Proceedings of the Twenty-Eighth AAAI Conference on Artificial Intelligence Q-Intersection Algorithms for Constraint-Based Robust Parameter Estimation Clement Carbonnel Gilles Trombettoni LAAS-CNRS LIRMM Universite´ Toulouse, France Universite´ Montpellier, France [email protected] [email protected] Philippe Vismara Gilles Chabert LIRMM LINA SupAgro Montpellier, France EMN Nantes, France [email protected] [email protected] Abstract simple harmonic motion whose angular frequency is ! := pm=k. On this example, the input is m, the output is !, the Given a set of axis-parallel n-dimensional boxes, the q- p intersection is defined as the smallest box encompassing all parameter is k and we have f(x; p) = x=p. We perform the points that belong to at least q boxes. Computing the q- pairs of measurements (xi; yi), each measurement being as- intersection is a combinatorial problem that allows us to han- sociated with some uncertainty. For instance, attaching to dle robust parameter estimation with a numerical constraint the spring different loads of given masses mi, one obtains a programming approach. The q-intersection can be viewed as set of measurements !i of the frequencies. a filtering operator for soft constraints that model measure- In many applications uncertainty is modeled by a Gaus- ments subject to outliers. This paper highlights the equiva- sian noise.1 In the interval constraint programming ap- lence of this operator with the search of q-cliques in a graph whose boxicity is bounded by the number of variables in the proach, uncertainty is modeled by a bounded error (Walter constraint network. We present a computational study of the and Piet-Lahanier 1993). In this context, one can enforce q-intersection. We also propose a fast heuristic and a sophisti- consistency algorithms on the ith measurement constraint: cated exact q-intersection algorithm. First experiments show yi − i ≤ f(xi; p) ≤ yi + i that our exact algorithm outperforms the existing one while our heuristic performs an efficient filtering on hard problems. to get a filtered/contracted box [p]i that rigorously encloses the true value of the parameters. The overall approximation T is then obtained by a simple intersection: p 2 i[p]i. This 1 Introduction filtering approach is nice because it gives a safe enclosure of The combinatorial q-intersection operator can handle a spe- the parameters, regardless of the model. However, it is not cific class of numerical (real-valued) Constraint Satisfaction robust to outliers, i.e., points that correspond to experimental Problems. In numerical CSPs, the domain of the n variables errors and lead to an empty intersection. Another idea is to is an n-dimensional box (a Cartesian product of n intervals). assume that at least q measurements are valid, the other ones The q-intersection appears in the context of parameter es- being potential outliers. The q-intersection studied in this timation, a fundamental task in control theory, robotics and paper replaces the previous intersection by a union of all the autonomous systems, where the state (position, velocity, etc) intersections obtained with q measurements. More formally, n has to be estimated from noisy sensor data. The purpose of given a set S of boxes and an integer q, we say that p 2 R is parameter estimation is to estimate the unknown “internal” a (S; q)-intersection point if p belongs to at least q boxes in parameters of a system from a set of measurements. A sys- S. The q-intersection of S is thus defined as follows (where tem can be thought here as any physical process that trans- IR is the set of all intervals over R). forms inputs into outputs. The estimation is made from the Definition 1. Let S be a set of boxes of IRn. The q- measurements of outputs obtained with different inputs. intersection of S, denoted by \qS, is the box of smallest Using a constraint programming approach, each measure- perimeter that encloses the set of (S; q)-intersection points. ment provides a constraint system y = f(x; p) where x For instance, the box in dotted lines in Fig. 1–b is the is the input vector, y is the output vector, f is a vector of 4-intersection of the 10 two-dimensional boxes (in plain real-valued functions and p is a vector of unknown param- eters. As an illustrative example, consider a simple spring 1When the noise is described by a probability distribution, the for which the constant k is the parameter to be determined. parameter estimation problem is tackled by statistical methods (see It is well-known that a mass m attached to the spring has a e.g., (Beck and Arnold 1977)). If the function f is linear, the least squares method gives the maximum-likelihood estimator (by the Copyright c 2014, Association for the Advancement of Artificial Gauss-Markov theorem). Otherwise, no clear property holds on the Intelligence (www.aaai.org). All rights reserved. result, which motivates the constraint programming approach. 