CN113008240B - Four-wheel independent drive intelligent electric vehicle path planning method based on stable domain - Google Patents
Four-wheel independent drive intelligent electric vehicle path planning method based on stable domain Download PDFInfo
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Abstract
The invention discloses a four-wheel independent drive intelligent electric vehicle path planning method based on a stable region, which comprises the following steps: establishing a nonlinear seven-degree-of-freedom nonlinear vehicle model, wherein the seven-degree-of-freedom comprises a longitudinal direction, a lateral direction, a horizontal swing and 4 wheels; obtaining a stable domain of the four-wheel independent drive electric automobile based on the established nonlinear seven-degree-of-freedom vehicle model; and planning a path based on the obtained stable domain. The path planning method for the four-wheel independent drive electric vehicle provided by the invention not only can meet the daily driving requirements of the intelligent electric vehicle, but also has the characteristics of good working condition adaptability, high path planning accuracy, strong fault-tolerant capability and the like under emergency working conditions such as emergency collision avoidance, high-speed driving and the like, fully exerts the advantages of the four-wheel independent drive electric vehicle compared with the traditional vehicle or a centralized electric vehicle, fully and tightly combines the intelligent driving layer and the chassis control layer of the four-wheel drive electric vehicle, and improves the safety and the high efficiency of the electric vehicle in the driving process.
Description
Technical Field
The invention belongs to the field of design and manufacture of new energy automobiles, relates to a path planning technology of a four-wheel independent drive intelligent electric automobile, and provides a path planning method of the four-wheel independent drive intelligent electric automobile.
Background
With the increasing of automobile holding capacity, a series of problems such as frequent road traffic accidents, aggravated urban traffic jam, environmental pollution and the like are increasingly highlighted, the automobile industry meets the challenge of 'new and fourteen-style' and the automobile electromotion, networking, intellectualization and sharing are taken as important breakthrough of the future trend of the development of the automobile industry and the bottleneck of the current industry, so that the intelligent driving vehicle is taken as a research hotspot of researchers and engineers at home and abroad. The intelligent driving vehicle is different from a common indoor wheeled robot and is driven in a complex traffic environment at a high speed, and when the intelligent driving vehicle is driven at a high speed and enters a destabilization state, even if a vehicle-mounted stability control system is involved, the stable driving of the vehicle can not be recovered frequently. However, if the capabilities of information processing, environmental perception and the like of the intelligent driving vehicle are fully utilized, the driving state of the intelligent driving vehicle in a period of time in the future is predicted by utilizing a dynamic model of the vehicle, so that the optimal path meeting nonlinear dynamic constraints such as yaw and sideslip, road boundary constraints and environmental geometric constraints is planned in advance, and the vehicle is controlled to track the planned path within the vehicle body stability range, so that traffic accidents are effectively avoided, and higher safety and high efficiency are endowed to the vehicle.
At present, electric automobiles are generally centralized, the design of a centralized electric automobile track planning and tracking control algorithm is conservative, even though the optimal path is planned by fully considering dynamic constraints, the vertical and horizontal movement behaviors of the vehicles need to accord with the kinematic relationship due to the kinematic characteristics of the centralized electric automobiles, so that the centralized electric automobiles have important bottleneck which cannot be broken through in the face of complex traffic environments, and traffic accidents occur due to the fact that the centralized electric automobiles do not decelerate or cannot directly avoid obstacles when meeting emergency obstacle working conditions in the driving process.
