Abstract:
A hybrid electric vehicle includes a powertrain controller and an anti-lock braking system (ABS) controller. The powertrain controller modulates the torque delivered by an internal combustion engine, a generator, and a motor to deliver a desired torque to two drive wheels. The ABS controller modulates the braking torque exerted by brakes on each of the four wheels. During modest braking events with good traction, the motor recaptures vehicle kinetic energy. During heavy braking and/or poor traction, the ABS controller and motor controller each respond to speed sensor signals to modulate the motor and brake torques to minimize stopping distance. The motor torque responds more quickly than the brake torque such that the frequency of oscillation is higher for the combined system than for an independent ABS system.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure pertains to a method of operating a hybrid electric vehicle to reduce the stopping distance on limited traction surfaces. 
       BACKGROUND 
       [0002]    The distance required to stop a vehicle is improved if the braking torque at each wheel is maintained near the level corresponding to he maximum friction force available between the tire and the road surface. If he braking torque exceeds this level, he wheel locks-up and slides along the surface. Since the coefficient of friction decreases when he wheel is sliding as opposed to rolling, braking distance increases when wheels are allowed to lock-up. To improve braking performance, many vehicles are equipped with anti-lock braking systems (ABS). When an ABS senses wheel lock-up, it intervenes to apply a lower braking torque than commanded by the driver. 
         [0003]    In order to reduce fuel consumption, some vehicles, called hybrid electric vehicles, are equipped with electric motors in addition to the gasoline or diesel powertrain. One of the ways that the electric motor reduces fuel consumption is through regenerative braking When the driver steps on the brake pedal, the powertrain uses the electric motor to apply a braking force instead of the friction brakes generating electricity that is stored in a battery. The stored power is then used later to propel the vehicle reducing the power that must be generated by burning fuel. However, if the electric motor exerts enough braking force to lock-up the wheels, then the ABS will not be able to restore fraction by reducing the torque of the friction brakes. 
       SUMMARY OF THE DISCLOSURE 
       [0004]    A hybrid electric vehicle has four wheels each of which is equipped with a hydraulically actuated friction brake and a speed sensor. An anti-lock brake system controller monitors the speed sensors and reduces the brake torque in response to an indication of tire slip and then increases the brake torque in response to an indication of regained traction. An electric motor drives two of the vehicle wheels through a differential. A powertrain controller monitors the speed sensors associated with the driven wheels and reduces the motor torque (in absolute value) in response to an indication of tire slip and then increases the motor torque in response to an indication of regained traction. The electric motor responds more quickly than the hydraulic brake actuators. The cycle of increasing and decreasing torque results in oscillating torques with given frequencies. The faster response of the electric motor results in a higher frequency than hydraulic brakes acting alone, such as occurs on the non-driven wheels. 
         [0005]    Wheel slip may be indicated by a negative rate of change of wheel speed below a threshold value. Alternatively, wheel slip may be indicated by a wheel speed that differs by more than a threshold value from an expected wheel speed based on vehicle speed and tire radius. Vehicle speed may be estimated, for example, by averaging the speeds of non-slipping wheels. Similarly, regained traction may be indicated by positive rate of change of wheel speed above a threshold value. Alternatively, regained traction may be indicated by a wheel speed that is within a threshold value of an expected wheel speed based on vehicle speed and tire radius. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic representation of a hybrid electric vehicle powertrain. 
           [0007]      FIG. 2  is a schematic representation of an anti-lock braking system. 
           [0008]      FIG. 3  is a schematic representation of a controller. 
           [0009]      FIG. 4  is a set of graphs illustrating the operation of an anti-lock braking system during a deceleration without intervention from the hybrid powertrain. 
           [0010]      FIG. 5  is a set of graphs illustrating the operation of an anti-lock braking system during a deceleration with participation of the hybrid powertrain. 
           [0011]      FIG. 6  is a flow chart illustrating the method of operation with participation of the hybrid powertrain. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
         [0013]      FIG. 1  is a schematic representation of a power-split type hybrid vehicle. Solid lines represent mechanical connections among components. Lines with long dashes represent electrical power connections among components. Lines with short dashes represent signal connections. This configuration is called a power-split because planetary gear set  20  splits the power flowing from the engine to the wheels into a mechanical power flow path and an electrical power flow path. Planetary gear set  20  includes sun gear  22 , ring gear  24 , and carrier  26  which rotate about a common axis. A number of planet gears  28  are supported for rotation with respect to carrier  26  and mesh with both sun gear  22  and ring gear  24 . 
