Patent Abstract:
Sensing elements that quickly and accurately determine if a liquid or gas is present around the sensing elements are disclosed. These sensing elements find particular application in identifying the location of the cavity wall in which a supercavitating vehicle is operating, relative to the vehicle. In certain embodiments signal emitting elements carried on the vehicle emit signals towards the presumed position of the cavity wall, and sensing elements carried on the vehicle receive the emitted signals after they are reflected off of the cavity wall. The sensing elements identify the location where the reflected signal is received, and based on this identified location, the location of the cavity wall is determined. In alternative embodiments, sensing elements are positioned along fins extending outward with respect to the hull of the vehicle, and the sensors sense the presence of liquid or gas.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of prior filed co-pending U.S. application No. 60/695,337, filed on Jun. 30, 2005, the content of which is incorporated fully herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to methods and systems for sensing the presence or absence of liquid or gas around a sensor and, more particularly, for sensing and tracking the location of the wall of the cavity formed about a supercavitating vehicle and, even more particularly, to the sensing and identifying the location of the cavity wall relative to an underwater supercavitating vehicle without measuring the time-of-flight of an optical or RF signal. 
   2. Description of the Related Art 
   The U.S. Navy has funded long-running research programs for controlling supercavitating projectiles and vessels, referred to herein generically as supercavitating vehicles. Some of this work extends back to the 1940&#39;s and 1950&#39;s. The non-linear and high-speed nature of supercavitation makes control of supercavitating projectiles and vessels difficult. 
   Operating and controlling a supercavitating vehicle in an optimal manner involves limiting friction exerting drag on the vehicle. As is well known, a supercavitating vehicle operates within a cavity formed around the vehicle and contact between the supercavitational vehicle and the wall of the cavity increases the friction and thus the drag exerted on the vehicle. Thus, it is important to be able to extract and measure where the wall of the cavity is located, so that the vehicle can be operated in a manner that minimizes contact between the vehicle and the cavity wall. Stable guidance of the vehicle is critically dependent upon maintenance of the cavity so as to limit the friction exerted on the vehicle, and this guidance is dependent upon having quick and accurate information about the location of the cavity wall relative to the vehicle at all times. Thus, it is desirable to have a method for sensing and tracking the location of the cavity wall quickly and accurately. 
   SUMMARY OF THE INVENTION 
   The present invention pertains to sensing elements that quickly and accurately determine if a first changing media of a first index of refraction or a second changing media of a second index of refraction is present around the sensing elements. These sensing elements find particular application in identifying the location of the cavity wall in which a supercavitating vehicle is operating, relative to the vehicle, particularly where the first media is a liquid and the second media is a gas. In certain embodiments signal emitting elements carried on the vehicle emit signals towards the presumed position of the cavity wall, and sensing elements carried on the vehicle receive the emitted signals after they are reflected off of the cavity wall. The sensing elements identify the location where the reflected signal is received, and based on this identified location, the location of the cavity wall is determined. In alternative embodiments, sensing elements are positioned along fins extending outward with respect to the hull of the vehicle, and the sensors sense the presence of liquid or gas. Sensors sensing gas identify portions of the fin that are located within the cavity, and sensors sensing liquid identify portions that are located beyond the cavity wall. This enables quick and accurate location of the location of the cavity wall. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the known concept of creating a cavity around a supercavitational vehicle; 
       FIG. 2  illustrates a first embodiment of the present invention; 
       FIG. 3  illustrates a configuration of the embodiment discussed in  FIG. 2  in which more detail is provided regarding the sensing element; 
       FIG. 4  illustrates an alternative embodiment, whereby a fiber optic bundle conveys the light received after reflection off of the cavity wall to the bank of photo-resistive diodes; 
       FIG. 5  illustrates an alternative embodiment for sensing the location of a cavity wall relative to the vehicle; 
       FIG. 6  illustrates an alternative embodiment for sensing the presence or absence of water along the fin; and 
       FIG. 7  illustrates an example of a typical processing circuit that can receive the outputs from the light-sensitive receivers and utilize this information to determine the cavity wall location and, if desired, control guidance of the vehicle. 
   

   DETAILED DESCRIPTION 
   The present invention is a method and system for sensing the presence of changing media having differing indices of refraction, e.g., gas or liquid, around a sensor, and in a preferred embodiment, this information is used for monitoring the location of the cavity wall surrounding a supercavitational vehicle, relative to that vehicle. The examples illustrated herein all pertain to underwater vessels where the vessel is operating in water and the cavity is formed by the absence of water created by a cavitation. However, it is understood that the sensors of the present invention can be used in any environment where the sensor is in contact with media having differing indices of refraction; air and water are used for the purpose of example. It is contemplated that a shock boundary between two gaseous media (as would be found in a supercavitating missile operating in the earth&#39;s atmosphere) could be detected using the principles of the present invention. 
