Variable Structure Systems Theory Research Articles

Robust autopilot design for ship steering in waves

Nowadays, autopilots have been used widely in ship steering, while the robustness of the autopilots designed on the basis of the adaptive control theory is need to be enhanced. This paper presents a theoretical frame of a robust autopilot using the theory of Variable Structure Systems (VSS). Having computed the wave forces and moments acting on a ship, a Variable Structure Control System (VSCS) is investigated for the autopilot of a ship steering in random sea. A kind of nonlinear switching plane and a modified control law are employed to remove the frequent back-and-forth steering. Simulations show that the proposed simple control system is robust to the modeling errors and changes of environment and speed, so it is worthy to be applied to real ship steering/control.

Variable structure robust flight control system for the F-14

In this paper, a flight control law for a simplified F-14 aircraft model is designed based on variable structure control (VSC) theory. For m-input, q-output linear uncertain systems (q<m), a VSC law is derived. For the derivation of the control law, a choice of a sliding surface is made so that the zero dynamics of the system are stable. The linear lateral and longitudinal dynamics of the F-14 are decoupled. This allows design of lateral and longitudinal flight controllers separately. An application of variable structure system (VSS) theory to control of lateral dynamics is presented. A control law is derived for the control of roll angle, lateral velocity, and yaw rate. The chosen sliding surface for the controller design is a linear function of the tracking error, its derivative, and the integral of the tracking error. Simulation results are presented to show that in the closed-loop system, precise bank angle trajectory tracking and regulation of lateral velocity and yaw rate can be accomplished in spite of aerodynamic parameter uncertainty.

Variable structure system theory based disturbance identification and its applications

Based on the VSS theory, this paper presents a new method of estimating the disturbance for minimum-phase (with respect to the relation between the disturbance and output ) continuous-time dynamical systems with arbitrary relative degrees. By using the transfer function method, first, the higher order filter of the disturbance is estimated, then, the lower order filter of the disturbance is estimated inductively. Eventually, the disturbance is estimated. The estimated disturbance is employed to construct a VSS-type state observer. Further, the estimated disturbance and the state observer are applied to a controller to place the desired stable poles and to cancel the disturbance. A design example and simulation results are presented to show the practicality and effectiveness of the proposed algorithm.

Experimental study of a power conditioning system using sliding mode control

The theory of variable structure systems with sliding mode control has been used to develop a power conditioning system. An experimental system has been developed, and digital simulation of both the power and control systems has been performed. The good agreement between the analytical study and the experimental and simulation results confirms the validity of the proposed control system.

Control of seismic-excited buildings using active variable stiffness systems

It has been demonstrated that active variable stiffness (AVS) systems may be effective for response control of buildings subjected to earthquake excitations. The applications of active variable stiffness systems involve nonlinear control in which control theories for linear systems are not applicable. Based on the theory of variable structure system (VSS) or sliding mode control (SMC), control methods are presented in this paper for applications of active variable stiffness systems to seismic-excited buildings. In addition to full-state feedback controllers, general static output feedback controllers as well as simple output feedback controllers using only collocated sensors are presented. The principle of active variable stiffness control is interpreted based on the concept of the dissipation of hysteretic energies. Simulation results indicate that the control methods presented are robust and the performance of static output feedback controllers is comparable to that of the fullstate feedback controllers. Simulation results further indicate that the active variable stiffness systems, using locking and unlocking devices, are effective in reducing the interstorey drifts of seismicexcited buildings. However, the floor acceleration of the building may increase significantly, depending on the structure design and earthquake excitation.

Robust tracking control of mechatronic arms

A robust tracking control scheme based on variable structure systems (VSS) theory is presented to cope with the uncertainties and parameter variations in mechatronic arm dynamics. A modification of VSS is used to remove its restrictions with regard to chattering and required control efforts. By blending VSS with a self-organizing controller (SOC), a sliding mode self-organizing controller (SLIMSOC)scheme has been developed. In this scheme, both control actions and performance evaluation are executed using the distance from the desired sliding surface and rate of approach to it. Comparisons are drawn and it is shown that the inherent robustness properties of variable structure systems are retained while the undesirable chatter motion of the sliding mode is eliminated. The results are illustrated by applications of SLIMSOC on a direct drive SCARA type of robot.

ADVANCED HVDC LINK CONTROL FOR DAMPING POWER SYSTEM OSCILLATIONS

ABSTRACT This paper presents a new approach to the design of an integrated AC/ DC system using variable structure systems theory based on the concepts of sliding mode control. The sliding mode control constrains the system motion to a state trajectory and provides greater robustness and insensitivity to plant parameter variations and is practically realisable. The paper highlights the designs of DC link current regulator and gamma modulation controllers for fast transient stability of an AC/ DC system.

