The Unmanned Port Security Vessel an Autonomous Platform for Monitoring Ports and Harbors

The Unmanned Port Security Vessel: An Autonomous Platform for Monitoring Ports and Harbors Vincent Howard, Jonathon Mefford, Lee Arnold, Brian Bingham Mechanical Engineering Department University of Hawaii at Manoa Honolulu, HI, USA Abstract—This paper describes the development of the Unmanned Port Security Vessel (UPSV), a small autonomous surface vehicle designed to support maritime domain awareness in port and harbor environments. The UPSV is capable of rapidly producing fine resolution, shallow-water bathymetry maps using a multibeam sonar, detecting chemical threats using an on board mass spectrometer and monitoring oceanographic parameters using off-the-shelf instruments. In this paper we discuss the significance and scientific advantages of sensor fusion on a small autonomous robotic platform, as well as detailing the control methods chosen for such a vehicle. Command and control of the robotic platform is implemented using the lightweight communications and marshalling (LCM) library, a general purpose software infrastructure for sensor integration and feedback control. Based on this foundation we have built a flexible software architecture that includes mission planning, real-time navigation and control, payload management and telemetry to support a variety of maritime domain awareness missions. Using the developed software and control methods, the UPSV is capable of autonomously generating high fidelity bathymetric maps while simultaneously conducting chemical surveys with a mass spectrometer. We demonstrate the utility of the software architecture and hardware integration through results from both simulation and experiments. We present the results of a set of field trials to evaluate the performance of the UPSV in operational settings. These experiments include system identification for estimation and control, feedback control performance evaluation, demonstration of autonomous high-resolution bathymetric mapping and demonstration of in-situ chemical sensing. The performance of the UPSV’s control system is characterized with the presented navigation data, and the results of the bathymetric surveys and in-situ chemical sensing are detailed. The implementation of this functionality on a small, deployable platform is discussed in the context of similar projects on unmanned surface vehicles.

I.

INTRODUCTION

Ports and harbors play a crucial role in every economy, and disruption to such infrastructure can be extremely detrimental. Marine structures face both natural and manmade threats; ensuring the operation and security of these structures in an expedient and efficient manner is absolutely vital after disasters. Ports may be hindered by fallen debris and damaged

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Richard Camilli Applied Ocean Physics and Engineering Woods Hole Oceanographic Institution Woods Hole, MA, USA structures or rendered too dangerous for humans due to chemical, biological, or radioactive agents. Being able to detect submerged obstacles and test for dangerous compounds can greatly help officials effectively assess the situation and determine the best course of response. Unmanned surface vehicles provide a safe, efficient way to evaluate ports and harbors with minimal manpower needed to operate, alleviating resources for other recovery tasks. The US Navy recognized some of the advantages unmanned surface vehicles (USV) offered with use dating back to the 1940’s [22]. While not a new concept, USV’s gained renewed popularity in the science community with work conducted in the early 1990’s by MIT’s Sea Grant College Program [9]. These unique platforms have evolved into many forms and have taken on a wide variety of tasks. Bathymetric mapping emerged on MIT’s first vehicles and remains prominent for many USV systems [9, 16, 20]. While bathymetric surveying remains popular, recent work has explored the advantages of using autonomous surface craft for data collection and hydrological surveying. Vehicles such as SESAMO, ASMV, and various other systems have been developed with a focus on water sampling and environmental testing [19, 18, 13, 14, 8]. USV’s have also found use in disaster recovery, with the Sea-RAI being the first known vehicle utilized as a disaster response vehicle [6]. These roles and many others prove USV’s to be an adaptable platform suitable for many tasks. The Unmanned Port Security Vessel (UPSV), shown in Fig. 1, is a versatile platform designed for maritime domain awareness in port and harbor environments, including but not limited to first responder situations following a natural disaster or a terrorism event. The system is capable of autonomous bathymetric mapping and hydrological surveying using onboard sonar and chemical sensors. Bathymetric data can ensure ports are clear of debris and water analysis can check for harmful chemical agents from munitions or spills. A unique software architecture allows for easy modification of the scientific package and vehicle behavior, simplifying the addition or removal of instruments. The UPSV is modular, allowing for easy transportation and deployment by a two-man crew, ideal in post disaster situations where resources are limited and infrastructure may be damaged. The system is also

