Troubleshooting power steering pump noise and vibration issues Part 5: Paths identification by Jerry Nessler

Generally, in passenger cars interior SPL, frequencies below 800 Hz are associated with the structure borne paths and above 800 Hz the airborne path will dominate. To identify the dominant path, it is necessary to know 1) the sensitivity functions relating the excitation levels to the interior SPL and 2) the associated excitation levels. The excitation levels (such as power steering pump forces, fluid dynamic pressures and under hood sound pressure) are quantified using a pump test stand as described earlier. The sensitivity functions for each path are measured in the vehicle and are hence vehicle dependent.

The acoustic path sensitivity function is the Acoustic Transmission Loss (ATL) function between the engine compartment and the passenger compartment. The ATL function is typically measured using speakers placed in the engine compartment and measuring the averaged Front of Dash (FOD) SPL and the interior SPL using broadband random excitation. A ratio of the Power Spectra of the Averaged FOD SPL and interior SPL (also referred to as pDRE/pa) is an estimate of the ATL function. The ATL function in vehicles increases with frequency, typically approaching (in today's vehicles) around 50-60 dB at frequencies above 3-4 kHz. The ATL along with the FOD SPL generated by the pump are used to predict the Interior SPL due to the airborne path.

The structure borne sensitivity function is measured using impact excitation. The pump is removed from the vehicle and frequency response functions are measured between the pump attachment locations and the interior SPL (these transfer functions are also referred to as pDRE/F). The structure-borne sensitivity functions are then multiplied by the measured forces from the pump stand to predict the interior SPL contribution from the structure borne path. If the forces are not available, an alternate approach is to use the measured acceleration levels at the pump attachment locations. A sensitivity function between acceleration at the power steering pump and the interior SPL (pDRE/A) can be measured using an electrodynamic exciter. The exciter can be used to enforce an RMS acceleration level in the frequency range of interest at the pump mounting location and the frequency response function calculated between interior SPL and pump attachment acceleration in a particular direction. This SPL/Acceleration frequency response function obtained from artificial excitation times the measured order acceleration function at the problem order will yield an estimate of the interior SPL due to the structure borne path in a particular direction due to that order. All three directions can be measured and summed together to derive the total structure-borne contribution to the interior SPL, which is then compared to the measured interior SPL to determine if the measured levels can be predicted by the structure borne path.

As explained in the previous article, the interior SPL is made up of a summation of all the paths (air-, structure- and fluid-borne) and the fluid path cannot be isolated as easily as the airborne and structure borne paths using artificial excitation. To measure the fluid borne sensitivity function using the pump excitation in the vehicle, the pump must be operating and all three paths exist simultaneously. One approach that has been used successfully to measure the fluid sensitivity function relies on a separate source or a slave vehicle to supply the hydraulic power to the power steering system, thereby isolating the fluid path. Characterizing the fluid path involves measuring the pump discharge pressure pulsations at the orders of the pump and the interior SPL and calculating a sensitivity function by a ratio of the orders. The frequency range of the fluid path is typically below 800 Hz, so the sensitivity function can be calculated using the ratio of the 10th orders only. This ratio of operating responses assumes that all the interior SPL is due to the fluid path. An alternate approach to measure the fluid path sensitivity function has been developed by inserting an electro-hydraulic servo valve in series with the power steering pump. The power steering pump supplies the fluid flow with the engine at idle and the servo valve is used to vary the flow to the rotary control valve in the steering gear. Swept sine excitation is used to measure the fluid path sensitivity function from 100 Hz to 700 Hz by controlling the dynamic pressure at the steering gear to be 10 psi over the frequency range of interest. The sensitivity function is then calculated between the Interior SPL and the steering gear inlet pressure.