CN112549987B - Automobile inter-wheel differential steering method based on driving-braking composite control - Google Patents

Automobile inter-wheel differential steering method based on driving-braking composite control Download PDF

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CN112549987B
CN112549987B CN202011382848.7A CN202011382848A CN112549987B CN 112549987 B CN112549987 B CN 112549987B CN 202011382848 A CN202011382848 A CN 202011382848A CN 112549987 B CN112549987 B CN 112549987B
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braking
wheels
driving
brake
force
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CN112549987A (en
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刘军
王菁菁
刘皓皓
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Jiangsu University
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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
    • 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
    • B60L15/2009—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 for braking
    • 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
    • B60L15/2036—Electric differentials, e.g. for supporting steering vehicles
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60T—VEHICLE BRAKE CONTROL SYSTEMS OR PARTS THEREOF; BRAKE CONTROL SYSTEMS OR PARTS THEREOF, IN GENERAL; ARRANGEMENT OF BRAKING ELEMENTS ON VEHICLES IN GENERAL; PORTABLE DEVICES FOR PREVENTING UNWANTED MOVEMENT OF VEHICLES; VEHICLE MODIFICATIONS TO FACILITATE COOLING OF BRAKES
    • B60T13/00—Transmitting braking action from initiating means to ultimate brake actuator with power assistance or drive; Brake systems incorporating such transmitting means, e.g. air-pressure brake systems
    • B60T13/74—Transmitting braking action from initiating means to ultimate brake actuator with power assistance or drive; Brake systems incorporating such transmitting means, e.g. air-pressure brake systems with electrical assistance or drive
    • B60T13/745—Transmitting braking action from initiating means to ultimate brake actuator with power assistance or drive; Brake systems incorporating such transmitting means, e.g. air-pressure brake systems with electrical assistance or drive acting on a hydraulic system, e.g. a master cylinder
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
    • B62D—MOTOR VEHICLES; TRAILERS
    • B62D11/00—Steering non-deflectable wheels; Steering endless tracks or the like
    • B62D11/001—Steering non-deflectable wheels; Steering endless tracks or the like control systems
    • B62D11/003—Electric or electronic control systems
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
    • B62D—MOTOR VEHICLES; TRAILERS
    • B62D11/00—Steering non-deflectable wheels; Steering endless tracks or the like
    • B62D11/02—Steering non-deflectable wheels; Steering endless tracks or the like by differentially driving ground-engaging elements on opposite vehicle sides
    • B62D11/04—Steering non-deflectable wheels; Steering endless tracks or the like by differentially driving ground-engaging elements on opposite vehicle sides by means of separate power sources
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/60—Other road transportation technologies with climate change mitigation effect
    • Y02T10/72—Electric energy management in electromobility

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  • Engineering & Computer Science (AREA)
  • Transportation (AREA)
  • Mechanical Engineering (AREA)
  • Power Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Combustion & Propulsion (AREA)
  • Automation & Control Theory (AREA)
  • Regulating Braking Force (AREA)

Abstract

The invention discloses a driving-braking composite control-based differential steering method between wheels of an automobile, belongs to the field of differential steering control, and builds a slip ratio s according to the theoretical basis of Julie1And a driving force FxObtaining a variation relation between the slip ratio and the driving force; build up slip ratio s and braking force FxbObtaining the change relation between the slip ratio and the braking force; the VCU performs composite control on a hub motor driving system of 4 wheels and a wheel side braking system of 4 wheels according to the signals, controls the slip ratio or the slip ratio in a certain range by utilizing the change relationship of the slip ratio and the driving force and the change relationship of the slip ratio and the braking force and further restrains the longitudinal force in the composite control so as to realize the driving, braking and steering of the automobile. The invention provides the yaw moment required by the vehicle steering, so that the steering is more accurate, and the stability of the vehicle is improved.

Description

Driving-braking composite control-based differential steering method between wheels of automobile
Technical Field
The invention relates to the field of automobile steering systems, in particular to an automobile inter-wheel differential steering method based on driving-braking composite control.
