A distributed wheel drive heavy-duty tractor driving stability control method

Abstract

The application discloses a kind of distributed wheel edge drive heavy tractor driving stability control method, it is related to vehicle brake control technical field, comprising the following steps: S1, the two-degree-of-freedom three-axle vehicle handling stability model of heavy tractor is established, and centroid side angle expected value and yaw angular velocity expected value are solved;S2, according to yaw angular velocity threshold value and stability index, the stability of vehicle is judged;S3, if vehicle is in stable domain outside, then based on model predictive control theory, three-axle vehicle prediction model is established, and the additional steering angle and additional yaw moment of front wheel that meet control target are decided;If in stable domain, then model predictive controller does not participate in work;S4, the additional steering angle directly acted on front steering wheel is decided;By rule driving force control distribution method, additional yaw moment of whole vehicle is distributed to each drive wheel, and the additional driving moment of each drive wheel is obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of vehicle safety control, in particular to a distributed wheel drive heavy tractor driving stability control method. BACKGROUND

[0002] The distributed wheel drive vehicle has the characteristics that each drive wheel can be independently driven and braked, and the torque of each drive wheel can be autonomously distributed within a certain range. Therefore, compared with the centralized drive vehicle, it has more reliable, coordinated and flexible advantages.

[0003] In the vehicle active safety control technology, the direct yaw moment control system of the distributed wheel drive vehicle can correct the motion state of the vehicle by controlling the longitudinal force of each drive wheel to make the inside and outside wheels appear differential to generate additional yaw moment. Therefore, many current driving stability control methods fully utilize the advantages of distributed drive vehicles in active safety control.

[0004] However, for example, the distributed drive vehicle driving stability control method disclosed in Chinese Patent No. CN112644455A, and the distributed drive vehicle steering stability control system and its control method disclosed in Chinese Patent No. CN112644457A, the research on the driving stability control method of the distributed drive vehicle in the prior art is mainly applied in the control field of two-axle vehicles, and there are few applications in the field of three-axle or more-axle heavy commercial vehicles. In order to fill this technical gap, the present application provides a distributed wheel drive heavy tractor driving stability control method. SUMMARY

[0005] The present application provides a distributed wheel drive heavy tractor driving stability control method, which mainly aims to solve the problems existing in the prior art.

[0006] The present application adopts the following technical solutions:

[0007] A distributed wheel drive heavy tractor driving stability control method, comprising the following steps:

[0008] Step S1, according to the driving operation of the driver and the state of the vehicle, a two-degree-of-freedom three-axle vehicle handling stability model of the heavy tractor is established, considering the steady-state steering and the adhesion limit state of the vehicle, so as to obtain the mass center side slip angle expectation value And the yaw rate expectation value ;

[0009]

[0010] In the formula: ; ; ; ; is the distance from the center of mass of the vehicle to the front axle; is the distance from the center of mass of the vehicle to the middle axle; is the distance from the center of mass of the vehicle to the rear axle; is the front wheel steering angle; 、 、 are the front wheel, middle wheel and rear wheel comprehensive cornering stiffness respectively; is the total mass of the vehicle; is the longitudinal speed of the center of mass of the vehicle; is the road adhesion coefficient;

[0011] Step S2, based on the two-degree-of-freedom three-axle vehicle handling stability model, yaw rate deviation , center of mass side slip angle and center of mass side slip angle velocity are calculated, and the stability of the vehicle is determined according to the yaw rate threshold value and the stability index , and the calculation formula of the stability index is:

[0012]

[0013] In the formula: is the slope of the stability boundary line; is the intercept of the boundary line;

[0014] Step S3, if the vehicle is outside the stable domain, a three-axle vehicle prediction model is established based on the model predictive control theory, and the front wheel additional steering angle and additional yaw moment that meet the control target are determined according to the deviation of the actual value of the driver's target vehicle speed, yaw rate / actual value of the center of mass side slip angle and the expected value of the yaw rate / expected value of the center of mass side slip angle ; if it is in the stable domain, the model predictive controller does not work;

[0015] Step S4, the determined front wheel additional steering angle directly acts on the front steering wheel; the additional yaw moment of the whole vehicle is distributed to each driving wheel through a rule-based driving force control distribution method, and the additional driving moment of each driving wheel is obtained.

