CN113022551B - Control rod steer-by-wire control method - Google Patents

Control rod steer-by-wire control method Download PDF

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CN113022551B
CN113022551B CN202110328476.8A CN202110328476A CN113022551B CN 113022551 B CN113022551 B CN 113022551B CN 202110328476 A CN202110328476 A CN 202110328476A CN 113022551 B CN113022551 B CN 113022551B
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angle
front wheel
vehicle
steering
beta
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CN113022551A (en
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谢正超
张睿
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South China University of Technology SCUT
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60W—CONJOINT CONTROL OF VEHICLE SUB-UNITS OF DIFFERENT TYPE OR DIFFERENT FUNCTION; CONTROL SYSTEMS SPECIALLY ADAPTED FOR HYBRID VEHICLES; ROAD VEHICLE DRIVE CONTROL SYSTEMS FOR PURPOSES NOT RELATED TO THE CONTROL OF A PARTICULAR SUB-UNIT
    • B60W30/00—Purposes of road vehicle drive control systems not related to the control of a particular sub-unit, e.g. of systems using conjoint control of vehicle sub-units
    • B60W30/02—Control of vehicle driving stability
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60W—CONJOINT CONTROL OF VEHICLE SUB-UNITS OF DIFFERENT TYPE OR DIFFERENT FUNCTION; CONTROL SYSTEMS SPECIALLY ADAPTED FOR HYBRID VEHICLES; ROAD VEHICLE DRIVE CONTROL SYSTEMS FOR PURPOSES NOT RELATED TO THE CONTROL OF A PARTICULAR SUB-UNIT
    • B60W50/00—Details of control systems for road vehicle drive control not related to the control of a particular sub-unit, e.g. process diagnostic or vehicle driver interfaces
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
    • B62D—MOTOR VEHICLES; TRAILERS
    • B62D5/00—Power-assisted or power-driven steering
    • B62D5/04—Power-assisted or power-driven steering electrical, e.g. using an electric servo-motor connected to, or forming part of, the steering gear
    • B62D5/0457—Power-assisted or power-driven steering electrical, e.g. using an electric servo-motor connected to, or forming part of, the steering gear characterised by control features of the drive means as such
    • B62D5/046—Controlling the motor
    • B62D5/0463—Controlling the motor calculating assisting torque from the motor based on driver input
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60W—CONJOINT CONTROL OF VEHICLE SUB-UNITS OF DIFFERENT TYPE OR DIFFERENT FUNCTION; CONTROL SYSTEMS SPECIALLY ADAPTED FOR HYBRID VEHICLES; ROAD VEHICLE DRIVE CONTROL SYSTEMS FOR PURPOSES NOT RELATED TO THE CONTROL OF A PARTICULAR SUB-UNIT
    • B60W50/00—Details of control systems for road vehicle drive control not related to the control of a particular sub-unit, e.g. process diagnostic or vehicle driver interfaces
    • B60W2050/0001—Details of the control system
    • B60W2050/0019—Control system elements or transfer functions
    • B60W2050/0028—Mathematical models, e.g. for simulation
    • B60W2050/0031—Mathematical model of the vehicle

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  • Engineering & Computer Science (AREA)
  • Transportation (AREA)
  • Mechanical Engineering (AREA)
  • Automation & Control Theory (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Human Computer Interaction (AREA)
  • Steering Control In Accordance With Driving Conditions (AREA)

Abstract

The invention discloses a control method for controlling the steer-by-wire of a control lever, which comprises the following steps: acquisition joystick angle deltadAnd by a rotation angle factor fδAnd a speed factor fuCorrecting and calculating the ideal steady-state front wheel steering angle
Figure DDA0002995477120000011
Calculating a reference yaw angular velocity omega according to a linear two-degree-of-freedom reference modelrAnd a reference centroid slip angle betar(ii) a Detecting whether the deviation between the actual yaw velocity and the centroid slip angle and the reference value reaches a stable threshold value, and enabling the ideal front wheel steering angle if the deviation does not reach the stable threshold value
Figure DDA0002995477120000012
If the ideal front wheel rotation angle reaches the threshold value, correcting the ideal front wheel rotation angle by using a stability control program based on model prediction, and calculating the braking torque of each wheel; and the steering execution motor and the braking system execute actions according to the instructions. The invention can simplify the steering operation, improve the operation stability, save the space in the vehicle, reduce the collision damage and easily realize the switching between the automatic driving mode and the manual driving mode.

