Control method suitable for integration of four-wheel independent driving and driving rear wheel steering

Abstract

The invention belongs to the field of vehicle stability control, and particularly relates to a control method suitable for integration of four-wheel independent drive and active rear wheel steering. Comprising the steps of establishing a whole vehicle dynamics model suitable for controller design, performing stable region division by combining front and rear wheel side slip angle phase planes, setting an ideal yaw velocity and an ideal side slip angle, designing a feedforward and PID feedback integrated controller, performing torque distribution and the like. According to steering wheel rotation angle input, corresponding parameters of the vehicle are combined, the vehicle stability coefficient, the ideal yaw velocity and the side slip angle are calculated, the feedforward and PID feedback integrated controller inputs the calculated rear wheel rotation angle into the controlled vehicle, the calculated additional yaw moment obtains the basic driving moment based on pedal information, and the vehicle stability coefficient, the ideal yaw velocity and the side slip angle are calculated. Torques corresponding to four wheels are input to a controlled vehicle through torque distribution, so that the purpose of integrated control is achieved, and the vehicle handling stability is improved.

Description

Technical Field

[0001] The present invention belongs to the field of vehicle stability control, and particularly relates to a control method applicable to the integration of four-wheel independent drive and active rear-wheel steering. Background Art

[0002] With the development of domain controllers, it provides the hardware conditions for the application of integrated control strategies for new energy vehicles. The four wheels of a four-wheel independent drive electric vehicle can independently control torque and speed, and thus can achieve more precise vehicle dynamics control to improve vehicle handling stability. In addition, applying active rear-wheel steering angle control can also improve vehicle handling stability. Since the control systems are different but the expected control effects are the same, if the two systems are not uniformly controlled, during the specific control process, it may cause interference between the systems and even conflicts, affecting the vehicle handling stability. Therefore, there is an urgent need to design an integrated control method applicable to four-wheel independent drive and active rear-wheel steering electric vehicles. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention provides a control method applicable to the integration of four-wheel independent drive and active rear-wheel steering, including the following steps:

[0004] Step 1: Establish a vehicle dynamics model suitable for controller design:

[0005] Adopt an extended two-degree-of-freedom vehicle dynamics model that combines rear-wheel steering angle and additional yaw moment;

[0006] Further, the space state expression of the extended two-degree-of-freedom vehicle model is as follows:

[0007]

[0008] Wherein, B 12 = 0, C f is the equivalent cornering stiffness of the front axle, C r is the equivalent cornering stiffness of the rear axle, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass, δ f is the front-wheel steering angle, δ r is the rear-wheel steering angle, ΔM z is the additional yaw moment, ω r is the vehicle yaw rate, β is the sideslip angle of the center of mass, I z is the vehicle moment of inertia, v x is the longitudinal velocity of the center of mass, and m is the vehicle mass.

[0009] Step 2. Divide the stability region by combining the phase planes of the front and rear wheel sideslip angles:

[0010] Combine the tire sideslip characteristics to divide the phase planes of the front and rear wheel sideslip angles into a stable region, a transition region, and an unstable region, with the origin as the center of the circle for all three regions;

[0011] Furthermore, the stable region has a radius of R1 = max(α f,lin , α r,lin ), where α f,lin represents the tire sideslip angle corresponding to the boundary point between the linear region and the non-linear region of the front wheel lateral force, which is affected by the road surface adhesion coefficient; α r,lin represents the tire sideslip angle corresponding to the boundary point between the linear region and the non-linear region of the rear wheel lateral force, which is affected by the road surface adhesion coefficient;

[0012] The transition region has a radius of R2 = max(α f,sat , α r,sat ), where α f,sat represents the tire sideslip angle corresponding to the maximum front wheel lateral force, and α r,sat represents the tire sideslip angle corresponding to the maximum rear wheel lateral force;

[0013] The unstable region has a radius of , where (α fs , α rs ) are the saddle point coordinates closer to the origin.

[0014] Define the vehicle stability coefficient as ξ, and the calculation formula for the vehicle's real-time stability coefficient is as follows:

[0015] If R3 > R2:

[0016]

[0017] If R3 ≤ R2:

[0018]

[0019] Among them, R0 represents the position of the vehicle's driving state in the phase plane, which is calculated by the following formula:

[0020]

[0021] Among them, α f , α r are the front and rear wheel sideslip angles respectively;

[0022] Normalize the stability coefficient ξ to convert it into a coefficient between 0 and 1. The normalized stability coefficient is expressed as:

[0023]

[0024] Among them, ξ0 represents the stability coefficient after normalization processing.

[0025] Step 3: Setting the ideal yaw rate and the ideal sideslip angle of the center of mass:

[0026] Based on the stability coefficient, set the handling yaw rate and the stability yaw rate, and fuse the handling yaw rate and the stability yaw rate to obtain the ideal yaw rate;

[0027] Based on the stability coefficient, set the handling sideslip angle of the center of mass and the stability sideslip angle of the center of mass, and fuse the handling sideslip angle of the center of mass and the stability sideslip angle of the center of mass to obtain the ideal sideslip angle of the center of mass;

[0028] Make the ideal yaw rate and the ideal sideslip angle of the center of mass have different expressions in different stability intervals.

[0029] Furthermore, the setting of the handling yaw rate is as follows:

[0030] By reasonably setting K us , a y,max , the handling yaw rate ω rhd under the steady-state condition is obtained, and its specific expression is as follows:

[0031]

[0032] Among them, K us is the degree of understeer, is the maximum lateral acceleration in the linear region, a y,max is the maximum lateral acceleration that the vehicle can achieve; a y is the lateral acceleration, v x is the longitudinal speed of the center of mass, δ f is the front wheel steering angle, L is the wheelbase, δ fdyn0 is the dynamic steering angle;

[0033] Performing Laplace transform on the linear two-degree-of-freedom differential equation of the vehicle, the target of the handling yaw rate represented by the transfer function is expressed as follows:

[0034]

[0035] Among them, is the transfer function of the yaw rate expected value filter, s is the complex frequency variable in the Laplace transform, is the natural frequency; is the damping ratio; is the time constant;

[0036] The setting of the stability yaw rate is as follows:

[0037] Integrating the two - degree - of - freedom vehicle model gives the steady - state yaw rate ω rsd , and its specific expression is as follows:

[0038]

[0039] In the formula, μ is the road surface adhesion coefficient, g is the acceleration due to gravity, and v x is the longitudinal velocity of the center of mass;

[0040] Establish its response transfer function, and the target expression of the steady - state yaw rate in combination with the transfer function is as follows:

[0041]

[0042] Fusing the target of the handling yaw rate and the target of the steady - state yaw rate using the vehicle stability coefficient and the road surface adhesion coefficient gives the ideal yaw rate target as:

[0043]

[0044] Among them, ξ0 is the stability coefficient after normalization, and f(μ) represents a piece - wise function, and the function form is:

[0045]

[0046] Furthermore, the set value of the handling center - of - mass sideslip angle is as follows:

[0047] The expression of the steady - state center - of - mass sideslip angle is:

[0048]

[0049] Among them, β is the center - of - mass sideslip angle, l f is the distance from the front axle to the center of mass, l r is the distance from the front axle to the center of mass, m is the vehicle mass, v x is the longitudinal velocity of the center of mass, L is the wheelbase, C r is the equivalent cornering stiffness of the rear axle, K us is the degree of understeer, δ f is the front - wheel steering angle;

[0050] The limit of the center - of - mass sideslip angle β lim is:

[0051] β lim = tan -1 (0.02μg)

[0052] Among them, μ is the road surface adhesion coefficient, and g is the acceleration due to gravity;

[0053] Steady-state maneuvering centroid sideslip angle β hd Expressed as:

[0054] β hd =min(|β|,β lim )sign(β)

[0055] The Laplace transform of the vehicle's linear two-degree-of-freedom differential equation is performed, and the side slip angle target of the control centroid combined with the transfer function is expressed as:

[0056]

[0057] Where G β (s) is the transfer function of the sideslip angle expected value filter, ω nβ is the natural frequency, ζ β is the damping ratio, τ β is the time constant;

[0058] The stability center of mass side slip angle target is set to 0, that is:

[0059]

[0060] The ideal center of mass sideslip angle target is obtained by integrating the vehicle stability coefficient with the control center of mass sideslip angle target and the stability center of mass sideslip angle target:

[0061]

[0062] Among them, f β (ξ0) is a piecewise function, which can be expressed as follows:

[0063]

[0064] Step 4: Design of integrated controller with feedforward and PID feedback:

[0065] First, a feed-forward derivation is performed;

[0066] The active rear wheel steering angle feedforward value for suppressing the sideslip angle is derived based on the model, and then the additional yaw moment feedforward value is derived considering the active rear wheel steering angle feedforward value.

[0067] Furthermore, the feedforward derivation is as follows:

[0068] The active rear wheel angle feedforward derivation model is derived based on the form of the extended two-degree-of-freedom vehicle model after adding the yaw moment. The active rear wheel angle feedforward decision quantity is as follows:

[0069]

[0070] Among them, δ rFF is the active rear wheel steering angle feedforward value, lf is the distance from the front axle to the center of mass, l r is the distance from the front axle to the center of mass, v x is the longitudinal velocity of the center of mass, m is the mass of the whole vehicle, C f is the equivalent cornering stiffness of the front axle, C r is the equivalent cornering stiffness of the rear axle, L is the wheelbase, K us is the degree of understeer, δ f is the front wheel steering angle;

[0071] After obtaining the feedforward value of the active rear wheel steering angle, based on the extended two-degree-of-freedom vehicle model and considering the feedforward value of the active rear wheel steering angle, the feedforward value of the additional yaw moment is derived; the feedforward decision variable of the additional yaw moment is as follows:

[0072]

[0073] where, ΔM zFF is the feedforward value of the additional yaw moment, δ r is the rear wheel steering angle;

[0074] Ignoring the transient characteristics of the ideal center of mass sideslip angle target, the feedforward decision variable of the active rear wheel steering angle is:

[0075]

[0076] where, is the ideal center of mass sideslip angle target, f β (ξ0) is a piecewise function.

[0077] Secondly, combined with the vehicle speed, the control of the feedforward decision variable is carried out, and the feedforward expression of the rear wheel steering angle is corrected so that the controller has different expressions under different vehicle speeds and different stability intervals; the corrected feedforward is expressed in the following form:

[0078] When the vehicle speed is less than the critical vehicle speed:

[0079]

[0080] When the vehicle speed is greater than the critical vehicle speed:

[0081]

[0082] where, f(ξ0) is expressed in the following form:

[0083]

[0084] ξ0 is the stability coefficient after normalization; k is the calibration parameter, and μ is the road adhesion coefficient.

