Monitoring ground vibration

Tescia - Ground Vibration Monitoring

Many high technology applications and processes are very sensitive to vibration. Among these processes are semiconductor R&D and production, precision metrology, long-beam-path laser applications, high energy physics, medical surgery facilities and some biotechnology research.

Sensitive processes

Typically, these applications require specially designed facilities to accommodate heating, cooling, humidity, and vibration isolation requirements. In the case of semiconductors, the smaller their size, the more critical the effects of vibration. Many semiconductor fabrication machines must now be placed on active vibration isolation platforms, or the machines themselves have built-in vibration isolation. In both cases, machine manufacturers set minimum vibration criteria for proper operation of their machines.

Buildings are subjected to many sources of vibration – traffic, construction, pumps, pedestrians walking, to name just a few. After many years of research, the industry has tried to understand the physics of floor vibration and how it affects machinery. In the diagram below, you can see vibration criterion/criteria (VC) curves. These curves are represented in velocity. In velocity, the curve for values ranging from 8 Hz to 80 Hz is a flat line. Signals between 4 Hz and 8 Hz are on a line, indicating constant acceleration starting at 8 Hz. These curves are general criteria and in most cases are used as guidelines. Most vibration sensitive machinery is provided with vibration curves that allow the machine to operate properly. Often, these machines are mounted on top of vibration isolation platforms, for which the platform manufacturer will also provide a vibration curve that the floor must maintain for proper operation of the installed machinery. In these types of installations, the curve is typically more than just two lines. The red line below is a typical curve given by a vibration isolation platform, and which will guarantee proper operation of the device.

VC-curve 

Floor monitoring – a practical solution

HBK has been providing systems for monitoring vibration for many years. Tescia, a new software platform has been designed to monitor and record acceleration, velocity or displacement. Used with HBK’s LAN-XI data acquisition hardware, Tescia can interface directly to seismic accelerometers. The Tescia software can integrate or double integrate the acceleration to create velocity and displacement signals. Tescia can monitor the velocity of the accelerometers visually using either FFTs or 1/nth-octave displays. A standard method is to monitor the 1/3-octave plots for each accelerometer being monitored. As well as visual monitoring, Tescia can simultaneously check all frequencies of each signal to determine if their level exceeds a set of user-defined reference profiles. Each channel can have a different profile and time above profile programmed to create a trigger. This allows sustained warning levels and/or instantaneous alarm levels, which can create a trigger. Triggers can be used to send emails, text, start recordings with pre-triggers and create analysis. For many floor monitoring applications, VC curves are used, but in many cases, for example, a vibration isolator, a curve is given based on the device on the isolator and the capabilities of the isolator itself. Tescia has no problem monitoring these more complicated profiles.

 

Just one out of many examples

In the screenshot below, you can see an example of a simple VC monitoring configuration in Tescia. This Tescia application monitors the 1/3-octaves of a VC curve line in the spectrum. When the signal goes above the red line, a recording with a 20 second pre-trigger is started, and a screen capture is sent via text or email to a phone or computer. Tescia can monitor hundreds of channels simultaneously, where each channel can have separate exceedance criterion. This example monitors 12 channels using the same criterion. Tescia gives users access to data recorded while the system continues to monitor each channel. The Tescia system does not have to be stopped to gain access to the times series or frequency base analysis at the triggered events. As demonstrated, Tescia is the perfect tool for hands free, long-term vibration monitoring, providing users with a rich amount of capability to analyze what happened during a triggered event.

Floor monitoring app note 

Example of 6-channel floor vibration monitoring showing exceedance of vibration criteria.
The lower part of the screen dump shows vibration level over time.

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