Abstract:
A method of controlling the lateral position of a towed body in water where the body is equipped with effectors for effecting at least aileron and rudder control. A lateral correction is defined by a distance measured perpendicular to the towed body&#39;s direction of travel. A first command indicative of an aileron control is determined that would cause the effectors to move the body laterally of the direction of travel by an amount equal to the lateral correction. A second command indicative of a rudder control is determined that would cause the effectors to move the body laterally of the direction of travel by an amount equal to the lateral correction. The first and second commands are combined to define a combined command that is applied to the effectors so that the optimum aileron and rudder controls are implemented simultaneously.

Description:
ORIGIN OF THE INVENTION  
       [0001] The invention described herein was made in the performance of official duties by employees of the Department of the Navy and may be manufactured, used, licensed by or for the Government for any governmental purpose without payment of any royalties thereon. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to towed body systems such as those equipped with onboard sensors capable of sensing parameters of an underwater area, and more particularly to a method of controlling the lateral motion and position of such towed bodies.  
         BACKGROUND OF THE INVENTION  
         [0003]    Underwater searches and surveys are typically carried out by towing a sensor platform through the water. Many underwater sensors have a relatively small range and field-of-view. Consequently, a towed sensor platform carrying one or more sensors must be positioned almost exactly over a target or object of interest, hereinafter referred to simply as the “target”.  
           [0004]    Generally, a target&#39;s location is not known precisely. Therefore, it may not possible to plan a tow track that passes precisely over a target. Furthermore, the towed body&#39;s motion can be disrupted by water currents and wave action. In an attempt to address this problem, towed systems may have a forward-looking sonar that helps to reacquire the target shortly before reaching it. However, the range of the forward-looking sonar is limited. Thus, the advance time provided by the sonar detection is often too short to maneuver the entire system (i.e., the tow craft and towed body sensor platform) to assure that the sensing instrument passes over the target. If the target is missed (i.e., the target does not fall within the towed sensor(s) field-of-view), the entire maneuver must be repeated. This can be very time-consuming, depending on the system, the length of the tow cable, and the sea state conditions.  
         SUMMARY OF THE INVENTION  
         [0005]    Accordingly, it is an object of the present invention to provide a method of controlling the lateral position of a body as it is towed through the water.  
           [0006]    Another object of the present invention is to provide a method of quickly effecting lateral movement of a body as it is towed through the water.  
           [0007]    Still another object of the present invention is to provide a method of controlling the lateral movement of a towed body equipped to effect aileron, elevator and rudder control.  
           [0008]    Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.  
           [0009]    In accordance with the present invention, a method of controlling the lateral position of a towed body in water where the body is equipped with effectors for effecting aileron, elevator and rudder control. The body is pulled through the water along a known direction of travel and in a defined frame of reference having one axis along the known direction of travel. A lateral correction is defined by a distance measured perpendicular to the one axis in the frame of reference. A first command indicative of an aileron control is determined that would cause the effectors to move the body laterally of the direction of travel by an amount equal to the lateral correction. A second command indicative of a rudder control is determined that would cause the effectors to move the body laterally of the direction of travel by an amount equal to the lateral correction. The first and second commands are combined to define a combined command that is applied to the effectors so that the optimum aileron and rudder controls are implemented simultaneously. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a schematic diagram of a towed body system in which the body being towed is to be laterally controlled/repositioned in accordance with the present invention; and  
         [0011]    [0011]FIG. 2 is a flow diagram of the lateral position control method according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    Referring now to the drawings, and more particularly to FIG. 1, a towed body system includes a tow craft  10 , a tow body  12  and a tow line  14  tethering tow body  12  to tow craft  10 . Tow craft  10  pulls tow body  12  through a water environment along a nominal tow track  16 . The actual track  17  traversed by tow body  12  can deviate slightly from track  16  due to water currents, disturbances in the motion of tow craft  10 , etc. However, in most instances, this deviation is minor and will not have much impact on the method of the present invention.  
         [0013]    Tow body  12  is equipped with a variety of control effectors (e.g., control surfaces, jets, etc.) that can effect aileron control actions for roll control, rudder control actions for yaw or heading control, and elevator control actions for pitch control. For example, tow body  12  can have tail fins  12 A along with a depressor wing  12 B (as illustrated) as its control effectors. Tail fins  12 A can be installed in a number of well known configurations including cruciform, x-tail and inverted-Y configurations. Depressor wing  12 B can be a fixed wing pre-set to a non-zero angle to generate lift, or a movable wing capable of being adjusted in terms of its (lift) angle. Accordingly, it is to be understood that the particular hardware configuration of the control effectors is not a limitation of the present invention.  
