2012 IEEE/SICE International Symposium on System Integration (SII)
Kyushu University, Fukuoka, Japan
December 16-18, 2012

High-precision three-dimensional laser measurement system bycooperative multiple mobile robots
Yongjin Jeong, Yoonseok Pyo, Yumi Iwashita, Tsutomu Hasegawa, and Ryo Kurazume
Abstract— This paper presents a high-precision threedimensional laser measurement system of an architectural
structure by cooperative multiple mobile robots. This system
is composed of three mobile robots, that is, a parent robot
and two child robots. The parent robot is equipped with athree-dimensional laser scanner, attitude sensor, total station,
and auto-leveling device. On the other hand, the child robots
are equipped with six corner mirrors. The parent robot moves
and stops repeatedly, and measures a three-dimensional architectural shape using the equipped laser scanner at several
positions. Meanwhile, the child robots also move and stop
alternately, and act as mobile landmarks for the positioningof the parent robot. By replacing or newly installing several
devices/mechanisms, the precision of the proposed system
becomes incomparably higher than our previous system. We
report the system achieves quite high accuracy of 0.03 0.05 %of targets’ size through indoor/outdoor experiments. We apply
the proposed technique for the shape measurement system oftunnels under construction and verify that the accuracy of thedeveloped system is as high as a conventional ground-fixed laser
scanner.

Parent robot,
P-cleChild robot 2
Child robot 1

Fig. 1.

parent-child robot system is utilized for an accurate robot
positioning technique named as “the Cooperative Positioning
System (CPS)” [5]. Owing to this technique, the positions
where the robots acquired the shape and/or the texture of
the target are identified quite accurately, which is beyond
comparison with other conventional localization techniquessuch as odometry or IMU. Complex and time-consuming
post-processing procedures such as Iterative Closest Point(ICP) algorithm for range image registration are not requiredsince all the positions where the shapes and/or the textures
are measured are accurately known by the CPS. This function
makes the process of three-dimensional modelling much
simpler and more easy-to-use.
Although this system has been developed for the purposeof digital preservation of cultural properties at first, thissystem is useful for a variety of other application fields.
For instance, civil engineering is one of the promising
applications. Especially, in a tunnel construction process,three-dimensional geometrical information has been widely
utilized in recent years. During tunnel construction, an
accurate cross-sectional shape of a tunnel after primaryand secondary linings is required to confirm wall thicknessor to estimate amount of spray concrete. Several systems
and devices for a tunnel shape measurement have beendeveloped so far, such as (i) ground-fixed three-dimensionallaser scanner [6],[7],[8],[9], and (ii) mobile mapping systemusing a vehicle equipped with laser scanners [10],[11]. As
for (i), several sensing devices are sold in the market suchas Leica ScanStation C10 and TOPCON GTP 1500, andhigh-accurate and high-resolution geometrical information
has been in constant use. However, the measurement processusing these special devices takes many man-hours and needsa few days for data processing. Therefore they are notsuitable for an on-site and real-time evaluation of a tunnel
construction process just after drilling operation or primaryand secondary linings. On the other hand, the system in (ii)realizes a high-speed scanning and modeling. However, since

I. INTRODUCTION
In this decade, three-dimensional geometrical informationof a large-scale real object has been widely utilized in several
applications such as civil engineering, architecture and urban
design, and topographic mapping. Digital preservation ofhistorical objects, which is a technique to preserve geometrical and photometrical information of historical heritages andtreasures which are close to be missed or weathered, has also
been attempted in several sites [1],[2].
We also have been developing a robot system for digitalarchiving of historical properties consisting of multiple mobile robots (Fig.1) [3],[4]. In this system, three mobile robots
equipped with laser sensors and digital cameras move arounda large-scale architectural structure alternately, and acquire
object information such as shapes and textures at multiple
view points. As an example of digital preservation by the
proposed system, the Dazaifu Tenmangu shrine and several
cultural properties were successfully reconstructed as virtual
reality models [4].
The biggest characteristic of this system is the use of multiple mobile robots cooperatively. A parent robot is equippedwith a total station which is an optical distance meter for
surveying, laser scanner, digital camera, and etc., and twochild robots are equipped with corner mirrors on each. This
Y. Iwashita and R. Kurazume are with Graduate Faculty of Information
Science and Electrical Engineering, Kyushu University, 744 Motooka, Nishiku, Fukuoka 819-0395, Japan kurazume@ait.kyushu-u.ac.jp
J. Yongjin is with Graduate School of Information Science and ElectricalEngineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 8190395, Japan

