Part II F´olaga: a light low cost Autonomous Underwater Vehicle for coastal oceanography

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Part II

F´

olaga: a light low cost

Autonomous Underwater

Vehicle for coastal

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Chapter 3 The Autonomous

Underwater Vehicle F´

olaga

F´olaga is the Italian name of a shallow water aquatic bird that swim

on the water surface and dives to catch fish. Inspired by this bio-logical behaviour, in the past years the research group, which I have been working with, has prototyped a class of Autonomous Under-water Vehicles (AUVs) for oceanographic sampling in coastal areas: these vehicles, collectively named after the bird, are characterized by ease of operability and transport and very low cost of produc-tion. The first vehicles belonging to such class could navigate at the sea surface and dive along the vertical axis at selected points to gather salinity and temperature samples, without underwater ma-noeuvrability. In the most recent prototype, specific technologies have been developed to include buoyancy and attitude control, in order to achieve full manoeuvrability also underwater, making it fea-sible the deployment of the vehicle in a much wider range of shallow

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water applications, while preserving its low cost. Moreover, concur-rent use of vehicles belonging to this class can eﬃciently be exploited in rapid environmental assessment through cooperative concurrent mapping of oceanographic quantities as described in the next part.

3.1 Introduction

A considerable increase in field experimentations and operational ap-plications of Autonomous Underwater Vehicles (AUVs) has been wit-nessed in the last years. Thanks to several success stories, AUVs are on their way to reach the technological maturity and the widespread diffusion enjoyed by their robotic predecessors, i.e. Remotely Oper-ated Vehicles (ROVs). However, the gap in number of applications, costs and market availability between the two classes of unmanned underwater vehicles has not been filled yet. Some years ago, in [18] the researchers involved in the Fólaga project, observed that two severe limitations in the diffusion of AUVs in operational scenarios are those of vehicle cost and user-friendliness. Still in [18], it was ar-gued that the great majority of existing AUVs has been originated directly from research prototypes, whose development was led by the general research interest of increasing the vehicle autonomy and not by specific mission needs; the absence of a well defined mission goal at the design stage has favoured prototypes able to account for several potential missions, eventually leading to conservative de-sign choices, as for navigational capabilities, depth ranges, allowable payloads, energy consumption, safety systems. This has led to ex-pensive final products; moreover, AUV operation still requires in

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3.1 Introduction

each mission a team of trained engineers, knowledgeable with the system design, in stark contrast with ROV operations. In [18], [19], [20] an attempt to reverse the trend has been proposed as for the design and subsequent realization and field test of an autonomous vehicle to be employed as a sensor platform in oceanographic cruises devoted to the investigation of ocean mesoscale motions in shallow coastal waters. Focusing on the mission has allowed to come out with a very simple and yet effective design, with great reduction in costs: the resulting vehicle exhibits similarities with autonomous gliders together with some standard AUV features. In particular, the vehicle could carry a CTD package as payload; it navigates on the sea surface when in transit from one measuring station to an-other, and it submerges vertically when on station to perform the measurement. When on the surface, the vehicle has continuous GPS contact and land-station contact through a mobile phone link. The land station link allows for on-line modification of the mission re-quirements and for almost real-time data transmission. Due to this peculiar behaviour (navigate on the surface and dive when needed by the mission), and for its range of action limited to shallow water areas, the vehicle was named Fólaga, the Italian name of the coot

aquatic bird. Since there have been several evolution of the original design, we now indicate with the F´olaga name the generic class of

vehicles which respond to the above speciﬁed behaviour.

