Dynamics and nonlinear effects of a compact near-zero frequency vibration isolator with HSLD stiffness and fluid damping enhancement

Aiming to isolate disturbance vibration for heavy payloads with low frequency, a novel hydro-pneumatic near-zero frequency vibration isolator is proposed, which possesses high-static and low-dynamic (HSLD) stiffness. And different from most isolators existing previously, a nonlinear damping strategy realized by fluid damping mechanism is implemented into the device in order to enhance vibration isolation performance. Due to that the natural frequency of isolation system is close to zero and loading capacity can be adjusted by the gas pressure, the proposed device is termed as pneumatic near-zero frequency (NZF) vibration isolator. To obtain the primary resonance response for the harmonically forced nonlinear system, the averaging method is employed first and verified numerically. A direct current (d. c.) term is added in the analytical trial solution, which reflects the asymmetry existed in dynamic response. As a result, multiple steady solutions are observed and the induced amplitude jump appears. Then the stability of periodic response is carried out by using the Jacobin matrix approach. It is concluded that increases of all types damping including linear, square and friction damping are able to shrink the unstable region and further reduce the possibility of occurrence of amplitude sudden-jump in primary resonance. More accurately, the critical border which can separate the jump and none jump zones is found analytically and numerically, and design criterion for avoidance of sudden-jump is developed. After that, equivalent damping analysis gives a new insight into how nonlinearity effects of each damping dominate vibration isolation transmissibility in resonance and effective isolation frequency band. It is shown that the square type damping based on fluid mechanism can suppress the resonance response and transmissibility and does not decrease the effectiveness in isolation band, which will bring the favorable advantage for NZF isolator in practical application.