A method and device for determining a vehicle's stable cornering speed limit

Abstract

The present invention discloses a method for quickly determining the limit cornering speed of a vehicle for stable cornering, comprising the following steps: S1-1: constructing a two-degree-of-freedom nonlinear vehicle dynamics model to generate a phase plane space; S1-2: constructing a phase plane space database; S1-3: designing a closed-loop control strategy for trajectory tracking control of vehicle dynamics simulation software; S1-4: finding the maximum speed of the vehicle passing through the scene without deviating from the road and with the vehicle body stable, which is the limit cornering speed of the vehicle for stable cornering. The present invention explores the critical cornering speed index for vehicle body stability and no deviation from the road under curve conditions, which is of great significance for actually guiding vehicle operation.

Description

Technical Field

[0001] The present invention relates to the field of automobile control, and in particular to a method and a device for determining a limit cornering speed for a vehicle to corner stably under control. Background Art

[0002] The limit speed refers to the maximum speed at which a vehicle can pass through a preset scenario without deviating from the road or rolling over. In actual scenarios, the limit speed is often difficult to obtain through experiments. This is because at the limit speed, the vehicle often operates in extreme conditions and is prone to danger. At the same time, conducting large-scale experiments in actual scenarios is time-consuming and laborious.

[0003] The judgment of the speed limit involves the stability of both the vehicle body and the trajectory. Currently, the commonly used methods for judging vehicle body stability are bifurcation theory, Lyapunov function and phase plane method. Bifurcation theory and Lyapunov function rely on the formula derivation and mathematical solution of the vehicle model. In high-dimensional vehicle models, complex mathematical operations are required to estimate the saddle point (bifurcation theory method) or determine the function form (Lyapunov function method). The phase plane method does not rely on mathematical solutions. The stability boundary can be determined by simulation methods to analyze the stability of the vehicle. It is easy to expand to high-degree-of-freedom models, but the current methods are often limited to a single phase plane (considering the center of mass sideslip angle-yaw rate phase plane or the front wheel slip angle-rear wheel slip angle phase plane separately). In the case of nonlinear vehicle dynamics, the stability of the vehicle in different phase planes is inconsistent. It is difficult to comprehensively consider the multi-dimensional stability of the vehicle using a single phase plane. Summary of the invention

[0004] Based on the methods and shortcomings described in the background, the main purpose of the present invention is to provide a method for determining the maximum cornering speed for a vehicle to stably corner, which can combine simulation to give the maximum cornering speed under LQR control at which the vehicle does not deviate from the road boundary and the vehicle body does not become unstable.

[0005] The technical problem of the present invention is solved by the following technical solutions:

[0006] A method for determining a limiting cornering speed for a vehicle to corner stably comprises the following steps:

[0007] S1-1: Construct a two-degree-of-freedom nonlinear vehicle dynamics model to generate the phase plane space;

[0008] S1-2: Constructing a phase plane spatial database;

[0009] S1-3: Design closed-loop control strategies for trajectory tracking control in vehicle dynamics simulation software;

[0010] S1-4: Find the maximum speed at which the vehicle passes through the scene without deviating from the road and the vehicle body is stable. This is the maximum cornering speed for the vehicle to turn stably.

[0011] Preferably, in step S1-1, the integral relationship between the sideslip angle of the center of mass, the yaw angular velocity, the front wheel slip angle, and the rear wheel slip angle is obtained by modeling the yaw characteristics and tire characteristics of the vehicle.

[0012] Preferably, step S1-2 includes the following steps:

[0013] S1-2-1. Define the streamline stability boundary through the divergence and convergence characteristics of the trajectory in the phase plane;

[0014] S1-2-2. Traverse the speed, road adhesion coefficient and front wheel steering angle within a certain range and with a certain accuracy to build a phase plane space database.

[0015] Preferably, step S1-3 includes: designing an LQR controller and applying it to trajectory tracking control of CarSim.

