CA3043073A1

CA3043073A1 - Electric actuator for a marine vessel

Info

Publication number: CA3043073A1
Application number: CA3043073A
Authority: CA
Inventors: Noam Davidson; Richard Redfern; Edmond Astafanus; Eric Fetchko; Mark DYCK; Anson Chan; Ian Carlson; Ray Wong
Original assignee: Marine Canada Acquisition Inc; Yamaha Motor Co Ltd
Current assignee: Dometic Marine Canada Inc; Yamaha Motor Co Ltd
Priority date: 2018-05-14
Filing date: 2019-05-13
Publication date: 2019-11-14

Abstract

An electric actuator for imparting steering movement to a propulsion unit of a marine vessel includes a housing and an output shaft reciprocatingly received by the housing.
There is a motor assembly disposed within the housing. The motor has a rotor assembly and a stator. Rotation of the rotor assembly causes the output shaft to translate axially relative to the rotor assembly and causes the output shaft to reciprocate relative to the housing. There is a coupling assembly mounted to the housing. The coupling assembly coupling the electric actuator to the propulsion unit of the marine vessel.
There is a steering control unit mounted to the housing. The steering control unit drives the motor to cause the rotor assembly to rotate and the output shaft to reciprocate relative to the housing.

Description

ELECTRIC ACTUATOR FOR A MARINE VESSEL
BACKGROUND OF THE INVENTION
Field of the Invention [0001] The present invention relates to an actuator and, in particular, to an actuator for a marine steering system for steering a marine vessel.
Description of the Related Art

[0002] United States Patent Application Publication No. 2017/0106959 which was published on April 20, 2017, in the name of Davidson et al., discloses an electric actuator comprising a housing and an output shaft reciprocatingly received by the housing. There is a screw assembly disposed within the housing and coupled to the output shaft.
The screw assembly includes a plurality of annular rollers and a central screw received by the annular rollers. The annular rollers are rotatable about the central screw. There is a motor which includes a stator and a rotor. The rotor has an inner bore which engages the annular rollers.
Rotation of the rotor causes the central screw to translate axially relative to the rotor and the output shaft to reciprocate relative to the housing.
SUMMARY OF THE INVENTION

[0003] It is an object of the present invention to provide an actuator for a marine steering system for steering a marine vessel.

[0004] There is accordingly provided an electric actuator for imparting steering movement to a propulsion unit of a marine vessel. The actuator includes a housing and an output shaft reciprocatingly received by the housing. There is a motor disposed within the housing. The motor has a rotor assembly and a stator. Rotation of the rotor assembly causes the output shaft to translate axially relative to the rotor assembly and causes the output shaft to reciprocate relative to the housing. There is a coupling assembly mounted to the housing. The coupling assembly may couple the actuator to the propulsion unit of the marine vessel. There is a steering control unit mounted to the housing. The steering control unit drives the motor to cause the rotor assembly to rotate and the output shaft to reciprocate relative to the housing.

[0005] The actuator may also include an electrical connector which electrically connects the steering control unit to the stator. There may be an opening in the housing and .. the electrical connector may extend through the opening to electrically connect the steering control unit to the stator. An interior of the steering control unit may be sealed. The electrical connector may include a commutation sensor board which senses a position of the rotor assembly and a motor connection which provides power to the stator.
The commutation sensor board may include three sets of sensors with each set of sensors having .. redundant and offset sensors. A first sensor of each set of sensors may be positioned for advanced timing of the stator in a first direction and a second sensor of each set of sensors may be positioned for advanced timing of the stator in a second direction. The first direction and the second direction may be opposite directions and may respectively correspond to opposite steering directions based on rotation of the rotor assembly.

[0006] The actuator may further include a brake. The brake may include an electromagnetic circuit and a brake pad. The electromagnetic circuit may include a magnet and a coil assembly. The magnet may generate a magnetic field which pulls the brake pad to an engaged position. The brake pad may be actuated to a released position when the coil assembly is energized by an electric current. The brake pad may engage a first friction surface and a second friction surface. The first friction surface and the second friction surface may be annular and concentric. The coil assembly may be an annular coil assembly which is rotatable within the housing for alignment purposes. The brake may pad may be actuated to a released position when the coil assembly is energized by an electric current between a first threshold current and a second threshold current. There may be a magnetic retainer received within a body and there may be an air gap between the magnetic retainer and the body to set brake strength and current thresholds.

[0007] The actuator may still further include an end gland at each end of the housing.
Each end gland may include an annular seal within a floating seal housing and each floating seal housing may be sealed by an 0-ring. The floating seal housing may include a radial lip with a sharp edge which functions as a scraper to scrape debris from the output shaft.
The floating seal housing may include a heel which is concentric with the output shaft and the heel moves radially with the output shaft as the heel is side loaded.

[0008] The actuator may yet still further include an absolute position sensor senses a steering position and a relative position sensor senses a position of the motor. A steering control unit may calibrate the relative position sensor based on a signal of the absolute position sensor. Calibration of the relative position sensor based on a signal of the absolute sensor initializes an accumulative position which accumulates a relative position as the actuator moves over time. The steering control unit may compare a signal of the absolute position sensor and the accumulative position for fault detection. The steering control unit may initialize the accumulative position of the relative position sensor with the absolute position sensor when the actuator is started-up. The steering control unit may monitor the accumulative position and the steering position during a change in steering direction to determine backlash. The steering control unit may monitor backlash during initial steering movement and compensate for backlash by reinitializing the accumulative position. The steering control unit may restrict a steering range of the actuator after initialization of the accumulative position and before compensating for backlash. The steering control unit may use the accumulative position for position control of the actuator. The steering control unit may use a redundant sensor for position control of the actuator if the absolute position sensor or the relative position sensor fails.

[0009] There is also provided an actuator for imparting steering movement to a propulsion unit of a marine vessel. The actuator comprises a housing and an output shaft reciprocatingly received by the housing. There is a motor disposed within the housing. The motor has a rotor and a stator. The rotor has a magnetic field and the stator has a magnetic field. Rotation of the rotor causes the output shaft to translate axially relative to the rotor and causes the output shaft to reciprocate relative to the housing. There is a sensor for sensing a position of the rotor. A steering control unit energizes the stator to align the magnetic field of the stator with the magnetic field of the rotor based on a position of the rotor sensed by the sensor for sensing the position of the rotor to hold a position of the rotor. The motor may be at least a two-phase motor and the steering control unit may energize the stator by applying a PWM to generate a magnetic field that aligns with and holds the positon of the rotor. The steering control unit may apply a minimum PWM to create a holding torque which counters a back driving torque as a function of at least one of marine vessel speed, engine RPM and steering position. The steering control unit may increase or decrease the minimum PWM applied in order to counter the back driving torque. The steering control unit may determine a magnitude and rate of PWM
increase based on a signal from a position sensor sensing back driving. The steering control unit may determine a magnitude and rate of PWM increase based on a signal from a torque sensor sensing back driving torque.

[0010] There is further provided a marine steering system comprising a propulsion unit having a tiller and an actuator for imparting steering movement to the tiller of the propulsion unit. The actuator includes a housing and an output shaft reciprocatingly received by the housing. A motor causes the output shaft to reciprocate relative to the housing. A power source supplies electric power to the motor and a sensor senses a capacity of the power source. A steering control unit limits the maximum electric power supplied to the motor by controlling the voltage to the motor relative to the capacity of the power source. The steering control unit may limit the maximum electric power supplied to the motor by increasing or decreasing PWM based on whether the PWM is in a first zone, a second zone, or a third zone. The steering control unit may decreases PWM when the PWM
is in the first zone. The steering control unit may increase or decrease PWM
when the PWM is in the second zone. The steering control unit may prevent increasing PWM when the PWM is in the third zone.

