Abstract:
A hybrid-electric vehicle and method of controlling a hybrid-vehicle is provided. The vehicle includes a traction battery, at least one electric-machine, and a controller. The controller is configured to alter a voltage between the battery and the electric-machine based on a measured voltage of the battery or the high voltage bus. In response to an indication that the measured voltage is faulty during a drive cycle, the controller instead alters the voltage between the battery and the electric machine based on a substituted battery voltage signal such that the electric-machine remains operable for the drive cycle.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional Application No. 61/695,589 filed Aug. 31, 2012, the disclosure of which is incorporated in its entirety by reference herein. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a system for controlling an electric machine in an electric vehicle. 
     BACKGROUND 
     Battery electric vehicles (BEVs) include a fraction battery that is rechargeable from an external electric power source and powers the electric machine. Hybrid electric vehicles (HEVs) include an internal combustion engine, one or more electric machines, and a traction battery that at least partially powers the electric machine. Plug-in hybrid electric vehicles (PHEVs) are similar to HEVs, but the traction battery in a PHEV is capable of recharging from an external electric power source. These vehicles are examples of vehicles that are capable of being at least partially driven by an electric machine. 
     In these vehicles, if a failure of a component necessary for electric propulsion is detected, several actions may be necessary to ensure the safety of the vehicle occupants. Since shutdown of the entire vehicle may be undesirable, limited operation strategy (LOS) modes can be implemented to enable the operator of the vehicle to continue to drive while individual components are disabled. 
     SUMMARY 
     In one embodiment of the disclosure, a vehicle is provided including a fraction battery, at least one electric-machine, and a controller. The controller is configured to alter a voltage between the battery and the electric-machine based on a measured voltage of the battery. In response to an indication that the measured voltage is faulty during a drive cycle, the controller instead alters the voltage between the battery and the electric machine based on a substituted battery voltage signal such that the electric-machine remains operable for the drive cycle. 
     In another embodiment, the controller comprises a variable voltage converter. 
     In another embodiment, the measured voltage is faulty when the measured voltage exceeds a threshold range. 
     In another embodiment, the vehicle also includes at least one sensor for providing the measured voltage signal of the battery voltage. 
     In another embodiment, in response to the faulty measured voltage battery voltage signal, the substitute battery voltage signal is provided by a battery control unit. 
     In another embodiment, the alternate battery voltage signal is provided from the battery control unit through a vehicle control area network (CAN). 
     In one embodiment of the disclosure, a vehicle is provided including a fraction battery, a high voltage bus to provide high voltage to least one electric-machine and a controller. The controller is configured to alter a voltage between the battery and the electric-machine based on a measured voltage of the high voltage bus. In response to an indication that the measured voltage is faulty during a drive cycle, the controller instead alters the voltage between the battery and the electric machine based on a substituted battery voltage signal such that the electric-machine remains operable for the drive cycle. 
     In another embodiment, the controller comprises a variable voltage converter. 
     In another embodiment, in response to the faulty measured signal of the high voltage bus, the variable voltage converter is set to a mode that prevents providing a boost voltage the at least one electric machines. 
     In another embodiment, the substituted battery voltage signal is equal to a measured battery voltage signal. 
     In another embodiment, the measured voltage is faulty when the measured voltage exceeds a threshold range. 
     In another embodiment, the vehicle also includes at least one sensor for providing the measured voltage signal of the high voltage bus. 
     In one embodiment of the disclosure, a method of controlling a hybrid vehicle is provided. The method includes varying a voltage between a traction battery and an electric-machine based on a measured voltage signal. In response to an indication that the measured voltage signal is faulty during a drive cycle, the method substitutes an alternate voltage signal. The method then varies the voltage between the battery and the electric machine based on the alternate voltage signal such that the electric-machine remains operable for the drive cycle. 
     In another embodiment, if the measured voltage signal is based on a battery voltage, substituting the alternate battery voltage signal provided by a battery control unit. 
     In another embodiment, the alternate battery voltage signal is provided from the battery control unit through a vehicle control area network (CAN). 
