ZARCO – AN AUTONOMOUS CRAFT FOR UNDERWATER SURVEYS
Nuno C uz, An´ıbal Ma os, S´e gio Cunha, S´e gio Sil a
Faculdade de Engenha ia da Uni e sidade do Po o
Rua D . Robe o F ias
4200-465 Po o
Po ugal
{nac uz,anibal,se gio,s ui}@ e.up.p
Keywo ds: Au onomous Su ace C a , High Resolu ion Sensing.
Abs ac
The au onomous su ace c a (ASC) Za co is a small size c a de eloped a Po o Uni e si y and designed
o pe o m au onomous missions mainly in i e dams and es ua ine en i onmen s, ei he se ing as a
mo ing na iga ion beacon o unde wa e ehicles o collec ing high esolu ion in e e ome ic syn he ic
ape u e sona (In-SAS) da a. This pape desc ibes he main sys ems o Za co and p esen s p elimina y
esul s conce ning he pe o mance o he on-boa d con ol sys em, ob ained in es missions. Ano he
pape p esen ed in his con e ence add esses he In-SAS sys em.
1 In oduc ion
This pape desc ibes he Au onomous Su ace C a Za co ( igs. 1,7). Za co is a small size (1.5 m long)
ca ama an ype essel. In i s basic con igu a ion, he ehicle weigh s a o al o 50 kg, and has an addi ional
payload capaci y o ano he 50 kg. The ehicle is elec ically powe ed and can ope a e a speeds up o 3
kno s. I ca ies an on-boa d compu e esponsible o he execu ion o au onomous o emo ely con olled
missions, o he eal ime compu a ion and o he s o age o collec ed payload da a. A WiFi link connec s
Za co o a sho e s a ion, allowing o he emo e con ol o he boa and he supe ision o i s au onomous
ope a ion.
The pape s a s wi h a desc ip ion o he in e nal s uc u e o Za co, a he mechanical, elec ical and
compu a ional le els. Then, a desc ip ion o he p ocedu e o p og amming and execu ing au onomous
missions is add essed. Finally, esul s conce ning he pe o mance o Za co con ol sys em a e p esen ed.
2 Za co Subsys ems
2.1 Mechanical s uc u e
Za co ha dwa e a chi ec u e ollows a highly modula app oach. The mechanical s uc u e is based on
COTS anodized aluminum elemen s, o ming a igid ame ha a aches o he la e al pon oons by a se
o snap bu ons, esul ing in a ca ama an a angemen . The pon oons a e made o molded polye hylene
and each one has a ne buoyancy o 50 kg.
P opulsion is p o ided by wo elec ical h us e s loca ed a he ea o he mechanical s uc u e. These
h us e s a e based on s anda d olling mo o s, ypically a ailable o small boa s, and p o ide a maximum
h us exceeding 250 N. In Za co, special wa e igh junc ion boxes we e designed so ha he mo o s can
ecei e powe and commands om he compu a ional module, ins ead o he adi ional manual con olle s.
Fo anspo a ion, he pon oons a e de ached om he igid ame and a special moun ing adap e allows
he sha s o he mo o s o change in o a ho izon al posi ion, aligned wi h he es o he ame. All he
Figu e 1: Za co CAD model.
mechanical s uc u e can hus be anspo ed in he unk o a ca and since no ools a e equi ed o
eassemble i , Za co is eady o ope a e wi hin a ew minu es upon a i al a he mission si e.
2.2 Elec ical componen s
As a as elec ical sys ems a e conce ned, he base e sion has wo sepa a e modules, each on i s own
wa e igh enclosu e ha sea s on he aluminum ame: an ene gy module in he on and an elec onics
module in he ea ( ig. 1). This physical sepa a ion has wo main ad an ages as compa ed o a single
enclosu e: i s , i is e y easy o swap he ene gy module by a new one when he ba e ies a e exhaus ed
and, second, i he e is he need o a con inuous long mission, i is possible o edesign he ene gy module,
eplacing he ba e ies by o he models based on echnologies wi h be e powe o olume a ios.
