Steering system and method for reducing the power converted in an electrically assisted steering system

Abstract

Method for reducing the power dissipated in an electrically assisted steering system (1) comprising the steps: - Determining (100) a signal to reduce power, and responding to it - Reducing (200) the proportions (power / level) of a first frequency range (I) in a control signal (S) f ) of the electric assistance, wherein a limit (f g ) of the frequency range (I) depending on a predefined measure for the reduction of power (P2) is selected, characterized in that the signal for the reduction of power responds to - user input and / or - a current or predicted temperature is provided within the steering system (1), in particular in an electrical component, preferably an electric motor (2) or a control unit (4) for controlling the electric motor (2).

Description

State of the art

[0001] The present invention relates to an electrically assisted steering system and a device according to the preamble of patent claim 2 and a method according to the preamble of patent claim 1 for reducing a power converted in such a steering system.

[0002] Series-produced electromechanical steering systems (EPS) typically have a function for deactivating or reducing steering assistance in the event of increased thermal stress on the steering system or the EPS control unit. Thermal stress can occur particularly, but not exclusively, in electrical components and can be detected using suitable sensors. When increased thermal stress is detected, state-of-the-art EPS systems reduce the assistance power by a percentage. For example, in a first stage, the assistance power is reduced to 80% and in a second stage to 60% of the full assistance power. If the load continues to increase or does not decrease, overheating can be gradually prevented in this way, up to the point where the steering assistance is completely deactivated.This reduction in steering assistance is very clearly perceived by the driver. The steering forces required increase considerably. Depending on the level of reduction, warning lights are used in the instrument cluster to inform the driver about the restricted function or malfunction of the steering system. Furthermore, steering systems are currently being developed in which the steering assistance power is not requested in the form of a control from the assistance motor, but rather a closed-loop control (e.g. of the assistance force (see DE 10 2010 036 655 A1) or the manual torque, see S. Grüner: "Control of electronic power steering systems: state of the art and future concepts", in Chassis.Tech Plus, Munich, 2011). This control leads to a significant improvement in the feedback characteristics of the steering. For this purpose, the transmission behavior between external excitation on the chassis and the steering wheel is improved by an electric motor.A disadvantage of this control is that the torque requested by the assist motor is constantly being adjusted according to the desired steering feel. These adjustments make the system more "active" overall. In other words, energy conversion is significantly increased compared to a conventionally controlled EPS system.

[0003] DE 41 42 580 A1 discloses a method for operating an electric power steering system with a servo motor. According to the invention, the speed of the servo motor is selected depending on at least one of the following variables: vehicle speed, steering torque, engaged gear, operating state of the vehicle's internal combustion engine, and operating state of the power steering system. In this way, wear, fuel consumption, and noise emissions, among others, can be favorably influenced.

[0004] DE 198 43 861 A1 discloses a method and a system for preventing temperature increases in a motor vehicle, comprising a temperature sensing device that is thermally coupled to an electronic circuit or a braking system. In response to a temperature threshold being exceeded, a measure is initiated to prevent an unauthorized temperature increase.

[0005] DE 10 2010 036 655 A1 discloses a method for operating a steering system using a control loop to regulate the steering assistance torque, wherein the control loop takes into account the inertia of the steering column. This enables the assistance behavior to be controlled in response to the driver's hand force, since the acceleration behavior of the steering column is taken into account.

[0006] DE 11 2007 002 144 T5 discloses a control device for a steering system which adapts parameters of a steering system in a frequency-dependent manner depending on an electrical voltage provided by the vehicle battery.

[0007] It is an object of the present invention to improve electromechanically assisted steering systems with regard to their power conversion. Disclosure of the invention

