Profiling of hot surfaces by pulsed time-of-flight laser range finder techniques

Profiling of hot surfaces by pulsedtime-of-flight laser range finder techniques

Kari Mdtta, Juha Kostamovaara, and Risto Myllyla

The possibilities for using the pulsed time-of-flight (TOF) laser radar technique for hot refractory liningmeasurements are examined, and formulas are presented for calculating the background radiationcollected, the achievable signal-to-noise ratio (SNR), and the measurement resolution. An experimentallaser radar device is presented based on the use of a laser diode as a transmitter. Results obtained underreal industrial conditions show that a SNR of 10 can be achieved at measurement distances of up to 15-20m if the temperature of the converter is 1400 0C and the peak power of the laser diode used is 10 W. Thesingle-shot resolution is about 60 mm (sigma value), but it can be improved to millimeter range byaveraging techniques over a measurement time of 0.5 s. A commercial laser radar profiler based on theexperimental laser radar device is also presented, and results obtained with it in real measurementsituations are shown. These measurements indicate that it is possible to use the pulsed TOF laser radartechnique in demanding measurement applications of this kind to obtain reliable data on the lining wearrate of a hot converter in a steel works.

Key words: Range finders, laser optics, thickness measurement, radar, optical.

Introduction

Pulsed time-of-flight (TOF) laser distance measure-ment techniques based on semiconductor laser diodeshave been developed extensively for industrial inspec-tion purposes in recent years.1' 2 The fact that themeasurement point is known is an important advan-tage in profiler applications, as the transmitter beamcan be focused on a small spot by means of a singlelens. The size of the optics can be reduced because ofthe shorter wavelength of the light compared withmicrowaves, and this enables small, even handheldlaser systems to be constructed. The TOF techniquemakes it possible to achieve a larger linear dynamicmeasurement range with smaller construction dimen-sions than those achieved with the triangulationmethod.3 The radiation source in the pulsed TOFtechnique is usually a semiconductor laser diode,which is stable and long lived and allows a pulse-repetition rate of several kilohertz, which meansreliable and faster measurement than with devices

K. Mddtta and J. Kostamovaara are with the Electronics Labora-tory, Department of Electrical Engineering, University of Oulu,SF-90570 Oulu, Finland. R. Myllyla is with the Technical Re-search Centre of Finland, Optoelectronics Laboratory, P.O. Box202, SF-90570 Oulu, Finland.

Received 24 February 1992.0003-6935/93/275334-14$06.00/0.© 1993 Optical Society of America.

that use the other TOF principle, modulation of acontinuous wave (the CW technique). The pulsedTOF technique always gives an exact result for anobject at an unknown distance, whereas the CWtechnique usually requires the distance to be mea-sured at at least two modulation frequencies toproduce an exact result.4 The transmitted laserpulses used in the pulsed TOF technique are usuallynarrow, which means that a high peak power of up totens of watts can be achieved while the averageoptical power level remains low. This condition andthe well-defined beam are important points if measure-ments are to be carried out in environments where alarge number of workers are present. If the receivedsignal-to-noise-ratio (SNR) is small, so that the single-shot resolution is not good enough, resolution can beimproved as far as the millimeter range by using anaveraging technique without the measurement timebecoming unnecessarily long.

One important field of application for this tech-nique concerns profiling measurements in industry,e.g., measurement of the shape of huge steel blocks inshipyards,5 which can be done to an accuracy of a fewmillimeters before assembly. This means savings inassembly time and costs. Another application inwhich benefits can be achieved, especially in terms of afaster measurement time, is described in this article.This application is concerned with checking the thick-ness of hot refractory linings in steel works.6 7 8

5334 APPLIED OPTICS / Vol. 32, No. 27 / 20 September 1993

In the modern LD-KG (Linz Donawitz-KawasakiGas) steelmaking process adopted at the Raahe SteelWorks in Finland in 1984, the LD converter in whichthe steel is produced is a large, cylindrical steel vessellined with heat-resistant bricks. The diameter ofthe converter body at the Raahe Steel Works is 5.4 mand its depth is 7.1 m; the lining thickness variesbetween 60 and 100 cm and is thickest on the bottomof the converter. A typical campaign time for theconverter is approximately 1400-1600 heats, whichtakes roughly 50 days, after which the converter mustbe relined, at a cost of roughly $250,000. To achievemaximum savings, the converter should be used foras long as possible without the risk of burnoutbecoming too high. These requirements indicate theneed for an effective, reliable means of checking thelining thickness, which should make it possible tomeasure between tapping and recharging to preventunnecessary costs and time-consuming delays in pro-duction. The total profiling time should also be asshort as possible, preferably less than 10 min, toprevent the converter from cooling down too much.This means that the temperature of the lining duringmeasurement will be high, usually between 1100 and1400 C, which induces severe background radiationand noise problems.

The possibilities for applying TOF techniques tothe profiling of hot surfaces are discussed here.First the theory of the measurement method isexamined as far as the amount of background radia-tion, noise, and achievable SNR are concerned, andthen the basic construction of the experimental laserradar device, which contains an optomechanical mea-suring head and distance measurement electronics, ispresented. After this, basic measurement resultsobtained in the laboratory and in real industrialenvironments are shown, and finally a commerciallaser profiler, LR-2000, based on the prototype de-scribed in this article is presented, and some realmeasurement results obtained in hot converters arequoted.

Signal and Noise

Background Radiation

Many problems arise when one is carrying out TOFlaser profiling measurements in a hot converter.First the high temperature introduces backgroundradiation that detracts from the SNR and the single-shot resolution of the measurement or increases themeasurement time. Second the speed of light isdependent on the refractive index of the air, which isa function of temperature,9 and this variation in the

refractive index may alter the transit time of the laserpulses and cause instability as the temperature of theair in front of the target varies. Possible tempera-ture gradients along the path of the measurementbeam can also cause local variations in the refractiveindex, which may disturb the beam and cause instabil-ity in the measurements. Third the temperature ofthe air decreases further away from the hot lining ofthe converter, and if the surface is measured at anangle, the laser beam can be refracted because therefractive index changes. This may cause nonlinear-ity in the results at different measurement angles.Also, a huge amount of dust and smog exists in frontof the converter immediately after tapping, and thiscan attenuate the received signal and scatter some ofit back to the receiver, which can take the form of anunwanted stop pulse before the real one.

The radiance of an ideal blackbody can be calcu-lated by integrating Planck's law of blackbody radia-tion over a desired wavelength interval. The radi-ance of a real target is obtained by multiplying thespectral radiance of the blackbody by the emissivity Eof the surface. The e value is usually a function oftemperature and wavelength, 0 but in this article it isassumed to be a constant (0.8), as no exact estimate ofits variation as a function of temperature is available.An example of the radiance of a graybody, LB, and thewavelength at which LB is at its maximum, Xp, arepresented in Table 1, calculated around the wave-length of a GaAs single heterojunction laser diode,906 nm.

