Drifting control method for rear-drive vehicle based on steer-by-wire

Abstract

The invention is suitable for the technical field of vehicle control, and particularly relates to a rear-drive vehicle drift control method based on steer-by-wire, and the method comprises the steps: activating a drift function, and selecting a drift mode; according to the drifting mode selected by the user, judging a drifting starting condition, and when the drifting starting condition is met, prompting the user; when a user determines to perform drifting, constructing a three-degree-of-freedom vehicle dynamics model, calculating a drifting steady-state balance point, and outputting an expected drifting balance point; when drifting begins, drifting starting assistance is carried out, drifting process control is carried out in the drifting process, and drifting process control comprises longitudinal control and transverse control. According to the invention, the driver is actively helped to complete the reverse hitting action, a certain active operation space for the driver is reserved on the basis of reducing the operation difficulty, and the driving fun of man-machine interaction is reserved.

Description

Technical Field

[0001] The present invention belongs to the technical field of vehicle control, and particularly relates to a drift control method for a rear-wheel drive vehicle based on steer-by-wire Background Art

[0002] Currently, the control of drifting generally requires manually turning off the chassis electronic stability system (such as ESP / TCS, etc.). The control of drifting generally relies on the driver's precise control of steering and throttle, with high operation difficulty and strong dependence on driving experience. During the conventional drifting starting process, as the tire force of the rear wheels saturates, the driver needs to counter-steer to enter the drifting state. However, counter-steering goes against the subjective operation intuition, and the speed, timing, and magnitude of counter-steering are the keys to entering the drifting state, which requires a large amount of practice. After entering the drifting process, the driver needs to continuously adjust the direction to maintain the drifting process. During the drifting process, the longitudinal slip of the rear wheels is strongly coupled with the steering angle of the front wheels. Therefore, it is necessary to precisely control the steering angle of the front wheels according to the force condition of the rear wheels, which undoubtedly increases the difficulty of the driver's coordinated control of the throttle and steering. The existing electronic control systems for drift control are generally auxiliary systems. Due to the limitation of the steering mechanical structure, they mainly coordinate the control for drive torque distribution, with limited system control degree and high requirements for the driver's experience. Summary of the Invention

[0003] The purpose of the present invention is to provide a drift control method for a rear-wheel drive vehicle based on steer-by-wire, aiming to solve the problem of the drift control method for a rear-wheel drive vehicle based on steer-by-wire.

[0004] The present invention is implemented as follows. A drift control method for a rear-wheel drive vehicle based on steer-by-wire, the method comprising:

[0005] Activating the drift function and selecting a drift mode, the drift mode including a free drift mode, a semi-active drift mode, and an active drift mode;

[0006] Judging the starting drift conditions according to the selected drift mode by the user, and giving a prompt to the user when the starting drift conditions are met;

[0007] When the user determines to perform drifting, constructing a three-degree-of-freedom vehicle dynamics model, calculating the drift steady-state equilibrium point, and outputting the expected drift equilibrium point;

[0008] When starting to drift, performing starting drift assistance, and performing drift process control during the drifting process, the drift process control including longitudinal control and lateral control.

[0009] Preferably, the starting drift conditions are that the vehicle speed reaches a preset vehicle speed and the vehicle is performing a steady-state circular motion within a preset radius range, and at this time, a prompt is given to the user through an indicator light.

[0010] Preferably, in the step of constructing a three-degree-of-freedom vehicle dynamics model, the center of mass sideslip angle β, yaw rate γ and longitudinal velocity V x A three-degree-of-freedom dynamic model is constructed for the state variables. The three-degree-of-freedom dynamic model is expressed as:

[0011]

[0012] in, and They are the sideslip angle β, yaw rate γ and longitudinal velocity V x The first derivative of yf is the lateral force of the front wheel, F yr is the lateral force of the rear wheel, m is the mass of the vehicle, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass, I z is the moment of inertia, and δ is the front wheel steering angle.

[0013] Preferably, in the step of constructing a three-degree-of-freedom vehicle dynamics model, a tire brush model is selected to describe the tire longitudinal force and lateral force, and the tire brush model is expressed as:

[0014]

[0015] Among them, α f is the front wheel slip angle, α r is the rear wheel slip angle, V y is the lateral velocity at the center of mass, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass; C α is the tire cornering stiffness, μ is the friction coefficient between the ground and the tire, F z is the tire vertical force, α is the tire side slip angle, α sl is the critical slip angle of the tire.

[0016] Preferably, in the step of calculating the drift steady-state equilibrium point and outputting the expected drift equilibrium point, for a nonlinear system, let:

[0017]

[0018] The steady-state condition is obtained:

[0019]

[0020] According to the relationship between the vehicle state and the turning radius in steady state, the expected drift equilibrium point can be obtained by the simultaneous equations:

[0021] The expected equilibrium point is: β', γ', V x ', λ' and δ', where β', γ', Vx β’, γ’ and δ’ are the expected values of the centroidal sideslip angle β, yaw rate γ, longitudinal velocity V x , tire slip ratio λ, and front wheel steering angle δ, respectively.

