Deceleration control apparatus and method for automotive vehicle

0001The present invention relates to an apparatus and method for performing deceleration control on an automotive vehicle in accordance with a turning state of the vehicle.

0002<patcit id="pcit0001" dnum="JPH10278762A"><text>Japanese Laid-Open Patent Publication No. H10-278762</text></patcit> discloses a deceleration control apparatus for an automotive vehicle, which is configured to estimate a road friction coefficient, determine a maximum limit of vehicle lateral acceleration based on the estimated road friction coefficient, and then, performs deceleration control on the vehicle in such a manner that the actual vehicle lateral acceleration does not exceed the maximum lateral acceleration limit, so as to attain stable, smooth vehicle behavior according to road conditions and prevent a vehicle rollover during turning.

0003In the above-disclosed type of deceleration control apparatus, the maximum lateral acceleration limit is based only on the estimated road friction coefficient. There thus arises a case where the maximum lateral acceleration limit is set lower than an appropriate value due to an underestimation of the road friction coefficient. In such a case, the acceleration of the vehicle becomes limited even under a driver's accelerator pedal operation at the time the vehicle shifts from a curved road to a straight road. This results in driver discomfort.

0004It is therefore an object of the present invention to provide an apparatus and method for performing deceleration control on an automotive vehicle in accordance with a turning state of the vehicle without causing driver discomfort.

0005<patcit id="pcit0002" dnum="US6081761A"><text>US 6,081,761</text></patcit> discloses an automatic deceleration control apparatus for a vehicle for securing appropriate tire grip performance. <patcit id="pcit0003" dnum="US6081761A"><text>US-A-6 081 761</text></patcit>, considered as closest prior art, shows an apparatus for performing deceleralion control on an automobile vehicle in accordance with a turning state of the vehicle comprising means for detecting an actual lateral acceleration of the vehicle and means for estimating a friction coefficent of a road on which the vehicle is traveling; and means for calculating a target deceleration. <patcit id="pcit0004" dnum="US6405116B"><text>US 6,405,116</text></patcit> and <patcit id="pcit0005" dnum="EP0874149A"><text>EP 0,874,149</text></patcit> disclose automotive deceleration control systems. <patcit id="pcit0006" dnum="EP0819912A"><text>EP 0,819,912</text></patcit> discloses a vehicle driving condition prediction and warning device.

0006According to a first aspect of the present invention, there is provided an apparatus for performing deceleration control on an automotive vehicle in accordance with a turning state of the vehicle, comprising: means for determining an actual yaw rate of the vehicle; means for detecting an actual lateral acceleration of the vehicle; means for detecting an accelerator pedal depression of the vehicle; means for estimating a friction coefficient of a road on which the vehicle is traveling; means for calculating a target vehicle speed based on the estimated road friction coefficient, the detected actual lateral acceleration and the detected accelerator pedal operation; means for calculating a target deceleration from the target vehicle speed; and means for controlling engine throttle opening and brake fluid pressure of the vehicle according to the target deceleration.

0007According to a second aspect of the present invention, there is provided a method for performing deceleration control on an automotive vehicle, comprising in accordance with a turning state of the vehicle: determining an actual yaw rate of the vehicle; detecting an actual lateral acceleration of the vehicle; detecting an accelerator pedal depression of the vehicle; estimating a friction coefficient of a road on which the vehicle is traveling; calculating a target vehicle speed based on the estimated road friction coefficient, the detected actual lateral acceleration and the detected accelerator pedal depression; calculating a target deceleration from the target vehicle speed; and controlling engine throttle opening and brake fluid pressure of the vehicle according to the target deceleration.

0008The other objects and features of the invention will also become understood from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

0009<figref idref="f0001">FIG. 1</figref> is a schematic view of a vehicle equipped with a deceleration controller according to first through fourth embodiments of the invention.

0010<figref idref="f0002">FIG. 2</figref> is a block diagram of the deceleration controller of <figref idref="f0001">FIG. 1</figref>.

0011<figref idref="f0003">FIG. 3</figref> is a flowchart of a main control program of the deceleration controller according to the first embodiment of the invention.

0012<figref idref="f0003">FIG. 4</figref> is a block diagram of a yaw rate calculation unit of the deceleration controller of <figref idref="f0001">FIG. 1</figref>.

0013<figref idref="f0004">FIG. 5</figref> is a flowchart of a target vehicle speed calculation subroutine program of the deceleration controller according to the first embodiment of the invention.

0014<figref idref="f0004">FIG. 6</figref> is a flowchart of a control signal output subroutine program of the deceleration controller according to the first embodiment of the invention.

0015<figref idref="f0005">FIG. 7</figref> is a flowchart of a target vehicle speed calculation subroutine program of the deceleration controller according to the second embodiment of the invention.

0016<figref idref="f0005">FIG. 8</figref> is a flowchart of a target vehicle speed calculation subroutine program of the deceleration controller according to the third embodiment of the invention.

0017<figref idref="f0006">FIG. 9</figref> is a flowchart of a target vehicle speed calculation subroutine program of the deceleration controller according to the fourth embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

0018The present invention will be described in detail by way of the following first to fourth embodiment, in which like parts and portions are designated by like reference numerals so as to omit repeated explanations thereof.

