Nicolas Minvielle Larousse

Marie-Christine Bailly-Maître

Ore grinding in the Middle Ages: the example of Brandes-en-Oisans (Isère, France)

Introduction

Across the millennia, mills have played the role of 'power transformers' for multiple purposes: milling and grinding food, of course (grain, olives, etc.), and for activities unrelated to food such as tanning and metallurgy as well.

The silver coins that fed most of the economy of the west in the Middle Ages were produced by means of a long process in which mills were used to grind ore extracted from the mines. The activity was carried out on large industrial sites like the Argenteria de brandis. Brandesen-Oisans (fig. 1) was a mining community, a part of the Dauphiné Principality, fully devoted to the extraction and treatment of several argentiferous lead veins between the 12th and 14th centuries (Bailly-Maître and Dupraz 1994.). The site, which comprised a village with some 80 dwellings, fortifications, a parish church and large industrial areas, has been excavated by the CNRS since 1977. So far, it has revealed more than 206 millstones, plus fragments and chips, the remains of the ore mills that operated on the plateau at that time as links in a large chain of mineralurgical operations. The ore leaving the galleries was trapped in gangue consisting mainly of barite and quartz. A process was required to separate the precious metal from this waste rock and several methods were implemented, such as crushing, decrepitation and grinding. While crushing was done manually and decrepitation was done by fire, grinding was mechanised by water-mills. However, instead of pouring grain onto the millstones, the miners cast ore and gangue onto them and recovered a powder that was then treated in washing and decantation basins to collect even the smallest gram of silver.

Very few documents describe this activity and this is no doubt the reason for which the mills were only acknowledged as a result of the excavation of several mining sites. Archaeologists have addressed the subject, first separately [START_REF] Benoit | La mine de Pampailly, XVe-XVIIIe siècles, Brussieu-Rhône. Document d'archéologie en Rhône-Alpes[END_REF]Domergue et al. 1997, 48-61;Bailly-Maître 2002, 224) and then collectively to summarise acquired knowledge and set the foundations for future research in an article published in 1997 for the Aix-en-Provence symposium (Benoit et al. 1997, 62-69). This study starts with three main themes. The first one will focus on the production of millstones, their extraction and shaping, as shown by fragments that still show traces of cutting, and also by the rough-outs found on the plateau. The second theme will then evoke the 'memory' of grinding, in particular by analysing traces of wear on the grinding surfaces of millstones. The third theme consists of establishing the context in which the millstones were used in the mills, proposing interpretation schemes based on elements noted by observing the millstones and also through the contribution of other types of sources.

From quarry to millstone 1.1 Millstone grit in Brandes-en-Oisans

The Brandes plateau, composed of various outcrops and innumerable blocks, was worked to supply the millstones required for the mills on the site. The miners used the millstone grit over and over since it provided them with stones they could extract on location, without the difficulty and cost of transporting stones weighing several hundreds of kilos. Rather than working rocky massifs, however, they preferred to extract their millstones from blocks dispersed across the plateau. These blocks were the remains of glaciers or had crashed down from the slopes of the surrounding summits and many of them were indeed the right size for shaping millstones (Figs. 2 and3).

The millstone grit quarries of Brandes are highly representative of 'village millstone grits' for local production, situated as close as possible to the mills (Belmont 2006, 77). Ore mills are no different from grain mills in this respect. Our vision is not optimal though, since few blocks of this sort have been preserved. They were probably fully whittled down by extraction and the few minute scraps created by cutting are most likely to have been dispersed or reused.

Whatever the case may be, whether the millstone grits were obtained locally or further away, this practice is not an archaism, as it might be presumed in view of the period of the operations and use (13th century AD), and given that the industry was destined to expand and become specialised. As regards the grain mills of the neighbouring community of Huez, for example, millstones continued to be extracted in this manner over the centuries that followed, up to the 19th century (AD 38, 7 S 1/1, 1809 survey on mills in France. Cited by Alain Belmont, Atlas des meulières de France et d'Europe, 2006, http://meuliere.ishlyon.cnrs.fr/ index.htm). There are several reasons for this. First of all, it would appear that the financial investment was much too high for the intended use. But, considering the fact that the Huez inhabitants remained faithful to this type of quarry for wheat mills for at least 600 years, it is important to turn our attention to the quality of the stone.

Rocks

The quality of the millstone rocks cannot be emphasized too strongly. It has now been established that this point was of key importance (Belmont 2006, 89-113.). Certainly, the millstones used to grind ore did not necessarily meet the same requirements as grain millstones, but the elements that break away from them, as small as they may be, nevertheless add pollution to an ore that is supposed to be in the enrichment phase. Millstone wear is another factor to be considered. The faster the millstone wears out, the faster it needs to be changed.

The millstones from Brandes are shaped from two main types of clast (Fig. 4): 56.4% conglomerate and 33% sandstone. Added to this are a few gneiss and granite inclusions (4.3%), but they remain marginal. These are indeed the rocks that form the blocks scattered across the plateau, apart from the gneiss and granite which come from the rocky massifs (This classification was carried out by Stéphane Rouméjon (Université Joseph Fourrier, Grenoble I) to whom we are deeply grateful). Figure 3. Rough-out 2009-1-009.

Is there a constant to be detected in the rocks chosen by the miners? Nothing is less certain. Resistance tests can provide part of the answer, provided that a comparative approach is used. In 2005, Yves Orengo, of the Laboratoire Interdisciplinaire de Recherche Impliquant la Géologie et la Mécanique (LIRIGM) of the Université Joseph Fourier (Grenoble I) conducted tests on three samples from the millstone grit B 106 excavation that took place during the 2004 campaign. The samples are a siliceous microconglomerate. The results are summarised in the table below (Fig. 5):

For each of the samples analysed, the compression strength (col. 3) is greater than 110 mega Pascal. On a scale used for the tests carried out on a number of European millstone grits (Fabre et al. 2006, 91-97.), the Brandes samples are situated beside the silex of La Ferté-sous-Jouarre (80 MPa) or Corfélix (240 MPa) in the Brie region (France). Hence, this is a high quality rock. Moreover, this rock was used equally for both ore and grain. This dual usage is visible on three grain millstones made from the same conglomerate and used for the Brandes miners' food. Caution is advisable however as regards the characteristics of these millstones. We do not know exactly which qualities were required for the rocks intended to grind ore. Other mining works, for their part, such as those in Pampailly (Rhône -France) (Benoit 1997, 70.), or Castel Minier (Ariège -France) have revealed granite millstones (Benoit et al. 1997, 62-69.).

The diversity of the cases encountered shows that the miners tended to make do with the rocks at their disposal.

Whether the sandstone and conglomerate of Brandes, or the granite of Pampailly, what these rocks have in common is that the majority are present in situ. It was not as much about making choices as it was about adapting to the situations encountered.

Extraction techniques

The methods used to extract millstones are fully representative of the various quarries studied in Europe (Anderson 2003, 49-50;Jaccotey and Milleville 2008, 16-23;[START_REF] Belmont | Les carrières de meules de Claix, lieudit Les Meulières[END_REF]. We can differentiate however between the blocks used to extract several millstones and the blocks intended to produce a single item.

First, there was the cutting out of a ring-shaped channel to outline the future millstone. This operation was carried out using a unique tool with a circular tip slightly more than 1 cm in diameter, in three cuts (Fig. 6.): the first one as close as possible to the rough-out, the second 10 cm further along and the last one in the middle, after the second cut was made. This method, which in all likelihood involved the oblique 'thrown percussion' or 'pecking' technique, is common in the millstone grits studied. The squaring off of the edges of the blocks was done using rougher tools, which fractured the rock. The millstone makers worked step by step in order to 'test' the rock. They tackled it in different ways and in different locations to make sure it was worthwhile to continue working on it. If the rock yielded at any time, they did not pursue their art any further, so as not to waste time fully shaping a part of the millstone whereas the other part was bound to fracture.

Regarding the blocks cut for the production of a single millstone, the cutting method differs slightly. The various steps involved in the cutting of these blocks proposed by Gary Adams and Gérard Serres are fully applicable to the rough-outs of Brandes (6 steps of cutting. Cited by Hockensmith 2006, 193-194;Serres unpublished, 1995. Diagram produced from the medieval ore millstones in Peyrusse-le-Roc (Aveyron -France)). First, the gross block is roughly hewn to a shape that is rounded at best or at least plane-parallel. This step can consist initially of rough work, leaving corners, after which the operation is finalised on the edges with a sledge hammer or a pick.

There is little trace of this phase still visible on the rough-outs present, since the millstone makers continued their work. However, on the 2009-1-09, a clean cut can be observed on the bottom of the block, attesting to the first actions of the tradesmen of Brandes (Fig. 7). At the second step, the reduced block was cut, much more rigorously this time in order to give it its rounded shape. Again, on the 2009-1-09, traces of a pick or chipping chisel 1 cm wide resulting from this operation are visible on the major part of the edge of the block (Fig. 8). Then, the upper surface of the block was flattened. Apart from preparing a possible grinding surface, this task was also used to test the rock.

If the rock proved to be inappropriate for cutting, it might break and by testing, the millstone makers could avoid wasting their time.

The methods used to extract millstones for ore grinding are therefore no different from those intended for the milling of grain. The millstones could only be adapted for specialised use after extraction.

Shaping

The coring out of the eye is a step still common to both usages. In the centre of the potential millstone, the millstone maker would first cut a circular vein that was progressively enlarged with the aim of being sunk in as work progressed.

The task was carried out using a tool with a round tip 7 mm in diameter as shown by the marks left at the bottom of the incision made on rough-out 2009-1-10 (Fig. 9). Once the incision was made, the millstone cutter removed the first section in order to complete the circle, still using the same tool. The eye was only completed after the future millstone was the right thickness.

