Abstract:
A sensor system for determining the position of a gas-liquid interface on a surface includes a light source and photo detectors spaced along the surface and capable of detecting reflections from the light source. The photo detectors are joined to a processor which determines if the detectors have received a reflection when the gas phase of the gas-liquid interface contacts the surface. An optical guide can be provided having the light source and detectors positioned therein. The guide is positioned in the region of the gas-liquid interface. The guide can utilize total internal reflection to distribute light to the detectors where light strikes a gas-guide interface whereas light striking a liquid-guide interface is refracted through the interface.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND TO THE INVENTION 
     (1) Field of Invention 
     The invention relates to a sensor system for remotely detecting a gas-liquid interface on a solid body. 
     (2) Description of the Prior Art 
     Determining the location of a liquid contact location is a common problem when dealing with gas-liquid interfaces. This problem occurs in determining the level of liquid in a tank, location of a waterline on a vessel or buoy, or positioning of a cavity about a high-speed underwater vehicle. 
     There is a trade-off between sensor spatial resolution and complexity. A sensor array sized to cover a large area must reduce the overall number of sensors or use a prohibitively complex processing system. For example, pressure sensors are a common method of detecting the presence of either gas or liquid at a single point. Making these measurements over a large area requires an array of sensors which can be tedious to work with and consume large amounts of processing time. 
     Sensors based on electrical conductance/resistance changes are also known in the art, but are also generally limited to small surface areas, which makes them impractical for large-scale applications such as vessels or structures. 
     Another type of sensor, an optical sensor, utilizes the principles of total internal reflectance to distinguish a gas-liquid interface. Several prior art patents describe sensors that make use of the principle of total internal reflection for detecting the presence of moisture or dirt on a vehicle windshield (e.g., U.S. Pat. No. 6,052,196 to Pientka et al., U.S. Pat. No. 6,232,603 to Nelson, U.S. Pat. No. 6,084,519 to Coulling et al.). While these sensors typically are incorporated into a feedback control system, they only measure locally and do not provide specific information about the location of contact of gas-liquid interfaces with the surface. Moreover, the sensors are not configured to provide information for a larger area. 
     It is desireable to have sensors which can accurately determine the location of a gas-liquid interface over a large surface. As previously mentioned, dynamically and remotely detecting the location of a gas supercavity on a supercavitating vehicle is vital for vehicle control and model validation, yet a reliable method of remotely detecting the closure location is lacking. Another application is detecting the location of contact of a free-surface on a ship hull or buoy, critical for vessel control and safety. Finally, application of such a sensor to a floating platform or other marine structure would provide important information on the structure&#39;s submergence, preventing it from being raised or lowered beyond safety margins or structural limits. 
     It is desirable to have a sensor which can monitor a gas-liquid interface and continuously provide feedback over a large area without increasing the need for the number of sensors. 
     It is further desirable to have a device which has the sensitivity to collect data variations from single point to a large surface area. 
     It is further desirable to have a device which may utilize such data in a feedback loop to control a cavitator or other object. 
     SUMMARY OF THE INVENTION 
     Accordingly, a sensor system for determining the position of a gas-liquid interface on a surface includes a light source and photo detectors spaced along the surface and capable of detecting reflections from the light source. The photo detectors are joined to a processor which determines if the detectors have received a reflection from the gas phase of the gas-liquid interface. An optical guide can be provided having the light source and detectors positioned therein. The guide is positioned in the region of the gas-liquid interface. The guide can utilizes total internal reflection to distribute light to the detectors where light strikes a gas-guide interface whereas light striking a liquid-guide interface is refracted through the interface. 
     The present invention is intended to be configurable so as to measure the dynamic contact location of a gas-liquid interface either at a local point, along a linear dimension of the body or over a surface area of the body. The signal received from the sensor can be incorporated into a feedback control loop to provide information about the separation, closure and/or cavity about the solid body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the accompanying drawings in which are shown an illustrative embodiment of the invention, wherein corresponding reference characters indicate corresponding parts, and wherein: 
         FIG. 1  shows one embodiment of a sensor for detecting the location of cavity closure on a supercavitating body; 
         FIG. 2  shows one embodiment of a sensor for detection of a gas-liquid interface location on a ship hull; 
         FIG. 3  shows an exemplary embodiment of a single source, multi-detector sensor; 
         FIG. 4  shows a graph representing reflectance as a function of angle of incidence for sample materials; 
         FIG. 5  shows an exemplary embodiment of a sensor for detection of a gas-liquid interface in multiple dimensions; 
         FIG. 6  shows an exemplary embodiment of a light detecting element; 
         FIG. 7  shows an embodiment of an alternate sensor system; and 
