Patent Publication Number: US-7220156-B2

Title: Outboard motor steering control system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application claims priority under 35 USC 119 based on Japanese patent application No. 2004-340071, filed Nov. 25, 2004, the entire disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an outboard motor steering control system. 
   2. Description of the Related Art 
   In recent years, outboard motor steering control systems have been developed that eliminate the need for mechanical connection between the steering wheel and the steering mechanism of the outboard motor, as taught, for example, by Japanese Laid-Open Patent Application No. 2002-187597, particularly paragraphs 0022, 0025 and 0027 and FIG. 1. The outboard motor steering system taught by the reference is equipped with an actuator for steering the outboard motor and a rotation angle sensor for detecting the rotation angle of the steering wheel. The system controls the steering angle of the outboard motor by regulating the drive current to be supplied to the actuator based on the detected rotation angle. 
   Higher accurate control of the outboard motor steering angle to a desired value (i.e., a desired steering angle matched to the detected rotation angle of the steering wheel) can be achieved, for example, by additional implementation of feedback control on the technique set out in the reference. One specific way of achieving such control is to provide a steering angle sensor for detecting the steering angle of the outboard motor in addition to the rotation angle sensor for detecting the rotation angle of the steering wheel and control operation of the actuator so as to eliminate the error between the detected and desired steering angle values. However, this has a problem in that steering becomes impossible when the steering angle sensor fails. 
   SUMMARY OF THE INVENTION 
   An object of this invention is therefore to overcome this disadvantage and to provide an outboard motor steering control system that enables steering of an outboard motor to be continued even when a steering angle sensor for detecting the steering angle of the outboard motor fails. 
   In order to achieve the object, this invention provides a system for controlling steering of an outboard motor adapted to be mounted on a stem of a boat and having an internal combustion engine powering a propeller, comprising: an actuator steering the outboard motor relative to the boat; a rotation angle sensor detecting a rotation angle of a steering wheel installed at the boat; a plurality of steering angle sensors each detecting a steering angle of the outboard motor relative to the boat; a controller determining a drive current to be supplied to the actuator based on the detected rotation angle and at least one of the detected steering angles and supplying the determined drive current to the actuator to control operation of the actuator; an engine speed sensor detecting a speed of the engine; a steering angle estimator estimating the steering angle of the outboard motor relative to the boat, based on the determined drive current and the detected engine speed; and a steering angle sensor failure detector detecting failure of the steering angle sensors; wherein the controller determines the drive current based on the detected rotation angle and the estimated steering angle, when all of the steering angle sensors are detected to have failed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the invention will be more apparent from the following description and drawings in which: 
       FIG. 1  is an overall schematic view of an outboard motor steering control system according to an embodiment of the invention; 
       FIG. 2  is an enlarged partial sectional view of portions around a swivel shaft shown in  FIG. 1 ; 
       FIG. 3  is a block diagram showing the configuration of the outboard motor steering control system shown in  FIG. 1 ; 
       FIG. 4  is a block diagram showing the operation of the system, more specifically the processing performed for controlling the operation of an electric steering motor shown in  FIG. 1 ; 
       FIG. 5  is a graph showing characteristics of drive current with respect to a steering angle and engine speed, which are stored in an ECU shown in  FIG. 4 ; 
       FIG. 6  is a block diagram, similar to  FIG. 4 , but showing the processing performed for controlling the operation of the electric steering motor when all of steering angle sensors shown in  FIG. 1  have been detected as failed; and 
       FIG. 7  is a flow chart showing the operation of the system, more specifically processing performed for controlling the operation of the electric steering motor shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An embodiment of an outboard motor steering control system according to the present invention will now be explained with reference to the attached drawings. 
     FIG. 1  is an overall schematic view of an outboard motor steering control system according to an embodiment of the invention. 
   In  FIG. 1 , reference numeral  10  indicates an outboard motor. As illustrated, the outboard motor  10  is mounted on the stern (transom) of a boat (hull)  12 . 
