Planetary excavators face unique and extreme engineering constraints relative to terrestrial counterparts. In space missions mass is always at a premium because it is the main driver behind launch costs. Lightweight operation, due to low mass and reduced gravity, hinders excavation and mobility by reducing the forces a robot can effect on its environment.
This thesis shows that there is a quantifiable, non-dimensional threshold that distinguishes the regimes of lightweight and heavy excavation. This threshold is crossed at lower weights for continuous excavators (bucket-wheels, bucket chains, etc.) than discrete excavators (loaders, scrapers, etc.). The lightweight threshold relates payload ratio (weight of regolith payload collected to empty robot weight), excavation resistance (force imparted on an excavator by cutting and collecting soil), and excavation thrust (force supplied by an excavator that is available for cutting soil).
Experiments and simulation herein show that payload ratio governs productivity of lightweight excavators. Reducing weight (due to low mass, reduced gravity, or both) decreases an excavator's thrust to resistance ratio, especially in cohesive soils. There is a predictable regime in the operating space where this ratio is low enough that it limits an excavator's payload ratio and, ultimately, productivity. Discrete excavators cross into this regime more readily than continuous excavators, because soil accumulation on their blades increases their excavation resistance.
This research introduces novel experimentation that for the first time subjects excavators to gravity offload (a cable pulls up on the robot with 5/6 its weight, to simulate lunar gravity) while they dig. A 300~kg excavator offloaded to 1/6~g successfully collects 0.5~kg/s using a bucket-wheel, with no discernable effect on mobility. For a discrete excavator of the same weight, production rapidly declines as rising excavation resistance stalls the robot; in total the discrete bucket collects less than 20~kg of regolith. These experiments demonstrate that discrete excavation crosses the lightweight threshold under conditions where continuous excavation does not. They also suggest caution in interpreting low gravity performance predictions based solely on testing in Earth gravity.
This work develops a novel robotic bucket-wheel excavator. It features unique direct transfer from a bucket-wheel to a high payload ratio dump bed, as well as a high traction and high speed mobility system. Past lightweight excavator prototypes were too slow or carried too little regolith payload. Some used bucket-wheels or bucket-ladders to dig continuously, but transported regolith using exposed chains or conveyors that would not withstand harsh lunar conditions.