An existing ISO standard frequency-domain method for measurement of speed of sound in a hydraulic pipeline is enhanced and extended in this article to include in situ measurement of pressure transducer calibration factors. Transducer mounting stresses are shown to cause variations in the calibration factors, and the proposed method can be used to eliminate these uncertainties, consequently improving the accuracy of the speed of sound. 95% confidence ranges in the speed of sound of less than ±0.1% have been achieved, and such high precision cannot be achieved by other practical methods. The method can also been extended to estimate viscosity and mean flow velocity, but accuracy is less good. Novel time-domain versions of the method are introduced. These may be valuable for real-time monitoring, and changes in speed of sound or calibration factor can be tracked with minimal delay. Some examples showing the effect of sudden aeration are presented; a sudden drop in speed of sound is apparent.
The nature of digital hydraulic systems may cause severe fluid-borne noise problems because of the pulsed nature of the flow. An effective method to reduce the noise that does not impair the system performance and efficiency is needed. This article reports on initial investigations of an active valve for pressure pulsation attenuation in switched inertance hydraulic systems (SIHS) based on in-series and by-pass structures. The in-series structure represents a valve arranged in line between the SIHS and the load providing a controlled pulsating pressure drop, whilst for the by-pass structure the valve was arranged in parallel with the load providing a controlled pulsating bleed-off flow. A high-performance piezoelectric valve was used as the active controller. Adaptive notch filters with the filtered-X least mean square algorithm were applied for pressure pulsation attenuation, while a frequency-domain least mean square filter was used for secondary path identification. Simulated and experimental results show that excellent cancellation was achieved using the proposed methods, which have several advantages over passive noise control systems. Comparison of the in-series and by-pass structures is discussed in terms of system performance, robustness and advantages. The proposed control structures are very promising for fluid-borne noise cancellation in fluid power systems or other fluid systems with severe noise or vibration problems.
Amongst many other high performance flow control applications, servo valves are used to control aero engines by metering the fuel delivered from the fuel pump. Conventionally, a fuel metering servo valve has a pilot stage with an electromagnetic torque motor moving a flapper which differentially restricts a pair of nozzles to create a hydraulic signal (i.e. a pressure difference). These valve pilot stages use mature, optimised technology such that to achieve improvements requires a novel approach. Torque motors in particular present reliability and manufacturing difficulties, and news solutions should ultimately allow a reduction in manual assembly and set-up, improve repeatability, and eliminate failures associated with fine wire devices. In this paper, a pilot stage actuated by piezoelectric ring benders is proposed, designed, built and tested, and test results are compared with a model used to predict pressure-flow characteristics. A particular challenge is the need to include redundancy, and thus a pair of ring benders is used, allowing isolation between duplicated electrical control channels. Another challenge is the mounting of the ring bender, which has to flex to allow the outer edge of the ring bender to deform, yet be stiff enough to adequately react against generated forces. O-ring mounts made from three different elastomer materials are compared in this study. In aerospace, an added complication is the large range of fuel temperature; F70 fluorosilicone O-rings have been chosen with this in mind, and successfully demonstrated in the range −50 °C to +180 °C. With one active and one inactive ring bender to simulate a failure condition, the new dual lane pilot stage achieves +/−50 μm displacement under test, giving control port flows up to +/−0.6 l min−1, and a control port pressure variation of 40 bar using a 100 bar supply pressure difference (supply minus return pressure). This research establishes that a piezoelectric aero engine fuel valve is feasible, and in particular, that piezoelectric ring bender actuators with elastomeric mountings are highly suited to this application.
Single-affiliation systems are observed across nature and society. Examples include collaboration, organisational affiliations, and trade-blocs. The study of such systems is commonly approached through network analysis. Multilayer networks extend the representation of network analysis to include more information through increased dimensionality. Thus, they are able to more accurately represent the systems they are modelling. However, multilayer networks are often represented by rank-4 adjacency tensors, resulting in a N2M2 solution space. Single-affiliation systems are unable to occupy the full extent of this space leading to sparse data where it is difficult to attain statistical confidence through subsequent analysis. To overcome these limitations, this paper presents a rank-3 tensor representation for single-affiliation systems. The representations is able to maintain full information of single-affiliation networks in directionless networks, maintain near full information in directed networks, reduce the solution space it resides in (N2M) leading to statistically significant findings, and maintain the analytical capability of multilayer approaches. This is shown through a comparison of the rank-3 and rank-4 representations which is performed on two datasets: the University of Bath departmental journal co-authorship 2000-2017 and an Erdos-Renyi network with random single-affiliation. The results demonstrate that the structure of the network is maintained through both representations, while the rank-3 representation provides greater statistical confidence in node-based measures, and can readily show inter- and intra-affiliation dynamics.