     In another embodiment, if the measured voltage signal is based on a high voltage bus voltage, an alternate high voltage signal is substituted for the faulty measured voltage signal and set equal to a battery voltage signal. A variable voltage controller is set to a mode that prevents providing a boost voltage to at least one of the electric machines. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a hybrid-electric vehicle according to one embodiment of the disclosure; 
         FIG. 2  is a block diagram illustrating an example of a control system of the vehicle of  FIG. 1 ; 
         FIG. 3  is a schematic illustration of a portion of the vehicle of  FIG. 1 ; 
         FIG. 4  is a schematic illustration of a Variable Voltage Converter (VVC) of  FIG. 3 ; 
         FIG. 5  is a flow chart of an algorithm implemented in a control system of the vehicle of  FIG. 1  according to one embodiment of the disclosure; 
         FIG. 6  is a flow chart of another algorithm implemented in the control system of the vehicle of  FIG. 1  according to one embodiment of the disclosure; 
         FIG. 7  is a flow chart of another algorithm implemented in the control system of the vehicle of  FIG. 1  according to one embodiment of the disclosure; 
         FIG. 8  is a flow chart of another algorithm implemented in the control system of the vehicle of  FIG. 1  according to one embodiment of the disclosure; 
         FIG. 9  is a flow chart of another algorithm implemented in the control system of the vehicle of  FIG. 1  according to one embodiment of the disclosure; and 
         FIG. 10  is a flow chart of another algorithm implemented in the control system of the vehicle of  FIG. 1  according to one embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     Referring to  FIG. 1 , a hybrid-electric vehicle  10  is illustrated having a power-split powertrain. A vehicle control system  12  is provided, and can generally be referred to as a controller. The vehicle control system  12  controls the power distribution in the powertrain or driveline of the vehicle  10 . 
     The vehicle  10  includes a traction battery  14 . The battery  14  has a two-way electrical connection, such that the battery  14  receives and stores electric energy through regenerative braking, for example. The battery  14  also supplies the energy to an electric machine, such as an electric traction motor  16 . 
     Although the control system  12  of the vehicle  10  is illustrated in  FIG. 1  as a single controller, such a control system can include more than one controller, as desired. For example, a separate battery control module can directly control the battery  14 . Furthermore, a separate motor control module can be directly connected to the motor  16  and to the other controllers in the vehicle  10 . It should be understood that all contemplated controllers in the vehicle  10  can be referred to as a “controller”, and the vehicle control system  12  is not necessarily limited to only one controller. Separate additional controllers and their hierarchy will be described in more detail in  FIG. 2 . 
     An inverter  15  is provided to converts direct current (DC) from the battery into alternating current (AC) for powering the electric machine. The inverter  15  may also selectively enable/disable electrical flow from the battery  14  to the motor  16 . Alternatively, during regenerative braking, the inverter  15  converts AC from the electric machine into DC such that electric power is stored in the battery  14 . 
     An internal combustion engine  18  is also a power source for the vehicle  10 . The vehicle control system  12  controls the operation of engine  18 . Both the motor  16  and the engine  18  are capable of powering a transmission  20  that ultimately delivers torque to the wheels of the vehicle  10 . 
     The engine  18  delivers power to a torque input shaft  22  that is connected to a planetary gear set  24  via a one way clutch. The input shaft  22  powers the planetary gear set  24 . The planetary gear set  24  includes a ring gear  26 , a sun gear  28 , and a planetary carrier assembly  30 . The input shaft  22  can be driveably connected to the carrier assembly  30  which, when powered, can rotate the ring gear  26  and/or the sun gear  28 . The sun gear  28  can be driveably connected to a generator  32 . The generator  32  may be engaged with the sun gear  28 , such that the generator  32  may either rotate with the sun gear  28 , or can be disengaged so that the generator  32  does not rotate with the sun gear  28 . Like the motor  16 , the generator  32  may be referred to as an electric machine which, when utilized in other vehicle powertrain configurations, is capable of both generating electric power and providing motive power. 
     When the engine  18  is driveably coupled to the planetary gear set  24 , the generator  32  generates energy as a reactionary element to the operation of the planetary gear set  24 . Electric energy generated from the generator  32  is transferred to the battery  14  through electrical connections  36 . The battery  14  also receives and stores electric energy through regenerative braking, in known fashion. The battery  14  supplies the stored electric energy to the motor  16  for operation. The portion of the power delivered from the engine  18  to the generator  32  may also be transmitted directly to the motor  16 . The battery  14 , motor  16 , and generator  32  are each interconnected in a two-way electric flow path through electrical connections  36 . The vehicle control system  12  controls the components in the powertrain to provide proper torque distribution to the wheels. 
     It should be understood that the motor  16  and the generator  32  can both be referred to as an electric machine. Each electric machine can operate as a generator by receiving torque from the engine  18  and supplying AC voltage to the inverter  15 , whereby the inverter  15  converts the voltage into DC voltage to charge the battery  14 . Each electric machine can also operate as a generator by utilizing regenerative braking to convert the braking energy of the vehicle into electric energy to be stored in the battery  14 . Alternatively, each electric machine can operate as a motor whereby the electric machine receives power from the inverter  15  and the battery  14  and provides torque through transmission  20  and ultimately to the wheels  58 . 
     The inverter  15  selectively powers the motor  16  and the generator  32 . The inverter  15  can include a motor inverter for selectively disabling the motor  16 , and a generator inverter for selectively disabling the generator  32 . 
