CN110466602B - Time-sharing four-wheel steering system of electric automobile driven by hub motor and control method thereof - Google Patents
Time-sharing four-wheel steering system of electric automobile driven by hub motor and control method thereof Download PDFInfo
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- CN110466602B CN110466602B CN201910820810.4A CN201910820810A CN110466602B CN 110466602 B CN110466602 B CN 110466602B CN 201910820810 A CN201910820810 A CN 201910820810A CN 110466602 B CN110466602 B CN 110466602B
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Classifications
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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
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
- B62D—MOTOR VEHICLES; TRAILERS
- B62D11/00—Steering non-deflectable wheels; Steering endless tracks or the like
- B62D11/02—Steering non-deflectable wheels; Steering endless tracks or the like by differentially driving ground-engaging elements on opposite vehicle sides
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B62—LAND VEHICLES FOR TRAVELLING OTHERWISE THAN ON RAILS
- B62D—MOTOR VEHICLES; TRAILERS
- B62D6/00—Arrangements for automatically controlling steering depending on driving conditions sensed and responded to, e.g. control circuits
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
- Y02T10/00—Road transport of goods or passengers
- Y02T10/60—Other road transportation technologies with climate change mitigation effect
- Y02T10/72—Electric energy management in electromobility
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- Steering-Linkage Mechanisms And Four-Wheel Steering (AREA)
- Steering Control In Accordance With Driving Conditions (AREA)
Abstract
The invention discloses a time-sharing four-wheel steering system of an electric automobile driven by an in-wheel motor and a control method thereof, wherein the system comprises a detection module, a steering mode selection switch, a vehicle speed tracking module, a vehicle controller, a front wheel steering-by-wire control module, a rear wheel differential steering control module and a rear wheel steering locking module; when the selected steering mode is four-wheel steering, the rear wheels are unlocked, and the front wheels finish independent steering in a mode of steer-by-wire and rear wheel differential steering. The beneficial effects are that: the steering device not only can switch front wheel steering and four-wheel steering according to the requirements of a driver and provide proper road feel for the driver, but also has the advantages of simple structure, accurate steering, good robustness and the like; and the maneuverability of the vehicle can be improved under the low-speed working condition, and the maneuvering stability of the vehicle can be improved under the high-speed working condition.
Description
Technical Field
The invention relates to a steering control system and a steering control method, in particular to a time-sharing four-wheel steering system of an electric automobile driven by an in-wheel motor and a control method thereof, and belongs to the technical field of electric automobiles.
Background
The independent driving and steering of the wheels are usually realized in a distributed driving mode, and the independent driving and steering of the wheels can be divided into wheel side motor driving and wheel hub motor driving according to motor driving modes. The wheel motor driving is to install the motor system near the wheels and drive the wheels through the wheel reducer; in the in-wheel motor driving, an in-wheel motor is arranged in a wheel hub to directly drive a wheel. The hub motor driven automobile integrates driving and transmission in the hub, so that mechanical parts such as a clutch, a transmission shaft, a differential mechanism and the like required by the traditional automobile are omitted, the mechanical structure of the automobile is greatly simplified, and the space utilization rate of the automobile is improved. Meanwhile, the control of the hub motor drive only needs a motor and a controller, so that the delay influence of mechanical structures such as an electromagnetic valve, a hydraulic pipeline and the like is avoided, the real-time performance and the precision are good, and various control schemes can be fully utilized.
An Active Four-Wheel Steering (4 WS) system can effectively improve the maneuvering flexibility of a vehicle at a low speed, the Steering stability at a high speed and the path tracking problem of an intelligent vehicle by actively controlling the front and rear Wheel angles. However, since the rear wheels participate in steering, and a common control idea is to make the centroid slip angle of the vehicle zero, it is inevitable that the driver's feel is different from that of driving a conventional front-wheel steering vehicle. Therefore, the advantages of the active four-wheel steering system are fully exerted, and meanwhile, good road feel is fed back, so that a driver can pay attention to the fact that the driver is careful when driving the electric vehicle; in addition, uncertainty such as mass, mass center position, tire cornering stiffness and the like which change along with working conditions can occur in the running process of an actual vehicle, so that the design of a reasonable control strategy to improve the robustness of system response is also a problem to be solved.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provide a time-sharing four-wheel steering system of an electric automobile driven by an in-wheel motor and a control method thereof, which not only can switch front-wheel steering and four-wheel steering according to the requirements of a driver and provide proper road feel for the driver, but also have the advantages of simple structure, accurate steering and the like, and have good robustness.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
a time-sharing four-wheel steering system of an in-wheel motor driven electric vehicle, comprising:
the detection module is used for detecting the running parameters of the electric automobile and inputting the running parameters to the whole automobile controller; wherein the driving parameters comprise steering wheel rotation angle signals, pedal position signals of an accelerator pedal, driving vehicle speed signals, front wheel rotation angle signals and rear wheel rotation angle signals;
a steering mode selection switch for a driver to manually select a steering mode including front wheel steering and four wheel steering, and inputting the selected steering mode to the vehicle controller;
the vehicle speed tracking module is used for calculating the running speed and the required front wheel driving moment of the electric vehicle according to the pedal position signal of the accelerator pedal, controlling the wheel hub motor to execute the running vehicle speed signal of a driver and inputting the running speed to the whole vehicle controller;
The whole vehicle controller is used for receiving the running speed, transmitting a steering wheel angle signal to the front wheel drive-by-wire steering control module when the selected steering mode is front wheel steering, and outputting a rear wheel locking signal to the rear wheel steering locking module; when the selected steering mode is four-wheel steering, calculating and obtaining ideal front and rear wheel turning angles required by the four-wheel steering and outputting a rear wheel unlocking signal to a rear wheel steering locking module, wherein the ideal front and rear wheel turning angles comprise the ideal front wheel turning angle and the ideal rear wheel turning angle, the ideal front wheel turning angle is transmitted to a front wheel steer-by-wire control module, and the ideal rear wheel turning angle is transmitted to a rear wheel differential steering control module;
the front wheel drive-by-wire steering control module is used for driving the front wheel steering executing motor to steer the front wheel according to the steering wheel angle signal and the front wheel steering angle signal or the ideal front wheel steering angle, providing corresponding steering road feel for a driver and feeding back the steering road feel to the whole vehicle controller;
the rear wheel differential steering control module is used for calculating rear wheel differential moment according to an ideal rear wheel steering angle and driving a rear wheel hub motor to steer the rear wheel according to the rear wheel differential moment;
and the rear wheel steering locking module is used for driving the steering locking mechanism to turn the rear wheel back according to the rear wheel locking signal and the rear wheel steering angle signal, and preventing the rear wheel from steering after the rear wheel is turned back, and driving the steering locking mechanism to release the steering locking according to the rear wheel unlocking signal.
