Mouna Fradi
email: mouna.fradi@isae-supmeca.fr

Faïda Mhenni
email: faida.mhenni@isae-supmeca.fr

Raoudha Gaha
email: raoudha.gaha@utc.fr

Abdelfattah Mlika
email: abdelfattah.mlika@eniso.u-sousse.tn

Jean-Yves Choley
email: jean-yves.choley@isae-supmeca.fr

Conflict Management for Mechatronic Systems Design

Keywords: mechatronic design, collaboration, multidisciplinary design, conflict management, category theory

Due to the multidisciplinary and complex nature of mechatronic systems, collaborative design becomes necessary in order to reduce the design cycle time and cost. This collaboration involves the use of numerous languages and expert models in order to represent distinct views on the same system. Consequently, conflicts among these models are likely to happen and need to be detected and resolved carefully. In order to capture and handle these conflicts in an easy and simple way, a unifying formalism based on mathematical concepts is required. Thus, we suggest the use of Category Theory (CT), motivated by its mathematical and formal foundation to handle conflicts in a formal way. This work presents a new framework able to check and manage conflicts in collaborative design. This framework has two key features. First, to detect the conflicts between the expert models in a formal way based on category theory concepts. And second, to manage these conflicts in order to ensure the coherence of the system under design. Our approach is illustrated through its application to an Electronic Throttle Body (ETB) from the automotive industry since it involves the collaboration among several expert domains.

I. INTRODUCTION

Mechatronic systems result from the synergic integration of electrical, mechanical, information technology and computer system [START_REF] Mhenni | Collaborative mechatronic design and systems engineering: an educational experiment with karren[END_REF]. This multiplicity of domains renders adopting collaborative processes necessary in order to ensure efficient communication between the different expert models (EMs) relative to each domain [START_REF] Mhenni | Sysml extensions for safetycritical mechatronic systems design[END_REF]. Since these models are performed by several stakeholders, they may present some conflicts that need to be handled carefully [START_REF] Dávid | Engineering process transformation to manage (in) consistency[END_REF]. This can be reached through creating a common formalism supporting conflict detection as well as resolution [START_REF] Zhu | Towards a categorical framework for verifying design and implementation of concurrent systems[END_REF]. The formal framework provides, on one hand, the detection of potential contradictions occurred among the expert models, and on the other hand, the resolution of these conflicts.

The choice of the appropriate technique for this issue is critical for attaining the objectives of efficient conflict detection and resolution. In this context, we propose to address this challenge through the use of a mathematical foundation, that is, category theory (CT). This theory, invented in the early 1940s, provides a structured and organized representation of thoughts using a set of objects related among each other through morphisms [START_REF] Mabrok | Category theory as a formal mathematical foundation for model-based systems engineering[END_REF], [START_REF] Spivak | Category theory for scientists[END_REF]. Our main goal remains in handling conflicts between data with fine level of granularity (constraints and parameters) using the formalism of CT.

The present article is organized as follow. Section II presents a review of related works. Section III provides the details of the proposed framework. Then, an application to an Electronic Throttle Body (ETB) is provided in section IV. Finally, section V concludes this paper.

II. LITERATURE SURVEY

A. Conflict management methodologies

The notion of "Consistency management" has appeared in the model-driven software engineering domain by Finkelstein et al. [START_REF] Finkelstein | Inconsistency handling in multiperspective specifications[END_REF]. Taylor et al. [START_REF] Medvidovic | Software architecture: foundations, theory, and practice[END_REF] suggested that the inconsistency can be described as a contradiction or a conflict between different model elements. The authors proposed five forms of this contradiction: name, interface, behavioral, interaction and refinement inconsistencies. However, only interaction and interface inconsistencies will be studied in the present paper. Interface inconsistency can be defined as mismatching terminologies or values between model elements. This conflict type can be presented when the same parameter has different values or when an "electrical connection", for example, is named "electrical wiring" in another model. However, interaction inconsistency appears when a constraint is not respected. Inconsistencies due to incoherent values or a non-respect of constraints will be considered in our study. Nonetheless, the remaining types of conflicts are out of the scope of the present research work. Several approaches are presented in the related literature. The relevant ones are described below.

Model synchronization approaches have been presented in order to manage conflicts. In this context, Legendre et al. [START_REF] Legendre | Toward model synchronization between safety analysis and system architecture design in industrial contexts[END_REF] proposed a model synchronization-based approach. This approach deals with the synchronization of safety analysis architecture design and assessment issues. The synchronization consists in identifying inconsistencies obtained from the confrontation of the different points of view. These conflicts are handled by finding compromises that evolve progressively the different models. An industrial case study is used to validate manually the proposed approach in this work. Consequently, some efforts have been suggested by Berriche et al. [START_REF] Beriche | Towards model synchronization in model driven engineering of mechatronic systems[END_REF], [START_REF] Berriche | Towards model synchronization for consistency management of mechatronic systems[END_REF] in order to automate as well as formalize model synchronization for mechatronic systems. This approach makes use of Query View Transformation (QVT) standard as a support of abstraction and concretization operations. The authors dealt with automated consistency checking. Nonetheless, conflict resolution is still based on a manual process. Model synchronization aims at maintaining consistency among distinct models. However, this approach does not deal with conflict management between fine granularity data, which is the main objective of the present paper.

Another interesting solution has been proposed to deal with conflict management using rules and patterns. Hence, Herzig et al. [START_REF] Herzig | An approach to identifying inconsistencies in model-based systems engineering[END_REF], [START_REF] Herzig | A conceptual basis for inconsistency management in model-based systems engineering[END_REF] make use of rules and patterns in order to automate conflict management in the context of Model-Based Systems Engineering (MBSE). The authors proposed to represent heterogeneous models through graphs that can be queried using patterns in order to detect as well as define inconsistencies types. Analogously, Feldmann et al. [START_REF] Feldmann | Managing inter-model inconsistencies in model-based systems engineering: Application in automated production systems engineering[END_REF] presented a new approach aiming at handling conflicts. A graphical modeling language is developed in this proposition to detect conflicts between design models. The use of rules in these approaches makes them a flexible solution due to the possibility of modifying these rules without changing the entire conflict management system. Nevertheless, such approaches are based on graphs which can overlook the practice insights and may not keep the right meaning of models used in the collaboration [START_REF] Kuang | Towards a formal reactive autonomic systems framework using category theory[END_REF].