0019Each of the first to fourth embodiments refers to a four-wheel vehicle equipped with brake fluid pressure control unit 1, engine throttle control unit 3 and deceleration controller 10 as shown in <figref idref="f0001">FIG. 1</figref>. Brake fluid pressure control unit 1 is disposed between a master cylinder and wheel cylinders of the vehicle and controls pressures of brake fluid supplied to the respective cylinders of front-left wheel 2FL, front-right wheel 2FR, rear-left wheel 2RL and rear-right wheel 2RR of the vehicle. Although the brake fluid is normally pressurized by the master cylinder and supplied to each wheel cylinder in response to a driver's brake pedal depression, brake fluid pressure control unit 1 operates under a brake fluid pressure control signal from deceleration controller 10 (irrespective of a driver's brake pedal operation) to control the brake fluid pressure. Herein, brake fluid control unit 1 may employ a fluid pressure control circuit configuration suitable for use in antiskid control or traction control. Engine throttle control unit 3 operates under its own control or under a throttle opening control signal from deceleration controller 10, to control an engine throttle opening of the vehicle.

0020There is further provided in the vehicle various detecting units including: yaw rate sensor 11 to detect an actual yaw rate φ' of the vehicle by measurement (hereinafter referred to as a "yaw rate sensor value"); steering angle sensor 12 to detect a steering (wheel) angle δ of the vehicle, wheel speed sensors 13FL, 13FR, 13RL and 13RR to detect rotation speeds V_Wi (i = FL, FR, RL and RR) of vehicle wheels 2FL, 2FR, 2RL and 2RR (hereinafter referred to as "wheel speeds"), respectively; accelerator pedal sensor 14 to detect a depression θth of an accelerator pedal of the vehicle; and lateral acceleration sensor 15 to detect an actual lateral acceleration Yg of the vehicle (hereinafter occasionally referred to as a "lateral acceleration sensor value"). These sensors 11-12, 13FL, 13FR, 13RL, 13RR and 14-15 output signals responsive to the detected vehicle parameters to deceleration controller 10.

0021Deceleration controller 10 performs deceleration control on the vehicle in accordance with a turning state of the vehicle on the basis of the detection signals from the sensors 11-12, 13FL, 13FR, 13RL, 13RR and 14-15.

0022As shown in <figref idref="f0002">FIG. 2</figref>, deceleration controller 10 includes: road friction coefficient estimation unit 20 that estimates the friction coefficient µ of a road on which the vehicle is traveling; yaw rate determination unit 21 that determines a yaw rate φ* of the vehicle, for use in the following operation processing, with reference to the steering angle δ, the wheel speeds V_Wi (i = FL, FR, RL and RR) and the yaw rate sensor value φ'; lateral acceleration limit setting unit 25 that sets a lateral acceleration limit Yg* for the vehicle; target vehicle speed calculation unit 22 that calculates a target vehicle speed V* based on the yaw rate φ*, the lateral acceleration Yg and the road friction coefficient µ; target deceleration calculation unit 23 that calculates a target deceleration Xg* from the target vehicle speed V*; and deceleration control unit 24 that controls brake fluid pressure control unit 1 and engine throttle control unit 3 under the brake fluid pressure control signal and the throttle opening control signal, respectively, so as to achieve the target deceleration Xg*. Although road friction coefficient estimation unit 20 estimates the road friction coefficient µ by calculation in the first to fourth embodiments, the method for calculation of the road friction coefficient µ is not particularly limited. For example, the road friction coefficient µ can be calculated by computing a slip ratio from the wheel speeds V_Wi (a ratio between the front and rear wheel speeds), obtaining a relational formula between the slip ratio and the vehicle acceleration or deceleration, and then, determining a slope of the relational formula as disclosed in <patcit id="pcit0007" dnum="US6577941B"><text>U.S. Patent Publication No. 6,577,941</text></patcit>. Road friction coefficient estimation unit 20 may be provided separately from deceleration controller 10. Instead of providing road friction coefficient unit 20, a road friction coefficient sensor can alternatively be mounted onto the vehicle so as to estimate the road friction coefficient µ by measurement. One example of road friction coefficient sensor is discussed in <patcit id="pcit0008" dnum="US6266600B"><text>U.S. Patent No. 6,266,600</text></patcit>.

0023In the first embodiment, the vehicle deceleration control is performed through the following control program, as shown in <figref idref="f0003">FIG. 3</figref>, by timer interruptions at established intervals.

0024Deceleration controller 10 first enables yaw rate determination unit 21 to determine the vehicle yaw rate φ* at step S1. Yaw rate determination unit 21 has estimation section 31 and selection section 32 as shown in <figref idref="f0003">FIG. 4</figref>. Estimation section 31 estimates an actual yaw rate φ" of the vehicle (hereinafter referred to as a "yaw rate estimation value") by calculation based on the steering angle δ and the wheel speeds V_Wi (i = FL, FR, RL and RR). The yaw rate estimation value can be obtained by e.g. calculating a traveling speed of the vehicle from the wheel speeds V_Wi and estimating the vehicle yaw rate based on the steering angle δ and the vehicle traveling speed according to a known method. Then, estimation section 31 outputs the yaw rate estimation value to selection unit 32. Selection unit 32 selects the higher one of the yaw rate sensor value φ' and the yaw rate estimation value φ" and outputs the thus-obtained actual yaw rate selection value φ* (> 0) to target vehicle speed calculation unit 22. Generally, the yaw rate estimation value φ" can be obtained faster than the yaw rate sensor value φ'. The use of the raw rate estimation value φ" in the vehicle deceleration control thus results in quick control response. In low-friction road conditions, however, there are some vehicle behavior modes (such as a slow-spin mode) in which the actual yaw rate of the vehicle becomes increased even under little steering. For this reason, the higher one of the yaw rate sensor value φ' and the yaw rate estimation value φ" is selected and used in the subsequent operation processing, so as to allow early intervention of the vehicle deceleration control.

