Patent Publication Number: US-2018052235-A1

Title: Optical Navigation for Underwater Vehicles

Description:
STATEMENT OF GOVERNMENT INTEREST 
     FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT 
     The United States Government has ownership rights in this invention. Licensing inquiries may be directed to Office of Research and Technical Applications, Space and Naval Warfare Systems Center, Pacific, Code 72120, San Diego, Calif., 92152; telephone (619)553-5118; email: ssc.pac.12@navy.mil. Reference Navy Case No. 103,105. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of Invention 
     This disclosure relates to optical navigation and, more particularly, to optical navigation for underwater vehicles. 
     Description of Related Art 
     Underwater navigation presents challenges for vehicles. Underwater navigation is not feasible for a typical global positioning system (GPS) as these systems cannot operate underwater. The radio frequency signals that are typically necessary for GPS are attenuated by water. Therefore, the location of an underwater vehicle may not be known until the vehicle resurfaces for GPS navigation or visual confirmation. Accordingly, a means to track location between known points is required for location accuracy. Given the current availability of navigation tools for underwater use, the cost has been prohibitive for many uses. When an underwater vehicle submerges, location metrics such as from GPS and other communication methods are lost. At this point, the underwater vehicle must rely on onboard sensors to maintain location accuracy. 
     Prior art methods for underwater navigation include using an Inertial Measurement Unit (IMU), Doppler Velocity Log (DVL), or acoustic communication with surface floats or subsea clumps. The cost of these sensors can be on the order of at least tens of thousands of dollars. In addition, these sensors are delicate and subject to damage, and may require active logistics support to accomplish the task via surface or underwater reference locators. Typical additional costs when acquiring and adapting the above-mentioned devices include customizing proprietary programming, non-recurring engineering cost associated with feature implementation, and support hardware. 
     In addition, an IMU is very sensitive to shock and may not be reliable. A DVL works through acoustic means and may be sensitive to fouling as its sensors are exposed to seawater. IMUs and DVLs also don&#39;t report position, so their solution needs to be integrated with respect to time, so even the highest end sensor will experience navigation “drift”. Other acoustic means using known reference sources are limited by range, are noisy (not covert) and require a lot of energy. 
     Computer mouse technology is well proven and accurate for local telemetry and is achieved for a very low cost. Therefore, it should be considered for underwater telemetry. It is very robust with high reliability, and can be made easily programmable through commonly available means. It works by performing image processing algorithms to determine the offset of features between multiple images taken with the mouse&#39;s optical sensor. It typically uses a standard LED or laser in the red-to-infrared spectrum to illuminate a scene. The return images are retrieved through a set focal length lens. When a surface is within close proximity (approximately 0-6 inches), LED is sufficient to illuminate the surface and the sensor can achieve high accuracy tracking. 
     Though the sensor is capable of taking measurements with ambient light, it can be shown that the accuracy diminishes with lower light conditions. By using a laser or other light source, the measurement field can be illuminated such that the sensor can more easily detect differences in the images and track movement. Because a laser can focus on a given point on the measured surface (hereafter called “ground”), given the proper lens geometry, the sensor can track telemetry in a similar manner to its more conventional desktop use. 
     The typical mouse sensor has a near focus, narrow field of view lens that is physically very close to the light source and the ground. This geometry is preserved in its application because the sensor and light source are always at a constant distance from the ground (i.e. the mouse is physically on the ground). This, however, is impractical for underwater navigation as the ground is very seldom flat. 
     There is a need for incorporation of a low-cost mouse sensor into a system for low-cost optical navigation for underwater vehicles. This new system should address the aforementioned shortcomings of using a mouse sensor system that was designed for a computer. 
     BRIEF SUMMARY OF INVENTION 
