CN113547928B - Dual-motor four-wheel drive electric vehicle torque distribution method considering tire slippage - Google Patents

Dual-motor four-wheel drive electric vehicle torque distribution method considering tire slippage Download PDF

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CN113547928B
CN113547928B CN202110795840.1A CN202110795840A CN113547928B CN 113547928 B CN113547928 B CN 113547928B CN 202110795840 A CN202110795840 A CN 202110795840A CN 113547928 B CN113547928 B CN 113547928B
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vehicle
motor
efficiency
torque
model
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CN113547928A (en
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曹开斌
胡明辉
翟钧
傅春耘
杨官龙
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Chongqing University
Chongqing Changan Automobile Co Ltd
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Chongqing University
Chongqing Changan Automobile Co Ltd
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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
    • 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
    • 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

Abstract

The invention relates to a torque distribution method of a double-motor four-wheel-drive electric vehicle considering tire slippage, and belongs to the field of intelligent control of new energy vehicle chassis. The method comprises the following steps: s1: building a complete vehicle dynamics and each component model, including a vehicle dynamics model, a battery model, a motor model, a tire model and an electric drive system loss model; s2: analyzing dynamic constraint changes of front and rear axle load transfer, and obtaining an active anti-skid control strategy by combining the attachment condition of the whole vehicle in running; s3: comprehensively considering the electric drive system loss caused by frequent switching of working modes due to the change of working conditions to obtain an optimal efficiency curved surface of the whole vehicle drive system; s4: and constructing a multi-target double-motor four-wheel-drive torque distribution control strategy with optimal overall vehicle efficiency by considering the dynamic changes of front and rear axle charge transfer and adhesion characteristics. On the premise of ensuring the safety and stability of the vehicle, the invention obviously reduces the energy consumption and the mode switching frequency, improves the economy of the vehicle and improves the comfort of the whole vehicle.

Description

Dual-motor four-wheel-drive electric vehicle torque distribution method considering tire slippage
Technical Field
The invention belongs to the field of intelligent control of new energy automobile chassis, and relates to a torque distribution method for a double-motor four-wheel drive electric automobile with optimal overall efficiency by considering tire slip.
Background
With the increasing social environmental protection requirements, higher requirements are put forward on the electric automobile from the energy-saving perspective. On the premise that the existing battery technology is not a major breakthrough, in order to further improve the economy of the pure electric vehicle, the efficiency of the whole system is optimized by methods such as lightweight design or high-efficiency electric drive system configuration, and the like, so that the longest driving range is realized. In the aspect of selection of the configuration of a driving system, compared with the traditional single-motor four-wheel drive driving mode, the front and rear shaft double-motor driving mode omits a front and rear shaft transmission system, reduces mechanical loss and maintenance quality, and simultaneously can control the adhesion characteristic of wheels by directly adjusting the driving force of each driving motor, so that the whole vehicle can obtain the maximum driving force; compared with the characteristics of complex driving control, large spatial arrangement difficulty, high cost and the like of a distributed hub motor, the front-rear-axle dual-motor driven electric automobile gradually becomes one of the focuses of the automobile industry. Torque distribution control of a front-axle and rear-axle double-motor-driven electric automobile is a key technology for intelligent control of a new energy automobile chassis, and plays a decisive role in the performance of the whole automobile. Therefore, the key technical research of intelligent control of the new energy automobile chassis is developed, and the method has very important significance for transformation of the automobile industry and improvement of the research and development level of the new energy automobile.
The safety stability and the economical efficiency of the vehicle are important indexes of the performance of the whole vehicle, and the performance of the whole vehicle is directly determined by the distribution control of the driving torque of the vehicle. At the present stage, with the aim of economy as a research target, a torque distribution strategy for optimizing the efficiency of a motor or a system is provided, and whether the torque distribution meets the current driving wheel adhesion condition under the strategy is not mentioned. If the torque distribution is not reasonable, the driving force of partial wheels is larger than the maximum adhesive force provided by the ground due to excessive torque output, the wheels can slip, and therefore the ABS or ASR anti-skid system can be triggered by the vehicle to ensure the running stability of the vehicle. The change of the energy consumption of the whole vehicle caused by the change of the attachment condition of the whole vehicle caused by the change of the front and rear axle loads under the dynamic working condition is not considered. Under good road surfaces, torque distribution strategies proposed with economic optimization goals if ABS or ASR anti-skid systems are frequently triggered, under the control of these passive anti-skid strategies, although wheel slip is inhibited, excessive torque output increases overall vehicle energy consumption, suggesting that the optimization strategy is not reasonable. Meanwhile, the prior art provides a curved surface with optimal efficiency of a driving system with the aim of economy, divides different corresponding working modes under different working condition points, and improves the economy of a vehicle by selecting different working modes; but neglects the drag loss, excitation loss, switching loss and the like of the front and back motors caused by the frequent switching of the working modes due to the change of the working conditions. Therefore, the efficiency curve of the driving system used in the prior art is not necessarily the optimal efficiency, which further influences the formulation and control of the torque distribution strategy, thereby limiting the space for improving the economic performance of the vehicle.