0025At step S2, deceleration controller 10 enables lateral acceleration limit setting unit 25 to set the lateral acceleration limit Yg* according to the following expression (1):<maths id="math0001" num="(1)"><math display="block"><mi mathvariant="italic">Yg</mi><mo>*</mo><mo>=</mo><mi mathvariant="italic">Yga</mi></math><img file="EP1577148B1_D0001.tif" /></maths> where Yga is a fixed limit value of target lateral acceleration (e.g. approximately 0.45 G) up to which the vehicle can travels stably on a curved road without developing a spin, a drift out or a turnover (rollover).

0026At step S3, deceleration controller 10 enables target vehicle speed calculation unit 22 to calculate the target vehicle speed V*.

0027The target vehicle speed V* is calculated through the following subroutine shown in <figref idref="f0004">FIG. 5</figref> in the first embodiment.

0028At step S31, target vehicle speed calculation unit 22 reads various vehicle parameters including the road friction coefficient µ, the lateral acceleration sensor value Yg, the lateral acceleration limit Yg*, the yaw rate selection value φ* and the engine throttle opening Acc_bs corresponding to the accelerator pedal depression θth (hereinafter referred to as a "base throttle opening"). If the vehicle has an adaptive cruise control system or a speed control system so as to automatically control the accelerator, the base throttle opening Acc_bs may be set corresponding to such an automatic accelerator control operation.

0029Target vehicle speed calculation unit 22 judges at step S32 whether the accelerator pedal depression θth is larger than a given threshold, i.e., whether the base throttle opening Acc_bs is larger than a given accelerator operation threshold Ath (e.g. 5%).

0030If Acc_bs ≤ Ath (No at step S32), it is judged that the accelerator pedal is not operated by a driver. Then, program control proceeds to step S33. At step S33, target vehicle speed calculation unit 22 gives the target vehicle speed V* by the following expression (2):<maths id="math0002" num="(2)"><math display="block"><mi>V</mi><mo>*</mo><mo>=</mo><mi>μ</mi><mo>×</mo><mi mathvariant="italic">Yg</mi><mo>*</mo><mo>/</mo><mi>φ</mi><mo>*</mo></math><img file="EP1577148B1_D0002.tif" /></maths> where µ is the estimated road friction coefficient; φ* is the yaw rate selection value determined at step S1; and Yg* is the lateral acceleration limit set at step S2. As is evident from the expression (2), the calculated target vehicle speed V* decreases as the road friction coefficient µ becomes lower, as the lateral acceleration limit Yg* becomes lower and as the yaw rate selection value φ* becomes higher.

0031On the other hand, it is judged that the accelerator pedal is operated by a driver if Acc_bs > Ath (Yes at step S32). Program control then proceeds to step S34. At step S34, target vehicle speed calculation unit 22 selects the higher one of the lateral acceleration sensor value Yg and the result of multiplication of the road friction coefficient µ by gravitational acceleration <i>g</i> as indicated by the following expression (3):<maths id="math0003" num="(3)"><math display="block"><mi mathvariant="italic">Yg</mi><mspace width="1em" /><mi>max</mi><mo>⁢</mo><mi /><mo>=</mo><mi>max</mi><mfenced><mi>μ</mi><mo>×</mo><mi>g</mi><mo>,</mo><mi mathvariant="italic">Yg</mi></mfenced></math><img file="EP1577148B1_D0003.tif" /></maths> where max () is a functional operator for selecting the higher one of the values inside the parentheses. At step S35, target vehicle speed calculation unit 22 gives the target vehicle speed V* by the following expression (4):<maths id="math0004" num="(4)"><math display="block"><mi>V</mi><mo>*</mo><mo>=</mo><mi mathvariant="normal" /><mo>⁢</mo><mi mathvariant="italic">Yg</mi><mspace width="1em" /><mi>max</mi><mo>×</mo><mi mathvariant="italic">Yg</mi><mo>*</mo><mo>/</mo><mi>φ</mi><mo>*</mo></math><img file="EP1577148B1_D0004.tif" /></maths> where φ* is the yaw rate selection value determined at step S1; Yg* is the lateral acceleration limit set at step S2; and Ygmax is the higher one of the lateral acceleration sensor value Yg and the multiplication product µ × <i>g</i> selected at step S34. It is clear from comparison of the expressions (2) and (4) that the multiplication product Ygmax × Yg* is used in the expression (4) in place of the lateral acceleration calculation value µ × Yg* used in the expression (2). This makes it possible to reflect the lateral acceleration sensor value Yg through the target vehicle speed V* such that the target vehicle speed V* increases with the lateral acceleration sensor value Yg when the road friction coefficient µ is estimated to be lower than its actual value in a situation that the vehicle actually undergoes high lateral acceleration.

0032Upon determination of the target vehicle speed V* at step S33 or S35, control exits from the timer interruption subroutine and returns to the main routine.

0033Next, deceleration controller 10 enables at step S4 target deceleration calculation unit 23 to calculate the target deceleration Xg* according to the following expression (5):<maths id="math0005" num="(5)"><math display="block"><mi mathvariant="italic">Xg</mi><mo>*</mo><mo>=</mo><mi>K</mi><mo>×</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>V</mi><mo>/</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>t</mi></math><img file="EP1577148B1_D0005.tif" /></maths> where ΔV is a difference between the actual vehicle speed V and the target vehicle speed V* (ΔV = V - V*); Δt is a predetermined time (to reduce the vehicle speed difference ΔV to zero); and K is a gain factor. As is evident from the expression (5), the calculated target deceleration Xg* increases as the vehicle speed difference ΔV becomes larger i.e. as the difference between V and V* becomes positively increased.

0034The target deceleration Xg* may alternatively be given by the following expression (6) in consideration of the increment of vehicle speed difference:<maths id="math0006" num="(6)"><math display="block"><mi mathvariant="italic">Xg</mi><mo>*</mo><mo>=</mo><mfenced><mi>K</mi><mo>⁢</mo><mn>1</mn><mo>×</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>V</mi><mo>+</mo><mi>K</mi><mo>⁢</mo><mn>2</mn><mo>×</mo><mi>d</mi><mo>⁢</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>V</mi></mfenced><mo>/</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi>t</mi></math><img file="EP1577148B1_D0006.tif" /></maths> where dΔt is the difference between the present vehicle speed difference value ΔV and the past vehicle speed difference value ΔVz (dΔt = ΔV - ΔVz). This makes it possible to calculate the target deceleration Xg* so as to decelerate the vehicle instantaneously under fast steering.

0035At step S5, deceleration controller 10 enables deceleration control unit 24 to output brake fluid pressure and throttle opening control signals to control brake fluid pressure control unit 1 and engine throttle control unit 3, respectively, in such a manner that the vehicle reaches the target vehicle speed V* and the target deceleration Xg*.

0036The brake fluid pressure and throttle opening control signals are output through the following subroutine shown in <figref idref="f0004">FIG. 6</figref> in the first embodiment.

0037At step S51, deceleration control unit 24 first reads the target deceleration Xg* and the base throttle opening Acc_bs.

0038At step S52, deceleration control unit 24 judges whether the target deceleration Xg* is larger than zero. If Xg* > 0 (Yes at step S52), it is judged that the vehicle is to be decelerated. Then, program control proceeds to step S53. If Xg* ≤ 0 (No at step S52), it is judged that the vehicle is to be accelerated. Program control then proceeds to step S58.

0039At step S53, deceleration control unit 24 sets a deceleration control flag "Flag" to the ON position (Flag = ON) indicating the intervention of the vehicle deceleration control.

0040At step S54, deceleration control unit 24 decrements the target throttle opening Acc by a given amount ΔAdn at each sampling as indicated by the following expression (7):<maths id="math0007" num="(7)."><math display="block"><mi mathvariant="italic">Acc</mi><mo>=</mo><mi mathvariant="italic">Acc</mi><mo>-</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi mathvariant="italic">Adn</mi></math><img file="EP1577148B1_D0007.tif" /></maths> The initial value of the target throttle opening Acc is set to the base throttle opening Acc_bs.

0041At step S55, deceleration control unit 24 judges whether the present target throttle opening Acc is smaller than zero. If Acc &lt; 0 (Yes at step S55), program control proceeds to step S56. If Acc ≥ 0 (No at step S55), program control directly proceeds to step S57.

0042At step S56, deceleration control unit 24 sets the target throttle opening Acc to 0 (zero) to thereby prevent the target throttle opening Acc from becoming smaller than zero. Program control then proceeds to step S57.

0043Deceleration control unit 24 controls at step S57 brake fluid pressure control unit 1 so as to increase the brake fluid pressure to a predetermined level.

0044In this way, the vehicle is decelerated by the throttle control operation at steps S54 to S56 and by the brake control operation at step S57 when the target deceleration Xg* is larger than zero (Xg* > 0). After step S57, control exits from the timer interruption subroutine and then returns to the main routine.

0045When the target deceleration Xg* is smaller than or equal to zero (Xg* ≤ 0), deceleration control unit 24 judges at step S58 whether the deceleration control flag has been set to the ON position. If Flag = OFF (No at step S58), it is judged that the vehicle deceleration control has never intervened before. Then, program control proceeds to step S59. If Flag = ON (Yes at step S58), it is judged that the vehicle deceleration control has intervened. Program control then proceeds to step S60.

0046At step S59, deceleration control unit 24 resets the target throttle opening Acc to the base throttle opening Acc_bs as indicated by the following expression (8):<maths id="math0008" num="(8)."><math display="block"><mi mathvariant="italic">Acc</mi><mo>=</mo><mi mathvariant="italic">Acc_bs</mi></math><img file="EP1577148B1_D0008.tif" /></maths> After step S59, control exits from the timer interruption subroutine without performing throttle and brake control operations and returns to the main routine.

0047At step S60, deceleration control unit 24 controls brake fluid pressure control unit 1 so as to decrease the brake fluid pressures.

