Guglielmo S. Aglietti

Harnessing High-Altitude Solar Power

As an intermediate solution between Glasers satellite solar power (SSP) and ground-based photovoltaic (PV) panels, this paper examines the collection of solar energy using a high-altitude aerostatic platform. A procedure to calculate the irradiance in the medium/high troposphere, based on experimental data, is described. The results show that here a PV system could collect about four to six times the energy collected by a typical U.K.-based ground installation, and between one-third and half of the total energy the same system would collect if supported by a geostationary satellite (SSP). The concept of the aerostat for solar power generation is then briefly described together with the equations that link its main engineering parameters/variables. A preliminary sizing of a facility stationed at 6 km altitude and its costing, based on realistic values of the input engineering parameters, is then presented.

Microvibrations induced by a cantilevered wheel assembly with a soft-suspension system

Microvibration management onboard spacecraft with high stability requirements has drawn increasing interest from engineers and scientists, and this paper discusses a reaction wheel design that allows a significant reduction of mid- to high-frequency microvibrations and that has been practically implemented in industry. Disturbances typically induced by mechanical systems onboard a spacecraft (especially rotating devices such as reaction wheel assemblies and momentum wheel assemblies) can severely degrade the performance of sensitive instruments. Traditionally, wheel-induced high-frequency (over 100-200 Hz) vibrations, generated by a combination of phenomena from bearing noise to dynamic amplifications due to internal resonances, are especially difficult to control. In this paper, the dynamic behavior of a newly designed wheel assembly, with a cantilevered flywheel configuration supported by a soft-suspension system, is investigated. The wheel assemblys mathematical model is developed and later verified with vibration tests. Wheel-assembly-induced lateral and axial microvibrations are accurately measured using a seismic-mass microvibration measurement system, which represents an alternative to typical microvibration measurement setups. Finally, the performance of this wheel assembly in terms of microvibration emissions is compared with a traditional design (with a rigid suspension) through comparison of frequency spectra, and it is shown that this design produces significantly lower vibrations at high frequency. Copyright

A modeling technique for active control design studies with application to spacecraft microvibrations

Microvibrations, at frequencies between 1 and 1000 Hz, generated by on board equipment, can propagate throughout a spacecraft structure and affect the performance of sensitive payloads. To investigate strategies to reduce these dynamic disturbances by means of active control systems, realistic yet simple structural models are necessary to represent the dynamics of the electromechanical system. In this paper a modeling technique which meets this requirement is presented, and the resulting mathematical model is used to develop some initial results on active control strategies. Attention is focused on a mass loaded panel subjected to point excitation sources, the objective being to minimize the displacement at an arbitrary output location. Piezoelectric patches acting as sensors and actuators are employed. The equations of motion are derived by using Lagranges equation with vibration mode shapes as the Ritz functions. The number of sensors/actuators and their location is variable. The set of equations obtained is then transformed into state variables and some initial controller design studies are undertaken. These are based on standard linear systems optimal control theory where the resulting controller is implemented by a state observer. It is demonstrated that the proposed modeling technique is a feasible realistic basis for in-depth controller design/evaluation studies.

Accuracy of simplified printed circuit board finite element models

Electronic components are prone to failure due to shock or vibration loads. To predict when this failure may occur it is necessary to calculate the vibration response of the printed circuit board (PCB); this is most usually achieved through use of simplified finite element (FE) models. The accuracy of these FE models will be mainly dependant on various sources of error, including: manufacturing variability, which will cause supposedly identical printed circuit boards to behave differently (including variability in materials and assembly, as well as dimensional tolerances); inaccuracy in the model input parameters, which is caused by either the modelling assumptions used or poor measurement technique; and errors in the solution process (e.g. linear solutions in non-linear situations). This paper investigates experimentally the contribution of these effects, this is achieved by first looking at measurement of input parameters and to what accuracy a PCB can reasonably be modelled, and then secondly measuring the extent of manufacturing and assembly induced variability. When these contributions have been defined, it will be possible to assess the confidence in any FE PCB model.

Study of the Dynamics of Three-Dimensional Tape Spring Folds

One of the most significant drivers in satellite design is the minimization of mass to reduce the large costs involved in the launch. With technological advances across many fields, it is now widely known that very low-mass satellites can perform a wide variety of missions. However, the satellite power requirement does not reduce linearly with mass, creating the need for efficient and reliable small satellite deployable structures. One possible structural solution for this application is tape springs. Tape springs have been previously studied in many countries for space applications focusing on two-dimensional systems. This work studies the possible impact of using tape springs folded in three dimensions. By first analytically determining the static moments created, simple deployment models can be constructed for tape springs in free space. Then, determining the impact of these moments about an array fold line allows the creation of a dynamic model of an array that is directly comparable to the two-dimensional system. The impact of the three-dimensional fold can then be determined.

