Method and apparatus for adaptive control of traction drive units in a hybrid vehicle

A hybrid electric vehicle having an energy generation system, an energy storage system and at least one, and preferable two or more, traction drive units includes a controller for controlling operation of vehicle systems. The controller may adaptively control traction by one or more traction drive units to better propel the vehicle. The controller may, for example, prevent unnecessary wheel slip, allow a traction profile to match a desired profile, and may be used to assist in turning of the vehicle. A method of adaptive control is also provided.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to methods and apparatus for adaptively controlling two or more traction drive units installed in a vehicle, including relative direction, torque, speed, power or other operational states.

2. Description of Related Art

The desire for cleaner air has caused various federal, state and local governments to adopt or change regulations requiring lower vehicle emissions. Furthermore, elevated fuel costs prompt consumer action to obtain vehicles for personal or business operations that consume less fuel or operate more efficiently.

Electric vehicles have been developed that produce zero emissions. Electric vehicles are propelled by an electric motor that is powered by a battery array on board the vehicle. The range of electric vehicles is limited as the size of the battery array which can be installed on the vehicle is limited. Recharging of the batteries can only be done by connecting the battery array to a power source. Electric vehicles are not truly zero emitters when the electricity to charge the battery array is produced by a power plant that bums, for example, coal.

Hybrid electric vehicles have also been developed to reduce emissions. Hybrid electric vehicles include an internal combustion engine and at least one electric motor powered by a battery array. In a parallel type hybrid electric vehicle, both the internal combustion engine and the electric motor are coupled to the drive train via mechanical means. The electric motor may be used to propel the vehicle at low speeds and to assist the internal combustion engine at higher speeds. The electric motor may also be driven, in part, by the internal combustion engine and be operated as a generator to recharge the battery array.

In a series type hybrid electric vehicle, the internal combustion engine is used only to run a generator that charges the battery array. There is no mechanical connection of the internal combustion engine to the vehicle drive train. The electric traction drive motor is powered by the battery array and is mechanically connected to the vehicle drive train.

Conventional internal combustion engine vehicles control propulsion by increasing and decreasing the flow of fuel to the cylinders of the engine in response to the position of an accelerator pedal. Electric and hybrid electric vehicles also control propulsion by increasing or decreasing the rotation of the electric motor or motors in response to the position of an accelerator pedal. Electric and series type hybrid electric vehicles may be unable to accelerate properly if the power available from the battery or batteries and/or genset is insufficient.

Conventional internal combustion engine vehicles may also include systems to monitor the slip of a wheel or wheels to thereby control the internal combustion engine and/or the brakes of the vehicle to reduce the slip of the wheel or wheels.

SUMMARY OF THE INVENTION

The invention provides methods and apparatus for adaptively controlling the operation of one or more traction drive units in a hybrid vehicle.

In hybrid electric vehicles, it is necessary to control the speed and torque of the electric motor or motors to control the slip of the wheels. According to aspects of the invention, the control of the speed, power, direction and torque of the traction drive units (when more than one is employed) allows for the addition of a range of functionality and benefits, including traction control, tight turning, reduced output operation, and others.

An exemplary embodiment of a hybrid electric vehicle according to aspects of the invention includes an energy generation system, an energy storage system receiving power at least from the energy generation system, and at least one, preferably two, traction drive units receiving power from the energy storage system. The vehicle is adaptively controlled so that the operation of each of the at least one traction drive unit may be specified as a result of conditions of various vehicle inputs and external inputs and of system states and conditions.

According to an exemplary embodiment, a method for determining the control of the traction drive units of a hybrid electric vehicle having an energy generation system, an energy storage system receiving power at least from the energy generation system, and at least two traction drive units receiving power from the energy storage system, consists of comparing the states of the various traction control units, monitoring the conditions of various vehicle and external inputs and of system states and conditions, determining the control of each of the various traction control units, and generating commands based upon the determined controls to operate the traction drive units in accordance with the parameters of the determined control method.

