Global Chassis Control and Braking Control using Tyre Forces Measurement

Abstract

Mobility and traffic safety is a major concern in the society today. Many accidents take place because the vehicle is not following the trajectory that the driver desires. To avoid such accidents, an increasing number of active safety systems are introduced in modern vehicles. Still, most of the time, those systems are designed independently, which can lead to non-optimal performances and unexpected interactions. To properly consider the coupling between the different actuators like the brakes and steering, coupling coming for example from the nonlinear tyre dynamics, a Global Chassis Control strategy is necessary. This thesis defines a new 2-level framework for Global Chassis Control. At the chassis level, the distribution of the total control action to all the actuators is formulated as a constrained convex optimization problem, independent from uncertain parameters. As the driver is constantly changing his inputs to the vehicle, the solution of the optimization should be constantly updated. For that purpose, a new continuous optimization method, the Hybrid Descent Method, has been developed. This method defines a dynamical system such that its trajectory converges, first to a solution allowed by the constraints, and then to the optimum of the related optimization problem. In order to introduce the right number of degrees of freedom in the optimization, the actuators are grouped into building blocks depending on their coupling with the different tyre forces. Tyre constraints are handled in a realistic way, without the common assumption that the whole tyre characteristic in known long in advance. At the wheel level, the control of the tyre forces is made simple and robust to uncertainties in the road conditions thanks to the use of tyre force measurement. Such tyre forces measurement is at the moment too expensive to be implemented. In current production vehicles, only sensors measuring motion are available, like accelerometers or gyroscopes. At the moment, SKF is developing a technology enabling the measurement of the load in their bearings. This technology allows the introduction of cheap force sensors in all vehicles, which will allow the implementation of new control algorithms like those presented in this thesis. A particularly critical problem in vehicle dynamics control is to get the largest possible braking force out of the tyres while maintaining vehicle stability. This thesis contributes to the development and experimental validation of Anti-lock Braking Systems (ABS). As a result of a collaboration with CNRS/Supelec, Paris, France, a theoretical hybrid ABS based on wheel deceleration has been improved to handle actuation delay; and a new wheel slip regulator based on a cascaded approach has been implemented. Furthermore, new algorithms using force measurement are shown to be simpler to tune, better performing and more robust to road conditions than acceleration-based alternatives. The first ABS algorithm directly considering the loss of potential to generate lateral tyre forces because of braking is an achievement of this thesis. All algorithms for straight-line braking have been validated on the tyre-in-the-loop experimental facility, which was made suitable for ABS testing during this research.