WO2022266824A1 - 一种转向控制方法及装置 - Google Patents

一种转向控制方法及装置 Download PDF

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Publication number
WO2022266824A1
WO2022266824A1 PCT/CN2021/101398 CN2021101398W WO2022266824A1 WO 2022266824 A1 WO2022266824 A1 WO 2022266824A1 CN 2021101398 W CN2021101398 W CN 2021101398W WO 2022266824 A1 WO2022266824 A1 WO 2022266824A1
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WO
WIPO (PCT)
Prior art keywords
steering
vehicle
coordination
tire
ratio
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PCT/CN2021/101398
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English (en)
French (fr)
Inventor
郑敏
崔臣
周勇有
罗杰
Original Assignee
华为技术有限公司
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Application filed by 华为技术有限公司 filed Critical 华为技术有限公司
Priority to CN202180007073.1A priority Critical patent/CN115715263A/zh
Priority to PCT/CN2021/101398 priority patent/WO2022266824A1/zh
Publication of WO2022266824A1 publication Critical patent/WO2022266824A1/zh

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
    • B60L15/00—Methods, circuits, or devices for controlling the traction-motor speed of electrically-propelled vehicles
    • B60L15/20—Methods, circuits, or devices for controlling the traction-motor speed of electrically-propelled vehicles for control of the vehicle or its driving motor to achieve a desired performance, e.g. speed, torque, programmed variation of speed
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60W—CONJOINT CONTROL OF VEHICLE SUB-UNITS OF DIFFERENT TYPE OR DIFFERENT FUNCTION; CONTROL SYSTEMS SPECIALLY ADAPTED FOR HYBRID VEHICLES; ROAD VEHICLE DRIVE CONTROL SYSTEMS FOR PURPOSES NOT RELATED TO THE CONTROL OF A PARTICULAR SUB-UNIT
    • B60W30/00—Purposes of road vehicle drive control systems not related to the control of a particular sub-unit, e.g. of systems using conjoint control of vehicle sub-units
    • B60W30/02—Control of vehicle driving stability
    • B60W30/045—Improving turning performance
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
    • B62D—MOTOR VEHICLES; TRAILERS
    • B62D6/00—Arrangements for automatically controlling steering depending on driving conditions sensed and responded to, e.g. control circuits

Definitions

  • the present application relates to the technical field of intelligent vehicles, in particular to a steering control method and device.
  • a steering control method in order to make the vehicle travel according to a predetermined path and trajectory, there is a steering control method in which autonomous steering and differential steering work together to track the trajectory. Specifically, in this method, when only the front When the wheel autonomous steering cannot suppress the lateral deviation within a small range (that is, the lateral deviation is too large), use differential steering (Differential Steering, DS) to assist the front wheel autonomous steering, so that the vehicle can reliably follow the predetermined path travel.
  • differential steering Different Steering
  • the autonomous steering and the differential steering work together to track the trajectory, and the coordinated function of the autonomous steering and the differential steering is not fully utilized.
  • the lateral response speed is slow and the tracking accuracy is low.
  • the present application provides a steering control method and device, etc., so as to improve trajectory tracking accuracy.
  • the first aspect of the present application provides a steering control method, which includes the following content: obtain the tire force range; determine the steering coordination rate according to the tire force range, the steering coordination rate indicates the steering amount generated by the vehicle's autonomous steering system and the vehicle's differential steering The weight of the steering quantity generated by the system; obtain the yaw rate error; determine the target wheel angle and the target yaw moment according to the yaw rate error and the steering coordination rate; A control command for generating a target wheel angle and a control command for causing the differential steering system to generate a target yaw moment.
  • the steering coordination rate that is, the weight of autonomous steering and differential steering
  • the tire force can be fully utilized, the lateral response speed can be improved, the turning radius can be reduced, and the trajectory tracking accuracy can be improved.
  • the steering coordination ratio is determined by considering the tire force range, the steering stability and safety of the vehicle can also be improved under extreme driving conditions (low road adhesion coefficient, high-speed driving, etc.).
  • the steering control method proposed in this embodiment uses the dual systems of the autonomous steering system and the differential system for coupled steering control, and has redundant functions, which can improve the driving safety of the vehicle under extreme working conditions.
  • the differential steering system can work to improve the driving safety of the vehicle.
  • the road surface adhesion coefficient and/or the load variation of multiple wheels of the vehicle are obtained; and the tire force range is determined according to the road surface adhesion coefficient and/or the load variation.
  • the tire force range can be reliably planned by considering the road surface adhesion coefficient and the load variation of the wheel to determine the tire force range.
  • one or more of the vehicle s acceleration, roll motion state parameters, or pitch motion state parameters in the horizontal, vertical, and vertical directions of the vehicle is acquired; according to the acceleration, roll motion state parameters, or pitch One or more of the motion state parameters determine the amount of load change.
  • the load change amount can be determined reliably by considering the vehicle's acceleration in the horizontal, vertical, and vertical directions, a roll motion state parameter, or a pitch motion state parameter to determine the load change amount.
  • determining the steering coordination ratio according to the tire force range may specifically include: obtaining the tire lateral relative utilization ratio and a preset basic coordination ratio, and the tire lateral relative utilization ratio indicates that the lateral tire force is relative to the total tire Force ratio, the basic coordination ratio is the initial parameter of the steering coordination ratio; the steering coordination ratio is determined according to the basic coordination ratio, the tire lateral relative utilization ratio and the tire force range.
  • the amount of steering can typically be determined by the turning radius or yaw angle.
  • the second aspect of the present application provides a vehicle steering control device, which includes a processing module and a transceiver module.
  • the processing module is used to obtain the tire force range and yaw rate error, determine the steering coordination rate according to the tire force range, and the steering coordination rate indicates the vehicle The weight of the steering amount generated by the autonomous steering system and the steering amount generated by the vehicle's differential steering system, and determine the target wheel angle and target yaw moment according to the yaw rate error and the steering coordination rate;
  • the transceiver module is used to send the first control command , the first control command includes a control command for making the autonomous steering system generate a target wheel angle and a control command for making the differential steering system generate a target yaw moment.
  • the processing module is specifically configured to acquire the road surface adhesion coefficient and/or the load variation of multiple wheels of the vehicle; and determine the tire force range according to the road surface adhesion coefficient and/or the load variation.
  • the processing module is specifically configured to obtain one or more of the vehicle's acceleration in the horizontal, vertical, and vertical directions, a roll motion state parameter, or a pitch motion state parameter; One or more of the pitch motion state parameters or the pitch motion state parameters determine the amount of load variation.
  • the processing module is specifically configured to obtain the tire lateral relative utilization rate and the preset basic coordination rate, the tire lateral relative utilization rate indicates the ratio of the lateral tire force to the total tire force, and the basic coordination rate
  • the steering coordination ratio is the initial parameter of the steering coordination ratio; the steering coordination ratio is determined according to the basic coordination ratio, the tire lateral relative utilization ratio and the tire force range.
  • the amount of steering can be determined by the turning radius or yaw angle.
  • the third aspect of the present application provides a computing device, which includes a processor and a memory, where the memory stores program instructions, and when the program instructions are executed by the processor, the processor executes any one of the methods described in the first aspect.
  • a fourth aspect of the present application provides a computer-readable storage medium, which stores program instructions. When executed by a computer, the program instructions cause the computer to execute any one of the methods described in the first aspect.
  • a fifth aspect of the present application provides a computer program product, which includes program instructions, and when executed by a computer, the program instructions cause the computer to execute any one of the methods described in the first aspect.
  • FIG. 1 is a schematic structural diagram of a vehicle to which a steering control method according to an embodiment of the present application is applied;
  • FIG. 2 is a flowchart of a steering control method provided by an embodiment of the present application
  • Fig. 3 is a schematic structural block diagram of a steering control device provided by an embodiment of the present application.
  • Fig. 4 is a schematic structural diagram of an electronic control unit provided by an embodiment of the present application.
  • FIG. 5 is an explanatory diagram of a steering control method provided by an embodiment of the present application.
  • FIG. 6 is a schematic illustration of vehicle states involved in trajectory tracking control in an embodiment of the present application.
  • FIG. 7 is a schematic illustration of an attachment ellipse involved in an embodiment of the present application.
  • Fig. 8 is a schematic illustration of the coordination rate range involved in an embodiment of the present application.
  • Fig. 9 is a schematic diagram of force analysis of vehicle body movement involved in the description of an embodiment of the present application.
  • Figure 10 is a schematic illustration of the tire force range involved in an embodiment of the present application.
  • FIG. 11 is a schematic diagram of the architecture of a steering system (of a vehicle) to which the steering control method according to an embodiment of the present application is applied.
  • a steering control method in order to make the vehicle travel according to a predetermined path and trajectory, there is a steering control method in which autonomous steering and differential steering work together to track the trajectory. Specifically, in this method, when only When the front wheel autonomous steering cannot suppress the lateral deviation within a small range (that is, the lateral deviation is too large), the differential steering is used to assist the front wheel autonomous steering, so that the vehicle can reliably follow the predetermined path.
  • the autonomous steering and the differential steering work together to track the trajectory, and the coordinated function of the autonomous steering and the differential steering is not fully utilized.
  • the lateral response speed is slow and the tracking accuracy is low.
  • an embodiment of the present application provides a steering control method to improve trajectory tracking accuracy.
  • FIG. 1 is a schematic structural diagram of a vehicle to which a steering control method according to an embodiment of the present application is applied.
  • the vehicle 100 is a distributed drive vehicle, and in-wheel motors 120 are respectively arranged in four wheels 110 thereof, and the in-wheel motors 120 drive and brake the wheels 110 .
  • the in-wheel motors 120 can be independently controlled (the motor controller is not shown in the figure), so that the torques received by the coaxial wheels 110 are different, so as to generate differential torques and cause the vehicle 100 to produce differential steering. That is, in-wheel motor 120 constitutes a differential steering (Differential Steering, DS) system.
  • DS differential Steering
  • in-wheel motors 120 may also be replaced, and wheel-side motors may be arranged near the four wheels 110, and the wheel-side motors are respectively connected to the wheels 110 through transmission mechanisms, so that the wheels 110 can be driven. with braking.
  • the method in the embodiment of the present application may also be applicable to other types of vehicles.
  • the vehicle 100 further includes a steering wheel 20 , a torque and rotation angle sensor 30 , a steering motor 40 , a clutch 70 , a reduction mechanism 50 , a steering gear 60 and a steering controller 90 .
  • the steering wheel 20 is used for steering operations by the driver.
  • the torque and rotation angle sensor 30 is used to detect the rotation angle of the steering wheel 20 and the received torque.
  • the steering motor 40 is used to drive the steering wheel 20 to rotate.
  • the reduction mechanism 50 is used to transmit the deceleration of the rotation of the steering motor 40 to the steering wheel 20 .
