Patent Publication Number: US-2010109437-A1

Title: Battery pack disconnection method for a hybrid vehicle

Description:
TECHNICAL FIELD 
     The present invention is directed to a failure-mode battery disconnect method for a hybrid vehicle electrical system including a high voltage battery pack and an engine-driven electric machine operable in generating and motoring modes, and more particularly to a control method for taking the battery pack off-line while permitting continued operation of the engine and electric machine. 
     BACKGROUND OF THE INVENTION 
     The fuel efficiency of a motor vehicle can be considerably enhanced with a hybrid system including an electric machine coupled to the engine, a high voltage battery pack, and a power electronics system for interconnecting the electric machine, the battery pack and the electrical loads of the vehicle. The electric machine is operable in a generating mode to charge the battery pack and supply power to various electrical loads, and in a motoring mode to crank the engine and to augment the engine power output. Various drive arrangements can be used to propel the vehicle. For example, the engine can be coupled to the drive wheels through a conventional drivetrain, and/or one or more electric propulsion motors can be used. 
       FIG. 1  illustrates an example of a hybrid vehicle system including an engine  10  that is mechanically coupled to a set of drive wheels  12  through a transmission (T)  14  and differential gearset (DG)  16 . The hybrid vehicle system includes an AC electric machine  18 , a main 120-volt battery pack  20 , and a power conversion system  22 . The electric machine  18  is selectively operable in generating and motoring modes, and is mechanically coupled to the engine  10 , either directly or by way of a drive belt. The power conversion system  22  includes a high voltage DC bus  24 , a bus capacitor  26  for maintaining the bus voltage, a relay  28  coupling the positive side of high voltage bus  24  to the battery pack  20 , an inverter  30  coupling high voltage bus  24  to the electric machine  18 , a DC-to-DC converter  32  coupling high voltage bus  24  to a low voltage DC bus  34 , and a Power Control Unit (PCU)  36  for controlling the operation of relay  28 , inverter  30  and DC-to-DC converter  32 . The low voltage DC bus  34  is used to supply power to various  12 -volt electrical loads  38  of the vehicle, and an auxiliary  12 -volt storage battery  40  is coupled to the low voltage bus  34  for maintaining the bus voltage and temporarily supplying power to the loads  38  in the event of a system failure. 
     Under certain failure mode conditions such as battery pack over-temperature, it is necessary to disconnect battery pack  20  from the high voltage bus  24 . The relay  28  serves as the disconnect device, but first the PCU  36  powers down the inverter  30  and DC-to-DC converter  32  to minimize the current that the relay  28  must break, and to prevent load-dump transient voltages. However, once the battery pack  20  is off-line, there is insufficient reserve electrical power to re-activate the electric machine  18  and the only source of power for the electrical loads  38  is the auxiliary storage battery  40 . Unfortunately, this significantly limits the failure-mode range of the vehicle because certain electrical loads such as the engine ignition system are required for continued operation of engine  10 . Accordingly, what is needed is a way of disconnecting the battery pack  20  from the high voltage bus  24  without having to forego the generating capability of the electric machine  18 . 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved control methodology for an engine-driven electric machine of a hybrid vehicle electrical system for enabling continued operation of the vehicle electrical system under failure mode conditions that require disconnection of the battery pack from the electrical system. At the onset of a failure mode condition requiring disconnection of the battery pack, the electric machine is operated as a generator and controlled in accordance with a first mode of operation such that its output substantially matches the instant electrical load requirements of the vehicle without supplying charging current to the battery pack. When the battery pack current falls below a near-zero threshold, a relay is activated to disconnect the battery pack from the electrical system, and the electric machine is then controlled in accordance with a second mode of operation for maintaining the operating voltage of the electrical system at a specified value. In each mode of operation, the control is dynamically compensated for changes in the speed and efficiency of the electric machine and changes in the electrical load requirements of the vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an exemplary hybrid vehicle electrical system and powertrain, including an engine, a high voltage battery pack, an engine-driven electric machine operable in generating and motoring modes, and a microprocessor-based Power Control Unit; and 
