Brian Claus

Development of an Auxiliary Propulsion Module for an Autonomous Underwater Glider

A low-power propeller-based propulsion module has been developed to augment the buoyancy engine of a 200 m Slocum electric glider. This device is introduced to allow new behaviours such as horizontal flight and faster overall speeds to expand the existing operational envelope of underwater gliders. The design of the system is optimized for use at the typical horizontal glider speed of 0.3 m/s. Before integration into the glider the stand-alone propulsion module has been tested in a small flume tank to verify the systems performance. The validity of a previously published hydrodynamic model of the glider at zero angle of attack was verified by conducting drag measurements at various flow velocities at full scale in a larger flume tank. Self-propulsion tests were also performed to establish the performance of the glider with the new propulsion module in the larger flume tank and in the university tow tank. The results from these tests show that the new propulsion module is able to match the performance of the conventional glider for full depth profiles and to exceed it for limited depth profiles.

Analysis and development of a buoyancy-pitch based depth control algorithm for a hybrid underwater glider

The hybrid glider augments a Slocum electric glider with a propeller based propulsion device enabling new modes of operation. One of the new modes available is constant depth flight. The glider has two mechanisms which lend themselves to a control scheme for depth control, a ballast system and an internal mass shifting mechanism for pitch control. This paper examines the use of a ballast depth controller and a pitch based depth controller. The detailed implementation of both controllers is described and experimental results are presented.

Terrain-aided Navigation for an Underwater Glider

A terrain-aided navigation method for an underwater glider is proposed that is suitable for use in ice-covered regions or areas with heavy ship traffic where the glider may not be able to surface for GPS location updates. The algorithm is based on a jittered bootstrap algorithm, which is a type of particle filter that makes use of the vehicles dead-reckoned navigation solution, onboard altimeter, and a local digital elevation model DEM. An evaluation is performed through postprocessing offline location estimates from field trials that took place in Holyrood Arm, Newfoundland, overlapping a previously collected DEM. During the postprocessing of these trials, the number of particles, jittering variance, and DEM grid cell size were varied, showing that convergence is maintained for 1,000 particles, a jittering variance of 15 m2, and a range of DEM grid cell sizes from the base size of 2i¾?m up to 100i¾?m. Using nominal values, the algorithm is shown to maintain bounded error location estimates with root-mean-square RMS errors of 33 and 50i¾?m in two sets of trials. These errors are contrasted with dead-reckoned errors of 900i¾?m and 5.5 km in those same trials. Online open-loop field trials were performed for which RMS errors of 76 and 32 m- were obtained during 2-h-long trials. The dead-reckoned error for these same trials was 190 and 90i¾?m, respectively. The online open-loop trials validate the filter despite the large dead-reckoned errors, single-beam altitude measurements, and short test duration.

Experimental flight stability tests for the horizontal flight mode of a hybrid glider

The stability of a glider during flight in the horizontal plane is examined. An existing drift-at-depth behaviour is used to control the depth and pitch of the vehicle while the auxiliary propulsion module provides the motive force. Experimental depth controller tests are presented here for fresh water. Additionally, the experimental horizontal flight results in a fresh water test tank are presented. These results show the initial ability of the hybrid glider to maintain a stable path during horizontal flight.

Design and evaluation of a magnetically-geared underwater propulsion system for autonomous underwater and surface craft

This paper will cover the work performed in the Autonomous Ocean Systems Lab (AOSL) at Memorial University of Newfoundland in the design, construction and evaluation of a high performance under water propulsion system. A small 400W thruster was developed for testing and experimentation while a 3kW thruster was designed specifically for the Sea Dragon, an unmanned surface craft (USC) currently being built by the AOSL. Each thruster consists of three main components: a brushless DC motor, a custom propeller, and a magnetic gearing system. A duct was incorporated as a means to improve hydrodynamic performance and to protect the propeller. All subsystems are purpose-built to maximize their efficiency at the desired operational point. A detailed computer model was developed in MATLAB Simulink in order to predict thruster performance. OpenProp was used to design a number of efficient propellers and were tested using the 400W thruster in an open water flume tank. The propellers did not perform as well as expected so additional experiments were performed in an effort to identify the source of discrepancy. It was determined that propeller surface finish and blade thickness have a significant impact on performance. More importantly it was concluded that the quality of flow in the flume tank was not suitable for accurate testing. Bollard tests may provide more accurate results in the future. The larger 3kW thruster was designed with a greater focus on efficiency. A parametric study was performed in order to identify an ideal propeller geometry that would suit the needs of the Sea Dragon. A custom DC motor was designed such that its efficiency is maximized in the operating regime of the vessel. Simulated results indicate that the overall efficiency of the unit to be between 0.5 and 0.6 which is very attractive for unmanned, untethered vehicles. The thruster will be tested in a tow tank and then onboard the Sea Dragon.