  • the clutch 70 is disposed between the steering motor 40 and the reduction mechanism 50 , and is used to control the connection between the driving motor 40 and the reduction mechanism 50 .
  • the steering gear 60 is used to convert the rotation of the steering wheel 20 into linear motion or the like to drive the two front wheels 110 to rotate.
  • the steering controller 90 is used to control the steering motor 40 and the clutch 70 according to the driver's operation on the steering wheel 20 or according to the instructions of the vehicle domain controller (Vehicle Domain Controller, VDC) 10 described later.
  • the steering controller 90 can be electronically controlled.
  • Control unit electronic control unit, ECU
  • the torque and rotation angle sensor 30, the steering motor 40, the clutch 70, the reduction mechanism 50, the steering gear 60, the steering controller 90, etc. constitute an electric power steering system (Electronic Power Steering, EPS).
  • the EPS system also includes structural elements such as a vehicle speed sensor.
  • the EPS system has a power assist function to assist the driver's steering operation, and also has an autonomous steering function to actively steer the wheels 110 according to the instructions of the controller (such as the vehicle domain controller 10). Therefore, it can be said that The EPS system constitutes the autonomous steering system.
  • a four-wheel steering (4WS) system may also be employed.
  • the vehicle 100 has a vehicle domain controller 10, which is used to provide services for vehicle components in the body domain and vehicle components in the chassis domain, wherein the vehicle components in the body domain Components include door and window lift controllers, electric rearview mirrors, air conditioners, central door locks, etc.
  • Vehicle components in the chassis domain include vehicle components in the braking system, vehicle components in the steering system, and vehicle components in the acceleration system, such as the accelerator.
  • the vehicle domain controller 10 is also responsible for the overall control function of the differential steering system and the autonomous steering system. , it is also possible to make the differential steering system and the autonomous steering system work together (simultaneously). By making the differential steering system and the autonomous steering system function together to perform steering control, it is possible to obtain effects such as improvement of vehicle stability during steering.
  • the steering control method provided by an embodiment of the present application will be described below with reference to FIGS. Applied in automatic driving, it can also be used in manual driving as an auxiliary driving function.
  • Fig. 2 is a flow chart of a steering control method provided by an embodiment of the present application.
  • the steering control method is executed by a control device.
  • the control device is a vehicle domain controller.
  • the vehicle domain controller may include a functional module for implementing vehicle dynamics control.
  • the function module executes the above-mentioned steering control method.
  • the steering control method may also be executed by a controller that is independent from the vehicle controller and is used to realize vehicle dynamics control.
  • the steering control method provided by the embodiment of the present application is described in detail below, and the method may specifically include the following content:
  • the vehicle dynamics related parameter information includes the heading angle obtained by the vehicle's inertial measurement unit (Inertial Measurement Unit, IMU) Roll angle ⁇ Roll , pitch angle ⁇ Pitch , longitudinal acceleration a x , lateral acceleration a y , vertical acceleration a z , and yaw rate r estimated by calculation, road surface adhesion coefficient ⁇ , etc.
  • IMU Inertial Measurement Unit
  • the ESP system receives the angle signal from the steering wheel torque angle sensor, combined with the vehicle speed signal, it can estimate the vehicle body yaw rate value under the vehicle speed and steering wheel angle.
  • the inertial measurement unit is a device that measures the three-axis attitude angle (or angular rate) and acceleration of an object.
  • an inertial measurement unit includes three single-axis accelerometers and three single-axis gyroscopes.
  • the accelerometer detects the object
  • the acceleration signals of the three axes are independent in the carrier coordinate system, while the gyroscope detects the angular velocity and acceleration of the object in the three-dimensional space.
  • the estimation of road surface adhesion coefficient is briefly described below.
  • the road surface adhesion coefficient is equal to the ratio of the tire longitudinal force to the vertical load.
  • the estimation of the road surface adhesion coefficient is the estimation of the maximum adhesion rate, and there is a ⁇ -s curve relationship between the adhesion coefficient ⁇ and the tire slip rate s, and the slip rate can be determined by the wheel speed , vehicle speed, ground force and other signals are estimated and calculated, and combined with the longitudinal acceleration, the adhesion coefficient can be calculated.
  • the yaw rate r and the road surface adhesion coefficient ⁇ can be estimated by the vehicle domain controller, or obtained by the vehicle domain controller from other controllers.
  • the tire force range indicates an allocatable range of tire force.
  • the road surface adhesion coefficient will affect the tire force, so the tire force range can be determined according to the road surface adhesion coefficient.
  • the change of the vertical load on the wheel will change the tire force range of the wheel, so the tire force range can be determined according to the variation of the vertical load.
  • the variation of the vertical load can be determined according to one or more of the vehicle's acceleration in the horizontal, vertical and vertical directions, the state parameter of the rolling motion (roll angle) or the state parameter of the pitching motion (pitch angle).
  • the real-time vertical load of each wheel is determined according to the variation of the vertical load, and the tire force range of each wheel is determined according to the real-time vertical load.
  • Steering coordination rate (sometimes simply referred to as coordination rate) can also be called the control coefficient of the autonomous steering controller and differential steering controller, indicating the weight of autonomous steering and differential steering, and can also be said to indicate the vehicle's autonomous steering system.
  • the amount of steering is weighted with the amount of steering produced by the vehicle's differential steering system.
  • the steering amount here can be determined by the steering radius or the yaw angle.
  • the basic coordination rate is an initial parameter of the steering coordination rate, which can be preset according to experiments or experiences, for example.
  • S5. Determine the target wheel angle and target yaw moment according to the yaw rate error and the steering coordination rate.
  • the target wheel angle and differential steering produced by the autonomous steering system can be determined based on the yaw rate error and combined with the weights of autonomous steering and differential steering.
  • the steering coordination rate that is, the weight of autonomous steering and differential steering
  • the tire force can be fully utilized, the lateral response speed can be improved, the turning radius can be reduced, and the trajectory tracking accuracy can be improved.
  • the steering coordination ratio is determined by considering the tire force range, the steering stability and safety of the vehicle can also be improved under extreme driving conditions (low road adhesion coefficient, high-speed driving, etc.).
  • the steering control method proposed in this embodiment does not need to add additional on-board hardware (such as the hardware required for in-wheel motors to drive electric vehicles).
  • the threshold value of the vehicle state quantity is corrected, and the steering coordination rate is determined on this basis, thus improving the driving safety of the vehicle.
  • the steering control method proposed in this embodiment uses the dual systems of the autonomous steering system and the differential system for coupled steering control, and has redundant functions, which can improve the driving safety of the vehicle under extreme working conditions.
  • the differential steering system can work to improve the driving safety of the vehicle.
  • the method of this embodiment can reduce the driver's operations and reduce the driving burden.
  • the target center-of-mass sideslip angle can also be determined.
  • the target wheel rotation angle and yaw moment can be determined according to the yaw rate error and the center-of-mass sideslip angle error. In this way, the vehicle can be turned Maintain a stable stance and improve occupant comfort.
  • Fig. 3 is a schematic block diagram of the structure of a steering control device provided by an embodiment of the present application.
  • the steering control device is used to execute the steering control method described with reference to FIG. 2 .
  • the steering control device 200 includes a processing module 210 and a transceiver module 220.
  • the processing module 210 can be used to execute the above S2-S5, and the transceiver module 220 can be used to execute the above S1 and S6.
  • the steering control device can be composed of an electronic control unit ECU, which refers to a control device composed of integrated circuits for realizing a series of functions such as analyzing, processing and sending data.
  • ECU electronice control unit
  • FIG. 4 an embodiment of the present application provides an ECU, which includes a microcomputer, an input circuit, an output circuit, and an analog-to-digital (A/D) converter.
  • A/D analog-to-digital
  • the main function of the input circuit is to preprocess the input signal (such as the signal from the sensor), and the processing method is different for different input signals.
  • the input circuit may include an input circuit that processes analog signals and an input circuit that processes digital signals.
  • the main function of the A/D converter is to convert the analog signal into a digital signal. After the analog signal is preprocessed by the corresponding input circuit, it is input to the A/D converter for processing and converted into a digital signal accepted by the microcomputer.
  • the output circuit is a device that establishes a connection between the microcomputer and the actuator. Its function is to convert the processing results sent by the microcomputer into control signals to drive the actuators to work.
  • the output circuit generally uses a power transistor, which controls the electronic circuit of the actuator by turning on or off according to the instructions of the microcomputer.
  • Microcomputer includes central processing unit (central processing unit, CPU), memory and input/output (input/output, I/O) interface, CPU is connected with memory, I/O interface through bus, can communicate with each other through bus exchange.
  • the memory may be a memory such as a read-only memory (ROM) or a random access memory (RAM).
  • the I/O interface is a connection circuit for exchanging information between the central processor unit (CPU) and the input circuit, output circuit or A/D converter. Specifically, the I/O interface can be divided into a bus interface and a communication interface .
  • the memory stores programs, and the CPU calls the programs in the memory to execute the steering control method described in the embodiment corresponding to FIG. 2 .
  • the embodiment of the present application provides a computing device, the computing device includes a processor and a memory, and the memory stores program instructions, and when the program instructions are executed by the processor, the steering control method described in the corresponding embodiment in FIG. 2 is executed.
  • the embodiment of the present application also provides a computer-readable storage medium (memory) and a computer program product included in the computing device.
  • Fig. 5 is an explanatory diagram of a steering control method provided by an embodiment of the present application.
  • x represents the longitudinal direction of the vehicle
  • y represents the lateral direction of the vehicle
  • z represents the vertical direction of the vehicle.
  • f indicates front
  • r indicates rear
  • l indicates left
  • r indicates right.
  • the " ⁇ ” on the letter means differential
  • one " ⁇ ” means first order differential
  • two " ⁇ ” means second order differential.
  • ⁇ Roll represents the roll angle
  • the steering control method of this embodiment includes the following contents.
  • S10 Obtain vehicle dynamics-related parameter information for subsequent path planning, and use the trajectory tracking controller (model) established based on the steering dynamics model to determine the target front wheel rotation angle and target yaw moment.
  • the vehicle dynamics related parameter information includes the heading angle obtained by the vehicle's inertial measurement unit Roll angle ⁇ Roll , pitch angle ⁇ Pitch , longitudinal acceleration a x , lateral acceleration a y , vertical acceleration a z , and yaw rate r obtained through calculation and estimation, road surface adhesion coefficient ⁇ , etc.
  • the yaw rate r and the road surface adhesion coefficient ⁇ can be estimated by the vehicle domain controller, or obtained by the vehicle domain controller from other controllers.
  • S20 According to the real-time position of the vehicle and other vehicle state signals, for example, adopt the preview control algorithm to realize lateral displacement and heading angle tracking, and obtain the target yaw rate and the target center-of-mass sideslip angle.
