Method and system for steering the wheels of a vehicle

Abstract

A controller, a system, and a method for steering wheels of a vehicle. The vehicle includes a steering system associated with a front wheel and a rear wheel, and adapted to control steering of the front wheel separately from steering of the rear wheel. A steering request to drive the vehicle in a target trajectory is received, and an updated front-axle steering-angle is determined based on the received target trajectory. A longitudinal speed, a longitudinal acceleration, and a yaw rate of the vehicle are obtained, and a target rear-axle steering angle is computed. A maximal rear-steering rate is determined, such that yaw acceleration of the vehicle is within a pre-determined yaw acceleration threshold. The front and rear wheels are steered to the updated front- and rear-axle steering-angles at a rate of change that is not greater than the maximal rear-steering rate.

Description

[0001] METHOD AND SYSTEM FOR STEERING THE WHEELS OF A VEHICLE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to methods and systems of steering rear wheels of a vehicle during a turn, and particularly to methods and systems for steering the rear wheels to minimize yaw acceleration of the vehicle during transient cornering maneuvers.

[0004] BACKGROUND OF THE INVENTION

[0005] Implementations of rear wheels steering may require rear wheels steering control logic to define steering radius and dynamics parameters. It is known to use feedforward rear wheel steering techniques to aid maneuverability during steady state low speed cornering maneuvers, in which the rear wheels are typically steered in an opposite direction to that of the front wheels. These techniques also augment vehicle stability at moderate to high-speed steady state maneuvers, in which the rear wheels are typically steered in the same direction, or parallel, to the steering of the front wheels. However, in highly transient cornering events, transition between rear-steering in the opposite direction and rear-steering in the parallel direction may lead to an undesirable yaw motion being added to the vehicle. Additionally, a rapid change in the rear steering angle, such as for example when pulling away from a T-junction, may lead to higher yaw acceleration. Such yaw motion or acceleration results in the driver feeling unstable and being required to exert additional effort to maintain the stability of the vehicle. The yaw motion or acceleration is exacerbated in vehicles which deploy large opposite rear-wheel steer-angles for low-speed maneuverability and which also transition to parallel steer at moderate speeds to augment vehicle stability such as high sided delivery vehicles.

[0006] There is thus a need in the art for a method for controlling the steering of the rear wheels of the vehicle in a manner that reduces or eliminates undesirable yaw vehicle motion during transient cornering events, while ensuring the lateral stability of the vehicle at differing vehicle speeds.

[0007] SUMMARY OF EMBODIMENTS OF THE INVENTION

[0008] Some embodiments of the invention relate to methods and systems for steering the rear wheels to minimize yaw acceleration of the vehicle during transient cornering maneuvers. There is thus provided, in accordance with an embodiment of the teachings herein, a method for steering wheels of a vehicle, the vehicle including a steering system associated at least one front wheel and at least one rear wheel of the vehicle, the steering system adapted to control steering of the at least one front wheel separately from steering of the at least one rear wheel. The method includes the steps of: a. while the at least one front wheel is steered to a current front-axle steering-angle and the at least one rear wheel is steered to a current rear-axle steering angle, the current front-axle steering-angle and the current rear-axle steering-angle having a current steering-ratio therebetween, receiving a steering request to drive the vehicle in a target trajectory; b. determining an updated front-axle steering -angle based on the received target trajectory; c. obtaining a longitudinal speed, a longitudinal acceleration, and a yaw rate of the vehicle; d. computing a target rear-axle steering angle; e. determining a maximal rear-steering rate reflecting a maximal allowed rate of change in the rear-axle steering angle, such that yaw acceleration of the vehicle is within a pre-determined yaw acceleration threshold; and f. steering the at least one front wheel to the updated front-axle steering -angle and the at least one rear wheel to the updated rear-axle steering-angle, at a rate of change that is not greater than the maximal rear-steering rate.

[0009] In some embodiments, the determining at step b includes determining the updated frontaxle steering -angle to be defined in absolute terms.

[0010] In some embodiments, the determining at step b includes determining the updated frontaxle steering -angle to be defined relative to the current front-axle steering-angle.

[0011] In some embodiments, the computing at step d is based on the longitudinal speed of the vehicle, or only on the longitudinal speed of the vehicle.

[0012] In some embodiments, the computing at step d includes computing the target rear-axle steering -angle to be defined in absolute terms.

[0013] In some embodiments, the computing at step d includes computing the target rear-axle steering -angle to be defined relative to the current rear-axle steering -angle.

[0014] In some embodiments, the computing at step d is independent of a current yaw rate of the vehicle.

[0015] In some embodiments, the computing at step d includes computing a target steering-ratio between the updated front-axle steering-angle and an updated rear-axle steering-angle. In some embodiments, the determining at step e includes determining, as the maximal rear-steering rate, an upper limit of the target steering-ratio to be within a computed maximal allowed change in the target steering-ratio with respect to a current steering-ratio, such that the yaw acceleration of the vehicle is within the pre-determined yaw acceleration threshold. In some embodiments, the method further includes computing an actual rear-axle steeringangle, based on the updated front-axle steering-angle, which can be accomplished within the maximal rear-steering rate. In some such embodiments, the steering at step f includes steering the at least one rear wheel to the actual rear-axle steering angle.

[0016] In some embodiments, the computing of the actual rear-axle steering-tingle is based on a steering angle difference and a distance travelled.

[0017] In some embodiments, the computing of the actual rear-axle steering-angle is based on the trajectory created by front wheels, such that rear-axle steering is computed to follow the same trajectory.

[0018] In some embodiments, the computing of actual rear-axle steering-angle is based on the computing of the target rear-axle steering angle, which is updated following a variable time delay, or periodically.

[0019] In some embodiments, the computing of the actual rear-axle steering-angle is based on tour segment scheduling maps with steering overlay, where segments are created based on the front-axle steering-angle, and yaw rate gain or yaw error.

[0020] In some embodiments, the method further includes, prior to step e, obtaining the predetermined yaw acceleration threshold.