[0022] Preferably, in the semi-active drift mode, when performing drift assist, after the vehicle enters the steady-state circular motion and the driver deeply presses the accelerator pedal, the rear axle starts to slide. The theoretical front wheel steering angle is obtained according to the dynamic equation. The active front wheel steering control module controls the active front wheel steering to complete the counter-steering according to the deviation value, and the end of the counter-steering means the end of the drift assist.

[0023] Preferably, in the active drift mode, after the vehicle enters the steady-state circular motion, the driver deeply presses the accelerator pedal, and the deep press threshold is set as a calibration parameter. When the accelerator pedal depth exceeds the threshold, the accelerator pedal opening only serves as a switch quantity, and the vehicle does not respond to specific opening requests. Once the opening exceeds the threshold, the system automatically intervenes to complete the drift assist. When the accelerator pedal is completely released, it is considered that the drift function terminates and the driver takeover is restored.

[0024] Preferably, in the semi-active drift mode, the objective of longitudinal control is the rear wheel slip ratio λ and the longitudinal velocity V x , which is controlled by the driver. After the driver counter-steers the steering wheel, the vehicle enters the non-steady-state drift process. At this time, the degree of rear axle side slip is controlled by the driver. The expected rear axle slip ratio is calculated according to the initial steady-state circular motion vehicle speed and curvature radius. According to the expected rear axle slip ratio and the current rear axle slip ratio, with the driver output torque T r as the target, the expected torque is calculated by PID. According to the expected torque, the accelerator pedal opening corresponding to the expected torque is inversely resolved by looking at the drivability MAP. The accelerator pedal opening corresponding to the expected torque is the expected accelerator pedal opening. The user is prompted according to the difference between the expected accelerator pedal opening and the actual accelerator pedal opening.

[0025] Preferably, in the semi-active drift mode and the active drift mode, during the lateral control process, the driver only needs to maintain the steering wheel angle, and the front wheel steering angle will be actively adjusted by the active front wheel control module. The specific lateral control quantity is the front wheel steering angle δ. The three-degree-of-freedom vehicle dynamics model combined with the tire brush model calculates the theoretical front wheel steering angle δ according to the difference between the expected centroidal sideslip angle β', expected yaw rate γ' and the actual centroidal sideslip angle β, yaw rate γ, and the actual rear axle slip ratio λr, longitudinal velocity Vx as inputs, and corrects it with PID control according to the difference Δδ between the theoretical front wheel steering angle and the actual front wheel steering angle.

[0026] Preferably, in the active drift mode, during longitudinal control, when the driver deeply steps on the accelerator pedal and exceeds the set threshold during steady-state circular driving, both the longitudinal control function and the lateral control function are activated and coordinated. The longitudinal control variables are the rear-wheel slip ratio λ and the longitudinal speed Vx. The three-degree-of-freedom vehicle dynamics model combined with the tire brush model calculates the control torque Tr based on the difference between the desired rear-wheel slip ratio λ and the actual rear-wheel slip ratio λ as the input.

[0027] The drift control method for a rear-wheel drive vehicle based on steer-by-wire provided by the present invention can achieve starting drift assistance, actively help the driver complete the counter-steering action, actively correct the front-wheel steering angle after entering the drift process, reduce the operation difficulty, and guide the driver to control the accelerator pedal opening through the prompt system to maintain the drift process. The process that requires the driver to precisely coordinate the throttle and steering to complete the drift action is simplified to only need to follow the throttle response according to the system prompt to complete the starting drift and maintain the drift. On the basis of reducing the operation difficulty, a certain space for the driver's active operation is reserved, and the driving pleasure of human-machine interaction is retained. Brief Description of the Drawings

[0028] Figure 1 It is the first schematic diagram of the display interface provided by the embodiment of the present invention;

[0029] Figure 2 It is the second schematic diagram of the display interface provided by the embodiment of the present invention;

[0030] Figure 3 It is the functional schematic diagram of different drift modes provided by the embodiment of the present invention;

[0031] Figure 4 It is the flowchart of the semi-active drift mode provided by the embodiment of the present invention;

[0032] Figure 5 It is the flowchart of the active drift mode provided by the embodiment of the present invention;

[0033] Figure 6 It is the schematic diagram of the working principle of the semi-active drift mode provided by the embodiment of the present invention;

[0034] Figure 7 It is the first part of the schematic diagram of the working principle of the active drift mode provided by the embodiment of the present invention;

[0035] Figure 8 It is the second part of the schematic diagram of the working principle of the active drift mode provided by the embodiment of the present invention;

[0036] Figure 9 It is the model schematic diagram of the vehicle drift process provided by the embodiment of the present invention. Detailed Embodiments

[0037] To make the objectives, technical solutions and advantages of the present invention more clear and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0038] It can be understood that the terms "first", "second", etc. used in this application may be used herein to describe various elements, but unless otherwise specified, these elements are not limited by these terms. These terms are only used to distinguish a first element from another element. For example, without departing from the scope of this application, the first xx script may be referred to as the second xx script, and similarly, the second xx script may be referred to as the first xx script.

