Abstract:
A controller and control strategies minimize shift shock in a hybrid electric vehicle during a downshift during regenerative braking by maintaining the transmission input speed substantially linear when the transmission input speed is slowing. The controller and the control strategies control the regenerative braking torque during a downshift occurring during regenerative braking in such a way that the transmission input speed is maintained substantially linear when the transmission input speed is slowing during a torque phase of the downshift.

Description:
TECHNICAL FIELD 
     The present invention relates to controlling regenerative braking torque in a hybrid vehicle powertrain during a ratio change of the transmission that occurs during regenerative braking. 
     BACKGROUND 
     A hybrid vehicle powertrain includes an electric machine such as an electric motor, wherein torque produced by an engine is supplemented with torque produced by the motor. The combined engine and motor torques are transferred to vehicle drive wheels through a transmission. 
     In a modular hybrid transmission (“MHT”) configuration, the engine is connectable to and dis-connectable from the motor by a disconnect clutch with the motor configured to directly drive the input shaft of the transmission. The engine, the disconnect clutch, the motor, and the transmission are connected sequentially in series when the engine is connected to the motor by the disconnect clutch. 
     During a process referred to as regenerative braking in a hybrid powertrain, charging a high-voltage battery during vehicle braking collects the potential and kinetic energy stored in the braking vehicle. During regenerative braking, required wheel braking torque is allocated between friction brakes and the motor, which acts as a generator. The amount of wheel braking torque required as the vehicle brakes is apportioned in real time by a controller between the hydraulic, mechanical friction braking hardware and the electric powertrain regenerative braking. The apportionment of wheel braking torque between friction braking torque and regenerative braking torque (i.e., negative input torque via the motor) is balanced through the braking process to achieve as much regeneration as possible to thereby improve fuel economy. 
     SUMMARY 
     Embodiments of the present invention are directed to a controller and control strategies which minimize shift shock in a hybrid electric vehicle during a downshift during regenerative braking by maintaining the transmission input speed substantially linear when the transmission input speed is slowing. The controller and the control strategies control the regenerative braking torque during a downshift occurring during regenerative braking in such a way that the transmission input speed is maintained substantially linear when the transmission input speed is slowing during a torque phase of the downshift. 
     In an embodiment, a system having a transmission and a controller is provided. The transmission includes multiple gear ratios for driving a drive wheel. The controller is configured to effect a downshift between gear ratios during regenerative braking while maintaining the transmission input speed substantially linear when the transmission input speed is slowing. 
     The system may further include a motor configured to supply a regenerative braking torque to the transmission. The controller may limit the regenerative braking torque to counteract variations, which tend to be caused by regenerative braking, in the transmission input speed when the transmission input speed is slowing. The controller may limit the regenerative braking torque during a torque phase of the downshift to counteract the variations. For instance, the controller may limit the regenerative braking torque by slowly increasing the regenerative braking torque over time in lieu of immediately increasing the regenerative braking torque. 
     The system may further include a friction brake system configured to supply a friction braking torque to the drive wheel. The controller may vary the friction braking torque according to the regenerative braking torque such that the summation of the friction braking torque and the regenerative braking torque corresponds to a required wheel braking torque. For instance, the controller may slowly decrease the friction braking torque as the regenerative braking torque is slowly increased such that the summation of the friction braking torque and the regenerative braking torque is maintained constant. 
     In an embodiment, a method for a hybrid vehicle powertrain system including a transmission for driving a drive wheel through multiple gear ratios is provided. In the method, a downshift between the gear ratios during regenerative braking is effected. A transmission input speed signal indicative of the transmission input speed is measured. The transmission input speed signal is maintained substantially linear while the transmission input speed is slowing during the downshift. 
     In an embodiment, a method in a hybrid vehicle powertrain system including an automatic transmission with multiple gear ratios includes effecting a downshift between the gear ratios during regenerative braking and maintaining the transmission input speed substantially linear while the transmission input speed is slowing during the downshift. 
