DRIVE SYSTEM WITH SLIP-LIMITED ELECTRIC DIFFERENTIAL DRIVE UNIT

Abstract

Slip-limited electric drive system (10) for a motor vehicle, comprising: an induction motor (12), comprising the induction motor (12): a stator (24); a first rotor (26a) which is arranged for rotation in the stator (24), wherein the first rotor (26a) is associated with a first wheel (20) of the vehicle; a second rotor (26b) arranged for rotation in the stator (24), wherein the second rotor (26b) is associated with the second wheel (22) of the vehicle and is able to rotate in the stator (24) independently of the first rotor (26a), wherein each of the first and second rotors (26a, 26b) can slip relative to the other when necessary when the vehicle is turning; and a traction converter and control system (14) for monitoring an angular velocity of each of the first and second rotors (26a, 26b), which determines which of the first and second rotors (26a, 26b) is rotating more slowly than the other and which generates a signal for controlling the induction motor (12) according to the respective first or second rotor (26a, 26b) that has the lower speed, so that a torque signal is generated according to the rotor (26a, 26b) that has the lower speed.

Description

AREA

[0001] The present disclosure relates to propulsion systems for electric vehicles, and more particularly to a propulsion system utilizing an induction motor for driving a pair of wheels of a vehicle, and wherein the induction motor includes fully independent and uncoupled induction rotors housed in a common stator for independently driving the pair of wheels without the need for a conventional mechanical differential. BACKGROUND

[0002] This section provides background information related to the present disclosure that is not necessarily prior art.

[0003] Today's electric vehicles, such as electric-powered automobiles, can employ an induction motor to provide driving power to a pair of wheels. Typically, the front wheels are driven, but in some cases, all four wheels of the vehicle are driven: one motor to drive the front wheels and a separate motor to drive the rear wheels. It is also possible to drive the vehicle's rear wheels using a single electric motor. In either case, today's electric-powered vehicles typically require the use of a mechanical differential to receive the output from the electric motor and couple the driving power to the driver and passenger wheels of the vehicle. The driven wheels could be the vehicle's front wheels or they could be the vehicle's rear wheels.

[0004] The differential is necessary because the driver and passenger wheels must rotate at different angular velocities when the vehicle corners. However, the use of a mechanical differential adds cost, complexity, and weight to the vehicle. The mechanical differential also introduces mechanical losses due to friction within the differential, which reduces the power delivered to the vehicle's wheels.

[0005] One way to avoid the use of a mechanical differential is to use two completely separate motors to independently drive a pair of wheels (either front or rear) of the vehicle. This implementation typically requires two independent power converters, one for each motor, due to the need to drive the wheels at different angular velocities when the vehicle corners. Obviously, this option also suffers from the disadvantages of additional cost and complexity due to the need for the second power converter.

[0006] Another possibility is the use of an electronic differential, as described in document US 5 172 784 A. SUMMARY

[0007] This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all of its features.

[0008] In one aspect, the present disclosure relates to a limited-slip electric drive system for a motor vehicle. The system may comprise an induction motor, the induction motor including a stator. The induction motor may further include a first rotor disposed for rotation within the stator, the first rotor associated with a first wheel of the vehicle, and a second rotor disposed for rotation within the stator. The second rotor may be associated with a second wheel of the vehicle and capable of rotating within the stator independently of the first rotor. Each of the first and second rotors is capable of slipping relative to the other as needed when the vehicle is cornering.The system may further include a traction converter and control system for monitoring an angular velocity of each of the first and second rotors and for determining which of the first and second rotors is rotating slower than the other. The traction converter and control system may also be operable to generate a signal for controlling the induction motor according to one of the first or second rotors having a lower speed, such that a torque signal is generated according to the rotor having the lower speed.

