Abstract:
A limited slip electric drive system for a motor vehicle is disclosed. The system uses an induction motor having a stator, and first and second rotors disposed for independent rotation within the stator. The rotors are each independently associated with one wheel of the vehicle and are able to slip when the vehicle goes around a turn. A traction inverter and control system monitors angular speeds of the rotors and determines which of the rotors is turning slower than the other, and controls the induction motor in accordance with the rotor having a lower speed so that a torque signal is generated in accordance with the rotor having the lower speed.

Description:
FIELD 
     The present disclosure relates to drive systems for electric vehicles, and more particularly to a drive system that makes use of an induction motor for driving a pair of wheels of a vehicle, and where the induction motor includes fully independent and uncoupled induction rotors housed within a common stator, for driving a pair of wheels independently without the need for a conventional mechanical differential. 
     BACKGROUND 
     This section provides background information related to the present disclosure which is not necessarily prior art. 
     Present day electric vehicles, such as electrically powered automobiles, may employ an induction motor for providing a motive force to a pair of wheels. Typically the front wheels are powered, but in some instances all four wheels of the vehicle are powered: one motor for powering the front wheels and a separate motor for powering the rear wheels. It is also possible to power the rear wheels of the vehicle using a single electric motor. In any event, present day electrically powered vehicles typically require the use of a mechanical differential for receiving the output from the electrical motor and coupling the motive drive force to the driver and passenger side wheels of the vehicle. The driven wheels could be the front wheels of the vehicle or they could be the rear wheels of the vehicle. 
     The differential is needed because of the requirement that the driver and passenger side wheels need to rotate at different angular speeds when the vehicle is turning. The use of a mechanical differential, however, adds cost, complexity and weight to the vehicle. The mechanical differential also introduces mechanical losses which reduce the power made available to the wheels of the vehicle because of frictional losses within the differential. 
     One option for avoiding the use of a mechanical differential is by using two fully separate motors to independently drive a pair (either front or rear) of wheels of the vehicle. In this implementation typically two independent inverters are required, one for each motor, because of the need to be able to drive the wheels at different angular speeds when the vehicle is turning. As will be appreciated, this option also suffers from the drawbacks of additional cost and complexity because of the need for the second inverter. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     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, with the induction motor including a stator. The induction motor may also include a first rotor disposed for rotation within the stator, the first rotor being 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 able to rotate within the stator independently of the first rotor. Each of the first and second rotors is able to slip relative to the other as needed when the vehicle is travelling around a turn. The system may also include a traction inverter and control system for monitoring an angular speed of each of the first and second rotors, and determining which of the first and second rotors is turning slower than the other. The traction inverter and control system may also operate to generate a signal for controlling the induction motor in accordance with one of the first or second rotors having a lower speed, so that a torque signal is generated in accordance with the rotor having the lower speed. 
     In another aspect the present disclosure relates to a limited slip electric drive system for a motor vehicle. The system may comprise 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 which is also disposed for rotation within the stator. The second rotor may be associated with a second wheel of the vehicle and able to rotate within the stator independently of the first rotor such that each of the first and second rotors is allowed to slip relative to the other when needed as the vehicle is travelling around a turn. The system may further include a first encoder associated with the first rotor and configured to detect a speed of the first rotor. A second encoder may be included which is associated with the second rotor and configured to detect a speed of the second rotor. The system may also include a traction inverter and control system. The traction inverter and control system may include a slip frequency calculation system for calculating a real time slip frequency of the induction motor. The traction inverter and control system may also include an encoder selection system responsive to signals from the first and second encoders and may be configured to determine and select for use a speed signal from the one of the first and second encoders that has a lower speed. The traction inverter and control system may operate to control the induction motor so that a torque signal is generated in accordance with the rotor having the lower speed. 
     In still another aspect the present disclosure relates to a method for forming a limited slip electric drive system for a motor vehicle. The method may comprise arranging a first rotor for rotation within a stator of an induction motor, the first rotor being associated with a first wheel of the vehicle on a driver side of the vehicle. The method may further comprise arranging a second rotor for rotation within the stator, the second rotor being 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 being able to rotate within the stator independently of the first rotor, and further such that the first and second rotors are able to rotate at different speeds as needed when the vehicle is travelling around a turn. The method may further comprise using a traction inverter and control system to monitor an angular speed of each of the first and second rotors while the vehicle is travelling, to determine which of the first and second rotors is turning slower than the other. The traction inverter and control system may also operate to control the induction motor so that so that a torque signal is generated in accordance with the rotor having the lower speed. 
