Vehicle steering system control system

Abstract

A control system (104) for a steering system (101) of a vehicle (100) includes one or more controllers that receive (600) a vehicle speed signal indicative of a current vehicle speed, receive (602) a steering input signal output corresponding to a requested steering angle of the steering wheel, determine from the received vehicle speed signal whether the current vehicle speed has reached zero, and output (606) a control signal to control the steering angle of the steered road wheels (103, 106) to rotate towards a straight ahead condition if the current vehicle speed remains zero and is within a time T that has elapsed since the current vehicle speed reached zero.

Description

[Technical field]

[0001] The present invention relates to a control system for a steering system of a vehicle, a steering system, a vehicle, and a method for controlling a steering system of a vehicle. [Background technology]

[0002] Some vehicle steering systems are electronically controlled, meaning there is no mechanical link between steering demand and steering output. Such systems are known as "steer-by-wire" systems. One example of a steer-by-wire is a rear-wheel steering vehicle. Such steering systems are known to provide different benefits to the vehicle depending on the vehicle speed. At high vehicle speeds, the rear wheels are steered in phase with the front wheels, promoting vehicle stability. At low vehicle speeds, the rear wheels are steered out of phase with the front wheels, improving maneuverability.

[0003] Turning the wheels while the vehicle is stationary (known as dry steering) can have undesirable consequences, such as excessive tire wear and increased load on the steering actuator. Furthermore, dry steering may not be possible if there is too much friction between the wheels and the surface they are resting on or if the vehicle mass is large.

[0004] One approach is to prevent the wheels from turning below a certain threshold speed. The drawback of this approach is that the vehicle loses the maneuverability benefits provided by rear-wheel steering (RWS) systems at low speeds. Summary of the Invention

[0005] It is an object of the present invention to address one or more of the shortcomings associated with the prior art.

[0006] Aspects of the present invention relate to a control system for a steering system of a vehicle, a steering system, a vehicle, and a method for controlling a steering system of a vehicle.

[0007] According to one aspect of the present invention, there is provided a control system for a steering system of a vehicle, the control system comprising one or more controllers, configured to: receive a vehicle speed signal indicative of a current vehicle speed, receive a steering input signal corresponding to a requested steering angle of a steered wheel, determine from the received vehicle speed signal a time when the current vehicle speed reaches zero, and output a control signal for controlling a steering angle of the steered wheel such that the steered wheel is controlled to rotate towards a straight ahead condition if the current vehicle speed remains at zero and the elapsed time since the current vehicle speed reached zero is within a time period T.

[0008] The present invention provides a control system operative to return the steered wheels of a vehicle toward a straight ahead condition, provided that the control system is within a time period T from the time the vehicle speed reaches zero. The control system further receives a steering input signal corresponding to a steering angle requested by a driver. During the time period T, the steered wheels may be steered toward the requested steering angle so long as the requested steering angle directs the steered wheels toward the straight ahead condition. If the requested steering angle is away from the straight ahead condition, the control system may ignore the received steering input signal output.

[0009] The one or more controllers may collectively include: at least one electronic processor configured to access and execute instructions thereon to determine if a current vehicle speed has reached zero, and an electrical output configured to output a control signal to a steering actuator of a steering system.

[0010] The control system may be configured to output a control signal to control the steering angle of the steering wheels to rotate the wheels towards the requested steering angle if the current vehicle speed remains at zero, the time since the current vehicle speed reached zero is within the time period T, and the requested steering angle is closer to a straight ahead condition than the current steering angle. Thus, if the time condition is met and the requested angle is closer to a straight ahead condition than the current wheel position, the control system attempts to steer towards the requested angle. If the requested steering angle is beyond the straight ahead condition, the control system may be configured to rotate the steering wheels towards the requested steering angle, but stop the rotation control signal once the wheels reach the straight ahead condition. If the requested steering angle is further away from the straight ahead condition than the current steering angle, the control system may be configured to simply hold the current wheel position. Once the end of the time period T is reached, the control system may be configured to hold the steering wheels in the position reached. The straight ahead condition referred to here is understood to mean a position where the steering wheels are pointing straight ahead, and thus in normal use would command the vehicle to continue in the current direction / heading.

