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7/27/2019 Vibration Signal Fundamentals
1/19
Vibration Signal Fundamentals Page 2
Rev 0 T00024
2008 General Electric Company. A ll rights reserved.
Measuring Machine Vibration
Proximity probes measure distance
Between probe and shaft
Non contacting
Magnetic energy absorbed proportionalto distance
While there are many types of proximity probes manufactured by Bently Nevada, the
most commonly used proximity probes measure distances between the probe tip and
the shaft over an 80 mil range and changes in distance cause changes in output Volts dc
200mV/mil.
Radio Frequency(RF)
The signal is generated without contacting the shaft by measuring the amount of
magnetic energy that is absorbed in the shaft via eddy currents. When the shaft is
close to the probe, more eddy currents are generated. When the shaft is farther away
(within effective range), less eddy currents are generated. The loss of energy reduces
the amplitude of the RF generated in the Proximity System. The proximity probe
system measures the energy lost through eddy currents in terms of voltage, the
amplitude of the RF signal.
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2008 General Electric Company. A ll rights reserved.
Small Gap / Large Gap
RF SIGNAL 0
RF SIGNAL
+10
-10
0
+12
-12
Once the probe is close enough to cause eddy currents to flow in a conductive material
the RF signal is affected in two ways:
1. Amplitude is at a MINIMUM when distance (Gap) between probe and target material
(Target) is at a MINIMUM. Maximum eddy current flow occurs.
2. Amplitude is at a MAXIMUM when distance (Gap) between probe and targetmaterial is at a MAXIMUM. Minimum eddy current flow occurs.
Change in distance over a given range occurs at 200mV/mil
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2008 General Electric Company. A ll rights reserved.
Changing Gap
RF SIGNAL 0
If the target is moving SLOWLY within the RF field, the signal amplitude
INCREASES or DECREASES SLOWLY. If the target is moving RAPIDLY within the
RF field, the signal amplitude INCREASES or DECREASES RAPIDLY. Oscillatory
movement of the target causes the RF signal to modulate.
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2008 General Electric Company. A ll rights reserved.
Demodulator Operation
DEMODULATORINPUT
PROXIMITOROUTPUT
0
0
AC peak topeak
DC Gap
The demodulator circuit deals with slowly or rapidly changing signal amplitude in the
same way. If the target is not oscillating, as might be the case with a thrust probe, the
Proximitor output is a constant DC voltage, called the gap. If the target is oscillating
(gap changing slowly or rapidly) the Proximitors output is a varying DC voltage (AC)
shown above by a sine wave. If the probe is observing vibration, the Proximitor will
provide both a DC (gap) and an AC (vibration) component in the output signal. Whenthe shaft is vibrating the DC component represents the average position of the shaft. A
typical system frequency response is from 0 Hz (DC) to 10 kHz. Newer transducer
systems, such as the 3300XL proximity system have responses up to 12 kHz.
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2008 General Electric Company. A ll rights reserved.
Timebase Waveform
The signal generated, when viewed on an
oscilloscope or System 1 Displaypresentation, is called a TimebaseWaveform
TIMEAMPLITUDE
Peak
to
Peak
When we plot this signal against time, we get a timebase waveform. The peak to
peak amplitude for a Proximity Transducer System is simply how close, and then how
far away the shaft is from the probe in its vibration cycle.
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2008 General Electric Company. A ll rights reserved.
Timebase Waveform
The cycling component of the signal is
called the AC (alternating current)component.
The average of the AC signal is called theDC component, or gap.
If there is no vibration, the DC componentprovides a simple distance measurement.
The AC or alternating current signal is the vibration signal generated by the
instantaneous change in distance from the transducer to the shaft. The DC or direct
current component (also known as the gap) is the average distance from the probe to
the shaft (in terms of probe voltage). If the shaft is not actually vibrating, the DC
component is the actual distance between the probe and the shaft, once again, in terms
of voltage.
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2008 General Electric Company. A ll rights reserved.
Casing Transducer Example
Casing transducers(acceleration and velocity)can also generatevibration signals.
