CN112668095B - Design method of active front wheel steering controller - Google Patents
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Abstract
The invention relates to the field of active safety control of vehicles, in particular to a design method of an active front wheel steering controller. The method comprises the following steps: s1: determining a basic motion equation and a state space equation of a vehicle model; s2: obtaining a desired yaw rate ω from a vehicle model d And a desired slip angle beta d The method comprises the steps of carrying out a first treatment on the surface of the S3: for the maximum desired yaw rate omega d max And a maximum desired slip angle beta d max Correcting; s4: setting a combined deviation e of the yaw rate and the centroid slip angle of the vehicle 1 Introducing integral term into the switching function to obtain sliding mode switching function s 1 The method comprises the steps of carrying out a first treatment on the surface of the S5: obtaining a first order derivative function of a sliding mode switching functionS6: calculating a switching control angle u of a vehicle sw And equivalent control angle u eq The control angle of the final active front wheel steering controller is delta AFS ＝u sw +u eq . The controller designed based on the method can improve the response speed of the vehicle steering system, can adapt to the stability state of the vehicle, and stabilizes the vehicle under the conditions of larger interference and poor stability.
Description
Technical Field
The invention relates to the field of active safety control of vehicles, in particular to a design method of an active front wheel steering controller.
Background
With the continuous improvement of the living standard of people, the quantity of the resident automobiles in China is continuously increased. Related researches find that the automobile loses transverse stability to be a big cause of frequent traffic accidents; it is often difficult for a driver to accurately judge whether the vehicle is in a dangerous unstable state according to his own driving experience. If the vehicle itself can find the dangerous state in time and early warn the driver, or can perform active lateral stable control after judging the dangerous state, most traffic accidents can be avoided. This is one of the latest research directions in the technical field of active safety control of vehicles, and is also an important basis for the automatic driving technology of vehicles, and various automobile manufacturers and research institutions have already achieved a great deal of technical results in the field.
The method effectively judges the transverse stable state of the vehicle to avoid the vehicle from being out of control, and has important significance for ensuring the safety of a driver. However, in the prior art, there are still a number of problems associated with the identification of the lateral steady state of the vehicle during operation. On one hand, the existing classification method based on the transverse stability data set is difficult to determine an initial clustering center, the processing difficulty of data is high, and calculation load is caused on a system; meanwhile, the processing result cannot accurately reflect the real-time running state of the automobile, and the accurate judgment of the stability of the automobile is also affected. On the other hand, the traditional transverse stability judging method does not comprehensively consider parameters affecting the transverse stability of the vehicle, only analyzes a certain number of characteristic quantities representing the transverse stability of the vehicle to obtain transverse stability criteria, the data base is relatively weak, and the selected characteristic quantities often cannot comprehensively represent the transverse stable state of the vehicle. Paper [ phase plane method-based vehicle running stability judging method [ J ]. Liu Fei, xiong Lu, deng Lvhua, et al, university of North China university of science (Nature science edition), 2014,42 (11) ] discloses a vehicle running stability judging method, which establishes a centroid side deflection angle-centroid side deflection angle speed phase diagram according to a phase plane analysis method, adopts an improved five-eigenvalue diamond method to divide a vehicle lateral stability area, and draws a vehicle running all-condition phase diagram based on a Matlab programming environment. This solution has various drawbacks as described above.
In addition, the collection and the processing of the characteristic parameters can improve the hardware cost and the running cost of the vehicle; the collection of the vehicle operation characteristic information should also take into account its contribution to the accurate determination of the lateral stability of the vehicle, so as to determine the most necessary and effective vehicle operation characteristic data, which is also one of the problems to be solved.
On the basis of solving the recognition of the lateral stability of the vehicle, a more effective vehicle control method is needed to be selected, the unstable state of the lateral stability of the vehicle is adjusted and controlled in a targeted manner, and a regulation strategy is executed in a targeted manner according to different types of the stable state of the vehicle, so that a better regulation effect is obtained. Obviously, there is also a difficulty in achieving the above object by simply coordinating the existing active front steering control (AFS) and direct yaw moment control (DYC), so that a more effective control manner of combining AFS and DYC needs to be designed based on the vehicle lateral stability recognition result.
Disclosure of Invention
In order to overcome the problems in the prior art, the design method of the active front wheel steering controller provided by the invention has the advantages that the response speed of a vehicle steering system can be improved by the controller designed based on the method, the vehicle stability state can be adapted, and the vehicle can be stabilized under the conditions of larger interference and poor stability.
