Technical Reports, Virginia Center for Autonomous Systems

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  • User Manual for Bioflyer Database
    Allen, David (2015-11-30)
    In order to compare the results of the new integrated morphology and gait optimization to diverse examples from biology, a comprehensive and easily accessible database is required. Unfortunately, no such database is publicly available, so one was developed. This database includes data from various sources. In addition, a text-based interface that utilizes MATLAB was developed that allows for easy access and editing the database. Additionally this interface can be used to create plots. These plots allow for the data to be sorted so that trends can be observed. These trends can be used to identify trends that form commonalities between diverse species as well trends that show difference between different fight strategies.
  • Under-actuated Controllability for Spacecraft Rendezvous
    Rogers, Andrew; Woolsey, Craig A.; McGwier, Robert W. (Virginia Tech, 2014-06-27)
    In this report, we examine the controllability of a particular form of the equations of motion for spacecraft formation flying. These equations, the Tschauner-Hempel equations, rescale the formation flying equations to a domain in which the true anomaly is the independent variable. Using this form, we are able to compute an explicit, closed-form Gramian matrix for the period of one full orbit at arbitrary eccentricity. We do this for two cases: 1) the case in which there are three inputs to the system as well as 2) the restricted case where authority only exists in the in-track and cross-track directions. This Gramian is invertible and as a result the system is controllable for both cases. Since the transformation between the time-domain, linear equations of motion and the Tschauner-Hempel equations is bijective, we conclude that the linear equations of motion are also controllable.
  • Design and testing of a pneumatically propelled underwater glider for shallow water
    Wolek, Artur; Gode, Tejaswi; Woolsey, Craig A.; Quenzer, Jake; Morgansen, Kristi A. (Virginia Center for Autonomous Systems, 2015-10-28)
    This report details the design and testing of a pneumatically propelled underwater glider. The vehicle was designed as a platform for motion control experimentation, and to explore the use of novel actuator designs to improve performance in shallow water and significant currents. The glider’s pneumatic buoyancy engine is capable of rapidly inflating an elastomeric bladder to 5 liters. (This displacement is an order of magnitude greater than that of legacy buoyancy engine designs.) The buoyancy engine was shown to operate reliably at 25 m depth. However, the compressibility of the bladder and associated change in tank weight (from exhausting air with each dive) presented significant challenges in trimming the vehicle. The attitude of the glider is controlled by translating and rotating a semi-annular mass. Because of the geometry of this mechanism, the glider is not restricted to a range of roll attitudes (i.e. the glider has unlimited roll authority and can “flip over”). By flipping over the glider may employ asymmetric hydrodynamic surfaces while preserving the same flow-relative geometry during both descents and ascents. Such asymmetric hydrodynamic surfaces (e.g. cambered hydrofoils, dihedral, wing twist) may be used to improve efficiency and performance. The ability to operate in both upright and inverted orientations requires reducing the contribution of the rigid body (minus the moving mass) to the bottom heaviness of the vehicle. A moving acoustic long-baseline ranging system was developed to position the glider while it was underway. The performance of this system was characterized experimentally in terms of ping success rate for various transducer geometries and depths in a shallow-water, rocky bottom lake.
  • Exploration of under-ice regions with ocean profiling agents (EUROPA)
    Allen, David W.; Jones, Matthew; McCue-Weil, Leigh S.; Woolsey, Craig A.; Moore, William B. (Virginia Center for Autonomous Systems, 2013-09-14)
    Europa is an incredibly enticing target for exploration – the nearest reaches of what may be a vast new "habitable zone" of interior oceans warmed and stirred by tidal forces. Decades of NASA and National Academy studies including the most recent planetary science decadal survey have affirmed the preeminence of Europa as a destination for astrobiology research. This report provides a comprehensive technology roadmap and an assessment of current state of the art and future technologies to enable an under-ice mission to Europa. In this study, the authors provide an overview of key mission objectives, a profile of Europa, and a mission overview. The authors then delve into a discussion of the key fundamental science objectives and design tradeoffs to arrive at a comprehensive science traceability matrix and value system for design of a multi-vehicle, under-ice mission to Europa. The current state of the art is assessed and design alternatives discussed. The report culminates in a concept of operations for the mission and a recommended mission architecture utilizing three surface units, each deploying a single cryobot, with each cryobot carrying three biologically inspired, gliding under-ice hydrobots equipped with sensor packages that will characterize the physical and chemical state of Europa’s ocean over its entire depth.