In the original design [18], [19], the F´olaga motion in diving was

due to jet-pumps oriented along the vertical (z -heave) axis of the ve-hicle, and actuated in a controlled way in order to maintain a stable asset of the vehicle in the surge (x ) direction during the dive. Though eﬀective, the system did not allow for any underwater

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manoeuvrabil-ity, and was not very efficient from an energetic point of view. While lack of manoeuvrability during the dive could be of no concern, since the mission did not require it, an increase in the energetic efficiency of the diving system was welcome. In the most recent version of the Fólaga class, the Fólaga III, diving jet pumps have been replaced by two internal mechanical modules that change the buoyancy and the asset of the vehicle; propulsion, either at the surface or underwater, is solely given by the stern jet-pump propellers. In this way, while preserving the simplicity in design and the low cost of the overall system (still in the range of aboute10,000 as cost in parts, payload excluded), the energy saving performance of the vehicle during dive has been greatly increased, and also the system is now capable of underwater manoeuvrings, at least from the actuation system point of view. The similarity with gliders is enhanced, in the Fólaga III, since the gliders also dive with changes in buoyancy and asset. How-ever, some important distinctions have to be outlined, in order to differentiate the Fólaga vehicles from standard oceanographic glid-ers. First of all, the change in attitude of the Fólaga vehicles is due to a change in the relative positions of the centre of mass and of the centre of buoyancy of the vehicle, and not through actua-tion of control surfaces (e.g., fins), as in gliders. The Fólaga has no control surfaces whatsoever. Moreover, the Fólaga has autonomous navigation capabilities, with its own propulsion system, while the gliders, moving by exploitation of the density gradients in the water mass, have a motion constrained by the local oceanographic condi-tions. Finally, since without own propulsion there is much less need of power consumption, all else being equal the gliders have a much longer endurance with respect to the Fólaga vehicles.

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3.2 F´olaga design and evolution

The low cost of the Fólaga vehicles makes it affordable even to budget-limited organizations (as University Centres, for instance) the availability of more than one such vehicle. This in turn opens up realistic possibilities of setting up cooperative missions performed by a team of AUVs. Adaptive algorithms for oceanographic sampling mission to be performed by a team of Fólaga class AUVs will be described in the next part of this thesis.

The chapter is organized as follows: in the next section the de-velopment history and basic design features of the Fólaga vehicle are reviewed. In section 3.3 field data from past experimental tests are reported for completeness, and the manoeuvrability characteristics of the past versions of the vehicle analyzed. In section 3.4 the exper-imental performance during dive by the Fólaga III is presented and compared with that of previous versions; the data in this section are taken from an oceanographic sampling operation in the marine pro-tected area of the Portofino Mountain, in the Ligurian Sea. Finally conclusions are given.

3.2 F´

olaga design and evolution

The original Fólaga design (Fólaga I [18]) consisted in a two-cylinders vehicle, with jet flow pumps at the stern for surge propulsion, at the bow for steering, and one at middle-vehicle among the two cylinders, mounted aligned with the vertical axis, for diving propulsion; verti-cal diving maintaining an horizontal asset was passively guaranteed by a careful distribution of the weight inside the cylinders. Fólaga I is depicted in figure 3.1, together with a picture of the diving pump

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in action during operation.

A subsequent version [19], [20] removed the module at middle-vehicle, and mounted two diving propulsion pumps one at the stern, and one at the bow of the vehicle. Asset during dive was feedback controlled and, since the diving pumps could be actuated indepen-dently, asset was guaranteed by proper modulation of the commands to the pumps. A Fólaga II class vehicle has been delivered to the the Spanish Oceanographic Institute IMEDEA, who has been col-laborating with our team from the very beginning with trial test and assistance on the oceanographic specifications of the mission. In fig-ure 3.2 the Fólaga II is shown, emphasizing the main differences with the Fólaga I.

For both Folaga I and Fólaga II the forward propulsion system is based on jet flow pump. The on-board electronics for communica-tion and Guidance, Navigacommunica-tion and Control (GNC) is built through PC-104 boards and with a navigation sensor suites composed by compass, inclinometers, depthmeter, GPS receiver (for surface nav-igation). The control law defined in [21] for steering of unicycle-like vehicles has been adapted to the Fólaga case, taking into account actuator limitations [22]. The vehicle installed payload has been a CTD, although the vehicle can carry different payloads of approxi-mately same size and dimensions, like multiparameter probes, acous-tic sensors, etc. The Fólaga II vehicle had a length of 2.20 m and approximately 30 Kg of weight, with an internal cylinder diameter of 0.125 m.