To achieve an estimate of the background radiationcollected from a hot target, a schematic diagram ofthe typical receiver optics of a laser profiler con-structed with one positive lens is presented in Fig. 1.The optics consists of a detector of area AD at adistance s, from the lens. The signal reflected fromthe target is collected on the detector by the receiverlens, with an area of AR and a focal length R. Thebackground radiation is reduced by a narrow-bandoptical interference filter located either in front of thereceiving lens or between the lens and the detector.The receiving optics are adjusted in a such way thatthe lens forms an image of the detector of area A, at adistance sl' from itself. This image can be eitherreal or virtual, which means that the focus distancecan be from minus infinity to plus infinity. Thedistances between the detector and lens and theimage and lens can be calculated according to theGaussian lens equation.

Table 1. Radiance of the Graybody L8 and Peak Wavelength AP at Different Temperatures(X = 906 nm, AX = 886 nm ... 926 nm = 40 nm and e = 0.8)

Temperature (C)

800 900 1000 1100 1200 1300 1400 1500 1600

xp(pm) 2.70 2.47 2.28 2.11 1.97 1.84 1.73 1.63 1.55LB(Wsr- 1 m-2 ) 2.3 8.1 23.4 58.3 127.9 254.1 465.1 795.1 1283.7

20 September 1993 / Vol. 32, No. 27 / APPLIED OPTICS 5335

A

small as possible. This product can be expressed as afunction of the area and focal length of the receiverlens, the detector diameter, and the focus distancewith the formula

AR ARAD(S1' -f)ARflI = A 12 S1'2f2R)2Detector

ARAD

* 2

( fR

Fig. 1. Schematic diagram of the receiver optics.

Calculation of the background radiation, PB, isbased on the formula

(2)

Equation (2) implies that a change in the focusdistance from 1 m to infinity, for example, willincrease the product AR fI by only a factor of approxi-mately 1.3 if the focal length of the receiver lens is120 mm. Thus the focus distance of the receiveroptics can be freely adjusted without having mucheffect on the received background radiation.

A,PB TRLBARPI = TRLBAR 71 1 = TRLBAIfIR, (1)

where TR is the transmission of the receiver optics.Equation (1) can easily be understood to be true if thehot target is at the distance of the image plane of thereceiver lens, but it is in fact valid at all targetdistances. If the position of the target changes, thedetector and its image remain fixed, and the detectorcan be thought to be replaced at the position of itsimage. Because the solid angle that collects radia-tion, QR = AR/si' 2 , and the area of the detector image,AI, are independent of the position of the target, thebackground power collected remains constant even ifthe distance between the radiating target and theimage of the detector changes. The situation is thesame as if the detector is looking at the radiatingtarget through a hole of distance s' and area AR.The assumption that the background power receivedis independent of the target distance as long as thefocus distance remains constant was confirmed bymeasuring the optical radiation received from a hotoven at a temperature of approximately 800 C withthe optics shown below. The results are given inFig. 2.

To reduce the background radiation by opticaldesign methods, the product AR fl should be kept as

z0It

0

z0

0C,

1.4

1.2

1.0 4

0.8

0.6

0.40 2 4 6 8 10 12 14 16

DISTANCE [m]

Fig. 2. Measured background radiation in a hot oven as a functionof distance at focus distances of the receiver optics of 6 m (0), 8 m(0), 10 m (A), and 12 m (x).

Total Noise of the System and Its Contribution toMeasurement ResolutionThe resolution of a TOF measurement is determinedby the SNR of its detection. A simplified formula bywhich the distance resolution, (URCL, of a single shotmeasurement can be estimated is"

c un 0.35cURCL = 2 du/dt 2BSNR

where c is the speed of light in a vacuum, u, is the rootmean square value (rms) of the noise, du/dt is theslope of the timing pulse at the moment of timing,and B is the bandwidth of the timing pulse. Theslope of the timing pulse in Eq. (3) is approximated bythe peak value of the signal us divided by the rise timetr of the signal and the rise time is approximated bythe formula tr = 0.35/B. The SNR is the signal-to-noise ratio us/u,. If the number of measurementsto be averaged is N, then the resolution in Eq. (3) isimproved by a factor of 1/V/N.

Equation (3) can be applied directly only to con-stant level detection, and Eq. (3) assumes that thenoise in the reference level is small compared withthat in the signal. Timing discrimination in pulsedTOF laser radar devices is determined by a constantfraction timing discrimination (CFD) principle ofsome kind,' 2 in which the timing is performed bydividing the incoming pulse into two parts, delayingone part in a such way that the timing moment occurswhen the falling edge of the undelayed pulse and therising edge of the delayed pulse cross, at a momentwhen the amplitude of these pulses is approximatelyhalf of the maximum value. In this detection princi-ple the rms value of the noise in the rising edge isroughly the same as that in the falling edge at themoment of timing. If the noise levels in the leadingand trailing edges are uncorrelated, the distanceresolution of the CFD, URCF, can be estimated to be

C TaL2

+ UT2

(URCF -2 (I duL/dt + I duT/dtl) (4


a a..l.....l.........l............|

sS l|

Se

(3)

4

(4)

where uL, UT, duL/dt, and duT/dt are the rms noises ofthe leading and trailing edges of the timing pulses andthe slopes of these edges, respectively. If UL = UT

an and the slopes of the leading and trailing edges areequal, then the distance resolution of the CFD detec-tion is URCL/V' -

Equation (4) should also be applied to the detectionof the start signal. If the noises in the start and stoppulses can be assumed to be uncorrelated and if thetiming moment in the start channel is also deter-mined based on the CFD principle described above,the total resolution CR can be approximated by sum-ming the distance resolution of the start and stopchannels as random variables:

0.35c L 1 1 1/2

2 V2B (SNRSTART)2 (NSTOP)2 (5)

where the signal bandwidths in the start and stopchannels are assumed to be equal. Any other jittersources that reduce the resolution, such as the single-shot resolution of the time-interval measurementmust also be summed as a random variable in Eq. (5)to obtain the total resolution.

The TOF range finding systems involve three mainnoise sources that reduce the SNR: the signal itself,the noise in the electronics (preamplifier and ava-lanche photodiode), and background-induced noise.The summary of each noise source is shown in Table2, reduced to current noise in the input of thepreamplifier. Typical numerical values for each noisecomponent are shown in Table 3.