   A dashboard  14  of the boat  12  is installed with a steering wheel  16  that is freely rotated by the operator. A plurality of, specifically three rotation angle sensors  20   a ,  20   b  and  20   c  are installed near a rotary shaft (hereinafter referred to as “steering shaft”)  18  of the steering wheel  16  and produce outputs or signals indicative of the rotation angle of the steering wheel  16 . The rotation angle sensors indicated by  20   a ,  20   b  and  20   c  will be called the “first rotation angle sensor”, “second rotation angle sensor” and “third rotation angle sensor”, respectively. 
   The outboard motor  10  is equipped with an internal combustion engine  24  (hereinafter referred to as “engine”) at its upper portion. The engine  24  comprises a spark-ignition gasoline engine. An electronic control unit (ECU)  26  that comprises a microcomputer is disposed near the engine  24 . The outboard motor  10  is equipped at its lower portion with a propeller  30 . The output of the engine  24  is transmitted to the propeller  30  such that the propeller  30  is rotated to generate thrust that propels the boat  12  in the forward and reverse directions. The outboard motor  10  is further equipped with an electric steering motor (actuator)  34  that is connected to a steering shaft (hereinafter referred to as “swivel shaft”)  32 . 
     FIG. 2  is an enlarged partial sectional view of portions around the swivel shaft  32  shown in  FIG. 1 . 
   As shown in  FIG. 2 , the outboard motor  10  is equipped with stern brackets  36  fastened to the stern of the boat  12 . A swivel case  40  is attached to the stern brackets  36  through a tilting shaft  38 . The swivel shaft  32  is housed in the swivel case  40  to be freely rotated about a vertical axis. The upper end of the swivel shaft  32  is fastened to a frame of the outboard motor  10  via a mount frame  42  and the lower end thereof is also fastened to the frame of the outboard motor  10  via a connecting member (not shown). 
   The upper portion of the swivel case  40  is installed with the steering motor  34 . The output shaft of the steering motor  34  is connected to the mount frame  42  via a speed reduction gear mechanism  44 . Specifically, a rotational output generated by driving the steering motor  34  is transmitted via the speed reduction gear mechanism  44  to the mount frame  42  such that the outboard motor  10  is steered about the swivel shaft  32  as a rotational axis to the right and left directions (i.e., steered about the vertical axis). The maximum steering angle of the outboard motor  10  is 30 degrees to the left and 30 degrees to the right. 
   The explanation of  FIG. 1  will be resumed. A plurality of, specifically three steering angle sensors  46   a ,  46   b  and  46   c  are installed near the swivel shaft  32  and produce outputs or signals indicative of steering angle of the outboard motor  10 . The steering angle sensors indicated by  46   a ,  46   b  and  46   c  will be called the “first steering angle sensor”, “second steering angle sensor” and “third steering angle sensor”, respectively. An engine speed sensor  48  is installed near a crank shaft (not shown) of the engine  24  and produces an output or a signal indicative of speed of the engine  24 . 
   The dashboard  14  of the boat  12  is installed or provided with, in addition to the steering wheel  16 , a lever, etc., that are to be manipulated by the operator to input instructions to change a shift (gear) position, to regulate the engine speed or the like. Although the outboard motor  10  is also equipped with an actuator that drives a shift mechanism in response to an instruction of shift change, another actuator that opens or closes a throttle valve of the engine  24  in response to an instruction of speed regulation and the other components, they are not directly related to this invention and thereby omitted in  FIG. 1 . 
     FIG. 3  is a block diagram showing the configuration of the system shown in  FIG. 1 . 