[0009] In another aspect, the present disclosure relates to a limited-slip electric drive system for a motor vehicle. The system may include an induction motor, and the induction motor may include a stator and a first rotor disposed for rotation within the stator. The first rotor may be associated with a first wheel of the vehicle. A second rotor may be included, also disposed for rotation within the stator. The second rotor may be associated with a second wheel of the vehicle and capable of rotating within the stator independently of the first rotor, such that each of the first and second rotors may slip relative to the other as needed when the vehicle is cornering. The system may further include a first position sensor connected to the first rotor and configured to detect a speed of the first rotor.A second position sensor may be included, associated with the second rotor and configured to detect a speed of the second rotor. The system may further include a traction converter and control system. The traction converter and control system may include a slip frequency calculation system for calculating a slip frequency of the induction motor in real time. The traction converter and control system may further include a position sensor selection system responsive to signals from the first and second position sensors and configured to determine and select for use a speed signal from the one of the first and second position sensors having a lower speed. The traction converter and control system is operable to control the induction motor such that a torque signal is generated according to the rotor having the lower speed.

[0010] In yet another aspect, the present disclosure relates to a method for forming a limited-slip electric drive system for a motor vehicle. The method may include disposing a first rotor for rotation within a stator of an induction motor, the first rotor associated with a first wheel of the vehicle on a driver's side of the vehicle. The method may further include disposing a second rotor for rotation within the stator, the second rotor associated with a second wheel of the vehicle on a passenger side of the vehicle and rotating about a common axis with the first rotor, the second rotor further capable of rotating within the stator independently of the first rotor, and further such that the first and second rotors are capable of rotating at different speeds as needed when the vehicle is cornering.The method may further include using a traction converter and control system to monitor an angular velocity of each of the first and second rotors while the vehicle is traveling to determine which of the first and second rotors is rotating slower than the other. The traction converter and control system may also be operable to control the induction motor so that a torque signal is generated according to the rotor having the lower speed.

[0011] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. DRAWINGS

[0012] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations and are not intended to limit the scope of the present disclosure. Fig. 1 is a general block diagram of an embodiment of a drive system according to the present disclosure; and Fig. 2 is a general control diagram showing various components of the system of Fig. 1 together with operations performed in determining a wheel speed for use for each pair of wheels driven by the drive system.

[0013] Corresponding reference numbers identify the respective parts in the various views of the drawings. DETAILED DESCRIPTION

[0014] Exemplary embodiments will now be described more fully with reference to the accompanying drawings.

[0015] Referring to Fig. 1 illustrates a propulsion system 10 according to one embodiment of the present disclosure. System 10, in this example, may include an induction motor subsystem 12 and may also include a traction power converter and control system 14 (hereinafter simply "power converter system 14"). Power converter system 14 may include an electronic controller 16 and is in communication with a processor-based engine control unit ("ECU") 18 of the vehicle. Induction motor subsystem 12 is used simultaneously to provide motive power for driving a left wheel 20 and a right wheel 22 of an automotive vehicle, such as a car or truck.However, it is immediately understood that system 10 is not limited to use with automobiles alone, and indeed, it is expected to find applicability to any type of wheeled vehicle in which wheels are driven by an electric motor and the vehicle needs to be propelled while cornering. The teachings of the present disclosure may even find applicability in connection with any type of device powered by an electric motor for which two simultaneously driven elements having slightly different angular velocities must be produced.

[0016] The induction motor subsystem 12 in this example includes a stator 24 housing two completely independent rotors 26a and 26b. The rotors 26a and 26b rotate about a common longitudinal axis but are decoupled from each other and can thus rotate freely at different angular velocities, such as when the vehicle is cornering and the outer wheel 20 or 22 has a higher angular velocity than the inner wheel.

[0017] Rotor 26a has an output shaft 28a, and rotor 26b has an output shaft 28b. The output shaft 28a may be supported by, and its speed monitored by, a sensor / bearing assembly forming a position encoder 30a, while the rotor 26b may be supported by, and its speed monitored by, a sensor / bearing assembly forming a position encoder 30b. The output shaft 28a may alternatively be coupled to a planetary gear reduction system 32a, while the output shaft 28a may alternatively be coupled to a separate planetary gear reduction system 32b. The planetary gear reduction systems 32a and 32b may be coupled to the axles 34a and 34b associated with the wheels 20 and 22 and may provide a driving force to each wheel according to the output from the wheel's associated planetary gear reduction system 32a or 32b.It is understood that the planetary gear reduction systems 32a and 32b may not be required in some applications. However, it is expected that the planetary gear reduction system 32a and 32b will likely be required for automotive applications.

