Entre culture artisanale et culture industrielle : quelle politique de formation à l'innovation dans les professions du bâtiment ?

L'innovation dans la construction : un développement problématique

Dans le secteur professionnel du bâtiment, les stratégies de diffusion des innovations technologiques et industrielles, produits ou procédés, affrontent des obstacles spécifiques aux activités de la construction : -Quand le produit n'est pas prescrit ou conseillé... -Quand les procédures de pose ne sont respectées que partiellement... -Quand la compatibilité des produits est imparfaite... L'ensemble de ces facteurs contribue à l'émergence de litiges, donc à l'insatisfaction des clients et des professionnels du secteur, alors que la qualité des produits n'est pas nécessairement en cause.

C'est dans ce contexte professionnel que notre équipe de recherche (1) a été sollicitée par le centre national de formation aux produits du bâtiment pour concevoir un dispositif de recherche dont le but est d'aider les acteurs de la profession à définir les objectifs et les moyens d'une politique de formation à l'innovation dans les professions du bâtiment.

Premiers éléments d'analyse

Dans le secteur de la construction, le produit fini, une bâtisse par exemple, résulte de la mise en oeuvre et de l'articulation d'éléments et de matériaux fabriqués en des lieux et des logiques industrielles spécifiques : terres cuites, menuiseries, équipements électriques, bétons, moquettes, etc.. A la différence des produits manufacturés comme un appareil électroménager ou une automobile, la conception et l'assemblage du produit fini, celui qui donne un sens à l'acte de production, n'est du ressort d'aucune des filières industrielles impliquées dans la fabrication des éléments de la construction. Sous la responsabilité du maître d'oeuvre, les cas de figure sont nombreux entre la livraison clés en main d'une ville à l'Arabie Saoudite par le groupe Dumez et la mise en oeuvre par le particulier, lui-même, d'éléments achetés dans une grande surface. Triomphe de la logique industrielle d'une part ; de l'autre, puissance de la grande distribution. Entre ces deux extrêmes, les activités de construction sont caractérisées par la mise en oeuvre de matériaux et l'assemblage de produits industriels ou artisanaux par des professionnels appartenant à des corps de métiers différents. A cette pluralité des corps de métiers, nous devons rajouter la diversité de la taille et des modes d'organisation des entreprises : artisans, 1 PME et groupes industriels. Enfin diversité des maîtres d'oeuvre : architectes bâtisseurs, bureaux d'études, promoteurs, entreprises du Btp, groupement d'artisans, particuliers... C'est dans ce contexte que le développement des innovations technologiques rencontre, selon les industriels du secteur, quatre catégories d'obstacles :

-1 -Des obstacles au moment de l'étude des travaux quand le produit n'est pas prescrit ou conseillé, ou bien lorsqu'intégré, son usage n'est pas pertinent.

-2 -Des obstacles liés aux réseaux commerciaux qui ne possèdent pas toujours les connaissances nécessaires à l'optimisation de leurs actions.

-3 -Des obstacles au moment de la mise en oeuvre quand les procédures de pose ne sont que partiellement suivies par manque de qualification ou de formation.

-4 -Des obstacles liés à la compatibilité imparfaite des produits qui peut être l'objet de difficultés à l'occasion de la pose quand elle n'est pas source de dysfonctionnements.

Ces observations recouvrent deux difficultés qu'une première analyse permet de distinguer. L'une, la compatibilité imparfaite, renvoie à l'analyse fonctionnelle des produits et appelle une solution technique fondée sur une recherche développement qui pourrait être articulée à un observatoire de la pose comme le propose le centre national de formation aux produits du bâtiment. La mission de l'observatoire serait d'étudier les pratiques sur les chantiers, puis de traiter et d'analyser l'ensemble des données afin de transmettre aux bureaux d'étude des industriels les informations nécessaires à l'amélioration technique des produits.

La seconde renvoie à la spécificité du développement des innovations industrielles dans le secteur du bâtiment. C'est qu'ici, l'échange entre l'offre et la demande ne se noue pas directement autour d'un produit que le client sélectionne comme lorsqu'il choisit un vêtement ou une voiture. En fait, le client paie -en coopérant souvent -la conception et l'organisation d'un ensemble d'opérations qui produit un bien matériel qu'il utilisera ensuite. C'est-à-dire que la réalisation effective de l'innovation est étroitement liée à la prestation de l'ensemble de la chaîne des professionnels de la construction. Dès lors, une conception trop rigide de l'articulation entre activité de conception, de production et de commercialisation, devient un obstacle au développement de l'innovation. Les pratiques obligent à penser l'interaction, d'où le concept de l'observatoire de la pose qui prend tout son sens dans la perspective générale de doter l'ensemble de la profession d'une capacité réflexive en organisant la rétroaction entre chacun de ses maillons. Mais cet effort serait vain sans la prise en compte que dans le bâtiment, l'innovation industrielle suppose une innovation au niveau des services puisqu'il ne s'agit pas seulement de vendre un produit nouveau, mais aussi de transformer des pratiques professionnelles. D'où le nécessaire développement d'une politique d'information et de formation de l'ensemble des professionnels.

Les hypothèses de travail et les objectifs poursuivis

La définition d'une politique de formation suppose la définition des buts poursuivis et la manière concertée, concrète, de les atteindre. A cette fin, l'enquête scientifique vise, dans une première étape, l'analyse des conditions matérielles et symboliques du développement des innovations technologiques dans le secteur de la construction. Nous formulons l'hypothèse que l'industrialisation croissante du secteur, le développement des innovations et les évolutions corrélatives du marché affectent non seulement les gestes professionnels, mais aussi, plus fondamentalement, la définition des rôles et fonctions de chacun des acteurs de la profession. En conséquence, c'est non seulement la structure de la distribution et de l'organisation du travail qui est en cours de mutation, mais aussi celle de ses revenus matériels et symboliques.

Ainsi, pour ne prendre qu'un exemple, dans une logique industrielle, l'artisan est davantage conçu comme un installateur que comme un homme de l'art. Cette évolution détermine les nouvelles conditions d'exercice du métier vécu. Pour une grande part, l'analyse des besoins du client, les réponses techniques et l'offre de prix obéissent à des langages formalisés produits de la logique industrielle. C'est-à-dire que leur étude et leur définition échappent en grande partie aux hommes de l'art. De ce point de vue, le bilan est négatif puisque le développement des innovations technologiques et l'industrialisation croissante de la branche conduisent au remplacement du savoir-faire traditionnel par des gestes professionnels qui sont déterminés par les langages formalisés des procédures industrielles. De plus, les artisans ne maîtrisent que partiellement l'offre commerciale puisqu'ils peuvent se trouver confrontés à la demande de produits nouveaux mis sur le marché par les industriels que, pour des raisons techniques ou commerciales, ils ne souhaitent pas mettre en oeuvre.

Pourtant, cette évolution ne conduira pas nécessairement à instrumentaliser le secteur de l'artisanat au service de l'industrie. Ce serait oublier que l'artisanat repose aussi sur les activités de service. Conseil et aide à la conception définissent tout autant l'activité de l'artisan que le geste professionnel. Comme l'ont montré les travaux de Jean-Claude Abric, la créativité et la personnalisation du produit sont des éléments fondamentaux de la représentation sociale des artisans et de l'artisanat dans la population. Dès lors, et d'autant plus que la diversité des produits croîtra, nous pouvons raisonnablement supposer que les métiers évolueront vers une différenciation croissante entre deux pôles : d'une part, une expertise technique dans la mise en oeuvre comme installateur des produits industriels, et d'autre part, une activité de conseil de plus en plus importante pour aider les clients dans le choix de la solution technologique adaptée à leur situation. Dans cette perspective, le bilan n'est plus négatif, car les métiers ne s'apprêtent pas à être dépouillés par l'industrialisation du secteur, mais à vivre une profonde mutation.

