Christian Stammen

Selbstverstärkende hydraulische Bremse

Fahrerassistenzsysteme benotigen eine elektrische Schnittstelle zur Bremssteuerung auf niedrigem Leistungsniveau. Gleichzeitig soll aus Sicherheitsgrunden eine mechanische Ruckfallebene vorgesehen werden. Am Institut fur Fluidtechnische Antriebe und Steuerungen der RWTH Aachen wird eine selbstverstarkende hydraulische Bremse entwickelt, die diesen Zielkonflikt lost. Neben dem besonders geringen Energiebedarf zeichnet sie sich durch die Regelbarkeit des tatsachlichen Bremsmoments aus. Die dargestellten Messergebnisse demonstrieren das Entwicklungspotenzial der Bremse fur Kraft- und Nutzfahrzeuge.

Investigation of Different Methods to Measure the Entrained Air Content in Hydraulic Oils

The fluids used as pressure media in fluid power systems are often polluted with undesired air bubbles. This entrained air affects the system behaviour, stability and safety. Knowledge of the amount of entrained air inside a hydraulic fluid plays a decisive role in predicting the system behaviour. In addition, this information is necessary when a system or components should be optimised to obtain better air release properties. The content of entrained air highly depends on the static pressure as air is always dissolved in hydraulic pressure fluids up to a certain equilibrium condition.In this paper, different physical principles (optical, mechanical and electrical) are presented to determine the amount of entrained air in an oil-hydraulic system. Starting from these theoretical ideas, different methods are selected and corresponding test devices are designed and built up. These devices are experimentally investigated by including entrained air into a commonly used mineral oil in a hydraulic system. The tested devices are based on different physical principles. In the end, the methods are compared against each other in terms of accuracy of the results and effort to perform these measurements.Copyright

An analytic thermodynamic model for hydraulic resistances based on CFD flow parameters

Abstract This article illustrates the development of an analytic lumped parameter thermo-hydraulic model for a wide range of hydraulic resistance geometries based on mass flow. The relevant flow parameters such as the contraction coefficient in case of laminar flow separation are derived from CFD simulations. Furthermore, the consideration of cavitation effects can be included. State of the art in lumped parameter simulations of hydraulic circuits utilise volume-flow based equations like the orifice equation, which is extended for a wide variety of geometries and flow conditions including the transition from laminar to turbulent flow by adjusting the discharge coefficient based on empirical equations or lookup tables. The same situation persists for laminar flow description. In this case the Hagen-Poiseuille equation is often used in conjunction with correction factors based on the Reynolds number to regard the transition of laminar to turbulent flow. However, in practical applications the use of different equations for various flow conditions and geometries is cumbersome. Furthermore, in the widely used volume based flow description, the absolute pressure dependency of mass flow due to density changes and critical flow at which cavitation occurs is not accounted for until now. Without consideration of these influences a mass conservative modelling and thus high model precision is not possible. The overall goal of the proposed model is to increase accuracy of hydraulic system simulation tools and to support usability by simplifying parameterisation on basis of dimensions available from data sheets. The results of this study are obtained analytically as well as empirically by means of CFD simulations. Moreover, a large number of performed simulations support the understanding of fundamental effects in hydraulic resistance flow.

CFD Simulations and Experiments of the Dispersed Two-Phase Flow Through Hydraulic Orifices

The classical approach in one dimensional fluid power systems simulation considering the flow through hydraulic resistances is to imply a single-phase flow of an incompressible fluid. This assumption leads to significant errors in cases when the system is polluted with entrained air. In this paper the classical orifice equation describing the incompressible flow based on the Bernoulli’s equation is analysed and transferred to allow the calculation of the existence of a second disperse gaseous phase. The basic objective hereby is the examination of the different behaviour caused by a laminar, turbulent or laminar-to-turbulent transition flow field as well as its ranges.Therefore Computational Fluid Dynamics (CFD) simulations of the single-phase flow and the two-phase flow through a hydraulic orifice are performed and the calculated discharge coefficients parameterizing the orifice equation are compared. To assure the validity of the simulations the one-phase and two-phase flow is measured using a new designed test bench. Finally the calculated, simulated and measured results are compared.© 2013 ASME

Fundamentals of Mass Conservative System Simulation in Fluid Power

This article illustrates the development of a dynamic system simulation tool for fluid power on basis of mass flows. The goal is to increase the predictability and efficiency of system simulation tools in fluid power. State of the art simulation tools make use of simplified differential equations. Especially in closed systems or long-term simulations, the volume flow based approach leads to significant variations of simulation results as balancing of flow parameters and its integrations to potentials lead to a violation of the equation of continuity. However, with a mass flow and energy conservative approach we obtain a clear and physically correct model implemented in the simulation tool DSHplus. The new basis of calculation enables further implementation of thermo-hydraulic and multi-phase flow models such as cavitation or particle transport into the concentrated parametric system simulation.© 2009 ASME

The self-energising hydraulic brake

Driver assistance systems require for low-power electrical interfaces for braking control. At the same time for safety reasons a mechanical fallback level should be provided. At the Institute of Fluid Power Drives and Controls of the RWTH Aachen University (Germany) an innovative Self-Energising Hydraulic Brake is being developed to solve this conflict of objectives. Besides its low power consumption it features a closed loop control of the actual braking torque. The presented measuring results demonstrate the brake’s development potential for cars and utility vehicles.

Fluid effects in small gaps - A challenge for sensor application

This paper is motivated by the difficulty of pressure distribution measurement in the small gap between valve spool and housing or sleeve. The research project “Design of Balancing Grooves for Hydraulic Valves using Analysis of Flow and Pressure Distribution” at IFAS aims at providing a design tool for balancing grooves. It is based on the idea that the application of modern simulation techniques together with experimental results enhances knowledge about necessity and design of balancing grooves. Main influence on the pressure distribution is the gap height profile. Although valve spool and sleeve can be examined separately, it cannot be concluded to the geometrical information about the gap between these parts as accurate as needed to verify the simulation model. The paper describes the development of miniature pressure sensors made of manganin wire. Measurements showing the current results are presented. As an outlook, the integration of gap height measurement into the pressure sensors is proposed.Copyright