     FIG. 1  illustrates the known concept of creating a cavity around a supercavitational vehicle. Referring to  FIG. 1 , a vehicle  100  (e.g., a torpedo traveling through water) has a cavitator  102  attached at the front of the vehicle  100 . In a well known manner, cavitator  102  creates an air cavity  106  surrounding the vehicle  100 . A cavity wall  108  defines the border between the air cavity  106  and the fluid in which the vehicle  100  is traveling. Fins  104  extend away from vehicle  100  in a well known manner and are utilized for stabilizing and controlling the vehicle  100 . It is understood that the vehicle illustrated in  FIG. 1  is schematic in nature and is not to scale, but is instead utilized to identify the various parts of the structure and their relationship to the air cavity  106 . 
     FIG. 2  illustrates a first embodiment of the present invention. A light source  210  (e.g., a laser, LED, or the like) is configured into the hull of vehicle  100  and directs a light beam at a predetermined angle away from the vehicle  100 . In a preferred embodiment the light source  210  comprises a laser, because this embodiment makes use of the Snell&#39;s law of reflection, i.e., that the angle of incidence of a light beam is equal to the angle of reflection of the light beam. As such, since a laser can be more specifically directed to a point of reflection, a laser will result in a more accurate result. An LED, while functional for this purpose, has a more multi-directional emission. 
   A series of light-sensitive receivers illustrated collectively as sensing element  212  of  FIG. 2  are positioned along the hull of vehicle  100  such that light reflecting off of cavity wall  108  will be received by one or more of the light sensors in sensing element  212 . The exact positioning of sensing element  212  can be determined in a known manner based upon the angle at which the light source  210  emits its light and the estimated maximum and minimum distances between the vehicle  100  and the cavity wall  108 . These maximum and minimum distances can be determined based upon the operational specifications of vehicle  100 . Further, the distance along the hull between the light source  210  and each of the light-sensitive receivers comprising sensing element  212  are known and these values are stored in a processor (not shown) on board vehicle  100 , which processor is configured to receive and process data signals from the light-sensitive receivers. 
   As shown in  FIG. 2 , three different cavity wall positions, cavity wall position  208   a , cavity wall position  208   b , and cavity wall position  208   c , are illustrated by dotted lines. At any given moment there will only be a single cavity wall; however, since vehicle  100  is traveling in fluid, the position of the cavity wall relative to the vehicle  100  will fluctuate, and this fluctuation is illustrated by the three cavity wall positions  208   a ,  208   b , and  208   c.    
   The basic operation of the configuration shown in  FIG. 2  is now described. The light source  210  emits a light beam  214 . In a well known manner, upon the light beam  214  striking the cavity wall, a significant portion of the light beam  214  is reflected back towards the vehicle  100 . For example, as shown in  FIG. 2 , if the cavity wall is located at position  208   a , light source  214  will travel up to the cavity wall at position  208   a  and then reflect back towards sensing element  212  as reflected beam  216   a . If the cavity wall is closer to the vehicle at position  208   b , the light beam  214  will be reflected back towards sensing element  212  as reflected beam  216   b . Finally, if the cavity wall is at location  208   c , the light beam  214  is reflected back towards sensing element  212  as reflected beam  216   c.    
   As can be seen from  FIG. 2 , the location of the cavity wall will determine where on sensing element  212  the light beam is reflected. By determining where along sensing element  212  the light beam is received (i.e., identifying which of the light-sensitive receivers receives the reflected light beam), the processor can be used to calculate the approximate perpendicular distance between the vehicle  100  and the cavity wall, referred to herein as the “standoff distance”. Specifically, the standoff distance SD can be calculated using the formula SD=(X/2)*tan(theta), where X is the distance along the hull from the light source  210  to the light-sensitive receiver receiving the reflected beam, and theta is the angle between the light beam  214  and the hull. Since each light-sensitive receiver will have a unique value of X (distance along the hull from light source  210  to the light-sensitive receiver), the value of SD can be calculated easily and quickly. 
