Date: Jul 15, 2010 Author: Joe Singleton Source: MDA (
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by Joe Singleton/jsingleton@nttc.edu
Night-vision goggles, building security access devices, and law-enforcement reconnaissance systems could become smaller and lighter, if the promise of an MDA-funded cryogenic technology for infrared imaging comes to fruition.
To reduce the size and weight of infrared imaging components, Virtual AeroSurface Technologies, Inc. (VAST; Atlanta, GA), is developing a miniaturized pulse-tube refrigerator to cool sensors.
The agency originally funded VAST in 2008 through concurrent SBIR and STTR Phase II contracts—both joint projects with Georgia Tech and Raytheon—to develop a pulse tube and compressor for a miniaturized, solid-state, hermetically sealed, thermo-acoustic refrigeration device. MDA had interest in such technology for its potential as a more efficient alternative to the power-gobbling thermoelectric coolers used on missile defense platforms. VAST was able to parlay knowhow from the MDA work to develop its current miniaturized refrigerator offering.
Sensor-cooling refrigerators are integral components of high-end infrared (IR) camera systems—reducing the thermal noise and enabling the camera's high sensitivity. VAST's miniaturized pulse-tube refrigerator should fit the bill for what IR camera makers seek. The company's goal is to develop a refrigeration device capable of cooling a focal plane array to 160 K. Its small size—about one-tenth the size and weight of competing small-scale cryocoolers, and small enough to fit in an enclosure the size of a 10-cubic-centimeter syringe—adds to its appeal.
The company's work has focused on an improving a technology that already existed: the pulse tube refrigerator, a thermo-acoustic refrigeration device that uses pressure waves to produce cold and hot points within a pulse tube (essentially a tube filled with helium gas). A pulse tube includes no moving parts, although a compressor attached to the end of the tube creates pressure oscillations that drive the device. Because there are no moving parts, there is minimal vibration at the interface between the tube and the sensor being cooled. Less vibration means a more reliable sensor.
For the compressor part of their system, VAST researchers have reduced its size by developing coils that can be packed more densely than standard wire-wrapped coils. The VAST coils are fabricated using microelectromechanical systems (MEMS) techniques. VAST researchers also are developing their compressor to operate efficiently at the rates necessary for a small pulse tube. The company has determined that a small pulse tube requires the compressor to run at a higher frequency of strokes—the reciprocating motion cycles within a compressor. Most competing, large-scale cryocoolers have compressors that run at about 60 Hertz. VAST's miniaturized product requires a frequency of at least 200 Hertz.
VAST President Tom Crittenden said the company's cryocoolers can be designed, based on the application, to cool sensors within a large temperature range—from 70 K to 200 K—through a process known as "staging." Staging involves cooling different sensors at different temperatures, in an orderly manner, allowing for a net reduction of energy needed to run the system. No other small-scale cryocooler offers such a dynamic range of temperature options or the ability to stage cooling operations, according to Crittenden.
VAST's cryocooling technology also offers solid-state dependability. Crittenden said other cryocoolers do not have the 10-plus-years lifespan of a VAST cooler.
The company continues to fine-tune its miniaturized refrigerator and is still seeking partners or system integrators.
