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* Corresponding author. Tel.: #358-9-5489-8780; fax: #358-9-

5489-8782.

E-mail address: hannu.makela@navitec." (H. MaKkelaK ).

Control Engineering Practice 9 (2001) 573}583

Development of a navigation and control system for an autonomousoutdoor vehicle in a steel plant

Hannu MaKkelaK*, Thomas von Numers

Navitec Systems Oy, Nikkarinkuja 5, 02600 Espoo, Finland

Received 5 May 2000; accepted 20 December 2000

Abstract

This paper describes the navigation and control system for an autonomous guided outdoor vehicle (AGV) which is used to

transport heavy steel slabs in a steel plant area. The vehicle has unconventional kinematics. It has six axles all pair wise steerable by

three bogie structures. The weight of the machine is 17,000kg and its load may weigh up to 95,000 kg. The navigation is based on

a fusion of dead reckoning and transponder positioning. The transponders are passive and are buried in the ground every 5 }10 m

along the routes of the AGV. A wireless communication has been built to connect the vehicle control system and a remote control

station. This article concentrates mainly on describing the navigation system including the kinematics and position control as well as

the safety system and the remote control system. 2001 Elsevier science td. All rights reserved.

Keywords: Autonomous vehicle; Gyroscopes; Kinematics; Navigation system; Position control; Position measurement; Transponder

1. Introduction

Inductive wire guidance has been the traditional navi-gation means for an autonomous guided outdoor ve-

hicles (AGVs) in factories. The major drawback of this

navigation method is that all changes in the routes re-

quire moving or adding a wire to the desired path.

Several other methods have been developed during the

last two decades to simplify the change of the production

layout and the routes in the factory.

In more advanced methods positioning is typically

based on active or passive arti"cial beacons or natural

landmarks. Knowing the distance or direction to at least

three known beacons makes it possible to calculate the

position. For example (Tsumura, Fujiwara, & Hashi-moto, 1984) have introduced a positioning concept based

on re#ective beacons. Commercial navigation systems

based on this idea have been available for several years

(NDC, 2001; Everett, 1995). The positioning may also

rely on sonic waves (Kauppi, SeppaK , & Koskinen, 1992)

or radio beacons. Also a millimeter-wave radar has been

used to detect arti"cial beacons (Durrant-Whyte, 1996).

Durrant-Whyte describes an experimental AGV system

with resembling kinematics. The AGV has a steerable

wheel at each of its corners. The paper gives the kin-ematic equations of the AGV and describes the position-

ing system. In that case the dead reckoning has been

supported by a position measurement system based on

radar technology. The merit of this method is the immun-

ity of positioning to environmental disturbances. As in

this article the fusion of di!erent types of position

measurements takes place using a Kalman "lter.

Another method of using passive arti"cial beacons is

to bury RF tags in the ground and detect them with

special hardware. This method, where the vehicle navi-

gates between the beacons relying on dead reckoning is

often called free ranging on grid (FROG). At each beaconthe drift of dead reckoning may be corrected (Van Brus-

sel, Van Helsdingen, & Machiels, 1988). Here, the merit

of this method is the immunity of positioning to environ-

mental disturbances and also the availability of proven

commercial products.

Natural landmarks have also aroused interest. For

example, James Crowley, one of the pioneers of mobile

robotics, studied world modeling and navigation in an

unknown environment (Crowley, 1985). The modeling

and navigation are based on distance measurement using

ultrasonics.

0967-0661/01/$- see front matter 2001 Elsevier Science Ltd. All rights reserved.PII: S 0 9 67 - 0 6 6 1 ( 0 1 ) 0 0 0 1 4 - 4

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Fig. 1. The AGV.

Fig. 2. The AGV driven under a cassette and lifting the cassette.

