Active damping for the control of flexible structures

Abstract

Active damping can be generated in a number of ways, the most obvious one being applying a force to the structure which is proportional to the velocity at a point on the structure. Another technique is to actively increase the damping of a structure by using a passive damping element i.e. dissipate the energy of vibration by increasing the relative motion of the structure with respect to the passive damping element. A practical way to achieve this is by using constrained layer damping (CLD). It is well known that CLD is effective because of the relative motion of the structure and the constraining layer which produces extra shear in a layer of viscoelastic material. The work described in this paper is concerned with controlling the strain of the constraining layer of a composite structure in such a way as to enhance the shear generated in the viscoelastic material and hence improve the overall damping of the composite structure. If the control law selected forces the constraining layer to move 180 degrees out of phase with the structure then the viscoelastic damping layer undergoes maximum shear and active damping is achieved. The results have indicated that this concept of active damping produces very effective levels of vibration suppression. In the case of cantilever beams the first two modes can be almost eliminated when velocity feedback of the beam tip is used. The results show that the addition of active control and passive damping in a single structure combines the advantages of passive damping in the higher modes and active control in the lower modes. In addition active damping as defined in this paper produces a fail safe mechanism in case of instability occurring in the feedback system since passive damping is always present.