[0008] The above-mentioned object is achieved according to the invention by a method having the features of claim 1, a device having the features of claim 2 and a steering system having the features of claim 9. The method according to the invention serves to reduce the power implemented in an electrically assisted steering system. For this purpose, in a first step, a signal for reducing the power is determined. This can, for example, be a signal in response to a user input (e.g. for activating an ECO mode) or a signal from a temperature sensor. Alternatively, a temperature monitoring system can also computationally determine or even predict a temperature. In response to the determined signal, components of a first frequency range in a control signal for the electrical assistance are reduced, wherein a limit of the frequency range is selected as a function of a predefined measure for the reduction in power.Based on the control signal, an electric drive is activated to assist the steering system. In addition to low-frequency components, which primarily induce a quasi-static steering angle, high-frequency components are used in the current technology to improve the feedback from the road surface or chassis to the steering system and ultimately to the driver. Frequencies between 10 Hz and 30 Hz in the control signal are primarily associated with the feedback. The lower the frequencies in the control signal, the more likely they are to be associated with a static assistance signal for setting a current (macroscopic) steering angle.By positioning a cutoff frequency within the control signal according to the invention, which is dependent on the determined signal, slight power reductions can be achieved exclusively at the expense of a direct driving feel and positive feedback characteristics, while in response to the need for a greater reduction in power, increasing comfort losses are accepted. Since, depending on the road surface conditions and the operating state of the vehicle, the high-frequency components are sometimes perceived only to a very limited extent or not at all by the user, the invention offers the possibility of implementing power reductions with the least possible or even no user experience. The signal interpreted for the inventive power reduction can - as indicated at the beginning - have different sources.For example, a user can express a wish by operating a control element to put their vehicle into an economical operating mode (ECO mode). This signal is used by the present invention to reduce controller components above the cut-off frequency to a predefined extent. Alternatively or additionally, a current temperature can be measured (e.g. using appropriate temperature sensors) or derived from measured secondary variables (e.g. current, voltage, power, average steering angle / average steering angle change per unit of time, or similar). In addition, a prediction of a temperature that will occur in the near future can be made in order to be able to react to excess temperature as early as possible and to carry out the necessary measures essentially unnoticed by the user.While the electrical components of the steering system according to the invention are not the only location for temperature detection, it can generally be assumed that, due to electrical losses, they will, on the one hand, reach a critical temperature early on, and, on the other hand, that deactivating the control system at these components will have a particularly positive effect. Furthermore, electronic components are generally more temperature-sensitive than mechanical parts of a steering system. Furthermore, temperature detection at such elements can often be performed using existing hardware, so that additional hardware is not required to implement the present invention.

[0009] The subclaims show preferred developments of the invention.

[0010] Preferably, the components of the first frequency range can be reduced by a first amount, while components of a second (and lower) frequency range are reduced by a second amount or remain completely unchanged. The first amount and / or the second amount can, for example, be a predefined percentage of an originally provided steering assistance power in a respective frequency range. Alternatively, it is possible to use predefined power levels (e.g. in watts) as the first / second amount, so that the heat input is specifically reduced by corresponding amounts by reducing the control signal. In other words, the invention provides two "adjustment screws" for the power reduction according to the invention: On the one hand, the cut-off frequency between the first range and the second range can be lowered in order to reduce the power more significantly.In addition, it is possible to further reduce the power converted within a predefined frequency range (e.g. by adjusting a maximum amplitude or reducing all amplitudes of the control signal).

[0011] It has been found advantageous to reduce the first variable within a first (higher) frequency range more than the second variable within a second (lower) frequency range. The units of the first and second variables can be identical, while their values ​​or magnitudes are different according to the invention. This increases the degree of freedom in power reduction without disrupting the user experience.

[0012] Preferably, the cutoff frequency of the control signal can be selected such that components above the cutoff frequency essentially cause inertia compensation of mechanical components of the steering system. Thus, deactivating the control here causes a higher inertia in the system, resulting in reduced feedback. However, the steering forces necessary for good steering behavior are not impaired.

[0013] A further possibility is to reduce the high-frequency components, in particular via a modified reference variable, in particular a rack force, or a more sluggish control. A modified reference variable can, for example, be a filtering of a force (or torque) used as an input signal of the steering system according to the invention. For adaptation, the reference variable can, for example, be low-pass filtered. The signal from a force transducer or a calculated / estimated rack force (whereby the dynamics of the estimation or the type of calculation algorithm are also changed so that no separate filter is necessary) on the rack is sent for this purpose, for example, to the steering control (electronic control unit), within which low-pass filtering can be carried out. Alternatively, the control signal can also be smoothed and / or edge steepness reduced.Below the threshold for the first magnitude reduction, the components primarily contribute to setting the desired steering angle. Frequencies for such a function are generally a few hertz and can therefore be described as low-frequency. This achieves less inertia compensation with lower power conversion in the corresponding frequency range, while the loss of comfort remains essentially unaffected.