Additionally, parameters and constraints-based approaches has been proposed in the literature. If, during collaborative design process, a violation of some constraints occurs, the conflict should be, carefully, handled. To do so, Badin et al. [START_REF] Badin | Knowledge configuration management for product design and numerical simulation[END_REF] presented a new model called the Knowledge Configuration Model (KCModel). It consists of Knowledge Configuration (KC), User Configuration (UC) and Information Core Entity (ICE). In KCModel, ICEs encapsulates Knowledge, which groups parameters and constraints. The ICE structure helps in detecting conflicts through the interdisciplinary constraints. Several research works present new approaches using KCModel such as [START_REF] Mcharek | Knowledge sharing for mechatronic systems design and optimization[END_REF], [START_REF] Fradi | Design of an electronic throttle body based on a new knowledge sharing engineering methodology[END_REF]. However, KCModel showed its limits in conflict handling. Based on the main concepts of KCModel, Mcharek et al. [START_REF] Mcharek | Collaborative design process and product knowledge methodology for mechatronic systems[END_REF] developed a new model named Collaborative Design Process and Product Knowledge (CDPPK). The main objective of CDPPK model is to reorganize parameters in order to ensure efficient collaboration. CDPPK model is inspired by the KCModel structure proposed in [START_REF] Badin | Knowledge configuration management for product design and numerical simulation[END_REF]. The authors make use of ICE concept and improved their model by proposing a new entity, that is, Design Prod-uct Knowledge (DPK). DPK contains all the ICEs and the collaboration results. This model suggests resolving conflicts following a manual manner which can be considered as a timeconsuming task.

As aforementioned, our main goal consists in formalizing the detection and handling of conflicts occurred between parameters and constraints during collaborative process of mechatronic systems. However, constraint and parameterbased approaches do not provide useful methods to resolve conflicts. Therefore, we propose to integrate the concept of rules and patterns in constraint and parameter methodologies since using rules and patterns will make the process more flexible. However, graphs used in such approaches can overlook the right meaning of the EMs. Thus, formal mathematical foundations become necessary to keep the right meaning and to carry out the intuitions of the practice. CT, the powerful mathematical tool, will be used in this paper to overcome the drawbacks of applying graphs to detect and manage conflicts and to provide a formal framework.

B. Category theory related approaches

Category theory was introduced by Eilenberg and MacLane to explore the relationship among algebra and topological field [START_REF] Mabrok | Category theory as a formal mathematical foundation for model-based systems engineering[END_REF]. A category can be formally defined as a collection of objects and morphisms. Considering a Category C consisting in [START_REF] Roman | An introduction to the language of category theory[END_REF] (see Fig. 1):

• objects (e.g., X, Y, Z).

• A collection of morphisms (e.g., f, g, h) relating the objects among each other.

• An identify morphism 1 X : X → X. This morphism must respect the following property : f

• 1 X = f = 1 Y • f.
• A composition morphism g • f: X → Z, where f: X → Y and g: Y → Z.

• An associative property if the composition exists. Considering the morphisms f, g and h, where f: X → Y, g: Y: → Z and h: X → Z, then h

• (g • f) = (h • g) • f.
A functor F: C → D between two categories C and D is a mapping from C to D while preserving structure. Here, the functor is considered as a mathematical truth conductor [START_REF] Roman | An introduction to the language of category theory[END_REF].

The powerful mathematical structure of CT is applied in several research works in different domains such as systems engineering and collaborative design. A categorical framework is presented by Zhu and Li [START_REF] Zhu | Towards a categorical framework for verifying design and implementation of concurrent systems[END_REF] in order to design and implement concurrent systems. This approach makes use of functors to check consistency between design categorical models and implementation categorical models. Furthermore, in an extended work presented by Zhu et al. [START_REF] Zhu | Representation of categorical specification of self-configurations in reactive autonomic systems framework[END_REF], CT was used to formally specify Reactive Autonomic Systems (RAS) and its configuration workflow. The proposed categorical approach can help to check errors in RAS. Additionally, Kibret et al. [START_REF] Kibret | Category theoretic based formalization of the verifiable design process[END_REF] proposed to formalize the Verifiable Design Process (VDP) using CT. Here, the different models of the VDP are presented through categories. The VDP parts are defined using objects and morphisms. The categorical structures of CT are used to define the different abstraction layers of the VDP. In this research paper, CT will provide a formalism that supports conflict checking and management. Compared to the aforementioned works, our proposition represents fine granularity data using the mathematical theory instead of representing the global system. Thus, manipulating mechatronic systems parameters and constrains will facilitate conflict management.

III. PROPOSED APPROACH

Actually, it is common practice for mechatronic collaborative design to make use of a wide range of expert models [START_REF] Ammar | Architectural design of complex systems using set-based concurrent engineering[END_REF]. Such models are interrelated which may present some conflicts between parameters of these models. The occurred conflicts must be detected and solved at an early stage in order to avoid activity re-execution. Hence, a conflict management methodology is presented in the present study. Our proposition provides a formal conflict checking and resolution in collaborative design. The proposed methodology is based on four main steps, as shown in Fig. 2 : (1) Creating a common representational formalism for expert models based on CT, (2) Defining consistencies using rule-based techniques, (3) Identifying conflicts by querying category patterns and (4) Resolving detected conflicts.

