Péton Field measurement tecchniques and instrumentation for torsional vibrations determination

FIELD MEASUREMENT TECHNIQUES AND
INSTRUMENTATION FOR TORSIONAL VIBRATIONS
DETERMINATION
Grislin C 1, Péton N 1, Cousin G 2, Denisot N 2
1
GE Oil & Gas Digital Solutions, 44303 Nantes, France, [email protected]
2
OROS, 23 chemin des Près, Inovallée 4403, 38944 Meylan Cedex,
[email protected]
Abstract
Torsional vibration excitation in rotating machinery can cause system
reliability issues or even catastrophic failures. All rotating machineries
undergo some degrees of torsional vibration during operations. The torsional
vibration is not as easily identified as the translational vibrations, due to
lacking of simple and direct measurement devices.
However, if left
uninspected, torsional vibrations could do as much damage as that from
translational vibrations. Typical damages developed under excessive
torsional vibrations include shaft cracks, coupler cracks, gear wear, gear tooth
failures, key failures, shrink fit slippage, etc. Therefore, torsional vibration
detection and monitoring becomes an important step in rotating machinery
condition monitoring, especially for those machines driven by a variable
frequency drive (VFD), a pulse width modulation motor (PWM), or a
synchronous motor (SM), etc. To detect the torsional vibration of the rotating
machinery, several methods have been developed and/or improved.
Commonly used methods include strain gauge based methods, torsiographs,
tachometer frequency modulation based methods, laser vibrometer based
methods, and zero-crossing detection based methods. On site, traditionally,
the torsional vibration is detected by a phase demodulation process to the
signals generated by tooth wheels or optical encoders. This demodulation
based method has a few unfavorable issues: the installation of the tooth
wheels needs to interrupt the machinery normal operation; the installation of
the optical barcode is relatively easier; however, it suffers from short term
survivability in harsh industrial environments. The geometric irregularities in
the tooth wheel and the end discontinuity in the optical encoder will
sometimes introduce overwhelming contaminations from shaft order response
and its harmonics.
In addition, the Hilbert Transform based phase
demodulation technique has inevitable errors caused by the edge effect in
FFT and IFFT analyses. Fortunately, in many industrial rotating machinery
applications, the torsional vibration resonant frequency is usually low and the
Torsional Vibration Symposium 2017, Salzburg, Austria
keyphasor® and/or encoder for speed monitoring is readily available. Thus, it
is feasible to use existing hardware for torsional vibration detection.
INTRODUCTION
Torsional vibrations in the rotating machinery are not easily perceived as the
translational vibrations in stationary structures. However, the importance of
the torsional vibrations is as high as that of the translational vibrations. The
torsional vibrations usually cannot be directly sensed and transmitted for
further data analysis. However, if left uninspected, torsional vibrations could
do as much damage as that from translational vibrations. Typical damages
developed under excessive torsional vibrations include shaft cracks, coupler
cracks, gear wear, gear tooth failures, key failures, shrink fit slippage, etc.
(IG-02-4, December, 2002, Wachel, 1993, Howes, October 28th / 29th, 2008).
In addition, the coupling between the torsional and lateral vibrations may
cause additional damage to the stationary structures as well [10].
Methods for detecting torsional vibrations of the rotating machinery have been
limited. Historically, only a few methods have been successfully put into
practical uses in industries. Typical industrial applications of those methods
include strain gauge based methods [2, 1, 11], torsiographs [6, 7], tachometer
frequency modulation based method [7, 11, 4], laser vibrometer based
method [5, 13], and the time interval measurement (TIM) based method [15,
14, 3, 12]. Each method has its own pros and cons. For detailed analysis of
each method, please refer to reference [8].
Torsional vibration detection using time interval measurement (TIM) is the
most active approach and received a great deal of attention in recent years.
There are numerous papers in the public domain on the research and
application of the TIM method. The TIM approach utilizes the pulse train
signal as in the phase/frequency demodulation method. Instead of using
demodulation, the TIM based method calculates the time interval between the
consecutive pulses first. The variation of the time intervals is then directly
related to the shaft torsional vibration. Vance first described the TIM method
in detail in the late 1980s [15]. Since then, there have been many different
variations of algorithm development as well as successful applications.