Disclosure of Invention
In view of the above, the invention aims to provide a torque distribution method for a dual-motor four-wheel-drive electric vehicle considering tire slip, which combines the attachment condition of the whole vehicle running, considers the dynamic constraint change of front and rear axle load transfer, and comprehensively considers the loss problems of front and rear motor dragging loss, excitation loss, switching loss and the like caused by frequent switching of working modes due to the change of working conditions, so that the obtained optimal efficiency curved surface of the whole vehicle driving system can better reflect the real situation of a real vehicle, and can provide better decision basis for the formulation and control of a torque distribution strategy; meanwhile, on the premise of not triggering the control of an ABS or ASR anti-skid system, the phenomenon of excessive wheel slip is reduced as much as possible, an active anti-skid effect is achieved, the safety and stability of the vehicle are guaranteed, the increase of the energy consumption of the whole vehicle caused by excessive torque output is avoided, and the economy of the vehicle is further improved.
In order to achieve the purpose, the invention provides the following technical scheme:
a torque distribution method of a double-motor four-wheel-drive electric vehicle considering tire slip specifically comprises the following steps:
s1: building a complete vehicle dynamics and each component model, including a vehicle dynamics model, a battery model, a motor model, a tire model and an electric drive system loss model;
s2: analyzing dynamic constraint changes of front and rear axle load transfer, and obtaining an active anti-skid control strategy by combining the attachment condition of the whole vehicle in running;
s3: comprehensively considering the loss of an electric drive system (the loss problems of dragging loss of front and rear motors, excitation loss, step torque transient response actual power loss and the like) caused by frequent switching of working modes due to the change of working conditions to obtain an optimal efficiency curved surface of the whole vehicle drive system;
s4: and constructing a multi-target double-motor four-wheel-drive torque distribution control strategy with optimal overall vehicle efficiency by considering front and rear axle load transfer and dynamic change of adhesion characteristics.
Further, in step S2, obtaining an active anti-skid control strategy specifically includes: under the condition that the real-time road adhesion coefficient can be obtained, the vehicle dynamic model considering the load transfer of the front and rear axles is used for obtaining the output torque T corresponding to the maximum adhesion force which can be provided by the front/rear axles mf_safe 、T mr_safe (ii) a Then the maximum peak torque T of the front/rear motor mf_max 、T mr_max In contrast, the smaller one is used as an external characteristic constraint boundary, so that dynamic constraint is performed on the dynamic property of the vehicle, and an active anti-skidding effect is achieved.
Further, in step S2, the constraint condition of front and rear axle load transfer and the adhesion condition of the whole vehicle running specifically include:
Figure GDA0003836277700000021
Figure GDA0003836277700000022
wherein, F zf For front axle vertical loads, F zr For rear axle vertical loads, T mf_safe 、T mr_safe Motor shaft end output torque, eta corresponding to maximum adhesion force which can be provided by front/rear shafts respectively tran In order to achieve the efficiency of the transmission system,
Figure GDA0003836277700000031
is the road surface adhesion coefficient, i f As front axle rotation ratio, i r Is the rotation ratio of the rear axle; t is a unit of mf 、T mr Actual torque output for front/rear motor shaft ends, T mf_max 、T mr_max The front/rear motor maximum peak torque.
Further, in step S3, obtaining an optimal efficiency curved surface of the entire vehicle driving system specifically includes: according to the working condition, comprehensively considering a loss model of the electric drive system and an efficiency difference model of a boundary area, taking economy and switching frequency as targets, analyzing influence factors of mode boundary bandwidth, and carrying out boundary optimization; and then analyzing the energy consumption and switching frequency of all working condition points in each section of vehicle speed under different boundary bandwidths to perform multi-target boundary optimization, thereby obtaining an optimal overall efficiency curved surface of the whole vehicle driving system.
Further, the expression of the boundary area efficiency difference model is as follows: Δ η = f (T, n, η) 1 ，η 2 ) Where T is the current total demand torque, n is the rotational speed, η 1 To front motor efficiency, η 2 For rear motor efficiency.
Further, the dynamic change of the dynamic constraint of the whole vehicle is comprehensively considered, and the comprehensive efficiency of the whole vehicle driving system under different (front-drive, rear-drive and four-drive) driving modes is obtained by combining the efficiencies of a battery, a motor and a transmission system;
the overall efficiency depends on the speed, acceleration, battery efficiency, motor efficiency and transmission system efficiency;
taking the efficiency of the whole vehicle as a target function max eta sys Obtaining the optimal torque distribution of front and rear motors with optimal vehicle running efficiency under each driving mode, and further obtaining the mode division rule of the vehicle; the constraint condition taking the whole vehicle efficiency as a target function is as follows:
Figure GDA0003836277700000032
wherein SOC is a state of charge, SOC min At a minimum cut-off charge state, P bat Is the battery power, P bat_max For maximum battery discharge power, T mf 、T mr For actual output torque at the shaft ends of the front/rear motors, T mf_max 、T mr_max Maximum peak torque, T, of the front/rear motor mf_safe 、T mr_safe Motor shaft end output torque n corresponding to the maximum adhesion force which can be provided by the front/rear shafts respectively f 、n r Front/rear motor speeds, n f_max ，n r_max The maximum rotational speeds of the front and rear motors, respectively.
Further, the control strategy of mode switching is: according to the comprehensive efficiency of the whole vehicle driving system in different modes, obtaining different mode division rules under different vehicle speeds and different required torques, namely the running interval of each mode; the operating mode is then automatically selected based on the change in speed and total required torque.