A. Step 1: Creating a CT-based formalism for expert models

Because of expert models heterogeneity, a common formalism becomes necessary. Models are defined as collection of interrelated elements. Thus, graphs provide a formalism generic enough to represent models. However, using graphs in models abstraction may ignore some insights of practice and influence the right meaning of models. Hence, we make use of CT concepts, motivated by its powerful formalization basis. The mathematical theory provides a common formalism of EMs and carries out practice intuitions. Here, parameters are defined as categories. Each category will contain the evolution of the corresponding parameter from an EM to another. The parameter evolution form an EM to another one, is represented as a set of objects related through morphisms. Our CT-based approach can be introduced as a 7-tuple F = {EM n , C m , O p , OL p , OL p , M q , M L q }. EM n refers to the different expert models (n is the number of the expert models). Parameters used in the different EMs are defined as categories (C m ) (m refers to the total number of parameters). In each category, the instances of the corresponding parameter are considered as objects (O p ) (Here, p is the number of objects in a category). These objects have an identity morphism (OI p ) and labels (OL p ). Parameter instances are related among each other through morphisms (M q ) and have different labels (M L q ) (q is the total number of morphisms in a category). An excerpt of the common formalism based on CT is shown in Fig. 3. Fig. 3.a illustrates the evolution of a parameter P 1 from the first expert model (EM 1 ) to the second one (EM 2 ), whereas, Fig. 3.b refers to the evolution of P 2 from EM 1 to EM 3 . These categories should conform to the formal definition of category mentioned beforehand. Thus, we have to prove the existence of identity, composition and associativity. Let P 1 E M 1 and P 1 E M 2 two objects representing different instances of the parameter P 1 such that EM 1 interacts with EM 2 through the morphism "activity 1". Let V 1 E M 1 and V 2 E M 2 two objects to illustrate P 1 values in EM 1 and EM 2 . Each object has an identity morphism, which aims Fig. 3. An excerpt of parameters categories [START_REF] Fradi | Conflict resolution in mechatronic collaborative design using category theory[END_REF]. at representing its internal evolution. We represent this morphism through the looped arrow (see Fig. 3) only for one object to avoid cluttering the representation of categories. P 1 E M 1 evolves to P 1 E M 2 and P 1 E M 2 has a value V 2 E M 2 . Then, P 1 E M 1 can have the value V 2 E M 2 indirectly through P 1 E M 2 which highlights the morphism of composition. Let "activity 1", "allocate value 2" and "allocate value 3" be three morphisms where "activity 1": P 1 E M 1 → P 1 E M 2 , "allocate value 2": P 1 E M 2 → V 2 E M 2 and "allocate value 3":

P 1 E M 1 → V 2 E M 2 . It is clear that activity 1 • (allocate value 2 • allocate value 3) = (activity 1 • allocate value 2) • allocate value 3.
This verification must be applied to all the created categories in order to ensure the respect of the mathematical definition of a category.

B. Step 2: Defining consistencies using rule-based techniques

The second step of our process remains in defining consistency rules (CRs) in order to describe the constraints to be respected during the design process. A consistency rule can take the following form: CR 1 = " P 1 E M 2 must be equal or less than P 1 E M 1 ". If one of CRs is not respected, a conflict is detected and must be handled.

C. Step 3: Identifying conflicts by querying category pattern

In this step, the conflicts must be detected. Based on the CRs created previously, the existence of conflicts can be described through a category pattern. These patterns aim at illustrating the conflicts that can exist during the design process. By mapping these patterns categories against each parameter category, conflicts can be captured. This mapping is formally achieved through functors as illustrated in Fig. 4. The morphism between m 1 and m 2 in the category pattern represents the non-respect of a CR. Fig. 4. A mapping between pattern and parameter categories [START_REF] Fradi | Conflict resolution in mechatronic collaborative design using category theory[END_REF].

The image of y 1 , y 2 , m 1 and m 2 through the functor G in pattern category are respectively

P 1 E M 1 , P 1 E M 2 , V 1 E M 1 and V 2 E M 2 in P 1 category
. By means of this mapping, consistency verification among P 1 values becomes simple and formal. We need, only, to create a functor from pattern category toward the parameter category to detect conflicts.

D. Step 4: Resolving detected conflicts

Once conflicts are detected, an action to handle them must be taken by the project manager. conflict handling can be either tolerating, ignoring or resolving it. The project manager can tolerate a conflict by proposing a slight deviation form the required values or ignore this conflict. However, resolving the conflict means informing the concerned engineers of the detected conflict in order to ameliorate their expert models until finding the appropriate trade-off between them. Let be P 1 E M 1 and P 1 E M 2 , two instances of the rise time defined in requirement model and in the multi-physical model respectively. This conflict can be handled by tolerating it: " If P 1 E M 2 -P 1 E M 1 ≤ α then tolerate", where α is the tolerance measure defined by the decision maker.

Thanks to the proposed methodology, it is possible to detect and manage conflicts in the context of mechatronic design. Our approach is considered flexible thanks to the use of CRs, which can be modified or removed without influencing the whole process. In addition, conflict checking is based on formal and mathematical concepts of CT, which helps in carrying out the practice intuitions. To demonstrate the capacity of our approach, an application will be presented in the following section.

IV. CASE STUDY

The Electronic Throttle Body (ETB) is defined as a mechatronic system that regulates the airflow entering to the engine to modify its output torque [START_REF] Mcharek | Collaborative design process and product knowledge methodology for mechatronic systems[END_REF]. ETB system contains a controller, a DC motor, a reducer and a failsafe system composed of two springs, a throttle plate and a position sensor as illustrated in Fig. 5 adopted from [START_REF] Mcharek | Multiphysical design approach for automotive electronic throttle body[END_REF]. Fig. 5. Electronic Throttle Body architecture adopted from [START_REF] Mcharek | Multiphysical design approach for automotive electronic throttle body[END_REF].

The motivation behind using the ETB system to validate our methodology is due to its multi-disciplinary nature which helps in illustrating the conflict detection and resolution concept. Thus, our proposed methodology, described beforehand, will be applied to the ETB system.

A. Step 1: Creating a CT-based formalism for expert models

A common formalism for the expert models involved in ETB system design is created in this step. We consider four EMs to illustrate our methodology : (1) a requirement model (EM R ) that identifies the requirements defined for ETB design process, (2) a multi-physical model (EM M P ) created to perform dynamic analysis, (3) a Commercial Off-The-Shelf (COTS) components model (EM C ) which contains the characteristics of ETB system components and (4) a 3D model (EM 3D ) that verifies the mechanism integration. These models will be further described in the following.

(EM R ) is created using SysML language within Magic Draw software [START_REF] Magicdraw | Dassault systèmes[END_REF] in order to specify the requirements to be respected during the ETB design process as described in Fig. 6.