Background
As the atmospheric pollution degree becomes more and more serious, the measures which must be taken at present are energy conservation, emission reduction and environmental pressure relief. In order to reduce the pollution of the exhaust emission of the automobile to the air, the development of the new energy automobile industry is a necessary way for the country, wherein the hub motor driven pure electric automobile is more widely concerned in various fields. As a novel driving form, the hub motor driven electric automobile improves the transmission efficiency of the power of the whole automobile, optimizes the chassis structure of the whole automobile, can realize a complex driving form, and develops a large amount of research in various colleges and universities at home and abroad.
In both the two-rear-wheel drive and the four-wheel drive, the wheel hub motor is used for direct drive, a mechanical differential and a transmission mechanism are not needed to be installed, the advantages of high response speed of electrical components and short time constant can be exerted, and when an automobile turns, the driving distances of the left rear wheel and the right rear wheel are different, but the time is the same, so that the problem of differential speed exists. How to guarantee the difference of the speeds of the two sides and control the tangential reaction force between the four wheels and the road surface, so that the automobile obtains accurate yaw moment, the purpose of steering is achieved, and the stable running state of the automobile is guaranteed.
Aiming at the problem of differential speed, the patent provides an automobile inter-wheel differential steering method adopting driving and braking composite control. The automobile achieves steering because the automobile obtains a yaw moment, and the patent enables the automobile to obtain a proper yaw moment by utilizing the interaction between the four wheels and a road surface, namely, through controlling the driving torque (controlled by a hub motor) and the braking torque (wheel side brake) of the four wheels of the automobile.
Disclosure of Invention
In order to solve the existing technical problems, the invention provides a driving-braking composite control-based automobile inter-wheel differential steering method. The driving-braking-based composite control method is applied to an automobile inter-wheel differential steering system, and steering accuracy and vehicle stability are ensured.
The technical scheme of the invention is realized as follows: a driving-braking composite control-based differential steering method between wheels of an automobile comprises the following steps:
s1, establishing a slip rate S according to the theoretical basis of Julie1And a driving force FxObtaining a variation relation between the slip ratio and the driving force;
s2, establishing slip ratio S and braking force FxbObtaining the change relation between the slip ratio and the braking force;
s3, an electronic accelerator pedal signal, an electronic brake pedal signal and an electronic steering wheel signal reflect the intention of a driver, a VCU performs compound control on a hub motor driving system of 4 wheels and a wheel side braking system of 4 wheels according to the signals, and the driving, braking and steering of the automobile are realized by controlling the slip ratio or the slip ratio in a certain range and further restricting the longitudinal force in the compound control by utilizing the change relation of the slip ratio and the driving force and the change relation of the slip ratio and the braking force.
Further, the specific process of S1 is: assuming that the tread is an elastic band, only the tyre is considered in the adhesion zone s1And FxThe relationship between the two is that under the action of driving torque, the front end of the tread will be groundedLongitudinal deformation e0Then the longitudinal deformation at x from the front end is: e ═ e0+xε＝(λt+ x) ε, wherein λtIs the longitudinal deformation coefficient, and epsilon is the longitudinal strain; the driving force generated by the attachment region before point x is
Figure BDA0002810025680000021
Wherein k istanIs the tangential stiffness of the tread;
considering only the fully attached state, s1The range is 0-20%, and the driving force F isxAnd slip ratio s1In a linear relationship, such that
Figure BDA0002810025680000022
Then it can be obtained:
Figure BDA0002810025680000023
and is provided with
Figure BDA0002810025680000024
Wherein epsilon is longitudinal strain, e is longitudinal deformation from the front end x, l is tire grounding length, omega is tire angular velocity, u is vehicle running speed, and r is wheel radius;
by substituting formula (2) for formula (1), it is possible to obtain:
Fx＝kts1(3)。
further, the specific process of S2 is:
order to
Figure BDA0002810025680000025
Wherein, FxbIs the braking force of the ground,
Figure BDA0002810025680000026
for adhesion, FZIn order to provide a normal reaction force of the ground against the wheel,
Figure BDA0002810025680000027
is the adhesion coefficient;
the ratio of the ground braking force to the vertical load is the braking force coefficient
Figure BDA0002810025680000028
Then
Figure BDA0002810025680000029
Wherein, FxbIs ground braking force, and W is vertical load;
when the tire is in pure rolling, the slip rate is within the range of 0-20%, the method comprises the following steps
Figure BDA0002810025680000031
Wherein c isxIs the elastic modulus of a unit area tire in the circumferential tangential direction; s is slip rate, omega is tire angular velocity, l is tire ground contact length; meanwhile, the optimal slip ratio is between 15% and 20%.
