Fabrice Flipo

Theoretical positions around the ecological implications of ICTs

7 Diderot) L'émergence du numérique dans les années 1970 à 1990 a suscité des positions théoriques assez classiques. Les positions socialistes et marxistes classiques voient dans l'avènement des TIC une possible nouvelle forme d'exploitation du travail (ce qui sera nommé « digital labor » mais qui est anticipé par Hardt & Negri ou par Gorz sous d'autres noms tels que « technofascisme » etc.) mais aussi une manifestation de la ruse de l'histoire, au cours de laquelle le capitalisme fournit les outils de son propre dépassement, ainsi André Gorz, qui voit dans les TIC une technologie-carrefour, ou Negri et Hardt, qui l'ont vu comme abolissant la valeur ; Gorz a même écrit que la sortie du capitalisme avait commencé, dès 1983 (également en 2007). Marx avait en effet anticipé l'avènement du marché mondial, les TIC peuvent être vues comme n'en étant que l'outil permettant d'y parvenir, d'une certaine manière, avec la transnationalisation de l'économie, l'intégration de tous dans l'échange, comme le suggère le Manifeste ; la nouveauté ne vient que des modalités concrètes de mise en oeuvre, mais sur le fond rien n'est réellement surprenant. L'intuition de Marx est confirmée par exemple par la thèse de Benedict Anderson, qui fait jouer à la vitesse de communication un rôle déterminant dans la construction des nations, à partir du pluralisme des situations locales. Il suffit de prolonger la thèse : avec le numérique le globish va s'imposer. Les analyses plus écologistes sont partagées, entre une lecture à la Gorz et les analyses d'Ellul ou d'Illich et qui se traduiraient plutôt dans les positionnements d'associations militantes telles que Pièces & Main-d'Oeuvre ou Technologos : le numérique n'est que la dernière manifestation du sacré technologique, auquel tout doit être sacrifié, même la planète. Le numérique représente 3 % des émissions de GES mondiales et n'a pas du tout « dématérialisé la croissance », contrairement à ce que défendent les thèses de Negri et Hardt ou les thèses dominantes de la Commission Européenne, des grandes entreprises ou d'une partie des économistes (ainsi Sylvie Faucheux). Alors que la colonisation du monde vécu (Gorz, Habermas) par les TIC continue, et qu'elles sont chaque jour plus en passe de s'installer en situation de monopole radical (Illich), avec toutes les difficultés de protection que cela implique, la voracité des industries continue de croître, tout comme la masse de leurs déchets.

The emergence of the digital world in the 1970s gave rise to fairly classic theoretical positions. Classical socialist and Marxist positions see the advent of ICT as a possible new form of labor exploitation (which will be called "digital labor" but is anticipated by Hardt & Negri or Gorz under other names such as " technofascism "etc.) but also a manifestation of the "cunning of reason", in which capitalism provides the tools for its own overtaking, as well as André Gorz, who sees ICT as a crossroad technology, or Negri and Hardt, who saw it as abolishing value, and therefore paving the way to an exit of capitalism; Gorz even wrote in 1983 that the exit of capitalism has began (also in 2007). Marx had indeed anticipated the venue of a global market, ICTs can be seen as a necessary tool to achieve the transnationalization of the economy, the integration of all in exchange, as the Communist Manifesto suggests; the novelty comes only from the concrete methods of implementation, but basically nothing is really surprising. Marx's intuition is confirmed, for example, by the thesis of Benedict Anderson, for whom the speed of communication plays a determining role in the construction of nations. Withe the same thesis, digital technologies are paving the way for the domination of globish as the lingua franca for the world. Theoretical analyses more sensitive to ecological issues are more divided, between a gorzian reading and others such as Ellul or Illich which would be closer to associations such as Pièces & Main d'Oeuvre or Technologos, for whom the digital is the last manifestation of the technological sacred, to which everything must be sacrificed, even the planet. Digital represents 3% of global GHG emissions and has not at all "dematerialized the growth", contrary to what is argued by Negri and Hardt's theses or the dominant theses of the European Commission, big companies or most of the economists (thus Sylvie Faucheux). While the colonization of the lived world (Gorz, Habermas) by ICTs continues, and that they are every day more and more in situation of radical monopoly (Illich), with all the difficulties that implies, the voracity of industries continues to grow, just like the mass of their waste.

Dans cette communication nous nous intéressons au statut des implications écologiques du numérique, dans la perspective plus générale du débat autour de l'écologie politique, dans une perspective d'émancipation. Nous nous intéressons plus particulièrement à quatre auteurs, qui font jouer aux TIC un rôle important : Hardt et Negri, André Gorz, Jacques Ellul et Ivan Illich.

In this paper we are interested in the status of the ecological implications of digital, in the more general perspective of the debate around political ecology, in a perspective of emancipation. We are particularly interested in four authors who make ICTs play an important role: Hardt and Negri, André Gorz, Jacques Ellul and Ivan Illich.

Dans Empire Negri & Hardt théorisent un ordre global ou du monde transnational, un pouvoir biopolitique capable d'organiser les rapports sociaux, un pouvoir d'ordre mais aussi d'exception dans ses interventions de police. En termes de production la transformation des secteurs primaires et secondaires est achevée ; l'enjeu est celui des services c'est-à-dire d'un travail immatériel caractérisé par la communication, l'affectivité, l'information et la connaissance (p. 349). L'autoroute de l'information est devenue l'intermédiaire universel (p. 362). C'est un changement de paradigme : on passe de la production au réseau (p. 371). L'Empire et le capitalisme ont donc conduit à l'émergence d'un travail immatériel qui ouvre sur de nouvelles formes de subjectivation, et donc peut-être d'insubordination (p. 55). Les pouvoirs de la science, de la connaissance, de l'affect et de la communication principalement constituent notre virtualité anthropologique et qui se déploie à la surface de l'Empire (p. 441). C'est la fin de l'histoire moderne et de la préhistoire du capitalisme. L'enjeu est celui d'un nouveau pouvoir constituant, à l'échelle globale ; en quoi les auteurs se différencient des localismes et des nationalismes de gauche. Les difficultés sont là, cependant, car si le potentiel est là, les auteurs observent qu'à l'âge de la communication les mouvements ne communiquent pas entre eux (pp. 88-89), aussi l'enjeu stratégique est celui de la définition d'un ennemi commun et d'un langage commun. Le postmodernisme est jugé très confus dans ses solutions, à l'exemple d'Homi Bhabha. Le marché est postmoderne depuis longtemps, il rassemble les différences (p. 195). Une nouvelle notion de « commun » émerge sur le terrain de l'égalité virtuelle des autoroutes de l'information. La possibilité d'une alternative mondiale réside dans le pauvre (p. 203). La multitude est le nom commun des pauvres (p. 203). La multitude mobile doit arriver à la citoyenneté mondiale (p437). Nous assistons à une « communisation » de la multitude. Negri et Hardt soulignent la différence entre émancipation et libération chez Marx : l'émancipation vise l'entrée de nouvelles nations dans la société impériale ; « la libération, au contraire, signifie la destruction des frontières et des schémas de migration forcée, la réappropriation de l'espace et le pouvoir pour la multitude de déterminer la circulation mondiale et le mélange des individus et des populations » (p. 438).

In Empire Negri & Hardt are theorizing a global or transnational world order, a biopolitical power capable of organizing social relations, a power of order but also of exception in its police interventions. In terms of production, the transformation of the primary and secondary sectors is said to be complete; the challenge is that of services, that is, immaterial work characterized by communication, affectivity, information and knowledge (p.349). The so-called "information highways" are seen as an universal medium (p.362). Authors see a paradigm shift: from production to the network (p. 371). The Empire and capitalism have therefore led to the emergence of an immaterial work that opens up new forms of subjectivation, and thus perhaps insubordination (p. 55). The powers of science, knowledge, affect, and communication are primarily our anthropological potentialities that unfold on the surface of the Empire (p.441). It is the end of modern history and of prehistory of capitalism. The challenge is that of a new constituent power on a global scale; authors are strongly differenciating themselves from left localisms and nationalisms. The difficulties are there, however, because if the potential is there, the authors observe that at the age of communication the movements do not communicate with each other (pp. 88-89); an issue is also the definition of a common enemy and a common language. Postmodernism is considered to be very confusing in its solutions, like Homi Bhabha. The market is postmodern for a long time, it brings together the differences (p.195). A new notion of "common" emerges in the field of virtual equality of information highways. The possibility of a global alternative lies in the poor; the multitude is the common name of the poor (p.203). From the mobile multitude must emerge a world citizenship (p437). We are witnessing a "communisation" of the multitude. Negri and Hardt emphasize the difference between emancipation and liberation in Marx: emancipation aims at the entry of new nations into imperial society; "Liberation, on the other hand, means the destruction of borders and patterns of forced migration, the reappropriation of space and the power for the multitude to determine the global circulation and the mixing of individuals and populations" (p.438) .

