Designing the Controller of a Servo Valve by Simulation
Nicolae VASILIU*, Ina COSTIN, Constantin CALINOIU,
Daniela VASILIU, Marius D. BONTOS
University POLITEHNICA of Bucharest, Fluid Power Laboratory,
313 Splaiul Independentei, Sector 6, RO 060042 Bucharest, Romania
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* Corresponding author
Abstract: The paper presents the research activities aiming to design the controller of a high flow electro-hydraulic servo valve needed for the power stage of the speed governors controlling high power hydraulic turbines operating under low oil pressure. This is the common case of any Kaplan turbine, but the refurbishing of the old Francis turbines needs the same valve configuration. Design problems, simulation methods and experimental researches are briefly presented. From an industrial point of view, the main idea of the new concept is the use of high-quality industrial electro-hydraulic and electronic components only, in order to obtain good performances even under a low-pressure supply. This target generated a new approach of the design by eliminating the pipes between the flow control stages. A detailed model of a high power servomechanism containing a three-stage non-linear electro hydraulic proportional servo valve was designed by simulation, taking into account the real geometry of the metering spool windows. The valve dynamic behaviour was simulated with SIMULINK and AMESIM, and finally the results were compared with some preliminary laboratory measurements. The simulated and the real responses for different inputs were found in good agreement.
Keywords: Simulation, controller fine-tuning, high power electro hydraulic servo valves, speed governors.
CITE THIS PAPER AS:
Nicolae VASILIU, Ina COSTIN, Constantin CALINOIU, Daniela VASILIU, Marius D. BONTOS, Designing the Controller of a Servo Valve by Simulation, Studies in Informatics and Control, ISSN 1220-1766, vol. 25(1), pp. 51-58, 2016.
Some important applications of the hydraulic control systems require very large flow, and special dynamic performance. A typical practical case can be found in the field of the earthquake simulators of the important buildings, fly simulation tables, automotive dynamics simulators etc. The variation of the speed and the power during the operation of a hydropower unit is achieved by adjusting the water flow that passes through the wicket gates. High accuracy electro hydraulic servo mechanisms actuate these control elements. The architecture of the power control system directly depends on hydropower unit size, specific speed, and the required dynamic performance. The last one depends on the quality requirements of the electrical power produced by the unit. The static and the dynamic forces that appear during the hydropower unit operation are rather large. The pistons of the hydraulic cylinders have rather large diameters, while the oil pressure is usually in the range 20-160 bar. The shut down time of a hydropower unit in case of a damage is usually lower than 10 s.
The current requirements regarding the quality of the electrical power provided by hydropower units are extremely strict, and meeting them requires speed governors to have a better accuracy than 2 mHz. This performance condition requires very precise positioning of the hydraulic cylinders rods, keeping a reasonable stability reserve for the speed governor. For medium size hydropower units, these contradictory requirements can be satisfied by means of a two-stage proportional flow servo valve (Figures 1 and 2), which has critical lap two-slope flow characteristics (Figure 3). Each metering land of the spool of such a flow control valve has rectangular slots (Figure 4). The servo valve dynamics (Figure 5) strongly depends on the supply pressure. The amplitude of the input signals is also an important parameter. A small supply pressure and a high turbine nominal output lead to a three stages servo valve. For example, the two servomotors acting the wicket gates of the KAPLAN turbines of the IRON GATES I Hydropower Station from Danube River have a diameter of 600 mm, and a stroke of 1,200 mm. The servomotor stroking the runner blades has 3,120 mm diameter and a stroke of 300 mm.
The pressure supply is about 40bar, and the emergency shutdown occurs in about 6 seconds. This performance requires a huge oil flow supplied by two control valves with three stages. The final stage is about 200 mm diameter. In such cases, the designer needs always a proper mathematical model and detailed simulations in order to optimize a complex architecture , , .
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