  • MPC trajectory tracking control is a kind of optimal control problem dedicated to decomposing a longer time span, or even infinite time, into several shorter time spans, or optimal control problems with a limited time span, and to a certain extent A control method that still pursues the optimal solution.
  • Model predictive control consists of three elements: predictive model, online rolling optimization, and feedback correction.
  • S50 Based on the adhesion coefficient of the road surface, plan the dynamic distribution area of the tire force according to the acceleration of the vehicle in the horizontal, vertical and vertical directions and the load transfer effect caused by the roll and pitch motions.
  • S70 Determine the required EPS torque according to the target front wheel rotation angle, determine the required driving torque of each wheel according to the target yaw moment, and finally complete the closed-loop control of the motor.
  • S70 may be executed by the controller of the EPS system and the controller of the in-wheel motor respectively.
  • S20 According to the real-time position of the vehicle and other vehicle state signals, for example, adopt the preview control algorithm to realize lateral displacement and heading angle tracking, and obtain the target yaw rate and the target center-of-mass sideslip angle.
  • FIG. 6 is a schematic illustration of vehicle states involved in trajectory tracking control in an embodiment of the present application.
  • the S-shaped curve in the figure is the target path of the vehicle, and CG is the center of mass of the vehicle.
  • ⁇ ( ⁇ ) represents the curvature at T on the path, which is related to the arc length from this point to the starting point.
  • the lateral error and heading error of the vehicle are expressed as follows:
  • the lateral error and heading error in the above formula are globally asymptotically stable and converge to zero, while meeting the stability requirements of the vehicle.
  • the control objective is:
  • the target yaw rate is obtained through backstepping algorithm design as follows:
  • k 1 and k 2 represent weight coefficients, which are constants greater than zero. k 1 and k 2 need to satisfy the condition k 2 >k 1 v x to determine the asymptotic stability of lateral error and heading error.
  • the preview error can be expressed as follows:
  • L is the preview distance
  • e a is the preview error
  • a small angle assumption is made for the heading angle error
  • the target value of the lateral velocity is designed to tend to be 0, namely:
  • the target side slip angle of the center of mass is zero.
  • S30 Based on the yaw rate error, the center of mass side slip angle error, and the steering coordination rate, use the established model predictive control (MPC) trajectory tracking controller for front wheel steering and differential steering to determine the target front wheel angle and Target yaw moment.
  • MPC model predictive control
  • the front wheel steering dynamics model is established.
  • the front wheel autonomous steering takes the steering angle ⁇ f as the system input, and the vehicle dynamics equation is expressed as follows:
  • a non-linear brush tire model will be used to calculate the longitudinal and lateral forces of the tire
  • the state quantity is:
  • the input amount is:
  • differential steering dynamics model is established, and the differential torque Mz is defined as the system input.
  • differential torque Mz is defined as the system input.
  • the differential steering system can be expressed as follows:
  • J f , J r represent the equivalent yaw moment of inertia of the front axle and rear axle respectively
  • d f , d r represent the equivalent damping of the front axle and rear axle respectively
  • ⁇ Af , ⁇ Ar represent the front and rear tires respectively
  • the aligning torque, ⁇ Ff and ⁇ Fr represent the friction torque of the front and rear tires respectively.
  • F 1 and F 2 are the driving forces of the two front wheels
  • F 3 and F 4 are the driving forces of the two rear wheels
  • a and b are the distances from the front and rear axles to the center of mass respectively.
  • the state quantity is:
  • the input amount is:
  • the vehicle dynamics model based on front wheel active steering and differential steering is uniformly written in the following nonlinear time-varying form:
  • the differential steering system can be realized for both front and rear axles, and its state and input are:
  • ⁇ x(k+1) A k ⁇ x(k)+B k ⁇ u(k)
  • x p+1,k A p,k x p,k +B p,k u p,k +d p,k (3-9)
  • ⁇ U is the control input increment
  • N p and N c are the number of prediction steps and control compensation, respectively
  • Q, R, and ⁇ are the corresponding weight coefficients
  • ⁇ is the relaxation factor
  • the objective function is similarly designed as:
  • the optimal control increment sequence at the current moment can be obtained as follows:
  • the first item of the optimal control increment sequence is used as the current actual control quantity of the system, and finally the rolling optimization is performed to obtain the result of the entire control sequence.
  • ⁇ f,test and M Z,test represent the front wheel rotation angle and differential torque input of the steering circle experiment respectively.
  • the weight of the front wheel steering can be determined:
  • ⁇ b ,1 represents the basic coordination rate of the front wheel EPS in the circular motion lateral tracking control, and is the initial parameter of the steering coordination rate.
  • the control states of the two systems should be constrained, so that the vehicle does not have to perform front wheel steering and differential braking control at the same time under certain working conditions.
  • the driving condition does not consider the acceleration process, and then according to different working conditions and the estimated vehicle posture, the vehicle driving state is determined as shown in Table 1.
  • r is the yaw rate
  • ⁇ f is the front wheel angle
  • r t is the threshold value of the yaw rate
  • ⁇ t is the threshold value of the front wheel angle.
  • K is the stability factor, which characterizes the steady-state response in the vehicle handling stability experiment, as follows:
  • m represents the mass of the vehicle
  • l f represents the distance from the front axle to the center of mass of the vehicle
  • l r represents the distance from the rear axle to the center of mass of the vehicle
  • k 1 represents the cornering stiffness of the front axle
  • k 2 represents the cornering stiffness of the rear axle
  • the control states of the Front-wheel Steering (FS, Front-wheel Steering) controller and the Differential Steering (DS) controller are obtained.
  • the front wheels When controlling the steering of the front wheels, the front wheels will be subjected to the lateral force provided by the ground to generate a certain turning angle, and the differential steering controls the driving torque difference between the left and right wheels of the coaxial four-wheel independent drive electric vehicle, that is, the differential torque, not only generates lateral
  • the pendulum motion can also act on the steering system to drive the wheels to rotate at a certain angle.
  • the forces acting on the tires from the ground include lateral force (lateral force, lateral force) and longitudinal force, both of which satisfy the attachment ellipse relationship, as shown in Figure 7, in addition to being affected by the magnitude of the driving force and braking force, the lateral force It is also related to the side slip angle of the tire. The larger the side slip angle, the greater the cornering force.
  • ⁇ f,2 represents the relative utilization ratio of the tire in the transverse direction
  • F yi is the cornering force on tire i
  • F xi is the longitudinal force on tire i
  • the limit value of transverse and longitudinal tire force F yi,lim , F xi,lim is obtained from the tire attachment ellipse. Refer to Figure 7 for a schematic view of the tire attachment ellipse.
  • F yi is the cornering force on tire i (in this embodiment, there are 4 tires, so i is a positive integer less than or equal to 4).
  • the cornering force is proportional to the side slip angle, when When it is in the non-linear region, it is obtained through the calibrated data look-up table, and its expression is:
  • whether it belongs to the linear region or the nonlinear region can be judged according to the size of the slip angle. In addition, it can also be judged in combination with the size of the side slip angle, the state of the road surface and the state of the tires (tire pressure, etc.).
  • ⁇ c,1 represents the control coefficient of the front wheel steering controller in the coordinated control
  • ⁇ c,2 represents the control coefficient of the differential steering controller in the coordinated control.
  • the range of coordination rate can be obtained as shown in Figure 8.
  • the ellipse line represents the boundary of the tire force range
  • the hatched line represents the change range of the tire force range boundary, that is, due to the different vehicle load and road adhesion coefficient, the tire force that the vehicle can exert is different, so there is such a boundary change .
  • the solid vector line on the left corresponds to the base coordination rate.
  • the solid vector line on the right corresponds to the steering coordination rate of a working condition (such as under normal driving conditions, without exceeding the dynamic boundary), and the dotted vector line corresponds to the steering coordination rate of front wheel steering and differential steering under extreme conditions (such as When turning quickly at a large corner, a large lateral force needs to be increased).
  • the embodiment of the present application is based on the road adhesion coefficient , considering the motion posture of the car body, and planning the tire force distribution area (tire force range) in real time.
  • the vertical load F zi affects the tire force range of the vehicle.
  • the embodiment of the present application considers the impact of the X, Y, and Z three-dimensional movement on the load transfer, mainly including the acceleration in the horizontal, vertical, and vertical directions of the vehicle, as well as the roll and pitch motions.
  • Figure 9 is a schematic diagram of the force analysis of the vehicle body motion. , and then establish the relational expression of the load transfer (load variation) due to the movement of the vehicle body.
  • the lateral acceleration a y results in a load transfer to the left and right wheels as:
  • I x is the moment of inertia around the x-axis (longitudinal axis), is the roll angular acceleration.
  • the longitudinal acceleration a x causes the load transfer on the front and rear axles to be:
  • the estimated values of the vertical loads of the four wheels are:
  • m is the mass of the vehicle, specifically, it can be the sprung mass (unloaded mass), or the actual mass (non-unloaded mass) estimated in real time.
  • the boundary of the attachment ellipse changes with the change of the load
  • the dynamically distributable area of tire force obtained by ⁇ F zfl is shown in Figure 10.
  • the ellipse line represents the boundary of the tire force range
  • the hatched area represents the variation range of the tire force boundary.
  • the right vector line corresponds to the resultant force of the braking force and the cornering force
  • ⁇ is the angle between the resultant force and the braking force.
  • the vector line on the left corresponds to the resultant force after considering the vertical force (an example).
  • the maximum lateral force and maximum longitudinal force of the tire can be obtained as:
  • the MPC controller described in S30 enables the vehicle to track the target yaw rate and the target center-of-mass side slip angle, but it cannot be used under some extreme conditions (low ground adhesion coefficient, uneven force on each tire) Provide enough tire force to achieve the target yaw rate, so in order to improve driving stability, combined with the front and rear axle tire force limits, the yaw rate is constrained:
  • F yf,lim and F yr,lim are the maximum lateral force of the front and rear axles respectively:
  • the sideslip angle of the center of mass can be constrained to ensure that the side slip angle of the center of mass is not too large.
  • the relationship between the side slip angle of the center of mass and the side slip angle of the front and rear tires is:
  • the side slip angle of the center of mass can be kept within a reasonable range by limiting the side slip angle of the tire:
  • the limit values of the above-mentioned vehicle state signals are also fed back to the MPC trajectory tracking controller in S30, and converted into inequality constraints for rolling optimization solution.
  • S40 and S50 can be regarded as one process.
  • the objective function mainly includes three parts, the first term is the sum of the corner errors within the control domain time, the second term is the sum of the torque errors within the control domain time, and the third term is the sum of the load rates of each tire.
  • ⁇ f (k+i) represents the front wheel angle error value at time k+i
  • ⁇ M z (k+i) represents the yaw moment error value at time k+i
  • R represents the corresponding weight matrix
  • N c represents the step size of the control domain.
