VEHICLE BEHAVIOR CONTROL APPARATUS

A vehicle behavior control apparatus includes: a variable roll stiffness device configured to change roll stiffness of a first axle being one of a front axle and a rear axle and roll stiffness of a second axle being another of the front axle and the rear axle; and a controller. The controller is configured to control the variable roll stiffness device so as to increase the roll stiffness of the first axle in accordance with a lateral acceleration in at least a low acceleration range. The controller is also configured to execute a control process of controlling the variable roll stiffness device so as to increase the roll stiffness of the second axle when the lateral acceleration increases to a high acceleration range beyond the low acceleration range, or a vehicle stability control for reducing at least one of oversteer and understeer of the vehicle is abnormal.

CROSS-REFERENCES TO RELATED APPLICATION

The present disclosure claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2023-137101, filed on Aug. 25, 2023, which is incorporated herein by reference in its entirety.

BACKGROUND

Technical Field

The present disclosure relates to a vehicle behavior control apparatus.

Background Art

JP 2023-051026 A discloses a roll control apparatus for a vehicle. This roll control apparatus increases control amounts of actuators on both sides of a first axle and a second axle so as to increase roll stiffness of the first axle and the second axle (i.e., a rear axle and a front axle) in accordance with an increase in a lateral acceleration. When the lateral acceleration increases to a high acceleration range beyond a low acceleration range, the roll control apparatus reduces a gain of the control amount of the actuator on the first axle side with respect to the lateral acceleration in accordance with an increase in the lateral acceleration such that a roll stiffness distribution ratio of the first axle does not exceed a predetermined value.

SUMMARY

According to the technique described in JP 2023-051026 A, it is possible to reduce a bias in roll stiffness distribution in the high acceleration range and, as a result, reduce the occurrence of unintended oversteer or understeer. However, with the technique of lowering the gain for controlling the roll, a large roll may be likely to occur in the high acceleration range. Moreover, when the lateral acceleration is low but a vehicle stability control is abnormal, the bias in roll stiffness distribution may be a factor that makes it easy to cause unintended oversteer or understeer depending on, for example, road surface conditions.

The present disclosure has been made in view of the problem described above, and an object thereof is to provide a vehicle behavior control apparatus that can reduce a bias in roll stiffness distribution while reducing a large roll according to the lateral acceleration or taking into consideration the state of a vehicle stability control.

A vehicle behavior control apparatus according to the present disclosure includes a variable roll stiffness device and a controller. The variable roll stiffness device is configured to change roll stiffness of a first axle being one of a front axle and a rear axle of a vehicle and roll stiffness of a second axle being another of the front axle and the rear axle. The controller is configured to control the variable roll stiffness device so as to increase the roll stiffness of the first axle in accordance with a lateral acceleration acting on the vehicle in at least a low acceleration range of ranges of the lateral acceleration. The controller is also configured to execute a control process of controlling the variable roll stiffness device so as to increase the roll stiffness of the second axle when the lateral acceleration increases to a high acceleration range beyond the low acceleration range, or a vehicle stability control for reducing at least one of oversteer and understeer of the vehicle is abnormal.

According to the present disclosure, when the lateral acceleration increases to the high acceleration range beyond the low acceleration range, the bias in the roll stiffness distribution can be reduced without depending on the technique of lowering the roll control gain (that is, while the occurrence of a large roll according to the lateral acceleration is reduced). Also, according to the present disclosure, when the lateral acceleration is in the low acceleration range but the vehicle stability control is abnormal, unintended oversteer or understeer can be reduced because the bias in the roll stiffness distribution is reduced by the control process.

DETAILED DESCRIPTION

1. Vehicle Behavior Control Apparatus

FIG.1is a diagram illustrating a configuration of a vehicle10equipped with a behavior control apparatus according to an embodiment. As shown inFIG.1, the vehicle10has four wheels14including a left front wheel14FL and a right front wheel14FR on a front axle16F, and a left rear wheel14RL and a right rear wheel14RR on a rear axle16R. In the vehicle10, for example, the left and right wheels14FL and14FR are steered wheels.