For outdoor navigation the GPS has been the choice

for some years. With current technology one can get

a position within a few centimeters of accuracy at the rate

of at least 5 Hz. The major drawback of the real timekinematic GPS positioning is the relatively long startup

time of the kinematic mode and especially the sensitivity

of the system to occlusions. Positioning, for example, in

a forest or close to buildings may be impossible.

This paper describes the navigation and control sys-

tem of a heavy outdoor vehicle, later called AGV (see

Fig. 1). The system consists of three main parts. The

remote control system is used to give new missions to the

AGV, to monitor the operation of the AGV and to store

the log data of the operation. The control system on

board the AGV is both physically and conceptually

divided into two parts. The navigation system is respon-

sible for path handling, positioning, position control,obstacle avoidance and radio communication. The

lowest level part of the control hierarchy is the motion

control module which is the interface to the actuators

and most of the sensors. The motion control module

contains some safety features and the steering angle and

speed controllers. The AGV may be driven not only

automatically but also manually. In the manual mode it

operates on the y-by-wire'-principle using the motion

control module to read the joysticks and drive the

machine.

The machine operates 24 h/day. For an outdoor ap-

plication with very high requirements of reliability andavailability one should choose such methods and compo-

nents for navigation and control that are of proven

technology and commercially available. Since the routes

of the AGV may vary every now and then, the changes of

routes must be quick and thus the traditional wire guid-

ance was out of the question. In addition, the drawbacks

of the GPS mentioned before made this option impos-

sible. A laser based positioning system, which would have

been commercially available, was considered to be too

susceptible to the environmental conditions (rain, snow,

dust, long bearing to the re#ectors). The "nal choice was

a combination of a dead reckoning and beacon detection

system. The dead reckoning here is based on measuring

the direction of the vehicle body with a "ber optic gyro

and the distance travelled with two odometers attached

to two of the front wheels. The beacon system consists of

an antenna on board the AGV, a position calculation

unit and passive transponders buried into the ground

along the routes every 5}10 m. When the antenna passesover a beacon the position calculation unit gives the

transversal deviation of the beacon from the center line of

the antenna. Thus, the position of the AGV relative to the

beacon can be used to correct the drift of the dead

reckoning position estimate. Although the intermediate

distance of the beacons is relatively short the AGV has

not to cross all of them. Small variations into the routes

are possible as long as there is a beacon to be detected

every 30 m.

2. The application

The AGV has been built for carrying heavy steel slabs

(up to 95,000 kg) inside the steel plant area. The items are

lifted by cranes onto cassettes that are then carried by

the AGV to the next site in the process. Until now the

cassettes have been transported by manually driven ter-

minal tractors, which pull or push a translifter carrying

the cassette. Both the manual translifter and the AGV are

driven under the cassette to be transported. The con-

struction of the machine is such that its body is lifted

hydraulically about 30 cm to lift the cassette, which nor-

mally stands with its legs on the ground (see Fig. 2). The

sideways margin between the AGV's frame and the cas-sette is about$4 cm.

The current route network of the AGV is according to

Fig. 3. The distance from loading site to unloading site is

about 500 m. There are six places available for the cas-

settes to be loaded. The AGV can take any of them in

a given sequence. The operator of the crane loads the

cassettes with steel slabs. He has a terminal by which he

informs the remote control system which cassette has

been "lled. The operator also gives the destination area

for the cassette. He tells the AGV to take the load to

either of the two areas. It is up to the remote control

system to decide to which of the free places in the given

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Fig. 3. The route network of the AGV.

Fig. 4. AGV structure, top view.

area the cassette is taken. The remote control system alsogives the AGV an order to take on an empty cassette

before returning to the loading site. If no work task is

available the AGV moves to the waiting site near the

loading area. If fueling, service or repair is needed, the

AGV is able to drive automatically to the service site.