[0016] Further, in step S1, the two-degree-of-freedom three-axle vehicle handling stability model adopts three lateral elastic tire supports on the ground, and a three-axle heavy tractor vehicle model with two degrees of freedom of lateral and yaw, and the differential equation is:

[0017]

[0018] Where: is the vehicle's moment of inertia; is the sideslip angle of the vehicle's center of mass; is the lateral acceleration of the vehicle's center of mass; is the vehicle's yaw angular velocity; is the vehicle's yaw angular acceleration.

[0019] Furthermore, in step S2, the stability judgment standard of the vehicle is:

[0020] (1) If the stability index , then the car is judged to be unstable;

[0021] (2) If the stability index And the yaw rate deviation When , it is determined that the car is unstable;

[0022] (3) If the stability index And the yaw rate deviation When the vehicle is stable, the vehicle is determined to be stable and the vehicle stability is continuously monitored.

[0023] Furthermore, in step S3, a two-degree-of-freedom three-axle vehicle handling stability model for a heavy-duty tractor with an additional yaw moment is used as a prediction model of the model predictive control theory, and its differential equation is:

[0024]

[0025] Where: is the additional yaw moment.

[0026] Furthermore, in step S4, the rule-driven force control allocation method is as follows:

[0027] (1) When it is necessary to correct the left understeer or right oversteer condition: the right wheel increases the driving torque, which generates a positive 1 / 4 additional yaw moment value; the left wheel reduces the driving torque, which generates a negative 1 / 4 additional yaw moment value;

[0028] (2) When it is necessary to correct the left oversteer or right understeer condition: the left wheel increases the driving torque, which generates a positive 1 / 4 additional yaw moment value; the right wheel reduces the driving torque, which generates a negative 1 / 4 additional yaw moment value.

[0029] Furthermore, the additional yaw moment is completed according to the rule-based driving force control distribution method. After the control distribution, it should be further verified whether the sum of the driving torques of each driving wheel is equal to the total target driving torque:

[0030]

[0031] In the formula: is the output torque of each wheel assembly of the wheel-side motor; is the rolling radius of each drive wheel; wherein is the left wheel of the middle shaft, is the right wheel of the middle shaft, is the left wheel of the rear shaft, is the right wheel of the rear shaft; is the total target driving torque, which is calculated by the driving controller and obtained from the controller through the CAN bus.

[0032] Compared with the prior art, the present application has the beneficial effects that:

[0033] The main innovation of the present application is to establish a two-degree-of-freedom three-axle vehicle handling stability model for heavy-duty tractors, and on this basis, based on the online real-time control characteristics of the model predictive control and the rule-based driving force distribution method, additional yaw moment MPC control and driving force distribution are performed, so as to facilitate real-time control of the handling stability of the distributed wheel-side driving heavy-duty tractor, and improve the driving stability control efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is the control flowchart of the present application.

[0035] Figure 2 is the two-degree-of-freedom three-axle vehicle handling stability model of the present application.

[0036] Figure 3 is the MPC control principle block diagram of the present application. DETAILED DESCRIPTION

[0037] The specific embodiments of the present application will be described below with reference to the accompanying drawings. In order to fully understand the present application, many details are described below, but the present application can be implemented without these details for those skilled in the art.

[0038] The present application discloses a driving stability control method for a distributed wheel-side driving heavy-duty tractor. The main innovation of the control method is to establish a two-degree-of-freedom three-axle vehicle handling stability model for heavy-duty tractors, to comprehensively consider the stability index under nonlinear working conditions, and to perform front wheel additional angle and additional yaw moment MPC control based on the good online real-time performance of model predictive control, and to realize the distribution of additional yaw moment of each distributed wheel-side driving by using a rule-based driving force distribution method with small calculation amount and good real-time control performance, so as to maximize the driving stability and safety of the distributed wheel-side driving heavy-duty tractor.

