HIGH-FREQUENCY AC BATTERY HEATING WITH VOLTAGE COMMANDS

Abstract

A method for heating a battery pack of an electrified powertrain system with an electric motor involves determining the battery pack's temperature. In response to the battery temperature falling below a predetermined threshold, the method initiates a self-heating mode for the battery pack. This includes injecting a high-frequency DC voltage waveform, which minimizes the output torque and prevents the electric motor's rotor from rotating. As a result of the voltage injection, an AC waveform is applied to the battery pack. A controller includes a temperature sensor configured to determine the battery pack's temperature and a processor configured to execute the method.A motor vehicle comprises the control system, an electrified powertrain system with the battery pack and an electric motor, and road wheels connected to and driven by the electric motor.

Description

INITIATIONThe present disclosure relates to electrical systems having an electrochemical battery pack connected to an electric motor powered by alternating current (AC) via an inverter, e.g., a traction inverter module of an automobile, and to computer-based control methods for internally heating the battery cells of the battery pack using a high frequency alternating current waveform.Modern hybrid electric vehicles and fully electric motor vehicles have one or more electric drive motors. Each drive motor is powered by a controlled discharge of a lithium ion battery pack or set of battery chemistry suitable for use. In cold ambient conditions, the battery cells of the battery pack are typically heated to a certain temperature prior to being charged outside the vehicle. However, the motor vehicle may not have its own fluid thermal management system (TMS), or the heating behavior of an existing TMS may be suboptimal for a particular charging scenario or the prevailing weather conditions.DESCRIPTIONDescribed herein are electrical circuit topologies and associated control methods for heating an electrochemical battery pack of an electrified powertrain system, e.g., a hybrid electric vehicle or a battery electric vehicle. The present solutions enable selective high frequency alternating current (AC)-based internal heating ("self-heating") of the battery pack. In particular, self-heating is initiated by transmitting in-axis (d-axis) and quadrature-axis (q-axis) voltage commands to an inverter module (PIM) of the electrified powertrain system, where the PIM is in turn connected to an electric drive motor. In certain cases, zero voltage commands may also be used, e.g., when the drive motor has a neutral connection point.When the d-axis voltage command is manipulated according to the disclosure, the q-axis voltage command is set to zero. As a result of the d-axis voltage commands, a high frequency alternating current waveform is thus generated, which is then applied to the electrode terminals of the battery pack to heat it. That is, the present approach includes generating unbalanced AC currents in the traction motor that ultimately result in the AC current in the battery pack. As part of the present control strategy, an angular position of a rotor of the above-mentioned traction motor may be controlled in a closed loop to minimize residual rotor torque that might otherwise result from slight deviations from zero in the q-axis voltage command, and / or reluctance torque or magnetic torque could be generated over a total voltage command to maintain a desired rotor position.In particular, a method for heating a battery pack of an electrified propulsion system having at least one electric motor includes determining a temperature of the battery pack via a temperature sensor, where the temperature is treated herein as a measured battery temperature. In response to a set of input conditions, the method includes performing a self-heating mode of the battery pack via an electronic controller including injecting a high frequency AC voltage waveform onto a d-axis of the electric motor, i.e., via the terminals of the electric motor. This is done via a d-axis voltage command in conjunction with a zero quadrature axis (q-axis) voltage command to heat the battery pack. The input conditions may include the measured battery temperature being below a lower temperature threshold.Embodiments of the method may include applying an AC waveform to the electrode terminals of the battery pack during the self-heating mode in response to the high frequency AC waveform being injected into the d-axis of the electric motor.The method could include detecting a threshold rotation of a rotor of the electric motor and adjusting, via the electronic controller, a total voltage command in response to the threshold rotation of the rotor.Applying the high frequency AC waveform to the d-axis of the electric motor may include generating a d-axis pulsating voltage command via a position control routine of the rotor. The generation of the d-axis pulsating voltage command includes the use of a linear combination of a varying d-axis voltage command and / or the use of a constant d-axis voltage command. Generating the pulsating DC voltage command may also include using a varying displacement voltage command and / or a constant displacement voltage command.The electronic controller includes a current regulator in one or more embodiments. The method may, in one embodiment, comprise selectively increasing an injection frequency of the d-axis voltage injection and a bandwidth of the current regulator via the electronic controller. The method can also comprise the changeover from the voltage regulation of the electric motor to the current regulation of the electric motor if an injection frequency of the d-axis voltage injection lies within the bandwidth of the current regulator.The method disclosed herein may also include determining a control angle of the electric motor during the auto-heating mode and then using the control angle to maintain an angular position of a rotor of the electric machine during the auto-heating mode.The method may include calculating a reluctance torque when the control angle exceeds