CN105751919B - A kind of four-wheel wheel hub electric vehicle anti-skid control method - Google Patents

Abstract

The invention discloses an anti-skid control method for a four-wheel hub electric vehicle, which calculates the optimal slip rate of the wheel in real time through a road surface recognition algorithm, and calculates the expected rotational speed of the wheel based on the optimal slip rate of the wheel. Then, according to the state of the wheel, the compensation torque of the wheel is calculated; wherein, if the wheel slips, the desired wheel speed is taken as the control target, and the compensation torque is calculated through the PID controller of the wheel speed. If the wheel does not slip, The compensation torque is zero; at the same time, the vehicle speed control takes the desired vehicle speed as the control target, and calculates the command torque of the motor according to the vehicle speed controller; finally, the aforementioned compensation torque and command torque are added and input to the motor to realize four Drive skid control for in-wheel hub electric vehicles.

Description

A method for anti-slip control of four-wheel hub electric vehicles

technical field

The invention belongs to the technical field of electric vehicles, and more specifically relates to an anti-skid control method for four-wheel hub electric vehicles.

Background technique

Four-wheel hub motor electric vehicles have become one of the hot spots in the development of electric vehicles due to the characteristics of independently controllable four-wheel drive torque and easy measurement of torque and speed. When a four-wheel hub motor electric vehicle is running on a low-adhesion road surface, especially when accelerating, the output torque of the motor may exceed the torque corresponding to the maximum adhesion force that the road surface can provide. When this happens, the difference between the wheel speed and the vehicle speed will increase rapidly, the wheel will slip, and the slip rate will enter the unstable area from the stable area, resulting in the decrease of the adhesion between the electric vehicle and the road surface, resulting in occurrence of security incidents. In recent years, the research on electric vehicles with four-wheel hub motors has focused on the realization of basic control functions, and there are few in-depth studies on the anti-skid control methods for electric vehicles with four-wheel hub motors.

At present, the methods of driving anti-skid control mainly include logic threshold, PID control, fuzzy control and other methods, among which PID control method is widely used. However, the above methods all regard the optimal slip ratio of the wheel as a fixed value, but in the actual driving process of the vehicle, the optimal slip ratio of the wheel changes with the change of the road surface condition. In order to improve the control effect of the driving anti-skid control method, this paper introduces the road surface recognition algorithm into the driving anti-skid control method, and calculates the optimal slip ratio of the wheel in real time according to the road surface recognition results. At the same time, aiming at the problem that the control parameters are difficult to be determined in the driving anti-slip method based on PID control, a PID control parameter tuning method is given.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art, and provide a four-wheel hub electric vehicle anti-skid control method, which uses the optimal slip ratio of the wheels to enhance the stability and safety of the electric vehicle, and then achieves drive anti-skid control.

In order to achieve the above-mentioned purpose of the invention, the present invention provides a four-wheel hub electric vehicle anti-skid control method, which is characterized in that it comprises the following steps:

(1) Obtain the optimal slip ratio s _{opt_ij} of the wheel

(1.1), calculate the real-time slip rate s _ij of the wheel in the driving process of the electric vehicle;

Among them, ω _ij is the wheel speed, r is the wheel radius, v is the vehicle speed, ij=fl, fr, rl, rr, respectively represent the left front wheel, right front wheel, left rear wheel, right rear wheel;

(1.2), according to the wheel drive moment balance principle, calculate the utilization adhesion coefficient u _ij of each wheel;

Among them, T _ij is the driving force of the wheel, J is the moment of inertia of the wheel, Wheel angular acceleration, F _{z_ij} is the wheel load, and it satisfies:

Among them, m is the full load mass of the vehicle, g is the acceleration of gravity, l _f and l _r are the distances from the center of mass of the vehicle to the front and rear axles, respectively;

(1.3), according to the real-time slip rate s _ij of the wheel, using the relationship between the standard road surface adhesion coefficient and the wheel slip rate, calculate the adhesion coefficient u _{t_ij} under the standard road surface;

