Rapid Recovery for Systems with Scarce Faults

Our goal is to achieve a high degree of fault tolerance through the control of a safety critical systems. This reduces to solving a game between a malicious environment that injects failures and a controller who tries to establish a correct behavior. We suggest a new control objective for such systems that offers a better balance between complexity and precision: we seek systems that are k-resilient. In order to be k-resilient, a system needs to be able to rapidly recover from a small number, up to k, of local faults infinitely many times, provided that blocks of up to k faults are separated by short recovery periods in which no fault occurs. k-resilience is a simple but powerful abstraction from the precise distribution of local faults, but much more refined than the traditional objective to maximize the number of local faults. We argue why we believe this to be the right level of abstraction for safety critical systems when local faults are few and far between. We show that the computational complexity of constructing optimal control with respect to resilience is low and demonstrate the feasibility through an implementation and experimental results.Comment: In Proceedings GandALF 2012, arXiv:1210.202

Control and Mechanism Interaction for Ground-Based Testing of Space Structures

In the ground-based validation testing, the adverse effect of terrestrial conditions such as a gravitational force interferes with the dynamic behavior of space structures. A suspension system is developed to assess the structural characteristics in a simulated zero-gravity environment. Using a mechanisms approach, the synthesis of a noncircular disk with a torsional spring at its rotational axis is designed to counteract the gravitational force of test structures during the testing. The multibody dynamics of a flexible steel beam carried on a rigid trolley has been investigated. The system is constructed in such a way that the rapid and large-angle slewing maneuver is performed by means of hybrid rotational/translational motions. A flexible one-beam structure and a flexible two-beam structure with such noncircular gears will be investigated. The varying gear ratio of noncircular gears is specified to produce varying output speeds so as to tune the rapid slewing maneuver while suppressing structural vibration. One optimization technique based on the Generalized Reduced Gradient Method is employed to determine the optimal design of the controllers as well as the noncircular gears for vibrational suppression during the rapid slewing maneuvers. The numerical simulations are implemented to evaluate the effectiveness of the integrated design of control and mechanism for the slewing maneuvers of flexible space structures. Based on Lyapunov\u27s stability criterion, the stability analysis of space structures leads to the design of a Lyapunov-based controller that yields a stable closed-loop system. Such a controller is developed by combining a linear part and a nonlinear part for the rotational/translational maneuver. The simulations of three kinds of nonlinear dynamic systems are performed to verify the usefulness of Lyapunov-based nonlinear feedback control. Two types of beam-like flexible space structures, i.e., the flexible one-beam structure on a trolley and the flexible articulated two-beam structure on a trolley, are simulated to implement maneuvering tasks of position control while suppressing the structural vibration simultaneously. An inverted pendulum is stabilized through its Lyapunov-based nonlinear controller to confirm the feasibility of such a nonlinear controller for unstable systems

Toughening polyamide 6 nanocomposites with maleic anhydride grafted polyethylene octene

Rubber toughened nanocomposites consisting of ternary blends of polyamide 6 (PA 6), maleic anhydride grafted polyethylene octene (POEgMAH) and organoclay montmorillonite (MMT) were prepared by melt compounding followed by injection moulding. The organoclay content was kept constant at 4 wt% while the POEgMAH content was varied between 5 to 20 wt%. The mechanical properties were studied through tensile, flexural and impact properties. The scanning electron microscope (SEM) and X-ray diffraction (XRD) were used to examine the morphology of the nanocomposties. The results showed that, the incorporation of 4 wt% organoclay significantly increased the stiffness and strength but at the expense of the toughness. Izod impact measurement indicated that the addition of POEgMAH led to a significant improvement in the impact strength of the nanocomposites. X-ray diffraction analysis (XRD) revealed that an intercalation organoclay silicate layer structure was formed in rubber-toughened PA6 nanocomposites. SEM study revealed a two-phase morphology where POE, as droplets was dispersed finely and uniformly in the PA6 matrix