Autonomous Control of Mobile Robots- An Approach to an Orienteering Robot Kazuo Tani, Nobumasa Shirai, Os 阻U Matsumoto, Kiyoshi Komoriya
Mechanical Engineering Laboratory
Namiki, Tsukuba, Ibraki 305 Japan
Susumu Tachi
Research Center for Advanced Science and Technology, University of Tokyo
Komaba, Me郡iro-ku, To』cyo 153 Japan
Abstract: This paper describes the latest achievements of the research ofautomonous control of mobile robots carried out at the Mechanical EngineeringLaboratory. The functions required of the mobile robot are: navigation using
the environment model, obstacle detection, and rough road negotiation, which
are just those required of a human orienteering runner. To materialize thefunctions in a wheeled mobile robot, we conducted the research using anenvironment 皿ap and a robot with ultrasonic and laser sensors, an opticalobject scanner, and a four-wheeled robot with active suspensions. Combining
these technologies, we hope to realize an orienteering robot.
Key Words: Autonomous Control, Wheeled Robot, Orienteering Robot, Navigation,
Environment Map, Active Suspension.
1.

INTRODUCTION

Conventional mobile robots are mainly usedin factories and hospitals where theenvironment of the robot is well definedin advance. Particularly for efficient
locomotion OJl flat level floors, wheeled
robots are frequently used which trackcontinuous marks or cables laid out on thefloors.
The recent demands for the mobile robots
are their use in rough terrain, such as
forests, farms, disaster areas, andconstruction areas, where the environmentof the robot is defined only vaguely.
Sticking in this case to a wheeled robot
favouring its efficient locomotion on aflat ground, we require following
capabilities of a wheeled mobile robot.
The task of the mobile robot is to moveautonomously in an environment without
prior experienοe.
(1) The robot is given a map, i.e.,
structured information of the environment.
The robot first plans its path to thegiven destination on the map, and thenfollows the path as it senses the environ­ment using sensors, compares the acquired
information to the map, and estimates its
location in the map.
(2) If there is an obstacle in the way,
the robot detects it and avoids thecollision with it by detouring from thepredetermined path.
(3) The robot runs over uneven grounds andat the same time suppresses the shake andvibration of its body for the safety ofthe payload and for the stability of theenvironment sensors.
The capabilities look like those requiredof a human who enjoys the sports oforienteering. To materialize them in arobot is the image of our research.
A lot of studies have been made worldwideof the ele皿ent technologies and systematization for such robots. A皿ong them are
NAVLAB at Carnegie-Mellon University
Robotics Institute [1] and DARPA ALV atMartin Mar・E・ietta Denver Aerospace [2] forvisual navigation including road follow­
ing, and Yamabico at the University .of
Tsukuba for model based navigation withobstacle avoidance [3]. Also in the areaof wheeled vehicles for uneven grounds, apneumatic active suspension for a racing
car at Lotus [ 4) and a hillside tractor
for forestry use at the Forestry andForest Products Research Institute [5].
Our research of automonous control ofmobile robots was conducted at the Mechan­ical Engineering Laboratory as part of theLarge Scale National Project of Advanced
Robot Technology promoted by the Ministryof International Trade and Industry.

-317 ー唱百o富晶国TThe concept of our research is shown inFig. L We carried out the research under
three subthemes: (1) Navigation using theenvironment model, (2) Obstacle detection,and (3) Rough road negotiation. The
results of the research will be describedin the following chapters.
2. NAVIGATION USING THE ENVIRONMENT MODEL
An autonomous navigation control system ofa 皿obile robot using an environment model
was built and experimented.
2.1 Map Making and Path Planning
The mobile robot is given the model or themap of the environment where it is about
to move as a priori knowledge. The map
should represent the geometry of the freespace where the robot is allowed to pass,distinguish it from the obstructed space,and contain the information of thelandmarks, which are the targets the robot
can recognize using its environment
sensors. The map also has to be a kind ofa database the robot can refer to easily.
In our study we assume the robot environ­ment is a building floor and have made themap in two-dimensional representation.
The free space is decomposed into connect­
ing quadrangles. A side of a quadranglewhich is not shared with another quad­
rangle is a border of the free space, andsome of the border sides are walls. Walls
and wall edges are registered as land皿arks.
The centers of the quadrangles and thecenters of the shared sides are registeredas candidate subgoals for path planning.

Fig. 10�rn a畠A 国 T

In planning the path, possible sequencesof subgoals are considered starting withthe subgoal nearest the present robot
position and ending with the one nearest
the given destination. Among them an
optimum sequence is selected as the pathbased on the criteria such as the minimum
total distance.
2.2 Experimental SystemMobile robot
The mobile robot used in the experiment,
shown in Fig. 2, is a tricycle with two
independent driving wheels on the rightand left and an active caster with
controllable wheel direction in the rearto keep the robot stably upright. The
driving wheels are driven by DC servo
motors with harmonic gear reduction. The
rotation of each driving wheel is detectedby a rotary encoder with a resolution of
1/20000 of a revolution.
The robot has external sensors to sense
the environment, n 咽ely four ultrasonicrange sensors and a laser range sensor.
Three of the ultrasonic sensors aremounted on the front, right and left sidesof the robot. The remaining ultrasonicsensor and the laser sensor are mounted ona rotary head placed on the l!PPer deck of
the robot whose direction is servocontrolled.

