Steer-by-wire steering system and electric motor

Abstract

The invention relates to a steer-by-wire steering system for a motor vehicle, having an electric motor for indirectly or directly displacing a steering rod, wherein the electric motor is designed as a high-cogging-torque motor and has an electric machine with a predefined number of phases and an inverter which is in effective connection thereto and has an intermediate circuit capacitor, wherein the inverter is designed as an inverter with an integrated step-up converter. The invention also relates to an electric motor having an inverter with an integrated step-up converter.

Description

[0001] Steer-by-wire steering and electric motor

[0002] The present invention relates to the field of controlling electric motors for vehicle steering systems, in particular rear axle steering systems with steer-by-wire technology.

[0003] One way to operate a steer-by-wire steering system, for example, a highly efficient rear-axle steering system, is to use an actuator with a spindle drive with little or no self-locking. To minimize or eliminate the displacement of a spindle-shaped steering rod due to forces acting in the chassis within the actuator when switched off or de-energized, an electric motor with a high cogging torque can be used as the drive motor. This can be referred to as a high-cogging-torque motor (HCTM) and is described, for example, in DE 10 2019 204 913 A1.

[0004] Furthermore, DE 10 2012 106 455 A1 discloses a steering system for a motor vehicle, comprising an electric motor for the indirect or direct displacement of a steering rod, wherein the electric motor comprises an electric machine with a predetermined number of phases and an inverter with an intermediate circuit capacitor that is operatively connected thereto.

[0005] Even in the event of a fault, with the control unit deactivated and the motor phases deenergized, the high cogging torque ensures that the wheels are held in position and the set wheel steering angle remains unchanged or only changes to a tolerable extent. External dynamic forces (lateral forces, longitudinal forces) acting on the steering system are compensated by the cogging torque of the electric motor and lead to no, or only a minimal, change in the position of the steering rod or spindle.

[0006] One problem with operating an electric motor with high cogging torque is that fluctuations in the output torque lead to undesirable effects in terms of noise and vibration (NVH). Therefore, a special control system is used for the electric motor. The goal is to compensate for the fluctuating cogging torque of the motor and generate a constant output torque. This control system is called anti-cogging control or anti-cogging control (ACC). To achieve a constant output torque of the electric machine, high-frequency current components are superimposed on the three-phase motor currents. The generation of these currents requires an additional voltage component, which the control unit must provide. The magnitude of this additional voltage component is proportional to the motor inductance (L), the magnitude of the current to be injected (i), and the frequency (f).The frequency of the currents is proportional to the engine speed, which significantly increases the engine's voltage requirements at high speeds. Since the maximum available voltage of the control unit is limited by the vehicle electrical system (e.g., 12V), the usable engine speed decreases, and the operating range of the electric motor is restricted.

[0007] A further disadvantage is that, when the axial adjustment force of the actuator is required, a high drive torque is needed at the spindle nut with a spindle with a high thread pitch. This means that, for the same mechanical gear ratio, the drive motor also tends to have a higher drive torque than in existing systems. Such motors are also referred to as high-torque motors. A disadvantage of high-torque motors is that, due to their design, they often have poor field weakening capabilities. This means that the speed-Z torque characteristic of the motor drops sharply with increasing speeds in the field weakening range. A wider speed range with constant power output is therefore not possible. The motor's output power drops sharply at high speeds.

[0008] If a high-torque motor with high cogging torque and anti-cogging control (ACC) is used for a steer-by-wire steering system, both disadvantages add up, and the operating range of the machine is severely restricted. The invention therefore aims to counteract these disadvantages.

[0009] This object is achieved by the features of the independent claims. Advantageous embodiments are the subject of the dependent claims. A steer-by-wire steering system for a motor vehicle is proposed, comprising an electric motor for the indirect or direct displacement of a steering rod. The electric motor is designed as a high-cogging-torque motor and has an electric machine with a predetermined number of phases and an inverter with an intermediate circuit capacitor that is operatively connected thereto. The inverter is designed as an inverter with an integrated boost converter.