EM M P is a multi-physical model based on Modelica language using Dymola environment [START_REF] Dymola | Dassault systèmes[END_REF] as shown in Fig. 7. Once the multi-physical model is created, the existing Commercial Off-The-Shelf (COTS) components will be chosen conforming within the results of simulation. This model (EM C ) highlights the characteristics of the selected components as show in Table . I.

The last EM is a 3D model within CATIA software (EM 3D ) [START_REF] Catia | Dassault systèmes[END_REF]. It is established to verify the integration of the selected components in the allocated area (see Fig. 8).

In this paper, a well-defined order to create the EMs must be taken into account. The first phase in the ETB design remains in developing the requirement model (EM R ). Then, the multiphysical model (EM M P ) is established. Furthermore, the COTS components in (EM C ) are selected based on EM M P . Finally, the 3D model (EM 3D ) will provide a verification of selected components integration. At this level, the required EMs for ETB system design are established. Hence, the common representational formalism is needed. The CT-based formalism illustrates the evolution of ETB system parameters. Here, we represent parameters as categories and its different instances as objects. The evolution of this parameter form an EM to another one is established through morphisms. For each parameter a category is created. An excerpt of some parameters categories is illustrated in Fig. 9. The "required power" category is represented in Fig. 9.a. The requirement model (EM R ) provides the first value. This value evolves into new values in EM M P model and the COTS model (EM C ). The identity morphism of each parameter is represented through a looped arrow and highlights its internal evolution. Similarly, Fig. 9.b describes the evolution of the "rise time" parameter (Rt) from the requirement model to the multi-physical model. All the parameters needed for ETB system design are listed in Table II.

B. Step 2: Defining consistencies using rule-based techniques

The second step in our approach validation consists in defining a set of consistency rules to illustrate the constraints that have to be respected during the ETB design. An excerpt of consistency rules in ETB system is given in the following form: CR 1 = P w E M M P must be equal or less than P w E M R , CR 2 = rg E M C must be equal to rg E M M P , CR 3 = M ass E M C must be equal or less than M ass E M R , CR 4 = Rt E M M P must be equal or less than Rt E M R , CR 5 = .....

C. Step 3: Identifying conflicts by querying category pattern

Using the CRs defined previously, category patterns have to be created to describe the non-respect of the constraints (see Fig. 4). At this level, all what we need is to match, through functors, the category patterns against each parameter category in order to detect the conflicting values of each parameter. In Table II the conflicting values are shown through the highlighted rows. An example of a mapping among patterns and parameter categories is illustrated in Fig. 10. This mapping will be repeated for all parameters categories.

D. Step 4: Resolving detected conflicts

In this last step, the detected conflicts are managed. As mentioned beforehand, three handling actions (i.e., tolerating, resolving and ignoring actions) can be taken according to the previous experiences and knowledge of the project manager. Table III contains the handling actions taken to manage conflicts appeared during the ETB system design.

The project manager tolerates the conflicts occurred in the "static error" and "global mass" parameters. Moreover, the value mismatch of "cost" and "inertia" parameters is ignored. However, the conflict between the values of the motor power and length parameters are resolved through a close exchange between the involved engineers in the conflict until finding the appropriate trade-off among them.

Fig. 1 .

 1 Fig. 1. Category theory basic concepts [20].

Fig. 2 .

 2 Fig. 2. Proposed conflict resolution process.

Fig. 6 .

 6 Fig. 6. SysML requirement diagram of the ETB system.

Fig. 7 .

 7 Fig. 7. Multi-physical model of the ETB.

Fig. 9 .

 9 Fig. 9. Required power (Pw) and Rise time (Rt) categories in ETB system.

TABLE I COMMERCIAL

 I OFF-THE-SHELF (COTS) MODEL (EM 3 ) (AN EXCERPT OF DC MOTOR CHARACTERISTICS EXTRACTED FROM[START_REF] Maxon | Maxon group[END_REF]).

	Values at nominal voltage
	Nominal voltage	12 V
	No load speed	8130 rpm
	No load current	320 mA
	Nominal speed	7610 rpm
	Nominal torque	77.7 mN.m
	Stall torque	2080 mN.m
	Max. efficiency	86%

Fig. 8. 3D model of the ETB using Catia environment.

TABLE III HANDLING ACTIONS TO MANAGE CONFLICTS

Conflicting values

Handling action

CONCLUSION AND FUTURE WORK

A new methodology has been proposed in this research paper in order to manage conflicts during mechatronic systems design. This methodology was based on a combination of fine granularity data and pattern-based approaches. Category Theory was applied in this methodology as a tool for knowledge unification and formalization. CT provided a formal conflict checking using the functor concept in order to map parameter and pattern categories. However, during the conflict management process, parameters dependency was overlooked, which is considered as a critical issue in mechatronic systems. Furthermore, decisions are taken considering the project manager experiences. This makes conflict resolution centralized on its decision.

In future works, an attempt to use CT in parameters dependency representation will be made. The integration of decision-making methods in the presented methodology will be also explored in order to facilitate conflict resolution.


The city through the senses

Jean-Paul Thibaud

Urban ecology of the senses

The contemporary city is undergoing numerous changes which are redefining its appearance and accompanying the emergence of new frameworks of sensitivity. The magnitude of such transformations is such that the term "city" is becoming more and more obsolete and it is now more appropriate to talk about "urban" instead. Such profound changes to urban life call for new theoretical perspectives and new intelligibility models to describe them. Among these, we can list an increasing interest in the sensory environment of inhabited spaces. Recent publications bear witness to this and increasingly focus on perception, landscape, sensations, the body, ambiances and other terms directly related to the ordinary experiences of city dwellers. Although it is surely an exaggeration and premature to speak of a sensory turning point in contemporary research, we can nonetheless identify the increasing presence of such topics in the social sciences. Put another way, aesthetic issues are no longer seen as secondary or ancillary; they are well on the way to becoming a key element in current urban thought. This approach, in which the body and senses are allowed to exist, scorns strict disciplinary divisions and operates within a very broad scientific spectrum. Whether one wants to study the fit between the sensory and the social, update cultural perception schemata, write a history of sensibilities, take measure of lived space, design architecture for the senses, rethink the place of the senses in philosophical thought or derail common perception through artistic performance, all involve reference to experience and pay particular attention to the sensory register. In brief, a whole collection of paths are open, which intersect and complement each other in the development of a sensory ecology of everyday life in the widest sense of the term. It is as if we are witnessing a fundamental shift that is redefining how we think about the current world around us.