         [0014]    By way of illustrative example, it will be assumed that tow body  12  is equipped with one or more onboard sensor systems (not shown) for detecting, imaging, localizing and/or classifying a target  100  which must fall within the sensor(s) field-of-view. However, it is to be understood that, in general, the present invention provides a method for the lateral position of any type of tow body. If target  100  is beyond the sensor(s) field-of-view, tow body  12  must be repositioned laterally relative to tow track  16  before reaching target  100 . Thus, the goal of the present invention is to effect lateral control of tow body  12  so that it will deviate from tow track  16  to a revised track  18  that captures target  100  within the tow body&#39;s sensor field-of-view.  
         [0015]    A towed body having control effectors as described above can move laterally in one of two ways:  
         [0016]    (1) If tow body  12  is rolled, the lift force produced by depressor wing  12 B will be angled, resulting in a lateral force. Therefore, although aileron (roll) control is normally provided by tail fins  12 A, they only have an indirect effect on lateral motion as they are only effective when depressor wing  12 B is deflected. If the wing deflection is small (i.e., if tow body  12  is operating shallow water), the effect of tail fins  12 A on aileron controlled lateral motion is weak. Even at deeper depths, the achievable lateral displacement is limited because the if tow body  12  rolls too much, target  100  will end up outside the sensor field-of-view. For these reasons, lateral control using aileron control is limited in its effectiveness.  
         [0017]    (2) The rudder control using tail fins  12 A produces a yaw angle that results in lateral displacement. This is the most effective way of achieving lateral motion. That is, large lateral position changes are possible using rudder control. This is especially true at shallow depths.  
         [0018]    The above description suggests that lateral control of tow body  12  should be done using only rudder control. However, the problem of moving tow body  12  laterally onto revised track  18  does not simply involve moving tow body  12  a certain distance. Rather, the problem also requires that tow body  12  change its position fast enough so that target  100  ends up in the tow body&#39;s sensor field-of-view before tow body  12  passes target  100 . The present invention is based on the discovery that lateral positioning control performance improves by using both rudder and aileron control simultaneously. Tow body  12  moves laterally faster in response to roll motion than to yaw motion. That is, aileron control of tow body  12  results in a very quickly executed roll maneuver. The resulting lateral force on depressor wing  12 B due to the roll motion complements the rudder control by making tow body  12  move laterally faster than with rudder control only. Thus, combining the aileron and rudder control allows tow body  12  to achieve the dual goals of moving off-track enough to position itself close to or over target  100 , and doing so quickly enough to reach the lateral position before passing target  100 .  
         [0019]    The control function of the present invention consists of three required elements:  
         [0020]    1) A function that calculates the off-track position of target  100  relative to track  16 . This off-track position becomes the control system&#39;s lateral position command.  
         [0021]    2) A feedback control system that determines the ordered aileron control as a function of off-track error (i.e., the difference between the lateral position of tow body  12  relative to track  16  and the position of target  100 ), roll angle and roll rate.  
         [0022]    3) A feedback control system that determines the ordered rudder control as a function of off-track error, track-relative yaw angle and yaw rate.  
         [0023]    Another control function may be required if the depth of tow body  12  must be maintained/controlled. Such depth control can be required for certain types of onboard sensors such as an electro-optic camera. If required, the depth control function is a feedback control system that maintains the depth or altitude of tow body  12  above the bottom as it moves laterally. This ensures that the proper sensor altitude above target  100  is maintained. Without this control feature, tow body  12  could change depth as it moves, which may degrade the performance of some types of sensors.  
         [0024]    The present invention assumes that the positions of tow body  12  and target  100  can be measured in a Cartesian (X-Y) coordinate system, with the X axis pointing north and the Y axis pointing east as illustrated in FIG. 1. This coordinate system is defined herein as the local reference frame. Also, it is assumed that motion measurements (e.g., roll, pitch, heading, and the corresponding angular rates, and depth or altitude) of tow body  12  are available. Measurement sensor/systems for providing these quantities are well known in the art and need not be described herein. Finally, it is assumed that nominal track  16  of tow body  12  is known. Track  16  is described by the local reference frame&#39;s X-Y coordinates for any point on track  16  and the track heading relative to north. The only restriction is that this point must be sufficiently close to target  100  so that the earth&#39;s surface between this point and target  100  can be assumed to be flat. The track direction is the same as the direction of travel of tow body  12 .  
         [0025]    The present invention&#39;s calculations are carried out in a track-relative Cartesian (X-Y) coordinate system referred to herein as the track reference frame. The origin of this system is located at a point  16 A on track  16 . The track reference frame&#39;s positive X T  axis points along track  16  in the travel direction of tow body  12 . The positive Y T  axis is perpendicular to the X T  axis and points to the right of track  16 . The transformation of coordinates between the two reference frames is a well-known procedure described in any linear algebra textbook. See, for example, “Elementary Linear Algebra” by S. I. Grossman, Wadsworth Publishing Company, Inc. 1980, pp. 217-218. This transformation will be described briefly below.  
