Seamus D. Garvey

A combined genetic and eigensensitivity algorithm for the location of damage in structures

Abstract Genetic algorithms have been the subject of considerable interest in recent years, since they appear to provide a robust search procedure for solving difficult problems. Due to the way the genetic algorithm explores the region of interest it avoids getting stuck at a particular local minimum and locates the global optimum. The genetic algorithm is slow in execution and is best applied to difficult problems. This paper applies a genetic algorithm to the problem of damage detection using vibration data. The objective is to identify the position of one or more damage sites in a structure, and to estimate the extent of the damage at these sites. The genetic algorithm is used to optimize the discrete damage location variables. For a given damage location site or sites, a standard eigensensitivity method is used to optimize the damage extent. This two-level approach incorporates the advantages of both the genetic algorithm and the eigensensitivity methods. The method is demonstrated on a simulated beam example and an experimental plate example.

Automatic choice of measurement locations for dynamic testing

This paper examines the problem of choosing an optimum set of measurement locations for experimental modal testing and suggests criteria whereby the suitability of the chosen locations can be assessed. Two methods of coordinate selection are used: one based on Guyan reduction and the other on the Fisher information matrix. Each begins with a detailed finite element model of the structure being tested. Both procedures reduce this model by one degree of freedom at a time until the number of degrees of freedom in the reduced model equals the number of measurement locations required. The choice of the eliminated coordinates is generally automatic, and the coordinates of the reduced model are those used for modal testing

Parameter subset selection in damage location

Methods to locate damage in structures, using a finite element model and low frequency measured vibration data, have attracted considerable interest. A large number of parameters are required to ensure that the damage location and mechanism may be modelled by at least one set of parameter values. Generally the identified parameter values are not unique and extra information must be incorporated into the identification. The finite element model of a damaged structure is likely to be in error at only a small number of sites. This is equivalent to requiring that only a subset of parameters are in error, and leads to the methods of subset selection. The standard method uses the sensitivity matrix based on the initial finite element model to choose the parameter subset. Many residuals used for damage location are nonlinear functions of the parameters, and this paper examines the relationship between the subset selection and the iteration required for the parameter estimation. Also measurements are often taken ...

Shape and cost analysis of pressurized fabric structures for subsea compressed air energy storage

In this article, three different methods are presented for finding the deformed shape of pressurized fabric structures underwater. The methods are used here to analyse the shape and cost of ‘energy bags’, inflatable bags that can be anchored to the seabed and used for subsea compressed air energy storage. First, a system of coupled ordinary differential equations is derived which can be solved to find the shape of an inextensible axisymmetric membrane. Then finite-element analysis (FEA) of an axisymmetric natural shape bag is carried out using cable elements, giving the deformed shape of an extensible axisymmetric membrane. Finally, a full three-dimensional FEA is presented which includes cable and membrane elements. A simple optimization is also used to minimize the cost per unit of energy stored in an axisymmetric natural shape energy bag, and it is shown that if only materials costs are taken into account, the cost of surface is approximately equal to the cost of meridional reinforcement in the optimum-sized bag.

Practical Implementation of the Bridge Configured Winding for Producing Controllable Transverse Forces in Electrical Machines

It is now evident that the value of endowing rotating machines with the capability to produce transverse forces in addition to their normal torque-producing function can be extremely high. The bridge configured winding is a cost-saving connection scheme for electrical machines to exploit transverse magnetic forces on the rotors. The winding scheme retains the machines three-phase terminals such that ordinary motor inverters can be employed for the normal torque-producing function while providing separate terminals for transverse force action. This paper describes the practical implementation of such a winding connection in a conventional electric machine and demonstrates that controllable transverse forces can be produced.

Analysis of flexible fabric structures for large-scale subsea compressed air energy storage

The idea of storing compressed air in submerged flexible fabric structures anchored to the seabed is being investigated for its potential to be a clean, economically-attractive means of energy storage which could integrate well with offshore renewable energy conversion. In this paper a simple axisymmetric model of an inextensional pressurised bag is presented, along with its implementation in a constrained multidimensional optimization used to minimise the cost of the bag materials per unit of stored energy. Base pressure difference and circumferential stress are included in the optimization, and the effect of hanging ballast masses from the inside of the bag is also considered. Results are given for a zero pressure natural shape bag, a zero pressure bag with circumferential stress and hanging masses, and a nonzero pressure bag with circumferential stress and hanging masses.