[0021] In some embodiments, the vehicle includes two front wheels, and the steering at step f is in accordance with Ackermann geometry.

[0022] In some embodiments, the vehicle includes two rear wheels, and the steering at step f is in accordance with Ackermann geometry.

[0023] In some embodiments, the computing at step d, and / or the computing of the actual rearaxle steering angle, is independent of a sideslip angle of the vehicle.

[0024] In some embodiments, the determining of the maximal rear-steering rate includes dynamically changing the maximal rear-steering rate and / or the rear-steering angle as the longitudinal speed of the vehicle changes.

[0025] There is further provided, in accordance with an embodiment of the teachings herein, a controller for control of a steering system of a vehicle, the steering system being functionally associated with at least one front wheel and at least one rear wheel of the vehicle, the steering system adapted to control steering of the at least one front wheel separately from steering of the at least one rear wheel. The controller includes one or more processors and a non-transitory computer readable storage medium for instructions execution by the one or more processors. The non-transitory computer readable storage medium has stored: (i) instructions, to be carried out while the at least one front wheel is steered to a current front-axle steering-angle and the at least one rear wheel is steered to a current rearaxle steering angle, the current front-axle steering-angle and the current rear-axle steeringangle having a current steering-ratio therebetween, to receive a steering request to drive the vehicle in a target trajectory;

[0026] (ii) instructions to determine an updated front-axle steering-angle based on the received target trajectory;

[0027] (iii) instructions to obtain a longitudinal speed, a longitudinal acceleration, and a yaw rate of the vehicle;

[0028] (iv) instructions to compute a target rear-axle steering angle;

[0029] (v) instructions to determine a maximal rear-steering rate reflecting a maximal allowed rate of change in the rear-axle steering angle, such that yaw acceleration of the vehicle is within a pre-determined yaw acceleration threshold; and

[0030] (vi) instructions to steer the at least one front wheel to the updated front-axle steeringangle and the at least one rear wheel to the updated rear-axle steering-angle, at a rate of change that is not greater than the maximal rear-steering rate.

[0031] In some embodiments, the instructions to determine the updated front-axle steering-angle include instructions to determine the updated front-axle steering-angle to be defined in absolute terms.

[0032] In some embodiments, the instructions to determine the updated front-axle steering-angle include instructions to determine the updated front-axle steering-angle to be defined relative to the current front-axle steering-angle.

[0033] In some embodiments, the instructions to compute of the target rear-axle steering angle include instructions to compute the target rear-axle steering angle based on the longitudinal speed of the vehicle.

[0034] In some embodiments, the instructions to compute the target rear-axle steering angle include instructions to compute the target rear-axle steering angle to be defined in absolute terms.

[0035] In some embodiments, the instructions to compute the target rear-axle steering angle include instructions to compute the target rear-axle steering angle to be defined relative to the current rear-axle steering -angle.

[0036] In some embodiments, the instructions to compute of the target rear-axle steering angle include instructions to compute the target rear-axle steering angle independently of a current yaw rate of the vehicle. In some embodiments, the instructions to compute the target rear-axle steering angle include instructions to compute a target steering-ratio between the updated front-axle steeringangle and an updated rear-axle steering-angle. In some embodiments, the instructions to determine the maximal rear-steering rate include instructions to determine, as the maximal rear-steering rate, an upper limit of the target steering-ratio to be within a computed maximal allowed change in the target steering-ratio with respect to a current steering-ratio, such that the yaw acceleration of the vehicle is within the pre-determined yaw acceleration threshold.

[0037] In some embodiments, the non-transitory computer readable storage medium further has stored instructions to compute an actual rear-axle steering-angle, based on the updated front-axle steering -angle, which can be accomplished within the maximal rear-steering rate. In some embodiments, the instructions to steer include instructions to steer the at least one rear wheel to the actual rear-axle steering angle.

[0038] In some embodiments, the instructions to compute the actual rear-axle steering-angle include instructions to compute the actual rear-axle steering-angle based on a steering angle difference and a distance travelled.

[0039] In some embodiments, the non-transitory computer readable storage medium further has stored instructions to obtain the pre-determined yaw acceleration threshold.

[0040] In some embodiments, the vehicle includes two front wheels, and the instructions to steer include instructions to steer the front wheels in accordance with Ackermann geometry.

[0041] In some embodiments, the vehicle includes two rear wheels, and the instructions to steer include instructions to steer the rear wheels in accordance with Ackermann geometry.

[0042] In some embodiments, the instructions to compute the target rear-axle steering angle include instructions to compute the target rear-axle steering angle and / or the actual rear-axle steering -angle independently of a sideslip angle of the vehicle.

[0043] In some embodiments, the instructions to compute the target rear-axle steering angle include instructions to compute the target rear-axle steering angle based only on the longitudinal speed of the vehicle.

[0044] In some embodiments, the instructions to determine the maximal rear-steering rate include instructions to dynamically change the maximal rear-steering rate and / or the rear-steering angle as the longitudinal speed of the vehicle changes.

[0045] There is further provided, in accordance with an embodiment of the teachings herein, a system for control of steering of a vehicle having at least one front wheel and at least one rear wheel, the system including: a. a steering system functionally associated with the at least one front wheel and the at least one rear wheel, the steering system adapted to control steering of the at least one front wheel separately from steering of the at least one rear wheel; and b. the controller described herein, functionally associated with the steering system.

[0046] There is additionally provided, in accordance with an embodiment of the teachings herein, a vehicle including: at least one front wheel; at least one rear wheel; and the system described herein, functionally associated with the at least one front wheel and the at least one rear wheel.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains. In case of conflict, the specification, including definitions, will take precedence.

[0048] As used herein, the terms “comprising”, “including”, "having" and grammatical variants thereof are to be taken as specifying the stated features, integers, steps or components but do not preclude the addition of one or more additional features, integers, steps, components or groups thereof. These terms encompass the terms "consisting of' and "consisting essentially of' .

