EP4688552A1

EP4688552A1 - Vessel motion control based on propeller slip

Info

Publication number: EP4688552A1
Application number: EP24706014.8A
Authority: EP
Inventors: Mats RYDSTRÖM; Johan TELL; Leo Laine; Lars Wik; Peter TUULIAINEN; Mats Jonasson
Original assignee: Volvo Penta AB
Current assignee: Volvo Penta AB
Priority date: 2023-04-04
Filing date: 2024-02-15
Publication date: 2026-02-11
Also published as: WO2024208476A1; CN120981389A; SE2350394A1

Abstract

A control system (140, 600, 1100) for controlling at least one marine drive unit (110) on a vessel (100), the control system (140, 600, 1100) being configured to obtain data indicative of a speed through water, STW, (V W , V A ) of the vessel (100), the control system (140, 600, 1100) being configured to determine a propeller slip (λ) of the at least one marine drive unit (110) based on a propeller speed (ω P ) of the drive unit (110) and on the speed through water (V W ) of the vessel (100), where the control system (140, 600, 1100) is configured to obtain a relationship (400) between drive unit thrust (F X ) and propeller slip (λ), and where the control system (140, 600, 1100) is configured to obtain data indicative of a desired thrust to be generated by the drive unit (110), and to determine a target propeller slip corresponding to the desired thrust, based on the relationship (400) between drive unit thrust (F X ) and propeller slip (λ), the control system (140, 600, 1100) being arranged to control the propeller speed (ω P ) of the marine drive unit (110) to generate the target propeller slip and to maintain the propeller slip (λ) below a predetermined propeller slip limit (λ lim ).

Description

VESSEL MOTION CONTROL BASED ON PROPELLER SLIP

TECHNICAL FIELD

This disclosure relates generally to motion control for marine vessels such as leisure craft and smaller commercial vessels. In particular aspects, the disclosure relates to computer- implemented methods of propulsion control and general motion management of marine vessels. Control input devices for electric marine drive units are also disclosed. Although the disclosure may be described with respect to a particular vessel, the disclosure is not restricted to any particular type of boat or ship.

BACKGROUND

Marine vessels such as leisure craft and smaller commercial vessels can be powered by one or more propellers. The propellers are driven by respective power sources, such as combustion engines or electrical machines. Energy efficiency of the marine propulsion system is becoming more and more important, both for sustainability reasons and cost reasons. Comfort is another important aspect, in particular with regard to noise and vibration.

US20020185046A1 and WO2008155448A1 both describe marine drive unit control systems that implement functions which limit the propeller slip of the drive unit so as to lie below a propeller slip limit. This way negative effects from, e.g., cavitation can be managed.

Despite the work done to-date, there is a desire for marine propulsion systems which are both energy efficient and which provide comfortable boat rides.

SUMMARY

There is disclosed a control system for controlling at least one marine drive unit on a vessel which may seek to mitigate one or more of the above-mentioned issues. The control system is configured to obtain data indicative of a speed through water (STW) of the vessel, such as a speed of the hull of the vessel through the water and/or a speed of one or more propellers of the vessel through the water. The control system is also configured to determine a propeller slip of the marine drive unit based on a propeller speed of rotation of the drive unit and on the speed through water of the vessel and/or propeller. The control system is furthermore configured to obtain a relationship between drive unit thrust and propeller slip, to obtain data indicative of a desired thrust to be generated by the drive unit, and to determine a target propeller slip corresponding to the desired thrust, based on the relationship between drive unit thrust and propeller slip. The control system then controls the propeller speed of the marine drive unit to generate the target propeller slip so as to provide the desired thrust. This way of controlling the drive unit can result in a more efficient propulsion and may also result in a more consistent vessel behavior in response to thrust commands at different vessel speeds.

The control system can for instance be arranged to obtain the relationship between drive unit thrust and propeller slip as a predetermined analytical function or a collection of predetermined analytical functions. The relationship between drive unit thrust and propeller slip may also be preconfigured as a look-up table (LUT), and in some cases the LUT can be used as a complement to one or more predetermined analytical functions.

The control systems described herein are furthermore arranged to control the propeller speed of the marine drive unit to maintain the propeller slip below a predetermined propeller slip limit. This means that the drive unit operation is always operated at rotation speeds that are not associated with excessive propeller slip, which is normally an inefficient mode of operation to be avoided. Consequently, aspects of the disclosure may seek to ensure that the marine drive unit is not operated at too high propeller slips where, e.g., cavitation may be detrimental to performance, and where unwanted noise and vibration is generated. The wear on the propeller is also reduced by the techniques disclosed herein since excessive cavitation is avoided in this manner. The control systems disclosed herein provide both energy efficient and comfortable propulsion. The propeller slip limit is preferably configured at a value where further increase in propeller slip does not yield any significant increase in propeller thrust. This way the drive unit implements a type of back off in propulsion power from an inefficient high power operating point, where more power only gives an increase in propeller slip without any significant yield in terms of increased propeller thrust.

Aspects of the present disclosure also relate to control units which map an operator thrust input command to a corresponding propeller slip via the relationship between drive unit thrust and propeller slip. This way the operator may experience a more consistent behavior from the vessel in response to a thrust command, which is less dependent on the current speed of the vessel through the water.

According to some aspects, the marine drive unit comprises an electric machine arranged to provide a target axle speed upon request from the control system. The control systems disclosed herein are particularly advantageous to use with electric machines that can adjust axle speed with low latency and with high control bandwidth, i.e., that respond to a request for change of axle speed quickly and accurately. The propeller speed can therefore be kept close to the target propeller speed regardless of disturbances from variation in sea and wind conditions.

The control system may for instance be arranged to receive the data indicative of speed through water from a speed log arranged on the vessel. Speed logs are often present as part of the standard sensor set-up on most boats. It is an advantage that no additional sensors are required in order to implement the techniques disclosed herein, since additional sensors drive cost and system complexity. However, other data sources can also be used to receive the data indicative of speed through water. Speed over ground data from a global positioning system arranged on the vessel and/or from a sonar sensor system can for instance be used for the purpose of determining or at least estimating the speed through water of a boat.