0048At step S61, deceleration control unit 24 judges whether the brake control operation has completed. If Yes at step S61, program control proceeds to step S62. If No at step S61, program control directly proceeds to step S63.

0049At step S62, the deceleration control unit 24 increments the target throttle opening Acc by a predetermined amount ΔAup at each sampling, for throttle recovery, as indicated by the following expression (9):<maths id="math0009" num="(9)."><math display="block"><mi mathvariant="italic">Acc</mi><mo>=</mo><mi mathvariant="italic">Acc</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">Δ</mi><mo>⁢</mo><mi mathvariant="italic">Aup</mi></math><img file="EP1577148B1_D0009.tif" /></maths> After that, program control proceeds to step S63.

0050At step S63, the deceleration control unit 24 checks whether the throttle recovery has completed. If the throttle recovery has completed so that the throttle opening recovers to the base throttle opening Acc_bs (Yes at step S63), program control proceeds to step S64. If the throttle recovery has not completed (No at step S63), control exits from the timer interruption subroutine and returns to the main routine for continuation of the throttle recovery.

0051At step S64, the deceleration control unit 24 resets the deceleration control flag to the OFF position. Then, control exits from the timer interruption subroutine and returns to the main routine.

0052It is now assumed that the vehicle is traveling on a straight road on the condition that the target deceleration Xg* is set lower than or equal to zero (Xg* ≤ 0). In this straight-running state, program control goes from step S52 to step S58 upon judgment of "No" at step S52 in the control signal output subroutine. If the vehicle deceleration control has never intervened such that the deceleration control flag remains at the initial OFF position (Flag = OFF), control goes from step S58 to step S59. The target throttle opening Acc is set to the base throttle opening Acc_bs (corresponding the accelerator pedal depression θth) at step S59. Accordingly, the vehicle keeps on running in response to a driver's accelerator pedal operation without intervention of the vehicle deceleration control.

0053It is next assumed that the vehicle subsequently comes to a curved road and shifts from the above straight-running state to a turning state. When the target vehicle speed V* decreases with increase in vehicle yaw rate, the target deceleration Xg* becomes higher than zero (Xg* > 0). Program control goes from step S52 to step S53 upon judgment of "Yes" at step S52 in the control signal output subroutine. The deceleration control flag is set to the ON position (Flag = ON) at step S53, and then, the vehicle is decelerated by the throttle control operation at steps S54 to S56 (in which the target throttle opening Acc gradually decreases from the base throttle opening Acc_bs) and by the brake control operation at step S57 (in which the brake fluid pressure increases to the predetermined level). Through such deceleration control, the vehicle can be prevented from overspeeding during turning. This vehicle deceleration control can be performed without giving an abrupt deceleration feeling to the driver, as the target throttle opening Acc is decremented by the given amount ΔAnd at each sampling even under a driver's accelerator pedal operation.

0054The ease of intervention of the vehicle deceleration control increases with decrease in the road friction coefficient µ when the accelerator pedal is not operated during turning of the vehicle. Under no (undetectable) accelerator pedal operation, control goes from step S32 to step S33 in the target vehicle speed calculation subroutine. The target vehicle speed V* is calculated at step S33 using the road friction coefficient µ according to the expression (2) so that the target vehicle speed V* decreases with the road friction coefficient µ, thereby leading to a higher target deceleration Xg*. In the subsequent control signal output subroutine, control goes from step S52 to step S53 upon judgment of "Yes" at step S52. The vehicle is then gradually decelerated by the throttle control operation and by the brake control operation, so as to show smooth vehicle turning behavior.

0055By contrast, control goes to steps S34 and S35 in the target vehicle speed calculation subroutine when the accelerator pedal is operated. The target vehicle speed V* is calculated at step S35 using the select-high value Ygmax according to the expressions (3) and (4). If the road friction coefficient µ is estimated to be lower than its actual value, the lateral acceleration sensor value Yg is selected as Ygmax in a situation that the vehicle actually undergoes high lateral acceleration during turning. This leads to a higher target vehicle speed V* and a lower target deceleration Xg* so as to limit the intervention of the vehicle deceleration control and avoid such driver discomfort that the vehicle cannot be accelerated in response to a driver's accelerator pedal operation.

0056It is very likely that the vehicle deceleration control deteriorates in control response at the time the vehicle shifts from a low-friction road to a high-friction road because the road friction coefficient µ is estimated by calculation. Accordingly, there has been a problem that the driver has a stall feeling for a longer time under vehicle deceleration control of the earlier technology in which the target vehicle speed is only determined based on the road friction coefficient. In the first embodiment, however, the target vehicle speed V* is calculated based on the higher one of the lateral acceleration sensor value Yg and the multiplication product µ × <i>g</i> according to the expressions (3) and (4) under vehicle deceleration control. The intervention of the vehicle deceleration control becomes thus limited quickly upon selection of the lateral acceleration selection value Yg even though the estimation of the road friction coefficient µ is slow in response to changes in road friction conditions.

