Active shape control of optical membranes structures

The objective is to develop a control approach for the active shaping of large membrane structures using active materials. Membrane mirrors are thin, flexible optical surfaces for satellite lenses that are lightweight and can be stored compactly. These types of mirrors are expected to enhance inflatable space technology by providing quality optical imaging while reducing current weight and storage requirements. Membrane mirrors, however, require active surface control for proper imaging and are subject to various disturbances (including large thermal gradients and internally induced excitation) that can lead to degraded performance. Active materials, such as piezoceramics, have been proposed as a means of providing disturbance rejection via distributed actuation; the idea is to attach a number of actuators along the outer rim of the optic and actively control them to produce a desired motion. However, unlike typical piezoelectrically actuated structures, due to the properties of the membrane, the introduction of the piezoceramic significantly alters the dynamic behavior of the membrane structure. Here we propose a novel method of simultaneous shape and vibration control based on the dynamical theory of partly specified motion. The basic idea behind this method is to solve for the open-loop control input from the membrane-actuator model that achieves the approximate desired shape. The open-loop control is then augmented by a feedback loop that accounts for uncertainties due to modeling errors and external vibration disturbances. We propose to conduct numerical simulations and a simple small-scale experiment to test the control method. Active membrane shaping is an important problem for the employment of new ultralightweight, ultra-large and deployable telescope architectures. NASA JPL is at the forefront of this effort with several ongoing projects, including the Dual Anamorphic Reflector Telescope (DART); this new telescope architecture consists of a single aperture formed from two cylindrical parabolic reflectors. The system is ideally suited to use tensioned membranes for the reflective surface. The problem of active shape control remains a challenge and is an ongoing effort at JPL. The research effort proposed here is intended to provide a means to foster interaction with NASA JPL and to develop collaborative relations. The results generated from this work will be useful in forming shared objectives with JPL with the ultimate goal of pursuing collaborative research projects. As indicated in the attached letter, Dr. Greg Agnes of NASA JPL has expressed interest in discussing future collaborations in the area of adaptive structures technology. The proposed effort will provide a basis for successful interaction.