Further, in step S4, the constructed multi-target dual-motor four-wheel drive torque distribution control strategy specifically includes: according to the vehicle dynamics and each part model built in the step 1 and the active anti-skid control strategy obtained in the step 2, the driver model analyzes the total required torque through the position of a driving/braking pedal, and mode division is carried out through the optimal efficiency curved surface of the vehicle driving system obtained in the step 3 according to the current vehicle speed and the required torque; considering dynamic change of front and rear axle load transfer, front/rear torque distribution T is carried out on total required torque under the current mode f_req 、T r_req And then the output torque T corresponding to the maximum adhesion force that can be provided by the front/rear axle mf_safe 、T mr_safe And maximum torque T that can be provided by external characteristics of the front/rear motor mf_max 、T mr_max Comparing to obtain the actual output torque T of the front and rear shafts mf 、T mr (ii) a Outputting the vehicle speed and the angular speeds of front and rear wheels in the current state through a vehicle dynamics model, and then judging the slip ratio; and if the current slip rate is smaller than the optimal slip rate corresponding to the road surface adhesion coefficient, entering the next moment so as to play an active anti-slip role.
The invention has the beneficial effects that:
1) According to the built dynamic model of the whole vehicle and the front and rear axle systems, the dynamic constraint change of the front and rear axle load transfer is analyzed, and an active anti-skid control strategy is provided by combining the running attachment condition of the whole vehicle, so that the phenomenon of excessive wheel skid is reduced as much as possible on the premise of not triggering the anti-lock braking system (ABS) or ASR (driving anti-skid system, only driving wheels are controlled) anti-skid system, and the safety and stability of the vehicle are ensured.
2) The invention comprehensively considers the problems of the loss of the dragging loss, the excitation loss, the switching loss and the like of the front motor and the rear motor caused by the frequent switching of the working modes due to the change of the working conditions, and the obtained optimal efficiency curved surface of the whole vehicle driving system can reflect the real situation of the real vehicle better and can provide better decision basis for the formulation and the control of the torque distribution strategy.
3) According to state variables such as vehicle speed, required torque and SOC, the influence rule of electric drive system loss caused by mode switching and the influence rule of boundary region efficiency difference between modes on mode boundary bandwidth determination is emphatically analyzed; and analyzing the energy consumption and the switching frequency of all working points in each section of vehicle speed under different boundary bandwidths by taking the economy and the switching frequency as targets to perform multi-target boundary optimization.
4) According to the multi-target double-motor four-wheel-drive torque distribution control strategy method for optimizing the overall efficiency of the whole vehicle by considering the front and rear axle load transfer and the dynamic change of the attachment characteristic, a driver model analyzes the total required torque through the position of a driving/braking pedal, and mode division is performed through an optimal overall efficiency curved surface of a whole vehicle driving system according to the current vehicle speed and the required torque; and (4) taking dynamic changes of front and rear axle load transfer into consideration, carrying out front and rear torque distribution on the total required torque in the current mode, and comparing the total required torque with the output torque under the active anti-skid control strategy to obtain the actual output torque of the front and rear axles. On the premise of guaranteeing the safety and stability of the vehicle, the energy consumption is obviously reduced, the economy of the vehicle is improved, the mode switching frequency is greatly reduced, and the comfort of the whole vehicle is improved.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims thereof.
Drawings
For a better understanding of the objects, aspects and advantages of the present invention, reference will now be made to the following detailed description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic view of a dual-motor four-wheel drive electric vehicle;
FIG. 2 is a schematic diagram of a longitudinal/vertical dynamics model of a whole vehicle;
FIG. 3 is a forward drive and four-drive torque distribution response process;
FIG. 4 is an active antiskid control strategy that takes into account the axle load transfer and adhesion condition dynamic constraints;
FIG. 5 illustrates the maximum output torque provided by the shaft ends of the front axle drive mode motor;
FIG. 6 illustrates the maximum output torque provided by the shaft end of the rear axle drive mode motor;
FIG. 7 illustrates the maximum output torque provided by the shaft end of the four wheel drive mode motor;
FIG. 8 is the overall optimum combined efficiency for the vehicle at each mode of 60% SOC;
FIG. 9 is a 60% SOC mode division;
FIG. 10 shows the mode switching frequency under various operating conditions;
FIG. 11 is a pattern division of a 60% SOC with pattern boundaries;
FIG. 12 illustrates the mode selection corresponding to the operating point;
FIG. 13 is a comparison of WLTC (550 s-900 s) unbounded versus bounded mode switching;
FIG. 14 is a curved surface of optimal efficiency of the entire vehicle drive system;
FIG. 15 is a torque distribution control strategy that takes into account the axle load shift and adhesion condition dynamic constraints.
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.
Referring to fig. 1 to 15, in the embodiment, for the design of the coordinated controller for torque distribution and driving anti-skid of the front-rear independent driving four-wheel drive electric vehicle, the specific torque distribution method mainly includes the following steps:
step 1: building a complete vehicle dynamics and each component model;
the two-motor four-wheel drive electric automobile researched by the embodiment is characterized in that a driving motor is respectively arranged on a front axle and a rear axle, the driving torque of the shaft end of the motor is transmitted to a differential mechanism and a half axle through a speed reducer to drive wheels on the left side and the right side of the front axle and the rear axle, and a power source is provided by a battery pack. In order to meet the driving torque requirement of the whole vehicle driving system, the driving torque of each driving motor is directly adjusted through the MCU, and 3 driving modes of independent work of front and rear motors and simultaneous work of double motors are realized. Fig. 1 is a schematic structural diagram of a dual-motor four-wheel drive electric vehicle.