Other features of the invention will become apparent as the following description proceeds and upon reference to the drawings.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring toFIG. 1, an exemplary embodiment of a hybrid electric vehicle10according to the invention includes a plurality of wheels11,12,13, and14and a vehicle chassis15. The wheels13and14are coupled to traction drive units50and60, respectively, through gear boxes52and62, respectively. In the exemplary embodiment, these traction drive units50and60may be electric drive motors, however, other devices capable of producing tractive effort, such as hydrostatic-type drives, may be used. The wheels13and14are independently mounted to respective suspension components, such as swing arms. In this embodiment, the wheels13and14are not coupled together by an axle. In other embodiments, the wheels13and14may be coupled together, for example, by an axle.

The wheels13and14may be either the front wheels or the rear wheels of the vehicle10. In this embodiment, the wheels11and12are not driven and may be coupled together by an axle. In other embodiments, the wheels11and12may be driven.

Four wheel speed sensors11′–14′ are provided for sensing the rotational speed of each wheel11–14, respectively. Any known or subsequently developed sensor may be used. For example, speed sensors11′–14′ could be inductive-type pickup sensors, such as commonly used in vehicle anti-lock braking systems to detect wheel speed.

In an exemplary embodiment of the vehicle according to the invention, the vehicle10is a bus having an occupancy capacity in excess of100. However, it should be appreciated that the vehicle may be a bus of a smaller capacity or that the vehicle may be a smaller passenger vehicle, such as a sedan. In various exemplary embodiments, the vehicle may be any size and form currently used or later developed.

The traction drive units50and60are powered by an energy storage device500, such as a battery array30, and are controlled by drive motor controllers51and61, respectively. It will be appreciated that other energy storage devices, such as ultracapacitors, flywheels, and the like might be employed alone or in combination in the energy storage device500. According to an exemplary embodiment of the vehicle10, the traction drive units50and60are synchronous, permanent magnet DC brushless motors. Each electric drive motor is rated for 220 Hp and 0–11,000 rpm. The maximum combined power output of the electric drive motors is thus 440 Hp. However, this invention is not limited to permanent magnet DC brushless motors, and other types of electric drive motors, such as AC induction motors, or other types of traction drives can be used.

The hybrid electric vehicle10is preferably a series type hybrid electric vehicle that also includes an energy generation device400, which in an exemplary embodiment may include an internal combustion engine300and a generator310that is driven by the internal combustion engine300. The internal combustion engine300may be powered by gasoline, diesel, or compressed natural gas. It should be appreciated, however, that the internal combustion engine300and generator310may be replaced by a fuel cell, turbine or any other number of alternatives for creating usable electric power. According to an exemplary embodiment of the invention, the internal combustion engine300may be a 2.5 liter Ford LRG-425 engine powered by compressed natural gas. The engine300is operated to produce 70 Hp. It should be appreciated that the power of the engine300may be increased by increasing the RPM of the engine300and decreased by decreasing the RPM of the engine300. Other internal combustion engines can of course be utilized.

In this exemplary embodiment, the generator310is a DC brushless generator that produces, for example, 240–400 VAC. In an exemplary embodiment of the vehicle10, the generator is operated to produce 345 VACduring certain drive modes. An output shaft of the internal combustion engine300is connected to the generator310and the AC voltage of the generator310is converted to a DC voltage by a generator controller320. However, this invention is not limited to permanent magnet DC brushless generators, and other types of electric generators, such as AC induction generators, or other types of generators can be used. The converted DC voltage charges the energy storage device500. The energy storage device500may include, for example, 26 deep cycle, lead-acid batteries of 12 volts each connected in series. It should be appreciated, however, that other batteries, such as nickel cadmium, metal hydride or lithium ion, or that other energy storage devices, such as capacitors, ultracapacitors, or flywheels may be used and that any number of batteries or other devices may be employed, as space permits. Depending upon the load on the vehicle10, the battery array voltage ranges between 240 and 400 VDC.