La question écologique est rapidement écartée. Les auteurs reprennent des positions de ce que Postone appelle le « marxisme classique », reconnaissant qu'un jour les ressources seront épuisées, mais que la seule solution est la destruction du capitalisme (p. 284). Mais ce n'est pas encore le cas, et trois raisons sont données à cela : le capitalisme est devenu écologique cad rapports conservationnistes à son envt ; les ressources sont plus abondantes que prévu ; enfin un saut qualitatif s'est produit dans l'ordre des machines, avec l'avènement de machines faites par des machines (p. 332). Les auteurs rappellent que Marx reconnaissait que le conflit entre machines et ouvriers était un faux conflit : « Il fallait à la fois du temps et de l'expérience avant que les travailleurs n'apprissent à distinguer entre le machinisme et son emploi par le capital, donc transférer leurs attaques des instruments matériels de production à la forme de société qui utilise ces instruments » (p. 442). Ils s'appuient sur Deleuze & Guattari pour soutenir qu'il faut accélérer la mondialisation du capitalisme et non lui résister (p. 239).

The ecological question is quickly dismissed. The authors take positions of what Postone calls "classical Marxism," recognizing that one day resources will be exhausted, but that the only solution is the destruction of capitalism (p.284). But this is not yet the case, and three reasons are given for this: capitalism has become ecological that is capable of integrating a conservationist perspective; resources are more abundant than expected; finally, a qualitative leap has taken place in the order of machines, with the advent of machines made by machines (page 332). The authors recall that Marx strongly underlines that the conflict between machines and workers was a false conflict: "It took both time and experience before the workers learned to distinguish between mechanization and its use by capital, hence to transfer their attacks from the material instruments of production to the form of society using these instruments "(p.442). They rely on Deleuze & Guattari to argue that the globalization of capitalism must be accelerated and not resisted (p.239).

La revue Multitude a récemment publié le Manifeste accélérationniste qui va dans le même sens. Les auteurs reconnaissent l'existence d'un réchauffement climatique et d'autres problèmes écologiques mais ils en appellent à Marx en tant que penseur « paradigmatique » de « l'accélérationnisme », reconnaissant que, « malgré toute son exploitation et toute sa corruption, le capitalisme constituait le système économique le plus avancé de son temps. Ses acquis ne demandaient pas à être renversés pour revenir à un état antérieur, mais à être accélérés au-delà des contraintes de la forme de valeur capitaliste ». Lénine est également évoqué, pour qui le socialisme « est impossible sans la technique du grand capitalisme, conçue d'après le dernier mot de la science la plus moderne, sans une organisation d'État méthodique qui ordonne des dizaines de millions d'hommes à l'observation la plus rigoureuse d'une norme unique dans la production et la répartition des produits. Nous, les marxistes, nous l'avons toujours affirmé ; quant aux gens qui ont été incapables de comprendre au moins cela (les anarchistes et une bonne moitié des socialistes-révolutionnaires de gauche), il est inutile de perdre même deux secondes à discuter avec eux. » (Sur l'infantilisme de gauche). Pour les auteurs le capitalisme ne saurait être l'agent d'une véritable accélération, et si « la gauche politique doit avoir un avenir, c'est dans la mesure où elle saura le mieux embrasser cette tendance accélérationniste refoulée ».

The journal Multitude has recently published the Acceleratization Manifesto which goes in the same direction. The authors acknowledge the existence of global warming and other ecological problems but they call on Marx as a "paradigmatic" thinker of "accelerationism," recognizing that "despite all its exploitation and corruption, capitalism was the most advanced economic system of its time. His achievements did not require to be overthrown to return to a previous state, but to be accelerated beyond the constraints of the capitalist value form ". Lenin is also evoked, for whom socialism "is impossible without the technique of great capitalism, conceived according to the last word of the most modern science, without a methodical organization of state that orders tens of millions of men to the most rigorous observation of a single standard in the production and distribution of products. We Marxists have always affirmed it; as for people who have been unable to understand at least that (anarchists and a good half of left-wing socialrevolutionaries), it is useless to lose even two seconds to arguing with them. (On the left infantilism). For the authors, capitalism can not be the agent of a real acceleration, and if "the political left must have a future, it is insofar as it will know best how to embrace this pent-up accelerationist tendency".

Dans Commonwealth (2010) Negri et Hardt ont sensiblement évolué, sous la pression de la réalité des mouvements sociaux. Il est toujours question de multitude mais la perspective mondiale et le réseau se sont sinon effacés du moins ont été profondément relativisés, au regard d'autres enjeux, qui peuvent être incompatibles avec la perspective de « l'accélération ». Dans les mouvements radicaux en effet Seattle et le réseau ont laissé la place au zapatisme et à la déclaration des Droits de la Nature à Cochabamba. Les penseurs se voient contraints de reconnaître une actualité au précapitalisme, telle qu'elle se donne à voir dans les mouvements sociaux. Leur interprétation de Marx évolue, voire change radicalement : ils voient maintenant une « ambivalence » de la tradition marxiste par rapport à la modernité : d'un côté centralité du mouvement ouvrier et soutien au développement industriel, de l'autre le soutien aux indigènes etc. et à la critique de l'industrie (p. 133). Lénine et surtout Mao sont désormais dépeints en critiques de la modernité (p. 137). Marx aurait souligné les vertus des communautés paysannes russes (p. 139). Ils concluent à une relation étroite entre modernité et antimodernité, citant Garcia Linera (p. 169). L'idée d'un secteur primaire et secondaire dépassés est reléguée au profit de la notion de « communs » défendue par Dardot et Laval, entendue comme richesse commune du monde matériel (air eau etc.), ce qui inclut les connaissances et les codes mais ceux-ci n'ont pas le privilège qui leur était accordé 10 ans plus tôt. Les luttes boliviennes sont jugées emblématiques d'un trait essentiel de l'altermodernité : son ancrage dans le commun. Trois champs d'enquête sont jugés essentiels pour définir l'altermodernité : l'exploration de voies alternatives au sein des Lumières européennes ; les mouvements de travailleurs et la résistance à la « colonialité » qui pour le coup prend une tournure spécifique, là où Homi Bhabha était raillé dix ans plus tôt pour la superficialité de ses analyses. Ils ne se départissent pas complètement d'une lecture positive du capitalisme et de l'accumulation : l'économie pousse à la « la métamorphose des sujets sociaux à travers l'éducation et l'apprentissage de la coopération, de la communication et de l'organisation des rencontres sociales ; et donc une accumulation progressive de commun. C'est ainsi que le capital creuse sa tombe, car en poursuivant ses intérêts propres et en essayant d'assurer sa survie, il est obligé d'encourager la puissance et l'autonomie de la multitude productive » (p. 441), « Si l'accumulation des puissances franchit un certain seuil, la multitude émergera, dotée de la capacité de dominer la richesse commune de manière autonome ». Désormais le « devenir-prince » de la multitude est « le processus par lequel la multitude apprend l'art de s'autogouverner et invente des formes démocratiques d'organisation sociale durables » (p. 442). « La forme-multitude n'est pas une clé magique qui ouvre toutes les portes mais elle formule de manière adéquate un vrai problème politique et se présente comme le modèle pour y répondre : elle est composée d'un ensemble de singularités sociales autonomes, capables, ensemble, en construisant leurs actions selon les chemins parallèles d'un réseau horizontal, de transformer la société » (p. 170).

In Commonwealth (2010) Negri and Hardt have evolved significantly under the pressure of social movements. Multitude is still at stake but the emergence of a global emancipatory subject and the centrality of the network have been deeply relativized, considering other issues, which may be incompatible with the prospect of "acceleration". In radical movements indeed Seattle and the network have given way to zapatism and the declaration of the Rights of Nature in Cochabamba. Authors are forced to recognize the actuality of precapitalism, as it can be seen in these social movements. Their interpretation of Marx evolves or even changes radically: they now see an "ambivalence" of the Marxist tradition in relation to modernity: on the one hand, the centrality of the labor movement and support for industrial development, on the other, support for the indigenous people and criticism of industry (p.133). Lenin and especially Mao are now portrayed as critics of modernity (p.137). Marx is said to have emphasized the virtues of Russian peasant communities (p.139). They conclude that there is a close relationship between modernity and antimodernity, quoting Garcia Linera (p.169). The idea of an outdated primary and secondary sector is relegated to the benefit of the notion of "common" defended by Dardot and Laval, understood as common wealth of the material world (air water etc.), which includes knowledge and codes but they do not have the privilege granted to them 10 years ago. The Bolivian struggles are considered emblematic of an essential trait of the altermodernity: its anchoring in the common. Three fields of inquiry are considered essential to define the altermodernity: the exploration of alternative paths within the European Enlightenment; the workers' movements and the resistance to "coloniality" which, for the time being, takes a specific turn, where Homi Bhabha was taunted ten years earlier for the superficiality of his analyzes. They do not abandon completely a positive reading of capitalism and accumulation: the economy paves the way to a "metamorphosis of social subjects through education and learning cooperation, communication and the organization of social gatherings; and therefore a progressive accumulation of common. This is how capital digs its grave, because by pursuing its own interests and trying to ensure its survival, it is obliged to encourage the power and autonomy of the productive multitude ". If the accumulation of powers crosses a certain threshold, the multitude will emerge, endowed with the capacity to dominate the common wealth autonomously". Therefore the "becoming-prince" of the multitude is "the process by which the multitude learns the art of self-governance and invents sustainable democratic forms of social organization" (p.442). "The form-multitude is not a magic key that opens all the doors but it adequately formulates a real political problem and presents itself as the model to answer it: it is composed of a set of autonomous social singularities, able together, by building their actions along the parallel paths of a horizontal network, to transform society "(170).