Active Axial-Magnetomotive Force Parallel-Airgap Serial Flux Magnetic Bearings

The maximum force capability of conventional active magnetic bearings is limited since the magnetic flux density in the iron is limited due to saturation. We introduce in this paper a new concept of active magnetic bearings that exploits high bearing force from a number of parallel ironless stator and permanent-magnet rotor discs. We optimized the design of the bearing by solving tens of thousands of finite-element models in nested loops. We commissioned an experimental rig comprising a set of interleaved bearing discs and a 3 degrees-of-freedom mechanical platform to verify its force production capability.

Structural capacity and the 20 MW wind turbine

Abstract This article addresses a generic and fundamental question: given the specification for a structure, how can one assess or even express its capacity without first designing it? The question is motivated by the consideration of how wind turbines (offshore wind turbines in particular) may scale up further. An underlying premise is that the wind-turbine designs that have evolved from 50 kW to 5 MW may warrant re-examination to check whether scaling effects demand new approaches at larger scales. Since several in the industry are already setting sights on 8 MW and even 10 MW machines as extrapolations of current designs, this article considers the development of a 20 MW (232 m diameter) machine. The conventional three-bladed arrangement on a slender tower is examined for notional 5 and 20 MW machines for similar rated wind conditions. Initially, the article considers the structural capacity requirements for conventional designs at both 5 and 20 MW and how these can be reduced substantially – using a floating framework in place of a tower and by deploying cable bracing on the rotor. When tower bending and the non-productive blade bending moments have thus been addressed, the turbine torque itself emerges as a major cause of cost. The turbine torque directly influences the costs of blades, pitch and yaw bearings, direct-drive generators (if present), gearbox (if present), and main shaft, and it is confirmed that in all cases, the total amounts of material required are at least proportional to the turbine torque for conventional designs. An alternative wind-turbine configuration is examined. In this, gravity is co-opted as an ally rather than an enemy, and power conversion takes place entirely within the rotor by allowing gravity to move bodies within the rotor. The concept is illustrated for a 20 MW machine and shown to be entirely workable in principle. A strong feature of this proposed new machine configuration is that the costs are almost exclusively structural and can be quantified in terms of the uniform currency of structural capacity. The scalability of this concept becomes clear by observing that the mean velocities of the moveable masses relative to the main rotor structure are virtually independent of machine scale.

The Specific Load Capacity of Radial-Flux Radial Magnetic Bearings

This paper addresses the specific load capacity of radial-flux radial magnetic bearings and provides some insight into what values are achievable and how these values depend on various parameters such as air-gap thickness, allowable temperature rise in the coils, certain heat-transfer coefficients, and certain de-rating factors associated with the distribution of the bearing load in time and space. We define the specific load capacity of a radial magnetic bearing as the ratio of the largest sustainable root-mean-square radial force to the total self-weight of all parts necessary for the electromagnetic function of the bearing. We show that it is possible to push the limit of the specific load capacity of naturally cooled bearings up to around 35:1 with present-day materials, for a wide range of practical bearing sizes. This figure is still very small compared with the capacity of mechanical bearings.

Use of geometric algebra: compound matrices and the determinant of the sum of two matrices

In this paper we demonstrate the capabilities of geometric algebra by the derivation of a formula for the determinant of the sum of two matrices in which both matrices are separated in the sense that the resulting expression consists of a sum of traces of products of their compound matrices. For the derivation we introduce a vector of Grassmann elements associated with an arbitrary square matrix, we recall the concept of compound matrices and summarize some of their properties. This paper introduces a new derivation and interpretation of the relationship between p–forms and the pth compound matrix, and it demonstrates the use of geometric algebra, which has the potential to be applied to a wide range of problems.