3. Vehicle kinematics

The kinematic equations of the AGV must be known

in order to calculate proper control actions for its steer-

ing to keep the machine on the path. A point called

navigation point is selected on the vehicle body. Thenavigation point is actually the point that is controlled to

move along the reference path. In principle, the position

of the navigation point may be chosen freely but there are

some factors, which should be taken into account. The

mechanical structure of the AGV is such that the position

of the navigation point has an e!ect on the stability of the

position control and also on the dead reckoning posi-

tioning. The navigation point was chosen to be in the

middle of the rear axles at the center line of the vehicle

body since this point guarantees a stable position control

for both reversing and driving forwards. Also, the kin-

ematic equations for this point are relatively simple. Thenavigation point is constant and does not depend on the

driving direction.

The kinematics of the AGV is very unconventional.

The AGV has two front axles with four wheels and four

rear axles with eight wheels (see Fig. 4). The number of

wheels is high and the wheels are wide because the

pressure of a wheel against the ground should be within

reasonable limits even with the maximum load. All the

front wheels are equipped with a hydraulic traction mo-

tor. The axles are attached to a common bogie, which

may be rotated relative to the body of the AGV. The

AGV is steered by turning the bogie along with the two

front axles, not by turning the separate wheels. Since thelength of the vehicle body is more than 15 m, its steerabil-

ity is quite poor. The maximum steering angle of the front

wheels is 453 but still the turning radius (rin Fig. 4) would

be more than 17 m when turning only the front wheels.

The rear wheels are also steerable to reduce the steering

radius and especially to prevent the rear wheels from

sliding sideways when turning. Both two pairs of rear

axles are attached to turning plates, which may be turned

independently. Due to the restrictions of the mechanical

construction the rear axles may only be turned 153. This

reduces the minimum turning radius to 14 m.

The dead reckoning positioning based solely on gyro

and odometers would require that the AGV moves in thedirection of its frame. However, the steering system en-

ables the AGV to move sideways when turning the rear

axles in the same direction. Since the gyro does not

measure the direction of motion but the direction of the

vehicle body the dead reckoning positioning does not

detect the sideways motion. For this reason and also for

steering angle control, all the steerable axles are equipped

with sensors measuring the steering angle. The dead

reckoning equations take this motion into account.

The selected steering system is not easy to model

accurately. However, the main objectives of the mechan-

ical design have been the simplicity and reliability. Inaddition, the same steering system is of proven techno-

logy since it is in use in the corresponding manual ma-

chines. The kinematics may be simpli"ed by replacing the

four rear axles by two and by replacing the two front

axles with one. Each of the imaginary axles is in the

middle of the two axles which they represent (see Fig. 5a).

For each turning radius, the angles for the wheel axles

are calculated as follows:

cPCD"

1

rPCD

, (1)

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Fig. 5. (a) Simpli"ed kinematics, (b) sideways motion calculation.

"arctan (cPCD

r

), (2)

"arctan (cPCD

r

), (3)

where rPCD

is the reference turning radius, cPCD

is reference

curvature of the path, r

, r

are distances of the rear and

front axles to the navigation point, and , are streering

angles of the front and middle axles.

In an ideal case the magnitude of the turning angle of

the middle axle would be the same as that for the rear

axle but with the opposite sign ("!). In practice, the

steering controllers for the axles are not ideal and the

angle does not necessarily correspond to the front angle

. In this case the navigation point is shifted from its

nominal position and the angle can be calculated as

follows:

rP"

(lDK

sin(/4#))cos

sin(!), (4)

rKB"r

Ptan, (5)

rPCB"rKB!(r#r), (6)

"arctanrPCBr, (7)

where r

is the distance of the middle axle to the navi-

gation point, is streering angle of the middle axle, rP

is

real curvature relative to the shifted navigation point,

rKB

is real middle axle distance (from shifted navigation

point), rPCB

is real rear axle distance (from shifted navi-

gation point), and lDK

is distance between front and

middle axles.