     Additional objects, features, and advantages of embodiments of the present invention will become more readily apparent from the following detailed description when taken in conjunction with the drawings, wherein like reference numerals refer to corresponding parts. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of an exemplary hybrid vehicle powertrain in accordance with an embodiment of the present invention; and 
         FIGS. 2A and 2B  illustrate graphs of a downshift event in the powertrain of  FIG. 1  from a high gear configuration to a low gear configuration with transmission input speed dipping effects compensated for by limiting regenerative braking torque from the electric motor. 
     
    
    
     DETAILED DESCRIPTION 
     Detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the present invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may 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. 
     Referring now to  FIG. 1 , a block diagram of an exemplary powertrain system  10  for a hybrid electric vehicle in accordance with an embodiment of the present invention is shown. Powertrain system  10  includes an engine  20 , an electric machine such as an electric motor/generator  30  (“motor”), a multiple-ratio automatic transmission  50 , and a friction braking system. 
     Engine  20  has an output shaft  22  connectable to and dis-connectable from an input shaft  24  of motor  30  through an engine clutch  32  (i.e., a disconnect clutch  32 ). Motor  30  has an output shaft  42  connectable to and dis-connectable from an input shaft  44  of transmission  50  through a motor clutch  52  (i.e., a launch clutch  52 ). Although clutches  32 ,  52  are described and illustrated as hydraulic clutches, other types of clutches such as electro-mechanical clutches may be used. 
     Transmission  50  includes multiple gear ratios and is connected to a drive shaft  54  (i.e., an output shaft of transmission  50 ). Output shaft  54  is connected to a differential  56 . Left and right drive wheels  60 ,  62  are connected to differential  56  through left and right axles  64 ,  66 . With this arrangement, transmission  50  transmits a powertrain output torque  68  to drive wheels  60 ,  62 . Wheels  60 ,  62  are provided with friction brakes  70  for applying a braking force to slow the vehicle. 
     Engine  20  may be connected to a starter motor  34  used to start engine  20 . Starter motor  34  is connected to a traction battery  36  through wiring  38  so as to be also used as a generator to produce electric energy for storage in battery  36 . When running, engine  20  can provide power to starter motor  34  so that starter motor  34  generates electric energy for storage in battery  36 . Motor  30  is also linked to battery  36  through wiring  53 . 
     Engine  20  is a primary source of power for powertrain system  10  and battery  36  is a secondary source of power for powertrain system  10 . Engine  20  is an internal combustion engine such as a gasoline, diesel, or natural gas powered engine. Engine  20  generates a first input torque  76  (i.e., an engine torque) that is supplied to motor  30  when engine  20  and motor  30  are connected via engine clutch  32 . To drive the vehicle with engine  20 , at least a portion of first input torque  76  passes from engine  20  through engine clutch  32  to motor  30  and then from motor  30  through motor clutch  52  to transmission  50 . Engine  20  also provides power through engine clutch  32  to motor  30  so that motor  30  can act as a generator and produce electric energy for storage in battery  36 . 
     Depending on the particular operating mode of the vehicle as will be detailed further below, motor  30  either sends power to battery  36  or converts electric energy stored in battery  36  into a second input torque  78  (i.e., a motor torque). Second input torque  78  is also sent to transmission  50  through motor clutch  52 . When generating electrical power for storage in battery  36 , motor  30  obtains power either from engine  20  in a driving mode or from the inertial mass of the vehicle as motor  30  acts as a brake in what is referred to as a regenerative braking mode. 
     As described, engine  20 , engine clutch  32 , motor  30 , motor clutch  52 , and transmission  50  are connectable sequentially in series as illustrated in  FIG. 1 . As such, powertrain system  10  represents a modular hybrid transmission (“MHT”) configuration in which engine  20  is connected to motor  30  by engine clutch  32  with motor  30  being connected to transmission input shaft  44  by motor clutch  52 . 