Further, the specific process of S3 is:
according to different input signals, firstly, the working conditions are judged, and the working conditions are mainly divided into three types: 1) the pure driving working condition is only an electronic throttle signal; 2) the pure braking condition is only an electronic brake pedal signal; 3) steering working conditions;
if the vehicle is in the working condition 1), when only an electric signal input by an electronic accelerator pedal is available, the VCU receives the signal and obtains a target rotating speed by using a preset control strategy, the motor controller controls the hub motor to reach the target rotating speed and feeds back the target rotating speed to the VCU to receive the actual rotating speed of the wheel, whether the wheel slip rate is in a given range is judged according to a formula (2), if the wheel slip rate is not in the given range, the driving force on the side is reduced, the slip rate is reduced, the VCU redistributes the speeds of the four wheels, and the steps are repeated until the slip rates on the two sides are controlled in the given range;
if the vehicle is in the working condition 2), only the electric signal output by the electronic brake pedal is output, the VCU receives the signal and transmits the signal to the hydraulic brake system through the CAN bus by using a preset control strategy, the four brakes generate braking force to wheels to finish the braking process, and if the vehicle passes the braking process, the VCU transmits the signal to the hydraulic brake system through the CAN busIn the course, according to formula
Figure BDA0002810025680000032
When the slip ratio of one side is detected to fall outside the optimal slip ratio, reducing the braking force of the side and reducing the slip ratio;
if in condition 3), the target yaw rate is obtained based on the steering intent, and the VCU controls the slip rates of the inner and outer wheels based on the target yaw rate.
Further, the method also comprises the following steps of,
if the braking signal and the steering signal exist at the same time, the wheels on the inner side and the outer side are controlled to brake, the driving is not effective, the slip rate of the wheels on the inner side is increased and the slip rate of the wheels on the outer side is reduced by controlling the braking; if the actual yaw velocity is less than the target yaw velocity, the braking moment of the inner side wheels is further increased, the slip rate of the inner side wheels is improved, the vehicle can obtain larger yaw moment, the actual yaw velocity is increased, namely, when the actual yaw angle is greater than the target yaw angle, the slip rate of the inner side wheels is reduced;
if the driving signal and the steering signal are input simultaneously, the brake is not in effect at the moment, a large rotation angle is set when the steering angle is larger than 40 degrees, and a small rotation angle is set when the steering angle is lower than 40 degrees, and if the driving signal and the small rotation angle signal are input simultaneously, the slip rate of the inner side wheel is large, and the slip rate of the outer side wheel is small by controlling the driving of the inner side wheel and the outer side wheel, so that the steering purpose is achieved; if a driving signal and a large turning angle signal are simultaneously input, the yaw moment required by the turning angle cannot be obtained only by the driving force of the wheels, at the moment, the aim of steering is fulfilled by adding a braking force and the driving force together, so that the slip rate of the inner side wheels is increased, the slip rate of the outer side is increased, and the slip rates are controlled within 20 percent, so that the speed difference between the inner side wheels and the outer side wheels is increased, and the automobile can obtain enough yaw moment.
