Discontinuous control strategy for rare-earth-free powertrain system in electric vehicle applications

Abstract

A method for operating a powertrain system of an electric vehicle includes: determining a torque demand for a rare-earth free electric machine to propel the electric vehicle; commanding, an inverter to apply an alternating current (AC) power to stator windings of the rare-earth free electric machine to cause the rare-earth free electric machine to operate in an over-torque mode, generating an output torque in excess of the torque demand; and commanding the inverter to operate the rare-earth free electric machine to operate in a coasting mode. An average torque produced by the rare-earth free electric machine over a given time period including one or more instances of the over-torque mode and the coasting mode approximates the torque demand.

Description

DISCONTINUOUS CONTROL STRATEGY FOR RARE-EARTH-FREE POWERTRAIN SYSTEM IN ELECTRIC VEHICLE APPLICATIONSCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This PCT International Patent Application claims the benefit of and priority to U. S.Provisional Patent Application Serial No. 63 / 532,976, filed August 16, 2023, titled “Discontinuous Control Strategy For Rare-Earth-Free Powertrain System In Electric Vehicle Applications,” the entire disclosure of which is hereby incorporated by reference in its entirety.FIELD

[0002] The present disclosure relates generally to a method and system for controlling a motor drive. More specifically, the method and system of the present disclosure may be used for controlling a motor drive coupled to an electric motor configured as a traction motor for propelling a motor vehicle, such as a passenger car or truck.BACKGROUND|0003| A powertrain system has a significant impact on the driving range and cost of a battery electric vehicle (BEV). Such powertrain systems include one or more traction machines, such as electric motors. The traction machine of a powertrain system most often operates in the low torque region when the vehicle is driven on a drive cycle. Vehicles may spend a significant portion of their operation in the low torque region, particularly during long intervals of cruising at constant speed, where extra torque is not required for acceleration. Hence, this mode of operation is a vital target for improving vehicle range or reducing the battery requirement of an electric vehicle.SUMMARY

[0004] The present disclosure provides a method for operating a powertrain system of an electric vehicle. The method includes: determining a torque demand for a rare-earth free electric machine to propel the electric vehicle; commanding, an inverter to apply an alternating current (AC) power to stator windings of the rare-earth free electric machine to cause the rare-earth free electric machine to operate in an over-torque mode, generating an output torque in excess of the torque demand; and commanding the inverter to operate the rare-earth free electric machine to operate in a coasting mode. An average torque produced by the rare-earth free electric machine over a given time period including one or more instances of the over-torque mode and the coasting mode approximates the torque demand.

[0005] The present disclosure also provides a powertrain system of an electric vehicle. The powertrain system includes: a rare-earth free electric machine configured to propel the electric vehicle; an inverter configured to apply an alternating current (AC) power to stator windings of the rare-earth free electric machine; and a controller. The controller is configured to: determine a torque demand for the rare-earth free electric machine to propel the electric vehicle; command the inverter to apply the alternating current (AC) power to stator windings of the rare-earth free electric machine to cause the rare-earth free electric machine to operate in an over-torque mode, generate an output torque in excess of the torque demand; and command the inverter to operate the rare- earth free electric machine to operate in a coasting mode. An average torque produced by the rare- earth free electric machine over a given time period including one or more instances of the overtorque mode and the coasting mode approximates the torque demand.(0006] These and other aspects of the present disclosure are disclosed in the following detailed description of the embodiments, the appended claims, and the accompanying figures.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Further details, features and advantages of designs of the invention result from the following description of embodiment examples in reference to the associated drawings.10008[ FIG. 1 shows a schematic block diagram of a system, in accordance with an aspect of the present disclosure;

[0009] FIG. 2 shows a graph representing system efficiency of an example wound-field synchronous machine (WFSM) powertrain;

[0010] FIGS. 3-4 show graphs of a jerk profile and combined vehicle speed and torque profiles for Pulse and glide (PnG) control with a jerk of up to 0.28 meters per second cubed (0.28 m / s3);

[0011] FIGS. 5-6 show graphs of a jerk profile and combined vehicle speed and torque profiles for PnG control with a jerk of up to 0.89 meters per second cubed (0.89 m / s3);

