Julien Cohen

A class of structured P2P systems supporting browsing

Browsing is a way of finding documents in a large amount of data which is complementary to querying and which is particularly suitable for multimedia documents. Locating particular documents in a very large collection of multimedia documents such as the ones available in peer to peer networks is a difficult task. However, current peer to peer systems do not allow to do this by browsing.

In this report, we show how one can build a peer to peer system supporting a kind of browsing. In our proposal, one must extend an existing distributed hash table system with a few features : handling partial hashkeys and providing appropriate routing mechanisms for these hash-keys. We give such an algorithm for the particular case of the Tapestry distributed hash table.

Introduction

Browsing is a means to access information which is complementary to querying. A browsing system is useful if it allows the user to find quickly the document she is looking for among a very large set of documents. In current peer to peer (P2P) systems, browsing is not supported and the user has to make with keyword queries. This kind of queries is convenient for textual data but not for multimedia documents.

In this paper, we show that one can easily extend distributed hash tables (DHTs), which are simple and efficient P2P systems, to support browsing. To illustrate this, we build a personal image collection sharing application supporting browsing. We also show with an example how the routing mechanisms of DHTs can be extended to ensure that browsing makes a convenient use of the network.

Browsing and peer to peer

Characteristics of fully decentralized peer to peer systems (without any kind of server or super-peer) make browsing difficult to achieve. Unstructured systems like Gnutella are not quick enough to answer queries due to the flooding method for locating data. Indeed, browsing is an interactive process between the user and the system and needs short latency. On the opposite, structured systems (DHTs) are very fast but the query mechanism is too poor to deal with document properties. Nevertheless, we will show how to extend that query mechanism while preserving most of the good properties of DHTs.

Providing complex queries in DHTs

In DHTs, document identifiers are hashed by classical hash functions and routing mechanisms make it possible to associate a unique machine in the system to each hash key. Hash keys then play the role of logical addresses. In that setting, one must know the exact identifier of the object she is looking for to compute its hash key in order to locate it in the system. This clearly does not allow search by content.

To solve this problem, we use combinatorial hashing [START_REF] Knuth | The art of computer programming, volume 3 / Sorting and Searching[END_REF]. Let us explain this. First, instead of hashing an identifier, we hash a description, i.e. a set of properties, of the documents. This means that all the documents with the same properties have the same hash key. One can then ask to locate all the documents with some desired properties since she can compute the corresponding hash key. Second, instead of hashing the whole property descriptor, we hash each property separately into small hash keys and we concatenate them into a large hash key. Then, hash keys being structured into sub-hash keys, we can replace some slices with wildcards to handle all hash keys with a given value for some properties of interest.

The first of these two points is theoretically sufficient to allow search by content since hash keys with wildcards can be compiled into a set of simple hash keys. However, that solution is not convenient in practice because in general the corresponding set of descriptors is very large and it would lead to flooding the network. This is why structuring hash keys is useful as it will allow to make a convenient use of the network by the means of relevant locating mechanisms.

2 Descriptors, hash keys, wildcards

Descriptors

We assume documents are described by vectors of properties called descriptors. All the descriptors in the system must have the same structure : the same number of components, component at a same position representing a same property, with a shared meaning for the possible values of a component. Descriptors are usually extracted automatically even if it can be done manually. We accept One has to keep in mind the goal of browsing when choosing the descriptor space. Indeed, we use descriptors to denote classes of documents among which the browsing takes place. First, the properties descriptors hold must be meaningful for the user. For instance, for personal image collections, users usually do not make a distinction between GIF and JPEG images. Second, the range of values for each property must be coarse grained. For instance, if we consider the brightness of an image, 256 possible values is too much. A choice of 3 values (dark, medium, bright) could be better for browsing. Of course, the choice of the descriptor space largely depends on the intended application and on the available technologies for the analyze of the considered type of documents.

In the following, we call digits the components of descriptors and we call range of a digit the set of possible values for the corresponding property.

Let us give two examples of descriptor spaces for an application of sharing personal image collections.

Toy example.

The considered properties are the brightness in several characteristic areas of the picture. For each area, the mean brightness is normalized into the {0, 1} space. The areas are the center, the bottom and the top area, as pictured in figure 1. This choice of areas is justified by the fact that on many pictures, the subject is near the center and is surrounded by its environment. Here a descriptor is then a sequence of 3 binary digits, 000 meaning a dark picture and 111 meaning a bright picture.

Second example.

In this second example, descriptors are composed of average values for red, green, and blue components of the same three areas of the picture. Descriptors then have 9 digits. In addition, these values are normalized into integers between 0 and 3. With this setting, digits are encoded onto 2 bits and descriptors onto 18 bits.

If we consider the sunset picture of figure 1, the descriptor is 000220320, that is 0, 0, 0 for the bottom area (black), 2, 2, 0 for the middle area (yellow), and 3, 2, 0 for the top area (orange). For the flamingo picture, the descriptor is 223322223, that is 2, 2, 3 for the bottom area (light blue), 3, 2, 3 for the middle area (light red and blue) and 2, 2, 3 for the top area.

Hash keys

In distributed hash tables, hash keys are used as logical addresses in the system. These addresses are sequences of bits of a chosen length. Descriptors have to be transformed into hash keys. (We could also have extended a DHT to accept descriptors as logical addresses which gives approximately the same result). For each digit of descriptors, if n is the size of the range, we set a bijection from that range to the set of integers between 0 and n -1. These bijections are used as "hash" functions for the digits of the descriptors. A hash key is then the sequence of the hashed digits.

Mapping descriptors to hash keys allows to map each descriptor to a logical address of the system (we take as set of logical addresses the set of possible hash keys). Then, to locate all the documents having a given descriptor, one just has to send a message to the corresponding logical address (in DHTs, the system allows to deliver a message to a machine knowing only its logical address). Since there is a bijection between descriptors and hash keys, we will consider in the rest of this paper that these notions are the same.

Note that in Tapestry [START_REF] Zhao | Tapestry: An infrastructure for fault-tolerant wide-area location and routing[END_REF], the DHT we use as an example, the digits of the tuples are originally in a same base, which is a power of 2, but this can be easily relaxed to any range of integers (or to any finite set equipped with a total order), which does not need to be the same for all the digits of the tuples. So the ranges of the different digits of the tuple can be chosen independently to describe different sorts of properties of the documents.

Discussion.

Unlike traditional hash keys, descriptors of available documents are not uniformly distributed in the key space. This could break the balance of the system since a small number of machines could be responsible for the location of popular types of documents. Depending on the particular application, this could be a major problem. However, we can notice that :

• Each node is responsible for several descriptors, possibly a large number (depending on the number of participants and on the size of the descriptor space). So each machine could be responsible of popular and unpopular descriptors.

• Location queries are light requests compared to the cost of transferring a document and the documents are not located on the node which is responsible of their location.

• DHTs usually use a cache of location to allow responsible nodes to be helped by their logical neighbors to answer location queries.

These remarks make us think that in most cases the unbalance of the descriptors is negligible.

