Anne Auger

Nikolaus Hansen

Jorge Perez Zerpa

Raymond Ros

Marc Schoenauer

J M Perez Zerpa

Empirical comparisons of several derivative free optimization algorithms

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Continuous Optimization Problems (COPs) aim at finding the global optimum (or optima) of a real-valued function (aka objective function) defined over (a subset of) a real vector space. COPs commonly appear in everyday's life of many scientists, engineers and researchers from various disciplines, from physics to mechanical, electrical and chemical engineering to biology. Problems such as model calibration, process control, design of parameterized parts are routinely modeled as COPs. Furthermore, in many cases, very little is known about the objective function. In the worst case, it is only possible to retrieve objective function values for given inputs, and in particular the user has no information about derivatives, or even about some weaker characteristics of the objective function (e.g. monotonicity, roughness, . . . ). This is the case, for instance, when the objective function is the output of huge computer programs ensuing from several years of development, or when experimental processes need to be run in order to compute objective function values. Such problems amount to what is called Black-Box Optimization (BBO).

Because BBO is a frequent situation, many optimization methods (aka search algorithms) have been proposed to tackle BBO problems, that can be grossly classified in two classes: (i) deterministic methods include classical derivative-based algorithms, in which the derivative is numerically computed by finite differences, and more recent Derivative Free Optimization (DFO) algorithms [START_REF] Scheinberg | Recent progress in unconstrained nonlinear optimization without derivatives[END_REF], like pattern search [START_REF] Torczon | On the convergence of pattern search algorithms[END_REF] and trust region methods [START_REF] Powell | The NEWUOA software for unconstrained optimization without derivatives[END_REF]; (ii) stochastic methods rely on random variables sampling to better explore the search space, and include recently introduced bio-inspired algorithms (see Section 3).

However, the practitioner facing a BBO problem has to choose among those methods, and there exists no theoretical solid ground where he can stand to perform this choice, first because he does not know much about his objective function, but also because all theoretical results either make simplifying hypotheses that are not valid for real-world problems, or give results that do not yield any practical outcome. Moreover, most of BBO methods require some parameter tuning, and here again very little help is available for the practitioner, who is left with a blind and timeconsuming test-and-trial approach.

In such context, this paper proposes an experimental perspective on BBO algorithms comparisons. Rigorous procedures to compare the results of different BBO algorithms have been proposed [START_REF] Suganthan | Problem definitions and evaluation criteria for the CEC 2005 special session on real-parameter optimization[END_REF], taking into account the stochastic nature of many of them, and giving fair chances to each one of them. However, a critical issue in such experiments is that of the benchmark suite. And because no set of real-world problems can be guaranteed to cover all possible cases of difficult COPs, the approach that has been chosen here is to build artificial test functions with some precise characteristics that are known to be possible sources of difficulty for optimization (e.g. illconditioning, non-separability, non-convexity, ruggedness, . . . ). Such experimental results could then be cautiously generalized, leaving only a few good-performing algorithms in each specific context. Of course, in real-life BBO situations, it is assumed that nothing is known about the objective function. However, the user sometimes has some partial information (e.g. because his problem is known to be similar to other better-known problems) that might lead him to decide for a BBO method that is (experimentally) known to perform well, 'in vitro', in his precise situation. But on the other hand, assuming absolutely nothing is known in advance about the objective function, running the champion algorithms as identified in perfectly controlled environment might give him some information about his function (e.g. if numerical gradient-based algorithms perform 100 times better than all other methods, his problem is probably very similar to a quadratic problem). This paper is a first step toward such 'in vitro' results. Next, in Section 2, some characteristics of the objective function are surveyed that are known to make the corresponding BBO problem hard. Section 3 introduces the algorithms that will be compared here. Section 4 then introduces the test bench that illustrates the different difficulties highlighted in Section 2, as well as the experimental conditions of the comparisons. The results are presented and discussed in Section 5, and the paper ends with a summary in Section 6. This paper is organized as follows: in Section 2, some characteristics of the objective function are surveyed that are known to make the corresponding BBO problem hard. Section 3 introduces the algorithms that will be compared here. Section 4 then introduces the test bench that illustrates the different difficulties highlighted in Section 2, as well as the experimental conditions of the comparisons. The results are presented and discussed in Section 5.

What makes a search problem difficult?

In this section, we discuss problem characteristics that are especially challenging for search algorithms.

Ill-conditioning

The conditioning of a problem can be defined as the range (over a level set) of the maximum improvement of objective function value in a ball of small radius centered on the given level set. In the case of convex quadratic functions ( f (x) = 1 2 x T Hx where H is a symmetric definite matrix), the conditioning can be exactly defined as the condition number of the Hessian matrix H, i.e., the ratio between the largest and smallest eigenvalue. Since level sets associated to a convex quadratic function are ellipsoids, the condition number corresponds to the squared ratio between the largest and shortest axis lengths of the ellipsoid. Problems are typically considered as ill-conditioned if the conditioning is larger than 10 5 . In practice we have seen problems with conditioning as large as 10 10 .

Non-separability An objective function f (x 1 , . . . , x n ) is separable if the optimal value for any variable x i can be obtained by optimizing f ( x 1 , . . . , x i-1 , x i , x i+1 , . . . , x n ) for any fixed choice of the variables x 1 , . . . , x i-1 , x i+1 , . . . , x n . Consequently optimizing an n-dimensional separable objective function reduces to optimizing n one-dimensional functions. Functions that are additively decomposable, i.e., that can be written as f (x) = ∑ n i=1 f i (x i ) are separable. One way to render a separable test function non-separable is to rotate first the vector x, which can be achieved by multiplying x by an orthogonal matrix B: if x → f (x) is separable, the function x → f (Bx) might be non-separable for all non-identity orthogonal matrices B.

Non-convexity Some BBO methods implicitly assume or exploit convexity of the objective function. Composing a convex function f to the left with a monotonous transformation g : R → R can result in a non-convex function, for instance the one-dimensional convex function f (x) = x 2 composed with g(.) = sign(.)|.| 1/4 becomes the non-convex function |.|.

In this paper we will quantitatively assess the performance dependency on the conditioning of objective functions, investigate separable and non-separable problems as well as study the dependence of the performances in the convexity of the problem.

Algorithms tested

This section introduces the different algorithms that will be compared in this paper. They have been chosen because they are considered to be the champions in their category, both in the deterministic optimization world (BFGS and NEWUOA) and in the stochastic bio-inspired world (CMA-ES, DE and PSO). They will also be a priori discussed here with respect to the difficulties of continuous optimization problems highlighted in Section 2.

The algorithms

BFGS is a well-known quasi-Newton algorithm. It has a proven convergence to a stationary point provided the starting point is close enough from the solution, and the objective function is regular. Because it is blindly used by many scientists facing optimization problems, the Matlab R version of BFGS fminunc has been used here, with its default parameters. The algorithm is stopped whenever the objective improvement in one iteration is less than 10 -25 . Of course, in BBO contexts, the gradients have to be computed numerically.