  • ⁇ represents the relaxation factor, which ensures that the optimization problem is solvable.
  • Inequality constraint 6-2-3) means that the force balance of each tire force is considered, and ⁇ lim is the limiting coefficient; formula 6-2-4) means that the longitudinal force is constrained by comprehensively considering the performance of the in-wheel motor and road adhesion conditions; the above optimization problem It can be transformed into a Quadratic Programming (QP) problem for solution, for example, by an effective set method or an interior point method.
  • QP Quadratic Programming
  • the quadratic programming problem is a nonlinear programming problem with constraints
  • the objective function f(x) is a quadratic function with a simple form, which can be solved by using the general method for solving nonlinear programming, and has a specific solution method.
  • the steering coordination rate can be determined, which can be provided to the MPC trajectory tracking controller in S30 for determining the target front wheel rotation angle and target yaw moment.
  • the steering coordination rate that is, the weight of autonomous steering and differential steering
  • the tire force can be fully utilized, the lateral response speed can be improved, the turning radius can be reduced, and the trajectory tracking accuracy can be improved.
  • the steering coordination ratio is determined by considering the tire force range, the steering stability and safety of the vehicle can also be improved under extreme driving conditions (low road adhesion coefficient, high-speed driving, etc.).
  • the steering control method proposed in this embodiment does not need to add additional on-board hardware (such as the hardware required for in-wheel motors to drive electric vehicles).
  • the threshold value of the vehicle state quantity is corrected, and the steering coordination rate is determined on this basis, thus improving the driving safety of the vehicle.
  • the steering control method proposed in this embodiment uses the dual systems of the autonomous steering system and the differential system for coupled steering control, and has redundant functions, which can improve the driving safety of the vehicle under extreme working conditions.
  • the differential steering system can work to improve the driving safety of the vehicle.
  • FIG. 11 is a schematic diagram of the architecture of a steering system (of a vehicle) to which the steering control method according to an embodiment of the present application is applied.
  • the system architecture mainly includes the following modules:
  • Signal processing module mainly includes detection and collection, parameter estimation, identification and other modules, the purpose of which is to obtain information related to steering control. Specifically, it includes obtaining the real-time horizontal and vertical position X, Y of the vehicle through camera acquisition, and obtaining the heading angle through IMU Roll angle ⁇ Roll , pitch angle ⁇ Pitch , longitudinal acceleration a x , lateral acceleration a y , and vertical acceleration a z , the parameter estimation module mainly obtains the side slip angle ⁇ of the center of mass and the yaw rate r, and the identification module obtains the road adhesion in real time. Coefficient ⁇ .
  • the signal processing module may be used to execute the content described in S10 above.
  • the parameter estimation module and the identification module can also be integrated in the VDC.
  • Center of mass sideslip angle estimation Based on the two-degree-of-freedom model of the vehicle, it can be simplified to a linear estimation model in which the lateral speed is the state, and the state value of the lateral speed can be estimated by using the linear Kalman filter method Finally, according to the relationship between the sideslip angle of the center of mass and the longitudinal and lateral speeds, it can be calculated
  • ESP system receives the angle signal from the steering wheel angle sensor, combined with the vehicle speed signal, it can estimate the yaw rate value of the vehicle body under the vehicle speed and steering wheel angle.
  • the estimation of the road surface adhesion coefficient is the estimation of the maximum adhesion rate, and there is a ⁇ -s curve relationship between the adhesion coefficient ⁇ and the tire slip rate s, and the slip rate can be estimated by signals such as wheel speed, vehicle speed and ground force Calculated, combined with the longitudinal acceleration can be calculated to get the adhesion coefficient.
  • Path planning module Assuming that the vehicle is traveling at a constant speed during the trajectory tracking process, the path tracking target can specifically consider the lateral tracking accuracy, that is, to pursue the minimum lateral error and heading angle error, and use the preview control algorithm to obtain the target yaw angular velocity, Center of mass slip angle.
  • the path planning module may be used to execute the content described in S20 above.
  • MPC trajectory tracking module Based on the vehicle dynamics model of front wheel steering and differential steering, a dual-system coordination rate model predictive controller is established to control the yaw rate and the sideslip angle of the center of mass to the target value to meet the lateral direction of trajectory tracking. Tracking requirements, the controller solves to get the target front wheel angle and target yaw moment