The four-wheel independent drive electric automobile takes a hub motor as a power unit, and a novel electric automobile framework with independent drive, brake and steering functions is considered by domestic and foreign researchers to be one of the most potential electric automobile frameworks. Because the mechanical structure limitation is eliminated, the motion of the automobile in all directions can be actively interfered by driving, braking, steering and a suspension subsystem actuator, the control of the tire force vector can be realized by reasonably distributing control instructions of a plurality of actuating mechanisms, and compared with a centralized electric automobile, the control dimension of the four-wheel independent drive electric automobile is higher, so that the appearance of the four-wheel independent drive electric automobile provides a new 'challenge' for redesigning an intelligent driving framework. The dynamics characteristic of the four-wheel independent drive electric automobile is different from that of a centralized electric automobile, the inherent kinematic constraint on the structure can be broken through, and a plurality of domestic and foreign researches show that the stable boundary of the four-wheel independent drive electric automobile is widened, and the control performance is improved compared with that of the traditional automobile. The running stability and the motion reliability of the vehicle are important criteria for evaluating the intelligent level of the four-wheel independent drive electric vehicle, and how to fully utilize the redundant actuator to carry out advanced motion planning is one of the key problems for realizing the intelligence of the four-wheel independent drive electric vehicle and is a necessary path for intelligently driving the vehicle in a complex traffic environment. Under the large background that the intelligent development of automobiles meets the requirements of multiple use scenes, the upper-layer path planning is very important by fully utilizing the stable domain of the four-wheel independent drive electric automobile, and the relationship between the intelligent driving layer and the chassis control layer of the four-wheel independent drive electric automobile is inseparable and cannot be isolated. How to reveal the influence mechanism of the stable region of the new power configuration vehicle of the four-wheel independent drive electric automobile, and redesigning a motion planning and tracking control framework by using the stable region becomes an important problem of the current four-wheel independent drive intelligent electric automobile.
Disclosure of Invention
The invention provides a path planning method for a four-wheel independent drive intelligent electric vehicle, which is used for carrying out path planning on the basis of fully utilizing a stable domain of the four-wheel independent drive electric vehicle, wherein a path planning algorithm can meet daily driving requirements of the intelligent electric vehicle, and also has the characteristics of good working condition adaptability, high path planning accuracy, strong fault-tolerant capability and the like under emergency working conditions such as emergency collision avoidance, high-speed driving and the like, the advantages of the four-wheel independent drive electric vehicle compared with a traditional vehicle or a centralized electric vehicle are fully exerted, an intelligent driving layer and a chassis control layer of the four-wheel drive electric vehicle are fully and tightly combined, and the safety and the efficiency of the electric vehicle in the driving process are improved.
The technical scheme adopted by the invention for solving the technical problem is as follows:
a four-wheel independent drive intelligent electric vehicle path planning method based on a stable domain is characterized by comprising the following steps:
establishing a nonlinear seven-degree-of-freedom nonlinear vehicle model, wherein the seven-degree-of-freedom comprises a longitudinal direction, a lateral direction, a horizontal swing and 4 wheels;
obtaining a stable domain of the four-wheel independent drive electric automobile based on the established nonlinear seven-degree-of-freedom vehicle model;
and planning a path based on the obtained stable domain.
Through the technical scheme, compared with the prior art, the invention has the following beneficial effects:
the method can solve the limitation and conservation of the traditional vehicle and the centralized electric vehicle, enriches the theories related to the intellectualization of the four-wheel independent drive electric vehicle by researching the influence mechanism of the stability domain of the four-wheel independent drive electric vehicle, discloses the influence mechanism of the stability domain of the four-wheel independent drive electric vehicle, and can meet the daily driving requirement of the intelligent electric vehicle by a path planning algorithm.
The invention also has the characteristics of good working condition adaptability, high path planning accuracy, strong fault-tolerant capability and the like under emergency working conditions of emergency collision avoidance, high-speed driving and the like, fully exerts the advantages of the four-wheel independent drive electric automobile compared with the traditional automobile or a centralized electric automobile, fully and tightly combines the intelligent driving layer and the chassis control layer of the four-wheel drive electric automobile, and improves the safety and the high efficiency of the electric automobile in the driving process.
Drawings
FIG. 1 is a schematic illustration of the stability domain of a centroid yaw-rate phase plane in an example of the present invention.
FIG. 2 is a flow chart of a stability domain analysis in an embodiment of the present invention.
FIG. 3 is a system block diagram of an embodiment of the present invention.
The following further describes the practice of the present invention in conjunction with the accompanying drawings.
Detailed Description
The present invention will now be described in further detail with reference to the accompanying drawings. These drawings are simplified schematic views illustrating only the basic structure of the present invention in a schematic manner, and thus show only the constitution related to the present invention.
