The Controller Output Observer: Estimation of Vehicle Tire Cornering and Normal Forces

A controller output observer is used for estimating specific outputs from a physical system through the use of alternative outputs that are measured. In most cases these estimated signals can be measured directly using sensors. However, some outputs are either not possible to measure directly or alternative outputs are just easier to measure. This paper focused on a method to estimate immeasurable quantities, in near real time, through the use of physical models and measured quantities. This can be done through the use of classical observers as introduced by Luenberger in 1964 ("Observing the state of a Linear System," IEEE Trans. Mil. Electron., 8, pp. 74-80) However, since observers estimate the system states, one would have to again estimate the sought after outputs by another method, such as a constitutive relationship. The method proposed here shows that certain signals can be estimated directly by using a controller but without using a constitutive law. These estimated quantities are inputs to a model of the real system. In other words, these inputs drive the observer model. A promising use for the controller output observer is estimation of tire forces in vehicles. This is because tires are very difficult to model accurately, and even if the tire is modeled accurately, the surface conditions of the road must be known to predict tire forces. The controller output observer does not require a model of the tires. The method is tested using a vehicle model. Real measurements from a vehicle are used to show that the method succeeds in estimating quantities from the real vehicle.