application notes

The Laser Velocity Transducer

Its Principles and ApplicationsBOO266-11

The Laser Velocity TransducerIts Principles and Applications

by Mark Serridge, Brüel & Kjær

IntroductionThere will always be occasions when

it is impossible or undesirable to actu-ally mount a vibration transducer ontoa vibrating object. It is for those occa-sions that Brüel & K j æ r in conjunc-tion with The Institute of Sound andVibration Research (ISVR) at South-ampton University, England, has de-signed the Laser Velocity-TransducerSet Type 3544. Its simplicity of useand rugged construction means thatthe user need not concern himself withthe state-of-the-art laser technologyused during every measurement: tothe user the Type 3544 simply repre-sents a highly accurate and versatilevibration transducer.

The Laser Velocity Transducer SetType 3544 consists of the Laser Veloc-ity Transducer Type 8323 and thePower Supply Type 2815. By simplyattaching a small piece of retroreflec-tive tape to the surface of the vibrat-

ing object, and aiming the Type 8323at it, vibration velocity and displace-ment at that point can be measured.

Type 3544 has evolved from the re-quirement of today’s industries for amethod of measuring vibration veloci-ty without contacting the vibratingsurface. For example, the loudspeakerindustry has long needed to measurethe vibration of light diaphragmswithout the mass-influence of an ac-celerometer; the aerospace and auto-mobile industries are familiar with theproblems of mounting accelerometerson very hot surfaces: these industriesare also familiar with the time andcost of tapping holes and mountingaccelerometers on an engine in a test-cell. In many industries, such as thepower industry, there is often a needto measure lateral and axial vibrationsof rotating shafts. The requirementfor a transducer which could be simply

pointed at any surface, and which toldthe vibration level at that surface hasalways been recognized.

With the development of the Type3544 Brüel & Kjær has taken a basicphysical principle - the Doppler Shift- and blended this with classical opti-cal principles and more recent Lasertechnology, and successfully satisfiedthe requirements of the industriesmentioned above. The Type 3544 is afully portable vibration transducerwhich the user can simply point at avibrating object and measure its vibra-tion level.

How It WorksThe heart of the Type 3544 is a low

power (< 2 mW) Helium-Neon Laser.This produces the characteristic red

Measurement

Velocity Output4

Signalprocessing

Retroreflective

FM signal

Fig. 1. The arrangement of the electronic and optical components within the Type 8323

light beam commonly associated withlasers. The arrangement of the princi-ple components within the Type 8323is shown in Fig.1.

The beam leaving the laser is splitinto two parts by using a beamsplitter.One of the beams is directed at thetest object. The other beam (the refer-e n c e beam) is directed at a rotatingdisc via one fixed and one moveableprism. Back scattered light from therotating disc and the test object returnon-axis with the incident beams andare mixed in the beamsplitter. Thislight is then directed towards a photo-detector. The frequency tracking elec-tronics after the photodetector firsttrack the Doppler frequency shiftcaused by the vibrating surface andthen produce a voltage proportional tothe surface velocity in the direction ofthe beam.

Why Use Laser Light?Laser light is used instead of any

other because it has special properties,and in particular a property calledtemporal coherence. This means thatwe can divide and recombine the laserlight such that a well defined phaserelationship still exists between thebeams upon recombination, eventhough they may have travelled differ-ent distances. The maximum path dif-ference that can be tolerated beforethe phase relationship is destroyed iscalled the coherence length. The lightlevel adjust control on the back of theType 8323 varies the distance trav-elled by the reference beam within theinstrument, and so ensures the pathdifference is not longer than the co-herence length, assuming that the tar-get is within the specified distancefrom the Type 8323.

Because the Type 8323 is a safe lowpower laser not employing focusing,the maximum distance to the target isabout 0,8m. The advantage of a non-focused beam is that the Type 8323requires no lengthy setting up proce-dure.

The neutral density filter ensuresthat the power output of the Type8323 is less than lmw and brings theinstrument into Safety Class 2.

