Nicolas Perry
email: nicolas.perry@irccyn.ec-nantes.fr

Magali Mauchand

Alain Bernard
email: alain.bernard@irccyn.ec-nantes.fr

COSTS MODELS IN DESIGN AND MANUFACTURING OF SAND CASTING PRODUCTS

Keywords: cost management, enterprise -product -cost modelling, cost entity

In the early phases of the product life cycle, the costs controls became a major decision tool in the competitiveness of the companies due to the world competition. After defining the problems related to this control difficulties, we will present an approach using a concept of cost entity related to the design and realization activities of the product. We will try to apply this approach to the fields of the sand casting foundry. This work will highlight the enterprise modelling difficulties (limits of a global cost modelling) and some specifics limitations of the tool used for this development. Finally we will discuss on the limits of a generic approach.

Introduction

In the early Seventies, studies in the United Kingdom and in the United States highlight the strategic role of the design activities. The conclusions lead both companies and authorities towards new approaches in order to improve the economic performances of companies. At the end of the Eighties, the paramount role of the quality in the design was reinforced in the United States by the Made-in-America report from the MIT "Commission on the Productivity". The Improving Engineering Design confirmed these conclusions in 1991: Designing for Competitive Advantage report, from the United State Nation Research Council "Engineering Design Theory and Methodology". As resumed by Perrin [START_REF] Perrin | Cohérence, pertinence et évaluation économique des activités de conception[END_REF], the design phase is the key factor of the product development process. The ability to product new products with a high quality, a low cost and witch fit with the customer requests is fundamental to improve the nation competitiveness. Consequently, the costs (and cost management from the early design to the end delivery) become as important as the other technical requests.

Due to the global market and the worldwide competition, reactivity and agility are the only way to maintain the enterprise competitiveness. This can be characterized by the ability to change its products and/or processes in very short times and at minimal cost. The cost control, at the early step of design, becomes a key factor of success, since this phase fixes an average up to 70 to 80% of the end product costs (depending on the kind of production).

Moreover, the costs distribution (respectively direct and non direct) is changing: more time and services are dedicated to the studies for smaller products batches and shorter product life. The former fees sharing-out methods, the analytic or analogical cost-accounting methods, no longer give efficient results. Then, thanks to studies from CAM-I (Computer Aided Manufacturing-International) and authors like Johnson and Kaplan, the increasing gap between "traditional methods" of cost estimation and the new management requirements were highlighted.

All these works lead to new approaches integrating the complete cost and spread accounting methods based on the enterprise activities (ABC for instance). French economist, G.Perrin [START_REF] Perrin | Prix de revient et contrôle de gestion par la méthode GP[END_REF], since the Sixties, also developed a method based on one single cost-inductors through all the steps of the product development process (Added Value Unit method). We implemented such a costing management in a French sand casting foundry in order to allow a several-level management, based on indicators linked to the exact costs of the product to be delivered [START_REF] Bernard | Optimization of complete design process for sand casting foundry[END_REF][START_REF] Perry | CE : the Vision for the future generation in research and applications[END_REF]. During this PHD thesis we validated the concepts but also the methodology required for a complete numerical traceability.

The work that will be presented in this paper concerns this former study and uses a concept called cost entity [START_REF] Mida | L'approche entité coût pour l'estimation des coûts en production mécanique[END_REF]. It includes several concepts: the cost inductors from the activity based accounting methods, the feature from the CAD and the homogeneity from the analytical cost accounting. Consequently, in order to define a cost entity, it is necessary to fill in several attributes linking technical and economical variables. The product model uses the concept of manufacturing feature. The cost are evaluated on the base of specific knowledge and reasoning models with the tool "Cost Advantage", giving information on costs to the CAD model. This is adding a cost semantic level to the CAD model. This models (called costgramme) put the expertise of the manufacturing cost available to the designer. Some models, dedicated to the sand casting production of primary parts, were created with the wish to evaluate a meta-model that could be deployed in all the sand casting industries. Thus, the goals of this study are, on the one hand, to create the more generic as possible model related to the sand casting job, and on the other hand, to determine how they are transposable from a company to another (or from a production line to another…). So we will have to define and discuss which are the limits of the concepts from the triptych product/process/cost, and what level of detail is necessary to implement the methodology in most of the industrial environment.

This study is realized in collaboration with Cognition Europe, software developer of the tool Cost Advantage, and based on an industrial experimentation in the already-mentioned SMC Colombier Fontaine casting enterprise.

Value and cost management

The concept of cost or value strongly evolved with the context. Today, the selling price depends on the price the customer is ready to pay according to the value that it appreciates. Consequently, the margin is not used to calculate the selling price, it results from it. The selling margin and price must be defined beforehand; they determine an "objective maximum cost", known as "objective cost".

According to Perrin the indicators relating to the costs are mainly to be classified among the results' indicators [START_REF] Perrin | Cohérence, pertinence et évaluation économique des activités de conception[END_REF]. The two criteria considered are on one side the cost of the product and on the other side the respect of the budget:

-the respect of the product cost is proposed within the framework of the development of the target costing methods (management by cost-target) and ABC (Activity Based Costing). The idea is to fix a target cost as an objective to which one must make correspond the cost of the product. The indicator then used, is the relationship between the effective and the objective product cost, -the respect of the budgets, in particular of the studies' budgets, is the second element to be taken into account at the financial level. The indicator is then naturally the ratio between the exceeding and the initially fixed budget.

The contextual industrial evolutions change the nature of the costs in the company. Formerly, the final costs included a large majority of direct costs (often about 70%), i.e. directly assigned to the products (labour or raw material). The other costs could be the subject of global distributions; the choice of the scale hardly influenced the result. The method of the cost accounting with indirect expenses shared through a fixed percentage, can be admitted as long as their proportion is not too big. But the proportions are reversed, the direct costs do not constitute often any more that 30 % of the total cost. Not only the indirect expenses, according to odd sharing keys, create an arbitrary factor in the calculation of the costs, but it make the control difficult. These two concerns gave place to many researches that ended up in deeply modifying the systems of costs evaluation.

Let us analyse the product life cycle and the impact of decisions in term of generated costs. The early phase of design implies between 70 and 85% of the product cost and, at the end of the detailed design the margin on the final cost is thus very limited (cf. Fig 1). Contrary, the evolution of the costs really engaged by the company are very limited in these upstream phases. However, it is there that the control of the costs should be really exerted.

Consequently, the more the product is defined, the less the cost reduction is easy. The modification becomes increasingly expensive. There is thus an obvious economic interest to have a product optimisation at the early step of the process. The profits potentialities are Fig. 1 : Cost evolution during the PLM [START_REF] Alcouffe | Standardisation et évaluation des activités de conception dans le secteur aérospatial[END_REF] significant and the committed costs weak. The design choices, inexpensive in terms of resources consumption can be very expensive in production, when it becomes extremely difficult to carry out modifications [START_REF] Bellut | Estimer le coût d'un projet[END_REF][START_REF] Mevellec | Le coût global, nouvelle frontière du calcul de coûts[END_REF].

Design to Cost

The design to cost can be defined as a principle of action aiming the establishment of rigorous objectives. It allows compromises between performance and cost [START_REF] Delafollie | Analyse de la Valeur[END_REF][START_REF] Petitdemange | La maîtrise de la valeur » La gestion de projet et l'ingénierie simultanée -Paris-La Défense[END_REF]. The "cost" constraint becomes capital and this data should be managed on the same level than technical performances. Then the cost objectives become constraints and the technical performances, variables.

The interest to integrate the cost management at the design phase is shared either by the customer and the seller since it makes it possible to control the development according to the exact needs for the future users (it incites in the search of new ideas required by the economic constraints), in addition, it makes possible to prepare and organize its production very early and to better control its margin [START_REF] Giannopoulos | Embedded systems software cost estimating within the concurrent engineering environment[END_REF]. This method of design takes again the steps of activities or tasks with a focus on the costs limits. This led to an extension of the design time due to the successive iterations needed to agree on the solutions after negotiation (technical or economic). This leads to cross the functions within the company and can be used jointly with methods of costs analysis like ABC. This also allows a very thorough analysis of the product functions linked to its costs [START_REF] Kieffer | Méthodes du pilotage des systèmes de production[END_REF].

One can however point out that the design phase is longer. But, the debugging step (preproduction) is no longer disturbed by modifications as it occurs in a traditional step. We then accept in fact "to waste" time with the design phase, therefore with the quasi-certainty to regain it downstream before the delivery date.

The Cost Entity concept and the modelling logic

The aim of our study is to sharply manage the costs (direct and indirect) during the production of sand casting parts. As illustrated previously, it is imperative to give a tool to the engineers of the engineering and design department in order to help them to control the costs of the parts design. In collaboration with the company Cognition Europe, and on the basis of the tool Cost Advantage, we work on the costs models to apply in the case of the steel sands casting parts. We are based on a preceding work, proposing an integrated approach for the sand foundry, realized within the framework of a thesis in partnership with SMC Colombier Fontaine (France) of group AFE Metal [START_REF] Perrin | Prix de revient et contrôle de gestion par la méthode GP[END_REF]. This work allowed formalizing the base of knowledge trade necessary to the control of the product life cycle in a foundry company. In addition, we validated an approach, a methodology and a deployment leading to ensure an exact knowledge of the parts costs and their impact on the output of the company [START_REF] Perry | CE : the Vision for the future generation in research and applications[END_REF].

