CN116859952B - Longitudinal compound control method and system for vehicle fleet based on second-order continuous sliding mode - Google Patents

Abstract

The present invention discloses a longitudinal composite control method and system for a fleet based on a second-order continuous sliding mode, and relates to the field of automobile control technology. The method comprises: constructing a vehicle longitudinal dynamics model of a target fleet including actuator dynamics; based on the vehicle longitudinal dynamics model, and using a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding mode surface, dynamically controlling the headway deviation of the target fleet under a non-zero initial deviation condition, so that the headway of the target fleet is the expected value while ensuring the chord stability of the fleet system. The present invention can ensure the finite-time stability of the estimation error, and adopts a continuous control signal to avoid the chattering problem caused by the discontinuous control signal of the prior art solution, thereby improving the performance of the fleet longitudinal control system.

Description

Longitudinal compound control method and system for vehicle fleet based on second-order continuous sliding mode

Technical Field

The present invention relates to the technical field of vehicle control, and in particular to a method and system for longitudinal composite control of a vehicle fleet based on a second-order continuous sliding mode.

Background technique

With the progress of modern automobile manufacturing industry, people's travel and the transportation of goods have become more convenient. While bringing convenience to people, it also brings serious traffic congestion and other problems. The common causes of traffic congestion can be attributed to two aspects: macro and micro. From a macro perspective, it is because of the imbalance between supply and demand, and the speed of infrastructure construction cannot keep up with the speed of traffic demand growth, which leads to a long-term shortage of highway traffic in key areas; from a micro perspective, it is because of traffic events that lead to reduced traffic capacity, inappropriate traffic flow control measures, and traffic shocks caused by uncontrolled driving behavior. In order to further enhance the synergistic relationship between macro and micro, domestic and foreign scholars have proposed networked vehicles. From a macro perspective, networked vehicles can complete dynamic traffic flow scheduling and better meet the balance between traffic supply and demand; from a micro perspective, networked vehicles can better achieve vehicle-to-vehicle/vehicle-to-road coordination, respond to traffic events in a timely manner, reduce the possibility of traffic shocks and spread, and thus reduce or even eliminate traffic congestion.

The related convoy longitudinal following control algorithms include the sliding mode variable structure control (SMC) algorithm, which is an effective method to solve the parameter uncertainty and external disturbance in the system. Based on the SMC algorithm, many important results have been achieved in the longitudinal control of connected vehicles. However, an inherent disadvantage of SMC is jitter. In order to reduce the impact of jitter on the performance of the convoy system, a related technical solution proposes an adaptive SMC control algorithm. More recently, for a convoy system affected by actuator dynamic uncertainty, when a vehicle-to-vehicle communication failure occurs in the convoy and the acceleration of the leading vehicle is unavailable, the acceleration of the leading vehicle and the uncertainty in the system are considered as lumped disturbances, and an interference observer is used for estimation, and then a composite control algorithm based on interference compensation and SMC is designed. However, the interference observer used in the above technical solution is asymptotically stable, not finite-time stable. In addition, the control signal used is a discontinuous control signal, which may lead to potential system performance degradation.

Summary of the invention

The purpose of the present invention is to provide a vehicle fleet longitudinal composite control method and system based on a second-order continuous sliding mode, which can ensure the finite time stability of the estimation error, avoid the chattering problem caused by the discontinuity of the control signal by designing a continuous control signal, and improve the longitudinal control performance of the vehicle fleet system.

To achieve the above object, the present invention provides the following solutions:

A longitudinal composite control method for a convoy based on a second-order continuous sliding mode, comprising:

Constructing a vehicle longitudinal dynamics model of a target platoon including actuator dynamics; the desired headway of the platoon is described using a fixed headway strategy;

Based on the vehicle longitudinal dynamics model, and by using a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding surface, the headway deviation of the target fleet is dynamically controlled under a non-zero initial deviation condition, so that the headway of the target fleet is at an expected value while ensuring the chord stability of the entire fleet system.