Reflective TapeBecause the Type 8323 contains a

low power laser without focusing

Fig.2 Retroreflective tape attached to the hot exhaust system of a car (photograph courtesyof The Ford Motor Company (UK) Ltd.)

lenses it is necessary to treat the tar-get surface in some way to ensure suf-ficient back scattered intensity at thephotodetector. The easiest solution isto apply a small piece of the suppliedretroreflective tape (DU0164) to thepoint of measurement. The tape canwithstand the extremely high tem-peratures found on hot exhaust sys-tems as shown in Fig.2.

The tape consists of densely popu-lated glass spheres bonded in a medi-um. Each individual sphere acts as alight scatterer. If the tape is appliedaround the circumference of a rotatingshaft, the photodetector sees the backscattered light from the same discretescatterers which pass through thebeam with every revolution of theshaft. The same signal repeats withevery revolution. This fact is impor-tant and is used later when analyzingthe pseudo-random noise sourceswithin the instrument, since the Type8323 has a rotating disc inside it toback scatter the reference beam andfrequency shift it.

Glass spheres will always reflectlight back in the direction from whichit came. This means that the laserneed not be perpendicularly incidenton the surface in order to get back

scattering into the Type 8323. It isalways the velocity component in thedirection of the beam which is mea-sured.

An alternative to the tape is thepaint (3M Type7210), which containsglass spheres. The principle is thesame although it does not work as wellas the tape.

The Doppler EffectWhen light is scattered back from a

vibrating target it will undergo a fre-quency shift proportional to the veloc-ity of the target. This is known as theDoppler Effect. As the target movestowards the light source the back scat-tered light undergoes an increase infrequency; as the target moves awaythe back scattered light undergoes alowering of frequency. Evidentally, ifthe target is vibrating, the frequencyof the back scattered beam will be fre-quency modulated at the so calledDoppler frequency. The Doppler fre-quency is directly proportional to thevelocity of the target. Therefore, totrack this Doppler frequency is tohave a direct measure of the target’svelocity relative to the motion of thelight source.

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The Reference BeamThe frequency of the back scattered

light is of the order of 1015Hz which isfar too high in order to directly detecttypical Doppler frequency modula-tions of the order of 106Hz. So the firststep to take is to mix the scatteredlight with a reference beam and directthe two at the photodetector, wherethey heterodyne. The current from thephotodetector then becomes modulat-ed at the Doppler frequency. So indi-rectly tracking the Doppler frequencylike this produces a voltage propor-tional to the target velocity.

Frequency ShiftingThe system described so far is of

limited use in measuring vibration ve-locity. This is because vibrating sur-faces during a vibration cycle have in-stants of zero velocity. When this hap-pens we get no instantaneous Dopplershift and hence no voltage output.This leads to an ambiguity about thedirection of the velocity; in vibrationmeasurements, of course, the directionof the velocity is essential.

The problem is overcome by fre-quency shifting the reference beam byan amount of the order of one MHz.The Type 3544 achieves this by direct-ing the reference beam at a reflectiverotating disc placed at an angle to thereference beam. This adds a constantDoppler shift to the reference beam.This means that when the target ob-ject is stationary we still get an outputat the photodetector, caused by theconstant Doppler shift from the disc.The instrument can then be calibratedto give OV at this frequency. LowerDoppler frequencies can then be madeto give a negative voltage and higher

Fig.3 The rear panel of the Type 8323,showing the controls for “Range”, and“Light level Adjust”, together with the“Light Level” meter

3,456

Fig.4. The frequency shift of the reference beam is obtained by using a rotating disc which canrun at two speeds, enabling the selection of two different measurement ranges.

Doppler frequencies to give a positivevoltage. Now we can measure vibra-tion velocities without direction ambi-guities. This arrangement is easy tocalibrate and self-aligning.

The user can select two disc speeds -- 15Hz and 4Hz giving frequencyshifts of 3,456MHz and 0,922 MHz.This has the effect of optimizing thedynamic range of the Type 3544, andis achieved by using the “Range”switch on the back of the Type 8323(see Fig. 3). We can measure lower lev-els of vibration on the O-O,2 m/s range.

Dynamic RangeThe upper limit of the dynamic

range is set by the maximum Dopplerfrequency which can be tracked. Thelower limit is set by the smallestchanges in Doppler frequency whichcan be detected by the frequencytracker.

