Ashok Iyer

Nonlinear decoupling sliding mode control and attitude control of spacecraft

Control of a class of uncertain nonlinear systems which can be decoupled by state-variable feedback is considered. A variable-structure-control (VSC) law is derived so that in the closed-loop system the output variables asymptotically track given output trajectories in spite of any uncertainty in the system. On the basis of this result, a control law is derived for the attitude control of an orbiting spacecraft in the presence of uncertainty using reaction jets. The controlled outputs are the three Euler angles which describe the orientation of the spacecraft relative to an orbital frame. Simulation results are presented to show that, in the closed-loop system, precise attitude control is accomplished in spite of the uncertainty in the system. >

Variable structure slewing control and vibration damping of flexible spacecraft

Abstract The question of attitude control and elastic mode stabilization of a spacecraft (orbiter) with “beam-tip mass” type payload is considered. It is assumed that bounded but unknown disturbance torques are acting on the spacecraft. Based on variable structure system theory, a discontinuous three-axis moment control law is derived to control the attitude of the spacecraft. Although, this control law accomplishes attitude trajectory tracking, it excites the elastic modes of the beam. A modal velocity feedback design is presented to damp the elastic oscillations using additional actuators at the tip of the beam. Simulation results are presented to show that rotational maneuvers and vibration stabilization can be accomplished in the closed-loop system in spite of disturbance torques and uncertainty in the system.

MFDs of spinning satellite and attitude control using gyrotorquers

Questions related to the matrix fraction descriptions (MFDs) and the minimal realizations of the transfer matrix of a spinning satellite system and control system design using output feedback are considered. The control torques are generated using gyrotorquers, and the output variables are the attitude angles. Two minimal state-space representations of the system, namely, a controller-form and an observer-form realization, are obtained from a right and left MFD of the transfer matrix, respectively. Using these canonical realizations, analytical expression for the feedback matrices of the controller and the observer as functions of the system parameters are obtained. The poles of the closed-loop system are invariant with respect to the spin rate of the satellite. Simulation results are presented to show that precise attitude control is accomplished in the closed-loop system using output feedback. >

Sliding mode of control of flexible spacecraft under disturbance torque

A control system design is presented for large angle rotational manoeuvres of a spacecraft-beam-tip body (an antenna or a reflector) configuration. Although this approach is applicable to three-axis manoeuvres, for simplicity only single-axis control is treated here. It is assumed that an unknown but bounded disturbance torque acts on the spacecraft. A sliding mode attitude control law is derived for slewing of the space vehicle. This slewing control law requires only the attitude error and its derivative for feedback. It does not need any information on the elastic motion of the system. For the damping of the elastic motion, a stabilizer is seperately designed based on the asymptotically decoupled system describing the elastic deflections in two orthogonal planes. Simulation results are presented to show that precise large rotational manoeuvres can be performed using an attitude controller and elastic mode stabilizer in spite of the uncertainty in the system.

Detumbling and reorientation maneuvers and stabilization of NASA SCOLE system

The questions of rotational maneuver and vibration stabilization of the NASA Spacecraft Control Laboratory Experiment (SCOLE) system is considered. The mathematical model of the SCOLE system includes the rigid body dynamics as well as the elastic dynamics representing transverse and torsional deformations of the elastic beam connecting the orbiter and end body (reflector). For the rotational maneuver, a new control law (orbiter control law) is derived using an orbiter input torque vector. Detumbling and reorientation maneuvers of the SCOLE system are accomplished using this control law; however, this excites the elastic modes of the beam. The orbiter control law asymptotically linearizes the flexible dynamics. Using the linearized model, a linear feedback control law is designed for vibration suppression. An observer is designed for estimating the state variables using sensor outputs which are also used for the synthesis of the control law. Simulation results are presented to show that in the closed-loop system detumbling and reorientation maneuvers can be accomplished and the effect of control and observation spillover is insignificant. >

Variable structure attitude control and elastic mode stabilization of flexible spacecraft

The question of attitude control and elastic mode stabilization of a spacecraft (orbiter) with a beam-tip-mass-type payload is considered. It is assumed that bounded but unknown disturbance torques are acting on the spacecraft. Using variable-structure-system theory, a discontinuous three-axis moment control law is derived to control the attitude of the spacecraft. Although this control law accomplishes attitude trajectory tracking, it excites the elastic modes of the beam. A modal velocity feedback design that damps the elastic oscillations using additional actuators at the tip of the beam is presented. Simulation results are presented to show that rotational maneuvers and vibration stabilization can be accomplished in the closed-loop system in spite of disturbance torques and uncertainty in the system.<<ETX>>

Sliding mode control of flexible spacecraft under disturbance torque

The authors present a control system design for large-angle rotational maneuvers of a spacecraft-beam-tip body (an antenna or a reflector) configuration. It is assumed that an unknown but bounded disturbance torque is acting on the spacecraft. A sliding mode attitude control law is derived for the slewing of the space vehicle. This slewing control law requires only attitude error and its derivative for feedback and does not need any information on the elastic motion of the system. For the damping of the elastic motion, a stabilizer is designed separately based on the asymptotically decoupled system describing the elastic deflections in two orthogonal planes. Simulation results are presented to show that precise large rotational maneuvers can be performed using the attitude controller and the elastic mode stabilizer in spite of the uncertainty in the system.<<ETX>>

Nonlinear rotational maneuver and vibration damping of NASA SCOLE system

Abstract We treat the question of large rotational maneuver and vibration stabilization of NASA Spacecraft Control Laboratory Experiment (SCOLE) system. The mathematical model of SCOLE system includes the dynamical equations for rigid body slew maneuver and three-dimensional vibration of the flexible beam and the reflector with an offset mass. The design approach taken here is to decompose the rigid mode control from vibration stabilization. Feedback input (Shuttle torque)-output (attitude angles) map linearization technique is used for designing attitude control system for large-angle slewing. Linearization of the input-output (i-o) map is accomplished by nonlinear inversion theory. It is shown that attitude control system asymptotically decouples the flexible dynamics and a linear feedback law is easily designed for vibration suppression. For the synthesis of the control law an observer is designed. Simulation results are presented to show that in the closed-loop system large angle maneuvers can be accomplished using only the measured state variables.

Nonlinear excitation and governor control using variable structures

Abstract Modern microprocessor capabilities permit the control designer to consider using relatively complicated nonlinear control algorithms, which would have been considered impractical in the past. This paper presents the results of a study of the variable structure control technique for the design of excitation and governor controllers for a power system. Control laws for rotor angle and field flux are derived. The closed loop system is shown to be asymptotically stable. The system can be transferred to a new operating condition corresponding to any desired terminal voltage Vt, and tie-line power Ptie.

Nonlinear adaptive attitude control of satellite using gyrotorquers

A nonlinear adaptive control law for the attitude control of satellites using gyrotorquers is presented. The derivation of the control law does not require any information on the system dynamics and the environmental disturbance torques acting on the satellite. A simplified control law for spinning satellites is derived. The controller requires only attitude tracking error and its derivative for feedback. Simulation results are presented which show that, in the closed-loop system, precise attitude control can be accomplished in spite of the uncertainty in the system.<<ETX>>