Abstract:
A method for compensating a steering system exhibiting dead-zone characteristics estimates the dead-zone parameters of the steering system and establishes a dead-zone inverse function utilizing the estimated parameters. The dead-zone inverse is utilized to transform steering orders in the dead-zone. Parameters of the dead-zone inverse are continuously up-dated with the utilization of the transformed steering order, the existing dead-zone parameters, and the present heading.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention pertains to automatic steering control of marine vessels, and more particularly to compensation for steering systems that exhibit dead-zone characteristics.  
         [0003]     2. Description of the Prior Art  
         [0004]     Marine high-speed craft frequently use waterjets for combined propulsion and steering. These systems utilize paddles that straddle the waterjets. When the paddles are rotated the water streams from the jets are deflected causing the craft to change direction. These devices, however, may exhibit a steering dead-zone. Helm orders for small angular turns produce small angular rotations of the paddles with a concomitant small or zero deflection of the water stream, which may be insufficient to steer the craft. This creates a major control problem, particularly for an autopilot in a course maintaining mode wherein it is continuously making small course corrections. The problem is exacerbated for high speed craft, which are especially prone to instability in maintaining heading due to the dead-zone problem. An uncompensated or improperly compensated dead-zone always results in reduced heading keeping accuracy, longer and less fuel efficient voyages, and a less comfortable ride for passengers and crew.  
         [0005]     A dead-zone is characterized by four parameters: the extent of the dead-zone band on either side of zero plus a slope in each direction respectively extending from the ends of the dead-zone. Some prior art marine autopilots for high speed craft utilize the inverse of the dead-zone function to provide dead-zone compensation. These systems, however, require dead-zone parameters which are functions of the speed of the craft, sea conditions, and other factors. At each installation on a vessel, the operator or engineer must go through trial and error searches to establish dead-zone parameters for various operating conditions. The determination of the dead-zone parameters is labor intensive and require trained personnel. Further, during operation the prevailing conditions must be established before the appropriate parameter is applied. The selection of the parameters assumes that the dead-zone is symmetric about zero and the slopes in either direction are known. Additionally, it assumes that the dead-zone characteristic does not change with changes in speed or other vessel changes such as loading. Requiring an operator or engineer to determine the four parameters by trial and error and the operator to constantly update them based on changes in ship operating characteristics is too demanding to be at all practical.  
         [0006]     Other prior art system set-up strategies reduce the water jet nozzle diameter to increase the diameter of the water flow, thus providing a greater water stream coverage of the paddles and a greater turning moment and/or offsetting the outboard waterjets in outboard direction so that for small turning angles there is a reduced, but non-zero turning moment from one water jet. Either of these solutions result in reduced propulsion efficiency and performance and, therefore, are not desired by the vessel operator.  
       SUMMARY OF THE INVENTION  
       [0007]     In accordance with the present invention an adaptive inverse is provided by iterating estimates of all four dead-zone parameters thereby implementing the dead-zone inverse for the prevailing operating characteristics. The iterations do not require any manual input by the operator or any performance reducing adjustments to the water jet propulsion. It compensates for asymmetric and time varying dead-zones without encumbering the operator.  
         [0008]     The invention will now be described in greater detail with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a graphical representation of a dead-zone function.  
         [0010]      FIG. 2  is a graphical representation of a dead-zone inverse.  
         [0011]      FIG. 3  is a block diagram of a preferred embodiment of the invention.  
         [0012]      FIG. 4  is a block diagram of a transfer function generated by an autopilot controller and ship dynamics combination.  
         [0013]      FIG. 5  is a plot of a simulated ship&#39;s response to a course order change without dead-zone compensation.  
         [0014]      FIG. 6  is a plot of a simulated ship&#39;s response to a course order change with adaptive dead-zone compensation.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     A representation of a steering system response to a steering command is shown in  FIG. 1 . Upon reception of a heading order r(t), a helm order V(t) is established, represented by the ordinate  11 , and a rudder turning moment u(t), represented by the abscissa  13 , is generated in response to the helm order. The relationship between helm order and turning moment is defined by a curve represented by the line  15 . As the vehicle approaches the desired course, the helm order and turning moment are reduced until the helm order achieves a small or zero value to which the system does not respond. At this time it enters the dead-zone  17 . Throughout the dead-zone the small error value persists and the ship continues along a course that crosses the desired one. The ship continues along the residual error course until the difference between the ship&#39;s heading and the desired course establishes an error large enough to remove the system from the dead-zone. The new detectable error has a sign that is opposite that of the detectable error prior to the entry into the dead-zone and turning moments back towards the desired course are generated along a curve representative by the line  19 . The system will eventually enter the dead-zone again and the zig-zag course, limit cycle, shown in  FIG. 5 , will continue.  
         [0016]     Assuming the curves  15  and  19  are straight lines, the dead-zone function DZ(t), provides a turning moment u(t), which may be described mathematically follows: 
 
