Abstract:
An ordinary target driving force is generated against a detected value of the vehicle operator depression of an accelerator pedal and a detected value of the vehicle speed. The ordinary target driving force is a driving force required to keep the vehicle rolling on a flat horizontal road at the detected value of vehicle speed with the detected value of the vehicle operator depression of the accelerator pedal. A running resistance increment, i.e., an increase of running resistance from a standard running resistance, is calculated. A preliminary driving force correction is determined in response to the running resistance increment. The preliminary driving force correction is subjected to variation. The preceding old value of a driving force correction is subtracted from a current value of the preliminary driving force to give a variation in driving force correction. This variation is limited to fall in a range between an upper and a lower limit to give a limited driving force correction variation. The preceding old value of the driving force correction is added to the limited driving force correction variation to give the driving force correction. This driving force correction is added to the ordinary target driving force to give a corrected target driving force. To realize the corrected target driving force, engine torque and CVT ratio are adjusted.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a driving force control for an automotive vehicle. 
     BACKGROUND OF THE INVENTION 
     The term “standard running resistance” is herein used to mean any force, which opposes the motion of an automotive vehicle which is driven to keep on rolling over the surface of a flat road having 0% gradient at a constant vehicle speed. The term “running resistance” is herein used to mean any force that opposes the motion of an automotive vehicle, which is driven to keep on rolling over the surface of a road at a constant vehicle speed. Running resistance is equal to standard resistance if an automotive vehicle is driven to keep on rolling over the surface of a flat road having 0% gradient at a constant vehicle speed. Running resistance increases and becomes greater than standard resistance if the automotive vehicle is accelerated to increase speed from the constant vehicle speed. The term “acceleration resistance” is herein used to mean this increment or difference in running resistance that has occurred due to acceleration. Running resistance is greater when the automotive vehicle is driven to keep rolling over the surface of a flat road having gradient greater than 0% at a constant vehicle speed than standard resistance for the same vehicle speed. The term “gradient resistance” is used to mean this increment or difference in running resistance. 
     JP-A 9-242862 discloses a vehicle control system in which a gear ratio of an automatic transmission is selected in response to road gradient, throttle opening degree and vehicle speed. In order to estimate road gradient of a road, over which the vehicle is rolling, a road gradient torque (Tα) is determined by subtracting from a driving torque (To) a sum of a flat road running resistance torque (Tr) and an acceleration resistance torque (Tα). A characteristic of variation of flat road running resistance torque (Tr) against variation of vehicle speed is mapped. This mapped data are retrieved using a current reading point of vehicle speed to give a value of flat road running resistance torque (Tr). 
     JP-A10-329585 discloses technique to enhance transient response to a step-like change in target driving force, which occurs when, for example, an accelerator pedal is depressed greatly. The target driving force is defined as required output on vehicle driving axle for keeping the vehicle rolling on the surface of a road having road gradient at a vehicle speed with a depressed position of an accelerator pedal. Thus, the target driving force is determined in response to accelerator depressed position, vehicle speed, and road gradient. A change in the target driving force over an interval between the current and preceding control cycle is determined and used to select an appropriate time series waveform. This waveform gives time series data of variation of target driving force. Engine torque control and a CVT ratio control are carried out to accomplish the target driving force. 
     JP-A 8-200112 discloses a driving force control system wherein, during traction control, a wheel slip determines a target engine torque and operator depression of an accelerator pedal determines a correction amount. This correction amount is added to the target engine torque to determine a corrected target engine torque. An electronically controlled throttle is controlled to cause the engine to produce the corrected target engine torque. 
     Each of the above-mentioned known systems is satisfactory to some extent. However, a need remains to provide a driving force control system that can cope with a temporal drop in running resistance. If, with the target driving force kept constant in response to constant operator power demand, such temporal drop in running resistance occurs, the vehicle body may be subjected to shock caused by disturbance owing to this temporal drop in running resistance. 
     An object of the present invention is to improve a driving force control system of the type wherein an increase in running resistance is used to determine a correction amount in driving force and the determined driving force correction is added to an ordinary target driving force to give a corrected driving force, such that a temporal drop in running resistance may not cause any substantial shocks. 
     SUMMARY OF THE INVENTION 
     U.S. patent application Ser. No. 09/518,691, filed Mar. 3, 2000 entitled “Driving Force Control With Gradient Resistance Torque Dependent Correction Factor” is pending and has been assigned to the same assignee to which the present application is to be assigned. This United States Patent Application claims priority based on Japanese Patent Application No. 11-58289 filed in Japan on Mar. 5, 1999. 
