Comprehensive Performance Study Of Semi-active Suspension Considering Invariant Point Properties

The comfort and safety are key concerns for the automobile consumers in addition to the price.These performances affect the passengers’ ride experience and personal safety directly.Therefore,they attract more and more attentions from automobile manufacturers and researchers.As a key system of the vehicle,the suspension has direct effects on ride comfort,handling and safety.Hence,it is important to optimize the suspension performance to improve the vehicle quality as well as the consumer’s acceptance.After nearly four decades of development,the semi-active suspension has attracted a lot of attentions from automobile manufacturers and researchers for its good performance,low energy consumption,reliability and other advantages.The semi-active suspension could improve vehicle performance by adjusting the damping.However,the semi-active suspension could hardly deal with the conflicting between ride comfort and safety which is the inherent characteristic of the suspension.Therefore,it is of great significance to study the performance conflicting and explore the method to improve the vehicle comprehensive performance.The three main questions concerned in this study are: 1.What causes the suspension performance conflicting;2.How to balance different performance of the semi-active suspension under existing performance constraints;3.How to break the existing performance constraints to improve the semi-active suspension comprehensive performance.With regard to the three questions,the main work of this study are as follows:Firstly,the models of the road excitation,controllable damper and vehicle are built.By analyzing the working principle of controllable dampers,the general hysteretic model is built,which could express different damping characteristics of the compression and rebound travel.The quarter vehicle is validated by the experiment.Secondly,three kinds of invariant points are derived by the vehicle model,which are the fixed invariant points(FIPs),the invariant points of the linear passive suspension(LIPs)and the invariant points of the semi-active suspension(SIPs).The effect of the invariant points on the suspension dynamics performance,especially the effects on the performance conflicting,are analyzed through the simulation.It is proved that the FIPs could cause the conflicting between ride comfort and safety as well as ride comfort and suspension deflection.In addition,the FIPs also restricts the comfort improvement around the second order resonance.The damping requirements the for good performance at different frequency ranges are analyzed based on the properties of the LIPs.At the middle and low frequencies,ride comfort and safety could be improved simultaneously through the damping control.At high frequencies,the required damping for ride comfort is conflicting to that for safety,which could not be solved by the semi-active control.The properties of the SIPs show that large adjustable damping range is beneficial to the semi-active suspension.Once the adjustable damping range reaches a special value,the suspension performance could be improved by decreasing the minimum damping.Thirdly,a frequency-division control is developed based on the properties of the invariant points.This control algorithm could identify the vibration frequencies in real time with a frequency selector.Then,different control methods are conducted in corresponding frequency ranges.According to the properties of the LIPs,the mixed sky-hook & ADD(SH-ADD)control algorithm is applied in the frequency range below the forth invariant points LIP-4 to improve ride comfort and safety simultaneously.In the frequency range beyond LIP-4,passive damping is adopted to balance ride comfort and safety.As a result,the proposed control algorithm can improve the comprehensive performance up to 6.6%.Fourthly,a composite vibration-suppression method is proposed based on the properties of the invariant points.This method combines the semi-active suspension and semi-active tuned mass damper(STMD)to improve suspension performance.The STMD is used to get rid of the constraint of the invariant point as well as share the resonance energy of the unsprung mass.Then,the sliding mode controller is selected as the controller of the vehicle suspension and TMD for its excellent stability and outstanding performance with two or more control objectives.As a result,the proposed structure can eliminate the performance constraint of FIP-1 as well as improve ride comfort across the whole frequency spectrum.The composite vibration-suppression method could improve the ride comfort by 36.1% and the comprehensive performance by 11.7%.At last,the application of the two proposed methods in the full car model is studied.Meanwhile,a quarter vehicle test rig is developed to validate the two methods.The result show that frequency-division control could balance different performance of the vehicle.And the composite vibration-suppression method could improve the sprung mass vibration and body posture,while keeps better safety than other semi-active suspensions.The experiment results verify the control effect,which agree well with the simulation results.The main innovations in this dissertation includes:1.The influence of the invariant points on vehicle dynamics performance is studied,which perfects the invariant points theory.The performance conflicting of the suspension is analyzed based on the invariant points.2.The frequency-division control is developed based on the invariant points,which could balance different vehicle performance under the exsiting constraints of the invariant points.3.The composite vibration-suppression method is proposed based on the invariant points,which break the existing performance constraint(FIP-1)of the suspension.Consequently,the comprehensive suspension performance is improved across the whole frequency spectrum.