Derek G. Tilley

A tabu search method for the optimisation of fluid power circuits

Abstract This paper describes the development of an efficient algorithm for the optimization of fluid power circuits. The algorithm is based around the concepts of Tabu search, where different time-scale memory cycles are used as a metaheuristic to guide a hill climbing search method out of local optima and locate the globally optimum solution. Results are presented which illustrate the effectiveness of the method on mathematical test functions. In addition to these test functions, some results are presented for real problems in hydraulic circuit design by linking the method to the Bathfp dynamic simulation software. In one such example the solutions obtained are compared to those found using simple steady state calculations.

Control of a hydropneumatic active suspension based on a non-linear quarter-car model

Abstract It is extremely difficult to maintain simultaneously a high standard of ride, handling, and body control in a vehicle with a conventional passive suspension. However, it is well known that active suspensions provide a possible solution to this problem, albeit with additional cost and weight. This paper describes the design and analysis of a hydropneumatic slow active suspension. The design is based on hydropneumatic suspension components taken from a commercial system. A non-linear quarter-car model is developed, which includes a gas strut model developed in a previous study and a non-linear dynamic flow control valve model. A hybrid control strategy is proposed for the disturbance rejection and self-levelling requirements. The disturbance rejection control is based on limited state feedbacks and the linear quadratic method plus a Kalman filter that is used to optimize the performance index. The self-levelling control employs a proportional, integral, and derivative (PID) control strategy. Practical issues, such as power consumption, controller robustness, and valve dynamics, are also investigated in this paper. Simulations show that the proposed system has good performance and robustness.

Piezoelectrically actuated hydraulic valve design for high bandwidth and flow performance

The performance of hydraulically actuated machine systems could be improved with the use of valves that have high bandwidth and high flowrates under low pressure drops. Although high flowrates can be achieved using very large spool strokes and/or diameters, the overall bandwidth of the valve will be reduced. Research has therefore been undertaken on a prototype valve design incorporating the Horbiger plate principle, which utilizes multiple metering edges to allow high flowrates to be obtained at low pressure drops and small poppet displacements. The valve is directly activated using a piezoelectric actuator to achieve a fast dynamic response. Valve performance is assessed using a mathematical model that includes the piezoelectric actuator and power amplifier, the supply flow, fluid squeeze forces, end stop response, and valve mechanical components. The steady state relationship between valve flow, force and pressure drop, and the fluid inertance, were determined using computational fluid dynamics software. The simulation model has been validated using test data obtained from experimental tests undertaken on a prototype valve. Good agreement is obtained between the predicted and measured results and it is shown that the valve is capable of opening or closing fully in less than 1.5 ms, and can pass a flow of 65l/min at a pressure drop of 20 bar.

The use of plane wave theo ry in the modelling of pressure ripples in hydraulic systems

This paper discusses the application of plane wave theory to the modelling of pressure ripples generated by positive displacement pumps in high-pressure hydraulic systems. These ripples can lead to structural vibration of pipework and associated components and hence result in airborne noise. A clear understanding of the mechanism of pressure ripple generation is essential in the development of quiet systems. The characteristics of the pump and the circuit components have a considerable influence on the amplitude of the pressure fluctuations. Methods of evaluating these characteristics are described and typical results presented. It is shown that, from a knowledge of these characteristics, the pressure ripple levels at any position in a circuit may be accurately determined using plane wave theory.

A multimedia approach to fluid power system design

The design of fluid power systems is a multidisciplinary activity requiring not only considerable background knowledge and experience but also the use of numerous design resources. Current design resources are fragmented and offer no structured guidance. The paper describes the first year of a three-year project to create an integrated multimedia environment for fluid power system design and instruction using Java and other Internet based technologies. Prototype software is under development and has been tested on a number of different platforms.

Piezoelectric Actuation in a High Bandwidth Valve

To expand the operational capabilities of hydraulically actuated systems the development of new valves capable of enhanced flow rates and bandwidth performance is required. In previous work, the solution to achieving desired flow rates was to increase valve spool size and travel, but this is at the expense of the dynamic response. To increase flow without restricting dynamic performance, this paper proposes the use of multiple metering edges achievable using the Hörbiger plate valve principle, actuated by piezoelectric means. Experimental results from a piezoelectric actuated valve are then used to demonstrate improved performance in both static and dynamic operation.