[0012] FIGS. 7-8 show graphs of a jerk profile and combined vehicle speed and torque profiles for PnG control with a jerk of up to 50 meters per second cubed (50 m / s3);10013 [ FIG. 9 shows a combined graph displaying motor speed and torque profiles for PnG control with a jerk of up to 0.1 meters per second cubed (0.1 m / s3);[0014[ FIG. 10 presents a 3-dimensional graph showing PnG control variable effects on percentage loss reduction at 30 mph cruising speed, including percentage loss reduction (%) as a function of pulse period (s) and duty cycle (%);|0015| FIG. 11 presents a 3-dimensional graph showing PnG control variable effects on speed ripple at 30 mph cruising speed, including speed ripple (mph) as a function of pulse period (s) and duty cycle (%); and

[0016] FIG. 12 shows a flow chart of steps in a method for operating a powertrain system of an electric vehicle, in accordance with some embodiments of the present disclosure.DETAILED DESCRIPTION

[0017] Referring to the drawings, the present invention will be described in detail in view of following embodiments.

[0018] To overcome the technical limitations of the existing approaches, a control technique is proposed in the form of a method for operating a powertrain system of an electric vehicle (EV).

[0019] Pulse and glide (PnG) vehicle control is a driving technique that is used to improve powertrain efficiency and increase vehicle range. The technique involves repeatedly accelerating the vehicle to a certain speed, and then releasing the accelerator and allowing the vehicle to coast. This reduces the amount of energy that is consumed by the vehicle by operating the powertrain in high efficiency regions when a low load torque command is required. Pulse and glide can be used in a variety of different driving situations, including highway driving and city driving, but it is especially useful when operating the vehicles for extended periods of time at a constant speed. Therefore, the benefits of improved fuel efficiency are most pronounced during conditions such as highway driving.

[0020] FIG. 1 shows a block diagram of powertrain control system 10 of an electric vehicle (EV), in accordance with an aspect of the present disclosure. The powertrain control system 10 is provided in a vehicle 12 having four wheels 14. The system 10 includes an inverter 20 having one or more pairs of solid-state switches 21, such as field effect transistors (FETs) configured to switch current from a DC power supply 22 and to generate an AC power upon a set of motor leads 24. A DC link bus 23 conducts current between the DC power supply 22 and the inverter 20 and definesa DC-link voltage VDC. The motor leads 24 transmit electrical power between the inverter 20 and stator windings 27 of an electric motor 26.

[0021] The electric motor 26 may be free of rare-earth materials. The electric motor 26 may include, for example, a wound field synchronous machine (WFSM), a synchronous reluctance machine (SynRM), or an induction machine. In some embodiments, the electric motor 26 may include a permanent magnet synchronous machine (PMSM) having permanent magnets that are free of rare-earth materials. The electric motor 26 is shown as a 3-phase machine, however, the electric motor 26 may have any number of phases. For example, the electric motor 26 may be single-phase machine, a 3-phase machine, or a higher-order multiphase machine. The electric motor 26 may be used as a motor, a generator, or as a motor / generator that functions as both a motor and a generator. The electric motor 26 may be coupled to one or more of the wheels 14 of the vehicle 12 for driving the vehicle 12. In some embodiments, the system 10 of the present disclosure may have a non-vehicular application, such as for motor control in industrial or manufacturing applications.

[0022] A current sensor 28 is arranged to measure current in one of the motor leads 24. In some embodiments, and as shown on FIG. 1, the current sensor 28 measures an A-phase current ia on a corresponding one of the motor leads 24. However, the system 10 may measure current on any one or more of the motor leads 24. The system 10 may include other sensors, such as voltage sensors configured to measure voltages upon or between the motor leads 24.

[0023] The system 10 of FIG. 1 also includes an electronic control unit (ECU) 30 in communication with the current sensor 28 to measure the currents in the motor leads 24. The ECU 30 may also be in functional communication with the inverter 20 to control operation of the inverter 20 and / or to monitor parameters measured by sensors associated with the inverter 20. The ECU 30includes a processor 32 coupled to a storage memory 34. The storage memory 34 stores instructions, such as program code for execution by the processor 32, in an instruction storage 36. The storage memory 34 also includes data storage 38 for holding data to be used by the processor 32. The data storage 38 may record, for example, values of the parameters measured by the current sensor 28 and / or the outcome of functions calculated by the processor 32.

