Department of Mechanical Engineering

Fluid power and fluid systems PhD theses

Fundamental concepts associated with hydraulic seals for high bandwidth actuation

Arthur Bullock, 2010


This thesis is concerned with issues relating to the development of an active sealing system for hydraulic actuators where the sealing elements can be radially extended and retracted to vary the friction and leakage characteristics. In order to determine the feasibility of the active sealing concept it is necessary to establish that varying the seal geometry may achieve useful improvements in the friction-leakage trade-off and that a practical method of achieving this seal extension can be realised. Experimental and simulation approaches for seal friction prediction have been developed and active seal prototypes produced to demonstrate the concept.

Experiments were carried out to measure the constant velocity friction for single-lip and double-lip seals over a range of sliding speeds and sealed pressures with special consideration applied to the instroke-outstroke direction dependence. Additional experiments were performed with sinusoid motion to provide an indication of the transient friction characteristics. Friction was shown to increase towards the end of the outstroke cycle and decrease once the instroke motion began.

Tribology simulations were produced based on the results of a FEA simulation of the rod-seal contact pressure. Empirical friction-load relationships and novel contact mechanics approaches for high loads were considered. Simulations based on the Reynolds equation including standard inverse EHL theory and the GW-average Reynolds lubrication are also presented. Experimental agreement could be improved if loading is assumed to transfer to the fluid to maintain a fluid film. A hysteresis friction model was also developed in attempt to improve the prediction of speed dependent friction.

Two active seal prototypes were produced, each with an adjustable external pressure supplied to the outer circumference of the sealing element. Constant velocity friction measurements for different external pressures and the transient response following step changes in this pressure are presented.


Fuel pump motor-drive systems for more electric aircraft

Grzegorz Skawinski, 2010


The fuel systems fitted to the current generation of civil transport aircraft are rather complicated, due to the presence of multiple tanks, pumps, valves and complex pipeline systems. During fuel transfer between the tanks, when controlling the aircraft centre of gravity or engine feed and refuel operations, a number of pumps and valves are involved resulting in complex pressure and flow interactions. In order to minimise the pressure surges during sudden system changes and flow overshoot during fuel transfer and refuelling, different motor drive system control strategies have been investigated.

It is proposed that the current control method of electrically driven centrifugal-type pumps could be replaced by improved open and closed loop strategies where the flow overshoot can be minimised and pressure surges reduced. Steady-state and dynamic models of an AC induction motor drive and typical aircraft fuel system pipework components have been developed. The validation of these models has been performed using experimental data obtained from a fuel test rig constructed at the University of Bath using water as the working fluid.

The simulation results have been shown to agree well with those from experimentation. In addition, the induction motor has been modelled based on its physical properties using the Finite Element Method software MEGA. The investigated fuel system has been described in linear terms and its behaviour has been identified. It is shown that the system dynamic behaviour can be controlled/improved using well established closed loop proportional- integral control. An open loop technique of simultaneous pump and valve control has been proposed and validated using experimental results, resulting in a reduction of both the transient pressure surges and flow overshoot during sudden valve closures, showing significant performance improvements.

Improved closed loop control strategies for the pump drive system have also been developed in simulation. These are based on adaptive proportional-integral-derivative and fuzzy logic control strategies.


Modelling and experimental investigation into the performance of a ball valve from an aircraft fuel system

Andrew Roberts, 2010


The performance of an aircraft fuel system ball valve has been assessed using various modelling techniques and experimental methods. The performance largely splits into two parts. The first part deals with the steady state characteristics and internal flow field for fixed valve positions. The second part looks at the dynamic performance of pressure surge and flow overshoot by closing the valve using a linear profile.