[0085] Finally, PID feedback is adopted to control the output of the additional yaw moment; when using the additional yaw moment to control the yaw rate, the yaw rate feedback control quantity based on PID control is expressed in the following form:

[0086]

[0087] Among them, ΔM zFB (k) is the discrete yaw moment feedback value, is the proportional coefficient, is the integral coefficient, is the differential coefficient, is the error, and Δt is the sampling period.

[0088] Step Five: Torque Distribution:

[0089] Based on the rear wheel steering angle and the additional yaw moment obtained in the above Step Four, reasonable distribution control is carried out; the rear wheel steering angle is directly used for control; the additional yaw moment needs to be distributed to the four wheels and then controlled. The distribution method adopts the average distribution method to distribute and control the four-wheel torque of the vehicle. The specific distribution formula is as follows:

[0090]

[0091] Among them, T fl 、T fr 、T rl 、T rr are the executive torques of the front left wheel, front right wheel, rear left wheel, and rear right wheel respectively; d f is the front axle track; d r is the rear axle track; R e is the tire rolling radius; T fl,basic 、T fr,basic 、T rl,basic 、T rr,basic are the four-wheel basic analysis torques when the driver steps on the accelerator respectively. These torques are all equal, and their sum is denoted as T basic ; the torques obtained above are used to control the output of the four-wheel torque of the vehicle and control the vehicle simultaneously with the rear wheel steering angle to achieve the purpose of integrated control.

[0092] Advantages of the present invention:

[0093] The present invention provides a control method applicable to the integration of four-wheel independent drive and active rear-wheel steering. The driver operates the steering wheel. According to the steering wheel angle input and in combination with the corresponding parameters of the vehicle, the vehicle stability coefficient, the ideal yaw angular velocity, and the ideal centroidal side slip angle are calculated and input into the "feedforward and PID feedback" integrated controller. This controller inputs the calculated rear-wheel angle into the controlled vehicle. The calculated additional yaw moment is based on the basic driving moment obtained from the pedal information, and the torques corresponding to the four wheels are input into the controlled vehicle through torque distribution to achieve the purpose of integrated control. In the stable region, when the control method of the present invention is adopted, the yaw angular velocity gain of the vehicle is significantly improved, and at the same time, the lateral acceleration is amplified to a certain extent, improving the vehicle's handling performance in the stable region; in the unstable region, when the amplitude of the steering wheel angle is at the trough, the yaw angular velocity of the vehicle controlled by the control method of the present invention is significantly suppressed, and the vehicle has good retention ability after emergency obstacle avoidance, and the vehicle stability is significantly improved. Brief Description of the Drawings

[0094] Figure 1 It is a schematic diagram of the overall flow of the control method in the present invention.

[0095] Figure 2 It is a schematic diagram of the overall architecture of the control method in the present invention.

[0096] Figure 3 It is a schematic diagram of an extended two-degree-of-freedom vehicle model for controller design in the present invention.

[0097] Figure 4 It is a schematic diagram of the division of the stable region of the front and rear wheel side slip angles in the phase plane in the present invention.

[0098] Figure 5 It is a schematic diagram of the simplified front and rear wheel side slip characteristics in the present invention.

[0099] Figure 6 It is a schematic diagram of the relevant characteristics of the whole vehicle when the original vehicle is actuated in the stable region in the present invention.

[0100] Figure 7 It is a schematic diagram of the relevant characteristics of the whole vehicle when the control is turned on and the vehicle is actuated in the stable region in the present invention.

[0101] Figure 8 It is a schematic diagram of the relevant characteristics of the whole vehicle when the original vehicle is actuated in the unstable region in the present invention.

[0102] Figure 9 It is a schematic diagram of the relevant characteristics of the whole vehicle when the control is turned on and the vehicle is actuated in the unstable region in the present invention. Detailed Embodiment

[0103] As Figure 1-2 shown, a control method applicable to the integration of four-wheel independent drive and active rear-wheel steering provided by the present invention includes the following steps:

[0104] Step 1: Establish a vehicle dynamics model applicable to controller design:

[0105] Due to the controller computing power limitation and the requirement of vehicle model accuracy, the extended two-degree-of-freedom vehicle dynamics model with rear wheel steering angle and additional yaw moment is adopted in the present invention.

[0106] Further, as Figure 3 shown, the lateral yaw dynamics equation of the vehicle is:

[0107]

[0108] In the formula, F yf , F yr are the lateral reaction forces of the ground on the front and rear wheels respectively, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass, δ f is the front wheel steering angle, δ r is the rear wheel steering angle, ΔM z is the additional yaw moment, ω r is the vehicle yaw angular velocity, β is the center of mass sideslip angle, I z is the vehicle moment of inertia, and v x is the longitudinal velocity of the center of mass;

[0109] [[ID=3N]]Considering that the front wheel steering angle δ f is small, so the sideslip angles of the front and rear wheels are small, and the tire sideslip characteristics are in the linear region, then there are:

[0110]

[0111] In the formula, C f is the equivalent sideslip stiffness of the front axle, C r is the equivalent sideslip stiffness of the rear axle; α f , α r are the sideslip angles of the front and rear wheels respectively, expressed as:

[0112]

[0113] Further arranging, the space state expression of the extended two-degree-of-freedom vehicle model is obtained as follows:

[0114]

[0115] Among them, B 12 = 0,

[0116] Step 2: Divide the stability region by combining the sideslip angle phase planes of the front and rear wheels:

[0117] Input relevant parameters of the vehicle, combine the phase plane of the front and rear wheel sideslip angles, divide the stable state region, and normalize the results of the region division. The specific steps are as follows:

[0118] Determine the saddle point positions in the phase plane of the front and rear wheel sideslip angles. The left and right saddle point coordinates are expressed in the following form:

[0119]

[0120] Among them, S l and S r are the coordinates of the left saddle point in the phase plane and the coordinates of the right saddle point in the phase plane respectively. α fl , α rl , α fr , α rr are the abscissa of the left saddle point, the ordinate of the left saddle point, the abscissa of the right saddle point, and the ordinate of the right saddle point respectively;

[0121] The actual positions of the left and right saddle points are determined by the two-dimensional coordinates formed by the front and rear wheel sideslip angles. The front and rear wheel sideslip angles are related to the yaw rate and the sideslip angle of the center of mass. The yaw rate and the sideslip angle of the center of mass at the saddle point are expressed as:

[0122]

[0123] Among them, ω rsl and ω rsr are the yaw rates corresponding to the left saddle point and the yaw rates corresponding to the right saddle point respectively. a x is the longitudinal acceleration, μ is the road surface adhesion coefficient, g is the gravitational acceleration, β sl and β sr are the sideslip angles of the center of mass corresponding to the left saddle point and the sideslip angles of the center of mass corresponding to the right saddle point respectively. α f,sat represents the tire sideslip angle corresponding to the maximum lateral force of the front wheel. It is affected by the road surface adhesion coefficient, and the two are approximately linearly related, that is:

[0124] α f,sat = e1μ + e2

[0125] f1, f2, f3, and f4 in the above formula represent different fitting parameters of the phase plane respectively, and the specific expressions are as follows:

[0126]

[0127] f2 = e4 + e5δ f μ + e6v x

[0128]

[0129]

[0130] Among them, e1 to e 10 are parameters to be identified. The left and right saddle point coordinate data under different vehicle speeds, front wheel steering angles, road surface adhesion coefficients, and longitudinal accelerations can be fitted through the Matlab toolbox.

[0131] Furthermore, the left and right saddle point coordinates can be obtained as follows:

[0132]

[0133] The present invention divides the front and rear wheel sideslip angle phase plane into a stable region, a transition region, and an unstable region in combination with the above-mentioned tire sideslip characteristics, and all three regions are centered on the origin.

[0134] As Figure 4 shown, within the stable region, with R1 = max(α f,lin , α r,lin ) as the radius, where α f,lin represents the tire sideslip angle corresponding to the boundary point between the linear region and the non-linear region of the front wheel lateral force, which is affected by the road surface adhesion coefficient, and the two are approximately linearly related, that is:

[0135] α f,lin = e 11 μ + e 12

[0136] e 11 and e 12 are parameters to be identified, and can also be obtained through the identification of the Matlab toolbox;

[0137] α r,lin represents the tire sideslip angle corresponding to the boundary point between the linear region and the non-linear region of the rear wheel lateral force, which is affected by the road surface adhesion coefficient.

[0138] The transition region takes R2 = max(α f,sat , α r,sat ) as the radius, α f,sat represents the tire sideslip angle corresponding to the maximum front wheel lateral force, and α r,sat is the tire sideslip angle corresponding to the maximum rear wheel lateral force; the unstable region takes as the radius, and (α fs , α rs ) is the saddle point coordinate closer to the origin.

[0139] Define the vehicle stability coefficient as ξ, and the calculation formula for the real-time vehicle stability coefficient is as follows:

[0140] If R3 > R2:

[0141]

[0142] If R3 ≤ R2:

[0143]

[0144] Wherein, R0 represents the position of the vehicle driving state in the phase plane, which is calculated by the following formula:

[0145]

[0146] The stability coefficient ξ is normalized to a coefficient between 0 and 1. The normalized stability coefficient is expressed as:

[0147]

[0148] Wherein, ξ0 represents the stability coefficient after normalization.

[0149] Step 3: Set the ideal yaw rate and ideal sideslip angle of the center of mass:

[0150] The handling and stability of the yaw rate and the sideslip angle of the center of mass are set respectively, and the yaw rate fusion and the sideslip angle fusion of the center of mass are carried out respectively, so that the ideal yaw rate and the sideslip angle of the center of mass have different expressions in different stability intervals.

[0151] First, the ideal yaw rate is set, which is divided into the handling yaw rate and the stability yaw rate;

[0152] The setting of the handling yaw rate is as follows:

[0153] The understeer degree K us is characterized as follows:

[0154]

[0155] Wherein, L is the wheelbase, m is the vehicle mass, l f is the distance from the front axle to the center of mass, l r is the distance from the front axle to the center of mass, C f is the equivalent cornering stiffness of the front axle, C r is the equivalent cornering stiffness of the rear axle;

[0156] The front wheel steering angle is expressed as:

[0157]

[0158] Wherein, R is the turning radius, δ fkin is the Ackermann steering angle, δ fdyn is the dynamic steering angle, a y is the lateral acceleration.

[0159] The relationship between the dynamic steering angle and the lateral acceleration is expressed in the following form:

[0160]

[0161] Among them, represents the maximum lateral acceleration in the linear region, and a y,max represents the maximum lateral acceleration that the vehicle can achieve; Express a y in the following form:

[0162]

[0163] Among them, δ f is the front wheel steering angle, v x is the longitudinal velocity of the center of mass, and δ fdyn0 is the dynamic steering angle;

[0164]

[0165] By reasonably setting K us , a y,max , the handling yaw rate ω rhd under steady-state conditions is obtained, and its specific expression is as follows:

[0166]

[0167] The linear two-degree-of-freedom differential equation of the vehicle is Laplace-transformed and rearranged by moving terms to obtain the required transfer function. The handling yaw rate target combined with the transfer function is expressed in the following form:

[0168]

[0169] Among them, is the transfer function of the yaw rate expectation filter, s is the complex frequency variable in the Laplace transform, is the natural frequency; is the damping ratio; is the time constant; By reasonably setting the transient response characteristics of the handling yaw rate can be characterized.