0057It is finally assumed that the vehicle exits from a curved road so that the target deceleration Xg* is again set smaller than or equal to zero (Xg* ≤ 0). In the control signal output subroutine, control goes from step S52 to step S58. As the vehicle deceleration control has intervened, the deceleration control flag is in the ON position (Flag = ON). Then, control goes from step S58 to S60 upon judgment of "Yes" at step S58. The brake fluid pressures are adjusted into a decreasing direction at step S60, and then, the throttle opening gradually increases to the base throttle opening Acc_bs corresponding to the accelerator pedal depression θth at step S62. When the engine throttle opening fully recovers, the deceleration control flag is set to the OFF position so as to terminate the vehicle deceleration control.

0058As described above, target vehicle speed calculation unit 22 has a first target vehicle speed calculating section (corresponding to step S33) for giving a first vehicle speed value by dividing the multiplication product of the road friction coefficient µ and the lateral acceleration limit Yg* by the yaw rate φ*, a second vehicle speed calculating section (corresponding to steps S34-S35) for giving a second target vehicle speed value by selecting the higher one of the lateral acceleration sensor value Yg and the multiplication result µ × <i>g</i> and then dividing the multiplication product of the thus-obtained select-high value Ygmax (= Yg or µ × <i>g</i>) and the lateral acceleration limit Yg* by the yaw rate φ*, and a correcting section (corresponding to step S32) for switching between the first and second target vehicle speed values to correct the target vehicle speed V* depending on the accelerator pedal depression θth and the lateral acceleration sensor value Yg and thereby adjust the amount of deceleration control on the vehicle in the first embodiment. In other words, the target vehicle speed V* is calculated based on the higher one of the lateral acceleration sensor value Yg and the multiplication product µ × <i>g</i> when the accelerator pedal depression θth is detected. It is therefore possible to correct the target vehicle speed V* to a higher value upon selection of the lateral acceleration sensor value Yg as Ygmax so as to limit the intervention of the vehicle deceleration control and avoid giving a stall feeling to the driver, even though the road friction coefficient µ is estimated to be lower than its actual value, in a situation that the vehicle undergoes lateral acceleration in response to a driver's accelerator pedal operation during shifting from a turning state to a straight-running state. If no lateral acceleration is applied by a driver's accelerator pedal operation, the target vehicle speed V* is calculated based on the road friction coefficient µ and the lateral acceleration limit Yg*. The target vehicle speed V* is also calculated based on the road friction coefficient µ and the lateral acceleration limit Yg* when the accelerator pedal depression θth is undetectable. In such cases, it is possible to allow the intervention of deceleration control on the vehicle in accordance with road friction conditions and attain stable, smooth vehicle turning behavior.

0059Next, the second embodiment will be described below with reference to <figref idref="f0005">FIG. 7</figref>. The second embodiment is similar to the first embodiment, except that the target vehicle speed V* is given upon selection of the higher one of the target vehicle speed values calculated based on the road friction coefficient µ and the lateral acceleration sensor value Yg, respectively, when the accelerator pedal is operated, in the target vehicle speed calculation subroutine.

0060At step S311, target vehicle speed calculation unit 22 first reads the road friction coefficient µ, the lateral acceleration sensor value Yg, the lateral acceleration limit Yg*, the yaw rate selection value φ* and the base throttle opening Acc_bs.

0061At step S312, target vehicle speed calculation unit 22 calculates the target vehicle speed V* from the road friction coefficient µ, the lateral acceleration limit Yg* and the yaw rate selection value φ* according to the above expression (2).

0062At step S313, target vehicle speed calculation unit 22 judges whether the base throttle opening Acc_bs is larger than a given accelerator operation threshold Ath. If Acc_bs > Ath (Yes at step S313), program control proceeds to step S312. If Acc_bs ≤ Ath (No at step S313), control exits from the timer interruption subroutine and returns to the main routine.

0063At step S314, target vehicle speed calculation unit 22 gives an alternative possible target vehicle speed value V1 by the following expression (10):<maths id="math0010" num="(10)"><math display="block"><mi>V</mi><mo>⁢</mo><mn>1</mn><mo>=</mo><mi mathvariant="italic">Yg</mi><mo>/</mo><mi>φ</mi><mo>*</mo></math><img file="EP1577148B1_D0010.tif" /></maths> where φ* is the yaw rate selection value; and Yg is the lateral acceleration sensor value. It is clear from comparison of the expressions (2) and (10) that the lateral acceleration sensor value Yg is used in the expression (10) in place of the lateral acceleration calculation value µ × Yg* used in the expression (2). This makes it possible to reflect the lateral acceleration sensor value Yg* through the target vehicle speed V* and set the target vehicle speed V* irrespective of the road friction coefficient µ and the lateral acceleration limit Yg*.

0064At step S315, target vehicle speed calculation unit 22 compares the alternative possible target vehicle speed V1 with the target vehicle speed V* to judge whether the alternative possible target vehicle speed V 1 is higher than the target vehicle speed V*. If V1 > V* (Yes at step S315), program control proceeds to step S316. If V1 ≤ V* (No at step S315), control exits from the timer interruption subroutine and directly returns to the main routine.

0065At step S316, target vehicle speed calculation unit 22 sets the target vehicle speed V* to the target vehicle speed value V1 as indicated by the following expression (11):<maths id="math0011" num="(11)."><math display="block"><mi>V</mi><mo>*</mo><mo>=</mo><mi>V</mi><mo>⁢</mo><mn>1</mn></math><img file="EP1577148B1_D0011.tif" /></maths> After that, control exits from the timer interruption subroutine and returns to the main routine.