Let us start with the concept of cost entity and context, which are our modelling bases.

Costs Entities

A Cost Entity is a grouping of costs associated with the resources consumed by an activity. The general condition is due to the homogeneity of the resources, which makes it possible to associate a single inductor: the entity cost [START_REF] Mida | L'approche entité coût pour l'estimation des coûts en production mécanique[END_REF]. The model allows expertise formalisation, knowledge capitalisation and to have, at the early design phase, some information about the production step. Moreover, it helps the communication between all collaborators during the product life cycle.

Contexts

The contexts specify defined entities in three levels in our model. The first is defined in a process level, the second on a material level and the last is directly related to the feature. This context is a cross between a process, a material and a feature, connected to an environment (cf. Fig. 2).

Fig.2 : Sand casting Modelling

Sand casting modelling

Based on this analysis we created a generic model using Cost Advantage Software. The first step closely defines the production process dedicated to this industry. The master parameters acting upon the product cost must be identified in order to enrich the cost semantic of the model.

The generic approach quickly highlights the problems of the contexts characteristics. How to define a significant cost for the part and with which level of detail in order to be generic?

We will not answer this question, but we will present the paths or solutions we used.

First, let us describe the sand casting organisation through the part life cycle in the enterprise. This model is based on the SMC Colombier Fontaine foundry (France), from AFE Metal group. We will reduce our cost model to the primary parts through its life cycle in the enterprise and focus on the production phase, from the sand elaboration, the tooling machining and the parts perfecting.

Step induced by the use of Cost Advantage

Figure 3 presents a functional view of the process with the compound (raw material, tooling) and the elements needed to manufacture a part linked with the major indicators dealing with the final cost (loss, scrap ratio, production rate). Figure 4 represents a transposition under the concepts of Cost Advantage of this model gathering the three levels of entities defined in the software. To illustrate this, for example, the mould, the tooling and the cores are components required to carry out the assembly named moulding by the operation (feature) of remould. It is thus necessary to define the final part, to carry out the two assemblies, which are the moulding then the casting.

In term of model design, the functional view identifies the assemblies needed, it is then necessary to define the components and choose and define the related operations. An ascending step must be practiced, starting with the components up to the definition of the assemblies. The costs calculation is presented in figure 5 and the structure of the implemented data in figure 6. Calculations are simply taking into account volumes of material, rates of production, losses and the machine and labour costs. To say nothing about the difficulty in knowing the exact parameters, the problems we met were model organization, more than process modelling.

Pi Pi è è ce_Brute ce_Brute

Parach

Fig.5: Process cost structure sample

The rules of calculation then implemented will make possible to the future user to inform only the relevant data about its study. Indeed, only the operational process, rates, dimensions, numbers of cores (etc.) will be required (or deduced directly in a CAD software) to allow an automatic calculation of the cost of the part according to its particular characteristics.

Discussion

The principal remark on this model, implemented with Cost Advantage platform, comes to that it doesn't integrate the global aspect of the costs management. For instance, the indirect share due to the development and the design of the tools (master pattern, cores boxes…) isn't taken in account.

But let us keep in mind the framework of use of such a tool: to help the designer to achieve a cost objective related to expected technical functions. He can then propose design modifications (joint section, cores, quality…) or process (number of parts in the mould…) to achieve its goal. Moreover, when the whole partners validate this design, the parameters costs are fixed (in agreement or not with the objective laid down initially), the cost is well known and will not be any more a consequence of later decisions.

During this work we have identified a principal difficulty, which for this modelling impact the multiplicity of the elements on the availability for their characterization and for their organization. Even if the manufacturing sand casting process seems simple, it uses many components (alloy, cores, mould…), and we limited the definition in term of model refinement since each one of these components could be the subject of a finer modelling. We tried to choose this limitation in order to represent the general process without going closer into enterprise specificities. For example, the cores or fusion process of realization are not completely defined since depending on the machines, uses and other specificities of the workshop. In fact this remark is shareable for all the components entering into the realization of the finished part. But we think that we defined a basic minimal skeleton, transposable from one company to another using the sand casting process.

Conclusion

To conclude on this work, we started to apprehend logic of cost oriented modelling through a tool using the concept of cost entity. In order to ensure a generic aspect of our work, we deliberately limited the details of the operations, components and assemblies. Indeed, the development of these elements takes into account many parameters that it seemed to us initially overflowing to define.

We thus defined a structure that we think minimal, as well as indicators necessary to evaluate the whole costs without the indirect part. The two next steps of this study are in the first hand among other sand casting companies, to apply this modelling, configure the model with the existing processes and informing the exact values of the indicators. But also in the other hand, to calibrate the model and the results on real studies already done. These two last steps allow comparing the effectiveness of the various companies and could be used as Benchmark. A foreseeable difficulty is the possibility of reaching this information. Moreover, these factors are often managed in a total cost accounting. As a result, the efficient indicators may be sunk in a not very transparent accounting system or may be aggregated with not relevant others.

The other significant continuation to give to this work is the taking into account of the global costs mainly related to the indirect shares (structural). Our introduction puts forward the lack of management of these aspects and our first approach did not give place to a better control of these factors. However the work is done and the workers must be paid (designer, maintenance, buyers, logistics…) even if their work is not as well managed through a cost management system. A better specification (by the means of indicators, of metric) of the tools design phases, tools lifespan, etc. could integrate a real cost of the complete series. The question of the relevance of the tool used for this type of approach arises then. Some solutions come from the use of single or very limited number of cost inductors such as the time and they define a minimal global enterprise cost per hour to balance its financial objectives. Such an approach allow a multi-level management of the parts impact and give real time information to asses the enterprise objectives and manage strategies and operational decisions.

Fig. 3 :Fig. 4 :

 34 Fig.3: Process view of a sand casting production cycle

Fig. 6 :

 6 Fig.6: Structure of data with Cost Advantage


Jean-Paul Viricelle
email: viricelle@emse.fr

Sorina-Nicoleta Udroiu

Geoffroy Gadacz

Michèle Pijolat

Christophe Pijolat

Development of Single Chamber Solid Oxide Fuel Cells (SCFC)

Keywords: BSCF, Fuel Cell, GDC, LSM, Single Chamber, SOFC, Solid Oxide Fuel Cell, SSC

teaching and research institutions in France or abroad, or from public or private research centers.

