Abstract:
A vehicle in which propulsion can be distributed between first and second axles includes: a first electric motor coupled to the first axle and a second electric motor coupled to the second axle. An electric control unit (ECU) coupled to the motors causes electrical energy to be generated by the first motor in response to the ECU determining that a wheel speed of at least one wheel associated with the first axle exceeds the vehicle speed and causing electrical energy to be supplied to the second motor in response to electrical energy being generated in the first motor.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to vehicle control systems that enhance vehicle stability and performance. 
     2. Background Art 
     Stability-control systems are increasingly being used in automotive vehicles. In some prior two driven axle systems, a mechanical coupling is provided between the front and rear axles of the vehicle. In the event that one or both of the wheels associated with one of the driven axle lose traction, the coupling apparatus, which is normally uncoupled, is commanded to couple the two axles so that torque is redistributed between a primary axle and a secondary axle. Although such a mechanical system provides improved performance compared to a purely braking approach such as with anti-lock braking systems or traction control system, a mechanical system has several disadvantages. There is a delay between the time that the traction loss is detected and the mechanical coupler actually redistributes torque from the spinning wheels of the primary axle to the wheels of the secondary. In situations such as encountering a patch of ice, in which road surface conditions can change very rapidly, a mechanical system is incapable of effecting a change in torque distribution sufficiently fast. Furthermore, due to frictional losses through the mechanical coupler, the sum of the torques supplied to the two axles is somewhat less than what the powertrain supplies to the primary axle. Thus, when the mechanical coupler is invoked, there is a drop in longitudinal performance of the vehicle, which may be particularly noticeable during acceleration. The ability of a mechanical system to redistribute torque may be limited in torque transfer capacity and further hampered by environmental influences, such as temperature. 
     SUMMARY 
     A system to distribute propulsion in a vehicle has first and second axles coupled to the vehicle, a first motor coupled to the first axle, a second motor coupled to the second axle, wheel speed sensors coupled to vehicle wheels, a vehicle speed sensor, and an electronic control unit (ECU) electronically coupled to motors, the wheel speed sensors, and the vehicle speed sensor. The ECU causes electrical energy to be generated by the first motor and causes electrical energy to be supplied to the second motor in response to the ECU determining that at least one wheel associated with the first axle is spinning. The electrical energy generated by the first motor may be provided directly to the second motor and possibly supplemented by a battery coupled to the motors. The wheels sensors may be part of an anti-lock braking system. 
     A method to distribute propulsion in a vehicle includes monitoring wheel spin for wheels associated with a first axle of the vehicle, extracting electrical energy from a first motor coupled to the first axle when a first axle wheel is spinning, and providing electrical energy to a second motor coupled to a second axle of the vehicle in response to the extracting electrical energy from the first motor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic representation of a hybrid electric vehicle (HEV); 
         FIG. 2  is a flowchart of a method for redistributing torque between the axles according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated and described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. Those of ordinary skill in the art may recognize similar applications or implementations consistent with the present disclosure, e.g., ones in which components are arranged in a slightly different order than shown in the embodiments in the Figures. Those of ordinary skill in the art will recognize that the teachings of the present disclosure may be applied to other applications or implementations. 
     In  FIG. 1 , one embodiment of a hybrid electric vehicle (HEV)  10  is shown schematically. Rear wheels  12  are coupled via rear axle  16  with a rear axle motor  18 . Rear axle  16  has a differential  19 . Front wheels  14  are coupled to front axle  20 . A differential and final drive gear set  22  are coupled to front axle  20 . The vehicle powertrain system is coupled to differential  22  through a transmission  24 . Transmission  24  is coupled to a front axle motor  28  via a clutch  28 . Front axle motor  28  is coupled to an internal combustion engine  34  via a clutch  32 . Front axle motor  28 , in the arrangement shown in  FIG. 1 , can be called an integrated starter generator (ISG) because it can be used to spin up engine  34  for starting purposes. Depending on the exact configuration, it is likely that all of the powertrain components cannot be coupled end to end within the width of HEV  10 . In the embodiment shown in  FIG. 1 , a chain drive  30  is provided between engine  34  and front axle motor  28  such that engine  34  rotates along a first axis and front axle motor  28  and transmission  24  rotate along a second axis substantially parallel to the first axis. The configuration in  FIG. 1  illustrates simply one HEV configuration. There are many alternatives for configuring HEV which do not depart from the scope of the present disclosure. HEV  10  shows an arrangement in which internal combustion engine  34  is coupled to the front wheels. In another embodiment, engine  34  is coupled to the rear axle. In yet another embodiment, the vehicle is an electric vehicle, sometimes called a battery-only electric vehicle (BEV). Front and rear axle motors  28  and  18  can operate as motors providing torque to the associated axle or as generators absorbing torque from the associated axle, i.e., providing a braking force on wheels associated with the axle. 
