A Novel Actuated Composite Tape-spring for Deployable Structures

This paper presents a novel composite tape-spring that is suitable for use as a structural element of deployable space structures. The tape-spring uses a combination of material orthotropy and prestress to achieve the unique property known as neutral stability. A consequence of this property is that the tape-springs can be partially rolled and they neither want to unroll or rollup. Previously investigated composite tape-springs always proceed towards their manufactured geometry once the rolling process has been started. This paper documents a procedure to fabricate neutrally stable tape-springs. An analysis methodology is presented that predicts neutral stability in appropriately prestressed laminates. The analysis predictions are shown to agree with observations. Finally, the ability of neutrally stable tape-springs to undergo large elastic deformations with diminishingly small forces is exploited in the development of actuated tape-springs. Analytical and experimental results are presented. Introduction This paper presents a novel composite tape-spring that is suitable for use as a structural element of deployable space structures. The tape-spring has the unique property that it is neutrally stable; as shown in the sequence of photographs in Figure 1, the tapespring is static in a continuum of positions without external forces to hold it. This class of structure is referred to here as a Neutrally Elastic Mechanism (NEM) because its behavior is functionally equivalent to typical sliding contact joint mechanisms. A consequence of this neutral stability is that diminishingly small forces are required to roll or unroll the tape-spring. The tape-spring can therefore be actuated (controlled rolling and unrolling) with relatively small and low force actuators bonded to the tape-spring surface. Two actuation options are investigated here: NiTi shape memory allow and PVDF piezoelectric film. Such actuated NEM tape-springs offer four prominent features for deployable space structures. First, they allow controlled deployment. Similarly, they allow controlled retraction. Third, their neutral stability allows for zero stiffness isolators and actuators. Finally, they allow mass efficient (with respect to stiffness and strength) structures in the fully deployed configuration. Several planar deployable structures that have potential to benefit from NEM tape-springs have recently been investigated; Lockheed Martin is developing a rolled solar array, and JPL has investigate various structures with rolled tubes. These structures could be constructed from actuated NEM tape-spring frames for a lightweight, controlled deployment system. The tape-springs could also be used in controlled deployment trusses. A third application is vibration isolators. The neutral stability of NEM tape-springs enables an actuator with mechanical continuity (no sliding parts) and theoretically zero stiffness. A final application is flex harnessing. Elastic high conductivity beryllium-copper wires could be configured in a NEM tape-spring such that there is low parasitic deployment torque, regardless of temperature. This paper presents a description of NEM tapespring construction, an analytical model of the tapesprings based on classical laminate theory, and finally, experimental results from efforts to actuate NEM tapesprings with NiTi shape memory alloy and PVDF piezoelectric film bonded to the tape-spring surface. NEM tape-springs achieve their unique properties through a material pre-stress, similar to how metal tapesprings have been made to exhibit interesting properties with engineered pre-stresses. As will be shown, the orthotropic materials under investigation here allow for stable deformations not possible with isotropic materials. The current tape-springs are in marked contrast to the bi-stable composite tape-springs that have been of recent interest. Such tape-springs are * Engineer, Dynacs Military & Defense, Inc., Member AIAA, [email protected] † Professor of Structural Engineering, University of Cambridge, Associate Fellow AIAA, [email protected] Copyright © 2004 by Thomas W. Murphey. Published by the American Institute of Aeronautics, Inc. with permission.