Several methods of TIM have been practically tried out on the field. The first
one has been using a real-time OROS analyzer, using the toothed wheel
signal. The second one uses the same equipment, but from an Optel Thevon
optical probe with a zebra tape. Also, GE Bently Nevada developed a process
for torsional vibration detection using the raw signal recorded from the Bently
Nevada ADRE 408 DSPi and using a multi pulses per revolution. The last
method uses once-per-revolution pulse train from the keyphasor® to identify
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the torsional vibrations. All these methods have their positive and negative
aspects so the goal is to understand which method is best to use in a case.
However, the TIM based method is not without issues. Typically, it requires
the pulse train generator and detector to be accurate in signal generating and
detecting, otherwise, the shaft fundamental frequency component and its
harmonics will overwhelm the torsional vibration signature. Although this
geometry variation based effect can be alleviated by a synchronous
averaging technique [(Resor, 2005)], the speed variation in the practical
rotating machinery is still a problem since the traditional TIM based approach
requires a constant speed operation to get the reference for determining the
torsional vibration. Recently authors used a speed extract approach, which
uses piecewise regression or wavelet filtering, which extended the TIM based
torsional vibration extraction to aggressive speed variation cases, such as in
speed-up or coast-down processes [8].
TIME INTERVAL MEASUREMENT APPLIED TO
TURBOMACHINERY
Steam Turbine generator test
Three methods of TIM have been practically tried out on the field, using two
different algorithms. All these methods have been experimented on a steam
turbine generator unit installed on the GE Oil & Gas test bench facility in
Florence, Italy for an acceptance test. Torsional vibration modes were
previously calculated as in Figure 1, so comparison of the torsional natural
frequency could be made with experimental results.
When looking at the calculated modes there is a first one at 18.09 Hz (system
mode) and then bucket modes (2nd and 3rd). The first mode is the first
fundamental rigid torsional frequency of the train. The first mode would
generally be the mode that is looked after, because it is a sub-synchronous
frequency, so with a resonance during a transient event and because it has
the maximum of energy.
For the first mode, the nodal point is at the coupling. This mode is not
expected to be influenced when the bucket flexibility is considered. This mode
is expected to be most sensitive to the torsional stiffness changes at coupling.
This mode is also characterized by just one nodal point along the axial length
of the full train.
By considering the TIM method, as opposed as the stress analysis method,
the best location for the velocity measurement instrumentation would be at
the anti-nodal point.
Torsional Vibration Symposium 2017, Salzburg, Austria
Figure 1: Theoretical torsional vibration modes identified with the model
Three types of encoders were used on this test:
 First, existing toothed wheel at machine train’s end using
permanently installed magnetic pickups. Timing wheel has 60
equally spaced teeth.
 Second, keyphasor® signal (one pulses per revolution) from
proximity probe was recorded
 Third type of encoder is a temporary installed zebra tape (126
pulses per revolution) that was also installed on the rigid coupling,
where shaft was free to access
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Figure 2: Pictures showing timing wheel on bearing 1 of the machine on the left and
zebra tapes of sensor #2 located at the coupling between the steam turbine and the
generator on the right.
Real time processing of the data
First a 4-channels OROS analyzer (Figure 3, Left)) is used to perform a realtime measurement of the torsional vibration. The device has an integrated
time interval encoder, high speed timer / counter with clock rates up to 6MHz
that are used for encoding segment detection from the zebra tape or a
toothed wheel. Both sensors were implemented to the steam turbine to
compare results at three different locations
Figure 3: 4-channel OROS analyzer, using over-sampled channels on the left and
ADRE System from GE Bently Nevada on the right (front and back view).
To measure the torsional natural frequency of the rotor during operation at
steady state, excitation of the rotor is done by a sudden load change of the
generator. Different load steps were performed but load rejection was the
best excitation.