[0010] The proposed steer-by-wire steering system with an HCT motor can prevent movement of the steering rod when the control unit is deactivated due to the cogging torque. Combining it with an inverter with an integrated boost converter allows the system voltage at the electric motor to be increased without additional components (DC / DC converter), thus counteracting the otherwise resulting disadvantage of a sharp drop in output power at high speeds of the HCT motor with ACC function.

[0011] In one embodiment, the inverter with integrated boost converter is formed in such a way that a battery voltage is tapped between a terminal of the intermediate circuit capacitor of the inverter and a common voltage tap of the phases of the electric machine of the electric motor.

[0012] In one embodiment, the electrical machine is a three-phase electrical machine, and the three phases are connected in a star topology.

[0013] In one embodiment, the steer-by-wire steering is designed as a rear-axle steering system, which is present in addition to the front-axle steering system in a motor vehicle.

[0014] Furthermore, an electric motor for use in a steer-by-wire steering system of a motor vehicle is provided, wherein the electric motor is designed as a high-cogging-torque motor and has an electric machine and an inverter with an intermediate circuit capacitor that is operatively connected thereto, wherein the inverter is designed as an inverter with an integrated boost converter. Further features and advantages of the invention emerge from the following description of exemplary embodiments of the invention, with reference to the figures that show details according to the invention, and from the claims. The individual features can each be implemented individually or in groups in any desired combination in a variant of the invention.

[0015] Preferred embodiments of the invention are explained in more detail below with reference to the accompanying drawings.

[0016] Figure 1 shows a schematic plan view of a rear axle with steer-by-wire steering of a motor vehicle.

[0017] Figure 2 shows a circuit arrangement in the electric motor according to the state of the art.

[0018] Figure 3 shows a circuit arrangement in the electric motor with an inverter with integrated boost converter for use in an HCT motor of a steer-by-wire steering system according to an embodiment of the present invention.

[0019] Figure 4 shows diagrams of motor operating ranges when using different electric motors and inverters.

[0020] In the following descriptions of the figures, the same elements or functions are provided with the same reference symbols.

[0021] The schematic representation according to Fig. 1 shows a vehicle axle in plan view, represented here as a rear axle with a subframe 2, which is attached to a vehicle body or forms such a body and is connected to the body of a motor vehicle. However, the invention is not limited to a rear axle. The wheels 5 and 6 are articulated to the subframe 2 by means of control arms 3, 4 as part of a wheel suspension. An actuator 10 of a steer-by-wire steering system 12 is arranged on the subframe 2. The actuator 10 is fastened to the subframe 2 by its housing 22. In the present embodiment, the actuator 10 has a continuous steering rod 24 designed as a spindle, which is guided through the housing 22 of the actuator 10. The drive motor in the form of an electric motor 29 is arranged axially parallel to the steering rod 24, but can also be designed in a different way, e.g. as a hollow shaft motor.At the ends of the steering rod 24, tie rods 23, 25 are articulated via articulated connections 27, 28, which are each articulatedly connected to a wheel carrier 7 of the wheels 5 and 6 at the end facing away from the actuator 10. The axial displacement of the steering rod 24 is achieved by means of a rotation / translation converter, which can be designed, for example, as a motion thread of a spindle drive. When the electric motor 29 rotates, a spindle nut, which is fixedly mounted in the housing 22 and rotatably mounted, is set in rotation about its longitudinal axis by means of a drive belt without slippage. The spindle nut has an internal thread which engages the external thread of a spindle (= steering rod 24) and forms a motion thread.Depending on the direction of rotation of the spindle nut, a linear displacement of the steering rod 24, which is secured against rotation about its longitudinal axis, occurs in one direction or the other along its longitudinal axis depending on the direction of rotation of the electric motor 29. The longitudinal axes of the spindle nut and spindle run at least parallel and ideally coincide. It is obvious that an axial displacement, i.e. a displacement of the steering rod 24 along the longitudinal axis in one direction or the other, results in a change in the wheel steering angle 8, 9 because the tie rods 23, 25 represent a positive connection between the wheel 5, 6 or wheel carrier 7 and the actuator 10. To steer the wheels 5, 6, these are pivoted about their vertical axis via the wheel carrier 7 to the wheel suspension 3, 4. In other words, the actuator 10 is a device that enables operation of the steering system 12.

