Lotfi Beji

Morphology and photoluminescence studies of electrochemically etched heavily doped p-type GaAs in HF solution

Abstract Porous GaAs layers have been produced by electrochemical anodic etching of (100) heavily doped p-type GaAs substrate in HF solution. Scanning electron microscopy revealed the presence of etch pits ranging in size from 0.01 to 2 μm and they were strongly dependent on the electrochemical etching conditions. The etch pits chemical composition consists of O, Ga and As whereas the porous structure consists predominantly of GaAs as performed by energy dispersive X-ray analyzer. Typical porous structure with pores diameter ranging from 15 to 50 nm has been obtained. Room temperature photoluminescence (PL) investigations reveal the presence of two and in one case three PL bands besides the PL band of the started GaAs. Peaks wavelengths positions were approximately located in 600–900 nm range. The PL bands peaks wavelengths positions depend on the electrochemical etching conditions and they were approximately unchanged with increasing temperature. However, their PL intensity increased slowly with increasing temperature and tend to saturate. The observed PL bands were explained by the quantum confinement effects in GaAs nanocrystallites.

Tracking control of trim trajectories of a blimp for ascent and descent flight manoeuvres

A blimp is a small airship that has no metal framework and collapses when deflated. It belongs to family of unmanned aerial vehicles (UAVs). In this paper we address the problem of designing tracking feedback control of an underactuated autonomous UAV. The ascent and descent flight conditions as one in which the rate of change (of magnitude) of the airships state vector is zero and the resultant of the applied forces and moments is constant lead to trimmed flight trajectories. The subject of the tracking control is to stabilize the engine around the planned flight. Using a combined integrator backstepping approach and Lyapunov theory, the stability results are local and overcome the minimum number of actuators (inputs) with respect to the blimps six degrees of freedom. Considering physic limits in UAVs, other trimmed flights are investigated and compared.

Smooth control of an X4 bidirectional rotors flying robot

A particular structure of a four rotors mini-flying robot where two rotors are directional (X4 bidirectional rotors) is presented in this paper. The two internal degree of freedom leads to a transformation between the equivalent system of the control-inputs and the rotor force-inputs which is not a diffeomorphism. This makes our system different from that of the classical flyer robot (X4 flyer). The dynamic model involves five control inputs which will be computed to stabilize the engine with predefined trajectories. A path-like flying road, described as straight-lines with rounded corners permits to prove the effectiveness of the proposed static feedback control law.

Trajectory Generation and Tracking of a Mini-Rotorcraft

We present in this paper the stabilization (tracking) with motion planning of the six independent configurations of a mini unmanned aerial vehicle equipped with four streamlined rotors. Naturally, the yaw-dynamic can be stabilized without difficulties and independently of other motions. The remaining dynamics are linearly approximated around a small roll and pitch angles. It will be shown that the system presents a flat output that is likely to be useful in the motion generation problem. The tracking feedback controller is based on receding horizon point to point steering. The resulting controller involves the lift (collective) time derivative for what flatness and feedback linearization are used. Simulation tests are performed to progress in a region with approximatively ten-meter-buildings.

Motion generation and adaptive control method of automated guided vehicles in road following

Dynamics of automated guided vehicles (AGVs) are described by a nonlinear nonholonomic model with two inputs: the rear axle torque and the steering angle torque. This model uses integrated longitudinal and lateral behavior. The first part of this paper is concerned with motion generation, taking into account kinodynamics and motors constraints. Usual kinematics constraints are not always sufficient to provide feasible trajectories; thus, we focus on velocity limitation and the motors current and slew rate constraints. Optimal velocity is determined for AGVs along a specified path with a known curvature. The main result concerns the realistic situation when the parameters of the model describing the movement of the vehicle are not well known. A nonlinear strategy is proposed to ensure control of the vehicle even if the knowledge of the AGVs constant parameters is not perfect. The proof of the main result is based on the Lyapunov concept and the proposed results are illustrated by simulations and some comments.

Stabilization of a nonlinear underactuated autonomous airship-a combined averaging and backstepping approach

A strategy to design a time-varying stabilizing controller of the position and the orientation of an underactuated autonomous airship is proposed. The dynamic modelling of the airship involves six equations with only three inputs. This airship cannot be stabilized to a point using continuous pure-state feedback law. However, the stabilization problem is solved with an explicit homogeneous time-varying control law, based on an averaging approach. We prove that the origin of the system is locally exponentially stable.

Co-leaders and a flexible virtual structure based formation motion control

The motion control of multi-robots in formation using a Flexible Virtual Structure Approach (FVSA) is proposed. The dynamic model of n agents in formation is developed and sufficient conditions to the desired shapes stability over time are given. Inspired by a shepherd who supervises his troop by controlling the elements on the border, thus, he is able to control all the remainder of the troop. To control the formation shape, one defines control laws for co-leaders, selected from the border, which permits to control motions of the remaining formation agents. The strategy depend strongly on two objectives, on one hand performing an obstacles free motion and on the other avoiding collision among the agents. The Lyapunov technique is used to construct the control law ensuring obstacles avoidance for the agents on the border.

Tracking control of a parallel robot in the task space

The tracking control problem of a parallel robot including the electrical actuator dynamics is addressed in the task space. For the electrically-actuated robots, we design a nonlinear control law for armature input voltages. The control technique consists of a cartesian tracking control and a force convergent control. The model obtained is in standard form to allow the application of singular perturbation methods. To validate the proposed corrective controller, a passivity concept and singular perturbation techniques are combined successfully. Simulation results show a good behavior of the proposed task space tracking controller.

Stabilization of stick-slip oscillations Integrating fluid injection in oilwell drillstring system

To address security issues in oilwell drillstring system, the drilling operation handling which is in generally not autonomous but ensured by an operator may be drill bit destructive or fatal for the machine. In drilling process, the drillstring control at the right speed is not sufficient and a rate of fluid mud is necessary. This paper studies coupling between torsional vibrations, as a wave equation, the drillstring slider displacement, and the fluid flow distribution. The design consists to couple the fluid behavior ODE and the drillstring torsional vibration phenomena PDE. In presence of damping in the wave equation, we show only a PDE-ODE system stability. Further, investigation of the wave equation without damping term coupled with the fluid dynamics is reduced to a difference equation, and the designed controller leads to an asymptotic stability of the torsional variable, the downhole pressure, and the annulus pressure. Simulation results are presented to illustrate the PDE-ODE coupled system stability.

Bottomhole pressure stabilizing observer-based controller in tunnel drilling system

Managed Pressure Drilling (MPD) in oil-gas industry remains a challenge problem. As the annulus is completely clogged, upstream choke pressure measurement can be used to control the well downhole pressure. To reinforce the tunnel face, MPD becomes difficult as the annulus is totally open. In this paper, we propose a finite dimensional model to all fluid phenomena between the main pump till the fluid outlet through the annulus. The translational dynamic of the sonic-head carrying the drillstring is coupled to mud dynamics. We construct a stabilizing observer-based controller to the bottomhole pressure in analyzing all the system variables. From the perturbation theory, we show that the perturbed system is stable at the origin. The theoretical results are supported by simulation.