The next step consisted of making the notch in the millrynd. The widths vary from 4.5 cm to 7 cm for the 2009-1-38 (Fig. 10). As for lengths, they are seldom preserved in their entirety. For the 2009-1-38, for example, one is 29.5 cm long and another is slightly more than 28 cm. The thickness varies from 2 to 3 cm, no notches being fully regular. Finally, the furrowing of the millstones is clearly similar here for the grinding of ore. Although during Antiquity, the grinding surfaces of grain millstones could be furrowed or at least covered by multiple grooves (Belmont 2006, 41), this process appears to have been abandoned afterwards, only reappearing at the end of the 18th century (Amouric 1984, 272;Perry 198, 155). In the meantime, the millers preferred to peck at their stones randomly. The furrowing technique, however, did not disappear forever during the Middle Ages. It was adapted for the grinding of ore not only in Brandes, but throughout the mining works equipped with such artifices. The furrowing of wheat millstones is a more complicated task than the cutting out of furrows for ore, but the skilled Medieval tradesmen were familiar with and mastered both types of know-how as they used them both at the same time.

A high degree of consistency is noted in the method used to cut out the furrows. The quarry workers appear to have preferred a particular type of tool fitted with a circular tip ranging from about 5 mm to 1 cm in diameter. In a few more isolated cases, they may also have used rougher tools such as chipping chisels, leaving marks measuring more than a centimetre. They began by cutting out a central recess which they would then enlarge by one or two shallower cuts using a pick (Fig. 11).

Several types of furrowing are observed on the ore millstones. Factors such as the number of furrows, their distribution on the millstone and their dimensions are taken into account. This typology is in the process of being drawn up using a diachronic approach (Lepareux-Couturier and Jodry 2010; Minvielle Larousse and Bailly-Maître 2010).

In making a final assessment, we can state that ore millstone quarries do not differ from those that produced wheat millstones. The methods and steps of extraction were similar as well. From squaring off to cutting out the notch for the millrynd, all of the Brandes tradesmen's actions are representative of a more general context. Only the size of the furrows and, perhaps, the thickness truly differentiate ore millstones, the variations in diameters being too fine to opt for one of the two uses.

After having compared the methods of shaping ore millstones with those used for wheat millstones, we are also in a position to link up millstone making techniques with quarrying techniques. This parallel has already been made, centred more on the way of working underground extractions (Bailly-Maître 2006, 163-170.). We propose to extend these considerations to encompass the entire chain of operations. The tool marks left by the tradesmen are clearly reminiscent of the tools used in mining work: tapers, sledge hammers, chipping chisels, picks, etc. The miners used their skills and their tools in the open air to shape the millstones which we are now going to present in detail.

Millstones, 'memory' of grinding

As regards traces left on the grinding surfaces, ore millstones have a sizeable advantage over wheat millstones, for two main reasons. The first concerns the material precipitated into the eye of the upper millstone.

Grain is no rival to rock and always ends up being cleaned out by the asperities, without leaving any evident traces. Ore, on the other hand, mixed with gangue, is a much tougher adversary for millstones. The results are, admittedly, practically the same, with a fine powder at the outlet, but in the meantime the millstones were lacerated by the passage of ore which left thousands of indelible marks, traces that will enable us to better understand the rotation and operation of the grinding mills. The second reason is related to millstone maintenance. By regularly dressing the grain millstones, the millers also eliminated the minute traces of the previous grinding. Miners, though, did not do the same for their ore stones, thereby leaving a multitude of testimony. To propose an interpretation of the traces of grinding, we will first analyse the surfaces of fragments before more closely examining the ridges.

Intensive use

It is a useful statement of the obvious to say that the rotation of millstones progressively wears away the grinding surface. What is remarkable about this phenomenon is its intensity. The millstones that have been preserved are worn down to the chord. By extension, it is likely that ore millstones received no intermediate treatment as grain mills did.

Not content to mercilessly whittle down one side of the millstone, when the millstone became unsuitable for grinding, rather than re-employ it differently, the miners sometimes preferred to turn it over in order to extend its service life. Out of a corpus of 84 millstone fragments (excluding small fragments and rough-outs), 14 items, i.e. 16.6% were used successively on both sides. Among these items, despite a high proportion of undetermined ones (a flat surface or fragments still too small), three points stand out. First of all, no fragments are convex on both sides at the same time. The use of the millstone as a bedstone on both sides is therefore to be eliminated. However, two specimens are concave on both sides: runner stones that were always turned over to serve the same purpose. The rest of the time though (6 items), they are concave on one side and convex on the other. In this case, dual usage is indeed to be considered, but in an order that is still to be defined. Regarding grain mills, normally runner stones arriving at the end of their service lives are re-employed as bedstones, which have less stringent quality demands (Belmont 2006, 39.). But the grinding of ore does not have to meet the same constraints as the grinding of grain, so the question is still open. The study of the millstones concerned here has not answered it, due to similar wear on both sides. It is still likely that the miners worked not according to habit but according to the conditions encountered in particular cases.

The consequence of such intensive usage is the inexorable reduction of millstone thickness. From an initial 30 cm, if we can trust the rough-outs or the almost-new halfmillstone still in context on the right bank of the source (Fig. 12), much more modest figures are found for most of the fragments. The chart below (Fig. 13) summarises the data and highlights a distribution of maximum fragment thicknesses of between 5 and 15 cm. The initial thickness of the millstone, which may appear very large at first glance, is actually in line with an average. In the 14th century, the bread baking stones purchased by the Lords of the Dauphiné were massive blocks that could be up to 2 feet thick (66 cm) for bedstones and 1.5 feet (49.5 cm) for runner stones (AD 38, 8B 83, Accounts of the Lord's domain of Claix 1382-1383.).

It would appear to have been established that the requirements for ore millstones were not the same as those for grain millstones. What remains to be found is if this type of millstone management was intentional, without any compromising of ore enrichment, or, to the contrary, if it shows that the miners were growing away from the process.

Ridges

The other visible manifestation and testimony of grinding on the millstones are ridges, discreet or obvious traces of the passage of ore and gangue (Fig. 14).

The immense majority of millstones or millstone fragments that have been preserved display these ridges. Out of a group of 84 items, 72 have ridges, i.e. 85.7%. This figure is even liable to be increased, since it only accounts for ridges that are visible to the naked eye or those appearing on tracing paper (Regarding the methodology of the acquisition of ridges: Minvielle Larousse and Bailly-Maître 2010). Generally speaking, the marks become finer from the centre of the millstone towards the edge as shown in the example of the 2009-1-39 (Fig. 14.). This is directly linked to the distance between and relative placement of the rotating millstone in relation to the bedstone. First of all, the shape of the stones, convex for the bedstone and concave for the runner stone, prepare for this flow. The runner millstones are placed in such a way that the material poured into the eye is accompanied as it is reduced. The distance between the millstones is greater at the centre of the stones, since the ore module is at its largest, and it is then smaller towards the outside so as to reduce the ore. Marks of grinding surfaces result from this arrangement. Near the eye, the ridges are wide and rough, or even absent.. They become finer towards the outside as the millstones move closer together, through the breast and finally the edge to make the ore become similar to flour. Moreover, this is exactly the same procedure as the one used for grain, minus the ridges cut by a material much harder than rye or oats.

All of these steps are found almost systematically on all of the whole millstones, half millstones and millstone segments. The fragments are classified according to their position on the millstone, by centre, breast and edge (Fig. 15). The first observation that can be made upon reading the graph is that a large majority of the fragments have been identified without difficulty. A mere 13.10% of them have remained outside classification for various reasons, related in particular to too few samples to be able to take a decision. Nevertheless, in most cases, there is hesitation between the edge and the breast. And these same edges and breast account for the lion's share. With 32.14% of edges and 17.86% of breast, the 2.38% of centres is very much a minority. This is all the more valid if double zones are added (15.48%). As a result, even if segments and whole millstones are added up, centres and eyes are underrepresented. It makes sense for the millstone to become more fragmented at the outer edge, which is more fragile than the massive central part. This does not mean however that centres would not be found as well, fewer of them certainly, but here there is a virtual absence. Nor can this difference be totally ascribed to salvaging and successive reuse. This point deserves further attention, which it will be given further on.

Once the classification is carried out, close and attentive examination of the ridges shows that under cover of surface consistency, the ridges are actually highly unequal and recut in many locations. The examples are infinite and an example is illustrated from fragment 2007-0-1515 (Fig. ). An initial 3 mm wide ridge is very clearly perceived and it splits a first time. The lower ridge then disappears a bit further on while the upper ridge, which had started as a fine bevel, progressively widens and ends up splitting and finally evaporating. The ridges are therefore not rigorously concentric. They evolve as the millstone rotates over time.

Here, we have seen changes of direction, creation and destruction, but many fragments display ridges that have been whittled down by repeated use.

The grinding surfaces have therefore been kept 'in memory' with a quantity of information on grinding techniques, as well as on the workmen's method and actions. The ridges underwent a complex evolution, linked directly to the operation of the mills. They were furrowed consistently by the first rotations, but then continued grinding ended up by whittling them down and segmenting them. Since the axis of rotation was more or less flexible, the matching of ridges between the two millstones was not ensured for long and the ore finished cutting out the summits it had initially formed, sometimes in a direction that had more or less shifted. All of these observations have only been made macroscopically. It therefore clearly appears that a study of this type would call for a much more detailed approach than ours. Microscopic analyses would no doubt provide a number of additional elements in the quest for detail that is inherent in tribology. As an example, micro centre samples taken from antique ore millstones from the Les Forges domain (hand mill) have revealed grain still embedded at the end of a ridge that has been characterised chemically to define the type of material ground (Domergue et al. 1997, 48-61). Having said that, based on observations made, we are in a position to propose a number of interpretation schemes regarding ore grinding in Brandes.

