Harmonic torque ripple reduction at low motor speeds

Methods and systems are provided for reducing torque ripple in an electric motor. A method comprises receiving a torque command and determining a cancellation current command based on the torque command. The method further comprises generating a harmonic cancellation command based on the cancellation current command, wherein the harmonic cancellation command compensates for a phase shift and an attenuation introduced by a current regulated control module coupled to an inverter coupled to the electric motor. The method further comprises providing the harmonic cancellation command to the current regulated control module, wherein the current regulated control module is configured to control the inverter in response to the harmonic cancellation command and the torque command.

TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally to electric motor drive systems, and more particularly, embodiments of the subject matter relate to methods and apparatus for reducing torque ripple in electric motors utilized in electric and hybrid-electric vehicle drive systems.

BACKGROUND

In vehicles using electric traction motors, alternating current (AC) motor drives are used to provide a requested torque to the motor shaft. Most motor drives attempt to provide a balanced set of purely sinusoidal currents to the motor stator windings to produce a constant torque with no distortion or ripple. However, due to practical design constraints of the AC motor, torque ripple exists even with purely sinusoidal stator current excitation. Torque ripple may cause speed ripple, excite driveline resonances, or produce other undesirable effects. In the case of a vehicle, torque ripple may produce vehicle oscillations or noise.

In some situations, the torque ripple generated by a motor can be reduced by making mechanical changes to the motor design, such as the winding configuration, stator tooth geometry, rotor barrier geometry, and rotor skewing. However, there is a trade-off between torque ripple and torque density of the motor. Therefore, in all practical applications, the motor produces some torque ripple when supplied by a sinusoidal current. Passive damping methods, such as the addition of structural reinforcement or sound dampening materials to the vehicle, may be utilized to reduce some of the adverse affects of torque ripple and mitigate acoustic noise. However, these damping methods can be costly and do not directly address the problem of torque ripple produced by the motor.

In the case of hybrid or electric vehicles, higher level supervisory controllers may employ algorithms such as active damping which attempt to modulate the torque command provided to the AC motor drive in order to minimize excitation of driveline resonances due in part to the torque ripple. However, these algorithms typically cannot operate at very low speeds, as they can not differentiate between the modulation of the driver requested torque command and oscillations induced by the torque ripple of the AC motor. Alternative techniques attempt inject a harmonic cancellation current in the fundamental synchronous frame. However, these techniques fail to address the effects of current regulator bandwidth limitations on the system, which could possibly lead to increased torque ripple.

BRIEF SUMMARY

A method is provided for reducing torque ripple in an electric motor. The electric motor is coupled to an inverter, which is coupled to a current regulated control module, wherein the inverter is configured to drive the electric motor. The method comprises receiving a torque command and determining a cancellation current command based on the torque command. The method further comprises generating a harmonic cancellation command based on the cancellation current command, wherein the harmonic cancellation command is generated to compensate for a phase shift and an attenuation introduced by the current regulated control module, and providing the harmonic cancellation command to the current regulated control module, wherein the current regulated control module is configured to control the inverter in response to the harmonic cancellation command and the torque command.

A method is provided for reducing torque ripple in an electric motor in response to a torque command. The electric motor is coupled to an inverter, which is coupled to a current regulated control module, wherein the inverter is configured to drive the electric motor. The method comprises determining a harmonic cancellation command based on the torque command, wherein the harmonic cancellation command is adjusted to compensate for a phase shift introduced by the current regulated control module, and providing the harmonic cancellation command to the current regulated control module.

An apparatus is provided for a controller for reducing torque ripple in an electric motor in response to a torque command, wherein the electric motor is coupled to an inverter. The controller comprises a current regulated control module, wherein the current regulated control module generates control signals for the inverter in response to the torque command, and a harmonic cancellation command block coupled to the current regulated control module. The harmonic cancellation command block is configured to generate a harmonic cancellation command for the current regulated control module to reduce a torque ripple harmonic, wherein the harmonic cancellation command compensates for a phase shift introduced by the current regulated control module.

