D. Nigel Johnston

A Switched Inertance Device for Efficient Control of Pressure and Flow

This paper reports on an initial investigation of a switched inertance device (‘SID’). Using this device, flow and pressure can be varied by a means that does not rely on dissipation of power. The device can provide a step-up or step-down of pressure or flowrate, analogous to a hydraulic transformer. Simulated and experimental results on a prototype device show a promising performance. The device could potentially provide very significant reduction in power consumption over conventional valve-controlled systems, provided that noise issues and some other practical problems can be overcome.Copyright

Behavioural prediction of hydraulic step-up switching converters

In this paper the fundamental principles of energy-conservative hydraulic control based on the fluid inertance principle are discussed and a detailed analysis of step-up switched-inertance control is presented. A non-loss system comprising an inertance tube and switching valve is modelled and its operational curves are presented as a reference for an ideal behaviour. Considering the load loss at both the tube and the PWM switched valve, a linear mathematical model for the step-up switched-inertance hydraulic system is presented which describes the pressure response as a function of the PWM duty cycle. Mathematical expressions of the flow rates through the tube and the supply and return ports as well as the system efficiency are also presented. A system prototype is evaluated on a test rig and the experimental data compared with the theoretical results, demonstrating the model accuracy. The proposed model simplifies the analysis process for step-up switching converters and thus their restrictions and potential can be investigated more quickly.

Measured Dynamic Properties of Flexible Hoses

A method for measuring the impedance matrix and the dynamic properties of a liquid-filled flexible hose is described in this paper. Dynamic hose properties are presented for a wide range of hose types. Nylon-reinforced hoses are shown to have considerably lower bulk moduli and stiffnesses than steel-reinforced hoses. The dynamic bulk moduli and stiffnesses are shown to be significantly and consistently higher than the static values. These results may be used to give an estimate of representative properties for a hose, based on its maximum pressure rating and its type of reinforcement.

Measurement of Wave Propagation Characteristics of Flexible Hoses

A method for measuring the impedance matrix and the dynamic properties of liquid-filled flexible hose is described in this paper. Dynamic hose properties are presented for a wide range of hose types. Nylon-reinforced hoses are shown to have considerably lower bulk moduli and stiffnesses than steel-reinforced hoses. The dynamic bulk moduli and stiffnesses are shown to be significantly and consistently higher than the static values. These results may be used to give an estimate of representative properties for a hose, based on its maximum pressure rating and its type of reinforcement.Copyright

Prediction of Hydraulic Inductance

The dynamic response, stability and fluid-borne noise characteristics of fluid power components and systems can be strongly influenced by the inertia or ‘hydraulic inductance’ of the fluid in passageways, which are often of complex geometry. The hydraulic inductance is a parameter that has often proved to be very difficult to quantify accurately, either theoretically or experimentally. This paper presents a method of numerical calculation of the hydraulic inductance in a passageway. The method is simple to apply and can be applied to geometries of arbitrary complexity. A simple way of using a Computational Fluid Dynamics package for calculating hydraulic inductance is also demonstrated. Results are presented for a simple cylindrical orifice, a simple spool valve and a conical poppet valve. The effect of the inductance on the response of a poppet valve is demonstrated.Copyright

The Influence of Passive Valve Characteristics on the Performance of a Piezo Pump

In this study, a piezoelectric stack actuator is used to oscillate a piston in a single cylinder pump. The pump is intended to directly supply a hydraulic actuator for motion control, and power output of about 1kW is targeted. Flow rectification is achieved by the use of passive check valves. The valve resonant frequency is found to have a significant effect on output flow. The expected increase in pump flow rate with driving frequency has been confirmed in simulation to hold true in a certain frequency range only. In addition, check valve size and therefore orifice area has to be adequate in order not to prohibitively restrict flow. Valve spring stiffness and valve mass need to be simultaneously optimized for the area of the valve to achieve the highest flow rate.Calculations indicate that there is a power limitation due to the high current demand and also a high temperature rise for a large continuously operated piezo stack. Thus the piezo pump appears more promising for smaller scale applications, and those that require intermittent power (i.e. a low duty cycle).Copyright

Modeling for a High-Bandwidth High-Flow Valve Design Based on Horbiger Plate Operation

In order to expand the operational capabilities of hydraulically actuated systems the development of new valves to allow of enhanced flow rates and bandwidth performance is required. Previously, the technical challenge in developing such valves was the need for large spool strokes to achieve the desired flow rates. However, this would then hinder the dynamic response of the valve. To increase flow without reducing dynamic performance it is proposed that the use of multiple metering edges is appropriate. This is achievable using the Horbiger plate valve principle and direct connection to a piezoelectric actuator. This paper examines the design criteria associated with such a valve. Simulations undertaken as part of its construction and design show that improved flow rates can be achieved. Results from these simulations are then included in orifice equations to further predict flow as a function of plate separation. Finally a simulation was undertaken to determine the total forces acting on the valve, and the forces on the piezoactuator were found to be compressive under normal conditions, which should lead to predictable and stable operation.Copyright

PREDICTION OF HYDRAULIC INERTANCE USING ACOUSTIC MEASUREMENTS AND CFD MODELLING

Fluid inertia within passageways of hydraulic components is known to have a significant impact on their dynamic response and fluid-borne noise characteristics. This inertance is often hard to quantify either theoretically or experimentally due to the complex nature of component geometries, and because it is related to dynamic, not steady-state behaviour. Previous studies have used the secondary source method to determine the impedance, of which the inertance is an important parameter, of components such as positive displacement pumps and valves. A simple acoustic test for predicting the inertance of a component is proposed. The component must have a direct connection between inlet and outlet, and as such is directly applicable to valves and accumulators. Results were compared to theory for known components, including uniform pipes, and with predictions made using the commercial computational fluid dynamics (CFD) package ANSYS CFX, using the analogy of steady state flow through a porous passageway of identical geometry. In general, good agreement between acoustic measurements and CFD predictions was obtained for a number of ball valves, gate valves and an accumulator poppet valve.Copyright

Efficient Methods for Numerical Modelling of Laminar Friction in Fluid Lines

An improved method for simulating frequency-dependent friction in laminar pipe flow using the Method of Characteristics (MOC) is proposed. It has a higher computational efficiency than previous methods whilst retaining a high accuracy. By lumping the frequency-dependent friction at the ends of the pipeline the computational efficiency can be improved further, at the expense of a slight reduction in accuracy. The technique is also applied to the Transmission Line Method (TLM) and found to give a significant improvement in accuracy over previous methods, whilst retaining a very high computational efficiency.Copyright

Modelling and analysis of hydraulic step-down switching converters

In this study, a steady state analysis of step-down converter systems, considering the load losses in the inertance tube and switched valve, is presented. The model describes the behaviour of the average load pressure as a function of the pulse-width modulated duty cycle. The steady state expressions for the load flow rate, high and low supply flow rates, and system efficiency are also discussed. A system prototype was developed and tested to evaluate the model accuracy. The system parameters (e.g. tube diameter and length and switching frequency) were analysed to predict the best system configuration. The study describes how the system efficiency is influenced by these parameters. The model presented allows the ideal parameter combination for maximum efficiency to be determined. It can be used for the preliminary design of switching converters, and a further time or frequency analysis can be performed for system optimization.