1-s2.0-s0168169907001834-main.pdf

8/20/2019 1-s2.0-S0168169907001834-main.pdf

1/13

c o m p u t e r s a n d e l e c t r o n i c s i n a g r i c u l t u r e 6 0 ( 2 0 0 8 ) 250–262

a v a i l a bl e a t w w w . s c i en c e d i r e c t . co m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p a g

Comparison of two 2D laser scanners for sensing object

distances, shapes, and surface patterns

Kyeong-Hwan Lee, Reza Ehsani ∗

University of Florida, IFAS, Agricultural and Biological Engineering Department, Citrus Research and Education Center,

700 Experiment Station Road, Lake Alfred, FL 33850, USA

a r t i c l e i n f o

Article history:

Received 25 April 2007

Received in revised form

10 July 2007

Accepted 17 August 2007

Keywords:

Laser scanner

Measurement drift

Measurement accuracy

a b s t r a c t

Laser scanners are increasingly used in automation and robotic applications as a sensing

device for navigation and safety. They have agricultural applications in measuring plant

growth rate, tree volume, tree count, 3D imaging, and pattern recognition. Laser scanners

are commercially available, but there is very little published information on characteristics

and performance of these laser scanners. This study compared two laser scanners, the Sick

LMS200 and the Hokuyo URG-04LX, for measurement drift over time, the effect of material

and color on measurement accuracy, and the ability to map different surface patterns.

Measurement drift over time was studied by determining the distance between the laser

scanner sensor and a stationary object at different fixed distances and angles. Distance

measurements over time fluctuated with a peak-to-peak value of 10–20mm. The settling

time, which is the time required for the averaged distance data to reach a stable level,

increased when measurement distance increased but for a given distance, the settling time

remained constant for different angles. At the measurement angle of 90◦, the settling times

for the LMS200and the URG-04LX for 50%of the maximum scanner measurement distances

were 53 min and 70 min, respectively. Therefore, to obtain accurate distance measurements,

thelaserscannersshould bewarmed upfor theduration of thesettlingtimebefore recording

measurement data.

The measured distance for soft material objects, such as a styrofoam plate and a sheet

of dry sponge, was longer than the actual distance. For shiny objects, such as orange tree

leaves, transparencyfilm, anda stainlesssteel plate,the measurement distance wasshorter

than actual distance. At the measurement angle of 90◦, the difference between the longest

andshortest measured distance (90% of the maximum scanner measurement distance) was

21.3mm for the LMS200 and 29.7mm for the URG-04LX. At the measurement angle of 45◦,

this difference increased to 73.2 mm for the LMS200; the URG-04LX was not able to detect

any objects at 45◦.

The surface shapes of a cylindrical pipe, a folded cardboard plate with a square-shaped

valley, and a folded cardboard plate with a V-shaped valley were well-depicted by the laser

scanner. For the object with a V-shaped valley with a true depth of 6.1 cm, the averaged

depths measured by the LMS200 and URG-04LX were 6.8 cm and 3.6 cm, respectively. The

larger discrepancy in the URG-04LX depth measurement may be caused by the relatively

lower angular resolution of the URG scanner, compared to that of the LMS scanner.

© 2007 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +1 863 956 1151x1228; fax: +1 863 956 4631.E-mail address: [email protected] (R. Ehsani).

0168-1699/$ – see front matter © 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.compag.2007.08.007

mailto:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.compag.2007.08.007http://localhost/var/www/apps/conversion/tmp/scratch_5/dx.doi.org/10.1016/j.compag.2007.08.007mailto:[email protected]

8/20/2019 1-s2.0-S0168169907001834-main.pdf

2/13

c o m p u t e r s a n d e l e c t r o n i c s i n a g r i c u l t u r e 6 0 ( 2 0 0 8 ) 250–262 251

1. Introduction

For agricultural and industrial applications, the distance to a

targetobject is a valuablemeasurement because it canbe used

for determining a variety of other measurements. For exam-

ple, objects in an open space can be detected and counted by

measuringthe distancesto the objects. Even thepositions andshapes of the objects can be obtained. Distance measurement

can be useful information in generating the surface topogra-

phy of a target object, like for example, fruit trees. The 3D

image of an object can be also reconstructed using distance

measurements, obtained by moving a sensor in a 2D plane.

With advances in sensing technology, various types of sen-

sors for distance measurement have been developed. Among

them, sensors which use lasers have shown dominance. A

laser is a light source device which converts external energy

into electromagnetic radiation. The word LASER came from

an acronym for light amplification by stimulated emission of

radiation, but it is now accepted as a single word. What differ-

entiates a laser from other light sources is that a laser beamhas a single wavelength, a same phase, and high energy den-

sity. Thus, a laser beam can travel to quite a long distance in

a straight line, maintaining a narrow beam. Because of this

characteristic, lasers are commonly used as a sensing source

for distance measurement.

A laser scanner, which is also called a laser radar or a laser

range finder, is a non-contact optical device that measuresthe

distance to an object in a scanning field using a pulsed laser

beam. The scanner’s measurement is based on the time-of-

flight (TOF) principle. A laser source inside the scanner emits

a pulsed laser beam. If this beam hits an object, part of the

beam is reflected back to thescannerand hits a detector inside

the scanner. The time between transmission and receptionof the pulsed signal is directly proportional to the distance

between thescannerand the object. Thelaser pulseis diverted

sequentially with a specific angular interval using an internal

rotating mirror. Thus, a fan-shaped two-dimensional scan is

made of the surrounding area.