Wi-Fi An enna
Wi-Fi An enna
Po Mo o
Po Mo o
S b d Mo o
S b d Mo o
GPS An enna
GPS An enna
GPS An enna
GPS An enna
Spa e Po s
Figu e 2: Elec ical componen s.
In he ene gy module, he e is a se o deep-cycle lead-acid ba e ies, a echnology commonly used in
elec ical wheelchai s and gol ca s. These ba e ies ha e modes powe o olume (and weigh ) a io
bu hey a e a ailable o he shel a e y easonable p ices, as opposed o o he echnologies, such as
Nickel-me al Hyd ide o Li hium-Ion. In he case o Za co, he o al ene gy a ailable on boa d is 625Wh
(a 12V). The ASC au onomy is g ea ly dependan on he payload ins alled on boa d and he eloci y
p o ile equi ed o he mission, bu i is usually in he o de o 6 o 10 hou s o ope a ion. In he case
ha longe missions a e equi ed, a spa e ene gy module can easily be swapped in he ield. Since he ime
equi ed o ully echa ge he ba e ies is 6-8 hou s, i is i ually possible o ha e inde ini e missions wi h
wo se s o ene gy modules. The ba e ies may be echa ged wi hou opening he enclosu e, since any gas
accumula ion will be eleased o he en i onmen ia a en plug. The ene gy module also holds elec onic
p o ec ion ci cui s, wi h ol age and cu en moni o ing, and he main swi ch ha commands he powe
o he on-boa d elec onics.
The elec onics module con ains he main compu e , he na iga ion senso s and he mo o powe con-
olle s. The enclosu e has se e al spa e connec o s o p o ide ene gy and communica ions wi h payload
senso s and allow o u u e upg ades.
The main compu e is based on he PC-104 echnology. In he basic con igu a ion, he s ack includes
a powe supply boa d, he CPU boa d and a communica ion boa d o in e ace wi h he o he de ices
(na iga ion senso s and mo o con olle s). Addi ional boa ds can be ins alled o in e ace wi h speci ic
payload sys ems. A solid s a e disk s o es he on-boa d so wa e and is also used o log da a collec ed
du ing ehicle missions, and a long ange WiFi link p o ides communica ions o sho e.
Cu en ly, he na iga ion sys em is composed by 2 L1 GPS boa ds (µBlox RCB-LJ) and a digi al compass
module wi h il senso s (PNI TCM2.0). Al hough su icien o he i s ehicle es s and some demo op-
e a ions, his senso package does no p o ide he accu acy equi ed by some o he en isaged applica ions.
Fo ha , an Ine ial Measu ing Uni will be in eg a ed in he nea u u e, and he GPS boa ds will be
eplaced by L1+L2 RTK ecei e s.
2.3 Payload
As a as payload capabili y is conce ned, he e is physical space o an ex a enclosu e in he aluminum
ame, wi h maximum base dimensions o 500 ×300 mm. Na u ally, his can be inc eased by eplacing
some o he aluminum elemen s o he ame by longe ones.
3 Sho e s a ion
In an ope a ional scena io, a sho e s a ion based in a lap op compu e is used o emo ely con ol Za co
and o moni o i s beha io while pe o ming au onomous missions. The sho e s a ion is connec ed o
he ehicle h ough a Wi-Fi link, gua an eeing a wide band connec ion. The ehicle is equipped wi h an
omni-di ec ional an enna while he sho e s a ion can be connec ed o an omni-di ec ional o a sec o ial
an enna, depending on ope a ion a ea and loca ion o he sho e s a ion. In any case, high gain (12 o 24
dBi) an ennas a e always used, which allows communica ion anges exceeding 2 km.
A g aphical in e ace [4] ( ig. 3), unning on he sho e s a ion lap op, communica es wi h he on-boa d
so wa e o send commands o he ehicle and ecei e eal ime da a om i . Recei ed da a include ehicle
posi ion, a i ude and eloci ies, gene al s a us da a, such as ba e y ol ages and powe consump ion,
as well as au onomous ope a ion ela ed da a. A s anda d joys ick can also be a ached o he lap op o
emo ely ope a e he ehicle.
