Andrew Plummer

Robust Adaptive Control for Hydraulic Servosystems

The application of an indirect (self-tuning) adaptive controller to an electro-hydraulic positioning system is described. The underlying control method is pole placement, with the addition of a demand filter to allow noise effects to be reduced without degrading closed-loop performance. Recursive least squares is used to estimate the plant parameters, but the data is pre-filtered to reduce bias. A novel covariance trace limiting algorithm provides estimator reliability despite periods of insufficient excitation. Off-line system identification is employed to help controller design for the electro-hydraulic servosystem. The resulting controller performs well, and adapts rapidly to changes in load stiffness and supply pressure.

Model-in-the-Loop Testing:

Abstract Physical testing is used extensively to characterize mechanical systems. However, in many cases, mathematical models are now available that adequately describe the behaviour of part of the test specimen. Thus, test systems can be conceived which split the specimen into a physical part, and a virtual part, i.e. a real-time computer simulation. This has the potential to enhance convenience and reduce cost. The term ‘model-in-the-loop’ (MiL) has been used in the automotive industry to describe this concept. In this paper, a general framework for MiL system analysis is described. Two detailed examples are given. These concern the integration of numerical aerodynamic and tyre models into car test rigs. In the first case, the analytical results are validated by data from a real system. It is clear that particular demands are placed on the actuation and sensing systems if the numerical and real parts of the specimen are to interact correctly to give a realistic response for the complete system. Acceptable realism is achieved in the case of the aerodynamic model example. However, the results are less satisfactory in the tyre model example where both actuator response and sensor noise are limiting factors.

Determination of optimal parameters for a hydraulic power take-off unit of a wave energy converter in regular waves

Wave energy has the potential to be a major provider of renewable energy, especially in the UK. However, there is the major problem of producing efficient devices for a wide variety of sites with different operating conditions. This article addresses the time domain modelling of a heaving point absorber connected to a hydraulic power take-off (PTO) unit in regular waves. Two cases for the hydraulic PTO unit are considered: an ideal model and a model containing losses. Component losses are included to give a more accurate prediction of the maximum power production and to discover if the parameters to optimize the device change when losses are included. The findings show that both cases are optimized by varying the size of the hydraulic motor and the optimal size is only dependent on wave period and the trend is the same for both cases. Results also showed that to maximize the power produced for both cases, there is an optimal force that the unit produces, which can be derived from theory. Finally, power reduction as a result of the hydraulic losses is also observed with efficiencies reducing at larger wave heights.

Decoupling pole-placement control, with application to a multi-channel electro-hydraulic servosystem

A multivariable controller, applicable to coupled multi-channel servosystems, is described. The controller is designed from an estimated matrix fraction description plant model, and allows the channels to be decoupled with individually specified pole positions. The controller is applied to a two-channel electro-hydraulic servosystem, and demonstrates a significant reduction in interaction compared to non-decoupling controllers.

Optimisation and control of a hydraulic power take-off unit for a wave energy converter in irregular waves

The optimisation of a wave energy converter hydraulic power take-off for sea states of varying wave amplitude, direction and frequency is a significant problem. Sub-optimal configuration can result in very inefficient energy conversion, so understanding the design trade-offs is key to the success of the technology. This work focuses on a generic point absorber type wave energy converter. Previous work by the authors has considered the optimisation of this device for regular waves to gain an understanding of the fundamental issues. This work extends the analysis to the more realistic case of irregular waves. Simulations are performed using an irregular wave input to predict how the power take-off will operate in real sea conditions. Work is also presented on a motor speed control strategy to maintain the maximum flow of electrical power to the grid, assuming the use of a doubly fed induction generator. Finally, the sizing of key components in the power take-off is considered in an attempt to maximise power take-off efficiency and generated power.