Among them, t=1, 2, 3, 4, 5, 6 represent 6 kinds of standard road surfaces, C _{1_t} , C _{2_t} , C _{3_t} are parameters related to standard road surfaces;

(1.4), calculate the weight coefficient x _t under the standard road surface;

Among them, ε is a positive number infinitely close to 0;

(1.5), calculating the optimum slip ratio s _{opt_ij} of the wheel;

Among them, s _{opt_t} is the optimal slip ratio of the wheel under the standard road surface;

(2), calculate the expected wheel speed ω _{ref_ij} of the wheel

(3) Use the compensation torque T _{e_ij} output by the PID controller to control the wheel speed

Compare the current wheel speed ω _ij with the expected wheel speed ω _{ref_ij} , if the wheel speed ω _ij is greater than the expected wheel speed ω _{ref_ij} , that is, ω _ij > ω _{ref_ij} , the wheel will slip, and use the PID controller to control the wheel speed ω The difference e _ij between _ij and the expected wheel speed ω _{ref_ij} is controlled to obtain the compensation torque T _{e_ij} ;

Among them, k _{p_ij} is the proportional coefficient of the PID controller, k _{i_ij} is the integral coefficient of the PID controller, and k _{d_ij} is the integral coefficient of the controller;

If the wheel speed is less than or equal to the desired wheel speed, that is, ω _ij ≤ ω _{ref_ij} , the wheel does not slip, and the PID controller outputs compensation torque T _{e_ij} =0;

(4), calculate the command torque T _{com_ij} of each motor

(4.1), calculate the total command torque T _com ;

Among them, e is the deviation between the desired vehicle speed and the actual vehicle speed, k _{p_v} , k _{i_v} , and k _{d_v} are the proportional, integral and differential constants in the vehicle speed control, respectively;

(4.2), the total command torque T _com is evenly distributed to each motor, and the command torque T _{com_ij} of each motor is obtained;

(5), calculate the driving torque T _{d_ij}

In the motor of an electric vehicle, first sum the compensation torque T _{e_ij} and the command torque T _{com_ij} to obtain the input torque of the limiting module Then, limit the processing through the limiting module to obtain the driving torque T _{d_ij} of the motor;

Among them, T _max is the maximum output torque of the hub motor;

Finally, the drive torque T _{d_ij} is input to each motor, and the anti-skid control is performed by controlling the speed of the electric vehicle.

Further, the transfer function C(s) of the PID controller is:

Among them, s is the frequency domain operator, k _{p_ij} is the proportional coefficient of the PID controller, k _{i_ij} is the integral coefficient of the PID controller;

In wheel speed control, the transfer function G(s) of the hub motor of the controlled object can be expressed as:

Among them, T is the motor time constant, J is the moment of inertia of the wheel;

From formulas (12) and (13), it can be obtained that the closed-loop transfer function of wheel speed control is:

From formula (14), it can be obtained that the characteristic equation of the wheel speed control system is:

JTs ³ +Js ² +k _{p_ij} s+k _{i_ij} = 0 (15)

In order to ensure the stability of wheel speed control, the Laws criterion can be obtained:

Therefore, the values of the proportional parameter k _{p_ij} and the integral parameter k _{i_ij} of the PID controller in the wheel speed control should satisfy:

The purpose of the invention of the present invention is achieved like this:

The invention provides an anti-skid control method for a four-wheel hub electric vehicle, which calculates the optimal slip rate of the wheel in real time through a road surface recognition algorithm, and calculates the expected rotational speed of the wheel based on the optimal slip rate of the wheel. Then, according to the state of the wheel, the compensation torque of the wheel is calculated; wherein, if the wheel slips, the desired wheel speed is taken as the control target, and the compensation torque is calculated through the PID controller of the wheel speed. If the wheel does not slip, The compensation torque is zero; at the same time, the vehicle speed control takes the desired vehicle speed as the control target, and calculates the command torque of the motor according to the vehicle speed controller; finally, the aforementioned compensation torque and command torque are added and input to the motor to realize four The drive anti-skid control of wheel-hub electric vehicles; therefore, the present invention can ensure that the wheels run in a stable range during the driving process of the vehicle, thereby improving the stability and safety of the vehicle; secondly, the present invention also adopts the parameter setting method of the PID controller , to solve the problem that the controller parameters in the drive anti-skid control system need to be repeatedly debugged in actual engineering.