I. Introduction

Solid oxide fuel cells (SOFC) have been studied intensively over the past two decades. Main efforts deal with the reduction of working temperature from 1,000 to around 600 °C in order to reduce the cost due to the requirement of robust materials with adequate electrical and electrochemical properties, thermal stability and due to the technological problems, especially sealing between anodic and cathodic chambers.More recently, a new type of device named 'Single Chamber Solid Oxide Fuel Cells' (SCFC) has been proposed. In such device, contrary to conventional SOFC, anode and cathode are exposed to a gas mixture of fuel (hydrocarbon) and oxidant (air) so that no more sealing with electrolyte is necessary. Their operating principle is based on the different catalytic activities of anode and cathode: ideally, the anode has to be active for the oxidation of fuel while the cathode should only present a strong electro-activity for oxygen reduction. Most significant and initial developments of SCFC devices were performed by Hibino and co-workers who proposed a review on SCFC in Reference [1]. In the first development of SCFC devices [2][3][4], materials used in conventional SOFC were used, namely YSZ for electrolyte, Ni-YSZ cermet anode and LSM (La 0.8 Sr 0.2 MnO3) cathode. As for conventional SOFC, such materials are now more and more replaced in order to significantly reduce the operating temperature while increasing or, at least maintaining performances. YSZ electrolyte is substituted by doped ceria oxide either with Gadolinium (GDC) or samarium (SDC). Most of the anodes are constituted by a cermet of previous electrolyte with nickel. For cathode, the trend is to use doped perovskites with cobalt, either Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-d (BSCF, [5]) or Sm 0.5 Sr 0.5 CoO 3 (SSC, [6][7][8]), or La 0.5 Sr 0.5 CoO 3 (LSC, [9]). With such SCFC devices, depending on the cell microstructure, geometry and flow conditions, maximum power densities of 700 mW cm -2 at 600 °C have been reached [7]. However, such high values are not so easily obtained: even for fixed materials, all experimental details such as flow geometry [8] or the way of the SCFC initialisation [10] for example, have significant influence on the performances. The aim of this study was to investigate the influence of experimental parameters, especially flow composition, in our testing bench conditions. More comprehensive studies on SCFC working principle were reported by Hibino and coworkers [1] and Haile and co-workers [7]. In the present paper, we present our recent results concerning SCFC developments, with GDC electrolyte, Ni-GDC anode and LSM or SSC or BSCF as cathode. Materials characterisations and SCFC evaluations were performed under propane/oxygen mixtures. Among possible hydrocarbons, propane was chosen because it can be cracked at lower temperature than methane, and it can also produce more hydrogen by partial oxidation due to its higher carbon number (Equation ( 1)). . M Ma at te er ri ia al ls s, , S SC CF FC C P Pr re ep pa ar ra at ti io on n The electrolyte material was made using a starting commercial powder ( ® NexTech Materials Ltd) of gadolinium doped ceria (Ce 0.9 Gd 0.1 O 1.95 named GDC), having a specific area around 9 m 2 g -1 . The particle size is in the range 0.1-10 µm with a mean size (d 50 ) around 0.5 µm. The powder was pressed into pellets (100 MPa, 2 min) and sintered at 1,350 °C for 4 h with an intermediate step at 1,200 °C during 6 h. Temperature rates were 10 °C min -1 for both heating and cooling. Final pellets have a diameter of 19 mm and a thickness of 0.8 mm. Their relative density is around 92%. Anode cermet was prepared from a mixture of 60 wt% NiO (Sigma-Aldrich) and 40 wt% GDC. The initial NiO powder having a specific area around 3-4 m 2 g -1 was strongly agglomerated and was ball-milled in ethanol. The solvent was then evaporated while stirring and the resulting powder was passed through a 100 µm screen sieve. The NiO powder particles size was then comparable to the one of GDC powder. The two powders were then mixed and ground in a mortar and pestle. Then, commercial binder and solvent were added to obtain an ink for screen-printing. The ink was homogenised with an ® Ultra-Turrax dispersion tool. The anode layer of 6 × 6 mm 2 was then deposited by screen-printing technology on the previously described GDC pellet. To study the influence of electrode thickness, the number of deposited layer was varied, with an intermediate drying step at 130 °C during 10 min between each deposit. The deposited films were fired at 1,200 °C for 2 h, resulting in a porous anode layer with a thickness of either around 10 µm (1 layer), 20 µm (2 layers) or 40 µm (4 layers). In order to activate the anode, reduction of nickel oxide was performed by thermal treatment in hydrogen. This part is detailed in Section III.1. Cathode was prepared using a similar process as for the anode. The powders were passed though a 100 lm screen sieve, and then mixed with a binder and a solvent to obtain an ink. Cathode layers were also deposited using screen-printing. Three materials were used: La 0.8 Sr 0.2 MnO 3 (named LSM) and Sm 0.5 Sr 0.5 CoO 3 (named SSC), commercial powders from Nex-Tech Materials Ltd, and an in-house powder supplied by IRCELYON laboratory (France), Ba 0.5 Sr 0.5 Co 0.2 Fe 0.8 O 3 (named BSCF). Considering their thermal stability, these cathodes were fired at 1,200, 1,000 and 950 °C for LSM, BSCF and SSC, respectively. Anode and cathode were successively deposited and fired as reported previously, on the same side of the electrolyte pellet. The space between the two square electrodes is around 1 mm. For SCFC performances tests, a gold current collector was deposited on the cathode. These collectors consist in a grid (line space of 0.5 mm) deposited by screen-printing, using a commercial gold paste. It was fired after deposition, at 900 °C during 20 min. An SCFC prototype can be seen in Figure 1. I II I. .2 2. . M Ma at te er ri ia al l C Ch ha ar ra ac ct te er ri is sa at ti io on n, , F Fu ue el l C Ce el ll l P Pe er rf fo or rm ma an nc ce e T Te es st t Prior to fuel cell performances tests, materials were characterised in regards of their electrical conductivity, catalytic activity and thermal stability. Electrical conductivity of anode and cathode was measured according to Van Der Pauw method [11]. Particular samples supporting only the anode or the cathode on the GDC pellet were prepared, using the same elaboration process and parameters as described in previous part. Four thin platinum wires were stuck to the border of the studied layer with a platinum paste. These wires were connected to conventional electrical equipment to monitor current and voltage according to Van Der Pauw methodology. For cathode materials, measurements were conducted in air in the range 20-600 °C. For anode, to avoid nickel re-oxidation, airpropane mixture was set in the bench after the reduction process made in hydrogen. Catalytic activity of cathode materials was investigated in a fixed bed reactor, towards propane conversion. The same quantity of cathode material in the form of powder dispersed in alumina was introduced to have comparative results. Propane conversion was measured. Propane conversion was measured thanks to a mass spectrometer coupled with a micro-gas chromatograph (IGC-MS) placed at the bench outlet. Effectively, as the SCFC principle is based on the difference of catalytic activity of anode and cathode, cathode materials have to be the less active as possible with the hydrocarbon; they should only present a strong electrochemical activity towards oxygen reduction. Anode and cathode thermal stability was investigated in the SCFC running conditions, after exposure to air-propane mixtures, by X-ray diffraction. For cathodes, same characterisations were performed after the hydrogen treatment of the anode in order to control if they can withstand it. SCFC performances were measured in a simple test bench consisting in a tubular furnace with a quartz tube. Electrical contacts are achieved thanks to gold wires in mechanical contact with gold collector on the cathode, and directly on the anode. In situ activation of the anode was performed under a gas flow of 2% hydrogen balanced with argon. Then, after purging the system with nitrogen, the propane-air mixture was introduced. Three parameters were mainly investigated: total gas flow, C 3 H 8 /O 2 ratio and the temperature. The gas flow was varied in the range 15-50 L h -1 : it is constituted of air-propane mixture and, for safety reasons, of an additional nitrogen flow in order to decrease the oxygen content to 12% in the bench. For example, total gas flow of 45 L h -1 was composed of 30 L h -1 of air-propane mixture and 15 L h -1 of nitrogen. Effectively, in air, the explosive domain of air-propane mixtures is in the range 2.2-10% of propane. The decrease of oxygen content reduces this area which disappears for oxygen content lower than 12%. Another important parameter is air-propane ratio which was investigated in the range 10-16 vol% in propane, corresponding, respectively, to C 3 H 8 /O 2 ratio in the range 0.53-0.95. It must be noted that the corresponding ratio for partial oxidation of propane (Equation ( 1)) is 0.66. SCFC were tested in the temperature range 580-650 °C. These temperatures correspond to the regulated value of the furnace; depending on gas conditions, strong exothermic effect (overshoot of around 50 °C) can occur during propane oxidation. If not specified, the reported temperatures are the furnace temperatures. A gas chromatograph and an infrared gas analyser performed gas analysis at the outlet of the test bench. The polarisation curves of the SCFC devices were simultaneously measured in order to correlate performances with experimental conditions.

III. Results

I II II I. .1 1. . A

An no od de e C Ch ha ar ra ac ct te er ri is sa at ti io on n III. 1

.1. Anode Reduction

The as-prepared NiO-CGO cermet anode has to be reduced before use in SCFC device. As in situ reduction in air-propane mixture could not be easily achieved, a pre-treatment in H2 (2% balanced in Ar) was conducted. In order to define the adequate temperature range, thermogravimetric experiments were conducted with NiO and GDC powders. These powders were previously fired in air at 1,200 °C during 2 h, as done for the screen-printed layers. Results reported in Figure 2 show that NiO begins to reduce at around 400 °C, with a total reaction reached around 800 °C corresponding to the theoretical weight loss of 21.4%. For GDC, a significant weight loss attributed to partial reduction of cerium IV in cerium III is observed from 600 °C. Taking into account previous results, various reduction conditions of the anode in the same gas have been tested in the temperature range 400-600 °C, with varying duration from 30 min to few hours. Optimum condition to obtain a total reduction of NiO in our test bench, as shown by X-ray diffraction analysis (Figure 3), was found for a treatment of 600 °C during 30 min.

III.1.2. Anode Conductivity In order to check if the Ni-GDC anode has a satisfying electronic conductivity, conductivity measurements were performed as described in Section II.2. Results can be seen in Figure 4.

Immediately after reduction at 600 °C, the anode is stabilised under a nitrogen flow and the temperature is decreased to 520 °C. During this period (domain (a) in Figure 4), the conductivity is quite high around 900 S cm -1 . However, as soon as nitrogen is replaced by the flow of air-propane (C 3 H 8 /O 2 ratio equal to 0.66), at 520 °C, the conductivity begins to decrease (domain b). The temperature is then raised to 535 °C, and step by step up to 570 °C. At 535 °C, the conductivity comes back to its initial value and keeps a nearly constant and high value in the range 535-570 °C (domain c). Same experiment (not shown in Figure 4) was done with decreasing temperature and a similar behaviour was observed. At temperature lower than 530 °C, the conductivity begins to collapse. After cooling back to room temperature, a green colour of the anode was observed at the surface, indicating a re-oxidation of nickel. These results validate the preparation process of the anode, but indicate that, for SCFC operating in our experimental conditions, temperature must be kept higher than 520 °C to avoid nickel re-oxidation. A thermodynamic study was performed to check these experimental observations. A good agreement was obtained: calculations of the free Gibbs energy indicate that, among possible reactions of nickel and nickel oxide in the studied system, the predominant reaction at temperature lower than 535 °C is the re-oxidation of nickel (Equation (2)). At higher temperature, reduction of nickel oxide by propane is favoured (Equation (3)). (2)

3 8 2 1/3C H +NiO Ni+CO+4/3H  (3) 4 I
II II I. .2 2. . C Ca at th ho od de e C Ch ha ar ra ac ct te er ri is sa at ti io on n III.2.1. Cathode Conductivity Electrical conductivity of the three studied cathode material, LSM, SSC and BSCF was measured under air in the range 20-600 °C. Values reported in Figure 5 point out that SSC is the more conducted cathode with conductivity around 650 S cm -1 at 600 °C. The value for LSM and BSCF are, respectively, around 130 and 10 S cm -1 at 600 °C. The difference of electrical conductivity and especially the low value measured for BSCF cathode may be explained by the morphology of the layers: SEM observations (Figure 6) clearly point out that BSCF layer is quite more porous than SSC and LSM layers.

