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The Figure shows the block diagram. The PCE-VM 25 supplies the IEPE Sensor with 2 mA constant current. At the sensor output, a vibration acceleration proportional
AC voltage arises, which is amplified in the instrument to produce an optimum level. The gain switch-over takes place automatically. The subsequent analogue/digital
converter
is a Sigma-Delta converter with 24 bit resolution.
Further signal processing, such as filtering, integration (for calculating velocity and displacement from acceleration) as well as the RMS and peak value rectification is
carried out in the micro controller. The micro controller also controls the graphic display, the infrared thermometer, the rotation speed sensor, the USB communication
and storing of measurements.
3. Measuring Ranges
3.1. Measurable Vibration Quantities
The PCE-VM 25 can display the vibration quantities acceleration, velocity and displacement. Velocity and displacement are generated by single or double integration of
the sensor acceleration signal.
Furthermore, various frequency ranges can be selected.
The display rate adjusts itself according to the selected quantity in order to ensure that the RMS does not fluctuate even at lower frequencies. The following table shows
the display rates.
Measurand
Frequency Range
Display Rate
Sample Rate
Vibration acceleration
0,2 Hz – 10 kHz
5.6 s
23.438 kHz
Vibration acceleration
3 Hz – 1 kHz
1.4 s
2.930 kHz
Vibration acceleration
1 kHz – 10 kHz
1.4 s
23.438 kHz
Vibration velocity
2 Hz – 1 kHz
1.4 s
2.930 kHz
Vibration velocity
10 Hz – 1 kHz
1.4 s
11.719 kHz
Vibration displacement
5 Hz – 200 Hz
2.8 s
2.930 kHz
3.2. Vibration Acceleration
The PCE-VM 25 has the following frequency ranges for vibration acceleration:
• 0.2 Hz to 10 kHz: full frequency bandwidth of the accelerometer
• 3 Hz to 1 kHz: lower frequency acceleration
• 1 kHz to 10 kHz: high frequency vibration only
In this way specific signal components can be measured whilst others are attenuated. For example, when taking measurements on machines, at a frequency of 1 kHz
to 10 kHz predominantly the running noise of rolling bearings can be monitored, whilst vibrational unbalances are attenuated.
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