978-1-4673-1497-8/12/$31.00 ©2012 IEEE

Three-dimensional laser measurement system, CPS-V

198

Total station

the GPS is not available in a tunnel and thus the positioning
has to depend on the odormetry or IMU, it is quite hard toacquire highly accurate and reliable information by a mobile
mapping system.
In this paper, we attempt to apply the developed threedimensional measurement system to the civil engineering. To
achieve required high-precision specification, several devices
are replaced or installed to improve the performance ofthe previous system, and finally the accuracy of the threedimensional geometrical modelling is drastically improved
up to 0.03% of targets’ size. We also apply the improved
system for the geometrical modelling of a tunnel underconstruction and confirm the performance of the proposedsystem for three-dimensional tunnel construction process
management.
For creating a three-dimensional environmental
map, several SLAM systems have been proposed[12],[13],[14],[15][16]. However, in general, these systems
are based on stochastic approach and accumulated errors
for both mapping and localization are compensated whena same area is observed repeatedly. On the other hand, theproposed system is based on deterministic approach. In
addition, even though the positioning and modelling errors
are accumulated gradually, these errors are incomparably
smaller than other SLAM-based techniques even if the area
is observed only once. Moreover, in our best knowledge, no
SLAM technique which is as accurate as our system (0.03%) has not been proposed so far[17].
In Section 2, we will introduce the improved system which
achieves a drastic improvement in terms of the modelingaccuracy. In Section 3, several experimental results in atunnel will be reported for the purpose of performanceverification of the developed system.

Auto-leveling system

Parent robot

Laser scanner
Remote contollerChild robot

Child robot

Fig. 2.

The developed tunnel shape measurement system
TABLE IIS PECIFICATION OF LMS 151 (SICK AG)
View angle
Angular resolution
Systematic error
Statistical error
Scanning frequency
Max. range

270◦
0.25◦ /0.5◦
±30mm
±12mm
25Hz/50Hz
50m

The time-of-flight 1-axis laser scanner placed on the upper
body of the parent robot acquires a cross-sectional shape by
scatting slit-like laser, and thus, by rotating a turn table onwhich the scanner is mounted, a three-dimensional shape forwhole directions can be obtained as shown in Figs.3 and 4.

Turn table
1-axis laser range
finderII. M ULTIPLE ROBOT SYSTEM FOR HIGH ACCURACY
Slit-like laser

TUNNEL SHAPE MEASUREMENT

A. System overview
Fig. 3.

Figure 2 shows the newly developed system consisting of
one parent robot and two child robots. The parent robot isequipped with a total station for land surveying (TOPCON,
GPT-9005A, Table I), auto-leveling system (Risumu, AS21), 1-axis laser scanner (SICK, LMS-151, Table II),1-axis
rotation table (Chuo-Seiki, ARS-136-HP), and 2-axes inclinometer (Applied Geomechanics Inc., MD-900-TS, Table
III). On the other hand, the child robots are equipped withsix corner mirrors (TOPCON, prism unit A3) and an infraredbeacon (TOPCON RC-3) for controlling a total station on
each.

Scanning system by 1-axis laser scanner and turn table

On the other hand, the positions of parent and child robots
are determined alternately by the total station on the parent
robot utilizing the Cooperative Positioning System (CPS)[4],[5]. Figure 5 shows the fundamental procedure for the
robot localization by the CPS. First, we assume that theinitial position of the parent robot is measured or defined
beforehand.
Step 1 The child robots 1 and 2 are moved and stopped.
Step 2 The parent robot measures the distances and the

TABLE IS PECIFICATION OF GPT-9005A (TOPCON)
Range
Angular resolution
Accuracy (distance)
Accuracy (angle)

TABLE IIIS PECIFICATION OF MD-900-TS (A PPLIED G EOMECHANICS I NC .)1.3 ∼ 3, 000m0.5′′ /1′′±2mm + 2ppm × Distance
1′′

Range
Resolution
Repeatability
Hysteresis

199

±25◦
0.004◦
0.01◦ (= 36′′ )0.02◦ (= 1′ 12′′ )laser scanner, the three-dimensional model of a large-scale
object can be obtained without any convergent calculations
such as ICP.

