Method for providing improved driveability for a vehicle

A method for controlling torque delivery in a vehicle powertrain using an enhanced limited operating strategy. The strategy is implemented when a powertrain controller fails to respond properly to a driver command for traction wheel torque whereby a modified wheel torque at vehicle traction wheels under driver control is made available.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a vehicle powertrain in which provision is made for delivering motive power to vehicle traction wheels in the event of a powertrain control signal failure.

2. Background Art

It is known design practice in the automotive industry to establish a limited power operating strategy for a vehicle powertrain in the event of a powertrain control malfunction. This involves the use of a vehicle creep mode during a so-called “quit-on-the-road” (QOR) event. Upon a loss of a driver-activated acceleration control signal for the powertrain, the powertrain creep mode of operation will permit the operator of the vehicle to maneuver the vehicle off a roadway. This operating strategy is known in the automotive industry as a limited operating strategy (LOS). It is initiated by a loss of signal, which may be identified by the same acronym.

The LOS drive mode is used in situations involving accelerator pedal faults and other faults in which the vehicle system controller will not respond to accelerator pedal movement by the vehicle driver. It is known practice to implement an LOS drive mode in conventional powertrain designs by commanding an engine throttle plate to a fixed throttle angle, thereby commanding a fixed limited power from the engine. This provides a limited wheel torque with a calibrated torque delivery delay.

In the case of a hybrid electric vehicle of the type described, for example, in U.S. Pat. No. 6,994,360, a conventional LOS drive mode is not available because of the powertrain architecture for that type of hybrid electric vehicle. Known methods for implementing an LOS drive mode in response to accelerator pedal signal faults or other similar faults in a hybrid electric vehicle powertrain control system require special strategies to improve the performance of the LOS drive mode.

SUMMARY OF THE INVENTION

The invention comprises what may be described as a “super-creep” strategy. It is designed to use a closed-loop control that would allow a vehicle to respond to a powertrain control failure by permitting the vehicle to be driven, for example, up roadway grades that would not be available using known LOS strategies that involve commanding the engine throttle plate to a fixed angle. The strategy of the present invention uses an estimated wheel torque and a torque feedback variable to determine an amount of feed-forward torque to be delivered to vehicle traction wheels in order to achieve a desired vehicle acceleration rate. It will allow the operator to control vehicle acceleration under a wide range of environmental or road conditions while providing smooth, predictable acceleration.

Unlike conventional LOS operation, which commands a “clipped” or limited amount of torque, the super-creep mode of operation of the present invention controls the amount of powertrain torque commanded to be delivered based on a feedback control using calculated vehicle acceleration rates. An acceleration rate request is converted to an output shaft torque request. The acceleration rate can be calibrated so that it will vary based upon vehicle speed.

The strategy of the present invention uses the brake pedal to provide a brake override feature. This feature will permit brake pedal application to cancel the torque commanded to be delivered by the powertrain during LOS operation. In this way, the driver will be able to maintain control of vehicle speed by using the brake pedal. For this reason, the strategy of the present invention will include an initial test of brake pedal input to verify that braking torque is available.

The super-creep strategy of the present invention, will limit maximum torque delivered to the powertrain to a calibratable limit. It provides a maximum torque clip based on a calibratable speed limit.

The super-creep strategy of the invention, once it is initiated, is non-recoverable so that the driver will not be surprised if the normal function of the control system will unexpectedly return to normal following LOS mode operation, when full torque would be delivered to the traction wheels. Normal function would return, if appropriate, during the next key cycle. Further, the strategy will reduce the chance of a wheel spin on road surfaces with a low friction coefficient. The strategy will monitor the acceleration rate so that the acceleration requested by the controller will not exceed a desired amount (e.g., 0.15 g). The control feedback feature of the invention will adjust the amount of torque required to move the vehicle at a calibratable acceleration.

In executing the strategy of the invention, an acceleration request by the driver will be converted to an output shaft torque request. A calibratable rate of acceleration based on vehicle speed will change for different speeds. After the acceleration request is converted to an output shaft torque request, it is sent to a torque control feature of the control system.

The invention may be applied to vehicles with a powertrain other than a hybrid electric vehicle powertrain of the type disclosed herein. For example, it may be applied to a vehicle with a powertrain powered only by a conventional internal combustion engine, by a fuel cell, by an electric motor and battery system, etc., wherein desired wheel torque is determined using electronic controls.

