Characteristic Of Fluid Damper And Its Effect On The Whole-Spacecraft Vibration Isolation

Compared to other damping, the fluid damping provides a higher load capacity with a smaller displacement. And the stiffness and damping coefficient of the isolator are completely independent. The idea of Whole-Spacecraft Vibration Isolation (WSVI) is to put an isolator into or use it to replace a payload attach fitting, which is a section connecting the launch vehicle and the spacecraft, and can effectively improve the dynamic environment of the satellite. The Multi-Actuator Vibration Isolation Platform (MVIP), in which the fluid damper is selected as actuator, is an important method of WSVI. To provide the references to the design of the fluid damper and platform, in this dissertation, based on the fluid damper and its application in MVIP, the nonlinear elements which affect the characteristics of the fluid damper are investigated both theoretically and experimentally.With the research of the mechanism of fluid damping, the expression of the fluid damping force is established. And to meet the requirement on the actuator of MVIP, some types of the fluid damper is designed, analyzed and tested.During the experiment of fluid damper with a piston, we found that the performance of damper is different with different excitation amplitude. Based on the bi-linear hysteresis model which is used to describe the friction force in the damper, the nonlinear dynamic equation of the damper is established and solved by the harmonic balance method. And the effect of the friction on the performance of the damper is investigated. Moreover, considering the effect of the friction, the nonlinear dynamic models of an MVIP with eight actuators (OVIP) are established with two kinds of the passive actuator: the two-parameter model and the three-parameter model. The results show that no matter what in single damper, or OVIP, the friction interface has two states: sticking and slipping. Under the small excitation amplitude, the friction force behaves as spring and causes a shift of resonant frequency.From the experiment of the fluid damper with bellows, it is found that the characteristic of the damper behaves as a two degree-of-freedom system. So a two degree-of-freedom model, which accounts for the mass and the nonlinear damping of the fluid, is established. Furthermore, the dynamic equation of the OVIP which is composed of this type of damper is developed. And the effect of the fluid mass and the nonlinear damping on the performance of the isolator is discussed. The results indicate that when the damping is small, the effect of the fluid mass must be taken into account, which will cause a resonance in high frequencies and thus reduce the performance of vibration isolation. If the damping is large enough, the effect of the fluid mass can be negligible and the model of damper can be simplified as the three-parameter model.The energy transformation and heat exchange are the major characteristics of a fluid damping. Based on the temperature-viscosity characteristics of the fluid, the interaction effect of the fluid damping force and the fluid temperature is studied. And the thermodynamic equation of the damper, which includes a heat balance equation, is established. The Short Time Fourier Transform is applied to analyze the dynamic performance in the time and frequency domain for dealing with the time variance and the nonlinearity of the system induced by the effect of viscous heating. It is found that with the past of the time, the temperature of the damper increased, and thus resulted in a shift of the resonance frequency and the changes of the resonance amplitude, until the system reaches a dynamic balance.In the actual condition, many nonlinear factors operate simultaneously and couple with each other. So the combining effect of viscous heating and friction force in the damper with a piston is investigated. And it is concluded from the numerical results that when the friction force is taken into account, the effect of the viscous heating on the performance of vibration isolation can be negligible. In consideration of the effect of the viscous heating, the thermodynamic model of the damper with bellows is developed based on the two degrees-of-freedom system. Analysis shows that with a small damping coefficient, the effect of the viscous heating can be neglected. But in the condition of a large damping coefficient, the effect of viscous heating must be taken into account. In addition, the property of system is same as the condition of the single degree-of-freedom.