Further, the step S3 further includes: two sets of independent electronic hydraulic braking systems are adopted and are respectively connected with the brake wheel cylinders of the wheel-side disc brakes (6) of the two wheels on the left side and the two wheels on the right side through hydraulic pipelines, and the braking torques of the disc brakes of the two wheels on the left side and the two wheels on the right side are respectively and independently controlled;
if the first set of hydraulic braking system fails, the second set of hydraulic braking system can still control the braking force of the wheels, and if the first braking circuit fails, the second hydraulic braking system can still work normally; when a driver steps on a brake pedal, a brake pull wire and a pull wire lever act, an input ejector rod correspondingly moves towards a brake master cylinder, a brake signal is transmitted to a vehicle control unit through a CAN bus, the vehicle control unit obtains brake force required by a brake according to a control algorithm and transmits the brake force to a motor, an electric push rod senses an electric signal transmitted by the motor and then applies thrust to the ejector rod, the ejector rod pushes a piston in the brake master cylinder under the combined action of the acting force of the electric push rod and the acting force converted from manpower to the push rod, the piston and the motor generate brake hydraulic pressure under the combined action, and the brake hydraulic pressure is respectively transmitted to brake wheel cylinders of front and rear wheel-side disc brakes through hydraulic pipelines connected to two sides of the brake master cylinder to form two hydraulic circuits.
The beneficial results of the invention are as follows: two sets of hydraulic braking systems are adopted to form double-loop hydraulic pipeline independent control, and the braking forces of the four wheels are respectively controlled by using the left brake and the right brake, so that the adjustability of the braking forces of the four wheels is realized; build up slip s and drive FxAnd slip ratio and braking force coefficient, as compared to driving a single control differential steering. Through the driving and braking combined control differential steering system, when the steering angle is too large, the composite control of the driving force and the braking force is realized, the yaw moment required by the steering of the vehicle is provided, the steering is more accurate, and the stability of the vehicle is improved.
Drawings
FIG. 1 is a block diagram of a differential steering control system of the present invention
FIG. 2 differential steering control system force diagram
FIG. 3 Structure of electro-hydraulic brake System
FIG. 4 is a graph of road surface adhesion coefficient versus wheel slip ratio
Detailed Description
The present invention will be described in further detail below by way of examples with reference to the accompanying drawings, which are illustrative of the present invention and are not to be construed as limiting the present invention.
Referring to fig. 1, a block diagram of a differential steering control system is shown, and the differential steering control system comprises an electric four-wheel hub motor-driven automobile which is provided with an electronic accelerator pedal output speed signal, an electronic brake pedal output speed signal, an electronic steering wheel output steering signal, an instrument panel for displaying the speed, the mileage, battery related information and the like, and is characterized in that the electric automobile is provided with a control device, a driving device, a power device and a braking device, the control device comprises four motor controllers, EHBS controllers in two electronic hydraulic control systems and a master controller (VCU) of the whole automobile, and the four motor controllers acquire signals transmitted by the VCU so as to control the driving of four wheels of the automobile; the power device comprises a power battery and a Battery Management System (BMS), provides power required by automobile running for the hub motor controller and the hub motor, and monitors and manages the battery; the driving device and the braking device are respectively in communication connection with the control device through a CAN bus; the braking device comprises two sets of Electronic Hydraulic Braking Systems (EHBS) and electronic hydraulic braking systems, and the two sets of electronic hydraulic braking systems are respectively connected with braking wheel cylinders of wheel-side disc brakes of two wheels on the left side and two wheels on the right side through hydraulic pipelines. The driving device comprises four hub motors which are respectively arranged on the wheels with 4.
The braking device comprises two sets of Electronic Hydraulic Braking Systems (EHBS) which respectively and independently control the braking torque of the disc brakes of the two wheels on the left side and the two wheels on the right side.
The differential steering system can realize steering through a driving and braking composite control method, so that the automobile obtains a proper yaw moment, and a steering effect meeting driving intentions is obtained.
The two sets of Electronic Hydraulic Brake Systems (EHBS) can form double-loop hydraulic pipeline independent control, and realize independent control and adjustment of braking force of wheels on two sides.