Le dispositif méthodologique

Le dispositif méthodologique proposé s'appuie sur trois volets clairement distingués :

1 -L'étude et la caractérisation des mutations en cours. 2 -L'analyse du dispositif de formation en place. 3 -Une formation-développement des acteurs de la formation.

L'étude des mutations en cours et l'analyse du dispositif de formation en place seront étayées par des études de cas visant principalement la description de situations concrètes afin de mettre en évidence, derrière la variété des situations observées, des points communs, des invariants. Ces descriptions confronteront l'analyse économique des faits aux discours des acteurs impliqués dans les processus de développement des innovations techniques dans les industries de la construction. Nous limiterons notre approche à cinq produits industriels nettement différenciés dans les secteurs de la terre cuite, de l'aluminium, de l'équipement électrique et des produits dérivés du plâtre. Ces deux phases de l'enquête donneront lieu à la rédaction d'un rapport dont la conclusion proposera les orientations, les objectifs et les moyens d'une politique de formation adaptée au développement des innovations industrielles dans le secteur de la construction.

La phase n° 3 consistera à soumettre à la critique des responsables de la formation dans le secteur de la construction les résultats de l'étude. Fondée sur un usage social de la méthode scientifique, cette lecture critique remplira une double fonction : 1 / tester l'acceptabilité des résultats auprès des spécialistes de la formation dans le bâtiment, 2 / accompagner les acteurs dans la mise au point d'un dispositif concret de formation. C'est-à-dire, non seulement fixer des objectifs de formation, mais aussi veiller à leur opérationnalisation tout en précisant clairement que la formation échappe au schéma « fins-moyens », c'est-à-dire que la formation comme la politique, à la différence de la technologie, n'est jamais assurée des effets des actions qu'elle engage (Cf. Imbert, 1994). Nous en déduirons que la méthode scientifique ne peut permettre de déterminer une solution optimale de formation, même si son apport reste essentiel pour éclairer et guider l'action.

Les résultats attendus

Au moment où nous rédigeons l'article, la lenteur des procédures administratives a retardé le démarrage de l'enquête de terrain. C'est pourquoi notre apport sera ici limité à l'exposé des principaux résultats attendus. Mais, c'est avec plaisir que nous communiquerons les résultats définitifs et détaillés aux lecteurs qui nous en feront la demande.

-extension des parts de marché de la grande distribution dans la commercialisation des matériaux ce qui entraîne un développement des ventes directes aux particuliers en contribuant ainsi à développer le sentiment d'insécurité économique de certains artisans, -industrialisation croissante du secteur qui tend à déposséder les artisans au profit des industriels de la plus-value liée aux savoir-faire , -« prolétarisation » d'une partie de l'artisanat due à l'extension des marchés sous-traités, -mais dynamisme des petites entreprises -et incompressibilité du secteur des métiers au-delà d'un certain seuil, -les artisans sont donc les partenaires incontournables de toute politique de développement de l'innovation technologique.

3 / La formation-produit (2) : une réponse partielle ? -A l'articulation des logiques commerciales et pédagogiques, la formationproduit est une réponse pertinente comme le montrent des secteurs de pointe tels que l'aéronautique ou la santé, mais insatisfaisante tant dans la forme particulière qu'elle présente dans le bâtiment, que dans son articulation actuelle aux dispositifs existants de formation initiale et continue.

Notes :

(1) Dirigé par le Professeur Jean Ferrasse, le Centre d'Observation des Systèmes Emploi, Formation et Développement local (COSEFD) est une équipe de recherche intégrée au Centre de Recherche en Education, Formation et Insertion de l'Université de Toulouse-le-Mirail, membre du pôle européen de recherche en sciences humaines et sociales.

(2) La formation-produit renvoie à un ensemble de pratiques fortement différenciées qui oscillent entre réunions d'information technico-commerciale, formation à un produit spécifique et organisation de séminaires animés conjointement par des chercheurs universitaires, des spécialistes du développement industriel et des représentants d'un secteur socioprofessionnel. Si la formation-produit est un concept pratiqué différentiellement, sa théorie régionale du point de vue de la recherche en sciences de l'éducation reste à construire Bibliographie ABRIC, J.-C., (1985). L'artisan et l'artisanat : analyse du contenu et de la structure d'une représentation sociale. Bulletin de psychologie, tome XXXVII ; n° 366, p. 861-875.

CALLON, M., LAREDO, P., RABEHARISOA, V., (1997). Que signifie « innover » dans les services ? Une triple rupture avec le modèle de l'innovation industrielle. La recherche, n° 295, février 1997295, février , p. 34-36. IMBERT, F. (1994)). Les métiers impossibles et les impasses du schéma « finsmoyens ». L'année de la recherche en sciences de l'éducation. PUF, 1994, p.153-174.

  BIENNALE DE L'ÉDUCATION ET DE LA FORMATION PARIS : LA SORBONNE -LE CNAM 15-18 avril 1998 Entre culture artisanale et culture industrielle : quelle politique de formation à l'innovation dans les professions du bâtiment ? Daniel GUY, Maître de Conférences COSEFD-CREFI, Université de Toulouse-le-Mirail Centre d'Observation des Systèmes Emploi, Formation & Développement Local

/ Un secteur en crise : -difficultés économiques, -conditions de travail et de sécurité qui restent encore archaïques dans de nombreuses entreprises, -image sociale dégradée, -qualification de la main d'oeuvre globalement insatisfaisante, -dégradation de la rémunération des salariés, -précarisation des emplois (CDD + travail intérimaire), -développement des créations d'entreprises dont le motif principal est la création d'un emploi et non le développement d'une unité économique, -glissement d'une part du marché en dehors du secteur légal du travail.

/ Crise et renouveau du secteur des métiers : -reconfiguration des métiers, -émergence de nouvelles activités, -quasi disparition d'activités traditionnelles,


Jean-Philippe Béja

THE FLY IN THE OINTMENT ? CHINESE DISSENT AND US-CHINA RELATIONS

Since the early eighties, transnational forces, non-governmental organisations, and all kinds of non-State actors which constitute the "international civil society" (B.Badie), have played an increasingly important role in the shaping of international relations. Political refugees, a phenomenon which goes back to the nineteenth century, can be considered as a precursor of these forces. Whether in the thirties --when German political opponents who took refuge in Western Europe or the USA put pressure on the governments of their host country to act tough toward Hitler's regime --, or after World War II, --when Spanish or Portuguese refugees in France acted to isolate the dictatorships that had taken power in their country -refugees have tried to intervene in the Foreign policy decision making process of their host countries. In the nineties, these pressures have been further legitimized by the positive image enjoyed by non-State forces in politics, whether domestic or international. The Chinese émigré dissident movement is a good example of these new developments.

The 1989 pro-democracy movement has been the largest spontaneous popular movement in China since 1949. It has given rise to a large group of militants who tried to set up autonomous organisations. Although students represented the chore of the protest leaders, many non conformist intellectuals supported their actions and took part in the demonstrations.

And for the first time since 1949, an autonomous federation of workers, distinct from the official unions, was founded on Tiananmen Square1 .

After the June 4th, 1989 Tiananmen massacre, and for the first time since the foundation of the People's Republic of China (PRC), a substantial number of political activists succeeded in leaving their motherland. They had in common a perception of the nature of the communist regime, an experience of political participation in a very repressive regime, and an exposure to the international media which had comprehensively covered the movement, and made its leaders icons.

A few months after they left China, the student and intellectual leaders of the movement decided to create a political organization which meant to regroup all the militants who had left China for political reasons since the foundation of the PRC. In September 1989, all the activists who, in the United States, had founded opposition journals or organizations (such as the alliance for Chinese democracy), the main dissident intellectuals who had taken refuge abroad, and the many leaders of the Tiananmen pro-democracy movement who had taken refuge in France, took part in the creation of the Federation for a Democratic China (FDC).

