A control system for maintaining vehicle stability requires a side slip angle value that shows the vehicle attitude. From viewpoint of cost, in the system, the angle is detected using an estimation method based on the vehicle model from sensors. There are many types of running environment changes, for example, the number of passengers, the tire exchange, the climate such as snow, road shape as such cant, etc. Therefore, errors in the vehicle model and sensor's signal are generated. The subject is how precise estimation is achieved by suppression of the effect of errors. The size of system must be small because it is used in a vehicle's microcomputer.

A robust observer based on the vehicle equation is constructed. It is not expressed using the state variables, so it can easily express non-linear characteristics such as the tire differences. Also, its size is small. Moreover, each correction system is constructed for the changes in which the above correction ability is poor. These changes include any change in the coefficient of road surface friction, the generation of sensor offset on road inclination, and spin that can not be used in the vehicle model.

It is confirmed that the system can estimate the slip angle with a high accuracy under the various running environment changes.
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