Investigation and Optimization of Hydraulic Step-down Switched Inertance Converters with Non-uniform Inertance Tubes

In hydraulic systems with multiple actuators, difficulty can arise with matching the load requirements with the supply power from the system's pump. To get the desired performance at the individual loads, restrictive type valves are used to control pressure or flow by throttling the flow over a spool orifice, creating considerable power loss. This causes hydraulic systems powered off the same pump to be inefficient. Switched inertance hydraulic systems are a new technology in the field of fluid power that convert pressure and flow more efficiently than using restrictive type hydraulic valves. The step-down, or "buck", converter considered exclusively in this thesis has the ability to reduce pressure and increase flow rate to a load. The system is constructed using a digital hydraulic valve and check valve connected to the pressure supply and system reservoir respectfully. Following the valves, the system has an inertance tube, a long piece of uniform hydraulic line where fluid inertia is built up. The inertance tube also causes pressure wave propagation effects to occur since the length of the line is typically long. The performance of switched inertance converters are largely governed by the performance of the switching valve. An ideal switched inertance converter is 100% efficient at converting pressure and flow, however this would require the valve to actuate at extremely high frequency and switch instantaneously fast. This is not realizable as real valves operate up to a maximum of a couple hundred Hertz, and take a finite time to open and close, on the order of milliseconds. One of the main losses of a buck converter is the power loss across the switching valve as it transitions from open to closed and vice versa. This loss arises from the throttling of flow over the valve opening during actuation. The research presented in this thesis looks at mitigating this loss, as well as the viscous friction loss within the inertance tube. These losses can be reduced by using an inertance tube of variable shape, a new idea introduced very recently. A shaped inertance tube is a fluid pipeline with varying cross sectional area over its length, as compared to uniform inertance tubes which have constant cross sectional area. The current gap in the research is that the tube design is not fully optimized leaving room for potential improvements in identifying better dimensions, or perhaps finding a more optimal shape. Models for computing fluid transients in uniform lines are well developed, however modelling fluid dynamics in shaped inertance tubes is an area that has not been studied as extensively. The research presented proposes a computer model for simulating fluid transients in tapered transmission line segments using the transmission line method (TLM). The current research gap in modelling tapered transmission lines is that previous models are difficult to simulate in the time domain, have poor accuracy, and have a limited range of applicability. The proposed TLM model looks to mitigate these shortcomings. When connected in succession, the tapered TLM can model shaped inertance tubes for application to hydraulic buck converters. The proposed model shows improved agreement to a numerical solution of the Navier-Stokes equations than the previous models on the topic. Validation of the model is also gained though analysis of the dynamic response in the frequency domain. With the model now available to simulate shaped inertance tubes, a buck converter system is defined with equations presented for dynamic simulation. Initial simulations of the buck converter using parameters and design from previous research showed unoptimized performance operating at an efficiency of 47.8% for a system using a uniform inertance tube. The main objective was to optimize the shape of the inertance tube to realize increased performance using simulation studies. Genetic and pattern search algorithms were used to optimize the dimensions of the inertance tube with the goal of maximizing system efficiency while maintaining the same load. As a baseline, the uniform intertance tube design was optimized, and realized an efficiency of 64.1%, performing significantly better than the unoptimized uniform inertance tube. Further optimizations added an increasing number of tapered sections to describe the arbitrary shape of an inertance tube, up to 4 tapered segments. Significant efficiency increases were realized when using shaped inertance tubes. The best tube design increased system efficiency over 6% compared to the uniform design at a value of 70.2%. Other optimizations showed improvements in efficiency over the traditional design by reducing both valve and frictional losses in the system. The research presents a novel inertance tube design, containing a uniform section of high inertance followed by a diverging tapered section followed by another uniform section at larger diameter and low resistance. This design also proposes the idea of potential noise reduction due to the suppression of pressure fluctuations at the load