John Ringwood - Profile on Academia.edu (original) (raw)

Papers by John Ringwood

The body contains a bewildering array of regulatory systems which maintain homeostasis. There is ... more The body contains a bewildering array of regulatory systems which maintain homeostasis. There is considerable difficulty in isolating a single control loop for analysis, due to the interactions with other systems/loops. One important such regulatory loop is the baroreflex, and baroreflex sensitivity is a characteristic open-loop parameter which can help us to assess the health of the baroreflex. A diverse range of methods have been proposed to determine baroreflex sensitivity from experimental data. Unfortunately, there appears to be little consistency of result among the different methods and some explanation can be found in the nature of the problem: In most cases, an attempt is being made to determine open-loop measures from a system operating in closed-loop, subject to poor excitation. In this paper we propose a strict procedure, based on a rigourous mathematical framework, from which reliable estimates of baroreflex sensitivity can be obtained. A comparison with other methods for baroreflex sensitivity estimation, using the EuroBaVar data set, is performed.

In this paper, we investigate the differences between two control strategies for a two-device lin... more In this paper, we investigate the differences between two control strategies for a two-device linear array of wave energy converters (WEC) for device spacings of 4 to 80 times the device diameter. The WECs operate in heave only and are controlled in real time. The control strategies, called the independent device and global array control, estimate the excitation forces and calculate the optimum vertical velocity trajectory and reactive power take off force to achieve the velocity for a given sea state. The independent device controller assumes that each device is in isolation and only computes the excitation forces based on the incident wave, whereas the global array controller does take the radiation and diffraction effects from the other devices into account. Whatever controller is used in the simulations, all hydrodynamic coupling effects are accounted for.

Physical constraints need to be considered in the design of control systems for an oscillating bo... more Physical constraints need to be considered in the design of control systems for an oscillating body Wave Energy Converter (WEC). In the case of a hydraulic Power Take Off (PTO) unit, such constraints include the length of the stroke of a hydraulic piston or the maximum pressure permitted in the hydraulic circuit. In the paper, two types of WEC are considered: A 1-body point absorber oscillating in heave and a 2-body point absorber also oscillating in heave. A procedure for the analysis of the constraints is presented. It provides sufficient conditions for the satisfaction of both constraints and/or the violation of at least one constraint. The procedure is based on the discretization of the equations of motion of the WECs by approximating the forces and the velocities with a linear combination of basis functions. A special case is presented in which truncated Fourier series are used for the approximation. It is shown that the constraints can be interpreted as a geometrical object in a finite dimensional vector space, and that the study of sufficient conditions for the satisfaction of both constraints and for sufficient conditions for the violation of at least one constraint can be seen as the study of the intersection between these geometrical objects. It is also shown that the method allows the study of the effect of constraints on the amount of produced energy.

IEEE Transactions on Sustainable Energy, 2015

Benefits of up-wave measurements in linear short-term wave forecasting for wave energy applications

2014 IEEE Conference on Control Applications (CCA), 2014

… and Control in …, 2003

Several coupled and nonlinear controlling mechanisms are involved in the regulation of blood pres... more Several coupled and nonlinear controlling mechanisms are involved in the regulation of blood pressure. The possible presence of chaos in physiological signals has been the subject of some research. In this study, blood pressure signals were analysed using a range of nonlinear time series analysis techniques. Individual effectors of blood pressure were either experimentally removed or enhanced, so that the controlling mechanisms that are responsible for the chaotic nature of the signals may be identified by chaotic analysis of the signals. The level of chaos varied across the different experimental conditions, showing a distinct decrease from control conditions to all other experimental conditions.

The paper is dedicated to the study of non-linear waves acting on three dimensional surface-pierc... more The paper is dedicated to the study of non-linear waves acting on three dimensional surface-piercing structures. A time-domain non-linear potential-flow model has been developed to predict the largeamplitude motions of devices in a prescribed seaway. While earlier models have been developed for single wave energy devices, this paper addresses multi-body devices, with particular application to the WAVEBOB wave energy device.

Plasma processing is used in a variety of industrial systems, including semiconductor manufacture... more Plasma processing is used in a variety of industrial systems, including semiconductor manufacture (deposition and etching) and accurate control of the impedance matching network is vital if repeatable quality is to be achieved at the manufacturing process output. Typically, impedance matching networks employ series (tune) and parallel (load) capacitors to drive the reflection coefficient on the load side of the network to zero. The reflection coefficient is normally represented by real and imaginary parts, giving two variables to be controlled using the load and tune capacitors. The resulting problem is therefore a nonlinear, multivariable control problem. Current industrial impedance matching units employ simple single-loop proportional controllers, which take no account of interaction between individual channels and, in many cases, may fail to tune altogether, if the starting point is far away from the matching point. A hierarchical feedback controller is developed which, at the upper level, performs a single-loop tuning, but with the important addition of a variable sign feedback gain. When convergence to a region in the neighbourhood of the matching point is achieved, a dual single-loop controller takes over, which gives fine tuning of the matching network.

