Rugged IR camera withstands special treatment

San Diego, CA--Like James Bond, Special Forces operatives get to play with lots of high-tech gadgets. Take the LOTIS-D (Laser Overlay Thermal Imaging System-Digital) infrared camera. Developed by Science Applications International Corporation (SAIC) for the Office of Special Technology (OST), the dual-band high-resolution system operates at 3 to 5µm and 1.06µm wavelengths.

It functions as a passive night-vision telescope and can peer through smoke and fog at ranges as great as 3 km. Or, using its built-in 1.06µm laser, it can serve as a target designator with the ability to overlay the laser spot on the thermal image. External RS-170 or RS-232 outputs allow users to capture images to file or for transmission.

Tipping the scale at 9.6 lbs, the digital device is less than half the size and weight of its analog-only predecessor. That weight is especially impressive considering that the OST-supplied core components weigh 7 lbs, excluding circuit boards, control panel, and mil-spec protective housing. "If all I did was wrap 0.060-inch-thick aluminum around the envelope, I was already a couple of pounds over spec," says Brett Sanders, senior mechanical engineer for SAIC's Military Products Division.

Sanders' housing needed to contain a dual-band 200-mm f2 catadioptric lens, a Stirling-engine cryogenic cooler supporting a 256 × 256-pixel platinum-silicide focal-plane array chilled to about 70K for improved thermal resolution, batteries, power supply, control panel, and digital and analog electronics. The unit would also have to survive a 3-ft fall to concrete, vibration testing, and battle environments that included tropical jungles and moonlit raft rides launched from surfaced submarines.

A plastic case would have been light, but not strong enough. So Sanders turned to that strength-to-weight champion, carbon-epoxy composite. He analyzed prior housing designs and saw that engineers had elected to mount the lens and Stirling engine separately. Because of the critical relationship between the two components, this solution required a stiff and heavy structure.

Instead, Sanders created a light but rigid magnesium "metering structure" to which both the lens and Stirling cooler mount. The tough but compliant housing attaches only to the lens and not the cooler. "It's like an egg crate," he says. "You allow for flexure of the housing to take up impacts and loads." This approach produces a shock-isolated sub-assembly, eliminates relative movement between the two components, and maintains a precise focal length.

An extra strip of composite wraps around the unit over the lap joint between the housing's upper and lower halves. Although the resulting package is strong enough to stand on, the housing weighs just 2.3 lbs.

Sanders redesigned the CRT viewfinder as well. "Originally, it would light your face up as you started to look through it," he explains, "not too good for Special Forces." A past solution made use of a molded rubber eyepiece that opened when pressed against the face. This design was difficult to produce and tiresome to use.

As an alternative, Sanders designed a spring-loaded sliding sleeve that, when pressed, triggers a microswitch and activates the image on the CRT. By turning the eyepiece outer barrel, users can lock the eyepiece in the ON position, if desired.

Just as in many other fast-track projects, computer design tools played a role in shortening the design cycle. On day one, Sanders put together a detailed Microsoft Excel spreadsheet that included every screw, washer, and gasket. It proved so accurate that the final design matched the estimate to within 0.10 lb.

The housing, keypads, eyepiece, and other components were designed with AutoCAD's Advanced Modeling Extension (AME). Sanders converted the AME surfaces to wireframes and exported the geometry to Cadkey's Fastsurf program to drive a CNC milling machine that created molds for the housing.

Additional details…Contact Brett Sanders, SAIC, 4161 Campus Point Court., Bldg. E, San Diego, CA 92121, (619) 458-3847.