[0011] The steering system may be a rear wheel steering system.

[0012] The time period T may be a predetermined time value stored in the memory of one or more controllers. Additionally, the time period T comprises an adjustable time value. Thus, the time period during which the vehicle control system operates to return the steering wheels to a straight ahead state may vary depending on, for example, the terrain setting, the environment in which the vehicle is located, user preferences, etc. If the vehicle comprises an electric vehicle, the time period T may be adjusted depending on the state of charge of the vehicle, for example, the higher the state of charge, the longer the time period and the lower the state of charge, the shorter the time period T may be.

[0013] Conveniently, in one embodiment, the time period T may comprise a time period of zero to two seconds starting from the time the current vehicle speed reaches zero.

[0014] At the end of the time period T, the control system may be configured to stop outputting the control signal and hold the steering angle at its time=T value. If the displacement of the steered wheels is large when the current vehicle speed reaches zero, this means that the steered wheels may not return to a straight ahead state by the end of the time period T. In other words, if the angular displacement of the wheels is large at the end of the time period T, the steered wheels may not return to the aforementioned straight ahead state.

[0015] The control system may further be configured to receive a drive mode signal (commonly referred to as a "driver mode") indicative of a drive mode of the vehicle, and not output a control signal depending on the drive mode signal. When the vehicle is in a particular drive mode, for example an "emergency home" mode, the control system may be configured to not output a control signal for controlling the steering angle of the steered wheels.

[0016] According to a further aspect of the present invention there is provided a steering system comprising a control system according to the above aspect. The steering system may comprise a steering actuator receiving a control signal output from the control system.

[0017] The invention extends to a vehicle comprising a steering system comprising a control system according to the above aspects. The vehicle may be a rear-wheel steer vehicle or an all-wheel steer vehicle.

[0018] According to a further aspect of the present invention, there is provided a method of controlling a steering system of a vehicle, comprising the steps of receiving a vehicle speed signal indicative of a current vehicle speed, receiving a steering input signal corresponding to a requested steering angle of the steered wheels, determining from the received vehicle speed signal a time when the current vehicle speed becomes zero, and if the current vehicle speed remains zero and is within a time period T since the current vehicle speed became zero, controlling the steering angle of the steered wheels so that the steered wheels are rotated towards a straight ahead condition.

[0019] The invention extends to a non-transitory computer readable medium comprising computer readable instructions which, when executed by a processor, cause the performance of a method according to the above aspect of the invention.

[0020] It is expressly contemplated that within the scope of this application, the various aspects, embodiments, examples and alternatives described in the preceding paragraphs, claims and / or the following description and drawings, in particular their individual features, may be employed independently or in any combination. Features described in relation to one embodiment are applicable to all embodiments, except where such features are incompatible. [Brief description of the drawings]

[0021] One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings.

[0022] [Figure 1] FIG. 1 shows a top view of a vehicle embodying the present invention.

[0023] [Diagram 2] FIG. 2 shows a top view of another vehicle embodying the invention.

[0024] [Diagram 3] FIG. 3 is a block diagram illustrating a system that enables steering of the vehicle of FIGS.

[0025] [Figure 4] FIG. 4 is a plan view of a vehicle traveling at a relatively high speed.

[0026] [Diagram 5] FIG. 5 is a plan view of a vehicle traveling at a relatively low speed.

[0027] [Figure 6] FIG. 6 illustrates a method according to an embodiment of the present invention.

[0028] [Figure 7] FIG. 7 illustrates the method of FIG. 6 in more detail. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] A vehicle 100 embodying the invention is shown in a top view in Figure 1. Vehicle 100 is a vehicle configured for use on various types of terrain, both road and off road. In this embodiment, vehicle 100 is a four-wheel drive vehicle, although it will be understood that many of the features of vehicle 100 described below are also applicable to front-wheel drive or rear-wheel drive vehicles.

