Control device for controlling an electric motor in a motor vehicle steering system

Abstract

Control device for controlling an electric motor (5) of a motor vehicle steering system (1) comprising: - a power supply path (12) for supplying the control device (10) with electric current from a voltage source (11), - an inverter (13) connected to the power supply path (12) for electronically commutating the control of the electric motor (5) at an output terminal (14) of the control device (10), - a motor controller (15) and a gate driver unit (16, 16') for controlling the inverter (13) depending on a driver steering request (17), - a capacitor (18) connected in parallel to the inverter (13) for stabilizing the supply voltage present in the power supply path (12) during operation of the control device (10), - a current measuring device (19) connected in series with the capacitor (18), and - a switching device (20) connected in series with the capacitor (18), characterized in thatthat the control device (10) comprises a separate microcontroller (22) for controlling the switching device (20) with at least one input (221) and one output (222), wherein a current measurement signal (21, 21') received by the current measuring device (19) can be supplied to the input (221) and the microcontroller (22) is configured to output a switching signal (23) at the output (222) depending on the current measurement signal (21, 21'), which can be supplied to the switching device (20).

Description

The invention relates to a control device for controlling an electric motor of a motor vehicle steering system according to the preamble of claim 1. A control device of this type is known from DE 10 2019 200 091 A1. The known control device incorporates a protective mechanism to disconnect it from the connected power source in the event of a short circuit within the device. Such a protective mechanism is particularly advantageous when the control device is powered independently of the mains electricity supply by a mobile power source, such as a battery. For example, a short circuit in a vehicle steering control unit can cause a drop in the vehicle battery voltage, discharging it within a short time to such an extent that the other electrical components of the vehicle's electrical system can no longer operate properly. Control devices for operating the electric motor of a steering actuator in a motor vehicle steering system typically include an inverter for phase-correct electronic commutation of the DC voltage supplied by the voltage source. These control devices also typically include at least one capacitor connected in parallel to the inverter to accommodate the rapidly fluctuating current demand of the inverter. Faulty conditions in these capacitors can lead to serious limitations in the operability of the vehicle's steering system and therefore pose a significant risk to the vehicle's occupants. Basically, three fault conditions are conceivable for the capacitors: short circuit, open circuit, and altered capacitance. While the latter two fault conditions allow for some residual functionality of the electric motor and can be easily detected by the motor controller software, a short circuit in the capacitor leads to a blockage or braking torque of the electric motor, as the capacitor creates a direct connection between the terminals of the voltage source and short-circuits the electric motor. To release the blockage of the electric motor and prevent the vehicle battery from discharging, a protective mechanism is required in the control unit that can detect these faults and react by disconnecting the affected capacitor from the voltage source via a switching element. When selecting the switching element and its control, it must be considered that the resulting short-circuit currents can reach the order of 1000 A within a period of approximately 100 µs. Therefore, either switching elements designed for such high currents must be used, or the control of the switching elements must ensure that the switching element opens before the short-circuit current exceeds its maximum switching current. For example, MOSFETs cannot be switched on once the short-circuit current exceeds the MOSFET's avalanche current level. DE 10 2016 203 504 A1 describes a method and a control unit for operating an electric motor, wherein the motor is controlled by means of an inverter for controlling motor coils. An input voltage is checked for falling below a first voltage value, and a diagnostic current dependent on an input current is checked for exceeding one or more current values. If these checks are successful within a specific time window, one or more shutdown measures are initiated. DE 10 2018 124 906 A1 discloses a motor vehicle steering system with an electric motor and a control unit, which has at least two redundant control paths for controlling the electric motor. An imbalance detection device compares the electrical currents of both control paths and, if an imbalance is detected, disconnects the faulty control path. Each control path has an inverter, a current measuring device, and a switching element for this purpose. With the faulty control path disconnected, the electric motor can continue to be controlled by means of the fault-free control path. In DE 10 2019 200 091 A1, MOSFETs or IGBTs are used as switching elements. To ensure sufficiently fast control of the switching elements, the control circuit is implemented as a hard-wired circuit made up of individual components. Disadvantages include the high complexity and space requirements of this circuit during manufacturing, as well as the limited operational flexibility of hard-wired control circuits. The