[0014] According to a further aspect of the present invention, a device is proposed which is configured to carry out the method described above within a steering system. The device can, for example, comprise sensors for determining the signal to reduce power. Optionally, sensors for detecting temperature signals can also be present. In addition, the device comprises an evaluation unit by means of which the method steps of a method described above can be carried out, whereby an electric motor external to the device or contained in the device receives control signals modified according to the invention. The device can, for example, be contained in a steering system, which is additionally protected as a whole according to the present invention.Steering systems typically comprise a steering column, a rack, a gear between the rack and the steering column, and an electric motor for providing assistance to the user. The features, combinations of features, and the resulting advantages correspond to those discussed in connection with the method according to the invention, so that reference may be made to the above explanations to avoid repetition.

[0015] One way to determine whether a reduction in the power converted within the steering system is necessary is to implement an electronic load model that determines the energy input into the steering system by evaluating the temporal progression of thermal and mechanical system variables. In other words, it can be determined based on experience and on variables linked to energy input whether a reduction in the control signal appears necessary. In a system with manual or torsion bar torque control, it is not simply possible to switch off the controller, since this also stabilizes the control loop. However, it is possible to modify the reference variables (e.g. rack force) so that a less dynamic torque is requested from the assistance motor.It is also possible to switch to a slower controller, which has a damping effect on the system, thus reducing the system dynamics. In this way, the shutdown concept presented here can also be implemented using only one control loop or one assist motor.

[0016] The greatest advantage of the steering system according to the invention is that it is possible to reduce the thermal load on the steering system without significantly restricting comfort for the driver. In the prior art, these limitations are so significant that a warning light may have to be illuminated in the instrument cluster. A system according to the invention, however, enables an early response by reducing the energy input. If signs of impending thermal overload of the system are detected, the control component can be switched off until the system has stabilized thermally again. The control component is then switched on again (e.g., imperceptibly for the driver). Furthermore, the invention enables systems in which the controller is only activated in a sport mode selectable by the driver.Here, the system can be protected in such a way that the driver can no longer select this sport mode above a certain temperature / load limit. In another version of the system, the driver can selectively deactivate the control component using a switch. This can also be linked to other settings on the steering and other control units ("ECO mode"). Due to the overload protection according to the invention, depending on the system design and vehicle configuration, it may also be possible to design the steering components in such a way that the power reserve can be smaller, which can lead to cost savings. Short description of the drawings

[0017] Embodiments of the invention are described in detail below with reference to the accompanying drawings. In the drawings: Fig. 1 a schematic overview of components of a steering system according to the invention; Fig. 2 a schematic diagram of a control signal versus frequency; and Fig. 3 a flowchart illustrating steps of an embodiment of a method according to the invention. Embodiments of the invention

[0018] Fig. 1 shows a steering system 1 in which steering torques M T via a steering column 6 and a gear 12 to a rack 5. The rack 5 transmits forces to wheels 11 of a steerable axle, in return for which the wheels 11 transmit a wheel force F R from the road to the rack 5. An electric motor 2 is used to amplify the (generally low-frequency) signals originating from the steering wheel 7 to the wheels 11 and, on the other hand, to transmit the wheel force F originating from the wheels 11 Rto the steering wheel 7. For the latter component of a controller signal, a controller 9 is provided, which outputs high-frequency controller signals to an electronic control unit (ECU) 4 as a steering assistance unit via a switch 8 provided according to the invention. The ECU as the control unit 4 is configured to supply the electric motor with signals, in response to which the electrical assistance force is generated within the steering system 1. To determine a temperature at critical components, the ECU has a first temperature sensor 3a and the electric motor 2 has a second temperature sensor 3b. If the ECU receives a signal to reduce the power converted in the steering system 1, the switch 8 opens, in response to which high-frequency controller components of the controller 9 are not taken into account by the ECU.In this way, both the electrical power conversion and the mechanical power conversion within the steering system 1 are reduced. Wear and fuel consumption are reduced accordingly.