Proximity probes are not the only way to capture a vibration signal. An acceleration
transducer or a velocity (seismic) transducer can capture casing vibration data from a
machine. These devices translate the machine vibration directly to a complex
waveform similar to those discussed in previous pages. However, casing transducers do
not provide rotor position information, and they measure rotor movement only
indirectly. Units for casing motion are 0-peak instead of peak-peak.
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2008 General Electric Company. A ll rights reserved.
Phase Reference Signal
0
0
-V
-V ONEREVOLUTION
ONEREVOLUTION
0
0
The proximity probe can also be used to indicate when a shaft has completed one
rotation.
If a notch or a projection is provided at one location on a shaft, a proximity probe will
show a significantly changed signal when the notch or projection passes under theprobe.
This large signal change indicates that the notch/projection has come back to a position
under the probe. This point will allow the System 1 platform to be given a starting and
ending point, which will in turn indicate one revolution of the shaft.
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2008 General Electric Company. A ll rights reserved.
Timebase Waveform with PhaseReference
More useful information provided with
Phase Reference. Balancing
Phase reference signal creates blank-bright on waveform.
Called a Keyphasor
A Keyphasor once-per-turn reference provides much more information about
vibration activity. It lets us know the angular location (phase) of the vibration motion
in relation to the shaft rotation. This is especially useful for diagnostic activities such as
balancing a rotor. As stated, the original use of the Keyphasor was to turn off the
oscilloscope beam at the point of Keyphasor passage, and allow it to be turned on
after passage. This presents the characteristic blank-bright spot that can be seen onthe timebase and other plots.
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2008 General Electric Company. A ll rights reserved.
Timebase Waveform Example
0 360
CombinedVibration signal
andKeyphasor signal
Keyphasor signal
0 0
Once the phase reference (Keyphasor) signal and the timebase signal are combined,
information about where the shaft vibration motion is at any given time can be
displayed. Remembering that each timebase waveform only shows the output of one
probe, this is a one-dimensional view of the shaft. In the example shown here, the
rotor shaft makes its closest approach to the vibration measurement probe soon after
Keyphasor passage.
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2008 General Electric Company. A ll rights reserved.
Absolute Phase
0 360
Vibration
Signal
Time PhaseLag
Keyphasor
Signal
Degrees of
Rotation
0 0
Absolute phase angle of a vibrating shaft can be found by using the timebase plot. The
absolute phase is the number of degrees of vibration cycle from when the Keyphasor
fires (once-per-turn reference pulse) to the first positive peak in the vibration signal. It
is by definition a phase lag angle.
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Absolute Phase Rules
Phase lag measured from reference point
(Keyphasor pulse) on shaft to the firstpoint of closest approach to probe
Requires reference signal and a vibrationsignal
Filtered waveform
As mentioned before, finding out how soon (what time, in degrees) the signal reaches
its first positive peak after the Keyphasor has fired will greatly support further
analysis. So the first formal analysis process is called Absolute Phase. As stated
before, this is simply the time in degrees after the Keyphasor signal to the first
positive peak of the vibration signal. The same rule for absolute phase is also used for
velocity and acceleration transducers.
The supporting rules for Absolute Phase are as follows:
Two signals are required, one reference signal (the Keyphasor), and one
vibration signal.
A filtered vibration signal is used and the filtered signal frequency must be an
integer multiple of the reference signal.
The absolute phase is measured from when the reference signal occurs and is
therefore always a lag angle, measured from 0 to 360 degrees.
The 0 degrees location is defined as the point on the shaft under the reference
vibration transducer when the reference signal occurs.
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2008 General Electric Company. A ll rights reserved.
Absolute Phase Measurement
Vibration
Signal
Keyphasor
Signal
0 360
Degrees of
Rotation
Phase Lag
0 0
Again, the Absolute Phase (or phase lag) for this situation is measured in degrees from
when the Keyphasor occurs to the first positive peak of the vibration cycle following
the pulse.
What is the absolute phase angle of this condition?
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2008 General Electric Company. A ll rights reserved.