IMU / Compass Housing

Multibeam m Sonar Figure 1. UPSV performing a bathymetric and hydrollogical survey in shallow water near Honolulu Harbor.

compact with a shallow draft making it suittable for shallow water areas and harbor surroundings not easiily accessible by larger, manned vehicles. All of these features make the UPSV ideal for disaster response while also rremaining easily adaptable to various scientific missions. II.

UPSV DESIGN

A. Hardware Architecture The UPSV utilizes a twin hull design simillar to many other USV’s to provide both stability and a largge work area for hardware. A third pontoon is used to house a vvariety of sensors and provide a hydrodynamic housing, but dooes not offer any buoyancy. The three hulls are made off Low Density Polyethylene (LDPE) to reduce overall weiight and provide good corrosion resistance in the saltwater ennvironment. The frame is made of aluminum tubing, providiing strength and reasonable corrosion resistance with a deck pplatform made of King Starboard to provide a durable, corrosion resistant base for mounting electronics. The electronics hardware is housed in ttwo PVC boxes mounted to the top platform. One box houses ttwo 12 V lithium ion battery packs wired in series to powerr the electronics, along with two motor controllers, one for eeach motor. The motors are Minn Kota Endura C2 55 lb thrusst trolling motors

Figure 3. Dissolved oxygen, conductivity, and pressure ssensors attached to the front of the UPSV sub frame.

Figure 2. Multibeam sonar located near ceenter of gravity of UPSV with watertight container moun nted above.

and are mounted to the aluminum frame. The motors can be y or easily removed for rotated up and out of the way transportation. Power for the moto ors is run in parallel from two lead acid batteries, one houseed in each outer hull, with power distributed to the motor conttrollers in the first box. The second box houses a small, embed dded mini-ITX x86 system used to integrate the various senssors, control software, and wireless hardware. Also housed in the second box are a Trimble ProXT GPS receiver and a FreeWave MM2 900 MHz c The Trimble GPS radio for remote command and control. system utilizes an external Trimble Tempest Antenna mounted F radio uses a high directly to the platform, while the FreeWave gain, 5dBi, 33” tall antenna also o mounted directly to the platform. An Imagenex 837B “DeeltaT” Multibeam Sonar is mounted to the center of the vehicle, below the main frame of nge is from 0.5m to 100m the body. The DeltaT working ran depth [32]. A special sub frame waas built to house the DeltaT along with other sensors that need to be either submerged or in c and a MicroStrain contact with the water. A digital compass 3DM-GX2 IMU are mounted in a watertight box directly above the DeltaT to provide feedback of the vehicle orientation. Attaching the IMU direectly to the DeltaT reduces the calibration necessary for atttitude correction of the multibeam sonar data. Three water chemistry sensors are also nsist of an Aanderaa Data mounted to the sub frame and con Instruments 4117 Pressure Sensor, 4319 Conductivity Sensor, and 4330/4330F Oxygen Optode. Fig. 2 shows the chemical he sub frame while Fig. 3 sensors attached to the front of th shows the DeltaT attached to the cen nter of the sub frame. B. Software Architecture U follows a modular The software design for the UPSV structure. Drivers for each of the t sensors are written in standard C and are responsible for the t interpretation of the RS232 output from each of the sensorss. The actual command and control software is written in the Python programming language and is composed of an esttimator which processes the sensor data to produce an estim mate of vehicle state, the controller which is comprised of o the various behaviors