[0039] As Figure 1 shown, a distributed wheel drive heavy-duty tractor driving stability control method, specifically comprising the following steps:

[0040] Step S1: According to the driver's driving operation and the vehicle state, a two-degree-of-freedom three-axle vehicle handling stability model of the heavy-duty tractor is established, considering the steady-state steering and the adhesion limit state of the vehicle, so as to obtain the expected value of the vehicle motion response, i.e. the expected value of the mass center side slip angle and the expected value of the yaw rate .

[0041] Specifically, the driver's driving operation and the vehicle state include but are not limited to the steering wheel angle operated by the driver, the vehicle mass center longitudinal speed, the vehicle mass center lateral speed, the vehicle mass center position, the wheel side slip angle, the side slip stiffness, the vehicle yaw rate, the vehicle longitudinal speed, the vehicle lateral speed and the vehicle mass center side slip angle, etc. Among them, the vehicle longitudinal speed, the vehicle lateral speed and the mass center side slip angle cannot obtain signals through sensors, so it is necessary to estimate through the vehicle speed estimator and the mass center side slip angle estimator.

[0042] As Figure 2 shown, the two-degree-of-freedom three-axle vehicle handling stability model established in this embodiment according to the driver's driving operation and the vehicle state is taken as an example of a two-degree-of-freedom front-wheel steering three-axle heavy-duty tractor. It is a vehicle model supported by three laterally elastic tires on the ground, with two degrees of freedom in lateral and yaw directions. The differential equation of the two-degree-of-freedom three-axle vehicle handling stability model of the heavy-duty tractor is:

[0043] (1)

[0044] In the formula: is the total mass of the vehicle; is the moment of inertia of the vehicle; is the vehicle mass center side slip angle; is the vehicle mass center longitudinal speed; is the vehicle mass center lateral speed; is the vehicle mass center lateral acceleration; is the vehicle yaw rate; is the vehicle yaw angular acceleration; is the distance from the vehicle mass center to the front axle; is the distance from the vehicle mass center to the middle axle; is the distance from the vehicle mass center to the rear axle; is the wheelbase of the front and rear axles; , , are the front wheel, middle wheel and rear wheel side slip angles, respectively; is the front wheel steering angle; , , are the front, middle and rear wheel cornering stiffness respectively.

[0045] At steady state, the vehicle is driving safely, so the steady state response values of yaw rate and side slip angle of the three-axle vehicle handling stability model are taken as the reference values for control. At steady state, the response yaw rate and side slip angle are both constant values, i.e. the vehicle side slip acceleration and the vehicle yaw angular acceleration , the steady state values of yaw rate and side slip angle can be calculated from the above differential equation (1) as follows:

[0046] (2)

[0047] wherein: ; ; ; .

[0048] When the tires are in a saturated state, i.e. the vehicle is in an adhesion limit state, the steady state response values of the two-degree-of-freedom reference model are not suitable as reference values, and the reference values need to be replaced by boundary values at this time.

[0049] The calculation formula of the boundary value of yaw rate is:

[0050] (3)

[0051] wherein: is the road adhesion coefficient.

[0052] The calculation formula of the boundary value of side slip angle is:

[0053] (4).

[0054] Therefore, considering the steady state steering and adhesion limit state of the vehicle at the same time, the expected value model of the control system is:

[0055] (5)

[0056] wherein: is the expected value of side slip angle; is the expected value of yaw rate.

[0057] Step S2: obtaining the yaw rate deviation based on the two-degree-of-freedom three-axle vehicle handling stability model of the heavy-duty tractor , the side-slip angle of the center of mass , and the side-slip angular velocity of the center of mass , and according to the yaw rate threshold value and the stability index , the stability of the automobile is determined. This step includes the following sub-steps:

[0058] Step S2.1: inputting the longitudinal velocity of the center of mass of the automobile , the front wheel steering angle , and the current road adhesion coefficient into the two-degree-of-freedom three-axle automobile handling stability model in step 1 to obtain the yaw rate deviation , the side-slip angle of the center of mass , and the side-slip angular velocity of the center of mass .