a calibrated value, and then repositioning the rotor using the reluctance torque. Alternatively, the method may also include calculating a magnetic torque when the control angle exceeds a calibrated value, and then repositioning the rotor using the magnetic torque.Also disclosed herein is an electronic controller for use with an electrified powertrain system having a battery pack and an electric motor. In one possible embodiment, the electronic controller comprises a temperature sensor and a processor. The temperature sensor is configured to measure the temperature of the battery pack. The processor in communication with the temperature sensor receives the temperature of the battery pack from the temperature sensor as a measured battery temperature. In response to a set of input conditions including the measured battery temperature being below a lower temperature limit, the electronic controller is configured to perform a self-heating mode of the battery pack including applying a high frequency AC voltage waveform to the d-axis of the electric motor via a d-axis voltage command in conjunction with a q-axis voltage command of zero to thereby heat the battery pack.A motor vehicle having road wheels and an electrified powertrain system having a battery pack and an electric motor, the latter having a rotor connected to one or more of the road wheels, is also disclosed herein. The motor vehicle also includes an electronic controller having a temperature sensor configured to determine the temperature of the battery pack and a processor in communication with the temperature sensor. In this particular embodiment, the processor is configured to receive the temperature of the battery pack as a measured battery temperature. In response to a series of input conditions, including the measured battery temperature being below a lower temperature limit, a self-heating mode of the battery pack is performed by commanding the injection of a high frequency AC waveform into the d-axis of the electric motor. This is done via a d-axis voltage command in conjunction with a zero q-axis voltage command and is used to warm the battery pack by applying an AC waveform to the electrode terminals of the battery pack.The above features and advantages, as well as other features and advantages of the present teachings, will be readily apparent from the following detailed description of some of the best modes and other embodiments for carrying out the present teachings as defined in the appended claims when considered in connection with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 shows a representative motor vehicle with an electrified powertrain system in which a battery pack is warmed via the control methodology described in detail herein. FIG. 2A is a circuit diagram showing an example electronic controller that may be used for performing aspects of the self-heating methodology of the present disclosure that may be used for rotor positioning / locking using reluctance torque. FIG. 2B is a circuit diagram showing an alternative embodiment of the electronic controller of FIG. 2A that may be used for rotor positioning / locking using magnetic torque. FIG. 3 shows a representative circuit topology for feeding a pulsating current waveform to the battery pack via a common axis (d-axis) command, according to an aspect of the disclosure. FIGS. 4A, 4B, and 4C are vector diagrams that together illustrate an application of a pulsating current output. FIGS. 5A, 5B, and 5C are control variables corresponding to FIGS. 4A, 4B, and 4C, respectively. FIG. 6 is a simplified illustration of a rotor of an electric motor within the electrified powertrain system of FIG. 1. FIGS. 7A-7D are representative timing diagrams of parameters in accordance with the present disclosure. FIG. 8 is a flow chart describing an example method for self-heating the battery pack of FIG. 1.The present disclosure may be modified or embodied in alternative forms, with representative embodiments shown in the drawings and described in detail below. The inventive aspects of the present disclosure are not limited to the disclosed embodiments. Rather, the present disclosure is intended to cover alternatives falling within the scope of the disclosure as defined by the appended claims.DETAILED DESCRIPTIONReferring now to the drawings, wherein like reference numerals correspond to like or similar components throughout the several figures, an electrified powertrain system 10 is illustrated in FIG. 1. The electrified powertrain system 10, which may be used onboard a motor vehicle 12 in one or more embodiments, includes a high voltage traction battery set (B HV) 14. The traction battery set 14 is connected to a DC bus 15 via a main battery contact set 16. An inverter module ("DC / AC") 18, hereinafter referred to simply as inverter 18, is electrically connected to the traction battery 14 via the DC bus 15, wherein the illustrated open state of the contact set 16 corresponds to a disconnected state of the traction battery 14.In the embodiment shown in FIG. 1, an electric drive motor (M E) 20 is connected to the AC side of the inverter 18. Although only a single electric drive motor 20 and a single inverter 18 are shown for simplicity, the present disclosure may be extended to powertrains with multiple inverters and multiple motors, as will be appreciated by those skilled in the art. Thus, a direct current voltage (VDC) applied to the direct current bus 15 is converted into an alternating current voltage (VAC) suitable for energizing individual phase windings of the electric drive motor 20. When energized, a rotor 20R of the electric drive motor 20 generates an output torque (T O), the output torque (T O) being directed via an output member 200 coupled to the rotor 20R. The rotation of the output member 200 is finally transmitted to one or more drive axles 17 and finally to one or more driven road wheels 21 connected to the drive axles 17, wherein two driven road wheels 21 are shown in FIG. 1 for the sake of simplicity.HIGH FREQUENCY AC self-heating mode: The AC-based internal heating of the traction battery 14 ("self-heating") contemplated herein requires