[0049] BRIEF DESCRIPTION OF THE FIGURES

[0050] Figs. 1A and IB are schematic illustrations of implementations of a system for steering a vehicle according to embodiments of the disclosed technology;

[0051] Figs. 2A and 2B are flow charts of two exemplary methods of steering a vehicle according to embodiments of the disclosed technology;

[0052] Fig. 3 is a graphic representations of data flow during implementation of the method of Fig. 2B according to embodiments of the disclosed technology; and

[0053] Fig. 4 shows exemplary plots of wheel steering using the methods of the disclosed technology.

[0054] DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

[0055] The invention, in some embodiments, relates to methods and systems for steering the rear wheels to minimize yaw acceleration of the vehicle during transient cornering maneuvers.

[0056] There is thus provided, in accordance with an embodiment of the teachings herein, a method for steering wheels of a vehicle, the vehicle including a steering system associated at least one front wheel and at least one rear wheel of the vehicle, the steering system adapted to control steering of the at least one front wheel separately from steering of the at least one rear wheel. In accordance with the method, while the at least one front wheel is steered to a current front-axle steering-angle and the at least one rear wheel is steered to a current rear-axle steering angle, the current front-axle steering -angle and the current rear-axle steering -angle having a current steeringratio therebetween, receiving a steering request to drive the vehicle in a target trajectory. An updated front-axle steering-angle is determined based on the received target trajectory. A longitudinal speed, a longitudinal acceleration, and a yaw rate of the vehicle are obtained, and a target rear-axle steering angle is computed. A maximal rear-steering rate, reflecting a maximal allowed rate of change in the rear-axle steering angle, is determined, such that yaw acceleration of the vehicle is within a pre-determined yaw acceleration threshold. Finally, the at least one front wheel is steered to the updated front-axle steering-angle and the at least one rear wheel is steered to the updated rear-axle steering-angle, at a rate of change that is not greater than the maximal rear-steering rate.

[0057] For convenience, in the context of the description herein, various terms are presented here. To the extent that definitions are provided, explicitly or implicitly, here or elsewhere in this application, such definitions are understood to be consistent with the usage of the defined terms by those of skill in the pertinent art(s). Furthermore, such definitions are to be construed in the broadest possible sense consistent with such usage.

[0058] When used in this specification and in the claims appended hereto, the word “vehicle” is to be understood as referring to a vehicle having one or more front wheels, and one or more rear wheels. Non-limiting examples of a vehicle, according to this definition, are a vehicle with motive power provided by an onboard engine, and an ‘electric vehicle’ powered, when in motion, by one or more electric motors and a battery or other energy storage device onboard. The battery need not be provided with the vehicle, or installed in the vehicle, unless and until the vehicle is in motion. The word ‘vehicle’ can also be understood as encompassing a “vehicle platform” comprising at least a chassis (or other ‘reference frame’ to which wheel assemblies can be mounted) as well as one or more front wheels and one or more rear wheels. A ‘vehicle platform’ need not necessarily comprise, at the time of providing the vehicle platform, all of the accoutrements required for transport of passengers and / or cargo such as vehicle-body components or interior furnishings.

[0059] The term “controller” as used herein means a computing device configured for monitoring, controlling, regulating and / or actuating one or more components, systems or subsystems. A controller should be understood to include any or all of (and not exhaustively): one or more processors, one or more computer-readable media, e.g., transient and / or non-transient storage media, communications arrangements, a power source and / or a connection to a power source, and firmware and / or software. Controllers (and control units) can be programmed in advance, e.g., by having program instructions stored in the computer-readable media for execution by one of more processors of the controller. Thus, a controller ‘configured’ to perform a function is equivalent herein to the controller being programmed, i.e., having access to stored program instructions for execution, to perform the function.

[0060] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

[0061] Reference is now made to Figs. 1A and IB, which are schematic illustrations of implementations of a system for steering a vehicle according to embodiments of the disclosed technology.

[0062] As seen in Figure 1A and IB, a vehicle 10, which typically includes a vehicle platform 11 (e.g. a reference frame, a chassis, support platform), and a vehicle capsule (e.g. vehicle body, tophat, cargo box, cabin, passenger capsule), and is controlled by a vehicle-controller 18 (e.g. ECU, VCU).

[0063] In some embodiments, vehicle 10 further includes a plurality of wheel assemblies, here shown as four wheel assemblies: 20fr and 20fl (front right and front left wheel assemblies, respectively) and 20rr, and 20rl (rear right and rear left wheel assemblies, respectively). Vehicle 10 has a front end 50 and a rear end 52. A wheelbase of the vehicle, defined as the distance between the front wheels and the rear wheels, is marked by Lwb in Fig. 1 A.

[0064] A steering system is associated with wheel assemblies 20fr, 20fl, 20rr, and 20rl and includes at least one front steering assembly and at least one rear steering assembly, such that front wheels 20fr and 20fl are steered independently from rear wheels 20rr and 20rl. In some embodiments, the steering system, or each of the steering assemblies, may be associated with a steering -controller, adapted to control operation of the steering system. In some embodiments, the steering-controller is functionally associated with vehicle-controller 18, and is adapted to receive input from vehicle-controller 18.

[0065] In the embodiment of Fig. 1A, a central controller is functionally associated with subcontrollers 64a and 64b, each of which is associated with one of a front steering assembly 32a and a rear steering assembly 32b. As such, sub-controller 64a is configured to control steering of front wheels 20fr and 20fl, and sub-controller 64b is configured to control steering of rear wheels 20rr and 20rl.

[0066] In some embodiments, and as illustrated, the central controller may be vehicle-controller 18, such that sub-controllers 64a and 64b function as the steering-controller 33 (illustrated for example in Fig. IB).

[0067] In some other embodiments, the central controller may include a central steeringcontroller, equivalent to steering-controller 33 of Fig. IB, and sub-controllers 64a and 64b are additional steering-controllers, each associated with a specific steering assembly.

[0068] In some other alternative embodiments, vehicle platform may include dedicated subcontrollers 64 for each of wheel assemblies 20fr, 20fl, 20rr and 20rl.