[0019] Fig. 2 shows a frequency diagram of a possible controller signal S (f), which is determined by a cutoff frequency f g is divided into a first frequency range I and a second frequency range II. While the first frequency range I is above f g is located, the second frequency range II is between 0 Hz and f gA first double arrow P1 illustrates the possibility of adjusting the strength of a reduction in the controller signal S(f) in the first frequency range I. A second double arrow P2 illustrates the possibility of adjusting fewer or additional frequency ranges according to the double arrow P1 in order to adjust the electrically induced power within the steering system. Of course, additional frequency ranges (not shown) can be defined to achieve a smoother gradation between a strong power reduction in a first frequency range and essentially unchanged control in a second frequency range II.

[0020] Fig.3 shows a flowchart illustrating steps of an exemplary embodiment of the method according to the invention. In step 100, a signal for reducing the power within the steering system 1 according to the invention is determined. In response to this, in step 200, a first portion of a control signal S(f) is reduced, which portion belongs to a first frequency range I of the control signal S(f). In this way, the power converted in the steering system 1 is reduced without the steering assistance experiencing any functional impairment. Only when a need for a further reduction in the converted power is determined in step 300 is the limit f g of the frequency range is shifted towards lower frequencies, whereby further frequency ranges of the control signal are attenuated in order to further reduce the input of electrical power into the steering system 1.

[0021] Even if the aspects and advantageous embodiments of the invention have been described in detail with reference to the exemplary embodiments explained in conjunction with the attached drawing figures, modifications and combinations of features of the illustrated exemplary embodiments are possible for those skilled in the art without departing from the scope of the present invention, the scope of protection of which is defined by the attached claims. List of reference symbols 1 steering system 2 electric motor 3a, 3b Temperature sensor 4 Control unit ECU 5 rack 6 Steering column 7 Steering wheel 8 switches 9 controls 11 wheels 12 gearboxes 100, 200, 300 process steps I First frequency range II Second frequency range F R Wheel force f g Cutoff frequency M M Engine torque M T Steering column torque T1, T2 influence of the control signal S(f) S(f) control signal

Claims

[1] Method for reducing a power converted in an electrically assisted steering system (1) comprising the steps: - Determining (100) a signal to reduce power, and in response thereto - Reducing (200) components (power / level) of a first frequency range (I) in a control signal (S f ) of the electrical support, with a limit (f g ) of the frequency range (I) is selected depending on a predefined measure for the reduction of the power (P2) characterized by that the signal to reduce power in response to - a user input and / or - a current or predicted temperature is provided within the steering system (1), in particular in an electrical component, preferably an electric motor (2) or a control unit (4) for controlling the electric motor (2). [2] Device for reducing a power converted in an electrically assisted steering system (1) comprising: - means (3) for determining a signal for reducing (100) the power, and - a control unit (4) which is arranged - components of a first frequency range (I) in a control signal (S f ) of the electric support, and - a border (f g ) of the frequency range (I) depending on a predefined degree of power reduction, characterized by that the signal to reduce power in response to - a user input and / or - a current or predicted temperature is provided within the steering system (1), in particular in an electrical component, preferably an electric motor (2) or a control unit (4) for controlling the electric motor (2). [3] Method according to claim 1 or device according to claim 2, characterized by , that - the reduction (200) of the components of the first frequency range (I) is carried out by a first value, and - Portions of a second, lower frequency range (II) are reduced by a second amount or remain unchanged. [4] Method or device according to claim 3, characterized by that the application of the first size results in a greater reduction than the application of the second size. [5] Method or device according to one of claims 3 or 4, characterized by that the first variable is a complete shutdown of control signals (S f ) in the first frequency range (I). [6] Method according to one of claims 1, 3, 4 or 5 or device according to one of claims 2 to 5, characterized by that the limit (f g) of the first frequency range (I) is essentially chosen so that components above the limit (f g ) bring about an inertia compensation of mechanical components (5, 6, 7) of the steering system (1), and the components are reduced in particular via a modified reference variable or via a more sluggish control. [7] Method according to one of claims 1 or 3 to 6 or device according to one of claims 2 to 6, characterized by that the limit (f g ) of the first frequency range (I) is essentially chosen so that components below the limit (f g ) essentially contribute to setting the steering angle desired by the user. [8] Method or device according to one of the preceding claims, characterized by that the temperature is predicted based on measured values ​​and / or an operating state history. [9] Steering system comprising a device according to one of claims 2 to 8.

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