A control method for rear-wheel active steering

Abstract

A control method for rear-wheel active steering, which relates to a wheel steering control method. Obtain basic vehicle-road parameters, and derive a reference model from a two-degree-of-freedom model built based on a non-linear tire side force model, including a front wheel slip angle calculation module, a front wheel side force calculation module, an expected yaw rate dynamics, a lateral acceleration calculation module, an expected rear wheel side force calculation module, and an integrator 1. Design a rear wheel steering angle resolver, including a side tire force model, a PI controller, an integrator 2, and a rear wheel steering angle calculation module. Combine the reference model and the rear wheel steering angle resolver into a rear-wheel active steering controller and apply it to the vehicle system, with the front wheel steering angle and the longitudinal vehicle speed as inputs, and output the rear wheel steering angle. Without using the yaw rate or the sideslip angle of the center of mass as a feedback quantity, only the front wheel steering angle and the longitudinal vehicle speed need to be collected, which can ensure the performance under the extreme working conditions where the tire force approaches saturation.

Description

Technical Field

[0001] The present invention relates to a method for controlling wheel steering, in particular to a control method for active rear-wheel steering, and belongs to the technical field of vehicle motion control. Background Art

[0002] With the development of vehicle chassis electronic control systems and autonomous driving technologies, more and more vehicles are equipped with an active rear-wheel steering device (abbreviation: ARS), which is integrated with the front-wheel steering system and can keep the sideslip angle of the vehicle's center of mass basically zero, improving the vehicle's dynamic response characteristics and handling stability to the steering wheel angle input.

[0003] Currently, the control methods for rear-wheel steering can be divided into open-loop control and closed-loop control. Among them, the feedback signals of the closed-loop control include yaw rate and sideslip angle of the center of mass. However, the processing of the noise of the yaw rate sensor will cause delay, and the sideslip angle of the center of mass cannot be directly measured and can only be obtained through estimation. At present, there is no generally recognized and commonly used estimation method that takes into account both accuracy and cost. The control method of open-loop control uses a linear model design and cannot guarantee the performance under the extreme conditions where the tire force approaches saturation. Summary of the Invention

[0004] To solve the deficiencies in the background art, the present invention provides a control method for active rear-wheel steering, which does not require the yaw rate or the sideslip angle of the center of mass as feedback quantities, and only needs to collect the front-wheel steering angle and the longitudinal vehicle speed, and can guarantee the performance under the extreme conditions where the tire force approaches saturation.

[0005] To achieve the above object, the present invention adopts the following technical solution: A control method for active rear-wheel steering includes the following steps:

[0006] Step 1: Obtain basic vehicle-road parameters

[0007] Including the vehicle mass M, the distance l from the center of gravity to the front axle f , the distance l from the center of gravity to the rear axle r , the yaw moment of inertia I zz and the road surface adhesion coefficient μ;

[0008] Step 2: Establish a non-linear tire lateral force model

[0009] F y = Dsin[Carctan{Bα - E(Bα - arctan Bα)}]

[0010] where F y represents the tire lateral force, α represents the tire sideslip angle, and B, C, D, and E are parameters related to the road surface adhesion coefficient μ;

[0011] Step 3: Establish a two-degree-of-freedom model

[0012]

[0013]

[0014] Among them, r represents the yaw rate, v y represents the lateral velocity, F yf represents the front-wheel lateral force, F yr represents the rear-wheel lateral force, a y represents the lateral acceleration, v x represents the longitudinal vehicle speed;

[0015] Step Four: Establish a reference model

[0016] Set to 0, and we can get:

[0017]

[0018]

[0019] Combining the above formulas, the dynamic equation for the desired yaw rate is as follows:

[0020]

[0021] And the calculation equation for the desired rear-wheel lateral force is as follows:

[0022] F yrd = Ma y - F yf

[0023] Based on this, the reference model is established, including:

[0024] Front-wheel side slip angle calculation module, which calculates the front-wheel side slip angle according to the front-wheel steering angle δ f , the longitudinal vehicle speed v x and the desired yaw rate rd output by integrator 1:

[0025]

[0026] Front-wheel lateral force calculation module, which calculates the front-wheel lateral force according to the front-wheel side slip angle α f output by the front-wheel side slip angle calculation module, and uses the non-linear tire lateral force model to calculate the front-wheel lateral force:

[0027] F yf = Dsin[Carctan{Bα f - E(Bα f - arctan Bα f )}]

[0028] Desired yaw rate dynamics, based on the lateral force of the front wheels F output by the front wheel lateral force calculation module yf , the longitudinal vehicle speed v x and the desired yaw rate rd output by integrator 1, use the dynamic equation of the desired yaw rate to calculate the derivative of the desired yaw rate:

[0029]