Whereas the Federation's objective consisted mainly in mobilizing the population in China itself in order to put an end to "One-Party rule", it also recognized the importance of political action in the West, and especially the need to convince Western governments to put pressure on Peking leaders for the democratization of China. For most of the activists present in Paris, the West meant the US, and therefore, most of those who had taken refuge in France decided to emigrate to the United States. Was the émigré dissident community going to become an effective lobby of the type of the Taiwan lobby or the Cuban lobby? Thirteen years later, the overall picture is, at best, mixed.

I have decided not to include the Tibetan dissidents in the category of Chinese dissident.

Although Tibetan activists have been in touch with Chinese dissidents, their agenda, as the Uyghurs', is quite different. I also decided not to talk about the Falungong, which has very specific ways to influence public opinion and decision makers. I am aware that these decisions might sound arbitrary to many.

PROLOGUE: THE CONTEXT OF US-CHINA RELATIONS

In the last two decades, US-China relations have kept developing and their importance in American domestic politics has been growing, especially since 1989. The end of the cold war has made China, the last largest communist country, a possible adversary of the United States. Besides, during the 13 years that have elapsed since the Tiananmen massacre, Sino-US trade has developed exponentially and the trade deficit, of 10.5 billion in 1990, has reached more than 80 billion US dollars 2 in 2000. The development of China and the changes in the international situation have made Sino-US relations an important issue in American Foreign policy. As such, it has played an important part in the 1992 presidential election, during which Bill Clinton denounced incumbent President Bush's "amoral position" for having failed to emphasize the importance of Human rights in China. During the decade that elapsed since then, the China question has been a bone of contention for various political forces in the US.

Numerous actors intervene: business interests represented by the US-China Chamber of

Commerce lobby in favour of a warm relationship between the two countries; the Taiwan lobby, which is very well structured and has a long-standing relationship with various forces on Capitol Hill and in the military establishment militates for a tough attitude towards Peking; the main Unions, quite powerful with the Democratic party, try to protect American workers from competition by Chinese cheap labor, to name just a few of the actors. We shall try to asses how much Chinese dissidence has been able to become an actor in a game which is characterized by a multiplicity of actors, most of which are much more structured and influential than it is.

The picture is complicated by the fact that both parties have changed attitudes over the years, with the Democrats denouncing the Republicans' compromising attitude in the early 90s whereas George Bush Jr denounced president Clinton's concept of "strategic partnership" during the campaign of 2000. In this fluctuating situtation, it is all the more difficult for a newly emerging force to act efficiently. It must evaluate the political situation quickly, and chose the fields of action with care.

I. IS THERE A CHINESE DISSIDENT LOBBY?

The émigrés' influence depends on the strength of organizations, personalities

1) The Difficulty to Structure a Political Organization:

1990-1993: Inspired by Sun Yat-sen's precedent in the early twentieth century, the leaders of the Tiananmen movement decided to unify all the branches of the overseas dissident movement to make it the voice of the broad-based Tiananmen pro-democracy movement.

However, the political inexperience of most activists, the difference of experience between generations, and their scarce knowledge of the way Western political systems function made cooperation difficult. Although there were few differences in terms of analysis of the Chinese political system, the founders of the Federation were at a loss as to how push for the democratization of the motherland: the absence of a network of supporters, of r elay organisations in the PRC reinforced the dissidents'isolation. The fact that people's discontent had not given rise to more demonstrations after June 4 th made it difficult for the new Federation to claim that it represented the wishes of the Chinese people. The activists could not agree on which social group should lead the struggle for democracy, and suspicion ran high between the student leaders and the intellectuals. This division was reflected in the leadership structure where the first chairman and vice-chairman had difficulty to work together.

A second constraint weighed on the efficiency of the FDC: the ignorance of English, and of the ways of the American political system limited the influence of its members. The choice of political scientist Yan Jiaqi3 , who had led the Autonomous Federation of Intellectuals during the Tiananmen movement, especially, was a mistake: to these drawbacks, one must add the lack of personal charisma. More popular figures with the media such as Wuer Kaixi, Chai Ling, Li Lu, who had been stars during the events in Peking, knew how to deal with the media, but they competed for attention rather than collaborated. Therefore, they were not able to build up on their image by strengthening their links with decision makers in the NSC, the State Department and on Capitol Hill. Within the FDC, relations between students and intellectuals, and between the students themselves led to a dramatic rise of factionalism, disputes, denunciations, severely hurting the image of the dissident movement as a whole.

Its isolation from China, where it could not count on organized support or underground organizations, its inability to establish a political program which might appeal to the population in the motherland, and its deteriorating image of infighting and corruption in the Chinese and Overseas Chinese populations, weakened its capacity to appear as an alternative, albeit distant, to CCP power.

Nevertheless, the obvious legitimacy crisis faced by the CCP in China, the sympathy that the "victims of the Tiananmen" massacre enjoyed with American opinion, gave a the dissident movement an undeniable importance, especially in the first half of the 1990s, even though the FDC had some difficulty to achieve its goal of being considered a viable alternative to the Communist Party. During the first few months of 1990, the student leaders excelled at mobilizing their contacts with the media in order to exert influence on decision makers. The conferences they gave in universities and their popularity with the Chinese residents of the United States gave them some weight: they were often consulted on Chinese affairs.

2) Initial Success

In the early years after the Tiananmen movement, and with the end of the cold war, Chinese dissidents could build upon the immense goodwill which existed in the American public, especially thanks to the continued interest of the media for the Chinese opposition movement.

The movement as a whole, despite its deep divisions, was vocal and its voice was respected in the area of Sino-US relations. Its leaders were consulted by some members of Congress. In June 1990, President Bush announced his support for MFN. In July, the House passed a bill linking the renewal to the freedom of all those jaildc for their participation in the 1989 movement. The Senate added more conditions, such as the suppression of import of prison labor made goods, and voted it. President Bush vetoed the bill. But the emigré movement's position had been instrumental in the decision made by Congress. The demonstration they organized attracted attention, and they drew a lot of support from the American public 6 .

When Bill Clinton was elected at the White House, he tried to adopt an alternative policy toward China, hoping to exert pressure on the Peking governement so that it relaxed its rule and freed political prisoners if it wanted the renewal of the MFN. During the first year of President Clinton's rule, the White House regularly sought the opinions of major dissidents on this subject. Relations were established, and the prospectives for the emergence of a "dissident lobby" were bright.

However, the White house's interest decreased when in 1994, the President decided to delink the granting of MFN from the human rights situation. This decision, added to an increase of factional infighting, resulted in a weakening of the dissidents' influence.

3) Fragmentation and the importance of personalities

The factionalism which plagued the FDC since its creation became increasingly serious with the passing of years, and was a factor of the decreasing importance of Chinese overseas democratic opposition in the eyes of American decision makers. Most of the student leaders of the Tiananmen movement, either didn't join (such as Chai Ling who arrived in the US later than the others, in May 1990), or did not feel obliged to act in the name of the federation. At the FDC second congress in the fall of 1990, Yan Jiaqi and Wuer Kaixi were not re-elected, and they acted increasingly as individuals rather than as militants of a group. Although this personalized action had a certain degree of efficiency, especially with the media, in the long run, it became counter productive as individuals tended to enhance their personal positions, and to try to weaken the others' images. These individual initiatives contributed to weaken the organisations which had been created earlier.

After the failed attempt at unification between the Chinese Alliance for Democracy and the Federation for a Democratic China in January 1993, the various factions started to leave the organization and to create new parties: the Chinese Social Democratic Party, the Justice Party, the Liberal Democratic Party etc. regrouped no more than a few dozens of members who had no following in the motherland. This multiplication of parties dealt a mortal blow to the political organization of overseas dissent as such.

This fragmentation was reinforced by the star system: student leaders especially used their social capital with the media to try to influence policy-making on US-China relations. In the second half of the nineties, networking by individuals with politicians in Congress, (Wuer Kaixi, then Wei Jingsheng with Senator Jesse Helms, and California representative Nancy Pelosi7 for example), or famous journalists and academics, replaced the collective pressure exerted in the name of the dissident movement as a whole.

