DETAILED DESCRIPTION OF AN EXEMPLARY EMBODIMENT

[0018] Disclosed herein in an exemplary embodiment is steering system employing control-by-wire technology to enhance the directional control capabilities of marine craft. Control-by-wire technology eliminates the mechanical linkages in systems by sensing desired inputs such as helm position and generates commands to drive an output device. The output device may be an electric motor, actuator, hydraulic actuator, and the like, as well as combinations including at least one of the foregoing, which is responsive to the commands and manipulates a steerable member such as a rudder and hereinafter denoted rudder.

[0019] As stated earlier, prior mechanisms for directional control of a watercraft employ a mechanical interconnection while the other end is attached to the steerable member. One advantage in having a direct connection to a steerable member is that the operator receives tactile feedback via the steering linkages through to the helm control and the phase relationship between the operator's input and the responses is substantially fixed. For example, if the watercraft changes directions while it is moving, the operator will feel resistance in the helm and the response of the steerable member follows inputs at the helm. With a steer-by-wire system, since the mechanical link between the helm and the rudder(s) is inoperative/eliminated, what the driver feels at the helm is highly tunable. Therefore, the steering system may exhibit variable desirable tactile feedback to the operator. At the same time, with the elimination of the mechanical connection, the phase relationship between the driver's helm angle input and the torque felt by the driver can change significantly.

[0020] Advantageously, a control-by-wire architecture of an exemplary embodiment as disclosed herein allows the angle between the helm angle and the steerable member to be variable. Features/functions of this embodiment include, but are not limited to providing resistive torque or feedback to the operator that may be programmed to enhance steering tactile feedback (feel). Additionally, an autopilot function for direction control and guidance may readily be integrated with or without movement of the helm when active. Additional features of an exemplary embodiment include low speed directional control (docking, no wake speeds, and the like) enhancements. Steer-by-wire facilitates implementations that operate multiple steering devices concurrently.

[0021] Referring now to FIG. 1 , an exemplary control-by-wire watercraft control system 10 is depicted. An exemplary watercraft control system 10 includes, but is not limited to a helm control system 12 , a direction control system 14 , and a master control unit 16 . The helm control system 12 includes a helm position sensor 18 to detect the position and movement of a helm 20 or any equivalent operator input device and sends a helm position signal 22 to the master control unit 16 . The helm control system 12 may optionally include a helm torque sensor 24 to detect the torque applied to the helm and sends a helm torque signal 26 to the master control unit 16 . The master control unit 16 combines the information of the helm position signal 22 helm torque signal 26 , with a watercraft speed signal 28 from a watercraft speed sensor 29 , and rudder position signal 30 from a rudder position sensor 32 that detects the position of the rudder 15 in the direction control unit 14 . Using these input signals, the master control unit 16 produces a directional command signal 34 that is sent to the direction control system 14 . In addition, a helm command signal 36 optionally, may be transmitted to the helm control system 12 . It will be appreciated, as described further herein, that the helm control system 12 may employ either a passive torque control (e.g., as an brake and open loop) or active torque control (e.g., with an motor and either open or closed loop). Moreover, it will be appreciated that the inclusion of a torque sensor 24 may be a function of implementation for a given embodiment. For example, if the position sensor is located at a position away or “downstream” from a compliant member (as may be employed for a torque sensor) then the position sensor information and torque information is needed to ascertain the true position of the helm 20 .

[0022] It will be appreciated, that the helm control system 12 , master control unit 16 , and direction control system 14 are described for illustrative purposes. The processing performed throughout the system may be distributed in a variety of manners. For example, distributing the processing performed in the master control unit 16 among the other processes employed may eliminate the need for such a component or process as described. Each of the major systems may have additional functionality that will be described in more detail herein as well as include functionality and processing ancillary to the disclosed embodiments. As used herein, signal connections may physically take any form capable of transferring a signal, including, but not limited to, electrical, optical, or radio. Moreover, conventional position/force control of actuators, servos, and the like often utilize a feedback control system to regulate or track to a desired position/force. The control law maybe a proportional, integrative or derivative gain on the tracking error or may be a more sophisticated higher-order dynamic. In either case, the feedback measurement is the actual position/force and in some cases, it's derivatives.

