Dynamic Analysis and Characterization of Conventional Hydraulic Power Supply Units

Hydraulic power units operated as constant supply pressure systems remain to be widely used in the industry, to supply valve controlled hydraulic drives etc., where the hydraulic power units are constituted by variable pumps with mechanical outlet pressure control, driven by induction motors. In the analysis of supplied drives, both linear and rotary, emphasis is commonly placed on the drives themselves and the related loads, and the supply system dynamics is often given only little attention, and usually neglected or taken into account in a simplified fashion. The simplified supply system dynamics used in such analyzes is often justified by short supply lines and/or the utilization of accumulators near valve inlets, accounting for the majority of possible supply pressure variations. Such considerations are reasonable in many test benches, where the supply pressure variations are small enough such that limited impact on the drive dynamics is observed. Such ideal properties however, are not necessarily present in industrial hydraulic applications for various reasons, with the most common being large volumes of supply lines. Long supply lines, hence large supply line volumes, between the supply system and drives will reduce the flow-to-pressure gain of the supply system, and hence increase the time constant of the supply pressure dynamics. A consequence of this may be large variations in the supply pressure, hence large variations in the pump shaft torque, and thereby the induction motor load torque, with possible excitation of the induction motor dynamics as a result. In such cases, the coupled dynamics of the pressure controlled pump and induction motor may influence the supply pressure significantly, possibly affecting the dynamics of the supplied drives, especially in cases where pilot operated valves with internal pilot supply are used. This paper is concerned with the analysis and characterization of the coupled pump-induction motor dynamics, confined to hydraulic power units constituted by an axial piston pump with mechanical outlet pressure control, driven by an induction motor operated at grid conditions. Furthermore, a simplified general model representation of the coupled dynamics is established, accounting for the entire dominating dynamics of the supply unit. Results demonstrate the accuracy of the simplified model representation.