  • Control-Oriented Planar Motion Modeling of Unmanned Surface Vehicles
    Sonnenburg, C.; Gadre, Aditya; Horner, D.; Krageland, S.; Marcus, A.; Stilwell, Daniel J.; Woolsey, Craig A. (Virginia Center for Autonomous Systems, 2010)
    This technical report describes a comparison of experimentally identified dynamic models for the planar motion of an unmanned surface vehicle (USV). The objective is to determine a model which is (1) sufficiently rich to enable effective model-based control design, (2) sufficiently simple to allow straight forward parameter identification, and (3) sufficiently general to apply to a variety of hullforms and actuator configurations. Starting from a three degree-of-freedom nonlinear model obtained from physical principles, we consider five simplified variants that include four linear models and two nonlinear models for low speed operation. The first linear model comes from linearizing the full planar boat dynamics about a straight constant speed. A first order steering model relates steering angle to turn rate. A second order steering model relates steering angle to turn rate and sideslip angle. A first order speed model relates throttle setting to forward speed. The two nonlinear models are derived from potential flow around a simple shape. Linear damping and quadratic damping are included in each nonlinear model respectively. To identify parameters for these models, data must be collected that show the dynamic and steady-state relationships between inputs and outputs. Using these data sets, simple models that satisfy the three given criteria are identified for three types of unmanned surface vehicle: a rigid hull inflatable boat with an outboard engine, a rigid hull inflatable boat with a waterjet propulsion system, and a small pontoon boat with two electric thrusters.
  • Vehicle Dynamics in Currents
    Woolsey, Craig A. (Virginia Center for Autonomous Systems, 2011)
    Vehicles operating in non-uniform flow fields are subject to forces and moments that are not captured by kinematic motion models. These effects are even greater when the mass of the displaced fluid is commensurate with the mass of the vehicle, as is the case for maritime vehicles and airships. Following along the lines of a recent paper by Thomasson, this report presents a dynamic model for the motion of a rigid vehicle in a non-uniform flow. The flow field is assumed to be irrotational, comprising a steady, non-uniform component and an unsteady, uniform component. As Thomasson suggests, rotational flow effects can be incorporated by modifying the vehicle's angular rate when computing viscous forces and moments. These equations have a variety of applications for modeling, simulation, and design, a few of which are listed at the end of the report.
  • Dynamics & Control of Underwater Gliders II: Motion Planning and Control
    Mahmoudian, N.; Woolsey, Craig A. (Virginia Center for Autonomous Systems, 2010)
    This paper describes an underwater glider motion control system intended to enhance locomotive efficiency by reducing the energy expended by vehicle guidance and control. In previous work, the authors derived an approximate analytical expression for steady turning motion by applying regular perturbation theory to a sophisticated vehicle dynamic model. Using these steady turn solutions, including the special case of wings level glides, one may construct feasible paths for the gliders to follow. Because the turning motion results are only approximate, however, and to compensate for model and environmental uncertainty, one must incorporate feedback to ensure precise path following. This report describes the development and numerical implementation of a feedforward/feedback motion control system for a multi-body underwater glider model. Since the motion control system relies largely on steady motions, it is intrinsically efficient. Moreover, the nature of the steady turn approximations suggests a method for nearly energy-optimal path planning.
