DE102009049055A1 - Method and system for the electric drive of a vehicle - Google Patents

Abstract

The invention relates to the operation of a multi-phase electrical machine provided as a vehicle drive, including the generation of PWM pulses to control switching elements of an inverter to provide phase currents (In, Iv, Iw) for the electrical machine, whereby for each phase current (In, Iv , Iw) a respective PWM pulse sequence (Pu, Pv, Pw) is generated with a predetermined pulse frequency (fpwm = 1 / Tpwm). In order to reduce acoustic interference, the pulse frequency (fpwm) of at least one of the PWM pulse trains (Pu, Pv, Pw) is continuously varied during operation according to the invention.

Description

The present invention relates to methods and systems for the electric drive of vehicles, and more particularly to an operating method according to the preamble of claim 1 and to an electric drive system according to the preamble of claim 8.

Such a method and such a system are for example from DE 10 2008 052 923 A1 known. According to this prior art, a three-phase three-phase motor is provided for driving a motor vehicle. An inverter powered by a vehicle electrical battery provides the three electrical phase currents required to operate the motor. For this purpose, electrical switching elements of the inverter by pulse width modulated pulses, briefly "PWM pulses" driven, wherein the measures provided for the phase currents PWM pulse train are generated with a predetermined as "switching frequency of the inverter" pulse frequency.

The concrete choice of the pulse frequency of the inverter results in the following conflict in practice:
On the one hand, it is advantageous if the pulse frequency is selected to be as large as possible, so that a desired (ideal) temporal phase current profile can be approximated as well as possible by the phase current actually provided by the inverter. If z. B. a sinusoidal phase current waveform in a polyphase three-phase motor is to be provided at maximum motor speed with a phase current frequency of about 100 to 200 Hz, so this requires a PWM pulse frequency, which is well above said phase current frequency. From a practical point of view, for example in order to avoid harmonic components of the motor torque which are disturbing in the drive train, the PWM pulse frequency should even be several orders of magnitude greater than the phase current frequency. The ratio between the pulse rate (the PWM pulse train) and the phase current frequency is often referred to as the pulse ratio. In typical motor controls, for example, a pulse ratio of about 10 ¹ to 10 ^{3 is} provided. In the above example, this results in a pulse frequency which is already in the kHz range (the larger, the better).

On the other hand, the pulse frequency should be as small as possible in order to minimize the electrical switching losses inevitably associated with each switching operation of the inverter. This aspect is very important for the energy efficiency of the relevant electric drive system.

In the above-mentioned known electric drive system, a value of the pulse frequency of 8 kHz is proposed as a compromise, wherein in a very specific operating situation, namely at above a predetermined torque level drive torque, the inverter is set to a contrast lowered pulse rate. Thus, in terms of energy efficiency, the choice of pulse frequency is advantageously made as a function of the instantaneous engine operating situation.

However, in the known electric drive system remains a serious disadvantage, which is that the pulse rate is always in the audible audible range, resulting in practice (eg., In so-called hybrid vehicles or pure electric vehicles for road traffic) as annoying buzzing or Making pipes noticeable.

Due to the mechanical design of the vehicle-related mechanical resonances can this z. B. by the driver as disturbing perceived effect even more.

It is an object of the present invention to improve a method and a system of the type mentioned above with respect to the generation of acoustic noise.

This object is achieved according to the invention by an operating method according to claim 1 and an electric drive system according to claim 8. The dependent claims relate to advantageous developments of the invention.

The operating method according to the invention is characterized in that the pulse frequency of at least one of the PWM pulse sequences is continuously varied in operation.

Instead of a rigid pulse rate, which is lowered at best in a very specific engine operating situation, an operationally continuous variation of the pulse rate is provided to reduce the acoustic interference in the invention. Due to the continuous variation, the acoustic interference spectrum is distributed over a larger frequency range, whereby the corresponding acoustic interference amplitudes are reduced accordingly. Moreover, the continuous variation reduces the risk of mechanical resonances since the "excitation frequency" is constantly being changed. The remaining noise is distributed over a larger frequency range and significantly reduced, in particular with regard to human-acoustic perception.

The invention can be particularly advantageous for road vehicles with pure electric drive or hybrid drive (from internal combustion engine and electric machine), but is by no means limited to these applications.

The electric drive may comprise one or more electrical machines, of which at least one, in particular all, can be controlled in accordance with the invention.