[0018] The power converter system 14 receives angular velocity signals from the sensor / bearing assemblies and the electronic controller 16 can be used to process these signals in determining a three-phase AC drive signal in real time, which is applied via a suitable power bus 36 to the coil windings (in Fig. 1 not visible) may be applied to the stator 24 to commutate the induction motor subsystem 12. The electronic controller 16 may also communicate with the vehicle's engine ECU 18 via a suitable communications bus 38 (e.g., a control area network bus) and may utilize information from various sensors in generating the three-phase AC drive signal applied to the induction motor subsystem 12.

[0019] Referring to Fig.2, a control diagram 100 is shown illustrating various operations and subsystems that the system 10 may use in generating the three-phase AC drive signal for the motor subsystem 12. The power converter system 14 may include a power converter section 40 and optionally a field-oriented control (FOC) system 42 and optionally a slip frequency calculation system 44 and optionally a position sensor selection control subsystem 46.

[0020] The ECU 18 provides a flux command 48 and a torque command 50 to the FOC 42 in the form of DC voltage signals, as well as to the slip frequency calculation system 44. The FOC 42 may use the flux command 48, the torque command 50, along with a real-time rotor angle signal (θr) 52, and optionally a voltage and / or current feedback signal 54, to generate three-phase AC voltages 56 that are applied as drive signals to the power converter section 40. It should be understood that the power converter may be a conventional power converter utilizing MOSFETs or insulated-gate bipolar transistors (IGBTs) as switching devices to control the application of the drive signals in a manner to commutate the motor subsystem 12. Such circuits are well known in the industry, and therefore, no additional description is provided for the power converter section 40.The power converter section 40 sequentially applies the drive signals, as needed, to various coils wound on the stator 24 of the motor subsystem 12 to commutate the motor subsystem 12. The position sensors 30a and 30b provide speed signals to indicate the rotational speed of each of the rotors 26a and 26b, respectively, in real time. The position sensors 30a and 30b output the speed signals in real time to the position sensor selection control subsystem 46.

[0021] The position sensor selection control subsystem 46 may include one or more control algorithms that select the rotor (26a or 26b) currently having the lower rotational speed, and it may then output a corresponding speed signal, which is ultimately used to determine the angular position signal (θr). For example, if the vehicle is moving forward while cornering clockwise, the rotor, in this example rotor 26b on the passenger side of the vehicle, will have a lower speed, and the rotor 26a on the driver side of the vehicle will have a higher angular velocity, because its wheels must rotate faster when the vehicle corners clockwise. The opposite would be true if the vehicle cornered counterclockwise.In this example, since rotor 26b has the lower angular velocity of the two rotors 26a and 26b, its velocity is used by the position sensor selection control subsystem 46. The slip frequency calculation system 44 uses the torque and flux commands to calculate the slip frequency for the motor subsystem 12 in real time and provides a signal associated with the real-time calculated rotor slip velocity (ω). sr ). The signal can then be summed with the output from the position sensor selection control subsystem 46 at summing node 58, and the summed output can be applied to an integrator 60. The integrator 60 produces the rotor angle signal 52, which is then output to the FOC 42, in real time.

