Express and non-invasive dynamic testing of Integrated Dewar-Detector-Cooler Assemblies

Ina concept of Integrated Dewar-Detector-Cooler Assembly, the entire cold assembly (substrate, infrared focal plane array, cold shield and cold filter) is mounted upon the distal (cold) tip of the thin-walled metal cold finger having no mechanical contact with the warm shroud of the evacuated Dewar. This forms a slender, tip-mass cantilever, the dynamic properties of which depend, amongst other factors, on the nonlinear interaction (rattling) of the cold finger tube with displacer cartridge arranged slidably inside the cold finger bore. The resonant frequency of such a weakly nonlinear and lightly damped mechanical system usually falls within the range of operational vibration profiles. This results in extremely high susceptibility to vibrational extremes. Explicit knowledge of damping ratios and resonant frequencies is, therefore, critical for predicting dynamic responses under adverse environmental conditions. Unfortunately, because of the above-mentioned nonlinearities, analytical assessments are not feasible. Experimental evaluation involves making mockups instrumented with contactless sensors; the entire procedure is quite laborious, expensive, requires special equipment/skills, is not always accurate and is not applicable to evacuated and sealed operational IDDCA. The authors suggest a non-invasive and express approach to evaluating the above mentioned modal properties. The procedure relies on measuring the local frequency response function (accelerance) of the cold finger base and applying a curve-fitting procedure for extracting the dynamic properties of Focal Plane Array (modal mass, frequency and damping). Theoretical predictions are supported by a full-scale experimentation.