CN114407673A - Torque control method of electric four-wheel drive vehicle based on slip rate - Google Patents
Torque control method of electric four-wheel drive vehicle based on slip rate Download PDFInfo
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- B60W40/02—Estimation or calculation of non-directly measurable driving parameters for road vehicle drive control systems not related to the control of a particular sub unit, e.g. by using mathematical models related to ambient conditions
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- B60L2220/00—Electrical machine types; Structures or applications thereof
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- B60L2220/42—Electrical machine applications with use of more than one motor
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- B60L2220/00—Electrical machine types; Structures or applications thereof
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- B60L2220/44—Wheel Hub motors, i.e. integrated in the wheel hub
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- B60L2240/00—Control parameters of input or output; Target parameters
- B60L2240/10—Vehicle control parameters
- B60L2240/12—Speed
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- 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
- B60L2240/00—Control parameters of input or output; Target parameters
- B60L2240/40—Drive Train control parameters
- B60L2240/42—Drive Train control parameters related to electric machines
- B60L2240/423—Torque
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- B60L2240/00—Control parameters of input or output; Target parameters
- B60L2240/40—Drive Train control parameters
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Abstract
The invention discloses a slip rate-based torque control method of an electric four-wheel drive vehicle, which comprises the following steps of calculating a target wheel edge slip rate and a target inter-axle slip rate; calculating the actual wheel side slip rate and the actual inter-axle slip rate; checking the road surface state and estimating a tire-road surface longitudinal adhesion limit; judging the serious grade of vehicle slippage; a feedback control torque required for vehicle slip rate control is calculated. The invention has the following beneficial effects: and in the motion limit range of the vehicle, the driving or braking torque of a front axle or a rear axle or a left axle or a right axle is reasonably calculated according to a control target, and the slip rate control of the vehicle under various driving conditions is quickly and accurately realized.
Description
Technical Field
The invention relates to the field of vehicle power control, in particular to a torque control method of an electric four-wheel drive vehicle based on a slip rate.
Background
The four-wheel independent drive electric automobile based on the hub motor has the advantages that the four wheels are independently controllable in torque, the power performance of the automobile can be improved through torque control distribution, and the control freedom degree is increased. In addition, the motor can be used for driving and braking, and compared with the traditional internal combustion engine and hydraulic braking system, the torque response speed and the control precision are higher, so that the performance of a power control system is improved. Therefore, four-wheel independent drive electric vehicles have obvious advantages in power control, and are gradually a research hotspot in recent years.
Wheel torque control distribution is an important aspect of dynamics control of a four-wheel-drive electric vehicle, and at present, research on the torque control distribution mainly comprises optimization targets of safety and energy conservation, for example, safety control indexes are represented by stability control generalized force tracking errors, tire force utilization adhesion coefficients and the like, and energy conservation indexes are represented by energy efficiency of a driving system. Then, the optimal control method is used for obtaining expected tire force of each wheel, and then expected wheel torque is obtained through the wheel radius and is used as an input command of an actuator to realize dynamic control.
In the wheel torque control distribution, the wheel torque is obtained by the dynamic characteristics of the tire so that the desired tire force can be obtained, and therefore the dynamic characteristics of the tire cannot be ignored. In the current research on wheel torque distribution, the main focus is on the control distribution stage of tire force, and the process of realizing tire force by controlling wheel torque is usually ignored. And one of the main parameters of tire force control is slip ratio. When the wheels slip unexpectedly in the starting or driving process, the slip ratio control function can ensure the normal driving of the vehicle or realize the escape by recalculating the driving torque of the front and rear shafts (or the left and right wheels) on the premise of keeping the stability of the vehicle. Therefore, it is a popular technique to determine the slip ratio, such as the technique disclosed in chinese patent CN102114782B and chinese patent CN 109383469B.
