FUNA Lunch Talk:

Ben Jewell

Ohio University and NRAO

Investigation of a Structural Health Monitoring System for ALMA 12-Meter Antennas

August 31

12:10PM, Room 230, NRAO, Edgemont Road


In February 2007, it was determined that a joint in one of the quadrapod legs supporting the subreflector of the North American 12-meter antenna had experienced a failure involving an adhesive bond between a CFRP member and an INVAR insert. This joint failure was distorting the shape of the primary reflector to the extent that astigmatism in the surface was observed. However, the joint failure could have potentially caused serious structural damage to the antenna or personnel injury during fast motion activities. What is of particular concern to NRAO engineers is that we have no means of knowing exactly how long this damage existed in the quadrapod structure. The summer project investigated methods for equipping the antennas with a "Structural Health Monitoring System" (SHMS). Of particular interest will be what structural information could (or should) be monitored, what equipment would be necessary, and how often the monitoring or assessment of the SHMS data would need to be done.

This summer project has focused on the resonant frequencies of the GBT and ALMA antennas. We have used the three-axis accelerometer system developed for the GBT as a test bed for this analysis. We have found that the data must first undergo a "clean-up" process to remove instrumental drifts and the slew and switching accelerations of observations. We report on the practicalities of automating the resonant frequency analysis and suggest how a working system for ALMA antennas and the GBT can be employed.