The Hydraulic Buck Converter - Concept and Experimental Results

The idea of so called hydraulic transformers, i.e. of devices that can rise or lower a hydraulic pressure without much energetic losses, is by no means new. To a large extent, the proposed technical realizations of this hydraulic principle have been based on combinations of adjustable pumps and motors (see, e.g., /1/). Also Achten’s celebrated hydro transformer /2/ belongs to this category of transformer concepts based on variable displacement machines. Other, physically different approaches for realizing a hydraulic transformer are hydraulic switching converters which adopt analog concepts from electrical engineering. To have an energetic advantage over resistance control such switching converters must employ some inertia (or inductivity) effects. The simplest realization of such a converter stems from Gall and Senn /3/. Their concept is based on the load inertia and is rather limited in its performance if both, smoothness of the generated motion and efficiency are relevant. In /4/ a converter concept is presented which exploits the inertia of a hydraulic machine. Switching converter concepts based on hydraulic inductivities have been discussed in /5, 6, 7/. The hydraulic Buck converter is the simplest hydraulic switching converter and is nearly a one to one transfer of the electrical Buck converter to hydraulics. Some specific aspects of this converter, namely the modeling of wave propagation phenomena in the pipe, the control of linear drives with this converter, and the cavitation at the entrance to the pipe and means to avoid cavitation, have been studied in /8,9,10/. But so far, no description of its basic properties by simple mathematical models has been given. This and basic experimental results which prove the feasibility of this hydraulic converter are addressed in this paper.