[0069] In some other alternative embodiments, vehicle platform may include dedicated steering sub-controllers 64 for each of wheel assemblies 20fr, 20fl, 20rr and 20rl, the steering subcontrollers 64 receiving steering instructions from the central controller.

[0070] The structure illustrated in Fig. 1A may be suitable for steer-by-wire applications (e.g. steering by sub-controllers 64a / 64b providing electronic signals to the steering assemblies), as shown, but would also be suitable for vehicles in which the front wheels are mounted on a front axle and the rear wheels are mounted on a rear axle, and the axles are mechanically controlled by the sub-controllers.

[0071] Turning to Fig. IB, it is seen that in some embodiments, vehicle 10 may include one or more sensors or sensing assemblies 34, which sense characteristics and parameters of the vehicle and / or of the vicinity of the vehicle, and which are associated with one or more controllers of the vehicle. In some embodiments, and as explained in further detail hereinbelow, sensor(s) 34 may provide to vehicle-controller 18 and / or to steering-controller 33 input indicative of a required direction of steering, facilitating dynamic steering of the vehicle, and particularly of the rearwheels, as described herein.

[0072] Sensors 34 may include imaging or image capturing sensors, such as cameras, radar, lidar, and the like. Such imaging or image capturing sensors may provide input relating to objects in the vicinity of the vehicle, such as the presence of obstacles or another vehicle.

[0073] Sensors 34 may include proximity sensors, which provide input relating to the proximity of vehicle 10 to another object, such as another vehicle or an obstacle.

[0074] Sensors 34 may include a location system receiver, such as a receiver for signals of GPS, GLONASS, GALILEO, and the like, which provide input relating to the location, on the globe, of vehicle 10, and / or information relating to the route to be taken by vehicle 10 and the terrain conditions along that route. Sensors 34 may include one or more of an accelerometer, a gyroscope, and an inertial measurement unit.

[0075] Sensors 34 may include angle sensors, such as a steering wheel angle sensor, wheel-angle input sensors, and the like.

[0076] In the embodiment of Fig. IB, a central controller, such as vehicle-controller 18, is functionally associated with steering-controller 33, which in turn controls steering assemblies 32fr, 32fl, 32rr, and 32rl, each of which is configured to steer a corresponding wheel 20fr, 20fl, 20rr and 20rl.

[0077] Optionally, in some embodiments, each of the wheels is steered independently of other wheels, for example as would be the case when the wheels are each mounted onto a separate VCM, for example as described in PCT Application Publication No. WO2022 / 254382, filed June 2, 2022 and entitled “VEHICLE CORNER MODULES AND SYSTEMS AND METHODS FOR INSTALLATION THEREOF” which is incorporated by reference as if fully set forth herein.

[0078] Controlling of steering assemblies 32fr, 32fl, 32rr, and 32rl may be by steering subcontroller or control sub-unit associated with or located within each of steering assemblies 32fr, 32fl, 32rr, and 32rl. Controlling of steering assemblies 32fr, 32fl, 32rr, and 32rl may be by steering sub-controller or control sub-unit associated with or located within each of wheel assemblies 20fr, 20fl, 20rr, and 20rl.

[0079] As seen in Fig. IB, sensor 34 provides input to the central controller, which input is used by the sub-controller(s). Sensor 34 is illustrated as being disposed within both rear overhang sides of the vehicle. However, such sensors may be provided on one or any sides of the vehicle and / or on the rear end of the vehicle. Additionally, sensors 34 may be provided in the embodiment of Fig. 1A, and may provide input to the central controller or to the sub-controllers.

[0080] The steering system of vehicle 10 may include any or all of the mechanical and / or electrical components required for steering, i.e., pivoting the wheel(s) of the vehicle around a steering axis, including, and not exhaustively: a steering actuator, steering rods, steering system controller or control unit, steering inverter and wheel-angle sensor.

[0081] In some embodiments, vehicle-controller 18 receives steering instructions as electrical (including electronic) inputs from the vehicle, e.g., from a driver-operated steering mechanism or an autonomous steering unit, and carries out the instructions by causing, responsively to the received instructions, the motion of a steering rod, e.g., via a steering actuator, to effect the turning of the wheel(s), for example, by regulating a current and voltage transmitted to the steering actuator and / or transmitting high-level instructions to a steering-system controller. Such steering system may be a steer-by-wire (SBW) system, in which the steering instructions are transmitted electrically to the steering actuator.

[0082] In some embodiments, vehicle 10 may be a vehicle using Vehicle Comer Modules (VCMs), for example as described in PCT Application Publication No. WO2022 / 254382, filed June 2, 2022 and entitled “VEHICLE CORNER MODULES AND SYSTEMS AND METHODS FOR INSTALLATION THEREOF” which is incorporated by reference as if fully set forth herein.

[0083] Reference is now made to Figs. 2A and 2B, which are flow charts of exemplary methods of steering a vehicle according to embodiments of the disclosed technology, and to Fig. 3, which is a graphic representation of data flow during implementation of the method of Fig. 2B. The description of Figs. 2A to 3 is provided with respect to the system of Fig. 1A. However, it is to be appreciated that the methods of Figs. 2A and 2B and the data flow of Fig. 3 may be implemented using any suitable vehicle having a steering system which separates between steering of the front wheels and steering of the rear wheels, as described hereinabove.

[0084] The methods of Figures 2A and 2B assume that vehicle 10 has a current state, in which front wheels 20fl and 20fr are steered to a current front-axle steering-angle and rear wheels 20rl and 20rr are steered to a current rear-axle steering angle. A current steering -ratio is defined as a ratio between the current front-axle steering-angle and the current rear-axle steering angle.