PARTICULAR DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

For the purpose of describing an operating environment of a hybrid electric vehicle powertrain which include a controller programmed to use the strategy of the present invention, reference will be made toFIG. 1. A power-split hybrid electric vehicle powertrain shown inFIG. 1includes an engine that functions as a first power source, and a second power source that includes at least one electric motor and high voltage battery. These power sources establish a mechanical power flow path and an electrical power flow path. A mechanical power flow path delivers engine power to vehicle traction wheels by controlling generator speed, whereby the powertrain may act in a manner similar to a continuously variable transmission where vehicle speed changes do not depend upon engine speed changes. The combination of the motor, the generator and the planetary gearing, illustrated inFIG. 1, define as an electro-mechanical, continuously-variable power flow path.

FIG. 1shows a vehicle system controller (VSC)10, a battery12and a transmission14, together with a motor-generator subsystem and a control area network (CAN). An engine16, controlled by controller10, distributes torque through torque input shaft18to transmission14.

The transmission14includes a planetary gear unit20, which comprises a ring gear22, a sun gear24, and a planetary carrier assembly26. The ring gear22distributes torque to step ratio gears comprising meshing gear elements28,30,32,34and36. A torque output shaft38for the transaxle is drivably connected to vehicle traction wheels40through a differential-and-axle mechanism42.

Gears30,32and34are mounted on a countershaft, the gear32engaging a motor-driven gear44. Electric motor46drives gear44, which acts as a torque input for the countershaft gearing.

The battery12delivers electric power to the motor through power flow path48. Generator50is connected electrically to the battery and to the motor in known fashion, as shown at52.

When the powertrain battery12is acting as a sole power source with the engine off, the torque input shaft18and the carrier assembly26are braked by an overrunning coupling53. A mechanical brake55anchors the rotor of generator50and the sun gear24when the engine is on and the powertrain is in a parallel drive mode, the sun gear24acting as a reaction element.

InFIG. 1, the vehicle system controller10receives a signal63from a transmission driver-actuated range selector (PRNDL). The signal is distributed to transmission control module67, together with a desired wheel torque, a desired engine speed and a generator brake command, as shown at71. A battery contactor or switch73is closed after vehicle “key-on” startup. The controller10issues a desired engine torque request to engine16, as shown at69, which is dependent on a sensor output65from accelerator pedal position sensor (APPS).

A brake pedal position sensor BPPS distributes a wheel brake signal61to the controller. The transmission control module issues a generator brake control signal to generator brake55. It also distributes a generator control signal to generator50.

As mentioned previously, there are two power sources for the driveline. The first power source is a combination of the engine and generator subsystems, which are connected together using the planetary gear unit20. The other power source involves only the electric drive system, including the motor, the generator and the battery, where the battery acts as an energy storage medium for the generator and the motor.

A typical vehicle may include a dual track accelerator pedal position sensor that will output a voltage based on accelerator pedal position.FIG. 1ashows at Track1and Track2the slope for each accelerator pedal position sensor track. The vehicle control system determines the final accelerator pedal position based on a minimum voltage for each track.

FIG. 1ashows a different slope for each track. The purpose of the differences in slope is to provide the vehicle operator with an indication that one of the sensors has failed. With the loss of one pedal position sensor, the vehicle may enter a limited operating mode. With the loss of two accelerator pedal position sensors, the vehicle will enter the LOS super-creep mode.

The present invention would be implemented, in the case of a hybrid electric vehicle powertrain of the type shown inFIG. 1, when both accelerator pedal position sensors fail and the track1relationship and the track2relationship are both unavailable. The present invention will avoid a so-called “quit-on-the-road” (QOR) event in which the vehicle is severely limited in operation. The super-creep will make available an improved limited operating strategy (LOS). Before the strategy of the present invention is implemented, several accelerator pedal LOS strategy entry conditions must be satisfied. These entry conditions will be described subsequently with respect toFIGS. 4, 5 and 6.

After the entry conditions are satisfied, a temporary estimated wheel torque is calculated as shown at80inFIG. 2. This is accomplished by multiplying the vehicle acceleration rate in miles per hour per second by the acceleration torque to wheel torque conversion factor in Newton-meters per miles per hour per second. The temporary estimated wheel torque is delivered to switch92, shown inFIG. 2. Switch92receives a vehicle speed input, as shown at86. If the vehicle speed signal is positive, then the temporary estimated wheel torque will be positive. On the other hand, if the vehicle speed signal is negative, then the temporary estimated wheel torque will be negative. If the vehicle speed is zero, then the temporary estimated wheel torque will be zero. The sign of the vehicle speed (plus or minus) becomes the sign of the temporary estimated wheel torque.

The temporary estimated wheel torque is transferred to a summing point98where the temporary estimated wheel torque, which can be plus or minus, is combined with a signal indicating mechanical rolling friction losses at the wheels, as shown at100. Those losses are calibrated using a known calibration technique.