     Depending on whether engine clutch  32  and motor clutch  52  are engaged or disengaged determines which input torques  76  and  78  are transferred to transmission  50 . For example, if engine clutch  32  is disengaged, then only second input torque  78  from motor  30  is supplied to transmission  50 . If both clutches  32 ,  52  are engaged, then both first and second input torques  76 ,  78  from engine  20  and motor  30 , respectively, are supplied to transmission  50 . Of course, if input torque for transmission  50  is only desired from engine  20 , both clutches  32  and  52  are engaged, but motor  30  is not energized, such that first input torque  76  from engine  20  is only supplied to transmission  50 . 
     Transmission  50  includes several planetary gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of a plurality of friction elements in order to establish the desired multiple drive ratios. For instance, the friction elements of transmission  50  can be constituted by an on-coming friction element  72  (i.e., an on-coming clutch (“OCC”)), an off-going friction element  73  (i.e., an off-going clutch (“OGC”)), and a forward clutch  74 . The friction elements of transmission  50  are controllable through a shift schedule that connects and disconnects certain elements of the planetary gear sets of transmission  50  to control the ratio between the transmission output and the transmission input. 
     Transmission  50  is automatically shifted from one ratio to another based on the needs of the vehicle. Transmission  50  then provides powertrain output torque  68  to output shaft  54 . Powertrain output torque  68  ultimately drives drive wheels  60 ,  62 . The kinetic details of transmission  50  can be established by a wide range of transmission arrangements. Transmission  50  is an example of a transmission arrangement for use with embodiments of the present invention. Any multiple ratio transmission that accepts input torque(s) from an engine and/or a motor and then provides torque to an output shaft at the different ratios is acceptable for use with embodiments of the present invention. 
     Powertrain system  10  further includes a powertrain control unit  80  and a brake control unit  85 . Control units  80  and  85  collectively constitute a vehicle system controller. Based on repositioning a brake pedal  92 , the driver of the vehicle provides a total braking torque requirement signal  94  when the driver wants to slow the vehicle. The more the driver depresses pedal  92 , the more wheel braking torque is requested. Brake control unit  85  apportions the total wheel braking torque between (i) a powertrain braking torque signal  95  (which represents the amount of torque to be obtained by regenerative braking) and (ii) a friction braking torque signal  96  (which represents the amount of torque to be obtained through friction brakes  70 ). 
     Brake control unit  85  provides friction braking torque signal  96  to friction brakes  70  for the friction brakes to apply the friction braking torque to the drive wheels. Brake control unit  85  provides powertrain braking torque signal  95  to powertrain control unit  80 . In response, powertrain control unit  80  sends a motor torque signal  98  to motor  30  representing the requisite amount of motor torque to be provided by regenerative braking. In turn, motor  30  generates the regenerative braking torque and thereby supplies a negative input torque to transmission  50 . 
     Powertrain control unit  80  receives torque ratio signals  101  from a transmission control unit (not shown) associated with transmission  50  regarding shifting from one speed ratio to another, such as during a gear shift. Powertrain control unit  80  also receives a powertrain torque signal  99  representing an amount of total powertrain torque  68  from a control unit (not shown) which calculates total powertrain torque  68 . Powertrain control unit  80  sends an engine torque signal  100  to engine  20  indicating how much engine torque is required at a given time. 
     Transmission  50  has an input torque limit for each gear. Transmission  50  protects itself by publishing its input torque limit to the vehicle system controller. This insures that the controller accordingly controls engine  20  and motor  30  such that the input torque limit of transmission  50  is not exceeded by the combined engine and motor torques. 
     As described above, the driver depresses brake pedal  92  to brake the vehicle. A regenerative braking event begins when brake pedal  92  is depressed. During the regenerative braking event, motor  30  provides the input torque to transmission  50 . In this case, the input torque is a regenerative braking torque (i.e., a negative input torque via the motor which acts as a generator whereby battery  36  is charged with collected potential and kinetic energy stored in the braking vehicle). Transmission  50  uses the motor torque to provide at least part of the wheel braking torque. (The other component of the wheel braking torque is the friction braking torque as described above.) 