[0023] Referring to FIG. 2 , the helm control system 12 is a control system (in this instance closed loop, but not necessarily so) that uses the helm position signal 22 as sensed from the helm position sensor 18 as the feedback signal. Optionally, the helm torque signal 26 is also utilized in an exemplary embodiment, the helm command signal 36 is received from the master control unit 16 ( FIG. 1 ) into the helm control unit 40 where the signal is compared to the helm torque signal 26 . For example, a simple method of comparison is simply to subtract one signal from another. A zero result indicates that the desired torque is being applied. A compensation process 240 ( FIG. 8 ) may be employed in the helm control unit 40 to maintain stability of the helm dynamics unit 42 . The compensation process 240 ( FIG. 8 ) is used to provide stability of the helm control system 12 at sufficient gains to achieve bandwidth greater than 3 Hz. In the case, of each local loop (helm and rudder) the bandwidth of each affects the stability of the overall system. If either direction and/or helm control systems, 14 and 12 respectively, have low bandwidth, over all stability is reduced and compensation on a higher level is required. A torque command signal 44 is then passed to the helm dynamics unit 42 as needed to comply with the helm torque command signal 36 . The helm dynamics unit 42 contains the necessary elements to provide a reaction torque to the operator as well as a torque sensor 24 to provide the feedback, torque signal 18 to the helm control unit 40 as well to the master control unit 16 ( FIG. 1 ), and a helm position sensor 18 that produces and sends the helm position signal 22 . Generally, reaction torque will be imparted to the operator by an electric motor coupled to the helm 20 . However, other configurations are possible. Preferred reaction torque motors are those with reduced torque ripple, such as are described in detail in commonly assigned U.S. Pat. No. 6,498,451, entitled TORQUE RIPPLE FREE ELECTRIC POWER STEERING, filed Sep. 6, 2000, the disclosures of which are incorporated by reference herein in their entirety. It is noteworthy to appreciate that a torque ripple free motor is desirable, but not required for this invention. Either type will work with the invention as disclosed and described. Finally, once again, while an exemplary embodiment has been described employing a motor to provide a reaction torque to the operator, a simple brake that provides resistance to motion or a brake and return spring (to provide a centering force) may also be utilized.

[0024] In another exemplary embodiment, resistive torque may be applied to the helm control system 12 in the case of a motor (not shown) attached to the helm 20 in the helm dynamics unit 42 to provide a center or straight ahead feel to the operator. This torque is referred to as active torque feedback. In addition, optionally, resistive passive torque may also be applied. For example, passive torque may be applied with a friction brake (not shown), optionally part of helm dynamics unit 42 . This resistive force could be a function of helm 20 displacement from center as measured by the helm position sensor 18 (or rudder position from center), a detent at center, or of some other load on the watercraft control system 10 . This would allow the operator to always know where center of the helm 20 control is regardless of the speed of the watercraft.

[0025] In another exemplary embodiment, the motor or brake (of the helm dynamics unit 42 ) can be used to communicate that the operator has reached an end of travel for the control input. For example, (in the case of variable ratio) an end of travel (e.g., stop) may be indicated by increasing the force when the helm 20 moves (commands a travel) beyond a selected limit, for example, the maximum travel of the rudder 15 (yet may not have reached another physical control travel stop). Advantageously, this end of travel stop may vary as the variable ratio changes. For instance, if in a selected configuration, the rudder 15 travel is +/−40 degrees, and the ratio can vary from 2:1 to 15:1 (helm 20 control degrees: rudder degrees) the helm 20 stops would vary from +/−80 degrees to +/−600 degrees. Additionally, the variation of the stops may be controlled depending upon a selected mechanical configuration. For example, in an exemplary embodiment, and for a configuration where the brake (not shown) and the helm position sensor 18 are located on the lower side of the helm torque sensor 24 , as the operator approaches a stop, the helm control system 12 may increase the torque and stop further movement in a given direction. In this embodiment, the helm torque sensor 24 would be monitored to determine the direction of helm torque signal 26 . If the helm torque signal 26 is in a direction to increase the helm control angle (from center), the brake may remain locked. If the helm torque signal is in the direction to decrease the helm 20 control angle (from center), the command to the brake may be decreased.

[0026] In yet another exemplary embodiment, the brake may be mounted on the lower side (away from the operator input at the helm) of the torque detector (an apparatus that facilitates measurement of the torque applied to the helm 20 , such as a t-bar) and the helm position sensor 18 is mounted on the upper side (“closer” to the operator input at the helm) of the t-bar no electrical helm torque sensor 24 would be required and the torque sensor 24 could be optional. In this embodiment, the brake control would be a function of helm position signal 22 as measured by the helm position sensor 18 . In this instance the electrical components for torques sensing need not be employed, but the t-bar or compliant member between the brake and helm 20 would be employed along with the position sensor 18 being located on the side of the t-bar closest to the helm 20 .

