The Hydraulic Buck Converter - Conceptual Study and Experiments

Dissertation, Johannes Kepler Universität, Linz, 2012.

Thesis Cover
ISBN 978-3-99033-059-3


The main advantage of hydraulic drive technology is its unrivalled force and power density. Small size hydraulic actuators can drive huge forces, but, hydraulics has the blemish of bad efficiency if proportional control is applied. This is a major reason why hydraulics loses ground to other drive technologies in terms of competitiveness. Furthermore, proportional or servo valves are often highly sophisticated, fairly expensive and sensitive components. The importance of energy efficiency increases steadily due to rising energy costs and the demand for environmental friendly technology.

„Digital Hydraulics“ is an attempt to give an answer to this demand on simpler, more efficient, cheaper and more reliable hydraulic actuators. Two research groups, the Department of Intelligent Hydraulics and Automation (IHA) at the Tampere University of Technology in Finland and the Institute of Machine Design and Hydraulic Drives (IMH) at the Johannes Kepler University in close cooperation with the Austrian Center of Competence in Mechatronics (ACCM) in Linz started around the turn of the century to intensively work on such types of hydraulic systems. While the IHA is concentrating on hydraulic digital-analogue converters, the IMH deals with fast switching hydraulic systems like, for instance, the hydraulic buck converter. Both research groups transfer concepts from digital electronics to hydraulics. This has not been possible for long, because of missing qualified components. For some years now, fast enough valves are available as prototypes to realise switching converter concepts in hydraulics.

This work focuses on the hydraulic buck converter. Basically, many other converter principles from electrical engineering like, for instance, the boost-, boost buck- or the resonance converter can be realised in hydraulics as well. Hydraulic switching converters represent dynamic systems, which require adequate mathematical models, control, fast valves and sensors. Hence, the proper design of such devices requires a mechatronic approach.


This thesis concerns the Hydraulic Buck Converter (HBC), which is a concept transferred from electric drive technology to hydraulics. In contrast to resistance control the HBC enables an energy efficient operation of hydraulic drives. This type of power control belongs to the class of fast switching hydraulic systems. Conventional resistance control uses proportional valves, which are responsible for the bad efficiency, since the surplus of pressure is dissipated by the metering of the valve. Moreover, proportional valves are expensive and sensitive against oil contamination. Fast switching hydraulic drives employ digital switching valves with a response time in the range of 1 ms. Because of their simple design such valves can be produced at low costs and, furthermore, they are robust against oil contamination. HBCs according to the current stage of switching valve technology operate at a switching frequency in the range of 100 Hz in pulse-width-mode. The fluid in a pipe inductance is accelerated by the switching of a valve for a certain pulse time of the corresponding duty ratio, which is the control input of the converter. The occurring spill-over of kinetic energy of the fluid is used to draw oil from a lower to a higher pressure level, which results in a higher efficiency of the configuration. Furthermore, the HBC is even able to recuperate energy.

In this work the concept of the hydraulic buck converter is investigated. The potential of this drive technology is discussed by a number of simulations and experiments. The fast switching of the valves provokes high pressure pulsations in the hydraulic pipe system, which are often unwanted at the load. Thus, for a sound understanding of the dynamic behaviour of an HBC wave propagation in the fluid must be taken into account. To smoothen the excited pressure fluctuation due to the switching process a pulsation damper, e.g. a hydraulic accumulator, is applied. Also the dynamic behaviour of such accumulators plays a crucial role in hydraulic switching control. Due to the application of gas loaded hydraulic dampers for pressure attenuation a loss of stiffness in the load system occurs. The resulting softness of the system can be compensated by certain control strategies at least partially. A corresponding analysis is carried out and a flatness based controller design for a fast switching hydraulic drive is proposed in this work. The resulting non-linear controller designs are evaluated by simulations and by a number of experiments. Finally, an outlook of further developing steps of the HBC is presented.