         [0026]    If (x L ,y L ) represents the X-Y coordinates of any point in the local reference frame, the coordinates of the same point in the track reference frame, (x T ,y T ) are given by 
           x   T =cos(ψ T ) ( x   L   −x   o   L )+sin(ψ T ) ( y   L   −y   o   L )  (1) 
           y   T =−sin(ψ T ) ( x   L   −x   o   L )+cos(ψ T ) ( y   L   −y   o   L )  (2) 
         [0027]    In these equations, (x o   L ,y o   L ) are the coordinates of the track origin point  16 A and ψ T  is the track heading with respect to north.  
         [0028]    Referring additionally now to FIG. 2, one embodiment of the method of the present invention will be explained for the scenario illustrated in FIG. 1. The various measured parameters are indicated at the left of each block. At block  200 , the off-track error Δy is determined using the positions of tow body  12  and target  100 , along with the position of track  16  and track heading ψ T . The off-track error is defined as the difference between the Y coordinate locations of tow body  12  (traveling on track  16 ) and target  100  expressed in the track reference frame. Thus, if (x t   T ,y t   T ) and (x b   T ,y b   T ) represent, respectively, the target and tow body positions in the track reference frame, the off-track error Δy is given by 
           Δy=y   t   T   −y   b   T   (3) 
         [0029]    The off-track error Δy (depicted in FIG. 1) is the distance that tow body  12  must move laterally to position itself over target  100 .  
         [0030]    The present invention determines the aileron control command that would generate enough roll of tow body  12  required to move tow body  12  over target  100 . Many feedback control design methods are available to perform this function without departing from the scope of the present invention. One method described herein by way of example is based on the classical Proportional-Integral-Differential (PID) feedback control technique. For a description of the PID feedback control technique, see for example, “Modern Control Engineering” by K. Ogata, Prentice-Hall, Inc., 1970.  
         [0031]    In general, the PID control technique consists of a sequence of nested feedback loops. The outer loops generate control commands for the inner ones. The innermost loop computes the actuation command. In each loop, the computed signals are proportional to some error, or to the integral over time of some error, or to the rate of change of the error.  
         [0032]    At block  202 , the aileron control command δ A  can be determined using the nested-loop PID control technique. In determining the aileron control command, the PID controls outermost feedback loop computes a commanded/desired roll angle to achieve zero off-track error. This loop equation can be written as 
         φ com   =K   yA   Δy+K   iyA ∫(Δ y ) dt   (4) 
         [0033]    where φ com  denotes the commanded roll angle for tow body  12  and Δy is the off-track error computed in equation (3). The parameters K yA  and K iyA  are feedback gains which may be constant or variable. These gains are determined using standard closed-loop control design methods as is well known in the art.  
         [0034]    The commanded roll angle φ com  passes to an intermediate feedback loop which generates a commanded roll rate. This loop equation can be written as 
         φ com(rate)   =K   φA (φ com −φ)+ K   lφA ∫(φ com −φ) dt   (5) 
         [0035]    where φ com(rate)  is the commanded roll rate of tow body  12 , and φ is the actual measured roll of tow body  12 . The parameters K φA  and K iφA  are feedback gains. Finally, the innermost feedback loop computes the required aileron control command based on the commanded roll rate. This loop equation can be written as 
         δ A   =K   pA (φ com(rate) −φ rate )  (6) 
         [0036]    where δ A  is a commanded aileron angle, Orate is the measured roll rate of tow body  12 , and K pA  is a feedback gain.  
         [0037]    At block  204 , the rudder control command δ R  can be determined using the nested-loop PID control technique. The present invention determines a rudder control command required to achieve zero off-track error by only yawing tow body  12 . As in the case of the aileron control, any feedback control scheme can be used to design this function. The method described next is also a classical PID technique.  
         [0038]    In determining the rudder control command, the outermost feedback loop computes a commanded heading angle (relative to nominal track  16 ) that is needed to move tow body  12  over target  100 . The loop equation can be written as 
         ψ com   =K   yR   Δy+K   iyR ∫(Δ y ) dt   (7) 
         [0039]    where ψ com  denotes the commanded heading angle for tow body  12 , Δy is the off-track error computed in equation (3), and the parameters K yR  and K iyR  are feedback gains which may be constant or variable. These gains are determined using standard closed-loop control design methods.  