  • the MPC trajectory tracking module can be used to execute the content in S30 above.
  • Coordination control module comprehensively consider the basic coordination rate, the relative utilization rate and the vehicle attitude state to determine the coordination rate range.
  • a comprehensive objective function J1 is established.
  • X, Y, Z three-dimensional motion on load transfer determine the tire force distribution area, and provide boundary constraints for the coordination rate range.
  • a coordination rate optimization model is constructed, which is converted into a quadratic programming problem and solved to obtain the optimal coordination rate of the dual system (front wheel steering, differential steering).
  • the coordination control module may be used to execute the content described in S40-S60 above.
  • the above-mentioned path planning module, MPC trajectory tracking module, coordination control module, and vehicle attitude estimation module are all integrated in the VDC. Calculated in real time, the control command front wheel angle ⁇ f,c and the driving torque command T i for distributing the yaw moment M z,c to the four wheels are finally obtained.
  • Motor control strategy module the front wheel autonomous steering controller completes the closed-loop control of the corner after receiving the corner, and the hub motors of the four wheels complete the closed-loop control of the torque after receiving the VDC torque command T i .
  • the motor control strategy module can be used to execute the content in S70 above. In addition, some or all functions of the motor control strategy module can also be integrated in the VDC.
  • the embodiment of the present application also provides a vehicle including the above-mentioned system architecture, the steering control device, the computing device, etc. described in the above-mentioned embodiments.
  • first”, “second”, “third” and other similar terms in the description and claims are only used to distinguish similar objects, and do not represent a specific ordering of objects, and they can be used interchangeably if permitted. in a particular order or sequence.
  • the labels involved to indicate the specific content of the method, such as S10, S20..., etc. do not necessarily mean that they will be executed in the order of the labels, and the front and rear sequences can be interchanged or executed at the same time if allowed.
  • the term “comprising” used in the description and claims should not be interpreted as being limited to what is listed thereafter; it does not exclude the presence of other elements or steps.

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Abstract

一种车辆转向控制方法及装置，该方法包括：根据路面附着系数与车辆的实时载荷转移确定轮胎力范围，根据轮胎力范围确定自主转向与差动转向的转向协调率，转向协调率指示自主转向与差动转向的权重，根据该转向协调率以及横摆角速度误差等确定目标车轮转角与目标横摆力矩，将目标车轮转角提供给自主转向系统，将目标横摆力矩提供给差动转向系统。该方法为基于轨迹跟踪的自主转向与差动转向的转向协调控制方法，其能够提高轨迹跟踪精度。

Description

一种转向控制方法及装置 技术领域
本申请涉及智能车辆技术领域，特别涉及一种转向控制方法及装置。
背景技术
作为车辆控制方法，为了使车辆按照预定路径和轨迹行驶，存在如下一种使自主转向与差动转向共同作用来进行轨迹跟踪的转向控制方法，具体而言，在该方法中，当仅执行前轮自主转向不能使侧向偏差抑制在较小范围内(即侧向偏差过大)时，用差动转向(Differential Steering，DS)来辅助前轮自主转向，从而能够使车辆能够可靠地按照预定路径行驶。
然而，上述转向控制方法中，仅在自主转向产生的侧向偏差较大的情况下使自主转向与差动转向共同发挥作用来进行轨迹跟踪，未充分发挥自主转向与差动转向的协调作用，在某些行驶工况(例如高速行驶工况或者其他极限工况)下侧向响应速度较慢，跟踪精度较低。
发明内容
本申请提供一种转向控制方法及装置等，以能够提高轨迹跟踪精度。
本申请第一方面提供一种转向控制方法，其包括如下内容：获取轮胎力范围；根据轮胎力范围确定转向协调率，转向协调率指示车辆的自主转向系统产生的转向量与车辆的差动转向系统产生的转向量的权重；获取横摆角速度误差；根据横摆角速度误差与转向协调率确定目标车轮转角与目标横摆力矩；发送第一控制指令，第一控制指令包括用于使自主转向系统产生目标车轮转角的控制指令与用于使差动转向系统产生目标横摆力矩的控制指令。
采用如上的转向控制方法，由于根据轮胎力范围确定转向协调率即自主转向与差动转向所占的权重，因此，可充分发挥轮胎力，提高侧向响应速度，降低转弯半径，提高轨迹跟踪精度。
另外，由于考虑了轮胎力范围来确定转向协调率，因而在极限行驶工况(路面附着系数较低、高速行驶等工况)下也能够提高车辆的操纵稳定性、安全性。
另外，本实施例提出的转向控制方法，利用自主转向系统与差动系统双系统进行耦合转向控制，具备冗余功能，可提高应对极限工况下的车辆行驶安全性。此外，例如当自主转向系统失效或发生故障时，能够由差动转向系统工作，提高车辆行驶的安全性。
作为第一方面的一个可能的实现方式，在上述方法中，获取路面附着系数和/或车辆的多个车轮的载荷变化量；根据路面附着系数和/或载荷变化量确定轮胎力范围。
考虑路面附着系数与车轮的载荷变化量来确定轮胎力范围，可以可靠地规划轮胎力范围。
作为第一方面的一个可能的实现方式，获取车辆的横纵垂三方向的加速度、侧倾运动状态参数或俯仰运动状态参数中的一项或多项；根据加速度、侧倾运动状态参数或俯仰运动状态参数中的一项或多项确定载荷变化量。
考虑车辆的横纵垂三方向的加速度、侧倾运动状态参数或俯仰运动状态参数来确定载荷变化量，能够可靠地确定载荷变化量。
作为第一方面的一个可能的实现方式，根据轮胎力范围确定转向协调率具体可以包括：获取轮胎横向相对利用率与预设的基础协调率，轮胎横向相对利用率指示横向轮胎力相对于总轮胎力的比率，基础协调率是转向协调率的初始参数；根据基础协调率、轮胎横向相对利用率与轮胎力范围确定转向协调率。
转向量典型地可以由转向半径或横摆角确定。
本申请第二方面提供一种车辆转向控制装置，其包括处理模块与收发模块，处理模块用于，获取轮胎力范围与横摆角速度误差，根据轮胎力范围确定转向协调率，转向协调率指示车辆的自主转向系统产生的转向量与车辆的差动转向系统产生的转向量的权重，根据横摆角速度误差与转向协调率确定目标车轮转角与目标横摆力矩；收发模块用于发送第一控制指令，第一控制指令包括用于使自主转向系统产生目标车轮转角的控制指令与用于使差动转向系统产生目标横摆力矩的控制指令。
作为第二方面的一个可能的实现方式，处理模块具体用于，获取路面附着系数和/或车辆的多个车轮的载荷变化量；根据路面附着系数和/或载荷变化量确定轮胎力范围。
作为第二方面的一个可能的实现方式，处理模块具体用于，获取车辆的横纵垂三方向的加速度、侧倾运动状态参数或俯仰运动状态参数中的一项或多项；根据加速度、侧倾运动状态参数或俯仰运动状态参数中的一项或多项确定载荷变化量。
作为第二方面的一个可能的实现方式，处理模块具体用于，获取轮胎横向相对利用率与预设的基础协调率，轮胎横向相对利用率指示横向轮胎力相对于总轮胎力的比率，基础协调率是转向协调率的初始参数；根据基础协调率、轮胎横向相对利用率与轮胎力范围确定转向协调率。
转向量可以由转向半径或横摆角确定。
本申请第二方面的技术效果与第一方面中描述的基本相同，这里不再重复描述。
本申请第三方面提供一种计算设备，其包括处理器与存储器，存储器存储有程序指令，程序指令当被处理器执行时使得处理器执行第一方面中描述的任一种方法。
本申请第四方面提供一种计算机可读存储介质，其存储有程序指令，程序指令当被计算机执行时使得计算机执行第一方面中描述的任一种方法。
本申请第五方面提供一种计算机程序产品，其包括有程序指令，程序指令当被计算机执行时使得计算机执行第一方面中描述的任一种方法。
本申请的这些和其它方面在以下(多个)实施例的描述中会更加简明易懂。
附图说明
图1为本申请一个实施例的转向控制方法所应用的一种车辆的结构示意图；
图2为本申请一个实施例提供的一种转向控制方法的流程图；
图3为本申请一个实施例提供的转向控制装置的结构示意框图；
图4为本申请一个实施例提供的电子控制单元的一种结构示意图；
图5为本申请一个实施例提供的转向控制方法的说明图；
图6是本申请一个实施例中的轨迹跟踪控制涉及的车辆状态的一种示意说明图；
图7是本申请一个实施例中涉及的附着椭圆的示意说明图；
图8是本申请一个实施例中涉及的协调率范围的示意说明图；
图9为本申请一个实施例的描述中涉及的车体运动受力分析简图；
图10为本申请一个实施例中涉及的轮胎力范围的示意说明图；
图11为本申请一个实施例的转向控制方法所应用的一种(车辆的)转向系统架构示意图。
具体实施方式
作为车辆的控制方法，为了使车辆按照预定路径和轨迹行驶，存在如下一种使自主转向与差动转向共同作用来进行轨迹跟踪的转向控制方法，具体而言，在该方法中，当仅执行前轮自主转向不能使侧向偏差抑制在较小范围内(即侧向偏差过大)时，用差动转向来辅助前轮自主转向，从而能够使车辆能够可靠地按照预定路径行驶。