The system of the invention is further arranged to: the steering locking mechanism comprises a return motor and two groups of steering mechanisms which are symmetrically arranged on the frame and have identical structures, the return motor is connected with the rear wheel steering locking module, and the two groups of steering mechanisms are connected with the return motor and respectively act on the left rear wheel and the right rear wheel;
the steering mechanism comprises an electromagnetic clutch, a return spur gear, a locking pull rod, a front beam rod, a first steering knuckle arm and a second steering knuckle arm which are sequentially connected, an electromagnet which is lapped on the surface of the locking pull rod and is arranged on the frame, steering main pins which are connected with one end of the first steering knuckle arm and one end of the second steering knuckle arm, an upper cross arm and a lower cross arm which are arranged on the frame, and a locking pull rod displacement sensor which is used for detecting the rotation angle of the rear wheel and is positioned on the locking pull rod; the electromagnetic clutch is connected with the positive return motor and the rear wheel steering locking module, the locking pull rod is connected with the positive return gear through the rod surface rack, the locking pull rod is connected with the front beam rod, the front beam rod is connected with the first steering knuckle arm through the spherical hinge, the two ends of the steering main pin are respectively connected with the upper cross arm and the lower cross arm through the spherical hinge, the electromagnet is used for generating suction force after being electrified and is attracted onto the locking pull rod so that the locking pull rod is locked on the frame, and the locking pull rod displacement sensor is used for detecting displacement parameters between the locking pull rod and the vehicle body in real time and feeding the displacement parameters back to the whole vehicle controller to determine the rear wheel steering angle.
The system of the invention is further arranged to: the front wheel steering executing motor makes the front wheel turn through driving the front wheel steering driving medium, the driving front wheel steering driving medium is including the left front wheel knuckle arm, left front wheel steering tie rod, rack, steering gear, right front wheel steering tie rod and the right front wheel knuckle arm that link to each other in proper order, left front wheel knuckle arm acts on left front wheel, right front wheel knuckle arm acts on right front wheel, steering gear links to each other with the front wheel steering executing motor.
The system of the invention is further arranged to: the detection module comprises a torque sensor and a steering wheel angle sensor for detecting a steering wheel, a pedal position sensor for detecting an accelerator pedal, and a rack displacement sensor for detecting a front wheel angle and positioned on a rack.
The invention also provides a control method of the time-sharing four-wheel steering system of the wheel hub motor driven electric automobile, which runs in the time-sharing four-wheel steering system of the wheel hub motor driven electric automobile and comprises the following steps:
1) The whole vehicle controller acquires the driving parameters acquired by the detection module and the steering mode selected by the driver and input by the steering mode selection switch, wherein the driving parameters comprise a steering wheel rotation angle signal, a pedal position signal driving vehicle speed signal of an accelerator pedal, a front wheel rotation angle signal and a rear wheel rotation angle signal;
2) The vehicle speed tracking module calculates the running speed and the required front wheel driving moment of the electric vehicle according to the pedal position signal of the accelerator pedal, controls the wheel hub motor to execute the running vehicle speed signal of the driver, and inputs the running speed to the whole vehicle controller;
3) During running of the vehicle, the whole vehicle controller judges whether the steering mode selected by the driver is front wheel steering or four-wheel steering;
if the steering mode is front wheel steering, adopting a front wheel steering by wire and rear wheel steering locking mode, executing the steps 4) to 5), and returning to the step 1);
if the steering mode is four-wheel steering, adopting a front wheel steering by wire and rear wheel differential steering mode, executing the steps 6) -8), and returning to the step 1);
4) When the selected steering mode is front wheel steering, the whole vehicle controller transmits steering wheel angle signals to a front wheel drive-by-wire steering control module, and outputs rear wheel alignment signals to a rear wheel steering locking module so that rear wheels align and lock rear wheel steering;
5) The front wheel steering control module drives the front wheel steering execution motor to steer the front wheels according to the steering wheel angle signal and the front wheel steering angle signal, provides corresponding steering road feel for a driver, and feeds back the steering road feel to the whole vehicle controller; meanwhile, the rear wheel steering locking module drives the rotary locking mechanism to align the rear wheel and prohibit the rear wheel from steering according to the rear wheel locking signal and the rear wheel steering angle signal;
6) When the selected steering mode is four-wheel steering, the whole vehicle controller calculates and obtains ideal front and rear wheel turning angles required by the four-wheel steering and outputs a rear wheel unlocking signal to a rear wheel steering locking module, wherein the ideal front and rear wheel turning angles comprise the ideal front wheel turning angle and the ideal rear wheel turning angle, the ideal front wheel turning angle is transmitted to a front wheel steer-by-wire control module, and the ideal rear wheel turning angle is transmitted to a rear wheel differential steering control module;
7) The front wheel steering control module drives the front wheel steering execution motor to steer the front wheels according to the ideal front wheel steering angle, provides corresponding steering road feel for a driver, and feeds back the steering road feel to the whole vehicle controller; meanwhile, the rear wheel steering locking module drives the slewing locking mechanism to release steering locking according to a rear wheel unlocking signal;
8) The rear wheel differential steering control module calculates rear wheel differential moment according to the ideal rear wheel steering angle, and drives the rear wheel hub motor to steer the rear wheel according to the rear wheel differential moment.
The method of the invention is further provided with: the vehicle speed tracking module searches a relation curve between the longitudinal speed and the position of the accelerator pedal, which is preset in the system, according to signals acquired by a pedal position sensor of the accelerator pedal, so that the running speed of the electric vehicle is obtained.
The method of the invention is further provided with: the whole vehicle controller calculates and obtains ideal front and rear wheel steering angles required by four-wheel steering, specifically,
the vehicle controller inputs the collected steering wheel angle to a reference model to obtain an ideal yaw rate and a centroid side deflection angle, and controls the yaw rate and the centroid side deflection angle of the decoupled linear four-wheel steering model by adopting an asymptotic tracking controller to track the ideal yaw rate and the centroid side deflection angle, so that the ideal front and rear wheel steering angles of the four-wheel steering vehicle are obtained; the reference model is a linear two-degree-of-freedom vehicle model with neutral steering characteristics after the centroid slip angle is subjected to amplitude reduction and filtering.
The method of the invention is further provided with: the state equation of the reference model is that,
let x be d ＝[γ d β d ] T ，u d ＝[δ f ]，
The reference model is expressed as
in the formula ,
wherein ,γd Yaw rate, beta, of the reference model d Is the centroid slip angle of the reference model, m is the automobile mass, u x For longitudinal speed at the centre of mass of the car, delta f Is the front wheel angle, l fd Is the distance from the centroid to the front axis, l rd Is the distance from the centroid to the rear axis, k f For cornering stiffness, k of the front wheel r For cornering stiffness of rear wheels, I z The moment of inertia of the vehicle to the Z axis;
The amplitude reduction filter is carried out on the centroid side deflection angle, the transfer function of the adopted amplitude reduction filter is,
wherein eta is a gain coefficient, zeta is a damping coefficient and omega n Is a circular frequency;
by adjusting the circular frequency omega n To adjust the magnitude of the centroid slip angle.