     The vehicle  10  can also include a variable voltage converter (VVC)  60 , or also referred to as a boost converter, for varying voltage between the battery  14  and the motor  16  and the generator  32 . The VVC  60  is used to boost the battery  14  voltage to a higher voltage. The higher voltage in a hybrid-electric drivetrain system can be used for multiple purposes such as torque capability optimization for electric machines, system loss optimization, and well other hybrid-electric system optimization. The VVC  60  allows the vehicle  10  to use a smaller battery pack with lower voltage while maintaining the functionally associated with the higher voltage. A smaller battery pack may have advantages such as lower cost, smaller size and less packaging restraints, for example. The VVC  60  will be described in more detail in  FIG. 3  and  FIG. 4 . 
     The vehicle  10  may be powered by the engine  18  alone, by the engine  18  and generator  32  alone, by the battery  14  and motor  16  alone, or by a combination of the engine  18 , battery  14 , motor  16  and generator  32 . In a mechanical drive mode, or a first mode of operation, the engine  18  is activated to deliver torque through the planetary gear set  24 . The ring gear  26  distributes torque to step ratio gears  38  comprising meshing gear elements  40 ,  42 ,  44 , and  46 . Gears  42 ,  44 , and  46  are mounted on a countershaft, and gear  46  distributes torque to gear  48 . Gear  48  then distributes torque to a torque output shaft  50 . In the mechanical drive mode, the motor  16  may also be activated to assist the engine  18  in powering the transmission  20 . When the motor  16  is active in assisting, gear  52  distributes torque to gear  44  and to the countershaft. 
     In an electric drive mode (EV mode), or a second mode of operation, the engine  18  is disabled or otherwise prevented from distributing torque to the torque output shaft  50 . In the EV mode, the battery  14  powers the motor  16  to distribute torque through the step ratio gears  38  and to the torque output shaft  50 . The torque output shaft  50  is connected to a differential and axle mechanism  56  which distributes torque to traction wheels  58 . The vehicle control system  12  controls each of the battery  14 , motor  16 , engine  18  and generator  32  to distribute torque to the wheels  58  in either the mechanical drive mode or the EV mode according to driver torque demands. 
     As previously described, there are two power sources for the driveline. The first power source is the engine  18 , which delivers torque to the planetary gear set  24 . The other power source involves only the electric drive system, which includes the motor  16 , the generator  32  and the battery  14 , where the battery  14  acts as an energy storage medium for the generator  32  and the motor  16 . The generator  32  may be driven by the planetary gear set  24 , and may alternatively act as a motor and deliver power to the planetary gear set  24 . 
     It should be understood that while a power-split powertrain is illustrated in the vehicle  10 , the vehicle  10  can include many other configurations. As such, it is contemplated that individual components of the powertrain may differ to suit various particular applications. For example, in another configuration that does not include a planetary gear set  24 , an electric machine (motor/generator) can be provided to operate as a generator by receiving torque from the engine or regenerative braking, while the same electric machine can also operate as a motor by receiving power from the traction battery and providing torque through the transmission. Other vehicle configurations of vehicle powertrains and implementations of electric machines are contemplated, and are therefore considered to be within the scope of the present disclosure. 
     Referring to  FIG. 2 , a block diagram illustrating a vehicle control system  12  within the vehicle  10  is shown. A driver inputs a request  62 , such as pressing the accelerator to input an acceleration request or pressing the brake pedal to input a braking request, for example. The driver inputs  62  are received by a vehicle system controller (VSC)  64 . The VSC  64  processes these driver inputs  62  and communicates commands throughout the vehicle  10 . 
     The vehicle control system  12  may be electrically connected to with various subsystems in the vehicle  10  and acts as an overall controller of the vehicle  10 . The VSC may be electrically connected to and communicate with various subsystems through a vehicle network  65 . The vehicle network  65  continuously broadcasts data and information to the vehicle-based systems. The vehicle network  65  may be a controlled area network (CAN) bus used to pass data to and from the VSC  64  and other various controllers or subsystems or components thereof. For example, as shown in  FIG. 2 , the VSC  64  can be connected to a hybrid control unit (HCU)  66 , a battery control module (BCM)  72  and an engine control unit (ECU)  68  through the vehicle network  65 . 
     The HCU  66  that controls the hybrid-specific components in the vehicle  10 , such as the motor  16 , the generator  32 , the battery  14  and/or the inverter  15 . The HCU  66  is communicatively connected to the ECU  68  such that the HCU  66  may command the ECU  68  to control the engine  18  in various manners. A battery control module (BCM)  72  may also communicate with the HCU  66 . The BCM  72  may receive commands from the HCU  66  and controls the power distribution of the battery  14 . 
     The HCU  66  is also communicatively connected to a motor/generator control unit (MGCU)  70 . The MGCU  70  communicates with the HCU  66  through a Serial Peripheral Interface (SPI)  71 . SPI  71  is four-wire serial bus. The SPI  71  is an extremely simple hardware interface and is not limited to any maximum clock speed, thus enabling potentially high throughput. The MGCU  70  receives commands from the HCU  66  and controls the motor  16 , the generator  32 , and VVC  60 . As further illustrated in  FIG. 2 , the MGCU  70  is communicatively connected to inverter  15 . The motor/generator inverters  15  receive commands from the MGCU  70  and open and close switches within the inverter  15  to enable and disable power flow to and from the electric machines. 