     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 
       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 high level block diagram of one embodiment of a drive system in accordance with the present disclosure; and 
         FIG. 2  is a high level control diagram illustrating various components of the system of  FIG. 1  along with operations that are performed in determining a wheel speed to use for each one of a pair of wheels being drive by the drive system. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     Referring to  FIG. 1 , a drive system  10  is shown in accordance with one embodiment of the present disclosure. The system  10  in this example may include an induction motor subsystem  12 , and may also include a traction inverter and control system  14  (hereinafter simply “inverter system  14 ”). The inverter system  14  may include an electronic controller  16  and is in communication with a processor controlled vehicle engine control unit (“ECU”)  18 . The induction motor subsystem  12  is used simultaneously to provide a motive drive force to drive a left wheel  20  and a right wheel  22  of an automotive vehicle such as a car or truck. However, it will be appreciated immediately that the system  10  is not limited to use with only automobiles, and in fact is expected to find utility on any type of wheeled vehicle where the wheels are driven by an electric motor and the vehicle needs to be propelled while making turns. The teachings of the present disclosure may even find utility in connection with any type of device which is powered by an electric motor, and for which two simultaneously driven elements having slightly different angular speeds need to be produced. 
     The induction motor subsystem  12  in this example includes a stator  24  in which two fully independent rotors  26   a  and  26   b  are housed. The rotors  26   a  and  26   b  rotate about a common longitudinal axis but are uncoupled from one another, and thus are free to rotate at different angular speeds, such as when the vehicle is making a turn and the outside wheel  20  or  22  has a higher angular speed than the inside wheel. 
     Rotor  26   a  has an output shaft  28   a  and rotor  26   b  has an output shaft  28   b . Output shaft  28   a  may be supported by, and its speed monitored by, a sensor/bearing assembly forming an encoder  30   a , while rotor  26   b  may be supported by, and its speed monitored by, a sensor/bearing assembly forming an encoder  30   b . The output shaft  28   a  may alternatively be coupled to a planetary gear reduction system  32   a , while output shaft  28   a  may alternatively be coupled to a separate planetary gear reduction system  32   b . The planetary gear reduction systems  32   a  and  32   b  may be coupled to axles  34   a  and  34   b  associated with the wheels  20  and  22 , and provide a drive force to each wheel in accordance with the output from the wheel&#39;s associated planetary gear reduction system  32   a  or  32   b . It will be appreciated that in some applications the planetary gear reduction systems  32   a  and  32   b  may not be required. However, it is expected that the planetary gear reduction system  32   a  and  32   b  will likely be required for automotive applications. 
     The inverter system  14  receives angular speed signals from the sensor/bearing assemblies and the electronic controller  16  may be used to process these signals when determining a real time, three-phase AC voltage drive signal, which may be applied via a suitable power bus  36  to the coil windings (not visible in  FIG. 1 ) on the stator  24  to commutate the induction motor subsystem  12 . The electronic controller  16  may also communicate with the engine ECU  18  of the vehicle via a suitable communications bus  38  (e.g., a controller area network bus), and may use information from various sensors in generating the three phase AC drive signal applied to the induction motor subsystem  12 . 
     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 voltage drive signal for the motor subsystem  12 . The inverter system  14  may include an inverter section  40 , and optionally a field oriented control system (FOC)  42 , and optionally a slip frequency calculation system  44 , and optionally an encoder selection control subsystem  46 . 
     The ECU  18  provides a flux command  48  and a torque command  50  to the FOC  42  in the form of direct current (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 (Or)  52 , and optionally a voltage and/or current feedback signal  54 , to generate three phase AC voltages  56  which are applied as drive signals to the inverter section  40 . As will be appreciated, the inverter may be a conventional inverter that makes use of MOSFETs or insulated gate bipolar junction transistors (IGBTs) as the switching devices to control 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 shall be provided for the inverter section  40 . The inverter section  40  applies the drive signals sequentially to various coils wound on the stator  24  of the motor subsystem  12  as needed to commutate the motor subsystem  12 . Encoders  30   a  and  30   b  provide speed signals to indicate the real time rotational speed of each of the rotors  26   a  and  26   b , respectively. The encoders  30   a  and  30   b  output their real time speed signals to the encoder selection control subsystem  46 . 