Descriptors with wildcards, hash keys with wildcards

We use * wildcards (read star wildcards) instead of some digits in descriptors to denote a property which value has not been fixed. Descriptors with wildcards can be seen from two points of view. First, it can denote a set of descriptors. For instance, 0*0 denotes the set {000, 010}. Second, it can denote the values of some properties, in particular to represent what kind of document the user is looking for. For instance, 0*0 represent the properties "dark bottom" and "dark top" in our toy example. We will see that the browsing process builds a descriptors with wildcards that expresses users query, but the user does not need to see that descriptor. In the rest of the paper, "descriptor with wildcards" means "descriptor that can have wildcards".

We say that a descriptor is denoted by a descriptor with wildcards d if it can be obtained by replacing the wildcards of d by appropriate values.

As for descriptors, we will use hash keys with wildcards (this notions depends on the type of DHT, for Tapestry, a wildcard replaces a digit). Wildcards in descriptors are mapped to wildcards in hash keys.

Of course, the hard work is hidden from the user. From the system point of view, the browsing process roughly consists in building iteratively a descriptor (possibly with wildcards) expressing user's needs. The descriptor with wildcards under construction, called the current descriptor, evolves at each browsing step. The process starts with a current descriptor containing only * wildcards and progresses by replacing wildcards by values. Each step in the process consists in:

1. Retrieving the representation of some documents which are representative of the documents which are denoted by the current descriptor.

2. Displaying them.

3. Allowing the user express a preference on the displayed documents.

4. Deciding based on that input which wildcard to replace with which value.

Making the corresponding change in the current descriptor

The loop ends when the current descriptor has not any wildcard left, or when the user is satisfied with the current result.

The visual representation of documents is defined is section 3.2. A notion of representativeness is proposed in section 3.4. The strategy for retrieving the locations of representative documents given a descriptor with wildcards is described in section 4.4. A simple way for making the user choose the evolution of the current descriptor is proposed in section 3.5. The retrieval of all the documents at the end of the process is discussed in section 4.3.

Miniatures

During the browsing process, it is not necessary to have a whole document to show to the user, a short description allowing her to preview it and which is cheaper to transfer is sufficient. We call this short version a miniature. Miniatures can be thumbnails for pictures, storyboards for videos, summaries for texts, extracts for audio and so on. During the browsing process, we only use miniatures. A machine which publishes a document must also provide its miniature.

The browsing graph

The browsing process can be described by a finite state machine where each state corresponds to a descriptor with wildcards (the current descriptor). The state machine for our toy example is pictured in figure 2. We call this state machine the browsing graph. The browsing graph, which is a lattice, only depends on the descriptor space.

Illustrative documents, samples, representative sets

Displaying one document for each descriptor denoted by a descriptor with wildcards would make too much information to retrieve, to display and to handle for the user. We make the assumption that displaying only one document for each direct successor of the current descriptor in the browsing graph is sufficient. Let us formalize this idea.

Illustrative documents.

We say a document is illustrative for a descriptor with wildcards d when its descriptor is denoted by d.

Samples.

Given a descriptor with wildcards d, we call sample a set of documents that contains for each descriptor with wildcards which is a direct successor of d at least one illustrative document and which does not contain any document which is not illustrative for d.

Illustrativeness and the browsing graph.

We have the following property: a document is illustrative for an descriptor with wildcards d if and only if the descriptor of that document is reachable from d in the browsing graph. This shows that displaying a sample is sufficient to allow the user to reach any descriptor that is denoted by the current descriptor by browsing.

3.4.4 Representative sets of descriptors.

Finally, we introduce the notion of representative set of descriptors which is used in our routing algorithm to retrieve samples. Given an descriptor with wildcards d, a set S of descriptors is said to be representative when (d[i] denotes the digit of d in position i):

• for each i, if d[i] = * , then for each descriptor e in S we have e

[i] = d[i],
• for each i such that d[i] = * and for each value v in the range of the digit in position i, there is a descriptor e in S such that e[i] = v.

Having one document for each descriptor of a representative set of descriptors (for d) constitutes a sample (for d).

User interface

A document with descriptor d ′ is illustrative for all the descriptors from which d ′ is reachable in the browsing graph. So the documents of a sample for a given descriptor with wildcards are illustrative for several next states. For instance, with **0 as current descriptor, a document described by 010 can illustrate 0*0 and *10. We then have to choose to display several times a same miniature or not. Doing so can be acceptable if the miniatures are cleverly organized on the screen. But simple solutions may also work. For instance, we can label the miniatures with the different possibilities it illustrates as pictured in figure 3 with 0 * * (dark bottom) as current descriptor. Each document then has one label for each wildcard in the current descriptor. The user acts by selecting a label attached to a document, fixing the corresponding digit with its value in that document. Notice that in the example of figure 3, the sample is not minimal. When the sample which is retrieved is not minimal, which will generally occur with the mechanism we propose in section 4.4, some properties are illustrated several times. We think that as long as the number of miniatures is not too large, this is not disturbing for the user.

Routing, query resolution 4.1 Use of a distributed hash table

We need the classical services of distributed hash tables (publishing, locating, dynamical set of participants...) and we do not modify them. As already said, we could simply reuse a DHT by choosing the right hash function and compiling hash keys with wildcards into sets of hash keys, but adding a routing mechanism for wildcards ensures a better use of the network.

In this section, we show how to add that routing mechanism into the Tapestry DHT [START_REF] Zhao | Tapestry: An infrastructure for fault-tolerant wide-area location and routing[END_REF]. Our approach is not restricted to Tapestry but the routing mechanism must be adapted to each kind of DHT.

Choice of the DHT

Although our global setting is independent of the DHT, the choice of the DHT has effects on the whole application. One of the main advantages of Tapestry is probably that it takes locality into account which is important in our case because multimedia documents are relatively large files and because browsing involves an interaction requiring low latency. Additionally, Tapestry hash keys are structured into digits of several bits which can be associated to the digits of our descriptors.

Chord [START_REF] Stoica | Chord: A scalable peer-to-peer lookup service for Internet applications[END_REF]) does not seem to be a good candidate compared to Tapestry since it does not have these two properties. The routing system of CAN [START_REF] Ratnasamy | A scalable content-addressable network[END_REF] could be interesting because the digits can be resolved in a random order, meaning that the fixed values of descriptors can be resolved first, leaving the wildcards for the end, resulting in less logical messages in the system. But CAN does not take the locality into account.

Total resolution

When the browsing process has ended, the nodes responsible for the descriptors denoted by the current descriptor are contacted and they send to user's system a list of locations for all the corresponding documents. Then user's system can use the received physical addresses to query the documents or their miniatures.

We add a specific routing algorithm for finding all the nodes responsible of the denoted classes. Let us consider a message m = d, Total , o, i where d is a descriptor with wildcards (more precisely a hash key with wildcards), Total is a symbol meaning that a total resolution is queried, o is the physical (ip) address of the originator of the query and i is the number of digits that have already been resolved in the routing process. For a node receiving this message, the behavior is the following: If n is the number of digits in descriptors, after n logical hops the messages have the form d ′′ , T otal, o, n (each with a different d ′′ ), where the descriptors d ′′ have no wildcards. Such a message arrives at the node which is responsible for the descriptor d ′′ . Then, for each of these messages, the node responsible for d ′′ sends to o a list of locations for the documents published with d ′′ as descriptor.

