Order track in Labview with MIO E plug-in dacq cards
|
|
Order track in Labview with MIO E plug-in dacq cards |
|
This web page concerns people wishing to implement a package
to monitor vibrations in rotating machinery using standard software and
hardware from National Instruments. A demo executable application without
storage options and limited to 10 minutes' operation illustrates the real-time
capabilities of such a solution. It was tested in LBV6i and 6.02
using nidaq 6.8 and 6.9 with general-purpose cards like AT MIO E 10 2 and
1, PCI MIO E 1 and PCI 6204. Key to such a endeavor is to resort to standard
PC plug-in MIO E cards to perform what is called synchronous sampling.
The same hardware can be re-used for other applications than only
vibrations. It opens the door to Labview developers and their creativity
in using the numerous Labview tools to process vibration and vibroacoustic
signatures and, finally, to customize their product.
In order to clarify the vibration jargon, the next section is a selected glossary of terms borrowed from web sites published by some leading manufacturers of such monitoring systems: Bently Nevada, Dymac and CSI. Most of the jargon can be translated into Labview programs.
Another outstanding link is the forum maintained by the reliability
magazine over balancing/vibration/alignment.
A basic tool to address vibrations in rotating machinery is phase-locked
synchronous sampling. This basically consists of sampling vibrations at
equally spaced angular positions of the shaft indexed from a phase reference
(0°) on the shaft surface, at variable shaft rpms. If you can perform
this task in real-time without loss of samples, you get a sort of "portal"
for Labview to implement and customize all the features of existing
monitoring systems for rotating machinery.
Here is a short glossary of related terms with pictures obtained from Labview.
|
|
|
| This represents a partial list of terms and definitions used in the field of rotating machinery measurement, monitoring, design, and analysis. These definitions are not universal, but are expressed in the context of this specialized field. In some instances, Bently Nevada Corporation has redefined terms from other technical fields for its own purposes. | |
| Absolute phase | Absolute phase is the time between the flash (trigger position on time signal) and the first positive peak of the vibration signal. Time is always measured from left to right on a display. This time difference can now be converted to represent a phase angle that corresponds to a defined position on the shaft if the shaft's rotate speed or sampling frequency is known. |
| Phase reference | A signal used in signal analysis that for
rotating machinery represents a once-per-revolution pulse. This pulse can
be related to a fixed position on the shaft. This position is known as
the 0-degree position on the shaft, and all phase measurements made by
a system that uses this pulse as a reference, will calculate it's phase
angles with reference to this fixed position on the shaft.
(also see trigger keyphasor® Bently Nevada, phase reference) |
| Vector Filter | An electronic instrument that measures the amplitude and phase lag angle of, primarily, 1X and 2X components of a vibration signal. The vector filter uses a Keyphasor&Reg signal to automatically adjust its band-pass filter center frequency to coincide with the shaft rotative speed, or its first harmonic (double frequency), especially under transient rotor speed conditions. A simulated Keyphasor® signal input allows the instrument to also track other vibration frequency components, or to generate a swept spectrum plot. |
| Keyphasor | A signal used in rotating machinery measurements, generated by a transducer observing a once-per-revolution event. The keyphasor signal is used in phase measurements for analysis and balancing. (Keyphasor is a Bently Nevada trade name.) |
| Synchronous Sampling | In a DSA, it refers to the control of the
effective sampling rate of data; which includes the processes of external
sampling and computed resampling used in order tracking.
This is implemented in the vi below. This not be confused with another NI solution ( here ) where data acquisition is clocked and indexed with an incremental encoder linked to the shaft revolutions. Some encoders generate power of 2 pulses per rev for FFT implementations. In retrofit solutions or where such encoders are not possible this solution is hard to implement. That is the reason why manufacturers of vibration monitoring systems drop it. Mail me for more details. The NI solution has antialiasing filters. Such filters for MIO cards are described here. |
| DSA | Dynamic Signal Analyzer (a common vi in Labview
examples)
Vibration analyzer that uses digital signal processing and the Fast Fourier Transform to display vibration frequency components. DSAs also display the time domain and phase spectrum, and can usually be interfaced to a computer (implicit with vi) |
| Fast Fourier Transform (FFT). | A computer (or microprocessor) procedure
for calculating discrete frequency components from sampled time data. A
special case of the Discrete Fourier Transform where the number of samples
is constrained to a power of 2.
Actually DFT in Labview are for buffers with a number of samples not a power of 2, while FFT are only for buffers with a power of 2 samples. This means that synchronous sampling for rotating machines must sample vibration a power of 2 per rev over a power of 2 revs for FFT processing. Otherwise DFT automatically executes in Labview with a substantial loss of CPU time. |
| Seismic Transducer | Seismic Transducer. A transducer that is
mounted on the case or housing of a machine and measures casing vibration
relative to free space. Accelerometers and velocity transducers are seismic.