[0059] Specifically, this step can first obtain the yaw rate expectation value , the yaw rate of the automobile , and the side-slip angle of the center of mass in combination with the above formulas (1)-(5), then obtain the yaw rate deviation according to the absolute difference between the yaw rate expectation value and the yaw rate of the automobile , and further derive the side-slip angular velocity of the center of mass according to the side-slip angle of the center of mass .

[0060] Step S2.2: inputting the side-slip angle of the center of mass and the side-slip angular velocity of the center of mass into the stability index formula to obtain the stability index . The specific method of obtaining the calculation formula of the stability index in this step includes the following sub-steps:

[0061] Step S2.2.1: obtaining a second-order autonomous system based on the two-degree-of-freedom three-axle automobile handling stability model of the heavy tractor:

[0062] (6)

[0063] Under the given longitudinal velocity of the center of mass and the front wheel steering angle , assign different initial values to the above formula, draw the phase trajectory of the system, and obtain the side-slip angle of the center of mass - the side-slip angular velocity of the center of mass phase plane diagram.

[0064] Step S2.2.2: dividing - The method of constructing the phase plane diagram builds the boundary equation of the stability domain of the vehicle as:

[0065] (7)

[0066] In the formula, is the slope of the stability boundary line; is the intercept of the boundary line; and The values of and are related to the influencing factors of the phase plane.

[0067] Step S2.2.3: The shortest distance from the state point of the vehicle in the stability domain to the boundary of the stability domain is defined as the instability degree, and the formula of the stability degree index is further obtained as:

[0068] (8)

[0069] - In the phase plane, when the state point (x, y) satisfies , the vehicle is in the stability domain of the phase plane; when the state point (x, y) satisfies , , the vehicle is in the instability region. Step S23: The stability of the vehicle is determined according to the yaw rate threshold value

[0070] and the stability degree index , and specifically, the stability judgment criterion of the vehicle is: (1) If the stability degree index

[0071] , it is determined that the vehicle is unstable;

[0072] (2) If the stability degree index and the yaw rate deviation , it is determined that the vehicle is unstable;

[0073] (3) If the stability degree index and the yaw rate deviation , it is determined that the vehicle is stable, and the stability of the vehicle is continuously monitored.

[0074] Wherein, is the yaw rate threshold value, and different vehicle speeds correspond to different yaw rate threshold values.

[0075] Step S3: If the vehicle is outside the stability domain (i.e., the vehicle is determined to be unstable through step S2), a three-axle vehicle prediction model is established based on the model predictive control theory, and the actual value of the yaw rate / the actual value of the center side slip angle​ and yaw rate expectation value / center of mass side slip angle expectation value of the deviation, the front wheel additional steering angle satisfying the control target is determined and additional yaw moment ; if in the stable region (i.e. the vehicle is determined to be stable by step S2), the model predictive controller does not work.

[0076] As shown in Figure 3 , if out of the stable region, the specific processing procedure includes the following steps:

[0077] Step S3.1: a two-degree-of-freedom three-axle vehicle handling stability model with additional yaw moment for heavy tractor is used as the prediction model of model predictive control, and its differential equation is:

[0078] (9)

[0079] In the formula: is the total mass of the vehicle; is the moment of inertia of the vehicle; is the vehicle center of mass side slip angle; is the vehicle center of mass longitudinal velocity; is the vehicle center of mass lateral velocity; is the vehicle center of mass lateral acceleration; is the vehicle yaw rate; is the vehicle yaw angular acceleration; is the distance from the vehicle center of mass to the front axle; is the distance from the vehicle center of mass to the middle axle; is the distance from the vehicle center of mass to the rear axle; is the wheelbase of the front and rear axles; is the front wheel steering angle; , , are the front wheel, middle wheel and rear wheel comprehensive cornering stiffness respectively; is the additional yaw moment.

[0080] Taking the state vector as , the input vector as , and the output vector as , the prediction model of formula (9) is written as the state equation:

[0081] (10)

[0082] In the formula: ; ; ; ; For the lateral acceleration of the vehicle's center of mass.

