Method for controlling an electric motor in a steer-by-wire steering system, wheel steering actuator unit for a steer-by-wire steering system, motor vehicle

Abstract

The invention relates to a method for controlling an electric motor in a steer-by-wire steering system, to a wheel steering actuator unit for a steer-by-wire steering system, and to a motor vehicle. The method for controlling the electric motor comprises the following steps: - receiving a first control command; - determining current operating parameters of the electric motor; - determining an energy balance prognosis of the electric motor, in particular a value of a current generated by the electric motor in a generator mode, on the basis of the first control command and the operating parameters; - checking whether the energy balance value prognosis lies in a permissible range; and - generating and sending a second control command, the execution of which causes an energy balance of the electric motor to lie in the permissible range.

Description

[0001] Method for controlling an electric motor in a steer-by-wire steering system, wheel steering actuator unit for a steer-by-wire steering system, motor vehicle

[0002] The invention relates to a method for controlling an electric motor in a steer-by-wire steering system. Furthermore, the invention relates to a wheel steering actuator unit for a steer-by-wire steering system and a motor vehicle.

[0003] In a steer-by-wire steering system, the steered wheels and the steering wheel are not mechanically but electrically coupled. The respective steering wheel position is detected by a steering angle sensor, and a corresponding control command is transmitted to a roadwheel actuator. The roadwheel actuator is designed to change the position of the steered wheels in accordance with the control command via a steering actuator driven by an electric motor. To create a realistic steering feel despite the lack of a mechanical connection between the steering wheel and the steered wheels, a steer-by-wire steering system can incorporate a feedback actuator that generates appropriate resistance during steering movements.

[0004] In some situations, a torque acts on the rotor of the wheel steering actuator's electric motor, causing the electric motor to operate in generator mode, i.e. generate electrical current. Such generator mode can occur, for example, during dynamic steering maneuvers where the rotor must be braked sharply to change direction in order to execute the respective control commands. In this case, the rotor initially continues to rotate in its previous direction due to its inertia. When the rotor is braked to reverse direction, energy is absorbed, generating an electrical current that can flow back into the vehicle electrical system. External forces acting on the steered wheels from outside the vehicle can also cause the electric motor to operate in generator mode due to the coupling of the wheels to the wheel steering actuator.

[0005] A current flow from the wheel steering actuator into the vehicle's electrical system and / or an inverter located therein is generally undesirable. Although under certain conditions, the additional power generated can be compensated by other power consumers, such as batteries, this requires high-capacity batteries. If the currently available capacity is insufficient, overvoltages and voltage spikes can occur in the vehicle's electrical system, potentially damaging electrical systems connected to the electrical system.

[0006] The invention is based on the object of providing an improved control method for a steer-by-wire system, particularly with regard to the electrical current flow generated by an electric motor of the wheel steering actuator unit operating in generator mode. The invention is further based on the object of providing a corresponding wheel steering actuator unit and a motor vehicle.

[0007] According to the invention, the object is achieved with regard to the method by the subject matter of claim 1. With regard to the wheel steering actuator unit and the motor vehicle, the object is achieved by the subject matter of claims 6 and 8, respectively. Preferred embodiments are specified in the corresponding subclaims.

[0008] Specifically, the problem is solved by a method for controlling an electric motor in a steer-by-wire steering system. The method comprises the following steps:

[0009] - Receiving a first control command;

[0010] - Determining current operating parameters of the electric motor;

[0011] - determining an energy balance forecast of the electric motor based on the first control command and the operating parameters, in particular a value of a current generated by the electric motor in a generator mode based on the first control command and the operating parameters;

[0012] - Check whether the energy balance value forecast is within an acceptable range; and

[0013] - Generating and sending a second control command, the execution of which ensures that the energy balance of the electric motor is within the permissible range.

[0014] The invention is based on the idea of ​​determining an energy balance forecast of the electric motor on the basis of a first (to be executed) control command and current operating parameters of the electric motor.

[0015] The energy balance prediction may be one or more numerical values ​​indicating an energy balance of the electric motor, assuming that the electric motor is controlled according to the first control command. In particular, the energy balance may be a value of a current (in particular, the current intensity) generated by the electric motor in generator mode. In particular, a positive value may indicate that the electric motor is generating electrical power, i.e., operating in generator mode, while a negative value indicates that the motor is acting as a power consumer, i.e., not in generator mode.

[0016] By checking whether the energy balance forecast is within a permissible range and generating and transmitting the second control command, the method according to the invention can prevent the electric motor from feeding an impermissibly high electrical current into the vehicle electrical system in generator mode. This can prevent damage to other components connected to the vehicle electrical system.

[0017] The permissible range can be defined in particular by two threshold values.

[0018] The second control command can, in particular, be generated such that the current generated in generator mode is less than or equal to a threshold value that defines the permissible range. In particular, the second control command can reduce a requested rotor torque compared to the first control command to such an extent that the electric motor does not operate in generator mode when the second control command is executed.

[0019] Thus, the operation of the electric motor in generator mode is advantageously not fundamentally prevented, but advantageously only modified in such a way that a value of the generated current lies within a permissible range.