[0039] As Figure 3 、 Figure 4 and Figure 5 shown, a rear-wheel drive vehicle drift control method based on steer-by-wire is provided in an embodiment of the present invention. The method includes:

[0040] Activate the drift function and select a drift mode, where the drift mode includes a free drift mode, a semi-active drift mode, and an active drift mode;

[0041] According to the selected drift mode by the user, determine the starting drift conditions, and when the starting drift conditions are met, prompt the user;

[0042] When the user determines to perform a drift, construct a three-degree-of-freedom vehicle dynamics model, calculate the drift steady-state equilibrium point, and output the expected drift equilibrium point;

[0043] When starting to drift, perform starting drift assistance, and perform drift process control during the drift. The drift process control includes longitudinal control and lateral control.

[0044] In the embodiment of the present invention, as Figure 1 and Figure 2 shown, a UI interface is set on the vehicle. The drift function is activated through the UI interface to select a drift speed, and then the vehicle steadily travels in a circle at the set speed. At this time, the UI interface should dynamically display the wheel angle, the radius of the circle being traveled, the driving trajectory, the actual throttle pedal opening, the expected throttle pedal opening, and the drift status indicator icon.

[0045] The drift function settings are divided into: fully open (free drift mode), semi-intervention (semi-active drift mode), full intervention (active drift mode), and a drift vehicle speed limit is set for semi-active and active drifts (the purpose is to preset the speed to ensure the expectation and control of the drift vehicle speed and ensure the safety of the drift process).

[0046] As Figure 3As shown in the figure, the associated settings are as follows: If the free drift mode is selected, the electronic stability system and the traction control system should be turned off; if the semi-active drift mode is selected, the electronic stability system and the traction control system should be turned off and the lateral control function should be activated; if the active drift mode is selected, the electronic stability system and the traction control system should be turned off and the lateral control function and the active drift mode activation should be activated.

[0047] Drift start condition judgment (for semi-active drift mode): If the vehicle speed reaches the set speed and the vehicle makes a steady-state circular turn at a certain radius, it is considered that the drift start condition is met, and the UI (Drift Ready) indicator light is lit.

[0048] Drift start condition judgment (for active drift mode): If the vehicle speed reaches the set speed and the vehicle makes a steady-state circular turn at a certain radius, it is considered that the drift start condition is met, and the UI (Drift Ready) indicator light is lit.

[0049] Steady-state drift parameter determination (for semi-active drift mode):

[0050] As Figure 9 shown, a three-degree-of-freedom dynamic model with the sideslip angle at the center of mass, yaw rate, and longitudinal velocity as state variables is established based on the three-degree-of-freedom vehicle dynamics model:

[0051]

[0052] Among them, and are the first derivatives of the sideslip angle at the center of mass β, yaw rate γ, and longitudinal velocity V x respectively; F yf is the lateral force of the front wheels, F yr is the lateral force of the rear wheels, m is the total vehicle mass, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass, I z is the moment of inertia, and δ is the front wheel steering angle.

[0053] The tire brush model is selected to describe the longitudinal force and lateral force of the tire:

[0054]

[0055]

[0056] Among them, α f is the front wheel sideslip angle, α r is the rear wheel sideslip angle, V y is the lateral velocity at the center of mass, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass; C αis the tire cornering stiffness, μ is the friction coefficient between the ground and the tire, F z is the tire vertical force, α is the tire side slip angle, α sl is the critical slip angle of the tire.

[0057] Drift steady-state equilibrium point solution:

[0058] For nonlinear systems, let:

[0059]

[0060] The steady-state condition is obtained:

[0061]

[0062] According to the relationship between the vehicle state and the turning radius in steady state, the expected drift equilibrium point can be obtained by the simultaneous equations: The expected equilibrium point is: β', γ', V x ', λ' and δ', where β', γ', V x ', λ' and δ' are respectively the sideslip angle β, yaw rate γ, and longitudinal velocity V x , the expected values ​​of tire slip λ and front wheel steering angle δ.

[0063] The determination of steady-state drift parameters for the active drift mode is the same as that for the semi-active drift mode and will not be described in detail.

[0064] During the drift assist process, the drift assist (for semi-active drift mode (such as Figure 6 Shown)):

[0065] When the vehicle enters the steady-state circle, the driver steps on the accelerator deeply. At this time, the rear axle begins to slide. The system obtains the theoretical front wheel angle according to the dynamic equation. The active front wheel angle control module controls the active steering of the front wheel to complete the reverse steering according to the deviation value (theoretical and current front wheel angle). The completion of the reverse steering is the end of the drift assistance. There are two characteristic points here: 1. When the system intervenes to calculate the theoretical front wheel angle; 2. The transition from the auxiliary drift to the steady-state drift. For feature 1, it is judged by the size of the slip rate |λ|. This value needs to be calibrated and determined according to the vehicle type; for feature 2, the end of the active reverse steering of the front wheel is the end of the auxiliary drift process, and the process prompts the driver through the dynamic display of the front wheel angle on the UI interface. Since the reverse steering of the front wheel angle without a mechanical structure does not cause the driver's steering wheel to move, how the steering wheel hand force operated by the driver changes when the front wheel angle changes is not within the scope of discussion of the present invention. The purpose of the present invention is to achieve active steering of the front wheel without causing the driver to notice while maintaining the current steering wheel angle, so as to reduce the difficulty of the driver's operation.