     This United States Patent Application has proposed a driving force control system that includes an ordinary target driving force generator that generates an ordinary target driving force (tTd#n), and a running resistance increment generator that generates a running resistance increment (RESTRQ). The ordinary target driving force (tTd#n) is given after retrieving a map using accelerator pedal opening (APO) that is equivalent to operator&#39;s depression of the vehicle&#39;s accelerator pedal and vehicle speed (VSP). The proposed driving system further includes a driving force correction generator that determines a driving force correction (ADDFD) in response to the running resistance increment (RESTRQ), and a corrected target driving force generator where the driving force correction (ADDFD) is added to the ordinary target driving force (tTd#n) to produce a corrected target driving force (tTd). This corrected target driving force (tTd) is used to determine a target engine torque (tTe) and a target CVT ratio (tRATIO). 
     Referring to FIG. 7, let us now assume the case where the automobile travels against actual running resistance that is unaltered. In FIG. 7, at moment t 0 , a wheel slippage occurs on a manhole cover, and at moment t 2 , the wheel slippage ceases. This causes a temporal drop between t 0  and t 2  in actual running resistance increment. In the proposed driving force control system, this temporal drop is reflected in running resistance increment (RESTRQ) after a delay that is unavoidable. Thus, the corresponding temporal drop in running resistance (RESTRQ) begins at moment t 1  and terminates at moment t 3 . In response to this temporal drop in running resistance increment (RESTRQ), the driving force increment (ADDFD) drops for a temporary period t 1 -t 3  as shown by the dotted line in FIG.  7 . This temporal drop in driving force increment causes a temporal drop in corrected target driving force (tTe), so that automobile is subjected to an undesired change in acceleration as shown by the dotted line in FIG.  7 . This undesired change in acceleration produces shocks. 
     Accordingly, the present invention aims at improving the driving force control system of the above kind such that occurrence of substantial shocks, which may be induced by a temporal change in running resistance increment, is prevented or at least reduced. 
     According to one aspect of the present invention, there is provided a driving force control system for an automotive vehicle powertrain including a prime mover and an automatic transmission, the driving force control system comprising: 
     a first sensor to detect the vehicle&#39;s operator demand on driving force to drive the vehicle; 
     a second sensor to detect a predetermined parameter indicative of vehicle speed of the vehicle; and 
     a microprocessor that is programmed to be operative to determine a target value indicative of driving force in response to the vehicle&#39;s operator demand on driving force and the vehicle speed, 
     to determine a running resistance increment, 
     to determine a preliminary correction in response to the determined running resistance increment, 
     to subtract a preceding value of correction from the determined preliminary correction to give a variation, 
     to limit said variation between upper and lower limits to give a limited variation, 
     to add the preceding value of correction to the limited variation to give a current value of correction, and 
     to correct the determined target value with the current value of correction. 
     According to another aspect of the present invention, there is provided a driving force control system for an automotive vehicle powertrain including a prime mover and an automatic transmission, the driving force control system comprising: 
     a first sensor to detect the vehicle&#39;s operator demand on driving force to drive the vehicle; 
     a second sensor to detect a predetermined parameter indicative of vehicle speed of the vehicle 
     a target value generator to determine a target value indicative of driving force in response to the vehicle&#39;s operator demand on driving force and the vehicle speed; 
     a running resistance increment generator to determine a running resistance increment that represents an increase in running resistance from a standard running resistance; 
     a correction generator to determine a correction in response to the running resistance increment; 
     a corrected target value generator to correct the determined target value with the correction, 
     said correction generator being operative to calculate a differential, with respect to time, of the correction and operative to tune the calculated differential, and also operative to alter the correction by the tuned differential. 
     According to still another aspect of the present invention, there is provided a driving force control method for an automotive vehicle powertrain including a prime mover and an automatic transmission, the driving force control method comprising: 
     detecting the vehicle&#39;s operator demand on driving force to drive the vehicle; 
     detecting a predetermined parameter indicative of vehicle speed of the vehicle; 
     determining a target value indicative of driving force in response to the vehicle&#39;s operator demand on driving force and the vehicle speed; 
     determining a running resistance increment that represents an increase in running resistance from a standard running resistance; 
     determining a correction in response to the running resistance increment; 
     correcting the determined target value with the correction; 
     calculating a differential, with respect to time, of the correction; 
     limiting the calculated differential to give a limited differential; and 
     altering the correction by the limited differential. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an automotive vehicle having driving wheels, a powertrain including an engine and an automatic transmission, and a powertrain control module (PCM). 
     FIG. 2 is a control diagram, illustrating a first preferred implementation according to the present invention. 
     FIG. 3 is a graphical representation of characteristic of variation of a preliminary driving force correction (TADDFD) against variation of a running resistance increment (RESTRQ). 
     FIG. 4 is a graphical representation of a conversion map stored in a limited criteria used to limit a value (JGDTADO) representing a change in running resistance increment (RESTRQ) during time elapsed between the preceding cycle and the current cycle. 
     FIG. 5 is a flow chart of a main control routine implementing the present invention. 
     FIG. 6 is a flow chart of a control routine to calculate a driving force correction (ADDFD). 
