A Review of Underwater Robotics, Navigation, Sensing Techniques

A Review of Underwater Robotics, Navigation, Sensing Techniques and Applications Swagat Chutia1, 2, Nayan M. Kakoty1 and Dhanapati Deka2 1

Embedded Systems and Robotics Lab 2 Biomass Conversion Lab

Tezpur University, Assam, INDIA

[email protected] ABSTRACT The focus of this paper is to review the history of underwater robotics, advances in underwater robot navigation and sensing techniques, and an emphasis towards its applications. Following an introduction, the paper reviews development of the underwater robots since the mid 19th century to recent times. Advancements in navigation and sensing techniques for underwater robotics, and their applications in seafloor mapping and seismic monitoring of underwater oil fields were reviewed. Recent navigation and sensing techniques in underwater robotics has enabled their applications in visual imaging of sea beds, detection of geological samples, seismic monitoring of underwater oil fields and the like. This paper provides a recent review of underwater robotics in terms of history, navigation and sensing techniques, and their applications in seafloor mapping and seismic monitoring of underwater oil fields.

Keywords Underwater robotics; navigation; sensing techniques ______________________ * Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee. Request permissions from [email protected]. AIR '17, June 28-July 2, 2017, New Delhi, India © 2017 Association for Computing Machinery. ACM ISBN 9781-4503-5294-9/17/06…$15.00 https://doi.org/10.1145/3132446.3134872

1. Introduction The ocean covers about two-thirds of the earth and has a great effect on the future existence of all human beings. About 37% of the world’s population lives within 100 km of the ocean [1]. The ocean is generally given less importance or overlooked as our prime focus lies on land and atmospheric issues. Mankind has not been able to explore the full depths of the ocean and its abundant living and non-living resources. Underwater robotics is by far the best option to explore and harness the vast energy which lies amidst the water bodies. The recent advances in underwater robotics are opening a myriad opportunity for the researchers and industries. Situations which earlier seemed to be impossible to solve due to technical limitations, are now solvable because of the advancement in the navigation and sensing technology of underwater robotics in the recent decade [2]. Extensive use of manned submersibles and remotely operated vehicles are

currently limited to a few applications because of very high operational costs, operator fatigue, and safety issues [3]. The demand for advances in underwater robot technologies is growing and will eventually lead to fully autonomous, specialized, reliable underwater robotic vehicles. In recent years, various research efforts have increased autonomy of the vehicle and minimized the need for the presence of human operators [4]. Unmanned underwater vehicles are either tethered which are called remotely operated vehicles (ROVs) or untethered which are known as autonomous underwater vehicles (AUVs). ROVs are further classified as observation-class, working-class and special use vehicles [5]. ROVs can also be classified depending upon their size, weight, operating depth and operating power. The motivation for reviewing the advancements in navigation and sensing techniques for underwater robotics is that underwater robots used are highly dependent on their ability to sense and respond to their environment for their exploration activities. They can operate in a previously unmapped environment with unpredictable disturbances and threats [6]. Moreover underwater robots can handle harsh underwater environment and are capable of overcoming static as well as dynamic obstacles. In view of the future scope of underwater robotics, use of the advances in navigation and sensing techniques is of utmost importance. This leads to the needs of reviewing the recent advances in the area and an approach for less explored applications. Following a brief history of underwater robotics, this paper reviews the recent advancement in navigation and sensing techniques and application in seafloor mapping and seismic monitoring of underwater oil fields.

2. History of Underwater Robotics The use of underwater robotic vehicles was dated back to 1950’s. The Royal Navy of Great Britain employed an underwater vehicle for recovering of torpedoes and removal of underwater sea mines. In 1960’s, a Cable-Controlled Underwater Recovery Vehicle (CURV) intended for rescue and recovery operations in a deepsea was realized by US Navy that were succeeded by CURV II and CURV III recovery vehicles in early 1970’s. With the development and growth of offshore industry in 1980’s and 90’s, the trend of military Remotely Operated Vehicles (ROVs) shifted towards the underwater oil and gas exploration, education and ocean science research [7]. Underwater robotic vehicles have been in service to the human beings for exploring oceanic world and dealing with underwater deployments since the mid 19th century. Although scientific literature does not pin point the first underwater robotic vehicle developed historically, the CUTLET ROV (shown in Fig. 1) has been categorically highlighted as a pioneer remotely operated underwater vehicle which was developed and introduced by the Royal Navy in 1950’s to recover practice torpedoes [7].