In this exemplary embodiment, an electronic control unit (ECU)200includes a programmable logic controller (PLC)210and a master control panel (MCP)220. The MCP220receives input from various sensors and provides the connection to outputs in the vehicle10and the PLC210executes various programs to control, for example, the energy generation device400, the energy storage device500, the traction drive units50and60, and the motor controllers51and61.

Although not shown in the drawings, the vehicle10includes a cooling system or cooling systems for the energy generation device500, the energy storage device400, and the traction drive unit controllers51and61. The cooling system may be a single system which includes a coolant reservoir, a pump for pumping the coolant through a heat exchanger such as a radiator and a fan for moving air across the heat exchanger or a plurality of cooling systems similarly constructed. The ECU200controls the cooling systems, including the pumps and the fans, to perform a heat shedding operation in which the heat generated by the engine300, the controllers320,51, and61, the energy storage device500, and various other systems is released to the atmosphere. Any acceptable means and methods for cooling the vehicle components may be utilized.

As shown inFIG. 2, the coils of the generator310are connected to the generator controller320by leads311,312, and313. The generator controller320includes two switching insulated or isolated gate bipolar transistors (IGBT)330per phase of the generator310and their corresponding diodes. In an exemplary embodiment including a three phase generator310, the generator controller320includes 6 IGBT330and six corresponding diodes.

The PLC210controls each IGBT330of the generator controller320to control the conversion of the AC voltage of the generator310to the DC voltage for charging the battery array30. The PLC210may switch one or more of the IGBT330's off when the SOC of the battery array30reaches an upper control limit, to stop the conversion of the AC voltage to DC voltage and prevent overcharging of the battery array30.

According to an exemplary embodiment of the invention, the engine300runs continuously during operation of the vehicle10and continuously turns the shaft315of the generator310. The PLC210switches each IGBT330on and off via high speed pulse width modulation (PWM) to control charging of the battery array30. It should be appreciated however that the PLC210may control the charging of the battery array30by turning the engine300on and off, or in the alternative, by changing the RPM's of the engine300.

A possible control circuit for the traction drive units50and60is illustrated inFIG. 3, and includes the motor controllers51and61. The motor controllers51and61receive power from the battery array30and distribute the power to the traction drive units50and60, by switches B1–B6of pulse width modulation (PWM) inverters54and64. The PWM inverters54and64generate AC current from the DC current received from the battery array30. The battery current IBis distributed by the switches B1–B6, for example IGBT, of the PWM inverters54and64into motor currents I1, I2, and I3for driving the motors50and60.

The motor controllers51and61distribute the battery current IBvia the switches B1–B6by factoring feedback from position sensors53and63and encoders56and66that determine the timing or pulsing of electromagnets of the motors50and60. The pole position sensors53and63determine the pole positions of the permanent magnets of the motors50and60and the encoders56and66determine the phase angle. It should be appreciated that each pair of pole position sensors53and63and encoders56and66, respectively, may be replaced by a phase position sensor and the phase change frequency may be read to determine the speed of rotation of the electric motors50and60.

The motor controllers51and61calculate the motor connector voltages U12, U31, and U23based on the rotary velocity and the known flux value of the motors50and60between the motor connectors. The operating voltage of the inverters54and64is then determined by the rectified voltages of the diodes of the switches B1–B6or by the voltage Ui of an intermediate circuit including a capacitor C. If the voltage Ui becomes larger than the battery voltage UB, uncontrolled current may flow to the battery array30. Voltage sensors55and65determine the voltage Ui and the motor controllers51and61compare the voltage Ui to the battery voltage UB. The motor controllers51and61activate the switches B1–B6to cause magnetizing current to flow directly to the motors50and60to avoid unnecessary recharging of the battery array30.