     A downshift of transmission  50  is typically requested when collecting regenerative braking energy as the higher input speed of transmission  50  allows for greater regenerative braking power, usually at higher efficiencies. The standard shift schedule is typically modified during a regenerative braking event to downshift sooner so that more power can be collected at a better efficiency. The input speed of transmission  50  is pushed higher when transmission  50  downshifts because of the speed ratio increase. 
     During a downshift the pressure of OGC  73  is calculated based on the input torque. The driver can instantly increase or decrease the desired wheel braking by depressing brake pedal  92  differently and therefore the input torque can instantly increase or decrease in response. If either the regenerative braking event has not progressed to the maximum available input torque (e.g., the maximum negative regenerative braking torque) or the driver desires more braking, then the change in the desired input torque relative to the actual input torque (i.e., the desired input torque versus the negative regenerative braking torque) may be too fast for OGC  73  to hold before OCC  72  is engaged. This results in the transmission input speed dipping too low or even stopping. 
     Control strategies in accordance with embodiments of the present invention limit the actual input torque (i.e., the negative regenerative braking torque) from changing too quickly once a downshift has been initiated. For instance, the torque limit of transmission  50  may be employed to limit the input torque from changing too quickly once a downshift has been initiated. As described above, transmission  50  has an input torque limit which is the amount of input torque that transmission  50  is capable to transmit at steady state. Setting the input torque limit to the current input torque level ensures that the calculated pressure of OGC  73  can hold the input torque as OGC  73  waits for OCC  72  to engage. This is meaningful during this time if either the desired wheel braking torque increases or the regenerative braking torque is not at its final value. After initially limiting the input torque, the input torque limit can be removed at a rate at which OGC  73  can respond, thus increasing the regenerative braking energy captured. This effectively keeps the transmission input speed at appropriate levels; reducing perceived drivability and possible clutch wear issues. 
     Turning now to  FIGS. 2A and 2B , there is shown an embodiment setting forth a control strategy for preventing the input torque from changing too quickly during a downshift event occurring during regenerative braking. In particular, the regenerative braking torque is limited (e.g., slowly increased over time) to thereby prevent transmission input speed dipping effects. In each of  FIGS. 2A and 2B , an exemplary downshift from a 2 nd  gear to a 1 st  gear is presented, with the X-axis representing time and the Y-axis representing either transmission input speed, actual input torque, driver commanded input torque (e.g., total braking required torque (BRK CMD TQ)), input torque limit, or clutch pressure depending on the particular curve of interest. 
     With reference to  FIGS. 2A and 2B , a downshift  203  is shown having five basic phases. A first or boost phase  204  is where OCC  72  is boosted to fill its friction element actuator quickly while OGC  73  has its pressure set to a value just sufficient to hold input torque. In a second or start phase  205 , OGC  73  continues to hold transmission  50  in its current gear ratio while OCC  72  is still stroking. In a third or torque phase  206 , OGC  73  begins releasing, reducing its torque capacity, and OCC  72  continues increasing its torque capacity. This results in the input torque transferring from OGC  73  to OCC  72 . In fourth or inertia phase  208 , OCC  72  continues to control the transmission input speed up to the new speed ratio. In a fifth or end phase  210 , the downshift is completed. 
       FIG. 2A  further shows a plot of the speed of transmission input speed  220  (i.e., the speed of transmission input shaft  44  in RPM), regenerative braking torque  230  (i.e., the actual negative motor torque), driver commanded input torque  240 , and transmission torque limit  250 , which are all plotted as a function of time during the shift.  FIG. 2B  further shows a plot of transmission input speed  220 , the pressure (psi)  260  of OCC  72 , and the pressure (psi)  270  of OGC  73 . 
     As described above, the regenerative braking event initiates when the driver depresses brake pedal  92 . During the regenerative braking event, the controller apportions the overall wheel braking torque between (i) the friction braking torque provided by friction brakes  70  and (ii) the regenerative braking torque (i.e., negative input torque) provided via motor  30 , which acts as a generator. Thus, during the regenerative braking event, motor  30  provides the input torque which is in the form of a negative input torque. 