[0020] Overall, the method according to the invention thus makes it possible to detect an energy backflow of the wheel steering actuator that is harmful to the vehicle and to advantageously control the actuator precisely in such a way that the harmful energy backflow is prevented.

[0021] In one embodiment, the operating parameters of the electric motor indicate at least one of the following variables: rotor speed, rotor velocity, torque, force acting on the rotor.

[0022] In particular, not only operating variables resulting from the engine control itself (especially rotor speed, rotor rpm, torque, etc.) can be included, but also external variables, in particular an (external) force acting on the rotor. This can arise, in particular, from mechanical influences on the steered wheels, especially a collision of the wheels with an obstacle.

[0023] One or more of the mentioned variables can also be derived from the first control command, in particular a torque which is required to execute the first control command.

[0024] Determining the energy balance forecast may, in particular, be based on a comparison between the current operating parameters of the motor and the corresponding operating parameters that would result from the execution of the first control command. In particular, a large difference between the instantaneous rotor torque and the torque required to execute the first control command may indicate that the electric motor would operate in generator mode upon execution of the first control command.

[0025] In a further embodiment, the second control command is advantageously generated such that a deviation of the second control command from the first control command is minimal.

[0026] The deviation can be determined in particular based on the difference between the corresponding operating parameters, in particular torque, rotor speed, rotor position, etc.

[0027] This offers the advantage that the intervention in the steer-by-wire system is minimal in the sense that the driver of the vehicle does not perceive any change in the steering behavior due to the modification of the first control command or the sending of the second control command.

[0028] In a further embodiment, the method further comprises determining a current value flowing in the electric motor. Additionally or alternatively, the energy balance forecast is further determined based on the determined current value.

[0029] The current value can represent an indicator of the current energy balance of the electric motor. Thus, according to this embodiment, the energy balance forecast can be determined based on an additional reference value, which increases the precision of the energy balance forecast and thus of the method. In a further embodiment, determining the energy balance forecast comprises looking up a lookup table containing a mapping of control commands and operating parameters to energy balance forecasts. The mapping contained in the lookup table can, in particular, be specific to the respective electric motor.

[0030] This embodiment offers the advantage that even higher-dimensional, non-linear relationships between operating parameters and the energy balance (or forecast) of the electric motor can be modeled in advance and stored in a control unit's memory. These relationships can be efficiently retrieved from the lookup table to control the motor, without computationally intensive processing steps.

[0031] The object is further achieved by a wheel steering actuator unit for a steer-by-wire steering system, wherein the wheel steering actuator unit has the following:

[0032] - an electric motor; and

[0033] - a control unit.

[0034] The control unit is designed to perform the following steps:

[0035] - Receiving a first control command;

[0036] - Determining current operating parameters of the electric motor;

[0037] - determining an energy balance forecast of the electric motor, in particular a value of a current generated by the electric motor in a generator mode, based on the first control command and the operating parameters;

[0038] - Check whether the energy balance value forecast is within an acceptable range;

[0039] - Generating and sending a second control command, the execution of which ensures that the energy balance of the electric motor is within the permissible range.

[0040] In one embodiment, the wheel steering actuator unit further comprises a current limiting element configured to limit the return flow of current from the electric motor to a vehicle electrical system when the electric motor operates in a generator mode.

[0041] This further increases the reliability of the wheel steering actuator unit with respect to undesirable current backflow from the electric motor in generator mode.

[0042] The object is further achieved by a motor vehicle having a wheel steering actuator unit as described above. The advantages mentioned in connection with the method according to the invention also apply correspondingly to the wheel steering actuator unit according to the invention and the motor vehicle according to the invention. The features described in this context are also disclosed in connection with the wheel steering actuator unit and the motor vehicle, and vice versa. In particular, the control unit of the wheel steering actuator unit can be designed to perform the calculation and control of the electric motor according to the described method according to the invention.

[0043] The invention is explained in more detail below using an embodiment with reference to the attached schematic figures.

[0044] Show

[0045] Fig. 1 is a schematic view of a steer-by-wire steering system;

[0046] Fig. 2 is a schematic view of a steer-by-wire steering system with control according to the invention according to an embodiment; and

[0047] Fig. 3 shows a speed curve of a rack driven according to the control system according to the invention.

[0048] Figure 1 shows some components of a steer-by-wire steering system. The components of the steering system are described to the extent necessary for understanding.

[0049] The steer-by-wire steering system includes a steering wheel 9 connected via a steering shaft to a steering wheel angle detection device and a feedback actuator 8. The steering wheel angle detection device is configured to detect the steering angle of the steering wheel 9 specified by the driver and transmit it to the control unit 3 of the wheel steering actuator unit.

[0050] In addition to the control unit 3, the wheel steering actuator unit includes the electric motor 1, which is designed to drive the rack 6 to reach a predetermined position. In this way, the steered wheels 7 can be adjusted to the desired angle. Figure 2 shows a section of a steer-by-wire steering system similar to the system shown in Figure 1.

[0051] In particular, Figure 2 shows a wheel steering actuator unit comprising the following: an electric motor 1 for driving the rack 6; a sensor unit 2 configured to detect the position of the rack 6; and a control unit 3 configured to carry out the inventive method for controlling the electric motor 1.