[0011] There is still further provided a marine steering system comprising a first propulsion unit and a second propulsion unit. The first propulsion unit has a tiller, an actuator for imparting steering movement to the tiller of the first propulsion unit, and a position sensor for sensing a steering position of the first propulsion unit.
The second propulsion unit has a tiller and an actuator for imparting steering movement to the tiller of the second propulsion unit, and a position sensor for sensing a steering position of the second propulsion unit. A steering control unit controls the actuator of the first propulsion unit to impart steering movement to the first propulsion unit based on relative steering positions of the first propulsion unit and the second propulsion unit. The steering control unit controls the actuator of the first propulsion unit by slowing down steering movement of the first propulsion unit relative to the steering position of the second propulsion unit.

[0012] The steering control unit may also control the actuator of the first propulsion unit to impart steering movement to the first propulsion unit based on an engine target rudder angle position scheduler. The steering control unit may control the actuator of the first propulsion unit to pause steering movement to the first propulsion unit based on relative steering positions of the first propulsion unit and the second propulsion unit. The steering control unit may control the actuator of the first propulsion unit to impart steering movement to the first propulsion unit in one of a first state in which the actuator is allowed to operate up to a maximum speed, a second state in which the actuator is allowed to operate up to a maximum speed limit determined by a reduce speed curve, and a third state in which the actuator is prevented from moving further in a first direction. The steering control unit may control the actuator of the first propulsion unit to impart steering movement of the first propulsion unit based on a direction of the steering movement. The steering control unit may control the actuator of the first propulsion unit to impart steering movement of the first propulsion when the first propulsion unit is moving towards the second propulsion unit. The transition between said states is based on the relative steering positions of the first propulsion unit and the second propulsion unit.

[0013] There is yet still further provided a marine steering system comprising a first propulsion unit and a second propulsion unit. The first propulsion unit has a tiller, an actuator for imparting steering movement to the tiller of the propulsion unit, and a position sensor for sensing a steering position of the first propulsion unit. The second propulsion unit has a tiller and an actuator for imparting steering movement to the tiller of the second propulsion unit, and a position sensor for sensing a steering position of the second propulsion unit. There is a device for setting a toe angle between the first propulsion unit and the second propulsion unit. A steering control unit receiving a steering position command, wherein the steering control unit may modify the toe angle based on the steering position command. The steering control unit modifies the toe angle and an engine target rudder angle for each of the first propulsion unit and the second propulsion unit based on the toe angle. The engine target rudder angle may be dependent on a vessel target rudder angle.
BRIEF DESCRIPTIONS OF DRAWINGS

[0014] The invention will be more readily understood from the following description of the embodiments thereof given, by way of example only, with reference to the accompanying drawings, in which:

[0015] Figure 1 is a perspective view of a marine vessel provided with a plurality of propulsion units each having an electric actuator;

[0016] Figure 2 is a perspective view of a first side of an electric actuator of one of the propulsion units of Figure 1;

[0017] Figure 3 is a perspective view of a second side of the electric actuator of one of the propulsion units of Figure 1;

[0018] Figure 4 is an exploded, perspective view of the electric actuator of Figures 2 and 3;

[0019] Figure 5 is a longitudinal, sectional view of the electric actuator of Figures 2 and 3;

[0020] Figure 6 is a fragmentary and exploded, perspective view of the electric actuator of Figures 2 and 3;

[0021] Figure 7 is a perspective view of a stator of the electric actuator of Figures 2 and 3;

[0022] Figure 8A is a perspective view of an electrical connector of the electric actuator of Figures 2 and 3;

[0023] Figure 88 is another perspective view of an electrical connector of the electric actuator of Figures 2 and 3;

[0024] Figure 9 is a plan view of an inside of a cover for a steering control unit of the electric actuator of Figures 2 and 3;

[0025] Figure 10 is an exploded, perspective view of a rotor assembly for the electric actuator of Figures 2 and 3;

[0026] Figure 11 is an exploded, perspective view of an output shaft for the electric actuator of Figures 2 and 3;

[0027] Figure 12 is an exploded, perspective view of a brake for the electric actuator of Figures 2 and 3;

[0028] Figure 13 is a fragmentary, exploded perspective view of the brake of the electric actuator of Figures 2 and 3;

[0029] Figure 14 is another fragmentary, exploded perspective view of the brake of the electric actuator of Figures 2 and 3;

[0030] Figure 15 is a partially sectional, elevation view of the brake for the electric actuator of Figures 2 and 3 in an engaged position;

[0031] Figure 16 is an enlarged view of an air gap of the brake taken from Figure 15;

[0032] Figure 17 is a partially sectional, elevation view of the brake for the electric actuator of Figures 2 and 3 in a released position;

[0033] Figure 18 is an enlarged view of the air gap of the brake taken from Figure 17;

[0034] Figure 19 is an exploded perspective view of an end gland for the electric actuator of Figures 2 and 3;

[0035] Figure 20 is a partially sectional, elevation view of the end gland for the electric actuator of Figures 2 and 3;

[0036] Figure 21 is an enlarged view of a portion of a floating seal housing of the end gland taken from Figure 20;

[0037] Figure 22 is a cross-sectional view taken across line 22-22 of Figure 2;

[0038] Figure 23 is a block diagram of the steering control unit of the electric actuator of Figures 2 and 3;

[0039] Figure 24 is a schematic showing the initialization logic of a sensing method .
employed by the electric actuator of Figures 2 and 3;

[0040] Figure 25 is a graph showing PWM duty cycle as a function of time when the steering control unit energizes the stator to align the magnetic field of the stator with the magnetic field of the rotor in the electric actuator of Figures 2 and 3;

[0041] Figure 26 is a schematic showing the stator and a stationary rotor when the holding torque is greater than or equal to the external torque that is being applied to the electric actuator of Figures 2 and 3;

[0042] Figure 27 is a schematic showing the stator and a back driving rotor when a holding torque is less than the external torque that is being applied to the electric actuator of Figures 2 and 3;

[0043] Figure 28 is a schematic showing the stator and a stopping rotor when the holding torque has been increased to counter the external torque that is being applied to the electric actuator of Figures 2 and 3;

[0044] Figure 29 is a state diagram showing the states of the steering control unit as the steering control unit controls the holding torque to hold the position of the actuator of Figures 2 and 3;

[0045] Figure 30 is a block diagram showing the logic of the current limiting method employed by the electric actuator of Figures 2 and 3;

[0046] Figure 31 is a graph showing a three zone current limiting scheme for the electric actuator of Figures 2 and 3;

[0047] Figure 32 is a block diagram showing the logic of a three stage process to prevent engine collision employed by the actuator of Figures 2 and 3;

[0048]
Figure 33 is a graph showing a target actuator position scheduler for the propulsion units of the marine vessel of Figure 1;

[0049]
Figures 34A and 34B are schematics showing the port engine slowing down due to a load and the port center engine slowing down to prevent engine collision;

[0050] Figure 35 is a graph showing a pause line of an engine and slow down line of an engine relationship with an adjacent engine of the marine vessel of Figure 1;
[00511 Figure 36 is a state diagram of the three stage process to prevent engine collision employed by the electric actuator of Figures 2 and 3; and [0052]
Figure 37 is a variable toe angle diagram of the three stage process to prevent engine collision employed by the electric actuator of Figures 2 and 3.
DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
[0053]
Referring to the drawings and first to Figure 1, there is shown a marine vessel 10 which is provided with a plurality of propulsion units which, in this example, are in the form of four outboard engines, namely, a port engine 12, a port center engine 14a, a starboard center engine 14b, and a starboard engine 16. However, the propulsion units may be any number or form of propulsion units in other examples. The marine vessel 10 is also provided with a control station 18 that supports a steering wheel 20 mounted on a helm 22, a control head 24, and a joystick 26. The control station 18 is conventional and allows the port engine 12, the port center engine 14a, the starboard center engine 14b, and the starboard engine 16 to be steered using either the steering wheel 20 and the helm 22 or the joystick 26 as disclosed in PCT International Application Publication Number WO
2013/1123208 Al which is incorporated herein by reference. The control station 18 further includes a first display interface 28 and a second display interface 30. In this example, the first display interface is a display interface which displays navigational information and the second display interface is a display interface which displays onboard system information.

