Aerotel Chopper

Time period

Summer 2002 to December 2002

Project description

The Aerotal System is an airborne Lidar instrument based out of the NASA Goddard Space Flight Center and is jointly operated by GSFC and NASA Langley. As the laser light leaves the aircraft, it passes through a window that separates the cabin environment from the outside.

Some laser light reflects off this window as well as the atmosphere near the aircraft. These near reflections are not wanted but are still gathered by the telescope and can saturate the detectors. This makes the detectors less sensitive when the desired signal from farther distances reaches the telescope.

To combat this situation, Welch Mechanical Designs (WMD) used multiple approaches. The first solution was to gate the detectors. With this approach, special electronics turned the detectors off for the short time it took the laser beam to move a kilometer away from the aircraft. However, this approach did not work well for a very weak return signal since the unpowered detector could still show saturation effects when the power was returned to the detector.

The second solution required blocking all collected light from reaching the detector. For this system, a chopper blade spun and crossed just in front of the field stop of the telescope. The tangential velocity and the size of the beam of light determined how rapidly the detectors saw the incoming beam of light appear once the chopper passed the field stop. This system spun at 12,000 RPM to achieve a turn on time of __microseconds.

This system was extremely challenging to build due to the aerodynamic effects that were not initially considered. The original solid spinning disk had too much surface area and the maximum rotation rate was approximately 9,000 RPM before the motor's capacity was reached. The final chopper blade has a greatly reduced surface area to allow it to run stably just above the required 12,000 RPM.

This system was used successfully during the SOLVE mission in early 2003. The customer says the chopper improved the quality of their data and allowed them to take data at ranges that were not possible compared to previous missions.

Challenges and lessons learned

During this project we learned a great deal about aerodynamic effects acting on a high-speed disk. What was originally thought to be a negligible higher-order effect was actually a dominant effect. This project has taught us to be more aware of all the effects acting on a design. This will help to improve the quality of all designs we do in the future.

Future

WMD is currently researching methods to shrink the size of the chopper while increasing the rotation rate. This will improve packaging in future systems while maintaining or even decreasing turn on times.