 u ( t )= m   l   [V ( t )+ b   l   ] V ( t )≦ b   l  
 
 u ( t )=0  b   l   &lt;V ( t )&lt; b   r  
 
 u ( t )= m   r   [V ( t )− b   r   ] V ( t )≧ b   r  
 
 where m l  and m r  are the slopes of lines  15  and  19 , respectively, and b l  and b r  are abscissa intercepts of lines  15  and  19  respectively. Though the equation indicates discontinuities at b l  and b r , the system actually enter the dead-zone smoothly, as indicated by the arcs  21  and  23 . 
 
         [0017]     To compensate for the undesired effect of the dead-zone, a dead-zone inverse function DI(t), shown in  FIG. 2 , is implemented which provides a new helm order V′(t). Designating the desired effective turning order as u d (t), this new helm order may be represented as:  
           V   ′     ⁡     (   t   )       =               u   d     ⁡     (   t   )       +       m   r     ⁢     b   r           m   r       ⁢           ⁢       u   d     ⁡     (   t   )         &gt;   0         
           V   ′     ⁡     (   t   )       =       0   ⁢           ⁢       u   d     ⁡     (   t   )         =   0         
           V   ′     ⁡     (   t   )       =               u   d     ⁡     (   t   )       +       m   l     ⁢     b   l           m   l       ⁢           ⁢       u   d     ⁡     (   t   )         &lt;   0         
 
         [0018]     Refer now to  FIG. 3 , wherein a block diagram of the invention is shown. A heading order r(t) is given to the steering system  30 . The actual heading of the vessel y(t) is subtracted from the heading order r(t) in a differencing network  31 , wherefrom the difference Δh is coupled to an autopilot controller  33 . When a dead-zone is encountered, a dead-zone steering signal u d (t) is coupled to an adaptive dead-zone inverse function generator  35  wherefrom a new heading order r′(t) is provided to the steering control system having a dead-zone characteristic DZO  37 . In response to the new heading order r′(t), dead-zone characteristic  37  of the steering control system causes a new turning moment u′(t) to operate on the ship dynamics  39 , thereby establishing the actual heading y(t).  
         [0019]     If the parameters of the dead-zone inverse function were known exactly at all times, the steering system  30  would need no further correction and the vessel would be turned to the desired course in response to the r(t) command. These parameters, however, are initially estimated and corrections must be made for proper system operation.  
         [0020]     To establish corrections to the initial estimated parameters m r (t), m r b r (t), m l (t), and m l b l (t), dead-zone inverse function generator  35  couples these parameters to a vector generator  41  wherein a vector of these parameters is formed and the transpose,  θ   d   T (t)=[m r (t)m r b r (t) m l (t) m l b l (t)] is provided. Additionally, new heading order r′(t) is coupled to a second vector generator  43  which generates a vector  
             ω   _     d     ⁡     (   t   )       =     [               ⁢       xr   ′     ⁡     (   t   )                       ⁢   x                   ⁢       (     x   -   1     )     ⁢       r   ′     ⁡     (   t   )                         ⁢     1   -   x             ]         
 
 where: x=1 when r′(t)&gt;0 and x=0 when r′(t)&lt;0. The transposed vector  θ   d   T (t) and the vector  ω   d (t) are coupled to a multiplier  45  wherein the multiplication  θ   d   T (t)  ω   d (t) is performed to provide a scalar value 
 
β= m   r ( t ) r ′( t )+ m   r   b   r ( t ) r ( t )&lt;0 
 
β=− m   l ( t ) r ′( t )+ m   l   b   l ( t ) r ( t )&gt;0 
 
 The vector  ω   d (t), scalar value β, and the new heading order r′(t) are respectively coupled to identical transform function generators W m (s)  47 - 1 ,  47 - 2 , and  47 - 3  in a function operator  48 , wherefrom the vector  ζ (t), the scalar β′, and the transformed heading order r′ m (t) are obtained. 
 