[0170] The stability yaw rate is set as follows:

[0171] The stability yaw rate target functions to ensure the stability of the vehicle and avoid excessive steering of the vehicle, which may cause danger. Based on this, during the setting process of the stability yaw rate target, only the lateral force of the vehicle's front wheels is saturatedly limited, and its side slip characteristics are described as linear and non-linear; while the lateral force of the vehicle's rear wheels is not limited, and its side slip characteristics are only described as linear. In this way, excessive steering of the vehicle can be avoided at the target level. AsFigure 4 as shown

[0172] To set the lateral forces on the front and rear axles obtained by simplifying the tire cornering characteristics for the stability yaw rate:

[0173]

[0174] F yrs = C r α r

[0175] In the formula, F yfs and F yrs are the lateral forces on the front and rear axles respectively, C f is the equivalent cornering stiffness of the front axle, C r is the equivalent cornering stiffness of the rear axle, α f,tip1 and α f,tip2 are the tire cornering angles corresponding to the turning points of the front-wheel cornering characteristics when the road adhesion coefficient is μ, α f is the cornering angle of the front-axle wheels, α r is the cornering angle of the rear-axle wheels, F zf is the vertical load of the front-axle wheels;

[0176] By performing integral operations on the two-degree-of-freedom vehicle model, the stability yaw rate ω rsd under steady-state conditions is obtained, and its specific expression is as follows:

[0177]

[0178] In the formula, μ is the road adhesion coefficient, g is the acceleration due to gravity, and v x is the longitudinal velocity of the center of mass;

[0179] To characterize the transient characteristics of the stability yaw rate target, a response transfer function is established for it, and the stability yaw rate target expression combined with the transfer function is as follows:

[0180]

[0181] Using the vehicle stability coefficient and the road adhesion coefficient to fuse the handling yaw rate target and the stability yaw rate target, the final ideal yaw rate target is:

[0182]

[0183] Among them, ξ0 is the stability coefficient after normalization processing, and f(μ) represents a piecewise function, and the function form is:

[0184]

[0185] Next, the ideal centroid slip angle is set as follows:

[0186] Under steady-state conditions, for a two-degree-of-freedom vehicle model, The expression for the steady-state centroid slip angle is as follows:

[0187]

[0188] where β is the centroid slip angle, l f is the distance from the front axle to the centroid, l r is the distance from the front axle to the centroid, m is the vehicle mass, v x is the longitudinal velocity of the centroid, L is the wheelbase, C r is the equivalent cornering stiffness of the rear axle, K us is the degree of understeer, δ f is the front wheel angle. Considering the road adhesion limit, based on experience, the limit of the centroid slip angle β lim is taken as:

[0189] β lim = tan -1 (0.02μg)

[0190] The steady-state handling centroid slip angle β hd is expressed as:

[0191] β hd = min(|β|, β lim ) sign(β)

[0192] Taking the Laplace transform of the vehicle's linear two-degree-of-freedom differential equation and rearranging the terms, the required transfer function is obtained. Combining with the handling property of the transfer function, the centroid slip angle target is:

[0193]

[0194] In the formula, G β (s) is the transfer function of the desired slip angle filter, ω nβ is the natural frequency, ζ β is the damping ratio, τ β is the time constant; by reasonably setting ω nβ , ζ β , τ β , the transient response characteristics of the centroid slip angle can be characterized.

[0195] To simplify the description and achieve the final stability control, the present invention sets the stable centroid slip angle target to 0, that is:

[0196]

[0197] Fuse the vehicle stability coefficient with the handling centroidal side slip angle target and the stability centroidal side slip angle target to obtain the final ideal centroidal side slip angle target as follows:

[0198]

[0199] Among them, f β (ξ0) is a piecewise function and is expressed in the following form:

[0200]

[0201] Step 4: Design of an integrated controller for feedforward and PID feedback:

[0202] First, perform feedforward derivation; first, derive the feedforward value of the active rear wheel steering angle that suppresses the centroidal side slip angle based on model derivation, and then derive the feedforward value of the additional yaw moment considering the feedforward value of the active rear wheel steering angle.

[0203] Use the form of the extended two-degree-of-freedom vehicle model after degenerate additional yaw moment as the feedforward derivation model for the active rear wheel steering angle; let β take the ideal centroidal side slip angle target set in Step 3 Eliminate ω in the formula r , and obtain the feedforward decision variable of the active rear wheel steering angle as follows:

[0204]

[0205] Among them, δ rFF is the feedforward value of the active rear wheel steering angle, l f is the distance from the front axle to the centroid, l r is the distance from the front axle to the centroid, v x is the longitudinal speed of the centroid, m is the vehicle mass, C f is the equivalent cornering stiffness of the front axle, C r is the equivalent cornering stiffness of the rear axle, L is the wheelbase, K us is the degree of understeer, δ f is the front wheel steering angle;

[0206] After obtaining the feedforward value of the active rear wheel steering angle, based on the extended two-degree-of-freedom vehicle model, considering the feedforward value of the active rear wheel steering angle, derive the feedforward value of the additional yaw moment; let ω r take the ideal yaw rate target set in Step 3 Eliminate β in the formula, and obtain the feedforward decision variable of the additional yaw moment as follows:

[0207]

[0208] Among them, ΔM zFF is the feedforward value of the additional yaw moment, δ ris the rear wheel steering angle;

[0209] If the transient characteristics of the ideal sideslip angle target are ignored, it can be approximately considered that the ideal sideslip angle target Then the feedforward decision-making quantity of the active rear wheel steering angle is:

[0210]

[0211] Among them, is the ideal sideslip angle target, and f β (ξ0) is a piecewise function, and β hd is the steady-state handling property sideslip angle.