Experimental Investigation of Tape Springs Folded in Three Dimensions

One of the most significant drivers in satellite design is the minimization of mass to reduce the large costs involved in the launch. With technological advances across many fields, it is now widely known that very low-mass satellites can perform a wide variety of missions. However, there is a need for small, efficient, area deployment devices. One possible structural solution for such devices is tape springs. Previous work on tape spring hinges has focused on two-dimensional folds; however, applications exist that incorporate three-dimensional tape spring folds. The properties of three-dimensional tape spring folds are experimentally investigated using a specially designed test rig. The rig is first used to produce comparative two-dimensional data before being used to analyze more complex three-dimensional folds.

Dynamic Response of a High-Altitude Tethered Balloon System

This paper illustrates a procedure to calculate the response of a tethered spherical aerostat to gusts, including the effect of structural nonlinearity and accounting for some of the fluid–structure interaction between the aerostat and tether line. The procedure developed and presented here is based on a full three-dimensional dynamic finite element model, with aerodynamic loads calculated from the relative velocity between a time-varying input airflow and resulting structural velocities. Exact solutions for the static response and a simplified dynamic model, both developed to validate the results of the procedure illustrated in this paper, are also derived and described in detail. The dynamic responses to gusts are compared with the equivalent steady-state solution to assess the approximations of the static solutions. Particular emphasis is placed on the output rotation of the aerostats to quantify disturbances on the pointing stability produced by gusts.

Satellite multi-functional power structure: Feasibility and mass savings

A multi-functional structure saves mass from a spacecraft by incorporating other functional subsystems into the structure. By using the structural properties of a non-structural element, inert structure may be eliminated, and the requirement to allot internal volume to the subsystem in question is removed. The current paper describes a multi-functional structure based on the secondary power system. By using commercially available plastic lithium-ion cells to form the core of a sandwich panel, inert mass is eliminated from both the structure and from the battery enclosure. The feasibility of the proposed multi-functional structure is demonstrated though vibration testing on a single cell, and the successful manufacture of a test panel. The work goes on to quantify the potential mass savings that may be achieved by using a multi-functional structure of this type. By varying a set of spacecraft attributes, the study identifies that small spacecraft with high power requirements have the potential to gain the most benefit from using a multi-functional structure of this type.

A study of tape spring fold curvature for space deployable structures

Abstract Tape springs, defined as thin metallic strips with an initially curved cross-section, are an attractive structural solution and hinge mechanism for small satellite deployable structures because of their low mass, low cost, and general simplicity. They have previously been used to deploy booms and array panels in various configurations that incorporate a two-dimensional deployment of the tape. However, applications currently exist that incorporate three-dimensional tape springs folds. To accurately model the deployment of an appendage mounted with tape spring hinges, it is necessary to accurately model the opening moments produced from the material strains in the tape spring fold. These moments are primarily a function of curvature. This publication uses a photographic method to analyse the curvature assumptions of two-dimensional tape spring folds and to define the curvature trends for three-dimensional tape spring folds as a basis for calculating the opening moment. It is found that although a variation in the curvature can be seen for three-dimensional tape spring folds, its effect is secondary to the tape thickness tolerance. Therefore, constant curvature models are concluded to be accurate enough for general tape fold applications.

Model building and verification for active control of microvibrations with probabilistic assessment of the effects of uncertainties

Abstract Microvibrations, generally defined as low-amplitude vibrations at frequencies up to 1 kHz, are of critical importance in a number of areas. It is now well known that, in general, the suppression of such microvibrations to acceptable levels requires the use of active control techniques which, in turn, require sufficiently accurate and tractable models of the underlying dynamics on which to base controller design and initial performance evaluation. Previous work has developed a systematic procedure for obtaining a finite-dimensional state-space model approximation of the underlying dynamics from the defining equations of motion, which has then been shown to be a suitable basis for robust controller design. In this paper, the experimental validation of this model prior to experimental studies is described in order to determine the effectiveness of the designed controllers. This includes details of the experimental rig and also the use of methods for assessing the safety of the resulting structure against uncertain parameters.