The Fólaga III, the latest version of the Fólaga class, sailed first in 2006, and it had a major change in the design of the diving phase. The two-cylinders structure, that was devised to allow for easy

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trans-3.2 F´olaga design and evolution

Figure 3.1: On the top: the F´olaga I design, with two cylinder sec-tions. On the middle: the vehicle ashore. On the bottom: The vehicle in operation activating the middle jet pump at the beginning of a dive.

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Figure 3.2: On the top: the F´olaga II vehicle. On the middle: front view of the bow, with the CTD probe and the two lateral pumps for steering. On the bottom: side view of the bow part, with the system electronics behind the closing cap, the jet pump for diving oriented along the vertical just behind the steering pumps, and the CTD probe.

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portation of the two separable mechanical parts, was substituted with a single, shorter cylinder; moreover, the vertically aligned jet pumps were removed to increase the efficiency of the diving phase with a different diving system (in particular, by diminishing the energy consumption during dive, the overall system endurance is en-hanced). The new diving system implemented in Fólaga III consist in the mechanical system to change buoyancy and attitude of the vehicle by moving its internal ballast. In this way it is possible to control the whole system attitude during dive, and to acquire ma-noeuvrability during dive. There is no additional cost in parts for the new diving system, in the sense that the cost in parts of Fólaga III is substantially equivalent to that of Fólaga II. The Folaga III length is 1.80 m, with the same weight of Fólaga II. Fólaga III is depicted in Figure 3.3.

The communication and mission management modules of the var-ious Fólaga versions have not undergone important changes since the first design. Communication exchanges among the system compo-nents are implemented through TCP/IP procedures. In particular, communications take place between the vehicle and the user inter-faces on the surface station. On the surface station there are two different interfaces: one is the system engineer interface, which gives information on the vehicle current state: measured geographical po-sition and orientation, propeller commands, alarm state, GPRS link monitor, mission setting (next desired location) and it allows, when the Fólaga works on open loop, to remotely operate the vehicle send-ing direct control to the propellers. The system engineer interface module receives the data from the AUV by a GPRS link, stores them in an incrementally log file and also communicate them to the

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Figure 3.3: On the top: the F´olaga III vehicle. On the middle: The main design characteristics of the vehicle. On the bottom: Close up of main design characteristics for battery displacement (attitude change) and ballast chamber (buoyancy change).

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oceanographic mission manager interface, which will be described

later on in this section.

After vehicle diving, when communication is re-established, the system engineers interface receives the oceanographic data, stores them on hard drive and pass them to the mission manager by LAN. The overall communication topology is depicted in figure 3.4. The mission manager has been designed as an high-level interface for the oceanographer end-user to plan a mission, monitor the vehi-cle trajectory under navigation, display the oceanographic data as they become available after a measurement, play back navigation and oceanographic data. In order to fulfil these tasks in an intu-itive way for the users, the mission manager has been built through a Geographical Information System (GIS). In particular, a mission can be planned by defining over the displayed geographical chart way points and mission points, including requested diving depths. The GIS interface can display additional prior information relevant to the mission, as for instance bathymetry. Mission editing, saving, etc. are also available features. An example of the mission planner interface is in Figure 3.4. The planned mission is downloaded to the vehicle system through the system engineer interface link. Once a mission is started, the navigation data from the vehicle are dis-played by the mission manager over the same geographical charting information, and also over the planned via points and measurement points. The GIS system can manage missions with cooperative ve-hicles (multi-veve-hicles missions), where the mission goal is defined in terms of accuracy in the reconstruction of the environmental field to be sampled, as described in part 3.

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Figure 3.4: On the top: communication architecture among the F´olaga, the control interface and the mission interface. On the

bot-tom left: oceanographic mission manager examples: mission planner

with possibility of indicating measurement points (red symbol ) and way-points (yellow star ), editing, etc.; On the bottom right: the mis-sion monitoring panel of the oceanographic mismis-sion manager. The current vehicle position is displayed (red sysmbol of the vehicle), to-gether with the past vehicle trajectory (red, continuos), the desired via-points (connected through a yellow, dotted straight lines) and the geographical charting layer. Field data from June 2004 ﬁeld test in Cala Major.

