Adaptive cruise control system for vehicle

[0002] The present invention relates to an adaptive cruise control (ACC) system for an automotive vehicle, and more particularly to an ACC system which controls a driving force and/or braking force of the vehicle so as to bring an inter-vehicle distance between a preceding vehicle and a host vehicle closer to a target inter-vehicle distance.

[0003] A typical ACC system is arranged to detect an inter-vehicle distance and to control a vehicle speed or driving/braking force so as to bring the inter vehicle distance closer to a target value. Such an ACC system employs one of an A-type method of employing a vehicle-speed sensor and generating a vehicle speed command and a B-type method of directly calculating the driving force command. The A-type method can provide an inter-vehicle distance control system where the affect of disturbance is eliminated by a robust design of a vehicle speed control system. However, this A-type method is required to further improve a control accuracy under an extremely low speed region. On the other hand, the B-type method can accurately perform the control under the low speed region, but it is required to simultaneously satisfy riding comfort and robustness against disturbances. The inventors of the present invention have proposed an ACC system which employs both of the A-type method adapted under a high-speed region and the B-type method adapted under a low-speed region.

[0004] However, this ACC system is further required to improve the responsibility of the ACC under a low-speed region against the increase of load to the system.

[0005] It is therefore an object of the present invention to provide an improved adaptive cruise control system which enables a controlled vehicle (host vehicle) to accurately execute a following control in the whole vehicle speed region including an extremely low speed region without adding an inter-vehicle distance control system for a low vehicle speed region.

[0006] An adaptive cruise control (ACC) system according to the present invention is installed to a host vehicle and comprises a vehicle speed detector detecting a vehicle speed of the host vehicle, a distance detector detecting an inter-vehicle distance between a preceding vehicle and the host vehicle, a driving/braking force generator generating driving/braking force according to a signal indicative of a target driving torque, and a controller connected to the vehicle speed detector, the distance detector and the driving/braking force generator. The controller calculates a target inter-vehicle distance between the preceding vehicle and the host vehicle, calculates a target vehicle speed and a first target driving torque based on the inter-vehicle distance and the target inter-vehicle distance, calculates a second target driving torque based on the host vehicle speed and the target vehicle speed, generates a torque select signal based on the host vehicle speed, selects one of the first and second target driving torques as the target driving torque based on the torque select signal, matches the selected target driving torque with a previous target driving torque when the target driving torque is changed, and outputs the control signal indicative of the target driving torque to said driving/braking force generator.

[0007] FIG. 1 is a schematic view showing a construction of a first embodiment according to the present invention.

[0008] FIG. 2 is a block diagram showing a construction of an ACC controller of FIG. 1.

[0009] FIG. 3 is a block diagram showing a construction of an inter-vehicle distance control section of FIG. 2.

[0010] FIG. 4 is a block diagram showing a construction of a vehicle speed control section of FIG. 2.

[0011] FIG. 5 is a block diagram showing a construction of a driving torque control section of FIG. 2.

[0012] FIG. 6 is a graph showing an engine operation map employed for obtaining a throttle opening from an engine torque.

[0013] FIG. 7 is a graph showing a relationship between an engine speed and the engine torque under a condition that the throttle opening is zero.

[0014] FIG. 8 is a flowchart showing a following control executed by the ACC controller.

[0015] FIGS. 9A to 9C are time-charts of the host vehicle speed, the torque select flag and the target driving torque during the selecting of target driving torque under a decelerating condition of the host vehicle.

[0016] FIGS. 10A to 10C are time-charts of the host vehicle speed, the torque select flag and the target driving torque during the selecting of target driving torque under a accelerating condition of the host vehicle.

[0017] FIG. 11 is a block diagram showing a construction of the ACC controller of a second embodiment.

[0018] FIG. 12 is a flowchart showing the following control executed by the ACC controller of the second embodiment.

[0019] FIG. 13 is a flowchart showing a torque deviation decreasing process executed at step S37 of the flowchart of FIG. 12.

[0020] FIG. 14 is a block diagram showing a construction of the ACC controller of a third embodiment.

[0021] Referring to FIGS. 1 to 10C, there is shown a first embodiment of an adaptive cruise control (ACC) system employed in a rear-wheel drive vehicle in accordance with the present invention. This ACC control system mainly executes a preceding-vehicle following control for following a preceding vehicle ahead of the rear-wheel drive vehicle (host vehicle).

[0022] As shown in FIG. 1, the host vehicle comprises front wheels 1FR and 1FL functioning as a driven wheel and rear wheels 1RR and 1RL functioning as a driving wheel. An internal combustion engine 2 is connected through an automatic transmission 3, a propeller shaft 4, a final reduction gear 5 and an axle 6 to rear wheels 1RL and 1RR to transmit rotational power of engine 2 to rear wheels 1RL and 1RR. A brake actuator 7 constituted by a disc brake unit is installed to each of front and rear wheels 1FR, 1FL, 1RR and 1RL. Brake actuator 7 is of a hydraulic control type, and a brake control unit 8 controls brake hydraulic pressure applied to each brake actuator 7. The brake control unit 8 varies a hydraulic pressure according to a depression degree of a brake pedal 8a, and varies the hydraulic pressure according to a brake pressure command P_BRfrom an adaptive cruise control (ACC) controller 20. An engine output control unit 11 is installed to engine 2 to control an output of engine 2. A throttle actuator 12 is installed to engine 2 and varies an opening of a throttle of engine 2. Engine output control unit 11 controls the throttle actuator 12 according to a depression degree of an accelerator pedal 11aand a throttle opening command θ_Routputted from ACC controller 20.

[0023] A vehicle speed sensor 13 is installed to an output shaft of automatic transmission 3 to detect a host vehicle speed V_Sof the host vehicle. Vehicle speed sensor 13 is constituted by an electromagnetic pickup which detects a positional change of a rotor gear attached to the output shaft of automatic transmission 3. Further, an inter-vehicle distance sensor 14 is installed to a frond and lower portion of a vehicle body, and detects an inter-vehicle distance between the host vehicle and a preceding vehicle traveling ahead of the host vehicle. Inter-vehicle distance sensor 14 is of a radar type which sweepingly emits laser beam in the forward direction and receives the reflection beam thereof to detect a distance to an obstacle reflecting the laser beam.

[0024] ACC controller 20 receives a detection signal outputted from vehicle speed sensor 13, a detection signal outputted from inter-vehicle distance sensor 14. When ACC controller 20 catches a preceding vehicle, ACC controller 20 controls the inter-vehicle distance so as to bring the distance closer to a target inter-vehicle distance determined by ACC controller 20. When ACC controller 20 does not catch the preceding vehicle, ACC controller 20 outputs brake hydraulic pressure command P_BRand a target throttle opening θ_Rto brake control unit 8 and engine output control unit 11, respectively, to control host vehicle speed V_Sat a set-speed V_SETset at a vehicle speed setting device 20aby a driver of the host vehicle.

[0025] ACC controller 20 comprises a microcomputer and peripheral devices of the microcomputer and further comprises a control block in the form of software previously stored therein as shown in FIG. 2. This control block shown in FIG. 2 is constituted by a distance signal processing section 21, a vehicle speed signal processing section 30, an inter-vehicle distance control section 40, a vehicle speed control section 50, a driving torque changeover deciding section 60, a driving torque changeover control section 61, a torque selecting section 62, and a driving torque control section 70. Distance signal processing section 21 measures a time period from a moment of sweepingly emitting laser beam from inter-vehicle distance sensor 13 to a moment of receiving reflected beam of the preceding vehicle and calculates inter-vehicle distance L between the preceding vehicle and the host vehicle on the basis of the measured time period. Vehicle speed signal processing section 30 measures a cycle period of a vehicle speed indicative pulse outputted from vehicle speed sensor 13 and calculates host vehicle speed V_Son the basis of the measured cycle period.