According to some aspects the control system is arranged to control a plurality of marine drive units on the vessel, such as a port drive unit and a starboard drive unit, or more than two drive units. In this case the control system can be configured to obtain the data indicative of speed through water separately for at least two drive units out of the plurality of drive units and thus determine separate propeller slips for the drive units. This improves the accuracy of the propeller slip control when there are two or more drive units since the two or more drive units may travel at different speeds through the water, e.g., when the vessel is turning. The control system can optionally also be arranged to control a port side drive unit and a starboard side drive unit by configuring a lower propeller slip limit for an outer drive unit of a turning maneuver out of the port side drive unit and the starboard side drive unit compared to a propeller slip limit for an inner drive unit of the turning maneuver out of the port side drive unit and the starboard side drive unit. The outer drive unit may be operated at reduced draught due to the roll motion induced by turning and may therefore be more sensitive to developing cavitation compared to the inner drive unit which is then operated at increased draught compared to nominal draught. These aspects may also be accounted for when controlling propeller slip, e.g., by reducing the propeller slip limit of the outer drive unit compared to the propeller slip limit of the inner drive unit during a turning maneuver.

According to some aspects, the control system is arranged to control a marine drive unit having an adjustable propeller pitch. In this case the control system can be arranged to control the propeller speed and the propeller pitch of the marine drive unit to generate a target propeller slip, based on a predetermined relationship between efficiency, motor axle speed, and applied torque of an electric machine. This way propulsion efficiency can be increased even more, which is an advantage. Given a desired amount of propeller thrust, and a speed of the vessel through the water, the control system can determine a desired propeller slip (based on the relationship between propeller thrust and propeller slip), and consequently control both propeller speed and pitch in order to obtain an efficient drive unit operating point.

The control system may furthermore comprise a centralized vessel motion management (VMM) module arranged to request a propeller slip from at least one motion support device (MSD) control module. A centralized VMM module or function is in a position to perform further optimization in case a desired maneuver can be achieved in more than one way, using more than one drive unit. Steering on a multi drive unit vessel can, for instance, often be achieved using both propeller angle (steering of the drive unit in different azimuth angles) and difference in thrust which generates yaw motion by the vessel.

The centralized VMM module may implement functions for dynamic adaptation of the configured propeller slip limits during turning maneuvers. The centralized VMM module may for instance be configured to automatically increase the propeller slip limit for the outer propeller during a turning maneuver compared to the inner propeller during the same turning maneuver, due to the differences in draught during the turning maneuver. This way the propeller speeds will be modulated as the vessel undertakes a turning maneuver.

The above aspects, accompanying claims, and/or examples disclosed herein above and later below may be suitably combined with each other as would be apparent to anyone of ordinary skill in the art. Additional features and advantages are disclosed in the following description, claims, and drawings, and in part will be readily apparent therefrom to those skilled in the art or recognized by practicing the disclosure as described herein. There are also disclosed herein control units, computer systems, computer readable media, and computer program products associated with the above discussed technical benefits.

Aspects of the present disclosure also relate to a control input device which can be used to control a marine drive unit. The control input device is arranged to obtain an input command from an operator indicative of a desired vessel speed or acceleration to be generated, e.g., via a lever or a joystick device. The control input device then translates the input command into an equivalent propeller slip vale, which is then used to control the speed of the propeller based on the current speed of the vessel through the water.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of aspects of the disclosure cited as examples.

Figure 1 illustrates an example marine vessel,

Figure 2 explains the pitch of a propeller,

Figure 3 is a graph showing generated tyre force as function of tyre slip on a road vehicle,

Figure 4 is a graph illustrating example relationships between propeller slip and force,

Figure 5 is a graph illustrating an example boat speed and propeller slip during launch,

Figure 6 schematically illustrates example components of a marine propulsion system,

Figure 7 schematically illustrate aspects of an example vessel control system,

Figure 8A illustrates a vessel with two drive units during a yaw motion maneuver,

Figure 8B shows vessel speeds and propeller slips during a yaw motion maneuver,

Figure 9 exemplifies a relationship between electric motor torque, speed, and efficiency,

Figure 10 shows an example drive unit throttle control device,

Figure 11 is a schematic diagram of an exemplary computer system, Figure 12 shows an example computer program product, and

Figure 13 is a flow chart illustrating methods.

DETAILED DESCRIPTION

The present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. The disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness. Like reference character refer to like elements throughout the description. Aspects set forth below represent the necessary information to enable those skilled in the art to practice the disclosure.

Figure 1 schematically illustrates a marine vessel 100, in this case a boat powered by two drive units 110 attached to the transom of the hull 120 of the boat 100. Each drive unit comprises a propeller arrangement 130. The propeller arrangement of the drive units 110 discussed herein may, e.g., be a single propeller arrangement or a dual propeller arrangement, often referred to as a duo-prop arrangement. The propeller arrangement 130 in Figure 1 is a duo-prop arrangement. The present disclosure is not limited to any particular type of marine vessel, nor to any particular type of propeller arrangement. The teachings herein may be used with both outboard drive units and inboard drive units, as well as with both pulling and pushing propellers. The drive units may be attached to the transom of the vessel 100 as illustrated in Figure 1 or extend down from an underside of the hull 120. The drive units may be rotatable, i.e., provide steering function, or rotationally fixed relative to the hull 120.

Electric machines and hybrid electric power sources are becoming more and more common in marine vessels. An advantage with electric machines is that the axle speed of the drive unit can be controlled rapidly and with high accuracy. Another advantage is that electric machines allow very quick changes in applied torque. Generally, the propeller speed can be adjusted very rapidly and with high accuracy if the propeller is driven by an electric machine or a hybrid electric drive unit. The forward direction of the vessel 100 will be referred to herein as the x-direction, while the lateral direction perpendicular to the forward direction is referred to as the y-direction. The vertical direction is the z-direction, and orthogonal to both x- and y-directions. Velocity in the forward or longitudinal direction of the marine vessel 100 is referred to herein as v_x, while lateral velocity is referred to as v_y. Yaw motion about the z-axis will be referred to as m_z, as shown in Figure 1.