[0030] 1 also shows diagrammatically a steering system 101 arranged to enable steering of the vehicle 100. The system 101 includes an actuator 102 arranged to effect steering of rear road wheels 103 of the vehicle 100, and also includes a control system 104 including control means in the form of a controller 105 for controlling the operation of the actuator 102.

[0031] In this embodiment, the front road wheels 106 of the vehicle 100 are steered by a mechanism 107 that includes a steering wheel 108 connected to a pinion 109 via a steering column 110. The pinion 109 engages a rack 111 that is connected to a steering knuckle 112 by a tie rod 113.

[0032] The rear road wheels 103 are steerable by a mechanism 114 actuated by an actuator 102. In this embodiment, the actuator 102 is arranged to drive a second pinion 115 associated with a second rack 116 which applies a force to a steering knuckle 117 of the rear road wheels 103 via a tie rod 118.

[0033] The steering input or position sensor 119 is configured to sense the orientation of the steering wheel 108 and provide a signal to the controller 105 indicative of the orientation of the steering wheel 108, and therefore of the front road wheels 106. The controller 105 is configured to provide an output signal to the actuator 102 to steer the rear road wheels 103 in response to the signal received from the steering input sensor 119. However, the output signal provided to the actuator 102 also depends on other signals received by the controller 105, as will be explained in more detail below. It is noted that the steering input signal output from the position sensor corresponds to a driver-demanded steering angle of the steered road wheels (103, 106) of the vehicle 100.

[0034] In FIG. 2, an alternative vehicle 100 embodying the present invention is shown, in which a system 101 allows for "steer-by-wire" of all road wheels 103, 106 of the vehicle 100. The vehicle 100 of FIG. 2 has much in common with the vehicle 100 of FIG. 1, and is labeled with the same reference numerals. Thus, like the vehicle 100 of FIG. 1, the vehicle 100 of FIG. 2 comprises a steering system 101 comprising a pinion 109 and a rack 112 arranged to actuate a steering knuckle 112 via a tie rod 113 to steer the front road wheels 106. A first actuator 102 is arranged to drive a second pinion 115 associated with a second rack 116 which applies a force to a steering knuckle 117 of the rear road wheels 103 via a tie rod 118.

[0035] 2, however, the pinion 109 for driving the front road wheels 106 is driven by a second actuator 202. The steering wheel 108 is mounted on a rotatable shaft 201 but is not mechanically connected to the pinion 109. Instead, in addition to providing a signal to the actuator 102 for causing steering of the rear road wheels 103, the controller 105 is also configured to provide a signal to the second actuator 202 for causing steering of the front road wheels 106 in dependence on a signal received from a steering input sensor 119 located on the shaft 201 of the steering wheel 108.

[0036] In an alternative embodiment, the vehicle 100 has steer-by-wire front wheels similar to that of FIG. 2, but the rear road wheels 103 are non-steerable.

[0037] The steering system 101 of FIG. 1 and the steering system 101 of FIG. 2 are illustrated by the block diagram shown in FIG. 3. FIG. 3 also illustrates some exemplary vehicle systems that may be in communication with the steering system 101. The control system 104 comprises a controller 105 that includes an electronic processor 301 and an electronic memory device 302 that stores instructions 303 that cause the processor 301 to execute the methods described below and output signals to the first steering actuator 102 to steer the rear road wheels 103. In the case of the vehicle 100 of FIG. 2, the processor 301 also provides signals to the second steering actuator 202 to steer the front road wheels 106. Although only one controller, processor, and memory device are illustrated in FIG. 3, it will be understood that the control system 104 may comprise multiple controllers 105, and each controller 105 may comprise multiple processors 301 and / or multiple electronic memory devices 302, such that the processing described below may be distributed across multiple processors.

[0038] As well as receiving signals from the steering input sensor 119, the control system 104 also receives signals from wheel speed sensing means 304 indicative of the rotational speed of each road wheel 103, 106. The wheel speed sensing means 304 may consist of wheel speed sensors, each arranged to measure the rotational speed of a respective one of the road wheels 103, 106 and provide the rotational speed values ​​directly to the controller 105. Alternatively, the wheel speed sensors may form part of another system, such as an anti-lock braking system (not shown), comprising a control unit arranged to receive signals from the wheel speed sensors and provide wheel speed values ​​to the controller 105.