For the application of slip ratio control, the torque control distribution of the four-wheel-drive electric vehicle is of great significance based on the slip ratio control and considering the dynamic characteristics of tires. For example, chinese patent CN108394313B discloses a torque control allocation method, which performs slip rate calculation and tire force control allocation, then determines whether the tire is in a stable state according to the slip rate, and if the vehicle is in a stable state, solves the target slip rate through a tire inverse model, performs slip rate control, and implements wheel torque allocation; if the automobile is in an unstable state, controlling the wheel torque by using a sliding mode extreme value search algorithm to realize the optimal slip ratio and the maximum tire force. The technology has the defects that a complicated tire model or a sliding mode extreme value searching algorithm needs to be solved in the target slip ratio calculation process, an optimal value is difficult to be solved in a convergent manner in a short time in the dynamic driving process of the vehicle, and clustering and accurate control need to be carried out in combination with the actual slip ratio condition of the vehicle.
Disclosure of Invention
The invention aims to solve the technical problems and provides a torque control method of an electric four-wheel drive vehicle based on a slip ratio.
The purpose of the invention is realized by the following technical scheme:
a torque control method of an electric four-wheel drive vehicle based on slip ratio comprises the following steps:
s1, calculating a target wheel side slip rate and a target inter-axle slip rate according to parameters such as vehicle speed, wheel speed, longitudinal acceleration, peak road surface adhesion coefficient, wheel load, tire longitudinal slip stiffness and the like;
s2, calculating the actual wheel side slip rate and the actual inter-axle slip rate according to parameters such as vehicle speed, wheel speed, longitudinal acceleration, yaw angular velocity, front wheel turning angle and wheel base;
s3, checking the road surface state and estimating a tire-road surface longitudinal adhesion limit value;
s4, judging the serious grade of the vehicle slip;
s5, calculating feedback control torque required by vehicle slip ratio control, defining a torque distribution mode and a distribution proportion according to the structural form of the electric four-wheel drive system, and implementing front and rear axle torque distribution for the vehicle type with a single motor of a front axle and a rear axle; for the vehicle type with a single front axle motor and double rear axle motors, front and rear axle torque distribution and independent rear axle left and right wheel torque distribution are implemented; for a four-wheel motor-driven vehicle model, four-wheel torque independent distribution is implemented.
Preferably, in step S1, the slip ratio control is performed by controlling the slip ratio of the traveling wheel from the unstable state region to the stable state region, and taking the road surface adhesion coefficient into consideration while making full use of the influence of the wheel loaddes＝f(μmax,Fz) And (4) performing table lookup calculation, wherein the input quantity of the table is the peak road adhesion coefficient and the wheel load, and the output quantity is the target wheel edge slip rate.
Preferably, in step S1, the target inter-axle speed difference is calculated according to the front and rear axle target torque, the four wheel speed mean value, and the tire longitudinal slip stiffness under the current open-loop control, and the target inter-axle slip ratio is calculated according to the following formula in combination with the current vehicle speed, wherein the formula is as follows:
wherein: v. offl-left front wheel speed;
vfr-right front wheel speed;
vrl-left rear wheel speed;
vrr-right rear wheel speed;
v-vehicle speed;
Cx-tire longitudinal slip stiffness;
the four wheel speeds are measured by sensors and the vehicle speed is estimated from the drive motor speed.
Preferably, in step S2, the actual wheel slip ratio is calculated from the four wheel speeds and the vehicle speed, and is corrected by the longitudinal acceleration signal to obtain a corrected actual wheel slip ratio:
λ＝f(vfl,vfr,vrl,vrr,V,ax)
wherein: a isx-longitudinal acceleration.
Preferably, in step S2, the actual inter-axle speed difference base value is calculated from the average value of the front and rear axle speeds, the corrected actual inter-axle speed difference is calculated from the front wheel angle, the wheel base, and the yaw rate when the vehicle is turning, and the actual inter-axle slip ratio is calculated in combination with the current vehicle speed according to the following formula:
wherein: v. offl-left front wheel speed;
vfr-right front wheel speed;
vrl-left rear wheel speed;
vrr-right rear wheel speed;
v-vehicle speed;
l-wheelbase;
delta-front wheel corner;
Vdiff-actual interaxial speed difference;
λdiff-actual inter-axis slip ratio.