Optionally, the expected headway of the target fleet is expressed as:

in, represents the expected headway of vehicle i at time t; _vi (t) represents the speed; /> represents a predefined fixed time interval; Δ _i represents the headway distance when vehicle i is stationary.

Optionally, the headway deviation of the target fleet is expressed as:

where τ _i represents the lag time of the actuator to achieve the required acceleration; κ _i represents the ratio of the required acceleration that vehicle i can achieve; u _i (t) represents the control input; Ω _i (t) represents the lumped disturbance; a _i (t) represents the acceleration of vehicle i; a _i-1 (t) represents the acceleration of the preceding vehicle; δ _i (t) represents the parameter uncertainty or external disturbance in the system; δ _i (t) is unknown but bounded, and satisfies |δ _i (t)|<δ, where δ is a positive constant. represents a predefined fixed time interval; ε _i (t) represents the headway deviation; /> and θ _i are system parameters.

Optionally, based on the vehicle longitudinal dynamics model, a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding mode surface are used to dynamically control the headway deviation of the target vehicle fleet under a non-zero initial deviation condition, so that the headway deviation of the target vehicle fleet is a desired value while ensuring the chord stability of the entire vehicle fleet system, specifically including:

Based on the vehicle longitudinal dynamics model, the finite time disturbance observer is used to dynamically estimate the lumped disturbance to obtain a lumped disturbance estimation result;

According to the lumped interference estimation result, the second-order continuous sliding mode algorithm and the coupled sliding mode surface, the headway deviation of the target fleet is controlled under the non-zero initial deviation condition, and a composite control strategy is designed, including adjusting the headway deviation to tend to 0 over time, so that the headway of the target fleet is the expected value while ensuring the chord stability of the entire fleet system.

Optionally, the method further includes: designing and analyzing a controller under the non-zero initial deviation to solve the problem of large instantaneous engine/brake torque caused by the non-zero initial deviation, specifically including:

Design the controller under non-zero initial deviation, setting:

in, is the virtual headway deviation at time t; ε _i (t) is the actual headway deviation at time t; ε _i (0) is the headway deviation at the initial moment; /> is the initial value of the first-order derivative of ε _i (t),/> is the initial value of the second-order derivative of ε _i (t); _ni is the convergence rate to be designed, and exp(·) represents the natural exponential function;

Design a finite-time disturbance observer for estimation as follows:

in, ψ ₁ =ε _i (0),/> x _i,0 (t), x _i,1 (t), x _i,2 (t) are Ω _i (t),/> The estimated value of are the first-order derivatives of _xi,0 (t), _xi,1 (t), _xi,2 (t) respectively; is the virtual headway deviation at time t; ε _i (t) is the actual headway deviation at time t; ε _i (0) is the headway deviation at the initial moment; _ni is the convergence rate to be designed; a _i (t) represents the acceleration of vehicle i; /> and θ _i are system parameters; b is the Lipshitz constant of Ω _i (t); _mi0 , _mi1 , _mi2 are the positive observer gains to be designed; sign(·) is the sign function; sig ^c (·) = sign(·)|·| ^c . The estimation error is defined as:

Then the dynamics of the estimation error is obtained as:

Optionally, the design of the composite control strategy includes adjusting the headway distance deviation to tend to 0 over time so that the headway distance of the target fleet is an expected value while ensuring the chord stability of the entire fleet system, specifically including:

The control objective of the designed controller is to adjust the headway deviation ε _i (t) to zero over time while ensuring the chord stability of the entire fleet. In order to make the headway deviation ε _i (t) converge to 0, the sliding surface is designed Where α is a positive constant to be designed; the following coupled sliding surface is introduced to ensure the chord stability of the entire fleet:

Where 0<|β|≤1 is the weight coefficient, and the following relationship between _Si (t) and _Si (t) can be obtained:

S _i (t) = Bs _i (t)

in,

S(t)＝col(S _i (t),i佬N),s(t)＝col(s _i (t),iN),N＝1,2,L,N is the set of following vehicles in the convoy.

