Miniature Digital Hydraulic Valve System - Pilot operated design with fast response and high flow capacity

In digital hydraulic valve systems, a metering edge consist of a number of parallel connected on/off valves and they can be utilized for throttling control of hydraulic actuators, similarly to spool type proportional and servo valves. Compared to the commonly utilized servo valves, digital valve systems enable several improvements to the energy efficiency and actuator control performance of throttling control. Even though throttling inherently causes energy losses, these losses can be significantly reduced with independent metering enabled by digital valve systems. Digital valve systems are also generally robust, fault tolerant and can have a faster response than the best-performing servo valves. Utilizing digital valve systems is currently not widespread partly due to a lack of on/off valves suitable for assembling them. Manufacturing very fast and especially small digital valve systems is at the moment not possible utilizing commercial components. The research presented in this thesis aims to improve digital valve technology by developing a miniature on/off valve with fast response and high flow capacity as well as a digital valve system utilizing these miniature valves. This thesis presents a design for a fast-acting pilot operated hydraulic on/off valve with a volume of approximately 3 cm3 and a response time of less than 2 ms. In addition to designing the hydraulic components, developing such a valve involved designing a miniature solenoid actuator with a response time of approximately 0.3 ms and suitable control electronics for it. This thesis analyzes the effect of several parameters on the response of the solenoid actuator utilizing finite element modeling as well as measurements with a prototype. Also the effect of utilizing control electronics with reverse current boosting capability on the turn-off response time of the solenoid actuator is modeled and measured. In addition, the fundamental benefits and limitations of miniaturizing solenoid actuated poppet valves are analyzed. The developed on/off valve is utilized to assemble a small digital hydraulic valve system with dimensions similar to those of commercially available NG6 subplate mounted servo valves. Two additive manufacturing methods, laminated object manufacturing and selective laser melting, are evaluated for realizing the complex manifold geometry of the valve system. Improving the manifold geometry with computational fluid dynamics simulations and the versatility of selective laser melting resulted in a valve system with a flow capacity exceeding that of the best-performing similar sized commercial servo valves by 200 percent. Simultaneously, the response time of the valve system is significantly faster than that of the reference servo valves.