Force feedback steering unit for a motor vehicle and motor vehicle

Abstract

The invention relates to a force-feedback steering unit for a motor vehicle, comprising a steering actuator, a control unit, an electric drive unit, and a brake unit, wherein the steering actuator can be operated by an operator to input a steering command, a steering resistance torque can be generated by means of the electric drive unit and / or the brake unit to provide the operator with a driving feel, and an actual base friction torque of the brake unit effective when operating the steering actuator is subject to random change. The invention further relates to a motor vehicle.

Description

The invention relates to a force-feedback steering unit for a motor vehicle for conveying a driving sensation to the operator. The invention further relates to a motor vehicle with such a force-feedback steering unit. Common force feedback steering systems for motor vehicles typically consist of a drive motor and a brake to generate steering resistance for the operator at a steering actuator, such as a steering wheel, by applying appropriate torques from the drive motor and / or the brake unit. In addition to force feedback steering systems with only a drive unit, systems with both a drive unit and a brake unit are frequently used to allow for a smaller and therefore more cost-effective drive unit design. DE 10 2008 021 848 A1 discloses a method and a system for taking into account static and dynamic friction in a system subject to friction, in particular a steering system. For this purpose, a friction model is created that describes the dependence of the frictional force on the speed of at least one moving part and is stored in the system. To update the friction level, at a defined speed, in particular v = 0, the friction stored in the model is compared with the actually measured friction, whereby their difference is determined as the current friction offset. This offset is then added to the model-based friction at different speeds to compensate for aging and wear effects of the system. DE 10 2004 001 764 A1 discloses a method for operating an electronic steering system in which a steering value for at least one front wheel is first determined and transmitted as a corresponding signal to a wheel system with at least one motorized actuator. A control algorithm defines a target torque value to which a friction compensation value is added to compensate for the impairment of the steering value caused by mechanical friction. The corrected values ​​are then used to control the front wheels, resulting in a more precise implementation of the desired steering movement. Korean patent application KR 10-2019-0066995 A describes a method for generating a steering torque return sensation in a steer-by-wire system, in which a reaction motor and a magnetorheological brake (MR brake) are controlled together. A control unit acquires steering and vehicle speed information, uses this to determine a target reaction torque for the motor – which remains identical for forward and reverse steering – and additionally determines a target friction torque for the MR brake. The motor torque is increased proportionally to the vehicle speed and depending on the steering angle, with a characteristic curve that is initially steep and then becomes flatter, while the friction torque decreases from a midpoint value with increasing steering angle, also with two different slopes.By simultaneously controlling the motor and the MR brake, the total restoring torque follows predefined characteristic curves, resulting in hysteresis between the steering and restoring movements. This approach is intended to improve the subjective steering feel and increase stability when the steering wheel is free, without requiring a mechanical connection between the steering wheel and the wheels. In particular, the braking unit, for example an electrically actuated magnetic brake or a passive Coulomb friction element, can be subject to fluctuations depending on factors such as ambient temperature or weather conditions, causing the basic frictional torque of the braking unit to change randomly. This results in an uneven and artificial steering feel for the operator. The purpose of the invention is to improve the state of the art. The problem is solved by a force-feedback steering unit for a motor vehicle according to claim 1 and by a motor vehicle according to claim 10. Advantageous embodiments of the invention are listed in the dependent claims. A key concept of the invention is that the steering actuator is driven by the electric drive unit, so that the randomly varying actual frictional torque is effective. The actual frictional torque of the brake unit is then determined by a measuring unit that records it while the steering actuator is being driven. An additional torque can then be calculated, which is necessary to compensate for the actual frictional torque by superimposing it on the electric drive unit. This compensates for the torque in the direction of the target frictional torque of the brake unit, particularly to keep this torque as constant as possible for the operator and thus create the most realistic steering feel. This compensates for the random variation in the actual frictional torque of the force-feedback steering unit caused by the applied additional torque.The following terms are explained in this context: A "force feedback steering unit" describes, in particular, an electrically driven steering unit, such as those used in motor vehicles, where corresponding steering inputs are transmitted via electrical signals to, for example, a drive motor of the vehicle's steering system. The force feedback steering unit can simulate steering forces adapted to the respective driving situation and represent them to the operator as if the operator were driving a mechanically steered vehicle. For this purpose, a "steering actuator," such as a steering wheel or another actuator like a steering yoke, a control lever, or a similar device, is provided, at which the operator, for example, a driver, makes the corresponding steering inputs.A "control unit" takes over corresponding control tasks of the force feedback steering unit and can, for example, be used to control the respective components of the steering unit and also perform additional control tasks. An "electric drive unit" serves to apply torques to the steering actuator by means of an electric drive, while a "brake unit" serves as a mechanical brake to apply corresponding torques to the steering actuator. By means of the electric drive unit and / or the brake unit, a "steering resistance torque", i.e. a torque which is directly perceptible and noticeable to the operator, can be generated to convey a driving sensation. The brake unit exhibits a "basic friction torque" which is