Because β≠0, B is reversible, and when S _i (t) tends to 0, S _i (t) also tends to 0;

Then, the following composite controller can be designed:

in, η _i1 and η _i2 are normal constants to be designed. Under the action of the designed composite controller, it can converge to 0 asymptotically and the entire fleet is chord-stable.

If Ω _i (t) is differentiable and The Lipshitz constant b holds true when the controller parameters satisfy:

Under the action of the designed controller, the headway deviation of each vehicle in the convoy system converges asymptotically to 0. When 0<|β|≤1, the entire convoy is chord-stable;

Where, χ＝max{βl _i,2 (t)+l _i+1,2 (t),βl _N,2 (t)}, i＝1,2,…,N-1, and l _i,2 (t) is the upper bound of the rate of change of e _i,1 (t).

The present invention also provides a longitudinal composite control system for a vehicle fleet based on a second-order continuous sliding mode algorithm, comprising:

A model building module is used to build a vehicle longitudinal dynamics model of a target fleet of actuator dynamics; the dynamic change of the expected headway of the vehicle longitudinal dynamics model is described by using a fixed headway strategy;

The convoy control module is used to dynamically control the headway deviation of the target convoy under non-zero initial deviation conditions based on the vehicle longitudinal dynamics model and using a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding mode surface, so that the headway of the target convoy is the expected value while ensuring the chord stability of the entire convoy system.

According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects:

The present invention discloses a longitudinal composite control method and system for a fleet based on a second-order continuous sliding mode. The method includes first constructing an actuator of a longitudinal dynamics model of a vehicle, which is used to describe the dynamic change of the expected headway spacing, and then dynamically controlling the headway spacing deviation of the target fleet based on the longitudinal dynamics model of the vehicle, using a finite-time disturbance observer, a second-order continuous sliding mode algorithm, and a coupled sliding mode surface, so that the headway spacing of the target fleet is the expected value while ensuring the chord stability of the entire fleet system. The technical solution can ensure the finite-time stability of the estimation error, and avoid the chattering problem caused by the discontinuity of the control signal by realizing the continuity of the control signal, thereby improving the performance of the fleet longitudinal control system.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

FIG1 is a schematic diagram of a flow chart of a longitudinal composite control method of a vehicle fleet based on a second-order continuous sliding mode according to the present invention;

FIG2 is a schematic diagram of a headway deviation test based on a second-order continuous sliding mode algorithm under a non-zero initial deviation of a homogeneous vehicle fleet in this embodiment;

FIG3 is a schematic diagram of a sliding surface experiment based on a second-order continuous sliding mode algorithm under a non-zero initial deviation of a homogeneous fleet in this embodiment;

FIG4 is a schematic diagram of a control input experiment based on a second-order continuous sliding mode algorithm under a non-zero initial deviation of a homogeneous fleet in this embodiment;

FIG5 is a schematic diagram of the simulation of the headway deviation of a heterogeneous fleet based on a second-order continuous sliding mode algorithm under non-zero initial deviation in this embodiment;

FIG6 is a schematic diagram of sliding surface simulation based on a second-order continuous sliding mode algorithm under non-zero initial deviation of a heterogeneous fleet in this embodiment;

FIG7 is a schematic diagram of a control input simulation test based on a second-order continuous sliding mode algorithm under non-zero initial deviation of a heterogeneous fleet in this embodiment.

Detailed ways

The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

The purpose of the present invention is to provide a vehicle fleet longitudinal composite control method and system based on a second-order continuous sliding mode, which can ensure the finite time stability of the estimation error, avoid the chattering problem caused by the discontinuity of the control signal by realizing the continuity of the control signal, and improve the longitudinal control performance of the vehicle fleet.

In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

As shown in FIG1 , the present invention provides a longitudinal composite control method for a vehicle fleet based on a second-order continuous sliding mode, comprising:

Step 100: Construct a vehicle longitudinal dynamics model of a target fleet including actuator dynamics; the desired headway of the fleet is described using a fixed headway strategy.

