Lightweight Design and Test of Electric Experimental Car

A light-weight electric experimental vehicle is designed. According to the light-weight design requirements, the power battery, the battery manager and the driving Motor controller are designed and selected, the chassis frame, anti-rolling frame, steering system and driving system are designed and validated with CAD and CAE software respectively. On this basis, we have carried out the production, assembly and commissioning of the real vehicle. The test results show that the electric experimental vehicle has high practical value and can meet the requirements of experimental teaching. It is worthy of further study, performance optimization and application.

The requirements are as follows:1. The trolley shall meet the normal driving of a driver with a height of 180 cm and a weight of 100 kg, and shall be able to ensure that there will be no rollover and collision during driving; It can be drived normally on mountains, dirt roads, stones, slippery roads, etc.2.The output (nominal) voltage of the whole vehicle driving battery shall not exceed DC 60V, and the working voltage of the control system shall be DC 12V; The power system must be sufficiently safe and completely insulated from the chassis and any conductive parts. 3.The power source of the whole vehicle can only be the power battery, which shall meet the requirements that the external output voltage is lower than 60V.

Design and Selection of "Three Electricity"
The core technology of electric vehicle is "three electricity", including electric drive, battery and controller. The functions of the "three electricity" modules designed in this paper are based on the control logic of the electric vehicle, and the components are designed by using the exclusive accessories for the high voltage and low voltage of the electric vehicle to the greatest extent. It will step down the high voltage of electric vehicles properly and use dc-60v voltage to ensure the safety of drivers; At the same time, in the vehicle control system, the high-voltage power on and power off process is controlled in strict accordance with the control logic of the electric vehicle; It can also collect the temperature, single voltage, total voltage, current and other data information of power battery and display it in detail in the instrument.
Lithium iron phosphate high-performance and high-power single battery with stable performance and wide application is selected as the power battery. At the same time, the internal power battery BMS system collects the information of single battery voltage, battery pack temperature, total voltage and current of power battery, and calculates the SOC value. The BMS system sends the information of single battery voltage, battery pack temperature, total power battery voltage, current and SOC value 3 through the bus, which is convenient for the driver to intuitively grasp the power battery status. The overall dimensions of the power battery are shown in figure 2, and the specific parameters are shown in table 1.  The whole vehicle controller collects the information of accelerator pedal, vehicle speed, brake pedal, power battery voltage, temperature, SOC, gear, etc., outputs a torque and vehicle driving direction information to the motor controller through calculation, and the drive motor controller receives this information to control the vehicle operation; In the process of vehicle charging, detect the connection state of vehicle charging gun, battery power information, etc., and control vehicle charging; Meanwhile, in the process of vehicle power on and off and charging, the vehicle power on and off process is controlled according to the new energy vehicle control logic. The specific dimensions are shown in figure 3, and the specific parameters are shown in table 2.  DC-DC and high voltage distribution functions are integrated into the drive motor controller to form the power control unit PEU (power control unit). 12V voltage is used when the whole vehicle is running, and the low-voltage battery needs to be charged by the power battery when the vehicle is running. The model of DC-DC module is NES-75-12, and the specific parameters are shown in table 3. Its external connection plugs are high-voltage and low-voltage connectors of new energy vehicles, and its flow capacity, safety protection ability and reliability are greatly guaranteed. It integrates motor controller MCU, current sensor, shunt, charging relay, etc. to centrally distribute the high-voltage power supply. At the same time, it receives the information of the whole vehicle controller to control the torque and direction of the driving motor. Overall dimension (with connector)(long) × wide × High) 570mm×315mm×150mm

Lightweight Design of Vehicle Chassis Frame
The chassis frame is designed. The chassis of lightweight vehicle is welded with aluminum alloy square tube Al6063. The material is shown in table 4. The chassis frame is modeled with UG software, and the established model is imported into CAE module. The design dimension and structure of chassis frame are shown in figure 4.   We set the material properties in the CAE module. The chassis frame is welded, and the surface adhesion is used to simulate the solder joint. The automatic meshing mode is adopted to mesh the frame, which can refine the nodes and adjust the number of cells. After the mesh generation, the finite element model of the frame is established. The frame is divided into 955703 nodes and 478355 elements, and the mesh quality is good.
Through the modal analysis of the chassis frame, we test whether the natural frequency of the frame coincides with the external excitation frequency of the frame, resulting in resonance; The seventh order modal natural frequency of the frame is obtained by CAE analysis. The modal analysis results of the frame are shown in table 5.
The sixth order modal frequency of the frame is 140HZ, the minimum displacement is 0.198mm, and the maximum displacement is 0.588mm. Through analysis, the frame will not affect the service life of the structure due to serious resonance.

