Abstract:
An optical noninvasive pressure sensing assembly capable of accurately indicating relatively minute pressure changes. The present invention generally includes a light source, such as a Light Emitting Diode (LED) or normal room illumination, positioned to reflected light off of a surface of a membrane. The membrane is in contact with the fluid in which the pressure is to be measured so that changes in the pressure in the fluid cause movement of the membrane. A charge coupling device (CCD) camera captures light reflected off of the membrane and the reflected light is analyzed to determine the relative movement of the membrane based on the change in the pattern of the reflected light.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to pressure sensors and more specifically to noninvasive pressure sensing.  
           [0002]    Various devices have been developed over the years for measuring or sensing the pressure in a volume of fluid. Many of these devices have a load cell containing probe or other sensing apparatus that must physically contact the fluid being measured. While in many mechanical applications (for example, an oil pressure sensor used on an internal combustion engine), physical contact between the probe and the fluid raises no particular concerns, such contact is undesirable in medical applications where the fluid may be a virally or microbially contaminated biological fluid. Under these circumstance, if the probe is allowed to contact the biological fluid, the probe must either be discarded or sterilized prior to reuse. Therefore, in medical applications, it is important that the pressure sensor not contact the fluid being measured.  
           [0003]    Several noninvasive pressure sensors have previously been disclosed in U.S. Pat. Nos. 1,718,494, 2,260,837, 2,510,073, 2,583,941 and 3,805,617, the entire contents of which are incorporated herein by reference. These devices use a metal disk moving within the electromagnetic field of an energized coil to sense pressure changes. As the iron disk moves closer or farther from the coil, the current flow through the coil varies, and these current fluctuations can be used to calculate pressure changes. While these devices are satisfactory for measuring relatively large pressure changes, more minute pressure changes do not cause the current to fluctuate to a sufficient degree to provide an accurate and reliable indicator of pressure variation.  
           [0004]    Other pressure sensors avoid contacting the fluid being tested by using a test chamber separated into two parts by a flexible diaphragm. The fluid volume being measured is contained on one side of the chamber and the pressure sensor is in communication with the second side of the chamber. Any increase or decrease in the fluid pressure causes the diaphragm either to expand into the second side of the chamber or to be pulled into the fluid part of the chamber, thereby increasing or decreasing the pressure in the second side of the chamber an amount corresponding to the change in fluid pressure in the first side of the chamber. While these diaphragm type pressure sensors do not invade the test fluid and can be used to detect relatively small pressure changes, the accuracy of such sensors relies to a great extent on the compliance or elastic properties of the diaphragm, properties that can be hard to control during manufacture and that may change over time as the diaphragm is repeatedly stretched and relaxed.  
           [0005]    Another noninvasive pressure sensor described in PCT Publication No. WO 93/24817 (corresponding to U.S. Pat. No. 5,392,653) uses a flexible diaphragm with an attached magnet. By attaching an iron disk to the diaphragm, the diaphragm is mechanically coupled to the transducer. In order for the transducer to measure the pressure accurately, the diaphragm is extremely flexible. Nevertheless, variations in the flexibility of the diaphragm affect the accuracy of the pressure measurements. In addition, this assembly relies on firm contact between the magnet and the transducer, variations of which will also affect the accuracy of the pressure measurement. Another noninvasive pressure sensor is disclosed in PCT Publication No. WO 99/23463. This pressure sensor includes a pressure chamber separated from the pressure transducer by a thin, compliant membrane. As with the previously described device, this device relies on the use of a bulky and relatively expensive load cell and stepper motors to position the load cell against the diaphragm  
           [0006]    Accordingly, a need continues to exist for an inexpensive, reliable and accurate pressure sensor capable of detecting relatively small pressure changes in a fluid without contacting the fluid.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The present invention improves upon prior art pressure sensors by providing an optical noninvasive pressure sensing assembly capable of accurately indicating relatively minute pressure changes. The present invention generally includes a light source, such as a Light Emitting Diode (LED) or normal room illumination, positioned to reflected light off of a surface of a membrane. The membrane is in contact with the fluid in which the pressure is to be measured so that changes in the pressure in the fluid cause movement of the membrane. A charge coupling device (CCD) camera captures light reflected off of the membrane and the reflected light is analyzed to determine the relative movement of the membrane based on the changes in the pattern of the reflected light.  
           [0008]    Accordingly, one objective of the present invention is to provide an optical noninvasive pressure sensing assembly.  