然而，上述转向控制方法中，仅在自主转向产生的侧向偏差较大的情况下使自主转向与差动转向共同发挥作用来进行轨迹跟踪，未充分发挥自主转向与差动转向的协调作用，在某些行驶工况(例如高速行驶工况或者其他极限工况)下侧向响应速度较慢，跟踪精度较低。
有鉴于此，本申请一个实施例提供一种转向控制方法，以能够提高轨迹跟踪精度。
在描述该转向控制方法之前，先描述一下应用该转向控制方法的一种车辆的相关结构。
图1为本申请一个实施例的转向控制方法所应用的一种车辆的结构示意图。如图1所示，该车辆100为分布式驱动汽车，在其4个车轮110中分别配置有轮毂电机120，由轮毂电机120对车轮110进行驱动与制动。另外，轮毂电机120可以被独立控制(电机控制器未图示)，而使同轴的车轮110间受到的力矩不同，如此来产生差动力矩，使车辆100产生差动转向。即，轮毂电机120构成了差动转向(Differential Steering，DS)系统。
另外，作为分布式驱动汽车的其他例子，也可以代替轮毂电机120，而在4个车轮110附近分别设置轮边电机，轮边电机分别通过传动机构与车轮110连接，而能够对车轮110进行驱动与制动。另外，本申请实施例的方法还可以适用于其他类型的车辆。
另外，如图1所示，车辆100还包括方向盘20、转矩与转角传感器30、转向电机40、离合器70、减速机构50、转向器60与转向控制器90。方向盘20供驾驶员进行转向操作。扭矩与转角传感器30用于检测方向盘20的转角与受到的扭矩。转向电机40用于驱动方向盘20转动。减速机构50用于将转向电机40的转动减速传递给方向盘20。离合器70设置在转向电机40与减速机构50之间，用于控制驱动电机40与减速机构50之间的连接的通断。转向器60用于将方向盘20的转动变换为直线运动 等而驱动两个前轮110进行转动。转向控制器90用于根据驾驶员对方向盘20的操作或者根据后述的整车域控制器(Vehicle Domain Controller，VDC)10的指令，控制转向电机40与离合器70，转向控制器90可以用电子控制单元(electronic control unit，ECU)构成。转矩与转角传感器30、转向电机40、离合器70、减速机构50、转向器60与转向控制器90等构成了电动助力转向系统(Electronic Power Steering，EPS)。该EPS系统除了包括上述结构要素外，还包括车速传感器等结构要素。
该EPS系统具有对驾驶员的转向操作进行辅助的助力功能，此外，还具有根据控制器(例如整车域控制器10)的指令主动地使车轮110进行转向的自主转向功能，因此，可以说EPS系统构成了自主转向系统。作为自主转向系统的其他例子，还可以采用四轮转向(four-wheel steering，4WS)系统。
另外，如图1所示，车辆100具有整车域控制器10，整车域控制器10用于为车身域的车辆零部件以及底盘域的车辆零部件提供服务，其中，车身域的车辆零部件包括门窗升降控制器、电动后视镜、空调、中央门锁等。底盘域的车辆零部件包括制动系统中的车辆零部件、转向系统中的车辆零部件、加速系统中的车辆零部件，比如油门等。
另外，整车域控制器10还承担差动转向系统与自主转向系统的整体控制功能，在其控制下，当需要控制车辆100进行转向时，可以分别使差动转向系统与自主转向系统发挥作用，也可以使差动转向系统与自主转向系统共同(同时)发挥作用。通过使差动转向系统与自主转向系统共同发挥作用来执行转向控制，能够获得提高转向时的车辆稳定性等效果。
下面参照图2-图4等对本申请一个实施例提供的转向控制方法进行描述，该转向控制方法是基于轨迹跟踪的使自主转向系统与差动转向系统两者相协调的转向协调控制方法，可以应用在自动驾驶中，也可以作为辅助驾驶功能应用在人工驾驶中。
图2为本申请实施例提供的一种转向控制方法的流程图。该转向控制方法由控制装置执行，在本实施例中，该控制装置是整车域控制器，具体而言，该整车域控制器可以包括用于实现车辆动力学控制的功能模块，由该功能模块执行上述转向控制方法。另外，作为其他实施例，该转向控制方法也可以由独立于整车控制器、用于实现车辆动力学控制的控制器执行。
下面对本申请实施例提供的转向控制方法进行详细描述，该方法具体可以包括如下内容：
S1，获取车辆动力学相关信息。这些信息包括通过车辆的摄像头等获取的车辆实时横纵向位置。车辆动力学相关参数信息包括通过车辆的惯性测量单元(Inertial Measurement Unit，IMU)获得的航向角
Figure PCTCN2021101398-appb-000001
侧倾角θ Roll、俯仰角θ Pitch、纵向加速度a x、侧向加速度a y、垂向加速度a z，还包括通过计算估计得到的横摆角速度r、路面附着系数μ等。关于横摆角速度的估计，ESP系统接受来自方向盘转矩转角传感器的角度信号，再结合车速信号，可估计出在该车速、方向盘转角下应有的车身横摆角速度值。这里，惯性测量单元是测量物体三轴姿态角(或角速率)以及加速度的装置，一般的，一个惯性测量单元包含了三个单轴的加速度计和三个单轴的陀螺，加速度计检测物体在载体坐标系统独立三轴的加速度信号，而陀螺检测物体在三维空间中的角速度和加 速度。
下面简要描述路面附着系数的估计。路面附着系数等于轮胎纵向力与垂直载荷的比值，路面附着系数的估计就是最大的附着率的估计，而附着系数μ与轮胎滑移率s存在μ-s曲线关系，滑移率可通过轮速、车速及地面力等信号进行估计计算获得，再结合纵向加速度可计算得到附着系数。
另外，横摆角速度r、路面附着系数μ可以由整车域控制器估计得到，也可以由整车域控制器从其他控制器获取。
S2，根据车辆实时位置信号确定侧向位移误差和航向角误差，根据侧向位移误差和航向角误差确定路径规划，获得目标横摆角速度。具体而言，例如，假设在轨迹跟踪过程中车辆以恒定车速行驶，路径跟踪目标可专门考虑侧向跟踪精度，即追求侧向误差、航向角误差为最小，例如采用预瞄控制算法得到目标横摆角速度。
S3，基于路面附着系数和/或垂向载荷(有时也简称为载荷)变化量，规划轮胎力动态范围(有时也简称为轮胎力范围)。轮胎力范围指示轮胎力的可分配范围。路面附着系数对轮胎力会产生影响，因而可以根据路面附着系数确定轮胎力范围。另外，车轮受到的垂向载荷的变化会改变该车轮的轮胎力范围，因而可以根据垂向载荷的变化量来确定轮胎力的范围。垂向载荷的变化量可以根据车辆的横纵垂三方向的加速度、侧倾运动状态参数(侧倾角)或俯仰运动状态参数(俯仰角)中的一项或多项来确定。根据垂向载荷的变化量确定各车轮的实时垂向载荷，根据实时垂向载荷确定各车轮的轮胎力范围。
S4，根据基础协调率与轮胎力范围确定转向协调率。转向协调率(有时也简称为协调率)也可以称为自主转向控制器与差动转向控制器的控制系数，指示自主转向与差动转向的权重，也可以说是指示车辆的自主转向系统产生的转向量与所述车辆的差动转向系统产生的转向量的权重。这里的转向量可以由转向半径或横摆角确定。基础协调率是转向协调率的初始参数，例如可以根据实验或者经验预先设定。
S5，根据横摆角速度误差与转向协调率确定目标车轮转角与目标横摆力矩。在S4中确定了转向协调率即自主转向与差动转向的权重后，即可基于横摆角速度误差，结合自主转向与差动转向的权重，确定自主转向系统产生的目标车轮转角与差动转向系统产生的目标横摆力矩。可以理解，横摆角速度误差根据目标横摆角速度与当前横摆角速度获得。
S6，发送基于目标车轮转角与横摆力矩的控制指令。即，根据目标车轮转角与目标横摆力矩，生成并发送控制指令，该控制指令包括用于使自主转向系统产生目标车轮转角的控制指令与用于使差动转向系统产生目标横摆力矩的控制指令。
采用如上的转向控制方法，由于根据轮胎力范围确定转向协调率即自主转向与差动转向的权重，因此，可充分发挥轮胎力，提高侧向响应速度，降低转弯半径，提高轨迹跟踪精度。
另外，由于考虑了轮胎力范围来确定转向协调率，因而在极限行驶工况(路面附着系数较低、高速行驶等工况)下也能够提高车辆的操纵稳定性、安全性。
另外，本实施例提出的转向控制方法不必额外增加车载硬件(例如轮毂电机驱动电动汽车所需的硬件)。
再者，在本实施例中，考虑路面附着条件和/或垂直载荷转移，对车辆状态量阈值进行修正，在此基础上确定转向协调率，如此，提高了车辆行驶安全性。
另外，本实施例提出的转向控制方法，利用自主转向系统与差动系统双系统进行耦合转向控制，具备冗余功能，可提高应对极限工况下的车辆行驶安全性。此外，例如当自主转向系统失效或发生故障时，能够由差动转向系统工作，提高车辆行驶的安全性。
另外，在应用于人工驾驶的情况下，本实施例的方法可以减少驾驶员的操作，降低驾驶负担。
可选地，在S2中，还可以确定目标质心侧偏角，在S5中，根据横摆角速度误差与质心侧偏角误差来确定目标车轮转角与横摆力矩，如此，可以使车辆在转向时保持平稳的姿态，提高乘员的舒适感。
图3为本申请一个实施例提供的转向控制装置的结构示意框图。该转向控制装置用于执行参照图2描述的转向控制方法。如图3所示，该转向控制装置200包括处理模块210与收发模块220，处理模块210可以用于执行上述S2-S5，收发模块220可以用于执行上述S1、S6。
另外，该转向控制装置可以用电子控制单元ECU构成，电子控制单元ECU是指由集成电路组成的用于实现对数据的分析处理发送等一系列功能的控制装置。如图4所示，本申请实施例提供了一种ECU，该ECU包括微型计算机(microcomputer)、输入电路、输出电路和模/数(analog-to-digital，A/D)转换器。
输入电路的主要功能是对输入信号(例如来自传感器的信号)进行预处理，输入信号不同，处理方法也不同。具体地，因为输入信号有两类：模拟信号和数字信号，所以输入电路可以包括处理模拟信号的输入电路和处理数字信号的输入电路。
A/D转换器的主要功能是将模拟信号转变为数字信号，模拟信号经过相应输入电路预处理后输入A/D转换器进行处理转换为微型计算机接受的数字信号。
输出电路是微型计算机与执行器之间建立联系的一个装置。它的功能是将微型计算机发出的处理结果转变成控制信号，以驱动执行器工作。输出电路一般采用的是功率晶体管，根据微型计算机的指令通过导通或截止来控制执行元件的电子回路。
微型计算机包括中央处理器(central processing unit，CPU)、存储器和输入/输出(input/output，I/O)接口，CPU通过总线与存储器、I/O接口相连，彼此之间可以通过总线进行信息交换。存储器可以是只读存储器(read-only memory，ROM)或随机存取存储器(random access memory，RAM)等存储器。I/O接口是中央处理单元(central processor unit，CPU)与输入电路、输出电路或A/D转换器之间交换信息的连接电路，具体的，I/O接口可以分为总线接口和通信接口。存储器存储有程序，CPU调用存储器中的程序可以执行图2对应实施例描述的转向控制方法。
由上可知，本申请实施例提供了一种计算设备，计算设备包括处理器与存储器，存储器中存储有程序指令，程序指令当被处理器执行时执行图2对应实施例描述的转向控制方法。另外，本申请实施例还提供了该计算设备包括的计算机可读存储介质(存储器)与计算机程序产品。
下面参照图5对本申请一个实施例提供的转向控制方法进行描述。
图5为本申请一个实施例提供的转向控制方法的说明图。
首先对下面的描述中使用的标记进行简单的描述。在下面的描述中，关于三维方向，如无特别说明，x表示车辆的纵向，y表示车辆的横向，z表示车辆的垂向。另外，关于前后左右，f表示前，r表示后，l表示左，r表示右。字母上的“·”表示微分，一个“·”表示一阶微分，两个“·”表示二阶微分。例如，θ Roll表示侧倾角，
Figure PCTCN2021101398-appb-000002
为侧倾角加速度。另外，如无特别说明，说明书全文中的符号的含义是一致的。
如图5所示，本实施例的转向控制方法包括以下内容。
S10：获取车辆动力学相关参数信息，以便后续进行路径规划、利用基于转向动力学模型建立的轨迹跟踪控制器(模型)确定目标前轮转角和目标横摆力矩。
这里，车辆动力学相关参数信息包括通过车辆的惯性测量单元获得的航向角
Figure PCTCN2021101398-appb-000003
侧倾角θ Roll、俯仰角θ Pitch、纵向加速度a x、侧向加速度a y、垂向加速度a z，还包括通过计算估计得到横摆角速度r、路面附着系数μ等。横摆角速度r、路面附着系数μ可以由整车域控制器估计得到，也可以由整车域控制器从其他控制器获取。
S20：根据车辆实时位置及其他车辆状态信号，例如采用预瞄控制算法实现侧向位移及航向角跟踪，获得目标横摆角速度与目标质心侧偏角。
S30：基于横摆角速度误差与质心侧偏角误差，并结合下文详述的转向协调率，利用建立的前轮转向与差动转向的模型预测控制(Model predictive control，MPC)轨迹跟踪控制器，确定目标前轮转角和目标横摆力矩，并输出基于此的指令，以使自主转向系统产生目标前轮转角，使差动转向系统产生目标横摆力矩。
