Title: Vibro-acoustic analysis of an electric power steering system for passenger comfort mitigation
The vibro-acoustic performance of electrically powered steering systems (EPS) is an essential comfort related requirement in today’s automotive development. This will get particularly relevant for electrically driven vehicles, where noises from the steering system are not masked by a classical combustion engine anymore. Meeting EPS acoustic requirements on vehicle level demands a systematic integration process and knowledge of both the structural-dynamic excitations inside the gear as well as their propagation to the passenger’s ears. While the latter can be determined experimentally, the internal gear excitations may only be obtained using computer simulation models.
In this paper, we present a combined approach on predicting vehicle interior noise during a parking maneuver using (a) simulated internal gear excitations, (b) a structural model of the gear, the subframe, and the connection between both, and (c) experimentally measured Transfer Path Analysis data. For the excitation model, it was necessary to simulate the structural-dynamic behavior of the steering gear for the full rack stroke of 150mm. Therefore, the gear mechanics was first modeled in a multibody dynamics software using both rigid and flexible bodies, and then exported to a system level simulation environment. The electric drive was first modeled using Finite Elements (FE) techniques and then represented via lookup tables in the system level simulation environment, together with the torque and angle sensor that measures the steering wheel input. During the parking maneuver load case, time histories of forces at various bearing interfaces were extracted as excitations for the structural model.
The simulated excitations were fed into an assembled finite element structural model of the steering gear, mounted with fixed joints and rubber bushings on the vehicle’s subframe. Both the gear and subframe models were merged to an assembly structural FE model. The pre-calculated Frequency Response Functions (FRF) characterizing the stand-alone steering gear and subframe were then coupled to establish the FRFs for the assembled structure. The response of the complete system to the previously simulated excitation was calculated at the mounting interfaces. The noise pressure level at the passenger’s ear was then estimated by combining the system response with the measured vehicle Noise Transfer Functions (NTFs). The described complete “gear to ear” approach allows for an early estimation of the steering gear’s operational noise, as well as performing sensitivity analyses that allow for the identification of critical parameters. The results section of this paper shows exemplarily the influence of electric drive orders and bushing stiffness on the operational noise.