CASPER is a screw-propelled discrete scooper excavation robot, and is shown in Figure \ref{357248} and Figure \ref{541285}. It is driven by a system of four counter-rotating Archimedes screw pontoons. Each of these pontoons are driven by an internal 12 V DC motor which is sealed against sand ingress. The pontoons serve primarily as the mobility system, but also play a key role in the excavation system. The excavation system consists of two major components, the pontoons, and the ramp. The pontoons churn up the material and propel the ramp forward, while the actuated ramp collects the material and rotates to deposit the material into the collection bin. The majority of CASPER parts were 3D printed in polyethylene terephthalate glycol (PETG) on an fused deposition modeling (FDM) 3D printer. 
    A Raspberry Pi was used to control all craft functions and collect experimental data. The screw pontoons were each powered by a 75:1 DC gear motor (Pololu; item #4846) and the ramp was actuated using a 488:1 DC gear motor (Polulu; item #3723). The ramp required a very high gear ratio motor because it needed to be able to maintain the desired angle while experiencing very high drag forces from the granular media. Each motor comprised of a motor, gearbox, encoder, current sensor, and a motor controller for speed control. The craft was tethered to and powered by an external power supply providing a constant voltage of 12 V to the system. A proportional-derivative (PD) controller was utilized to regulate pontoon speeds to ensure accurate speed control and was tuned such that the craft consistently achieved steady state in approximately 0.9 - 1.3 seconds for all screw angular velocity settings. This guaranteed that the craft would be at steady state when the ramp excavation procedure began at 2 seconds after run start.