[0026] Inter-vehicle distance control section 40 calculates a target vehicle speed V* for maintaining inter-vehicle distance L at target inter-vehicle distance L* on the basis of inter-vehicle distance L calculated at distance signal processing section 21 and host vehicle speed VS calculated at vehicle speed signal processing section 30. Inter-vehicle distance control section 40 further calculates a first target driving torque T_W1*. Vehicle speed control section 50 calculates a second target driving torque T_W2* on the basis of target vehicle speed V* calculated at inter-vehicle distance control section 40. Driving torque changeover deciding section 60 decides a changeover of the driving torque on the basis of host vehicle speed V_Scalculated at vehicle speed signal processing section 30 and outputs a torque select signal S_Tindicative of the decision result. Drive-shaft torque changeover control section 61 calculates a third target driving torque T_W3* on the basis of torque select signal S_Toutputted from driving torque changeover deciding section 60 and first and second target driving torques T_W1* and T_W2*. Further, driving torque changeover control section 61 executes the matching of second and third target driving torques T_W2* and T_W3* when the content of torque select signal S_Tchanges. Torque selecting section 62 selects one of second and third target driving torques T_W2* and T_W3* and sets the selected one as a target driving torque T_W*. Driving torque control section 70 calculates target throttle opening θ_Rfor controlling throttle actuator 12 and target brake hydraulic pressure P_BRfor controlling brake actuator 7 on the basis of target driving torque T_W*, and outputs target throttle opening θ_Rand target brake hydraulic pressure P_BRto engine output control unit 11 and brake control unit 8, respectively.

[0027] Inter-vehicle distance control section 40 comprises a target inter-vehicle distance setting section 42 and an inter-vehicle distance control calculating section 43. Target inter-vehicle distance setting section 42 calculates target inter-vehicle distance L* between the preceding vehicle and the host vehicle on the basis of host vehicle speed V_Sinputted from vehicle speed processing section 30. Inter-vehicle distance control calculating section 43 calculates target vehicle speed V* for bringing inter-vehicle distance L closer to target inter-vehicle distance L* on the basis of inter-vehicle distance L and host vehicle speed V_S.

[0028] Target inter-vehicle distance setting section 42 calculates target inter-vehicle distance L between the host vehicle and the preceding vehicle from host vehicle speed V_San a time duration (time gap) T₀for reaching the host vehicle at a position apart from the preceding vehicle by the distance L₀[m] and the following equation (1):

[0029] L*=V_S×T₀+L_S,  (1)

[0030] where L_Sis an inter-vehicle distance when the vehicle is stopping. By employing the concept of this time gap, it becomes possible to increase the inter-vehicle distance according to the increase of the vehicle speed.

[0031] Further, inter-vehicle distance control calculating section 43 calculates first target driving torque T_W1* and target vehicle speed V* employed for execute the following of a preceding vehicle while keeping inter-vehicle distance L at target distance L*, on the basis of inter-vehicle distance L, target inter-vehicle distance L* and host vehicle speed V_S. More specifically, as shown in FIG. 3, inter-vehicle distance control calculating section 43 is arranged to calculate the relative speed ΔV from the equation (2) below, in a manner of calculating a change rate of inter-vehicle distance L by treating inter-vehicle distance L by means of a band-pass filter 43a.

[0032] ΔV=K₀·s·L/(s²+K₁·s+K₀),  (2)

[0033] where K₀and K₁are filter constants, and S is Laplace operator.

[0034] Assuming that the vehicle speed control system can be approximated by a first-order lag system which has a time constant τ_V(=1/ωV) representative of a response of host vehicle speed V_Swith respect to the target vehicle speed V*, the inter-vehicle distance control system is constructed as shown in FIG. 3, and the transfer characteristics from target inter-vehicle distance L* to actual inter-vehicle distance L is expressed by the following equation (3): $\begin{array}{cc}L=\frac{1}{s}\left\{V_T-\frac{\omega V}{s+\omega V}{V}^{*}\right\}.& \left(3\right)\end{array}$

[0035] Therefore, target vehicle speed V* is obtained by adding a target relative speed ΔV* to preceding-vehicle speed V_T, where target relative speed ΔV* is calculated from inter-vehicle distance L and relative speed ΔV, as follows.

[0036] V*=V_T+ΔV*,  (4)

[0037] where preceding-vehicle speed V_Tis obtained by adding relative speed ΔV to host vehicle speed V_S, as follows:

[0038] V_T=V_S+ΔV.  (5)

[0039] By substituting equation (4) into equation (3), the transfer function from target relative speed ΔV* to actual inter-vehicle distance L is expressed by the following equation (6): $\begin{array}{cc}L=\frac{1}{s}\left\{\frac{s}{s+\omega V}{V}_T-\frac{\omega V}{s+\omega V}\Delta V^{*}\right\}.& \left(6\right)\end{array}$

[0040] Next, in order to set relative speed ΔV under stationary state at zero by bring actual inter-vehicle distance L closer to target inter-vehicle distance L* with respect to the controlled object of equation (6), target relative speed ΔV* is calculated from the following equation (7):

[0041] ΔV*=K_L(L*−L)+K_D·ΔV.  (7)

[0042] Since equation (7) is a target value of the relative speed, target vehicle speed V* of the host vehicle is calculated by adding preceding vehicle speed V_Tto target relative speed ΔV* as follows:

[0043] V*=ΔV*+V_T.  (8)

[0044] FIG. 4 shows a block diagram of an inter-vehicle distance control system in case that the inter-vehicle distance is controlled based on the above-mentioned control rule. At this time, the transfer characteristic from target inter-vehicle distance L* to actual inter-vehicle distance L is expressed by the following equation (9): $\begin{array}{cc}\begin{array}{c}L=\frac{\omega V·K_L}{s^2+\omega V\left(1+K_D\right)s+\omega V·K_L}{L}^{*}+\\ \frac{s}{s^2+\omega V\left(1+K_D\right)s+\omega V·K_L}{V}_T,\end{array}& \left(9\right)\end{array}$

[0045] where K_Dis a relative speed gain, and K_Lis an inter-vehicle distance gain.

[0046] By properly setting relative speed gain K_Dand inter-vehicle distance gain K_Lof equation (9), it becomes possible to change a pole of equation (9) and to set the following response characteristic at a desired characteristic. More specifically, the calculation of equations (7) and (8) is executed in the block diagram of FIG. 3. That is, an inter-vehicle distance deviation (L*−L) is calculated by subtracting actual inter-vehicle distance L from target inter-vehicle distance L* at a subtracter 43b. The product of deviation (L*−L) and inter-vehicle distance gain K_Lis obtained at a multiplier 43c. At a subtracter 43d, the target relative speed ΔV* is calculated by subtracting the value obtained at the multiplier 43cfrom a value obtained by multiplying relative speed ΔV and relative speed gain K_Dat a multiplier 43e. Further, relative speed ΔV and host vehicle speed V_Sare added to target relative speed ΔV* at an adder 43fand an adder 43g. The sum (ΔV+V_S) of the added values corresponds to preceding vehicle speed V_T. By these calculations in the inter-vehicle distance control calculation section 43, target vehicle speed V* is calculated. Hereinafter, it will be proved that the calculation process of target vehicle speed V* in the inter-vehicle distance control calculation section 43 can calculate a value equivalent to an acceleration/deceleration command value of the vehicle.