If we look more closely at research into cities, it quickly becomes apparent that this general trend provides fertile territory for reflection. After Henri Lefebvre's focus on "practico-sensory realms" in the city, there are many proposals which aim to introduce the inhabitant body into urban sensations. To this effect, walking is often taken as the starting point for thought and allows for the city dweller's sensory relationship with his or her close environment to be examined. Consequently, we can describe ways of living in the city and reveal the affective power of places by asking questions about the "social imaginary". The senses thus become the starting point par excellence for inhabitant expression. Alternatively, focus is directed more towards the social habits of city dwellers to describe their various strategies and habits. Activities as banal as walking or sitting down are minutely observed, with the utmost attention paid to the sensory context in which, and from which they occur. Others are interested in public sociability and in the layout of social interactions. The exchange of glances thus plays an extremely important role, testing polite avoidance between passers-by and testing the civility of the human eye. Finally, it is the very materiality of the city that is screened by sensory perception, revealing and detailing the "lived features" of built environments. Using perception appears to be not only possible but actually inevitable for those seeking to capture and restore the concrete nature of the urban experience; depending on the context, it takes a more sensory or cognitive approach.

As the planet's environmental problems become increasingly evident and urgent and the public nature of urban spaces is largely challenged, it seems more important than ever to embark on in-depth reflection about the sensory city. It is not a matter of droning on about energy problems, climatic, political, economic, demographic or technical issues, but rather about how we inhabit the urban world, fit in and experience it in everyday life. The contention here is that the senses are the backdrop of an inhabiting experience, a focal point between ecologies of society, of the mind, and of the environment (Guattari, 1989). The challenge is vast since it involves understanding how large-scale changes to urban territories are embodied in, and spread through everyday life. If the sensory field can claim to be relevant here, it is above all because it is one of the most present and immediate expressions of an environment undergoing change. This involves elaborating an aesthetic paradigm for renewed thought about urban ecology. One of the salient issues is thus to create an urban ecology of the senses which provides access to the socio-aesthetic framework of ordinary experience. How does contemporary urban space harness the senses of city dwellers? How does sensory experience work both as an analyser and operator of current urban change? What does thought about the sensory city tell us about current life forms and ways of living together? What conceptual and methodological tools can we use to begin fieldwork in this area? Such questions have given rise to several currents of thought which we shall briefly introduce. Without attempting to be exhaustive, we shall discuss three main currents which all take a sensory approach to the city:

the aesthetic of modernity which draws on the consequences of changes to large cities at the start of the last century;

environmental aesthetics particularly attentive to the place and role of nature in living spaces;

the aesthetic of ambiances concerned with the affective tonality of architectural and urban spaces.

Before briefly introducing each of the above, let us note that the term aesthetic must be understood here in its original meaning of aesthesis, i.e., perception by the senses and not only as judgement of taste or philosophy of beauty.

The aesthetic of modernity

Urban thinking has been marked for over a century by various types of research into the sensory experience of city dwellers. We can trace the origins of this orientation back to the pioneering work of Georg Simmel, particularly his essay on the metropolis and mental life, followed by the works of Siegfried Kracauer and Walter Benjamin. Despite major differences between these three authors, they do share common traits and each has sought to combine, in their own way, sociological thought and an aesthetic approach. By emphasising the way urban environmental changes affect the city dweller's experience and perception and, in doing so, modify the human sensorium [START_REF] Simay | d'une ville à l'autre[END_REF], these authors paved the way for an urban ecology of the senses which has continued to develop right up to the present. We know the arguments used at the start of the last century to describe the aestheticisation process of modern cities: the over-stimulating nature of metropolises, the dulling of city dwellers' senses, the onset of distracted reception, the dominance of vision over the other senses and the loss of communicable experience in favour of an aesthetic of shock. Such phenomena shaped the way the sensory relations of large city dwellers were reconfigured at the start of the 20 th century.

Such research has the merit of showing that a specifically urban sensory ecology indeed existed, with its own properties and characteristics. It was by focusing on large European cities -Berlin and Paris in particular -that such authors were able to think about the transformation of the structures of modern experience. It was not a matter of analysing the underlying reality of the sensory world and making it an autonomous field free of all determination, but rather of connecting it to its conditions of material, technical, social, cultural or historical potential. As Walter [START_REF] Benjamin | oeuvre d'art à l'époque de sa reproductibilité technique[END_REF] has stated, "The manner in which human sense perception is organized, the medium in which it is accomplished, is determined not only by nature but by historical circumstances as well."

In other words, we cannot describe the sensory experience of the city without also explaining the conditions which nurture it and make it possible. The challenge is to show how a change in sensibility occurred at the start of the last century, how a mass aesthetic was born and affected the various changes at work in large cities.

Siegfried Kracauer and Walter Benjamin took a more critical approach and were more overtly political than Georg Simmel; whilst Kracauer happily delved into the empirical reality of the period, Benjamin preferred the archaeology of modernity. But all three authors developed a common approach which showcased the microscopic and concrete phenomena of everyday life. New means of perception in the urban milieu were embedded in the facts and gestures of passers-by; they occurred in the most banal situations and crystallised in the built infrastructure of cities. A whole mosaic of phenomena which shaped and filled the sensory world was thus reviewed: for example, the city dweller's reserve which protects from excess stimulation, silent face-offs in the tram which reveal the importance of the eye, the furtive greetings of early-morning taxi drivers learning to respond to the acceleration of exchanges, the abundance of shows in Berlin revealing the cult of distraction, the phantasmagoria of covered passageways devoted to the world of goods, the effects of close-up shots and slow motion in nascent cinema, and training spectators in the experience of being shocked. In brief, the most ordinary details of urban life encompassed the sensibility of an entire era.