The invention provides a driving-braking composite control-based differential steering method between wheels of an automobile, which comprises the following steps:
s1, establishing a slip rate S according to the theoretical basis of Julie1And a driving forceFxAssuming that the tread is an elastic band, only the tyre is considered in the adhesion zone s1And FxThe relationship between the two is that under the driving torque of the tire, the longitudinal deformation e of the tread grounding front end is generated0Then the longitudinal deformation at x from the front end is: e ═ e0+xε＝(λt+ x) ε, wherein λtIs the longitudinal deformation coefficient, and epsilon is the longitudinal strain; the driving force generated by the attachment region before point x is
Figure BDA0002810025680000051
Wherein k istanIs the tangential stiffness of the tread;
this patent only considers the full attachment state, s1The range is 0-20%, and the driving force F isxAnd slip ratio s1In a linear relationship, such that
Figure BDA0002810025680000052
Then it is possible to obtain:
Figure BDA0002810025680000053
and is provided with
Figure BDA0002810025680000054
Wherein epsilon is longitudinal strain, e is longitudinal deformation from the front end x, l is tire grounding length, omega is tire angular velocity, u is vehicle running speed, and r is wheel radius;
by substituting formula (2) for formula (1), it is possible to obtain:
Fx＝kts1 (3)；
s2, establishing slip ratio S and braking force FxbThe model of (2);
Figure BDA0002810025680000061
wherein, FxbIs the braking force of the ground,
Figure BDA0002810025680000062
for adhesion, FZIn order to provide a normal reaction force of the ground against the wheel,
Figure BDA0002810025680000063
is the adhesion coefficient;
the ratio of the ground braking force to the vertical load value is used as the braking force coefficient
Figure BDA0002810025680000064
Then
Figure BDA0002810025680000065
Wherein, FxbIs ground braking force, and W is vertical load;
when the tire is in pure rolling, the slip rate is within the range of 0-20%, the method comprises the following steps
Figure BDA0002810025680000066
Wherein c isxIs the elastic modulus of a unit area tire in the circumferential tangential direction; s is slip rate, omega is tire angular velocity, l is tire ground contact length; meanwhile, the optimal slip ratio is between 15% and 20%;
s3, an electronic accelerator pedal signal, an electronic brake pedal signal and an electronic steering wheel signal reflect the intention of a driver, and the VCU performs composite control on a hub motor driving system of 4 wheels and a wheel side braking system of 4 wheels according to the signals to realize driving, braking, steering and the like of the automobile;
according to different input signals, firstly, the working conditions are judged, and the working conditions are mainly divided into three types: (1) the pure driving working condition is only an electronic throttle signal; (2) the pure braking condition is only an electronic brake pedal signal; (3) steering working conditions;
if the vehicle is in the working condition (1), only when an electric signal is input by an electronic accelerator pedal, the VCU receives the signal and obtains a target rotating speed by using a preset control strategy, at the moment, the motor controller controls the hub motor to reach the target rotating speed and feeds back the target rotating speed to the VCU to receive the actual rotating speed of the wheel, whether the wheel slip ratio is in a given range is judged according to a formula (2), if the wheel slip ratio is not in the given range, the driving force on the side is reduced, the slip ratio is reduced, the VCU redistributes the speeds of the four wheels, and the steps are repeated until the slip ratios on the two sides are controlled in the given range;
if the vehicle is in the working condition (2), only the electric signal input by the electronic brake pedal is available, the VCU receives the signal and transmits the signal to the hydraulic brake system through the CAN bus by using a preset control strategy, the four brakes generate braking force to wheels to finish the braking process, and if the vehicle is in the process, the braking process is finished according to the formula
Figure BDA0002810025680000067
When the slip ratio of one side is detected to fall outside the optimal slip ratio, reducing the braking force of the side and reducing the slip ratio;
if the vehicle is in the working condition (3), obtaining a target yaw rate (derivative value of the target course angle) according to the steering intention, and controlling the slip rates of the inner side wheels and the outer side wheels by the VCU according to the target yaw rate;
if the braking signal and the steering signal are simultaneously provided, the inner side wheel and the outer side wheel are controlled to brake, the driving is not effective, the slip rate of the inner side wheel is increased, and the slip rate of the outer side wheel is reduced by controlling the braking. If the actual yaw velocity is less than the target yaw velocity, the braking moment of the inner side wheels is further increased, the slip rate of the inner side wheels is improved, the vehicle obtains a larger yaw moment, the actual yaw velocity is increased, namely, when the actual yaw angle is greater than the target yaw angle, the slip rate of the inner side wheels is reduced.