Millstones in their mills

'L'impossible description technique' is the title chosen by Aline Durand to propose a typology of Carolingien mills in the Languedoc region (Durand 2002, 33.). Despite the poverty of sources during the early Middle Ages, this observation has subsequently been widely contradicted in the development that has since taken place. Yet, understanding a discreet building and a complex technology is not easy, even if such artifices literally abound in western towns and rural areas (Regarding the distribution of mills in rural areas, refer to: Pichot 2002, 260.). Although, in the Oisans, nearly each parish, including Brandes, had at least one wheat mill available, there was evidently not the same demand for ore mills. De facto, the sources reporting on their activity are truncated and leave this type of artifice in the dark. It will be necessary to use every detail of every source, whether textual, iconographic or archaeological, in order to endeavour to shed light on certain aspects of the issue. To do so, we will first analyse the only tangible remnants of the mills, namely their millstones. Secondly, we will propose an interpretation based on iconographic sources combined with the historical context. And thirdly, we will situate ore grinding within its process, mineralurgy, in order to understand the role of the ore mill in Brandes.

Segmented millstones?

Side by side with monolithic millstones, it would appear that the miners also used segmented millstones.

The subject of segmented millstones, addressed by Henri Amouric in 1984, is a thorny issue that is continually being revisited. At the time he wrote his thesis, no examples prior to the 19th century were known for segmented millstones, regardless of the type (Amouric 1984, 266-267). Since then, the first references have been pushed back further and further as historians look into the question. Still in 1984, Dorothée Kleinmann reported on the appearance of this new type of millstone between 1760 and 1775 [START_REF] Kleinmann | La fabrication de meules à Cinq-Mars la Pile (Indre-et-Loire)[END_REF](Kleinmann , 1107(Kleinmann -1116)). The perspective of the late 18th century would appear to receive the assent of researchers. For Jean Bruggeman, who studied the millstones of Flanders for the symposium in La Ferté-sous-Jouarre in 2003, there is no possible doubt: 'Medieval millstones are always monolithic. Black basalt stones were still used in the centuries that followed. White stones were used too, up to the end of the 18th century. But bedstones could be made of several irregular-shaped pieces. They were bonded with plaster, held together by an iron or wooden band and sometimes rested on a bed of cemented bricks' (Bruggeman 2003, 232). However, Benoît Deffontaine, at the same symposium, dated these stones back yet another 150 years or so thanks to fortunate textual references to 'milling stones' in 1647, 1652 and 1680. According to him, 'nothing prevents us from believing [that production] could even date back to before the first half of the 17th century' (Deffontaine 2003, 297-298). Alain Belmont, again at the La Ferté symposium, reported on an agreement dated 3 May 1452 whereby 'a merchant from Rouen named Robert Le Cornu agreed to convey one or more ships loaded with 35 millstones, 5 eyes, 100 tiles and a tombstone to Normany' (Belmont 2003, 282). The abovementioned tiles obviously came from the famous quarry of La Ferté. That is the extent of research at the present time. Remember, however, that this brief review of the question only concerns wheat millstones.

In Brandes, one fragment of the corpus stands out (Fig. 17).

At first glance, it is a core fragment, since a piece of the eye and millrynd notch have been saved. Below the eye, a ridged surface is present, displaying very fine ridges, of the edge type. This is atypical, but may remain possible under multiple conditions. However, the outstanding element is the direction of the ridges in relation to the eye. They are completely reversed and it is not possible for the axis of rotation to have simply been modified, since the difference is far too great. This would indeed appear to be a 'piece' of a millstone and not just a fragment.

In the light of this assumption, we must reconsider certain cases such as these half-millstones, photographed in the context in which they were abandoned at the beginning of the 20th century (Fig. 18). The break is very clear for the three visible items. It is therefore also possible that these were millstones put together from two segments.

The miners would appear to have used all of the methods at their disposal. If, and only if this assumption proves to be true, some ore millstones would have to be seen as an assembly of extremely diverse pieces of stone, difficult to connect to any typology whatsoever as such. When a piece was worn, it was replaced straight away by another one cut for the occasion, or sometimes even by a used piece that was recut. This ranges from modest-sized pieces to half-millstones. There would appear to be no general de facto trend in the matter. This possibility would explain the module of most of the fragments preserved and thereby the small numer of known monolithic millstones for operations that lasted nearly 150 years. It would also explain the distribution of the same fragments: many edges and breasts for very few centres. On the other hand, this amounts to saying that it is impossible to estimate a minimum number of items (MNI) for the millstones that were used in Brandes. Several points can be put forward against these elements:

1. Firstly, the existence of a single explicit fragment may appear rather insufficient. It is true that its presence is just an exception, in the end. An exception, but not one that confirms the rule; instead it raises questions.

2. Secondly, it is known that considerable salvaging of millstones was done on the plateau. As soon as the operations were abandoned in the mid-14th century, the neighbouring community of Huez was certain to have hurried over to salvage the millstones in good condition to take them down to the mills of the Sarenne, treat them and 're-dress' them to make them more suitable for grinding wheat. This type of salvaging has gone on up to the present day, with the original stones becoming objects to decorate the gardens of the Alpe d'Huez resort. There were many more monolithic millstones in situ. However, we cannot conclude that there were no segmented millstones simply by the presence of monolithic millstones. Both forms may very well have cohabited.

3. Thirdly, a question is raised by this operating method in relation to the way of holding the pieces together. Customarily, they are bonded together with plaster and made solid with an iron or wooden band, or with masonry mountings. However, so far, we have found nothing of this sort on the millstones. The ends of some of them are even so irregular that it may be wondered how strapping could have been put in place. There are no traces of iron fasteners in the fragments either. 4. Lastly, we are not familiar so far with any rough-outs cut out for this process. But we saw above that this is a more general problem. Up to now, only three millstone grit quarries have been recognized. Nonetheless, this is an element that can contribute considerably in terms of understanding the fragments. Provided, of course, that their working is not confused with that of ashlar.

The question therefore remains open and can only be resolved by the contribution of other comparable mining works, lacking new elements from Brandes. Regarding the chronology of the process, this would move back its date of appearance by more than 150 years. Reference is made to circular millstones, but was there not another type of mill that already used segmented millstones some 1400 years earlier? In was in 1987 on the Island of Délos that 'G. Siebert discovered the remains of a dislocated mill on the ground of one of the rooms of the main floor of the Maison des Sceaux' (Brunet 1997, 29-38). It was equipped with millstones in separate parts assembled by clamps for the rotating stone and simply held in a wooden frame for the bedstone. Of course, these are cone-shaped millstones like in the mills found in the bakeries of Pompei, and not circular, but this process had already been conceived of by man, probably back in the middle of the 3th century BC.

Mills

The sole representation of an ore mill comes to us from Georgius Agricola for the 16th century, in his De re metallica [START_REF] Agricola | De re metallica. Bâle, Forben and Bischoff[END_REF]. In this work, he proposes a functional though highly schematic model, without being truly attached to the proportions of the component parts (Fig. 19).

Several comments can be made about this representation. First of all, there is Agricola's desire to be didactic. All of the parts are shown clearly, sometimes to the detriment of reality. For example, the eye of the millstone is out of proportion to the millstone. It is true that on the proper scale, it would have been nearly invisible for his readers. The same is observed for the diameters of the millstones, which are much too small. This representation must therefore be seen as a model more theoretical than practical. Nevertheless, nothing is missing and no frills have been added. There is no doubt that Agricola knew his subject well. On the other hand, it is unlikely that this type of mill existed in Brandes. The wheel was the stumbling block. It is what determines the mechanism that follows and how water is to be brought within its range. In other words, it is the wheel that makes the mill. For Brandes, it is necessary to look at a much wider technological context. Claude Rivals has shown that the breakdown between horizontal wheels ('rodets') and vertical wheels noticeably matches the cultural areas of the 'langue d'oïl' and the 'langue d'oc' [START_REF] Rivals | Le moulin et le meunier[END_REF]. A majority of horizontal wheels are used in the south whereas vertical wheels blossom in the north. There are always exceptions, but on the whole, the various regional studies carried out before and after these findings have not challenged this fact, the reasons for which are still relatively unknown, but, on the contrary, have confirmed it [START_REF] Amouric | Moulins et meuniers en Basse-Provence occidentale du Moyen-âge à l'ère industrielle[END_REF]. This distinction was moreover reported several centuries ago with Belidor, who spoke in 1739 of the mills of Provence and the Dauphiné region customarily built with a horizontal wheel (Bélidor 1739, 301). Regarding the Dauphiné and hence the Lord›s domain of Oisan to which the village belonged, the registers of the 'Chambre des comptes et des maîtres des oeuvres' [Chamber of accounts and contractors] have led to same conclusion (Minvielle Larousse 2009, 130-132). The operating method for this mill was simpler than as described by Agricola. Henri Amouric even speaks of a 'childish mechanism': a gear point with wheel and trundle, but a single engine shaft linking the horizontal wheel and the rotating millstone by means of the millrynd (Amouric 1984, 130).

This gives a process comprising a flume, a holding basin with its weir, running stone, mill and tail race to finish. ). This point is important since in the field, a hiatus may be formed in this way between the major hydraulic facilities and the place of grinding.

These ore mills are difficult to observe by excavation because of the materials used (wood), the relatively small size of the structures and the salvaging of iron parts after they were abandoned for use in other mills. In this respect, the quest for details in excavations is essential.

Mills and ore enrichment

Mills are only one item among others in the mining context. Based on this, their place in the system and the role they play are called into question. In fact, the industrial installations of Brandes must only have included a small number of mills. When this is put into perspective with the moderate number of fragments, the possibility of counting segmented millstones and remnants in the field, all of this combined with the period of activity of the mine, we are able to propose this conclusion. Moreover, the situation appears to be the same in Pampailly where documents mention during the middle of the 15th century, barely 'three mills to grind the ore and gangue in Cosne, Brussieu and Vernay' (Benoit 1997, 70).