III.2.2. Cathode Catalytic Activity

As reported previously, the efficiency of a cathode in a SCFC device is based on its low catalytic activity in regards of hydrocarbon conversion. Figure 7 represents propane conversion measured by the µGC-MS equipment at the outlet of the catalytic test bench versus temperature, for the three studied cathode material. Gas composition was set at a C 3 H 8 /O 2 ratio equal to 0.66, thus corresponding to stoichiometric conditions of partial oxidation (Equation ( 1)). Results show a strong catalytic activity of LSM and SSC material in the range 450-600 °C, quite higher than the one of BSCF. For this material, propane conversion increase slowly versus temperature but remains lower than 20% at 600 °C, whereas it reaches, respectively, around 50 and 80% for SSC and LSM. Hence, considering catalytic activity criteria, BSCF is the more adequate cathode material for SCFC device.

III.2.3. Cathode Thermal Stability

Although interesting performances have been obtained with the studied cathode materials used in SCFC, the problem of SSC and BSCF stability has been reported previously [9,10]. Hence, it is necessary to investigate the thermal stability of the three cathode materials both in hydrogen (conditions of anode reduction) and air-propane (SCFC test conditions) gas flow. After a thermal treatment similar to the anode reduction one, 600 °C during 30 min in H 2 (2% balanced in Ar), SSC and BSCF materials are degraded (Figure 8 for SSC). In similar conditions (30 min in H 2 ), LSM material is stable according to XRD analysis. Then, the stability of the three cathodes was also investigated in air-propane mixture at 630 °C, without prior exposure to the reduction pretreatment in hydrogen. Effectively, other processes have been proposed in the literature [10] to avoid anode initialisation by hydrogen pretreatment, in order to prevent cathode degradation. However, in our conditions, SSC and BSCF materials still present degradation, even if it appears less drastic than in hydrogen. LSM did not present any degradation in the same conditions. In our actual preparation process of the SCFC device, both anode and cathode are deposited on the GDC support before the anode reduction treatment. Moreover, long-term stability of BSCF and SCC materials is questionable in airpropane mixtures, at least in our experimental conditions. Hence, for the present study, it was only possible to use LSM material as a cathode, although we have confirmed that it is not the more adequate one, in regards of a too high catalytic activity for propane conversion. I II II I. . Preliminary Tests with a 20 lm Thick Anode SCFC devices were tested in our bench as described in Section II.2. Preliminary tests were conducted with the sample prepared with a 20µm thick anode, a C 3 H 8 /O 2 ratio equal to 0.55 and a total flow of 45 L h -1 . Typical variation of the open circuit voltage (OCV) versus time and sample temperature can be seen in Figure 9. After the in situ reduction of the anode at 600°C, the sample temperature is set at 580 °C and the air-propane mixture is introduced. At this temperature, the OCV is quite low, in the range 100-200 mV. Then, the temperature is increased to find the optimum value corresponding to the highest OCV. Figure 9 shows that the OCV increases sharply from around 620 °C with also increasing exothermic effect, but the OCV presents some strong oscillations. It tends to stabilise around 500 mV at 700 °C (sample temperature; furnace is at around 650 °C). If the temperature is more increased, a sudden reduction of OCV value is observed, which is reversible with temperature decrease. Hence, for this sample (20µm thick anode) and in the present experimental conditions, the optimum sample temperature is 700 °C. In a previous paper [12], we have shown that the optimal conditions are strongly dependant on experimental parameters. Voltage-current characteristic measured at these optimum conditions for this sample is reported in Figure 10. The effective area considered for calculation of current and power densities, taking into account our planar geometry (Figure 1), is the section of the electrolyte between the two electrodes, equal to 4.8 mm 2 (6 × 0.8 mm 2 ). A maximum power density of around 10 mWcm -2 is obtained. Characterisations of anode and cathode materials after the SCFC evaluation with various conditions were carried out by X-ray diffraction. For LSM, no significant modification could be detected. For Ni-CGO anode, results are more various depending on the history of the SCFC device. A partial nickel re-oxidation is sometimes observed, but it is not clear if it occurs during the fuel cell test, or during cooling down to ambient after test, which would be in agreement with observations made during anode conductivity characterisation (Section III.1.2). In other conditions, especially higher C3H8/O2 ratio (0.95), carbon deposition is observed on the anode.

III.3.2. Influence of Experimental Parameters on Outlet Gas Composition

The influence of C 3 H 8 /O 2 ratio and of total gas flow was investigated in more details thanks to gas analysis at the bench outlet. Thick anode sample (40µm) were used is this part of the study. For this sample, the optimum SCFC temperature was shifted to 620 °C in conditions similar to previous ones (C 3 H 8 /O 2 ratio equal to 0.55 and a total flow of 45 L h -1 ). Hence, this sample temperature was fixed for following tests. Influence of C 3 H 8 /O 2 ratio with a fixed total flow of 45 L h -1 is shown in Figure 11. The concentration (vol%) of main constituents detected by the gas chromatograph are reported. Taking into account that the desired reaction is partial oxidation for CO and H 2 production (Equation ( 1)), comments are focused on these gases. Figure 11 shows that, with increasing C 3 H 8 /O 2 ratio, CO and H2 production decreases, contrary to concentrations of propane conversion sub-products. SFC performances were also observed to decrease, both OCV and power densities with increasing C 3 H 8 /O 2 ratio, starting from a value of 0.53. The influence of total gas flow in the range 12-45 L h -1 was then investigated at 620 °C and C 3 H 8 /O 2 ratio fixed at 0.53. Figure 12 indicates that CO and H2 production increases with increasing flow higher than 25 L h -1 . The same trend was also observed for SCFC performances. The polarisation curve measured for our optimum conditions, 620 °C, C 3 H 8 /O 2 ratio 0.53 and total gas flow of 45 L h -1 can be seen in Figure 13. Main curve is obtained with a 40 lm thick anode, while the insight shows measurements performed with a device having a 10µm thick anode. For the first one (40µm thick anode), OCV is around 780 mV, and the maximum power density reaches 18 mW cm -2 . Hence performances are improved compared to previous tested device with a 20µm thick anode (Figure 10). Moreover, insight of Figure 13 shows that the 10µm thick anode device exhibits almost no more power density (0.035 mW cm -2) . Observations of SCFC samples tested in our optimum conditions (Figure 14) reveals that the thin anode (10µm) is totally degraded with a black colour compared to grey colour of safe nickel anode. Strong exothermic effect due to high anode reactivity can explain this phenomenon. Hence, for fixed gas conditions (composition, flow), the thickness of the anode is of crucial importance for SCFC performances.

IV. Conclusion

An SCFC device with a simple coplanar geometry of the electrodes was developed. The feasibility of operating in air-propane mixture was confirmed. However, with the actual configuration of SFC device (LSM-GDC-Ni) and test bench (furnace and gas feed geometry), limited performances were recorded. The maximum power density that we could obtain is around 20 mW cm -2 . Considering the too high catalytic activity of LSM towards hydrocarbon conversion, and the low studied temperature around 600 °C, SSC and BSCF cathode materials should lead to higher performances compared to LSM. However, stability of SSC and BSCF materials in air-propane mixtures could be hardly achieved. For nickel anodes, in our test conditions, a minimum temperature of 535 °C was required to avoid its re-oxidation. Moreover, a minimum thickness is also probably required, regarding strong exothermic effects during propane oxidation. In order to improve SCFC performances, investigations on anode behaviour in running conditions is still continued, and modelling of the reactor geometry and chemical reactions rates around cathode and anode is required to optimise the experimental conditions.

  3 3. . ( (N Ni i--C CG GO O/ /C CG GO O/ /L LS SM M) ) S SC CF FC C P Pe er rf fo or rm ma an nc ce e T Te es st ts s III.3.1.

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Julien Bénabès
email: julien.benabes@irccyn.ec-nantes.fr

Fouad Bennis
email: fouad.bennis@irccyn.ec-nantes.fr

Emilie Poirson
email: emilie.poirson@irccyn.ec-nantes.fr

Yannick Ravaut
email: yannick.ravaut@fr.thalesgroup.com

Accessibility in Layout Optimization

Keywords: Design optimization, Layout problems, Accessibility

Component and facility layout plays an important role in the design and usability of many engineering products and systems as mechanical design, facility layout, process plan, management and architecture including ship compartment layout. In real-world applications, all designer's requirements can not be formulated as simple mathematical expressions. The accessibility to components is one of these requirements which can not easily be taken into account in the layout optimization process. Firstly, this accessibility deals with the required vacant space located around the component and used to guarantee the correct mechanism of the component. This space is described, in this paper, as a virtual component with particular attributes. Moreover, the accessibility requirement means that each component has to be attainable from the container's entry. In order to consider it in the layout optimization process, designers usually use their expertise to modify the layout problem description and satisfy a priori all the accessibility specifications. This paper proposes an alternative approach based on the integration of the accessibility to components in the layout problem formulation. The accessibility is considered as a design constraint or an objective of the optimization problem. This method is applied on a two dimensional real-world problem which deals with the layout optimization of components inside a shelter. Some research leads are also described in order to use this approach for the three dimensional layout problems.

