Equipment: A microscope mounted from the ceiling of a surgery room. The special microscope was used by the surgeon during micro-surgery operations which involved replanting severed fingers and toes.
Symptoms: The chief surgeon complained that the image was jittery and that it was very tiring to operate under those conditions, particularly when the scope was set for its maximum magnification.
Test Data and Observations: The scope was set to its greatest magnification and printed material was placed on the operating table. Vibration was clearly noticeable, just as the surgeon had indicated. Vibration spectra were taken on both the table and the microscope. The levels on the table were very low across the spectrum. However, the levels on the microscope were significant. A view of the vibration spectra revealed that peaks were present at 225 CPM and 435 CPM. To trace the source of the vibration, levels were measured on the top of the microscope’s isolator and on the structural steel supporting the isolator . It was discovered that the levels on the isolator were seven times higher than on the steel support.
This meant that instead of isolating the microscope from the structural vibration, the isolators were actually amplifying the vibration which was present on the I-beam. To determine the cause of the amplification, an impact test was performed on the microscope to determine its natural frequencies. It was found that the natural frequencies of the scope on its isolation system matched the vibration that was present on the I-beam.
Conclusion and Recommendations: Isolators perform their isolation function by creating a system with a natural frequency tuned much lower than the expected disturbing frequency. This in turn creates a mechanical low pass filter, which will not pass the higher frequencies. A problem can, however, occur if a low frequency is present near the low tuned natural frequency of the isolated system. Instead of isolating the frequency, the isolators will then actually amplify the levels. The solution in this case was to ground out the isolators. When the isolators were grounded out, the levels on the scope dropped to acceptable levels. The frequencies that had been present were due to isolators on fans above the room being tuned to the same frequencies as the microscope. Flow excitation in the fans excited the fans’ isolated natural frequencies, which were transmitted through the structural steel and amplified by the microscopes isolators. Grounding of isolators should only be tried if nothing else works. When the isolators are grounded, higher frequencies, if any are present, will obviously pass through. In this particular case, grounding out the isolators didn’t introduce any significant higher frequency vibrations.