     FIG. 7 are timing chart of variations of RESTRQ, ADDFD and acceleration (α) after occurrence of a temporary drop in actual running resistance increment. 
     FIG. 8 is a flow chart of a control routine similar to FIG. 6, illustrating a second preferred implementation according to the present invention. 
     FIGS. 9A and 9B illustrate vehicle speed dependent upper and lower limits used in a third preferred implementation according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawings, FIG. 1 is a schematic view of a passenger automobile installed with a driving force control system implementing the present invention. 
     The automobile has a powertrain including a prime mover in the form of an internal combustion engine  101  and an automatic transmission  103 , and a powertrain control module (PCM)  50 . Output from the engine  101  is transmitted via the automatic transmission  103  to driving wheels. The PCM  50  controls engine torque of the engine  101  and a speed ratio, a ratio between a transmission input shaft speed and a transmission output shaft speed, of the automatic transmission  103  in such a manner as to cause the powertrain to produce driving force desired. 
     An accelerator pedal position detector in the form of an accelerator pedal opening sensor  105  is operatively connected to a manually operable accelerator, such as for example, an accelerator pedal, to feed operator demand on driving force to the PCM  50 . The accelerator pedal opening sensor  105  detects an accelerator position and generates an APO signal indicative of the detected accelerator position. This APO signal is fed as an input to the PCM  50 . The vehicle operator depresses the accelerator pedal to express driving force demand. In this example, the APO signal is indicative of driving force demand, i.e., operator demand on driving force, and the accelerator pedal opening sensor  105  is a sensor to detect vehicle&#39;s operator demand on driving force. Naturally, any other form of sensor may be employed for this purpose. The automatic transmission  103  has plurality of ranges that may be selected by a range select lever  107 . An inhibitor switch  108  is operatively connected to the range select lever  107  to detect which range is being selected and generates a select signal indicative of the range being selected by the select lever  107 . The select signal is fed as an input to the PCM  50 . A vehicle speed sensor  11  detects a predetermined parameter indicative of the vehicle speed and generates a vehicle speed signal VSP. The vehicle speed sensor  11  may take any form as long as it could output signal indicative of the vehicle speed. The vehicle speed signal VSP is fed as an input to the PCM  50 . A crankshaft angle sensor, not shown, generates an engine speed signal NRPM. The engine speed signal NRPM is fed as an input to the PCM  50 . 
     Based on input signals including the above-mentioned input signals, the PCM  50  conducts adjustment of engine torque of the engine  101  and adjustment of the ratio within the automatic transmission  103  to produce driving torque transmitted to the driving wheels. The adjustment of engine torque may be made by varying one of or any combination of fuel injection quantity Tp, intake air flow rate Qa, and spark timing. 
     To adjust the intake air flow rate Qa, an electronically controlled throttle valve  102  is disposed in an intake passage of the engine  101 . In response to a throttle valve opening command from the PCM  50 , a throttle control module (TCM) adjusts the position of the throttle valve  102 . 
     The automatic transmission  103  includes a continuously variable transmission (CVT) that can alter a ratio continuously in response to a ratio command from the PCM  50 . The PCM  50  multiplies a predetermined constant with the vehicle speed VSP to give a transmission output shaft speed No. An input shaft speed sensor  12  detects revolution speed of the transmission input shaft and generates an input shaft speed signal Nin indicative of the detected speed of the transmission input shaft. The input shaft speed signal Nin is fed as input to the PCM  50 . The PCM  50  calculates a ratio RATIO Nin/No and determines the ratio command and applies it to a ratio control mechanism of the CVT  103  to match a target ratio tRATIO that is determined by the PCM  50 . The CVT may be of the V belt type or the toroidal type. Rotation of the output shaft of the automatic transmission  103  is transmitted via a final-drive to the vehicle driving wheels. The final-drive has a fixed ratio. 
     The PCM  50  is in the form of a microprocessor that includes a CPU, a ROM, a RAM, and an input/output device. 
     The engine  101  is equipped with, as accessories, a compressor of an air conditioner  120  and an oil pressure pump  121  of a power steering unit. 
     In order for the PCM  50  to gather information as to the amount of engine output consumed by the accessories, the CPU inputs a pressure of refrigerant detected by a liquid pressure sensor, not shown, attached to the compressor  120 , and oil pressure detected by an oil pressure sensor attached to the oil pressure pump  121 . The CPU determines whether or not the accessories are in operation based on the magnitudes of the corresponding detected pressures. 
     Referring now to FIG. 2, a description is made on driving force control carried out within the PCM  50 . 
     FIG. 2 is a control block diagram of the driving force control. It includes an ordinary target driving force generator (OTDFG)  1 , a running resistance increment generator (RRIG)  2 , a preliminary driving force correction generator (PDFCG)  3 , a corrected target driving force generator (CTDFG)  6 , a target engine torque generator (TETG)  7 , and a target ratio generator (TRG)  8 . It also includes a limit criteria  4 . 
     The OTDFG  1  inputs APO and VSP. The OTDFG  1  includes a memory storing a predetermined tTd#n vs. (APO, VSP) map that defines various target values indicative of ordinary target driving force tTd#n at various values of VSP with various values of APO. Each target value tTd#n exhibits ordinary driving force needed to accomplish a desired traveling performance of a vehicle on a flat road having 0% gradient. The OTDFG  1  performs a table look-up operation of the map using APO and VSP to determine an ordinary target driving force tTd#n and provides the determined ordinary target driving force tTd#n to the CTDFG  6 . 
     Thus, tTd#n can be expressed as 
     