determined by its internal design. This basic friction torque can be perceived as the "actual basic friction torque," meaning the real, existing basic friction torque that is subject to random changes. For example, this actual basic friction torque fluctuates depending on external conditions such as temperature or humidity, so the operator initially perceives a different actual basic friction torque, for instance, when starting to drive. In contrast, a "target basic friction torque" is the basic friction torque that should be present, for example, as a constant basic friction torque, so that the operator perceives a correspondingly smooth steering feel. The steering actuator is driven by the electric drive unit, for example by a predetermined drive current, so that the steering actuator is then, for example, driven rotationally. Using the example of a rotating steering wheel, the randomly varying or randomly occurring actual frictional torque then becomes effective. A "measuring unit" can then be used to determine the actual basic friction torque. For example, this measuring unit is mounted on the steering actuator or on a shaft of the steering actuator, so that the actual basic friction torque can be determined as the effective torque. Various measuring devices that reliably record the actual basic friction torque can be used for this purpose. In a "processing unit," such as a microprocessor or transistor circuit, an additional torque is calculated based on the deviation of the determined actual frictional torque from the expected target frictional torque. This calculation can be performed with the aim of ensuring that the additional torque compensates for part or all of the difference between the actual and target frictional torques. The goal of this calculation is to dimension the additional torque in such a way that the actual frictional torque is matched as closely as possible to the expected target frictional torque. The "control" of the electric drive unit then describes in particular the application of an additional drive current to the electric drive unit, so that the additional torque is imposed on the steering actuator by the electric drive unit and thus the determined actual basic friction torque is regulated to the expected target basic friction torque.The calculation of the additional torque can be based on a difference, a weighted difference, or a proportional difference between the determined actual friction torque and an expected target friction torque. The "difference" need not necessarily be mathematically exact, but rather an approximate difference as closely as possible to ensure the actual friction torque is balanced against the target friction torque. A "weighted difference" can, for example, take into account the rotational speed of the steering actuator, the steering position based on a measured steering angle, or other factors to prevent system oscillations or to accommodate specific driving situations.Different operating points of the force feedback steering unit can be taken into account and weighted differently; the same applies to a "proportional difference", where, for example, only a portion of the calculated difference is used to determine the additional torque. To utilize existing components of the force feedback steering unit, the actual basic friction torque can be determined by evaluating a drive torque of the electric drive unit, in particular a drive current of the electric drive unit that is proportional to the drive torque. For example, the drive current of the electric drive unit can be used to obtain an indirect measurement signal regarding the actual basic friction torque. The actuation of the steering actuator and the determination of the actual basic friction torque can be carried out automatically, especially before the start of a journey, after the end of a journey and / or during a journey interruption. To avoid unwanted torque on the steering actuator while driving, the actual base friction torque is determined in appropriate situations, preferably when the vehicle is stationary. For example, a corresponding routine in the control unit can be programmed to drive the steering actuator when the vehicle ignition is switched on or when the vehicle is activated before driving begins, in order to determine the actual base friction torque. The same procedure can be performed after a journey, for example, to measure and store compensation or experience values, and / or during a break in driving, such as at a traffic light. Additional measurement signals from the vehicle can be integrated, for example, by only driving the steering actuator to determine the actual base friction torque when the operator's hands are off the steering wheel. According to one embodiment, the direction of rotation of the steering actuator is detected by means of a rotation direction detection system, whereby the control of the electric drive unit with the additional torque is carried out based on the detected direction of rotation of the steering actuator. This prevents, for example, the application of the additional torque in a drive direction opposite to the desired direction of action, which would further impair the steering feel due to the additional torque. The corresponding steering resistance torque is generated, in particular, linearly dependent on the rotation angle of the steering actuator, wherein the steering resistance torque is generated, in particular, with a hysteresis offset. According to the invention, the basic friction torque is kept constant, or the actual basic friction torque is adjusted as closely as possible to the target basic friction torque. By generating the steering resistance torque linearly dependent on a rotation angle, and in particular with a hysteresis offset when the direction of rotation is reversed, a mechanical steering system is simulated as closely as possible, and the operator is given the feeling of a mechanically implemented steering system. According to a further embodiment, a compensation value can be determined based on a temperature measurement, a humidity measurement and / or a wear detection, whereby the additional torque is adjusted based on the compensation value, so that a temperature influence, a humidity influence and / or a wear influence, in particular after starting a journey or after an interruption of the journey, is compensated. The brake unit is in particular a friction brake unit, in particular an electrically controlled friction brake unit, in particular a magnetrheological brake unit or a magnetrheological powder brake unit. The electric drive unit is in particular an electric motor, especially an electronically commutated electric motor. In another aspect, the task is solved by a motor vehicle, in particular a passenger car or a truck, with a force-feedback steering