Natural Oscillations of Underactuated Cable-Driven Parallel Robots - Complete Experiments Data

Underactuated Cable-Driven Parallel Robots (CDPR) employ a number of cables smaller than the degrees of freedom (DoFs) of the end-effector (EE) that they control. As a consequence, the EE is underconstrained and preserves some freedoms even when all actuators are locked, which may lead to undesirable oscillations. This paper proposes a methodology for the computation of the EE natural oscillation frequencies, whose knowledge has proven to be convenient for control purposes. This procedure, based on the linearization of the system internal dynamics about equilibrium configurations, can be applied to a generic robot suspended by any number of cables comprised between 2 and 5. The kinematics, dynamics, stability, and stiffness of the robot free motion are investigated in detail. The validity of the proposed method is demonstrated by experiments on 6-DoF prototypes actuated by 2, 3, and 4 cables. Additionally, to highlight the interest of this modelling strategy in a robotic context, we apply it to the trajectory planning of a 6-Dof 4-cable CDPR by means of a frequency-based method (multi-mode input shaping) and experimentally compare the latter with traditional non-frequency-based motion planners.