Troubleshooting power steering pump noise and vibration issues Part 4: Source Characterization by Jerry Nessler

The characteristics of the power steering pump that determine the level of excitation that will be applied to the vehicle are sound power, reaction forces and the pressure ripple. These pump characteristics are load and speed dependent. A pump test stand can be used to maintain the operating parameters such as load, speed and fluid temperature constant during the testing. The test stand also can provide an acoustic environment that is free of reflections within certain frequency limits, thus allowing for the measurement of sound power using an array of microphones. The sound power of the pump is used to predict the front of dash (FOD) SPL. The sound power measurement is done at a fixed load condition over the speed range of the pump and sound power computed at the harmonics of the power steering pump. For a 10 vane pump, the pump orders 10th-60th are typically extracted. The sound pressure at the FOD is calculated from the sound power order functions and the acoustic properties in the engine compartment.

For the structural excitation, the reaction forces between the power steering pump and the test stand can be measured using triaxial load cells. The load cells are placed in series with the power steering pump mounted in the test stand and forces are measured during loaded speed sweeps of the pump. The reaction force orders 10th-60th are extracted as a function of pump RPM and they are used to represent the excitation associated with the structure borne path. The reaction forces at each mounting location are then summed in each direction to obtain set of forces in the axial, lateral and vertical directions. These summed pump stand order functions are multiplied by the measured vehicle structure borne path sensitivity functions in each direction to predict the interior SPL for the structure path.

For the fluid borne path excitation, the pressure pulsations generated by the pump on the test stand are measured using pressure transducers in the discharge line. The fluid system impedance seen by the pump ideally needs to approximate that of an anechoic termination, that is with no reflected pressure waves traveling back to the source. This may be difficult to achieve in practice on the test stand due to the presence of elements such as oil cooler, filter, and valves which change the impedance seen by the pump. This effect can be identified by comparing the profile of the 10th order of the pump measured in the test stand to the one measured in vehicle, where the 10th order is typically independent from speed and therefore frequency. The pressure ripple orders 10th-60th are extracted as a function of speed and load. The pressure ripple order functions are used with the vehicle sensitivity function between Interior SPL and pressure ripple at the pump discharge measured in the vehicle to predict the interior SPL due to the fluid path.

Figure 1 summarizes the procedure followed to compute the contributions from the different paths.