Description of drawings

Fig. 1 is a kind of antiskid control schematic diagram of four-wheel hub electric vehicle of the present invention;

Fig. 2 is a kind of antiskid control flowchart of four-wheel hub electric vehicle of the present invention;

Fig. 3 is a flow chart of road recognition;

Figure 4 is a block diagram of the electric vehicle motor control algorithm based on PID control;

Fig. 5 is a comparison chart of simulation results with and without anti-skid control;

Fig. 6 is a comparison chart of wheel slip rate simulation results when using and not using anti-skid control;

Fig. 7 is a graph showing the simulation results of the electric vehicle's driving trajectory with and without anti-skid control.

Detailed ways

Specific embodiments of the present invention will be described below in conjunction with the accompanying drawings, so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when detailed descriptions of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

Example

Fig. 1 is a schematic diagram of anti-skid control of a four-wheel hub electric vehicle according to the present invention.

In this embodiment, as shown in FIG. 1 , a driving anti-slip control system for a four-wheel hub electric vehicle includes: a road surface recognition module, a desired wheel speed calculation module, a wheel drive anti-skid control module, a vehicle speed control module and a limiter module.

Among them, the road surface recognition module estimates the road surface on which the electric vehicle is driving in real time, and obtains the optimal wheel slip ratio s _{opt_ij} .

The expected wheel speed calculation module calculates the expected wheel speed ω _{ref_ij} of the wheel according to the optimal wheel slip ratio s _{opt_ij} , the vehicle speed v and the wheel radius r obtained from road surface identification.

The wheel drive anti-slip control module obtains the compensation torque T _{e_ij} of the wheel according to the state of the wheel. If the wheel speed ω _ij is greater than the expected wheel speed ω _{ref_ij} , that is, ω _ij > ω _{ref_ij} , the wheel slips, and the PID controller is used to control the difference e _ij between the wheel speed ω _ij and the wheel expected wheel speed ω _{ref_ij} , to obtain Compensation torque T _{e_ij} ; if the wheel speed is less than or equal to the expected wheel speed, that is, ω _ij ≤ ω _{ref_ij} , the wheel does not slip, and the drive anti-skid control module outputs compensation torque T _{e_ij} =0.

The vehicle speed control module calculates the total command torque T _com of the wheels according to the difference between the expected vehicle speed v _ref and the actual vehicle speed v, and then distributes the total command torque to each wheel through the average distribution control algorithm to obtain the command torque of each wheel moment T _{com_ij} .

The limiting module limits the sum T _{d_ij} of the compensation torque T _{e_ij} and the command torque T _{com_ij} , which is to be compared with the maximum output torque T _max of the motor. When T _{d_ij} is greater than T _max , the limiting module outputs, otherwise it outputs T _{d_ij} .

Fig. 2 is a flow chart of anti-skid control of a four-wheel hub electric vehicle according to the present invention.

In the present embodiment, as shown in FIG. 2 , a method for anti-skid control of a four-wheel hub electric vehicle of the present invention includes the following steps:

S1. Obtain the optimal slip ratio s _{opt_ij} of the wheel

S1.1. Calculate the real-time slip rate s _ij of the wheel during the driving process of the electric vehicle;

Among them, ω _ij is the wheel speed, r is the wheel radius, v is the vehicle speed, ij=fl, fr, rl, rr, respectively represent the left front wheel, right front wheel, left rear wheel, right rear wheel;

S1.2. Calculate the utilization adhesion coefficient u _ij of each wheel according to the principle of wheel driving torque balance;

Among them, T _ij is the driving force of the wheel, J is the moment of inertia of the wheel, Wheel angular acceleration, F _{z_ij} is the wheel load, and it satisfies:

Among them, m is the full load mass of the vehicle, g is the acceleration of gravity, l _f and l _r are the distances from the center of mass of the vehicle to the front and rear axles, respectively;

In this embodiment, the value of m is 2150 kg, the value of the acceleration of gravity is 9.8, the value of the wheel radius is 0.3262 m, and the values of l _f and l _r are both 1.35 m.

