Suresh M. Joshi

Adaptive state feedback and tracking control of systems with actuator failures

Direct adaptive-state feedback control schemes are developed for linear time-invariant plants with actuator failures with characterizations that some of the plant inputs are stuck at some fixed or varying values which cannot be influenced by control action. Conditions and controller structures for achieving plant-model state matching in the presence of actuator failures are derived. Adaptive laws are designed for updating the controller parameters when both the plant parameters and actuator-failure parameters are unknown. Closed-loop stability and asymptotic-state tracking are ensured. Simulation results show that desired system performance is achieved with the developed adaptive actuator failure compensation control designs.

Adaptive actuator failure compensation for nonlinear MIMO systems with an aircraft control application

A direct adaptive approach is developed for control of a class of multi-input multi-output (MIMO) nonlinear systems in the presence of uncertain failures of redundant actuators. An adaptive failure compensation controller is designed which is capable of accommodating uncertainties in actuator failure time instants, values and patterns. A realistic situation is studied with fixed grouping of actuators and proportional actuation within actuator groups. The adaptive control system is analyzed, to show its desired stability and asymptotic tracking properties in the presence of actuator failure uncertainties. As an application, such an adaptive controller is used for actuator failure compensation of a twin otter aircraft longitudinal model, with design conditions verified and control structure and adaptive laws developed for a nonlinear aircraft dynamic model. The effectiveness of adaptive failure compensation is demonstrated by simulation results.

Robust attitude stabilization of spacecraft using nonlinear quaternion feedback

This paper considers the problem of three-axis attitude stabilization of a rigid spacecraft. A nonlinear control law which uses the feedback of the unit quaternion and the measured angular velocities is proposed and is shown to provide global asymptotic stability. The control law does not require the knowledge of the system parameters and is, therefore, robust to modeling errors. The significance of the control law is that it can be used for large-angle maneuvers with guaranteed stability. >

Brief Adaptive actuator failure compensation for parametric strict feedback systems and an aircraft application

Adaptive actuator failure compensation for parametric-strict-feedback systems is studied under different system structure conditions. Adaptive state feedback control schemes are developed, which ensure asymptotic output tracking and closed-loop signal boundedness. An adaptive control scheme is applied to a twin otter aircraft longitudinal nonlinear dynamics model in the presence of unknown failures in a two-segment elevator servomechanism. Simulation results verify the effectiveness of adaptive actuator failure compensation for desired system performance.

Brief An adaptive control scheme for systems with unknown actuator failures

A state feedback output tracking adaptive control scheme is developed for plants with actuator failures characterized by the failure pattern that some inputs are stuck at some unknown fixed values at unknown time instants. New controller parametrization and adaptive law are developed under some relaxed system conditions. All closed-loop signals are bounded and the plant output tracks a given reference output asymptotically, despite the uncertainties in actuator failures and plant parameters. Simulation results verify the desired adaptive control system performance in the presence of actuator failures.

Comparison of controller designs for an experimental flexible structure

Control system design and hardware testing are addressed for an experimental structure displaying the characteristics of a typical large flexible spacecraft. The practical aspects associated with designing and implementing various control design methodologies for a real system are described, and the results are given. The design methodologies under investigation include linear-quadratic-Gaussian (LQG) control, static and dynamic dissipative control, and H/sub infinity / optimal control. The merit of each design is based on its capacity for vibration suppression, its stability robustness characteristics with respect to unmodeled dynamics, and its ease of design and implementation. Among the three controllers considered, it is shown, through computer simulation and laboratory experiments, that the dynamic dissipative controller gives the best results.<<ETX>>

On a class of marginally stable positive-real systems

A class of marginally stable positive real systems is defined which is less restrictive than the previously published definitions of strictly positive real systems. A minimal realization and state-space characterization of such systems are presented, and it is proven that controllers belonging to this class robustly stabilize positive real plants.

Sensor/actuator placement for flexible space structures

A new approach for the placement of sensors and actuators in the active control of flexible space structures is developed. The approach converts the discrete nature of sensor and actuator positioning problems to a nonlinear programming optimization through approximation of the control forces and output measurements by spatially continuous functions. The locations of the sensors and actuators are optimized in order to move the transmission zeros of the system further to the left of the imaginary axis. This criterion for sensor/actuator placement can be useful for optimal regulation and tracking problems, as well as for low authority controller designs. Two performance metrics are considered for the optimization and are applied to the sensor/actuator positioning of a large-order flexible space structure. >

Design of failure-accommodating multiloop LQG-type controllers

This note investigates the stability of multiloop LQG-type controllers in the presence of actuator and sensor outages. For the linear quadratic state feedback (LQSF) regulator case, it is proved that the closed-loop stability can be maintained in the presence of certain actuator failure states by inserting appropriate constant gains in the control loops. The results are also applicable (via duality) to the design of state estimators which maintain stability in face of sensor failures, provided that the failure state is known or detected. This regulator-estimator combination yields an LQG-type controller which is tolerant to certain actuator and sensor failure states.

On the design of the dissipative LQG-type controllers

The design of dissipative linear-quadratic-Gaussian-type compensators for positive real plants is considered. It is shown that if the noise covariance matrices (used as weighting matrices) satisfy certain conditions, the compensator has a strictly positive real transfer function matrix. The stability of the resulting closed-loop system is guaranteed regardless of modeling errors as long as the plant remains positive real. In view of this property, the controller is expected to be useful for vibration suppression in large flexible space structures.<<ETX>>