The vehicle10includes suspensions20FL,20FRA,20RLA, and20RRA. The suspensions20FL,20FRA,20RLA, and20RRA respectively suspend the left front wheel14FL, the right front wheel14FR, the left rear wheel14RL, and the right rear wheel14RR from a vehicle body12.

The suspension20FL of the left front wheel14FL is a non-active suspension, and includes a spring22FL and a shock absorber24FL.

The suspension20FRA of the right front wheel14FR is an active suspension (more specifically, full active suspension), and includes an actuator26FR in addition to a spring22FR and a shock absorber24FR. The actuator26FR is configured to actively apply a control force in the vertical direction between the vehicle body12and the right front wheel14FR. The actuator26FR is, for example, electric or hydraulic.

The suspensions20RLA and20RRA for the rear axle16R are also active suspensions. The suspensions20RLA and20RRA respectively include actuators26RL and26RR in addition to springs22RL and22RR and shock absorbers24RL and24RR. The actuators26RL and26RR are configured to actively apply control forces in the vertical direction between the vehicle body12and the left and right wheel14RL and14RR for the rear axle16R, respectively. The actuators26RL and26RR are, for example, electric or hydraulic.

The vehicle10is equipped with a controller30. The controller30is configured to acquire signals from sensors40mounted on the vehicle10. The sensors40include sensors configured to measure physical quantities related to the behavior of the vehicle10, such as one or more acceleration sensors, one or more vehicle height sensors, one or more sprung mass acceleration sensors, and one or more wheel speed sensors. The controller30is configured to control the actuators26FR,26RL, and26RR.

The controller30includes a processor32and a memory34coupled to the processor32. The processor32is configured to execute various processes related to a vehicle behavior control of the vehicle10. The vehicle behavior control includes at least a vehicle roll control described below. The memory34is configured to store various kinds of information necessary for the processor32to execute the various processes. For example, the memory34stores a program36executable by the processor32and various kinds of information related to the program36. The vehicle behavior control is realized by the processor32executing the program36.

The various processes related to the vehicle behavior control of the vehicle10may include a process related to the following vehicle stability control. The vehicle stability control is control for reducing at least one of oversteer and understeer of the vehicle10. The vehicle stability control includes, for example, controlling at least one of a drive device and a brake device of the vehicle10such that the controller30detects skidding of the vehicle10using the sensors40and reduces the skidding. The drive device is, for example, at least one of an electric motor and an internal combustion engine.

2. Vehicle Roll Control

In the present embodiment, the rear axle16R corresponds to a “first axle”, and the front axle16F corresponds to a “second axle”. The pair of left and right active suspensions20RLA and20RRA for the rear axle16R and the single active suspension20FRA for the front axle16F function as a “variable roll stiffness device” that changes the roll stiffness of the rear axle16R and the roll stiffness of the front axle16F. Further, the “vehicle behavior control apparatus” according to the present embodiment includes the variable roll stiffness device and the controller30. The vehicle roll control according to the present embodiment is executed by the controller30that controls the variable roll stiffness device.

First,FIG.2is a diagram conceptually showing a basic configuration of the vehicle roll control according to the present embodiment.FIG.2shows a modeled vehicle10, a lateral acceleration (i.e., lateral G) ayacting on the gravity center11of the vehicle10, and control forces Frland Frrfor roll reduction that are applied by the variable roll stiffness device. The left side ofFIG.2shows a state in which the lateral acceleration ayis low, and the right side shows a state in which the lateral acceleration ayis increased. Further,FIG.2shows a graph indicating changes in roll stiffness distribution ratios for the front axle16F and the rear axle16R before and after the lateral acceleration ayincreases.