Preferably, in step S3, the "checking road surface state and estimating tire-road surface lateral adhesion limit" includes:
s31, judging the road surface state through vehicle slip rate calculation or image recognition, and dividing the road surface state into a high typical attachment road surface, a medium typical attachment road surface and a low typical attachment road surface by adopting a clustering algorithm;
and S32, according to the vehicle motion state and the driving intention, estimating the tire-road lateral adhesion limit value in the current and future specific time periods by using a vehicle dynamics formula.
Preferably, in step S4, the vehicle slip severity level includes single-wheel slip, single-axle slip, single-side wheel slip, cross-wheel slip, three-wheel slip, and four-wheel slip, and the specific process of "determining the vehicle slip severity level" includes: and (4) performing cluster analysis according to the actual wheel side slip rate and the actual inter-axle slip rate calculated in the step (S2), and triggering a corresponding slip rate control function if the judgment result shows that a certain vehicle has serious slip.
Preferably, the slip ratio control function is to calculate the longitudinal adhesion force limit value of each wheel according to a friction ellipse theory as a boundary value of the slip ratio control torque distribution.
Preferably, in step S5, the slip ratio control function includes,
controlling the inter-shaft slip rate: carrying out PID control according to the target inter-axle slip rate and the actual inter-axle slip rate, wherein the control output quantity is used for adjusting the torque of the front and rear axle motors, and the control is first-stage closed-loop control;
controlling the wheel edge slip rate: performing PID control according to the slip rate deviation of the wheel with the most serious slip, wherein the control output quantity is used for limiting the total required torque, and the control output quantity is the second-stage closed-loop control;
controlling the slip ratio between wheels: PID control is performed according to slip ratio deviation between the coaxial left and right wheels, and the device is suitable for an electric four-wheel drive system with a torque vectoring function.
Preferably, the slip rate control is carried out according to a PID control algorithm, the input quantity of the controller is the deviation of the target slip rate and the actual slip rate, and the output quantity of the controller is the closed-loop control torque T required by the slip rate controlfbThe calculation formula is as follows:
wherein: lambda [ alpha ]err-a slip rate deviation value;
Kp-P term control coefficients;
Ki-I term control coefficient;
Kd-D term control coefficient;
Tfb-PID feedback control calculates torque.
The invention has the following beneficial effects: and in the motion limit range of the vehicle, the driving or braking torque of a front axle or a rear axle or a left axle or a right axle is reasonably calculated according to a control target, and the slip rate control of the vehicle under various driving conditions is quickly and accurately realized.
Drawings
FIG. 1: is a schematic flow chart of a preferred embodiment of the invention.
FIG. 2: the torque calculation and distribution diagram of the preferred embodiment of the invention is shown.
FIG. 3: the effect of the vehicle slip rate control according to the preferred embodiment of the invention is shown schematically.
Detailed Description
Objects, advantages and features of the present invention will be illustrated and explained by the following non-limiting description of preferred embodiments. The embodiments are merely exemplary for applying the technical solutions of the present invention, and any technical solution formed by replacing or converting the equivalent thereof falls within the scope of the present invention claimed.
During vehicle take-off or acceleration, if the wheel driving force exceeds the road-tire longitudinal traction limit, the drive wheels will slip. According to the research result of the industry, if the slip rate is more than 20%, the wheel will slip, and the vehicle running is affected, so the slip rate of the pulley is controlled within 20% through the slip rate control, and the pulley is enabled to enter a stable rotation state. It is therefore desirable to implement slip-rate based torque control.
