Enrico SILANI
Date of final exam: 05/05/2004E-mail: silani@elet.polimi.it
Tutor: Prof. S. Bittanti, Politecnico di Milano
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Active and semi-active suspension control strategies in road vehicles___________________________________________________________________________________________________________
Advisor:
Prof. S.M. Savaresi, Politecnico di Milano
Summary of the thesis
In the last few years, extended research has been performed by car manufacturers in order to improve the handling, safety and comfort of ground vehicles. As a result, modern cars are equipped with many electronic automatic control devices. Among them the slow-active and semi-active controlled suspensions system is probably one of the most critical and challenging control system loop. Controlled suspensions are likely to become quickly a standard in top-range cars, due to the large benefits they can provide. Semi-active suspensions (i.e. closed-loop control of shock-absorbers) are already largely used on newly-designed cars, whereas only a bunch of cars are equipped with genuine active suspensions yet. The main goal of semi-active or active suspensions is typically comfort-oriented (reducing vertical accelerations) and/or safety-oriented (keeping the tire in contact with the road surface), but they can be employed also to improve the car performance. This thesis treats different aspects of an automotive suspensions system: mainly control strategies, but also fault-tolerant filtering and modeling.
The first part of this thesis is dedicated to the presentation of a “full-vehicle” model (6-degrees-of-freedom in the chassis movement and 2-degrees-of-freedom in each of the wheel-suspension sub-systems movement), aimed at simulation and evaluation of active chassis control systems for handling, safety and comfort enhancements. Special focus is put on inclusion of the dynamics of load transfer, which are of importance in active yaw-control and roll-over prevention.
In the main part of this thesis the attention is focused on the wheel-suspension sub-system. The problems considered are:
- Design and analysis of control strategies for semi-active suspensions in road vehicles.
The most commonly used control framework is the well-known Sky-Hook (SH) damping. Two-state or linear approximation of the SH concept are currently implemented on commercially-available vehicles. The goal of this thesis is to analyze the optimality of SH-based control algorithms, and to propose an innovative control strategy, named Acceleration-Driven-Damper (ADD) control. It is shown that ADD is optimal in the sense that it minimizes the vertical body acceleration (comfort objective) when no road-preview is available. This control strategy is extremely simple; it requires the same sensors of the SH algorithms, and a simple two-state active damper. In order to assess and to compare the closed-loop performance of the SH and ADD control strategies, both a theoretical and a numerical analysis of performance are proposed. The numerical analysis is made with an evaluation method based on the describing-function.
- Sensor fault-tolerant filtering for slow-active suspensions in road vehicles.
As the suspension system of an automotive vehicle is responsible for driving comfort and safety, a fault-free operation of the whole system is demanded. In general, the suspensions controller needs an estimation of some unknown state variables. As a result, a fault-tolerant estimator of these not directly measurable state variables is demanded. A fault-tolerant estimator is obtained from combining the concept of fault detection, isolation and identification together with estimation. In this thesis, the attention is focused only on the design of the best possible filter in case of total breakdown of a single or multiple sensors. The information about the time occurrence and location of the sensor fault is supposed to be given by an external fault detection and identification (FDI) scheme. Starting from a physical mathematical model of the considered active vehicle suspension implemented in a quarter car test rig, a bank of Kalman filters has been designed and its performance evaluated with test-rig data.
- Model identification of MR-dampers for semi-active vehicle dynamics control.
The topic of this work is the identification of a high-precision model for magnetorheological (MR) dampers (shock-absorbers). An active MR-damper can be seen as a non-linear system, where the inputs are the damper elongation and the command current; the current is the control input which modulates at high-bandwidth the damping ratio through the variation of a magnetic field. The output is the force delivered by the damper. Among the broad set of application where MR-dampers can be used, this work focuses on MR-dampers fro semi-active vehicle dynamics control. High-precision models of MR-dampers can be designed using two different model classes: gray-box models (also called semi-physical models) and black-box models. Both approaches are considered in this work. All the main problems and issues of the identification procedure are presented and discussed: experiment design, data-acquisition, signal pre-processing, model structure and performance index selection, optimization and parameter estimation, model validation and software implementation. State-of-the-art and genuinely innovative model structures are considered for analysis and comparison.
The last part of this thesis is dedicated to the analysis of the improvements on the maximum lateral acceleration achievable on a modern road vehicle by means of “performance-oriented” active yaw control and active suspensions. The main idea explored herein is to reduce lateral load transfers in order to keep the tires around their nominal working region. The reduction of the lateral load transfer is typically achieved by suitably tuning anti-roll bars and suspensions. In this thesis it is shown that: 1) there is a trade-off in the roll-bars tuning between lateral acceleration and lateral stability; this trade-off can be alleviated by active yaw control; 2) an alternate and very effective way of improving lateral acceleration is to use active suspensions to force zero or even negative roll. In order to find out quantitative results, on the basis of the mathematical dynamic model of a car developed in the thesis, both a steering-pad quasi-static analysis and a dynamic analysis are carried out. The dynamic analysis is performed using an intermediate black-box modeling step, based on subspace-based identification techniques.
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