The basic configuration of the vehicle roll control for the vehicle10having the suspension configuration shown inFIGS.1and2is as follows. That is, the control forces Frland Frrin opposite phases are generated by the actuators26RL and26RR on the side of the rear axle16R provided with the active suspensions20RLA and20RRA on both the left and right wheels14RL and14RR. More specifically, when, for example, the lateral acceleration ayin the right direction acts on the vehicle10as shown inFIG.2, the control force Frlin the downward direction is applied to the left rear wheel14RL by the actuator26RL, and the control force Frrin the upward direction is applied to the right rear wheel14RR by the actuator26RR. As a result, in the vehicle10having the suspension configuration described above, it is possible to generate a roll moment for reducing the roll caused by the lateral acceleration aywithout causing heave and pitching. In other words, the roll stiffness of the rear axle16R (first axle) can be increased with respect to the lateral acceleration ay.

Moreover, according to the basic configuration of the vehicle roll control, the control forces Frland Frrapplied to the left and right wheels14RL and14RR are increased in accordance with the lateral acceleration ayas will be described below in detail. That is, the variable roll stiffness device is controlled by the controller30so as to increase the roll stiffness of the rear axle16R in accordance with an increase in the lateral acceleration ay. InFIG.2, the lengths of arrows indicating the direction of the lateral acceleration ayindicate the magnitude of the lateral acceleration ay. The lengths of arrows indicating the directions of the control forces Frland Frrindicate the magnitudes of the control forces Frland Frr, respectively.

According to the basic configuration of the vehicle roll control, as shown inFIG.2, the roll stiffness distribution ratio for the rear axle16R increases when the roll stiffness of the rear axle16R increases in accordance with an increase in the lateral acceleration ay. That is, the bias in the roll stiffness distribution ratio increases with an increase in the lateral acceleration ay. This may lead to the occurrence of unintended oversteer or understeer when the vehicles turns with high lateral acceleration ay. In this regard, by lowering a roll control gain in the high acceleration range, it is possible to reduce the occurrence of the unintended oversteer or understeer. However, according to this method, a large roll becomes likely to occur in the high acceleration range. Further, it is desirable that the measures for reducing the bias in the roll stiffness distribution are taken in consideration of the state of the vehicle stability control.

Accordingly, the processes executed by the controller30in relation to the vehicle roll control according to the present embodiment include a “control process”. In the control process, when the lateral acceleration ayincreases to a high acceleration range R2beyond a low acceleration range R1or when the vehicle stability control is abnormal, the controller30controls the “variable roll stiffness device” so as to increase the roll stiffness of the front axle16F (second axle).

FIG.3is a flowchart illustrating an example of processes related to the vehicle roll control including the control process according to the present embodiment. The processes of this flowchart are repeatedly executed by the controller30(processor32) while the vehicle10is traveling.

First, in step S100, the controller30acquires the lateral acceleration ayusing the sensors40. Specifically, the lateral acceleration ayis an estimated value acquired from, for example, the steering angle and the vehicle speed. However, the method of acquiring the lateral acceleration ayis not particularly limited. The lateral acceleration aymay be, for example, a sensor value measured by an acceleration sensor.

Then, in step S102, the controller30determines whether or not the lateral acceleration ayacquired in step S100is equal to or higher than a designated threshold value ayt. That is, it is determined whether the lateral acceleration ayis in the high acceleration range R2or in the low acceleration range R1.

More specifically, the high acceleration range R2corresponds to an acceleration range near the turning limit of the vehicle10. Therefore, the lateral acceleration aybeing lower than the threshold value ayt(i.e., being in the low acceleration range R1) means that there is a margin with respect to the turning limit. When there is this margin, unintended oversteer or understeer, or large roll does not occur. Therefore, in step S102, the degree of margin of the lateral acceleration aywith respect to the turning limit is determined. For example, it is simply determined that, when the lateral acceleration ayis equal to or higher than the threshold value ayt(that is, in the high acceleration range R2), the degree of margin with respect to the turning limit is small. In addition, when an estimated friction coefficient of a road surface on which the vehicle10is traveling can be calculated, the threshold value aytmay be determined in consideration of the estimated friction coefficient in order to determine the degree of margin with respect to a limit lateral acceleration according to the estimated friction coefficient.

