Characteristics Analysis And Application Study Of The Quasi-zero Stiffness Isolator Using Air Spring

Excellent vibration isolation performance of the vehicle-mounted stretcher support system is of great significance in improving the ride comfort of the patient and increasing the quality of the ambulance.Recent advances of the quasi-zero stiffness(QZS)isolator and the application of vibration isolation techniques in the vehicle-mounted stretcher support system are presented.On the base,a novel QZS isolator of adjustable loads is designed to satify the variable isolated mass of vibration isolation system,which is resulted by the changing amouts,quantities and positions of the patients.Firstly,the characteristics and vibration isolation performance of the air spring quasi-zero stiffness isolator(ASQZSI)are studied theoretically and experimentally.Then,the ambulance-mounted stretcher support vibration isolation system is established by applying the ASQZSI in a certain field emergency ambulance.Lastly,the vibration isolation performance of the system is evaluated by carring out the road test.The main work and conclusions are as follows:(1)Design of the ASQZSI and study on its static characteristics.Based on the theory about combined positive and negative stiffness,a novel QZS isolator capable of adjustable loads is designed using air spring,and its static characteristics are discussed.The parameters of the ASQZSI for keeping its QZS characteristics are obtained by analyzing its static characteristics.The results show that increasing the length of the connecting rod and the distance between the connection point of load platform and fixed base of horizontal air spring when initial air pressure of air spring remaining constant accordingly extends displacement range under QZS characteristics around the equilibrium point position.When structure parameters of the ASQZSI are constant,the minimum stiffness of the ASQZSI is determined by initial air pressure of both horizontal air springs and vertical air spring.When initial air pressure of vertical air spring is remained constant,increasing air pressure of horizontal air springs can improve its negative stiffness in vertical direction,and accordingly lower stiffness of the ASQZSI.However,the negative stiffness happened to the ASQZSI when the initial air pressure of horizontal air springs increasing up to a certain value.When loads on the ASQZSI varying,the good QZS characteristics are kept by adjusting air pressure on both vertical and horizontal air springs.(2)Study on dynamics characteristics of the ASQZSI.The nonlinear dynamics characteristics of the ASQZSI are studied based on its static characteristics for getting its nonlinear characteristics.The nonlinear dynamic equations of the air spring quasi-zero stiffness vibration isolation system(ASQZSVIS)under harmonic displacement excitation are established,and its steady-state approximate solution is obtained by using the Harmonic Balance Method.The effects of the excitation amplitude,damping ratio and structural parameters on the ASQZSVIS and the single air spring vibration isolation system(SASVIS)are studied by using absolute displacement transmissibility as estimated index.In contrast to the SASVIS,the ASQZSVIS isolate much more vibration with lower frequency under the same excitation amplitude,and it also exhibits better isolation performance when excitation frequency is larger than its jump-down frequency.Properly controlling excitation amplitude benefits the ASQZSVIS on bettering isolation performance with low-frequency.Increasing the damping ratio can decrease maximum value of the absolute displacement transmissibility and instable scale of both vibration isolation systems,and when the damping ratio increasing up to a certain value,the max value of the absolute displacement transmissibility for the ASQZSVIS disappeared,which indicates that proper damping will effectively better isolation performance of the ASQZSVIS.If increasing the value of both ? and ?,the ASQZSVIS will gain much lower starting isolation frequency to achieve much better isolation performance in low frequency range.(3)Development of the ASQZSI and its experimental research.The ASQZSI principle prototype is developed based on previous theoretical research.Then,static and dynamics experiments for the prototype are carried out.The static experiment results show that the stiffness of the ASQZSI being smaller than single air spring in the whole displacement range when equal loads being at static mechanics equilibrium position for the ASQZSI and the single air spring isolator,which indicates that the ASQZSI can not only keep higher static stiffness but also have smaller dynamic stiffness.Especially,the stiffness approaching zero in a wide range of displacement around the static equilibrium position shows that the ASQZSI exhibits QZS characteristics.When loads varying at static equilibrium position due to change of the initial air pressure of vertical air spring,adjusting initial air pressure of horizontal air springs can make isolator keep QZS characteristics,which indicates the ASQZSI can achieve better vibration isolation effect for the system with loads changeable.Meanwhile,the harmonic sweeping excitation vibration experiment under different excitation amplitude and the discrete harmonic excitation vibration experiment under different excitation amplitude and frequency are carried out.The results show that the ASQZSVIS has lower resonance frequency,starting isolation frequency and the peak value of maximum transmissibility than the SASVIS.Due to existence of larger damping,the ASQZSVIS exhibits linear characteristic.Also,starting isolation frequency and resonance frequency of the system decrease with the rise on the excitation amplitude,while transmissibility of the resonance frequency increases with the rise on the excitation amplitude.Additionally,the shock excitation experiments of half-sine pulse and final peak saw tooth pulse are also carried out.The experimental results show that the ASQZSVIS has good attenuatipn effect on shock excitation,and its shock resistance performance is better than the SASVIS.(4)Application study of the ASQZSI in a certain type of ambulance.The ambulance-mounted stretcher support vibration isolation system is established by applying the ASQZSI in a certain field emergency ambulance.Then,the ride comfort experiments of patients in recumbent and sitting position are carried out.The experimental results show that the vibration isolation system gains good vibration isolation effect in both cases of one patient in recumbent position being transported in the ambulance and three patients in sitting position being transported on the asphalt road and the country dirt road,which also improves that the ASQZSI has good performance on vibration isolation characteristics with different loads in the vibration isolation system.At the same time,comfortable level is estimated at “the most comfortable” level with one patient in recumbent position and three patients in sitting position being transported on the asphalt road at different ambulance speeds.Comfortable level for patients in sitting position is estimated at “more comfortable” level except “comfortable” level at the ambulance speed of 50km/h in the country dirt road.By theoretical research,experiment analysis and application research of the ASQZSI,the reasonability of the design,feasibility of the manufacture and effectiveness of the application are verified.Application of the ASQZSI for the vehicle-mounted stretcher support system of the ambulance has great practical significance and application value in improving the ride comfort of the ambulance.