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Information Society Technologies Sixth Framework Programme

Vehicle State Observation

Complex automotive control systems rely heavily on accurate and reliable information about the state of the vehicle and its environment. While certain state variables, such as yaw rate, are measured directly, others, such as the vehicle's side slip angle, cannot be measured due to high sensor costs. The use of model based state estimators is therefore necessary to provide estimates of unmeasured states and achieve sensor fusion when there are redundant or distributed sensors. In addition, it provides a mechanism to achieve safe and graceful degradation of performance when there is temporary or permanent errors in some of the vehicle sensors. With the vehicle dynamics being nonlinear observability will depend on state and inputs of the system. As a matter of fact external disturbances such as the friction coefficient between tyre and road or the inclination of the road are only conditionally observable, depending on the driving situation.

Nonlinear observers

Recently, there has been significant progress in the design of nonlinear observers , which provides an attractive alternative to the EKF due to such properties as simplicity of design, reduced computational effort, stability and separation. Some of the advantages of nonlinear observers compared to the EKF are:
  1. The use of a fixed observer feedback (injection term) avoids covariance tuning in terms of Riccati equations.
  2. There are no covariance estimate equations to update, reducing the number of ODEs from n + n(n+1)/2 to n, where n is the number of states.
  3. One benefit of avoiding the covariance estimation equations is that the recursive algorithm becomes numerically less sensitive and easier to implement in fixed point arithmetic, which is essential in automotive embedded control units. \item The state and covariance resetting problem in EKF can be avoided. The implementation and experimental validation of nonlinear observers in mechanical applications such as navigation and ship control shows that they are more simple and intuitive to tune than the EKF and leads to reduced commission time, cost, and less computational and memory requirements.
    1. Beyond these technological advantages, an important point is that there exist several methods and principles for the proof of uniform global asymptotical/exponential stability for the non-linear observer error dynamics, including nonlinear dissipative forces such as friction and bias models (necessary due to sensor drift), sensor scale factor and sensor misalignments (mounting). Nonlinear separation principles and cascaded design techniques for combined nonlinear control/observer design has been developed and implemented in some applications. The closed loop is proven to be uniformly globally asymptotically stable under similar assumptions as in the linear case. These theoretical issues are becoming increasingly important from a practical engineer's point of view since in complex safety-critical automotive control systems a theoretical foundation is likely to avoid design errors and reduce the time used for development and testing. The nonlinear observer approach provides a computationally less complex solution, which has advantages both in terms of hardware costs and software verification. We conclude that the potential advantages of using nonlinear observers in automotive applications are significant. A combination of EKF and nonlinear observers can solve the global convergence problem while maintaining the benefits of tuning and monitoring local performance in a well established way.

Vehicle Active Safety

Integrated Chassis Control

Control Design

Vehicle State Observation

DaimlerChrysler AG Hamilton Institute University of Glasgow Lund University SINTEF