When the lateral acceleration ayis lower than the threshold value ayt(step S102; No), the controller30determines in step S104whether or not the vehicle stability control is abnormal. The technique of determining whether or not the vehicle stability control is abnormal is not particularly limited, and any known determination method may be used.

When the determination result in step S104is No (that is, when the lateral acceleration ayis lower than the threshold value aytand the vehicle stability control is normal), the processing proceeds to step S106. Hereinafter, the time when the processing proceeds to step S106in this way is simply referred to as “normal time”.

In step S106, the controller30calculates required control forces Ffli, Ffri, Frli, and Frrifor the respective wheels14(i.e., the four wheels14including the left front wheel14FL without the actuator26) in the vehicle roll control for the normal time. The required control forces Ffli, Ffri, Frli, and Frriare control forces required for the respective wheels14in order to realize a required roll moment Mr.

The required roll moment Mris a roll moment required for reducing the roll behavior by the vehicle roll control, and is expressed by the following Equation 1. In Equation 1, α is a roll control gain, β1 is a feedback control gain (FB control gain) β for roll velocity of a sprung mass of the vehicle10(i.e., sprung mass roll velocity), ϕ is a roll angle, and s is a Laplace operator. The second term on the right side of Equation 1 represents a feedback term for the sprung mass roll velocity. By having this feedback term, the required roll moment Mrcorresponding to the lateral acceleration aycan be calculated while the sprung mass roll velocity is brought close to a desired target value. It should be noted that the equation for calculating the required roll moment Mrmay have, for example, only the first term on the right side of Equation 1. That is, the required roll moment Mrmay be simply the product of the lateral acceleration ayand the roll control gain α.

The controller30calculates the required roll moment Mrin accordance with Equation 1, and converts (distributes) the calculated required roll moment Mrinto the required control forces Ffli, Ffri, Frli, and Frrifor the four wheels14in accordance with the following Equation 2. In Equation 2, Tfis a front wheel tread, and Tris a rear wheel tread. The required control forces Ffli, Ffri, Frliand Frriare positive when upward forces are required. The required roll moment Mris positive when a moment to lower the right side of the vehicle body12and lift the left side thereof is required.

When the lateral acceleration ayis equal to or higher than the threshold value ayt(step S102; Yes), or when the vehicle stability control is abnormal (step S104; Yes), the processing proceeds to step S108.

In step S108, the controller30calculates the required roll moment Mrin accordance with the following Equation 3. Equation 3 is different from Equation 1 in the FB control gain β with respect to the sprung mass roll velocity. Specifically, in Equation 3, a value greater than the FB control gain β1 in Equation 1 is set as a FB control gain β2.

In step S108, the controller30converts (distributes) the required roll moment Mrinto the required control forces Ffli, Ffri, Frli, and Frrifor the four wheels14in association with the control process described above. To be specific, the controller30calculates the required control forces Ffli, Ffri, Frland Frrifor the four wheels14by the following Equation 4. According to Equation 4, in comparison with Equation 2, the roll moment generated in the front axle16F (second axle) for realizing the required roll moment Mris increased by an increase amount Mrf. In the example of the vehicle10shown inFIG.1, the suspension20FL for the left front wheel14FL is a non-active suspension. Therefore, the increase amount Mrfis reflected in the required control force Ffrifor the right front wheel14FR having the actuator26FR.

The controller30determines the increase amount Mrfsuch that the bias in the roll stiffness distribution is reduced. To be specific, the controller30determines the increase amount Mrfby the following method, for example. That is, the controller30calculates the increase amount Mrfin accordance with the following Equation 5 when the lateral acceleration ayis equal to or higher than the threshold value ayt. In Equation 5, γ is a designated gain. By using Equation 5, when the lateral acceleration ayincreases in the high acceleration range R2(that is, when the lateral acceleration ayapproaches the turning limit), the roll moment generated on the side of the front axle16F increases. In other words, when the lateral acceleration ayincreases, the roll stiffness distribution shifts toward the front axle16F. In addition, when the processing proceeds to step S108because the lateral acceleration ayis lower than the threshold value aytbut the vehicle stability control is abnormal, the controller30may calculate the increase amount Mrfby multiplying the lateral acceleration ayitself by a designated gain.

