Switchmode hydraulic power supply is a new kind of energy-saving pressure converting system, which is originally proposed by the authors. It is mainly applied in multiple-actuator hydraulic systems, and installed between hydraulic pump and actuators (one switchmode hydraulic power supply for one actuator). It can provide pressure or flow rate that is adapted to the consumption of each actuator in the system by boosting or bucking the pressure, with low power loss, and conveniently, through high-speed switch valves, just like a hydraulic pressure transformer. There are two basic types of switchmode hydraulic power supply: pressure boost and pressure buck. Their structures and working principles are introduced. The dynamic characteristics of two typical types of switchmode hydraulic power supply, the pressure boost type and the pressure buck type, were analyzed through simulations and experiments. The performances were evaluated, and improvements on the efficiency of switchmode hydraulic power supply were proposed.

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Switch-mode hydraulic power supply is a hydraulic pressure converting unit made of some distributed hydraulic components, which can boost or buck hydraulic pressure steplessly, with low power loss (about 20%) and continuous flow-rate[1][2]. There are two types of switch-mode hydraulic power supply. One is pressure boost type and the other is pressure buck type. For the pressure boost power supply, changing of the pressure is realized through instantaneous braking of the large inertia load in the hydraulic inductor. For the buck power supply, changing of the pressure is realized through pulse flow-rate and low-pressure hydraulic complement (see "Switch-mode Hydraulic Power Supply Theory", 2005 ASME, IMECE-FPST No.79019)[2]. Because the output pressure is determined by the load, pressure buck is still requisite in pressure boost power supply. At the same time the system is unstable and with low efficiency. To deal with the problem that the pressure boost type switch mode hydraulic power supply is unfit for the low pressure load, the principle and the structure of a compounded switch-mode hydraulic power supply are proposed in this paper. In the compounded switch-mode hydraulic power supply, a pressure buck power supply is cascaded after a pressure boost power supply. At the same time, the output hydraulic capacitor of the pressure buck power supply and the input hydraulic capacitor of the pressure boost power supply are removed, which leads to the direct connection of the hydraulic inductors of the two power supplies Because of the same working principles of the two power supplies, one of the hydraulic inductors can be removed. Pressure boost and pressure buck are realized through the synchronically control of the two high - speed switch valves using PWM signal. No matter the outer load determined pressure is higher or lower than the pump pressure, compounded switch-mode hydraulic power supply can provide the proper power (not flow rate) matching actuators' consumption through regulating the duty ratio of the control signal. Therefore the optimal energy -saving is realized. Experimental research shows that the compounded switch-mode hydraulic power supply can realize a continuous bucking and boosting pressure with different duty ratio and the whole efficiency is at least 80%.

A novel seawater pressure energy conversion system that utilizes seawater pressure to generate electric energy has been studied in this paper. The energy conversion system utilizes the pressure difference between the pressurized seawater and the empty pressure container to drive hydraulic motor and the coaxially coupled generator to generate electric energy. The output electric energy has been recorded by the data logger throughout the process. However, the actual conversion efficiency from seawater pressure energy to electric energy reaches only 63.8%. Moreover, the electric power supply time of the conversion system has mainly been limited by the holding capacity of the empty pressure container. The low conversion efficiency and the short electric power supply time have limited its application in the long-term in-situ observatory stations. Therefore, a new hydraulic driving system consisting of high-speed on/off valves will be applied to the system not only to improve the conversion efficiency but also to extend the electric power supply time. In the new hydraulic driving system, the input flow rate of the system has been largely reduced due to the recycle of the flow through the hydraulic motor. Thus the conversion efficiency has been improved and the electric power supply time has been extended. In the current study, the analysis is conducted at various duty ratios of PWM (Pulse Width Modulation) signal which is provided to highspeed on/off valves so as to obtain maximum conversion efficiency. Research shows the optimum duty ratio and the maximum conversion efficiency at the optimum duty ratio can be theoretically calculated when the properties of the system are known. Simulation results have demonstrated the influence of duty ratio on conversion efficiency. Although the conversion efficiency of the system with the new hydraulic driving system has not increased significantly, the reduction of the input flow rate has largely extended the electric power supply time which is very useful for long-term in-situ observatory stations.