Electro-hydraulic pressure-control valves are used in many applications, such as manufacturing equipment, agricultural machinery, and aircrafts to name a few. A traditional electro-hydraulic pressure-control valve regulates an output pressure for a corresponding input current by balancing solenoid force, spring force, and regulated pressure force. This results in a repeatable steady-state pressure output that is nearly proportional to the input current. This is helpful in open loop applications when one wants to achieve a consistent output pressure for a corresponding input current. The transient pressure response, however, is highly sensitive to the component tolerances and manufacturing processes as well as the fluid properties in the regulated volume, such as bulk modulus, viscosity, density, and aeration. These properties are often unknown in a system and can vary significantly from system to system and also during use in a typical application, making controllability difficult. Since there is variation in the steady-state pressure output for a given valve population, these valves are often calibrated in the end system to better achieve the desired output. This helps, but there is variation in this process, and also variation within a single valve over life. So although various attempts are made to minimize steady-state error, it will always exist and therefore closed loop control is desirable. Unfortunately, attempts at closed loop control of a traditional pressure-control valve often yield unacceptable and inconsistent performance. This is due to the sensitivity of the transient response to system characteristics, primarily fluid and mechanical properties of the regulated control port volume. The transient performance sensitivity of the valve can be reduced by de-coupling the regulated pressure dynamics from the spool dynamics. This will conversely increase the sensitivity of the steady state performance; however this can be solved through the implementation of a closed loop controller. In this paper a dynamic model is developed for a traditional pressure-control valve and different pressure-control valves without the traditional pressure balancing force. The new valve models are validated experimentally and then used to compare the performance characteristics of the valves. Linear analysis is performed on the validated models to further illustrate the impact of the system properties. The objective of this work is to develop a pressure-control valve with more consistent transient performance characteristics that are less sensitive to the system parameters so that a closed loop controller can be developed for the valve.
A Review of Capillary Pressure Control Valves in Microfluidics
