Zarco - An autonomous craft for underwater surveys

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)