Angular Velocity Feedback Research Articles

Nonlinear Predictive Control of Spacecraft

A new approach for the control of a spacecraft with large angle maneuvers is presented. This new approach is based on a nonlinear predictive control scheme which determines the required torque input so that the predicted responses match the desired trajectories. This is accomplished by minimizing the norm-squared local errors between the predicted and desired quantities. Formulations are presented which use either attitude and rate tracking or attitude-tracking solely. The robustness of the new controller with respect to large system uncertainties is also demonstrated. Finally, simulations results are shown which use the new control strategy to stabilize the motion of the Microwave Anisotropy Probe spacecraft. Introduction The control of spacecraft for large angle slewing maneuvers poses a difficult problem. Some of these difficulties include: the governing equations have highly nonlinear characteristics, control rate and saturation constraints and limits, and incomplete state knowledge due to sensor failure or omission. The control of spacecraft with large angle slews can be accomplished by either open-loop or closed-loop schemes. Open-loop schemes usually require a predetermined pointing maneuver and are typically determined using optimal control techniques, which involve the solution of a two-point boundary value problem (e.g., see the time optimal maneuver problem [1]). Also, open-loop schemes are sensitive to spacecraft parameter uncertainties and unexpected disturbances [2]. Closed-loop systems can account for parameter uncertainties and disturbances, and thus provide a more robust design methodology. In recent years, much effort has been devoted to the closed-loop design of spacecraft with large angle slews. Wie and Barber [3] derive a number of simple control schemes using quaternion and angular velocity (rate) feedback. Asymptotic stability is also shown by Copyright c 1997 by John L. Crassidis. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. using a Lyapunov function analysis for all cases. Tsiotras [4] expands upon these formulations by deriving simple control laws based on both a Gibbs vector parameterization and a modified Rodrigues parameterization each with rate feedback (for a complete survey of attitude parameterizations, see [5]). Lyapunov functions are shown for all the controllers developed in [4] as well. Other full state feedback techniques have been developed which are based on sliding mode (variable structure) control, which uses a feedback linearizing technique and an additional term aimed at dealing with model uncertainty [6]. This type of control has been successfully applied for large angle maneuvers using a Gibbs vector parameterization [7], a quaternion parameterization [8], and a modified Rodrigues parameterization [9]. Another robust control scheme using a nonlinear H∞ control methodology has been developed by Kang [10]. This scheme involves the solution of Hamilton-Jacobi-Isaacs inequalities, which essentially determines feedback gains for the full state feedback control problem so that the spacecraft is stabilized in the presence of uncertainties and disturbances. Another class of controllers involves adaptive techniques, which update the model during operation based on measured performances (e.g., see [6]). An adaptive scheme which estimates for external torques by means of tracking a Lyapunov function has been developed by Schaub et. al. [11]. This method has been shown to be very robust in the presence of spacecraft modeling errors. The aforementioned techniques all utilize full state knowledge (i.e., attitude and rate feedback). The problem of controlling a spacecraft without full state feedback becomes increasingly complex. The basic approaches used to solve this problem can be divided into methods which estimate for the unmeasured states using a filter algorithm, or methods which develop control laws directly from output feedback. Filtering methods, such as the extended Kalman filter, have been successfully applied on numerous spacecraft systems without the use of rate-integrating gyro measurements (e.g., see [12]-[14]). An advantage of these methods is that the attitude may be estimated by using only one set of vector attitude measurements (such as magnetometer measurements). However, these methods are usually

On the Design of a Nominal Motion Regime of an Arm with Two or One Flexible Links

A method for the synthesis of a nominal motion regime for a flexible planar manipulator with one or two links is proposed. This regime is not far from time-optimal. Our experimental robot is planar two-link arm actuated by two drives. We can study separately each link. The control law for drive torques consists of two parts: the commanded nominal feedforward torque and the linear angular position and velocity feedback. In the experiments, the designed control algorithm has been successfully implemented.