If the input of the driving signal and the steering signal are simultaneously available, the brake is not effective. The steering angle is set to be a large angle when the steering angle is larger than 40 degrees and a small angle when the steering angle is lower than 40 degrees. If a driving signal and a small rotation angle signal are input simultaneously, the inner side wheel and the outer side wheel are controlled to drive, so that the slip rate of the inner side wheel is large, and the slip rate of the outer side wheel is small, and the purpose of steering is achieved; if a driving signal and a large turning angle signal are simultaneously input, the yaw moment required by the turning angle cannot be obtained only by the driving force of the wheels, at the moment, the aim of steering is fulfilled by adding braking force and the driving force, the inner wheels can be independently controlled by the brakes of the inner wheels, so that the slip rate of the inner wheels is increased, the slip rate of the outer wheels is increased, and the slip rates are controlled within 20 percent, so that the speed difference between the inner wheels and the outer wheels is increased, and the automobile can obtain enough yaw moment.
Further, the step S3 further includes: two independent hydraulic systems are adopted, if the first hydraulic braking system fails, the second hydraulic braking system can still control the braking force of the wheels, and if the first braking circuit fails, the second hydraulic braking system can still work normally. When a driver steps on a brake pedal, a brake pull wire and a pull wire lever CAN act, an input ejector rod correspondingly moves towards a brake main cylinder direction, a brake signal is transmitted to a vehicle control unit through a CAN bus, the vehicle control unit obtains brake force required by a brake according to a control algorithm and transmits the brake force to a motor, an electric push rod senses an electric signal transmitted by the motor and then applies thrust to the ejector rod, the ejector rod pushes a piston in the brake main cylinder under the combined action of the acting force of the electric push rod and the acting force converted from manpower to the push rod, brake hydraulic pressure is generated under the combined action, and the brake hydraulic pressure is respectively transmitted to brake cylinders of front and rear wheel-side disc brakes through hydraulic pipelines connected to the two sides of the brake main cylinder to form two hydraulic circuits.
FIG. 2 is a diagram of a differential system steering control system, in which when steering occurs, driving force and braking force are applied to the inner and outer wheels, and a yaw moment is generated due to the resultant force of longitudinal force and the lateral force applied to the inner and outer wheels, so that the driving direction of the vehicle is changed; fig. 3 is a schematic structural diagram of an electronic hydraulic brake system, which mainly includes a motor (1), an electric push rod (5), an upper bracket (6), a lower bracket (2), a push rod (7), a brake master cylinder (11), a brake cable (4) and a brake lever (3), wherein the brake master cylinder is provided with a first piston (8) and a second piston (10), which are connected through a spring (9) to push brake fluid into a brake pipeline; FIG. 4 is a graph showing the relationship between the road surface adhesion coefficient and the wheel slip rate, and the longitudinal adhesion coefficient μ when the drive wheel slip rate increases from zerodEnlarging; when the slip ratio of the driving wheel reaches Kw_optThe longitudinal adhesion coefficient also increases to the maximum value mud_max(ii) a When the slip ratio of the driving wheel exceeds Kw_optAfter that, the longitudinal adhesion coefficient mudGradually decreases. Coefficient of lateral adhesion mudDecreases as the drive wheel slip increases.
In summary, the invention relates to an inter-wheel differential steering method based on driving-braking composite control, belongs to the field of differential steering control, and particularly relates to an inter-wheel differential steering method based on driving-braking composite control. The electric four-wheel drive automobile comprises an electric four-wheel drive automobile provided with an electronic accelerator pedal, an electronic brake pedal, an electronic steering wheel and an instrument panel, wherein the electric automobile is provided with a control device, a driving device, a power device and a braking device; the power device comprises a power battery and a Battery Management System (BMS), provides power for the control device, and monitors and manages the battery; the driving device and the braking device are respectively in communication connection with the control device through a CAN bus; the braking device comprises two sets of Electronic Hydraulic Braking Systems (EHBS) which are connected with brakes mechanically linked on wheels through hydraulic pipelines. Permanent magnet direct current brushless wheel hub motors and wheel disc brakes are uniformly arranged on the four driving wheels. The driving-braking composite control based inter-wheel differential steering system is provided with braking control on the basis of the conventional driving control, improves the steering sensitivity and the operation stability of a vehicle, is simultaneously provided with two sets of electronic hydraulic braking systems, respectively controls the braking torque of brakes acting on two wheels on the left side and two wheels on the right side of the vehicle, forms two independent braking systems, and can realize two hydraulic circuits of front wheels and rear wheels on the same side of the vehicle because a braking main cylinder controlled by each set of electronic hydraulic braking system is double-circuit. Further improving the braking safety of the whole vehicle.