[0047] Since the second term of right side in equation (3) is host vehicle speed V_S, an equation (10) below is obtained by substituting equations (4) and (5) into this relation as to host vehicle speed V_S. $\begin{array}{cc}\begin{array}{c}{V}_S=\omega V·V^{*}/\left(s+\omega V\right)\\ =\omega V\left(V_s+\Delta V+\Delta V^{*}\right)/\left(s+\omega V\right)\end{array}& \left(10\right)\end{array}$

[0048] By coordinating equation (10), the following equation (11) is obtained.

[0049] V_S=ωV(ΔV+ΔV*)/s  (11)

[0050] It is noted from equation (11) that the value obtained by multiplying ωV and the sum of relative speed ΔV and target relative speed ΔV* corresponds to the acceleration/deceleration command value. Therefore, first target driving torque T_W1* outputted from inter-vehicle distance control calculating section is expressed by the following equation (12):

[0051] T_W1*=ωV(ΔV+ΔV*)·M·RW,  (12)

[0052] where M is a weight of vehicle, and RW is a tire radius.

[0053] Accordingly, by supplying the addition output (ΔV+ΔV*) of the adder 43fto the multiplier 43hand by multiplying a value (ωV·M·RW) indicative of a vehicle specification with the output (ΔV+ΔV*) as shown by the block diagram of FIG. 3, first target driving torque T_W1* is calculated.

[0054] Vehicle speed control section 50 is arranged to calculate second target driving torque T_W2* for bring host vehicle speed V closer to target vehicle speed V*. Vehicle speed control section 50 is arranged as shown by a block diagram of FIG. 4. Herein, it is assumed that the transfer lag of the driving torque control system is negligible. First, By calculating a running resistance F_RES, which is the sum of air resistance and rolling resistance, based on an equation (13) below and by previously adding the running resistance F_RESinto the controlled object, each running resistance is compensated.

[0055] F_RES=μA·SV·V_S²+μR·M·g,  (13)

[0056] where μA is air resistance coefficient, SV is frontal projected area, μR is rolling resistance coefficient, M is vehicle weight, and g is gravitational acceleration.

[0057] Grade resistance F_DHis estimated from the calculation of an equation (14) below, target driving force F* and host vehicle speed V_S.

[0058] F_DE=H(s)·M·s·V_S−H(s)·F*,  (14)

[0059] where H(s) is a low-pass filter which has a stationary gain of “1”.

[0060] Target driving force F_W* is calculated by subtracting grade resistance estimate F_DHfrom driving force command value F_WRobtained by multiplying vehicle speed gain K_SPand a deviation of target vehicle speed V* and host vehicle speed V_S. This calculation of target driving force F_W* functions to eliminate interference of the grade resistance and so on with respect to the transfer characteristic from driving force command value F_WRto host vehicle speed V_S.

[0061] F_W*=F_WR−F_DE  (15)

[0062] By adding running resistance estimate F_RESto target driving force F_W* calculated by equation (15) and by multiplying the tire radius to this sum as expressed by an equation (16) below, second target driving torque T_W2* is calculated.

[0063] T_W2*=RW(F_W*−F_RES)  (16)

[0064] Assuming that disturbance to the transfer characteristic from driving force command value F_WRto host vehicle speed V_Sshown in FIG. 4 was eliminated by the running resistance compensation, transfer characteristic G_Vis represented by the following equation (17):

[0065] G_V(S)=V_S(S)/F_WR(S)=1/M·s.  (17)

[0066] A driving force command F_WRreceiving this transfer characteristic G_V(s) as a new controlled object is calculated based on the following equation (18):

[0067] F_WR=K_SP(V*−V_s).  (18)

[0068] Accordingly, the transfer characteristic from target vehicle speed V* to actual vehicle speed V_Sis represented by an equation (19) below, and therefore it becomes possible that the response of the vehicle speed control system is adjusted to the desired response characteristic.

[0069] V_S=(K_SP/M)V*/(s+K_SP/M)  (19)

[0070] In the block diagram of FIG. 4, subtracter 51 calculates the vehicle speed deviation by subtracting host vehicle speed V_Sfrom target vehicle speed V*. Multiplier 52 calculates driving force command value F_WRby multiplying vehicle speed gain K_SPto the vehicle speed deviation. Grade resistance estimating section 54 calculates grade resistance estimate F_DHbased on target driving force F_W* and host vehicle speed V_Sand from equation (14). Subtracter 53 calculates target driving force F_W* by subtracting grade resistance estimate F_DHcalculated at grade resistance estimating section 54 from the driving force command F_WR. Adder 55 obtains the sum of target driving force F_W* and running resistance F_RES. Multiplier 56 calculates second target driving torque T_W2* by multiplying a tire radius RW to the sum obtained at adder 55.

[0071] Driving torque changeover deciding section 60 generates a torque select signal S_Tfor selecting the target driving torque. The decision as to whether torque select signal S_Tis generated is executed by deciding whether host vehicle speed V_Sis smaller than or equal to a vehicle speed which ensures a vehicle speed detecting time by which a safety margin of the vehicle speed control system becomes greater than a predetermined value. It is assumed that it is necessary that the time lag due to the vehicle speed detection is smaller than or equal to a value Tmg(s) in order to ensure a predetermined safety margin Gmg(dB) of the vehicle speed control system, and that an interval of the vehicle speed pulses becomes greater than a value Tmg(s) when the vehicle speed is smaller than or equal to a value Vmg (km/h). If so, when the vehicle speed is smaller that value Vmg (km/h), the safety margin cannot be ensured, and therefore the changeover of the target driving torque is executed. If an electromagnetic pickup is employed, changeover decision vehicle speed Vmg (km/h) may be set at a vehicle speed at which a vehicle speed pulse voltage cannot become detected. This decision has been previously calculated, and a changeover vehicle speed V_CHGfor generating the torque changeover has been preset. Further, in order to avoid the torque changeover from being frequently executed in the vicinity of changeover vehicle speed V_CHG, a hysteresis is provided. More specifically, the hysteresis is set such that the theoretical value of the torque select signal S_Tis set at “1” when V_S≦V_CHG, and that the theoretical value is set a “0” when V_S>V_CHG+ΔV_CHG, where ΔV_CHGis a hysteresis width.

[0072] The driving torque changeover control section 61 calculates a third target driving torque T_W3* based on torque select signal S_Toutputted from the driving torque select deciding section 60 and first target driving torque T_W1*. More specifically, by deceleration of the vehicle speed of the preceding vehicle from a condition that the theoretical value of torque select signal S_Tis “0”, when host vehicle speed V_Sbecomes smaller than or equal to predetermined changeover vehicle speed V_CHGand when torque select signal S_Tis turned to “1”, first target driving torque T_W1*(k) is selected instead of second target driving torque T_W2(k−1) at torque selecting section 62. Although the second target driving torque T_W2* has been set upon taking account of the running resistance including road grade, first target driving torque T_W1* has been set without taking account of the running resistance. Therefore, a deviation between first and second target driving torques T_W1* and T_W2* increases according to the increase of the road grade. By calculating a torque deviation ΔT_W1between second and first target driving torques T_W2* and T_W1* according to an equation (20) below, by latching the deviation and by adding torque deviation ΔT_W1to first target driving torque T_W1*, third target driving torque T_W3corresponded to second target driving torque T_W2* is calculated. Further, since the feedback of grade resistance estimate F_DHat vehicle speed control section 50 provides a function as same as the integral thereof, grade resistance estimate F_Eis set at “0” by outputting a reset signal S_Rindicative of a theoretical value “1” to grade resistance estimating section 54 and resetting an inner variable employed in the calculation of the grade resistance at grade resistance estimating section 54.