DETAILED DESCRIPTION

Technologies and concepts discussed herein relate to reducing torque ripple in electric motor drive systems. As used herein, the meaning of subscription and superscription is as follows:

Subscript d and q: Quantity in the d-q frame. The d-q frame of reference, in Cartesian coordinates, is synchronous with the rotation of a rotor within the electric motor.

Subscript s: Quantity in the stator windings of the electric motor.

Superscript e: Quantity in the rotating (synchronous) frame.

Superscript r: Quantity related to the machine rotor.

Superscript *: Quantity which is commanded.

Referring toFIG. 1, in an exemplary embodiment, an electric motor system5includes, without limitation: an electric motor10, an energy source11, an inverter12, an electronic control system14, a current command generator block16, a harmonic cancellation command block18, a summing junction19, a current regulated control module20, and a resolver system22.

In an exemplary embodiment, the electric motor10is coupled to the inverter12which is coupled to the energy source11. The electronic control system14is coupled to the current command generator block16and the harmonic cancellation command block18. The current command generator block16may be further coupled to the energy source11. The output of the current command generator block16and output of the harmonic cancellation command block18feed the summing junction19, which also receives an output from the current regulated control module20. The output of the summing junction19is coupled to the current regulated control module20. The current regulated control module20may be further coupled to the electric motor10, and it feeds the summing junction19to create a feedback loop. The resolver system22is coupled to the electric motor10, and is further coupled to the harmonic cancellation command block18and the current regulated control module20to provide information regarding operation of the electric motor10.

In an exemplary embodiment, the electric motor10is a three-phase alternating current (AC) electric machine having a rotor and stator windings. In various embodiments, the electric motor10may be an internal permanent magnet (IPM) motor, an induction motor, a synchronous reluctance motor, or another suitable motor as will be understood. Further, it should be understood that the subject matter discussed herein is not limited to three-phase machines, and may be adapted for any number of phases. In an exemplary embodiment, the energy source11provides electrical energy and/or voltage to the inverter12for driving the electric motor10. The energy source11may comprise a battery, a fuel cell, an ultracapacitor, or any other suitable energy source known in the art. The electric motor10operates in response to voltage applied to the stator windings from the inverter12, which creates torque-producing current in the stator windings. In an exemplary embodiment, the inverter12provides pulse-width modulated (PWM) voltage signals to each phase of the stator windings and may comprise a plurality of transistor switch pairs for modulating the voltage provided, as is understood in the art.

In an exemplary embodiment, the electronic control system14may include various sensors and automotive control modules, electronic control units (ECUs), or at least one processor and/or a memory which includes instructions stored thereon (or in another computer-readable medium) for carrying out the processes and methods as described below. Although not shown, the electronic control system14may be coupled to additional vehicle components, as will be appreciated in the art. In an exemplary embodiment, the electronic control system14generates a torque command (T*) in response to a request for torque, and provides the torque command to the current command generator block16and the harmonic cancellation command block18.

In an exemplary embodiment, the current command generator block16generates a synchronous stator current command (Ise*) at a fundamental electrical frequency (fe) to operate the electric motor10with the commanded torque. In accordance with one embodiment, the synchronous stator current command is realized as two components relative to the d-q reference frame, Idse* and Iqse*, discussed in greater detail below (seeFIG. 2). In an exemplary embodiment, the current command, Ise*, is based on the torque command (T*), the energy source voltage (VDC), the angular velocity of the motor (ωr), and possibly other operating parameters of the electric motor system5. The current command generator block16provides the current command Ise* to the summing junction19.

In an exemplary embodiment, the harmonic cancellation command block18provides a second current command (Is—He*) to the summing junction19to cancel torque ripple harmonics in the electric motor10for one or more multiples of the fundamental electrical frequency. In accordance with one embodiment, the harmonic cancellation command may be realized in the d-q reference frame as Ids—He* and Iqs—He* discussed in greater detail below (seeFIG. 2). The harmonic cancellation command block18is configured to perform additional tasks and functions as described in greater detail below.

