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Passengers' frequent requests are for less Noise, Vibration and Harshness (NVH) in the vehicle compartment. This and the reduction of noise and vibration levels from major sources like the engine necessitate better performance of other sources of noise and vibrations in a vehicle. Some of these sources are the hydraulic circuits including the power steering system. Fluid pulses or pressure ripples, generated typically by a pump, become excitation forces to the structure of a vehicle or the steering gear and represent a considerable source of discomfort to the vehicle passengers. Current power steering technology attenuates this ripple along the pressure line connecting the pump to the steering gear. Finding the optimum design configuration for the components (hose, tuner, tube, and others) has been a matter of experiencebased trial and error. This paper is a part of a program to simulate and optimize fluid borne noise in hydraulic circuits. INTRODUCTION A fluid borne noise program is being developed. This program uses a database of experimentally characterized pumps, hoses and steering gears, where a test rig is built and used to develop such databases according to recent ISO standards [1]. These data are then used in simulation software to determine the optimal design configuration for a specific application. This optimization procedure determines, among other things, finding the hose, tube, and tuner lengths in a power steering configuration for a specific driving condition. The most frequent driving conditions in which the power steering noise is noticeable include: 1. Idle no load conditions. This is usually encountered while the driver is waiting in his vehicle (for example, in front of a traffic light). The driver usually leaves the engine running at 700 to 1000 rpm. Under this condition, no load is applied at the power steering system. The pressure in the fluid line can be up to 5 bar at the pump outlet to keep the system running and overcome energy losses. Drivers may experience this condition for extended periods in a traffic jam. 2. Parking conditions. These are encountered in the parking lot. The driver here is applying some load on the power steering system that will make the pressure reach up to 80 bar, while the engine is running at 800-1100 rpm. The duration of this condition is usually for several seconds (5 10 seconds). 3. Driving around corners in town. The driver usually runs the engine at speeds from 1500 to 2000 rpm, and puts the steering system under pressure values near 40 bar. The duration of this condition is approximated in seconds. 4. Steady state driving. This is a condition where the drivers experience when driving on the highway, where the engine is running between 1500 to 2500 rpm. The steering pressure is approximately 5 bar at the pump outlet and may reach 10 bar when changing lanes. 5. Accelerating while parking. This is experienced when the driver drives his vehicle in the parking lot fast. The driver, under this condition, will be driving his vehicle at 1500 rpm and putting up to 80 bar of pressure in the power steering system The last two conditions are usually investigated for loading and durability and rarely studied for noise. The reason is that engine noise levels under these conditions exceeds power steering noise levels. The first three conditions described above are considered the most critical. They produce objectionable noise levels. As can be seen, for these conditions the steering system experiences pressure values from 5 to 80 bar and engine speeds of 800 to 2500 rpm. With these values in mind, the system is usually investigated under the controlled conditions described in Reference 8. The engine usually drives power steering pumps at similar speeds. Typical pumps contain 10 pumping elements (vanes). This results in critical frequencies of the tenth order and their harmonics. Among other things, hoses, hoses with inserts or tuners, restrictors and other devices are used to attenuate the fluid ripple. The effect of hose parameters on the hydraulic system performance was studied earlier [6,7]. In this paper, certain important tuner parameters will be studied for their effects on the whole system NVH performance. This information should be valuable for design and research engineers involved in power steering systems. It should be mentioned that extensive correlation studies are being performed to correlate the theory established here with experiment. POWER STEERING SYSTEM MODEL The governing relations for input pressure and flow and output pressure and flow are needed for each component at any given frequency to assemble the system. We will first study the impedance of each of these components and then concatenate the system. |
