Generating a catch-up engine torque in a steer-by-wire system

Abstract

Steer-by-wire steering system, comprehensive: a steering wheel actuator (10) that provides a commanded position for a road wheel actuator (20) of a vehicle (100); the road wheel actuator (20), which moves a rack of the vehicle (100) into a rack position based on the position commanded by the steering wheel actuator (10); and a controller designed to: to generate a steering wheel torque command, wherein the steering wheel actuator (10) generates a feedback torque based on the steering wheel torque command, to calculate a following error based on a difference between the commanded position and the rack position, to determine a catch-up engine torque value using the trailing error; and to modify the steering wheel torque command using the catch-up motor torque, wherein the steering wheel actuator (10) generates a torque amount which is essentially a sum of the feedback torque and the catch-up motor torque, wherein the controller sends the steering wheel torque command to the steering wheel actuator (10) which generates the torque amount at a steering wheel.

Description

[0001] The present invention relates to a steer-by-wire steering system, a method for a steer-by-wire steering system and a computer program product. BACKGROUND

[0002] An electric power steering (EPS) system typically includes components such as the steering wheel, steering column, rack and pinion gear, electric motor actuator, etc. The EPS assists the driver in steering a vehicle by providing the necessary torque assistance. This assistance torque is based on a torque applied by the driver. In steady-state conditions, the driver's torque and the assistance torque act against the rack and pinion force generated by the interaction between the tires and the road.

[0003] A typical steer-by-wire (SbW) system includes a wheel actuator unit (RWA) and a steering wheel actuator unit (HWA). Unlike an EPS, the two units in an SbW system are mechanically separate and communicate via a CAN (Controlled Area Network) interface (or other similar digital communication protocols). The HWA receives a rack force signal from the RWA unit to generate a corresponding torque sensation for the driver. Alternatively, the steering wheel angle and vehicle speed can also be used to generate the desired torque sensation for the driver. The angle signal from the HWA unit is sent to the RWA unit, which performs position control to regulate the rack travel.

[0004] US 2003 / 0055546 A1 discloses a steer-by-wire control system comprising a master control unit that receives a steering wheel position signal, a road wheel force signal, a feedback torque sensor signal, and a vehicle speed signal and uses them to generate and output a steering wheel torque command signal. US 2007 / 0144814 A1 teaches a steering response system in which a torque applied to the steering wheel is determined based on a position sensor signal and a vehicle rotation sensor signal. DE 10205632 A1 describes a method for steering a motor vehicle in which a steering wheel torque is determined as a function of the difference between a rotation angle Δ VA,ist of a valve actuator, i.e., an angle measured by a rotary angle sensor on the valve actuator, and a detected pinion angle Δ Ritzel,istis regulated. DE 10 2017 115 850 A1 relates to a method for controlling a steer-by-wire steering system in which a feedback signal is issued to the steering input device in response to the driver's request and a driving condition of the motor vehicle. SUMMARY

[0005] It is an object of the invention to provide an improved steer-by-wire steering system as well as a method and computer program product for a corresponding one.

[0006] This task is solved by the subject matter of independent claims. Advantageous further developments are the subject matter of dependent claims.

[0007] According to one or more embodiments, a steer-by-wire steering system includes a steering wheel actuator that provides a commanded position to a road wheel actuator of a vehicle. The steer-by-wire steering system further includes the road wheel actuator, which moves a rack of the vehicle to a rack position based on the position commanded by the steering wheel actuator. The steer-by-wire steering system further includes a controller that generates a steering wheel torque command, wherein the steering wheel actuator generates a feedback torque based on the steering wheel torque command.

[0008] The controller further calculates a following error based on a difference between the commanded position and the rack position. Using this following error, the controller also determines a catch-up motor torque value. The controller further modifies the steering wheel torque command using the catch-up motor torque, with the steering wheel actuator generating a torque value that is essentially the sum of the feedback torque and the catch-up motor torque. The controller sends the steering wheel torque command to the steering wheel actuator, which then generates the torque value at the steering wheel.

[0009] According to one or more embodiments, a method for a steer-by-wire steering system includes transmitting a commanded position by a steering wheel actuator to be received by a road wheel actuator. The method further includes positioning a rack by the road wheel actuator to a rack position based on the commanded position. The method further includes generating a feedback torque by the steering wheel actuator based on a steering wheel torque command, wherein the feedback torque corresponds to one or more forces acting on the rack. The method further includes calculating a following error by a controller based on a difference between the commanded position and the rack position. The method further includes determining a catch-up motor torque value by the controller using the following error.The method further includes modifying the steering wheel torque command using the catch-up motor torque, wherein the steering wheel actuator generates a torque amount that is essentially a sum of the feedback torque and the catch-up motor torque, wherein the steering wheel torque command is sent to the steering wheel actuator, and wherein the torque amount is generated by the steering wheel actuator at a steering wheel.