         [0040]    The commanded heading angle ψ com  passes to an intermediate feedback loop which generates a commanded heading rate. This loop equation can be written as 
         ψ com(rate)   =K   ψR (ψ com −ψ+ψ T )+ K   iψR ∫(ψ com −ψ+ψ T ) dt   (8) 
         [0041]    where ψ com(rate)  is the commanded heading rate of tow body  12 . The parameters ψ and ψ T  are, respectively, the measured heading of tow body  12  (i.e., heading of actual track  17  with respect to north) and the heading of track  16  with respect to north. The parameters K ψR  and K lψR  are feedback gains. Finally, the innermost feedback loop computes the required rudder control command based on the commanded heading rate. This loop equation can be written as 
         δ R   =K   rR (ψ com(rate) −ψ rate )  (9) 
         [0042]    where δ R  is a commanded rudder angle, ψ rate  is the measured heading rate of tow body  12 , and K rR  is a feedback gain.  
         [0043]    If needed, the present invention&#39;s elevator control function is a feedback control loop that enables tow body  12  to maintain depth or altitude as it moves laterally to position itself over target  100 . This control is needed when tow body  12  must be positioned at a fixed depth relative to target  100 . For example, such depth control is required if tow body  12  houses an electro-optic camera (not shown). However, other types of sensors (e.g., a magnetic moment sensor) may not necessarily require use of depth control.  
         [0044]    At block  206 , the elevator control command δ E  can also be determined using the nested-loop PID control technique. The PID control equations described next are an example of an elevator control method. The control function again consists of three nested loops. The outermost feedback loop calculates a commanded pitch angle as a function of depth or altitude position error. As mentioned above, it is assumed that tow body  12  can determine its own depth and that the desired depth of tow body  12  is known. The outermost feedback loop equation for depth control can thus be written as 
         θ com   =K   z   Δ+K   iz   ∫Δdt   (10) 
         [0045]    where θ com  denotes the commanded pitch angle for tow body  12 , Δ is the depth or altitude position error, and the parameter K z  is a feedback gain which may be constant or variable.  
         [0046]    The commanded pitch angle θ com  passes to an intermediate feedback loop which generates a commanded pitch rate. This loop equation can be written as 
         θ com(rate)   =K   θ (θ com −θ)+ K   iθ ∫(θ com −θ) dt   (11) 
         [0047]    where θ com(rate)  is the commanded pitch rate of tow body  12 , θ is the measured pitch angle of tow body  12 , and the parameters K θ  and K iθ  are feedback gains. The commanded pitch rate is passed to the innermost feedback loop to compute the required elevator angle. This loop equation can be written as 
         δ E   =K   q (θ com(rate) −θ rate )  (12) 
         [0048]    where δ E  is a commanded elevator angle, θ rate  is the measured pitch rate of tow body  12 , and K q  is a feedback gain.  
         [0049]    The commanded aileron angle δ A , commanded rudder angle δ R  and, if needed, the commanded elevator angle δ E  are combined at block  208  to yield a comprehensive command for controlling the entirety of the tow body&#39;s control effectors. In the illustrated example, the two (or three) commanded angles δ A  and δ R  (and, if needed, δ E ) are combined to yield the command angles for tail fins  12 A. The mathematical combining operation is not necessarily a straight addition operation as the particular combining operation depends on the convention that the towed body&#39;s design uses to define positive and negative tail fin deflections. For example, if tail fins  12 A are defined by an inverted-Y tail fin configuration having left, right and top tail fins, the corresponding angles δ LEFT , δ RIGHT  and δ TOP  combine the commanded aileron angle δ A , commanded rudder angle δ R  and commanded elevator angle δ E  as follows:  
               δ   RIGHT     =       -     δ   A       -       (   0.5   )          δ   R       -     δ   E               (   13   )                 δ   LEFT     =       -     δ   A       -       (   0.5   )          δ   R       +     δ   E               (   14   )                 δ   TOP     =       -     δ   A       +     δ   R               (   15   )                               
 
         [0050]    Obviously, for other tail fin configurations, there will be different conventions.  
         [0051]    The equations of the lateral control procedure described herein can be implemented by analog or digital means. In an analog mode, a mechanical or electrical circuit evaluates the equations to produce continuous updates of the tail fin commands. More commonly, the equations will be implemented in a digital computer or digital signal processor. In this case, the equations are evaluated in a cycle. The rate of cycle execution depends on the dynamic characteristics of tow body  12  and its control effectors, and is determined using standard control design methods as is well known in the art. In each cycle, the digital controller receives/processes motion measurements of tow body  12 , position and orientation of track  16  and the location of target  100 , in order to output the control effector commands.  
         [0052]    The advantages of the present invention are numerous. A towed body can be optimally positioned to conduct its survey/data collection functions. Such positioning is achieved by determining commands for the tow body&#39;s control effectors significantly reducing the time required to complete a survey by decreasing the chances that the target will be out of the sensor field-of-view as the target is passed. The control method is effected without the need to maneuver the tow craft.  
         [0053]    Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.