With regard to the design of a multi-phase electric machine as well as the inverter used for this purpose can be used advantageously to known concepts of the prior art. In this respect, it is not contrary to the use of the invention, if the electrical machine in question is also operable as an electrical generator, for example, for regenerative energy recovery during braking (in general: delay) of the vehicle.

The electric machine can be, in particular, a multi-phase, in particular three-phase, three-phase motor or motor / generator.

The inverter may be implemented in a simple manner as a bridge circuit formed a plurality of times in accordance with the number of phases, such as, for example, a bridge circuit. B. in the above DE 10 2008 052 923 A1 shown. The switching elements are preferably designed as switching transistors, optionally with freewheeling diodes connected in parallel.

Each of the pulse frequencies provided for the PWM pulse sequences may have a time average z. B. in the range of 5 to 15 kHz.

In one embodiment, it is provided that the pulse frequency is varied within a fixed frequency range, for example.

The upper limit and / or the lower limit of this frequency range preferably differs by a maximum of 20%, in particular a maximum of 10%, from the time-averaged pulse frequency. In a specific embodiment, the upper limit and / or the lower limit are not fixed but predetermined as a function of a current engine operating state (eg speed, torque, etc.).

In one embodiment, it is provided that the pulse frequency is varied periodically.

It is preferred that the period of the pulse-frequency variation is less than 100 ms, in particular less than 50 ms, and / or that the frequency of the pulse-frequency variation is less than 1/10, in particular less than 1/100 of the time-averaged pulse frequency.

In a specific embodiment of the periodic pulse frequency variation, it is provided that the value of the pulse frequency is varied sinusoidally (over time). According to another specific embodiment, it is provided that the pulse frequency is varied in a triangular manner. In general, it is preferred that the periodic pulse frequency characteristic is symmetrical, ie has identical "positive and negative half-waves", as described, for example, in US Pat. B. for a sinusoidal or a symmetrical (not sawtooth) triangular shape is the case.

Alternatively or in addition to a periodic pulse frequency variation, it is considered that the pulse frequency is varied in a random manner.

For example, it can be provided for this purpose that the pulse rate is changed according to a so-called "random walk" method under the boundary condition of a predetermined maximum pulse frequency (upper limit) and a predetermined minimum pulse frequency (lower limit). In one embodiment of this "random walk" method, the pulse rate (or equivalently: the temporal pulse interval) is changed after every nth pulse in a random manner, for. For example, a particular frequency increment or frequency decrement, where n denotes a small natural number (eg, n in the range of 1 to 4, either fixed, or also chosen randomly from change to change). The increment or decrement provided with each change in the pulse frequency can also be selected randomly but preferably in a fixed predetermined range.

In a further development of the "random walk" method with predetermined upper and lower limits of the pulse rate, it is provided that, when the upper limit for the further course of the random change is reached, the probability for a decrement is greater than the probability for an increment Reaching the lower limit these two probabilities are reversed, d. H. for an increment a higher probability than for a decrement is provided. Such "biased" randomness results in a similar pulse rate variation to a strictly periodic pulse rate variation, but involves an additional random element. The preloaded probabilities and the associated increment or decrement values may be z. B. are chosen so that the statistical expectation of a "frequency" of the pulse rate variation meets the above explained for a periodic pulse rate variation criteria.

If the "random walk" method is carried out in real time, ie during the operation of the electric machine to be controlled, this requires a more or less large one Computational effort (including the generation of random numbers). Calculations must be carried out within a relatively short period of time - depending on the selected (time-averaged) pulse rate. This effort can z. B. be justified if one or more parameters of the "random walk" (probabilities, increments, decrements, pulse rate limits) of one or more only available in real time operating parameters of the electric machine (eg current speed, etc.) or the relevant Vehicle (eg current driving time speed, etc.) should be dependent.

In particular, in the event that no such dependence is to be provided, a variant of the "random walk" method is preferred in which the randomly varied pulse rate is not calculated in real time, but already calculated in advance (or determined in another way ) and stored in a memory device. The pulse frequency variation to be performed in real time can then be carried out very simply on the basis of information which is read from the relevant memory device (eg memory device in an electronic control unit). However, a previously stored "random sequence" can only have a finite length. In this case, however, such a sequence can be retrieved repeatedly in practice to realize a longer lasting pulse rate variation.