By researching the influence mechanism of the stable region of the four-wheel independent drive electric automobile, the intelligent related theory of the four-wheel independent drive electric automobile is enriched, the influence mechanism of the stable region of the four-wheel independent drive electric automobile is disclosed, the path planning algorithm can not only meet the daily driving requirement of the intelligent electric automobile, and under the emergency working conditions of emergency collision avoidance, high-speed driving and the like, the system also has the characteristics of good working condition adaptability, high path planning accuracy, strong fault-tolerant capability and the like, the electric automobile generally takes a centralized type as a main part, the track planning and tracking control algorithm design of the centralized electric automobile is conservative, even if the optimal path is planned by fully considering the dynamic constraint, because of the kinematics characteristics of the centralized electric automobile, the longitudinal and transverse movement behaviors of the vehicle need to conform to the kinematics relationship, therefore, in the face of a complex traffic environment, the centralized electric automobile has an important bottleneck which cannot be broken through. Therefore, the application aims to provide a four-wheel independent drive intelligent electric vehicle path planning method based on a stable domain, improve the conservatism and the limitation of the traditional path planning algorithm, and provide an intelligent solution for the four-wheel independent drive electric vehicle.
Example (b):
based on the defects of the prior art, as shown in fig. 2, the method for planning a path of an intelligent electric vehicle based on four-wheel independent drive of a stable domain provided by the present application includes the following steps, which are implemented based on the architecture shown in fig. 2, and fig. 1 is a schematic diagram of a stable domain of a centroid yaw angle-yaw rate phase plane.
The first step is as follows: establishing a seven-degree-of-freedom nonlinear vehicle model which comprises 7 degrees of freedom including longitudinal, lateral, horizontal and 4 wheels, wherein the tire model selects a Dugoff tire model, and parameter identification is carried out on the mass of the whole vehicle, the horizontal swinging moment of inertia of the whole vehicle, the rolling radius of the tire, the moment of inertia of the tire, the longitudinal rigidity of the tire and the cornering rigidity of the tire through a real vehicle test. And after obtaining the parameters, comparing the established seven-degree-of-freedom nonlinear vehicle model with output curves of the four-wheel independent drive passenger vehicle, wherein the output curves comprise yaw angular velocity, mass center and lateral deflection angle and lateral acceleration output curves.
Step 11, establishing a seven-degree-of-freedom vehicle model according to the four-wheel independent drive electric automobile, wherein the expression is as follows:
wherein m is the total mass of the vehicle,
in the form of a longitudinal acceleration, the acceleration,
as lateral acceleration, vxIs the longitudinal velocity, vyIs the lateral velocity, r is the yaw rate, IZFor moment of inertia about the z-axis, Σ Fx、∑Fy、∑MZRespectively the sum of the total longitudinal force, the total lateral force and the total yaw moment of the vehicle.
Step 12, Σ Fx、∑Fy、∑MZRespectively as follows:
∑Fx＝(Fxfl+Fxfr)cosδf-(Fyfl+Fyfr)sinδf+Fxrl+Fxrr
∑Fy＝(Fxfl+Fxfr)sinδf+(Fyfl+Fyfr)cosδf+Fyrl+Fyrr
where i ═ F, r denotes front and rear wheels, respectively, j ═ l, r denotes left and right wheels, respectively, and FxijIs a tire longitudinal force, FyijThe lateral force of the tire, a is the distance from the front axle to the center of mass, b is the distance from the rear axle to the center of mass, and lf、lrRespectively the wheel base of the front axle and the rear axle of the vehicle.
And step 13, the longitudinal slip ratio of each wheel is respectively as follows:
where i ═ f, r denotes front and rear wheels, respectively, j ═ l, r denotes left and right wheels, respectively, and λijIs the longitudinal slip ratio, R, of the tirewIs the rolling radius of the wheel, omegaijIs the wheel speed.
And step 14, respectively setting the slip angles of the tire as follows:
where i ═ f, r denote front and rear wheels, respectively, j ═ l, r denote left and right wheels, respectively, and αijIs the tire slip angle and beta is the centroid slip angle.
And step 15, respectively setting the vertical loads of the wheels as follows:
where i ═ F, r denotes front and rear wheels, respectively, j ═ l, r denotes left and right wheels, respectively, and FzijThe vertical load of the tire, beta is the vehicle mass center slip angle, h is the height from the mass center to the ground, and g represents the gravity acceleration.
And step 16, establishing a nonlinear seven-degree-of-freedom vehicle model, wherein the nonlinearity is expressed on a vehicle tire model, the tire model is a Dugoff tire model, and the tire force and the maximum slip angle of the tire when the tire slip force is saturated are respectively obtained according to the tire model:
wherein, CxFor longitudinal stiffness, i ═ f, r denotes front and rear wheels, respectively, j ═ l, r denotes left and right wheels, respectively, CiIs the tire cornering stiffness, mu is the road adhesion coefficient, alphasijThe maximum tire cornering angle at which the tire cornering power is saturated.
