Fault-tolerant operation of hybrid electric vehicle

A hybrid drive system has a battery and a combustion engine for energy sources. The system has a traction motor, a generator, a variable voltage converter (VVC), a motor inverter, a generator inverter, a bus coupling the VVC to the inverters, and a controller. The controller regulates engine speed, motor torque, and generator torque. The engine speed is determined according to a driver torque demand. In normal conditions, 1) the controller regulates the engine speed by modifying a generator torque command, and 2) the bus voltage is regulated using the VVC and battery. When the controller detects a fault in which the battery and VVC become unavailable for regulating the bus voltage, then the controller regulates a motor inverter power output to match a sum of a generator inverter power output and an estimated power loss of the inverters in order to regulate the bus voltage.

CROSS REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates in general to powersplit hybrid vehicle drive systems, and, more specifically, to operation of the drive after a fault occurs in the connection to or functioning of the battery and/or variable voltage converter.

One type of hybrid electric vehicle powertrain is the powersplit drive system of the type disclosed in U.S. Pat. No. 8,425,377 and U.S. Pat. No. 7,686,723, both incorporated herein by reference. The powersplit drive system has two sources of power. The first source includes an internal combustion engine and the second source is a combination of an electric motor, a generator, and a storage device such as a battery pack. Engine power is divided into two power flow paths at any generator speed and vehicle speed. Engine speed is controlled by the generator, whereby the engine speed can be decoupled from the vehicle speed within the allowed speed range of the generator. This mode of operation is called positive powersplit when the generator is generating electrical power using mechanical power input from the engine.

Because of the mechanical properties of the planetary gearset, the generator can distribute power to the planetary gearset to drive the vehicle. This mode of operation is called “negative powersplit”. The combination of a generator, a motor and a planetary gearset thus can be considered to have electrical continuously variable transmission (e-CVT) characteristics.

A generator brake can be activated so that engine output power is transmitted with a fixed gear ratio to the torque output side of the powertrain through a mechanical path only. The first power source can only produce forward propulsion of the vehicle since there is no reverse gear. The engine requires either generator control or application of a generator brake to transmit output power for forward drive. When the second power source is active, the electric motor draws power from the battery and drives the vehicle independently of the engine for both forward drive and reverse drive. In addition, the generator can draw power from the battery and drive against a one way clutch on the engine power output shaft to propel the vehicle in a forward direction. This mode of operation is called “generator drive mode”. A vehicle system controller coordinates the two power sources so that they work together seamlessly to meet a driver's torque demand without exceeding powertrain system limits. The vehicle system controller allows continuous regulation of engine speed for any given vehicle speed and power request. The mechanical power flow path provides efficient power delivery through the planetary gearset to the driveshaft.

By optimizing operation of the internal combustion engine (ICE) and by implementing regenerative braking, a hybrid electric vehicle with the powersplit architecture significantly improves fuel economy. Because of the large number of additional vehicle components (e.g., variable voltage converter (VVC), motor inverter, generator inverter, battery, motor, and generator), however, overall system cost is increased and additional concerns over vehicle reliability and durability are introduced.

Fault conditions that should be monitored for include electrical faults such as a malfunction of the VVC, the battery, the battery contactors (i.e., relays), or cables which interrupt the flow of electrical power into or out of the battery. Any of these malfunctions would block in ability of the VVC to regulate the voltage level on the high voltage (HV) bus between the VVC and the inverters. Consequently, a significant overvoltage could appear at the HV bus. To prevent such an overvoltage, detection of one of these faults has been used to trigger a shutdown of the vehicle drive system. It would be desirable to obtain a fault-tolerant operation wherein a shutdown of the vehicle drive is avoided during a VVC fault. It is further desirable to achieve such fault-tolerant operation without incurring a significant increase in system cost.