The speed of the vessel 100 through the water can be measured by a vessel control unit 140 using a speed log 150. Several different types of marine speed logs are known, such as electromagnetic logs, Doppler logs, impeller-based logs, Pitometer logs, and acoustic corelation logs.

The speed of the vessel through water may be purely longitudinal, but most likely there is also some drift, i.e., there is a v_y component in addition to the v_x component in the speed. When discussing propeller slip, it is normally the speed of the flow of water passing the propeller in direction of the propeller axle that matters. The speed through water in direction of the propeller axle at a given drive unit will be referred to herein as v_w. This speed v_w is a function of v_x, v_y, and m_z. It is also a function of the steering angle of the marine drive unit in case the drive unit is a steered (rotatable) drive unit. The drive unit can also have an elevation angle, i.e., a thrust direction separated from the horizontal plane by an angle. The present disclosure can be generalized to non-zero elevation angles in a straight-forward manner. The speed v_w can normally be approximated well enough by v_x, and in many cases also by the speed over ground, which can be obtained from, e.g., a satellite positioning system receiver or a sonar system.

Figure 2 schematically illustrates a single propeller 200 having a propeller diameter D and a pitch P. The pitch P is the distance travelled by the propeller as it is being screwed into an imagined solid medium. The rotation velocity of the propeller is denoted a>_P.

Longitudinal wheel slip X_x for a tyre on a road vehicle such as a truck or car can be defined as where R is an effective wheel radius in meters, a>_x is the angular velocity of the wheel (where the subscript x is for indicating that the wheel normally travels in the longitudinal direction), and v_x is the longitudinal speed of the wheel in the coordinate system of the wheel (relative to the road). Wheel slip is, in essence, a speed difference measured between the wheel and the vehicle. If the wheel accelerates in rotational motion faster than the road vehicle accelerates over the road surface, then the wheel slip increases. Wheel slip during acceleration is positive while wheel slip during braking is negative. In order for a wheel or tyre on a road vehicle to produce a tyre force, slip must occur. This is true for both acceleration and for braking. For smaller slip values the relationship between wheel slip and generated force is approximately linear due to the fact that there is full adhesion in the tyre contact patch and no sliding occurs, where the proportionality constant is often denoted as the slip stiffness of the tyre. The normal force F_z acting on the tyre of a wheel is key to determining some important vehicle properties. For instance, the normal force to a large extent determines the achievable longitudinal tyre force F_x by the wheel since, normally, F_x < F_z, where /J. is a friction coefficient associated with a current road friction condition.

An inverse tyre model is a model of wheel behavior which describes tyre force generated in longitudinal direction in the rolling direction as function of wheel slip. Figure 3 is a graph 300 showing an example of tyre force as function of longitudinal wheel slip X. The longitudinal tyre force F_x shows an almost linearly increasing part 310 for small wheel slips, followed by a part 320 with more non-linear behavior for larger wheel slips. At this stage part of the contact patch is sliding relative to the road surface. It is noted that there is a peak 330, beyond which the tyre force declines with wheel slip, i.e., if the wheel speed is increased beyond a given wheel speed, the generated tyre force will decline. The main reason for the declining characteristics is that sliding friction is lower than the stiction friction. Control of a road vehicle in the region 320 may be difficult and is therefore often avoided, and it is also undesired from an energy efficiency point of view as well as from a tyre wear perspective. Operation in the region 320 is often also associated with a reduced comfort level for the passengers in the road vehicle. Therefore, a wheel slip limit is is often imposed in order to ensure that the wheel does not operate in the undesired region 320, at least not for prolonged periods of time.

The longitudinal slip X_x of a propeller can be defined in a manner analogous to that of a tyre on a road vehicle as the difference between the pitch P of the propeller divided by the time for one revolution and the speed of the propeller through the water (STW) in direction of the propeller axle, i.e., v_x if the boat is travelling straight ahead, and a combination v_w of v_x, v_y, )_z if the boat is maneuvering. The propeller speed through water (v_P) without slip is the speed of the propeller if the propeller is seen as a screw being screwed into a solid medium, i.e., the pitch distance P divided by the time for one complete revolution of the propeller (27r/m_P).

If the propeller is accelerated the slip increases and vice versa. Propeller slip can be both positive (during acceleration) and negative (during retardation). Other alternative propeller slip definitions may be used with the same or similar effect, such as discretized approximations of the above expression.

It has been realized that a similar relationship as that in Figure 3 exists between propeller thrust and propeller slip for many propeller and hull shape combinations. As the propeller is accelerated the propeller slip increases and it starts to generate thrust. At first thrust increases with propeller slip, such that the higher the propeller slip the higher the thrust. Again, analogous to the situation with road vehicle tyres - without propeller slip there cannot be any thrust acting on the hull of a marine vessel. However, once the propeller slip has increased beyond a critical point, cavitation starts to develop, and a situation akin to the non-linear region 320 is seen where more propeller speed results in reduced thrust (or at least reduced thrust coefficient) and/or increased discomfort due to generation of noise and vibration. Propeller wear caused by the cavitation also sets in as propeller slip goes beyond a given propeller slip value.

Because a ship’s propeller rotates through the water at an angle of attack, a pressure side and a suction side are created on each of the propeller’s blades. The faster the blades cut through the water, i.e., the faster it rotates relative to the STW, the lower the pressure on the suction side of the blades becomes. At a certain propeller speed relative to STW, the pressure on the suction side becomes so low that the water flowing over that area evaporates, forming vapor bubbles or voids. When this happens, the efficiency of the propeller operation starts to decline. The effects of cavitation on propulsion performance can be considerable. Cavitation usually starts at the blade tips and spread gradually over the entire blades as the propeller loading is increased. When cavitation has extended to about 0.75 of the radius it is found there is a considerable loss in thrust followed by a reduction in torque which means in practice that, there will be a marked increase in revolutions for a given power. Since the thrust breakdown proceeds more rapidly than the change in torque there can be a considerable loss in efficiency. Noise and vibration are other undesired effects which result if cavitation becomes too pronounced.

