François Léonard

Dynamic Modelling and Stability Analysis of Model-Scale Helicopters Under Wind Gust

The purpose of this paper is to present the stabilization (tracking) with motion planning of a reduced-order model having 3 Degrees Of Freedom (DOF). This last one represents a scale model helicopter mounted on an experimental platform. It deals with the problem of disturbance reconstruction acting on the autonomous helicopter, the disturbance consists in vertical wind gusts. The objective is to compensate these disturbances and to improve the performances of the control. Consequently, a nonlinear simple model with 3DOF of a helicopter with unknown disturbances is used. Three approaches of robust control are compared via simulations: a robust nonlinear feedback control, an active disturbance rejection control based on a nonlinear extended state observer and backstepping control.

Robust nonlinear control and stability analysis of a 7DOF model-scale helicopter under vertical wind gust

The helicopter manoeuvres naturally in an environment where the execution of the task can easily be affected by atmospheric turbulences, which lead to variations of its model parameters. The originality of this work relies on the nature of the disturbances acting on the helicopter and the way to compensate them. The disturbance consists in vertical wind gusts. Here, a 7DOF nonlinear Lagrangian model of VARIO helicopter with unknown disturbances is used. This model presents quite interesting control challenges due to nonlinearities, aerodynamical forces, underactuation and its dynamics are non-minmum phase. Two approaches of robust control are compared via simulations: an approximate feedback linearization and an active disturbance rejection control (ADRC) using the approximate feedback linearization procedure. Several simulations show that adding the observer can compensate the effect of disturbances.

Experimental external force estimation using a non-linear observer for 6 axes flexible-joint industrial manipulators

This paper proposes a non-linear observer to estimate not only the state (position and velocity) of links but also the external forces exerted by the robot during Friction Stir Welding (FSW) processes. The difficulty of performing this process with a robot lies in its lack of rigidity. In order to ensure a better tracking performance, the data such as real positions, velocities of links and external forces are required. However, those variations are not always measured in most industrial robots. Therefore, in this study, an observer is proposed to reconstruct those necessary parameters by using only measurements of motor side. The proposed observer is carried out on a 6 DOF flexible-joint industrial manipulator used in a FSW process.

First-order optimal reduced-delay sample-data holds

A data-holding scheme whose phase delay is significantly less than that of the usual zero-order hold (ZOH) is presented. The improvement is achieved by means of simple additional software, and a standard ZOH device is employed.

Non-Linear Observer-Based Control of Flexible-Joint Manipulators Used in Machine Processing

This paper proposes to use a non-linear observer to build the state and the external force of flexible manipulator robots during their machining (composite materials) processes or Friction Stir Welding (FSW) processes. These two different processes have a problem in common: the flexibility of the robot can not be neglected, that is to say, the errors due to the deformation of the links should be taken into account. However, in most industrial robots, the real positions and velocities of each link are not measured, so in this study, an observer is proposed to reconstruct the real angular positions and velocities of links by using the measured angular positions and the velocities of actuators. A simulation by Matlab/Simulink has been carried out with a 2 axis Robot during its machining processes: the proposed observer showed great performances in estimating the state of the robot (position and velocity). Then, in order to improve the tracking accuracy in the tool frame, the state of the external force along the forward direction (x) and its normal direction (y) are required, while they are also not measured by our robot. A disturbance observer has been added to reconstruct the processing force. A good precision during the proposed processes have been obtained using the latter. This study contributes to solve the problem from the point of view of accuracies during the machining processes.Copyright

Robustness and Safe Sampling of Distributed-Delay Control Laws for Unstable Delayed Systems

In the control of delayed systems by a finite spectrum assignment (FSA) , in the control law, the integral over the time delay of a function of past control appears. This assignment is in fact available for continuous delayed process independently of the stability of the latter, which is very interesting since Smith predictor is usually only used with stable processes. Nevertheless, in case of FSA control implementation, this integral control should be sampled so that spectrum assignment is not necessarily preserved and an unstable discrete closed loop can be obtained , . In this technical note, FSA integral control robustness with respect to prediction time uncertainty is analyzed for an unstable continuous linear system. A transformation approach is also proposed to understand the effects of different ways of sampling control laws. In a last part, a case study shows how Simpson approximation of integral control law leads to an unstable digital closed loop.

Nonlinear Discrete Observer for Flexibility Compensation of Industrial Robots

Abstract This paper demonstrates the solutions of digital observer implementation for industrial applications. A nonlinear high-gain discrete observer is proposed to compensate the tracking error due to the flexibility of robot manipulators. The proposed discrete observer is obtained by using Euler approximate discretization of the continuous observer. A series of experimental validations have been carried out on a 6 DOF industrial manipulator during a Friction Stir Welding process. The results showed good performance of discrete observer and the observer based compensation has succeed to correct the positioning error in real-time implementation.

Robust and Active Trajectory Tracking for an Autonomous Helicopter under Wind Gust

High levels of agility, maneuverability and the capability of operating in degraded visual environments and adverse weather conditions are the new trends of helicopter design nowadays. Helicopter flight control system should make these performance requirements achievable by improving tracking performance and disturbance rejection capability. Robustness is one of the critical issues which must be considered in the control system design for such highperformance autonomous helicopter, since any mathematical helicopter model, especially those covering large flight envelope, will unavoidably have uncertainty due to the empirical representation of aerodynamic forces and moments. The purpose of this chapter is to present the stabilization (tracking) with motion planning of a reduced-order helicopter model having 3DOF (Degrees Of Freedom) (see Fig.1). This last one represents a scale model helicopter mounted on an experimental platform. It deals with the problem of disturbance reconstruction acting on the autonomous helicopter, the disturbance consists in vertical wind gusts. The objective is to compensate these disturbances and to improve the performances of the control. Consequently, a nonlinear simple model with 3DOF of a helicopter with unknown disturbances is used. Three approaches of robust control are then compared via simulations: a robust nonlinear feedback control, an active disturbance rejection control based on a nonlinear extended state observer and a backstepping control. Design of control of autonomous flying systems has now become a very challenging area of research, as shown by a large literature (Beji & Abichou, 2005) (Frazzoli et al., 2000) (Koo & Sastry, 1998). Many previous works focus on (linear and nonlinear, robust, ...) control, including a particular attention on the analysis of the stability (Mahony & Hamel, 2004), but very few works have been made on the influence of wind gusts acting on the flying system, whereas it is a crucial problem for out-door applications, especially in urban environment: as a matter of fact, if the autonomous flying system (especially when this system is relatively slight) crosses a crossroads, it can be disturbed by wind gusts and leave its trajectory, which could be critical in a highly dense urban context. In (Martini et al., 2005) and (Martini et al., 2007a), three controllers (nonlinear,

Improving Surface Roughness in Robotic Grinding Process

This paper presents an attempt to robotize the grinding process and to overcome grinding vibrations and chattering. The objective is to have a finished workpiece with a high quality of the final surface. In order to achieve that, we started by choosing the right strategy to grind the workpiece that has uneven initial surface. Then, a well-known model of the process is used in order to simulate the grinding of a metallic workpiece. The robot is supposed rigid and does not contribute in the flexibility of the system. The only flexibility that was taken into consideration is that of a pneumatic actuator used to control and reduce vibrations. Its dynamic behavior is approximated using a second degree transfer function.

Robotic Friction Stir Welding Path Planning with Deflection Compensation Using B-Splines

Robotic Friction Stir Welding (RFSW) is an innovative process which allows the joining of aluminum materials with robots. The main drawback of using robots to perform friction stir welding is that the end effector of the robots deviates in position and orientation during welding. This is due to the high forces induced by the process and the weak stiffness of the robots. Thus, the compensation of the deviations must be taken into account in an RFSW path planning. In this paper, a methodology based on B-splines curves is proposed to generate offline a welding path with the compensation of the elastic deformation of a robot in order to achieve a high quality welding of three-dimensional workpieces. The methodology is validated on a Kuka robot KR500-2MT which performs a sinusoidal welding path defined on a cylindrical surface. The experiment shows the effectiveness of the methodology and allows to reduce significantly (about \(88\%\)) the lateral end effector deviation.