Research On Nonlinearity And Damping Control Strategy Of Hydro-Pneumatic Suspension

Suspension system is an important vehicular sub-system which determines ride comfort ability and road handling ability. But the performance of traditional passive suspension is not able to match the requirement of the off-road vehicle high mobility. In this paper, the nonlinear damping and stiffness characters of the hydro-pneumatic suspension were deeply studied in terms of theory and test. Based on the theoretical analysis, a full vehicle principle prototype equipped with hydro-pneumatic suspension had been built successfully and the road test was also implemented too. Then the established full vehicle dynamical model with hydro-pneumatic suspension was validated and modified according to the bench and road test data. Finally, the damping control strategies were fully studied based on the modified dynamical model. This paper provides good technology reference for the application of damping control strategies on the semi-active hydro-pneumatic suspension vehicle. The main research work and conclusions are as follows:(1)A quarter car suspension dynamical model was established, which contains a nonlinear hydraulic damper model and a nonlinear pneumatic spring model. Through analyzing the damper parameters'effect on the damping character, a method was proposed that the damping character can be controlled by control the open pressure of the relief valve. Through analyzing the pneumatic spring parameters'effect on the stiffness character, a method was proposed that the stiffness character can be controlled by control the effective working volume of the pneumatic spring. Finally the hydraulic damper model and the pneumatic spring model were validated and modified according to bench test data separately, which provided a reliable basis for the following simulation research work.(2)A full vehicle dynamical model with hydro-pneumatic suspension was established and validated. Then the steer and brake handling ability determined by damping character and damper parameters was analyzed through simulation. Another hydro-pneumatic suspension model with twin-accumulator was established. Through the frequency domain analysis of its equivalent linear model, the matching proposal for the twin-accumulator was defined. Finally the full vehicle handing ability based on the twin-accumulator stiffness control was studied. (3)Aiming at the nonlinearity of the hydro-pneumatic suspension, the feedback linearization method was proposed to precisely linearise the hydro-pneumatic suspension model, then a linear optimal controller for damping control would be designed based on this linearised model. Two different output functions were designed, which would result in full linearization and partial linearization separately, then two optimal controllers were also separately designed based on the fully and partially linearised model. The effectiveness of these two feedback linearization optimal controller was analyzed through simulation. Finally, the simulation results show that a suitable output function is the key factor that determines the control performance.(4)Aiming at the nonlinearity of the hydro-pneumatic suspension, a model reference sliding mode controller was proposed as the damping control strategy. A skyhook model was adopted as the reference model, the response error between the controlled object and reference model was used to design the sliding surface. By controlling the damping force, the response of the controlled object would follow that of the reference model as much as possible. A neural network was also introduced to on-line adjust the switching control gain of the sliding mode control, so that the chatting problem could be weakened. The effectiveness of the model reference sliding mode control and feedback linearization optimal control was compared by simulation. The simulation results show that the comprehensive performance of the model reference sliding mode control is much better than that of the feedback linearization optimal control and more suitable for the damping control application on the hydro-pneumatic suspension system.(5)According to the features of the full vehicle sprung mass vibration, an observer based full vehicle model reference sliding mode controller was proposed in this paper. A 7 degrees of freedom full vehicle vibration model with nonlinear hydro-pneumatic suspension was established. And this model was validated and modified with the road test data of that principle prototype to make the simulation results credible. A full vehicle skyhook model was adopted as reference model. Three sliding model controllers were designed separately corresponding to the pitch, roll and vertical vibration of the full vehicle sprung mass. Then the control output of these three controllers was transformed to the controlled damping force on the four suspension units. A Romberg state observer was designed to observe the vibration states of the sprung mass. The comparison between the observed data and the test data show that the designed observer is able to observe the vibration of the sprung mass. Finally, the simulation results show that the proposed observer based full vehicle model reference sliding mode controller can decline the pitch, roll and vertical vibration at the same time and improve the full vehicle ride comfort ability effectively.(6)The structure proposal for the full vehicle hydro-pneumatic suspension system was proposed and the principle prototype was built successfully. Meanwhile, the electronic hardware and program for the control system were well prepared. The road test contrast between principle prototype and original vehicle was implemented in professional test site. The ride comfort test shows that under the random road input and pulse road input, the principle prototype enjoys a better ride comfort ability than the original vehicle. And also, under the pylon and double lane change manoeuvre, the principle prototype shows better handling ability than the original vehicle.