Vibration Isolation and Reduction Based on Theory of Variable Structure Systems. 2nd Report. Multi-Degree-of-Freedom Vibration System.

By controlling spring stiffness, vibration reduction and isolation of a multi-degree-of-freedom mass-spring system are achieved in this study. The control algorithm of the spring stiffness is based on the variable structure systems (VSS) theory, The stiffness-controllable spring is achieved by using two straight bars that are hinged to the mass along a line perpendiculal to the direction of mass motion and an axial force is applied to these bars. Since the axial force causes a restoring force which is equivalent to that of the spring. the stiffness is controlled by the axial force. In numerical simulations and experiments, the settling time of the impulse response and the displacement transmissiblility are investigated. The results show that the proposed method is effective in suppressing shock motion and in isolating vibration transmitted from the floor.

Sliding Mode Control for Nonlinear and Hysteretic Structures

Control methods based on the theory of variable structure system or sliding mode control are presented for applications to seismically excited nonlinear and hysteretic civil engineering structures. These control methods are robust with respect to parametric uncertainties of the structure. The controllers have no adverse effect should the actuator be saturated due to unexpected extreme earthquakes. Emphasis is placed on continuous sliding mode control methods that do not have possible chattering effects. Static output feedback controllers using only the measured information from a limited number of sensors installed at strategic locations are also presented. When a controller is installed in each story unit of a building, a complete compensation of the structural response can be achieved and the response state vector can be reduced to zero. Among the contributions of this paper are the establishment of saturated controllers and controllers for static output feedback. The robustness of the control methods, the applications of the static output controllers, and the control effectiveness in case of actuator saturation are all demonstrated by numerical simulation results. Simulation results indicate that the performance of the sliding mode control methods is remarkable.

Hybrid control of seismic‐excited bridge structures

AbstractRecently, several hybrid protective systems have been explored for applications to seismic‐excited bridge structures. In particular, two types of aseismic hybrid protective systems have been shown to be quite effective: (i) rubber bearings and variable dampers (or actuators), and (ii) sliding bearings and actuators. In this paper, control methods are presented for these hybrid protective systems. The control methods are based on the theory of variable structure system (VSS) or sliding mode control (SMC). Emphasis is placed on the static (direct) output feedback controllers using only the information measured from a few sensors without an observer. Simulation results demonstrate that the control methods presented are robust with respect to system parametric uncertainties and the performance is quite remarkable. Sensitivity studies are conducted to evaluate the effectiveness of hybrid protective systems and passive sliding isolators for reducing the response of seismic‐excited bridge structures. The advantages of each protective system are demonstrated by simulation results for a wide range of earthquake intensities.

Decentralized Adaptive Control of a Directly Driven Hydraulic Manipulator: Part 1: Theory

A new approach to adaptive control of manipulators is presented in this paper. The proposed controller for each individual axis is of the model reference type, designed through the use of variable structure systems theory. A novel feature of the controller is the introduction of a series-parallel model of the model-following error. The use of this model ensures system stability even if the manipulator design parameters or payload bounds are exceeded. Chattering of the system, associated with variable structure systems, is eliminated by arranging for the control objective to be physically achievable.

Design of a perturbation estimator using the theory of variable-structure systems and its application to magnetic levitation systems

A perturbation estimator using the theory of variable structure systems is proposed to enhance the robustness of a pole-placement controller design. In its ideal form, the pole-placement design using feedback-linearization technique achieves a desired performance in nonlinear time-varying systems. However, its performance deteriorates rapidly with the presence of disturbance and parametric uncertainties, referred to as perturbation. The estimate generated by the proposed perturbation estimator is incorporated as an additional input to rectify the uncertainties in the nominal control model of the pole-placement design. The proposed scheme requires neither the measurement of the time derivative of the state vector nor the precise knowledge of system parameters, hut rather the bounds on system perturbation. Chatter and the adverse effects of conservative bounds on system perturbation, often encountered in conventional sliding-mode control (SMC), are alleviated for the controlled plant by the proposed scheme. The benefits of this scheme are demonstrated in this study practically on a magnetic levitation system and its performance is compared with that of the conventional SMC scheme. >

Subject index of volume 33

This paper presents the design of a new nonlinear expert controller for single-machine infinite-bus and multimachine power systems. The controller uses the concepts of fuzzy linguistic control for normal and abnormal operating conditions of the power system. A nonlinear supervisory controller based on variable-structure system theory is designed to augment the linguistic fuzzy controller for severe transient disturbances when there is a possibility of the stability being lost. This robust nonlinear expert controller is applied to a single-machine infinite-bus system and some other selected machines in a multimachine power system subject to a variety of transient disturbances. The damping performance of this controller is found to be significantly better than the traditional fuzzy and conventional power system stabilizers.