intended for the system, and a mission input file detailing the specific objectives that are delivered to the controller. One of the innovations of this software architecture is the interfacing method by which the drivers communicate with the command and control software. This sensor integration and feedback control is achieved through use of the general purpose software infrastructure called the Lightweight Communications and Marshalling (LCM) library [33]. The driver software outputs data on various LCM channels which in turn are being continuously monitored and interpreted as appropriate by the control software. Once the estimate of the state is determined and the appropriate behavior and guidance responses determined, control feedback is output in the form of thrust commands over LCM channels which are used by the motor controller drivers to command the motors. This architecture is described in more detail in a previous publication [23]. Within the command and control software is a custom library of feedback control and estimate behaviors having a modular design that allows for easy modification of the system behaviors. Currently, this list of behaviors allows the UPSV to perform mission tasks including helm control, velocity control, waypoint following, and line following. In addition, the algorithms for these behaviors are interchangeable, allowing for rapid testing and evaluations of new implementations. For bathymetric and chemical surveying, achieving desired waypoints and following tracking lines are the foundational autonomous functions. If it is desired to employ the UPSV system for additional tasks, new behaviors may be coded into the software with relative ease. Current mission behaviors utilize PID control methods for commanding the vehicle. A manual tuning approach based on field-testing has been used for determining appropriate values for the various gains. C. Dynamic Model for Command and Control A simple dynamic model was used to simulate the behavior of the UPSV and to help predict the command and control characteristics of the system in the real-world environment. This model was implemented with LCM communication to enable hardware-in-the-loop simulation for developing feedback control, estimation and autonomous behavior functions. To illustrate this model, we compare simulation results with experimental data from a short bathymetric field survey. The state of the system is defined as follows:

(1)

respect to the y-axis in this coordinate system, with clockwise movement being positive. In order to increment these variables, an estimate of the vehicle’s linear and rotational velocity must be derived from the given thrust commands, taking into account predicted values of linear and rotational drag forces. The following equations are used: (2)

sin

(3) (4) (5) (6)

Where dt is the time interval between steps, k is the time step, v is the velocity, t is the thrust, b is the linear drag, m is the mass of the vehicle, τ is the torque, bt is the rotational drag, and i is the moment of inertia of the system. Once the position, velocity, and heading are incremented the controller interprets the new state of the vehicle and generates a new set of thrust commands. D. Mass Spectrometer Payload Currently, the UPSV is equipped only with pressure, conductivity, temperature, and dissolved oxygen sensors. The incorporation of a TETHYS in-situ mass spectrometer is, however, planned for the near future. TETHYS is a self-contained membrane inlet mass spectrometer that is optimized for deployment on surface and subsurface marine platforms. The instrument’s double focusing analyzer provides high sensitivity, long-term stability, and specificity. Its low power consumption (25W) enables autonomous operation with battery power. Minimum detection limits typically are on the order of parts-per-billion with the overall instrument response time of less than 10 seconds for concentration quantification at a 95% confidence interval [25]. This enables high spatial and temporal resolution mapping of dissolved chemical distributions, including anthropogenic pollutants, on spatial scales as small as 1 meter. TETHYS instruments are in routine use aboard submersible vehicles for offshore oil spill cleanup [26, 27], as well as investigation of deep ocean hydrates and cold seeps [28, 29]. Research is currently underway using the TETHYS instruments for in-situ detection of chemical weapon agents in marine environments [30]. III.

where positive x and y represent meters East and North of a defined origin respectively, as described by the Alvin XY coordinate system [24]. θ represents the vehicle heading with

cos

EXPERIMENTAL SETUP AND FIELD TESTING

A number of field trials were designed and conducted at various locations around Oahu, Hawaii. These tests were designed to assess and evaluate the mapping and chemical sensing capabilities of the UPSV prototypes. The original field trial of the USPV aimed to test structure capability,

