State Decoupling & Stability Considerations in Electro-Hydraulic Variable-Speed Drive Networks

The recent introduction of so-called electro-hydraulic variable-speed drive networks offers actuation of multi-cylinder systems by few component types, no throttle losses, small reservoir volume and provide for significantly improved efficiencies compared to valve controlled systems. Furthermore, such drive networks provide a more flexible hydraulic power distribution and potentially fewer components and lower installed power as compared to standalone electro-hydraulic variable-speed drives. These features are realized by interconnecting cylinder chambers by electro-hydraulic variable-speed drives at a system level, or even by short-circuiting cylinder chambers where the load permits this, while also sharing the electric supply. Hence, such drive networks are interconnected both electrically and hydraulically. Especially the latter feature renders electro-hydraulic variable-speed drive networks highly coupled systems, and individual control of piston motion or force can generally not be realized by the individual electro-hydraulic variable-speed drives in such systems. Hence, conventional control methodologies cannot be directly applied in such systems. The presented study considers decoupling of the cylinder motion/forces and the system pressure level by analytical methods, and a case study suggests effective decoupling by means of pressure and position measurements, cylinder and pipe/hose dimensions as well as control parameters. The stability boundaries related to the control parameters are assessed and it is shown that conventional controls combined with the proposed decoupling method provide for individual control of system pressure level and piston motions.