Shady: Robust Truss Climbing with Mechanical Compliances

1 Motivation Many large terrestrial structures—towers, bridges, construction scaffolds—are sparse assemblies of rigid bars connected together at structural nodes. This is also true of many in-space structures such as antennae, solar panel supports, and space-station members. A long-term application of truss climbing robots is automated assembly, repair, and inspection of such truss-like structures: one or more climbing robots could grip the bars and locomote about the truss, conveying sensors, tools, or construction materials. The robot could then either carry out the desired task on its own or cooperate with a human [1, 7]. Truss climbing is a special case of structure climbing, with some particular challenges. Many previous structure climbing robots, e.g. as in Pack et al [9], and others referenced therein, are intended to climb on assemblages of 2D planar surfaces. Only a few structure climbing robots, such as the mechanism described by Nechba et al in [7], are also designed for climbing on truss-like structures where the members are more nearly 1D links. The penalties for uncertainty are potentially higher for truss climbing than for climbing on planar surfaces. Consider foot placement. On a large 2D surface , foot placement can be resilient to significant parallel-plane misalignment, usually does not require strong certainty of the perpendicular distance to the surface (as the foot can often be extended until it hits the surface), and is similarly tolerant of orientation uncertainties. However, the comparable task in truss climbing—gripping a thin structural member starting from a nearby but uncertain spatial pose—can be much more sensitive: even small translation and orientation misalignments can result in a weak or missed grip. We propose a minimalist mechanical structure with several specific and intentional mechanical compliances (springs), detailed below, to address these challenges. Combined with wide-opening grippers and proprioceptive control algorithms, also detailed below, we hypothesized that these compliances could allow significant translation and orientation misalignment and still succeed in a firm and certain grip.