sensor integration, and heading response software. Subsequent tests included the addition of waypoint and line following software, new GPS, IMU, and FreeWave radio hardware, the multibeam mapping system, and the chemical sensing array. The most recent test was a culmination of these updates, with a system that has proven to be more reliable and robust with each field trial. This test was conducted at Ke’ehi Lagoon Park, which provided a testing ground offering satisfactory protection from the elements and marine traffic, as well as adequate water depth and surrounding geography. Previous testing at this location had yielded important insight into the bathymetric mapping capabilities of the DeltaT hardware, as well as the configuration requirements. As a result, two consecutive “lawn-mower” pattern missions were planned to test the capabilities of the UPSV system, with particular consideration given to mapping and chemical sensing requirements. To this effect, Google EarthTM mapping service was used in conjunction with developed software to plan out the missions, with survey lines spaced three meters apart as seen in Fig. 4. The first and smaller of the two surveys was carried out in a shallow inlet with approximate average water depth of two meters. The second survey was conducted just outside of the inlet in slightly deeper water, and was roughly perpendicular to the first survey. A sufficient number of survey lines were used to make a satisfactory map of the topography, as well as to gain the desired water characteristics and chemical data. Each of these missions was carried out autonomously while simultaneously collecting data from all of the sensors. IV. RESULTS Data was analyzed after the completion of the field surveys and post-processing has yielded several important results.

These results are broken into the three main areas of interest pertaining to the UPSV system: Navigation and Control, Bathymetric Surveys, and Chemical Sensing and Sensor Fusion. A. Navigation and Control Navigational data was collected from the onboard GPS, IMU, and compass continuously during the survey missions, being simultaneously logged by the UPSV computer and sent to computers on shore for real-time observation. This realtime observation was accomplished through the use of Google EarthTM mapping service in conjunction with developed software, which allowed for the viewing of a recent history of the UPSV position as well as the current position. This tracking software is demonstrated in Fig. 5.

Figure 5. Google Earth TM used in conjunction with developed software for real-time tracking and observation of the UPSV.

The actual path of the UPSV during the missions is compared to the results of the simulations to ensure that we

Figure 4. Ke’ehi Lagoon Park test site, with defined mission paths and origin displayed.

are able to accurately predict the motion of the vehicle in a real world environment within a certain degree. These comparisons are shown in Fig. 6 and Fig. 7. As can be seen from the figures, the simulations provide a satisfactory measure of predicting UPSV dynamics and control in the actual field trials. As expected, the results of the simulations are smoother and more homogenous for each of the lines that were followed, as they are not subjected to the same environmental and real world disturbances that the vehicle is. Also, further tuning of the PID controller used to control the vehicle will aid in following lines more closely, reducing line overshoot, and responding to environmental disturbances. B. Bathymetric Surveys Data collected by the DeltaT multibeam sonar system during the two surveys was archived on the UPSV’s onboard

hard drive to be analyzed on shore after the surveys. Using the MB-System [31] open-source1 software suite developed by the Monterey Bay Aquarium Research Institute (MBARI), the raw log files were processed using the program’s core mapping algorithms along with the editing and visualization software. In particular, the function MBclean was used to remove any outlying data points or anomalies that would otherwise affect the creation of an accurate map. Fig. 8 shows a swath plot of the data collected during the two missions conducted at Ke’ehi Lagoon Park. The actual path of the UPSV is shown in black, and the bathymetric survey data collected by the multibeam sonar system is represented by beam swaths laid out perpendicular to the path of the vehicle. Geographical coordinates are displayed on the edges of the figure. Using the legend it can be seen that depths of around two meters were primarily encountered during the first survey, whereas depths of up to seven meters were observed during the second survey. Using the same MB-System suite a 1 meter grid size topographical data map was generated, as seen in Fig. 9. This figure allows the topographical features to be distinguished much more clearly, including an underwater shelf discovered during the second survey.

Figure 6. Comparison of simulation and field test for survey mission 1. Position is measured in the AlvinXY coordinate system.

Figure 8. Swath projection from Ke’ehi Lagoon Park surveys with the navigation overlay

Figure 7. Comparison of simulation and field test for survey mission 2. Position is measured in the AlvinXY coordinate system.

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Figure 11. Conductivity of water at survey site 2. The colorbar indicates measured conductivity y in mS/cm.