Upper limitThe upper limit is quite easy to

specify as it depends solely on themaximum frequency “swing” whichcan be accommodated by the frequencytracker. Fig. 4 illustrates this point.Large velocities produce frequencyshifts which swing down close to theDC level. Consequently, the higherdisc speed is used when we measurehigh velocities (the lm/s range) be-cause this gives more room for theDoppler swing, and the slower speed isused to measure lower velocities (the0,2m/s range). The slower speed pro-duces less Doppler shift of the refer-ence beam. The Type 3544 has a maxi-mum vibration velocity measurementcapability of lm/s peak.

An obvious advantage of the Type3544 is that since it is not in contactwith the target it cannot be damagedby measuring shocks.

Lower limitThe use of the rotating disc to pro-

duce a frequency shift has several ad-vantages over alternative techniquessuch as Bragg cells and rotating dif-fraction gratings - cost and robustnessbeing two major factors. Frequencytracking the Doppler frequency of therotating disc produces a random noisepattern which repeats itself exactlywith each revolution. A pseudo-ran-dom noise signal is produced with aspectrum as shown in Fig. 5. Hencethe spectrum of the noise consists ofpeaks at the fundamental and har-monics of the rotation speed. The low-er measurement limit of the instru-ment is limited by this noise pattern.

An effect called Doppler frequencybroadening is associated with thisnoise floor.

Doppler Frequency BroadeningIdeally, when the Type 8323 is

pointed at a stationary target a con-stant Doppler shift from the rotatingdisc should be measured by the detec-tor. In practice, this is not quite thecase, and the detector output varies intime with both amplitude and phaseand a noise floor is created. (This isnot caused by disc speed variations,which are negligible).

The term broadening refers to thewidening of the Doppler frequencycomponent. The Doppler frequencycomponent from the disc appears as apeak up to several kHz wide. Conse-quently, low target velocities causing

5

Fig.5. The noise spectra of the Type 3544 in 4 different modes. Top left is with 0,2m/s range (velocity). Top right is 0,2m/s range (displace-ment). Bottom left is 1 m/s range (velocity) and bottom right is 1 m/s range (displacement). The component at 16OHz is the vibrationfrom a calibrated vibration exciter giving lOmm/s. This is used as an indication of where the noise floor lies in terms of actual velocityand displacement amplitudes.

Doppler shifts of similar magnitudeswill go undetected by the frequencytracker.

There are several causes of broaden-ing which are quite complex, but needonly be mentioned briefly here;

When using a rotating disc to fre-quency shift the reference beam, thespatial distribution of scatterers re-peats with each revolution. The fluc-tuations of the intensity on the photo-detector therefore repeat with eachrevolution of the disc, and this is whythe spectrum of the noise floor of theType 3544 is that of the pseudoran-dom noise signal just described.

The laser spot impinges on a finitearea on the disc and consequentlythere is a velocity gradient across the

6

laser spot. All the particle scattererspresent in the spot will not be movingat the same velocity and so will pro-duce different Doppler shifts andbroaden the Doppler frequency com-ponent of the reference beam.

Each particle scatterer on the disconly remains in the beam for a finiteperiod of time, before the next particlemoves in. Each particle produces a“burst” of scatter. Because we don’thave a continuous signal at the photo-detector, this time limitation causes afrequency broadening.

Two Rotating Disc SpeedsBecause the noise floor of the in-

strument contains defined peaks, if weare interested in specific single fre-quency vibrations which do not coin-cide with the peaks, we can measure

down to lower levels at all frequenciesin between the peaks. To aid this theuser can chose between two discspeeds as mentioned. This is achievedby using the “Range” switch on theback of the Type 8323 as describedunder the section “Frequency Shift-ing”. Hence if the frequency of the lowlevel vibration you wish to measurecoincides with one of the peaks in thenoise, simply move the peaks in thenoise. A narrow band frequency ana-lyzer analyzer such as a Brüel & KjærType 2032, 2033 or 2515 helps whenmeasuring vibration levels close to thenoise floor.