A "mixed form" of a real-time calculation and a previously performed calculation is also conceivable. For example, sequences of random numbers for use in determining probabilities and / or increments and decrements may be predetermined and stored to simplify the real-time computation in part. In another variant, it is provided that a plurality of different pulse-frequency variations were predetermined and stored (which differ, for example, with respect to their pulse-frequency upper limit and / or pulse-frequency lower limit), wherein in real time it is then merely determined which of the stored pulse-frequency variations is to be used (eg depending on current operating parameters).

The special feature of the invention is that the frequency of at least one of the PWM pulse sequences is continuously varied in operation, with which the relevant PWM pulse train (s) is (are) generated. The "pulse rate" can z. B. be defined by the temporal positions of the rising edges of the PWM pulses. Deviating from the pulse rate could also be defined by another, each pulse immanent temporal position, ie z. B. by the temporal positions of the trailing edges, or z. B. by the time positions pulse centers.

In a development of the invention, it is provided that at the same time the pulse frequencies of at least two of the PWM pulse sequences are varied in the same way in order to synchronize the pulses of these PWM pulse sequences to one another. In particular, the pulse frequency of all PWM pulse trains (that is to say, for example, the three PWM pulse trains in the case of a three-phase three-phase motor) for synchronizing all PWM pulse trains can be simultaneously varied in this way.

Such, common for two or more PWM pulse trains pulse rate can, for. B. define the temporal positions of the leading edges of the PWM pulses, so that these rising edges for all PWM pulse trains are on a common "temporal grid". Such a synchronization of the PWM pulse sequences with one another has advantages with regard to the load of the drive energy source (eg traction battery or intermediate circuit capacitor) and tends to even out the temporal torque curve of the electrical machine. In addition, such a synchronization may simplify the effort required to generate the pulse sequences (eg by means of a microcontroller).

The electric drive system according to the invention is characterized in that the control device provided for generating the PWM pulses is designed to continuously vary the pulse frequency of at least one of the PWM pulse trains in an operational manner.

With regard to the manner of this pulse-frequency variation, it is possible to fall back on all the above-described special features and embodiments, individually or in combination.

In an embodiment, it is provided that the control device carries out the generation of the PWM pulse trains and the pulse frequency variation in a program-controlled manner. For this purpose, the control device z. B. as a processor-controlled electronic control unit containing a microcontroller or be designed as a functionality of such a control device.

The generation of the PWM pulse sequences, ie in particular the actual pulse width modulation, as well as the pulse rate variation (equivalent: pulse interval variation) provided according to the invention can advantageously be carried out by a common software algorithm, which uses sensor-determined machine operating parameters as input variables (in particular instantaneous values of the phase currents and / or phase voltages ) and operating parameters (in particular eg Accelerator pedal position, etc.) receives to generate output variables, which in particular include control signals for generating the PWM pulse trains or the PWM pulse trains themselves.

Alternatively or in addition to the above-described generation of the PWM pulse sequences and the pulse rate variation in a program-controlled manner, it can also be provided that this pulse train generation and / or pulse frequency variation is realized by means of a "hard-wired logic" (or "programmable logic"). This can z. B. the burden of a processor in a programmable controller are advantageously reduced.

The invention will be further described by means of embodiments with reference to the accompanying drawings. They show:

1 a block diagram of an electric drive system for a hybrid vehicle,

2 a more detailed block diagram of some components of the electric drive system of 1 .

3 a timing diagram illustrating a conventional provision of a phase current through an inverter,

4 a timing diagram for illustrating a control of the inverter according to the invention,

5 a plot of the pulse rate as a function of time, according to a first embodiment,

6 a plot of the pulse rate as a function of time, according to a second embodiment, and

7 a plot of the pulse rate as a function of time, according to a third embodiment.

1 illustrates an electric drive system 10 comprising an electric motor 12 , one of an electric traction battery 14 supplied inverters ("inverters") 16 and a controller 18 for controlling the inverter 16 ,

The electric drive system 10 is integrated in this example in a drive system of a hybrid vehicle, in which the vehicle drive in addition to the electric motor 12 also an internal combustion engine 20 , z. As a gasoline engine or diesel engine is provided in a drive train of the vehicle in question (so-called parallel hybrid).

In known manner, which will not be discussed in detail here, the vehicle drive can be done according to various modes of operation in which a wheelset 22 (For example, front wheels or rear wheels of a motor vehicle) either by the internal combustion engine 20 or by the electric motor 12 or driven in combination by both drive sources. When decelerating the vehicle, z. B. during active braking, a regenerative energy recovery may be provided, wherein the electric motor 12 is operated as an electric generator to convert kinetic energy of the vehicle into electrical energy and into the traction battery 14 einzuspeichern. With regard to the various possibilities with regard to the arrangement of the drive sources and their coupling (eg via controllable couplings, gearboxes, etc.), as well as the activation of the aforementioned operating modes, reference is made to the relevant prior art.