1) Vehicle dynamics model
Analyzing the plane motion of the dual-motor four-wheel drive electric automobile on a ground coordinate system, neglecting the lateral force and the yaw moment, and considering that the vertical loads on the left side and the right side are equal, as shown in fig. 2, O is the position of the center of mass of the whole automobile, and b and c are the longitudinal distances from the center of mass of the whole automobile to the front shaft and the rear shaft respectively. As the vehicle has the phenomenon of axle load transfer in the moving process, the stress conditions of the front axle and the rear axle of the vehicle are respectively researched, and the inertia force and the moment generated when the vehicle moves longitudinally and vertically are comprehensively analyzed.
The longitudinal dynamic equation of the whole vehicle is as follows:
F x -F R ＝mδa (1)
Figure GDA0003836277700000061
wherein, F xf As front axle wheel end driving force, F xr Is the driving force of the rear axle wheel end, m is the whole vehicle prepared mass, f is the rolling resistance coefficient, C D The method is characterized in that the method is an air resistance coefficient, A is a windward area, m is a finished vehicle prepared mass, a is a finished vehicle acceleration, delta is a finished vehicle rotating mass conversion coefficient, theta is a gradient, and v is a finished vehicle speed.
In the running process of the vehicle, the vertical load and the running resistance of the front and rear axles are influenced by the change of the lift force, meanwhile, the longitudinal acceleration can also cause the longitudinal transfer of the load, and the change of the load of the front and rear axles directly influences the optimal torque distribution of the front and rear motors, so that the important significance is brought to the further research on the energy consumption of the whole vehicle and the stability of the vehicle. The vertical loads of the front axle wheel, the rear axle wheel and the whole vehicle are respectively analyzed.
The expression of the front axle vertical load and the dynamic equation is as follows:
Figure GDA0003836277700000062
wherein, F zf For front axle vertical loads, F Rf The running resistance of the front axle, C the distance from the center of mass to the rear axle, L the wheelbase, h the height of the center of mass, and C Lf Is the lift coefficient of the front axle;
the rear axle vertical load and the dynamic equation expression are as follows:
Figure GDA0003836277700000063
wherein, F zr For rear axle vertical loading, F Rr Rear axle running resistance, J er Equivalent moment of inertia of the rear axis, b is the distance from the center of mass to the front axis, C Lr Is the lift coefficient of the rear axle;
2) Build up driver model
The present embodiment employs PID control to simulate driver operation, with target vehicle speed and actual vehicle speed as control inputs, and pedal opening as an output signal. The PID control formula is:
Figure GDA0003836277700000071
wherein, K p Is a proportionality coefficient, K i Is the integral coefficient, K d Is a differential coefficient, K aw As coefficient of anti-saturation, e v Is a target vehicle speed errorDifference, e out The error of the pedal opening is represented by y, the acceleration is represented by the positive pedal opening, the deceleration is represented by the negative pedal opening, and the pedal opening is [ -1,1]Within a range, so introduce y sat The function is used as a constraint boundary with a small output.
Since the vehicle total required torque is positively correlated with the pedal opening, the variation relationship of the pedal opening and the total required torque is defined as follows:
T req ＝y·T max (n m )＝y·[T max_mf (n m )+T max_mr (n m )] (6)
wherein, T max (n m ) Is a rotational speed n m The sum of maximum torques which can be provided by the front motor and the rear motor; t is a unit of max mf (n m ) Is a rotational speed n m Maximum torque, T, that the motor can deliver before time max_mr (n m ) Is a rotational speed n m The maximum torque that the motor can send out after the time.
3) Building battery model
The charging and discharging characteristics of the battery pack are influenced by temperature, charging and discharging current and temperature, and in order to simplify a battery pack model, the environmental temperature of the battery pack is assumed to be 25 ℃;
the power of the battery pack is as follows:
P b ＝P m_in ＝EI-I 2 R 0 (7)
wherein, P b Is the charge and discharge power of the battery pack, E is the electromotive force of the battery pack, I is the charge and discharge current, R 0 Is equivalent internal resistance;
solving equation (8) can obtain the charging and discharging current as follows:
Figure GDA0003836277700000072
the state of charge of the battery pack is as follows:
Figure GDA0003836277700000073
wherein, SOC initial Is the initial state of charge of the battery, SOC (t) is the state of charge at time t, C b Is the rated capacity of the battery pack.
4) Building motor model
Considering that the motor response has a certain delay, an inertia link is adopted to represent the response characteristic of the motor:
Figure GDA0003836277700000081
wherein, T m Represents the motor output torque in Nm; t is cmd Represents a target torque of the motor in Nm; t is c Is a time constant; s complex variable.
The power is transmitted among the motor, the reducer and the wheels through gear engagement, and for a single motor, the conversion relation of the rotating speed, the torque and the power is as follows:
Figure GDA0003836277700000082
Figure GDA0003836277700000083
Figure GDA0003836277700000084
wherein n is m Is the motor speed, P m_out For the output power of the motor, P m_in For input power of the motor, η m (T m ，n m ) The working efficiency of the motor under corresponding torque and rotating speed is realized.
5) Vehicle tyre model
Assuming that the specifications of the 4 wheels are the same, the stress balance diagram of the front and rear driving wheels is shown in fig. 2. The balance equation of the driving wheel rotating moment under the wheel coordinate system is
J w a ij ＝T dij -F xij r-F zij fr (14)
Wherein, J w Is the moment of inertia (kg. M) of the wheel 2 )，T d Is the wheel driving torque (N.m), r is the wheel rolling radius (m), ij represents fl, fr, rl, rr;
the calculation formula of the longitudinal slip rate of the tire is
Figure GDA0003836277700000085
Where ω is the angular velocity (rad/s) of tire rotation, r ω Is the radius of the tire, S d For driving the slip of the tyre, S b The tire braking slip ratio is obtained.
