VC Vibration Criteria Assessment

The module translates measured site vibration into the class language used by equipment suppliers and acceptance specifications. Suppliers typically state site requirements such as “the floor must meet VC-C”, and clean room acceptance uses the same set of curves. The measured 1/3-octave velocity spectrum and all 19 criterion curves are drawn on one logarithmic chart, so it is clear which band lies near which curve, whether it exceeds it and by how much.

Screen layout

The 19 criterion curves

From VC-A to VC-G the velocity limit halves from class to class (about −6 dB per class). Each curve can be shown or hidden; the target class is drawn bold, and the curves are colored on a gradient from strict to lenient.

Automatic statistics and color-coded results

Compliance rate and margin cells are colored green (clear compliance), yellow (marginal) or red (exceeded). The final class is rated excellent (VC-E to VC-G), good (VC-C to VC-D), fair (VC-A to VC-B) or poor (exceeded), so readers unfamiliar with VC classes can understand the result.

Supporting functions

• Exponential averaging with adjustable count (default 10) for stable, repeatable readings.
• The 1/3-octave spectrum can be shown as points or as a line.
• Stay on top for comparison with other windows.
• Chinese and English interface.
• Export all data: the statistics table and the velocity spectra of all channels open in Excel.

Clean room and precision building acceptance

New semiconductor, optoelectronic and precision laboratory buildings must show before handover that the micro-vibration environment meets the process class. The module gives the VC class and minimum margin per 1/3-octave band, a result accepted by the acceptance party and a basis for corrective work.

Site assessment before equipment installation

Purchase contracts for electron microscopes, lithography or exposure equipment and laser interferometers often specify a site class such as VC-D. Measuring the site before delivery confirms the compliance rate and minimum margin and reveals non-compliant bands before the equipment arrives.

Source identification and vibration control

Peak frequency and the energy shares point to the source: a high low-frequency share usually relates to traffic or microtremor, a high mid- or high-frequency share to plant equipment and piping. Measured before and after treatment, the curve comparison and margin in dB quantify the improvement.

Long-term monitoring

With live acquisition, exponential averaging keeps the class reading stable, and the stay-on-top window supports continuous monitoring of critical areas.

Training and demonstration

All 19 curves on one screen, with a one-step target change and a description of suitable equipment for each class, explain clearly what a VC class means.

Processing chain

Why velocity

The octave scale divides the spectrum into bands of constant frequency ratio. Each 1/3-octave band extends from f × 2^(−1/6) to f × 2^(1/6), a width of about 23 % of the center frequency. The energy of the lines in the band is summed and the RMS taken to give the band velocity (RMS, µm/s). Precision equipment is mainly sensitive between 1 and 100 Hz, where the effect of floor vibration on precision processes correlates better with velocity; the VC curves therefore use RMS velocity. Acceleration and displacement are also calculated with a = v · 2πf and d = v / (2πf), so the three values can be compared.

Class determination: weakest-link rule

The final class is found by checking the 21 bands from the strictest class (VC-M) to the most lenient (ISO workshop). If any band exceeds a class limit, the final class drops to the next class. The final class is the strictest class met by all bands at once. This follows the acceptance practice that any band exceeding the limit means non-compliance.

Compliance rate and minimum margin

Compliance rate = bands meeting the limit ÷ valid bands × 100 % Minimum margin = min[20 · log10(limit / measured)]

Both refer to the selected target class (for example VC-C for an equipment installation), counting only bands where the class is defined. The minimum margin is how many dB the most critical band has left before the limit. In practice at least 6 dB is preferred, so the spectrum keeps a margin as conditions change; 0–6 dB is marginal and a negative value means exceeded.

Special curves

• The four ISO curves: constant velocity at higher frequencies, extended at constant acceleration in the 4–6.3 Hz range (equal-perception correction).
• NIST-A: VC-E limit above 20 Hz; between 1 and 20 Hz a constant displacement of 0.025 µm converted to equivalent velocity in each band, reflecting displacement sensitivity at very low frequencies.
• NIST-A1: 3 µm/s constant velocity below 4 Hz; 0.75 µm/s from 4 to 100 Hz.

• All 19 criterion curves built in, from workshops to nanometer laboratories, without tables or manual drawing.
• Three results on one screen — final class, compliance rate and minimum margin — answer whether the site qualifies and by how much.
• Color-coded results that readers without VC knowledge can understand.
• One-step comparison with the target class, with per-band compliance and the margin of the most critical band.
• Velocity, acceleration and displacement calculated together to identify the nature of the source.
• Energy shares point to traffic-type or equipment-type sources.
• Eight channels evaluated in parallel for a complete floor survey in one acquisition.
• Automatic conversion of g, m/s², mm/s² and in/s², with warnings for invalid units.
• Exponential averaging for repeatable results in monitoring and repeat acceptance tests.
• Excel export of parameters, statistics and all spectra for acceptance reports.
• Shares the data chain with the spectrum and order tracking functions of the T8 platform.

1. Select channels (the first enabled channel is selected by default).
2. Select the target class required for installation or acceptance (default VC-C); its curve is drawn bold.
3. Refresh; the chart and table update. Enable exponential averaging for stable readings.
4. Read the result in the last three columns of the table: compliance rate (colored), minimum margin (dB) and final class (excellent, good, fair or poor).

Recommended settings

Interpreting the result

First the peak frequency: in which band the energy lies, compared with known sources such as air handling, pumps or traffic. Then the energy shares: a high low-frequency share points first to traffic or microtremor, a high mid- or high-frequency share to plant equipment. Then the margin: the band with a margin close to 0 dB is the weakest link and the target of corrective work. Reports should give both the final class and the minimum margin, for example “final class VC-C (good), minimum margin 4.2 dB”.

Data export

The export contains a parameter header (sampling rate, frame points, lines, analysis frequency, resolution), the 13-column statistics table and the 1/3-octave velocity spectra of all channels, opened in Excel with automatic column widths.