[0052] For power supply, electric motor 1 is connected to a vehicle electrical system (not shown), which is fed by battery 5. Inverter 4 is connected between electric motor 1 and battery 5.

[0053] Figure 3 shows the influence of the control method according to the invention on the basis of the speed curve of a rack driven by an electric motor of a wheel steering actuator unit (see also Figures 1 and 2).

[0054] The diagram shown in Figure 3 shows the speed v_rack of the driven rack over time t.

[0055] The curve v, shown as a solid line, shows the speed curve without using the method according to the invention, ie with unchanged execution of the corresponding control commands. The course of the curve v includes

[0056] - a first phase from time 0 to time tl, in which the rack is accelerated approximately uniformly (positively);

[0057] - a second phase from time t1 to time t2, in which the speed of the rack is constant; and

[0058] - a third phase from time t2 to time t3, in which the rack is accelerated approximately uniformly (negatively), ie decelerated.

[0059] The speed profile shown can, for example, be caused by a dynamic steering maneuver with right / left direction changes, which requires that the rack must first be driven in one direction (first and second phase), and then suddenly driven in the opposite direction, which requires braking of the constantly accelerated rack (third phase).

[0060] Without the inventive control of the electric motor, a strong negative acceleration (braking) of the rack and the electric motor coupled to it takes place in the third phase, which causes the electric motor to operate in generator mode and feed a high electrical current back into the vehicle network.

[0061] Curve v', shown as a dashed line, shows the velocity profile using the method according to the invention. In the first and second phases, the velocity profile is identical to curve v.

[0062] In the third phase, the generation and transmission of the second control command to the electric motor's control unit intervenes in the electric motor's control system, reducing the magnitude of the negative acceleration of the electric motor in the third phase. Consequently, the third phase of curve v' ends only at time t4 (instead of t3). In other words, the second control command causes the electric motor to decelerate less.

[0063] This is done at least by an amount that ensures that the electric motor, if it is operating in generator mode at all when the second control command is executed, only generates an amount of current that is within a permissible range.

[0064] In Figure 3, the inventive control of the electric motor only affects the third phase (braking of the rack), since it is assumed in the context of the exemplary embodiment that, with unchanged control, the energy balance of the electric motor would operate in the impermissible range only in this phase, i.e., an excessive current would be generated. However, the electric motor can also operate in generator mode under other operating conditions, particularly during rotor acceleration, and thus generate an impermissibly high current, which can be prevented or reduced by the inventive control method.

[0065] The invention is not limited to steering wheels, but is generally applicable to steering devices, including other types of handlebars. In addition to the steering wheel described above, steering devices in general are also encompassed. The invention is also not limited to the human-machine interface, but also encompasses steering systems that enable autonomous driving. Reference numerals list

[0066] 1 electric motor

[0067] 2 Position sensor

[0068] 3 Control unit 4 Inverter

[0069] 5 Battery

[0070] 6 rack

[0071] 7 steered wheel

[0072] 8 Wheel steering actuator 9 Steering wheel v, v' speed curves

Claims

Claims 1. A method for controlling an electric motor (1) in a steer-by-wire steering system, the method comprising the following steps: - Receiving a first control command; - Determining current operating parameters of the electric motor (1); - determining an energy balance forecast of the electric motor (1), in particular a value of a current generated by the electric motor (1) in a generator mode, based on the first control command and the operating parameters; - Check whether the energy balance value forecast is within an acceptable range; and - generating and sending a second control command, the execution of which causes an energy balance of the electric motor (1) to be within the permissible range.

2. Method according to claim 1, characterized in that the operating parameters indicate at least one of the following variables: rotor speed, rotor velocity, torque, force acting on the rotor.

3. Method according to one of the preceding claims, characterized in that the second control command is generated such that a deviation of the second control command from the first control command is minimal.

4. Method according to one of the preceding claims, characterized in that the method further comprises determining a current value flowing in the electric motor (1), and / or determining the energy balance forecast further takes place based on the determined current value.

5. Method according to one of the preceding claims, characterized in that the determination of the energy balance forecast comprises a lookup in a lookup table which contains an assignment of control commands and operating parameters to energy balance forecasts.

6. Wheel steering actuator unit for a steer-by-wire steering system, the wheel steering actuator unit comprising the following: - an electric motor (1); and - a control unit (3), wherein the control unit (3) is designed to carry out the following steps: - Receiving a first control command; - Determining current operating parameters of the electric motor (1); - determining an energy balance forecast of the electric motor (1), in particular a value of a current generated by the electric motor (1) in a generator mode, based on the first control command and the operating parameters; - Check whether the energy balance value forecast is within an acceptable range; and - generating and sending a second control command, the execution of which causes an energy balance of the electric motor (1) to be within the permissible range.

7. Wheel steering actuator unit according to claim 6, characterized by a current limiting element which is designed to limit the return flow of a current from the electric motor (1) to a vehicle electrical system when the electric motor (1) operates in a generator mode.

8. A motor vehicle having a wheel steering actuator unit according to claim 6 or claim 7.

Images