A GPS e e ence s a ion migh also be connec ed o he lap op in o de o p o ide di e en ial co ec ions
o he on-boa d GPS de ices. These co ec ions a e also sen h ough he Wi-Fi link, hus a oiding he
need o a dedica ed link.
4 On-boa d So wa e
The on-boa d so wa e [4] is one o he mos impo an componen s o he Za co sys em. I is esponsible
o he au onomous ope a ion o he boa and can also be used o con ol he payload sys em and egis e
he ga he ed da a. I was de eloped in C++ and uns on a s anda d Linux ope a ing sys em.
The so wa e is composed by a se o modules o ganized as depic ed in igu e 4. This igu e shows he
hie a chical s uc u e wi h he so wa e in e ace modules a he lowes le el. A a in e media e le el
Figu e 3: Remo e console applica ion.
lie he na iga ion and con ol modules, which a e esponsible o he eal ime es ima ion o he ehicle
s a e and o he eal ime compu a ion o he ehicle ac ua ion, espec i ely. A he op le el esides
he supe ision module. Besides being esponsible o he communica ion wi h he sho e s a ion, ei he
execu ing emo e commands o sending back ele an da a, i moni o s he ehicle beha io and deals wi h
unexpec ed e en s. A black box da a logging sys em egis e s all da a ela ed o he ehicle mo ion on a
lash disk connec ed o he CPU boa d. This sys em can also be used o egis e payload da a, i necessa y.
GPS
GPS Compass
Compass IMU
IMU Mo o s
Mo o s
Na iga ion
Na iga ion Con ol
Con ol
Supe ision
Da a logging
Figu e 4: On-boa d so wa e modules.
All he so wa e modules un as independen Linux p ocesses. In his way, no only he sys em modula i y
and obus ness a e inc eased bu also i s debugging and eco e y om unexpec ed e en s is much mo e
simple. The communica ion be ween he modules elies on he exchange o messages, using he Use
Da ag am P o ocol. This allows connec ionless da a exchanges, wi h educed p ocessing o e head, as
equi ed is his kind o applica ions.
4.1 So wa e in e ace modules
The na iga ion senso s and mo o con ol boa ds a e con olled by specially de eloped so wa e in e ace
modules. Besides dealing wi h he speci ici ies o he ha dwa e in e aces o he di e en physical de ices,
hus c ea ing an abs ac ion laye , hese modules also p o ide in e aces o con igu e and moni o he
beha io o each de ice.
4.2 Na iga ion module
As al eady men ioned, he na iga ion module is esponsible o compu e in eal ime he s a e o he ehicle
(posi ion, a i ude and linea and angula eloci ies). This es ima ion is pe o med by an ex ended Kalman
il e based algo i hm [2], ha can be con igu ed o e lec di e en se s o a ached senso s and sys ems.
Cu en ly, na iga ion elies only on he da a p o ided by he wo µBlox RCB-LJ ecei e s and he TCM2.0
module. The ou pu a es o hese senso s a e 4 Hz and 16 Hz, espec i ely.
4.3 Con ol module
The con ol module is esponsible o he compu a ion in eal ime o he mo o ac ua ion, acco ding o
he mission ha he ehicle mus pe o m. Fo ha pu pose, he con ol sys em implemen s a se ies o
con ol loops associa ed wi h each di e en maneu e . A each con ol cycle, he con ol sys em compu es
he ehicle ac ua ion and es s he comple ion o he cu en maneu e . I he comple ion condi ions a e
me , a new maneu e is s a ed.
Fo con ol pu poses, i is ad an ageous o decompose he po and s a boa d h us e s ac ua ion in
common and di e en ial modes, acco ding o
Upo =Ucommon +Udi
2
Us a boa d =Ucommon −Udi
2
since, in his way, he linea and angula mo ions o he ehicle can be decoupled.