这里，MPC轨迹跟踪控制是一种致力于将更长时间跨度、甚至于无穷时间的最优化控制问题，分解为若干个更短时间跨度，或者有限时间跨度的最优化控制问题，并且在一定程度上仍然追求最优解的控制方法。模型预测控制由三个要素组成：预测模型、在线滚动优化、反馈校正。
S40：根据前轮转向与差动转向两者的基础协调率、轮胎的横向相对利用率、车姿状态，决策协调率范围，同时协调率边界受下面详述的轮胎力动态分配区域所约束。
S50：基于路面附着系数，根据车辆横、纵、垂三方向的加速度以及侧倾、俯仰运动造成的载荷转移影响，规划轮胎力动态分配区域。
S60：根据基于转向协调率以及其他约束条件构建的协调控制优化模型，进行求解确定转向协调率(即求解获得最优协调率)。
S70：根据目标前轮转角确定EPS需求力矩，根据目标横摆力矩确定各个车轮的需求驱动力矩，最终电机完成闭环控制。这里，S70可以由EPS系统的控制器和轮毂电机的控制器分别执行。
下面对S20-S60中的内容进行更加详细的描述。
S20：根据车辆实时位置及其他车辆状态信号，例如采用预瞄控制算法实现侧向位移及航向角跟踪，获得目标横摆角速度与目标质心侧偏角。
具体的算法设计如下：
图6是本申请一个实施例中的轨迹跟踪控制涉及的车辆状态的一种示意说明图。图中S形的曲线为车辆的目标路径，CG是车辆的质心。参照图5，定义目标路径起 点弧长为σ＝0，则T时刻点距离起点的弧长表示如下：
Figure PCTCN2021101398-appb-000004
上式中，ρ(σ)表示路径上T处的曲率，与该点距起点的弧长有关。
车辆的侧向误差、航向误差表示如下：
Figure PCTCN2021101398-appb-000005
使上式中的侧向误差和航向误差全局渐近稳定并收敛到零，同时满足车辆的稳定性要求。设计轨迹跟踪控制器时，控制目标即为：
Figure PCTCN2021101398-appb-000006
以侧向误差和航向误差的全局渐进稳定为目标，通过反步(backstepping)算法设计得到目标横摆角速度如下：
Figure PCTCN2021101398-appb-000007
上式中，k 1、k 2表示权重系数，为大于零的常数。k 1、k 2需要满足条件k 2＞k 1v x以确定侧向误差和航向误差的渐进稳定性。
另外，预瞄误差可以表示如下：
e a＝e+L sinψ    (2-5)
上式中，L为预瞄距离，e a为预瞄误差，对航向角误差做小角度假设，将上式简化为：
e a＝e+Lψ    (2-6)
对比公式(2-4)和(2-6)，可得L＝1/k 1，则目标横摆角速度可进一步表示为：
Figure PCTCN2021101398-appb-000008
另外，将侧向速度的目标值设计为趋于为0，即：
Figure PCTCN2021101398-appb-000009
由于质心侧偏角为侧向速度与纵向速度的比值，当纵向速度一定时，目标质心侧偏角为零。
S30：基于横摆角速度误差与质心侧偏角误差、转向协调率，利用建立的前轮转向与差动转向的模型预测控制(Model predictive control，MPC)轨迹跟踪控制器，确定目标前轮转角和目标横摆力矩。
首先描述轨迹跟踪控制器的设计。
首先，建立前轮转向动力学模型，前轮自主转向将转向角δ f作为系统输入，车辆动力学方程表示如下：
Figure PCTCN2021101398-appb-000010
非线性刷子轮胎模型将用来计算轮胎的纵向力和侧向力；
其中，状态量为：
x 1＝[r β] T
输入量为：
u 1＝δ f
之后，建立差动转向动力学模型，定义差动扭矩M z作为系统输入，差动转向系统可表示如下：
Figure PCTCN2021101398-appb-000011
上式中，J f、J r分别表示前轴、后轴等效横摆转动惯量，d f、d r分别表示前轴、后轴等效阻尼，τ Af、τ Ar分别表示前、后轮胎回正力矩，τ Ff、τ Fr分别表示前后轮胎摩擦力矩。
车辆动力学方程表示如下：
Figure PCTCN2021101398-appb-000012
M z＝M f+M r＝(F 2-F 1)l c+(F 4-F 3)l c
上式中，F 1、F 2是两个前轮的驱动力，F 3、F 4是两个后轮的驱动力，a、b分别是前、后轴到质心的距离。
另外，上式中，状态量为：
x 2＝[r β δ f δ r] T
输入量为：
u 2＝M z
基于前轮主动转向和差动转向的车辆动力学模型，统一写成如下非线性时变形式：
Figure PCTCN2021101398-appb-000013
针对分布式驱动汽车，除了前轮EPS实现自主转向，前、后双轴都可实现差动转向系统，其状态与输入量为：
Figure PCTCN2021101398-appb-000014
考虑轨迹跟踪误差，满足跟随横摆角速度目标值r *即可使其渐进稳定；侧向稳定性需要系统满足跟随质心侧偏角目标值β *，因此定义系统输出：
y＝[r β] T   (3-6)
将上述(3-4)状态方程进行离散化后的系统模型：
x k+1-x k＝T□f(x k,u k)   (3-7)
简化计算：针对非线性模型的当前时刻k的状态量x(k)和控制输入u(k-1)都需要进行线性化处理：
Δx(k+1)＝A kΔx(k)+B kΔu(k)
Figure PCTCN2021101398-appb-000015
得到离散化的线性时变预测模型：
x p+1,k＝A p,kx p,k+B p,ku p,k+d p,k    (3-9)
针对前轮自主转向：MPC控制过程中追求以最小转角输入获得最佳的轨迹跟踪效果，构建目标函数：
Figure PCTCN2021101398-appb-000016
上式中，ΔU为控制输入增量，N p、N c分别为预测步数和控制补偿，Q、R、ρ为对应的权重系数，ε为松弛因子。
针对差动转向轨迹跟踪，同理设计目标函数为：
Figure PCTCN2021101398-appb-000017
基于实时计算的转向协调率，为了获得最佳轨迹跟踪效果，追求最小转角与横摆 力矩，建立多目标优化模型如下：
min J＝λ cJ 1(x(k),u 1(k-1),ΔU(k))+(1-λ c)J 2(x(k),u 2(k-1),ΔU(k))
Figure PCTCN2021101398-appb-000018
基于上述优化模型，可以求解得到当前时刻最优控制增量序列为：
Figure PCTCN2021101398-appb-000019
之后将最优控制增量序列的第1项作为系统当前的实际控制量，最后进行滚动优化，可得到整个控制时序的结果。
S40：根据前轮转向与差动转向两者的基础协调率、轮胎的横向相对利用率、车姿状态，决策协调率范围，同时协调率边界受下面详述的轮胎力动态分配区域所约束。
首先，预先进行对比前轮EPS与差动转向(DS)的实验。具体而言，分别输入一定前轮转角与差动横摆力矩，进行不同车速下圆周运动，记录实际驾驶转向半径R 1(v x)，R 2(v x)将输入(前轮转角、横摆力矩)分别进行归一化，得到各自单位输入下的转向半径：
Figure PCTCN2021101398-appb-000020
上式中，δ f,test，M Z,test分别表示转向圆周实验的前轮转角与差动力矩输入。
因此，针对转向控制中两者控制效果，可确定前轮转向所占权重：
Figure PCTCN2021101398-appb-000021
上式中，ω b,1表示前轮EPS在圆周运动侧向跟踪控制中的基础协调率，是转向协调率的初始参数。
在不同行驶状态下时应对两系统控制状态加以约束，以使车辆在一定工况下不必同时进行前轮转向与差动制动控制。行驶工况不考虑加速过程，然后根据不同工况以及估计的车姿，确定车辆行驶状态如表1所示。
表1车辆不同行驶状态表
Figure PCTCN2021101398-appb-000022
上表中，r为横摆角速度，δ f为前轮转角，r t为横摆角速度门限值，δ t为前轮转角门限值。K为稳定性因素，表征汽车操纵稳定性实验中的稳态响应，如下式：
Figure PCTCN2021101398-appb-000023
上式中，m表示汽车质量；l f表示前轴到车辆质心的距离；l r表示后轴到车辆质心的距离；k 1表示前轴侧偏刚度；k 2表示后轴侧偏刚度；L表示车辆轴距：L＝l f+l r。
根据表1中的车辆行驶状态，例如按照表2，获得前轮转向(FS，Front-wheel Steering)控制器、差动转向(DS)控制器的控制状态。
表2双系统(FS/DS)可控状态表
Figure PCTCN2021101398-appb-000024
基于上表中的控制器状态，在不同车辆行驶状态下，定义前轮转向、差动转向两控制策略状态系数η s,1,η s,2，两者为0-1变量(取值为0或1)。
控制前轮转向时，前轮会受到地面提供的侧向力产生一定转角，而差动转向通过控制四轮独立驱动电动汽车同轴左右两轮的驱动力矩差，即差动力矩，不仅产生横摆运动，还可作用于转向系统驱动车轮一定角度转动。地面对轮胎的作用力有侧偏力(侧向力、横向力)和纵向力，两者满足附着椭圆关系，如图7所示，侧偏力除了受驱动力和制动力大小的影响，还与轮胎侧偏角有关，侧偏角越大，侧偏力则越大。考虑到前轮转向产生轮胎侧偏力，而差动转向的驱动力矩差受控于轮胎纵向力，结合轮胎横、纵向受力的极限值，建立轮胎横向相对利用率表达式为：
Figure PCTCN2021101398-appb-000025
上式中，ω f,2表示轮胎横向相对利用率，F yi为轮胎i所受侧偏力，F xi为轮胎i所受的纵向力，横、纵向轮胎力极限值F yi,lim，F xi,lim根据轮胎附着椭圆获得。轮胎附着椭圆示意图可以参照图7。
F yi为轮胎i(本实施例中，轮胎为4个，因此i为小于等于4的正整数)所受侧偏力，当轮胎处于线性区时，侧偏力与侧偏角成正比，当其处于非线性区时，通过已标定的数据查表获得，其表达式为：
Figure PCTCN2021101398-appb-000026
上式中，属于线性区还是非线性区可以根据侧偏角的大小判断。此外，还可以结合侧偏角的大小、路面的状态和轮胎的状态(胎压等)来进行判断。
根据(目标)差动力矩M z及轮距d，可得各轮胎的纵向力F xi满足如下关系：
Figure PCTCN2021101398-appb-000027
综合基础协调率、轮胎横向相对利用率及车姿状态确定前轮转向、差动转向两系统控制的协调率表达式为：
Figure PCTCN2021101398-appb-000028
上式中，λ c,1表示协调控制中前轮转向控制器的控制系数，λ c,2表示协调控制中差动转向控制器的控制系数。结合下文提及的轮胎力动态分配区域，可得协调率范围如图8所示。图8中，椭圆线表示轮胎力范围的边界，阴影线表示轮胎力范围边界的变化范围，即由于车辆负载及道路附着系数不同，车辆可发挥的轮胎力是不同的，所以存在这样的边界变化。左侧实线矢量线对应基础协调率。右侧实线矢量线对应一种工况(如正常行驶工况下，未超过动态边界)的转向协调率，虚线矢量线对应极限工况下前轮转向与差动转向的转向协调率(如大转角快速转向时，需要提高大的侧向力)。
S50：基于路面附着系数，根据车辆横、纵、垂三方向的加速度以及侧倾、俯仰运动造成的载荷转移影响，规划轮胎力动态分配区域(轮胎力范围)。
为了进行前轮转向与差动转向的协调控制，需要精确获取道路提供给轮胎的最大侧向力、纵向力，其为计算协调率的可行解提供边界约束，因此本申请实施例基于道路附着系数，考虑车体运动姿态，实时规划轮胎力可分配区域(轮胎力范围)。
垂直载荷F zi影响车辆的轮胎力范围。本申请实施例考虑X、Y、Z三维方向运动对载荷转移造成的影响，主要包括车辆横、纵、垂三方向的加速度以及侧倾、俯仰运动，图9为车体运动受力分析简图，接下来建立由于车体运动造成载荷转移(载荷变化量)的关系表达式。
侧向加速度a y导致左右车轮的载荷转移为：
Figure PCTCN2021101398-appb-000029
侧倾运动导致左右车轮的载荷转移为：
Figure PCTCN2021101398-appb-000030
上式中，I x为绕x轴(纵向轴)的转动惯量，
Figure PCTCN2021101398-appb-000031
为侧倾角加速度。
纵向加速度a x导致前后轴的载荷转移为：
Figure PCTCN2021101398-appb-000032
基于上述侧倾运动造成的载荷转移，同理可得俯仰运动造成的前后轴载荷转移为：
Figure PCTCN2021101398-appb-000033
上式中，
Figure PCTCN2021101398-appb-000034
为俯仰角加速度。
考虑垂直方向的加速度a z产生的垂直载荷转移为：
ΔF″″′ z＝-ma z      (5-5)
综上，可得四个车轮垂直载荷的估计值为：
Figure PCTCN2021101398-appb-000035