0066In the second embodiment, target vehicle speed calculation unit 22 has a first target vehicle speed calculating section (corresponding to step S312) for giving a first vehicle speed value by dividing the multiplication product of the road friction coefficient µ and the lateral acceleration limit Yg* by the yaw rate φ*, a third vehicle speed calculating section (corresponding to step S314) for giving a third target vehicle speed value by dividing the lateral acceleration sensor value Yg by the yaw rate φ*, and a correcting section (corresponding to steps S313 and S315-S316) for switching between the first and third target vehicle speed values to correct the target vehicle speed V* depending on the accelerator pedal depression θth and the lateral acceleration sensor value Yg and thereby adjust the amount of deceleration control on the vehicle in a similar manner as in the first embodiment. In other words, the target vehicle speed V* is calculated based on the lateral acceleration sensor value Yg according to the expression (10) when the accelerator pedal depression θth is detected in a situation that the vehicle undergoes high lateral acceleration, so that the target vehicle speed V* increases with the lateral acceleration Yg regardless of whether the road friction coefficient µ is underestimated. It is thus possible to perform deceleration control on the vehicle without giving a stall feeling to the driver at the time the vehicle shifts from a turning state to a straight-running state. If there is no increase in lateral acceleration under low-friction road conditions even tough the accelerator pedal depression θth is detected, the target vehicle speed V* is calculated using the road friction coefficient µ and the lateral acceleration limit Yg* according to the expression (2). The target vehicle speed V* is also calculated using the road friction coefficient µ and the lateral acceleration limit Yg* according to the expression (2) when the accelerator pedal depression θth is undetectable. In such cases, it is possible to allow the intervention of deceleration control on the vehicle in accordance with road friction conditions and attain stable, smooth vehicle turning behavior.

0067The third embodiment will be described below with reference to <figref idref="f0005">FIG. 8</figref>. The third embodiment is similar to the first and second embodiments, except that the target vehicle speed V* is given upon selection of the higher one of the target vehicle speed values calculated based on the lateral acceleration sensor value Yg and the select-high value Ygmax, respectively, when the accelerator pedal is operated, in the target vehicle speed calculation subroutine.

0068At step S321, target vehicle speed calculation unit 22 reads the road friction coefficient µ, the lateral acceleration sensor value Yg, the lateral acceleration limit Yg*, the yaw rate selection value φ* and the base throttle opening Acc_bs.

0069At step S322, target vehicle speed calculation unit 22 judges whether the base throttle opening Acc_bs is larger than a given accelerator operation threshold Ath. If Acc_bs ≤ Ath (No at step S322), program control proceeds to step S323. If Acc_bs > Ath (Yes at step S322), program control proceeds to step S324.

0070At step S323, target vehicle speed calculation unit 22 calculates the target vehicle speed V* from the road friction coefficient µ, the lateral acceleration limit Yg* and the yaw rate selection value φ* according to the above expression (2).

0071At step S324, target vehicle speed calculation unit 22 gives a first alternative possible target vehicle speed value V 1 from the lateral acceleration sensor value Yg and the yaw rate selection value φ* by the above expression (10).

0072At step S325, target vehicle speed calculation unit 22 selects the higher one of the lateral acceleration sensor value Yg and the multiplication product µ × <i>g</i> as indicated by the above expression (3).

0073Then, target vehicle speed calculation unit 22 gives at step S326 a second alternative possible target vehicle speed value V2 by the following expression (12):<maths id="math0012" num="(12)"><math display="block"><mi>V</mi><mo>⁢</mo><mn>2</mn><mo>=</mo><mi mathvariant="italic" /><mo>⁢</mo><mi mathvariant="italic">Yg</mi><mspace width="1em" /><mi>max</mi><mo>×</mo><mi mathvariant="italic">Yg</mi><mo>*</mo><mo>/</mo><mi>φ</mi><mo>*</mo></math><img file="EP1577148B1_D0012.tif" /></maths> where φ* is the yaw rate selection value; Yg* is the lateral acceleration limit; and Ygmax is the higher one of the lateral acceleration sensor value Yg and the multiplication product µ × <i>g</i> selected at step S325.

0074At step S327, target vehicle speed calculation unit 22 makes a comparison between the first and second alternative possible target vehicle speed values V 1 and V2 to judge whether the second alternative possible target vehicle speed value V2 is higher than the first alternative possible target vehicle speed value V1. If V2 > V1 (Yes at step S327), program control proceeds to step S328. If V2 ≤ V 1 (No at step S327), program control proceeds to step S329.

0075At step S328, target vehicle speed calculation unit 22 sets the target vehicle speed V* to the second alternative possible target vehicle speed value V2. After that, control exits from the timer interruption subroutine and returns to the main routine.

0076At step S329, target vehicle speed calculation unit 22 sets the target vehicle speed V* to the first alternative possible target vehicle speed value V1. Control then exits from the timer interruption subroutine and returns to the main routine.