[0073] ΔT_W1=T_W2*−T_W1*(k)  (20)

[0074] T_W3*(k)=ΔT_W1+T_W1*(k)  (21)

[0075] In reverse, when torque select signal S_Twas switched from “1” to “0”, torque selecting section 62 selects second target driving torque T_W2*(k) instead of third target driving torque T_W3*(k−1). Therefore, torque deviation ΔT_W2between third and second target driving torques T_W3*(k) and T_W2*(k) is calculated according to an equation (22) below. Further, torque deviation ΔT_W2is outputted to grade resistance estimating section 54, and the theoretical value of reset signal S_Ris set at “0”. Therefore, second target driving torque T_W2* outputted from vehicle speed control section 50 is offset to the negative direction by an initial value F_DHOwith respect to previous second target driving torque T_W2* as represented by an equation (23) below, and is adjusted to third target driving torque T_W3*.

[0076] ΔT_W2=T_W2*(k)−T_W3*(k)  (22)

[0077] T_W3*(k)=T_W2*(k)−F_DHO  (23)

[0078] Further, torque selecting section 62 selects second target driving torque T_W2* outputted from vehicle speed control section 50 when the theoretical value of torque select signal S_Tinputted from driving torque changeover deciding section 60 is “0” and outputs second target driving torque T_W2* to driving torque control section 70 as target driving torque T_W*. Further, torque selecting section 62 selects third target driving torque T_W3* outputted from vehicle speed control section 50 when the theoretical value of torque select signal S_Tis “1”, and outputs third target driving torque T_W3* to driving torque control section 70 as target driving torque T_W*. Driving torque control section 70 calculates throttle opening command θ_Rand brake hydraulic pressure command P_BRfor realizing target driving torque T_W* selected at the torque selecting section, as shown in FIG. 5.

[0079] Assuming that a torque amplification ratio of a torque converter is R_T, a gear ratio of the automatic transmission 3 is R_AT, a gear ratio of a differential is R_DEF, engine inertia is J_E, and an engine speed is N_E, a relationship among driving torque T_W, engine torque T_Eand brake torque T_BRis represented by the following equation (24):

[0080] T_W=K_GEAR{T_E−J_E(dN_E/dt)}−T_BR,  (24)

[0081] where K_GEAR=R_T·R_AT·R_DEF.

[0082] Accordingly, throttle opening command θ_Rfor generating target engine torque T_E* is calculated by calculating target engine torque T_E* with respect to target driving torque T_W* from an equation (25) below and by retrieving an engine map sown in FIG. 6.

[0083] T_E=J_E(dN_E/dt)+T_W*/K_GEAR  (25)

[0084] Herein, if throttle opening command θ_Ris greater than or equal to “0”, actual driving torque T_Wis brought closer to target driving torque T_W* only by engine torque. On the other hand, if throttle opening command θ_Ris smaller than “0”, the throttle opening is recognized as “0” and the brake operation quantity for bringing the driving torque to target driving torque T_W* is calculated upon taking account of the driving torque outputted by engine 2.

[0085] That is, the distribution control rule of target engine torque T_E* and target brake torque T_BR* is summarized as follows:

[0086] (A) When θ_R>0, T_BR*=0. Therefore, equation (24) under the condition (A) is represented by the following equation (26):

[0087] T_W=K_GEAR{T_E−J_E(dN_E/dt)}.  (26)

[0088] Accordingly, target engine torque T_E* obtained by equation (27) may be generated with respect to target driving torque T_W*.

[0089] T_E*=J_E(dN_E/dt)+T_W*/K_GEAR  (27)

[0090] Herein, since target brake torque T_BRis “0”, brake hydraulic pressure command P_BRis “0”.

[0091] (B) When θ_R≦0, engine torque T_EOunder a condition of θ_R=0 is calculated based on engine speed N_Eand with reference to the engine torque map shown in FIG. 7. Accordingly, equation (24) under this condition (B) is represented by the follow equation (28):

[0092] T_E*=K_GEAR{T_EO−J_E(dN_E/dt)}−T_BR.  (28)

[0093] Therefore, target brake torque T_BR* with respect to target driving torque T_W* is represented by the following equation (29):

[0094] T_BR*=−T_W*+K_GEAR{T_EO−J_E(dN_E/dt)}.  (29)

[0095] Assuming that a bake cylinder area is A_B, a rotor effective radius is R_B, and a pad friction coefficient is μB, the brake hydraulic command P_BRindicative of the brake operation quantity with respective to the target brake torque T_BR* is represented by the following equation (30):

[0096] P_BR=(1/K_BT)·T_BR*,  (30)

[0097] where K_BT=8·A_B·R_B·μ_B.

[0098] Accordingly, driving torque control section 70 is arranged as shown by a block diagram of FIG. 5. More specifically, a divider 71 receives target driving torque T_W* and calculates a target engine torque T_W* by dividing target driving torque T_W* by a coefficient K_GEAR. A throttle opening calculating section 72 receives target engine torque T_E* and calculates throttle opening command θ_Rbased on target engine torque T_E* and engine speed N_Eand with reference to an engine characteristic map shown in FIG. 6. A limiter 73 receives the throttle opening command θ_Rand limits the throttle opening command θ_Rwithin a range from zero to a maximum throttle opening controllable by throttle actuator 12. Limiter 73 outputs limited throttle opening command θ_Rto engine output control unit 11. A subtracter 74 calculates target brake torque T_BR* by subtracting target driving torque T_W* from a value K_GEAR{T_EO−J_E(dN_E/dt)} which is a second term of a right side in equation (29). A divider 76 receives target brake torque T_BR* calculated at subtracter 74 and calculates brake hydraulic pressure command P_BRby executing the calculation of equation (30). A limiter 77 limits brake hydraulic pressure command P_BRwithin a range from zero to a maximum braking pressure controllable by brake actuator 7 and outputs limited brake hydraulic pressure command P_BRto brake control unit 8.

[0099] The following control processing executed by ACC controller 20 of the first embodiment according to the present invention will be discussed with reference to a flowchart of FIG. 8. This following control processing is executed as a timer interruption process at predetermined intervals (10 msec) with respect to a predetermined main program.

[0100] At step S1, ACC controller 20 reads host vehicle speed V_Sprocessed by vehicle speed signal processing section 30, and decides whether or not host vehicle speed V_Sis smaller than or equal to changeover vehicle speed V_CHG. When the decision at step S1 is affirmative (V_S≦V_CHG), the routine proceeds to step S2 wherein ACC controller 20 sets host vehicle speed V_Sat zero (VS=0). When the decision at step S1 is negative (V_S>V_CHG), the routine proceeds to step S3.

[0101] At step S3, ACC controller 20 calculates target inter-vehicle distance L* by executing the calculation of equation (1) based on host vehicle speed V_S.

[0102] At step S4, ACC controller 20 calculates relative speed ΔV by executing the band-pass filter treatment as to equation (2) based on inter-vehicle distance L detected by inter-vehicle distance sensor 14.

[0103] At step S5, ACC controller 20 calculates target vehicle speed V* by executing the calculations of equations (7) and (8), and calculates first target driving torque T_W1* by executing the calculation of equation (12).