The second step is that: the obtained nonlinear seven-degree-of-freedom vehicle model is used for subsequent theoretical analysis, and the influence of different front wheel corners, longitudinal vehicle speeds, road adhesion coefficients and yaw moment intervention on a vehicle body stability domain is analyzed by drawing different yaw angular speeds-mass center slip angle phase plane diagrams, so that the complex dynamics law of sudden change of topological structures of balance points, saddle points and trajectories in the phase plane diagrams is summarized. According to the invention, the phase plane diagrams of different front wheel corners, longitudinal vehicle speeds, road adhesion coefficients and yaw moment interventions are finally obtained by drawing the phase plane diagrams, and the change law of the stable region of the four-wheel independent drive electric vehicle is summarized by drawing a large number of phase plane diagrams.
And step 21, a phase plane method is an effective tool for analyzing the stability of the nonlinear system, so that the beta-r phase plane graph with different front wheel corners, longitudinal vehicle speeds, road adhesion coefficients and intervening yaw moments is finally obtained by drawing the beta-r phase plane graph, and the change rule of the stable region of the four-wheel independently-driven electric vehicle is summarized by drawing a large number of phase plane graphs.
And step 22, drawing a phase plane diagram when the front wheel rotation angle is equal to 0 and no yaw moment is involved.
The longitudinal vehicle speed and the road adhesion coefficient are respectively set according to the parameters of 20m/s and 0.7, while the front wheel turning angle is kept unchanged and no yaw moment is involved.
And 221, when the front wheel corner is 0 and no yaw moment intervenes, dividing a stable region of the vehicle in the current state in a drawn beta-r phase plane, wherein the stable region is formed by four straight lines and forms a parallelogram shape, the four straight lines form a boundary of the stable region, the boundary divides the whole region into a stable phase track and an unstable phase track, and the maximum yaw angular velocity and the minimum yaw angular velocity are respectively represented as:
at step 222, the maximum and minimum centroid slip angles are expressed as the maximum and minimum front and rear tire slip angles, respectively.
Wherein i ═ f, r denote front and rear wheels, respectively, j ═ l, r denote left and right wheels, respectively,
is the maximum mass at each tire slip angleThe angle of the side-to-side deviation of the center,
is the minimum centroid slip angle at each tire slip angle,
is the centroid slip angle under the maximum and minimum front wheel slip angles,
is the centroid slip angle under the maximum and minimum rear wheel slip angles, a is the distance from the front axle to the centroid, b is the distance from the rear axle to the centroid, vxIs the longitudinal velocity, r is the yaw rate, δfFor angle of rotation of front wheel, λijIs the longitudinal slip ratio of the tire, alphaijIs the tire slip angle.
And 223, drawing vehicle phase plane diagrams under different combination working conditions of the corner of the front wheel and the longitudinal speed on the basis of the stable region division, and dividing the stable region under the combination of the current corner of the front wheel and the longitudinal speed according to the stable region division formula.
And step 23, drawing a yaw velocity-mass center slip angle phase plane diagram when the front wheel rotation angle is not equal to 0 and no yaw moment is involved.
And 231, keeping the longitudinal vehicle speed and the road adhesion coefficient unchanged without the intervention of a yaw moment, dividing a stable domain according to a previous stable domain division formula when the front wheel corner is 10 degrees, and then drawing a phase plane diagram according to the previous longitudinal vehicle speed and road adhesion coefficient parameter setting.
232, obtaining adhesion ellipse curves of different vertical loads and lateral deflection angles of the tire according to a tire force bench test, simplifying the adhesion ellipse into the following formula by theoretical analysis, calculating the limit value of the longitudinal force of each tire,
wherein i ═ f, r denote front and rear wheels, respectively,j ═ l, r denote left and right wheels, respectively, Fmax|xijIs the longitudinal force saturation value of the tire, FyijIs the cornering power of the current tyre.