Further, the control process according to the present embodiment may include a “distribution process”. In an example involving the distribution process, the controller30calculates the required control forces Ffli, Ffri, Frli, and Frrifor the four wheels14so as to reduce, on the side of the rear axle16R, a roll moment having the same magnitude as the roll moment increased on the side of the front axle16F.

To be more specific, Mrrland Mrrrare decrease amounts in roll moments generated at the two wheels14RL and14RR on the side of the rear axle16R (first axle), respectively. The increase amount Mrfand the two decrease amounts Mrrland Mrrrhave a relation expressed by the following Equation 6. That is, the decrease amounts Mrrland Mrrrare determined such that the sum of the decrease amount Mrrland the decrease amount Mrrrbecomes equal to the increase amount Mrf. Therefore, according to Equation 4 expressed above, the roll moment having the same magnitude as the roll moment increased on the side of the front axle16F is decreased on the side of the rear axle16R.

Additionally, with respect to the distribution process, broadly speaking, the ratio of each of the decrease amounts Mrrland Mrrrto the increase amount Mrfmay be freely set. However, the ratio may be determined using techniques described in the following Section3regarding “Specific Examples of Distribution Process”.

In step S110after step S106or S108, the controller30controls the actuators26FR,26RL, and26RR for the three wheels14based on the required control forces Ffli, Ffri, Frli, and Frricalculated in step S106or S108.

Specifically, in step S110, the controller30converts the required control forces Ffli, Ffri, Frli, and Frriinto required control forces Ffr, Frl, and Frrfor the three wheels14having the actuators26. For this conversion, for example, a technique described in JP 2023-047810 A can be used. The conversion using this technique is outlined as follows.

With respect to the conversion described above, the controller30first uses the following Equation 7 to convert the required control forces Ffli, Ffri, Frli, and Frriinto required values for three modes at gravity center. The three modes at gravity center are motion modes for a heave force, a roll moment, and a pitch moment acting on the gravity center11of the vehicle10. The required values for the three modes at gravity center are a required heave force Fht, a required roll moment Mrt, and a required pitch moment Mptshown in Equation 7. In Equation 7, lfand lrare distances between the gravity centers of the front axle16F and the rear axle16R, respectively. In addition, the required roll moment Mrtincludes the required roll moment Mrfor the vehicle roll control according to the present embodiment. It should be noted that the required values for the three modes at gravity center may include a required value for any vehicle behavior control other than the vehicle roll control.

With respect to the conversion described above, the controller30then uses the following Equation 8 to convert the required values for the three modes at gravity center into the required control forces Ffr, Frl, and Frrfor the three wheels14, i.e., the required control forces for the three actuators26.

In step S110, the controller30controls the actuator26FR such that the vertical control force applied to the right front wheel14FR becomes equal to the required control force Ffr. Similarly, the controller30controls the actuator26RR such that the vertical control force applied to the right rear wheel14RR becomes equal to the required control force Frr, and controls the actuator26RL such that the vertical control force applied to the left rear wheel14RL becomes equal to the required control force Frl. By controlling the respective actuators26FR,26RR, and26RL based on the required control forces Ffr, Frl, and Frrconverted from the required values for the three modes at gravity center in this way, desired behaviors including all of the roll, pitch, and heave are realized in the vehicle10.

In addition, the control of the actuators26described above is applied to the actuators26configured to perform force (torque) control. Unlike this control example, in an example in which actuators26configured to perform position (angle) control are provided for the three wheels14, the controller30may control the three actuators26as follows. That is, the controller30calculates position control amounts of the three actuators26that respectively satisfy the required control forces Ffr, Frl, and Frrconverted by Equation 8. Then, the controller30performs the position control of the three actuators26in accordance with the calculated position control amounts.