       
           tTd#n= MAP[ APO, VSP]   (1). 
       
     
     The RRIG  2  calculates an increase in running resistance from a standard value of running resistance to give a running resistance increment RESTRQ. 
     The RRIG  2  includes a driving torque generator (EDFG)  21 , a standard running resistance generator (SRRG)  22 , and an acceleration resistance generator (ARG)  23 . 
     The DTG  21  inputs Tp and NRPM. The DTG  21  includes a memory storing a predetermined ENGTRQ vs., (Tp, NRPM) map that defines various values of engine torque to be produced by the engine  101  against various combinations of values of Tp and values of NRPM. The DTG  21  performs a table look-up operation of this map using Tp and NRPM to determine an engine torque ENGTRQ. It multiplies the determined ENGTRQ with a current speed ratio RATIO established within the CVT  103  and a torque transmission ratio τ RATIO established within a torque converter to give an driving torque TRQOUT transmitted to the transmission output shaft. 
     The driving torque TRQOUT can be expressed as 
     
       
           TRQ OUT= ENGTRQ× RATIO×τRATIO  (2). 
       
     
     The SRRG  22  inputs VSP. The SRG B 22  includes a memory storing a predetermined RLDTRQ vs., VSP map that defines various value of standard running resistance RLDTRQ against various values of VSP. The standard running resistance RLDTRQ is indicative of a value resulting from converting the standard running resistance force to the resistance torque transmitted to the transmission output shaft. 
     The standard running resistance RLDTRQ can be expressed as 
     
       
           RLDTRQ= MAP[ VSP]   (3). 
       
     
     The ARG  23  inputs vehicle acceleration GDATA [m/s 2 ] that is derived as the first time derivative of VSP or as a measure of an accelerometer. Vehicle weight WV, tire radius rTIRE [m] and inverse of the final reduction ratio zRATIO are stored as reference data in the ARG  23 . The ARG  23  determines an acceleration resistance GTRQ as a product of GDATA, WV, rTIRE, and zRATIO as expressed as 
     
       
           GTRQ=G DATA× WV×r TIRE× z RATIO  (4). 
       
     
     Using the equation (4), the vehicle acceleration GDATA is converted to the acceleration resistance torque of the transmission output shaft. 
     The RRIG  2  calculates a sum RLDTRQ and GTRQ and subtracts the sum from TRQOUT to give the running resistance increment RESTRQ. The RRIG  2  provides RESTRQ to the PDFCG  3 . 
     The running resistance increment RESTRQ can be expressed as 
     
       
           RESTRQ=TRQ OUT−( RLDTRQ+GTRQ )  (5). 
       
     
     The PDFCG  3  inputs RESTRQ and determines a preliminary driving force correction TADDFD. The PDFCG  3  includes a memory storing a predetermined TADDFD vs., RESTRQ map as illustrated by the fully drawn line in FIG.  3  and performs a table look-up operation of the stored map using RESTRQ to determine the preliminary driving force correction TADDFD. 
     The preliminary driving force correction TADDFD, which is expressed in terms of the same dimension [N] as the target value tTd#n, is set less than a value resulting from converting RESTRQ to running resistance force. This relation can be expressed as 
     
       
           T ADDFD&lt; RESTRQ/z RATIO/ r TIRE  (6). 
       
     
     This exhibits that TADDFD is always less than 100% of the converted value from RESTRQ. The TADDFD is fed to an arithmetic point  31  for creating a driving force correction ADDFD. A delay  9  is provided to feed a preceding value of driving force correction ADDFDold, which was created during the preceding cycle of arithmetic operation, to the arithmetic point  31 . At the arithmetic point  31 , a variation JGDTADD is given by subtracting ADDFDold from TADDFD. The variation JGDTADD represents a change in ADDFD over a time elapsed between the preceding arithmetic operation cycle and the current arithmetic operation cycle on the assumption that TADDFD is used as the current ADDFD. The variation JGDTADD can be expressed as 
     
       
           JGDTADD=TADDFD−ADDFDold   ( 7 ). 
       
     
     The variation JGDTADD is fed to a limit criteria or tuner  4 . The limit criteria  4  has a predetermined upper limit DLTFPLS and a predetermined lower limit DLTFMNS, and a conversion map to convert JGDTADD into a value between the upper and lower limits DLTFPLS and DLTFMNS. The converted value is generated as a tuned variation DLTF and fed to a summation point  5 . 
     The preceding value ADDFDold is fed also to the summation point  5 . At the summation point  5 , the driving force correction ADDFD is given by adding ADDFDold to DLTF and can be expressed as 
     
       
           ADDFD=ADDFDold+DTLF   (8). 
       
     
     The driving force correction ADDFD is fed to the CTDFG. The CTDFG  6  adds ADDFD to tTd#n to give a target driving force tTd. The target driving force tTd can be expressed as 
     
       
           tTd=tTd#n+ADDFD   (9). 
       
     
     The CTDFG  6  provides tTd to a target engine torque generator (TETG)  7  and also to a target ratio generator (TRG)  8 . 
     The TETG  7  receives RATIO, rTIRE, and zRATIO as well as tTd and determines a target engine torque tTe after calculating the following equation: 
     
       
           tTe=tTd×r TIRE× z RATIO÷RATIO  (10). 
       
     
     The TETG  7  provides tTe to the engine  101 . In order to realize tTe, the TCM  51  determines the position of the electronically controlled throttle valve  102 , a control section of the engine  101  determines Tp and spark timing. 
     The TRG  8  receives VSP as well as tTd and determines a target speed ratio tRATIO using VSP and tTd. The TRG  8  has a memory storing a predetermined tRATIO vs., (tTd, VSP) map that defines various values of tRATIO against various combinations of values of VSP and values of tTd. In determining tRATIO, the TRG  8  performs a table look-up operation of this predetermined map using VSP and tTd. The TRG  8  provides tRATIO to a ratio control mechanism of the CVT  103 . The ratio control mechanism adjusts RATIO within the CVT  103  to tRATIO. 
     FIG. 3 illustrates the TADDFD vs., RESTRQ map that is stored in the PDFCG  4 . TADDFD is set against RESTRQ and used to compensate for a shortage in acceleration. 
     The fully drawn interconnected line segments shown in FIG. 3 illustrate the TADDFD vs., RESTRQ map used in the PDFCG  4 . 
     Over values of RESTRQ not greater than a first predetermined value RES#TLEV 1 , zero is set as TADDFD. During operation of the vehicle when the variation of RESTRQ is less than or equal to the first predetermined value RES#TLEV 1  and thus small, TADDFD is zero, thus preventing occurrence of any unexpected driving force correction due to, for example, an error in calculating RESTRQ, a small variation in wind against the vehicle or a small variation in running resistance derived from a gradual gradient change. 
     Thus, if 0≦RESTRQ&lt;RES#TLEV 1 , then TADDFD=0. 
     Next, over values of RESTRQ greater than RES#TLEV 1  but not greater than a second predetermined value RES#TLEV 2 , TADDFD can be expressed as 
     
       
           TADDFD= 0.5× RESTRQ/z RATIO/ r TIRE  (11). 
       