     The encoder selection control subsystem  46  may include one or more control algorithms that select the rotor ( 26   a  or  26   b ) that has the lower rotational speed at the moment, and then may output a corresponding speed signal that will eventually be used to determine the angular position (Or) signal. For example, if the vehicle is moving forward turning clockwise, then the rotor, in this example rotor  26   b  on the passenger side of the vehicle, will have the lower angular speed, and the rotor  26   a  on the driver side of the vehicle will have a higher angular speed because its wheel needs to turn faster when the vehicle is making a clockwise turn. The opposite would occur if the vehicle was making a counterclockwise turn. So in this example, since rotor  26   b  has the lesser angular speed of the two rotors  26   a  and  26   b , then its speed will be used by the encoder selection control subsystem  46 . The slip frequency calculation system  44  uses the torque and flux commands to calculate the real time slip frequency for the motor subsystem  12  and to output a signal relating to the real time calculated slip speed of the rotor (ω sr ). This signal may then be summed with the output from the encoder selection control subsystem  46  at summing node  58  and the summed output applied to an integrator  60 . The integrator  60  produces the real time rotor angle signal  52  which may then be output to the FOC  42 . 
     It will be appreciated that the system  10  is predicated on the fact that the torque acting on the rotor of a conventional induction motor is created by a slip frequency, or more specifically by the difference between the synchronous speed of the magnetic field caused by selective energization of the stator windings and the rotational speed of the rotor. The slip frequency is commonly expressed (or measured) in revolutions per minute (RPM). The measured slip will increase with increasing load, thus providing an increasing torque. In a traction application the torque is commanded and subsequently generated by controlling the slip frequency within the stator  24  winding. Feedback for an induction machine is typically obtained through a velocity signal from an incremental encoder associated with the rotor. With the system  10 , the use of two independent rotors  26   a  and  26   b  allows two encoders  30   a  and  30   b  to be used to independently determine the speeds of the two rotors. Since absolute position is not required, the two rotors  26   a  and  26   b  within the single stator  24  can “slip” with respect to each other. In other words, one rotor  26   a  or  26   b  is able to turn at a faster angular speed than the other when the vehicle is turning a corner, and where the inside wheel will be rotating at a slower angular speed, and therefore requiring greater torque, than the outside wheel. Even though the rotors ( 26   a  and  26   b ) are split into 2 sections within the stator  24 , there is only one slip frequency that can be controlled in the stator  24  by the inverter system  14 . When the vehicle is traveling in a straight line both rotors  26   a  and  26   b  are turning at the same velocity and are seeing the same slip frequency, as if the rotors were one unit. When the vehicle begins to turn, then one rotor ( 26   a  or  26   b ) will begin to have a higher velocity that the other. Assuming the control algorithm running in the controller  16  selects the rotational velocity of the slower spinning rotor, then the faster spinning rotor ( 26   a  or  26   b ) will see a smaller slip frequency compared to the other and subsequently less torque will be generated on that rotor shaft. If the faster spinning rotor ( 26   a  or  26   b ) continues to increase its velocity, at some point it will generate zero torque when its slip frequency is equal to the stator&#39;s  24  rotating electric field. This condition could occur in low traction conditions where one tire is on a lower friction surface than the other tire. With a standard open differential all the torque would go to the spinning tire resulting in zero tractive force to move the vehicle. In the present invention all the torque is transferred to the slower spinning tire to provide the best possible tractive force to move the vehicle. The system  10  thus operates as a mechanical differential with a limited slip differential function without the need for a costly mechanical differential. 
     It should also be noted that in this example stator  24  contains a conventional distributed 3 phase winding (or other conventional winding of various phases or distribution architectures). Meaning that stator  24  could be interchanged with an induction motor with a single continuous rotor and operate with similar single output shaft performance. No special winding configurations are implemented or needed within the stator  24 . 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It 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.