the selective application of a high frequency AC current to the traction battery 14. However, the maximum bandwidth of a typical current regulator used in the control of AC motors is often below 200 Hz and is therefore not sufficient for optimum AC-based self-heating of the traction battery pack 14. That is, the greater the difference between the signal frequency and the available bandwidth of the controller, the greater the resulting error between the desired and actual d-axis values.A method 100, the exemplary embodiment of which is described below with reference to FIG. 8, is therefore encoded as computer readable instructions executed by an electronic controller (C) 50 in communication with the electrified powertrain system 10 of FIG. 1 to solve this potential issue. The performance of the method 100 includes selectively using an in-phase (d-axis) oscillating voltage command in conjunction with a zero quadrature axis (q-axis) voltage command. Finally, as a result of the manipulation of the d-axis and q-axis voltage command ("dq-axes"), a high frequency alternating current is applied to the electrode terminals of the traction battery pack 14 to heat the traction battery pack 14, as described herein with reference to FIGS. 2-7D.As shown in FIG. 1, representative motor vehicle 12 may include a low voltage / auxiliary battery (B LV) 22, e.g., a 12-15 V lead acid battery. The auxiliary battery 22 may be connected to an auxiliary power module ("DC / DC") 24, i.e., a DC-DC converter, connected to the battery pack 14 as shown. By operating the auxiliary power module 24, the relatively high voltage on the DC bus 15, e.g., 60 V-300 V or more, may be reduced to 12-15 V suitable for powering various low voltage electrical devices onboard the motor vehicle 12.As will be appreciated by those skilled in the art, the inverter 18 includes solid state power switches (not shown), e.g., IGBTs or MOSFETs. The individual ON / OFF conduction states of the various power switches are controlled by the electronic controller 50 or other dedicated processor via pulse width modulation (PWM) or other suitable circuitry during the AC self-heating mode described herein. The DC bus 15 may be connected to several other DC voltage devices, possibly during the performance of the method 100. However, charging of the traction battery pack 14 may not be permitted by the electronic controller 50 when a battery temperature of the traction battery pack 14, e.g., measured by a temperature sensor 14S or estimated, calculated, or otherwise determined, remains below a predetermined lower temperature limit suitable for charging, e.g., about 32-35° F. In this case, the electronic controller 50 would execute computer readable instructions embodying the method 100 to thereby initiate the self-heating mode described herein.Within the scope of the present disclosure, the electronic controller 50 includes one or more processors 52 and a memory 54. Each processor 52 is configured to receive or generate input signals (CC I). The input signals (CC I) may include a measured battery temperature (T BAT), as transmitted from the temperature sensor 14S, a mode signal (CC 10), indicative of a current operating mode of the electrified powertrain system 10, e.g., the speed and on / off state of the motor vehicle 12, and a measured rotor position (θ R) from a rotational position sensor 20S of the electric drive motor 20, e.g., a resolver, encoder, or Hall effect sensor. The rotor position (θ R) is thus an indicator of the actual angular position of the rotor 20R and a measurement from which the electronic controller 50 could determine other relevant control values such as the motor speed, the electrical angle, etc.In response to receiving the input signals (CC I) the electronic controller 50 is configured to execute the method 100 as one or more algorithms or command sets, and the electronic controller 50 finally sends output signals (CCo) to the inverter 18 as part of the method 100. The output signals (CCo) include ON / OFF state commands or PWM signals for controlling the conduction state of individual power switches (not shown) of the inverter 18. computer readable code or instructions for implementing the method 100 may be stored in accessible, non-transitory portions of the memory 54, where the memory 54 embodies a computer readable storage medium, e.g., magnetic or optical media, CD-ROM, and / or solid state / semiconductor memory (e.g., various types of RAM or ROM).The term "controller" and related terms such as control module, module, controller, controller, processor, and similar terms refer to one or various combinations of application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), electronic circuits, central processing units, e.g., microprocessors and associated non-transitory memory components in the form of memories and memory devices (read-only, programmable read-only, random access, hard disk drives, etc.). Non-transitory components of the memory 54 are those capable of storing machine readable instructions in the form of one or more software or firmware programs or routines, combinational logic circuits, input / output circuits and devices, signal conditioning and buffer circuits, and other components that can be accessed by the processor(s) 52 to provide the described functionality.The input / output circuits and devices include analog-to-digital converters and related devices that monitor inputs from sensors, these inputs being monitored at a preset sampling frequency or in response to a triggering event. Software, firmware, programs, instructions, control routines, code, algorithms, and similar terms refer to controller-executable instruction sets including calibrations and look-up tables. Each controller executes control routine(s) to provide the desired functions. The routines may be executed at regular intervals, e.g., at 50-100 microseconds (ms) intervals during ongoing operation. Alternatively, the routines may be executed in response to the occurrence of a triggering event.D-AXIS VOLTAGE COMMANDS: As known in the art, d-axis and q-axis commands are used in a motor control context as adjustable