[0024] A gearbox 42 is driven by a shaft 40 of the electric motor 26 and transmits torque to two of the wheels 14 for propelling the vehicle 12. In some embodiments, the gearbox 42 provides a fixed gear ratio between the shaft 40 and the wheels 14. Alternatively, the gearbox 42 may provide two or more different gear ratios between the shaft 40 and the wheels 14.]0025[ A clutch 44, which may also be called a decoupling clutch, is connected between the electric motor 26 and the gearbox 42. Alternatively, the clutch 44 may be disposed between the gearbox 42 and the wheels 14. The clutch 44 may be engaged to transmit torque therebetween or disengaged to prevent torque transmission therethrough.

[0026] In some embodiments, the clutch 44 may be utilized with an electric motor 26 that is configured as a permanent magnet synchronous machine (PMSM). Alternatively or additionally, the clutch 44 may be utilized with an electric motor 26 that is configured as a WFSM.

[0027] The electric motor 26 shown on FIG. 1 is configured as a WFSM and includes a rotor winding 50 that rotates with the shaft 40. A power regulator 52 receives power from the DC power supply 22 and supplies a regulated power to the rotor winding 50. The power regulator 52 is in functional communication with the controller 30 and is configured to apply a given voltage to the rotor winding 50 and / or to supply a given current or a given power to the rotor winding 50, based on a command from the controller 30. In some embodiments the power regulator 52 may supply a DC power to the rotor winding 50 which may be transmitted to the rotor winding 50 usingone or more slip rings (not shown). Alternatively, the power regulator 52 may supply an AC power that may be transmitted to the rotor winding 50 via a rotary transformer (not shown). AC power induced in a secondary winding of the rotary transformer may be converted to DC power by a rectifier that rotates with the shaft 40 and for supply to the rotor winding 50.|0028| The pulse and glide control scheme of the present disclosure may include utilizing the clutch 44 to selectively decouple the electric motor 26 from one or more wheels 14 of the vehicle 12. For example, the electric motor 26 is coupled to the axle and generates an output torque in excess of the torque demand; then the electric motor 26 is decoupled from the axle using the clutch 44 to initiate a coasting mode. During the coasting mode, with the electric motor 26 decoupled from the wheels 14, the electric motor 26 may be driven to a reduced speed, thereby minimizing the creation of excess back-EMF, and then the inverter 20 may be switched off. For example, with the electric motor 26 decoupled from the wheels 14, the electric motor 26 may be brought down to zero speed to shut off the inverter 20, while the vehicle 12 continues coasting. The electric motor 26 is then brought up to speed, and the clutch 44 is actuated to re-couple the electric motor 26 to transmit torque to the wheels 14 and to perform a next iteration of an overtorque mode, with the electric motor 26 producing and transmitting output torque to one or more wheels 14 of the vehicle 12. This sequence is repeated and the torque profiles may be determined as described in the present disclosure to obtain enhancements in overall efficiency. For example, the torque profiles may be determined based on optimizations of pulse period, duty cycle and jerk limitations.|0029| FIG. 2 shows a graph representing system efficiency of an example wound-field synchronous machine (WFSM) powertrain. As shown in FIG. 2, operating efficiencies of an electric vehicle powertrain in the low torque region are much lower than the peak efficiency.Powertrain Architecture for PnG Control Strategy

[0030] Permanent magnet synchronous machines (PMSM) are widely used in EVs because of their high efficiency and torque density characteristics when compared to other machines. However, when operating in a field weakening (FW) mode, the rectified back-emf is larger than the DC-link voltage VDC. TO address this issue, the inverter may be commanded to supply a d-axis command to weaken the field and maintain the machines operation at the voltage limit. Disabling the power stage would result in an uncontrolled current flow from the machine into the battery, resulting in an uncontrolled braking torque on the load. In addition, a PMSM may experience a significant core loss due to the magnetic flux in the rotor while coasting and delivering zero output torque. These drawbacks may make the PMSM less suitable for the proposed PnG control strategy. The PnG technique may be used with SynRM motors due to the absence of a rotor excitation. However, SynRMs suffer from reduced power densities and poor FW operation. The present disclosure proposes using an electric machine that is free of rare-earth materials used in conventional PMSMs.