The characteristics of tires have been studied in detail from various viewpoints domestically and abroad. The magic formula has a large influence because of high simulation accuracy. Therefore, the nonlinear tire model is adopted by the invention to describe the mechanical characteristics of the tire more accurately.
Figure GDA0003836277700000091
Wherein s is longitudinal slip ratio, B is stiffness factor, C is curve shape factor, D is curve peak factor, E is curve curvature factor, B 0 -b 8 Is the main parameter coefficient of the magic formula.
6) Loss model for electric drive system
(1) Drag losses
Because the whole vehicle electric drive system has 3 drive modes, namely front drive, rear drive and four-wheel drive, when the VCU drives the rear axle according to the current working condition decision, the required torque of the rear motor is the total required torque, and the required torque of the front motor is 0; when the current motor gets into 0 torque mode, because whole car electric drive system is in the power-on running state, so there is certain power loss in front axle drive system, and changes along with the change of rotational speed, obtains the front axle through the test data fitting that the rotary drum test bench surveyed and pulls the loss:
P loss_fkz ＝f(n)＝a 1 n+b 1 (17)
wherein, a 1 As fitting coefficient, b 1 Is a constant.
When the VCU drives the front shaft according to the current working condition decision, the required torque of the front motor is the total required torque, and the required torque of the rear motor is 0; when the rear motor enters a 0-torque mode, because the whole vehicle electric drive system is in a power-on running state, the rear axle drive system also has certain power loss and changes along with the change of the rotating speed, and the rear axle dragging loss is obtained through the fitting of test data measured by the rotary drum test bed:
P loss_rkz ＝f(n)＝a 2 n+b 2 (18)
wherein, a 2 As fitting coefficient, b 2 Is a constant.
(2) Excitation loss
In the double-motor front-rear shaft driving configuration researched by the invention, the front motor adopts an asynchronous motor, and the rear motor adopts a permanent magnet synchronous motor. Therefore, when the current motor receives a command to enter a torque mode, the asynchronous motor needs an excitation process, and excitation loss generated by the asynchronous motor in the process is not negligible. The excitation loss of the front motor is obtained by fitting test data measured by a rotary drum test bed:
P loss_flc ＝f(n)＝a 3 n+b 3 (19)
wherein, a 3 As fitting coefficient, b 3 Is a constant.
(3) Step torque transient response power loss of electric drive system
The selection of the vehicle drive mode varies according to the operating conditions. When the driving mode is changed frequently, namely the front driving mode, the rear driving mode and the four-driving mode are switched frequently. When the front drive is switched to the rear drive, as shown in FIG. 3, at t 0 ～t 2 The total required torque provided by the front motor over time, at t 1 The time is provided by a rear motor; whether the motor exits or enters, the motor response is changed; while in this variation, the front motor is at t 0 ～t 2 Power consumption in time, t, with the front motor 0 ～t 1 Power consumption during time and after motor at t 1 ～t 2 The sum of power consumption in time is unequal, and the transient response power loss of the step torque of the electric drive system caused by frequent mode switching is related to the current required torque and the current rotating speed and cannot be ignored; therefore, the test data measured by the rotary drum test bed can be fitted to obtain the relation expression of the step torque transient response power loss of the electric drive system, and the step torque transient response power loss of the electric drive system corresponding to other mode switching can be obtained in the same way.
P loss_onoff ＝f(T q ，n) (20)
And 2, step: analyzing dynamic constraint changes of front and rear axle load transfer, and providing an active anti-skid control strategy by combining the attachment condition of the whole vehicle during running;
because the longitudinal axle load transfer exists in the running process of the vehicle, the dynamic property of the vehicle is constrained by the road adhesion conditions of the front wheels and the rear wheels and the maximum peak torque characteristics of the front motor and the rear motor while the dynamic equation of the whole vehicle is satisfied. Therefore, in the case where the required power provided by the battery pack can be satisfied, whether the actual output torque T of the vehicle can satisfy the vehicle required torque T _ need The constraint condition expression is as follows:
Figure GDA0003836277700000101
Figure GDA0003836277700000102
wherein, F zf For front axle vertical loads, F zr For rear axle vertical loads, T mf_safe 、T mr_safe Motor shaft end output torque, eta corresponding to maximum adhesion force which can be provided by front/rear shafts respectively tran In order to be efficient in the transmission system,
Figure GDA0003836277700000103
is the road surface adhesion coefficient, i f As front axle rotation ratio, i r Is the rotation ratio of the rear axle; t is mf 、T mr For actual output torque at the shaft ends of the front/rear motors, T mf_max 、T mr_max The front/rear motor maximum peak torque.
According to the dynamic model of the whole vehicle and the front and rear axle system built in the step 1, the dynamic constraint change of front and rear axle load transfer is analyzed, and an active anti-skid strategy is provided by combining the adhesion condition of the whole vehicle in driving, as shown in fig. 4. Under the condition that the real-time road adhesion coefficient can be obtained, the vehicle dynamic model considering the load transfer of the front axle and the rear axle is used for obtaining the output torque T corresponding to the maximum adhesion force which can be provided by the front axle and the rear axle mf_safe 、T mr_safe (ii) a Then the maximum torque T provided by the external characteristics of the front and the rear motors mf_max 、T mr_max And in contrast, the smaller side is used as an external characteristic constraint boundary, so that the dynamic property of the vehicle is dynamically constrained to play an active anti-skidding role.
