1. Introduction
Europe is coming under increased pressure due to its near-complete dependency on the import of mineral raw materials. At the same time, there are an estimated 30,000 inactive mining sites, a considerable number of which still contain raw materials, currently in critical demand [
1]. Among these sought-after raw materials there are metallic and industrial minerals, construction materials, and base metals, such as cobalt, gallium, indium, and a range of rare earths necessary for IT appliances [
2]. Therefore, there is increased interest for re-opening some of these abandoned mine sites.
Under normal operation conditions, surface and groundwaters filter into the mined tunnels and must be constantly removed to maintain a safe working environment. Once a mine is permanently closed, the dewatering systems cease to operate, and without the existence of any drainage, the tunnels become permanently submerged. Most of these mine sites, presently submerged, are more than a century old, and the information available regarding the structural layout of the tunnels is limited and imprecise, if not totally lacking. The network of tunnels inside a mine can be extremely complicated, so surveying and prospecting by conventional methods such as human divers, can result dangerous or even lethal in unknown deep mine conditions [
1]. Therefore, the use of robotic platforms such as underwater vehicles to explore these sites and gather valuable geological and mineralogical information, can help reducing risks for humans and provide important information for determining whether re-opening a mine is plausible and economically feasible.
Underwater vehicles are an active area of research due to their potential applications in a variety of fields, such as maritime security, marine archaeology, search, and rescue. The most common applications of underwater robotics include ocean mining exploration [
3], autonomous sea floor mapping [
4,
5], and data gathering [
6,
7,
8]. It is worth noting that most of the research on underwater robots addresses open water environments, where the restrictions on shapes and sizes of the UVs are not strict. Therefore, most UVs adopt an elongated shape to minimize drag in surge (forward motion) which is the dominant motion in open water.
Nevertheless, for autonomous exploration and 3D mine mapping of flooded mines, these elongated shapes are not efficient. The underwater robot will have to navigate within complex structures of mine tunnels and galleries with dimensions in the range of 1.5 m × 3 m, with the oldest mines tunnel sections as small as 1.5 m × 1 m. As such, the robot must be sufficiently small and agile to navigate the mine corridors and tight turns of the mine system. Furthermore, mine tunnels may contain objects or debris left after closure, which could obstruct paths or become entangled in the AUVs propellers and permanently disable the robot which implies certain design requirements aimed at limiting protruding elements to avoid becoming snared. By adopting a less common streamlined spherical shape design, attitude motions can be made in-place, minimizing the number of moving parts and protruding elements while obtaining high stability and flexibility along with a zero degree turn radius for high maneuverability and lower drag forces.
Although spherical hull designs for underwater vehicles are not commonly found in recent literature, because of the preferred bullet-shape hull, such as the MAYA AUV [
9], developed by the National Institute of Oceanography in India, several works can be found which took advantage of this particular shape. The UV design patented by the authors in [
10] is one of the first spherical underwater robots where the spherical shape of the prototype is used to obtain lower drag forces during operation. It is composed of four small engines that help stabilize and maneuver the vehicle, and a single engine that is in the center of the sphere for propulsion of the UV. The University of Hawaii developed ODIN-III [
11], a prototype robot with a hollow metal sphere housing of 0.315 m in radius and a propulsion system that consisted of 8 screw propellers fixed outside the hull. This platform has been used to test adaptive control methods together with disturbance observers [
12]. A micro AUV of 0.075 m in radius and 6 propellers around the hull was developed by the authors of [
13,
14] for monitoring sub-surface cluttered environments as in nuclear storage ponds.
Screw propeller thrusters have been generally used for propulsion of underwater vehicles, nevertheless, other types of propulsion methods such as water-jet thrusters have been designed and tested for spherical UV. The robot developed in [
15] is a prototype that has a spherical hull with a diameter of 0.40 m and uses 4 vector water-jets as propulsion to maneuver, which is based on the design patented in [
16]. Although this UV has 4 vector water-jet inside, it is only capable of performing motions in 3 DOF [
17,
18,
19]. The authors in [
20] make use of the spherical UV SUR-II, which is equipped with water-jet thrusters as propulsion, to design and develop attitude stabilization methods as well as buoyancy control with a variable ballast tank. Whereas, spherical AUVs with reaction wheel internal actuators have been designed and developed in [
21,
22]. One key disadvantage of these UV designs is the presence of external propulsion systems which could become entangled with objects encountered during operation. Taking into account the benefits and drawbacks of these systems, the UV design in this work shown in
Figure 1, has a manifold thruster configuration where the propulsion elements are embedded into the spherical hull to avoid foreign objects from damaging the propellers and effectively eliminating the possibility of ensnarement.
The work presented in this paper describes a scaled prototype UV, realized to study, develop, and compare control strategies, to be later deployed, with suitable adaptations, in the actual robotic explorer for the UNEXMIN project, hereby named UX-1. Here, we focus on three aspects: the mechanical and electrical design of the spherical UV prototype, the derivation of the equations of motion that describe the dynamics of the system and the implementation of control methods for performance analysis in real underwater experiments. The rest of this paper is organized as follows:
Section 2 introduces the mechanical and electronic design of the UV, while in
Section 3 the 6 DOF equations of motion for an UV are derived.
Section 4 explains the control system implemented on the UV, while the underwater experimental tests and discussions are reported in
Section 5. Finally, conclusions and future works are presented in
Section 6.
6. Conclusions
In this paper, the design and control of a scaled prototype spherical underwater vehicle for flooded mine tunnel exploration was presented. The mechanical and electrical designs have been validated, as well as a novel pendulum mechanism for passive pitch control and a manifold thruster configuration for propulsion. The 6 DOF equations of motion of the underwater vehicle have been derived and simplified to reduced ordered models of longitudinal and lateral states. Surge, heave, pitch and yaw motions were tested in real underwater position hold experiments to evaluate the performance of the State FL controller and compare these to a baseline PID controller.
Results demonstrate that the spherical underwater vehicle was able to achieve all the required motions and that the State FL control method outperformed the classical control scheme in terms of lower overall errors and faster transient responses. Thus, The FL control algorithm tested experimentally in this work will be extended and implemented on the UX-1 robot for field testing in real underwater mine scenarios.
One of the unique features expected of this UV is the ability to stabilize in a completely nose-down (−90°) or nose-up (+90°) orientation which increases maneuverability and aids navigation tasks. Pitching the vehicle while navigating in the forward or backward direction, offers the possibility of adjusting to any slope found without the need for extra actuation of the UV heave motion. Due to the limitations of the scaled prototype used, in this work the 90 degrees pitch movement was not tested. Improvements will be made to achieve this range of pitch motion in future tests. Nevertheless, the pitch control results validate the pendulum mechanism design implemented in this work as an appropriate solution for the pitch stabilization and control of a spherical UV.
Future work on the development of this prototype can be divided in three areas: parameter identification, software, and hardware. To evaluate the effects of uncertainties in the model parameter calculations, used for these initial tests, experimental identification tests will be performed to obtain accurate model parameters to improve the controllers’ overall performance and explore parameter uncertainty adaptation limits with the current control method. The work to be performed on improving the hardware includes testing low frequency communication modules for wireless operation, testing the lateral system dynamics generated by the manifold system configuration, and obtaining the full expected motion of the pitch using the pendulum mechanism. The software to be implemented for future works includes the implementation and validation of additional advanced control techniques such as sliding mode control and Adaptive Control and sensor fusion algorithms to enhance the localization of the UV in underwater tests. Additionally, the redundancy in the use of 8 motors for propulsion will be tested by simulating motor failures during operation and evaluating the reliability of the design.