unit according to one of the previously designated and described configurations. The invention will now be explained in more detail using exemplary embodiments. Fig. 1 shows a schematic representation of a force-feedback steering unit for a motor vehicle in a sectional side view, Fig. 2 shows a frontal view of the force-feedback steering unit of Fig. 1 with a view of a steering actuator designed as a steering wheel, Fig. 3 shows a schematic block diagram of a control system for the force-feedback steering unit of Figs. 1 and 2, and Fig. 4 shows a diagram with a target function and two actual functions with respect to a basic frictional torque of the force-feedback steering unit of Figs. 1 and 2. A force-feedback steering unit 101 is installed in a motor vehicle, for example, a passenger car (vehicle not shown), and serves to guide the driver in a direction of travel. For this purpose, the force-feedback steering unit 101 has a steering wheel mounted on a shaft 105, acting as a steering actuator 103. The shaft 105 with the steering actuator 103 is housed in an exemplary mechanism 107, which includes, for example, rolling bearings and other necessary components, such as slip rings for contacting a horn button (each not shown). The shaft 105 then runs along a steering axis 181 through a brake unit 121, which is designed as a magnetic powder brake, and terminates in a drive unit 123, which is designed as an electric motor.The drive unit 123 is rotationally fixed to the shaft 105 and thus to the steering actuator 103, while the brake unit 121 can exert a braking torque on the shaft 105 when the brake unit 121 is electrically activated. It should be noted that the arrangement of steering actuator 103, shaft 105, brake unit 121, and drive unit 123 is shown here as a geometric example and may be arranged differently in various steering systems, for example, also coupled by means of a gearbox. A computing unit serves as a control unit 131 and is connected to the drive unit 123 and the brake unit 121 respectively via control cables 133 and 135 and includes, for example, a microprocessor and the necessary electrical components with a corresponding power supply for controlling the drive unit 123 and the brake unit 121. The steering actuator 103 can be rotated by an operator by a rotation angle 205, in this case a steering angle, for example to specify a change of direction in the vehicle. It should be noted that the steering actuator 103 is movable around a center position 201 in an oscillation 203 with the drive unit 123. A control scheme 301 shows the processes for controlling and regulating the force-feedback steering unit 101 by means of the computing unit 131. The control scheme 301 can be implemented in the computing unit 131, for example, in electronic hardware or in software. An input variable 303, for example, represents the specified torque value, which is determined by corresponding electronic systems in the vehicle to provide the driver or operator with the appropriate steering feel for the respective driving situation. A resulting control deviation 305 is then fed into a controller 307, whereby in the initial situation it is assumed that the control deviation 305 corresponds to the input variable 303 (see below for further explanation). The controller 307 then specifies a drive current 309, which is then converted into a drive torque 315 by the drive unit 311 (equivalent to the electric motor drive unit 123). The drive torque 315 acts on the steering actuator 317 (equivalent to the steering actuator 103) and is perceived by the operator as steering wheel torque 319.A disturbance variable 331, for example an additional torque due to a user's hand or due to variable friction in the system, can, for example, influence the torque at the steering wheel 317, whereby this disturbance variable 331 is then corrected by repeating the control loop shown above. During a corresponding driving pause, particularly before starting a journey, after starting a journey, or during a journey interruption, feedback 321 of the steering wheel torque 319 can be sent to a measuring unit 323. For example, the drive current 309 of the drive unit 311 is evaluated, while the steering actuator 317 is driven by the drive unit 311 in an oscillating motion around the oscillation 203. In this operating state, an actual base torque of the brake unit 121 is recorded and stored in the measuring unit 323. In normal driving operation, for example when the vehicle is set in motion again after a stop at the traffic light, the corresponding stored measured value of an actual basic torque that deviates from a target basic torque is then forwarded to a subtraction node 325 by performing a corresponding subtraction from the input variable 303, so that the input variable 303 is constantly superimposed with the deviation of the target basic torque, and the operator at the steering actuator 317 feels the impression of a constant basic torque. To perform a corresponding measurement, a test reference parameter 341 can be applied to the controller 307, which then activates the operating state of the measurement and applies a corresponding drive current 309 to the drive unit. Reference is further made to diagram 401, which has a diagram area 403, an abscissa 405, and an ordinate 407. The abscissa 405 shows the rotation angle of the steering actuator 103 or 317, while the ordinate 407 shows the basic torque of the steering actuator 103 or 317. A target function represents the target basic friction torque 411 and shows corresponding desired torques of, for example, 0.7 Nm along a change in the steering wheel's direction of rotation from lock to lock. One actual function shows an actual basic friction torque 415, another actual function shows an actual basic friction torque 417, for example, due to cold or heat, and thus represent deviating torques over corresponding steering angles. Based on the measurement and compensation described above, a corresponding additional torque 416 or 418 is then applied to compensate in the direction of the target basic friction torque 411. Reference symbol list 101 Force feedback steering unit 103 Steering actuator 105 Shaft 107 Mechanics 121 Brake unit 123 Drive unit 131 Control unit 133 Control cable 135 Control cable 181 Steering axle 201 Center position 203 Oscillation 205 Rotation angle 301 Control scheme 303 Input variable 305 Control deviation 307 Controller 309 Drive current 311 Drive unit 315 Drive torque 317 Steering actuator 319 Steering wheel torque 321 Feedback 323 Measuring unit 325 Subtraction node 331 Disturbance variable 341 Test reference variable 401 Diagram 403 Diagram area 405 Abscissa 407 Ordinate 411 Target basic friction torque 415 Actual basic friction torque 416 Additional torque 417 Actual basic friction torque 418 Additional torque