   As shown in  FIG. 3 , the rotation angles θsw 1 , θsw 2  and θsw 3  of the steering wheel  16  detected by the first to third rotation angle sensors  20   a ,  20   b  and  20   c  are inputted to the ECU  26 . The steering angles θs 1 , θs 2  and θs 3  of the outboard motor  10  detected by the first to third steering angle sensors  46   a ,  46   b  and  46   c  and the engine speed NE detected by the engine speed sensor  48  are also inputted to the ECU  26 . 
   Based on the inputted outputs θsw 1 , θsw 2  and θsw 3  of the first to third rotation angle sensors  20   a ,  20   b  and  20   c  and outputs θs 1 , θs 2  and θs 3  of the first to third steering angle sensors  46   a ,  46   b  and  46   c , the ECU  26  determines or detects whether any of the rotation angle sensors and steering angle sensors has failed. 
   The ECU  26  determines or regulates a drive current Cd to be supplied to the steering motor  34  based on the inputted outputs of the sensors  20   a ,  20   b ,  20   c ,  46   a ,  46   b  and  46   c  and controls the operation of the steering motor  34  to regulate the steering angle of the outboard motor  10 . 
     FIG. 4  is a block diagram showing the operation of the system, more specifically the processing performed for controlling the operation of the steering motor  34 . A general explanation of the control of the operation of the steering motor  34  is explained with reference to  FIG. 4  in the following. The processing represented by the block diagram of  FIG. 4  is carried out when at least one of the first to third steering angle sensors  46   a ,  46   b  and  46   c  operates normally. The troubleshooting processing is explained later. 
   As shown in  FIG. 4 , the ECU  26  is equipped with a desired steering angle determining unit  26   a  and a regulation unit  26   b . The regulation unit  26   b  is equipped with a controller  26   b   1  and feedforward circuit  26   b   2 . 
   The desired steering angle determining unit  26   a  inputs the rotation angle θsw of the steering wheel  16  (more exactly, one of the rotation angles θsw 1 , θsw 2  and θsw 3  detected by the first to third rotation angle sensors  20   a ,  20   b  and  20   c ). The desired steering angle determining unit  26   a  determines a desired steering angle θds based on the inputted rotation angle θsw. 
   The controller  26   b   1  of the regulation unit  26   b  inputs the error or difference between the desired steering angle θds determined in the desired steering angle determining unit  26   a  and the detected steering angle θs of the outboard motor  10  (the feedback signal; more exactly, one of the steering angles θs 1 , θs 2  and θs 3  detected by the first to third steering angle sensors  46   a ,  46   b  and  46   c ). 
   The controller  26   b   1  determines the drive current (current command value) Cd to be supplied to the steering motor  34  based on the inputted error. Specifically, it determines the drive current Cd so that the steering motor  34  is operated in the direction for eliminating the error between the desired steering angle θds and the detected steering angle θs. The controller  26   b   1  controls the operation of the steering motor  34  by supplying the determined drive current Cd to the steering motor  34 , thereby rotating the swivel shaft  32  to control the steering angle θs of the outboard motor  10  to the desired steering angle θds. 
   So as long as at least one of the first to third steering angle sensors  46   a ,  46   b  and  46   c  operates normally, the operation of the steering motor  34  can be controlled by determining the drive current Cd based on the detected rotation angle θsw of the steering wheel  16  and the steering angle θs of the outboard motor  10 , thereby controlling the steering angle θs of the outboard motor  10  to the desired steering angle θds (feedback control). 
   The engine speed NE detected by the engine speed sensor  48 , the drive current Cd of the steering motor  34  determined by the controller  26   b   1  and the steering angle θs detected by one of the first to third steering angle sensors  46   a ,  46   b  and  46   c  are inputted to the feedforward circuit  26   b   2  of the regulation unit  26   b . The feedforward circuit  26   b   2  stores the inputted drive current Cd as characteristics with respect to the steering angle θs and engine speed NE. 
     FIG. 5  is a graph showing an example of the characteristics of the drive current Cd with respect to the steering angle θs and engine speed NE. 