For the steady state flow, a 3D CFD model was used to characterize the flow coefficient to within 10% of the measured data. The variation of the flow coefficient with angle is largely dominated by viscous effects, pressure recovery and jet contraction in that order with increasing angle. An accurate and efficient 2D model of the flow coefficient used 1/12th of the computing time of the equivalent 3D model. The 2D model was then used to look at the effect of orifice radius and it was shown that the Coanda effect has little influence on the trend in the flow coefficient. Following this, a steady state CFD model was used to predict the maximum velocity from LDV data to within 12.5%. With the LDV measurements, the presence of shear layers in the flow field creates jet contraction and is a possible source of cavitation.

For the dynamic flows, a method for determining the valve fluid inertance was presented, which followed an analogy between a porous medium and the valve fluid inertance. However, the effect of this inertance on the measured pressure surge was small due to the small percentage of the total system fluid inertance that the valve fluid inertance represents. For flow rate measurement, the two transducer technique is proposed and this method is an accurate post processing technique and can predict overshoot to within less than 2% of the modelled valve. A curve fitting method was also used to characterize the surge pressure and volume overshoot across a range of test conditions, the characterized surge pressure being within 6% of the measured values.


Modelling and analysis of pressure pulsations in hydraulic components and systems with particular reference to pump fault diagnosis

Mengeng Yang, 2009


Vane pumps are simple in principle and can be mass produced inexpensively, making them well suited to the automotive industry. They also have many other applications, such as in the chemical industry and food industry. A common type of damage to a vane pump is cavitation erosion on the side plates. If this damage is not detected in time, it could cause failure of the pump, which depending on the type of system may have safety implications, and in some cases a high cost from lost production whilst the system is shut down. This kind of damage is common on other types of pumps such as gear pumps and piston pumps. So a practical method for fault diagnosis of hydraulic pumps is required which does not necessitate removal of a pump from the working system.

This thesis presents a method of detecting and identifying cavitation damage on vanepump side plates via pump flow ripple. Power steering vane pumps are used for this study, although the principles may also be applicable to other types of vane pump, and indeed to piston and gear pumps. The investigation has been done through measurement and simulation. A numerical model of a vane pump is described, and simulated cavitation damage is introduced into the model. This damage is shown to have a clear effect on the simulated flow ripple. The pump flow ripple has also been measured experimentally using the Secondary Source Method, and artificial damage has been introduced into the pump. The damage is shown to have a clear effect on the measured flow ripple, consistent with the simulation results.

Whilst the secondary source method enables the measurement of flow ripple in laboratory conditions, it is generally impracticable for in-situ measurement for condition monitoring. Therefore the Deduced Flow Method (DFM) was developed to determine the pump flow ripple. The method achieved good sensitivity and accuracy for identifying cavitation damage in vane pumps using just one pressure transducer and one optical trigger sensor.


Active valve and pump technology: modelling and control of variable-speed trim transfer pumps in aircraft fuel systems

Lewis Boyd, 2008


The current generation of Airbus long-range civil transport aircraft actively control the centre of gravity of the aircraft by adjusting the fuel distribution between the horizontal tail surface and the forward tanks in order to minimise cruise drag. Here, it is proposed that the current on-off control method could be replaced by a variable flow rate, provided by a variable speed centrifugal pump. The impacts of this at the aircraft level in terms of cruise fuel burn reduction, valve operation cycle reduction and power consumption are investigated here using an extension to an existing fuel system simulation package and a generic aircraft fuel system definition. It is shown that using such a control system reduces fuel burn and the number of valve cycles, which could translate into a reduction in operating costs. The benefit of changing the controller to use tailplane trim angle directly rather than inferred centre of gravity position is assessed, and is shown to further reduce the fuel burn. It is suggested that such centre of gravity could provide significant benefits over the existing method.

