We take vibration measurements on existing structures for a number of reasons: a) to verify a vibration complaint, b) to assess ambient vibration levels to determine whether it is within the specifications of a sensitive equipment or a prospective tenant, c) to verify and update the engineering models we use to design effective structural solutions. We also conduct vibration site surveys to assess the ambient vibration levels and use the data to estimate the vibration levels of a future building proposed on that site.
A series of vibration tests investigated a surgical microscope that was experiencing excessive vibrations. On multiple occasions, neurosurgical cases had been disrupted due to the vibrations of the microscope. The task was to determine the source(s) of the vibrations and then propose solutions to eliminate or alleviate unwanted vibration levels.
The microscope manufacturer did not provide any specific vibration limits. Generic vibration criteria available in literature, however, stated that the limit should be 1,000 micro-in/s (mips) in root-mean-square (RMS) for the floors of microsurgery rooms. No vibration criteria specific to the eye of the microscope was found in the literature.
First, potential vibration sources were determined. This included human walking, the internal electronics of the microscope itself, the intensity of the air flow inside, and other medical equipment used in the building. Next, accelerometers were installed on the floor and on the microscope at multiple locations, and vibrations were recorded for one of the vibration sources at a time in a controlled manner.
The largest vibration levels measured at the microscope eye were around 21 Hz. Disruptive vibration levels were observed only when the motor of a condenser water pump operated at 1,260 RPM on the mechanical floor.
The pump controller was reprogrammed to skip the frequencies near 21 Hz which eliminated the resonance problem, hence, the vibration issue was solved with minimal cost.
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