NEWUOA (NEW Unconstrained Optimization Algorithm) [START_REF] Powell | The NEWUOA software for unconstrained optimization without derivatives[END_REF] is a DFO algorithm using the trust region paradigm. NEWUOA computes a quadratic interpolation of the objective function in the current trust region, and performs a truncated conjugate gradient minimization of the surrogate model in the trust region. It then updates either the current best point or the radius of the trust region, based on the a posteriori interpolation error. The implementation by Matthieu Guibert (http://www.inrialpes.fr/bipop/people/guilbert/newuoa/newuoa.html) has been used.

An important parameter of NEWUOA is the number of points that are used to compute the interpolation. As recommended [START_REF] Powell | The NEWUOA software for unconstrained optimization without derivatives[END_REF], 2n + 1 points have been used. Other parameters are the initial and final radii of the trust region, respectively governing the initial 'granularity' and the precision of the search. After some preliminary experiments, values of 100 and 10 -15 were chosen. [START_REF] Rechenberg | Evolutionsstrategie: Optimierung Technischer Systeme nach Prinzipien der Biologischen Evolution[END_REF] algorithm: from a set of 'parents', 'offspring' are created by sampling Gaussian distributions, and the best of the offspring (according to the objective function values) become the next parents. The Covariance Matrix Adaptation [START_REF] Hansen | Completely derandomized self-adaptation in evolution strategies[END_REF] uses the path that has been followed by evolution so far to (i) adapt the step-size, a scaling parameter that tunes the granularity of the search, by comparing the actual path length to that of a random walk; (ii) modify the covariance matrix of the multivariate Gaussian distribution in order to increase the likelihood of beneficial moves. A single Gaussian distribution is maintained, centered at a linear combination of the parents. CMA-ES proposes robust default parameters: the population size is set to 4 + 3 log(n) and the initial step-size to a third of the parameters range. The version used here (Scilab 0.92) implements weighted recombination and rank-µ update [START_REF] Hansen | The CMA evolution strategy: a comparing review[END_REF] (version 0.99 is available at http://www.lri.fr/~hansen/cmaes_inmatlab.html).

CMA-ES is an Evolution Strategy (ES)

PSO (Particle Swarm Optimization) [START_REF] Kennedy | Particle swarm optimization[END_REF] is a bio-inspired algorithm based on the biological paradigm of a swarm of particles that 'fly' over the objective landscape, exchanging information about the best solutions they have 'seen'. More precisely, each particle updates its velocity, stochastically twisting it toward the direction of the best solutions seen by (i) itself and (ii) some parts of the whole swarm; it then updates its position according to its velocity and computes the new value of the objective function. A Scilab transcription of the Standard PSO 2006, the latter available at PSO Central http://www.particleswarm.info/, was used with its default settings.

Differential Evolution (DE) [START_REF] Storn | Differential evolution -a simple and efficient heuristic for global optimization over continuous spaces[END_REF] borrows from Evolutionary Algorithms (EAs) the paradigm of an evolving population. However, a specific 'mutation' operator is used that adds to an individual the difference between two others from the population. Standard uniform crossover is also used. The implementation posted by the original authors at http://www.icsi.berkeley.edu/ ~storn/code.html was used here. However, the authors confess that the results highly depend on the parameter tuning. They propose 6 possible strategies, and extensive experiments (3 × 288 trials) on a moderately ill-conditioned problem lead us to consider the "DE/local-to-best/1/bin" strategy, where the difference vector is computed between a random point and the best point in the population. Also, the use of crossover seemed to have little beneficial impact on the results, so no crossover was used, thus making DE rotationally invariant. Moreover, the population size was set to the recommended value of 10n, the weighting factor to F = 0.8.

Invariances

Some a priori comparisons can be made about those algorithms, related to the notion of invariance. Indeed, invariances add to the robustness of an algorithm: functions belonging to the same equivalence class with respect to some invariance property will look exactly the same for an algorithm that is invariant under the transformation defining this equivalence class. Two sets of invariance properties are distinguished, whether they regard transformations of the objective function value or transformations of the search space. First, all comparison-based algorithms are invariant under monotonous transformations of the objective function, as comparisons are unaltered if the objective function f is replaced with some g • f for some monotonous function g. All bio-inspired algorithms used in this paper are comparison-based, while the BFGS and NEWUAO are not.

Regarding transformations of the search space, all algorithms are trivially invariant under translation of the coordinate system. But let us consider some orthogonal rotations: BFGS is coordinate-dependent due to the computation of numerical gradients. NEWUOA is invariant under rotation when considering the complete quadratic model, i.e. built with 1 2 (n + 1)(n + 2) points. This variant is however often more costly compared to the 2n + 1 one -but the latter is not invariant under rotation. The rotational invariance of CMA-ES is built-in, while that of DE depends whether or not crossover is used -as crossover relies on the coordinate system. This was one reason for omitting crossover here. Finally, PSO is (usually) not invariant under rotations, as all computations are done coordinate by coordinate [START_REF] Hansen | PSO facing non-separable and ill-conditioned problems[END_REF][START_REF] Wilke | Comparison of linear and classical velocity update rules in particle swarm optimization: Notes on scale and frame invariance[END_REF].

Test functions and experimental setup

Test functions The benchmark functions tested are given in Table 1. The functions are tested in their original axis-parallel version (i.e. B is the identity and y = x), and in rotated versions, Table 1: Test functions with coordinate-wise initialization intervals and target function value, where y := Bx implements an angle-preserving, linear transformation, i.e. B is orthogonal.

Function α Initialization f target f elli (x) = ∑ n i=1 α i-1 n-1 y 2 i [1, 10 10 ] [-20, 80] n 10 -9 f Rosen (x) = ∑ n-1 i=1 α (y 2 i -y i+1 ) 2 + (y i -1) 2 [1, 10 8 ] [-20, 80] n 10 -9 f 1/4 elli (x) = ∑ n i=1 α i-1 n-1 y 2 i 1/4 [1, 10 10 ] [-20, 80] n 10 -9
where y = Bx. The orthogonal matrix B is chosen such that each column is uniformly distributed on the unit hypersphere surface [START_REF] Hansen | Completely derandomized self-adaptation in evolution strategies[END_REF], fixed for each run. The Ellipsoid function f elli is a convex-quadratic function where the parameter α is the condition number of the Hessian matrix that is varied between 1 and 10 10 in our experiments. If α = 1 the Ellipsoid is the isotropic separable sphere function. The function f 1/4 elli has the same contour lines (level sets) as f elli , however it is neither quadratic nor convex. For α = 1, the functions f elli and f 1/4 elli are separable if and only if B = I. The Rosenbrock function f Rosen is non-separable, has its global minimum at x = [1, 1, . . . , 1] and, for large enough α and n, has one local minimum close to x = [-1, 1, . . . , 1], see also [START_REF] Shang | A note on the extended rosenbrock function[END_REF]. The contour lines of the Rosenbrock function show a bent ridge that guides to the global optimum (the Rosenbrock is sometimes called banana function) and the parameter α controls the width of the ridge. In the classical Rosenbrock function, α equals 100. For smaller α, the ridge becomes wider and the function becomes less difficult to solve. We vary α between one and 10 8 .