Lightweight Design of Vehicle Roll Cage
The experimental car designed in this paper adopts space truss frame, and the designed frame is The basic loads borne by the electric vehicle frame include: body shell mass, cabin mass (including passengers), motor mass, power battery mass and frame weight. According to the static equivalence principle, the load on the vehicle can be regarded as evenly distributed, the motor mass is added to the corresponding parts in the form of concentrated load, and the dead weight of the frame is realized by applying vertical downward gravity acceleration to the frame. The basic load of the frame is shown in figure 6: 1. Steering gear 2. Driver 3. Power battery 4. Drive motor Considering the actual working conditions of electric vehicle, two typical working conditions are selected for frame analysis, namely bending condition and torsion condition.
In the finite element analysis, we should not only fully reflect the actual situation of the model, but also ensure that the frame does not have rigid displacement. Under bending condition, the vehicle runs faster, the safety factor is 1.7 [4], and the allowable stress of the frame is 138 MPa. The constraint conditions of the frame under full load bending condition are shown in table 6. The deformation distribution and stress distribution of the frame under bending condition are obtained through finite element solution, as shown in figure 7. It can be seen from the figure that the maximum displacement of the frame occurs at the cross beam in the middle of the frame, and the maximum displacement is 1.413mm. The middle of the frame is the main load-bearing part, and the load is relatively large. The maximum stress of the frame is 52.89Mpa, which is less than the allowable stress of the frame 138Mpa. The stress values of other parts of the frame are generally small, which shows that the strength of the frame is sufficient.  This paper mainly simulates the situation that the left front wheel of the vehicle is suspended. Due to the low speed under torsional condition, the safety factor is 1.2, and the allowable stress of frame structure is 196Mpa. The restraint conditions of the frame under full load torsion condition are shown in table 7. After analysis, the deformation distribution and stress distribution of the frame under torsional condition are obtained, as shown in figure 8. It can be seen from the figure that the maximum displacement of the frame under torsional condition is 2.79mm, which is located at the left front wheel in the middle of the frame. Because the simulated working condition is that the left front wheel is suspended, the deformation is reasonable, and the frame stiffness is sufficient compared with the allowable deflection of the frame of 7.35 mm [5]. The overall maximum stress of the frame is 75.8018Mpa, which is still less than the allowable stress of the frame 196Mpa, and there is room for lightweight optimization.

Modal Analysis of Frame
The significance of modal analysis of the frame is to see whether the natural frequency of the frame coincides with the external excitation frequency of the frame, resulting in resonance. Resonance

ATAMI 2021
Journal of Physics: Conference Series 2185 (2022) 012046 IOP Publishing doi:10.1088/1742-6596/2185/1/012046 8 phenomenon will greatly reduce the service life of the structure. The sixth order modal natural frequency of the frame is obtained through UG-CAE software analysis, and the modal vibration mode of the frame is shown in figure 9. Pure electric vehicles will receive several kinds of excitation during actual driving [6]: 1) Vehicles will receive excitation from the road during driving, which is about within 0～20 Hz; 2) The excitation brought by the suspension is generally 1～3 Hz; 3) The excitation caused by wheel imbalance is generally 1～30 Hz. The dynamic influence of the low-order vibration mode on the components is greater than that of the high-order performance [7]. The natural frequency of the frame shown in the figure is 176Hz, which is significantly greater than the excitation frequency caused by the imbalance of the road, suspension and wheels. The analysis shows that the frame can effectively avoid resonance.

Design of Driving System
The basic requirements for driving system design are as follows: 1. The suspension can fully withstand the impact and does not interfere with the steering system during operation. It is connected with the frame by screw link, and the link bolt grade is grade 8.8. Wheel bolts and nuts adopt self-locking nuts, and the screw rod is exposed for 3 turns of threads.
2. The wheel is exposed, that is from the top view, the upper part of the front and rear wheels is not obscured by any object, and the overall contour of the front and rear wheels is not obscured by any object [8].
3. The size of the rim is 10 inches, and the front and rear rims are the same. 4. The clearance between brake caliper and rim shall be at least 10mm. 5. The reducer and chassis (rear axle) are connected by bolts. The reducer adopts single-stage deceleration with a reduction ratio of 4:3.
The system adopts double wishbone suspension structure. The assembly static finite element analysis of double wishbones is carried out. The analysis results are shown in figure 10. Simulate the extreme working condition of the vehicle driving on a flat road and restrict the pin shaft of the mounting hole. When a load of 2000N is applied, the maximum unit stress is 88.11Mpa. The suspension is welded with 6063 aluminum alloy steel pipe. According to the material manual, the yield strength of aluminum 6063 is greater than 170Mp, and the safety factor is considered. The overall maximum stress of the frame is 88.1059Mpa, the allowable stress of the suspension is 180MPa, and there is room for lightweight optimization.

Conclusion
This paper takes the design of lightweight electric vehicle as the theme, and carries out the design on the premise of ensuring safety. The research conclusions are as follows: 1. From the aspects of material lightweight and structure lightweight, the overall design of the experimental car is carried out by means of three-dimensional modeling, finite element analysis and topology optimization.
2. It focuses on the lightweight design of vehicle chassis frame, anti roll frame and suspension, the modal analysis of vehicle frame, and the design and selection of "three electricity" system.
3. The experimental car is assembled and debugged. The experiment shows that the car meets the requirements of lightweight design.
4. The developed lightweight electric experimental car has high practical value and plays a great teaching role. It is worthy of further research, performance optimization and popularization and application.