     With reference to  FIGS. 1 and 2A , at the initiation of the regenerative braking event, the controller generates a motor torque signal  98  indicative of driver commanded input torque  240  to be provided by regenerative braking. The controller sends motor torque signal  98  to motor  30 . In turn, motor  30  provides the regenerative braking torque to transmission  50 . As shown in  FIG. 2A , the magnitude of the regenerative braking torque from motor  30  is increased until the regenerative braking torque is equal to the driver commanded input torque. The magnitude of the regenerative braking torque is then held constant in accordance with the driver commanded input torque as the regenerative braking event proceeds towards downshift event  203 . Notably, the magnitude of the regenerative braking torque could be increased in accordance with an increased driver commanded input torque as the magnitude of transmission torque limit  250  is relatively larger than regenerative braking torque  230  prior to downshift event  203  as shown in  FIG. 2A . 
     The controller also generates friction braking torque signal  96  indicative of the amount of torque to be obtained through friction brakes  70 . As noted, the wheel braking torque corresponds to the summation of the friction braking torque and the regenerative braking torque. 
     During the entire regenerative braking event, the friction braking torque can be controlled accordingly such that the desired wheel braking torque is obtained at all times. For instance, both the friction braking torque and the regenerative braking torque can be increased to meet an increase in the desired wheel braking torque. This is what occurs at the initiation of the regenerative braking event. 
     As another example, the friction braking torque can be increased in lieu of increasing the regenerative braking torque to meet an increase in the desired wheel braking torque. The regenerative braking torque can then be increased slowly over time with the friction braking torque correspondingly decreasing slowly over time such that the wheel braking torque remains constant per the increased desired wheel braking. As explained below, this may occur during torque phase  206  pursuant to control strategies in accordance with embodiments of the present invention. Thus, according to the control strategies, in response to the driver instantly increasing the desired braking by further depressing brake pedal  92 , the friction braking torque in lieu of the regenerative braking torque is increased to immediately meet the increase in the desired wheel braking torque. Subsequently, the regenerative braking torque is increased slowly to an increased value at which the regenerative braking torque accommodates the increase in the desired braking. As the regenerative braking torque is increased, the friction braking torque is correspondingly decreased such that the wheel braking torque matches the desired wheel braking. 
     In another scenario pursuant to control strategies in accordance with embodiments of the present invention, the regenerative braking torque is increased slowly over time as the regenerative braking event progresses with the friction braking torque correspondingly decreasing slowly over time such that the wheel braking torque remains constant per constant desired braking. In this case, there may not be any further increase in the braking desired by the driver. As explained below, this case may also occur during torque phase  206  pursuant to control strategies in accordance with embodiments of the present invention. 
     After the initiation of the regenerative braking event, downshift event  203  then begins with boost phase  204  followed by start phase  205 . In both boost phase  204  and start phase  205 , the regenerative braking torque  230  remains constant as the driver commanded input torque  240  remains constant. However, the controller reduces the input torque limit of transmission  50  to have a lower magnitude. As a result, the magnitude of transmission torque limit  250  is lower in both phases  204 ,  205  than prior to boost phase  204  as shown in  FIG. 2A . In particular, the input torque limit is decreased (in magnitude) such that the regenerative braking torque cannot be increased (in magnitude) without violating the input torque limit. That is, the input torque limit is decreased such that regenerative braking torque  230  cannot cross over transmission torque limit  250  without violating the input torque limit. Consequently, the regenerative braking torque is not increased further regardless of whether there is an increase in the magnitude of the driver commanded input torque. In this case, during phases  204 ,  205 , the driver commanded input torque is constant such that the regenerative braking torque remains constant and lower than the input torque limit established by the transmission torque limit  250 . Any increase in the wheel braking torque during this time would be accommodated by a corresponding increase in the friction braking torque. 
     Downshift  203  then proceeds to torque phase  206 . Torque phase  206  begins with the controller increasing the magnitude of the driver commanded input torque  240 . It is noted that this connection between driver demanded braking and the timing of the torque phase occurs in this example coincidentally at the same time as the change in the driver demand. The change in the driver demand does not cause the torque phase to begin. The driver commanded input torque may be increased in response to the wheel braking torque increasing in response to the driver further depressing brake pedal  92 . Alternatively or additionally, the magnitude of the driver commanded input torque may be increased with the intention of increasing the regenerative braking (with a corresponding reduction in the friction braking to maintain the desired wheel braking torque) such that more regenerative braking energy is collected as the regenerative braking event progresses. 