Making the eye blink—modelling the operation of the Gateshead Millennium Bridge:

Abstract This paper considers the system dynamics associated with a hydraulic system moving a large flexible structure, namely the Gateshead Millennium Bridge. The hydraulic components possess a range of non-linear characteristics, which become coupled to the bridge structural dynamics. The bridge structure is lightly damped and may be represented by a linear model, provided that the amplitudes of vibrational modes are small enough to maintain positive cable tensions. The limiting amplitudes were known at the design stage and the hydraulic circuit had to ensure that these were not exceeded during operation, which may include various conditions of wind loading, emergency stops and variability of components. System design was achieved through an extensive iterative process of simulation that included all operational procedures to eliminate problematic circuits that would have caused instability or excessive bridge oscillations. The actual bridge and coupled hydraulic circuits underwent a series of commissioning trials and no undue problems were experienced. This indicated that the simulation procedures developed greatly reduced the risks associated with a unique structure actuated by a bespoke hydraulic circuit.

Development of a cardiopulmonary mathematical model incorporating a baro-chemoreceptor reflex control system.

This article describes the development of a comprehensive mathematical model of the human cardiopulmonary system that combines the respiratory and cardiovascular systems and their associated autonomous nervous control actions. The model is structured to allow the complex interactions between the two systems and the responses of the combined system to be predicted under different physiological conditions. The cardiovascular system model contains 13 compartments, including the heart chambers operating as a pump and the blood vessels represented as distensible tubes configured in a serial and parallel arrangement. The accurate representation of the hemodynamics in the system and the good fit to published pressure and flow waveforms gave confidence in the modelling approach adopted for the cardiovascular system prior to the incorporation of the baroreflex control and the respiratory models. An improved baroreceptor reflex model is developed in this research, incorporating afferent, central and efferent compartments. A sigmoid function is included in the efferent compartment to produce sympathetic and parasympathetic nerve outflow to the effector sites. The baroreflex action is modelled using physiological data, its interaction with the chemoreflex control is explained and the simulation results presented show the ability of the model to predict the static and dynamic hemodynamic responses to environmental disturbances. A previously published respiratory model that includes the mechanics of breathing, gas exchange process and the regulation of the system is then combined with the cardiovascular model to form the cardiopulmonary model. Through comparison with published data, the cardiopulmonary model with the baro–chemoreflex control is validated during hypoxia and hypercapnia. The percentage difference between the predicted and measured changes in the heart rates and the mean arterial pressures are within 3% in both cases. The total peripheral resistance correlates well for hypoxia but is less good for hypercapnia, where the predicted change from normal condition is around 7% compared with a measured change of 23%. An example showing the application of the proposed model in sport science is also included.

Piezoelectric actuators for screw-in cartridge valves

Research has been undertaken to explore the possibility of using smart materials for the actuation of screw-in cartridge valves. Four types were considered and a piezoelectric (PZT) stack was selected as the most appropriate actuator. The concept of a spool valve piloted by a PZT valve was chosen for investigation. A mathematical model of a hydraulic system with a spool valve piloted by a PZT valve is developed and the results obtained from simulation studies are assessed. Experimental tests were undertaken using a prototype PZT valve. The test results obtained confirm that the PZT valve was able to switch a high flow spool valve and the results show similar trends to those obtained from the simulations. Although effective, the current cost of the PZT valve is much higher than that of a conventional solenoid valve that it may one day replace. There are also issues to be addressed that relate top temperature dependence of the PZT valve operation. A solution is given to improve the response of the spool.

Modelling of a Novel Gas Strut Using Neural Networks

The strut is one of the most important components in a vehicle suspension system. Since it is highly non-linear it is difficult to predict its performance characteristics using a physical mathematical model. However, neural networks have been successfully used as universal ‘black-box’ models in the identification and control of non-linear systems. This approach has been used to model a novel gas strut and the neural network was trained with experimental data obtained in the laboratory from simulated road profiles. The results obtained from the neural network demonstrated good agreement with the experimental results over a wide range of operation conditions. In contrast a linearised mathematical model using least square estimates of system parameters was shown to perform badly due to the highly non-linear nature of the system. A quarter car mathematical model was developed to predict strut behavior. It was shown that the two models produced different predictions of ride performance and it was argued that the neural network was preferable as it included the effects of non-linearities. Although the neural network model does not provide a good understanding of the physical behavior of the strut it is a useful tool for assessing vehicle ride and NVH performance due to its good computational efficiency and accuracy.Copyright