Ces thèses conforteraient un Jacques Ellul pour qui ce qui aliène la modernité est le transfert du sacré à la technologie (in La technique ou l'enjeu du siècle (1954) (1973), chapter 3," it is not the technique that enslaves us but the sacred transferred to the technique " 316). Ellul is not a technophobe as it is sometimes said, but he believes that the technique or technology is located outside the field of criticism. Aristocrats and technocrats are above the law. The technique is defined as the ideal of a mechanization of everything (1954,10); "It is this" one best way "which is strictly the technical medium and it is the accumulation of these means that gives a technical civilization" (1954,19). Ellul criticizes the belief in the technical solutions that actually result in a complexity of the problems rather than their resolution, like the ecological problem that believes with the technical "solutions" (Bluff 71). And innovation that covers everything else, says Ellul in 1988, is the computer (The Bluff, 18). André Gorz est un autre théoricien ayant été conduit à accorder une place centrale aux TIC. Dès 1967 dans Le socialisme difficile il commence à évoquer la crise de la rémunération à la quantité de travail, du fait de l'importance de la connaissance. Après avoir été existentialiste il est marqué à cette époque par une vision marxiste classique, quoiqu'avec un volet critique de la consommation assez marqué (Trentin -« néocapitalisme »). La Critique de la division du travail (1973) insiste sur la montée de l'automatisation des tâches. Le livre comprend une interview de Jay Forrester, l'un des rédacteurs du rapport du MIT au Club de Rome, qui voit venir une entreprise flexible, dans laquelle chacun négocierait son adhésion à une structure mouvante. A la suite de Trentin Gorz se démarque de « la plupart des marxistes » et ne pense pas que les forces productives soient « idéologiquement neutres et leur développement » « intrinsèquement positif » (p. 251). Il veut montrer trois choses : 1/ que le capitalisme développe des forces productives qui détruisent, nient et dissimulent les potentialités libératrices 2/ qu'une partie des forces productives est improductive voire parasitaire 3/ la contradiction entre forces productives et rapports de production ne s'explicite pas spontanément, aussi la critique doit-elle être interne aux FP, dans la perspective d'un au-delà. Science et technique ne peuvent donc pas être considérées comme idéologiquement « neutres », elles portent l'empreinte de l'idée que la bourgeoisie se fait de leur fonctions et leurs fins. La fonction des travailleurs dans ce domaine est donc à la fois technique et idéologique. Les écoles d'ingénieur servent à produire un type particulier d'individus « intégrés ». Cette critique n'est pas écologiste dans un premier temps mais elle s'y ouvre dans les années 1970. Dans Écologie & Politique 1978 Gorz affirme que « sans la lutte pour des techniques différentes, la lutte pour une société différente est vaine », à l'exemple du nucléaire. « Bien sûr, on objectera qu'il n'est pas possible de changer les outils sans changer la société et que, pour cela, il faut prendre le pouvoir d'Etat. Cette objection est juste à condition qu'on n'en conclue pas que le changement de société et de prise du pouvoir d'Etat doivent précéder les changements technologiques. Car sans changement des outils, le changt de société restera formel et illusoire ».

André Gorz is another theorist who gives a central place to ICT. From 1967 in The difficult socialism he evokes the crisis of the remuneration to the amount of work, because of the growing importance of the knowledge. His existentialist period is closed at the time, he's marked by a classical Marxist vision, albeit with a critical vision of consumption (following Trentin's analysis -"neocapitalism"). The Critique of the Division of Labor (1973) emphasizes the rise of automation. The book includes an interview with Jay Forrester, one of the editors of the MIT's report to the Club of Rome, who sees a flexible company coming up, in which everyone negotiates for membership in a changing structure. Following Trentin Gorz's position differ from "most Marxists" because he does not think that the productive forces are "ideologically neutral and their development" "intrinsically positive" (251). He wants to show three things: 1 / that capitalism develops productive forces that destroy, deny and conceal some liberating potentialities 2 / that part of the productive forces is unproductive or even parasitic 3 / that the contradiction between productive forces and production relations not explicit spontaneously, so the criticism must be internal to productive forces, in the perspective of a beyond. Science and technology can not therefore be considered as ideologically "neutral"; they bear the imprint of the bourgeoisie's idea of their functions and their ends. The function of workers in this field is therefore both technical and ideological. Engineering schools are used to produce a particular type of "integrated" individuals. This criticism cannot be called "ecologist" at first but Gorz is more and more open on the issue in the 1970s. In Ecology & Politics 1978 Gorz states that "without struggle for different techniques, the struggle for a different society is futile", referring to the example of nuclear power. "Of course, it will be objected that it is not possible to change the tools without changing the society and that, for that, it is necessary to take the power of State. This objection is just as long as it is not concluded that the change of society and the seizure of state power must precede technological change. Because without change of tools, the change of society will remain formal and illusory.

Cette critique s'approfondit dans Adieux au prolétariat -au-delà du socialisme, 1980. Trentin reste isolé, Gorz perd espoir dans le mouvement ouvrier et conclut à la nécessité de dissoudre aussi bien le marxisme que la classe ouvrière. Il évolue vers la thèse explicitement néostartrienne d'une société duale, avec d'un côté le travail, activité sérielle, hétéronome et de l'autre l'autonomie, qui ne peut se produire qu'en dehors. La microinformatique va tenir une place de choix. Dans Les chemins du paradis (1983) Gorz estime que la microinformatique nous emmène vers la fin du travail. « A la différence des mégatechnologies de la période industrialiste, qui faisaient obstacle à un développement décentralisé enraciné dans les communautés de base, l'automatisation en effet, est en elle-même socialement ambivalente. Alors que les mégatechnologies étaient des technologies-verrou, la micro-électronique est une technologie-carrefour : elle n'interdit ni n'impose un type de développement », elle mène aussi bien à une hyper centralisation qu'à l'autogestion ou à la centralisation autogérée. Il voit sur ce point une convergence avec Touraine pour qui l'alternative est entre des sociétés autonomes ou des sociétés programmées (p. 73). Dans tous les cas l'intérêt pour le travail baisse, d'où une désaffection qui sape les bases idéologiques et éthiques de l'industrialisme. Gorz en tire sa proposition d'un revenu à vie 20 000 heures (soit 2,5j/semaine), à réaliser à volonté au cours de sa vie. Gorz a la conviction que l'informatique ne peut pas inaugurer un nouveau cycle d'accumulation capitaliste, elle va plutôt raréfier le travail, réduire la consommation de ressources (p. 70). Il semble tout aussi acquis que Negri et Hardt aux promesses capitalistes en matière écologique. Lui aussi estime que les Grundrisse ont bien anticipé la situation, trouvant dans le fragment sur les machines l'idée d'un travail vivant qui tendrait vers zéro. Ceci conforte en outre l'idée suivant laquelle si le communisme est le stade où le plein développement des forces productives est accompli et où la tâche n'est plus de produire plus mais d'organiser l'économie, alors « nous avons virtuellement atteint ce stade » (p. 99). C'est une affirmation qu'il reprendra vers la fin de sa vie.

This critique deepens in Farewell to the proletariat -beyond socialism, 1980. Trentin remains isolated, Gorz loses hope in the labor movement as an emancipatory force and concludes the need to dissolve both Marxism and the working class. It evolves towards the explicitly neo-sartrian thesis of a dual society, with on the one hand labor, serial, heteronomous activity and on the other, autonomy, which can only occur outside. Microcomputing is a key part of this. In The Roads to Paradise (1983) Gorz believes that microcomputing means the end of work. "Unlike the megatechnologies of the industrialist period, which hindered decentralized development rooted in grassroots communities, automation in fact is in itself socially ambivalent. While megatechnologies were lock-technologies, microelectronics is a crossrads-technology: it does not prohibit or impose a type of development", it could lead to hyper centralization as well as selfmanagement or self-managed centralization. He sees on this point a convergence with Touraine for whom the alternative is between autonomous societies or programmed societies (p.73). In any case, subjective implication in work diminishes, because of its heteronomic character, hence a disaffection that undermines the ideological and ethical bases of industrialism. This context leads Gorz to his proposal of a lifetime income of 20,000 hours (2.5j / week), to be achieved at will during his life. Gorz is convinced that ICT can't pave the way to a new cycle of capitalist accumulation, it will rather diminish the quantity of work and reduce the consumption of resources (p.70). Gorz seems as convinced by capitalist ecological promis as Negri and Hardt. He too believes that the Grundrisse have anticipated the situation, finding in the Fragment on machines the idea of a living work that tends to zero. This further reinforces the idea that if communism is the stage where the full development of the productive forces is accomplished and the task is no longer to produce more but to organize the economy, then "we have virtually achieved this stage "(page 99). This is an affirmation that he will take again towards the end of his life.