Three concepts for variable displacement gear and gerotor pumps were proposed. CAD models and physical prototypes were built for demonstrating and understanding the concepts. One concept was taken to the stage of a working prototype. This was constructed using commonly available rapid prototyping techniques. Though it was a crude prototype, the device was found to be able to pump effectively, and the flow varied as expected when the displacement was varied. The proposed concepts may be suitable for mass market applications such as automotive power steering and engine lubrication, and could provide very significant energy savings.
This article reports on experimental investigations of a switched inertance hydraulic system (SIHS), which is designed to control the flow and pressure of a hydraulic supply. The switched system basically consists of a switching element, an inductance and a capacitance. Two basic modes, a flow booster and a pressure booster, can be configured in a three-port SIHS. It is capable of boosting the pressure or flow with a corresponding drop in flow or pressure respectively. This technique makes use of the inherent reactive behaviour of hydraulic components. A high-speed rotary valve is used to provide sufficiently high switching frequency and minimise the pressure and flow loss at the valve orifice, and a small diameter tube is used to provide an inductive effect. In this article, a flow booster is introduced as the switched system for investigation. The measured steady state and dynamic characteristics of the rotary valve are presented, and the dynamics characteristics of the flow booster are investigated in terms of pressure loss, flow loss and system efficiency. The speed of sound is measured by analysis of the measured dynamic pressures in the inertance tube. A detailed analytical model of a SIHS is applied to analyse the experimental results. Experimental results on a flow booster rig show a very promising performance for the SIHS.
Abstract Digital Displacement pumps (DDP) are variable displacement radial piston pumps. Each cylinder comprises of a solenoid operated low pressure check valve and a passive high pressure check valve. Variable displacement is achieved by altering the number of active cylinders, and the proportion of the stroke for which the cylinder is pressurised. DDP has been demonstrated to be quieter and possess a significantly less tonal sound quality than traditional variable displacement axial piston swashplate pumps[1]. However, as the industry continues to trend towards battery electric driven excavators, the existing masking noise produced by conventional diesel combustion engines will cease to exist. Consequently, the hydraulic system will be the most significant noise source on the machine, driving the need to further refine pump Noise, Vibration & Harshness (NVH) performance. Passive check valves produce a characteristic in-cylinder pressure overshoot as upstream pressure has to be sufficient to overcome downstream pressure forces, plus resistive spring and damping forces responsible for holding the valve in place. Within a DDP, the transient nature of a pressure overshoot is very effective at exciting several different high frequency responses of the pump, most noticeably fluid-borne noise, and consequently radiated noise. This paper presents a method for incorporating a damping mechanism within a piston which reduces pressure overshoot, allowing an investigation into the relationship between pressure overshoot, fluid-borne noise and radiated noise. Modelling methodology is presented demonstrating a reduction in pressurisation rate and overshoot during the pumping cycle. Several key design variables are shown to give a large range in tuning scope. Initial test results are presented, demonstrating good model correlation and improvement in NVH performance.
Abstract Digital Displacement® Pump (DDP) is a variable displacement radial piston pump, driven by a central crankshaft, wherein each cylinder can be independently controlled. With each crank rotation a given cylinder may idle, operate a full pumping stroke or operate a partial stroke. Variable displacement is achieved by utilising one of, or a combination of, these control options. The fundamental design differences between DDP and traditional axial piston pump determine different mechanical and fluid forcing is generated, producing a distinctly different Noise, Vibration and Harshness (NVH) characteristic. DDP is the first major disruptor in the field of hydraulic pumps and as such the difference in human perception should be addressed and understood. This paper presents a comparison of the airborne noise radiated from DDP096 and two axial piston pumps, in a semi-anechoic chamber and on two identical 20-tonne excavators. The test chamber data shows that across a wide range of speeds and pressures, DDP is on average 3.5 dB quieter than a comparable axial piston pump. Coupled with a significant reduction in Prominence Ratio, DDP has on average a 12% better Articulation Index, whilst the transient nature of a radial piston pump shows detriment in other psychoacoustic metrics. The observed trends are also apparent when pumps are compared on working excavators.
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