         [0021]     Referring to  FIG. 4 , transform generator W m (s)  47  provides a closed loop transfer function of the ships steering system without considering the dead-zone effect. The input signal i(t) to transfer function generator and its output signal o(t), representative of the ship&#39;s heading, are coupled to a subtraction circuit  47   a , wherein o(t) is subtracted from i(t) and the difference is coupled to a simulation of the ship&#39;s autopilot controller  47   b . The output signal of the autopilot controller  47   b  is coupled to a model of the ship&#39;s dynamic system  47   c  which provides the output signal that is feed back to the difference network  47   a . The relationship between i(t) and o(t) is the same as the relationship between r m (t) a heading order given to the autopilot and the resulting heading y m (t) established by the steering system without dead-zone compensation.  
         [0022]     The vector  ζ (t), resulting from the transformation of  ω   d (t), is coupled from the transformation generator  47 - 1  to a multiplier  49  and multiplied therein  θ   d   T (t) to provide a scalar κ=  θ   d   T (t)  ζ (t). β′, the result of the W m (s)  47 - 2  transformation of β, is subtracted from κ in subtraction circuit  51  to obtain the scalar ξ(t)=κ−β′, while the heading y(t) is subtracted from r′ m (t), the result of the W m (s)  47 - 3  transformation of r′(t), in subtraction circuit  53  to obtain e(t)=r′ m (t)−y(t). ξ(t) is coupled to a parameter update computer  55 , which provides updated parameters for the dead-zone inverse function, and to an addition circuit  57  wherein it is added to e(t) to provide ε(t) which is also coupled to the computer  55 , as is the vector  ζ (t) to complete the input values to the calculator  55  from which the dead-zone function parameter up-date is computed.  
         [0023]     Computer  55  provides the parameter up-date vector  θ ′ d (t) with the utilization of the following equation: 
 
  θ ′ d ( t )=Γ d   ζ ( t ) 
 
 in which Γ d  is a diagonal matrix: 
 
Γ d   =pdiag[γ   1  γ 2  γ 3  γ 4 ]
 
 where p is a scalar function given as:  
       p   =       ɛ   ⁡     (   t   )         1   +           ζ   _     T     ⁡     (   t   )       ⁢       ζ   _     ⁡     (   t   )         +       ξ   2     ⁡     (   t   )               
 
 and γ 1 , γ 2 , γ 3 , and γ 4  are positive constants chosen by the operator to give good estimation performance. These constants may be altered at any time to fine tune the operation. 
 
         [0024]     Refer to  FIG. 5  wherein a plot of a simulated autopilot controlled  20  degree heading change is given for a ship with dead-zone parameters 
 
m r =1.0, b r =2.0, m l =0.75, and b l =−1.5 
 
 The desired turn is shown by the curve  61 . An inspection of curve  61  indicates that the desired course is achieved in approximately 100 seconds. Without dead-zone compensation the actual course change is represented by the curve  63 . An inspection of this curve indicates that the ship takes an undulating course about the desired heading with maximum course errors of approximately ± 100 . 
 
         [0025]     A simulation of a course change for the same dead-zone parameters with dead-zone compensation is shown in  FIG. 6 . In this simulation the initial dead-zone inverse parameters were chosen as: 
 
m r =1.0, b r =0.0, m l =1, and b l =0.0 
 
 which is a straight line with a 45° slope. Though these parameters represent an ideal situation, they do not provide the inverse function for the simulated dead-zone parameters given above. The resulting performance is represented by the curve  65 . An inspection of this curve indicates that, by using the adaptive parameter updates, the desired final heading is achieved in about 400 seconds with maximum course deviations of only ±1°. 
 
         [0026]     While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation. Thus the scope of the invention is determined by the appended claims and that changes may be made within the purview thereof without departure from the true scope and spirit of the invention in its broader aspects.