Accelerometers generates very high frequencies compared to velocity sensors and eddy probes. Antialiasing filters must filter these HF out for a sound vibration monitoring. Use of these filters with MIO cards is described here. |
| Proximity Probe
Eddy Probe. |
A noncontacting device which measures the displacement motion and position of an observed surface relative to the probe mounting location. Typically, proximity probes used for rotating machinery measurements operate on the eddy current principle, and measure shaft displacement motion and position relative to the machine bearing(s) or housing. |
| Dual Probe | A transducer set consisting of a proximity
probe and velocity transducer installed radially at the same point (usually
in a common junction box on the machine bearing. Four separate measurements
are provided by this
transducer system. The proximity probe measures: (1) shaft relative radial position within the bearing clearance, (2) shaft dynamic motion relative to the bearing, (3) machine casing absolute vibration measured by the velocity transducer, and (4) shaft absolute motion represented by the summation of the velocity signal integrated to displacement and added to the shaft relative signal. Definition of Bently. Other manufacturers replace velocity sensors with accelerometers and integrate their signal twice instead of once to recover displacements. Nice thing to do in Labview with filters etc.. |
| Spectrum | Commonly a presentation of the amplitudes
of a signal's frequency components versus their frequencies. Or the frequency
content of a signal.
Trivial in Labview. |
| Full Spectrum Plot | An enhanced spectrum plot produced by using
the timebase waveforms from XY transducers to calculate the amplitudes
of the forward and reverse (backward) frequency components. Advocated
by Bently Nevada and useful with pairs of eddy probes but much less so
with seismic sensors.
Trivial in Labview (use complex FFT instead of real-valued ones). See either cascade or intensity charts. |
| Campbell Diagram
Spectral Map. |
A diagram used in rotating machinery design.
A tool for selecting and checking shaft operational rotative speeds and
other possible forcing function frequencies against the spectrum of natural
frequencies to avoid resonances. The X axis represents the various possible
excitation frequencies, i.e., rotative speed (1X), oil whirl (.40X to .48X),
blade or vane passing frequencies, gear mesh frequencies, etc. The Y axis
represents the rotor lateral and torsional natural frequencies. The term
is sometimes used incorrectly to describe the cascade plot and waterfall
plot.
A three-dimensional plot of the vibration amplitude spectrum versus another variable, usually time or rpm. X axis can be expressed in orders instead of frequencies if synchronous sampling is used. |
| Vibration vector | For various orders of the shaft vibration
(one or two basically), vector whose amplitude equals that of the harmonic
and the phase is the corresponding absolute phase.
Requires synchronous sampling like the vi of this page. |
| Polar Plot /Format | A graphical format consisting of a center
reference point surrounded by concentric circles for representing the graphs
range. Vector information is plotted in this format by using magnitude
and phase, as a function of rotational speed. This plot is also sometimes
incorrectly called a Nyquist plot.
Basically the vibration vector as the rpm varies. See examples in High Spot glossary entry below. |
| Heavy Spot | The angular location of the imbalance vector
at a specific lateral location on a shaft. The heavy spot typically does
not change with rotational speed.
Once it is known (at least its absolute phase) one can perform some sort of quick balancing, if need be. |
| High Spot | The angular location on the shaft directly
under the vibration transducer at the point of closest proximity. The high
spot can move with changes in shaft dynamics (e.g., from changes in speed).
An example of such a high spot through a critical speed. Polar plots typically show these behavior. The name of the game is sometimes to observe the high spot behavior in a polar plot through a resonance to obtain the absolute phase of the heavy spot. |
| Vibrations norms | From seismic sensors in rms velocity between
10 Hz and 500 (1000)Hz. (ISO-10816)
From dual probes, sMax for shaft absolute vibrations or relative (ISO-7919 or 10817) All advanced monitoring systems display the vibration severity in terms of these norms plus many additional features. That is their bottom line. |
| Smax | This is a trend display of the amplitude
of the maximum deflection of the shaft, and the phase of this deflection,
according to time. Each time a Orbit display is produced. The orbit has
a maximum deflection indicated on it. This deflection is a vector with
magnitude and phase. If the magnitude and phase of this deflection is displayed
over a period of time, the display is called a Smax plot.
This implies synchronous sampling (Smax relates to the harmonic 1 of the running speed) and dual probes. Immediate with full FFT (complex-valued to get full spectra with an x-y pairs of dual probes). Smax with all frequencies in theorbit signal is much more intricate to calculate (akin to the traveling salesman's problem). |
| Etc. | Consult glossaries of Dymac Bently and CSI for further definitions. |
| Etc | |
Now you can vary speed manually and quite rapidly without losing any sample with this synchronous sampling vi. Very occasionally, there will be some loss with automatic recovering due to CPU speed to handle interrupts. You will also see the ctr backlog increase for very high rpm. It tells you how far behind in revs your synchronous sampling scheme is running.
