Optical methods for non contact measurements of membranes

Structures for space applications very often suffer stringent mass constraints. Lightweight structures are developed for this purpose, through the use of deployable and/or inflatable beams, and thin-film membranes. Their inherent properties (low mass and small thickness) preclude the use of conventional measurement methods (accelerometers and displacement transducers for example) during on-ground testing. In this context, innovative non-contact measurement methods need to be investigated for these stretched membranes. The exact measurement requirements obviously depend upon the specific application: sun shield, solar array, antenna or others, with sizes ranging from one to several dozens of meters. The surface nature of the reflector can be specular reflective, diffusing or transparent. Although there is a continuous effort from the optical metrology community to develop and improve metrology techniques, this paper demonstrates that there is no universal method to test membrane space-structures. The object of the present project is to perform a review of existing measurement systems able to fulfil the measurement requirements: dot-projection videogrammetry (static measurements), stereo-correlation (dynamic and static measurements), fringe projection (wrinkles) and 3D laser scanning vibrometry (dynamic measurements). Therefore, minimum requirements were given for the study in order to have representative test articles covering a wide range of applications. We present test results obtained with the different methods on our test articles.