Introduction

Layout problem is inherently a multidisciplinary task [START_REF] Giassi | Multidisciplinary design optimization and robust design approaches applied to concurrent design[END_REF]. It covers all the aspects of the product design life cycle from the conceptual to the detailed stage and makes necessary the collaboration between experts of technical and economical disciplines. In layout design literature, one finds some different definitions of layout problems [START_REF] Cagan | A survey of computational approaches to the three-dimensional layout problems[END_REF][START_REF] Yi | Vehicle configuration design with a packing genetic algorithm[END_REF]. The key idea is always the same: given a set of free form components and an available space, a layout problem consists of finding the best arrangement (location and orientation) of components satisfying geometrical and functional constraints and achieving design objectives. This generic definition can be adapted to all real-world applications. For example, Drira et al. [START_REF] Drira | Facility layout problems: A survey[END_REF] and Wascher et al. [START_REF] Wäscher | An improved typology of cutting and packing problems[END_REF] have adapted the definition of a layout problem to their respective research domain i.e. facility layout design and cutting and packing problems. One finds multiple search algorithms to solve layout optimization problems in two or three dimensions. Traditional optimization approaches for three dimensional layout problems are described by Jonathan Cagan et al. [START_REF] Cagan | A survey of computational approaches to the three-dimensional layout problems[END_REF]. They use genetic algorithms [START_REF] Yi | Vehicle configuration design with a packing genetic algorithm[END_REF], simulated-annealing algorithms [START_REF] Szykman | Constrained three-dimensional component layout using simulated annealing[END_REF] or extended pattern search algorithms [START_REF] Su | An extended pattern search algorithm for threedimensional component layout[END_REF]. Most search algorithms are developed for a specific problem and they provide an effective optimization strategy for it. Therefore, they are not generic and can not be adapted to a lot of layout problems. Some of recent studies deal with the search of efficient generic algorithms for solving layout problems [START_REF] Jacquenot | 2d multi-objective placement algorithm for free-form components[END_REF]. In most layout problems, the design constraints and the objectives are multiple: non-overlap, alignment of components, mass distribution, minimum or maximum distance between components. Moreover, in most real-world applications, the translation of all designer's requirements into simple mathematical expressions is a very hard task. These requirements need specific computation processes which can be time consuming. This paper focuses on one of these particular layout requirements, which deals with the accessibility to components from the container's entry. This designer's requirement can be found in some real-world applications as facility layout problems (to be sure that facilities are accessible from the plant's entry) or the mechanical assemblies (to be sure that a part can be accessible for maintenance). In fact, in a layout problem, the designer has two possibilities in order to take into account the accessibility to components:

-by inserting his expertise in the problem description in order to guarantee the accessibility to all components (for example by adding a free corridor in the middle of a room), -by integrating the accessibility requirement in the problem formulation, by inserting the accessibility to components in the design constraints or the objectives. The first approach can easily be used by the designer. However, this method does not encourage the innovation and the search of new solutions because the designer, by modifying the problem description, angles the search of feasible designs (it means that respect all the design constraints) to some existing solutions a priori intended by the designer. Consequently, this paper points up the second approach and proposes two strategies in order to consider the accessibility to components as a constraint or an objective of the layout optimization process. As far as we know, this approach has not been developed for layout problems yet. However, the different methods performed for path planning problems, especially in robotics, can be used. Trajectory planning deals with the determination of the optimal path in a workspace, where the start and end points of the trajectory are predefined and fixed. A review of trajectory planning techniques is given in [START_REF] Ata | Optimal trajectory planning of manipulators: a review[END_REF]. In layout design problems, the accessibility to components has to be represented by a numerical value in order to rank the designs according to this accessibility. The methods, proposed in this paper, achieve this requirement. This paper is organized as follows: section 2 presents two methods for characterizing the accessibility to layout components in a layout optimization process. The next section describes the optimization simulation realized on a real-world application and introduces the concept of "virtual component" used to define the space of accessibility of a layout component. The application deals with the layout problem of a shelter [START_REF] Bénabès | An interactive-based approach to the layout design optimization[END_REF]. Eight components have to be located in the shelter including four electrical and energetic cabinets, two desks and two electrical boxes. The layout optimization problem of this shelter is a three dimensional optimization problem. Because the cabinets are full height of the shelter and prevent a superposition of elements, the model is simplified in two dimensions. This simplified model is represented in the figure 1. Actually, sections 4 and 5 are dedicated to an outlook on future work and the conclusion of this paper.

Accessibility analysis

This section presents two approaches for computing the accessibility to components in order to consider the problem of accessibility as a constraint or an objective of a two dimensional layout optimization problem. In fact, the two methods return a value that characterizes the accessibility to components inside the container. The two methods are explained and compared, according to the layout problem of the shelter.

"Inner-Fit Polygon"-based approach

This method uses the computation of the Inner-Fit Polygon (assimilated to IFP in the next sections). The IFP is derived from the No-Fit Polygon (assimilated to NFP in the next sections) [START_REF] Bennel | A comprehensive and robust procedure for obtaining the no-fit polygon using minkowski sums[END_REF]. The NFP determines all arrangements that two arbitrary polygons may assume such that the shapes touch but so that they can not be moved closer together without intersection. For the layout problem of the shelter, firstly let us consider that the employee, who usually works in the shelter, is modeled by a circle C , with a radius equal to R . The center of the circle is represented by a point A . The shelter is modeled by a rectangular polygon P and the components, located inside the shelter, are considered in two dimensions as holes in this rectangle. The method suggests calculating the IFP between the circle C and the polygon P , it means all the feasible positions of the point A such as the circle C is inside the shelter and do not overlap with the components located in the shelter. The IFP can be composed of several polygons. Then, the polygon of the IFP that is linked to the entry of the shelter is selected. For each component, the distance i D between the center of the component's entry i and this polygon is measured. If the distance is equal to the radius of the circle C , the component is then accessible from the shelter's entry. Otherwise, the employee can not reach to this component from the shelter's entry. The figure 2 illustrates this IFP-based approach. The components are represented by hatched rectangles and their entry is symbolized by a thicker line. In order to integrate this accessibility requirement in the layout optimization process, we compute the value Acc, defined by: ] ,..,

1 [ , ) max( n i for R D Acc i ∈ - =
(1) where n represents the number of components taken into account in the computation of the accessibility requirement.

For the simulations which have been realized and whose results are presented in the next sections, the diameter of the circle C , that represents the employee, is fixed to 50 cm. The underlying fluctuation range step used to model the circle C is fixed to 10°.

Path optimization-based approach

This approach is based on a path optimization process that is realized between two points: the point A that is located at the center of the shelter's entry and the point B , located at the center of the component's entry. The path is composed of p n butt joint rectangular sections. These rectangles represent the path taken by the employee inside the shelter, in order to reach to the component from the shelter's entry. The path optimization process is an under constrained single-objective problem. The constraint deals with the respect of non-overlap constraints between the path and the components. Because we consider this layout problem in two dimensions and because of the rectangular shape of the path and the components, we can formulate the overlap between a section j of the path and a component i as the area intersection between the two elements. It is mathematically formulated as: In order to characterize the accessibility to a component, the sum of non-overlap constraints, between the path and all the components located inside the shelter, is computed:

)] 2 , 2 max( ) 2 , 2 min( , 0 max[ )] 2 , 2 max( ) 2 , 2 min( , 0 max[ fi fi pj pj fi fi pj pj fi fi pj pj fi fi pj pj ij L y L y L y L y l x l x l x l x A - - - + + × - - - + + = (2)
) ( 1 1 ∑ ∑ = = = = = n i i n j j ij i p A A (3)
If this sum is equal to zero, the component is then accessible from the entry of the shelter. It is also important to mention that the designer has to realize a path optimization simulation for each component. In order to integrate this accessibility requirement in the layout optimization process, we compute the value Acc, defined by:

∑ = = = n i i i A Acc 1 (4)
where n represents the number of components taken into account in the computation of the accessibility requirement. The figure 3 illustrates this path optimization-based approach, for the computation of the accessibility for the layout components 4 and 6. For the simulations which have been realized and whose results are presented in the next sections, the path is composed of 5 rectangular sections. The algorithm, used in the path optimization process, stops after a certain number of iterations, which is fixed to 100 in this paper. The vector of initial optimization variables is fixed to [L, L, L, 0, 0, 0], where L is arbitrarily chosen by the designer. For each layout configuration, the two methods are used to compute the accessibility to six components located inside the shelter. These components are those which are represented in the figure 2 and the figure 3 with a number ranged from 1 to 6, it means the cabinets and the desks. For each layout design, the calculation time and the accessibility value for each component are saved. The accessibility values obtained for each approach show that the two methods detect, for each layout configuration the same components accessible from the shelter's entry. The two methods are then comparable according to the relevance of the results. The difference between the two methods is underlined with the calculation time. The simulations, realized on these ten layout designs, show that the mean calculation time of the IFP-based approach (0,41s) is shorter than the mean calculation time of the path optimization-based approach (0,99s). This difference can have a significant repercussion on the global calculation time of the layout optimization process. Moreover, these simulations show that, for the IFP-based approach, the calculation time does not fluctuate much, so that the global calculation time of the layout optimization process can be predicted. The calculation time of the path optimization-based approach fluctuates more, because of the optimization process. It means that the convergence of the path optimization process depends on the arrangement of the components inside the container. Actually, the IFP-based approach is used for the two dimensional layout optimization process of the shelter, described in the next section.

Application

This section is dedicated to the layout optimization process of the shelter. The main objective of this problem is to find the best arrangement (location and orientation) of eight layout components in a shelter, satisfying geometrical and functional constraints and achieving design objectives. The accessibility to components is one of these design objectives.