This trend towards personalization of dissent was reinforced by the successive liberations of famous dissidents from Chinese jails, obtained thanks to international pressure. These liberations often followed by a measure of exile. Once, the newly liberated activists had arrived in the United States, each of them t ried to build up his position as the main representative of Chinese dissent and struggled to be received at the White house, in order to enhance his position in the emigré community. The unequal treatment they enjoyed from the media or from politicians provoked deep conflicts within the community. The liberation of Wei Jingsheng --a political dissident who, as soon as 1978, had demanded the establishment of a democracy, and had spent 17 years in jail -and his subsequent expulsion to the United States caused some hope for the unification of the dissident associations. Very well known in the United States, helped by all the organizations which had militated in favour of his liberation, he was considered a heroe and was received at the White House. His international prestige, his image among Chinese dissidents, his undeniable charisma put him in a very good position to become the spokesman for the whole movement. But his attempts failed and he became the target of attacks from all sides. Nevertheless, he was able to maintain contacts with politicians in Congress, and has been lobbying hard for the United

States to adopt a tough position towards Peking.

The trouble with the personalization of the movement is that the main personalities have been unable to unite on the setting of an agenda, even on some major questions such as the granting of MFN to China: for example, whereas Wang Xizhe, another activist of the 1979 Democracy Wall movement, and later, Wei Jingsheng, opposed such a measure, Fang Lizhi, known as the "Chinese Sakharov" and later Wang Dan, a student leader of the 1989 prodemocracy movement favored it.

This factionalism puts even the most supportive American politicians in an awkward position.

For example, in 1996, 58 congressmen signed a letter asking the Nobel Peace Price Jingsheng as a fake opponent. These events have had a very negative effect on politicians who wished to support the overseas opposition movement.

There are, however, opportunities for Overseas Chinese dissidents to display their unity. One such opportunity is the annual demonstration organized in Washington, New York and major cities for the commemoration of the June 4 th , 1989 Tiananmen massacre. On this occasion, they express the grief that the Chinese people is prohibited to show in China, and perform the important role of carrying on the memory of these tragic events.

To sum up, the Chinese opposition movement in the United States has been unable to create an efficient, unified organization, capable or setting an agenda of priorities and push it with the executive and legislative branches of the US Government. Even the most famous dissidents personalities, despite their social capital with the media and with concerned politicians, have had difficulty exerting influence for fear of denunciation by their peers.

But the environment in which they act is a very difficult one. In contrast with the Cuban lobby, they cannot count on a large community of compatriots who have fled the country for political reasons, and have enjoyed long time relations with politicians in the US. And, to the difference of the Taiwanese lobby, they do not represent a government, and lack the financial resources which could help enhance their efficiency.

II. THE RISE OF SPECIFIC ORGANIZATIONS: A HIGHER LEVEL OF EFFICIENCY

Seeing the opposition's influence was dwindling in China, the Chinese émigrés concentrated their limited means on lobbying American decision makers. The second part of the 1990s saw the rise of the influence of Emigré Chinese NGOs. Chinese dissidents, HRIC, on the impetus of its president Liu Qing, has kept from being caught into dissident in-fighting. Therefore, it enjoys reasonably good relations with the various opposition organisations and is fed with information from their networks. e) Human rights as an area of Foreign policy: HRIC's growing influence can be explained by the fact that concern for Human Rights violations has become an area of Foreign policy.

1) Human Rights in China

When American politicians visit China, when Chinese politicians visit the US, the American side hands a list of political prisoners that it would like to see freed. Before every important visit, human rights organisations are consulted by the State Department. Human Rights in China is considered a credible NGO, and its input enjoys a high degree of esteem.

2) China Labour Bulletin

Building upon his experience as the leader of the Beijing Workers Autonomous Federation during the Tiananmen pro-democracy movement, Han Dongfang, who emigrated to the US in 1992, has been working to help protect workers'interests in China. Helped by Human rights watch, which had worked at his liberation, Han has been presented as China's Lech Walesa.

In 1993, he tried to go back to the PRC, but was arrested in Canton and sent to Hong Kong.

3) The Laogai foundation

Thanks to his knowledge of the American political system and to his extensive network in the world of politics and the trade unions, Harry Wu has been able to raise a number of specific issues with the media, which have been relayed by politicians. Harry Wu became famous for a film he shot in China's Reform through Labor camps. He obtained the support of CBS and his program had a large echo. Having himself spent nineteen years in a labor camp, he has dedicated his life to the struggle against "laogai" (in Chinese: reform through labor).

His action concentrates mainly on specific issues, for which he lobbys intensely. His links with Senator Jesse Helms have allowed him to have the question of prison labor made goods raised in Senate. He won his major successes with the Customs Department who in actually a laogai camp. The administration went to court and in 1994, the Judge declared that the motors had in effect been made by prison labor 10 .

He has also launched a campaign against the trafficking of organs of executed prisoners by the Chinese governement. The Laogai foundation that he founded is regularly consulted by the US congress, and sends information to the various branches of the American administration.

III. SOME CASES OF EMIGRÉ INTERVENTIONS 1) Granting of MFN or PNTR

As we have seen above, in the latter half of the 1990s, the dissident community has had difficulty to adopt a unified position on the need to grant MFN to China or not. However, the whole community has seized the opportunity of the annual debate in Congress to raise the question of China's human rights situation. Every year in late May and early June, dissidents were able to testify, organize demonstrations, and lobby representatives in order to put pressure on the administration so that it raised the question of human rights in its dialogue with China. They also lobbyied hard with the media. With the help of the labor unions, which --worried by the threat that the influx of Chinese cheap goods was putting on American labor --were increasingly hostile to the granting of MFN to China, the dissidents were vocal in their condemnation of Big Business attitudes. Some representatives, such as Nancy Pelosi, who often seeks out the opinion of overseas dissidents, put a strong case for the linkage of MFN to an improvement in the human rights situation. It would be excessive to say that the dissidents alone succeeded in obliging the various administrations to demand prisoners'liberations as a show of goodwill by the Chinese authorities, but they have played their part. They have used the American forces hostile to MFN (or PNTR, or entry into WTO) to advance their agenda, but it is also undeniable that they have been used by these forces to strengthen their position. In these cases, they are presented as the voice of the Chinese people.

2) The UN Human Rights Commission in Geneva

Every March, all the groups of Chinese overseas dissidents are present at the session of the UN Human Rights Commission which takes place in Geneva : HRIC, Wei Jingsheng, the 10 Wu, Harry, Retour au laogaii, Paris, Bellefond, 1997, p.227. Laogai Foundation, etc. They are very active in the corridors, trying to persuade the US administration to sponsor a motion condemning China's human rights records, and lobbying various countries' delegations to vote in favor. On this occasion, most dissidents have usually been able to achieve a minimum degree of collaboration. The radical discourse of Wei Jingsheng usefully shoulders Han Dongfang's more moderate behavior; NGOs such as HRIC or the Laogai Foundation try to convince delegates by providing reliable information on Human rights violations. In some cases, the dissidents'discourse has a considerable impact on US delegate's behaviour. For example, in 1999 in Geneva, the US delegate to the Commission echoed a declaration Wei Jingsheng had made in 1998 in the same setting.

Then, he had declared: "this is precisely the time when support from our friends is most needed". The Ambassador, as if echoing his words, declared : "When Chinese dissidents have the courage to stand alone, we must have the courage to stand with them. We urge China to engage in genuine dialogue with its disssidents and the Dalai lama, not just members of this Commission" 11 . This shows that, although they cannot exert a constant pressure on the administration, their ideas can help shape some aspects of American Foreign policy.

Of course, theirs is simply one voice among others, but it carries some weight in the field of Human rights.

3) The creation of Radio Free Asia

The Bush endorsed the project, but it took until 1996 to set up the station. As Nancy Pelosi said, an independent radio station was deemed necessary to break the stranglehold of dictatorship: shows that the US government believes that the dissidents represent a voice which deserves to be heard.