IEEE Transactions on Semiconductor Manufacturing, 2010

Semiconductor wafer etching is, to a large extent, an open-loop process with little direct feedba... more Semiconductor wafer etching is, to a large extent, an open-loop process with little direct feedback control. Most silicon chip manufacturers rely on the rigorous adherence to a "recipe" for the various etch processes, which have been built up based on considerable historical experience. However, residue buildup and difficulties in achieving consistent preventative maintenance operations lead to drifts and step changes in process characteristics. This paper examines the particular technical difficulties encountered in achieving consistency in the etching of semiconductor wafers and documents the range of estimation and control techniques currently available to address these difficulties. An important feature of such an assessment is the range of measurement options available if closed-loop control is to be achieved.

2012 IEEE International Conference on Control Applications, 2012

This paper provides a stability analysis for a system that captures wave energy in order to produ... more This paper provides a stability analysis for a system that captures wave energy in order to produce potable water. The system, introduced in [1], is a Wave Energy Converter (WEC) of the point-absorber type coupled to a hydraulic Power Take-Off (PTO) that converts wave energy into pressure. Previous work has used a partial state-feedback controller with integral action and feed-forward to provide good nominal control behaviour. Although open-loop stability was proven in [1], no guarantees of closed-loop stability were given; in this paper we provide such guarantees for a class of controllers, of which the controller proposed in [1] is a special case.

2009 3rd International Conference on Signals, Circuits and Systems (SCS), 2009

Fixed speed actuators have a characteristic nonlinear transient response. While this may not pres... more Fixed speed actuators have a characteristic nonlinear transient response. While this may not present a problem for single-input, single-output (SISO) control systems containing such actuators, the mixture of different fixed speed actuators is usually problematic in multivariable control applications. This paper presents two methods which allow such nonlinear actuators to present a linear response and therefore allow their incorporation in linear multivariable control design methods. The first method is based on a closed-loop describing function approximation, while the second uses an equivalent nonlinearity concept to linearise the relay which represents the fixed-speed limitation. A simulation study, demonstrating the performance of both schemes in a multivariable control setting, is presented.

2009 3rd International Conference on Signals, Circuits and Systems (SCS), 2009

The past decade has seen the proliferation of elearning and distance learning programs across a w... more The past decade has seen the proliferation of elearning and distance learning programs across a wealth of discipline areas. In order to preserve maximum Àexibility in outreach, student assessment based exclusively on remotely submitted work has become commonplace. However, there is also growing evidence that e-learning also provides increased opportunity for plagiarism, with obvious consequences for learning effectiveness. This paper reports on the development of a prototype student authentication system, designed for use with a graduate e-learning program. The proposed system can be used to authenticate telephone-based oral examination which can, in turn, be used to con¿rm a student's ability in relation to submitted assignments and on-line test results. The prototype low-cost system is shown to be suf¿ciently accurate to act as an effective deterrent against plagiarism.

Particle filters for remaining useful life estimation of abatement equipment used in semiconductor manufacturing

2010 Conference on Control and Fault-Tolerant Systems (SysTol), 2010

Prognostics is the ability to predict the remaining useful life of a specific system, or componen... more Prognostics is the ability to predict the remaining useful life of a specific system, or component, and represents a key enabler of any effective condition-based-maintenance strategy. Among methods for performing prognostics such as regression and artificial neural networks, particle filters are emerging as a technique with considerable potential. Particle filters employ both a state dynamic model and a measurement model,

Control, Forecasting and Optimisation for Wave Energy Conversion

Proceedings of the 19th IFAC World Congress, 2014

MUSHROOM TUNNEL CLIMATE: NEURAL NETWORKS COMPARED

II Modelling Plant Growth, Environmental Control and Farm Management in Protected Cultivation, 1998

Stochastic search in data-based modelling of dynamic systems

Irish Signals and Systems Conference 2004, 2004

2012 IEEE International Conference on Control Applications, 2012

Plasma etch is a semiconductor manufacturing process during which material is removed from the su... more Plasma etch is a semiconductor manufacturing process during which material is removed from the surface of semiconducting wafers, typically made of silicon, using gases in plasma form. A host of chemical and electrical complexities make the etch process notoriously difficult to model and troublesome to control. This work demonstrates the use of a real-time model predictive control scheme to control plasma etch rate in the presence of disturbances to the ground path of the chamber, which are representative of maintenance events. Virtual metrology (VM) models, using plasma impedance measurements, are used to estimate the plasma etch rate in real time for control, with a view to eliminating the requirement for invasive measurements. The VM and control schemes exhibit fast set-point tracking and disturbance rejection capabilities. Etch rate can be controlled to within 1% of the desired value. Such control represents a significant improvement over openloop operation of etch tools, where variances in etch rate of up to 5% can be observed during production processes due to disturbances in tool state and material properties.