     Previous hybrid-electric vehicles used one control module to control the motor, generator and VVC. Within the control module, one microcontroller was used to control the motor and another microcontroller to control the generator while a third controller controlled the VVC. However, it was found to be difficult to control the VVC when it was separated from the motor/generator and it was found to be too slow in conveying information from the motor or generator to the VVC controller in the HCU. Therefore, it is advantageous to control the VVC, the motor, the generator and the respective inverters from one controller such as the MGCU shown in  FIG. 2 . 
     A hierarchy of controllers is thus provided in the illustration shown in  FIG. 2 . Other hierarchies of controllers are contemplated without deviating from the scope of the present disclosure. For example, the VSC  64  may directly communicate with the MGCU  70  without the presence of an HCU  66 . Other configurations are contemplated that would be beneficial for different particular vehicle architectures. 
     The vehicle control system  12  controls each of the controllers, according to requested torque and power demands. It should again be understood that more or less controllers than those described herein are contemplated, and one or more of these controllers can communicatively cooperate to accomplish certain tasks. Any and all of these controllers or combination thereof can simply be referred to as a “controller”. 
     Referring now to  FIG. 3  and  FIG. 4 , a schematic diagram of a portion of hybrid-electric vehicle  10  and vehicle control system  12  is described in more detail. As previously discussed, the VVC  60  is communicatively connected and controlled by the MGCU. Further, the VVC  60  is connected to the motor/generator inverter. Specifically, the VVC  60  is used to boost the battery  14  voltage to a higher level voltage in a HEV drivetrain system for multi-purposes such as, but not limited to, torque capability optimizations for electric machines and system loss optimization. 
     The battery  14  is connected to the VVC  60  along an input side  76 . The battery  14  supplies a low voltage to the VVC  60 . The VVC  60  then boosts the low voltage from the battery  14  into a higher voltage and outputs the higher voltage to an output side  78 . The output side  78  of the VVC  60  is supplying the high voltage to the high voltage bus  36  for use by the inverter  15  and subsequently the motor  16  and generator  32 . As shown in  FIG. 3 , the motor  16  and generator  32  may each have a separate inverter  15 . While the VVC  60  is described as having an input side and an output side, it should be noted that in a motoring mode, the path flows from the battery through the VVC to the high voltage bus. Conversely, in a regeneration mode, the path is reversed. 
     A sensor  80  is located between the battery  14  and VVC to measure to the voltage signal along the input side  76  of the VVC  60 . More specifically, the sensor  80  provides a voltage signal indicating the voltage from the battery  14 . A second sensor  82  is located along the output side  78  between the VVC  60  and the inverters  15 . The sensor  82  provides a signal indicating voltage from the high voltage bus  36 . The sensors  80 ,  82  provide a signal indicative of the measured voltage along the input side  76  and the output side  78  respectively. During normal operating conditions, the measured voltage signal is from the sensors  80 ,  82  is within an appropriate specified range. However, if the measured voltage signal from the sensors deviates from the appropriate specified range, this may indicate that a fault has occurred or that one of the sensors  80 ,  82  has failed. 
     Referring to  FIG. 4 , a schematic diagram of the VVC  60  circuitry is illustrated. The VVC  60  generally consists of an inductor  84 , two power switches  86  and  88 , and related gate drive circuits  90  as shows in  FIG. 4 . The two power switches  86 ,  88  are composed of an insulated-gate bipolar transistor  92 ,  94  and an anti-parallel diode  96 ,  98 . As shown in  FIG. 4 , the switches are arranged as an upper switch  86  and lower switch  88 . 
     The circuitry arrangement of the VVC allows for bi-directional power flow depending on the vehicle requirements, such as motoring or regeneration, for example. For example, when the upper switch  86  is closed and the lower switch  88  is open, power flows in one direction through the anti-parallel diode  96 . Similarly, if the upper switch  86  is open and the lower switch  88  is closed, power flows in one direction through the anti-parallel diode  98 . However, when both the upper switch  86  and lower switch  88  are closed, bi-directional power flow occurs and generates voltage boost. The boost voltage generated is output to the inverter  15  which controls the motor  16  and generator  32 . As discussed previously, by allowing voltage boost through the VVC  60 , the vehicle may be able to have a smaller battery pack, thereby saving cost, and battery packaging space, for example. 
     Certain fault conditions can be detected by one or more of the controllers that may indicate a fault in one of the powertrain components, such as the motor  16 , the generator  32 , the VVC  60  or the inverter  15 . When a fault in one of the components is detected, a limited operation strategy (LOS) can be implemented to enable the operator of the vehicle to continue to drive while certain individual components are disabled. This prevents complete shutdown of the vehicle  10 , which can be undesirable to drivers. The fault conditions that may cause the vehicle  10  and/or vehicle control system  12  to enter a LOS mode can include temperature, current and/or voltage of a powertrain component being outside an acceptable threshold. A fault condition may be caused by a transient event and may be only temporary; however, a reading of a value outside a threshold can cause the vehicle control system  12  to command an individual shutdown of that component, while commanding a LOS mode to allow the operator of the vehicle  10  to continue driving. 