[0030] Lateral acceleration calculation module, based on the longitudinal vehicle speed v x and the desired yaw rate rd output by integrator 1 to calculate the lateral acceleration:

[0031] a y =F d v x

[0032] Desired rear wheel lateral force calculation module, based on the lateral acceleration a output by the lateral acceleration calculation module y and the lateral force of the front wheels F output by the front wheel lateral force calculation module yf , use the calculation equation of the desired rear wheel lateral force to calculate the desired rear wheel lateral force:

[0033] F yrd =Ma y -F yf

[0034] Integrator 1, integrates the derivative of the desired yaw rate output by the desired yaw rate dynamics, and outputs the desired yaw rate r d ;

[0035] Step Five: Design the rear wheel steering angle resolver

[0036] Including:

[0037] Lateral tire force model, based on the rear wheel slip angle α output by integrator 2 r , use the non-linear tire lateral force model to calculate the rear wheel lateral force:

[0038] F yr =Dsin[Carctan{Bα r -E(Bα r -arctan Bα r )}]

[0039] PI controller, based on the rear wheel lateral force F calculated by the lateral tire force model yr and the desired rear wheel lateral force F output by the reference model yrd The rear wheel lateral force deviation e between them outputs the derivative of the rear wheel slip angle to integrator 2, and jointly adjusts with integrator 2 to output the rear wheel slip angle α r ,

[0040] Rear wheel steering angle calculation module, based on the longitudinal vehicle speed v x , the desired yaw rate r output by the reference model d and the rear wheel sideslip angle α output by the integrator 2 r to calculate the rear wheel steering angle:

[0041]

[0042] Step Six: Combine the reference model in Step Four and the rear wheel steering angle resolver in Step Five into a rear wheel active steering controller and apply it to the vehicle system.

[0043] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention does not require the yaw rate or the sideslip angle of the center of mass as the feedback quantity, avoiding the problems of sensor noise processing and state estimation, which is beneficial to practical applications. It adopts an open-loop structure and mainly consists of two parts: a reference model and a rear wheel steering angle resolver. It only needs to collect the front wheel steering angle and the longitudinal vehicle speed. The reference model is derived from a two-degree-of-freedom model built based on a non-linear tire lateral force model, and is used to calculate the front wheel lateral force and the desired yaw rate under ideal conditions, and then calculate the desired rear wheel lateral force that makes the sideslip angle of the center of mass zero. The rear wheel steering angle resolver calculates the rear wheel sideslip angle, and finally calculates the rear wheel steering angle, which helps to reduce the transient and steady-state sideslip angles of the center of mass simultaneously and can ensure the performance under the extreme working conditions where the tire force approaches saturation. Brief Description of the Drawings

[0044] Figure 1 is a schematic structural diagram of the present invention applied to the vehicle system;

[0045] Figure 2 is a schematic structural diagram of the rear wheel active steering controller of the present invention;

[0046] Figure 3 is a schematic structural diagram of the reference model of the present invention;

[0047] Figure 4 is a schematic structural diagram of the rear wheel steering angle resolver of the present invention. Detailed Embodiments

[0048] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present invention.

[0049] As Figures 1 to 4 shown, a control method for rear wheel active steering includes the following steps:

[0050] Step 1: Obtain basic vehicle-road parameters, including vehicle parameters and road parameters

[0051] The vehicle parameters include the vehicle mass M, the distance l from the center of gravity to the front axle f , the distance l from the center of gravity to the rear axle r and the yaw moment of inertia I zz , and the road parameters include the road surface adhesion coefficient μ;

[0052] Step 2: Establish a non-linear tire lateral force model

[0053] Establish the non-linear tire lateral force model according to the road surface adhesion coefficient μ, and adopt the Magic Formula as follows:

[0054] F y =Dsin[Carctan{Bα - E(Bα - arctanBα)}]

[0055] where, F y represents the tire lateral force, α represents the tire side slip angle, B, C, D and E are parameters related to the road surface adhesion coefficient μ, and the specific relationship is common knowledge in the art. See the literature Pacejka H. Tyre and Vehicle Dynamics[M]. Butterworth-Heinemann, 2002.;

[0056] Step 3: Establish a two-degree-of-freedom model

[0057] Establish the two-degree-of-freedom model according to the vehicle parameters and the non-linear tire lateral force model as follows:

[0058]

[0059]

[0060] where, r represents the yaw rate, v y represents the lateral velocity, F yf represents the front wheel lateral force, F yr represents the rear wheel lateral force, a y represents the lateral acceleration, v x represents the longitudinal vehicle speed;

[0061] Step 4: Establish a reference model

[0062] The reference model takes the front wheel steering angle δ f as the input, the desired yaw rate r d as the state variable, and the desired rear wheel lateral force F yrd as the output,