Laser scanners are becoming a common sensing device to

aid the steering device to avoid obstacles, and in mapping

environments for use in roboticsand agricultural applications.

Jiménez et al. (1999) built a laser scanner-based measurement

system to recognize fruits in field tree conditions, consider-

ing it as a sensing device for a fruit-harvesting robot. The

scanner provided the distance to an object and the attenu-

ation of laser signal which occurred in the round-trip travelto the object. The information obtained was merged to rec-

ognize the fruit and find the final fruit position. Hebert (2000)

compared the characteristics of several range sensing tech-

nologies used in robotics. The measurement range of a laser

scanner using the TOF principle was relatively long, com-

pared to other technologies. The scanner provided relatively

stable, accurate measurements under hostile environmental

conditions such as fog, dust, or smoke. Monta et al. (2004)

built a three-dimensional sensing system, composedof a laser

scanner and a scanner table moving vertically, for an agricul-

tural robot. The sensing system could detect objects such as

tree trunks, branches, and leaves in a vineyard, and calculate

the diameter of the tree trunk and the distance between the

tree trunks. Kise et al. (2005) presented an obstacle detection

and identification algorithm of a laser scanner-based sens-

ing system for autonomous agricultural vehicles. The sensing

system was capable of detecting a moving object within a

semicircle of an 8 m radius and reconstructing a 2D silhou-

ette of the obstacle progressively in real time. Subramanian et

al. (2006) developed machine vision and laser scanner-based

guidance systems to navigate a tractor through the alleywayof a citrus grove and compared the performance of these

systems. They reported that the laser scanner-based guid-

ance was the better guidance system for straight and curved

paths.

Measuring the surface topography of soil and plants or

knowing the shape of an object is important for many pre-

cision agriculture applications. For this, laser scanners have

shown great potential. Darboux and Huang (2003) developed

a laser scanning system composed of two diode lasers and

a digital camera to measure soil surface microtopography.

Gonzalez et al. (2007) demonstrated the capability of a 3D

laser scanning system, which consisted of a laser transmitter

and two cameras, in describing the evolution of an under-water sediment bed in real time. Ehsani and Lang (2002)

developed a laser scanner-based plant volume measurement

system. The system was able to measure plant volume and

height, indicating the possibility to measure the biomass

and leaf area index of the plant. Wei and Salyani (2004,

2005) showed the potential of a laser scanner for simulta-

neous measurement of tree canopy height, width, volume,

and foliage density. While applications of laser scanners in

automation, robotics, and agriculture are increased signifi-

cantly in the recent years, very little published information

is available on characteristics and performance of these laser

scanners.

2. Objectives

The overall goal of this study was to analyze and compare the

characteristics of two commercially available laser scanners.

The specific objectives were: (i) to test distance-measurement

drifts over time at different measurement distances and

angles, (ii) to examine the effect of different materials and

colors of target objects on distance measurements, and (iii)

to measure accuracy in mapping the surface patterns of the

objects of different shapes.

3. Materials and methods

Two laser scanners, LMS200 (Sick Ag, Germany) and URG-04LX

(Hokuyo Automatic Co., Japan), were used forthe tests (Fig. 1a).

Their specifications, provided from the manufacturers, are

shown in Table 1. The LMS200 has a longer measurement dis-

tance, larger size, and is heavier compared to the URG-04LX.

It also has the ability to change angular resolution. To control

thescanners anddownload the measurements to a computer,

a computer program was written using LabVIEW (National

Instruments Co., Austin, TX).

8/20/2019 1-s2.0-S0168169907001834-main.pdf

3/13

252 c o m p u t e r s a n d e l e c t r o n i c s i n a g r i c u l t u r e 6 0 ( 2 0 0 8 ) 250–262

Fig. 1 – Laser scanners (a) and steel stand (b).

3.1. Laser scanners

3.1.1. LMS200

The light source of the LMS200 is a pulsed infrared laser of

905nm, notvisible to thehuman eyes.It operatesin eithermm

mode or cm mode. Themaximum measurement distancesare

8m in the mm mode and 80m in the cm mode. The LMS200

hastwo scanningrangeoptions: (i) from 40◦ to140◦ with angu-

lar resolutions of 0.25◦, 0.5◦, and 1◦, and (ii) from 0◦ to 180◦

with angular resolutions of 0.5◦ and 1◦. The times for scan-

ning one cycle are 53.28 ms, 26.64ms, and 13.32 ms at 0.25◦,

0.5◦, and 1◦ angular resolutions, respectively, in both scan-

ning ranges. Thescannerrequires about 13.32ms forone cycle

rotation of an internal mirror with a 1◦ step. To achieve 0.25◦

and 0.5◦ angular resolutions, the 1◦ step is shifted to 0.25◦ and

0.5◦ at the start of the mirror wheel rotation, respectively, and

four and two mirror rotations are required. For this reason, a

scan with an angular resolution of 0.5◦ takes as twice as long

as a scan with an angular resolution of 1◦; and a scan with

an angular resolution of 0.25◦ takes four times as long. The

laser scanner can communicate with a computer via a serial

port at a baud rate of 9.6kbit/s, 19.2 kbit/s, and 38.4 kbit/s. For

high-speed datatransmission, the manufactureroffersspecial

high-speed data interface cards. With the cards, the baud rate

can increase up to 500 kbit/s. Our test results indicated that

some of the collected data were occasionally lost when com-

munication took place at 500 kbit/s via a PCMCIA high-speed

RS422 interface card (CSM GmbH, Germany), while no data

was lost at the slower speed of 38.4 kbit/s. Thus, for more reli-

able communication between the scanner and the computer,

a 38.4 kbit/s data transfer rate was chosen for the experiment.