[0104] At step S6, ACC controller 20 decides whether reset flag RS is set at 1 or not. When the decision at step S6 is affirmative (RS=1), the routine proceeds to step S7 wherein grade resistance estimate F_DHis set at zero (F_DH=0), and proceeds then to step S9. When the decision at step S6 is negative (RS=0), the routine proceeds to step S8 wherein ACC controller 20 calculates grade resistance estimate F_DHby executing the calculation of equation (14).

[0105] At step S9, ACC controller 20 calculates second target driving torque T_W2* by executing calculations of equations (13), (15) and (16).

[0106] At step S10, ACC controller 20 decides whether or not host vehicle speed V_Sis smaller than or equal to changeover vehicle speed V_CHG. When the decision at step S10 is affirmative (V_S≦V_CHG), the routine proceeds to step S11 wherein a present torque select flag TS(k) is set at 1 (TS(k)=1). When the decision at step S10 is negative (V_S>V_CHG), the routine proceeds to step S12 wherein ACC controller 20 decides whether or not host vehicle speed V_Sis greater than the sum of changeover vehicle speed V_CHGand hysteresis width ΔV_CH. When the decision at step S12 is affirmative (V_S>V_CHG+ΔV_CHG), the routine proceeds to step S13 wherein present torque select flag TS(k) is set at 0 (TS(k)=0). When the decision at step S12 is negative (V_S>V_CHG+ΔV_CHG), the routine proceeds to step S14 wherein present torque select flag TS(k) is set at a previous torque select flag TS(k−1) (TS(k)=TS(k−1)).

[0107] At step S15 following to the execution of step S11, S13 or S14, ACC controller 20 decides whether or not torque select flag TS(k) is set at 1. When the decision at step S15 is affirmative (TS(k)=1), the routine proceeds to step S16. When the decision at step S15 is negative (TS(k)=0), the routine proceeds to step S21.

[0108] At step S16, ACC controller 20 decides whether or not it is put in a torque select time where previous torque select flag TS(k−1) is set at 0. When the decision at step S16 is affirmative (TS(k−1)=0), the routine proceeds to step S17 wherein torque deviation ΔT_W1is calculated by executing calculation of equation (20). After the execution of step S17, the routine proceeds to step S18. When the decision at step S16 is negative (TS(k−1)=0), the routine proceeds to step S18.

[0109] At step S18, ACC controller 20 calculates third target driving torque T_W3* by executing the calculation of equation (21).

[0110] At step S19, ACC controller 20 sets reset flag RS at 1 (RS=1).

[0111] At step S20, ACC controller 20 calculates throttle opening command θ_Rand brake hydraulic pressure command P_BRby executing the calculations of equations (24) to (30) and outputs throttle opening command θ_Rand brake hydraulic pressure command P_BRto engine output control unit 11 and brake control unit 8, respectively. Thereafter, the present routine is terminated, and the routine returns to the predetermined main program.

[0112] On the other hand, when the decision at step S15 is negative (TS(k)=0), the routine proceeds to step S21 wherein ACC controller 20 decides whether or not it is put in a torque select time where previous torque select flange TS(k−1) is set at 1. When the decision at step S21 is affirmative, the routine proceeds to step S22 wherein torque deviation ΔT_W2is calculated by executing calculation of equation (22).

[0113] At step S23 following to the execution of step S22, ACC controller 20 sets torque deviation ΔT_W2as an initial value F_DHOemployed for calculating the grade resistance, and calculates second target driving torque T_W2* by executing the calculations of equations (13), (15) and (16) on the basis of initial value F_EHO.

[0114] At step S24 following to the negative decision at step S21 or execution of step S23, ACC controller 20 sets second target driving torque T_W2* as target driving torque T_W*. Thereafter, the routine proceeds to step S25 wherein reset flag RS is reset at 0. Then, the routine proceeds to step S20.

[0115] In the following control processing shown in FIG. 8, the processing at step S3 corresponds to target inter-vehicle distance calculating means, the processing at steps S4 and S5 corresponds to vehicle speed control means, the processing at steps S10 to S14 corresponds to driving torque deciding means, the processing at step S15 to S19 corresponds to driving torque select and match means, and the processing at step S20 corresponds to driving/braking force generating means.

[0116] It is now assumed that a set switch of the following control is put in an ON state, and therefore the following control is being executed, and host vehicle speed V_Sis put in a normal running condition where host vehicle speed V_Sis greater than changeover vehicle speed V_CHG, and the host vehicle is decelerated according to the deceleration of the preceding vehicle, as shown in FIG. 9A.

[0117] Under this condition, at step S3 of the processing of FIG. 8, target inter-vehicle distance L* is calculated, and at steps S4 and S5, target vehicle speed V* and first target driving torque T_W1* are calculated. At this time, the preceding vehicle is decelerated, and the inter-vehicle distance L detected by inter-vehicle distance sensor 14 is smaller than target inter-vehicle distance L*. Therefore, first target driving torque T_W1* takes a negative value as shown by a broken line of FIG. 9C. Next, at step S8, grade resistance estimate F_DHis calculated, and at step S9 second target driving torque T_W2*, which takes a negative value smaller than first target driving torque T_W1* due to running resistance as shown in FIG. 9C, is calculated.

[0118] Since host vehicle speed V_Sis greater than changeover vehicle speed V_CHGor the sum of changeover vehicle speed V_CHGand hysteresis ΔV_CHG, the following control routine proceeds to step S13 wherein torque select flag TS is reset (TS(k)=0). Then, the routine proceeds through steps S15 and S21 to step S24 wherein second target driving torque T_W2* calculated at step S9 is set as target driving torque T_W*. Although throttle opening θ indicative of negative value is calculated based on target driving torque T_W* with reference to the engine torque map of FIG. 7, the throttle opening θ0 calculated is limited at zero by limiter 73. Accordingly, throttle opening command θ_Rcommanding zero is outputted to engine output control unit 11, and therefore the throttle opening is controlled at zero by means of throttle actuator 12.

[0119] Simultaneously, target driving torque T_W* is negative, throttle opening θ is negative and throttle opening command θ_Ris zero. Accordingly, engine torque T_EOcalculated with reference to the engine torque map of FIG. 7 takes negative value. Further, engine brake torque T_EBrepresented by K_GEAR{T_EO−J_E(dN_E/dt)} takes a negative value, and therefore target brake torque T_BR* calculated from equation (29) takes a positive value. Brake hydraulic pressure command P_BRis calculated by executing the calculation of equation (30) based on target brake torque T_BR* and is outputted to brake control unit 8. Therefore, brake control unit 8 controls the brake pressure of brake actuator 7, and therefore braking state of brake actuator 7 is maintained and the deceleration condition of the host vehicle is continued.

[0120] When host vehicle speed V_Sbecomes smaller than or equal to changeover vehicle speed V_CHGat the time t1 due to the reason that the preceding vehicle is decelerated and travels at extremely low speed due to approaching traffic snarl, the measurement accuracy of host vehicle speed V_Sdegrades due to elongation of detection pulse interval outputted from vehicle speed sensor 13. However, since host vehicle speed V_Shas been set at zero by the execution of steps S1 and S2, target inter-vehicle distance L* calculated at step S3 is set as inter-vehicle distance L_Sduring vehicle stopping state.

[0121] At time t1, first target driving torque T_W1* calculated at step S5 on the basis of target inter-vehicle distance L* and actual inter-vehicle distance L is greater than second target driving torque T_W2*, which calculated at step S9 and is feedback controlled as to grade estimate F_DH, by torque deviation ΔT_W1, as shown in FIG. 9C.