The method of the invention is further provided with: the linear dynamics model adopted by the linear four-wheel steering model is that
In the equation(s) used in the present invention,
wherein ,lf Distance from front axle to centroid, l r Distance from the rear axis to the center of mass;
the linear four-wheel steering model is subjected to input-output decoupling,
the transfer function adopted is
wherein ,
introducing parameter u 1 、u 2 To realize decoupling of the yaw rate gamma and the centroid side deviation angle beta, and the yaw rate gamma obtained after decoupling is relative to u 1 Transfer function and centroid slip angle beta with respect to u 2 The transfer functions of (a) are G respectively 11(s) and G22 (s)；
Then there is
If and only ifAt the time, the yaw rate and the centroid slip angle are controlled by the variable u 1 and u2 Thus, it is possible to obtain,
wherein ,a0 ＝(a 12 b 21 -a 22 b 11 )(a 21 b 12 -a 11 b 22 +a 11 b 12 -a 12 b 22 )，a 1 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 11 b 12 b 21 -a 12 b 21 b 12 ，a 2 ＝b 11 b 22 -b 21 b 12 ，b 0 ＝(a 21 b 12 -a 11 b 22 )(a 12 b 21 -a 22 b 11 )，b 1 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 12 b 21 b 22 -a 22 b 11 b 22 ，b 2 ＝b 22 b 11 ，b 3 ＝(a 11 b 12 -a 12 b 22 )(a 21 b 12 -a 11 b 22 )，b 4 ＝-a 21 b 2 12 +a 11 b 22 b 12 +a 11 b 12 b 22 -a 12 b 2 22 ，b 5 ＝-b 12 b 22 ，b 6 ＝(-a 12 b 21 +a 22 b 11 )(a 21 b 11 -a 11 b 21 )，b 7 ＝-a 12 b 2 21 +a 22 b 11 b 21 -a 21 b 2 11 +a 11 b 11 b 21 ，b 8 ＝-b 11 b 21 ，b 9 ＝(a 21 b 12 -a 11 b 22 )(a 12 b 21 -a 22 b 11 )，b 10 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 12 b 21 b 22 -a 22 b 11 b 22 ，b 11 ＝b 22 b 11 ；
Let the transfer function of the asymptotic tracking controller be G C1 (s) the asymptotic tracking controller adopts a P, PI, PD or PID controller;
for the control equation e (t) =γ d (t)-γ(t)＝γ d (t)-G C1 (t)G 11 (t) e (t) performing a pull-type transformation,
obtaining the product
Is provided with
Then
Feature polynomial d of its 2 nd-order system g (s)d c (s)+n g (s)n c (s) =0 reduced to d 2 s 2 +d 1 s+d 0 ＝0；
wherein ,d2 ＝1+b 11 k d ，d 1 ＝b 11 k p +(-a 22 b 11 +a 12 b 21 )k d -(a 11 +a 22 )，
d 0 ＝a 11 a 22 -a 21 a 12 +k p (-a 22 b 11 +a 12 b 21 )；
Determining parameters of an asymptotic tracking controller for controlling the yaw rate and the centroid slip angle according to a route decision table and a route-Hurwitz stability criterion;
wherein the Routh decision table isIn->
The Routh-Hurwitz stability criterion is that the coefficients of the characteristic polynomials of the 2 nd order system are all positive or all negative.
The method of the invention is further provided with: the rear wheel differential steering control module calculates the rear wheel differential moment according to the ideal rear wheel steering angle, specifically,
establishing a dynamic model, taking an ideal front and rear wheel corner as an input two-degree-of-freedom four-wheel steering vehicle as a reference model, and designing a sliding mode variable structure controller to obtain a rear wheel differential moment for driving rear wheels to steer;
the method comprises the following steps:
a) The state equation of the front wheel drive-by-wire steering and the rear wheel differential steering without considering the following rotation angle of the wheels is that,
wherein beta is the vehicle mass center side deflection angle, gamma is the vehicle yaw rate,for vehicle yaw acceleration, +.>Is the vehicle mass center side deflection angular velocity, m is the vehicle mass, u x For the longitudinal speed at the centroid of the car, lf is the distance from the front axle to the centroid，l r Delta is the distance from the rear axle to the centroid f For front wheel rotation angle, k f For cornering stiffness, k of the front wheel r For cornering stiffness of rear wheels, I z D is the rotational inertia of the vehicle to the Z axis r R is the effective radius of the tire;
b) Let state space variable x (t) = [ βγ ]] T The input is the front and rear wheel driving torque difference, i.e. u (T) =delta T, and the state equation of the front wheel steer-by-wire and rear wheel differential steering is converted into
wherein ,
c) Definition of the slip plane s=γ - γ d +ξ(β-β d )，
Wherein, xi is the weight coefficient, gamma d Yaw rate, beta, as reference model d A centroid slip angle of the reference model;
the derivative is obtained by the method,
in the formula ,
wherein ,yaw acceleration for reference model, +.>The centroid slip angular velocity is the reference model;
due toBounded, then->Wherein->Is a constant;
the approach law with saturation function is adopted to obtain the controller as
Compared with the prior art, the invention has the following beneficial effects:
the time-sharing four-wheel steering system of the electric automobile driven by the hub motor and the control method thereof can switch front wheel steering and four-wheel steering according to the requirements of a driver, provide proper road feel for the driver, and have the advantages of simple structure, accurate steering and the like, and have good robustness. When the selected steering mode is front wheel steering, the front wheels are controlled by the front wheel steer-by-wire control module to perform traditional front wheel steering, and the rear wheels are locked back. When the selected steering mode is four-wheel steering, the rear wheels are unlocked, the front wheels finish independent steering in a traditional steering and rear wheel differential steering mode, and the rear wheels are independently driven and combined with a differential steering technology, so that the maneuverability of the vehicle can be effectively improved under a low-speed working condition, and the occurrence of tail flicking and sideslip when the vehicle turns can be effectively reduced at a high speed; at the same time, the active safety and the steering stability of the motor vehicle can be improved.
The foregoing is merely an overview of the present invention, and for the purpose of providing a better understanding of the present invention, the present invention is further described below with reference to the accompanying drawings.
Drawings
Fig. 1 is a schematic structural diagram of a time-sharing four-wheel steering system of an electric vehicle driven by an in-wheel motor according to the present invention;
FIG. 2 is a graph showing yaw rate at low speed simulated by an embodiment of the method of the present invention;
FIG. 3 is a graph showing the centroid slip angle at low speed simulated by an embodiment of the method of the present invention;
FIG. 4 is a graph showing the x and y positions at low speed simulated by an embodiment of the method of the present invention;
FIG. 5 is a graph showing torque at low speed simulated by an embodiment of the method of the present invention;
FIG. 6 is a graph showing yaw rate at medium speed as simulated by an embodiment of the method of the present invention;
FIG. 7 is a graph showing yaw rate at medium speed as simulated by an embodiment of the method of the present invention;
FIG. 8 is an illustration of the x and y positions at medium speed as simulated by an embodiment of the method of the present invention;
FIG. 9 is a graph showing torque at medium speed simulated by an embodiment of the method of the present invention;
FIG. 10 is a graph showing yaw rate at high speed simulated by an embodiment of the method of the present invention;
FIG. 11 is a graph of centroid slip angle at high speed simulated by an embodiment of the method of the present invention;
FIG. 12 is a graph of the x and y positions at high speed as simulated by an embodiment of the method of the present invention;
FIG. 13 is a graph showing torque at high speed simulated by an embodiment of the method of the present invention.
In the figure: 1-left front wheel knuckle arm; 2-left front wheel tie rod; 3-racks; 4-steering gears; 5-steering execution motor and speed reducer; 6-a rack displacement sensor; 7-a rear wheel hub motor; 8-steering wheel angle sensor; 9-road feel simulation motor and speed reducer; 10-a torque sensor; 11-steering wheel; 12-a front wheel steer-by-wire module; 13-a whole vehicle controller; 14-a rear wheel differential steering control module; 15-left rear wheel; 16-toe-in bar; 17-upper cross arm; 18-a kingpin; 19-spherical hinge; 20-lower cross arm; 21-a locking pull rod displacement sensor; 22-return spur gear; 23-an electromagnetic clutch; 24-aligning the motor; 25-locking tie rod; 26-first knuckle arm one end, 27-electromagnet, 28-frame, 29-second knuckle arm.
Detailed Description
The invention will be further described with reference to the drawings.
The invention provides a time-sharing four-wheel steering system of an electric automobile driven by an in-wheel motor, as shown in figure 1, comprising:
the detection module is used for detecting the running parameters of the electric automobile and inputting the running parameters into the whole automobile controller 13; wherein the driving parameters comprise steering wheel rotation angle signals, pedal position signals of an accelerator pedal, driving vehicle speed signals, front wheel rotation angle signals and rear wheel rotation angle signals;
A steering mode selection switch for the driver to manually select a steering mode including front wheel steering and four wheel steering, and input the selected steering mode to the whole vehicle controller 13;
the vehicle speed tracking module is used for calculating the running speed and the required front wheel driving moment of the electric vehicle according to the pedal position signal of the accelerator pedal, controlling the wheel hub motor to execute the running vehicle speed signal of the driver, and inputting the running speed to the whole vehicle controller 13;
the whole vehicle controller 13 is used for receiving the running speed, transmitting a steering wheel angle signal to the front wheel steer-by-wire control module 12 when the selected steering mode is front wheel steering, and outputting a rear wheel locking signal to the rear wheel steering locking module; when the selected steering mode is four-wheel steering, calculating and obtaining ideal front and rear wheel turning angles required by the four-wheel steering and outputting a rear wheel unlocking signal to a rear wheel steering locking module, wherein the ideal front and rear wheel turning angles comprise the ideal front wheel turning angle and the ideal rear wheel turning angle, the ideal front wheel turning angle is transmitted to a front wheel steer-by-wire control module, and the ideal rear wheel turning angle is transmitted to a rear wheel differential steering control module;
The front wheel steer-by-wire control module 12 is used for driving the front wheel steering executing motor 5 to steer the front wheel according to the steering wheel angle signal and the front wheel angle signal or the ideal front wheel angle, providing corresponding steering road feel for a driver through the road feel simulation motor and the speed reducer 9, and feeding back the steering road feel to the whole vehicle controller 13;
the rear wheel differential steering control module 14 is used for calculating rear wheel differential moment according to the ideal rear wheel steering angle and driving the rear wheel hub motor 7 to steer the rear wheel according to the rear wheel differential moment;
and the rear wheel steering locking module is used for driving the steering locking mechanism to turn the rear wheel back according to the rear wheel locking signal and the rear wheel steering angle signal, and preventing the rear wheel from steering after the rear wheel is turned back, and driving the steering locking mechanism to release the steering locking according to the rear wheel unlocking signal.
As shown in fig. 1, the steering locking mechanism includes a positive return motor 24 and two sets of steering mechanisms symmetrically mounted on the frame 18 and having identical structures, the positive return motor 24 is connected to the rear wheel steering locking module, and the two sets of steering mechanisms are connected to the positive return motor 24 and respectively act on the left rear wheel 15 and the right rear wheel.
The steering mechanism comprises an electromagnetic clutch 23, a return spur gear 22, a locking pull rod 25, a toe-in rod 16, a first steering knuckle arm 26 and a second steering knuckle arm 29 which are sequentially connected, an electromagnet 27 which is lapped on the surface of the locking pull rod 25 and is arranged on a frame 28, a steering main pin 18 which is connected with one end of the first steering knuckle arm 26 and one end of the second steering knuckle arm 29, an upper cross arm 17 and a lower cross arm 20 which are arranged on the frame 28, and a locking pull rod displacement sensor 21 which is used for detecting the rotation angle of a rear wheel and is positioned on the locking pull rod 25; the electromagnetic clutch 23 is connected with the positive return motor 24 and is connected with the rear wheel steering locking module, the locking pull rod 25 is connected with the positive return gear 22 through a rod surface rack, the locking pull rod 25 is connected with the front beam rod 16, the front beam rod 16 is connected with the first knuckle arm 26 through a spherical hinge 19, two ends of the steering main pin 18 are respectively connected with the upper cross arm 17 and the lower cross arm 20 through the spherical hinge 19, the electromagnet 27 is used for generating suction force after being electrified and is attracted onto the locking pull rod 25 so that the locking pull rod 25 is locked on the frame 28, and the locking pull rod displacement sensor 21 is used for detecting displacement parameters between the locking pull rod 25 and the vehicle body in real time and feeding the displacement parameters back to the vehicle controller 13 to determine the rear wheel steering angle.
As shown in fig. 1, the front wheel steering actuator 5 steers the front wheel by driving a front wheel steering transmission including a left front wheel steering arm 1, a left front wheel steering rod 2, a rack 3, a steering gear 4, a right front wheel steering rod, and a right front wheel steering arm that are sequentially connected, the left front wheel steering arm 1 acting on the left front wheel, the right front wheel steering arm acting on the right front wheel, and the steering gear 4 being connected to the front wheel steering actuator 5.
As shown in fig. 1, the detection module includes a torque sensor 10 and a steering wheel angle sensor 8 for detecting a steering wheel 11, a pedal position sensor for detecting an accelerator pedal, and a rack displacement sensor 6 for detecting a front wheel angle and located on a rack 3.
The invention also provides a control method of the time-sharing four-wheel steering system of the wheel hub motor driven electric automobile, which is operated in the time-sharing four-wheel steering system of the wheel hub motor driven electric automobile shown in figure 1 and comprises the following steps:
1) The whole vehicle controller acquires the driving parameters acquired by the detection module and the steering mode selected by the driver and input by the steering mode selection switch, wherein the driving parameters comprise a steering wheel rotation angle signal, a pedal position signal driving vehicle speed signal of an accelerator pedal, a front wheel rotation angle signal and a rear wheel rotation angle signal;
2) The vehicle speed tracking module calculates the running speed and the required front wheel driving moment of the electric vehicle according to the pedal position signal of the accelerator pedal, controls the wheel hub motor to execute the running vehicle speed signal of the driver, and inputs the running speed to the whole vehicle controller;
3) During running of the vehicle, the whole vehicle controller judges whether the steering mode selected by the driver is front wheel steering or four-wheel steering;
if the steering mode is front wheel steering, adopting a front wheel steering by wire and rear wheel steering locking mode, executing the steps 4) to 5), and returning to the step 1);
if the steering mode is four-wheel steering, adopting a front wheel steering by wire and rear wheel differential steering mode, executing the steps 6) -8), and returning to the step 1);
4) When the selected steering mode is front wheel steering, the whole vehicle controller transmits steering wheel angle signals to a front wheel drive-by-wire steering control module, and outputs rear wheel alignment signals to a rear wheel steering locking module so that rear wheels align and lock rear wheel steering;
5) The front wheel steering control module drives the front wheel steering execution motor to steer the front wheels according to the steering wheel angle signal and the front wheel steering angle signal, provides corresponding steering road feel for a driver, and feeds back the steering road feel to the whole vehicle controller; meanwhile, the rear wheel steering locking module drives the rotary locking mechanism to align the rear wheel and prohibit the rear wheel from steering according to the rear wheel locking signal and the rear wheel steering angle signal;
6) When the selected steering mode is four-wheel steering, the whole vehicle controller calculates and obtains ideal front and rear wheel turning angles required by the four-wheel steering and outputs a rear wheel unlocking signal to a rear wheel steering locking module, wherein the ideal front and rear wheel turning angles comprise the ideal front wheel turning angle and the ideal rear wheel turning angle, the ideal front wheel turning angle is transmitted to a front wheel steer-by-wire control module, and the ideal rear wheel turning angle is transmitted to a rear wheel differential steering control module;
7) The front wheel steering control module drives the front wheel steering execution motor to steer the front wheels according to the ideal front wheel steering angle, provides corresponding steering road feel for a driver, and feeds back the steering road feel to the whole vehicle controller; meanwhile, the rear wheel steering locking module drives the slewing locking mechanism to release steering locking according to a rear wheel unlocking signal;
8) The rear wheel differential steering control module calculates rear wheel differential moment according to the ideal rear wheel steering angle, and drives the rear wheel hub motor to steer the rear wheel according to the rear wheel differential moment.
In the method, the vehicle speed tracking module searches a relation curve between the longitudinal speed and the position of the accelerator pedal, which is preset in the system, according to the signals acquired by the pedal position sensor of the accelerator pedal, so as to obtain the running speed of the electric vehicle.
In the method, the whole vehicle controller calculates and obtains ideal front and rear wheel steering angles required by four-wheel steering, specifically, the whole vehicle controller inputs the collected steering wheel angles to a reference model to obtain ideal yaw rate and centroid side deflection angles, and the yaw rate and centroid side deflection angles of the decoupled linear four-wheel steering model are controlled by adopting an asymptotic tracking controller to track the ideal yaw rate and centroid side deflection angles, so that the ideal front and rear wheel steering angles of the four-wheel steering vehicle are obtained; the reference model is a linear two-degree-of-freedom vehicle model with neutral steering characteristics after the centroid slip angle is subjected to amplitude reduction and filtering.
In the method of the invention, the state equation of the reference model is that,
let x be d ＝[γ d β d ] T ，u d ＝[δ f ]，
The reference model is expressed as
in the formula ,
wherein ,γd Yaw rate, beta, of the reference model d Is the centroid slip angle of the reference model, m is the automobile mass, u x For longitudinal speed at the centre of mass of the car, delta f Is the front wheel angle, l fd Is the distance from the centroid to the front axis, l rd Is the distance from the centroid to the rear axis, k f For cornering stiffness, k of the front wheel r For cornering stiffness of rear wheels, I z The moment of inertia of the vehicle to the Z axis;
the amplitude reduction filter is carried out on the centroid side deflection angle, the transfer function of the adopted amplitude reduction filter is,
Wherein eta is a gain coefficient, zeta is a damping coefficient and omega n Is a circular frequency;
by adjusting the circular frequency omega n To adjust the magnitude of the centroid slip angle.
In the method of the invention, the linear dynamics model adopted by the linear four-wheel steering model is that
In the equation(s) used in the present invention,
wherein ,lf Distance from front axle to centroid, l r Distance from the rear axis to the center of mass;
the linear four-wheel steering model is subjected to input-output decoupling,
the transfer function adopted is
wherein ,
introducing parameter u 1 、u 2 To realize decoupling of the yaw rate gamma and the centroid side deviation angle beta, and the yaw rate gamma obtained after decoupling is relative to u 1 Transfer function and centroid slip angle beta with respect to u 2 The transfer functions of (a) are G respectively 11(s) and G22 (s)；
Then there is
If and only ifAt the time, the yaw rate and the centroid slip angle are controlled by the variable u 1 and u2 Thus, it is possible to obtain,
wherein ,a0 ＝(a 12 b 21 -a 22 b 11 )(a 21 b 12 -a 11 b 22 +a 11 b 12 -a 12 b 22 )，a 1 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 11 b 12 b 21 -a 12 b 21 b 12 ，a 2 ＝b 11 b 22 -b 21 b 12 ，b 0 ＝(a 21 b 12 -a 11 b 22 )(a 12 b 21 -a 22 b 11 )，b 1 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 12 b 21 b 22 -a 22 b 11 b 22 ，b 2 ＝b 22 b 11 ，b 3 ＝(a 11 b 12 -a 12 b 22 )(a 21 b 12 -a 11 b 22 )，b 4 ＝-a 21 b 2 12 +a 11 b 22 b 12 +a 11 b 12 b 22 -a 12 b 2 22 ，b 5 ＝-b 12 b 22 ，b 6 ＝(-a 12 b 21 +a 22 b 11 )(a 21 b 11 -a 11 b 21 )，b 7 ＝-a 12 b 2 21 +a 22 b 11 b 21 -a 21 b 2 11 +a 11 b 11 b 21 ，b 8 ＝-b 11 b 21 ，b 9 ＝(a 21 b 12 -a 11 b 22 )(a 12 b 21 -a 22 b 11 )，b 10 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 12 b 21 b 22 -a 22 b 11 b 22 ，b 11 ＝b 22 b 11 ；
Setting the transfer function of an asymptotic tracking controller as GC 1(s), wherein the asymptotic tracking controller adopts P, PI, PD or PID controllers;
for the control equation e (t) =γ d (t)-γ(t)＝γ d (t)-G C1 (t)G 11 (t) e (t) performing a pull-type transformation,
obtaining the product
Is provided with
Then
Feature polynomial d of its 2 nd-order system g (s)d c (s)+n g (s)n c (s) =0 reduced to d 2 s 2 +d 1 s+d 0 ＝0；
wherein ,d2 ＝1+b 11 k d ，d 1 ＝b 11 k p +(-a 22 b 11 +a 12 b 21 )k d -(a 11 +a 22 )，
d 0 ＝a 11 a 22 -a 21 a 12 +k p (-a 22 b 11 +a 12 b 21 )；
Determining parameters of an asymptotic tracking controller for controlling the yaw rate and the centroid slip angle according to a route decision table and a route-Hurwitz stability criterion;
Wherein the Routh decision table isIn->
The Routh-Hurwitz stability criterion is that the coefficients of the characteristic polynomials of the 2 nd order system are all positive or all negative.
In the method of the invention, the rear wheel differential steering control module calculates the rear wheel differential moment according to the ideal rear wheel steering angle, specifically,
establishing a dynamic model, taking an ideal front and rear wheel corner as an input two-degree-of-freedom four-wheel steering vehicle as a reference model, and designing a sliding mode variable structure controller to obtain a rear wheel differential moment for driving rear wheels to steer;
the method comprises the following steps:
a) The state equation of the front wheel drive-by-wire steering and the rear wheel differential steering without considering the following rotation angle of the wheels is that,
wherein beta is the vehicle mass center side deflection angle, gamma is the vehicle yaw rate,for vehicle yaw acceleration, +.>Is the vehicle mass center side deflection angular velocity, m is the vehicle mass, u x Is the longitudinal speed at the centroid of the automobile, l f Distance from front axle to centroid, l r Delta is the distance from the rear axle to the centroid f For front wheel rotation angle, k f For cornering stiffness, k of the front wheel r For cornering stiffness of rear wheels, I z D is the rotational inertia of the vehicle to the Z axis r R is the effective radius of the tire;
b) Let state space variable x (t) = [ βγ ]] T The input is the front and rear wheel driving torque difference, i.e. u (T) =delta T, and the state equation of the front wheel steer-by-wire and rear wheel differential steering is converted into
wherein ,
c) Definition of the slip plane s=γ - γ d +ξ(β-β d )，
Wherein, xi is the weight coefficient, gamma d Yaw rate, beta, as reference model d A centroid slip angle of the reference model;
the derivative is obtained by the method,
in the formula ,
wherein ,yaw acceleration for reference model, +.>The centroid slip angular velocity is the reference model;
due toBounded, then->Wherein->Is a constant;
the approach law with saturation function is adopted to obtain the controller as
Simulation analysis:
when the steering mode is four-wheel steering, the four-wheel steering is used as a reference model, and the front wheels are independently steered in a traditional steering and rear wheel differential steering mode, so that simulation analysis is performed.
And taking different vehicle speeds and front wheel steering angles as inputs, taking the front wheel steering angles and the rear wheel steering angles as outputs, performing decoupling calculation, and then simulating parameter curves such as yaw rate, centroid slip angle and the like.
At low speeds, u=5 m/s, v=5 m/s, front wheel angle δ is selected f ＝0.2094rad，
After decoupling, delta is obtained f1 ＝0.2092rad，δ r1 ＝-0.0005454rad。
At medium speed, u=15 m/s, v=15 m/s, front wheel rotation angle δ is selected f ＝0.0721rad，
After decoupling, delta is obtained f1 ＝0.07194rad，δ r1 ＝-2.505e-06rad。
At high speeds, u=25 m/s, v=25 m/s, front wheel angle δ is selected f ＝0.0454rad，
After decoupling, delta is obtained f1 ＝0.04512rad，δ r1 ＝-0.0001544rad。
And obtaining graphs such as yaw rate, centroid slip angle and the like through simulation by taking the corresponding vehicle speed and the decoupled front and rear wheel rotation angles as inputs, as shown in fig. 1 to 12.
As can be seen from fig. 1 to 12, the front-wheel conventional steering and rear-wheel differential steering model curves are substantially identical to the four-wheel steering model curves, which illustrates that the method can well realize four-wheel steering.
As is clear from fig. 1, 5 and 9, the yaw rate steady value increases with an increase in vehicle speed, and is stabilized between 3.3rad/s and 3.8 rad/s. Meanwhile, as can be seen from fig. 1, the front-wheel conventional steering and the rear-wheel differential steering at a low speed are smoother than the yaw rate stabilizing values of the four-wheel steering model.
As can be seen from fig. 2, 6 and 10, the centroid slip angle steady value decreases with an increase in vehicle speed. The centroid slip angle stable value at low speed is 0.058rad; the steady value of the centroid slip angle at medium speed is-0.03 rad; the centroid slip angle stability value at high speed is-0.078 rad. Meanwhile, as can be seen from fig. 2, the conventional steering of the front wheels and the differential steering of the rear wheels at low speed are more stable than the steady value of the centroid side deflection angle of the four-wheel steering model.
As can be seen from fig. 3, 7 and 11, the x and y position curves of the front and rear differential steering models coincide with the x and y position curves of the four-wheel steering models, and the maximum values of the x and y positions increase with an increase in vehicle speed.
As can be seen from fig. 4, 8 and 12, the torque value becomes smaller and stabilizes within a certain range as the vehicle speed increases.
Simulation results show that the steering characteristics of the front wheels are ideal by adopting the traditional steering and rear wheel differential steering rotation mode, so that the rear wheels are independently driven and combined with the differential steering technology, the vehicle maneuverability can be effectively improved under the low-speed working condition, and the occurrence of tail flick and sideslip when the vehicle turns can be effectively reduced at high speed; at the same time, the active safety and the steering stability of the motor vehicle can be improved.
The foregoing has outlined and described the basic principles, features, and advantages of the present invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, and that the above embodiments and descriptions are merely illustrative of the principles of the present invention, and various changes and modifications may be made without departing from the spirit and scope of the invention, which is defined in the appended claims. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (9)
1. A time-sharing four-wheel steering system of an in-wheel motor driven electric vehicle, comprising:
the detection module is used for detecting the running parameters of the electric automobile and inputting the running parameters to the whole automobile controller; wherein the driving parameters comprise steering wheel rotation angle signals, pedal position signals of an accelerator pedal, driving vehicle speed signals, front wheel rotation angle signals and rear wheel rotation angle signals;
A steering mode selection switch for a driver to manually select a steering mode including front wheel steering and four wheel steering, and inputting the selected steering mode to the vehicle controller;
the vehicle speed tracking module is used for calculating the running speed and the required front wheel driving moment of the electric vehicle according to the pedal position signal of the accelerator pedal, controlling the wheel hub motor to execute the running vehicle speed signal of a driver and inputting the running speed to the whole vehicle controller;
the whole vehicle controller is used for receiving the running speed, transmitting a steering wheel angle signal to the front wheel drive-by-wire steering control module when the selected steering mode is front wheel steering, and outputting a rear wheel locking signal to the rear wheel steering locking module; when the selected steering mode is four-wheel steering, calculating and obtaining ideal front and rear wheel turning angles required by the four-wheel steering and outputting a rear wheel unlocking signal to a rear wheel steering locking module, wherein the ideal front and rear wheel turning angles comprise the ideal front wheel turning angle and the ideal rear wheel turning angle, the ideal front wheel turning angle is transmitted to a front wheel steer-by-wire control module, and the ideal rear wheel turning angle is transmitted to a rear wheel differential steering control module;
The front wheel drive-by-wire steering control module is used for driving the front wheel steering executing motor to steer the front wheel according to the steering wheel angle signal and the front wheel steering angle signal or the ideal front wheel steering angle, providing corresponding steering road feel for a driver and feeding back the steering road feel to the whole vehicle controller;
the rear wheel differential steering control module is used for calculating rear wheel differential moment according to an ideal rear wheel steering angle and driving a rear wheel hub motor to steer the rear wheel according to the rear wheel differential moment;
the rear wheel steering locking module is used for driving the steering locking mechanism to enable the rear wheel to turn right according to the rear wheel locking signal and the rear wheel steering angle signal, and preventing the rear wheel from steering, and driving the steering locking mechanism to release steering locking according to the rear wheel unlocking signal;
the steering locking mechanism comprises a return motor and two groups of steering mechanisms which are symmetrically arranged on the frame and have identical structures, the return motor is connected with the rear wheel steering locking module, and the two groups of steering mechanisms are connected with the return motor and respectively act on the left rear wheel and the right rear wheel;
the steering mechanism comprises an electromagnetic clutch, a return spur gear, a locking pull rod, a front beam rod, a first steering knuckle arm and a second steering knuckle arm which are sequentially connected, an electromagnet which is lapped on the surface of the locking pull rod and is arranged on the frame, steering main pins which are connected with one end of the first steering knuckle arm and one end of the second steering knuckle arm, an upper cross arm and a lower cross arm which are arranged on the frame, and a locking pull rod displacement sensor which is used for detecting the rotation angle of the rear wheel and is positioned on the locking pull rod; the electromagnetic clutch is connected with the positive return motor and the rear wheel steering locking module, the locking pull rod is connected with the positive return gear through the rod surface rack, the locking pull rod is connected with the front beam rod, the front beam rod is connected with the first steering knuckle arm through the spherical hinge, the two ends of the steering main pin are respectively connected with the upper cross arm and the lower cross arm through the spherical hinge, the electromagnet is used for generating suction force after being electrified and is attracted onto the locking pull rod so that the locking pull rod is locked on the frame, and the locking pull rod displacement sensor is used for detecting displacement parameters between the locking pull rod and the vehicle body in real time and feeding the displacement parameters back to the whole vehicle controller to determine the rear wheel steering angle.
2. The time-sharing four-wheel steering system of an in-wheel motor-driven electric vehicle according to claim 1, wherein: the front wheel steering executing motor makes the front wheel turn through driving the front wheel steering driving medium, the driving front wheel steering driving medium is including the left front wheel knuckle arm, left front wheel steering tie rod, rack, steering gear, right front wheel steering tie rod and the right front wheel knuckle arm that link to each other in proper order, left front wheel knuckle arm acts on left front wheel, right front wheel knuckle arm acts on right front wheel, steering gear links to each other with the front wheel steering executing motor.
3. The time-sharing four-wheel steering system of an in-wheel motor-driven electric vehicle according to claim 1, wherein: the detection module comprises a torque sensor and a steering wheel angle sensor for detecting a steering wheel, a pedal position sensor for detecting an accelerator pedal, and a rack displacement sensor for detecting a front wheel angle and positioned on a rack.
4. A control method of a time-sharing four-wheel steering system of an in-wheel motor driven electric vehicle, characterized by operating in the time-sharing four-wheel steering system of an in-wheel motor driven electric vehicle according to any one of claims 1 to 3, comprising the steps of:
1) The whole vehicle controller acquires the driving parameters acquired by the detection module and the steering mode selected by the driver and input by the steering mode selection switch, wherein the driving parameters comprise a steering wheel rotation angle signal, a pedal position signal driving vehicle speed signal of an accelerator pedal, a front wheel rotation angle signal and a rear wheel rotation angle signal;
2) The vehicle speed tracking module calculates the running speed and the required front wheel driving moment of the electric vehicle according to the pedal position signal of the accelerator pedal, controls the wheel hub motor to execute the running vehicle speed signal of the driver, and inputs the running speed to the whole vehicle controller;
3) During running of the vehicle, the whole vehicle controller judges whether the steering mode selected by the driver is front wheel steering or four-wheel steering;
if the steering mode is front wheel steering, adopting a front wheel steering by wire and rear wheel steering locking mode, executing the steps 4) to 5), and returning to the step 1);
if the steering mode is four-wheel steering, adopting a front wheel steering by wire and rear wheel differential steering mode, executing the steps 6) -8), and returning to the step 1);
4) When the selected steering mode is front wheel steering, the whole vehicle controller transmits steering wheel angle signals to a front wheel drive-by-wire steering control module, and outputs rear wheel alignment signals to a rear wheel steering locking module so that rear wheels align and lock rear wheel steering;
5) The front wheel steering control module drives the front wheel steering execution motor to steer the front wheels according to the steering wheel angle signal and the front wheel steering angle signal, provides corresponding steering road feel for a driver, and feeds back the steering road feel to the whole vehicle controller; meanwhile, the rear wheel steering locking module drives the rotary locking mechanism to align the rear wheel and prohibit the rear wheel from steering according to the rear wheel locking signal and the rear wheel steering angle signal;
6) When the selected steering mode is four-wheel steering, the whole vehicle controller calculates and obtains ideal front and rear wheel turning angles required by the four-wheel steering and outputs a rear wheel unlocking signal to a rear wheel steering locking module, wherein the ideal front and rear wheel turning angles comprise the ideal front wheel turning angle and the ideal rear wheel turning angle, the ideal front wheel turning angle is transmitted to a front wheel steer-by-wire control module, and the ideal rear wheel turning angle is transmitted to a rear wheel differential steering control module;
7) The front wheel steering control module drives the front wheel steering execution motor to steer the front wheels according to the ideal front wheel steering angle, provides corresponding steering road feel for a driver, and feeds back the steering road feel to the whole vehicle controller; meanwhile, the rear wheel steering locking module drives the slewing locking mechanism to release steering locking according to a rear wheel unlocking signal;
8) The rear wheel differential steering control module calculates rear wheel differential moment according to the ideal rear wheel steering angle, and drives the rear wheel hub motor to steer the rear wheel according to the rear wheel differential moment.
5. The control method of the time-sharing four-wheel steering system of the in-wheel motor-driven electric vehicle according to claim 4, characterized by: the vehicle speed tracking module searches a relation curve between the longitudinal speed and the position of the accelerator pedal, which is preset in the system, according to signals acquired by a pedal position sensor of the accelerator pedal, so that the running speed of the electric vehicle is obtained.