The signal level at the timing moment is scaledfrom the peak value of Ps by use of the factor kTp inthe calculation of the signal noise. If the timingpoint is at a half of the peak amplitude of the pulse, afirst approximation to the value of kTp is 0.5 if thebandwidth of the receiver channel is roughly thesame as that of the optical pulse or higher. Theprimary response Ro of the silicon AP diode is 0.44A/W at a wavelength of 900 nm, if the quantumefficiency is 60%. The term F(M) is the excess noisefactor of the AP diode, which is a function of the APDgain M. There are several formulas for the excessnoise factor F(M) in the literature, of which the onegiven by the manufacturer of the diode13 is used inthese calculations:

F(M) = 0.98(2 - - + 002M (6)

Table 2. Noise Sources of the TOF-Laser Radar

Noise Component Noise Value

Signal-induced noise ins2 2qBnPskTpRoM 2 F(M)AP diode noise ind2

2qBn[Ids + IdbM 2F(M)]

Preamplifier noise ina2 2qB.IB + RF

Background-radiation-induced 2qBnPBRoM 2F(M)noise inb2

Table 3. Typical Noise Levels (pA/V/i)

Noise Component

, i,.< inb

Ps = 14.7 Ps = 147 ind Cjn=O Cin=2 T= T=nW nW C30902 pF pF 7600C 1400-C

7.4 23.5 0.23 2.3 3.4 6.6 88

The value of the signal noise in Table 3 was calculatedwith two values of Ps, corresponding to ratios of thepeak signal current to the RMS value of the electron-ics noise current of 10 and 100. The terms k, q, andBn in Table 2 correspond to Bolzmann's constant, theelectron charge, and the noise bandwidth of thereceiver electronics, respectively.

Noise in the AP diode is caused by the detector darkcurrent, which contains two parts: Ids, which is notmultiplied, and db, the multiplied part.14 Theamount of noise contributed by the dark currentdepends on the active area of the diode, so that diodeswith a small area (0.2 mm2), like C30902E, have noisethat is about one tenth of that of the preamplifier andcan thus be neglected.

The noise contribution of the transimpedance-typepreamplifier arises from four sources: the base cur-rent B, the base spreading resistor and collectorcurrent of the first transistor stage, I, and thefeedback resistor of the preamplifier itself, RF. If thetotal capacitance in the input of the amplifier, Ci,, issmall, the dominant noise sources in the preamplifierwill be IB and RF.15

The reflected current noise, i/B, in the input ofthe preamplifier is thus

-= [2q(kTpPs + PB)RoM 2F(M) + ina]1/2. (7)

When the target temperature increases, noise fromthe background radiation becomes the dominant noisecomponent, whereas below approximately 760 C thenoise contribution of the electronics is the dominantone, with a value of about 6.6 pA/\ 4H- in thisexperimental system.

Received Signal as a Function of DistanceThe optical signal, Ps, received at the detector isusually approximated as a function of distance by theradar equation, which gives a good approximationover long distances. In this case the whole signalthat reaches the receiver lens goes on to the detector,reduced only by the transmission of the receiveroptics. Over short distances only a fraction of thesignal collected by the receiver lens goes on to thedetector, because the image of the illuminated targetdoes not usually fit entirely on the detector surface.


The signal received at the detector surface, Ps(x), canbe approximated at all distances x by the modifiedradar equation:

PPT ' 'FRAR -2xPSWx = 2 T(x)eYx

where p is the reflectivity of a diffusely reflectingtarget in the direction of the normal of the surface, PTis the optical power of the laser diode, and TT is thetransmission of the transmitter optics. The expo-nent term represents a coefficient by which the signalis attenuated in the round trip for atmosphericreasons.' 6 The constant y, an extinction coefficient,represents the attenuation factor, which is usuallyassumed to be zero except when one is measuringdistances over hundreds of meters or if atmosphericconditions are very severe, because of dust, smog, orsmoke, for example. The term T(x) in Eq. (8) repre-sents the effect of the fixed optics on the signal leveland can be assumed to have a value of 1 only if thedetector sees the whole area illuminated by thetransmitter. As explained above, this is usually thecase with long distances and also with short distancesif the transmitter and receiver optics can be focusedand their beams can be kept overlapping at themeasurement distance.

The value of T(x) is dependent on the distance,usually in a highly complicated manner depending onthe construction and adjustments of the optics.This is impossible to evaluate in symbolic formbecause the transmitter beam profile is not uniformat out-of-focus distances, and the receiver usuallysees only a fraction of it at short distances if paraxialoptics are concerned. To calculate T(x), the radiancecaused by the transmitter and the solid angle atwhich the receiver is seen must be determined, andthe product of these two must be integrated over thewhole area from which the receiver optics collects thereflected signal. Details of this calculation, which isbest carried out in numerical form by means of acomputer program, are presented in Ref. 17. As anexample, three simulated curves (y = 0) with differ-ent optical adjustments, together with the 1/x2 curveobtained by the radar [Eq. (8)] when T(x) = 1, areshown as a function of distance in Fig. 3. The scaleis set to 1 at a distance of 8.5 m, where the curve Areaches its maximum, and the other two signal curvesare scaled with respect to this value. The adjust-ment of the optics clearly has a great effect on theamount and dynamics of the received signal in themeasurement area of 3-30 m, for example.

The signal current is in the input of the preampli-fier can be obtained from Eq. (8) by multiplying it bythe response of the AP diode:

is(x) = Ps(x)MRO. (9)

1.2

1.0

-J08

5E 0.8(9

we 0.6

(8) M 04

0.2

0.0 - -- I -' I

0 5 10 15 20 25 30

DISTANCE [m]

Fig. 3. Calculated beam overlap function with three adjustmentsof the transmitter and receiver optics: A, The transmitter andreceiver focus distance is 10 m, and the optical axes cross at 10m. B, The transmitter and receiver focus distance is 24 m, andthe optical axes cross at 10 m. C, The transmitter and receiverfocus distance is 24 m, and the optical axes cross at 24 m. D, Thereceived signal observes the l/x2 rule when it is fitted to curve A at10 m or to curve C at 24 m.

Estimation of the Signal-to-Noise Ratio

The SNR at the detection level can be expressed bycombining Eqs. (7) and (9):

SNR(x) = is(x)Zn

MRoPs(x)[2q(kTPPS(x) + PB)RoM 2F(M)B. + ina2 Bn]'/2

(10)

In real measurement situations in iron works, thedominant noise component when the temperature isabove 760 0C is the background-induced noise. Itcan be seen from Eqs. (1), (8), and (10) that in thiscase the SNR is proportional to (AR/flI)1/ 2 . To in-crease the SNR, the transmitter spot should be madeas small as possible, so that the receiver beam, andthus 1., can be made small. This can be done byincreasing the focal length of the transmitter lens,but at the same time its diameter must be increasedto avoid numerical aperture (NA) losses in the trans-mitter optics. The diameter of the active area of theradiating source can be reduced, of course, but theoutput power usually decreases, too. On the re-ceiver side, any increase in the diameter of thereceiver optics will increase the SNR. The NA of thedetector sets the upper limit for the diameter of thereceiver lens unless its focal length is increased,which will then reduce the receiver beam diameter.

Construction of an Experimental Laser Radar

Construction of the Distance Measurement ElectronicsThe main parts of the distance measurement electron-ics are a laser transmitter, two receiver channels forstart and stop pulses, a time-to-digital converter(TDC), and a pP-based control unit. A simplifiedschematic diagram of the electronics is shown in Fig.4, a more detailed description is presented in Ref. 6,and typical electrical parameters are summarizedbriefly in Table 4.


RS-232C

Fig. 4. Block diagram of the distance measurement electronics.