[0010] According to one or more embodiments, a computer program product includes a storage device containing computer-executable instructions, wherein the computer-executable instructions, when executed by one or more processing units, cause the processing units to adjust a feedback torque generated by a steer-by-wire steering system. Adjusting the feedback torque involves generating the feedback torque based on a steering wheel torque command by a steering wheel actuator, wherein the feedback torque corresponds to one or more forces acting on a rack of a vehicle. Adjusting the feedback torque further involves calculating a following error based on a difference between a position commanded by the steering wheel actuator and a rack position.Adjusting the feedback torque further involves determining a catch-up motor torque value using the following error. Adjusting the feedback torque further involves modifying the steering wheel torque command using the catch-up motor torque, causing the steering wheel actuator to produce a torque amount that is essentially the sum of the feedback torque and the catch-up motor torque. Adjusting the feedback torque further involves sending the steering wheel torque command to the steering wheel actuator, which produces the torque amount at a steering wheel.

[0011] These and other advantages and features will become clearer from the following description in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig.Figure 1 is an exemplary embodiment of a steer-by-wire steering system according to one or more embodiments; Fig. Figure 2 presents a flowchart of an exemplary method for generating a catch-up motor torque according to one or more embodiments; and Fig. Figure 3 illustrates the “dead zone” used in an example scenario according to one or more embodiments. DETAILED DESCRIPTION

[0012] The subject matter, which is considered the invention, is specifically described and detailed in the claims at the end of the description. The foregoing and other features and advantages of the invention will become apparent from the following detailed description in conjunction with the accompanying drawings.

[0013] The terms module and submodule, as used herein, refer to one or more processing circuits such as an application-specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory executing one or more software or firmware programs, a combinational logic circuit, and / or other suitable components that provide the described functionality. As will be noted, the submodules described below can be combined and / or further subdivided.

[0014] The present application relates generally to steering systems and describes in particular one or more techniques for an SbW system to provide synchronous responses to steering maneuvers performed by a vehicle driver.

[0015] Referring now to the figures in which the invention is described with reference to specific embodiments, without limiting them, it is evident in Fig. Figure 1 shows a SbW system 40 in a vehicle 100. It should be noted that the SbW system 40 shown and described can be used in an autonomous or semi-autonomous vehicle or in a more conventional vehicle. It is understood that the SbW system 40 shown is an exemplary block diagram and that the SbW system 40 may include additional components beyond those shown here in one or more embodiments. The SbW system 40 includes a steering wheel actuator (HWA) 10 and a road wheel actuator (RWA) 20.

[0016] The HWA 10 includes one or more mechanical components of a physical system 12, such as a steering wheel (handwheel), a steering column, and a motor / inverter, which is attached to the steering column either via a gearbox or a direct drive system. The HWA 10 also includes an HWA controller 14, e.g., a microcontroller, which controls the operation of the mechanical components of the physical system 12. The HWA controller 14 receives and / or generates torque via the one or more mechanical components of the physical system 12.

[0017] The RWA includes one or more mechanical components of a physical system 24, such as a rack and / or pinion, which are coupled to a motor / inverter via a ball nut / ball screw assembly (gearbox), and the rack is connected to the vehicle's road wheels / tires via tie rods. Accordingly, the physical system 24 can include one or more road wheels of the vehicle 100. The RWA 20 includes an RWA controller 22, e.g., a microcontroller, which controls the operation of the mechanical components of the physical system 24. The RWA controller 22 receives and / or generates torque via the one or more mechanical components of the physical system 24.

[0018] The HWA controller 14 and the RWA controller 22 are coupled via electrical connections that enable the sending and receiving of signals. As mentioned herein, a controller can comprise a combination of the HWA controller 14 and the RWA controller 22, or one of the dedicated microcontrollers.