In an exemplary embodiment, the current regulated control module20provides measured current (Ise) feedback from the electric motor10to the summing junction19. The output of the summing junction19is a current error command (Is—erre*) that represents Ise*+Is—He*−Ise. In accordance with one embodiment, the current regulated control module20generates three-phase voltage commands based on the current error command, Is—erre*. The current regulated control module20provides three-phase voltage commands to the inverter12to produce the commanded torque in the electric motor10. In an exemplary embodiment, the current regulated control module20regulates the current in the electric motor10, as described in greater detail below.

Referring now toFIG. 2, in an exemplary embodiment, the electric motor system5is implemented in the d-q reference frame. In an exemplary embodiment, the current regulated control module20includes a synchronous frame current regulator24, a synchronous to stationary transformation block26, a two to three phase transformation block28, a three to two phase transformation block30, and a stationary to synchronous transformation block32. The resolver system22includes a resolver34and a resolver to digital converter36.

In an exemplary embodiment, the output of the summing junction19is coupled to the synchronous frame current regulator24. The synchronous frame current regulator24is coupled to the synchronous to stationary transformation block26, which is coupled to the two to three phase transformation block28, which in turn is coupled to the inverter12. The three to two phase transformation block30is coupled to the electric motor10and to the stationary to synchronous transformation block32, which is coupled to the summing junction19to create a feedback loop as shown.

Referring again toFIG. 2, in an exemplary embodiment, the current error command is provided to the synchronous frame current regulator24. The synchronous frame current regulator24regulates the motor current by providing voltage commands such that the measured current in the electric motor10tracks the current command (Ise*+Is—He*). The outputs of the synchronous frame current regulator24are intermediate voltage commands Vdse* and Vqse*, which are processed by the synchronous to stationary transformation block26, which uses rotor position θr(e.g., from the resolver system22as described below) to transform the voltage commands from the synchronous reference frame to the stationary reference frame in accordance with conventional coordinate transformation. The outputs of the synchronous to stationary transformation block26are the stationary frame two phase alpha/beta voltage commands Vα* and Vβ*. The alpha/beta voltage commands are then passed to the two to three phase transformation block28, which converts the alpha/beta voltage commands to the equivalent three-phase signals Va*, Vb*, and Vc*. The 3-phase stationary frame voltage commands Va*, Vb*, Vc* are the operational control signals passed to the inverter12, which processes the voltage commands and applies the commanded voltages to stator windings of the electric motor10.

In an exemplary embodiment, two (or three) stator phase currents are sensed and passed to the three to two phase transformation block30. The three to two phase transformation block30converts the three phase currents Ia, Iband Icto equivalent two phase alpha/beta currents Iαand Iβ. The stationary to synchronous transformation block32transforms (using rotor position θrwhich may be provided by the resolver system22as described below) the alpha/beta currents to synchronous frame quantities Idseand Iqse, which are fed back to the summing junction19as shown to create a current control feedback loop. The outputs of the summing junction19(i.e., the current error commands) are the synchronous frame error signals, which are provided to inputs of the synchronous frame current regulator24.

In an exemplary embodiment, the resolver system22comprises a resolver34coupled to the electric motor10. The output of the resolver34is coupled to a resolver to digital converter36. The resolver to digital converter36generates a digital representation of the rotor position, which is provided to the synchronous to stationary transformation block26, the stationary to synchronous transformation block32, and the harmonic cancellation command block18.

Referring again toFIG. 2, in an exemplary embodiment, the resolver system22measures the rotor position (θr) and the motor speed (nr) and provides the measured values to other system components. The resolver34(or similar speed sensing device) senses the position of the rotor and, thereby, derives the speed of the electric motor10. The resolver to digital converter36converts the signals from the resolver34to digital signals (e.g., a digital motor speed signal and a digital rotor angular position signal), as is understood in the art. The resolver system22may provide digital representations of rotor position and motor speed to the harmonic cancellation command block18, the current regulated control module20, and/or other system components as described herein.