     The present disclosure addresses the needs noted above by providing an underwater vehicle and method for underwater navigation. In accordance with one embodiment of the present disclosure, the underwater vehicle is capable of operating within close proximity to an underwater ground. The underwater vehicle includes an optical navigation system. The optical navigation system comprises a pressure housing that includes, disposed within the pressure housing: a sensor capable of taking images; a light source configured to produce a light beam that is offset from the sensor lens. The light source is further configured to reflect light directly into the field of view of the sensor. The navigation system also includes a processor, operably coupled to the sensor. The processor is configured to execute instructions. A memory, operably coupled to the processor and sensor, stores processor-executable instructions and images taken with the sensor. When executed, the instructions cause the processor to determine the offset of features between at least two images taken with the optical sensor. The instructions cause the processor to determine a distance traveled based on the offset between the at least two images. 
     These, as well as other objects, features and benefits will now become clear from a review of the following detailed description, the illustrative embodiments, and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and form a part of the specification, illustrate example embodiments and, together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  illustrates an underwater vehicle and an optical navigation system in accordance with one embodiment of the present disclosure. 
         FIG. 2  illustrates an exploded view of components of a system for optical navigation of underwater vehicles, in accordance with one embodiment of the present disclosure. 
         FIG. 3A  illustrates an exterior view of the system in  FIG. 2  optical navigation of underwater vehicles, in accordance with one embodiment of the present disclosure. 
         FIG. 3B  illustrates a cross-sectional view of the system for optical navigation of underwater vehicles, in accordance with one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The optical navigation system and method disclosed herein achieve two-dimensional (2D) navigation telemetry for underwater vehicles by leveraging open source programming and low cost commercial off-the-shelf (COTS) technology. 
     Disclosed herein is an underwater vehicle with an optical navigation system that is disposed within a pressure housing. Also disclosed herein is a method for optical navigation for underwater vehicles. The optical navigation system and method include a sensor that takes images of an ocean floor or other underwater ground, through a sensor lens. A light source produces a light beam that is offset from the sensor lens. The light source reflects light directly into the field-of-view of the sensor. The field of view may feature the ocean floor. The sensor takes multiple images which are received by software that is stored in memory that resides within the housing. The software, which may be feature detection software, is executable by a processor. When executed, the software causes the processor to determine the offset of features between at least two images taken with the sensor. In this manner, navigation information may be derived. This navigation information may include a vehicle&#39;s two-dimensional position, especially when a compass is used for a fixed reference. In addition, for underwater vehicles, the information could include surge (front-back motion) and sway (side-to-side motion) which may occur as a result of wave motion. The optical navigation system disclosed herein could be adapted for use with land vehicles. 
     Referring now to  FIG. 1 , illustrated is an underwater vehicle to which the optical navigation system has been attached. The optical navigation system  110  is mounted to the underside of underwater vehicle  120 . In lieu of the underwater vehicle  120  shown in  FIG. 1 , the optical navigation system  110  may be used with other underwater vehicles. For example, autonomous underwater vehicles may be used to perform underwater survey missions. The missions may include detection and mapping of obstacles that pose a hazard to navigation for water vessels. These obstacles may include debris, rocks and submerged wrecks. Other underwater vehicles may be manned, e.g., vehicles transporting scientists for exploratory purposes. Numerous other examples exist for underwater vehicles or other objects or bodies that can be used with the present disclosure. The vehicle, other object or person needs to be capable of operating underwater within close proximity to underwater ground, or the water&#39;s floor. 
