Francisco Azevedo

Modelling Digital Circuits Problems with Set Constraints

A number of diagnostic and optimisation problems in Electronics Computer Aided Design have usually been handled either by specific tools or by mapping them into a general problem solver (e.g. a propositional Boolean SAT tool). This approach, however, requires models with substantial duplication of digital circuits. In Constraint Logic Programming, the use of extra values in the digital signals (other than the usual 0/1) was proposed to reflect their dependency on some faulty gate. In this paper we present an extension of this modelling approach, using set variables to denote dependency of the signals on sets of faults, to model different circuits problems. We then show the importance of propagating constraints on sets cardinality, by comparing Cardinal, a set constraint solver that we implemented, with a simpler version that propagates these constraints similarly to Conjunto, a widely available set constraint solver. Results show speed ups of Cardinal of about two orders of magnitude, on a set of diagnostic problems.

Computerising a guideline for the management of diabetes

This paper reports an experience of computerising a clinical guideline for the management of non-insulin-dependent diabetes mellitus (NIDDM). The guideline, designed by the European NIDDM Policy Group is being used in a National Programme for Diabetes supported by the Portuguese Ministry of Health, who is keen to supporting its widespread use by general practitioners, namely in computerised form. The paper presents the main characteristics of the prototype that was implemented within the European project Prestige, and was developed according to the Prestige Protocol Model. The model is briefly described, together with the generic architecture that supports it. Then the main design decisions of the prototype are explained, regarding the modelling of a general practitioner workflow during a typical consultation and the user interface, two key issues for obtaining acceptance from the users. The limitations of the system are discussed and a number of directions are outlined in order to circumvent such limitations, and broaden the scope of applicability of the system.

Colored Nonograms: An Integer Linear Programming Approach

In this paper we study colored nonogram solving using Integer Linear Programming. Our approach generalizes the one used by Robert A. Bosch which was developed for black and white nonograms only, thus providing a universal solution for solving nonograms using ILP. Additionally we apply a known algorithm to find all solutions to a puzzle. This algorithm uses a binary cut to exclude already known solutions. Finally we compare the performance of our approach in solving colored nonograms against other approaches, namely the iterative and the brute-force ones, pointing to a research direction of developing a hybrid method combining the iterative approach with ILP.

Interaction of Constraint Programming and Local Search for Optimisation Problems

In this paper we show, for the specific problem of test pattern optimisation, that adapting constraint propagation with results obtained from local search outperforms the use of each of these techniques alone. We show that a tool we developed to solve this problem using such approach with multivalued logics achieves better results than those obtained with a highly efficient tool based on an integer linear programming formulation over a SAT model.

An attempt to dynamically break symmetries in the social golfers problem

A number of different satisfaction and optimisation combinatorial problems have recently been approached with constraint programming over the domain of finite sets, for increased declarativity and efficiency. Such problems where one tries to find sets of values that satisfy some conditions, often present much symmetry on variables and values. In particular, the social golfers problem encompasses many possible symmetries. Allowing symmetric solutions increases search space unnecessarily, thus multiplying solution time. Therefore, ordering constraints have been proposed and incorporated in set solvers. However, such constraints are imposed statically in the global problem model and are unable to detect symmetries that still occur in sub-problems after a partial labelling. In this paper we discuss how to overcome this and present an approach that sequentially labels variables avoiding such symmetries by dynamically disallowing the assignment of other values from the same equivalence class in the golfers problem. Experimental results show that this approach outperforms previous ones, recently achieved by the constraint programming community, namely over sets. Unfortunately, the current method is incomplete and may loose solutions. Nevertheless, results are correct and show that similar techniques can be used efficiently to obtain faster solutions.

Deductive Diagnosis of Digital Circuits

In this paper we present an efficient deductive method for addressing combinational circuit diagnosis problems. The method resorts to bottom-up dependencies propagation, where truth-values are annotated with sets of faults. We compare it with several other logic programming techniques, starting with a naive generate-and-test algorithm, and proceeding with a simple Prolog backtracking search. An approach using tabling is also studied, based on an abductive approach. For the sake of completeness, we also address the same problem with Answer Set Programming. Our tests recur to the ISCAS85 circuit benchmarks suite, although the technique is generalized to systems modelled by a set of propositional rules. The dependency-directed method outperforms others by orders of magnitude.

GRASPER: constraint reasoning with graphs

In this paper we present GRASPER, a graph constraint solver based on set constraints. GRASPER is a constraint logic-based reasoning framework aiming to provide a powerful, efficient and intuitive framework for modelling and solving hard combinatorial problems by introducing graph variables. We specify GRASPERs core and higher level constraints and make use of it to model a problem in the context of biochemical networks showing promising results, when compared to an existing similar solver, for different search heuristics.

Lazy Constraint Imposing for Improving the Path Constraint

In this paper we propose a lazy constraint imposing mechanism for improving the path constraint in GRASPER, a state-of-the-art graph constraint solver, having obtained very promising results in terms of both time and space in solving an interesting problem in the Biochemistry subject area, in comparison with CP(Graph), the state-of-the-art solver.

Generation of Test Patterns for Differential Diagnosis of Digital Circuits

Objective. In a faulty digital circuit, many (single) faulty gates may explain the observed findings. It may be not practical (e.g. in VLSI chips) to test points in the circuit other than their input and output bits, so constraint technology was used to obtain input patterns that identify single faulty gates. However, two alternative faulty gates often have a large number of test patterns, but only a few, if any, differentiate them. We focus on obtaining test patterns that discriminate alternative faulty gates. Technique. The technique of [2] to generate a test pattern for a faulty gate introduces new values, d and notd, and assigns d to the gate output. New truth tables cope with the new values, and the patterns generated force a d (or notd) to reach an output bit

How environmental factors impact line performance, field results from Southwest Europe

Long series of field data obtained from exploitation and maintenance of electricity grids from 10kV to 400kV have shown the prevalence of environmental factors as the primary influence on the performance of over-head lines in Southwest Europe. Vegetation and clearance, lightning and birds come at the top of concerns; forest fires, fog and pollution follow and broad similarities were observed across territories and voltage levels. The main environmental factors affecting grids are introduced and a strategy to establish the nexus of cause that binds environmental factors, effects on power lines observed during inspections and grid events is proposed. Field data for distribution and transmission grids are presented to illustrate some of the hypotheses for causal links.