4) Obtaining American media coverage

As we have noted earlier, the Tiananmen pro-democracy movement of 1989 enjoyed very extensive media coverage, and the student leaders became stars of the day. After they arrived in the United States, they were regularly interviewed by the major networks and their opinion was sought out by the New York Times, the Washington Post, etc. Until the mid 90s, every time a major event took place in China, or when there was a Sino-American meeting, papers carried declarations by major dissidents. It was also the case during the protracted negotiations which led to China's entry in the WTO. The overseas dissidents' status has evolved from "voice of the Chinese opposition, of the silenced Chinese people", to that of providers of expert opinions. Wang Dan, who has earned a reputation of moderation, is particularly appreciated by journalists, whereas Wei Jingsheng's positions, which tend toward extremism, are still sought out out of respect for his personal experience.

Famous dissidents'opinion is regularly quoted on issues such as repression of dissent, the death penalty, and the surge of nationalism in the PRC. Although no organisation is ever consulted as such, dissident personalities have been able to build upon the capital of sympathy they enjoy, and, despite the versatility of media attention, are now regarded as a permanent resource to be used, much in the way as are academics. This continuation of media coverage helps the overseas dissident Chinese community to remain an actor on the Sino-US political scene, a fact regularly denounced by the official Chinese press.

CONCLUSION

In the last decade, the US Chinese NGOs have won a status and are now regularly consulted by Congress and by the Executive on bilateral Sino-American issues. With the exception of HRIC, they are mostly one-man shows, and their influence is very much dependent on the credibility of their operators. On the contrary, the influence of political organizations has been waning, and since the 1994 de-linkage of MFN and human rights, they have not been consulted by the American authorities. Factional in-fighting has prevented Chinese dissidents to maximize the efficiency of their pressure. The inability of their political organizations to unite and define a common agenda, their isolation from the political situation in China, and the tendency of their leaders to put personal feuds ahead of the interest of the cause have seriously hampered the emergence of a Chinese emigré lobby.

Chinese dissident political organizations have become almost insignificant: they have lost all kind of influence in the motherland, and are unable to exert any pressure whether in the realm of the media or in politics. They are not consulted as such by the administration, and when some individuals are sometimes sought out, it is just to know their personal position.

The various political parties programs are not considered relevant either by the media or by decision makers, and nobody regards them as an alternative to Communist Party Power.

Although Chinese dissidence had a strong capital of goodwill after the Tiananmen massacre, its inability to set up an organised lobby, its tendency to factionalism, have prevented it to become a substantial player in the elaboration of Us policy towards the PRC.

The integration of the human rights factor in Foreign policy during the last decade -a phenomenon which is not linked to Sino-US relations -has made for the emergence of Chinese NGOs which lobby on specific issues. They have become a permanent actor who is regularly consulted, allowing the voice of the Overseas Chinese pro-democracy movement to be heard in the organs of power.

Given the scattered characteristic of the Overseas Chinese community, its scarcity of resources, its small numbers, and its rather recent character, this is not a small achievement.

When Chinese dissidents arrived in the United States, they had absolutely no knowledge of the way the American system functioned, they couldn't count on a politicized community of emigrants, and their only capital was the media coverage they had enjoyed during the two months of the Tian'anmen movement. It is well-known that this capital is pretty volatile. The fact that in little more than a decade, they were able to set up human rights organizations, and issue oriented associations which are regularly consulted, even when the international context is not favorable (for example, the concentration of US Foreign policy on the war on terror), must be put to their credit.

However, on the eve of President Jiang Zemin's official trip to the United States in October 2002, not a single Chinese dissident was freed from jail seems to indicate that the safeguarding of dissidents' rights is not a top priority of the Bush administration.

For

  example, Democratic representative for California Nancy Pelosi (who since has become the Democratic Party whip at the House of Representatives) was particularly active in her opposition to the Chinese Governement. In September 1991 she went to Peking and deployed a banner on Tiananmen Square with the folllowing inscription, in Chinese and in English: « To those who died for democracy in China » 4 . In January 1993, she attended the third Congress of FDC. On the other end of the political spectrum, Chinese dissidents have also established a good working relationship with Republican Representative Christopher Cox 5

  committee to give the Price to Wei Jingsheng. But soon, other dissidents in China, Ren Wanding and Xu Wenli -both active during the 1979 Democracy Wall movement and jailed for their participation--relayed by overseas organisations (the Justice Party) denounced Wei

Founded

  in March 1989 by Chinese scholars and prominent American China specialists, Human rights in China has been through an impressive development. To the contrary of the political organizations, HRIC, reinforced by the arrival of Liu Qing --a Chinese dissident who spent ten years in jail for having made public the minutes of Wei Jingsheng's 1979 trial, and having denounced the Chinese judiciary system in a samizdat 8 --HRIC's efficiency was also accrued by its straight collaboration with Human Rights Watch. The association's influence kept growing during the last decade thanks to the following factors. important asset of Chinese dissidents. 8 Liu Qing's book was smuggled out of jail and published in French in 1981. Liu Qing, J'accuse devant le tribunal de la société, Paris, Robert Laffont, 1981. a) A high quality of information: HRIC has become a very valuable source of information on the human rights situation for academics, think tank researchers, and politicians interested in China. Thanks to its network of informers in the PRC, this NGO does not suffer from the same isolation from the Chinese public as do political organizations. Victims and their relatives inform HRIC of human rights violations. The fact that it is run mostly by Chinese activists reinforces its credibility. b) A high level of research: HRIC has numerous researchers, permanent or on contract, who write serious reports on subjects linked to human rights violations, but also on some social and political aspects of the reality of the PRC. These reports are widely used by the media, academics and politicians. c) A wide network in governement and Congress: The credibility and the quality of its information, as well as its early collaboration with HRW, have provided HRIC with a wide network of contacts on Capitol Hill and at the White House. It is regularly consulted by the State Department, as are NGOs such as Amnesty International and HRW, when it prepares its annual report on Human rights, and by the Congressional Executive Commission on China which was created after the granting of PNTR to the PRC. It was especially so during the first year after Bill Clinton won his first mandate in 1992. As he had emphasized that his policy toward China would be very different from what had been Bush's, "human rights advocates were given unprecedented visibility in the State Department, and the Clinton administration appeared prepared to use the theme not only to campaign, but to govern" 9 . Shattuck, then assistant secretary of state for human rights and humanitarian affairs, met every week with representatives of AI, Freedom house and other such NGOs. d) Keeping out of Chinese dissident movement bickering: Although mainly manned by

  crumbling of the Berlin Wall and the end of the Cold War have shown the importance of the diffusion of information. Many observers have emphasized the role of Radio Free Europe in the delegitimation of the European Communist regimes. This idea has been pursued by Chinese dissidents, and many well known individuals, as well as the Federation for a Democratic China, during its years of existence, have been lobbying for the creation of a free Radio Station which would broadcast to China. Their preoccupation met with some Congressmen's who thought that the end of the communist regime in China was a prerequisite for the emergence of a new international order. Members of the emigré Chinese community actively liaised with these concerned politicians. As soon as 1991, President

Table 5 : China's Trade with the United States ($ billion)

 5

	: US Exports: US imports: 15.22, Balance :-10.47 Total: 20.03
		1990	1996	1997	1998	1999	2000 Q1 2000 Q1 2001
	US Exports	4.81	12.0	12.8	14.3	13.1	16.3	3.2	4.4
	US Imports	15.22	51.5	62.5	71.2	81.8	100.1	20.0	22.4
	Total	20.03	63.5	75.3	85.4	94.9	116.4	23.2	26.8
	Balance	-10.47	-39.5	-49.7	-56.9	-68.7	-83.8	-16.7	-18.0

Source : Us-China Business Council, http://www.uschina.org/press/tradetable.html

  He then founded the China Labour Bulletin with the help of the Hong Kong Confederation of Trade Unions and the support of AFL-CIO. Since 1996, he has been working simultaneously for Radio Free Asia. Han's is very much a one man show, but the extent of his network in China, his fame among workers thanks to his radio program, make him the destination of very valuable first hand information on workers conditions, and on workers'movement. Han also helps Chinese workers push for the implementation of Chinese law.