     As shown in  FIGS. 2A and 2B , transmission input speed  220  decreases during boost phase  204 , start phase  205 , and torque phase  206 . During inertia phase  208 , an increase occurs in transmission input speed  220  as the gear ratios are shifted from higher gear  271  to lower gear  272 . 
     In a conventional control system, when the magnitude of driver commanded input torque  240  is increased in torque phase  206  as shown in  FIG. 2A , an increase in regenerative braking torque  230  correspondingly occurs as represented by dotted line  232 . Consequently, if the increase is too large over a small period of time, then transmission input speed dipping or slowing effects will, if not compensated for, show up as a dip  222  in transmission input speed  220 . This is because the change in regenerative braking torque  230  may be too fast for OGC  73  to hold before OCC  72  is engaged. This dip may be felt as a shift shock. 
     According to control strategies in accordance with embodiments of the present invention, when the magnitude of driver commanded input torque  240  is increased in torque phase  206  as shown in  FIG. 2A , the regenerative braking torque  230  is slowly increased over time until the regenerative braking torque equals the magnitude of the driver commanded input torque (which happens, in this example, in inertia phase  208 ). Slowly increasing regenerative braking torque  230  results in transmission input speed  220  not having dip  222 . Therefore, in accordance with an aspect of the present invention, motor torque signal  98  applied to motor  30  representing the requisite amount of motor torque to be provided by regenerative braking is regulated such that the regenerative braking torque is slowly increased over time, thereby avoiding transmission input speed dipping or stopping effects during a downshift with regenerative braking. 
     The regenerative braking torque  230  is slowly increased in conjunction with the transmission torque limit  250  being increased as shown in  FIG. 2A . As such, the transmission torque limit  250  is used to limit or prevent the regenerative braking torque  230  from increasing too fast. 
     If the magnitude of the driver commanded input torque  240  was increased in response to an increase in the desired braking, then the controller controls the friction braking system to provide a corresponding increased friction braking torque. In this event, the friction braking torque in lieu of the regenerative braking torque is increased to immediately meet the increase in the desired wheel braking torque. The controller controls motor  30  to slowly increase the regenerative braking torque until the regenerative braking torque by itself accommodates the increase in the desired wheel braking. The controller controls the friction braking system to correspondingly decrease the friction braking torque as the regenerative braking torque is increased such that the wheel braking torque matches the desired wheel braking. 
     Similarly, if the magnitude of the driver commanded input torque  240  was increased to increase the collected regenerative braking energy as the regenerative braking event progresses over time, then controller controls motor  30  to increase the regenerative braking torque and controls the friction braking system to correspondingly decrease the friction braking torque such that the wheel braking torque matches the desired wheel braking. 
     In turn, inertia phase  208  begins. In inertia phase  208 , the input speed of transmission  50  is pushed higher as transmission  50  downshifts because of the speed ratio increase. 
     As described, a downshift occurring during a regenerative braking event is a type of shift for the transmission where negative torque is transmitted from the road to the motor during the event. The transmission torque limit is used by the transmission to perform non-regenerative shifts. Control strategies in accordance with embodiments of the present invention extend the application of the transmission torque limit to a downshift occurring during a regenerative braking event. Using the transmission input torque (also called the “slow” torque limit) as described above, the OCC is reasonably guaranteed to hold the input torque across the clutch. By lowering the transmission torque limit at a set rate the amount of regenerative braking energy captured can be increased as the regenerative braking event progresses or as more wheel braking is desired. When using the transmission torque limit the transmission is telling the rest of the system its input torque is limited in some fashion. When using the transmission torque limit, the braking system can “see” the system limitation and fills with friction braking torque to provide the desired wheel braking. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the present invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the present invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the present invention.