For the implementation depicted inFIG. 2, the harmonic cancellation command block18generates Ids—He* and Iqs—He* in response to the torque command and other system parameters. Both outputs of the harmonic cancellation control block18are fed to the summing junction19. Thus, the harmonic cancellation control block18influences the operation of the current regulated control module20.

Referring again toFIG. 2, the current control feedback loop introduces a bandwidth frequency limitation (i.e., a cutoff frequency) to the frequency response of the current regulated control module20. The bandwidth frequency (fbw) is governed in part by the maximum switching frequency of the PWM inverter12and sampling rate of the electric motor system5, which are two factors limiting the achievable bandwidth of the current regulated control module20. In an exemplary embodiment, the bandwidth frequency may range from 300-500 Hz, although in other embodiments, the bandwidth frequency may be 1 kHz or greater. The bandwidth limitation of the current regulated control module20introduces attenuation and phase shift which affects the current regulated control module20frequency response. This limits the ability of the synchronous frame current regulator24to faithfully track the harmonic cancellation command when the command frequencies approach or exceed fbw.

Referring now toFIG. 3andFIG. 4, in an exemplary embodiment, the harmonic cancellation command block18generates a harmonic cancellation command, Is—He*, to reduce torque ripple caused by an identified ripple harmonic, H. In an exemplary embodiment, the harmonic cancellation command block18includes, without limitation: a cancellation current command block38, a magnitude compensation block40, a phase compensation block42, a deactivation block44, and a rectangular conversion block46. The harmonic cancellation command block18may receive as inputs a torque command (T*), rotor position (θr), motor speed (nr), electrical angular velocity (ωe=2πfe), current regulator bandwidth (ωbw), and/or the ripple harmonic to be cancelled (H). As shown inFIG. 4, the harmonic cancellation command block18may be realized and/or implemented in the d-q reference frame.

In the illustrated embodiment, the cancellation current command block38is coupled to the magnitude compensation block40and the phase compensation block42. The output of the magnitude compensation block40is coupled to the input of the deactivation block44. The output of the deactivation block44and the output of the phase compensation block42are coupled to the rectangular conversion block46. As shown inFIG. 2, the output of the rectangular conversion block46is coupled to the summing junction19. In an exemplary embodiment, the magnitude compensation block40compensates for the attenuation of the current regulated control module20and the phase compensation block42compensates for the phase shift of the current regulated control module20, as described in greater detail below.

Referring now toFIG. 5, in an exemplary embodiment, the harmonic cancellation command block may be configured to perform a torque ripple reduction process500and additional tasks, functions, and operations described below. The various tasks may be performed by software, hardware, firmware, or any combination thereof. For illustrative purposes, the following description may refer to elements mentioned above in connection withFIGS. 1-4. In practice, the tasks, functions, and operations may be performed by different elements of the described system. It should be appreciated any number of additional or alternative tasks may be included, and may be incorporated into a more comprehensive procedure or process having additional functionality not described in detail herein.

Referring again toFIG. 5, and with continued reference toFIG. 3andFIG. 4, in an exemplary embodiment, a ripple harmonic to be cancelled is identified, H (task502). In accordance with one embodiment, H is an integer multiple representing a harmonic of the fundamental electrical frequency that causes torque ripple. In an exemplary embodiment, the ripple harmonic may be predetermined based on motor operating characteristics and preconfigured in the harmonic cancellation command block18. In accordance with another embodiment, the ripple harmonic may be produced external to the harmonic cancellation command block18(i.e., provided to the harmonic cancellation command block18from the electronic control system or another vehicle control module). In an exemplary embodiment, the harmonic cancellation command block18generates the harmonic cancellation command based on the identified ripple harmonic.