Rather than offering an inward-looking system, the approach consisted in multiplying the microscopic scenes of everyday life to sketch the portrait of modern sensory culture. The city was thus seized by "phenomena of mediocre importance" (G. Simmel), "discrete surface manifestations" (S. Kracauer) and "dialectical images" (Benjamin) which needed to be observed, described and understood. In other words, not an urban ecology of the senses that takes an omniscient approach, but rather an approach based on the strangeness of everyday situations. Yet, if the authors were disenchanted with modernity, this was revealed more in images and forms than in concepts. Hence the particular attention paid to the ways of describing the urban experience. Thence the new types of writing tested to reproduce phenomena such as dispersal and distraction, raw sensations and the violent commotions which constituted the atmospheres of the period. Fragments, collages, essays, chronicles, serial writing and citations were all formal proposals which allowed for a reading of the shock aesthetic at work in modern metropolises. Methods of exposure which were much more in phase with the very structure of the urban experience replaced linear and monological discourse. This surely lay at the juncture between the social analysis of modernity and the narration of urban ambiances.

Environmental aesthetics

A second, more recent approach which has primarily been used in research in the English-speaking world treats the environment as a possible field for aesthetic experience. Such research also aims to move beyond the aesthetics of fine art and does not judge specific works or isolated objects in their entourage, but rather within the living spaces in which human beings are immersed. Strongly influenced by the 18 th century aesthetics of nature, environmental aesthetics developed over the last third of the 20 th century under the influence of citizens' increasing ecological sensitivity. Taking the natural environment as its model, it is most concerned with the art of landscapes and gardens, wild spaces and agricultural areas. Whilst this approach first used the multiple facets of the aesthetics of nature, it gradually began to look at urban and built environments, as well as into the vast field of environmental art. Research as diverse as that addressing Disney World fantasies, the vision of motorists in movement, interior design, walking in the city or shopping in a shopping centre has emanated from this tradition (Berleant, [START_REF] Carlson | On Aesthetically Appreciating Human Environments[END_REF]. It is obvious that the term "environment" is no longer restricted to the natural world, but now also covers a whole range of ordinary situations. A recent development in environmental aesthetics has led to its connection with the aesthetics of everyday life (Light and Smith, 2004).

Far from being homogenous, environmental aesthetics have thus far contributed to two relatively distinct perspectives: one which is predominantly cognitive, with an emphasis on the diverse knowledge involved in the environmental experience, and one which is predominantly sensory and more closely connected with the immediate, affective and multi-sensorial nature of such an experience. While the former is more focused on the role of cognitive frameworks in aesthetic appreciation of the environment and on the importance of scientific knowledge and cultural traditions, the latter tends to highlight the contextual character of any aesthetic experience, on the immersion of the perceiving subject within the surrounding world in which he or she is involved. Thus, Cheryl [START_REF] Foster | The Narrative and the Ambient in Environmental Aesthetics[END_REF] has distinguished between "narrative" and "ambient" environmental aesthetics.

It is Arnold Berleant who laid the foundations of this second, sensory and pragmatic approach when he developed the joint notions of continuity and engagement. The notion of continuity involves challenging the dualist approach which separates the mind from the body, natural from cultural and the human being from the environment. As Berleant (1992) has argued in the tradition of John Dewey, "There is no external world. There is no outside. There is no internal refuge in which I can protect myself from unfriendly external forces either. … People and environment are continuous." The environment is not a simple container or an external entity which can be studied independently of the experience it creates. In this perspective, the human being is necessarily connected with the world of which he or she is a part. It is as such that we can talk of aesthetic engagement, which is one of the key notions of environmental aesthetics. Rather than conceiving the subject as an uninvolved observer of the world it observes, the subject must be seen as an active participant engaged in the situations that it confronts. Corporeal immersion involving an attitude of active involvement thus replaces a distancing mechanism based on a disinterested attitude. It must however be noted that the idea of aesthetic engagement is the subject of much debate within environmental aesthetics. Some authors refuse to oppose contemplation and participation too radically and to systematically favour the latter to the detriment of the former [START_REF] Leddy | The Nature of Eveyday Aesthetics[END_REF].

Finally, we must point up the direct connection between environmental aesthetics and the environmentalist movement as it emerged in the United States in the 1960s. Although it is no longer only about contributing to conservation and protecting the natural environment, this ecological and voluntary orientation nonetheless remains very present in research devoted to purely urban issues. Noting the deterioration in the environment, environmental aesthetics attempts to isolate the conditions necessary for a happy urban existence. Although the urban environment is bursting with numerous aesthetic resources and potentiality, this does not necessarily mean that it is always capable of providing a full and complete experience. Of course, particular attention is given to the place of nature in the city, but, as Arnold Berleant has shown, we must also recognise the extent of the urban ecosystem's complexity and seek to examine its functional, imaginary, metaphysical and cosmic dimensions. Numerous proposals have thus been formulated to identify the criteria required for a quality urban environment understood in aesthetic terms: an environment which encourages freedom to act, provides a multi-sensory experience and seeks to involve city dwellers closely. Since it takes a meliorist approach which aims to improve urban conditions, environmental aesthetics is thus forced to address ethical issues as well. One field of thought is debating how the urban environment could harbour and express positive values, or, as Allen [START_REF] Carlson | On Aesthetically Appreciating Human Environments[END_REF] has suggested, "look as it should."

The aesthetic of ambiances

A third aesthetic approach deals with urban and architectural ambiances. Heavily influenced by phenomenology and focusing on the built and material aspects of inhabited spaces, the ambiances field has developed apace over the past twenty years and is comprised of a double movement.

The first movement -that of determination -involves clarifying and defining the notion of ambiance. The notion's approach has become more complicated over time and has given rise to a number of reformulations. The classic vision of "controlling ambiances" involved in propagating signals in built spaces and defining a built environment from a purely physical point of view has been replaced by a more interdisciplinary conception which puts sensory perception and aesthetic experience back at the forefront. The social sciences are involved and work alongside the design sciences and engineering sciences. Put briefly, ambiance is thus defined as space-time experienced in sensory terms. More qualitative and open, this new intelligibility model of the notion of ambiance was shaped over time and developed its own categories of analysis (sound effects, ambient objects, sensory configuration), in situ investigation methods (commented approaches, recurrent observation, acoustic reactivation, sensory ethnography) and modelling tools (declarative modelling, morphodynamic models, inverse simulation) (Les Cahiers de la Recherche Architecturale, 1998;Grosjean, Thibaud, 2001).