object of the present invention is to provide a control device for controlling an electric motor of a motor vehicle steering system, which can monitor the current flow in a capacitor connected in parallel to the inverter of the control device in a responsive and flexible manner and is at the same time simple and compact in design. This problem is solved by a control device for controlling an electric motor of a motor vehicle steering system with the features of claim 1. This creates a control device for controlling an electric motor of a motor vehicle steering system, comprising a power supply path for supplying the control device with electrical current from a voltage source, an inverter connected to the power supply path for electronically commutating the electric motor at an output terminal of the control device, a motor controller, and a gate driver unit for controlling the inverter depending on a driver steering input. The control device further includes a capacitor connected in parallel to the inverter for stabilizing the supply voltage present in the power supply path during operation of the control device, a current measuring device connected in series with the capacitor, and a switching device connected in series with the capacitor.According to the invention, the control device further comprises a separate microcontroller for controlling the switching device, with at least one input and one output, wherein a current measurement signal acquired by the current measuring device can be supplied to the input. The microcontroller is configured to output a switching signal at the output, depending on the current measurement signal, which can be supplied to the switching device. By using a dedicated microcontroller to control the switching device, it is possible to ensure a sufficiently short response time to interrupt a short-circuit current through the capacitor in the event of a fault. At the same time, the number of additional individual components required for controlling the switching device is significantly reduced. Furthermore, using a separate microcontroller to control the switching device offers the advantage over implementing the same functionality within the motor controller that it eliminates the need to adjust the motor controller's performance to ensure the required response time for reliably interrupting short-circuit currents.Furthermore, the possibility of modifying the signal processing in the microcontroller via software enables flexible operating scenarios for current monitoring in the capacitor, such as current limiting during a switch-on process. Preferably, the switching device in the control device according to the invention is formed by a semiconductor switching element, such as a MOSFET or IGBT. In preferred embodiments, the microcontroller has a smaller processing width and / or a shorter clock speed than the motor controller. To perform the task of monitoring the current flow in the capacitor, a microcontroller with a smaller processing width compared to the motor controller, for example, 8 bits, can be used. The smaller processing width enables shorter clock speeds with a comparatively simpler chip design. Therefore, using such a microcontroller results in faster, simpler, and more robust monitoring of the capacitor currents in the control device. In preferred embodiments, the microcontroller is configured to output an opening command to the switching device as a switching signal when the current measurement signal exceeds a predefinable threshold. In these embodiments, short-circuit monitoring of the capacitor in the control device is implemented. An advantage is that the threshold can be adjusted, for example, via software, using a microcontroller. Thus, the threshold can be predefinable by the motor controller, for instance. By specifying a very high or infinite threshold, the short-circuit monitoring can also be temporarily deactivated. In particular, it is preferred that the microcontroller be configured to detect whether the threshold has been exceeded before issuing the open command, based on at least two consecutive evaluations of the current measurement signal. For example, filtering of interference pulses can be provided, preferably based on two to five evaluations of the current measurement signal. In this way, random, temporary threshold exceedances can be distinguished from genuine error events, thereby increasing the robustness of the control system. Preferably, the microcontroller is configured with a response time for issuing the opening command that is less than 20 µs, preferably less than 15 µs. Providing a response time in this range ensures that no significant discharge of the voltage source and / or blocking of the electric motor occurs. Furthermore, such a response time is advantageous in conjunction with MOSFETs as switching elements, since the short-circuit current during this response time is generally insufficient to trigger an avalanche effect in the MOSFET. In preferred embodiments, a signal amplifier is interposed between the current measuring device and the microcontroller to amplify the current measurement signal. By adjusting the amplification of the current measurement signal, an amplified current measurement signal can be generated that is adapted to a permissible voltage range at the input of the microcontroller. The signal amplifier is preferably located within the gate driver unit. This allows for advantageous utilization of the existing component functionalities of the gate driver unit and reduces the number of additional individual components. The robustness of the control device is further increased. Preferably, at least one gain