IIn recent years, numerous applications of the variable-speed variable-displacement (VS-VP) hydraulic power unit have been investigated. Especially in a valveless drive system, the necessary hydraulic power could be supplied effectively through a flexible motor speed and pump displacement ratio. This paper presents a control strategy to improve the overall efficiency of a valveless transmission system by regulating motor speed and the displacement ratio simultaneously based on the data of combined overall efficiency maps. In this way, the interaction of the servo motor and hydraulic pump – and whether it affects the overall efficiency of the system – are studied. A numerical simulation is implemented for a series of typical working points of three types of hydraulic power units. The results show that the proposed control strategy achieves better efficiency values in almost all simulation points compared with single-variable power units (variable speed unit (VSFP) and variable displacement unit (FS-VP)). In particular, at a low pressure and low flow rate points, the efficiency of the proposed system is improved by up to 6% and 25%, respectively, compared with the VS-FP and FS-VP power units.
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Phan et al. (2021) studied this question.
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