As shown inFIG. 3, each motor controller51and61receives control data from the ECU200through a controller area network (CAN). The ECU200can communicate with the various sensors and the motor controllers51and61by, for example, DeviceNet™, an open, global industry standard communication network.

Referring toFIG. 4, each motor controller51and61includes a control unit101including a field axis current and torque axis current detector102. The detector102calculates the torque axis current Itand the field axis current Ifof each motor50and60by executing a 3-phase, 2-phase coordinate transfer from the input of the pole position sensors53and64and the encoders56and66. The torque axis current Itand the field axis current Ifcalculated by the detector102are input to a field axis current and torque axis current control unit103. The current control unit103receives a field axis current reference value Ifreffrom a field axis current reference control unit104and receives a torque axis current reference value Itreffrom a torque axis current reference control unit105.

The reference control units104and105determine the current reference values Ifrefand Itrefby comparing a torque reference value Tref(which is determined by the position of an accelerator pedal of the vehicle) with the actual rotational velocity determined by an rpm calculator106that receives input from the encoders56and66. A 2/3 phase changer107receives input from a phase calculator108and calculates the 3-phase AC reference values by performing a 2-phase/3-phase coordinate transformation. A PWM control unit109generates a PWM signal by comparing the 3-phase reference values with a triangular wave signal which is input to the PWM inverters54and64.

Referring toFIG. 5, the relationship between the power generated, the power stored, and the power consumed over time, by the series hybrid electric vehicle10according to the invention will be explained.

Power is consumed from the energy storage device500by the traction drive units50and60during acceleration of the vehicle10to a cruising speed. As shown inFIG. 5, the vehicle10reaches cruising speed at time t1which corresponds to a peak power Ppeakof the traction drive units50and60. The peak power Ppeakthe traction drive units50and60is dependent on the driving mode (discussed below) of the vehicle10selected by the operator. In the exemplary embodiment of the invention in which the traction drive units are each 220 HP electric motors50and60, the peak power Ppeakconsumed by the electric motors50and60is 440 Hp.

The power consumption (traction effort) of the electric motors50and60during acceleration is represented by the curve below the horizontal axis and the area defined by the curve below the horizontal axis between the times t0and t2represents the total power consumption of the vehicle10during acceleration. In the event that the SOC of the energy storage device500(battery array30) is insufficient to achieve the cruising speed, the ECU200controls the motor controllers51and61to limit the peak power Ppeakthe electric motors50and60may draw from the energy storage device500(battery array30). After the vehicle10has accelerated to cruising speed, the traction effort of the electric motors50and60may be reduced between the time t1and the time t2, and the power consumption by the electric motors50and60may also be reduced.

The cruising speed of the vehicle10is maintained between the time t2and the time t3. In this embodiment, during the time between t2and t3, the energy generation device400is operated to produce power Pgenhigher than the power consumption (traction effort) of the electric motors50and60necessary to maintain the vehicle's cruising speed. The differential in power between the traction effort and the power generated Pgenis stored in the battery array30.

The power Pgengenerated by the energy generation device400in this embodiment is dependent on the rpm of the engine300and a user demand signal sent to the energy generation device400that is controlled by the ECU200. The ECU200controls the engine300to generally maintain the rpm of the engine300, and the power generated Pgen, constant. However, it should be appreciated that the ECU200may control the engine300to reduce or increase the rpm of the engine300, and thus the reduce or increase, respectively, the power generated Pgen.

The power generated Pgenby the energy generation device400may be reduced if the SOC of the energy storage device500approaches an upper control limit at which the energy storage device500(battery array30) may become overcharged. The power generated Pgenby the energy generation device400may be increased if the SOC of the energy storage device500(battery array30) approaches a lower control limit at which the battery array30would be unable to drive the electric motors50and60with enough torque to propel the vehicle10. In an exemplary embodiment of the vehicle10in which the engine300is a 2.5 liter Ford LRG-425 engine powered by compressed natural gas, the power generated Pgenis 70 Hp.