The sideways motion of the AGV body, which cannot

be detected by the gyro, is calculated as follows: (see

Fig. 5b):

sRP?LQ"s sin , (8)

sJMLE"s cos, (9)

smRP?LQ"

sJMLE

(tan #tan)

2, (10)

hL"arctan

sRP?L

sJMLE

, (11)s

L"(s

RP?L#s

JMLE, (12)

wheresRP?LQ

is the transversal motion of the middle point

of the front axle, sJMLE

the longitudinal motion of the

middle point of the front axle, smRP?LQ

the transversal

motion of the middle point of the middle axle, hL

the

corrective term for the direction of motion of the navi-

gation point, and sL

the real motion increment of the

navigation point.Thus, the result of Eqs. (11) and (12) gives the speed

vector for the navigation point to be used in the dead

reckoning calculations.

4. Vehicle position measuring system

The basis of the navigation system is dead reckoning.

It calculates the position and heading of the vehicle

relative to the starting point by integrating the speed

vector. The speed vector is obtained by measuring the


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distance travelled and the direction of motion. As de-

scribed in the previous chapter the steering angles must

also be measured to calculate the sideways motion which

cannot be detected by the gyro.

The distance travelled by the vehicle is measured using

optical encoders attached to two of the front wheels of

the AGV. The measurement is simple and reliable. The

error is usually quite small, typically less than 0.5% of thedistance. To achieve this the tire wear must be compen-

sated using the known position of the transponders. The

encoders are attached to the wheels on the same axle and

the distance travelled in the direction of the wheels is the

mean of the values given by the encoders. In practice, the

measurement is less accurate in curves than when driving

straight. The accuracy in curves depends especially on

the friction of the ground. This is due to the mechanical

structure of the steering system of the front wheels which

forces the tires to slide sideways when turning.

The orientation of the body of the AGV is measured by

a"

ber optic gyro. Its drift is about 303

/h after an initia-lization time of 10 s. The gyro yields only the angular

rotation rate which has to be integrated to obtain the

orientation. To "x the orientation to the co-ordinate

system in use one has to initialize the heading and the

position. This is achieved by an external beacon

system.

In addition to position and heading initialization, the

external beacons are also used to correct the accumu-

lated error of the dead reckoning. The principle is that

the position of the vehicle is measured relative to a "xed

known point which gives a new accurate position. The

measurement of the direction of motion can be corrected

when the vehicle has travelled through two beacons,since the measured direction of motion can be compared

to the vector from beacon 1 to beacon 2. This procedure

takes place in the initialization phase where the AGV is

manually driven through two beacons.

In outdoor conditions the beacons must endure harsh

environmental stress. They must be passive, small, inex-

pensive, service free and robust. There exists at least one

type of beacon ful"lling these needs: passive LC trans-

ponders, which will be activated and detected using an

antenna on board the AGV.

The transponder detection system chosen for the AGV

can detect transponders at a height of 0.4 m maximum,while the nominal reading height is 0.2 m. A transponder

being passed is activated by an electromagnetic "eld

generated by the transponder antenna on board the

AGV. The transponder then emits a counter "eld which

is detected by the antenna. Identity information is in-

cluded in the signal of the transponders. A separate

position calculation unit calculates the sideways devi-

ation of the transponder at the moment the center line of

the antenna passes the transponder. The transponder

system is interfaced to the navigation computer via a ser-

ial line.

The accuracy of the beacon detection system is in

practice about 2 cm both in the sideways direction and in

the direction of motion. The measurement is combined

with the position estimate of the dead reckoning using

the following equations (Rintanen, MaKkelaK , Koskinen,

Puputti, & Ojala, 1996):

xLR

(t)"f(xL(t), u(t))#K(t), (13)

yL(t)"g(xL(t)). (14)

The state vector

x(

x"y(

x (15)

y

x

y

consists of two separate dead-reckoning displacement

and heading estimators. The "rst (x(y

() is based on

the cumulative heading change provided by the gyro.

