Journal of Control
Year:2010 | Volume:3 | Issue:4|Papers Count:8

In this paper, a study was done of the modeling and parameter identification of a rotary electrohydraulic servo system in presence of noise and structural uncertainties. The mathematical model accounted for all the system dynamics, apart from few trivial assumptions that were put together to alleviate the complexity of the expressions. The behavior of the spool dynamics in servo-valve is modeled with an appropriate 2nd-order transfer function. In fact, electro-hydraulic systems are known to be highly nonlinear and non differentiable due to many factors, such as leakage, friction, and especially, the fluid flow expression through the servo-valve. Then the system is written in linear parameters (LP) form and continuous-time least-squares estimation method is used to parameter identification of the system. Furthermore, the constant parameters of the valve can be identified using frequency response methods. In comparison with similar works, the experimental results present significant reduction in identification time. The method is validated with the nonlinear model of the system and substituting the procured parameters in the model.

In this paper, we present a new method for designing higher order sliding mode controller (HOSMC) with adaptive switching gain by defining a new PI sliding surface and employing a linear state feedback. The objective is to force the outputs of a nonlinear SISO system and their derivatives up to a certain order, track the states of a linear system with desired properties. The main property of proposed controller is that it does not need an upper bound for the uncertainty and moreover, the switching gain increases and decreases according to the system circumstances by employing an adaptation procedure. Then, chattering is removed completely by using the HOSMC with a small switching gain. Finally, we have used the proposed method to control and synchronize of chaotic uncertain systems.

Flexible behaviors in new aerospace structures can lead to a degradation of their control and guidance system and undesired perfomance. The objectives in the current work are to analyze the vibration resulting from the propulsion force on a Flexible Launch Vehicle (FLV) modeled as a follower force on a free-free beam with proportional damping, study its effects on the oscillation of the actuators, and develop an approach to reduce these oscillations. To pursue these objectives, the stability of the beam model is first studied using the Ritz method. It was determined that the proportional damping consisting of those of internal (material) and external (viscous fluid) result in a change in the critical follower force. The rigid dynamics of a FLV in the pitch channel was then modeled and modified using the vibrational model of the device for the same channel. A new dynamic model and an adaptive control system for the FLV was then developed, allowing the aerospace structure to run on its maximum bearable propulsion force with the optimum effects on the oscillation of its actuators. Simulation results show that such a control model provides an effective way to reduce the undesirable oscillations of the actuators.

In this paper, an active approach for designing Fault Tolerant Controller (FTC) is proposed. This aprroach utilizes the idea of dynamical observer for simultaneous estimation of system states and faults. The main advantage of the dynamical observer is looking at the simultaneous estimation of system states and faults purely as a robust control problem. In this proposed approach, the controller has a fixed structure and there is no need to reconfiguration of the controller to accommodate or compensate the effect of the faults occurred in the system which makes the proposed approach practical for real systems. The control law is a linear function of the system states and estimated faults and is designed to keep the closed loop system asymptotically stable and the performance of the closed loop system in an acceptable level in the presence of faults and disturbances. A constructive algorithm based on Linear Matrix Inequality (LMI) is presented for FTC design. The merit of the proposed control scheme has been verified by the simulation on the four-tank process subjected to the actuator faults.

This paper is concerned with bounded real criterion for linear neutral delay systems. Two new delay-dependent bounded real lemmas (BRLs) are obtained in this paper, in which, Lyapunov theory is used to derive the first delay-dependent representation for BRL. Using a descriptor model transformation of the system and a new Lyapunov-Krasovskii functional, a less conservative bounded real lemma is obtained compared to that of the first BRL. Then sufficient conditions for the system to possess an H¥-norm less than a prescribed level, is given in terms of a linear matrix inequality (LMI). The significant advantage of the derived bounded real lemmas is their efficiency in designing H¥ controller for the closed-loop neutral systems when delayed term coefficients depend on the controller parameters. Numerical examples are given which illustrate the effectiveness of our proposed BRLs.

In this paper a new method for compensating time delay is presented to improve the robustness of Smith method relative to the model error. This method is based on the dominant gain concept. In this method by knowing of the maximum error and use of the dominant gain concept for an added function, open loop control system is regulated, so that all of the zeros in the open loop, arising from the delay parameter are located in the left half plane. In this way the nonminimum phase characteristic of the open loop will become eliminated. Based on the dominant gain concept, the requirement of the method is that the gain of the added function becomes equal or higher compared to the other gains, at all or a wide high frequency range. The gain of the added function is determined so that the open loop transfer function phase is limited between zero and -180 degrees. This constraint is put to guarantee that no right half plane zero exists in the open loop transfer function.

An output feedback adaptive fuzzy model following controller is proposed for a class of MIMO nonlinear uncertain systems. The unknown nonlinear functions are approximated by fuzzy systems based on universal approximation theorem, where both the premise and the consequent parts of the fuzzy rules are tuned with adaptive schemes. Thus prior knowledge and the number of fuzzy rules for designing fuzzy systems are decreased effectively. In practical situations the states of the nonlinear systems are fully or partially not known, the proposed approach does not need the availability of the states and uses an observer to estimate the states. To cope with fuzzy approximation error and external disturbances an adaptive discontinuous structure is used to make the controller more robust, while due to adaptive mechanism attenuates chattering effectively. All the adaptive gains are derived via Lyapunov approach thus asymptotic stability of the closed-loop system is guaranteed. The approach is applied to stabilize the Chen's chaotic system with uncertain dynamics and amid significant disturbances. Analysis of simulations reveals the effectiveness of the proposed method in terms of coping well with the uncertainties while maintaining asymptotic convergence.