以左前轮为例，定义其三维运动引起的动态垂直载荷变化为ΔF zfl，即：
Figure PCTCN2021101398-appb-000036
考虑动态载荷转移，结合附着椭圆，得到轮胎所受的侧、纵向力满足：
Figure PCTCN2021101398-appb-000037
上式中，m是车辆质量，具体而言可以是簧上质量(空载质量)，也可以实时估算的实际质量(非空载质量)。
另外，由上式可得，附着椭圆的边界随载荷的变化而改变，由ΔF zfl得出的轮胎力动态可分配区域如图10所示。图10中，椭圆线表示轮胎力的范围边界，阴影线区域表示轮胎力边界的变化范围。右侧矢量线对应制动力与侧偏力的合力，θ是合力与制动力的夹角。左侧矢量线对应考虑了垂向力后的合力(一个例子)。
基于图10中的椭圆，可得轮胎的最大侧向力、最大纵向力为：
F yfl,lim＝μF zflsinθ，F xfl,lim＝μF zflcosθ    (5-9)
在S30中描述的MPC控制器，使车辆可以跟踪目标横摆角速度和目标质心侧偏角，但在一些极限工况(地面附着系数较低、各轮胎力受力不均等工况)条件下不能提供足够的轮胎力以达到目标横摆角速度，因此为了提高驾驶稳定性，结合前后轴轮胎力极限值，对横摆角速度进行约束：
Figure PCTCN2021101398-appb-000038
上式中，F yf,lim,F yr,lim分别为前、后轴最大侧向力：
Figure PCTCN2021101398-appb-000039
另外，在低摩擦系数或者高曲率路径的工况中，可以对质心侧偏角进行约束以确保质心侧偏角不至于过大，质心侧偏角与前、后轮胎侧偏角关系式为：
Figure PCTCN2021101398-appb-000040
可通过限制轮胎侧偏角保证质心侧偏角在合理范围内：
α i≤α i,lim＝f(F yi,lim,k i)     (5-13)
另外，将上述车辆状态信号(横摆角速度、侧偏角)的界限值还反馈给S30中的MPC轨迹跟踪控制器，转化成不等式约束进行滚动优化求解。
另外，S40与S50可以视为一个处理。
S60：根据基于转向协调率以及其他约束条件构建的协调控制优化模型，进行求解确定转向协调率(即求解获得最优协调率)。
首先对协调控制优化模型的设计进行描述。
为了追求最小的前轮转角与横摆力矩，且保证各轮胎受力小，基于协调率建立综合目标函数J 1。
Figure PCTCN2021101398-appb-000041
目标函数主要包括三部分，第一项为控制域时间内的转角误差和，第二项为控制域时间内的力矩误差和，第三项为各个轮胎负荷率和。
上式中，Δδ f(k+i)表示k+i时刻的前轮转角误差值；ΔM z(k+i)表示k+i时刻的横摆力矩误差值；R表示对应的权重矩阵；N c表示控制域步长。
结合S50中建立的横摆角速度、侧偏角阈值表达式以及其他约束，构建等式与不等式的约束条件，最终建立带约束的优化模型：
Figure PCTCN2021101398-appb-000042
上式中，ε表示松弛因子，保证优化问题可解。不等式约束6-2-3)表示考虑各个 轮胎力受力平衡，ξ lim为限制系数；式6-2-4)表示综合考虑轮毂电机性能以及道路附着条件对纵向力进行约束；上述的优化问题可转化成二次规划(Quadratic Programming，QP)问题进行求解，例如通过有效集法或者内点法进行求解。这里，二次规划问题是一种带约束的非线性规划问题，目标函数f(x)是二次函数，它形式简单，既可以使用求解非线性规划的一般方法求解，又有特定的解法。
使用上述优化模型，可以确定转向协调率，将其提供给S30中的MPC轨迹跟踪控制器，以用于确定目标前轮转角与目标横摆力矩。
采用本实施例的转向控制方法，由于根据轮胎力范围确定转向协调率即自主转向与差动转向的权重，因此，可充分发挥轮胎力，提高侧向响应速度，降低转弯半径，提高轨迹跟踪精度。
另外，由于考虑了轮胎力范围来确定转向协调率，因而在极限行驶工况(路面附着系数较低、高速行驶等工况)下也能够提高车辆的操纵稳定性、安全性。
另外，本实施例提出的转向控制方法不必额外增加车载硬件(例如轮毂电机驱动电动汽车所需的硬件)。
再者，在本实施例中，考虑路面附着条件和/或垂直载荷转移，对车辆状态量阈值进行修正，在此基础上确定转向协调率，如此，提高了车辆行驶安全性。
另外，本实施例提出的转向控制方法，利用自主转向系统与差动系统双系统进行耦合转向控制，具备冗余功能，可提高应对极限工况下的车辆行驶安全性。此外，例如当自主转向系统失效或发生故障时，能够由差动转向系统工作，提高车辆行驶的安全性。
图11为本申请一个实施例的转向控制方法所应用的一种(车辆的)转向系统架构示意图。
如图11所示，该系统架构主要包括以下模块：
信号处理模块：主要包括检测采集、参数估计、辨识等模块，目的是获取转向控制相关信息。具体包括通过摄像头采集获取车辆实时横纵向位置X,Y，通过IMU获取航向角
Figure PCTCN2021101398-appb-000043
侧倾角θ Roll、俯仰角θ Pitch、纵向加速度a x、侧向加速度a y、垂向加速度a z，参数估计模块主要获取质心侧偏角β、横摆角速度r，而辨识模块实时获取路面附着系数μ。信号处理模块可以用于执行上述S10中描述的内容。参数估计模块与辨识模块还可以集成在VDC中。
下面对质心侧偏角估计方法、横摆角速度估计方法与路面附着系数的获取方法给出简要的示例。
质心侧偏角估计：基于车辆二自由度模型，可简化为侧向速度为状态的线性估计模型，利用线性卡尔曼滤波方法可以估计出侧向速度状态值
Figure PCTCN2021101398-appb-000044
最后根据质心侧偏角与纵向、侧向速度之间的关系，可计算
Figure PCTCN2021101398-appb-000045
横摆角速度估计：ESP系统接受来自方向盘转角传感器的角度信号，再结合车速信号，可估计出在该车速、方向盘转角下应有的车身横摆角速度值。
路面附着系数：路面附着系数的估计就是最大的附着率的估计，而附着系数μ与轮胎滑移率s存在μ-s曲线关系，滑移率可通过轮速、车速及地面力等信号进行估计计算获得，再结合纵向加速度可计算得到附着系数。
路径规划模块：假设在轨迹跟踪过程中车辆以恒定车速行驶，路径跟踪目标可专门考虑侧向跟踪精度，即追求侧向误差、航向角误差为最小，采用预瞄控制算法得到目标横摆角速度、质心侧偏角。该路径规划模块可以用于执行上述S20中描述的内容。
MPC轨迹跟踪模块：基于前轮转向与差动转向的车辆动力学模型，建立双系统协调率模型预测控制器，将横摆角速度、质心侧偏角控制到目标值即可满足轨迹跟踪的侧向跟踪要求，控制器求解得到目标前轮转角
Figure PCTCN2021101398-appb-000046
与目标横摆力矩
Figure PCTCN2021101398-appb-000047
该MPC轨迹跟踪模块可以用于执行上述S30中的内容。
协调控制模块(模型)：综合考虑基础协调率，相对利用率及车姿状态三者确定协调率范围。为了追求最小的前轮转角与横摆力矩，以及轮胎负荷率建立综合目标函数J1。考虑X、Y、Z三维运动对载荷转移的影响，确定轮胎力可分配区域，为协调率范围提供边界约束。最终基于目标函数并结合其他约束条件，构建协调率优化模型，转化成二次规划问题进行求解获得双系统(前轮转向、差动转向)最优协调率。该协调控制模块可以用于执行上述S40-S60中描述的内容。
另外，为了减少控制器数量以及硬线连接，并充分发挥域控制器强大的计算能力，将上述的路径规划模块、MPC轨迹跟踪模块、协调控制模块以及车姿估计模块等都集成在VDC中进行实时计算，最终得到控制指令前轮转角δ f,c，以及将横摆力矩M z,c分配给四个车轮的驱动力矩指令T i。
电机控制策略模块：前轮自主转向控制器接收转角后完成转角闭环控制，四个车轮的轮毂电机接收VDC力矩指令T i后完成力矩闭环控制。该电机控制策略模块可以用于执行上述S70中的内容。另外，电机控制策略模块的一部分或全部功能还可以集成在VDC中。
另外，不言而喻，本申请实施例还提供包括上述系统架构、以及上面实施例所描述的转向控制装置、计算设备等的车辆。
应理解，本申请的各个实施例中，如果没有特殊说明以及逻辑冲突，不同的实施例之间的术语和/或描述具有一致性、且可以相互引用，不同的实施例中的技术特征根据其内在的逻辑关系可以组合形成新的实施例。
此外，说明书和权利要求书中的“第一”、“第二”、“第三”等类似用语，仅用于区别类似的对象，不代表针对对象的特定排序，在允许的情况下可以互换特定的顺序或先后次序。所涉及的表示方法具体内容的标号，如S10、S20……等，并不表示一定会按标号顺序执行，在允许的情况下可以互换前后顺序，或同时执行。说明书和权利要求书中使用的术语“包括”不应解释为限制于其后列出的内容；它不排除还存在其它的元件或步骤。

Claims (13)

  1. 一种转向控制方法，其特征在于，包括：
    获取轮胎力范围；
    根据所述轮胎力范围确定转向协调率，所述转向协调率指示车辆的自主转向系统产生的转向量与所述车辆的差动转向系统产生的转向量的权重；
    获取横摆角速度误差；
    根据所述横摆角速度误差与所述转向协调率确定目标车轮转角与目标横摆力矩；
    发送第一控制指令，所述第一控制指令包括用于使所述自主转向系统产生所述目标车轮转角的控制指令与用于使所述差动转向系统产生所述目标横摆力矩的控制指令。
  2. 根据权利要求1所述的转向控制方法，其特征在于，所述获取轮胎力范围，具体包括：
    获取路面附着系数和/或所述车辆的多个车轮的载荷变化量；
    根据所述路面附着系数和/或所述载荷变化量确定所述轮胎力范围。
  3. 根据权利要求2所述的转向控制方法，其特征在于，所述获取所述车辆的多个车轮的载荷变化量，具体包括：
    获取所述车辆的横纵垂三方向的加速度、侧倾运动状态参数或俯仰运动状态参数中的一项或多项；
    根据所述加速度、所述侧倾运动状态参数或所述俯仰运动状态参数中的一项或多项确定所述载荷变化量。
  4. 根据权利要求1-3中任一项所述的转向控制方法，其特征在于，所述根据所述轮胎力范围确定转向协调率，具体包括：
    获取轮胎横向相对利用率与预设的基础协调率，所述轮胎横向相对利用率指示横向轮胎力相对于总轮胎力的比率，所述基础协调率是所述转向协调率的初始参数；
    根据所述基础协调率、所述轮胎横向相对利用率与所述轮胎力范围确定所述转向协调率。
  5. 根据权利要求1-4中任一项所述的转向控制方法，其特征在于，所述转向量由转向半径或横摆角确定。
  6. 一种车辆转向控制装置，其特征在于，包括处理模块与收发模块，
    所述处理模块用于，获取轮胎力范围与横摆角速度误差，根据所述轮胎力范围确定转向协调率，所述转向协调率指示车辆的自主转向系统产生的转向量与所述车辆的差动转向系统产生的转向量的权重，根据所述横摆角速度误差与所述转向协调率确定目标车轮转角与目标横摆力矩；
    所述收发模块用于发送第一控制指令，所述第一控制指令包括用于使所述自主转向系统产生所述目标车轮转角的控制指令与用于使所述差动转向系统产生所述目标横摆力矩的控制指令。
  7. 根据权利要求6所述的转向控制装置，其特征在于，所述处理模块具体用于，
    获取路面附着系数和/或所述车辆的多个车轮的载荷变化量；
    根据所述路面附着系数和/或所述载荷变化量确定所述轮胎力范围。
  8. 根据权利要求7所述的转向控制装置，其特征在于，所述处理模块具体用于，
    获取所述车辆的横纵垂三方向的加速度、侧倾运动状态参数或俯仰运动状态参数中的一项或多项；
    根据所述加速度、所述侧倾运动状态参数或所述俯仰运动状态参数中的一项或多项确定所述载荷变化量。
  9. 根据权利要求6-8中任一项所述的转向控制装置，其特征在于，所述处理模块具体用于，
    获取轮胎横向相对利用率与预设的基础协调率，所述轮胎横向相对利用率指示横向轮胎力相对于总轮胎力的比率，所述基础协调率是所述转向协调率的初始参数；
    根据所述基础协调率、所述轮胎横向相对利用率与所述轮胎力范围确定所述转向协调率。
  10. 根据权利要求6-9中任一项所述的转向控制装置，其特征在于，所述转向量由转向半径或横摆角确定。
  11. 一种计算设备，其特征在于，包括处理器与存储器，所述存储器存储有程序指令，所述程序指令当被所述处理器执行时使得所述处理器执行权利要求1-5中任一项所述的方法。
  12. 一种计算机可读存储介质，其存储有程序指令，其特征在于，所述程序指令当被计算机执行时使得所述计算机执行权利要求1-5中任一项所述的方法。
  13. 一种计算机程序产品，其特征在于，包括有程序指令，所述程序指令当被计算机执行时使得所述计算机执行权利要求1-5中任一项所述的方法。
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