Mode Control Element Research Articles

Spacecraft attitude control based on generalised dynamic inversion with adaptive neural network

AbstractThis paper proposes a robust generalised dynamic inversion (GDI) control system design with adaptive neural network (NN) estimation for spacecraft attitude tracking under the absence of knowledge of the spacecraft inertia parameters. The robust GDI control system works to enforce attitude tracking, and the adaptive NN augmentation compensates for the lack of knowledge of the spacecraft inertia parameters. The baseline GDI control law consists of a particular part and an auxiliary part. The particular part of the GDI control law works to realise a desired attitude dynamics of the spacecraft, and the auxiliary part works for finite-time stabilisation of the spacecraft angular velocity. Robustness against modeling uncertainties and external disturbances is provided by augmenting a siding mode control element within the particular part of the GDI control law. The singularity that accompanies GDI control is avoided by modifying the Moore-Penrose generalised inverse by means of a dynamic scaling factor. The NN weighting matrices are updated adaptively through a control Lyapunov function. A detailed stability analysis shows that the closed loop system is semi-global practically stable. For performance assessment, a spacecraft model is developed, and GDI-NN control is investigated for its attitude control problem through numerical simulations. Simulation results reveal the efficacy, robustness and adaptive attributes of proposed GDI-NN control for its application to spacecraft attitude control.

An enhanced distributed control-theoretic time synchronization protocol using sliding mode control for wireless sensor and actuator network

<span>Time synchronization is very important in a wireless sensor and actuator network (WSAN) as it provides a common time notation to the WSAN. To handle the time synchronization issue, this paper presented an enhanced distributed control-theoretic time synchronization protocol for wireless sensor and actuator networks, named Time Synchronization using Distributed Observer algorithm with Sliding mode control element (TSDOS). In this protocol, an augmented sliding mode control element is used for robustification purpose. From the theoretical convergence analysis and simulation results presented in this paper, it proved that the proposed TSDOS is the best if compared to another two algorithms from the literature in terms of cumulative integral absolute error.</span>

High power two-color orbital angular momentum beam generation using vertical external cavity surface emitting lasers

We report the design and experimental results for a two-chip T-cavity vertical external cavity surface emitting laser utilized for two-color collinear generation of Hermite-Gaussian and Laguerre-Gaussian (LG) transverse modes. A combination of intracavity mode-control elements and an external astigmatic mode converter was used to achieve high power LG modes. By incorporating intracavity birefringent filters in each arm of the T-cavity, wide wavelength tuning in excess of 12 nm of each mode is demonstrated. Output power exceeding 1.5 W is measured for all the modes.

Generation of high-power spatially structured beams using vertical external cavity surface emitting lasers.

In this paper, we demonstrate the generation of high-power and spatially structured beams using vertical external cavity surface emitting lasers (VECSEL). At the fundamental wavelength, an intracavity mode-control element is first employed to generate a range of Hermite-Gaussian (HG) modes in a linear cavity. The same HG modes are then excited and frequency doubled in a V-cavity geometry to generate a rich variety of high-power spatially structured beams. The results compare well with our numerical modeling.

Robust Generalized Dynamic Inversion Quadrotor Control

This paper presents the two-loops Robust Generalized Dynamic Inversion (RGDI) Quadrotor control system design. The outer loop of the system provides pitch and roll attitude commands to the inner loop, which in turns generates the tilting angles that are required to control the quadrotor position in the horizontal inertial plane in addition to the required reaction torque and thrust for yaw attitude and altitude tracking, respectively. The generalized inversion singularity is avoided by augmenting dynamic scaling factors within the Moore-Penrose Generalized Inverses (MPGIs) in both loops, resulting in stable generalized inversions and position/attitude tracking. Additionally, Sliding Mode Control elements are augmented in both loops to robustify the system against instability and tracking performance deterioration due to dynamic scaling and parametric uncertainties. Numerical simulations are conducted on a six DOFs Quadrotor mathematical model under nominal and perturbed flight conditions, demonstrating the efficacy of RGDI Control in Quadrotor control applications.