[0054] Each of the port engine 12, the port center engine 14a, the starboard center engine 14b, and the starboard engine 16 is provided with an electric actuator which steers each engine. Figures 2 and 3 show an electric actuator 40 of the port engine 12. It will be understood by a person skilled in the art that the electric actuators for the port center engine 14a, the starboard center engine 14b, and the starboard engine 16 are substantially identical in structure and function to the electric actuator 40 for the port engine 12.
The electric actuators for the port center engine 14a, the starboard center engine 14b, and the starboard engine 16 are accordingly not described in detail herein. The electric actuator 40 includes a housing 42 as well as an output shaft 44 which is reciprocatingly received by the housing 42. It will be understood by a person skilled in the art that, when the electric actuator 40 is mounted on the port engine 12, axial movement of the output shaft 44 is inhibited relative to the marine vessel 10 while the housing 42 reciprocates linearly along the output shaft 44 and relative to the marine vessel 10. This relative linear movement of the housing 42 imparts a steering force to a tiller 46 of the port engine 12 and thereby causes the port engine 12 to be steering in a conventional manner.
[0055] There is a coupling assembly 48 mounted on the housing 42. The coupling assembly 48 is a ball joint assembly, in this example, and allows the housing 42 to be coupled to the tiller 46 of the port engine 12. There is also a steering control unit 50 mounted on the housing 42. The steering control unit 50 is accordingly integral with the electric actuator 40 as opposed to being elsewhere on the marine vessel 10 as is conventional. There are inputs, for example input 52, which allow the steering control unit 50 to be in communication with the control station of the marine vessel 10.
Making the steering control unit 50 integral with the electric actuator 40 simplifies wiring, by eliminating the need for a number of wires/harnesses required in conventional systems, and .. reduces voltage drop between the steering control unit and the actuator.
[0056] Figure 4 is an exploded view of the electric actuator 40 and shows a cover 54 of the steering control unit 50 which covers electrical components within an interior 56 of the steering control unit 50 which is integral with the actuator housing 42 in this example.
A gasket 58 is employed between the cover 54 and the interior 56 of the steering control unit 50 to seal the steering control unit 50 and the actuator housing 42. The electric actuator 40 further includes the following general components, namely, a stator 60, a rotor assembly 62, and a brake 64 which are disposed within an interior 66 of the housing 42 as best shown in Figure 5. The housing 42 is sealed at opposite ends by an end gland 68 and an end gland 70. The electrical components of the steering control unit 50 are generally separated from the components disposed within the housing 42. However, with reference to Figure 6, an electrical connector 72 extends through an opening 74 in the housing 42 to electrically connect the stator 60 to the steering control unit 50. A gasket 76 is employed about the electrical connector 72.
[0057] The stator 60 includes a plurality of segments, for example segments 78a and 78b, which are arranged in a generally annular formation and, in this example, are initially retained in the generally annular formation by a retaining ring 80. The segments 78a and 78b are received by the housing 42 and the retaining ring 80 is removed. The segments may then be retained in the generally annular formation in the housing 42 by potting about the stator with epoxy (not shown). The epoxy is sealed within the interior 66 of the housing 42 by the gasket 76. The segments may initially be arranged in the generally annular formation with play to allow the stator 60 to adjust to different sized housings. Each of the segments has a separate electrical winding coil, for example, coils 82a and 82b which are shown respectively for segments 78a and 78b. This arrangement allows for reduced end turns. The stator 60 also includes a lead frame 84, best shown in Figure 7, with a plurality of openings, for example openings 86a and 86b, through which respective coils 82a and 82b are crimped and wired to the steering control unit 50 by the electrical connector 72.
The lead frame 84 of the stator 60 also includes a portion 88 with a plurality of openings 90a, 90b, and 90c, shown in Figure 7, which receive respective fasteners 92a, 92b, and 92c to allow for alignment and connection of the stator 60 with the electrical connector 72. The electrical connector 72 receives dowel pins 93a and 93b which allow for alignment of the stator 60 with the electrical connector 72. The electrical connector 72 also receives a plurality of fasteners 94a, 94b, 94c and 94d which hold the electrical connector in place.
This arrangement separates electrical components within the interior 56 of the steering control unit 50 from the components within the interior 66 of the housing 42.