Step 200: Based on the vehicle longitudinal dynamics model, and using a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding surface, the headway deviation of the target fleet is dynamically controlled under a non-zero initial deviation condition, so that the headway of the target fleet is at an expected value while ensuring the chord stability of the entire fleet system.

The specific process is as follows: based on the vehicle longitudinal dynamics model, the finite time disturbance observer is used to dynamically estimate the lumped disturbance to obtain the error dynamic estimation result. According to the lumped disturbance estimation result, the second-order continuous sliding mode algorithm and the coupled sliding mode surface, the headway deviation of the target fleet is controlled under the non-zero initial deviation condition, and a composite control strategy is designed, including adjusting the headway deviation to tend to 0 over time, so that the headway of the target fleet is the expected value while ensuring the chord stability of the entire fleet system.

The expected headway distance changes dynamically, which is expressed as:

In the formula, represents the expected headway of vehicle i at time t; _vi (t) represents the speed; /> represents a predefined fixed time interval; Δ _i represents the headway distance when vehicle i is stationary.

The headway deviation of the target fleet is expressed as:

where τ _i represents the lag time of the actuator to achieve the required acceleration; κ _i represents the ratio of the required acceleration that vehicle i can achieve; _ui (t) represents the control input; Ω _i (t) represents the lumped disturbance; _ai (t) represents the acceleration of vehicle i; _ai-1 (t) represents the acceleration of the preceding vehicle; δ _i (t) represents the parameter uncertainty or external disturbance in the system; δ _i (t) is unknown but bounded, and satisfies |δ _i (t)|<δ.

On the basis of the above technical solution, the following embodiments are provided:

Step 1: Establish a vehicle longitudinal dynamics model considering the actuator dynamics, and choose to adopt a fixed headway strategy to describe the dynamics of the desired headway.

Step 2: Design and analyze the controller under non-zero initial deviation to solve the problem of large instantaneous engine/brake torque caused by non-zero initial deviation.

Step 3: Use numerical simulation to illustrate the effectiveness of the designed control algorithm and the correctness of the theoretical analysis.

Furthermore, a fixed time interval strategy is adopted in step 1, specifically:

in, represents the expected headway of vehicle i at time t; _vi (t) represents the speed; /> represents a predefined fixed time interval; Δ _i represents the headway distance when vehicle i is stationary.

The vehicle longitudinal dynamics model considering the actuator dynamics is established, specifically:

where a _i ( t ) represents the acceleration of vehicle i at time t, τ _i represents the lag time of the actuator to achieve the required acceleration, κ _i represents the ratio of vehicle i to achieve the required acceleration, u _i ( t ) represents the control input, δ _i ( t ) represents the parameter uncertainty or external disturbance in the system, and δ _i ( t ) is unknown but bounded, and satisfies |δ _i ( t )|<δ.

For vehicle i, the headway deviation ε _i (t) and the speed difference Δv _i (t) between the vehicle and the preceding vehicle can be expressed as:

_Δvi (t)＝vi _-1 (t) _-vi (t) (4)

where p _i (t) represents the actual headway between vehicle i and the preceding vehicle.

Taking the first-order derivatives on both sides of (2) yields:

Further, we can get:

Substituting formula (2) into formula (6), the dynamics of the expected headway distance can be obtained as:

in, δ _i (t) is unknown but bounded, and satisfies |δ _i (t)|<δ. To reduce communication, this embodiment adopts a unidirectional communication topology, that is, vehicle i only obtains the information of the vehicle behind it through wireless communication, and does not communicate with the vehicle in front of it. For this reason, the acceleration of the front vehicle a _i-1 (t) is regarded as an external disturbance. In the following controller design process, Ω _i (t) is regarded as a lumped disturbance.

