Research On Mixed Logic Dynamic Modeling And Control Strategy Of Air Suspension

Electronically controlled air suspension(ECAS)can automatically adjust the suspension stiffness,damping and body height,leaving the vehicle with favouable adaptive capacity for the complex driving conditions,which is beneficial to the performance improvement.However,with the disturblances of random excitation,differences in front and rear suspension parameters,and uneven distribution of payload,inappropriate control strategies will cause large roall and pitch angles during vehicle height adjustment,resulting in the unstable vehicle body posture.Therefore,the ECAS control strategy has become a research hotspot in the related fields.The working process of the ECAS includes multiple working modes of discrete events.The control strategy realizes the discrete switching of the working modes by controlling the on-off statuses of the solenoid valve(SV),thus controlling the air mass flow rates flowing into and out of the air spring and then adjusting the vehicle height.The control difficulties of vehicle height oscillations and overshoots,body posture instability,vehicle performance deterioration and frequent switching of the SV are studied.In addition,the hybrid control system of the ECAS venicle height and damping adjustment(VHDA)is designed to provide relevant technical support for the related field research.Firstly,the detailed analysis of the working process is conducted and the modelling assumptions are proposed.Based on the thermo-dynamic principle,the expressions describing the relationships among the air pressures,the displacements and the air mass flow rates of the air spring are derived.Combined with the vehicle system dynamics,the nonlinear vibration charcteristics of the magnetorheological(MR)damper and the nonlinear flow properties of the SV,the nonlinear model of the ECAS VHDA are established.Secondly,on the basis of the control requirements,the logical relationships between the vehicle height mode switching and the corresponding SV on-off statuses are analyzed.According to the transfer conditions between the different mode switchings,the discrete event set reflecting the evolution of the system continuous dynamic process is established,then the coupling and the interaction among the continuous dynamic process evolution,the discrete event set and the switching conditions are revealed.Thirdly,combined with the actual working properties,the nonlinear components are reasonably linearized.By defining the discrete events about the logical relationships between the SV on-off statuses and the piecewise approximate boundary constraints based on the propositional logic,the coupling relationships between the discrete events and the continuous dynamic processes are described when adopting the hybrid system descriptions language(HYSDEL),then the standard mixed logical dynamical(MLD)model are obtained.By comparing with the nonlinear model,the accuracy of the MLD model is verified.Fourthly,considering the hybrid properties of the system control variables,a layered control strategy is proposed,that is,the SV on-off status control approach is designed to realize the accurate tracking of the vehicle height based on the hybrid automaton at the upper layer,while the MR damper current control strategy based on the hybrid model predictive control(HMPC)is studied to achieve the optimal control law of the MR damper and enhance the vehicle performance.Finally,the ECAS VHAD system controller is established based on the proposed layered control and the traditional PID control,then the control ecffectiveness of the vehicle height and the body posture is compared.By analyzing the internal causes and summarizing the relevant conclusions,we hope to provide relevant research experience for the hybrid system control in the ECAS and other related fields.