A Novel Equivalent Continuous Metering Control With a Uniform Switching Strategy for Digital Valve System

Pulse number modulation (PNM) combined with pulse width modulation (PWM) control is an effective solution to improve the resolution of digital valve systems. However, the numerous discrete variables that use parallel on / off valves cause difficult control coordination and uneven switching. To address this issue, this article defines the equivalent spool displacement of the digital flow control unit by the number of PNM-controlled valves and the duty cycle of PWM-controlled valves to replace multiple discrete variables and develops the equivalent continuous metering control method. Furthermore, a uniform switching control strategy is proposed for the PWM-controlled valve using a uniformly distributed permutation for each on / off valve. The proposed control methods are verified by simulation on the built mathematical model of the equal-coded digital valve system. Experimental results for the displacement control of a hydraulic cylinder at 1 rad/s show that the average error of the equivalent continuous metering control is about 0.236 mm and the dispersion index reaches 20%, while the uniform switching control strategy achieves 80% with an average error of 0.215 mm. Simulated and experimental results demonstrate that the equivalent continuous metering control with a uniform switching strategy can almost evenly distribute switching numbers without compromising the accuracy of the displacement control.Peer reviewe

High-Linear Digital Hydraulic Valve Control by an Equal Coded Valve System and Novel Switching Schemes

This study proposes a novel digital hydraulic valve system using multiple equal size on/off valves and a circulating switching control, with an aim to increase the resolution and the linearity of the digital hydraulic valve systems. The solution is founded on the equal coded valve system concept which represents a recent development in the digital hydraulic valve technology. The circulating switching control algorithm is used to overcome non-linearities occurring in the typical non-circulating switching control and to decrease the operating frequency of single on/off valves. As a result, a substantial improvement in tracking control performance is demonstrated with 8+8 parallel connected valves. The results verify that compact, robust and high-performance valve control can be realized.acceptedVersionPeer reviewe

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

Equal coded digital hydraulics in low temperatures using water-glycol solution

In this thesis work, effects of arctic temperatures for digital hydraulic system was researched. Hydraulic medium was environmentally friendly and additive free 50 wt% water-glycol solution. Dynamics of on/off valves, steady-state properties of on/off valves and system’s hydraulic natural frequency were qualities under inspection. These qualities were chosen as they are key qualities for digital hydraulics. On the other hand, this research was conducted to determine usability of commercially avail-able on/off valves, when controller applying multi-valve pulse modulation and multi-valve pulse frequency modulation is used. Another objective was to find out, which method might be preferable to compensate the changing properties of system to accuracy of control. Research was conducted as simulation and experimental study. Simulations were minor part of the research and were performed to validate the implemented controller. Experimental study was done using a hydraulic system that composed of equal coded digital valve system and a hydraulic cylinder that was used to move a mass in horizontal direction. Experimental research can be separated to two different parts. In first part, changes in system properties were investigated in temperature interval of ˗20°C…30°C. Second part of the experimental research was testing the controller in different temperatures. Controller was tested with three different cylinder strokes (15 mm, 150 mm and 300 mm) in three different temperatures (˗20°C, 0°C and 30°C). Results of this research indicate that dynamics of on/off valves slows down and volume flow in steady-state conditions decrease in low temperatures. Hydraulic natural frequency of the system increased in low temperatures. Results attained using the controller were comparable to results achieved in reference study. This indicates that commercially available valves are usable in multi-valve modulation control. In addition, results attained in low temperature using the controller suggest that correct way to increase control accuracy in low temperatures is to compensate slowing dynamics of valves