Step 233, it is thus possible to obtain a tire with a longitudinal force variation range of:
-Fmax|xfl≤F′xfl≤Fmax|xfl
-Fmax|xfr≤F′xfr≤Fmax|xfr
-Fmax|xrl≤F′xrl≤Fmax|xrl
-Fmax|xrr≤F′xrr≤Fmax|xrr
at step 234, the torque distribution mode of the four-wheel independent drive electric vehicle is an average distribution mode, that is, the longitudinal forces of the tires on the front axle and the rear axle on the left side and the right side are the same, so the longitudinal force ranges of the tires on the left side and the right side can be expressed as follows:
wherein i ═ F, r denote front and rear wheels, respectively, j ═ l, r denote left and right wheels, respectively, and F'xijThe value of the longitudinal force substitution for the tire for the phase plane plot is the value of the change in the longitudinal force of the tire, FxiThe tire longitudinal forces on the left and right sides.
In step 235, the additional direct yaw moment is calculated as:
ΔMZ＝Fxl(lf+lr)/4-Fxr(lf+lr)/4
wherein, Δ MZIs an additional direct yaw moment value. When Δ MZAfter the intervention, the influence on the body stability region, i.e. yaw rate-centroid slip angleAnd (3) dynamic laws of topological structure mutation of balance points, saddle points and trajectories of the phase plane diagram, and drawing a stable region range according to the following linear equation, wherein the maximum and minimum yaw angular velocities are respectively expressed as:
at step 236, the maximum and minimum centroid slip angles are expressed as:
comparing the influence of the parameter changes of the front wheel turning angle, the longitudinal speed and the road surface adhesion coefficient on the vehicle stable region, when the influence of the intervention of the yaw moment on the stable region in the current vehicle state, the intervention of the yaw moment enables the yaw angular velocity value and the centroid slip angle of the current vehicle to be enveloped by the stable region again, and therefore the path planning of the following four-wheel independent drive intelligent electric vehicle is fully combined with the stable region to carry out algorithm design.
And step 24, calculating the maximum yaw angular speed and the centroid slip angle when different longitudinal speeds, road adhesion coefficients, front wheel corners and additional direct yaw moments of the vehicle intervene, dividing a stable region according to the calculation result, and obtaining the following linear equations, wherein the regions enveloped by the straight lines are the stable region.
Wherein r is yaw velocity, beta is centroid slip angle, rmax、rminIs the maximum and minimum steady state yaw rate of the vehicle,
is the centroid slip angle under the maximum and minimum front wheel slip angles,
the center of mass slip angle is the maximum and minimum side slip angle of the rear wheel.
The third step: the design of the path planning algorithm, the driving path of the driver in the steering collision avoidance or lane change behavior in the driving process can be obtained by fitting according to a high-order polynomial, a unitary quintic polynomial curve is established, the path planning algorithm is designed on the basis of a stable domain and needs to meet certain basic criteria, the basic criteria comprise safety, high efficiency, boundary conditions of path curvature, boundary conditions of a path function expression with displacement as a parameter independent variable and boundary conditions of a path function expression with time as a parameter independent variable, and finally the unitary quintic polynomial curve meeting the conditions is obtained.
Step 31, fitting by using a unary quintic polynomial curve, wherein the unary quintic polynomial is expressed as:
y(x)＝a0+a1x+a2x2+a3x3+a4x4+a5x5
a Cartesian coordinate system is established according to the right-hand rule, and the x axis coincides with the center line of the road and points to the advancing direction of the vehicle.
And step 32, obtaining the following conditions according to the boundary conditions of the path:
y(xa)＝ya
wherein y (x) is a univariate fifth order polynomial,
and
first and second derivatives, y, of a univariate fifth-order polynomial, respectivelyaTransverse displacement of the path of travel for steering to avoid collisions or to change lanes, xaLongitudinal displacement of the travel path for steering to avoid collision or lane change.
And step 33, simultaneously, the path planning also needs to meet certain basic criteria, including safety, high efficiency, boundary conditions of path curvature, boundary conditions of path function expressions with displacement as parameter independent variables, and boundary conditions of path function expressions with time as parameter independent variables, wherein K is the path curvature, theta is the heading angle of the vehicle, and the positive angle is specified to be the clockwise direction.
Step 331, therefore, according to the boundary condition of the path and the requirement of the planning index, a desired path planning expression can be calculated:
332, the invention realizes the vehicle steering collision avoidance or lane change by the active steering and differential steering of the front wheels, when the vehicle starts from the initial point coordinate (x) of the mass center of the current lane0,y0) Coordinate (x) of center of mass point to reach target lane by completing steering processa,ya). Changing the independent variable of the expression into time, and assuming that the longitudinal speed of the vehicle is not changed in the steering process of the vehicle, changing x into vxSubstituting t into the above equation, obtaining a functional expression of the lateral displacement with time as an independent variable as:
wherein t is more than 0 and less than ta，taThe time for the vehicle to complete steering to avoid collision or change lanes is provided. Because of yaAnd taDetermining the starting point and the ending point of the planned path and the curvature of the curve, and determining the undetermined parameters to be solved from the aspects of steering mechanism execution constraint, vehicle body stability region constraint, road boundary constraint and environment geometric constraint.