[0085] At an initial step S100, a maximum allowed yaw acceleration, or an upper yaw acceleration threshold, is determined or obtained. This threshold may be dependent on parameters of the vehicle (e.g., vehicle center of gravity), of the wheels, or of the driver, for example provided as an initial input. In some embodiments, the maximum allowed yaw acceleration may be dependent on a mode of use of the vehicle (e.g., a comfort mode, a casual use mode, or a sports mode). In some embodiments, the maximum allowed yaw acceleration may also depend on the road conditions. Initial step SI 00 may be carried out once for multiple implementations of the method.

[0086] At step S102, a steering request is received, indicating that vehicle 10 should be driven in a target trajectory. One example of a target trajectory may be from a stationary state, e.g. out of a parking location, in which the vehicle is parked with the current front- and rear-axle steeringangles. Another example of a target traj ectory may be a dynamic state of a second traj ectory which is different than a first driving trajectory utilizing the current front- and rear-axle steering-angles. In some embodiments, the steering request may be triggered by a mechanical steering request, provided by an operator of the vehicle, e.g. mechanically turning a steering wheel, which results in the controller 18 or the steering-controller 33 receiving the steering request. In some embodiments, such as for example in autonomous vehicles, the steering request may be triggered by an electronic component, such as a sensor or camera providing input to controller 18, indicating that the vehicle must be steered in the target trajectory.

[0087] At step S104, an updated front-axle steering-angle is determined or computed, in accordance with the target trajectory. The updated front-axle steering-angle typically includes a magnitude of the angle and a direction of steering. The updated front-axle steering -angle may be defined in absolute terms (i.e., from a situation in which the front wheels 20fr and 20fl are parallel to the lateral sides of chassis 11), or may be defined relative to the current front-axle steeringangle, for example of a delta to be steered from the current angle to the updated angle. In some embodiments, the front-axle steering-angle is determined based on steering input provided by the driver.

[0088] At step S106, the current longitudinal speed, current longitudinal acceleration, and current yaw rate of the vehicle are obtained.

[0089] Subsequently, the target rear-axle steering-angle is to be computed. This computation can be carried out in different ways, one of which is shown in Fig. 2A and another of which is shown in Fig. 2B.

[0090] As seen in Fig. 2A, at step S108a, the target rear-axle steering -angle is computed directly. The computed rear-axle steering angle is computed based on the target trajectory received at step S102, and on the longitudinal speed of the vehicle obtained at step S106. However, in accordance with the disclosed technology, the target rear-axle steering-angle is not based on the current yaw rate of the vehicle. This ensures that the computation of the target rear-axle steering -angle can be a feed-forward computation, not depending on the current yaw rate or current acceleration.

[0091] The target rear-axle steering-angle includes a magnitude of the angle and a direction of steering. The target rear-axle steering-angle may be defined in absolute terms (i.e., from a situation in which the front wheels 20rl and 20rr are parallel to the lateral sides of chassis 11), or may be defined relative to the current rear-axle steering-angle, for example of a delta to be steered from the current angle to the updated angle.

[0092] At step SI 10a, a maximal rear-steering rate for reaching the target rear-axle steering -angle is determined. Stated differently, the determined value is a maximal allowed rate of change in the rear-axle steering angle. The maximal rear-steering rate ensures that the yaw acceleration of the vehicle remains within the yaw acceleration threshold obtained at step SI 00.

[0093] At an optional step SI 12a, an actual rear-axle steering -angle, which can be accomplished within the maximal rear-steering rate while the vehicle is moving at the current longitudinal speed and within a predetermined duration (e.g. one second), is computed based on the target rear-axle steering -angle. It is to be appreciated that the actual rear-axle steering-angle may be smaller than the target rear-axle steering -angle. For example, if the maximal rear-steering rate is a rate of 7 degrees per second, and the target rear-axle steering-angle is 10 degrees, the actual rear-axle steering -angle would reach 10 degrees over time such time rear steering rate never exceeds 7 degrees per second at any point in time, if the conditions of the vehicle haven’t changed. However, if the longitudinal velocity or longitudinal acceleration of the vehicle have changed in the meantime, it may not be necessary to steer the rear-axle to 10 degrees, because a newer target rearaxle steering-angle, computed based on the newer vehicle velocity, is different from the previously computed 10 degrees.

[0094] In some embodiments, the computing of the actual rear-axle steering -angle is based on the trajectory created by front wheels, such that rear-axle steering is computed to follow the same trajectory.

[0095] In some embodiments, the computing of actual rear-axle steering-angle is based on the computing of the target rear-axle steering angle, which is updated following a variable time delay, or periodically.

[0096] In some embodiments, the computing of the actual rear-axle steering-angle utilizes a differential equation of a steering angle difference and a distance travelled to compute the rearaxle steering -angle.

[0097] In some embodiments, the computing of the actual rear-axle steering-angle is based on four segment scheduling maps with steering overlay, where segments are created based on the front-axle steering-angle, and yaw rate gain or yaw error.

[0098] At step SI 14a, a suitable command is sent to the front axle, or to each of front wheels 20fr and 20fl, to steer the front wheels to the updated front-axle steering-angle computed at step S106. Similarly, a suitable command is sent to the rear axle, or to each of rear wheels 20rr and 20rl, to steer the rear wheels to the actual rear-axle steering-angle computed at step SI 12a. The steering of each of the front wheels and / or rear wheels may be based on Ackermann geometry. In embodiments in which step SI 12a was omitted, the command provided at step SI 14a steers the rear wheels to the target rear-axle steering-angle, without exceeding the maximal rear-steering rate. As such, the steering command may not result in reaching the target rear-axle steering -angle, but rather to the maximal rear-axle steering-angle possible given the maximal rear-steering rate.

[0099] Subsequently, the flow returns to step SI 02 for receiving a further steering request.

[0100] In some embodiments, steps S102 to S108a may be carried out in any desired order. As such, in some embodiments, the flow may return to step S104 or S106 prior to returning to step

[0101] S102. The flow of Fig. 2B differs from that of Fig. 2A in that the rear-axle steering-angle is computed based on a steering ratio between the front-axle steering-angle and the rear-axle steering-angle, rather than being computed directly. A change of this ratio is limited, or constrained, to ensure that the maximal allowed yaw acceleration is not exceeded.