[0063] Derive the dynamics of the desired yaw rate based on the zero sideslip angle of the center of mass. Since the sideslip angle of the center of mass It can be derived that v y = 0. Therefore, in the two-degree-of-freedom model, is set to 0, and we can obtain:

[0064]

[0065]

[0066] Combining the above formulas, the dynamic equation for the desired yaw rate is obtained as follows:

[0067]

[0068] And the calculation equation for the desired rear-wheel lateral force is as follows:

[0069] F yrd = Ma y - F yf

[0070] Based on this, a reference model is established. Combining Figure 3 as shown, it includes a front-wheel slip angle calculation module, a front-wheel lateral force calculation module, a desired yaw rate dynamics, a lateral acceleration calculation module, a desired rear-wheel lateral force calculation module, and an integrator 1, a total of 6 modules. Among them:

[0071] The front-wheel slip angle calculation module calculates the front-wheel slip angle according to the front-wheel steering angle δ f , the longitudinal vehicle speed v x , and the desired yaw rate r d output by the integrator 1:

[0072]

[0073] The front-wheel lateral force calculation module calculates the front-wheel lateral force according to the front-wheel slip angle α f output by the front-wheel slip angle calculation module, using a non-linear tire lateral force model:

[0074] F yf = Dsin[Carctan{Bα f - E(Bα f - arctan Bα f )}]

[0075] The desired yaw rate dynamics calculates the derivative of the desired yaw rate according to the front-wheel lateral force F yf output by the front-wheel lateral force calculation module, the longitudinal vehicle speed v x , and the desired yaw rate rd output by the integrator 1, using the dynamic equation for the desired yaw rate:

[0076]

[0077] The lateral acceleration calculation module calculates the lateral acceleration according to the longitudinal vehicle speed v x and the desired yaw rate r output by the integrator 1 d as follows:

[0078] a y = r d v x

[0079] The desired rear-wheel lateral force calculation module calculates the desired rear-wheel lateral force by using the calculation equation of the desired rear-wheel lateral force according to the lateral acceleration a output by the lateral acceleration calculation module y and the front-wheel lateral force F output by the front-wheel lateral force calculation module yf :

[0080] F yrd = Ma y - F yf

[0081] The integrator 1 integrates the derivative of the desired yaw rate dynamically output by the desired yaw rate, and outputs the desired yaw rate r d ,

[0082] So far, the reference model has calculated the current desired yaw rate r d and the desired rear-wheel lateral force F yrd ;

[0083] Step Five: Design a rear-wheel steering angle resolver

[0084] The rear-wheel steering angle resolver takes the longitudinal vehicle speed v x , the desired yaw rate rd and the desired rear-wheel lateral force F output by the reference model yrd as inputs, and takes the rear-wheel steering angle δ yrd to achieve the desired rear-wheel lateral force F r as the output. As shown in combination with Figure 4 , it includes a lateral tire force model, a rear-wheel steering angle calculation module, a PI controller, and an integrator 2, a total of 4 modules, where:

[0085] The lateral tire force model calculates the rear-wheel lateral force by using the non-linear tire lateral force model according to the rear-wheel slip angle α r output by the integrator 2:

[0086] F yr = Dsin[Carctan{Bα r - E(Bα r - arctan Bα r )}]

[0087] The PI controller consists of a proportional link and an integral link, and calculates the lateral force F of the rear wheels according to the lateral tire force model yr and the desired lateral force F of the rear wheels output by the reference model yrd The lateral force deviation of the rear wheels between them e , outputs the derivative of the rear wheel sideslip angle to the integrator 2, and jointly adjusts with the integrator 2 to output the rear wheel sideslip angle α r , controls the lateral force F of the rear wheels output by the lateral tire force model yr to track the desired lateral force F of the rear wheels output by the reference model yrd , which is equivalent to calculating the desired lateral force F yrd The corresponding rear wheel sideslip angle α r ;

[0088] The rear wheel steering angle calculation module calculates the rear wheel steering angle according to the longitudinal vehicle speed v x , the desired yaw rate r output by the reference model d and the rear wheel sideslip angle α output by the integrator 2 r :

[0089]

[0090] So far, the rear wheel steering angle resolver has calculated the rear wheel steering angle δ yrd to achieve the desired lateral force F of the rear wheels r ;

[0091] Step six: Combine the reference model in step four and the rear wheel steering angle resolver in step five into a rear wheel active steering controller, as shown in Figure 2 , and it can be applied to the vehicle system, as shown in Figure 1 .