Fig 1: CUTLET ROV (Adopted from [7])

Fig 4: CURV III (Adopted from [8])

CUTLET’s main frame was made of aluminum and it was equipped with lights and camera. An electric motor was used for its manipulation. To grip the target objects, a metal claw was mounted on the arms attached to the main frame as end effectors. This vehicle remained operational till 1980’s. XN-3 underwater vehicle of US Navy is also considered amongst the early developments, which were further modified in 1960s as CURV and shown in Fig 2.

Among all the CURV vehicles, CURV III was the most sophisticated CURV series operating at 6000 feet under water with a weight of 5.85 tons using fiber-optic support. To interact with the targets, it was equipped with continuous transmission frequency modulated (CTFM) sonar sensors, TV cameras, digital cameras and two manipulators. The recent advances in navigation and sensing techniques for underwater robots have motivated designers and engineers to build far superior underwater robotic vehicles such as VICTOR 6000 and PEMEX’s ROV. VICTOR 6000 (shown in Fig. 5) project started in 1992 by a French research organization and was completed in 1997. It was a 6000m underwater vehicle with 0.77m/s speed. It was a 4 ton vehicle with six thrusters and 8000m tether with 6 optical fiber cables. Real time system was employed for the vehicle control with 2 VEM computers, one each in the vehicle and another on the surface unit. Vehicle was equipped with 2 pilot cameras, 3CCD camera, sonar, altitude and pressure depth sensors, a 7- function arm for manipulation with a 5-function arm for grasping applications [8].

Fig 2: CURV (Adopted from [8]) It was the first time that ROVs gained significant recognition [8] because CURV was used in human life recovery operations in Mediterranean Sea. The vehicle was facilitated with onboard hydraulic power system, operations and maintenance van, and acoustic tracking system for navigation. Electrical power for the system was provided through a diesel generator. The vehicle offered remote maneuvering with six degrees of freedom (DoF) for depth, altitude and heading using altimeter and depth-meter sensors. Later, the CURV was tailored to develop CURV II (as in Fig. 3) and a series of advanced ROVs including CURV II-B, CURV II-C, and CURV III (Fig. 4).

Fig 3: CURV II (Adopted from [8])

Fig 5: VICTOR 6000 (Adopted from [9])

Fig 6: PEMEX’s ROV (Adopted from [9])

PEMEX’s ROV (as shown in Fig.6) is consisted of surface unit, launching unit, tether management unit and the vehicle. The vehicle was designed to operate at the depth of 2000 m with six thrusters, 5-function hydraulic manipulator and a 3-phase power supply of 440VAC. The manipulator was operated from the surface unit using a joystick. It was equipped with depth sensor, compass, altimeter, rate gyros, sonar, 3 cameras and 4 lights. The velocity of the vehicle was 0.55 m/s vertically, 1.25m/s in forward and 1m/s in reverse direction. It was mainly designed to monitor underwater oil field [9].

3. Sensing Technology Navigation and sensing are the key features of an underwater robot. Some of the widely used sensors in underwater robotics are depth sensor, proximity sensor, two-axis and three-axis magnetic sensors, roll and pitch sensor, angular rate sensors, three-axis gyrocompasses, etc. The motivation for improving underwater vehicle navigation arises from the need to expand the capabilities of these underwater robotic vehicles and further increase their value to oceanography [10]. This part of the paper reviews the existing navigation and sensing techniques used in underwater robotic vehicles. Some of the navigation sensors that are used in underwater robotics are tabulated in table 1. Table 1: Sensors for underwater Vehicle navigation [11] Sensors