The technical scheme provided by the invention is as follows:
a design method of an active front wheel steering controller comprises the following steps:
s1: based on a 2-degree-of-freedom linear vehicle model, a basic motion equation and a state space equation of the vehicle model are determined on the assumption that the front wheel rotation angle is smaller and the vehicle speed is constant;
s2: obtaining a desired yaw rate ω from a vehicle model d And a desired slip angle beta d ；
S3: considering the influence of the road surface friction coefficient mu, the maximum expected yaw rate omega d max And a maximum desired slip angle beta d max Correcting;
s4: setting a combined deviation e of the yaw rate and the centroid slip angle of the vehicle 1 Introducing integral term into the switching function to obtain sliding mode switching function s 1 ；
S5: solving a first derivative of the integral sliding mode switching function to obtain a first derivative function of the sliding mode switching functionSimplifying the first order derivative function;
s6: bringing a two-degree-of-freedom dynamics equation to a first derivative of the sliding mode switching functionUnder the condition that the slip form reaching condition is met, calculating the switching control rotation angle u of the vehicle sw And equivalent control angle u eq The control angle of the final active front wheel steering controller is delta AFS ＝u sw +u eq 。
Further, in step S1, the basic equation of motion of the vehicle model is:
in the above formula: beta is the slip angle;for angular velocity of slip, I z The moment of inertia around the z-axis is the moment of inertia of the automobile; ω is the yaw rate and,is yaw acceleration; k (k) 1 Is the total cornering stiffness of the front tyre; k (k) 2 Is the total cornering stiffness of the rear tyre; u is the longitudinal speed of the automobile; delta f Is the front wheel corner;a is the distance from the centroid to the front axis; b is the distance from the centroid to the rear axis; m is the mass of the automobile;
the state space equation is:
in the equation(s) used in the present invention,
C＝[1 1]，
in the above-described matrix of the matrix,
further, in step S2, the desired yaw rate ω obtained based on the vehicle model d And a desired slip angle beta d The following formulas are respectively shown:
wherein ,
in the above formula, L is the wheelbase of the vehicle; v is the vehicle lateral speed; u is the longitudinal speed of the automobile; delta f Is the front wheel corner; a is the distance from the centroid to the front axis; b is the distance from the centroid to the rear axis; m is the mass of the automobile; k (k) 1 Is the total cornering stiffness of the front tyre; k (k) 2 Is the total cornering stiffness of the rear tyre.
Further, in step S3, the test is performedTaking into account the influence of the road surface friction coefficient, the maximum expected yaw rate ω d max And a maximum desired slip angle beta d max The corrected calculation formula is as follows:
in the above formula, mu is the friction coefficient of the road surface, u is the longitudinal speed of the automobile, and a is the distance from the mass center to the front axle; b is the distance from the centroid to the rear axis; m is the mass of the automobile; l is the wheelbase of the vehicle.
Further, in step S4, the vehicle yaw rate and the centroid slip angle are combined with the deviation e 1 The calculation formula of (2) is as follows:
e 1 ＝(ω d -ω)+η(β d -β)；
in the above formula, ω is yaw rate; omega d A desired yaw rate; beta d Is the desired slip angle; beta is the slip angle; η is a system adjustment parameter, η is a constant having a positive value, and represents a ratio of the centroid slip angle and the yaw rate to participate in the control.
Further, the integral sliding mode switching function is:
in the above, e 1 The combined deviation of the yaw rate and the centroid side deviation angle of the vehicle; t is time; c is a sliding mode control and regulation parameter;
the sliding mode switching function is:
in the above formula, ω is yaw rate; omega d A desired yaw rate; beta d Is the desired slip angle; beta is the slip angle; t is time; and c is a sliding mode control adjustment parameter.
Further, in step S5, the first derivative function of the sliding mode switching function is:
wherein, for simplifying the expression, it is assumed that there is a variable Ω, let
Ω＝c[(ω d -ω)+η(β d -β)]，
The first order derivative function transforms into:
further, in step S6, the system switching control coefficient u sw The calculation process of (2) is as follows:
to be in two-degree-of-freedom dynamics equationCarry-in to first order derivative function->In (1), the following steps are obtained:
let the above formula be 0, then the equivalent control angle u can be solved eq ：
Further, the system switch control angle u sw The following are provided:
in the above expression, η is a constant having a positive value, and represents the ratio of the centroid slip angle to the yaw rate involved in the control.
Further, the control steering angle δ of the active front wheel steering controller AFS Calculated from the following formula:
δ AFS ＝u eq +u sw ；
in the above, u sw For system switching control angle u eq The rotation angle is equivalently controlled.
The design method of the active front wheel steering controller provided by the invention has the following beneficial effects that:
1. the active front wheel steering control can be based on the existing steer-by-wire system of the vehicle, other hardware structures are not required to be designed independently, the implementation is convenient, and the cost is low.
2. Through designing an equivalent control corner item, the response speed of a vehicle steering system can be improved, and buffeting of sliding mode control is reduced; by designing a switching control corner item, errors can be eliminated, and integral saturation can be restrained; the combination of the equivalent control corner term and the switching control corner term can improve the robustness of control.
