Electro-hydraulic servosystems are utilized in various industrial fields, not to speak of control of machine tools. In the electro-hydraulic servosystems, hydraulic motors are used as actuators.
There exists solid friction in the sliding parts inside the hydraulic motors. This solid friction results in sticky rotation of the hydraulic motors in very low speed region, in other words, the hydraulic motors cannot rotate smoothly in that region.
Needless to say, this sticky rotation of the motors is undesirable from the viewpoint of control.
The purpose of this research is to rotate a hydraulic servomotor, which consists of a spool valve and an axial piston motor, as slow as possible by applying dither on the shaft of the axial piston motor.
In related fields, there are a few published papers on the dither and on stick-slip motion.
The fomer ones discuss on the dither for removing dead zone caused by static (solid) friction. in other words, they discuss about the dither effect on removing the solid friction only when mean velocity of the motor is zero.
The latter ones are related to the analyses of the stick-slip motion, but they don't show the way to remove the stick-slip motion of the motors by the dither.
In the first part of this paper, necessary velocity amplitude and frequency of the dither ta remove equivalently the solid friction at the various mean velocity of the motor are investigated theoretically.
On the other hand, it is generally impossible to generate the dither whose amplitude and frequency are arbitrary. So, in the second part of the paper, conditions which are necessary to enable generation of the dither are discussed theoretically.
In the final part of the paper, experimental results of the low speed characteristics of a hydraulic servomotor are shown.
In this experiment, the servomotor was rotated with appropriate dither obtained from the above mentioned theoretical results, and it was clarified that minimum velocity of the servomotor was much lower than the one without the dither.

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