Method and device for controlling an electric motor

Abstract

The invention relates to a method for controlling an electric motor (2) by providing a control signal (Cmot) for a motor driver (3) which operates the electric motor (2) by providing a motor current (Iu,v,w) for the electric motor (2) in accordance with the control signal (Cmot), wherein for driving the electric motor (2) the motor current (Iu,v,w) flowing from the motor driver (3) to the electric motor (2) is formed from a DC current (I) flowing from an electric battery (4) to the motor driver (3), and wherein the method comprises: receiving signals representative of an actual torque (M) provided by the electric motor (2), a requested torque (Mreq), an actual DC current (I), and a maximum allowable DC current (Imax); setting a setpoint torque (Mset) as the minimum of a maximum allowable torque (Mmax) and the requested torque (Mreq); and providing the control signal (Cmot) such that the actual torque (M) provided by the electric motor (2) is adjusted to the setpoint torque (Mset). For ensuring in a simple manner that in such a controlling of an electric motor a preset limitation of the actual DC current to a maximum allowable DC current is complied with, according to the invention the maximum allowable torque (Mmax) is determined based on the maximum allowable DC current (Imax), the actual DC current (I) and the actual torque (M). The invention further proposes a corresponding controlling device (10).

Description

[0001] Description

[0002] Method and device for controlling an electric motor

[0003] The present invention relates to a method for controlling an electric motor according to the preamble of claim 1 and a device for controlling an electric motor according to the preamble of claim 5.

[0004] Such methods and devices for controlling an electric motor with power supply from an electric battery are known in the prior art, as for example for controlling an electric motor which is arranged in a vehicle for the propulsion of the vehicle.

[0005] Such a known method serves to control the electric motor by providing a control signal for a motor driver which is coupled to the electric battery and which operates the electric motor by providing a motor current for the electric motor in accordance with the control signal.

[0006] For driving the electric motor, the motor current flowing from the motor driver to the electric motor is formed from a DC current flowing from an electric battery to the motor driver ("driving mode"). Optionally, for recovering electric energy from the electric motor to the electric battery, a DC current flowing from the motor driver to the electric battery can be formed from a motor current flowing from the electric motor to the motor driver ("recovering mode").

[0007] The known method comprises: receiving a signal representative of an actual torque provided by the electric motor, receiving a signal representative of a requested torque, receiving a signal representative of an actual DC current, receiving a signal representative of a maximum allowable DC current, setting a setpoint torque as the minimum of a maximum allowable torque and the requested torque, and providing the control signal for the motor driver such that the actual torque provided by the electric motor is adjusted to the setpoint torque.

[0008] Since the motor current for driving the electric motor is formed from a DC current originating from an electric battery, and an electric battery has a limited performance in terms of the current strength of a battery discharging current, the current strength of the DC current flowing to the motor driver for driving the electric motor must be limited to a certain maximum value in order to protect the electric battery from overloading.

[0009] The same applies in the event that a recovery of electric energy is provided. Also in this case (recuperation mode) the current strength of the charging current flowing to the electric battery must be limited to a certain maximum value in order to protect the battery from overloading.

[0010] Another reason for the need to limit the current strength of the DC current to a certain maximum value can be that the battery is connected to the motor driver via semiconductor components such as transistor switches for controllably connecting and disconnecting the electric battery from a DC input side (DC link) of the motor driver, and that these semiconductor components have a limited current strength performance.

[0011] Hereinafter, the above mentioned maximum value of the current strength of the DC current is referred to as the "maximum allowable DC current". A suitable maximum allowable DC current is dependent on the actual condition of the electric battery (e.g. temperature, SOH, SOC, etc.) and particularly in automotive applications may suitably set also in dependence on the past, the actual and predicted future operating conditions of the battery.

[0012] Accordingly, for the controlling methods that are subject of the present invention, the maximum allowable DC current can for example be determined based on a (e.g. sensor-based) determination of the physical condition of the electric battery and thus be provided for use in the controlling method. In particular in automotive applications, it is known to employ a so-called battery management system (BMS) to monitor and control the electric battery and to use the BMS amongst other things to determine and thus provide a signal representative of the maximum allowable DC current.