   As shown in  FIG. 5 , the drive current Cd is determined so that it increases with increasing engine speed NE and increasing steering angle θs. This is because when the engine speed NE rises (i.e., the boat speed rises) and/or the steering angle θs increases, the resulting rise in water flow resistance causes the drive current Cd of the steering motor  34  required for steering the outboard motor  10  to increase. 
   It should be noted that the graph of  FIG. 5  represents the magnitude of the drive current Cd required to change the steering angle θs a unit angle per unit time as a function of the steering angle θs and engine speed NE. Insofar as at least one of the first to third steering angle sensors  46   a ,  46   b  and  46   c  operates normally, the characteristics shown in the graph of  FIG. 5  is updated based on the values newly inputted by the feedforward circuit  26   b   2 . 
     FIG. 6  is a block diagram, similar to  FIG. 4 , but showing the processing performed for controlling the operation of the electric steering motor  34  when all of the first to third steering angle sensors  46   a ,  46   b  and  46   c  have been detected as failed. 
   When it is detected that all of the first to third steering angle sensors  46   a ,  46   b  and  46   c  have failed, as shown in  FIG. 6 , the detection values of the first to third steering angle sensors  46   a ,  46   b  and  46   c  are not used to control the operation of the steering motor  34 . 
   Specifically, the desired steering angle θds determined in the desired steering angle determining unit  26   a  is outputted to the controller  26   b   1  immediately (in other words, without being subtracted by the detected steering angle θs). Further, an estimated value of the steering angle θs (hereinafter referred to as “estimated steering angle”) θes is inputted to the controller  26   b   1  from the feedforward circuit  26   b   2 . As mentioned above, the feedforward circuit  26   b   2  stores the drive current Cd as the characteristics with respect to the steering angle θs and engine speed NE. As a result, the current steering angle θs can be estimated from the magnitude of the drive current Cd supplied to the steering motor  34  and the current engine speed NE. 
   The controller  26   b   1  determines the drive current Cd based on the inputted desired steering angle θds and estimated steering angle θes, and then controls the operation of the steering motor  34  by supplying the determined drive current Cd thereto. Thus when it is detected that all of the first to third steering angle sensors  46   a ,  46   b  and  46   c  have failed, the drive current Cd is determined based on the detected rotation angle θsw of the steering wheel  16  and the estimated steering angle θes of the outboard motor  10 , thereby controlling the operation of the steering motor  34  so as to control the steering angle θs of the outboard motor  10  to the desired steering angle θds. 
     FIG. 7  is a flowchart showing the operation of the system, more specifically the flow of the processing for controlling the operation of the steering motor  34 , detecting failure of the first to third steering angle sensors  46   a ,  46   b  and  46   c , and conducting related operations. The illustrated program is executed at predetermined intervals in the ECU  26 . 
   The processing of the flowchart of  FIG. 7  will now be explained. First, in S 10 , it is determined whether the values of the rotation angles θsw 1 , θsw 2  and θsw 3  of the steering wheel  16  detected by the first to third rotation angle sensors  20   a ,  20   b  and  20   c  are all equal (or nearly equal). 
   When the result in S 10  is YES, the program goes to S 12 , in which all of the first to third rotation angle sensors  20   a ,  20   b  and  20   c  are determined to operate normally, and the output θsw 1  of the first rotation angle sensor  20   a  is determined as the current or present value of the rotation angle θsw of the steering wheel  16 . The purpose of this processing is to select from among the outputs of the plurality of (three) rotation angle sensors the output of a normally operating sensor, thereby ensuring that the selected output accurately represents the rotation angle of the steering wheel  16 . From this it follows that the output determined as the current value of the rotation angle θsw in S 12  need not necessarily be the output θsw 1  of the first rotation angle sensor  20   a  but can instead be the output θsw 2  of the second rotation angle sensor  20   b  or the output θsw 3  of the third rotation angle sensor  20   c.    
