Technological innovations advance the state of the art for low size, weight, and power (SWAP) cryocoolers for high operating temperature infrared imaging. The authors present essentials of mechanical design, outcomes of theoretical modeling/optimization and initial testing for a cost-effective low SWAP split Stirling crycooler.
In а 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 author suggests 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.
The recent advent of high operating temperature infrared detectors allowed operating them at temperatures in excess of 150K without compromising performance indices typical of their 77K predecessors. These substantially relaxed cooling constraints along with a fundamentally higher coefficient of thermodynamic performance called for the development of a next-generation of low size, weight, and power cryocoolers purposely tailored for such high operating temperature detectors. Unfortunately, the most up-to-date low size, weight, and power cryocoolers are no more than downscaled and slightly lower-priced replicas of their low temperature predecessors. They commonly rely on gaseous helium as the working agent, metal stacked-screens regenerative heat exchangers, “moving magnet” dual-piston compressors, pneumatic expanders driven by differential pistons, mechanical springs, etc. Because of the inherent limitations, the currently achieved reliability and cost indices still prevent the broad use of cooled infrared imaging in the price-conscious and highly competitive commercial segment of the infrared imaging marketplace. From the very moment of its inception in 2018, CryoTech focused on disruptive innovation, enabling drastic reduction of ownership costs by deploying alternative, cost-effective technological solutions. The list of novelties includes but is not limited to: alternative working agent, low-cost microfiber regenerative heat exchanger, “moving iron” single-piston compressor with optional tuned dynamic counterbalancer, rodless pneumatic displacer, magnet springs, etc. The authors present the outcomes of the full-scale feasibility study and prototype life testing.
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