[0013] In automotive applications, for determining the maximum allowable DC current, in addition data originating from vehicle electronics, for example from a vehicle control device (as e.g. a "vehicle control unit" (VCU)) can be used or at least taken into account. In known motor controlling methods that are of interest here, the torque provided by the electric motor is set to a target value, which hereinafter is also referred to as the setpoint torque. Essentially, this is the main task of the motor control. If there is no risk of overloading the electric battery, the setpoint torque could, for example, correspond to a requested torque (in the sense of a desired torque).

[0014] However, the aforementioned maximum allowable DC current cannot straightforward be taken into account in such a controlling method based on the control of torque. Therefore, a "maximum allowable torque" is determined from the specified maximum allowable DC current and other operational parameters, and then the setting torque for the actual drive of the motor is determined as the minimum of the maximum allowable torque and the requested torque.

[0015] Due to this, the actual torque provided by the electric motor can be smaller than the actual requested torque. However, the method advantageously avoids that the DC current flowing from or to the electric battery exceeds the preset permissible limit (maximum allowable DC current).

[0016] In the prior art, in order to determine the maximum allowable torque, typically in a complicated combination of open and closed loop PID control, at least the actual rotational speed of the electric motor, the actual DC voltage (battery voltage) and the actual DC current are taken into account in addition to the maximum allowable DC current.

[0017] However, this determination is complex and often difficult to perform with high accuracy. In particular, calibrating a PID controller for the entire operating range of an electric motor is a tedious task. Furthermore, in particular due such a special calibration, it is difficult to generalize this concept to different combinations of electric motors and pertaining motor drivers.

[0018] It is an object of the present invention to provide a novel way for ensuring in a simplified manner that in a controlling of an electric motor of the above mentioned type a preset limitation of the actual DC current to a maximum allowable DC current is complied with.

[0019] According to the present invention, this object is solved by a controlling method according to claim 1 and a controlling device according to claim 5. Advantageous embodiments and further developments of the invention are defined by the dependent claims and are derivable from the following description.

[0020] Starting from a method according to the preamble of claim 1 , the method according to the invention is characterized in that the maximum allowable torque is determined based on the maximum allowable DC current, the actual DC current and the actual torque.

[0021] The determination of the maximum allowable torque based on the actual torque (in combination with the maximum allowable DC current and the actual DC current) as proposed by the invention surprisingly enables a simple and nevertheless comparatively reliable (accurate) determination of the maximum allowable torque for the entire operating range of the electric motor.

[0022] With the invention, instead of using a compex determination of the maximum allowable torque as in the prior art (e.g. with tedious calibration of an PID controller, etc.), a much more simple calculation allows to determine this maximum allowable torque.

[0023] The functional principle of the invention is based on the following considerations.

[0024] An efficiency q of an electric motor in the driving mode, i.e. when energy is transferred from the battery to the motor, can be expressed as follows: q = (M * co) / (U * I) wherein: q is the efficiency in the current operating point,

[0025] M is the motor torque in the current operating point (actual torque), co is the angular speed of rotation (rotor speed of the motor) in the current operating point,

[0026] U is the DC voltage in the current operating point (actual battery voltage), I is the DC current in the current operating point (actual DC current).

[0027] With the equation above and assuming that the efficiency q remains the same, the maximum allowable torque Mmax for a given maximum allowable DC current Imax can be calculated as follows:

[0028] Mmax = (q * U * Imax) / co wherein:

[0029] Mmax is the maximum allowable torque, Imax is the maximum allowable DC current.

[0030] If in this equation q is substituted by the q from the first equation, it results:

[0031] Mmax = (Imax / 1) * M wherein:

[0032] Mmax is the maximum allowable torque,

[0033] Imax is the maximum allowable DC current,

[0034] I is the actual DC current,

[0035] M is the actual torque provided by the motor.

[0036] Although the efficiency q usually depends strongly on the operating point, if the calculation recursion in the method (e.g. the above calculation algorithm) is high enough, the operating point does not change significantly between two recursions. Therefore, the method of determining Mmax according to the invention provides a highly useful result.

[0037] Notably, the last equation also applies to the recovering mode, i.e. when energy is transferred from the motor to the battery. This follows from an analogous consideration of an efficiency of the electric motor in the recovering mode under analogous assumptions.

[0038] Advantageously, the invention therefore provides a control for electric motors that can be employed very universally (for different types of motors).