[0083] Further, the prediction model is discretized and rewritten in the form of an incremental model:

[0084] (11)

[0085] where, ; ; , is a discrete matrix.

[0086] According to the principle of predictive control, the initial condition is the latest measurement value, and the future process is dynamically predicted based on the prediction model. The prediction time domain is set as , the control time domain is set as , and .

[0087] represents the state prediction of at time , and the QUOTE after the symbol " | " represents the current time. is the output, is the control increment, and outside the control time domain.

[0088] Define the vector:

[0089]

[0090]

[0091] For the future system , the output prediction is calculated using the following equation:

[0092] (12)

[0093] where, ; ;

[0094] Step S3.2: Based on the above prediction results, the following equation is solved by rolling optimization:

[0095] (13)

[0096] where, is the bias penalty coefficient matrix, is the control increment penalty coefficient matrix, To predict the expected output value in the prediction horizon, i.e. the expected yaw rate value based on the two-degree-of-freedom three-axis model and the expected value of the side-slip angle of the center of mass , .

[0097] The algorithm assumes that the expected value in the prediction horizon is constant and equal to the current value, i.e. The equation is solved to obtain the following optimal control output sequence:

[0098] (14)

[0099] where the control deviation can be calculated online.

[0100] According to the model predictive control principle, the algorithm assumes that the model predictive control expected value is constant in the prediction horizon, and the expected value is the reference model output value. The controller performs optimization calculation according to the penalty coefficient. In the objective function, the weighting coefficients are adjusted by weighting the tracking deviation and the control amount, so as to ensure reasonable control.

[0101] Step S4: The additional steering angle of the front wheels decided directly acts on the front steering wheels; the additional yaw moment of the whole vehicle is distributed to each driving wheel through a rule-based driving force control distribution method to obtain the additional driving moment of each driving wheel, and the sum of the driving moments of each driving wheel is ensured to be equal to the total target driving moment.

[0102] In this step, the additional yaw moment is controlled according to the following rule-based driving force control distribution method:

[0103] (1) When the left steering deficiency or the right steering excess condition needs to be corrected: the right wheels increase the driving moment, and each generates a positive 1 / 4 additional yaw moment value; the left wheels decrease the driving moment, and each generates a negative 1 / 4 additional yaw moment value.

[0104] (2) When the left steering excess or the right steering deficiency condition needs to be corrected: the left wheels increase the driving moment, and each generates a positive 1 / 4 additional yaw moment value; the right wheels decrease the driving moment, and each generates a negative 1 / 4 additional yaw moment value.

[0105] At the same time, a threshold control is set in the distribution algorithm, and negative values of the driving moment of each wheel are not allowed.

[0106] After the additional yaw moment ​​After the control distribution, it should be further verified that the sum of the driving torques of the driving wheels is equal to the total target driving torque, specifically, the longitudinal driving force of the tire of each driving wheel The wheel-side motor output torque control can be performed as follows:

[0107] (15)

[0108] In the formula: is the total output torque of the wheel-side motor of each wheel assembly; is the rolling radius of each driving wheel; wherein is the left wheel of the intermediate shaft, is the right wheel of the intermediate shaft, is the left wheel of the rear axle, is the right wheel of the rear axle; is the total target driving torque, which can be calculated by the driving controller and obtained from the controller through the CAN bus.

[0109] Additional yaw moment The longitudinal driving force of the control input is expressed as follows:

[0110] (16)

[0111] In the formula, ; is the wheelbase of the intermediate shaft; is the wheelbase of the rear axle.

[0112] The above is only a specific embodiment of the present application, but the design concept of the present application is not limited thereto, and any non-essential modification of the present application using this concept shall be deemed to be an act of infringing the protection scope of the present application.