S1.3, the function curve expression proposed by Burckhardt can well describe the relationship between the road surface adhesion coefficient u and the wheel slip s rate, and its functional relationship is:

Substituting the real-time wheel slip rate s _ij into the formula (4), according to the process shown in Figure 3, the adhesion coefficient u _{t_ij} under the standard road surface can be calculated;

Among them, t=1, 2, 3, 4, 5, 6, representing 6 kinds of standard road surfaces, namely: ice, snow, wet cobblestone, wet asphalt, dry cement and dry asphalt 6 standard road surfaces; C _{1_t} , C _{2_t} , C _{3_t} is a parameter relevant to the standard road surface, in the present embodiment, as shown in table 1;

Table 1 is the relevant parameters of the standard road surface;

pavement C ₁ C ₂ C ₃ s _opt ice 0.05 306.39 0.001 0.03 Snow 0.1946 94.129 0.0646 0.06 wet pebbles 0.4004 33.7080 0.1204 0.14 wet asphalt 0.8570 33.822 0.3470 0.13 dry cement 1.1973 25.168 0.5373 0.16 dry asphalt 1.2801 23.99 0.52 0.17

Table 1

Six standard pavement u-s curves can be obtained from formula (5) and Table 1;

S1.4, calculate the weight coefficient x _t under the standard road surface;

Among them, ε is a positive number infinitely close to 0;

In this embodiment, ε takes a value of 0.00001.

S1.5. Calculating the optimal slip ratio s _{opt_ij} of the wheel;

Among them, s _{opt_t} is the optimal slip ratio of the wheel on the standard road surface, which can be found from Table 1; S2. Calculate the expected wheel speed ω _{ref_ij} of the wheel

S3. Using the compensation torque T _{e_ij} output by the PID controller to control the wheel speed

Compare the current wheel speed ω _ij with the expected wheel speed ω _{ref_ij} , if the wheel speed ω _ij is greater than the expected wheel speed ω _{ref_ij} , that is, ω _ij > ω _{ref_ij} , the wheel will slip, and use the PID controller to control the wheel speed ω The difference e _ij between _ij and the expected wheel speed ω _{ref_ij} is controlled to obtain the compensation torque T _{e_ij} ;

Among them, k _{p_ij} is the proportional coefficient of the PID controller, k _{i_ij} is the integral coefficient of the PID controller, and k _{d_ij} is the integral coefficient of the controller;

If the wheel speed is less than or equal to the desired wheel speed, that is, ω _ij ≤ ω _{ref_ij} , the wheel does not slip, and the PID controller outputs compensation torque T _{e_ij} =0;

In this embodiment, for a four-wheel hub motor electric vehicle, wheel speed control can be equivalent to motor speed control, and the transfer function block diagram of the motor vector control algorithm based on the PID control algorithm is shown in FIG. 4 .

Among them, the transfer function C(s) of the PID controller is:

Among them, s is the frequency domain operator, k _{p_ij} is the proportional coefficient of the PID controller, k _{i_ij} is the integral coefficient of the PID controller;

In wheel speed control, the transfer function G(s) of the hub motor of the controlled object can be expressed as:

Among them, T is the motor time constant, J is the moment of inertia of the wheel;

From formulas (11) and (12), it can be obtained that the closed-loop transfer function of wheel speed control is:

From formula (13), it can be obtained that the characteristic equation of the wheel speed control system is:

JTs ³ +Js ² +k _{p_ij} s+k _{i_ij} = 0 (14)

In order to ensure the stability of wheel speed control, the Laws criterion can be obtained:

Therefore, the values of the proportional parameter k _{p_ij} and the integral parameter k _{i_ij} of the PID controller in the wheel speed control should satisfy:

In this embodiment, the torque motor time constant T of the motor is 0.7, the value of k _{p_ij} is 200, and the value of k _{i_ij} is 50.