The transmitter consists of a pigtailed semicondu6-tor laser diode and pulsing electronics based onavalanche breakdown of a specially selected transistor.The laser is inserted inside a box, the temperature ofwhich is stabilized by a Peltier element. This im-proves the stability of the distance measurement withrespect to temperature.

The start channel consists of a pigtailed P-I-N-typephotodiode, a transimpedance preamplifier, two post-amplifiers, and a CFD to convert analog pulses toaccurate emitter-coupled logic (ECL)-level start pulses.Optical start pulses are generated in the measuringhead and converted to current pulses by the PINphotodiode.The signal bandwidth of the receiver chan-nel is matched to that of the optical laser pulse, andthe amplification of the start channel is adjusted so

Table 4. Typical Values of the Electronics Parameters

Parameter Value

Optical power of the laser (PT) 10-20WFWHM of the optical pulse 10 nsRise time of the optical pulse 3.5 nsWavelength of the laser 906 nmFWHM of the laser wavelength 2 nmTypical pulse repetition rate of the laser 4 kHzElectrical bandwidth of the receiver (B) 100 MHzNoise bandwidth of the receiver (B ) 122 MHzInput noise of the preamplifier (inn) 6.6 pA/^/iDynamic range of the electronic gain control 25 dBWalk error (1:4 dynamic range) < ±3 mmPrimary response of the AP diode, Ro 0.44 A/WGainoftheAPdiodeM(PB = OW) 113Excess noise factor of the AP diode, F(M) 4.2

that the signal level in the CFD is at its optimumvalue.

The stop channel includes a pigtailed silicon ava-lanche photodiode, a preamplifier, gain control elec-tronics, two postamplifiers, and the CFD. An APdiode is used because the signal level received fromthe converter is too low for a PIN photodiode. Gaincontrol is performed electrically and has a dynamicrange of 25 dB, which is usually sufficient whenmaking measurements in actual factory environments.If more attenuation is needed, the processor lowersthe gain of the AP diode by reducing the bias voltage.

The timing moment in the start and stop channelCFD is based on crossing of the leading and trailingedges of the delayed and direct timing pulses, respec-tively, so that timing takes place not at the peaksignal level but at the half-amplitude point. A shortECL-timing pulse of about 10 ns is produced by theCFD and is transmitted to the TDC at the timingmoment as a start or stop pulse. The typical walkerror of this kind of CFD is within 3 mm in adynamic range of 1:4. Distance measurement ispossible if the ratio of the peak value of the signal tothe RMS value of the noise from the electronics isgreater than approximately 10. With a lower ratiothere are too many unwanted stop pulses in thereceiver channel caused by noise, which slows downreliable distance measurement too much.

The time interval between the start and stop pulsesis measured with the TDC, which is a fast, accurate,stable time-interval measuring device that uses adigital counting technique together with an analoginterpolation method.18


Construction of the Optical Measurement HeadThe optomechanical measuring head of the experimen-tal laser radar consists of paraxial optics with aseparate transmitter and receiver channel. Paraxialoptics were chosen because they have a better trans-mittance than coaxial optics, thus improving theinherently low SNR of measurements involving hotrefractory linings. The schematic diagram of theoptics is shown in Fig. 5, a detailed description of themeasurement head can be found in Ref. 6, and typicaloptical parameters are summarized in Table 5.

The pulses from the laser diode are guided to themeasuring head by an optical fiber, and the lightemerging from the fiber is collimated and focused by ahigh-quality achromatic lens. A fraction of the trans-mitter light pulse is picked up as a start pulse. Thisoptically generated start pulse improves the stabilityand resolution of the system relative to an electricallygenerated start pulse. An optical interference filteris placed in front of the transmitter fiber to reduce theheating effect of a hot target. The diameter andfocal length of the receiver lens are the same as thoseof the transmitter lens. The stop fiber feeds thelight to the AP diode of the stop channel receiver, andin front of it there is a narrow-band interference filterthat deflects most of the background radiation away.A heat-resistant glass shield is placed in front of thetransmitter and receiver lenses to protect the opticsfrom dust and other mechanical hazards present inindustrial environments.

The total losses in the transmitter optics consist ofNA mismatch losses between the transmitter fiberand lens and transmission losses in the interferencefilters, glass shield, and air-glass surfaces of thetransmitter lens, and total losses in the receiveroptics consist of losses in the glass shield, air-glasssurfaces of the receiver lens, and interference filterand connection losses between the receiver fiber anddetector. The detector is placed inside a receptacleto which the receiver fiber is connected with a fiberconnector. The fiber-diode transmission, TFD, is cal-

OPTICALINTERFERENCEFILTER

Table 5. Parameters of the Optical Measurement Head

Parameter Value

TransmitterFocal length of the transmitter lens ( fT) 120 mmDiameter of the transmitter lens 50 mmFocus distance of the transmitter 10 mDiameter of the transmitter fiber 0.3 mm

ReceiverFocal length of the receiver lens (fR) 120 mmDiameter of the receiver lens 50 mmFocus distance of the receiver 24 mDiameter of the receiver fiber 0.4 mmBeam overlapping distance 12 m

Interference FilterWavelength 906 nmbandwidth (FWHM) 40 nmbandwidth (10% transmission) 80 nm

Triinsmission of the opticsTotal transmission of the transmitter optics (rT) 0.5Total transmission of the receiver optics (QrR) 0.32Transmission of the shield glass 0.86Transmission of the air-glass surface 0.96Transmission of the fiber-diode connection (TFD) 0.50Transmission of the interference filter 0.78

culated from the core diameter of the fiber, thediameter of the active area of the detector (0.5 mm),the distance between the fiber and the detector (1.3mm), and the NA of the receiver fiber (0.3, 1/e 2-intensity level).

The diameter of the measurement beam varies as afunction of the measurement distance. The diame-ter has a minimum of 25 mm at the focus distance ofthe transmitter, 10 m. Variations in the reflectivityproperties and undulations of the target surfaceinside the measurement beam area can cause errorsin distance measurement of the order of some tens ofmillimeters. The finite size of the measurementspot rounds off any sharp stepwise variations in thesurface and smooths out any undulations of thesurface. The effect of these variations of the surface

ROBAX SHIELD GLASS

SEMI-TRANSPARENT(93 %) MIRRORI

TRANSMITTERFIBER 300um

START RECEIVERFIBER 400um

- DIFFUSE-a{ REkLECTING SURFACE

BOTH LENSES:DIAM: 50 mmF: 120 mmHIGH QUALITYACHROMATS

_ uSTOP RECEIVERFIBER 400um

OPTICALINTERFERENCEFILTER

Fig. 5. Schematic diagram of the optomechanical measuring head.


GM=

can be estimated if the beam intensity profile, theshape of the laser pulse in the time domain, and theprinciple of the timing discrimination method areknown.

Measurement Results under Laboratory ConditionsResults regarding the resolution, linearity, and stabil-ity of the experimental distance measurement elec-tronics are presented below with respect to tempera-ture and time as measured under stable laboratoryconditions.

