Assessment of laser range finders in risky environments

Jose Pascoal, Lino Marques and Anıbal T. de AlmeidaInstitute for Systems and Robotics

University of Coimbra3030-290 Coimbra, Portugal

{zefranc, lino, adealmeida}@isr.uc.pt

Abstract—This paper characterizes four commercial LaserRange Finders (LRF) while operating under adverse conditions,namely: low visibility, and multiple types of target surfaces, in-cluding different optical properties, angles and radiant surfaces.The study considered two 2D LRF commonly used in mobilerobotics: the Sick LMS-200 and the Hokuyo URG-04LX, andtwo punctual industrial LRF: the Ifm effector O1D100 and theSick DT60. Based on the results obtained, a set of conclusions andrecommendations are taken considering the utilization of LRF inmobile robots operating in risky and adverse environments, likefirefighting applications.

I. INTRODUCTIONAutonomous mobile robot navigation can only be achieved

if a robot can accurately sense its environment in order toestimate its localization and the position of the obstaclesaround it. This problem is currently addressed by SimultaneousLocalization And Mapping (SLAM) algorithms, but in orderto be effective, these methods require accurate range data.In optimal operating conditions, Laser Range Finders (LRF)are an excellent choice to use in mobile robots to providethis type of data [2]. A LRF is a device which uses a laserbeam in order to determine the distance to a reflective object.The most common form of laser range-finder operates on thetime-of-flight principle by sending a laser pulse in a narrowbeam towards the object and measuring the time taken by thepulse to be reflected off the target and returned to the sender.For optimal operation, these sensors need environments withhigh visibility and target surfaces with good reflectivity forany orientation (ideally white Lambertian surfaces), becomingfrequently unusable when this is not the case.

The performance of commercial LRFs has already beencharacterized by others. For example, Hebert and Krotkov [3]characterized the range and angular accuracy and precision oftwo 3D LRFs: an Erim and a Perceptron. Luo and Zhang [4]characterized an Acuity AccuRange 4000 in terms of the in-fluence to environmental light level, and target surface opticalproperties and orientation. Ye and Borenstein [5] characterizeda Sick LMS200 and Alwan et al. [1] characterized a HokuyoPBS-03JN. This paper expands the previous works with abroader sample of models - two scanning LRFs and twopunctual LRFs - and a broader sample of testing conditionsparticularly relevant for firefighting applications includingsmoking environments and radiant surfaces.

II. EXPERIMENTAL SETUPTable I describes some of the major characteristics of the

sensor used in this test.

Fig. 1. Picture showing the four sensors used in these tests.

Several environments were set up in order to characterizethe performance of the multiple range sensors evaluated inthis study. This study considered two 2D LRF commonlyused in mobile robotics: the Sick LMS-200 and the HokuyoURG-04LX, and two punctual industrial LRF: the Ifm effectorO1D100 and the Sick DT60 (see Figure 1 for a picture of thefour LRFs tested in this study).

The experimental tests were made inside an enclosed testingspace with 4x3x0.5 m3.

The punctual LRFs were connected to a 14 bit resolution NIUSB 6009 data acquisition board and the measured data wasobtained with the sensors configured to 4!!20 mA currentoutput mode using a 251,01! resistor with 20 ppm/"C. Alldata was recorded using Matlab and the Data AquisitionToolbox.

The scanning LRFs have been connected to a computerusing RS232 protocol and the data was recorded in a logfile using Player-Stage software as a server. This softwaretakes approximately 9 samples per second using the LMS200driver with 180" scan and angular resolution of 0.5 degreeincrements. The URG-04LX driver was configured to takeapproximately 10 samples per second and 3 degrees of scan-ning angle. For static tests, the target surfaces were placed 2

