Method for determining a friction parameter which characterises a friction present in a steering system, steering system, and steering system comprising the vehicle
The method determines a friction parameter in steering systems to adapt the friction model and actuator inputs, ensuring consistent steering feel and improved performance across temperature and wear conditions.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- ROBERT BOSCH GMBH
- Filing Date
- 2025-11-12
- Publication Date
- 2026-05-21
AI Technical Summary
Existing steering systems fail to maintain consistent steering feel across varying operating conditions, particularly at low temperatures, leading to compromised performance.
A method to determine a friction parameter within a steering system by measuring hysteresis and adapting the steering system's friction model using a Kalman filter to maintain consistent steering feel, adjusting actuator inputs and damping behavior based on friction changes.
Ensures a consistent steering feel for users by compensating for friction variations due to temperature and wear, improving system performance and safety.
Smart Images
Figure EP2025082815_21052026_PF_FP_ABST
Abstract
Description
[0001] R.413430
[0002] - 1 -
[0003] Description
[0004] title
[0005] Method for determining a friction parameter that characterizes existing friction within a steering system, a steering system and a steering system encompassing the vehicle
[0006] State of the art
[0007] The present invention relates to a method for determining a friction parameter that characterizes existing friction within a steering system, a steering system and a vehicle comprising the steering system.
[0008] Parameters of steering software, which represent the steering feel for a user of a steering system, are applied under different operating conditions, particularly temperatures. The prevailing temperature has a significant influence on the friction within the steering system and thus also on the steering feel. This application demonstrates that under normal conditions, especially at normal temperatures, for example, in the range of 5 to 40°C, a desired steering feel can be achieved. At lower temperatures, for example, below -5°C or below -10°C, particularly during a cold start when a vehicle with a steering system has been standing outside overnight in low temperatures, the steering feel achieved by the aforementioned application no longer meets the desired requirements.Therefore, a compromise is made in the application of the steering feel, with the aim of achieving an acceptable steering feel at low temperatures by minimizing the reduction in steering feel at normal temperatures.
[0009] Therefore, a steering system is desirable that provides a consistent steering feel for the user, regardless of operating conditions. R.413430
[0010] - 2 -
[0011] Disclosure of the invention
[0012] This is achieved by a method, a steering system and a vehicle comprising the steering system according to the independent claims.
[0013] The method for determining a friction parameter that characterizes existing friction within a steering system comprises: acquiring a first value of a parameter of the steering system, wherein the parameter, for example a steering torque, influences a control variable of the steering system that characterizes a steering angle of the steering system, wherein the first value of the parameter is acquired before a change in the direction of variation of the control variable; detecting the change in the direction of variation of the control variable; acquiring a second value of the parameter, in particular one that is present, when the change in the direction of variation of the control variable is detected; determining a hysteresis value that characterizes existing hysteresis of the steering system, depending on the first value and the second value.in particular by subtracting the second value from the first value; determining an estimate of the existing hysteresis depending on a nominal friction within the steering system, which is determined by a friction model encompassed by the steering system; determining the friction parameter depending on the hysteresis value and the estimate, for example by subtracting the estimate from the hysteresis value. The friction within the steering system is essentially influenced by friction of a steering actuator, for example a rack and pinion actuator. Further influences on friction within the steering system can be caused, for example, by axle kinematics coupled to the steering system. In this context, the steering torque comprises a hand torque applied by a user of the steering system and an output torque applied by an actuator, for example a servo drive of the steering actuator.which is used to drive the steering system. The steering system's control variable influences the trajectory of a vehicle comprising the steering system. The control variable can be, for example, a steering angle to be set, a position of a rack, or a rotor position of the steering system's actuator. The phrase "before a change in the direction of variation" refers in particular to a point in time, R.413430.
[0014] - 3 -
[0015] In particular, the last point in time before the manipulated variable becomes static. This is the case, for example, when, mathematically speaking, the gradient of the manipulated variable reaches zero. If this gradient changes its sign, this indicates a change in the direction of variation of the manipulated variable. For example, in a hypothetical time series of the manipulated variable, the point in time before a change in the direction of variation is the point before the first time derivative of the series reaches zero, i.e., before the manipulated variable exhibits a slope over time. The change in the direction of variation is detected, for example, when the gradient of the manipulated variable, or of a quantity that is operatively related to the manipulated variable, especially a correlated quantity, changes its sign. This operative quantity could be, for example, a rotor position or rotor angle of the steering actuator, or a position of the rack.Changing the direction of variation of the manipulated variable means in particular a deflection followed by a subsequent steering input of the steering system, i.e. a change of steering direction.