When manually ope a ed, bo h he common and di e en ial mode commands a e ob ained om he joys ick
a ached o he sho e s a ion. When in au onomous mode, hey a e compu ed by he con ol sys em wi h
an upda e a e o 10 Hz.
5 Au onomous Ope a ion
The au onomous ope a ion o Za co is based on he sequen ial comple ion o a se o elemen a y maneu e s
— he ehicle mission. Missions a e de ined in ex iles which a e ans e ed o he ehicle and p ocessed
by i s con ol sys em.
A ypical mission ile, as shown in ig. 5, is composed o wo main pa s. The i s con ains he de ini ion
o poin s in he a ea o ope a ion which a e ele an o he mission. These poin s can be de ined ei he in
absolu e coo dina es o ela i e o o he de ined poin s.
The second pa o he ile con ains he lis o maneu e s ha he ehicle mus execu e. Besides di ec
ac ua ion maneu e s, mainly used o es ing, he p incipal ehicle maneu e is he [LineTo]. I consis s
in he execu ion o a ec ilinea ajec o y a a cons an speed. The de ini ion o each [LineTo] maneu e
con ains he de ini ion o he eloci y and o he des ina ion poin . The s a poin o each maneu e
is implici ly de ined as he las poin o he p e ious one. I is also possible o include an Ea lyEnd
pa ame e ha de ines he dis ance om he inal poin a which he cu en maneu e is conside ed
comple ely execu ed.
Mission iles can be edi ed ei he manually o using he ehicle console. They a e uploaded o he ehicle
using he g aphical in e ace, whe e he use can also s a , s op and pause he execu ion o a mission.
6 Line T acking Algo i hm
In his sec ion we p esen he majo concep s behind he design o he line acking algo i hm. Since he
ehicle is mainly in ended o mo e along ec ilinea ajec o ies, his is he mos impo an con olle .
When acking a gi en line, he common mode ac ua ion Ucommon, used o con ol he linea ehicle
eloci y, can ei he be de ined a p io i, esul ing in an open loop eloci y con ol, o be he ou pu o a
eloci y eedback con ol. Fo mos applica ions, i is ad an ageous o use open loop eloci y con ol, since
% Za co sample mission ile
[De ini ions]
[Poin ]
name = middle
la lon = 41N3.6019 8W27.2990
[Poin ]
name = uppe
base = middle
o se RD = 50 137
[Poin ]
name = lowe
base = middle
o se RD = 50 -43
[Mission]
[LineTo]
name = uppe
mo o s = 50
[LineTo]
name = lowe
mo o s = 25
[LineTo]
name = lowe
o se RD = 50 -133
mo o s = 75
[LineTo]
name = uppe
o se RD = 50 -133
mo o s = 100
[LineTo]
name = uppe
mo o s = 50
Figu e 5: Typical mission ile.
i esul s in smoo he ehicle mo ions. The di e en ial mode ac ua ion Udi mus ob iously be ob ained
om a line acking algo i hm, in o de o ensu e ha he ehicle desc ibes he desi ed pa h, as desc ibed
below.
Desi ed ajec o y
β
d
Figu e 6: Line acking e o s.
When d i ing he ehicle owa ds a gi en line, he mos impo an quan i ies o ake in o accoun a e he
o - ack e o dand he heading e o β, as shown in igu e 6. The goal o he line acking algo i hm
is o d i e d o ze o. Usually, due o wa e cu en s, wind, mino misalignmen s o h us e s and small
assymme ies o he ehicle hull, i is no possible o d i e he β o ze o, a he same ime.
The line acking algo i hm conside ed he e is based on a wo s age con olle . A he ou e le el, a heading
e e ence is gene a ed, based on he di ec ion o he line being acked and also on he o - ack e o . A
he inne le el, a heading con olle is esponsible o he de e mina ion o he di e en ial con ol o he
ehicle. In his way, he equa ions de ining he line acking algo i hm a e gi en by
ψ e =ψ0−a c an(kyd+kll)
Udi =kψ(ψ e −ψ)−k
whe e ψis he ehicle heading, =˙
ψ,˙
l=d, and ψ0is he heading o he acked line.