According to the invention, the application scenes of torque control based on the slip rate are divided into the following categories according to the slip condition of each wheel:
single wheel slip (e.g., pit escape);
single axle wheel slip (e.g., front axle gravel and rear axle asphalt);
one-sided wheel slip (e.g., left snow and ice on right asphalt pavement);
cross wheel slip (e.g., cross rough road);
three wheels slipping (e.g., gobi road);
all-wheel skidding (e.g., on icy and snowy roads).
On the basis, the core idea of the invention is to monitor the change of the wheel slip rate in real time and estimate the tire-road surface adhesion limit, reduce the driving force of the slipping wheel as much as possible, distribute more driving force to the wheel with large adhesion, gradually realize getting rid of difficulties and recover to the normal driving state.
As shown in fig. 1 and fig. 2, the invention discloses a torque control method of an electric four-wheel drive vehicle based on slip ratio, comprising the following steps:
s1, calculating a target wheel side slip rate and a target inter-axle slip rate according to parameters such as vehicle speed, wheel speed, longitudinal acceleration, peak road surface adhesion coefficient, wheel load, tire longitudinal slip stiffness and the like;
s2, calculating the actual wheel side slip rate and the actual inter-axle slip rate according to parameters such as vehicle speed, wheel speed, longitudinal acceleration, yaw angular velocity, front wheel turning angle and wheel base;
s3, checking the road surface state and estimating a tire-road surface longitudinal adhesion limit value;
s4, judging the serious grade of the vehicle slip;
s5, calculating feedback control torque required by vehicle slip ratio control, defining a torque distribution mode and a distribution proportion according to the structural form of the electric four-wheel drive system, and implementing front and rear axle torque distribution for the vehicle type with a single motor of a front axle and a rear axle; for the vehicle type with a single front axle motor and double rear axle motors, front and rear axle torque distribution and independent rear axle left and right wheel torque distribution are implemented; for a four-wheel motor-driven vehicle model, four-wheel torque independent distribution is implemented.
In step S1, the slip ratio control is performed to control the slip ratio of the traveling wheel from the unstable state region to the stable state region, and to take full advantage of the road surface adhesion coefficient while considering the influence of the wheel load, by the function λdes＝f(μmax,Fz) And (4) performing table lookup calculation, wherein the input quantity of the table is the peak road adhesion coefficient and the wheel load, and the output quantity is the target wheel edge slip rate. Calculating a target axle speed difference according to the target torque of a front axle and a rear axle, the wheel speed mean value of four wheels and the longitudinal slip stiffness of the tire under the current open-loop control, and calculating a target axle slip rate according to the following formula in combination with the current vehicle speed, wherein the calculation formula is as follows:
wherein: v. offl-left front wheel speed;
vfr-right front wheel speed;
vrl-left rear wheel speed;
vrr-right rear wheel speed;
v-vehicle speed;
Cx-tire longitudinal slip stiffness;
The four wheel speeds are measured by sensors and the vehicle speed is estimated from the drive motor speed.
In step S2, the actual wheel slip ratio is calculated according to the four wheel speeds and the vehicle speed, and is corrected by the longitudinal acceleration signal to obtain the corrected actual wheel slip ratio:
λ＝f(vfl,vfr,vrl,vrr,V,ax)
wherein: a isx-longitudinal acceleration.
In step S2, an actual inter-axle speed difference base value is calculated from the average of the front and rear axle speeds, and when the vehicle is turning, a corrected actual inter-axle speed difference is calculated from the front wheel rotation angle, the wheel base, and the yaw rate, and an actual inter-axle slip ratio is calculated from the following formula in combination with the current vehicle speed, wherein the calculation formula is as follows:
wherein: v. offl-left front wheel speed;
vfr-right front wheel speed;
vrl-left rear wheel speed;
vrr-right rear wheel speed;
v-vehicle speed;
l-wheelbase;
delta-front wheel corner;
Vdiff-actual interaxial speed difference;
λdiff-actual inter-axis slip ratio.