3. In the design process, the control of the yaw rate and the slip angle is considered at the same time, and the proportion of the mass center slip angle and the yaw rate participating in the control is adjusted through the distribution coefficient, so that the control precision of the yaw rate and the slip angle can be improved.
4. The vehicle runs in a complex environment, is more interfered by the environment, and the integral equivalent sliding mode control based on the sliding mode idea has insensitivity to the interference, so that the vehicle can be stabilized under the conditions of larger interference and poor stability.
Drawings
Fig. 1 is a flowchart of a vehicle lateral stability determination and control process in the present embodiment 1;
fig. 2 is a graph showing the change of the steering wheel angle value of the vehicle with time in the CarSim vehicle simulation software of the embodiment 1;
fig. 3 is a decision chart obtained after clustering of the vehicle lateral stability data set in the present embodiment 1;
FIG. 4 is a schematic diagram showing the distribution of data points of the vehicle lateral stability classification in the present embodiment 1;
fig. 5 is a schematic diagram of the structure of the neural network in the present embodiment 1;
fig. 6 is a design flow chart of the active front wheel steering controller in this embodiment 1;
fig. 7 is a design flowchart of the direct yaw moment controller in the present embodiment 1;
FIG. 8 is a graph showing the variation of the vehicle front wheel steering angle input value with time during the Carsim/Simulink joint simulation test of embodiment 1;
fig. 9 is a comparison curve of the expected and actual values of the yaw rate of the vehicle over time in the joint simulation test of the present embodiment 1;
FIG. 10 is a graph showing the comparison of expected and actual values of the vehicle slip angle with time in the joint simulation test of the present embodiment 1;
fig. 11 is a flowchart of a vehicle lateral stability determination and control process employing a reduced attribute data set in this embodiment 2;
FIG. 12 is a graph showing the training error rate of the extended neural network according to the training period in example 2;
fig. 13 is a comparison curve of the output class and the actual class of the extended neural network in this example 2 as the test data increases.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
Example 1
As shown in fig. 1, the vehicle lateral stability determination and control method provided by the present embodiment includes the following processes:
1. data set acquisition
In the embodiment, when the fact that the real vehicle collects various data in the vehicle transverse stability data set is considered, a large potential safety hazard exists, and when the vehicle encounters an emergency, personal safety of a driver can be possibly damaged. Therefore, the present embodiment adopts the vehicle simulation software CarSim to collect the characteristic attribute values that characterize the lateral stability of the vehicle. CarSim is used as parameterized vehicle dynamics software, and has the advantages of simplicity in operation, high simulation speed, time saving, high reliability and the like. In the embodiment, a typical vehicle driving condition can be designed in CarSim to obtain the lateral stability data of the vehicle.
In order to comprehensively evaluate the state of the lateral stability of the vehicle, the embodiment comprehensively considers each characteristic attribute representing the lateral stability of the vehicle and selects the longitudinal speed u and the steering wheel rotation angle delta s Lateral velocity v, centroid slip angle beta, roll angle ψ, roll angle velocity ω ψ Yaw rate ω, lateral acceleration a y Lateral load transfer rate L of front wheel TR1 Lateral load transfer rate L of rear wheel TR2 A total of 10 parameters are used as characteristic attributes.
Wherein the front wheel lateral load transfer rate L TR1 And rear wheel lateral load transfer rate L TR2 Calculated from the following formulas:
L TRx ＝(F ZLx -F ZRx )/(F ZLx +F ZRx ),x＝1,2
in the above, F ZL1 Representing the vertical load of the left front wheel, F ZL2 Representing the vertical load of the left rear wheel, F ZR1 Representing the vertical load of the front right front wheel, F ZR2 Representing the vertical load of the right rear wheel.
In this embodiment, the vehicle lateral stability test of the ISO standard is referred, and the situation that the vehicle lateral stability is gradually deteriorated is further designed, and the steering wheel angle change curve after a lot of adjustment is shown in fig. 2. 10 simulation conditions with the longitudinal speed of the vehicle being 30km/h to 120km/h and the step length being 10km/h are set. 10 attribute values characterizing the stability of the vehicle were collected as one data point every 0.05 seconds, with a simulation time of 10 seconds at each longitudinal speed, resulting in a vehicle transverse attribute dataset. The attribute dataset contains 2000 data points, each data point containing 10 attribute values.
2. Lateral stability clustering
According to the obtained attribute data set, the embodiment adopts a fast search and density peak algorithm (CFSDPF) to cluster the lateral stability of the vehicle, and finds the category number of the lateral stability of the vehicle. CFSDPF is a new density-based clustering algorithm that automatically obtains the number of categories from the collected data set without determining the initial cluster center. The core idea of CFSDPF is that the local density of class centers is higher than the local density of its nearest neighbors. The algorithm utilizes the decision diagram to determine the clustering center, can quickly find the density peak value of the data set with any shape, and effectively distributes non-center sample points. The basis of clustering is that the density of cluster centers is greater than the density of its surrounding neighbors, and for each data point, CFSFDP first calculates its local density and its distance from other data points with local densities.