Timebase Display
Here is an example of a 1X timebase plot
found in System 1 Display
The absolute phase of a filtered vibration signal in this case 1X can be read as part of
the vector description of the vibration. Here the positive peak of vibration occurs 255
degrees after the Keyphasor fires.
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2008 General Electric Company. A ll rights reserved.
Timebase Display
Two cursors allow difference comparison of time
and amplitude
Another tool provided by System 1 Display allows the user to locate the cursor in one
location, and then double-click the cursor. The next time the user clicks the plot
another comparison cursor is added to the screen.
When observing unfiltered timebase plots the absolute phase cannot be measured eventhough the Keyphasor is on the plot. This does allow us to see the relationship of
peaks that repeat with each revolution of the shaft.
The two peaks identified are separated by 307.68 ms. Which is equal to a frequency of
3.25 Hz or 195 cpm. This is the same as the speed of the machine, 195 rpm
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2008 General Electric Company. A ll rights reserved.
Relative Phase
Relative Phase is also found using theTimebase plot
Relative Phase is the time difference indegrees of vibration cycle between onevibration signal and another
Relative phase requires two vibration probes
A second important analysis tool is the Relative Phase of two signals. This is a
comparison of which signal leads the other, and by how much in degrees of vibration
cycle.
Obviously, since it is a comparison, two probes (or two signal sources) are needed forthe process.
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2008 General Electric Company. A ll rights reserved.
Relative Phase Rules
Requires two signals, to be compared
Signals must be the same frequency
Signals must have the same units
Either signal can be the reference
The maximum difference is 180o, eitherleading or lagging
Two signals are required. These two probes may be located orthogonally (90 degrees
apart) on the same bearing, or may be located at different points on the machine.
Any two signals can be compared as long as they follow the guidelines given in the
slide; from there, various analyses and diagnostic procedures can be implemented.
The signals must be the same frequency and have the same units for the comparison to
be valid. Either signal can be used as the reference. In other words, one signal can be
said to be leading the other, or the other signal can be said to be lagging the first.
Once again, any two equal points on the signal waveforms can be used to compare time
differences, but using the waveform peak is convenient and traditional. In order to
maintain consistency and accuracy about which peak arrives first, the peaks closest to
each other are the items of interest. No matter how the two signals are compared, the
analysis can never show more than a 180-degree difference between the two signals.
As a final note, the Keyphasor signal is not required to determine relative phase.
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2008 General Electric Company. A ll rights reserved.
Relative Phase Measurement
0 360
3600
RELATIVEPHASE
ONE CYCLESignal A
(Y)
Signal B(X)
ONE CYCLE0 0
0 0
In this case, it appears that Signal A peaks first. However, the first peak of Signal B
shows more than a 180-degree difference since the first peak of Signal A. Where the
two peaks are closer together (less than 180 degrees), it can be seen that Signal B peaks
first, and Signal A peaks second, or lags.
What is the relative phase of this condition?
What is a different way to specify the relative phase of this condition?
Remember that a relative phase analysis is not complete unless it provides two pieces of
information:
1)which signal leads or lags, and
2) by how much. In addition, the phase difference between the two can never be
more than 180 degrees.
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Timebase Display
Again, double cursors support comparison
Given the following information, what is the relative phase angle between the X and Y
signals shown here?
Machine is operating at a speed of 6989 rpm, therefore 116.483 rev/sec 1/116.483 =
8.585 ms for 1 revolution and since this vibration 1 at a 1X frequency, 1 cycle of
vibration also takes 8.585 ms.
Using the double cursors we see that 2.19 ms of time passed from when the IB Horz
transducer peaked and the IB Vertical peaked. 2.19 ms is 25.5% of a complete cycle
(360 degrees). 25.5% of 360 degrees is ~92degrees so we could estimate that IB
Horizontal leads the IB vertical by ~92 degrees.
Of course it would have been easier to observe the absolute phase angles (and
remembering the difference would be relative phase and that relative phase must be
between 0 and 180 degrees) and seen the Horizontal at 316 degrees and the Vertical at
49 degrees we would see the relative phase as Horizontal leads Vertical by 93 degreeswhich is more exact.
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