[0016] Preferably, step S1-3 comprises:

[0017] S1-3-1, constructing a curved road scene;

[0018] S1-3-2, construct an LQR controller based on the linear two-degree-of-freedom dynamics model to obtain the optimal state feedback matrix;

[0019] S1-3-3, and then obtain the optimal steering angle input.

[0020] Preferably, step S1-4 includes: gradually increasing the cornering speed of CarSim under LQR control, and finding a limiting speed for the vehicle to pass the scene without deviating from the road and with the vehicle body stable.

[0021] Preferably, step S1-4 comprises:

[0022] S1-4-1. Compare the vehicle position coordinates output by CarSim with the road boundary coordinate sequence in the curved road model to determine whether the vehicle has deviated from the road;

[0023] S1-4-2, evaluate the vehicle body stability;

[0024] S1-4-3. Use the grid search method to search for the limit cornering speed and determine the limit cornering speed for the vehicle to turn stably.

[0025] Preferably, step S1-4-2 includes:

[0026] S1-4-2-1. Use the speed, front wheel steering angle output by CarSim and the road adhesion coefficient in the road model to search in the phase plane space database and obtain the stable boundary of the phase plane space under the corresponding working conditions;

[0027] S1-4-2-2, using the velocity neighborhood in the phase plane space database to determine the range of the stable boundary of the phase plane space, and dividing the phase plane space into stable regions;

[0028] S1-4-2-3, quantitatively evaluate vehicle stability using the relative position and shortest distance between the vehicle state point and the boundary of the stable region of the phase plane;

[0029] S1-4-2-4. Use the envelope area of ​​the phase plane to weight the stability results in a single phase plane to obtain a comprehensive evaluation result of the vehicle stability in the phase plane space.

[0030] A computer readable medium stores a computer program, wherein the computer program can be read and executed by a computer to implement the method described.

[0031] A device for determining a limiting cornering speed for a vehicle to corner stably comprises a processor and a memory, wherein a computer program is stored in the memory, and the computer program can be read and executed by the processor to implement the method described.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The method for determining the limit cornering speed of a vehicle for stable cornering of the present invention is a virtual testing method based on the simulation domain, which can solve the problems of high cost, long test cycle and high risk in actual road testing. Through simulation, the vehicle can be tested for passing through a scene at all possible speeds, so as to determine the limit cornering speed of the vehicle in the scene. Among them, in terms of vehicle body stability evaluation, the stability analysis in a single phase plane is extended to stability analysis in the phase plane space, avoiding the problem of inconsistency in stability evaluation of a single phase plane due to high nonlinearity. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a flow chart of a method for determining a limiting cornering speed for a vehicle to corner stably according to the present invention;

[0035] Figure 2 It is a specific flow chart of the method for quickly determining the limit cornering speed of a vehicle in stable cornering under LQR control of the present invention;

[0036] Figure 3 is a schematic diagram of a two-degree-of-freedom nonlinear vehicle dynamics model used in the present invention to generate a phase plane space;

[0037] Figure 4 Schematic diagram of a linear two-degree-of-freedom dynamic model considering road constraints used in the LQR control of the present invention;

[0038] Figure 5 It is a flow chart for evaluating vehicle stability using phase plane space. DETAILED DESCRIPTION

[0039] The present invention is further described below with reference to the accompanying drawings and in combination with preferred embodiments. It should be noted that the embodiments and features in the embodiments of the present application can be combined with each other without conflict.

[0040] It should be noted that the directional terms such as left, right, up, down, top, and bottom in this embodiment are merely relative concepts or are based on the normal use status of the product, and should not be considered as restrictive.

[0041] The present invention is described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can also be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0042] Under extreme conditions, the vehicle has a high degree of nonlinearity, and the use of a single phase plane to evaluate the vehicle body stability is prone to the situation where one phase plane is stable while another phase plane is unstable. In view of this, the present invention extends the stability evaluation in a single phase plane to the stability evaluation in the phase plane space, avoiding the problem of inconsistency in the stability evaluation of a single phase plane caused by the high degree of nonlinearity, and obtaining consistent evaluation results.

