An experimental study on the use of unstable electrohydraulic valves for control

Single stage valves have their main spools stroked directly by solenoid actuators. They are cheaper and more reliable. Their use, however, is restricted to low bandwidth and low flow rate applications due to the limitation of the solenoid actuators. In a previous paper, a way proposed to alleviate the need for large solenoids in single stage valves by inducing spool instability using the transient flow forces, hence, improve the spool agility. In this paper, we study the underlying premise that the transient flow forces can be controlled by the "damping length". Models for both steady and transient flow forces that include viscous effects are developed. Various models are analyzed, compared using CFD analysis and correlated to experiments. It was found that the model for the spool dynamics using the various flow force models are consistent with experimental results as long as viscous effects are taken into account. Both magnitude studies and experiments show that "damping lengths" and transient flow forces significantly affect spool agility. CFD studies also indicate that viscosity is an important factor to consider while modeling fluid flow forces.