   When the result in S 10  is NO, the program goes to S 14 , in which it is determined whether the values of the output θsw 1  of the first rotation angle sensor  20   a  and the output θsw 2  of the second rotation angle sensor  20   b  are equal (or nearly equal). When the result in S 14  is YES, i.e., when it can be concluded that the output θsw 3  of the third rotation angle sensor  20   c  differs from the other two outputs θsw 1  and θsw 2 , the program goes to S 16 , in which the third rotation angle sensor  20   c  is determined to have failed and the output θsw 1  of the first rotation angle sensor  20   a  (optionally the output θsw 2  of the second rotation angle sensor  20   b ) is determined as the rotation angle θsw of the steering wheel  16 . 
   The third rotation angle sensor  20   c  is determined to have failed in S 16  based on the reasoning that when, among the outputs of the three rotation angle sensors  20   a ,  20   b  and  20   c , only the output of the third rotation angle sensor  20   c  is of a different value, the probability of the third rotation angle sensor  20   c  having failed is high. 
   When the result in S 14  is NO, the program goes to S 18 , in which it is determined whether the values of the output θsw 1  of the first rotation angle sensor  20   a  and the output θsw 3  of the third rotation angle sensor  20   c  are equal (or nearly equal). When the result in S 18  is YES, i.e., when it can be concluded that the output θsw 2  of the second rotation angle sensor  20   b  differs from the other two outputs θsw 1  and θsw 3 , the program goes to S 20 , in which the second rotation angle sensor  20   b  is determined to have failed and the output θsw 1  of the first rotation angle sensor  20   a  (optionally the output θsw 3  of the third rotation angle sensor  20   c ) is determined as the rotation angle θsw of the steering wheel  16 . The second rotation angle sensor  20   b  is determined to be faulty in S 20  based on reasoning that similar to that in S 16 . 
   When the result in S 18  is NO, the program goes to S 22 , in which it is determined whether the values of the output θsw 2  of the second rotation angle sensor  20   b  and the output θsw 3  of the third rotation angle sensor  20   c  are equal (or nearly equal). When the result in S 22  is YES, i.e., when it can be concluded that the output θsw 1  of the first rotation angle sensor  20   a  differs from the other two outputs  74  sw 2  and θsw 3 , the program goes to S 24 , in which, based on reasoning similar to that in S 16  and S 20 , the first rotation angle sensor  20   a  is determined to have failed and the output θsw 2  of the second rotation angle sensor  20   b  (optionally the output θsw 3  of the third rotation angle sensor  20   c ) is determined as the rotation angle θsw of the steering wheel  16 . 
   Once the rotation angle θsw of the steering wheel  16  has been determined, the program goes to S 26 , in which it is determined whether the values of the steering angles θs 1 , θs 2  and θs 3  of the outboard motor  10  detected by the first to third steering angle sensors  46   a ,  46   b  and  46   c  are all equal (or nearly equal). 
   When the result in S 26  is YES, the program goes to S 28 , in which all of the first to third steering angle sensors  46   a ,  46   b  and  46   c  are determined to operate normally, and the output θs 1  of the first steering angle sensor  46   a  is determined as the current or present value of the steering angle θs of the outboard motor  10 . The reasoning here is similar to that in S 12  explained earlier. That is, the purpose of this processing is to select from among the outputs of the plurality (three) of steering angle sensors the output of a normally operating sensor, thereby ensuring that the selected output accurately represents the steering angle of the outboard motor  10 . Similarly, the output determined as the current value of the steering angle θs in S 28  need not necessarily be the output θs 1  of the first steering angle sensor  46   a  but can instead be the output θs 2  of the second steering angle sensor  46   b  or the output θs 3  of the third steering angle sensor  46   c.    
