This work presents a Negative-Positive Stiffness System (NPSS) based on a set of two double acting pneumatic linear actuators (PLA). The NPSS is added to a single degree of freedom system which consists of a sprung mass and an air spring. One end of each PLA is jointed to the sprung mass while the other end is jointed to the vibrating frame. In addition, the PLAs are symmetrically arranged so that they remain horizontal while the sprung mass is in static conditions. Depending on the pressurised chamber (rear or front), the vertical component of the force applied by the PLAs will favour or oppose vibration of the sprung mass, which leads to a reduction or increase of the dynamic resonance frequency of the overall system. The proposed adaptive control strategy allows to achieve a noteworthy vibration isolation of the sprung mass when the system is subjected to a chirp type signal. The decision to switch the pressure from the rear to the front chambers (or vice versa) is taken in accordance to the transmissibility functions of the system. These tests showed improvements regarding sprung mass isolation in comparison to the passive system without NPSS.
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