[0039] Figures 4 and 5 show plan views of a vehicle 100 traveling at a relatively high speed and a relatively low speed, respectively. In both Figures 4 and 5, the front road wheels 106 are rotated about 15 degrees relative to the longitudinal axis 1001 of the vehicle 100, causing the vehicle 100 to turn left. In Figure 4, the current speed of the vehicle 100, as determined from the wheel speed sensing means 304, exceeds a threshold speed, and as a result, the rear road wheels 103 are steered in phase with the front road wheels 106. That is, the front road wheels 106 are being rotated to the left, so the rear road wheels 103 are also being rotated to the left. As is well known, steering the rear road wheels 103 in phase with the front road wheels 106 increases the stability of the vehicle 100, which is advantageous when traveling at high speeds.

[0040] In Figure 4, the rear road wheels 103 are only steered to the left by about 1.5 degrees, i.e. one tenth of the angle turned by the front road wheels 106. The ratio of the steering angle of the rear road wheels 103 to the steering angle of the front road wheels is referred to herein as the gain value. Thus, in this example the gain value for the steering of the rear road wheels is +0.1 (=1.5 / 15).

[0041] 5, the current speed of the vehicle 100 is below a threshold speed, which results in the rear road wheels 103 being steered out of phase with the front road wheels 106. That is, the rear road wheels 103 are turning to the right because the front road wheels 106 are turning to the left. Stability of the vehicle 100 is not an issue at low speeds, and as is well known, steering the rear road wheels 103 out of phase with the front road wheels 106 makes the vehicle 100 more agile.

[0042] The rear road wheels 103 are steered to the right by approximately 3 degrees, i.e. one fifth of the angle turned by the front road wheels 106. Thus, in this example the gain value for the steering of the rear road wheels is -0.2 (=-3 / 15), i.e. although the absolute value of the gain value (0.2) is higher than the gain value for speeds above the threshold speed, the gain value is negative because the rear road wheels 103 are rotating out of phase with the front road wheels 106.

[0043] Depending on the driver's driving style and the type of terrain on which the vehicle 100 is traveling, a particular set of vehicle characteristics ("drive mode") may be most appropriate. For example, a particular accelerator pedal map may be more appropriate than others, as well as a particular transmission map and a particular set of stability control settings. To allow the user to select the settings that best suit the selected driving style or particular terrain, the vehicle 100 also includes a user input device (UID) 311 configured to allow the user to indicate the selected drive mode to the vehicle control system 310. For example, when traveling on a paved road, the user may select a standard mode (or normal mode), and the vehicle control system 310 controls the ECU 307, the TCU 308, and the SCU 309 to operate in a mode that is appropriate for the surface of the paved road. Alternatively, the user may select another mode, such as a grass, gravel, or snow mode for traveling on terrain with a low coefficient of friction, or a sand mode for traveling on a deformable surface such as sand that provides a very low coefficient of friction, or a rock driving mode for traveling on rough surfaces with a high coefficient of friction. In response to such user instructions, the vehicle control system 310 controls the ECU 307, TCU 308, and SCU 309 to operate in a mode appropriate for the indicated type of terrain. The mode selected through use of the user input device 311 may also be provided to the controller 105 and used to determine the signals provided to the first steering actuator 102 and / or the second steering actuator 202.

[0044] User input device 311 may consist of a switch or set of switches, a touch screen device, or any other electrical or electronic device suitable for allowing a user to provide an indication of the mode they wish to select.

[0045] The vehicle control system 310 may include a terrain estimation system (TES) 306. Such systems are known and described in the applicant's published UK patent GB2492655B and US patent application US2014350789A1. The terrain estimation system 310 is configured to select a drive mode that is the most appropriate mode for the subsystems 307, 308, 309 based on measurements indicative of the terrain over which the vehicle 100 is traveling, and enables the vehicle control system 310 to automatically control the subsystems 307, 308, 309 to operate in the selected mode.