TABLE ILASER RANGE FINDERS’ SPECIFICATIONS

Sensor Sick LMS200 Hokuyo URG-04LX Sick DT60 IFM O1D100Dimensions (mm) 155x210x156 50x50x70 38x99x104 42x52x59Total Weight (g) 4500 160 202 N.A.Scanning Range (") 180 240 punctual punctualAngular Resolution 1/0.5/0.25" 0.36" – –Range (m) 80 0.2 - 4 0.2 to 5.3 0.2 to 10Max. Error ±7.5mm ±10 mm or 1% of range ±10mm ±15mmWavelength (nm) 905 785 655 650Interface RS422/RS232 USB/RS232 4!20 mA; 0!10V 4!20 mA; 0!10VPower consumption 650mA@24V 550mA @5V < 170 mA @ 11 to 30V < 150 mA @ 18 to 30VOperating temperature ("C) 0 to +50 -10 to +50 -25 to +55 -10 to +60Approximated Cost (e) 4000 2500 750 250

C

B

A

2,00 m

0,50

m

0.16

m

D

Fig. 2. Setup to test the sensors at multiple accurate distances.

meters away from the lasers in the top of a translation axis,16 centimetres above the ground (see Figure 2).

For the linearity and sensitivity tests, a target surface(A) was placed 2 meters away from the LRFs, a precisiontranslation axis (D) was used with the LRFs (B) placed 16centimetres above the ground on the top of its actuator (seeFigure 2). The translation rail (Micro-Controle GV 88) has arange of 1200 millimetres and is actuated by a stepping motor(C) that provides 0.1 millimetres per 1 step (Micro-ControleUE72). This stepping motor was controlled by a TechnosoftIntelligent Servo Drive IDM680-8EI able to provide up top256 microsteps per step.

The environmental and the sensor surface temperatureswere monitored with Texas Instruments TMP102 temperaturesensors. These sensors interface to a higher level host throughSMBus (in our case a Gumstix1 single board computer runningLinux) and provide temperature measurements with 0.0625"Cresolution and ±0.5"C accuracy in the range of !25"C to+85"C.

The tests were made in an approximately constant lumi-nosity of 200 lux (measured with a ISO-TECH ILM350 lightmeter).

Unless otherwise specified, 2000 samples were gatheredfor each test performed. It was found that this number ofmeasurements was sufficient to characterize the statistics ofall the sensors analysed in this study.

A. Warm-up timeWhen a range sensor is turned on, its internal temperature

increases until equilibrium between the power consumption

1http://gumstix.com

and thermal dissipation to the environment is reached. Duringthis process, called warm-up, the average output of a LRFcan drift several millimetres (see Figures 3 to 6). This familyof measurements was made during about 2 hours for eachsensor inside an acclimatized laboratory with an approximatelyconstant temperature of about 20"C. A temperature sensorwas used to monitor the environmental temperature near theLRF and another temperature sensor was thermally coupledto the LRF case under test. A white MDF2 target surfacelocated 2 meter far from the LRF was used. All the laserswere maintained disconnected from power for at least 5 hoursbefore the beginning of these tests.

Figure 3 shows that the URG-04LX reaches a stationarystate about 40 minutes after starting the test. During this time,the sensor output drifts almost 2 cm. This time is far betterthan the warm-up time for the LMS200 that takes about 2hours to stabilize and drifts more that 2 cm during that period(see Figure 4).

0 10 20 30 40 50 60 7020

21

22

23

24

25

26Laser case and environment temperature

Time (min)

Tem

pera

ture

(o C)

LASER CASEENVIRONMENT

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5x 104

1.95

2

Distance Measurement

Samples

Dist

ance

(m)

Fig. 3. URG-04LX range drift during warm-up.

The punctual LRFs are much faster to stabilize (about 10minutes) and show less drift (about 1 cm). A curiosity in thetwo LRFs tested is their opposing behaviour during warm-up:while the DT60 increases its output, the O1D100 decreases

2Medium Density Fiberboard

0 20 40 60 80 100 12018

20

22

24

26Laser case and environment temperature

Time (min)

Tem

pera

ture

(o C)

LASER CASEENVIRONMENT

0 1 2 3 4 5 6x 104

2.06

2.07

2.08

2.09

2.1

2.11Distance Measurement

Samples

Dist

ance

(m)

Fig. 4. LMS200 range drift during warm-up.

0 5 10 15 20 25 30 35 40 45 5020

21

22

23

24

25

26Laser case and environment temperature

Time (min)

Tem

pera

ture

(o C)

LASER CASEENVIRONMENT

0 0.5 1 1.5 2 2.5 3x 105

2.41

2.42

2.43

2.44

2.45

2.46Distance Measurement

Samples

Volta

ge (V

)

Fig. 5. DT60 range drift during warm-up.

