Vehicle transfer robot stability control method

Abstract

The invention discloses a stability control method for a vehicle transfer robot. The method comprises the following steps: obtaining an ideal yaw velocity according to a longitudinal vehicle speed and a rear wheel rotation angle; according to the difference value between the ideal yaw velocity and the actual yaw velocity, an additional yaw moment and an additional rear wheel turning angle are respectively decided; according to the real-time stable state of the transfer robot, coordination control weights of the ARS control area and the DYC control area are distributed; a coordination control weight coefficient is decided, so that the required additional rear wheel turning angle and additional yawing moment output quantity are respectively calculated, and the additional yawing moment is distributed to each wheel; and the additional rear wheel steering angle and the additional yawing moment after coordination control weight coefficient distribution are distributed to a steering motor and hub motors of all wheels and are output. According to the invention, active steering and direct yawing moment coordination control and vehicle body posture adjustment under dangerous working conditions are realized through the coordination controller, so that the stability of the vehicle transfer robot under complex operation working conditions of ports and wharfs is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of vehicle stability control, and in particular relates to a stability control method for a vehicle transfer robot. Background Art

[0002] The vehicle transfer robot is one of the important achievements of the intelligent development of modern urban transportation, and its stability control is the key to ensure the safe operation of the vehicle transfer robot. In the process of transporting vehicles, the vehicle transfer robot needs to move accurately in a limited space, and the vehicle itself is heavy. Without good stability control, the robot may tilt, shake or even overturn during operation, which will not only cause damage to the vehicle, but also pose a threat to the surrounding environment and personnel safety. Through in-depth research on stability control, the vehicle transfer robot can maintain stable operation under various complex working conditions, effectively reduce safety risks, and provide reliable safety protection for vehicles and personnel. The stable operation state can ensure that the robot accurately performs parking tasks according to the preset path and speed, and reduce repeated adjustments and downtime caused by unstable factors. This can not only shorten the access time of the vehicle and improve the vehicle turnover rate of the parking lot, but also enhance the operating efficiency of the entire parking system and alleviate the problem of urban parking congestion. Furthermore, stability control helps to extend the service life of the vehicle transfer robot. Under stable operating conditions, the mechanical structure and drive system of the vehicle transfer robot are subject to less impact and wear, thereby reducing the loss rate of parts, reducing maintenance costs and equipment replacement frequency. This is of great significance for reducing the operating costs and maintenance costs of parking lots.

[0003] Traditional vehicle transfer equipment has certain limitations in stability control: traditional multi-axis transfer platforms mostly achieve steering control through mechanical linkage, the turning radius is easily limited and lacks the ability to adjust the height of independent wheel groups, the center of gravity is easily offset on bumpy roads, the power system is concentrated on the front axle, and the rear axle has no active control capability. The intelligent AGV transfer device uses a single-axis or dual-axis drive, and its load-bearing capacity is limited. Steering depends on the front wheel steering mechanism, and there is a trajectory following error under complex paths. The suspension system lacks an active compensation mechanism, and cargo is prone to tilt when driving on a slope. Hydraulic lifting transport equipment also has the problem of slow response speed of the lifting mechanism (≥500ms) and cannot adapt to dynamic road conditions in real time. Existing vehicle transfer equipment is mostly targeted at specific or good road conditions, and does not have a structure and corresponding control method for complex road conditions. Summary of the invention

[0004] In order to solve the above-mentioned problems existing in the prior art, the present invention provides a stability control method for a vehicle transfer robot, which performs stability analysis of the vehicle transfer robot under various complex working conditions in ports and terminals, and realizes active steering and direct yaw moment coordinated control and vehicle body posture adjustment under dangerous conditions through a coordinated controller, so as to improve the stability of the vehicle transfer robot under complex operating conditions in ports and terminals, and is particularly suitable for application scenarios of vehicle transfer in roll-on / roll-off ships in ports and terminals.

[0005] The objective of the present invention is achieved through the following technical solutions:

[0006] A vehicle transfer robot stability control method, characterized by comprising:

[0007] S1. Using the two-degree-of-freedom reference model, the ideal yaw rate is obtained according to the longitudinal vehicle speed and the rear wheel steering angle;

[0008] S2. According to the difference between the ideal yaw rate and the actual yaw rate, the sliding mode variable structure control algorithm is used to determine the additional yaw torque. and additional rear wheel turning angle ; Step S2 comprises:

[0009] S21. The actual yaw angular velocity and the ideal yaw rate The deviation is input to the direct yaw moment controller, and the sliding film variable structure control algorithm is used to determine the additional yaw moment ;

[0010] S22. The actual yaw angular velocity and the ideal yaw rate The deviation is input to the direct yaw moment controller, and the sliding mode control is used to determine the additional rear wheel steering angle , and then transmit the determined turning angle signal to the steering motor;

[0011] S3. The coordination controller determines the boundary of the stable area of ​​the transfer robot according to the real-time stable state of the transfer robot, and allocates the coordinated control weights of the ARS control area and the DYC control area; determines the suspension height adjustment state by detecting whether the roll angle of the transfer robot is greater than the safety threshold;

[0012] S4. The coordination controller determines the coordination control weight coefficient, and allocates the coordination control weight coefficient to the active rear wheel steering controller and the direct yaw moment controller, so that they respectively calculate the required additional rear wheel steering angle and additional yaw moment output, and use a dynamic allocation method based on quadratic programming to allocate the additional yaw moment to each wheel; the suspension height controller allocates each suspension height adjustment amount;

[0013] S5. distribute the additional rear wheel steering angle and additional yaw moment after coordinated control weight coefficient allocation to the steering motor and the wheel hub motors of each wheel and output them; distribute the suspension height adjustment amount allocated to each wheel to the suspension height adjustment motor of each wheel and output them.

[0014] Furthermore, the step S1 comprises:

[0015] From the vehicle's two-degree-of-freedom motion model, the state equation is obtained:

[0016]

[0017] Ideal yaw rate in steady-state steering conditions for:

[0018]

[0019] in, , +

[0020] Adhesion coefficient of road surface The maximum yaw rate satisfies:

[0021]

[0022] In summary, the ideal yaw rate of the vehicle is expressed as:

[0023]

[0024] In the formula, is a symbolic function; , are the cornering stiffness of the front and rear axles, respectively; , are the distances from the center of mass to the front axle, intermediate axle, and rear axle respectively; is the sideslip angle of the center of mass; is the yaw rate; u and v are the longitudinal and lateral vehicle speeds respectively; is the rear wheel turning angle; is the moment of inertia around the Z axis; is the vehicle mass; is the rear wheel turning angle; L is the wheelbase.

[0025] Furthermore, the step S21 includes:

[0026] Considering the effect of the additional yaw moment on the two-degree-of-freedom reference model, the state space equation is obtained:

[0027]

[0028] In the formula, , are the front and rear axle cornering stiffnesses, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, respectively. is the sideslip angle of the center of mass, is the yaw rate, u and v are the longitudinal and lateral speeds respectively, is the rear wheel turning angle, is the moment of inertia about the Z axis, is the additional yaw moment;

[0029] According to the above formula, we can get:

[0030] + +

[0031] The sliding film surface expression of the sliding mode controller is:

[0032]

[0033] In the formula, is the yaw rate deviation, is a positive weighting coefficient;

[0034] The derivative is:

[0035]

[0036] Further we get:

[0037] + +

[0038] Select an exponential approach rate:

[0039]

[0040] Where k>0, is the isokinetic approach term, The speed at which the system approaches stability;

[0041] Substituting the above control rate into the vehicle yaw motion model after adding the yaw moment, the additional yaw moment expression is obtained as:

[0042]

[0043] To reduce chattering, replace the sign function with a saturation function:

[0044]

[0045] In the formula, >0 is the boundary layer thickness;

[0046] Final additional yaw moment The control rate is:

[0047]

[0048] In the formula, .

[0049] Furthermore, the step S22 includes:

[0050] The sliding surface used for the additional rear wheel steering angle is defined as:

[0051]

[0052] In the formula, is the yaw rate deviation, is a positive weighting coefficient;

[0053] The derivative is:

[0054]

[0055] From the two-degree-of-freedom model state equation, we know:

[0056] +

[0057] In the formula, , are the front and rear axle cornering stiffnesses, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, respectively. is the sideslip angle of the center of mass, is the yaw rate, u and v are the longitudinal and lateral speeds respectively, is the rear wheel turning angle, is the moment of inertia around the Z axis;

[0058] Use constant approach rate:

[0059]

[0060] The larger the k value, the faster the approach speed and the greater the jitter generated;

[0061] Combining the above formulas, we get:

[0062]

[0063] Use a saturation function instead of a sign function:

[0064]

[0065] is the boundary thickness; the rear wheel steering angle output by the final sliding mode control is:

[0066]

[0067] At this time, the additional rear wheel turning angle It is the deviation between the rear wheel steering angle after the sliding film control output and the initial steering angle.

[0068] Furthermore, the step S3 comprises:

[0069] S31. Coordinate the controller to determine the stability area and divide the control area of ​​the transfer robot:

[0070] use The phase plane method is used to determine the vehicle stability of the coordinated controller. The phase plane divides the vehicle stability region by two straight lines symmetrical about the origin as boundary lines;

[0071] The boundary of the stable region is expressed as:

[0072]

[0073] In the formula, , is the boundary coefficient of the stable region;

[0074] When the vehicle's center of mass sideslip angle and the center of mass sideslip angle angular velocity satisfy the above formula, the vehicle is considered to be in a stable state;

[0075] exist The control areas of each controller in the phase plane are divided into: ARS control area, ARS and DYC coordinated control area, and DYC control area;

[0076] The boundary equation of the stable region divided according to each control area is:

[0077]

[0078] In the formula, , is the control boundary weight coefficient, 0< <1, ; The smaller it is, the smaller the area controlled by ARS alone; The larger it is, the larger the coordinated control area between ARS and DYC is;

[0079] S32. Detect whether the roll angle is greater than a threshold value and determine the suspension height adjustment state:

[0080] The IMU sensor is used to detect whether the roll angle and pitch angle of the transfer robot are greater than the set safety threshold, so as to determine whether the suspension height needs to be adjusted.

[0081] Furthermore, the step S4 comprises:

[0082] S41. The coordinated controller determines the coordinated control weight coefficient and calculates the required additional rear wheel steering angle and additional yaw moment output respectively;

[0083] S42. The additional yaw moment is distributed by a dynamic allocation method based on quadratic programming, and the torque difference between each wheel is controlled by the wheel hub motor to form an additional yaw moment about the center of mass;

[0084] S43. The height adjustment amount of each suspension is based on the tilting direction, and the suspension height controller allocates the corresponding suspension height lifting motor to increase or decrease the suspension height.

[0085] Furthermore, the step S41 includes:

[0086] The ARS control weight coefficient is defined as , the DYC control weight coefficient is ;

[0087] When ARS control is performed alone, ;

[0088] When DYC control is performed alone, ;

[0089] When ARS and DYC are coordinated, the sigmoid function is used to coordinate the control weight coefficient:

[0090]

[0091] The total expression of ARS control weight coefficient can be obtained:

[0092]

[0093] In the formula, , is the boundary coefficient of the stable region; , is the control area boundary weight coefficient, 0< <1, ;

[0094] The additional rear wheel steering angle after coordinated control allocation is obtained With additional yaw moment , as follows:

[0095]

[0096] The weighted additional yaw moment is obtained and the weighted additional rear wheel steering angle , through the coordinated control coefficient Realize coordinated control of direct yaw moment control and active rear wheel steering control.

[0097] Further, the step S42 includes:

[0098] Dynamic allocation takes the lowest comprehensive utilization rate of the six wheels or the maximum tire stability margin as the optimization objective function, and uses the quadratic programming algorithm to find the optimal solution;

[0099] The tire utilization expression is as follows:

[0100]

[0101] Where i=1,2,3,4,5,6, respectively representing the left and right wheels of the front axle, the left and right wheels of the intermediate axle, and the left and right wheels of the rear axle;

[0102] Without considering the influence of the tire lateral force, only the longitudinal force of the tire is considered, and the objective function is simplified to:

[0103]

[0104] The yaw moment output by the upper controller and the total longitudinal force of the vehicle and the motor torque should satisfy the following equality constraints:

[0105]

[0106] In the formula, is the tire rolling radius; , …is the torque of each wheel motor, , ...is the longitudinal force of each wheel, , fr, ml, mr, rl, rr are the left and right wheels of the front axle, the left and right wheels of the intermediate axle, and the left and right wheels of the rear axle respectively; d is the wheelbase;

[0107] The output torque of the drive motor is limited by the peak torque of the motor itself Affected by the road adhesion coefficient, the drive motor torque satisfies the following inequality constraints:

[0108]

[0109] Add yaw moment The formula for stability judgment is transformed into a quadratic programming problem for solution. The standard form of quadratic programming is:

[0110]

[0111]

[0112] In the formula, , ,

[0113] ,

[0114] The above quadratic programming problem is solved by the interior point algorithm to obtain the torque distribution results of each wheel.

[0115] Furthermore, the step S43 includes:

[0116] The suspension height of each wheel is adjusted in real time through the suspension height adjustment motor to control the body roll angle within a safe range;

[0117]

[0118] In the formula, is the difference between the body roll angle and the safety threshold, is the roll angle of the vehicle body, is the safety threshold of the roll angle;

[0119] The definition of roll angle is that the roll angle is positive when it is counterclockwise along the front direction of the vehicle and negative when it is clockwise. >0, the left suspension height is increased and the right suspension height is decreased; when When <0, the right suspension height is increased and the left suspension height is decreased;

[0120] Adjustment of suspension height:

[0121]

[0122] Where d is the wheelbase, is the suspension height adjustment amount;

[0123] The suspension height distribution controller receives the required suspension height adjustment amount from the suspension height controller After that, the height distribution of each wheel suspension is as follows:

[0124]

[0125] In the formula , They are the high-speed adjustment values ​​of the suspension for the front axle, intermediate axle and rear axle respectively; 1 and 2 represent the left and right wheels respectively.

[0126] The present invention has the following advantages:

[0127] The present invention provides a vehicle transfer robot stability control method, which takes into account the robustness of the control algorithm and adopts a sliding film algorithm to calculate an additional yaw moment and an additional rear wheel turning angle, thereby ensuring the effective operation of the stability control system during actual vehicle operation.

[0128] The present invention adopts hierarchical control of upper and lower layers: the upper layer controller is a coordination controller, which determines the boundary of the stable area of ​​the vehicle according to the real-time stable state of the vehicle, so as to reasonably allocate the coordination control weights of ARS and DYC, so that each controller can switch to the working area in time; the lower layer controller is the control system execution layer, which allocates the coordination control weight coefficient to the active rear wheel steering controller and the direct yaw moment controller according to the control weight output by the upper layer coordination controller, so that they can calculate the required additional rear wheel steering angle and additional yaw moment output respectively, so that the vehicle can be restored to a stable driving state. The controller computing power is saved and the control efficiency and control effect are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0129] 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 or the description of the prior art will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0130] Figure 1 This is a flowchart of a vehicle transfer robot stability control method according to an embodiment of the present invention;

[0131] Figure 2 Schematic diagram of the kinetic model used in the embodiments of the present invention;

[0132] Figure 3 is a schematic diagram of a two-degree-of-freedom reference model used in an embodiment of the present invention;

[0133] Figure 4 A schematic diagram of a tire mechanical property model used in an embodiment of the present invention;

[0134] Figure 5 Schematic diagram of the phase plane stable region in an embodiment of the present invention;

[0135] Figure 6 This is a schematic diagram of dividing the working areas of each controller into the phase plane stable area according to an embodiment of the present invention;

[0136] Figure 7 Schematic diagram of the vehicle body angle of the vehicle transfer robot when it is dangerously tilted in an embodiment of the present invention. DETAILED DESCRIPTION

[0137] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0138] The vehicle transfer robot involved in the present invention is driven by the power motor (with steering motor) of the rear axle to drive the front four axles and the car carried thereon to move forward. Since the front and rear plates are fixedly connected by die casting, the stability analysis can be appropriately simplified: the original five-axis structure is simplified to a three-axis structure, and the rear axle can be turned, which reduces the workload while ensuring the control effect. The vehicle transfer robot involved in the present invention is described as "vehicle" in the following. The present invention is described below in conjunction with the accompanying drawings.

[0139] like Figure 1 As shown, this embodiment is a vehicle transfer robot stability control method, comprising:

[0140] S1. Using the two-degree-of-freedom reference model, the ideal yaw rate is obtained based on the longitudinal vehicle speed and the rear wheel steering angle.

[0141] like Figure 2 As shown in the figure, the vehicle transfer robot model is simplified into a three-degree-of-freedom model, which is the longitudinal motion along the x-axis, the lateral motion along the y-axis, and the yaw motion around the z-axis. The simplified three-degree-of-freedom vehicle model can well reflect the lateral and longitudinal motion characteristics of the vehicle. The x-axis represents the longitudinal motion direction of the vehicle, the y-axis represents the lateral motion direction of the vehicle, and γ represents the yaw motion of the vehicle.

[0142] The longitudinal motion equation of the vehicle is:

[0143]

[0144] The lateral motion equation of the vehicle is:

[0145]

[0146] The equation of motion of the vehicle about the z-axis is:

[0147] - ) - ) - ) +( - ) ] +( + ) -( ) ) -( )

[0148] Among them, m is the vehicle mass, is the rear wheel turning angle, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, d is the wheelbase, u and v are the longitudinal and lateral speeds, respectively. is the yaw angular velocity, is the moment of inertia about the Z axis, , , Respectively represent the longitudinal forces of the tires on the front axle, intermediate axle, and rear axle (i=1, 2 represent the left and right wheels, respectively), , , Represent the lateral forces of the tires on the front axle, intermediate axle, and rear axle respectively.

[0149] The rubber viscoelastic structure of the tire itself leads to complex nonlinear mechanical properties when it moves under complex working conditions. Therefore, in vehicle dynamics control simulation, it is very important to establish a suitable tire model to analyze the mechanical properties of the tire. Figure 4 As shown, based on the Magic Formula tire model, the relationship between tire force and sideslip angle, slip rate and load can be known:

[0150] Considering the tire side slipping condition in a turn, ignoring the influence of the tire camber angle, the tire only slips in the lateral direction, and the expression of the Magic Formula tire lateral force mechanical characteristic curve when the tire is only subjected to lateral force can be obtained:

[0151]

[0152] The various factors satisfy the following relationship:

[0153]

[0154] In the formula, is the lateral force of the tire, are the curve shape factor and curvature factor, respectively. is the peak factor, is the stiffness factor, is the vertical force on the tire.

[0155] Considering that the tire is only subjected to longitudinal force during driving and the tire only slips in the longitudinal direction, the expression of the Magic Formula tire longitudinal force mechanical characteristic curve when the tire is only subjected to longitudinal force can be obtained:

[0156]

[0157] The various factors satisfy the following relationship:

[0158]

[0159] In the formula, is the longitudinal force of the tire, are the curve shape factor and curvature factor, respectively. is the peak factor, is the stiffness factor, is the tire vertical force, , , … are the tire fitting parameters obtained from tire test data. The tire fitting parameter values ​​in this embodiment are shown in the following table:

[0160]

[0161] During actual driving, the tires are simultaneously subjected to longitudinal force and lateral force. At this time, the relationship between tire force and sideslip angle, slip rate and load can be expressed by the following formula:

[0162]

[0163] Where: , , , is the longitudinal force, is the lateral force, is the slip angle, is the slip rate.

[0164] The tire slip rate s can be calculated by the following formula:

[0165]

[0166] In the formula, is the tire rolling radius, is the tire angular velocity, is the longitudinal speed.

[0167] Vertical load on tire It can be calculated by the following formula:

[0168]

[0169] In the formula, is the mass, g is the acceleration due to gravity; is the distance between the vehicle's center of mass and the ground, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, respectively. , , They represent the longitudinal forces of the tires on the front axle, intermediate axle, and rear axle respectively (i=1, 2 represent the left and right wheels respectively), d is the track width, and L is the wheelbase.

[0170] Since the vehicle transfer robot of the present invention has only the rear axle with an inputtable turning angle Here we can assume that the tire slip angles of the front axle and the intermediate axle are the same, so we can make appropriate simplifications, such as Figure 3 As shown, the two-degree-of-freedom motion model obtains the following equation:

[0171]

[0172] Written in the form of state equation:

[0173]

[0174] In the formula, , are the front and rear axle cornering stiffnesses, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, respectively. is the sideslip angle of the center of mass, is the yaw rate, u and v are the longitudinal and lateral speeds respectively, is the rear wheel turning angle, is the moment of inertia about the Z axis.

[0175] The ideal yaw rate in steady-state steering condition derived from the above formula is for:

[0176]

[0177] in, , +

[0178] Adhesion coefficient of road surface The maximum yaw rate is limited to:

[0179]

[0180] In summary, the ideal yaw rate of the vehicle is:

[0181]

[0182] In the formula, is a symbolic function.

[0183] S2. Input the difference between the ideal yaw rate and the actual yaw rate into the direct yaw moment controller and the active rear wheel steering controller, and use the sliding mode variable structure control algorithm to determine the additional yaw moment respectively. and additional rear wheel turning angle .

[0184] S21. The actual yaw rate and the ideal yaw rate The deviation is input to the direct yaw moment controller, and the sliding film variable structure control algorithm is used to determine the additional yaw moment .

[0185] Direct yaw moment control is to control the vehicle's yaw by adjusting the longitudinal forces of different wheels to form an additional yaw moment rotating around the center of mass. Therefore, the effect of the additional yaw moment on the two-degree-of-freedom reference model should be considered, and the state space equation should be modified as follows:

[0186]

[0187] In the formula, , are the front and rear axle cornering stiffnesses, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, respectively. is the sideslip angle of the center of mass, is the yaw rate, u and v are the longitudinal and lateral speeds respectively, is the rear wheel turning angle, is the moment of inertia about the Z axis, is the additional yaw moment.

[0188] According to the above formula, we can get:

[0189] + +

[0190] The sliding film surface expression of the sliding mode controller is:

[0191]

[0192] In the formula, is the yaw rate deviation, is a positive weighting coefficient;

[0193] The derivative is:

[0194]

[0195] Bring in:

[0196] + +

[0197] Select an exponential approach rate:

[0198]

[0199] Where k>0, is the isokinetic approach term, The speed at which the system approaches stability;

[0200] Substituting the above control rate into the vehicle yaw motion model after adding the yaw moment, the additional yaw moment expression is obtained as:

[0201]

[0202] To reduce chattering, replace the sign function with a saturation function:

[0203]

[0204] In the formula, >0 is the boundary layer thickness;

[0205] Final additional yaw moment The control rate is:

[0206]

[0207] In the formula, .

[0208] Stability is determined based on the Lyapunov function.

[0209] S22. The actual yaw rate and the ideal yaw rate The deviation is input to the direct yaw moment controller, and the sliding mode control is used to determine the additional rear wheel steering angle , and then transmits the determined turning angle signal to the steering motor to maintain vehicle stability.

[0210] Similar to the sliding film control used for the additional yaw moment, the sliding surface used for the additional rear wheel steering angle is defined as:

[0211]

[0212] In the formula, is the yaw rate deviation, is a positive weighting coefficient.

[0213] The derivative is:

[0214]

[0215] From the state equation of the two-degree-of-freedom model, we can know that:

[0216] +

[0217] In the formula, , are the front and rear axle cornering stiffnesses, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, respectively. is the sideslip angle of the center of mass, is the yaw rate, u and v are the longitudinal and lateral speeds respectively, is the rear wheel turning angle, is the moment of inertia about the Z axis.

[0218] Use constant approach rate:

[0219]

[0220] The larger the k value, the faster the approach speed and the greater the jitter generated.

[0221] Combining the above formulas, we can get:

[0222]

[0223] Similarly, the saturation function is used instead of the sign function:

[0224]

[0225] The boundary thickness is generally 0.05, and the rear wheel steering angle output by the final sliding mode control is:

[0226]

[0227] At this time, the additional rear wheel turning angle It is the deviation between the rear wheel steering angle after the sliding film control output and the initial steering angle.

[0228] S3. The coordination controller determines the stable area boundary of the transfer robot according to the real-time stable state of the transfer robot, and allocates the coordinated control weights of the ARS control area and the DYC control area; by detecting whether the roll angle of the transfer robot is greater than the safety threshold, the suspension height adjustment state is determined.

[0229] S31. Coordinate the controller to determine the stability area and divide the control area of ​​the transfer robot:

[0230] In order to give full play to the advantages of the active rear-wheel steering control system and the direct yaw moment control system, a coordinated controller is designed to transform the two control systems from separate control to integrated control, and coordinate the working tasks and working timing between the two control systems according to the stability of the vehicle to ensure that the vehicle can achieve optimal yaw stability control under different driving conditions.

[0231] 1) Stability determination

[0232] like Figure 5 As shown, this embodiment adopts The phase plane method is used to determine the vehicle stability of the coordinated controller. Phase plane, the vehicle stability region is divided by two straight lines symmetrical about the origin. These two straight lines are the boundary lines between the stable region and the unstable region, such as Figure 5 shown.

[0233] The boundary of the stable region can be expressed as:

[0234]

[0235] In the formula, , is the stable region boundary coefficient; specifically, is the slope of the stable region boundary, is the intercept of the stability region boundary with the axis.

[0236] When the vehicle's sideslip angle at the center of mass and the angular velocity of the sideslip angle at the center of mass satisfy the above formula, that is, the vehicle is in the stable area A between the two red straight lines in Figure 5, and the vehicle is considered to be in a stable state; otherwise, the vehicle is in the unstable area C outside the two straight lines, and the vehicle loses stability.

[0237] 2) Control area division

[0238] Active rear wheel steering control only works well in the linear working area of ​​the tire. DYC control intervention is required for nonlinear areas. The division of the phase plane stable area should also be consistent with the working area of ​​the tire. Figure 6 As shown, in The control areas of each controller in the phase plane are divided into: ARS control area, ARS and DYC coordinated control area, and DYC control area.

[0239] The road adhesion coefficient has the greatest impact on the change of the phase plane stable area. The boundary coefficient of the stable area can be obtained according to different road adhesion coefficients. , , the boundary equation of the stable region divided according to each control region is expressed as:

[0240]

[0241] In the formula, , is the control boundary weight coefficient, 0< <1, . The smaller it is, the smaller the area controlled by ARS alone; The larger the value is, the larger the coordinated control area between ARS and DYC is. , The solution can be taken as the longitudinal speed and front wheel angle as input, and the boundary weight coefficient , Specifically solve the fuzzy controller for the output.

[0242] S32. Detect whether the roll angle is greater than a threshold value and determine the suspension height adjustment state.

[0243] GB 7258-2017 stipulates that the stable roll angle of a passenger car body is no more than 35°. Combined with the specific analysis of the vehicle transfer robot in this embodiment, the safety threshold is set at 20°. The IMU sensor is used to detect whether the roll angle and pitch angle of the transfer robot are in a dangerous state (greater than the set safety threshold) to determine whether the height of each suspension needs to be adjusted. An instantaneous excessive roll angle generally occurs when the transfer robot passes through potholes, protrusions and other working conditions on the road. At this time, the active adjustment of the suspension height can maintain the body's stable posture and maintain vehicle stability.

[0244] S4. The coordination controller determines the coordination control weight coefficient and allocates the coordination control weight coefficient to the active rear-wheel steering controller and the direct yaw moment controller, so that they can calculate the required additional rear wheel steering angle and additional yaw moment output respectively, and use the dynamic allocation method based on quadratic programming to allocate the additional yaw moment to each wheel, so that the vehicle can be restored to a stable driving state; reasonably allocate the height adjustment amount of each suspension to restore the transfer robot to a stable state.

[0245] S41. The coordinated controller determines the coordinated control weight coefficient and calculates the required additional rear wheel steering angle and additional yaw moment output respectively:

[0246] like Figure 1 As shown, in this embodiment, the ARS control weight coefficient is defined as , the DYC control weight coefficient is ; When ARS control is performed alone, ; When DYC control is performed alone, ; When ARS and DYC are coordinated, the sigmoid function is used to coordinate the control weight coefficient:

[0247]

[0248] Therefore, the overall expression of the ARS control weight coefficient is as follows:

[0249]

[0250] In the formula, , is the stable region boundary coefficient; specifically, is the slope of the stable region boundary, is the intercept between the boundary of the stable region and the axis, , is the control area boundary weight coefficient, 0< <1, .

[0251] According to the above analysis, the additional rear wheel turning angle after coordinated control distribution can be obtained. With additional yaw moment , as follows:

[0252]

[0253] The weighted additional yaw moment is obtained and the weighted additional rear wheel steering angle , through the coordinated control coefficient Realize coordinated control of direct yaw moment control and active rear wheel steering control.

[0254] S42. The additional yaw moment is distributed by a dynamic distribution method based on quadratic programming, and the additional yaw moment about the center of mass is formed by controlling the torque difference between each wheel through the wheel hub motor:

[0255] Dynamic allocation is also called optimal allocation. It takes into account the mechanical characteristics of the tires and the constraints of the actuators, and dynamically adjusts the torque of each wheel to give full play to the longitudinal force of each wheel and improve the adhesion of the tire. Dynamic allocation considers the lowest comprehensive utilization rate of the six wheels or the maximum tire stability margin as the optimization objective function, and uses the quadratic programming algorithm for optimal solution. The expression of tire utilization is as follows:

[0256]

[0257] Wherein, i=1,2,3,4,5,6 represent the left and right wheels of the front axle, the left and right wheels of the intermediate axle, and the left and right wheels of the rear axle respectively.

[0258] In practice, the lateral force of the tire cannot be directly controlled due to conditions. Without considering the influence of the lateral force of the tire, only the longitudinal force of the tire is considered to improve the efficiency of the torque distribution controller, and the objective function is simplified to:

[0259]

[0260] The yaw moment output by the upper controller and the total longitudinal force of the vehicle and the motor torque should satisfy the following equality constraints:

[0261]

[0262] In the formula, is the tire rolling radius; , …is the torque of each wheel motor, , ...is the longitudinal force of each wheel, , fr, ml, mr, rl, rr represent the left and right wheels on the front axle, the left and right wheels on the middle axle, and the left and right wheels on the rear axle respectively; d is the wheelbase.

[0263] The output torque of the drive motor is limited by the peak torque of the motor itself. and road adhesion coefficient, select is 320N.m, that is, the drive motor torque should satisfy the following inequality constraints:

[0264]

[0265] Add yaw moment The formula for stability judgment is transformed into a quadratic programming problem for solution. The standard form of quadratic programming is:

[0266]

[0267]

[0268] In the formula, , ,

[0269] , .

[0270] The above quadratic programming problem can be solved by the interior point algorithm to obtain the torque distribution results of each wheel.

[0271] S43. The height adjustment amount of each suspension is based on the tilt direction, and the corresponding suspension height lifting motor is allocated by the suspension height controller to raise or lower the suspension height:

[0272] For the adjustment of suspension height, the coordination controller realizes the stability control of the vehicle by dividing the control area of ​​each controller in the stable area and coordinating the control weight coefficient. Under extreme working conditions, such as when passing through a large uneven road surface, the body roll angle increases instantly and exceeds the safety threshold of the body roll angle (the safety threshold is 20° in this embodiment). At this time, the vehicle may be in danger of rollover. The suspension height of each wheel is adjusted in real time through the suspension height adjustment motor, which is superior to the hydraulic adjustment speed, to control the body roll angle within a safe range.

[0273]

[0274] In the formula, is the difference between the body roll angle and the safety threshold, is the roll angle of the vehicle body, is the safety threshold of the roll angle°

[0275] The definition of roll angle is that the roll angle is positive when it is counterclockwise along the front of the vehicle and negative when it is clockwise. >0, the left suspension height is increased and the right suspension height is decreased; when When <0, the right suspension height is increased and the left suspension height is decreased.

[0276] like Figure 7 As shown, at this time <0, adjustment of suspension height:

[0277]

[0278] Where d is the wheelbase, is the suspension height adjustment amount.

[0279] The suspension height distribution controller receives the required suspension height adjustment amount from the suspension height controller After that, the height distribution of each wheel suspension is as follows:

[0280]

[0281] In the formula , They are the suspension high-speed adjustment values ​​for the front axle, intermediate axle, and rear axle (i=1, 2 represent the left and right wheels, respectively)

[0282] S5. distribute the additional rear wheel steering angle and additional yaw moment after coordinated control weight coefficient allocation to the steering motor and the wheel hub motors of each wheel and output them; distribute the suspension height adjustment amount allocated to each wheel to the suspension height adjustment motor of each wheel and output them.

[0283] This step is to match the additional rear wheel steering angle and the distributed additional yaw torque signal with the characteristics of the steering motor, wheel hub motor and suspension height adjustment motor, and convert them into corresponding electrical signals to control the output of the motor and achieve stability control.

Claims

1. A vehicle transfer robot stability control method, characterized in that: include: S1. Using the two-degree-of-freedom reference model, the ideal yaw rate is obtained according to the longitudinal vehicle speed and the rear wheel steering angle; S2. According to the difference between the ideal yaw rate and the actual yaw rate, the sliding mode variable structure control algorithm is used to determine the additional yaw moment and additional rear wheel turning angle ; Step S2 comprises: S21. The actual yaw rate and the ideal yaw rate The deviation is input to the direct yaw moment controller, and the sliding film variable structure control algorithm is used to determine the additional yaw moment ; S22. The actual yaw angular velocity and the ideal yaw rate The deviation is input to the direct yaw moment controller, and the sliding mode control is used to determine the additional rear wheel steering angle , and then transmit the determined turning angle signal to the steering motor; S3. The coordination controller determines the boundary of the stable area of ​​the transfer robot according to the real-time stable state of the transfer robot, and allocates the coordinated control weights of the ARS control area and the DYC control area; determines the suspension height adjustment state by detecting whether the roll angle of the transfer robot is greater than the safety threshold; S4. The coordination controller determines the coordination control weight coefficient, and allocates the coordination control weight coefficient to the active rear wheel steering controller and the direct yaw moment controller, so that they respectively calculate the required additional rear wheel steering angle and additional yaw moment output, and use a dynamic allocation method based on quadratic programming to allocate the additional yaw moment to each wheel; the suspension height controller allocates each suspension height adjustment amount; S5. distribute the additional rear wheel steering angle and additional yaw moment after coordinated control weight coefficient allocation to the steering motor and the wheel hub motors of each wheel and output them; distribute the suspension height adjustment amount allocated to each wheel to the suspension height adjustment motor of each wheel and output them.

2. A vehicle transfer robot stability control method as claimed in claim 1, characterized in that: The step S1 comprises: From the vehicle's two-degree-of-freedom motion model, the state equation is obtained: ； Ideal yaw rate in steady-state steering conditions for: ； in, , + ; Adhesion coefficient of road surface The maximum yaw rate satisfies: ； In summary, the ideal yaw rate of the vehicle is expressed as: ； In the formula, is a symbolic function; , are the cornering stiffness of the front and rear axles, respectively; , are the distances from the center of mass to the front axle, intermediate axle, and rear axle respectively; is the sideslip angle of the center of mass; is the yaw rate; u and v are the longitudinal and lateral vehicle speeds respectively; is the rear wheel turning angle; is the moment of inertia around the Z axis; is the vehicle mass; L is the wheelbase.

3. A vehicle transfer robot stability control method as claimed in claim 1, characterized in that: The step S21 comprises: Considering the effect of the additional yaw moment on the two-degree-of-freedom reference model, the state space equation is obtained: ； In the formula, , are the front and rear axle cornering stiffnesses, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, respectively. is the sideslip angle of the center of mass, is the yaw rate, u and v are the longitudinal and lateral speeds respectively, is the rear wheel turning angle, is the moment of inertia about the Z axis, is the additional yaw moment; According to the above formula, we can get: + + ； The sliding film surface expression of the sliding mode controller is: ； In the formula, is the yaw rate deviation, is a positive weighting coefficient; The derivative is: ； Further we get: + + ； Select an exponential approach rate: ； Where k>0, is the isokinetic approach term, The speed at which the system approaches stability; Substituting the above exponential approach rate expression into the vehicle yaw motion model after adding yaw moment, the additional yaw moment expression is obtained as follows: ； To reduce chattering, replace the sign function with a saturation function: ； In the formula, >0 is the boundary layer thickness; Final additional yaw moment The control rate is: ； In the formula, .

4. A vehicle transfer robot stability control method as claimed in claim 1, characterized in that: The step S22 comprises: The sliding surface used for the additional rear wheel steering angle is defined as: ; In the formula, is the yaw rate deviation, is a positive weighting coefficient; The derivative is: ; From the two-degree-of-freedom model state equation, we know: + ; In the formula, , are the front and rear axle cornering stiffnesses, , are the distances from the center of mass to the front axle, the middle axle, and the rear axle, respectively. is the sideslip angle of the center of mass, is the yaw rate, u and v are the longitudinal and lateral speeds respectively, is the rear wheel turning angle, is the moment of inertia around the Z axis; Use constant approach rate: ; The larger the k value, the faster the approach speed and the greater the jitter generated; Combining the above formulas, we get: ; Use a saturation function instead of a sign function: ; is the boundary thickness; the rear wheel steering angle output by the final sliding mode control is: ; At this time, the additional rear wheel turning angle It is the deviation between the rear wheel steering angle after the sliding film control output and the initial steering angle.