     
     In this equation, RESTRQ is divided by zRATIO to give torque on the driving wheel shaft, and this torque is divided by the tire radius rTIRE to convert the dimension from torque [Nm] to force [N], and 50% of the force given by this conversion is set as TADDFD. This percentage is not limited to 50% and may take an appropriate value less than 100%. The remaining portion of RESTRQ left unconverted is not translated into TADDFD, leaving a room for the vehicle operator to participate the driving force correction by depressing the accelerator pedal, thus providing a natural acceleration fit to the vehicle operator&#39;s demand. 
     Thus, if RES#TLEV 1 ≦RESTRQ≦RES#TLEV 2 , then TADDFD=0.5×RESTRQ/zRATIO/rTIRE. 
     Over values of RESTRQ greater than RES#TLEV 2 , TADDFD is kept at a predetermined value ADDFDLMmax. 
     Thus, if RESTRQ&gt;RES#TLEV 2 , then TADDFD=ADDFDLMmax. 
     During operation with RESTRQ greater than RES#TLEV 2 , the preliminary driving force increment TADDFD is limited to ADDFDLMmax. 
     FIG. 4 illustrates a DLTF vs., JGDTADD map used at the limit criteria  4 . Using this map, the limit criteria  4  limits JGDTADD as follows: 
     
       
           DLTFMNS≦JGDTADD≦DLTFPLS   (12). 
       
     
     If JGDTADD is greater than the lower limit DLTFMNS and less than the upper limit DLTFPLS, DLTF=JGDTADD holds. 
     Accordingly, the variation JGDTADD is prohibited from increasing beyond the upper limit DLTFPLS and dropping below the lower limit DLTFMNS and generated as the tuned variation DLTF. 
     The upper limit DLTFPLS, which is disposed on increasing side of tTd, defines an upper limit of acceleration, while the lower limit DLTFMNS, which is disposed on deceasing side of tTd, defines an upper limit of the absolute value of deceleration. The relationship is as follows: 
     
       
         (Absolute value of  DLTFPLS )&lt;(Absolute value of  DLTFMNS )  (13). 
       