Also funny is the behavior of the synchronous sampling while launching
other Windows tasks (Word, Netscape). This may cause an increase of the
ctr backlog. If it is fairly high, try to switch your phase-locked
sine test signal to triangular back and forth. The transitions sine/traing
on the display for occur somewhat later. Buffer activity can be monitored
on the vi and modulate tasks to keep it from overflowing.
Synchronous sampling is real-time in the sense that no sample is
lost due to the latency caused by these Windows tasks.
Again try to end tasks like Power Save and fast search that Windows
systematically launches in default configurations. After all, you may not
need them in this context. On the other hand, you might want to use Word
to type your report on-line from the current measurements and use software
to capture pics from the front panel. With a PII, this is feasible while
the synchronous sampling goes on.
Order track specs cluster of inputs (controls):
Channel scan: string (see LV doc for meaning). In this case, four
input analog signals are monitored.
Samples by (per) Rev: a power of 2 for frequency analyses with FFT. Allows order track up to half this value. Here: up to 8th harmonic of the running speed (rpm).
#revs: number of successive revs to retrieve each time for order track analysis. Order track resolution is the inverse of it. Here 1/8 of the shaft rpm. Higher #revs values decrease the rate of display refreshes. With slow graphics cards, this helps synchronous sampling reach higher rpms without overflowin the ctr backlog.
Time base (of MIO card): either 100 KHz (here) or 20 MHz. 100KHz
is recommended if the range of rpm (see below) starts lower than 80 rpm.
 |
Select range of rpm. Can be as low as 5 rpm
to monitor eccentricities on slow drives on big turbines.
If actual rpm exceeds twice max_rpm--> error message Quality order track: high will increase oversmapling rate to get more accurate phase for vibration vectors. Concurrent W tasks: up --> larger buffers. However, try to end such tasks as antivirus andother on-line safeguard programs like McAfee and/or Norton. Also powersave and findfast. |
Order track specs cluster of inputs (controls): |
Channel list: not used. Will be auttomatically set by sensor configuration
(below
Samples by (per) Rev: a power of 2 for frequency analyses with FFT. Allows order track up to half this value. Here: up to 8th harmonic of the running speed (rpm). #revs: number of successive revs to retrieve each time for order track analysis. Order track resolution is the inverse of it. Here 1/8 of the shaft rpm. Higher #revs values decrease the rate of display refreshes. Note that the order track is performed rev-wise. At very low speeds, it may be that if selected to 16 revs, it may correspond to two sets of the same successive 8 revs, to 4 sets of 4 successive revs all alike, etc, depending on the value of these low rpms. This decreases the response time of the vibration monitoring which otherwise would be too long in such applications as monitoring shaft eccentricities prior to launching a big turboset. For example, at 15 rpm, 16 revs would last 1 minute to complete. Time base (of MIO card): either 100 KHz (here) or 20 MHz. 100KHz is recommended if the range of rpm (see below) starts lower than 80 rpm. HP Hz: high-pass cut-off frequency of digital filters used to digitally (doubly) integrate (accelerometer)velocity pickup signals (see below) |
 |
This table allow the user to select up to 16 different
vibration pick ups amongst the four following types:
acc(elerometer) with sensitivities in M/sec**2/V vel(ocity Pickups) in mm/sec/V dis(placement) or eddy probes in µ/V none: any synchronous signal like pahse currents etc. Slope
MaxV sets the preamp of the MIO cards for the ADC not to saturate beyond the value entered. If one expects smaller responses from the pickup, please specifya lower value for MaxV to get dynamics. Plane #: integer Where? Cross section where sensors are along the shaft line.
Consistency checks:
The number of data acquisition channels (here 11) , the data acquisition rate will be automatically computed from this table. In case the rate exceeds the the dacq card capabilities---> message (OK here but some other guide to identify the "faulty" plane and stop. Color codes are common to all displays in other Tabs.
|
 |
Info Mesure (measurement) a string that gets stored into files to
identify the location aznd machine. Storage not implemented in demo version.
deltime: the time tick for trending. Here one second. This establishes a constant time base for the order track which has none since it depends upon the rpm and the order track specs. |
| Class of machine | With a reminder of their scope of application.
Only bearing vibrations, although this could easily extended to shaft vibrations. |
 |
Speed gradient in rpm/min to indicate whether the shaft speed undergoes soem variations beyond which storage is recommended. Speed in rpm. Note that the red square is for dstp transfer over
internet (not impelmented but already tested)
STOP or GO.
Grid Max (µ, could eb mils for USA): full scale for trending
of H1 (Tab Trensing&Aalarm), Polar dispay; Intensity charts for spectra
or full spectra, etc.
For cascade full scales
Not implemented in demo.. |
For people familiar with labview and its Application builder and drivers this is rather easy to do.
If you have interest in this or other related products, please
contact us.
-----
----------