     FIG. 3  illustrates a configuration of the embodiment discussed in  FIG. 2  in which more detail is provided regarding the sensing element  212 . Referring to  FIG. 3 , a laser  320  is utilized as the light source and projects light beam  214  out away from the vehicle as described previously. Sensing element  212  comprises a plurality of photo-resistive diodes  322   a - 322   i . Each of the photo-resistive diodes  322   a - 322   i  is coupled to a processing element  323  (connections omitted for simplicity), the function of which is described in more detail below. Thus, whichever of the photo-resistive diodes  322   a - 322   i  receives the reflected light beam from the cavity wall will sense a threshold level of received light that is significantly higher than those received by the remaining photo-resistive diodes. Accordingly, with knowledge of the angle at which light beam  214  leaves vehicle  100 , relative to the vehicle, and knowledge of which of the photo-resistive diodes is currently receiving the reflected beam, a simple calculation can be made to determine the standoff distance between the cavity wall and vehicle  100 . It is understood that although photo-resistive diodes are illustrated herein, numerous alternatives for the light-sensitive receiver will be apparent to a designer of ordinary skill in the art and such alternatives are covered by the appended claims. 
     FIG. 4  illustrates an alternative embodiment, whereby a fiber optic bundle comprising, in this example, optical fibers  424   a - 424   i , convey the light received after reflection off of the cavity wall to the bank of photo-resistive diodes  322   a - 322   i . The operation is otherwise identical to that of  FIG. 3 . With respect to  FIGS. 3 and 4 , it will be understood that nine photo-resistive diodes and/or photo-resistive diode/optical fiber pairs are shown for the purpose of example and that these numbers may be increased or decreased depending upon the needs of a particular designer. 
     FIG. 5  illustrates an alternative embodiment for sensing the location of a cavity wall relative to the vehicle  100 . In this embodiment, sensors referred to herein as “dome sensors” are situated along at least one of the fins  104  projecting outward from vehicle  100 . In the illustration of  FIG. 5   a , four such dome sensors  530 - 536  are illustrated, with details of the dome sensors  530 - 536  being illustrated in  FIG. 5   b . It is noted that, although not shown, electrical connections are utilized to connect the dome sensors to the processing circuitry  523  to enable transmission of the output of each dome sensor to the processing circuitry so that the presence or lack thereof of a liquid or a gas in contact with the dome sensors can be ascertained. Further, although there are four dome sensors shown, it is understood that in most instances there would likely be many more dome sensors to increase the resolution of the sensing of the location of the cavity wall. 
   Referring to  FIG. 5   b , each dome sensor includes a light source  540  (e.g., an LED, laser, etc.) and a light sensitive receiver  542  (e.g., photo diode, photo transistor, etc.). A dome  538 , made of glass, plastic, ceramic, or any other material that will allow light to pass therethrough, extends outward from the fin  104 , such that the dome  538  contacts any gas or liquid in contact with the portion of the fin  104  on which the dome sensor is situated. If desired, optical fibers can be situated between the light source  540  and the light receiving element  542  to direct the light to and from the dome  538 . The dome  538  is a fixed media that forms a reflective/refractive interface with a changeable media (e.g., a first changeable media such as water, a second changeable media such as gas, etc.). 
   The operation of the dome sensor is as follows. Light source  540  emits a light beam  544 . When the dome  538  is in contact with water or other liquid a large portion of the light beam  544  refracts out into the liquid (illustrated by dotted line  546 ) and thus is not reflected back to the light sensing element  542 . However, in situations where there is no liquid in contact with the dome  538 , the light beam  544  reflects off the inside of the dome  538  and is received at light-sensitive receiver  542  (illustrated by line  548 ). Since there will be significantly more light received at light-sensitive receiver  542  when there is no liquid present outside of the dome  538 , the processing circuitry is able to identify when a liquid is present (sensing of a level of light below a predetermined threshold), and when a liquid is not present (sensing of a level of light at or above a predetermined threshold). Accordingly, an indication of a liquid being present indicates that the particular dome sensor indicating the presence of the liquid is beyond the cavity wall (i.e., it is in the liquid). However, dome sensors that are within the cavity will sense the presence of air (or the lack of water), indicating they are within the cavity. Therefore, it is possible to identify approximately where along the fin  104  the border between the cavity and the water exists, thereby identifying the approximate location of the cavity wall. 
     FIG. 6  illustrates an alternative embodiment for sensing the presence or absence of water along the fin  104 . Referring to  FIG. 6 , a series of optical fibers  650 ,  652 ,  654 , and  656  are shown. Each optical fiber comprises a loop of fiber which originates within the vehicle  100 , travels along fin  104  to a particular location along the edge of fin  104 , has a bent portion extending beyond, or flush with, fin  104  and then returns back to vehicle  100  (in  FIG. 6  the bent portion is shown as extending beyond the fin; the bent portion can instead be flush with the fin so as not to protrude out from the fin). This configuration defines multiple paths from the vehicle to an outer edge of fin  104  and back to the vehicle. In the example of  FIG. 6  there are four such optical fiber elements shown; however, it is understood that in most configurations there will be many more such elements and the more elements there are, the better the resolution of the sensing of the location of the cavity wall. 
