Using steady flow force for unstable valve design: Modeling and experiments

In single stage electrohydraulic valves, solenoid actuators are usually used to stroke the main spools directly. They are cheaper and more reliable than multistage valves. Their use however is restricted to low bandwidth and low flow rate applications due to the limitation of the solenoid actuators. Our research focuses on alleviating the need for large and expensive solenoids in single stage valves by advantageously using fluid flow forces. For example, in a previous paper we proposed to improve spool agility by inducing unstable transient flow forces by the use of negative damping lengths. In the present paper, how steady flow forces can be manipulated to improve spool agility is examined through fundamental momentum analysis, CFD analysis, and experimental studies. Particularly, it is found that two often ignored components-viscosity effect and nonmetering momentum flux, have strong influence on steady flow forces. For positive damping lengths, viscosity increases the steady, flow force, whereas for negative damping lengths, viscosity has the tendency to reduce steady flow forces. Also, by slightly, modifting the non-metering port geometry, the non-metering flux can also be manipulated to reduce steady flow force. Therefore, both transient and steady flow,forces call be used to improve the agility, of single stage electrohydraulic valves. Experimental results confirm the contributions of both transient and steady flow force in improving spool agility.