The existing toothed wheel of the steam turbine had two spare magnetic
sensors that could be used for the torsional test. The signal was directly
Torsional Vibration Symposium 2017, Salzburg, Austria
recorded by the analyzer. The most critical part of the measurement is the
acquisition of the instantaneous speed, and especially the tooth pulse
detection. The waveform signal of the toothed wheel is displayed on the top of
the Figure 4. Below, the pulse signal converted by the analogue-to-digital
counter is displayed. The last graph is the time-based signal of the torsional
velocity. This graph represents the instantaneous velocity variation of the
shaft, expressed in Deg/s
Figure 4: Time base signal, counter and angular velocity of a toothed wheel signal
and FFT spectrum of the instantaneous speed of the toothed wheel signal
during the same period
From this velocity oscillation around the average shaft speed, a Fourier Fast
Transform can be performed by the real-time analyzer, in order to display the
frequency spectrum of the torsional vibration. This spectrum shows two
components: the torsional vibration frequency and a synchronous component
(1X). The torsional vibration frequency identified on the spectrum can be
compared to the one found by calculation of the theoretical model, Table 1.
The comparison shows a perfect match between the model and the
measurement, which brings confidence in the reliability of the system.
Theoretical natural torsional frequency
18.04 Hz
Measured natural torsional frequency
18.4 Hz
Table 1: Comparison of natural torsional frequency between model and
measurement
This method allows us to identify the torsional mode in real-time. However,
this method suffers of the insufficient teeth spacing precision. This geometric
error introduces a synchronous component that should be considered as a
noise. Unfortunately, it is not possible to cancel this component with this
method. As a result, this could be a problem when both components perfectly
match.
The torsional natural frequency is clearly identified after resonance excitation,
following the load rejection. A waterfall graphic displays multiple frequency
spectrum of the torsional vibration in respect of time.
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Torsional Vibration Symposium 2017, Salzburg, Austria
Postprocessing of the data
The GE Bently Nevada machinery diagnostics team also developped its own
Matlab based software in order to identify torsional vibration modes. This
software post-processes the timebase signal of the pulse train in order to
measure the time interval between two pulses. The timebase signal is
recorded with an ADRE 408 DSPi (Figure 3 Right)using the raw mode
capability, which digitized the signal at 128 KHz sampling rate and with 24 –
bit A/D conversion. The signal is then exported to a tabular format to be read
by the Matlab program. The shaft period/frequency is isolated, and the
torsional vibration is extracted by comparing the identified average shaft
speed and the instantaneous shaft speed.as displayed in Figure 5, in order to
determine speed deviations and torsional vibrations.
Figure 5: Instantaneous speed overlaid with the average speed of the shaft during a
load rejection test on the steam turbine on the left. Torsional vibration of the teethed
wheel during the load rejection time interval on the right
This method can be validated by comparison with the method using the
OROS equipment since both results give the same torsional frequency.
However, due to the speed variation, and geometry error, the frequency
spectrum is not representing the real torsional vibration spectrum. In order to
get real torsional vibration only, a synchronous averaging is applied. The
result of the transformation is displayed Figure 6, showing that only torsional
frequency is left visible on the spectrum.
Torsional Vibration Symposium 2017, Salzburg, Austria
Figure 6: Torsional vibration spectrum after synchronous averaging of the teethed
wheel during the load rejection time interval
This solution is proven for torsional vibration identification and can be easiliy
implemented on the field.
Toothed wheel are often mounted on units, using induction sensors. These
sensors are usually designed for overspeed detection or speed control and
are critical for the machine operation. As a result, unless there is a spare
sensor mounted on the toothed wheel, it is not recommended to connect a
device in parallel of these sensors.
In industrial rotating machinery, a Keyphasor® is usually preinstalled on the
shaft for measurement of absolute phase and can be supplementary used for
shaft speed detection. It is also the case that, in many rotor systems, the first
torsional natural frequency is lower than the shaft speed. This opens the
opportunity of using shaft once-per-revolution pulse train only to identify the
torsional vibrations in rotating machinery systems.