Robust generalized dynamic inversion based control of autonomous underwater vehicles

A novel two-loop structured robust generalized dynamic inversion–based control system is proposed for autonomous underwater vehicles. The outer (position) loop of the generalized dynamic inversion control system utilizes proportional-derivative control of the autonomous underwater vehicle’s inertial position errors from the desired inertial position trajectories, and it provides the reference yaw and pitch attitude angle commands to the inner loop. The inner (attitude) loop utilizes generalized dynamic inversion control of a prescribed asymptotically stable dynamics of the attitude angle errors from their reference values, and it provides the required control surface deflections such that the desired inertial position trajectories of the vehicle are tracked. The dynamic inversion singularity is avoided by augmenting a dynamic scaling factor within the Moore–Penrose generalized inverse in the particular part of the generalized dynamic inversion control law. The involved null control vector in the auxiliary part of the generalized dynamic inversion control law is constructed to be linear in the pitch and yaw angular velocities, and the proportionality gain matrix is designed to guarantee global closed-loop asymptotic stability of the vehicle’s angular velocity dynamics. An additional sliding mode control element is included in the particular part of the generalized dynamic inversion control system, and it works to robustify the closed-loop system against tracking performance deterioration due to generalized inversion scaling, such that semi-global practically stable attitude tracking is guaranteed. A detailed six degrees-of-freedom mathematical model of the Monterey Bay Aquarium Research Institute autonomous underwater vehicle is used to evaluate the control system design, and numerical simulations are conducted to demonstrate closed-loop system performance under various types of autonomous underwater vehicle maneuvers, under both nominal and perturbed autonomous underwater vehicle system’s mathematical model parameters.

Optimized irregular structures for spatial- and temporal-field transformation

In a bounded region such as a waveguide, a mode is an eigensolution of the electromagnetic-wave equation with particular boundary conditions imposed. Conversion from one mode to another through a mode converter could be accomplished by using a scatterer located within the waveguide. Generalizing to an arbitrary domain, either within a waveguide or in unbounded media, a field transformer converts one spatial field to another as a function of frequency. Mode-control elements usually employ periodic structures and a special case of this mode conversion is filtering (amplitude and phase control) with the same input and output mode. We have developed a new kind of field transformer which uses aperiodic structures. Specific designs are arrived at through numerical optimization of a cost function representing the mode transformation. Rather than effecting field transformations using a series of small geometry perturbations, our concept forcefully changes the field using an optimized structure. The optimization process allows consideration of a number of electrical and mechanical issues, such as efficiency, bandwidth, size, and amenability to manufacture. We present designs for microwave mode converters using our optimized irregular structure concept and compare them with those achieved using a periodic coupled-mode concept. These designs show dramatic improvements in performance and physical size, while incorporating a dimensional constraint and sensitivity analysis that provides for ease in fabrication.

Enhanced single transverse mode oscillation by an intracavity circular hollow glass waveguide

We present in this paper a theoretical and experimental investigation of laser transverse mode control in a stable resonator using a circular hollow glass waveguide (i.e. a capillary) as mode control element. The resonator round trip losses for linearly polarized hybrid hollow glass waveguide eigenmodes, EH 1 m , are calculated and they are compared with those of eigenmodes, TEM 0 m , of a laser resonator using an intracavity circular aperture. Theoretical analysis shows that a capillary resonator has the following two advantages over an aperture adapted resonator: (a) low loss for fundamental transverse mode and at the same time, (b) high degree of mode selection. Thus the use of a capillary in a resonator is expected to enhance the oscillation of the fundamental transverse mode and to quench higher-order modes. The theoretical results are supported by the experiment performed both with a pulsed dye laser and with a Q-switched Nd: YAG laser, as the output powers of the fundamental transverse mode for the liquid and solid lasers are all enhanced due to the replacement of an intracavity pinhole with an intracavity capillary. The experimentally determined optimal waveguide parameters are also in good agreement with our theory.