Essential to the present invention is the manner of driving the inverter 16 realized operation of the electric motor 12 , This Ansteuererung is therefore discussed in more detail below.

The control device 18 receives operating commands c and controls on this basis, taking into account sensory detected operating parameters p of the hybrid drive system, in particular the inverter 16 and optionally z. B. also the internal combustion engine 20 ,

The operating commands c can z. B. include an accelerator pedal position, which is representative of acceleration or deceleration requests of the driver.

The sensor-determined operating parameters p include in particular those with reference to the 2 still described electrical characteristics, which in the range of the electric motor 12 and / or the inverter 16 be detected by appropriate sensor devices.

2 shows in more detail the structure of the electric drive system 10 according to an embodiment.

In this example, the electric motor 12 designed as a three-phase AC motor and a line arrangement 24 with the inverter 16 connected. In operation of the engine 12 leads the line arrangement 24 (positive or negative) phase currents Iu, Iv and Iw.

The inverter provided for providing these phase currents Iu, Iv and Iw 16 contains a triple formed bridge circuit of switching transistors T1 to T6. These Bridge circuit is connected via a so-called intermediate circuit capacitor C1 from the battery voltage U of the traction battery 14 provided.

By means of a PWM (Pulse Width Modulation) control of the switching transistors T1 to T6 known in principle, desired time profiles of the phase currents Iu, Iv and Iw can be approximated. This is done by appropriately switching on and off the switching transistors T1 to T6 whose control inputs (in this case eg gate terminals) are supplied with a corresponding PWM pulse train Pu, Pv or Pw for each of the phase currents.

The control device 18 generates the PWM pulse trains Pu, Pv, Pw in a program-controlled manner by means of a microcontroller 26 , in which a corresponding control program (algorithm) expires.

The microcontroller 26 is with an interface block 28 connected, in turn, with the inverter 16 connected is.

As shown in the figure, are in the range of the inverter 16 Measurements of the intermediate circuit voltage and the phase currents and / or phase voltages by means of a measuring device 30 of the interface module 28 performed and sent to the microcontroller 26 communicated.

Taking into account these operating parameters p and on the basis of the likewise supplied operating commands c of the vehicle user, the microcontroller calculates 26 for motor operation suitable pulse trains Pu, Pv, Pw and outputs this via a driver device 32 of the interface module 28 to the inverter 16 out.

3 illustrates the example of an assumed ideally temporally sinusoidal phase current Iu (dashed lines drawn) the conventional principle of approximation of this ideal course by the switching transistors T1 and T2 driving PWM pulse train Pu (In 3 t denotes the time).

The temporally successive individual pulses of the pulse train Pu are generated at a fixed predetermined pulse frequency fpwm, corresponding to the reciprocal of a fixed pulse interval Tpwm (fpwm = 1 / Tpwm). To approximate the ideal sinusoidal current curve, the pulse widths are suitably modulated.

The solid line in FIG. 1 represents the resulting actual phase current profile Iu. This results from the type and arrangement of the pulses and the (typically inductive) electrical properties of the relevant electrical machine.

In 3 For the sake of clarity of the illustration, a very large pulse interval Tpwm (measured on the period of the sinusoidal shape of the current) is shown. In practice, a typical pulse rate is about 10 kHz, corresponding to a pulse interval Tpwm of about 0.1 ms. The frequency of the current is typically on the order of about 100 Hz, so that each period of the sinusoidal shape in fact by about 100 pulses, that is much more than in 3 represented, is formed.

The temporally equidistant successive rising edges of the pulse sequence Pu condition in the generated phase current Iu the "lower jaws" of the illustrated actual current waveform. These abrupt changes in current lead in practice to acoustic interference containing in particular z. B. a humming or whistling on the selected pulse frequency (eg., 10 kHz).

To avoid such interference is the controller 18 ( 2 ) according to the invention designed to continuously vary the pulse frequency fpwm of at least one of the PWM pulse trains Pu, Pw, Pv. Such a variation of the pulse rate is exemplary 4 illustrated.

4 shows in the upper part the approximation of an again dashed lined ideal course through the again drawn through actual phase current Iu. The lower part of 4 is a corresponding representation for another, here the phase current Iv.