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3.3 Surface navigation of F´olaga II

3.3 Surface navigation of F´

olaga II

In this section we report data from the vehicle surface navigation acquired with the purpose of estimating the Fólaga speed at full propulsion power, and the curvature radius at full speed. The data in this section are taken from an engineering test performed at the gulf of Cala Major during the summer 2004. The Fólaga was in tele-operation mode, and was steered through a ”figure of 8” manoeuver: with full propulsion power, the vehicle rudder has been kept in suc-cession, at three minutes interval, at midship, full left, midship, full right, midship, full right, midship full left. Note that in this case the vehicle navigates in open loop. Of course, in the Fólaga case, the rudder action is obtained through the steering pumps. GPS data were collected by the vehicle during the operation, at 1 Hz sam-pling. The measured vehicle trajectory is reported in 3.5, where the numbered sections indicate the portions of the trajectory in which the rudder was at midship, i.e, the sections that, in absence of per-turbation, should correspond to straight lines. It is clear from the navigation data that during the test there was a non-negligible effect of external perturbations. This has made necessary the estimation of such external disturbances. They will be in the following referred as ”currents”, since this is considered the most likely perturbation effect in the area. However, in general, perturbations may also be due to the combined action of other environmental effects, as wind, wave motions, etc.

In order to estimate the current eﬀect, a kinematic model is em-ployed, with a reference system as illustrated in 3.5. In the follow-ing, u is the unknown vehicle surge speed, v is the vehicle measured

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Figure 3.5: On the top: F´olaga GPS-measured navigation during the maneuverabilty test. Numbered sections indicate the trajectory portions with midship rudder. On the bottom Reference system for

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velocity (from GPS navigation data), c is the unknown current per-turbation. The compass reading gives, with the same GPS sampling, the measurement of the angle σ(t). The relation among u, v and c at any time instant is:

1 0 cos(σ(t)) 0 1 sin(σ(t)) ⎡ ⎢ ⎣ cx(t) cy(t) u ⎤ ⎥ ⎦ = vx(t) vy(t) (3.1) It’s a well known fact that exclusively knowing the GPS data on position, and the vehicle not having a sensor for measuring the Surge velocity relative to the water, we can not univocally determine the speed of the current and the vehicle, as there are inﬁnite combina-tions that can satisfy the equacombina-tions system (3.1) at every instant. Anyway, considering the surge speed u and the current speed c as a constant along the portion of trajectory where the rudder was held at midship, hypothesis compatible with the environmental proper-ties, a least square system in the three unknowns cx, cy, u can be

built for each trajectory segment:

Σ ⎡ ⎢ ⎣ cx cy u ⎤ ⎥ ⎦ = V (3.2) where the matrices Σ and V are built as follows:

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Σ = ⎡ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎣ 1 0 0 1 cos(σ(t₁)) sin(σ(t₁)) 1 0 0 1 cos(σ(t₂)) sin(σ(t₂)) ... ... ... ⎤ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎦ V = ⎡ ⎢ ⎢ ⎢ ⎢ ⎢ ⎢ ⎣ vx(t1) vy(t1) vx(t2) vy(t2) ... ⎤ ⎥ ⎥ ⎥ ⎥ ⎥ ⎥ ⎦ (3.3) The most critical part in these procedure has been the estimate of the values in V. In fact, the vehicle velocity has been estimated from finite differences of the GPS position data, and subsequent low pass and moving average filtering. The results obtained as for the estimate of the current effects are reported in 3.6. It can be seen that the estimated current vector exhibits some spatial variability; this is compatible with a current following the coast line, since the sea trial took place in a gulf (see 3.4 for a crude approximation of part of the coast line in Cala Major). Once the current velocity has been taken into account, the average vehicle speed estimated from all the trajectory portions analyzed is u = 0.4 m/s, i.e.,slightly less than 1 knot.

