A Low Cost Prototypal Robotic Platform for Underwater Survey in Shallow Water

A small, innovative robotic platform for underwater surveys in shallow water is proposed. The platform integrates a commercially available, remotely operated, underwater micro vehicle and a specifically designed and constructed actuated buoy. The mechatronic structure and the control architecture of the platform are described together with its functional features. Tests to validate the implemented constructive solutions are briefly reported, together with functional tests to assess usability in geolocalizing underwater targets.

in a controlled environment. A future phase of work will follow in order to construct an advanced version of the platform that can undergo more demanding field tests.
The paper is organized as follows. In Section 2, the general mechatronic structure of the robotic platform is described. The underwater component is a commercially available micro-ROV, while the surface component is an actuated buoy that has been designed and constructed for this specific application. The buoy structure is illustrated in details, together with the functionalities of the overall platform. Section 3 describes the Graphical User Interface that is used to operate the platform. Section 4 briefly describes the tests that have been carried on to assess viability and practical implementability of the proposed solutions. In addition, the possibility to employ the platform in geolocalizing underwater targets has been evaluated by performing a series of tests in a controlled environment. Finally, Section 5 contains conclusions and a brief illustration of future developments. The same robotic platform is also considered and briefly described in [13], where the focus is on the development of a 4G connection that enables the platform to operate in harbour and coastal area under the coverage of the 4G mobile network.

Mechatronic Structure
The general mechatronic structure of the robotic platform at issue is described in Figure 1

The micro-ROV
The underwater component of the robotic platform is a micro-ROV that is remotely operated through the control console from the shore control station. It is equipped with on-board batteries, so to avoid the need of supplying power through the umbilical.
This feature has the advantage of making possible the use of a thin umbilical for data and commands exchange, which minimally affects the maneuverability of the platform. The main task of the micro-ROV is that of carrying a video camera for visual inspection of underwater structures. The video stream is transferred through the umbilical and the wireless connection from the micro-ROV, to the buoy and then to the console. It can be displayed on the PC monitor for guidance and on-line inspection by the operator, as well as recorded for possible off-line processing [14]. Further data from navigation sensors are transmitted in the same way. Commands for the vehicle's thrusters are generated through the console by means of the GUI using the PC mouse or a joystick and transmitted via the wireless communication link and the umbilical. In assembling the prototypal robotic platform, we used the micro-ROV provided with the OpenROV v2.8 KIT [15]. The vehicle has two horizontal thrusters and one vertical thruster that actuate three degrees of freedom (surge, heave and yaw). Its dimensions are about 30cm x 20cm x 15cm; in standard configuration it weighs about 2.6kg in air and it is slightly positive buoyant in water; its depth rating is 100m and its maximum forward speed is 2kn. It is endowed with IMU, depth sensor and an HD Video Camera, whose mounting allows tilt movements within +/-60 degrees from the horizontal position. The electrical motors of the thrusters are driven by an internal control board that generates the required PWM signals. The control board has a built-in, static IP address that is used for connecting the control console with it. The nominal battery life, using rechargeable lithium batteries, is about 150min.
In addition to the sensors included in the OpenROV v2.8 KIT, a simple sensor has been installed inside the case that contains the control board to detect possible water infiltrations. As the control board integrates in its structure an Arduino Mega board, this water detector consists of a commercially available Arduino shield. The internal control board is coupled with an Ethernet switch Tenda

The bouy
The surface component of the robotic platform is a small cylindrical, actuated buoy that consists of a PVC tube with a sealed endcap at one side and a removable one at the other, with a length of 60cm and a diameter of 12cm. The buoy is equipped with two battery operated horizontal thrusters (Turnigy DST-1200 Brushless Outrunner motors with three-blade propeller) at one end (stern) that actuate two degrees of freedom (surge and yaw). The main task of the buoy is that of acting as a bridge for communication between In the Automatic Guidance Mode, remote guidance is disabled and the thruster are activated, under specific circumstances, by the traction of a tow cable that connects the bouy to the micro-ROV. The presence of a tow cable is necessary to keep the distance between the micro-ROV and the bouy smaller than the length of the umbilical, so to avoid stressing it. Moreover, its arrangement forces the buoy to move autonomously toward the (projection onto the sea surface of the) position of the micro-ROV when a traction force is exerted, so that the micro-ROV is not required to tow the buoy when it is moving. This is obtained by letting the cable pass through a small pulley that is mounted at one end (bow) of the buoy. The shape of the bouy facilitate the rotation around a vertical axis when traction is exerted by the cable, so that the buoy orients its longitudinal axis towards the (projection onto the sea surface of the) position of the micro-ROV. One end of the tow cable is attached to a spring that elongates due to the traction and elongation is detected by Arduino UNO processes the sensor's signal by comparing it with a given threshold and, if this is exceeded, it activates, for a fixed duration, the thrusters, moving the buoy in the surge direction. In this way, the bouy keeps autonomously in the vicinity of the micro-ROV, at a maximal distance that depends on the length of the tow cable and the depth at which the micro-ROV is moving ( Figure 2).

