O'Connor, William

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O'Connor, William
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O'Connor, William

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Experimental modal analysis of violin and similar thin plates by added point masses

2012, O'Connor, William, Hanson, Thomas J.

Novel methods are proposed to measure the modal properties of thin plates, such as free violin plates (prior to assembly), simply and inexpensively, by measuring certain changes when a small mass is added to the resonating plate. Iso-amplitude contours and mode shapes can easily be plotted. Modal mass, stiffness and damping can also be inferred. Underlying theory is developed, and experimental and numerical modelling methods of validation are briefly outlined.

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Development of the Ground Segment Communication System for the EIRSAT-1 CubeSat

2021-05-05, Marshall, Fergal, Murphy, David, Salmon, Lana, O'Callaghan, Derek, Doyle, Maeve, Reilly, Jack, Dunwoody, Rachel, Erkal, Jessica, Finneran, Gabriel, Fontanesi, Gianluca, Kyle, Jack, Mangan, Joseph, Thompson, Joseph W., Walsh, Sarah, de Faoite, Daithí, Hanlon, Lorraine, McKeown, David, O'Connor, William, Wall, Ronan, McBreen, Sheila, Greene, Derek

The Educational Irish Research Satellite (EIRSAT-1) is a student-led project to design, build and test Ireland’s first satellite. As part of the development, a ground segment (GS) has also been designed alongside the spacecraft. The ground segment will support two-way communications with the spacecraft throughout the mission. Communication with the satellite will occur in the very high frequency (VHF) and the ultra high frequency (UHF) bands for the uplink and downlink respectively. Different modulation schemes have been implemented for both uplink and downlink as part of the GS system. Uplink incorporates an Audio Frequency Shift-Keying (AFSK) scheme, while downlink incorporates a Gaussian Minimum Shift-Keying (GMSK) scheme. In order for the spacecraft to successfully receive a telecommand (TC) transmitted from the ground station, a framing protocol is required. AX.25 was selected as the data link layer protocol. A hardware terminal node controller (TNC) executes both the AX.25 framing and the AFSK modulation. Keep It Simple Stupid (KISS) framing software was developed to allow data to be accepted by the TNC. A software defined radio (SDR) approach has been chosen for the downlink. GNURadio is software that allows flowcharts to be built to undertake the required signal processing of the received signal, the demodulation of the signal and the decoding of data. This paper provides a detailed account of the software developed for the ground segment communication system. A review of the AX.25 and KISS framing protocols is presented. The GNURadio flowcharts that handle the signal processing and data decoding are broken down and each constituent is explained. To ensure the reliability and robustness of the system, a suite of tests was undertaken, the results of which are also presented.

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Wave-based control of under-actuated flexible structures with strong external disturbing forces

2015-03-18, O'Connor, William, Habibi, Hossein

Wave-based control of under-actuated, flexible systems has many advantages over other methods. It considers actuator motion as launching a mechanical wave into the flexible system which it absorbs on its return to the actuator. The launching and absorbing proceed simultaneously. This simple, intuitive idea leads to robust, generic, highly efficient, precise, adaptable controllers, allowing rapid and almost vibrationless re-positioning of the system, using only sensors collocated at the actuator-system interface. It has been very successfully applied to simple systems such as mass-spring strings, systems of Euler-Bernoulli beams, planar mass-spring arrays, and flexible three-dimensional space structures undergoing slewing motion. In common with most other approaches, this work also assumed that, during a change of position, the forces from the environment were negligible in comparison with internal forces and torques. This assumption is not always valid. Strong external forces considerably complicate the flexible control problem, especially when unknown, unexpected or unmodelled. The current work extends the wave-based strategy to systems experiencing significant external disturbing forces, whether enduring or transient. The work also provides further robustness to sensor errors. The strategy has the controller learn about the disturbances and compensate for them, yet without needing new sensors, measurements or models beyond those of standard wave-based control.

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Animal models of traumatic brain injury : a critical evaluation

2011-05, O'Connor, William, Smyth, Aoife, Gilchrist, M. D.