[0066] Drift assist (for active drift mode (such as Figure 7 and 8 Shown)):

[0067] When the vehicle enters the steady state of circular motion, the driver can step on the accelerator deeply. The deep-step threshold is set as a calibration parameter, and the specific value is adjusted according to different vehicle models and product definitions. After exceeding the threshold, the throttle opening only acts as a switch quantity, and the vehicle does not respond to specific opening requests. Once the opening exceeds the threshold, the system automatically intervenes to complete the drifting assist. Releasing the accelerator pedal completely is considered the termination of the drifting function, and the driver takeover will be restored. There are two features here: 1. The trigger of the full drifting function is based on the UI (DriftReady) light being lit first, and then the driver steps on the accelerator deeply. After exceeding the limit threshold, the vehicle enters the active drifting mode; 2. Releasing the accelerator completely can be judged that the driver hopes to exit the drifting function; the specific implementation is comprehensively judged by the enabling module according to the input front wheel angle, vehicle speed, and throttle opening.

[0068] During the drifting process, the vehicle is controlled for the drifting process:

[0069] For the semi-active drifting mode: Longitudinal control process:

[0070] The longitudinal control targets are the rear wheel slip ratio λ and the longitudinal speed Vx. These variables are both controlled by the driver, but the system designs a prompt system to assist the driver in control to reduce the driver's operation difficulty.

[0071] After counter-steering, the vehicle enters the non-steady state drifting process. At this time, the degree of rear axle sideslip is controlled by the driver. Too small or too large sideslip cannot maintain the turning curvature radius of the initial steady state circular motion for circular drifting. Therefore, a throttle opening estimation system and a driver reminder system are designed. The system calculates the expected rear axle slip ratio based on the initial steady state circular motion vehicle speed and curvature radius. The expected AP estimation module calculates the expected torque according to the expected rear axle slip ratio and the current rear axle slip ratio, with the driver output torque Tr as the target basis using PID. The expression of the expected torque is:

[0072] T r =k p Δλ r +k I ∫Δλ r dt+k f λ rd

[0073] Then, according to the expected torque, the corresponding AP (accelerator pedal) opening of the expected torque is reversely analyzed by looking at the drivability MAP diagram. The AP opening corresponding to the expected torque is the expected AP opening, and Δλ r is the difference between the expected rear axle slip ratio and the actual rear axle slip ratio.

[0074] The implementation steps for reverse-analyzing the desired AP opening through the desired torque input to the drivability MAP look-up table are as follows: The desired AP estimation module calculates the desired torque and sends it to the VCU together with the current longitudinal speed Vx, requesting to obtain the AP opening. The VCU looks up the table based on Tr and Vx and sends the corresponding AP opening value to the desired AP estimation module; the desired AP estimation module independently creates a relevant MAP diagram based on the VCU parameters; 3. The desired AP estimation module directly maps the VCU drivability MAP to this module.

[0075] As Figure 1 and Figure 2 shown, driver prompt: Judge the feedback direction based on the desired AP% and the actual AP%. If the actual AP is less than the desired value, prompt that the throttle opening is too small; otherwise, prompt that it is too large. Prompt scheme: 1. Cockpit voice prompt (too small: prompt to increase the throttle opening, too large: prompt to decrease the throttle opening); 2. Display prompt information in the UI interface; 3. Display the desired AP% information and the actual AP% information in the UI interface, both in the form of dynamic sliding. The driver can dynamically track the desired AP% to achieve the steady-state drift target. The above three prompts can be separated or coexist. Regarding the magnitude of the AP% difference: For 1, three voice prompt intervals can be set (prompt words can be: please increase the throttle opening slightly, please increase the throttle opening, please increase the throttle opening significantly; the situation for decreasing the prompt is similar); for 2, considering that the driver needs to concentrate on the ground information, only two gears of increasing / decreasing the throttle are set; for 3: Instead of directly displaying the difference, compare and display the desired AP% and the actual AP%, and guide the driver in a dynamic following manner.

[0076] Lateral control process: During the lateral control process, the driver only needs to maintain the steering wheel angle, and the front wheel angle will be actively adjusted by the active front wheel control module. Specifically: The lateral control quantity is the front wheel angle δ. The three-degree-of-freedom model combined with the tire model calculates the theoretical front wheel angle δ based on the differences between the desired β', desired γ' and the actual β, γ, and the actual λr, Vx as inputs. The active front wheel control module uses PID control to correct based on the difference Δδ between the theoretical front wheel angle and the actual front wheel angle.

[0077] The calculation formula for the theoretical front wheel angle is:

[0078] δ = k p Δδ + k I ∫Δδdt + k f δ d

[0079] For the drift process control, its characteristics are also reflected in that the lateral control is actively completed by the system, and the driver only needs to maintain the initial steady-state steering wheel angle for turning the circle. The longitudinal control is mainly carried out by the driver. Through theoretical calculation and the human-machine interaction system, the driver is prompted to further stabilize the control to achieve the goal of stable drift control.

[0080] In the active drift mode, the active drift process control and the starting drift assistance can be regarded as the same process and no longer distinguished.