setpoints or control "knobs" or "handles" that the electronic controller 50 can access in controlling the flux and torque settings of the electric drive motor 20. Geometrically, the d-axis and the q-axis are single phase representations of the flux contribution of separate sinusoidal phase magnitudes occurring at the same angular velocity. Specifically, the d-axis is in-phase with the permanent magnetic field of the rotor 20R, and is the axis through which the flux is mainly generated. The current in the d-axis may be affected with voltage commands for the d-axis, as described below, when the self-heating mode is executed.The torque generation of the electric drive motor 20 takes place mainly on its q-axis, which is typically aligned with the rotating field of the stator, i.e. 90° phase shifted with the magnetic field of the rotor 20R. During the present self-heating mode, therefore, the q-axis voltage command is set to zero to minimize torque generation and resulting movement of the motor vehicle 12. The q-axis command is used in engine control strategies to affect torque generation. However, during the self-heating mode under consideration herein, the q-axis voltage command is intentionally set to zero and held at that value for the duration of the self-heating mode. Despite this control action, visual variations in the q-axis currents may occur, and other techniques for minimizing or preventing movement of the rotor 20R will be described below. The d-axis voltage command, in turn, is controlled to supply a high frequency AC component to the traction battery pack 14 and to produce an imbalance between the output phases of the electric drive motor 20 during the self-heating mode.When the rotor 20R of the electric drive motor 20 does not rotate, i.e., is maintained at zero speed, there is no cross-coupling effect between the d-axis and the q-axis (due to the counter electromotive force of the permanent magnet flux and the stator induced flux). Therefore, the d-axis voltage command may be derived here by an open loop. Because of the non-linearities of the inductance (saturation), the voltage commands could be defined here by look-up tables rather than calculating them in real time. Nevertheless, minor asymmetry in the electric drive motor 20 of FIG. 1 could still produce small cross-coupling effects between the d-axis and the q-axis, even when the rotor 20R is at a standstill, using the control actions contemplated herein to counteract such effects.When using rotor position sensors, e.g., position sensor 20S shown in FIG. 1, position error could also result in low residual torque during the contemplated d-axis voltage regulation and the resulting d-axis current injection. Because rotation of the rotor 20R is not desired upon heating the traction battery pack 14, the present control approach could optionally include generating a d-axis pulsating voltage command via a position control routine for the angular position of the rotor 20R as shown in the example control logic 50L and 150L, respectively, in FIGS. 2A and 2B, as described below. In this way, it is ensured that a negligible torque is exerted on the rotor 20R during the self-heating mode in order to avoid a possible movement of the motor vehicle 12.ZERO SEQUENCE VOLTAGE COMMAND: A ZERO SEQUENCE VOLTAGE COMMAND MAY BE APPLIED IN ONE OR MORE CONSTRUCTIONS OF THE ELECTRIC DRIVE MOTOR 20 TO GENERATE A BALANCED SET OF PHASE VOLTAGES. In a balanced three-phase system, the individual phase voltages may be represented, for example, with complex numbers, wherein the zero voltage represents a voltage signal that is applied simultaneously to all three electrical phases, i.e., phases a, b and c. Application of a balanced voltage signal causes a current to flow through the stator windings of such a machine, which in turn generates a rotating magnetic field having a net magnetic field of zero in each direction. Zero voltage commands that do not contribute to the generation of mechanical torque may be selectively used herein to change the magnetic field generated in the electric traction motor 20 during the self-heating mode under consideration. That is, although a null point is typically not present in an AC motor designed for use in the electrified powertrain system 10, such motor design could be used in some cases, as opposed to household electrical systems. In such a case, the zero sequence voltage command provides another control handle that could be used for optimal performance of the self-heating mode.As can be seen from FIGS. 2A and 2B, the implementation of the method 100 by the electronic controller 50 of FIG. 1 can take place via a control logic 50L (FIG. 2A ) or FIG. 150L (FIG. 2B ). A simplified representation of the electrified powertrain system 10 of FIG. 1 includes the traction battery set 14, the inverter 18, and the electric drive motor 20 with associated resistors (R 1, R 2), a capacitor (C 1), and an inductance (L 1) that represent the equivalent resistance, capacitance, and inductance of the various circuit components, such as an DC link capacitor, the traction battery set 14, and the associated wiring. The electronic controller 50 shown in FIG. 1 could be programmed with computer readable instructions embodying control logic 50L or 150L, which in the non-limiting configuration of FIGS. 2A and 2B comprises a voltage command block 55 and a closed loop position control block 58.In voltage command block 55, values for the d-axis voltage command (V d), the q-axis voltage command (V q) and optionally the zero voltage command (V 0) are selected. The q-axis voltage command is set to zero, i.e., V q= 0, to effectively prevent generation of torque as mentioned above. The zero voltage command (V 0) is also set to zero, but may also be set to a non-zero value in certain embodiments of the electric drive motor 20 having a neutral connection point. A transformation block 57 performs a conversion from a dq reference frame to an abc reference frame such that a representative three-phase voltage command is provided to the inverter 18 in a nominal abc reference frame as part of the output signals (CCo), e.g., as PWM signals from an