Regenerative braking occurs between the times t3and t4when the vehicle10decelerates after release of the accelerator pedal or when the vehicle10travels on a downhill slope at a constant speed. During regenerative braking, the traction drive units50and60function as generators and current is supplied to the energy storage device500, such as battery array30, by the traction drive units50and60. The power generated Pbrakingduring regenerative braking is stored in the energy storage device500.

The power generated by the energy generating device400during maintenance of the cruising speed and the power generated by regenerative braking Pbrakingis represented by the curve above the horizontal axis and the area A2defined by the curve above the horizontal axis represents the total energy creation and storage of the vehicle10during maintenance of the cruising speed and regenerative braking.

The power Pgenof the energy generation device400and the regenerative braking power Pbrakingare controlled by the ECU200to substantially equal the energy consumption (traction effort) of the traction drive units50and60during acceleration. In other words, the area A1defined by the curve below the horizontal axis is equal to the area A2defined by the curve above the horizontal axis. The ECU200controls the traction effort of the traction drive units50and60(including the peak power Ppeak) and the power generated Pgenso that the power generated and the power stored do not exceed the power consumed, and vice versa, so as to maintain the SOC of the energy storage device500(battery array30) within a range of control limits. The ECU200controls the power generated Pgenand the traction effort of the traction drive units50and60so that the ampere hours during energy consumption do not exceed the thermal capacity of the energy storage device during power creation and storage.

As discussed above, in certain operational modes, the energy generation device400operates to produce power greater than the power consumption of the traction drive units50and60. In various exemplary embodiments, the power output by the energy generation device400declines as the SOC of the energy storage device500approaches a high level SOC. The energy storage device500is not fully charged, but managed to a SOC level predetermined to maximize the battery life and to accommodate the power requirements of the electric drive motors50and60. Thus, it should be appreciated that the energy storage device500can be maintained at any SOC level less than the maximum SOC level. By keeping the energy storage device500at less than the maximum SOC, the energy storage device500is less likely to experience mechanical or thermal failure due to overcharging.

Furthermore, the ECU220can determine the SOC of the battery array30over a period of time to determine if there are any trends in the SOC level. The trend can be an overall reduction, increase, or maintaining of the SOC of the energy storage device500over a predetermined period of time. The ECU220can then adjust the energy requirement of the energy generation device400accordingly.

An exemplary method and embodiment for adaptively controlling the state of charge SOC of the energy storage device500is disclosed in U.S. Pat. No. 6,333,620, the contents of which are hereby incorporated by reference herein in its entirety.

The control of the electric drive motors is accomplished by sending a command signal to the electric drive motor controller unit attached to each drive motor. The ECU200uses a control algorithm such as the exemplary embodiment described below to determine the appropriate command to deliver to the motor controller. Depending upon the type and design of the motor controller device, the command signal supplied to the motor controller may be a torque command signal, a speed command signal, a position command signal, or other type of command signal to indicate the desired operation to the motor controller. An exemplary embodiment of a motor controller device and method of controlling an electric motor in torque, speed and position output is disclosed in U.S. patent application Ser. No. 20020096375, the contents of which are hereby incorporated by reference herein in its entirety.

Exemplary embodiments for controlling the hybrid electric vehicle10will be explained with reference toFIGS. 6–12. The subroutines illustrated inFIGS. 6–12may be automatically executed concurrently at predetermined times, intervals, or locations during operation of the vehicle10, by internal or remote signal to the ECU220, or executed manually.