The estimated current ﬁeld has been employed also in the turning portions of the trajectory in order to estimate the vehicle turning radius at full speed. Let s(t) be the position of the vehicle at time

t. Then: s(t) = t 0 v(τ )dτ = s₀(t) + t 0 c(τ )dτ (3.4) where s₀is the vehicle position without the current eﬀects (nomi-nal trajectory). Reminding that we are assuming a constant current

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Figure 3.6: On the top: Estimated current field (modulus and orien-tation). On the bottom: Effective vehicle trajectory (dark blue) and nominal turning trajectory (light red ) after correction for current effects.

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during the turnings, then the curve s₀(t) can be estimated from the data and from the previous current estimate through equation (3.4). This has led in particular to the determination of the nominal turning trajectories of figure 3.6. Vehicle turning radius has been estimated by fitting a circumference to both nominal trajectories, and taking the average of both estimated radii. The final estimate of the vehicle turning radius at full speed is r = 23.8 m.

3.4 Diving performance of F´

olaga III

In this section some experimental results are reported on the diving performance of the Fólaga III vehicle, and compared with the perfor-mance of the previous versions when jet pumps were employed also for diving. The data have been collected in March 2007 as part of an oceanographic trial in the Marine Protected Area of the Portofino Mountain, in the Ligurian Sea, South-East from Genova. The Fólaga III vehicle has gathered CTD data in the area accordingly to the oceanographic needs, in particular with the task of determining the thermocline depth. To meet the goal, the oceanographers have pro-grammed each dive at a maximum depth of 20 m (some dive at shallower depth were also included). The oceanographic results of the trial will be discussed elsewhere. Here it is of concern he evalu-ation of the diving performance of the vehicle, with respect to those of the previous versions. In particular, as reported in [18], diving speed of the Fólaga I was about 0.071 m/s. Fólaga II data con-firmed the performance of Fólaga I. As for Fólaga III, the Portofino experiment has shown a remarkable reproducibility of the data: in

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3.5 Conclusion

particular the depth vs. time profile of each dive are almost undis-tinguishable from each other, except at the turning point (beginning of immersion, reaching of the depth and begin re-emersion), where some more notable variation may be seen. A typical depth vs. time profile of the Fólaga III is shown in Figure 3.7. The diving speed of the vehicle is constant, and it can be calculated as 0.15 m/s (more than twice than Fólaga I and II) in immersion and 0.075 in emer-sion. It has to be noted that the vehicle was in any case diving with a prescribed horizontal asset (the pitch angle is maintained close to 0 ° pitch), and that oceanographic data were acquired in both immersion and emersion phases; the slower emersion was a require-ment from oceanographers, and buoyancy in the emersion phase was controlled accordingly. Note also how the vehicle, after reaching the prescribed measurement depth, gently continues the dive while beginning changing its buoyancy for the emersion phase.

3.5 Conclusion

The development and design choices of a low-cost, light weight AUV, the Flaga, for coastal oceanography mission has been described, from the first prototype until the last more efficient and powerfull Fólaga III. The Fólaga concept, its specific design and realization, the com-munication and mission planning software, and the monitoring soft-ware, available on all versions, have been reported. Field data have been presented to evaluate some manoeuvrability characteristics of the vehicle, in particular, the surface performances of the second version of the vehicle have been shown and analyzed, while, as for

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0 100 200 300 400 500 0 10 20 Depth [m] Time [s] 0 100 200 300 400 500 −50 0 50 Pitch [°] Time [s] 0 100 200 300 400 500 −400 −200 0 200 400 Ballast [g] Time [s] 0 100 200 300 400 500 −50 0 50 Displacement [mm] Time [s]

Figure 3.7: Data from a dive in vertical diving (zero-pitch) mode, with different gain setting in the descending and resurfacing phase; from top to bottom: depth, pitch angle, ballast and displacement; ballast and displacement are the differential ballast and displacement with respect to the trimming configuration.

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3.5 Conclusion

the F´olaga III is concerned, the divining performance have been discussed and copared to the ones of the prevuois vehicles. The autonomous underwater vehicles preseted in this chapter are also well suited to be used in environmental monitorning mission, where the monitoring task can beneﬁt from the employment of a team of cooperative vehicles, as will be described in part 3.