Development of a Parallel Link Crane: Modeling and Control of a System With Unilateral Cable Constraints

A parallel link crane with positioning and orientation capabilities has been developed. This paper presents a dynamic model of this cable-controlled system with unilateral cable constraints. The dynamic model is developed using multi-body dynamics framework and the multiple point impact problem solved using linear complementarity theory. The sources that induce a loss of constraint in the system are identified and a control strategy that would maintain the unilateral constraints has been developed. In the event of loss of cable tension, a feedback control strategy that would recover from that state without inducing slackness in any of the other cables and would minimize the impulsive forces at the moment of imposition of constraint is also presented. The feedback control strategy can be generalized to problems in catching and in controlling impacts.