CN109684777B - Electric formula car frame design method - Google Patents

Abstract

The invention specifically relates to a design method for the frame of an electric formula racing car. In the early stage of design, a human-machine test bench is built to obtain human-machine parameters, and the vehicle parameters are determined based on the overall layout of the vehicle. The system is calculated through suspension kinematics and dynamics simulation. Suspension hard point position, estimate the frame hard point position; use the frame parameters and hard point positions to build an initial frame model; optimize the frame model and perform finite element torsional stiffness analysis to obtain the ideal frame model; determine Carry out working condition analysis on the frame model to verify the structural strength and safety performance under different working conditions; after the working condition analysis, conduct constrained modal analysis on the model to obtain the modal parameters of the frame and verify whether resonance phenomena will occur. And based on the analysis results, the model was optimized to obtain the final frame model; the frame welding fixture was designed to complete the physical welding. The invention greatly reduces the frame design cycle, improves the rationality of the design, and enables the designed frame to better meet the performance of the entire vehicle.

Description

A design method for electric formula racing car frame

Technical field

The invention belongs to the technical fields of structural design and computer-aided engineering, and specifically relates to a design method of an electric formula racing car frame.

Background technique

The China Formula Electric Car Competition started in 2013 and is an expansion and supplement of the China Formula Car Competition. According to the technical rules issued by the competition organizing committee, a single-person racing small car was designed, manufactured and tuned within one year. Its biggest feature is that the design and manufacturing of the car must comply with the technical rules, including the main structure of the frame. , battery box design, module power, motor power and other important parameters to ensure the safety of the car and the fairness of the event.

As the largest structural component independently designed and manufactured by the Formula E racing car, the frame of the formula racing car is the carrier of each assembly of the vehicle and plays an important role in arranging the overall layout and connecting the assemblies. The design of the frame takes high strength, high stiffness, comfort and lightweight as the design goals. At present, formula racing car frames mostly adopt a steel tube truss frame structure, which is processed and welded by steel tubes. There are also a few schools that use carbon fiber monocoque designs. During the design process, there are problems such as non-standard design process and unreasonable theoretical application. There is no overall design consideration and most of them are subjective and improvised, resulting in a long design and processing cycle and many problems. The most important thing is that the frame design cannot meet the requirements. Vehicle performance requirements.

Contents of the invention

In view of the above-mentioned existing problems, the present invention proposes a frame design method for a formula electric racing car to solve the problem of irregular and unreasonable frame design of a formula electric racing car.

In order to achieve the above objects, the specific technical solutions of the present invention are as follows: a design method for a formula electric racing car frame, including the following steps:

1) Determine the frame parameters and the position of the frame’s hard points;

2) Build the initial frame model T using human-machine parameters, vehicle parameters, and frame hard point positions;

3) Add auxiliary rods to the initial frame model T, adjust the rod dimensions, and obtain the adjusted frame model T’, where the rod dimensions include pipe diameter and wall thickness;

4) Use finite element analysis to analyze the torsional stiffness of the frame model T’ and obtain the ideal frame model T1;

5) Perform working condition analysis on the ideal frame model T1 to obtain the ideal safe frame model T2;

6) Conduct finite element modal analysis on the ideal safety frame model T2 to obtain the final frame model T3;

7) Design the welding fixture for the final frame model T3.

Further, the specific method for determining the frame parameters and frame hard point positions in step 1) above is as follows:

1.1) Carry out human-machine experiments in the early stage of design, build a human-machine experiment platform, use the simulator operation performance and the driver's subjective feelings as the evaluation criteria, and use the driver's human-machine database as an objective reference to obtain human-machine parameters, in which the human-machine parameters include : Distance d ₁ between the main ring and the front ring, height h ₁ of the main ring, height h ₂ of the front ring, distance d ₂ between the brake accelerator pedal and the main ring, inclination angle γ of the brake accelerator pedal, height h ₃ of the steering wheel center point And the distance d ₂ from the main ring, the cockpit width l ₁ ;

1.2) Determine the front and rear load ratio k;

1.3) Calculate the vehicle wheelbase L;

1.4) Determine the wheelbase L _s ;

1.5) Determine the position of the suspension hard point, and use the suspension positioning parameters to perform kinematics and dynamics simulations to determine the position of the suspension hard point. The positioning parameters include camber angle α ₁ , inclination angle α ₂ , caster angle α ₃ , and toe-in α ₄ ;

1.6) Determine the position of the hard point of the frame. The position of the hard point of the frame is obtained based on the position of the hard point of the suspension and the relative position of the hard point of the frame and the hard point of the suspension. The relative amount is determined by the size of the lifting lugs connecting the suspension and the frame. .

Further, the specific method for obtaining the ideal frame model T1 in step 4) above is as follows:

4.1) Use finite element to calculate the torsional stiffness value K _n of the frame model T'. The calculation formula is as follows:

In the formula, K _n is the torsional stiffness value calculated by each model, F is the support reaction force, d is the distance between the two hard points of the suspension, ΔA and ΔB are the forced displacements of the two hard points;

4.2) Calculate the comprehensive frame torsional stiffness K of the frame model T’. The calculation formula is as follows:

In the formula, K _f is the torsional stiffness of the front suspension under torsion, K _r is the torsional stiffness of the rear suspension under torsion, a is the front wheelbase, b is the rear wheelbase, and L is the wheelbase;

4.3) Determine whether K>K ₀ and m<m ₀ are satisfied. If satisfied, go to step 4.4), otherwise go to step 3);

4.4) Determine whether it is satisfied If satisfied, the ideal frame model T1 is obtained, otherwise go to step 3).

Further, the working condition analysis of the ideal frame model T1 in step 5) above includes the following steps:

5.1) Straight-line working condition analysis;

5.2) Analysis of uniform speed cornering conditions;

5.3) Accelerate working condition analysis;

5.4) Analysis of braking conditions.

Further, in step 6) above, finite element modal analysis is performed on the ideal safety frame model T2, and the specific method to obtain the final frame model T3 is as follows:

6.1) Fixed and constrained 8 hard points on the front and rear suspension of the frame;

6.2) Apply inertial force load to the frame;

6.3) Carry out simulation analysis to obtain the first 8 mode frequencies and vibration shapes;

6.4) Compare the obtained 8th-order modal frequency with the external excitation frequency of the motor and other components, and observe whether the frequencies coincide with resonance. If resonance occurs, return to step 3) to change the size of the members; otherwise, the final frame model T3 is obtained. .

Further, the specific method for calculating the vehicle wheelbase L in step 1.3) above is as follows:

1.3.1) Calculate the vehicle front axle load M _tf , the formula is as follows:

In the formula, M _i is the mass of each main component, _Xi is the x-axis distance of each main component relative to the center of mass, L is the wheelbase, and N represents the number of components;

1.3.2) Calculate the vehicle rear axle load M _tr , the formula method is as follows:

M _tr =M ₀ -M _tf

In the formula, M ₀ is the mass of the vehicle (when the driver is fully loaded);

1.3.3) Calculate the vehicle wheelbase L, the formula is as follows:

Further, the straight-line working condition analysis in step 5.1) above includes the following steps:

5.1.1) Four hard points of the rear suspension are fixed and constrained. The left hard point y of the front suspension is constrained in the z direction and releases the degree of freedom in the x direction. The right hard point y is constrained in the z direction and releases the degree of freedom in the x direction;

5.1.2) Apply a load to the frame, where the load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame, and the load size is the respective weight multiplied by the dynamic load factor;

5.1.3) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under linear working conditions;

5.1.4) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than ξ ₀ , increase the wall thickness and pipe diameter of the rod in the unsafe position , go to step 3), otherwise it means that the safety requirements are met under straight-line operating conditions, go to step 5.2).

Further, the analysis of uniform speed cornering conditions in step 5.2) above includes the following steps:

5.2.1) Fixed constraints test all hard points inside, restrict the degree of freedom in the z direction on the outside, and release the degree of freedom in the xy direction;

5.2.2) Calculate the maximum centripetal acceleration a _rmax . The calculation formula is as follows:

In the formula, a _rmax is the maximum centripetal acceleration, r is the turning radius, and t _min is the fastest lap time;

5.2.3) Apply an inertial force load to the frame. The load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame. The load size is the product of the inertial force of the component and the maximum centripetal acceleration. , in which the inertial force includes the self-weight of the vehicle frame, the inertial force of the driver, the inertial force of the electric drive system, and the inertial force of the battery box;

5.2.4) Apply secondary forces to the frame, which come from the rear suspension triangular rocker arm, rear suspension spring and front suspension triangular rocker arm;

5.2.5) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under uniform speed cornering conditions;

5.2.6) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than ξ ₀ , increase the wall thickness of the rod and pipe at the unsafe position. diameter, go to step 3), otherwise the safety requirements are met under constant speed cornering conditions, go to step 5.3).

Further, the acceleration working condition analysis in step 5.3) above includes the following steps:

5.3.1) Fixed and constrained the four hard points of the rear suspension, restricted the four hard points of the front suspension in the yz direction, and released the degree of freedom in the x direction;

5.3.2) Calculate the maximum acceleration a _max required for acceleration conditions. The calculation formula is as follows:

In the formula, S is the distance covered by straight-line acceleration, t is the acceleration time, q is the classical coefficient, and the simplified model is equal to a _max ;

5.3.3) Apply an inertial force load to the frame, where the load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame. The load size is the product of the inertial force of the component and the maximum acceleration, where The inertial force includes the self-weight of the vehicle frame, the inertial force of the driver, the inertial force of the electric drive system, and the inertial force of the battery box;

5.3.4) Apply secondary forces to the frame, which come from the rear suspension triangular rocker arm, rear suspension spring and transmission bracket;

5.3.5) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under accelerated conditions;

5.3.6) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than ξ ₀ , increase the wall thickness of the rod and pipe at the unsafe position. diameter, go to step 3), otherwise the safety requirements are met under accelerated conditions, go to step 5.4).

Further, the braking condition analysis in step 5.4) above includes the following steps:

5.4.1) Fixed constraints on all hard points, that is, four-wheel lock state;

5.4.2) Calculate the braking deceleration a _z required for braking conditions. The calculation formula is as follows:

In the formula, v ₀ ² is the starting braking speed, and Z is the braking distance;

5.4.3) Apply an inertial force load to the frame. The load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame. The load size is the product of the inertial force of the component and the braking deceleration. , in which the inertial force includes the self-weight of the vehicle frame, the inertial force of the driver, the inertial force of the electric drive system, and the inertial force of the battery box;

5.4.4) Apply secondary forces to the frame, which come from the rear suspension triangular rocker arm, rear suspension spring and front suspension triangular rocker arm;

5.4.5) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under braking conditions;

5.4.6) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than ξ ₀ , increase the wall thickness and pipe diameter of the rod in the unsafe position , go to step 3), otherwise the safety requirements are met under acceleration conditions, and the ideal safety frame model T2 is obtained.

Compared with the existing technology, this invention provides a set of rigorous and reasonable processes and theoretical methods for the design of the electric formula racing car frame starting from the early ergonomic experiments and ending with the design of the frame fixture model, which greatly reduces the time required for frame design. cycle, improves the performance of the frame, supplements and improves the current theoretical research on FSEC frames, and is practical.

Description of the drawings

Figure 1 Flowchart of the design method of the electric formula racing car frame.

Figure 2 Schematic diagram of the frame model and hard point locations.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. It should be pointed out that the technical solution and design principles of the present invention are described in detail below only with an optimized technical solution, but the protection scope of the present invention does not Limited to this.

The above-described embodiments are preferred implementations of the present invention, but the present invention is not limited to the above-described implementations. Without departing from the essence of the present invention, any obvious improvements, substitutions or modifications that can be made by those skilled in the art can be made without departing from the essence of the present invention. All modifications belong to the protection scope of the present invention.

Figure 1 shows the flow chart of the frame design method for an electric formula racing car; it includes the following steps:

1) Determine the frame parameters and the position of the frame hard points. The frame parameters include human-machine parameters and vehicle parameters. Among them, the human-machine parameters include: the distance d ₁ between the main rings and the front rings, the height h ₁ of the main rings, the front Ring height h ₂ , distance d ₂ between the brake accelerator pedal and the main ring, brake accelerator pedal inclination angle γ, steering wheel center point height h ₃ and distance d ₂ from the main ring, cockpit width l ₁ , the vehicle parameters include the vehicle Wheelbase L, wheelbase L _s and front-to-rear load ratio, the specific method is as follows:

1.1) Carry out human-machine experiments in the early stage of design, build a human-machine experiment platform, use the simulator operation performance and the driver's subjective feelings as the evaluation criteria, and use the driver's human-machine database as an objective reference to obtain human-machine parameters. The human-machine parameters include : Distance d ₁ between the main ring and the front ring, height h ₁ of the main ring, height h ₂ of the front ring, distance d ₂ between the brake accelerator pedal and the main ring, inclination angle γ of the brake accelerator pedal, height h ₃ of the steering wheel center point And the distance d ₂ from the main ring, the cockpit width l ₁ ;

1.2) Determine the front and rear load ratio k. In this embodiment, the load ratio k is 48:52. When the front and rear load ratio of the vehicle is maintained at 48:52, the vehicle will have good handling stability.

1.3) Calculate the wheelbase L of the vehicle. The wheelbase is determined according to the load of the vehicle. The lowest point of the seat is the human-machine H point, which is the x-direction position of the center of mass of the vehicle. The specific calculation method is as follows:

1.3.1) Calculate the vehicle front axle load M _tf as follows:

In the formula, M _i is the mass of each main component, _Xi is the x-axis distance of each main component relative to the center of mass, L is the wheelbase, and N represents the number of components;

1.3.2) Calculate the vehicle rear axle load M _tr as follows:

M _tr =M ₀ -M _tf

In the formula, M ₀ is the mass of the vehicle (when the driver is fully loaded);

1.3.3) Calculate the wheelbase L of the vehicle as follows:

In this embodiment, taking into account the requirements of FSEC competition rules, the wheelbase of the racing car is at least 1525mm. The wheelbase of the racing car is large, and the mass of the frame is bound to increase. Under the idea of lightweighting, the whole vehicle is arranged as compactly as possible, and the axle is initially determined. distance 1570mm;

1.4) Determine the wheelbase L _s ; the wheelbase is determined according to the rules. The smaller wheelbase is not less than 75% of the larger wheelbase. The wheelbase has a great impact on the total mass, total size, and handling stability of the car. As the wheelbase increases, the stiffness of the suspension roll angle increases, and the mechanical and kinematic performance of the suspension becomes better, which is also more beneficial to the layout of the rear cabin. However, it should not be too large, otherwise it will lead to redundant space and unreasonable increase in mass. In this embodiment, the front wheelbase is selected to be 1210mm and the rear wheelbase is 1180mm;

1.5) Determine the position of the suspension hard point. Use the suspension positioning parameters to perform kinematics and dynamics simulations to determine the position of the suspension hard point. The positioning parameters mainly include camber angle α ₁ , inclination angle α ₂ , caster angle α ₃ , and toe-in. α ₄ etc.;

1.6) Determine the position of the hard point of the frame, and obtain the position of the hard point of the frame according to the position of the hard point of the suspension and the relative position of the hard point of the frame and the hard point of the suspension; in this embodiment, the position of the hard point of the frame and the position of the hard point of the suspension The relative quantity range is: x direction (-10, 20), y direction (30, 40), z direction (-5, 5), and the hard point position is shown in Figure 2;

2) Build the initial frame model T using human-machine parameters, vehicle parameters, and frame hard point positions;

According to the distance d ₁ between the main ring and the front ring, the height h ₁ of the main ring, the height h ₂ of the front ring, the distance d ₂ between the brake and accelerator pedals and the main ring, the inclination angle γ of the brake accelerator pedal, and the height h of the steering wheel center point ₃ and the distance d ₂ from the main ring, the cockpit width l ₁ , the wheelbase L, the wheelbase L _s and the position of the hard points of the frame are used to build the model. The hard points are connected with rods and connected to the front ring of the main ring and the front ring. At the partition position; the frame model includes leg compartments, seat compartments and rear compartments; the leg compartment structure is the frame part before the front ring, providing operating space for the driver's legs and feet. Specifically, there are front bulkheads, front bulkhead support rods, front rings, front ring support rods and other auxiliary rods. The braking system, steering system and front suspension system are integrated in the leg compartment. Rods are set at the bottom of the leg compartment to carry the steering. Assembling, the lifting lugs are welded on the front bulkhead to fix the brake cylinder. On the basis of meeting the rules of 915 in the regulations, to further save space, two parallel steel pipes in the x direction at the bottom are lap-welded to support the brake floor. In order to better support the frame at the shock absorber, a crossbar is added to the upper part of the leg well as a support. As for the front ring, in order to take care of the driver's driving vision, the height of the front ring should be reduced as much as possible, but the height must be ensured to meet the height of the leg compartment rule plate and the steering wheel does not exceed the height of the front ring when the steering wheel is turned down; the cockpit refers to the frame part between the main ring and the front ring. In the cabin design, the side anti-collision structure is designed according to the classic triangular structure required by the rules. The upper part of the side bar is added as a rod that fits the car body, so that the driver can press the car body on both sides of the driver to exit when escaping. The cabin requires that the distance between the body side box and the upper part of the upper side anti-collision bar should not exceed 10mm; the rear cabin is the part of the frame behind the main ring, which provides an installation environment for the battery box, motor transmission assembly and electronic control system. The design of the rear cabin requires a moderate distance in the y direction to meet the installation and operation space of the battery box. Two 4mm thick steel bars are welded to the bottom of the rear cabin as a support for the battery box. The battery box is mechanically connected to the battery box through 10 corner codes. On the frame. The relevant rod structure is arranged at the rear to provide welding positions for the differential bracket and the motor;

3) Add auxiliary rods to the initial frame model T, adjust the dimensions of the rods, including pipe diameter and wall thickness, and obtain the adjusted frame model T’;

4) Use finite element analysis to analyze the torsional stiffness of the frame model to obtain the ideal frame model T1; the torsional stiffness of the frame is an important item in the frame evaluation index. The torsional stiffness of the frame refers to the torsional stiffness of the frame when it is subjected to vertical loads. Depending on the degree of torsional deformation, the frame model can be simplified into a simply supported beam, with the fulcrums being the hard points of the front and rear suspension frame. The frame and suspension are matched. During the movement, if the frame stiffness is lower than the suspension roll angle stiffness, the frame will be torsion, causing safety risks to the frame structure and reducing the vehicle's handling stability. Therefore The actual torsional stiffness of the frame is required to be higher than the roll angle stiffness of the suspension, and there must be enough margin. The specific methods are:

4.1) Use finite element to calculate the torsional stiffness value K _n of the adjusted frame model T'; the analysis method is to constrain the hard points of one of the front and rear suspensions in the finite element analysis software, and impose a forced displacement of 1mm up and down on the other side. The support reaction force F is obtained, and the torsional stiffness of the frame is calculated from this. The torsional stiffness value calculated by each model is calculated as follows:

In the formula, K _n is the torsional stiffness value calculated by each model, F is the support reaction force, d is the distance between the two hard points of the suspension, and ΔA and ΔB are the forced displacements of the two hard points.

4.2) Calculate the comprehensive frame torsional stiffness K of the adjusted frame model T’. K is jointly derived from the torsional stiffness under the front and rear suspension constraints. The specific formula is as follows:

In the formula, K _f is the torsional stiffness under torsion of the front suspension, K _r is the torsional stiffness of the rear suspension under torsion, a is the front wheelbase, b is the rear wheelbase, and L is the wheelbase.

In the specific implementation, the material is defined in the finite element software, the model is imported, the pipe fittings are assigned, and the constraint conditions are applied. First, the torsional stiffness value under torsion of the front suspension is calculated, and the four hard points of the upper, lower, left and right rear suspension are fixed and constrained. To limit the degree of freedom, impose a forced displacement of 1 mm on each of the upper and lower hard points of the front suspension, calculate the support reaction force, and enter it into the formula to calculate K _f . In this embodiment, d ₁ =404 mm, through simulation The maximum support reaction force F = 1632.55N is obtained, and the calculated K _f = 2325.306N·m/deg; then the torsional stiffness value under torsion of the rear suspension is calculated, and the four hard points on the upper, lower, left and right front of the front suspension are fixed and restrained. Limit its degree of freedom, apply a forced displacement of 1 mm each to the upper and lower hard points behind the rear suspension, calculate the support reaction force, and enter it into the formula to calculate K _r . In this embodiment, d ₂ = 506 mm, through simulation The maximum support reaction force F = 1269.5N, K _r = 2836.5N·m/deg, is entered into formula (4) to obtain the total stiffness K. In this embodiment, a = 753.6mm, b = 816.4mm,

4.3) Determine whether K>K ₀ and m<m ₀ are satisfied. If satisfied, go to step 4.4), otherwise go to step 3). In this embodiment, K ₀ =2000N·m/deg, m ₀ =28kg;

4.4) Calculate torsional stiffness per unit mass

4.5) Determine whether it is satisfied If the ideal frame model T1 is satisfied, otherwise go to step 3). In this embodiment, p ₀ =81N·m/(kg·deg);

5) Conduct working condition analysis on the ideal frame model T1 to obtain the ideal safe frame model T2; check the strength of the frame under various typical working conditions, and optimize and strengthen the weak parts of the frame to ensure that it meets the requirements of racing Requirements during driving; among them, working conditions include straight-line working conditions, uniform speed cornering conditions, acceleration working conditions and braking conditions.

5.1) The main consideration for straight-line conditions is the strength and bending conditions of the car under full load conditions. In this case, in addition to bearing its own gravity, the frame also has to bear the gravity of the battery box, motor transmission system, steering system and other devices as well as the rider, so the frame is required to have high strength and stiffness. It is assumed here that the quality of other small parts does not have much impact on this working condition and can be ignored. The simulation analysis method of the straight line working condition is as follows:

5.1.1) Fix and constrain 4 hard points on the rear suspension. The hard point locations are shown in Figure 2. Considering the approximate linear motion of the racing car, the hard point on the left side of the front suspension is y, constrained in the z direction, and the degree of freedom in the x direction is released. On the right side Hard point y, z direction constraint, release x direction freedom.

5.1.2) Apply a load to the frame. The load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame. The load size is the weight of each multiplied by the dynamic load factor. The dynamic load factor in the specific implementation 2. In addition, there are other main components such as gravity and motor torque; in this embodiment, 5 modules are arranged in the battery box, and each module uses 18650 ternary lithium batteries, 7 in parallel and 20 in series, plus Various electrical components, the total weight of the battery box is conservatively estimated to be 55Kg, and its force acts on the two 4mm thick steel bars at the bottom of the battery box. The motor is a 228 motor, plus the planetary gear reduction mechanism, sprocket and differential. The electric drive assembly weighs 34Kg. The force is exerted on the steel pipe welded by the relevant lifting lugs. The driver weighs 70Kg. Multiplied by the dynamic load factor, the effect On the steel pipe welded to the seat lifting lugs, the rest is the weight of the frame and the motor torque. The motor torque is 240N·m, which acts on the motor bracket;

5.1.3) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under linear working conditions;

5.1.4) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than 1.8, go to step 3) and add the member wall at the unsafe position. Thickness and pipe diameter; otherwise, it means that the requirements are met under straight-line conditions, and go to step 5.2);

5.2) The main considerations in the uniform speed cornering condition are the main effects on the frame truss structure in the lateral direction under the maximum centripetal acceleration cornering condition, including the centrifugal acceleration inertia force of the frame and the inertia of each major large-mass component. Force, the same as the straight line condition, the inertial force is realized by multiplying the dynamic load factor. It is assumed that the working condition is a left turn, and the influence of other small components is ignored. The force of the maximum centrifugal acceleration mainly acts on the frame through the suspension wishbone, the lateral force is the main force, and the lateral displacement is the main constraint. When analyzing simulation calculations, the specific operations are as follows:

5.2.1) The fixed constraint tests all hard points inside, restricts the degree of freedom in the z direction on the outside, that is, the right side, and releases the degree of freedom in the xy direction.

5.2.2) Calculate the maximum centripetal acceleration a _rmax . The calculation formula is as follows;

In the formula, a _rmax is the maximum centripetal acceleration, r is the turning radius, t _min is the fastest lap time, and after calculation, a _rmax =18m/s ² ;

5.2.3) Apply an inertial force load to the frame. The load calculation method is the product of the inertial force of each component and the maximum lateral acceleration. The inertial forces include the self-weight of the frame, the driver's inertial force, the inertial force of the electric drive system, and the inertia of the battery box. force, in addition to other main components such as gravity and motor torque; in this embodiment, the driver's inertia force load is 70×2×1.8g=2520N, and the electric drive system's inertia force load is 34×2×1.8g= 1224N, the inertia force load of the battery box is 55×2×1.8g=1980N, in addition to the main component gravity of 3060N, and the motor torque of 240N·m;

5.2.4) Secondary forces are exerted on the frame, respectively from the rear suspension triangular rocker arm, the rear suspension spring and the front suspension triangular rocker arm; in this embodiment, the force situation of the rear suspension triangular rocker arm is F _x =0, F _y = 432.54N, F _z = -212.94N; the force of the rear suspension spring is F _x = 366.26N, F _y = 2168N, F _z = -935.25N; the force of the front suspension triangular rocker arm is F _x = 0, F _y =600.05N, F _z =1610.2N;

5.2.5) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under uniform speed cornering conditions;

5.2.6) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than 1.8, go to step 3) and add the member wall at the unsafe position. Thickness and pipe diameter; otherwise, it means that the requirements are met under uniform speed cornering conditions, go to step 5.3);

5.3) The above-mentioned acceleration conditions mainly consider the stress of the frame under the maximum acceleration of the racing car in a straight line. The acceleration inertia force of the frame and the inertia force of each major large-mass component act in the longitudinal direction. At the maximum When accelerating, the inertial force of the frame is balanced with the longitudinal force of the suspension system acting on the frame, and this is used as the basis for calibration. It is assumed that other small mass components have little impact on the overall effect. The force of maximum linear acceleration mainly acts on the frame through the suspension wishbone, the longitudinal force is the main force, and the longitudinal displacement is the main constraint. When analyzing simulation calculations, the specific operations are as follows:

5.3.1) Fixed and constrained the four hard points of the rear suspension, restricted the four hard points of the front suspension in the yz direction, and released the degree of freedom in the x direction.

5.3.2) Calculate the maximum acceleration a _max required for acceleration conditions. The calculation formula is as follows:

_amax = _qaavg

In the formula, S is the distance covered by straight-line acceleration, t is the acceleration time, a _avg is the average acceleration during the acceleration process, q is the classic coefficient, and the simplified model is equal to a _max ; in this embodiment, q is 1.5, and the straight-line acceleration time Taking 4.33s, S taking 75m, the calculation shows a _max = 1.2g;

5.3.2) Apply an inertial force load to the frame. The load calculation method is the product of the inertial force of each component and the maximum lateral acceleration. The inertial force includes the self-weight of the frame, the driver's inertial force, the inertial force of the electric drive system, and the battery box inertial force. , in addition to the other main components gravity and motor torque; in this embodiment, the driver's inertial force load is 70×2×1.2g=1680N, and the electric drive system's inertial force load is 34×2×1.2g=816N , the inertia force load of the battery box is 55×2×1.2g=1320N. In addition, the gravity of the main components is 3060N and the motor torque is 240N·m;

5.3.4) Secondary forces are exerted on the frame, respectively from the rear suspension triangular rocker arm, rear suspension spring and transmission bracket; in this embodiment, the force situation of the rear suspension triangular rocker arm is F _x = 0, F _y = 324.06N, F _z =-159.54N; the force of the rear suspension spring is F _x =274.4N, F _y =1624.2N, F _z =-700.68N; the force of the upper left bracket of the transmission is F _x =2784.4N, F _y = 0, F _z = -5071.34N; the stress of the upper right bracket of the transmission is F _x = -393.8N, F _y = 0, F _z = 717.23N; the stress of the lower left bracket of the transmission is F _x = -5188.8N, F _y =0, F _z =-6693.5N; the stress of the lower right bracket of the transmission is F _x =733.8N, F _y =0, F _z =946.6N;

5.3.5) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under accelerated conditions;

5.3.6) Compare the simulation results with the material yield strength σ _s and other mechanical properties, and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than 1.8, go to step 3) and add the member wall at the unsafe position. Thickness and pipe diameter; otherwise, it means that the requirements are met under accelerated working conditions, go to step 5.4);

5.4) Under the above braking conditions, the inertial force of the frame truss and the force of the suspension system acting on the frame are in a balanced state. This is used as a theoretical basis for calibration. The longitudinal direction acts on the deceleration inertia of the frame. force and the inertial force of each main large-mass component. It is assumed that the effect of other small-mass components is very small, and the inertial force of the small-mass component has little effect. The force of maximum linear deceleration mainly acts on the frame through the suspension wishbone, the longitudinal force is the main force, and the longitudinal displacement is the main constraint. When analyzing simulation calculations, the specific operations are as follows:

5.4.1) Fixed constraints on all hard points, that is, four-wheel lock state;

5.4.2) Calculate the braking deceleration a _z required for braking conditions. The calculation formula is as follows:

In the formula, v ₀ ² is the starting braking speed, Z is the braking distance, and through calculation, a _z =1.6g can be obtained.

5.4.3) Apply an inertial force load to the frame. The load calculation method is the product of the inertial force of each component and the braking acceleration. The inertial force load includes the self-weight of the frame, the driver's inertial force, the inertial force of the electric drive system, and the battery box's inertial force. , in addition to the other main components gravity and motor torque; in this embodiment, the driver's inertia force load is 70×2×1.6g=2240N, and the electric drive system's inertia force load is 34×2×1.6g=1088N , the inertia force load of the battery box is 55×2×1.6g=1760N. In addition, the gravity of the main components is 3060N and the motor torque is 240N·m.

5.4.4) Secondary forces are exerted on the frame, respectively from the rear suspension triangular rocker arm, the rear suspension spring and the front suspension triangular rocker arm; in this embodiment, the force situation of the rear suspension triangular rocker arm is F _x =0, F _y = 102.84N, F _z = -50.63N; the force of the rear suspension spring is F _x = 87.085N, F _y = -515.47N, F _z = -222.37N; the force of the front suspension triangular rocker arm is F _x =0, F _y =527.9N, F _z =1416.6N.

5.4.5) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under braking conditions;

5.4.6) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than 1.8, go to step 3) and add the member wall at the unsafe position. thickness and pipe diameter; otherwise, it means that the requirements are met under accelerated conditions, and the ideal safety frame model T2 is obtained;

6) Conduct finite element modal analysis on the ideal safety frame model T2 to obtain the final frame model;

Modal analysis is an important part of the dynamic characteristics analysis of vehicle technology. Through analysis, the modal frequency and mode shape of the structure can be obtained. When a racing car drives on a track, uneven road surfaces and engine vibrations will excite the frame. If the excitation frequency is the same as a certain natural frequency of the car, resonance will occur, which may affect the mechanical performance of the racing car or even damage the car. frame structure. Therefore, it is necessary to analyze the natural frequency and vibration shape of the frame to provide reference for the structural design of the frame. There are two types of modal analysis: free mode and constrained mode. In order to better fit the reality, the constrained modal analysis method is used. The specific operations are:

6.1) Fixed and constrained 8 hard points on the front and rear suspension of the frame.

