Research On Multi-mode Switching Control Of Semi-active Air Suspension Hybrid System

Because the semi-active air suspension could achieve active controls of body height and damping compared with traditional suspensions, it has played a significant role in improving the ride comfort, driving safety and fuel economy of the vehicle in the driving process and become a research hotspot of vehicle engineering. Along with the increasing maturity of researches about air spring and adjustable damping shock absorber, the design of control system has become a bottleneck to realize the control requirements and improve adaptive capacity of semi-active air suspension.This dissertation took a car with air suspension integrating variable damping and variable body height as the objects of study. By analyzing the working principles and running states of the vehicle system, the nonlinear semi-active air suspension system was described as an aggregation of continuous dynamic process behaviors constrained by the physical properties of suspension system and discrete dynamic behaviors such as multi-condition enable/disable switching inputs and multi-condition control outputs. According to the mixture and influence between the two different behaviors, the main tasks of the dissertation were to design the control strategy of semi-active air suspension, analyze the hybrid dynamic behaviors of the suspension system and then realize the corresponding control.A semi-active air suspension hybrid system was built in the dissertation firstly. By analyzing the control mode and performance characteristics and studying the hybrid characteristic of semi-active air suspension, the damping control process was described as a hybrid system. Based on hybrid system theory, the functional model structure and implementation model structure of semi-active air suspension hybrid system were built, which laid the foundation of control.A suitable control mode and switching parameters of semi-active air suspension were determined secondly. According to the control feature of semi-active air suspension hybrid system, a multi-model switching control was studied. The body height control was divided into three control modes, such as the high, the middle and the low. The parameters of modes’switch were determined based on the road conditions and the speed changes of vehicle. Because of the coupling relationship between the body height control and the damping self-adjustment, a new control strategy was put forward, which set the principles ensuring the priority of body height switching, then decoupled the body height control and damping control, and realized the stability of body height adjustment by fuzzy control algorithm finally. Because the vehicle height couldn’t be adjusted in steering condition, the new control strategy added a steering control mode on the base of three body height control modes.A local controller of semi-active air suspension hybrid system was designed thirdly. The adjustable height of air spring was determined by offset frequency characteristic of suspension, and on this basis, a nonlinear dynamics vehicle model with semi-act’ve air suspension was then established. According to different damping control targets in accord with different body height modes under straight-line driving condition, three corresponding damping force fuzzy-PID control algorithms were designed. At the same time, the damping force control based on fuzzy neural network under steering condition was studied. The effectiveness of the control algorithms was verified by the performance simulation of the local controllers.A switching supervisory control method of semi-active air suspension hybrid system was proposed then in this dissertation. A multi-condition performance evaluation index system of semi-active air suspension was built, which could analyze the control performance of the unsupervised switching system. According to the instability and oscillation problem caused by the output jumps of local controllers during switching process, a switching process supervisory controller of hybrid system was designed based on fuzzy theory. The system’s final control inputs obtained as the weighted sum of the local controller outputs could realize the smooth transitions between different control modes of semi-active air suspension hybrid system. The effectiveness of the designed supervisory controller was verified by simulation.Finally, with the single chip microcomputer MC9S08A, a multi-mode switching controller of semi-active air suspension hybrid system was explored and the control system’s software and hardware were designed then. Under local conditions, real vehicle road tests were carried out individually to verify the ride comfort and handling stability, and the test results were analyzed and studied then.The research results showed that the semi-active air suspension hybrid system built in the dissertation and its corresponding multi-mode switching supervisory control strategy could not only meet the suspension control requirements under different driving conditions, but also ameliorate the vibration and shock performance in switching process of different modes. The overshoot amplitude of suspension performance in switching process could be reduced by19.4%in maximum. Compared with original car control, in the random rode tests under local conditions, the semi-active air suspension hybrid control of system could improve the ride comfort by11.15%on average. Additionally, in the pylon course slalom tests, the mean values of body side angles, yawing rates and lateral accelerations were respectively reduced by6.18%,7.24%and5.43%on average. The vehicle’s manipulation stability with semi-active air suspension hybrid system reached and transcended the control effectiveness of the original car control system.