Abstract
Recently, electro-hydraulic servovalves have come to be used widely in hydraulic control, but the complexity of their construction is unfavourable. Needless to say, servovalves without any moving parts are desirable from the viewpoint of fast-response, efficiency and reliability.
So, the purpose of this research, as the first step to the desirable servovalves, is to develope a new servovalve adopting a piezoelectric flapper directly actuated by an electric signal. A piezoelectric flapper, adopted instead of a conventional torque motor and a flapper, consists of piezoelectric slabs and a steel slab. As the result of the preliminary experiments, it was clarified that this flapper could be practically used if the adverse effect of its inherently strong hysteresis is removed.
In this research in order to give the flapper equivalently linear characteristics, the input signal to the flapper is given in the form of a pulse width modulated wave. However, in this case the output of the pulse width modulator consists of the various frequency components depending on the input and the carrier frequency. It is well-known that the signal cannot be accurately transmitted, when the ratio of the carrier frequency to the input frequency becomes small. So, one of the most important points of this paper is to establish the conditions of the accurate signal transmission in the P.W.M. mode operation.
As the results of the experiments on the new servovalve, it has become clear that in spite of the strong hysteresis, the new flapper has equivalently linear characteristics, when driven by the pulse width modulated wave from the modulator. Futhermore it has been clarified theoretically and experimentally that for accurate signal transmission through the modulator, the carrier frequency must be higher than seven times the highest frequency of the input signal components.