Problem description

The layout problem of the shelter has already been described by Benabes et al. in [START_REF] Bénabès | An interactive-based approach to the layout design optimization[END_REF]. The particularity of the problem description is that the layout components can be classified in two categories:

-the "material" components: have a mass and can not overlap with other material components. Layout components are generally considered as material components in layout literature [START_REF] Grignon | A GA based configuration design optimization method[END_REF][START_REF] Su | An extended pattern search algorithm for threedimensional component layout[END_REF], -the "virtual" components: have no mass and can overlap with some material or virtual components, according to the designer's requirements. The passages, described in the layout problem presented in [START_REF] Lee | An improved genetic algorithm for facility layout problems having inner structure walls and passages[END_REF], can be considered as virtual components. As shown in the figure 1, the virtual components are used to define the spaces of accessibility of some components (cabinets and desks). They are represented by dotted rectangles. For example, the space of accessibility of a cabinet is the required space to insert some materials into a cabinet. Two spaces of accessibility can overlap, considering that operations of material loading are not simultaneously made.

Problem formulation

The layout problem of the shelter is an under constrained multi-objective problem. 24 design variables are defined, it means that each layout component is represented by 3 optimization variables : the coordinates of each element (a continued variable along X axis and an other one along Y axis) and the orientation (one discrete variable along Z axis). The design constraints are non-overlap constraints, meaning only geometrical constraints. These non-overlap constraints are estimated by the computation of the area intersection between components. Moreover, the layout problem of the shelter is a multi-objective optimization problem. The decision on the preferences between objective functions is delayed so that the designer can use the Pareto-front in order to select the most appropriate solution. Three design objectives have been defined for this layout problem:

-one objective to minimize the distance between the center of gravity of components and the geometric center of the shelter, in order to balance the masses inside the shelter (O1), -one objective to maximize the distance between the energetic networks (cabinet 1) and the electrical networks (cabinet 3, cabinet 4, electrical box 2), -one objective to evaluate the accessibility to each component from the shelter's entry (O3). This accessibility is computed by the IFP-based approach.

Layout optimization algorithm

As most real-world applications, the layout problem of the shelter is a complex problem. The geometrical shape of the layout components is simple. The detection of collision between elements is not time consuming. However, the density of the problem is important if we consider the material and the virtual components. Moreover, this problem is also complex because of the problem formulation. The designed space, meaning the set of feasible designs, is parceled. In order to pass to a feasible region to an other one, traditional gradient-based optimization approaches can not be used. Instead, stochastic algorithms, as the genetic algorithm, have to be employed. Consequently, we decide to use the layout optimization approach proposed by Jacquenot et al. in [START_REF] Jacquenot | 2d multi-objective placement algorithm for free-form components[END_REF]. This optimization strategy uses a Genetic Algorithm coupled with a Separation Algorithm. The structure of this generic algorithm is very close to a generational genetic algorithm. The separation algorithm is nested in the genetic algorithm, and modifies components positions so that proposed solutions respect non-overlap constraints. Before evaluating a solution, the algorithm checks if placement constraints are satisfied. If so, the different objectives of the solution are evaluated and the algorithm moves to the next solution. Otherwise, the separation algorithm is run and modifies the solution so that placement constraints are respected. The solution is then evaluated. The optimization process is illustrated in the figure 5.

Interactive decision making

In design optimization, the designer is always the only person who is able to make the final decision on optimal solutions. The optimization strategy described in this paper suggests using an interactive environment in order to allow the designer to:

-select a Pareto-optimal solution generated by the algorithm, -visualize the solution in two or three dimensions, -manually and locally modify the solution, by changing the position and the orientation of some components and visualizing the new values of the constraints and the objectives which result from these modifications. The main objective of this interactivity with the designer is to improve the performances of the optimal solutions proposed by the algorithm, by inserting in the decision making the personal judgment of the designer. This interactivity between the designer and the optimization process needs an efficient graphical and numerical environment. The figure 5 represents an example of interactive environment which allows the designer to visualize the set of Pareto-optimal solutions. The designer can click on a point on the graph and visualize the layout design, the constraints and the objectives related to this solution.

Results

This subsection presents the results obtained by using the optimization strategy previously described. Firstly, the hybrid optimization algorithm has been randomly initialized by 200 individuals Then, after 100 iterations, the algorithm has computed 20 000 designs. Among these designs, 72 are Pareto-optimal "variants". A design j is a new variant if it differs from the design i by at least one of the following criteria:

-one of the components of the layout has been displaced from at least Δ mm along one of the axis ( Δ is set to 500 mm in this application), -one of the components has been rotated, -the minimum difference between the objective values of the two designs is bigger than a limit for example fixed to 10 cm. As it is explained in the previous subsection, the designer has to select one of these Pareto-optimal variants. For example, let us consider that the designer chooses the design represented in the figure 5 and the figure 6 (b). Then, by locally modifying the position of some components, according to the personal judgment of the designer, the solution represented in the figure 6 (c) is obtained.

Outlooks

This paper proposes two approaches to take into account the accessibility to components in the layout problem formulation. These two approaches are different. One method uses the Inner-Fit-Polygon (IFP), whereas the other one performs a path optimization process for each design. One of the main outlooks is the adaptation of these approaches to the three dimensional layout problems. On the one hand, in order to use the IFP-based approach, the method has to be modified. A suggestion is to transform the problem into a two dimensional problem. Firstly, according to the designer's requirements, the components which are located below a certain plan, fixed by the designer, can be deleted for the computation of the accessibility to components. It means that the designer is able to step over a component if the component is, for example, a small object fixed to the floor. By the same way, the components located above a certain plan can be deleted. It means that the designer is able to pass below these components. These components can be for example a ceiling light or a ventilator. Then, all the other components have to be projected in two dimensions and the IFP-based approach, previously described, can be used. This approach can be used for most three dimensional layout problems. On the other hand, in order to use the path optimization-based approach, a suggestion is to transform the rectangular sections of the path into parallelepipeds. Then, the method, adapted to three dimensional applications, has to compute intersection volumes between the components and the path's sections. As it is explained for the path optimization-based method in two dimensions, if the intersection volume between a component and the path is equal to zero, then the component is accessible from the container's entry. Moreover, it is important to mention that the accessibility requirement that guarantees that all components are accessible from the container's entry differs from the accessibility condition which guarantees that a component can be taken out from the container. One finds this second requirement in mechanical assemblies, when the designer wants to maintain a part. Consequently, the strategies, presented in this paper, have to be adapted to these specific layout problems.

Conclusion

Accessibility in layout optimization can have two different meanings. Firstly, this accessibility can refer to the space located near a layout component and required to insert some materials into the component. The layout optimization strategy, proposed in this paper, defines this space of accessibility as a "virtual" component. Secondly, accessibility means also that all components have to be accessible from the container's entry. This paper proposes two methods to consider this accessibility requirement as either a design constraint or an objective of the optimization problem. These approaches suggest simplified models in order to compute the accessibility to layout components, without being time consuming. Then, it allows the designer to use these methods in layout optimization processes. Actually, this paper proposes innovative approaches which allow the designer to explore more alternative solutions to his layout problem. It is a very good way to encourage the innovation. The designer is not forced to modify the problem description and the optimization algorithm can find original solutions.

Figure 1 :

 1 Figure 1: Description of the shelter in 2D

Figure 2 :

 2 Figure 2: Illustrations of the IFP-based approach 2.2 Path optimization-based approachThis approach is based on a path optimization process that is realized between two points: the point A that is located at the center of the shelter's entry and the point B , located at the center of the component's entry. The path is composed of p n butt joint rectangular sections. These rectangles represent the path taken by the employee inside the shelter, in order to reach to the component from the shelter's entry. The path optimization process is an under constrained single-objective problem. The constraint deals with the respect of non-overlap constraints between the path and the components. Because we consider this layout problem in two dimensions and because of the rectangular shape of the path and the components, we can formulate the overlap between a section j of the path and a component i as the area intersection between the two elements. It is mathematically formulated as:

  respectively define the coordinates and the dimensions (width and length) of the section j of the path. By the same way, respectively define the coordinates and the dimensions (width and length) of the component i . The design objective is to minimize the length of the path, it means the sum of the lengths of the different sections of the path. The design variables are the variables that drive the position of the rectangular sections of the path, it means a continued parameter for the length of the rectangle and a discreet variable for its orientation which can take four values: 0°, 90°, 180° or 270°. Moreover, the last two sections result from the position of the variables in this path optimization problem.

Figure 3 :

 3 Figure 3: Illustrations of the path optimization-based approach 2.3 Comparison of the two approachesThis subsection compares the two approaches, previously described, on ten different layout designs of the shelter. For each layout configuration, the two methods are used to compute the accessibility to six components located inside the shelter. These components are those which are represented in the figure2and the figure3with a number ranged from 1 to 6, it means the cabinets and the desks. For each layout design, the calculation time and the accessibility value for each component are saved. The accessibility values obtained for each approach show that the two methods detect, for each layout configuration the same components accessible from the shelter's entry. The two methods are then comparable according to the relevance of the results. The difference between the two methods is underlined with the calculation time. The simulations, realized on these ten layout designs, show that the mean calculation time of the IFP-based approach (0,41s) is shorter than the mean calculation time of the path optimization-based approach (0,99s). This difference can have a significant repercussion on the global calculation time of the layout optimization process. Moreover, these simulations show that, for the IFP-based approach, the calculation time does not fluctuate much, so that the global calculation time of the layout optimization process can be predicted. The calculation time of the

Figure 4 :

 4 Figure 4: Hybrid layout optimization algorithm[START_REF] Bennel | A comprehensive and robust procedure for obtaining the no-fit polygon using minkowski sums[END_REF]

Figure 5 :

 5 Figure 5: Interactive environment for decision making

Figure 6 :

 6 Figure 6: Graphical resultsIf we consider the values of the three objectives formulated in this layout problem, this solution is better than the initial one, proposed by the designer and represented in the figure6 (a). It is important to mention that this initial configuration is an intuitive solution which has been generated only by considering geometric aspects. The table1represents the values of the three objectives for the three solutions illustrated in the previous figure.