As described above, according to the control process of the present embodiment, when the lateral acceleration ayincreases to the high acceleration range R2beyond the low acceleration range R1, the roll stiffness of the front axle16F (second axle) is increased. As a result, in the high acceleration range R2, the bias in the roll stiffness distribution can be reduced without depending on the technique of reducing the roll control gain (that is, while reducing the occurrence of a large roll according to the lateral acceleration ay). Also, according to the present embodiment, when the lateral acceleration ayis in the low acceleration range R1but the vehicle stability control is abnormal, the roll stiffness of the front axle16F (second axle) is increased. As a result, when the lateral acceleration ayis in the low acceleration range R1but the vehicle stability control is abnormal, unintended oversteer or understeer can be reduced by the control process executed to reduce the bias in the roll stiffness distribution, and the stability of the vehicle10can be improved.

Moreover, according to the distribution process included in the control process, the increase in the roll stiffness of the front axle16F is executed based on the relations of Equations 4 and 6 (see step S108). That is, part of the control forces to be generated by the pair of left and right actuators26RL and26RR (first actuators) on the side of the rear axle16R in order to realize the required roll moment Mris distributed to the actuator26FR (second actuator) on the side of the front axle16F. Broadly speaking, the control process may be executed so as to simply increase the roll moment generated on the side of the front axle16F without being accompanied by the distribution process. In contrast, by including the distribution process, the bias in the roll stiffness distribution can be effectively reduced when the roll moment generated on the side of the front axle16F is increased by the same amount, as compared with an example in which the distribution process is not included.

Moreover, according to the present embodiment, when the roll stiffness of the front axle16F is increased by the control process, the FB control gain β with respect to the sprung mass roll velocity is increased (β2>β1). As a result, when the vehicle roll control is performed using the actuators26for the three wheels14with the control process, changes in the sprung mass roll velocity can be converged more quickly and the bias in the roll stiffness can be reduced.

Furthermore, according to the control process of the present embodiment, the increase in the roll stiffness of the front axle16F in the high acceleration range R2is executed based on the relation of Equation 5 (see step S108). That is, when the lateral acceleration ayis higher, the increase amount Mrfbecomes greater and the roll stiffness of the front axle16F is increased. Therefore, the bias in the roll stiffness can be appropriately reduced in accordance with an increase in the lateral accelerations ayin the high acceleration range R2.

3. Specific Examples of Distribution Process

First, a specific example EX1 of the distribution process (see step S108) will be described. In this specific example EX1, the controller30distributes, to the actuator26FR (second actuator), part of the control force to be generated by the actuator26RR (first actuator) of the right rear wheel14RR of the rear axle16R located on the same side as one wheel (right front wheel14FR) of the front axle16F having the actuator26FR.

More specifically, as expressed by Equation 9, the decrease amount Mrrrof the right rear wheel14RR is set to be equal to the increase amount Mrf. Therefore, from the relation of Expression 5 expressed above, the decrease amount Mrrlof the left rear wheel14RL becomes 0 as expressed by Expression 10. As a result, part of the control force to be generated by the actuator26RR of the right rear wheel14RR (i.e., the control force according to the decrease amount Mrrr) is distributed to the actuator26FR as a control force according to the increase amount Mrf.

From the viewpoint of preventing the vehicle10from overturning, a lower sprung mass gravity center height is better. According to the specific example EX1, it is possible to distribute, to the second actuator (e.g., actuator26FR), part of the control force to be generated by the first actuator (e.g., actuator26RR) on the side of the rear axle16R while preventing the force in the heave direction from being generated. More specifically, according to the distribution of the specific example EX1, the pitch behavior is generated in accordance with the output of the roll moment, but no force is generated in the heave direction. Therefore, it is possible to reduce the behavior in which the sprung mass gravity center height increases, which is the behavior that may lead to the overturning. Thus, according to the specific example EX1 (the same applies to specific examples EX2 and EX3 described below), it is possible to prevent the overturning while reducing unintended oversteer or understeer in the high acceleration range R2. In addition, the decrease amounts Mrrland Mrrrmay be determined from Equation 5 while satisfying the relation of “decrease amount: Mrrr>Mrrl” instead of the specific example EX1. The effect of reducing heave can be obtained also by this relation.

