Concurrent multi-link deployment of a gravity-assisted underactuated snake robot for aircraft assembly

This paper presents algorithms for concurrent deployment of multiple links of a gravity-assisted underactuated robot arm. The joints of the hyper-articulated arm have no dedicated actuators, but are activated with gravity. By tilting the base link appropriately, multiple unactuated links may be steered simultaneously to desired angular positions. This underactuated arm design was motivated by the need for a compact snake-like robot that can go into aircraft wings and perform assembly operations using heavy end-effecters. The dynamics of the unactuated links are essentially 2(nd) order nonholonomic constraints, for which there are no general control algorithms. We perform a controllability analysis to establish the feasibility of multi-link positioning using the available inputs, viz., the biaxial tilts of the base link. We propose a feed-forward control algorithm for simultaneous positioning of multiple links. We also propose an intermittent feedback control scheme to compensate for disturbances acting on the system. We built a 4 link prototype where the base is tilted using a Stewart Platform. The proposed control schemes are implemented on our prototype system. The experimental results indicate the efficacy of the control schemes.