Universität Karlsruhe (TH): Development and test of nonlinear lateral vehicle dynamics control
High speed in a bend, suddenly appearing obstacles, different road surfaces - there are many reasons why a driver loses control of his vehicle and goes off the track. The Institute of Industrial Information Technology in Karlsruhe researches nonlinear control procedures using CarMaker in order to avoid the crashing of a vehicle in critical lateral dynamic situations.
Automotive development teams have worked out many approaches to avoid a vehicle losing its stability in the lateral direction by means of a control intervention in the vehicle dynamics. The Electronic Stability Program (ESP), for example, decelerates the wheels individually in order to hold the vehicle on the track. Active front and rear steering are other possibilities to help the driver control the vehicle. At the Institute of Industrial Information Technology (IIIT) of Karlsruhe the effectiveness of the different control procedures is examined fundamentally. The researchers analyze the mode of operation of controller actions for each intervention individually, and in combination, as well as developing new approaches for lateral vehicle dynamics control.
Side-Slip Angle as a key variable
To evaluate the lateral vehicle dynamics the side-slip angle plays an important role. It describes the angle between the longitudinal axis and the direction of movement of the vehicle. A high absolute value or a rapidly changing side-slip angle indicates a critical driving situation in terms of the lateral vehicle dynamics. Anne von Vietinghoff, scientific assistant at the IIIT, regards the identification of the side-slip angle as a central point for the development of lateral dynamic control concepts. She says "If you can succeed in determining the side-slip angle in a reliable way, critical driving situations can be identified and the driving behavior can be actively influenced."
However, there are currently no sensors available that suit for the measurement of the side-slip angle for the use in series vehicles. For this reason the IIIT examines methods to determine the side-slip angle on the basis of models. The researchers from Karlsruhe concentrate on nonlinear observers and extended Kalman filters.
A nonlinear double-track model is the basis for the estimation of the side-slip angle because - in contrast to the single-track model - it describes the lateral vehicle's dynamics accurately up to the limits of stability. The four wheel speeds, the yaw rate, as well as the longitudinal and the lateral acceleration are used as measurable output parameters. The validation of the side-slip angle determination is conducted with CarMaker. First results show that the Extended Kalman Filter is capable to reliably estimate the side-slip angle from values obtained by measurements in series vehicles.
Virtual test drives with CarMaker
Based on the side-slip angle the researchers from Karlsruhe develop approaches for lateral dynamics control. The basis for the control design is also a nonlinear double-track model. CarMaker is used for the model validation as the parameters coming from CarMaker are taken as reference values. The engineers of the IIIT furthermore use CarMaker to test the control concepts in a virtual vehicle. Anne von Vietinghoff: "All important parameters of the vehicle that are in a large part not measurable at a real car are available with CarMaker. The models can be examined in detail and the influence on the controllers can be analyzed precisely."
To study the lateral vehicle dynamics in a realistic way different test drives are performed with CarMaker. Thus the control concepts can be tested in reproducible test maneuvers (e.g. ISO lane change, J-Turn or Fish-hook). Especially the closed-loop validation plays an important role because it takes into account the reaction of the driver. After all, the usefulness of a controller concept has to be guaranteed also for different drivers. "The configurable driver model of CarMaker is very interesting as how the performance of the controller depends on the driver, can be analyzed" says von Vietinghoff.
Due to the reproducibility of the virtual test drives the control algorithms and the impact of the different controller interventions can be studied systematically. Identical driving maneuvers are therefore performed to compare the effectiveness of ESP, front steering and rear steering individually and in combination with each other. This research has shown that in particular the rear steering enables a very effective impact on the side-slip angle.
Conclusion and outlook
The vehicle dynamics simulation with CarMaker is an important method for the Institute for Industrial Information Technology in Karlsruhe to study elementary and systematically nonlinear models and approaches for the control of lateral dynamics. The researchers have already derived controller concepts that promise a considerable stabilization of the lateral vehicle dynamics.
The engineers of the IIIT plan further research of control procedures in the future. One of the objectives is to develop a controller that only intervenes in critical driving situations. Besides this, the tire model of the underlying double-track models will be enhanced. For this, a physical tire model that was presented by Mohamed Kamel Salaani at the SAE World Congress 2007 will be compared with the Magic Tyre Formula of Pacejka.
The validation of the models and the controls by means of the vehicle dynamics simulation will play an important role in these projects. Thus, CarMaker will continue to support the research objectives of the IIIT.