Fractional Describing Function Analysis of PWPF Modulator

Pulse-width pulse-frequency (PWPF) modulators are widely used in spacecraft thruster control. Their dynamic characteristic is still lack of effective analysis tools. This paper presents a fractional describing function method to describe the frequency characteristics of PWPF. A frequency-dependent gain and phase shift are clearly described by fractional-order expression, and the fractional-order behaviors depict the nonlinear properties of PWPF modulators. This fractional describing function method can also be applied to other kinds of modulators

Target tracking and source seeking using unmanned aerial vehicles with input constraints

The potential applications of unmanned aerial vehicles (UAVs) in both military and civilian fields have spurred the research on UAV control. Although great progresses in this area have been made by researchers worldwide, practical and theoretical challenges are still abundant. This thesis focuses on the problems of target tracking and source seeking using UAVs subject to input constraints. Ground target tracking is an important application of UAVs. Due to UAV inherent motion constraints, the fixed-wing UAV generally can not stay on top of the ground target. Standoff tracking is one possible solution to target tracking. In the first part of the thesis, a single UAV is used to achieve moving ground target tracking. A guidance law is introduced to generate a desired relative course for the UAV. Based on the desired relative course, a saturated heading rate controller is proposed to achieve standoff target tracking. A rigorous global stability proof of the system with the proposed controller is provided. As the ground target velocity may be unknown, a bounded adaptive observer is designed to estimate the unknown constant target velocity and background wind. Ground target could be friendly or adversarial. For adversarial target tracking or hazardous target tracking, safety and tracking performance are two critical issues for the UAV. In order to achieve adversarial target tracking, the initial condition of the UAV is analyzed, and two types of tracking scenarios are defined: exposure avoidance and minimum exposure time. A bang-bang controller is proposed to achieve target tracking in both scenarios. Advances in sensing, communication and control systems make it possible for multiple UAVs to carry out missions cooperatively. A circular formation strategy for target tracking is proposed in this thesis. Firstly, a guidance law based heading rate controller is designed for circular tracking. A notion called temporal phase is introduced for multi-UAV cooperation. Based on the temporal separation, a bounded variable airspeed controller is developed to achieve equal temporal separation. Source seeking is a potential application of multi-UAV cooperation. In the past decades, single vehicle is successfully used for source seeking. However, the time cost and extreme movement are two challenging issues to be addressed. Cooperation of multiple UAVs makes it possible to attain better source seeking performance. This thesis proposes a leader-follower formation strategy for source seeking. Based on the measurement of all UAVs, the scalar field gradient at the leader location is estimated using least-squares method. After that, an adaptive observer is designed to estimate the source velocity. By using the estimated gradient and source velocity, a heading rate controller for the leader is developed to achieve level tracking. Then, heading rate controller for follower UAVs is also proposed to achieve circular formation around the leader. Finally, conclusions and some recommendations are presented.ELECTRICAL and ELECTRONIC ENGINEERIN

Adversarial ground target tracking using UAVs with input constraints

This paper deals with the problem of adversarial ground target tracking using Unmanned Aerial Vehicles (UAVs) subject to input constraints. For adversarial ground target tracking, tracking performance and UAV safety are two important considerations during tracking controller design. In this paper, a bang-bang heading rate controller is proposed to achieve circular tracking around the target. Exposure avoidance of the UAV to the target and minimizing the exposure time are studied respectively in terms of the initial state of the UAV. The performance of the proposed controller in both cases is also analyzed. Simulation results demonstrate the effectiveness of the proposed approach

A New Navigation Function Based Decentralized Control of Multi-Vehicle Systems in Unknown Environments

This paper deals with navigation for a group of vehicles while avoiding collisions and ensuring global network connectivity in unknown environments using a new decentralized navigation function. It is pointed out that the traditional navigation function is not effective in the situation where vehicles work in a large environment. It is shown that in this situation velocity of the vehicle would be extremely small, which is not realistic in practical applications. This paper proposes a new decentralized navigation function with a novel goal function based on which a decentralized control law that is along the negative gradient of the decentralized navigation function is derived. Finally the proposed decentralized control law is applied in a multi-vehicle navigation scenario. Based on the properties of the proposed navigation function and dual Lyapunov theorem, a sufficient condition is derived for vehicles to converge to regions surrounding their corresponding goal positions in a collision-free and connectivity-keeping manner. Simulation results demonstrate the efficacy of the proposed method.Accepted versio

Cooperative moving path following for multiple fixed-wing unmanned aerial vehicles with speed constraints

This paper is to address a cooperative moving path following (CMPF) problem, in which a fleet of fixedwing unmanned aerial vehicles (UAVs) are required to converge to and follow a desired geometric moving path while satisfying prespecified speed and spatial constraints. A representative application of the CMPF problem is the challenging mission scenario where a group of UAVs are tasked to track a moving ground target. The proposed methodology is based on the insight that a vehicle can follow a given path only through attitude control, thus leaving its speed as an extra input to be used at the coordination level. To deal with moving path following (MPF) of a single UAV, a non-singular control law is derived to steer the vehicle along the desired moving path which avoids the singularity problem in the previous MPF strategy. For multi-UAV coordination, a pursuit strategy is employed with the introduction of a virtual leader. To account for speed constraints and collision avoidance, conditions are derived under which the combined MPF and multi-UAV coordination closed-loop system is asymptotically stable while speed and spatial constraints are satisfied. Further simulation has been performed to demonstrate the effectiveness of the proposed method.Accepted versio

Ground target tracking using UAV with input constraints

This paper provides a solution to the problem of ground target tracking using an unmanned aerial vehicle (UAV) with control input constraints. Target tracking control with input constraints is an important and challenging topic in the study of UAVs. In order to achieve precise target tracking in the presence of constant background wind and target motion, this paper proposes a saturated heading rate controller based on a guidance vector field while the airspeed is held constant. This proposed approach guarantees the global convergence of the UAV to a desired circular orbit around a target. To estimate unknown constant background wind and target motion, an adaptive observer with bounded estimate is developed. Simulation results demonstrate the effectiveness of the proposed approach

Integral-type sliding mode fault-tolerant control for attitude stabilization of spacecraft

Two fault-tolerant control (FTC) schemes for spacecraft attitude stabilization with external disturbances are proposed in this brief. The approach is based on integral-type sliding mode control strategy to compensate for actuator faults without controller reconfiguration. First, a basic integral-type sliding mode FTC scheme is designed so that sliding manifold can be maintained from the very beginning. Once the system enters the sliding mode, the dynamics of the closed-loop system with actuator fault is identical to that of the nominal healthy system. Second, the integral-type sliding mode fault-tolerant controller is incorporated with adaptive technique to accommodate actuator faults so that the required boundary information can be relaxed. The effectiveness of the proposed schemes against actuator faults is demonstrated in simulation.Accepted versio

Self-Assembled BODIPY Nanoparticles for Near-Infrared Fluorescence Bioimaging

In vivo optical imaging is an important application value in disease diagnosis. However, near-infrared nanoprobes with excellent luminescent properties are still scarce. Herein, two boron–dipyrromethene (BODIPY) molecules (BDP-A and BDP-B) were designed and synthesized. The BODIPY emission was tuned to the near-infrared (NIR) region by regulating the electron-donating ability of the substituents on its core structure. In addition, the introduction of polyethylene glycol (PEG) chains on BODIPY enabled the formation of self-assembled nanoparticles (NPs) to form optical nanoprobes. The self-assembled BODIPY NPs present several advantages, including NIR emission, large Stokes shifts, and high fluorescence quantum efficiency, which can increase water dispersibility and signal-to-noise ratio to decrease the interference by the biological background fluorescence. The in vitro studies revealed that these NPs can enter tumor cells and illuminate the cytoplasm through fluorescence imaging. Then, BDP-B NPs were selected for use in vivo imaging due to their unique NIR emission. BDP-B was enriched in the tumor and effectively illuminated it via an enhanced penetrability and retention effect (EPR) after being injected into the tail vein of mice. The organic nanoparticles were metabolized through the liver and kidney. Thus, the BODIPY-based nanomicelles with NIR fluorescence emission provide an effective research basis for the development of optical nanoprobes in vivo