| Magneto-rheological fluid (MRF) is known as smart materials, whose yield stress and viscosity can reversely change between the Newtonian liquid and the Bingham fluid when subjected to or removed from a magnetic field. MR damper (MRD) utilizing MRF possesses highly controllable yield stress, as well as the characteristics of low power consumption and fast response. Therefore, MRD has been widely used in the semi-active structural control applications. However, output damping force of MRD shows serious hysteresis characteristics, owing to the movement of inherent flux, lattice dislocation and the plastic slip in the MRF. The hysteresis may lead to the nonlinear dynamic such as bifurcation and chaos in the MRD applied system, especially severely in the new type of MR suspension system. The present study on decoupling and hysteresis nonlinear control of vehicle MR suspension system, a hot topic widely concerned by scholars around the world, has not yet obtained a breakthrough. The research in this thesis is based on two projects supported by the National Natural Science Foundation of China, which aims to build up the systematic theory of vehicle semi-active MR suspension system. The innovative content of the dissertation is as following:1. The modified Boucwen force-velocity (F-v) model of which the drive current and hysteresis factor are separated is proposed, based on common F-v hysteresis model for the MRD. The proposed model can be used for inverse calculation of drive current in real engineering application, due to the separation of the drive current and hysteresis factor. Model parameters identification based on vibration test bench is conducted. Both of the calculated and experimental results show that the proposed Boucwen F-v model can accurately describe the hysteresis characteristics and the damping force output characteristic of the MRD.2. The systematical nonlinear analysis method for 2-degree-of-freedom (2-DoF) MR suspension system is proposed based on chaos theory. The stabilization of equilibrium is discussed for the MR suspension in the passive way and the typical semi-active control mode, respectively. Global nonlinear characteristics under harmonic excitation are represented from the two-parameters phase plane. With the bifurcation diagram and corresponding Lyapunov exponent spectrum diagrams, phase trajectory and power spectrum density (PSD) diagrams, the complex dynamical behaviors and oscillating mechanism of alternating periodic oscillations, quasi-periodic oscillations and chaotic oscillations with different profiles of road excitation, as well as the dynamical evolutions to chaos by period-doubling bifurcations, saddle-node bifurcations and reverse period-doubling bifurcations is revealed. The results lay a theoretical foundation for further chaos control of the MR suspension system.3. A novel sliding mode control (SMC) strategy, with the reference of the ideal state of vehicle suspension, is put forward for chaotic vibration control of 2-DoF 1/4 MR suspension frame system. The SMC controller can easily realize the fast tracking of the ideal sliding surface by combing the traditional PID control strategy. Comparing with the existing controller such as linear feedback control, force tracking control, the proposed SMC controller has obvious advantages of clear physical significance, easy implementation, fast response speed, etc. The experimental study is conducted on the self-built hardware in the loop (HIL) test platform of MR suspension system, the results prove the effectiveness of the proposed SMC controller. With the control, the chaotic vibration of the MR suspension system is effectively suppressed into the stable periodic orbit, and vibration isolation performance of the MR system is improved.4. A novel structural decoupling method for the full-vehicle 7-DoF MR suspension system is proposed, to solve the complexity of controller design for the four coupling 2-DoF MR sub-suspension system. The analytical expression of the coupling force between four MR sub-suspension systems is deduced. The new type of dual controllable damper semi-active suspension structure is further applied in the full vehicle MR suspension system to offset the inhibition of coupling damping force, and thus realizing the purpose of decoupling. The proposed SMC controller is further applied to the four decouple MR sub-suspension systems independently. Under harmonic excitation, smooth pulse and the measured road spectrum excitation, comprehensive performances of the decoupled semi-active MR suspension system are systematically analyzed. Then the HIL test research is conducted. The results show that the decoupled full vehicle MR suspension system can be excellently controlled, owing to the independent semi-active control of four MR sub-suspension systems. The controller design for the full-vehicle semi-active suspension is greatly simplified. And comprehensive performances of both ride comfort and handling stability are effectively enhanced. |
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