[0122] Since torque select flag TS is set at 1 as shown in FIG. 9B by the execution of steps S10 and S11 of the following control process of FIG. 8, the routine proceeds from step S15 to step S16 wherein torque select flag TS is changed from 0 to 1. Therefore, the routine proceeds to step S17 wherein torque deviation ΔT_W1taking a negative value is calculated by executing the calculation of equation (20). Then, the routine proceeds to step S18 wherein third target driving torque T_W3*(k) is calculated by adding torque deviation ΔT_W1to first target driving torque T_W1*(k) and is set as target driving torque T_W*(k). Next, the routine proceeds to step S19 wherein reset flag RS is set at 1 (RS=1). Then, the routine proceeds to step S20 wherein throttle opening command θ_Rand brake hydraulic pressure command P_BRare outputted.

[0123] At this time, since third target driving torque T_W3*(k) is put in a torque matching condition where it is matched with second target driving torque T_W2*, the changeover of target driving torque T_W* from second target driving torque T_W2*(k−1) to third target driving torque T_W3*(k) is smoothly executed without generating torque fluctuation. Accordingly, it is possible to maintain previous braking state without generating change of deceleration.

[0124] Thereafter, when the preceding vehicle continues extremely low-speed traveling due to traffic snarl, in the following control processing of FIG. 8 after the predetermined time period elapsed, the routine proceeds from step S6 to step S7 since reset flag RS has been set at 1. Accordingly, the internal variable for the running resistance calculation in the vehicle speed control is reset, and grade estimate F_DHis set at zero. Although second target driving torque T_W2* calculated at step S9 takes a value greater than third target driving torque T_W3*, target inter-vehicle distance L* is kept at stopping-state inter-vehicle distance L_SAccordingly, first target driving torque T_W1* is calculated so as to keep inter-vehicle distance L_S, and third target driving torque T_W3* obtained by adding torque deviation ΔT_W1to first target driving torque T_W1* is set as target driving torque T_W*. Consequently, the following control under extremely low-speed is continued without depending on inaccurate host vehicle speed V_S.

[0125] When the host vehicle travels at this extremely low-speed due to traffic snarl and when the vehicle speed is generated by creeping phenomenon of automatic transmission 3, if inter-vehicle distance L detected by inter-vehicle speed sensor 14 becomes greater than target inter-vehicle distance L* set at stopping-state inter-vehicle distance L_S, third target driving torque T_W3*(k) takes a positive value, as shown in FIG. 10C. Further, throttle opening command θ_Rincreases according to the change of third target driving torque T_W3*(k), and the throttle valve is slightly opened by throttle actuator 12.

[0126] Thereafter, at time t11, the preceding vehicle starts accelerating, and inter-vehicle distance L becomes greater than target inter-vehicle distance L* according to the acceleration of the preceding vehicle. Accordingly, third target driving torque T_W3*(k) increases as shown by a continuous line of FIG. 10A, and second target driving torque T_W2*(k) increases, as shown by a broken line of FIG. 10A, so as to become greater than third target driving torque T_W3*(k). However, since host vehicle speed V_Sis kept at a vehicle speed smaller than changeover vehicle speed V_CHG, third target driving torque T_W3* is continuously set as target driving torque T_W*, and the acceleration control of the host vehicle is executed on the basis of this target driving torque T_W*.

[0127] Thereafter, when host vehicle speed V_Sbecomes greater than changeover vehicle speed V_CHGat time t12, the routine of FIG. 8 proceeds from step S10 to step S12. However, since host vehicle speed V_Sis still smaller than the sum of changeover vehicle speed V_CHGand hysteresis width ΔV_CHG, the routine proceed to step S14 wherein torque select flag TS is maintained at 1, and therefore the acceleration control of the host vehicle is continued by employing third target driving torque T_W3*(k).

[0128] Thereafter, when host vehicle speed V_Sbecomes greater than the sum of changeover vehicle speed V_CHGand hysteresis width ΔV_CHGat time t13, the routine of FIG. 8 proceeds from step S12 to step S13 wherein torque select flag TS(k) is reset at 0 (TS(k)=0). Therefore, the routine proceeds from step S15 to step S21 wherein affirmative decision is made since previous torque select flag TS(k−1) is set at 1 (TS(k−1)=1). Then, the routine proceeds to step S22 wherein torque deviation ΔT_W2is calculated by subtracting third target driving torque T_W3*(k) from second target driving torque T_W2*(k) according to equation (22). Next, the routine proceeds from step S22 to step S23 wherein torque deviation ΔT_W2is set as initial value F_DHOemployed in the grade resistance calculation of equation (14). Further, at step S23, target driving force F is calculated based on initial value F_HOby executing the calculation of equation (15), and second target driving torque T_W2*(k) is calculated by executing the calculation of equation (16).

[0129] Second target driving torque T_W2*(k) is the sum of grade resistance estimate F_DHOand previous second target driving torque T_W2*(k−1) calculated at step S9 under a condition of F_DH=0. Therefore, it becomes possible to match second target driving torque T_W2*(k) with third target driving torque T_W3*(k) in torque matching. This enables smooth changeover of target driving torque T_W* from third driving torque T_W3*(k−1) to second driving torque T_W2*(k). Therefore, accelerating condition is continued without generating change of acceleration.

[0130] Thereafter, when the processing shown in FIG. 8 is executed after the predetermined time elapsed, since reset flag RS has been reset at 0 (RS=0), the routine of FIG. 8 proceeds from step S6 to step S8 wherein grade resistance estimate is calculated based on initial value F_DHO. Accordingly, grade resistance estimate F_DHis gradually converged to a value corresponding to the road grade, and the control condition of the host vehicle is returned to the normal following condition.

[0131] With the thus arranged embodiment according to the present invention, when host vehicle speed V_Sis greater than changeover vehicle speed V_CHGand is accurately detected by vehicle speed sensor 13, ACC controller 20 selects second target driving torque T_W2*. On the other hand, when host vehicle speed V_Sis smaller than or equal to changeover vehicle speed V_CHG, ACC controller 20 selects third target driving torque T_W3* which is not affected by host vehicle speed V_Sdetected by vehicle speed sensor 13. Therefore, the following control is smoothly executed. Further, since the torque changeover is executed after the torque matching for matching second target driving torque T_W2* and third driving torque T_W3* is executed, the torque selecting is smoothly executed. Furthermore, since ACC controller 20 is arranged to provide first target driving torque T_W1* employed as a basis of third target driving torque T_W3* at inter-vehicle distance control section 40, it is not necessary to separately prepare an inter-vehicle distance control system which is not affected by the vehicle speed. This greatly contributes to suppressing storage capacity for programs and/or calculation load of ACC controller 20 from increasing.

[0132] Referring to FIGS. 11 and 12, a second embodiment of the ACC system according to the present invention will be discussed. The second embodiment is characteristically arranged to omit the process for calculating third target driving torque T_W2* and the process for setting the initial value for the calculation of the grade resistance estimate and to correct the torque deviation from the target driving torque selected at torque-changeover time.

[0133] More specifically, as shown in FIG. 11, driving torque select control section 61 shown in FIG. 2 is omitted in the second embodiment. Accordingly, first target driving torque T_W1* calculated at inter-vehicle distance control calculating section 43 and second target driving torque T_W2* calculated at vehicle speed control section 50 are inputted to a torque select and match control section 63. When torque select signal S_Tinputted from driving torque changeover deciding section 60 is zero (S_T=0), first target driving torque T_W1* is selected. When torque select signal S_Tis 1 (S_T=1), second target driving torque T_W1* is selected. Further, during the torque changeover, the torque matching between former and later target driving torques is executed.

[0134] ACC controller 20 executes the following control processing shown in FIG. 12 instead of the following control of FIG. 8 at predetermined intervals (10 msec) as a timer interruption processing.

[0135] This following control processing of the second embodiment is modified by replacing step S17 of FIG. 8 with step S31 where torque deviation ΔT_W3is calculated by subtracting first target driving torque T_W1* calculated at step S5 from previous target driving torque T_W*(k−1). Similarly, step S22 of FIG. 8 is replaced with step S32 wherein torque deviation ΔT_W4is calculated by subtracting second target driving torque T_W2* calculated at step S9 from previous target driving torque T_W*(k−1). Step S18 of FIG. 8 is replaced with step S33 wherein target driving torque T_W* is calculated by adding torque deviation ΔT_W3and first target driving torque T_W1*, and step S23 of FIG. 8 is replaced with step S34 wherein target driving torque T_W* is calculated by adding torque deviation ΔT_W4and second target driving torque T_W2*. Further, following to steps S31 and S32, step S35 for resetting a condition flag OS at 0 and step S36 for setting condition flag OS at 1 are provided, respectively. Furthermore, there is provided step S37 between steps S21 and S34 as shown in FIG. 12. At step S37, ACC controller 20 executes a torque-deviation decreasing process for gradually decreasing torque deviation ΔT_Wby executing a subroutine. The other steps of the flowchart of FIG. 12 are the same as those of FIG. 8 and are designated by same reference numerals. Therefore, the explanation thereof is omitted herein.

[0136] The torque-deviation decreasing process executed at step S37 is particularly shown by a flowchart of FIG. 13. First, at step S38, ACC controller 20 decides whether or not condition flag OS indicative of an offset condition of second target driving torque T_W2* is set at 1. When the decision at step S38 is negative (OS=0), this subroutine jumps to a return step to terminate the present subroutine. When the decision at step S38 is affirmative (OS=1), the subroutine proceeds to step S39 wherein ACC controller 20 decides whether torque deviation ΔT_Wis positive or not. When the decision at step S39 is affirmative (ΔT_W>0), the subroutine proceeds to step S40 wherein new torque deviation ΔT_W4is calculated by subtracting a predetermined value ΔT_WSfrom present torque deviation ΔT_W4(ΔT_W4=ΔT_W4−ΔT_WS). Then, the subroutine proceeds from step S40 to step S41 wherein ACC controller 20 decides whether new torque deviation ΔT_W4is negative or not. When the decision at step S41 is affirmative (ΔT_W4<0), the subroutine proceeds to step S42 wherein torque deviation ΔT_W4is set at 0 (ΔT_W4=0). Thereafter, the subroutine proceeds from step S42 to step S43 wherein condition flag OS is reset at 0 (OS=0), and further proceeds to the return step to return to the routine of FIG. 12. When the decision at step S41 is negative (ΔT_W4≧0), the subroutine proceeds to the return step to terminate the present subroutine.

[0137] On the other hand, when the decision at step S39 is negative (ΔT_W4≦0), the subroutine proceeds to step S44 wherein new torque deviation ΔT_W4is calculated by adding predetermined value ΔT_WSto present torque deviation ΔT_W4(ΔT_W4=ΔT_W4+ΔT_WS). Then, the subroutine proceeds from step S44 to step S45 wherein ACC controller 20 decides whether new torque deviation ΔT_W4is positive or not. When the decision at step S45 is affirmative (ΔT_W4>0), the subroutine proceeds to step S42. When the decision at step S45 is negative (ΔT_W4≦0), the subroutine proceeds to the return step to terminate the present subroutine. After the execution of the subroutine of FIG. 13, the program routine of the FIG. 13 proceeds to step S34.

[0138] In the processing of FIGS. 12 and 13, processing at steps S10 to S14 corresponds to driving torque deciding means, processing at steps S15, S16, S19, S21, S25 and S31 to S45 corresponds to torque select and match means.

[0139] With the thus arranged second embodiment according to the present invention, as is similar to the first embodiment, when host vehicle speed V_Sis greater than changeover vehicle speed V_CHGand when the host vehicle is decelerated, torque select flag TS is reset at 0. Therefore, the routine of FIG. 12 proceeds from step S15 through step S21 to step S37 wherein the torque deviation decreasing process is executed. However, when a relatively long time period elapsed from the moment when toque select flag TS is changed from 1 to 0 (that is, previous accelerating period), torque deviation ΔT_W4is set at 0, and condition flag OS is reset at 0. Therefore, second target driving torque T_W2* is put in a non-offset condition.

[0140] Accordingly, second target driving torque T_W2*, which is calculated at step S9 upon taking account of the running resistance including road grade, air resistance and rolling resistance, takes a negative value (T_W2<TW1*) and is selected as target driving torque T_W2*. Therefore, the throttle opening is fully closed and the braking condition is continued through the control of the braking pressure of brake actuator 7.

[0141] Under this braking condition, when host vehicle speed V_Sbecomes smaller than changeover vehicle speed V_CHG, that is, when the host vehicle travels at extremely low-speed, torque select flag TS(k) is set at 1 (TS(k)=1), Further, the routine of FIG. 12 proceeds through steps S15 and S16 to step S31 wherein torque deviation ΔT_W3is calculated by subtracting first target driving torque T_W1* from previous target driving torque T_W*(k−1). Accordingly, torque deviation ΔT_W3takes a negative value, and the routine proceeds to step S35 wherein condition flag OS is set at 1 (OS=1). Further, the routine proceeds to step S33 wherein target driving torque T_W* is calculated by adding torque deviation ΔT_W3to first target driving torque T_W1*. The sum of torque deviation ΔT_W3to first target driving torque T_W1* takes a value equal to previous second target driving torque T_W2*(k−1) and is selected as target driving torque T_W* instead of second target driving torque T_W2*. Therefore, the changeover of the target driving torque is smoothly executed without generating change of deceleration during this torque changeover, and therefore the following control for maintaining the predetermined inter-vehicle distance is continued without being affected by inaccurate extremely-low host vehicle speed V_S.

[0142] When host vehicle speed V_Sbecomes greater than the sum of changeover vehicle speed V_CHGand hysteresis width ΔV_CHGfrom the extremely low-speed traveling condition, due to the acceleration of the preceding vehicle, torque select flag TS(k) is set at 1 (TS(k)=1), and therefore the routine of FIG. 12 proceeds from step S15 through step S21 to step S32 wherein negative torque deviation ΔT_W4is calculated by subtracting positive second target driving torque T_W2* calculated at step S9 from previous target driving torque T_W*(k−1), that is, the sum of first target driving torque T_W1* and torque deviation ΔT_W3. Further, the sum of second target driving torque T_W2* and torque deviation ΔT_Wcalculated at step S34 becomes smaller by torque deviation ΔT_Wand therefore the sum is matched with previous target driving torque T_W*(k−1). Therefore, as is similar to the first embodiment, the torque changeover to second target driving torque T_W2* is smoothly executed without generating acceleration change.

[0143] Thereafter, if accelerating condition is maintained, by the processing of step S13, the routine proceeds from step S21 to step S37 wherein the torque deviation decreasing process shown in FIG. 13 is executed. Therefore, torque deviation ΔT_W4is brought closer to zero by each timer interruption cycle. And if torque deviation ΔT_W4becomes smaller than 0, torque deviation ΔT_W4is set at zero (ΔT_W4=0). Accordingly, target driving torque T_W* calculated at step S34 takes a value where an offset amount of second target driving torque T_W2* is cancelled, and therefore the following control is accurately executed.

[0144] With the thus arranged second embodiment according to the present invention, when target driving torque T_W* is selected from one of first and second target driving torques T_W1* and T_W2* to the other, torque deviation ΔT_W3or ΔT_W4between former target driving torque T_W*(k−1) and later (new) target driving torque T_W1* or T_W2* is calculated, and the calculated deviation is added to later (new) target driving torque T_W1* or T_W2* to match the torque between the selecting former and later torques. Therefore, the torque changeover is smoothly executed without generating change of acceleration and deceleration.

[0145] Although the first and second embodiments have been shown and described such that host vehicle speed V_Sis set at zero, when host vehicle speed V_Sbecomes smaller than or equal to changeover vehicle speed V_CHGand therefore host vehicle speed V_Sbased on the detection signal of vehicle speed sensor 13 becomes inaccurate, the invention is not limited to this and may be arranged such that target inter-vehicle distance L* is set at a proper value greater than stopping-state inter-vehicle distance L_Swhen host vehicle speed V_Sbecomes smaller than changeover vehicle speed V_CHG.

[0146] Further, although the first and second embodiments according to the present invention have been shown and described such that vehicle speed sensor 13 detects a rotation speed of an output side of automatic transmission 3, it will be understood that the invention is not limited to this and may be arranged such that the vehicle speed is obtained by detecting wheel speeds of front wheels 1FL and 1FR and by multiplying the average of the wheel speeds and a tire circumference. Further, an estimated vehicle speed, which possibly suppresses the influence of anti-brake lock control, may be calculated by detecting wheel speeds of four wheels 1FL, 1FR, 1RL and 1RR.

[0147] Referring to FIG. 14, a third embodiment of the ACC system according to the present invention will be discussed. The third embodiment is particularly arranged to further comprise a fail-safe function for adapting to abnormality of the host vehicle speed detecting system in addition to the construction of the first embodiment. More specifically, the third embodiment is arranged such that a two-input OR circuit 80 is installed between driving torque changeover deciding section 60 and driving torque changeover control section 61, as shown in FIG. 14. One of two inputs of OR circuit 80 is connected to driving torque changeover deciding section 60, and the other of two input is connected to a vehicle speed abnormality detecting section 81 which detecting the abnormality of vehicle speed sensor 13. Vehicle abnormality detecting section 81 monitors host vehicle speed V_Sinputted from vehicle speed processing section 30. When the detection signal indicative of host vehicle speed varies within a predetermined variation width, vehicle abnormality detecting section 81 decides that the host vehicle speed detecting system including vehicle speed sensor 13 and vehicle speed signal processing section 30 is operating normally and outputs an abnormality detection signal AS set at 0 (AS=0) to OR circuit 80. However, when the vehicle speed detection signal suddenly varies to zero over a predetermined variation width, vehicle abnormality detecting section 81 decides that the host vehicle speed detecting system is operating abnormally and outputs abnormality detection signal AS set at 1 (AS=1) to OR circuit 80.

[0148] With this third embodiment according to the present invention, when the host vehicle speed detecting system including vehicle speed sensor 13 is operating normally, abnormality detecting signal AS outputted from vehicle-speed abnormality detecting section 81 is set at 0 indicative of normality of the host vehicle speed detecting system. Accordingly, driving torque changeover control section 61 and torque selecting section 62 execute the selecting of the target driving torque by selecting one of second and third target driving torques T_W2* and T_W3* on the basis of torque select signal T_Soutputted from driving torque changeover deciding section 60 and by executing the torque matching.

[0149] When host vehicle speed V_Sinputted to vehicle speed abnormality detecting section 81 suddenly decreases from a normal value to zero due to trouble of the vehicle speed detecting system, such as breaking of wire, vehicle speed abnormality detecting section 81 decides that the vehicle speed control system is put in an abnormal state and outputs abnormality detection signal AS set at 1 (AS=1) to OR circuit 80. Therefore, driving torque select control section 61 calculates torque deviation ΔT_W1by executing the calculation of equation (20) under a condition where second target driving torque T_W2* taking account of running resistance is selected on the basis of host vehicle speed V_S. Further, driving torque changeover control section 61 calculates third target driving torque T_W3* by adding torque deviation ΔT_W1to first target driving torque T_W1*. Simultaneously, torque selecting section 62 selects third target driving torque T_W3* as target driving torque T_W*.

[0150] Even under this condition, third target driving torque T_W3* is smoothly matched with second target driving torque T_W2*. Therefore, the changeover to third target driving torque T_W3* is smoothly executed without generating change of acceleration and deceleration.

[0151] That is, when the host vehicle speed detecting system is put in the abnormal state, by the processing at step S3 of FIG. 8 for calculating target inter-vehicle distance L*, it is decided whether or not abnormal detection signal AS is 1. When AS=0, target inter-vehicle distance L* is calculated from equation (1). When AS=1, it is decided that the vehicle speed control system is put in abnormal state, and target inter-vehicle distance L* is kept at the previous value, or is set at a relatively long value, so as to prevent the inter-vehicle distance from being set at an excessively small value.

[0152] Although the third embodiment according to the present invention has been shown and described such that vehicle speed abnormality detecting section 81 detects the abnormality of the vehicle speed control system due to breaking of wire, it will be understood that the abnormality is not limited to this and may be decided as abnormal when host vehicle speed V_Sis kept a value greater than an upper limit for a predetermined time due to short-circuit in the vehicle speed control system or when the change rate of the vehicle speed during deceleration is excessively large due to the short-circuit. Further, if a longitudinal acceleration sensor is installed to the vehicle equipped with the system according to the present invention, the abnormality detection may be executed by comparing the integral of longitudinal acceleration detected by the longitudinal acceleration sensor and host vehicle speed V_S.

[0153] While the third embodiment has been shown and described such that the fail-safe function as to the vehicle speed abnormality is added to the first embodiment, it will be understood that the present invention is not limited to this, and the fail-safe function may be added to the second embodiment.

[0154] Further, although the first to third embodiments according to the present invention have been shown and described such that inter-vehicle distance control calculating section 43 calculates relative speed ΔV by executing the band-pass filter treatment of inter-vehicle distance L, it will be understood that the present invention is not limited to this and may be arranged such that relative speed ΔV is calculated by executing a high-pass filer treatment or is directly measured by means of a Dopplar radar.

[0155] Furthermore, although the first to third embodiments according to the present invention have been shown and described such that automatic transmission 3 is installed to the output of engine 2, it will be understood that the present invention is not limited to this and may be arranged to install a continuously variable transmission to the output of engine 2.

[0156] Although the first to third embodiments have been shown and described such that disc brake 7 is employed as brake actuator 7 and brake control unit 8 controls the braking force of brake actuator 6 to properly generate braking force, it will be understood that the present invention is not limited to this and may be adapted to an electric vehicle which employs an electric motor as an actuator of a braking system and is arranged to generate regenerative braking force by controlling electrical output to the electric motor.

[0157] Furthermore, although the embodiments according to the present invention have been shown and described to be adapted to a rear-wheel drive vehicle, the present invention may be adapted to a front-wheel drive vehicle. Further, although the explanation has been made as to a case that engine 2 is employed as a rotation drive source, the present invention is not limited to this and may employ an electric motor or may be adapted to a hybrid vehicle employing an internal combustion engine and an electric motor.

[0158] The entire contents of Japanese Patent Application No. 11-226824 filed on Aug. 10, 1999 in Japan are incorporated herein by reference.

[0159] Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teaching. The scope of the invention is defined with reference to the following claims.