         FIG. 8  shows an exemplary embodiment of a single-detector sensor. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     For the purpose of promoting an understanding of the present invention, references are made in the text to exemplary embodiments of gas-liquid interface sensors and sensor systems. It should be understood that no limitations on the scope of the invention are intended by describing these exemplary embodiments. One of ordinary skill in the art will readily appreciate that alternate but functionally equivalent sensors and systems may be used. The inclusion of additional elements may be deemed readily apparent and obvious to one of ordinary skill in the art. Specific elements disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one of ordinary skill in the art to employ the present invention. 
     It should be understood that the drawings are not necessarily to scale; instead, the drawings emphasize the principles of the invention. In addition, in the embodiments depicted herein, reference numerals in the various drawings refer to identical or near identical structural elements. 
     The following terms are used throughout this disclosure. “Feedback control mechanism” refers to a mechanism which utilizes input data to dynamically control a system. “Guide” refers to a three-dimensional structure capable of transmitting and/or directing light. The term “optical component” refers to a component which alters or processes a light beam. Optical components may include, but are not limited to, beam splitters, prism couplers and optical filters. The term “photosensitive component” refers to a component capable of sensing light or other electromagnetic energy. Photosensitive components may include, but are not limited to, photodetectors, photographic plates and particle detectors. As used herein, the term “supercavitating vessel” refers to a vessel which is designed to move through an underwater environment with a gaseous bubble thereabout. 
     In  FIG. 1 , there is shown a supercavitating vehicle  10 . A cavitator  12  is positioned at the nose of vehicle  10 . In use, vehicle  10  generates a cavity  14 . Typically cavity  14  is generated by a drop in pressure as the vehicle  10  travels through an underwater environment. Cavity  14  begins at a separation region  16  at the terminal edge of cavitator  12 . Cavity  14  extends along vehicle  10  to a closure  18  where the cavity  14  terminates. Closure  18  can be along vehicle  10 , as shown here, or behind vehicle  10 . 
     A sensor system is adhered, attached or embedded along the length of a supercavitating vehicle  10 . Sensor system detects the location at which the cavity gas-liquid interface or closure  18  contacts vehicle  10 . Sensor system includes a light source  20  and a plurality of light sensing elements  22 . Light sensing elements  22  are positioned along a surface of vehicle  10  and are joined to a control computer  24  within vehicle  10 . Light sensing elements  22  provide a signal to computer  24  indicating whether the surface is locally in contact with a liquid or a gas. Contact with a gas is expected to yield a signal indicating the presence of light from light source  20 , and contact with a liquid is expected to yield a signal indicating a lower level of light. Light sensing element  22  signals can be incorporated into a feedback control loop governed by computer  24 . 
     Control computer  24  monitors signals from elements  22 . Control computer  24  can be pre-programmed to continuously monitoring the gas-liquid body contact location represented by shifts in element  22  signals. Control computer  24  can determine the gas-liquid body contact location continuously or at any given time by determining the location of the signal transition. By way of specific, non-exclusive examples, this information can be utilized in control computer  24  for modifying control of the cavitator, modifying control of other systems which may affect location of the gas-liquid interface, displaying a continuously updated visual representation of the gas-liquid body contact location on a user interface, or storing the data for later analysis. 
       FIG. 2  shows another embodiment that relates to detecting the location of contact of a wetted surface. A cross-sectional view of a vessel or buoy hull  30  is shown. A gas-liquid boundary  32  between air  34  and water  36  contacts an intermediate portion of hull  30 . A sensor system is joined to hull  30 . A light source  20  is positioned in the nominally gas region  34  and light sensing elements  22  extend along the hull  30  crossing the expected location of the gas-liquid boundary  32 . Regular spacing of elements  22  is not required as long as the location of each element  22  is known. Elements  22  can be in communication with a computer  24 . 
     One general embodiment of sensor is shown in  FIG. 3  where a sensor is positioned on a body  40 . The sensor consists of a single light source  20  that provides light  42  at an angle of incidence θ i  within a guide  44  having a width, length L and thickness h. The thickness may be constant or variable along the length of the sensor. The guide  44  is constructed of a translucent material having an index of refraction n s . A plurality of light detecting units  22  having one or more optical components and one or more photosensitive components are arranged along the length of the sensor at or near the bottom of the guide  44 . Detecting elements  22  and electronics can be provided in a backing material  46  that can be attached, adhered or embedded to body  40 . 
     The method of detection of the contact location  18  of the gas-liquid interface  32  involves placement of the sensor on or near the surface of the body  40  such that a non-opaque optical path exists between the sensor and the environment. The sensor material has an index of refraction, n s , that is larger than the indices of refraction of the both the liquid (n l ) and the gas (n g ). The critical angles associated with the guide-gas interface and guide-liquid interface are given, respectively, as 
                     θ     c     s   -   g         =       sin     -   1       ⁡     (       n   g       n   s       )               (   1   )               
and
 
