Method for operating an electronic power stage

Abstract

Method for operating an electric auxiliary or power steering system with an n-phase electric motor (108) for initiating an auxiliary torque supporting the steering force or for additively superimposing a steering angle applied by the driver with an additional steering angle, and a control unit (130) which generates corresponding control signals in the form of a PWM pattern for the purpose of electronically commutation of the electric motor (108) and which is provided for influencing switching times of pairs of semiconductor switches (202) arranged in bridge branches, each comprising a high-side and a low-side semiconductor switch, and wherein the semiconductor switches (202) are thermally connected to a cooling element or housing (201),characterized by the following process steps: - Determination of the temperature (TG) of the base plate - Determination of the temperature (TJ) of a semiconductor switch (202) - Determination of the temperature difference (TD) between the base plate and semiconductor switch (202), and - a modification of the modulation of the PWM pattern such that the heat generated in the at least one semiconductor switch (202) or the at least one pair of semiconductor switches is limited or reduced, so that the determined temperature difference (TD) does not exceed a predefinable limit (THUB,max).

Description

[0001] The invention relates to a method for operating an electronic power stage in an electric power steering system according to the preamble of the main claim. State of the art

[0002] DE 102 005 045 715 A1 discloses a method for operating an electronic power unit in an electric power steering system. This method proposes a method for limiting the temperature of the power unit, in which the power loss of a bridge component is calculated, and the resulting temperature increase at the component is determined. The temperature of a base plate of the power unit is then added to this temperature increase. If the determined temperature of the bridge component exceeds a certain maximum temperature, the output torque of the electric power steering motor is reduced.

[0003] In publication US 2004 / 0124808A1, a motor control is proposed in which the boundary layer temperature of a semiconductor switch of an electronic output stage is determined and subsequently compared with a predetermined temperature limit, the aim of the comparison being to set the instantaneous boundary layer temperature equal to or less than the temperature limit in the event that the boundary layer temperature exceeds the temperature limit.

[0004] In the German application with file number 10 2008 054744.1, which was not published at the time of filing, a method for operating an electronic power unit is described, the disclosure content of which is included here.

[0005] Typically, at low speed and high current in a servo motor of an electric power steering system, the temperature of the semiconductor switches driving the electric motor rises rapidly. The absolute magnitude of this rise is usually limited and is primarily determined by compliance with the maximum permissible transistor temperature. Therefore, with low starting transistor temperatures, the difference between the starting temperature (e.g., 0°C) and the final temperature (e.g., 150°C) can be large. Large temperature fluctuations are detrimental to the service life of the power transistors and also stress the design of the power output stage, particularly its interconnection technology.

[0006] The object of the present invention is to achieve safe operation and an extension of the expected service life of an electronic power unit of an electric auxiliary or external power steering system.

[0007] This problem is solved by the characterizing features of claim 1. Advantageous embodiments of the method are specified in the dependent claims.

[0008] According to the invention, the following process steps are carried out: – Determination of the temperature of the base plate TG – Determination of the temperature of a semiconductor switch TJ – Determination of the temperature difference TD between the base plate TG and semiconductor switch TJ, and – a modification of the PWM pattern is carried out such that the heat generated in at least one semiconductor switch or the at least one pair of semiconductor switches is limited or reduced, so that the determined temperature difference TD does not exceed a predefinable limit THUB,max.

[0009] This has a positive effect on the expected overall service life of the steering system. In addition, it makes it possible to optimize and therefore cost-effectively dimension the power stage.

[0010] Fig. 1 shows schematically the structure and arrangement of an electric power steering system and its system components in a motor vehicle according to the prior art.

[0011] A driving direction request initiated by the driver at the steering handle 101 in the form of a steering angle ΔS is transferred to a steering gear 107 of the electric power steering system 106 by means of a steering column consisting of the two steering shaft sections 102 and 103. The rotational movement of the steering column is converted into a translational movement ZS of a rack 109 and transmitted by means of tie rods 110 to the steered wheels 111 of an axle to set a wheel steering angle ΔR. A torsion element 105 is arranged between the low-torque side steering shaft section 102 and the high-torque side steering shaft section 103, by means of which a torque can be detected. The torsion element 105 is designed as a torsion bar with a known torsion bar stiffness, on which an angle difference ΔHM between the input and output side steering shaft section 102, 103 can be detected by means of a torque and / or combined angle difference sensor 104.The torsion element 110 has an inherent stiffness (torsion bar stiffness). This quantity is also a measure of the force acting on the steering shaft 103. Hand torque MH, which is perceived by the driver at the steering handle 101. To initiate a steering torque MS, which assists the driver's steering force, an electric motor 108 is provided, which is operatively connected to the rack 109 via a gear stage. An electronic power control unit (PCU) 110 energizes the phases U, V, W of the electric motor EMot 108 according to a target torque MSS specified by a control unit 130.