Systematic design and stability analysis of a fuzzy logic controller

Although the fuzzy logic controller has been adopted in many engineering applications, its performance is not guaranteed since there is no definite stability analysis. It may be the main factor that we hesitate to adopt the FLC in critical applications. In this paper, a guideline of systematic analysis and design of an FLC is presented for guaranteeing the stability and/or the performance. Based on the well-developed variable structure system theory, we account for the relationship between the tracking accuracy of a control system and the I/O fuzzy set membership shapes of the FLC. It is also possible to explain the performance or the stability of commonly used FLCs through the proposed guideline for the design and analysis.

Design of nonlinear expert supervisory controllers for power system stabilization

This paper presents the design of a new nonlinear expert controller for single-machine infinite-bus and multimachine power systems. The controller uses the concepts of fuzzy linguistic control for normal and abnormal operating conditions of the power system. A nonlinear supervisory controller based on variable-structure system theory is designed to augment the linguistic fuzzy controller for severe transient disturbances when there is a possibility of the stability being lost. This robust nonlinear expert controller is applied to a single-machine infinite-bus system and some other selected machines in a multimachine power system subject to a variety of transient disturbances. The damping performance of this controller is found to be significantly better than the traditional fuzzy and conventional power system stabilizers.

Average modelling of current-mode controlled converters: instability predictions

A stable DC-DC convener example is presented for which conventional average models predict instability. It is shown that the formalism of variable structure system theory provides a systematic procedure for deriving an average model which does not suffer this limitation. The new and conventional models are compared.

Vibration Control Of A Magnetic Gear System By Using Sliding Mode Control

A new magnetic gear system with an actuator made by an electromagnet is proposed. Vibration due to the system parameter variations or external disturbance is absorbed by changing the magnetic fluxes in this electromagnet, and the controller is designed by variable structure system theory. Theoretical expressions for the transmitted torque, magnetic spring constant and control force versus electric current in the electromagnet are derived. Simulations of this magnetic gear system have been carried out by using sliding mode control and the results are compared with those obtained by the PID control method. The experimental results for the sliding mode controller confirm the theoretical analyses.

Cartesian trajectory control of a flexible manipulator using sliding mode

A sliding mode control algorithm is presented for Cartesian trajectory control of a three-axis flexible manipulator by using the theory of variable structure systems. The manipulator has two rigid links and the third link is elastic. A parameterization of the Cartesian coordinates of a point close to the end-effector position is suggested. Using these coordinates as output variables, a discontinuous output control law is derived based on the variable structure theory for tracking reference Cartesian trajectories. To regulate the end point to a fixed position, a linear stabilizer is designed to damp the elastic vibration. Simulation results are presented to show the robustness of the proposed controller in spite of the parameter uncertainty in the arm dynamic model.

Variable structure control of robot manipulators in cartesian space with efficient computation

The predominant concern in control of a robot arm is to make the movement of the endeffector track desired motion. In this paper, a Cartesian Space (Operational Space) robot controller which uses the Newton Euler (NE) formulation is presented. The control is designed by the model based Cartesian Space computed torque technique (CTT) and Variable Structure System (VSS) theory. The cartesian space CTT linearizes and decouples robot dynamics in cartesian space, while the VSS component is utilized to eliminate effects of parameter uncertainty. The control can guarantee the convergence of tracking error, and the computation of this control when incorporating the NE form of the dynamics is outlined. Simulation results of this controller when applied to the first three joints of a PUMA 560 arm are presented.

Vibration Isolation and Reduction by Theory of Variable Structure Systems.

By controlling spring stiffness, vibration isolation and reduction of the one-degree-of freedom system are achieved in this study. The control algorithm of the spring stiffness is based on the variable structure system (VSS) theory, and the theory is developed to consider the spring stiffness limitation and delay of switching time. The stiffness-variable spring is realized by a straight bar which is hinged to the mass at right angles to the direction of mass motion, and the axial force is applied at the end of the bar. The axial force causes restoring force equivalent to the spring, and the stiffness is varied by controlling the axial force. In the numerical simulations and experiments, the settling time of the impulse response and the displacement transmissibility are investigated. The results show that this system is effective in suppressing shock motion and isolating vibration transmitted from the floor.