sensors

Article

High Accuracy Buoyancy for Underwater Gliders:

The Uncertainty in the Depth Control

†

Enrico Petritoli, Fabio Leccese * and Marco Cagnetti

Science Department, Università degli Studi “Roma Tre”, Via della Vasca Navale n. 84, 00146 Rome, Italy;

e_petritoli@libero.it (E.P.); ing.marco.cagnetti@gmail.com (M.C.)

* Correspondence: fabio.leccese@uniroma3.it; Tel.: +39-06-5733-7347

† This paper is an extended version of our paper published in Petritoli, E.; Leccese, F; Cagnetti, M. A High

Accuracy Buoyancy System Control for an Underwater Glider. In Proceedings of the second IEEE

International Workshop on Metrology for the Sea (MetroSea), Bari, Italy, 8–10 October 2018.

Received: 8 March 2019; Accepted: 12 April 2019; Published: 17 April 2019

Abstract:

This paper is a section of several preliminary studies of the Underwater Drones Group

of the Universit

degli Studi “Roma Tre” Science Department: We describe the study philosophy,

the theoretical technological considerations for sizing and the development of a technological

demonstrator of a high accuracy buoyancy and depth control. We develop the main requirements and

the boundary conditions that design the buoyancy system and develop the mathematical conditions

that deﬁne the main parameters.

Keywords:

uncertainty; buoyancy; depth control; accuracy; AUV; glider; autonomous; underwater;

vehicle

1. Introduction

This paper is part of several preliminary studies by Underwater Drones Group (UDG) of the

Science Department of the Universit

degli Studi “Roma Tre”, which is developing an advanced

Autonomous Underwater Vehicle (AUV) for the exploration of the sea at high depths. The ﬁnal aim

of the project is to create a platform for underwater scientiﬁc research that can accommodate a wide

range of diﬀerent payloads.

We will examine the buoyancy system and evaluate its sizing; then we will illustrate the

technological solution we have come up with in order to realize the hydraulic system to be assembled

in the Underwater Glider Mk. III (see Figure 1)[1–5].

(a) (b)

Figure 1. Perspective view of the Underwater Glider Mk. III. (a) Rear/port; (b) front/starboard.

Sensors 2019, 19, 1831; doi:10.3390/s19081831 www.mdpi.com/journal/sensors

123

Sensors 2019, 19, 1831

1.1. The Underwater Glider

1.1.1. AUV Evolution

The exploration of the underwater world has always been one of mankind’s dreams: submarines

and bathyscaphes (for extreme depths) have been developed to study the “deep blue”. Due to obvious

dangers, the human exploration can take place only for very short periods and very limited areas: for

these reasons, the exploration of the sea has immediately been drawn towards unmanned automatic

systems [6–8].

An AUV is a vehicle that travels underwater without requiring input from an operator; this means

that it must be equipped with a “brain” that regulates and coordinates its position, its depth and its

speed: moreover, it is able to collect and store data from the payload. One of the ﬁrst realizations was

the Autonomous LAgrangian Circulation Explorer (ALACE) system, a buoy that was able to vary its

buoyancy and therefore its depth. Although it possessed a great endurance, it only could be employed

for great depths and in open sea—the consequences of these limitations are evident.

The next step was the use of Remote Operated Vehicles (ROVs). These, thanks to the constant

development of electronic miniaturization, are extremely high performing vehicles for short-lasting

marine operations, but the require the constant presence of a support vessel.

The need to get rid of the randomness of the currents has led to the natural development of the

underwater glider concept [9–15].

1.1.2. The Underwater Glider

An underwater glider is a vehicle that, by changing its buoyancy, moves up and down in the

ocean like a proﬁling ﬂoat [

]. It uses hydrodynamic wings to convert vertical motion into horizontal

motion, moving forward with very low power consumption [

–

]. While not as fast as conventional

AUVs, the glider, using buoyancy-based propulsion, oﬀers increased range and endurance compared

to motor-driven vehicles and missions may extend to months and to several thousands of kilometres

in range. An underwater glider follows an up-and-down, sawtooth-like mission proﬁle providing data

on temporal and spatial scales unavailable with previous types of AUVs [23–27].