[0102] As seen in Fig. 2B, at step S108b, a target steering ratio, defined between the front-axle steering-angle determined in step S104 and a target rear-axle steering-angle, is computed based on the target traj ectory received at step SI 02, and on the longitudinal speed of the vehicle obtained at step S106. However, in accordance with the disclosed technology, the target steering ratio is not based on the current yaw rate of the vehicle. This ensures that the computation of the target rear-axle steering-angle and / or of the target steering ratio can be a feed-forward computation, not depending on the current yaw rate or current acceleration.

[0103] At step SI 10b, a maximal allowed rate of change, in the target steering ratio, computed at step S108b, is determined, in order to ensure that the yaw acceleration of the vehicle remains within the yaw acceleration threshold obtained at step S100.

[0104] At step SI 11b, the target steering ratio of step S108b is constrained or limited, by the maximal allowed change in the target steering ratio determined at step SI 10b, to obtain a steering ratio threshold to be used during steering of the vehicle.

[0105] At step SI 12b, an actual rear-axle steering-angle, which can be accomplished while maintaining the steering ratio threshold given the front-axle steering angle, and while the vehicle is moving at the current longitudinal speed, is computed. It is to be appreciated that the actual rearaxle steering-angle may be smaller than the target rear-axle steering-angle, as explained hereinabove. As an example, the rear-axle steering-angle can be computed as the product of the front-axle steering angle and the target steering ratio, or as a signed portion of the front-axle steering angle.

[0106] At step SI 14b, a suitable command is sent to the front axle, or to each of front wheels 20fr and 20fl, to steer the front wheels to the updated front-axle steering-angle computed at step S104. Similarly, a suitable command is sent to the rear axle, or to each of rear wheels 20rr and 20rl, to steer the rear wheels to the actual rear-axle steering-angle computed at step SI 12b. The steering of the front and rear wheels uses Ackermann geometry. The command sent to each of the front and rear axles, or to each of the front and rear wheels, includes the target angle for steering (in relative or absolute terms), and the rate of steering to be used when rotating the rear wheels.

[0107] Subsequently, the flow returns to step S102 for receiving a further steering request, or to any one of steps S104, S106, or S108b, as explained hereinabove. It is to be appreciated that, in some embodiments, computation of the target rear-axle steering angle and of the actual rear-axle steering angle is independent of a sideslip angle of the vehicle.

[0108] Turning to Fig. 3, it is seen that arrows represent data, being collected or provided, and boxes represent computational steps. The data flow of Fig. 3 correlates to the method of Fig. 2B, as described herein.

[0109] As such, box 120 represents computation of the front-axle steering-angle, based on the steering request received as input 122, for example as described hereinabove with respect to step S104 of Fig. 2B. The output 124 of box 120 is a front-axle steering-angle, provided to box 126 for further processing prior to delivery to the wheels.

[0110] Box 130 receives the longitudinal speed of the vehicle as input 132, and computes a target steering ratio, as described with respect to step S108b of Fig. 2B, which is provided as output 133.

[0111] The longitudinal speed of the vehicle in input 132 is provided also to box 134. Box 134 further receives the longitudinal acceleration of the vehicle as input 136, the yaw rate of the vehicle as input 138. Box 134 is further aware of, or has reference to, the maximal allowed yaw acceleration. In some embodiments, the maximal allowed yaw acceleration is provided to box 134 as a further input. Box 134 computes a maximal allowed rate of change in the steering ratio, based on the longitudinal speed of the vehicle, while retaining the yaw acceleration within the allowed threshold, as described with respect to step SllOb above. This maximal allowed rate of change of the steering ratio is provided as output 142.

[0112] The target steering ratio of output 133 and the maximal allowed rate of change in the steering ratio of output 142 are provided to box 144, which computes a steering ratio to be used during steering of the vehicle, as described with respect to step SI 1 lb, which is provided as output 146.

[0113] The front-axle steering-angle of output 124, and the steering ratio of output 146, are provided to box 148, which computes an actual rear-axle steering-angle to be used, which is based on the front-axle steering-angle and ensures that the steering ratio is not exceeded, as described with respect to step SI 12b. This actual rear-axle steering angle is provided as output 150 to box 126, for further processing.

[0114] Box 126 provides, as its output 152, instructions for steering each of the front and rear axles, or each of the four wheels of the vehicle, based on the received computed front-axle and rear-axle steering angles, and based on the Ackermann geometry. These instructions are then used by the suitable controllers for actually steering each of the axles and / or wheels as required. It is a particular feature of the present invention that the methods of Fig. 2A-3 are methods of dynamic steering, which dynamically adjusting the steering of the vehicle, in real-time, to improve steering thereof and reduce jolts stemming from excessive yaw acceleration during steering. Such dynamic steering is advantageous for its increase in ease of use for the driver, while ensuring driving within the target trajectory. Without wishing to be bound by theory, the Inventors hypothesize that the behavior of the vehicle, using the methods of Figs. 2A-3, would be smoother and less jerky, and therefore would be easier for the driver to adjust to and to feel comfortable with, despite the steering being different from what the driver typically experiences.

[0115] Reference is now made to Fig. 4, which shows exemplary plots of wheel steering using the methods of the disclosed technology. Each of the plots shown in Fig. 4 lists the time in the X-axis, and all the plots relate to the same use and steering of the vehicle. The plots were generated using a suitable vehicle having four wheels, each of which can be steered independently. During the experiment, a driver held a steering wheel at a constant 250 degree angle, and accelerates the vehicle from rest to 30 kph. The experiment was carried out twice - with and without the method of the disclosed technology.

[0116] As seen in plot I, the vehicle starts out stationary, and at approximately 2.3 seconds from the start begins accelerating, at a relatively fixed rate, until passage of approximately 5.3 seconds.