The clustering process of the "fast search and density peak algorithm" is as follows:
assume that the lateral stability dataset is:
in the above, x i Data point, data point x i Data point x j The distance between them is expressed as:
d ij ＝dist(x i ,x j )；
the local density of the data points is calculated using the formula:
in the above, d c Is an empirically determined cutoff distance, in this embodiment, d c ＝0.0029。ρ i Representing those in S and x i Is less than d c Is a point of (2);
wherein, the function χ (x) in the local density calculation formula is:
when x is i When the local density of (a) is maximum, calculating the data point and x in S i Is greater than the maximum distance theta i The calculation formula is as follows:
θ i ＝max j (d ij )；
in the above formula, max is a function of calculating the maximum value;
when x is i When the local density of (2) is not the maximum, then the ratio x in S is calculated i Is a local density of data points and x i Is the minimum distance theta of (2) i The calculation formula is as follows:
in the above formula, min is a function for calculating the minimum value;
after CFSDPF clustering, each point in the dataset can be clustered with ρ i and θi To represent. From ρ i and θi The two-dimensional graph drawn as the abscissa and ordinate is called a decision graph, and the category center and the category number can be determined from the decision graph.
And after normalizing the transverse attribute data set, adopting a CFSDPF algorithm to obtain a classification result. The cutoff distance dc=0.0029 is set.
In this example, the decision chart including all the data points is shown in fig. 3, and it can be intuitively seen from the decision chart that the values of ρ and θ of three data points are larger, and the CFSDPF algorithm can accurately identify three class centers.
As shown in table 1, the present embodiment defines these three categories as "absolute stable", "near stable", and "almost unstable", respectively.
Table 1: lateral stability classification results
Fig. 4 shows 10 lines of data points, representing 10 longitudinal speed vehicle stability changes, respectively. The driving situation of the vehicle with an initial longitudinal speed of 30-120km/h is shown from top to bottom. The data points at the beginning of each line indicate that the vehicle is in an "absolute steady" state, which means that the vehicle is in an absolute steady state, as the vehicle is traveling in a straight line during this time. As the steering wheel turns, the lateral stability of the vehicle gradually becomes "nearly unstable", i.e., the vehicle nearly loses stability, but the vehicle is still controllable. The point at the end of the curve indicates that the vehicle is in a "near steady" state when the longitudinal speed is 30km/h, indicating that the vehicle is stationary even when the steering wheel is rotated at a large angle during slow speed. The faster the longitudinal speed at the same steering wheel angle, the earlier the point of instability is characterized, indicating that the longitudinal speed has a significant impact on lateral stability. The remaining black dots are halo dots, representing data points at the edges of the category.
3. Lateral stability identification
The present embodiment employs an Extended Neural Network (ENN) to identify vehicle lateral stability based on clustered vehicle lateral attribute datasets.
ENN was developed in the extension theory, which assumes that a thing is named N, the property of the thing is h, the value of the property h is g, and the property of the thing can be described by the variable r= (N, h, g).
For the vehicle lateral stability identification in the present embodiment, N represents the vehicle lateral stability, h represents an attribute that characterizes the vehicle lateral stability, and g represents an attribute value.
Assuming n attributes are present, then there are:
g j ＝<a n ,b n >(j＝1…n)；
in the above, g j Representing the variation range of each attribute, a n Representation changeLeft end point of chemical range, b n Indicating the right end of the range of variation.
Let N be d Data points, variable R can be expressed as:
in the above formula, i= … N d ，N i Is the ith data point, g ij ＝<a ij ,b ij >Is attribute h i The range of variation of the values of (a), called classical domain, a ij Represents the left end point of the range of variation, b ij Indicating the right end of the range of variation.
ENN is a product of the combination of the extension theory with neural networks. It uses the extension to measure the similarity between data and class centers. The schematic structure of ENN is shown in fig. 5, a being the input layer and b being the output layer. Neurons of the input layer receive attributes of the input data points and the output layer is used to represent the class of the input data points. There are two connection weights between the input and output neurons.A lower bound representing the connection weight between the j-th input neuron and the k-th output neuron,/->Representing the upper bound of the connection weight.