If d[i] is not a wildcard.

Sample resolution

We add a sample resolution service to the DHT to be able to retrieve samples at low cost, that is without contacting all the nodes responsible for all denoted classes. Let us consider as an example the descriptor with wildcards d = 0 * 23 * 012 * (9 digits between 0 and 3), and a node n 0 asking for a sample for d. (We randomly consider that Tapestry resolves digits from left to right.) A first message is sent from n 0 to a neighbor node n 1 with a 0 as first digit (the digit on the left) in its address, with d in the message. After this routing step, the first digit has been resolved, n 1 now has to resolve the second digit, which is a * wildcard. Then the following five new descriptors are created from d, with a new type of wildcards used only during the routing process:

0 ⊥ 2 3 * 0 1 2 * 0 0 2 3 ⊥ 0 1 2 ⊥ 0 1 2 3 ⊥ 0 1 2 ⊥ 0 2 2 3 ⊥ 0 1 2 ⊥ 0 3 2 3 ⊥ 0 1 2 ⊥
While we use the * wildcard to mean roughly "all", the bottom (⊥) wildcard means "any", that is, during the routing process, the query can be routed to a neighbor with any value for that digit.

Each of these new descriptors gives a new message to be routed. When considering a ⊥ wildcard as the digit on which to route, a node can replace it by the value it wishes, in particular the best choice is to replace it by the corresponding value in its self logical address to save a logical hop (route to self). Let us look at the general algorithm.

The algorithm for retrieving samples.

We consider the following message: m = d, Sample, o, i . The behavior of a node with logical address a receiving it is as follows: This mechanism ensures that the messages reach the nodes responsible of a (not minimal) representative set of descriptors. When all the digits have been resolved, each node receiving the message sends the location of (at least) one document having the appropriate descriptor to o, which is then used to get a miniature copy of that document to be displayed2 .

If d[i] is not a wildcard. The message is routed normally. If d[i] = * . A new
If for each possible descriptor there is a document in the system having that descriptor, then this routing mechanism ensures that the set of nodes that is finally contacted is able to provide a correct, yet not minimal sample.

Limits of the algorithm.

A problem may occur when some descriptors are not represented. For instance suppose the current descriptor is **0 and the sample request reaches 010. If the class 010 is empty, the browsing interface may not have a miniature to display for the 0*0 vertex whereas 000 could contain some documents. In such a case, an additional mechanism has to be set to retrieve documents to represent the concerned descriptors. This means that some documents may take more time than others to be displayed. However:

• In a very large system, depending on the choice of the descriptor function, empty classes might be rare.

• The user does not have to wait until all the documents are displayed to make her choice to continue the browsing process.

• If a direct successor is not represented, the user can still reach the desired class following another path. In our example, 010 can be reached by following *10.

• Some documents are representative for several direct successors, then we have in fact several representatives for each direct successor, then that direct successor could still be represented by another miniature.

• If a node in the graph does not have any illustrative documents in the system, it seems acceptable that it should not be accessible by browsing.

For these reasons, we think this problem does not invalidate our approach.

Related work

Multimedia browsing systems. The work of Loisant et al. [START_REF] Loisant | Browsing a Classification of an Image Collection[END_REF][START_REF] Martinez | Browsing image databases with Galois' lattices[END_REF] is a good representative of the state of the art in multimedia browsing systems. It is based on a clustering of the documents that is done before the browsing process. This pre-analysis allows to build classes which are more relevant than the ones we use. However, in order to build that clustering, the system needs to have access to all the descriptors. For this reason, that system cannot be easily used in a peer to peer framework. Moreover, when new documents are added to the system, the clustering may have to be modified, which raises new difficulties. Another difficulty in using such a system is the mapping between clusters and logical addresses. In our system this mapping is canonical. With dynamical or ad hoc clusters, another DHT layer would be necessary to make the translation.

Search by content in P2P systems. Search by content is available in some (decentralized, structured) peer to peer systems and could have been considered instead of using a simple DHT. Most of these systems are based on an indexing layers (see the CANDy system [START_REF] Bauer | Bringing efficient advanced queries to distributed hash tables[END_REF] for instance). We did not retain this possibility because of the two DHT layers implying a double latency. Particular features of the systems also have to be taken into account. For instance in CANDy a sequential process is involved to solve queries, meaning that the more the query is complex, the longer it takes to be resolved, which is clearly not convenient for us. In our system, the sub-queries are resolved in a parallel way and the response time is the same as in the original DHT system.

Conclusion and Future work

Our contribution is to show that structured peer to peer systems supporting browsing can be built. For achieving this we use semantic hash keys instead of classical hash keys to provide fast access by content, with a minor risk of unbalance. We also have developed particular routing mechanisms to quickly retrieve sets of documents relevant to the browsing process. Many points are left to the designer of the final application. In particular, the validity of our approach largely depends on the design of the descriptor space and of the user interface. We have given some clues to guide that design.

A strong constraint on our system is that the structure of the descriptors, that is the set of the properties describing the documents, must be shared among users. This is not convenient for open system where any kind of document can be shared, but it is convenient for specific applications such as library catalogs or photography agency catalogs.

Further studies are necessary to validate our approach formally, in particular the short latency and the minimal balance of the system have to be confirmed. We could also study how our requirements on the descriptor space limits the possibilities of the browsing interface.

Simple features can be directly introduced in our setting : the possibility to follow edges in the two directions in the browsing graph or the use of intelligent features in the automatic extraction of the descriptors (for instance, the coordinates of the center area of an image can be dynamically computed for each image). Some other nice features need a bit more of work, for instance we could give several descriptors to a same document (or use fuzzy descriptors or interval descriptors) to cope with the fact that two very similar documents may have different descriptors. Finally, some features correspond to open problems, in particular choosing the clustering of the documents dynamically in a peer to peer way as new documents arrive and disappear from the system (for instance, if all the pictures are very bright, the best choice for describing brightness is not the same as if all the pictures are very dark or if they are well balanced in the brightness space).

Figure 1 :

 1 Figure 1: Example of division of an image into areas

Figure 2 :

 2 Figure 2: Browsing graph

Figure 3 :

 3 Figure 3: Example of a possible browsing interface

4. 4 . 4

 44 Justification of this algorithm.This relatively complex algorithm to get samples is justified by the fact that it is generally far less expensive than getting one document for each class denoted by an open descriptor: O(w.b) nodes have to be contacted instead of O(b w ), where w is the number of * wildcards and b is the number of possible values of a digit (when the digits have the same range).

