Passive nonlinear actuators for deploying mesh antennas

Abstract

Deployable mesh antennas have dominated high-resolution earth observations and telecommunications, benefitting from their high package ratios and low areal densities. They are usually folded compactly for launch and unfolded to the working configuration once in orbit. This unfolding process is carried out by driving units, composed of pretensioned springs and/or motors, which transfer the energy stored in passive springs and/or outputted by active motors into the potential energy of the antenna. A previous energetic analysis has revealed that the potential energy remains low during the initial deployment stage and grows sharply in the late stage when the structure members are stressed almost to the fully deployed configuration. Therefore, the force required to deploy a mesh antenna increases over the process of unfolding, exhibiting negative stiffness. However, the output force of a normal linear spring has positive stiffness. To tackle this conflict, we proposed, designed, and developed a new type of passive actuator with nonlinear force-displacement curves to meet the specific demand of unfolding mesh antennas. This was achieved through the combined use of a torsional spring and a convex cam. For the desired force displacement curve, the cam profile was theoretically formulated as an optimization problem, the solution of which was numerically obtained by the sequential quadratic programming algorithm. The experimental result showed that the designed cam could generate the target force. We also compared the unfolding processes of a mesh antenna via simulations, driven by three kinds of springs with nearly the same stored energy; one has positive stiffness, one has zero stiffness and the other one has negative stiffness. Results confirmed that the negative-stiffness spring is more suitable for unfolding the mesh antenna. Moreover, the passive nonlinear actuators developed here might also be used in other applications where nonlinear force is needed.