[0016] Hysteresis describes, for example, a difference between the steering torque required when turning and the steering torque required when turning. In this context, "turning" refers specifically to a steering movement towards a neutral position. A neutral position is characterized, for example, by the fact that the vehicle, which includes the steering system, follows a straight-ahead trajectory. Due to axle kinematics, which, for example, represent a functional connection between the steering system and the vehicle's tracking elements, such as the wheels, the steering torque required when turning can be lower or higher than when turning. This difference can be described as hysteresis. For example, the axle kinematics may have a restoring force, which results in a lower steering torque required when turning than when turning.This effect can be described, for example, as restoring behavior. In this context, the hysteresis comprises a component attributable to the restoring force and a component attributable to friction within the steering system. The restoring force is essentially constant and known. However, for the application of the method, it does not necessarily need to be known. The hysteresis of a steering system is described, for example, in R.413430.
[0017] - 4 -
[0018] Within a nominal operating range of the steering system, the hysteresis can be determined and identified through tests or models. This determined hysteresis includes, for example, the contribution of the restoring force and the contribution of any applied nominal friction. A difference between the hysteresis value and the estimated value can therefore be attributed to a change in the frictional behavior or the friction within the steering system, for example, induced by a temperature change or wear. This method can determine and provide information on this change in the frictional behavior of the steering system.
[0019] The procedure may include adapting at least one parameter of the friction model depending on the determined friction characteristic, for example by means of a Kalman filter.
[0020] This adapts the steering system's friction model to the current state of the system. The adapted friction model improves the steering system, particularly the performance of model-based control algorithms. Furthermore, it reduces application effort.
[0021] It can be implemented that the estimated value of the existing hysteresis is determined based on nominal frictions, which are calculated by the friction model based on the first and second values. The first and second values serve as input variables, and in particular, a nominal friction value is determined for the first value and a nominal friction value for the second value. Based on these nominal frictions, the estimated value of the existing hysteresis can be determined. This allows for an efficient and effective estimation of the value.
[0022] It may be provided that the steering system includes a steering actuator and that a determined input variable of a steering controller regulating the steering actuator is adapted depending on the determined friction characteristic, in particular that it is corrected for any variation in friction within the steering system characterized by the friction characteristic. By correcting the input variable of the steering controller for the determined friction or variation in friction within the steering system, a target hand torque becomes friction-independent. This means, for example, that the R.413430
[0023] - 5 -
[0024] The steering feel for the user of the steering system remains constant even when friction within the steering system changes. Therefore, the user experiences no change in steering feel, particularly with changes in temperature.
[0025] It can be provided that, depending on the determined friction parameter, a return behavior caused by an external force acting on the steering system is adapted by means of an active return generated by a steering actuator. This makes the return behavior, perceived by the user, for example, independent of friction. The steering feel is thus improved. For example, increased friction within the steering system causes reduced return behavior, as the increased friction counteracts the return force and vice versa.
[0026] It can be provided that the drive torque of the steering actuator is scaled to adapt the return behavior, whereby the drive torque is increased with increasing friction within the steering system, as characterized by the friction parameter, and decreased with decreasing friction within the steering system. Thus, the return behavior can be influenced efficiently and effectively.
[0027] It can be designed so that, depending on the motion state of the vehicle comprising the steering system and depending on the friction characteristic, a damping behavior is adapted to a user's hand input to the steering system. This improves the steering feel for the user. For example, at high vehicle speeds, even the slightest movements by the driver should not lead to a change in the vehicle's trajectory. Especially at high speeds, this effect can be achieved, for example, by adapting the damping behavior, perhaps by means of a damping torque generated by the steering actuator. This makes the steering system safer.
[0028] The steering system, in particular an electronic power steering system, comprises a steering actuator, a steering controller, and a computing unit, the computing unit being configured to perform a method as described above. R.413430
[0029] - 6 -
[0030] The vehicle includes a steering system as described above.
[0031] Further embodiments are shown in the drawing and the following description. The drawing shows:
[0032] Figure 1 shows a flowchart of a procedure for determining a friction parameter that characterizes the friction present within a steering system;
[0033] Figure 2a shows the curve of a control variable of the steering system;
[0034] Figure 2b shows the curve of a characteristic parameter of the steering system;
[0035] Figure 3 shows a schematic representation of a vehicle comprising the steering system;
[0036] Figure 1 shows a flowchart of a method 100 for determining a friction parameter 18 that characterizes the friction present within a steering system 300. The friction parameter 18 can be expressed, for example, as a relative change or as an absolute value. The method 100 comprises acquiring 102 a first value 26 of a parameter 22 (Fig. 2b) of the steering system 300 (Fig. 3), wherein the parameter 22 influences a manipulated variable 20 (Fig. 2a) of the steering system 300. The parameter 22 is, for example, a steering torque of the steering system 300. The manipulated variable 20 characterizes a steering angle of the steering system 300 and is, for example, a rack position, a rotor position of an actuator, or a rotor angle of the actuator of the steering system 300. The actuator is, for example, a servo drive of the steering system 300.
