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LUTRON VT8204
The vibration sensor supplied with the VT8204 is a piezo-electric accelerometer
and is widely used in industry for machine vibration measurement.
Principles of operation:
Please refer diagram below. An electrical output is produced by the
accelerometer when the piezo-electric element is squeezed between the seismic
mass and the base. In basic terms – when the accelerometer is moved vertically
in an up and down motion the piezo-electric material compresses/extracts and
experiences a force, in turn it generates an electric charge between its surfaces.
This electric charge is proportional to the amount of acceleration/deceleration the
unit experiences. For those wishing to educate themselves further on the
fundamentals of this type of accelerometers please refer to attachment
Accelerometer.pdf (please keep in mind this is a generalized document not
specific to the Lutron sensor).
The electrical signal produced by the accelerometer is digitized in the hand held meter
and can now be used to display acceleration levels on the LCD display. In the case of
the VT8204 we view this in either “g” or “m/s²” when using the metric scale, 1g being the
equivalent to 9.81m/s ². With the understanding the sensor is an accelerometer and can
only produce an output due to acceleration/deceleration one might ask how does the
meter display velocity and displacement? This function is carried out by the process of
differentiation, the meters internal microprocessor calculates both velocity and
displacement by using the measured acceleration and the frequency the sensor is
oscillating at. Differentiation involves a multiplication by frequency, this means the
vibration acceleration at any frequency is proportional to the velocity times the
frequency. It is a similar result for displacement - the vibration velocity at any frequency
is proportional to the displacement times of the frequency. For a given displacement, if
the frequency is doubled, the velocity will also double and if the frequency is increased
tenfold, the velocity is also increased by a factor of ten.
To add further clarity, if the sensor produces a constant force of 1g at varying
frequencies we can easily see the relationship to velocity and displacement.
Frequency Acceleration Velocity Displacement
(Hz)
(g) (mm/s) (mm)
1000 1 1.56 0.00025
100 1 15.6 0.025
10 1 156 2.5
1 1 1560 250
*All above readings in peak
Notes from Lutron
The vibration sensor and the vibration cable are the very high impedance units,
so the vibration measurement procedures should be executed very carefully.
During the measurement, hand or the fingers cannot touch the cable or the
socket, only use the hand to hold the instrument if necessary [much better to
clamp in place]
For the precision vibration measurement, if possible, we should use tape to fix
the cable on a solid place to prevent it from moving
The above operation procedures should be consideration for the displacement
measurement especially because the displacement circuit is the high gain
compared with the measurement of acceleration and velocity
Summary
If velocity and displacement is of particular interest then use at frequencies
above 10 Hz is desirable). During handling/mounting of the sensor for low
frequency (below 10Hz), movement of the sensor/cable will affect the meter
readings and they should not be considered accurate.
Lutron state a Frequency Range of 10Hz to 1kHz and a calibration point of
160Hz in the specification area of the manual. We recommend to have the
anchor point of the cable on the fixture that is being measured (as per following
page). This will allow the cable to move in the same motion as the sensor – and
forces on the sensor and cable will now have a better chance of staying equal,
this is more critical at lower frequencies
Anchorage
Point
It is best to keep the
loop of cable as
short as allowable
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