The LMS200 ran in the mm mode and scanned target objects

in the range of 40–140◦ with an angular resolution of 0.25◦.

3.1.2. URG-04LX

The URG-04LX uses a semiconductor laser beam of 785 nm to

measuredistance. It hasa fixed scanningrange of 60–300◦ with

a 0.36◦ angular resolution. Its maximum measurement dis-

tance is 4 m when an object is white paper. The scanner and

external devices caninterface with each other viaa RS232 port

Table 1 – Specifications of the LMS200 and URG-04LX

LMS200 URG-04LX

Maximum measurement distance (m) 8 (mm mode), 80 (cm mode) 4

Scanning angle (◦) 180 and 100 (selectable) 240

Angular resolution (◦) 0.25, 0.5, and 1 (selectable) 0.36

Scanning time (ms/cycle) 53, 26, and 13 at 0.25◦, 0.5◦, and 1◦ angular

resolution, respectively

100

Measurement resolution (mm) 10 1

Measurement error (mm) ±20 (mm mode), ±40 (cm mode) ±10: up to 1 m distance; 1% of distance:

1–4m distance

Data interface and transfer rate RS232 and RS422 (9.6kbit/s, 19.2kbit/s,

38.4 kbit/s, and 500kbit/s)

RS232 (19.2 kbit/s, 57.6 kbit/s, 115.2 kbit/s,

500 kbit/s, and 750 kbit/s), USB 2.0

(12 Mbit/s)

Supply voltage (VDC) 24±15% 5±5%

Current consumption (mA) 830 500

Weight (kg) 4.5 0.16

External dimensions (mm) 185 (W )×156 (L)×210 (H) 50 (W )×50 (L)×70 (H)

8/20/2019 1-s2.0-S0168169907001834-main.pdf

4/13


with a baud rate of 19.2–750kbit/s and a USB port with a baud

rate of 12 Mbit/s. In thisstudy, the scannercommunicated with

the computer via a USB port.

3.2. Experimental methods

3.2.1. Drift testExperiments were designed to test measurement drifts of the

laser scanners over a given period of time. The laser scan-

ners were placed on a stand at a height of 96cm. A steel stand

(Fig. 1b) was made to place a target object at a certain distance

with a constant height. Thestand was designedwith the capa-

bilityof attaching a targetobject to thebackside of theopening

square on the stand. A laser beam passes through the open-

ing (25cm×25cm) and hits the surface of the target object.

For the tests, a sheet of white paper (30 cm×30 cm) was used

as the target object.

The stands were placed at angles ( ) of 45◦, 90◦, and

135◦, facing the scanners (Fig. 2a). The intended distances

(D) between the object and the scanners were 0.8 m, 4.0 m,and 7.2m for the LMS200, and 0.4m, 2.0 m, and 3.6m for the

URG-04LX,which are10%,50%,and 90%of themaximum mea-

surement distances of the scanners, respectively. It was hard

to measure thetrue distance between theobject andthe scan-

ners because the laser beam detector, which is the reference

point on the scanner for distance measurement, is inside the

sealed case of the scanners. Therefore, to keep intended dis-

tancesconstant for each test, a new reference point wasmade

on the outside of the scanner housing. A point at which a

power plug and a communication plug meet was set as the

new reference point on theLMS200 (Fig. 1a). On the URG-04LX,

a marker indicating the front of the scanner was used as the

new reference point (Fig. 1a).At 50% of the maximum scanner measurement distances,

distance data were collected at 45◦, 90◦, and 135◦ simultane-

ously every second for 4 h. This test was repeated at 10% and

90% of the maximum measurement distances. These experi-

ments were conducted in a room illuminated with fluorescent

lamps. The light intensity of the room was between 650 lx

and 700 lx, and the temperature was between 12◦C and 20 ◦C.

Before the tests, the power of the scanners was shut off for

about 10 h.

3.2.2. Test on objects of different materials and colors

For examining the effect of different materials and col-

ors of objects on distance measurement, 11 target objects

(30cm×30 cm) were used. The materials included five sheets

of colored paper with a 0.5 mm thickness (white, blue, yellow,

red, and black), a 20-mm thick laminated wood plate, a 2-mm

thickstainless steel plate, a 26-mmthick styrofoamplate,a 25-

mm thick sheet of sponge, a 4-mil thick sheet of transparency

film, and a sheet made of orange tree leaves that were affixed

on a transparency film using double-sided tape without any

free space.

The three intended distances from the scanner to the

object used in the experiment for drift measurement were

also used in this experiment. The target objects were placedat

angles ( ) of 45◦ and 90◦ with same distance (D) to the scanner

(Fig. 2b). The objects were turned towards the front so that the

effect of an incidence angle from the laser beam to thesurface

of the object on distance measurement could also be studied.