[0046] The TES 306 receives signals from a terrain sensing means 312, which is made up of a variety of different sensors and devices, to provide information indicative of the type of surface the vehicle 100 is traveling on. The terrain sensing means 312 may include the IMU 305, the wheel speed sensing means 304, the steering input sensor 119, and other sensors (not shown), such as an ambient temperature sensor, an atmospheric pressure sensor, an engine torque sensor, a brake pedal position sensor, an accelerator pedal position sensor, a vehicle height sensor, and the like. The various outputs from the terrain sensing means 312 are used by the terrain estimation system 310 to derive a number of terrain indicators. For example, the vehicle speed is derived from the wheel speed sensors, the wheel acceleration is derived from the wheel speed sensors, the longitudinal forces on the wheels are derived from the IMU 305, and the torque at which wheel slip occurs (if wheel slip occurs) is derived by detecting yaw, pitch, and roll from the motion sensors of the IMU 305. The terrain indicators are then processed to determine the likelihood that each of the different drive modes is appropriate, thereby determining the most appropriate mode for operation of the subsystem. In the automatic mode, the terrain estimation system 310 continuously determines, for each mode, the probability that it is appropriate, and depending on which alternative mode has a consistently higher probability than the currently selected control mode, the vehicle control system 310 commands the subsystems to operate according to that alternative mode.

[0047] The mode determined automatically by the terrain estimation system 306 or selected through use of the user input device 311 may also be provided to the controller 105 and used to determine the signals provided to the first steering actuator 102 and / or the second steering actuator 202.

[0048] The first steering actuator 102 is operable to provide sufficient torque to turn the rear road wheels 103 of the vehicle 100 at low and high speeds, as previously described in relation to Figures 3 and 4.

[0049] As the vehicle 100 slows down, the rear road wheels 103 may be returned to a straight ahead condition depending on the determined time value at which the vehicle speed reaches zero. A control system 104 for performing such operations is described in the applicant's UK patent GB1809351.8.

[0050] In some driving situations, such as rapid deceleration when the vehicle is approaching an intersection, the road wheels 103, 106 may not return completely to a straight state even when the vehicle speed reaches zero. In other driving situations, the road wheels may or may not have returned to a straight state when the vehicle speed reaches zero, but the driver turns the steering wheel 108, resulting in a steering input signal being received by the control system 104 from the steering input sensor 119.

[0051] As described in relation to Figures 6 and 7, according to an embodiment of the present invention, the control system operates to output a control signal for controlling the steering angle of the steering wheel in accordance with the logic flow and conditions defined below.

[0052] Referring to FIG. 6, a method for controlling a steering system 101 of a vehicle 100 according to an embodiment of the present invention is shown.

[0053] In step 600, the controller 105 in the control system 104 receives a vehicle speed signal from the wheel speed sensing means 304. The vehicle speed signal indicates the current speed of the vehicle 100.

[0054] In step 602, the controller 105 receives a steering input signal output from a steering input sensor 119 which senses the orientation of the steering wheel 108. The signal received from the sensor 119 is indicative of the orientation of the steering wheel 108 and therefore the orientation of the front road wheels 106. The position of the actuator 102 relative to the rear road wheels 103 indicates the angle of the rear road wheels 103, with an actuator position of 0 mm corresponding to a straight ahead condition.

[0055] It should be noted that the signals received by the controller 105 in steps 600 and 602 above are received sequentially, and the numbering of the various steps does not indicate an order of the steps.

[0056] In step 604, the controller 105 determines whether the current vehicle speed has reached zero, and in step 606, if the current vehicle speed remains at zero and the time since the current speed reached zero is within the time period T, a control signal is output from the control system 104 to the actuator (102, 202) to control the steering angle of the steered road wheels (103, 106) to rotate the steering wheel towards a straight ahead condition.

[0057] The processing sequence within the controller 105 corresponding to steps 604 and 606 above is shown in more detail in FIG.