Figure 4 illustrates some example relationships between thrust, i.e., propulsion force, and propeller slip that can be seen for some propeller design and hull shape combinations. There is an approximate linear region 410 where increasing propeller speed results in increase propeller thrust. If the propeller speed relative to the speed of the vessel through water is increased beyond this range, a peak range 420 is entered where the propeller operates at peak efficiency. Beyond this range a region 430 is seen where increased propeller speed is detrimental to both propulsion force and energy efficiency.

In the first example 440, propulsion force increases with propeller slip up to a point where no more thrust is obtained, regardless of how much the propeller speed is increased. In this case is does not make sense to increase propeller speed beyond a certain point, since the additional increase only gives rise to inefficiency and often also discomfort manifested as noise and vibration. Propeller wear may also increase in the region 430.

In the second example 450 the propeller design and the hull shape together are such as to result in an approximately linear increase in propulsion force with increasing propeller speed up to a point where cavitation starts to be so pronounced as to act detrimental to the thrust. The thrust then starts to decrease if the propeller is rotated faster relative to the surrounding water.

In the third example 460 the effects of cavitation are more dramatic. In this case the propeller design and hull shape combination are such as to result in a catastrophic cavitation which results in a near total loss of propulsion force.

In all examples, the generated noise and vibration is at first relatively small, and then increases sharply as the propeller slip reaches high enough values. Notably, each scenario has a propeller slip limit A_xl, A_x2, _x3 below which efficient and predictable operation is to be expected, and beyond which a more unpredictable and less efficient operation is more likely, where noise and vibration are also more pronounced. Hence, for each combination of propeller design and hull shape, there is a desired propeller slip range of operation, beyond which inefficient and/or uncomfortable operation ensues.

As mentioned, the relationship between thrust and propeller slip depends on many factors such as propeller design, hull shape, temperature, air pressure and sea conditions. However, an approximate relationship between propeller thrust and propeller slip can often be established from computer simulation, mathematical analysis, and/or practical experimentation in a straight-forward manner. Different amounts of propeller slip can be generated, and the thrust can be measured for each propeller slip value. The measured and/or simulated thrust levels can then be plotted against the propeller slip values to obtain a relationship like those in Figure 4.

It has been realized that some marine drive units are better controlled based on propeller slip or, equivalently, based on propeller speed relative to the STW, compared to control based propeller axle torque as is the more or less prevailing control method today. At the very least, significant advantages may be obtained if a propeller slip limit is implemented, where the marine drive unit is prevented from operating at a propeller slip beyond a predetermined slip limit. Since operation beyond the slip limit rarely brings any benefits when it comes to boat handling, propulsion efficiency, or drive component wear. The present disclosure therefore presents a control system 600 and corresponding methods for controlling a marine drive unit 110 on a vessel 100, such as a leisure craft or a smaller commercial vessel, i.e., a ferry, small freight boat, or the like. The control system 600 is configured to obtain data indicative of a speed through water (STW) v_w of the vessel 100. The STW may be obtained using a traditional speed log and/or via other means, such as a global positioning system receiver, as mentioned above. The relevant speed through water here is the speed through water measured along the extension direction of the propeller axis, i.e., the speed of the water that the propeller sees. The data indicative of a speed through water may also be related to the speed through water in front of the propeller, often referred to as v_A. This speed is often lower than the STW, since the hull pulls an amount of water with it as it travels through the water. The two are at least approximately related as v_A = v_w (1 — w), where w is the boat wake number. Different hulls have different wake numbers. The STW of the vessel is normally a combination of longitudinal, lateral and yaw motion, although it can often be approximated well enough by the longitudinal speed of the hull through the water, i.e., v_x. The control system, depending on the available sensor data, may be configured to determine this speed through water using a single log, or a collection of different sensors. The control system 600 is configured to determine a propeller slip of the marine drive unit based on a propeller speed a>_P of the drive unit 110 and on the speed through water v_w of the vessel 100. This propeller slip is indicative of how fast the propeller spins in relation to the speed of the water passing the propeller, the faster the propeller spins relative to the speed of the water passing the propeller the higher the slip is. If the boat is almost stationary, then a relatively small propeller speed may give rise to large slip, while a boat moving fast through the water requires a substantial propeller speed to generate the same propeller slip. Very large propeller slips can often be seen during launch of the vessel if the drive unit is powered by an electric machine, since the applied torque by an electric machine is almost instantaneous in response to a control command from the helm and does not build up over a transient time period as is normally the case with combustion engines. The control system 600 is arranged to control the propeller speed of the marine drive unit 110 to maintain the propeller slip below a predetermined propeller slip limit _(im, to ensure that the propeller operates in the desired region 410 or in the peak thrust region 420, and not in the undesired region 430 discussed in connection to Figure 4.

In some cases, such as in the first example 440 in Figure 4, the propeller thrust saturates with propeller slip. This means that at some point a further increase in propeller speed will not give any significant extra thrust. A control system that is configured to obtain data indicative of a desired thrust to be generated by the drive unit 110, and to determine a target propeller slip corresponding to the desired thrust, based on the relationship between drive unit thrust and propeller slip, can be configured with a propeller slip limit in vicinity of the saturation point, such as at _x3 or even at A_x2. The control system will then increase propeller speed in response to an increase in desired thrust, up to the point of the propeller slip limit where further increase in desired thrust will not result in increase in propeller speed, unless the speed through water decreases.