Fig. 4.

Scanned three-dimensional data in outdoor environment(1) Child robots moves

azimuth and elevation angles to the child robots 1
and 2, and identifies their positions.
Step 3 The parent robot moves and stops.
Step 4 The distances and the azimuth and elevation angles
to child robots 1 and 2 are then measured, and theposition of the parent robot is calculated using thetriangular surveying technique.
Step 5 Repeat Steps 1 through 4 until the target position
is reached.
The details of the above procedures are shown in [4]. By
using the CPS, the robots are able to identify their positions
with high accuracy even in unknown and rough terrains.
From the experimental results conducted so far, a typicalaccuracy of the CPS is known as 0.3% of total travel distanceof the robot and it is shown that the accuracy of the CPS
is much higher than the odormetry (> 10%) or IMU-based
systems [5].
In general, when we acquire a three-dimensional shapeof a large-scale object by a range sensor such as a laser
scanner or a stereo camera, the range sensor is placed inseveral positions around the object, and acquire many partial
range images of the object from each position to avoid
occlusions. The obtained partial range images are, however,represented in local coordinate systems at each position.
Therefore, these partial range images must be transformed
to the global coordinate system defined previously using the
ICP or other post-processing procedures. However, fullyautomated alignment processing is quite difficult if the
distances between the measurement positions are large or
the partial range images are not sufficiently overlapped eachother, and human intervention is required in these cases.
On the other hand, if all the measurement positions by
the range sensor are identified accurately beforehand, thesecomplex post-processing procedures are not necessary andthe aligned three-dimensional model is obtained directly
by applying a simple coordinate transformation calculation
between the local and global coordinate systems.
In the three-dimensional measurement system developed
so far[4], the accurate positioning and the modeling with theaccuracy of 0.3 % of targets’ size are realized according to
the above strategy. The detailed procedure is shown in Fig. 5.
The range image taken at each position by a laser scanner istransformed from each local coordinate system to the global
coordinate system according to the position and orientationof the robots measured by the CPS as shown in Fig.6. By
repeating the positioning by the CPS and the scanning by the(3) Parent robot moves

Fig. 5.

(2) Positioning of child robots(4) Positioning of parent robot
and measurement by LRF

Three-dimensional measurement using CPS and laser scanner

Measured
local shape 1

Measured
local shape 2

Measured
local shape 3

Coordinate transformation by CPS data
Aligned modelFig. 6. Coordinate transformation by CPS information for creating aligned
three-dimensional model

B. System improvement
The developed system utilizes the total station, which iscommonly used in civil engineering, and thus this system
is easy to use for workers in construction fields. Sincealigned range images are obtained directly without postprocessing procedures, the difference between the measured
and designed shapes of the tunnel can be evaluated on-sitejust after drilling operation or primary and secondary linings.
However, in our previous system [4], the accuracy of thethree-dimensional modeling is about 0.3% of the targets’
size and thus, for instance, it may cause an error of 150
mm at the halfway point of a 100 meter tunnel. Obviously
this performance is not enough as a sensing system of thetunnel shape, which requires 50 mm accuracy in general.
Therefore, in order to improve the accuracy of the system,
several devices used in the previous system are replaced asfollows:
1) The total station was replaced from AP-L1 (TOPCON)
to GPT-9005A (TOPCON)
200