     Referring now to  FIG. 5 , one embodiment of the LOS mode is illustrated at  100 . A diagnostic is performed on each of the motor  16 , the generator  32 , the VVC  60  and the inverter  15  associated with each of the electric machines. The diagnostic determines if the LOS mode is necessary such that a temporary disabling of that component should be commanded. The MGCU  70  begins performing the diagnostic, as represented by block  102 . Next, as represented by block  104 , it is determined whether or not a fault condition exists in the motor and/or motor inverter. If there is such a fault condition, an LOS counter is increased, as represented by block  106 . The LOS counter can be a single digit counter or identification means. Once the LOS counter is increased by one, as represented by block  108 , a motor temporary disablement flag is flagged as TRUE. The motor is requested to be disabled, as represented by block  110 . The motor may be disabled by opening switches in the motor inverter or opening another switch associated with the motor. 
     If there is no fault condition in the motor and/or motor inverter, it is then determined whether the LOS counter is greater than zero, as represented by block  112 . If the LOS counter is not greater than zero, the motor temporary disablement flag is flagged as FALSE, and the motor is requested to be enabled, or continued to be enabled, as represented by block  116 . 
     However, if the LOS fault counter is greater than zero, then the LOS counter is decremented or decreased, as represented by block  118 . After the LOS counter has been reduced, it is determined whether the fault counter has reached zero, as represented by block  120 . If the LOS counter is zero, then the method proceeds to set the flag for motor disablement to FALSE, as represented by block  114 , and the motor is requested to be enabled, or continued to be enabled, as represented by block  116 . If the fault counter remains greater than zero, then the method proceeds again to disable the motor, as represented by blocks  108 ,  110 . Finally, the TRUE and FALSE flags are sent to the vehicle control system, as represented by block  122 . Based on the information sent to the vehicle control system, the vehicle can act according to the description provided with reference to  FIG. 6 . 
     By requiring that the fault counter is equal to zero, as represented by block  120 , the control system ensures that even if it is determined that there is no fault condition in the motor and/or the motor inverter, the motor will continue to be temporarily disabled for a period of time if the LOS fault counter is still above zero. This allows the diagnostic to continue to run multiple times while reducing the LOS fault counter each time the diagnostic is run, until the counter reaches zero. Multiple checks of the motor and/or the motor inverter are therefore conducted while the motor is disabled before re-enabling the motor if no fault condition is detected. 
     The diagnostic performed by the MGCU works to temporarily disable the motor in the event a fault condition is detected. While the motor is temporarily disabled, the vehicle operates in a temporarily reduced power mode. However, if the fault condition only exists for a short amount of time (e.g., under 1 second), the LOS mode be discontinued and the motor will be re-enabled quickly, thus reducing the disturbances felt by the operator of the vehicle. It should be understood that the entire diagnostic can be accomplished in less than one second, such as in twenty micro-seconds, and thus the time the motor is temporarily disabled may not be detected by the operator of the vehicle. 
     As shown in  FIG. 5 , the LOS mode  100  and the performing of diagnostics operation, as represented by block  102 , is performed for the generator and the VVC as well as the motor. The diagnostics are accomplished for the motor, the generator, the associated inverter and the VVC generally simultaneously such that a check for fault conditions is continuous in each component. The MGCU can thus temporarily disable any or all of the motor, the generator, or the VVC. It is contemplated that the diagnostics can also be accomplished for other components, such as the engine. 
       FIG. 6  illustrates a flowchart  200  of another embodiment of the LOS mode implemented by a controller, or the vehicle control system. As previously described, the MGCU sets a flag for either TRUE or FALSE to temporarily disable or enable the motor, as represented by blocks  108  and  114  respectively. The TRUE and/or FALSE flags are received by the vehicle control system from the MGCU, as represented by block  202 . If the flag is FALSE, then the vehicle control system commands the MGCU to return to the diagnostic check  102 , as represented by block  204 . 
     However, if the flag is TRUE, then a determination is made as to whether the motor has been temporarily disabled for at least a threshold time, as represented by block  206 . If the motor has been disabled for at least the threshold time, then the motor can be permanently disabled for the current key cycle, as represented by block  208 . In one embodiment, the threshold time may be approximately one second, such that the motor will be permanently disabled during the current key cycle if it has been temporarily disabled for at least one second. However, any suitable threshold time is contemplated and the threshold time may be variable depending on other factors. A key-cycle may also be referred to as a drive-cycle or power-cycle and is the time from when the vehicle is powered on, i.e. key-on, until the vehicle is shutdown during a key-off. During a new key cycle, the motor, or any faulted device such as the VVC or inverter, can be re-enabled, as will be described with reference to  FIG. 7 . 