inverter control unit or PWM control block 56.In particular, the block diagram of FIG. 2A also illustrates the position control block 58, which is used in one or more embodiments for positioning / locking the rotor with the aid of the reluctance torque, as described in more detail below. At this time, the electronic controller 50 of FIG. 1 determines, with the control logic 50L, a difference between a reference angle (θ e, ref) ( block 60), i.e., an initial value, and the measured electrical angle (θ e) ( block 51), i.e., where P is the number of pole pairs and θ m is the mechanical angle between the rotor 20R and the stator (not shown). Thus, a hypothetical embodiment of the electric drive motor 20 with two pole pairs (P=2) has an electrical angle which is equal to the mechanical angle. The result may be processed via a proportional-integral-derivative (PID) control block 64 and then integrated in integrator block 66 to generate and introduce a small error value (θ e, err_gen). This generated error value (θ e, err_gen), which generates residual torque on the rotor 20R of FIG. 1 to maintain its angular position, is subtracted from the measured electrical angle (θ e) ( block 51) at node (N1), which change over time, i.e., ω(t), is fed to transformation block 57 to track changes in electrical angle in real time. Another node (N2) could be used to correct for the sensory offset error (θ e,err) in determining (θ e) as is conventional in the art.Using the example control logic 50L of FIG. 2A, the electronic controller 50 of FIG. 1 may slowly increase the amplitude of the d-axis voltage command (V d) to allow the position control to respond if the voltage feed-in generates residual torque. The regulation of the rotor 20R is achieved with the position regulation block 58, but can also assume alternative topologies. These may include, for example, the PID control block 64 including zero gains, a PID controller in series with the integrator block 66, or the use of lead / lag compensators, or other suitable techniques. The output signals of the PWM control block 56 ultimately control the output phase voltages (V abc, 0), the phase currents (I abc) and the commanded torque (T q), these values being included in the output signals (CCo), or as separate signals.The particular control action of generating a pulsating d-axis current (I d), when performed as part of the self-heating mode described herein, could include the use of a linear combination of a varying d-axis voltage command (V d) and / or a constant d-axis voltage command (V d) and possibly the use of a varying zero sequence voltage command (V 0) and / or a constant zero sequence voltage command (V 0). The zero sequence voltage command (V 0) could be added regardless of whether the electric drive motor 20 has a neutral connection point. However, embodiments of the electric drive motor 20 that do not have such a neutral connection point do not have zero current circulation, as can be appreciated by those skilled in the art.FIG. 2B shows control logic 150L as an alternative to control logic 50L of FIG. 2A. The control logic 150L may use magnetic torque instead of reluctance torque for positioning and locking the rotor. Since the i q- current is not high frequency for rotor positioning and locking, a current regulator can be used directly for this low frequency component, even if the bandwidth of the controller 50, which operates as a current controller, is limited. In parallel, the d-axis voltage feed described herein remains responsible for AC heating of the traction battery pack 14.With respect to the circuit topology and control logic 50L of FIG. 2A, FIG. 2B uses blocks 55 and 58 revised and adds a current regulator block 59. Here, the above-described block 58 determines a q-axis current setpoint (i q*) based on the output of the integrator 66, e.g., via a look-up table, and feeds the q-axis current setpoint (i q*) to the current regulator 59, e.g., a limited bandwidth PID regulator. The output of block 55 is added to the output of current regulator block 59 and the sum is fed into block 57. In other words, rather than generating a small error value (θ e,err_gen) as in FIG. 2A and subtracting this value from the output of block 51, the current regulator block is used directly. Such an approach could be used to control the magnetic torque, as described below, for rotor positioning / locking purposes during AC heating of the traction battery pack 14.Referring briefly to FIG. 3, in some implementations, the zero voltage command (V 0), which is shown with a zero value in FIGS. 2A and 2B, could instead have a non-zero value, e.g., using logic 70 performing a linear combination of varying and / or constant voltage commands 72 as mentioned above. A similar combination may be performed to generate the d-axis voltage commands (V d). A corresponding switching block 74A and 74B could be controlled to output a desired resultant waveform for the d-axis voltage commands (V d) and the zero voltage command (V 0) as shown.In block 75 of FIG. 3, the q-axis voltage command (V q) remains set to zero as in FIGS. 2A and 2B. The PWM control of the electric drive motor 20 of FIG. 2 via the inverter 18 is via the PWM control block 56 of FIGS. 2A and 2B using the transformation block 57. if the electric drive motor 20 does not have a neutral point, as is often the case with vehicle drives, an infinite impedance and a lack of zero current circuit results. Therefore, the zero sequence voltage commands (V 0) may be added and controlled to non-zero values if desired, regardless of whether the electric drive motor 20 has a neutral connection point.FIGS. 4A, 4B, and 4C show the d-axis, q-axis, and representative a-axis, b-axis, and c-axis of a nominal abc frame of reference for an unlimited three-phase embodiment of the electric drive motor 20 (FIGS. 1, 2A, and 2B ). FIG. 4A shows the d-axis voltage component (non-zero) and the zero q-axis components used herein during the self-heating mode.A corresponding amplitude diagram of the dq variable is shown in FIG. 5A, a similar diagram of the abc variable is seen in FIG. 5B. The nominal amplitude is shown on the vertical