     
     It has been recognized that, with the same absolute value of change in driving force, the change in acceleration direction and the change in deceleration direction cause the vehicle operator to feel different deviations from what he/she has anticipated. This recognition has provided a support for setting of the above relationship. 
     With the same absolute value, deceleration causes the vehicle operator to feel less deviation from what he/she has anticipated than acceleration does. Setting the absolute value of lower limit DLTFMNS greater than that of upper limit DLTFPLS enables quick correction of driving force without causing the vehicle operator to feel little deviation from what he/she has anticipated, providing good derivability and enhanced control performance. 
     FIG. 5 is a flow chart of a main control routine implementing the present invention. FIG. 6 is a flow chart of a sub-routine to illustrate arithmetic operation to determine ADDFD. These routines are stored in the ROM of the microprocessor that forms the PCM  50 . The CPU of the PCM  50  executes the main control routine at regular interval of 10 milliseconds. 
     At step S 1 , the CPU inputs VSP, APO, and NRPN. 
     At step S 2 , the CPU determines tTd#n by performing a table look-up operation, using APO and VSP, of the tTd#n vs., (APO, VSP) map illustrated in FIG.  2 . 
     At step S 3 , the controller  3  determines ENGTRQ by performing a table look-up operation, using TP and NRPN, of the ENGTRQ vs., (Tp, NRPM) map illustrated in FIG. 2 calculates a product of ENGTRQ, RATIO, and τRATIO to give TRQOUT. 
     At step S 4 , the CPU determines RLDTRQ by performing a table look-up operation, using VSP, of the RLDTRQ vs., VSP map illustrated in FIG.  2 . 
     At step S 5 , the CPU determines GTRQ after calculating a product of GDATA, WV, rTIRE, and zRATIO. 
     At step S 6 , the CPU determines RESTRQ after subtracting (RLDTRQ+GTRQ) from TRQOUT. 
     At step S 7 , the CPU determines TADDFD by performing a table look-up operation, using RESTRQ, of the TADDFD vs., RESTRQ map illustrated in FIG.  3 . 
     At step S 8 , the CPU executes the sub-routine shown in FIG. 6 to determine ADDFD as the sum of ADDFDold and DLTF. 
     At step S 9 , the CPU determines tTd by adding ADDFD to tTd#n. 
     The corrected target driving force tTd thus determined as explained above is fed to the TETG  7  and also to TRG  8 . 
     Referring to the flow chart of FIG. 6, a description is made on how to derive ADDFD from JGDTADD. 
     At step S 10  in FIG. 6, the CPU determines JGDTADD by subtracting ADDFDold from TADDFD. 
     At step S 11 , the CPU determines whether or not JGDTADD is greater than 0 (zero). The fact that JGDTADD is greater than 0 means it is increasing. The fact that JGDTADD is not greater than 0 means it is zero or decreasing. If JGDTADD is greater than 0, the routine proceeds to step S 12 . 
     At step S 12 , the CPU determines whether or not JGDTADD is less than or equal to the upper limit DLTFPLS. If this is the case, the routine proceeds to step S 16 . If this is not the case, the routine proceeds to step S 13 . 
     At step S 13 , the CPU sets DLTF equal to the upper limit DLTFPLS. At step S 16 , the CPU sets DLTF equal to JGDTADD. 
     If, at step S 11 , the CPU determines that JGDTADD is not greater than 0 (zero), the routine proceeds to step S 14 . 
     At step S 14 , the CPU determines whether or not JGDTADD is greater than or equal to the lower limit DLTFMNS. If this is the case, the routine proceeds to step S 16  where DTLF is set equal to JGDTADD. If this is not the case, the routine proceeds to step S 15 . 
     At step S 15 , the CPU sets DLTF equal to the lower limit DLTFMNS. 
     After step S 13  or S 16  or S 15 , the routine proceeds to step S 17 . At step S 17 , the CPU determines ADDFD by calculating the sum of ADDFDold and DLTF. 
     At the next step S 18 , the CPU updates ADDFDold with ADDFD. After step S 18 , the control returns to the main control routine at step S 9 . 
     The preceding description clearly indicates that the variation of driving force correction ADDFD is determined by DLTF that is equal to JGDTADD within a range having the upper and lower limits DLTFPLS and DLTFMNS (see FIG.  4 ). FIG. 2 clearly illustrates that a change in RESTRQ causes a change JGDTADD. Under the condition as illustrated in FIG. 7, a temporary drop in RESTRQ causes a drop in JGDTADD at moment t 1 . But, this drop in JGDTADD is limited at the lower limit DLTFMNS. Subsequently, at moment t 3 , an increase in RESTRQ causes an increase in JGDTADD. But, this increase in JGDTADD is limited at the upper limit DLTFPLS. In other words, when the temporal drop in RESTRQ with a considerably great depth, the variation of the driving force correction ADDFD is determined by DLTF and confined within the range having the upper and lower limits DLTFPLS and DLTFMNS Accordingly, as illustrated byu the fully drawn line in FIG. 7, a drop in ADDFD caused by the temporary drop in RESTRQ is shallow and a change in acceleration a is small. 
     If a temporary change in RESTRQ occurs due to noise interference and/or arithmetic operation error, a rapid change in ADDFD is prevented in the same manner. Thus, suppressing rapid acceleration not anticipated by the vehicle operator, operation to give corrected driving force tTd is carried out smoothly top ensured enhanced drivability. 
     Naturally, a moderate change in RESTRQ allows ADDFD to follow this change without any delay because the variation in JGDTADD falls in the range between the upper and lower limits DLTFPLS and DLTFMNS. 
     The flow chart of FIG. 8 is a sub-routine similar to FIG.  6  and illustrates a portion of the second preferred implementation according to the present invention. The flow chart of FIG. 8 is substantially the same as that of FIG. 6 except the provision of logic (steps S 20 , S 21 , S 22  and S 23 ) inserted between the steps S 10  and S 11 . This logic is intended to selectively prohibit limiting or tuning of JGDTADD. 
     After step S 10 , the routine proceeds to step S 20 . At step S 20 , the CPU determines whether or not any change has occurred in operation state of accessory attached to the engine  101 . 
     As before mentioned in connection with FIG. 1, the CPU determines whether or not the compressor  120  of the air conditioner is in operation by detecting pressure of refrigerant and the oil pressure pump  121  of the power steering unit is in operation by detecting oil pressure. 
     If, at step S 20 , there is a change in operation state of the compressor  120  or the oil pressure pump  121 , the routine proceeds to step S 21 . At step S 21 , the CPU sets a predetermined initial value SMTIM 0  at a switch timer SMTIM. Then the routine proceeds to step S 22 . If, at step S 20 , a change in operation state is not determined, the routine proceeds to step S 22 . 
     At step S 22 , the CPU determines whether or not the timer SMTIM is equal to 0 (zero). If this is the case, the routine proceeds to step S 11 . 
     If, at step S 22 , the CPU determines that the timer SMTIM is not equal to 0 (zero), the routine proceeds to step S 23 . At step S 23 , the CPU decreases the content of timer SMTIM by 1 (one). Then, the routine proceeds to step S 16 . At step S 16 , the CPU sets DLTF equal to JGDTADD. 
     From the preceding description, it is readily seen that JGDTADD that is variable dependent on RESTRQ is not limited and used as it is to vary the driving force correction ADDFD until elapse of the content of timer SMTIM after occurrence of change in operation state of accessory. 
     There is a delay from the moment when a change in operation state of accessory to the moment when a change in actual running resistance is reflected as a change in RESTRQ. The initial value SMTIM 0  of the timer SMTIM has been set after due consideration of this delay. In this example, the setting of the initial value SMTIM 0  is such that time required to decrease SMITIM 0  to 0 (zero) is less than 1 second. 
     A change in operation state of an air conditioner or a power steering unit causes a rapid change in running resistance. Against such a rapid change in running resistance, a quick correction of driving force is preferred for the purpose of reducing the amount of deviation from what the vehicle operator anticipates. 
     If, for example, an air conditioner or a power steering unit is put into operation state, a rapid increase in RESTRQ causes deceleration or drop in acceleration, which are not anticipated by vehicle operator. According to the second preferred implementation, JGDTADD is not limited for the predetermined period of time after the air condition or power steering unit has been put into operation, a rapid correction of driving force is made. 
     Referring to FIGS. 9A ands 9 B, a description is made on the third preferred implementation. This third preferred implementation is substantially the same as the first or second preferred implementation except the fact that the upper and lower limits DLTFPLS and DLTFMNS are not predetermined fixed values and they are variable depending on vehicle speed VSP as shown in FIGS. 9A and 9B. 
     At low vehicle speeds, there is the tendency that the vehicle operator feels shocks if ADDFD varies quickly. However, as intermediate and high vehicle speeds, such tendency decreases. 
     Accordingly, the upper limit DLTFPLS increases from zero level gradually as vehicle speed VSP increases as shown in FIG. 9A, and the lower limit DLTFMNS decreases from zero level gradually as vehicle speed VSP increases as shown in FIG.  9 B. Setting the upper and lower limits DLTFPLS and DLTFMNS in the illustrated manner provides smooth correction of driving force over extended operation range without causing the vehicle operator to feel any deviation from what he/she has anticipated. 
     At vehicle speed higher than a predetermined value Vx, the absolute value of upper limit DLTFPLS and the absolute value of lower limit DLTFMNS stay at maximum values DLTFPLSmax and DLTFMNSmax, respectively. 
     In the preceding description of the first and second preferred implementations, the target value tTd#n has been expressed in terms of the vehicle driving force. This target value tTd#n may be a predetermined parameter indicative of the vehicle driving force. Examples of the predetermined parameter are driving wheel shaft torque and transmission output shaft torque. 
     If tTd#n is a target value of the driving wheel shaft torque, TADDFD and ADDFDLM are expressed in terms of torque on the driving wheel shaft. In this case, TADDFD can be expressed as 
     
