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Synchronization usually starts with atachomete r, which provides a pulseor an integral number of pulses foreach revolution. This signal indicates

that the ma chine has finished onecycle and is beginning the next. Asingle tach pu lse indicates when therotating machine has reached aparticular angular position. Aftercapturing two tach p ulses, you candetermine rotating speed by countingclock cycles between the tach pulses.A third p ulse will then t ell you if themachine is changing speed. Using thetach pulse to trigger the DSA syn-chronizes the machine and themeasurement.

DSAs us e an analog-to-digital con-

verter to collect a block of data. Inregular spectr um analysis, this blockof data consists of voltage valuesspaced at regular time intervals.When you star t gathering a block ofsuch points, you also determine thefinish time. This is one of the d ifficul-ties with orde r ana lysis. If themachine is changing speed, how do

you fill the block just as you get thenext tachometer pulse? Threetechniques are commonly used.

Shaft encoders are electro-opticaldevices that generate thousands ofdigital pulses per revolution, gatingtime samples into the data block.

A ratio synthesizer and tracking filteremulates a sha ft encoder with aliasprotection.

With the digital resampling technique,the DSA digitizes data at a very highrate, co llecting and storing tightlyspaced t ime samples. As tach pulsesarrive, the analyzer resamples the t imepoints into correctly spaced data.

With each of these three te chniquesthe goal is a set of points evenlyspaced b y shaft position, not by time(Figure 1). This yields data in therevolution domain, rather than in thetime domain. Again, the key benefithere is that the measurement ratetracks the rotational speed of themachine.

Frequency spectrum vs .order spectrumThe FFT process transforms timedomain data to the frequencydomain, creating a spectrum . Signalsthat are p eriodic (repetitive) in thetime domain appear as peaks in thefrequency domain. In order analysisthe FFT transforms the revolutiondomain data into an order spectrum.Signals that ar e periodic in therevolution domain appear as peaks inthe order domain. For example, if avibration peak occu rs twice everyrevolution at the same sh aft position,a peak appears at the second order inthe order spectrum.

What do order spectra look like?Figure 2 is an FFT spectrum map of

an automo bile engine run-up testfrom 665 to 3995 RPM. Figure 3 is anorder spectrum map of the samemeasurement. The Y-axis in bothmaps is amplitude. The X-axis isfrequency for the spectrum map andorders of rotation for the order map.

Order Analysis

Equal Time Spacing Equal Position Spacing

Vibration Transducer

Tachometer

MAR SPL

M E A S U E M E N

DYNAMIC SIGNAL ANALYZER

MAR

M E A S U E M E N

DYNAMIC SIGNAL ANALYZER

Vibration Transducer

35670A 35670A

Shaft PositionS a m p l e s

S y n c h r o n i z i n gsamples to shaftposition gives aconstant number ofpoints per cycle.

Figure 1:Time DomainS a m p l e s

If samples aregathered at equaltime intervals, thenumber of samplesper cycle wil l vary.

Analyzing the health and behavior ofrotating machinery is a k ey applica-tion for dynamic signal analyzers(DSAs). Rotating machines produce

repetitive vibrations and acousticsignals related to r otational speed.These relationships are not alwaysobvious with standard dynamic signalanalysis, particularly with variationsin the rotational speed. A measure-ment tec hnique called order analysisis the secret to sorting out all themany signal components tha t arotating machine can generate.

Syn chroni zi ng the m easurementWith flexible settings for frequencyspan and r esolution, a typical DSAcan do a great job of isolating noise

and vibration components, as long asthe machine is oper ating at a fixedspeed. As the s peed chan ges, how-ever, the signals of interest shift up ordown in frequency, making analysisdifficult if not impo ssible. Synchron iz-ing the DSAs data collection with themachines rotat ional speed is there-fore a key step in order ana lysis.

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One obvious difference between thetwo maps is how the peaks line up.Each line of peaks on the order mapclearly indicates a relationship

between vibration and shaft pos ition;the peaks in the spectrum map aredifficult to relate to shaft speed . Themaximum amplitude in the order mapis at the 12th orde r and at 3815 RPM.This identifies the vibration in ter msof engine speed, indicating a compo-nent is be ing excited 12 times pe reach engine r evolution.

Order trackin gIn Figure 3, the 12th order appears tobe the mo st interesting, so you wantto examine that order and to ignorethe other orders. When you measure

one order and exclude the others, themeasurement is called an order track.Figure 4 shows this 12th orderamplitude versus RPM for the enginerun-up.

In the order track measurement, therelationship between the measuredorder vibration and the engine speedis clear. Order tracking helps youfocus on exact components and tomeasure their contribution to theoverall performance of a rotatingmachine.

Orders are essentially harmonics. Butunlike harmonics, many interestingorders are noninteger multiples ofthe 1st order. A speed reducer has anoutput shaft ord er vibration at lessthan the first o rder. An automobileengine has order components thatare higher ordered nonintegermultiples. These may be gear m eshrates, timing chain engagement orvalve action, for instance .

Appli cat i onsOrder tracking and order analysishave become widely acceptedrotating machinery measurements.

Devices ranging from gear motors togas turbines are t ested this way. Evendental tools have been d esigned usingorder an alysis. Some of the mor eunusual applications involve usingorder ana lysis to measure power linequality and for loudspea ker te sting.These applications take advantage ofthe harmonic nature of the integerorders.

Frequency spectrum map of 6 cylinder engine run-up

Order spectrum map of 6 cylinder engine run-up

Order track of 12th order, 6 cylinder engine run-up

Figure 2:A frequency mapreveals peaks but itis difficult to relatethem to shafts p e e d .

Figure 3:An order mapclarif ies ther e l a t i o n s h i p sbetween shaftspeed and vibrationa m p l i t u d e .

Figure 4:Order trackingclarif ies therelationship of aparticular shaftspeed to amplitudevs. rpm.