Design of a combined magnetic negative stiffness mechanism with high linearity in a wide working region

Combining magnetic negative stiffness mechanism (NSM) in parallel with positive stiffness has been considered to be an effective approach to realize the quasi-zero stiffness (QZS) characteristic, thus resolving the contradiction between high load capacity and (ultra-) low-frequency vibration isolation capability. However, the remarkable stiffness nonlinearity of common magnetic NSMs restricts the displacement region with reliable negative stiffness, resulting in considerable nonlinear behavior, poor vibration attenuation performance, and probable instability under large amplitude vibrations. A novel combined negative stiffness mechanism (CNSM) with attractive magnetic NSM (ANSM) and repulsive magnetic NSM (RNSM) in parallel is proposed in this paper. The stiffness nonlinearities of the ANSM and RNSM in the CNSM are counteracted through the parallel configuration such that the displacement region with reliable linear stiffness of the CNSM is widened by several times. An analytical model of the CNSM is established by the magnetic charge model and verified by simulation on ANSYS Maxwell. Parametric studies are then conducted to investigate the effects of design parameters on the stiffness characteristic, providing guidelines for the optimal design of the CNSM. Meanwhile, the stiffness and nonlinearity of the CNSM are compared with that of a single ANSM and RNSM. Static and dynamic experiments are finally conducted on the proposed test prototypes. Experimental results demonstrated the validity of the established model and the effectiveness of the CNSM in generating high linear stiffness within a wide displacement region and lowering the resonance frequency. Thus, the proposed CNSM can be applied in (ultra-) low-frequency vibration isolation under large amplitude excitations.