Considerations on the Plastic Structure of a UAV Payload Made by 3D Printing Technology

With the development of unmanned aerial vehicle (UAV) systems for a multitude of real-time applications, 3D printing technologies have been developed to make thermoplastic structures by fusing filament Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF). However, we consider that the realization of new technologies of experimental models / technological demonstrators / prototypes becomes profitable by using 3D printing technologies. The main aim of the paper is to highlight how the use of three types of materials, which are processed differently, influences the Von Mises stresses of the payload used for a UAV, with the mission of photographing and filming from high altitude.


Introduction
UAVs (Unmanned Aerial Vehicles) [1] are defined as generic air vehicles capable of operating autonomously, without a pilot on board [2]. The most common types of UAVs are multi-rotor, fixedwing, flap-wing, and hybrid wing systems [2] (Figure1). They are considered very useful for activities such as mapping, topography, telecommunications [3,4], surveillance, and agricultural management [5,6]. The development of technologies for the listed applications was based on innovations in structural and aerodynamic projects. Within the CERAS research center, payloads are designed and made for aerial drones, in order to develop collaborative robotic systems [7][8][9][10][11]. The need for this study is due to increasingly restrictive conditions for reducing carbon emissions [12,13]. Through the introduction of new lightweight materials and manufacturing technologies to enable the development of detection and control systems -payloads [14]. In other words, the use of additive manufacturing allows remote work, and the production of parts, close to the final requirements, consistently reduce emissions due to transport and delivery of parts to a desired location [15][16][17] (Figure 1).

Figure 1.
Hirrus 3D model of the drone and the paylaod components [9] *email: lucian.grigore64@gmail.com ( ) 1 1 where: f E și m E -represents the modulus of elasticity of the fiber and the matrix for the range of relevant sections, respectively is the volume fraction of the fiber.  As can be inferred, the critical breaking point of the adjacent fibers triggers the rupture of the pieces in the Textolite. The formation of such a cluster of adjacent fibers depends on the statistical distribution of fiber strengths and the transfer of local stresses in the vicinity of the breaking fibers. This cluster is influenced by the fiber-matrix interface.
The Textolite laminate is quasi-isotropic in nature, so that any crack can be highlighted immediately. For numerical modeling both cotton fibers and laminate are elastic and linear isotropic. The interface between cotton fibers and laminated resin was modeled with a linear softening law with equal critical energy (case of the law of elastic-plastic softening interface).
The component elements of the payload structure made of Textolite come from the processing of plate, bar and pipe type semi-finished products ( Figure 2). Analytical-numerical modeling of the stress of a thin plate requires solving three-dimensional elasticity systems of differential equations.

Considerations regarding the characteristics of the numerical modeling of the PET-G structure
PET-G is a co-polyester thermoplastic material that is deposited by the FDM method. The resulting product is characterized by reasonable tensile strength, impact resistance, durability and flexibility. About PETG it can be appreciated that some elongation of the material can be approximated in terms of tensile stress. On the other hand, it can be seen that it has the characteristics of a material that allows the absorption of a fairly large amount of energy, energy that can generate its own modes of vibration unfavorable on the structure of the payload.

Numerical modeling when using ABS
Finite element modeling aims to distribute the stresses in the structural parts of the payload made of ABS and test the capacity of the modulus of elasticity by comparing Young's predicted and experimental modulus. The geometry is transformed into a finite element mesh in which each element is represented by a cube-type structural element. Each element is defined by eight nodes and each node has three degrees of freedom corresponding to the displacement components (UX, UY, UZ) in the main directions X, Y and Z. The density of the network is adapted according to the degree of approximation of the results to the characteristics provided by the manufacturer.
An ABS with modulus of elasticity of 2 [GPa] and a Poisson's contraction coefficient of 0.394 was used. The simulation of the structure has the following test conditions: the upper end of the structure (the one that does not leave the resting place in the drone during the retraction); and the end provided with hinged doors through which the EO / IR (Electro-Optical / Infra-Red) filming module exits is free in the Z direction.

Kinematic and dynamic analysis of the Payload compartment door opening
The door has a parallel plane motion, the motion plane being yOz. Figure 5 and Figure 6 show two views of it. In Figure 6 the dimensions corresponding to the kinematic model shown in Figure 7 are highlighted.

Figure 5. Side view of the payload compartment door assembly
In point "A" is a steel shaft that rests in a curved seat that can slide along it, in point B there is a geometric configuration that allows this end to hang on the side of a closed toothed belt, mounted between two belt wheels. In (Figure 7) point A rests on the curve of the seat (C1) and B rests on the belt, which being stretched and undeformable during movement, represents a linear support (inclined line C2). Point C represents the center of mass and C 'is the projection of the center of mass on the segment AB ( Figure 8).
Curve C1 is given by the explicit form:  , respectively with respect to the three axes of the reference system. The moments of inertia with respect to the system originating in the center of mass, with the axes parallel to the axes of the global system are:

Determining the kinematics
A displacement of point B is imposed along the direction of the belt, which depends on the speed of the belt. This law is considered: , where:   The initial position of the door is "closed", point A having the coordinates (4; 6.72).
Using the length of the segment, determine the initial position of point B with the equations of the 2 support curves, resulting in coordinates (44.06; 20.50). Divide the time interval into "n" intervals, where, Oy coordinate is given by: The time variation of the angle of inclination of the segment AB, represents the angular velocity ( Figure 11) of the door assembly of the payload compartment.

Dynamic analysis
By isolating the payload compartment door assembly, in point B having a joint and in point A having a simple friction bearing, the situation is obtained from (Figure 12). In what follows, apart from the force of gravity, the terms are dependent on indices j, so, for simplicity, this index is omitted:  It follows from the impulse theorem: and the kinetic moment theorem written with respect to the center of mass and projected on the Oz axis, results: The following system of equations results from the expressions of forces and relations (11), (12): The system contains three unknown functions: hence the components of the normal reaction force ( Figure 13).

Finite element model
In the finite element model a double symmetry of the ( Figure 14) structure is considered. For plane 1 of symmetry, the boundary conditions are displacements along the zero axis Ox, and for plane 2, displacements along the zero axis OZ. It is observed that the maximum normal reaction force is obtained if the payload compartment door is closed, so at the beginning of the opening movement.
All nodes in the mounting holes of the rest of the drone structure have all zero displacements. The nodes in the contact area of the door with the drive belt are also locked. On the steel bolt entering the curved seat, mechanically equivalent forces are distributed with the values of the normal reaction corresponding to the initial moment. The component parts of the considered structure are made of ABS with longitudinal modulus of elasticity, tensile breaking stress, Poisson's ratio 0.394.
The structure was divided with tetrahedral elements. Figure 15 shows the division with elements and a detail of the contact area. In (Figure 16) is presented the field of total deformations and in ( Figure 17) the field of equivalent stresses in the contact area between the steel bolt and the curved seat of the ABS structure.

Conclusions
ABS have some advantages over Textolite and PET-G: greater resistance to thermal degradation; greater thermal stability; a lower statistical dispersion of ABS compared to PET-G; increased flexibility in 3D printing.
It is found that the stresses in the contact area do not exceed 1Mpa, thus being rational the use of plastics in the realization of the component parts of this assembly.
For the future, the action of aerodynamic forces that produce an increase in mechanical stresses must also be considered. Wear will be studied at the contact between two parts, one movable and the other fixed, which slides, and the movement is in the form of cycles. In particular, the traces left will be studied. When paying, the doors have metal elements that lead to bearing wear. A device will be built to simulate a number of movements of the metal elements on different plastics.