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

Abstract

This paper discusses the design and manufacturing of a thin polymer spherical adaptive reflector of diameter D=200 mm, controlled by an array of 25 independent electrodes arranged in a keystone configuration actuating a thin film of PVDF-TrFE in d31-mode. The 5 μm layer of electrostrictive material is spray-coated. The results of the present study confirm that the active material can be modelled by a unidirectional quadratic model and that excellent properties can be achieved if the material is properly annealed. The experimental influence functions of the control electrodes are determined by a quasi-static harmonic technique; they are in good agreement with the numerical simulations and their better circular symmetry indicates a clear improvement in the manufacturing process, as compared to a previous study. The low order optical modes can be reconstructed by combining the 25 influence functions; a regularization technique is used to alleviate the ill-conditioning of the Jacobian and allow to approximate the optical modes with reasonable voltages.

Adaptive Deployable Thin Spherical Shell Reflectors

“…This equation confirms that an electrostrictive material can be made piezoelectric by applying a bias electric field [18]. The method for determining the material properties (ε 1 , d 31 , Q 33 , P s ) has been described in [6,7]; they are reported in Table 1; the two values for P s refer to two independent methods of estimation. Notice that the value of the dielectric constant ε r has been found consistently smaller for the spray-coated samples than for the spin coated ones.…”

“…Our work [6,7] investigates the control a thin spherical polymer shell covered with a thin film of electrostrictive copolymer (PVDF-TrFE) used as in-plane strain actuator (according to the d 31 mode); the target shape is obtained by applying voltages to a set of independent electrodes. Earlier work has shown that strain actuators are very efficient to control a flat plate; they are widely used in Adaptive Optics (AO) (e.g., see [8]).…”

“…The behavior of the electrostrictive polymer was investigated in an earlier phase of the project [17], in the configuration described in Figure 2a. The active material consists of PVDF-TrFE (PVDF-TRFE FC25 from Piezotech, 0.12 g/mL in Acetone:Dimethylacetamide 95:5), annealed at 140 • C for 2 h. The material was applied by spin coating in [6] and by spray coating in [7] (spray coating can be easily scaled up to large sizes).…”

“…The sphere has a radius of 2.5 m; two mirrors have been built, one with a diameter of 100 mm, provided with a set of 7 independent electrodes, and one of 200 mm with 25 electrodes arranged in a keystone layout (Figure 4). The demonstrators were manufactured by MateriaNova according to the procedure described in [7]. The experimental determination of the influence functions of the electrodes is described in [6].…”

“…For illustration purpose, Figure 5 compares the cross sections of the measured influence functions of 6 electrodes of the same row (rotated successively by 60 • ) with a numerical simulation. More comprehensive results and details on the experimental setup are provided in [7].…”