Figure 9. Topography map generated from Ke’ehi Laagoon Park surveys

Data for both of these maps was gathered aautonomously by the UPSV and logged by the computer system m on board. This data was then post-processed along with naviggation data using an EKF. Ultimately, it is envisioned to impleement some form of real-time map generation and visualizatiion to aid firstresponders and other users of the system in a more expedient manner. C. Chemical Sensing and Sensor Fusion Real-time data was collected from thhe conductivity, pressure/temperature, and dissolved oxygen seensors during the missions. Similar to the navigation data colleection, this sensor data was simultaneously being logged annd sent to the computers on shore for real-time observation. The conductivity measurements gathereed during both surveys are shown in Fig. 10 and Fig. 11. As can be seen from the trends, the conductivity appears to be noticceably less in the shallowest regions of the inlet that was mapped. It appears to increase to approximately 54-55 mS/cm aand level off as distance from shore and depth is increased.

Figure 10. Conductivity of water at survey site 1. The colorbar indicates measured conductivity in mS/cm.

Temperature data from both surv veys is shown in Fig. 12 and Fig. 13. As might be expected, tem mperature decreases slightly as distance from shore increases. However, H it does not appear to decrease significantly when the depth increases to approximately seven meters in survey two.

Figure 12. Temperature of water at survey y site 1. The colorbar indicates measured temperature in degrees d Celsius.

Figure 13.

Temperature of water at surrvey site 2. The colorbar indicates measured temperature in degrees d Celsius.

The pressure data gathered was not of muuch significance, since all measurements were taken at the surfaace and there was not much variance. Of particular importance for this iteration of field tests was the incorporation of the ddissolved oxygen sensor. This implementation was a vital sstep towards the planned addition of a novel mass spectrometeer which will add significant value to the system as a whole. In-situ chemical sensing in conjunction with autonomous bathhymetric surveys is one of the key goals for this platform. Thhe data collected from the dissolved oxygen sensor during the ssurveys is shown in Fig. 14 and Fig. 15 below. Althouugh not entirely conclusive, it appears that dissolved oxygeen concentration decreases slightly as distance from shore andd depth increase. More tests with this sensor would serve tto further refine understanding and allow for a more accurate assessment to be made. V.

CONCLUSIONS AND FUTURE WORK

This paper discusses the progress made onn the Unmanned Port Security Vessel, which aims to bee a dependable autonomous surface vehicle for supporting pport security and maritime domain awareness. The operational capabilities of

the UPSV prototype were verified d through a series of field trials culminating in the most recent test, in which two autonomous bathymetric surveyss were conducted while simultaneously performing in-situ chemical c sensing and water analysis. ue to be refined through These capabilities will continu further testing and as new functionaality is added to the current configuration. Current plans inclu ude finer tuning of the PID control software governing the vehicle behavior. This will be oftware as well as manual accomplished using simulation so tuning techniques in the field, which w will also serve as a testing basis for enhancing bathym metric mapping capabilities. Such capabilities include finer reso olution maps and the ability to monitor mapping progress real-tim me. Addition of a novel mass spectro ometer will greatly increase the scientific value of this platform m in fulfilling its role in port security and maritime domain awarreness. The additional field testing planned for navigation and bathymetric survey refinement will also serve to test this new sensor. As the b upon, it is envisioned modular platform continues to be built that photographic and video imagin ng capabilities will be added to the system to aid in determin ning the status of marine infrastructure. In addition, overalll system health monitoring capabilities will be implemented and tested in the near future. GMENTS ACKNOWLEDG

The authors acknowledge the supp port from the University of Hawaii at Manoa, and would like to t thank all other members of the UPSV team and those who contributed. The work on the UPSV was supported in paart by a grant from the Department of Homeland Security y Science and Technology Directorate (2009-ST-061-MD001) NCES REFEREN [1]

Figure 14. Dissolved oxygen concentration of water at survey site 1. The colorbar indicates the measured dissolved oxygen llevel in mMol.

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Figure 15. Dissolved oxygen concentration of water at survey site 2. The colorbar indicates the measured dissolved oxygen llevel in mMol.

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