[0031] The present disclosure provides wound field synchronous machine (WFSM) that has an enhanced power density, efficiency, and FW performance in comparison to SynRMs and has added cost benefits to PMSMs due to the absence of rare earth materials. In contrast to PMSMs, WFSMs have the advantage of full control over the strength of the magnetic field in the rotor. Therefore, the field can be de-excited to mitigate the BEMF over voltage during FW operation at high speeds. This feature allows the inverter to be deactivated during coasting intervals in the PnG technique which significantly reduces the switching losses of the inverter. Furthermore, with a deexcited rotor, the machine theoretically experiences zero core losses during coasting to significantlyreduce the total electric motor losses. The principles of the present disclosure may be applied to other types of electric machines.WFSM Rotor Field Control Strategy

[0032] The field current in a WFSM is not able to instantly be set to zero due to a relatively large inductance of the rotor winding 50. When the zero-torque command is requested by the PnG method, the field current command may also be set to zero. The field current must decay down to zero and the inverter 20 must operate until the BEMF generated by the speed of the machine and residual field current is less than the system limitation. One strategy is to ensure that the field current is zero before opening the inverter switches. Alternatively, the speed and field current magnitude can be mapped to a BEMF voltage and the inverter can be shutoff once the BEMF is below the system limitation and then continue letting the field current decay to zero during the coasting period.Acceleration and Jerk Limitation Controllers

[0033] A primary consideration for the PnG control method is the potential for user discomfort due to the pulsating torque output of the powertrain. Vehicle acceleration and jerk are measurable parameters that have a direct impact on user discomfort. Discomfort increases almost linearly with acceleration magnitude, and acceleration is the most important predictor of discomfort. Therefore, to address user comfort as a design limitation in the PnG strategy, an acceleration limitation is considered. To reduce the user discomfort, an acceleration and jerk controller is implemented in the control software to maintain operation below the limits of a human’s perceived discomfort.

[0034] FIGS. 3-4 show graphs of a jerk profile and combined vehicle speed and torque profiles for Pulse and glide (PnG) control with a jerk of up to 0.28 m / s3. FIG. 4 includes a first plot102 representing achieved or actual speed of the vehicle 12, a second plot 104 representing desired or setpoint speed, a third plot 106 representing applied torque produced by the electric motor 26, and a fourth plot 108 representing required torque to maintain the desired actual speed of the vehicle 12. FIGS. 5-6 show graphs of a jerk profile and combined vehicle speed and torque profiles for PnG control with ajerk of up to 0.89 m / s3. FIG. 6 includes a fifth plot 112 representing achieved or actual speed of the vehicle 12, a sixth plot 114 representing desired or setpoint speed, a seventh plot 116 representing applied torque produced by the electric motor 26, and an eighth plot 118 representing required torque to maintain the desired actual speed of the vehicle 12. FIGS. 7-8 show graphs of ajerk profile and combined vehicle speed and torque profiles for PnG control with ajerk of up to 50 m / s3. FIG. 8 includes a ninth plot 122 representing achieved or actual speed of the vehicle 12, a tenth plot 124 representing desired or setpoint speed, an eleventh plot 126 representing applied torque produced by the electric motor 26, and a twelfth plot 128 representing required torque to maintain the desired actual speed of the vehicle 12.(0035] The resultant acceleration profile is determined and the acceleration weighting factors from ISO2631 are utilized to calculate an acceleration value that is perceived by the user. From testing data, a threshold of acceptable acceleration is determined and used as a constraint to select the optimal jerk limitations and acceleration constraints for the PnG torque profile. This allows the PnG method to be implemented with low frequencies in the 0.05-0.5 Hz range. However, the PnG method of the present disclosure may be implemented using other frequencies, such as frequencies above 0.5 Hz and / or below 0.05 Hz.Duty-Cycle and Pulse Period Optimization(0036] For each torque-speed operating point of the vehicle, the PnG method has a unique solution for two control variables, pulse period and duty cycle, to optimally minimize the total lossand obtain an acceptable level of speed ripple. The PnG torque pulses and resultant vehicle speed are displayed in FIG. 9. The pulse period dictates the frequency of the torque pulses, and the duty cycle represents the portion of the period that the electric motor is delivering torque as opposed to coasting. To ensure that the proper vehicle speed is maintained during operation, the average torque delivered by the powertrain must remain constant, irrespective of the duty cycle. Therefore, a reduced duty cycle requires a larger magnitude of torque to deliver the same average torque to the wheels in a given period. During the intervals of applied torque, the WFSM, inverter, and gearbox losses are present and experience a larger loss magnitude due to the increased torque requirement. While the vehicle is coasting, the rotor field of the WFSM is de-excited to eliminate core losses and avoid BEMF voltages that could exceed the voltage limitations of the system during FW operation.