The measured and calculated single-shot resolu-tions of the electronics as a function of the SNR in thestop channel are shown in Fig. 6. The SNR isexpressed as the ratio of the peak value of the signalcurrent to the RMS value of the noise current of theelectronics. The single-shot resolution of the TDC,6 mm (sigma value),' 8 is added to the calculatedresolution. We altered the value of the SNR duringmeasurement by using neutral-density filters in frontof the transmitter optics. The calculated and mea-sured resolutions are in good agreement, and theresolution at high SNR values is limited to approxi-mately 10 mm. The ratio of the peak value of thesignal current to the RMS value of the noise currentof the electronics in the start channel was approxi-mately 90.

The effect of averaging on the resolution can beseen in Fig. 7, where the measured resolution isplotted at two SNR values. The measured valuecorresponds to the theoretical 1/A/9 curve, as Nvaries from 1 to 4000. A small deviation from the11FN curve can be seen in both curves at large valuesof N. The resolution can be improved to a millime-ter range, however, if the SNR is more than 10 and ifN is approximately 1000.

The nonlinearity of the device as a function ofdistance is presented in Fig. 8. The measurementwas performed on a black lining brick with a reflectiv-ity, 0.06, corresponding. to that of the cool converterlining. The brick was moved along a scale in 0.5-isteps and was capable of being positioned with anestimated accuracy of ± 1 mm. The maximum signalwas collected at a distance of 10 m, and the variationin signal level was approximately 23 dB in the area

ES

E.az050Cowr

80

70

60

50

40

30

20

10

0

10 100 1000

SIGNAL-TO-NOISE RATIO IN STOP CHANNEL10000

100E

5 10

Cu

E

0Co

0

0.1 1 10 100 1000 10000

NUMBER OF MEASUREMENT RESULTS AVERAGED

Fig. 7. Resolution ofthe electronics as a function ofthe number ofmeasurements averaged, with SNR values of 10 (0) and 270 (l).

measured. The gain control electronics had an inter-nal delay that depended on the attenuation. Thisdelay was partly compensated for by the processor inFig. 8.

The stability of the electronics with respect toambient temperature is shown in Fig. 9. The temper-ature of the laser was stabilized to +20 C by thePeltier element. The drift of the electronics withouttemperature compensation is linear and is approxi-mately -1. 1 mm/'C. If the temperature of the laserhad not been stabilized, the drift would have beennonlinear, as presented in Ref. 6, because of thevariation in the shape of the optical pulse in the timedomain, for which the CFD cannot compensate.The temperature stability of the TDC of this device isbetter than -0.2 mm/'C, which means that oneprobable reason for the temperature drift is nonequaldrift between the start and stop channels. The driftwas found to be independent of the measurementdistance and the amplification of the receiver chan-nel, which means that we can easily compensate forit. Figure 9 also presents the drift of the devicewhen the temperature drift is compensated for by theprocessor according to a preprogrammed tempera-ture-drift table for the electronics. The stability ofthe temperature-compensated electronics is approxi-mately ±4 mm over the whole temperature range,-20-+40 C.

The stabilization time of the electronics after poweron is shown in Fig. 10. The measured distance isstabilized to an accuracy of ±5 mm within 5 min andto an accuracy of ± 1 mm within 20 min. The

30

20

1

z -100

-20

-30

Fig. 6. Calculated (A) and measured (1) single shot resolution ofthe electronics as a function of SNR (ratio of the peak value of thesignal to the rms value of the electronics noise).

0 5 10 15

DISTANCE [m]20 25 30

Fig. 8. Nonlinearity of the electronics.


t

1

40

30

20

E 0

o-10

-20

-30

-40 . .-30 -20 -10 0 10 20 30 40 50

AMBIENT TEMPERATURE [C]

Fig. 9. Temperature stability of the electronics with temperaturecompensation (0) and no temperature compensation (l).

temperature of the electronics rises approximately4 C during stabilization. The total drift duringstabilization is -20 mm, of which the TDC accountsfor 3.5 mm. The reason for the stabilization driftlies in the two-channel electronics. The start andstop channels have nonequal drift during thermalstabilization of the electronics.

The long-term stability of the electronics has beenmeasured over a time period of approximately 60 hr,and the drift in the measured distance was found toremain within 3 mm. The temperature of theelectronics varied periodically by approximately ± 3 Cduring measurement.

Measurement Results in Real Environments

The measurements described in this chapter weremade with the experimental laser radar device in anactual industrial environment, the Raahe Steel Worksof Rautaruukki Ltd., during normal operation.

Background Radiation

The background radiation collected by the optics wasmeasured with the optomechanical head describedearlier and with an optical power meter. The targetwas a hot converter, ad the distance to the bottomwas 10 m. Measurements were made at a rightangle to the bottom lining of the converter. Thebackground radiation was fed to the power meter viaan optical fiber of the same type as used in the stopchannel. The focus distance of the receiver optics

was 24 m. The temperature of the converter surfacewas measured with a pyrometer.

The measured and calculated background radia-tions in the receiver fiber are shown in Fig. 11 asfunctions of the temperature of the converter. Thebackground power was calculated with Eq. (1). Thetransmission of the receiver optics from the shieldglass to the output of the receiver fiber, without theconnection coefficient between the fiber and detector,was 0.67.

The measured and calculated values are not exactlythe same if the emissivity is 0.8, as was assumed.Instead an emissivity value of 0.7 gives results within± 10% of measured background radiation. The factthat the measured background radiation differs onlyslightly from the calculated value nevertheless provesthat the design of the optics is correct, for the opticscollects radiation only within the solid angle deter-mined by the diameter of the receiver fiber and thefocal length of the receiver lens. There is no excessradiation that reaches the measurement head fromoutside the solid angle and is transmitted to thedetector by internal reflections. The interferencefilter effectively blocks the background radiation,which lies beyond the passband.

Gain of the AP DiodeTo measure the dependence of the gain of the APphotodiode as a function of bias current, the receiverfiber was installed in the transmitter optics duringthe previous measurement, and the transmitter op-tics focused at the same distance as the receiveroptics, with their axes overlapping over a measure-ment distance of 10 m. Thus the optical powermeter and AP diode were looking at the same point inthe converter through identical optics. The depen-dence of the gain of the AP diode with respect toambient temperature was corrected by increasing thebias voltage by a factor of +0.7 V/C.

The gain of the AP diode M was calculated as afunction of the measured bias current, B, by the

30

20

10

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-10

-20

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25

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Z W

23 Eow

22 ,, j

21

5

z07a0

2030 35 40 45

4

3

2

01000 1100 1200 1300 1400

TEMPERATURE OF A CONVERTER [-C]

Fig. 10. Stabilization of the electronics after power on. Thetemperature drift of the distance is compensated for by theprocessor.

Fig. 11. Measured (El) and calculated background radiation in thehot converter. The emissivity of the surface is 0.6 (x), 0.7 (A),and 0.8 ().