L'arrivée de la microinformatique n'invalide pas entièrement la dimension écologique à laquelle Gorz avait adhéré dans les années 1970. Dans Capitalisme socialisme écologie 1991 Gorz affirme que « l'impératif écologique exige donc bien une décroissance de l'économie mais cette décroissance n'exige pas nécessairement de sacrifices ; elle exige seulement des renoncements » (p. 171) Il voit un changement de paradigme : « moins mais mieux » (p. 171). Ces thèses sont reprises dans Misère du présent, richesse du possible, Galilée, 1997 ainsi que dans L'immatériel -connaissance, valeur et capital, 2003. Gorz se montre à nouveau proche de Negri et de Hardt, évoquant la thèse du general intellect. Ecologica (2008) ne remet pas fondamentalement en cause cette perspective, bien que Gorz se mette à défendre le revenu de base, qu'il liait jusque-là à un travail. L'économie de la connaissance est vue comme mettant le capitalisme en crise. L'économie de la connaissance a « vocation » à être une économie de la mise en commun et de la gratuité, c'est-à-dire le contraire d'une économie (p. 21). Le hacker en est la figure paradigmatique (p. 25). La sortie du capitalisme a donc déjà commencé (p. 25) (Ecorev 2007). La décroissance est un impératif de survie, mais elle suppose une autre économie, d'autres rapports sociaux ; sans cella elle sera une économie de guerre (p. 26). De plus elle ne possède pas de traduction politique, et aucun gouvernement ne voudra la mettre en oeuvre. Cependant en raison de l'économie de la connaissance la décroissance de l'économie a déjà lieu et elle s'accentuera. La seule question est de savoir si elle prendra la forme de l'auto-org ou celle de la crise catastrophique. Une sortie du capitalisme et de l'industrialisme se dessine du côté de l'artisanat high tech et des digital fabricators (p. 119). Gorz estime que l'ouvrage de Robert Kurz Der Kollaps der Modernisierung, devenu une sorte de best-seller au Brésil, est prophétique, mais il dira également ne pas avoir eu le temps de s'approprier complètement l'école de la critique de la valeur, incarnée par Kurz, Postone et Jappe.

The arrival of microinformatics does not completely invalidate the ecological dimension Gorz had adhered to in the 1970s. In Capitalism socialism ecology 1991 Gorz states that "the ecological imperative therefore requires a decrease of the economy but this decrease does not necessarily require sacrifices; it requires only renunciations. "(p.171) He sees a paradigm shift:" less but better "(p.171). These theses are included in Misery of the Present, Richness of the Possible, 1997 and in The Intangible -Knowledge, Value and Capital, 2003. Gorz is again close to Negri and Hardt, evoking the thesis of the general intellect. Ecologica (2008) does not fundamentally question this perspective, although Gorz begins to defend the basic income, a revenue without a link to a job, a solution which he was discarding until then. The knowledge economy is still seen as putting capitalism in crisis, having a "vocation" to be an economy of pooling and free, that is to say the opposite of an economy as usually understood (p.21). The hacker is the paradigmatic figure (p.25). The exit of capitalism has already begun (page 25) (Ecorev 2007). Degrowth is an imperative for survival, but it supposes another economy, other social relations; without it, it will be a war economy (p.26). Moreover, it does not have a political support, and no government will want to implement it. However, due to the knowledge economy, the decline of the economy is already taking place and will be accentuated. The only question is whether it will take the form of self-management or that of the catastrophic crisis. Gorz sees an exit emerging from capitalism and industrialism through high-tech craft and digital fabricators (p.119). Gorz believes that Robert Kurz's book Der Kollaps der Modernisierung, which has become a kind of bestseller in Brazil, is prophetic, but he also says that he did not have the time to completely appropriate the school of value criticism, represented by Kurz, Postone and Jappe.

Gorz n'aura jamais reconnu de positivité au précapitalisme et ne se sera pas ouvert ni aux courants postmodernes ni aux analyses postcoloniales. Il reste marxiste dans la mesure où il adhère à la thèse de la ruse de lahistoire, suivant laquelle le capitalisme produit les moyens de son propre dépassement. Dans Capitalisme, socialisme, écologie, 1991 il s'en prend à l'utopie « désindustrialiste » des Verts les plus radicaux, estimant qu'elle « réactualise sous une forme régressive le projet de la société communiste. […] Le caractère pré-moderne de cette théorie éco-radicale réside en ce qu'elle n'envisage pas la naissance d'une société post-industrielle comme le résultat d'un développement par lequel le capitalisme se dépasserait luimême, mais comme une destruction due à des facteurs externes. La foi matérialiste-dialectique en un sens de l'histoire fait place à la foi quasi religieuse en la bonté (28) de la Nature et d'un ordre naturel, qu'il s'agirait de rétablir. Il y a donc entre les fondamentalismes « Vert » et religieux plus que des parentés circonstancielles. Il n'est pas exclu, d'ailleurs, que le fondamentalisme islamique ait recours à des armes biologiques ou nucléaires afin d'anéantir l'impie civilisation moderne avec sa propre technique scélérate. Du point de vue des fondamentalismes pré-modernes, tout le développement de la modernité a été, du début jusqu'à ce jour, un péché contre l'ordre naturel du monde. Son issue catastrophique obligera l'humanité à sa nécessaire conversion » (28 note 1 un fondamentaliste vert, Jürgen Dahl, a exposé cela dans Die Zeit 23 nov 1990 -idée que seule la pauvreté peut nous sauver). Et si on misait sur les sociétés modernes ? Question ouverte. (29) Dans Écologie & Politique 1978 Gorz affirmait déjà que « la lutte écologique n'est pas une fin en soi, c'est une étape ». Dans Adieux au prolétariat Gorz affirmait également qu'une société ne peut résorber entièrement d'hétéronomie que dans les sociétés monacales auxquelles Illich aspirait, selon lui. Ceci conduit Gorz à éluder les questions qui lui ont été posées à propos de l'impact écologique du numérique.

Gorz has never recognized any positivity for precapitalism and has also never been open to postmodern trends or postcolonial analysis. He remains Marxist insofar as he adheres to the thesis of the "cunning of reason", according to which capitalism produces the means of its own transcendence. In Capitalism, Socialism, Ecology, 1991 he attacks the "deindustrialist" utopia of the most radical Greens, believing that it "updates in a regressive form the project of the communist society. [...] The pre-modern character of this ecoradical theory lies in the fact that it does not envisage the birth of a post-industrial society as the result of a development by which capitalism would surpass itself, but as a destruction due to external factors. Materialistic-dialectical faith in one sense of history gives way to almost religious faith in the goodness (28) of Nature and a natural order, which it would be necessary to restore. There is therefore between the "Green" and religious fundamentalisms more than circumstantial kinships. It is not excluded, moreover, that Islamic fundamentalism has recourse to biological or nuclear weapons in order to annihilate the impious modern civilization with its own villainous technique. From the point of view of pre-modern fundamentalisms, the whole development of modernity has been, from the beginning to the present, a sin against the natural order of the world. Its catastrophic outcome will force humanity to its necessary conversion "(28 note 1 a green fundamentalist, Jürgen Dahl, exposed this in Die Zeit 23 Nov 1990 -idea that only poverty can save us). And what if we bet on modern societies? He asks. (29) In Ecology & Politics 1978 Gorz already affirmed that "the ecological struggle is not an end in itself, it is a stage". In Farewell to the proletariat Gorz also affirmed that a society can not be entirely reduced to heteronomy except in the monastic societies to which Illich aspired, according to him. This leads Gorz to avoid questions about the ecological impact of digital.

Ellul verrait à nouveau dans ce refus la marque du sacré, de ce qu'on ne peut critiquer. Un lecteur d'Ellul, Theodor Kasczynscki, alias Unabomber, appellera à l'action directe pour stopper cette avancée des systèmes numériques, ne voyant d'autre issue. Il provoqua la rédaction de cette célèbre lettre de Bill Joy, le fondateur de Java : Pourquoi l'avenir n'a pas besoin de nous, dans laquelle l'auteur soutient une position de moratoire, à la la limite d'une position luddite de destruction de la technique. Le débat avec les Luddites, ce mouvement de briseurs de machine auquel Marx fait allusion et qui se tint entre 1810 et 1812 dans les Midlands anglais, n'est pas terminé.

Ellul would see again in this refusal the mark of the sacred, of what can not be criticized. An Ellul reader, Theodor Kasczynscki, alias Unabomber, will call for direct action to stop this advance of digital systems, seeing no other way out. He caused the writing of this famous letter from Bill Joy, the founder of Java: Why the future does not need us, in which the author supports a position of moratorium, to the limit of a Luddite position of destruction of the technique. The debate with the Luddites, the movement of machine breakers to which Marx refers and which was held between 1810 and 1812 in the English Midlands, is not over.

  , réed. Economica 1990 pp. 130-134 ; "Le sacré dans le monde moderne"(1963), in revue Le Semeur n°2, pp. 24-36 ; Les nouveaux possédés(1973), chapitre 3, « ce n'est pas la technique qui nous asservit mais le sacré transféré à la technique » p. 316). Ellul n'est pas un technophobe comme on le dit parfois mais il estime que la technique ou technologie est située en-dehors du champ de la critique. Aristocrates et technocrates sont au-dessus des lois. La technique est définie comme l'idéal d'une mécanisation de tout (1954, p. 10) ; « c'est ce « one best way » qui est à proprement parler le moyen technique et c'est l'accumulation de ces moyens qui donne une civilisation technique » (1954, p. 19). Ellul critique la croyance dans les solutions techniques qui se traduisent en réalité par une complexification des problèmes plutôt que par leur résolution, à l'exemple du problème écologique qui croit avec les « solutions » techniques (Le Bluff p. 71). Et linnovation qui recouvre tout le reste, dit Ellul en 1988, c'est l'ordinateur (Le bluff, p. 18). These theses would support a Jacques Ellul's position for which what alienates the modernity is the transfer of the sacred to the technology (in The technique or the stake of the century (1954), reed Economica 1990 pp. 130-134; "The sacred in the modern world "(1963), in review Le Semeur No. 2, pp. 24-36; The new possessed


Marco Aggravi
email: marco.aggravi@irisa.fr

Florent Pausé
email: florent.pause@inria.fr.

