Development and Analysis of Additively Manufactured Non-Pneumatic Tires for Mars Rover

Mars rovers are developed using the most sophisticated technology. Competitions such as the University Rover Challenge (URC), European Rover Challenge (ERC), and Anatolian Rover Challenge (ARC) encourage students to become acquainted and experienced with cutting-edge technology. This paper focuses on the development and analysis of a modified version of a non-pneumatic tire (NPT) for the MIST Mars rover “Phoenix 2.0.” Recent studies show that NPT has a good potential to replace conventional tires. To optimize the tire design for the rover, many design aspects are considered. The additive manufacturing process is used to fabricate the tire model to ensure the proper geometrical shape and usage of material. Later, a static structural analysis is conducted to investigate the stress and deformation of the tire that it may experience during rover operation. The developed deformation and stress in this analysis are well protected by the honeycomb structures that are optimized from many design attempts. The result shows a positive indication of meeting the desired criteria that eventually results in the fabrication of the tire and participation in the ARC competition. This research will inspire others to contribute to the advancement of NPT in various circumstances where pneumatic tires perform inadequately.


Introduction
NASA has launched a total of five rovers to Mars and all of them are autonomously controlled as of this writing.The reason behind this unmanned operation is the difference in the atmospheric conditions between Earth and Mars.As humans are not ready yet to land on Mars, these vehicles need to be viable to continue their exploration for a long time.Hence, the components should be chosen to meet the required criteria and need minimal maintenance for the task.The tire plays a crucial role in the rover's mobility.So, tires must be more resilient since they are exposed to the roughness of the environment.Non-pneumatic tire (NPT) has good potential to be used in the Mars rover because of its shear band design, added suspension, and decreased rolling resistance [1].Besides, it will not blow out as it does not use air pressure like conventional tires.The Honeycomb structured NPTs are deemed to be the best alternative to conventional tires due to their uniform traction and reduced wear [2].The honeycomb spoke also gives better load-carrying capability.But it is not easy to fabricate this honeycomb spoke design by using conventional manufacturing processes [3] which shows the possibility of using 3D printing to create the complex shape of the NPT spoke.Additive manufacturing (AM) refers to a set of unique procedures that enable the direct fabrication of three-dimensional (3D) objects from computer-aided design (CAD) data [4].AM has many advantages over conventional machining operations.AM can produce complex geometry and organic prototypes in a noticeably short time.Again, multiple design iterations can be evaluated for optimization during the testing phase without interrupting the production line at a low cost.Nowadays a wide range of materials i.e., metals, composite materials, diverse types of plastics, and bioinks can be used in 3D printers [5].Before proceeding to the manufacturing stage, it is essential to conduct several analyses to ensure the desired characteristics and capabilities of the model [6] demonstrate the development of a methodology for evaluating the mechanical behavior of an NPT using experimental data and a numerical approach [7] employed the finite element method (FEM) to evaluate the structural integrity of their NPT model prior to its 3D printing.They subsequently integrated the fabricated model into their rover, which took part in the URC 2018 competition.URC and ARC are prestigious international university rover competitions where teams from all around the world put their rovers as well as their teamwork to test in a range of tasks [8].To experience the challenges, -Phoenix 2.0‖ a MIST Mars rover was equipped with all the tools to conduct missions in the best feasible way.An earlier version of this rover was named "Phoenix,‖ which participated in the URC 2021 [9].Thermoplastic polyurethane NPT tire has been equipped with -Phoenix 2.0," considering the recent advancement of additive manufacturing.To build a Mars rover, Tire is one of the most important parts.In previous years, the tire performance was not up to the expectation as it had stability and traction issues that hamper the operation.Other alternative approaches were taken for design consideration.However, none of them were able to meet the challenges required for the competition.That leads to the development of our wheel, meeting the following challenges:  Equipped with all gears, the rover should not exceed 50 kg and 60 kg of weight for URC and ARC, respectively. Must be able to withstand the rough terrain just like the Mars environment while maneuvering the rover. The driving system must be adequate to perform a maximum speed of 3 km/h in the above conditions. The stability of the rover in any position is necessary to do other tasks that involve the precise movement of the manipulator.

Design Parameters
The three-dimensional model of the tire was established using SolidWorks shown in Fig. 1.The spokes on the tire are hexagonal honeycomb structured with various cell geometries having uniform thickness.The integration of the honeycomb structure in the spoke is quite feasible as it absorbs energy through deformation and provides both support and suspension [10].For better traction, a helical-shaped tread was designed.This prevents the wear of the actual honeycomb structure.

Fabrication Process
The tire was fabricated using the Fused deposition modeling (FDM) printing process.The single material used in the tire was thermoplastic polyurethane (TPU) [13].TPU is a material that is popular due to its high strength, high elongation at break, and high elasticity [14].To support the overall tire structure, the first iteration of the tire was mounted with a stainless steel (SS) rim shown in Fig. 3 (a).The rim and tire were pressed together, and strong adhesive material (epoxy resin) was used to prevent them from sliding.After that metal wires were used to tighten the rim and the tire together to ensure better surface-to-surface adhesion.A DC-geared motor was connected to the flange that eventually drives the wheel.Topologically optimized triangular-shaped cut-out ensured weight reduction in rim structure without sacrificing its performance.In the second iteration, the steel rim was replaced by the hub motor shown in Fig. 3 (b).These two versions were put into use for two distinct contests, one for the URC and the other for the ARC.