0077In the third embodiment, target vehicle speed calculation unit 22 has a first target vehicle speed calculating section (corresponding to step S323) for giving a first vehicle speed value by dividing the multiplication product of the road friction coefficient µ and the lateral acceleration limit Yg* by the yaw rate φ*, a second vehicle speed calculating section (corresponding to steps S325-S326) for giving a second target vehicle speed value by selecting the higher one of the lateral acceleration sensor value Yg and the multiplication result µ × <i>g</i> and then dividing the multiplication product of the thus-obtained select-high value Ygmax (= Yg or µ × <i>g</i>) and the lateral acceleration limit Yg* by the yaw rate φ*, a third vehicle speed calculating section (corresponding to step S324) for giving a third target vehicle speed value by dividing the lateral acceleration sensor value Yg by the yaw rate φ*, and a correcting section (corresponding to steps S322 and S327-S329) for switching among the first, second and third target vehicle speed values to correct the target vehicle speed V* depending on the accelerator pedal depression θth and the lateral acceleration sensor value Yg and thereby adjust the amount of deceleration control on the vehicle in a similar manner as in the first and second embodiments. In other words, the target vehicle speed V* is calculated based the lateral acceleration sensor value Yg according to the expression (10) when the accelerator pedal depression θth is detected in a situation that the vehicle undergoes high lateral acceleration, so that the target vehicle speed V* increases with the lateral acceleration Yg regardless of whether the road friction coefficient µ is underestimated. It is thus possible to perform deceleration control on the vehicle without giving a stall feeling to the driver at the time the vehicle shifts from a turning state to a straight-running state. If there is no or little increase in lateral acceleration under low-friction road conditions even tough the accelerator pedal depression θth is detected, the target vehicle speed V* is calculated based on the road friction coefficient µ and the lateral acceleration limit Yg* according to the expressions (3) and (12). The target vehicle speed V* is calculated based on the road friction coefficient µ and the lateral acceleration limit Yg* according to the expression (2) when the accelerator pedal depression θth is undetectable. In such cases, it is possible to allow the intervention of deceleration control on the vehicle in accordance with road friction conditions and attain stable, smooth vehicle turning behavior.

0078To sum up, deceleration controller 10 is configured to correct the target vehicle speed V* depending on the accelerator pedal depression θth and the lateral acceleration Yg so as to decrease the amount of deceleration control on the vehicle and thereby limit the intervention of the vehicle deceleration control when the vehicle undergoes lateral acceleration in response to a driver's accelerator pedal operation in the first to third embodiment. Accordingly, the vehicle deceleration control can be performed appropriately, without giving a stall feeling to the driver, even if the road friction coefficient µ is underestimated or if the estimation of the road friction coefficient µ is slow in response to changes in road friction conditions. Deceleration controller 10 is further configured to limit the decrease in the amount of deceleration control on the vehicle when no lateral acceleration is applied even by a driver's accelerator pedal operation under low-friction road conditions and when no driver's acceleration pedal operation is made in the first to third embodiments. The intervention of the vehicle deceleration control can be thus allowed appropriately in accordance with road conditions so as to attain stable vehicle behavior.

0079The first and third embodiments may be modified so as to determine the select-high value Ygmax in keeping with a positive change (increase) thereof. For example, the fourth embodiment can be given as such a modification. The fourth embodiment is similar to the first embodiment, except that the target vehicle speed calculation subroutine additionally includes steps S331 and S332 subsequently to step S34 as shown in <figref idref="f0006">FIG. 9</figref>. At step S331, target vehicle speed calculation unit 22 makes a comparison between the current select-high value Ygmax and the previous select-high value Ygmaxz to judge whether the current select-high value Ygmax is higher than the previous select-high value Ygmaxz. If Ygmax ≤ Ygmaxz (No at step S331), target vehicle speed calculation unit 22 counts down the select-high value Ygmax by a given degree as indicated by the following expression (15) at step S332:<maths id="math0013" num="(15)."><math display="block"><mi mathvariant="italic" /><mo>⁢</mo><mi mathvariant="italic">Yg</mi><mspace width="1em" /><mi>max</mi><mo>=</mo><mi mathvariant="italic" /><mo>⁢</mo><mi mathvariant="italic">Yg</mi><mspace width="1em" /><mi>max</mi><mo>-</mo><mn>1</mn></math><img file="EP1577148B1_D0013.tif" /></maths> When there is a positive change in the select-high value Ygmax, the select-high value Yg is used as it stands. On the other hand, the select-high value Ygmax is adjusted according to the expression (15) when there is a negative change in the select-high value Ygmax. This makes it possible to secure a maximum possible target lateral acceleration and prevent hunting.

0080The entire contents of <patcit id="pcit0009" dnum="JP2004073125A"><text>Japanese Patent Application No. 2004-073125 (filed on March 15, 2004</text></patcit>) are herein incorporated by reference.

0081Although the present invention has been described with reference to specific embodiments of the invention, the invention is not limited to the above-described embodiments. Various modification and variation of the embodiments described above will occur to those skilled in the art in light of the above teaching. Target vehicle speed calculation unit 22 may additionally includes a lateral acceleration limit adjusting section to adjust the lateral acceleration limit Yg* to a larger value by e.g. adding a given value (Yg* = Yga + α) to the fixed lateral acceleration limit value Yga upon detection of a driver's accelerator pedal operation though the lateral acceleration limit Yg* is fixed at the given value Yga in the above embodiments. This also makes it possible to increase the target vehicle speed V* so as to limit the intervention of the vehicle deceleration control and avoid giving a stall feeling to the driver. The scope of the invention is defined with reference to the following claims.