To explain this, it is necessary to recall the purpose of mineralurgy: the whole process is to separate the ore from gangue. The smaller the module of the mix, the longer and more difficult it is for the miners. The 'flour' produced by the grinding of the ore and gangue is therefore often complicated to treat, even in the washing basins (Marconnet Figure 19. Representation of an ore mill according to Agricola. 2006). In Brandes, since the gangue is essentially barite, the use of mortar and strikers is sufficient to reduce it to powder, as witnessed by the numerous experiments carried out. The very high number of experiments points in favour of the massive use of these tools.

In these conditions, ore mills played only a complementary role in the enrichment of ore. The miners used them in certain cases, which still need to be precisely defined. Was it for a particular module or for a specific composition? Additional analyses will be useful to determine this. Whatever the case may be, we have observed a wide variety of techniques used for the grinding of ore. Mortar and strikers are by far the most used, followed by water-mills and then a third, more singular but also more marginal.

The corpus of millstone includes an alternative hand mill, with parallel 'furrows' made with a pick using the same method as described above (Fig. 20).

Conclusion

At the end of the study, it goes without saying that many questions have still not been answered and that further queries have arisen.

We have observed, in particular, that the extraction of ore millstones is in many ways similar to the extraction of wheat millstones, which shows that use of the methods was widespread. Generally speaking, the extraction or cutting techniques used by the millstone makers, miner and quarry workers are very similar to each other, with respect to both tools and actions. This raises the question of the specialisation of rock tradesmen in specific areas.

For Brandes, we do not know, for instance, if the extraction and cutting of millstones were done by a millstone making tradesman who was called in for the occasion or by the miners themselves. Regarding the use of millstones, it appears that they were not serviced regularly by the miners, apart from the cleaning out and recutting out of the furrows as the thickness of the millstone decreased. The immense majority of the items show grinding surfaces that are extremely smooth unlike wheat millstones which are dotted, whereas the context in which they were abandoned is the same. This point relating to the properties of the rocks, the maintenance of the millstones and the miners' grinding quality requirements can only be truly clarified by experimental archaeology. By taking into consideration the diversity of the methods linked to Medieval mineralurgy (crushing, decrepitation, grinding) and knowing that ore mills were probably not numerous in Brandes as in Pampailly, mechanical grinding, by means of watermills should not be viewed as an innovation in relation to the manual treatment undergone by the ore. They are situated in an auxiliary branch of the process and did not replace crushing, since they were not suitable for that work. Innovation would come much later with the stamp mill which transformed rotational motion into alternative motion by means of a camshaft. This device reproduced the same actions performed earlier by the miners equipped with strikers, but at a virtually industrial stage. Stamp mills appeared for the first time in the literature with the 'Graduel de Saint-Dié' produced between 1494 and 1513. But the camshaft is attested to at least in the 10th century and since the advances in archaeology regularly date technologies further back in time, there is nothing against prior use of these mills. Later excavation may clarify this point.

Figure 1 .

 1 Figure 1. Brandes-en Oisans.

Figure 2 .

 2 Figure 2. Millstone Grit B106.Figure3. Rough-out 2009-1-009.

Figure 4 .

 4 Figure 4. Distribution of types of rocks.

Figure 5 .

 5 Figure 5. Results of analyses carried out in 2005 by Yves Orengo (UJF, Grenoble I).

Figure 6 .

 6 Figure 6. Details of the ring-shaped channel of Millstone Grit B 106.

Figure 7 .

 7 Figure 7. Overview of the rough-out 2009-1-9.

Figure 8 .

 8 Figure 8. Details of the rough-out 2009-1-9.

Figure 9 .

 9 Figure 9. Rough-out of an eye on the 2009-1-10.

Figure 10 .

 10 Figure 10. Eye and millrind of the 2009-1-38.

Figure 11 .

 11 Figure 11. View of millstone 2009-1-52. It contained 16 furrows across the surface.

Figure 12

 12 Figure 12. 2009-1-36.

Figure 13 .

 13 Figure 13. Millstone thickness distribution chart.

Figure 14 .

 14 Figure 14. Negative view of the 2009-1-39.

  16

Figure 16 .

 16 Figure 16. Details of negative of 2007-0-1515.

Figure 15

 15 Figure 15 a, b, c and d. Views of a core (top left, 2009-1-39), a breast (top right, 2007-0-1515) and an edge (bottom left, 2007-0-1488) and distribution of fragments according to position on millstone (bottom right).

Figures

  Figures 17 a (top left), b (top right) and c (right). Views of fragment 2007-0-1504 and projection of the presumed axes of rotation.

Figure 18 .

 18 Figure 18. View of half-millstones in the context in which they were abandoned. Taken by Hyppolite Müller, at the start of the 20th century. Musée Dauphinois collection.

Figure 20

 20 Figure 20 a (above) and b (right). Views of 2009-1-56.

Acknowledgements

We are particularly grateful to: Timothy Anderson, Alain Belmont, Kathleen Lewis, David Peacock, Flavien Perraza, Francis Pierre, Stéphane Rouméjon, Gérard Serres and David Williams


Xiaowei Liu

Chunguang Suo

Yufeng Zhang

Haifeng Zhang

Weiping Chen

Xuebin Lu

APPLICATION OF MEMS TECHNOLOGY TO MICRO DIRECT METHANOL FUEL CELL

In view of micro fuel cells, the silicon processes are employed for microfabrication of the micro direct methanol fuel cell ( DMFC). Using the MEMS technology we have successfully made single  DMFC as small as 10mm×8mm×3mm. The main reason for the use of MEMS processes is the prospective potential for miniaturization and economical mass production of small fuel cells. The double side deep wet etching for the gas channel and fuel chamber preparation and the formation of porous silicon (PS) layers for electrode supports by electrochemical process are the key technologies to improve the MEMS-based  DMFC. Moreover, for the first time, the fabrication of electro-catalytic electrodes using electroless plating platinum and ruthenium (Pt-Ru) co-deposition method over PS substrates is introduced. Using the MEMS technology makes the batch fabrication of   DMFCs much easier and reduces the usage of rare metals.

INTRODUCTION

Recently, the demand for clean and renewable energy sources has significantly increased [START_REF] Wozniak | A micro direct methanol fuel cell demonstrator[END_REF]. Due to the rapidly growing portable electronics market and the needs for leisure, military or telecommunication equipment, miniature fuel cells (FCs) have been attracting more and more attention by many research groups to generate power for MEMS and IC devices [START_REF] Shah | Novel microfabrication approaches for directly patterning PEM fuel cell membranes[END_REF][START_REF] Pichonat | Development of porous silicon-based miniature fuel cells[END_REF][START_REF] Hobson | Modified Nafion 117 as an improved polymer electrolyte membrane for direct methanol fuel cells[END_REF][START_REF] Yen | A micro methanol fuel cell operating at near room temperature[END_REF][START_REF] Yamazaki | Application of MEMS technology to micro fuel cells[END_REF][START_REF] Shah | A PDMS micro proton exchange membrane fuel cell by conventional and non-conventional microfabrication techniques[END_REF]. The micro direct methanol fuel cell ( DMFC) which uses liquid methanol without a reformer is considered as the most promising type of fuel cell for small power-supply units because of their advantage of ambient condition operation, which is most essential for portable application [START_REF] Chang | Materials and processes for small fuel cells[END_REF][START_REF] Han | Direct methanol fuel-cell combined with a small back-up battery[END_REF][START_REF] Blum | Water-neutral micro direct-methanol fuel cell (DMFC) for portable applications[END_REF]. Liu et al. have reviewed the state-of-art on the silicon-based  DMFC [START_REF] Lu | Development and characterization of a silicon-based micro direct methanol fuel cell[END_REF].

However, much attention has been given to using the MEMS technology for fabrication of the current collector and flow channel of the micro fuel cells [START_REF] Yu | Fabrication of a miniature twin-fuel-cell on silicon wafer[END_REF]. Our research work includes not only the fabrication of the structure of the  DMFC, but also the preparation of the key components of the fuel cell, the electro-catalytic electrodes. Some papers [START_REF] Pichonat | A new proton-conducting porous silicon membrane for small fuel cells[END_REF][START_REF] Gold | Acid loaded porous silicon as a proton exchange membrane for micro-fuel cells[END_REF] reported that micro fuel cells containing porous silicon (PS) based membranes sandwiched between the electrodes as membrane electrolyte assemblies (MEA), which have inherent the conventional direct methanol fuel cells' disadvantages such as bulky in size, waste in rare metals, and difficulty in batch fabrications.

For the above-mentioned reasons, we provide a novel method of preparation electro-catalytic electrodes using electroless plating of rare metals on the porous silicon substrates as catalytic layers, which has many advantages may predigest the fabrication processes of the micro direct methanol fuel cells.

PRINCIPLE

A direct methanol fuel cell (DMFC) works through the principle of separating the oxidation of a fuel, such as the oxidation of methanol and the reduction of oxygen. Through electrochemical reactions, the oxidation and reduction processes take place at the anode and cathode, respectively. The methanol is oxidized and produces electrons, protons, and carbon dioxide. The electrons are released to an outer circuit at the anode and received through the same circuit at the cathode. Otherwise, the protons can transport through a proton exchange membrane (PEM) to the cathode, where to meet the electrons to form into water. Figure 1 illustrates the principle of a DMFC single cell.

Through the oxidation and reduction processes, the power of electricity is exported continually.

The

Preparation of  DMFC using MEMS technology

Figure 2 shows the process of a micro direct methanol fuel cell using the silicon process technology.

