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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. 

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.

knowledge, resource center, articles, data management, monitoring ground vibration
knowledge, resource center, articles, data management, monitoring ground vibration

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 0.5 Hz and 8 Hz are on a line, indicating constant acceleration for curves VC-A and VC-B. 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 also provides a vibration curve that the floor must maintain for proper operation of the installed machinery.

In these types of installations, the curves are typically more than just simple straight lines. The red line below is a typical curve given by a vibration isolation platform, and which guarantees proper operation of the device.

Floor monitoring: A practical solution

HBK has been providing systems for monitoring vibration for many years. Tescia®, a new[SC1]  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 velocity 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.

Example of a simple VC monitoring configuration in Tescia

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 six 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.

knowledge, resource center, articles, data management, monitoring ground vibration

Example of 6-channel floor vibration monitoring showing exceedance of vibration criteria.

The lower part of the screen dump shows the vibration level over time.

Tescia monitoring methods

Tescia software can be used to monitor a manufacturer’s floor. In this case putting triaxial accelerometers at the corners and edges of the building allows the system to create seismic measurements which are periodic. This periodic analysis can be used to show long-term, time-stamped nth-octave and narrow-band FFT values for each period, which could be as small as 10 seconds and as long as hours. This allows manufacturers to look at the influence of the vibration on their operation. Our software can create spectrum versus time inside an Excel® spreadsheet. This allows users to create reports quickly.

Tescia software can also be used simultaneously to monitor the floor or the base of the machine anywhere on the production floor. It can monitor all sensors and have different failure criterion for each channel of acceleration, velocity, or displacement. When a channel’s vibration goes above its individual profile, the software can put away special analysis data and time series recordings for further detailed analysis after the fact. This same triggered event can close a relay, email or text a group of people the analysis plot, which can warn the manufacturer that they may need to double check their semi-processing chain for errors.

Semiconductor Processing Floor

Semiconductor processing floor

Triaxial Monitoring Points (Yellow circles)
Semiconductor Processing Machines (Blue squares)

Measuring capabilities with HBK

The Tescia system uses HBK LAN-XI hardware, which allows the user to distribute the data acquisition of the accelerometer measurement out near the accelerometers. This reduces the noise and the use of expensive signal cables from the accelerometers to the measurement hardware. Common Ethernet cable can be run for 6- or 12-channel modules. Each module can also be put into a hermetically sealed box for extreme locations.

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