axis and the time (t) in seconds (s) on the horizontal axis of FIGS. 5A, 5B and 5C, wherein FIGS. 5A and 5B correspond to the vector diagram of FIG. 4C. FIG. 4B is also a vector diagram, particularly for the zero voltage command (V 0). described above. The vector diagram of FIG. 4C again represents the sum of the vector diagrams of FIGS. 4A and 4B, wherein the phase currents I a, I b and I c are shown in the amplitude representation of FIG. 5C. FIG. 5C, in turn, illustrates the unbalanced character of the phase currents, where an AC waveform is ultimately applied to the electrode terminals of the traction battery pack 14 of FIG. 1 for AC-based self-heating as part of the present method 100.ROTOR POSITIONING / LOCKING: In FIG. 6, a greatly simplified representation of the rotor 20R is shown, in which the south pole (S) and the north pole (N) are shown. The inertia of the rotor 20R and that of a load associated therewith, e.g., the drive axle(s) 17 and the road wheels 21 of FIG. 1, filters out high frequency electromagnetic torque components that may be present. The components of the magnetic torque average toward zero when the unbalanced high frequency alternating current is injected via the d-axis voltage commands according to method 100. However, reluctance torque will have a non-zero average when d-axis voltage feed is performed according to the present method 100. One aspect of the disclosure therefore includes the controlled use of this reluctance torque to maintain a desired angular position of the rotor 20R.The amplitude of the d-axis voltage commands used in method 100 may also be slowly increased to allow position control of rotor 20R to respond in the event of low torque generation during the auto-heating mode. Signal injection may be in a rotating or stationary frame of reference, either using αβ or abc voltage signals as is recognized in the art. Due to light harmonics, some rotor positions during injection of the high frequency AC waveform could be more immune to torque ripple than others. For example, the rotor 20R of FIG. 1 could be selectively placed in predetermined "best" angular positions upon starting of the motor vehicle 12.With respect to the use of reluctance torque to position the rotor 20R, the following equation may be used to determine the torque (T):The component ψ m i q reduces to zero because the average value of the q-axis current (i) q is zero when the high frequency alternating current is injected via the d-axis voltage commands. However, in some embodiments, the electronic controller 50 could command the injection of a DC component into the q-axis to affect torque over the ψ m i q- term. In addition, the q-axis and d-axis currents may be expressed as: where θ err is the angular difference between the dq-axis real reference frame aligned with the north pole of the rotor and the virtual reference frame, and the respective d-axis and q-axis current components are the real reference system, and the respective d-axis and q-axis current components are in the virtual reference frame of the controller 50. These terms, when multiplied together, are expressed as follows, considering that P is the number of poles of the electric drive motor 20. By generating a small error (θ err) in the virtual reference frame as explained above with reference to FIG. 2A, it is possible to generate a small average reluctance torque. This induced reluctance torque is then applied to the rotor 20R to maintain a desired angular position of the rotor 20R during the self-heating mode of the battery pack 14. Alternatively, the circuit topology of FIG. 2B could be used to generate a small average magnetic torque for positioning / locking the rotor 20R.Referring to FIGS. 7A through 7D, in some embodiments, the present method 100 results in a q-axis voltage setpoint of zero, i.e., V q= 0, and for purposes of this example, a zero voltage that is also zero, i.e., V 0= 0. However, the d-axis voltage command value tends to fluctuate over a representative time period (t) and a representative voltage range (±V d) as shown in FIG. 7A. For the same period of time, the current setpoint for the d-axis (I d) also varies as shown. Besides pure sine signals, other waveforms are also possible, e.g. triangular waves, square waves etc.As shown in FIG. 7B, the injection of a high frequency d-axis voltage command (V d), as shown in FIG. 7A, can result in small q-axis torque producing current (I q) variations 80. During self-heating of the traction battery pack 14 of FIG. 1 via the method 100, this current variation could result in slight rotation of the rotor 20R. Thus, in one or more embodiments, as set forth above with reference to FIGS. 2A or 2B, the angular position of the rotor 20R could be controlled to maintain the rotor 20R in a desired angular position and possibly first adjust the rotor 20R to such a position prior to the beginning of the self-heating mode.Ideally, the angular position of the rotor 20R does not change as the battery pack 14 of FIG. 1 warms up. However, the closed loop control response may have slight / insignificant movements within a calibrated window, e.g., + / - 5×10 -3 radians, as represented by the position curve 82 in FIG. 7C. FIG. 7D, in turn, illustrates the resulting battery current (i BAT) of curve 84 over the same interval, which battery current circulates in traction battery pack 14. Thus, the battery pack 14 heats up by the injected high frequency alternating current i d, which in turn is induced by the d-axis voltage command (V d) described above.An embodiment of the method 100 for heating the battery pack 14 of the electric propulsion system 10 of FIG. 1 is shown in FIG. 8, where the battery pack 14 is connected to the electric propulsion motor 20. FIG. 8 is illustratively organized into discrete process steps, code segments, or blocks, with each block of method 100 being executed and thus performed by the electronic controller 50 of FIG. 1.In block B 102, the method 100 includes determining a temperature (T BAT) of the traction battery 14 that may be performed by the temperature sensor 14S of FIG. 1. The measured battery temperature (T BAT) is then