In step 2, the controller is designed and analyzed under non-zero initial deviation to solve the problem of large instantaneous engine/brake torque caused by non-zero initial deviation, specifically:

In practical applications, non-zero initial deviation is inevitable. When a vehicle merges into a convoy or leaves a convoy, it will cause a non-zero initial deviation. Non-zero initial deviation may cause relatively large engine torque or braking torque. Each vehicle in the convoy is interconnected and coupled. The deviation caused by the disturbance acting on one vehicle may have an adverse effect on other vehicles, propagate backward along the convoy, reduce the following performance between vehicles in the convoy, and even cause the convoy string to be unstable. For this reason, the controller is designed and analyzed under non-zero initial deviation.

set up

Next, a composite control algorithm based on the finite time disturbance observer (FTDO) and the second-order continuous sliding mode algorithm is designed. The following FTDO is designed to estimate Ω _i (t):

in, ψ ₁ =ε _i (0),/> _mi0 , _mi1 , _mi2 are the positive observer gains to be designed; _xi,0 (t), _xi,1 (t), _xi,2 (t) are respectively/> Ω _i (t),/> The estimated value of .

This embodiment sets the estimated error:

Then the observer estimation error dynamics can be obtained as:

Select appropriate observer gain, and the estimated error e _i,1 (t) converges to 0 in a finite time. To adjust ε _i (t) to 0, design the sliding surface Where α is a positive constant to be designed. To ensure the stability of the queue chord of the entire fleet system, the following coupled sliding surface is designed:

In the formula, 0<|β|≤1 is the weight coefficient, and the following relationship between _Si (t) and _Si (t) can be obtained:

S _i (t) = Bs _i (t) (13)

in:

S(t)＝col(S _i (t),i佬N),s(t)＝col(s _i (t),iN),N＝1,2,L,N is the set of following vehicles in the convoy.

Because β≠0, B is reversible, and when _Si (t) approaches 0, _Si (t) also approaches 0. And vice versa. The following composite controller based on FTDO and second-order continuous sliding mode algorithm is designed:

in,

η _i1 and η _i2 are normal numbers to be designed.

So far, this embodiment can be concluded that:

If Ω _i (t) is differentiable and The Lipshitz constant b holds true, and the controller parameters are selected to satisfy:

Then, under the condition of non-zero initial deviation, the headway deviation of each vehicle in the convoy system converges asymptotically to 0 under the action of the composite controller. When 0<|β|≤1, the entire convoy is chord-stable. Where χ＝max{βl _i,2 (t)+l _i+1,2 (t),βl _N,2 (t)}, i＝1,2,…,N-1, l _i,2 (t) is the upper bound of the rate of change of e _i,1 (t).

Then, under the condition of non-zero initial deviation, the headway deviation ε _i (t) of each vehicle in the convoy system converges asymptotically to 0 under the composite control law (14) based on FTDO and second-order continuous sliding mode algorithm. And when β satisfies 0<|β|≤1, the entire convoy is chord-stable. Where, χ＝max{βl _i,2 (t)+l _i+1,2 (t),βl _N,2 (t)}, i＝1,2,…,N-1, l _i,2 (t) is the upper bound of the rate of change of e _i,1 (t).

In step three, numerical simulation is used to illustrate the effectiveness of the designed control algorithm and the correctness of the theoretical analysis, specifically:

Consider a convoy system consisting of 6 vehicles, including 1 pilot vehicle and 5 follower vehicles. The numerical simulation of two types of convoys, homogeneous and heterogeneous, is carried out using MATLAB simulation software, where the maneuvering process of the pilot vehicle is assumed to be:

In the following simulations, the constant time interval is taken as n _i ＝4+i*0.1,i＝1,2,L,5. In the simulation of homogeneous fleet, the parameters are set to κ _i ＝0.85,τ _i ＝0.25. In the simulation, δ _i (t) uses trigonometric functions, δ ₁ (t)＝2.5sint,δ ₂ (t)＝2sint+0.01,δ ₃ (t)＝1.5sin(2t)+0.03,δ ₄ (t)＝sin(5t)+0.03,δ ₅ (t)＝2.8sint. The FTDO parameters are set to _mi0 ＝3,m _i1 ＝1.5,m _i2 ＝1,i＝1,2,…,5. The controller parameters are set to b＝0.99,a＝6,h _i1 ＝2.1,h _i2 ＝11.2. When simulating heterogeneous fleets, the parameters of the vehicles in the fleet are shown in Table 1, and the corresponding FTFO and controller parameter settings are shown in Table 2. The simulation results of the convoy system under the non-zero initial deviation under the proposed control algorithm are shown in the figure. The horizontal axis in the figure represents time, the vertical axis of Figure 2 represents the headway deviation, the vertical axis of Figure 3 represents the sliding surface, the vertical axis of Figure 4 represents the control input, the vertical axis of Figure 5 represents the headway deviation, the vertical axis of Figure 6 represents the sliding surface, and the vertical axis of Figure 7 represents the control input. From the simulation diagram, it can be observed that under the non-zero initial error, the convoy system performs well under the proposed control algorithm.

Table 1 Vehicle parameters

Table 2 Simulation results

In addition, the present invention also provides a second-order continuous sliding mode algorithm longitudinal composite control system for a vehicle fleet, comprising:

The model building module is used to build a vehicle longitudinal dynamics model of a target fleet of actuator dynamics; the dynamic change of the expected headway of the vehicle longitudinal dynamics model is described by using a fixed time distance strategy.

The convoy control module uses a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding surface to dynamically control the headway error of the target convoy, so that the headway of the target convoy is the expected value while ensuring the chord stability of the entire convoy system.

The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

This article uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only used to help understand the core idea of the present invention. At the same time, for those skilled in the art, according to the idea of the present invention, there will be changes in the specific implementation methods and application scope. In summary, the content of this specification should not be understood as limiting the present invention.

Claims (2)