[0058] In step 700 , the controller 105 performs a wheel speed check based on the wheel speed signals received from the wheel speed sensing means 304 .

[0059] In step 702, the controller 105 determines from the vehicle speed check in step 700 that the current vehicle speed has reached zero.

[0060] In step 704, the controller 105 monitors how long the state of the current vehicle speed being zero has continued, and in step 706, the controller compares the required steering angle of the steerable road wheels (103, 106) with the actual position of the steering wheel (for example, the actuator positions 102, 202 or the position determined from the steering input sensor 119).

[0061] Thereafter, at decision point 708, the controller 105 checks whether the condition (current speed = zero, time (T) that the speed is zero is less than the time period T) is true.

[0062] If the condition T < T is not satisfied, the controller 105 controls in step 710 to hold the steering wheel at the current position (that is, the control signal output by the control system 104 maintains the steering wheel at the current position).

[0063] If the condition T < T is satisfied, the controller 105 proceeds to the next decision point 712, where the controller checks whether the required steering angle of the steerable road wheels (103, 106) is closer to the straight-ahead position than the current steering angle position.

[0064] If the required steering angle is further away from the straight-ahead position than the current steering angle, the controller 105 controls in step 714 to hold the steering wheel at the current position (that is, the steering wheel is maintained at the current position by the control signal output by the control system 104).

[0065] If the requested steering angle is closer to a straight ahead position than the current steering angle, the controller 105 controls the steering wheel to rotate toward a straight ahead state (i.e., the control signal output by the control system 104 controls the steering wheel to rotate toward a straight ahead state) in step 716. This straight ahead state may correspond to an actuator position of 0 mm.

[0066] It should be noted that the control signals output by the control system 104 and / or the controller 105 control the actuators (102, 202) to rotate the steering wheel.

[0067] The control system 104 continues to monitor both the current vehicle speed and the time since the vehicle speed reached zero. If either condition is no longer met, the control system 104 and / or the controller 105 will stop outputting control signals to the actuator (102, 202) configured to rotate the steering wheel and output control signals to hold the steering wheel in its position (e.g., if the time T exceeds the time period T) or control the steering wheel according to the selected driving mode (e.g., if the vehicle speed is no longer zero). The time period T is stored in the memory 302 of the controller 105 and may be an adjustable value that can be changed by the vehicle manufacturer, the vehicle service entity, or even the driver.

[0068] If the vehicle is an electric vehicle, the control of rotating the road wheels when the vehicle is stationary may depend on the state of charge of the vehicle. For example, the time period T may be set to a relatively longer period when the vehicle is fully charged or the state of charge is above a certain threshold than when the state of charge of the vehicle is low. The control system 104 may be configured to vary the time period T depending on the received state of charge of the vehicle. In one preferred embodiment, the time period T may include a period from T=0 seconds to T=2 seconds from the time the vehicle speed reaches zero.

[0069] As described above, the driving mode, either determined automatically by the terrain estimation system 306 or selected through use of the user input device 311, may be provided to the controller 105 and used to determine the signals provided to the first steering actuator 102 and / or the second steering actuator 202. In particular, in certain driving modes (e.g., some off-road driving modes), the control system 104 may not output a control signal even if the logical conditions described above in relation to FIG.

[0070] A numerical summary of FIG. 3 is shown in Table 1. A numerical summary of FIG. 6 is shown in Table 2. A numerical summary of FIG. 7 is shown in Table 3.

[0071] [Table 1]

[0072] [Table 2]

[0073] [Table 3]

[0074] The electronic processor 301 may be comprised of any suitable electronic processor (e.g., microprocessor, microcontroller, ASIC, etc.) configured to execute electronic instructions. The electronic memory device 302 may be comprised of any suitable memory device and may store therein or thereon various data, information, thresholds, look-up tables or other data structures, and / or instructions. In one embodiment, the memory device 302 stores information and instructions, such as software, firmware, programs, algorithms, scripts, applications, etc., that may control all or a portion of the methodologies described herein. The processor, or each electronic processor 301, may access the memory device 302 and execute and / or use the instructions and information therein to execute or implement some or all of the functions and methodologies described herein.

