vibration signal fundamentals

7/27/2019 Vibration Signal Fundamentals

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Vibration Signal Fundamentals Page 2

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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