Animal models are necessary to elucidate changes occurring after brain injury and to establish new therapeutic strategies towards a stage where drug efficacy in brain injured patients (against all classes of symptoms) can be predicted. In this review, six established animal models of head trauma, namely fluid percussion, rigid indentation, inertial acceleration, impact acceleration, weight-drop and dynamic cortical deformation are evaluated. While no single animal model is entirely successful in reproducing the complete spectrum of pathological changes observed after injury, the validity of these animal models including face, construct, etiological and construct validity and how the models constitute theories about brain injury is addressed. The various types of injury including contact (direct impact) and non-contact (acceleration/deceleration) and their associated pathologies are described. The neuropathologic classifications of brain injury including primary and secondary, focal and diffuse are discussed. Animal models and their compatibility with microdialysis studies are summarised particularly regarding the role of excitatory and inhibitory amino acid neurotransmitters. This review concludes that the study of neurotransmitter interactions within and between brain regions can facilitate the development of novel compounds targeted to treat those cognitive deficits not limited to a single pharmacological class and may be useful in the investigation of new therapeutic strategies and pharmacological testing for improved treatment for traumatic head injury.

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Wave-Based Attitude Control of Spacecraft with Fuel Sloshing Dynamics

2015-07-02, Thompson, Joseph W., O'Connor, William

Wave-Based Control has been previously applied successfully to simple underactuated flexible mechanical systems. Spacecraft and rockets with structural flexibility and sloshing are examples of such systems but have added difficulties due to non-uniform structure, external disturbing forces and non-ideal actuators and sensors. The aim of this paper is to extend the application of WBC to spacecraft systems, to compare the performance of WBC to other popular controllers and to carry out experimental validation of the designed control laws. A mathematical model is developed for an upper stage accelerating rocket moving in a single plane. Fuel sloshing is represented by an equivalent mechanical pendulum model. A wave-based controller is designed for the upper stage AVUM of the European launcher Vega. In numerical simulations the controller successfully suppresses the sloshing motion. A major advantage of the strategy is that no measurement of the pendulum states (sloshing motion) is required.

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Wave-like modelling of cascaded, lumped, flexible systems with an arbitrarily moving boundary

2011-06-20, O'Connor, William

This paper considers cascaded, lumped, flexible systems, which may be short and non-uniform, which are driven by an arbitrarily moving boundary. Such systems exhibit vaguely wavelike behaviour yet defy classical wave analysis. The paper proposes novel ways to analyse and model such systems in terms of waves. It presents two new wave models for non-uniform systems, one series and one shunt, defining their component wave transfer functions, and thereby providing a way to define, identify and measure component waves. Features of the models are compared. The series and shunt configurations are mutually consistent and can be combined into a single composite wave model. The models are exact, but elements within them remain arbitrary to some degree, implying slight differences in the wave decomposition of the system. Some good model choices are proposed and explored. Wave speed and wave impedance are briefly considered, as are ways to measure component waves. Implications are discussed.

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Wave-based control of non-linear flexible mechanical systems

2009-07, O'Connor, William, Ramos de la Flor, Francisco, McKeown, David, Feliu, Vicente

The need to achieve rapid and accurate position control of a system end-point by an actuator working through a flexible system arises frequently, in cases from space structures to disk drive heads, from medical mechanisms to long-arm manipulators, from cranes to special robots. The system’s actuator must then attempt to reconcile two, potentially conflicting, demands: position control and active vibration damping. Somehow each must be achieved while respecting the other’s requirements. Wave-based control is a powerful solution with many advantages over previous techniques. The central idea is to consider the actuator motion as launching mechanical waves into the flexible system while simultaneously absorbing returning waves. This simple, intuitive idea leads to robust, generic, highly efficient, adaptable controllers, allowing rapid and almost vibrationless re-positioning of the remote load (tip mass). This gives a generic, high-performance solution to this important problem that does not depend on an accurate system model or near-ideal actuator behaviour. At first sight wave-based control assumes superposition and therefore linearity. This paper shows that wave-based control is also robust (or can easily be made robust) to non-linear behaviour associated with non-linear elasticity and with large-deflection effects.

