David G. Watters

Shape control of large lightweight mirrors with dielectric elastomer actuation

Space-based astronomy and remote sensing systems would benefit from extremely large aperture mirrors that can permit greater-resolution images. To be cost effective and practical, such optical systems must be lightweight and capable of deployment from highly compacted stowed configurations. Such gossamer mirror structures are likely to be very flexible and therefore present challenges in achieving and maintaining the required optically precise shape. Active control based on dielectric elastomers was evaluated in order to address these challenges. Dielectric elastomers offer potential advantages over other candidate actuation technologies including high elastic strain, low power dissipation, tolerance of the space environment, and ease of commercial fabrication into large sheets. The basic functional element of dielectric elastomer actuation is a thin polymer film coated on both sides by a compliant electrode material. When voltage is applied between electrodes, a compressive force squeezes the film, causing it to expand in area. We have explored both material survivability issues and candidate designs of adaptive structures that incorporate dielectric elastomer actuation. Experimental testing has shown the operation of silicone-based actuator layers over a temperature range of -100 °C to 260 °C, suitable for most earth orbits. Analytical (finite element) and experimental methods suggested that dielectric elastomers can produce the necessary shape change when laminated to the back of a flexible mirror or incorporated into an inflatable mirror. Interferometric measurements verified the ability to effect controllable shape changes less than the wavelength of light. In an alternative design, discrete polymer actuators were shown to be able to control the position of a rigid mirror segment with a sensitivity of 1800 nm/V, suggesting that sub-wavelength position control is feasible. While initial results are promising, numerous technical challenges remain to be addressed, including the development of shape control algorithms, the fabrication of optically smooth reflective coatings, consideration of dynamic effects such as vibration, methods of addressing large-numbers of active areas, and stowability and deployment schemes.

Through-the-wall differential radar

We have designed and built an ultra-wideband differential radar that images targets moving behind walls, fences, trees, and other obstacles. We have demonstrated the differential-radar concept in anechoic, indoor, and outdoor environments. The outdoor demonstration proved the radars ability to detect people moving behind walls and illustrated its capability for suppressing high-level clutter from the radar imagery.

Design and performance of wireless sensors for structural health monitoring

Wireless sensors can be realized by integrating a sensor with a passive commercial radio-frequency identification (RFID) chip. When activated, the chip responds with a digitally encoded signal that not only identifies the sensor but also contains information about the sensor state. Two devices have been developed to date: a temperature-threshold indicator and a chloride-threshold indicator. This paper discusses basic concepts, design issues, and preliminary performance.

Wireless subsurface microsensors for health monitoring of thermal protection systems on hypersonic vehicles

Health diagnostics is an area where major improvements have been identified for potential implementation into the design of new reusable launch vehicles in order to reduce life cycle costs, to increase safety margins, and to improve mission reliability. NASA Ames is leading the effort to develop inspection and health management technologies for thermal protection systems. This paper summarizes a joint project between NASA Ames and SRI International to develop SensorTags, radio-frequency identification devices coupled with event-recording sensors, that can be embedded in the thermal protection system to monitor temperature or other quantities of interest. Two prototype SensorTag designs containing thermal fuses to indicate a temperature overlimit are presented and discussed.

Detection of chemical contraband using spectroscopic microwave imaging

We have developed and demonstrated a microwave technique for detecting high explosives, illegal drugs, and other chemical contraband in checked airline baggage. Our technique isolates suspicious materials using microwave tomography and identifies chemical contraband using microwave spectroscopy. Measurements in the frequency range 2 - 18 GHz indicate that microwave energy will penetrate nonmetallic suitcases and that contraband materials feature distinct dielectric spectra at these wavelengths. We have also formed microwave images of a soft-sided suitcase and its contents. After manually segmenting the microwave imagery, we successfully identified chemical simulants for both high explosives and illegal drugs.

Spatial modulation display using spatial light modulators

A spatial modulation display is described that permits the observation of a phantom or transparent image by several persons simultaneously and is suitable for medical imaging. The display uses spatial light modulators and large format convex lenses within a Schlieren optical system. The number of sectional images in a three-dimensional image is limited by the number of spatial light modulators. The display is electro-optical and requires no moving parts.