[0022] It will be appreciated that system 10 is based on the fact that the torque acting on the rotor of a conventional induction motor is created by a slip frequency, or more precisely, by the difference between the synchronous speed of the magnetic field caused by the selective excitation of the stator winding and the rotational speed of the rotor. Slip frequency is often expressed (or measured) in revolutions per minute (rpm). The measured slip increases with increasing load, thereby providing increased torque. In a traction application, torque is controlled and subsequently generated by controlling the slip frequency within the stator winding 24. Feedback for an induction machine is typically obtained by a speed signal from an incremental position sensor associated with the rotor.In system 10, the use of two independent rotors 26a and 26b allows the use of two position sensors 30a and 30b to independently determine the speed of the two rotors. Since no absolute position is required, the two rotors 26a and 26b can "slip" relative to each other within the single stator 24. In other words, one rotor 26a or 26b is capable of rotating at a faster angular velocity than the other when the vehicle is cornering, with the inner wheel rotating at a lower angular velocity and therefore requiring greater torque than the outer wheel. Even though the rotors (26a and 26b) are divided into two sections within the stator 24, there is only one slip frequency that can be controlled within the stator 24 by the power converter system 14.When the vehicle is traveling in a straight line, both rotors 26a and 26b rotate at the same speed and experience the same slip frequency, as if the rotors were a single unit. When the vehicle begins to turn, one rotor (26a or 26b) begins to have a higher speed than the other. Assuming the control algorithm executing in controller 16 selects the rotational speed of the slower-rotating rotor, the faster-rotating rotor (26a or 26b) will experience a lower slip frequency compared to the other, and as a result, less torque will be generated at that rotor shaft. As the faster-rotating rotor (26a or 26b) continues to increase its speed, it will generate zero torque at some point when its slip frequency is equal to the rotating electric field of stator 24.This condition could occur under low traction conditions when one tire is on a lower friction surface than the other. With a conventional open differential, all torque would go to the rotating tire, resulting in zero traction force to move the vehicle. In the present invention, all torque is transferred to the slower-rotating tire to provide the best possible traction force to move the vehicle. System 10 thus operates as a mechanical differential with a limited-slip differential function, without the need for a costly mechanical differential.

[0023] It should also be noted that in this example, the stator 24 contains a conventional, distributed, 3-phase winding (or other conventional winding with different phases or distribution architectures). This means that the stator 24 is driven by an induction motor with a single continuous rotor and operate with similar performance from a single output shaft. No special winding designs are implemented or required in stator 24.

[0024] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or limiting of the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but may be interchangeable and utilized in a selected embodiment, even if not specifically shown or described. They may also be varied in many ways. Such modifications are not to be regarded as a departure from the disclosure, and all such changes are intended to be included within the scope of the disclosure.