[0058] The electrical connector 72 is shown in greater detail in Figures 8A and 8B. The electrical connector 72 has a commutation sensor board with sensors which, in this example, is a Hall Effect sensor board 96 with an array of Hall Effect sensors. There are three sets of Hall Effect sensors 98a, 98b, and 98c, in this example, because the electric actuator 40 has a three-phase electric motor. Alternating current fluctuates according to a curve from a positive to negative state over time. Each component of the three-phase current follows the same pattern but are spaced apart timewise. Each current phase uses all three sets of Hall Effect sensors. Each set of Hall Effect sensors 98a, 98b, and 98c is coupled to a respective one of Hall Effect sensor connections 100a, 100b and 100c. The electric connector 72 also has a plurality of motor connections 102a, 102b and 102c and corresponding motor pins 103a, 103b, and 103c as well as a plurality of brake connections 104a and 104b and corresponding brake pins 105a and 105b.
[0059]
Referring now to Figure 9, an inner side 102 of the cover 54 of the steering control unit 50 is provided with a first connection 106 and a second connection 108. The first connection 106 connects to the Hall Effect sensor connections 100a, 100b and 100c as well as the brake pins 105a and 105b. The second connection 108 connects to the motor connections 102a, 102b and 102c. The steering control unit 50 is thereby connected to the motor which includes the stator 60, the rotor assembly 62, and the Hall Effect sensor board 96. The rotor assembly 62 is best shown in Figure 10 and includes a first rotor member 109 having a magnet sleeve 110 provided with a plurality of slots (not shown) which receive a plurality of magnets, for example magnets 111a and 111b, which are adhered to the magnet sleeve 110 in a generally annular formation. There are also retaining rings 113a and 113b that provide backup retention of the magnets 111a and 111b. The magnet sleeve 110 is mounted on a second rotor member 112 with an internally threaded bore 114.
There is an angular contact bearing 116 that allows the rotor assembly 62 to rotate. There is also a lock washer 118 and a lock nut 120.
[0060] The Hall Effect sensor board 96 senses the relative position of the magnets 111a and 11 lb and is timed to the stator 60. The Hall Effect sensor board 96 is advanced timed in each direction. The stator 60 is accordingly powered by motor connections 102a, 102b and 102c such that the magnetic field generated is slightly in front of the rotational position of the magnets 111a and 111b and the magnetic field pulls the magnets 111a and 111b to rotate the rotor assembly 62. Each set of Hall Effect sensors 98a, 98b, and 98c on the Hall Effect sensor board 96, as called out for a first set of the Hall Effect sensors in Figure 8A, has a first Hall Effect sensor 99a and a second redundant and offset Hall Effect sensor 99b.
A first one of the Hall Effect sensors in each set of Hall Effect sensors 98a, 98b, and 98c is positioned for advanced timing in a first rotational direction and a second one of the Hall Effect sensors in each set of Hall Effect sensors 98a, 98b, and 98c is positioned for advanced timing in a second rotational direction. The first rotational direction and the second rotational direction are opposite and respectively correspond to opposite steering directions of the marine vessel 10 based on the rotation of the rotor assembly 62. The use of two different Hall Effect sensors in each set of Hall Effect sensors 98a, 98b, and 98c allows each set of Hall Effect sensors 98a, 98b, and 98c to "soak up"
manufacturing tolerances while allowing advanced timing for the first rotational direction and the second rotational direction. Furthermore, if one of the Hall Effect sensors in any set of Hall Effect sensors 98a, 98b, and 98c fails then the other one of the Hall Effect sensors may function as a failsafe.
[0061] The rotor assembly 62, as shown in Figure 5, receives the output shaft 44 which is shown in greater detail in Figure 11 and, in this example, the output shaft 44 includes a roller screw assembly 122, a first shaft portion 124, and a second shaft portion 126. The roller screw assembly 122 has a central screw 128 and a plurality of rollers, for example, rollers 130a and 130b, which are able to rotate about the central screw in a planetary fashion but do not translate axially relative to the central screw 128. The rollers 130a and 130b are aligned by annular end plates 131a and 131b. The roller screw assembly 122 is received by the internally threaded bore 114 of the rotor assembly 62 which is shown in Figure 10.
Rotation of the rotor assembly 62 causes the output shaft 44, as shown in Figures 3 and 4, to reciprocate relative to the housing 42. When the electric actuator 40 is mounted on the port engine 12, axial movement of the output shaft 44 is inhibited relative to the marine vessel 10 while the housing 42 reciprocates linearly along the output shaft 44 and relative to the marine vessel 10. This relative linear movement of the housing 42 imparts a steering force to the tiller 46 of the port engine 12 and thereby causes the port engine 12 to be steered in a conventional manner. The output shaft 44 may also be rotated manually in an emergency by loosening a lock nut (not shown) which prevents rotation of the output shaft in normal operation.
[0062] The brake 64 of the electric actuator 40 is shown in greater detail in Figure 12.
The brake 64 has an annular body 140 which houses an electromagnetic circuit 142. The electromagnetic circuit 142 includes a magnet retainer 144, a magnet holder 146 which holds a plurality of magnets, for example, permanent magnets 148a and 148b, and a brake coil assembly 150. The magnet retainer 144 retains the magnets and magnetic field. The magnet retainer 144, the magnet holder 146 and the permanent magnets 148a and 148b are shown in greater detail in Figure 13. In other examples, there may be an annular magnetic ring. Referring back to Figure 12, the brake 64 further includes a hub 152 which is press-fitted to a bearing 154. The bearing 154 is loose-fitted to the annular body 140. The bearing 154 holds the annular body 140 concentric to the hub 152. This allows the brake 64 to align the rotor member 62 to the housing 42. The brake 64 may be sub-assembled and tested as a module in production before integrating with the rotor member 62 and the housing 42. A
retaining ring 156 acts as a backup retention. The bearing 154 allows the hub 152 to rotate with the rotor assembly 62. A plurality of flexures, for example arcuate flexures 158a, 158b, and 158c, are radially retained by and rotate with the hub 152. There is an annular brake pad 160 disposed between the electromagnet circuit 142 and the hub 152.
The brake pad 160 is coupled to the hub 152 by the flexures 158a, 158b and 158c. The brake pad 160 therefore also rotates with the hub 152. The hub 152, the brake pad 160, and the flexures 158a, 158b, and 158c are shown in greater detail in Figure 14.
[0063]
Figures 15 and 16 show the coil assembly 150 disposed in an annular space or annulus 162 between the body 140 and the magnet retainer 144. The annulus 162 is larger in cross section than the coil assembly 150. This allows the coil assembly 150 to move within the annulus 162 so that a connector 164 of the coil assembly 150 may be aligned with the electrical connector 72 to connect with the brake connections 104a and 104b, shown in Figure 8B, during assembly of the electric actuator 40. The movement of the coil assembly 150 within the annulus 162 may be rotary and/or radial movement.
There is also an air gap 166 between the body 140 and the magnet retainer 144.
[0064] The permanent magnets 148a and 148b generate magnetic fields which pull the brake pad 160 to an engaged position against a first friction surface 168 and a second friction surface 170. In this example, the first friction surface 168 is a shoulder of the body 140 and the second friction surface 170 is an annular surface of the magnet retainer 144.
The brake torque, or friction between the brake pad 160 and the first friction surface 168 and the second friction surface 170, is sufficient to prevent rotation of the hub 152 which is torsionally coupled to the rotor assembly 62. The brake 64 is accordingly normally in an engaged position, as shown in Figures 15 and 16, until the coil assembly 150 is energized to actuate the brake pad 160 to a released position.
[0065] Figures 17 and 18 show the brake 64 in the released position, when the coil assembly 150 is energized, and the coil assembly 150 generates a magnetic field which is sufficient to redirect the magnetic field generated by the permanent magnets 148a and 148b. This allows the flexures 158a, 158b, and 158c to bias the brake pad 160 away from the first friction surface 168 and the second friction surface 170 to the released position.
There is a brake pad stop 172 which, in this example, is an annular lip of the hub 152 that restricts movement of the brake pad 160 away from the first friction surface 168 and the second friction surface 170 when the brake 64 is in the released position so the hub 152 can rotate freely. The brake pad stop 172 sets a maximum distance of the brake pad 160 from the first friction surface 168 and the second friction surface 170 when the brake 64 is in the released position. The maximum distance is set to allow the brake 64 to re-engage.
[0066] The coil assembly 150 is energized by receiving an electric current. However, in this example, the brake 64 is only actuated to the released position if a current supplied to the coil assembly 150 is between a first threshold current and a second threshold current.
The first threshold current may be, for example, about 0.6 amps and the second threshold current may be about 1.2 amps, or the first threshold current may be about 0.8 amps and the second threshold current may be about 1.0 amps. This ensures consistent release of the brake 64 within the design limits of the current draw.
[0067] The air gap 166 between the body 140 and the magnet retainer 144 sets brake strength and current thresholds when the brake is in the released position.
This minimizes the electric current required to move the brake to the released position while maintaining a desired minimum brake torque. The brake torque may be lower than an output torque of the motor of the electric actuator 40. This allows the motor of the electric actuator 40 to override the brake 64 in the event of brake failure. The brake torque may be greater than a back driving torque. This allows the brake 64 to restrict movement of the output shaft 44 in the event of motor failure. This also allows the brake to restrict movement of an engine mounted on a marine vessel which is important during trailering of the marine vessel.
[0068] The brake 64 may also be provided with an H-bridge in the steering control unit for reversing the polarity of the voltage supplied to the coil assembly 150.
This increases the speed at which the brake 64 is actuated from the released position to the engaged position. This also increases the brake torque. There is an 0-ring 174 which mates against the rotor assembly to provide rotational dampening. There is also a support ledge 176 on the body 140 of the brake 64 which minimizes thermal expansion of the brake 64.
[0069] The end gland 68 is shown in greater detail in Figures 19 to 21. The end gland 68 includes an annular body 178. The annular body 178 receives a floating floating seal housing and a bushing 182. The bushing 182 is disposed between an inner seal 184 and an outer seal 186. The inner seal 184 is provided with an inner seal energizer 188 and the outer seal 186 is provided with an outer seal energizer 190. The outer seal 186 and the outer seal energizer 190 are received by a groove 192 in the floating floating seal housing. There is a seal, in this example, an 0-ring 194 disposed between the annular body 178 and the floating floating seal housing. The 0-ring 194 sits on a concave wall 198 of the floating seal housing. This centers the 0-ring 194 relative to the floating floating seal housing. The floating seal housing is provided with a first radial lip 200 and a second radial lip 202. The first radial lip 200 is resilient and facilitates the installation of the floating seal housing in the end gland 68. The second radial lip 202 has a sharp edge which functions as a scraper for scraping debris from the output shaft 44 and also provides stability and output shaft tracking. The sealing housing 180 is also provided with a chamfer 204 which facilitates the installation of the 0-ring 194. A back wall 206 of the floating seal housing is flush with the annular body 178 for stability. A back heel 208 of the floating seal housing is concentric with the output shaft 44 and is formed of a rigid low wear material. The 0-ring 194 allows the floating seal housing to move radially when subjected to loads against the output shaft 44. This reduces wear of the second radial lip 202 and back heel 208 of the floating seal housing.
[0070] Figure 22 shows the electric actuator 40 coupled to the tiller 46.
There is a magnetic position target 210 mounted on the coupling assembly 48 and absolute position sensors 212a and 212b mounted on the electric actuator 40 within the steering control unit 50. The absolute position sensors 212a and 212b sense a steering position based on a position of the magnetic position target 210. The steering position may be referred to as the steering angle. The steering control unit 50 and the absolute position sensors 212a and 212b move axially relative to the output shaft 44 when steering motion is imparted to the tiller 46. The coupling assembly 48 and magnetic position target 210 rotate when steering motion is imparted to the tiller 46. The coupling assembly 48 has a curved surface 214 which drives a curved surface 216 of the magnetic position target 210. The curved surface 214 and the curved surface 216 allow a rotational degree of freedom in the direction of the output shaft 44 axis to handle potential rotational misalignment due to manufacturing tolerances and external loading.
[0071] The absolute position sensors 212a and 212b signal the steering position to a microcontroller 218 disposed within the steering control unit 50 as shown in Figure 23.
There is also a relative position sensor 220 which signals a position of the motor (M) to the microcontroller 218. The absolute position sensors 212a and 212b are in the form of anisotropic magnetoresistance sensors, in this example, and the relative position sensor 220 is a Hall Effect Sensor, in this example, but any suitable absolute position sensor and relative position sensor may be used. The relative position sensor 220 may be one or more of the Hall Effect sensors on the Hall Effect sensor board shown in Figure 8A.
The steering control unit 50 calibrates an accumulative position using multiple points through the actuator stroke based on the relationship between the absolute position sensors 212a and 212b and the absolute position sensors. This map for the accumulative position is stored in a non-volatile memory 242. This initializes an accumulative position which signals an accumulative steering position to the microcontroller 218 and then accumulates relative position as the electric actuator 40 moves over time.
[0072] The accumulative position is initialized at start-up, as shown in Figure 24, and preferably when the motor and brake are not enabled. This allows for a high initialization accuracy of the accumulative position. Backlash can be measured by comparing a signal of the accumulative position and a signal of the absolute position sensors 212a and 212b at a single steering position after the steering position is approached from both sides. Backlash is primarily imposed at the interface between the tiller and the actuator, along the steering axis, where the absolute position sensors 212a and 212b are disposed. There is minimal backlash imposed along the output shaft axis where the relative position sensor 220 is disposed. The steering control unit 50 monitors backlash during initial steering movement to determine whether there has been significant steering movement to take up the backlash.
The steering control unit 50 compares a value of the absolute position sensors 212a and 212b to determine a magnitude of the backlash. The steering control unit 50 then .. compensates for the backlash by reinitializing the accumulative position.
The steering control unit 50 may restrict the steering range to take up the backlash and, after the backlash has been compensated for, allow the full steering range. The steering control unit 50 may also store the measured backlash in non-volatile memory 242 to monitor the change of backlash over time. The information stored in the non-volatile memory can be used to provide warning or service reminder. If the backlash has exceeded an allowable safety threshold, then the steering control unit 50 may reduce the performance of the actuator.
[0073] The use of a combination of the absolute position sensors 212a and 212b together with the relative position sensor 220 results in a triple redundancy in which each sensor is compared with the other sensors for fault detection. In this example, the steering control unit 50 compares a signal of the absolute position sensors 212a and 212b to the accumulative position for fault detection. The use of additional absolute position sensors and relative position sensors to provide additional accumulative positions will result in multiple redundancies based on the number of sensors used. For example, and with reference to Figure 23, a motor voltage sensor 236 may measure back EMF
waveforms of the stator phase, as the motor commutates, to determine a relative position of the motor.
This allows the steering control unit 50 to use the motor voltage sensor as an additional relative position sensor.
[0074] The steering position is continuously monitored by the steering control unit 50 and a PID position closed loop control is employed to minimize steering position error as is conventional. The steering control unit 50 energizes the stator 60 to generate a magnetic field 226 that is ahead of a magnetic field 228 of the rotor 62 which results in a rotating torque. This is conventional motor operation called commutation. The steering control unit 50 stops advancing the phase commutation once the steering position error has been minimized to a desired threshold error. The steering control unit 50 then energizes the stator 60 to align the magnetic field 226 of the stator 60 with the magnetic field 228 of the rotor assembly 62, as shown in Figures 25 to 28. The steering control unit 50 selects one of the three phases of the three-phase alternating current to apply the holding PWM.
This is best shown in Figure 25 which is a graph showing a PWM duty cycle applied to the stator 60 as a function of time.
[0075] The description above describes the accumulative position being initialized at startup, and reinitialized again after initial steering movement to compensate for backlash.
However, it is understood that the steering control unit 50 may choose to delay the initialization if certain initialization conditions are not met, or reinitialize again after certain specific trigger events. For example, after a failed sensor has recovered during operation.
[0076] Referring now specifically to Figure 25 and, with reference to states shown in Figure 29, the steering control unit 50 is in a drive state between time TO
and time Ti during which the steering control unit 50 applies a PWM so there is commutation. The desired steering position is reached at time Ti and the steering control unit transitions to a hold state during which the steering control unit applies a holding PWM. The steering control unit 50 applies a holding PWM by applying a positive PWM and current to a first phase of the three-phase current and a negative PWM and current to a second phase of the three-phase current. The holding PWM is relatively high when first applied at time Ti but is reduced at a controlled rate, during a torque reduce state through time T2, until a minimum holding PWM is reached at time T3 to restrict back driving in a hold state. The holding PWM creates a holding torque that counteracts the back driving torque, i.e. holds a position of the rotor assembly 62. The holding torque can be larger than the back driving torque without causing motor rotation. Figure 26 shows the holding torque opposing the back driving torque. The steering control unit 50 may set a minimum holding PWM as a function of at least one of marine vessel speed, engine RPM, and steering position.
Referring back to Figure 23, the marine vessel speed may be sensed by a marine vessel speed sensor 250 and the engine RPM may be sensed by an engine speed sensor 252.
[00771 However, back driving torque is directly proportional to rudder load and rudder load is a function of propeller torque, vessel dynamic, engine mounting configuration, rudder angle, engine speed, and marine vessel torque. The back driving torque may therefore fluctuate significantly especially during a turn and/or at high speeds. The back driving torque may therefore become greater than the holding PWM and holding torque which results in back driving. The phase can be changed to maintain the holding torque opposing the external torque and stop motor rotation. The steering control unit 50 monitors back driving according to the state diagram shown in Figure 29 and transitions the torque increases state. The minimum holding PWM may therefore be increased at a controlled rate, during a torque increase state starting at time T4, until the back driving torque is less than or equal to the holding PWM and back driving has stopped at time T5 and return to the hold state. There may also be a steering torque sensor 224 which senses steering torque.
The steering control unit 50 may determine a magnitude and rate of PWM
increase based on a signal from the steering torque sensor 224 which senses back driving torque. The steering control unit 50 may also determine a magnitude and rate of PWM
increase based on a signal from the relative position sensor 220 which may sense back driving.