Steady-state and dynamic models of centrifugal pumps, AC induction drives and typical aircraft fuel system pipework components are developed. These are validated against experimental data from a test rig of a representative system. Test rig simulation results are shown to agree well with those from experimentation. A new secondary noise source is developed for the dynamic analysis of the centrifugal pump, and a new acoustic experimental method is developed for the prediction of fluid inductance in pipework components. The results are compared against an existing CFD based method and show good agreement. The new method represents a much simpler experimental means of determining the effects of fluid inertia than the existing secondary source method. It is demonstrated that the dynamic behaviour of the centrifugal pump is insignificant when considering systems containing long pipes, and that steady-state pump models are sufficient for analysing their behaviour.

The pump models are generalised by non-dimensionalisation, in order to maximise their applicability to analysis of aircraft fuel systems. They are applied to a generic aircraft fuel system simulation, in order to model the behaviour of the system during a trim transfer. This is used to demonstrate the application of the proposed variable flow rate trim control system. The results of these simulations agree well with those used to demonstrate the benefits of the control system at the aircraft level. Concepts of system health monitoring tools are discussed with reference to the system simulations.


Active control of fluid-borne noise

Lin (Michael) Wang, 2008


Fluid-borne noise is one of the main components of hydraulic noise. The attenuation of it may have a significant effect on the cost in hydraulic systems. Standard passive silencers and dampers can be useful in reducing it in certain frequency ranges; however, these tend to be heavy, bulky and expensive. Active control algorithms, which are a comparatively recent means to reduce fluid-borne noise, can be applied to overcome this compromise.

The work presented in this thesis is the development of some active control algorithms utilised in a simple hydraulic system to cancel a number of harmonic orders of fluid-borne noise generated by a servo valve or a real pump. To realise cancellation, the filtered reference least mean square (FXLMS) adaptive control method is mainly presented. Furthermore, a fast response servo valve is applied as an actuator to generate a proper anti-noise flow signal in real time. For simplicity, an off-line identification method for the secondary path is applied in the time invariant working condition. Moreover, ripple reflection from both ends of the hydraulic circuit can give different effects under different working conditions. In order to execute the cancellation without any priori information about the dynamics of hydraulic system, the on-line secondary path identification method is discussed. However, in this algorithm an auxiliary white-noise signal applied for on-line method may contribute to residual noise and an extra computation burden can be added to the whole control system.

The performance of these control algorithms is firstly investigated via simulation in a hydraulic pipe model and the real time application on test rig using a servo valve as noise source. Finally, these schemes are realised in a simple hydraulic system with real pump noise source. The fluid-borne noise can be attenuated by about 20 dB with normal working conditions.


Modelling the human respiratory and cardiovascular systems

Jerry Lin, 2008


This thesis describes the development of a comprehensive human cardiopulmonary model that combines respiratory and cardiovascular aspects and their associated control actions.

A literature survey indicated a wide variation in the approaches used to model the cardiopulmonary system. The objective of the research reported in this thesis is to develop a comprehensive human cardiopulmonary model with detailed models of the respiratory and cardiovascular systems which is capable of modelling the interaction of the two systems and their responses under different physiological conditions.

The mathematical model developed for the cardiovascular system contains 13 segments including blood vessels and heart chambers. The heart serves as a hydraulic pump and the vessels are distensible pipes configured in a serial and parallel arrangement. The accurate representation of the hemodynamics in various parts of the system and the good fit to published pressure and flow waveforms provided confidence for the incorporation of the baroreflex control model and the respiratory model.

An improved baroreceptor reflex model was developed in this research as a three compartment model - afferent compartment, central compartment and efferent compartment. A sigmoid function was included in the efferent compartment to produce sympathetic and parasympathetic nerve outflow to the effector sites. The baroreflex action was modelled in a physiological manner and provides a clearer picture of how the baroreflex systems works. Simulation results presented show the ability of the model to predict the static and dynamic hemodynamic responses to environmental disturbances.

A respiratory model including the mechanics of breathing, gas exchange process and the regulation of the system was then developed and integrated with the cardiovascular model to form a complete cardiopulmonary model. The cardiopulmonary model was then validated using a number of different test conditions. Possible applications of the model were demonstrated and good agreement was obtained with published data.