     The algorithms described with reference to  FIG. 5  and  FIG. 6  provide for a diagnostics check of the motor, the generator, the VVC, or any other powertrain component. In short, if the particular powertrain component is detected to be operating under a fault condition, that component is temporarily disabled. Diagnostics continuously run on that component while it is temporarily disabled. If the component recovers from its fault condition or the fault was transient so that the component can operate under normal conditions within the time threshold, the component can be re-enabled. If, however, the component fails to recover from its fault condition within the time threshold, the component is permanently disabled during the current key cycle, and can only be re-enabled upon a new key cycle (e.g., turning the vehicle off and on). 
       FIG. 7  illustrates a flowchart  300  of another embodiment of the LOS mode implemented by a controller, or the vehicle control system. As represented by block  302 , the vehicle is requested to start, and a new key cycle is commanded. Initially the electric machines, including the motor, generator, as well as the VVC are disabled. A series of pre-start safety checks before initializing of the vehicle. 
     For example, the vehicle control system checks that the current sensor zeroing is complete, as represented by block  304 . The current sensor zeroing must be complete throughout the electric machines. The current sensors must have a zeroed reading while the current is zero in order to have accurate readings for when the current spikes during startup. Next, a self-test of the VVC is accomplished, as represented by block  306 . The VVC self test ensures that any faults within the VVC are detected and resolved. Also, a determination is made as to whether there are any torque failures present, as represented by block  308 . In other words, the available power and/or torque of the electric machines must be evaluated to determine whether any requested torque can be fulfilled by the electric machines. 
     The duty cycle commands provided to the electric machine are disabled or reset by the controller, as represented by block  310 . Resetting the duty cycle puts the electric machines in a safe mode, thus protecting the hardware. Only after the fault conditions are removed can the duty cycle commands be re-enabled, allowing the electric machine to be safely controlled. This is considered to be a “soft restart” in which no new key cycle necessary, rather than a “hard restart” in which the vehicle must be shut down. Finally, as represented by block  312 , any faults present in the hardware are determined before enabling the vehicle to start. 
     Once the pre-start safety checks are successfully completed, the vehicle starts and the electric machines can begin, as represented by block  314 . The electric machines are also fully enabled, and the vehicle can be driven. 
     During operation of the vehicle, the diagnostic algorithms described with reference to  FIG. 5  and  FIG. 6  are accomplished and represented as block  316 . The electric machines are continuously checked for faults such that any of them can be temporarily disabled, according to the methods previously described. 
     If a request to disable any of the electric machines is determined at block  316 , the electric machine is disabled, as represented by block  318 . In order to re-enable the electric machine, the controllers in the vehicle must accomplish a series of safety checks and safety processes before re-enabling the electric machine again at block  314 . The safety checks and processes allow the vehicle to continue driving and the electric machines to continue providing propulsion without having a key-cycle. 
     In one of the safety checks, as represented by block  320 , the controller determines whether a temporary disablement of the electric machine is still being required, as previously described in reference to block  110  of  FIG. 5 . If disabling in not being requested of the electric machines, the controller can determined whether any of the electric machines are requested to be in a shutdown or permanent disablement mode, as represented by block  322 . If the electric machines are not disabled, a torque fulfillment check is completed, as represented by block  324 . The torque fulfillment check is similar to the check performed with reference to block  308 . 
     Next, a power limiting and balancing check is completed, as represented by block  326 . In this check, the controller may determine whether a process is underway in which electrical power is limited to one of the electrical machines or that one of the electric machines does not have power or torque limit much greater than the other electrical machine. The power limiting mode will be described in more detail in  FIG. 8 . Finally, an over-current check is accomplished as represented by block  328 . The over current check determines whether any electric machine is supplied with or is outputting a current value over a given threshold. If all of the safety checks are satisfactory, the prestart/re-enabled checks at are completed starting again at block  302 , until the disabled electric machine is re-enabled at block  314 . 
       FIG. 8  illustrates a flowchart  400  of another embodiment of the LOS mode implemented by a controller, or the vehicle control system.  FIG. 8  describes a LOS mode where a power limiting mode is implemented when a fault condition is detected on one of the electric machines. In previous hybrid-electric vehicles, failures or fault conditions in a hybrid-electric vehicle drivetrain were difficult to mitigate without reducing vehicle performance while driving or stopping the vehicle completely and then required a key cycle to resume partial operation. A key cycle was necessary to properly maintain power balance with the faulted device. In a hybrid electric transmission, when a failure is detected on one of the devices, such as an electric machine, taking action on only the faulted device would result in a power imbalance and would lead to unstable performance and additional control-related faults. 