[0043] like Figure 1-5 As shown, an embodiment of the present invention provides a method for determining a limit cornering speed for a vehicle to corner stably, and the specific steps are as follows:

[0044] Step 1: Construct a two-degree-of-freedom nonlinear vehicle dynamics model to generate the phase plane space. Preferably, in step 1, by modeling the yaw characteristics and tire characteristics of the vehicle, the integral relationship of the center of mass sideslip angle, yaw angular velocity, front wheel slip angle, and rear wheel slip angle is obtained to generate the center of mass sideslip angle-yaw angular velocity, front wheel slip angle-rear wheel slip angle, center of mass sideslip angle-center of mass sideslip angular velocity three types of phase planes to constitute the phase plane space. Specifically, the tire characteristics of the vehicle use the Fiala tire model, and the vehicle dynamics model (i.e., the two-degree-of-freedom nonlinear vehicle dynamics model) focuses on the yaw modeling of the vehicle.

[0045] Among them, the two-degree-of-freedom nonlinear vehicle dynamics model is as follows Figure 3As shown in Figure 2, the model mainly measures the yaw characteristics of the vehicle, specifically:

[0046]

[0047]

[0048] in represents the vehicle's center of mass side slip angular velocity; m represents the vehicle's mass; V x Indicates the longitudinal speed of the vehicle; F yr Indicates the lateral force on the rear wheel; F yf represents the lateral force on the front wheel; r represents the yaw angular velocity of the vehicle; Indicates the vehicle's yaw angular acceleration; I z Indicates the vertical moment of inertia of the vehicle; l f Indicates the distance from the center of mass of the vehicle to the front wheel; l r Represents the distance from the vehicle's center of mass to the rear wheels.

[0049] Lateral force F on the tire y The relationship between the side slip angle α is given by the Fiala tire model, specifically:

[0050]

[0051] Among them C α represents the cornering stiffness of the vehicle tire; α represents the side slip angle of the vehicle tire; μ represents the road adhesion coefficient; F z Represents the vertical force on the vehicle tire. sl Indicates the tire slip angle when the tire is saturated.

[0052] The tire slip angle when the tire is saturated is given by the following formula:

[0053]

[0054] Vertical load on the front wheel F zf and the vertical load F on the rear wheel zr They are given by the following formulas:

[0055]

[0056]

[0057] Front wheel slip angle α f and the rear wheel slip angle α r They are given by the following formulas:

[0058]

[0059]

[0060] Among them, δ is the front wheel turning angle, and β is the vehicle's center of mass sideslip angle.

[0061] Step 1: Use the two-degree-of-freedom nonlinear vehicle dynamics model to calculate the center of mass sideslip angle-yaw rate (β-r), front wheel slip angle-rear wheel slip angle (α f -α r ), center of mass sideslip angle-center of mass sideslip angular velocity Three types of phase planes are generated to form a phase plane space. Specifically, by selecting different initial values ​​and using and The state equation is differentiated and calculated. Under different initial values, the ode45 function in Matlab is used to iteratively calculate different state parameter sequences to generate the center of mass sideslip angle-yaw rate phase plane; the front wheel slip angle-rear wheel slip angle phase plane is iteratively calculated by the front and rear wheel sideslip angle formula; the center of mass sideslip angle-center of mass sideslip rate is obtained by the aforementioned The state equation is calculated and iterated.

[0062] Step 2: Construct phase plane space database.

[0063] The specific process is as follows:

[0064] Step 201: Through the divergence and convergence characteristics of the trajectory in the phase plane, a streamlined stable boundary is simulated. The trajectory in the phase plane is a time series of parameters under different initial values, where V i The two outermost convergence trajectories are selected as stable boundaries. That is, in step 201, the outermost convergence curve is found, and the trajectory in the phase plane is directly used as the stable boundary (the trajectory is obtained by integral iteration, and the generation method is consistent with other methods (such as Lyapunov method, etc.), and the stable boundary is obtained by judging the convergence of each curve). Through step 201, the stable boundary in the stability analysis is given by numerical methods, avoiding formula derivation and mathematical solution (for example, Lyapunov method needs to derive formulas to find Lyapunov functions and then determine the stable boundary, and bifurcation theory needs to derive formulas and use Newton's analytical method to estimate the saddle node position and then determine the stable boundary), which is easy to expand to high-degree-of-freedom vehicle models.