communicated to the electronic controller 50, for example, via a controller area network (CAN) message bus. The method 100 continues to block B 104.In block B 104, the electronic controller 50 determines whether a predetermined set of input conditions is satisfied. The conditions considered in block B 104 could include, for example, the measured battery temperature (T BAT) being below a calibrated minimum temperature limit (T CAL) suitable for charging the battery pack 14. Although other temperature values may be used as the minimum temperature threshold (T CAL) a temperature threshold of about 32-35° F may be suitable in one or more embodiments. Other conditions could also be considered, including, but not limited to, receiving a user request for self-heating, e.g., via a cellular phone or touch screen (not shown), the motor vehicle 12 of FIG. 1 is parked in a powered-off state / stationary, etc. The method 100 proceeds to block B 106 if the input conditions are met, alternatively the method 100 proceeds to block B 102 if the input conditions are not met.Next, in block B 106, the method 100 performs the above-mentioned self-heating mode of the battery pack 14 via the electronic controller 50. This includes at least applying a high frequency AC waveform to the traction battery pack 14 via the d-axis voltage command (V d) in conjunction with a q-axis voltage command (V q) of zero, and controlling the position of the rotor 20R in a closed loop (see FIGS. 2A and 2B ) during the self-heating mode to minimize the output torque (T O) ideally to zero or within an existing lash tolerance. That is, the lash of the electric drive motor 20 may allow small movements of the rotor 20R without movement of the motor vehicle 12 such that engagement beyond such predetermined lash limits could occur.Block B 106 may include generating unbalanced AC waveforms in the electric motor via the electronic controller 50 and applying the AC waveform to the electrode terminals of the traction battery pack 14. The electronic controller 50 could have a bandwidth of less than about 200 Hz, in which case generating the unbalanced AC waveform may include generating an AC waveform having a frequency of at least 200 Hz to 1 kHz. As part of block B 106, the method 100 could also include sensing a threshold rotation of the rotor 20R, for example using the position sensor 20S of FIG. 1. in such a case, generating the unbalanced AC waveform may include adjusting, via the electronic controller 50, the feed angle in response to the threshold rotation, thereby counteracting the threshold rotation.Execution of the control routine at block B 106 of FIG. 8 could also include generating a d-axis pulsating voltage command (V d) via the electronic controller 50 such that negligible electromagnetic torque is generated during the auto-heating mode. As described above with reference to FIG. 3, this action could include using a linear combination of a varying and / or constant d-axis voltage command, and possibly using a varying and / or constant zero voltage command. The method 100 then proceeds to block B 108.Block B 108 includes determining whether the angular position (θ R) of the rotor 20R is within a predetermined tolerance of a calibrated position (θ CAL) during the self-heating mode, i.e., θ R ≈ θ CAL. The method 100 proceeds to block B 109 if the angular position is not within the predetermined tolerance, e.g., 5×10 -3 radians, as shown in FIG. 7C. Block B 108 may include determining a control angle during the auto-heating mode and then using the control angle to maintain a position of the rotor 20R for a duration of the auto-heating mode, e.g., when the control angle exceeds a calibrated value. The control angle used herein is the particular angle at which a PWM portion of the control signals (CCo) of FIG. 1 is applied to the electric drive motor 20. The method 100 alternatively continues to block B 110.In block B109 (GEN T R) the electronic controller 50 could generate reluctance torque as described above and then apply the reluctance torque to reposition the rotor 20R. Generating the reluctance torque may include generating a small difference from a virtual reference angle, e.g., 1-2°, in one or more embodiments. In one or more embodiments, block B 109 could include selectively applying a DC voltage waveform to the d-axis of the electric drive motor 20 to maintain the rotor 20R at a predetermined angular position. The method 100 then returns to block B 108.In block B 110 ("XC?"), the electronic controller 50 may determine whether an exit condition for exiting the method 100 is present. Representative conditions could be electrical faults or possibly a threshold rotation of the rotor 20R indicating that the motor vehicle 12 is traversing more than the allowable gear lash of the electric drive motor 20. If the motor vehicle 12 is equipped with front / rear cameras or sensors, the movement of the motor vehicle 12 (and the control of the rotor 20R) could be coordinated with the data from such sensors. The method 100 continues to block B 112 if such an error condition has been detected. Block B 102 is alternatively executed if the fault condition is not detected.Block B 112 ("EXT") includes exiting the method 100 and terminating the self-heating mode. In such a case, the method 100 can begin again with block B 102 on the next "key-off" cycle of the motor vehicle 12 or be interrupted until maintenance work is carried out depending on the type of fault ascertained in block B 110.The proposed method 100 of self-heating the traction battery pack 14 of FIG. 1 may be performed under various circumstances. For example, the method 100 could be performed when one or more high voltage loads, such as an AC compressor control module, auxiliary power module 24, or high voltage heaters (not shown), are operating on the DC bus 15 and connected to the traction battery pack 14. The method 100 may also be performed when the traction battery 14 is actively charged, e.g., by a DC fast charging operation. This can be done regardless of whether the HV load on the DC voltage bus 15 is operating or not.The electronic controller 50 could selectively