On the response of a shaken cable-suspended mass

A small mass is suspended by two cables which are attached at angles to a rigid frame. The frame is shaken harmonically in the horizontal direction, causing the mass to respond typically in an erratic manner as the cables alternately become taut and slack. Since the cables are assumed to be inextensional, they provide an effective impulsive force to the mass as they become taut, causing an elastic rebound (which is modeled using a coefficient of restitution) [Plaut and Farmer, 2000]. The mass is assumed to be a point mass, and motion takes place within a vertical plane. One of the practical motivations for this work is the potential for using moored buoyant devices in an attempt to mitigate wave energy. Simulations capture the unpredictable dynamic response of the mass, despite the relatively simplistic nature of the modeling. Special care is taken to accurately model the sudden transition when a cable instantaneously becomes taut. In the general case, the motion has chaotic characteristics and Poincare sampling provides a useful diagnostic tool [Virgin, 2000]. The experimental results also show the complex nature of the behavior. The motion of the center of mass is obtained using a video camera and the IMAQ image processing package within LabVDEW. The results agree qualitatively with simulation.