PALS-Based Analysis of an Airplane Multirate Control System in Real-Time Maude

Distributed cyber-physical systems (DCPS) are pervasive in areas such as aeronautics and ground transportation systems, including the case of distributed hybrid systems. DCPS design and verification is quite challenging because of asynchronous communication, network delays, and clock skews. Furthermore, their model checking verification typically becomes unfeasible due to the huge state space explosion caused by the system's concurrency. The PALS ("physically asynchronous, logically synchronous") methodology has been proposed to reduce the design and verification of a DCPS to the much simpler task of designing and verifying its underlying synchronous version. The original PALS methodology assumes a single logical period, but Multirate PALS extends it to deal with multirate DCPS in which components may operate with different logical periods. This paper shows how Multirate PALS can be applied to formally verify a nontrivial multirate DCPS. We use Real-Time Maude to formally specify a multirate distributed hybrid system consisting of an airplane maneuvered by a pilot who turns the airplane according to a specified angle through a distributed control system. Our formal analysis revealed that the original design was ineffective in achieving a smooth turning maneuver, and led to a redesign of the system that satisfies the desired correctness properties. This shows that the Multirate PALS methodology is not only effective for formal DCPS verification, but can also be used effectively in the DCPS design process, even before properties are verified.Comment: In Proceedings FTSCS 2012, arXiv:1212.657

NEW DEVELOPMENTS IN LOCAL CONFIGURATION INTERACTION THEORY

Many chemical phenomena, from the freezing point of water to the strength of a chemical bond, are determined by the distribution of electrons which make up matter. We can model and predict chemical phenomena by using quantum mechanics to determine the electrons' distribution (wavefunction). Multireference configuration interaction (MRCI) provides a flexible wavefunction for capturing crucial electron correlation effects. Unfortunately, MRCI's computational cost grows rapidly, O(N^6 ), limiting its application to small molecules. Over the last three decades, researchers have exploited the spatial locality of electron correlation to reduce the costs of correlated quantum chemistry methods like MRCI. The local electron correlation approximation removes insignificant long range correlations thereby reducing MRCI's cost. By so doing, local MRCI methods can be applied to much large molecules than canonical MRCI. In this thesis, previous efforts by Carter and co-workers applying the local correlation approximation to MRCI are expanded upon: both computational speedups and improved accuracy are considered. The state-of-the art local MRCI algorithm scales as O(N^3 ) which, while cheaper than conventional MRCI, scales rapidly with system size. Converting the previously serial local MRCI code to parallel code allows exploitation of multicore architectures common in modern CPUs. Replacing the Cholesky-decomposed two-electron integrals with cheaper density-fitted two-electron integrals reduces local MRCI's cost. These two advances don't effect the O(N^3 ) scaling, but rather reduce the scaling prefactor, thereby allowing simulation of larger molecules. MRCI's accuracy is hampered by the well-known size extensivity error, which grows with molecular size. We introduce previously proposed MRCI size- extensivity corrections to the O(N^3 ) local MRCI. Both a priori and a posteriori size extensivity corrections can be applied. However, a priori corrections can cause numerical instabilities in both canonical and local MRCI. We show that these instabilities are avoided crossings with spurious low energy states. This analysis suggests two different approaches to maintain a stable size extensivity correction. Finally, we improve the accuracy of local MRCI by optimizing the parameters controlling the local electron correlation. The combination of these developments provides a faster, more accurate method for modeling larger scale chemical phenomena than previously possible

Designing and verifying distributed cyber-physical systems using Multirate PALS: An airplane turning control system case study

Distributed cyber-physical systems (DCPS), such as aeronautics and ground transportation systems, are very hard to design and verify, because of asynchronous communication, network delays, and clock skews. Their model checking verification typically becomes unfeasible due to the huge state space explosion caused by the system's concurrency. The Multirate PALS ("physically asynchronous, logically synchronous") methodology has been proposed to reduce the design and verification of a DCPS to the much simpler task of designing and verifying its underlying synchronous version, where components may operate with different periods. This paper presents a methodology for formally modeling and verifying multirate DCPSs using Multirate PALS. In particular, this methodology explains how to deal with the system's physical environment in Multirate PALS. We illustrate our methodology with a multirate DCPS consisting of an airplane maneuvered by a pilot, who turns the airplane to a specified angle through a distributed control system. Our formal analysis using Real-Time Maude revealed that the original design did not achieve a smooth turning maneuver, and led to a redesign of the system. We then use model checking and Multirate PALS to prove that the redesigned system satisfies the desired correctness properties, whereas model checking the corresponding asynchronous model is unfeasible. This shows that Multirate PALS is not only effective for formal DCPS verification, but can also be used effectively in the DCPS design process. (C) 2014 Elsevier B.V. All rights reserved.111415sciescopu