The ENN-based lateral stability recognition process includes two parts, a training phase and a recognition phase as follows. The training process comprises the following steps:
(1) Defining a lateral stability dataset as:
in the above, N d The total data number; n in the present embodiment d ＝3000。
The i-th data point can be written as:
in the above formula, the category of the ith data point is p, and the data point has n attributes; n=10 in this embodiment;
(2) The classical domain for each lateral stability class is determined as an initial weight according to the following equation:
in the above formula, min is a function for calculating the minimum value, and max is a function for calculating the maximum value; k= … N c ，N c The total category number; n in the present embodiment c ＝3。
(3) The initial class center for each lateral stability class is calculated as follows:
Z k ＝{z k1 ,z k2 ,…,z kn }，
wherein ,
(4) The ith lateral stability data point is read along with the category p to which this data point belongs from the lateral stability data set of:
(5) Calculating data points by referring to a calculation formula of the extension distance in the extension theoryThe distance from the kth category is calculated as follows:
(6) Finding data pointsThe category o with the smallest distance from the kth category is such that the following holds:
ED io ＝min{ED ik }；
in the above formula, min is a function of calculating the minimum value, ED ik Representing data pointsDistance from the kth category;
if o=p, running step (7), otherwise re-running step (6);
(7) Updating the p-th class center and the o-th class center, wherein the updating formula is as follows:
the p-th class weight and the o-th class weight are respectively updated, and the update formulas of the p-th class weight and the o-th class weight are as follows:
in the above formula, η is a learning rate, which is determined by a priori knowledge, in this embodiment, η=0.01;
(8) Repeating the steps (3) - (6), if all the data in a certain period are trained, completing one training period, re-inputting the first data and performing the training of the next period;
(9) And ending the training process of the ENN if the designated training period or error rate is reached.
The training error rate is defined as follows:
in the above formula, E is training error rate, N M For error data of all training periods, N P All data for all training periods.
In this example, the specified training period is 100, and the specified training error rate E is less than or equal to 1x10 -6 。
In this example, the purpose of the training phase is to obtain the final weight of the updated lateral stability class. The training phase is completed and the recognition phase is entered.
The identification process comprises the following steps:
(1) Reading the final weight of the training stage;
(2) The initial category center for the dataset is calculated using the following formula:
Z k ＝{z k1 ,z k2 ,…,z kn }，
wherein ,
(3) Reading data points collected in real time and used for testing to obtain a transverse stability data set:
X t ＝{x t1 ,x t2 ,...,x tn }，
the distance between the data point for the test and each class is calculated as follows:
(4) Determining the category of the data point according to the calculation result of the step, and judging whether the data point meets the following conditions:
in the above equation, min is a function of calculating the minimum value,
when the distance between the data point and the o-th class meets the above formula, the data point is considered to belong to the o-th class;
(5) Sequentially identifying all data points, and if all test data points are identified, completing the identification process; if the identification of all the data points is not completed, the step (3) is carried out to continue the identification until the identification of all the data points is completed.
4. Coordinated control
The coordinated control process of the lateral stability of the vehicle in this example is completed by a coordinated controller composed of an Active Front Steering (AFS) controller and a direct yaw moment (DYC) controller. In this embodiment, the AFS controller and the DYC controller are respectively designed, and then the ratio of the two participating in control is adjusted by the adaptive adjustment coefficient, so as to form a coordination controller based on the AFS and the DYC in this example.
1. In this example, the AFS controller is designed based on the integral equivalent sliding mode control method, as shown in fig. 6, and the design process is specifically as follows:
the AFS controller is designed based on a 2-degree-of-freedom linear vehicle model, and assumes that the front wheel angle is small and the vehicle speed is constant.
The basic equation of motion of the model is as follows:
in the above formula: beta is the slip angle;for angular velocity of slip, I z The moment of inertia around the z-axis is the moment of inertia of the automobile; ω is the yaw rate and,is yaw acceleration; k (k) 1 Is the total cornering stiffness of the front tyre; k (k) 2 Is the total cornering stiffness of the rear tyre; u is the longitudinal speed of the automobile; delta f Is the front wheel corner; a is the distance from the centroid to the front axis; b is the distance from the centroid to the rear axis; m is the mass of the vehicle;
the above equation is written in the form of a state space equation as follows:
wherein ,
C＝[1 1]，
obtaining a desired yaw rate omega by the above model d And a desired slip angle beta d The method comprises the following steps of:
wherein ,
in the above formula, L is the wheelbase of the vehicle; v is the vehicle lateral speed; u is the longitudinal speed of the automobile; delta f Is the front wheel corner; a is the distance from the centroid to the front axis; b is the distance from the centroid to the rear axis; m is the mass of the vehicle; k (k) 1 Is the total cornering stiffness of the front tyre; k (k) 2 Is the total cornering stiffness of the rear tyre.
In practical cases, the desired values of the vehicle slip angle and yaw rate are limited by the road surface friction coefficient. Therefore, in the present embodiment, the maximum expected yaw rate ω is taken into consideration by the road surface friction coefficient d max And a maximum desired slip angle beta d max Expressed as:
in the above formula, mu is the friction coefficient of the road surface, u is the longitudinal speed of the automobile, and a is the distance from the mass center to the front axle; b is the distance from the centroid to the rear axis; m is the mass of the automobile; l is the wheelbase of the vehicle.