[0117] Plot II compares the yaw acceleration of the vehicle when using prior art steering methods, indicated by a dashed line, to the yaw acceleration of the vehicle using the method of Figs. 2B and 3, indicated by a solid line. As seen, the yaw acceleration remains the same in both methods until approximately 3.75 seconds, and then diverges as the vehicle speed increases. When using prior art methods, the yaw acceleration has very high variance - ranging between -12 and +45 degrees per secondA2, and is quite erratic, which could cause discomfort to the driver. By contrast, when using the method of Figs. 2B and 3, the yaw acceleration is constrained, and ranges between 0 and +15 degrees per secondA2, and the curve representing the yaw acceleration is much smoother, indicating smoother motion of the vehicle.

[0118] Plot III compares the front and rear steering angles, when using prior art steering methods, indicated by dashed lines, and when using the method of Figs. 2B and 3, indicated by solid lines. As seen, the front steering angle is the same regardless of the steering method used, and is at approximately 0.22 radian (approximately 12.5 degrees). By contrast, the rear steering angle varies greatly when using the prior art steering methods and when using the method of the disclosed technology. When using prior art methods, the rear steering angle decreases from 0 to -0.18 radians (-10 degrees) within less than one second, and the rises back to 0 over the next second and a half. This leads to a very strong peak in the plot, indicative of strong and rapid changes to the rear steering angle, which could be felt by the driver and cause them discomfort. By contrast, when using the methods of the disclosed technology, the rear steering angle remains between 0 and - 0.05 radians (-3 degrees) throughout the entire drive, thereby making the driver’s experience smoother.

[0119] Plot IV compares the front to rear steering ratio, when using prior art steering methods, indicated by dashed lines, and when using the method of Figs. 2B and 3, indicated by solid lines. As seen, the shape of plot IV is substantially similar to that of the rear wheel steering in plot III, merely using different units for measurement.

[0120] In conclusion, the plots shown in Fig. 4 demonstrate how the method of the disclosed technology significantly reduces the changes to the yaw acceleration and the required steering angles, thereby leading to a smoother and more comfortable drive.

[0121] CONCLUDING COMMENT

[0122] All references cited herein are incorporated by reference in their entirety. Citation of a reference does not constitute an admission that the reference is prior art.

[0123] It is further noted that any of the embodiments described above may further include receiving, sending or storing instructions and / or data that implement the operations described above in conjunction with the figures upon a computer readable medium. Generally speaking, a computer readable medium (e.g. non-transitory medium) may include storage media or memory media such as magnetic or flash or optical media, e.g. disk or CD-ROM, volatile or non-volatile media such as RAM, ROM, etc.

[0124] Having thus described the foregoing exemplary embodiments it will be apparent to those skilled in the art that various equivalents, alterations, modifications, and improvements thereof are possible without departing from the scope and spirit of the claims as hereafter recited. In particular, different embodiments may include combinations of features other than those described herein. Accordingly, the claims are not limited to the foregoing discussion.

Claims

WHAT IS CLAIMED IS:

1. A method for steering wheels of a vehicle, the vehicle including a steering system associated at least one front wheel and at least one rear wheel of the vehicle, the steering system adapted to control steering of the at least one front wheel separately from steering of the at least one rear wheel, the method comprising: a. while the at least one front wheel is steered to a current front-axle steering-angle and the at least one rear wheel is steered to a current rear-axle steering angle, the current front-axle steering-angle and the current rear-axle steering-angle having a current steering-ratio therebetween, receiving a steering request to drive the vehicle in a target trajectory; b. determining an updated front-axle steering -angle based on the received target trajectory; c. obtaining a longitudinal speed, a longitudinal acceleration, and a yaw rate of the vehicle; d. computing a target rear-axle steering angle; e. determining a maximal rear-steering rate reflecting a maximal allowed rate of change in the rear-axle steering angle, such that yaw acceleration of the vehicle is within a pre-determined yaw acceleration threshold; and f. steering the at least one front wheel to the updated front-axle steering-angle and the at least one rear wheel to the updated rear-axle steering-angle, at a rate of change that is not greater than the maximal rear-steering rate.

2. The method of claim 1, wherein the determining at step b comprises determining the updated front-axle steering-angle to be defined in absolute terms.

3. The method of claim 1, wherein the determining at step b comprises determining the updated front-axle steering-angle to be defined relative to the current front-axle steering -angle.

4. The method of any one of claims 1 to 3, wherein the computing at step d comprises computing the target rear-axle steering angle based on the longitudinal speed of the vehicle.

5. The method of claim 4, wherein the computing at step d is based only on the longitudinal speed of the vehicle.

6. The method of any one of claims 1 to 5, wherein the computing at step d comprises computing the target rear-axle steering -angle to be defined in absolute terms.

7. The method of any one of claims 1 to 6, wherein the computing at step d comprises computing the target rear-axle steering-angle to be defined relative to the current rear-axle steering -angle.

8. The method of any one of claims 1 to 7, wherein the computing at step d comprises computing the target rear-axle steering angle independently of a current yaw rate of the vehicle.

9. The method of any one of claims 1 to 8, wherein the computing at step d comprises computing a target steering-ratio between the updated front-axle steering-angle and an updated rear-axle steering-angle, and wherein the determining at step e comprises determining, as the maximal rear-steering rate, an upper limit of the target steering-ratio to be within a computed maximal allowed change in the target steering-ratio with respect to a current steering-ratio, such that the yaw acceleration of the vehicle is within the pre-determined yaw acceleration threshold.

10. The method of any one of claims 1 to 9, further comprising computing an actual rear-axle steering-angle, based on the updated front-axle steering-angle, which can be accomplished within the maximal rear-steering rate, wherein the steering at step f comprises steering the at least one rear wheel to the actual rear-axle steering angle.

11. The method of claim 10, wherein die computing of ths actual rear-axle steering-angle is based on a steering angle difference and a distance travelled.

12. The method of any one of claims 1 to 11, further comprising, prior to step e, obtaining the pre-determined yaw acceleration threshold.

13. The method of any one of claims 1 to 12, wherein the vehicle includes two front wheels, and the steering at step f is in accordance with Ackermann geometry.

