Troubleshooting power steering pump noise and vibration issues Part 3: Power steering noise identification by Jerry Nessler

For the case of objectionable power steering noise inside the vehicle, the measurements of most interest are not the ones of Sound Pressure Level (SPL) in the engine compartment or near the power steering pump. The vehicle interior SPL is the measurement made to determine the noise perceived by the vehicle occupants. The interior SPL is typically the metric used to determine acceptability. For example, if there is a perceived whine in the vehicle at a particular vehicle speed or maneuver, measurements in the engine compartment will be made along with the interior SPL. The SPL frequency spectrum will be calculated and the whine frequency identified. Knowing the speed of the power steering pump, the whine frequency can be compared to the harmonics of the pump to see if they are related. The data could be converted to the order domain to see if the whine frequency lines up with any of the pump harmonics as described earlier.

Once the objectionable whine has been identified as related to the power steering pump, the next step is to identify if there is a problem with the source (pump) and/or with the paths (i.e. a structural or acoustic resonance amplifying the excitation or insufficient transmission loss between the engine compartment and the passenger compartment). A schematic diagram of a Source-Path-Receiver model as it relates to power steering noise is shown in Figure 1.


Figure 1. Source-Path-Receiver Model of Power Steering System
The source is the power steering pump. The receiver is the occupant in the passenger compartment. As shown in figure 1, the SPL perceived by the receiver is a summation of three paths. The task confronting the test engineer is to decompose the system into source and paths and determine which path is dominant.

The source, which is the power steering pump, is seen as an acoustical, structural and fluid source and as such is characterized alternatively in terms of Sound Power, forces at the connection points and discharge pressure ripple.

The structural, acoustic and fluidborne paths are each characterized by measurement of sensitivity functions which are vehicle and speed dependent. The same pump may produce different interior sound pressure levels in two different vehicles depending upon the vehicle sensitivity functions.

The resulting interior SPL due to the power steering pump is predicted by multiplying the measured vehicle sensitivity functions by the pump generated SPL, reaction forces and discharge pressure. The predicted levels are a function of pump speed and will identify the speed ranges where a particular path is dominant.