.Autmnms1hnHrlSuneying<i~Gft'fltBaniTRaf Stlfan\Villianl;,PaulNewlmn,Som~ ,Jutio~HughDummt.Whyte Amtralian CearefirReId RoOOtia; University ofSydrey Abstract This paper presents work done as a step towards fully autonomous navigation using the Oberon submersible developed at the University of Sydney. The Oberon team is currently working on a project proposed by the Australian Institute of Marine Sciences (.AIMS).This orlaaisation is surveying sections of the Great Barrier Reef in order to study the growth and propagation of coral. They send a diver to record vioeo transects of the reef while holoing an unoerwater video camera at a height of approximately 30cm above the coral. The video images are then analysed to determine the growth and density of Figure 1 - Diver taking a visual transect of the reef various coral structures. This presents an ideal opportunity for the introduction of a small, submersible robot to perform the data collection In order to achieve some level of autonomous control of a task with some degJ:ee of autonomy. The task robotic vehicle the context in which the robot will be used must be well understood. The environment in which it has been divided into two sub-tasks: maintaining an~ altitude above the reef and following the linear will operate the. tasks that it will be required to undertake will all impact heavily on the sensors and transect. Independent behaviours and decoupled controllers provide a modular means of control schemes that are adopted. accomplishing these tasks. Both a sonar-based In the early development of any robotic system, approach and vision are being considered as it is important to find tasks that are achievable and at the methods with which to guide the robot along its same time provioe enough complexity to be interesting. intended course. The task that has been. proposed by the AIMS group has proven to be an ioeal one with which to begin work towards a truly autonomous submersible robot. It Introduction provides few restrictions on the method with which the Keeping a scientifically rigorous track of change on the task will be accomplished but sets clear goals to be Great Barrier Reef is an essential component of good reef achieved. management. ~e Great Barrier Reef survey project of The task itself can be divided up into two distinct the Australian Institute of Marine Sciences (AIMS), problems that must be solved - maintaining a fixed height initiated in 1991, is desiped to provide long-term above the reef and traversing the linear transect. Separate quantitative geographic data about corals, algae and behaviours and controllers are used to develop ·and marine life over the extent of the Great Barrier Reef. This accomplish each ofthese sub-tasks independently. It must data is an information source for the Great Barrier Reef have a stand-off behaviour that allows it to maintain a Marine Park Authority as well as a basis for studies of fixed altitude above the reef while it follows the transect abundance and population change in selected org~isms and collects data for later off-line analysis. on a large geographic scale. The robot must also have the ability to traverse AIMS divers are currently required to perform the section ofreef in a straight line and to be able to repeat broadscale surveys of coral densities and censuses of its path on subsequent missions. Without absolute .corals and marine life that form the basis for status position data available to the sub, it is necessary to sense reports. A set of reefs is monitored on an annual basis; features in the environment and to deduce the vehicle's others are sampled when specific issues arise. Currently, position from these features. There are a number of ways visual transect information is recorded using underwater in which this could be accomplished and two methods are video cameras held above the reef as the diver follows a currently being pursued - visual servoing on a line laid prescribed path (Figure 1). Subsequent analysis of the out along the coral and using sonar to navigate relative to recorded sequences is used to determine coral diversity targets mounted along the line. and gJ:owth patterns. This project presents an interesting By decoupling the control into the separate opportunity for the introduction of a small, autonomous subproblems of altitude control and line following, the subsea robot to perform the reef-surveying task. individual controller desian is greatly simplified. The behaviours to be exhibited by the robot can be developed independently of one another and later combined to allow the robot to meet its goals. 16 the wide beam of the sonar. The Oberon Vehicle The second sonar is a Tritech SeaKing. This sonar has two narrow beam sonar heads and is considerably The AUV project at the University of Sydney has focused faster than the Imagenex unit. It is mounted on top of the on the development of a mid-size submersible robotic sub and is used to scan the environment in which the sub vehicle called Oberon (Figure 2). This device is intended is operating. It can achieve 3600 scan rates on the order primarily as a research platform upon which to test a of 0.25 Hz. The information that is returned from this variety of sensing strategies and control methods. sonar is used to maintain a feature map of the. environment. Internal Sensors A pressure sensor is used to measure the depth of the sub below the surface of the ocean. This sensor provides a voltage signal proportional to the pressure and is sampled at high speeds by an analogue to digital· converter on the embedded controller. This sensor is used to control the depth ofthe sub. An Andrews Laser Gyro has been included in the Oberon robot to allow the robot's orientation to be determined. This sensor provides fairly acc.ate heading information and is used to control the heading of the sub. A tri-axial, Crossbow accelerometer is also present. Since the Oberon is a relatively slow-moving vehicle, this is used primarily as a tilt sensor. The Figure 2 -the 0beron· vehicle accelerations experienced by the sub are negligible and the accelerometers can be used to measure the inclination of the sub relative to the gravitational field. Embedded controller At the heart of the control of the robot is an embedded Camera control system. At the time of the experiments reported A small Pulnix camera is· used to provide video feedback here, the Oberon vehicle was controlled using a series of of the underwater scenes in which the robot operates. transputers. The Oberon robot has since been upgraded to This is a colour camera and sends the video signal to the use a CompactPCI system running Windows NT that surface via the tether. A Matrox Meteor card is then used interfaces directly to the hardware and is used to control to acquire the video signal for further image processing. the motion of the robot and to acquire the sensor data. This data is collated and sent to the surface where a Thrusters network of computers are used for further data processing There are currently 5 thrusters on the Oberon vehicle. and to display the information to provide the user with Three of these are oriented in the vertical direction while feedback about the state ofthe sub. the remaining two are directed horizontally. This gives A number of processes have been developed to the vehicle the ability to move itself up and down, control accomplish the tasks of gathering data from the robot's its yaw, pitch and· roll and move forwards and backwards. sensors, processing this data and reasoning about the Side to side motion is not possible with this thruster course of action to be taken by the robot. These processes arrangement. communicate asynchronously via a TCPIIP socket-based interface. Communications between the computers at the Solving the Problem surface and the sub are via a tether. This tether also provides power to the robot, a video cable for transmitting MaiDtaimDgAltitude abo'¥e Sea Floor video data and a serial line for control of the camera The firSt task that must be accomplished in order·to realise pan/tilt unit. While some effort might have been spent on the goals set out in this project is :the ability to maintain eliminating the tether, it was felt that the development of the robot at a certain altitude· above -the ·seafloor. The the navigational techniques was of more immediate robot must be able to detect the ground and maintain its interest. height in order to avoid colliding with the coral specimens (Figure 3). Sonar This task has been accomplished using. a Sonar is the primary sensor of·interest for the Oberon combination of the lmagenex Sonar and the depth sensor. project. Navigating autonomously underwater using these The robot maintains a particular depth using a simple PIO sonar sensors is one of the ultimate goals of the project controller based on the measurements returned by the [Newman and Durrant-Whyte, 1997]. There are currently depth sensor. The Sonar is then used to periodically two sonars on the robot. determine the desired depth by finding the altitude of the An Imagenex sonar unit has been mounted at the robot. front of the vehicle. It is positioned such that its scanning head can be used as a forward and downward looking beam. This enables the altitude above the sea floor as well as the proximity of obstacles to be determined using 17 The x-y coordinates of the ith pixel (xj, Yi) found are then fitted to a straight line using a linear regression algorithm (Equation 1).