Description

Control rod steer-by-wire control method
Technical Field
The invention belongs to the technical field of a steer-by-wire system in an automobile steering system, and particularly relates to a control method for controlling the steer-by-wire of a control lever.
Background
The current automobile automatic driving technology develops rapidly, and on automatic driving vehicles of L3 and L4 grades, sufficient activity space needs to be reserved for drivers, and convenience and safety of manual driving need to be guaranteed. The traditional steering wheel type mechanical steering system has large operation action, occupies more space, limits the installation of a safety airbag with higher specification, easily causes damage to a driver when instruments and steering column structures are collided, and is linked with a steering wheel, so that a vehicle stability control system cannot directly correct a steering angle; when the automatic driving system works, the interference of the movement of passengers in the vehicle to the automatic driving system is easy to cause.
In the chinese granted patent "a control method of steering-by-wire system angle gear ratio based on joystick" (201210385476.3), the steering angle of the steering wheel is found by integrating the steering angle of the joystick by the angle gear ratio, and no stability auxiliary measure is taken according to the vehicle motion state. When a vehicle runs on a wet road surface or is emergently steered, the vehicle is easy to sideslip or rotate suddenly due to the saturation of the tire lateral deviation force, and then the vehicle is out of control.
Disclosure of Invention
In order to overcome the defects in the prior art, the invention provides a control method for controlling the steer-by-wire of a control lever, which can correct a steering angle and improve the control stability.
In order to achieve the object of the present invention, the present invention provides a joystick steer-by-wire control method, comprising the steps of:
step 1, acquiring a control lever corner;
step 2, correcting according to the size of the steering angle of the operating lever and the current vehicle speed to obtain an ideal stable front wheel steering angle; the steering dead zone is set in the steering middle position, so that the steering load of a driver is reduced when the vehicle is in straight motion; calculating a speed correction factor f according to the current speeduThe steering transmission ratio under different vehicle speeds is changed, so that the steering gain is kept in a reasonable range under various driving conditions, and the limit position of the front wheel steering angle can be reached at low speed; calculating ideal steady-state front wheel steering angle
Figure BDA0002995477100000021
Step 3, calculating a reference yaw angular velocity and a reference mass center slip angle according to an ideal steady-state front wheel corner and a linear two-degree-of-freedom reference model of the vehicle;
step 4, judging whether the deviation between the current actual yaw velocity and the centroid sideslip angle and the reference value reaches a threshold value, and if the deviation reaches the threshold value, turning to step 5; otherwise, directly controlling a steering execution motor to track the rotation angle of the ideal stable front wheel;
and 5, correcting the steering angle by adopting a stability control program based on a model prediction algorithm and carrying out differential braking on the left wheel and the right wheel.
In a further development of the invention, the lever angle δdIs ± 40 °.
The invention is further improved, and the specific steps for obtaining the ideal steady-state front wheel turning angle in the step 2 are as follows:
step 2.1: calculating a corner correction factor f according to the size of the corner of the operating leverδ；
Step 2.2: calculating a speed correction factor f based on the current vehicle speedu；
Step 2.3: based on a corner correction factor fδVelocity correction factor fuSteering angle delta of joystickdObtaining the ideal steady-state front wheel turning angle. The calculation formula of the ideal steady-state front wheel steering angle is as follows:
Figure BDA0002995477100000022
in a further improvement of the present invention, the speed correction factor in step 2.2 is designed in the following way:
1) at low speed, the corner of the front wheel can reach the limit position, and the speed correction factor is the ratio of the limit corner of the front wheel to the corner limit of the operating lever;
2) maintaining the steering gain value K of the response from the joystick angle to the yaw rate at the medium and high speeds and less than the critical speedrConstant;
the velocity correction factor at this time is:
Figure BDA0002995477100000031
wherein K is the stability factor of the vehicle and l is the wheelbase;
3) maintaining the critical speed correction factor calculated in the condition 2) unchanged when the speed is higher than the usual vehicle speed range.