Experimental Setup

For each algorithm tested we conduct 21 independent trials of up to 10 7 function evaluations. For all algorithms, initial points have been sampled uniformly in the range [-20, 80] n . If, for BFGS, no success was encountered, the number of trials was extended to 1001. We quantify the performance of the algorithms using the success performance SP1 used in [START_REF] Hansen | Evaluating the CMA evolution strategy on multimodal test functions[END_REF], analyzed in [START_REF] Auger | Performance evaluation of an advanced local search evolutionary algorithm[END_REF], and also denoted as Q-measure in [START_REF] Feoktistov | Differential Evolution: In Search of Solutions. Optimization and Its Applications[END_REF]. The SP1 equals the average number of function evaluations for successful runs divided by the ratio of successful runs, where a run is successful if the target function value 10 -9 is reached before 10 7 function evaluations are exceeded. The SP1 is an estimator of the expected number of function evaluations to reach 10 -9 if the algorithm is restarted until a success (supposing infinite time horizon) and assuming that the expected number of function evaluations for unsuccessful runs equals the expected number of evaluations for successful runs.

Results

Results are shown for dimension 20. Results for 10 and 40-D (see Appendix) reveal similar trends.

Ellipsoid functions: dependencies In Figure 1 a remarkable dependency of the performance (SP1) on the condition number can be observed in most cases. The two exceptions are PSO on the separable functions and DE. In the other cases the performance declines by at least a factor of ten for very ill-conditioned problems as for CMA-ES. The overall strongest performance decline is shown by PSO on the rotated functions. NEWUOA shows in general a comparatively strong decline, while BFGS is only infeasible for high condition numbers in the rotated case, reporting some numerical problems. The decline of CMA-ES is moderate. For CMA-ES and DE the results are (virtually) independent of the given ellipsoidal functions, where CMA-ES is consistently between five and forty times faster than DE. For PSO the results are identical on Ellipsoid and Ellipsoid 1/4 , while the performance declines under rotation (left versus right figures) is very pronounced. A similar strong decline under rotation can be observed for NEWUOA on the Ellipsoid function for moderate condition numbers. BFGS, on the other hand, shows a strong rotational dependency on both functions only for large condition numbers ≥ 10 6 .

Separable Ellipsoid Function

Switching from Ellipsoid (above) to Ellipsoid 1/4 (below) only effects BFGS and NEWUOA. BFGS becomes roughly five to ten times slower. A similar effect can be seen for NEWUOA on the rotated function. On the separable Ellipsoid function the effect is more pronounced, because NEWUOA performs exceptionally well on the separable Ellipsoid function.

Ellipsoid functions: comparison On the separable Ellipsoid function up to a condition number of 10 6 NEWUOA clearly outperforms all other algorithms. Also BFGS performs still better than PSO and CMA-ES while DE performs worst. On the separable Ellipsoid 1/4 function BFGS, CMA-ES and PSO perform similar. NEWUOA is faster for low condition numbers and slower for large ones. For condition number larger than 10 6 , NEWUOA becomes even worse than DE.

On the rotated functions, for condition numbers larger than 10 3 , PSO is remarkably outperformed by all other algorithms. On the rotated Ellipsoid function for moderate condition numbers BFGS and NEWUOA perform best and outperform CMA-ES by a factor of five, somewhat more for low condition numbers, and less for larger condition numbers. For large condition numbers Rosenbrock Function CMA-ES becomes superior and DE is within a factor of ten of the best performance.

On the rotated Ellipsoid 1/4 BFGS and CMA-ES perform similar up to condition of 10 6 . NEWUOA performs somewhat better for lower condition numbers up to 10 4 . For larger condition numbers BFGS and NEWUOA decline and CMA-ES performs best.

Rosenbrock function On the Rosenbrock function NEWUOA is the best algorithm (Figure 2) . NEWUOA outperforms CMA-ES roughly by a factor of five, vanishing for very large values for the conditioning parameter α. For small α, BFGS is in-between, and for α > 10 4 BFGS fails. DE is again roughly ten times slower than CMA-ES. Only PSO shows a strong dependency on the rotation of the function and the strongest performance decline with increasing α.

Scaling behaviors

The scaling of the performance with search space dimension is similar for all functions (results not shown here for space reasons). CMA-ES, NEWUOA and PSO show the best scaling behavior. They slow down by a factor between five and ten in 40D compared to 10D. For BFGS the factor is slightly above ten, while for DE the factor is thirty or larger, presumably because the default population size increases linearly with the dimension.

Summary

In this paper we have conducted a comparison of BFGS, NEWUOA, and three stochastic bioinspired optimization methods in a black-box optimization scenario. The empirical study was conducted on smooth functions with varying condition number. Aside from gradients being not provided, we consider these functions as the favorite playgrounds of BFGS and NEWUOA. We find that NEWUOA performs exceptional on separable quadratic functions, it performs in all cases very well with moderate condition numbers, but shows a comparatively steep performance decline with increasing ill-conditioning. BFGS performs well overall, but shows a strong decline on very ill-conditioned non-separable functions. For DE, the parameters are difficult to tune and yet it performs overall poorly with the single best parameter setting on our small function set. With the chosen parameters, DE shows the strongest robustness to ill-conditioning though. PSO performs similar to CMA-ES on the separable problems, with an even weaker dependency on the conditioning. On non-separable problems PSO performs very poorly even on moderately ill-conditioned functions. Finally, CMA-ES generally outperforms DE and PSO, while up to a moderate function conditioning BFGS and NEWUOA are significantly faster in most cases. Due to their invariance properties, the performance results of CMA-ES and DE are the most stable ones and most likely to generalize to other functions.

A All Results

Separable Ellipsoid Function

Figure 1 :

 1 Figure 1: All ellipsoidal functions in 20D. Shown is SP1 (the expected running time or number of function evaluations to reach the target function value) versus condition number.

Figure 2 :

 2 Figure 2: Rosenbrock function. Shown is SP1 (the expected running time or number of function evaluations to reach the target function value) versus conditioning parameter α.

Figure 3 :

 3 Figure 3: Ellipsoid function. Shown is SP1 (the expected running time or number of function evaluations to reach the target function value) versus condition number.

Figure 4 :Figure 5 :

 45 Figure 4: Ellipsoid 1/4 function. Shown is SP1 (the expected running time or number of function evaluations to reach the target function value) versus condition number.

Acknowledgements We would like to acknowledge Philippe Toint for his kind suggestions, and Nikolas Mauny for writing the Scilab transcription of the Standard PSO 2006 code.


Olivier Cardin
email: olivier.cardin@univ-nantes.fr

Fouzia Ounnar
email: fouzia.ounnar@univ-amu.fr

André Thomas
email: andre.thomas@univ-lorraine.fr

Keywords: Agility, sustainability, cyber-physical systems, future industrial systems. I

published or not. The documents may come

manufacturing control concepts, and specifically the contribution of what is called nowadays cyber-physical systems (CPS) as a key enabler of future industrial systems [START_REF] Lee | A Cyber-Physical Systems architecture for Industry 4.0-based manufacturing systems[END_REF].

