AUV for Diver Assistance and Safety- Design and Implementation -

Nikola StilinovicFaculty of Electrical Engineering

and ComputingUniversity of Zagreb

Unska 3, 10000 Zagreb, CroatiaEmail: nikola.stilinovic@fer.hr

Dula NadFaculty of Electrical Engineering

and ComputingUniversity of Zagreb

Unska 3, 10000 Zagreb, CroatiaEmail: dula.nad@fer.hr

Nikola MiskovicFaculty of Electrical Engineering

and ComputingUniversity of Zagreb

Unska 3, 10000 Zagreb, CroatiaEmail: nikola.miskovic@fer.hr

Abstract—Diving as a profession and sport is and will be oneof the most dangerous disciplines known by man. Water is not anatural habitat for humans and people need equipment to breatheunderwater. Failure in the breathing apparatus, burst eardrum,decompression sickness and nitrogen narcosis are just a coupleof problems which can occur during an ordinary dive and resultin injuries, long-term illnesses or even death. The most commonway to reduce the risk of diving is to dive in pairs. Use of thebuddy system by scuba divers is a set of safety procedures thatare intended to improve divers’ chances of avoiding or survivingaccidents underwater. When diving in pairs, divers can cooperatewith each other and respond when uncommon situations occur.Having the ability to react before an unwanted situation happenswould improve diver safety. This paper proposes a mechanicalsystem which will be used with a scuba diver in a symbiotic link.The concept consists of a diver, an autonomous underwater robot(Buddy) and an autonomous surface robot (PlaDyPos) (Fig. 1) [1].

Keywords—AUV, ROS, vehicle design, diver tracking, marinesystems

I. INTRODUCTION

A diver interacts with the companion underwater robot thatmaneuvers in the vicinity of the diver and reacts accordingly todiver actions. The autonomous surface vehicle communicateswith the diver and the autonomous underwater robot viaacoustic communication and shares the acquired data with thecommand center. The autonomous surface robot is also used asa navigation aid for the diver and the autonomous underwaterrobot.

As mentioned, the diver is equipped with the acousticmodem which allows diver tracking and communication. Thediver tablet, located in a custom made underwater housing andstylus, gives the diver the possibility to exchange messageswith the command center and, for the first time, to seeits position on the map underwater. Full tablet functionalityis preserved with the underwater housing and stylus whichenables taking photos or videos, writing notes or playinggames, reading a book and watching videos during the ascent.The autonomous surface robot provides a precise GPS positionfor both the diver and the autonomous underwater robot. It isalso used as a diver buoy with the diver down flag since itis dynamically positioned to be above the diver. With fourthrusters in a X-configuration it is over-actuated so it canmove in every direction while maintaining desired heading.

Figure 1. The CADDY concept

The autonomous underwater robot (Fig. 2) which design andcontrol will be explained in details plays an important role inthis system. It is equipped with a high resolution multibeamsonar and a stereo camera for detecting the diver’s pose, anIMU and DVL for underwater navigation, a underwater tabletfor interaction with the diver and a low light camera withtilt capabilities for, e.g. mosaicking, looking back, etc. Specialattention is also given to the diver safety requirements.

Four thrusters are placed in a horizontal X-configurationallowing movement in every direction in the horizontal plane.Two thrusters are placed in front and back of the vehiclefor depth control and it is also possible to pitch the vehiclepowering the thrusters in opposite directions. The navigation,guidance and control (NGC) system is designed in ROS.Control is separated into a dynamic velocity controller andkinematic controllers for position and orientation. Position andattitude estimation incorporates asynchronous measurementsfrom available sensors like DVL, USBL to localize the vehicleduring underwater maneuvers. Additionally, the positions ofthe diver and surface platform are estimated on the vehicleduring acoustic data exchange for complete situational aware-ness. High-level behaviours (primitives), e.g. diver trackingand guidance are achieved by combining available kinematiccontrollers. In addition to the hardware description a brief

Figure 2. Autonomous underwater robot - Buddy

overview of the NGC architecture is given. This work isdivided in three main sections:

• Hardware construction

• Electric assembly

• Software implementation

II. HARDWARE CONSTRUCTION

The frame of the autonomous underwater vehicle is madeof an engineered plastic (polyacetal, POM-C). All pieces werecut from 9 mm thick sheet of plastic and screwed togetherwith stainless steel nuts and bolts. Hinges, lock nuts, flangesand mounts are made of Aluminum alloy. Overall dimensionsof the vehicle are 1150x700x750 mm. Three waterproof Alu-minum canisters contain electronics and the battery which willbe explained in section III.

The vehicle carries a large variety of sensors. For naviga-tion it uses:

• USBL - Autonomous surface platform and underwatervehicle have USBL beacons and diver has modembeacon. With the help of the GPS unit on autonomoussurface platform, vehicle can get its GPS coordinates,as well as coordinates of the diver [2].