In a further improvement of the present invention, the linear two-degree-of-freedom reference model of the vehicle in step 3 is as follows:
Figure BDA0002995477100000032
wherein m is the total vehicle mass, IzThe yaw moment of inertia of the vehicle, a and b are the distances from the front and rear axles to the mass center respectively, and Cf、CrThe equivalent cornering stiffness of the front axle and the rear axle, u is the component of the vehicle running speed in the longitudinal direction, omega is the yaw angular velocity, and beta is the centroid cornering angle.
In a further improvement of the present invention, the reference yaw rate in step 3 is obtained as follows:
Figure BDA0002995477100000033
in a further improvement of the present invention, the reference centroid slip angle in step 3 is determined as follows:
Figure BDA0002995477100000034
the tire adhesion limit constrains the magnitude of the centroid slip angle, and therefore:
Figure BDA0002995477100000035
get betar1、βr2The smaller of the absolute values is taken as the reference centroid side slip angle betar。
The invention is further improved, in the step 4, the judging method for judging whether to switch to the step 5 is as follows:
according to the current actual vehicle yaw rate omegavAnd a reference yaw rate ωrObtaining a yaw rate deviation e (ω) ═ ωc-ωrAccording to the current actual centroid slip angle betavAnd a reference centroid slip angle betarObtaining the deviation e (beta) of the centroid side deflection angle as | betac-βr|；
Setting a yaw angular speed deviation threshold value delta omega and a centroid sideslip angle deviation threshold value delta beta;
if e (omega) is more than or equal to delta omega or e (beta) is more than or equal to delta beta, activating a stability control program based on model prediction, and turning to the step 5; and if e (omega) < delta omega and e (beta) < delta beta, directly controlling the steering execution motor to track the rotation angle of the ideal steady front wheel.
In a further development of the invention, the stability control routine in step 5 determines the steering angle correction as follows:
two-degree-of-freedom non-linear prediction model for vehicle
Figure BDA0002995477100000041
First order the basic control quantity ub＝[δ0,0]' where xi is the system state quantity, delta0The equivalent front wheel rotation angle at the starting moment of the control period;
keeping the basic control quantity unchanged in the control period, and obtaining the recursion relation of the basic state quantity of the system;
obtaining a linear prediction model in the control period based on the recursion relation of the system basic state quantity;
according to a model prediction control algorithm, a quadratic programming cost function is established by taking a reference yaw velocity and a reference mass center lateral deviation angle as tracking target values, an optimal control sequence is solved in an optimized mode, instant control quantities, namely an ideal front wheel corner and a compensation yaw moment, are taken, and a steering execution motor is controlled to track the corrected ideal front wheel corner.
In a further improvement of the present invention, the stability control routine in step 5 determines the ideal braking torque for differential braking of the left and right wheels in the following manner:
defining the front and rear wheel load factors as Pf、PrLet Pf＝1-Pr；
Figure BDA0002995477100000051
Wherein alpha isf、αrSlip angles of the front and rear axles, respectively, neglecting wheel rotationInertia, setting the braking torque as
Figure BDA0002995477100000052
Figure BDA0002995477100000053
Wherein a is the distance from the front axle to the center of mass, c is the left and right wheel track, r is the rolling radius of the tire, M*In order to compensate for the yaw moment,
Figure BDA0002995477100000054
and
Figure BDA0002995477100000055
respectively, the ideal braking torques of the left front wheel, the right front wheel, the left rear wheel and the right rear wheel.
Compared with the prior art, the invention can realize the following beneficial effects:
(1) the control lever adopted by the invention can be operated by one hand, the volume of the control lever is greatly reduced compared with the traditional steering wheel, the space in front of the driver seat can be fully released, and the arrangement of an instrument panel and a front safety air bag is convenient.
(2) The steer-by-wire control provided by the invention can replace a steering column, so that secondary damage of the steering column to a driver in a collision accident is avoided; and under the automatic driving state, the steering angle of the control lever is decoupled with the steering angle of the front wheel, so that the interference of the movement of the passengers on the automatic driving system can be prevented.
