© Brüel & Kjaer Vibro ● S1079330.002 / V04 ● █ Page 131 of 188
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Brüel & Kjaer Vibro │ Instruction
VC-8000 MPS
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The maximum signal frequency that can be detected is 180,000 Hz. For example, If the machine
speed is 3,600 rpm and the event ratio is 40, the expected signal frequency (at running speed) will be
144,000 Hz.
20.3.3 Auto Threshold and Hysteresis
Auto threshold measures the peak-to-peak phase trigger signal and sets the threshold at the
midpoint. The auto threshold value is updated each revolution. Auto Threshold is the most typical
setting and normally works just fine.
Hysteresis is a dead-band region (centered on the threshold) that provides noise immunity. Half of the
hysteresis is applied above the threshold and half below. The default hysteresis value of 2.0 Vdc is
great for most applications.
Figure 20-4) Detecting a phase trigger pulse
20.3.4 Threshold (manual)
If the phase trigger is not triggering correctly, you may have to set the threshold manually. You will
need an oscillosope (or CMS data) to observe the behavior and range of the signal, and then you can
correclty set a manual threshold.
The configured threshold specifies the center of the triggering region as shown in Figure 20-4.
Typically, this is set at the midpoint of the signal pulse.
When Auto Threshold is enabled, the manual threshold value is ignored.
20.3.5 Trigger Type
Setting the triggering to “Notch” causes the Phase Trigger event to occur on a falling sensor signal.
Setting the triggering to “Projection” causes the Phase Trigger event to occur on a rising sensor
signal.
20.3.6 Phase Trigger Installation
Use caution when installing the Phase Trigger sensor. The probe’s surface target (on the shaft) must
be known. Is the probe currently viewing a notch, a projection, or a portion of the notch and a portion
of the shaft (50/50)? The Gap voltage will vary greatly depending on the probe’s surface target during
the installation process.
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