The distance data to the 11 objects were collected first at 90◦

for 90% of the maximum scanner measurement distances. The

experimental order of the objects was randomly determined,

and then the test was conducted again at 45◦ with the same

distance in thenew random experimentalorder of theobjects.

These tests were repeated at 10% and 50% of the maximum

scanner measurement distances.

In order to analyze the data, a multiplecomparison analysis

was conducted using the “multicompare” function in MAT-

LAB’s statistics toolbox (The MathWorks Inc., Natick, MA, Ver

5.1). The multicompare function follows Tukey’s procedure,

which is based on the Studentized range distribution.

3.2.3. Determining the laser beam spot size

Immediately after thetest on the objects of differentmaterials

andcolors at a certain angle anddistance, another experiment

was conducted to investigate the size of the laser beam spot

Fig. 2 – Schematic of the experimental setup (top view) (a) for drift measurement and (b) for testing objects of different

materials and colors.

8/20/2019 1-s2.0-S0168169907001834-main.pdf

5/13


Fig. 3 – Objects for generating surface patterns: (a) cylindrical pipe, (b) folded cardboard plate with square-shaped valleys,

and (c) folded cardboard plate with V-shaped valleys. Distances in cm.

at the same angle and distance. The square stand opening

was blocked using two sets of two sheets of white paper. By

moving one set of two sheets of paper horizontally from the

center to the left-hand side and right-hand side, respectively,

and the other set of two sheets of papers vertically from the

center to the top andbottom, respectively, a small rectangular

open area was made at the center of the opening. When the

laser beam, generated at a single angle, passed through the

rectangular opening, the papers were fixed. The rectangular

opening was considered as the approximate size of the laser

beam spot at the angle and distance.

3.2.4. Generating surface patterns of objects of different

shapes

Experiments for investigating the capability of the scanners ingenerating the surface patterns of objects of different shapes

were designed. For thetests, threeobjects shown in Fig.3 were

prepared: (i) a cylindrical pipe, (ii) a folded cardboard plate

with square-shaped valleys, and (iii) a folded cardboard plate

with V-shaped valleys. The size of the cardboard plates was

91cm (W )×15cm (H). These objects were put on a table with

adjustable height. The table was fixed at a height at which the

laser beam hit the mid-height of the objects. The objects were

also positioned where the laser beam, generated at 90 ◦, hit

the center of the objects. The intended distance between the

scanners and the objects was 100 cm.

To build the surface patterns of the objects, the parallel

distance (P) between the scanner and the object was calcu-lated from the measured distance (M) and the measurement

angle ( ) using the definitionof the sinetrigonometricfunction

(Fig. 4):

P =Msin (1)

The diameter of the cylindrical pipe and the width (D) of

the hill in the square-shaped object were obtained based on

the distances (R1 and R2) and measurement angles ( 1 and 2),

which were measurements at the right-most and left-most

edges of the objects, respectively (Fig. 5):

D =

R21 + R22 − 2R1R2 cos( 2 − 1) (2)

Fig. 4 – Geometry for obtaining the parallel distance

between the laser scanner and an object.

4. Results and discussion

4.1. Measurement drift

Distance measurements by the LMS200 over time at an

intended distance of 4.0m and three different angles (45◦, 90◦,

and 135◦) are shown in Fig. 6. The distance data fluctuatedwith a peak-to-peak value of about 20mm. The period of the

fluctuation at about 2 min of run time was in the range of

0.3–0.5 min. The period increased with run time, and was in

the range of 15–20min when the run time reached 200 min.

To examine the trend of measurement drift, the distance data

were averaged every 20 min. This time interval was selected

because the longest period of the fluctuation was close to the

time interval. The averaged distance data decreased with run

time until about 53min, andthen stayed at a constant level. In

this stable region, the averaged distance data at 45◦, 90◦, and

135◦ differed a little from each other. This might be caused by

the difference of the distance between the scanner and the

object at each angle.

8/20/2019 1-s2.0-S0168169907001834-main.pdf

6/13


Fig. 5 – Geometry for obtaining the width of an object.

The differences between the averaged distance data at

2min and that at 53min were 7.8mm, 7.7mm, and 5.7mm

at 45◦, 90◦, and 135◦, respectively. These were considered as

the measurement errors caused only by insufficient warm-up

time of the laser scanner. The effect of measurement angle on

measurementdrift wastrivial.The pattern of the averaged dis-

tance data over time at an intended distance of 4.0 m (Fig. 6)

was also observed in the data measured at 0.8 m and 7.2 m.The settling time, which is the time required for the averaged

distance data to reach a stable level, was different depending

on measurement distances.

Fig. 7 presents distance measurements by the LMS200 at

an angle of 135◦ for three intendeddistances (0.8 m, 4.0m, and

7.2m). Like the data measuredat a constantdistance fordiffer-

ent angles (Fig. 6), the distance measurements shown in Fig. 7

fluctuated. Again, the averaged distance data decreased with

runtime at the beginning of the scanner’s operation, and then

began to stabilize at a settling time. The averaged distances

in the stable regions deviated from the intended distances.

This may have been caused by disagreement between the

location of the new reference point on the scanners and thelocation of the laser beam detector inside the sealed case. In

addition, these could have been inaccurate distance measure-

ment using a tape measurement when the scanner and the

Fig. 6 – Distance measurements by the LMS200 at an intended distance of 4.0 m for the angles: (a) 45◦, (b) 90◦, and (c) 135◦.