[0081] For the lateral control: The method is the same as that in the semi-active drift mode and will not be elaborated here.

[0082] For the longitudinal control: When the driver enters the steady-state turning the circle and deeply steps on the accelerator pedal and exceeds the set threshold, the longitudinal control function and the lateral control function are activated simultaneously and controlled collaboratively.

[0083] Specifically: The longitudinal control quantity is the rear-wheel slip ratio λ and the longitudinal speed Vx. The three-degree-of-freedom model combined with the tire model calculates the control torque Tr based on the difference between the expected rear-wheel slip ratio λ and the actual rear-wheel slip ratio λ as the input. The actual algorithm uses PID control:

[0084] T r =k p Δλ r +k I ∫Δλ r dt+k f λ rd

[0085] The active drift process is the combined control of the lateral function and the longitudinal function. Its characteristics are: 1. By dividing the steady-state drift parameters, λr and Vx are the longitudinal control inputs, and λr, Vx, β, and γ are the lateral control inputs. Finally, the theoretical front-wheel steering angle δ and the torque Tr are obtained respectively to control the steering angle and the driving torque to complete the active control of the drift process; 2. The triggering of the lateral and longitudinal functions depends on the enabling signal of the enabling module to avoid the driver's panic caused by the sudden intervention of the system.

[0086] The main process of the present invention is: Select the drift mode and vehicle speed through the UI interface; the UI display interface can display the drift state information in real time; the voice prompt can guide the driver to operate the opening of the accelerator pedal.

[0087] Associated setting: By selecting the drift mode, the system actively turns off the electronic stability system, traction control system, etc., without manual shutdown.

[0088] Drift start condition judgment: It is characterized by the lighting of UI (Drift Ready). Actually, it is to judge whether the vehicle enters the steady-state circular driving, that is: driving in a steady-state circle at a certain radius according to the preset vehicle speed. Its significance is equivalent to presetting the longitudinal speed and driving trajectory for the drift control system. The system controls the vehicle to perform steady-state drifting at this speed and radius, which is equivalent to driving along the trajectory and at the speed expected by the driver.

[0089] Determination of steady-state drift parameters: Based on the vehicle three-degree-of-freedom model combined with the brush tire model, the expected equilibrium points: β’, γ’, V x ’, λ’ and δ’ are obtained when the first derivative of the system is 0.

[0090] Drift start assistance: For semi-active drift control:

[0091] The intervention timing of the active lateral control is judged by the rear wheel slip ratio |λ|, and the end of the drift start assistance is judged based on the completion of the front wheel counter-steering.

[0092] For active drift control:

[0093] The enabling condition for activating the combined lateral and longitudinal control is that the throttle pedal opening exceeds the threshold. The drift start process and the steady-state drift process are all actively controlled by the lateral and longitudinal functions, so there is no special distinction.

[0094] Semi-active drift process control:

[0095] Dividing steady-state parameters: The lateral inputs are λr, Vx, β, γ, expected β’, expected γ’. The front wheel steering angle δ is the lateral control target. The theoretical front wheel steering angle δ is obtained based on the three-degree-of-freedom model and the brush tire model, and then the active front wheel steering angle control module controls it through the PID algorithm. Longitudinally, it is controlled by the driver. The expected AP% estimation module calculates the expected AP% through the inputs of λr, Vx, and expected λr’ and prompts the driver to follow in different ways through the driver prompt system.

[0096] Active drift process control:

[0097] Dividing steady-state parameters: The lateral inputs are λr, Vx, β, γ, expected β’, expected γ’. The front wheel steering angle δ is the lateral control target. The theoretical front wheel steering angle δ is obtained based on the three-degree-of-freedom model and the brush tire model, and then the active front wheel steering angle control module controls it through the PID algorithm. The longitudinal inputs are λr, Vx, expected λr’, and expected vehicle speed Vx’. According to the difference between the expected slip ratio and the actual slip ratio, the control torque Tr is obtained through the PID algorithm. Thus, the dynamic control of the lateral and longitudinal directions is realized.

[0098] In a specific embodiment of the present invention, the whole operation process is as follows:

[0099] Open the screen settings, select to enable the semi-active drift mode, and slide the slider to select the vehicle speed;

[0100] Considering the non-linear and unstable characteristics of drift control, to ensure safety and facilitate the start of drifting, the set vehicle speed range here is set to: 5 km / h - 80 km / h.

[0101] If the semi-active drift mode is turned on, the system requests to turn off the electronic stability system, requests to turn off the traction control system, and the lateral drift control module is activated;

[0102] If the free drift mode is selected to be turned on, the electronic stability system and the traction control system should be turned off, and the lateral control function should be activated;

[0103] If the active drift mode is selected to be turned on, the electronic stability system and the traction control system should be turned off, and the active drift mode should be activated.