[0065] Step 202: Traverse the speed, road adhesion coefficient, and front wheel steering angle within a certain range and accuracy to construct a phase plane space database. Specifically, a phase plane space database covering all working conditions is obtained by traversing the speed, front wheel steering angle, and road adhesion coefficient. For example, the range and step size can be freely selected according to the variation range of the speed, road adhesion coefficient, and front wheel steering angle in the actual working condition, and the desired accuracy requirements. In the present invention, it is intended to cover conditions from conventional working conditions to extreme working conditions. The speed uses 0.5m / s-50m / s with a step size of 0.5m / s; the road adhesion coefficient uses 0.1-0.9 with a step size of 0.1; the front wheel steering angle uses -10deg-50deg with a step size of 0.5deg. In the phase plane space database, the speed, road adhesion coefficient, and front wheel steering angle are used as indexes to store the phase plane space stability boundary (i.e., the streamlined stability boundary) obtained by step 201 under the index combination.

[0066] Step 3: Design a closed-loop control strategy for the trajectory tracking control of the vehicle dynamics simulation software. Preferably, an LQR (linear quadratic regulator) controller is designed based on CarSim and applied to the vehicle's trajectory tracking control (i.e., an LQR controller is designed and applied to the trajectory tracking control of CarSim).

[0067] Specifically:

[0068] Step 301: Construct a curved road scene (i.e. Figure 5 The road boundary is expressed using a sequence of (x,y) coordinates. The road adhesion coefficient is input into the CarSim model (i.e. Figure 5 in the Vehicle Model section).

[0069] Step 302: construct an LQR controller. Specifically, construct an LQR controller based on a linear two-degree-of-freedom dynamics model to obtain an optimal state feedback matrix K=(k1 k2 k3 k4), and then obtain an optimal steering angle input.

[0070] Among them, the schematic diagram of the linear two-degree-of-freedom dynamic model is as follows Figure 4 As shown, the state transfer matrix with negative feedback control is briefly recorded as follows:

[0071]

[0072] The optimal front wheel steering angle control amount is given by the following formula:

[0073]

[0074] in e cg is the orthogonal straight line distance from the vehicle's center of mass to the edge of the road, θe is the heading error (θ e =θ-θ p (s), where θ p (s) is the tangent angle from the center of mass of the vehicle to the orthogonal point of the road edge, θ is the angle between the vehicle axis and the horizontal axis of the global coordinate system), δ is the front wheel turning angle, is the expected yaw rate at the current running speed and current position. A, B, and C are given by the linear two-degree-of-freedom dynamic model, and the meaning of each parameter is consistent with that in step 1.

[0075]

[0076]

[0077]

[0078] Define the following objective function:

[0079]

[0080] Where Q is the state weight coefficient, R is the control weight coefficient, and k marks the state sequence number. In view of the influence of road following error and heading error on the control quantity, the weight matrix Q is selected as follows:

[0081]

[0082] The input control variable is the front wheel steering angle, and the weight matrix R degenerates into a constant coefficient, and the value R is set to 1.

[0083] When the objective function J is minimum, the optimal state feedback matrix K is obtained, which is given by the following formula:

[0084] K=(R+B T PB) -1 B T PA

[0085] The matrix P satisfies the Riccati equation

[0086] P=A T PA-A T PB(R+B T PB) -1 B T PA+Q

[0087] The lqr function in MATLAB can be used to solve and obtain the optimal front wheel steering angle control value under the current vehicle state.

[0088] Step 4: Search for the limit cornering speed in the simulation domain, that is, use the grid search method to search for the maximum speed at which the vehicle passes through the scene without deviating from the road and the body is stable. This is the limit cornering speed for the vehicle to pass the corner stably. Specifically, it includes: gradually increasing the cornering speed of CarSim under LQR control, searching the speed from large granularity to small granularity through the grid search method, and obtaining the maximum speed at which the vehicle passes through the scene without deviating from the road and the body is stable.