8/20/2019 1-s2.0-S0168169907001834-main.pdf

7/13


Fig. 7 – Distance measurements by the LMS200 at 135◦ for the intended distances: (a) 7.2m, (b) 4.0 m, and (c) 0.8 m.

object were setup before the test. The settling times at 0.8 m,

4.0 m, and 7.2m distances were 32min, 53 min, and 137 min,

respectively, showing the settling time increases when the

measurement distance increases. The differences between

the averaged distance at 2 min of run time and that at the

settling times were 2.4mm, 5.7mm, and 14.1mm at 0.8m,

4.0 m, and 7.2 m, respectively. This shows a measurement

error causedby insufficientwarm-up time of thelaserscannerincreases when measurement distance increases.

From this experiment, it was shown that in order to mea-

sure the distance to an object accurately, the LMS200 needs

to warm up for some time before measurement. The required

warm-up time differs depending on measurement distance

andthe data measured shouldbe averaged over a specific time

interval.

Distance measurements by the URG-04LX at a single angle

of 135◦ for three intended distances of 0.4 m, 2.0 m, and 3.6 m

are shown in Fig. 8. Like the distance measurements by the

LMS200 (Fig. 7), the distance data shown in Fig. 8 also fluc-

tuated with a peak-to-peak value of about 10–15 mm. The

amplitude of the fluctuation tended to be larger when the

measurement distance increased, but the period of the fluc-

tuation was not recognizable. The distance data, averaged

every 20min, presented a different pattern with that of the

averaged distance data measured by the LMS200 (Fig. 7). The

averaged distances increased in the beginning of the scan-

ner’s operation at the intended distances of 0.4 m and 2.0 m,

which had settling times of 50 min and 70 min, respectively.

At an intended distance of 3.6 m, the averaged distance beganto decrease, reached a bottom limit at 70min, increased

until 111 min, and then stabilized. The averaged distances

at the settling times were quite different with the intended

distances. This might also have been caused by inaccurate

distance setup between the scanner and the object before

the test. The settling time was larger when the measurement

distance increased. The differences between the averaged dis-

tance at 2 min of run time and that at the settling times were

9.3 mm, 12.3mm, and 4.1 mm at intended distances of 0.4 m,

2.0 m, and 3.6 m, respectively. Thus, like the LMS200, the URG-

04LX shouldalso be warmed up forthe settlingtime beforethe

test to minimize measurement error, and the distance data

should be averaged. In the test with the URG-04LX at a single

8/20/2019 1-s2.0-S0168169907001834-main.pdf

8/13


Fig. 8 – Distance measurements by the URG-04LX at 135◦ for the intended distances: (a) 3.6 m, (b) 2.0 m, and (c) 0.4 m.

distance for three different angles, the distance measurement

was not affected by the angles.

4.2. Effect of different materials and colors of objects

on distance measurements

The previous experimental results demonstrated that some

warm-up settling time is required for the laser scanners toprovide stable distance measurements (Figs. 6–8). The highest

settling time for the LMS at a distance of 7.2m was 137min.

The longest period of the fluctuating distance data was about

15–20min in the stable region. Thus, to avoid error in the

distance measurement by insufficient warm-up time of the

scanners and a short data-sampling period, the tests were

started after running the scanners for 3h without data col-

lection. The scanners recorded 1000 readings on each object,

which roughly corresponded to one period of the fluctuating

distance data in the stable region.

Tables 2 and 3 show the mean and standard deviation

of distance measurements to each object by the LMS200 at

intended distances of 0.8 m and 7.2 m, respectively, and the

results of multiple comparison analysis at the distances. The

mean distances of the objects were sorted in an ascending

order of alphabet indexes in the multiple comparison col-

umn. When the objects have the same index, there is no

significant difference among them in a 95% confidence level.

In general, the shortest distance measurements were found

with shiny objects such as orange tree leaves, transparency

film, and a stainless steel plate. The longest distance mea-surements were found with objects made of soft materials

such as styrofoam and dry sponge. In particular, the distance

measurement to the transparent film was very sensitive to

measurement angles. At 45◦, thetransparent filmcouldnot be

detectedby the LMS200. Some portion of thelaser beam might

penetrate the film, and a large portion of the beam bounced

off the film might be deviated from the route to the scanner.

Thus, the amount of the laser beam returning to the scanner

may have been insufficient for the scanner to detect the tar-

get object. When a laser beam is directed towards an object

of high reflectivity such as a stainless steel plate, most of the

beam is bounced off the object immediately after hitting it

and comes back to the scanner. However, when the object has

8/20/2019 1-s2.0-S0168169907001834-main.pdf

9/13


Table 2 – Mean, standard deviation, and multiple comparison analysis of distance measurements to the different objects by the LMS200 at an intended distance of 0.8 m

Material 45◦ 90◦

Mean (mm) S.D. (mm) Multiple comparison* Mean (mm) S.D. (mm) Multiple comparison*

Tree leaves 788.4 3.8 a 788.2 3.3 d

Blue paper 789.4 3.8 b 787.5 4.7 cd

Black paper 790.2 1.7 c 786.3 5.0 c

Yellow paper 790.4 1.6 c 785.3 2.0 b

Red paper 791.8 5.0 d 787.2 4.8 cd

Wood plate 792.1 2.5 d 788.2 3.5 d

White paper 793.8 1.7 e 788.3 1.9 d

Steel plate 799.9 3.9 f 772.1 1.6 a

Styrofoam 802.3 1.6 g 796.7 1.9 f

Sponge 811.1 2.5 h 802.5 2.0 g

Transparency – – – 794.4 9.8 e

∗ “a–h” represent the alphabet indexes of the multiple comparison analyses. When the objects have the same index, there is no significant

difference. The confidence level of the multiple comparison analyses was 95%.

