Text | Gu Xuan said history
Edit | Gu Xuan said history
In the field of UAVs, rotary wing unmanned aerial vehicles (UAVs) have a wide range of applications in military and civilian applications, but there are still some challenges in the design process of UAVs, such as long design cycles, high manufacturing costs and difficult maintenance.
To address these issues, the researchers proposed an innovative approach to drone design aimed at obtaining a lightweight and easy-to-maintain drone framework that covers the entire process from configurable design to detailed design, and through a series of steps to achieve the goal of optimizing the design.
01
Configurable design
The design process takes configurable design as the starting point, determines the initial design scope of the UAV frame, adopts the topology optimization method based on inertial release theory, and transforms the initial geometric model into the actual UAV frame structure, which not only ensures the robustness of the structure, but also meets the requirements of lightweight.
In the design process, special consideration is also given to process design to improve the manufacturability and maintainability of the drone frame, and this comprehensive consideration of process factors helps ensure that the final design is not only superior in theory, but also feasible in actual manufacturing and maintenance.
To verify the durability and crashworthiness of the drone frame, a dynamic drop test was conducted, a step that allowed researchers to perform actual physical tests on the designed drone frame structure to ensure its performance in real-world environments.
During the configurable design phase of a rack, the first task is to determine an initial data that meets the requirements, including key parameters such as maximum take-off weight, maximum thrust-to-weight ratio, and payload.
These initial data will provide the basis for subsequent design work, which can effectively determine the geometry of the drone by analyzing the maximum thrust required for each propeller.
At this stage, the choice of brushless motors and propellers is particularly important, as they are closely related to the generation and transmission of thrust from the drone.
In fact, the drag of the drone is mainly due to the use of propellers, which is related to the speed constant (KV) of brushless motors.
In the design process, the specifications of the motor and propeller should be considered, and generally larger size propellers should be paired with motors with smaller KV values to enhance the thrust of multi-rotor UAVs.
Multi-rotor UAVs are usually divided into different types such as quadcopters, hexacopters and octoccopters according to the number of rotors, and the carrying capacity of octoccopter UAVs is usually higher than that of quadcopter UAVs.
The frame layout of multi-rotor UAV can generally be divided into "+" type and "X" type, in particular, "X" type frame due to its good controllability, widely used in agriculture and other fields, based on configurable design, the size parameters of the UAV frame can be obtained, so as to establish the initial design range.
The main goal of the configurable design phase is to identify a series of key parameters to provide an accurate basis for subsequent detailed designs, from maximum take-off weight to thrust-to-weight ratio, as well as propeller and motor specifications.
Different types of multi-rotor UAVs also need to choose the frame layout according to actual needs, and through this stage of efforts, the design team can better design and optimize the UAV in the follow-up work.
02
Topology optimization design
After completing the configurable design of the UAV frame, the next step is to adopt a topology optimization method based on inertial release theory to obtain the optimal material distribution of the rack, which can significantly shorten the design cycle compared to the traditional design method, and the optimization goal is to minimize the compliance of the structure under the constraints of the given available volume.
In the static analysis of unconstrained UAVs, the influence of inertial release must be considered, and the core idea of inertial release theory is to achieve a balance of forces by subtracting the calculated rigid body inertia from a given load vector.
Although the inertial release method can solve the force balance problem, rigid body motion still occurs in the structure, which can lead to matrix singularity, which in turn makes the structural stiffness unsolvable.
To solve this problem, support point constraints, similar to the concept of single-point constraints, must be introduced to eliminate the singularity of matrices, where no constraints are created and the effect of constraints on the local deformation of the structure and the path of force transfer can be ignored.
For the optimization problem of UAV racks, the design variable can choose the relative density of the unit, in the optimization process, the goal is to make the relative density of the unit close to 0 or 1, although the optimization result may contain many intermediate density elements, but it can be reduced by filtering methods to obtain more suitable results.
In the optimization problem for UAV racks, the key to the design variable is the relative density of the elements, when carrying out the optimization process, the main goal is to adjust the relative density of these elements to be close to 0 or 1, so as to achieve better structural performance, the actual optimization results may contain a certain number of intermediate density elements, which can be reduced by filtering methods, in order to avoid the singularity problem, a non-zero vector is selected as the minimum relative density vector (xmin).
In discretization of the design domain, the number of elements is expressed in nitrogen (N), the penalty power (usually denoted as p) and the prescribed volume fraction (denoted as f) are parameters that need to be considered in the optimization problem, V(x) represents the volume of the material, and V0 represents the volume of the design domain.
Taking the frame of the octarotor plant protection drone as an example to illustrate the process of topology optimization, aluminum alloy has great potential in lightweight design, so in the material selection of UAV frame, we chose aluminum alloy as the material.
The parameters of the initial structure and other components are determined based on the reference values of the DJI MG-1 plant protection drone, which uses DJI's state-of-the-art A3 flight controller, a solution tailored specifically for agricultural environments.
The models of the flight battery and nozzle are MG-12000 and XR11001, respectively, on the basis of which the basic parameters of the plant protection UAV are set, according to which the minimum wheelbase is calculated is 1468mm.
The initial design domain of the UAV frame included a design space with a mass of 5846 kg, which was meshed by 98,745 hexahedral or pentahedral units, each with an average size of 15 mm, it is worth noting that this design domain does not include components such as motors, propellers, medicine boxes and nozzles.
In this design, each propeller is capable of providing about 49.98 N of vertical upward thrust, and the payload of the drone rack is 18 kg, which includes components such as batteries, radar, etc.
Through the first topology optimization, significant optimization results were achieved, but there was still a problem of material redundancy, and further optimization was necessary to obtain new optimization schemes, and these optimization results were manually reconstructed using CAD software to reproduce the recommended shape of topology optimization.
Transforming the results of topology optimization into a manufacturable product while maintaining its optimized performance is challenging for traditional manufacturing techniques, and for the results of topology optimization, the researchers hope to replace solid cross-section truss structures with thin-walled cross-sections with high inertia-to-mass ratios.
Although there are some difficulties in manufacturing the optimization results due to the different thicknesses of each point, the results of these topology optimizations are still of great value in further process design.
After topology optimization, the optimization results need to be sized to obtain the best structural thickness to adapt to the manufacturing process of the drone frame, and this series of design and optimization steps will help to obtain a lighter, higher performance and easy-to-manufacture drone frame structure.
03
Process design
As a lightweight impact-resistant structure with high rigidity-to-mass ratio and mature manufacturing process, thin-walled structure is widely used in the engineering field, and when manufacturing UAV frames, in order to reduce manufacturing costs and complexity, it is often necessary to segment the frame in the early stages of design.
After optimization, the drone frame can be divided into 8 arms and 1 center hub, mainly composed of thin-walled components, stamping process with its efficient manufacturing speed and material utilization and other advantages, widely used in the manufacture of thin-walled parts, from the perspective of reducing manufacturing costs, stamping process is very suitable for the manufacture of UAV frames.
These eight arms are evenly distributed on the drone frame, while the diameter of the drone frame and center hub is 1468 mm and 628 mm, respectively, and the diameter of the motor mounting platform is 36 mm, each arm consists of a motor mounting platform, an upper panel, a lower panel, and ribs connecting the upper and lower panels.
The design of the ribs and center hub also adopts a simple thin-walled structure, and all components are designed as thin-walled structures to improve the maintainability of the drone frame.
In the design, the size and shape of the geometry also needed to be adjusted to obtain a lightweight thin-walled frame structure while maintaining the stiffness of the topology optimization results and improving manufacturability, and to avoid excessive connectors, the upper and lower panels of the adjacent two arms were designed as a one-piece structure.
And the small holes are also removed to ensure that the minimum size of each hole is not less than 10 mm, and the thickness of the ribs and panels is 3 mm and 2 mm, respectively.
A series of design and optimization steps that help create a lightweight and robust drone rack structure with a thickness of 2 mm for the upper and lower panels, 3 mm for the ribs connecting the upper and lower panels, and a mass of 2.332 kg for the entire rack.
At present, the connection process plays an important role in the manufacture of UAV frames, common connection methods include welding, riveting and bolted connections, etc., compared with welding and riveting, bolted connections have more advantages in large-volume UAV racks, such as easy disassembly, easy maintenance and transportation, and the choice of connector materials is more flexible.
In the construction process of the drone frame, bolted connections are selected as the main way to connect components, bolted connections are usually divided into three grades: A, B and C (coarse bolts), from the practical application and strength point of view, B class M3 hexagonal bolts are considered to meet the requirements of the drone frame.
There are eight connections between the different components, which are designed to be symmetrical structures to ensure a more uniform stress distribution.
In addition, from the point of view of manufacturing, the distance between the bolt and the edge of the connector also needs to be reasonably arranged, referring to the national structural design code, the diameter d0 of the hexagonal bolt hole in M3 is set to 3.2 mm, and the distance between the bolt and the edge of the connector shall not be less than 1.5 times d0, these well-designed connection methods ensure the strength and stability of the UAV frame.
04
Crashworthiness verification
In order to evaluate the crashworthiness of the UAV, the research team conducted a drop test and verified by finite element analysis, and its maximum horizontal operating speed was 4 m/s and the maximum flight altitude was 3 m under the normal operating state of the fully loaded plant protection UAV.
To simulate a fall at a maximum flight altitude, the commercial finite element software LS-DYNA was used, and the landing gear consisted of 20 mm diameter aluminum alloy round rods, in which the soil was reduced to an elastoplastic block.
During the simulation, the process of frame falling is omitted, and the velocity at the moment of collision is directly given, specifically, the speed of horizontal X is 4000 mm/s, and the vertical Z negative direction is 7668 mm/s.
The final geometric model includes about 100,000 elements, including shell elements and solid elements, these elements are used for the energy curve in the finite element calculation, through the simulation, a large amount of kinetic energy is converted into internal energy, according to the standard, the hourglass energy should account for within 5% of the total energy to ensure the reliability of the simulation results, the simulation results show that the energy conversion process is stable, and the hourglass energy is less than 5%.
From the simulation, it can be observed that the deformation of the UAV frame is relatively small, in the frame structure, the maximum stress appears at the center hub 1, and when the stress reaches the yield limit, plastic deformation occurs.
Although the center hub 1 failed in the dynamic drop test, this did not affect the subsequent replacement and repair, which was in line with the original design requirements, and these results showed that the structure of the drone showed good crash resistance in the drop test.