Although he is based in Hong Kong, Han's work is largely recognized in the United States, and besides the support of AFL-CIO, he is often consulted by Congressional committees, and displays a great efficiency at the UN Commission of Human Rights. Besides, he has been able to abort attempts by some leaders of the AFL-CIO at establishing relations with the official All Chinese Federation of Trade Unions.

"

  The vast majority of people in China still only hear what China's government wants them to hear, they only see what the government wants them to see, they only read what the government allows them to read...Those who speak out are arrested, exiled or killed.". RFA funded by Congress but is completely autonomous. Many of the regular chronicles are manned by dissidents such as journalist Liu Binyan, a founding member of the FDC, Han Dongfang, head of the China Labour Bulletin. It constitutes a forum for the Overseas dissident movement, and allows it to broadcast its ideas in China. The creation of RFA

11 Rosemary Foot, Rights Beyond Borders, Oxford, Oxford University Press, 2000, p.243.

is

On the 1989 pro-democracy movement, Cf. J.P.Béja, M.Bonnin, A.Peyraube, Le Tremblement de terrede Pékin, Paris, Gallimard, 1991.

For an English translation of Yan Jiaqi's main works, cf, Bachman, David, and Yang, Dali Yan Jiaqi and China's Struggle for Democracy, Armonke, M.E.Sharpe, 1994

Washington Post,

5/9/91 5 Nancy Pelosi is representative for San Francisco, and Christopher Cox for Orange County. Both areas have strong Chinese communities.

Lee, Ta-ling, Copper, John, The Bamboo Gulag: Human Rights in the PRC, Occasional Papers/Reprint Series in Contemorary Asian Studies, University ofMaryland, n°4, 1994 (123), pp.193-94

Nancy Pelosi has been very active in support of the democratization of China, and she has a good working relationship with most Chinese dissidents. Her position as the Democratic party Whip at the House of Representatives makes her a very

Dittmer, Lowell, « Chinese Human Rights and American Foreign Policy », The Review ofPolitics, Summer 2001, Vol.63, n°3, pp.437.


J Cortés
email: jcortes@laas.fr

T Siméon

Probabilistic Motion Planning for Parallel Mechanisms

Despite the increasing interest on parallel mechanisms during the last years, few researchers have addressed the motion planning problem for such systems. The few existing techniques lie onto a representation of the workspace of the mechanism (or its boundary). However, obtaining this representation is generally too difficult, only partial solutions exist for particular cases. In this paper we propose a general approach based onto probabilistic motion planning techniques. This approach does not need any modeling of the robot's workspace. It combines random sampling techniques with simple but general geometric algorithms that guide the sampling toward feasible configurations satisfying the closure constraints of the parallel mechanism. The efficiency and the generality of the method are demonstrated onto several complex mechanisms made up with serial or parallel associations of Stewart platforms, or created with several redundant robots manipulating an object.

I. INTRODUCTION

A parallel manipulator is a mechanism in which the end-effector is connected to the base by at least two independent kinematic chains [START_REF] Merlet | Parallel Robots[END_REF]. The most representative parallel manipulator is the six-degrees-of-freedom (d.o.f.) mechanism known as Stewart platform [START_REF] Stewart | A Platform with Six Degrees of Freedom[END_REF], [START_REF] Dasgupta | The Stewart Platform Manipulator: A Review[END_REF]. This definition can be also applied to more complex multi-loop mechanisms formed by several manipulators handling an object. In this paper we address the problem of planning collision-free motions for such general parallel mechanisms.

The few existing techniques for trajectory validation and motion planning of parallel mechanisms [START_REF] Merlet | Manipulateurs parallèles, 7eme partie: Vérification et planification de trajectoire dans l'espace de travail[END_REF] use a representation of the workspace of the mobile platform. The difficulty to compute such representations limits the generality of these approaches, like with deterministic motion planning techniques [START_REF] Latombe | Robot Motion Planning[END_REF] that first relied on an exact model of the collision-free configuration-space CS f ree of the mobile system. Probabilistic motion planning techniques [START_REF] Kavraki | Probabilistic Roadmaps for Path Planning in High-Dimensional Configuration Spaces[END_REF] do not need to build a model of the space where they are applied. This property was the key of their success during the last decade. However, the closure constraints of parallel mechanisms (i.e. multiple loops) remain an important challenge for probabilistic motion planning methods and the few existing approaches [START_REF] Lavalle | A Probabilistic Roadmap Approach for Systems with Closed Kinematic Chains[END_REF], [START_REF] Han | A Kinematics-Based Probabilistic Roadmap Method for Closed Kinematic Chains[END_REF], [START_REF] Cortés | A Random Loop Generator for Planning the Motions of Closed Kinematic Chains using PRM Methods[END_REF] are mostly limited to single-loop mechanisms. We present an extension of our work on motion planning for closed kinematic chains [START_REF] Cortés | A Random Loop Generator for Planning the Motions of Closed Kinematic Chains using PRM Methods[END_REF] using the PRM framework, to efficiently deal with multi-loop mechanisms such as general parallel robots.

A first application of the approach is to capture the self-collision-free workspace of the parallel mechanism into a small data structure (a random visibility roadmap [START_REF] Siméon | Visibility-Based Probabilistic Roadmaps for Motion Planning[END_REF]). Once computed for a given mechanism, this data structure can be used to generate in real-time valid motions avoiding self-collisions between the links of the mechanism. In presence of obstacles, the proposed approach also allows us to solve motion planning problems like the one illustrated in Figure 1 where the path to extract the ring mounted onto a Stewart platform from the "s-shaped" obstacle is computed in only a few seconds.

Section II first gives a brief overview of probabilistic motion planning techniques. In this same section, we discuss about the extension of these techniques to handle closed kinematic chains. Parallel mechanisms are presented in Section III. Our approach for sampling random configurations of such systems is explained in Sections IV and V. Results in Section VI show the generality of the method through different applications for various kinds of systems.

II. PROBABILISTIC MOTION PLANNING

Probabilistic motion planning techniques appeared in the last decade as an alternative to deterministic approaches. In particular, Probabilistic RoadMap (PRM) methods [START_REF] Kavraki | Probabilistic Roadmaps for Path Planning in High-Dimensional Configuration Spaces[END_REF] have been mostly developed. These techniques have demonstrated to be efficient and general tools for motion computing.

PRM Principle: The general PRM principle is to construct a graph (roadmap) that captures the topology of CS f ree . The nodes are randomly sampled configurations satisfying intrinsic conditions in this space (e.g. collisionfree). The edges are short feasible paths (local paths) linking "nearby" nodes.

PRM Variants: Several algorithms [START_REF] Amato | OBPRM: An Obstacle-Based PRM for 3D Workspaces[END_REF], [START_REF] Boor | The Gaussian Sampling Strategy for Probabilistic Roadmap Planners[END_REF], [START_REF] Wilmarth | MAPRM: A Probabilistic Roadmap Planner with Sampling on the Medial Axis of the Free Space[END_REF], [START_REF] Siméon | Visibility-Based Probabilistic Roadmaps for Motion Planning[END_REF], [START_REF] Bohlin | Path Planning using Lazy PRM[END_REF] have been proposed sharing this basic idea. These methods mostly differ from their sampling strategies. In particular, the visibility-PRM approach [START_REF] Siméon | Visibility-Based Probabilistic Roadmaps for Motion Planning[END_REF] is used in our solution. The algorithm building the graph only keeps the sampled configurations in two cases: when they link several connected components of the roadmap or when they can not be connected to any of these components. The main advantage is to compute a smaller roadmap which significantly decreases the number of calls to the local planner (the most expensive step of the roadmap construction) compared to other approaches. Figure 2 shows two roadmaps for the same 2D environment. The left one has been computed by a basic-PRM algorithm that keeps every valid sampled configuration. The right one, obtained by the visibility approach, encodes the same information in a much smaller structure.