[0212] Secondly, the discussion of the feedforward decision-making quantity is carried out; it can be seen from the above formula that in order to satisfy the sideslip angle of the center of mass being 0, when the vehicle speed is lower than a certain critical speed, the direction of the rear wheel steering angle is opposite to that of the front wheel steering angle; when the vehicle speed is higher than the critical speed, the direction of the rear wheel steering angle is the same as that of the front wheel steering angle.

[0213] When the vehicle speed is less than the critical speed and the vehicle is in the stable region: At this time, the rear wheel steering angle calculated by the classical proportion feedforward of ARS is opposite to the front wheel steering angle, which can increase the yaw rate gain. Therefore, ARS should participate in the control at this time, and together with TVC, improve the vehicle handling performance. As can be seen from the above, at this time ARS does not participate in the control, and the handling performance is completely responsible for by TVC, and the advantages of the rear steering actuator are not fully utilized at this time.

[0214] When the vehicle speed is less than the critical speed and the vehicle is in the transition region: At this time, the vehicle speed is less than the critical speed. Increasing the expression ratio of the classical proportion feedforward of ARS will have the effect of suppressing the sideslip angle of the center of mass. However, since the rear wheel steering angle calculated by the classical proportion feedforward is opposite to the front wheel steering angle at this time, the yaw rate gain will also be increased at this time. Actually, the yaw rate gain should be reduced at this time; as the expression of the classical proportion feedforward of ARS increases, it will lead to an increase in the feedforward of TVC, which may cause an overshoot of the yaw rate.

[0215] When the vehicle speed is less than the critical speed and the vehicle is in the unstable region: This working condition is a continuation of the previous working condition, so the performance at this time is also opposite to the expectation.

[0216] When the vehicle speed is greater than the critical speed, the above formula has good performance.

[0217] In order to fully utilize each actuator and ensure the vehicle handling stability, the feedforward expression of the rear wheel steering angle is corrected so that the controller has different expressions for different vehicle speeds and different stable intervals; combining with practical experience, the feedforward when the vehicle speed is lower than the critical speed is constructed in the following form:

[0218]

[0219] In the formula, f(ξ0) is expressed in the following form:

[0220]

[0221] Among them, f(ξ0) is a piecewise function, and ξ0 is the stability coefficient after normalization processing;

[0222] Considering the control requirements under different road surface attachments, a unified feedforward may cause control overshoot under certain road surfaces. Therefore, the feedforward formula is appropriately scaled according to the road surface attachment. The corrected feedforward is expressed in the following form:

[0223] When the vehicle speed is less than the critical vehicle speed:

[0224]

[0225] When the vehicle speed is greater than the critical vehicle speed:

[0226]

[0227] Among them, k is a calibration parameter, μ is the road surface adhesion coefficient, and f β (ξ0) is a piecewise function.

[0228] Finally, PID feedback is adopted to control the output of the additional yaw moment; the additional yaw moment is used to control the yaw rate. The yaw rate feedback control quantity based on PID control is expressed in the following form:

[0229]

[0230] Among them, ΔM zFB (k) is the discrete yaw moment feedback value, is the proportional coefficient, is the integral coefficient, is the differential coefficient, is the error, and Δt is the sampling period.

[0231] Step Five: Torque Distribution:

[0232] Based on the rear wheel steering angle δ r and the additional yaw moment ΔM z obtained in the above Step Four, reasonable distribution control is carried out; the rear wheel steering angle δ r is directly used for control; the additional yaw moment ΔM z needs to be distributed to the four wheels and then controlled.

[0233]

[0234] The present invention adopts the average distribution method for distribution to control the four-wheel torque of the vehicle. The specific distribution formula is as follows:

[0234]

[0235] Among them, T fl , T fr , T rl , T rr are the executive torques of the front left wheel, front right wheel, rear left wheel, and rear right wheel respectively; d f is the front axle track; d r is the rear axle track; R e is the tire rolling radius; T fl,basic , T fr,basic , T rl,basic , T rr,basic are the basic analysis torques of the four wheels when the driver steps on the accelerator. These torques are all equal, and their sum is denoted as T basic ; the torques obtained above are used to control the output of the four-wheel torque of the vehicle, and the vehicle is controlled simultaneously with the rear wheel steering angle to achieve the purpose of integrated control.