[0039] According to the invention, the maximum allowable torque can for example be calculated based on an equation containing the actual torque, and in particular for example an equation containing only the maximum allowable torque, the maximum allowable DC current, the actual DC current, and the actual torque. In an embodiment of the invention, the maximum allowable torque is calculated according to the above equation Mmax = (Imax / 1) x M.

[0040] In particular in the field of electrically driven vehicles, in a typical electric motor controlling device (e.g. "motor control unit" (MCU)), the actual torque provided by the electric motor is determined anyway, for example by means of a direct sensory detection with a torque sensor arranged at an output shaft of the motor or, for example, by means of a calculation (at least estimation) based on electrical operating parameters measured at the motor and / or at a corresponding motor driver. The actual torque determined in this way can advantageously be used in the invention for the purpose of determining the maximum allowable torque.

[0041] The above embodiment can also be modified within the scope of the invention in that the above calculation equation links the four relevant quantities (Mmax, Imax, I, M) in a mathematically modified form and / or a subsequent correction of the calculated Mmax as a function of one or more operating parameters is carried out after the calculation has been performed. An example of such an operating parameter is a temperature measured at the motor.

[0042] In an embodiment of the invention, the electric motor is a multi-phase AC electric motor (e.g. three-phase AC electric motor) and the motor driver is a bidirectional DC / AC converter for enabling electric current to flow from the electric battery in alternating fashion to each one of a plurality of windings of the electric motor, for driving the electric motor, and preferably also enabling electric current to flow in alternating fashion from each one of the plurality of windings to the electric battery, for recovering electric energy from the electric motor to the electric battery, respectively.

[0043] Alternatively, it is possible for the invention to provide for the controlling of other types of electric motor, such as for example an electric 6-phase AC motor (with the motor current in this case being formed as a 6-phase AC current). Furthermore, the invention can for example also provide for the controlling of an electric DC motor (with a DC motor current).

[0044] In the invention, the control signal provided for the motor driver determines the direction and extent of the energy transfer between the electric battery and the electric motor (and thus the extent of motor torque, positive in driving mode and negative in recovering mode).

[0045] In an embodiment of the invention, the control signal provided for the motor driver is a PWM (pulse-width-modulated) signal. In this case, the duty cycle of the PWM signal can in a known manner determine the extent of the energy transfer and thus the extent of motor torque. The PWM signal can be used for example for timed operation of semiconductor switches in the motor driver which provides the motor current for the electric motor. In a preferred embodiment of the invention, it is provided that by means of the controlling of the electric motor, both the driving mode and the recovering mode of the electric motor can be performed.

[0046] In an embodiment of the invention, the method for controlling the electric motor is carried out on board of a vehicle equipped with an electric motor for the propulsion of the vehicle for controlling this electric motor.

[0047] According to a further aspect of the invention the above mentioned object is solved by a controlling device for controlling an electric motor by providing a control signal for a motor driver which operates the electric motor by providing a motor current for the electric motor in accordance with the control signal, wherein for driving the electric motor, the motor current flowing from the motor driver to the electric motor is formed from a DC current flowing from an electric battery to the motor driver, and optional, for recovering electric energy from the electric motor to the electric battery, a DC current flowing from the motor driver to the electric battery is formed from a motor current flowing from the electric motor to the motor driver.

[0048] According to the invention, the controlling device comprises: an interface (e.g. digital bus system interface) for receiving at least a signal respresentative of an actual torque provided by the electric motor, a signal respresentative of a requested torque, a signal representative of an actual DC current, and a signal representative of a maximum allowable DC current, a setpoint torque setting portion for setting a setpoint torque as the minimum of a maximum allowable torque and the requested torque, a control signal providing portion for providing the control signal for the motor driver such that the actual torque provided by the electric motor is adjusted to the setpoint torque, a maximum allowable torque determining portion for determining the maximum allowable torque based on the maximum allowable DC current, the actual DC current and the actual torque. In an embodiment, the above mentioned interface for receiving said signals is at least partially formed as a digital signal interface for receiving the signals as data signals via a digital communication bus system (e.g. CAN bus, LIN bus, etc.).

[0049] In case of the controlling device for controlling the electric motor is arranged on board of a vehicle for carrying out the controlling method for the electric motor used for the propulsion of the vehicle, at least the signal (e.g. data) respresentative of the requested torque can be generated by means of a vehicle control device and provided (e.g. via said digital communication bus system) to the controlling device.

[0050] In an embodiment, such a vehicle control device further provides the signal representative of the maximum allowable DC current to the controlling device.

[0051] In an embodiment, the signal respresentative of an actual torque provided by the electric motor and / or the signal representative of the actual DC current is received by the controlling device from a sensor device or sensor arrangement of multiple sensors arranged at the electric motor and / or the corresponding motor driver.

[0052] The term "representative of ... " used in this context is to be interpreted broadly in the sense of the invention such that the respective signals or data received via the interface as such need not directly reflect the information about the relevant quantity (requested torque, maximum allowable DC current, actual torque, and actual DC current, respectively). In this case, however, the term "representative of ..." shall mean that the relevant quantity can be obtained by processing or analyzing the received signal or data.

[0053] The embodiments and specific details described here for the controlling method according to the invention can be provided, in an analogous manner, individually or in any combination, as embodiments or specific details of the controlling device according to the invention, and vice versa.

[0054] In an embodiment, the controlling device comprises one or a plurality of (communicatively interconnected) program-controlled electronic devices, each having a calculation unit for digital data processing and a memory for storing a control program for controlling the calculation unit, such that the method for controlling the electric motor is implemented with software by means of the controlling device. Against this background, each of the above-mentioned "portions" in the controlling device, i.e.the setpoint torque setting portion, the control signal providing portion, and the maximum allowable torque determining portion can be implemented in particular, for example, as respective functional components of a respective software which runs in the controlling device in order to carry out the method according to the invention.

[0055] In an embodiment of the controlling device, the maximum allowable torque determining portion is designed to determine the maximum allowable torque according to the equation Mmax = (Imax / 1) x M, wherein Mmax is the maximum allowable torque, Imax is the maximum allowable DC current, I is the actual DC current, and M is the actual torque.

[0056] In order to realize this embodiment, the maximum allowable torque determining portion can comprise at least a division unit and a multiplication unit in order to conduct the necessary calculations.

[0057] In a modified embodiment, the maximum allowable torque determining portion is designed to conduct the calculation by means of an equation which links the quantities Mmax, Imax, I, and M in a mathematically slightly modified form.

[0058] Further, a subsequent correction of the calculated Mmax as a function of at least one operating parameter (e.g. a temperature determined at the motor) may be carried out after the calculation. The calculations performed in the invention may also use look-up tables.

[0059] According to a further aspect of the invention, it is proposed an electric drive system comprising an electric motor, a motor driver for operating the electric motor by providing a motor current for the electric motor in accordance with a control signal, an electric battery, and a controlling device as described herein for controlling the electric motor by providing the control signal for the motor driver.

[0060] In such an electric drive system, the electric motor can be a multi-phase (e.g. three phase or e.g. six phase) AC electric motor and the motor driver can be a bidirectional DC / AC converter for enabling electric current to flow from the electric battery in alternating fashion to each one of a plurality of windings of the electric motor, for driving the electric motor, and preferably also enabling electric current to flow in alternating fashion from each one of the plurality of windings to the electric battery, for recovering electric energy from the electric motor to the electric battery, respectively.

[0061] According to a further aspect of the invention, it is proposed a vehicle equipped with an electric drive system of the type mentioned above, wherein the electric motor of the electric drive system is provided for the propulsion of the vehicle. The vehicle can be in particular an electrically or hybrid-electrically powered motor vehicle (e.g. BEV, HEV).

[0062] According to a further aspect of the invention, it is proposed a computer program product comprising software code for performing any of the controlling methods described therein when the software code is run on a program-controlled controlling device.

[0063] The invention will now be described by way of an exemplary embodiment with reference to the accompanying drawings, in which

[0064] Fig. 1 shows a block diagram of an electric drive system of a vehicle comprising, inter alia, an electric motor and a controlling device for controlling the electric motor, according to an embodiment,

[0065] Fig. 2 shows a control signal providing portion in the controlling device shown in Fig. 1 , and

[0066] Fig. 3 shows a maximum allowable torque determining portion in the controlling device shown in Fig. 1 .

[0067] Fig. 1 shows an example of an electric drive system 1 provided in a vehicle for the propulsion of the vehicle, wherein the drive system 1 comprises an electric motor 2 and a controlling device (motor controlling device) 10 for controlling the electric motor 2. In the operation of the vehicle, the controlling device 10 controls the electric motor 2 by generating and outputting a control signal Cmot to a motor driver 3.

[0068] The motor driver 3 operates the electric motor 2 by generating a motor current, which in the shown example is a three-phase AC current lu,v,w (i.e. comprising three phase currents lu, Iv and Iw), for the electric motor 2, which in the example is a three-phase AC motor. The motor driver 3 operates the electric motor 2 in accordance with the control signal Cmot supplied to the motor driver 3. The electric drive system 1 further comprises an electric battery 4 coupled to a DC link side of the motor driver 3, such that the battery 4 supplies a DC voltage (battery voltage) U and a DC current I to the DC link side of the motor driver 3 when operating the electric motor 2.

[0069] In the shown example, the motor driver 3 is is formed by a bidirectional DC / AC converter which in a driving mode of the electric motor 2 enables electric current I to flow from the electric battery 4 in alternating fashion (i.e. as the three-phase AC current lu,v,w) to each one of a plurality of windings of the electric motor 2. Thus, for driving the electric motor 2, a motor current lu,v,w flowing from the motor driver 3 to the electric motor 2 is formed from the DC current I flowing from the electric battery 4 to the motor driver 3.

[0070] Furthermore, the motor driver 3 (bidirectional DC / AC converter) in a recovering mode enables electric current to flow in alternating fashion (as the three-phase AC current lu,v,w) from each one of the plurality of windings to the electric battery (as the DC current I), for recovering electric energy from the electric motor to the electric battery and thus charging the electric battery 4 in a recuperation mode of the vehicle operation. Thus, in this operating mode a DC current I flowing from the motor driver 3 to the electric battery 4 is formed from a motor current lu,v,w flowing from the electric motor 2 to the motor driver 3.

[0071] In the shown example, the control signal Cmot provided for the motor driver 3 is a pulse-width-modulated (PWM) signal, wherein the timing and the duty cycle of the PWM signal Cmot determines the direction and extent of the torque provided by the electric motor 2 and for this purpose triggers the operation of semiconductor switches in the motor driver 3. In a manner known as such from the prior art of inverters, the motor driver 3 can for example be comprised of three half-bridge branches arranged parallel to one another and supplied by the DC link voltage U, at the center taps of which the three phase voltages and thus phase currents lu,v,w are provided for the motor 2.

[0072] As shown in Fig. 1 , various information is communicated to the motor controlling device 10 by means of signals (e.g. data signals) about a plurality of e.g. sensor-detected operating parameters from the "physical drive train" ranging from the battery 4 via the motor driver 3 to the motor 2, namely information about the actual DC current I (battery current), the actual battery voltage U, the actual phase currents lu,v,w, the actual torque M provided by the motor 2 and the actual rotational speed (and / or actual angle of rotation) co. This information I, U, lu,v,w, M, co, which may be obtained for example directly from the received signals and / or obtained indirectly, i.e. by processing such received signals in the motor controlling device 10, is used by the motor controlling device 10 to control the motor 2 in such a way that the torque M actually delivered by the motor 2 is continuously adjusted (in accordance with a motor controlling strategy provided by the motor controlling device 10 for this purpose, e.g. with a PID control or the like) to a value "Mset" determined by the motor controlling device 10 as a setpoint torque for this purpose under consideration of a requested torque "Mreq" inputted to the motor controlling device 10. The generation and use of these parameters Mset and Mreq will be explained in more detail below.

[0073] As shown in Fig. 1 , a battery management system (BMS) 5 monitors physical quantities or parameters indicating the actual condition of the electric battery 4 as for example in particular a temperature and a state of charge (SOC) of the electric battery 4 and provides corresponding information and / or information derived from the monitoring results to a vehicle control device 6.

[0074] In particular, the battery management system 5 determines an actual maximum allowable DC current Imaxraw, which shall be allowed to flow out from the battery 4 (when the motor 2 is operated in the driving mode) or to flow into the battery 4 (when the motor 2 is operated in the recovering mode), respectively, and communicates a corresponding signal (data) to the vehicle control device 6.

[0075] The the BMS 5 sets the maximum allowable DC current Imaxraw as a threshold value of the maximum current strength that can be applied in the momentary operating mode (motor driving mode or regenerating mode) without overloading the electric battery 4 and thus protecting it against damage.

[0076] The BMS 5 sets the maximum allowable DC current Imaxraw as a limit value of the (maximum) current strength of the DC current that can flow in the momentary operating mode (motor driving mode or regenerating mode) without overloading or even damaging the electric battery 4.

[0077] The vehicle control device 6 processes the information indicating Imaxraw received from the BMS 5 in order to communicate a corresponding value of a maximum allowable DC current Imax to the motor controlling device 10, which value Imax is thus taken into account by the motor controlling device 10 when controlling the motor 2.

[0078] The setting of the value Imaxraw by the BMS 5 focuses on the aspect of protecting the battery 4 from an overload due to an excessive current strength of a DC discharge current I flowing out of the battery 4 or a DC charge current I flowing into the battery 4. The vehicle control device 6, on the other hand, depending on the vehicle and a vehicle control strategy implemented with the vehicle control device 6 (e.g. implemented in a program-controlled manner), can still modify the value Imaxraw, in particular reduce its magnitude, before it is passed to the motor controlling device 10 as the final value Imax to be used for the limitation of the DC current I.

[0079] In the example of Fig. 1 of a vehicle controlled by a human driver, the electric drive system 1 comprises a number of suitable operating input units, such as an acceleration pedal, a brake pedal, etc., all of which are shown in Fig. 1 summarized as an operating device 7. Operating actions performed by the driver on the operating device 7 are communicated to the vehicle control device 6 by means of an operating detection signal, wherein the operating detection signal in particular communicates the value of a torque Mreqraw to the vehicle control device 6, which torque thus can be regarded as the "torque requested by the driver". In an alternative use case of an autonomous vehicle, the torque Mreqraw could be a torque requested by the corresponding autonomous vehicle control device and could be passed directly to the motor controlling device 10.

[0080] In Fig. 1 , the vehicle control device 6, depending on the vehicle control strategy implemented therein, can modify the value Mreqraw before it is passed to the motor controlling device 10 as the value Mreq. Namely, the motor torque Mreqraw requested by the driver (e.g. corresponding to the position of an acceleration pedal) does not necessarily have to be passed on 1 :1 as a target torque to the motor controlling device 10. Rather, the vehicle control device 6 may pass this target modified to the value Mreq to the motor controlling device 10, e.g. in the course of performing comfort and safety functionalities. Thusly, the final value Mreq is actually taken into account as a requested torque by the motor controlling device 10 when controlling the motor 2. However, depending on the concrete situation, it needs still to be modified by the motor controlling device 10 for taking into account a possible limitation of the motor torque M implied by the requirement that the DC current I must not exceed the maximum allowable DC current Imax. The motor controlling device 10 comprises a digital data signal interface for receiving inter alia the signals indicating the actual torque M, the requested torque Mreq, the actual DC current I and the maximum allowable DC current Imax.

[0081] The motor controlling device 10 further comprises a torque controlling unit 12 for providing the control signal Cmot for the motor driver 3, according to which the motor driver 3 operates the electric motor 2, i.e. adjusts the motor current lu,v,w.

[0082] Further, the motor controlling device 10 comprises a maximum allowable torque determining portion 15, which determines a maximum allowable torque Mmax based on the maximum allowable DC current Imax, the actual DC current I and the actual torque M, and which provides the determined value Mmax to the torque controlling unit 12.

[0083] The determination and taking into account of Mmax is a "vehicle" for taking into account the above-mentioned maximum allowable DC current Imax when the motor controlling device 10 controls the electric motor 2 based, for example, on a closed loop control of the motor torque.

[0084] The Fig. 2 and 3 show the torque controlling unit 12 and the maximum allowable torque determining portion 15 more detained.

[0085] Fig. 2 shows the torque controlling unit 12, which comprises a setpoint torque setting portion 13 and a control signal providing portion 14.

[0086] The setpoint torque setting portion 13 receives as an input the maximum allowable torque Mmax and the requested torque Mreq, and determines as an output a setpoint torque Mset as the minimum of the maximum allowable torque Mmax and the requested torque Mreq:

[0087] Mset = min (Mreq, Mmax)

[0088] This determined value of the setpoint torque Mset is provided to the control signal providing portion 14.

