Now showing 1 - 10 of 21
  • Publication
    Experimental modal analysis of violin and similar thin plates by added point masses
    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.
  • Publication
    Boundary-controlled travelling and standing waves in cascaded lumped systems
    (Elsevier, 2012-05) ;
    This paper shows how pure travelling waves in cascaded, lumped, uniform, mass-spring systems can be defined, established, and maintained, by controlling two boundary actuators, one at each end. In most cases the control system for each actuator requires identifying and measuring the notional component waves propagating in opposite directions at the actuator-system interfaces. These measured component waves are then used to form the control inputs to the actuators. The paper also shows how the boundaries can be actively controlled to establish and maintain standing waves of arbitrary standing wave ratio, including those corresponding to the classical modes of vibration of such systems with textbook boundary conditions. These vibration modes are achieved and maintained by controlled reflection of the pure travelling wave components. The proposed control systems are also robust to system disturbances: they react to overcome external disturbances quickly and so to re-establish the desired steady motion.
  • Publication
    Wave-Based Attitude Control of Spacecraft with Fuel Sloshing Dynamics
    (International Center for Numerical Methods in Engineering, 2015-07-02) ;
    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.
  • Publication
    Travelling waves in boundary-controlled, non-uniform, cascaded lumped systems
    (Elsevier, 2013-08) ;
    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.
      354Scopus© Citations 4
  • Publication
    Wave-Based Analysis and Control of Lump-Modeled Flexible Robots
    (IEEE, 2007-04)
    Flexible robots are frequently represented by lumped models. In the mechanics of lumped systems, wave concepts have been avoided, for good reasons, generally. In the control of lumped flexible systems, however, wave concepts prove very fruitful. This paper provides a foundation for the wave-based control application by exploring the validity and nature of wave concepts in lumped robotic systems. A new wave-based model of uniform mass-spring systems is proposed and verified. The model is exact but not unique. Useful simplifications and approximations are also presented. The model leads to control strategies for flexible robotic systems that are simple, powerful, robust, and generic. The wave approach also provides a new analysis tool and conceptual framework for lumped dynamic systems.
      546Scopus© Citations 72
  • Publication
    An alternative TLM method for steady-state convection-diffusion
    (Wiley Blackwell (John Wiley & Sons), 2009-09-03) ;
    Recent papers have introduced a novel and efficient scheme, based on the transmission line modelling (TLM) method, for solving one-dimensional steady-state convection–diffusion problems. This paper introduces an alternative method. It presents results obtained using both techniques, which suggest that the new scheme outlined in this paper is the more accurate and efficient of the two. Copyright © 2009 John Wiley & Sons, Ltd.
  • Publication
    Multibody domain decomposition for parallel processing: a wave-based approach to handling interface dynamics
    For many good reasons there is growing interest in ways to allow parallel processing of multibody dynamics problems. Some recent approaches include “Domain Decomposition” and “Divide and Conquer”. This paper explores a new approach, reported as work in progress, with initial, promising results. The strategy is an extension of work done on wave analysis of lumped systems in another context. In the approach, a larger system is subdivided into smaller subsystems, which are solved in parallel. Interconnection points are boundaries for each. Dynamic coupling across boundaries is handled in terms of transmitted and reflected motion components (or "waves"), in both directions, across the boundaries.
  • Publication
    EIRSAT-1 - The Educational Irish Research Satellite
    The Educational Irish Research Satellite, "EIRSAT-1", is a collaborative space project that aims to build, launch and operate the first ever Irish satellite. The EIRSAT-1 spacecraft is a 2U CubeSat incorporating three novel experiment payloads: GMOD, a gamma-ray detector; EMOD, a thermal management coating demonstration; and WBC, an attitude control algorithm. The spacecraft is currently under construction at University College Dublin and will be delivered to ESA in late 2019.
  • Publication
    Wave-based control of under-actuated flexible structures with strong external disturbing forces
    (Taylor and Francis, 2015-03-18) ;
    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.
      303Scopus© Citations 7
  • Publication
    Development of the Ground Segment Communication System for the EIRSAT-1 CubeSat
    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.