1. A longitudinal composite control method for a vehicle fleet based on a second-order continuous sliding mode, characterized by comprising: Constructing a vehicle longitudinal dynamics model of a target platoon including actuator dynamics; the desired headway of the platoon is described using a fixed headway strategy; Based on the vehicle longitudinal dynamics model, and using a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding mode surface, the headway deviation of the target convoy is dynamically controlled under a non-zero initial deviation condition, so that the headway of the target convoy is the expected value while ensuring the chord stability of the entire convoy system; The expected headway of the target fleet is expressed as: in, represents the expected headway of vehicle i at time t; _vi (t) represents the speed; /> represents a predefined fixed time interval; △ _i represents the headway distance when vehicle i is stationary; The headway deviation of the target fleet is expressed as: where τ _i represents the lag time of the actuator to achieve the required acceleration; κ _i represents the ratio of vehicle i to achieve the required acceleration; u _i (t) represents the control input; Ω _i (t) represents the lumped disturbance; a _i (t) represents the acceleration of vehicle i; a _i-1 (t) represents the acceleration of the preceding vehicle; δ _i (t) represents the parameter uncertainty or external disturbance in the system; δ _i (t) is unknown but bounded and satisfies |δ _i (t)|<δ, where δ is a positive constant. represents a predefined fixed time interval; ε _i (t) represents the headway deviation; /> and θ _i are system parameters; Based on the vehicle longitudinal dynamics model, and using a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding mode surface, the headway deviation of the target convoy is dynamically controlled under a non-zero initial deviation condition, so that the headway of the target convoy is the expected value while ensuring the chord stability of the entire convoy system, specifically including: Based on the vehicle longitudinal dynamics model, the finite time disturbance observer is used to dynamically estimate the lumped disturbance to obtain a lumped disturbance estimation result; According to the lumped disturbance estimation result, the second-order continuous sliding mode algorithm and the coupled sliding mode surface, the headway deviation of the target fleet is controlled under the non-zero initial deviation condition, and a composite control strategy is designed, including adjusting the headway deviation to tend to 0 over time, so that the headway of the target fleet is the expected value while ensuring the chord stability of the entire fleet system; The longitudinal compound control method of the vehicle fleet further includes: designing and analyzing a controller under the non-zero initial deviation to solve the problem of large instantaneous engine/brake torque caused by the non-zero initial deviation, specifically including: Design the controller under non-zero initial deviation, setting: in, is the virtual headway deviation at time t; ε _i (t) is the actual headway deviation at time t; ε _i (0) is the headway deviation at the initial moment; /> is the initial value of the first-order derivative of ε _i (t),/> is the initial value of the second-order derivative of ε _i (t); _ni is the convergence rate to be designed, and exp(·) represents the natural exponential function; The finite-time disturbance observer is designed as follows to estimate Ω _i (t): in, ψ ₁ =ε _i (0),/> x _i,0 (t), x _i,1 (t), x _i,2 (t) are respectively/> Ω _i (t),/> The estimated value of are the first-order derivatives of x _i,0 (t), x _i,1 (t), and x _i,2 (t);/> is the virtual headway deviation at time t; ε _i (t) is the actual headway deviation at time t; ε _i (0) is the headway deviation at the initial moment; _ni is the convergence rate to be designed; a _i (t) represents the acceleration of vehicle i; /> and θ _i are system parameters; b is/> Lipshitz constant; _mi0 , _mi1 , _mi2 are the positive observer gains to be designed; sign(·) is the sign function; sig ^c (·) = sign(·) | · ^c ; Definition/> Ω _i (t),/> The estimated errors are: Then the dynamics of the estimation error is obtained as: The design of the composite control strategy includes adjusting the headway distance deviation to tend to 0 over time, so that the headway distance of the target fleet is the expected value while ensuring the chord stability of the entire fleet system, specifically including: The control objective of the designed controller is to adjust the headway deviation ε _i (t) to 0 over time while ensuring the chord stability of the entire fleet. In order to make the headway deviation ε _i (t) converge to 0, the sliding surface is designed. Where α is a positive constant to be designed; the following coupled sliding surface is introduced to ensure the chord stability of the entire fleet: Where 0<|β|≤1 is the weight coefficient, and the following relationship between _Si (t) and _Si (t) can be obtained: S _i (t) = Bs _i (t) in, For the collection of following vehicles in a convoy; Because β≠0, B is reversible, and when S _i (t) tends to 0, S _i (t) also tends to 0; Then, the following composite controller can be designed: in, η _i1 and η _i2 are normal constants to be designed. Under the action of the designed composite controller, they can converge to 0 asymptotically and the entire fleet is chord-stable; If Ω _i (t) is differentiable and The Lipshitz constant b holds true when the controller parameters satisfy: Under the action of the designed controller, the headway deviation of each vehicle in the convoy system converges asymptotically to 0, and when 0<|β|≤1, the entire convoy is chord-stable; in, is the upper bound of the rate of change of e _i,1 (t). 