Milos Vukovic

STEAM - A mobile hydraulic system with engine integration

In recent years, research institutions worldwide have developed a number of new mobile hydraulic systems. Despite their improved energy efficiency, these systems have yet to gain market acceptance due to their related increase in component costs and decrease in robustness. At the Institute for Fluid Power Drives and Controls in Aachen, a new system for mobile machines, named STEAM (Steigerung der Energieeffizienz in der Arbeitshydraulik mobiler Arbeitsmaschinen), is being developed using inexpensive off-the-shelf components. The aim is to improve the total system efficiency by considering all the subsystems in the machine. This is done by integrating the internal combustion engine (ICE) into the hydraulic design process. By using a constant pressure system in combination with a low-cost fixed displacement pump the hydraulic system is designed to ensure the ICE experiences a constantly high load in a region of high efficiency, so-called point operation. To decrease the hydraulic losses incurred when supplying the linear actuators with flow, an additional intermediate pressure rail with independent metering edges is used. This enables various energy efficient discrete operating modes, including energy regeneration and recuperation.Copyright

Hydrostatic Transmission for Wind Turbines: An Old Concept, New Dynamics

Hydrostatic drives are commonly used in mobile machinery. A new application for this technology is the renewable energy sector, especially wind power. Despite using the same basic components the dynamics of these new drive systems are somewhat different compared to those used in mobile applications. In order to design an appropriate control system for a wind turbine it is necessary to understand these differences and how they affect the system. In this paper, the system behavior of a hydrostatic transmission for wind turbines is compared to commonly used hydrostatic drives in mobile machinery. The analysis begins by explaining that the characteristics of the loading acting on a turbine are fundamentally different to the load torque present in a standard application. Using mathematical models of both systems these differences are highlighted and discussed with special reference to how changes in system parameters can affect stability and lead to non-minimum phase behavior. These theoretical results are validated using measurements of a 1 MW hydrostatic transmission installed on a test bench.Copyright

Single Edge Meter Out Control for Mobile Machinery

To decrease throttling losses in mobile hydraulic systems a number of new system architectures have been introduced in recent years. The concept of independent metering (IM), developed in the seventies, shows a lot of promise [1]. By allowing the meter-in as well as meter-out edges to be controlled separately, additional operating modes are created allowing a more efficient adaptation of system pressure to load pressure [2, 3]. Despite these advantages IM circuits have yet to find their way into industrial applications. This is mainly due to the related increase in component costs and more demanding control strategies. Additionally, the effect of mode switching on actuator motion and operator comfort is still unclear and considered to be a challenge.The STEAM mobile hydraulic system currently being developed in Aachen, Germany uses a number of new features to improve total machine efficiency [4]. Among others is the use of a new independent metering circuit called single edge meter-out control. Unlike other IM configurations, only one proportional valve is used to control cylinder motion. This paper introduces the new concept and discusses its advantages.Copyright