[0037] The first value 26 of the parameter 22 is recorded before a change 30 (Fig. 2a) in the direction of variation of the manipulated variable 20. The change 30 is determined, for example, based on the direction of rotation of a rotor of the actuator by means of a rotor angle signal. The rotor angle signal describes, for example, a given rotor angle. R.413430
[0038] - 7 -
[0039] Method 100 comprises detecting 104 the change 30 in the direction of variation of the manipulated variable 20. For this purpose, the rotor angle signal is cleaned of measurement noise, for example, by means of a trailing filter. If the rotor angle signal no longer changes from a first time 24a (Fig. 2a) to a second time 24b (Fig. 2a), a value of the characteristic parameter 22, for example, the first value 26 of the first time 24a, is recorded, and the sign of a gradient of the rotor angle signal at the first time 24a is stored. The sign characterizes, in particular, a direction of rotation of the rotor. If the rotor angle signal changes from the second time 24b to a third time 24c, and the gradient of the rotor angle signal has a different sign than at the first time 24a, a change 30 in the direction of variation of the manipulated variable 20 occurs.
[0040] Method 100 comprises acquiring 106 a second value 28 of the parameter 22, in particular one that is present, when the change 30 in the direction of variation of the manipulated variable 20 is detected 104. In the example mentioned above, the second value 28 is acquired at the third time 24c. It can be provided that, in the case of a discrete signal of the manipulated variable 20 and the parameter 22, the first value 26 and the second value 28 are each acquired at intervals between which the change 30 of the manipulated variable 20 will have occurred. In particular, the first value 26 and the second value 28 are acquired at intervals that are closest to the detected change 30.
[0041] The method 100 comprises determining 108 a hysteresis value 14, which characterizes an existing hysteresis of the steering system 300, depending on the first value 26 and the second value 28, in particular by subtracting the second value 28 from the first value 26. For the subtraction, for example, the magnitudes of the first and second values 26, 28 can be used.
[0042] Method 100 comprises determining 110 an estimated value 16 of the existing hysteresis depending on a nominal friction within the steering system 300, which is determined by a friction model encompassing the steering system 300. The friction model models friction within the steering system. This friction can be influenced by various components of the steering system, such as a rack and pinion actuator R.413430.
[0043] - 8 -
[0044] including servo drive, as well as taking into account the influences of axis kinematics.
[0045] Parameters of the friction model are applied, for example, to a nominal state. The estimated value is determined specifically based on these applied parameters.
[0046] Method 100 comprises determining the friction parameter 18 as a function of the hysteresis value 14 and the estimated value 16, for example, by subtracting the estimated value 16 from the hysteresis value 14. Thus, for example, a deviation between a measured hysteresis determined from the first and second values 26, 28 and an estimated hysteresis is determined. This deviation is directly attributable to a variation in friction within the steering system, starting from the applied nominal state. The method therefore enables the determination of the existing friction or a change in friction within the steering system 300. The existing friction or the change in friction is characterized by the determined friction parameter 18.
[0047] It may be provided that the procedure Adapting 114 includes at least one parameter of the friction model depending on the determined friction characteristic 18, for example by means of a Kalman filter. This allows, for example, the application of the friction model or the steering system 300 to be updated and adapted to a given state of the steering system 300.
[0048] It can be provided that the estimated value 16 of the existing hysteresis is determined depending on nominal frictions, which are determined by the friction model depending on the first value 26 and the second value 28. For example, the estimated value 16 is determined depending on the first and second values 26, 28, in the form of steering torques, using the applied parameters of the friction model.
[0049] It can be provided that the steering system 300 includes a steering actuator 302 (Fig. 3) and a determined input variable 305 of a steering controller 306 (Fig. 3) controlling the steering actuator 302, depending on the determined R.413430
[0050] - 9 -
[0051] Friction parameter 18 is adapted, in particular to correct for variations in friction within the steering system 300 characterized by friction parameter 18. The steering actuator 302 can, for example, be a rack and pinion actuator with a servo drive for controlling a rack. The input variable is, for example, a determined rack force. This is corrected for the determined friction within the steering system 300 to obtain a friction- and temperature-independent rack force as an input signal for the steering controller 306. This is done by subtracting the torque caused by the determined friction from the total torque. By adapting the input signal for the steering controller, a target steering torque remains independent of friction.
[0052] It may be provided that, depending on the determined friction characteristic (18), a restoring behavior caused by an external force acting on the steering system (300) is adapted by means of an active return generated by a steering actuator 302.
[0053] It may be provided that, in order to adapt the return behavior, a drive torque of the steering actuator 302 is scaled, wherein the drive torque is increased in particular with increasing friction within the steering system 300 as characterized by the friction parameter 18 and is decreased with decreasing friction within the steering system 300.
[0054] It can be provided that, depending on the movement state of a vehicle 400 comprising the steering system 300 (Fig. 3) and depending on the friction characteristic 18, a damping behavior is adapted for a hand input of a user of the steering system 300.
[0055] Figures 2a and 2b show the curves of the manipulated variable 20 and the characteristic variable 22 over time 24. The change 30 of the variation symbolizes, for example, a deflection followed by a steering input. The steering input is characterized, for example, by a steering torque that has the opposite sign to a steering torque during deflection. This is illustrated, for example, in Figure 2b. The magnitude of the difference shown between the first value 26 and the second value 28, for example in the form of R.413430
[0056] - 10 -
[0057] Steering torques, characterized by the measured hysteresis or the hysteresis value 14.
[0058] Figure 3 shows a schematic representation of a vehicle 400 comprising the steering system 300. The steering system 300 is specifically designed as an electronic power steering system and includes the steering actuator 302, the steering controller 306, and a computing unit 200. The steering actuator 302 is, for example, designed as a rack and pinion actuator and comprises a servo drive 308 and a rack 310. The steering system 300 includes, for example, a steering hand control 312 by means of which the user of the steering system 300 can make hand inputs. The steering hand control 312 is coupled to the rack 310 by means of a steering column 314. Thus, the hand input directly causes a movement of the rack, which in turn moves the servo drive, or rather the rotor of the servo drive 308, and vice versa.
[0059] The vehicle 400 comprises wheels 402 which are operatively connected to the rack 310 via an axle, so that a movement of the rack 310 influences a position of the wheels 402 and thus a travel trajectory of the vehicle 400.
[0060] The computing unit 200 is configured to execute the procedure 100 according to the above descriptions. The computing unit 200 can be implemented centrally or decentrally, or embedded in a control unit.
Claims
R.413430 - 11 - Claims 1. Method (100) for determining a friction parameter (18) that characterizes existing friction within a steering system (300), comprising: Acquiring (102) a first value (26) of a characteristic parameter (22) of the steering system (300), wherein the characteristic parameter (22), for example a steering torque, influences a manipulated parameter (20) of the steering system (300) that characterizes a steering angle of the steering system (300), wherein the first value (26) of the characteristic parameter (22) is acquired (102) before a change (30) of a variation direction of the manipulated parameter (20); detecting (104) the change (30) of the variation direction of the manipulated parameter (20); Detect (106) a second value (28) of the parameter (22), in particular if the change (30) of the direction of variation of the manipulated variable (20) is detected (104); Determine (108) a hysteresis value (14) that characterizes any existing hysteresis of the steering system (300) depending on the first value (26) and the second value (28), in particular by subtracting the second value (28) from the first value (26); Determining (110) an estimated value (16) of the existing hysteresis depending on a nominal friction within the steering system (300) determined by a friction model encompassing the steering system (300); Determine (112) the friction parameter (18) depending on the hysteresis value (14) and the estimated value (16), for example by subtracting the estimated value (16) from the hysteresis value (14).
2. The method (100) according to claim 1, comprising: - Adapting (114) at least one parameter of the friction model depending on the determined friction characteristic (18), for example by means of a Kalman filter. R.413430 - 12 - 3. The method (100) according to one of the preceding claims, wherein the estimated value (16) of the present hysteresis is determined depending on nominal frictions which are determined by the friction model depending on the first value (26) and second value (28).
4. The method (100) according to one of the preceding claims, wherein the steering system (300) comprises a steering actuator (302) and a determined input variable (305) of a steering controller (306) controlling the steering actuator (302) is adapted depending on the determined friction characteristic (18), in particular to eliminate a variation of friction within the steering system (300) characterized by the friction characteristic (18).
5. The method (100) according to one of the preceding claims, wherein, depending on the determined friction characteristic (18), a restoring behavior caused by an external force acting on the steering system (300) is adapted by means of an active return generated by a steering actuator (302).
6. The method (100) according to claim 5, wherein a drive torque of the steering actuator (302) is scaled to adapt the return behavior, wherein the drive torque is increased in particular with increasing friction within the steering system (300) as characterized by the friction parameter (18) and is decreased with decreasing friction within the steering system (300).
7. The method (100) according to one of the preceding claims, wherein, depending on the friction characteristic (18) and, for example, depending on a state of motion of a vehicle (400) comprising the steering system (300), a damping behavior is adapted for a hand input of a user of the steering system (300).
8. Steering system (300), in particular an electronic power steering system, comprising a steering actuator (302), a steering controller (306) and a computing device (200), wherein the computing device (200) is configured to perform a method (100) according to claims 1 to 7. R.413430 - 13 - 9. Vehicle (400) comprising a steering system (300) according to claim 8.