Design of a Hydraulic-mechanical Load-sensing System Using Object-oriented Modelling and Simulation

This paper deals with the design of a hydraulicmechanical load-sensing system using computer modelling and simulation in an object-oriented programming environment. As initial, the scheme of a hydraulic-mechanical load-sensing system of Bosch GMbH has been taken. A modified scheme of the system is proposed. The object-oriented mathematical model of the system is composed. The approach is based on using multi-pole models and signal-flow graphs of functional elements, enabling methodical, graphical representation of mathematical models of large and complicated chain systems. In this way we can be convinced of the correct composition of models. The high-level programming environment NUT is used as a tool for modelling and simulation. A procedure of adjusting the multi-pole model is proposed. Modelling and simulation of separate objects, subsystems and the whole system are discussed. 1 HYDRAULIC-MECHANICAL LOAD-SENSING SYSTEM Fluid power systems, in which working pressure (pressure in pump output) is kept proportional to load, are called hydraulic load-sensing systems. Such systems are mainly used in mechanisms containing numerous drives to run with the purpose to save energy. The scheme of the hydraulic load-sensing system of Bosch GmbH is shown in Fig. 1, the scheme of hydraulicmechanical controller in Fig. 2 and the section of the valve block in Fig. 3. In Fig. 1, the variable displacement axial piston pump is driven by an electric motor M. Hydraulic-mechanical control of the pump volumetric flow is performed by control valve and hydraulic cylinder. The feeding chain of the hydraulic motor RVerbr contains tube RL-zu, pressure compensator RIDW, measuring valve RVW, check valve, meter-in throttle edge RSK-zu and connection elements. The output chain of the hydraulic motor RVerb contains a meter-out throttle edge RSK-r, and tube RL-ab. The device contains load-sensing pressure feedback with resistance RLS. Fig. 1 Scheme of the hydraulic–mechanical load-sensing system of Bosch GmbH. Proceedings 21st European Conference on Modelling and Simulation Ivan Zelinka, Zuzana Oplatková, Alessandra Orsoni ©ECMS 2007 ISBN 978-0-9553018-2-7 / ISBN 978-0-9553018-3-4 (CD) The scheme of a hydraulic-mechanical controller (Fig. 2) contains a spool valve (effective area AV) with inflow and outflow slots, constant resistor (volumetric flow QDr), positioning cylinder (effective area AZ), and swash plate with spring. Fig. 2 Scheme of a hydraulic-mechanical controller. Fig. 3 Section of the valve block. The main part of the valve block (Fig. 3) is a directional valve with measuring throttle edge RVW, meter-in throttle edge RVS-zu and meter-out throttle edge RSK-r. The valve block also contains pressure compensator with throttle edge RIDW and check valve with pressure drop 0.5 bars. Examples of modelling and simulation of this loadsensing system were discussed in (Grossschmidt et al. 2006). The load-sensing system is quite complicated and contains several feedbacks. Iterations that were used in the simulation process turned out to be unstable. The resulting graphs appeared to be not smooth. To get better results we need to improve the model. The main reason of instability is that the feeding pressure of the controller was taken from the output of a variable displacement hydraulic pump. But the pressure in the pump output depends on the resistance of the whole system. Pressure drops in connecting tubes of feedback chains affect the system behaviour as well. 2 MODIFIED LOAD-SENSING SYSTEM A modified scheme of load-sensing system is proposed, in which the controller has an independent constant pressure feeding. Feedback pressures have been taken directly from the measuring valve with pressure compensator RIDVW. The scheme of modified load-sensing system is shown in Fig. 4.