Claims

Force-feedback steering unit (101) for a motor vehicle, comprising a steering actuator (103, 317), a control unit (131), an electric drive unit (123, 311) and a brake unit (121), wherein the steering actuator (103, 317) can be operated by an operator to input a steering command, a steering resistance torque can be generated by means of the electric drive unit (123, 311) and / or the brake unit (121) to provide a driving feel to the operator, and an actual basic friction torque (415, 417) of the brake unit (121) effective when operating the steering actuator (103, 317) is subject to random change, wherein the force-feedback steering unit is configured such that the randomly changing actual basic friction torque (415, 417) is compensated for by the following steps: - Driving the steering actuator (103, 317) by means of the electric drive unit (123, 311), so that the steering actuator (103, 317) is driven and the randomly changed actual basic friction torque is effective,- Determining the actual basic friction torque using a measuring unit (323) while driving the steering actuator (103, 317), so that the actual basic friction torque (415, 417) is determined using the measuring unit (323), - Calculating an additional torque (416, 418) using a computing unit (131), wherein the additional torque (416, 418) is determined from a deviation of the determined actual basic friction torque (415, 417) from an expected target basic friction torque (411), so that the additional torque (416, 418) is determined, - Controlling the electric drive unit (123, 311) such that the additional torque (416, 418) is applied to the steering actuator (103, 317) to compensate for the determined actual basic friction torque (415, 417). The expected target basic friction torque (411) is applied, so that the steering actuator (103, 317) is subjected to the additional torque (416, 418), so that the random change of the actual basic friction torque (415, 417) of the force feedback steering unit by the applied additional torque (416,418) is regulated. Force feedback steering unit according to claim 1, characterized in that the calculation of the additional torque (416, 418) is carried out from a difference, from a weighted difference or from a proportional difference of the determined actual basic friction torque (415, 417) to an expected target basic friction torque (411). Force feedback steering unit according to claim 1 or 2, characterized in that the determination of the actual basic friction torque (415, 417) is carried out by evaluating a drive torque of the electric drive unit (123, 311), in particular a drive current (309) proportional to the drive torque, of the electric drive unit (123, 311). Force feedback steering unit according to one of the preceding claims, characterized in that the driving of the steering actuator (103, 317) and the determination of the actual basic friction torque (415, 417) are carried out before the start of a journey, after the end of a journey and / or during a journey interruption, in particular automatically. Force-feedback steering unit according to one of the preceding claims, characterized in that a direction of rotation of the steering actuator (103, 317) is detected by means of a direction of rotation detection, wherein the control of the electric drive unit (123, 311) with the additional torque (416, 418) is carried out on the basis of the detected direction of rotation of the steering actuator (103, 317) in a direction-dependent manner. Force feedback steering unit according to one of the preceding claims, characterized in that the steering resistance torque is generated in particular linearly dependent on a rotation angle (205) of the steering actuator (103, 317), wherein the steering resistance torque is generated in particular with a hysteresis offset. Force feedback steering unit according to one of the preceding claims, characterized in that a compensation value is determined on the basis of a temperature measurement, a humidity measurement and / or a wear detection and the additional torque is adjusted on the basis of the compensation value in such a way that a temperature influence, a humidity influence and / or a wear influence, in particular after starting a journey or after an interruption of a journey, is compensated. Force feedback steering unit according to one of the preceding claims, characterized in that the brake unit (121) is a friction brake unit, in particular an electrically controlled friction brake unit, in particular a magnetic rheological brake unit or a magnetic rheological powder brake unit. Force feedback steering unit according to one of the preceding claims, characterized in that the electric drive unit (123, 311) comprises an electric motor, in particular an electronically commutated electric motor. Motor vehicle, in particular passenger car or truck with a force feedback steering unit (101) according to any one of the preceding claims 1 to 9.

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