And the combined deviation of the yaw rate and the centroid side deflection angle of the vehicle is as follows:
e 1 ＝(ω d -ω)+η(β d -β)；
in the above formula, ω is yaw rate; omega d A desired yaw rate; beta d Is the desired slip angle; beta is the slip angle; η is a system adjustment parameter, η is a constant having a positive value, and represents a ratio of the centroid slip angle and the yaw rate to participate in the control.
Introducing an integral term into a switching function, and setting an integral sliding mode switching function as follows:
in the above, e 1 The combined deviation of the yaw rate and the centroid side deviation angle of the vehicle; t is time; c is a sliding mode control and regulation parameter;
further, the sliding mode switching function is:
in the above formula, ω is yaw rate; omega d A desired yaw rate; beta d Is the desired slip angle; beta is the slip angle; t is time; and c is a sliding mode control adjustment parameter.
And (3) obtaining a first derivative of the sliding mode switching function to obtain:
order the
Ω＝c[(ω d -ω)+η(β d -β)]；
Then
In two-degree-of-freedom dynamics equationCarry to->In (1), the following steps are obtained:
let the upper part be 0 to solve the equivalent control angle u eq ：
To ensure that the slip form arrival condition is met, a system switching control angle u is designed sw The following are provided:
in the above expression, η is a constant having a positive value, and represents the ratio of the centroid slip angle to the yaw rate involved in the control.
The control angle of the AFS controller is as follows:
δ AFS ＝u eq +u sw 。
2. the DYC controller in this example is designed based on a fast non-singular terminal sliding mode (NFTSM) control method; NFTSM control has fast limited time convergence and strong robustness.
As shown in fig. 7, the specific process of designing the DYC controller is as follows:
the basic equations of motion for the vehicle model are:
in the above formula, ω is the yaw rate, u is the longitudinal speed of the vehicle,to the angular velocity of the side deviation F y To sideways force, I z For the moment of inertia of the motor vehicle about the z-axis +.>For yaw acceleration, M DYC For additional yaw moment.
Differentiating the first expression and substituting the first expression into a second expression to obtain:
wherein ,
namely:is the actual slip angle beta and the expected slip angle beta d Is a difference in (2);
g β the unmodeled part of the vehicle is an unknown bounded function, which ranges from:
|g β |≤k′ 1
in the above, k' 1 G is g β Is defined by a boundary of (2);
slip angle beta and additional yaw moment M DYC The dynamics of the second order system are met, so the following second order system is set:
in the above, x 1 ，x 2 Representing the state of the second order system.
According to a fast nonsingular terminal sliding mode (NFTSM) control theory, setting a sliding mode surface of the NFTSM as follows:
in the above formula, alpha, gamma 1 、ξ、γ 2 The control parameters of the fast nonsingular terminal sliding mode are all the control parameters of the fast nonsingular terminal sliding mode.
Thus, the additional yaw moment is:
in the above, I z For the moment of inertia of the car around the z-axis, alpha, gamma 1 、ξ、γ 2 、k′ 1 、k′ 2 All are rapid nonsingular terminalsAnd controlling parameters of the end sliding mode.
In the control parameters of the fast nonsingular terminal sliding mode, alpha is more than 0, zeta is more than 0 and gamma is more than 0 1 ＞γ 2 And 1 < gamma 1 ＜2，，k′ 1 ＞0，k′ 2 ＞0。
The vehicle generates an additional yaw moment by differential braking of different wheels, and according to the front wheel rotation angle and the yaw moment direction, the vehicle can judge which wheel is braked. In this example, the braking force distribution strategy is shown in Table 2, where the braking force is shown in F T And l is the track of the vehicle.
TABLE 2 braking force distribution strategy
3. Based on the Active Front Steering (AFS) controller and the direct yaw moment (DYC) controller designed as described above, the coordination controller of the present embodiment is obtained, and in the coordination controller provided by the present embodiment, the additional front wheel rotation angle δ of the coordination controller is calculated * AFS And an additional yaw moment M * DYC The calculation formula is as follows:
in the above formula, λ is the adaptive adjustment coefficient. The value of lambda is determined by the three types of vehicle lateral stability states obtained in the clustering section.
In this embodiment, the coordination control strategy of the coordination controller is as follows.
(1) When the vehicle is in an absolute stable type, the coordination controller is not activated, so that the interference to a driver is avoided;
(2) When the vehicle is in the "near steady" category, the coordination controller activates, and the coordination controllers of the AFS and DYC are used to control the vehicle, respectively, at which point the value of the adaptive adjustment factor is determined by:
in the above, dist 1 Distance between the vehicle attribute and the "absolute stability" class center point; dist (dist) 2 Distance between the vehicle attribute and the center point of the "near destabilizing" category;
(3) When the vehicle is in the "almost unstable" category, the DYC controller is activated alone, and the AFS controller is turned off, at which time λ=0.
In the embodiment, the coordination control effect of the coordination controller is tested by adopting a Carsim/Simulink joint simulation, in the test process, a vehicle is designed to run on a dry road at a constant speed of 80km/h, the friction coefficient mu=0.85 of the road surface, and the front wheel angle input by a driver is shown in fig. 8; fig. 9 is a graph showing a comparison between the expected value and the actual value of the yaw rate in the model, and fig. 10 is a graph showing a comparison between the expected value and the actual value of the slip angle. As can be seen from fig. 9 and 10, the yaw rate and the slip angle of the simulated vehicle always track the desired value well under the control of the cooperative controller. It can thus be concluded that: the coordination controller provided in this example can minimize the deviation of the yaw rate of the vehicle from the desired value while keeping the value of the slip angle also around the desired value, achieving a good vehicle lateral stability control effect.
Example 2
Fig. 11 shows a flowchart of a vehicle lateral stability determination and control method in the present embodiment, which differs from embodiment 1 in that: in order to reduce the burden of data acquisition and processing, the embodiment appropriately reduces the acquired 10 attribute data, determines the attribute with the greatest influence on the lateral stability of the vehicle, and cuts off the attribute data with less influence on the lateral stability of the vehicle from the lateral stability data set.
Attribute reduction in this example is performed using a neighborhood rough set algorithm; the attribute reduction process is as follows:
defining a non-empty argument over a real set as:
U＝{x 1 ,x 2 ,…,x n }；
in the above, x i Is a data point representing the lateral stability of the vehicle;
defining A as an attribute set; d is the vehicle lateral stability category, namely decision attribute; c is a lateral stability characteristic, i.e., a conditional attribute.
Then there are:
a=c.u.d
Wherein, the neighborhood decision system is:
Ndt＝<U,A,D>，
for any x i E U, definition
σ B (x i )＝{x j |x j ∈U,Δ B (x i ,x j )≤σ}；
In the above, delta B Representing a distance function; sigma is x i Is a neighborhood radius of (2);
let M denote the neighborhood relation in U, then < U, M > constitutes the neighborhood space, and the lower approximation and the upper approximation of X on the neighborhood space < U, M > are expressed as:
MX＝{x i |σ(x i )∈X,x i ∈U}；
in the above-mentioned method, the step of,also known as X belongs to the neighborhood space<U,M>The positive field on the upper side of the frame,
defining the negative field of X in neighborhood space < U, M > as:
similarly, for the neighborhood decision system ndt= < U, a, D >, if the decision attribute D divides the domain U into M categories of lateral stability, namely:
(X 1 ,X 2 ,…,X M )(M＝3)；
then for any conditional attribute BDefining the lower and upper approximations of D with respect to B in the decision system is:
the dependence of D on B was further deduced as:
for a decision system<U,A,D>，In the sense of a word. If phi B-b (D)<φ B (D) The dependency of D on B becomes smaller after the deletion of attribute B from the system, which is important for the decision attribute set D and cannot be reduced. Conversely, if phi B-b (D)＝φ B (D) Indicating that after the attribute B is deleted from the system, the dependency of D on B is unchanged, and attribute B is redundant to the decision attribute set D and can be reduced.
The condition attribute B is a reduction of the attribute set a when B satisfies the following two conditions:
the present embodiment reduces 10 characteristic attributes of the lateral stability of the vehicle in embodiment 1 by the above-described method. The attribute reduction results are shown in table 3,
table 3: reduction of feature attributes
According to the reduction result:
front wheel load transfer rate L TR1 Rear wheel load transfer rate L TR2 Centroid slip angle beta, longitudinal speed u, yaw rate omega, steering wheel angle delta s The lateral inclination angle psi is the most influenced by 7 attributes on the lateral stability of the vehicle; the embodiment uses them to determine the lateral steady state of the vehicle, and simultaneously, three attributes are abandoned, thereby reducing the workload of data acquisition.
In this example, the attribute dataset, after reduction, contains 2000 7-dimensional data points, each dimension representing an attribute of lateral stability of the vehicle. Thus, the extended neural network consists of 7 input neurons and 3 output neurons.
In the training and identifying process of the extended neural network of the present example, a learning rate η=0.01 and the iteration number is set to 100, and each data point is sequentially input into the ENN model. The algorithm calculates the distance ED by ik It is determined whether to update the category center and the category weights. A portion of the data is used to train the ENN and the remaining data points are used to test the recognition effect of the trained ENN. The cyclic iteration of the ENN algorithm produces training errors, and fig. 12 shows the variation of the training error rate of ENN with increasing training period. The results show that the neural network is trained very quickly, and only four iterations are needed to reduce the error rate to 0.001. The error rate for 50 iterations is almost zero and convergence can be achieved. FIG. 13 shows a test for testingThe result of the comparison of the output categories of the data with their actual categories. The circled portion of the figure is the misclassified point. The curve comparison in the graph shows that the output type and the actual type of the extension neural network are almost completely consistent, which reflects that the vehicle lateral stability recognition method provided by the embodiment still has extremely high recognition accuracy after the attribute is reduced.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the invention.