In step S3, the "checking road surface state and estimating tire-road surface lateral adhesion limit value" includes the following specific steps:
s31, judging the road surface state through vehicle slip rate calculation or image recognition, and dividing the road surface state into a high typical attachment road surface, a medium typical attachment road surface and a low typical attachment road surface by adopting a clustering algorithm;
and S32, according to the vehicle motion state and the driving intention, estimating the tire-road lateral adhesion limit value in the current and future specific time periods by using a vehicle dynamics formula.
In the step S4, the vehicle slip severity level includes single wheel slip, single axle slip, single side wheel slip, cross wheel slip, three wheel slip and four wheel slip, and the specific process of "determining the vehicle slip severity level" is as follows: and (4) performing cluster analysis according to the actual wheel side slip rate and the actual inter-axle slip rate calculated in the step (S2), and triggering a corresponding slip rate control function if the judgment result shows that a certain vehicle has serious slip.
And the slip ratio control function is to calculate the longitudinal adhesion force limit value of each wheel according to a friction ellipse theory, and the longitudinal adhesion force limit value is used as a boundary value of slip ratio control torque distribution.
In step S5, the slip ratio control function in the normal mode includes,
controlling the inter-shaft slip rate: carrying out PID control according to the target inter-axle slip rate and the actual inter-axle slip rate, wherein the control output quantity is used for adjusting the torque of the front and rear axle motors, and the control is first-stage closed-loop control;
controlling the wheel edge slip rate: performing PID control according to the slip rate deviation of the wheel with the most serious slip, wherein the control output quantity is used for limiting the total required torque, and the control output quantity is the second-stage closed-loop control;
controlling the slip ratio between wheels: PID control is performed according to slip ratio deviation between the coaxial left and right wheels, and the device is suitable for an electric four-wheel drive system with a torque vectoring function.
Slip rate control is carried out according to a PID control algorithm, the input quantity of the controller is the deviation of the target slip rate and the actual slip rate, and the output quantity of the controller is the closed-loop control torque T required by the slip rate controlfbThe calculation formula is as follows:
wherein: lambda [ alpha ]err-a slip rate deviation value;
Kp-P term control coefficients;
Ki-I term control coefficient;
Kd-D term control coefficient;
Tfb-PID feedback control calculates torque.
Referring to fig. 3, the concrete slip ratio control according to the severity level of the vehicle slip is implemented as follows.
Single wheel slip: the phenomenon is usually found on a water pit or a mud pit road surface, and if the vehicle is in a front driving mode and the right front wheel slips, the front driving mode needs to be switched into a four-wheel driving mode or a rear driving mode, the driving force of the front wheel is reduced for getting rid of difficulty, and the driving force of the rear wheel is increased for driving; similar control strategies may be employed when other individual wheels slip.
Single-shaft wheel skidding: the phenomenon is common on a gravel-asphalt front and back spliced road surface, and if a vehicle is in a front driving mode and wheels of a front axle slip, the front driving mode needs to be switched to a four-wheel driving mode or a rear driving mode, the driving force of the front axle is reduced for escaping from difficulties, and the driving force of a rear axle is increased for driving; a similar control strategy may be employed when the rear axle wheels slip.
One-sided wheel slip: this phenomenon is often seen on ice-snow-asphalt left and right spliced road surfaces, and if the vehicle is in a four-wheel drive mode and the left wheel slips, if the four-wheel drive system supports torque distribution of the left and right wheels, the driving force needs to be distributed to the wheel with large adhesion force as much as possible for driving; if the four-wheel drive system does not support the torque distribution of the left wheel and the right wheel, the inter-axle slip rates of the front axle and the rear axle need to be compared, and the driving force of the axle with small slip rate is increased for driving; if the slip rates of the front and rear shafts are equal and the torque distribution of the left and right wheels cannot be carried out, torque limitation is required to reduce the driving force to be within the limit range of the adhesive force between the slipping wheels and the road surface; a similar control strategy may be employed when the right hand wheel is slipping.