  The message is routed normally as specified in the base routing mechanism of the DHT.If d[i] is a * wildcard. A new setof descriptors with wildcards is created by replacing d[i] by each possible value in the range of the digit at i in the descriptor space. Each of these new descriptors d ′ is used in a new message d ′ , T otal, o, i that has to be routed. Since d ′ [i] is not a wildcard, each message is routed normally as specified in the base DHT.

  set of descriptors is created by replacing d[i] by each possible value in the range of the digit in position i in the descriptor space and all by replacing all the other * wildcards by ⊥ wildcards. In addition, another descriptor is created by replacing d[i] by a ⊥ wildcard (the other wildcards are not modified). Each of these new descriptors d ′ is used in a new message d ′ , Sample, o, i that has to be routed. If d[i] = ⊥. Any value can be chosen to instantiate d[i]. Let d ′ be d where the digit in position i is replaced by a[i]. This new descriptor d ′ is used in a new message d ′ , Sample, o, i that has to be routed (the message is routed to self, saving a logical hop).

	4.4.2 Do we get samples?

Browsing, samples3.1 The browsing processFrom the user point of view, the browsing process is based on the following loop: 1. The system displays the representation of some documents satisfying the expressed constraints.2. Based on that display, if the user is satisfied, then she asks for previewing the corresponding documents, else she indicates to the system an additional constraint to take into account 1 .1 In order to provide a better browsing process, one should also consider the possibility of removing one of the constraints already set. However, we do not consider this in this report.

Or the contacted machines can directly request the nodes having miniatures to send them to o to save physical hops.


Sébastien Hanauer

Inès Massiot
email: ines.massiot@laas.fr

Adnen Mlayah

Franck Carcenac

Jean-Baptiste Doucet

Sélyan Beldjoudi

Ihar Faniayeu

Alexandre Dmitriev

Photothermal conversion of solar infrared radiation by plasmonic nanoantennas for photovoltaic-thermoelectric hybrid devices

Keywords: hybrid energy harvesting, thermoelectricity, photovoltaics, photothermal conversion, interface layer, thermoplasmonics

Photovoltaics have become one of the low-cost options for electricity generation.

However, the lack of absorption of the near-infrared and infrared solar spectrum intrinsically limits its efficiency. Here, we present an approach for photothermal conversion of solar infrared radiation in photovoltaic-thermoelectric systems using plasmonic nanoantennas. Through numerical calculations-driven shape engineering we identified Ni diabolo nanoantennas as efficient solar infrared spectrum harvesters. Nanofabrication with electron-beam lithography further revealed its impact on nanoantennas optical properties at single-nanoantenna level. In the large-scale low-cost approach, however, photothermal surfaces of nanocone plasmonic antennas, made with a simple and robust fabrication process, still deliver a significant 6.1°C temperature increase under solar 1 infrared illumination. The reported results pave the way towards the development of hybrid photovoltaic-thermoelectric systems with an optimal utilization of the solar spectrum.

One of the key issues at stake in a society full of energy-consuming devices is to design and build self-sufficient energy systems. A winning strategy could be to make the most out of ambient energy, including the most abundant energy source on Earth: solar. Solar photovoltaics is one of the lowest-cost options for electricity generation. [START_REF]Projected Costs of Generating Electricity[END_REF] However, no matter the photovoltaic material, the conversion efficiency of a solar cell is fundamentally limited by intrinsic losses. [START_REF] Hirst | Fundamental losses in solar cells[END_REF] As illustrated in Figure 1A, the efficiency of a single-junction solar cell (with an energy bandgap E g ) suffers from two main channels of losses, the thermalization of photons with an excess energy (E > E g ) and the lack of absorption of near-infrared photons (E < E g ). Several approaches have been developed to tackle these losses and to optimize the conversion of the solar spectrum by solar cells, [START_REF] Green | Energy conversion approaches and materials for highefficiency photovoltaics[END_REF] with the multijunction architecture for example. [START_REF] Geisz | Six-junction III-V solar cells with 47.1% conversion efficiency under 143 Suns concentration[END_REF] An original strategy that has emerged in the last decade to boost the efficiency of the single-junction solar cells is to construct a hybrid device combining photovoltaic conversion with thermoelectric generation. Though several reported experimental photovoltaicthermoelectric (PV-TE) devices demonstrated they indeed produce more power than a single solar cell, [START_REF] Wang | A novel high-performance photovoltaic-thermoelectric hybrid device[END_REF][START_REF] Park | Lossless hybridization between photovoltaic and thermoelectric devices[END_REF][START_REF] Lorenzi | Practical development of efficient thermoelectric -Photovoltaic hybrid systems based on widegap solar cells[END_REF] the main factor limiting the global efficiency of such PV-TE devices is still the low conversion efficiency of the thermoelectric generator (TEG) (below 8% at ambient temperature). [START_REF] Kelk Ltd | Product information on thermo modules[END_REF] To overcome this limit, one key aspect needs to be considered and has been largely overlooked until recently, namely the interface between the solar cell and the TEG. [START_REF] Wang | A novel high-performance photovoltaic-thermoelectric hybrid device[END_REF][START_REF] Kim | High-Performance Monolithic Photovoltaic-Thermoelectric Hybrid Power Generator Using an Exothermic Reactive Interlayer[END_REF][START_REF] Park | Performance of Hybrid Energy Devices Consisting of Photovoltaic Cells and Thermoelectric Generators[END_REF][START_REF] Zhang | A high performance photothermal film with spherical shell-type metallic nanocomposites for solar thermoelectric conversion[END_REF][12] Optimizing this interface is crucial in order to fulfill the PV-TE device two pri- have shown recently that using a highly conductive thermal material such as boron nitride at the PV-TE interface enables an efficient heat dissipation towards the TEG and, as a consequence, cooling down the solar cell (-7.7 • C under 1 sun). [START_REF] Park | Performance of Hybrid Energy Devices Consisting of Photovoltaic Cells and Thermoelectric Generators[END_REF] Furthermore, integrating a photothermal conversion layer to harvest the solar infrared radiation and converting it into heat allows to heat up the hot contact of the TEG and thus to boost its power output. [START_REF] Kim | High-Performance Monolithic Photovoltaic-Thermoelectric Hybrid Power Generator Using an Exothermic Reactive Interlayer[END_REF][START_REF] Park | Performance of Hybrid Energy Devices Consisting of Photovoltaic Cells and Thermoelectric Generators[END_REF] Up till now, only a handful of works have investigated the use of a photothermal interface in PV-TE devices and, to the best of our knowledge, none of them reported the specific spectral design of a photothermal interface to match the solar infrared spectrum for optimal light harvesting.

In this work, we propose a photothermal conversion layer with plasmonic nanoantennas aimed at harvesting solar infrared radiation in hybrid PV-TE devices. Our approach focuses on nanoantennas, that are designed and fabricated to achieve spectrally selective optical and thermal properties, thereby offering the possibility for fine tuning subject to the photovoltaic efficient and spectrally selective absorbers, in particular in the infrared range, 13-15 as they provide intense resonant optical properties at tunable frequencies. Photon absorption in metallic nanostructures can subsequently result in heat generation through non-radiative relaxation of hot carriers. Plasmonic nanostructured surfaces have thus been widely studied for photothermal conversion of solar radiation, with various applications, 16,17 including direct solar-heat conversion in TEG devices. 18,19 These metasurfaces are in general tailored to achieve perfect absorption in the visible spectral range (where the solar irradiance is the highest), often at the cost of a limited infrared absorption.

