Benefit of "Push-Pull" Locomotion for Planetary Rover Mobility

Abstract

As NASA's exploration missions on planetary terrains become more aggressive, a focus on alternative modes of locomotion for rovers is necessary. In addition to climbing steep slopes, the terrain in these extreme environments is often unknown and can be extremely hard to traverse, increasing the likelihood of a vehicle or robot becoming damaged or immobilized. The conventional driving mode in which all wheels are either driven or free-rolling is very efficient on flat hard ground, but does not always provide enough traction to propel the vehicle through soft or steep terrain. This paper presents an alternative mode of travel and investigates the fundamental differences between these locomotion modes. The methods of "pushpull" locomotion discussed can be used with articulated wheeled vehicles and are identified as "walking" or "inching/inch-worming". In both cases, the braked nonrolling wheels provide increased thrust.An in-depth study of how soil reacts under a rolling wheel vs. a braked wheel was performed by visually observing the motion of particles beneath the surface. This novel technique consists of driving or dragging a wheel in a soil bin against a transparent wall while high resolution, high-rate photographs are taken. Optical flow software was then used to determine shearing patterns in the soil. Different failure modes were observed for the rolling and braked wheel cases. A quantitative comparison of inching vs. conventional driving was also performed on a full-scale vehicle through a series of drawbar pull tests in the Lunar terrain strength simulant, GRC-1. The effect of tire stiffness was also compared; typically compliant tires provide better traction when driving in soft soil, however it's been observed that rigid wheels may provide better thrust when non-rolling. Initial tests indicate up to a possible 40% increase in pull force capability at high slip when inching vs. rolling.

UTOPUS Traction Technology: A new method for planetary exploration of steep and difficult terrain

“…Seeni et al (2008) provide a general discussion of mobility concepts for planetary surface exploration. Inching locomotion for exploration in difficult terrain has been discussed by Wettergreen et al (2010), Moreland et al (2011) and Creager et al (2012). The idea of inching locomotion itself goes back to at least Schreiner and Czako (1973).…”

Visualizing and Analyzing Machine‐soil Interactions using Computer Vision

“…This research [see also Creager et al () and Moreland et al ()] provides a better understanding of the thrust benefit of push‐rolling through full‐scale robot tests in the laboratory as well as soil flow visualization and analysis using SOFT.…”

“…Drawbar pull force being applied to Scarab through a load cell at rear wheel hubs (Creager et al., ).…”

“…Comparison of rolling versus inching for compliant and rigid wheels in loose GRC‐1 (Creager et al., ). For rolling, the load was increased approximately every 0.5 m; for inching, the load was held constant for at least two inching cycles, and then increased.…”

Interlocking Spikes for Extreme Mobility

“…Ship and land anchors often use lateral forces for penetration and extraction. Creager et al (2012) have shown that a push-pull device with independently articulated pairs of wheels can double the pull/weight ratio, presumably because braked wheels have higher static friction. Bover (2011) discovered that a push-pull device with frames that move independently along a common axis can use the resulting lateral forces to penetrate interlocking spikes into the ground and extract them from the ground.…”