Fig. 2. Fabrication process of a  DMFC These processes employed silicon micromachining techniques including thermal oxidation, optical lithography, wet etching, silicon anodization, physical vapor deposition, electroless plating, laser beams cauterization, and anodic bonding. The starting material were two pieces of 3 in. diameter, 500 m thick double-side polished silicon waters, one used as the anodic electrodes material, the other as cathodal electrodes.

First SiO 2 film is deposited on both the silicon substrates. To make fuel chambers and air flow channels, the SiO 2 over one side the wafers is patterned by photolithography. Second wet etching method is employed to finish the structure both in anodic and cathodal electrodes. Third porous silicon (PS) layer is obtained on the substrates by silicon anodic etching method. As cathodal electrodes the PS layer is on the side of structure side, as anodic electrodes the PS layer is over against the structure side of the silicon wafer. Fourth the Pt or Pt-Ru alloy is deposited by electroless plating over the anodic silicon substrate and cathodal silicon substrate, respectively. Fifth the methanol fuel channels through the anodic substrates are produced by laser beam cauterization, as well as the fuel inlet and outlet in the glass cover. Then the proton exchange membrane (PEM) is sandwiched between the anodic and cathodal substrates, at last the fuel cell is packaged and finished.

During the fabrication processes, the method of design and achievement of wet etching deep fuel chamber in one side of the anode is possible to expand the fuel containing room without a corresponding increase in the fuel cell overall volume. The depth of the fuel chamber is about 230  m. The advantages of using laser for fuel channels making are their precise location of the holes, their prompt service, and easy controlling of the size of the apertures. Geometric analysis reveals that the islands spreading across the chamber will help the fuel diffusion across the electrodes more symmetrical and the fuel can be used more effectively. The micro gas channels run through the cathodal substrate make the oxygen feed more sufficient than micro columns pattern. The PEM available is a kind of modified Nafion ® 117 membrane, we have other reports about the method of the membrane modification.

Preparation of silicon-based electro-catalytic electrodes by electrochemical method

The method of catalyst layer deposition reported here differs from previous work [START_REF] Shah | Novel microfabrication approaches for directly patterning PEM fuel cell membranes[END_REF][START_REF] Yamazaki | Application of MEMS technology to micro fuel cells[END_REF], with the specific method of electroless plating Pt-deposition and Pt-Ru co-deposition on the PS substrates. Both anodic and cathodal substrates obtained the PS on the surface of the silicon wafer facing the PEM.

The PS layer is formed by anodic etching in a HF-ethanol solution (HF:ethanol:water = 1:1:1) which is considered an effective and cheap fabrication method.

Catalyst being electroless deposited on PS brings A SEM micrograph of Pt-Ru plating on PS is shown in Figure 3. Compare with vapor deposition and sputtering method, electroless plating will enhance the adherence of a catalyst layer to a substrate and improve the uniformity of the deposition surface.

Prototype of single micro direct methanol fuel cell

The main four parts of the micro direct methanol fuel cell are the glass cover, silicon anode, PEM and silicon cathode. These parts are assembled to form the whole  DMFC. The schematic view of the MEMS-based micro fuel cell components is shown in figure 5. The active area of the  DMFC is about 0.64cm Xiaowei Liu, Chunguang Suo, Yufeng Zhang, Haifeng Zhang, Weiping Chen, Xuebin Lu Application of MEMS Technology to Micro Direct Methanol Fuel Cell was fed using pure oxygen from the cylinder at a measured flow rate using an electronic mass flow controller or was fed directly from the air. The fuel cell working system was kept in a constant temperature container controlled by a thermocouple to keep the operating temperature. Current-voltage characteristics were measured using two digital multimeters with variable load resistor. The generated current was passed through different loads and the cell voltage was recorded as a function of current once the steady state was achieved. Then the power density curve as a function of current was made.

RESULTS AND DISCUSSION

Figure 7 shows the current-voltage-power output of micro-fabricated  DMFC with MEA and electro-catalytic electrodes, respectively. The results were gained at room temperature with 2M methanol, and using air as oxygen fed. Both the MEA and electro-catalytic electrodes provide an 8mm×8mm contact area to the catalyst with the polymer electrolyte membrane. Fig. 7. I-V characteristic and corresponding power density of a  DMFC using MEA (1) compared to the performances of a  DMFC using electro-catalytic electrodes [START_REF] Shah | Novel microfabrication approaches for directly patterning PEM fuel cell membranes[END_REF] From the polarization curves and current-voltage characteristics we found that the performance of the micro single direct methanol fuel cell with electro-catalytic electrodes did not act as well as expect. The power density of the single cell reached only 2.5mW/cm 2 much lower than that single cell with traditional MEA (4.9mW/cm 2 ) at the same operation conditions. From the results it can be inferred that the scale-down problems were associated more with the catalyst layers than other reasons. Low fuel cell performances reflected a small active catalyst area compared to the whole area of the fuel cell. When integrated the four parts (glass, anode, PEM, cathode) into a whole micro fuel cell, the two substrates deposited with different catalyst may make bad contact with the PEM, thus cause the insufficient activity of the catalysts.

In order to gain information about the active catalyst surface area and the utilization of the catalyst (both anode and cathode), it is necessary to characterize the thin layer of electroless plating catalyst via physisorption (for accessible surface area) and chemisorption (for active site density). These measurements remain as our future research work.

CONCLUSION

The microfabrication processes which were developed with the progress of MEMS technology provide us a new prospect in the design of small fuel cells. The method of fabrication of electro-catalytic electrodes reported here differs from previous work. Approaches like porous silicon substrates and catalyst deposition using electrochemical anodic etching and electroless plating methods were investigated for improving the effective area of catalyst layer over the PS. The method showed promise and allowed novel catalyst configuration on the porous silicon substrate of the silicon-based  DMFC. Such electro-catalytic electrodes helped minimize the fuel cell size, save the usage of rare metals and reduce the contact resistances and at the same time increase the reactive area of the catalyst over the silicon electrode surface.

Fig. 1 .

 1 Fig. 1. Schematic principle of a DMFC single cell

  Suo, Yufeng Zhang, Haifeng Zhang, Weiping Chen, Xuebin Lu Application of MEMS Technology to Micro Direct Methanol Fuel Cell three-dimensional features, which increase the specific surface area within a stated volume. Additionally, as high as 95% the utilization ratio of noble metal in electroless plating solution leads economizing platinum and ruthenium. Depending on the operation conditions, perfect thickness (about 150nm-200nm) of the Pt or Pt-Ru alloy deposition can be easily obtained. In order to improve the compactness between catalyst and PS substrate, a thin film of titanium was sputtered on the PS before catalyst deposition.

Fig. 3 .

 3 Fig. 3. SEM graph of Pt-Ru plating on PS In order to tell components of the electroless plating film on the anodic substrate, EDX analysis was employed. The spectra results shows that the catalyst deposition is almost platinum and ruthenium without other impurity. By controlling the concentration of the Pt ion and Ru ion in the electroless plating solution, the different proportion of platinum and ruthenium in the catalyst layer can be easily gained. The EDX spectra of Pt-Ru catalyst deposition on the anodic PS substrate is shown figure 4.

Fig. 4 .

 4 Fig. 4. EDX spectra of Pt-Ru deposition on PS

2 .Fig. 5 .

 25 Fig. 5. Schematic view of the  DMFC assembly After micro fabrication using MEMS technology the final micro direct methanol fuel cell compared with a piece of coin is shown in the digital photograph of figure 6. The total size of the fuel size is about 10mm×8mm×3mm.

Fig. 6 .

 6 Fig. 6. Photograph of final  DMFC3.4 Fuel cell performance test set-upA custom-built testing system was used to test the micro fuel cell. A peristalsis pump is employed to control the elimination of methanol fuel. At cathode side the oxygen

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Ms. Cao Shanshan for helping with some experiments. The authors would also like to thank the following colleagues in the Materials Science and Technology College at HIT: Dr. Wu Peilian for EDX and SEM topography, Dr. Du Chunqiu for sputtering work.


Tianwei Zhang
email: [zhang@ynl.t.u-tokyo.ac.jp

Yoshihiko Nakamura
email: nakamura]@ynl.t.u-tokyo.ac.jp

PoseFusion: Dense RGB-D SLAM in Dynamic Human Environments

RGB-D Simultaneous Localization and Mapping (SLAM) in indoor environments is a hot topic in computer vision and robotics communities, and the dynamic environment is a remaining problem. Dynamic environments, which are often caused by dynamic humans in indoor environments, usually lead to the camera pose tracking method failure, feature association error or loop closure failure. In this paper, we propose a robust dense RGB-D SLAM method which efficiently detects humans and fast reconstructs the static backgrounds in the dynamic human environments. By using the deep learning-based human body detection method, we first quickly recognize the human body joints in the current RGB frame, even when the body is occluded. We then apply graph-based segmentation on the 3D point clouds, which separates the detected moving humans from the static environments. Finally, the left static environment is aligned with a state-of-the-art frame-to-model scheme. Experimental results on common RGB-D SLAM benchmark show that the proposed method achieves outstanding performance in dynamic environments. Moreover, it is even comparable to the performance of the related state-of-the-art methods in static environments.

Introduction

The task of Simultaneous Localization and Mapping (SLAM) is to estimate the visual sensor's pose and reconstructed the three-dimensional (3D) static backgrounds at the same time. Except for some special applications, most of the SLAM approaches assume that the robot works in static environments. The dynamic environment is a challenging problem for visual SLAM since that the foreground dynamic objects occlude static background features and then result in failing or wrong features corresponding. Humans are often considered as moving obstacles in indoor environments. Particularly, they may occlude visual features during the human-robot interactions.