         [0014]    Internal combustion engine  30  is drivably connected to carrier  26 . Sun gear  22  is driveably connected to generator  32 . Ring gear  24  is drivably connected to output shaft  34 . A driveable connection is established between two components if rotation of one component causes the other component to rotate at a proportional speed. In  FIG. 1 , the driveable connection between sun gear  22  and generator  32  is a solid shaft  36  whereas the driveable connection between ring gear  24  and output shaft  34  includes gears  38  meshing with gear  40 . Output shaft  34  is also driveably connected to traction motor  42  and differential  44 . Differential  44  transmits power to a left front wheel  46  and a right front wheel  48  while permitting slight variations in speed, such as when the vehicle turns a corner. 
         [0015]    Generator  32  and traction motor  42  are both reversible electrical machines capable of converting electrical energy into rotational mechanical energy and converting rotational mechanical energy into electrical energy. As illustrated in  FIG. 1 , generator  32  is an alternating current (AC) motor electrically connected to battery  50  via DC/DC converter and inverter  54 . Inverter  54  converts direct current (DC) to three phase alternating current in response to commands from powertrain controller  56 . The voltage level, frequency, and phase angle of the three phase alternating current determine the resulting torque level. Similarly, inverter  58  converts direct current to three phase alternating current for traction motor  42 . Alternatively, generator  32  and/or traction motor  36  may be DC motors. 
         [0016]      FIG. 2  is a schematic representation of the anti-lock brake (ABS) system. In addition to front wheels  46  and  48 , the vehicle has a left rear wheel  60  and a right rear wheel  62 . As illustrated, the rear wheels are not powered, but the rear wheels may be powered in some embodiments. Hydraulic brakes  64 ,  66 ,  68 , and  70  apply torque to wheels  46 ,  48 ,  60 , and  62  respectively in response to signals from brake controller  72 . Speed sensors  74 ,  76 ,  78 , and  80  measure the speeds of wheels  46 ,  48 ,  60 , and  62  respectively and communicate these speeds to brake controller  72 . 
         [0017]    As shown in  FIG. 3 , brake controller  72  and powertrain controller  56  communicate with one another via a controller area network (CAN)  82 . Specifically, brake controller  72  makes signals from speed sensors  74 ,  76 ,  78 , and  80  available to powertrain controller  56  via CAN  82 . Alternatively, brake controller  72  and powertrain controller  56  could be integrated into a single controller. 
         [0018]    When the driver presses a brake pedal, braking can be accomplished either by commanding negative torque from motor  42  or by commanding the brakes to apply torque to each of the wheels. For low levels of braking on surfaces with good traction, regenerative braking via motor  42  is preferable because the energy can be recovered and later used for propulsion. The motor torque is divided approximately equally between the two front wheels  46  and  48  by differential  44 . However, the brakes may be capable of generating more braking torque than motor  42  and are capable of applying a different level of torque to each of the four wheels. 
         [0019]    For high levels of braking or when the surface is slippery, brake controller  72  enters an anti-lock brake (ABS) control mode as illustrated in  FIG. 4 . The objective in ABS mode is to decrease the vehicle speed as rapidly as possible subject to the available wheel traction. The dotted line in the top graph  90  indicates the vehicle speed divided by the wheel radius. Controller  72  may infer this value by, for example, averaging the values of the wheel speed sensors. The solid line in the top graph  92  indicates the value of one of the speed sensors  74 ,  76 ,  78 , or  80 . The difference between these two lines at any point in time is the wheel slip at that moment. The middle graph in  FIG. 4  indicates the wheel acceleration  94 . Controller  72  may calculate this value by computing a time derivative of the wheel speed signal. The bottom graph shows the torque applied by the corresponding brake. 