How out 4 As can be seen, the pulse frequency fpwm is continuously reduced in the illustrated range of time t (meaning: the pulse interval Tpwm is continuously increased). Thus one recognizes an initially relatively small pulse distance (see, for example, Tpwm1), which increases more and more to the right (see, for example, Tpwm2). In the example shown, the pulse interval Tpwm is even increased from pulse to pulse. In the example shown, there is a linear increment of the pulse interval Tpwm.

In a later, in 4 However, not shown area of time t, the pulse frequency fpwm is increased again (accordingly, the pulse interval Tpwm again reduced).

Due to the operationally continuous variation of the pulse frequency fpwm, the acoustic interference is advantageously reduced.

In the example shown, the pulse frequency of both PWM pulse trains Pu and Pv in. At the same time Similarly, the pulses of these pulse trains Pu, Pv are synchronized with one another (see dashed vertical lines at the positions of the leading edges).

The 5 . 6 and 7 illustrate various possibilities or embodiments of the pulse rate variation.

In the 5 and 6 For example, a periodic variation of the pulse frequency fpwm is shown in each case (period of the variation: Tv). The variation is in 5 sinusoidal and in 6 Symmetrical-triangular, wherein the pulse frequency fpwm is varied within a frequency range, which is given by fixed upper and lower limits fmax and fmin.

In 7 the pulse frequency variation is also within a predetermined frequency range between an upper limit fmax and a lower limit fmin, but the variation is not provided periodically but in a random manner.

In all of the examples described above, the pulse rate variation may be based, at least in part, on pre-stored information to reduce or substantially eliminate the computational burden required in real time. Both periodic and non-periodic pulse rate variations may be e.g. B. in the form of suitable rows of numbers or tables in an electronic storage device are stored in advance, in which case during the operation of the electrical machine only to determine the manner of retrieval of this information.

As an alternative to a fixed specification, fmax and / or fmin could also be variable in the examples described above, for example. B. as a function of a current torque and / or a current speed of the electric motor 12 ,

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102008052923 A1 [0002, 0016]

Claims (9)

Method for operating a multi-phase electric machine provided as a vehicle drive ( 12 ) comprising generating PWM pulses for driving switching elements (T1-T6) of an inverter ( 16 ) for providing phase currents (Iu, Iv, Iw) for the electric machine ( 12 ), wherein for each phase current (Iu, Iv, Iw) a respective PWM pulse train (Pu, Pv, Pw) with a predetermined pulse frequency (fpwm) is generated, characterized in that the pulse frequency (fpwm) at least one of the PWM pulse trains (Pu, Pv, Pw) is continuously varied in operation. The method of claim 1, wherein the pulse rate (fpwm) is varied within a predetermined frequency range. Method according to Claim 2, wherein an upper limit (fmax) of the frequency range deviates by a maximum of 20, in particular a maximum of 10%, from a time-averaged pulse frequency. The method of claim 2 or 3, wherein a lower limit (fmin) of the frequency range by a maximum of 20%, in particular a maximum of 10% deviates from a time-averaged pulse rate. Method according to one of claims 1 to 4, wherein the pulse frequency (fpwm) is periodically varied. Method according to one of claims 1 to 4, wherein the pulse frequency (fpwm) is varied in a random manner. Method according to one of the preceding claims, wherein at the same time the pulse frequencies (fpwm) of at least two of the PWM pulse trains (Pu, Pv, Pw) are varied in the same way to the pulses of these PWM pulse trains (Pu, Pv, Pw) to each other synchronize. Electric drive system ( 10 ) for a vehicle, comprising - a multi-phase electric machine ( 12 ) in a drive train of the vehicle, - one of a vehicle electrical power source ( 14 ) supplied inverters ( 16 ) for providing phase currents (Iu, Iv, Iw) for the electric machine ( 12 ), - a control device ( 18 ) for generating PWM pulses (Pu, Pv, Pw) for driving switching elements (T1-T6) of the inverter ( 16 ), wherein for each phase current (Iu, Iv, Iw) a respective PWM pulse train (Pu, Pv, Pw) with a predetermined pulse frequency (fpwm) is generated, characterized in that the control device ( 18 ) is designed to continuously vary the pulse frequency (fpwm) of at least one of the PWM pulse trains (Pu, Pv, Pw). Electric drive system ( 10 ) according to claim 8, wherein the control device ( 18 ) performs the generation of the PWM pulse trains (Pu, Pv, Pw) and the pulse rate variation in a program-controlled manner.

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