1.1.3. The Mk. III Architecture

The Mk. III sub-glider has a cylindrical fuselage with a radome on the bow containing the

customizable payload and, on the other end, the hydrodynamic fairing. The vehicle does not have

moving surfaces: control is provided by the displacement of the battery package that varies the position

of the centre of mass. The wings aerofoil is based on the Eppler E838 Hydrofoil. The aerofoil has the

maximum thickness 18.4% at 37.2% chord and maximum camber 0% at 46.5% chord. The arrangement

of the internal sectors is visible in Figure 2a,b. The buoyancy system is contained in the buoyancy

control bay: it accommodates the buoyancy motor and the oil tank and provides longitudinal balance

to the system by adjusting the level in the reservoir. The bladder is contained in the hydrodynamic

fairing, in contact with the open water. The fairing is not a critical structural part—it has the task of not

disturbing the hydrodynamic ﬂow of the fuselage [28].

1.1.4. Conventions

We introduce, for clarity, the mathematical conventions and symbols that will be used in the

subsequent discussion (see Figure 3a,b): where:

• α (or ϕ) is the angle between the x axis and the N axis.

• β (or θ) is the angle between the z axis and the Z axis.

• γ (or ψ) is the angle between the N axis and the X axis.

124

Sensors 2019, 19, 1831

For the rotation matrix, we have:

⎛

⎜

⎝

0 −ζ

−ζ

⎞

⎟

⎠

(1)

where ζ is the parameter vector [29].

(a) (b)

Figure 2.

Underwater Glider Mk. III cutaway: (

) fuselage prospective section; (

) fuselage

sagittal section.

(a) (b)

Figure 3.

The Euler angles. (

) Body frame (blue) and reference frame (red); (

) The body frame

referred to the drone.

2. Materials and Methods

2.1. The Buoyancy System

2.1.1. Basic Concepts

Gliders are controlled through hydrostatics (vertical forces) and manipulate hydrostatic balances

in order to accomplish roll and pitch of the vehicle. Stability of the vehicle is a major critical factor:

125

Sensors 2019, 19, 1831

a stable vehicle has the centre of gravity below the centre of buoyancy. In this conﬁguration, the weight

of the vehicle creates a restoring moment to add stability to the vehicle. Roll and pitch on the glider

is accomplished by moving the battery pack. Figure 4 below displays a basic concept of a buoyancy

system for the glider [30–36].

Figure 4. Basic scheme of the buoyancy system.

The system is extremely simple: while descending, hydraulic ﬂuid moves from the external

inﬂatable bladder, which produces a high pressure in the internal reservoir, which is at a low pressure

through a valve: the decrease in volume of the bladder creates an increase in density, causing negative

buoyancy [37–44].

While ascending, hydraulic ﬂuid moves from internal accumulator to the external inﬂatable

bladder through the pump. The increase in volume creates a decrease in density causing positive

buoyancy. The seawater also ﬂushes out the open hydrodynamic fairing of the vehicle, aiding it to rise

to the surface. For neutral buoyancy, the vehicle must have a density equal to seawater [45–52].

2.1.2. The System Prototype

Our group has developed a technology demonstrator (see Figure 4) of the buoyancy system to

validate the related technology and then to test it. To reduce the force required to actuate the oil piston,

which pushes the oil in the bladder at high pressure, is necessary to reduce the piston surface (diameter)

and increase the stroke: so, the buoyancy engine resembles a “shotgun”. An open-loop stepper motor

was used to drive the screw inside the actuator that, in turn, pushes the piston. Two solenoid valves

regulate the ﬂow of oil into the bladder [53–57].

The ﬁrst problem was the occurrence of actuator buckling: under the push of the engine,

the probability of a part bending is high, thus deforming the thread and jamming the mechanism.

The problem was solved by constructing a rigid cage with four struts that support the piston’s push

load, leaving the screw only with the rolling friction load. In the early project development stages, the

workgroup was oriented to use a centrifugal pump for all drives: this technology however did not allow

us to create strong pressure diﬀerences; the need to use a more powerful engine was also highlighted

because the prevalence was too low: this would have led to an excessive battery consumption. The

second solution was to use volumetric pumps in order to obtain greater diﬀerences in pressure (even

ones considerably higher than needed). Unfortunately, these would require too much power and are

too heavy for our small vehicle [58–62].

At this stage of development, we have also thought to use the oil tank only as a passive ﬂuid

reservoir and trimmable counterweight of the payload and as an active actuator for longitudinal

stability. The long travel of the piston (forward or backward) ensures the necessary variation of the

bladder volume for manoeuvrability [63].

126