Closed loop control of a gravity-assisted underactuated snake robot with application to aircraft wing-box assembly

Abstract

Stable, nonlinear closed-loop control of a gravityassistedunderactuated robot arm with 2nd order non-holonomicconstraints is presented in this paper. The joints of the hyperarticulated arm have no dedicated actuators, but are activatedwith gravity. By tilting the base link appropriately, the gravitationaltorque drives the unactuated links to a desired angularposition. With simple locking mechanisms, the hyper articulatedarm can change its configuration using only one actuator at thebase. This underactuated arm design was motivated by the needfor a compact snake-like robot that can go into aircraft wingsand perform assembly operations using heavy end-effecters. Thedynamics of the unactuated links are essentially 2nd order nonholonomicconstraints, for which there are no general methodsfor designing closed loop control. We propose an algorithm forpositioning the links of an n-link robot arm inside an aircraftwing-box. This is accomplished by sequentially applying a closedloop point-to-point control scheme to the unactuated links. Wesynthesize a Lyapunov function to prove the convergence ofthis control scheme. The Lyapunov function also provides uswith lower bounds on the domain of convergence of the controllaw. The control algorithm is implemented on a prototype 3-link system. Finally, we provide some experimental results todemonstrate the efficacy of the control scheme.