The aims of this paper is to synthetize the general problems of intelligent manufacturing and demonstrate the principal contributions developed within the framework of this national workgroup 1 in four general research topics concerning intelligent manufacturing systems, namely agility, new technologies, sustainability and industrial dissemination.

This paper is organized as follows. First, the general scientific topics that need to be addressed for the development of future industrial systems are synthesized from an analysis of corresponding international roadmaps. Then, an overview of IMS² contributions is proposed and a critical evaluation of the research efforts of this group is conducted towards the international positioning relative to those problems. Finally, last section analyses the results with regards to these topics and elaborates a roadmap for future research in the community to cope with the challenges exposed before.

II. OVERVIEW OF RESEARCH GENERAL TOPICS ABOUT FUTURE INDUSTRIAL SYSTEMS

A. International roadmaps

Four long-term paradigms will guide the transformations that European manufacturing needs to undergo regarding Manufacturing vision 2030 [START_REF] Effra | Factories of the Future: Multi annual roadmap for the contractual PPP under Horizon 2020[END_REF]: factory and nature (green/sustainable); factory as a good neighbor (close to the worker and the customer); factories in the value chain (collaborative); factory and humans (human centered). Programs mainly based around FoF (Factories of the Future) or SPIRE (Sustainable Process Industry) and at a least extent EEB (Energy-efficient Buildings) are coordinating scientific and industry efforts into these directions. Similarly to European events, the "recovery plan" launched in US foresees a specific chapter dealing with "Manufacturing and green jobs" which establishes the setting-up of an "Advanced Manufacturing Fund" for the development of advanced manufacturing strategies. In order to support the operational implementation of this program, the "Council of Advisors on Science and Technology" suggested supporting important research initiatives in the advanced manufacturing sector related to development of new technologies and design methodologies supporting the production of highly innovative T products and materials [START_REF] Anderson | Report to the President on Ensuring American Leadership in Advanced Manufacturing[END_REF]. The American National Institute of Science and Technology (NIST) has expressed for the last decade several challenges related to future industry in a global roadmap considering the development of CPS. Recently, a public workgroup on CPS 2 has been settled in order to foster the activity around this concept.

French National Research Agency (ANR 3 ) draws every year a roadmap for "Stimulating industrial renewal". This roadmap is derived from the French National Research Strategy 4 . Since 2010, this holistic view of industry also took shape, addressing the concept of System Factory, connected to a worldwide supply chain, next to the developments of the technologies for virtual factories use. Same as NIST does, a highlight is defined on smart and connected factories, comprising topics such as generic information model and cybersecurity. Finally, flexibility and agility of systems are strongly connected to the development of innovative and effective processes and technologies. The "Factories of the Future" Project was also approved by the Interministerial Committee for Economic Planning (CIPE) within the Italian Research National Program (PNR) 5 . ActionPlanT (roadmap project of the strategic Germany initiative 'Industry 4.0' 6 ) has established a manufacturing roadmap for Horizon 2020, which outlines a bold vision, where megatrends and key ICT innovations can fulfil five underlying ambitions for European enterprises: ondemand, optimal, innovative, green, and human centered. This vision goes beyond the shop floor and focuses on the manufacturing enterprises and their collaborating stakeholders in the holistic supply chain. Based on the technological concepts of CPS, the Internet of Things, and the Internet of Services, Industry 4.0 provides the vision of Smart Factory [START_REF] Lee | A Cyber-Physical Systems architecture for Industry 4.0-based manufacturing systems[END_REF].

B. General topics of intelligent manufacturing

Table I positions these international roadmaps towards federative topics they address. The main interest of this positioning is to observe that the programs, although with different terminologies, have a consistent view of the different aspects where research and innovation efforts need to focus for efficiency and acceptability improvements of next generation industrial systems. From this perspective, it is possible to synthetize three general topics that shall lead intelligent manufacturing research efforts in the next few years, namely agility; new technologies; and sustainability, which need to be completed by a fourth one dealing with industrial dissemination that is transversally expressed throughout all the programs. This section intends to detail these topics and to draw a short overview of representative international granted projects aligned with the roadmaps. The presentation of these projects is made accordingly to the introduced three first topics. The necessary interoperability between industrial systems must also be dealt with in a 2 www.cpspwg.org 3 www.agence-nationale-recherche.fr/fileadmin/aap/2016/ANR-Plan-Action-2016-details-defis.pdf 4 www.enseignementsup-recherche.gouv.fr/cid78802/strategie-nationale-derecherche-bilan-des-travaux-des-10-ateliers.html#atelier3 5 www.miur.it/UserFiles/3239.pdf 6 www. actionplant-project.eu transversal way inside the projects relevant to the four topics mentioned above, but is not studied in this paper.

X X X X X NIST X X X X ANR X X X X X PNR X X X ActionPlanT X X X X X

A. IMS² in a nutshell

The WG IMS², as it was briefly introduced, is one among the 27 WGs of the CNRS research group MACS. It is composed of around 140 French members. It was created a decade ago to study service or manufacturing control systems and architectures characterized by a set of autonomous entities whose relationships are not necessarily integrated hierarchically. In other words, heterarchical relationships may exist, involving negotiation or cooperation mechanisms among entities. The group has inherited its theoretical foundations from artificial intelligence (AI), distributed AI, operations research, soft computing, control theory, discrete-event modeling and simulation, ambient intelligence, information science, robotics or mechatronics, to name just a few domains. Intelligence can be embedded into material, components, product, resources, or distant facilities, which allows these entities to participate actively in the global process efficiency. Holonic and multi-agents modellings are typical approach that can be used. Emerging optimization / multicriteria decision aid mechanisms (e.g., reinforcement learning, stigmergy, potential fields, swarming, analytic hierarchy/network process) can also be used efficiently and the existing technical solutions are credible today thanks to the CPS technologies. The typical issues addressed by this WG concern localization, performance management and robustness, proof of properties, sustainability, safety and security, interoperability with the existing information systems and deployment methodologies, to mention only a few. Typical applications are related not only to the production phase of the product life cycle, but also to the services involved in the supply chain activities, or in energy distribution and healthcare systems domains. The design, development, use, and recycling phases and the whole life cycle of products, services, and processes must be considered within the global context of product lifecycle management.

B. IMS² added value to the research community

The operational activities of the WG IMS² involve mainly: (i) Two or more meetings each year -mainly devoted to the French research community; international guest researchers may be invited; (ii) Co-supervision of PhD students; (iii) Establishment of consortiums for project calls (French or European calls); (iv) Transfer of scientific developments to scholars, industrialists, and engineering schools. Aside from classical meetings, there are many activities which the WG IMS² initiated and that would have not existed without this WG. One typical representative added value is the co-creation with a Romanian team from Bucharest of the SOHOMA workshop series. This action has led to publishing five books edited by Springer. Table V lists some representative addedvalue joint actions.