SUMMARY OF THE INVENTION

In one aspect of the invention, a hybrid drive system comprises a DC power source, an internal combustion engine, a traction motor, a generator, a variable voltage converter (VVC) selectably coupled to the battery, a motor inverter, a generator inverter, a bus coupling an output of the VVC to the inverters, and a controller regulating an engine speed, a motor torque, and a generator torque. The regulated engine speed is determined according to a driver torque demand. The controller detects a fault in which the battery and VVC are unavailable for regulating a bus voltage on the bus. Upon detecting the fault, the controller regulates a motor inverter power output to match a sum of a generator inverter power output and an estimated power loss of the inverters. As a result, the bus voltage is maintained at a reference level even though the battery and VVC are unavailable, and vehicle operation can continue using engine power alone.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring toFIG. 1, a hybrid electric vehicle10includes an internal combustion engine11and a transaxle12containing a traction motor13, generator14, and planetary gearset15. Motor13receives electrical power from a battery pack16via a VVC17and inverters18, and provides drive torque for vehicle propulsion under control of a transmission control module (TCM)20. Motor also functions as a generator for converting mechanical power into electrical power through regenerative braking.

Transaxle12has a powersplit configuration wherein motor13and generator14interact with ICE11via planetary gearset15which includes a sun gear21, a planet carrier22, and a ring gear23. Sun gear21is connected to an output shaft of generator14for receiving generator torque. Planet carrier22is connected to an output shaft of ICE11for receiving engine torque. Planetary gearset15combines the generator torque and the engine torque and provides a combined output torque at ring gear23. Transaxle12may also include a one-way clutch OWC and a generator brake24as known in the art.

Transaxle12includes a countershaft25having a first gear26, a second gear27, and a third gear28. Planetary output gear30is connected to ring gear23and meshes with first gear26for transferring torque between planetary gearset15and countershaft25. An output gear31is connected to an output shaft of motor13and meshes with second gear27for transferring torque between motor13and countershaft25. A transmission output gear32is connected to a transmission output shaft33. Output shaft33is coupled to a pair of driven wheels34through a differential35. Transmission output gear32meshes with third gear28for transferring torque between transaxle12and driven wheels34.

Battery16is a high voltage battery that is capable of outputting electrical power to operate motor13and generator14. Battery16also receives electrical power from motor13and generator14when they are operating as generators. Typically, battery16is a battery pack made up of several battery modules (not shown), wherein each battery module contains a plurality of battery cells (not shown). Other types of energy storage devices can also be used, such as capacitors and fuel cells (not shown) that supplement or replace battery16.

A battery energy control module (BECM)36receives input signals that are indicative of vehicle and battery conditions, such as battery temperature, voltage and current. BECM36calculates and estimates battery parameters, such as battery state of charge and the battery power capability. VVC17and inverters18are electrically connected between battery16and motor13and generator14. Depending on instantaneous operating mode of the drive system, VVC17pushes power from battery16to a high voltage (HV) DC bus for conversion by inverters18or from the HV DC bus to battery16, whereby the DC voltage on the bus is maintained within a desired range. TCM20controls motor13, generator14, VVC17, and inverters18.

Vehicle10includes a vehicle system controller (VSC)37that communicates with other vehicle systems and controllers for coordinating their function. Although it is shown as a single controller, the VSC37may include multiple controllers that may be used to control multiple vehicle systems according to an overall vehicle control logic, or software. VSC37communicates with other vehicle systems and controllers (e.g., BECM36and TCM20) over one or more communication buses (e.g., a CAN bus). VSC37receives a demand torque input (not shown) from a driver that represents an accelerator pedal position. VSC37provides various output signals (i.e., commands) for controlling driver operation, e.g., desired wheel torque, desired engine speed, and generator brake command. A contactor control signal determined the activation of battery contactor relays (not shown).

Vehicle10may include a braking system which has a brake system control module (BSCM)38that communicates with VSC37to coordinate regenerative braking. Vehicle10may further include an engine control module (ECM)39for controlling ICE11. VSC37provides a desired engine torque command to ECM39that is based on a number of input signals including the driver's demand for vehicle propulsion.

FIG. 2is an electrical schematic showing one preferred embodiment of an electric drive system useful with a powersplit hybrid drive. Battery16is coupled by contactor relay switches40and41to VVC17. A main linking capacitor42is connected to an output of VVC17establishing a positive bus43and a negative bus44. A motor inverter18A and a generator inverter18B are coupled between the buses. Inverters18A and18B are each comprised of a plurality of switching devices (such as insulated gate bipolar transistors, IGBTs) in a bridge configuration. Switches in inverters18A and18B and in VVC17are driven according to control signals from controller20in a conventional manner (e.g., using pulse width modulation).