E

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1500

I

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.... ill''i, ... i.... i....

equation

(11)M ' IBTFDPB R0

The calculated and measured bias currents of the APdiode are shown as a function of the temperature ofthe converter in Fig. 12, and the gain of the APphotodiode is shown as a function of the bias currentin Fig. 13.

The measured gain of the AP diode at a currentlevel of 10 ptA is only slightly more than the minimumvalue promised by the manufacturer on the diodedatasheet. The calculated gain would be higher ifthe transmission of the fiber-diode connection waslower, but as mentioned earlier, the given value wascalculated from the geometry of the connection.The decrease in the gain of the AP diode when thedc-bias current is increased from 10 to 80 A is 27%.A decrease in gain of this kind was ascertained later inlaboratory measurements carried out on a number ofAP diodes. According to the manufacturer, the re-sponse of the AP diodes is linear over a wide inputpower level, 0.01-10 nW, depending on the band-width and the quantum efficiency.' 9 Gain satura-tion can occur because of a number of effects; how-ever, the principal ones are voltage drop across theload resistance or bias circuitry, heating effects, andspace-charge effects. All three reasons seem to beprobable in this application.

The 80-[iA bias current level corresponds to aconverter temperature of 1405 C, which is achievedimmediately after the molten steel is poured out.The maximum allowable continuous dc reverse cur-rent of the AP diode is 200 A,' 3 which can beestimated to be achieved if the temperature of theconverter is about 1530 C and no bias current limita-tion is included in the bias circuitry.

Background Radiation-Induced NoiseThe background radiation-induced noise was alsomeasured at the same time as the gain of the APdiode. The radiation collected by the optics from thehot converter was guided to the AP diode via the stopfiber, and the noise level was measured at the outputof the stop channel amplifiers with a wideband rf

zCD

140

120

100

80

60

40

20

0

Fig. 13.current.

0 10 20 30 40 50 60 70 80 90

BIAS CURRENT [uA]

Gain of the AP photodiode as a function of its bias

millivolt meter. The measured and calculated noisecurves, reduced to the input of the preamplifier, arepresented in Fig. 14 as a function of the temperatureof the converter.

Three noise curves are shown in Fig. 14. The firstcurve was obtained from the calculated backgroundradiation, Eq. (1), after which the noise was calcu-lated with Eq. (7); the second curve was calculatedfrom the measured background radiation, as shownin Fig. 11, also with Eq. (7); and the third curve wascalculated from the measured electrical noise, scaledto the input of the preamplifier by dividing it by thebandwidth and gain of the preamplifier and postampli-fiers. In all these calculations the gain and excessnoise factor of the AP diode are corrected with respectto the bias current shown above.

All three noise curves are almost linear and identi-cal in slope, having only different amounts of offset.If the emissivity were expected to have a value of 0.7,the first and second curves would be identical. It canbe deduced by extrapolation from all the curves thatthe background-radiation-induced noise is equal tothe electronics noise 6.6 pA/JHIz at temperatures ofapproximately 800 C.

Signal-to-Noise Ratio

The SNR was measured withpulse power of approximately

a laser transmitter of10 W. The transmit-

100

1000

.:

in

0a-

100

10

1000 1100 1200 1300 1400 1500 1600TEMPERATURE OF A CONVERTER ['C]

Fig. 12. Measured () and calculated () bias current of the APphotodiode as a function of the temperature of the converter.

I

U0

z

80 -

60 -

40

20

1000 1100 1200 1300 1400TEMPERATURE OF A CONVERTER ['C]

1500

Fig. 14. Measured () and calculated noise scaled to the input ofthe preamplifier as a function of target temperature. Calculationsare based on the measured background radiation (A) or thecalculated background radiation ().


14

rE

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . - . . I . . .

. . . . . I . . . . I . . .

n o

ter and receiver optics were focused and their axeswere set to cross at 24 m and 10 m, respectively. Themeasurement distance was 10 m. The signal andnoise levels in the output of the stop channel amplifi-ers (input of the CFD) were measured with a wide-band oscilloscope and a rf millivoltmeter, respec-tively.

Measurement was begun immediately after tap-ping of the converter, at a temperature of approxi-mately 1400 "C. The received signal increased about28% during measurement, partly because of theincrease in the gain of the AP diode, whereas thesignal level varied randomly by approximately ± 10%with a frequency of approximately 1-2 Hz, probablyon account of the smoke and dust issuing from thehot converter.

The measured and calculated SNR values are pre-sented in Fig. 15. The signal level was calculated bythe radar equation, Eq. (8), at a distance of 10 m, inwhich case T(x) = 0.6, because the focus distance ofthe transmitter and receiver optics was 24 m ratherthan 10 m.

The SNR was approximately 15 immediately aftercommencing measurement, after which it increasedto approximately 55 by the end of measurementroughly 15 min later. The calculated SNR curve hasvalues about 4 times larger than the measured valuesover the entire temperature range, and because themeasured and calculated noise levels are roughly thesame, the reason must lie in the intensity of thereceived signal. The extinction coefficient y is as-sumed in these calculations to be zero, whereas if it isassumed to be 0.069 m-' an attenuation of 1/4 will beobtained in the signal. This attenuation could becaused by smoke or dust in the hot converter. Onthe other hand, the attenuation would probably notbe constant during measurement because the dustand smoke would disperse with time. The mostprobable reason is thus that the reflection coefficientof the hot target may vary depending on whether themeasurement point is the fire bridge itself or acoating of slag, other iron oxides, or even pure iron onthe target's surface. Test measurements under lab-oratory conditions and the modified radar equation,Eq. (8), give equal results when carrying out measure-ments on a target of known reflectivity.

Stability of the Distance Measurement

A stability measurement was performed to assess thedependence of the measured distance on the tempera-ture of the converter. The speed of light is depen-dent on the refractive index of the air, which dependson the air pressure, temperature, and humidity.9The flow of hot air out of the converter can cause themeasurement beam to refract, which can be seen as avariation in the measured distance.

The stability, signal level, and single-shot resolu-tion of measurements of a hot converter at a distanceof 14 m are shown in Figs. 16 and 17. The time 0corresponds to the point at which the converter wasturned just after tapping so that measurement be-came possible. The temperature of the converterwas approximately 1400 C initially and approxi-mately 1050 C by the end of the measurement.Reliable distance results were obtained 1.5 min afterbeginning the measurement because of the hugeamount of smoke and dust rising in front of theconverter, which caused a proportion of the signal toscatter back, to be detected in the oscilloscope in theform of an additional wide pulse of randomly varyingposition and amplitude in the receiver channel.

The measured distance drifted -5 mm during thefirst 5 min, after which the rate of drift decreased.This drift may be caused by variation in the refractiveindex of the air as the converter cooled down. All theresults remained within ±6 mm during the 22-minmeasurement period, but the signal level measuredby the system processor doubled during this time,partly as a result of an increase in the gain of the APdiode because the bias current decreased as theconverter cooled down, and partly because the smokeand dust dispersed, reducing the extinction coefficientand increasing the signal. The single-shot resolu-tion measured by the processor was approximately 50mm (sigma value) at first, which corresponds to theSNR value of 10 obtained from Fig. 6. By the end ofthe measurement the resolution had improved to 20mm, corresponding to a SNR value of 50.