Paolo Robuffo Giordano

Claudio Pacchierotti
email: claudio.pacchierotti@irisa.fr

Vibrotactile Stimuli

Keywords: Haptics and Haptic Interfaces, Wearable Robots, Telerobotics and Teleoperation

I. INTRODUCTION

W EARABLE haptic devices have shown promising results in the fields of robotic teleoperation [START_REF] Pacchierotti | Cutaneous haptic feedback to ensure the stability of robotic teleoperation systems[END_REF], [START_REF] Bimbo | Teleoperation in cluttered environments using wearable haptic feedback[END_REF], rehabilitation [START_REF] Bortone | Serious game and wearable haptic devices for neuro motor rehabilitation of children with cerebral palsy[END_REF], guidance [START_REF] Scheggi | Cooperative Navigation for Mixed Human-Robot Teams Using Haptic Feedback[END_REF], [START_REF] Culbertson | WAVES: A Wearable Asymmetric Vibration Excitation System for Presenting Three-Dimensional Translation and Rotation Cues[END_REF], and gaming [START_REF] Maisto | Evaluation of wearable haptic systems for the fingers in augmented reality applications[END_REF], [START_REF] Bye | Tricking the brain is the only way to achieve a total haptics solution[END_REF]. They can be easily worn by the human user and are able to provide effective, rich, localized, and private stimuli. Moreover, wearable interfaces can also easily enable the engaging of multicontact interactions and operations [START_REF] Scheggi | Touch the virtual reality: using the leap motion controller for hand tracking and wearable tactile devices for immersive haptic rendering[END_REF]. Finally, ungrounded haptic devices have also been proven to guarantee the stability and safety of haptic-enabled teleoperation loops [START_REF] Pacchierotti | Cutaneous haptic feedback to ensure the stability of robotic teleoperation systems[END_REF], [START_REF] Pacchierotti | Enhancing the performance of passive teleoperation systems via cutaneous feedback[END_REF].

As per the location of the wearable device, the forearm has been often chosen as an effective place for providing haptic guidance and feedback in various robotics-related applications, such as robotic teleoperation and human-robot interaction. For example, Bark et al. [START_REF] Bark | Rotational skin stretch feedback: A wearable haptic display for motion[END_REF] presented a wearable haptic feedback device able to impart rotational skin stretch to the hairy skin of the forearm. Subjects were able to use feedback from the device to control the positioning of a virtual object within ±6.5 degrees of the total range of motion. Stanley and Kuchenbecker [START_REF] Stanley | Evaluation of tactile feedback methods for wrist rotation guidance[END_REF] evaluated ten different forms of tactile feedback for wrist rotation guidance. The devices are capable of tapping, dragging across, squeezing, twisting, or vibrating against the user's wrist. Results showed that directional responses are fastest when direction is conveyed through the location of the tactile stimulus or steady lateral skin stretch. More recently, Casini et al. [START_REF] Casini | Design and realization of the CUFF-clenching upperlimb force feedback wearable device for distributed mechano-tactile stimulation of normal and tangential skin forces[END_REF], [START_REF] Bianchi | A fabric-based approach for wearable haptics[END_REF] developed a wearable device, called CUFF, for the haptic stimulation of the user's arm with pressure and stretch cues. It is composed of two DC motors attached to a fabric cuff worn around the arm. Scheggi et al. [START_REF] Scheggi | Cooperative Navigation for Mixed Human-Robot Teams Using Haptic Feedback[END_REF] presented a cooperative human-robot navigation system in which vibrotactile armbands provide the human wearer with information about the motion of a mobile robot, keeping the two agents in a soft formation. Chinello et al. [START_REF] Chinello | Design of a wearable skin stretch cutaneous device for the upper limb[END_REF] used four pulleys, worn evenly around the arm, to provide skin stretch at the palmar, dorsal, ulnar, and radial sides of the arm.

Within this context, this paper discusses a novel wearable haptic device and its evaluation in robotic teleoperation. The proposed device is able to provide cutaneous haptic feedback to the forearm through pressure, vibrotactile, and skin stretch haptic feedback, as shown in Fig. 1. The device consists of two servo motors moving an elastic belt placed in contact with the user's arm skin. Similarly to [START_REF] Casini | Design and realization of the CUFF-clenching upperlimb force feedback wearable device for distributed mechano-tactile stimulation of normal and tangential skin forces[END_REF], when the motors spin in opposite directions, the belt tightens or loosens around the user's arm, while when the motors spin in the same direction, the belt applies a shear force to the skin. Moreover, it also houses four vibrotactile motors, positioned evenly around the arm at 90 degrees from each other. The device weights 220 g for 115 × 122 × 50 mm of dimensions. Combining these three types of haptic stimuli (pressure, skin stretch, vibrotactile) in such a compact package, the device can provide a wide set of feedback information to the human wearer in a comfortable and unobtrusive way [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF]. We present the design and characterization of the device, as well as the results of two human-subject teleoperation experiments in a virtual environment. In the first experiment, participants were asked to control the motion of a simulated 6-degrees-of-freedom (6-DoF) robotic manipulator for grasping an object; in the second experiment, participants were asked to teleoperate the motion of a fleet of ten simulated drones along a given path. In both scenarios, the proposed haptic interface provided feedback information about the status of the slave robot(s) and of the given task.

II. THE HAPTIC DEVICE

A prototype of our device is shown in Fig. 1b, while Fig. 1a indicates its main parts. Referring to Fig. 1a, the device consists of an ABS static platform (A) that houses two HS-625MG servo motors (B) and two pulleys (C), a 6-cm-wide elastic fabric belt (D) that applies the requested stimuli to the skin, and four 307-100 Pico Vibe vibrotactile motors (E) positioned evenly around the arm at 90 degrees from each other. A strap band is used for securing the device on the forearm (see Fig. 1b).

The cutaneous device can provide pressure and skin stretch stimuli by moving the fabric band. When the two servo motors rotate in opposite directions, the belt is tightened or loosened, providing a force normal to the arm skin. On the other hand, when motors spin in the same direction, the belt applies a shear force to the arm skin. The servomotors are position controlled, which means that it is only possible to command a desired angle. The relationship between the commanded angle and belt displacement for each motor is ∆b c,i = r∆θ i , i = 1, 2, being r = 12.2 mm the radius of the servo motor pulley, ∆b c,i the commanded belt displacement due to the motion of motor i, and ∆θ i the i-th motor commanded angle, expressed in radians (see Fig. 1a). In our case, the two motors always rotate of the same amount, i.e., |∆θ

1 | = |∆θ 2 | and |∆b c,1 | = |∆b c,2 |.
As introduced above, the motion of the two motors results in either a belt lateral displacement ∆b l,c (if the motors rotate in the same direction) or in a tightening of the belt ∆b p,c (if the motors rotate in opposite directions),

∆b l,c = sgn(∆θ2)∆bc,2 if sgn(∆θ1) = sgn(∆θ2) ∆bp,c = 2 sgn(∆θ2)∆bc,2 if sgn(∆θ1) = sgn(∆θ2).
(1)

III. DEVICE CHARACTERIZATION

To understand how to correctly modulate the device motor inputs, we evaluated the (i) belt shearing effect on the skin when moving the servo motors in the same direction, the (ii) normal force applied on the skin when moving the servo motors in opposite directions, and the correspondent (iii) differential thresholds of perception. Moreover, we used the characterization of [START_REF] Aggravi | Haptic wrist guidance using vibrations for Human-Robot teams[END_REF] to effectively modulate the vibromotors inputs for guidance. These four mappings will be used in Sec. IV for controlling the device in the two experiments.

A. Skin stretch stimuli

This Section analyzes the real belt lateral displacement ∆b l,r resulting from a commanded belt lateral displacement ∆b l,c (see [START_REF] Pacchierotti | Cutaneous haptic feedback to ensure the stability of robotic teleoperation systems[END_REF]). In fact, although using an elastic fabric band guarantees high comfort and ease of use, it introduces uncertainties in the actuation system, i.e., ∆b l,c may differ from ∆b l,r . A human user (male, 22 years old, wrist 5.4 cm, arm 9.2 cm) was asked to wear our device on his right forearm, as shown in Fig. 2. The device laid on a table, with the belt facing up. The user was asked to keep his arm still during the experiment. Twelve black dots were drawn on his forearm, six between the device and the wrist and six between the device and the elbow. Each block of six dots was arranged in a 3×2 array, with a dot-to-dot distance of 2 cm. The belt was then commanded to move clockwise and counterclockwise around the arm, ∆b l,c = [-19.2, -12.8, -8.5, -4.3, 4.3, 8.5, 12.8, 19.2] mm, which means rotating both motors of [-90, -60, -40, -20, 20, 40, 60, 90] degrees (see [START_REF] Pacchierotti | Cutaneous haptic feedback to ensure the stability of robotic teleoperation systems[END_REF]). After each stretch stimulus, the experimenter took a picture of the setup, in order to measure the real displacement of the belt ∆b l,r and the stretch of the and skin stretch ∆s resulting from a commanded belt displacement ∆b l,c . To make the graph easier to read, we aggregated the data coming for neighboring points on the skin, showing their mean displacement three by three.

skin around the belt ∆s = [∆s 1 , . . . , ∆s 12 ] T . The device was well tighten to the arm for preventing any slippage of the belt w.r.t. the skin. Figure 3 shows the commanded belt displacement ∆b l,c vs. the real belt displacement ∆b l,r and the stretch of the twelve points on the skin ∆s. It is interesting to notice that proximal points follow more closely the motion of the belt. Moreover, medial commanded belt displacements (negative values in Fig. 3) result in larger skin stretches and in larger actual belt displacements with respect to lateral (positive) commanded belt displacements.