Claims (2)
1. The design method of the active front wheel steering controller is characterized by comprising the following steps of:
s1: based on a two-degree-of-freedom linear vehicle model, determining a basic motion equation and a state space equation of the vehicle model on the assumption that the front wheel rotation angle is smaller and the vehicle speed is constant; the basic equations of motion for the vehicle model are:
in the above formula: beta is the slip angle;for angular velocity of slip, I z The moment of inertia around the z-axis is the moment of inertia of the automobile; omega is yaw rate, +.>Is yaw acceleration; k (k) 1 Is the total cornering stiffness of the front tyre; k (k) 2 Is the total cornering stiffness of the rear tyre; u is the longitudinal speed of the automobile; delta f Is the front wheel corner; a is the distance from the centroid to the front axis; b is the distance from the centroid to the rear axis; m is the mass of the automobile;
the state space equation is:
in the equation(s) used in the present invention,
C＝[1 1]，
in the above-described matrix of the matrix,
s2: obtaining a desired yaw rate ω from a vehicle model d And a desired slip angle beta d The method comprises the steps of carrying out a first treatment on the surface of the Both are respectively shown in the following formulas:
wherein ,
in the above formula, L is the wheelbase of the vehicle; v is the vehicle lateral speed; u is the longitudinal speed of the automobile; delta f Is the front wheel corner; a is the distance from the centroid to the front axis; b is the distance from the centroid to the rear axis; m is the mass of the automobile; k (k) 1 Is the total cornering stiffness of the front tyre; k (k) 2 Is the total cornering stiffness of the rear tyre;
s3: considering the influence of the road surface friction coefficient mu, the maximum expected yaw rate omega dmax And a maximum desired slip angle beta dmax Correcting;
s4: setting a combined deviation e of the yaw rate and the centroid slip angle of the vehicle 1 Introducing integral term into the switching function to obtain sliding mode switching function s 1 ；
Vehicle yaw rate and centroid slip angle combined deviation e 1 The calculation formula of (2) is as follows:
e 1 ＝(ω d -ω)+η(β d -β)；
in the above formula, ω is yaw rate; omega d A desired yaw rate; beta d Is the desired slip angle; beta is the slip angle; η is an adjustment parameter of a steer-by-wire system of the vehicle, η is a constant with a positive value and represents the proportion of the centroid slip angle and the yaw rate to participate in control;
the sliding mode switching function is:
in the above, e 1 The combined deviation of the yaw rate and the centroid side deviation angle of the vehicle; t is time; c is a sliding mode control and regulation parameter;
the sliding mode switching function is:
in the above formula, ω is yaw rate; omega d A desired yaw rate; beta d Is the desired slip angle; beta is the slip angle; t is time; c is a sliding mode control and regulation parameter;
s5: switching integral sliding modeThe function obtains the first derivative to obtain the first derivative function of the sliding mode switching functionSimplifying the first order derivative function; the first order derivative of the sliding mode switching function is:
wherein, for simplifying the expression, it is assumed that there is a variable Ω, let
Ω＝c[(ω d -ω)+η(β d -β)]，
The first order derivative function transforms into:
s6: bringing the basic equation of motion of a two-degree-of-freedom linear vehicle model to a first order derivative of the sliding mode switching functionUnder the condition that the slip form reaching condition is met, calculating the switching control rotation angle u of the vehicle sw And equivalent control angle u eq The control angle of the final active front wheel steering controller is delta AFS ＝u sw +u eq ；
Equivalent control angle u eq The calculation process of (2) is as follows:
in basic equations of motion of a two-degree-of-freedom linear vehicle modelCarry-in to first order derivative function->In (1), the following steps are obtained:
let the above formula be 0, then the equivalent control angle u can be solved eq ：
Switching control steering angle u of vehicle steer-by-wire system sw The following are provided:
in the above expression, η is a constant having a positive value, and represents the ratio of the centroid slip angle to the yaw rate involved in the control.
2. The design method of the active front wheel steering controller according to claim 1, characterized in that: in step S3, the maximum expected yaw rate ω is taken into consideration by the influence of the road surface friction coefficient dmax And a maximum desired slip angle beta dmax The corrected calculation formula is as follows:
in the above formula, mu is the friction coefficient of the road surface, u is the longitudinal speed of the automobile, and a is the distance from the mass center to the front axle; b is the distance from the centroid to the rear axis; m is the mass of the automobile; l is the wheelbase of the vehicle.
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