6.2) Apply inertial force load to the frame. Considering the approximate linear motion of the racing car, in addition to the weight of the frame, there is also the weight of the driver and the weight of the engine. In addition, this working condition simulates the stress state of the racing car during movement, so it should be multiplied by a dynamic load factor. , in this embodiment, the dynamic load factor is taken to be 2. The main forces are the driver, battery box, electric drive system, frame weight, and motor torque. The load size and application position are the same as those under linear conditions.

6.3) Carry out simulation analysis to obtain the first 8 mode frequencies and vibration shapes;

6.4) Compare the obtained 8th-order modal frequency with the external excitation frequency of the motor and other components, and observe whether the frequencies overlap and cause resonance, causing damage to the frame structure. If resonance occurs, return to step 3) to change the size of the rod; if If no resonance phenomenon occurs, it means that the frame meets the requirements of modal analysis, and the final frame model T3 is obtained;

7) Design the welding fixture for the final frame model T3; in this example, 4040 aluminum profiles are used as the fixture material, and corner codes are used to fasten the rods. During the design process, the front partition, front ring, and main ring are used as the three A standard surface, using a frame structure to ensure integrity and reduce the impact of welding stress on the position of the steel pipe during the welding process. It mainly ensures the positions of the 16 hard points related to the frame and suspension and the positions of the front and rear shock absorber lifting lugs. Aluminum profiles and auxiliary fixtures are used to limit the degree of freedom of these important points to ensure processing accuracy. The frame model and the welding fixture are matched and observed in the 3D software to ensure the rationality of the fixture design.

Claims (4)

1. A design method for an electric formula racing car frame, which is characterized by including the following steps: 1) Determine the frame parameters and the position of the frame’s hard points. The specific method is as follows: 1.1) Carry out human-machine experiments in the early stage of design, build a human-machine experiment platform, use the simulator operation performance and the driver's subjective feelings as the evaluation criteria, and use the driver's human-machine database as an objective reference to obtain human-machine parameters, in which the human-machine parameters include : Distance d ₁ between the main ring and the front ring, height h ₁ of the main ring, height h ₂ of the front ring, distance d ₂ between the brake accelerator pedal and the main ring, inclination angle γ of the brake accelerator pedal, height h ₃ of the steering wheel center point And the distance d ₂ from the main ring, the cockpit width l ₁ ; 1.2) Determine the front and rear load ratio k; 1.3) Calculate the wheelbase L of the vehicle. The specific method is as follows: 1.3.1) Calculate the vehicle front axle load M _tf , the formula is as follows: In the formula, M _i is the mass of each main component, _Xi is the x-axis distance of each main component relative to the center of mass, L is the wheelbase, and N represents the number of components; 1.3.2) Calculate the vehicle rear axle load M _tr , the formula method is as follows: M _tr =M ₀ -M _tf In the formula, M ₀ is the mass of the vehicle (when the driver is fully loaded); 1.3.3) Calculate the vehicle wheelbase L, the formula is as follows: 1.4) Determine the wheelbase L _s ; 1.5) Determine the position of the suspension hard point, and use the suspension positioning parameters to perform kinematics and dynamics simulations to determine the position of the suspension hard point. The positioning parameters include camber angle α ₁ , inclination angle α ₂ , caster angle α ₃ , and toe-in α ₄ ; 1.6) Determine the position of the hard point of the frame. The position of the hard point of the frame is obtained based on the position of the hard point of the suspension and the relative position of the hard point of the frame and the hard point of the suspension. The relative amount is determined by the size of the lifting lugs connecting the suspension and the frame. ; 2) Build the initial frame model T using human-machine parameters, vehicle parameters, and frame hard point positions; 3) Add auxiliary rods to the initial frame model T, adjust the rod dimensions, and obtain the adjusted frame model T’, where the rod dimensions include pipe diameter and wall thickness; 4) Use finite element to analyze the torsional stiffness of the frame model T’ and obtain the ideal frame model T1. The specific method is as follows: 4.1) Use finite element to calculate the torsional stiffness value K _n of the frame model T'. The calculation formula is as follows: In the formula, K _n is the torsional stiffness value calculated by each model, F is the support reaction force, d is the distance between the two hard points of the suspension, ΔA and ΔB are the forced displacements of the two hard points; 4.2) Calculate the comprehensive frame torsional stiffness K of the frame model T’. The calculation formula is as follows: In the formula, K _f is the torsional stiffness of the front suspension under torsion, K _r is the torsional stiffness of the rear suspension under torsion, a is the front wheelbase, b is the rear wheelbase, and L is the wheelbase; 4.3) Determine whether K>K ₀ and m<m ₀ are satisfied. If satisfied, go to step 4.4), otherwise go to step 3); 4.4) Determine whether it is satisfied If satisfied, get the ideal frame model T1, otherwise go to step 3); 5) Perform a working condition analysis on the ideal frame model T1 to obtain the ideal safe frame model T2. The working condition analysis on the ideal frame model T1 includes the following steps: 5.1) Linear working condition analysis includes the following steps: 5.1.1) Four hard points of the rear suspension are fixed and constrained. The left hard point y of the front suspension is constrained in the z direction and releases the degree of freedom in the x direction. The right hard point y is constrained in the z direction and releases the degree of freedom in the x direction; 5.1.2) Apply a load to the frame, where the load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame, and the load size is the respective weight multiplied by the dynamic load factor; 5.1.3) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under linear working conditions; 5.1.4) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than ξ ₀ , increase the wall thickness and pipe diameter of the rod in the unsafe position , go to step 3), otherwise it means that the safety requirements are met under straight-line operating conditions, go to step 5.2); 5.2) Analysis of uniform speed cornering conditions, including the following steps: 5.2.1) Fixed constraints test all hard points inside, restrict the degree of freedom in the z direction on the outside, and release the degree of freedom in the xy direction; 5.2.2) Calculate the maximum centripetal acceleration a _rmax . The calculation formula is as follows: In the formula, a _rmax is the maximum centripetal acceleration, r is the turning radius, and t _min is the fastest lap time; 5.2.3) Apply an inertial force load to the frame. The load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame. The load size is the product of the inertial force of the component and the maximum centripetal acceleration. , in which the inertial force includes the self-weight of the vehicle frame, the inertial force of the driver, the inertial force of the electric drive system, and the inertial force of the battery box; 5.2.4) Apply secondary forces to the frame, which come from the rear suspension triangular rocker arm, rear suspension spring and front suspension triangular rocker arm; 5.2.5) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under uniform speed cornering conditions; 5.2.6) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than ξ ₀ , increase the wall thickness of the rod and pipe at the unsafe position. diameter, go to step 3), otherwise the safety requirements are met under constant speed cornering conditions, go to step 5.3); 5.3) Accelerate working condition analysis; 5.4) Analysis of braking conditions; 6) Conduct finite element modal analysis on the ideal safety frame model T2 to obtain the final frame model T3; 7) Design the welding fixture for the final frame model T3. 2. The design method of the electric formula racing car frame as claimed in claim 1, characterized in that in step 6) the specific method of performing finite element modal analysis on the ideal safety frame model T2 to obtain the final frame model T3 as follows: 6.1) Fixed and constrained 8 hard points on the front and rear suspension of the frame; 6.2) Apply inertial force load to the frame; 6.3) Carry out simulation analysis to obtain the first 8 mode frequencies and vibration shapes; 6.4) Compare the obtained 8th-order modal frequency with the external excitation frequency of the motor and other components, and observe whether the frequencies coincide with resonance. If resonance occurs, return to step 3) to change the size of the members; otherwise, the final frame model T3 is obtained. . 3. The design method of the electric formula racing car frame according to claim 1, characterized in that the acceleration condition analysis in step 5.3) includes the following steps: 5.3.1) Fixed and constrained the four hard points of the rear suspension, restricted the four hard points of the front suspension in the yz direction, and released the degree of freedom in the x direction; 5.3.2) Calculate the maximum acceleration a _max required for acceleration conditions. The calculation formula is as follows: In the formula, S is the distance covered by straight-line acceleration, t is the acceleration time, q is the classical coefficient, and the simplified model is equal to a _max ; 5.3.3) Apply an inertial force load to the frame, where the load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame. The load size is the product of the inertial force of the component and the maximum acceleration, where The inertial force includes the self-weight of the vehicle frame, the inertial force of the driver, the inertial force of the electric drive system, and the inertial force of the battery box; 5.3.4) Apply secondary forces to the frame, which come from the rear suspension triangular rocker arm, rear suspension spring and transmission bracket; 5.3.5) Carry out simulation calculations and calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under accelerated conditions; 5.3.6) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than ξ ₀ , increase the wall thickness of the rod and pipe at the unsafe position. diameter, go to step 3), otherwise the safety requirements are met under accelerated conditions, go to step 5.4). 4. The design method of the electric formula racing car frame according to claim 1, characterized in that the braking condition analysis in step 5.4) includes the following steps: 5.4.1) Fixed constraints on all hard points, that is, four-wheel lock state; 5.4.2) Calculate the braking deceleration a _z required for braking conditions. The calculation formula is as follows: In the formula, v ₀ ² is the starting braking speed, and Z is the braking distance; 5.4.3) Apply an inertial force load to the frame. The load position is the position where the frame, driver, motor, battery box, and reducer are in contact with the frame. The load size is the product of the inertial force of the component and the braking deceleration. , in which the inertial force includes the self-weight of the vehicle frame, the inertial force of the driver, the inertial force of the electric drive system, and the inertial force of the battery box; 5.4.4) Apply secondary forces to the frame, which come from the rear suspension triangular rocker arm, rear suspension spring and front suspension triangular rocker arm; 5.4.5) Carry out simulation calculations to calculate the total deformation cloud diagram, stress cloud diagram and safety factor diagram under braking conditions; 5.4.6) Compare the simulation results with mechanical properties such as material yield strength σ _s , and obtain the minimum safety factor ξ from the safety factor diagram. If the minimum safety factor ξ is less than ξ ₀ , increase the wall thickness and pipe diameter of the rod in the unsafe position , go to step 3), otherwise the safety requirements are met under acceleration conditions, and the ideal safety frame model T2 is obtained.