[0092] The control method of the present invention has the characteristics of an open-loop structure. It only requires the input of the front-wheel steering angle and the longitudinal vehicle speed, without the feedback of the yaw rate or the sideslip angle of the center of mass. It mainly consists of two main parts. One part is the reference model, which is used to calculate the front-wheel lateral force, the desired yaw rate, and the desired rear-wheel lateral force. The other part is the rear-wheel steering angle resolver, which is used to solve the rear-wheel steering angle. Among them, the reference model is derived from a two-degree-of-freedom model built based on the non-linear tire lateral force model. By setting the derivative of the lateral velocity in the two-degree-of-freedom model to 0, the dynamic equation about the desired yaw rate is derived. In the reference model, the front-wheel lateral force is calculated based on the front-wheel steering angle and used as the excitation of the dynamic equation of the desired yaw rate to obtain the dynamics of the desired yaw rate. At the same time, the desired rear-wheel lateral force is obtained by combining the front-wheel lateral force. The rear-wheel steering angle resolver calculates the corresponding rear-wheel sideslip angle based on the desired rear-wheel lateral force, and then combines it with the desired yaw rate to calculate the rear-wheel steering angle. The rear-wheel steering angle acts on the rear wheels of the actual vehicle, making the dynamics of the actual vehicle conform to the dynamics in the reference model, that is, the actual yaw rate is always equal to the desired yaw rate, and the actual sideslip angle of the center of mass is always equal to the desired sideslip angle, that is, it always remains at 0.

[0093] For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent conditions of the claims are intended to be included in the present invention. Any reference signs in the claims should not be regarded as limiting the claims involved.

[0094] In addition, it should be understood that although this specification is described in terms of embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A control method for rear-wheel active steering, characterized in that: It includes the following steps: Step 1: Obtain basic vehicle-road parameters including the vehicle mass M, the distance l from the center of gravity to the front axle f , the distance l from the center of gravity to the rear axle r , the yaw moment of inertia I zz and the road surface adhesion coefficient μ; Step 2: Establish a non-linear tire lateral force model F y = D sin[C arctan{Bα - E(Bα - arctan Bα)}] Among them, F y represents the lateral force of the tire, α represents the tire slip angle, and B, C, D, and E are parameters related to the road surface adhesion coefficient μ; Step 3: Establish a two-degree-of-freedom model where r represents yaw rate, v y represents lateral velocity, F yf represents the lateral force of the front wheels, F yr represents the lateral force of the rear wheels, a y represents lateral acceleration, v x represents longitudinal vehicle speed; Step 4: Establish a reference model Set to 0, we get: Combining the above formulas, the dynamic equation for the desired yaw rate is as follows: And the calculation equation for the desired rear-wheel lateral force is as follows: F yrd = Ma y - F yf Based on this, the reference model is established, including: Front wheel slip angle calculation module, based on the front wheel steering angle δ f , longitudinal vehicle speed v x and the desired yaw rate r output by integrator 1 d calculate the front wheel slip angle: Front wheel lateral force calculation module, according to the front wheel sideslip angle α output by the front wheel sideslip angle calculation module f , use the non-linear tire lateral force model to calculate the front wheel lateral force: F yf = D sin[C arctan{Bα f - E(Bα f - arctan Bα f )}] Desired yaw rate dynamics, based on the lateral force of the front wheels F output by the front wheel lateral force calculation module yf , longitudinal vehicle speed v x and the desired yaw rate r output by integrator 1 d , use the dynamic equation for the desired yaw rate to calculate the derivative of the desired yaw rate: Lateral acceleration calculation module, based on the longitudinal vehicle speed v x and the desired yaw rate r output by integrator 1 d calculate the lateral acceleration: a y = r d v x Desired rear wheel lateral force calculation module, based on the lateral acceleration a output by the lateral acceleration calculation module y and the front wheel lateral force F output by the front wheel lateral force calculation module yf , calculates the desired rear wheel lateral force using the calculation equation for the desired rear wheel lateral force: F yrd = Ma y - F yf Integrator 1 integrates the derivative of the desired yaw rate output from the dynamic output of the desired yaw rate and outputs the desired yaw rate r d ; Step 5: Design a rear-wheel steering angle resolver Including: Lateral tire force model, based on the rear wheel slip angle α output by the integrator 2 r , calculates the rear wheel lateral force using the non-linear tire lateral force model: F yr = D sin[C arctan{Bα r - E(Bα r - arctan Bα r )}] PI controller, the lateral force F of the rear wheels calculated according to the lateral tire force model yr and the desired lateral force F of the rear wheels output by the reference model yrd The lateral force deviation e of the rear wheels between them outputs the derivative of the rear wheel sideslip angle to the integrator 2, and jointly adjusts the output rear wheel sideslip angle α in combination with the integrator 2 r , Rear-wheel steering angle calculation module, based on the longitudinal vehicle speed v x , the desired yaw rate r output by the reference model d and the rear-wheel sideslip angle α output by integrator 2 r to calculate the rear-wheel steering angle: Step 6: Combine the reference model in Step 4 and the rear-wheel steering angle resolver in Step 5 into a rear-wheel active steering controller and apply it to the vehicle system.

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