S4. Calculate the command torque T _{com_ij} of each motor

S4.1. Calculate the total command torque T _com ;

Among them, e is the deviation between the desired vehicle speed and the actual vehicle speed, k _{p_v} , k _{i_v} , and k _{d_v} are the proportional, integral and differential constants in the vehicle speed control, respectively;

S4.2. Evenly distribute the total command torque T _com to each motor to obtain the command torque T _{com_ij} of each motor;

S5. Calculate the driving torque T _{d_ij}

In the motor of an electric vehicle, first sum the compensation torque T _{e_ij} and the command torque T _{com_ij} to obtain the input torque of the limiting module Then, limit the processing through the limiting module to obtain the driving torque T _{d_ij} of the motor;

Among them, T _max is the maximum output torque of the hub motor;

In this embodiment, the maximum output torque of the hub motor is 320Nm.

Finally, the drive torque T _{d_ij} is input to each motor, and the anti-skid control is performed by controlling the speed of the electric vehicle.

In this embodiment, the joint simulation of Carsim and Simulink is used to set the initial speed of the four-wheel hub electric vehicle to 5km/h, the expected speed to 60km/h, and the road surface adhesion coefficient to 0.1. The vehicle speed control with the driving anti-skid method is simulated, and the speed simulation results are shown in Figure 5. In the vehicle speed control without the driving anti-skid method, the vehicle speed gradually increases from 5km to the desired speed of 60km/h, and the vehicle speed reaches the desired speed in about 82s. The vehicle speed remains stable, and the overshoot of the vehicle speed in the control process is 36.6%. In the vehicle speed control including the driving anti-skid method, when the vehicle speed gradually increases from 5km to the desired speed of 60km/h, the vehicle speed reaches the desired speed within 20s and maintains Stable, and the vehicle speed overshoot in the control process is 0%.

In this embodiment, as shown in Figure 6, (a), (b), (c), (d) respectively represent the slippage of the left front wheel, right front wheel, left rear wheel and right rear wheel of the electric vehicle From the results of the rate simulation, it can be seen from the four images that in the vehicle speed control without driving anti-skid method, the slip rate of the four wheels rises rapidly and exceeds 0.9 within 1 second, and the wheel slips seriously, and with the increase of time, the wheel slips The slip rate cannot converge to the optimal slip rate, that is, the wheels are always in a state of out-of-control; while in the vehicle speed control including the driving anti-skid method, the wheel slip rate rises to 0.2 at the beginning, but under the intervention of the driving anti-skid, the wheel slips within 2s The slip ratio converges to the optimal slip ratio of 0.05, and the wheel slip state is well controlled.

In this embodiment, this embodiment also simulates the driving trajectory of the electric vehicle, and the simulation results are shown in Figure 7. In the vehicle speed control without the driving anti-skid method, the longitudinal displacement of the vehicle gradually increases with the movement of the vehicle, and at the same time , the vehicle produces a lateral movement, and finally, when the longitudinal displacement of the vehicle is 1000m, the lateral displacement of the vehicle is 3m, which means that the vehicle is sideways; and in the vehicle speed control with the driving anti-skid method, during the vehicle movement, as the vehicle As the longitudinal displacement increases, the lateral displacement of the vehicle is 0m, the vehicle does not deviate, and the vehicle can track the desired trajectory well. It can be seen from the simulation that the driving anti-skid control can improve the stability and safety of the vehicle.

Although the illustrative specific embodiments of the present invention have been described above, so that those skilled in the art can understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, As long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are included in the protection list.