Figure 5 shows a graph 500 that illustrates an example marine vessel launch operation in terms of propeller axle speed M_P and vessel speed v_w through water as function of time. A constant acceleration is assumed for this simplified illustrative example. The vessel speed through water v_w is shown by the solid line 510, and the propeller speed is shown by the dashed line 520. At time T_o the boat starts to accelerate from standstill in the water. The propeller speed is therefore increased to a point m₀ corresponding to a propeller slip which results in close to optimal thrust, i.e., a propeller slip somewhere in or around the region 420. The maximum propeller speed is illustrated by the dash-dotted line 530, and it is noted that the propeller is not accelerated immediately up to this maximum speed, since this would mean that the propeller-hull combination ends up in the undesired high-slip region 430 discussed above in connection to Figure 4. Instead, the propeller speed is increased gradually as the vessel picks up speed though the water, always ensuring that the speed difference between the propeller speed and the vessel speed through water, i.e., the propeller slip is kept close to the propeller slip that corresponds to the peak region 420 in Figure 4.

The techniques discussed herein are at the same time used to achieve a desired level of propeller thrust in an efficient manner by controlling the propeller slip to be at a value corresponding to the desired level of propeller thrust, as long as the resulting propeller slip is below the configured propeller slip limit. In particular, there is disclosed herein a control system 600 for controlling a propeller-based marine drive unit 110 on a vessel 100. The control system can be used separately from that configured to limit propeller speed to speeds below the propeller slip limit, and the two can also be combined with advantage. The control system 600 is configured to obtain data indicative of a speed through water v_w of the vessel 100, and also configured to obtain a relationship 400 between drive unit thrust F_x and propeller slip . The control system 600 is furthermore configured to obtain data indicative of a desired thrust to be generated by the drive unit 110, and to determine a target propeller slip corresponding to the desired thrust, based on the relationship 400 between drive unit thrust F_x and propeller slip . The control system 600 is then able to control propeller speed a>_P of the marine drive unit 110 to generate the target propeller slip which provides the desired thrust.

The relationship 400 between drive unit thrust F_x and propeller slip may be configured as a predetermined analytical function or a collection of predetermined analytical functions. These functions can be determined based on computer simulation or by practical experimentation, or by a combination of computer simulation and practical experimentation. Mathematical analysis can also be used in the derivation of these relationships. The relationships do not need to be very exact, approximate relationships, such as linear approximations, have been found to give good results in many cases. The relationship 400 between drive unit thrust F_x and propeller slip A can also be realized as a preconfigured look-up table (LUT), allowing translation from thrust to propeller slip and back by indexing into the LUT.

Figure 6 shows some example components of a control system 600 for controlling a propellerbased marine drive unit 110 on a vessel 100. A vessel operator provides control commands 615 to the system 600 from the boat helm 610. The control commands may comprise a desired acceleration a_req and a desired curvature c_req that the boat motion is to follow. The vessel operator is often a person using a steering input device and a throttle control, but it can also be a computerized controller that operates the boat on autopilot. So-called driver assistance systems are also known where the boat is controlled by a blend of human operator input and computer control.

The control commands 615 are received by a vessel motion management (VMM) function 620 that translates the high level commands into lower level actuator controls 625 sent to one or more motion support device (MSD) control units 630. The MSD control units then control physical actuators on the boat 100, such as propulsion 640, transmission 650 and steering 660, by issuing suitable control commands 645, 655, 665. The speed through water v_w is obtained from one or more STW sensors 670. These sensors often but not necessarily comprise a speed log arranged on the vessel 100 but may also comprise satellite positioning system receivers and sonar systems arranged to measure speed over ground, which is indicative of the STW, even though they are approximations. Thus, both the MSD control units 630 and the VMM function 620 may obtain information indicative of the speed of the boat through water. The system components are also able to determine the propeller speed a>_P, since the transmission setting T is known to the system 600 and also the output axle speed a>_A of the propulsion device 640. In case the propulsion device 640 comprises an electric machine, then the motor axle speed of the electric machine is inherently known from the motor drive circuit. The teachings herein are particularly suitable for use with electric machines that can be controlled to deliver a target axle speed in an accurate manner. In case the propulsion device is some other type of engine, such as a combustion engine, then the axle speed can be determined by an axle speed sensor or the like.

With reference also to Figure 7, the VMM function 620 continuously transforms the acceleration profiles a_req and curvature profiles c_req received from the helm 610 into control commands 631, 632 for controlling vessel motion functions, actuated by the different MSDs of the vessel 100, i.e., the different propulsion resources and steering resources on the vessel 100. Other systems on the vessel 100 may also be controlled by the VMM function, such as ballast tanks, interceptors, gyros, and the like.

The VMM function 720 performs vessel state or motion estimation 720, i.e., the VMM function 620 continuously determines a current vessel state at time t, s(t), comprising, e.g., position, speed, acceleration, and yaw motion by monitoring operations using various sensors 710 arranged on the vessel 100, such as the speed logs mentioned above, satellite positioning system receivers, and inertial measurement units (IMUs). The VMM function 620 optionally also performs motion prediction 730, i.e., estimates a future vehicle state s(t+T) at one or more time instants t+T in the future. Both motion estimation 720 and motion prediction 730 are generally known techniques, and several example implementations exist in the literature. These sub-functions will therefore not be discussed in more detail herein.

The result of the motion estimation 720 and the optional motion prediction 730, i.e., the estimated vehicle state s(t), and possibly also the predicted future vehicle state s(t+T), is input to a force generation module 740 which determines the required global forces to cause the vessel 100 to move according to the requested acceleration and curvature profiles a_req, c_req. This actuation may comprise both propulsion and steering, i.e., actuation of both propulsion units and rudders.

The required global force vector is in this example input to an MSD coordination module 750 which allocates forces and coordinates actuators. The coordinated actuators then together provide the desired lateral Fy and longitudinal Fx forces on the vessel, as well as the required moments Mz, to obtain the desired motion by the vessel 100. The MSD coordination module 750 optimizes MSD allocation with a primary objective to meet the required global forces. The MSD coordination module 750 outputs one or more target propeller slip values and one or more rudder angles {8_L}_{l=1 N}. Suppose for instance that an acceleration of the vessel is requested from the helm, then an increase in propeller slip is called for in order to generate an increased propeller thrust. However, the increase in propeller slip will not be so large as to result in inefficiency, i.e., the propeller slip will not pass beyond the propeller slip limit imposed by the system. In case a curvature is requested, then some steering may be applied, possibly in combination with a difference in propeller slips in case two or more drive units are installed on the vessel.

One or more interfaces 631, 632 pass from the VMM function 620 to the different MSD controllers on the boat, such as electric machine controllers, combustion engine controllers, and rudder controllers.

The VMM module 620 may be arranged to request a propeller slip from the MSD control module or modules 630, and/or a propeller speed a>_P from the MSD control module(s) 630.

Figure 8A illustrates an example boat that is executing a turning maneuver 800. This boat has two drive units, one on the starboard side and one on the port side. There are also two speed logs STW-L and STW-R configured to measure a speed through water for each of the drive units. With the propeller slip-based control methods discussed herein, it is noted that the propeller speed of the port-side propeller will be slightly lower compared to the starboard side propeller, since the starboard side propeller is travelling faster through the water due to the curvature. The two propellers are still operated at the same slip value = A, i.e., at the desired target propeller slip. This is illustrated in Figure 8B, which shows a graph of propeller speed, where the upper solid curve 860 is the propeller speed of the starboard side drive unit and the lower dashed curve 870 is the propeller speed of the port side drive unit. Since the two propellers are operating at the same speed, the thrust is about the same on both drive units. However, the VMM function may of course choose to allocate more or less propeller speed, i.e., a difference in propeller slip, in case a difference in thrust is wanted, e.g., to aid in steering.

Looking at the example in Figure 8A, it is understood that is may be advantageous to use separate speed logs when there are more than one drive unit on a vessel. According to some aspects, the control system is arranged to control a plurality of marine drive units 110 on the vessel 100. The control system 600, 1100 is configured to obtain the data indicative of speed through water v_w separately for at least two drive units out of the plurality of drive units.

According to some aspects, the control system is arranged to configure a lower propeller slip limit X°_lm for the outer drive unit 820 compared to the propeller slip limit ^l _lim for the inner drive unit 810 in a dual drive unit system performing a turning maneuver. Here, “inner” and “outer” refer to the radial position of the propellers when the vessel is executing the turning maneuver, as indicated in Figure 8. The outer propeller travels faster than the inner propeller, at least over ground, due to the longer distance covered by the outer propeller compared to the inner propeller. The difference between the two slip limits can, e.g., be configured as function of the curvature of the turning maneuver, or the roll angle of the vessel during the turning maneuver. This way the outer drive unit which is probably operating at reduced draught will not exhibit the same amount of cavitation due to the reduced draught, which may also be accounted for when configuring the propeller slips to use for a given turning maneuver.

The VMM module 620, having regard to the current depth of the propellers in a multi-propeller drive system, may dynamically configure propeller slip limits of the multi-propeller system, e.g., based on an estimated operating depth of the propellers in the multi-propeller system. An inner propeller during a turning maneuver will lie deeper in the water compared to the outer propeller and can therefore often support a larger propeller slip. The mapping between propeller slip limit and vessel motion state, such as yaw rate, roll angle, and the like, can be predetermined, e.g., as a LUT or the like.

Figure 9 is an efficiency map 900 of an example electric machine. The motor speed of the electric machine is plotted on the x-axis and the applied torque by the electric machine is plotted on the y-axis. The contour plot then shows efficiency of the electric machine, i.e., how much of the input power that is converted into motion and how much that is lost as heat etc. The shape of this map is similar for most electric machines. This example electric machine is able to provide a high torque for low axle speeds, and the maximum achievable applied torque then declines with axle speed. It is noted that, for each propeller speed there is a best applied torque. Hence, for a given speed 910, it is possible to find an applied torque 920 that maximizes the efficiency of the electric machine operation. Conversely, for a given applied (positive) torque, there is an axle speed that maximizes efficiency. According to some aspects, the control system 600 is arranged to control a marine drive unit 110 having an adjustable propeller pitch P. The control system 600 is arranged to control the propeller speed a>_P and the propeller pitch P of the marine drive unit 110 to generate a target propeller slip , based on a predetermined relationship 900 between efficiency, axle speed, and applied torque of an electric machine. According to an example, the propeller slip is first determined based on the motion requests received from the helm of the vessel. This target propeller slip can be translated into an equivalent motor axle speed using information about the transmission setting and the STW v_w of the vessel. Given this motor axle speed, the efficiency map can be consulted, and the best pitch can be found by a straightforward linear search. The electric machine will then generate the target propeller slip at an efficiency that is as high as possible given the request from the helm.

The VMM module 620, having regard to the efficiency map, may coordinate the propellers of a multi-propeller system so as to obtain a higher overall efficiency.

Figure 10 illustrates an example control input device 1000 according to the present teaching. Normally a thrust control input device such as that schematically illustrated in Figure 10 maps an operator input command to drive axle torque, which then translates to a propeller axle speed which is independent (or at least not intentionally governed) of the speed of the vessel through the water. The control input device 1000 instead maps the position of the control input device to a target propeller slip value, via the relationship between drive unit thrust and propeller slip discussed above, which is then controlled against. This way an operator command is mapped to propeller slip instead of torque as in most traditional marine propeller thrust control devices. The target propeller slip is preferably limited, such that a full forward thrust command or a full reverse thrust command is limited to the configured positive or negative propeller slip limits A positive side effect of the control input device 1000 is that the marine drive unit operation becomes more energy efficient, in particular if the propeller slip limits are configured at a value where more slip only gives small extra thrust. In more traditional control input devices, an operator desiring more thrust may be permitted to increase torque to a point where the extra torque only contributes to additional propeller slip and no significant extra generated thrust. In fact, the extra added torque by the operator may even in some case result in a decrease in obtained propeller thrust, as discussed above in connection to Figure 4.

Figure 11 is a schematic diagram of a computer system 1100 for implementing examples disclosed herein. The computer system 1100 is adapted to execute instructions from a computer-readable medium to perform these and/or any of the functions or processing described herein. The computer system 1100 may be connected (e.g., networked) to other machines in a LAN, an intranet, an extranet, or the Internet. While only a single device is illustrated, the computer system 1100 may include any collection of devices that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. Accordingly, any reference in the disclosure and/or claims to a computer system, computing system, computer device, computing device, control system, control unit, electronic control unit (ECU), processor device, processing circuitry, etc., includes reference to one or more such devices to individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. For example, control system may include a single control unit, or a plurality of control units connected or otherwise communicatively coupled to each other, such that any performed function may be distributed between the control units as desired. Further, such devices may communicate with each other or other devices by various system architectures, such as directly or via a Controller Area Network (CAN) bus, etc.

The computer system 1100 may comprise at least one computing device or electronic device capable of including firmware, hardware, and/or executing software instructions to implement the functionality described herein. The computer system 1100 may include processing circuitry 1102 (e.g., processing circuitry including one or more processor devices or control units), a memory 1104, and a system bus 1106. The computer system 1100 may include at least one computing device having the processing circuitry 1102. The system bus 1106 provides an interface for system components including, but not limited to, the memory 1104 and the processing circuitry 1102. The processing circuitry 1102 may include any number of hardware components for conducting data or signal processing or for executing computer code stored in memory 1104. The processing circuitry 1102 may, for example, include a general-purpose processor, an application specific processor, a Digital Signal Processor (DSP), an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a circuit containing processing components, a group of distributed processing components, a group of distributed computers configured for processing, or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. The processing circuitry 1102 may further include computer executable code that controls operation of the programmable device.

The system bus 1106 may be any of several types of bus structures that may further interconnect to a memory bus (with or without a memory controller), a peripheral bus, and/or a local bus using any of a variety of bus architectures. The memory 1104 may be one or more devices for storing data and/or computer code for completing or facilitating methods described herein. The memory 1104 may include database components, object code components, script components, or other types of information structure for supporting the various activities herein. Any distributed or local memory device may be utilized with the systems and methods of this description. The memory 1104 may be communicab ly connected to the processing circuitry 1102 (e.g., via a circuit or any other wired, wireless, or network connection) and may include computer code for executing one or more processes described herein. The memory 1104 may include non-volatile memory 1108 (e.g., read-only memory (ROM), erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), etc.), and volatile memory 1110 (e.g., random-access memory (RAM)), or any other medium which can be used to carry or store desired program code in the form of machineexecutable instructions or data structures, and which can be accessed by a computer or other machine with processing circuitry 1102. A basic input/output system (BIOS) 1112 may be stored in the non-volatile memory 1108 and can include the basic routines that help to transfer information between elements within the computer system 1100.

The computer system 1100 may further include or be coupled to a non-transitory computer- readable storage medium such as the storage device 1114, which may comprise, for example, an internal or external hard disk drive (HDD) (e.g., enhanced integrated drive electronics (EIDE) or serial advanced technology attachment (SATA)), HDD (e.g., EIDE or SATA) for storage, flash memory, or the like. The storage device 1114 and other drives associated with computer-readable media and computer-usable media may provide non-volatile storage of data, data structures, computer-executable instructions, and the like.

Computer-code which is hard or soft coded may be provided in the form of one or more modules. The module(s) can be implemented as software and/or hard-coded in circuitry to implement the functionality described herein in whole or in part. The modules may be stored in the storage device 1114 and/or in the volatile memory 1110, which may include an operating system 1116 and/or one or more program modules 1118. All or a portion of the examples disclosed herein may be implemented as a computer program 1120 stored on a transitory or non-transitory computer-usable or computer-readable storage medium (e.g., single medium or multiple media), such as the storage device 1114, which includes complex programming instructions (e.g., complex computer-readable program code) to cause the processing circuitry 1102 to carry out actions described herein. Thus, the computer-readable program code of the computer program 1120 can comprise software instructions for implementing the functionality of the examples described herein when executed by the processing circuitry 1102. In some examples, the storage device 1114 may be a computer program product (e.g., readable storage medium) storing the computer program 1120 thereon, where at least a portion of a computer program 1120 may be loadable (e.g., into a processor) for implementing the functionality of the examples described herein when executed by the processing circuitry 1102. The processing circuitry 1102 may serve as a controller or control system for the computer system 1100 that is to implement the functionality described herein.

The computer system 1100 may include an input device interface 1122 configured to receive input and selections to be communicated to the computer system 1100 when executing instructions, such as from a keyboard, mouse, touch-sensitive surface, etc. Such input devices may be connected to the processing circuitry 1102 through the input device interface 1122 coupled to the system bus 1106 but can be connected through other interfaces, such as a parallel port, an Institute of Electrical and Electronic Engineers (IEEE) 1394 serial port, a Universal Serial Bus (USB) port, an IR interface, and the like. The computer system 1100 may include an output device interface 1124 configured to forward output, such as to a display, a video display unit (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)). The computer system 1100 may include a communications interface 1126 suitable for communicating with a network as appropriate or desired.

The operational actions described in any of the exemplary aspects herein are described to provide examples and discussion. The actions may be performed by hardware components, may be embodied in machine-executable instructions to cause a processor to perform the actions, or may be performed by a combination of hardware and software. Although a specific order of method actions may be shown or described, the order of the actions may differ. In addition, two or more actions may be performed concurrently or with partial concurrence.

Figure 12 illustrates a computer readable medium 1210 carrying a computer program comprising program code means 1220 for performing the methods illustrated in Figure 13 and the techniques discussed herein, when said program product is run on a computer. The computer readable medium and the code means may together form a computer program product 1200.

Figure 13 is a flow chart illustrating an example method which summarizes at least some of the above discussion. There is illustrated a computer-implemented method for controlling a marine drive unit 110 on a vessel 100, the method comprising: obtaining SI, by processing circuitry of a computer system, data indicative of a speed through water v_w of the vessel 100, determining S2, by the processing circuitry, a propeller slip of the marine drive unit based on the propeller speed of the drive unit 110 and on the speed through water v_w of the vessel 100, and controlling S3, by the processing circuitry, the propeller speed a>_P of the marine drive unit 110 to maintain the propeller slip below a predetermined propeller slip limit _(im.

The operational steps described in any of the exemplary aspects herein are described to provide examples and discussion. The steps may be performed by hardware components, may be embodied in machine-executable instructions to cause a processor to perform the steps, or may be performed by a combination of hardware and software. Although a specific order of method steps may be shown or described, the order of the steps may differ. In addition, two or more steps may be performed concurrently or with partial concurrence. The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," "includes," and/or "including" when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure.

Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element to another element as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. It is to be understood that the present disclosure is not limited to the aspects described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the present disclosure and appended claims. In the drawings and specification, there have been disclosed aspects for purposes of illustration only and not for purposes of limitation, the scope of the inventive concepts being set forth in the following claims.

Claims

1. A control system (140, 600, 1100) for controlling at least one marine drive unit (110) on a vessel (100), the control system (140, 600, 1100) being configured to obtain data indicative of a speed through water, STW, ( _w, v_A) of the vessel (100), the control system (140, 600, 1100) being configured to determine a propeller slip ( ) of the at least one marine drive unit (110) based on a propeller speed (m_P) of the drive unit (110) and on the speed through water (v_w) of the vessel (100), where the control system (140, 600, 1100) is configured to obtain a relationship (400) between drive unit thrust (F_x) and propeller slip ( ), and where the control system (140, 600, 1100) is configured to obtain data indicative of a desired thrust to be generated by the drive unit (110), and to determine a target propeller slip corresponding to the desired thrust, based on the relationship (400) between drive unit thrust (F_x) and propeller slip ( ), the control system (140, 600, 1100) being arranged to control the propeller speed (m_P) of the marine drive unit (110) to generate the target propeller slip and to maintain the propeller slip ( ) below a predetermined propeller slip limit ( _(im).

2. The control system (140, 600, 1100) according to claim 1, where the marine drive unit (110) comprises an electric machine arranged to provide a target axle speed upon request from the control system (140, 600, 1100).

3. The control system (140, 600, 1100) according to claim 1 or 2, arranged to receive the data indicative of speed through water ( _w) from a speed log arranged on the vessel (100).

4. The control system (140, 600, 1100) according to any previous claim, arranged to receive the data indicative of speed through water (v_w) as speed over ground data from a global positioning system arranged on the vessel (100) and/or from a sonar sensor system.

5. The control system (140, 600, 1100) according to any previous claim, arranged to obtain the relationship (400) between drive unit thrust (F_x) and propeller slip ( ) at least in part as a predetermined analytical function or a collection of predetermined analytical functions.

6. The control system (140, 600, 1100) according to any previous claim, where the relationship (400) between drive unit thrust (F_x) and propeller slip ( ) comprises a preconfigured look-up table, LUT.

7. The control system (140, 600, 1100) according to any previous claim, where the control system is arranged to control a plurality of marine drive units (110) on the vessel (100), where the control system (140, 600, 1100) is configured to obtain the data indicative of speed through water ( _w) separately for at least two drive units out of the plurality of drive units.

8. The control system (140, 600, 1100) according to any previous claim, arranged to control a port side drive unit (810) and a starboard side drive unit (820), where the control system is arranged to configure a lower propeller slip limit ( _im) for a turning maneuver outer drive unit (820) out of the port side drive unit and the starboard side drive unit compared to a propeller slip limit ( j_im) for a turning maneuver inner drive unit (810) out of the port side drive unit and the starboard side drive unit.

9. The control system (140, 600, 1100) according to any previous claim, arranged to control a marine drive unit (110) having an adjustable propeller pitch (P) and comprising an electric machine that is controlled by the control system, where the control system (140, 600, 1100) is arranged to control the propeller speed (m_P) and the propeller pitch (P) of the marine drive unit (110) to generate a target propeller slip ( ), based on a predetermined relationship (900) between efficiency, motor axle speed, and applied torque of the electric machine.

10. The control system (140, 600, 1100) according to any previous claim, comprising a centralized vessel motion management, VMM, module (620) arranged to request a propeller slip ( ) from at least one motion support device, MSD, control module (630).

11. The control system (140, 600, 1100) according to any previous claim, comprising a centralized vessel motion management, VMM, module (620) arranged to request a propeller speed (m_P) from at least one motion support device, MSD, control module (630).

12. A marine vessel (100) comprising the control system (140, 600, 1100) of any of claims 1-11.

13. A computer-implemented method for controlling a marine drive unit (110) on a vessel (100), the method comprising: obtaining (SI), by processing circuitry of a computer system, data indicative of a speed through water, STW, (v_w) of the vessel (100), determining (S2), by the processing circuitry, a propeller slip ( ) of the marine drive unit based on a propeller speed (m_P) of the drive unit (110) and on the speed through water (v_w) of the vessel (100), obtaining a relationship (400) between drive unit thrust (F_x) and propeller slip ( ), obtaining data indicative of a desired thrust to be generated by the drive unit (110), determining a target propeller slip corresponding to the desired thrust, based on the relationship (400) between drive unit thrust (F_x) and propeller slip ( ), and controlling (S3), by the processing circuitry, the propeller speed (m_P) of the marine drive unit (110) to generate the target propeller slip and to maintain the propeller slip ( ) below a predetermined propeller slip limit ( _(im).

14. A computer program product (1100) comprising program code (1120) for performing, when executed by the processing circuitry, the method of claim 13. 15. A non-transitory computer-readable storage medium (1110) comprising instructions

(1120), which when executed by processing circuitry, cause the processing circuitry to perform the method of claim 13.