Research On Quasi-zero-stiffness Vibration Isolator With Asymmetric Stiffness And Damping Characteristics

There exists kinds of harmful vibration in the mechanical system and its working environment,which will affect the performance of precision instruments,reduce the riding comfort of construction machinery drivers and vehicle passengers,and destroy the concealment of military equipment.Therefore,it is necessary to control the vibration of the mechanical system and eliminate the harmful vibration.It is an effective vibration control method to install a vibration isolator between the isolation object and the vibration source.Traditional linear vibration isolators are simple in structure and widely used.However,they are faced with the contradiction between high load capacity(requiring high stiffness)and low-frequency vibration isolation(requiring low stiffness)under limited displacement deformation,which is difficult to meet the requirements of high vibration isolation performance.The nonlinear vibration isolators with high static and low dynamic stiffness properties have been widely studied for its good bearing capacity and low-frequency vibration isolation performance,and has been successfully employed in low-frequency and even ultra-low-frequency vibration isolation of precision instruments.Based on the National Key Research and Development Program "Research on the key technology of special chassis and suspension for high-mobility emergency rescue vehicles(including firefighting vehicles)"(No.2016YFC0802902),we innovatively design a passive quasi-zero stiffness vibration isolator with asymmetric stiffness and damping characteristics(called asymmetric quasi-zero stiffness vibration isolator,AQZS-VI),and study the static and dynamic characteristics of the AQZS-VI,systematically.The main contents and conclusions are listed as follows:(1)Structural design and static characteristics research of AQZS-VI.Based on the design strategy of constructing quasi-zero stiffness vibration isolator(QZS-VI)by parallel connecting positive and negative stiffness mechanisms,a negative stiffness mechanism(NSM)including L-shaped lever and cantilever leaf spring is designed,and an AQZS-VI is constructed by parallel connection of the NSM with linear springs.Considering the geometric nonlinear characteristics of the mechanical structure,the static models of the NSM and the AQZS-VI are established.By analyzing the static characteristics of the NSM,the influence of the three key structural parameters(the distance between the cantilever leaf spring and the horizontal guiding mechanism,the lengths of the long arm and the short arm of the L-shaped lever)on the static characteristics of the NSM is obtained,which provides guidance for the structure design of the NSM.Different from the symmetrical quasi-zero stiffness isolator,the restoring force provided by the NSM is greater than the one provided by the positive stiffness spring near the zero stiffness point,which helps to improve the static loading capacity of the isolator under small displacement deformation.Compared with the corresponding linear isolator(having the same static bearing capacity under the same displacement deformation)and another QZS-VI with three springs,the designed AQZS-VI has a wider displacement range of low stiffness and better static characteristics near the zero stiffness point.(2)The dynamic characteristics research of the AQZS-VI.The nonlinear damping force of the isolator is simplified as the linear viscous damping force.The dynamic models of the isolator are established under both of the force excitation and base displacement excitation.The Harmonic Balance Method is used to solve the main harmonic resonance response of the isolator.The stability of the response is analyzed using Floquet theory.The accuracy of the analytical solution is verified by the numerical method of Fourth order Runge-Kutta.The main harmonic resonance response,sub-harmonic resonance response and vibration transmissibility characteristics of the isolator are analyzed under both of the force excitation and base displacement excitation.The vibration isolation performance of the isolator is compared with that of the linear isolator and the QZS-VI with three springs.The analytical results show that compared with the corresponding linear isolator and the QZS-VI with three springs,1)under the force excitation,the AQZS-VI has lower isolation frequency and peak transmissibility,and the response in resonance region is more stable.However,after passing the resonance frequency,the force transmissibility of the AQZS-VI is higher than that of the QZS-VI with three springs;2)Under the base displacement excitation,the AQZS-VI has lower isolation frequency and peak transmissibility,and its stability is obviously better than that of the QZS-VI with three springs.(3)The research of nonlinear Coulomb friction damping characteristics of the AQZSVI.Considering the intrinsic nonlinearity of Coulomb friction and the structural geometric nonlinearity of the AQZS-VI,the nonlinear Coulomb friction damping force model of the isolator is established.The influence of the main structural parameters of the isolator on its friction damping characteristics is analyzed.The influence of nonlinear Coulomb friction on the amplitude-frequency response and vibration isolation performance of the isolator are analyzed under both of the force excitation and base displacement excitation.It can be obtained that 1)under the force excitation,the increase of Coulomb friction factor will reduce the force transmissibility in the resonance frequency range,increase the force transmissibility among the vibration isolation frequency,and reduce the resonance frequency of the isolator;2)under the base displacement excitation,Coulomb friction makes it happen for the isolator to move rigidly in low frequency region,that is,the load and the base move synchronously,increase the Coulomb friction factor can reduce the absolute displacement transmissibility in the resonance frequency range,even restrain or eliminate the occurrence of resonance phenomenon,and broaden the frequency range of rigid motion.(4)Research on dynamic characteristics of AQZS-VI under load imperfect.The dynamic model of the AQZS-VI considering load imperfect is established.The main resonance response characteristics and vibration isolation performance of the vibration isolator under both of the force excitation and the base displacement excitation are analyzed.The results show that under the condition of underload,increasing the absolute value of displacement offset can make the characteristics of vibration isolator gradually change from the harden stiffness to soften stiffness,at the same time,increases the resonance frequency of vibration isolator,and increases the force and displacement transmissibility of isolator in higher frequency.(5)Prototype design and experimental study of AQZS-VI.The physical prototype of AQZS-VI is designed and manufactured,and the static experiment,hammer excitation experiment and harmonic excitation experiment are carried out based on the prototype.The experimental results are obtained that 1)the statics model is feasible;2)there exists large Coulomb friction force in the prototype;3)the vibration isolation performance of the prototype is obvious under hammer excitation;4)under harmonic displacement excitation,the prototype has lower initial vibration isolation frequency and lower vibration transmissibility than the corresponding linear vibration isolator.