[0078] The holding PWM should preferably be minimized to a level that is sufficient to create a holding torque which counters the back driving torque, but not higher than necessary to avoid inefficiencies in current consumption. The minimum holding PWM may also be adaptively increased based on engine speed. This minimizes actuator position error and current draw to reduce the chances of engine overheating.
[0079] Referring back to Figure 23, in this example, there is a power source in the form of a battery 230 which supplies electric power to the electric actuator 40.
There are sensors, a voltage sensor 232 and a current sensor 234 in this example, which sense a state of the battery 230. The voltage sensor 232 and/or the current sensor 234 may sense a capacity of the battery 230. The voltage sensor 232 and/or the current sensor 234 may signal the microcontroller 218 regarding the capacity of the battery 230 and the steering control unit 50 limits the maximum electric power supplied to the motor by controlling the voltage to the motor relative to the capacity of the battery 230.
[0080] Battery current draw may be limited by employing a three zone current limiting .. scheme as shown in Figure 30. The three zone current limiting scheme uses a PWM limit to control the maximum PWM that can be applied to the motor. The steering control unit can provide any PWM up to the maximum PWM. The PWM limit is decreased by a pre-defined slew rate in a Reduce Zone. The purpose of the Reduce Zone is to reduce and limit the battery current below an allowable threshold. The PWM is allowed to increase and decrease according to a slew rate controlled PWM command in a Normal/Recovery Zone.
The purpose of the Normal/Recovery Zone is to allow the PWM to recover back to normal level and follow the PWM command. The PWM limit is prevented from increasing in a Hold Zone but the PWM may be decreased in the Hold Zone. The purpose of the Hold Zone is to hold or slow down current increase by preventing the PWM from increasing.
The Hold Zone reduces battery current oscillations and excessive voltage drops. This increases maximum battery current output. Figure 30 is a block diagram showing the logic of the current limiting method.

