Keywords: electrohydraulic servo-drive, low motion velocity, stepping motor.

Linear electro hydraulic servo-drives are built in and used as part of various industrial equipment for more than sixty years. They are distinguished by an ability to develope extremely large forces with a positioning accuracy of the order of a micrometer, ensuring, at the same time, very good dynamic features. The drives are characterised by very slow motion and large applied forces among others.They are used in presses, rolling mills, large machine tools, in mobile devices, such as building machinery, and in antennas and space telescopes where the observed objects must be followed with a high accuracy. These features are confirmed by many literature references cited in this paper. In order to ensure precise control of electro-hydraulic drives the very expensive servo-valves provided with torque motors or proportional valves are used. Operational characteristics of the stepping motors indicate that they are able to make very small angular motions (steps) with high frequencies. This enables their use in driving elements that control very small fluxes.

This paper presents and discusses the construction of a hydraulic valve controlled with a stepping motor, construction of the whole servo-unit, and a complete stand for verification tests provided with a servo-drive. A mathematical description is presented for a four-edge hydraulic amplifier controlled using the stepping motor and a hydraulic servo-motor. Their models have been built and simulation tests have been carried out with the use of the Matlab-Simulink software. The phenomena characteristic for slow fluxes and small motion velocity are considered in the model.

Based on theoretical considerations and the simulation and experimental tests of the electro-hydraulic servo-unit, with stepping motor, acting as a setting element, and electric feedback it is concluded that the use of the stepping motor for setting the travel of the slide of the hydraulic amplifier is possible and it enables very small and accurate slide displacements to be achieved.

Such a solution ensures that the flux control valve can be set with high precision and enables very slow motion of a servo-unit provided with such a valve. The minimum feasible permanent speed of the servo-unit considered amounted about to 2µm/s. Reduction of the minimum speed is limited by the low frequency of the digital measurement of low velocity and contamination of the liquid resulting in obliteration of small slots. The possibility of achieving a low velocity for the rotational motion of the slide is controlled by removal of the contamination accumulating at the edges of the working slots. Contamination particles of the dimensions exceeding the slot area may flow as a result of the temporary increase in the slot area as a result of slide oscillation. The amplitude and frequency of the oscillation is particularly important. Servo-units provided with electric feedback may ensure the high flexibility of the adjustment of the parameters and compensation of the nonlinearity.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £50 +P&P)