According to the active anti-skid control strategy, the maximum driving torque boundary which can be provided by the vehicle in three driving modes of front driving, rear driving and four driving is obtained. When the vehicle enters the mode1 and the front axle is driven independently, the load of the front axle is transferred backwards, the load of the front axle is reduced and is restrained by the ground adhesion condition, so that the maximum driving torque curve T provided by the front motor can be ensured under the condition that the front axle is not driven to rotate excessively mf_real As shown in fig. 5.
When the vehicle enters the mode2 and the rear axle is independently braked, the load of the rear axle is transferred forwards, the load of the rear axle is reduced and is simultaneously restrained by the ground adhesion condition, and the maximum driving torque curve T provided by the rear motor mr_real As shown in fig. 6; if the torque exceeds T mr_safe Excessive slippage of the rear axle occurs, resulting in loss of energy consumption.
When the vehicle is driven by four wheels, the influence of longitudinal axle load transfer and road adhesion condition on vertical load of each axle is comprehensively considered, the relation between effective output torque and acceleration-speed provided by front and rear motors under a certain road adhesion coefficient is obtained, and when the road adhesion coefficient is reachedCoefficient of performance
Figure GDA0003836277700000113
When the change occurs, the effective output torque provided by the front motor and the rear motor is changed. As can be seen from fig. 7, the maximum effective output torque that the motor can provide at each vehicle speed of the vehicle (solid line) is not equal to the sum of the front and rear motor maximum output torques in the ideal state (broken line). Therefore, if the torque distribution is performed in the ideal state (within the dotted line), when the torque exceeds the maximum available output torque (solid line), the wheels must slip excessively, resulting in a large loss of energy consumption.
And 3, step 3: comprehensively considering the problem of electric drive system loss caused by frequent switching of working modes due to the change of working conditions to obtain an optimal efficiency curved surface of the whole vehicle drive system;
how to distribute the torque between the front and rear axle motors of the double-motor four-wheel drive electric automobile directly influences the efficiency of the whole automobile system. The overall vehicle efficiency depends on the vehicle speed, acceleration, battery efficiency, motor efficiency and transmission system efficiency.
When the vehicle is driven, the overall efficiency of the whole vehicle is as follows:
Figure GDA0003836277700000111
wherein eta is mf 、η mr Front and rear motor efficiencies, η m_dis Total driving efficiency of front and rear motors, eta bat_dis Discharge efficiency of the battery pack, η tran For transmission system efficiency, η sys_dis The total efficiency of the whole vehicle driving system is improved.
When the vehicle brakes, the overall efficiency of the whole vehicle is as follows:
Figure GDA0003836277700000112
wherein eta m_chg For the total efficiency, eta, of equivalent braking energy recovery of front and rear motors bat_chg Efficiency of charging the battery, η _sys_chg For the whole vehicleEquivalent total efficiency of braking energy recovery.
Therefore, the overall vehicle efficiency is taken as the objective function max eta sys The optimal torque distribution of the front motor and the rear motor with optimal vehicle running efficiency under each driving mode can be obtained, and the mode division rule of the vehicle is further obtained. Equation (25) is a constraint condition that takes the overall vehicle efficiency as an objective function.
The constraint conditions are as follows:
Figure GDA0003836277700000121
wherein SOC is a state of charge, SOC min At a minimum cut-off charge state, P bat Is the battery power, P bat_max At maximum battery discharge power, n f 、n r Front/rear motor speeds, n, respectively f_max ，n r_max The maximum rotation speeds of the front and rear motors, respectively.
The comprehensive efficiency of the driving system of the whole vehicle under three driving modes of front driving, rear driving and four driving can be obtained by comprehensively considering the dynamic change of the dynamic constraint of the whole vehicle and combining the efficiencies of a battery, a motor and a transmission system. As shown in fig. 8, when the SOC is 60%, the driving system efficiency of the three curved surfaces in the figure are respectively the forward driving (FWD), the backward driving (RWD), and the four-wheel driving (AWD) modes, and are indicated by colors and arrows. By comparing the efficiency of the driving system in different modes at each operating point and selecting the optimal operating point, the comprehensive efficiency of the whole vehicle driving system in the full range of speed and total required torque can be obtained.
In order to achieve better overall vehicle system efficiency, and to meet the driving torque requirement considering the change of front and rear axle loads in combination with the dynamic change of vehicle dynamic constraint, it is necessary to make a feasible mode switching control strategy. According to the comprehensive efficiency of the whole vehicle driving system under different modes, three mode division rules under different vehicle speeds and different required torques, namely the running interval of each mode can be obtained. The operating mode is automatically selected based on changes in speed and total required torque.
Fig. 9 shows the operating region for each of the 3 modes at speed and total required torque conditions when the SOC is 60%. The vehicle selects a mode2 rear-drive mode in a low-speed and low-acceleration region, and selects a mode1 front-drive mode in a medium-speed/high-speed and low-acceleration region, so that the vehicle driving requirement can be met by a single-motor running mode in the low-acceleration region with the best overall efficiency; in order to obtain higher dynamic performance, the front motor and the rear motor work simultaneously to enter a mode3 four-wheel drive mode, and the requirement of vehicle dynamic performance is met with the best overall vehicle efficiency.
However, according to the above existing mode division, economic cycle condition test analysis such as NEDC, UDDS, CLTC, WLTC and the like is performed, and it is found that there is very high frequency mode switching under 4 different cycle conditions, as shown in fig. 10; this causes high frequency shocks that greatly reduce the ride comfort of the vehicle, and frequent mode switching not only increases energy consumption, but also places extremely high demands on the electric drive control technology. Therefore, the present embodiment proposes the mode boundary as the transition region of the mode switching, as shown in fig. 11, thereby reducing the switching frequency, improving the comfort of the vehicle ride, while avoiding an increase in energy consumption.
FIG. 12 is a partial enlargement of FIG. 11, and it can be seen from FIG. 12 that the last mode is maintained when operating point mode2 is stepped to boundary 0 mode; when the operating point continues to the yellow region mode3, the mode3 state is actually entered. Thus, the problem of frequent switching between the mode2 and the mode3 caused by the fact that the working point wanders only at the boundary can be greatly avoided. Meanwhile, the motor response requires a time, and if the mode maintenance time is shorter than the motor response time and the can bus signal transmission time, the selection and execution of the current mode are unnecessary, and the power consumption is also increased. As shown in fig. 13, the mode3 sustain time is extended, the switching frequency is reduced, and mode switching for a very short time is also avoided.
The influence factors of how to define the size of the mode boundary bandwidth are related to vehicle speed and torque fluctuation errors; different boundary bandwidths can obtain different comprehensive efficiency curved surfaces of the whole vehicle driving system. The larger the boundary bandwidth area is, the lower the mode switching frequency is, the smaller the electric drive loss is, and the lower the energy consumption is; but the mode boundary is the last mode, and the larger the area is, the larger the efficiency difference between the two modes is, and the poorer the economy is; the size of the efficiency difference value is related to the efficiencies of the two motor bodies; thus, the larger the mode boundary bandwidth region, the lower the mode switching frequency is, by definition, contradictory with respect to power consumption, both the electrical driving losses caused by mode switching and the difference in efficiency between the two modes.
In order to solve the problem, according to the working condition, comprehensively considering the electric drive system loss model established in the step 1 and a boundary region efficiency difference value model formula (26), taking economy and switching frequency as targets, analyzing the influence factors of the mode boundary bandwidth, and performing boundary optimization, as shown in table 1. And analyzing the energy consumption and switching frequency of all working condition points in each section of vehicle speed under different boundary bandwidths to perform multi-target boundary optimization, and obtaining an optimal comprehensive efficiency curved surface of the whole vehicle driving system as shown in fig. 14.
Boundary area efficiency difference
Δη＝f(T，n，η 1 ，η 2 ) (26)
Wherein T is the current total required torque, n is the rotation speed, eta 1 To front motor efficiency, η 2 For rear motor efficiency.
TABLE 1 specific control variables and influencing factors in Multi-objective optimization
Figure GDA0003836277700000131
And 4, step 4: constructing a multi-target double-motor four-wheel-drive torque distribution control strategy with optimal overall vehicle efficiency and taking forward and backward axle charge transfer and dynamic change of adhesion characteristics into consideration;
according to the dynamic model of the whole vehicle and the front and rear axle systems built in the step 1 and the active anti-skid control strategy provided in the step 2, as shown in fig. 15, a driver model analyzes total required torque through the position of a driving/braking pedal, and mode division is carried out according to the current vehicle speed and the required torque through the optimal comprehensive efficiency curved surface of the whole vehicle driving system obtained in the step 3; considering dynamic change of front and rear axle load transfer, front and rear torque distribution T is carried out on total required torque under the current mode f_req 、T r_req Then, with the front and rear axleOutput torque T corresponding to maximum adhesion force capable of being provided mf_safe 、T mr_safe And maximum torque T provided by external characteristics of front and rear motors mf_max 、T mr_max Comparing to obtain the actual output torque T of the front and rear shafts mf 、T mr (ii) a Outputting the vehicle speed and the angular speeds of front and rear wheels in the current state through a vehicle dynamics model, and then judging the slip ratio; and if the current slip rate is smaller than the optimal slip rate corresponding to the road surface adhesion coefficient, entering the next moment so as to play an active anti-slip role. Under this strategy, the phenomenon of skidding of the vehicle wheels generally cannot occur, and unless the vehicle wheels are under extreme-variable working conditions with extremely low road adhesion coefficients and extremely high power requirements, in order to guarantee the safety and stability of the vehicle, if the phenomenon of skidding occurs under the extreme working conditions, the ABS or ASR antiskid system is triggered to serve as the final guarantee.
In conclusion, the multi-target dual-motor four-wheel drive torque distribution control strategy method with optimal overall vehicle efficiency and considering dynamic changes of front and rear axle load transfer and adhesion characteristics provided by the embodiment has the advantages that on the premise of ensuring the safety and stability of the vehicle, the energy consumption is obviously reduced, the economy of the vehicle is improved, the mode switching frequency is greatly reduced, and the comfort of the overall vehicle is improved.
Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.

Claims (6)

1. A torque distribution method of a double-motor four-wheel-drive electric vehicle considering tire slip is characterized by comprising the following steps:
s1: building a complete vehicle dynamics and each component model, wherein the complete vehicle dynamics and each component model comprises a vehicle dynamics model, a battery model, a motor model, a tire model and an electric drive system loss model;
s2: analyzing dynamic constraint changes of front and rear axle load transfer, and obtaining an active anti-skid control strategy by combining the attachment condition of the whole vehicle in running; the constraint condition of front and rear axle load transfer and the adhesion condition of the whole vehicle running specifically comprise:
Figure FDA0003863959650000011
Figure FDA0003863959650000012
wherein, F zf For front axle vertical loads, F zr For rear axle vertical loads, T mf_safe 、T mr_safe Motor shaft end output torque eta corresponding to maximum adhesion force which can be provided by front/rear shafts respectively tran In order to achieve the efficiency of the transmission system,
Figure FDA0003863959650000013
is the road surface adhesion coefficient, i f As front axle rotation ratio, i r Is the rotation ratio of the rear axle, r ω Is the tire radius; t is a unit of mf 、T mr For actual output torque at the shaft ends of the front/rear motors, T mf_max 、T mr_max The maximum peak torque of the front/rear motor;
s3: comprehensively considering the dragging loss of the electric drive system, the excitation loss generated by an asynchronous motor and the actual power loss of the step torque transient response of the electric drive system caused by frequent switching of the working mode due to the change of the working condition, so as to obtain the optimal efficiency curved surface of the whole vehicle drive system;
s4: the method comprises the following steps of constructing a multi-target double-motor four-wheel drive torque distribution control strategy with optimal overall vehicle efficiency and taking forward and backward axle charge transfer and dynamic change of adhesion characteristics into consideration, and specifically comprises the following steps: according to the vehicle dynamics and each component model built in the step 1 and the active anti-skid control strategy obtained in the step 2, the driver model analyzes the total required torque through the position of a driving/braking pedal, and according to the current vehicle speed and the required torque, the optimal efficiency of the vehicle driving system obtained in the step 3 is obtainedCarrying out mode division on the curved surface; considering dynamic change of front and rear axle load transfer, front/rear torque distribution T is carried out on total required torque under the current mode f_req 、T r_req And then the output torque T corresponding to the maximum adhesion force that can be provided by the front/rear axle mf_safe 、T mr_safe And maximum torque T that can be provided by external characteristics of the front/rear motor mf_max 、T mr_max Comparing to obtain the actual output torque T of the front and rear shafts mf 、T mr (ii) a Outputting the vehicle speed and the angular speeds of front and rear wheels in the current state through a vehicle dynamics model, and then judging the slip ratio; and if the current slip rate is smaller than the optimal slip rate corresponding to the road surface adhesion coefficient, entering the next moment so as to play an active anti-slip role.
2. The torque distribution method of the dual-motor four-wheel drive electric vehicle according to claim 1, wherein in the step S2, an active anti-skid control strategy is obtained, and specifically comprises: under the condition of obtaining the real-time road adhesion coefficient, obtaining the output torque T corresponding to the maximum adhesion force which can be provided by the front/rear axle by considering the vehicle dynamic model of the front/rear axle load transfer mf_safe 、T mr_safe (ii) a Then the maximum peak torque T of the front/rear motor mf_max 、T mr_max In contrast, the smaller one is used as an external characteristic constraint boundary, so that the dynamic constraint is carried out on the vehicle dynamics, and the active anti-skidding function is achieved.
3. The torque distribution method of the dual-motor four-wheel drive electric vehicle as claimed in claim 1, wherein in step S3, obtaining an optimal efficiency curved surface of a vehicle driving system specifically comprises: comprehensively considering the dragging loss of an electric drive system, the excitation loss generated by an asynchronous motor and the efficiency difference value model of a boundary area and an actual power loss model of step torque transient response of the electric drive system caused by frequent switching of a working mode due to the change of the working condition according to the working condition, analyzing the influence factors of mode boundary bandwidth by taking economy and switching frequency as targets, and carrying out boundary optimization; and then analyzing the energy consumption and switching frequency of all working condition points in each section of vehicle speed under different boundary bandwidths to perform multi-target boundary optimization, thereby obtaining an optimal overall efficiency curved surface of the whole vehicle driving system.
4. The torque distribution method of the dual-motor four-wheel-drive electric vehicle as claimed in claim 3, wherein the expression of the boundary region efficiency difference model is as follows: Δ η = f (T, n, η) 1 ，η 2 ) Where T is the current total required torque, n is the rotational speed, η 1 Is the front motor efficiency, eta 2 For rear motor efficiency.
5. The torque distribution method for the double-motor four-wheel-drive electric vehicle according to claim 4, characterized in that the comprehensive efficiency of a whole vehicle driving system in different driving modes is obtained by comprehensively considering the dynamic change of the dynamic constraint of the whole vehicle and combining the efficiencies of a battery, a motor and a transmission system;
the overall efficiency depends on the speed, acceleration, battery efficiency, motor efficiency and transmission system efficiency;
taking the efficiency of the whole vehicle as a target function max eta sys Obtaining the optimal torque distribution of front and rear motors with optimal vehicle running efficiency under each driving mode, and further obtaining the mode division rule of the vehicle; the constraint condition taking the whole vehicle efficiency as a target function is as follows:
Figure FDA0003863959650000021
wherein SOC is a state of charge, SOC min At a minimum cut-off charge state, P bat Is the battery power, P bat_max For maximum battery discharge power, T mf 、T mr For actual output torque at the shaft ends of the front/rear motors, T mf_max 、T mr_max Maximum peak torque, T, of the front/rear motor mf_safe、 T mr_safe Motor shaft end output torque n corresponding to the maximum adhesion force which can be provided by the front/rear shafts respectively f 、n r Front/rear motor speeds, n, respectively f_max 、n r_max The maximum rotation speeds of the front and rear motors, respectively.
6. The torque distribution method for the dual-motor four-wheel drive electric vehicle according to claim 5, wherein the control strategy for mode switching is as follows: obtaining different mode division rules under different speeds and different required torques, namely running intervals of each mode according to the comprehensive efficiency of the whole vehicle driving system under different modes; the operating mode is then automatically selected based on the change in speed and total required torque.
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