Patent Application
20100109437
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.
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.
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.
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.
While the method of the present invention is disclosed herein in the context of the exemplary hybrid vehicle electrical system and powertrain of
The flow diagram of
Referring to
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
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.
