Design and advanced control of active suspension system with linear actuator

Abstract

Active suspension system (ASS) has been growing its popularity in industrial development and academic research since 1980s. The main advantage of electromagnetic ASS is its high performance in automotive applications. The ASS shows a high flexibility in control and provides enhanced comfort and safety to drivers and passengers when it is compared with the passive suspension. While compared to the pneumatic and hydraulic types, the ASS with electrical actuators can eliminate many mechanical components and perform faster response which in turn reduces the maintenance cost. Investigations into the electromagnetic ASS have been actively performed by worldwide researchers. Switched reluctance actuator (SRA) is a type of synchronous motors that has simple structure, low cost, robustness, and reliability characteristics. The linear motion type of SRA, linear switched reluctance actuator (LSRA), has inherent distinct advantages of high force and fault tolerance features, which align with the ASS operations to maintain equilibrium. The ASS with LSRA involves complex control. Variable reluctance and inherent nonlinear force characteristics are two major uncertainties that lead to implementation difficulty of LSRA control. Hence, an accurate position sensor is installed to the system for phase commutation and force control. A noise-free tracking mechanism is developed to acquire high precision position control. The ultimate objective of this project is to investigate and propose an effective nonlinear controller and position-tracking scheme for the quarter-car and full-car ASS equipped with LSRA. To achieve this target, the whole research work frame is divided into three phases. The first phase of research work is to investigate the linear actuator and develop the prototype of LSRA for the ASS. The magnetic path of the LSRA and its operation principle are reviewed. Based on the dimension and force requirement of the ASS, the specified magnetic circuit is modelled and analysed through finite element analysis (FEA). Moreover, the prototype model is developed and verified by experimental implementation through direct drive converter. Following the fabrication of the proposed LSRA, the experimental platform of the quarter-car ASS is built. The models of the quarter-car ASS and full-car ASS are developed for both force control of LSRA and nonlinear advanced control of ASS. The open-loop instability exists due to the external road disturbances which compensation algorithms are used to meet the standstill requirement; moreover, the coupling behaviour of the full-car ASS deteriorates the performance of the controller. To simplify the control scheme of the overall system, the ASS is divided into two subsystems, sprung part and unsprung part. The control issue focuses on the sprung part to improve the performance of the ASS. High accuracy position feedback is critical in this application. A novel tracking mechanism without the requirement of the system state equation, the so-called tracking differentiator (TD), is introduced to track the feedback displacement signal and to calculate its velocity directly through numerical method based on optimal control theory. The effectiveness of the TD is verified through position tracking of the sinusoidal road profile. A nonlinear proportional-derivative (PD) controller is developed for the quarter-car ASS. Both the simulation and experimental results illustrate that the electromagnetic active suspension control system can achieve a high motion performance and keep the system stable consistently with the proposed nonlinear PD controller. The developed nonlinear control method is more robust than that of linear PD controller under the variations of system parameters and road disturbances. The second phase of the research work is to improve the robustness of the whole suspension system by implementing the nonlinear controllers into the ASS. An integral sliding mode controller is examined. The integral item of integral sliding surface guarantees the stability of the nominal system at the initial time instant. In addition, adaptive mechanism is applied to the quarter-car ASS. An adaptive model following control method is used to determine the dynamic behaviour of the suspension system by predefining a reference model. The control input is generated through adaptive regulator based on partially known parameters and disturbances. Furthermore, combination of sliding mode technique and adaptation mechanism is attempted in this research to obtain the advantages of simplicity and robustness. The final phase of the research work is to propose both linear and nonlinear control algorithms for the full-car ASS. The full-car sprung mass subsystem is decoupled into three individual single-input single-output subsystems using decoupling transformation matrix. This results that the control complexity of the full-car active suspension is significantly simplified. The proposed control algorithms are categorised by two types of suspension models such that one is linear and the other is nonlinear. For the linear model, a Linear Model Following Control method is used to suppress the vertical vibration due to road profile. A proportional-derivative regulator is added to compensate the parameters variation and external disturbance. For the other one, a nonlinear sliding mode controller is applied. The discontinuous control laws of the sliding mode controller compensate the system uncertainties and unmodelled dynamics. The thesis presents a number of advanced control methods to successfully suppress the vertical oscillation of the ASS and it thus enhance the safety and comfort. It is confident that the new development will be applied to a vehicle for the next generation of mobility.