Nabil G. Chalhoub

A Dynamic Model and a Robust Controller for a Fully Actuated Marine Surface Vessel

A nonlinear six degree-of-freedom dynamic model has been developed for a marine surface vessel. The formulation closely follows the existing literature on ship modeling. It accounts for the effects of inertial forces, wave excitations, retardation forces, nonlinear restoring forces, wind and current loads. The model is used herein to predict the response of a ship in a turning-circle maneuver. Furthermore, a nonlinear robust controller has been designed based on a reduced-order version of the ship model, which only takes into consideration the surge, sway and yaw motions. The controller is formulated by implementing the sliding mode methodology. It considers the ship to be fully actuated. The simulation results, generated based on the reduced-order model of the ship, illustrate the robust performance and the good tracking characteristic of the controller in the presence of significant modeling uncertainties and environmental disturbances.

Dynamic analysis of a composite-material flexible robot arm

Abstract Increased demands for higher productivity and improved quality of goods have required industrial robots to operate at high speed with greater precision. To meet these demands, robots should be lightweight, quick, and accurate. In this study these requirements are satisfied by inclusion of structural flexibility in the dynamic model of the robotic manipulator and implementation of advanced composite materials in the structural design. The focus of this study is a three-dimensional, revolute, compositematerial robot arm. A displacement finite element dynamic model is employed which includes all the coupling terms between the rigid and flexible motions and takes into consideration the axial, in-plane, and out-of-plane transverse deflections. The material damping of the laminated flexible link in both transverse directions is considered. The digital simulation results clearly demonstrate the advantage of incorporating advanced composite materials in the structural design of robotic manipulators. The effect of flexible motion on the rigid body motion is proven to be very important. It is also shown that there is a significant difference between the behavior of the geometrically linear and nonlinear models. Effects of fiber orientation and material orthotropy on the bending stress and displacements are also assessed. It is demonstrated that the inclusion of material damping in the dynamic model is an important factor in the design of flexible robot arms made of advanced composite materials.

Dynamic modeling of a revolute-prismatic flexible robot arm fabricated from advanced composite materials

A dynamic model for a two degree-of-freedom planar robot arm is derived in this study. The links of the arm, connected to prismatic and revolute joints, are considered to be flexible. They are assumed to be fabricated from either aluminum or laminated composite materials. The model is derived based on the Timoshenko beam theory in order to account for the rotary inertia and shear deformation. These effects are significant in modeling flexible links connected to prismatic joints. The deflections of the links are approximated by using a shear-deformable beam finite element. Hamiltons principle is implemented to derive the equations describing the combined rigid and flexible motions of the arm. The resulting equations are coupled and highly nonlinear. In view of the large number of equations involved and their geometric nonlinearity (topological and quadratic), the solution of the equations of motion is obtained numerically by using a stiff integrator.The digital simulation studies examine the interaction between the flexible and the rigid body motions of the robot arm, investigate the improvement in the accuracy of the model by considering the flexibility of all rather than some of the links of the arm, assess the significance of the rotary inertia and shear deformation, and illustrate the advantages of using advanced composites in the structural design of robotic manipulators.

Design of a robust nonlinear observer for constrained systems

In control applications, a full knowledge of the state variables is generally required for the computation of the control signals. This often translates into using an observer to estimate the state variables that are not readily available through measurements. The focus of this work is to develop a nonlinear robust observer for constrained systems whose dynamics are governed by a set of highly nonlinear differential-algebraic (D-A) equations. For fairly complicated and nonlinear constraint equations, the substitution method is not feasible to eliminate the superfluous coordinates. Therefore, the D-A form of the equations of motion of the system has to be dealt with in the design of the observer. The current study presents a general procedure for developing a robust nonlinear observer capable of accurately estimating all the state variables of a constrained system, including the superfluous ones. To assess the performance of the proposed observer, the multi-body dynamics of a piston/connecting-rod/crankshaft mechanism for a single cylinder internal combustion engine is considered. The equations of motion account for both the rigid and flexible motions of the crank-slider mechanism. The simulation results illustrate the capability of the proposed observer in accurately estimating all the state variables of the system. They demonstrate the robustness of the observer to modeling uncertainties and external disturbances. Moreover, the estimated state variables are shown to satisfy the nominal constraint equations.

Guidance and control scheme for under-actuated marine surface vessels

A guidance and control approach is proposed in the present work for autonomous under-actuated marine surface vessels. The dynamic model considers the six rigid body degrees of freedom of the ship and one degree-of-freedom for the rudder dynamics. The formulation accounts for the effects of coriolis and centripetal accelerations, retardation forces, wave excitations forces, linear damping forces, nonlinear restoring forces, wind and current loads. The guidance scheme is based on the concepts of the variable radius line-of-sight (LOS) and the acceptance radius. It can handle large cross-track error while aiding the controller to quickly converge the ship to its desired trajectory. The control strategy consists of a two-layer controller, which is designed based on the sliding mode methodology. The simulation results demonstrate the robust performance of the integrated guidance and control system in spite of significant modeling uncertainties and environmental disturbances.