The invention applies the driving-braking composite control method to the automobile inter-wheel differential steering system, and mainly solves the problems of differential speed of the electric automobile during turning, instability of the automobile during differential steering, easy external interference on parameters and the like.
The above description of the present invention is intended to be illustrative. Various modifications, additions and substitutions for the specific embodiments described may be made by those skilled in the art without departing from the scope of the invention as defined in the accompanying claims.

Claims (5)

1. A method for differential steering between wheels of a vehicle based on driving-braking composite control is characterized by comprising the following steps:
s1, establishing a slip ratio S according to Julie' S theoretical basis1And a driving force FxObtaining a variation relation between the slip ratio and the driving force;
s2, establishing slip ratio S and braking force FxbObtaining the change relation between the slip ratio and the braking force;
s3, an electronic accelerator pedal signal, an electronic brake pedal signal and an electronic steering wheel signal reflect the intention of a driver, a VCU performs composite control on a hub motor driving system of 4 wheels and a wheel side braking system of 4 wheels according to the signals, and the driving, braking and steering of the automobile are realized by controlling the slip ratio or the slip ratio in a certain range and further restricting the longitudinal force in the composite control by utilizing the change relation of the slip ratio and the driving force and the change relation of the slip ratio and the braking force;
the specific process of S1 is as follows: assuming that the tread is an elastic band, only the tyre is considered in the adhesion zone s1And FxThe relationship between the two is that under the action of driving torque, the longitudinal deformation e of the front end of the tread is generated0Then the longitudinal deformation at x from the front end is: e ═ e0+xε＝(λt+ x) ε, wherein λtIs the longitudinal deformation coefficient, and epsilon is the longitudinal strain; the driving force generated by the attachment region before point x is
Figure FDA0003549063880000011
Wherein k istanIs the tangential stiffness of the tread;
considering only the full adhesion state, s1In the range of0 to 20%, at this time, driving force FxAnd slip ratio s1In a linear relationship, such that
Figure FDA0003549063880000012
Then it can be obtained:
Figure FDA0003549063880000013
and is provided with
Figure FDA0003549063880000014
Wherein epsilon is longitudinal strain, e is longitudinal deformation from the front end x, l is tire grounding length, omega is tire angular velocity, u is vehicle running speed, and r is wheel radius;
by substituting formula (2) for formula (1), it is possible to obtain:
Fx＝kts1 (3)。
2. the method for driving-braking composite control-based differential steering between wheels for a vehicle according to claim 1, wherein the specific process of S2 is as follows:
order to
Figure FDA0003549063880000021
Wherein, FxbIs a braking force for the ground,
Figure FDA0003549063880000022
for adhesion, FZIs the normal reaction force of the ground to the wheel,
Figure FDA0003549063880000023
is the adhesion coefficient;
the ratio of the ground braking force to the vertical load is the braking force coefficient
Figure FDA0003549063880000024
Then the
Figure FDA0003549063880000025
Wherein, FxbIs ground braking force, and W is vertical load;
when the tire is in pure rolling, the slip rate is within the range of 0-20%, the method comprises the following steps
Figure FDA0003549063880000026
Wherein c isxIs the elastic modulus of a unit area tire in the circumferential tangential direction; s is slip rate, omega is tire angular velocity, l is tire ground contact length; meanwhile, the optimal slip ratio is between 15% and 20%.