AS-21

2) The laser scanner was upgraded from LMS-200(SICK) to LMS151 (SICK)3) The corner mirror system was replaced from A5 type
(TOPCON, prism offset is 18mm) to A3 type (TOPCON, prism offset is 0mm) so that the inclinationof the corner mirrors does not affect the distancemeasurement by the total station.
In addition to these improvements, we attempted to reducethe compensation error of inclination of the total station,
which affects the measurement accuracy quite directly.
To measure the distance and the angles to the corner mirror
by the total station, the compensation of the inclination
angle of the total station is quite important since it affectthe measurement accuracy of the azimuth and elevation
angles directly. However, in the previous system [4], theinclination of the total station was estimated using the 2axes inclinometer (Applied Geomechanics Inc., MD-900-TS,
Table III) instead of the high-precision build-in inclinometerof the total station. This is because the available range ofthe build-in inclinometer of the total station is quite narrowas shown in Table V since the total station is mostly leveledby the adjustment mechanism of a tripod using the bubble
tubes in standard survey procedures.
To utilize the high-precision build-in inclinometer of thetotal station, we installed an automatic leveling device(Rizumu, AS-21, Fig. 7, Table IV) between the robot bodyand the total station as shown in Fig. 8. This device can adjust
the inclination of the total station with the accuracy of 4 seconds mechanically and automatically. Therefore, we changed
the measurement process by the total station as follows:
firstly, the inclination of the total station is adjusted by theleveling device mechanically, and then residual inclination
angles are compensated by the build-in inclinometer quite
precisely. Figure 9 shows the flowchart of data correction of
the improved system.
Figure VI shows the comparison of the inclination accuracy before and after using the automatic leveling device. We
think that this produces great improvement of the measurement accuracy of the total system.

Auto-leveling system
MD-900-TS

Two-axes attitude
sensorFig. 8. Two-axes attitude sensor (MD-900-TS) and auto-leveling system(AS-21)TABLE IV
S PECIFICATION OF AUTO - LEVELING DEVICE (AS-21, R IZUMU )Leveling accuracy
Max. range

Firstly, we evaluated the system accuracy in a long corridor with a loop as shown in Fig.10. Total distance ofthe corridor is 210 m. While the robots moved along thiscorridor, the parent robot scanned the environment aroundthe robot at 33 different positions and obtained 40,340,000
points. The number of movement of the parent robot is
38 times and the number of movements of the two childrobots are 7 and 8 times, respectively. The obtained threedimensional geometrical model is shown in Fig.11.
To evaluate the accuracy of the obtained model, we
compared the three-dimensional positions of the same featurepoints (the corner of the door) before and after the robots
moved along the loop as shown in Fig.12. The distance oftwo points was 98 mm (x:93mm, y:25mm, z:19mm) and theaccuracy was 0.054 % for the total travel distance of 180.9
m of the parent robot along this loop.
Finally, we compared the obtained map with the one
created by a standard SLAM technique utilizing a RaoBlackwellized particle filter [15]. Figure 13 shows two 2D
maps acquired by the SLAM and the proposed approach,respectively. For the SLAM approach, we put a 2D laser
range finder (TOP-URG, Hokuyo) on the robot at 300mm

Total station

Automatic leveling device

TABLE VS PECIFICATION OF INTERNAL ATTITUDE SENSOR IN TOTAL STATION(GPT-9005A, TOPCON)

Laser scannerFig. 7.

±10′′±4◦

Resolution
Max. range

Automatic leveling device

III. E XPERIMENTAL EVALUATION OF ACCURACY

5′′
±6′

TABLE VII MPROVEMENT OF INCLINATION ACCURACY

IMPROVEMENT

To evaluate the accuracy improvement of the developed
system, we conducted measurement experiments in several
environments.

Original (MD-900-TS)
Proposed (AS-21/GPT-9005A)

201

0.01◦
0.0013◦

Before improvement
Read pitch and yaw angles
from external attitude sensor
Read azimth, elevation angles,
and distance from total station
Data correction
70m
Loop

After improvement
Make total station levelby auto-leveling system

35m
xRead pitch and yaw angles
from internal attitude sensoryFig. 11.

Read azimth, elevation angles,
and distance from total station

Three-dimensional model of corridor

Error at the corner of the door is 98mm

Data correction
Fig. 9.

