A tunable 'negative' stiffness system for vibration control

Commonly used variable stiffness methods for vibration control are employed to alter a system's resonant frequency by increasing its stiffness. The concept of "negative" stiffness could be used to decrease the system stiffness; thus, reducing resonant systems' frequency. A tunable stiffness isolation device (TSID) with negative stiffness capability enables a controlled mass to be isolated in a large range of excitation frequencies. This study presents theoretical and experimental investigations of a tunable stiffness system with negative stiffness. The proposed system comprises two electromagnets, two rubber elements and a mass. The negative stiffness effect is obtained from a magnetic force which is nearly a linear function of amplitude in small vibrations. A finite element analysis is performed to obtain a relation between the magnetic force and geometrical dimensions, as well as electromagnets' characteristics. The force transmissibility of the system under different applied currents for a frequency range of 30 to 120Hz is investigated. The results show that the system's resonant frequency decreases with the increased applied magnetic field.