Claims

1. A method for controlling driving stability of a distributed wheel-driven heavy-duty tractor, characterized by: The steps include: Step S1: Establish a two-degree-of-freedom three-axis vehicle handling stability model for a heavy-duty tractor based on the driver's driving operation and the vehicle's state, while taking into account the vehicle's steady-state steering and adhesion limit state, and thereby obtain the expected value of the center of mass sideslip angle. and the expected value of the yaw rate ; Where: ; ; ; ; is the distance from the center of mass of the vehicle to the front axle; is the distance from the center of mass of the car to the center axis; is the distance from the center of mass of the vehicle to the rear axle; is the front wheel steering angle; 、 、 are the comprehensive cornering stiffness of the front, middle and rear wheels respectively; is the total mass of the vehicle; is the longitudinal velocity of the vehicle's center of mass; is the road adhesion coefficient; Step S2: Obtaining the yaw rate deviation based on a two-degree-of-freedom three-axis vehicle handling stability model , center of mass side slip angle and the center of mass sideslip angular velocity , and according to the yaw rate threshold and stability index Determine the stability of the car, stability index The calculation formula is: Where: To stabilize the slope of the boundary line; is the intercept of the boundary line; Step S3: If the vehicle is outside the stable region, a three-axis vehicle prediction model is established based on the model predictive control theory, and the actual value of the driver's target speed and yaw angular velocity is used to predict the vehicle's direction. / Actual value of the center of mass side slip angle and the expected value of the yaw rate / Expected value of the sideslip angle of the center of mass Deviation, decide the additional front wheel turning angle that meets the control target and additional yaw moment ; If it is in the stable region, the model predictive controller does not participate in the work; Step S4: The determined additional turning angle of the front wheel Acts directly on the front steering wheel; adds yaw moment to the vehicle through regular driving force control distribution method Distributed to each drive wheel, additional drive torque of each drive wheel is obtained.

2. The method for controlling driving stability of a distributed wheel-driven heavy-duty tractor according to claim 1, wherein: In step S1, the two-degree-of-freedom three-axle vehicle handling stability model uses a three-axle heavy-duty tractor vehicle model supported on the ground by three laterally elastic tires and having two degrees of freedom, lateral and yaw. The differential equation is: Where: is the vehicle's moment of inertia; is the sideslip angle of the vehicle's center of mass; is the lateral acceleration of the vehicle's center of mass; is the vehicle's yaw angular velocity; is the vehicle's yaw angular acceleration.

3. The method for controlling driving stability of a distributed wheel-driven heavy-duty tractor according to claim 1, wherein: In step S2, the vehicle stability judgment standard is: (1) If the stability index , then the car is judged to be unstable; (2) If the stability index And the yaw rate deviation When , it is determined that the car is unstable; (3) If the stability index And the yaw rate deviation When the vehicle is stable, the vehicle is determined to be stable and the vehicle stability is continuously monitored.

4. The method for controlling driving stability of a distributed wheel-driven heavy-duty tractor according to claim 1, wherein: In step S3, a two-degree-of-freedom three-axle vehicle handling stability model for a heavy-duty tractor with an additional yaw moment is used as a prediction model of the model predictive control theory, and its differential equation is: Where: is the additional yaw moment.

5. The method for controlling driving stability of a distributed wheel-driven heavy-duty tractor according to claim 1, wherein: In step S4, the rule-driven force control allocation method is as follows: (1) When it is necessary to correct the left understeer or right oversteer condition: the right wheel increases the driving torque, which generates a positive 1 / 4 additional yaw moment value; the left wheel reduces the driving torque, which generates a negative 1 / 4 additional yaw moment value; (2) When it is necessary to correct the left oversteer or right understeer condition: the left wheel increases the driving torque, which generates a positive 1 / 4 additional yaw moment value; the right wheel reduces the driving torque, which generates a negative 1 / 4 additional yaw moment value.

6. The method for controlling driving stability of a distributed wheel-driven heavy-duty tractor according to claim 1, characterized in that: The additional yaw moment is completed according to the rule-based driving force control distribution method After the control distribution, it should be further verified whether the sum of the driving torques of each driving wheel is equal to the total target driving torque: Where: Output torque of each wheel assembly of the wheel-side motor; is the rolling radius of each driving wheel; ,in For the center axis left wheel, For the right wheel of the middle axle, For the left wheel of the rear axle, For the right wheel of the rear axle; is the total target driving torque, which is calculated by the drive controller and obtained from the controller through the CAN bus.

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