Design and testing of a concave surface with changes in curvature using null-screens

This work presents the design of polynomial-based variable curvature surfaces. The design is based on a variable curvature polynomial that utilizes a sixth-degree polynomial. This surface exhibits a curvature change from concave to convex over a small section, acting as a barrier to slow the movement of the friction pendulum in seismic isolation systems. A second, similar surface was also designed, but the points where the curvature changes were selected to create two curvature changes, or two inflection points. This resulted in the surface changing from concave to convex and vice versa over a larger area than the first surface. Simulations of null screen construction were performed for these surfaces to design the experimental test device. These surfaces were fabricated using a stereolithography (SLA) 3D printer with a pixel size of 25µm, providing acceptable accuracy for surface fabrication. Surface characterization was performed using optical deflectometry with off-axis null screens.