   When the result in S 26  is NO, the program goes to S 30 , in which it is determined whether the values of the output θs 1  of the first steering angle sensor  46   a  and the output θs 2  of the second steering angle sensor  46   b  are equal (or nearly equal). When the result in S 30  is YES, i.e., when it can be concluded that the output θs 3  of the third steering angle sensor  46   c  differs from the other two outputs θs 1  and θs 2 , the program goes to S 32 , in which, based on reasoning similar to that in S 16 , for example, the third steering angle sensor  46   c  is determined to have failed and the output θs 1  of the first steering angle sensor  46   a  (or the output θs 2  of the second steering angle sensor  46   b ) is determined as the steering angle θs of the outboard motor  10 . 
   When the result in S 30  is NO, the program goes to S 34 , in which it is determined whether the values of the output θs 1  of the first steering angle sensor  46   a  and the output θs 3  of the third steering angle sensor  46   c  are equal (or nearly equal). When the result in S 34  is YES, i.e., when it can be concluded that the output θs 2  of the second steering angle sensor  46   b  differs from the other two outputs θs 1  and θs 3 , the program goes to S 36 , in which the second steering angle sensor  46   b  is determined to have failed and the output θs 1  of the first steering angle sensor  46   a  (or the output θs 3  of the third steering angle sensor  46   c ) is determined as the steering angle θs of the outboard motor  10 . 
   When the result in S 34  is NO, the program goes to S 38 , in which it is determined whether the values of the output θs 2  of the second steering angle sensor  46   b  and the output θs 3  of the third steering angle sensor  46   c  are equal (or nearly equal). When the result in S 38  is YES, i.e., when it can be concluded that the output θs 1  of the first steering angle sensor  46   a  differs from the other two outputs θs 2  and θs 3 , the program goes to S 40 , in which the first steering angle sensor  46   a  is determined to have failed and the output θs 2  of the second steering angle sensor  46   b  (or the output θs 3  of the third steering angle sensor  46   c ) is determined as the steering angle θs of the outboard motor  10 . 
   Once the steering angle θs of the outboard motor  10  has been determined, the program goes to S 42 , in which the drive current Cd to be supplied to the steering motor  34  is determined based on the determined rotation angle θsw of the steering wheel  16  detected by one of the three rotation angle sensors and the determined steering angle θs of the outboard motor  10  detected by one of the three steering angle sensors. Specifically, as has been explained with reference to the block diagram of  FIG. 4 , the desired steering angle θds is determined based on the determined rotation angle θsw of the steering wheel  16  and the drive current Cd is determined so that the steering motor  34  is operated in the direction for eliminating or decreasing the error between the set desired steering angle θds and the actual steering angle θs. 
   The program then goes to S 44 , in which the characteristics of the drive current Cd with respect to the steering angle θs and engine speed NE is updated based on the present and past values of the steering angle θs, engine speed NE and drive current Cd. Specifically, the magnitude of the drive current Cd required to change the steering angle θs a unit angle per unit time is calculated based on current and past values of the steering angle θs and drive current Cd and the calculated values are stored as representing the characteristics of the drive current Cd with respect to the steering angle θs and engine speed NE at that time. 
   Next, in S 46 , the determined drive current Cd is outputted to control the operation of the steering motor  34  so as to converge the steering angle θs to the desired steering angle θds. 
   When the result in S 38  is NO, i.e., when the outputs of the first to third steering angle sensors  46   a ,  46   b  and  46  are all different with each other, so that it becomes impossible to determine which, if any, of the sensors operates normally, the program goes to S 48 , in which it is determined that all of the sensors have failed and the operator is informed, visually or audibly, for instance, of the fact that the steering angle sensors have been detected as failed. At the same time, the operation of the actuator connected to the throttle valve of the engine  24  is controlled to reduce the throttle opening so as to lower the engine speed NE and stop the boat  12 . 
   Next, in S 50 , the estimated steering angle θes, namely the estimated value of the steering angle θs is determined based on the drive current Cd and engine speed NE. 
   The processing for determining the estimated steering angle θes will be explained. 