Table 1 :

 1 Numerical results

		Initial design (a)	Selected design (b)	Modified design (c)
	Obj. 1 (min,cm)	25,41	28,92	22,90
	Obj. 2 (max,cm)	604,88	587,56	669,03
	Obj. 3 (min,cm)	0	0	0

Acknowledgment

The authors would like to acknowledge THALES for the application study.


Jean-Marc Chovelon
email: jean-marc.chovelon@univ-lyon1.fr

A P Soldatkin

S V Dzyadevych

Tran-Minh Canh

Keywords: Lactate oxidase, Amperometric biosensor, Resydrol, Lactate, Wine, Electrochemical polymerization

Amperometric biosensor for lactate analysis in wine and must during fermentation

I. Introduction

Wine as an element of food industry at all stages of production should be exposed to technical and chemical control aimed at determination of components of must and wine which influence the final product quality [START_REF] Rusakov | Chemistry and Biochemistry of Wine[END_REF]. The formation and transformation of organic acids at must fermentation and wine production are of great importance in wine-making. Biochemical processes caused by yeast enzymes are significant to achieve necessary quality of wine materials. Knowledge on the mechanisms of formation of secondary products ensures their proper regulation toward wine enrichment in the beneficial components which determine wine race and flavour [START_REF] Rodopulo | Biochemistry of Wine-making[END_REF]. Organic acids also protect wine against bacterial diseases. In acid medium, redox processes are slower which inhibits wine ripening but prevents ferrophosphate turbidity.

The acids due to reaction with spirits form esters and thus take part in providing race of wine. However, high content of some acids influences wine flavour badly. An increased content of malic acid is considered to result in taste of green berries. Therefore, in practice significant importance has conversion by lactic bacteria of dicarboxylic malic acid into monocarboxylic lactic acid since the latter is characterized by softer taste and thus harmonizes wine taste. At the same time, an excess of lactic acid has also a negative effect on wine, especially if fermentation is caused by heterotrophic lactic bacteria because of formation of acetate, diacetyl and other substrates which spoil wine race [START_REF] Rodopulo | Biochemistry of Wine-making[END_REF]. Lactate formation at normal alcohol fermentation was proved by S. Durmishidze [START_REF] Durmishidze | Biochemistry of Vine[END_REF]. It is a secondary product of anaerobic fermentation, resulting from reduction of its predecessor, pyruvate, in the presence of NAD•Н 2 . Lactate can be also generated in another way -from glutamate and alanine [START_REF] Rodopulo | Biochemistry of Wine-making[END_REF]. Permanent, simultaneous and selective lactate monitoring (preferably, along with other key components) is important in wine-making since this parameter not only determines quality and special odour of wine but if necessary, allows interfering into fermentation efficiently, thus preventing significant economic losses. Actually, there are classic methods for lactate determination: liquid chromatography [START_REF]Office International de la Vigne et du Vin (OIV)[END_REF], colorimetry, and enzymatic methods [START_REF]Office International de la Vigne et du Vin (OIV)[END_REF] and [START_REF] Ribereau-Gayon | [END_REF]. However, they use rather complicated apparatus and depend on analyte volume, therefore alternative approach is required. Biosensors can be considered as promising alternative if they are inexpensive and reliable, provided simple, fast and cheap procedure, and, what is of essential importance, profitability [6] and [7]. Now a few biosensors are applied to test quality of raw material in food production despite the fact that they are advantageous in fast response and high specificity as well as in low cost and easy data processing. Therefore, development of biosensors assigned for control of food quality is now in focus of scientific research. Remarkable progress in this field revealed during recent years is a result of up to date achievements in micromechanics as well as in increasing sensitivity and quality of associated electric equipment. Lactate biosensors can be based on two enzymes, either lactate oxidase or lactate dehydrogenase [8], [9], [10] and [11]. However, the latter require the presence of cofactor NAD + which complicates analysis and increases its cost. Moreover, a supplementary enzyme, glutamate pyruvate transaminase, is necessary for efficient reaction, thus the price of analysis raises still more [12]. In work [13] and [14] authors described sensors based on lactate dehydrogenase for lactate determination which required presence of hexacyanoferrate (III) in the samples that also complicates analysis. The biosensors based on immobilized lactate oxidase (LOX) are preferable to those based on lactate dehydrogenase because, in contrast to the latter, there is a natural cofactor FAD in the LOX composition. A number of lactate biosensors have been described in literature, e.g. the biosensor based on lactate oxidase immobilized into polyphenilethylene diamine ultrathin polymer membrane for the lactate analysis in blood [15]. Because of a miniature sensor, the volume of electrochemical cell where the measurements were carried out, was only a few nanoliters. However, it had poor operational stability. There was some information [16] and [17] about the multisensors capable of determining simultaneously lactate and other components of physiological liquids. Standard hydrogels were used to form the polymer matrixes, operational stability was from two weeks to two months. Different polymer matrixes can be utilized for LOX immobilization into membrane [18], [19] and [20]. Recently it was shown that usage of 2,6-dihydroxynaphthalene copolymerized with 2-(4-aminophenyl)-ethylamine makes it possible to obtain high sensor sensitivity to lactate (10 μM) [18]. Using nylon matrix as a base of sensitive membrane other authors developed a lactate biosensor based on immobilized lactate oxidase with high sensitivity (2 μM) which operated stably for a week [21]. This work was aimed at development of the amperometric biosensor based on the platinum SensLab electrode (SensLab GmbH, Leipzig, Germany) with immobilized lactate oxidase for lactate determination in must during its fermentation and in wine.

II. Materials and methods

I II I. .1 1. . C

Ch he em mi ic ca al ls s Enzyme lactate oxidases from Pediococcus species with specific activity 31 U/mg and 49 U/mg were obtained from Sigma-Aldrich Chemie GmbH (Germany) and with specific activity 40 U/mg was obtained from Fluka (Switzerland). As the polymer matrix for enzyme deposition, Resydrol AY 498 w/35WA (Slutia Austria GmbH) was used. For electrochemical deposition the monomer 3,4-ethylenedioxythiophene (EDT) from Baytron M (Germany) and polyethylene glycol 1450 from Sigma-Aldrich Chemie GmbH (Switzerland) were used. Besides, we used the following chemicals: Na 2 HPO 4 •7H 2 O, KH 2 PO 4 , KCl, NaOH and lactate from Sigma-Aldrich Chemie GmbH (Germany). All chemicals were analytical grade. I II I. .2 2. . A Ap pp pa ar ra at tu us s All electrochemical experiments were performed using a conventional three-electrode system with the SensLab electrode (SensLab GmbH, Leipzig, Germany) which combines all three electrodes, i.e. platinum working, auxiliary and reference ones [22]. The electrodes produced were previously tested with regard to their reproducibility and reliability by cyclic voltamperometry within the potential range of 0-300 mV (speed of potential involute was 20 mV/s) using potentiostate PI-50-11 (Ukraine). Amperometric measurements were carried out in 5 ml electrochemical cell at a constant potential using the same potentiostate, programmer PR-8 (Ukraine) and VoltaLabTM 21 system (potentiostate/galvanostate PGP201 Radiometer, Copenhagen, Netherlands) controlled by the VoltaMaster 11 programme, production of Radiometer Co. (Copenhagen, Netherlands).

F Fi ig gu ur re e 1 1: : B Ba as si ic c s sc ch he em me e o of f P PE ED DT T p po ol ly ym me er ri iz za at ti io on n [ [2 23 3] ]

I II I. . Hydrophilic properties of the deposited polymer rise in this case. Whether it would be oxidation or reduction depends on the potential applied since it controls generation of a positive charge in the polymer structure. It is proved for PEDT that polyanions as oligonucleotides can be effectively kept in the polymer net which acts at weak electronic strength [25]. Comparison of polyacetylene with polypyrrole and polythiophene (all charged positively) demonstrates higher stability of the latter two which can be supposed to be a result of the presence of positive charges of nitrogen or sulfur A mixture of 10 -2 М 3,4-EDT, 10 -3 М polyethylene glycol and 35 mg/ml lactate oxidase prepared in 20 mM phosphate buffer, pH 6.2, was used for electrochemical polymerization. It was performed applying potential from + 0.2 to + 1.5 V at a speed of 0.1 V/s during 15 cycles.

The cyclic voltamperogramme of PEDT electrochemical synthesis (Figure 3) demonstrates decrease of peak values at each next cycle as a result of polymer film formation on the working electrode surface. Commercial resin Resydrol AY 498 w/35WA (Slutia GmbH Austria) was used as a polymer matrix for enzyme deposition. These resins mainly consist of acryl copolymers and acrylic ethers which determine solubility and protonation rate at definite pH [27] and [28].