     The optical navigation system  110  may take images of the ocean floor. Based on those images, the system  110  can determine the two-dimensional position of underwater vehicle  120 . The optical navigation system  110  can also determine surge motion has occurred based on how far front and/or back at least one of the images is from at least one other image. The optical navigation system  110  can determine how much sway motion has occurred based on how far sideways at least one of the images is from at least one other image. 
     As shown in  FIG. 1 , light beam  113  is emitted from the optical navigation system  110  via a light source (not shown) that is resident within the housing of the optical navigation system  110 . The light from light beam  113  is then reflected from the underwater ground  115  which, in this embodiment is a sea floor. The light is then received back into the optical navigation system  110  via a camera resident within the optical navigation system  110 . 
     Referring now to  FIGS. 1 and 2  together, the optical navigation system  110  includes a watertight pressure housing that includes a pressure body  210  and a pressure lid  220  to contain the elements of the optical navigation system  110 . The pressure body  210  and a pressure lid  220  may include a watertight seal provided by O-ring  225 . Multiple O-rings such as O-ring  225  may also be used. 
     Disposed within the pressure body  210  are an optical sensor  230  and a sensor lens  240 . The optical sensor  230  is capable of taking images through sensor lens  240 , and thus the line of sight of optical sensor  230  should be directed through sensor lens  240 . Optical sensor  230  may be a complementary metal-oxide-semiconductor (CMOS) sensor, an N-type metal-oxide-semiconductor (NMOS), a semiconductor charge coupled device (CCD) sensor or other sensor capable of taking digital images or capable of converting reflecting light back to a digital signal. 
     Still referring to  FIGS. 1 and 2  together, lens  240  may be a typical single lens reflex (SLR) lens with differing focal lengths. Lens  240  may be used to focus light reflected back into optical sensor  230  based on the distance of the optical navigation system  110  from underwater ground  115 . For purposes of the present disclosure, underwater ground  115  may include the bottom of an ocean or a sea, or a manmade body of water through which an underwater vehicle may travel. In the present illustration, underwater ground  115  is the sea floor. It may also be possible to implement the optical navigation system  110  without lens  240  where a laser beam is used for light source  250 . When light beam  113  is emitted from a laser as light source  250 , the emitted light may already be focused. 
     Still referring to  FIGS. 1 and 2  together, light source  250  produces a light beam  113  that may be offset from the sensor lens  240 . Light source  250  may be a standard LED or a laser in the red-to-infrared spectrum that illuminates the underwater ground  115 , or sea floor. When underwater ground  115  is within close proximity to light source  250 , a light emitting diode (LED) may be sufficient to illuminate the underwater ground  115  and the optical sensor  230  can achieve high accuracy tracking. Close proximity to underwater ground  115  may mean as little as approximately zero to six inches (0″-6″), and in some cases, as much as zero to eighteen inches (0″-18″). The light source  250  is positioned to reflect light directly into the field of view of the optical sensor  230 . In one example, the field of view may be thirty degrees (30°). The farther from the underwater ground  115  the light source  250  is positioned, the more distance covered by the field of view. 
     Still referring to  FIGS. 1 and 2  together, the optical sensor  230  is capable of taking measurements with ambient light. However, accuracy may be diminished with lower light conditions. If the optical sensor  230  incorporates a laser as light source  250 , the measurement field can be illuminated such that the optical sensor  230  can more easily detect differences in the images and track movement. Because a laser can focus on a given point on underwater ground  115 , given the proper lens geometry, the optical sensor  230  can track telemetry in a similar manner to its more conventional desktop use. 