parameter of the signal amplifier is adjustable by the motor controller. For example, the gain of the signal amplifier can be increased during a functional test of the short-circuit monitoring to ensure that the microcontroller outputs a switching signal to the switching device even at normal capacitor charging currents. Such a functional test can be performed, for example, when the control device is switched on. Further advantages of the invention arise when the microcontroller is configured to output a pulse-width modulated (PWM) signal to the switching device as a switching signal in order to limit the current flow in the capacitor. Even during normal operation of the capacitor, high charging currents occur at the capacitor, particularly when the vehicle's power steering is switched on. These high currents can cause damage or aging of the capacitor over time, reducing its service life. For this reason, it is advantageous to limit the current flow in the capacitor by means of a PWM switching signal. For example, after the vehicle's power steering is switched on, the microcontroller can apply a PWM switching signal with a predetermined duty cycle to the switching device for a predetermined period to limit the inrush current.Alternatively or additionally, it is also conceivable to regulate the capacitor current by adjusting the duty cycle of the pulse-width modulated signal depending on a deviation of the current measurement signal from a setpoint. In some embodiments, the on-times of the pulse-width modulated signal are in the range of 5 µs to 75 µs, preferably in the range of 10 µs to 60 µs, and particularly preferably in the range of 20 µs to 50 µs. Further embodiments of the invention can be found in the following description and the dependent claims. The invention is explained in more detail below with reference to the exemplary embodiments shown in the accompanying figures. Fig. 1 schematically shows a motor vehicle steering system with an electronic control device according to the invention for controlling an electric motor of the steering actuator of the motor vehicle steering system. Fig. 2 schematically shows the structure of the control device according to Fig. 1 in a detailed view. Fig. 3 schematically shows time profiles of currents and voltages occurring in the control device during a short-circuit fault of the capacitor. Figure 1 schematically shows the structure of a motor vehicle steering system 1. The motor vehicle steering system 1 has a steering input device 3 designed as a steering wheel, which is connected to a steering input sensor 4 via a steering shaft 2. The steering input sensor 4 is designed to determine a steering input 17 entered by the driver. The driver's steering input 17 is determined, for example, based on the steering wheel angle and / or steering torque. The steering shaft 2 extends to a steering gear 7, in which the steering input is converted into a corresponding translation of a rack 6. The translations of the rack 6 are transmitted via tie rods 9 to steered wheels 8 in order to set the wheel steering angle of the wheels 8 specified by the steering input. To assist in setting the desired wheel steering angle, an electric motor 5 acts as a steering actuator on the steering shaft 2 – or, in alternative embodiments, on the rack. The electric motor 5 is controlled by a control device 10 according to the invention, depending on the driver's steering input 17 received by the steering input sensor 4. The control device 10 is connected to a power supply 11. The power supply 11 is typically a vehicle battery. For communication with a higher-level vehicle control system and / or other vehicle components, the control device 10 may be provided with an additional input line, which could, for example, be a vehicle bus 26. The transmission of the driver's steering command 17 to the control device can also be carried out via a vehicle bus, either alternatively or additionally. Figure 1 shows an example of an electromechanical motor vehicle steering system. It is understood that the invention can be used in the same way in other vehicle steering systems, for example, a steer-by-wire steering system. Fig. 2 schematically shows the structure of the control device 10 for controlling the electric motor 5 of the vehicle steering system 1 (see Fig. 1). This control device 10 includes a power supply path 12 for supplying the control device 10 with electrical current from the voltage source 11. The power supply path 12 has a main branch 12.1 connected to the voltage source 11, which branches into an inverter branch 12.2 to supply an inverter 13 connected to the power supply path 12 and a capacitor branch 12.3, which supplies a capacitor 18 connected in parallel to the inverter 13. The inverter 13 is designed for the electronically commutated control of the electric motor 5, which can be connected to an output terminal 14 of the control device 10. To control the inverter 13 in response to the driver's steering input 26, the control device comprises a motor controller 15, typically designed as a microcontroller, and a gate driver unit 16. The gate driver unit 16 controls switching elements in the inverter according to specifications from the motor controller 15. The switching elements in the inverter are typically designed as MOSFETs, for which the respective gate electrode must be recharged to switch. The gate driver unit 16 applies the required voltages / currents to the switching elements at the intended switching time. Furthermore, the gate driver unit 16 preferably includes at least one signal processing unit to receive current measurement signals from a current measuring device 25, which detects