Abstract:
A method of braking an electric or hybrid vehicle  7  is provided. The vehicle ( 7 ) has electric regenerative brakes and friction brakes ( 94 ). In a normal braking situation regenerative brakes are applied. If a wheel ( 42 ) slip condition is sensed, a selectively operable clutch ( 76 ) tortionally connects an undriven axle ( 60 ) to the driven axles ( 44, 45 ) to alleviate the slip condition. The alleviation of the slip condition prevents the electric regenerative brakes from being shut off in favor of an antilock friction brake application.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to electric or hybrid electric vehicles. More specifically, the present invention relates to an electric or hybrid electric vehicle (HEV) having regenerative and friction brakes that utilize wheel slip data of the regeneratively braked axle to selectively tortionally connect a second axle with the first axle to extend the range of regenerative braking. 
     BACKGROUND OF THE INVENTION 
     The general principle of regenerative braking is recognized by manufacturers of electric and HEVs as a way to increase the overall efficiency of the vehicle. Regenerative braking seeks to recover the kinetic energy of the vehicle which is normally dissipated as heat through a normal hydraulic friction brake system, by operating the electric motor drive as a generator to restore the generated electricity to a battery or other energy storage device. However, regenerative braking has certain limitations. The maximum amount of regenerative braking torque available is not a constant, but is a function of a normal force at a tire&#39;s patch and a rolling surface coefficient of friction. If the regenerative brake torque demand exceeds the physics of this interface, wheel slip will be triggered. Most antilock braking systems (ABS) require that the brakes be applied and released on a given wheel at a rate that exceeds the frequency response of the typical electric motor. Many ABS require that the friction brakes be applied and released on a given wheel approximately a minimum of seven times a second. Accordingly, during an ABS incident, most electric and HEVs are programmed to shut off the regenerative braking and begin the friction braking. Shutting off the regenerative braking causes energy losses and a lowering of overall fuel economy. It would therefore be advantageous to provide a method of braking for an electric or a HEV wherein the incidents which require a termination of regenerative braking during an ABS mode of operation be minimized. This and other issues related to electric and HEVs are the subject of the following U.S. Pat. Nos. 4,962,969; 5,269,390; 5,294,191; 5,358,084; 5,378,053; 5,421,643; 5,450,324; 5,551,764; 5,573,312; 5,615,933; 5,632,534; 5,895,100; 6,070,689; 6,076,899; 6,086,166; and 6,099,089. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment of the present invention, when a regenerative braking situation occurs such as in a lift throttle or operator-signaled brake command, a motor generator regeneratively brakes the driven wheels of a primary driving axle of the vehicle. If a slip condition occurs wherein the driven wheels are locked by application of the regenerative brakes, a clutch which torsionally connects the secondary axle to the primary axle is engaged. Regenerative braking will then occur over all four wheels of the vehicle, effectively cutting in half the friction requirement for the primary driven wheels. Accordingly, in many cases the primary driven wheels will become unlocked and the ABS operation will not be required. Not using the ABS mode of operation allows regenerative braking to be maintained and accordingly, the fuel economy of the vehicle is enhanced. When the regenerative braking operation is over, the clutch between the two axles may be opened and the vehicle can continue onward in a two-wheel drive mode of operation. 
     An advantage of the present invention is to provide a method of regenerative braking for a four-wheel drive electric or HEV which also has conventional brakes for an antilock mode of operation. 
     A further advantage of the present invention is to provide a four-wheel drive vehicle which can stay in a regenerative braking operation for longer periods of time than previously possible. 
     Other advantages of the present invention will become more apparent to persons having ordinary skill in the art to which the present invention pertains from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a general hybrid electric vehicle configuration for the normally driven wheels. 
     FIG. 2 illustrates a planetary gear set for the vehicle show in FIG.  1 . 
     FIG. 3 illustrates a general four wheel drive hybrid electric vehicle configuration. 
     FIG. 4 illustrates a flowchart illustrating operation of the braking system of the vehicle shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 demonstrate a front wheel drive portion of a possible HEV configuration, specifically a parallel-series hybrid electric vehicle  7  (powersplit) configuration. 
     A planetary gear set  20  mechanically couples a gear carrier  22  to an internal combustion engine  24  via a one way clutch  26 . The planetary gear set  20  also mechanically couples a sun gear  28  to a generator motor  30  and a ring (output) gear  32 . The generator motor  30  is also mechanically linked to a generator brake  34  and is electrically linked to a battery  36 . A traction motor  38  is mechanically coupled to the ring gear  32  of the planetary gear set  20  via a second gear set  40  and is electrically linked to the battery  36 . The ring gear  32  of the planetary gear set  20  and the traction motor  38  are mechanically coupled to front drive wheels  42  via gear  59 , transaxle  43  and axle half shafts  44  and  45 . 