Sensing Principle

Acoustic Altimeter

Echo Sounding

Pressure Sensor

Piezoresistive, Piezoelectric

12 KHz LBL

Transponder

Inclinometer

Tilt sensor technology

Magnetic Compass

Earth's magnetic field

Gyro Compass

Torque induced gyroscopic precession

Acoustic Altimeter: An acoustic altimeter is also known as an underwater or subsea altimeter. It is primarily used to measure the altitude (height) of an object above the seafloor. They are also suited to various other applications including positioning, berthing and below surface monitoring. The underwater vehicle industry (remotely operated vehicles and autonomous underwater vehicles) is the primary user of altimeters [12]. Pressure Sensors: A pressure sensor is used to sense the pressure of gases and liquids. Some of the applications of pressure sensor are altitude sensing, flow sensing, level/ depth sensing, leak testing and ratio metric correction of transducer output [13]. The two basic sensing principles of pressure sensors are [9]: 1) Force collector types: These sensors are electronics in nature and use a force collector such as diaphragm, piston, bourdon tube, or bellows to measure strain or deflection due to applied force over an area i.e. pressure. 2) Other types: These types of electronic pressure sensors use other properties (such as density) to infer pressure of a gas, or liquid.

Long baseline (LBL) acoustic positioning sensor: LBL sensors are generally deployed around the perimeter of a work site. The LBL technique results in very high positioning accuracy and position stability that is independent of water depth [14]. They are generally employed for precision underwater survey work where the accuracy or position stability of ship-based positioning systems does not suffice [15]. Inclinometer: It is used for measuring angles of slope or tilt, elevation or depression of an object with respect to gravity. It is also used as a tilt meter, tilt indicator, slope alert, slope gauge, gradient meter, gradiometer, level gauge, level meter, declinometer, and pitch and roll indicator [16]. Inclinometers generate an artificial horizon and measure angular tilt with respect to this horizon. The tilt angle range and number of axes are the vital parameters to be considered for inclinometer sensing applications [16]. Magnetic compass: It is used for navigation and orientation that shows direction relative to the geographic cardinal directions [17]. Earth’s magnetic field is utilized by the magnetic compass to show the direction. True North-Seeking Three-Axis Gyrocompasses: A gyroscope is a device used primarily for navigation and measurement of angular velocity. Gyroscopes are available that can measure rotational velocity in 1, 2 or 3 directions. 3- axis gyroscopes are often implemented with a 3- axis accelerometer to provide a full 6 DoF motion tracking system. A gyrocompass consists of a rotor that is made to spin about an axis. Often the spinning rotor is gimbaled and allowed to move freely [18]. The earth’s rotation and earth’s gravitational field are employed by North-seeking gyrocompasses to determine the direction of local vertical and true North. True North-Seeking Three-Axis Gyrocompasses are however not used on non-military underwater vehicles due to their bulky size and high power consumption [18]. Two-axis and three-axis magnetic sensors: Most modern navigation magnetometer units incorporate an on-board microprocessor to provide a serial digital data output. These units are economic and can be relied upon. The two-axis and three-axis magnetic sensors are widely available in varieties in the market and when properly calibrated can provide heading accuracies accurately. They consume very less amount of power. Roll and pitch sensors: Roll axis is often termed as the longitudinal axis. It is an axis drawn through the body of the vehicle from tail to nose in the normal direction of movement. Pitch axis is also known as lateral axis or transverse axis. This axis runs from left to right of the vehicle. Roll and pitch sensors are used extensively in underwater robotic vehicle [19]. Low cost roll and pitch sensors are most commonly based upon measuring the direction of the acceleration due to gravity with pendulum sensors, fluid-level sensors, or accelerometers [20]. Angular rate sensors: Rate sensors measure angular rate directly, without integration in conditioning electronics. Some of the technologies related with angular rate sensors are electromechanical, vibrating structure gyroscopes including micro-electro-mechanical systems (MEMS) gyroscopes, ring laser gyroscopes, fiber optic gyroscopes, MHD sensors based on the magneto hydrodynamic effect, electrochemical sensors [21]. Optical gyroscopes remain the most accurate available angular rate sensors [19], yet their comparatively high cost and power

consumption has limited their use. Fiber optic gyroscopes (FOG) and ring-laser gyroscopes (RLG) can provide angular drift rates typically on the order of 0.1–10 per hour. Low-end FOG motion units employ FOGs, accelerometers, and flux-gate compasses to estimate angular position, angular velocity, and translational acceleration. Multiple sensor data fusion Multi sensor fusion system refers to the phase in integration process where there is a combination (or fusion) of different sources of sensory information into one representational format [29]. Multiple sensor data fusion (MSDF) systems can fuse information from complementary sensors, redundant sensors or even from a single sensor over a period of time. The advantages of fusion of sensor data are: reduction of uncertainty, rejection of noise, toleration of sensor failure, increase in resolution and the extension of sensor coverage. The MSDF techniques have been summarized into four main categories on the basis of the applications of the sensor fusion techniques: filtering and estimation, mapping-oriented, behavior oriented and machine learning [30].