In step 34, the path planning needs to meet certain basic criteria, including:
step 341, executing constraint by the steering mechanism, wherein the constraint of the steering mechanism exists in the steering process of the vehicle, namely the corner of the front wheel of the vehicle is within a certain range, the angular speed of the corner of the front wheel is within a certain range, and the determination of the execution constraint of the steering mechanism influences the parameter ya1While y is determineda1Influence of the parameter value on the path transverse displacement yaSo that the range of front wheel cornering angle and front wheel cornering angular velocity is expressed as:
wherein, deltafIs the turning angle of the front wheel,
angular velocity of the corner of the front wheel, (. delta.)f)min、(δf)maxThe minimum and maximum front wheel turning angles of the vehicle front wheel turning angle,
the minimum and minimum values of the angular velocity of the vehicle's front wheel angle.
Parameter ya1The relational expression of the value and the front wheel rotation angle is as follows:
vxttan(δf)min≤ya1≤vxttan(δf)max
wherein v isxLongitudinal speed, t time to steer to avoid collision or change lanes, (delta)f)min、(δf)maxThe minimum and maximum front wheel angles of rotation of the front wheels of the vehicle.
Step 342, vehicle body stability region constraint, wherein the vehicle path planning is to conform to the vehicle dynamics constraint, and the vehicle stability and the safety of passengers can be satisfied, therefore, the vehicle body stability region constraint starts from the stability region of theoretical analysis, and the range of the yaw rate and the centroid slip angle is obtained according to the stability region, so that the yaw rate and the centroid slip angle can be respectively expressed as:
in the formula, vxIs the longitudinal velocity，vyAs lateral velocity, ayIn the case of a lateral acceleration, the acceleration,
is the second derivative of the expression for the lateral displacement function with time as argument.
According to the yaw velocity and the mass center slip angle range of the stable region of the vehicle body, the longest completion time t of the steering process of the vehicle is calculatedrmaxAnd tβmaxThe determination of these 2 parameters influences the completion time t of the steering collision avoidance or lane change taking into account the body stability regionsTherefore, the yaw rate r, the centroid slip angle β and the steering completion time tsThe following relationship should be satisfied:
rmin≤r≤rmax
ta＝ts≥max[trmax,tβmax]
in the formula, rmaxAnd rminRespectively the maximum and minimum yaw rates in the stable region of the vehicle body,
is the centroid slip angle under the maximum and minimum front wheel slip angles,
is the centroid slip angle t under the maximum and minimum rear wheel slip anglesrmaxAnd tβmaxThe maximum value of the steering completion time obtained in the range of the yaw rate and the centroid slip angle in the stable region is obtained.
Step 343, road boundary constraint, which needs to be considered because the vehicle needs to meet the requirements of safety and traffic regulations in the process of steering collision avoidance or switching, and the geometric boundary of the vehicle needs to be ensured not to exceed the road boundary or be a lane line boundary after steering is completed, so the road boundary constraint can be expressed as:
wherein, ya2Transverse displacement, y, of the planned path determined to take into account road boundary constraintsvIs the lateral displacement of the vehicle from the lane line boundary along the axis of the vehicle in the x-axis direction, yeIs the lateral displacement of the vehicle from the road boundary along the axis of the x-axis, and W is the maximum width of the vehicle.
Transverse displacement yaThe values of (A) are as follows:
ya＝min(ya1,ya2)
wherein, yaTransverse displacement of the driving path for steering to avoid collision or to change lanes, ya1Transverse displacement of the planned path, y, determined to take account of steering mechanism execution constraintsa2The lateral displacement of the planned path determined for consideration of the road boundary constraints.