Cross wheel slip: this phenomenon is often seen on irregular cross-rough roads, assuming that the vehicle is in four-wheel drive mode and the left front wheel and the right rear wheel slip, if the four-wheel drive system supports torque distribution of the left and right wheels, it is necessary to distribute the driving force to the wheels with large adhesion force as much as possible for driving; if the four-wheel drive system does not support the torque distribution of the left wheel and the right wheel, the inter-axle slip rates of the front axle and the rear axle need to be compared, and the driving force of the axle with small slip rate is increased for driving; if the slip rates of the front axle and the rear axle are equal and the torque distribution of the left wheel and the right wheel cannot be carried out, torque limitation is required to reduce the driving force to be within the range of the adhesive force limit of the slipping wheel and the road surface, and the escaping can not be realized through software control under special conditions (such as the idle running of two wheels); a similar control strategy may be employed when the right front wheel slips and the left rear wheel slips.
Three wheels slip: this phenomenon is common on a sand-mixed gobi road surface, and if the vehicle is in a four-wheel drive mode and the front left wheel, the front right wheel and the rear right wheel slip, the four-wheel drive mode needs to be maintained (or switched to a two-wheel drive mode according to circumstances); if the four-wheel drive system supports torque distribution of left and right wheels of a rear axle, the driving force is distributed to the left and the rear wheels as much as possible, otherwise, torque limitation of the rear axle is required; similar control strategies may be employed when any other three wheels slip.
Slipping of all wheels: the phenomenon is usually seen on the full ice and snow road surface, the vehicle is in a four-wheel drive mode, the difference of slip rates of four wheels needs to be analyzed, the four-wheel drive mode is maintained (or switched to a two-wheel drive mode according to the situation), the driving force is distributed to the axle or the wheel with small slip rate as much as possible, and corresponding torque limitation is carried out; if the slip rates of the four wheels are equal and the road adhesion coefficient is low, the vehicle not only slips but also has a destabilization condition in the driving process, and at the moment, a driver needs to safely stop the vehicle and safely run after taking physical anti-slip measures (such as replacing anti-slip tires).
The invention has various embodiments, and all technical solutions formed by adopting equivalent transformation or equivalent transformation are within the protection scope of the invention.
Claims (10)
1. A torque control method of an electric four-wheel drive vehicle based on slip ratio is characterized by comprising the following steps: comprises the following steps of (a) carrying out,
s1, calculating a target wheel side slip rate and a target inter-axle slip rate according to parameters such as vehicle speed, wheel speed, longitudinal acceleration, peak road surface adhesion coefficient, wheel load, tire longitudinal slip stiffness and the like;
s2, calculating the actual wheel side slip rate and the actual inter-axle slip rate according to parameters such as vehicle speed, wheel speed, longitudinal acceleration, yaw angular velocity, front wheel turning angle and wheel base;
s3, checking the road surface state and estimating a tire-road surface longitudinal adhesion limit value;
s4, judging the serious grade of the vehicle slip;
s5, calculating feedback control torque required by vehicle slip ratio control, defining a torque distribution mode and a distribution proportion according to the structural form of the electric four-wheel drive system, and implementing front and rear axle torque distribution for the vehicle type with a single motor of a front axle and a rear axle; for the vehicle type with a single front axle motor and double rear axle motors, front and rear axle torque distribution and independent rear axle left and right wheel torque distribution are implemented; for a four-wheel motor-driven vehicle model, four-wheel torque independent distribution is implemented.
2. The method of claim 1The method is characterized in that: in the step S1, the function λ is useddes＝f(μmax,Fz) And (4) performing table lookup calculation, wherein the input quantity of the table is the peak road adhesion coefficient and the wheel load, and the output quantity is the target wheel edge slip rate.