[0027] It will further be appreciated that while particular sensors and nomenclature are enumerated to describe an exemplary embodiment, such sensors are described for illustration only and are not limiting. Numerous variations, substitutes, and equivalents will be apparent to those contemplating the disclosure herein. For example, while a torque sensor 24 and helm position sensor 18 are described to sense the helm torque signal 26 and helm position signal 22 , such description is illustrative. Any sensor and nomenclature which can be utilized to measure equivalent or similar parameters is also contemplated

[0028] Referring now to FIG. 3 , the direction control system 14 , like the helm control system 12 , is also a control system (once again, closed loop in this instance, but not necessarily) that in an exemplary embodiment employs rudder position as a feedback signal. There may be a direction control system 14 for each steerable member/rudder 15 (only one is shown). In an embodiment, within the direction control system 14 the directional command signal 34 is received from the master control unit 16 and compared with a rudder position signal 30 within the direction control unit 50 . A position command signal 52 is sent to the rudder dynamics unit 54 . The rudder dynamics unit 54 contains the necessary elements to manipulate the position of the rudder 15 as well as a rudder position sensor 32 to provide rudder position signal 30 indicative of the rudder position. It will be appreciated that the directional command signal 34 could be dependent upon additional sensors and functions. For example, rudder force may also be sensed and employed to enhance control functions of the control-by-wire system 10 . In an alternative embodiment a rudder force sensor 53 also located within rudder dynamics unit 54 . The rudder force sensor 53 detects and also measures the forces/loads exerted in the direction control system 14 and sends a rudder force signal 55 representative of the measured forces to rudder control unit 50 and the master control unit 16 ( FIG. 1 ). The rudder dynamics unit 54 includes hydraulic actuators, drive motors, and the like, which may be operated in either current or voltage mode, provided, in each case, sufficient stability margins are designed into the direction control system 14 with local loop (rudder control unit 50 /rudder dynamics unit 54 loop) compensators. In an embodiment, a bandwidth greater than 3 Hz has been shown to be desirable in either case.

[0029] Similarly once again, it will further be appreciated that while particular sensors are enumerated to describe an exemplary embodiment, such sensors and nomenclature are described for illustration only and are not limiting. Numerous variations, substitutes, and equivalents will be apparent to those contemplating the disclosure herein. For example, while a rudder force sensor 53 and rudder position sensor 32 are described to sense the rudder force signals 55 and rudder position signal 30 , such description is illustrative. Any sensor and nomenclature, which can be utilized to measure equivalent or similar parameters is also contemplated. Moreover, it will be appreciated that the rudder force sensor 53 may optional. For example, in the case of an alternative embodiment where the helm torque command is a function of position deviated from center of either the rudder 15 or of helm 20

[0030] Referring now to FIG. 3 as well, additional features for the steer-by-wire watercraft control system 10 may be considered in an exemplary embodiment adding one or more lateral thruster(s) 56 to the watercraft. The longitudinal (fore/aft) control of the watercraft could be controlled by the throttle position (not shown). For example, rudder 15 and/or outdrive directional control may be used in combination with lateral thruster(s) 56 . For example, in a docking mode, in an exemplary embodiment, the steerable member, in this instance, the rudder 15 could be held in a fixed position, e.g., straight ahead, and the function of the helm 20 i.e. commanded inputs thereto, could change to a yaw type of control where yaw rotation/lateral motion is facilitated via lateral thruster(s) 56 . Alternatively, the steerable member, in this instance rudder 15 could be configured to work in collaboratively with the lateral thruster(s) 56 to affect primarily lateral or yaw directional control. In this instance variable ratio control for the helm may be employed as disclosed herein to facilitate achieving the desired lateral/yaw control for a given motion of the helm 20 .

[0031] In yet another exemplary embodiment, control of the lateral thruster(s) 56 is integrated with the steering control of the helm 20 and helm control system 12 . The integrated steering control may be configured such that a lateral thruster(s) 56 operate under selected conditions to enhance steering with integrated lateral and yaw control of the watercraft. In an exemplary embodiment, the lateral thruster(s) 56 are configured to intermittently operate under the following conditions:

[0032] For a helm input of within a selected window of a number of degrees: 0% duty cycle i.e. hysteresis or a dead band; in an exemplary embodiment, twenty degrees is utilized;

[0033] For a helm control position exceeding a selected number of degrees: a duty cycle linearly increasing with helm position up to a travel stop, or a helm input is indexed into a look-up table for to facilitate employing a nonlinear duty cycle to the travel stops; in an exemplary embodiment, a window of five degrees is employed.

[0034] In yet another exemplary embodiment, the lateral thruster(s) 56 may be configured to operate with a helm input within a selected threshold of a travel stop. For example, within a selected number of degrees from an established helm travel stop.

[0035] It will be appreciated that because the steering response time of a vessel is relatively long, (in a controls system sense, in the area of about 10 seconds or more) the response duty cycle will also be relatively long to coincide with that of the watercraft.

[0036] The lateral thruster(s) 56 may also be configured to be responsive to other parameters. For example, in another exemplary embodiment, the lateral thruster(s) 56 operation varies as a function of a selected gear/drive e.g., forward, reverse, neutral, or as a function of mode, e.g., standard or non-docking (yaw control), transitional (combination of yaw and lateral control), docking lateral control.