Our original approach consists in the design of plasmonic nanoantennas for the specific photothermal conversion of the solar infrared radiation transmitted through a photovoltaic cell. The challenge is to design and fabricate simple nanostructures that exhibit optimal matching with the solar infrared spectrum (Figure 1A), particularly in the spectral range above the bandgap wavelength of the solar cell of interest (λ g ), i.e. λ g -2.5 µm. Our study thus used such spectral matching as a key figure of merit. Further, we chose the GaAs solar cell (E g =1.42 eV, λ g =0.875 µm), for its high PV conversion efficiency and low sensitivity to temperature. To validate our approach, cm-sized surfaces with nanoantennas are fabricated and thermally characterized.

As a first step, we modeled with discrete dipole approximation (DDA) 20 a single nanoantenna by a cubic mesh of interacting dipoles (see examples in Figure 2A-E). The shape, size and composition of the model nanoantennas are taken into account to calculate the absorption σ abs and scattering σ scat cross-sections in air, under two orthogonal linear polarizations (for complex refractive indices, see Figure S1, Supporting Information). This is followed by the performances estimates with two main figures of merit. First, the power absorbed by a single nanoantenna under solar infrared illumination, noted P abs , is calculated by weighing σ abs by the AM1.5G solar spectral irradiance (ϕ) and by integrating over the range 0.875-2.5 µm:

P abs = 2.5 µm 0.875 µm σ abs (λ)ϕ(λ)dλ (1)
Note that we assume a perfect transmission through the solar cell over the range 0.875-2.5 µm. However, this absorbed power is not always relevant for the performance of the photothermal interface. When designing an absorbing interface, the main factor to consider is the absorption per unit surface. For example, an array of small nanoparticles with low σ abs could in fact harvest more power than one larger particle for the same absorbing surface. So, we define the absorbed power per unit surface P S abs as the absorbed solar power normalized by the area of the particle's base S N P :

P S abs = P abs S N P (2)
A key parameter for the design of a photothermal interface, naturally, is the nanoantenna material. We have investigated the impact of the nanoantenna composition on P abs for different geometries (Figure S2, Supporting Information) and have chosen Ni as the best available, having in mind the value of the absorbed power and fabrication constraints, and also the potential material abundance.

Identifying the optimal nanoantenna geometry for a given application is a challenging task with a plethora of nanostructures from spheres and wires to complex star-like geometries reported so far. [START_REF] Kretschmer | Survey of Plasmonic Nanoparticles: From Synthesis to Application[END_REF][START_REF] Lu | Chemical Synthesis of Novel Plasmonic Nanoparticles[END_REF] Assessing the fitness for the solar infrared absorption, we scanned through rather simple geometries: nanopads, nanodisks, nanorods and truncated nanocones (Figures 2A-D). We also investigated the diabolo geometry (Figure 2E), its elongated shape and sharp angles being beneficial for high absorption in the near infrared region and its more complex structure offering additional degrees of freedom. For all these investigated structures, the lateral sizes varied between 50 nm and 600 nm, while the height was kept constant at 150 nm taking into account potential nanofabrication constraints. Indeed, for given lateral dimensions, the height of the fabricated structures is limited by the lift-off process. 1. Clearly, particles with a volume below 0.01 µm 3 have a better absorption per unit area due to, firstly, blue-shifting the plasmon resonance with a size reduction, and eventually better overlaping with the solar spectrum; and secondly, by increasing the absorption-over-scattering ratio with decreasing size. [START_REF] Young | Rayleigh scattering[END_REF] However, volumes below 0.005 µm [START_REF] Green | Energy conversion approaches and materials for highefficiency photovoltaics[END_REF] give poorer absorption efficiency.

The optical performances of nanoantennas are strongly influenced by their morphology (Figure 2F). On one hand, nanodisks, nanopads and nanocones exhibit a very similar behavior: P S abs is not affected by their dimensions for volumes above 0.005 µm 3 . On the other hand, P S abs shows a strong dependence on the dimensions of rod-like nanoantennas, reaching a maximum value of 1.23 nW.µm -2 for the antenna with L rod =400 nm and W rod = 50 nm.

An even higher P S abs is achieved with the diabolo shape, outperforming all other geometries (Table 1).

The model nanoantennas evaluated so far (Figure 2) have perfectly smooth surfaces and sharp corners. However, it is well established that nanofabrication introduces surface roughness and often a divergence from the nominal shape and dimensions. We choose to focus on the diabolo nanoantenna (L=540 nm, W =540 nm, l=300 nm and w=50 nm, Figure 3C) to evaluate how these nanodefects affect the optical properties of a nanostructure. Figures 3A-B show electron-beam lithography-fabricated 2D arrays of diabolo nanoantennas (period of 900 nm), where one directly notices, from the SEM and AFM measurements, the deviation of nanoantennas sizes (L=570 nm, W =490 nm, l=270 nm, w=80 nm, Figure 3A) and roughness (Figure 3B) from the nominal design. We thus create two 3D models, taking into account either the deviation in size only (Figure 3D), or both the deviation in size and surface roughness (Figure 3E), and quantify with DDA their absorption of the solar infrared radiation (Figure 3F). The absorption spectrum of the Ni diabolo with the measured dimensions is shifted to longer wavelengths compared to the nominal structure. As a result, the absorption power drops from 75 pW to 66 pW, highlighting the need of control of the lateral dimensions to reach the optimal absorption. Surface roughness, by contrast, has a moderate impact on the absorption spectrum of the particle, similar to what was evidenced in previous works. [START_REF] Trügler | Influence of surface roughness on the optical properties of plasmonic nanoparticles[END_REF][START_REF] Kildishev | Numerical Modeling of Plasmonic Nanoantennas with Realistic 3D Roughness and Distortion[END_REF] In our case, adding surface roughness induces a slight increase of the absorbed power (67 pW). While diabolo nanoantennas have superior absorption power, we further test the temperature evolution upon solar infrared illumination on much simpler nanoantennas (truncated nanocones) with yet reasonable optical performance (Table 1), that can be fabricated into several cm 2 arrays. [START_REF] Fredriksson | Hole-Mask Colloidal Lithography[END_REF][START_REF] Jonsson | Nanoplasmon-enabled macroscopic thermal management[END_REF][START_REF] Jönsson | Solar Transparent Radiators by Optical Nanoantennas[END_REF] In this way, we are able to measure the temperature resulting from the collective heating effect of a macroscopic array of interacting particles. We thus produced short-range-ordered arrays of truncated nanocone antennas (base diameter of 200 nm) of various metals on top of a glass/silver/alumina substrate (Figures 4A-B). The 100 nm-thick silver layer is used as a mirror reflecting radiations towards the nanoantennas, while the 80 nm-thick alumina layer acts as a dielectric spacer, thus creating a metal-dielectric-metal photonic stack. Using an infrared camera, we measure the temperature of the back side of the substrates (glass) under illumination from the front (antennas). Illuminating surfaces covered with 50 nm-high cones with a simulated AM1.5G solar spectrum, but limited to the 800-1500 nm range (Figure S4, Supporting Information), we earn temperature increase from 2°C (for Ag) to 5°C (for Al). Furthermore, we demonstrate a 7°C temperature increase when illuminating a sample covered with 200 nm-high Ni truncated nanocones (Figure 4C), which represents a noticeable temperature rise of 6.1°C compared to the reference substrate without nanoantennas (Figure 4D). We believe that even higher temperature could be achieved with 200 nm-high Al particles. Taking into account the surface density of particles, we calculated the temperature increase per particle (see Supporting Information for details). Our 50 nm-high nanocone antennas deliver from 0.5 nK (Ag) to 3.8 nK (Al) increase, while 200 nm-high Ni nanocones only show a 3.3 nK temperature raise. The superior macroscopic temperature achieved by the 200 nm-high Ni nanoantennas is then attributed to a reasonably efficient heating of each single nanoantenna combined with their high density on the surface.