Referring toFIG. 6, a left traction control subroutine for the electric motor50(left traction drive), in an exemplary embodiment in which the vehicle10is rear wheel drive, begins in step S700and proceeds to step S710where it is determined if the electric motor50is operating nominally. According to an exemplary embodiment of the invention, the electric motor50is determined to be operating nominally if the voltage and temperature of the electric motor50are within predetermined parameters. If the electric motor50is not operating nominally (S710: No), the control proceeds to step S720where a drive warning and/or faults are reset. The faults are error codes generated by the ECU200upon detection of abnormalities, such as a short circuit in an IGBT330or failure of an encoder56or66. The control then proceeds to step S730where it is determined if the electric motor50is operating nominally. If the electric motor is still not operating nominally (S730: No), the control proceeds to step S740where the electric motor50is shut down if required and torque is shifted to the right side by increasing the torque drive command to the electric motor60. The control then returns to the beginning in step S795.

If after resetting the drive warning and/or faults, it is determined that the electric motor50is operating nominally (S730: Yes), the control proceeds to step S750where it is determined if the electric motor60(right drive in the exemplary rear wheel drive vehicle10) is operating nominally. The electric motor60is determined to be operating nominally if the voltage and temperature of the electric motor60are within predetermined parameters. If the electric motor60is not operating nominally (S750: No), the control proceeds to step S760where torque is shifted to the left drive by increasing the drive to the electric motor50and increasing upper control limits of the torque and velocity of the electric motor50. The control then proceeds to step S770. If it is determined that the electric motor60is operating nominally (S750: Yes), the control proceeds directly to step S770.

In step S770, it is determined if adequate traction is maintained. Adequate traction is not maintained if excessive slippage is detected between a rear wheel13or14and a speed reference which is a value slightly higher than the speed of the front wheels11and12, which can be determined in various ways, such as by comparison of wheel speed sensors11′–14′. If adequate traction is not maintained (S770: No), the control proceeds to step S780where the drive to motors50and60is decreased until the speed of the wheels13and14matches the speed reference value. The control then returns to the beginning in step S795. If adequate traction is maintained (S770: Yes), the drives to the motors50and60are maintained in step S790. The control then returns to the beginning in step S795.

Referring toFIG. 7, a right traction control subroutine including steps S800–S895for the electric motor60(right drive) corresponds to the steps S700–S795of the left traction control subroutine shown inFIG. 6. The right drive is checked in steps S810and S830to determine if the electric motor60is operating nominally and the left drive is checked in step S850to determine if the electric motor50is operating nominally.

Referring toFIG. 8, another exemplary embodiment of a traction control subroutine for the vehicle10is described. The control begins at step S200and proceeds to step S210where the control examines the wheel speeds of the various wheel speed sensors11′–4′. The variable WHLSPD is set to the highest wheel speed value examined, and an average wheel speed AVGSPD is calculated from the remaining wheel speed inputs, not including the speed WHLSPD. In this embodiment, the value WHLSPD is the speed of the wheel most likely to be slipping, and AVGSPD represents the speed reference value of the vehicle. The control then proceeds to step S220. In step S220, it is determined if the wheel speed WHLSPD is greater than some percentage of the average wheel speed AVGSPD. In an exemplary embodiment, this value may be 105% of the average wheel speed AVGSPD. A value differential greater than some percentage would indicate a likely slipping wheel, and would be greater than that difference caused, for example, by an underinflated tire. If WHLSPD is greater than 105% of AVGSPD (S220: Yes) the control proceeds to S230. If WHLSPD is not greater than 105% of AVGSPD (S220: No) the control proceeds to S250and returns to the beginning. In step S230, the command to one or more of the traction drive motors50,60is reduced to allow traction to be regained and the wheel with WHLSPD to more closely match AVGSPD. The control then proceeds to S240, where it is determined if the value WHLSPD has been sufficiently reduced to be within some percentage of AVGSPD. In an exemplary embodiment, the value of WHLSPD is examined to determine if it is between 95% and 105% of the value of AVGSPD. If it is within 95% and 105% of the value of AVGSPD (S240: Yes) the control proceeds to step S250, where it returns to the beginning. If WHLSPD is not within the limit (S240: No) the control proceeds back to step S210, where WHLSPD is re-determined and the average AVGSPD is recalculated, re-starting the subroutine.