[0037] In one exemplary embodiment, with a selected gear in the forward position and non-docking mode (yaw control) the lateral thruster(s) 56 are configured to operate in the direction of steering e.g., helm turned to the left (port) then lateral thruster operates to push the bow of the watercraft to the left (while the rudder 15 control provides thrust of the stem to the right). In other words, the lateral thruster(s) operates to provide thrust in the opposite direction of the rudder control (yaw control).

[0038] In a docking mode, the lateral thruster(s) ( 56 ) operate to direct the watercraft in particular the bow, in the same direction as the stem propulsion (lateral control). In an exemplary embodiment, the gear position/selection is employed to select the desired lateral thruster(s) 56 direction. It will be appreciated that other variations and combinations of rudder directional control/lateral thruster(s) 56 control are conceivable.

[0039] In yet another additional embodiment, expanded functionality may be achieved for lateral/yaw control of a watercraft by employing an additional control input such as a joy stick, or push buttons providing a directional signal command 21 as part of the helm 20 that would command lateral control of the directional control system 14 to generate a position command to the rudder 15 of the rudder dynamics unit 54 and a lateral thrust command 23 to the lateral thruster(s) 56 , and thereby cause the rudder 15 to direct the watercraft to the left while the lateral thruster(s) 56 would provide thrust in the left direction, a control system would maintain close to zero yaw while the boat would travel in a lateral direction. For example, a joystick or push buttons could be utilized for yaw, & lateral/longitudinal directional control of the watercraft. Moreover, an additional lateral thruster 56 may be employed to facilitate pure lateral motion control, if some yawing motion is deemed undesirable.

[0040] On the other hand, while in a high-speed mode, the helm 20 control characteristics may be reconfigured to control the rudder 15 and directing drive thrust, with the lateral thruster(s) 56 disabled. In an exemplary embodiment, mode switching is automatic and transparent to the operator and is based on watercraft parameters, including but not limited to, speed of the craft and/or throttle position. In yet another exemplary embodiment, the lateral thruster(s) 56 discussed above could also be employed as an input approaches the above-mentioned stops. The input is the helm 20 , the stops are adjustable as in the variable ratio case, and as the helm 20 approaches a selected position, e.g., approximately 5 degrees from a stop the lateral thruster 56 would be turned on. For example, in an exemplary embodiment, when the helm is turned to the left, the lateral thruster(s) 56 may be turned on to provide thrust to the right direction causing the bow of the watercraft to move left. Similarly, when the helm is turned to the right, the lateral thruster(s) 56 may be turned on to provide thrust to the left direction causing the bow of the watercraft to move right. It will be appreciated that one or more lateral thruster(s) 56 may be employed. For example, in an exemplary embodiment, two lateral thrusters 56 are employed including interlocks to prevent simultaneous operation. Moreover multiple lateral thruster(s) 56 may be employed, with variable directional thrust in multiple directions.

[0041] In yet another exemplary embodiment control of the water craft and mode selection may be implemented employing a simple switched input. For example, in one embodiment a switched input is used to select “yaw” control as opposed to “lateral” control. Moreover, a switched input from the helm may be employed to select other operating modes including a variable ratio helm command as described herein. Advantageously, this provides a rather simple implementation for selected control functions and features.

[0042] Continuing with FIGS. 1, 3 , and 4 , in yet another exemplary embodiment, an inclination system 300 comprising an inclination sensors 310 a , in the fore and aft direction and 310 b in the port and starboard direction may be utilized to measure tilt of the watercraft for instances where a load is not centered on the center of gravity or to control plane time and application. Control of inclination is facilitated by an additional control process for trim 320 in the master control unit 16 , which generates a left and right trim command 322 and 334 respectively for I/O trim 336 , (in the case of an I/O drive) and trim tab control. In an exemplary embodiment, these functions are optionally a function of watercraft speed to facilitate implementation. For example, trim control could be disabled at low speed. In the case of port/starboard control, a closed loop control integrated with port/starboard inclination sensors 310 transmit an inclination signal 312 to the master control unit 16 . Process trim 320 in turn computes a trim commands 322 , and 324 to direct the stern trim tabs 332 and 334 and/or I/O trim 336 for port and starboard respectively. The trim tabs 332 and 334 may be controlled out of phase from each other to control port starboard tilt. Similarly, for fore/aft control, a closed loop control integrated with the fore/aft inclination sensor 310 and the stern trim tabs 332 and 334 respectively. In this instance, the trim tabs 332 , and 334 could be controlled in phase of each other to control fore/aft tilt.