     
       
         
           
             
               
                 
                   
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     Because the light beam is from light source  20  and contacts guide  44  at an angle of incidence θ i , light  42  travels within the sensor at the sensor-environment interface such that the incident light makes an angle θ i  with the surface normal vector. The incident angle θ i  is set such that
 
θ c     s-g   &lt;θ i &lt;θ c     s-l   .  (3)
 
     Therefore, total internal reflection will be achieved when the environment in contact with the sensor consists of gas  34 . When the environment consists of the liquid  36 , a portion of light  48  will escape into the liquid  36 , producing a measureable change in the amount of light reflected from the sensor-environment interface. Because the sensor incorporates light detecting elements  22 , which measure the change in the amount of light reflected, a change in light detected can indicate the presence of a liquid environment. 
     For the embodiment shown in  FIG. 3  the spatial resolution, d, of the sensor is given by
 
 d= 2 h  tan θ i .  (4)
 
     If the light source  20  in the embodiment shown in  FIG. 3  consists of a laser with circular beam of radius r, then the minimum allowable guide thickness is given by 
                   h   =       r     sin   ⁢           ⁢     θ   i         .             (   5   )               
The smallest achievable spatial resolution is then
 
     
       
         
           
             
               
                 
                   
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     Although to this point the light detecting units have remained generic, any detecting unit will necessarily consume a portion of the light incident upon it. Defining the initial light source power as P 0 , we can define the power remaining in the beam after contacting the n-th light detecting unit as:
 
 P′   n   =x·P   n-1   =x   n   ·P   0 .  (7)
 
where x is the fraction of initial power that remains in the beam after contacting the light detecting unit. Assuming a state of total internal reflection at the sensor-environment interface, the power incident upon the n th  detecting unit is then
 
 P″   n   =x   n-1   ·P   0 .  (8)
 
and the power sampled by n th  detecting unit is
 
 P   n =(1− x )· x   n-1   ·P   0 .  (9)
 
     The n-th detecting element must be capable of detecting this power. Furthermore, every detecting unit must be capable of detecting the change in the incident power when the environment in contact with the sensor changes from gas to liquid. Although the light will cease to be totally internally reflected at the location at which the liquid contacts the guide surface, a portion of the light will still be reflected. This portion depends on the angle of incidence and the refractive indices of the guide and the liquid. The reflectance, R, defined as the ratio of reflected power to incident power, is plotted in  FIG. 5  for representative refractive indices of n s =1.5, n l =1.333 and n g =1. The solid and dashed black curves correspond to the reflectance in a plane perpendicular and parallel to the plane of incidence for the guide-gas interface, respectively. Similarly, the gray curves correspond to the guide-liquid interface. The vertical black lines denote the critical angle associated with each interface. In order to maximize the sensitivity of the sensor, the incidence angle should be as close as possible to θ c     s-g    so that the amount of reflected power when liquid contacts the surface is minimized. Nonetheless, the reflectance increases slowly until an angle very close to the critical angle, thus providing relatively loose design constraints. 
     The minimum power resolution (i.e., smallest detectable change in power) of the sensor can be defined as
 