[0019] In one or more examples, the HWA controller 14 and the RWA controller 22 of the SbW system 40 communicate with each other via a CAN interface (or other similar digital communication protocols). Steering of the vehicle 100, equipped with the SbW system 40, is accomplished using the steering gear with an input shaft rotated by the RWA 20, such as a servo actuator. The RWA 20 receives an electronic communication signal indicating the rotation of the steering wheel by the driver. The driver steers the steering wheel to control the direction of the vehicle 100. The angle from the HWA 10 is sent to the RWA 20, which performs position control to direct a rack and pinion movement to steer the road wheel. However, due to the lack of a mechanical connection between the steering wheel and the road wheels, the driver receives no feel for the road (unlike with an EPS, as previously described) without torque feedback.

[0020] In one or more examples, the HWA 10, which is coupled to the steering column and steering wheel, simulates the driver's driving sensation on the road. The HWA 10 can apply tactile feedback in the form of torque to the steering wheel. The HWA 10 receives a rack and pinion force signal from the RWA 20 to generate a corresponding torque sensation for the driver. Alternatively, the steering wheel angle and vehicle speed can also be used to generate the desired torque sensation for the driver.

[0021] The HWA 10 and RWA 20 typically feature a "steering ratio" that determines how much the road wheel changes position when the steering wheel of the SbW system 40 is turned. In a steering system with a mechanical linkage between the steering wheel and the road wheel, a gearbox maintains this ratio. In the SbW system 40, the steering ratio is a preset value. However, in the SbW system 40, it is possible that the desired steering ratio will not always be maintained. Several common situations can cause this. Examples include exceeding the speed capacity of the RWA 20 during rapid driver input, entering a steering wheel angle that requires the road wheel to move beyond its lock position, overloading the RWA 20 (above a preset maximum threshold), initialization problems, and the like.

[0022] Failure to achieve the desired steering ratio has a number of undesirable effects on vehicle 100. Probably the most significant negative effect is that the SbW system 40 continues to attempt to move the road wheel according to the desired steering ratio after the driver has ceased steering. This results in a significant delay in the response of vehicle 100 and can create the impression that vehicle 100 has continued to steer itself.

[0023] Since the two systems, the HWA 10 and the RWA 20, are not mechanically linked, a delay between them can lead to a discrepancy between the driver's desired steering angle and the actual steering angle at the road wheels. This discrepancy can be caused by the driver controlling the HWA 10 faster than the RWA 20's ability to steer the road wheels, or it can occur due to a malfunction in one or both systems. When such a condition occurs, a delay in the vehicle's response is generated; that is, the vehicle does not follow the operator's input. This can cause the operator to feel "disconnected" from the road. The technical solutions described herein address these technical problems and make it possible to keep the operator's inputs synchronized with the vehicle's response.

[0024] In hydraulic power steering (HPS) and electric power steering (EPS) systems, the driver is mechanically connected to the vehicle's wheels, unlike in the SbW system. When the HPS or EPS system can no longer provide sufficient assistance to keep pace with the driver's inputs, the steering effort increases significantly. This is typically referred to as a "catch" or "catch-up" state, where the driver's inputs exceed the capabilities of the assistance system. The state in which this "catch-up" occurs depends on the steering speed and the load. The load depends on many factors, such as vehicle speed, payload, tire pressure, the coefficient of friction of the road surface (µ), and so on.

[0025] Existing techniques for addressing such technical problems include SbW systems that simulate the "catch" state by slowing down the operator through the programming of a damping term into the HWA 10. While this can slow down the driver's inputs, it does not represent actual driving conditions or system capacity. One or more embodiments of the technical solutions described herein overcome these shortcomings in existing techniques and accordingly enable improved generation of a catch-up motor torque to simulate the catch-up state in the SbW system 40.

[0026] Fig.Figure 2 presents a flowchart of a method for generating a catch-up motor torque for a SbW system according to one or more embodiments. The method includes, at 210, receiving a commanded position from the HWA 10. The commanded position is based on an input torque provided by the operator via the mechanical component of the physical system 12, for example, via the steering wheel. The commanded position is an indicator of a desired position of the rack (physical system 24) based on a steering ratio between the HWA 10 and the RWA 20.

[0027] The procedure at 220 also includes receiving the actual position of the rack. The actual position can be measured with a sensor. The actual position may deviate from the commanded position for various reasons described herein, such as a physical limit to where the rack can be positioned, a physical limit to the steering wheel, etc.

[0028] The procedure in section 230 involves calculating a following error using the commanded position and the actual position. In one or more examples, the following error is calculated by determining the difference between the commanded position and the actual position.

[0029] The procedure further includes, at 240, generating a steering wheel torque command according to the forces acting on the rack and on other components of the physical system 24. As already described, the steering wheel torque command provides the feedback due to the road surface or another physical force acting on the physical system 24.

[0030] The following error is compared to a predefined threshold at 250. If the following error is less than (or equal to) the predefined threshold, the steering wheel torque command is sent to the HWA 10 at 260, which generates a corresponding feedback torque amount for the operator. The feedback torque increases the resistance the operator experiences when maneuvering a steering wheel or other type of directional input device of the vehicle 100.

[0031] If the following error exceeds the specified threshold at 250, the catch-up motor torque is calculated at 270 instead. The catch-up motor torque is calculated based on the following error. In one or more examples, a value or magnitude of the catch-up motor torque to be generated is determined using a lookup table (Table 1). It is understood that the values ​​and units shown in Table 1 are exemplary and that the values ​​may differ in other examples. The lookup table may be based on a predefined relationship between the following error and the catch-up motor torque. In one or more examples, this relationship may be non-linear. In one or more examples, the catch-up motor torque is calculated dynamically based on a function that represents the ratio between the catch-up motor torque and the following error.The dynamic calculation uses the calculated following error as an input parameter. Following error (mm) 0 1 2 5 7 Catch-up motor torque (HwNm) 0 1 2 6 10

[0032] In one or more examples, the following error is compared with the threshold for generating the recovery motor torque to apply a "dead zone" to ensure that the SbW system 40 does not react to relatively small deviations in the following correlation between the HWA 10 and the RWA 20. The specified threshold determines how small the deviations can be before the steering wheel torque command is modified. Fig.Figure 3 illustrates the applied "dead zone" in an example scenario. Here, the following error in the range of + / - 0.5 does not cause a catch-up motor torque to be generated (range 310 in the diagram). For all other values ​​of the following error, a corresponding catch-up motor torque is generated, in this case via a linear function of the following error. It is understood that the function may be different and / or non-linear in other examples. Accordingly, the catch-up motor torque is determined in response to the following error being within a predefined range of values. The function can also be dynamic, i.e., its input can also include time. A specific example is the use of a low-pass filter in conjunction with a linear / non-linear static function. The low-pass filter can reduce the noise in the catch-up motor torque caused by noise in the following error.

[0033] The catch-up motor torque added to the steering wheel torque command presents the operator with increased resistance when maneuvering the SbW system 40. Consequently, the operator must exert more effort to maneuver the SbW system 40 due to the catch-up motor torque compared to the effort required to overcome the feedback torque, which corresponds to one or more forces acting on the vehicle's rack. The catch-up motor torque is limited to a predetermined maximum value to restrict the additional effort required from the operator.

[0034] With reference to Fig.In section 280, the procedure further involves adding the calculated catch-up motor torque to the feedback torque that the HWA 10 must generate. Accordingly, the steering wheel torque command is modified to add a catch-up motor torque command that is generated solely based on the catch-up motor torque value.

[0035] The modified steering wheel torque command is then sent to the HWA 10 at 260 to generate the feedback torque.

[0036] The above method can be implemented by the HWA controller 14 of the HWA 10 or by the RWA controller 22 of the RWA 20. Alternatively or additionally, the HWA controller 14 and the RWA controller 22 can implement the method for generating the catch-up motor torque by cooperation.

[0037] Alternatively or additionally, a separate catch-up motor torque module 50 generates (see Fig.1) The catch-up motor torque. The catch-up motor torque module 50 can be coupled with the HWA controller 14 and / or the RWA controller 22 to receive one or more input values ​​for generating the catch-up motor torque.

[0038] It should be noted that, while the description herein uses a position of the rack to explain one or more embodiments, the position used may in other embodiments be a road wheel position or a position associated with any other component controlled by the RWA 20.

[0039] One or more embodiments of the technical solutions described herein allow the operator inputs to a SbW system to be slowed down only under the conditions in which the RWA cannot maintain the position commanded by the HWA. There are a number of conditions that can cause the RWA to fail to keep pace with the commanded position, as described herein; regardless of the cause, the end result is an increase in the road wheel position tracking error. The road wheel tracking error is the difference between a target road wheel position and the actual position of the road wheel. The technical solutions described herein utilize the road wheel tracking error to trigger the simulation of the "catch-up effect."

[0040] One or more embodiments of the technical solutions described herein enable the operator to apply additional steering force when a steering wheel in a SbW system "overtakes" the road wheel. The technical solutions described herein simulate the missing mechanical connection between the steering wheel and the road wheel in an SbW system.