C. Research effort classification

The activity of the WG IMS² is presented and structured in the remainder of this document using an analysis framework. This classification is composed of three axes, representing what is impacted, how and for what purposes. First, a criteria based on the framework of [START_REF] Engels | A Method for Engineering a True Service-Oriented Architecture[END_REF] is used to represent what level of aggregation is considered, with the following possible values: (i) Component level; (ii) Machine level; (iii) Shop-floor level;

(iv) Enterprise level; (v) Logistic level; (vi) Energy system level. Second, a criteria based on the five categories (5C) proposed by [START_REF] Lee | A Cyber-Physical Systems architecture for Industry 4.0-based manufacturing systems[END_REF] is used: C1 -Connection; C2 -Conversion; C3 -Cyber; C4 -Cognition; C5 -Configuration. Finally, the third axis is based on the topics defined before, and illustrates the objective of the work, either in terms of agility, technologies or sustainability.

D. IMS² Contributions to future industrial systems general topics

Numerous early WG contributions concerned the study of the intelligent product (IP) concepts and applications that could be classified in {Shop-floor; C1; Agility} in the previously described framework. First of all, several experiments were conducted to prove the concept (to make the concept credible) of IP and product-driven systems, paving the way for CPS [START_REF] Trentesaux | Planning and Control of Maintenance, Repair and Overhaul Operations of a Fleet of Complex Transportation Systems: A Cyber-Physical System Approach[END_REF], [START_REF] Morel | Manufacturing enterprise control and management system engineering: paradigms and open issues[END_REF]. All these experiments led to highlighting of opportunities to implement this kind of systems [START_REF] Berger | Semiheterarchical Allocation and Routing Processes in FMS Control: A Stigmergic Approach[END_REF]. At the same time, to help implementing such solutions, several control-emulation system architectures were proposed [START_REF] Klein | Opportunities to reconsider decision making processes due to Auto-ID[END_REF], [START_REF] Pannequin | The performance of productdriven manufacturing control: An emulation-based benchmarking study[END_REF]. Consequently, a necessary step was to define the concepts, terminology and expose the relevant paradigm associated with this notion [START_REF] Sallez | The Lifecycle of Active and Intelligent Products: The Augmentation Concept[END_REF], [START_REF] Ounnar | HLA distributed simulation approaches for supply chain[END_REF]. Table VI displays an extensive overview of the contributions of the WG to this subject and also those developed in the remainder of this document.

1) IMS² contributions to the agility general topic

This paradigm got then formalized into more generic and innovative control architectures, enabling the use of tools and concepts inherited from the field of computer science in the industrial context. Typically, bio-inspired techniques are used to integrate the products' advanced detection and monitoring functionalities in the loop [START_REF] Pannequin | Another interpretation of stigmergy for product-driven systems architecture[END_REF]. The implementation of such models led to the problem of decision making, which was investigated in many ways, with stigmergic approaches [START_REF] Gamboa Quintanilla | Evolution of a Flexible Manufacturing System: From Communicating to Autonomous Product[END_REF], [START_REF] Pannequin | Stigmergy: a design pattern for product-driven systems[END_REF], potential fields [START_REF] Berger | Semiheterarchical Allocation and Routing Processes in FMS Control: A Stigmergic Approach[END_REF], multicriteria decision aid methods [START_REF] Ounnar | Customersupplier relationship management in an intelligent supply chain network[END_REF] or negotiations via contract-net protocol [START_REF] Borangiu | Distributed manufacturing control with extended CNP interaction of intelligent products[END_REF].

Many performance evaluations, commonly made using discrete-event simulation, were performed on academic testbeds, and a dedicated approach to generate them was proposed [START_REF] Haouzi | Contribution to reusability and modularity of manufacturing systems simulation models: Application to distributed control simulation within DFT context[END_REF]. The pertinence of including the reactivity enabled by the intelligence of the product into the classical production activity control, organized around a predictive scheduling of the orders to run in the manufacturing workshop, was highlighted [START_REF] Pach | ORCA-FMS: a dynamic architecture for the optimized and reactive control of flexible manufacturing scheduling[END_REF]. Therefore, the activities extended to the holonic paradigm, leading to the notion of heterarchical control of systems [START_REF] Aissani | Use of machine learning for continuous improvement of the real time heterarchical manufacturing control system performances[END_REF]. The HMS concept was more than 10 years old at that time, and therefore, the community mainly worked on the efficient implementation of these solutions. Similarly to the classical holonic reference architecture (PROSA [START_REF] Van Brussel | Reference architecture for holonic manufacturing systems: PROSA[END_REF], ADACOR [START_REF] Leitão | ADACOR: A holonic architecture for agile and adaptive manufacturing control[END_REF], HCBA [START_REF] Chirn | A holonic component-based approach to reconfigurable manufacturing control architecture[END_REF], or D-MAS [START_REF] Verstraete | Engineering Manufacturing Control Systems Using PROSA and Delegate MAS[END_REF]), several attempts have been made to propose generic control architectures: PROSIS, enabling an isoarchic control using real time and multicriteria decision rules [START_REF] Pujo | PROSIS: An isoarchic structure for HMS control[END_REF], VSM for HMS proposed a meta-model of HMS architecture systems [START_REF] Herrera | Viable System Model Approach for Holonic Product Driven Manufacturing Systems[END_REF], ORCA-FMS, enabling an online evolution of the control policy for each entity of the control architecture independently [START_REF] Pach | ORCA-FMS: a dynamic architecture for the optimized and reactive control of flexible manufacturing scheduling[END_REF] and H²CM for hybrid production systems [START_REF] Indriago | H2CM: A holonic architecture for flexible hybrid control systems[END_REF], all this towards {Shop-floor; C3; Agility} systems. The contribution of some members of the WG IMS² to the definition of an improved version of ADACOR can also be pointed out [START_REF] Barbosa | Dynamic selforganization in holonic multi-agent manufacturing systems: The ADACOR evolution[END_REF].

Considering manufacturing systems, some studies were also led in the field of reconfigurable manufacturing and assembly lines systems (RMS), one of them being described in the Industrial applications section [START_REF] Manceaux | Dynamic Rebalancing of an Assembly Line with a Reachability Analysis of Communicating Automata[END_REF]. Based on an industrial case, a generic framework denoted RAMS was proposed in order to model efficiently both the systems and the reconfiguration activities [START_REF] Chalfoun | Proposal for a Generic Model Dedicated to Reconfigurable and Agile Manufacturing Systems (RAMS)[END_REF], [START_REF] Chalfoun | Specification of a Reconfigurable and Agile Manufacturing System (RAMS)[END_REF]. Methodologies for the design of manufacturing systems enabling efficient reconfiguration methods were also proposed [START_REF] Benkamoun | Architecture Framework for Manufacturing System Design[END_REF].

