An extensive laboratory study was made of a slow active suspension pressure control, which can regulate the pressure in an actuator fitted to each wheel to control vehicle attitude. A proportional pressure reducing valve (PPRV) has been employed to optimize the vehicle attitude and tires' contact with road surfaces under any load and surface conditions. In response to the electric current to the proportional solenoid, the PPRV can control the actuator pressure. Two-stage PPRV's, which have been used in actual active suspension systems, give rise to relatively high energy loss compared with single-stage valves. This was due to the return flow at the pilot stage, which increased fuel consumption of the vehicle. Consequently the efficiency of the active suspension system was deteriorated. Furthermore, it was found that very small orifices in a two-stage PPRV often cause silting problems and malfunction of the valve.
In this study, a new type of single-stage PPRV was proposed as a substitute for the two-stage PPRV to overcome the afore-said problems. The stability characteristics of the proposed valve system was scrutinized via linearization and root locus method. The step response and parameter sensitivity with respect to the reduced pressure were analyzed to investigate the effects of variations in main system parameters on the system behaviors. Based on theoretical and experimental studies, the nominal values of proposed single-stage PPRV for slow active suspension systems were presented. The prototype valve showed good linearity and small hysteresis in the relationship of reduced pressure versus input voltage to the proportional solenoid. The damping piston, which was directly linked to the valve spool, could improve the valve stability effectively. Satisfactory reduced pressure responses were obtained in the step response experiments. To achieve the faster step response of the controlled pressure, the push rod size was increased and the nonsymmetric spool lands and sleeve were provided in the design of the single-stage PPRV. The redesigned prototype valve was tested in the experimental setup and the performance of the valve was compared with that of the two-stage PPRV.
It was found from the experimental results that the new single- stage PPRV could be used as a substitute for a two-stage PPRV in slow active suspension systems.