Abstract
Keywords - Steering system, Steering behaviour, Tyre model, Steer-by-Wire, State estimator
Abstract - Steering behaviour and steering feel are major aspects in which car manufacturers can distinguish their products with respect to their competitors. Present day conventional steering systems with hydraulic power assist are currently evolving into actively controlled electrical systems and, in the near future, Steer-by-Wire systems. One of the challenges for electrical steering systems and Steer-by-Wire systems is to achieve a steering feel similar or better than conventional systems. Due to a higher mass moment of inertia, higher friction in electrically powered systems and the absence of a mechanical link in Steer-by-Wire systems the steering feel has to be programmed in the control software.
This paper describes the development of a tyre slip angle and tyre force estimator, which provides the estimates of tyre forces and moments to the steering system algorithm. An extended Kalman filter was used to estimate the body slip angle based on ESP sensor signals (steering wheel angle, yaw rate and lateral acceleration). The estimated body slip angle was used to calculate front tyre slip angles. The estimated slip angles can be used for vehicle control applications towards global chassis control. For this research, they were used to construct tyre forces and moments which on their turn were used to calculate the force on the steering rack.
The estimated rack force was used as input to a Steer-by-Wire algorithm, which mimics the behaviour of a mechanical steering system with hydraulic power assist. The algorithm is based on a 2-DOF description of a rack and pinion steering system and uses steering wheel angle and tyre forces as inputs. The estimator was tested in road tests using a rear wheel driven compact car equipped with a high bandwidth Steer-by-Wire system. The results of the road tests show good correspondence between measured and estimated forces. The algorithm, using the estimated tyre forces and moments, resulted in a good steering feel.