     The planetary gear set  20  splits the engine  24  output energy into a series path from the engine  24  to the generator motor  30  and a parallel path from the engine  24  to the drive wheels  42 . Engine  24  speed can be controlled by varying the split to the series path while maintaining the mechanical connection through the parallel path. The traction motor  38  augments the engine  24  power to the drive wheels  42  on the parallel path through the second gear set  40 . The traction motor  38  also provides the opportunity to use energy directly from the series path, essentially running off power created by the generator motor  30 . This reduces losses associated with converting energy into and out of chemical energy in the battery  36  and allowing all the energy in engine  24 , minus conversion losses, to reach the drive wheels  42 . 
     A vehicle system controller (VSC)  46  controls many components in HEV  7  by connecting to each component&#39;s controller. The engine control unit (ECU)  48  connects to the engine  24  via a hardwire interface. The ECU  48  and VSC  46  can be based in the same unit, but are actually separate controllers. The VSC  46  communicates with the ECU, as well as a battery control unit (BCU)  50  and a transaxle management unit (TMU)  52  through a communication network such as a controller area network (CAN) (not shown). The BCU  50  connects to the battery  36  via a hardwire interface (not shown). The TMU  52  controls the generator motor  30  and traction motor  38  via a hardwire interface. 
     Referring additionally to FIG. 3, the vehicle  7  according to the present invention additionally has a rear axle  60 . Rear axle  60  has wheels  64  and half shafts  66 ,  68 . The half shafts  66 ,  68  are powered via a rear differential  70 . The rear differential  70  is connected to a drive shaft  74  by a rear axle electronic clutch  76 . The rear axle electronic clutch  76  can be a viscous-type clutch, which can additionally be electrically actuated upon command. The clutch  76  is normally engaged by a rotational difference between the rear wheels  64  and front wheels  42 . At other times the clutch is normally non-engaged. The drive shaft  74  is connected to a power take off unit  78 . The power take off unit  78  torsionally connects the drive shaft  74  with an output shaft from the transaxle  43 . 
     The vehicle  7  also has a conventional hydraulic braking system having a master cylinder  90  which is connected to an anti-skid or antilock brake module  92 . The brake module  92  allows the master cylinder  90  to be directly connected to foundation friction brakes  94  during normal brake operation. During ABS mode of operation, the brake module  92  will isolate the foundation brakes  94  from the master cylinder  90  and will then modulate the mode of operation. During ABS mode of operation, brake module  92  will be controlled by a brake controller  100 . The brake controller  100  will be in communication with wheel speed sensors (not shown) which monitor the front wheels  42  and rear wheels  64 . Brake controller  100  will be connected to the vehicle main communicative bus allowing it to communicate with other components as required, typically including the VSC  46 . 
     Referring to FIG. 4, in an operation due to a lift throttle incident (commonly referred to as compressive braking) or by virtue of an operator-commanded brake signal, regenerative braking will be applied to the primary driven front axle. The regenerative braking will occur due to the action of the traction motor  38 . If the driven wheels  42  are on a low friction surface such as ice or gravel, a slip condition can occur, meaning that the wheels are slipping on the road surface. The slip condition is also commonly referred to as a locked condition wherein the brakes (regenerative or friction) have locked the wheels  42  from rotation. A slip condition can be sensed by virtue of control parameters which may include vehicle speed, wheel speed, regenerative torque and undriven axle clutch torque. When a slip condition is sensed, the VSC  46  will sense if the undriven axle  60  electronic clutch  76  is disengaged. If the electronic clutch  76  is disengaged the VSC  46  will signal it to engage. The engagement of the electronic clutch  76  will cause the regenerative braking to occur not only against the driven axle (half shafts  44 ,  45 ) but also against the torque of the undriven wheels  42  via the undriven axle  60 . Accordingly the torque available to stop the driven wheels  42  will be halved and in many instances the driven wheels  42  will be relieved from their slip condition. Accordingly the slip condition indication for the wheels  42  will be relieved and regenerative braking can continue. In instances where both the front  42  and rear wheels  64  are on low friction surfaces the VSC  46  will signal the regenerative braking to be reduced. If reduction of the regenerative braking does not relieve the slip conditions the VSC  46  will signal the brake controller  100  to engage into an ABS mode causing the brake module  92  to isolate the master cylinder  90  and to pulsate the friction brakes  94 . When a slip condition is relieved, the brake controller  100  can signal for the brake module  90  to cease ABS actuation and regenerative braking can again be instigated by the traction motor  38 . 
     The above-described embodiment of the invention is provided purely for purposes of example. For instance, the present invention can also be used on primarily rear wheel drive vehicles. Additionally, the present invention can be utilized in a vehicle where regenerative braking is applied on a secondary non-driven wheeled axle and a clutch can be actuated to torsionally connect the non-driven wheeled axle to the primary driven wheeled axle to relieve a slip condition. Many other variations, modifications, and applications of the invention may be made.