[0041] Although the technical solutions have been described in detail in connection with only a limited number of embodiments, it should be readily apparent that the technical solutions are not limited to these disclosed embodiments. Rather, the technical solutions can be modified to incorporate any number of variations, changes, substitutions, or equivalent arrangements not yet described, but which are consistent with the spirit and scope of the technical solutions. Furthermore, although various embodiments of the technical solutions have been described, it is understandable that aspects of the technical solutions may only include some of the described embodiments. Accordingly, the technical solutions should not be considered limited to the foregoing description.

Claims

[1] Steer-by-wire steering system, comprising: a steering wheel actuator (10) that provides a commanded position for a road wheel actuator (20) of a vehicle (100); the road wheel actuator (20), which moves a rack of the vehicle (100) into a rack position based on the position commanded by the steering wheel actuator (10); and a controller designed to: to generate a steering wheel torque command, wherein the steering wheel actuator (10) generates a feedback torque based on the steering wheel torque command, to calculate a following error based on a difference between the commanded position and the rack position, to determine a catch-up engine torque value using the trailing error; and to modify the steering wheel torque command using the catch-up motor torque, wherein the steering wheel actuator (10) generates a torque amount which is essentially a sum of the feedback torque and the catch-up motor torque, wherein the controller sends the steering wheel torque command to the steering wheel actuator (10) which generates the torque amount at a steering wheel. [2] Steer-by-wire steering system according to claim 1, characterized by , that the controller determines the recovery motor torque in response to the following error exceeding a predetermined threshold. [3] Steer-by-wire steering system according to claim 1, characterized by , that the controller determines the catch-up motor torque in response to the fact that the following error lies within a predetermined range of values. [4] Steer-by-wire steering system according to claim 1, characterized by, that the recovery motor torque is determined using the drag error based on a lookup table. [5] Steer-by-wire steering system according to claim 1, characterized by , that the recovery engine torque causes an operator of the steer-by-wire steering system to provide an increased effort to maneuver the steer-by-wire steering system. [6] Steer-by-wire steering system according to claim 1, characterized by , that the recovery motor torque is limited to a predetermined maximum value. [7] Steer-by-wire steering system according to claim 1, characterized by , that the controller is part of the steering wheel actuator (10). [8] Steer-by-wire steering system according to claim 1, characterized by , that the controller is part of the road wheel actuator (20). [9] Method for a steer-by-wire steering system, the method comprising: Sending a commanded position by a steering wheel actuator (10) to be received by a road wheel actuator (20); Positioning a rack by the road wheel actuator (20) into a rack position based on the commanded position; Generating a feedback torque based on a steering wheel torque command by the steering wheel actuator (10), wherein the feedback torque corresponds to one or more forces acting on the rack; Calculating a following error by a controller based on a difference between the commanded position and the rack position; Determining a catch-up motor torque value by the controller using the following error; and Modifying the steering wheel torque command using the catch-up motor torque, wherein the steering wheel actuator (10) generates a torque amount which is essentially a sum of the feedback torque and the catch-up motor torque, wherein the steering wheel torque command is sent to the steering wheel actuator (10), wherein the torque amount is generated by the steering wheel actuator (10) at a steering wheel. [10] Method according to claim 9, characterized by , that the controller determines the recovery motor torque in response to the following error exceeding a predetermined threshold. [11] Method according to claim 9, characterized by , that the recovery motor torque is determined using the drag error based on a lookup table. [12] Method according to claim 9, characterized by, that the recovery motor torque causes an operator of the steer-by-wire system to provide an increased effort to maneuver the steer-by-wire system. [13] Method according to claim 9, characterized by , that the recovery motor torque is limited to a predetermined maximum value. [14] Computer program product comprising a storage device with computer-executable instructions stored therein, wherein the computer-executable instructions, when executed by one or more processing units, cause the processing units to adjust a feedback torque generated by a steer-by-wire steering system, wherein the adjustment comprises: Generating the feedback torque by a steering wheel actuator (10) based on a steering wheel torque command, wherein the feedback torque corresponds to one or more forces acting on a rack of a vehicle (100); Calculating a following error based on a difference between a position commanded by the steering wheel actuator (10) and a rack position; Determining a catch-up motor torque value using the trailing error; Modifying the steering wheel torque command using the catch-up motor torque, thereby causing the steering wheel actuator (10) to generate a torque amount that is essentially the sum of the feedback torque and the catch-up motor torque; and Sending the steering wheel torque command to the steering wheel actuator, which generates the torque amount at a steering wheel.

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