For LOX immobilization into polymer film Resydrol, 30 mg/ml LOX solution prepared in 10 mM phosphate buffer, pH 7.2, was mixed at a ratio of 1:3 with 300 μl of Resydrol dissolved in 1 ml of water. Due to homogeneity and low viscosity of the mixture obtained it could be easily collected and deposited on the working electrode surface. For polymerization, the sensor was maintained during 2 h at + 4 °С. Pasteurized unsulfizated must from the Rkaziteli grapes of 2003 year's harvest produced in the Sofia Perovskaya close corporation (Sevastopol, Crimea, Ukraine) was used for fermentation. 2% of yeast culture 1-527 from the National microorganisms collection of the Magarach Institute of grapes and wine was added to the must. The must fermentation was processed in the apparatus of periodic operation, volume of 10 l, sealed with a cotton pug under semi-anaerobic conditions at temperatures of 16-28 °С and 26 °С. The apparatus was thermostated by the І130 air thermostat. The fermentation must was stirred once or twice daily. The periodicity of sampling was the same, at the moment the must temperature was measured. The samples were centrifuged at 5000 rpm, the supernatant was further analyzed. Reference control measurement of the lactate concentration in must at fermentation and in wine was carried out by ion chromatography [START_REF] Rusakov | Chemistry and Biochemistry of Wine[END_REF] and [START_REF] Mekhuzl | International Book of methods of analysis of wine and must, Food industry[END_REF]. To obtain the calibration curve, certain aliquots of concentrated lactate solution were added to electrochemical cell. A wine sample of 5 μl volume, the lactate concentration in which to be measured was then introduced into the vessel, thus dilution ratio equaled 1000. The lactate concentration in wine sample was determined by the response obtained in about 1 min according to the calibration curve.

After obtaining each response, the sensor was washed with buffer solution up to output signal stabilization.

III. Results and discussion

The basic enzymatic reaction in lactate determination by amperometric biosensors based on lactate oxidase is:

2 2 2 pyruvate H O    LOX L lactate O   
The lactate hydrolysis is accompanied by accumulation of electrically active substance, hydrogen peroxide, whose decomposition results in generation of electrons registered by amperometric transducer.

In the work presented, the enzyme was immobilized onto transducer surface by two methods, by physical adsorption in Resydrol and by electrochemical deposition in PEDT.

Prior to the investigation, the biosensor should be optimized as to its analytical characteristics for further operation in real samples. As the first step, optimum pH of the biosensor based on immobilized LOX was defined (Figure 4). The results obtained were in accordance with those presented in literature, i.e. 7.0-7.5 [START_REF] Bardeletti | [END_REF], [31] and [32]. As seen in Figure 4, in our experiments optimal pH = 7.2 in case of Resydrol while it is 7.4 for PEDT, so the pH optimal value of lactate biosensor is not influenced by the method of immobilization. As has been earlier reported [33] and [34] the background electrolyte concentration in buffer as well as buffer capacity have no effect on working characteristics of amperometric glucose biosensor. Since this situation is typical for enzyme biosensors, no further study on the issue for lactate biosensor was carried out. The stability of the sensor based on immobilized LOX was studied. As a result, the response in case of LOX immobilized in Resydrol was shown to decrease by 2.5% daily (Figure 5, curve 1) while that for electrochemically deposited in PEDT -by 3% daily (Figure 5, curve 2), i.e. the immobilization method has no effect on the sensor stability.

F Fi ig gu ur re e 4 4: : R Re es sp po on ns se e d de ep pe en nd de en nc ce e f fo or r a am mp pe er ro om me et tr ri ic c b bi io os se en ns so or rs s b ba as se ed d o on n l la ac ct ta at te e o ox xi id da as se e i im mm mo ob bi il li iz ze ed d i in n R Re es sy yd dr ro ol l p po ol ly ym me er r b by y p ph hy ys si ic ca al l a ad ds so or rp pt ti io on n ( (1 1) ) a an nd d i in n P PE ED DT T p po ol ly ym me er r b by y e el le ec ct tr ro oc ch he em mi ic ca al l p po ol ly ym me er ri iz za at ti io on n ( (2 2) ) o n p pH H o of f s so ol lu ut ti

The developed amperometric biosensor based on LOX immobilized in Resydrol by physical adsorption was characterized in linear dependence of the biosensor response on lactate concentration within the range of 0.004-0.5 mМ, while for the biosensor based on LOX immobilized in PEDT by electrochemical deposition this range was 0.05-1.6 mМ (Figure 6). Consequently, the former has a narrower dynamic range of lactate determination at higher sensitivity as compared with the latter. This phenomenon can be elucidated by formation of a more compact membrane in case of the enzyme immobilization by electrochemical deposition which results in decrease of response, on the one hand, and in the range extension -on the other. el le ec ct tr ro od de eOptimization of the amperometric lactate biosensor sets in motion its application for the lactate analysis in real samples. As a subject of investigation four wine samples and 29 samples of must at various fermentation stages were taken. All of them were the production of the Magarach Institute of grapes and wine. The lactate concentration during fermentation and in finished wine was measured by two methods -the developed amperometric biosensor with immobilized lactate oxidase and ion chromatography. A good correlation of results for wine samples was showed (Table 1). However, for samples of must at various stages of fermentation some discrepancy of results was obtained, especially for first days of fermentation (Table 2). It could be connected with fact that at first days of fermentation a lactate concentration is very small, and sensitivity of amperometric biosensors is insufficient for low lactate concentrations.

IV Conclusion

On the purpose of development of amperometric biosensor for lactate determination, two methods of immobilization of lactate oxidase onto the surface of commercial platinum printed electrodes SensLab were compared. The sensor with LOX immobilized in polymer Resydrol by physical adsorption is shown to have both narrower dynamic range and higher sensitivity in comparison with the biosensor with LOX immobilized in polymer PEDT by electrochemical deposition. The analytical characteristics of these biosensors are studied, the immobilization method is shown to be of no influence on biosensor operational stability and pH optimal value. The lactate concentration both in must during its fermentation and in finished wine was analyzed. The results obtained by the amperometric lactate biosensor correlated with those measured by standard method of ion chromatography. Thus, the biosensor developed can be recommended in food industry to monitor and optimize the fermentation process and to control quality of finished wine.

  3 3. . E El le ec ct tr ro oc ch he em mi ic ca al l L LO OX X i im mm mo ob bi il li iz za at ti io on n o on nt to o s su ur rf fa ac ce e o of f S Se en ns sL La ab b e el le ec ct tr ro od de es s Electrochemical polymerization is of special interest due to its manufacturability. It allows choosing and preserving matrix size, shape and thickness, assures direct control over precipitation. Poly(3,4-ethylenedioxythiophene) (PEDT) is one of the polythiophenes, conductive polymers with new and noteworthy characteristics. They demonstrated faint conductivity, low charge alteration, high stability, and valuable ability of film formation. The EDT monomer, the Baytron commercial product, recently came into the market from Bayer AG Co. (Germany). The polymer form and mechanism of polymer structure modification are demonstrated in Figure 1 [23] and Figure 2 [24].

F

  Fi ig gu ur re e 3 3: : C Cy yc cl li ic c v vo ol lt ta am mp pe er ro og gr ra am mm me e o of f e el le ec ct tr ro oc ch he em mi ic ca al l s sy yn nt th he es si is s o of f p po ol ly y( (3 3, ,4 4--e et th hy yl le en ne ed di io ox xy yt th hi io op ph he en ne e) ) i in n 0 0. .0 02 2 M M p ph ho os sp ph ha at te e b bu uf ff fe er r, , p pH H 6 6. .2 2, , i in nv vo ol lu ut te e s sp pe ee ed d cy yc cl le e n nu um mb be er rs s. . I II I. .5 5. . L La ac ct ta at te e d de et te er rm mi in na at ti io on n i in n m mo od de el l s so ol lu ut ti io on ns s The substrate concentration was measured in 100 mM buffer solution KH 2 PO 4 -Na 2 HPO 4 •12H 2 O, рН 7.2, at room temperature in open vessel at intensive stirring. The concentrations were varied by addition of certain aliquots of concentrated solutions. I II I. .6 6. . L La ac ct ta at te e d de et te er rm mi in na at ti io on n i in n w wi in ne e a an nd d m mu us st t Lactate was measured in four samples of different wines produced under conditions of micromanufacture in the Magarach Institute of grapes and wine and the Magarach agricultural firm (Crimea, Ukraine) as well as in 29 samples of wine materials at various stages of their fermentation.

5 I

 5 II I. .7 7. . L La ac ct ta at te e m me ea as su ur re em me en nt t i in n w wi in ne e a an nd d m mu us st t b by y a am mp pe er ro om me et tr ri ic c b bi io os se en ns so or r The measurements were performed in an open vessel of intensively stirred 100 mM phosphate buffer solution, pH 7.2.

  homogenous film PEDT is fixed on the surface of the working electrode, electrochemically polymerized 3,4-ethylenedioxythiophene (EDT) upon neutral pH and temperature. Film is formed preferably in aqueous and, if possible, hydrophilic polymers of polyvinyl pyrrolidone (PVP) or polyethylene glycol type dissolved in water with electropolymerizing solution.

F Fi ig gu ur re e 2 2: : S Su ug gg ge es st te ed d m me ec ch ha an ni is sm m o of f m mo od di if fi ic ca at ti io on n o of f P PE ED DT T s st tr ru uc ct tu ur re e a at t e el le ec ct tr ro oc ch he em mi ic ca al l p po ol ly ym me er ri iz za at ti io on n [ [2 24 4] ]. . A

Acknowledgements

Part of this work was financially supported by National Academy of Sciences of Ukraine in the frame of Scientific and Technical Program "Sensors systems for medical-ecological and industrial-technological problems".