           [0009]    Another objective of the present invention is to provide a relatively inexpensive pressure sensing assembly.  
           [0010]    Still another objective of the present invention is to provide a pressure sensing assembly that can measure pressures less than ambient pressure.  
           [0011]    Still another objective of the present invention is to provide a pressure sensing assembly that will measure pressures different than ambient pressure.  
           [0012]    These and other advantages and objectives of the present invention will become apparent from the detailed description, drawings and claims that follow. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a schematical illustration of the pressure sensing assembly of the present invention.  
         [0014]    [0014]FIG. 2 is a schematical illustration of a first embodiment of the pressure sensing assembly of the present invention showing the pressure sensing membrane at ambient pressure.  
         [0015]    [0015]FIG. 3 is a schematical illustration of a first embodiment of the pressure sensing assembly of the present invention showing the pressure sensing membrane under a vacuum.  
         [0016]    [0016]FIG. 4 is a schematical illustration of a second embodiment of the pressure sensing assembly of the present invention.  
         [0017]    [0017]FIG. 5 is a schematical illustration of a third embodiment of the pressure sensing assembly of the present invention.  
         [0018]    [0018]FIG. 6A is a schematical illustration of a third embodiment of the pressure sensing assembly of the present invention showing the pressure sensing membrane at ambient pressure.  
         [0019]    [0019]FIG. 6B is a schematical illustration of a third embodiment of the pressure sensing assembly of the present invention showing the pressure sensing membrane under a vacuum.  
         [0020]    [0020]FIG. 7 is a plan view of a first grating that may be used with the third embodiment of the pressure sensing assembly of the present invention.  
         [0021]    [0021]FIG. 8 is a plan view of a second grating that may be used with the third embodiment of the pressure sensing assembly of the present invention.  
         [0022]    [0022]FIG. 9 is a schematical representation of a fourth embodiment of the pressure sensing assembly of the present invention.  
         [0023]    [0023]FIG. 10 is a top plan view of a chamber and membrane that may be used with the fourth embodiment of the pressure sensing assembly of the present invention.  
         [0024]    [0024]FIG. 11 is a schematical representation of one of the Moiré fringe formations used in the fourth embodiment of the pressure sensing assembly of the present invention.  
         [0025]    [0025]FIG. 12 is a schematical representation of another Moiré fringe formation used in the fourth embodiment of the pressure sensing assembly of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]    As can be seen in FIG. 1, pressure sensing assembly  10  of the present invention generally includes a light source, such as LED  12 , CCD  14 , pressure chamber  16  and flexible membrane  18 . Pressure chamber  16  containing a fluid having a pressure to be measured and may be made of any suitable material, such as metal, glass or plastic, may be of any suitable size or shape and contains a port (not shown) through which the pressure in chamber  16  may be varied. Diaphragm  18  may be made of any suitably compliant material having good dimensional stability, such as stainless steel. LED  12  may be any of a variety of commercially available LEDs, such as a super luminescent LED and directs light  20  toward membrane  18  through lens or lenslette array  22 . Light  24  reflected off of diaphragm  18  is captured by CCD  14 , which may be any of a variety of commercially available devices.  
         [0027]    As best seen in FIGS. 2 and 3, light  20  from LED  12  is focused by lens  22  at an angle onto membrane  18  so that light  20  is reflected off of membrane  18  as reflected light  24  and onto CCD  14 . When the pressure within chamber  16  is at or near ambient, as seen in FIG. 2, membrane  18  will be relatively flat, and the angle of reflected light  24  will be approximately the same as light  20 , causing the pattern of reflected light  24  falling on CCD  14  to be approximately the same as the pattern formed by light  20  emanating from lens  22 . When the pressure within chamber  16  is below ambient, as shown in FIG. 3, membrane  18  will be deflected inward (concave), causing reflected light  24  to be more tightly focused on CCD  14  than light  20  emanating from lens  22 . One skilled in the art will recognize that in a similar manner, pressures in chamber  16  above ambient will cause membrane  18  to be convex (not shown) and reflected light  24  falling on CCD  14  will be more scattered than light  20  emanating from lens  22 . The pattern of reflected light  24  can be captured by CCD  14  and analyzed using software well-known in the art and the relative position and movement of membrane  18  calculated. Alternatively, the pattern of reflected light  24  captured by CCD  14  can be analyzed using a wavefront measurement device, such as the one described in U.S. Pat. No. 6,460,997 B1 (Frey, et al.), the entire contents of which being incorporated herein by reference. The position of movement of membrane  18  directly relates to the pressure and pressure changes within chamber  16 .  