4. Navigation Technology The techniques that are currently used for the autonomous underwater vehicle navigation can be classified into three categories. Inertial navigation: Gyroscopic sensors are used to detect the acceleration of the underwater robot in inertial navigation. This technique is successor of the dead reckoning technique and is often combined with a Doppler velocity log (DVL) that can measure the vehicle’s relative velocity [10, 22]. Acoustic navigation: Acoustic navigation uses acoustic transponder beacons to allow the autonomous underwater vehicle to determine its position. The most common methods for autonomous underwater vehicle navigation are LBL that uses at least two, widely separated transponders and ultra short baseline (USBL) that generally uses GPS-calibrated transponders on a single surface vessel [10, 22]. Geophysical navigation: Geophysical navigation uses physical features of the underwater robot’s environment to produce an estimate of the location of the robot. Magnetic compass and Gyro compass are generally used in geophysical navigation [10, 22]. Table 2 describes the principles of various methods applied for underwater robot navigation. Table 2: Methods applied for underwater robot navigation Methods

Principles

Terrain-aided navigation for underwater robotics [23].

Scanning sonar is used to generate navigation estimate based on a simultaneous localization and mapping algorithm.

Doppler based navigation [11].

Bouncing a microwave signal off a desired target and analyzing how the object's motion has altered the frequency of the returned signal.

Combined Doppler/ LBL based navigation [11].

Complementary linear filters are utilized to combine low passed LBL position fixes with highpassed Doppler position fixes.

Dead Reckoning and inertial navigation [25].

The accelerations of the vehicle are integrated twice in time to derive the updated position.

Terrain-aided navigation for underwater robotics While many land-based robots use GPS or maps of the environment to provide accurate position updates for navigation, a robot operating underwater does not typically have access to this type of information. This is where terrain aided navigation comes in. It uses SONAR based on a simultaneous localization and mapping algorithm to navigate. Sonar targets are currently introduced into the environment in which the vehicle will operate in order to obtain identifiable and stable features. The OBERON vehicle designed and built at the Australian Centre for Field Robotics is an example of terrain aided navigation. The vehicle is equipped with two scanning low-frequency terrain-aiding sonar’s and a color CCD camera, together with bathymetric depth sensors, a fiber optic gyroscope and a magneto-inductive compass with integrated two-axis tilt sensor [8, 23]. Doppler based navigation The Doppler transducer unit has four downward looking beam transducers oriented at about 30 degree from the instrument vertical axis. A minimum of three beams are required. First the Doppler sensor measures the apparent bottom velocity along each of the beams. The velocity measurement in broadband dopplers is performed with an ensemble of one or more discrete pings employing the entire set of four beams [11]. The Doppler unit digitally processes the four ping responses to compute a 4x1 vector of velocities. Single ping velocity error standard deviation varies with beam frequency. Typical velocity error standard deviation is under 1% [11]. Combined Doppler/ LBL based navigation This method is used to take advantage of the incremental precision of the Doppler with the absolute precision of LBL. It uses complementary linear filters to combine low-passed LBL position fixes with high-passed Doppler position fixes. This system introduces a conventional magnetic heading compass and conventional gravitational roll/pitch sensors. In normal operation this system requires only standard LBL sea-floor transponders unlike the simple Doppler based navigation which requires additional fixed seafloor mounted continuous tone beacons [11]. The combined Doppler system provides significant improvement in vehicle navigation precision and update rate of over 12 kHz. It is particularly useful for real world ROV operations. Dead Reckoning and inertial navigation The longest established navigation technique is to integrate the vehicle velocity in time to obtain new position estimates [24]. Compass and water speed sensor are the key devices to measure the velocity components of a vehicle. The principal constrain is that the presence of an ocean tide will add a velocity component to the vehicle which is not detected by the speed sensor. In the vicinity of the shore, ocean currents can exceed 3.7 kilometer per hour. Consequently, dead reckoning for AUVs, operating at small speeds (5.5 km to 11 km per hour), involving water-relative speed measurements can generate extremely poor position results. It has been found that in inertial navigation systems, the accelerations of the vehicle are integrated twice in time to derive the updated position. Position drift rates for high quality commercial grade INS units are on the order of several kilometers per hour. Moreover, dead Reckoning and inertial navigation systems are costly and highly power consuming. The problem with exclusive reliance on dead reckoning or inertial navigation is that position