Integrated controller-observer system for marine surface vessels

A robust nonlinear controller has been designed to control the surge speed and the heading angle of a marine surface vessel. The control actions are carried out through the propeller and the rudder. Moreover, a nonlinear observer has been devised to accurately estimate the surge speed and the yaw angle and their time derivatives. Both the controller and the observer are designed based on a reduced-order model of the ship. However, their performances have been assessed on a six degree-of-freedom ship model, which accounts for the wave excitation, retardation forces, nonlinear restoring forces, wind and sea-current resistive loads. Furthermore, the model accounts for the physical limitations of both the rudder and the ship propulsion system. The simulation results demonstrate the capability of the integrated controller-observer system in providing a good tracking characteristic of the ship in spite of significant modeling imprecision and environmental disturbances.

Development of a Robust Nonlinear Observer for a Single-Link Flexible Manipulator

Accurate measurements of all the state variables of a given system are often not available due to the high cost of sensors, the lack of space to mount the transducers or the hostile environment in which the sensors must be located. The purpose of this study was to design a robust sliding mode observer that is capable of accurately estimating the state variables of the system in the presence of disturbances and model uncertainties. It should be emphasized that the proposed observer design can handle state equations expressed in the general form. The performance of the nonlinear observer is assessed herein by examining its capability of predicting the rigid and flexible motions of a compliant beam that is connected to a revolute joint. The simulation results demonstrate the ability of the observer in accurately estimating the state variables of the system in the presence of structured uncertainties and under different initial conditions between the observer and the plant. Moreover, they illustrate the deterioration in the performance of the observer when subjected to unstructured uncertainties of the system. Furthermore, the nonlinear observer was successfully implemented to provide on-line estimates of the state variables for two model-based controllers. The simulation results show minimal deterioration in the closed-loop response of the system stemming from the usage of estimated rather than exact state variables in the computation of the control signals.

Dynamic modeling of a laminated composite-material flexible robot arm made of short beams

High-precision assembly tasks are often performed by compact industrial robots with small work envelopes. The end-effector positional accuracy of compact robots can be accurately predicted by considering the links of the robot arm as short beams. In this study, a general procedure to derive a dynamic model for a revolute flexible robot arm, which takes into consideration the rotary inertia and shear deformation effects, is presented. Only the last link of the arm is considered to be flexible and assumed to be fabri cated from laminated composite materials. Hamiltons principle is used to derive the equations of motion. A displacement finite element model based on the Timoshenko beam theory is implemented to approximate the solution. The digital simulation studies predict the deflections at the end effector and examine the combined effects of rotary inertia and shear deformation. Further more, the improvement in the dynamic response of the robot arm resulting from the fabrication of the manipula tor from laminated composite materials is demonstrated.

Enhanced structural controllers for non-collocated systems

The present work investigates the adverse effects of non-collocated sensors and actuators on the phase characteristics of flexible structures and the ensuing implications on the performance of structural controllers. Two closed-loop systems are considered. The first one consists of a pinned-free flexible beam with the control torque applied at the pinned-end. The second one is a clamped-free deformable beam with the control moment generated by two piezoelectric actuators bonded at the top and bottom surfaces near the clamped-end. The phase angle contours for both systems were generated as functions of the normalized sensor location and the excitation frequency. They illustrate the loci of the imaginary open-loop zeros along with the resulting minimum and non-minimum phase regions of the systems as the sensors sweep the entire span of the beams. Two structural controllers are designed based on the sliding mode methodology and the active damping control strategy to attenuate the undesired in-plane transverse deformation of the pinned-free beam. The results have revealed three distinct regions for the sensor’s location whereby the performance of the sliding mode controller can be stable, unstable, or stable after incorporating a remedial action into the control algorithm based on the phase angle contour information of the open-loop system. On the other hand, the active damping controller eliminated the overall in-plane transverse deformation by both active damping and having the first two elastic modes being equal in magnitude and opposite in sign. The dissipative nature of this controller and the dependency of its gains on the mode shapes of the beam have yielded a robust and stable performance of the closed-loop system irrespective of the sensor location.

A Robust Self-Tuning Fuzzy Sliding Mode Observer

A nonlinear robust observer has been developed by combining the advantages of the variable structure systems (VSS) theory with those of the self-tuning fuzzy logic algorithm. The observer does not require an exact knowledge of the system dynamics or the construction of a rule-based expert fuzzy inference system. Instead, it requires the upper bounds on the modeling imprecision to be known. The observer design involves the derivation of inequality conditions that must be satisfied by the tuning parameters in order to ensure the convergence of the estimation process.The observer has been applied to estimate the state variables of an under-actuated marine vessel. The simulation results demonstrated the capabilities of the observer in providing accurate estimates of the state variables in spite of considerable modeling imprecision and significant environmental disturbances.Copyright