5. A vehicle transfer robot stability control method as claimed in claim 1, characterized in that: The step S3 comprises: S31. Coordinate the controller to determine the stability area and divide the control area of ​​the transfer robot: use The phase plane method is used to determine the vehicle stability of the coordinated controller. The phase plane divides the vehicle stability region by two straight lines symmetrical about the origin as boundary lines; The boundary of the stable region is expressed as: ; In the formula, , is the boundary coefficient of the stable region; When the vehicle's center of mass sideslip angle and the center of mass sideslip angle angular velocity satisfy the above formula, the vehicle is considered to be in a stable state; exist The control areas of each controller in the phase plane are divided into: ARS control area, ARS and DYC coordinated control area, and DYC control area; The boundary equation of the stable region divided according to each control area is: ; In the formula, , is the control boundary weight coefficient, 0< <1, ; The smaller it is, the smaller the area controlled by ARS alone; The larger it is, the larger the coordinated control area between ARS and DYC is; S32. Detect whether the roll angle is greater than a threshold value and determine the suspension height adjustment state: The IMU sensor is used to detect whether the roll angle and pitch angle of the transfer robot are greater than the set safety threshold, so as to determine whether the suspension height needs to be adjusted.

6. A vehicle transfer robot stability control method as claimed in claim 1, characterized in that: The step S4 comprises: S41. The coordinated controller determines the coordinated control weight coefficient and calculates the required additional rear wheel steering angle and additional yaw moment output respectively; S42. The additional yaw moment is distributed by a dynamic allocation method based on quadratic programming, and the torque difference between each wheel is controlled by the wheel hub motor to form an additional yaw moment about the center of mass; S43. The height adjustment amount of each suspension is based on the tilting direction, and the suspension height controller allocates the corresponding suspension height lifting motor to increase or decrease the suspension height.

7. A vehicle transfer robot stability control method as claimed in claim 6, characterized in that: The step S41 comprises: The ARS control weight coefficient is defined as , the DYC control weight coefficient is ; When ARS control is performed alone, ; When DYC control is performed alone, ; When ARS and DYC are coordinated, the sigmoid function is used to coordinate the control weight coefficient: ; The total expression of ARS control weight coefficient can be obtained: ; In the formula, , is the boundary coefficient of the stable region; , is the control area boundary weight coefficient, 0< <1, ; The additional rear wheel steering angle after coordinated control allocation is obtained With additional yaw moment , as follows: ; The weighted additional yaw moment is obtained and the weighted additional rear wheel steering angle , through the coordinated control coefficient Realize coordinated control of direct yaw moment control and active rear wheel steering control.

8. A vehicle transfer robot stability control method as claimed in claim 6, characterized in that: The step S42 comprises: Dynamic allocation takes the lowest comprehensive utilization rate of the six wheels or the maximum tire stability margin as the optimization objective function, and uses the quadratic programming algorithm to find the optimal solution; The tire utilization expression is as follows: ; Where i=1,2,3,4,5,6, respectively representing the left and right wheels of the front axle, the left and right wheels of the intermediate axle, and the left and right wheels of the rear axle; Without considering the influence of the tire lateral force, only the longitudinal force of the tire is considered, and the objective function is simplified to: ； The yaw moment output by the upper controller and the total longitudinal force of the vehicle and the motor torque should satisfy the following equality constraints: ； In the formula, is the tire rolling radius; , …is the torque of each wheel motor, , ...is the longitudinal force of each wheel, , fr, ml, mr, rl, rr are the left and right wheels of the front axle, the left and right wheels of the intermediate axle, and the left and right wheels of the rear axle respectively; d is the wheelbase; The output torque of the drive motor is limited by the peak torque of the motor itself Affected by the road adhesion coefficient, the drive motor torque satisfies the following inequality constraints: ； Add yaw moment The formula for stability judgment is transformed into a quadratic programming problem for solution. The standard form of quadratic programming is: ； ； In the formula, , , , ； The above quadratic programming problem is solved by the interior point algorithm to obtain the torque distribution results of each wheel.

9. A vehicle transfer robot stability control method as claimed in claim 6, characterized in that: The step S43 comprises: The suspension height of each wheel is adjusted in real time through the suspension height adjustment motor to control the body roll angle within a safe range; ； In the formula, is the difference between the body roll angle and the safety threshold, is the roll angle of the vehicle body, is the safety threshold of the roll angle; The definition of roll angle is that the roll angle is positive when it is counterclockwise along the front direction of the vehicle and negative when it is clockwise. >0, the left suspension height is increased and the right suspension height is decreased; when When <0, the right suspension height is increased and the left suspension height is decreased; Adjustment of suspension height: ； Where d is the wheelbase, is the suspension height adjustment amount; The suspension height distribution controller receives the required suspension height adjustment amount from the suspension height controller After that, the height distribution of each wheel suspension is as follows: ； In the formula, , They are the high-speed adjustment values ​​of the suspension for the front axle, intermediate axle and rear axle respectively; 1 and 2 represent the left and right wheels respectively.

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