(3) Through speed correction, the vehicle is enabled to be sensitive to steering at low speed, and large operation action is not needed; and the steering gain is limited to a proper magnitude level at high speed, so that the steering fault tolerance is improved.
(4) And a stability control program is fused, when the fact that the actual yaw velocity and the centroid slip angle of the vehicle have large deviation from the reference value is detected, the steering angle is corrected and the left wheel and the right wheel are subjected to differential braking, so that the operation stability under the extreme working conditions of emergency steering, road surface wet slipping and the like is further improved, and the condition that the vehicle generates sideslip or excited rotation under the extreme working conditions to cause out of control is reduced.
Drawings
FIG. 1 is a flow chart of the present joystick steer-by-wire control method;
FIG. 2 is a schematic diagram of the stability control routine based on the model prediction algorithm in step 5;
FIG. 3 is a plot of a steering angle correction factor versus joystick steering angle;
fig. 4 is a graph of the speed correction factor versus the travel speed.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
Referring to fig. 1, a method for controlling a steer-by-wire by using a joystick as a driver input device according to the present invention includes the following steps:
step 1, obtaining the rotation angle of the operating lever.
In one embodiment of the invention, a driver inputs a steering command through the left-right freedom degree of a joystick, and a steering angle sensor arranged below the joystick acquires the current joystick steering angle deltad。
In one embodiment of the present invention, the rotation angle sensor is a hall sensor. It will be appreciated that other types of sensors may be used in other embodiments.
In one embodiment of the invention, the joystick is rotated by a certain angle δdIs ± 40 °.
And 2, correcting according to the rotation angle of the operating lever and the current vehicle speed to obtain an ideal stable front wheel rotation angle.
In one embodiment of the present invention, the step specifically includes:
step 2.1: obtaining a rotation angle correction factor f according to the rotation angle of the operating leverδ。
Fig. 3 shows a relationship between the rotation angle correction factor and the joystick rotation angle.
As shown in fig. 3, in the range of-1 ° to 1 °, a steering dead zone is set, and the rotation angle correction factor is 0; the corner correction factor is 1 in the range of-40 ° to-5 ° and 5 ° to 40 °; in the range of-1 deg. to-5 deg. and 1 deg. to 5 deg., a sinusoidal transition is used, and the rotation angle correction factor is gradually increased from 0 to 1. The steering dead zone is set at the steering middle position, so that the steering load of a driver when the vehicle runs straight is reduced.
Step 2.2: calculating a speed correction factor f based on the current vehicle speedu。
The speed correction factor is designed to meet the following conditions:
1) at low speeds, i.e. speeds less than v1In time, the front wheel steering angle should be able to reach a limit position to ensure steering flexibility. The speed correction factor is the limit rotation angle delta of the front wheellimAnd joystick rotation angle limit.
In one embodiment of the present invention, v1Value v1＝10km/h。
In one embodiment of the invention, the limit turning angle delta of the front wheellimA value of 46 degrees and a joystick rotation angle limit value of 40, namely fu＝46/40＝1.15。
2) Velocity higher than v1And less than the critical velocity v2While maintaining steering gain value K of joystick to yaw rate responserIs constant.
The relationship between the speed correction factor and the steering gain is:
Figure BDA0002995477100000071
where K is the stability factor of the vehicle and l is the wheelbase.
In one embodiment of the present invention, v2＝118km/h。
Different vehicle types can have different steering gain values, and the same vehicle type can also select different steering gain values. The constraints of this value are related to handling stability and subjective drivability. In one embodiment of the present invention, the steering gain value k is determined by experiments for a specific vehicle typerTaking the value of 1.1963.
3) Velocity higher than v2While maintaining v calculated in the condition 2)2The velocity correction factor is unchanged.
Fig. 4 shows a speed correction factor versus the driving speed.