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Travelling waves in boundary-controlled, non-uniform, cascaded lumped systems

2013-08, O'Connor, William, Zhu, Ming

A companion paper in this conference considers travelling and standing waves in cascaded, lumped, mass-spring systems, controlled by two boundary actuators, one at each end, when the system is uniform. It first proposes definitions of waves in finite lumped systems. It then shows how to control the actuators to establish desired waves from rest, and maintain them despite disturbances. The present paper extends this work to the more general, non-uniform case, when mass and spring values are arbitrary. A special "bi-uniform" case is first studied, consisting of two different uniform cascaded systems in series, with an obvious, uncontrolled, impedance mismatch where they meet. The paper shows how boundary actuator control systems can be designed to establish, and robustly maintain, apparently pure travelling waves of constant amplitude in either the first or the second uniform section, in each case with an appropriate standing wave pattern in the other section. Then a more general non-uniform case is studied. A definition of a "pure travelling wave" in non-uniform systems is proposed. Curiously, it does not imply constant amplitude motion. It does however yield maximum power transfer between boundary actuators. The definition, and its implementation in a control system, involves extending the notions of "pure" travelling waves, standing waves, and input and output impedances of sources and loads, when applied to non-uniform lumped systems. Practical, robust control strategies are presented for all cases.

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Gantry crane control of a double-pendulum, distributed-mass load, using mechanical wave concepts

2013-07-01, O'Connor, William, Habibi, Hossein

The overhead trolley of a gantry crane can be moved in two directions in the plane. The trolley is attempting to control the motion of a suspended, rigid-body, distributed mass load, supported by a hook, modelled as a lumped mass, in turn connected to the trolley by a light flexible cable. This flexible system has six degrees of freedom, four variables describing the flexible, hanging load dynamics and two (directly controlled) input variables for the trolley position. The equations of motion are developed and the crane model is verified. Then a form of wave-based control (WBC) is applied to determine what trolley motion should be used to achieve a reference motion of the load, with minimum swing during complex manoeuvres. Despite the trolley's limited control authority over the complex, flexible 3-D dynamics, WBC enables the trolley to achieve very good motion control of the load, in a simple, robust and rapid way, using little sensor information, with all measurements taken at or close to the trolley.

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Quantitative MRI analysis of brain volume changes due to controlled cortical impact

2010-07-26, Colgan, Niall C., Cronin, Michelle M., Gobbo, Olivier L., O'Mara, S. M. (Shane M.), O'Connor, William, Gilchrist, M. D.

More than 85% of reported brain traumas are classified clinically as “mild” using GCS; qualitative MRI findings are scarce and provide little correspondence to clinical symptoms. Our goal, therefore, was to establish in-vivo sequellae of traumatic brain injury following lower and higher levels of impact to the frontal lobe using quantitative MRI analysis and a mechanical model of penetrating impact injury. To investigate time-based morphological and physiological changes of living tissue requires a surrogate for the human central nervous system. The present model for TBI was a systematically varied and controlled cortical impact on deeply-anaesthetized Sprague Dawley rats designed to mimic different injury severities. Whole-brain MRI scans were performed on each rat prior to either a lower or a higher level of impact and then at hourly intervals for five hours post-impact. Both brain volume and specific anatomical structures were segmented from MR images for inter-subject comparisons post-registration. Animals subjected to lower and higher impact levels exhibited elevated intracranial pressure (ICP) in the low compensatory reserve (i.e., nearly exhausted) and terminal disturbance (i.e., exhausted) ranges, respectively. There was a statistically-significant drop in cerebrospinal fluid of 35% in the lower impacts and 65% in the higher impacts at Hr5 in comparison to the sham control. There was a corresponding increase in corpus callosum volume starting from Hr1 of 60-110% and 30-40% following the lower and higher impact levels, respectively. A statistically significant change in the abnormal tissue from Hr2 to Hr5 was observed for both impact levels, with greater significance for higher impacts. Furthermore, a statistically significant difference between the lower impacts and the sham controls occurred at Hr3. These results are statistically substantiated by a fluctuation in the physical size of the corpus callosum, a decrease in the volume of CSF, and elevated levels of atrophy in the cerebral cortex.

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