The purpose of this estimate is to compensate for the

gyro drift. The second estimate (x y) is based on the

drift- and o!set-compensated heading#

.

is the

sum of the vehicle initial heading and gyro drift and

would be constant during the motion for an ideal gyro.

(x

y

) are the sums of the vehicle initial position and

dead-reckoning drift. (x

y

) would be constant for an

ideal dead-reckoning navigator. The input-, output- and

innovation-vectors are

u"[ ]2, (16)

y"[X > ]2, (17)

"[x

y

]2, (18)

where is the turning rate measured by the gyro, is the

speed of the vehicle, [X > ] is the estimated posture of

the vehicle and x

y

are the measured errors of

initial heading and position. The system equations are

g(x)"

x#x

x#x

x#x

, (19)

u

0u

cos x

u

sin x (20)f(x, u)"

u

cos(x#x

)

,

u

sin(x#x

)

0

0


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Fig. 7. Sideways motion of the navigation point.

coe$cient. Fig. 6a shows the test route where the trans-

ponders are marked with x. The installation accuracy of

the beacons was better than$1 cm. Fig. 6b illustrates

the compensated dead reckoning drift (x(

, x(

) and Fig. 6c

the compensated gyro drift. The compensation is cal-

culated at each detected beacon according to the Eqs.

(13)}(25). The position drift remains in the range of

$1 m and the heading error in the range of$1.53.Thus, the drift of dead reckoning is less than 0.2% of the

distance travelled, being even smaller than what was

expected.

5. Position control system

5.1. The route builder

Since there are several possible routes from any cas-

sette position at the loading site to any position at the

unloading site it would be impractical to store all thealternative routes separately. Instead all crossing points

and end points of the route network are depicted as

nodes, each node having a distinctive identi"cation num-

ber. A route segment is a sequence ofx- and y-points from

one node to another. Thus, any practical route may be

obtained by combining proper segments.

All the route segments to be driven are stored into the

memory of the VCS. A segment contains not only the

co-ordinates of each point but also the reference speed at

these points. After "nishing a task the next target node

for the vehicle is given by the remote control station via

radio. Since the navigation system knows the current

position of the AGV it can build a route to the givendestination by connecting appropriate route segments

using a special search algorithm. If there were several

alternative routes available the search algorithm would

take the shortest one. The algorithm is based on the idea

introduced by Dijkstra (1959).

5.2. The position controller

Originally the position controller had two modes of

driving the AGV. The docking mode was used only when

driving under a cassette. The normal mode was used at

other times. The sideways accuracy requirement for thedocking mode is about$4 cm based on the dimensions

of the cassettes and the AGV. Elsewhere the accuracy

requirement is such that the transponder antenna must

detect the next transponder with some safety margin. In

practice, the position control accuracy including the

position uncertainty must be within$10 cm. The di!er-

ence between the modes was that the sideways motion of

the vehicle body was allowed in docking mode while not

in normal mode. Sideways motion means a more e$cient

way of moving the navigation point sideways by turning

the rear wheels partly independently of the front wheels

(see Fig. 7). In this case the navigation point of the AGV

does not move in the direction of the frame as was

explained in Section 3.The position control of the normal mode calculated

the reference curvature for a "xed navigation point. The

"nal reference curvature was based on the reference cur-

vature of the trajectory, the current position error and

the heading error. The idea was to correct the heading

error and position error by steering the front wheels only.

The steering angle of the front wheels was calculated

based on the reference curvature and the angles of the

rear wheels were calculated such that all the axles had the

same center point of turning (see Fig. 4). Practical tests

proved that the normal mode did not work well. Because

of the length of the vehicle body, only slight sideways

position errors caused remarkably large steering maneu-vers of the front wheels. Since the transponder antenna is

in the middle of the frame of the AGV, the wide sideways

motion of the front part caused some transponders to be

missed. Due to these di$culties the normal mode of

position control was abandoned and the sideways cor-

rection using the rear wheels independently from the

front wheels was allowed.

