Gabriel Abba

Modeling and robust control of winding systems for elastic webs

The objective is to control a web transport system with winder and unwinder for elastic material. A physical modeling of this plant is made based on the general laws of physics. For this type of control problem, it is extremely important to prevent the occurrence of web break or fold by decoupling the web tension and the web velocity. Due to the wide-range variation of the radius and inertia of the rollers the system dynamics change considerably during the winding/unwinding process. Different strategies for web tension control and linear transport velocity control are presented. First, an H/sub /spl infin// robust control strategy which reduces the coupling between tension and velocity, is compared to the decentralized control strategy with proportional integral derivative (PID) controllers commonly used in the industry. Second, an H/sub /spl infin// robust control strategy with varying gains is shown to render the control more robust to the radius variations. Then, a linear parameter varying (LPV) control strategy with smooth scheduling of controllers is synthesized for different operating points and compared to the previous methods. Finally, this LPV control and the H/sub /spl infin// robust control strategy with varying gains are combined to give the best results on an experimental setup, for the rejection of the disturbances introduced by velocity variations and for the robustness to radius and inertia changes.

Visual servoing of a 6-DOF manipulator for unknown 3-d profile following

This paper presents the visual servoing of a six degrees of freedom (6-DOF) manipulator for unknown three-dimensional profile following. The profile has an unknown curvature, but its cross section is known. The visual servoing keeps the transformation between a cross section of the profile and the camera constant with respect to 6 DOE The position of the profile with respect to only five degrees of freedom can be measured with the camera since the image does not provide position information along the profile. The kinematic model of the robot is used to reconstruct the displacement along the profile, i.e., the sixth degree of freedom, and allows to control the profile-following velocity. Experiments show good accuracy for positioning at a sampling rate of 50 Hz. Two control strategies are tested: proportional-integral control and generalized predictive control (GPC). The visual servoing exhibits better accuracy with the GPC in simulations and in real experiments on a 6-DOF manipulator due to the predictive property of the algorithm.

Robust Control of Web Transport Systems

Abstract A web transport system including an unwinder, a winder and a traction motor has been modeled from the laws of mechanics and identified by parameter optimization. During the winding process the radius and the inertia of the rollers change in a large scale. The effect of these changes is minimized by a gain scheduling approach using a particularity of the plant. The type of controller appearing classically in industrial systems is usually decentralized and using PID controllers. In this paper we present a multivariable H ∞ robust control with gain scheduling that improves significantly the performances concerning the disturbance rejection and decouples web tension and web velocity. These results are validated on an experimental setup. A µ -analysis shows a good robustness to the elasticity modulus uncertainty of the web.

Real-Time Trajectory Compensation in Robotic Friction Stir Welding Using State Estimators

This brief demonstrates a method of real-time motion control for robotic friction stir welding (FSW). For some manufacturing processes, the lack of stiffness of industrial manipulators can cause a lack of precision and is, thus, problematic. During the processes that require significant forces, this error becomes the primary source of defects. This brief provides significant improvements using digital estimators. A compensator based on a discrete-time nonlinear observer and two other compensators that use only motor current and position measurements are proposed to compensate for the tracking error due to the deflection of the robot. Simulations and experiments on an industrial robot show the effectiveness of the three proposed compensators, which successfully attenuate the dynamic error in the case of a 2-D FSW process. Our adapted compensators provide accurate performance (~90% error reduction) for a robotized FSW welding setup.

Robust Nonlinear Control of a 7 DOF Model-Scale Helicopter Under Wind Gusts Using Disturbance Observers

Nowadays, high levels of agility, maneuverability and capability of operating in reduced visual environments and adverse weather conditions are the new trends of helicopter design. Helicopter flight control systems 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 domains, will unavoidably have uncertainty due to the empirical representation of aerodynamic forces and moments. Recently the control problems of unmanned scale helicopter have been attracted extensively attention of control researchers. As the helicopter can hover, it is used to implement many important flight missions such as rescue, surveillance, security operation, traffic monitoring, etc. However, helicopter, which is difficult to hover, is more complicated than other familiar control objects. Helicopter is dynamic unstable when it flights in hover mode at nearly zero forward speed. Moreover, the helicopter is open-loop unstable and most mathematical model contain a moderate-high degree of uncertainty models associated with neglected dynamics and poorly understood aeromechanical couplings. Therefore, it is very important to design a stable controller for unmanned helicopter. Many previous works focus on (linear and nonlinear, robust, ...) control (Beji and Abichou, 2005) (Frazzoli et al., 2000) (Koo and Sastry, 1998), including a particular attention on the analysis of the stability (Mahony and 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., 2008), thw controllers (an approximate feedback control AFLC and approximate disturbance observer AADRC) are designed for a nonlinear model of a 7 DOF helicopter using in its approximate minimum phase model. In (Pflimlin et al., 2004), a 3