14. The method of any one of claims 1 to 13, wherein the vehicle includes two rear wheels, and the steering at step f is in accordance with Ackermann geometry.

15. The method of any one of claims 1 to 14, wherein the computing at step d is independent of a sideslip angle of the vehicle.

16. The method of any one of claims 10 to 14, wherein the computing of the actual rear-axle steering -angle is independent of a sideslip angle of the vehicle.

17. The method of any one of claims 1 to 16, wherein the determining of the maximal rearsteering rate comprises dynamically changing the maximal rear-steering rate and / or the rearsteering angle as the longitudinal speed of the vehicle changes.

18. A controller for control of a steering system of a vehicle, the steering system being functionally associated with at least one front wheel and at least one rear wheel of the vehicle, the steering system adapted to control steering of the at least one front wheel separately from steering of the at least one rear wheel, the controller comprising: a. one or more processors; and b. a non-transitory computer readable storage medium for instructions execution by the one or more processors, the non-transitory computer readable storage medium having stored:(i) instructions, to be carried out while the at least one front wheel is steered to a current front-axle steering-angle and the at least one rear wheel is steered to a current rearaxle steering angle, the current front-axle steering-angle and the current rear-axle steeringangle having a current steering-ratio therebetween, to receive a steering request to drive the vehicle in a target trajectory;(ii) instructions to determine an updated front-axle steering-angle based on the received target trajectory;(iii) instructions to obtain a longitudinal speed, a longitudinal acceleration, and a yaw rate of the vehicle;(iv) instructions to compute a target rear-axle steering angle;(v) instructions to determine a maximal rear-steering rate reflecting a maximal allowed rate of change in the rear-axle steering angle, such that yaw acceleration of the vehicle is within a pre-determined yaw acceleration threshold; and(vi) instructions to steer the at least one front wheel to the updated front-axle steeringangle and the at least one rear wheel to the updated rear-axle steering-angle, at a rate of change that is not greater than the maximal rear-steering rate.

19. The controller of claim 18, wherein the instructions to determine the updated front-axle steering-angle comprise instructions to determine the updated front-axle steering-angle to be defined in absolute terms.

20. The controller of claim 18, wherein the instructions to determine the updated front-axle steering-angle comprise instructions to determine the updated front-axle steering-angle to be defined relative to the current front-axle steering -angle.

21. The controller of any one of claims 18 to 20, wherein the instructions to compute the target rear-axle steering angle comprise instructions to compute the target rear-axle steering angle based on the longitudinal speed of the vehicle.

22. The controller of claim 21 , wherein the instructions to compute the target rear-axle steering angle comprise instructions to compute the target rear-axle steering angle based only on the longitudinal speed of the vehicle.

23. The controller of any one of claims 18 to 22, wherein the instructions to compute the target rear-axle steering angle comprise instructions to compute the target rear-axle steering angle to be defined in absolute terms.

24. The controller of any one of claims 18 to 22, wherein the instructions to compute the target rear-axle steering angle comprise instructions to compute the target rear-axle steering angle to be defined relative to the current rear-axle steering-angle.

25. The controller of any one of claims 18 to 24, wherein the instructions to compute the target rear-axle steering angle comprise instructions to compute the target rear-axle steering angle independently of a current yaw rate of the vehicle.

26. The controller of any one of claims 18 to 25, wherein the instructions to compute the target rear-axle steering angle comprise instructions to compute a target steering-ratio between the updated front-axle steering-angle and an updated rear-axle steering-angle, and wherein the instructions to determine the maximal rear-steering rate comprise instructions to determine, as the maximal rear-steering rate, an upper limit of the target steering-ratio to be within a computedmaximal allowed change in the target steering-ratio with respect to a current steering-ratio, such that the yaw acceleration of the vehicle is within the pre-determined yaw acceleration threshold.

27. The controller of any one of claims 18 to 26, wherein the non-transitory computer readable storage medium further has stored instructions to compute an actual rear-axle steering -angle, based on the updated front-axle steering-angle, which can be accomplished within the maximal rearsteering rate, and wherein the instructions to steer comprise instructions to steer the at least one rear wheel to the actual rear-axle steering angle.

28. The controller of claim 27, wherein the instructions to compute the actual rear-axle steering-angle comprise instructions to compute the actual rear-axle steering-angle based on a steering angle difference and a distance travelled.

29. The controller of any one of claims 18 to 28, wherein the non-transitory computer readable storage medium further has stored instructions to obtain the pre-determined yaw acceleration threshold.

30. The controller of any one of claims 18 to 29, wherein the vehicle includes two front wheels, and wherein the instructions to steer comprise instructions to steer the front wheels in accordance with Ackermann geometry.

31. The controller of any one of claims 18 to 30, wherein the vehicle includes two rear wheels, and wherein the instructions to steer comprise instructions to steer the rear wheels in accordance with Ackermann geometry.

32. The controller of any one of claims 18 to 31, wherein the instructions to compute the target rear-axle steering angle comprise instructions to compute the target rear-axle steering angle independently of a sideslip angle of the vehicle.

33. The controller of any one of claims 27 to 31 , wherein the instructions to compute the actual rear-axle steering-angle comprise instructions to compute the actual rear-axle steering-angle independently of a sideslip angle of the vehicle.

34. The controller of any one of claims 18 to 33, wherein the instructions to determine the maximal rear-steering rate comprise instructions to dynamically change the maximal rear-steering rate and / or the rear-steering angle as the longitudinal speed of the vehicle changes.

35. A system for control of steering of a vehicle having at least one front wheel and at least one rear wheel, the system comprising: a. a steering system functionally associated with the at least one front wheel and the at least one rear wheel, the steering system adapted to control steering of the at least one front wheel separately from steering of the at least one rear wheel; and b. the controller of any one of claims 18 to 34, functionally associated with the steering system.

36. A vehicle comprising: at least one front wheel; at least one rear wheel; and the system of claim 35, functionally associated with the at least one front wheel and the at least one rear wheel.

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