     Still referring to  FIGS. 1 and 2  together, though light source  250  need not be a laser, a laser may be more effective for longer distances between the optical sensor  230  and ground  115 . Using a laser may minimize the illuminator&#39;s projection on the medium, thus minimizing backscatter. Wavelengths for light source  250  can be chosen such that backscatter from the water particulates are minimized, and less power is required to achieve high local illuminance values. As a general matter, higher wavelengths may tend to attenuate more and scatter more in sea water. Lasers with wavelengths in the green spectrum may work well in the water because they may propagate through the water. However, it should be considered whether green may propagate too well and be too light for the sensor  230 . Lasers having wavelengths in the red spectrum may also be a suitable fit. The power of the laser may also be taken into account in order to reduce attenuation in ways that are known in the art. 
     The ocean floor and other underwater ground areas are very seldom flat. Therefore, it may be desirable for the light source  250  and the optical sensor  240  to be on the same optical path. Ideally, when using a laser, the line of sight of the optical sensor  230  should be on the same axis as the beam path of the laser to eliminate any errors due to parallax. Parallax is a displacement or difference in the apparent position of an object when the object is viewed along two different lines of sight. Parallax may be measured by the angle or semi-angle of inclination between those two lines. 
     Light source  250  may be made to travel directly through the sensor lens  240  (bore sighting), or it may be mounted at a minimum slight offset, so that it can reflect light directly in the field of view of the optical sensor  230 . If the light is made to travel directly through the sensor lens  240 , this has the advantage of zero parallax so that distance is not an issue for alignment, only illuminance. 
     The sensor lens  240  may have a wider field of view or a larger depth of field to maintain low sensitivity to varying height. Two-dimensional (2D) telemetry is taken with the optical sensor  230  and calibrated through compass readings. A compass (not shown in  FIG. 1 ) may be provided onboard the underwater vehicle  120 . Commercially available compasses, which are cheap and robust, may be used to provide a fixed reference frame, including north, south, east and west coordinates. Thus, the compass may give a fixed geographical position for the underwater vehicle  120 . The compass may also include rotation, pitch and yaw data for further accuracy. The compass (not shown in  FIG. 2 ) may be operably coupled to the processor  245  and optical sensor  230 . 
     Circuit board  260  includes a processor  245  that is operably coupled to the optical sensor  230 . Processor  245  may be a digital signal processor. A power source  247 , e.g., a battery, may provide power to the optical sensor  30 , processor  245 , light source  250  and other components needing power. Circuit board  260  also includes a memory  235  that stores processor-executable instructions as well as images taken with the optical sensor  230 . Processor  245  should be of sufficient speed to process images and instructions for the optical navigation system  110  at the rate needed in order to determine image offsets at the rate necessary to accomplish 2-D navigation. Images of underwater ground  115  may be captured in continuous succession and compared with each other in order to determine how far the underwater vehicle  120  has moved. Memory  235  or other data storage medium should be of sufficient size to store multiple images over at least the course of a trip for the underwater vehicle. Memory  235  is operably coupled to processor  245 . When executed, the instructions in memory  235  cause the processor  245  to determine the offset of features between at least two images taken with the sensor  230 . Features may include any identifiable characteristic in the image, including any change in pixel. The features may include rocks, aquatic plants, changes in elevation, and any other feature that can translate to an identifiable pixel. Features can even be naked to the human eye, such as a multiple lighter colored pieces of sand next to multiple slightly darker colored pieces of sand. The features may also include different textures on the underwater ground  115  or sea floor. 
     A window  280  is disposed within the watertight pressure housing. Window  280  is configured to receive light emitted from the light source to the underwater ground  115 . The window  280  is further configured to receive light reflected back from the underwater ground  115  to a field of view of the optical sensor  230 . Bolts  290  or other securing means may secure the pressure lid  220  to the pressure body  210 . 
     Optical sensor  230  may be chosen, at least in part, based on its frame rate. The frame rate needed for optical sensor  230  may depend on the speed of the vehicle or other body on which the optical sensor  230  is mounted. 
     The frame rate needed for the optical sensor  230  may be determined according to the following equation: 
     