Some Real Lining Wear Measurements

After developing the experimental version of the laserradar device, we developed two new laser radar

20

250

200 -

WCo

0

S2

(3

150 -

100 -

50 -

0

1000 1100 1200 1300 1400 1500

TEMPERATURE OF A CONVERTER [C]

Fig. 15. Measured () and calculated () signal-to-noise ratio asa function of the temperature of the converter.

E

0c

15-

10-

5-

0-

-5 -

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0 5 10 15MEASUREMENT TIME [min]

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Fig. 16. Stability of actual measurements of a hot converter.Each measurement is a average of 1000 successive single-shot

results.


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m

1000

800

E

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600

400

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0 5 10 15 20

MEASUREMENT TIME (min]

Fig. 17. Measured signal and single shot resolutments of a hot converter.

instruments together with two Finnish ILtd. and Rautaruukki New Technology Imore advanced electronics and optics.sulted in the production of two comrradar devices, the older LR-1500 for po:ment in hot converters and the more recfor automatic profiling of the bottom re,converter.

Construction of the Laser Profiler LR-2000

The laser profiler consists of five units:chanical measurement head, step mot(measurement electronics, power supply, acompatible control computer. A drawinprofiler is provided in Fig. 18.

The distance measurement electronicthe electronics needed to measure the tra 10-ns laser pulse to a target and back.controller that communicates with thRS-232C serial bus. Signals betweenmeasurement electronics and the optiitransmitted by optical fibers. The opti(

Keyboard

driveGrphicspr ntIer

Bat teries

Fig. 18. LR-2000 laser profiler. Its height, widt160 cm, 55 cm, and 73 cm. The weight is about 12shields are not shown.

50 tains transmitter optics to collimate the laser pulse tothe target and receiver optics to collect part of the

40 reflected signal in the receiver fiber. The measure-ment head can be scanned with good accuracy and

0E high speed by means of two step motors, which0 contain an integrated package of a servo motor, gearI 20Z-3 box, angle encoder, and driver electronics. The scan-

10 ning motors communicate with the PC via an RS-cc 232C interface. The power supply provides dc volt-

0 ages for the other units and also includes batteries,25 which makes it possible to use the system for about 4

hr without a main supply. The control computerion of measure- includes an electroluminescent EGA-compatible flat

display, keyboard, disc drive, and program package tohandle all the measurement and analysis tasks. The

arms Nptel whole measurement system is installed in an equip-nn, Nopte ment cart that is easy for one person to move to thetd, based on measurement site. Heat shield elements for protec-Thi asr- tion against thermal radiation are also included, andiercial laser the weight of the whole system is approximately 120met measure- kg.:ent LRt2000 The measurement sequence is as follows. The3gion of a hot first step is to fix the coordinate system of the

measuring equipment to the object coordinate sys-tem, after which automatic measurement can beperformed according to a preprogrammed sequence.

an dptome- A manual measurement mode with manual scanningrd, stacATe and single-point measurement is also possible. Then a PC/A T results are displayed on a screen and stored for

gof the laser postprocessing and analysis. The wear rate of thes contain all lining is obtained by comparing the results with thoseansit time of obtained on first measuring the relined converter.

It has a pLPe PC via an Measurement Results Obtained in Normal Use of thethe distance LR-1 500 and LR-2000ial head are The LR-1500 has been in use for several years forcal head con- performing lining wear measurements at the Raahe

Steel Works of Rautaruukki Ltd.820 It does notsupport computer-controlled scanning and must be

Removable heat cover aimed manually at the desired point. It is designedMeasurement head for point measurements to obtain lining wear data on

the tuyere regions of converters. These are the mostcritical areas because the rate of wear is highest onaccount of the N 2/Ar gas blown into the molten steelduring the process. Two typical lining wear rates

Display during campaign times with two types of lining brickJoystick are presented in Fig. 19. The typical measurementControl computer distance was approximatley 10 m. The whole mea-

suring process for 6 tuyeres lasts about 20 min,including all preparations, calibration and data pro-

measuremen unit cessing.The more advanced model of the device, LR-2000,

Power unit with computer-controlled scanning, has been in usecontinuously for over a year at the Raahe SteelWorks. The profiler has been used to measure lining

Wheels wear in the bottoms of the converters and in criticalregions of the sides. Typical lining wear results areshown in Fig. 20. The solid curve shows the new

h and depth are lining profile of the converter, and the dashed curve0 kg. Thermal shows the lining wear rate after 849 tappings. The

upper panel shows a cross section about 2.5 m from


LINING WEAR RATE CAMPAIGN NUMBER 46EORICK: ANCARBON

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30-

0 200 400 6oo e00 1000 1200 0 400

NUMBER CF HEATSo TUYERE 1 TUYERE 2 0 TUYERE 3A TUYERE 4 X TUYERE 5 V TUYERE 6

LINING WEAR RATE CAMPAIGN NUMBER 56BRICK:RADEX K 418

0 200 400 600 00 1000 1200

14UMBER OF HEATSa TUY RE I + TUYERE 2 o TUYERE a TUYERE 4 X TUYERE 5 v TUYERE 6

19. Tuyere brick lengths during campaigns with two brickS.

the bottom of the converter seen from in front, andthe lower panel is the cross section in the middle ofthe converter, left side up. Because of the shape ofthe converter, it is impossible to measure the whole ofthe bottom and both sides in a single profiling period.Both pictures in Fig. 20 are combinations of resultsobtained in two measurement sequences measuredfrom different places in front of the converter.

The number of measurement points was approxi-mately 100 per measurement, the total time takenbeing approximately 10 min, including fixing of thecoordinates (1-2 min) and changing the position ofthe profiler. The time per measurement point isabout 0.5 s. This consists of the time required tomeasure the distance, which is approximately 310 msbecause 500 successive results were averaged, andreading and storing the result and turning the mea-surement head, which together take about 200 ms.The savings in measurement time compared with alaser profile based on the modulation of a continuouswave is obvious. The measurement speed of mostother devices is typically between 3 and 10 s/pointdepending on the temperature and reflective proper-ties of the lining. 7,2 '

Esc-itShms gi-otunons automatiuaoeI Fl-N.ID

Fig. 20. Typical converter profiling results. The grid scale inboth pictures is 500 mm.

1400

Discussion

The possibilities for measuring hot refractory liningwear rate by pulsed TOF techniques are examinedhere, including considerations of the collection ofbackground radiation and the signal-to-noise ratioand its effect on resolution. An experimental laserradar device has been constructed to verify the calcu-lations. The test system consists of distance mea-surement electronics and a separate optomechanicalmeasurement head connected to the electronics byoptical fibers. The distance measurement electron-ics consist of a semiconductor laser diode, two chan-nel receivers for the start and stop pulses, gaincontrol electronics, and a novel time-interval measur-ing system. The optomechanical measurement headis constructed with paraxial optics and separate trans-mitter and receiver channels to achieve maximumefficiency. The narrow-band optical interference fil-ter in the receiver channel reduces the high back-ground radiation from the hot converter and thusimproves the inherently low SNR.