(see Figures 5 and 6). A major conclusion of this set of testsis that all LRFs suffer from drift of 1 to 2 cm during a warm-uptime that can range from 10 minutes to 2 hours.

B. Linearity of range values

Accurate displacements inside the range of the positionerallow an estimation of the sensors non-linearity and infinites-imal range increments allow to estimate their sensitivity.

Using a Micro-Controle GV88 translation rail and the UE72stepping motor it was possible to estimate the precision of therange measurements and identify the error in range readings.Each sensor was moved from a distance of 1.5 metres to2.5 metres in 20 cm steps. In each stopping position 2000measurements were taken. The results from these tests areshown in Table II.

Figures 7, 8, 9 and 10 show the linearity of the range valuesand the standard deviation multiplied by a factor of 10, in orderto be more representative.

As we can see in Figure 10, the standard deviation fromthe O1D100 laser should be taken into account, since itis far bigger than the values for the other sensors testedin this paper. This test helped us to convert the punctual

0 5 10 15 20 25 30 35 40 45 5020

21

22

23

24

25

26Laser case and environment temperature

Time (min)

Tem

pera

ture

(o C)

LASER CASEENVIRONMENT

0 0.5 1 1.5 2 2.5 3x 105

1.7

1.75

1.8

1.85

1.9Distance Measurement

Samples

Volta

ge (V

)

Fig. 6. O1D100 range drift during warm-up.

TABLE IILINEARITY OF RANGE VALUES

Distance 1.5 1.7 1.9 2.1 2.3 2.5URG-04LX

Mean 1.5251 1.7157 1.9138 2.1161 2.3187 2.5138StDev 0.0018 0.0020 0.0020 0.0026 0.0018 0.0020

LMS200Mean 1.5811 1.7810 1.9823 2.1804 2.3792 2.5778StDev 0.0031 0.0038 0.0019 0.0043 0.0038 0.0038

DT60Mean Voltage 2.0248 2.1889 2.3517 2.5150 2.6783 2.8409Mean 1.5000 1.7000 1.9000 2.1001 2.3000 2.5000StDev 0.0054 0.0066 0.0065 0.0070 0.0069 0.0067

O1D100Mean Voltage 1.6055 1.6854 1.7662 1.8454 1.9228 2.0049Mean 1.5000 1.6995 1.9007 2.1010 2.2978 2.5000StDev 0.0943 0.0910 0.0899 0.0908 0.0887 0.0865

1.4 1.6 1.8 2 2.2 2.4 2.61.4

1.6

1.8

2

2.2

2.4

True distance (m)

Mea

sure

d di

stan

ce (m

)

Fig. 7. URG-04LX range and standard deviation for the linearity test

1.4 1.6 1.8 2 2.2 2.4 2.61.4

1.6

1.8

2

2.2

2.4

2.6

True distance (m)

Mea

sure

d di

stan

ce (m

)

Fig. 8. LMS200 range and standard deviation for the linearity test

1.4 1.6 1.8 2 2.2 2.4 2.61.8

2

2.2

2.4

2.6

2.8

3

True distance (m)

Mea

sure

d vo

ltage

(V)

Fig. 9. DT60 range and standard deviation for the linearity test

1.4 1.6 1.8 2 2.2 2.4 2.61.2

1.4

1.6

1.8

2

2.2

True distance (m)

Mea

sure

d vo

ltage

(V)

Fig. 10. O1D100 range and standard deviation for the linearity test

TABLE IIIINTERPOLATION VALUES FOR DISTANCE MEASUREMENT OF PUNCTUAL

LASERS

O1D100 DT60Distance (m) Mean Voltage Mean Voltage

1.5 1.6055 2.02481.7 1.6854 2.18891.9 1.7662 2.35172.1 1.8454 2.51502.3 1.9228 2.67832.5 2.0049 2.8409

lasers voltage to distance, using Matlab interpolation functionwith extrapolation, for each of the distances. Using a linearregression for the output of the analog sensors using the valuesof table II, the following transfer functions are obtained:

dDT 60 = 6,1349693#V !11,423313 (1)dO1D100 = 12,658228#V !19,316456 (2)

The interpolation for the following tests took in considera-tion the Table III values.