[0012] To determine the target value MSS of the support torque Ms, the control unit 130 receives information about the measured steering wheel torque MH or an angle difference value ΔΔHM as a key input. A steering function controller (LFR) 131 determines the target value MSS of the support torque from these and other vehicle-specific values. The current vehicle speed v is provided as another key input value for the LFR. Other values ​​can also be supplied to the LFR, such as vector-based vehicle dynamics values ​​v, a, which describe the current motion of the vehicle. These include, among others, the vehicle's longitudinal speed vx and the longitudinal and lateral acceleration ax and ay.

[0013] A subsequent control stage 132 performs field-oriented control (FOR) to generate pulse-width modulated control signals PWM_UVW, which are supplied to the PCU to drive a corresponding phase current I_UVW in the phases UVW of the electric motor 108. A further input variable of the FOR is the motor-side rotor position angle θ, as well as the measured phase currents I_UVW of the electric motor 108. By appropriately setting the target value, the assist torque MS can support or counteract the steering movement. This constitutes the primary function of a steering system of this type. In principle, additional functionalities can be implemented with such a system, in particular comfort (parking assistance, steering feedback) and safety functions (vehicle dynamics interventions, crosswind compensation).

[0014] To determine the driver's steering angle Δs, an index sensor 120 is arranged in an upper shaft section 102 of the steering column. Index information Si derived from the output signal of the sensor is supplied to the control unit 130.

[0015] A part of the index sensor 120 is rotationally fixed to the shaft section 102 and consists of at least one position marker 122, which is designed as a magnetic pole. During the rotational movement of the steering shaft, changes in rotational movement with respect to a stationary, magnetic field-sensitive sensor 121 can be detected.

[0016] By means of a vehicle bus system 141, which serves as an interface 140 to other systems belonging to the vehicle network, for example a higher-level vehicle control computer (FCC), the control unit 130 can receive external data and send internal, steering-specific data. Received data includes vehicle dynamics information suitable for describing the current movement of the vehicle. In particular, this includes the current longitudinal speed of the vehicle vx, the wheel speed information ΔR of the wheels, as well as parameters describing the yaw, pitch, and roll of the vehicle. Further received data includes information suitable for describing the current operating state of the vehicle: This includes information about the speed of the internal combustion engine, information about the ignition state, and the state of charge of the vehicle's electrical system.Accordingly, an operating mode is possible in which the control unit 130 receives the target support torque MSS as a specification from the FSR. Internal steering-specific information is provided as transmission data, which the control unit 130 can process, validate, calculate, and transmit due to its connection to steering-specific sensors. This essentially includes the current steering angle Δs, which the control unit can derive and validate based on the rotor position angle Δph and the index information Si from the index sensor 120. The information about the current steering angle can thus be evaluated by the FSR or other vehicle systems.

[0017] Fig. 2 schematically shows part of the structure of the electronic power unit in a steering system for controlling / commutating an electric motor. Electronic switches 202 for transferring electrical power losses dissipated as heat are mounted on a heat sink 201, which can also be part of the steering system housing. The switches are typically designed as FET semiconductor switches and arranged in a bridge circuit for controlling / commutating the motor current of a servo motor.

[0018] A temperature sensor 203 serves to measure the temperature TG of the heat sink 201. The pulse-width modulated control signals PWM_UVW, which are converted into complementary signals UHS, ULS, VHS, VLS, WHS, WLS, are supplied to the semiconductor switches at their control electrodes (gates) for controlling switching times.

[0019] Fig. 3 shows the internal structure of a power unit for controlling an electric motor EMot of an electric power steering system. Functionally, the power unit shown (reference numeral 112 in Fig. 1) consists of an electronic inverter WR. In the intermediate circuit ZK of the inverter, electrical storage elements for current and voltage are arranged, which are combined in block 301. The electric motor is arranged in the load circuit LK, with its three phases U, V, W connected in a B6 bridge such that they can be connected with low resistance to the positive side of the supply voltage Ubat in the case of the high-side switches and to the negative side of the supply voltage in the case of the low-side switches by means of two associated electronic switches S_UHS, S_ULS. The phases of the electric motor are connected to the phase outputs of the bridge branches at the center point, between the high-side and low-side semiconductor switches.A matching high-side and low-side semiconductor switch each form a semiconductor switch pair. Pulse-width modulated control signals (PMW_UVW) are provided for controlling the electronic switches. These signals are generated by a field-oriented control unit (FOR) and conditioned by special driver stages to control the semiconductor switch. The windings of the electric motor (EMot) are connected to a star point with a common reference potential (UN) within the motor. The motor is controlled relative to a star point voltage, which is normally approximately Ubat / 2. This allows for a symmetrical power reserve or a symmetrical modulation range. The design and basic operating principle are known from the prior art and serve only to illustrate the integration of the present invention.