     Continuing to refer to  FIG. 1 , wheels  12  and  14  are provided with traction sensors  36 , which are coupled to an ECU  38 . Traction sensors  36 , in one embodiment, are part of an anti-lock braking system (ABS). ABS compares vehicle speed with wheel speed. When the two differ by more than a predetermined amount, the wheel is determined to be spinning. ABS is simply one example; any suitable traction sensor can be used. 
     A battery  40  is coupled to rear axle motor  18  and front axle motor  28  to provide electrical energy or to absorb electrical, depending on operational mode. Battery  40  may also be electronically coupled to ECU  38  via sensors to monitor state of charge of the battery, battery health, etc. In one embodiment, battery  40  is a high voltage battery to facilitate large power extraction from or storage into the battery. Front axle motor  28  and rear axle motor  18  may be coupled directly via a switch  41  to provide electrical energy generated in one motor to the other. Switch  41  is controlled via ECU  38 . 
     In one embodiment, ECU  38  is coupled to a yaw rate sensor  42 , a sensor coupled to a steering wheel  44 , and a variety of other sensors  46 , such as a vehicle speed sensor, temperature sensors, transmission sensors, pressure sensors, and acceleration sensors. In embodiments without yaw rate sensor  42 , yaw rate may be estimated based on signals from other sensors  46 . 
     An HEV is shown in  FIG. 1 . In an alternative embodiment, the vehicle is an electric vehicle (EV) having a front axle motor and a rear axle motor. In such an embodiment, the following components are no longer included: clutch  26 , chain drive  30 , clutch  32 , and engine  34 . In some embodiments, transmission  24  is also not included. 
     Assume the engine is connected to the front axle and the front axle is the primary driven axle. The engine torque is T eng  and the front axle motor torque is T m     —     f . The total front axle traction torque is T eng +T m     —     f . Assuming the instantaneous friction capability at the front axle is F fric , which limits the front axle torque capacity to F fric ·R W , where R W  is the effective wheel radius. In a traction control situation, the driver requested propulsion torque T prop  is larger than F fric ·R W . The total torque at the front axle is reduced to avoid wheel slipping. As the electric motor coupled to the front axle can react quickly and provide a negative torque to the front axle, the motor torque is determined by T m     —     f =F fric ·R W −T eng . When T eng &gt;F fric ·R W , T m     —     f  is negative and the front axle motor is serving as a generator with the energy stored in the battery or provided directly to the rear axle motor. 
     To compensate for the traction loss: T prop −F fric ·R W , the propulsion torque is allocated to the free, or lightly loaded, rear axle wheels that have reserve friction. As a result, the new rear axle motor torque will be: T m     —     r   N =T m     —     r +(T prop −F fric ·R W ) where T m     —     r  is the existing propulsion/braking torque at the rear axle wheels. The new rear axle motor torque is contingent on not exceeding the rear axle wheel friction limit. As a result, available traction from the road is exploited to satisfy the driver&#39;s vehicle propulsion request. The energy to provide T m     —     r  can be provided either from the front axle regenerated electricity or from the battery. 
     A flowchart, according to an embodiment of the disclosure, is shown in  FIG. 2 , which starts in  200 . In regards to  FIG. 2 , the axles are referred to as first and second axles. In some embodiments, the first axle corresponds with rear axle  16  of  FIG. 1 , with second axle corresponding to front axle  20 . In other embodiments, the first axle corresponds to front axle  20  and the second axle corresponds to rear axle  16 . In block  202  of  FIG. 2 , it is determined whether a wheel associated with a first axle is spinning. If not, block  202  is continually checked until spinning is occurring to cause control to pass to block  204 . In block  204 , the motor coupled to the first axle is commanded to reduce torque to the first axle by an amount to stop spinning. The torque is reduced by operating the motor as a generator. Control passes to block  206  in which an amount of torque to supply to the second axle is computed that would maintain the longitudinal propulsion of the vehicle. An amount of electrical energy to provide such torque is determined. Next, in block  208  according to one embodiment, an amount of electrical energy that would lead to incipient spinning of a wheel associated with the second axle is determined. Alternatively, an amount of electrical energy that is a predetermined amount less than that which would lead to incipient spinning is determined to provide a safety factor. In block  210 , the lesser of the electrical energy computed in blocks  206  and  208  is commanded to the electric motor associated with the second axle. Control returns to block  202 . The flowchart of  FIG. 2  is active, in one embodiment, whenever the vehicle is moving, or in another embodiment, whenever longitudinal propulsion is commanded by the operator of the vehicle. 
     While the best mode has been described in detail, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. Where one or more embodiments have been described as providing advantages or being preferred over other embodiments and/or over background art in regard to one or more desired characteristics, one of ordinary skill in the art will recognize that compromises may be made among various features to achieve desired system attributes, which may depend on the specific application or implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. For example, it may be desirable to have an extensive set of sensors to provide an accurate assessment of the vehicle&#39;s movement. However, to maintain a desirable cost structure, a satisfactory estimation of some vehicle quantities may be ascertained by inferring from a lesser set of sensor data. The embodiments described as being less desirable relative to other embodiments with respect to one or more characteristics are not outside the scope of the disclosure as claimed.