Design and Control of Switched Inertance Hydraulic Converters

Student thesis: Doctoral ThesisPhD


Despite their high power density, conventional hydraulic systems used in fluid power industry are known for low energy efficiencies. Digital hydraulics is a competitive alternative for conventional servo or proportional valve-controlled hydraulic systems, with high energy efficiency, control flexibility, and potential to be more cost-efficient, robust, and less sensitive to contamination. The switched inertance hydraulic converter (SIHC) is a subdomain of digital hydraulics, which can control flow or pressure efficiently by means of digital switching instead of throttling. SIHCs have been systematically studied in theory and experiments to determine performance and characteristics in open-loop with a constant load. However, the investigation of the closed-loop control and optimisation of SIHCs for a time-varying load has been much more limited.

In this research, the design, control, and optimisation of an SIHC was undertaken to achieve good closed-loop performance and high efficiency for a time-varying load. A high-speed rotary valve was used to simultaneously control the switching frequency and switching ratio of SIHCs. The valve switching characteristics were analysed by using the analytical model of the valve switching orifice area, and numerical and Computational Fluid Dynamics (CFD) models considering fluid compressibility and throttling effect of the valve. The valve was used as a high-speed switching valve in an SIHC and analytical, numerical, and experimental work were conducted to investigate the open-loop performance and switching characteristics of the SIHC. The closed-loop performance of the SIHC was studied and a Model Reference Adaptive PID (MRA-PID) controller was proposed with the capability of automatically tuning the controller parameters. The controller has significantly improved the dynamic response of the SIHC under varying working conditions, with up to 51% and 61% reduction on overshoot and settling time, respectively, compared with a conventional PID controller. An efficient control of optimising the switching frequency and switching ratio has been implemented on the SIHC with a time-varying loading system, which achieves up to 10% efficiency improvement and up to 65% pressure ripple reduction, compared with using a non-optimised control.

The work has firstly validated the feasibility and advantages of simultaneously controlling the switching ratio and switching frequency of SIHCs, and achieved good open-loop and closed-loop performance with high energy efficiency with a time-varying load. As closed-loop control is usually required for accurate force or motion tracking, and time-varying loading condition are commonly found in hydraulic applications, this research constitutes an important contribution to the practical use of highly efficient SIHCs in fluid power industry to improve energy efficiency, save energy and reduce CO2 emissions.
Date of Award12 Oct 2022
Original languageEnglish
Awarding Institution
  • University of Bath
SponsorsChina Scholarship Council
SupervisorMin Pan (Supervisor) & Andrew Plummer (Supervisor)


  • Digital hydraulics
  • Switched Inertance Hydraulic Converters
  • Design and Control
  • Hydraulic systems
  • Fluid power

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