One of authors has shown in the previous paper that hard self-excited oscillations can occur in a hydraulic sevomechanism with an asymmetrically underlapped spool valve by delivering a step input beyond a certain critical value, even if the neutral position of the valve is stable.
In this paper, it is shown that there exists an absolutely stable region in the system, where no self-excited oscillation is induced by any input disturbance. To study the global stability of the system, the fundamental system equations containing marked nonlinearities are analyzed by a digital simulation procedure by using Runge-Kutta-Gill method.
Conclusions obtained are as follows:
(1) Stability of the system is divided into three parts, that is, (a) a soft self-excited region, (b) a hard self-excited region, and (c) an absolutely stable region.
(2) The local stability limit of the valve neutral position actually coincides with the absolute stability limit of the system for Λ<0.7, but they separate into two curves for Λ>0.7 and the hard self-excited region appears between them.
(3) To asymmetrize the valve underlap is effective not only to stabilize the valve neutral position, but also to make the system absolutely stable.
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