Controlled Periodic Motion in a Nonlinear System with Impulse Effects: Walking of a Biped Robot

Abstract The goal is to demonstrate a means to prove asymptotically stable walking in a planar, under actuated, five-link biped robot model. The analysis assumes a rigid contact model when the swing leg impacts the ground and an instantaneous double-support phase: under theses hypotheses, the robot is modeled by a dynamic nonlinear system and an impulse model. The controller induces finite-time stabilization of four of the robots five degrees of freedom, resulting in a reduced Poincare stability analysis that can be carried out by computing a one dimensional map.

Optimal walking of an underactuated planar biped with segmented torso

Recently, underactuated bipeds with pointed feet have been studied to achieve dynamic and energy efficient robot walking patterns. However, these studies usually simplify a robot torso as one link, which is different from a human torsos containing 33 vertebrae. In this paper, therefore, we study the optimal walking of a 6-link planar biped with a segmented torso derived from its 5-link counterpart while ensuring that two models are equivalent when the additional torso joint is locked. For the walking, we suppose that each step is composed of a single support phase and an instantaneous double support phase, and two phases are connected by a plastic impact mapping. In addition, the controlled outputs named symmetry outputs capable of generating exponentially stable orbits using hybrid zero dynamics, are adopted to improve physical interpretation. The desired outputs are parameterized by Bézier functions, with 5-link robot having 16 parameters to optimize and 6-link robot having 19 parameters. According to our energy criterion, the segmented torso structure may reduce energy consumption up to 8% in bipedal walking, and the maximum energy saving is achieved at high walking speeds, while leaving the criteria at low walking speeds remain similar for both robots.

Influence of the Wind Load in the Trolley-Payload System with a Flexible Hoist Rope

The anti-sway controllers are widely discussed due to the increasing requirements of crane automation in seaports. In this paper, the dynamic model of the trolley-payload pendulum system is put forward considering the flexibility and damping of the hoist rope as well as the wind load as the external excitation. As indicated from the simulation, the wind load increases both the static and fluctuating part of the response of sway angle; the flexibility of the hoist rope cannot be ignored especially near the destination of the final position of the payload. As inferred from the results, the sway angle is the main source of the position error of the payload in both horizontal and vertical direction.

Dynamic behavior of a trolley traction system with a flexible driving rope

The rope traction driven trolleys are widely used in modern port machineries especially on the quayside cranes since it has so many benefits compared with a self-driving system. However, the system is more complex for automatic control since: (1) the system has different mass distribution varies according to different load cases; (2) the ropes equivalent stiffness depends on the current position of the trolley. In this paper, kinemics and dynamic model of the trolley traction system considering the interaction between the trolley position and the stiffness of the system are put forward. Several numerical experiments based on the systems model considering the influence of trolley positions and payload weights are tested. The results indicate the great influence in stiffness and mass of the system are greatly changed during operation. A controller with scheduled gains adjusting gains in both position and velocity circle to the most suitable according to current payload weight and trolley position is introduced and tested. The behavior shows the effectiveness of the controller with scheduled gains.

Identification of physical parameters including ground model parameters of walking robot rabbit

Control in robotics needs more and more precise models of the mechanical parts of the structure and especially for a complex system such as a biped robot. An important but difficult aspect of this work is the modeling of the mechanical loss due to friction in the transmission chain from motor to axis. The loss of each part is defined as a sum of three terms, one constant, another depending only on speed and the last one depending on the torque transmitted to the part. The robot joint kinematic chain is modeled with three elements: the motor, the gearbox and a rotational joint at the leg. The results show good adequacy between measurement and simulation with the proposed identification method in comparison with a classic least square identification method.