       
         
           
             
               
                 
                   
                     
                       
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             θ=FOV of optical sensor 
             β=% frame overlap needed for Digital Image Correlation (DIC) 
             H=height of optical sensor from reflecting surface 
             FPS=Frames per second of optical sensor 
             V=velocity of vehicle. 
           
         
       
    
     The return images may be received via sensor lens  240 , which may have a set focal length. 
     Digital image correlation and tracking and/or image processing algorithms may be used to determine the offset of features between multiple images taken with the optical sensor  230 . Digital image correlation and tracking is an optical method that uses tracking and image registration techniques for accurate two-dimensional and three-dimensional measurements of changes in images. An example of a digital image correlation technique is cross-correlation to measure shifts in data sets. Another example of a digital image correlation technique is deformation mapping, wherein an image is deformed to match a previous image. 
     Feature detection algorithms are an example of the type of image processing algorithm that may be used. Feature detection algorithms are known in the art. Examples of feature detection algorithms can be found in the following publication: Jianbo Shi and C. Tomasi, “Good features to track,” Computer Vision and Pattern Recognition, 1994. Proceedings CVPR &#39;94., 1994 IEEE Computer Society Conference on, Seattle, Wash., 1994, pp. 593-600. 
     Some feature detection algorithms receive an image, divide it into segments and look for features, texture and surfaces as markers. For example, if a camera zooms in to a small square, e.g., a sandy bottom, pixels will show distinctions between portions of the sandy bottom. Markers such as these may be compared in subsequent images to see how far a vehicle has traveled. Memory  235  may also be operably coupled to a compass (not shown in  FIG. 2 ) onboard the underwater vehicle so that the memory  235  receives data from the compass. In this manner, the compass data may be used to provide an absolute position for the underwater vehicle. 
     Still referring to  FIGS. 1 and 2  together, The distance traveled can be determined based on the focal length of the optical sensor  230 . If the height of sensor  230  in relation to underwater ground  115  is fixed, and the optical sensor  230  outputs pixels, the pixels could be converted to a value in feet or inches. The distance traveled will depend on how high the optical sensor  230  is from underwater ground  115 . If there are known data points as far as height, then the distance traveled can be extrapolated/interpolated based on that known data. For example, at twelve inches (12″) from underwater ground  115 , a ten-pixel movement may translate to three inches (3″) of travel. Therefore, this data can be interpolated so that a twenty-pixel movement may translate to six inches (6″) of travel. 
     Also by way of example, if we know what the distance traveled would be if we were six inches (6″) from underwater ground  115  and eight inches (8″) from underwater ground  115 , we may be able to interpolate that data to reach a conclusion as to distance traveled if we were seven inches (7″) from underwater ground  115 . Generally, the closer to the water&#39;s floor, the less the vehicle has traveled. Feature detection algorithms, which may be obtained as COTS items, take information such as this into account. 
     Referring now to  FIGS. 3A and 3B  together,  FIG. 3A  illustrates an exterior view of the optical navigation system, while  FIG. 3B  illustrates a cross-sectional view of the optical navigation system. As shown in  FIGS. 3A and 3B  together, optical navigation system  110  includes a pressure body  210  and a pressure lid  220 . Bolts  290  or other securing means may secure the pressure lid  220  to the pressure body  210 . On the interior of pressure body  210  and pressure lid  220  may reside the sensor  230 , memory  235 , sensor lens  240 , processor  245 , light source  250  and circuit board  260 . Pressure body  210  and pressure lid  220  aid in keeping internal components sensor  230 , memory  235 , sensor lens  240 , processor  245 , light source  250 , and circuit board  260  protected from the pressure that can occur at significant subsea depths. Such pressures may be particularly strong near a sea floor or ocean floor. 
     Circuit board  260  and light source  50  may be mounted onto the interior of pressure body  210 , or otherwise disposed within pressure body  210 , using a number of means known in the art, including hard mounting, brackets, and foam. Mounted on circuit board  260  may be sensor  230 , memory  235 , sensor lens  240 , processor  245  and power source  247  (e.g., a battery). 
     When used underwater, it is the intention of this system to work where measurement can be taken close to the ground. Because of optical challenges with visibility and backscatter due to turbidity, distances of less than a meter from ground are expected for subsea use. However, this technology could be adapted as an alternative navigation source to any vehicle traveling over ground where the distance is known such as land vehicles. 
     Additionally it can be used where ambient light can be utilized for image processing, and the distance can be taken as optical infinity, such as day use for aerial vehicles, or where ground lights can be used as the tracking points during night flight. 
     The invention can take on alternate embodiments. In this invention&#39;s first embodiment, ground refers to the sea floor, however it is not limited to this. Ship hull inspection, pipeline inspection, etc. could also apply. Also, for vehicles that require an operational depth that is not near ground, the user could modify their vehicle&#39;s mission to submerge near the seafloor, navigate a 2D position, then float up to its desired working depth. 
     Another embodiment could be for land survey or mapping utilizing the high accuracy of this system. 
     Another embodiment could be as a cheap alternative for land or air speed utilizing the low cost of this system to eliminate the lens of the laser, the sensor or both. Autofocus could be implemented to account for varying measurement distance. Multiple systems could be used in tandem to reduce error for turbid conditions. Different colored lasers or alternative light sources could be used based on mission conditions for better performance or covert operations. 
     The present system incorporates proven, reliable components such as circuit boards, sensors and lasers have proven to be very high. The system may be provided using COTS, easy to use items. The present system eliminates the requirement for acoustic measurements. Therefore, operation can be made active while still maintaining a covert signature to listening devices. Because it does not use acoustic devices, the system has a comparatively lower energy cost. 
     The foregoing description of various preferred embodiments have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The example embodiments, as described above, were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.