[0037] Therefore, the conduction and core losses of the WFSM are absent during vehicle coasting. Furthermore, the absence of the field excitation allows the inverter to be deactivated, drastically reducing the switching losses of the inverter. However, the mechanical losses of the gearbox and the WFSM remain since the axle continues to rotate during coasting. This develops a tradeoff between the reduced mechanical losses during coasting and the increased electrical losses during acceleration.

[0038] FIG. 9 shows a combined graph displaying motor speed and torque profiles for PnG control with a jerk of up to 0.1 m / s3. FIG. 9 includes a thirteenth plot 132 representing achieved or actual speed of the vehicle 12, a fourteenth plot 134 representing desired or setpoint speed, a fifteenth plot 136 representing applied torque produced by the electric motor 26, and a sixteenth plot 138 representing a required torque to maintain the desired actual speed of the vehicle 12. FIG. 10 presents a 3 -dimensional graph showing PnG control variable effects on percentage lossreduction at 30 mph cruising speed, including percentage loss reduction (%) as a function of pulse period (s) and duty cycle (%). FIG. 11 presents a 3-dimensional graph showing PnG control variable effects on speed ripple at 30 mph cruising speed, including speed ripple (mph) as a function of pulse period (s) and duty cycle (%);|0039| The results in FIG. 10 display that the pulse period has no significant impact on the percentage loss reduction of the powertrain components but directly correlates to the ripple in the speed profile of the vehicle, as seen in FIG. 11. Reduced pulse periods have less speed variation from the desired speed setpoint, and higher pulse periods provide longer durations of acceleration and deceleration that cause the speed to deviate further from the setpoint, as shown FIG. 11. If the pulse period is small enough, the speed ripple can be minimized to a point where the vehicle operator cannot feel the effects of the pulsating torque profile. The duty cycle has a direct impact on both the speed ripple and the percentage loss reduction. Smaller duty cycles have increased torque magnitudes and higher rates of acceleration. This again causes the vehicle to coast longer and increase the speed variation from the desired setpoint. As discussed, the duty cycle directly manipulates the tradeoff between the reduced mechanical losses during coasting and the increased electrical losses during acceleration which is visible in FIG. 10.Applications in Unmanned Autonomous Electric Vehicle[0040| The rare earth free powertrain architecture and PnG control strategy described in the present disclosure is compatible with any electric vehicle, including passenger vehicles. An additional application is autonomous vehicle such as cargo vehicles or any unmanned vehicle where user discomfort is not a concern. In unmanned vehicles, the PnG strategy may be operated with any pulse period or duty cycle, and without regard to jerk and acceleration limitations for user comfort, thus providing further improvements in efficiency.Implementation Using Offline Configured Look-Up-Tables (LUTs)

[0041] The PnG optimization of duty-cycle and pulse period considering jerk and acceleration limits is performed across the entire torque-speed operating region to define percentage system loss reduction in all areas of the torque-speed map. The optimal PnG duty cycle and pulse period trends across the torque-speed operating curve are analyzed and stored in an offline developed LUT for control implementation. The optimization procedure for each load point is described in the simulation case study described below. The speed and torque of the machine is an input into the LUT and the output is the duty-cycle and pulse period.(0042) The present disclosure provides a Pulse and Glide vehicle control strategy using WFSM or any rare-earth free machine powertrain architecture. In some embodiments, the system and method of the present disclosure may employ a WFSM rotor field winding control strategy to deactivate the inverter once a back-emf (BEMF) is below a predetermined value. In some embodiments, the system and method of the present disclosure may limit acceleration and / or jerk to minimize user discomfort caused by the pulsating torque profile. Such a control technique may employ low frequency pulses that still satisfy user discomfort requirements. In some embodiments, the system and method of the present disclosure may adjust a torque profile as a ramp, sinusoid, or another shape to satisfy the jerk limitation and to minimize user discomfort. In some embodiments, the system and method of the present disclosure may employ duty-cycle and pulse period control for maximizing potential percentage loss reduction and speed ripple variation. In some embodiments, the system and method of the present disclosure may be configured to minimize speed ripple through pulse period reduction to minimize or eliminate user perception of the PnG control strategy. In some embodiments, the powertrain and PnG control strategy may be provided in an autonomous vehicle, such as an unmanned delivery vehicle where user discomfortis not a concern. In some embodiments, the system and method of the present disclosure may include offline calibration of PnG duty-cycle and pulse periods, which may be provided across an entire operating map of the electric machine for LUT-based implementation in the control software

