Monarch Instrument Nova-Strobe vbx - Manual (Page 17)

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Once set up, you have to tune the strobe to set the center frequency of
the filters to work with the accelerometer. Using the Internal Mode,
adjust the flash rate of the strobe to stop motion of the target of
interest. Press the Input/Memory button until the LOCK icon is on.
The strobe will now use the signal from the accelerometer to control
the flash rate using the wide band filter lock. Press the Input/Memory
button again to go into the narrow band filter lock. The LOCK icon
will blink.
The Narrow and Wide Bandwidth filters can be used to optimize its
ability to lock onto the vibration signal at any speed. The Wide
bandwidth will allow the strobe to track the signal over a fairly wide
range of speed change and should be used when tracking the fundamental
frequency (in simple systems).
On machines such as a gear drives, reciprocating engines, or any drive
with multiple fundamental frequency excitation, the strobe could
encounter difficulty tracking the designated signal, due to the tracking
filter’s normal bandwidth. If substantial levels of multiple fundamental
frequencies, or fundamentals with harmonics occur within a selected
bandwidth, the shaft reference may appear to oscillate, or drift
substantially, when viewed with the Nova-Strobe vbx. Also, if the
accelerometer cannot be positioned on the primary point of interest, a
fundamental frequency of something other than that of the primary
point of interest may be predominant. In these cases, use the Internal
(INT) Mode and flash rate adjust to stop the motion of the primary
point of interest, then use the Narrow Bandwidth LOCK Mode. The
filter is far more sensitive and will better be able to discriminate the
required signal. However, it will not be able to track over widely
varying speed changes, which will require retuning in the Internal (INT)
Mode.
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Example: Object Rotating at 5000 RPM
If the speed is outside the full scale range of the stroboscope (50,000
FPM), it can be measured using the method of harmonics and multipoint
calculation. Start at the highest flash rate and adjust the flash rate down.
You will encounter multiple images so be aware of these. Note the flash
rate of the first SINGLE image you encounter, call this speed “A”. Continue
decreasing the flash rate until you encounter a second SINGLE image.
Note this speed as “B”. Continue decreasing the speed until you reach a
third SINGLE image at speed “C”.
For a two point calculation the actual speed is given by:
RPM = AB/(A-B)
For a three point calculation: RPM = 2XY(X+Y)/(X-Y)2 where
X = (A-B) and
Y = (B-C)
If a Remote Optical Sensor or Magnetic Sensor is used to sense one pulse
per revolution (External mode), the readout will display directly in RPM
(FPM) without any adjustment required.
In instances when you can shut down the device and install a piece of
reflective tape, then an optical tachometer is easier to use for RPM
measurement. Stroboscopes must be used when you can’t shut down
the device. The human eye is not easily tricked into seeing a stopped
image by a stroboscope when the flash rate is slower than 300 FPM.
Therefore, stroboscopes are just about impossible to use below 300 FPM
for inspection or to measure RPM.
Stopped Image 1/4 times 1/2 times 1 time 2 times 3 times 4 times
Flash Rate (FPM) 1250 2500 5000 10000 15000 20000