FIG. 3shows normal power flow paths between the various elements of the powersplit powertrain. Power flow between various components is bi-directional, depending on an operating mode (e.g., regenerative braking). Engine power delivered to planetary gearset15can be split between generator14(e.g., for charging battery16or powering motor13) and intermediate shaft25(e.g., for rotating the wheels) by controlling the generator torque, for example. Motor torque may be controlled according to the generator torque and a driver's torque command (i.e., torque demand via an accelerator pedal). During normal (i.e., non-faulted) operating conditions, electrical power flows in to or out from battery16under control of the inverter system components (VVC17and inverters18). Using a measured voltage for the HV bus, the direction of power flow and the switching operation of VVC17are determined such that the HV bus voltage is regulated within a desired range.

Certain control operations utilized in a conventional drive system are shown in greater detail inFIGS. 4 and 5. Normal operation includes using a generator torque command to regulate engine speed, and a motor torque command is generated to meet the driver torque command. As shown inFIG. 4, electronic signals representing a driver torque command and a measured vehicle speed are provided to respective inputs of a multiplier45to produce a Driver Power Command which is coupled to an adding input of an adder47. A signal representing a battery state of charge (SOC) and the signals representing driver torque command and measured vehicle speed are provided to respective inputs of a battery SOC management block46which uses known methods to provide a Battery Power Request that is coupled to a subtracting input of summer47. The Driver Power Command represents a total power to be delivered to the vehicle wheels, and the output of summer47is an Engine Power Command. When battery SOC is high, then block46outputs a Battery Power Request which is positive (i.e., it is desired to utilize stored electrical power to propel the vehicle). When the Driver Power Command is low, then the vehicle may be driven using only battery power and the resulting Engine Power Command may be zero (i.e., engine off). At higher levels of Driver Power Command or when battery management results in a request for battery charging (i.e., Battery Power Request is negative), then the resulting Engine Power Command is positive. Using vehicle speed as an input, an engine optimization block48determines an Engine Speed Command that produces the desired engine power at a highest efficiency. In a divider50, the Engine Power Command is converted to an Engine Torque Command by dividing by a measured Engine Speed Feedback signal. The Engine Torque Command may be transmitted to an engine controller for use in regulating the engine.

FIG. 5illustrates a known manner of determining desired torques to be generated by the generator and motor in view of the engine torque and speed. Thus, the Engine Speed Command is coupled to a summing input of an adder51. The measured engine speed is coupled to a subtracting input of summer51. The error between the commanded and actual engine speed is input from summer51to an engine speed controller52which uses a conventional method for generating a Generator Torque Command (e.g., by modifying the Generator Torque Command, the engine speed is able to be regulated). The Generator Torque Command is used in a controller (e.g., the TCM) to control the generator inverter such that the desired generator torque is obtained. Engine speed controller52may be a proportional-integral controller, for example, and may be implemented within the TCM.

The Driver Torque Command is multiplied by a known gear ratio k2in an amplifier53and coupled to a summing input of an adder54. The Generator Torque Command and a generator speed feedback are input to a ring gear torque estimator55and a resulting torque estimate is multiplied by another known gear ratio k1in an amplifier56. The output of amplifier56is coupled to a summing input of adder54to produce the Motor Torque Command.

In the event that a fault occurs in which the battery and/or VVC become unavailable for regulating a bus voltage on the bus, it becomes necessary to cease use of the control strategy ofFIGS. 4 and 5.FIG. 6shows available power flow paths when the VVC is faulted. Battery16is no longer available to store excess power produced by generator14or motor13. When the VVC fault occurs, the normal control strategy creates a power mismatch between the motor inverter and the generator inverter. The resulting extra power from the generator would charge the HV DC link capacitor very quickly to an undesirable overvoltage (e.g., over 1000 V), which can trigger a system OverVoltage (OV) protection fault that causes a shutdown of the drive system.

To avoid an undesirable shutdown, the present invention employs a fault-tolerant control regime during such a VVC/battery fault which regulates the motor inverter output power such that it substantially matches the generator inverter output power. More particularly, a HV bus voltage balance can be achieved by matching the motor inverter power to the generator inverter output power plus estimates of power losses occurring in the motor inverter and generator inverter.