[0075] At least one memory device 302 includes a computer-readable storage medium (e.g., non-transitory or non-transitory storage medium) and may include any mechanism for storing information in a form readable by a machine or electronic processor / computing device, including, but not limited to, magnetic storage media (e.g., floppy disks), optical storage media (e.g., CD-ROM), magneto-optical storage media, read-only memory (ROM), random access memory (RAM), erasable and programmable memory (e.g., EPROM and EEPROM), flash memory, or any other type of medium for storing such information / instructions.

[0076] The exemplary controller 105 comprises at least one electronic processor 301 configured to execute electronic instructions stored in at least one memory device 302 that, when executed, cause the electronic processor(s) 301 to perform the methods described above. However, the present invention is not limited to being implemented by programmable processing devices, and at least some, and in some embodiments all, of the functions and / or method steps of the present invention may likewise be implemented by non-programmable hardware, such as non-programmable ASICs, Boolean logic circuits, etc.

[0077] It will be understood that various changes and modifications can be made to the present invention without departing from the scope of the present invention.

Claims

1. 1. A control system for a steering system of a vehicle, comprising: one or more controllers; receiving a vehicle speed signal indicative of a current vehicle speed; receiving a steering input signal corresponding to a requested steering angle of the steered wheels; determining from said received vehicle speed signal when a current vehicle speed becomes zero; if the current vehicle speed remains zero and the time since the current vehicle speed became zero is within a predetermined time T, outputting a control signal to control a steering angle of the steered wheels to rotate toward a straight ahead condition.

2. the one or more controllers: at least one electronic processor configured to access and execute instructions of at least one electronic memory device to determine when the current vehicle speed has reached zero; 10. The control system of claim 1, further comprising: an electrical output configured to output a control signal to a steering actuator of the steering system.

3. The control system includes:

3. The control system of claim 1, further configured to output a control signal for controlling a steering angle of the steered wheels to rotate toward the requested steering angle when the current vehicle speed remains zero, the time since the current vehicle speed reached zero is within a predetermined time T, and the requested steering angle is closer to a straight ahead state than the current steering angle.

4. 4. A control system according to claim 1, wherein the steering system is a rear wheel steering system.

5. A control system according to claim 1 , wherein the predetermined time T is a predetermined time value stored in a memory of the one or more controllers.

6. The control system of claim 1 , wherein the predetermined time T comprises an adjustable time value.

7. The vehicle is an electric vehicle, 7. The control system of claim 6, wherein the predetermined time T is adjusted depending on the state of charge of the vehicle.

8. 6. The control system of claim 1, wherein the predetermined time T comprises a time from zero to two seconds starting from when the current vehicle speed reaches zero.

9. 9. A control system according to claim 1, wherein after the predetermined time T, the control system is configured to stop outputting the control signal and to hold the steering angle at the value of the predetermined time T.

10. The control system comprises: A drive mode signal indicating a drive mode of the vehicle is received. The control system according to claim 1 , configured not to output the control signal depending on the drive mode signal.

11. A steering system comprising a control system according to any one of claims 1 to 10 and a steering actuator.

12. A vehicle comprising a control system according to any one of claims 1 to 10 or a steering system according to claim 11.

13. 13. A vehicle as claimed in claim 12, wherein the vehicle is a rear road wheel steering vehicle or a rear and front road wheel steering vehicle.

14. 1. A method of controlling a steering system of a vehicle, comprising: receiving a vehicle speed signal indicative of a current vehicle speed; receiving a steering input signal corresponding to a requested steering angle of a steered wheel; determining from the received vehicle speed signal when the current vehicle speed reaches zero; If the current vehicle speed remains zero and the time since the current vehicle speed reached zero is within a predetermined time T, the steering angle of the steered wheels is controlled to rotate toward a straight ahead condition.

15. A non-transitory computer readable medium comprising computer readable instructions that, when executed by a processor, cause performance of the method of claim 14.

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