[0043] FIG. 4 shows a more detailed view of the master control unit 16 and particularly the processes executed therein. The master control unit 16 receives the helm position signal 22 and helm torque signal 26 from the helm control system 12 . This helm position signal 22 , the helm torque signal 26 and the watercraft speed signal 28 are utilized to generate and output the rudder directional command signal 34 within a position control process 60 of the master control unit 16 . Moreover, the helm position signal 22 , optional rudder force signal 55 , helm torque signal 26 and watercraft speed signal 28 are utilized to generate and output the helm torque command signal 36 within a torque control process 70 of the master control unit 16 . The torque control process 70 and position control process 60 form outer loop controls for the helm control system 12 and direction control system 14 respectively. The master control unit 16 as well as any controller functions may be distributed to the helm control system 12 and direction control system 14 . The master control unit 16 is disposed in communication with the various systems and sensors of the control-by-wire system 10 . Master control unit 16 (as well as the helm control unit 40 and rudder control unit 50 ) receives signals from system sensors, quantify the received information, and provides an output command signal(s) in response thereto, in this instance, for example, commands to the subsystems and to the helm dynamics unit 42 and rudder dynamics unit 54 respectively. As exemplified in the disclosed embodiments, and as depicted in FIGS. 2 and 8 , one such process may be determining from various system measurements, parameters, and states the appropriate force feedback for compensating a helm control system 12 , another may be determining from various system measurements, parameters, and states the appropriate position feedback for compensating a direction control system 14 .

[0044] In order to perform the prescribed functions and desired processing, as well as the computations therefore (e.g., the control algorithm(s), and the like), the controllers e.g., 16 , 40 , 50 may include, but not be limited to, a processor(s), computer(s), memory, storage, register(s), timing, interrupt(s), communication interface(s), and input/output signal interfaces, and the like, as well as combinations comprising at least one of the foregoing. For example, master control unit 16 may include signal input signal filtering to enable accurate sampling and conversion or acquisitions of such signals from communications interfaces. Additional features of master control unit 16 , the helm control unit 40 , and rudder control unit 50 and certain processes therein are thoroughly discussed at a later point herein.

Master Control Processes

[0045] Referring to FIG. 5 , the torque control process 70 performs several processes for generating the helm torque command signal 36 . These processes include, but are not limited to an active damping process 72 , compensation 74 , and a feel process 76 . These processes utilize as inputs; the rudder force signal 55 , watercraft speed signal 28 , the helm torque signal 26 , and the helm position signal 22 , to generate the helm torque command signal 18 as an output. The first process is the active damping process 72 , which utilizes one or more of: the watercraft speed 28 , the helm torque signal 26 , and may employ the helm position signal 22 , rudder force signal 55 (if utilized) in various combinations to generate a damping torque command signal 73 . The active damping process 72 provides the opportunity to control the damping of the control-by-wire-system 10 dynamically as a function of watercraft operational parameters. It will be appreciated that active damping employed with a passive torque control in the helm control system 12 will be able to add damping. However, with an active torque control utilized in the helm control system 12 , damping may be readily added or subtracted from the system. In an exemplary embodiment, the active damping process generates an increasing desired damping command signal with increasing watercraft speed as indicated by the watercraft speed signal 28 , decreasing helm torque as detected by the feedback torque sensor signal 36 , and increasing rate of change of helm position signal 20 . A damping torque command signal 73 is sent to a compensation process 74 of the torque control process 70 .

[0046] The compensation process 74 may include, but is not limited to, frequency based filtering to manipulate the spectral content of the damping torque command signal 73 to ensure control-by-wire overall system loop stability. Moreover, the compensation process 74 is configured to maintain system stability in the event the bandwidth of the control loops within the helm control system 12 or direction control system 14 decrease. Finally, the compensation process 74 manipulates the damping torque command signal 73 to modify the spectral content of sensed force feedback to the watercraft operator. The compensation process 74 outputs the compensated torque command signal 75 to the feel process 76 . It will be appreciated that if passive torque control is used in the presence of non-linear plant dynamics compensation such as compensation process 74 may also be necessary. As stated earlier such compensation may include, but not be limited to, scaling, scheduling, frequency based manipulation, and the like of the damping torque command signal 73 .

[0047] Continuing with FIG. 5 , and moving now to the feel process 76 , which includes several sub-processes for generating the helm torque command signal 36 . The first sub-processes of one exemplary embodiment being the assist sub-process 78 , which generates an assist torque command signal 79 as a function of watercraft speed and the rudder force signal (if rudder force is not used, the sub-process may be simplified or not employed). In an exemplary embodiment, the assist sub-process 78 indexes the rudder force signal initiated, compensated torque command signal 75 and watercraft speed signal into a set of one or more torque look-up tables (not shown) yielding an assist torque command signal 79 . Alternatively, where more than one look-up table is used, the look-up table resultants are preferably blended based upon a ratio dependent upon the watercraft speed signal 28 . For example, two lookup tables might be used, one for low speeds, and one for high speeds. As the watercraft speed signal 28 increases, the table for high speeds becomes increasingly dominant in the blend over the table for low speeds. Generally, it may be desirable for the assist process 78 to provide increasing assist torque as a function of watercraft speed increases. Assist forces may be formulated/evidenced as a decrease in the steering assisting force to allow the operator to feel more of the steering load or as in an exemplary embodiment, the commanded torque to the operator is increased to cause the operator to feel additional steering load at the helm 20 . It will be appreciated that the assist function is optionally employed if the steering system is configured to detect the load of the direction control system 14 . In the instance where position is utilized to provide a force (tactile feedback) to the operator the assist function is optional and not needed.