[0081] The zones of the three zone current limiting scheme may be based on the battery current as shown, for example, in Figure 31. The Reduce Zone is when the battery current is greater than 98 amps. The Normal/Recovery Zone is when the battery current is less than 93 amps. The Hold Zone is when the battery current is between 93 amps and 98 amps.
Figure 31 also shows an example of the three zone current limiting scheme being employed. The PWM and battery current is allowed to increase in the Normal/Recovery Zone during time period Ti. The PWM is held steady in the Hold Zone during time period T2. The PWM decreases in the Reduce Zone during time period T3. This results in a drop in motor voltage (Vin) and, a drop in motor current drop leads to a drop in battery current (Is), which achieves the goal of battery current limit. The battery current (Is) continues to drop as a result of the sharp torque and current ripple during time period T4.
The PWM
however remains constant. When the battery current (Is) reaches the Normal/Recovery Zone, during time period T5, the PWM increases slightly until the PWM and battery current stabilize in the Hold Zone in time period T6.
[0082] The steering control unit 50 may also employ a three stage process to reduce engine speed and a pause actuator movement to prevent engine collision as shown in Figure 32. Each of the engines has its own engine target rudder angle position scheduler, as shown in Figure 33, which is generated during a scheduler step. The engine target rudder angle schedule is based on a vessel target rudder angle steering position command.
When all engines operate on their respective engine target rudder angle schedules there should be no engine collision as the scheduler is defined by collision points which themselves are defined by engine geometry, distances between engines, and steering angles.
[0083]
However, a given engine may experience more load and may slowdown as shown in Figures 34A and 34B. In Figure 34A the engines are starting to turn and the port engine 12 and the inner port engine 14a operate on their respective scheduler there should be no engine collision as the schedulers have the same engine target rudder angle O.
However, in Figure 34B the port engine 12 slows down due to a load. There is accordingly a slowdown step which includes ratiometric slowing down of the inner port engine 14a in response to a separation distance between a slowdown line and a pause line shown in Figure 35.
[0084]
Figure 35 shows a pause line of an engine and slow down line relationship with an adjacent engine on the starboard side. The Y-axis is a steering position of the engine and the X-axis is a steering position of the adjacent engine. The steering control unit 50 uses the map to determine where to slow down and pause steering movement of the engine.
For example, if the adjacent engine is the starboard engine, and the steering position of the adjacent engine is at 300, the port side engine can only travel to 22 before it has to slow down, and travel to 24 before it has to pause steering. However, if the same port engine is steering towards port direction, there is no restriction to slow down or pause steering.
The inner port engine 14a slows down relative to the steering position of the port engine 12. There is also a pause step in which a brake of the actuator or the holding PWM is applied to restrict movement of the actuator.
[0085] The leading engines and the following engines exhibit different behaviours and only the following engines will reduce engine speed and stop actuator movement. The steering control unit 50 monitors the position of its respective engines as well as the relative position of adjacent engines in order to reduce engine speed and stop actuator movement as the engines come close to the target actuator positions. Referring back to Figure 23, for example, the steering control unit 50 may determine the relative position of the engines based on the absolute position sensors 212a and 212b and relative position sensor 220, as described above, or the steering control unit 50 may determine the relative position of the engines based on a position sensor 254 of the adjacent engine. Figure 36 shows that the steering control unit operates the actuator in three states. There is a Normal State in which the actuator is allowed to operate up to a maximum speed without restriction.
There is a Reduce Speed State in which the actuator is allowed to operate up to a maximum speed limit determined by a reduce speed curve. The maximum speed limit is also direction dependent in the Reduce Speed State. There is a Wait State in which the actuator is prevented from moving further in a first direction. However, the actuator is allowed to move in a second direction which is opposite to the first direction in the Wait State.

[0086] The distance between adjacent engines is used to trigger the transition between the Normal State, the Reduce Speed State, and the Wait State. The distance at which to transition back to a state that allows a higher maximum speed can be the same or larger than the distance to transition to the lower state. If there is no adjacent actuator on a given side, the actuator will always be in this state when moving towards that side.
This three stage process allows for smaller separation distance between engines which allows more engines to be mounted on a marine vessel.
[0087] Figure 37 is a variable toe diagram showing that a toe angle logic of the steering control unit 50 is vessel target rudder angle 260 dependant. The toe angle 262 is at its maximum value and equal to the toe setting when vessel target rudder angle is 00. In this example, the toe angle modifies the engine target rudder angle for each engine, and ratiometrically reduce to a minimum toe value at the two hardstops (i.e.
vessel target rudder angle = +/-300). In this example, the minimum toe value is defined as 00 at hardstop. A
negative toe angle indicates toe in, and a positive toe angle indicates toe out. This is beneficial especially for big engines with small separation distance. For those systems at hardover steering, a non-zero toe value will reduce the steering range for the outer engines in a turn, and thus may affect steering performance such as turn radius.
However, in other examples, other parameters may be used.
[0088] It will be understood by a person skilled in the art that many of the details provided above are by way of example only, and are not intended to limit the scope of the invention, which is to be determined with reference to the following claims.

Claims

What is claimed is:

1. An electric actuator for imparting steering movement to a propulsion unit of a marine vessel, the electric actuator comprising:
a housing;
an output shaft reciprocatingly received by the housing;
a motor disposed within the housing, the motor including a rotor assembly and a stator, rotation of the rotor assembly causing the output shaft to translate axially relative to the rotor assembly and causing the output shaft to reciprocate relative to the housing;
a coupling assembly mounted to the housing, the coupling assembly coupling the electric actuator to the propulsion unit of the marine vessel; and a steering control unit mounted to the housing, the steering control unit driving the motor to cause the rotor assembly to rotate and cause the output shaft to reciprocate relative to the housing.

2. The electric actuator as claimed in claim 1 further including:
an electrical connector which electrically connects the steering control unit to the motor; and an opening in the housing, wherein the electrical connector extends through the opening to electrically connect the steering control unit to the stator and an interior of the steering control unit is sealed.

3. The electric actuator as claimed in claim 1 further including:
an electrical connector which electrically connects the steering control unit to the motor, the electrical connector including a commutation sensor board which senses a position of the rotor assembly and a motor connection which provides power to the stator; and an opening in the housing, wherein the electrical connector extends through the opening to electrically connect the steering control unit to the stator and an interior of the steering control unit is sealed.

4. The electric actuator as claimed in claim 3 wherein the commutation sensor board includes three sets of sensors with each set of sensors having redundant and offset sensors, wherein a first sensor of each set of sensors is positioned for advanced timing of the stator in a first direction and a second sensor of each set of sensors is positioned for advanced timing of the stator in a second direction, the first direction and the second direction are opposite and respectively correspond to opposite steering directions based on rotation of the rotor assembly.

5. The electric actuator as claimed in claim 1 further including a brake, the brake including:
an electromagnetic circuit having a magnet and a coil assembly; and a brake pad, wherein the magnet generates a magnetic field which pulls the brake pad to an engaged position, and the brake pad is actuated to a released position when the coil assembly is energized by an electric current.

