Kamal Youcef-Toumi

A Time Delay Controller for Systems With Unknown Dynamics

This paper focuses on the control of systems with unknown dynamics and deals with the class of systems described by ¿=f(x,t)+h(x,t)+B(x,t)u+d(t) where h(x,t) and d(t) are unknown dynamics and unexpected disturbances, respectively. A new control method, Time Delay Control (TDC), is proposed for such systems. Under the assumption of accessibility to all the state variables and their derivatives, the TDC is characterized by a simple estimation technique of the effect of the uncertainties. This is accomplished using time delay. The control systems structure, stability and design issues are discussed for linear time-invariant and single-input- single-output systems. Finally, the control performance was evaluated through both simulations and experiments. The theoretical and experimental results indicate that this control method shows excellent robustness properties to unknown dynamics and disturbances.

Analysis and Design of a Direct-Drive Arm With a Five-Bar-Link Parallel Drive Mechanism

The direct-drive arm that has no gears between motors and their loads have several important advantages including no backlash, small friction, and high mechanical stiffness. The arm mechanism, however, becomes extremely massive, when each motor is directly attached to its joint along a serial linkage mechanism. The complicated dynamics resulting from varying inertia, interactions, and nonlinearities, is also more prominent than that of a robot with gears. This paper describes a lightweight arm mechanism with invariant and decoupled inertia characteristics. Instead of having motors at serial joints, a parallel drive mechanism with a closed-loop five bar linkage is utilized. The dynamic behavior of this mechanism is analyzed and the condition for the elimination of the interactions and nonlinearities in the mass properties is derived. The decoupled and invariant arm dynamics significantly reduces the complexity of controlling the direct-drive arm. In the latter half of the paper, a prototype robot developed on this basis is described. By using high torque brushless motors which were specially designed for the direct-drive robot, top speed and maximum acceleration were increased by an order-of-magnitude to about 10 m/s and 5 G, respectively.

Input/Output Linearization Using Time Delay Control

A control procedure that uses Time Delay Control to achieve input/output linearization of a class of nonlinear systems is presented. The control system is characterized by a simple algorithm and enhanced robustness properties in comparison with current control algorithms. The paper first reviews the fundamentals of input/output linearization. The use of Time Delay Control is then shown to result in an exact linear system for sufficiently small delay time. Modified controllers for systems with a low-pass filter are also investigated. Simulation results show that the algorithm works well with measurement noise. The controller is also tested on a single-link flexible arm to show the effectiveness of the simple algorithm in the control of complicated systems.

Impact and force control

Robot manipulators and drive systems can experience instability or poor control performance after impacting with an environment. The authors present an analytical model for impact which is experimentally validated step-by-step. Extensive simulations and experiments are conducted to explain impact phenomena for the case of a force feedback control of a single-axis drive system. The experimental tests were conducted on a manipulator drive system which consists of a motor, a transmission, a link, a force sensor, and a movable environment. The results are based on an energy method and presented concisely in dimensionless form. To this end, a small number of dimensionless groups are used to characterize the impact behavior through simulations and tests. It is shown that integral force compensation with velocity feedback improves force tracking and reject impacts. It is also revealed that impact response can be tuned by selecting a favourable dimensionless ratio of force to approach velocity.<<ETX>>

Design of Machines With Compliant Bodies for Biomimetic Locomotion in Liquid Environments

The aim of this work is to investigate alternative designs for machines intended for biomimetic locomotion in liquid environments. For this, structural compliance instead of discrete assemblies is used to achieve desired mechanism kinematics. We propose two models that describe the dynamics of special compliant mechanisms that can be used to achieve biomimetic locomotion in liquid environments. In addition, we describe the use of analytical solutions for mechanism design. Prototypes that implement the proposed compliant mechanisms are presented and their performance is measured by comparing their kinematic behavior and ultimate locomotion performance with the ones of real fish. This study shows that simpler, more robust mechanisms, as the ones described in this paper, can display comparable performance to existing designs.

Analysis of Linear Time Invariant Systems With Time Delay

Time Delay Control has recently been suggested as an alternative scheme for control of systems with unknown dynamics and unpredictable disturbances. The proposed control algorithm does neither require an explicit plant model nor depend on the estimation of specific plant parameters. Rather, it uses information in the recent past to directly estimate the unknown dynamics at any given instant, through time delay. In earlier papers, analysis and implementation of Time Delay Controller for nonlinear systems were discussed. This paper analyzes the continuous Time Delay Controller for a class of linear time-invariant (LTI) systems. LTI systems under Time Delay Control are described by LTI differential difference equations. Results of stability analysis are obtained using the properties of linear time-delayed systems. These results provide useful guidelines in design of the Time Delay Controller. The convergence of closed loop system error to zero for certain classes of inputs and disturbances when the system is stable is also established. A discussion is presented comparing Time Delay Control with Repetitive Control. Illustrative examples are included.

Coupling in piezoelectric tube scanners used in scanning probe microscopes

A new model for tube scanners used in scanning probe microscopes (SPM), and particularly in atomic force microscopes (AFM), is presented. The model captures the coupling between motion in different axes as well as a bending motion due to a supposedly pure extension of the tube. In addition, the effect of coupling on the AFM cantilever dynamics is presented in a revised version of our model in El Rifai, and Youcef-Toumi, (2000). It is shown that due to coupling, the bending mode becomes observable from the AFM cantilever deflection sensor output. This is contrary to the ideal uncoupled case. Consequently, to avoid exciting the first bending mode, a large reduction in feedback bandwidth is required (a factor of 35 for the commercial AFM under consideration). As a result, ringing might occur during scanning at relatively low scan speeds, few Hertz, which will introduce artifacts in the image. Furthermore, in scanning a 4 /spl mu/m step, an estimated change of 12% will result in the steady state probe-sample force between the top and bottom of the step. This adversely affects the scanner calibration for large heights, adding to the nonlinear sensitivity of the piezoelectric material. The results presented within are supported by experimental data.

Fabrication of large area nanostructured magnets by interferometric lithography

Patterned arrays of magnetic elements may be useful as media for high density magnetic storage applications. Interferometric lithography has been used to fabricate arrays of cobalt and nickel pillars with periods of 200 nm over areas of 5 cm/spl times/5 cm using a UV laser. This provides an economical and rapid method for manufacturing particle arrays.

Impact and Force Control: Modeling and Experiments

Robot manipulators and drive systems can experience instability or poor control performance after impacting with an environment. This paper presents an analytical model for impact which is experimentally validated step-by-step. Extensive simulations and experiments are conducted to explain impact phenomena for the case of a force feedback control of a drive system. The results are based on an energy method and presented concisely in dimensionless form. To this end, a small number of dimensionless groups are used to characterize the impact behavior through simulations and tests. The study shows that an integral force compensation with a velocity feedback improves force tracking and reject impacts

Dynamic Analysis and Control of High Speed and High Precision Active Magnetic Bearings

The successful operation of actively controlled magnetic bearings depends greatly on the electromechanical design and control system design. The function of the controller is to maintain bearing performance in the face of system dynamic variations and unpredictable disturbances. The plant considered here is the rotor and magnetic bearing assembly of a test apparatus. The plant dynamics consisting of actuator dynamics, rigid rotor dynamics and flexibility effects are described. Various components of the system are identified and their corresponding linearized theoretical models are validated experimentally