Flowchart of data collection measured by total stationin height from the floor and captured 2D horizontal shapesof the environment continuously. To evaluate the accuracyof the laser measurement itself, we did not utilize the error
compensation by the loop closer in both maps. Consequently,
since the corridor which consists of similar and featurelesswalls is a tough environment for SLAM, the obtained 2D
map contained the error of 1598 mm after moving the loop,
which is about 10 times larger than the error of 98 mm by the
proposed technique. Note that the loop closer technique canbe applied to both approaches. However, a refined shape may
be different from an actual shape if the error in an original
shape is large.

Fig. 12.

Measured shapes before and after a long distance movement

robots are moved 343 m including the different of heightof 5 m around a building, and the parent robot scanned theenvironment at 20 positions. The obtained three-dimensionalmodel is shown in Fig.15 and the path of the parent robot
is illustrated in Fig.16, respectively. The error of the samefeature points before and after the movement around thebuilding is 116mm (x:-47mm, y:72mm, z:-78mm) and 0.034% of total travel distance of the parent robot.
From these experiments, we verified that the accuracy of
the three-dimensional modeling is drastically improved from
0.3% in the previous system[4] to 0.054 % or 0.034 % of
the targets’ size.
IV. E XPERIMENTS IN A TUNNEL
To evaluate the system performance and the applicability
for the tunnel construction management, we conducted theexperiments in an actual tunnel shown in Fig. 17. The length,
the inclination, and the cross-sectional area are 80m, 0.3%,
and 77.6m2 , respectively. Figure 18 shows the parent andchild robots in the measurement experiment.

Path of robots

A. Accuracy evaluation with designed shape
Fig. 10.

Firstly, we compared the measured and the designed
shapes of the tunnel. However, to compare them, the robot
and the global (tunnel) coordinate systems must be aligned
precisely. To do so, we placed corner mirrors at two known
anchoring points in the tunnel and determined the initial

Corridor for indoor experiment

Next, we conducted the experiments in an outdoor environment (Fig.15). In this experiment, the parent and child202

Error (1598mm)

130m

z
x

y

Error 0.116m(a) Map by the SLAM approach [15]Error (98mm)
Fig. 15.

Measured shapes and errors in outdoor environment(127.6, 17.7, 4.9)5
43
z [m] 21
0(25.9, 37.7, 1.4)

(113.2, -15.0, 4.9)(b) Map by the proposed approach
Fig. 13.

Start & Goal(-14.4, -0.3, 0.0)
40 30
140
100120
20 10
60 80
0 -10
20 40
0
y [m]
x
[m]
-20
-20

Comparison with proposed technique and SLAM approach

Fig. 16.

Path of parent robots

B. Accuracy evaluation by comparing with a conventionallaser scannerFig. 14.

The designed shape does not always match precisely withthe actual shape. Therefore, we evaluated the difference
between the actual shape obtained by a conventional groundfixed laser scanner (TOPCON GLS-1000, Table VIII) and themeasured shape.
The calculated error distribution is shown in Fig.22 andTable IX, and the histogram of the error is shown in Fig.23
and Table X. In Fig.22, the areas with red and blue colors
indicate that these areas include a large error (10 ∼ 50mm)
and the areas with green involve a small error(< 10mm).
From the error distribution, we can see that the areas around
30 m from the entrance have rather large errors which are

Buildings for outdoor experimentposition and orientation of the parent robot. Then we scanned
the three-dimensional tunnel shape at 11 positions along thetunnel by the developed system. The trajectories of the robots
are shown in Fig.19. It took about 30 minutes to measure
the whole shape of the tunnel and about 424 million threedimensional points were acquired.
In fact, there are many obstacles in the tunnel such as construction vehicles or drilling machines as shown in Fig.20.
Therefore, we chose about 348 million points which areplaced higher than 2 meters from the ground and evaluatedthe different of both shapes. In addition, the area from the
7.5m to 17.5m from the entrance was covered by a sheet andthis area was removed from the evaluation. The difference
distribution between the measured and the designed shapes isshown in Fig.21 and Table VII. In Fig.21, the areas with red
and blue colors indicate that these areas include a large error(10 ∼ 50mm) and the areas with green show a small error(< 10mm). The experimental result shows that the RMS
error in whole area between the measured and the designed
shapes is 32.2mm.

