May-Win L. Thein

Quasi-Continuous Higher Order Sliding-Mode Controllers for Spacecraft-Attitude-Tracking Maneuvers

This paper studies higher order sliding-mode-control laws to deal with some spacecraft-attitude-tracking problems. Quasi-continuous second- and third-order sliding controllers and differentiators are applied to quaternion-based spacecraft-attitude-tracking maneuvers. A class of linear sliding manifolds is selected as a function of angular velocities and quaternion errors. The second method of Lyapunov is used to show that tracking is achieved globally. An example of multiaxial attitude-tracking maneuvers is presented, and simulation results are included to verify and compare the practical usefulness of the various controllers.

Position, Orientation and Velocity Detection of Unmanned Underwater Vehicles (UUVs) Using an Optical Detector Array

This paper presents a proof-of-concept optical detector array sensor system to be used in Unmanned Underwater Vehicle (UUV) navigation. The performance of the developed optical detector array was evaluated for its capability to estimate the position, orientation and forward velocity of UUVs with respect to a light source fixed in underwater. The evaluations were conducted through Monte Carlo simulations and empirical tests under a variety of motion configurations. Monte Carlo simulations also evaluated the system total propagated uncertainty (TPU) by taking into account variations in the water column turbidity, temperature and hardware noise that may degrade the system performance. Empirical tests were conducted to estimate UUV position and velocity during its navigation to a light beacon. Monte Carlo simulation and empirical results support the use of the detector array system for optics based position feedback for UUV positioning applications.

Optical Detector Array Design for Navigational Feedback Between Unmanned Underwater Vehicles (UUVs)

Designs for an optical sensor detector array for use in autonomous control of unmanned underwater vehicles (UUVs), or between UUVs and docking station, are studied in this paper. Here, various optical detector arrays are designed for the purpose of determining and distinguishing relative 5 degrees-of-freedom (DOF) motion between UUVs: 3-DOF translation and 2-DOF rotation (pitch and yaw). In this paper, a numerically based simulator is developed to evaluate varying detector array designs. The simulator includes a single light source as a guiding beacon for a variety of UUV motion types. The output images of the light field intersecting the detector array are calculated based on detector hardware characteristics, the optical properties of water, and expected noise sources. Using the simulator, the performance of planar and curved detector array designs (of varying size arrays) are analytically compared and evaluated. Output images are validated using empirical in situ measurements conducted in underwater facilities at the University of New Hampshire, Durham, NH, USA. Results of this study show that the optical detector array is able to distinguish relative 5-DOF motion with respect to the simulator light source. Furthermore, tests confirm that the proposed detector array design is able to distinguish positional changes of 0.2 m and rotational changes of 10 ° within 4-8 m range in x-axis based on given output images.

Characterization of optical communication in a leader-follower unmanned underwater vehicle formation

As part of the research to development an optical communication design of a leader-follower formation between unmanned underwater vehicles (UUVs), this paper presents light field characterization and design configuration of the hardware required to allow the use of distance detection between UUVs. The study specifically is targeting communication between remotely operated vehicles (ROVs). As an initial step in this study, the light field produced from a light source mounted on the leader UUV was empirically characterized and modeled. Based on the light field measurements, a photo-detector array for the follower UUV was designed. Evaluation of the communication algorithms to monitor the UUV’s motion was conducted through underwater experiments in the Ocean Engineering Laboratory at the University of New Hampshire. The optimal spectral range was determined based on the calculation of the diffuse attenuation coefficients by using two different light sources and a spectrometer. The range between the leader and the follower vehicles for a specific water type was determined. In addition, the array design and the communication algorithms were modified according to the results from the light field.

Distance detection of Unmanned Underwater Vehicles by utilizing optical sensor feedback in a leader-follower formation

This paper proposes an optical detection system between a leader and a follower Unmanned Underwater Vehicle, specifically Remotely Operated Vehicles (ROVs). Cost efficient photodetectors and a single LED light source are used to develop distance detection algorithms to detect translational motion in x-and y-axis directions. Analytical simulations are performed where light is modeled as a first order Gaussian function and integrated into the nonlinear ROV dynamics. The stability of a proportional derivative (PD) controller is shown via Lyapunov stability, as in Fossen [7]. Both leader and follower ROV motions are simulated and experimental results from the distance detection algorithm are shown for proof of concept. In this stage of research, all experiments are performed out of water. Initial results indicate that the proposed detection system shows promise as a precursor stage to underwater testing.