       
           TADDFD= 0.5× RESTRQ/r TIRE  (9′). 
       
     
     The target engine torque tTe can be expressed as 
     
       
           tTe=tTd×z RATIO÷RATIO  (8′). 
       
     
     If tTd#n is a target value of the transmission outputl shaft torque, TADDFD and ADDFDLM are expressed in terms of torque on the transmission output shaft. In this case, TADDFD can be expressed as 
     
       
           TADDFD= 0.5× RESTRQ   (9″). 
       
     
     The target engine torque tTe can be expressed as 
     
       
           tTe=tTd÷ RATIO  (8″). 
       
     
     In the preceding description, both the engine torque and the ratio are controlled based on tTd to accomplish the driving force expressed by tTd. The manner of accomplishing tTd is not limited to this example. It is possible to control the engine torque based on tTd and to control the ratio without any reference to tTd. 
     In the first preferred implementation, the acceleration resistance torque GTRQ is incorporated in the RRIG  2  (see FIG.  2 ). In addition to the acceleration resistance torque GTRQ, the RRIG  2  may incorporate the accessory dependent load as referred to in the second preferred implementation. Further, gradient road dependent resistance and wind dependent resistance may be incorporated. 
     In the preceding description on the preferred implementations, the running resistance increment generator (RRIG)  2  is described in connection with FIG.  2 . For full understanding of the RRIG  2 , reference should be made to a pending U.S. patent application Ser. No. 09/518,459, filed Mar. 3, 2000, entitled “Process of Forming Standard Resistance Values and Vehicle Control Using Same”, and claims priority based on Japanese Patent Application No. 11-58291 filed in Japan on Mar. 5, 1999. This pending United States Patent Application is hereby incorporated by reference in its entirety and commonly assigned herewith. Particular reference is made to FIG. 2 illustrating a driving torque generator (DTG)  2 , a standard resistance generator  3 , and a summation point to make subtraction of RLDTRQ from TRQALL to give RESTRQ. 
     While the present invention has been particularly described, in conjunction with the preferred implementations, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention. 
     The content of disclosure of Japanese Patent Application No. 11-103692, filed Apr. 12, 1999 is hereby incorporated by reference in its entirety.