Simulation and Experimental Investigations of a Hydraulic Stepper Drive

Abstract

This paper presents simulation and experimental results of an energy saving hydraulic stepper drive prototype. Different concepts, advantages and the mechanical design of such kind of stepper drive were discussed in a previous publication. The excellent efficiency, the possibility of energy recuperation, and the control by switching and check valves only, may help to open new applications for hydraulic drives. Also the flow rate can be controlled rather directly by adjusting the switching frequency. This characteristic makes the sensorless position and speed control relatively easy. The drive is realized by a hydraulic cylinder piston unit which displaces a defined fluid quantum by the limited forward stroke of the piston controlled by a fast switching valve. This end to end motion of the piston in its cylinder generates a precise, incremental motion of an additional load cylinder; this enables a sensorless position control. Energy saving is achieved by storing the pressure surplus intermediately in the kinetic energy of the piston to displace a part of the fluid quantum without hydraulic energy from the supply line. A detailed simulation model of a stepper drive including transmission lines, flow channels, hydraulic accumulators and valve dynamics is applied to analyze the experimental results. This dynamic model in connection with the prototype allows to identify the potential for improvement. The different ways to improve the behavior are reviewed, in particular concerning energy losses: bearing friction, leakages in gaps, pressure losses and backflow through check valves. The measured dynamic characteristics and the energy efficiency are presented and compared to the simulation results. The preliminary results showed that the energy efficiency can be drastically increased by a better piston sealing and guidance system and faster check valves. Hence, the development of a fast plate type check valve for the hydraulic stepper drive is also proposed in this study.