In this step, we allow the shaft speed to be a variable with respect to time, but
the variation frequency is assumed to be lower than the lowest torsional
modal frequency. This is a reasonable assumption for most of industrial
applications. Therefore, the shaft period/speed variation has no frequency
overlap with that from torsional vibrations. Under such assumptions, we may
use low-pass filter to isolate the shaft period/frequency. However, in many
cases, the low-pass filter cut-off frequency needs to be much lower than the
sampling rate. A direct digital low-pass filter design may cause computation
instability. In the tool, we used two approaches to isolate the shaft
period/frequency: a wavelet decomposition based method and a piece-wise
regression method.
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Once the shaft period/frequency is isolated, the torsional vibration is extracted
by comparing the identified shaft speed and the instantaneous shaft speed.
Since there is no pulse geometric variation error in a once-per-revolution
sensor, such as the tooth gear spacing error and optical barcode end joint
error, it is expected to have less shaft order related noise contamination in the
identified torsional vibrations.
The torsional vibration is converted in terms of shaft angles, just like it was
done with multiple pulses per revolution signal.
Many techniques have been tried out to effectively measure the
instantaneous angular velocity. Temporary solutions must be also used in
some cases where toothed wheel and keyphasor® cannot be used.
An optical encoder can be mounted on the shaft, away from the nodal point.
This optical encoder can be an optical probe pointing to a zebra tape with
reflecting stripes. The transducer output is a high or low voltage (0 to 5V)
depending on whether it detects “white” or “black” region of the transducer
segments. If the machine train is not instrumented with a keyphasor®, it is
possible to mark the zebra tape with a missing tooth by blacking out up to 3
pulses. Timebase signal collected from the zebra tape, Figure 7, shows the
signal does not always have the same amplitude and is not as clean as the
toothed wheel signal.
Figure 7: Time base signal of a zebra tape, showing the imperfections of the tape
Torsional Vibration Symposium 2017, Salzburg, Austria
Instrumentation related issues
Instrumentation is probably the most sensitive part of the measurement. The
Zebra Encoder tape allows for torsional vibration measurements to be taken
on the machine in question. To do this the tape needs to be fitted and then
observed with either an optical or laser tachometer. Among the most often
problems met on site, are:
 The probe bracket should be stiff enough to be always parallel to the
shaft. One possible cause of the signal amplitude variation is the
vibration of the probe itself due to lack of enough stiffness of the
bracket.
 As the zebra tape is self-adhesive (sticky backed) then the area of the
shaft in question must be clean prior to any application to guarantee a
good hold. Therefore, any surface corrosion, dirt, grease or any other
contaminates must be removed from the entire circumference of the
shaft area intended for mounting. This is commonly done with a wire
brush, emery paper and alcohol free wipes. Although the tools may
differ depending on machine location and size, the aim of providing a
clean mounting surface remains the same. Once the shaft surface is
clean, mounting of the zebra tape can proceed.
 It is important to try and keep the tape as close to perpendicular as
possible. This does not have to be excessively precise but checking
the distance from a datum (like a bearing housing, shaft collar, etc.) to
confirm the tape is being fitted the same distance at regular points
along the shaft is one way of confirming this. In some cases, it may be
preferable to use a Tailor’s tape to create a checked datum line first to
fit the zebra tape against, as shown below. This temporary tape is
removed once the zebra tape is in place. The tape will need to be cut
so that it correctly fits the shaft circumference (Figure 8). To determine
where to cut the tape place the tape around the shaft and mark were
the two ends will meet on the long end of the tape. The preference is
for the tape to meet exactly without the two ends overlapping or falling
short. However, it is preferable to fall slightly short if need be as an
overlap can cause spurious signals. At this point cut the tape and
make a note of how many stripes are present on the cut length. This
number is needed later for signal processing.
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Figure 8: Alignment and length

Junction of the tape can be an issue too as it can be seen on Figure 9,
since overlaping the tape can introduce a gap error. Ideally, the best
way to make the junction is to tape edge-to-edge. However, this is not
practically possible since there is always a thin gap between both ends
of the tape. The shaft beeing reflective, this could add a false pulse
into the signal. That’s the reason why a missing tooth can be introduce
at that area, as seen on Figure 10. Missing teeth are generally created
with black paint and can include one to three pulses, as seen Figure 9.
