Damping of a thin-walled honeycomb structure using energy absorbing foam

Abstract

This paper presents researches into the application of energy absorbing foams as a passive damping medium to enhance the controllability for fast steering (tip-tilt) of lightweight mirrors. The direct relationship between passive damping and control highlights the merits of energy absorbing foams that have shown significant attenuation in high mode resonant frequencies (typically above 1 kHz) without adding significant mass. Studies by these researchers suggest that attenuation is coupled to standing waves through the foam. In this case study these foams have been applied to a 300 mm diameter aluminum platen that represents the final mirror geometry. In practice, mirrors for our application will be made from silicon carbide. The platen is designed with an open-back honeycomb structure and high-speed machined from 6160-T7 aluminum. Three dominant mode shapes were observed at approximately 1140, 1350 and 2100 Hz with high ‘ Q’. In general, the platen or any adaptive structure may require high-bandwidth dynamic control. Fundamentally, the high ‘ Q’ values limit the control dynamics. This paper shows the benefits of energy absorbing foams as a passive damping medium for large lightweight structures. The foams are strategically placed inside the honeycomb pockets at the back of the mirror and experimental investigations demonstrate damping coefficients up to ξ = 0.12 (factor of 60 improvement) compared to a ξ ∼ 0.002 with no added passive damping in the system. Furthermore, the energy absorbing foam only adds 6% additional mass to the overall lightweight platen. As a result of this high damping coefficient, the Q values were attenuated by 25–50 dB for all high-frequency modes and as a result the controller parameters (i.e. such as PID) may be further optimized and provide faster settling times. Finally, the results obtained from modal testing, the relationship of damping and controller gains and potential benefits of this technique for large, lightweight structures are discussed.