Claims (2)

1. a four-wheel hub electric vehicle anti-skid control method, is characterized in that, comprises the following steps: (1) Obtain the optimal slip ratio s _{opt_ij} of the wheel (1.1), calculate the real-time slip rate s _ij of the wheel in the driving process of the electric vehicle; Among them, ω _ij is the wheel speed, r is the wheel radius, v is the vehicle speed, ij=fl, fr, rl, rr, respectively represent the left front wheel, right front wheel, left rear wheel, right rear wheel; (1.2), according to the wheel drive moment balance principle, calculate the utilization adhesion coefficient u _ij of each wheel; Among them, T _ij is the driving force of the wheel, J is the moment of inertia of the wheel, Wheel angular acceleration, F _{z_ij} is the wheel load, and it satisfies: Among them, m is the full load mass of the vehicle, g is the acceleration of gravity, l _f and l _r are the distances from the center of mass of the vehicle to the front and rear axles, respectively; (1.3), according to the real-time wheel slip rate s _ij , using the relationship between the standard road surface adhesion coefficient and the wheel slip rate, calculate the adhesion coefficient u _{t_ij} under the standard road surface; Among them, t=1, 2, 3, 4, 5, 6 represent 6 kinds of standard road surfaces, C _{1_t} , C _{2_t} , C _{3_t} are parameters related to standard road surfaces; (1.4), calculate the weight coefficient x _t under the standard road surface; Among them, ε is a positive number infinitely close to 0; (1.5), calculating the optimum slip ratio s _{opt_ij} of the wheel; Among them, s _{opt_t} is the optimal slip ratio of the wheel under the standard road surface; (2), calculate the expected wheel speed ω _{ref_ij} of the wheel (3) Use the compensation torque T _{e_ij} output by the PID controller to control the wheel speed Comparing the current wheel speed ω _ij with the expected wheel speed ω _{ref_ij} , if the wheel speed ω _ij is greater than the expected wheel speed ω _{ref_ij} , that is, ω _ij > ω _{ref_ij} , the wheel slips, and the PID controller is used to control the wheel speed ω The difference e _ij between _ij and the expected wheel speed ω _{ref_ij} is controlled to obtain the compensation torque T _{e_ij} ; Among them, k _{p_ij} is the proportional coefficient of the PID controller, k _{i_ij} is the integral coefficient of the PID controller, and k _{d_ij} is the integral coefficient of the controller; If the wheel speed is less than or equal to the desired wheel speed, that is, ω _ij ≤ ω _{ref_ij} , the wheel does not slip, and the PID controller outputs compensation torque T _{e_ij} =0; (4), calculate the command torque T _{com_ij} of each motor (4.1), calculate the total command torque T _com ; Among them, e is the deviation between the desired vehicle speed and the actual vehicle speed, k _{p_v} , k _{i_v} , and k _{d_v} are the proportional, integral and differential constants in the vehicle speed control, respectively; (4.2), the total command torque T _com is evenly distributed to each motor, and the command torque T _{com_ij} of each motor is obtained; (5), calculate the driving torque T _{d_ij} In the motor of an electric vehicle, first sum the compensation torque T _{e_ij} and the command torque T _{com_ij} to obtain the input torque of the limiting module Then, limit the processing through the limiting module to obtain the driving torque T _{d_ij} of the motor; Among them, T _max is the maximum output torque of the hub motor; Finally, the drive torque T _{d_ij} is input to each motor, and the anti-skid control is performed by controlling the speed of the electric vehicle. 2. a kind of anti-skid control method for four-wheel hub electric vehicles according to claim 1, is characterized in that, during the wheel speed control in described step (3), the transfer function C (s) of PID controller is: Among them, s is the frequency domain operator, k _{p_ij} is the proportional coefficient of the PID controller, k _{i_ij} is the integral coefficient of the PID controller; In wheel speed control, the transfer function G(s) of the hub motor of the controlled object can be expressed as: Among them, T is the motor time constant, J is the moment of inertia of the wheel; From formulas (12) and (13), it can be obtained that the closed-loop transfer function of wheel speed control is: From formula (14), it can be obtained that the characteristic equation of the wheel speed control system is: JTs ³ +Js ² +k _{p_ij} s+k _{i_ij} = 0 (15) In order to ensure the stability of wheel speed control, the Laws criterion can be obtained: Therefore, the values of the proportional parameter k _{p_ij} and the integral parameter k _{i_ij} of the PID controller in the wheel speed control should satisfy: