Constraints identification for vibration control of time-varying boundary conditions systems

The focus of this paper is on modal identification and updating techniques for a mechanical system characterized by time-varying boundary conditions. In these systems it is very difficult to develop effective robust controllers in reason of the large variations of the dynamic model parameters. In most mechanical systems, such as NC machines, model parametric variations are due to changes in the position of a physical constraint. This fact determines continuous changes in system dynamics (system natural frequencies and vibration modes) while we may assume that constraint properties (such as stiffness and damping coefficients) remain unaltered. Nevertheless constraint properties can't be determined analytically so it is essential to perform experimental tests. Assuming that a FEM model of the mechanical system may well reproduce its free-free dynamics (without physical constraints) we may easily identify constraints mechanical properties. If these properties are satisfactorily known we may relate system dynamic properties to the effective position of the moving constraint(s) and therefore to perform realistic dynamic simulations and develop useful control systems for vibrations suppression. In this paper the preliminary results of the studies on the active vibrations control of a ball-screw moving the carriage of a NC wood machine tool are briefly described. In particular we want to present how the elastic properties of the constraints sustaining the screw and the elastic properties of the interface between screw and nut have been extracted starting from some experimental data.