Of the multitude of available control techniques, modal control is a favourite amongst structural dynamicists because of its representation in modal coordinates. The term modal control is used to describe a wide variety of control techniques which find their origin in a description of the system through the main coordinates, defined by the modes of vibration of the system. This approach stems from the consideration that the response of a mechanical system to a disturbance is the sum of the independent responses of its vibrational modes. This motivates the desire to design a control that does not alter these mode shapes, but allows to change the natural frequency and the damping of each mode. In active vibration control the purpose is to increase the damping of modes interested in the vibratory phenomenon. The paper shows how stability, spillover effects, system controllability and sensors and actuators position are all linked to the analysis of the controlled system damping matrix and to the possibility that the forces introduced by the control is non-dissipative. Theoretical aspects are supported by numerical simulations.

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