Generally, a SLAM framework can be divided into a front-end and a back-end part. The task of the front-end is to extract environment features, which are used for image or point clouds alignment. The back-end part takes care of maintaining the graph structure, saving keyframe information, loop finding and dealing with camera pose drift. In [START_REF] Saputra | Visual SLAM and Structure from Motion in Dynamic Environments: A Survey[END_REF], Saputra et al. survey the dynamic SLAM methods by the year 2016. They classify the existed dynamic SLAM methods to three types based on different application and outputs: Robust Visual SLAM, Dynamic Object Segmentation, and Joint Motion Segmentation and Reconstruction. Follow this taxonomy, we propose a novel dense RGB-D SLAM method for dynamic humans environments based on human motion segmentation and static environment reconstruction.

Our method works in multiple humans dynamic environment, it efficiently detects humans and fast reconstructs the static environments. We detect moving objects by integrating the OpenPose [START_REF] Cao | Realtime multi-person 2d pose estimation using part affinity fields[END_REF] into our front-end algorithm, which is an advanced deep learning based human detection method for a 2D color image. Dynamic point clouds are then removed by using the Min-Cut Segmentation [START_REF] Golovinskiy | Min-cut based segmentation of point clouds[END_REF]. Finally, we input the static environment point clouds to a state-of-the-art dense RGB-D SLAM framework, ElasticFusion [START_REF] Whelan | ElasticFusion: Dense SLAM without a pose graph[END_REF] for static environment reconstruction. As a combination of OpenPose and ElasticFusion, our method is named as PoseFusion, which indicate that this SLAM framework applies human pose detection and frame-tomodule schemes. Our method is tested on the well known Freiburg RGB-D SLAM dataset dynamic serials [START_REF] Sturm | A benchmark for the evaluation of RGB-D SLAM systems[END_REF], and it achieved the smallest camera trajectory error compared to other state-of-the-art dynamic SLAM methods.

Related Works

Dynamic SLAM Methods

In dynamic SLAM survey [START_REF] Saputra | Visual SLAM and Structure from Motion in Dynamic Environments: A Survey[END_REF], Saputra et al. divide the exited dynamic SLAM and visual odometry by their fundamental techniques, such as background/foreground initialization, deep learning, optical flow, ego-motion constraints, geometry constraints, and feature based. In this section, we only introduce recent works for RGB-D dense reconstruction, which are similar to our setting. 1. EF [START_REF] Whelan | ElasticFusion: Dense SLAM without a pose graph[END_REF] and Kintinous [START_REF] Whelan | Kintinuous: Spatially extended kinectfusion[END_REF] are extended from KinectFusion [START_REF] Newcombe | KinectFusion: Real-time dense surface mapping and tracking[END_REF], which is an early dense RGB-D reconstruction framework. [START_REF] Newcombe | KinectFusion: Real-time dense surface mapping and tracking[END_REF], [START_REF] Whelan | ElasticFusion: Dense SLAM without a pose graph[END_REF], [START_REF] Whelan | Kintinuous: Spatially extended kinectfusion[END_REF] perform great in static environments. They adopt a module maintaining scheme. Different from the others, EF is a state-of-the-art method designed for static environments within slightly dynamic scenes. It can handle small-scale environment changing since it benefits from the deformation graph based non-rigid module fusion. When a slightly changed scene occurs, EF can fuse it into the saved key-frame modules and ignore small scene changing. 2. Co-Fusion [START_REF] Rnz | Co-fusion: Real-time segmentation, tracking and fusion of multiple objects[END_REF] (CF) is a state-of-the-art approach for tracking and reconstructing multiple moving objects using EF framework. However, CF has to first reconstruct the map in a static environment, and then they enable the dynamic object detection and tracking abilities within that reconstructed map. 3. Jaimez et al. [START_REF] Jaimez | Fast odometry and scene flow from RGB-D cameras based on geometric clustering[END_REF] proposed an odometry method named The joint visual odometry and scene flow (SF). SF localizes the moving camera in dynamic scenes by segmenting intensity point clouds into a number of clusters (called super-voxels), These above three SLAM methods are advanced in light dynamic environments. Same to CF and JF, the proposed method is also based on EF. Our idea is similar to SF, that is to decouple the motion of camera from the motion of moving humans. The difference is that SF tries to separate the foreground from the background by comparing the moving speeds of all segments, while we separate moving human segments by using OpenPose.

The Human Detection Methods

OpenPose [START_REF] Cao | Realtime multi-person 2d pose estimation using part affinity fields[END_REF] estimates the body joints as feature points of human beings such as wrists and shoulders in real time from a single RGB image. It is to estimate each joint position by deriving Part Confidence Maps using trained Convolutional Neural Networks. Moreover, [START_REF] Cao | Realtime multi-person 2d pose estimation using part affinity fields[END_REF] can estimate the vector fields between connected two joints, which make it robust to the occlusions no mater form self body nor multiple humans' bodies. OpenPose provide us the probable positions of human body joints in the 2D RGB image, thus we are able to quickly label the position of humans in 3D Point Clouds Data (PCD).

Multiple Humans Detection in Dynamic Environment

The Flowchart of proposed PoseFusion is illustrated in Fig. 1. PoseFusion take the RGB and Depth image pair as input. The RGB image is first used for body joints estimation using OpenPose, which tells the likelihood of the human joint positions on the input image. When the input image f is given, the feature map is extracted via CNN network and then output data:

[h h h × w w w] f (1)
in which h h h is the detected human bodies, at a maximum 15, which means Open-Pose can detect at maximum 15 humans within one frame. The w w w is the list of estimated joints, it presents a probability map on the RGB image plane which indicates the existence likelihood of at maximum 18 human joint. In matrix 1, each element has three components: u, v, p. They are the image pixel coordinates (u, v) and the existence likelihood (p ∈ (0, 1]) of the human body joint.

The estimated joint points are converted from 2D to 3D using the pinhole camera model and then, they are used for labeling humans' positions in the PCD. This processing is the red arrow in Fig. 1, and note that the green points in the point clouds stand for the projected joint positions. Then, the PCD with these green joints are inputted to Min-Cut point foreground segmentation.

Min-Cut [START_REF] Golovinskiy | Min-cut based segmentation of point clouds[END_REF] is a Graph-Cut [START_REF] Boykov | Graph cuts and efficient n-d image segmentation[END_REF] based method for segmenting objects in point clouds. Graph-cut treats every single point as a vertex and vertices are connected with their neighbors by edges. Given some vertices as foreground priors, it cuts the foreground object out of the background points by computing the weights of the edges. T apply Min-Cut, we use the human joints from Equation. 1 as foreground prior, we assign two edge weights in min-cut: the edge smooth cost C and background penalty P.

C = e -( len

σ ) 2 (2)
in which len is the length of the edge, obviously, the father away the vertices are, the more is the probability the edge will be cut. The σ is a user defined parameter. The background penalty is to weight the points connected with the foreground points. In which, for a joint point J(J x , J y , J z ), we set an input parameter rad as the maximum horizontal (X-Y plane) radius of foreground objects. Then, for a neighbor point (x, y, z) of J, its background penalty is:

P = (x -J x ) 2 + (y -J y ) 2 r (3)
As the pose points are labeled on the human body, we set the foreground rad as 20 cm, σ as 0.25 which draws a good segmentation performance. After Min-Cut, the background segments are converted to static depth images, which are the inputs for the following static environment reconstruction, together with the original RGB image. Finally, a clean static environment reconstruction is achieved through frame-to-module map fusions. As the frame grows, the first row reconstruction is gradually completed, the RGB viewpoint moves as the camera moves, the point clouds segmentation is also changing. There is no foreground cluster in the 4th image, the second row, since the guy walks out of the scene.

Experiments Setup and Results

Our method is evaluated with the Freiburg SLAM benchmark, which contains a dynamic SLAM series (fr3 series). This benchmark is wildly used for dynamic scenes comparison. The experiment is shown in Fig. 2, which take fr3/walk xyz (in which the camera moved by a man walks in x, y, and z translations) as an example. Each fr3 dataset contains several hundred color (RGB) and Depth (D) image pairs, which can be transformed into RGB-D PCD frames. For instance, there are 827 RGB-D frames in fr3/walk xyz, these RGB-D PCD frames can be played as a video.

In Fig. 2, the first row shows the reconstructed maps, and the second row shows the respective input RGB images and their foreground-background segmentation results. As the frame number grows, the first row reconstruction is gradually completed, the RGB viewpoint moves as the camera moves, the point clouds segmentation is also changing. In the fourth image of the second row, there is no foreground cluster since the people walk out of the camera view. These experiment setting intuitively indicate the dynamic ability of SLAM methods. One can compare the moving object detection and removal abilities throw checking when and how many moving object ghost shadows are integrated into the reconstructed scenes in the first row. One comparison with EF and PF is given in Fig. 4.

We compare our PoseFusion (PF) method with three state-of-the-art dynamic SLAM methods: Scene Flow (SF) method from [START_REF] Jaimez | Fast odometry and scene flow from RGB-D cameras based on geometric clustering[END_REF], ElasticFusion (EF) [START_REF] Whelan | ElasticFusion: Dense SLAM without a pose graph[END_REF] and Co- Fusion (CF) [START_REF] Rnz | Co-fusion: Real-time segmentation, tracking and fusion of multiple objects[END_REF]. All of them are implemented from their open source repositories. StaticFusion (JF) [START_REF] Scona | StaticFusion: Background Reconstruction for Dense RGB-D SLAM in Dynamic Environments[END_REF] is the most recent method and its code is not opened yet.