3. The method of claim 2, wherein: in step S1, a target inter-axle speed difference is calculated according to the front and rear axle target torque, the four wheel speed mean value, and the tire longitudinal slip stiffness under the current open-loop control, and a target inter-axle slip ratio is calculated according to the following formula in combination with the current vehicle speed, wherein the calculation formula is as follows:
wherein: v. offl-left front wheel speed;
vfr-right front wheel speed;
vrl-left rear wheel speed;
vrr-right rear wheel speed;
v-vehicle speed;
Cx-tire longitudinal slip stiffness;
the four wheel speeds are measured by sensors and the vehicle speed is estimated from the drive motor speed.
4. The method of claim 3, wherein: in step S2, the actual wheel slip ratio is calculated according to the four wheel speeds and the vehicle speed, and is corrected by the longitudinal acceleration signal to obtain the corrected actual wheel slip ratio:
λ＝f(vfl,vfr,vrl,vrr,V,ax)
wherein: a isx-longitudinal acceleration.
5. The method of claim 4, wherein: in step S2, an actual inter-axle speed difference base value is calculated from the average of the front and rear axle speeds, and when the vehicle is turning, a corrected actual inter-axle speed difference is calculated from the front wheel rotation angle, the wheel base, and the yaw rate, and an actual inter-axle slip ratio is calculated from the following formula in combination with the current vehicle speed, wherein the calculation formula is as follows:
wherein: v. offl-left front wheel speed;
vfr-right front wheel speed;
vrl-left rear wheel speed;
vrr-right rear wheel speed;
v-vehicle speed;
l-wheelbase;
delta-front wheel corner;
Vdiff-actual interaxial speed difference;
λdiff-actual inter-axis slip ratio.
6. The method of claim 1, wherein: in step S3, the "checking road surface state and estimating tire-road surface lateral adhesion limit value" includes the following specific steps:
s31, judging the road surface state through vehicle slip rate calculation or image recognition, and dividing the road surface state into a high typical attachment road surface, a medium typical attachment road surface and a low typical attachment road surface by adopting a clustering algorithm;
and S32, according to the vehicle motion state and the driving intention, estimating the tire-road lateral adhesion limit value in the current and future specific time periods by using a vehicle dynamics formula.
7. The method of claim 1, wherein: in the step S4, the vehicle slip severity level includes single wheel slip, single axle slip, single side wheel slip, cross wheel slip, three wheel slip and four wheel slip, and the specific process of "determining the vehicle slip severity level" is as follows: and (4) performing cluster analysis according to the actual wheel side slip rate and the actual inter-axle slip rate calculated in the step (S2), and triggering a corresponding slip rate control function if the judgment result shows that a certain vehicle has serious slip.
8. The method of claim 7, wherein: and the slip ratio control function is to calculate the longitudinal adhesion force limit value of each wheel according to a friction ellipse theory, and the longitudinal adhesion force limit value is used as a boundary value of slip ratio control torque distribution.
9. The method of claim 1, wherein: in step S5, the slip ratio control function includes,
controlling the inter-shaft slip rate: carrying out PID control according to the target inter-axle slip rate and the actual inter-axle slip rate, wherein the control output quantity is used for adjusting the torque of the front and rear axle motors, and the control is first-stage closed-loop control;
controlling the wheel edge slip rate: performing PID control according to the slip rate deviation of the wheel with the most serious slip, wherein the control output quantity is used for limiting the total required torque, and the control output quantity is the second-stage closed-loop control;
controlling the slip ratio between wheels: PID control is performed according to slip ratio deviation between the coaxial left and right wheels, and the device is suitable for an electric four-wheel drive system with a torque vectoring function.
10. The method of claim 9, wherein: slip rate control is carried out according to a PID control algorithm, the input quantity of the controller is the deviation of the target slip rate and the actual slip rate, and the output quantity of the controller is the closed-loop control torque T required by the slip rate controlfbThe calculation formula is as follows:
wherein: lambda [ alpha ]err-a slip rate deviation value;
Kp-P term control coefficients;
Ki-I term control coefficient;
Kd-D term control coefficient;
Tfb-PID feedback control calculates torque.
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