Investigated are PID-type controllers that use noncollocated sensory feedback for damping vibrations of elastic structures with high friction in the actuators. Based on a two-degree-of-freedom spring-mass system, it is demonstrated how the control-lability of such systems can be considerably improved by feedback of noncollocated measurements. Friction in the actuators leads to undamped, residual vibrations that cannot be controlled because the occurring forces do not suffice to overcome static friction. The boundaries of this uncontrollable domain are derived in the state space. A positive feedback of noncollocated sensory signals renders a stronger coupling of the motions of the actuators to structural deformations, resulting in a considerable reduction of the amplitudes of uncontrollable vibrations. Additionally, steady state errors can be significantly reduced because higher controller gains can be realized. It is shown that the time behavior of the deformations of an elastic structure can be predicted in good approximation by a linear model that neglects friction if the proposed controller is used.

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