To evaluate these four SLAM methods, we compare their absolute trajectory error (ATE) and relative pose error (RPE). ATE is well-suited for measuring the per- .21 cm error. The dynamic performance of proposed is as good as the EF's static performance. Note that, the left chair's ghost shadow in PF reconstruction is not a wrong alignment, it is because the people moved that chair to that place for several seconds, which can be observed in our supplement video.

formance of visual SLAM systems. In contrast, the RPE is well-suited for measuring the drift of a visual odometry system, for example, the drift per second. The ATE directly measures the difference between points of the ground truth and the estimated trajectory. The RPE computes the error in the relative motion between pairs of timestamps. Table 1 and Table 2 show the root-mean-square error (RMSE) of translate ATE (cm) and RPE (cm/s). All of these six datasets are dynamic scenes, but the first three: fr3/sit static; sit xyz and sit halfsphere are low dynamic scenes, while the other three are high dynamic scenes. From these two Tables, one can obviously find that our PF method achieved the smallest estimation errors in highly dynamic situations. In the light dynamic scenes, EF achieved the best performances, since it is designed for static environments. Fig. 4 intuitively indicate the performances of EF and PF. Big trajectory error of EF results in obvious wrong map reconstruction. The wall, the table, and the desktops are not aligned, as well as people shadows corrupt the final map of (a). While (b) is reconstructed by PF, which is well aligned. The dynamic performance of PF is as good as the EF's static performance. Note that, the left chair's ghost shadows in PF reconstruction is not wrong alignments, they are reconstructed since that the people moved that chair to that places for several seconds, which can be observed in our supplement video.

Conclusions

In this paper, we propose a novel dense RGB reconstruction approach for the dynamic human environments. We combine the advanced deep learning based human body detection method and static dense SLAM approach to deal with the dynamic environment problem using dense RGB-D data. We compare the proposed method with EF [START_REF] Whelan | ElasticFusion: Dense SLAM without a pose graph[END_REF], CF [START_REF] Rnz | Co-fusion: Real-time segmentation, tracking and fusion of multiple objects[END_REF], and SF [START_REF] Jaimez | Fast odometry and scene flow from RGB-D cameras based on geometric clustering[END_REF]. Our approach significantly decreases the alignment error and archive 3.21 cm average ATE in TUM benchmark, while EF results in a 67.78 cm ATE. Moreover, We not only propose one approach for a particular dynamic environment setting but also provide a framework for solving SLAM "Type II Error", which means the front-end part doesn't "reject" the wrong features of the moving objects so that the moving shadows are integrated to key-frames on the back-end, thus breakdown loop closing. According to the proposed, one can integrate advanced learning-based object recognition methods to handle dynamic environments. For instance, involving vehicle recognition methods for high way scenes, and including semantic labeling methods for service robots in house and office scenes.

Acknowledgement

This work was supported by JSPS Grants-in-Aid for Scientific Research (A) 17H06291.

Fig. 1

 1 Fig.1PoseFusion Flowchart: Firstly input RGB images to OpenPose[START_REF] Cao | Realtime multi-person 2d pose estimation using part affinity fields[END_REF] to detect human joints. Then we project joint points to the PCD and followed by foreground removal. Finally, static backgrounds are feed into reconstruction.

Fig. 2

 2 Fig.2Experiment Setting: RGB-D PCD frames are played as a video. The first row shows the reconstructed maps, the second row shows the respectively RGB input and foreground-background segmentations. As the frame grows, the first row reconstruction is gradually completed, the RGB viewpoint moves as the camera moves, the point clouds segmentation is also changing. There is no foreground cluster in the 4th image, the second row, since the guy walks out of the scene.

Fig. 3

 3 Fig. 3 Evaluation of the Proposed PoseFusion compare to ElasticFusion, both are the result of fr3/walk xyz. (a) and (c) are absolute trajectory error (ATE) and the relative pose error (RPE) of PF, while (b), (d) are ATE and RPE of EF. PF achieves very small trajectory error, average 3.21 cm (short red line segments in (a)), while EF gets big ATE, long red line segments in (b). (c) and (d) indicate PF achieves about ten times smaller RPE than EF.

Fig. 4

 4 Fig.4EF and PF Scene Reconstruction of fr3/walk xyz. (a) is the result of EF, 67.78 cm absolute trajectory error makes an obvious wrong reconstruction. The wall, the table, and the desktops are not aligned and multiple people shadow corrupt the final map. While (b) is reconstructed by PF, which is well aligned with only 3.21 cm error. The dynamic performance of proposed is as good as the EF's static performance. Note that, the left chair's ghost shadow in PF reconstruction is not a wrong alignment, it is because the people moved that chair to that place for several seconds, which can be observed in our supplement video.

Fig. 3 and

 3 Fig. 4 show the comparison between PF and EF run on the fr3/walk xyz dataset. Inside the former figure set, (a) and (c) are ATE and RPE of PF, while (b), (d) are ATE and RPE of EF. PF achieves very small trajectory error, average 3.21 cm (difference of the same time-stamp are shown as the short red line segments in (a)), while EF gets big ATE, which can be seen as the long red line segments in (b). (c) and (d) indicate that PF achieves around ten times smaller RPE than EF.

Table 1

 1 Translate ATE RMSE (cm)

	Dynamic DataSet [5]	SF	EF	CF	PF
	fr3/sit static	8.67	3.18	2.93	3.18
	fr3/sit xyz	7.65	0.90	1.49	0.90
	fr3/sit halfsphere	50.21	30.18	27.58	2.31
	fr3/walk static	81.07	25.13	18.77	6.87
	fr3/walk xyz	65.31	67.78	37.11	3.21
	fr3/walk halfsphere	79.97	47.90	21.23	4.65

Table 2

 2 Translate RPE RMSE (cm/s)

	Dynamic DataSet [5]	SF	EF	CF	PF
	fr3/sit static	2.71	1.12	0.81	1.19
	fr3/sit xyz	9.61	3.90	4.93	3.97
	fr3/sit halfsphere	41.10	29.19	23.80	5.19
	fr3/walk static	27.77	20.33	18.79	3.77
	fr3/walk xyz	35.32	21.78	37.11	2.11
	fr3/walk halfsphere	69.74	77.91	31.32	4.50


X C Shan

Z F Wang

R Maeda

Y F Sun

M Wu

J S Hua

A SILICON-BASED MICRO GAS TURBINE ENGINE FOR POWER GENERATION

This paper reports on our research in developing a micro power generation system based on gas turbine engine and piezoelectric converter. The micro gas turbine engine consists of a micro combustor, a turbine and a centrifugal compressor. Comprehensive simulation has been implemented to optimal the component design. We have successfully demonstrated a silicon-based micro combustor, which consists of seven layers of silicon structures. A hairpin-shaped design is applied to the fuel/air recirculation channel. The micro combustor can sustain a stable combustion with an exit temperature as high as 1600 K. We have also successfully developed a micro turbine device, which is equipped with enhanced micro air-bearings and driven by compressed air. A rotation speed of 15,000 rpm has been demonstrated during lab test. In this paper, we will introduce our research results major in the development of micro combustor and micro turbine test device.

INTRODUCTION

As part of the effort in developing micro power generation systems, miniaturization of gas turbine engine using MEMS technology is proposed by MIT [START_REF] Hua | Millimeter-scale MEMS gas turbine engines[END_REF][START_REF] Epstein | Power MEMS and micro engines[END_REF][START_REF] Mehra | Development of a high power density combustion system for a silicon micro gas turbine engine[END_REF]. Tohoku University has also fabricated three-dimensional micro turbines using micro milling [START_REF] Isomura | Component development of micromahined gas turbine generators[END_REF][START_REF] Isomura | Development of high speed micro-gas bearing for 3dimensional micro-turbine machines[END_REF]. In micro heat engine system, both the heat loss and chamber wall cooling in the combustor are critical problems. A design of recirculation gas flow jacket in micro combustor was reported in [START_REF] Mehra | Development of a high power density combustion system for a silicon micro gas turbine engine[END_REF], which had limited effects in preheating the fuel/air mixture and cooling the combustor sidewall.

Our research aims to develop a micro power generation systems based on micro gas turbine engine and a piezoelectric converter, as illustrated in Fig. 1 [START_REF] Shan | Optimal Design of a Micro Gas Turbine Engine[END_REF]. The micro gas turbine engine is composed of a centrifugal compressor, a combustor and a radial inflow turbine. The piezoelectric converter is to produce electricity from the rotation of the turbine, which links to the piezoelectric element.

In this paper, we present our research on micro combustor, which consists of seven layers of silicon structures. An extended gas path is applied in micro combustor design, which has been proved to be effective for sustaining high temperature in combustion chamber and for cooling the chamber sidewall. Stable combustion is sustained in the micro chamber, and exit temperature has achieved as high as 1600 K [START_REF] Shan | Studies on a micro combustor for gas turbine engines[END_REF]. We have also developed a turbine device with enhanced micro airbearings, which has achieved a rotation speed of 15,000 rpm. In this paper, we will introduce the concept of micro power generator, the system design of micro gas turbine engine, and some investigations on critical components.

THE ENGINE SYSTEM DESIGN

We aim to use hydrogen fuel in our micro gas turbine engine. The simplified configuration of the micro power generator is illustrated in Fig 2 . The gas turbine engine has a dimension of 21.5 mm × 21.5 mm. It will be made from seven layers of silicon wafers with a total thickness of about 4.4 mm after assembly. Both of the compressor and turbine consist of centrifugal blades with two-dimensional profiles.

Centrifugal Com pressor

Com bustor

Micro Gas Turbine

Com bustor

Piezo Converter

Fig. Fig. 2 Implementation of the micro gas turbine engine

In the system design, the inlet airflow rate, compressor ratio and consumption rate of hydrogen fuel are targeted at 0.36 g/s, 4:1 and 17 g/h, respectively. The adiabatic efficiencies of compressor and turbine are estimated to be 0.65 and 0.75, respectively. The efficiency and pressure recovery coefficient of combustor are predicted to be 0.74 and 0.92, respectively [START_REF] Shan | Optimal Design of a Micro Gas Turbine Engine[END_REF]. The net power output of the designed gas turbine engine is expected to be 39 watts.