To mount the zebra tape all that is needed is to remove the protective
packaging and place the tape adhesive side down onto the shaft using
the datum as a reference to make sure the tape is fitted perpendicular.
Run a clean hand over the tape slowly as you fit it to make sure no air
bubbles or creases form during fitting. However, once the tape ends
are next to one another it is common to have a single abnormal length
stripe. This should be, or made to be, one extra-long black stripe. To
do this use a marker pen followed by a length of black electrical tape to
color then secure the tape ends, as illustrated in Figure 10 (top left). If
the tape will be left fitted for extended periods or exposed to adverse
conditions (weather, temperature) it is recommended to use additional
adhesive tape on both side of the zebra tape to secure it. When doing
this, it is preferable to trap the tape used previously to secure the ends
of the zebra tape under it, as on Figure 10(bottom left). When fitting the
additional adhesive tape, it is preferable to mount it on an equally clean
surface and to take note of the shaft direction of rotation when deciding
which end to put under the other, Figure 10 (bottom right). This should
be considered as having the joint facing away from the direction of
travel reduces windage effects on the joint and therefore reduces the
likelihood of any problem being made worse by windage.
Torsional Vibration Symposium 2017, Salzburg, Austria
3 missing teeth
Figure 9: Missing teeth counting showing error due to tape overlap
Figure 10: Missing teeth black painted on the zebra tap. Once the tape ends
are next to one another it is common to have a single abnormal length stripe.
This should be, or made to be, one extra-long black stripe. To do this use a
marker pen followed by a length of black electrical tape to colour then secure
the tape ends, as illustrated on the right.

When using a laser, it should be no closer than 15cm from the shaft
and at an angle (Figure 11). Not directly perpendicular to the shaft, as
below. The laser tacho will work best if the tape is well lit however care
must be taken to use a consistent light source such as a light or LED.
Not a light force that can flicker, such as a florescent light. This must
be avoided as a flickering or changing light will cause amplitude
modulation which in turn can cause problems when post processing
the data
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Figure 11: Mounting the laser.
Comparison of each TIM methods: positive and negative aspects
Toothed
wheel
Keyphasor®
Zebra tape
Positive
Already mounted on the
machine train
Clean signal
Already mounted on the
rotor
Can be mounted on
other location with laser
keyphasor®
Can be implemented on
multiple location
Negative
Need to measure from
a spare sensor
No choice of location
Typical applications
Large
machinery
acceptance test
during
Limited in frequency
resolution and span
Any machinery for torsional
natural frequency identification
below half rotating speed
Signal quality issue
Machine must be shut
down for mounting the
instrumentation
Machinery without toothed
wheel
access
and
with
torsional natural frequency
higher than half the rotating
speed. Also, can be used for
stress analysis of a coupling
Table 2: comparison of each TIM methods
CONCLUSION
TIM method is a powerful way to identify torsional natural frequency. Several
methods of analysis and instrumentation exist and choosing the right one is
critical. Most of the cases applied to turbomachinery where the 1st torsional
mode is below half of the operating speed frequency, the keyphasor® offers
then an interesting choice. This method offers good results for torsional
vibration identification but instrumentation is difficult to set up. However, it
offers good resolution and flexibility for the location plane. Also, it is
mandatory to implement a signal processing algorithm like a synchronous
averaging on the measurement to effectively remove the irregularities caused
by the timing wheel or barcode spacing.
Torsional Vibration Symposium 2017, Salzburg, Austria
This opens another field of torsional vibration measurement which focuses on
the stress caused by the shaft torque. Indeed, by measuring the
instantaneous velocity on both sides of the coupling, it is possible to measure
the twist angle after a torque is applied on the driven machine (like a load
rejection).
A stress analysis generated on the nodal point of the rotor (typically the
coupling) could also be done. It can be done with the TIM method using two
TIM planes on both sides of the nodal point. It would require then the use of
two multiple pulses-per-revolution signal as a zebra tape.
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