[0020] Fig. 4a shows the modification of the modulation method according to the invention in a first variant in which switching operations are omitted.

[0021] Fig. 4b shows the modification of the modulation method according to the invention in a second variant, in which an increasing symmetrization of the switching pattern takes place. In the two figures, the control signals PWM_U, PWM_V, PWM_W assigned to the three phases U, V, W of the electric motor are shown with respect to a symmetry AS and a pulse period TPWM, corresponding to the fundamental frequency 1 / TPWM of the inverter. A logical state “High” here means that a semiconductor switch S_UHS, S_VHS, S_UHS of a related semiconductor switch pair is in a conductive, low-resistance state and its complementary switch S_ULS, S_VLS, S_WLS is in a non-conductive, high-resistance, non-conductive state. The application of the methods according to the invention is shown here using the example of the control signal PWM_U corresponding to phase U, which has been identified as the currently relevant load bridge branch Ln. In the Fig. 4a and Fig.4b represents the signal waveform PWM_UVW as an unmodified waveform of a momentary commutation situation according to a specific modulation method, and the signal waveform PWM_U'V'W (dashed switching edges) as a modified waveform according to the two variants of the method according to the invention.

[0022] Fig. 4a visualizes the first basic method: The switching losses PV,el,Schalt caused by physical processes in the semiconductor material in the corresponding switches S_UHS and S_ULS of its bridge branch are reduced to zero by setting the switching edges and / or switching times so that no switching operations are triggered within one TPWM period of the inverter's fundamental frequency. This can also be achieved, for example, by directly influencing control parameters in the FOR unit. With respect to the phase identified as the current load bridge branch Ln (here phase U), the switching edges are completely eliminated by appropriately modifying the associated signal. Since the high-side and low-side switches of a phase are controlled in opposite directions, this means that no switching-related power load or switching-related power loss is generated in any of the switches.For control engineering reasons, the switching times of the remaining phases must be adjusted accordingly.

[0023] Fig. 4b visualizes the second basic method according to the invention: Here, the modulation is modified to increasingly symmetrize the PWM pattern of a semiconductor switch pair belonging to a bridge branch of the identified load bridge branch Ln (here phase U). This results in a progressively more even distribution of the power load and power dissipation of the high-side and low-side switches S_UHS and S_ULS compared to conventional modulation within the bridge branch. The symmetrization of the PWM pattern in the identified load bridge branch Ln can be progressively symmetrical until the switching operations of the semiconductor switch pair belonging to a phase within one PWM period (TPWM) are completely eliminated in the bridge branches assigned to the remaining n – 1 phases (here one of phases V or W). An "ideal" symmetrization of the power distribution between the high-side and low-side semiconductor switches of a branch is achieved when the duty cycle is 50%.According to the invention, it is provided that in the course of symmetrization the situation of the power distribution. The system continuously improves, but the balancing is only carried out up to a certain power distribution ratio. This increasing balancing results in no switching operations occurring in one of the remaining bridge branches. This ratio typically deviates from the "ideal" value of the test ratio. Nevertheless, this approach achieves a better power distribution in the identified load bridge branch.

[0024] The variants shown in Figs. 4a and 4b demonstrate the application of the methods according to the invention with respect to symmetrical PWM generation with an axis of symmetry AS. In principle, both variants shown can also be applied to non-symmetrical PWM generation.

[0025] The invention relates to a method for operating an electric auxiliary or power steering system with an n-phase electric motor for introducing an auxiliary torque supporting the steering force or for additively superimposing a steering angle imposed by the driver with an additional steering angle, and a control unit which generates corresponding control signals in the form of a PWM pattern for the purpose of electronic commutation of the electric motor and which are provided for influencing switching times of semiconductor switch pairs arranged in bridge branches, each comprising a high-side and low-side semiconductor switch, and wherein the semiconductor switches 202 are thermally connected to a cooling or housing body 201.

[0026] The method according to the invention comprises the following steps: – Determination of the temperature of the base plate TG – Determination of the temperature of a semiconductor switch TJ – Determination of the temperature difference TD between the base plate TG and semiconductor switch TJ, and – a modification of the PWM pattern is carried out such that the heat generated in the at least one semiconductor switch 202 or the at least one pair of semiconductor switches is limited or reduced, so that the determined temperature difference TD does not exceed a predefinable limit THUB,max.