the currents flowing through the phases of the electric motor. The processed current measurement signals are made available to the motor controller 15 for motor control. Capacitor 18 serves to stabilize the supply voltage present in the power supply path 12 during operation of the control device 10. A current measuring device 19 and a switching device 20 are connected in series with capacitor 18. The switching device 20 is preferably formed by a MOSFET or IGBT. The current measuring device is, for example, designed as a shunt resistor. In Fig. 2, the switching device 20 is arranged downstream of the capacitor 18 in the capacitor branch 12.3. It is understood that in other embodiments of the invention, the switching device 20 can be arranged at any other location in the capacitor branch 12.3 or even in the main branch 12.1. For controlling the switching device 20, the control device 10 has a separate microcontroller 22, independent of the motor controller 15. The microcontroller 22 has at least one input 221 and one output 222. A current measurement signal 21', acquired by the current measuring device 19, can be supplied to the input 221. The microcontroller 22 is further configured to output a switching signal 23 at its output 222, depending on the current measurement signal 21', which can be supplied to the switching device 20. In the illustrated embodiment, a signal amplifier 24 is interposed between the current measuring device 19 and the microcontroller 22 to amplify the current measurement signal 21. The current measurement signal 21' amplified by the signal amplifier 24 is thus supplied to the microcontroller 22. In alternative embodiments, however, it is also conceivable to supply the current measurement signal 21 directly to the microcontroller 22. In such cases, a signal amplifier can either be omitted, or its functionality can be integrated into the current measuring device 19 or the microcontroller 22. The arrangement shown in Fig. 2 is particularly preferred, in which the signal amplifier 24 is located in the gate driver unit 16'. Conventional gate driver units 16, 16' already have several signal amplifiers for processing the current measurement signals from the current measurement unit 25. Of these signal amplifiers present in the gate driver unit 16', one selected signal amplifier 24 can be used to amplify the current measurement signal 21. In this way, a very robust design is achieved with few additional components. At least one gain parameter of the signal amplifier 24 can be adjusted by the motor controller 15. For example, the gain factor and / or the offset of the signal amplifier 24 can be adjusted. A signal line from the motor controller 15 to the signal amplifier 24 is provided in Fig. 2 for supplying the gain parameters. Preferably, the microcontroller 22 has a smaller processing width, for example 8 or 16 bits, and / or a shorter clock speed than the motor controller 15. A clock speed of 250 µs is typically sufficient for the motor controller 15 to control the electric motor 5, whereas such a clock speed cannot guarantee a sufficiently short response time for monitoring the capacitor current. Therefore, by using a separate microcontroller 22 for monitoring the capacitor current, a costly performance upgrade of the motor controller 15 can be avoided using simple electronic components. The separate microcontroller 22 reduces the load on the motor controller 15, enabling it to operate within tighter time tolerances when controlling the motor. Fig. 3 shows various simulated measurement signals during the operation of the control device 10 according to the invention when a short circuit fault occurs in the capacitor 18. Fig. 3 relates to an embodiment of the control device 10 according to the invention, in which the microcontroller 22 is configured to output an opening command 23' to the switching device 20 as a switching signal 23 when the current measurement signal 21, 21' exceeds a predefinable threshold value S. The top row of Fig. 3 shows a short-circuit signal SC, which, by means of a rectangular pulse, illustrates the duration of the simulated short circuit in the capacitor 18 from time t1 to time t4. The second row shows a total current I1 flowing in the main branch 12.1 of the power supply path. This total current I1 consists of a current flowing in the inverter branch 12.2 (not shown) and a current I2 flowing in the capacitor branch 12.3, which is plotted in the third row of Fig. 3. Finally, the bottom row of Fig. 3 shows the source-drain voltage at the switching element 19. As can be seen in Fig. 3, the short circuit at time t1 causes a rapidly increasing current I2 in the capacitor branch 12.3, which first exceeds the predetermined threshold S at time t2. The microcontroller 22 is configured with a response time T, after which an opening command 23' is issued at time t3, opening the switching device 20. After the switching device 20 opens at time t3, the currents I1 and I2 drop – oscillating due to the (residual) capacitances and inductances present in the circuit. The reaction time T is preferably less than 20 µs, preferably less than 15 µs. During such a period, a short-circuit current in the capacitor branch 12.3 typically reaches an order of magnitude of approximately 100 A. Such currents can still be interrupted by semiconductor switching elements 20 such as MOSFETs without an avalanche effect occurring. The microcontroller 22 is preferably configured to detect whether the threshold S has been exceeded before issuing the open command 23', based on at least two consecutive evaluations of the current measurement signal 21, 21'. Therefore, within the response time T, the