[0089] The control signal providing portion 14 provides the PWM control signal Cmot for the motor driver 3 such that the actual torque M provided by the electric motor 2 is continuously adjusted to the value of the setpoint torque Mset. This can be implemented for example in a closed loop control of the motor torque M.

[0090] As regards the concrete design and function of the torque controlling unit 12 with the control signal providing portion 14, within the framework of the present invention it is possible to make use of concepts which as such are known from the prior art. For example, although not explicitly described herein, the torque controlling unit 12 can comprise an controller (e.g. PI controller) for a closed loop control of the motor torque M (in order to adjust it to the target value Mset). Further, the torque controlling unit 12 can comprise for example a slew rate limiter for a smooth setting of Mmax and / or M even during state transitions.

[0091] However, an important and specific feature of the electric drive system 1 (Fig. 1 ) and the motor controlling device 10 used therein is the portion 15 for determining the maximum allowable torque Mmax and the fact that this portion 15 determines Mset based on the values of the maximum allowable DC current Imax, the actual DC current I and the actual torque M, as illustrated in Fig. 3.

[0092] Fig. 3 shows the the maximum allowable torque determining portion 15, which comprises a division unit 16 and a multiplication unit 17 in order to conduct a calculation of dividing (Imax by I) and a calculation of multplicating (Imax x I with M). Thus, in the shown example the maximum allowable torque Mmax is calculated by the maximum allowable torque determining portion 15 according to the equation Mmax = (Imax / 1) x M. In a modified embodiment, the portion 15 can be designed to conduct the calculation of Mmax by means of an equation which links the values Mmax, Imax, I, and M in a mathematically modified form. Further, a subsequent correction of the calculated Mmax as a function of at least one operating parameter may be provided by the controlling device 10.

[0093] In summary, with the invention and the described exemplary embodiments, it is advantageously foreseen to calculate the maximum allowable torque Mmax based on determined (at least estimated) values of the DC link current I, the actual torque M and the maximum allowed value of the DC current Imax at each operating conditions. The concept can be generalized to many types of motors and motor drivers. A preferred use of a controlling method and a controlling device as described herein is the control of an electric motor which is installed in a vehicle for the propulsion of the vehicle. List of reference signs

[0094] 1 electric drive system

[0095] 2 electric motor

[0096] 3 motor driver (inverter)

[0097] 4 electric battery

[0098] 5 battery management system

[0099] 6 vehicle control unit

[0100] 7 operating device

[0101] 10 motor controlling device

[0102] 12 torque controlling unit

[0103] 13 setpoint torque setting portion

[0104] 14 control signal providing portion

[0105] 15 maximum allowable torque determining portion

[0106] I actual DC current

[0107] U actual DC voltage (battery voltage) lu,v,w actual motor current

[0108] M actual torque (provided by the electric motor) co speed of rotation (rotor speed of the motor)

[0109] Mreqraw requested torque (requested by driver) Mreq requested torque (requested by driver) Mset setpoint torque

[0110] Imaxraw maximum allowable DC current (determined by BMS)

[0111] Imax maximum allowable DC current (determined by motor controlling device)

Claims

Claims1 . A method for controlling an electric motor (2) by providing a control signal (Cmot) for a motor driver (3) which operates the electric motor (2) by providing a motor current (lu,v,w) for the electric motor (2) in accordance with the control signal (Cmot), wherein for driving the electric motor (2), the motor current (lu,v,w) flowing from the motor driver (3) to the electric motor (2) is formed from a DC current (I) flowing from an electric battery (4) to the motor driver (3), and optional, for recovering electric energy from the electric motor (2) to the electric battery (4), a DC current (I) flowing from the motor driver (3) to the electric battery (4) is formed from a motor current (lu,v,w) flowing from the electric motor (2) to the motor driver (3), wherein the method comprises: receiving a signal respresentative of an actual torque (M) provided by the electric motor (2), receiving a signal representative of a requested torque (Mreq), receiving a signal representative of an actual DC current (I), receiving a signal representative of a maximum allowable DC current (Imax), setting a setpoint torque (Mset) as the minimum of a maximum allowable torque (Mmax) and the requested torque (Mreq), providing the control signal (Cmot) for the motor driver (3) such that the actual torque (M) provided by the electric motor (2) is adjusted to the setpoint torque (Mset), c h a r a c t e r i z e d in that the maximum allowable torque (Mmax) is determined based on the maximum allowable DC current (Imax), the actual DC current (I) and the actual torque (M).