[0048] Another sub-process employed in the feel process 76 is the return sub-process 80 . If an optional active torque control loop control is employed a return sub-process 80 may be utilized. The return sub-process 80 generates a return to center torque command 81 to drive the helm and the control-by-wire system 10 to neutral or center under particular operating conditions based upon the current helm position as indicated by the helm position signal 22 and the watercraft speed as indicated by the watercraft speed signal 28 . Similar to the assist sub-process 78 , the return sub-process 80 may employ one or more lookup tables, which, in this case, are indexed by the helm position signal 22 . In an exemplary embodiment, the return sub-process 80 indexes the helm position signal 22 and watercraft speed signal 28 into a set of one or more lookup tables yielding a return to center torque command 81 . Alternatively, where more than one look-up table is used, the look-up table resultants may be blended based upon a ratio dependent upon the watercraft speed signal 28 . For example, two lookup tables might be used, one for low speeds, and one for high speeds. As the watercraft speed signal 28 increases, the table for high speeds becomes increasingly dominant in the blend over the table for low speeds. Generally, it may be desirable for the return sub-process 80 to provide increasing return torque as a function of watercraft speed increases. The final processing of the feel 76 process is to combine the assist torque command 79 (if generated) and the return to center torque command 81 (if generated) and thereby generating the helm torque command signal 36 . In an embodiment, the combination is achieved via a summation at summer 82 .

[0049] It should be appreciated that several embodiments are described some including additional sensor information and therefore additional processing function(s) e.g. rudder force. It should be further appreciated that an embodiment of the torque control process disclosed above could be as simple as braking, passive damping alone active damping 72 alone, an assist sub-process 78 alone, a return sub-process 80 alone, and the like as well as any combination including at least one of the foregoing.

[0050] Referring now to FIG. 6 , the position control process 60 includes, but is not limited to several sub processes that are used in the calculation of the directional command signal 34 . The position control unit 60 may include, but not be limited to, a variable ratio process 62 , and a directional command process 66 . In an exemplary embodiment, the variable steering ratio process 62 receives the helm position signal 22 and the watercraft speed signal 28 . The helm position signal 22 , and the watercraft speed signal 28 are used as inputs to a three dimensional look-up table to generate a variable steering ratio signal 64 . The resulting variable steering ratio signal 64 is passed to the directional command process 66 . In another exemplary embodiment, a variable ratio process 62 may be employed, which is further scheduled as a function of the helm position. For example, during the first few degrees of helm motion, the ratio may be greater than for other inputs. Since watercraft generally exhibit slow response especially at slow speeds, variable ratio as a function of helm position provides an advantage in handling and controllability by increasing the response of the watercraft to small inputs about center of helm position.

[0051] The directional command process 66 provides theta correction, that is, to correct the commanded rudder position to reflect the actual position of the helm 20 correctly. It may be appreciated that such a correction may only be needed for situations where the helm control system 12 includes a torque motor to provide a reaction torque to the operator in response to a movement of the rudder 15 . However, the operator does not necessarily permit the helm 20 to turn (although he feels the reaction torque). The helm torque signal 26 provides an effective, relative position measurement under the abovementioned conditions. This relative position measurement is used by the directional command process 66 to account for the motor to helm difference and compensate the helm position signal 22 accordingly. The effect of the rudder 15 moving without the helm moving is undesirable so an angle correction is provided and a theta-corrected, directional command signal 34 is generated. It is noteworthy to further understand that theta correction is only needed if the helm position sensor 22 for the helm 20 is located such that a compliant member (t-bar or compliant torque sensor 24 ) in the actuator implementation of the helm dynamics unit 42 is between the helm position sensor 33 and at the helm 20 .

[0052] It will be further appreciated that the correction identified above is a resultant of a selected implementation. In other implementations for an exemplary embodiment, such as where the helm control is simpler e.g., a brake for holding the helm 20 as opposed to a motor for providing reaction torque as described herein.