use a voltage control approach of the present disclosure. For example, the controller 50 could switch from the voltage controller to the current controller based on the input frequency of the d-axis voltage inputs. This could mean that current regulation is switched over if the injection frequency is determined to be within the existing bandwidth of the current regulator controller 59, e.g. on the basis of a threshold comparison.In the current control, the feeding frequency and the bandwidth of the electronic controller 50 functioning as a current controller may be selectively increased to improve the control. For example, the bandwidth of the current regulator can be varied by adjusting the regulator gains, e.g., the proportional and integral gains. The gain values of the electronic controller 50 may be determined using look-up tables, with the input to the look-up table being the highest frequency component of the d-axis current command. Alternatively, algebraic functions such as linear, nonlinear, polynomial, and piecewise linear functions may be used to define each regulator gain based on the input (highest frequency component of the d-axis current command).Among other advantages, the method 100 of FIG. 8 solves the limited AC feed problem for battery self-heating at higher frequencies, while minimizing rotation of the rotor 12R due to excessive residual torque. By using the method 100, the traction battery 14 can also be heated uniformly more quickly than in conventional methods, for example before charging. In this way, implementation of the present solutions may help improve the operating range of the motor vehicle 12 of FIG. 1 without requiring additional hardware. These and other advantages of the present disclosure will be appreciated by those skilled in the art in view of the foregoing disclosureThe detailed description and the drawings or figures are supporting and describing the present teachings, but the scope of the present teachings is defined solely by the claims. While some of the best modes and other embodiments for carrying out the present teachings have been described in detail, there are various alternative designs and embodiments for carrying out the present teachings that are defined in the appended claims.

Claims

An electronic controller for use with an electrified powertrain system having a battery pack and an electric motor, comprising: a temperature sensor configured to measure a temperature of the battery pack; and a processor in communication with the temperature sensor, the processor configured to: receive the temperature of the battery pack as a measured battery temperature; and in response to a set of input conditions including the measured battery temperature being below a lower temperature limit, perform a self-heating mode of the battery pack including injecting a high frequency alternating current (AC) voltage waveform into a common axis (d-axis) of the electric motor via a d-axis voltage command in conjunction with a quadrature axis (q-axis) voltage command of zero, thereby heating the battery pack.The electronic controller of claim 1, wherein the processor is further configured to: apply an AC waveform across electrode terminals of the battery pack in response to applying the high frequency unbalanced AC voltage waveform via the electric motor, thereby heating the battery pack during the self-heating mode.The electronic controller of claim 2, wherein the processor is configured to detect a threshold rotation of a rotor of the electric machine and adjust a total voltage command via the processor in response to the threshold rotation.The electronic controller of claim 3, wherein the processor is configured to generate a d-axis pulsating stress command such that negligible torque is applied to the rotor during the self-heating mode.The electronic controller of claim 3, wherein the processor is configured to generate the d-axis pulsating voltage command using a linear combination of a varying d-axis voltage command and / or using a constant d-axis voltage command.The electronic controller of claim 3, wherein the processor is configured to generate the d-axis pulsating voltage command using a varying zero voltage command and / or using a constant zero voltage command.The electronic controller of claim 1, wherein the processor is configured to determine a control angle during the auto-heating mode and maintain a desired angular position of a rotor of the electric machine for a duration of the auto-heating mode using the control angle.The electronic controller of claim 7, wherein the processor is configured to: determine when the control angle exceeds a calibrated value; calculate a reluctance torque or a magnetic torque when the control angle exceeds the calibrated value; and command the reluctance torque or the magnetic torque to reset the rotor.An automotive vehicle, comprising: road wheels; an electrified powertrain system having a battery pack and an electric motor, the electric motor having a rotor connected to one or more of the road wheels; and an electronic controller having: a temperature sensor configured to determine a temperature of the battery pack; and a processor in communication with the temperature sensor, the processor configured to: receive the temperature of the battery pack as a measured battery temperature; and responsive to a set of input conditions including the measured battery temperature being below a lower temperature limit, performing a self-heating mode of the battery pack by commanding a high frequency alternating current (AC) voltage waveform to a common axis (d-axis) of the electric motor via a d-axis voltage command in conjunction with a quadrature axis (q-axis) command of zero to heat the battery pack, thereby causing an alternating current waveform to be applied across electrode terminals of the battery pack.The motor vehicle of claim 9, wherein the electronic controller is configured to determine a control angle during the auto-heating mode and maintain an angular position of the rotor for a duration of the auto-heating mode using the control angle.

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