Novel Water Hydraulic On/Off Valves and Tracking Control Method for Equal Coded Valve System

Water hydraulics is an interesting alternative to oil hydraulics since it uses a clean, environment friendly, and non-inflammable pressure medium. However, the availability of good control valves, which are essential components in many hydraulic systems, is limited in water hydraulics owing to the challenging characteristics of water. This, in turn, considerably limits the application of water hydraulics. In this thesis, a new digital hydraulic valve technology is investigated in order to promote the applicability of this clean technology; the method using equal-size on/off valves is preferred.

This equal coding method suffers from a lack of suitable valves. Thus, a fast-acting and low-power-consuming miniature valve is developed. The valve development is based on previous experience with oil hydraulic miniature valves and is carried out mainly with heuristic methods, by using simplified electromagnetic equations and scaling. However, the prototypes show good properties for the intended purpose. As another challenge, a high number of these valves are needed in order to achieve sufficient resolution for demanding control. To decrease this requirement, the circulating switching control method is proposed, which can increase the effective flow resolution of the valve system using the existing valves. The method divides the switching duty equally among the valves and can diminish the drawbacks that exist in typical non-circulating switching control.

As the main goal, a water hydraulic servo axis was implemented using the developed miniature valves and the control method. The control system comprises a simple model based valve controller as the lower level part and a filtered P-controller as the upper level motion controller. Excellent tracking and positioning accuracy was achieved with a valve system having 16 miniature valves in total and using this relatively simple controller structure. This verifies that high-performance water hydraulic motion control can be realized with a reasonable effort using on/off seat valves. This gives hope for increasing the applicability of water hydraulics.