Step 2.3: based on a corner correction factor fδVelocity correction factor fuAngle delta of the operating leverdObtaining the ideal steady-state front wheel turning angle. The calculation formula of the ideal steady-state front wheel steering angle is as follows:
Figure BDA0002995477100000081
step 3, calculating a reference yaw angular velocity omega according to the ideal steady-state front wheel rotation angle and a linear two-degree-of-freedom reference model of the vehiclerAnd a reference centroid slip angle betar。
In one embodiment of the invention, a linear two-degree-of-freedom reference model of a vehicle is derived from two-degree-of-freedom vehicle dynamics equations
Figure BDA0002995477100000082
Wherein m is the total vehicle mass, IzThe yaw moment of inertia of the vehicle, a and b are the distances from the front and rear axles to the mass center respectively, and Cf、CrThe equivalent cornering stiffness of the front axle and the rear axle, u is the component of the vehicle running speed in the longitudinal direction, omega is the yaw angular velocity, and beta is the centroid cornering angle.
Therefore, the calculation formula of the reference yaw rate is as follows:
Figure BDA0002995477100000083
centroid slip angle based on reference model:
Figure BDA0002995477100000091
the tire adhesion limit constrains the magnitude of the centroid slip angle, and therefore:
Figure BDA0002995477100000092
where g is the acceleration of gravity. Reference centroid slip angle betarGet betar1、βr2The smaller of the absolute values.
Step 4, judging whether the deviation between the current actual yaw velocity and the centroid sideslip angle and the reference value reaches a threshold value, and if the deviation reaches the threshold value, turning to step 5; otherwise, directly controlling the steering executing motor to track the rotation angle of the ideal steady-state front wheel.
In one embodiment of the invention, the actual vehicle yaw rate ω is based on the current actual vehicle yaw rate ωvAnd the actual centroid slip angle betavCalculating deviations e (ω) ═ ω from the respective reference values, respectivelyv-ωr|,e(β)＝|βv-βr|；
And setting a yaw angular speed deviation threshold value delta omega and a centroid slip angle deviation threshold value delta beta. And if e (omega) is more than or equal to delta omega or Ee (beta) is more than or equal to delta beta, activating the stability control program based on model prediction, and turning to the step five, wherein the vehicle may have larger lateral acceleration or run on a wet road surface, the tire slip angle is larger, and the slip force enters a non-linear region. The deviation of the yaw angular velocity and the centroid slip angle from a reference value is limited by correcting the steering angle and compensating the yaw moment, so that the driving characteristic is close to a linear state, and the vehicle is prevented from being out of control; if e (omega) < delta omega and e (beta) < delta beta, omitting the step 5, and directly controlling the steering execution motor to track the ideal steady-state front wheel rotation angle.
In one embodiment of the invention, the current actual vehicle yaw rate ωvAnd the actual centroid slip angle betavIs obtained through sensor acquisition or state estimation.
And 5, correcting the steering angle by adopting a stability control program based on a model prediction algorithm, carrying out differential braking on the left wheel and the right wheel to compensate the yaw moment, limiting the deviation of the yaw velocity and the mass center slip angle from a reference value, and preventing the vehicle from being out of control.
In one embodiment of the present invention, the calculation of the steering angle correction amount and the compensation yaw moment by the stability control program based on the model prediction algorithm is performed by:
establishing a two-degree-of-freedom nonlinear prediction model of the vehicle:
Figure BDA0002995477100000101
wherein f iscIs a function of the cornering power and the cornering angle of the tyre, the lower corner indices f, r representing the front and rear wheels respectively, δ being the steering angle of the front wheels, M being the yaw moment generated by the differential braking of the tyre. Taking the state quantity xi ═ beta, omega]', control quantity u ═ δ, M]', the above model can be abbreviated as
Figure BDA0002995477100000104
Basic control quantity ub＝[δ0,0]′，δ0The equivalent front wheel angle at the beginning of the control cycle. The basic control quantity is kept unchanged in a prediction time domain, and a 4-order Runge-Kutta formula is applied to obtain the recurrence relation of the basic state quantity of the system, namely
Figure BDA0002995477100000102
Wherein ξbFor estimated quantities of state, TsIs the control period of the system, h1、h2、h3、h4Is an intermediate variable of the longge-kutta formula, at the present timePerforming first-order Taylor expansion at the working point to obtain a linear prediction model in the control period
Figure BDA0002995477100000103
Where N is the serial number of the variable, NpFor the prediction horizon of the model prediction algorithm, d (n) is an intermediate variable, and