low reflectivity such as sponge, a large portion of the laser

beam is absorbed on the surface of the object for some time,and then returns to the scanner. Thus, the flight time of the

laser beam to an object of low reflectivity is longer than one of

high reflectivity, and the distance measurement to the object

of low reflectivity is larger. At an intended distance of 0.8 m

(Table 2), the differences between the longest distance mea-

sured on the soft objects and the shortest distance measured

on the shiny objects were 22.7 mm and 30.4 mm at 45◦ and

90◦, respectively; at an intended distance of 7.2 m (Table 3),

the differences at 45◦ and 90◦ were 73.2mm and 21.6mm,

respectively.

The effect of color on distance measurement was more

sensitive at 45◦ than at 90◦. At an intended distance of 0.8 m,

the differences between the longest and shortest distanceson the colored papers at 45◦ and 90◦ were 4.4 mm and 3.0 mm,

respectively; at an intended distance of 7.2 m, the differences

at 45◦ and 90◦ were 5.0 mm and 2.0 mm, respectively. The

results showed that the effect of the measurement distances

on distance measurement of the different colored objects was

minor.

The mean and standard deviation of distance measure-

ments to the different objects by the URG-04LX at intendeddistances of 0.4m and 3.6 m and the results of multiple com-

parison analysis are shown in Tables 4 and 5. As observed in

Tables 2 and 3, the shortest distance measurements were also

found with shiny objects and the longest distance measure-

ments were also found with soft objects. The URG-04LX could

not detect transparency film at 45◦ for an intended distance

of 0.4 m. The capability of the URG-04LX was not enough to

detect any objects at 45◦ for an intended distance of 3.6 m.

At an intended distance of 0.4m (Table 4), the differences

between the longest distance observed at the soft objects and

the shortest distance at the shiny objects at 45◦ and 90◦ were

55.4 mm and21.6mm, respectively. At an intended distance of

3.6 m (Table 5), the differences at 90◦ were 29.7mm. The mea-surement angles at an intended distance of 0.4 m might not

influence on the results of the multiple comparison analysis

for the different colored papers. However, at a measurement

angle of 90◦, theresult of multiple comparison analysis for the

colored papers at an intendeddistance of 0.4m might be more

sensitive than that at an intended distance of 3.6 m.

Table 3 – Mean, standard deviation, and multiple comparison analysis of distance measurements to the different objects by the LMS200 at an intended distance of 7.2 m



Tree leaves 7168.6 2.7 a 7188.0 5.0 cd

Steel plate 7169.3 4.8 a 7178.2 4.8 b

Wood plate 7173.1 4.5 b 7192.0 2.1 f

Blue paper 7175.1 4.0 c 7189.4 1.5 e

Black paper 7175.2 4.9 c 7187.9 2.4 cd

Yellow paper 7176.3 3.8 d 7187.4 2.4 c

Red paper 7178.6 4.5 e 7188.4 2.6 cde

White paper 7180.2 2.9 f 7188.8 2.4 de

Styrofoam 7185.2 6.7 g 7197.7 3.3 g

Sponge 7241.8 5.3 h 7197.4 2.3 g

Transparency – – – 7176.1 3.0 a



8/20/2019 1-s2.0-S0168169907001834-main.pdf

10/13


Table 4 – Mean, standard deviation, and multiple comparison analysis of distance measurements to the different objects by the URG-04LX at an intended distance of 0.4 m



Steel plate 360.2 4.1 a 442.6 2.5 i

Black paper 397.9 2.2 b 424.0 1.8 d

Tree leaves 398.8 2.2 c 424.8 1.7 e

Wood plate 399.0 2.2 c 421.9 1.8 b

Blue paper 399.4 2.2 c 424.3 2.0 d

Yellow paper 400.5 2.2 d 427.2 1.8 f

Red paper 402.7 2.2 e 422.9 1.8 c

White paper 405.7 2.4 f 425.0 1.6 e

Sponge 412.6 2.0 g 428.7 1.7 g

Styrofoam 415.6 2.0 h 437.5 1.8 h

Transparency – – – 415.9 2.8 a



Table 5 – Mean, standard deviation, and multiplecomparison analysis of distance measurements to thedifferent objects by the URG-04LX at an intendeddistance of 3.6m

Material 90◦

Mean (mm) S.D. (mm) Multiplecomparison*

Steel plate 3643.3 3.3 a

Blue paper 3647.6 2.9 b

Red paper 3648.3 2.9 c

Yellow paper 3648.6 3.0 cd

Black paper 3649.1 2.9 cde

White paper 3649.5 2.9 cde

Tree leaves 3649.9 3.4 e

Wood plate 3652.3 2.9 f Styrofoam 3658.1 2.8 g

Transparency 3666.6 2.8 h

Sponge 3673.0 2.9 i

∗ “a–i” represent the alphabet indexes of the multiple compari-

son analyses. When the objects have the same index, there is

no significant difference. The confidence level of the multiple

comparison analyses was 95%.