344, the environmental geometric constraint needs to monitor the motion state of the front static and dynamic obstacles in real time, estimate the position of the front obstacles, and solve the time t for the vehicle to turn to avoid collision or change lanes if the position of the front obstacles is known and the path planning needs to meet the requirement of the geometric boundary constraint of the front obstaclesa：
xa＜vxta
Wherein x isaFor transverse displacement of steering lane change, vxAs longitudinal velocity, R1Is the vehicle steering radius at the center of mass, R2Steering radius at the apex of the body contour, y, in the direction of vehicle travel and close to the obstaclebIs a line extending from the left boundary of the obstacle to the center line of the center of mass of the vehicleΔ d is the safety margin set.
Step 35, the steering mechanism executes the constraints, the body stability region constraints, the road boundary constraints and the environment geometric constraints to determine the y needed to be solvedaAnd taAnd finishing the whole path planning process.
It will be understood by those skilled in the art that, unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the prior art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
The meaning of "and/or" as used herein is intended to include both the individual components or both.
The term "connected" as used herein may mean either a direct connection between components or an indirect connection between components via other components.
In light of the foregoing description of the preferred embodiment of the present invention, many modifications and variations will be apparent to those skilled in the art without departing from the spirit and scope of the invention. The technical scope of the present invention is not limited to the content of the specification, and must be determined according to the scope of the claims.
Claims (6)
1. A four-wheel independent drive intelligent electric vehicle path planning method based on a stable domain is characterized by comprising the following steps:
establishing a nonlinear seven-degree-of-freedom nonlinear vehicle model, wherein the seven-degree-of-freedom comprises a longitudinal direction, a lateral direction, a horizontal swing and 4 wheels;
obtaining a stable domain of the four-wheel independent drive electric automobile based on the established nonlinear seven-degree-of-freedom vehicle model;
performing path planning based on the obtained stable domain;
the path planning expression is:
wherein t is the time for steering to avoid collision or change lanes, and t is more than 0 and less than ta；taTime for completing steering collision avoidance or lane change of the vehicle; y isaTransverse displacement of the driving path for steering collision avoidance or lane change; c. C1、c2、c3Determining the coefficient of the expected path expression according to the boundary condition of the path and the requirement of a planning index;
said yaAnd taDetermining by a steering mechanism execution constraint, a vehicle body stability region constraint, a road boundary constraint and an environment geometric constraint; determining the yaAnd taThe method comprises the following steps:
and (3) steering mechanism execution restraint: parameter y for determining steering mechanism execution constrainta1The relational expression of the value and the front wheel rotation angle is as follows:
vxttan(δf)min≤ya1≤vxttan(δf)max
wherein v isxLongitudinal speed, t time to steer to avoid collision or change lanes, (delta)f)min、(δf)maxMinimum and maximum front wheel corners for the vehicle front wheel corner;
and (3) vehicle body stability region constraint: obtaining the range of the yaw angular velocity and the centroid slip angle according to the stable region, and calculating the longest completion time t of the vehicle steering process under the vehicle body stabilityrmaxAnd tβmaxRespectively obtaining the ranges of the yaw velocity and the centroid slip angle under the vehicle body stability region, and solving according to a function expression taking the steering time as an independent variable;
the yaw rate and the centroid slip angle are respectively expressed as:
in the formula, vxIs the longitudinal velocity, vyAs lateral velocity, ayIn the case of a lateral acceleration, the acceleration,
the second derivative of the expression of the transverse displacement function with time as an independent variable;
yaw rate r, centroid slip angle beta and steering completion time tsThe following relation is satisfied:
rmin≤r≤rmax
ta＝ts≥max[trmax,tβmax]
in the formula, rmaxAnd rminRespectively the maximum and minimum yaw rates in the stable region of the vehicle body,
is the centroid slip angle under the maximum and minimum front wheel slip angles,
is the centroid slip angle t under the maximum and minimum rear wheel slip anglesrmaxAnd tβmaxThe maximum value of the steering completion time obtained in the range of the yaw velocity and the centroid slip angle under the stable region;
and (3) road boundary constraint: the road boundary constraint is expressed as:
wherein, ya2Transverse displacement, y, of the planned path determined to take into account road boundary constraintsvIs the lateral displacement of the vehicle from the lane line boundary along the axis of the vehicle in the x-axis direction, yeThe lateral displacement of the vehicle from the road boundary along the axis of the vehicle in the x-axis direction, wherein W is the maximum width of the vehicle;
and (3) constraint of environment geometry: according to the geometrical boundary constraint of the front obstacle, the time t for the vehicle to finish steering collision avoidance or lane change is solveda：
xa＜vxta
Wherein x isaFor transverse displacement of steering lane change, vxIs the longitudinal velocity;
wherein R is1Is the vehicle steering radius at the center of mass, R2Steering radius at the apex of the body contour, y, in the direction of vehicle travel and close to the obstaclebThe distance between the left boundary of the obstacle and the extension line of the center of mass of the vehicle is delta d, and the delta d is a set safety margin;
transverse displacement yaThe values of (A) are as follows:
ya＝min(ya1,ya2)
wherein, yaTransverse displacement of the driving path for steering to avoid collision or to change lanes, ya1Transverse displacement of the planned path, y, determined to take account of steering mechanism execution constraintsa2The lateral displacement of the planned path determined for consideration of the road boundary constraints.