Concept of autonomous control
of a mobile robot.
-318-Fig. 2

Mobile robot"MELBOY ” ．

The ultrasonic sensors can detect thedistance to a wall facing nearly rightwith an accuracy of about 3 mm. The laser
sensor has a laser emitting diode whose
tilt is controllable and a CCD C 姐era
sensor to detect the beam spot made on theobject surface. As the tilt is servひ ed sothat the spot image comes in the center ofthe CCD, the distance to the object is
calculated fro皿 the tilt by triangulation.
The computer on board the robot is acombination of CPU 80286 and NDP 80287 andperforms tasks such as monitoring therobot hardware status, controlling
actuators, sampling environment
information, using external sensors,interpreting and executing co皿mand givenby the host computer, and logging data.
Host computer
The host computer, a combination of CPU
80386 and NDP 80387, performs the overall
control of the total system and interfaces
the system with the operator. It
communicates with the on-board co皿puter
via-a wireless modem.
The software of the host computer is themain bqdy of the total control system andconsists of three parts.
(1) Th。皿an －皿achine interface part dis­
plays the status of the robot and the hostcomputer and accepts commands from theoperator. It also displays the environ­ment model at the path planning and theestimated robot position and the resultsof the sensor infor皿ation processing
during the run. (2) The data management
part performs maintenance of the environ­ment model, logs the moving process data,and communicates with the mobile robot.

(3) The motion planning part, the essen­tial part of the autonomous control, plans
the path and actions of the robot, if
necessary according to the operator ’scommands, and make commands to the robot.
2.3 Navigation Control
Position estimation
The robot position is recognized mainly
based on the dead reckoning using thewheel rotations detected by the encoders.
In this method the estimation error
accumulates, so it should be cancelled byrecognizing landmarks using environment
sensors and referring them to the 皿ap. In
the experi皿ent, two kinds of landmarks areused: walls and wall edges.
The robot recognizes the wall position bymeasuring the distances to a wall with theultrasonic sensor as it proceeds, andcompares the measured result with the wall
information registered in the map. If
their difference is smaller than thepredetermined value, the estimation ofcurrent robot position is corrected in thedirection perpendicular to the wall.
Fig. 3 shows the recognition of the wallusing the ultrasonic sensor in apreliminary experiment of avoiding thecollision with the wall.
The robot recognizes the wall edge using
the laser sensor as it stops where it getsin the range of the target edge. The
laser sensor scans horizontally byservoing the rotary head and series ofrange measurements are sampled for theboth sides of the wall edge. The position
and the direction of the edge relative tothe robot, as well as the angle of theedge, are calculated using the data.

edge recognitionactual wall

front ofrob t b dy。

。

4. m、，2.

。

edge p sition (0.56, 2.43) m
92.2 degedge angle
1 m。

ー 1.

mobile rob t

10

1.
center ofrobot b dy。

Fig. 4 Recognition of the wall edge
using the laser range sensor.

Fig・ 3 Recognition of the wall using
the ultrasonic range sensor.
-319 ー

回目2

1畠A国T

The computer program for the autonomous
control of a mobile robot must process theinformation acquired by internal andexternal sensors appropriately to decide
the movement actions. To complete theprogram requires thorough tests or
debugging to drive out errors. To test
the program using the real robot in thereal environment accompanies danger of acollision between the robot and the object
in the environment. Testing it by meansof software simulation is a possible way
to avoid the danger, but it still remains
risky just to transfer the simulation
result to the reality. So we propose, as
the intermediate step, the experimental
system of a mobile robot using the virtualenvironment in which the real robot iscontrolled in a simulated environment [6].
Fig. 6 shows the concept of the system.

If there is a good coincidence between thecalculation result and the map information,
the estimation of the robot position iscorrected. Fig. 4 shows an example of therecognition of a wall edge using the laser
sensor.
Trajectory control
For the trajectory control of the robot,
we have adopted the method in which thespeeds of the both driving wheels arecontrolled so that the velocity vector of
the robot aims at the next subgoal. Whenthe estimation of the robot position iscorrected, the velocity vector is modified
accordingly to lead the robot onto thecorrect path. At the start of the move,the robot is told its exact position.
Fig. 5 shows part of the environment map
used in the experi皿ent and the trajectoryof the estimated robot position.

In this experimental system, the position
and the movement of the robot are 皿easured
from outside and located in the virtualenvironment. The sensor signals the robot
would acquire if the virtual environment
were real are si皿ulated and given to therobot. The robot works based on thesesignals using the s 担e control logics asin the real environment. We expect inthis method the danger of collision ismini皿ized and extensive debugging isattained.

2.4 Experimental System of a Mobile Robot
Using the Virtual Environment＝－‘t[...ー‘

landmark(wall edge)．．．．．．与．－

wall!
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へ....且／
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子、 ー

2－・・＋ーconnection
betweenquadrangles

・＠開閉冊。．．．
．

virtual environment

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手4－… ＋一一一・匂盟幽阻司ト一一ーー－·. －ー－�\free spacequadrangl 巴l-\\+\t'

・＋・・……ー・・・＠田輔副＠一一 一 ・・十 一－〔ずr4・・・z

目場四’P-(t)l+ + start+i・・··－・－．・ー・ー・・・司－＇·回目・・”“・

φ_ Position esti 『nation is correctedusing wa11 recognition.
削伽n制f『『lt』sing wa 11 edge recog 『1ition.

real-time motion
measurement system• Recognized land 『『1arks (wall edges)

Fig. 50�rn

The environment map and thetrajectory of the estimatedrobot position.

0邑A国 T

Fig. 6-320 ーConcept of the experimental system
of autonomous control of a mobile
robot using the virtual environment.