Design and evaluation of a model for power steering

Abstract: This report presents the results and findings gathered during the master thesis work performed by Madeleine Jareblom at Scania CV AB in Södertälje. Scania is a developer and manufacturer of trucks and buses. They always strive to improve their testing and simulations for current and future products.  In order to predict the outcome or to analyse the performance of components or systems, simulation models are a powerful tool to use. Realistic and adaptive models are of great interest for Scania and this thesis will cover a steering system performance simulation capability. A steering system consist of a hand wheel, steering gear and linkages down to the steering wheels. The purpose of a steering gear is to ease the resistance a driver has to apply in order to steer the wheels.  There are a lot of different steering systems that are being used in vehicles these days but one of the more common ones is Electric Hydraulic Power Steering (EHPS) which this paper will cover. The simulation model is of an open loop design for an EHPS system that takes input data and divide them into separate calculation blocks. Static variables and data from vehicle measurements were used, together with some assumptions such as; front wheel steering, only applicable for trucks, no roll inclination, 2- or 3-axel vehicles and some static angels in the suspension geometry. During the construction phase of the model, calibrations were performed and there it was found that the friction in the system was load dependent. When the model was calibrated the final output results consisted of; calculated trajectory of the vehicle, the required output torque from the steering gear, input and output power of the steering system together with the consumed energy. From the results of the trajectory output it could be seen that the model is sensitive to small deviations of the handwheel angle and possible road banking. This entailed that the calculated trajectory did not resemble the driven path gathered from vehicle test data. The calculated output torque from the model, was validated with the use of measured pressure and supplier specifications for the steering gear. The results indicated that the calculation model could capture torques from the steering gear close to reality. It could be concluded that the model could be used to describe resulting torques, output and input power together with the consumed energy.