A Simple Switching Control Structure for Improving the Steady-State Error of a Full-Digital Water-Hydraulic Cylinder Position Control

Jyh-Chyang Renn1,a, Shigeru Ikeo2 and Chin-Yi Cheng1,b

1Department of Mechanical Engineering, National Yunlin University of Science and Technology, Douliou, Yunlin, Taiwan.

2Department of Mechanical Engineering, SOPHIA University, Chiyoda-ku, Tokyo, Japan


In this paper, a full-digital closed-loop water-hydraulic cylinder position control system with reduced steady-state error is proposed and simulated. Firstly, the conventional binary coding system together with two sets of proportional digital flow control unit (DFCU) is chosen for this study. One DFCU consists mainly of four parallel-connected 2/2 water-hydraulic switching valves with different but multiple flow-rate outputs and serves as the key linear control element in the positioning system. The major fault of the DFCU, however, is its nonlinear saw-toothed flow-rate characteristic which generally results in an undesirable steady-state error. Therefore, a novel methodology to reduce the steady-state error is developed in this paper. The basic idea is to reduce the opening areas of all four switching valves in a DFCU by applying lower input excitation voltage to the coils. Consequently, without any hardware modification, an alternative saw-toothed flow-rate characteristic with higher resolution but less maximal flow-rate is available in the steady-state response. In details, after reaching the quasi steady-state response, the control scheme is then switched to the higher resolution mode so that the steady-state error can be effectively reduced. Finally, from the simulation results, it is proved that the steady-state error is significantly reduced by using the proposed novel switching control structure as compared to the utilization of a conventional binary coding controller.

Keywords: Full-Digital Control, Water-hydraulics, Cylinder, Position Control.

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