Rotating Target SurfacesThe ability of laser vibrometers to

measure low level velocities is depen-

dent on the target’s surface velocitycharacteristics. If the particle scattererswithin the laser beam spot only have avelocity in the direction of the inci-dent laser beam then the laser specklepattern which they form on the photo-detector surface remains essentiallystationary and no Doppler frequencybroadenings occur.

However, if the scatterers move outof the beam and are replaced by oth-ers, such as when measuring on a ro-tating component, the speckle patternon the photodetector will move andproduce a broadening of the Dopplerfrequency associated with the shaft vi-bration we are trying to measure. Thishas the effect of significantly raisingthe level of the noise floor of laservibrometers which employ Bragg Cellsor other acousto-optic frequency shift-ing devices. However, for the Type3544, the addition of a moving specklepattern to the one already producedby the rotating disc has negligible ef-fect on the lower limit and only fur-ther peaks associated with the rota-tional speed of the shaft will appear.

The speckle pattern produced bythe target surface will also move onthe photodetector if the particle scat-terers “tilt” within the laser spot. Inpractice, and particularly at high am-plitudes of vibration, all target sur-faces have an inherent tilt associatedwith motion. This means that thepractical advantages of Bragg Cell de-vices are lost, while the change in thenoise floor of the Type 3544 is negliblein these circumstances.

Because changes to the lower mea-surement limit of the Type 3544 dueto target rotation are slight, and de-pend on the rotational speed, it is notpossible to accurately specify the noisefloor of the instrument under suchconditions and consequently thespecifications of the instrument areonly concerned with targets which donot rotate.

Shaft VibrationsThe Type 3544 can be used to mea-

sure lateral vibrations (unbalance) ofrotating shafts. Unlike proximityprobes, the laser does not interpretany changes in the geometry of theshaft as a vibration. Imperfections onthe shafts surface will not influencethe measurement. The laser will onlydetect vibrations of the centre of rota-tion of the shaft, in the direction ofthe laser beam. This may not be intu-itively obvious. The following mathe-matical proof will help.

Centre of rotation

8323

Fig.6. The Type 3544 does not interpret changes in the distance to the surface of an irregular-ly shaped target as a vibration. When correctly aimed, only axial vibrations will bedetected

Consider a shaft with an irregulargeometry, as shown in Fig.6. The shaftis rotating at an angular frequency ofw, corresponding to a tangential veloc-ity of V at the point where the laserbeam is incident, at a distance r fromthe axis of rotation, and at a verticaldistance d below it.

The tangential velocity can be re-solved into two components V, andV,, perpendicular to each other. Thecomponent in the direction of thebeam, V,, is then expressed as

V, = Vsinrv where LY is as indicated.

Since V = rw, then .

V, = rwsinrv

r can also be expressed in terms of LYand d, as

dr=-simr

Therefore, V, = - = dw

which is wholly independent of r.

Consequently, for a shaft with noaxial vibration any Doppler shift inthe laser beam is proportional only tothe shaft rotation speed w, which is aDC component, and the distance “offaxis” of the laser beam. The laser doesnot interpret the changing distance tothe target surface as a vibration.

A practical point results from thisanalysis. When making axial vibrationmeasurements on rotating shafts it isimportant to ensure that d is zero ornearly zero by carefully aligning thelaser beam so that it lies in a imagi-nary straight line passing through thecentre of the shaft. This is importantsince the DC rotation component canoften have considerably larger magni-tude than the axial vibration itself,and in some cases may overload theType 8323.

Frequency RangeUnlike the majority of

transducers, the Type 3544__vibrationdoes not

suffer any electrodynamic frequencyrestrictions such as resonances andnon-linearities of moving parts. Thereis a high frequency cut-off built intothe electronics at 20 kHz. Besides that,if there is sufficient velocity at anyfrequency below 20 kHz, (includingDC) it can be measured with the Type3544. Otherwise, the frequency rangeof the Type 3544 is inextricably linkedwith its noise spectrum as can be seenin the specifications supplied with theProduct Data Sheet, and the measure-ment range nomogram shown on theback page.

The Type 2815 contains an integra-tor to convert the velocity output ofthe Type 8323 into a displacement sig-nal, enabling measurements of vibra-tion displacements.

In velocity mode the Type 3544 isDC-coupled and hence measurementsof DC velocities are possible. In dis-placement mode the Type 3544 has a0,3Hz lower cut-off frequency. This isa result of the integration network.

