Abstract:
Braking control for a hybrid vehicle provides both service and regenerative mode braking for the driven wheels. A hybrid drive system is coupled to the driven wheels to provide traction power and which is capable of operating in a regenerative braking mode. The service brakes are provided by pneumatically actuated service brakes coupled to the driven wheels. Braking is initiated conventionally using an operator controlled brake actuator. A pressure regulator is placed in a pneumatic brake actuation line coupled from the operator controlled brake actuator to the pneumatically actuated service brakes for the driven wheels. The pressure regulator initially closes during braking, preventing operation of the service brakes up to the limit of the ability of the hybrid drive system to absorb torque for regenerative braking. When the torque limit for the hybrid drive system is reached, the regulator opens the actuation line progressively allowing the service brakes to supplement the hybrid drive system. During loss of traction events regenerative braking is discontinued to avoid interference with operation of anti-lock braking of the vehicle&#39;s service brakes.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The invention relates to brake system control of a motor vehicle, and more particularly to control of braking where a vehicle includes regenerative braking capability and conventional service brakes. 
         [0003]    2. Description of the Problem 
         [0004]    Various types of hybrid and electric vehicles obtain higher operating efficiencies and extend operating range by using regenerative braking. During regenerative braking a vehicle&#39;s kinetic energy is captured converted to a form amenable to storage. For example electrical energy may be stored in capacitors or subjected to conversion to potential chemical energy and stored in batteries or capacitors, or the energy may be stored mechanically by compressing a fluid. Later, the stored energy can be used to propel the vehicle. In the case of electrical power it can be applied as electricity to a traction motor, and on a hydraulic hybrid vehicle the working fluid can be applied to a pump under pressure. Regenerative braking may operate to supplement or replace operation of the conventional service brakes, in a fashion similar to an engine brake or retarder in the drive line on a conventional vehicle. The torque absorbed for regeneration supplements the braking torque requested by the driver by use of the brake pedal. Absent compensating brake pedal resistance, this results in the vehicle stopping faster for a given brake pedal input and biases the braking force to the drive axle(s). 
         [0005]    In a full hybrid or electric vehicle, the vehicle&#39;s electric traction motor doubles as the electrical generator which can be coupled to be driven by the wheels. On a hydraulic hybrid vehicle a pump may be coupled to the driveline. Typically only some of the wheels are driven, and thus capable of being coupled to the electric traction motor/generator or hydraulic pump when it is operating in its generating/storage mode. Thus, on either type of vehicle, a portion of the braking torque will come from the service brakes mounted with non-driven wheels, though braking force is biased toward the drive axles as they receive both service brake torque and regeneration torque while the non-drive axle(s) receive only service brake torque. Consideration may be given the issue of anti-lock braking systems (ABS) which distribute braking force to maintain braking stability. 
         [0006]    U.S. Pat. No. 6,454,365 describes a braking force control system for a vehicle incorporating hydraulic service brakes and regenerative braking for the vehicle&#39;s drive wheels. The &#39;365 patent provides a braking controller which generates a target braking force for front and rear wheels of the vehicle. Initially the controller applies regenerative braking in attempting to meet the target braking force levels. If regenerative braking proves insufficient to meet braking target levels, friction service brake operation is added to any wheels not supplying the target level of braking torque. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention provides a braking system for a motor vehicle. A plurality of wheels are coupled to a motor which provides traction power for propelling the vehicle and regenerative braking for slowing or stopping of the motor vehicle. Pneumatically actuated service brakes are further coupled to the drive wheels to provide slowing or stopping of the motor vehicle. An operator controlled brake actuator connects air from a compressed air source to a pneumatic brake actuation line to pneumatically actuate the service brakes for the driven wheels. A pressure regulator is disposed in the pneumatic brake actuation line. A brake controller is provided which is responsive to operation of the operator controlled brake actuator for closing the pressure regulator in the pneumatic brake actuation line up until the torque limit of the motor operating in the regenerative braking mode. An anti-lock braking system controller may be further provided responsive to indications of limited traction for overriding closure of the pressure regulator in the pneumatic brake actuation line to open the pressure regulator and providing for cessation of regenerative operation of the hybrid drive system. The control functions are implemented by incorporating pressure transducers in the driven wheel, service-brake, pneumatic actuation line. These are located both upstream and downstream from the pressure regulator for the line. The upstream transducer signal indicates occurrences of actuation of the brake actuator. The downstream transducer confirms operation of the pressure regulator. 
         [0008]    Additional effects, features and advantages will be apparent in the written description that follows. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The preferred mode of use, further objects and advantages of the present disclosure, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
           [0010]      FIG. 1  is a brake circuit schematic illustrating the modifications used to implement one embodiment of the invention. 