   Shown within the dotted line circles in  FIG. 6  are exploded views of the exposed bent element  651  of fiber  650  and the ends  658  and  660  of fiber  650 . A light source  662  is situated at the outbound end  658  of fiber  650  and inputs light thereto in a well known manner. The light travels along outbound portion  658  until it reaches the bent element  651 , which is exposed outside of, or flush with, the fin  104  such that it is in contact with any liquid or gas that is in contact with fin  104  at that point. The bent element  651  forms a reflective/refractive interface with changeable media (e.g., water, air, etc.) coming in contact therewith. If a liquid is in contact with the bent element  651 , light traveling along outbound path  658  will refract out into the water and thus minimize the amount of light that continues along fiber  650  down the inbound path  660 . However, in the absence of water, light traveling along outbound path  658  will continue around the bent element  651  and be returned along inbound path  660  to a light-sensitive receiver element  664 . The sensor of  FIG. 6  utilizes the known property of optical fibers that light can leak from bends in the fiber. The boundary between two transparent media having different indices of refraction (in this example, there will be either a fiber/air interface or a fiber/liquid interface) will refract and reflect light differently, depending on the particular types of media. The measurable quantity of light returning on fiber is modulated by the change in the external medium in an identifiable way, allowing the type of media to be discerned as described above with respect to the dome sensor. 
     FIG. 7  illustrates an example of a typical processing circuit that can receive the outputs from the photodiodes and utilize this information to determine the cavity wall location and, if desired, control the guidance of the supercavitational vehicle. It is understood that this circuit is presented for the purpose of example only and that there are multiple other configurations that can be utilized to perform this function. 
   The output of each light-sensitive element ( 702  in  FIG. 7  is representative of each photodiode or other light-sensing element) is input to a wideband photodiode amplifier  704  which converts the photodiode current into an amplified voltage. Threshold device  706  (e.g., a comparator) compares the voltage output from the wideband photodiode amplifiers to predetermined voltage references set for each sensor. A logic 1 is output from the threshold device  706  only if its threshold is exceeded. Accordingly, until the light received at a particular light-sensing element  702  is of a level which will output a current that, when amplified by the wideband photodiode amplifier  704  exceeds the threshold level, there will be a logic 0 output from threshold device  706 . Therefore, if water is present, a logic 0 will be output, and if air water is present, a logic 1 level will be output. 
   The output of each threshold device  706  is input to processor  708 . Processor  708  is configured to identify which light-sensitive receivers are sensing the presence of water and which are sensing the presence of gas. Data regarding the location of each sensor is stored in processor  708 , and thus a determination can be made as to the location of the cavity wall. As the cavity wall moves, different light-sensitive receivers receive the reflected light, and hence the correspondence between the photo detection and cavity wall location changes accordingly. 
   The time to complete processing and make steering adjustments in a supercavitating vehicle varies from 100 μsec for speeds of 20 m/s to less than 2 μsec for speeds of 1000 m/s. These calculations assume a maximum displacement of 2 mm before correction occurs. The sensors described herein can have response times as low as 1 μsec or less. Each of the sensors give a robust indication of the proximity of the cavity wall, in a very short period of time. 
   A control system utilizing the sensors of the present invention can be a guidance control processor  710  which receives the data from processor  708  that discriminates between the various media around each sensor and thus can determine the location of the cavity wall relative to the vehicle, and guidance control processor  710  can then actuate the control fins on the supercavitating projectile or vessel. This configuration can use a classical approach to control system design, for example, the system described in Dzielski and Kurdila (“A Benchmark Control, Problem for Supercavitating Vehicles and an Initial Investigation of Solutions,” Pennsylvania State University and University of Florida). Alternatively, the control system could take a much more neural network approach, so that the guidance control processor is really only a collection of “neural synapses” such as an animal nervous system ganglian or simple insect brain, as described in Zbikowski (“Sensor-Rich Feedback Control,”  IEEE Instrumentation and Measurement Magazine , Vol. 7, No. 3, pp. 19-26). This neural network type of control system has been described by Zbikowski as a “sensor-rich system” and not “actuator-rich”, since as many sensors as desired can be utilized to monitor the proximity of the supercavitating cavity wall without increasing the number of actuators or control fins. The advantage of this type of control system is that it is conceptually simple and relatively easy to implement in hardware and software. 
   While there has been described herein the principles of the invention, it is to be understood by those skilled in the art that this description is made only by way of example and not as a limitation to the scope of the invention. Accordingly, it is intended by the appended claims, to cover all modifications of the invention which fall within the true spirit and scope of the invention.

Technology Classification (CPC): 1