Design and optimisation of a compliant leg module for enhanced tractive performance of a lunar nano-rover

Abstract

Extra-terrestrial rovers play a vital role in the acquisition of scientific data on celestial bodies, offering a reduced risk and cost compared to human exploration. In pursuit of mapping the lunar surface, a swarm of nano-rovers can collectively form a network and cover extensive areas. The Lunar Zebro, a hexapod nano-rover, employs an innovative C-shaped leg module that combines the efficiency of a wheel with the stability and climbing capability of a leg. The current C-shaped leg module features a fully rigid design, for which tractive performance could be improved by increasing the soil contact area. Due to the extreme thermal environment on the Moon, this improvement must be achieved using compliant mechanisms and space-graded material. Consequently, the research goal is to design a compliant leg module to enhance the tractive performance of the Lunar Zebro on lunar terrain, employing an analytical optimisation model that combines compliant system behaviour with the mechanics of leg-soil interaction.
The design process employs a morphological analysis along with the ACRREx method, in which different subproblem solutions are combined into multiple concepts. The performance of these concepts is evaluated against selection criteria, leading to the creation of the final concept by integrating the design features associated with high criteria performance. The final concept is translated into an analytical model utilising Castigliano’s theorem to describe the compliant behaviour and Wong’s terramechanics theorem for deformable wheels to describe the leg-soil interaction. A set of design variables impacting the system’s compliant behaviour is defined, for which the tractive performance is optimised while confined by constraints that ensure reliability and durability across various movements and scenarios. The optimisation process is conducted using Sequential Quadratic Programming, due to the non-linearity of the objective function and constraints. An optimal set of design variables is identified for both the middle and outer legs, resulting in not only a significant enhancement of tractive performance but also an increase in tractive efficiency and reduction of leg sinkage. Although concerns are raised about contact surface wear at the optimal set of design variables, a minimal amount of compliance, while staying in the feasible region of the design variables, enhances the tractive performance of the compliant leg compared to the original rigid leg. A final leg design is presented that incorporates modifications informed by research findings.