,  30 . On the basis of this dependency, the pressure difference may be determined by analyzing the electric signal generated by the phototransistor  22 . 
     By a respective design of the light blocker  36  and/or the inclination of the reflection surfaces  40 ,  42 , the characteristic of the pressure sensor may be adjusted within a certain scope. Also, the spring  34  influences of course the characteristic of the pressure sensor and is therefore selected depending on the application. 
     In summary, it may be said that the pressure sensor can be assembled very easily and cost effectively and is nevertheless suitable for use in many fields of application.