2. The method for planning a stable domain-based four-wheel independent drive intelligent electric vehicle path according to claim 1, wherein the method for obtaining the stable domain of the four-wheel independent drive electric vehicle based on the established nonlinear seven-degree-of-freedom vehicle model comprises the following steps:
drawing different yaw angular velocities-mass center slip angle phase plane diagrams, analyzing the influence of different front wheel corners, longitudinal vehicle speeds, road adhesion coefficients and yaw moment intervention on a vehicle body stability region, and obtaining the change rule of the four-wheel independent drive electric vehicle stability region.
3. The method for planning a path of an intelligent four-wheel independent drive electric vehicle based on a stable region according to claim 2, wherein the stable region of the four-wheel independent drive electric vehicle is a region enveloped by the following straight lines:
wherein r is yaw velocity, beta is centroid slip angle, rmax、rminIs the maximum and minimum steady state yaw rate of the vehicle,
is the centroid slip angle under the maximum and minimum front wheel slip angles,
the center of mass slip angle is the maximum and minimum side slip angle of the rear wheel.
4. Stability-based according to claim 3The method for planning the path of the localized four-wheel independent drive intelligent electric vehicle is characterized in that the maximum yaw angular velocity rmaxAnd minimum yaw rate rminRespectively expressed as:
where i ═ f, r denotes front and rear wheels, respectively, j ═ l, r denotes left and right wheels, respectively, and λijmaxIs the maximum longitudinal slip ratio, v, of 4 tires at the current timexThe longitudinal speed is g, the gravity acceleration is g, and the road adhesion coefficient is mu;
the maximum centroid slip angle and the minimum centroid slip angle are respectively expressed as:
wherein i ═ f, r denote front and rear wheels, respectively, j ═ l, r denote left and right wheels, respectively,
is the maximum centroid slip angle at each tire slip angle,
is the minimum centroid slip angle at each tire slip angle,
is the centroid slip angle under the maximum and minimum front wheel slip angles,
is the centroid slip angle under the maximum and minimum rear wheel slip angles, a is the distance from the front axle to the centroid, b is the distance from the rear axle to the centroid, vxIs the longitudinal velocity, r is the yaw rate, δfFor angle of rotation of front wheel, λijIs the longitudinal slip ratio of the tire, alphaijIs the tire slip angle.
5. The stable domain-based four-wheel independent drive intelligent electric vehicle path planning method according to claim 1, characterized in that: the established nonlinear seven-degree-of-freedom vehicle model expression is as follows:
wherein m is the total mass of the vehicle,
in the form of a longitudinal acceleration, the acceleration,
as lateral acceleration, vxIs the longitudinal velocity, vyIs the lateral velocity, r is the yaw rate, IZFor moment of inertia about the z-axis, Σ Fx、∑Fy、∑MZRespectively the sum of the total longitudinal force, the total lateral force and the total yaw moment of the vehicle.
6. The stable domain-based four-wheel independent drive intelligent electric vehicle path planning method according to claim 1, characterized in that:
the tire model of the established nonlinear seven-degree-of-freedom vehicle model is a Dugoff tire model, and the tire force and the maximum slip angle of the tire when the tire slip force is saturated obtained according to the tire model are respectively as follows:
wherein, CxFor longitudinal stiffness, i ═ f, r denotes front and rear wheels, respectively, j ═ l, r denotes left and right wheels, respectively, CiIs the tire cornering stiffness, mu is the road adhesion coefficient, alphasijIs the maximum tire cornering angle, λ, at which the tire cornering power is saturatedijIs the longitudinal slip ratio of the tire, FxijIs a tire longitudinal force, FyijFor tyre side force FzijIs the vertical load of the tire.
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