Research On The Characteristics Of A Nonlinear Isolator With A Curved-Spring-Roller Mechanism

During disaster relief,vehicle-mounted precision medical devices may be damaged due to austere or complicated enviomental and road conditions as well as vehicle violent vibration induced by engine transmission system,so that the effiency of disaster relief is decreased greatly.It's proved two-stage vibration isolator or platform contributes to enhancing the intactness and reliability of the vehicle-mounted devices by vibration isolation.In this paper,based on high-mobility offroad vehicle chassis the nonlinear vibration isolator for vehicle-mounted precision devices has its static characteristics as well as dynamic characteristics at ideal or nonideal states explored comprehensively,and a nonlinear isolator with curved mount-spring-roller mechanism is designed and undergoes the trials for static and dynamic characteristics as well as actual application.The paper falls into the following parts:(1)Study on the static characteristics of the curved mount-spring-roller mechanism nonlinear isolator and its optimization.Taking the curved mount-spring-roller mechanism as a negative stiffness,in parallel with a vertical positive spring,the nonlinear isolator with quasi-zero stiffness characteristics can be obtained.Its mechanical model can be gotten based on the law of conservation of energy,and the expression of the restoring force and the stiffness with the displacement of the isolator are achieved,then the zero stiffness condition around the equilibrium position is obtained,and the effect of the configurative parameters on the restoring force and the stiffness are also studied.Then the optimization model with the non-dimensional displacement as the objective function is established,and the optimized configurative parameters are obtained through genetic algorithm.In the end,the restoring force of the isolator is approximated using the Taylor expansion to a cubic stiffness.The conclusions are just as: the quasi-zero stiffness can be got with appropriate configurative parameters,and on the basis of satisfying with the dimensional limitation,the isolator should be designed as big as enough roller and curved mount radiuses,and the increasing of the pre-compression amount of the horizontal springs is helpful to reduce the dynamic stiffness of the isolator and increase the isolation performance.Changing the pre-compression amount of the vertical spring,the loading mass of the isolator can be changed,and its application range can be enlarged accordingly,in which there is a better isolation performance.(2)Dynamic characteristics of the nonlinear isolator in an ideal state.On the basis of Newton's second Law,the general nonlinear dynamic equitation of the isolation system with quasi-zero stiffness is set up,and the following dynamic analysis is based on this equitation.In this section,the steady state response and the absolute displacement transmissibility of the nonlinear system under a harmonic displacement excitation are determined through the average method,and the effects of the excitation amplitude,the damping ratio,the cubic stiffness coefficient and the configurative parameters of the isolator on the isolation system response and the transmissibility are studied.The response of the isolation system under different excitation signals such as the harmonic excitation,the multiple excitation,the random excitation and the shock excitation,are calculated by the Runge-Kutta Method.The following conclusion can be decided: the nonlinear isolation system is a hardening stiffness system,and its frequency response curve bends to the right,the system response may involve in jumping phenomenon.In the high frequency region,with a bigger excitation amplitude,a smaller damping ration or a bigger cubic stiffness coefficient,the system response may have a bigger response peak value,even to an infinite response value.The maximum of the excitation amplitude should be limited in a proper region,the damping ratio should be regulated appropriately,and the cubic stiffness coefficient should be reduced as far as possible.The nonlinear system has evidently more excellent isolation performance than the equivalent linear system under the harmonic excitation,the multiple excitation,the random excitation and the shock excitation signal.(3)Dynamic characteristics of the nonlinear isolator in an imperfect state.Considering the imperfect load and stiffness of the nonlinear isolation system under a vertical displacement excitation from the base of the van,that is transmitted to the isolated equipment,the nonlinear differential equations of motion for the isolation system are established accordingly.The solutions of the two nonlinear equations are deduced by the Harmonic Balance(HB)method,a useful approximate analytic method for the nonlinear vibration equation.The approximate solutions are verified by the numerical solutions obtained from the Runge-Kutta method,a normal numerical integration method for the nonlinear equation.The absolute displacement and acceleration transmissibility are defined,and the frequency responses of the systems are emphasized,the effects of the imperfect load and stiffness on the dynamic response characteristics of the systems are obtained.It can be concluded from the simulation results: If only the load is imperfect,i.e.the isolation system is an overloaded system or underload system,the system still has a better isolation performance for the low-frequency vibration isolation than the equivalent linear system and the ideal system,and it can be adapted with the excitation amplitude increasingly.If only the stiffness is imperfect,the dynamic characteristics of the system is similar to the ideal system,and the low frequency vibration isolation becomes better with the stiffness change reducing.If the load and the stiffness are also imperfect,the dynamic characteristics of the system is characterized by the similar characteristics with the only imperfect-loaded system,and the system having the minimum resonance frequency can be obtained with some stiffness change and some overload or underload mass.To get a better isolation performance,it should be in a positive stiffness for the nonlinear isolation system if it cannot be determined whether the static equilibrium position of the system is the minimum stiffness point with its load mass in using.(4)Prototype design and the related experiments studies.The prototype of the nonlinear isolator is developed and assembled based on the theoretical analysis,and then the two experimental platforms are established for the static and dynamic experiments respectively.The experiments are carried out as follows: the static experiments,the vibration experiments and the test for the shock excitations and the random excitation from C1 road spectrum.The experimental results indicate that the actual restoring force and displacement curve of the proposed prototype is in accordance with the theoretical result very well,nevertheless there are some deviations on the linear springs for the materials and machining precision factors.The vibration experiments are accomplished in overload or underload system for the ideal quasi-zero system can hardly get.For the proposed prototype,the resonance frequency and its respective transmissibility peak value reduce first and then increase with the excitation amplitude becoming bigger.This regulation agrees very well with the theoretical results.Compared with the equivalent linear system,the overload and underload systems get a smaller dynamic stiffness.In other words,the nonlinear system has a better low frequency isolation performance even if used in an imperfect state.Under actual shock excitations or random excitations,a very good shock isolation,a high shock attenuation rate and an excellent isolation for random excitations can be obtained in the experiments.(5)Travelling experiments for the nonlinear isolation platform on the EQ2050 chassis.An on-board precision instruments isolation platform is developed using the proposed nonlinear isolator,it's mounted on the high-mobility chassis EQ2050,and then the travelling experiments are carried out.The vibration can be isolated by the vibration isolation platform greatly when the vehicle travels on the asphalt road and cross-country road with different speed,the vibration isolation efficiency is about 54.5% to 76.8%.From the experiment results,it can be known that the vibration from the base of the van can be isolated very well through the proposed vibration isolation platform,the two-stage vibration isolation effects are evident and well done.On the static and dynamic analysis of the nonlinear isolator,the dynamic characteristics analysis of the proposed isolator under the ideal or nonideal state,the nonlinear isolator prototype is developed and assembled.The theoretical analysis results are verified by experiments in this paper.The method using in this paper can provide an effective reference for the similar research,and it is significant for the research on the two-stage vibration isolation of the on-board precision medical device.