Test measurements in laboratory and industrialenvironments show that the achievable distance mea-surement resolution of a pulsed TOF laser radardevice can be estimated to an accuracy better than afew percent with the formulas presented here. Thebackground radiation collected is independent of themeasurement distance and depends only on the dimen-


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sioning and adjustment of the receiver optics. Theformulas estimate this to an accuracy of ±10%, butthe accuracy to which the noise in the receiverchannel can be estimated is somewhat lower, 15%,because of variation in some parameters, such asemissivity of the hot target.

The pulsed TOF laser radar devide is capable ofmeasuring distances reliably if the ratio of the peaksignal to the noise of the electronics in the input tothe CFD is greater than approximately 10. ThisSNR can be achieved at measurement distances at upto 15-20 m if the temperature of the target is 1400 "Cand the optical peak power of the laser is at least 10W. The single-shot distance resolution is approxi-mately 60 mm (sigma value) if the SNR is 10, butresolution can be improved to the millimeter level byaveraging, with the measurement time still remain-ing short, approximately 0.5 s. The measurementtime is roughly one tenth of the time required by theCW technique. The total accuracy of this experimen-tal device is limited to ± 10 mm because of nonlineari-ties caused by the electronic gain control and temper-ature drift related to the electronics with separatereceiver channels for start and stop pulses.

A new commerical laser profiler, LR-2000, hasrecently been developed based on the experimentalversion described above. This again uses the pulsedTOF principle, the main advantage of which ascompared with the CW technique is its shorter mea-surement time. The LR-2000 has been used success-fully at Raahe Steel Works for 1 year for measuringthe lining wear rates of three converters. No con-verter breakouts have occurred during this time.Exact knowledge of the wear rate in each campaignhas provided good data for selecting the optical brickmaterial, thereby increasing converter productivity.The pulsed TOF laser radar device or modifications ofit may prove applicable in the future to other demand-ing industrial measurement situations requiring cen-timeter-class accuracy.

This project was carried out in cooperation withtwo Finnish firms, Noptel Ltd. and Rautaruukki NewTechnology Ltd. We thank them for their financialand technical support. We also thank the RaaheSteel Works of Rautaruukki Ltd. and Heikki Tuomi-koski for the opportunity to carry out measurementsin real industrial environments. Hannu Jokinenand Jukka Kauniskangas helped us during the mea-surements in Raahe and prepared the actual liningwear measurement results, for which particularthanks are due.

References1. M. Manninen, "Task-oriented approach to interactive control

of heavy-duty manipulators based on coarse scene descrip-tion," Acta Polytech. Scand. Math. Comput. Sci. Ser. 42, 1-81(1984).

2. J. Kostamovaara, K. Mdattd, M. Koskinen, R. Myllyla, "Pulsed

laser radars with high-modulation-frequency in industrialapplications," in LaserRadarVII: AdvancedTechnologyforApplications, Richard J. Becherer, ed., Proc. Soc. Photo-Opt.Instrum. Eng. 1633, 114-127 (1992).

3. T. A. Clarke, K. T. V. Grattan, and N. E. Lindsey, "Laser-based triangulation techniques in optical inspection of indus-trial structures," in Optical Testing and Metrology III:RecentAdvances in Industrial Optical Inspection, C. P. Grover,ed., Proc. Soc. Photo-Opt. Instrum. Eng. 1332, 474-485(1990).

4. D. E. Smith, "Electronic distance measurement for industrialand scientific applications," Hewlett-Packard J. 31, 3-10(1980).

5. M. Manninen and J. Jaatinen, "Productive method and systemto control dimensional uncertainties at final assembling stagesin ship production," J. Ship Product. 8, 244-248 (1992).

6. K. Mdattd, J. Kostamovaara, and R. Myllyld, "On the measure-ment of hot surfaces by pulsed time-of-flight laser radartechniques," in Industrial Inspection II, D. W. Braggins, ed.,Proc. Soc. Photo-Opt. Instrum. Eng. 1265, 179-191 (1990).

7. L. Green, "Condition monitoring-its impact on plant perfor-mance at British Steel, Scunthorpe Works," Ironmak. Steel-mak. 17, 355-362 (1990).

8. K. Mdatta, J. Kostamovaara, and R. Myllyld, "A laser range-finder for hot surface profiling measurements, in Laser Tech-nologies in Industry, 0. D. D. Soares, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 952, 356-364 (1988).

9. J. F. Ready, Industrial Applications of Lasers (Academic, NewYork, 1978).

10. R. Siegel and J. R. Howell, Thermal Radiation Heat Transfer(Hemisphere, New York, 1981).

11. G. Bertolini, "Pulse shape and time resolution," in Semicon-ductor Detectors, G. Bertolini and A. Coche, eds. (North-Holland, Amsterdam, 1968), pp. 243-276.

12. J. Kostamovaara, "Techniques and devices for positron life-time measurement and time-of-flight laser rangefinding,"Acta Univ. Oulu 37, 1-69 (1986).

13. C30902E, C30902S, C30921E, and C30921S Data sheet (RCA,Inc., Electro Optics, Vaudreuil, Canada, 1988).

14. H. Kressel, Semiconductor Devices for Optical Communica-tion (Springer-Verlag, Berlin, 1980).

15. R. G. Meyer and R. A. Blauschild, "A wide-band low-noisemonolithic transimpedance amplifier," IEEE J. Solid-StateCircuits SSC-21,530-533 (1986).

16. RCA Corporation, RCA-Electro-Optics Handbook, tech. ser.EOH-11 (RCA, Inc., Lancaster, Pa., 1974).

17. J. Wang, K. Matta, and J. Kostamovaara, "Signal powerestimation in short range laser radars, in ICALEO'91:Optical Sensing and Measurement Symposium (Laser Insti-tute of America, Toledo, Ohio, 1992), pp. 16-26.

18. K. Maatta, J. Kostamovaara, and R. Myllyld, "Time-to-digitalconverter for fast, accurate laser rangefinding, in IndustrialInspection, D. W. Braggins, ed., Proc. Soc. Photo-Opt. In-strum. Eng. 1010, 60-67 (1988).

19. P. P. Webb, R. J. McIntyre, and J. Conradi, "Properties ofavalanche photodiodes," RCA Rev. 35, 234-278 (1974).

20. M. Karppinen, J. Kostamovaara, R. Myllyla, M. Seppanen, andK. Maattd, "A novel laser rangefinder system for the profilemeasurements of refractory linings," in Proceedings of theUNITECR'89 Meeting, L. J. Trostel, Jr., ed. (American Ce-ramic Society, Westerville, Ohio, 1989), 1340-1353.

21. G. Klages, "Measurement of refractory lining in metallurgicalvessels," Metall. Plant Technol. 12, 32-41 (1989).


Documents

Profiling of hot surfaces by pulsed time-of-flight laser range finder techniques