Abstract
Despite being widely used in several applications, commercially available spool valves, both servovalves and proportional valves, are inefficient components because they cause high power consumption due to the large pressure drops across the metering orifices. A recent research field aims at substituting spool valves with on/off valves having high switching frequency (changing state between open and closed in a few milliseconds or less) and producing low pressure drops, in order to make the so-called digital hydraulics possible. In spite of the advantages that it could provide, digital hydraulics does not have significative industrial applications yet, because of the difficulty in manufacturing such high frequency on/off valves. Hence, this paper performs a feasibility study of an on/off poppet-type valve actuated directly by a commercially available ring stack, which is a multilayer piezo-actuator capable of generating very high actuation forces needed for this application. Modulation of the average flow can be achieved by changing the duty cycle of the pulse width modulation (PWM) signal driving the piezostack. An inertance tube could also be used to smooth flow pulsation. The simulations obtained using a detailed Simulink model show that high switching frequency and very effective flow modulation can be obtained with this valve architecture along with low pressure drops and high flow rates, thus making it potentially suitable for digital hydraulics. The disadvantages of this single stage architecture are the large dimensions of the piezo stacks, and the high current generated because of both the high capacitance of the piezo stack and the high frequency switching. However, large-scale production of these components could help to reduce the costs, and the simulations show that limiting the maximum current to 10 A still provides good regulation.
Highlights
Proportional and servovalves are employed in several industrial and aeronautical applications
A proportional valve is usually a single stage configuration in which the spool is directly moved by solenoids; whereas a servovalve employed in aircraft is usually a two-stage configuration, in which the spool is actuated by a pilot stage providing hydraulic amplification
Its success will depend on the development of novel high switching ON/OFF valves
Summary
Proportional and servovalves are employed in several industrial and aeronautical applications. In aircraft, flight control surfaces (e.g. elevators, ailerons, rudders, flaps, etc.) are usually actuated using a servovalve, because of the faster response and lower weight than a proportional valve. A proportional valve is usually a single stage configuration in which the spool is directly moved by solenoids; whereas a servovalve employed in aircraft is usually a two-stage configuration, in which the spool is actuated by a pilot stage providing hydraulic amplification. The main difference between a proportional valve and a servovalve (two stage or single stage) is that the latter is provided with a bushing sleeve, which allows better manufacturing tolerances, lower overlaps, and faster response [1, 2]
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