[0089] Specifically:

[0090] Step 401: Compare the vehicle position coordinates output by CarSim with the curved road model (i.e. Figure 5 The road boundary coordinate sequence in the "road model" in the vehicle model is compared to determine whether the vehicle has deviated from the road.

[0091] Step 402: Perform vehicle body stability evaluation. The specific process is as follows: Figure 5 As shown, including:

[0092] Step 402-1: Retrieve the stable boundary of the phase plane space under the corresponding working condition in the phase plane space database through the vehicle state and the road model. Specifically, the speed, front wheel steering angle output by CarSim, and the road adhesion coefficient in the road model are used to search in the phase plane space database constructed in step 2, and the stable boundary of the phase plane space under the corresponding working condition is obtained. Step 402-2: Determine the range of the stable boundary of the phase plane space using the speed neighborhood in the phase plane space database, and divide the stable region of the phase plane space. Near the stable boundary, the nonlinearity of the vehicle is strong, and there is a certain error in the two-degree-of-freedom nonlinear model. This error is inevitable, resulting in the inaccuracy of the stable boundary defined in step 201, so a fuzzy region is designed as the critical stable region using the speed neighborhood. The speed neighborhood refers to two phase plane spaces in the phase plane space database that are separated from the specified phase plane space by a speed step. The area between the stable boundaries of the two phase plane spaces is defined as the critical stable region, and the expansion of the stable boundary is achieved to obtain a more conservative stable region.

[0093] Step 402-3: Quantitatively evaluate the vehicle stability using the relative position and shortest distance between the vehicle state point and the boundary of the stable region of the phase plane. Vehicle state parameters: center of mass slip angle, yaw velocity, front wheel slip angle, rear wheel slip angle, center of mass slip angle angular velocity correspond to a state point in the center of mass slip angle-yaw velocity phase plane, front wheel slip angle-rear wheel slip angle phase plane, center of mass slip angle-center of mass slip angle velocity phase plane, respectively. When the state point is located inside the stable region of the phase plane, the vehicle state in the phase plane is evaluated as stable; when the state point is located inside the critical stable region and unstable region of the phase plane, the vehicle state in the phase plane is evaluated as unstable. Calculate the shortest distance between the state point and the boundary of the stable region as the quantitative evaluation result of the stability of the vehicle state at that moment in the phase plane. For unstable states, the quantitative evaluation result takes the opposite number (negative value) of the shortest distance.

[0094] Step 402-4: Use the envelope area of ​​the phase plane to weight the stability results in a single phase plane to obtain a comprehensive evaluation result of vehicle stability in the phase plane space, that is, use the normalized weighted method to unify the inconsistent results that may exist in different phase planes to obtain a consistent evaluation result. The positive and negative evaluation results correspond to the stability and instability of the state respectively.

[0095] Step 403: Use the grid search method to search for the limit cornering speed. Specifically, first gradually increase the vehicle's cornering speed with a larger granularity, and find the speed range where the vehicle becomes unstable through the judgment method of steps 401 and 402; adjust the vehicle's cornering speed with a smaller granularity within the range to determine the limit speed of the vehicle's stable cornering.

[0096] Step 4: By traversing the vehicle's cornering speed under LQR control, the stability quantitative evaluation model and road constraints are used to comprehensively evaluate the parameter sequence of the vehicle during driving, and the maximum cornering speed of the vehicle under LQR control is obtained.

[0097] An embodiment of the present invention further provides a computer-readable medium storing a computer program, which can be read and executed by a computer to implement the method for determining the limiting cornering speed for stable cornering of a vehicle as described in the above embodiment.

[0098] An embodiment of the present invention also provides a device for determining a maximum cornering speed for a vehicle to corner stably, which includes a processor and a memory, wherein the memory stores a computer program, and the computer program can be read and executed by the processor to implement the method for determining the maximum cornering speed for a vehicle to corner stably as described in the above embodiment.

