Polytec PSV-500, Hardware Manual

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4-3
4 Making Measurements
When there is an extremely weak optical signal, a permanently Overrange
display can be caused by noise due to the system. If switching on the tracking
filter (refer to
SECTION
4.2.2) does not remedy this problem, check the
following:
•
Have you focused the laser optimally?
•
Can you treat the surface of the object under investigation for optimizing
the reflectivity ?
•
Can you change the stand-off distance ?
4.2.2 Tracking Filter
The tracking filter can be used to improve the signal-to-noise ratio of the
signal from the sensor head. This filter bridges brief dropouts. These dropouts
occur due to the speckle nature of the light scattered back from the object.
Thus the tracking filter increases the optical sensitivity of the vibrometer.
There are four tracking filter settings available: Slow , Medium , Fast , and Off .
With the Off setting you can switch off the tracking filter.
The different tracking filters have different maximum accelerations. The lower
the maximum acceleration, the higher the optical sensitivity.
With the Fast setting, the tracking filter is always adjusted to the highest
possible acceleration values that result from the bandwidth set and the
selected measurement range. Thus, the Fast setting is suitable for all
velocities and frequencies that may occur within the measurement range.
With the Medium setting, the tracking filter is optimized for a maximum
acceleration of 10% of the Fast setting. If the acceleration does not exceed
this value, the Medium setting has a better effect than Fast . It is suitable for
noise excitations, as generally in this case the maximum accelerations of the
measurement ranges are not fully used.
With the Slow setting, the tracking filter is optimized for a maximum
acceleration of 1% of the Fast setting. It is suitable if the velocity range is
nearly saturated by a low-frequency distortion and at the same time the actual
high-frequency wanted signal only has a low amplitude. This is particularly the
case with measurements on rotating parts. The Slow setting has the highest
optical sensitivity.
In addition, the different tracking filter settings are designed for different cutoff
frequencies.
The tracking filter can also be used for bandwidths that exceed the respective
cutoff frequencies of Slow , Medium , and Fast . This is particularly useful for
measurements in the time domain as in this case, to obtain a better time
resolution, you often select a higher bandwidth than it would be required by
the signals. Signal components above the respective cutoff frequency are
damped with 15 - 30 dB / decade.
Setting Cutoff frequency
Fast 1 MHz
Medium 500 kHz
Slow 200 kHz