In conclusion, we demonstrate a library of plasmonic nanoantennas for photothermal conversion of solar infrared radiation. We identify diabolo nanoantennas as the most promising, leading to the highest efficiency of infrared solar light harvesting. We highlight the importance of taking into account the fabrication constraints in the design of the practical nanoantennas. We further experimentally demonstrate a 6.1°C temperature rise (compared to a reference substrate) under solar infrared illumination with a cm 2 -sized surface of Ni nanocone antennas. This demonstrates the viability of our plasmonic nanoantenna strategy for the infrared radiation conversion by a photothermal interface. We intend to achieve larger temperature increases with denser antenna arrays of diabolo-shaped nanoantennas made using large-area replication techniques such as nanoimprint lithography. We foresee such interface to be straightforwardly integrated in a complete photovoltaic-thermoelectric device, eventually leading to self-sufficient microsystems that use hybrid conversion of solar energy into electricity for power supply. The advantage of our nanoantenna-based approach, as compared to the thin film interface previously used in PV-TE, 9,10 is the decoupling of heat generation and thermal diffusion properties. Indeed, the heat sources are the nanoantennas, which have a small height that renders their thermal resistance negligible, while the thermal distribution in the hybrid system can be regulated by selecting the thermal conductivity of the substrate and encapsulant. Moreover, the use of nanolithography techniques offers control over the position of the particles, which could in the future be leveraged to design systems in which the heat sources are located directly on top of the thermoelectric elements, precisely where the temperature needs to be maximized. Alternatively, our photothermal interface could prove useful for applications requiring direct solar-thermal conversion, as is the case for solar nanoantenna radiator-based smart windows. [START_REF] Jönsson | Solar Transparent Radiators by Optical Nanoantennas[END_REF] We also believe that our design (nanoantenna material, geometrical parameters) could be tuned to match other parts of the solar spectrum, thus being suitable for a broad range of photovoltaic materials.

Energy Materials and Solar Cells 2020, 215, 110666.

Figure 1 :

 1 Figure1: Principle of a hybrid PV-TE device including a photothermal interface. (A) Spectrum of the AM1.5G solar irradiance at the earth's surface showing the energy converted by a GaAs solar cell with E g = 1.42 eV (green) and the intrinsic losses in the cell (thermalization, blue; lack of absorption, red) with the corresponding loss processes schematized in the inset. (B) Schematics of a hybrid photovoltaic-thermoelectric device, including a photothermal interface to convert solar infrared radiation into heat.

Figure 2 :

 2 Figure 2: DDA-based numerical simulations of the optical absorption of various nanoantennas. (A-E) 3D representation of the models used for the DDA calculations: (A) pad, (B) rod, (C) cone, (D) disk and (E) diabolo geometries, their varying parameters marked. (F) Absorbed solar power per unit surface P S abs calculated for 150 nm-thin isolated Ni nanoantennas with different shapes and dimensions.

Figure 3 :

 3 Figure 3: Impact of fabrication defects on the optical properties of Ni diabolo nanoantennas. (A) SEM and (B) AFM images of diabolo-shaped Ni nanoantennas on Si substrate. 3D views of (C) the smooth diabolo with nominal dimensions, (D) the smooth diabolo with measured dimensions and (E) a realistic diabolo with measured dimensions and surface roughness. (F) Calculated absorption spectra of the three structures.

Figure 4 :

 4 Figure 4: Large-scale photothermal interface. (A) Photograph of one of the fabricated cm 2sized photothermal interfaces. (B) SEM image of the top surface of a photothermal interface showing a quasi-disordered array of Ni truncated nanocones. (C) Schematics of a nanocone antenna on Ag/alumina. (D) Infrared images of the back sides of the photothermal interface with 200 nm-high Ni nanoantennas and the reference sample (no antennas), when illuminated from the front side. The region across which the temperature is averaged is depicted by the dashed line. (E) Temperature increase of the nanocone antennas surface under AM1.5G, 800-1500 nm, 254 W/m 2 illumination for various nanoantenna materials (dashed lines are guides for an eye).

  For comparison, Park et al. earlier demonstrated 5.1°C temperature increase with a broadband thermal interface material (BN-rGO) integrated in a complete PV-TE device. 10 Kim et al. used a 5 µm thick acrylic film with low thermal conductivity that reached a temperature increase of 30°C under illumination with the full solar spectrum. 9

( 12 )

 12 Park, Y.; Cho, K.; Kim, S. Performance Prediction of Hybrid Energy Harvesting Devices Using Machine Learning. ACS Applied Materials & Interfaces 2022, 14, 11248-11254. (13) Brown, L. V.; Yang, X.; Zhao, K.; Zheng, B. Y.; Nordlander, P.; Halas, N. J. Fan-Shaped Gold Nanoantennas above Reflective Substrates for Surface-Enhanced Infrared Absorption (SEIRA). Nano Letters 2015, 15, 1272-1280. (14) Mitobe, D.; Suzuki, Y.; Shimada, T. Local enhanced site in surface enhanced infrared absorption with gold nano particle array by Rigorous coupled-wave analysis. Journal of Physics Communications 2020, 4, 115009. (15) Bagheri, S.; Zgrabik, C. M.; Gissibl, T.; Tittl, A.; Sterl, F.; Walter, R.; De Zuani, S.; Berrier, A.; Stauden, T.; Richter, G.; Hu, E. L.; Giessen, H. Large-area fabrication of TiN nanoantenna arrays for refractory plasmonics in the mid-infrared by femtosecond direct laser writing and interference lithography [Invited]. Optical Materials Express 2015, 5, 2625. (16) Liu, G.; Xu, J.; Chen, T.; Wang, K. Progress in thermoplasmonics for solar energy applications. Physics Reports 2022, 981, 1-50. (17) Cheng, P.; Wang, D.; Schaaf, P. A Review on Photothermal Conversion of Solar Energy with Nanomaterials and Nanostructures: From Fundamentals to Applications. Advanced Sustainable Systems 2022, 6, 2200115. (18) Pan, Y.; Tagliabue, G.; Eghlidi, H.; Höller, C.; Dröscher, S.; Hong, G.; Poulikakos, D. A Rapid Response Thin-Film Plasmonic-Thermoelectric Light Detector. Scientific Reports 2016, 6, 37564.