[0053] It is important to note that all the examples provided herein relate to a watercraft having a single steerable rudder 15 . However, this type of system could be easily extended to a watercraft that requires one or more rudders to be steered independently and simultaneously by adding additional direction control units 14 . Moreover, as previously discussed, in watercraft employing additional steerable members e.g. rudder, additional functionality may be implemented. For example, in an alternative embodiment, two or more steerable members may be employed to facilitate low speed maneuvering such as docking and the like. It is evident with multiple steerable members, a watercraft's thrust may be directed in multiple directions to facilitate yawing or lateral maneuvering.

Direction Control System

[0054] Referring now to FIGS. 3 and 7 depicting a simplified block diagram of a direction control system 14 in an exemplary embodiment of the position control implementation and specifically addressing the processing therein. The control functions implemented by the rudder control unit 50 (as discussed earlier as part of the direction control system 14 ) are used to control the rudder position of the steering system 10 via the rudder dynamics unit 54 , (also discussed earlier). The position control functionality of the rudder control, optionally, may be augmented by force compensation, which is based on the load experienced by the plant, in the example herein, the rudder dynamics unit 54 or the direction control system 14 .

[0055] FIG. 7 depicts a simplified diagram of an algorithm 100 that implements an exemplary process for rudder position control and optionally force compensation thereto. The rudder control unit 50 of the direction control system 14 performs several processes for generating the rudder position command signal 52 . These processes utilize as inputs the directional command signal 34 and the helm position signal 22 to ultimately generate the rudder position command signal 52 as an output. In FIG. 7 , the directional command signal 34 is scaled by a selected variable ratio at gain 110 to formulate a desired rudder position signal 112 . The desired rudder position signal 112 is compared with the actual rudder position as indicated by the rudder position signal 30 at summer 120 to generate a rudder position error 122 . The rudder position error 122 , may, optionally, be applied to a position compensation process 130 to formulate a compensated rudder position command 132 , which may then once again be scaled at gain 140 to formulate a rudder position command signal 142 which may be output as the rudder position command signal 52 . In an alternative embodiment, the rudder force 55 may be scheduled or scaled at gain 150 to formulate a force compensation signal 152 . The force compensation signal 152 , may, optionally, be applied to a force compensation process 170 to formulate a compensated force signal 172 , which may then once again be scaled if necessary. The compensated force signal 172 may be combined with the position command signal 142 at summer 160 to formulate a force compensated rudder position command signal 52 and thereafter applied to the rudder plant dynamics unit 54 .

[0056] The position compensation process 130 includes, but is not limited to, frequency based filtering to manipulate the spectral content of the compensated rudder position command signal 132 to ensure direction control system 14 , loop stability. Similarly, the force compensation process 170 includes, but is not limited to, frequency based filtering to manipulate the spectral content of the force compensation signal 172 to ensure direction control system 14 , loop stability. Finally, for an alternative embodiment, the combination of the rudder position command signal 142 and the force compensated signal 172 operate in conjunction to modify the spectral content of sensed force feedback and position and ensure direction control system 14 , loop stability. It should also be noted that the figures herein may depict additional and optional elements, connections, interconnections and the like. It will be appreciated that such configurations are commonly employed for implementation of a selected control configuration. For example, transport delays may be employed to ensure that data time coherency is addressed. Likewise, scaling may be employed to address unit conversions and the like.

[0057] A benefit of the alternative embodiment for algorithm control process 100 is that the addition of force compensation has a stabilizing effect on the direction control system 14 . This effect is beneficial in that the load (force) feedback in position control exhibits a dampening effect on the system. Therefore, a desired gain margin may readily be achieved via a conventional position control. Advantageously, this allows the conventional control to focus on providing enhanced performance under varying conditions. Yet, another way of looking at the stability enhancements to the direction control system 14 is improvement in the free control oscillations. A more stable system would damp out such oscillations more rapidly than a less stable system. The addition of force feedback in the position control coupled with other control system tuning reduces the tendency of the system to exhibit free control oscillations.

[0058] Another benefit is that the alternative embodiment of control process 100 including force compensation is that it preserves the desired dynamic behavior of the closed loop rudder system under varying loads. When a steering load is applied and both embodiments are optimized for this load, both will exhibit comparable performance. However, when the load is lowered, (e.g., low speed, rudder centered) degradation in the performance of the embodiment with position control alone results. However, there is no degradation in the performance the control system when the alternative embodiment is employed. Similarly, when the load is raised (e.g., high speed, turning,) once again, degradation in the performance of the position control is observed while there is no degradation in the performance of the control system when the alternative embodiment is employed. This effect is beneficial in that the load (force) feedback exhibits a robustness enhancement on the system.