The mentioned PRM techniques are called multiplequery. Once the roadmap is computed, motion planning queries are solved by connecting the start and goal configurations to the graph and searching a path in it. Other algorithms dedicated to solve simple planning queries have been developed from the same principles than PRM (e.g. RRT [START_REF] Kuffner | RRT-Connect: An Efficient Approach to Single-Query Path Planning[END_REF], SBL-PRM [START_REF] Sánchez | A Single-Query Bi-Directional Probabilistic Roadmap Planner with Lazy Collision Checking[END_REF]).

All techniques above require the generation of random configurations of the mechanism. This is a trivial process in the case of open kinematic chains. On the contrary, when the mechanism contains loops, samples must be gen- erated into a variety instead of a configuration-space. The difficulty to compute (and to connect) such configurations remains a challenge for the application of probabilistic motion planners.

PRM and Closure Constraints: Only a few works extending the PRM framework to deal with closed-chain mechanisms can be found in the literature [START_REF] Lavalle | A Probabilistic Roadmap Approach for Systems with Closed Kinematic Chains[END_REF], [START_REF] Han | A Kinematics-Based Probabilistic Roadmap Method for Closed Kinematic Chains[END_REF], [START_REF] Cortés | A Random Loop Generator for Planning the Motions of Closed Kinematic Chains using PRM Methods[END_REF]. The approach in [START_REF] Cortés | A Random Loop Generator for Planning the Motions of Closed Kinematic Chains using PRM Methods[END_REF] demonstrates good performance onto complex 3D closed chains involving tenths of d.o.f.. Each single-loop in the mechanism is broken (as initially proposed in [START_REF] Han | A Kinematics-Based Probabilistic Roadmap Method for Closed Kinematic Chains[END_REF]) into two chains (passive and active). The random node generation combines a sampling technique called Random Loop Generator (RLG) with forward kinematics for the active chain and inverse kinematics for the remaining (passive) part of the loop in order to force the closure. When computing the edges, the local planner is limited to act onto the active joints. The passive part of each loop follows the motion of the rest of the chain using point to point inverse kinematics.

The main interest of RLG is that it produces random samples for the active chain that have a high probability to be reachable by the passive part. The algorithm in [START_REF] Cortés | A Random Loop Generator for Planning the Motions of Closed Kinematic Chains using PRM Methods[END_REF] performs well on independent single-loops and was also applied to some cases of multi-loops. However, this approach requires an extension to efficiently handle more general closed-chain mechanisms. Parallel mechanisms are a more complex instance that presents particular interest.

APi A P 2 AP1 A B 1 A B 2 A B i Mi M2 M1 F P F B P B Fig. 3.
General description of a parallel mechanism.

III. PARALLEL MECHANISMS

Description: A parallel mechanism is composed of a base B, a platform P and n kinematic chains M i linking them. We call A Bi and A Pi the frames corresponding to the connections of each M i to B and P respectively. F B and F P are the frames associated with B and P (see Figure 3).

The spatial situation of P (usually called pose) is defined by a vector q P ={x, y, z, θ x , θ y , θ z }. The three firts elements represent the position of F P with respect to F B . The orientation is given by three consecutive rotations around the axes of F P 1 . The platform is considered to be the end-effector of a parallel mechanism. Hence, an equivalence can be established between poses of P and points of the workspace of the system.

Workspace: The workspace WS P of a parallel mechanism is usually computed from the workspaces WS Mi of the chains M i and the dimensions related to P. The difficulty is that WS P can not be decoupled into two three-dimensional (graphically representable) sub-spaces because of the dependence between position and orientation of the end-effector. Therefore, only sub-sets of the workspace may be represented. Most of the existing works are limited to the determination of some particular sections of the positional workspace with constant orientation of the platform [START_REF] Gosselin | Stiffness Mapping for Parallel Manipulators[END_REF], [START_REF] Masory | Workspace Evaluation of Stewart Platforms[END_REF]. Other techniques compute the feasible rotations of the platform around a fixed point [START_REF] Merlet | Determination of the Orientation Workspace of Parallel Manipulators[END_REF].

Configuration: The configuration of a parallel mechanism is defined by the joint values of the chains M i (and the pose of P). Configurations satisfying the closure constraints could be easily computed from a model of its workspace. Hence, a feasible pose q P could be directly obtained from this representation. The spatial situation of the connection-frames A Pi w.r.t. the A Bi would be given by q P , and the configuration of the chains M i linking these frames could be then computed by inverse kinematics techniques.

However, modeling the workspace of a general parallel mechanism remains an open problem [START_REF] Merlet | Parallel Robots: Open Problem[END_REF]. In next section, we describe an algorithm that generates random configurations of a general parallel mechanism without requiring the explicit computation of WS P .

IV. RANDOM CONFIGURATION SAMPLING

FOR PARALLEL MECHANISMS We propose a general approach that combines random sampling techniques with simple geometric operations for generating random configurations of parallel mechanisms. Spherical shells approximating the WS Mi are used to progressively compute the pose q P of P. The algorithm first generates the position parameters of q P and then it computes the rotation parameters. Such obtained pose of P correspond to random samples in a conservative approximation of WS P . Then, the existence of a feasible configuration is checked for each chain M i linking A Bi and A Pi . The configuration of the parallel mechanism is kept when all the M i connect the base and the platform, else the process is iterated. Next paragraphs detail the main features of the approach.

WS Mi Approximation: In a similar way than in [START_REF] Cortés | A Random Loop Generator for Planning the Motions of Closed Kinematic Chains using PRM Methods[END_REF], spherical shells bounding the reachable workspace (only in position) of the chains M i are used. A tradeoff between accuracy and computing time justifies this choice. A spherical shell is defined by the intersection 1 The approach is valid for other representations of the orientation (e.g. Euler angles).

F Π Π Cxy A B 1 A B 2 A B 3 A B 4
Fig. 4. Illustration of the computation of Cxy.

of the volume between two concentric spheres and a cone whose vertex coincides with their common center. Spherical shells are used in collision detection as boundary volumes of objects because of the fast computation of their intersections [START_REF] Krishnan | Spherical Shell: A Higher Order Bounding Volume for Fast Proximity Queries[END_REF]. Parameters characterizing the spherical shell are derived from the features of the chains M i . The center is the origin of the frame A Bi . The external and internal radii correspond respectively to the maximum and minimum extension of the chain. The axis of the cone cutting the full shell is a vector associated with A Bi (normally its z-axis). The half-opening angle is the maximum angle between this axis and the vector passing through the origins of A Bi and A Pi .

Platform Position: Given the fixed spatial situation of the A Bi in F B , a plane Π can be computed by interpolating the position of the frame origins (when there are more than two). We next explain the method to generate the position of the platform w.r.t. the frame F Π associated with this plane.

First, a rectangle C xy approximating the orthogonal projection of WS P on Π is computed as follows. The spherical shell approximating each WS Mi is augmented by the distance from A Pi to F P . The projection of the external portion of sphere on Π corresponds in general to an ellipse. C xy is the rectangle bounding the intersection of these ellipses. Figure 4 illustrates this process in the case of four chains M i . For clarity purpose, we have only represented the external surface of the augmented shell of the chain M 1 .

The generation of a pose of P begins by randomly sampling a point p xy in C xy . Then, the intersection of the line perpendicular to Π passing through p xy with each one of the augmented spherical shells is computed. (when one or several volumes are not intersected, a new point p xy must be sampled). The result of this operation are one or several intervals in z (relative to Π) for each M i . The intersection of such intervals represents a conservative approximation of the set of reachable positions of the platform for a given p xy . The z coordinate of the origin of F P is generated by randomly sampling in this set. Platform Orientation: For a given position of P, its orientation is generated by progressively computing the three elementary rotations. We next explain the process for one rotation.