Computational Setup
As the system is under static conditions, a static structural analysis was conducted in ANSYS 2021 R2.For the material properties a new material named -TPU Material'' was defined with the given data in Table III.The geometry file was saved by SolidWorks 2020 in Parasolid (*x_t;*x_b) format, which was later imported to Ansys.In the model, the material was assigned to individual parts accordingly.The user-defined coordinate system was implemented at the center of the tire for symmetrical analysis.
The connection between The SS rim and the inner surface of the tire was assigned to be bonded.The tetrahedrons method is used along with the patch-conforming algorithm.Tetrahedrons are threedimensional elements where each element has three degrees of freedom.Thus, an appropriate mathematical model was generated for analysis.The element size of 5 mm allowed the mesh to be finer shown in

Results and Discussion
The analysis is performed on NPT for different force configurations to justify the strength and proper deformation of the tire before fabrication.Fig. 6. shows the maximum equivalent (von Mises) stress of 1.5284 MPa.The red portion experiences the highest amount of localized stress.The color contour in the figure helps to identify the stress distribution easily.The total maximum deformation corresponding to this stress is 3.4219 mm in react to 147.15 N of reaction force shown in Fig. 7. Due to this deformation, there will be additional surface area, increasing tire traction while also providing suspension to the rover.The simulation's output slightly deviates from the actual situation.The deformation in the physical model was higher compared to the simulation result, measuring 4 mm (0.4 cm) depicted in Fig. 9.This measurement disparity resulted from some limitations in the real-world environment.Like, the weight is not equally distributed in the four wheel especially when a robotic manipulator is attached to the rover.Surface condition like inclined surfaces creates an uneven distribution of weight in tires as we are using a rocker-bogie suspension mechanism.The minor deviation observed in the result suggests a strong validation of the numerical analysis by the experimental data.However, in this analysis, only static conditions are considered.The transient structural analysis may be conducted to demonstrate the performance of the tire in running conditions. In this study an NPT for Mars rover is additively manufactured.This method facilitates faster unit production as well as minimum wastage of material. From numerical studies, the maximum stress and deformation are 1.5284 MPa and 3.4219 mm, respectively.This deformation is acceptable in our scenario as it provides enough suspension and carries the rover's weight. The thickness of the honeycomb structures and the width of the tire are optimized based on the simulation to have the best performance-to-weight ratio.In future work, various geometrical parameters of the honeycomb structures can be modified to get the optimized result for specific loads and environmental conditions.

Acknowledgment
This paper is the outcome of a collaborative project between the Department of Mechanical Engineering (ME) and the Department of Computer Science and Engineering (CSE) of the Military Institute of Science and Technology (MIST).The development of Phoenix 2.0, incorporating nonpneumatic tires, was funded by the CSE department.We would like to express our sincere gratitude to the faculty advisors, especially Md Abdur Razzak, Dr. Muhammad Nazrul Islam, Dr. Md Akhtaruzzaman, and Shah Md.Ahasan Siddique, as well as the team members, for their constant support throughout the whole project.

Figure 1 .
Figure 1.3D model of non-pneumatic tire with dimensions The parameters that are considered for the design of the honeycomb spokes:  Cell length (L): The volume of the honeycomb spoken is affected by this parameter.A longer length results in a larger honeycomb volume [11]. Cell width (H): This setting influences the volume of an individual honeycomb.A wider width produces a greater volume of each honeycomb and brings single honeycombs closer together [11]. Cell angle (θ): This parameter influences how flexible the structure is [12]. Cell thickness (t): This parameter impacts the weight of the tire as well as the flexibility of the structure [12].

Figure 3 .
Figure 3. Fabricated model of (a) NPT with SS rim (b) NPT with hub-motor

Fig. 4 .
The number of nodes was 223117 and the elements were 112081 [1].

Figure 4 .
Figure 4. Model after meshing3.2.Boundary ConditionFixed constraint was applied to the surface of the inner hub.The force applied was 147.15 N, equaling ¼ of the rover's weight (60/4 kg × 9.81 m/s2 = 147.15N) acting from ground to tire as a reaction force.As a result, honeycomb structures deform and absorb the force[15].

Figure 5 .
Figure 5. Applied boundary conditions for the model

Figure 9 .
Figure 9. Deformation measurement (a) before loading and (b) after loading This study helps the plausible development of a customized NPT for our Mars rover, which ultimately contributes to our third-place finish in ARC 2022 held in Turkey.

Figure 10 .
Figure 10.-Phoenix 2.0‖ during the competition5.ConclusionNPTs have a good potential over conventional pneumatic tires to be used in planetary vehicles like Mars rovers.This technology will create a new horizon for the future exploration of the planet.In this study an NPT for Mars rover is additively manufactured.This method facilitates faster unit production as well as minimum wastage of material. From numerical studies, the maximum stress and deformation are 1.5284 MPa and 3.4219 mm, respectively.This deformation is acceptable in our scenario as it provides enough suspension and carries the rover's weight. The thickness of the honeycomb structures and the width of the tire are optimized based on the simulation to have the best performance-to-weight ratio.In future work, various geometrical parameters of the honeycomb structures can be modified to get the optimized result for specific loads and environmental conditions.

Table 1 .
Design Parameters

Table 3 .
Material Properties