Next, in the specific example EX2, the decrease amounts Mrrland Mrrrare set as follows in accordance with the sign of the increase amount Mrf. That is, the downward control force Frlor Frrof the control forces Frland Frrof the first actuators configured to generate the roll moment on the side of the rear axle16R corresponds to a force acting in a direction to lower the sprung mass gravity center height and also corresponds a force acting in a direction to prevent overturning of the vehicle10due to the action of the lateral accelerations ay.

Accordingly, in the specific example EX2, when the increase amount Mrfis negative, the controller30sets the decrease amounts Mrrland Mrrrin accordance with the relations of Equations 9 and 10 described above. On the other hand, when the increase amount Mrfis positive, the controller30sets the decrease amounts Mrrland Mrrrin accordance with the relations of the following Equations 11 and 12, that is, the relations opposite to the relations of Equations 9 and 10. This makes it possible to realize the distribution process that can further effectively lower the sprung mass gravity center height. In addition, the decrease amounts Mrrland Mrrrmay be determined from Equation 5 while satisfying the relation of “decrease amount: Mrrl>Mrrr” instead of the specific example EX2. The effect of lowering the sprung mass gravity center height can be obtained also by this relation.

The distribution process that involves considering the sign of the increase amount Mrfis not limited to the specific example EX2, and may be executed as in the following specific example EX3.FIG.4is a diagram used to describe the specific example EX3 of the distribution process. That is, in the distribution process in which the sign of the increase amount Mrfis considered, when the increase amount Mrfis positive, the decrease amount Mrrlaccording to the increase amount Mrfhaving the same absolute value may be set to be larger than when the increase amount Mrfis negative. To be specific, the decrease amounts Mrrland Mrrrmay be set in accordance with the relation shown inFIG.4, for example. InFIG.4, τ is a ratio of (the absolute value of) the decrease amount Mrrrto (the absolute value of) the increase amount Mrf. Therefore, when the ratio τ is 1, the decrease amount Mrrris equal to the increase amount Mrf. In the example shown inFIG.4, when the increase amount Mrfis negative, the ratio τ is constant at 1, that is, the decrease amount Mrrlis constant at 0. On the other hand, when the increase amount Mrfis positive, the ratio τ linearly decreases from 1 to 0 with an increase in the increase amount Mrf. Accordingly, the decrease amount Mrrllinearly increases from 0 to the same magnitude as the increase amount Mrf. The effect of lowering the sprung mass gravity center height can be obtained also by the specific example EX3.

4. Other Embodiments

In the flowchart shown inFIG.3, when the vehicle stability control is normal but the lateral acceleration ayis in the high acceleration range R2, the control process using Equation 4 is executed. Instead of this example, when the vehicle stability control is normal, the process for the normal time using Equation 2 may be executed regardless of the level of the lateral acceleration ay, and, only when the vehicle stability control is abnormal, the control process may be executed.

Moreover, with respect to the calculation of the required roll moment Mr, a feedback term for the time derivative of the lateral acceleration aymay be used instead of the feedback term for the sprung mass roll velocity (see Equation 1 or 3). Also, similarly to the example of the sprung mass roll velocity, when the roll stiffness of the front axle16F is increased by the control process, a feedback control gain for the time derivative may be increased.

Furthermore, in the example shown inFIG.1, the suspension20FL for the left front wheel14FL is a non-active suspension that does not have the actuator26. Instead of this example, the suspension20FR for the right front wheel14FR may be a non-active suspension. In addition, unlike the example shown inFIG.1, the front axle16F may be the “first axle” and the rear axle16R may be the “second axle”. In this example, the suspension20RL or20RR on the side of the rear axle16R is a non-active suspension.