Polytec PDV-100, Руководство пользователя

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4-2
4 Making Measurements
4.2 Selecting Suitable Settings
In
SECTION
4.2.1 to
SECTION
4.2.3 you will find information on how to configure
the right controller settings for velocity measurements as well as to set the
high and low pass filters.
4.2.1 Setting the Velocity Measurement Range
The PDV offers three velocity measurement ranges, whose full scale values
are shown in mm / s (20 mm / s, 100 mm / s, and 500 mm / s) on the display in line
Velo . When using the digital output, the full scale value is needed as a
reference for the numerical value transmitted. If you divide the full scale
values by four, corresponding to the output voltage swing of ± 4 V, you get the
respective scaling factors in
(5 , 25 and 125 ).
To optimize the signal-to-noise ratio, as a general rule the smallest possible
measurement range should be used which is not exceeded under the given
measurement conditions. Coming close to exceeding a measurement range
(94% of the full scale) is shown by a circular symbol on the right next to the
measurement range display. A brief appearance of the overrange indicator
does not necessarily mean the measurement signal is causing overrange as
noise peaks caused by brief disruptions of the optical signal level can trigger
the display. However when using the filters, it is also possible that an internal
overload outside the frequency pass band has occurred. In this case the
overrange indicator would appear without overrange being apparent in the
output signal. Despite this, the next highest measurement range would have
to be selected to avoid waveform distortions.
4.2.2 Setting the Low Pass Filter
The PDV is equipped with a switchable, digital low pass filter, which you can
activate or deactivate if necessary. Unlike as usual with analog filters, the
three selectable cutoff frequencies 1 kHz, 5 kHz and 22 kHz mark the upper
frequency limit for precise amplitude measurements (flatness ± 0,1 dB) but not
the − 3 dB point. The changeover to the stop band has a steep frequency roll-
off of − 120 dB / dec, resulting in an effective suppression of noise at higher
frequencies. The cutoff frequency of the low pass filter is set on the display in
the line LP . On the technical specifications as for example the frequency
response of the low pass filter please refer to
SECTION
7.2.2.