2) IMS² contribution to the general topic of the development of new technologies

As stated in Table VI, numerous academic performance evaluations of the proposed control architectures and implementation were executed. In the early years, the WG was created around the laboratories having a strong activity of platform development, aiming at implementing CPS at size 1. These academic workshops are built around automated transfer loops conveying products on pallets around 3 to 12 machines, manual, robotic, or automated, as illustrated in Fig. 1. The intelligence is either embedded on the pallet or container (RFID tag [START_REF] Dubromelle | RFID synchronisation for ambient services in a holonic and isoarchic control model[END_REF][59], embedded CPU [START_REF] Sallez | Semi-heterarchical control of FMS: From theory to application[END_REF], Arduino chips [START_REF] Gamboa Quintanilla | Evolution of a Flexible Manufacturing System: From Communicating to Autonomous Product[END_REF], WSN mote [START_REF] Pujo | Wireless Holons Network for Intralogistics Service[END_REF], etc.) or in the IP (RFID tag embedded in wood or textile raw material [START_REF] Kubler | Embedding data on 'communicating materials' from context-sensitive information analysis[END_REF]) defining {Machine; C3; Agility} systems. Many performance evaluations and benchmarking were performed, both in the job-shops and flow-shops. As demonstrated in Table VII, the main contributions were focused on the design of innovative control and the associated decision making techniques. In addition, the respective involved teams also took the opportunity to develop new CPS hardware. The idea is generally to design active transporters or products thanks to Auto-ID technologies (barcode, RFID, etc.). The objective is to carry the computation and operation abilities with the product and make them capable of interacting with their environment, making their own decisions, and executing these by themselves. For example, a low-cost and low-consumption pallet equipped with Arduino, Zigbee, batteries, and steering actuators was developed on an Elcom-based conveyors system [START_REF] Gamboa Quintanilla | Evolution of a Flexible Manufacturing System: From Communicating to Autonomous Product[END_REF], a pull control based on the RFID associated with products was tested [START_REF] Pannequin | Another interpretation of stigmergy for product-driven systems architecture[END_REF], and a control solution for manufacturing systems based on the WSN technology was proposed to explore new ways of improving the embedded intelligence on holonic entities [START_REF] Pujo | Wireless Holons Network for Intralogistics Service[END_REF]. 3) IMS² contribution to the sustainability general topic Sustainability in manufacturing can be dealt with in many ways [START_REF] Thomas | Are Intelligent Manufacturing Systems Sustainable?[END_REF]. The main focus that the WG IMS² treated is relative to the definition of choreographies able to decrease the environmental impact of manufacturing [START_REF] Trentesaux | Sustainability in Manufacturing Operations Scheduling: Stakes, Approaches and Trends[END_REF], [START_REF] Pach | Reactive and energy-aware scheduling of flexible manufacturing systems using potential fields[END_REF], [START_REF] Giret | Sustainability in manufacturing operations scheduling: A state of the art review[END_REF] (i.e. {Shop-floor; C3; Sustainability}). In particular, this activity is aiming at defining innovative scheduling methods able to optimize both the productivity and the energy consumption of the systems. Another research axis is dealing with the production of energy itself, either from traditional [START_REF] Indriago | H2CM: A holonic architecture for flexible hybrid control systems[END_REF] or renewable sources [START_REF] Ounnar | Intelligent Control of Renewable Holonic Energy Systems[END_REF] (i.e. {Energy system; C3; Sustainability}). This axis aims at increasing the flexibility of the control of these systems in order to cope with the specificities of these systems. In traditional systems, a generic and fractal reference architecture was developed [START_REF] Indriago | H2CM: A holonic architecture for flexible hybrid control systems[END_REF] in order to ease the development of control systems for such production facilities, usually of very large sizes. The flexibility added to the orchestration of the system also enables the control to cope more easily with the numerous disturbances happening on the system. The development of the energy distribution grids is also a matter of great importance with the management of energy coming from renewable sources. A decentralized control energy system multi-sources and multi-users was thus proposed [START_REF] Ounnar | Intelligent Control of Renewable Holonic Energy Systems[END_REF]. Indeed, the random nature of power generation requires a high flexibility of the distribution management in order to avoid systematically storing the energy.

4) Key results of IMS² in the general topic of industrial dissemination

After validation of the scientific proposals, implementations in industrial context were deployed. The historical field of research of the WG being the industrial systems, the first implementations took place in the workshops of local industries willing to handle the complex dynamic systems where classical scheduling did not find any satisfying solution (Table VIII).

Then, a holonic-based production activity control (PAC) replaced a hoist scheduling problem solver in an aeronautic parts surface treatment line [START_REF] Chové | Hoist Scheduling Problem: Coupling reactive and predictive approaches[END_REF], in the same PAC domain a re-scheduling approach was proposed for the car supply industry [START_REF] Pannequin | Stigmergy: a design pattern for product-driven systems[END_REF], a Holonic Manufacturing Execution System was implemented in a Latin-American car glasses manufacturer [START_REF] Blanc | A holonic approach for manufacturing execution system design: An industrial application[END_REF], the holonic principles were deployed to monitor a complex transportation system [START_REF] Le Mortellec | Embedded holonic fault diagnosis of complex transportation systems[END_REF] with Bombardier who led the SURFER project with the objective of a {Machine; C4; Agility} system, the dynamic behavior of the scheduling procedure for maintenance tasks in petroleum industry was enhanced by the use of heterarchical approaches [START_REF] Aissani | Dynamic scheduling of maintenance tasks in the petroleum industry: A reinforcement approach[END_REF], and a scheduling of a production line using the AHP method was tested in vascular prostheses industry [START_REF] Thomas | Are Intelligent Manufacturing Systems Sustainable?[END_REF]. To help in decision making processes in Just in Time contexts, several intelligent approaches were proposed, such as "active kanbans" [START_REF] Klein | An emulation-based execution control system evaluation tool: application to a furniture manufacturing group[END_REF] or reactive demand flow technology systems [START_REF] Haouzi | Contribution to reusability and modularity of manufacturing systems simulation models: Application to distributed control simulation within DFT context[END_REF], [START_REF] Manceaux | Dynamic Rebalancing of an Assembly Line with a Reachability Analysis of Communicating Automata[END_REF] for line reconfiguration towards a {Shop-floor; C3; Agility} system, and also multicriteria decision aid methods taking into account contradictory interests in a {Shop-floor; C3; Agility} system [START_REF] Ounnar | Pull control for job shop: holonic manufacturing system approach using multicriteria decision-making[END_REF]. The implementation of IP in trucks for parcel delivery is indeed very promising [START_REF] Brahim-Djelloul | Product-oriented transportation using RFID technology: A simulationbased study[END_REF], and many potential benefits are foreseen in the contexts such as the Physical Internet [START_REF] Pach | Routing Management in Physical Internet Crossdocking Hubs: Study of Grouping Strategies for Truck Loading[END_REF]. A major breakthrough in the classical implementation of HMS was also the industrial case of fault diagnosis of trains [START_REF] Le Mortellec | Embedded holonic fault diagnosis of complex transportation systems[END_REF], proving that the potential of fractality of holons was indeed pertinent in the case of large-scale systems.