In an exemplary embodiment, the harmonic cancellation command block is configured to receive a torque command, T* (task504). The torque command may be provided to the cancellation current command block38from the electronic control system or another control module within a vehicle in response to a user request for torque (i.e., a driver of a vehicle depressing an accelerator pedal).

In an exemplary embodiment, the cancellation current command block38determines a cancellation current command based on the torque command (task506). The cancellation current command block38provides a cancellation current command for the harmonic frequency (H*fe) based on the torque command and predetermined torque characteristics of the electric motor. The torque ripple characteristics of the electric motor are a complex function of the motor design, including stator and rotor lamination geometry and the winding configuration. In an exemplary embodiment, the predetermined torque characteristics may be determined either empirically, or by finite element analysis (FEA). In accordance with one embodiment, the cancellation current command block38determines the cancellation current command by obtaining the cancellation current command from a lookup table39containing stored current cancellation commands corresponding to a range of possible input torque commands. In another embodiment, the cancellation current command block38may determine the cancellation current command by performing a polynomial curve fitting operation on the torque command. In the exemplary embodiment, the cancellation current command is a polar quantity having a magnitude (M) and a cancellation phase angle (φ), as labeled inFIG. 4.

In an exemplary embodiment, the harmonic cancellation command block18generates the harmonic cancellation command by modifying the magnitude of the cancellation current command based on the identified ripple harmonic frequency to compensate for the attenuation of the current regulated control module (task508). The current regulated control module may be modeled as a single pole low pass filter, with a pole at the bandwidth frequency (ωbw=2πfbw), wherein the gain of the current regulator has a frequency response

G⁡(ω)=11+(ωωbw)2.
Accordingly, the attenuation increases (or the magnitude of the gain decreases) as command frequencies provided to the current regulated control module increase (i.e., ω→ωbw). In an exemplary embodiment, to compensate for attenuation, the magnitude compensation block40generates an adjusted magnitude (M′) by multiplying the magnitude by a compensating factor, such that

M′=M×1+(H·ωeωbw)2.
Referring toFIG. 4, the magnitude compensation block40may include a gain compensation operation block41coupled to a first multiplier43which receives the cancellation current command magnitude M as an input. The gain compensation operation block41evaluates

1+(H·ωeωbw)2
and provides the result to a first multiplier43to produce M′. In accordance with one embodiment, the bandwidth frequency of the current regulated control module may be predetermined and the magnitude compensation block40may be preconfigured accordingly.

In an exemplary embodiment, the harmonic cancellation command block18generates the harmonic cancellation command by modifying the cancellation phase angle to compensate for the phase shift of the current regulated control module (task510). As discussed above, the current regulated control module may be modeled as a single pole low pass filter, with a pole at the bandwidth frequency (ωbw=2πfbw), such that the phase shift of the current regulated control module is governed by

γ=tan-1⁡(H×ωeωbw).
Referring toFIG. 4, the phase compensation block42may include a phase correction block45which evaluates

tan-1⁡(H×ωeωbw)
to produce γ. Additionally, the location of the identified ripple harmonic must be accounted for by adding a harmonic phase angle corresponding to the harmonic position, governed by H×θr, which may be generated by a second multiplier47. In an exemplary embodiment, the phase compensation block42modifies the cancellation phase angle by adding a compensating phase shift and a harmonic phase angle, wherein the harmonic cancellation phase angle is governed by φ′=φ+γ+H×θr.

Referring again toFIG. 4, in an exemplary embodiment, the deactivation block44is configured to disable the harmonic cancellation command if the motor speed (nr) is greater than a threshold speed (nth) (task512,514). In this regard, the deactivation block44may be implemented with a scaling factor block49and a third multiplier51which receives M′ as an input. The scaling factor block49calculates a scaling factor dnwhich is provided to the third multiplier51to disable the harmonic cancellation command as speed increases. As the speed increases, the fundamental electrical frequency increases (fe), which causes the frequency of the harmonic cancellation command (i.e., H×fe) to approach the switching frequency of the inverter. At some point the ratio of the harmonic frequency to the switching frequency reaches a level where the harmonic cancellation command is no longer effective because of pulse ratio (ratio of the harmonic frequency to the output PWM frequency) restrictions. Accordingly, the harmonic cancellation command is deactivated as speed increases.