The resultant temporary estimated wheel torque is transferred, as shown at102, to multiplier104, where a wheel torque to output shaft torque conversion factor106, which can be precalibrated, is multiplied by the temporary estimated wheel torque. The conversion factor would be the gear ratio of the vehicle axle assembly, including differential42inFIG. 1. If the axle ratio is one-to-one, the temporary estimated wheel torque, multiplied by the value of unity, will produce an unchanged output at104.

The final estimated wheel torque is passed through a low pass filter108, which acts as a buffer to eliminate transient torque peaks.

One of the driver inputs to the controller is a selection of reverse drive or forward drive. If the PRNDL position is reverse, the software will select an enhanced foot off pedal (FOP) calibratable look-up table or map. See, for example,FIG. 2b. If the PRNDL position is drive or low, the software will select a drive-enhanced foot off pedal (FOP) pedal map look-up table or map. See, for example,FIG. 2a. Each look-up table or map will deliver a corresponding torque value based on a corresponding vehicle speed.

FIG. 2aindicates that the maximum vehicle speed in the super-creep mode can be calibrated in a typical powertrain application. The torque progressively increases as the vehicle speed decreases, as shown at120inFIG. 2a. The corresponding map for reverse drive, shown inFIG. 2b, indicates that the maximum speed can be calibrated as shown in the look-up calibration table. As reverse drive speed approaches zero, the feed-forward torque becomes greater, as shown at122. The feed-forward torque developed at116or118is compared at summing point120to the estimated wheel torque to determine a torque error, as shown at122.

The torque error, a controller software loop delta time and an integral gain value for output shaft speed, shown at124, are combined at multiplier126to produce a torque feedback term, shown at128. The calibrated integral gain value for output shaft speed is obtained by a calibrated integral gain look-up table130, which has an output shaft speed input shown at132.

The torque feedback term at128is combined with the final torque feedback shown at134. This is indicated at summing point138. The feedback torque at134is clipped at136between a lower limit term and a feedback torque plus a feedback term upper limit after the feedback torque and the torque feedback term are combined at summing point138.

The feedback torque at134and a feed-forward torque shown at140, which is developed by the feed-forward torque look-up tables116and118, are added at summing point142to develop a final output shaft wheel torque.

The overall control strategy, which includes the strategy described with reference to the block diagram ofFIG. 2, is shown in the flow diagram ofFIG. 3. This overall strategy includes super-creep strategy entry conditions, which will be described with reference toFIGS. 4-6. After the entry conditions are satisfied, the strategy shown in the block diagram ofFIG. 2will be executed. This will be described with reference toFIGS. 7 and 8.

Referring first toFIG. 3, the overall control strategy routine for calculating output shaft wheel torque begins with an inquiry at step144as to whether the powertrain is conditioned by the operator for “park” or “neutral”. If the powertrain is conditioned for “park” or “neutral”, there is no need to execute the super-creep strategy of the invention. Thus, the output shaft wheel torque, the feed-forward torque, the feedback torque and the estimated wheel torque are set to zero. If the inquiry at144indicates that the powertrain is conditioned for reverse drive or forward drive, the super-creep strategy of the invention may be used.

At action block146, a check will be made to determine whether the entry conditions for the super-creep strategy of the invention are satisfied. this step is carried out also at153inFIG. 4. Then, an inquiry is made at148regarding whether the flag indicating that the driver's foot is off the brake pedal is true. If it is true, then the super-creep strategy for enhanced performance may be carried out, as shown at150. Otherwise, the driver demand torque tables with accelerator pedal input would be used by the driver in the usual fashion, as shown at152. If the strategy routine proceeds to step150inFIG. 3, the strategy routine ofFIG. 7is carried out.

FIG. 4shows at153the beginning steps of the routine for checking the entry conditions for the super-creep strategy of the invention. This is the step included also inFIG. 3at146. It initiates the strategy routine inFIG. 4. When the super-creep strategy is initialized, as shown at154, it is determined at decision block156whether the super-creep strategy state is off. Hereafter, the super-creep strategy state may be referred to as a foot-off pedal state (FOP).

Following the inquiry at156, it is determined at decision block158whether the brake is applied. If it is applied, it is determined at decision block160whether the brake apply flag is true and whether the brake status flag is verified to be okay. If the brake error flag is false, but the control area network brake signal flag is true, the result of the inquiry at160is positive, as shown at162. This would be followed by enabling the FOP strategy at action block164. If the inquiry at160is negative, the routine will proceed to the entry condition strategy ofFIG. 6, which will be described subsequently, as shown at166. If the inquiry at156is true, and the inquiry at157is true, the FOB state is set to check brake at159before the routine continues as shown inFIG. 6. TheFIG. 6routine is carried out also if the inquiry at157is not true.

InFIG. 5, the inquiry at164is repeated, as shown at168. If the FOP state is not enabled, that information is confirmed at decision block174inFIG. 5, and execution of the strategy of the invention will not occur, as shown at176. If the inquiry at174is positive, the disabled FOP state is confirmed at178. If the brake check at158inFIG. 4indicates that the FOB state flag is off at176, the routine moves toFIG. 6. Likewise, if the strategy routine proceeds to block160inFIG. 4and the condition described in block160is either true or false, the strategy moves toFIG. 6at194.

If the FOP state is enabled, a super-creep mode flag is latched to “full” and the super-creep mode set to feed-forward; or if the super-creep mode is set to “off”, as shown at180, the FOP state becomes disabled, as shown at182.

If the inquiry at180is negative, a confirmation is made at184regarding whether the super-creep mode flag is latched to full and the super-creep mode is set to off. A positive response will result in setting the FOP state equal to the FOP state disabled status, as shown at186, the routine will move toFIG. 6. If the confirmation at184is negative, an inquiry is made at188regarding whether the brake status is faulted. If it is not faulted, the routine will proceed to the entry condition strategy ofFIG. 6, as shown at190. If it is faulted, the strategy will set the FOP state flag to FOP state disabled, as shown at192, and it will move to the routine ofFIG. 6.

If the FOP state strategy flag is enabled, as indicated by the decision block196inFIG. 6, the FOP state strategy is enabled. If the inquiry at194is negative, the FOP state strategy is not available. Thus, the FOP state enable flag is set to false, the FOP state is set to FOP off, the FOP state feed forward torque is set to zero, the FOP state feedback torque is set to zero and the estimated output shaft wheel torque is set to zero, as indicated by action block198. Simultaneously, the super-creep mode flag is latched in the super-creep mode as shown at200.

If the entry conditions described with reference toFIGS. 4-6result in a setting of the super-creep mode flag being latched to the super-creep mode at200inFIG. 6, the routine will enter the super-creep mode strategy ofFIG. 7, which as previously mentioned, is illustrated in block diagram form inFIG. 2.

The super-creep mode routine will occur when the brake applied flag is true, or the parking brake flag is true, or the control area network (CAN) brake pedal flag is true and the brake pedal error flag is false. This is illustrated at202inFIG. 7. It then is determined at204whether the operator has conditioned the powertrain for forward drive or for reverse drive.

If the driver has selected reverse, reference will be made by the controller to the reverse enhanced FOP pedal map ofFIG. 2b. If the powertrain is conditioned for forward drive, reference will be made by the controller to the drive-enhanced FOP pedal map ofFIG. 2a. The maps ofFIGS. 2band 2aare indicated, respectively, by numerals206and208inFIG. 7.

The information obtained fromFIG. 2aor2bis used in the inquiry at decision block210. If it is determined at decision block210that the brake is applied, there will be no feed-forward torque, as shown at212. If the brake is off on the other hand, the routine will use the feed-forward torque pedal map shown at206or208. This is indicated at214inFIG. 7.

A summary of the status of each entry condition is shown at block216ofFIG. 7. This information is stored in ROM memory of controller10and used in executing the strategy ofFIG. 8.

The control routine will proceed as shown inFIG. 8, where it is confirmed at218that the brake is applied, that the vehicle acceleration rate is greater than a calibrated value or that the super-creep mode flag is set to feed-forward. If that is true, the feedback torque at134inFIG. 2would be zero.

At action block220, the error at122inFIG. 2is calculated. It then is determined at222whether the driver has selected reverse drive or forward drive. This was explained previously with respect to the block diagram ofFIG. 2where switch112and feed-forward torque tables are shown for reverse drive and forward drive at116and118, respectively. If reverse drive has been selected, maximum and minimum torque values are determined at action block224using the table116inFIG. 2. If forward drive has been selected, upper and lower torque limits are determined at action block226using the pedal map shown at118inFIG. 2.

After the upper and lower torque limits are determined for reverse or forward drive, and after the torque feed-back term shown at128inFIG. 2is calculated based on the product of loop delta time, torque error and integral gain value. The resulting torque feedback term is combined with feedback torque as shown at128inFIG. 2. That value is clipped between upper and lower saturation values at action block228.

The output shaft wheel torque is calculated by adding feedback torque to feed-forward torque at action block230. This was described previously with respect to summing point142inFIG. 2.

Although an embodiment of the invention has been described, it will be apparent to persons skilled in the art that modifications may be made without departing from the scope of the invention. All such modifications and equivalents are intended to be governed by the following claims.