Fig. 17.

Target tunnel

TABLE VIIE RRORS AGAINST THE DESIGN SHAPE
Number of points
RMS error

203

3,482,477
32.2mmtimber supports which are placed with an interval of 125mm.
Therefore, we can say that the accuracy of the proposedsystem fits the desired precision as a measurement deviceof a tunnel shape.
TABLE VIIIS PECIFICATION OF GLS-1000 (TOPCON)
Fig. 18.

Measurement experiment in tunnel

Range
Accuracy (distance)
Accuracy (angle)
Max. range
Scanning speed

80m
Parent robot(0.7,3.1)

12.3m(81.6,-1.0)

1 ∼ 150m
4mm(σ)/1 ∼ 150m
6′′330m
3000points/sec

17.2m

y

x

Child robot 2

Child robot 1
Fig. 19.

Travel paths of the robots [m]0+50larger than 50mm. On the other hand, the error is less than
50mm in other areas. The areas with large errors may becaused by a following reason: the parent robot moved arather long distance in this area as shown in Fig.19, andthe calibration error between the total station and the laser
scanner affected severely the measurement accuracy. Notethat in the most-recent system this problem has been solved
by a precise calibration process utilizing a laser pointer andan infrared camera.
In addition, Figs.24 and 25 show the cross-sectional shapes
and its close-up figure at 50m position from the entrance.
From the experimental results, it is verified that themeasurement error is less than 50mm in the area of 91.7%of the whole shape. In general, the standard accuracy of
the tunnel construction process is 50mm at the location of00Fig. 22. Shape comparison between the proposed system and the conventional laser scanner (TOPCON, GLS-1000) [mm]TABLE IXRMS ERROR BETWEEN THE PROPOSED SYSTEM AND THE
CONVENTIONAL LASER SCANNER (TOPCON, GLS-1000)

Number of points
RMS error

Number of points

12

2,808,262
29.6mm

x10 5

8
4
0
0

Fig. 20.

204060
Error [mm]

80

100

Fig. 23. Error histogram between the proposed system and the conventionallaser scanner (TOPCON, GLS-1000)

Obstacles in the tunnel

V. C ONCLUSION-500This paper presented a high-precision three-dimensional
measurement system of an architectural structure by multiple
mobile robots, and its application for the shape measurementof a tunnel under construction. This system is composed ofthree mobile robots consisting of a parent robot equippedwith a total station and a laser scanner, and two child robots
equipped with six corner mirrors for each. All the robots
move and stop repeatedly and alternately, and the parent
robot measures the three-dimensional shape using the laser

+50

+50 0
0

-50

Surfaces are covered by sheets

Fig. 21.
[mm]

Shape comparison between the design and the measured shapes

204

TABLE X
ACCUMULATIVE ERROR DISTRIBUTION
Error [mm]Ratio [%]

∼10
35.2

∼20
61.8

∼30
77.5

∼40
86.4

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∼50
91.7

8
developed
scanner
design7
Fig.25

6Hieght [m]

5
4
3
2
1
0
-1
-8

-6

Fig. 24.

-4-20
Width [m]

2

4

6

8

Cross-sectional shape at 50m position

scanner at several positions. To improve the measurement
accuracy to fit the need as a tunnel shape measurement
device, several devices were replaced or newly installed tothe previous system. The experimental results showed thesystem achieves quite high accuracy of 0.03 % of targets’size. Experimental results in a tunnel showed that the RMS
error of the developed system is 29.6 mm for a 80 m tunneland has enough performance as a three-dimensional tunnel
shape measurement system with low-cost devices comparedwith a conventional ground-fixed laser scanner.
The present study was supported in part by a Grant-in-Aid
for y Scientific Research (B) (23360115).
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7.5

7.4

Hieght [m]scanner

design7.3
developed

7.2

7.1

7

Fig. 25.

-0.2

-0.1

0
Width [m]

0.1

0.2

Close-up of cross-sectional shape at 50m position

205