2. A longitudinal composite control system for a vehicle fleet based on a second-order continuous sliding mode algorithm, characterized by comprising: A model building module is used to build a vehicle longitudinal dynamics model of a target fleet of actuator dynamics; the dynamic change of the expected headway of the vehicle longitudinal dynamics model is described by using a fixed headway strategy; A convoy control module is used to dynamically control the headway deviation of the target convoy under non-zero initial deviation conditions based on the vehicle longitudinal dynamics model and using a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding mode surface, so that the headway deviation of the target convoy is the expected value while ensuring the chord stability of the entire convoy system; The expected headway of the target fleet is expressed as: in, represents the expected headway of vehicle i at time t; _vi (t) represents the speed; _ti ^* represents the predefined fixed time interval; △ _i represents the headway of vehicle i when it is stationary; The headway deviation of the target fleet is expressed as: where τ _i represents the lag time of the actuator to achieve the required acceleration; κ _i represents the ratio of vehicle i to achieve the required acceleration; u _i (t) represents the control input; Ω _i (t) represents the lumped disturbance; a _i (t) represents the acceleration of vehicle i; a _i-1 (t) represents the acceleration of the preceding vehicle; δ _i (t) represents the parameter uncertainty or external disturbance in the system; δ _i (t) is unknown but bounded and satisfies |δ _i (t)|<δ, where δ is a positive constant. represents a predefined fixed time interval; ε _i (t) represents the headway deviation; /> and θ _i are system parameters; Based on the vehicle longitudinal dynamics model, and using a finite-time disturbance observer, a second-order continuous sliding mode algorithm and a coupled sliding mode surface, the headway deviation of the target convoy is dynamically controlled under a non-zero initial deviation condition, so that the headway of the target convoy is the expected value while ensuring the chord stability of the entire convoy system, specifically including: Based on the vehicle longitudinal dynamics model, the finite time disturbance observer is used to dynamically estimate the lumped disturbance to obtain a lumped disturbance estimation result; According to the lumped disturbance estimation result, the second-order continuous sliding mode algorithm and the coupled sliding mode surface, the headway deviation of the target fleet is controlled under the non-zero initial deviation condition, and a composite control strategy is designed, including adjusting the headway deviation to tend to 0 over time, so that the headway of the target fleet is the expected value while ensuring the chord stability of the entire fleet system; The longitudinal compound control method of the vehicle fleet further includes: designing and analyzing a controller under the non-zero initial deviation to solve the problem of large instantaneous engine/brake torque caused by the non-zero initial deviation, specifically including: Design the controller under non-zero initial deviation, setting: in, is the virtual headway deviation at time t; ε _i (t) is the actual headway deviation at time t; ε _i (0) is the headway deviation at the initial moment; /> is the initial value of the first-order derivative of ε _i (t),/> is the initial value of the second-order derivative of ε _i (t); _ni is the convergence rate to be designed, and exp(·) represents the natural exponential function; The finite-time disturbance observer is designed as follows to estimate Ω _i (t): in, ψ ₁ =ε _i (0),/> x _i,0 (t), x _i,1 (t), x _i,2 (t) are respectively/> Ω _i (t),/> The estimated value of are the first-order derivatives of x _i,0 (t), x _i,1 (t), and x _i,2 (t);/> is the virtual headway deviation at time t; ε _i (t) is the actual headway deviation at time t; ε _i (0) is the headway deviation at the initial moment; _ni is the convergence rate to be designed; a _i (t) represents the acceleration of vehicle i; /> and θ _i are system parameters; b is/> Lipshitz constant; _mi0 , _mi1 , _mi2 are the positive observer gains to be designed; sign(·) is the sign function; sig ^c (·) = sign(·) | · ^c ; Definition/> Ω _i (t),/> The estimated errors are: Then the dynamics of the estimation error is obtained as: The design of the composite control strategy includes adjusting the headway distance deviation to tend to 0 over time, so that the headway distance of the target fleet is the expected value while ensuring the chord stability of the entire fleet system, specifically including: The control objective of the designed controller is to adjust the headway deviation ε _i (t) to 0 over time while ensuring the chord stability of the entire fleet. In order to make the headway deviation ε _i (t) converge to 0, the sliding surface is designed. Where α is a positive constant to be designed; the following coupled sliding surface is introduced to ensure the chord stability of the entire fleet: Where 0<|β|≤1 is the weight coefficient, and the following relationship between _Si (t) and _Si (t) can be obtained: S _i (t) = Bs _i (t) in, For the collection of following vehicles in a convoy; Because β≠0, B is reversible, and when S _i (t) tends to 0, S _i (t) also tends to 0; Then, the following composite controller can be designed: in, η _i1 and η _i2 are normal constants to be designed. Under the action of the designed composite controller, they can converge to 0 asymptotically and the entire fleet is chord-stable; If Ω _i (t) is differentiable and The Lipshitz constant b holds true when the controller parameters satisfy: Under the action of the designed controller, the headway deviation of each vehicle in the convoy system converges asymptotically to 0, and when 0<|β|≤1, the entire convoy is chord-stable; in, is the upper bound of the rate of change of e _i,1 (t).