Design investigation of a coreless tubular linear generator for a Moball: A spherical exploration robot with wind-energy harvesting capability

Moball is a wind-driven spherical robot equipped with sensors for in-situ observation of scientifically important and windy environments, e.g., the Earth's polar regions, Mars, and Saturn's moon Titan. More importantly, Moball incorporates an internal triaxial set of linear electromagnetic generators which can be used to harvest wind energy for long-duration self-sustained operation, or to bias its' wind-driven motions as a form of steering. This paper describes our process to optimize the design of a coreless tubular linear generator for Moball so as to improve energy generation and motion control capabilities with the minimal moving generator mass. The performance of three different types of movers was analyzed with the help of finite element analysis. We determined a final optimized structure and its' dimensions involving a single dipole PM and novel slope-shaped back-irons. A prototype of a single-axis linear generator with a length of 0.8 m was fabricated and assembled. Drop and rotating tests were performed to measure the generated power with this machine. The maximum generated power in the rotating test was 1.05 W at 19 rpm when the load resistance was 40 Ω. The experimental results agreed well with our model predictions. The paper concludes with an overview of the current Moball prototype and ongoing work. The design process developed in this paper can serve as a guideline for future design of energy scavenging systems for robots.