         FIG. 2  is flow diagram representing a control methodology according to this invention that is implemented by the microprocessor-based Power Control Unit of  FIG. 1  under failure mode conditions requiring disconnection of the battery pack. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the method of the present invention is disclosed herein in the context of the exemplary hybrid vehicle electrical system and powertrain of  FIG. 1 , it should be understood that the described method is applicable to any hybrid vehicle electrical system including a battery pack and an engine-driven electric machine. Virtually all hybrid vehicle electrical systems include a battery pack for storing electrical energy and an engine-driven electric machine selectively operable in a generating mode to charge the battery pack and supply power to various electrical loads, and in a motoring mode to crank the engine and to augment the engine power output. And in all such configurations, there is the possibility of a failure mode condition that requires disconnection of the battery pack from the vehicle electrical system—for example, from the high voltage bus  24  in the configuration of  FIG. 1 . But the disconnect relay  28  (or comparable disconnect device) cannot safely or reliably interrupt the connection if the battery pack current is too high, and even in cases where the connection can be interrupted, the interruption can cause load-dump transient voltages severe enough to damage electronic components coupled to the high voltage bus  24 . The conventional approach of powering down the inverter  30  and DC-to-DC converter  32  to minimize the battery pack current so that the battery pack  20  can be reliably disconnected results in the undesired situation in which there is insufficient reserve electrical power to re-activate the electric machine  18 . And with the auxiliary storage battery  40  as the only source of power for the electrical loads  38 , the vehicle range will be substantially curtailed, particularly in hybrid configurations that utilize electric propulsion motors. In contrast, the method of the present invention minimizes the battery pack current by controlling the electric machine  18  so that its output substantially matches the instant electrical load requirements of the vehicle without supplying charging current to the battery pack  20 . And once the battery pack  20  is safely disconnected, the electric machine  18  is controlled in a manner to dynamically maintain the operating voltage of the electrical system at a specified value to enable continued normal operation of the vehicle until the engine  10  is turned off. 
     The flow diagram of  FIG. 2  outlines the control method of the present invention, and as such, describes a software routine periodically executed by the PCU  36  of  FIG. 1  in response to detection of a failure mode requiring disconnection of the battery pack  20 . The disclosed control assumes that the inverter  30  includes a controller configured to accept a desired torque input and control the machine winding currents accordingly. In the motoring mode, the desired torque input indicates the desired torque output of the machine  18 ; and in the generating mode, the desired torque input indicates a load torque that is supplied to the machine  18  by engine  10  for producing electric power. 
     Referring to  FIG. 1 , the PCU  36  receives inputs  42  pertaining to a number of system-related parameters pertaining to the method of the present invention, including the battery pack current I_BP, the voltage HV_BUS of high voltage bus  24 , the engine speed ES, the DC-to-DC converter input current I_DCDC, and the state RELAY_STATE of relay  28 . And for purposes of this invention, the PCU  36  outputs the desired machine mode (i.e., generating or motoring) and the desired torque DES_TQ to the inverter  30 . 
     In general, the desired torque output DES_TQ is iteratively calculated both before and after disconnection of the battery pack  20  as follows: 
         DES   —   TQ =( PWR   —   DCDC+PWR   —   CNTL )/( ES*GEN   —   EFF )   (1) 
     where PWR_DCDC is the input power of DC-to-DC converter  32 , PWR_CNTL is a power control term, and GEN_EFF is the generating efficiency of the electric machine  18 . The converter input power POWER_DCDC is computed according to the product of the bus voltage HV_BUS and the converter input current I_DCDC. The generating efficiency GEN_EFF of the electric machine  18  can be determined by table look-up as a function of its rotational speed (which is proportional to, or equal to, engine speed ES depending on the powertrain configuration) and the desired torque output DES_TQ. 
     The power control term PWR_CNTL in equation (1) is formulated so that the desired torque output DES_TQ will satisfy the mode-specific control objective in addition to satisfying the low voltage load requirement. In the operating mode prior to battery pack disconnection, the control objective is to drive the battery pack current to zero. While in theory, this can be achieved by exactly matching the output of electric machine  18  with the current electrical load requirements, the power control term PWR_CNTL is formulated as an integrator term PWR_INT for driving the battery pack current I_BP to zero. Preferably, the integrator term PWR_INT for this operating mode is formulated as follows: 
         PWR   —   INT=PWR   —   INT _LAST−( INT _GAIN*LOOP_TIME* HV _BUS* I   —   BP )   (2) 
     where PWR_INT_LAST is the previous value of PWR_INT, INT_GAIN is a calibrated integrator gain term, and LOOP_TIME is the program loop time for updating PWR_INT. 
