CASPER yields a lower excavation rate than other discrete excavators, such as the Backhoe (100 kg/hr \cite{van_Susante_2010}) and Cratos (900kg/hr \cite{greer2013cratos}) systems. However, CASPER delivers a serviceable excavation rate at a much lower system mass (~80kg for both Backhoe and Cratos) and much lower power requirements (~200 W for Backhoe and ~100 W for Cratos) while traveling at similar speeds. Furthermore, CASPER yields a nearly identical ETR to the Backhoe system. Based on the available power, excavation rate, system mass, and velocity data \cite{van_Susante_2010,Just_2020}, the Backhoe system has an ETR of 2.01. This is remarkably close to CASPER's maximum ETR of 1.97-2.06. From these comparisons, it can be concluded that although CASPER delivers lower excavation rates than other discrete excavation systems, it does so with a much lower system mass and lower power requirements. In addition, the novel ETR parameter makes it clear that CASPER provides performance as a mobile excavator as larger systems in a much smaller package. This implies that CASPER might be better suited for extra-terrestrial applications where power and mass must be minimized. Moreover, for applications where higher excavation rates are needed, CASPER can be scaled up to accommodate those needs. However, further studies are warranted to extend current helical granular scaling laws to excavation systems \cite{Thoesen_2020b}.

Conclusion

    The purpose of this investigation was to characterize the excavation and mobility performance of the CASPER robot, a novel discrete scooper hybrid excavation platform. CASPER is propelled by four counter rotating Archimedes screws. These screws serve as both the mobility system, as well as augmenting the excavation system with the front screws churning up the granular media in order to be discretely scooped into the collection bin via the ramp mechanism. Four major configurations of CASPER were investigated, consisting of narrow and wide variants of inward and outward churning front screws.
    The major performance parameters measured included excavation rate, power consumption, velocity, cost of transport, and a new parameter, the excavation transport rate. These dependent variables were measured over a range of the independent variables of screw angular velocity, ramp load time, ramp angle, and front screw configuration. It was found that CASPER had a peak excavation rate of around 30 kg/hr at the optimal settings of 40 RPM and 4 seconds of load time. The operational power of CASPER ranged from approximately 10-15 W for 20 RPM cases up to 20-25 W for the 40 RPM cases and exhibited very little variation with respect to load time. The velocity performance exhibited similar trends to the power consumption, with velocities ranging from 5-18 cm/s for 20-40 RPM with very little variation with load time. The cost of transport results indicated that higher angular velocities are more efficient for mobility, insofar as they move quicker for a given amount of power. It is important to recognize that these velocity and power data are highly specific to the Quikrete silica sand that the craft was tested in. In planetary regolith analogues, which have smaller, cohesive, and more angular particles, the power consumption would be expected to increase, and the velocity would be expected to decrease \cite{thoesen2020comparative}. In other words, the harsher geological properties of regolith found on planetary bodies will have an adverse impact on the mobility performance. The most critical parameter investigated, the excavation transport rate, aims to quantify a mixture of mobility and excavation performance that is normalized by the craft mass to enable clearer comparison between existing systems. The ETR results indicated that the optimal configuration of CASPER was a 45 degree ramp angle, 40 RPM angular velocity, with a load time of 4 seconds with the wide inward churning configuration of the front screw pontoons. This configuration had the best combination of excavation rate, power consumption, and velocity. The outward churning configurations were ineffective due to the fact that they did not create the raised section of pre-churned material in front of the ramp that the inward churning configurations did. According to the ETR results, the narrow and wide configurations were very similar, with the narrow configuration having a slight advantage at lower ramp angles. This is likely due to the height of the elevated wake behind the front pontoons being slightly higher from the close proximity of the pontoons, which allows the ramp to make contact at a lower angle.
    Some promising avenues of future work have been identified during this investigation. First and foremost, increasing the scale of CASPER and operating it in a field environment would be key in determining if this architecture would be suitable for extra-terrestrial ISRU and terrestrial mining applications. One major avenue of upgrading CASPER's capability would be adding a navigation system in order to precisely control the direction of travel and correct for any deviations due to the lateral forces induced by the excavation ramp. Furthermore, adding a system to measure the excavated mass in real time would enable an investigation on how the dynamics and mobility performance of the craft is impacted as it takes on mass. Ultimately, this investigation has shown a great deal of promise for discrete scooper excavation systems, and has shown that Archimedes screw propulsion systems can substantially augment the excavation capabilities as long as the screws rotate in the proper direction to churn material towards the excavation system.

Acknowledgements

The authors would like to thank the members of the ASU BIRTH Lab for their assistance and Arizona State University for funding.  

Conflict of interest

The authors report no conflicts of interest.