A P i I θ k i k ax is o f F P A B i
The rotation of P around an axis k produces a circular motion of each A Pi . The intersection of the spherical shell approximating each WS Mi with the circle generated by its corresponding A Pi is computed. The result is the set I θ k i of values of the rotation parameter making A Pi reachable (in position) by M i (considering our approximation). Figure 5 illustrates this operation. When the circle intersects all the shells, the value of the rotation θ k is obtained by randomly sampling in the interval(s) resulting of the intersection of the I θ k i sets. The process is iterated when any of the computed intersections is null.

M i Configuration & Validation:

The conservativeness of the approach is essential in order to guarantee that WS P will be completely sampled. However, the validity of each configuration must be tested. A configuration is valid when the platform's pose induces feasible joint values of all the chains M i .

For parallel manipulators (Stewart platform type) obtaining the configuration of the M i for a given pose of P is straightforward due to the simple nature of these chains. In the general context this process is more complex. Each triplet {A Bi ,A Pi ,M i } is treated as a closed kinematic chain. When M i is a non-redundant system, its configuration can be directly obtained by inverse kinematics. In case of redundancy, we use the RLG algorithm presented in [START_REF] Cortés | A Random Loop Generator for Planning the Motions of Closed Kinematic Chains using PRM Methods[END_REF] to generate it.

Note that this approach also allows to handle particular cases such as mechanisms where the position of the platform is fixed w.r.t. the base and/or the rotational mobility is limited (i.e. rotating only around one or two axes).

V. COMPLEX MECHANISMS

The presented approach has been extended to handle more complex systems obtained by the associations of parallel mechanisms. It has been also adapted to a particular case of highly-redundant chains M i .

Associations: When n parallel mechanisms are connected in series (see left image in Figure 8), each platform P i , i=1..n-1, becomes the base for the next platform. The process of generation and validation of the configuration is progressively achieved for each platform starting from the base (P 1 ) to the top (P n ). When the sampled pose of a given P i is not valid, the process does not re-start completely. It is only iterated from P i-1 . When several parallel mechanisms are disposed in parallel (see right image in Figure 8) they form a "main" parallel system. Each mechanism can be considered as a chain M i of the main system. Therefore, their platforms become passive elements of the whole mechanism.

Mobile M i bases: Such case occurs for example when the M i chains correspond to mobile manipulators (i.e. articulated system composed of an arm mounted on a mobile base [START_REF] Khatib | Coordination and Decentralized Cooperation of Multiple Mobile Manipulators[END_REF], see Figure 10). Spherical shapes are suitable to bound the reachable workspace of the arm, but not for the whole mobile manipulator. We have extended the approach to the case where the A Bi can freely move on parallel planes. While computing q P , these frames are considered to be placed at the position that maximizes the variation of each parameter. Then, feasible random configurations of the chains M i are computed by RLG.

VI. RESULTS

The approach has been implemented into the motion planning software Move3D [START_REF] Siméon | Move3D: A Generic Platform for Path Planning[END_REF]. In this section we comment some of the obtained results for very different parallel mechanisms. Numerical results correspond to tests performed with a Sun Blade 100 workstation.

Self-collision-free motions: The first experiment aims at demonstrating the performance of the approach to compute self-collision-free motions of the Stewart platform. The roadmap computed for this mechanism can be used to generate such motions in real-time. The left image in Figure 6 shows an example of the self-collision configurations to be avoided. The graph illustrated in the other image of this figure was computed by the visibility-PRM [START_REF] Siméon | Visibility-Based Probabilistic Roadmaps for Motion Planning[END_REF] approach in 22 seconds. It only contains one connected component made up with 11 configurations. This small graph covers more than the 99.99% of the robot workspace. The roadmap construction required the generation of 17442 configurations of the mechanism, of which 2328 were found to be collision-free. Using our sampling strategy, 30840 platform's poses were tested for the generation of valid configurations (more than 50% of success). With similar tests performed using standard random sampling techniques to generate the platform's pose2 , less than a 2% of the samples produced valid configurations of the mechanism. This illustrates the important gain (about 25 times faster) using the proposed sampling approach. Motion planning for Stewart platforms: The second example illustrated by Figure 7 shows a constrained motion planning problem for the Stewart platform. The figure shows the start and goal configurations and the trace of the solution path. Note that the motion requires extreme deformations of the mechanism. A graph containing this solution was computed in 60 seconds. Once computed, it allows to process motion planning queries in some hundredths of second. The two images in Figure 8 correspond to motion planning problems involving associations of parallel mechanisms. The manipulator of the left image is a model of the Logabex-LX4 [START_REF] Charentus | Modélisation et commande d'un robot manipulateur redondant composé de plusieurs plateformes de Stewart[END_REF]. The arm is composed of four Stewart platforms connected in series. Motion planning queries solving problems where the manipulator with the grasped bar changes from one to another opening of the bridge were computed by RRT algorithm [START_REF] Kuffner | RRT-Connect: An Efficient Approach to Single-Query Path Planning[END_REF] in a few seconds. The right image illustrates an example where two sets of three Stewart platforms cooperate in a assembly task. This type of association as been proposed in [START_REF] Chai | Designing a Stewart Platform-based Cooperative System for Large Component Assembly[END_REF] for the manipulation of large objects. The motion to assemble the two puzzle-like parts was computed in only 15 seconds.

Parallel systems including manipulator arms:

The two last examples show the generality of the approach. In both cases, the mechanism consists of several robotic arms grasping an object. The problem illustrated in Figure 9, where four 6R manipulators have to unhook an object and to insert it into the cylindrical axis, was solved using RRT in less than 1 second. The last example (see Figure 10 combines two types of difficulty. First, the system composed by the three holonomic mobile manipulators and the piano is a very complex parallel mechanism (9 d.o.f. for each M i chain). Also, the complexity of the scene makes the validation of collision-free configurations and local paths harder. A graph that permits to rapidly compute any feasible motion in this scene was computed using the visibility-PRM approach in about 5 minutes. In this example, the redundancy of the manipulators (M i chains) is treated by the RLG algorithm as explained in Section V.

VII. CONCLUSIONS

The proposed approach allows to extend the PRM framework to efficiently handle complex mechanisms with multiple loops. Our aim is to reach the highest level of generality. The approach can deal with the most general definition of parallel mechanisms and its efficacy was demonstrated onto complex examples (e.g. serial/parallel associations of Stewart platforms, parallel system with redundant chains). A possible improvement of this approach could be to integrate constraints for avoiding singular configurations along the trajectory [START_REF] Dasgupta | Singularity-Free Path Planning for the Stewart Platform Manipulator[END_REF], [START_REF] Merlet | Parallel Robots: Open Problem[END_REF].

We are currently investigating the application of our closed-chain PRM approach to highly articulated mechanisms encountered in molecular models. Hence, tools for analyzing the motion of loops in protein structures [START_REF] Fiser | Modeling of Loops in Protein Structures[END_REF] should help biologists to better understand the important processes such as protein-ligand or protein-protein interactions and protein folding.

Fig. 1 .

 1 Fig. 1. Example of motion planning problem for a Stewart platform.

Fig. 2 .

 2 Fig. 2. Basic-PRM and visibility-PRM in the same 2D environment.

Fig. 5 .

 5 Fig. 5. Illustration of the computation of I θ k i .

Fig. 6 .

 6 Fig. 6. Illustration of the self-collision-free visibility-PRM computed for a Stewart platform.

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Fig. 7 .

 7 Fig. 7. Sequence of the solution motion for the s-bar problem.

Fig. 8 .

 8 Fig. 8. Examples of associations of Stewart platforms.

Fig. 9 .

 9 Fig. 9. Four robotic arms manipulate an object.

Fig. 10 .

 10 Fig. 10. A piano mover problem with three cooperating mobile arms.

Samples are taken in a six-dimensional-box bounding the space of feasible poses of P.

Acknowledgments: This paper has greatly benefited from discussions with Jean-Pierre Merlet and Lluís Ros.