Referring toFIG. 9, an exemplary embodiment of a speed control routine for the vehicle10is described. The control begins at step S300and proceeds to step S310, where a vehicle speed reference value SPD is calculated by measuring the wheel speeds determined at the wheel speed sensors11′–14′ and computing an average value, with erroneous or false readings (due to errors, faults, wheel slippage or other captured events) eliminated from the average. The control then proceeds to step S320, where it is determined if the value SPD is less than the value MAXSPD. In an exemplary embodiment, the value of MAXSPD is determined by the selection of an operator input such as a switch that indicates the maximum desired speed range of the vehicle. It will be appreciated that many additional methods of determining the value of MAXSPD are available, including an external signal, a default limit of the traction drive unit, a preset value in controller memory, or the selection of a particular driving or performance mode of the vehicle10. An exemplary description of a method to select a driving or a performance mode is detailed in co-pending U.S. patent application Ser. No. 10/795,348 filed Mar. 9, 2004, the contents of which are hereby incorporated by reference herein in its entirety. If the value SPD is less than the value MAXSPD (S320: Yes) the control proceeds to step S340, where the current driving speed is maintained. The control then proceeds to step S350, where it returns to the beginning. If the value SPD is not less than the value MAXSPD (S320: No) the control proceeds to step S330, where the command to the drive motors is reduced to at or below the reference value MAXSPD. The control then proceeds back to step S310, where the vehicle speed reference SPD is re-calculated and the subroutine begins again.

Referring toFIG. 10, an exemplary embodiment of a special turning subroutine for the vehicle10is described. In an exemplary embodiment, the vehicle is rear wheel drive. The control begins at step S400and proceeds to step S405, where the turning mode and direction are determined. A special turning mode is determined by referencing the various input values supplied to the vehicle controller. In an exemplary embodiment, a tight turning mode might be selected by the triggering of a switch attached to the input shaft of the steering column of vehicle10. A switch triggered by the clockwise rotation of the steering column would indicate a right hand turn, a switch triggered by the counter-clockwise rotation of the steering column would indicate a left hand turn. Of course it will be appreciated that other inputs or combinations of inputs may be used to trigger a tight steering mode or other specialty steering modes, and that a triggered switch is only one possible indicator of a tight turning mode.

The control then proceeds to step S410, where it is determined if the turning mode is a tight left hand turning mode. If the turning mode is not a tight left hand turning mode (S410: No) the control proceeds to step S435. If a tight left hand turning mode is active, (S410: Yes) the control proceeds to step S415, where it is determined if the driving direction is forward. If the driving direction is forward (S415: Yes) the control proceeds to step S420, where the drive command to the drive motor50(left hand) is issued in reverse, proportional to the forward command issued to the drive motor60(right hand). The control then proceeds to step S465, where it returns to the beginning. If the driving direction is determined to not be forward (S415: No), the control proceeds to step S425, where it is determined if the driving direction is reverse. If the driving direction is reverse (S425: Yes) the control proceeds to step S430, where the drive command to the drive motor50(left hand) is issued in forward, proportional to the reversing command issued to the drive motor60(right hand). The control then proceeds to step S465, where it returns to the beginning. If the driving direction is determined to not be reverse (S425: No), the control proceeds to step S435.

Although an exemplary embodiment reverses the relative rotation of the left and right motors50,60, it is equally possible to achieve improved steerability of the vehicle by inducing a relative speed differential between the motors. This may be achieved by turning off one of the motors, with the other being driven, or by allowing one motor to be driven faster than the other so as to allow one corresponding driven wheel to rotate faster to facilitate turning of the vehicle10.

