The problem being investigated involves a noise-quality issue on a power steering application, when a sudden change of steering wheel angle generates an unwanted steering system noise or “Squawk.” This phenomenon is mostly observed during parking maneuvers, especially at lock positions and when the hydraulic fluid reaches a critical temperature on the specific application.
The objective of the work to solve this noise-quality issue was to first identify the cause and then eliminate the Squawk noise. There were several constraints:
No change could be made in the properties or type of hydraulic fluid used due to specification requirements;
Steering wheel valve torsion bar characteristic (torque vs. angle) needed to be maintained within specification for ride and handling purposes; and,
In addition to the mentioned constraints, a high capability of noise elimination generated by the production tolerances and dispersion has been considered.
The first step in this investigation is to identify and evaluate the noise components related to the Squawk. This is accomplished by the use of subjective noise measurement, requiring “Head Acoustic” and associated signal processing. Different designs of experiments demonstrate the influence of secondary parameters on the variation of the noise.
Further investigation into the root cause of this steering system Squawk noise phenomenon requires in-vehicle study and analysis of additional physical parameters. These include, for example, dynamic pressure sensors on the cylinder lines, radial and torsional vibration of the valve torsion bar, and vibration transmissibility measurements using accelerometer sensors on the steering rack assembly and vehicle chassis.
The examination of the noted parameters shows a consistent correlation between the transient signals. This correlation analysis identifies the hydraulic excitation physical components and the torsional response behavior of the system.
The severity of the noise can be evaluated and the generating mechanisms identified by observing the effects of modifying the signal behavior of the physical parameters.
The source of the noise can also be eliminated in the defined performance characteristics of the steering system by implementing the following: