Study of Drift in an Asymmetric Linear Hydraulic Motor

It is well known that in a hydraulic servomechanism the ram exhibits in the open-loop a linear motion in addition to its usual oscillatory motion for a sinusoidal input. This phenomenon, called drift, gives rise to an apparent instability of the system and the study of the dynamic behaviour becomes difficult. A theoretical investigation into the causes and remedy of this phenomenon is made on a loaded system with an asymmetric linear hydraulic motor controlled by an open-centre three-way spool valve. The original non-linear system equation in dimensionless form is derived taking into account the effects of leakage, compressibility (both fixed and varying) and area ratio, and solved numerically by the fourth-order Runge–Kutta method. It is shown that the determination of the reverse flow boundary is essential prior to the study of drift, since the rate of drift will be maximum near the boundary frequency. It is also shown that while the effect of compressibility due to the change in initial volume is mainly responsible for drift for an area ratio of 1 : 2, the effect of area ratio itself is predominant for an area ratio not equal to 1 : 2. It is observed that in most of the cases the drift can be removed by giving a proper input shift to the valve. A method of calculating the input shift required is suggested and the effects of different system parameters on its magnitude are studied.