2630 (a) A set S of 2-dimensional (b) The dashed box is \4S. boxes Zones that belong to at least 4 Figure 2: Principle of the grid algorithm for q=2, n=2. Cells boxes are darkened. contained in q boxes are dotted. The dashed box is \2S. Figure 1: Illustration of q-intersection for q=4, n=2. lines). The q-intersection allows a (soft) constraint pro- gramming approach to parameter estimation. It handles soft numerical constraint networks where at least q numerical constraints must be verified. Each subsystem yi − i ≤ f(xi; p) ≤ yi + i corresponding to one measurement con- tracts the box (filters the domain) using consistency meth- ods, e.g., numeric constraint propagation (Lhomme 1993; Figure 3: Principle of projective filtering for q=2, n=2. The 2 Benhamou et al. 1999). Then, an operator computes the q- method outputs the dashed box, a gross overestimate of \ S. intersection of all the filtered boxes. For some applications, these two steps are called at each node of a search tree. The problem of parameter estimation with outliers in a there exists a cell C containing x that is included in at least bounded-error context has been introduced first in (Jaulin, q boxes. Therefore, the algorithm iterates through the cells, Walter, and Didrit 1996), where the q-intersection has also and computes the minimum-perimeter bounding box of all been formalized. Our paper presents the first theoretical the cells that are included in at least q boxes. The overall n study of q-intersection, along with efficient incomplete and complexity is O(np(2p − 1) ). This algorithm is polyno- exact algorithms to handle this problem. mial if n is fixed, but not in the general case. In practice the grid algorithm is rather slow, even for small values of n. Moreover, the q-intersection operator is Intervals, boxes and orderings A closed interval is a set typically used to filter the search space at each node of a of the form [a; b] = fx 2 R : a ≤ x ≤ bg. For an in- search tree. Hence, if exact q-intersection algorithms are terval I = [a; b], we use the notation I = a and I = b. too slow for this task, computing a reasonable enclosure of An n-dimensional box is a Cartesian product of n intervals, \qS may be satisfactory (provided it can be done in poly- i.e., an element of IRn. The ith coordinate of a box B (an nomial time). This can be achieved using a method first in- interval) is denoted by B[i], and its perimeter is defined as troduced in (Chew and Marzullo 1991), called here projec- Pn tive filtering jBj = i=1 B[i] − B[i]. We call direction a couple (i; o), , that solves the problem on each dimension in- where i is a dimension and o 2 {−; +g is the orientation. dependently. Formally, for each dimension i, the algorithm For a box B, we denote Ld(B) the left bound of B w.r.t. d, computes the projection S[i] of the boxes and solves the q- intersection problem on S[i]. Since S[i] is a set of intervals where Ld=(i;+)(B) = B[i] and Ld=(i;−)(B) = B[i]. Sym- each \qS[i] can be computed in polynomial time (a special- d metrically, we define R (B), the right bound of B w.r.t. ized version of the grid algorithm is O(p log(p))). It is clear d=(i;+) d=(i;−) q q Q q d, where R (B) = B[i] and R (B) = B[i]. that (\ S)[i] ⊆ (\ S[i]), so i2(1::n)(\ S[i]) is a valid We define the order ≥o on R as ≥ if o = +, and ≤ if enclosure of \qS. The overall complexity is O(np log(p)). o = −. Given a direction d = (i; o) and a set of boxes Figure 3 illustrates the algorithm. This method has no guar- S = fBigi2N, we denote by >d the strict order on S such anteed approximation ratio but can be fairly efficient on d d that Bi >d Bj if and only if (L (Bi) >o L (Bj)) or problems with few parameters (Jaulin and Bazeille 2009). d d (L (Bi) = L (Bj)) ^ (i > j). The q-intersection operator A straightforward exact al- 2 Theoretical analysis gorithm for computing the q-intersection is the grid algo- In this section we give a brief overview of the complexity of rithm introduced in (Jaulin and Bazeille 2009). The core idea problems closely related to the q-intersection operator. is described in Figure 2: using the projections of the boxes Given a set S of boxes, the intersection graph of S is the onto every axis, one can generate a grid of (2p − 1)n cells, graph whose vertices are in one-to-one correspondence with where p is the number of boxes and n is the number of di- the boxes in S and in which two vertices are adjacent if and mensions.