The development of a position control system for a shape memory alloy (SMA) wire actuator using an electrical resistance feedback is presented in this paper. A novel control scheme is implemented to eliminate the need for a position sensor to achieve stable and accurate positioning by utilizing the actuator's electrical resistance feedback. Experiments are conducted to investigate the relationship between electrical resistance and displacement using an SMA wire test setup. Due to the highly nonlinear behavior of the SMA actuator, a neural network is employed to model the relationship and to predict the position of the actuator using only the electrical resistance. Feedback control of the SMA is achieved by using a proportional-derivative (PD) controller. Experimental results demonstrate that the proposed position control system achieves good control performance without using a position sensor.

No abstract

Minimizing vibrations of a exible spacecraft actuated by on-off thrusters is a challenging task. This paper presents the rst study of pulse-width pulse-frequency modulated thruster control using command input shaping. Input shaping is a technique that uses a shaped command to ensure zero residual vibration of a exible structure. Pulse-width pulse-frequency modulation is a control method that provides pseudolinear operation for an on-off thruster. The proposed method takes full advantage of the pseudolinear property of a pulse-width pulse-frequency modulator and integrates it with a command shaper to minimize the vibration of a exible spacecraft induced by on-off thruster ring. Compared to other methods, this new approach has numerous advantages: 1) effectiveness in vibration suppression, 2) dependence only on modal frequency and damping, 3) robustness to variations in modal frequency and damping, and 4) easy computation. Numerical simulations performed on an eight-mode model of the Flexible Spacecraft Simulator in the Spacecraft Research and Design Center at the U.S. Naval Postgraduate School demonstrate the ef cacy and robustness of the method.

This paper presents the results of positive position feedback (PPF) control and linear-quadratic Gaussian (LQG) control for vibration suppression of a flexible structure using piezoceramics. Experiments were conducted on the US Naval Postgraduate School's flexible spacecraft simulator (FSS), which is comprised of a rigid central body and a flexible appendage. The objective of this research is to suppress the vibration of the flexible appendage. Experiments show that both control methods have unique advantages for vibration suppression. PPF control is effective in providing high damping for a particular mode and is easy to implement. LQG control provides damping to all modes; however, it cannot provide high damping for a specific mode. LQG control is very effective in meeting specific requirements, such as minimization of tip motion of a flexible beam, but at a higher implementation cost.

analysis 14 yields the bounds°m ax¸°¸°min where, to rst order in the small ±,°m ax;min ¼ rAgain, these quantities will be more sensitive if the moments of inertia are close than if they are widely spaced. For a vehicle with nominal inertias satisfying I 1 D 2I 3 and I 2 D 1:5I 3 , and the 1% normalized inertia perturbations considered earlier, arctan°can vary from 31.8 to 38.7 deg, a total range of 6.9 deg. Using Eqs. (13) and (14) together allows the possible variations in absolute separatrix directions to be quanti ed as a function of the inertia error bound ±, thus providing the desired insight into sizing ±y as a function of ±. ConclusionsThis Note has derived a design procedure that prevents a spacecraft with a stuck-on thruster from experiencing a large net linear accelerationand, hence, a signi cant perturbationto its orbit. It was shown that such an acceleration is most likely to arise for a thruster that generates a torque that is nominally about the intermediate axis of an asymmetric vehicle, but that is in reality slightly offset from it. The simple technique that was developed to avoid this makes use of a small shift in thruster position. Expressions were derived to allow this shift to be sized to produce robust results despite small uncertainties in the geometry and mass properties of the vehicle.