Figure BDA0002995477100000111
establishing a cost function
Figure BDA0002995477100000112
In which ξr＝[βr,ωr]′，Δu(n)＝u(n)-u(n-1)，
Figure BDA0002995477100000113
Is a weighting factor of the output quantity of the system,
Figure BDA0002995477100000114
are the weighting coefficients of the control increments. Defining the following quadratic programming problem;
Figure BDA0002995477100000115
subj.to umin≤u(n)≤umax
Δumin≤Δu(n)≤Δumax
n＝,1,…,Nu-1
in the formula, NuIs the control time domain of the model prediction algorithm. According to a model predictive control algorithm, solving the quadratic programming problem in each control period to obtain a control sequence in a prediction time domain, and taking a first column of control quantity
Figure BDA0002995477100000116
Control steering executive motor to track corrected ideal front wheel steering angle
Figure BDA0002995477100000117
And tracking the ideal direct yaw moment M by differential braking*。
When the rear axle of the vehicle is about to sideslip, braking force is applied to the front outer wheel to restrain; when the front axle of the vehicle is about to sideslip, the front wheel and the rear inner wheel are braked simultaneously. Defining the front and rear wheel load factors as Pf、Pr
Figure BDA0002995477100000118
Wherein alpha isf(n)、αr(n) slip angles of the front and rear wheels, P0Can take a value of 0.5 to 0.7. Let Pf＝1-Pr. Neglecting the rotational inertia of the wheels, setting the braking torque as
Figure BDA0002995477100000121
Figure BDA0002995477100000122
Wherein c is the left and right wheel track,
Figure BDA0002995477100000123
and ideal braking torques for the left front wheel, the right front wheel, the left rear wheel and the right rear wheel, respectively. The brake system adjusts the pressure of the brake wheel cylinder of each wheel to achieve the ideal value.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims (8)

1. A control method for controlling steer-by-wire of a joystick is characterized by comprising the following steps:
step 1, acquiring a control lever corner;
step 2, correcting according to the size of the steering angle of the operating lever and the current vehicle speed to obtain an ideal stable front wheel steering angle;
step 3, calculating a reference yaw angular velocity and a reference mass center slip angle according to an ideal steady-state front wheel corner and a linear two-degree-of-freedom reference model of the vehicle;
step 4, judging whether the deviation between the current actual yaw velocity and the centroid sideslip angle and the reference value reaches a threshold value, and if the deviation reaches the threshold value, turning to step 5; otherwise, directly controlling a steering execution motor to track the rotation angle of the ideal stable front wheel;
step 5, correcting a steering angle by adopting a stability control program based on a model prediction algorithm and carrying out differential braking on the left wheel and the right wheel;
the stability control routine in step 5 determines the steering angle correction amount as follows:
two-degree-of-freedom non-linear prediction model for vehicle
Figure FDA0003475681320000011
First order the basic control quantity ub＝[δ0,0]' where xi is the system state quantity, delta0The equivalent front wheel rotation angle at the starting moment of the control period is u, and the component of the running speed of the vehicle in the longitudinal direction is u;
keeping the basic control quantity unchanged in the control period, and obtaining the recursion relation of the basic state quantity of the system;
obtaining a linear prediction model in the control period based on the recursion relation of the system basic state quantity;
according to a model prediction control algorithm, taking a reference yaw velocity and a reference mass center lateral deviation angle as tracking target values, establishing a quadratic programming cost function, optimally solving an optimal control sequence, taking instant control quantities, namely an ideal front wheel corner and a compensation yaw moment, and controlling a steering execution motor to track the corrected ideal front wheel corner;
the determination method of the ideal braking torque when the stability control program performs differential braking on the left wheel and the right wheel in the step 5 is as follows:
defining the front and rear wheel load factors as Pf、PrLet Pf＝1-Pr；
Figure FDA0003475681320000021
Wherein alpha isf、αrRespectively the slip angles of the front and rear axles, neglecting the rotational inertia of the wheels, and setting the braking torque as
Figure FDA0003475681320000022
Figure FDA0003475681320000023
Wherein a is the distance from the front axle to the center of mass, c is the left and right wheel track, r is the rolling radius of the tire, M*In order to compensate for the yaw moment,
Figure FDA0003475681320000024
and
Figure FDA0003475681320000025
respectively, the ideal braking torques of the left front wheel, the right front wheel, the left rear wheel and the right rear wheel, and delta is the front wheel steering angle.