     To avoid power imbalance and unstable performance, flowchart  400  in  FIG. 8  describes the process where the vehicle enters the LOS mode upon failure of one of the electric machines while driving, and continuing to operate the hybrid electric drivetrain with the second electric machine without requiring a key cycle. The process described in flowchart  400  allows the control system to quickly balance power when one of the electric machines fails and allows the other electric machine to continue to provide propulsion to the vehicle. 
     Initially, as represented by block  402 , the control system detects a failure in a first hybrid-electric drivetrain device and disables the device in response to the fault condition. The failure may occur in one of the electric machines, or the associated inverters. In response to the fault condition and the disabled device, the control system initiates a power limiting mode, as represented by block  404 . Initially, the power limiting mode is conducted at a high speed execution rate. The high speed rate may be an execution rate of 100 microseconds, for example. 
     While still operating in a high speed execution rate, the control system temporarily disables the second device, as represented by block  406 . The VVC is also temporarily set to a bypass mode, as represented by block  408 . The bypass mode of the VVC allows the high voltage from the electric machines to be quickly dissipated to the low voltage input side of the VVC. A fault may also be displayed for the driver, in response to the failure, as represented by block  410 . 
     Once the VVC is in the bypass mode and the second device is temporarily disabled, the control system can initiate a power limiting mode, as represented by block  412 . The power limiting mode allows the control system to function at a lower speed execution rate. The lower execution rate may allow the control system to complete a more extensive diagnostic evaluation of the system. 
     The control system may initiate the power limiting mode after a threshold time so that the high voltage to dissipate and so that the devices are not threatened with over-voltage, which could cause more failures. The threshold time may be as short as 20 milliseconds, or any suitable threshold time that is enough time for dissipation of the high voltage. The control system initiates the lower speed execution rate so that additional diagnostics can be performed. 
     Once the low speed power limiting mode is initiated, the control system re-enables the second device that is not faulted. However, the second device is re-enabled in a torque limited mode, as represented by block  414 . In the torque limited mode, torque on the functional device is limited on the functional device based on vehicle operation. The maximum torque in the LOS mode is limited based on the following formula:
 
τ max =( I   max   ×V   battery )/ω
 
     In other words, the maximum torque in the LOS mode is limited based on the maximum allowable current of the high voltage bus in LOS mode multiplied by the voltage from the battery divided by the speed of the second device. In one embodiment, the maximum allowable current in the LOS mode is a fixed value, such as 150 Amps. The battery voltage can be variable. 
     Once the functional device is re-enabled in a functional mode, the control system checks to determine if the first device is still faulted or disabled, as represented by block  416 . If the first device is being requested to be disabled by the MGCU or the HCU, or continues to have a failure, a power limiting time counter is incremented, as represented by block  420 . However, if there is no fault and there is no disablement request from one of the controllers, the control system can exit the power limiting mode, as represented by block  424 . By exiting the power limiting mode, the control system also exits the low speed execution rate. 
     The control system then resets the power limiting time counter to zero, as represented by block  426 . Once the power limiting time counter is set to zero and the LOS mode is cleared, the control system can also re-enable the first and second devices, as represented by block  428 . Re-enabling the devices includes exiting any torque limiting mode and returning to normal function. 
     On the other hand, if the first device still has a faulted condition, or the MGCU or HCU are requesting that the device is disabled or set to an LOS mode, the control system determines if the power limiting time counter is greater than a threshold value, as represented by block  432 . 
     If the power limiting time counter has exceeded a threshold value, the low speed diagnostic mode is exited, as represented by block  434 . Then the torque limiting mode is permanently maintained for the functional device, as represented by block  436 . By maintaining the torque limiting mode, the device is permanently disabled. In some embodiments, the device may only be permanently disabled until a new key-cycle of the vehicle. The device may be set as permanently disabled for several reasons, including the method described in  FIGS. 5-7 . 
       FIG. 9  illustrates a flowchart  500  of another embodiment of the LOS mode implemented by a controller, or the vehicle control system. A high voltage battery signal, also known as the HVBATT signal, is received by the controller, such as the MGCU, as represented by block  502 . The high voltage battery signal is measured by the sensor along the input side of the VVC. Based on the high voltage battery signal provided by the sensor, the controller determines if the high voltage battery signal is valid, as represented by block  504 . The high voltage battery signal is valid if the signal is within an acceptable range. 
     If the HVBATT signal is valid, the controller continues to use then the high voltage battery signal, as represented by block  506 , and the VVC may function normally, such as where VVC provides voltage boost output to the inverters and electric machines, as described in  FIG. 3  and  FIG. 4 . 
     However, if the high voltage battery signal is outside of the acceptable range, the signal is determined to be invalid. When the high voltage battery signal is invalid, then the signal is set to a fault condition, as represented by block  508 . When the high voltage battery signal is set to the fault condition, the MGCU communicates with the HCU to determine whether there is an alternative signal available to provide the high voltage battery signal in order to prevent shut down or short of the electric machines and the vehicle. 