A novel piezohydraulic aerospace servovalve. Part 1: design and modelling

Servovalves are compact, accurate and high-bandwidth modulating valves widely used in aerospace, defence, industrial and marine applications. However, manufacturing costs are high due to high part count and tight tolerances required, particularly in the first stage of the valve, and due to manual adjustments required as part of the set-up process. In this research, a novel servovalve concept is investigated that has the potential to be more cost-effective. In particular, for the first time, a piezoelectric first-stage actuator is developed to move a servovalve spool using the deflector jet principle; this is especially suited to aerospace actuation requirements. In the new valve, the conventional electromagnetic torque motor is replaced by a multilayer bimorph piezoelectric actuator. The bimorph deflects a jet of fluid to create a pressure differential across the valve spool; hence the spool moves. A feedback wire is used to facilitate proportional spool position control via mechanical feedback. The bimorph is directly coupled to the feedback wire and is immersed in hydraulic fluid. A high-order non-linear model of the valve has been developed and used to predict valve static and dynamic characteristics and is described in this article. This makes use of stiffness constants derived analytically for the bimorph-feedback wire assembly and cross-referenced to finite element analysis predictions. The model of the flow force acting on the deflector is an approximation of the force characteristic found from computation fluid dynamic analysis. The measured characteristics of the prototype valve are in good agreement with the simulation results and prove that the operational concept is viable.

Theoretical and experimental studies of a switched inertance hydraulic system

A switched inertance hydraulic system uses a fast switching valve to control flow or pressure and is potentially very efficient as it does not rely on dissipation of power by throttling. This article studies its performance using an analytical method which efficiently describes the system in the time domain and frequency domain. A lumped parameter model and a distributed parameter model have been used for investigation using different parameters and conditions. The analytical models have been validated in experiments and the results on a prototype device show a very promising performance. The proposed analytical models are effective for understanding, analysing and optimizing the characteristics and performance of a switched inertance hydraulic system.

An electro-hydrostatic actuator for hybrid active-passive vibration isolation

A novel electro-hydrostatic actuator (EHA) for active vibration isolation has been designed, modelled and tested. The EHA consists of a brushless DC motor running in oil and integrated with a bidirectional gear pump, driving a hydraulic cylinder. The actuator is designed to be integrated into a flexible strut connecting a helicopter rotor hub and fuselage, to provide isolation at the dominant rotor vibration frequency of around 20 Hz. The resonant frequency of the EHA is tuned to provide some passive vibration isolation. Active control increases the isolation performance by compensating for damping losses, and provides isolation over a broader range of frequencies. Tests on a prototype demonstrated a four-fold reduction of the root-mean-square transmitted force and a near elimination at the fundamental frequency. The advantages of the resonant EHA are a wider range of operating frequencies than a purely passive system, and lower power consumption than a purely active system.

Numerical study of roll motion of a 2-D floating structure in viscous flow

In the present study, an open source CFD tool, OpenFOAM has been extended and applied to investigate roll motion of a 2-D rectangular barge induced by nonlinear regular waves in viscous flow. Comparisons of the present OpenFOAM results with published potential-flow solutions and experimental data have indicated that the newly extended OpenFOAM model is very capable of accurate modelling of wave interaction with freely rolling structures. The wave-induced roll motions, hydrodynamic forces on the barge, velocities and vorticity fields in the vicinity of the structure in the presence of waves have been investigated to reveal the real physics involved in the wave induced roll motion of a 2-D floating structure. Parametric analysis has been carried out to examine the effect of structure dimension and body draft on the roll motion.

Experimental Investigation of a Switched Inertance Hydraulic System With a High-Speed Rotary Valve

This paper 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 (inertance), 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 behavior of hydraulic components. A high-speed rotary valve is used to provide sufficiently high switching frequency and to minimize the pressure and flow loss at the valve orifice, and a small diameter tube is used to provide an inductive effect. In this paper, 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 an SIHS is applied to analyze the experimental results. Experimental results on a flow booster rig show a very promising performance for the SIHS.