In step S435, it is determined if the turning mode is a tight right hand turning mode. If the turning mode is not a tight right hand turning mode (S435: No) the control proceeds to step S460. If a tight right hand turning mode is active, (S435: Yes) the control proceeds to step S440, where it is determined if the driving direction is forward. If the driving direction is forward (S440: Yes) the control proceeds to step S445, where the drive command to the drive motor60(right hand) is issued in reverse, proportional to the forward command issued to the drive motor50(left hand). The control then proceeds to step S465, where it returns to the beginning. If the driving direction is determined to not be forward (S440: No), the control proceeds to step S450, where it is determined if the driving direction is reverse. If the driving direction is reverse (S450: Yes) the control proceeds to step S455, where the drive command to the drive motor60(right hand) is issued in forward, proportional to the reversing command issued to the drive motor50(left hand). The control then proceeds to step S465, where it returns to the beginning. If the driving direction is determined to not be reverse (S450: No), the control proceeds to step S460. In step S460, the control has determined that neither a tight left hand turning mode, nor a tight right hand turning mode have been selected. The control then maintains the normal driving mode and operates the vehicle normally. The control then proceeds to step S465, where it returns to the beginning.

Again, although an exemplary embodiment reverses the relative rotation of the left and right motors50,60, it is equally possible to achieve improved steerability of the vehicle by inducing a relative speed differential between the motors. This may be achieved by turning off one of the motors, with the other being driven, or by allowing one motor to be driven faster than the other so as to allow one corresponding driven wheel to rotate faster to facilitate turning of the vehicle10.

Referring toFIG. 11, an alternative power shifting control subroutine for the electric motors50,60is provided. The process begins at step S500and proceeds to step S510where it is determined if the electric motor50(left side) and/or electric motor60(right side) is operating nominally. According to an exemplary embodiment of the invention, the electric motors50,60are determined to be operating nominally if the voltage and temperature of the electric motors50,60are within predetermined parameters. If any of the electric motors50,60are not operating nominally (S510: No), the control proceeds to step S520where a drive warning and/or faults are reset. The faults are error codes generated by the ECU200upon detection of abnormalities, such as an overtemperature condition or overspeed operation. The control then proceeds to step S530where it is determined if the electric motors50,60are operating nominally. If either electric motor is still not operating nominally (S530: No), the control proceeds to step S540where the malfunctioning electric motor (50or60) is shut down, if required, and torque is shifted to the other side (i.e., the other electric motor) by increasing the torque drive command to the other electric motor (50or60). The control then returns to the beginning in step S550.

If after resetting the drive warning and/or faults, it is determined whether the electric motors50,60are operating nominally (S530: Yes), the control proceeds to step S550where the control returns to the beginning.

If necessary, the control can call the traction subroutine beginning in step S200, as detailed above. The use of the traction control subroutine verifies that the increased torque delivered to either electric motor50,60does not allow the wheel to overcome friction and spin uncontrollably, causing a safety hazard.

It will be appreciated that in performing the process outlined in the flow chart ofFIG. 11, the individual electric motors50,60can be monitored in parallel or sequentially, in any order, in steps S510and S530.

It will be appreciated by those skilled in the art that the ECU can be implemented using a single special purpose integrated circuit (e.g., ASIC) having a main or central processor section for overall, system-level control, and separate sections dedicated to performing various different specific computations, functions and other processes under control of the PLC. The ECU also can be a plurality of separate dedicated or programmable integrated or other electronic circuits or devices (e.g., hardwired electronic or logic circuits such as discrete element circuits, or programmable logic devices such as PLDs, PLAs, PALs, DSPs or the like). The ECU can be implemented using a suitably programmed general purpose computer, e.g., a microprocessor, microcontroller or other processor device (CPU or MPU), either alone or in conjunction with one or more peripheral (e.g., integrated circuit) data and signal processing devices. In general, any device or assembly of devices on which a finite state machine capable of implementing the flowcharts shown inFIGS. 6-12and described herein can be used as the ECU. A distributed processing architecture can be used for maximum data/signal processing capability and speed.