[0099] The above embodiment is for determining the critical speed of the vehicle for stable cornering by LQR control. In other variant examples, other closed-loop control strategies may also be designed, such as PID (Proportional Integral Derivative) control.

[0100] The above contents are further detailed descriptions of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art of the present invention, several equivalent substitutions or obvious variations can be made without departing from the concept of the present invention, and the performance or use is the same, which should be regarded as belonging to the protection scope of the present invention.

Claims

1. A method for determining a vehicle's maximum cornering speed for stable cornering, characterized in that: The steps include: S1-1: Construct a two-degree-of-freedom nonlinear vehicle dynamics model to generate the phase plane space; S1-2: Constructing a phase plane spatial database; S1-3: Design closed-loop control strategies for trajectory tracking control in vehicle dynamics simulation software; S1-4: Find the maximum speed of the vehicle passing through the scene without deviating from the road and the vehicle body is stable, which is the maximum cornering speed of the vehicle for stable cornering; Step S1-4 includes: S1-4-1. Compare the vehicle position coordinates output by CarSim with the road boundary coordinate sequence in the curved road model to determine whether the vehicle has deviated from the road; S1-4-2, evaluate the vehicle body stability; S1-4-3. Use the grid search method to search for the limit cornering speed and determine the limit cornering speed for the vehicle to turn stably.

2. The method for determining the maximum cornering speed for a vehicle to pass a corner stably according to claim 1, characterized in that: In step S1-1, the integral relationship between the sideslip angle of the center of mass, the yaw angular velocity, the front wheel slip angle, and the rear wheel slip angle is obtained by modeling the yaw characteristics and tire characteristics of the vehicle.

3. The method for determining the limit cornering speed for a vehicle to pass a corner stably according to claim 1, characterized in that: Step S1-2 includes the following steps: S1-2-1. Define the streamline stability boundary through the divergence and convergence characteristics of the trajectory in the phase plane; S1-2-2. Traverse the speed, road adhesion coefficient and front wheel steering angle within a certain range and with a certain accuracy to build a phase plane space database.

4. The method for determining the maximum cornering speed for a vehicle to pass a corner stably according to claim 1, characterized in that: Step S1-3 includes: designing an LQR controller and applying it to trajectory tracking control of CarSim.

5. The method for determining the limit cornering speed for a vehicle to pass a corner stably as claimed in claim 4, characterized in that: Step S1-3 includes: S1-3-1, constructing a curved road scene; S1-3-2, construct an LQR controller based on the linear two-degree-of-freedom dynamics model to obtain the optimal state feedback matrix; S1-3-3, and then obtain the optimal steering angle input.

6. The method for determining the limit cornering speed for a vehicle to pass a corner stably as claimed in claim 1, characterized in that: Step S1-4 includes: gradually increasing the cornering speed of CarSim under LQR control, and finding the limiting speed for the vehicle to pass the scene without deviating from the road and with the vehicle body stable.

7. The method for determining the limit cornering speed for a vehicle to pass a corner stably as claimed in claim 1, characterized in that: Step S1-4-2 includes: S1-4-2-1. Use the speed, front wheel steering angle output by CarSim and the road adhesion coefficient in the road model to search in the phase plane space database and obtain the stable boundary of the phase plane space under the corresponding working conditions; S1-4-2-2, using the velocity neighborhood in the phase plane space database to determine the range of the stable boundary of the phase plane space, and dividing the phase plane space into stable regions; S1-4-2-3, quantitatively evaluate vehicle stability using the relative position and shortest distance between the vehicle state point and the boundary of the stable region of the phase plane; S1-4-2-4. Use the envelope area of ​​the phase plane to weight the stability results in a single phase plane to obtain a comprehensive evaluation result of the vehicle stability in the phase plane space.

8. A computer-readable medium, characterized in that It stores a computer program, which can be read and executed by a computer to implement the method according to any one of claims 1 to 7.

9. A device for determining a vehicle's maximum cornering speed for stable cornering, characterized in that: The method comprises a processor and a memory, wherein the memory stores a computer program, and the computer program can be read and executed by the processor to implement the method according to any one of claims 1 to 7.

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