Proceedings of the 18th IFAC World Congress, 2011

"Plasma etching is a semiconductor manufacturing process during whic... more "Plasma etching is a semiconductor manufacturing process during which material is removed from the surface of silicon wafers using gases in plasma form. A host of chemical and electrical complexities make the etch process notoriously difficult to model and troublesome to control. This work demonstrates the use of a real-time model predictive control scheme to maintain a consistent plasma electron density in the presence of disturbances to the ground path of the chamber. The electron density is estimated in real time using a virtual metrology model based on plasma impedance measurements. Recursive least squares is used to update the controller model parameters in real time to achieve satisfactory control of electron density over a wide operating space."

IET Irish Signals and Systems Conference (ISSC 2010), 2010

Optical Emission Spectroscopy (OES) is a non-intrusive plasma diagnostic technique that can be us... more Optical Emission Spectroscopy (OES) is a non-intrusive plasma diagnostic technique that can be used to measure the chemical changes in a plasma that is increasingly being considered for monitoring and control of plasma etch processes. In the practice of collecting OES data for plasma etching, it is inevitable that noise is included in the measurements. The existence of noise can destroy signals or at least make the identification and interpretation of signal patterns unreliable, hence appropriate filtering and pre-processing of the data is needed prior to application of automated feature extraction and analysis techniques. In the absence of a priori knowledge of the noise characteristics and system bandwidth the selection of an appropriate noise suppression filter bandwidth is a challenging problem. This paper explores the characteristics of the noise inherent in OES measurements and proposes a systematic method for establishing a suitable noise filter bandwidth based on the auto-correlation and cross-correlation analyses of the filtered signals and their residuals.

The body contains a bewildering array of regulatory systems which maintain homeostasis. There is ... more The body contains a bewildering array of regulatory systems which maintain homeostasis. There is considerable difficulty in isolating a single control loop for analysis, due to the interactions with other systems/loops. One important such regulatory loop is the baroreflex, and baroreflex sensitivity is a characteristic open-loop parameter which can help us to assess the health of the baroreflex. A diverse range of methods have been proposed to determine baroreflex sensitivity from experimental data. Unfortunately, there appears to be little consistency of result among the different methods and some explanation can be found in the nature of the problem: In most cases, an attempt is being made to determine open-loop measures from a system operating in closed-loop, subject to poor excitation. In this paper we propose a strict procedure, based on a rigourous mathematical framework, from which reliable estimates of baroreflex sensitivity can be obtained. A comparison with other methods for baroreflex sensitivity estimation, using the EuroBaVar data set, is performed.

In this paper, we investigate the differences between two control strategies for a two-device lin... more In this paper, we investigate the differences between two control strategies for a two-device linear array of wave energy converters (WEC) for device spacings of 4 to 80 times the device diameter. The WECs operate in heave only and are controlled in real time. The control strategies, called the independent device and global array control, estimate the excitation forces and calculate the optimum vertical velocity trajectory and reactive power take off force to achieve the velocity for a given sea state. The independent device controller assumes that each device is in isolation and only computes the excitation forces based on the incident wave, whereas the global array controller does take the radiation and diffraction effects from the other devices into account. Whatever controller is used in the simulations, all hydrodynamic coupling effects are accounted for.

Physical constraints need to be considered in the design of control systems for an oscillating bo... more Physical constraints need to be considered in the design of control systems for an oscillating body Wave Energy Converter (WEC). In the case of a hydraulic Power Take Off (PTO) unit, such constraints include the length of the stroke of a hydraulic piston or the maximum pressure permitted in the hydraulic circuit. In the paper, two types of WEC are considered: A 1-body point absorber oscillating in heave and a 2-body point absorber also oscillating in heave. A procedure for the analysis of the constraints is presented. It provides sufficient conditions for the satisfaction of both constraints and/or the violation of at least one constraint. The procedure is based on the discretization of the equations of motion of the WECs by approximating the forces and the velocities with a linear combination of basis functions. A special case is presented in which truncated Fourier series are used for the approximation. It is shown that the constraints can be interpreted as a geometrical object in a finite dimensional vector space, and that the study of sufficient conditions for the satisfaction of both constraints and for sufficient conditions for the violation of at least one constraint can be seen as the study of the intersection between these geometrical objects. It is also shown that the method allows the study of the effect of constraints on the amount of produced energy.

