Rotor Orientation Optimization for Direct 6 Degree of Freedom Control of Multirotors

Standard multirotor designs typically arrange their propellers in the same plane, which results in an under-actuated system whose pose cannot track an arbitrary trajectory over time in 6 DoFs. Some researchers have explored 6-DoF control authority for multirotors with six or more rotors to resolve this drawback. This paper presents a multi-objective optimization (MOO) algorithm to determine the rotor tilt angles for the fixed-tilt configuration by inputting desired frame parameters and performance characteristics. While existing tilt-optimization methods seek to optimize a common inward tilt and twist for each rotor resulting in a two parameter search space, we explore an optimization method that covers independent orientation of each rotor resulting initially in 12 parameters. It then shows how the search space can be reduced to five parameters without impacting the global minimum and explore the inherent isomerism that results in the solution. The proposed objective function for the optimization is constructed to be easily understood and tuned during product design. This paper also compares the proposed optimization result with other fixed-tilt hexrotor layouts claiming to realize full actuation, and quantifies the improved control authority of the new design with experiments via both a test jig and in-flight testing.