The second movement -that of differentiation -involves suggesting an alternative to other approaches to the sensory environment of cities and this is how ambiance differs from all other constraint-, functional-, comfort-, or landscape-based approaches. It is a fifth axis to seeking transversal connections between sensory, spatial and social forms. By proceeding as such, the ambiental approach attempts to free itself from normative environmental perspectives; it distinguishes itself from an excessively positivist approach and from a strictly psycho-physical orientation; it emphasises the activity of the perceiving subject and the role of social customs in the sensory design of built infrastructure and focuses attention on the affective tonalities of urban life. One of the fundamental aspects of the notion of ambiance is that it postulates the unity of the sensory world rather than first dissociating the senses and then seeking to reunite them later. Hence the difficult question of inter-sensory and synesthetic phenomena which make up an ambiance [START_REF] Böhme | Über Synästhesien / On Synaesthesiae[END_REF]. Through its various contributions, ambiance has set itself apart from neighbouring fields of knowledge and has formulated a defining set of questions (Amphoux, Thibaud, Chelkoff, 2004).

The aesthetic of ambiances is part of the general context of urban and architectural research. The objective is clearly very ambitious since it involves nothing less than reviewing the aesthetic discipline itself, in the light of the notion of ambiance. Whether in the research of Gernot Böhme in Germany or Jean-François Augoyard in France, the two main founders of this aesthetic, the goal is to return to a phenomenological theory of sensory perception in order to have a means of conveying urban atmospheres. To this effect, both authors agree on the need to challenge the traditional division between subject and object. Defined as the moral and material atmosphere surrounding a place or person, ambiance is precisely a notion which challenges such a divide and pushes for its deconstruction. Thence, Jean-François Augoyard (2008) -along with Hermann Schmitz, one of the pioneers of this new phenomenology -has identified four operations at the root of this dichotomy: "objectivation (placing a perceived exteriority in front of a psychic interiority), psychologisation (autonomisation of the experience lived out by "the me"), reductionism (decomposition of what is felt into abstract elements), introjection (smoothing out, or even forgetting, the dividing process and the privatisation of the senses)". The role of the aesthetic of ambiances consists in suggesting an alternative to this way of thinking by showing how ambiance both precedes and is indissociable from the material properties of the environment and the affective state of the perceiving subject.

The notion of ambiance allows us to reintroduce the senses into the experience of living spaces and to characterise our ways of experiencing urban life but it also helps to conceive of and create urban and architectural spaces. It does not exist solely on the level of sensory reception, but also on the level of material production. Creators as diverse as Adolphe Appia with stage space, Michael Chekhov with theatre performance and Peter Zumthor with architectural design have shown the usefulness of atmospheres in the creative act. In each case, ambiance is what brings life to an environment and what gives it its overall effect and unique tone. As Michael Chekhov (1991) has stated, "Devoid of atmosphere, a performance becomes very mechanical". This comment is not only true for artistic creation, but also applies to the multiple inventions and creations of everyday life. In brief, ambiance creates poiesis as well as an aesthetic of built environments. The challenge is then to get a fit between these two dimensions and relate them to current changes to the city.

Sensitising the urban world

If a sensory approach to the city is proposed by the above three aesthetics, it also seeks more generally to permeate most research dealing with the design of contemporary urban space. There are few current urban theories and approaches which do not include the senses in one form or another in their discourse, whether they use them as a factor in urban governance, a criterion for urban planning or as a means of communicating a project. In other words, the advantage of a sensory approach resides not only in the path it forges towards a phenomenology of urban experience, it also makes perfect sense in creating the city itself. If this is the case, it is surely no coincidence given that the sensory environment lies at the juncture of city dwellers' quality of life, city socio-economic strategies and ecological problems.

One of the most striking aspects of current urban ecology is that it is increasingly based on a voluntary and intentional policy of sensitising the city and on explicit strategies for giving ambiance to urban spaces. Take the massive tendency (at least in rich countries) of landscaping the built environment, taking a scenographic approach to everyday places and animating public spaces, for example. Yet if we can discuss the creation of ambiances in urban spaces here, it is because these bases of involvement no longer operate solely within the material and physical framework of cities, but also on their sensory and immaterial components. Inhabited spaces are no longer designed solely from a visual point of view, they also tend to be designed based on sound, light, smell, heat and air as well. Projects which aim to transform the urban environment increasingly involve the full range of sensory mechanisms. It is as if the predominance of the visual at work in the aesthetics of modernity is gradually moving towards a rebalancing of the senses. The last Venice Bienniale is a good illustration of a state of the art in the subject. Further, this acceptance of the multi-sensory dimension of urban spaces has been accompanied by increased attention to the affective tonalities of inhabited spaces. The urban sphere seems to be witnessing a dual movement of festive programming and draconian security measures across a wide spectrum ranging from "ecology of fear" (Mike Davis) to "ecology of enchantment" (Christine Boyer).

In this respect, the three aesthetics discussed above can work, each in its own way, towards better identifying the challenges and questions of such ambiance creation in the contemporary world. In giving the sensory environment a socio-historical dimension (the aesthetic of modernity), by introducing ethical questions into the aesthetic assessment of urban areas (environmental aesthetics) and in characterising our ways of experiencing and creating the sensory world (the aesthetic of ambiances), these approaches allow us to develop models for making sense of the urban changes underway. As such, they help us examine how ambiance influences the current urban ecology of the senses.

To conclude, let us mention a few research pointers which might stimulate thought on the future of the sensory city.