[0027] Another solution provides that the modulation of the PWM pattern is modified such that the control signals do not initiate any switching operations in the at least one pair of semiconductor switches.

[0028] Alternatively, it is provided that the PWM pattern is modified in such a way that an increasing symmetrization of the PWM pattern takes place, which as a result distributes the power load of the two semiconductor switches of the semiconductor switch pair increasingly equally.

[0029] Essentially, the modification in the modulation is intended to reduce or limit the effective electric current of the semiconductor switches.

[0030] It is advantageous if the modification in the modulation is carried out as a result of a change in the target values ​​for a motor torque MS or a motor current MI of the electric motor. It is intended to limit or reduce the motor current MI or the motor torque MS over a period of time.

[0031] A further advantageous development of the method provides that the modification in the modulation is carried out depending on the determined temperature difference value TD.

[0032] Another embodiment provides that the temperature of the semiconductor switch TJ and / or the temperature of the base plate TG is determined using a corresponding physical temperature model of the base plate or the semiconductor switch.

[0033] Furthermore, it is provided that an overall temperature model is used which takes into account the heat transfer between the semiconductor switches 203 and the heat sink 202.

[0034] The limit value of the temperature difference THUB,max is set to a value within a range of approximately 40°C to approximately 80°C. This allows for a sufficiently high power reserve without subjecting the semiconductor switches to exceptionally high thermal loads. QUOTES INCLUDED IN THE DESCRIPTION

[0035] This list of documents cited by the applicant was generated automatically and is included solely for the convenience of the reader. The list does not form part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0036] - DE 102005045715 A1

[0002] - US 2004 / 0124808 A1

[0003] - DE 102008054744

[0004]

Claims

[1] Method for operating an electric power steering system with an n-phase electric motor for introducing an auxiliary torque supporting the steering force or for additively superimposing a steering angle applied by the driver with an additional steering angle, and a control unit which generates corresponding control signals in the form of a PWM pattern for the purpose of electronically commutation of the electric motor and which are provided for influencing switching times of pairs of semiconductor switches arranged in bridge branches, each comprising a high-side and a low-side semiconductor switch, and wherein the semiconductor switches (202) are thermally connected to a cooling or housing body (201), characterized by the following process steps: – Determination of the temperature of the base plate (TG) – Determination of the temperature of a semiconductor switch (TJ) – Determination of the temperature difference (TD) between the base plate (TG) and semiconductor switch (TJ), and – a modification of the PWM pattern is carried out such that the heat generated in the at least one semiconductor switch (202 ) or the at least one pair of semiconductor switches is limited or reduced so that the determined temperature difference (TD) does not exceed a predefinable limit value (THUB,max). [2] Method according to claim 1, characterized in that the modulation of the PWM pattern is modified such that the control signals do not initiate any switching operations in the at least one pair of semiconductor switches. [3] Method according to claim 1, characterized in that the PWM pattern is modified such that an increasing symmetrization of the PWM pattern takes place, which as a result distributes the power load of the two semiconductor switches (3 ) of the semiconductor switch pair increasingly equally. [4] Method according to any one of the preceding claims 1 to 3, characterized in that the modification in the modulation causes a reduction or limitation of the effective electric current of the semiconductor switches (3 ). [5] Method according to any one of the preceding claims 1 to 4, characterized in that the modification in the modulation is carried out as a result of a change in the target specifications for a motor torque (Ms) or a motor current (MI) of the electric motor. [6] Method according to claim 5, characterized in that the motor current (MI) or the motor torque (Ms) is limited or reduced over a period of time. [7] Method according to one of claims 1 to 5, characterized in that the modification in the modulation is carried out depending on the determined temperature difference value (TD). [8] Method according to any one of the preceding claims 1 to 7, characterized in that the temperature of the semiconductor switch (TJ) and / or the temperature of the base plate (TG) is determined using a corresponding physical temperature model of the base plate or the semiconductor switch. [9] Method according to claim 8, characterized in that an overall temperature model is used which takes into account the heat transfer between semiconductor switch (3 ) and base plate (2 ). [10] Method according to any one of the preceding claims 1 to 9, characterized in that the limit value of the temperature difference (THUB,max) is set to a value within a range of approximately 40°C to approximately 80°C. [11] Control and regulating unit for operating an electric power steering system, characterized in that it is prepared for carrying out one of the methods claimed in claims 1 to 10.