         [0020]    Controller  72  adjusts the commanded torque based on formulas that depend on the state of traction for the wheel. In the first phase, called a marginally stable phase, wheel speed generally tracks vehicle speed with low levels of slip indicating that the tire has acceptable traction. During this phase, the controller gradually increases the torque command as shown at  96 . In  FIG. 4 , this is indicated by a ramp function. In practice, the controller may adjust the commanded torque at regular intervals such that the increase is performed in a series of discrete steps. At  98 , the tire loses fraction and an unstable decelerating mode begins. The controller may detect this mode transition, for example, by a wheel acceleration value that falls below a calibrateable threshold value. The controller may estimate wheel acceleration based on the difference between the current sensed wheel speed and the sensed wheel speed at a previous time such as the previous control loop. In the unstable decelerating mode, the controller decreases the commanded torque in an attempt to regain traction as quickly as possible, as shown at  100 . The rate of decrease may be limited by physical responsiveness limitations of the brake actuator. Once the brake torque declines sufficiently, the tire regains traction as shown at  102 . The controller enters an unstable accelerating mode in response to wheel acceleration exceeding a calibrateable threshold value or slip decreasing below a calibrateable threshold. In unstable accelerating mode, the controller gradually increases the commanded brake torque as shown at  104 . At  106 , the controller returns to the marginally stable mode and the process repeats. 
         [0021]    Due to the repeating nature of this process, the brake torque oscillates with a frequency determined by the oscillation period. The braking is most effective during the marginally stable phase and less effective during the unstable decelerating phase when the tire has lost its traction. Braking performance is maximized by decreasing the duration of each unstable decelerating and unstable accelerating mode. However, physical limitations of the hydraulic brake actuators limit their responsiveness and therefore limit the ability of the controller to rapidly reestablish traction. 
         [0022]    The braking performance can be enhanced by taking advantage of the more responsive nature of electric motor  42  relative to the brake actuators, as illustrated in  FIG. 5 . In the bottom graph, the torque of one of the front brakes  64  or  66  is shown as a solid line and the absolute value of the motor torque is shown as a dotted line. In marginally stable mode, the motor torque gradually increases as shown at  108 . In unstable decelerating mode, the motor torque decreases as shown at  110 . Since the motor responds more quickly than a hydraulic brake actuator, the motor torque begins to decrease faster than the brake torque and decreases at a faster rate. As a result, the wheel regains traction sooner than it would have without the motor contribution. In other words, the duration of the unstable decelerating mode is shorter. During unstable accelerating mode, the motor torque increases as shown at  112 . The motor torque begins increasing before the brake torque and increases at a faster rate than the brake torque which tends to decrease the duration of the unstable accelerating phase. Since the unstable decelerating phase and the unstable accelerating phase are both shorter, the oscillation period decreases and the frequency increases. In the powertrain configuration of  FIG. 1 , motor  42  only influences the front wheels. The rear brakes would continue to respond as shown in  FIG. 4 . Therefore, the frequency of oscillation of the front brakes is higher than the frequency of oscillation of the rear brakes. 
         [0023]    The method of  FIG. 5  does not require a supervisory level controller to coordinate the actions of the powertrain controller and the ABS controller. The powertrain controller need not communicate directly with the ABS controller. Although  FIG. 3  shows the two controllers communicating via a controller area network, the only information exchanged is the wheel speed sensor readings. Alternatively, both controllers could directly read the sensor outputs. Like the ABS controller, the motor controller may adjust the motor torque at regular intervals such that the motor torque changes in a series of discrete steps rather than a continuous ramp. Due to the faster response of the motor, the interval between control loops may be shorter than for the ABS controller. A shorter control loop interval has the added advantage of more accurate estimate of wheel acceleration. 
         [0024]    The method of  FIG. 5  is summarized in the flow chart of  FIG. 6 . After braking begins, the method monitors the speeds of the wheels at  120 . If, at  122 , all tires still have traction, then the motor torque is increased in absolute value at  112  and the brake torque is increased at  104 . This process repeats until a loss of traction is detected at  122 . Then, the motor torque is decreased in absolute value at  110  and the brake torque is decreased in absolute value at  100 , and monitoring continues at  124 . This process repeats until traction is regained as detected at  126 . Note that steps  120 ,  122 ,  124 , and  126  may be performed independently by both brake controller  72  and powertrain controller  56 . The rate of change of motor torque changes direction before the rate of change of brake torque due to the faster response time of the corresponding actuator. 
         [0025]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.