           [0011]      FIG. 2  is a brake circuit schematic illustrating an alternative embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Referring now to the drawings and in particular to  FIG. 1 , a brake system  10  for a medium or heavy duty vehicle is illustrated. Brake system  10  is illustrated as configured for a vehicle having a front axle and a rear axle (the axles are not shown), but may be applied to other configurations, such as vehicles with lift axles and other combinations of axles having driven and non-driven wheels. Associated with the front and rear axles are individual, wheel mounted, pneumatically actuated service brakes  104 . The rear wheels  94  have brake assemblies  106  which include a park or spring brake chamber  105  in addition to the service brake  104  while the front wheels  92  do not include a park brake. In addition, the rear wheels  94  are connected by a vehicle drive train  96  to a hybrid drive system, such as an electric traction motor or the preferred hydraulic drive system  90 , which can operate regeneratively to supply braking torque. The rear wheel  94  brake assemblies  106  provide service braking and park braking. In the configuration illustrated the rear wheels  94  are driven and the front wheels  92  are non-driven. 
         [0013]    The functioning of the parking brake is discussed here for the sake of completeness of description of the pneumatic brake actuation system. Control over the distinct parking and service brake functions of the rear wheel brake assemblies  106  are accomplished by having separate air ports  111   a  and  111   b  for the service brake chambers  104  and the spring brake chambers  105 , respectively. The service braking air port  111   a  allows air to be directed to the service brake chamber  104  to move brake pads (not shown) to stop the rear wheels. The park braking port  111   b  allows air to be directed to the spring brake chambers  105  to act counter internal springs which normally urge application of brake pads. When the parking brake is disengaged, compressed air holds the park brakes off and free movement of the rear wheels  94  is allowed. Air is delivered to a quick release valve (QRV)  31  along an air line  19   h  from a push pull double check valve (PPDC)  29  and a spring brake modulator valve  30  for delivery to the park brake chambers  105 . Air is also supplied to the spring brake modulator valve  30  from relay valve  430  along air line  19   m  from the primary tank  20  and along air line  19   f  from the foot actuated double valve  26  from the secondary tank  21 . The parking brake system makes use of the redundant compressed air sources (primary and secondary compressed air tanks  20 ,  21 ) to avoid unintended engagement of the parking brake system should one compressed air source fail. Air lines  19   f  and  19   g  supply air from the primary and secondary tanks  20 ,  21  through the double valve  26  to a push pull double check (PPDC) valve  29 . 
         [0014]    The pneumatic components in the brake system  10  are supplied with compressed from an air compressor  22 . Air compressor  22  supplies air via air line  19   a  though an air dryer  23  to a wet tank  24 . The wet tank  24  acts as a supply reservoir for both a primary air tank  20  and a secondary air tank  21 , which in turn supply the service and parking brake systems. Air lines  19   b  and  19   c,  respectively, deliver air from the wet tank  24  to the primary tank  20  and the secondary tank  21 . Check valves  25  are incorporated into air lines  19   b  and  19   c  allowing air to flow out from the wet tank  24  but not back into the wet tank. 
         [0015]    Primary air tank  20  and secondary air tank  21  are the direct sources of supply of pressurized air for brake system  10 . The primary air tank  20  supplies air for service braking for the rear wheels  94  and the secondary air tank  21  supplies air for service braking for the front wheels  92 . Since independent sources of air are used for the service brakes for the rear and front wheels  94 ,  92 , the service brake system is considered to be redundant. Air is routed from primary air tank  20  via air line  19   d  through a foot actuated double valve  26  upon depression of foot pedal  26   a.  On anti-lock braking system (ABS) equipped vehicles quick release valves  31  (QRVs) are used only for rear parking brake functions. ABS modulators  91  perform the QRV functions for the service brakes and are included in the air lines  19   j  and  19   e  which supply air to the brake assemblies  104 . For the rear brakes an air line  19   j  from the primary tank  20  to the rear wheel  94  brake assemblies  104  includes a relay valve  430  which is actuated by air from the food pedal  26  delivered along air line  19   d  as a pneumatic signal for applying air to the rear wheel service brakes  104 . Air from secondary air tank  21  is coupled to the service brakes  104  for the front wheels  92  for service braking via air line  19   e  through the double valve  26  upon depression of foot pedal  26   a.  The operation of the ABS modulators  91  is well known in the art. The ABS modulators  91  operate to modulate air pressure delivered to the service brakes  104  to distribute braking torque to the wheel best able to absorb it. 
         [0016]    In the brake system  10  as illustrated the rear wheels  94  are driven and the front wheels  92  are non-driven. One source of traction power for the rear wheels  94  is a hybrid drive system, preferably a hydraulic system  90 , which is mechanically connected to the rear wheels by drive line  96 . During braking hydraulic drive system  90  operates as a pump turned by the wheels  94 . In an electric traction motor system a motor operates as a generator. Thus service braking is supplemented by regenerative braking which is applied to the rear wheels  94 . During normal operation of the brake system  10 , rear wheel  94  braking torque should be supplied by the hybrid (hydraulic) drive system  90 , and not the service brakes  104 , in order to recapture as potential energy as much of the vehicle&#39;s kinetic energy as possible. 