A Parameterized Geometric Magnetic Field Calibration Method for Vehicles with Moving Masses with Applications to Underwater Gliders

The accuracy of magnetic measurements performed by autonomous vehicles is often limited by the presence of moving ferrous masses. This work presents a parameterized ellipsoid field calibration method for magnetic measurements in the sensor frame. In this manner, the ellipsoidal calibration coefficients are dependent on the locations of the moving masses. The parameterized calibration method is evaluated through field trials with an autonomous underwater glider equipped with a low power precision fluxgate sensor. A first set of field trials were performed in the East Arm of Bonne Bay, Newfoundland, in December 2013. During these trials, a series of calibration profiles with the mass shifting and ballast mechanisms at different locations were performed before and after the survey portion of the trials. Further trials were performed in the Labrador Sea in July 2014 with two reduced sets of calibration runs. The nominal ellipsoidal coefficients were extracted using the full set of measurements from a set of calibration profiles and used as the initial conditions for the polynomials, which define each parameterized coefficient. These polynomials as well as the sensor misalignment matrix were then optimized using a gradient descent solver, which minimizes both the total magnetic field difference and the vertical magnetic field variance between the modeled and measured values. Including the vertical field in this manner allows for convergence in spite of severe limitations on the platforms motion and for computation of the vehicles magnetic heading.

Energy optimal depth control for long range underwater vehicles with applications to a hybrid underwater glider

The efficient control of a vehicle’s depth is a ubiquitous need of underwater vehicles for long term deployments. Using standard hydrodynamic drag and lift formulations for a hybrid autonomous underwater glider (AUG) the performance penalty for long range underwater vehicles with a non zero buoyant force is shown to be significant. To this end, an energy optimal depth controller design methodology for a long range autonomous underwater vehicle is presented with applications to a propeller driven hybrid AUG during level flight. The method makes use of a reduced order linear model that has been validated from field data. The standard state space model is augmented with the state integral matrix and rewritten to the state error representation. The resulting representation is well suited to the computation of energy optimal gains for a linear quadratic regulator. Field demonstrations of the standard pitching depth controller and the energy optimal depth controller using the computed gains shows the reduced order model to be sufficient for the purpose of the controller design, improving on the standard pitching depth controller response, transport efficiency and ease of tuning.

Closed‐loop one‐way‐travel‐time navigation using low‐grade odometry for autonomous underwater vehicles

This paper extends the progress of single beacon one‐way‐travel‐time (OWTT) range measurements for constraining XY position for autonomous underwater vehicles (AUV). Traditional navigation algorithms have used OWTT measurements to constrain an inertial navigation system aided by a Doppler Velocity Log (DVL). These methodologies limit AUV applications to where DVL bottom‐lock is available as well as the necessity for expensive strap‐down sensors, such as the DVL. Thus, deep water, mid‐water column research has mostly been left untouched, and vehicles that need expensive strap‐down sensors restrict the possibility of using multiple AUVs to explore a certain area. This work presents a solution for accurate navigation and localization using a vehicles odometry determined by its dynamic model velocity and constrained by OWTT range measurements from a topside source beacon as well as other AUVs operating in proximity. We present a comparison of two navigation algorithms: an Extended Kalman Filter (EKF) and a Particle Filter(PF). Both of these algorithms also incorporate a water velocity bias estimator that further enhances the navigation accuracy and localization. Closed‐loop online field results on local waters as well as a real‐time implementation of two days field trials operating in Monterey Bay, California during the Keck Institute for Space Studies oceanographic research project prove the accuracy of this methodology with a root mean square error on the order of tens of meters compared to GPS position over a distance traveled of multiple kilometers.

Towards navigation of underwater gliders in seasonal sea ice

The suitability of the available navigational aids for underwater gliders for year round use in waters which experience seasonal sea ice is evaluated and a path towards an operational system on the Labrador Shelf is presented. The extent of ice coverage is generally found to be limited to the shelf areas and with a duration of up to 20 weeks. For a desired navigational accuracy of 100 meters over a potential trackline in from the shelf break and back out again, around 400 kilometers, a series of low frequency sound sources or geophysical navigational methods are proposed. Acoustic methods require more maintenance and are more prone to loss, while geophysical methods require additional evaluation in the operational region and potential digital elevation model refinement. A three phase strategy is proposed to enable under ice observations. The first phase involves operating the gliders in the ice free season over the proposed track-lines. This data collection phase would allow the evaluation of the available methods and build confidence for later under ice operations. The second phase involves the refinement of the available DEMs both bathymetric and magnetic to the degree that successful navigation by geophysical methods is achieved during the ice free season. Upon the success of the vehicles navigation without surface access during the ice free season, the third phase would commence, that of under ice observations.

Towards online terrain aided navigation of underwater gliders

A terrain aided navigation algorithm has been developed through off-line trials which is suited for operations on an underwater glider. This method has been developed to enable persistent measurements using underwater gliders in regions where surface access is difficult or not possible. The algorithm is based on a jittered bootstrap particle filter. During two sets of off-line trials composed of a 10 km straight line segment and a 90 km survey segment the method was limited to RMS errors of 25 m and 50 m respectively. Integration of the algorithm into an underwater glider was achieved through the addition of a separate single board computer which is interfaced to the payload computer to retrieve the vehicles dead-reckoning solution, attitude, depth and attitude. Navigation updates and a status flag are sent back to the vehicle which logs the estimates in open-loop trials or incorporates them for closed loop trials.