[0043] FIG. 12 shows a flow chart of steps in a method 200 for operating a powertrain system of an electric vehicle, in accordance with some embodiments of the present disclosure. The method 200 can be performed by the controller 30, in accordance with some embodiments of the present disclosure. As can be appreciated in light of the disclosure, the order of operation within the method is not limited to the sequential execution as illustrated in FIG. 12, but may be performed in one or more varying orders as applicable and in accordance with the present disclosure.[004 1 The method 200 includes determining, at step 202, a torque demand for a rare-earth free electric machine to propel the electric vehicle. For example, the controller 30 may determine an amount of torque required to maintain a given speed or to accelerate or decelerate the vehicle 12 depending on driving requirements, such as to maintain a given speed or to change to a new desired speed.

[0045] The method 200 also includes commanding, at step 204, for an inverter to apply an alternating current (AC) power to stator windings of the rare-earth free electric machine to cause the rare-earth free electric machine to operate in an over-torque mode, generating an output torque in excess of the torque demand. For example, the controller 30 may command the inverter 20 to generate and supply a given d-axis and q-axis AC power to stator windings of the electric motor 26.

[0046] The method 200 also includes commanding, at step 206, for the inverter to operate the rare-earth free electric machine to operate in a coasting mode. For example, the controller 30 may command the inverter 20 and / or the power regulator 52 to reduce a field current in the rotorwinding 50 or in another conductor of the electric motor 26. In the coasting mode, output switches of the inverter 20 may all be opened to allow the powertrain to spin freely with no excitation. In some embodiments, the coasting mode includes ceasing current supply to both the inverter 20 and to the electric motor 26, and the powertrain spins while only experiencing the mechanical friction losses of the motor and gearbox.

[0047] Operating a system, such as the powertrain control system 10, in accordance with the method 200 may cause the rare-earth free electric machine to produce an average torque over a given time period, including one or more instances of the over-torque mode and the coasting mode, which approximates the torque demand. In other words, the average torque over time may be substantially equal to the torque demand, thereby causing the vehicle to maintain a desired acceleration.

[0048] In some embodiments, the rare-earth free electric machine includes a wound-field synchronous machine (WFSM), and operating the rare-earth free electric machine in the coasting mode includes de-exciting a rotor winding of the WFSM.

[0049] In some embodiments, the method further includes: determining a field current in the rotor winding of the WFSM; and driving all output switches of the inverter to a non-conductive state in response to determining the field current in the rotor winding of the WFSM being less than a predetermined threshold current. In some embodiments, the predetermined threshold current is substantially equal to zero amps.

[0050] In some embodiments, the method further includes: determining a field current in the rotor winding of the WFSM; determining a back-EMF (BEMF) voltage of the WFSM based on a speed of the WFSM and based on the field current in the rotor winding; and driving all outputswitches of the inverter to a non-conductive state in response to determining the BEMF voltage of the WFSM being less than a predetermined threshold voltage.

[0051] In some embodiments, the method further includes: determining at least one of an acceleration and a jerk produced by the powertrain system; and adjusting at least one of a duty cycle or a pulse period of the over-torque mode to cause the powertrain system to satisfy at least one of: an acceleration limit for acceleration of the powertrain system, or a jerk limit for jerk of the powertrain system. Jerk describes a third derivative of position, or a change in acceleration over time. Jerk in the powertrain in excess of the jerk limit may be associated with user discomfort. Excessive jerk may cause damage to powertrain components.[00521 In some embodiments, the adjusting the at least one of the duty cycle or the pulse period includes adjusting a torque profile of the rare-earth free electric machine.

[0053] In some embodiments, the adjusting the torque profile of the rare-earth free electric machine includes adjusting the torque profile using one of a ramp, a sinusoidal function, or a custom shape function to satisfy the jerk limit and to minimize user discomfort.

[0054] In some embodiments, the method further includes adjusting at least one of a duty cycle or a pulse period of the over-torque mode to cause the powertrain system maintain a speed of the electric vehicle within a predetermined speed ripple threshold.[0055| In some embodiments, the predetermined speed ripple threshold corresponds to a user perception of speed variation.[0(156] In some embodiments, the electric vehicle is an unmanned vehicle.

