2.2. The Measuring Instrument
Figure 2 shows the block diagram. The VM25 supplies the IEPE Sensor with 2 mA
constant current. At the sensor output, a vibration acceleration proportional AC volt-
age arises, which is amplified in the instrument to produce an optimum level. The
gain switch-over takes place automatically. The subsequent analogue/digital con-
verter 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 communica-
tion and storing of measurements.
3. Measuring Ranges
3.1. Measurable Vibration Quantities
The VM25 can display the vibration quantities acceleration, velocity and displace -
ment. 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.
4
Figure 2: Block Diagram
IEPE
supply
Progr.
amplifier
A
D
Display
Memory
Keys
Sensor
ID
Power
supply
USB
RPM
sensor
Infrared
thermometer
Micro
controller