Table 1 :

 1 Optical performances of the best isolated Ni nanoantenna for each studied geometry, calculated by DDA numerical simulations. The geometrical parameters are defined in Figures2A-E.

	Geometry and dimensions	Absorbed solar power per antenna (pW )	Absorbed solar power per unit area (nW/µm 2 )	Absorption/scattering power ratio
	Diabolo (L=400 nm; W =300 nm; l=300 nm; w=50 nm)	46	1.42	1.81
	Rod (L rod =400 nm; W rod = 50 nm)	25	1.23	1.03
	Pad (L pad =300 nm)	29	0.32	0.35
	Truncated cone (D cone =400 nm; d cone =100 nm)	39	0.31	0.67
	Disk (D disk =350 nm)	30	0.31	0.48
	Figure 2F displays the calculated P S abs from Equation 2 for Ni nanoantennas with the dif-
	ferent geometries and dimensions, with the best performing summarized in Table

Acknowledgement

This work was supported by LAAS-CNRS micro and nanotechnologies platform, a member of the Renatech french national network, as well as by the French Agence Nationale de la Recherche under grant ANR-21-CE50-0003 (project HYDRES). The authors also thank the CEMES lab for sharing the IR camera used for thermal characterization. AD and IF acknowledge Swedish Research Council (VR) (Project No. 2017-04828) and Swedish Research Council for Sustainable Development (Formas) (Project No. 2021-01390).

Supporting Information Available

The following files are available free of charge.

• Supporting Information: Additional details on numerical simulations of isolated nanoantennas, fabrication methods and thermal characterization (PDF)


Valentin Lamarque
email: valentin.lamarque@lecnam.net

Estelle Chin

Julie Queheille

Olivier Buttelli

Multidisciplinary approach Ergonomics and Lean: articulation between performance, health and safety

Keywords:

In recent years, important changes in work have appeared in the industrial landscape. However, there is still work to be done, and we need to go further and faster, which is why we, at Safran Landing Systems, thought of combining ergonomics with continuous improvement, or Lean management. So through the One Safran Lean workshops, Ergonomics has shown that it is possible to transform this "top-down" approach into a participative "bottom-up" approach built jointly with operators, taking into account the actual work, to adapt prescriptions and better control the WAI (work as imagine) / WAD (work as done) gap. This approach, focused on the workers and the work as they actually do it, allowed us to take into account their professional culture, to collectively build their own rules regarding health and safety issues, while contributing to the overall performance of the work unit.

Introduction

In the industry, there is no need to prove the benefits of Lean management anymore nor, to talk about its limitations on worker health and safety (Bellies & Buchman, 2011;[START_REF] Bourgeois | Que fait l'ergonomie que le lean ne sait/ne veut pas voir ?[END_REF][START_REF] Morais | Articulation entre l'ergonomie et le lean manufacturing chez PSA[END_REF]. It is also not necessary to prove the benefits of the complementary nature of Ergonomics and Lean (op cit.). However, it must be noted that these Lean sites, located in a limited space and time and focused on industrial performance, still pay too little attention to issues of operator health and safety. Indeed, the link between working conditions and occupational risks on one hand, and the search for added value on the other, is not always analyzed and highlighted, while healthy operators working in safe conditions will be able to find and organize better the most efficient operating procedures, provided they have sufficient resources to cope with the constraints. So, as part of a 12-week Lean "One Safran" workshop, this text provides feedback on one of the possible links between Lean and Ergonomics, between performance and health in the workplace. This opportunity, which must be seized, is an opportunity to put back at the heart of the search for industrial performance, health and safety issues as well as professional culture, a guarantee of a successful and favorable intervention for workers.

A necessary collaboration

Ergonomists agree on the link between activity and culture [START_REF] Wisner | Quand voyagent les usines : essai d'anthropotechnologie[END_REF][START_REF] Rabardel | Les hommes et les technologies. Une approche instrumentale des technologies contemporaines[END_REF][START_REF] Béguin | Conduite de projet et fabrication collective du travail (Habilitation à diriger des recherches)[END_REF]. So, faced with a change of an organizational or technical nature "there is a need for appropriation and development of the activity" [START_REF] Béguin | Conduite de projet et fabrication collective du travail (Habilitation à diriger des recherches)[END_REF], for the preservation of the vital resources of the profession [START_REF] Clot | Le travail à coeur : pour en finir avec les risques psycho-sociaux[END_REF]. Taking into account the "professional culture" as a provisional but stable set of what is publicly shared (knowledge, know-how, judgments, meanings and ways of thinking)" (Nascimento, 2020) thus becomes essential in the face of a Lean approach that is still too top-down.

On the other hand, over time, the understanding of the gap between what is to be done (WAI), directed by rules, prescribers, and how it is done (WAD), in relation to experience, knowledge and know-how, has become more refined and diversified in French-speaking works to transform work situations [START_REF] Cuvelier | Sécurité réglée et/ou sécurité gérée : quand l'ingénierie de la résilience réinterroge l'ergonomie de l'activité[END_REF]. Nevertheless, the participatory nature of workers is essential and promotes, within acceptable limits, the control of the WAI (Work As Imagine)/WAD (Work As Done) gap [START_REF] Cuvelier | Sécurité réglée et/ou sécurité gérée : quand l'ingénierie de la résilience réinterroge l'ergonomie de l'activité[END_REF].

To do this, the ergonomic intervention has focused on a triple positioning -skills, instruments and collective -that will make it possible to control the WAI/WAD gap and to "manage it in a conscious and reasoned way" (Mollo & Nascimento, 2013, p.218), between the need for Lean rationalization and the will of Ergonomics to adapt work to Man, and to understand it in order to transform it. Indeed, studies show (Bellies & Buchman, 2011; Bourgeois, 2012; Morais & Aubineau, 2012) that work prescribed too much leads the operator towards constraints not thought of by management. A discussion must then be built around the characteristic notions of Lean (the role of standards, waste management, the "one best way, etc.), which beyond the effective time saving can contribute to a reduction of important "marge de manoeuvre" [START_REF] Coutarel | Marge de manoeuvre situationnelle et pouvoir d'agir: des concepts à l'intervention ergonomique[END_REF] and transform a resource into a constraint.

However, acting together with different methodologies and knowledge, makes it necessary to build a common and shared repository. Thus, the use of a plan of the workspace (or the use of rating grids) as an intermediate object [START_REF] Vinck | From Intermediary Object towards Boundary-Object[END_REF], facilitates the collective construction of future work situations, also based on the simulation of future activity. The meeting of this plurality of actors around a common object, makes possible the crossing of the experience of each one, between knowledge, and know-how, procedures and regulations (respectively WAI and WAD). Moreover, it is through the intervention of "immersed agents" in future simulated work situations that it is possible to anticipate future risks and benefits, based on the needs and constraints of existing work (Van Belleghem & Coll., 2018).

This approach is also a means of supporting the players towards acceptance of change, by acting on the prescriptive system (Van Belleghem, 2018), while rediscussing the trade-offs between quality, efficiency and safety, all of which is bathed in their professional culture.