[0057] In some embodiments, the method further includes determining at least one of a duty cycle or a pulse period of the over-torque mode based on the torque demand and the speed of the rare-earth free electric machine. Commanding the inverter to apply the AC power to thewindings of the rare-earth free electric machine may include commanding the inverter in accordance with the at least one of the duty cycle or the pulse period.

[0058] In some embodiments, determining the at least one of the duty cycle or the pulse period of the over-torque mode includes determining both of the duty cycle or the pulse period and the over-torque mode based on the torque demand and the speed of the rare-earth free electric machine.[0059| In some embodiments, determining the at least one of the duty cycle or the pulse period of the over-torque mode includes determining the at least one of the duty cycle or the pulse period of the over-torque mode based on a lookup table (LUT).

[0060] In some embodiments, the LUT is determined in advance and stored in a nontransient storage memory.[00611 The present disclosure also provides a powertrain system of an electric vehicle. The powertrain system includes: an electric machine configured to propel the electric vehicle; a clutch configured to selectively couple the electric machine to one or more wheels of the electric vehicle; an inverter having a plurality of output switches configured to apply an alternating current (AC) power to stator windings of the rare-earth free electric machine; and a controller. The controller is configured to: determine a torque demand for the rare-earth free electric machine to propel the electric vehicle; command the clutch to a torque-transmitting mode; command, with the clutch in the torque-transmitting mode, for the inverter to apply the alternating current (AC) power to stator windings of the electric machine to cause the electric machine to operate in an over-torque mode, generating an output torque in excess of the torque demand; command the clutch to a decoupled mode, with the electric machine decoupled from the one or more wheels; and operate, with the clutch in the decoupled mode, the electric machine in a coasting mode. An average torqueproduced by the electric machine over a given time period including one or more instances of the over-torque mode and the coasting mode approximates the torque demand.

[0062] In some embodiments, the electric machine includes a permanent magnet synchronous machine (PMSM), and operating the electric machine in the coasting mode includes commanding all of the plurality of output switches of the inverter to a non-conductive state. In some embodiments, the electric machine includes a wound-field synchronous machine (WFSM), and wherein operating the electric machine in the coasting mode includes de-exciting a rotor winding of the WFSM.(0063) The system, methods and / or processes described above, and steps thereof, may be realized in hardware, software or any combination of hardware and software suitable for a particular application. The hardware may include a general purpose computer and / or dedicated computing device or specific computing device or particular aspect or component of a specific computing device. The processes may be realized in one or more microprocessors, microcontrollers, embedded microcontrollers, programmable digital signal processors or other programmable device, along with internal and / or external memory. The processes may also, or alternatively, be embodied in an application specific integrated circuit, a programmable gate array, programmable array logic, or any other device or combination of devices that may be configured to process electronic signals. It will further be appreciated that one or more of the processes may be realized as a computer executable code capable of being executed on a machine readable medium.|0064] The computer executable code may be created using a structured programming language such as C, an object oriented programming language such as C++, or any other high- level or low-level programming language (including assembly languages, hardware descriptionlanguages, and database programming languages and technologies) that may be stored, compiled or interpreted to run on one of the above devices as well as heterogeneous combinations of processors processor architectures, or combinations of different hardware and software, or any other machine capable of executing program instructions.|0065| Thus, in one aspect, each method described above and combinations thereof may be embodied in computer executable code that, when executing on one or more computing devices performs the steps thereof. In another aspect, the methods may be embodied in systems that perform the steps thereof, and may be distributed across devices in a number of ways, or all of the functionality may be integrated into a dedicated, standalone device or other hardware. In another aspect, the means for performing the steps associated with the processes described above may include any of the hardware and / or software described above. All such permutations and combinations are intended to fall within the scope of the present disclosure.10066] The foregoing description is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.

Claims

CLAIMSWhat is claimed is:

1. A method for operating a powertrain system of an electric vehicle, comprising: determining a torque demand for a rare-earth free electric machine to propel the electric vehicle; commanding an inverter to apply an alternating current (AC) power to stator windings of the rare-earth free electric machine to cause the rare-earth free electric machine to operate in an over-torque mode, generating an output torque in excess of the torque demand; and operating the rare-earth free electric machine in a coasting mode, wherein an average torque produced by the rare-earth free electric machine over a given time period including one or more instances of the over-torque mode and the coasting mode approximates the torque demand.

