A New Approach to Improve Steering Response Development Using Bode Diagrams 2007-01-2548
Nowadays, vehicle steering development processes are mostly based on metrics which describe gains over steering wheel angle inputs. Most of these metrics are related to the yaw and lateral motion of the vehicle, for different velocities and curves situations. In practical terms, it is a correct approach, once steering feel is related to yaw motions and lateral accelerations. For example, given a certain steering wheel angle input, a higher yaw will generally provide a quicker response feel.
However, there are subjective aspects related to the steering response feel that are not completely captured by traditional metrics alone, which are generally based on response gains over steering wheel inputs.
This work presents a new steering evaluation plot, based in a simple maneuver (step steer) and in the sequence of the dynamic events that follow, which showed to be effective on providing additional analysis data that captures important aspects of the steering response subjective feel. In this proposed method, Bode plots are built for the following systems: 1- Steering wheel angle as input, yaw motion as output; 2- Yaw motion as input; lateral acceleration as output; 3-Lateral acceleration as input and roll motion as output. The gains of the three different transfer functions mentioned above, as well as the respective phases, are plotted and compared.
CAE models were used in order to build up the Bode plots. Vehicle measurements were carried out for CAE models correlation. Subjective evaluations were also conducted for different proposals in order to identify the steering response feel relationship to the results of the Bode plots.
As a conclusion, it was identified that there is strong subjective steering response feel relationship with the phase of the responses, not only to the gains. A vehicle with low yaw and lateral acceleration gains, but with small phase lags (showed at bode plots), was subjectively considered more agile than a vehicle with higher gains, but with bigger phase lags. This work also identifies the analysis potential of the proposed Bode plots in detecting vehicle nonlinear behaviors and in analyzing vehicle behaviors at different response domains (steady, middle and high slip angle maneuvers).