3. The method for driving-braking composite control-based differential steering between wheels for a vehicle according to claim 1, wherein the specific process of S3 is as follows:
according to different input signals, firstly, the working conditions are judged, and the working conditions are mainly divided into three types: 1) the pure driving working condition is only an electronic throttle signal; 2) the pure braking condition is only an electronic brake pedal signal; 3) steering working conditions;
if the vehicle is in the working condition 1), when only an electric signal input by an electronic accelerator pedal is available, the VCU receives the signal and obtains a target rotating speed by using a preset control strategy, the motor controller controls the hub motor to reach the target rotating speed and feeds back the target rotating speed to the VCU to receive the actual rotating speed of the wheel, whether the wheel slip rate is in a given range is judged according to a formula (2), if the wheel slip rate is not in the given range, the driving force on the side is reduced, the slip rate is reduced, the VCU redistributes the speeds of the four wheels, and the steps are repeated until the slip rates on the two sides are controlled in the given range;
if the electric brake is in the working condition 2), the VCU receives the signal and transmits the signal to the hydraulic brake system through the CAN bus by using a preset control strategy when only the electric signal output by the electronic brake pedal exists, the four brakes generate braking force on wheels to finish the braking process, and if the braking process is finished, the brake is carried out according to the formula
Figure FDA0003549063880000027
When the slip ratio of one side is detected to fall outside the optimal slip ratio, reducing the braking force of the side and reducing the slip ratio;
if in condition 3), the target yaw rate is obtained based on the steering intent, and the VCU controls the slip rates of the inner and outer wheels based on the target yaw rate.
4. The method for differential steering between wheels of a vehicle based on a combined drive-brake control as claimed in claim 3, further comprising,
if the braking signal and the steering signal exist at the same time, the wheels on the inner side and the outer side are controlled to brake, the driving is not effective, the slip rate of the wheels on the inner side is increased and the slip rate of the wheels on the outer side is reduced by controlling the braking; if the actual yaw velocity is less than the target yaw velocity, the braking moment of the inner side wheels is further increased, the slip rate of the inner side wheels is improved, the vehicle can obtain larger yaw moment, the actual yaw velocity is increased, namely, when the actual yaw angle is greater than the target yaw angle, the slip rate of the inner side wheels is reduced;
if the driving signal and the steering signal are input simultaneously, the brake is not in effect at the moment, a large rotation angle is set when the steering angle is larger than 40 degrees, and a small rotation angle is set when the steering angle is lower than 40 degrees, and if the driving signal and the small rotation angle signal are input simultaneously, the slip rate of the inner side wheel is large, and the slip rate of the outer side wheel is small by controlling the driving of the inner side wheel and the outer side wheel, so that the steering purpose is achieved; if a driving signal and a large turning angle signal are simultaneously input, the yaw moment required by the turning angle cannot be obtained only by the driving force of the wheels, at the moment, the aim of steering is fulfilled by adding a braking force and the driving force together, so that the slip rate of the inner side wheels is increased, the slip rate of the outer side is increased, and the slip rates are controlled within 20 percent, so that the speed difference between the inner side wheels and the outer side wheels is increased, and the automobile can obtain enough yaw moment.
5. The method for driving-braking composite control-based differential steering between wheels for a vehicle according to claim 1, wherein said step S3 further comprises: two sets of independent electronic hydraulic braking systems are adopted and are respectively connected with the brake wheel cylinders of the wheel-side disc brakes (6) of the two wheels on the left side and the two wheels on the right side through hydraulic pipelines, and the braking torques of the disc brakes of the two wheels on the left side and the two wheels on the right side are respectively and independently controlled;
if the first set of hydraulic braking system fails, the second set of hydraulic braking system can still control the braking force of the wheels, and if the first braking circuit fails, the second hydraulic braking system can still work normally; when a driver steps on a brake pedal, a brake pull wire and a pull wire lever CAN act, an input ejector rod correspondingly moves towards a brake main cylinder direction, a brake signal is transmitted to a vehicle control unit through a CAN bus, the vehicle control unit obtains brake force required by a brake according to a control algorithm and transmits the brake force to a motor, an electric push rod senses an electric signal transmitted by the motor and then applies thrust to the ejector rod, the ejector rod pushes a piston in the brake main cylinder under the combined action of the acting force of the electric push rod and the acting force converted from manpower to the push rod, brake hydraulic pressure is generated under the combined action, and the brake hydraulic pressure is respectively transmitted to brake cylinders of front and rear wheel-side disc brakes through hydraulic pipelines connected to the two sides of the brake main cylinder to form two hydraulic circuits.
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