   The drive current Cd to be supplied to the steering motor  34  and the engine speed NE at the time the drive current Cd is supplied (in other words, the value of the drive current Cd in the preceding cycle and the value of the engine speed NE in the preceding cycle) are used as address data for retrieving the change Δθs in the steering angle θs per unit time (per program execution cycle) from the characteristics shown in  FIG. 5 . The value obtained by adding the change Δθs to the most recent or latest θs (i.e., the value in the preceding cycle) detected by the steering angle sensor (when operating normally) is determined as the estimated steering angle θes (estimated value of the current steering angle θs). 
   When the value of the estimated steering angle θes in the preceding cycle is available (i.e., when the change Δθs is not being ascertained for the first time), the value of the estimated steering angle θes in the current cycle can be calculated by adding the change Δθs to the estimated steering angle θes in the preceding cycle. 
   Next, in S 52 , the drive current Cd is determined based on the rotation angle θsw of the steering wheel  16  detected by one of the three rotation angle sensors and the estimated steering angle θes determined in the foregoing manner. 
   Specifically, the desired steering angle θds is determined based on the rotation angle θsw and the drive current Cd is determined so that the steering motor  34  is operated in the direction for eliminating the error between the desired steering angle θds and the estimated steering angle θes. The program then goes to S 46 , in which the determined drive current Cd is outputted to control the operation of the steering motor  34  so as to make the steering angle θs of the outboard motor  10  equal to the desired steering angle θds. 
   When the result in S 22  is NO, i.e., when the outputs of the first to third rotation angle sensors  20   a ,  20   b  and  20   c  are all different from each other and it becomes impossible to determine which, if any, of the sensors is operating normally, the program goes to S 54 , in which it is determined that all of the sensors have failed and the operator is informed, visually or audibly, for instance, that the rotation angle sensors have failed. In addition, the operation of the steering motor  34  cannot be controlled because the desired steering angle θds is not able to be determined when it is not possible to detect the rotation angle of the steering wheel  16  accurately. The steps S 26  to S 52  are therefore all skipped. At the same time, the operation of the actuator connected to the throttle valve of the engine  24  is controlled to reduce the throttle opening so as to lower the engine speed NE and stop the boat  12 . 
   As explained in the foregoing, in the outboard motor steering control system according to this invention, a plurality of rotation angle sensors  20   a ,  20   b  and  20   c  are provided for detecting the rotation angle θsw of the steering wheel  16  and a plurality of steering angle sensors  46   a ,  46   b  and  46   c  are installed for detecting the steering angle θs of the outboard motor  10 , and the drive current Cd is determined based on the outputs of normally operating sensors thereamong, thereby controlling the operation of the steering motor  34 . This configuration enhances the reliability of the outboard motor steering system. 
   Moreover, the outboard motor steering system is configured to respond to detection that all of the multiple steering angle sensors  46   a ,  46   b  and  46   c  have failed by determining the estimated steering angle θes, namely the estimated value of the steering angle θs, based on the drive current Cd to be supplied to the steering motor  34  and the engine speed NE at the time the drive current Cd is supplied and determining or regulating the drive current Cd based on the estimated steering angle θes and the rotation angle θsw of the steering wheel  16 . Owing to this configuration, steering of the outboard motor  10  can be continued even when all of the steering angle sensors  46   a ,  46   b  and  46   c  have failed. This configuration further enhances the reliability of the outboard motor steering system. 
   Further, the outboard motor steering system is configured to store the drive current Cd as the characteristics with respect to the steering angle θs and engine speed NE and respond to detection that not all of the steering angle sensors  46   a ,  46   b  and  46   c  have failed (at least one operates normally) by updating the characteristics based on the drive current Cd to be supplied to the steering motor  34  and the detected engine speed NE and steering angle θs and respond to detection that all of the multiple steering angle sensors  46   a ,  46   b  and  46   c  are faulty by using the drive current Cd supplied to the steering motor  34  and the engine speed NE at that time to determine the estimated steering angle θes from the characteristics. The steering  43 , angle θs can therefore be accurately estimated unaffected by aging of, or characteristics peculiar to, the outboard motor concerned. This configuration therefore further enhances the reliability of the outboard motor steering system. 