Attitude control without angular velocity measurement: a passivity approach

It is well known that the linear feedback of the quaternion of the attitude error and the angular velocity globally stabilizes the attitude of a rigid body. In this note, we show that the angular velocity feedback can be replaced by a nonlinear filter of the quaternion, thus removing the need for direct angular velocity measurement. In contrast to other approaches, this design exploits the inherent passivity of the system; a model-based observer reconstructing the velocity is not needed. An application of the proposed scheme is illustrated for the robot control problem. Simulation results are included to illustrate the theoretical results.

Dynamical Boundary Control for Elastic Plates of General Shape

The control of transverse vibrations of elastic plates of general shape by feedback boundary control is formulated as an abstract evolution equation. Because the control acts locally on the boundary, which possesses a flanged rim with inertial properties of mass and bending moment, the analysis concerns dynamical controllability and stabilizability of a hybrid system. By the approach of energy decay inequalities and Hörmander’s global uniqueness theorem, it is shown that the system is strongly stabilizable by a locally supported damping feedback of boundary velocity and boundary angular velocity, and hence the system is approximately controllable.

Effect of Positive Angular Velocity Feedback on Torque Control of Hydraulic Actuator.

Output torque/force of a hydraulic actuator is generally affected by its driving speed. This may cause deterioration of the control accuracy of the system in which the plant is controlled through the driving torque/force control of the actuator. This study proposes a positive angular velocity feedback for compensation of the effect of the driving speed on the driving torque control of a rotary hydraulic actuator. The compensation scheme is experimentally applied to motion control of a one-link hydraulic robot which is controlled by means of the computed torque method. The experiments show that the compensation scheme is effective in isolating the driving torque of the actuator from its driving speed and slightly improves the tracking accuracy of the motion control.

Effect of Positive Angular Velocity Feedback on Torque Control of Hydraulic Actuator.

Output torque/force of a hydraulic actuator is generally affected by its driving speed. This may cause deterioration of the control accuracy of the system in which the plant is controlled through the driving torque/force control of the actuator. This study proposes a positive angular velocity feedback for compensation of the effect of the driving speed on the driving torque control of a rotary hydraulic actuator. The compensation scheme is experimentally applied to motion control of a one-link hydraulic robot which is controlled by means of the computed torque method. The experiments show that the compensation scheme is effective in isolating the driving torque of the actuator from its driving speed and slightly improves the tracking accuracy of the motion control.

Artificial-reflex stimulation for FES-induced standing with minimum quadriceps force

A control strategy is proposed to decrease quadriceps activity during standing. Modified on/off (or artificial reflex) control is used: a non-numerical or finite-state control scheme based on feedback of knee angle and angular velocity. The control strategy is evaluated in paraplegic patients in an experimental setup using transcutaneous stimulation. The stability of the system and its sensitivity to various control parameters are determined. It is concluded that the control scheme will enable reduction of muscle force independent of additional mechanical bracing or specific posture, and may result in continuous dynamic activation of muscle.

Improvement of power system transient stability by a thyristor controlled thermoelectric converter

An application of a thyristor controlled thermoelectric converter (TCTEC) to a power system is proposed. The purpose of using a TCTEC is basically for electric power generation in the normal operating phase; however, such a device will also improve transient stability. Owing to the EMF caused by the Seebeck effect acting to enhance power dissipation in the thermoelectric semiconductor resistance, a TCTEC can act as a power compensator during upswings and downswings with the help of feedback control. The performance of a TCTEC is superior to that of the conventional braking resistor in absorbing power during the voltage drop on the machine terminal bus. The effectiveness of a TCTEC was tested on a two-machine infinite-bus system with a digital computer. The simulation results show that a TCTEC with a rotor relative angular velocity feedback channel improves not only the first-swing stability, but also plays a very important role in providing damping and assuring stability throughout subsequent swings.