[0059] Another significant advantage realized by an alternative embodiment employing force feedback in a position control function for the direction control system 14 is that it does not negatively impact the system bandwidth as significantly as a pure rate based damping might. It is well known, that rate based damping may be employed in a typical control loop to maintain stability. In an exemplary embodiment and as applied to a watercraft steering system as disclosed here, system bandwidth has a significant impact on the steering feel at the helm. A higher bandwidth position control system/loop exhibits an ability to closely follow operator applied input and as a result generate the expected effort (load) as feedback. Conversely, a system lacking sufficient bandwidth may lag behind an applied input, resulting in undesirable response or worse, instability. Input impedance is a way of characterizing or observing the feel of the control-by-wire system 10 . The effect of reducing the bandwidth (from about ten Hertz to about one Hertz) of the position control system/loop on the overall input impedance.

Helm Control System

[0060] Another embodiment of the invention described herein addresses the abovementioned issues of tactile feedback and stability by using information about helm position to directly influence the torque felt by the driver. By using a properly shaped transfer function, the input impedance of the steering system can be manipulated over a wide range of operating characteristics to obtain the desired feel. Including helm position in determination of the torque felt by the operator provides the desirable coupling between helm position and helm torque. However, beyond the fixed coupling that a mechanical connection provides, this approach provides a tunable coupling that can be adjusted based upon operator preferences, system characteristics, or operating conditions to achieve the desired steering feel for the watercraft overall.

[0061] This approach results in helm position and the resulting torque felt by the operator being largely decoupled. From a helm feel perspective, it will be appreciated that there is a desirable phase relationship between helm angular position and helm response torque. This desirable phase relationship is not fixed (as would be the case with a mechanically linked system) and may actually not be always be achievable depending upon the parameters sensed to provide the torque feedback to the helm. Moreover, there is also a desirable torque magnitude felt by the operator (as a function of input frequency). As the magnitude of this desired torque increases, the potential for undesirable response and even instability increases especially if the helm is released. This results from the feedback torque provided by the motor to achieve the desired feel is being balanced (in off-center and steady state sense) with the operator's effort. Once the operator releases the helm, however, the torque provided by the motor accelerates the helm to center and possibly overshoots, depending on the magnitude of the initial torque. As this overshooting action is taking place, the hand wheel system sends the corresponding position signal to the rudders, and the rudders return to center. However, due to lack of resistance by the operator (and thus a helm overshoot,) the rudder 15 may overshoot, as well. Therefore, the rudder forces under such a condition switch direction, and thus, there again, the helm dynamics unit 54 motor switches the direction of its torque (in response to the sensed rudder force). This causes the helm to drive back toward center (from the opposite off-center position now), and an overshoot of center may take place, again. The overshoot and oscillations is known in the art as “free control oscillation”. Since these oscillations are due in part to lack of resistance by the operator, it is reasonable to add some kind of resistance or damping in the helm control system to address this phenomenon.

[0062] The addition of resistance may be sufficient for many applications, especially, where the load on the system has a predictable relationship to the system position (rotational or translational). In control system terms, this could be predicted by the location of the poles and zeros of the system or frequency response. A conventional control system could then be designed based on these dynamics.

[0063] However, in many systems, the load varies based on operating conditions even with the position and its derivatives kept the same. For example, in steering applications, the load on the steering system changes as a function of operation (lateral acceleration, watercraft speed etc) and watercraft properties. In such cases, the conventional control design is optimal for a given operating condition, but has reduced performance as the conditions change. Therefore, is may be advantageous to provide a control-by-wire system, which addresses the load on the system while still providing the assist forces and tactile feedback for the operator and reducing free control oscillation.

[0064] Referring once again to FIGS. 1 and 2 , as disclosed earlier, the helm control system 12 is optionally a closed loop control system that optionally utilizes helm torque as the feedback signal. A helm torque command signal 36 optionally responsive to the rudder force signal 55 as detected by rudder force sensor 53 and/or a rudder position signal 30 as detected by rudder position sensor 32 may be received from the master control unit 16 into the helm control unit 40 where the signal is compared to the helm torque signal 26 .

[0065] Continuing with FIG. 2 , in addition the abovementioned torque feed back, an additional compensation path may be added to the helm control unit 40 of the helm control system 12 to incorporate position feedback in the torque control loop (e.g., position feedback in a force control loop) of the helm control system 12 . The addition of the helm position signal 22 as feedback to the torque control functions provided by the helm control unit 40 , enhances operation of the torque control functions therein. An optional position compensation process compensates the helm position feedback for combination with the compensated torque command signal 44 . A position compensated torque command signal 44 is then passed to the helm dynamics unit 42 as needed to comply with the helm torque command signal 36 . The position compensated helm torque command 44 determines the helm torque felt by the operator as generated by the helm dynamics unit 42 . This results in a direct relationship between helm position and helm torque, which can be tuned to get the desired helm steering feel to the operator.

[0066] Turning now to FIG. 8 as well, a simplified block diagram depicting an implementation of a control algorithm 200 executed by a controller, e.g., the helm control unit 40 . Control algorithm 200 includes, but is not limited t