To optimize the dynamic range ofthe instrument when making wide-band measurements, use is made offilters built into the Type 2815 PowerSupply. The user can select betweentwo wide-band frequency ranges in ve-locity mode (0 to 2 kHz and 0 to20kHz). One frequency range is avail-able in displacement mode (0,3Hz to20 kHz).

Both a hand-held and atripod-mountableinstrument

The Type 3544 makes relative vi-bration measurements, i.e the vibra-tion of the target relative to itself. Al-though no special expensive surface-table is required to makemeasurements, for the majority of ap-plications the Type 3544 would beused mounted on its tripod supplied(UA0989). When low level, low fre-quency vibration measurements arerequired it it essential to mount theType 3544 on the tripod.

When the instrument is used as ahand-held tool, spurious low frequen-cy noise resulting from hand vibration,body-tremble and other environmen-tal effects is introduced into the mea-surement at frequencies below 30Hz,thus raising the lowest detectable lev-els at these frequencies. Then theType 3544 is less sensitive in the sub-30Hz range and unusable in the 0 to10 Hz range. Consequently, the use of ahigh pass filter is recommended if

wideband measurements are made us-ing the Type 3544 as a handheld tool.

By using a magnet an accelerometercan be attached to the back plate ofthe Type 8323. By measuring the vi-bration of the Type 8323 in this way itis possible to obtain the absolute levelof the vibration from the relative mea-surement given by the Type 8323.However, under normal circumstanceswith the the Type 8323 standing onsolid ground, its own vibrations arenegligible.

Operating DistanceFor optimum performance the Type

8323 should be placed between 20 and80cm from the target. Within thisrange it is possible to optimize themeasurement and improve the coher-ence by varying the distance travelledby the reference beam. This isachieved by mechanically reposition-ing a mirror inside via the “Light Lev-el Adjust” control on the rear panel asdescribed earlier. A simple meter indi-cates the coherence obtained.

Fig.7. The Type 3544 can be used to measure the vibration of rotat-ing surfaces. In this example the axial vibration drive shaft ofan engine is being investigated (photograph courtesy of theAutomotive Design Advisory Unit, Southampton, England)

Type 3544 is Simpleto Use

The Type 3544 marks the advent ofeasy to use laser measurement tech-nology, being extremely simple andfast to operate.

The Type 3544 consists of two parts;the velocity transducer Type 8323containing the laser (and its associat-ed electronics), and its Power SupplyType 2815 (containing the filters andintegration networks). The two areconnected by the cable AO0308. Thiscable carries both power and velocitysignals. Type 2815 gives a calibratedvoltage output proportional to velocityor displacement. This can be fed into awide variety of analysis instrumenta-tion.

The user simply attaches the retro-reflective tape to the target and pointsthe laser at it to obtain either a wide-band vibration measurement on avoltmeter or narrow-band informationon a frequency analyzer.

Fig.8. The Type 8323 is simple to operate. It is shown here mountedon the tripod supplied

8

The angled mirror Type UA0965can be attached to the front of theType 8323. This turns the beamthrough 90°, thus facilitating mea-surements on targets in awkward loca-tions.

PortabilityA unique concept in the deisgn of

the Type 3544 is its portability, beingcompact and battery powered.

The Type 3544 and all its accesso-ries is easily carried or wheeled alongin its sturdy protective carrying case,giving a total weight of 21 kg. TheType 2815 accepts the battery boxType QB0040, which contains 12 1,2Vrechargeable batteries. These can berecharged using the charger TypeZG0166. The Type 3544 can also bepowered from any suitable +12 to+ 15 V power supply with a 4A capabil-ity. The charge time for fully depletedbatteries is 20 hours. Charging is pos-sible while the instrument is in use.