error increases without bound as the distance travelled by the vehicle increases. The rate of increase is a function of ocean currents, the vehicle speed, and the quality of dead reckoning sensors [10].

5. Application of underwater robots In this section, application of underwater robotics for seafloor mapping and seismic monitoring of oil fields are reviewed. Seafloor mapping Developing visual image of the seafloor has always been a particular challenge to mankind. The first modern breakthrough in seafloor mapping came with the innovation of underwater sound projectors called “sonar,” which was used in World War I to detect enemy submarines and torpedoes. By the 1920s, the Coast and Geodetic Survey (the predecessor agency to NOAA’s National Ocean Service) was using sonar to map virtual image in deep water. During World War II, technological advancement in sonar and electronics led to much more improved systems that was able to capture precisely timed measurements of the seafloor in depths. These systems provided the primordial databases used to construct the first real maps of important features, such as the deep-sea trenches and mid-ocean ridges [24]. Autonomous underwater robot is used to gather a high resolution near-bottom dataset of bathymetry, magnetic, temperature and optical backscatter across the active tectonic and neo volcanic zone of the Southern East Pacific rise [25]. High resolution mapping of individual fault scarps, fissures, lava tubes, open-lava channels and lava pillars were obtained by the underwater robot. The underwater robot used was the Autonomous Benthic Explorer (ABE) of the Woods Hole Oceanographic Institute. ABE uses acoustic travel time from a 4 transponder network moored to the seafloor to determine its position during surveying. Imagenix 855 mechanically scanned pencil-beam sonar was used for data collection. Fig 7 and Fig 8 shows an underwater robot collecting sonar data and the surface model from the sonar data.

Fig 7: Underwater robot collecting sonar data [26].

Seismic monitoring of oil fields underwater A promising application for underwater robots or sensor networks is seismic monitoring for oil extraction from underwater fields [27]. Frequent seismic monitoring is of importance in oil extraction. Studies of variation in the reservoir over time are called “4-D seismic” and are useful for judging field performance and motivating intervention. Monitoring of underwater oil fields is a challenging task than monitoring terrestrial oil fields. This constraint is mainly because seismic sensors are not currently deployed in underwater fields. Instead, seismic monitoring of underwater fields typically involves an underwater robotic ship with a towed array of hydrophones as sensors and air cannon as the actuator. Underwater robot could overcome this challenge and help in seismic monitoring of the oil fields. Federal Mexican Oil Company developed an underwater robot to inspect pipelines, oil production units and other structures in deep waters [9]. It was mainly used for visual analysis of underwater structure and was named as PEMEX’s ROV (as shown in Fig 9).

Fig 9: PEMEX’s ROV for underwater oil field monitoring (Adopted from [9]) Conclusion: This paper reported a review on the underwater robotic vehicles and recent advancement in the field of navigation and sensing techniques. Underwater robotic vehicles were in use since the World War II. The pioneers of the underwater robots which are CUTLEY and CURV series were reviewed in technical terms. The navigation sensors such as acoustic sensors, inclinometer, magnetic compass, gyro compass are discussed. All the three basic navigation systems for underwater robot navigation which are inertial navigation, acoustic navigation and geophysical navigation were reviewed. The applications of underwater robotics in seafloor mapping and seismic monitoring of underwater oil fields are reviewed. The reported work shall be helpful to the mankind to understand the history of underwater robotics and their navigation and sensing techniques.

Acknowledgement This research is financially assisted under the Department of Biotechnology, Govt of India funded Indo-Brazil project entitled “Integrated Biorefinery Approach towards production of sustainable fuel and chemicals from Algal biobased systems” approval no. DBT/IC-2/Indo-Brazil/2016-19/04.

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