C. Range accuracyTo determine the range precision, the tests were made at

0.50, 1, and 2 meters away from the white MDF surface usinga Edmund Scientific rail with 2.10 meters and mountings fromthe same company, 10.5 centimetres above the ground. Thedistance to the target surface was carried with a tape measure.

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

True distance (m)

Mea

sure

d di

stan

ce (m

)

Fig. 11. URG-04LX range and standard deviation for the range accuracytest

As we concluded in II-B, the IFM O1D100 presents exces-sive standard deviation.

D. Influence of target colour and surface propertiesIn order to analyse the influence of the target surface optical

properties in the output of the LRFs, a set of eleven differentsurfaces was chosen. The colours and materials used werewhite MDF, a mirror, blue, red, yellow, white and black mattecoloured cardboard, black velvet and aluminium foil, blackplate and grey plate of aluminium, respectively. For a better

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

True distance (m)

Mea

sure

d di

stan

ce (m

)

Fig. 12. LMS200 range and standard deviation for the range accuracy test

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 20.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

True distance (m)

Mea

sure

d di

stan

ce (m

)

Fig. 13. DT60 range and standard deviation for the range accuracy test

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2−0.5

0

0.5

1

1.5

2

2.5

3

True distance (m)

Mea

sure

d di

stan

ce (m

)

Fig. 14. O1D100 range and standard deviation for the range accuracy test

TABLE IVTARGET COLOR AND SURFACE INFLUENCE ON SENSORS

Surface Color URG-04LX

LMS200 DT60 O1D100

Aluminium Black Mean 2.0151 2.0776 1.9945 1.9991StDev 0.0027 0.0016 0.0056 0.0918

Foil Mean 2.0212 2.0774 1.9986 2.0057StDev 0.0041 0.0037 0.0055 0.0931

Gray Mean 2.0234 2.0407 1.9891 2.0065StDev 0.0045 0.0013 0.0061 0.0801

Cardboard Black Mean 2.0293 2.0814 1.9994 2.0194StDev 0.0045 0.0059 0.0069 0.0929

Blue Mean 2.0058 2.0837 1.9943 2.0100StDev 0.0028 0.0035 0.0059 0.0909

Red Mean 2.0129 2.0837 1.9913 1.9988StDev 0.0022 0.0041 0.0059 0.0912

White Mean 2.0164 2.0764 1.9985 1.8064StDev 0.0022 0.0011 0.0056 0.0357

Yellow Mean 2.0172 2.0777 1.9990 1.8068StDev 0.0022 0.0036 0.0055 0.0367

MDF White Mean 2.0184 2.0795 1.9996 1.8063StDev 0.0022 0.0028 0.0052 0.0362

Mirror Mean 2.0465 2.0700 0.1048 0.8765StDev 0.0049 0.0038 0.0054 0.0278

Velvet Black Mean 4.0000 2.0798 5.3246 1.9164StDev 0 0.0038 0.0059 0.2679

TABLE VSTANDARD DEVIATION FOR TARGET ROTATION MEASUREMENTS

LRFs 0" 10" 20" 30" 40" 50" 60"URG-04LX

0.0020 0.0024 0.0029 0.0034 0.0037 0.0040 0.0040

LMS200 0.0027 0.0013 0.0020 0.0036 0.0042 0.0055 0.0055DT60 0.0061 0.0058 0.0055 0.0056 0.0058 0.0057 0.0058O1D100 0.0929 0.0922 0.0908 0.0921 0.0913 0.0917 0.0917

characterization of the LRFs behaviour for multiple surfaces,Lambertian and specular surfaces were chosen with differentreflectivity for the LRFs wavelength. The LRFs were placedperpendicular to the target surface. As can be seen from the re-sults obtained and presented in Table IV, the output distance isnot independent of the surface optical characteristics. Althoughthe output for all sensors is not changing significantly with thetarget colour, there is a noticeable influence according to thespecular characteristics of the surfaces. An exception is blackvelvet that had a very strong influence on the measurementsof the URG-04LX laser.