2. The method of Claim 1, wherein the rare-earth free electric machine includes a wound-field synchronous machine (WFSM), and wherein operating the rare-earth free electric machine in the coasting mode includes de-exciting a rotor winding of the WFSM.

3. The method of Claim 2, further comprising: determining a field current in the rotor winding of the WFSM; and driving all output switches of the inverter to a non-conductive state in response to determining the field current in the rotor winding of the WFSM being less than a predetermined threshold current.

4. The method of Claim 2, further comprising: determining a field current in the rotor winding of the WFSM; determining a back-EMF (BEMF) voltage of the WFSM based on a speed of the WFSM and based on the field current in the rotor winding; and driving all output switches of the inverter to a non-conductive state in response to determining the BEMF voltage of the WFSM being less than a predetermined threshold voltage.

5. The method of Claim 1, further comprising: determining at least one of an acceleration and a jerk produced by the powertrain system; and adjusting at least one of a duty cycle or a pulse period of the over-torque mode to cause the powertrain system to satisfy at least one of: an acceleration limit for acceleration of the powertrain system, or a jerk limit for jerk of the powertrain system.

6. The method of Claim 5, wherein the adjusting the at least one of the duty cycle or the pulse period includes adjusting a torque profile of the rare-earth free electric machine.

7. The method of Claim 6, wherein the adjusting the torque profile of the rare-earth free electric machine includes adjusting the torque profile using one of a ramp, a sinusoidal function, or a custom shape function to satisfy a jerk limit.

8. The method of Claim 1, further comprising: adjusting at least one of a duty cycle or a pulse period of the over-torque mode to cause the powertrain system maintain a speed of the electric vehicle within a predetermined speed ripple threshold.

9. The method of Claim 1, wherein the electric vehicle is an unmanned vehicle.

10. The method of Claim 1, further comprising determining at least one of a duty cycle or a pulse period of the over-torque mode based on each of the torque demand and a speed of the rare-earth free electric machine, and wherein commanding the inverter to apply the AC power to the stator windings of the rare- earth free electric machine includes commanding the inverter in accordance with the at least one of the duty cycle or the pulse period.

11. The method of Claim 10, wherein determining the at least one of the duty cycle or the pulse period of the over-torque mode includes determining both of the duty cycle and the pulse period based on each of the torque demand and the speed of the rare-earth free electric machine.

12. A powertrain system of an electric vehicle, comprising: a rare-earth free electric machine configured to propel the electric vehicle; an inverter having a plurality of output switches configured to apply an alternating current(AC) power to stator windings of the rare-earth free electric machine; and a controller configured to:determine a torque demand for the rare-earth free electric machine to propel the electric vehicle; command the inverter to apply the alternating current (AC) power to stator windings of the rare-earth free electric machine to cause the rare-earth free electric machine to operate in an over-torque mode, generating an output torque in excess of the torque demand; and operate the rare-earth free electric machine in a coasting mode, wherein an average torque produced by the rare-earth free electric machine over a given time period including one or more instances of the over-torque mode and the coasting mode approximates the torque demand.

13. The powertrain system of Claim 12, further comprising: a clutch configured to selectively couple the rare-earth free electric machine to one or more wheels of the electric vehicle, and wherein the controller is further configured to: command the clutch to a torque-transmitting mode; command, with the clutch in the torque-transmitting mode, for the inverter to apply the alternating current (AC) power to stator windings of the rare-earth free electric machine to cause the rare-earth free electric machine to operate in an over-torque mode, generating an output torque in excess of the torque demand; command the clutch to a decoupled mode, decoupling the rare-earth free electric machine from the one or more wheels; andoperate, with the clutch in the decoupled mode, the rare-earth free electric machine in the coasting mode wherein a speed of the rare-earth free electric machine is reduced and then the inverter is switched off.

14. The powertrain system of Claim 12, wherein the rare-earth free electric machine includes a wound-field synchronous machine (WFSM), and wherein operating the rare-earth free electric machine in the coasting mode includes de-ex citing a rotor winding of the WFSM.

15. The powertrain system of Claim 12, wherein the rare-earth free electric machine includes a permanent magnet synchronous machine (PMSM), and wherein operating the rare-earth free electric machine in the coasting mode includes commanding all of the plurality of output switches of the inverter to a non-conductive state.

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