         [0028]    As best seen in FIG. 4, in a second embodiment  100  of the pressure sensing assembly of the present invention, membrane  118  contains a plurality of grooves, pits or other non-reflective areas  119 . The pattern of areas  119  is projected onto CCD  114  by reflected light  124 , and this pattern may be analyzed to determine the pressure within chamber  116  as described above. The use of areas  119  allows the use of a conventional focusing lens  122  rather than a lenslette array to form light  120 , and such lens  122  may be formed as part of LED  112 .  
         [0029]    As best seen in FIGS. 5-8, in another embodiment of the present invention, LED  212  emits light  220  through grating  180  so as to form an image of grating  180  onto membrane  216  on chamber  216  that is reflected off of membrane  218  as reflected light  224  and on to CCD  214 . As seen in FIGS. 6A-6B, when the pressure within chamber  216  is at ambient, the pattern reflected of reflected light  224  onto CCD  214  will be roughly identical to the pattern on grating  180 . When the pressure within chamber  216  is below ambient, membrane  218  will bow in slightly, resulting in the pattern of reflected light  224  reflected on to CCD  214  to have a tighter grid spacing than grating  180 . This change in grid spacing may be analyzed to determine the pressure within chamber  216  as described above. One skilled in the art will recognize that other pattern(s) besides a rectangular grid may be formed in grating  180 . For example, as seen in FIG. 8, grid  181  may contain a plurality of random or patterned holes  185  in its structure. The pattern of holes  185  can be analyzed prior to measuring pressure in chamber  216  to help calibrate system  210  or to indicate when system  210  is not functioning properly.  
         [0030]    As best seen in FIGS. 9-12, in assembly  410  of a fourth embodiment of the present invention, grating  420  is made on the surface of membrane  400 . General illumination of the area of membrane  400  makes grating  420  visible to Scheimflug imaging optics lens  430 . The Scheimflug imaging optics is constructed by having optical axis  431  of lens  430  aligned at an angle of 45° to the surface normal of membrane  400  and CCD  450 . With illumination, grating  420  is imaged by lens  430 , resulting in grating image  421  being formed. Image  421  has at least two useable Moiré fringe formations. The first method of Moiré fringe formation is formed by allowing image  421  to be formed directly on CCD  450  and allowing the receptor pixel array of CCD  450  form Moiré fringes. The Moiré image can then be extracted using appropriate software to read the pixel images, as shown in FIG. 12. The second method of Moiré fringe formation is placing second grating  440  near CCD  450  so as to form Moiré fringes on CCD  450 , as shown in FIG. 11. The angle between image  421  and grating  440 , as shown in FIG. 11, and the angle between image  421  and CCD  450 , as shown in FIG. 12, can be changed to adjust the Moiré fringe spacing and thus, the detection sensitivity of the pressure detection. The smaller the angle, the higher the sensitivity.  
         [0031]    Pressure sensing assembly  10 ,  100 ,  210  or  410  of the present invention allows the noninvasive measurement of pressure within a chamber. Provided that diaphragm  18 ,  118 ,  218  or  400  is made sufficiently large, contaminates on a portion diaphragm  18 ,  118 ,  218  or  400  will not prevent system  10 ,  100 ,  210  or  410  from detecting the pressure within chamber  16 ,  116 ,  216  or  316  as sufficient reflected light  24 ,  124  or  224  will reach CCD  14 ,  114 ,  214  or  450  from noncontaminated portions of diaphragm  18 ,  118 ,  218  or  400 . In addition, the use of a polarizer (not shown) will improve the signal to noise ratio because reflected light  24 ,  124  or  224  will be largely polarized while ambient light reflected off of membrane  18 ,  118 ,  218  or  400  will have no pronounced polarization. One skilled in the art will also recognize that by varying the thickness of diaphragm  18 ,  118 ,  218  or  400 , the focal power of lens  22 ,  122  or  430  and/or the relative positions of the components, the pressure range that can be detected by assembly  10 ,  100 ,  210  or  410  can be adjusted so that individual rays of light within light  20 ,  120  or  220  do not overlap.  
         [0032]    This description is given for purposes of illustration and explanation. It will be apparent to those skilled in the relevant art that modifications may be made to the invention as herein described without departing from its scope or spirit.