6. The electric actuator as claimed in claim 5 wherein the brake pad engages a first friction surface and a second friction surface.

7. The electric actuator as claimed in claim 6 wherein the first friction surface and the second friction surface are annular and concentric.

8. The electric actuator as claimed in claim 5 wherein the coil assembly is an annular coil assembly.

9. The electric actuator as claimed in claim 8 wherein the coil assembly is rotatable.

10. The electric actuator as claimed in claim 5 wherein the brake pad is actuated to a released position when the coil assembly is energized by an electric current between a first threshold current and a second threshold current.

11. The electric actuator as claimed in claim 5 wherein further including a magnetic retainer received within a body and an air gap between the magnetic retainer and the body.

12. The electric actuator as claimed in claim 1 further including an end gland at each end of the housing, each said end gland including an annular seal within a floating seal housing.

13. The electric actuator are claimed in claim 12 wherein the floating seal housing itself is sealed.

14. The electric actuator as claimed in claim 12 wherein the floating seal housing includes a radial lip with a sharp edge which functions as a scraper to scrape debris from the output shaft.

15. The electric actuator as claimed in claim 12 wherein the floating seal housing includes a heel which is concentric with the output shaft and moves radially with the output shaft.

16. The electric actuator as claimed in claim 1 wherein the rotor assembly includes:
a first rotor member including a sleeve and a plurality of magnets arranged in an annular formation on the first rotor member; and a second rotor member have an threaded bore, wherein the first rotor member is mounted about the second rotor member.

17. The electric actuator as claimed in claim 1 wherein the stator includes a plurality of segments arranged in an annular formation and each segment having a separate coil.

18. The electric actuator as claimed in claim 17 wherein the segments are maintained in an annular formation in the housing with potting.

19. An electric actuator for imparting steering movement to a tiller of a propulsion unit of a marine vessel, the electric actuator comprising:
a housing;
an output shaft reciprocatingly received by the housing; and a motor disposed within the housing, the motor including a rotor assembly and a stator, rotation of the rotor assembly causing the output shaft to translate axially relative to the rotor assembly and causing the output shaft to reciprocate relative to the housing;

a coupling assembly mounted to the housing, the coupling assembly coupling the electric actuator to the tiller of the propulsion unit of the marine vessel;
a magnetic position target mounted on the coupling assembly; and a position sensor mounted on the actuator, the position sensor sensing a position of the electric actuator based on a position of the magnetic position target.

20. The electric actuator as claimed in claim 19 further including a commutation sensor board which senses a position of the rotor assembly.

21. The electric actuator as claimed in claim 20 wherein the commutation sensor board includes three sets of sensors with each set of sensors having redundant and offset sensors, wherein a first sensor of each set of sensors is positioned for advanced timing of the stator in a first direction and a second sensor of each set of sensors is positioned for advanced timing of the stator in a second direction, the first direction and the second direction are opposite and respectively correspond to opposite steering directions based on rotation of the rotor assembly.

22. The electric actuator as claimed in claim 19 further including a brake, the brake including:
an electromagnetic circuit having a magnet and a coil assembly; and a brake pad, wherein the magnet generates a magnetic field which pulls the brake pad to an engaged position, and the brake pad is actuated to a released position when the coil assembly is energized by an electric current.

23. The electric actuator as claimed in claim 22 wherein the brake pad engages a first friction surface and a second friction surface.

24. The electric actuator as claimed in claim 23 wherein the first friction surface and the second friction surface are annular and concentric.

25. The electric actuator as claimed in claim 22 wherein the coil assembly is an annular coil assembly.

26. The electric actuator as claimed in claim 25 wherein the coil assembly is rotatable.

27. The electric actuator as claimed in claim 22 wherein the brake is pad is actuated to a released position when the coil assembly is energized by an electric current between a first threshold current and a second threshold current.

28. The electric actuator as claimed in claim 22 wherein further including a magnetic retainer received within a body and an air gap between the magnetic retainer and the body.

29. The electric actuator as claimed in claim 19 further including an end gland at each end of the housing, each said end gland including an annular seal within a floating seal housing.

30. The electric actuator are claimed in claim 29 wherein the floating seal housing itself is sealed.

31. The electric actuator as claimed in claim 29 wherein the floating seal housing includes a radial lip with a sharp edge which functions as a scraper to scrape debris from the output shaft.

32. The electric actuator as claimed in claim 29 wherein the floating seal housing includes a heel which is concentric with the output shaft and moves radially with the output shaft.

33. The electric actuator as claimed in claim 19 wherein the rotor assembly includes:
a first rotor member including a sleeve and a plurality of magnets arranged in an annular formation on the first rotor member; and a second rotor member have an threaded bore, wherein the first rotor member is mounted about the second rotor member.

34. The electric actuator as claimed in claim 19 wherein the stator includes a plurality of segments arranged in an annular formation and each segment having a separate coil.

35. The electric actuator as claimed in claim 34 wherein the segments are maintained in an annular formation in the housing with potting.

36. The electric actuator as claimed in claim 34 wherein the coupling assembly has a curved surface and the magnetic position target has a curved surface which is driven by the curved surface of the coupling assembly when the coupling assembly rotates.

37. A marine steering system comprising:
a propulsion unit having a tiller; and an actuator for imparting steering movement to the tiller of the propulsion unit, the actuator including:
a housing;
an output shaft reciprocatingly received by the housing;

a motor disposed within the housing, the motor including a rotor assembly and a stator, rotation of the rotor assembly causing the output shaft to translate axially relative to the rotor assembly and causing the output shaft to reciprocate relative to the housing;
a coupling assembly mounted to the housing, the coupling assembly coupling the actuator to the propulsion unit; and a steering control unit mounted to the housing, the steering control unit driving the motor to cause the rotor assembly to rotate and cause the output shaft to reciprocate relative to the housing.

38. An actuator for imparting steering movement to a tiller of a propulsion unit of a marine vessel, the actuator comprising:
a housing;
an output shaft reciprocatingly received by the housing;
a motor causing the output shaft to reciprocate relative to the housing;
an absolute position sensor which senses a steering position;
a relative position sensor which senses a position of the motor; and a steering control unit which calibrates the relative position sensor based on a signal of the absolute position sensor, wherein calibration of the relative position sensor based on a signal of the absolute position sensor initializes an accumulative position which accumulates a relative position as the actuator moves over time.

39. The actuator as claimed in claim 38, wherein the steering control unit compares a signal of the absolute position sensor and the accumulative position for fault detection.

40. The actuator as claimed in claim 38, wherein the steering control unit initializes the accumulative position of the relative position sensor with the absolute position sensor when the actuator is started-up.

41. The actuator as claimed in claim 38, wherein the steering control unit monitors the accumulative position and the steering position during a change in steering direction to determine backlash.

42. The actuator as claimed in claim 38, wherein the steering control unit monitors backlash during initial steering movement and compensates for backlash by reinitializing the accumulative position.

43. The actuator as claimed in claim 38, wherein the steering control unit restricts a steering range of the actuator after initialization of the accumulative position and before compensating for backlash.

44. The actuator as claimed in claim 38, wherein the steering control unit uses the accumulative position for position control of the actuator.

45. The actuator as claimed in claim 38, wherein the steering control unit uses a redundant sensor for position control of the actuator if the absolute position sensor or the relative position sensor fails.

46. An actuator for imparting steering movement to a tiller of a propulsion unit of a marine vessel, the actuator comprising:
a housing;

an output shaft reciprocatingly received by the housing;
a motor disposed within the housing, the motor having a rotor and a stator, the rotor having a magnetic field and the stator having a magnetic field, rotation of the rotor causing the output shaft to translate axially relative to the rotor and causing the output shaft to reciprocate relative to the housing;
a sensor for sensing a position of the rotor; and a steering control unit which energizes the stator to align the magnetic field of the stator with the magnetic field of the rotor, based on a position of the rotor sensed by the sensor for sensing the position of the rotor, to hold a position of the rotor.