[0022] For the proposed steering system, which is designed as a steer-by-wire steering system 12, an electric motor 29 with a high cogging torque is used, also referred to as an HCT motor for short, which is additionally equipped with an anti-cogging control (ACC). The cogging torque is used to keep the electric motor 29 stationary when the steering is at a standstill and also in the event of a fault (e.g., an interruption in the power supply), even when external forces act on the steering rod 24, so that the set wheel steering angle does not change as much as possible. The level of the cogging torque of the electric motor 29 is designed depending on the overall system, i.e., the motor vehicle, transmission, electric motor 29, and the associated components, so that when the electric motor 29 is at a standstill, any movement of the system (displacement of the steering rod 24) caused by external forces can be suppressed as much as possible, i.e., as little rotor movement as possible occurs.A change in the set wheel steering angle is thus minimized or prevented within permissible tolerances.

[0023] The cogging torque of an electric motor 29 is the torque required to overcome the interaction or attractive forces between the permanent magnets of the rotor and the stator slots (the iron core of the motor winding). This cogging torque is referred to in English as "cogging torque."

[0024] Electric motors 29 used in the automotive sector are electrically connected to a power source (not shown in the figures) in order to drive the electric machine of the electric motor 29. This also applies to steer-by-wire steering systems 12. Part of each electric motor 29 is also an inverter, i.e. a DC / AC inverter, which serves to convert the direct current (DC) output by the power source (e.g. a storage battery) to the electric motor 29 into an alternating current (AC) that can be used by the electric machine. The electric machine is designed as a PMSM (permanently excited synchronous machine) with a high cogging torque, referred to as an HCT motor (HCTM = high cogging torque motor) for short. Typically, an intermediate circuit capacitor ZK, also referred to as a DC link, is also provided between the power source and the power electronics, i.e. the inverter, and is electrically connected to the inverter.

[0025] Figure 2 shows a three-phase inverter 30, as used in electric motors 29 in the vehicle sector according to the prior art. In addition, further inverters or circuits can be provided and connected to the inverter 30 in order to fulfill various requirements. For example, boost converters or DC / DC converters can be provided and be operatively connected to the inverter 30. According to the invention, in a steer-by-wire steering system 12, the classic inverter 30 is replaced by an inverter 31 with an integrated boost converter, as disclosed, for example, in DE 10 2011 079 904 A1. The circuit topology of the drive comprising electric motor 29, inverter 31 with integrated boost converter and intermediate circuit (capacitor) ZK for a steer-by-wire steering system according to the invention is shown in Figure 3.Advantageously, the three phases of the electric motor 29 are connected in a star topology, i.e., with a center tap that serves as a common voltage tap UM for all phases. The special feature of an inverter 31 with an integrated boost converter is that the voltage tap of the battery voltage Ubat (of the DC energy source) is not located at the two terminals of the capacitor (intermediate circuit capacitor ZK), as was the case with a previously used inverter 30, see Figure 2. Rather, Ubat is tapped between a terminal of the intermediate circuit capacitor ZK and the electric machine, more precisely, a common voltage tap UM of the phases of the electric machine, as shown in Figure 3. This circuit arrangement combines a conventional inverter 30 (motor inverter) with an electronic boost converter in a single circuit arrangement.Thus, the DC link capacitor (ZK) is charged via the coils of the electric motor, which then serve as boost converters. This drive topology allows the usable motor voltage to be increased without additional components such as separate boost converters.