Clarifying how the notion of ambiance is used

It is becoming increasingly necessary to clarify how the notion of ambiance is to be used. Let us give two particularly revealing examples of creating ambiance in urban spaces: that of sensory marketing (retail atmospherics) and that of technological environments (ambient intelligence). As different as these two approaches are in their aims and scientific bases, they reveal the effectiveness of the notion of ambiance in creating new spaces which conform to today's changing world. Whether they use ambiance to develop new retail strategies and encourage buying behaviour [START_REF] Grandclément | Climatiser le marché. Les contributions des marketings de l'ambiance et de l'atmosphère[END_REF] or invent new perception mechanisms and automate everyday tasks (Wright et al., 2008), such endeavours offer technologies from the sensory environment adapted to carefully targeted objectives, turned towards functional strategies and especially attuned to economic opportunities in the contemporary world. We need to clearly distinguish between these two fields of action as their aims and effects are really very different.

Let us simply highlight the already operational nature of the ambiance notion in certain fields of activity which are helping to change the current urban world (e.g., new types of business and distribution, domotics, augmented reality, embedded technologies). We must however be careful as use of the ambiance notion can in no way be reduced to such strictly commercial and technical logics. Instead, the theory of ambiances actually allows us to put excessively utilitarian and instrumental uses into perspective by challenging our ways of inhabiting and creating today's world together. The issue is thus not so much to influence behaviour for commercial ends or even to give city dwellers some relief from the weight of their bodies, but rather to examine the meaning and consequences of such endeavours. In brief, in order to clarify what the notion of ambiance means, it is indispensable to discuss the scientific premises and strategic stakes as well as the ethical values which preside over its diverse uses.

Updating the city's socio-aesthetic challenges

The creation of ambiance in urban spaces is not devoid of socio-aesthetic factors which we need to update as far as possible. Working towards a sensory reading of city environments involves not only closely observing changes underway, but also taking a critical look at their effects and implications. In brief, it is a matter of learning lessons from the social and political nature of the "distribution of the sensible" (partage du sensible) (Jacques Rancière). Here, we are concerned with the future of urban public spaces and of conceptions about our ability to live in a shared world. This sensory application of the public nature of an urban space can be broken down in at least two ways. On the one hand, we may wonder whether certain urban planning aesthetic choices are also a means of "redistributing places and identities" (Rancière, 2000), or even of asserting the predominance of one segment of society. Whether we are talking about an "eco-health imaginary", clean urban planning or new social hygiene, it is as if the search for a sanitized environment occurs alongside the relegation of certain social categories deemed undesirable. On the other hand, the increasing control over city sensory environments -using light, sound, ventilation, odorising and other techniques and animation strategies -has tended to produce increasingly conditioned spaces and has left little place for the rituals of interaction between passers-by or opportunities for public improvisation. Does this new control over ambiances not run the risk of producing public spaces that are excessively neutralised, formatted and pacified, hence limiting opportunities for exchange and small run-ins between passers-by? Further, does an excessively artificial environment not lead to a relative loss of contact with reality, resulting in a sense of estrangement, a hallucinatory-type experience or anaesthetised perception? Such questions merely take those raised by the aesthetics of modernity a little further.

Reforming understanding about sensory ecology

Approaching urban spaces from a sensory perspective undoubtedly requires that new frameworks of thought be elaborated and applied. The notion of ambiance is a good example of great heuristic and operative potential that allows questions to be asked, and addresses issues heretofore ignored; yet it nonetheless remains difficult to define theoretically and to understand empirically. This is because ambiance cannot be reduced to a sum of specifically circumscribed factors but rather colours an entire situation by spreading around. Impregnation, radiation and contamination are in this sense key terms which allow us to define the phenomenon of sensory distribution. Like atmosphere or climate, ambiance works like a medium that blends the most diverse components in a situation into a single note and, in doing so, bestows them with an overall physiognomy and unity. The diffuse, undivided and intangible ambiental world replaces the carving up of reality into discrete entities. In brief, developing a sensory ecology presupposes the ability to free ourselves from the ethics of things and to think in terms of milieu and relations based on a logic of vagueness. Yet, as we have just seen, this involves challenging the traditional opposition between sensing subject and perceived object, since each is actually one side of the same coin. Although from very different horizons, several contemporary philosophers are focusing on relational, formless, envelope, immaterial or atmospheric thought, all categories which help break away from an overly dualist or substantialist orientation. This endeavour can involve a rereading of intangibles or of doxa (Anne Cauquelin), a spherological conception of human space (Peter Sloterdijk) or a major detour through Chinese philosophy (François Jullien). All attempt to highlight borrowings from the ambient world and from the sensory texture of our ways of living. All are paths working towards a new paradigm which will allow us to think about the indistinct foundations of contemporary ways of understanding the world in new light.

Towards a pragmatic ecology of the sensory city

The first three ideas, which attempted to clarify ambiance (what are its uses?), assess the practical consequences of the notion (what are its effects?) and review the thought frameworks upon which it is based (what are the developments?), have opened the way for a pragmatic ecology of the sensory city. To this effect, the issue is not so much to ask what an ambiance is or to try to define the notion once and for all, but rather to reflect on what it does and becomes, and what it is likely to affect and transform when it is physically experimented and tested in current situations. This is why the privatisation of public space and the conditioning of built environments were touched upon briefly. We can also ask how an ambiental approach opens new perspectives for sustainable environment issues, climate change or atmospheric pollution. After all, elements as diverse as air, water and plants are ambiental factors as well as environmental resources.

Connecting the aesthetics of urban and architectural ambiances with an ecology of natural and physical environments would surely allow for a better application and awareness of issues, as well as renewed understanding of the mechanisms for integrating such issues into the everyday lives of citizens. Beyond environmental activism which brings ecological problems into the public sphere, ambiances deliver sensory clues about the state and transformation of the planet's environment in a diffuse manner and in some respects help to guide attention. This too, is due to the operative nature of the ambiental notion, its ability to lead to other perspectives and to foreshadow new types of action. In a nutshell, it is nothing less than moving from contemplative knowledge to practical knowledge and making knowledge a field of experimentation rather than of representation. By placing sensory experience at the centre of urban environmental issues, this pragmatic ecology calls for an experimental approach to knowledge and enhanced recognition of the creativity of action.

Note

This article was previously published in French as « La ville à l'épreuve des sens » In Ecologies Urbaines. Olivier Coutard & Jean-Pierre Lévy (eds.), Editions Economica, Paris, 2010, pp. 198-213. Thanks to the publishers for authorizing us to publish this paper in English.