     Once the battery pack current I_BP falls within a calibrated current window, the relay  28  is activated to disconnect battery pack  20  from the high voltage bus  24 , and the control objective for electric machine  18  then becomes maintaining the voltage on high voltage bus  24  at a desired value HV_BUS_DES. This is achieved by formulating the power control term PWR_CNTL of equation (1) as a proportional-plus-integral control. The proportional term PWR_PROP is preferably determined by table look-up as a function of the bus voltage error (HV_BUS−HV_BUS_DES). And the integral term PWR_INT in this case is formulated as follows: 
         PWR   —   INT=PWR   —   INT _LAST+(LOOP_TIME* INT _GAIN)   (3) 
     where PWR_INT_LAST is the previous value of PWR_INT, LOOP_TIME is the program loop time for updating PWR_INT, and INT_GAIN is a integrator gain term preferably determined by table look-up as a function of the bus voltage error (HV_BUS−HV_BUS_DES). 
     Referring specifically to  FIG. 2 , the block  50  is first executed to determine whether relay  28  is open or closed, as indicated by the state of the RELAY_STATE flag. If the relay  28  is closed, the blocks  52  and  54  are executed to open relay  28  if the battery pack current I_BP is less than a specified current threshold. If the relay  28  is already open, the execution of blocks  52  and  54  is skipped. In either case, block  56  is then executed to determine if the entry conditions for the control are met. Specifically, the DC-to-DC converter  32  must be operating, and the engine  10  must be driving the electric machine  18 . If one or both of these conditions are not met, blocks  58  and  60  set DES_TQ to zero, and output DES_TQ to inverter  30 . However, if the conditions of block  56  are satisfied, block  62  computes the converter input power PWR_DCDC and block  64  determines whether relay  28  is open or closed. If the relay  28  is closed, blocks  60  and  66 - 70  are executed to define a first operating mode for which the control objective is to drive the battery pack current I_BP to zero. On the other hand, if the relay  28  is open, blocks  60  and  72 - 76  are executed to define a second operating mode for which the control objective is to maintain the voltage on high voltage bus  24  at the desired value HV_BUS_DES. 
     In the case where relay  28  is closed, the block  66  calculates the integrator term PWR_INT, block  68  determines the generator efficiency GEN_EFF, and block  70  calculates the desired torque output DES_TQ. The integrator term PWR_INT is calculated as a function of INT_GAIN, LOOP_TIME, HV_BUS, and I_BP according to equation (2) so that it continuously biases the battery pack current I_BP toward zero. The generator efficiency GEN_EFF is determined as a function of machine speed and torque, as mentioned above. And the desired torque output DES_TQ is calculated as a function of PWR_DCDC, PWR_CNTL, ES and GEN_EFF according to equation (1), where PWR_CNTL=PWR_INT. 
     In the case where relay  28  is open, the block  72  calculates the proportional and integral power terms PWR_PROP and PWR_INT, block  74  determines the generator efficiency GEN_EFF, and block  76  calculates the desired torque output DES_TQ. The proportional and integral power terms are both determined as a function of the bus voltage error (HV_BUS−HV_BUS_DES). As described above, the proportional power term PWR_PROP and the integrator gain term INT_GAIN are preferably determined by table look-up, and the integral power term PWR_INT is calculated according to equation (3). The combined effect of the proportional and integral power terms is to continuously drive the high bus voltage HV_BUS to the desired value HV_BUS_DES. As in the first control mode, the generator efficiency GEN_EFF is determined as a function of machine speed and torque. And the desired torque output DES_TQ is calculated as a function of PWR_DCDC, PWR_CNTL, ES and GEN_EFF according to equation (1), where PWR_CNTL=(PWR_PROP+PWR_INT). 
     In summary, the control methodology of the present invention present invention provides a safe and reliable way of disconnecting the battery pack  20  from the high voltage bus  24  without having to forego the generating capability of the electric machine  18 , thereby avoiding a walk-home condition and maintaining normal operation of the engine  10  and other vehicle electrical loads  38  until the engine  10  is turned off. Regardless of whether the control objective is minimizing battery pack current or maintaining the high bus voltage, the controls are configured to dynamically compensate for changes in the speed and efficiency of the electric machine  18  as well as for changes in the electrical load requirements. While the control method has been described in reference to the illustrated embodiment, it should be understood that various modifications in addition to those mentioned above will occur to persons skilled in the art. Accordingly, it is intended that the invention not be limited to the disclosed embodiment, but that it have the full scope permitted by the language of the following claims.