Δ P   n   =P   n (1− R )=(1 −R )·(1 −x )· x   n-1   ·P   0 .  (10)
 
Assuming N is the last sensor that can detect both P n  and ΔP n , then the length of the sensor can be written as:
 
 L=Nd.   (11)
 
       FIG. 5  depicts another embodiment of the sensor that utilizes a light source  20  with a network of light detectors  22  embodied in guide strips  44  about the body  40  of a vehicle. A cavity  32  is positioned about body  40 . Circuitry (not shown) can be positioned beneath sensor or within body  40 . This embodiment allows detection of the cavity position longitudinally and radially. 
     One embodiment of the light detecting unit  22  is shown in  FIG. 6 . At each light detecting unit  22  location, a beam splitter  50  is incorporated into the bottom of the guide  44 . Beam splitter  50  has an incident surface  52 , a partially reflective surface  54 , a transmission surface  56 , and a reflection surface  58 . Incident light  60  strikes the incident surface  52  of the beam splitter  50  at an angle of zero degrees to the normal. A portion  62  of the incident light  60  as given in equation (9) is transmitted through the beam splitter  50  to a photo-detector  64  that is positioned on the transmission surface  56  of the beam splitter  50 . As given by equation (7), the remainder of the incident light is reflected and directed towards the guide-environment interface  66  such that the angle of incidence that the reflected ray makes with the guide-environment interface  68  is 
                     θ   i   ′     =       π   2     -       θ   i     .               (   12   )               
Therefore, the next light detecting element  22  must be arranged at a different angle to ensure the incident surface of the beam splitter is normal to the incoming ray. If the initial incidence angle of the light is set to
 
                       θ   i     =     π   4       ,           (   13   )               
then each light detecting element  22  can be arranged in the same orientation.
 
       FIG. 7  shows another embodiment of the light detecting element  22  in which a beam splitter  70  is at or near bottom of the guide  44  away from the detected environment. The exterior of guide  44  is subjected to a gas-liquid interface  32  having a cavity closure  18  with the gaseous phase indicated at  34  and the liquid phase indicated at  36 . Light source  20  provides light  72  directed through guide  44  where it contacts a beam splitter  70 . Beam splitter  70  splits light  72  into a first portion  74  directed toward guide  44  surface and a second portion  76  for transmission to additional beam splitters  70 . If necessary, a mirror  78  can be incorporated in guide  44  for directing first portion  74  of light at the appropriate incident angle θ i . When the surface of guide  44  illuminated by first portion  74  of light is surrounded by cavity  34 , first portion  74  reflects on path  80 . Path  80  terminates at photo-detector  64  also positioned at the bottom of the guide  44 . The portion of light transmitted at the beam splitter  70  is incident upon another beam splitter  70  fixed somewhere along the length of the sensor. When the first portion illuminates the surface of guide  44  surrounded by a fluid  36 , first portion refracts on path  82 . This embodiment allows for variable spatial resolution along the length of the sensor, which may be changed depending upon the requirements of its application. 
     An alternative embodiment of the sensor is illustrated in  FIG. 8 . This embodiment consists of a single light source  20  and light detecting element  22  that is repositionable along the length of the guide  44  from a first position  82  to a second position  84 . In first position  82 , light  42  reflects from guide  44  to element  22 . In second position  84 , light  42  transmits through guide  44 . Cavity closure  18  can be detected by the lower light level at second position  84 . This embodiment provides the sensor with larger dynamic range because the signal is not split and potentially better spatial resolution. 
     The invention can be practiced other than as described herein. For example, the light source can have a specific frequency of light not ordinarily present in the environment, and the detector can be tuned to receive that specific frequency. This would limit interference from external light. 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.