Gianluca De Zanet
Academic and research departmentsSurrey Space Centre, Centre for Engineering Materials, Department of Electrical and Electronic Engineering, School of Mechanical Engineering Sciences.
In this paper, a homodyne single-beam interferometer for three degrees of freedom (DoF) measurement is used to assess the dimensional stability of a deployable telescope for small spacecraft. The interferometric system is based on a Michelson interferometer concept, and the number of components is kept to a minimum. The rig is composed of a HeNe laser at 632.8 nm, two lenses, a prism, a beamsplitter and a CMOS camera. This makes the setup very attractive for low-cost and low-complexity solutions, and its performance can be readily improved by upgrading the hardware according to need. The algorithm is based on the Discrete Fourier Transform (DFT) of the spatial interference pattern detected by a CMOS sensor. Spectral information on fringe density and orientation can be translated into both relative displacements and tilts. The system can easily measure displacements with nanometer resolution and angle variations with microrad resolution. The developed architecture was suitable to determine the thermal deformations of the optical payload. Maximum relative displacements of about 30 microns and angle variations of the order of 0.1 mrad were obtained experimentally, with good repeatability.
Space launch vehicles provide a very limited and expensive allowance for new satellites to be put into orbit, so that spacecraft manufacturers are subjected to stringent constraints in terms of volume. Moreover, the mass budget and the overall complexity of subsystems play a signifi- cant role, especially in the design of small platforms. Deployable structures address such issues as they require smaller volumes and allow for less complicated and lighter weight mechanisms. Extendable appendages such as composite STEM booms have thermal-mechanical behaviour that could be detrimental for in-orbit operations, as they can undergo deflection accompanied by possibly unstable thermal vibrations. For high accuracy applications, a prediction of the structural performance under space environment conditions is crucial. In this paper the interaction between composite slit tubes and Solar heat flux is studied through an analytical model and finite element simulations. The main motivation is to examine the feasibility of a support structure for a telescope secondary mirror featuring coilable booms. The possibility of scaling the subsystem from nanosatellites to bigger platforms could be appealing in the Earth Observation market. The effects of changing geometrical and material parameters will be explored, especially the impact of the number of plies, stacking sequence and the uncertainties related to the thermal properties of composites. Finally, a finite element model of the telescope assembly under Solar heat flux will be analysed.
Composite materials properties are affected by uncertainties that cannot be overlooked for accurate modelling predictions. In the present study, a novel implementation of statistical screening methods for sensitivity analysis on composites is proposed. The effect of uncertainties on the behaviour of the model is assessed rapidly and reliably. Despite their efficiency when models with several input factors are employed, screening approaches are rarely used in engineering. Two sampling strategies are explored, and the results for several case studies are shown and compared with statistical estimators from regression-based methods. It is shown that screening techniques manage to provide subsets of influential parameters for a variety of applications, including analytical and finite element models, with low computational cost.
Thin carbon fibre reinforced polymer (CFRP) tape-springs are attractive structures for use in space-based optical instruments because of their compact stowed form, and their high dimensional stability when deployed. In this paper we present, with examples, two inexpensive methods to assess the thermal expansion properties of tape-spring structures: one based on strain gauges to obtain coupon level values, and another based on laser interferometry for structure level measurements. The strain gauge technique is a versatile approach that exploits the thermal output characteristics of the sensors. The thermal expansion characterisation of thin-composite samples measured a longitudinal expansion of 4.44 ppm/C and transverse expansion 5.95 of ppm/C. The interferometry system is designed with a view to capturing the displacements and tilts that occur when a structure with a low thermal mass, like a tape-spring, experiences a rapid change in flux, as occurs in the space environment. The homodyne interferometer is developed for three degree-of-freedom (DoF) measurements with a resolution of 10^-8 m for distances and 10^-6 rad for angles. The interferometric setup is based on the classical Michelson architecture and consists of few inexpensive commercial optical components. The source is a 0.8 mW Helium-Neon laser with a wavelength of 632.8 nm. The other elements include two spherical singlets, a right-angle prism, a cubic beamsplitter and a CMOS camera. The recorded interference fringes are analysed by using an algorithm based on Discrete Fourier Transform (DFT). Spectral information on the light intensity signals can be used to determine relative displacements and tilts. The dimensional stability of an optical payload based on high-strain composites was tested. The telescope has a deployable Cassegrain design, which uses six extendable members for the separation of its secondary mirror. Axial deformations between 20-30 microns along with angle variations of the order of 0.1 mrad were recorded with good repeatability.