IEEE Transactions on Sustainable Energy, 2015

Benefits of up-wave measurements in linear short-term wave forecasting for wave energy applications

2014 IEEE Conference on Control Applications (CCA), 2014

… and Control in …, 2003

Several coupled and nonlinear controlling mechanisms are involved in the regulation of blood pres... more Several coupled and nonlinear controlling mechanisms are involved in the regulation of blood pressure. The possible presence of chaos in physiological signals has been the subject of some research. In this study, blood pressure signals were analysed using a range of nonlinear time series analysis techniques. Individual effectors of blood pressure were either experimentally removed or enhanced, so that the controlling mechanisms that are responsible for the chaotic nature of the signals may be identified by chaotic analysis of the signals. The level of chaos varied across the different experimental conditions, showing a distinct decrease from control conditions to all other experimental conditions.

The paper is dedicated to the study of non-linear waves acting on three dimensional surface-pierc... more The paper is dedicated to the study of non-linear waves acting on three dimensional surface-piercing structures. A time-domain non-linear potential-flow model has been developed to predict the largeamplitude motions of devices in a prescribed seaway. While earlier models have been developed for single wave energy devices, this paper addresses multi-body devices, with particular application to the WAVEBOB wave energy device.

Plasma processing is used in a variety of industrial systems, including semiconductor manufacture... more Plasma processing is used in a variety of industrial systems, including semiconductor manufacture (deposition and etching) and accurate control of the impedance matching network is vital if repeatable quality is to be achieved at the manufacturing process output. Typically, impedance matching networks employ series (tune) and parallel (load) capacitors to drive the reflection coefficient on the load side of the network to zero. The reflection coefficient is normally represented by real and imaginary parts, giving two variables to be controlled using the load and tune capacitors. The resulting problem is therefore a nonlinear, multivariable control problem. Current industrial impedance matching units employ simple single-loop proportional controllers, which take no account of interaction between individual channels and, in many cases, may fail to tune altogether, if the starting point is far away from the matching point. A hierarchical feedback controller is developed which, at the upper level, performs a single-loop tuning, but with the important addition of a variable sign feedback gain. When convergence to a region in the neighbourhood of the matching point is achieved, a dual single-loop controller takes over, which gives fine tuning of the matching network.

IEEE Transactions on Semiconductor Manufacturing, 2010

Semiconductor wafer etching is, to a large extent, an open-loop process with little direct feedba... more Semiconductor wafer etching is, to a large extent, an open-loop process with little direct feedback control. Most silicon chip manufacturers rely on the rigorous adherence to a "recipe" for the various etch processes, which have been built up based on considerable historical experience. However, residue buildup and difficulties in achieving consistent preventative maintenance operations lead to drifts and step changes in process characteristics. This paper examines the particular technical difficulties encountered in achieving consistency in the etching of semiconductor wafers and documents the range of estimation and control techniques currently available to address these difficulties. An important feature of such an assessment is the range of measurement options available if closed-loop control is to be achieved.

2012 IEEE International Conference on Control Applications, 2012

This paper provides a stability analysis for a system that captures wave energy in order to produ... more This paper provides a stability analysis for a system that captures wave energy in order to produce potable water. The system, introduced in [1], is a Wave Energy Converter (WEC) of the point-absorber type coupled to a hydraulic Power Take-Off (PTO) that converts wave energy into pressure. Previous work has used a partial state-feedback controller with integral action and feed-forward to provide good nominal control behaviour. Although open-loop stability was proven in [1], no guarantees of closed-loop stability were given; in this paper we provide such guarantees for a class of controllers, of which the controller proposed in [1] is a special case.

2009 3rd International Conference on Signals, Circuits and Systems (SCS), 2009

Fixed speed actuators have a characteristic nonlinear transient response. While this may not pres... more Fixed speed actuators have a characteristic nonlinear transient response. While this may not present a problem for single-input, single-output (SISO) control systems containing such actuators, the mixture of different fixed speed actuators is usually problematic in multivariable control applications. This paper presents two methods which allow such nonlinear actuators to present a linear response and therefore allow their incorporation in linear multivariable control design methods. The first method is based on a closed-loop describing function approximation, while the second uses an equivalent nonlinearity concept to linearise the relay which represents the fixed-speed limitation. A simulation study, demonstrating the performance of both schemes in a multivariable control setting, is presented.