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The operator at the consolle can therefore guide freely the micro-ROV, so to comply with the specifications and requirements of the inspection task, without the need of coordinating the behaviour of the bouy. The buoy is internally equipped with three water level sensors (located in the front, middle and rear section of the hull), that are commercially available as shields for Arduino UNO, to detect possible water infiltration through the endcaps.
The scheme of Figure 3 shows the devices that form the functional architecture of the buoy and how they are linked. Only one ESC and motor, instead of two, is represented and only one water sensor, instead of three, is represented. Power is supplied by a 12V and easiest one to implement for testing the functionality of the prototypal platform. A radio data link with a suitable transmission protocol can be used instead, while it is possible to employ the 4G or 5G network for operations in covered areas. The development and testing of a 4G connection for the buoy is described in [13].

Control Consolle/Graphical User Interface
The Graphical User Interface runs on a PC that is connected to the Wi-Fi LAN. It consists of two separate components:

1.
A dedicated GUI for remote control and monitoring of the micro-ROV.

2.
A dedicated GUI for remote control and monitoring of the buoy.
The operator can activate one component or the other according to the task he is performing or both components can be activated on two different PC that are connected to the LAN.

Micro-ROV GUI
The micro-ROV GUI is the Cockpit GUI that is included in

Buoy GUI
The bouy GUI has been designed and constructed for this specific application. It is conceived as an active webpage that gives to the operator the possibility to perform the following tasks:

3) Check the Hall effect sensor signal.
4) Select the operative mode.

5) Command the buoy motion in the Remote Guidance Mode.
Each task requires the exchange of suitable data and this has been obtained by implementing a client-server architecture in       A specific task in which the platform can be employed is that of geolocalizing underwater targets. To this aim, the platform has to be equipped with a GPS, to locate its position in terrestrial coordinates, and with an underwater positioning system, to locate the position of the underwater target with respect to the platform [16]. A scheme of the resulting glocalization system is shown in  Figure 7 shows the position of the buoy that is obtained by averaging the GPS measurements and the positions of the targets obtained by each acoustic measurement. In this way it is easy to evaluate qualitatively the degree of accuracy of the geolocalization procedure. It has to be noted that the standard deviation of GPS position data, which is 0.006m, increases the positioning error, although the overall performances remain satisfactory: the average slant range is 1.91m

Geolocalization of underwater targets
with a standard deviation of 0.09m and the position of the target is determined with less than 6x10 −6 degrees of uncertainty in latitude and less than 5x10 −6 degrees of uncertainty in longitude (less than 1m in all directions). In a second series of tests, the micro-ROV moved the transponder inside the deepest area of the pool. Figure 8 shows the trajectory reconstructed from measurements and the position of the bouy (obtained by averaging the GPS measurements) in WGS coordinates over a time period of 157s. By setting a measurement rate of 5/7Hz, i.e. one measurement each 1.4s, for the USB positioning system, it has to be noted that some measurements are lost along the Wi-Fi channel the connect the buoy with the control consolle, as shown by Figure 9. These occurrences correspond to longer tracts in the reconstructed trajectory, but it possible to see from Figure 8 that the uncertainty they induce is limited to very short paths.

Conclusion
A simple prototype has been constructed to experiment usability and performances of a small, low cost, easy to manage robotic platform for underwater survey and exploration in shallow water. The next step of the work consists in equipping the platform with a radio link that allows remote guidance over longer distances (1Km-3Km) and a GPS system that facilitates it.
As already mentioned, the buoy GUI will be modified in such a way to let the operator locate in real-time the bouy on a Google map by means of GPS data. Possibly, remotely operated small winches can be added to facilitate the management of the umbilical and of the tow cable by modifying their length. Further tests on the field will then be conducted. The envisaged final result is a novel robotic tool that will facilitate environmental protection, management of underwater installation and exploitation of marine resources in shallow water and littoral areas.