• IMU - 9 axis IMU gives attitude and assists theKalman filter with movement data.

• DVL - Gives altitude as well as velocity in X and Ydirection (speed over ground).

• GPS - Used for avigation on the surface.

Sensors for remote sensing are:

• Multibeam sonar - High frequency (3.0 and 1.8 MHz)sonar gives unprecedented high definition imagery of

the diver from 0.7 m to 5 and 15 m, depending onfrequency used.

• Stereo camera - 3-sensor multi-baseline monochromecamera is used for detecting divers pose when watervisibility is high.

• Low-light camera - 0.017 lx color camera with 180°tilt mechanism located on the rear end of the vehicleis used for mosaicking and as rear view camera.

A. Thrusters

The underwater vehicle uses six thrusters for moving infive degrees of freedom (DOF). Four horizontal thrusters setin a X-configuration allow surge, sway and yaw motion. Twovertical thrusters provide control of heave and pitch. Of-the-shelf commercial thrusters were fitted with a custom madecontroller, heat sink and mounts. For extra cooling capabilitythrusters were filled with mineral hydraulic oil.

B. Underwater tablet housing

This section provides the description of the dedicateddevices needed to interface the diver with the robotic platforms.Underwater tablet case was made of Polyacetal (POM-C) andtempered glass. It is situated in the front of the vehicle sothat the diver can communicate with it. It uses a customizedinductive pen for interaction. The main purpose of the casingis to hold a commercially available tablet that is used by thediver as an interface to the vehicle. The tablet case is designedin symmetrical form to implement 5 mm thick transparenttempered glass lids onto both sides of the housing. This allowsfor the use of the rear tablet camera and, due to stiffness oftempered glass, less sagging caused by external pressure. Ituses star-grip knobs for clamping. Rear lid is fastened andindependently sealed with the housing frame.

C. Diver safety

In order to ensure diver safety according to the require-ments, two types of safety kill switches are implemented inthe vehicle:

• Magnetic safety switches

• Haptic safety switch

There are five magnetic (reed) relays on the outer shell ofthe vehicle (Fig. 3). Two relays are in front, two on port andstarboard sides and one in at the back of the vehicle. Magneticsafety switches are directly connected to the solid state relaywhich powers the thrusters. Relays are connected in series; byremoving only one of the five magnets, the solid state relaycuts the power to the thrusters.

The haptic safety switch detects changes in IMU readings.If there is any deviation in one of the acceleration axes, e.g.diver pushes the vehicle back if it gets to close, the softwareautomatically stops the thrusters.

Figure 3. Kill switches installed on the vehicle

III. ELECTRIC ASSEMBLY

Buddy vehicle’s electronics is divided in three canisters:

• Battery canister

• Master canister

• Vision canister

A. Battery canister

Battery with nominal voltage of 46.8 V and nominalcapacity of 24.8 Ah is directly wired to 5, 12 and 24 V DC/DCregulators. Regulators are turned on or off via a magneticswitch which is controlled by a magnet outside the canister.When On/Off pins of DC/DC regulators are short circuitedto ground regulators are turned off. This is accomplishedby putting the magnet over the magnetic switch. When themagnet is released, DC/DC regulators are turned on. 24VDC/DC regulator feeds two 9V DC/DC regulators. 5V DC/DCregulator turns on the solid state relay used for disconnectingthe power to the motors via kill switches. The battery is bondedto one cap of the canister with double sided adhesive tape toprevent movement and rolling inside the canister. Through theother cap four underwater cables exit to connect the Masterand Vision canisters. There is an additional magnetic switchwhich is controlled by a magnet outside of the canister if inany case it is needed to completely turn off the battery or resetit if the battery management system (BMS) shuts itself downdue to over current, low voltage, etc.

B. Master canister

The master canister includes:

• Fiber optic converter for communication with thesurface.

Figure 4. Master cylinder setup

• Gigabit switch

• Ethernet relay board for powering up thrusters

• Ethernet relay board for powering up CPU, DVL,USBL and GSM module

• PC104 embedded PC

• IMU

• GPS

• Wi-Fi antenna

• GSM module

Inside the canister is a rack connected to one of the capsof the canister for easy assembly or disassembly of the wholecanister (Fig. 4). On the same cap there is a Wi-Fi antennawith a GPS module for communication and localization duringsurface operation. There is also an antenna for a GSM module.The GSM module can accept calls or SMS-es. After receivinga call or SMS the vehicle sends a SMS containing the currenttelemetry, e.g. GPS position, battery voltage, fault status, etc.The embedded PC is responsible for control of the BUDDYvehicle:

• Gathering and processing data from DVL, USBL,IMU, GPS

• Responding to commands and sharing telemetry datathrough fiber optic cable with the ground station fortesting purposes

• sending commands to the motors

Fiber optic cable is connected to a fiber optic converterwhich is then connected to gigabit Ethernet switch. Overall

intention is to have as much equipment as possible on theEthernet connection. This makes equipment easily accessiblefor the user and allows for independent use of equipment. Ifsome part of equipment is not on the Ethernet network, it isconnected to an embedded computer via a serial connection.