In the experiments of Sec. IV, to better secure the device on the forearm, we use a second stationary belt, which fixes the upper static platform to the proximal side of the arm (see Fig. 1b). While this solution is very simple and effective, it prevents part of the forearm from stretching properly (i.e., the area around points 10-12 in Fig. 2). As indicated in Sec. V, in the future, we will study different approaches for fixing the device onto the forearm, with the objective of eliciting the most effective sensations.

B. Pressure stimuli

This Section analyzes the applied squeezing action ϕ p given a commanded tightening of the belt ∆b p,c . The same human user as in Sec. III-A was asked to wear again our cutaneous device on his right forearm, as in Fig. 2. A FSR 402 force sensor was placed on the belt, opposite to the static platform housing the motors and in contact with the volar skin of the arm (near point b l,r in Fig. 2). The cutaneous device was then commanded to tighten the belt around the arm, shortening it every 1.5 mm from 0 to 76.5 mm, i.e., ∆b p,c = [0, 1.5, 3, . . . , 76.5] mm. After each squeeze stimulus, the experimenter saved the force ϕ p applied by the belt on the arm. The belt was kept in place for 5 s and the sensor values averaged. The starting force applied to the arm, i.e., when ∆b p,c = 0, was 2.5 N. This procedure was repeated three times and the resulting data averaged. Figure 4 shows the commanded belt squeeze ∆b p,c vs. the average squeezing force ϕ p .

C. Differential thresholds

The differential threshold, often referred to as JND, gives us information about how different two stimuli generated by our device need to be in order to be perceived as different by a human user. Ten participants took part in the experiment (two women and eight men). We evaluated the differential thresholds using the method of constant stimuli, both for the skin stretch and for the squeezing stimuli. This method involves the presentation of different stimulus levels (e.g., skin stretch) in random order. Unlike the methods of limits, this approach employs a nonsequential procedure: a step size and range of intensities is selected which, based upon previous experience, include the target threshold level. The subject's task is to say whether two stimuli are the different or the same. We tested the stretch JND at six standard stimuli, ∆b l,c = [-10.5, -6.3, -2.1, 2.1, 6.1, 10.5] mm, with a stepsize of 0.1∆b l,c between the considered standard stimulus ∆b l,c and the maximum tested value 1.5∆b l,c . Similarly, we tested the squeeze JND at four standard stimuli, ∆b p,c = [7.7, 23.0, 38.3, 53.7] mm, with a step-size of 0.1∆b p,c between the considered standard stimulus and the maximum tested value 1.5∆b p,c . Subjects were required to wear the cutaneous device as shown in Fig. 1b and tell the experimenter when the two provided stimuli felt different. Each participant performed 18 trials of the squeezing stimuli procedure, with four repetitions for each reference stimulus, and 12 trials of the skin stretch stimuli procedure, with two repetitions for each reference stimulus. The average JND was calculated by finding individual JNDs and then averaging them together. The device showed a differential threshold at the 75% level of 33.2% and 24.7% of the standard stimulus for the skin stretch and squeeze, respectively, which is in agreement with previous results in the literature [START_REF] Chinello | Design of a wearable skin stretch cutaneous device for the upper limb[END_REF], [START_REF] Pacchierotti | The hRing: a wearable haptic device to avoid occlusions in hand tracking[END_REF].

IV. EXPERIMENTAL EVALUATION

A. Experiment #1: shared-control of a robotic telemanipulator 1) Experimental setup: this first evaluation considers a shared-control teleoperation experiment in a virtual environment. It is inspired by [START_REF] Abi-Farraj | A visualbased shared control architecture for remote telemanipulation[END_REF], that describes the algorithmic design of a shared-control architecture for visual-based teleoperation. In [START_REF] Abi-Farraj | A visualbased shared control architecture for remote telemanipulation[END_REF], a grounded kinesthetic interface has been used for providing force cues, but no -even minor -evaluation was presented. The experimental setup is shown in Fig. 5. The master side consists of the human operator, wearing the proposed cutaneous device, and a Leap Motion controller in charge of tracking the position and orientation of the user's hand. The slave side consists of a 6-DoF Viper S850 robotic arm carrying a gripper, simulated using the popular V-REP environment, and two objects to grasp. The autonomous algorithm proposed in [START_REF] Abi-Farraj | A visualbased shared control architecture for remote telemanipulation[END_REF] is in charge of always keeping the manipulator gripper pointed towards the object to be grasped (2 DoF) with the aim of assisting the human operator in the grasping task. At the same time, the human operator is given the possibility to steer the slave end-effector along the remaining 4 null-space directions (w.r.t. the main pointing task) using the Leap Motion controller.

2) Methods: As explained, the autonomous algorithm controls 2 DoF of the robotic manipulator by keeping the gripper always oriented towards the object to be grasped. The gripper is therefore constrained to move on the surface of a virtual sphere centered at the object. The human operator can instead actuate the remaining 4 DoF in the null-space of the autonomous pointing task, that is, moving the gripper across the surface of the sphere, toward/away from the object (i.e., changing the radius of the sphere), and rotating the gripper along the pointing direction z towards the object (see Fig. 5). The haptic cues introduced in [START_REF] Abi-Farraj | A visualbased shared control architecture for remote telemanipulation[END_REF] are meant to inform the operator about how close the slave manipulator is to given constraints (singularities and joint limits in the considered scenario). These constraints are encoded in a global cost function H(q), with q ∈ R 6 being the manipulator joint configuration vector. The force cues τ ∈ R 4 , displayed on the 4 DoF actuated by the operator, are then obtained by projecting the joint motion caused by the i-th user command (i = 1, . . . , 4) onto the negative gradient of H(q): in this way, the operator is informed about the direction (in the space of the user commands) along which H(q) would diminish or, equivalently, along which would move away from the constraints (again, we refer to [START_REF] Abi-Farraj | A visualbased shared control architecture for remote telemanipulation[END_REF] for details). The operator can then easily react to the provided cues by adjusting the commanded motion in order to keep away from the constraints, an action that would be, otherwise, hard to infer and implement by simply looking at the slave manipulator.

In our case, we used the wearable device for providing the operator with the above-mentioned force cues τ = [ τ 1 , τ 2 , τ 3 , τ 4 ] T ∈ R 4 , being τ 4 the torque cue corresponding to the rotation of the gripper along the direction towards the object, and

τ l = [ τ 1 , τ 2 , τ 3 ]
T the remaining three cues corresponding to translations of the gripper along its x, y, z axes, respectively.

In particular, we chose to display τ l by means of vibrotactile cues, while τ 4 was provided through skin stretch stimuli. This choice is motivated by successful past experiences in providing similar information. Aggravi et al. [START_REF] Aggravi | Haptic wrist guidance using vibrations for Human-Robot teams[END_REF] used four vibrating motors at the wrist to guide an operator along a 3D path, while Chinello et al. [START_REF] Chinello | Design and evaluation of a wearable skin stretch device for haptic guidance[END_REF] used skin stretch at the wrist to convey rotation guidance. In what follows, we details how such haptic cues are obtained. Each component of τ is processed so that it is zeroed if lower, in absolute value, than a threshold th τ i = [ 0.01 N, 0.01 N, 0.01 N, 0.1745 Nm ], and it is constrained between maximal values T i = [ 15 N, 15 N, 15 N, 2/π Nm ], that is, with i = 1, . . . , 4,

τ * i = 0 if |τ i | ≤ th τ i max(min(τ i /T i , 1), -1) if |τ i | > th τ i .
With such regularized values for τ , we compute the frequencies for the vibrotactile motors and the angular values for the servo motors. For the former, we need to map six possible directions (left, right, top, down, backward, and forward) to the four available motors (Top, Right, Down, and Left, see Sec. II). Similarly to [START_REF] Aggravi | Haptic wrist guidance using vibrations for Human-Robot teams[END_REF], we directly map the left L and right R motors to the force cues on the y-axis of the gripper (see Fig. 5), that is

f R = 0, f L = -280τ * 2 if sgn(τ 2 ) < 0 f L = 0, f R = 280τ * 2 if sgn(τ 2 ) > 0 . (2)
Then, we use the top motor T to tell the user to both "go up" and "go forward", and the down motor D to tell the user to both "go down" and "go backward" [START_REF] Aggravi | Haptic wrist guidance using vibrations for Human-Robot teams[END_REF]. Of course, motors T and D need different vibration patterns for the two different directions they convey, i.e., "go up" should have a different pattern from "go forward", and "go down" should have a different vibration pattern from "go backward". This is achieved by changing the interval between each vibration. In particular, T will vibrate in bursts of β T = 100 ms for "go forward" and of β T = 200 ms for "go up". Similarly, D will vibrate in bursts of β D = 100 ms for "go backward" and of β D = 200 ms for "go down". To avoid sudden and possibly confusing changes between up/forward or down/backward stimuli, we weight the forward/backward feedback τ 1 more than the top/down feedback τ 3 by multiplying the former of δ = 1.5. Finally, to obtain vibration inputs f T , f D , β T , and β D , we compare τ 1 and τ 3 as follows

     f T = 280τ * 1 , β T = 100 ms if τ 3 = 0 and δτ 1 > th τ 1 or τ 3 = 0 and δτ 1 ≥ |τ 3 | f T = -280τ * 3 , β T = 200 ms if τ 1 = 0 and τ 3 < -th τ 1 or τ 1 = 0 and τ 3 < |δτ 1 | , (3)
     f D = -280τ * 1 , β D = 100 ms if τ 3 = 0 and δτ 1 < -th τ 1 or τ 3 = 0 and δτ 1 ≤ |τ 3 | f D = 280τ * 3 , β D = 200 ms if τ 1 = 0 and τ 3 > th τ 1 or τ 1 = 0 and τ 3 > |δτ 1 | , (4)
As per the stretch, the commanded angle for each servo motor comes directly from τ * 4 as ∆b l,c = 19.2 τ * 4 .

(

) 5
The desired velocity for the robot end-effector at time t, v ee (t), is obtained by tracking the user's hand with the Leap (e) Perceived effectiveness Fig. 6. Experiment #1. The task consisted of steering the robotic arm to grasp two target objects. Completion time, path followed by the robot, haptic actuation, gripper rotation error, and perceived effectiveness (with range from 1 to 9) provided a measure of performance. Mean and 95% confidence interval are plotted.

Motion in the following way. The keyboard works as a clutch and, when the space key is pressed, the current pose of the user's hand with respect to the Leap Motion base 3) is the rotation matrix between the Leap Motion frame and the hand frame, and p H ∈ R 3 is the position of the hand (see Fig. 5).

H(t) ∈ SE(3) is saved, H(0) = H(t)
As the hand moves, the desired end-effector velocity is calculated from the position of the hand p H (t) w.r.t. the initial value p H (0) saved when the user started pressing the clutch,

v ee (t) = B ee p H (t) -p H (0) ψ H (t) -ψ H (0) ,
where ψ H (t) is the yaw angle of the hand w.r.t. the Leap Motion frame, and B ee ∈ R 4×4 is a diagonal matrix of gains. This clutching approach allows the user to start the motion with the hand at an arbitrary position with respect to the Leap Motion, enabling him to pause, move to a more comfortable or suitable position, and then resume the control of the robot.

The main advantage of this position-velocity control mode (also used in [START_REF] Abi-Farraj | A visualbased shared control architecture for remote telemanipulation[END_REF]) is that the operator is not limited by the workspace of the Leap Motion. The V-REP simulation is shown in a screen placed in front of the user. The task consists of steering the robotic arm towards two objects to be grasped. Once the first object (placed on the right hand side of the manipulator) is reached, the second object becomes the target one, and the user is requested to steer the robotic arm towards it. The task started when the subject moved the robot for the very first time and finished when the subject reached the second object. As in [START_REF] Abi-Farraj | A visualbased shared control architecture for remote telemanipulation[END_REF], the grasping phase was not considered, and subjects were only requested to place the gripper close to the target object. However, subjects were also asked to orient the gripper such that it would be able to correctly grasp the objects. A video of the experiment is available as supplemental material. Each participant made six trials of the grasping task, with three randomized repetitions for each feedback condition: (C) cutaneous feedback provided by the wearable device, (N) no haptic feedback.

In condition C, the haptic feedback conveyed by our cutaneous device provides the human operator with information about the feasibility of the user's commands with respect to possible constraints of the robotic system, as detailed in (2)- [START_REF] Culbertson | WAVES: A Wearable Asymmetric Vibration Excitation System for Presenting Three-Dimensional Translation and Rotation Cues[END_REF]. Pressure (squeeze) feedback is not used in this experiment. In condition N, the human operator does not receive any haptic information. In both conditions, the subject is able to see the simulated environment on the screen. Trials were randomized to avoid any learning effect. Data from the same experimental condition and subject were averaged before the analysis [START_REF] Prattichizzo | Cutaneous force feedback as a sensory subtraction technique in haptics[END_REF].

3) Subjects: Fourteen participants took part in the experiment, including four women and ten men (age 21 -31). The experimenter explained the procedures and adjusted the setup to be comfortable for each user. One practice trial was allowed.

4) Results: In order to evaluate the effectiveness of our teleoperation system and the usefulness of wearable haptic feedback, we evaluated (i) the completion time, (ii) the robot path length, (iii) the amount of haptic stimuli provided during the task, and (iv) the perceived effectiveness of the different conditions. All data passed the Shapiro-Wilk normality test.

Figure 6a shows the average task completion time. A paired-samples t-test determined that the task completion time differed statistically significantly between conditions (t(13) = 2.312, p = 0.038, a = 0.05). Figure 6b shows the average linear motion covered by the robotic gripper during the task. A paired-samples t-test determined that this average linear motion differed statistically significantly between conditions (t(13) = 6.952, p < 0.001, a = 0.05). Figure 6c shows the percentage of time the cutaneous device was activated, either providing vibrations or skin stretch. This metric measures how much the user deviates from the constrained trajectory defined by the shared-control algorithm. In condition N, when users did not wear the cutaneous device, this metric indicates the percentage of time the cutaneous device would have been activated if worn. A paired-samples t-test determined that the percentage of time the cutaneous device was activated differed statistically significantly between conditions (t(13) = 2.358, p = 0.035, a = 0.05). Figure 6d shows the root-mean-square of τ * 4 , which indicates the error in following the gripper rotational cues during the task, provided through skin stretch feedback (see [START_REF] Culbertson | WAVES: A Wearable Asymmetric Vibration Excitation System for Presenting Three-Dimensional Translation and Rotation Cues[END_REF]). This metric assesses the benefits of the skin stretch guidance. A paired-samples t-test determined that this rotational error differed statistically significantly between conditions (t(13) = 2.899, p = 0.012, a = 0.05). Immediately after the experiment, subjects were also asked to report the effectiveness of both feedback conditions in completing the given task using bipolar Likert-type ninepoint scales. Fig. 6e shows the perceived effectiveness. A Wilcoxon signed rank test showed a statistically significant difference between the means of the two feedback conditions (z = 3.304, p = 0.001, a = 0.05). Finally, twelve out of fourteen subjects found using wearable haptic feedback to be the most effective at completing the grasping task. B. Experiment #2: teleoperation of Unmanned Aerial Vehicles 1) Experimental setup: this second evaluation considers an experiment of teleoperation of a fleet of ten quadrotors, simulated again using V-REP. It is inspired by [START_REF] Giordano | A passivity-based decentralized strategy for generalized connectivity maintenance[END_REF], [START_REF] Secchi | Bilateral Control of the Degree of Connectivity in Multiple Mobile-robot Teleoperation[END_REF], where it is presented the algorithmic design of a connectivitymaintenance strategy for telenavigating a group of quadrotors in a cluttered environment. However, no human-subject evaluation was presented. The connectivity-maintenance algorithm of [START_REF] Giordano | A passivity-based decentralized strategy for generalized connectivity maintenance[END_REF], [START_REF] Secchi | Bilateral Control of the Degree of Connectivity in Multiple Mobile-robot Teleoperation[END_REF] considers sensing/communication constraints among the robot pairs (max. range and line-of-sight occlusions) as well as other requirements, such as inter-robot and obstacle avoidance, and formation control. By properly designing the weights on the edges of the multi-robot sensing/communication graph, it is possible to encode the mentioned constraints and requirements into a single scalar quantity λ 2 , i.e., the second-smallest eigenvalue of the (state-dependent) Laplacian matrix associated to the interaction graph. In particular, the weights are designed so that if λ 2 = 0 the interaction graph is disconnected or a collision with an obstacle/robot has occurred, and λ 2 > 0 otherwise. The agents then implement a gradient-based action aimed at maintaining λ 2 (t) > 0 at all times despite the possible presence of additional motion commands (e.g., one or multiple quadrotors being steered by an external planner or a human operator). Finally, as explained in [START_REF] Secchi | Bilateral Control of the Degree of Connectivity in Multiple Mobile-robot Teleoperation[END_REF], the user can also set at runtime a minimum reference value λ d 2 > 0 for λ 2 , forcing the group to become more or less "connected" during motion by increasing or decreasing the minimum threshold λ d 2 . In our proposed experimental setup, shown in Fig. 7, the human user commands only one robot inside the formation, the leader, by providing a desired velocity 3D vector. The other robots are then forced to follow the leader by the enforced connectivity-maintenance controller since, otherwise, the graph would eventually become disconnected as the leader navigates in the environment. As explained, the connectivity-maintenance controller enforces, at the same time, both inter-robot and obstacle collision avoidance. We also note that the leader motion is influenced by the operator (who is commanding its desired velocity), but also by the connectivity maintenance action which is representative of the whole group status (e.g., if a robot in the group is lagging behind because of an obstruction, the leader would be automatically slowed down and eventually stopped to avoid breaking the graph connectivity). Therefore, by mapping (via force cues) the leader's response to the human commands, the operator is informed about the whole group status/mobility. As in Sec. IV-A, the master side consists of the human operator, wearing the proposed cutaneous device, and a Leap Motion controller.