microcontroller 22 preferably performs two to five measurement samples of the current measurement signal 21' and determines, for example, by means of a glitch filter, whether the threshold S has been exceeded due to a true fault event. The evaluation of the current measurement signal 21, 21' and / or the response time T can be adjusted via the software of the microcontroller 22. If a true fault cause must be assumed, the open command 23' is issued. The microcontroller 22 can further be configured to output a pulse-width modulated signal 23'' to the switching device 20 as a switching signal 23 in order to limit the current flow in the capacitor 18. The on-times of the pulse-width modulated signal 23'' can be in the range of 5 µs to 75 µs, preferably in the range of 10 µs to 60 µs, and particularly preferably in the range of 20 µs to 50 µs. The off-times can be in the range of 100 µs. The on-time pattern of the pulse-width modulated signal 23'' can be adapted via the software of the microcontroller 22. The use of a pulse-width modulated signal 23'' to limit the capacitor current can prevent fault conditions in the capacitor 18 and stabilize the voltage supplied by the voltage source 11, particularly during the switch-on process of the control device 10. Such voltage fluctuations could otherwise lead to malfunctions, such as a restart of the motor controller. Reference symbol list 1 Vehicle steering 2 Steering shaft 3 Steering input device 4 Steering input sensor 5 Electric motor 6 Rack and pinion 7 Steering gear 8 Wheels 9 Tie rods 10 Control device 11 Voltage source 12 Power supply path 12.1 Main branch 12.2 Inverter branch 12.3 Capacitor branch 13 Inverter 14 Output connector 15 Motor controller 16, 16' Gate driver unit 17 Driver steering request 18 Capacitor 19 Current measuring device 20 Switching device 21 Current measuring signal 21' Amplified current measuring signal 22 Microcontroller 23 Switching signal 23' Open command 23'' Pulse-width modulated signal 24 Signal amplifier 25 Current measuring device 26 Vehicle bus 221 Microcontroller input 222 Microcontroller output SC Short-circuit signal I1 Total current in main branch I2 Current in capacitor branch U Voltage at switching device T Response time

Claims

Control device for controlling an electric motor (5) of a motor vehicle steering system (1) comprising: - a power supply path (12) for supplying the control device (10) with electric current from a voltage source (11), - an inverter (13) connected to the power supply path (12) for electronically commutating the control of the electric motor (5) at an output terminal (14) of the control device (10), - a motor controller (15) and a gate driver unit (16, 16') for controlling the inverter (13) depending on a driver steering request (17), - a capacitor (18) connected in parallel to the inverter (13) for stabilizing the supply voltage present in the power supply path (12) during operation of the control device (10), - a current measuring device (19) connected in series with the capacitor (18), and - a switching device (20) connected in series with the capacitor (18), characterized in thatthat the control device (10) comprises a separate microcontroller (22) for controlling the switching device (20) with at least one input (221) and one output (222), wherein a current measurement signal (21, 21') received by the current measuring device (19) can be supplied to the input (221) and the microcontroller (22) is configured to output a switching signal (23) at the output (222) depending on the current measurement signal (21, 21'), which can be supplied to the switching device (20). Control device according to claim 1, characterized in that the microcontroller (22) has a smaller processing width and / or a smaller clock time than the motor controller (15). Control device according to claim 1 or 2, characterized in that the microcontroller (22) is configured to output an opening command (23') to the switching device (20) as a switching signal (23) when the current measurement signal (21, 21') exceeds a predefinable threshold value (S). Control device according to claim 3, characterized in that the microcontroller (22) is configured to determine the exceedance of the threshold value (S) before issuing the opening command (23') by means of at least two successive evaluations of the current measurement signal (21, 21'). Control device according to claim 3 or 4, characterized in that the microcontroller (22) is configured with a response time (T) for the output of the opening command (23') which is less than 20 µs, preferably less than 15 µs. Control device according to one of claims 1 to 5, characterized in that a signal amplifier (24) for amplifying the current measurement signal (21) is connected between the current measuring device (19) and the microcontroller (22). Control device according to claim 6, characterized in that the signal amplifier (24) is arranged in the gate driver unit (16'). Control device according to claim 6 or 7, characterized in that at least one gain parameter of the signal amplifier (24) is adjustable by the motor controller (15). Control device according to one of claims 1 to 8, characterized in that the microcontroller (22) is configured to output a pulse width modulated signal (23'') to the switching device (20) as a switching signal (23) in order to limit a current flow in the capacitor (18). Control device according to claim 9, characterized in that the on-times of the pulse width modulated signal (23'') are in the range of 5 µs to 75 µs, preferably in the range of 10 µs to 60 µs, particularly preferably in the range of 20 µs to 50 µs.

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