     As previously described, the HCU is able to communicate through the vehicle network, such as CAN. For example, the HCU is able to communicate with the BCM on the vehicle network to receive an alternate battery voltage signal from the BCM. The alternate battery voltage signal from the BCM may be a measured voltage that is measured within the BCM. Alternatively, the alternate battery voltage signal may be inferred from other battery readings in the BCM or other vehicle system controllers communicating with the vehicle network. 
     The MGCU determines if the alternate voltage signal provided from the vehicle network is valid, as represented by block  510 . The alternate battery voltage signal is considered to be a valid signal if the battery voltage is within an acceptable range. If the alternate battery voltage signal from the BCM is considered to be valid, then the alternate battery voltage signal is substituted in for the HVBATT, as represented by block  512 . By using the alternate voltage signal in place of the HVBATT, the VVC can continue to operate normally, as represented by block  514 . In normal operation, the VVC can provide voltage boost from the battery voltage on the input side to the inverter and electric machines on the output side. Therefore, with the on-the-fly substitution of the alternate signals, the electric machines can continue to operate normally, despite the faulted high voltage battery signal. 
     The vehicle may also display a fault for the driver, as represented by block  516 . The fault may be displayed as a wrench light informing the driver of the fault condition. The fault condition in the high voltage battery signal may be caused by a failure of the sensor. The display may indicate that the sensor needs to be replaced. 
     If the MGCU determines that alternate signal is not valid, the control system ignores the alternate signal, as represented by block  518 . The alternate signal may be invalid if it is beyond an acceptable range or threshold value, for example. If the alternate signal is invalid, it may indicate a secondary fault. 
       FIG. 10  illustrates a flowchart  600  of another embodiment of the LOS mode implemented by a controller, or the vehicle control system. A high voltage battery bus signal, also known as the HVDC signal, is received by the controller, such as the MGCU, as represented by block  602 . The high voltage bus signal is measured by the sensor along the output side of the VVC. Based on the high voltage bus signal provided by the sensor, the controller determines if the high voltage bus signal is valid, as represented by block  604 . The high voltage battery signal is valid if the signal is within an acceptable range. 
     If the HVDC signal is valid, the controller continues to use the high voltage bus signal, as represented by block  606 , and the VVC may function normally, such as where VVC provides voltage boost output to the inverters and electric machines, as described in  FIG. 3  and  FIG. 4 . 
     However, if the high voltage bus signal is outside of the acceptable range, the signal is determined to be invalid. When the high voltage bus signal is invalid, then the signal is set to a fault condition, as represented by block  608 . If the signal is considered invalid, the HVDC signal is set to fault and LOS mode is implemented to maintain function of the electric machines and to allow the operator of the vehicle to continue driving. 
     When the high voltage bus signal is set to the fault condition, the MGCU tries to substitute the high voltage battery signal, HVBATT from the input side of the VVC, for the high voltage bus signal. The MGCU determines whether the high voltage battery signal is valid, as represented by block  610 . It is contemplated that the controller may substitute the any HVBATT signal, such as the high voltage battery signal measured by the sensor or the alternate battery voltage signal provided to the MGCU from CAN via the HCU, as discussed above in  FIG. 9 . 
     As discussed previously, the high battery voltage signal is considered to be a valid signal if the battery voltage is within an acceptable range. If the high voltage battery voltage signal is considered to be valid, then the HVBATT, is substituted for the high voltage bus signal, as represented by block  612 . 
     When using the high voltage battery signal in place of the high voltage bus signal, the VVC can continues to operate but the VVC is set to a LOS mode, as represented by block  614 . In the LOS mode, the VVC is set to a bypass mode. In the bypass mode, the VVC is disabled from providing voltage boost, as represented by block  616 . The vehicle may also display a fault for the driver, as represented by block  618 . Again, the fault may be displayed as a wrench light informing the driver of the fault condition. The fault condition in the high voltage bus signal may be caused by a failure of the sensor. The display may indicate that the sensor needs to be replaced. 
     If the MGCU determines that HVBATT signal is not valid, the control system ignores the alternate HVBATT signal, as represented by block  620 . The alternate signal may be invalid if it is beyond an acceptable range or threshold value, for example. If the alternate signal is invalid, it may indicate a secondary fault in the HVBATT signal. 
     It should be understood that, while references have been made to disabling and enabling the motor, similar algorithms are contemplated to apply to the generator, the inverter and the VVC. In other words, if a fault condition is present in any of the motor, the generator, the inverter or the VVC, the methods described above can apply to any of these components and other powertrain components. 
     The processes, methods, or algorithms disclosed herein can be deliverable to/implemented by a processing device, controller, or computer, which can include any existing programmable electronic control unit or dedicated electronic control unit. Similarly, the processes, methods, or algorithms can be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on non-writable storage media such as ROM devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, CDs, RAM devices, and other magnetic and optical media. The processes, methods, or algorithms can also be implemented in a software executable object. Alternatively, the processes, methods, or algorithms can be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software and firmware components. 
     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.