The position controller initially developed for docking

under a cassette was taken into use for all parts of the

routes. The control method resembles the one described

in Durrant-Whyte (1996). Durrant-Whyte corrects the

position error by sideway motion of the AGV, i.e. turningthe rear and front wheels the same amount to the same

direction and the heading error by turning the rear and

front wheels the same amount to the opposite direction.

Also in this case the position controller actually consists

of two separate controllers, the position controller and

the heading controller. The heading error is de"ned ac-

cording to Fig. 8. To correct the current heading error,

a fraction of the error is added to the current curvature

which is based on the shape of the trajectory:

cPCD"c

RP#K

FeF

. (26)


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Fig. 10. Sensors for driving under a cassette.

Fig. 8. De"

nition of the heading error. Fig. 9. De"

nition of the current position error.

Eqs. (1)}(7) are now used to calculate the proper angles

for steering. The reference angle is used as such to

correct the heading error but an additional position

correction term is added to the steering angle of the rear

axles:

"#K@eQ , (27)

"#KAeQ , (28)

where eQ

is the position error in the sideways direction

and is calculated as follows (see Fig. 9):

A"!sin hP, (29)

B"cos hP, (30)

C"!AxP!By

P, (31)

eQ"Ax#By#C. (32)

5.3. Driving under a cassette

Since the sideways error margin is very small when

driving under a cassette the dead reckoning accuracy was

not su$cient to guarantee that the AGV would never hit

the cassette. Some extra sensors were needed to detect the

position of a cassette relative to the AGV (see Fig. 10).

These sensors were

E a laser scanner at the rear of the AGV,

E inductive switches at the rear corners of the frame of

the AGV,

E ultrasonic distance sensors in the middle of the frame,

E an inductive switch at the front of the frame.

The sensors are used as follows. When the AGV ap-

proaches a cassette the rear laser scanner, which is used

also for safety purposes, is put into a mode where it sends

180 distance measurements, one for every degree. Since

the approximate position of the cassette and the AGV are

known, an object in an interesting area is interpreted as

a potential leg of the cassette. If two legs are seen at

a certain distance from each other and in proper orienta-tion relative to the AGV it is believed to be a cassette and

a sideways correction is calculated. The correction takes

place with the rear wheels of the AGV and is such that

the rear part of the frame of the AGV is centered relative

to the cassette. The position control relative to the cas-

sette is started at a distance of 3 m from the cassette and is

stopped when the frame of the AGV goes under the

cassette. The laser scanner gives the fan of distances at

the rate of 5 Hz and the sideways correction is calculated

at the same rate. The orientation of the cassette relative

to the AGV is also calculated based on the position of the

legs of the cassette and the heading of the AGV is alignedaccordingly by turning the front wheels.

Using the laser scanner the rear part of the AGV drives

reliably under a cassette. Also, the orientation of the

AGV frame is corrected relatively well according to the

orientation of the cassette. The ultrasonic distance sen-

sors in the middle of the AGV are used to center the

frame according to the cassette. The correction for

centering the frame is done by turning the front wheels.

There are also inductive switches at the rear corners. If

the frame is too close to the cassette when driving under

it, the rear wheels are turned to center the rear part of the

frame based on the impulse of the switch. The amount of


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Fig. 11. AGV control system.

correction was tuned experimentally since no distance

measurement is available for continuous control when

using the switches.

The inductive switch at the front part of the frame is

used to detect the cassette when the AGV has driven long

enough under the cassette. When the switch detects the

cassette the motion is stopped and the cassette may be

lifted for transportation. In some cases the cassette maybe left manually to its position which normally is not the

correct one. The AGV is able to drive under a cassette

which is less than 0.4 m aside from its nominal position in

the sideways direction and at most 0.5m aside in the

driving direction.