2. The method according to claim 1 , wherein the maximum allowable torque (Mmax) is calculated according to the equationMmax = (Imax / 1) x M wherein:Mmax is the maximum allowable torque (Mmax), Imax is the maximum allowable DC current (Imax), I is the actual DC current (I), M is the actual torque (M).

3. The method according to claim 1 or 2, wherein the electric motor (2) is a multi-phase AC electric motor and the motor driver (3) is a bidirectional DC / AC converter for enabling electric current to flow from the electric battery (4) in alternating fashion to each one of a plurality of windings of the electric motor (2), for driving the electric motor (2), and enabling electric current to flow in alternating fashion from each one of the plurality of windings to the electric battery (4), for recovering electric energy from the electric motor (2) to the electric battery (4), respectively.

4. The method according to any of the preceding claims, wherein the method is carried out on board of a vehicle equipped with an electric motor (2) for the propulsion of the vehicle for controlling this electric motor (2).

5. A controlling device (10) for controlling an electric motor (2) by providing a control signal (Cmot) for a motor driver (3) which operates the electric motor (2) by providing a motor current (lu,v,w) for the electric motor (2) in accordance with the control signal (Cmot), wherein for driving the electric motor (2), the motor current (lu,v,w) flowing from the motor driver (3) to the electric motor (2) is formed from a DC current (I) flowing from an electric battery (4) to the motor driver (3), and optional, for recovering electric energy from the electric motor (2) to the electric battery (4), a DC current (I) flowing from the motor driver (3) to the electric battery (4) is formed from a motor current (lu,v,w) flowing from the electric motor (2) to the motor driver (3),wherein the controlling device (10) comprises: an interface for receiving at least a signal respresentative of an actual torque (M) provided by the electric motor (2), a signal respresentative of a requested torque (Mreq), a signal representative of an actual DC current (I), and a signal representative of a maximum allowable DC current (Imax), a setpoint torque setting portion (13) for setting a setpoint torque (Mset) as the minimum of a maximum allowable torque (Mmax) and the requested torque (Mreq), a control signal providing portion (14) for providing the control signal (Cmot) for the motor driver (3) such that the actual torque (M) provided by the electric motor(2) is adjusted to the setpoint torque (Mset), c h a r a c t e r i z e d in that the controlling device (10) further comprises a maximum allowable torque determining portion (15) for determining the maximum allowable torque (Mmax) based on the maximum allowable DC current (Imax), the actual DC current (I) and the actual torque (M).

6. The controlling device (10) according to claim 5, wherein the maximum allowable torque determining portion is designed to calculate the maximum allowable torque (Mmax) according to the equationMmax = (Imax / 1) x M wherein:Mmax is the maximum allowable torque (Mmax),Imax is the maximum allowable DC current (Imax),I is the actual DC current (I), M is the actual torque (M).

7. An electric drive system (1 ) comprising an electric motor (2), a motor driver(3) for operating the electric motor (2) by providing a motor current (lu,v,w) for the electric motor (2) in accordance with a control signal (Cmot), an electric battery (4),and a controlling device (10) according to claim 5 to 6 for controlling the electric motor (2) by providing the control signal (Cmot) for the motor driver (3).

8. The electric drive system (1 ) according to claim 7, wherein the electric motor (2) is a multi-phase AC electric motor and the motor driver (3) is a bidirectional DC / AC converter for enabling electric current to flow from the electric battery (4) in alternating fashion to each one of a plurality of windings of the electric motor (2), for driving the electric motor (2), and enabling electric current to flow in alternating fashion from each one of the plurality of windings to the electric battery (4), for recovering electric energy from the electric motor (2) to the electric battery (4), respectively.

9. A vehicle equipped with an electric drive system (1 ) according to claim 7 or 8, wherein the electric motor (2) of the electric drive system (1 ) is provided for the propulsion of the vehicle.

10. A computer program product comprising software code for performing the method of any of claims 1 to 4 when the software code is run on a program-controlled controlling device (10).

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