[0026] This circuit topology is used according to the invention in an HCT motor of a steer-by-wire steering system. The ability to increase the system voltage at the electric motor allows the speed range of the electric motor 29 to be significantly expanded, compensating for the disadvantages that would otherwise occur during operation of the HCT motor. The increase in the available system voltage can be used to provide optimal anti-cogging control with constant output torque even in the high speed range and to operate the electric motor at the permissible power limit over a wide speed range in the field-weakening range.Figure 4 shows three diagrams which show, by way of example, the motor operating ranges when using a PMSM (curve K1) or an HCTM (curve K2) with an inverter 30 according to the prior art, as shown in Figure 2, and an HCTM with an inverter 31 with an integrated boost converter (curve K3), as shown in Figure 3. The speed n of the electrical machine is shown on the X-axis. The top diagram shows the torque T on the Y-axis, the middle diagram the voltage U, and the bottom diagram the power P of the electrical machine.

[0027] It can be seen that when using standard inverters 30 with classic PMSM (curve K1 ), where no anti-cogging control (ACC) is necessary, the machine delivers a constant power in the field weakening range (from speed n2) and the machine can therefore be operated up to high speeds.

[0028] An HCTM high-torque motor with active ACC generates a higher torque in the basic control range (up to speed n1) (curves K2 and K3). In the field-weakening range (from speed n1) when operated with a standard 30-watt inverter (curve K2), the torque that can be generated drops significantly due to the increased voltage requirement of the ACC and the limited field-weakening capability of high-torque motors, causing the output power to drop to zero up to speed n3. The machine's speed operating range is severely restricted.

[0029] When using an HCTM with an inverter 31 with an integrated boost converter (curve K3), the increased system voltage provided allows the electric motor to be operated over a wider speed range (up to speed n4 compared to an HCTM with a standard inverter n3). The HCTM can be operated at a constant maximum power in the medium speed range (from speed n1), as with the PMSM.

[0030] A described electric motor 29, which is designed as an HCTM with an inverter 31 with an integrated boost converter, is used in a steer-by-wire steering system 12, which is provided, for example, in an actuator with an electric drive, advantageously in an electric rear-axle steering system, which is provided in addition to a front-axle steering system of a motor vehicle, in particular a passenger car. Thus, a steer-by-wire steering system 12 with an HCTM as the drive motor, as described above, is proposed, in which the HCTM is operated with an electric motor in which the inverter is designed as an inverter 31 with an integrated boost converter.

[0031] List of reference symbols

[0032] Subframe, vehicle body, 4 links, 6 wheels

[0033] Wheel carrier, 9 wheel steering angles

[0034] 10 Actuator

[0035] 12 Steering, steer-by-wire steering 2 Housing 3, 25 Tie rod 4 Steering rod

[0036] 27, 28 Articulated joints

[0037] 29 Electric motor

[0038] 30 inverters

[0039] 31 inverters with integrated boost converter

[0040] UM common voltage tap

[0041] Ubat battery voltage

Claims

Patent claims 1. Steer-by-wire steering system (12) for a motor vehicle, comprising an electric motor (29) for the indirect or direct displacement of a steering rod (24), wherein the electric motor (29) is designed as a high-cogging torque motor and has an electric machine with a predetermined number of phases and an inverter with an intermediate circuit capacitor (ZK) in operative connection therewith, characterized in that the inverter is designed as an inverter (31) with an integrated boost converter.

2. Steer-by-wire steering system (12) according to claim 1, wherein the inverter (31) with integrated boost converter is formed such that a battery voltage (Ubat) is tapped between a terminal of the intermediate circuit capacitor (ZK) of the inverter and a common voltage tap (UM) of the phases of the electric machine of the electric motor (29).

3. Steer-by-wire steering system (12) according to claim 1 or 2, wherein the electric machine is a three-phase electric machine and the three phases are connected as a star topology.

4. Steer-by-wire steering (12) according to one of the preceding claims, which is formed as a rear-axle steering system which is present in addition to the front-axle steering system in a motor vehicle.

5. Electric motor (29) for use in a steer-by-wire steering system of a motor vehicle, wherein the electric motor (29) is designed as a high-cogging torque motor and has an electric machine and an inverter (31) with an intermediate circuit capacitor (ZK) that is operatively connected thereto, wherein the inverter (31) is designed as an inverter (31) with an integrated boost converter.

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