The Naval Prototype Optical Interferometer (NPOI) is being constructed in Flagstaff, Arizona. The project manager and client for the Vibration Reduction Design is Dr. Nat White of Lowell Observatory. The basic concept of an interferometer is a series of mirrors (elevator cans) working in conjunction to capture a high resolution image. Small vibrations in these elevator cans are creating distorted images. These vibrations exceed 1 micron in amplitude. The object of the VRD team is to provide vibrational analyses of the elevator can utilizing raw data and finite element analysis. The target completion date of the entire project is May 4, 2001.
Analysis
A vibrational analysis was performed on the actual elevator can and the models. Vibration data was collected using a accelerometer to measure vibrations. Once the data was collected it was run through a Fourier Transform. A Fourier Transform gives the magnitude of acceleration forces at each frequency. The natural frequencies can be determined from this data. The actual elevator can had a natural frequency at 15 Hz and the models' natural frequencies ranged from 350-425 Hz. The data was then integrated twice to find the actual displacement of the tests. Before integrating, a high and low bandpass Butterworth digital filter was run on the data to remove any noise.
Recommendations
Using all of our forms of analysis, we recommend the use of a spring-mass or stiffening bar system to reduce the elevator can vibrations. Our first choice would be the stiffening bar due to its ability to not only quickly dampen out vibrations but to also reduce the initial displacement. The drawback is the stiffening bar must be attached to the can and also anchored to another concrete base. If another base cannot be implemented the spring mass system would be ideal. It would not reduce initial displacement but would quickly dampen them out.