5.4. Obstacle avoidance

The AGV is used in a closed plant area but its operat-

ing environment may not be isolated from other vehicles

or people. This is why the safety system for detecting

obstacles in the direction of the motion is vital. Thecontrol system needs sensors to give the distance to

obstacles around the vehicle and the vehicle must be able

to reduce its speed and stop if an obstacle is detected too

close to the vehicle, especially in the direction of the

motion. There are four scanning laser distance sensors

for detection of obstacles in the front, rear and on both

sides of the vehicle. The side sensors will detect moving

obstacles such as people approaching the AGV from the

side. The scanners are such that they can be programmed

to give a signal if an obstacle is detected in a warning area

or in a separate safety area. The size and shape of these

areas are programmable and typically the warning area is

larger than the safety area. The side sensors are mounted

high up, such that their fan is downwards covering the

whole length of the AGV. In the case of the side sensors

the warning area is not in use but the AGV is stopped if

an obstacle is detected in the safety area. The safety relays

from the distance sensors are connected both to the

motion controller and to the navigation controller to

increase the overall reliability of stopping when needed.The scanners in the front and rear are installed to

a movable frame so that their height is kept constant

independently of the height of the frame of the AGV. The

fan of these sensors is in a horizontal plane. They are

mounted to such a height that detecting an obstacle with

a minimum height of 20 cm is reliable. If an obstacle is

detected in a warning zone at a distance of 4 m the AGV

starts to decelerate. At the border of the safety area the

AGV stops until the obstacle has been removed. The

safety area extends to a distance of 2 m. Full braking

takes place if an obstacle comes into the safety area from

the side. A mechanical safety bumper at both ends of theAGV is the last guard and will stop the vehicle if the

bumper contacts an object.

6. AGV control system

6.1. Navigation unit

The AGV control system consists of two parts com-

municating with each other using a standard serial link

(see Fig. 11). The navigation computer is based on stan-

dard PC-104 components. In addition to a CPU-board


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Fig. 12. Remote control system.

(Pentium 133 MHz), it contains an extra serial interface

board as well as an analog and digital input/output

board and a DC/DC power board. The navigation-PC

interfaces with the four laser distance sensors, two ultra-

sonic distance sensors, three inductive switches, the

transponder system and the gyro. It also reads some

switches and buttons related to the operation modes and

the position initialization procedure. The communica-tion system with the remote control systems is realized

using commercial wireless Ethernet components.

6.2. Motion control unit

The motion control module is realized using a PLC. It

takes care of the low level inputs and outputs as well as

the low level controls of the AGV. The overall design

philosophy is such that the AGV can be moved manually

even if the navigation unit is not operative. This requires

that the steering and speed controllers are implemented

in the PLC since these controllers are active also in themanual mode. The motion control module interfaces to

more than 20 sensors and to 10 actuators including the

steering, throttle and hydraulic pump. The speed of the

AGV is controlled by the throttle which a!ects the rpm of

the diesel engine and by controlling the yield of the pump

which feeds the hydraulic traction motors. Hydraulic

cylinders are used as actuators for steering and the steer-

ing angle is measured by potentiometers. The position

controllers for steering as well as the speed controller are

of an ordinary PID type.

6.3. Remote control station

The remote control system is con"gured according to

Fig. 12. The core of the system is the remote control

station responsible for maintaining a cassette status

database, assigning tasks for the AGV and collecting

event logs for service and production performance fol-

low-up. Normally, there is no operator at the control

station. The everyday operators are the crane drivers

who access the system via two terminals connected to the

control station through Ethernet. The crane drivers in-

form the control station about system status changes as

they load and unload the cassettes for the AGV. When

a cassette is loaded the crane driver gives its destination

by the accuracy of the unloading area. The control sta-

tion then chooses the exact destination from the free

cassette positions in that unloading area. Accordingly,

the crane drivers inform the system when the cassettes

are unloaded. The control station then tells the AGV to

transport the empty cassette back to the loading site

when the position of the cassette "ts the transportationsequence. The activities of the AGV also modify the

database. Each time the AGV picks or leaves a cassette

the database is updated automatically. Events relevant to

production follow-up or AGV service are logged.