6. The control method of the time-sharing four-wheel steering system of the in-wheel motor-driven electric vehicle according to claim 4, characterized by: the whole vehicle controller calculates and obtains ideal front and rear wheel steering angles required by four-wheel steering, specifically,
the vehicle controller inputs the collected steering wheel angle to a reference model to obtain an ideal yaw rate and a centroid side deflection angle, and controls the yaw rate and the centroid side deflection angle of the decoupled linear four-wheel steering model by adopting an asymptotic tracking controller to track the ideal yaw rate and the centroid side deflection angle, so that the ideal front and rear wheel steering angles of the four-wheel steering vehicle are obtained; the reference model is a linear two-degree-of-freedom vehicle model with neutral steering characteristics after the centroid slip angle is subjected to amplitude reduction and filtering.
7. The control method of the time-sharing four-wheel steering system of the in-wheel motor-driven electric vehicle according to claim 6, characterized by: the state equation of the reference model is that,
let x be d ＝[γ d β d ] T ，u d ＝[δ f ]，
The reference model is expressed as
in the formula ,
wherein ,γd Yaw rate, beta, of the reference model d Is the centroid slip angle of the reference model, m is the automobile mass, u x For longitudinal speed at the centre of mass of the car, delta f Is the front wheel angle, l fd Is the distance from the centroid to the front axis, l rd Is the distance from the centroid to the rear axis, k f For cornering stiffness, k of the front wheel r For cornering stiffness of rear wheels, I z The moment of inertia of the vehicle to the Z axis;
the amplitude reduction filter is carried out on the centroid side deflection angle, the transfer function of the adopted amplitude reduction filter is,
wherein eta is a gain coefficient, zeta is a damping coefficient and omega n Is a circular frequency;
by adjusting the circular frequency omega n To adjust the magnitude of the centroid slip angle.
8. According to claimThe method for controlling a time-sharing four-wheel steering system of an electric vehicle driven by an in-wheel motor according to claim 7, wherein the method comprises the steps of: the linear dynamics model adopted by the linear four-wheel steering model is that
In the equation(s) used in the present invention,
wherein ,lf Distance from front axle to centroid, l r Distance from the rear axis to the center of mass;
the linear four-wheel steering model is subjected to input-output decoupling,
The transfer function adopted is
wherein ,
introducing parameter u 1 、u 2 To realize decoupling of the yaw rate gamma and the centroid side deviation angle beta, and the yaw rate gamma obtained after decoupling is relative to u 1 Transfer function and centroid slip angle beta with respect to u 2 The transfer functions of (a) are G respectively 11(s) and G22 (s)；
Then there is
If and only ifYaw rate and centroid yaw rateThe angle is controlled by the variable u 1 and u2 Thus, it is possible to obtain,
wherein ,a0 ＝(a 12 b 21 -a 22 b 11 )(a 21 b 12 -a 11 b 22 +a 11 b 12 -a 12 b 22 )，a 1 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 11 b 12 b 21 -a 12 b 21 b 12 ，a 2 ＝b 11 b 22 -b 21 b 12 ，b 0 ＝(a 21 b 12 -a 11 b 22 )(a 12 b 21 -a 22 b 11 )，b 1 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 12 b 21 b 22 -a 22 b 11 b 22 ，b 2 ＝b 22 b 11 ，b 3 ＝(a 11 b 12 -a 12 b 22 )(a 21 b 12 -a 11 b 22 )，b 4 ＝-a 21 b 2 12 +a 11 b 22 b 12 +a 11 b 12 b 22 -a 12 b 2 22 ，b 5 ＝-b 12 b 22 ，b 6 ＝(-a 12 b 21 +a 22 b 11 )(a 21 b 11 -a 11 b 21 )，b 7 ＝-a 12 b 2 21 +a 22 b 11 b 21 -a 21 b 2 11 +a 11 b 11 b 21 ，b 8 ＝-b 11 b 21 ，
b 9 ＝(a 21 b 12 -a 11 b 22 )(a 12 b 21 -a 22 b 11 )，b 10 ＝a 21 b 12 b 11 -a 11 b 22 b 11 +a 12 b 21 b 22 -a 22 b 11 b 22 ，b 11 ＝b 22 b 11 ；
Let the transfer function of the asymptotic tracking controller be G C1 (s) the asymptotic tracking controller adopts a P, PI, PD or PID controller;
for the control equation e (t) =γ d (t)-γ(t)＝γ d (t)-G C1 (t)G 11 (t) e (t) performing a pull-type transformation,
obtaining the product
Is provided with
Then
Feature polynomial d of its 2 nd-order system g (s)d c (s)+n g (s)n c (s) =0 reduced to d 2 s 2 +d 1 s+d 0 ＝0；
wherein ,d2 ＝1+b 11 k d ，d 1 ＝b 11 k p +(-a 22 b 11 +a 12 b 21 )k d -(a 11 +a 22 )，
d 0 ＝a 11 a 22 -a 21 a 12 +k p (-a 22 b 11 +a 12 b 21 )；
Determining parameters of an asymptotic tracking controller for controlling the yaw rate and the centroid slip angle according to a route decision table and a route-Hurwitz stability criterion;
wherein the Routh decision table isIn->
The Routh-Hurwitz stability criterion is that the coefficients of the characteristic polynomials of the 2 nd order system are all positive or all negative.
9. The control method of the time-sharing four-wheel steering system of the in-wheel motor-driven electric vehicle according to claim 4, characterized by: the rear wheel differential steering control module calculates the rear wheel differential moment according to the ideal rear wheel steering angle, specifically,
Establishing a dynamic model, taking an ideal front and rear wheel corner as an input two-degree-of-freedom four-wheel steering vehicle as a reference model, and designing a sliding mode variable structure controller to obtain a rear wheel differential moment for driving rear wheels to steer;
the method comprises the following steps:
a) The state equation of the front wheel drive-by-wire steering and the rear wheel differential steering without considering the following rotation angle of the wheels is that,
wherein beta is the vehicle mass center side deflection angle, gamma is the vehicle yaw rate,for vehicle yaw acceleration, +.>Is the vehicle mass center side deflection angular velocity, m is the vehicle mass, u x Is the longitudinal speed at the centroid of the automobile, l f Is the front axleDistance to centroid, l r Delta is the distance from the rear axle to the centroid f For front wheel rotation angle, k f For cornering stiffness, k of the front wheel r For cornering stiffness of rear wheels, I z D is the rotational inertia of the vehicle to the Z axis r R is the effective radius of the tire;
b) Let state space variable x (t) = [ βγ ]] T The input is the front and rear wheel driving torque difference, i.e. u (T) =delta T, and the state equation of the front wheel steer-by-wire and rear wheel differential steering is converted into
wherein ,
c) Definition of the slip plane s=γ - γ d +ξ(β-β d )，
Wherein, xi is the weight coefficient, gamma d Yaw rate, beta, as reference model d A centroid slip angle of the reference model;
the derivative is obtained by the method,
in the formula ,
wherein ,yaw acceleration for reference model, +.>The centroid slip angular velocity is the reference model;
due toBounded, then->Wherein->Is a constant;
the approach law with saturation function is adopted to obtain the controller as
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