© Brüel & Kjaer Vibro ● S1079330.002 / V04 ● █ Page 99 of 188
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Brüel & Kjaer Vibro │ Instruction
VC-8000 MPS
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13.3 Waveform Data Collection Triggers
Each UMM channel is in an infinite loop of continuously sampling waveforms. The “triggers” described
in this section are used by the UMM channel to determine when the sample is to be saved. If the
sample is to be saved, the UMM channel stores the waveform until it is polled (queried) for the
waveform.
In a 16P rack, the waveform polling interval is every 5 seconds. In an 8P (or 4P) rack, the interval is
every 2.5 seconds. If a UMM channel samples a new waveform (during the 5 second interval), only
the most significant waveform is saved.
Note that “Boost mode” is a special case as explained in the sections below.
13.3.1 Delta Time (Dynamic Collection Rate (Time))
The dynamic collection rate (also called delta-time) specifies the periodic rate for collecting
waveforms. When the dynamic collection rate time elapses, a waveform is saved. The default is 20
minutes.
13.3.2 Delta RPM (Dynamic Collection Rate (RPM))
Changes in machine speed during machine transients (start-up or shut-down) are used to trigger
waveform dynamic collection. Each channel has its own Delta RPM setting (default is 60 RPM).
In the VC-8000 the Delta RPM trigger is always enabled. In other words, even when the machine is in
steady state (not transient), the Delta RPM trigger is still looking for speed changes.
For machines that run with few speed variations the best practice is to set the Delta RPM at 2-3% of
the normal machine speed. For machines that operate with lots of speed variation you may need to
set a much higher Delta RPM, well above the normal variation in running speed. For example:
Normal Speed Delta RPM Comment
100 RPM more than speed variation
40,000 +/- 2000 RPM
200 RPM more than speed variation
Remember that Delta RPM is only one of the waveform triggers during a transient speed event. The I-
Factor will also trigger waveforms.
If Delta RPM is set too low, waveforms will be triggered constantly – during normal
(steady state) operation – degrading the performance of the system.
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