47. The actuator as claimed in claim 46, wherein the motor is at least a two-phase motor and the steering control unit energizes the stator by applying a PWM to at least one phase of the motor to generate a magnetic field that aligns with and holds -the position of the rotor.

48. The actuator as claimed in claim 47, wherein the steering control unit applies a minimum PWM to create a holding torque which counters a back driving torque.

49. The actuator as claimed in claim 48, wherein the steering control unit determines the minimum PWM as a function of at least one of marine vessel speed, engine RPM, and steering position.

50. The actuator as claimed in claim 48, wherein the steering control unit increases or decreases the minimum PWM applied in order to counter the back driving torque.

51. The actuator as claimed in claim 50, wherein the steering control unit determines a magnitude and rate of PWM increase based on a signal from a position sensor sensing back driving.

52. The actuator as claimed in claim 50 , wherein the steering control unit determines a magnitude and rate of PWM increase based on a signal from a torque sensor sensing a back driving torque.

53. A marine steering system comprising:
a propulsion unit having a tiller;
an actuator for imparting steering movement to the tiller of the propulsion unit, the actuator including a housing and an output shaft reciprocatingly received by the housing;
a motor causing the output shaft to reciprocate relative to the housing;
a power source supplying electric power to the motor;
a sensor for sensing a capacity of the power source; and a steering control unit which limits the maximum electric power supplied to the motor by controlling the voltage to the motor relative to the capacity of the power source.

54. The actuator as claimed in claim 53 wherein the steering control unit limits the maximum electric power supplied to the motor by increasing or decreasing PWM
based on whether the battery current is in a first zone, a second zone, or a third zone.

55. The actuator as claimed in claim 54 wherein the steering control unit decreases PWM when the battery current is in the first zone.

56. The actuator as claimed in claim 54 wherein the steering control unit increases or decreases PWM when the battery current is in the second zone.

57. The actuator as claimed in claim 54 wherein the steering control unit prevents increasing PWM when the battery current is in the third zone.

58. A marine steering system comprising:
a first propulsion unit having a tiller, an actuator for imparting steering movement to the tiller of the first propulsion unit, and a position sensor for sensing a steering position of the first propulsion unit;
a second propulsion unit having a tiller and an actuator for imparting steering movement to the tiller of the second propulsion unit, and a position sensor for sensing a steering position of the second propulsion unit; and a steering control unit controlling the actuator of the first propulsion unit to impart steering movement to the first propulsion unit based on relative steering positions of the first propulsion unit and the second propulsion unit, wherein the steering control unit controls the actuator of the first propulsion unit by slowing down steering movement of the first propulsion unit relative to the steering position of the second propulsion unit.

59. The marine steering system as claimed in claim 58, wherein the steering control unit controls the actuator of the first propulsion unit to impart steering movement to the first propulsion unit based on an engine target rudder angle position scheduler.

60. The marine steering system as claimed in claim 58, wherein the steering control unit further controls the actuator of the first propulsion unit to pause steering movement to the first propulsion unit based on relative steering positions of the first propulsion unit and the second propulsion unit.

61. The marine steering system as claimed in claim 58, wherein the steering control unit controls the actuator of the first propulsion unit to impart steering movement to the first propulsion unit in one of a first state in which the actuator is allowed to operate up to a maximum speed, a second state in which the actuator is allowed to operate up to a maximum speed limit determined by a reduce speed curve, and a third state in which the actuator is prevented from moving further in a first direction.

62. The marine steering system as claimed in claim 61, wherein the steering control unit controls the actuator of the first propulsion unit to impart steering movement of the first propulsion unit based on a direction of the steering movement.

63. The marine steering system as claimed in claim 61, wherein a transition between said states is based on the relative steering positions of the first propulsion unit and the second propulsion unit.

64. A marine steering system comprising:
a first propulsion unit having a tiller, an actuator for imparting steering movement to the tiller of the first propulsion unit, and a position sensor for sensing a steering position of the first propulsion unit;
a second propulsion unit having a tiller and an actuator for imparting steering movement to the tiller of the second propulsion unit, and a position sensor for sensing a steering position of the second propulsion unit;
a device for setting a toe angle between the first propulsion unit and the second propulsion unit; and a steering control unit receiving a steering position command, wherein the steering control unit modifies the toe angle based on the steering position command.

65. The marine steering system as claimed in claim 64, wherein the steering control unit modifies the toe angle and an engine target rudder angle for each of the first propulsion unit and the second propulsion unit based on the toe angle.

66. The marine steering system is claimed in claim 65, wherein the engine target rudder angle is dependent on a vessel target rudder angle.

67. A marine steering system comprising:
a propulsion unit having a tiller;
an actuator for imparting steering movement to the tiller of the propulsion unit, the actuator including a housing and an output shaft reciprocatingly received by the housing;
a motor causing the output shaft to reciprocate relative to the housing;
an absolute position sensor which senses a steering position;
a relative position sensor which senses a position of the motor; and a steering control unit which calibrates the relative position sensor based on a signal of the absolute position sensor, wherein calibration of the relative position sensor based on a signal of the absolute position sensor initializes an accumulative position which accumulates a relative position as the actuator moves over time.

68. The marine steering system as claimed in claim 67, wherein the steering control unit compares a signal of the absolute position sensor and the accumulative position for fault detection.

69. The marine steering system as claimed in claim 67, wherein the steering control unit initializes the accumulative position of the relative position sensor with the absolute position sensor when the actuator is started-up.

70. The marine steering system as claimed in claim 67, wherein the steering control unit monitors the accumulative position and the steering position during a change in steering direction to determine backlash.

71. The marine steering system as claimed in claim 67, wherein the steering control unit monitors backlash during initial steering movement and compensates for backlash by reinitializing the accumulative position.

72. The marine steering system as claimed in claim 67, wherein the steering control unit restricts a steering range of the actuator after initialization of the accumulative position and before compensating for backlash.

73. The marine steering system as claimed in claim 67 wherein the steering control unit uses the accumulative position for position control of the actuator.

74. The marine steering system as claimed in claim 67, wherein the steering control unit uses a redundant sensor for position control of the actuator if the absolute position sensor or the relative position sensor fails.

75. A marine steering system comprising:
a propulsion unit having a tiller;

an actuator for imparting steering movement to the tiller of the propulsion unit, the actuator including a housing and an output shaft reciprocatingly received by the housing;
a motor disposed within the housing, the motor having a rotor and a stator, the rotor having a magnetic field and the stator having a magnetic field, rotation of the rotor causing the output shaft to translate axially relative to the rotor and causing the output shaft to reciprocate relative to the housing;
a sensor for sensing a position of the rotor; and a steering control unit which energizes the stator to align the magnetic field of the stator with the magnetic field of the rotor based on a position of the rotor sensed by the sensor for sensing the position of the rotor.

76. The marine steering system as claimed in claim 75, wherein the motor is at least a two-phase motor and the steering control unit energizes the stator by applying a PWM to at least one phase of the motor to generate a magnetic field that aligns with and holds rotor position.

77. The marine steering system as claimed in claim 76, wherein the steering control unit applies a minimum PWM to create a holding torque which counters a back driving torque.

78. The marine steering system as claimed in claim 76 wherein the steering control unit determines the minimum PWM as a function of at least one of marine vessel speed, engine RPM, and steering position.

79. The marine steering system as claimed in claim 76, wherein the steering control unit increases or decreases the minimum PWM applied in order to counter a back driving torque.

80. The marine steering system as claimed in claim 79, wherein the steering control unit determines a magnitude and rate of PWM increase based on a signal from a position sensor sensing back driving.

81. The marine steering system as claimed in claim 79, wherein the steering control unit determines a magnitude and rate of PWM increase based on a signal from a torque sensor sensing back driving torque.

82. The marine steering system as claimed in claim 62, wherein the steering control unit controls the actuator of the first propulsion unit to impart steering movement of the first propulsion when the first propulsion unit is moving towards the second propulsion unit.