IV. CRITICAL EVALUATION OF IMS² CONTRIBUTION TO RESEARCH FIELD AND STRUCTURATION

This section intends to synthetize the efforts made by IMS² in comparison to the four introduced topics. As Table IX states, the first topic regarding agility in manufacturing was a great concern for the community, with over 80 reference papers in the last decade. The key results of IMS² deal with the definition of new holonic reference architectures, enabling this paradigm to cope with new manufacturing concerns [START_REF] Barbosa | Dynamic selforganization in holonic multi-agent manufacturing systems: The ADACOR evolution[END_REF], [START_REF] Pannequin | The performance of productdriven manufacturing control: An emulation-based benchmarking study[END_REF], [START_REF] Pach | ORCA-FMS: a dynamic architecture for the optimized and reactive control of flexible manufacturing scheduling[END_REF], [START_REF] Pujo | PROSIS: An isoarchic structure for HMS control[END_REF], [START_REF] Indriago | H2CM: A holonic architecture for flexible hybrid control systems[END_REF], [START_REF] Haouzi | Design and validation of a product-driven control system based on a six sigma methodology and discrete event simulation[END_REF], [START_REF] Klein | An emulation-based execution control system evaluation tool: application to a furniture manufacturing group[END_REF]. On an international point of view, these initiatives are the only ones that can be found in literature in the recent years, which constitutes important breakthroughs for future implementations. Indeed, these new architectures are meant to be more appropriate to future implementation concerns in terms of adaptability and changeability of the manufacturing context.

While the concept of CPS was always present in the different implementations, it was often based on the integration of existing technologies. This explains why very few papers presented innovations about the technology itself, aiming rather at demonstrating the breakthrough in the use of CPS. However, several developments were made on the subject of embedded intelligence, either directly inside the product [START_REF] Kubler | Embedding data on 'communicating materials' from context-sensitive information analysis[END_REF] or in the transporter of the product [START_REF] Berger | Personal Rapid Transit in an open-control framework[END_REF], [START_REF] Gamboa Quintanilla | Evolution of a Flexible Manufacturing System: From Communicating to Autonomous Product[END_REF], [START_REF] Pujo | Wireless Holons Network for Intralogistics Service[END_REF], [START_REF] Pach | An effective potential field approach to FMS holonic heterarchical control[END_REF]. These developments were reliable tools to enable proofs of the developed concepts but were not industrially disseminated. Also, the group is currently becoming interested in sustainability and energy footprint of agile manufacturing control, as the references listed in this field are quite recent and their number is constantly growing with time. Considering international literature on this point, a focus on human acceptability of future industrial systems is a major concern that is not but should be treated by the group.

Dissemination

++ <20 [START_REF] Haouzi | Design and validation of a product-driven control system based on a six sigma methodology and discrete event simulation[END_REF], [START_REF] Ounnar | Pull control for job shop: holonic manufacturing system approach using multicriteria decision-making[END_REF], [START_REF] Chové | Hoist Scheduling Problem: Coupling reactive and predictive approaches[END_REF], [START_REF] Le Mortellec | Embedded holonic fault diagnosis of complex transportation systems[END_REF] As exposed previously, industrial dissemination has always been a major pillar of IMS² activities. Those implementations constituted important proofs of the concepts developed in parallel: product-driven control, active products or holonic paradigm among others. In comparison to the previously described industrial projects, those initiatives are not integrated in larger research project, but are in direct application in the local workshops of the concerned industries. This specificity is favored by the distribution of the group members on the territory. On the other side, it is the size of the whole IMS² which increases the number of different industrial point of views and enhances the genericity of the developed concepts. Above these strictly thematic concerns, thanks to this original national dynamic, it has led to creating a real synergy between the French concerned labs. This synergy has materialized and led to common research actions. WG IMS² has now reached its first objective related to the construction of a stable core federation of French and international teams in the French-speaking countries (Romania, Canada, Morocco, Tunisia, Algeria, Belgium, Switzerland…), and this WG must now evolve. For that purpose, a process has been started to design a challenging roadmap for this WG. The strengths of the WG could be explained by the fact that each research team has its own platform embedding various types of Auto-ID technologies and control modes. On the other hand, IMS² has resulted in a lot of real industrial applications leading to induce the research inquiry with the accurate scale of the addressed topics. Moreover, the consortium teams have at least 10 to 15 years of experience on this subject based on complementary and/or similar research backgrounds (operations research, discrete event simulation, logistic and industrial management, traceability, and so on).

V. IMS² 2030 ROADMAP

The roadmap of the WG IMS² has been designed consistently with the needs expressed by the industrial partners working with its members. One main prospect outputted from various big projects concerns the way CPS can contribute to the optimization of the whole system life-cycle (e.g. LCC life-cycle cost). This optimization can be done inside a single phase of the system life cycle, often production and maintenance. It can also be done at a broader point of view, through different phases of the life cycle. Typically, one can consider different feedback loops from use, maintenance and disassembly phases to the design phase of new systems, being production resources and products, and to their overhaul during their use phase. This vision is aligned with the concept of "intelligent product". More, considering the whole system as a fleet of cooperative CPS (sub-systems) will ease the interaction with logistics and maintenance centers, enabling the integration of new organizational processes such as industrial internet or opportunistic maintenance. CPS oriented approach will also ease the capitalization of knowledge from aging and near retirement operators. Last, by extending the lifespan of systems and infrastructures, constructor will also orient their decision towards sustainability, including opportunistic energy savings, deconstruction and re-use of functional manufactured part whose history is known thanks to the CPS principles. This industrial testimony could be federated into the projection of the evolutions of complex industrial systems in the near future merging a vertical integration of the control with a horizontal one through interoperability of information based on the technological element, development of new technologies of ICT, environmental and actual cooperation with humans. IMS² 2030 roadmap has been established according to this projection. It has been structured consistently with the four introduced topics and is detailed in the following.

A. Agility of systems enhancement

In the next decade will occur the development of truly changeable systems, namely adaptive and reconfigurable flexible systems (RMS). During the last decade, our work mainly focused on reconfigurability at the level of the control architectures with predefined layout topologies (i.e. FMS) or in industrial contexts, enabling the definition of fully flexible reference architectures and their performance evaluation. The next step is to go further in the domain of development of reconfigurable control architectures based on variable topologies, dedicated to RMS that are being technologically available. These late implementations may exploit Automated Guided Vehicles (AGV). The arrival of other mobile entities in workshops such as drones gives new perspectives to the sensing capabilities of the system. However, a huge effort still needs to be brought to the coordination of these mobile entities and their sensing abilities in a semi-structured environment such as the shop floors. The effort on Intelligent Product is still to be carried, as the possession of information by the product is however an efficient way to enable a late and online customization of the manufacturing. This concept is meant to become the actual way to make distinct manufacturing systems interoperable considering the life cycle of products and all the other resources. To do so, the control systems of each system need to be able to operate with a common standard expressed a priori. The methodological aspects and the way agile IMS-based control architectures may be designed are also to be addressed. Indeed, many architectures exist, but few works have been done to propose how to deploy, implement, apply such architectures considering, for example, the costs, targeted performances, safety, etc. Typically, questions such as when, why, and how to give the decision power to objects in a distributed/ decentralized way or when it is a suitable time to give it to the ERP or to the objects are now crucial to address [START_REF] Li | Fuzzy decisionmaking method for product holons encountered emergency breakdown in product-driven system: an industrial case[END_REF]. Future industrial systems will also be impacted by the development of cloud technologies, and especially the notion of Cloud Manufacturing will actually take shape. In a cloud-based manufacturing system, the shop floor is classically controlled in its hard real-time layer by enhanced controllers and the near real-time functionalities are exported into the cloud. There, a multi-agent based control architecture is shared between different companies, introducing their own parameters for the connection to their actual shop floor. Doing so, they can benefit from the dissemination of efficient negotiation and optimization methods and pave the way for a real integration of the whole supply chain and collaboration between subcontractors. More specifically regarding logistics and supply chain, the scientific progress proposed by IMS 2 group will contribute to the deployment of the concept of Physical Internet through the establishment of an open, global, efficient and sustainable logistics system based on CPS.