  • Optimal Control of an Undersea Glider in a Symmetric Pull-up
    Kraus, R.; Cliff, Eugene M.; Woolsey, Craig A.; Luby, J. (Virginia Center for Autonomous Systems, 2008)
    An undersea glider is a winged autonomous undersea vehicle which modulates its buoyancy to rise or sink and moves its center of mass to control pitch and roll attitude. By properly phasing buoyancy and pitch control, an undersea glider rectifies the vertical motion caused by changes in buoyancy into forward motion caused by the lift force on the fixed wing. The characteristic "porpoising" motion is useful in oceanographic surveys and the propulsion method is extremely efficient - undersea gliders routinely operate for months without human intervention. Glider efficiency could be improved even further by addressing the phenomenon of "stall" (loss of lift) when a glider transitions from downward to upward flight. Because the stall phenomenon occurs asymmetrically over the vehicle's wing, it can cause directional errors which must be corrected at a corresponding energetic cost. This paper describes the formulation of a point mass model and its dynamic equations of motion. An optimal control formulation was designed using angle of attack and buoyancy as controls to investigate control scheduling methods for avoiding stall in a symmetric pull-up. The calculations were repeated using three different numerical solution techniques for comparison of the methodologies and results. The model was updated to include longitudinal rigid body dynamics and changed the control to the rate of change of the longitudinal center of gravity location. This model allowed for the inclusion of added mass effects due to fluid displacement.
  • Nonlinear Estimation with State-Dependent Gaussian Observation Noise
    Spinello, D.; Stilwell, Daniel J. (Virginia Center for Autonomous Systems, 2008)
    We consider the problem of estimating the state of a system when measurement noise is a function of the system's state. We propose generalizations of the iterated extended Kalman filter and of the extended Kalman filter that can be utilized when the state estimate distribution is approximately Gaussian. The state estimate is computed by an iterative root-searching method that maximize a maximum likelihood function. For sensor network applications, we also address distributed implementations involving multiple sensors.
  • Sensor Error Model for a Uniform Linear Array
    Gadre, Aditya; Roan, Michael J.; Stilwell, Daniel J. (Virginia Center for Autonomous Systems, 2008)
    We derive a measurement error model for a uniform linear array whose output is the bearing to a single narrowband acoustic source. The measurement error depends on various array as well as environmental parameters, which include the number of hydrophones in the array, spacing between adjacent hydrophones, frequency of the acoustic signal, speed of sound and signal-to-noise ratio. Most importantly, we show that the measurement error is a function of the true bearing from the array to the acoustic source.
  • Dynamics and Control of Underwater Gliders I: Steady Motions
    Mahmoudian, N.; Geisbert, J.; Woolsey, Craig A. (Virginia Center for Autonomous Systems, 2007)
    This paper describes analysis of steady motions for underwater gliders, a type of highly efficient underwater vehicle which uses gravity for propulsion. Underwater gliders are winged underwater vehicles which locomote by modulating their buoyancy and their attitude. Several such vehicles have been developed and have proven their worth as efficient long-distance, long-duration ocean sampling platforms. To date, the primary emphasis in underwater glider development has been on locomotive efficiency; maneuverability has been a secondary concern. The ultimate aim of our research is to develop optimal motion control strategies which enhance the natural locomotive efficiency of underwater gliders by minimizing the energy expended by the control system. Ambitious applications such as persistent undersea surveillance require not only efficient vehicles, but efficient guidance and control schemes. This technical report aims to develop a better understanding of glider maneuverability, particularly with regard to turning motions. As a preliminary step, we develop an approximate analytical expression for steady turning motion for a realistic glider model. The problem is formulated in terms of regular perturbation theory, with the vehicle turn rate as the perturbation parameter. The resulting solution exhibits a special structure that allows one to apply existing optimal path planning results for planar mobile robots. The ultimate result is a simple, energy-efficient motion control method for underwater gliders.