4.3. Determining the laser beam spot size

Table 6 shows the sizes of the laser beam spot, approximated

with a rectangle, for the LMS200 and URG-04LX at different

angles and distances. In the spot for the LMS200, the rect-

angle corresponded to a single laser beam spot. However,

in the spot for the URG-04LX, it corresponded to three laser

beam spots generated at consecutive three angles. The laser

beam could not be separated to a single beam with physical

methods.

The widths of the rectangles at 45◦ were greater than

those at 90◦. The ratios of the widths at 45◦ to the widths

at 90◦ were in the range of 1.63–1.73 for the LMS200 and

1.25–1.29 for the URG-04LX, but the heights of the rectangles

at both angles were almost the same. Both the width andthe height of the LMS beam spot increased when measure-

ment distance increased. In the URG beam spot, the width

was greater with increased distance, but the change in height

was minor. The linear regression analysis between the area

(Y ) of the laser beam spot and measurement distance (X) was

conducted (Eqs. (3)–(6)). The beam spot area increased linearly

with distance. The linear models for the LMS beam at 45 ◦

and 90◦ are:

Y = 6.70X− 3.52 (R2 = 0.98) (3)

Y = 3.86X− 1.63 (R2 = 0.99) (4)

Table 6 – Sizes of the laser beam spot, approximated with a rectangle, for the LMS200 and URG-04LX at three intendeddistances

Distance (m) Rectangle LMS200 URG-04LX

45◦ 90◦ 45◦ 90◦

0.8 for LMS200, 0.4

for URG-04LX

Width (cm) 2.0 1.2 1.5 1.2

Height (cm) 1.8 1.8 1.0 0.9

4.0 for LMS200, 2.0

for URG-04LX

Width (cm) 5.2 3.2 4.0 3.1

Height (cm) 3.8 3.9 0.8 0.9

7.2 for LMS200, 3.6

for URG-04LX

Width (cm) 8.3 4.8 – 4.8

Height (cm) 5.6 5.6 – 1.2

8/20/2019 1-s2.0-S0168169907001834-main.pdf

11/13


The linear models for the URG beam at 45◦ and 90◦ are:

Y = 1.46X+ 0.29 (R2 = 0.98) (5)

Y = 1.06X+ 1.08 (R2 = 1.00) (6)

4.4. Surface pattern of objects

Fig. 9 shows the surface pattern of a cylindrical pipe gen-

erated by the laser scanners. The pattern provided a sketch

of the surface shape of the pipe, but the resolution was not

enough to depict the pipe surface in detail. In the surface pat-

tern generated by the LMS200, the radii of the pipe obtained

by calculating the difference between the highest distance

at the edge and the shortest distance in the middle, and

obtained using the cosine law (Eq. (2)) were 3.5 cm and 3.7 cm,

respectively. The radii were 1.4 cm and 2.8 cm, respectively,

in the surface pattern generated by the URG-04LX. The radii

obtained in the pattern by the LMS were close to the true

radius of 3.5 cm, while the radii obtained in the pattern by the

URG were underestimated. The URG scanner has the lower

angular resolution of 0.36◦, compared to the LMS angular res-

olution of 0.25◦. Thus, the URG scanner might miss the right-

and left-most edges of the pipe, and this may explain the

reason why the measured radii are smaller than the true

radius.

The surface pattern of an object with flat hills and square-

shaped valleys generated by the laser scanners are shown in

Fig. 10. The pattern clearly reconstructed the hills and valleys.

The numbers of hills and valleys in the pattern agreed with

those in the actual object. In the pattern by the LMS200, the

widths of the hills and valleys in the middle, left-most, and

right-most were obtained using the cosine law. The widths

of the hills were 6.9cm, 6.1 cm, and 6.1 cm, respectively; and

the widths of the valleys were 6.1cm, 4.0cm, and 4.4cm,

respectively. The depths of the valleys were 7.2 cm, 7.5 cm,

and 7.8 cm, respectively. In the pattern by the URG-04LX, the

widths of the hills were 6.8cm, 5.0cm, and 5.6cm, respec-

tively; and the widths of the valleys were 6.0 cm, 4.6 cm, and

4.6 cm, respectively. The depths of the valleys were 7.2 cm,

6.6 cm, and 7.0 cm, respectively.

The measured widths of the hills in the middle were close

to the true width of 7.0cm. The widths of the hills at the leftand right sides were smaller than the true width. Since the

laser beam generated at 90◦ hit the middle of the object, and

the scanners were in parallel with the object, the resolution

of the surface pattern at the left and right sides became lower

than that in the middle. This might cause the smaller widths

at the left and right sides. The measured widths of the valleys

were much smaller than the true width of 7.0 cm. When the

laser beam was projected on the area of the valley, some of

the beam was blocked by the hill before it reached the valley.

Therefore, the measuredwidthsof the valleysbecame smaller.

The measured depths of the valleys were close to the true

depth of 7.0 cm.

Fig.11 shows the surface pattern of an object withV-shapedvalleys generated by the laser scanners. The pattern recon-

structed the surface shape of the object well. The numbers

of hills and valleys in the pattern agreed with those in the

actual object. The averaged depths of the valleys in the pat-

terns bythe LMS andURG were 6.8cm and3.6 cm,respectively.