B. Breakthrough in Technology

On the technological aspect, many advances might be expected on the shop floor level. First of all, the growing interest on vision and on wireless sensor networks and RFID technologies faced in the last few years is meant to lead to the development of a truly disseminated network of data, with a globalized access to information throughout the geometry and lifecycle of the objects. Such a concept clearly modifies the way the technology was designed at first. A new approach of these technological components needs to be defined in order to cope with these aspects of dissemination and distribution of information, considering the RFID tag as a member of a network of data of a CPS and not only as an autonomous carrier. The next few years should focus on the integration of the various results of emulation-based performance evaluations into innovative hardware of CPS, such as those recently introduced in [START_REF] Gamboa Quintanilla | Evolution of a Flexible Manufacturing System: From Communicating to Autonomous Product[END_REF] or [START_REF] Pujo | Wireless Holons Network for Intralogistics Service[END_REF].

A second aspect deals with the development of communication and interoperability standards and ontologies able to be recursively operated through the whole control architecture, from the actuator to the cloud [START_REF] Panetto | ONTO-PDM: Product-driven ONTOlogy for Product Data Management interoperability within manufacturing process environment[END_REF]. We believe that the adaptation of service-orientation and web-services approach, coming from computer science field, is going to be the next major breakthrough in the future industrial systems. At the lowest level, the interface between the service layer and the implementation methods needs to be more accurately defined. Initiatives and standards such as IEC 61499 or OPC UA are promising leads, but more generic and open standards could be preferably developed for a global interoperability of systems and products. A third aspect directly intervenes in the design of the controllers themselves, might they be robot controllers or classical PLC. The current invocation mechanism of programs and methods needs to be brought a step further in order to enrich the description of the methods and ease the self-discovery of the capabilities of the whole system. Furthermore, this is a necessary step for cobotic field, which is expected in full expand in the next few years. The collaboration between the human and the cobot being the base of its efficiency, a higher level of semantic is needed for enhancing the use of multi-purposes robots.

C. Sustainability

Environmental concerns, industrial ecology, diminishing non-renewable resources, stricter legislation and inflated energy costs, increasing consumer preference for environmentally friendly products will foster the development of sustainable and eco-friendly manufacturing systems. Efforts to develop sustainable manufacturing systems must jointly consider products, processes, and systems [START_REF] Giret | Sustainability in manufacturing operations scheduling: A state of the art review[END_REF], [START_REF] Thomas | Are Intelligent Manufacturing Systems Sustainable?[END_REF]. In a circular economy, a major function to address in the future is the recyclability phase of the product or of the system. A lot of scientific questions have to be addressed: what to recycle? When and where have components or sub-assemblies to be sent to recycling firm? How to manage the reverse logistic? Some research works have been launched [START_REF] Derigent | End-Of-Life information sharing for a circular economy : existing literature and research opportunities[END_REF], but huge opportunities still exist in this domain. Even if it can be considered as a "local" and low-level concern, a typical prospect that the IMS² WG aims to address soon concerns the scheduling function, which is one of the most critical function in manufacturing (i.e. with the most level of risk of [START_REF] Giret | Sustainability in manufacturing operations scheduling: A state of the art review[END_REF]), and to pay attention not only to energy, but also to other environmental footprints such as wastes and GHG, as well as other means required to realize the schedules, in a predictive and reactive way. In such as context, CPS can be useful since they can provide through cooperation and aggregation balanced predictive and reactive decisions, based on a global view of the manufacturing processes and relevant accurate, up-to-date information, from lower levels (production control, PLC) to business levels. A first widening of this scope is to link the scheduling function with other critical function at the manufacturing level, such as maintenance and safety studies. Indeed, elaborating a scheduling without any attention paid to maintenance may lead to risky, potentially hazardous thus unstainable situations. Meanwhile, even this first widening still deals with a single phase, the manufacturing phase. Thus, a second widening would concern the different phases of the product and the resources lifecycle, from their design to their recycling. The IMS² WG is convinced that CPS, as well as other close concepts such as "intelligent products" and "product-services system" PSS could be highly profitable. Social aspects are also seldom addressed in our community. They must be considered in the future activities of the WG IMS². This must be done at the operator level (e.g., the impact of scheduling on the welfare of operators, capitalization of operators' knowledge into CPS…) to help them in their daily decisions [START_REF] Klein | An emulation-based execution control system evaluation tool: application to a furniture manufacturing group[END_REF], [START_REF] Pach | Routing Management in Physical Internet Crossdocking Hubs: Study of Grouping Strategies for Truck Loading[END_REF], [START_REF] Mebarki | Evaluation of a New Human-Machine Decision Support System for Group Scheduling[END_REF], but also at a supervisory level. Indeed, CPS systems may have a drawback coming from their ability to self-adapt and self-reorganize themselves: the confidence level of the human supervisor or other human-in-the-loop dealing, communicating or cooperating with CPS must be carefully addressed.

VI. CONCLUSION

In the current economic and social context, the IMS² challenges must be targeted by more and more research teams and must be developed and applied in industrial, logistic, and service (health contexts, communicating networks, and so on) domains. In the current Internet of Things context, the IMS² principle could be easily implemented; the algorithms, heuristics, and all tools coming from the operational research community have to be adapted to take into account the distributed/centralized aspects of the architecture of the decision system. Emerging concepts as coordination, cooperation, consensus in agents / holons communities, have to be studied, and new models have to be proposed to result in a new way to take dynamic, predictive, reactive, and real time decisions.

Occidental factories of the future will implement these kinds of technologies and concepts. However, as previously discussed, even if the IMS² principles are accepted and chosen as future objectives in the international industrial and research communities, we have to note that French institutions and major automatic control and computer sciences labs are still too hesitant to support this kind of initiative and research subject despite the leading activities of the WG IMS² in that field. Thus, a growing role of the WG IMS² is to foster the intelligent manufacturing approaches in France in order to promote more and more French research labs working in that field. IMS² is an innovative structure in Europe, and the results of the first years of existence are promising for the future. Such similar structures are emerging, such as AIDI 8 in Italy for example, which emphasizes the impact perceived by the community to this kind of initiative.

Figure 1 .

 1 Figure 1. Four of the IMS² platforms

http://www.univ-valenciennes.fr/bench4star/

ACKNOWLEDGMENT

The authors thank the French CNRS and the MACS Research Group for their support to the creation and activities of the workgroup IMS².