2009 3rd International Conference on Signals, Circuits and Systems (SCS), 2009

The past decade has seen the proliferation of elearning and distance learning programs across a w... more The past decade has seen the proliferation of elearning and distance learning programs across a wealth of discipline areas. In order to preserve maximum Àexibility in outreach, student assessment based exclusively on remotely submitted work has become commonplace. However, there is also growing evidence that e-learning also provides increased opportunity for plagiarism, with obvious consequences for learning effectiveness. This paper reports on the development of a prototype student authentication system, designed for use with a graduate e-learning program. The proposed system can be used to authenticate telephone-based oral examination which can, in turn, be used to con¿rm a student's ability in relation to submitted assignments and on-line test results. The prototype low-cost system is shown to be suf¿ciently accurate to act as an effective deterrent against plagiarism.

Particle filters for remaining useful life estimation of abatement equipment used in semiconductor manufacturing

2010 Conference on Control and Fault-Tolerant Systems (SysTol), 2010

Prognostics is the ability to predict the remaining useful life of a specific system, or componen... more Prognostics is the ability to predict the remaining useful life of a specific system, or component, and represents a key enabler of any effective condition-based-maintenance strategy. Among methods for performing prognostics such as regression and artificial neural networks, particle filters are emerging as a technique with considerable potential. Particle filters employ both a state dynamic model and a measurement model,

Control, Forecasting and Optimisation for Wave Energy Conversion

Proceedings of the 19th IFAC World Congress, 2014

MUSHROOM TUNNEL CLIMATE: NEURAL NETWORKS COMPARED

II Modelling Plant Growth, Environmental Control and Farm Management in Protected Cultivation, 1998

Stochastic search in data-based modelling of dynamic systems

Irish Signals and Systems Conference 2004, 2004

2012 IEEE International Conference on Control Applications, 2012

Plasma etch is a semiconductor manufacturing process during which material is removed from the su... more Plasma etch is a semiconductor manufacturing process during which material is removed from the surface of semiconducting wafers, typically made of silicon, using gases in plasma form. A host of chemical and electrical complexities make the etch process notoriously difficult to model and troublesome to control. This work demonstrates the use of a real-time model predictive control scheme to control plasma etch rate in the presence of disturbances to the ground path of the chamber, which are representative of maintenance events. Virtual metrology (VM) models, using plasma impedance measurements, are used to estimate the plasma etch rate in real time for control, with a view to eliminating the requirement for invasive measurements. The VM and control schemes exhibit fast set-point tracking and disturbance rejection capabilities. Etch rate can be controlled to within 1% of the desired value. Such control represents a significant improvement over openloop operation of etch tools, where variances in etch rate of up to 5% can be observed during production processes due to disturbances in tool state and material properties.

Proceedings of the 18th IFAC World Congress, 2011

"Plasma etching is a semiconductor manufacturing process during whic... more "Plasma etching is a semiconductor manufacturing process during which material is removed from the surface of silicon wafers using gases in plasma form. A host of chemical and electrical complexities make the etch process notoriously difficult to model and troublesome to control. This work demonstrates the use of a real-time model predictive control scheme to maintain a consistent plasma electron density in the presence of disturbances to the ground path of the chamber. The electron density is estimated in real time using a virtual metrology model based on plasma impedance measurements. Recursive least squares is used to update the controller model parameters in real time to achieve satisfactory control of electron density over a wide operating space."

IET Irish Signals and Systems Conference (ISSC 2010), 2010

Optical Emission Spectroscopy (OES) is a non-intrusive plasma diagnostic technique that can be us... more Optical Emission Spectroscopy (OES) is a non-intrusive plasma diagnostic technique that can be used to measure the chemical changes in a plasma that is increasingly being considered for monitoring and control of plasma etch processes. In the practice of collecting OES data for plasma etching, it is inevitable that noise is included in the measurements. The existence of noise can destroy signals or at least make the identification and interpretation of signal patterns unreliable, hence appropriate filtering and pre-processing of the data is needed prior to application of automated feature extraction and analysis techniques. In the absence of a priori knowledge of the noise characteristics and system bandwidth the selection of an appropriate noise suppression filter bandwidth is a challenging problem. This paper explores the characteristics of the noise inherent in OES measurements and proposes a systematic method for establishing a suitable noise filter bandwidth based on the auto-correlation and cross-correlation analyses of the filtered signals and their residuals.