[0104] The driver starts a steady-state circular turn at the predetermined vehicle speed. If the vehicle speed and the turning radius are maintained unchanged, the UI (DriftReady) will be lit. At this time, when the drift condition is met and the driver deeply steps on the accelerator, the rear wheels start to slide, and the active steering will respond. To quickly light up the UI (Drift Ready) and avoid unnecessary false triggers, the time for maintaining the vehicle speed and the turning radius needs to be controlled here. Therefore, a table of vehicle speed and duration is set. When the duration is greater than or equal to the table time, the condition 1 for lighting up the UI (Drift Ready) is met. The table is as follows:

[0105]

[0106]

[0107] The time threshold here is a calibrated parameter and needs to be adjusted according to the actual vehicle. This is only for illustration. Considering that under a stable vehicle speed, due to insufficient driving experience of the driver, when the front-wheel steering angle input is too large, oversteering or understeering may occur in advance. Therefore, it is necessary to limit the minimum turning radius at each vehicle speed. Considering that the calculation of the dynamic turning radius is relatively complex, the lateral acceleration is selected here to characterize it, that is, the lateral acceleration threshold is set to 0.6g. When turning, regardless of the vehicle speed, as long as its maximum lateral acceleration does not exceed 0.6g, the corresponding turning radius is considered a valid input. The lateral acceleration threshold here is condition 2. Therefore, the driver's output torque is limited based on the lateral acceleration during steady-state circular turning. If the lateral acceleration is greater than 0.6g, the torque output will be limited until the lateral acceleration can be stabilized within 0.6g. Combining the two conditions of duration and lateral acceleration limit, if both conditions (1&2) are met, the UI (Drift Ready) will be lit and the active steering will respond to the rear-wheel side slip.

[0108] A three-degree-of-freedom dynamic model is established with the sideslip angle, yaw rate and longitudinal velocity as state variables:

[0109]

[0110] The tire brush model is used to describe the tire longitudinal force and lateral force

[0111]

[0112]

[0113] Drift steady-state equilibrium point solution:

[0114] For nonlinear systems, let:

[0115]

[0116] Then the steady-state condition can be obtained;

[0117] Right now:

[0118]

[0119] According to the relationship between the vehicle state and the turning radius in steady state, the expected drift equilibrium point can be obtained by the simultaneous equations: the expected equilibrium point is: β', γ', V x ', λ' and δ'; in the system, the three-degree-of-freedom model is established by the VehicleModel module, the brush tire model is established by the TireModel module, and the SignalProcess module is responsible for the transposition of the Vehicle input signal, the calculation of the slip rate and the calculation of the steady-state equilibrium point parameters. Rear wheel slip rate calculation:

[0120]

[0121] V vecicle Corresponding longitudinal speed Vx, V wheel is the average of the front wheel speeds FL and FR. The slip ratio λr is the actual slip ratio, and λr' is the theoretical calculated value.

[0122] 5. If the mode is selected as semi-active drift mode:

[0123] When the lighting condition of UI (Drift Ready) is satisfied, the driver should deeply step on the accelerator to make the rear axle start to slide. At this time, the system uses the rear wheel slip ratio |λ| to judge the intervention timing of the active lateral control. Here, |λ| is a calibrated parameter, and the threshold value needs to be determined according to the actual performance of different vehicle models. When the actual slip ratio |λr| is greater than or equal to |λ|, the active lateral control starts to intervene. At this time, the system calculates the theoretical front wheel angle δ based on the vehicle input signal, and the active front wheel angle control module updates the control target δ' according to the actual front wheel angle δ'. The steering system responds to update δ' to complete the counter-steering, then the drifting assist is completed, and the active counter-steering of the front wheels represents the end of the drifting assist. The judgment of the counter-steering is based on the judgment of the steering wheel angle and the front wheel angle direction. If the directions are opposite, it is considered as counter-steering, and the direction can be judged by the ± sign of the signal. If the signs are different, it is counter-steering.

[0124] If the mode is selected as the active drift mode:

[0125] Taking the throttle pedal opening exceeding the threshold value as the enabling condition for activating the combined lateral and longitudinal control, the entire drifting process and the steady-state drifting process are actively controlled by the lateral and longitudinal functions. Therefore, no special distinction is made for the drifting process.

[0126] If the mode is selected as the semi-active drift mode:

[0127] When the driver deeply steps on the accelerator and the rear axle starts to skid, the lateral system activates the active control based on the skid threshold value and completes the drifting assist through counter-steering. After the drifting assist is completed, the vehicle has the following characteristics: 1. A large sideslip angle of the center of mass, 2. Counter-steering of the front wheel angle, 3. A large skid of the rear wheel.

[0128] At this time, if the driver lacks experience and the rear wheel skids too much, it will enter an unstable circular drifting state, and the yaw rate will continue to increase, resulting in a drifting trajectory smaller than the preset value. Even worse, it may lead to SPIN. Therefore, it is necessary to give prompts and guidance to the driver to assist in controlling the steady-state drifting process.

[0129] The longitudinal control targets are the rear wheel slip ratio λ and the longitudinal speed Vx, and both variables are controlled by the driver. However, the system designs a prompt and guidance system to assist the driver in controlling to reduce the driver's operation difficulty. Therefore, a throttle opening estimation system and a driver reminder system are designed. The system calculates the expected rear axle slip ratio based on the initial steady-state circular driving speed and the curvature radius. The expected AP estimation module calculates the expected torque according to the difference Δλr between the expected rear axle slip ratio and the current rear axle slip ratio, with the driver output torque Tr as the target basis using PID.