MICRO COMBUSTOR

Design of micro combustor

The micro combustor [START_REF] Shan | Studies on a micro combustor for gas turbine engines[END_REF] utilizes the design of a static gas turbine engine, which means that the combustor has almost the same configuration as the gas turbine engine, as shown in Fig. 2, but without the rotating blades. The micro combustor is also based on seven layers of silicon wafers and uses hydrogen as the fuel. The function of each layer is expressed concisely as below.

Fuel is injected through a hole-array on the 2 nd wafer, and mixes with air and preheated as it flows through recirculation channel made up of the 3 rd to 6 th wafers. A hairpin-shaped channel on the 6 th wafer is designed to prolong the gas flow path in order to (i) preheat the fuel/air mixture well, (ii) efficiently cool down the outer wall of combustor, (iii) reduce heat loss via outer wall and (iv) sustain a stable flame. Then the mixture is injected into the combustion chamber through a set of flame holders on the 5 th wafer, reacts in the annular combustion chamber, and finally exhausts through the turbine vans on the 4 th wafer.

Comprehensive CFD (Computational Fluid Dynamics) simulations have been performed to investigate the mixing and combustion characteristics of hydrogen and air in the micro-combustor system. The simulation takes into account the coupling of fluid dynamics, heat transfer and detailed chemical kinetics. The fluid dynamics and heat transfers within the micro combustor, as well as the conjugated heat transfer in solid engine walls, are simulated using Fluent 6.0, and the detailed chemical kinetics of hydrogen-air combustion is expressed by DETCHEM as the user-defined functions of Fluent [START_REF] Hua | Numerical simulation of the combustion of hydrogen-air mixture in micro-scaled chambers[END_REF].

Fig. 3 shows some simulation results of temperature distribution inside the combustor, where the flow rate and equivalence ratio were set to be 0.15 g/sec and 0.6, respectively. The heights of combustor chamber were set to 0.6 mm and 1.0 mm, respectively. It was found that, when the equivalence ratio was as high as 0.6, the flame can be stable in the combustor for these cases. The temperature peak of flame and wall temperature and the combustion efficiency are very similar in these two cases. However, when the equivalence ratio was set at 0.5 and chamber height decreased to 0.6 mm, the flame can not be sustained in the combustor. The CFD simulations also show that the hairpin-shaped design of recirculation channel is effective for sustaining higher temperature inside combustion chamber and for cooling the outer walls.

Fabrication and characterization

The 7-layer implementation of micro combustor with a hairpin shaped recirculation channel has been successfully fabricated from silicon wafers. DRIE (Deep Reactive Ion Etching) is the major process used in the fabrication of micro combustor. Silicon wafers of 0.4 mm thick are used for the 1 st , 2 nd and 7 th layer, while wafers of 0.8 mm thick are used for the 3 rd to 6 th layers. Fig. 4 shows the fabricated structures before assembly, while Fig. 5 shows the cross-section of the micro combustor after assembly, which has a size of 21×21×4.4 mm 3 . The height of the combustor chamber was set to 0.8 mm at first, which To investigate the temperature profile during combustion, the assembled dies are assembled on a stainless steel fixture, which facilitates the connection with fuel/air supply and pressure sensors. Temperature was measured using a 0.5 mm-diameter K-type thermal coupler. Fig. 6 depicts the recorded temperature versus mass flow rate of air/fuel mixture when the equivalence ratio is kept at 0.8. The results illustrate that stable combustion can be sustained when mass flow rate is over 0.04 g/sec. The temperatures near the edge of the exit port range from 1400 to 1600 K.

MICOR TUBINE DEVICE

Design of micro turbine

Micro turbine, a critical part in micro gas turbine engine [START_REF] Hara | Infrared polarization modulator using a MEMS-based air turbine[END_REF][START_REF] Frechette | Demonstration of a microfabricated high-speed turbine supported on gas bearings[END_REF], is used for generating power and driving the compressor. In our design, the micro turbine is made up of three layers of silicon structure. Two acrylic plates are used for tubing and clamping. Fig. 7 shows the schematic diagram of our test rig for micro turbine. There are three flow paths driving for rotor, journal air bearing and thrust air bearing, respectively. The velocity vectors that express the relative flow of driving air at middle section of blades have been investigated by 3-dimensional CFD simulation. It can be seen from Fig. 8 that the airflow velocity in the boundary layer gradually increases from zero to the main flow velocity. The angle of airflow at rotating blade inlet X. C. Shan, Z. F. Wang, R. Maeda, Y. F. Sun, M. Wu and J. S. Hua A Silicon-Based Micro Gas Turbine Engine For Power Generation matches the blade profile. This ensures the improvement of efficiency and augmentation of output power. The design shown in Fig. 8 is under the assumption that the turbine is driven by exhausted gas from a combustor. When it is driven by compressed air at room temperature, the rotation speed and output power will be slightly decreased. To decrease friction and wear of micro turbine devices during high speed operation, the dynamic and selfacting thrust air bearings are designed and applied in our micro turbine. A pair of thrust air bearings is used for supporting the rotor from its top and bottom. A pump-in configuration with spiral grooves was applied for top thrust air bearing, and a pump-out configuration was applied for bottom bearing. Due to the miniaturized size and small weight of thrust plate, the axial load is mainly introduced by the pressure difference between the top and bottom thrust air bearings.

Guide van

Rotor blade

Fabrication of micro turbine

Similar to micro combustor, DRIE is also the major process for fabrication of micro turbine. However, the non-uniformity of DRIE process in term of etching rate over a 4-inch wafer can be as high as 5% under normal process condition. When etching the 2 nd wafer, such nonuniformity may introduce the mismatch of mass center and geometry center of rotors, which will cause collision between rotor and side wall.

To enhance the etching uniformity, we used prebonded silicon wafer, which was embedded with a 1.5 µm-thick SiO 2 layer. This oxide layer can serve as the etching stopper during DRIE process. The topographical map shown in Fig. 9 illustrates rotor profiles fabricated from a pre-bonded silicon wafer. X. C. Shan, Z. F. Wang, R. Maeda, Y. F. Sun, M. Wu and J. S. Hua A Silicon-Based Micro Gas Turbine Engine For Power Generation

Testing of the turbine device

The micro turbine was mounted on an acrylic test jig and connected to a gas distribution system for air-driven rotation testing. Fig. 11 shows the experimental setup. An optical fiber sensor is used to measure the rotation speed of the rotor blade, by sensing the periodic change of reflectivity. A stable rotation of 15,000 rpm is recorded during the experiments.

SUMMARY

Aiming towards a silicon-based micro gas turbine engine for power generation, we have investigated the system design, fabricated the micro combustor and micro turbine. The micro combustor consists of seven layers of silicon structures and uses a hairpin-shaped fuel/air recirculation channel for the extension of flow path. The experimental investigations demonstrate that the combustor can sustain a stable combustion with an exit temperature as high as 1600 K. We applied new structure of micro air-bearings in our micro turbine. A stable rotation with 15,000 rpm has been reported. Our future research will include the integration of turbine and compressor with micro combustor so as to implement the micro gas turbine engine.

Fig. 3

 3 Fig. 3 Temperature distribution inside micro combustor under different chamber heights

Fig. 4

 4 Fig. 4 Seven silicon dies of a micro combustor

Fig. 6

 6 Fig.6 Recorded temperatures at each position versus mass flow rate with a chamber height of 1.2 mm (#1: centre of gas exit; #0, #2-4: near exit edge)

Fig. 7

 7 Fig. 7 Design of micro turbine testing device. The turbine device consists of three layers of silicon structure and two acrylic plates. Silicon wafers of 0.4 mm thick are used for 1 st and 3 rd layers of wafers. A wafer of 0.8 mm thick is used for the 2 nd wafer, from which the rotor and stator are fabricated. The rotor has 17 blades with outer and inner diameters of 8.2 mm and 4.4 mm, respectively. The stator has 23 blades for guide vanes. The blade height is half of the wafer thickness.The velocity vectors that express the relative flow of driving air at middle section of blades have been investigated by 3-dimensional CFD simulation. It can be seen from Fig.8that the airflow velocity in the boundary layer gradually increases from zero to the main flow velocity. The angle of airflow at rotating blade inlet

Fig. 8

 8 Fig. 8 Turbine blade profiles under CFD simulation

Fig. 9

 9 Fig. 9 Topographical map of the rotor blades fabricated from a pre-bonded wafer A novel structure of thrust air-bearing was generated together with the micro turbine. The top bearing uses pump-in type grooves and was etched out from the bottom side of the 1 st wafer. The bottom bearing utilizes pump-out type grooves and is etched out from the top side of the 3 rd wafer. The DRIE process for fabricating thrust bearings has been specifically optimized. The etching depths of the grooves on the top and bottom bearings are 15 µm and 36 µm, respectively. Fig. 10 illustrates the fabricated pump-in spiral grooves for top thrust bearing.

Fig. 10

 10 Fig. 10 Pump-in spirals for top dynamic thrust bearing with a etch depth of 15 µm.

  Fig. 11 Lab setup for rotation measurements of micro air-driven turbine

For Power Generation Inlet Compressor Diffuser Rotor Vanes Blades Combustion Chamb er Roter Blades Roter Blades Turbine NGV Exhaust Nozzle

  1 Schematic of micro power generation system X. C. Shan, Z. F. Wang, R. Maeda, Y. F. Sun, M. Wu and J. S. Hua A Silicon-Based Micro Gas Turbine Engine

ACKNOWLEDGEMENTS

The authors would like to thank the Agency for Science, Technology and Research (A*Star), Singapore for financial support under the project No. 022 107 0011.