E. Angular influence

The influence to different target orientations was tested andthe target was the white MDF. A 60 degree rotation wascarried, 10 degrees per turn, using a UE 30 Micro-Controlestepping motor with 0.01 degrees per step, by the targetsurface. As shown in Figure 15, the target rotation modifiedthe values of the distance to target. It becomes noticeable thatthe range values increase like if the target surface was movingaway from the sensor. On the other hand, the variance is notaffected by this circumstance as shown on Table V.

0 10 20 30 40 50 60

1.99

1.995

2

2.005

2.01

2.015

2.02

2.025

Rotation Angle (o)

Dist

ance

(m)

URG−04LXLMS200DT60O1D100

Fig. 15. Lasers angular influence

Fig. 16. Setup for visibility test

F. Reduced visibility

To evaluate the behaviour of the LRFs under reducedvisibility, the sensors (3) were placed 2 meters away from atarget white MDF surface (1) inside an enclosed chamber witha small opening to the exterior (see Figure 16). The visibilitywas successively reduced by the injection of smoke producedby a Magnum 800 smoke machine (4) and the smoke washomogenized by means of a ventilator (5). An opacity sensormade with an infrared emitter and an infrared photodetectorwas monitoring the opacity inside the testing area (2). Eachtest started without smoke during the first minute and thensome smoke was injected during the next minute. During theremaining time of the test (28 minutes) the smoke was allowedto escape slowly by the opening, increasing the visibilityinside the testing chamber. This setup allowed to identify thebehaviour of each LRF in smoky environments and to identifythe threshold of visibility that allows to measure distances upto 2 meters away from white surfaces. The results of thesetests can be seen in Figures 17 to 20. From the tests it can beobserved that all sensors perform poorly in smoky conditions,but scanning sensors perform worse that the punctual ones. Itcan also be seen that at 2 metres distance, the sensors testedallow a reduction in visibility from 10 to 20%.

0 1 2 3 4 5 6x 104

0

0.5

1

1.5

2

2.5

3Infrared sensor data

SamplesVolta

ge o

n th

e In

frare

d se

nsor

0 1000 2000 3000 4000 5000 60000

1

2

3

4Distance Measurement

Samples

Dist

ance

(m)

Fig. 17. URG-04LX measurements for reduced visibility environment

0 1 2 3 4 5 6 7 8 9x 104

0

0.5

1

1.5

2

2.5

3Infrared sensor data

SamplesVolta

ge o

n th

e In

frare

d se

nsor

s

0 1000 2000 3000 4000 5000 6000 7000 8000 90000

2

4

6

8

10Distance Measurement

Samples

Dist

ance

(m)

Fig. 18. LMS200 measurements for reduced visibility environment

0 1 2 3 4 5 6 7x 104

0

0.5

1

1.5

2

2.5

3Infrared sensor data

SamplesVolta

ge o

n th

e In

frare

d se

nsor

s

0 1 2 3 4 5 6 7x 104

0

0.5

1

1.5

2

2.5Distance Measurement

Samples

Dist

ance

(m)

Fig. 19. DT60 measurements for reduced visibility environment

0 1 2 3 4 5 6 7x 104

0

0.5

1

1.5

2

2.5

3Infrared sensor data

SamplesVolta

ge o

n th

e In

frare

d se

nsor

s

0 1 2 3 4 5 6 7x 104

0

2

4

6

8

10Distance Measurement

Samples

Dist

ance

(m)

Fig. 20. O1D100 measurements for reduced visibility environment

G. Radiant surfacesA domestic radiant heater was used as a target radiant

surface. As with the previous tests, the target was placed 2meters away from the sensors. The temperature of the heaterwas adjusted with a variable transformer and a set of testswere made at three temperatures: environmental temperature,a medium hot temperature obtained with the heater suppliedat about 100 Volts, (tests made with a thermal radiation sensorgave an average temperature of 200 "C, but this temperatureshould be interpreted as the average temperature of the heatersurface, that includes the metallic protection grid), and a hightemperature with the heater supplied at full 230 V voltageand with the radiation elements presenting a bright orangecolour. Figures 21 to 24 show the behaviour of the sensorsto these testing conditions. It can be seen that the output ofthe sensors degrades with the surface temperature, becomingunusable when the surfaces are very hot.