Methodology

The ergonomic intervention took place in a cell of 2 machines which carry out the roughing of the raw parts, the first phase of the manufacture of the landing gear; as well as a maintenance station for the cutting tools. 7 operators divided into 2 shifts are assigned to the different workstations. First of all, the analysis of work situations (4 weeks) allowed to collect the real operating modes of each operator (manual handling, machine piloting, control and taking dimensions...) and to identify the professional risks associated with health, safety and work organization among the risks already identified, which could be treated and reduced during the Lean worksite. Occupational risks are of 3 categories: related either to general safety, chemical risk or ergonomics. Risks are evaluated and prioritized according to 4 levels of Criticality represented by colors (1/ Red: Major, 2/ Yellow: Significant, 3/ Green: Medium, 4/Blue: Minor). Assessments are performed by trained workers, using a rating grid developed for the Safran Group. For example, general risks are assessed by calculating criticality by taking into account the frequency of exposure to the hazard, the severity of the foreseeable damage and the level of control of the risk. For ergonomics, 3 dimensions are rated: posture (rating based on the NF EN 1005-4 standard); load handling based on the ISO 11228, NF EN 1005-2 and NIOSH lifting equation 1994 standards; repeatability based on the European standard NF EN 1005-5. The ergonomic analysis was based on an alternation of open and systematic observation methods (to understand the work activity, the interactions between operators/teams, the general organization of the cell, the operating strategies…), and informal interviews (to discuss in situation the intention, the reasons of these acts). In a second phase, 3 work sessions took place in multidisciplinary groups, which allowed to deepen the analysis of the work situations, as well as to find ways of improvement in a collective way. Discussions were conducted on the basis of a plan of the cell, representing the tools, furniture, parts and tool storage areas, as well as the flow of people and parts. The participatory work made it possible to define and set up a new implantation of the unit, as well as an action plan to reduce occupational risks. Some actions that were implemented during the Lean workshop were tested and verified in real working conditions, taking into account the variability of production conditions (hazards such as missing parts, urgent touch-ups, absence of colleagues...).

4

Working together and moving towards major improvements in working conditions and performance

The 12-week Lean workshop led to a reorganization of the initial space with the creation of dedicated areas and new work areas (activities not prescribed, but actually carried out), but also to an increase in workspace (+17m²) thanks to a rearrangement of the areas annexed to the two machines. In addition, the restructuring of space and traffic flows has made it possible to reduce operations without added value (manual handling and unnecessary travel), and to reorganize work without reducing the operator's resources needed to maintain his activity, helping to reduce professional risks (30% reduction in the risks of co-activity, falls, shocks, postures, load handling).

In fact, observations have shown that there was no distinction between the entry and exit of the part, thus generating a significant congestion of the work areas and the presence of cross-flows. The latter lead to a permanent co-activity between the operators, source of risks of shock, fall and unnecessary handling (risks that have so far been controlled by the workers thanks to a constant interaction during the transport of parts). These new traffic flows have been co-constructed with the operators, with the compromise between, on the one hand, clarifying procedures, reducing "lost time" and cross-flows and, on the other hand, taking into account the spatio-temporal needs and constraints of the different activities, by integrating the knowledge, know-how, but also the protection strategies, which they have already put in place.

We will take here as an example of these strategies, an off-machine operation to turn a landing gear leg, which requires the installation of bulky machining tools, where the clamps are difficult to access. The operation prescribed to be done in the machine is then performed by the operators, off-machine on the storage carriage in the middle of the two machines. The creation of a dedicated area for the operation (close to the machine but far from the co-activity zone), has made it possible to reduce the risk related to the activity, while taking into account the protection strategies built individually at the beginning and then shared within the collective.

Finally, in addition to the installation, many improvements concerning storage and tools were identified through action plans, some of which were carried out during the Lean workshop. Indeed, we were able to notice during our analysis that the operators of this work unit need a certain number of tools. They pace their activities: using slings to transport parts before and after machining, numerous measuring tools to accurately check all parts, clamping and cleaning tools throughout the machining phase. Special attention has therefore been paid to them to limit the risks during their use (adequate height, storage positioned according to the actual activity, restoration of their storage).

Contribution of the ergonomic approach

The gains identified at the end of the workshop showed an improvement in the overall performance of the work unit, while reducing the professional risks to which the operators were exposed. In addition to these benefits which have been objectified, the implementation of a participative approach with the whole team, operators and managers, focused on the real work has led to collectively accepted transformations.

More precisely, the ergonomic intervention in the Lean workshop showed the importance of taking into account the real work and the involvement of the operators as actors in the construction of their own health and safety, themselves linked to the performance of the work unit. In fact, performance is built collectively, and health and safety then becomes a lever for developing it. Then, the integration of all the actors has allowed to understand the activities as they are really carried out, and to valorize them in their professional culture.

By taking the example of the circulation flow of coins we could see that a participative approach allows to control the gap between WAI/WAD, favouring its adaptation by taking into account the work and the needs of the workers. Thus, it is no longer a question of seeing only a "maximum" time saving, but an "optimized" time saving according to the situations allowing the operator to have sufficient resources to intervene and react to the production hazards. In this workshop, the creation of dedicated areas throughout the circulation flow of the part protects the workers, limits risks (linked to co-activity, transport of parts, etc.) and it is all these gains that contribute to a better general organization of the unit.

Moreover, apart from the term "protection" commonly understood in the sense of "protective equipment" or "activity prescription" in risky situations, we have seen that the notion of protection refers to a dynamic individual and collective process, where it is readapted, "tamed" by the worker and put in his "hand", contributing to the importance of the WAD valorization. Indeed, rules are always at least interpreted, adjusted or appropriate to deal with the unexpected [START_REF] Cuvelier | Appropriation d'une stratégie opératoire : un enjeu du collectif de travail[END_REF], so it is essential to observe and adapt the prescriptions as they fit into the professional culture. The analysis of the knowledge, know-how and ways of thinking of the business lines has enabled us to promote this culture and to encourage the involvement of the players in this approach. It was also an opportunity for ergonomics to intervene directly in the design, thus limiting the risk of major modifications afterwards.

Conclusion

The operator constantly acts within a framework of resources and constraints, which he regulates in order to carry out an efficient action, at a lower mental and physical cost for him. In this sense, the will of Lean to increase the industrial performance, which in fact engages the activity of the first level workers, must take into account their experiences, knowledge and know-how in the field in order not to constrain the activity for a higher performance. Here, thanks to the analysis of the activity, in addition to being able to get closer to the real needs of the operators, we apprehend the professional culture in which it is inscribed. So, by immersing ourselves in the real activities developed in situ, we have targeted the reality of the work, such as a management of quality/performance/health/safety compromises, integrated into the professional culture, favouring the coherence of the new organisations, the capitalisation of new business rules. We then discussed this set of compromises, thus becoming a performance lever that could be justified in terms of health and safety. Indeed, beyond the simple control of the gap between WAI and WAD and the consideration of the professional culture, what was important to question in this lean workshop were the links between health, safety and performance at work. Indeed, we think that beyond an isolated intervention, this collaboration between Lean and Ergonomics should be generalized given the advantages of these two intervention methodologies.