2. According to the claimsThe joystick steer-by-wire control method according to claim 1 is characterized in that: steering angle delta of joystickdIs ± 40 °.
3. The joystick steer-by-wire control method according to claim 1, wherein the specific step of obtaining the ideal steady-state front wheel steering angle in step 2 is:
step 2.1: calculating a corner correction factor f according to the size of the corner of the operating leverδ；
Step 2.2: calculating a speed correction factor f based on the current vehicle speedu；
Step 2.3: based on a corner correction factor fδVelocity correction factor fuSteering angle delta of joystickdObtaining an ideal steady-state front wheel corner, wherein the calculation formula of the ideal steady-state front wheel corner is as follows:
Figure FDA0003475681320000031
4. a joystick steer-by-wire control method according to claim 3, wherein the speed correction factor in step 2.2 is designed as follows:
1) at low speed, the corner of the front wheel can reach the limit position, and the speed correction factor is the ratio of the limit corner of the front wheel to the corner limit of the operating lever;
2) maintaining the steering gain value K of the response from the joystick angle to the yaw rate at the medium and high speeds and less than the critical speedrConstant;
the velocity correction factor at this time is:
Figure FDA0003475681320000032
wherein K is the stability factor of the vehicle and l is the wheelbase;
3) maintaining the critical speed correction factor calculated in the condition 2) unchanged when the speed is higher than the usual vehicle speed range.
5. The joystick steer-by-wire control method of claim 1, wherein the linear two-degree-of-freedom reference model of the vehicle in step 3 is as follows:
Figure FDA0003475681320000033
wherein m is the total vehicle mass, IzThe yaw moment of inertia of the vehicle, a and b are the distances from the front and rear axles to the mass center respectively, and Cf、CrThe equivalent cornering stiffness of the front axle and the rear axle, u is the component of the vehicle running speed in the longitudinal direction, omega is the yaw angular velocity, and beta is the centroid cornering angle.
6. The joystick steer-by-wire control method according to claim 1, wherein the reference yaw rate in step 3 is obtained as follows:
Figure FDA0003475681320000041
where K is the stability factor of the vehicle, l is the wheel base, u is the component of the vehicle speed in the longitudinal direction,
Figure FDA0003475681320000042
the ideal steady-state front wheel turning angle.
7. The joystick steer-by-wire control method according to claim 1, wherein the reference centroid slip angle in step 3 is determined as follows:
Figure FDA0003475681320000043
the tire adhesion limit constrains the magnitude of the centroid slip angle, and therefore:
Figure FDA0003475681320000044
get betar1、βr2The smaller of the absolute values is taken as the reference centroid side slip angle betar；
Wherein u is the longitudinal component of the vehicle running speed, a and b are the distances from the front and rear axles to the mass center, m is the vehicle mass, CrThe equivalent yaw stiffness of the rear axle, l is the wheelbase, μ is the coefficient of friction, and g is the acceleration of gravity.
8. The joystick steer-by-wire control method according to claim 1, wherein in step 4, whether to shift to step 5 is determined by:
according to the current actual vehicle yaw rate omegavAnd a reference yaw rate ωrObtaining a yaw rate deviation e (ω) ═ ωc-ωrAccording to the current actual centroid slip angle betavAnd a reference centroid slip angle betarObtaining the deviation e (beta) of the centroid side deflection angle as | betac-βr|；
Setting a yaw angular speed deviation threshold value delta omega and a centroid sideslip angle deviation threshold value delta beta;
if e (omega) is more than or equal to delta omega or e (beta) is more than or equal to delta beta, activating a stability control program based on model prediction, and turning to the step 5; and if e (omega) < delta omega and e (beta) < delta beta, directly controlling the steering execution motor to track the rotation angle of the ideal steady front wheel.
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