0 200 400 600 800 1000 1200 1400 1600 1800 20002.06

2.07

2.08

2.09Radiant surface disconnected

Samples

Dist

ance

(m)

0 200 400 600 800 1000 1200 1400 1600 1800 20002.07

2.08

2.09

2.1Radiant surface at 100V

Samples

Dist

ance

(m)

0 200 400 600 800 1000 1200 1400 1600 1800 20002.05

2.1

2.15

2.2Radiant surface 220V

Samples

Dist

ance

(m)

Fig. 21. Radiant influence on URG-04LX.

III. CONCLUSIONSFour commercial Laser Range Finders widely used in mo-

bile robotics were tested in extreme environmental conditions,

0 200 400 600 800 1000 1200 1400 1600 1800 20002.11

2.12

2.13

2.14Radiant surface disconnected

Samples

Dist

ance

(m)

0 200 400 600 800 1000 1200 1400 1600 1800 20002.1

2.15

2.2Radiant surface at 100V

Samples

Dist

ance

(m)

0 200 400 600 800 1000 1200 1400 1600 1800 20007

8

9

10Radiant surface at 220v

Samples

Dist

ance

(m)

Fig. 22. Radiant influence on LMS200.

0 200 400 600 800 1000 1200 1400 1600 1800 20002

2.05

2.1Radiant surface disconnected

Samples

Dist

ance

(m)

0 200 400 600 800 1000 1200 1400 1600 1800 20002

2.05

2.1Radiant surface at 100V

Samples

Dist

ance

(m)

0 200 400 600 800 1000 1200 1400 1600 1800 20005.3

5.32

5.34

5.36Radiant surface at 220V

Samples

Dist

ance

(m)

Fig. 23. Radiant influence on DT60.

0 200 400 600 800 1000 1200 1400 1600 1800 20001.5

2

2.5Radiant surface disconnected

Samples

Dist

ance

(m)

0 200 400 600 800 1000 1200 1400 1600 1800 20001.5

2

2.5Radiant surface at 100V

Samples

Dist

ance

(m)

0 200 400 600 800 1000 1200 1400 1600 1800 20000

5

10

15Radiant surface at 220V

Samples

Dist

ance

(m)

Fig. 24. Radiant influence on IFM

particularly relevant for firefighting applications. In optimalconditions, namely good visibility, no interfering sources, andsurfaces with good reflectivity in the sensor direction, LRFsare excellent range sensors, particularly after warming-up, interms of linearity and accuracy. But in adverse environments,when the previous conditions are not met, LRFs provideerroneous or saturated outputs, becoming unusable as a rangesensor for robotics. The results published in this paper, al-though extensive, representing the culminate of several fulldays setting up different testing environments and gatheringmeasurements, are seen as a starting work in terms testingrange sensors in extreme environments. In the future, the au-thors intend to test the behaviour of other types of sensors, likesonars and microwave radars under this type of environments.

ACKNOWLEDGMENTS

This work was partially supported by the Portuguese Sci-ence and Technology Foundation (FCT/MCTES) by projectRoboNose, contract POSI/SRI/48075/2002 and by projectGUARDIANS contract FP6-IST-045269.

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[2] M.C. Amann, T. Bosch, M. Lescure, R. Myllyla, and M. Rioux. Laserranging: a critical review of usual techniques for distance measurement.Opt. Eng., 40(1):10–19, 2001.

[3] M. Hebert and E. Krotkov. 3D measurements from imaging laser radars:how good are they? Image and Vision Computing, 10(3):170–178, 1992.

[4] Xiujuan Luo and Hong Zhang. Characterization of acuity laser rangefinder. In 8th intl. Conf. on Control, Automation, Robotics and Vision.IEEE, 2004.

[5] Cang Ye and Johann Borenstein. Characterization of a 2-D laser scannerfor mobile obstacle negotiation. In Proc. IEEE Int. Conf. on Roboticsand Automation, 2002.