Type 3544 is SafeThe Type 3544 complies with Safe-

ty Class 2 of British Standard BS 4803- Radiation Safety of Laser Productsand Systems, IEC 825, and ANSI

Fig. 9. Portability i s offered viaa The Power Supply Type 2815 containing batteries for the Type8323, integration networks and wide-band filters

I IFig.10. The Type 3544 comes complete with a sturdy carrying case which can contain the Type 8323, the Type 8315 and all the accessories

including the tripod and spare battery box

Z136.1 (1980). Class 2 lasers such asthe Type 3544 have a target beampower of less than 1 mW and are “safefor accidental momentary viewing”.This means that the user should notstare directly into the beam. However,if by accident the laser beam shoulddirectly enter the eye, normal reflexblinking actions will be sufficient toprotect the lens and retina from allpossible injury. It is perfectly safe toexpose any other human tissue to thelaser light.

I \ I , LASER RADIATION I

Fig. 11 The Type 3544 is a Safety Class 2 la-ser device

Use With OtherInstruments

The output of the Type 2815 is avoltage-time signal proportional to thesurface velocity. For wideband mea-surements this signal can be fed into avoltmeter or measuring amplifier. Forfrequency analysis the output can befed into a digital or analogue analyzer.Brüel & Kjær manufacture a compre-hensive range of vibration measure-ment and analysis equipment for every

7006 + ZM 0053 for DC recording+ ZE 0299 for non-DC recording

I I 2815 I8323

Target

Dual ChannelReal-Tame Analyzer

Dual Channel Analyzer2032

Fig.12. Brüel & Kjær offer a comprehensive raequipment to follow the Type 3544

application. Some suggested instru-mentation setups are shown in Fig. 12.

Application AreasThe Type 3544 is not designed to

replace the accelerometer as the every-day all-purpose vibration transducer.

Fig. 13. Measuring differential vibration on a tyre as it spins (photo-graph courtesy of The Ford Motor Company (UK) Ltd.)

10

n g e of both wideband and narrowband analysis

Instead the Type 2544 is designed foruse in all occasions where it an accel-erometer cannot be used. Typical ap-plications are measurements on;

Light structures, hot structures, in-accesible parts, surfaces which mustnot be marked, very small surfaces,high voltage surfaces, radioactive sur-

Fig.14. Measuring the vibration of a rubber airhose in an engine(photograph courtesy of The Ford Motor Company (UK)Ltd.)

faces, living tissue, wet surfaces, rotat-ing surfaces (sides or ends of shafts),machines which would otherwise haveto be stopped to attach an accelerome-ter (no down-time required if paint isapplied to the rotating shaft), continu-ous surfaces moving with a DC veloci-ty (e.g. measurement of the speed of arotating disc), shock measurementsetc...

Such measurements may often berequired in the automobile, aerospace,power, and loudspeaker industries, asexamples.

Further Reading[l] The Laser Vibrometer - A Portable

Instrument, Neil Halliwell, Journalof Sound and Vibration, 1986, 107(3), pp 471-85

[2] Vibration Measurement using La-ser Technology, ISVR/SIRACourse Notes, available from SIRALtd., South Hill, Kent, England

Fig.16. Measuring unbalance of a flywheelin a diesel engine (photograph cour-tesy of the Automotiue Design Advi-sory Unit, Southampton, England)

Fig. 17 Investigating the vibration trans-mission characteristics of a CompactDisc player mounted on a vibrationexciter (photograph courtesy ofBang& Olufsen A/S Denmark)

Fig.15. Making an Operational Deflection Shape Measurement on an exhaust pipe by usingtwo lasers (photograph courtesy of The Ford Motor Company (UK) Ltd

Fig.18. Measuring the vibration of a tweeter diaphragm (photograph courtesy of Bang & O-lufsen A/S Denmark)

0.1 Hz 1 Hz 1OHz 1OOHz 1 kHz3% # ,2 00 /,.$;> 20 .,*I+’

1OkHz 100 kd

Velocity measurementrange nomogram Type 3544

10000

88035;

Fig.19. The frequency and dynamic measurement ranges (in velocity mode) of the Type 3544, indicating the possibilities when the outputthe Type 2815 is fed either into wideband analyzers such as voltmeters and measuring amplifiers, or into narrowband analyzers suchFFTs. Two areas are shown for narrowband analysis, depending on whether the frequencies of interest correspond to harmonics result-ing from the two rotating disc frequencies (4Hz and 15 Hz). The upper limit is the peak level, whereas the lower limits are RMS levels

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