The depth measured by the URG was much smaller than the

true depth of 6.1 cm. Since the URG scanner has a relatively

larger angular interval (0.36◦), compared to the angular inter-

val (0.25◦) of the LMS, it might miss the crests of the hills and

the bottom limits of the valleys. This might cause to under-

estimate the depth of the valley. This also can be confirmed

from the fact that the crests of the hills and the bottom limits

of the valleys in the pattern bythe LMS are sharper than thosein the pattern of the URG.

Fig. 9 – Surface pattern of a cylindrical pipe generated by the LMS200 and the URG-04LX.

8/20/2019 1-s2.0-S0168169907001834-main.pdf

12/13


Fig. 10 – Surface pattern of an object with flat hills and square-shaped valleys generated by the LMS200 and the URG-04LX.

Fig. 11 – Surface pattern of an object with V-shaped valleys generated by the LMS200 and the URG-04LX.

5. Conclusions

The characteristics of two commercially available laser scan-

ners, LMS200 and URG-04LX, were analyzed and compared

through several tests. The following conclusions can be drawn

from these tests:

• Distance measurementsby the laser scanners over run time

fluctuated with a peak-to-peak value of 10–20 mm. The dis-

tance data measured by the LMS200 showed a decreasing

pattern until a settling time, whereas that by the URG-04LX

showed an increasing pattern.

• The warm-up settling time was greater at a longer mea-

surement distance, but was not affected by measurement

8/20/2019 1-s2.0-S0168169907001834-main.pdf

13/13


angle. At a measurementangle of 135◦ for10%,50%, and90%

of the maximum measurement distances of the scanners,

the setting times of the LMS200 were 32min, 53min, and

137 min, respectively; those of the URG-04LX were 50min,

70 min, and 111 min, respectively.

• From distance measurements to objects of different materi-

als and colors, the longest measurements were found with

softobjectssuch as styrofoamand sponge; the shortestoneswere found with shiny objects such as orange tree leaves,

transparency film, and a stainless steel plate. The effect of

color on distance measurement was more sensitive at 45◦

than at 90◦; but the effect of measurement distances on

distance measurement of the different colored objects was

minor.

• At 90% of the maximum scanner measurement distances,

the differences between the longest measurement with the

soft objects and shortest measurements with the shiny

objects were 73.2 mm and 21.3mm at 45◦ and 90◦ for the

LMS200, respectively, and 29.7 mm at 90◦ for the URG-04LX.

The capability of the URG-04LX was not enough to detect

any objects at 45◦. The transparency film could not bedetected by either laser scanner at 45◦ for 10% and 50% of

the maximum scanner measurement distances.

• The size of the laser beam spot was approximated with

a rectangle. Both the width and height of the LMS beam

spot increased when measurement distance increased.

Regarding the URG beam spot, the width was greater with

increased distance, but the change of the height was minor.

Thebeam spot areas of both the scanners increasedlinearly

with distance.

• The surface patterns of different shapes of objects mapped

and reconstructed by the laser scanners depicted the sur-

face of the target objects well. From the surface pattern

of an object with V-shaped valleys, the averaged depths of the valleys in the patterns generated by the LMS and URG

were 6.8 cm and 3.6 cm, respectively. The depth measured

bythe URG was muchsmaller than the true depth of 6.1 cm.

This may have been caused by the relatively larger angular

interval of the URG scanner.

r e f e r e n c e s

Darboux, F., Huang, C., 2003. An instantaneous-profile laserscanner to measure soil surface microtopography. Soil. Sci.Soc. Am. J. 67, 92–99.

Ehsani, M.R., Lang, L., 2002. A sensor for rapid estimation of plantbiomass. In: Proceedings of the 6th International Conferenceon Precision Agriculture, Bloomington, MN.

Gonzalez, E.P., Diaz-Pache, F.S.-T., Mosquera, L.P., Agudo, J.P., 2007.Bidimensional measurement of an underwater sedimentsurface using a 3D-scanner. Opt. Laser Technol. 39,481–489.

Hebert, M., 2000. Active and passive range sensing for robotics.In: Proceedings of the 2000 IEEE International Conference onRobotics & Automation, San Francisco, CA,pp. 102–110.

Jiménez, A.R., Jain, A.K., Ceres, R., Pons, J.L., 1999. Automatic fruitrecognition: a survey and new results using range/attenuationimages. Pattern Recogn. 32 (10), 1719–1736.

Kise, M., Zhang, Q., Noguchi, N., 2005. An obstacle identificationalgorithm for a laser range finder-based obstacle detector.Trans. ASAE 48 (3), 1269–1278.

Monta, M., Namba, K., Kondo, N., 2004. Three-dimensionalsensing system using laser scanner. ASABE Paper No. 041158.ASABE, St. Joseph, MI.

Subramanian, V., Burks, T.F., Arroyo, A.A., 2006. Development of machine vision and laser radar based autonomous vehicleguidance systems for citrus grove navigation. Comput.Electron. Agric. 53, 130–143.

Wei, J., Salyani, M., 2004. Development of a laser scanner formeasuring tree canopy characteristics: phase 1. Prototype

development. Trans. ASAE 47 (6), 2101–2107.Wei, J., Salyani, M., 2005. Development of a laser scanner formeasuring tree canopy characteristics: phase 2. Foliagedensity measurement. Trans. ASAE 48 (4), 1595–1601.

1-s2.0-s0168169907001834-main.pdf

Documents