FIGS. 7 and 8illustrate respective changes made to the conventional control strategies ofFIGS. 4 and 5. InFIG. 7, a selector switch60couples either the Battery Power Request from SOC management block46or a value of zero to the subtracting input of adder47depending on the state of a Fault Detect signal. When the VVC fault is present then the battery is isolated (i.e., no power can flow in or out of the battery) and the battery request must be set to zero during the fault. InFIG. 8, a selector switch61outputs either the result from adder54or an output from a motor inverter DC balancing control62as the Motor Torque Command depending on the state of the Fault Detect signal.

Motor inverter DC balancing control62which controls the DC bus voltage by regulating the Motor Torque Command in shown in greater detail inFIG. 9. A desired reference voltage (Vac Ref) is subtracted from a measured value for the instantaneous voltage on the inverter bus (Vac Feedback) in an adder70. The difference (i.e., voltage error) is input to a DC voltage regulator71. Depending on the magnitude of the error (e.g., when it is outside a particular range around Vac Ref), regulator71generates a control signal which is coupled to an adding input of an adder72. Respective subtracting inputs of adder72receive an ISC Loss Estimate signal and a Generator Power Feedback signal. The Loss Estimate signal can be obtained from an empirically-derived lookup table based on various operating parameters of the drive system (e.g., magnitude of currents flowing in the inverters and the voltages present at the switching bridges to characterized the switching losses). The Generator Power Feedback signal can be measured at the output of the generator inverter. The output of adder72provides a Motor Power Reference which represents the magnitude of motor power consumption that consumes just enough of the excess power coming from the generator inverter to maintain the bus voltage within a desired voltage range. The Motor Power Reference is converted to a Motor Torque Command by dividing by a Motor Speed Feedback signal in a divider73.

FIG. 10shows a plots of the bus voltage using the conventional control strategies ofFIGS. 4 and 5. A VVC/battery fault occurs at a time ti. Since the bus voltage cannot be controlled by moving excess power into the battery, the bus voltage rises to unacceptable levels within a short time, and the vehicle drive system must be shut down. In contrast,FIG. 11shows a plots of the bus voltage using the improved control strategies ofFIGS. 7-9. A VVC/battery fault occurs at a time ti, but since the torque (i.e., power) generated by the generator is balanced by the torque load of the traction motor (plus the inverter power losses), the bus voltage is able to be maintained within the desired range. If sufficiently low, the inverter power losses can be ignored in some embodiments.

FIG. 12shows one preferred method of the invention. In step75, fault monitoring is performed. The monitored faults include any that may indicate that the battery and/or VVC are unavailable for regulating a bus voltage on the bus. A check is performed in step76to determine whether a VVC/battery-related fault has occurred. If not, then a controller for a powersplit hybrid drive system determines an engine speed command based on a driver torque demand, a vehicle speed, and a battery state of charge. An engine torque command is also determined in a conventional manner. In step78, a commanded generator torque is used to regulate a speed of the combustion engine (i.e., the determined engine speed determines the generator torque command). During this nominal operation wherein no faults are present, conventional operation of the VVC maintains a desired voltage on the high voltage bus in step79. In step80, a motor torque command is determined based on the driver demanded torque and the commanded generator torque.

If a VVC/battery-related fault is detected in step76, then the engine speed command and engine torque are determined in step81without regard to the battery state of charge. In step82, the commanded generator torque continues to be used to regulate the speed of the combustion engine. In this faulted condition, the voltage on the inverter bus is regulated to a predetermined reference voltage by commanding the motor torque to match the commanded generator torque plus estimated power loss of the inverters in step83. In particular, the method preferably regulates the motor inverter power output according to an error between the bus voltage and the predetermined voltage. The commanded motor torque command may be generated according to A) an error between the bus voltage and the predetermined voltage, B) a generator inverter power output, and C) optionally, according to the estimated power loss of the inverters.

In a preferred embodiment, the motor torque regulation is performed by a DC balancing control which comprises a voltage regulator providing a control signal according to a difference between the voltage at the inverter bus and the reference voltage whenever the bus is isolated from a vehicle battery. The DC balancing control includes a combiner which generates a motor inverter power target according to a difference between the control signal and a power output from the generator inverter. Consequently, a hybrid electric drive system can keep operating even under these fault conditions, which significantly decreases the severity of these faults and improves the system robustness and reliability. Moreover, this invention can be introduced without any added hardware components or cost.