   The present exemplary embodiment is thus configured to have a system for controlling steering of an outboard motor ( 10 ) adapted to be mounted on a stern of a boat ( 12 ) and having an internal combustion engine ( 24 ) powering a propeller ( 30 ), comprising: an actuator (electric steering motor  34 ) steering the outboard motor relative to the boat; a rotation angle sensor ( 20 ) detecting a rotation angle θsw of a steering wheel ( 16 ) installed at the boat; a plurality of steering angle sensors ( 46 ) each detecting a steering angle θs of the outboard motor relative to the boat; a controller (ECU  26 ; S 42 ) determining a drive current Cd to be supplied to the actuator based on the detected rotation angle and at least one of the detected steering angles and supplying the determined drive current to the actuator to control operation of the actuator; an engine speed sensor ( 48 ) detecting a speed of the engine NE; a steering angle estimator (ECU  26 ; S 50 ) estimating the steering angle θes of the outboard motor relative to the boat, based on the determined drive current and the detected engine speed; and a steering angle sensor failure detector (ECU  26 ; S 26  to S 40 , S 48 ) detecting failure of the steering angle sensors; wherein the controller determines the drive current based on the detected rotation angle θsw and the estimated steering angle θes, when all of the steering angle sensors are detected to have failed (S 52 ). 
   In the system, the steering angle estimator includes: a drive current characteristics determiner (ECU  26 ; S 44 ) determining characteristics of the drive current Cd with respect to the detected steering angle θs and the detected engine speed NE when not all of the steering angle sensors are detected to have failed; and estimates the steering angle θes based on the drive current Cd supplied to the actuator and the detected engine speed NE in accordance with the characteristics. 
   In the system, at least three of the steering angle sensors ( 46 ) are used ( 46   a ,  46   b ,  46   c ), and the steering angle sensor failure detector detects that one of the steering angle sensors has failed when outputs of other two steering angle sensors are equal (S 26  to S 40 ), but different than the output of the one sensor. 
   The system further includes: a plurality of the rotation angle sensors ( 20   a ,  20   b ,  20   c ) each detecting the rotation angle of the steering wheel installed at the boat; and the controller determines the drive current Cd based on at least one of the detected rotation angles and at least one of the detected steering angles (S 42 ). 
   The system further includes: a rotation angle sensor failure detector (ECU  26 , S 10  to S 24 , S 54 ) detecting failure of the rotation angle sensors. 
   In the system, at least three of the rotation angle sensors ( 20 ) are used ( 20   a ,  20   b ,  20   c ), and the rotation angle sensor failure detector detects one of the rotation angle sensors has failed when outputs of other two rotation angle sensors are equal (S 10  to S 24 )), but different than the output of the one sensor. 
   Although the outboard motor steering system described in the foregoing is explained as having three each of the rotation angle sensors for detecting the rotation angle θsw of the steering wheel  16  and the steering angle sensors for detecting the steering angle θs of the outboard motor  10 , the number of these sensors is not limited to three each. The number of rotation angle sensor may be one and that of the steering angle sensor may be four or more. 
   Although the steering actuator is exemplified as an electric motor in the foregoing, it can instead be a hydraulic actuator or any of various other kinds of actuators. When a hydraulic actuator is used, it suffices to determine the drive current to be supplied to operate the hydraulic pump based on the rotation angle θsw and the steering angle θs (or the estimated steering angle θes). 
   While the invention has thus been shown and described with reference to specific exemplary embodiments, it should be noted that the invention is in no way limited to the details of the described arrangements; changes and modifications may be made without departing from the scope of the appended claims.