Active Vibration Control For Vehicle Suspension Systems

Vehicle suspension systems are of vital importance for signifcantly improving pas-senger comfort and handling characteristics. A well-designed suspension system can pro-mote the whole performances of automobile chassis. With increased requirements forvehicle performances, traditional passive suspensions or semi-active suspensions are in-adequate in improving ride comfort or road holding, especially under those extreme poorroad conditions. In contrast, active suspension systems have a potential to improve theride comfort and vehicle maneuverability. In active suspensions, actuators are placed be-tween the car body and wheel-axle parallel to the suspension elements, and are able toboth add and dissipate energy from the system, which enables the suspension to controlthe attitude of the vehicle, to reduce the efects of braking and the vehicle roll duringcornering maneuvers to increase ride comfort and vehicle road handling. The present dis-sertation focuses on the control law design for vehicle active suspensions to improve ridecomfort and ride safety, where quarter-car model, half-car model and full-car model areestablished successively to be the control plants. The main researches are summarized asfollows.The control problem for quarter-car active suspensions in fnite frequency domain isinvestigated. Diferent from the traditional H_∞control approaches which design the con-trollers within the full frequency domain, this dissertation handles the H_∞control problemfor active vehicle suspensions in specifc frequency domain. By developing fundamentaltheory of disturbance attenuation control, the H_∞norm is reduced in concerned frequencyband to improve the drivers’ and passengers’ comfort. Compared with the full frequen-cy H_∞control technology, the proposed approach suppresses the road disturbances moreefectively for the concerned frequency range. Moreover, the necessary performance con-straints within the active suspension design are guaranteed in the whole time domain.Next, in view of the possible actuator input delay, the fnite frequency method is devel-oped to deal with the problem of suspension control with actuator input delay. In addition,state feedback control, which depends on the premise that all the state variables are on-line measurable, usually leads into higher cost and additional complexity, and sometimes,not all the state variables can be measured on-line. To response this situation, a dynamicoutput feedback control algorithm is proposed in this dissertation according to part of themeasured states. This dissertation proposes an adaptive backstepping control strategy for half-car un-certain active suspensions with hard constraints. An adaptive backstepping controller isdesigned to stabilize the attitude of vehicle and meanwhile improve ride comfort in thepresence of parameter uncertainties, where suspension spaces, dynamic tire loads and ac-tuator saturations are considered as time domain constraints. Furthermore, a referencetrajectory is planned to keep the vertical and pitch motions of car body to stabilize in pre-determined time, which helps adjust accelerations accordingly to high or low levels forimproving ride comfort.In response to the possible actuator saturation and actuator failures, the saturatedadaptive robust control strategy and the fault tolerant adaptive robust control approachare proposed to deal with the problems of actuator saturation and fault accommodationof active suspension systems. Comparative simulation studies are then given to verify theefectiveness of the proposed control approaches, in which one can see that the designedanti-windup controller and fault tolerant controller can reach good performances eventhough actuator saturation or failures happen.Difering from the existing results, in most of which the efect of actuator dynamic isneglected, this dissertation considers the electro-hydraulic systems as actuators to supplythe active forces into suspension systems. Furthermore, to overcome the explorationof terms problem existing in standard backstepping, a flter-based backstepping controlstrategy is subsequently proposed. Based on the above analysis and design, the problemof vibration suppression for full-car active suspension systems is investigated, whose aimis to stabilize the attitude of vehicle and meanwhile improve ride comfort. A full-carmodel is adopted and electro-hydraulic actuators with highly nonlinear characteristics areconsidered to form the basis of accurate control. In this part, the H_∞performance is in-troduced to realize the disturbance suppression by selecting the actuator forces as virtualinputs, and an adaptive robust control technology is further used to design controllerswhich help real force inputs track virtual ones. The resulting controllers are robust a-gainst both actuator parametric uncertainties and actuator uncertain nonlinearities, andthe following stability analysis for the closed-loop system is given within the Lyapunovframework.Experimental verifcations for quarter-car active suspensions are given to show thepractical application of the proposed control algorithms (fnite frequency H_∞control andadaptive robust control), from which the efectiveness and practical merits of the proposedmethod are verifed.