Electrostrictive PVDF-TrFE Thin Film Actuators for the Control of Adaptive Thin Shell Reflectors

Abstract

This paper presents the technology to control the shape of thin polymer doubly curved shell structures with a unimorph layer of strain actuators to achieve high quality, light-weight, foldable space reflectors. The selected active material is PVDF-TrFE deposited by spin coating; it is electrostrictive, isotropic and enjoys an excellent piezoelectric coefficient d 31 ≃ 15 pC/N when properly annealed, but has a nonlinear, quadratic behavior. The strain actuation is controlled by an array of segmented electrodes. The purpose of this study is to evaluate the material properties achieved in the manufacturing process. A simple, unidirectional model of electrostrictive material is considered and the material constants (electrostrictive constant Q 33 , piezoelectric constant d 31 , spontaneous polarization P s and poling strain S P ) are estimated from various static and dynamic experiments. The final part of the paper illustrates the control authority on a small demonstrator with seven independent electrodes and compares the experimental results with numerical finite element simulations.

Adaptive Shell Spherical Reflector Actuated with PVDF-TrFE Thin Film Strain Actuators

“…In the previous part of this study [10], it was found that the behavior of the thin film of PVDF-TrFE can be accounted for with a linear dielectric model:…”

“…The method for determining the material properties (ε 1 , d 31 , Q 33 , P s ) has been described extensively in Reference [10] for the spin-coated film. In the present study, it was decided to substitute the spin-coating of the PVDF-TrFE by spray-coating which can be easily scaled-up to reflectors of large size.…”

“…This can be realized in various methods depending on the configurations: with a group of thermal actuators for a truss antenna [8], with electrostatic actuators (acting out of plane) on a mesh supported antenna [7] or with an array of piezoelectric orthotropic actuators (acting in plane) glued on the back of the reflector [9]. The present work is a follow-up to Reference [10]; it is concerned with adding a thin film of electrostrictive copolymer material (PVDF-TrFE) on a polymer spherical shell substrate; an array of independent electrodes form a set of strain actuators (acting in plane). Two ways of application of copolymer have been investigated-spin-coating and spray-coating.…”

“…This paper reports on a small-scale technology demonstration project called "Multilayer Adaptive Thin Shell Reflectors for Future Space Telescopes" (MATS) developed on behalf of European Space Agency (ESA) in the framework of the General Support Technology Programme (GSTP) program. Preliminary results were published in Reference [10], with a demonstrator of diameter D = 100 mm, spin-coated, controlled with 7 independent electrodes. The present paper considers a reflector of D = 200 mm controlled with 25 independent electrodes.…”

“…The PVDF-TrFE is spray-coated rather than spin-coated in the smaller one, because it is more representative of what can be used on a large reflector. Numerical simulations have been reported in Reference [15]; the present paper reports on the experiments; it is organized as follows-Section 2 recalls the basic equations describing the behavior of an electrostrictive material and summarizes the methods used in Reference [10] to determine the main material properties of PVDF-TrFE thin films. Section 3 discusses the manufacture of the demonstrator.…”