[0236] After the above integrated controller design, the present invention has been verified by actual vehicles. From the comprehensive comparison of objective experimental data, as Figure 6 , Figure 7 shown, the steering wheel angle inputs when the control is turned on and off in the stable region are basically the same. When the control is turned on, the yaw rate gain of the vehicle is increased by about 13% compared with when the control is not turned on, and at the same time, the lateral acceleration is amplified to a certain extent, improving the vehicle's maneuverability in the stable region; as Figure 8 , Figure 9 shown, in the unstable region, the steering wheel angle inputs when the control is turned on and off are basically the same. When the amplitude of the steering wheel angle is at the trough, the yaw rate of the vehicle is significantly suppressed after the control is turned on, with an inhibition of about 14.2%. During the entire test cycle, the sideslip angle of the center of mass is maximally inhibited by about 37.5% after the control is turned on, and the lateral acceleration is maximally inhibited by about 4.6%. Moreover, the vehicle has good retention ability after emergency obstacle avoidance, and the vehicle stability is significantly improved.

Claims

1. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering, characterized in that: It includes the following steps: Step 1: Establish a vehicle dynamics model applicable to controller design: Adopt an extended two-degree-of-freedom vehicle dynamics model that combines rear wheel steering angle and additional yaw moment; Step 2: Divide the stability region by combining the front and rear wheel sideslip angle phase planes: Combine the tire sideslip characteristics to divide the front and rear wheel sideslip angle phase plane into a stable region, a transition region, and an unstable region, all with the origin as the center; Define the vehicle stability coefficient; Step 3: Set the ideal yaw rate and ideal sideslip angle of the center of mass: Through the stability coefficient, set the handling yaw rate and the stability yaw rate, and fuse the handling yaw rate and the stability yaw rate to obtain the ideal yaw rate as: wherein, is the ideal yaw rate, ξ0 is the stability coefficient after normalization, and f(μ) is a piecewise function, is the handling yaw rate, is the stability yaw rate; Through the stability coefficient, set the handling sideslip angle of the center of mass and the stability sideslip angle of the center of mass, and fuse the handling sideslip angle of the center of mass and the stability sideslip angle of the center of mass to obtain the ideal sideslip angle of the center of mass as: Among them, is the ideal centroidal sideslip angle, and f β (ξ0) is a piecewise function, is the handling centroidal sideslip angle, is the stability centroidal sideslip angle; Step 4: Design an integrated controller of feedforward and PID feedback: First, conduct feedforward derivation; Derive the feedforward value of the active rear wheel steering angle that suppresses the sideslip angle of the center of mass based on the model derivation, and then derive the feedforward value of the additional yaw moment considering the feedforward value of the active rear wheel steering angle; Secondly, combine the vehicle speed to control the feedforward decision-making quantity, and correct the feedforward expression of the rear wheel steering angle so that the controller has different expressions for different vehicle speeds and different stability intervals; Finally, adopt PID feedback to control the output of the additional yaw moment; Control the yaw rate with the additional yaw moment; Step 5: Torque distribution: Based on the rear wheel steering angle and the additional yaw moment obtained in the above Step 4, perform distribution control; The rear wheel steering angle is directly used for control; The additional yaw moment needs to be distributed to the four wheels and then controlled. The distribution method adopts the average distribution method for distribution, controls the four-wheel torque of the vehicle, and controls the vehicle simultaneously with the rear wheel steering angle to achieve the purpose of integrated control.

2. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering according to claim 1, characterized in that: The space state expression of the extended two-degree-of-freedom vehicle model described in Step 1 is as follows: Among them, C f is the equivalent cornering stiffness of the front axle, C r is the equivalent cornering stiffness of the rear axle, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass, δ f is the front wheel steering angle, δ r is the rear wheel steering angle, ΔM z is the additional yaw moment, ω r is the vehicle yaw rate, β is the sideslip angle at the center of mass, I z is the moment of inertia of the whole vehicle, v x is the longitudinal velocity of the center of mass, m is the mass of the whole vehicle.

3. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering according to claim 1, characterized in that: The stable region described in Step 2 has a radius of R1 = max(α f,lin , α r,lin ), where α f,lin represents the tire sideslip angle corresponding to the boundary point between the linear region and the non-linear region of the front wheel lateral force, which is affected by the road surface adhesion coefficient; α r,lin represents the tire sideslip angle corresponding to the boundary point between the linear region and the non-linear region of the rear wheel lateral force, which is affected by the road surface adhesion coefficient; The described transition region has a radius of R2 = max(α f,sat , α r,sat ), where α f,sat represents the tire sideslip angle corresponding to the maximum lateral force of the front wheel, and α r,sat represents the tire sideslip angle corresponding to the maximum lateral force of the rear wheel; The described unstable region is bounded by as the radius, where (α fs , α rs ) are the coordinates of the saddle point closer to the origin.

4. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering according to claim 1 or 3, characterized in that: Define the vehicle stability coefficient in Step 2 as ξ, and the calculation formula for the real-time vehicle stability coefficient is as follows: If R3 > R2: If R3 ≤ R2: Among them, R0 represents the position of the vehicle driving state in the phase plane, which is calculated by the following formula: where α f and α r are the sideslip angles of the front and rear wheels respectively; R1 is the radius of the stable region, R2 is the radius of the transition region, and R3 is the radius of the unstable region; Normalize the stability coefficient ξ, which is expressed as: Among them, ξ0 represents the stability coefficient after normalization.

5. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering according to claim 1, characterized in that: The handling yaw rate described in Step 3 is set as follows: By reasonably setting K us , a y,max , the handling yaw rate ω rhd under steady-state conditions is obtained, and its specific expression is as follows: Among them, K us is the understeer degree, is the maximum lateral acceleration in the linear region, a y,max is the maximum lateral acceleration that the vehicle can achieve; a y is the lateral acceleration, v x is the longitudinal velocity of the center of mass, δ f is the front wheel steering angle, L is the wheelbase, δ fdyn0 is the dynamic steering angle; Perform Laplace transform on the linear two-degree-of-freedom differential equation of the vehicle, and the handling yaw rate target combined with the transfer function is expressed as: Among them, is the transfer function of the yaw rate desired value filter, s is the complex frequency variable in the Laplace transform, is the natural frequency; is the damping ratio; is the time constant; The stability yaw rate described above is set as follows: Integrating the two-degree-of-freedom vehicle model gives the steady-state stability yaw rate ω rsd , and its specific expression is as follows: In the formula, μ is the road surface adhesion coefficient, g is the acceleration due to gravity, and v x is the longitudinal velocity of the center of mass; Establish a response transfer function for it, and the target expression of the stability yaw rate combined with the transfer function is as follows: Utilize the vehicle stability coefficient and the road adhesion coefficient to fuse the handling yaw rate target and the stability yaw rate target to obtain the ideal yaw rate target as: Among them, ξ0 is the stability coefficient after normalization, and f(μ) is a piecewise function, and its function form is:

6. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering according to claim 1, characterized in that: The handling sideslip angle of the center of mass described in Step 3 is set as follows: The expression of the steady-state sideslip angle of the center of mass is: Among them, β is the centroid sideslip angle, l f is the distance from the front axle to the centroid, l r is the distance from the front axle to the centroid, m is the vehicle mass, v x is the longitudinal speed of the centroid, L is the wheelbase, C r is the equivalent cornering stiffness of the rear axle, K us is the degree of understeer, δ f is the front wheel steering angle; Limit of centroidal sideslip angle β lim is as follows: β lim = tan -1 (0.02 μg) Among them, μ is the road adhesion coefficient, and g is the acceleration due to gravity; Steady-state handling property sideslip angle β hd Expressed as: β hd = min(|β|, β lim ) sign(β) Taking the Laplace transform of the linear two-degree-of-freedom differential equation of the vehicle and combining with the handling property of the transfer function, the target of the sideslip angle of the center of mass is expressed as: where G β (s) is the transfer function of the sideslip angle desired value filter, ω nβ is the natural frequency, ζ β is the damping ratio, τ β is the time constant; The target of the steady-state sideslip angle of the center of mass is set to 0, that is: Fusing the vehicle stability factor with the target of the handling sideslip angle of the center of mass and the target of the steady-state sideslip angle of the center of mass, the ideal target of the sideslip angle of the center of mass is: where f β (ξ0) is a piecewise function, and its functional form is:

7. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering according to claim 1, characterized in that: The feedforward in Step 4 is derived as follows: Based on the form after degenerating the additional yaw moment of the extended two-degree-of-freedom vehicle model as the feedforward derivation model of the active rear wheel steering angle; the feedforward decision variable of the active rear wheel steering angle is as follows: Among them, δ rFF is the feedforward value of the active rear wheel steering angle, l f is the distance from the front axle to the center of mass, l r is the distance from the front axle to the center of mass, v x is the longitudinal speed of the center of mass, m is the vehicle mass, C f is the equivalent cornering stiffness of the front axle, C r is the equivalent cornering stiffness of the rear axle, L is the wheelbase, K us is the degree of understeer, δ f is the steering angle of the front wheels; After obtaining the feedforward value of the active rear wheel steering angle, based on the extended two-degree-of-freedom vehicle model and considering the feedforward value of the active rear wheel steering angle, the feedforward value of the additional yaw moment is derived; the feedforward decision variable of the additional yaw moment is as follows: Among them, ΔM zFF is the feedforward value of the additional yaw moment, and δ r is the rear wheel steering angle; Neglecting the transient characteristics of the ideal target of the sideslip angle of the center of mass, the feedforward decision variable of the active rear wheel steering angle is: Among them, is the ideal centroid side slip angle target, and f β (ξ0) is a piecewise function.

8. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering according to claim 1 or 7, characterized in that: The corrected feedforward in Step 4 is expressed in the following form: When the vehicle speed is less than the critical vehicle speed: When the vehicle speed is greater than the critical vehicle speed: Among them, f(ξ0) is expressed in the following form: Among them, δ rFF is, ΔM zFF is, k is a calibration parameter, μ is the road surface adhesion coefficient, v x is the longitudinal speed of the center of mass, m is the vehicle mass, L is, C f is the equivalent cornering stiffness of the front axle, C r is the equivalent cornering stiffness of the rear axle, δ f is the front wheel steering angle, δ r is the rear wheel steering angle, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass, ξ0 is the stability coefficient after normalization, f(ξ0) is a piecewise function, and its function form is: f β (ξ0) is a piecewise function, and its functional form is: The feedback control quantity of the yaw rate based on PID control is expressed in the following form: Among them, ΔM zFB (k) is the discrete yaw moment feedback value, is the proportionality coefficient, is the integral coefficient, is the differential coefficient, is the error, and Δt is the sampling period.

9. A control method applicable to the integration of four-wheel independent drive and active rear-wheel steering according to claim 1, characterized in that: The distribution formula of the additional yaw moment in Step 5 is as follows: Among them, T fl , T fr , T rl , T rr are the executive torques of the front left wheel, front right wheel, rear left wheel, and rear right wheel respectively; d f is the front axle track; d r is the rear axle track; R e is the tire rolling radius; T fl,basic , T fr,basic , T rl,basic , T rr,basic are the four-wheel basic analysis torques when the driver steps on the accelerator. These torques are all equal, and their sum is denoted as T basic .

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