The de e mina ion o he ou pa ame e s ky,kd,kψand k , ha comple ely de ine he line acking con-
olle , depends on adeo s be ween line acking pe o mance, powe consump ion, mo ion smoo hness,
and makes use o a dynamic model o he ehicle mo ion [1, 3].
7 Vehicle Pe o mance
A he cu en s age o he p ojec only he basic na iga ion senso s a e ins alled on he ehicle (GPS and
magne ic compass wi h il senso s). This senso package does no allow o a e y p ecise posi ioning o he
ehicle, as equi ed o some o he in ended applica ions, as is he case o collec ion o high esolu ion In-
SAS da a. Howe e , i is al eady possible o execu e au onomous missions wi h he ehicle and cha ac e ize
i s dynamic beha io .
Figu e 7: Za co a C es uma dam.
Fo ha pu pose, a se ies o es s we e ca ied ou , i s in a long wa e ank and la e in a eal scena io.
Nex we p esen some esul s conce ning he pe o mance o he ehicle con ol sys em ob ained in es s
pe o med a he C es uma dam in i e Dou o. Figu e 8 shows he e olu ion o Za co while au onomously
desc ibing a 100 m ×50 m ec angle a eloci ies anging om 0.5 o 1.5 m/s, in he coun e clockwise
di ec ion.
−140 −120 −100 −80 −60 −40 −20 0 20
−60
−40
−20
0
20
40
60
80
100
Eas (m)
No h (m)
S a
Finish
Figu e 8: Sample mission.
The o e shoo s exhibi ed a he end o each line a e due he ac ha he ehicle only s a s acking a
new line a e c ossing he end o he p e ious one.
The pe o mance o he con ol sys em can be u he assessed conside ing he o - ack e o in he
execu ion o he [LineTo] maneu e s. Figu e 9 shows his e o o he i s 100 m line. Typically, as can
be seen in his igu e, he e o is almos always below 0.5 m, and i s oo mean squa e alue is in he
o de o 0.2 m.
0 10 20 30 40 50 60 70 80 90 100
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
dis ance (m)
o ack e o (m)
Figu e 9: Line acking pe o mance.
8 Conclusions and Fu u e Wo k
A he p esen s age o his p ojec , he Za co ASC has al eady been es ed and i s dynamical beha io
cha ac e ized. I has been showed ha i can pe o m au onomous missions, acking desi ed lines wi h
accep able accu acy. I s pe o mance is cu en ly cons ained by he na iga ion package ins alled on-
boa d, bu , in he nea u u e, he GPS ecei e s will be eplaced by a pai o L1+L2 RTK ecei e s, and
an ine ial measu emen uni will be ins alled on-boa d. These new senso s will ce ainly allow o a much
highe p ecision o he na iga ion sys em.
Acknowledgemen
This wo k was pe o med in he scope o p ojec s INCORP – Imp o ed Na iga ion wi h Coope a i e
Robo ic Pla o ms – unded by FCT and P og ama POSC (POSC / EEA-SRI / 59963 / 2004), and MUV
– Na iga ion and Con ol o Mul iple Unde wa e Vehicles – unded by FCT and P og ama POSI (POSI
/ SRI / 47351 / 2002).
Re e ences
[1] Fossen, T., 1994. Guidance and Con ol o Ocean Vehicles. John Wiley & Sons L d, London.
[2] Gelb, A., Kaspe J ., J., Nash J ., R., P ice, C., Su he land J ., A., 1996. Applied Op imal Es ima ion.
The MIT P ess, Camb idge.
[3] Ma os, A., 2005. Line T eacking o an Au onomous Su ace Vessel. INCORP P ojec Repo , FEUP,
Po ugal.
[4] Teixei a, J., Rod igues, P., Pin o, P., 2005. On-boa d Compu e and Remo e Console o an Au-
onomous Boa . FEUP, Po ugal. (in po uguese)