If manual machines change the order of the cassettes

so that the control station database is out of date, the

database can easily be modi"ed through a special modi"-

cation window on the control station. The correct state for

each cassette position is simply clicked with the mouse.

7. Practical experiences

The building of the software was started by program-

ming the PLC. This was necessary since even driving the

AGV manually requires the PLC. The interface to the

sensors and actuators was tested "rst, then the steering

controller was tuned while the front part of the AGV was

lifted up. The hydraulics for steering required some cha-

nges to speed it up but even after these changes the

steering was not too responsive. The speed controller was

relatively simple to program since no gears were to be

used. Originally, the speed controller was of type P but

later an integrating term was added to make the control

more accurate.The kinematics of the AGV proved to be very chal-

lenging. The real behavior of the AGV does not fully

obey the theoretical kinematic model when turning and

this slight discrepancy had to be modeled and taken into

account. When turning the front bogie the simpli"ed

model of Fig. 5a is not accurate enough. Actually the

steering had to be modeled as if the front axle of the

model would be linked to the frame of AGV, not directly

but with a short lever. This was due to the fact that when

turning, the front bogie did not turn around its center

point but the actual turning point was somewhat ahead.


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Some of the sensors caused a lot of trouble. The gyro

chosen initially turned out to be unreliable. In some cases

its drift changed quite rapidly which caused the AGV to

drive out of its path. The problem was greatly alleviated

by making the drift compensation adaptive. When pas-

sing a transponder the drift of the gyro was recalculated

and the compensation was changed accordingly. Even

with this procedure the AGV sometimes ran out of thepath. Each time the control system detected this situation

and safely stopped the motion. However, the gyro had to

be changed to a better, and more expensive model.

The scanners also caused a lot of extra stops. Safety is

a critical issue here and the system was built up in such

a manner that every time any of the four scanners in-

formed of an obstacle in the safety area the AGV was

stopped. Initially the scanners were too sensitive. They

detected small amounts of iron #akes dropping from steel

slabs on board. Dust, rain and snow#akes also caused

extra stops. The scanners were programmable and there

was a parameter which de"

nes how many consecutivetimes the scanner must hit an object in the same direction

and distance before the object is determined as an ob-

stacle. Changing this parameter did not help before the

scanners were changed to a model where this feature was

correctly implemented. The scanners were then relatively

immune to the above mentioned disturbances. Since

there is a lot of other activity in the working area of the

AGV, moving objects (vehicles, people) occasionally

stopped the AGV. The control system had to be changed

so that the motion continues after a delay when the

obstacle has been removed from the safety area.

After all the above mentioned corrections the navi-

gation and control system have been working withoutproblems. The navigation accuracy is within the require-

ments and the AGV is able to take also manually trans-

ported cassettes even if they are$40 cm aside from their

nominal positions. So far the safety system has not

failed to detect an obstacle and to stop the AGV when

necessary.

8. Conclusions

This paper gives an overview of the control and navi-

gation system of an outdoor AGV. The major require-ments for the system are reliable and accurate navigation,

a reliable safety system with collision avoidance and

navigation accuracy better than 4 cm. These require-

ments have to be met in a commercially realistic manner.

The navigation system is a combination of dead reckon-

ing and transponder positioning using commercial on-

the-shelf components and sensors. The machine is de-

signed to work in a nonisolated area requiring special

attention to the collision detection and safety system.Developing the system has been very challenging with

many problems related to, for example, modeling the

kinematics of the AGV. Reaching the required level of

overall reliability has been di$cult mainly because of

several problems with the sensors. Here, the demand of

ultimate safety has somewhat counteracted the require-

ment of availability. In this paper the major problems

have been solved and the navigation and control system

runs according to the requirements. What still has to be

done is to make the user interface simpler and more

#exible to use.

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