In an exemplary embodiment, the resolver system22may provide the motor speed, nrto the deactivation block44. In alternative embodiments, the motor speed may be calculated based on

nr=120⁢⁢fep,
where p is the number of poles in the electric motor. In an exemplary embodiment, the magnitude of the harmonic cancellation command is modified by multiplying the adjusted magnitude by a scaling factor to smoothly disable the harmonic cancellation command between a first speed (n1) and the threshold speed. For example, the scaling factor dnmay linearly vary from 1 at speed n1to 0 at speed nth(and speeds thereafter), such that for nth≧nr≧n1, M″=dn×M′.

In an exemplary embodiment, the rectangular conversion block46converts the harmonic cancellation command from a polar quantity (having magnitude M″ and phase φ′) to a rectangular quantity (task516). In an exemplary embodiment, the rectangular conversion block46generates the harmonic cancellation command in the d-q synchronous reference frame, wherein Ids—He*=M″ cos φ′ and Iqs—He*=M″ sin φ′. Referring toFIG. 4, the rectangular conversion block46may be realized as sine operator block53, a cosine operator block55, a fourth multiplier57and a fifth multiplier59. The sine operator block53and cosine operator block55each receive the harmonic cancellation phase angle φ′ and perform the sine and cosine function of φ′, respectively. The fourth multiplier57receives the output of the sine operator block53and M″ as inputs and produces Iqs—He*. The fifth multiplier59receives the output of the cosine operator block55and M″ as inputs and produces Ids—He*. The harmonic cancellation command block provides the harmonic cancellation command to the current regulated control module via the summing junction19(task518). The torque ripple reduction process500operates continually and the loop defined by tasks504,506,508,510,512,514,516, and518may repeat and respond to changes within the system, for example, changes to the torque command, T*, or the rotor position.

Referring now toFIGS. 6-11, for an exemplary case, when the harmonic cancellation command block is not active and a sinusoidal current shown inFIG. 6is provided to the motor, the motor torque exhibits a ripple component as shown inFIG. 7.FIG. 8illustrates a fast Fourier transform (FFT) of the torque ripple inFIG. 7which reveals torque ripple produced by a dominant harmonic of the fundamental electrical frequency corresponding to the 12thharmonic. As shown inFIG. 9, when the harmonic cancellation command block is enabled with H=12, the current provided to the motor is distorted rather than purely sinusoidal as a result of the harmonic cancellation current provided to the current regulated control module. In response to the harmonic cancellation command, the motor produces a smoother torque and torque ripple oscillations are notably reduced as shown inFIG. 10. As shown inFIG. 11, an FFT of the motor torque inFIG. 10reveals the harmonic cancellation command block with H=12 effectively cancels the 12thharmonic.

It should be understood that the exemplary embodiment shown is solely for the purpose of illustration, and the electric motor system may be adapted to include additional harmonic cancellation command blocks, or the harmonic cancellation command block may be adapted (i.e., by replicating the individual component blocks) in order to target additional harmonics and further reduce the torque ripple. Thus, in alternative embodiments, one or more harmonic cancellation command blocks may be utilized to generate commands to cancel multiple harmonics.

One advantage of the methods and/or systems described above is that the electric motor system produces a smoother torque at low motor speeds. Furthermore, compensating for the attenuation and phase shift introduced by the current regulated control module allows the harmonic cancellation command to be faithfully tracked by the synchronous frame current regulator as the fundamental electrical frequency increases. Other embodiments may utilize the systems and methods described above in different types of automobiles, different vehicles (e.g., watercraft and aircraft), or in different mechanical systems altogether, as it may be implemented in any situation where torque ripple harmonics exist.