C. Vision canister

This canister includes:

• Mini PC for acquisition of sonar, stereo and monocamera image

• Gigabit switch

• Ethernet relay board for powering up CPU, sonar andcameras

• 24/31 V DC/DC regulator for multibeam sonar

Electronics inside of the canister is assembled on a rackwhich is connected to one of the end caps for easy assembly.

IV. THE NGC SYSTEM

The NGC is designed in the Robot Operating System(ROS), [3]. ROS provides operating system services suchas hardware abstraction, low-level device control, message-passing between processes, and package management. ROSby itself is a soft-real time environment however integrationwith real-time operating systems is available. The frameworkis well established in land robotics and has a broad rangeof supported platforms and sensors. Lately, the emerging useof the framework in underwater vehicle competitions likeSAUC-E triggered higher usage in underwater robotics. Itprovides a platform for connecting application (ROS) nodes ina subscribe-publish manner. This inter-process communicationinfrastructure is well suited for modular development. Partsof the AUV control and navigation architecture are developedas ROS nodes, a self standing unit that has predefined inputsand outputs. In a sense a node is a black-box for other nodes,offering only a small interface for interaction. Exactly hownodes connect and interchange data in the background ishandled by ROS.

The control layout is structure into a cascade of speedcontrollers and higher-level controllers as shown in Figure 5.The speed controllers control the vehicle dynamics makingthe higher-level controllers independent of the vehicle modelparameters. The speed controllers are auto-tuned PI controllerswith a feed-forward action:

τ = Kp(ν∗ − ν) +Ki

∫(ν∗ν)dt+ τff (1)

where ν∗, ν and τff are the speed set-point, speed estimateand feed-forward term, respectively. The control output aregeneralized force and torque values denoted with τ . Thecontrol layout follow the same principles as presented in [4]but with extension to heave and pitch DOFs. Notice that fornon-linear dynamics the feed-forward term can be used forfeedback linearization. High-level controllers that act in theprimary loop of the cascade include position and orientationcontrollers. The controllers are design under the assumptionof an ideal secondary loop controller in order to decouple

Figure 5. Cascade control layout

dependence on vehicle model parameters from the high-levelcontroller auto-tuning. Controller are implemented as separateROS nodes but are all triggered by the navigation filter outputto achieve a consistent sampling rate across all nodes.

The ROS navigation node uses the thrust inputs and thedynamic model to estimate vehicle speeds and positions. Theestimated speeds are used by the controllers. The benefit of thisfilter is that the cascade control layout can be applied even incases where speeds are not directly measured. The EKF filteris designed to enable incorporation of additional asynchronousmeasurements from speed sensors without changing structure.This allows automatically fusing all available knowledge aboutthe vehicle state to increase the final navigation precisionwithout setting the requirement on the sensor suite.

V. CONCLUSION

This paper presents a concept with a diver, autonomous sur-face vehicle and autonomous underwater vehicle. Autonomousunderwater vehicle can follow or lead the diver, thus help-ing the diver while performing tasks underwater. Underwatercommunication and localization is available for all agents viaUSBL system. Autonomous underwater vehicle is describedin details with hardware and electric components for a betterinsight how system performs. The suggested software imple-mentation was already validated on multiple vehicles but theheave and pitch extensions remain to be tested.

ACKNOWLEDGMENT

This work is supported by the European Commissionunder the FP7–ICT project ”CADDY – Cognitive AutonomousDiving Buddy” under Grant Agreement No. 611373.

REFERENCES

[1] N. Miskovic and Z. Vukic and A. Vasilijevic and D. Nad and N. Stili-novic, Cognitive Autonomous Diving Buddy, Proceedings of the 5thConference on Marine Technology in memory of Academician ZlatkoWinkler, Rijeka : University of Rijeka, Faculty of Engineering, Crotia,2014.

[2] N. Miskovic and Z. Vukic and A. Vasilijevic, Autonomous Marine RobotsAssisting Divers, Lecture Notes in Computer Science - Computer AidedSystems Theory (EUROCAST 2013), 2013.

[3] Morgan Quigley and Brian Gerkey and Ken Conley and Josh Faust andTully Foote and Jeremy Leibs and Eric Berger and Rob Wheeler and NgAndrew, ROS: an open-source Robot Operating System, InternationalConference on Robotics and Automation, 2009.

[4] Miskovic, N. and Nad , E. and Stilinovic, N. and Vukic, Z., Guidanceand control of an overactuated autonomous surface platform for divertracking Control Automation (MED), 2013 21st Mediterranean Confer-ence on, 2013