[0130] T r =k p Δλ r +k I ∫Δλ r dt+k f λrd

[0131] Based on the desired torque, the opening of AP% (throttle pedal) corresponding to the desired torque is inversely analyzed by referring to the drivability MAP. The AP% corresponding to the desired torque is the desired AP%. The implementation method for inversely analyzing the desired AP opening by looking up the drivability MAP through the input of the desired torque is as follows: 1. The desired AP estimation module calculates the desired torque and sends it to the VCU together with the current vehicle speed signal Vx to request the AP opening. After looking up the table through Tr and Vx, the VCU sends the corresponding AP opening value to the desired AP estimation module; 2. The desired AP estimation module independently establishes a relevant MAP according to the VCU parameters; 3. The desired AP estimation module directly maps the VCU drivability MAP to this module.

[0132] Driver prompt:

[0133] Driver prompt system: Judge the feedback direction based on the desired AP% and the actual AP%. If the actual AP is less than the desired value, it prompts that the throttle opening is too small; otherwise, it prompts that it is too large. Prompt solutions: 1. Cockpit voice prompt (too small: prompt to increase the throttle opening, too large: prompt to decrease the throttle opening); 2. Display prompt information in the UI interface; 3. Display the desired AP% information and the actual AP% information in the UI interface, both in the form of dynamic sliding. The driver can dynamically track the desired AP% to achieve the steady-state drift target. The above three prompts can be separated or coexist. Regarding the magnitude of the difference in AP%: For 1, three voice prompt intervals can be set (the prompt words can be: please increase the throttle opening slightly, please increase the throttle opening, please increase the throttle opening greatly; the case of decreasing the prompt is similar); For 2, considering that the driver needs to concentrate on paying attention to the ground information, only two gears of increasing / decreasing the throttle are set; For 3: Instead of directly displaying the difference, the desired AP% and the actual AP% are compared and displayed to guide the driver in a dynamic following manner.

[0134] Lateral control process: During the lateral control process, the driver only needs to maintain the steering wheel angle, and the front wheel angle will be actively adjusted by the active front wheel control module. Specifically: The lateral control quantity is the front wheel angle δ. The three-degree-of-freedom model combined with the tire model calculates the theoretical front wheel angle δ based on the differences between the desired β', desired γ' and the actual β, γ and the actual λr, Vx as inputs. The active front wheel control module uses PID control to correct according to the difference Δδ between the theoretical front wheel angle and the actual front wheel angle.

[0135] δ = k p Δδ + k I ∫Δδdt + k f δ d

[0136] Combined with the above, the actual control process is as follows: After the start-up drift assistance ends, the vehicle enters the non-steady-state drift process. At this time, the lateral control tracks the preset trajectory by actively correcting the front-wheel steering angle. However, the lateral control is limited by physical limits. When the rear-wheel sideslip breaks through its limit, the drift will become uncontrollable until Spin or the drift is exited. Based on this, a driver prompting system is designed. The auditory / visual feedback system is used to guide the driver to increase or decrease the throttle pedal opening to maintain the steady-state drift and drift tracking performance, characterized by the desired AP%.

[0137] If the mode selection is the active drift mode:

[0138] When the driver deeply steps on the throttle pedal and exceeds the set threshold during the steady-state circular driving, the longitudinal control function and the lateral control function are activated simultaneously and cooperate for control. Here, the threshold is a calibrated value and should be adjusted according to the actual feedback of different vehicle models.

[0139] Specifically: The lateral inputs are λr, Vx, β, γ, the desired β', and the desired γ'. The front-wheel steering angle δ is the lateral control target. The theoretical front-wheel steering angle δ is obtained based on the three-degree-of-freedom model and the brush tire model, and then the active front-wheel steering angle control module controls it through the PID algorithm according to the theoretical front-wheel steering angle δ and the actual front-wheel steering angle δ'.

[0140] The longitudinal control variables are the rear-wheel slip ratio λ and the longitudinal speed Vx. The three-degree-of-freedom model combined with the tire model calculates the control torque Tr using the difference between the desired rear-wheel slip ratio λ and the actual rear-wheel slip ratio λ as the input. The actual algorithm uses PID control:

[0141] T r =k p Δλ r +k I ∫Δλ r dt+k f λ rd

[0142] In summary: The active drift process is the combined control of the lateral function and the longitudinal function. The method is as follows: 1. The steady-state drift parameters are divided. λr and Vx are the longitudinal control inputs, and λr, Vx, β, and γ are the lateral control inputs. Finally, the theoretical front-wheel steering angle δ and the torque Tr are obtained respectively to control the steering angle and the driving torque to complete the active control of the drift process; 2. The triggering of the lateral and longitudinal functions depends on the enabling signal of the enabling module to avoid driver panic caused by the sudden intervention of the system.

[0143] The above-described embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the appended claims.

[0144] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A drift control method for a rear-drive vehicle based on steer-by-wire, characterized in that: The method comprises: Activate the drift function and select a drift mode, wherein the drift mode includes a free drift mode, a semi-active drift mode and an active drift mode; According to the drift mode selected by the user, the drifting conditions are judged and the user is prompted when the drifting conditions are met; When the user decides to drift, a three-degree-of-freedom vehicle dynamics model is constructed, the drift steady-state equilibrium point is calculated, and the expected drift equilibrium point is output; When starting to drift, drift assistance is performed, and drift process control is performed during the drifting process. The drift process control includes longitudinal control and lateral control.

2. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 1, characterized in that: The drifting condition is that the vehicle speed reaches the preset speed and the vehicle is making a steady circle within the preset radius. At this time, the user is prompted by the indicator light.

3. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 1, characterized in that: In the step of constructing a three-degree-of-freedom vehicle dynamics model, the sideslip angle β, yaw rate γ and longitudinal velocity V x A three-degree-of-freedom dynamic model is constructed for the state variables. The three-degree-of-freedom dynamic model is expressed as: in, and They are the sideslip angle β, yaw rate γ and longitudinal velocity V x The first derivative of yf is the lateral force of the front wheel, F yr is the lateral force of the rear wheel, m is the mass of the vehicle, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass, I z is the moment of inertia, and δ is the front wheel steering angle.

4. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 3, characterized in that: In the step of building a three-degree-of-freedom vehicle dynamics model, the tire brush model is selected to describe the tire longitudinal force and lateral force. The tire brush model is expressed as: Among them, α f is the front wheel slip angle, α r is the rear wheel slip angle, V y is the lateral velocity at the center of mass, l f is the distance from the front axle to the center of mass, l r is the distance from the rear axle to the center of mass; C α is the tire cornering stiffness, μ is the friction coefficient between the ground and the tire, F z is the tire vertical force, α is the tire side slip angle, α sl is the critical slip angle of the tire.

5. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 1, characterized in that: In the step of calculating the drift steady-state equilibrium point and outputting the expected drift equilibrium point, for a nonlinear system, let: The steady-state condition is obtained: According to the relationship between the vehicle state and the turning radius in steady state, the expected drift equilibrium point can be obtained by the simultaneous equations: The expected equilibrium point is: β ’ , γ ’ 、V x ’ , ’ and δ ’ , where β ’ , γ ’ 、V x ’ , ’ and δ ’ They are the sideslip angle β, yaw rate γ, and longitudinal velocity V x , the expected values ​​of tire slip λ and front wheel steering angle δ.

6. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 1, characterized in that: In the semi-active drift mode, when drift assist is performed, after the vehicle enters a steady-state circle, the driver steps deeply on the accelerator and the rear axle begins to slide. The theoretical front wheel steering angle is calculated based on the dynamic equation. The active front wheel steering angle control module controls the active steering of the front wheels according to the deviation value to complete the reverse steering. The drift assist ends when the reverse steering is completed.

7. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 1, characterized in that: In active drift mode, when the vehicle enters a steady-state circle, the driver steps on the accelerator deeply. The threshold for deep stepping is set as a calibration parameter. When the accelerator depth exceeds the threshold, the throttle opening is only used as a switch quantity, and the vehicle does not respond to specific opening requests. Once the opening exceeds the threshold, the system automatically intervenes to complete drift assistance. When the accelerator pedal is completely released, the drift function is considered to have terminated and the driver will be able to take over again.

8. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 1, characterized in that: In semi-active drift mode, the longitudinal control targets are the rear wheel slip rate λ and the longitudinal speed V x , this variable is controlled by the driver. After the driver reverses the steering wheel, the vehicle enters the unsteady-state drift process. At this time, the rear axle side slip is controlled by the driver. The expected rear axle slip rate is calculated according to the initial steady-state round speed and curvature radius. According to the expected rear axle slip rate and the current rear axle slip rate, the driver outputs the torque T r The expected torque is calculated based on PID for the target, and the accelerator pedal opening corresponding to the expected torque is reversely analyzed by checking the drivability MAP diagram based on the expected torque. The accelerator pedal opening corresponding to the expected torque is the expected throttle opening, and the user is prompted based on the difference between the expected throttle opening and the actual throttle opening.

9. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 1, characterized in that: In semi-active drift mode and active drift mode, the driver only needs to maintain the steering wheel angle during lateral control, and the front wheel angle will be actively adjusted by the active front wheel control module. The specific lateral control amount is the front wheel angle δ. The three-degree-of-freedom vehicle dynamics model and the tire brush model use the expected center of mass sideslip angle β', the expected yaw rate γ' and the difference between the actual center of mass sideslip angle β, yaw rate γ and the actual rear axle slip rate λr and longitudinal speed Vx as inputs to calculate the theoretical front wheel angle δ, and use PID control to correct the difference Δδ between the theoretical front wheel angle and the actual front wheel angle.

10. The drift control method for a rear-wheel drive vehicle based on steer-by-wire according to claim 1, characterized in that: In active drift mode, during the longitudinal control process, when the driver enters a steady-state circle and steps deeply on the accelerator pedal and exceeds the set threshold, the longitudinal control function and the lateral control function are activated at the same time for coordinated control. The longitudinal control quantity is the rear wheel slip rate λ and the longitudinal speed Vx. The three-degree-of-freedom vehicle dynamics model combined with the tire brush model calculates the control torque Tr based on the difference between the expected rear wheel slip rate λ and the actual rear wheel slip rate λ as input.

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