         [0017]    During emergency braking, particularly where ABS operation comes into play, factors affecting vehicle control and the need for stopping the vehicle arise which may mitigate against the use of regenerative braking. Brake system  10  is modified to implement control over service brake operation and regenerative braking to better meet these potentially conflicting engineering requirements. Air line  19 d, connecting the foot actuated double valve  26  to the relay valve  430  (i.e., the air line transmitting a pneumatic signal from the foot-controlled valve to the relay valve for controlling application of pressure from the primary tank  20  to the rear service brakes  104  through the relay valve) is modified to include two pressure transducers, a primary transducer  80  and a feedback transducer  84 , with an intervening pressure regulator  82 . The pressure transducers  80 ,  84  are located in air line  19 d with the primary transducer  80  upstream from, and the feedback transducer  84  downstream from, the pressure regulator  82 . The pressure transducers  80 ,  84  report pressure readings to a hybrid brake controller  86 , from which the pressure difference across the modulator  82  can be determined. Additionally, pressure transducer  80  reports pressure readings in air line  19 d to a hybrid controller  88 . A control signal from the hybrid brake controller  86  is applied to modulator  82 . 
         [0018]    The hybrid drive system  90  is under the control of the hybrid controller  88 , which can set system  90  into a regeneration mode for operation as a pump or generator, depending upon the type of drive system, e.g. hydraulic, electric. A hydraulic drive system operates as a pump to increase pressure on a hydraulic fluid delivered through an energy storage device  76  embodied in an accumulator. The details of this arrangement are outside the scope of the present invention. 
         [0019]    Hybrid controller  88  communicates by one of various data network systems with the hybrid brake controller  86  and an ABS controller  74 . The hybrid controller  88  can report the amount of torque being absorbed by the drive system  90  during regenerative braking to the hybrid brake controller  86 . The hybrid brake controller  86  compares this with the degree of braking demanded as indicated by a pressure transducer  80 . In normal operation the hybrid brake controller  86  utilizes braking demand pressure as detected transducer  80  to demand regenerative braking from the drive system  90  up to the torque limit of its regenerative braking capacity. The front service brakes  104  are unaffected and operate normally. Once the torque limit of the drive system  90  is reached, the hybrid brake controller adjusts the pressure regulator  82  to allow actuation of the service brakes  104  for the rear wheels  94  to supplement the motor  90  braking. 
         [0020]    During ABS events the regenerative braking functionality of the drive system  90  is normally cancelled and the hybrid brake controller  86  instructed to allow normal service brake operation along air line  19   d  by opening modulator  82 . ABS controller  74  is connected to the hybrid controller  88  and the hybrid brake controller  86  to allow communication of the appropriate indication. ABS controller  74  also controls the modulation of ABS modulators  91  associated with the service brakes  104  for each wheel of the vehicle equipped with service brakes. ABS control over braking is provided over the service brakes  104  only. The object is that ABS operation is unaffected by the modifications to the brake system introduced by the invention. During an ABS event regulator  82  is opened. To confirm that the pneumatic braking system is operating conventionally, that is, as though no regenerative braking were available, the feedback pressure transducer in air line  19   d,  transducer  84 , should provide feedback indication to the hybrid brake controller  86  that pressure in air line  19   d  following regulator  82  closely matches the pressure measured by transducer  80  ahead of regulator  82 . 
         [0021]      FIG. 2  illustrates an embodiment of the invention applied to a 6×4 truck with a lift axle  114 . Service brakes  104  associated with wheels for the lift axle  114  have no associated park brake chambers. In addition, the lift axle is a non-driven axle, meaning no regenerative braking is produced from it. The service brakes  104  are actuated by a signal from an ABS control module  74  to relay valve  530 . A local auxiliary air tank  110  supplies the air to the relay valve  530  for operation of the service brakes  104  for lift axle wheels. ABS modulation of the brakes of the lift axle is not directly provided. During ABS events the brakes of the lift axle  114  may be lightly braked or not braked at all. 
         [0022]    The electronically controlled air pressure regulator  82  (located between the primary and feedback pressure transducers  80 ,  84 ) controls pressure in the primary air pressure signal line when the vehicle operator actuates the brake pedal  26 . When the vehicle operator is not requesting service brake application, this regulator is fully open (normally open). This allows for normal service brake function should there be a loss of power or control signal to the regulator. The hybrid brake controller determines how much air pressure is needed at the primary service brake relay valve to properly supplement the hybrid hydraulic regenerative braking torque up to the vehicle operator requested level. It sends a control signal to the electrically controlled air pressure regulator and monitors the signal from the second pressure transducer to ensure proper signal line air pressure to the primary service brake relay valve. The hybrid brake controller  86  control signal is disabled (the electronically controlled air pressure regulator allows full signal line pressure to pass unimpeded) during ABS active and other priority braking events. Under these conditions, full service braking capability is maintained and uninterrupted. The controller is also disabled when the ABS system is deactivated. The invention allows for increased regenerative braking efficiency because of the reduced or eliminated application of the service brakes on the axle(s) providing torque to the hybrid hydraulic drive system. The increase in regeneration efficiency will allow for greater availability of hydraulic launch assist from the hybrid hydraulic drive system, thus decreasing fuel consumption. This would be of significant benefit in vocations with frequent start and stop driving conditions. 
         [0023]    While the invention is described with reference to only a few of its possible forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.