Claims

[1] Slip-limited electric drive system (10) for a motor vehicle, comprising: an induction motor (12), the induction motor (12) comprising: a stator (24); a first rotor (26a) arranged for rotation in the stator (24), the first rotor (26a) being associated with a first wheel (20) of the vehicle; a second rotor (26b) arranged for rotation in the stator (24), the second rotor (26b) being associated with the second wheel (22) of the vehicle and being capable of rotating in the stator (24) independently of the first rotor (26a), each of the first and second rotors (26a, 26b) being capable of slipping relative to the other as needed when the vehicle is cornering; and a traction converter and control system (14) for monitoring an angular velocity of each of the first and second rotors (26a, 26b), determining which of the first and second rotors (26a, 26b) is rotating slower than the other, and generating a signal for controlling the induction motor (12) according to the respective first or second rotor (26a, 26b) having a lower speed, such that a torque signal is generated according to the rotor (26a, 26b) having the lower speed. [2] The system (10) of claim 1, wherein the traction converter and control system (14) includes a first position sensor (30a) associated with the first rotor (26a) and a second position sensor (30b) associated with the second rotor (26b). [3] The system (10) of claim 2, wherein the traction converter and control system (14) includes a position sensor selection control system (46) in communication with the first and second position sensors (30a, 30b) configured to detect which of the first and second rotors (26a, 26b) is rotating at a lower speed and to select a speed signal from the respective first and second rotors (26a, 26b) having the lower speed for use in determining a torque command (50) to be applied to the rotor (26a, 26b) having the lower speed. [4] The system (10) of claim 3, wherein the traction converter and control system (14) further includes a slip frequency calculation system (44) for calculating a slip frequency for the induction motor (12). [5] The system (10) of claim 4, wherein the traction converter and control system (14) further includes: a converter section (40); and a field-oriented control system (42) for receiving a flux command (48) and a torque command (50) and determining a three-phase AC voltage signal to be applied to the power converter section (40). [6] The system (10) of claim 5, wherein the field-oriented control system (42) further receives a signal representing a real-time rotor angle for the one of the first and second rotors (26a, 26b) determined to have the lower speed. [7] The system (10) of claim 1, further comprising: a first planetary gear reduction system (32a) drivable by the first rotor (26a); and a second planetary gear reduction system (32b) drivable by the second rotor (26b). [8] Slip-limited electric drive system (10) for a motor vehicle, comprising: an induction motor (12), the induction motor (12) comprising: a stator (24); a first rotor (26a) arranged for rotation in the stator (24), the first rotor (26a) being associated with a first wheel (20) of the vehicle; a second rotor (26b) arranged for rotation in the stator (24), the second rotor (26b) being associated with the second wheel (22) of the vehicle and being capable of rotating in the stator (24) independently of the first rotor (26a), so that each of the first and second rotors (26a, 26b) can slip relative to the other as needed when the vehicle is cornering; a first position sensor (30a) connected to the first rotor (26a) and configured to detect a speed of the first rotor (26a); a second position sensor (30b) connected to the second rotor (26b) and configured to detect a speed of the second rotor (26b); a traction converter and control system (14) comprising: a slip frequency calculation system (44) for calculating a slip frequency of the induction motor (12) in real time; and a position sensor selection system responsive to signals from the first and second position sensors (30a, 30b) and configured to determine and select for use a speed signal from the respective first or second position sensor (30a, 30b) having a lower speed; and wherein the traction converter and control system (14) is configured to control the induction motor (12) so that a torque signal is generated according to the one of the first and second rotors (26a, 26b) having the lower speed. [9] The system (10) of claim 8, wherein the traction converter and control system (14) further includes a field-oriented control system (42) for generating a three-phase AC drive signal. [10] The system (10) of claim 9, wherein the traction converter and control system (14) further includes a converter responsive to the field-oriented control system (42) for generating signals applied to the induction motor (12) to commutate the induction motor (12). [11] System (10) according to claim 8, further comprising: a first planetary gear reduction system (32a) driven by the first rotor (26a); and a second planetary gear reduction system (32b) driven by the second rotor (26b). [12] A method for forming a slip-limited electric drive system (10) for a motor vehicle, the method comprising: Arranging a first rotor (26a) for rotation in a stator (24) of an induction motor (12), the first rotor (26a) being associated with a first wheel (20) of the vehicle on a driver's side of the vehicle; Arranging a second rotor (26b) for rotation in the stator (24), the second rotor (26b) being associated with a second wheel (22) of the vehicle on a passenger side of the vehicle, and rotating about a common axis with the first rotor (26a), the second rotor (26b) further being capable of rotating in the stator (24) independently of the first rotor (26a) such that the first and second rotors (26a, 26b) are capable of rotating at different speeds as needed when the vehicle is cornering; Using a traction converter and control system (14) to monitor an angular velocity of each of the first and second rotors (26a, 26b) while the vehicle is cornering to determine which of the first or second rotors (26a, 26b) is rotating slower than the other; and Using the traction converter and control system (14) controlling the induction motor (12) such that a torque signal is generated according to one of the first and second rotors (26a, 26b) rotating slower than the other of the first and second rotors (26a, 26b). [13] The method of claim 12, wherein using the traction converter and control system (14) to control the induction motor (12) comprises using a field-oriented control system (42) to receive motor flux and motor torque command signals and generate a three-phase AC voltage signal for driving the induction motor (12). [14] The method of claim 13, wherein the traction and control system (14) for controlling the induction motor (12) further comprises using a power converter responsive to the three-phase AC drive signal to generate signals for commutating the induction motor (12). [15] The method of claim 14, wherein a first position sensor (30a) is connected to the first rotor (26a), wherein a second position sensor (30b) is connected to the second rotor (26b), and wherein using the traction and control system (14) to control the induction motor (12) further comprises using a position sensor selection control to receive signals from the first and second position sensors (30a, 30b) and using the signals from the first and second position sensors (30a, 30b) to determine which of the first and second rotors (26a, 26b) is rotating slower than the other of the first and second rotors (26a, 26b). [16] The method of claim 15, wherein the traction and control system (14) for controlling the induction motor (12) further comprises using a slip frequency detection system (44) for detecting a slip frequency of the induction motor (12) in real time. [17] The method of claim 12, further comprising: Using a first planetary gear reduction system (32a) arranged between the first rotor (26a) and the first wheel (20); and Using a second planetary gear reduction system (32b) arranged between the second rotor (26b) and the second wheel (22).

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