Relative Position Estimation and Control for Precision Formation Flying of Two Spacecraft Formations

A minimum configuration for a formation flying mission consists of two spacecraft. The two spacecraft are referred to as the leader spacecraft and follower spacecraft. The separation distance is controlled based on the requirements of the mission, which may vary from meters to kilometers or more. The purpose of this work is to develop an algorithm to estimate the relative position of the follower spacecraft with respect to the leader spacecraft to within sub-millimeter accuracy, applied specifically to close formation type missions with separations less than 100m. A sliding mode observer has a simple structure and is computationally less expensive than other noted estimation techniques. It is applied to the formation in conjunction with a passivity-based adaptive controller. The sliding mode observer is implemented using measurements from a vision-based navigation system. The resulting observer-based control system is shown to be robust against uncertainties and perturbations and shows reasonable performance under measurement noise.

SLIDING MODE FAULT DETECTION AND ISOLATION IN A SATELLITE LEADER/FOLLOWER SYSTEM

Abstract This paper proposes a fault detection and isolation scheme for a generic satellite leader/follower system. The measurement system is based on the so-called VISNAV scheme. The VISNAV outputs are processed in such a way as to obtain an accurate estimate of the relative position of the two space-craft and also to generate an alarm if a fault is detected in one of the beacons. The estimate of relative position is then used to drive a sliding mode observer which is used to robustly reconstruct actuators faults within the coupled system. The efficacy of this scheme is demonstrated on a ‘real world’ simulation.

Pose detection and control of multiple Unmanned Underwater Vehicles (UUVs) using optical feedback

This paper proposes pose detection and control algorithms in order to control the relative pose between two Unmanned Underwater Vehicles (UUVs) using optical feedback. The leader UUV is configured to have a light source at its crest which acts as a guiding beacon for the follower UUV which has a detector array at its bow. Pose detection algorithms are developed based on a classifier, such as the Spectral Angle Mapper (SAM), and chosen image parameters. An archive look-up table is constructed for varying combinations of 5-degree-of-freedom (DOF) motion (i.e., translation along all three coordinate axes as well as pitch and yaw rotations). Leader and follower vehicles are simulated for a case in which the leader is directed to specific waypoints in horizontal plane and the follower is required to maintain a fixed distance from the leader UUV. Proportional-Derivative (PD) control (without loss of generality) is applied to maintain stability of the UUVs to show proof of concept. Preliminary results indicate that the follower UUV is able to maintain its fixed distance relative to the leader UUV to within a reasonable accuracy.

An image processing approach for determining the relative pose of unmanned underwater vehicles

The use of a light source as a beacon is advantageous for the guidance and control of Unmanned Underwater Vehicles (UUVs). This approach allows a follower UUV to determine its relative pose (position and orientation) using low-cost commercial off the shelf (COTS) hardware (e.g., metal halide light sources). In order to design an effective detector unit for the follower UUV and predict its performance, a simulator program has been developed. The program simulates a light field using hardware and environmental parameters describing the light source, water properties, the detector unit geometry and electronic sensitivity. The simulator allows examination of different 3D detector array shapes of varying sizes (physical dimensions and number of detectors). It is convenient to present simulator output as an image, where each pixel represents the intensity logged by a corresponding detector. These image outputs are evaluated for the development of control algorithms for UUVs. Currently control algorithms assume that the water column is uniform with a background noise of known origin. Considered control algorithms are able to provide guidance based on relative intensity values, where the light field samples on the detector array resembles a Gaussian beam pattern. However, disturbances in the medium (e.g., sediment plume) may cause non-uniform distribution of the scatterers that distort the beam pattern. As a result, the control algorithms could misinterpret the acquired image and direct the follower UUV away from the guiding beam. The probability for such a situation increases with distance as the beam diverges. This paper suggests an alternative approach for the development of UUV control algorithms using calculations of various moments of the image (e.g., local Hessian estimations). This method allows the evaluation of the array performance with different array geometries and a varying number of detector elements.

On-board and/or ground-based gyroless accelerometer calibration for NASA's Magnetospheric MultiScale (MMS) Mission

Several methods are presented for the on-board and/or ground-based calibration of accelerometers for the spacecraft (s/c) of the NASA Magnetospheric MultiScale (MMS) Mission during mission operation. This feasibility study shows that a single lumped bias (represented by 3 parameters) is sufficient to represent all nine accelerometer calibration parameters for sensor bias, orthogonal misalignment and shift in accelerometer position with respect to the s/c center of mass. In this paper, three filters/observers are evaluated and compared for this purpose: the Extended Kalman Filter, the H-infinity filter, and the Sliding Mode Observer. All methods are shown to accurately determine lumped bias, so long as the s/c inertia tensor is well known. Analytical simulations show that these methods are able to correct accelerometer readings to within 1 micro-g of true acceleration.