2) Methods: The desired velocity for the leader at time t, qd 1 (t), is obtained using the same reasoning as in Sec. IV-A1. By clutching the user's hand position and giving the robot a velocity proportional to the distance between the current hand position p H (t) and its initial value p H (0), we have

qd 1 (t) = B q p H (t) -p H (0)
, where B q ∈ R 3×3 is a diagonal matrix of gains. The closure of the hand, similarly to [START_REF] Bimbo | Teleoperation in cluttered environments using wearable haptic feedback[END_REF], is instead computed for setting the value of the connectivity threshold λ d 2 . The V-REP simulation of the environment is shown in a screen in front of the user.

The task consists in following a predefined 3D path, passing through M = 20 via-points, P = {x 1 , . . . , x M }, x k ∈ R 3 . The position of these points is unknown to the user. After the second-last point x M -1 , the target path force the leader -and possibly the entire formation -to pass through an aperture in the virtual scene (between the two red walls in Fig. 7). To do so, the user needs to compact the team by increasing the desired connectivity value λ 2 by closing her/his hand. We set this target λ 2 value to be λ 2,final = 3. A video of the experiment is available as supplemental material. Similarly to Sec. IV-A, we considered two feedback conditions, V and CV.

In condition V, information about the location of the next viapoint is provided by a blue segment spawning from the leader robot toward the via-point (see the leader views in Fig. 7). The length of the segment increases linearly with the distance between the leader and the via-point: 0 m when the leader is on the via-point and 1 m when the leader is farer than 1 m from the via-point. In addition to navigation information, when users are trying to increase the value of λ 2 by closing their hands, the difference between the desired and current values of λ 2 is shown on the screen. In condition CV, we show again the blue segment pointing at the next via-point, as in condition V. However, we also provide guiding vibrotactile stimuli.

The considered guiding method is inspired by [START_REF] Aggravi | Haptic wrist guidance using vibrations for Human-Robot teams[END_REF]. Let q 1 ∈ R 3 be the 3D position of the leader robot and x * the next via-point to reach. Let us define the distance between q 1 and x * as d q = x * -q 1 and regularize it as in Sec. IV-A1,

d q,i = 0 if |d q,i | ≤ th d max(min(d q,i /D max , 1), -1) if |d q,i | > th d , i = { 1, 2, 3 },
with maximal distance D max = 1.5 m and distance threshold th d = 0.2 m. This threshold also determines the switching criterion between via-points: if the leader robot enters the 3D cube defined by th d , the new target via-point becomes the one that follows x * in P. When x * = x M and d q,i < th d , for any i = { 1, 2, 3 }, the task is completed. Although here we use vibrations to guide the leader robot along the predefined 3D path, we calculate the commanded vibromotor frequencies similarly to Sec. IV-A. Therefore, frequencies for motors left L and right R are

f L = 0, f R = -280d * q,2 if sgn(d q,2 ) < 0 f R = 0, f L = 280d * q,2 if sgn(d q,2 ) > 0 ,
while frequencies for motors top T and down D are

     f T = 280d * q,1
, β T = 100 ms if d q,3 = 0 and δd q,1 > th d or d q,3 = 0 and δd q,1 ≥ |d q,3 | f T = -280d * q,3 , β T = 200 ms if d q,1 = 0 and d q,3 < -th d or d q,1 = 0 and

d q,3 < |δd q,1 | ,      f D = -280d * q,1
, β D = 100 ms if d q,3 = 0 and δd q,1 < -th d or d q,3 = 0 and δd q,1 ≤ |d q,3 | f D = 280d * q,3 , β D = 200 ms if d q,1 = 0 and d q,3 > th d or d q,1 = 0 and d q,3 > |δd q,1 |

, with δ = 1.5 (see Sec. IV-A for details).

In addition to navigation information, as the user tries to increase the value of λ 2 by closing her/his hand, the cutaneous device provides a squeeze action ∆b p,c , proportional to the difference λ2 = λ d 2 -λ 2 between the desired and current value of λ 2 . It is indeed important to gradually increase λ d 2 . In fact, increasing it suddenly may lead to aggressive maneuvers of the robots, which may in turn have a negative effect on the overall stability of the formation. The commanded/desired value for λ 2 is defined as

λ d 2 = 3 o h , where o h ∈ [ 0 -1 ]
describes how much open the hand is (o h = 0 with a hand fully open and o h = 1 with a hand closed in a fist [START_REF] Bimbo | Teleoperation in cluttered environments using wearable haptic feedback[END_REF]). This leads to a commanded squeeze stimulus of

∆b p,c =      0 if λ2 < th λ min 76.5 2 (1 -cos(α λ λ2 + γ λ )) if th λ min ≤ λ2 < th λ max 76.5 if λ2 ≥ th λ max
with th λ min = 0.1, th λ max = 1, α λ = π/(th λ max -th λ min ), and γ λ = -α λ th λ min . This approach enables a smooth squeeze action, ranging from no squeeze when λ2 < th λ min to the maximum squeeze when λ2 ≥ th λ max (see [START_REF] Giordano | A passivity-based decentralized strategy for generalized connectivity maintenance[END_REF] for additional details on the construction of ∆b p,c ).

3) Subjects: Nine participants took part to the experiment, including two women and seven men (age 21 -31), all righthand dominant. The experimenter explained the procedures and spent about three minutes adjusting the setup to be comfortable. One practice trial was allowed.

4) Results:

To evaluate the effectiveness of our teleoperation system and the usefulness of wearable haptic feedback, we evaluated (i) the completion time, (ii) the overall path length, (iii) the activation time of the squeezing action, and (iv) the perceived effectiveness of the different conditions.

Average completion time for conditions V and CV was 359.8 s and 296.6 s, respectively (not shown in the figure). A paired-samples t-test determined that the task completion time did not differed statistically significantly between the conditions. Figure 8a shows the average trajectory length covered by the leader during the task. A paired-samples t-test determined that this metric differed statistically significantly between conditions (t(8) = 3.274, p = 0.011, a = 0.05). To evaluate the benefit of using the pressure/squeeze feedback, we also analyzed the percentage of time the squeezing action was activated (not shown in the figure). This metric indicates how often users commanded sudden increases of λ d 2 (i.e., high values for λd 2 ). In condition V, when users did not wear the cutaneous device, this metric indicates the percentage of time the squeezing action would have been activated if the cutaneous device was worn. A paired-samples t-test determined that in condition CV users requested significantly lower values for λd 2 than in condition V, leading to a smoother behavior of the robots (t(8) = 3.019, p = 0.017, a = 0.05). Immediately after the experiment, subjects were also asked to report the effectiveness of both feedback conditions in completing the given task using bipolar Likerttype nine-point scales. Fig. 8b shows the perceived effectiveness. A Wilcoxon signed rank test showed a statistically significant difference between the means of the two feedback conditions (z = 2.527, p = 0.012, a = 0.05). Finally, all subjects but one found using wearable haptic feedback to be the most effective at completing the teleoperation task.

V. DISCUSSION AND CONCLUSIONS

This paper presents a novel wearable haptic device for the forearm and its evaluation in two robotic teleoperation scenarios. The device is able to provide skin stretch, pressure, and vibrotactile stimuli. Two servo motors actuate an elastic fabric belt wrapped around the arm. The belt also houses four vibrotactile motors, positioned evenly around the arm at 90 degrees from each other. We carried out a detailed perceptual characterization of the device as well as two human-subjects teleoperation experiments, enrolling a total of 34 subjects.

Section IV-A considers a shared-control teleoperation experiment with a virtual 6-DoF robotic manipulator. Haptic feedback provided information about the constraints imposed by the shared-control algorithm on the motion of the slave robot. We evaluated the performance of the system either providing haptic feedback with our device (C) or no haptic feedback at all (N). Using our wearable device improved the performance of the task in all the considered metrics, and the vast majority of the subjects found condition C more effective. The largest improvement is seen in the trajectory length metric, showing that haptic feedback enables users to follow more closely the constrained motion of the robot and avoid problematic situations (e.g., workspace and joints limits). The second largest improvement is registered for the perceived effectiveness metric, proving that even inexperienced users can appreciate and effectively use our device. The difference between the two conditions in the other metric is less marked, but still statistically significant at the considered significance level. Finally, three subjects complained that, in condition N, it was sometimes hard to match the motion of their hand to the one of the simulated robot. In fact, in condition N, users have no direct information about the motion constraints of the slave robot. Therefore, the motion of the user's hand can significantly differ from the allowed motions of the slave robot, ending up in a very different and confusing configuration. This behavior was not registered in condition C, since the haptic feedback kept the human users close to the slave's allowed directions of motion.

Section IV-B considers the teleoperation of a fleet of ten virtual quadrotors. We evaluated the performance of the system providing either visual and haptic feedback together (CV) or visual feedback alone (V). Visual feedback consisted of a blue segment indicating on the screen the target direction of motion. Haptic feedback provided navigation information about the path to follow (vibrations) and the connectivity of the fleet (squeeze). Providing haptic feedback through our wearable device significantly improved the performance of the task in all the considered metrics but completion time. Seven subjects out of nine found condition CV more effective. The two subjects who preferred condition V did not like condition CV because their arm felt tired at the end of the experiment. Differently from Sec. IV-A, in this experiment we always provided navigation information through visual feedback (i.e., a segment pointing toward the target via-point). Haptic feedback then reinforced this information through vibrotactile stimuli provided by our device. All subjects reported that the task was easy to complete in both conditions. However, haptic feedback helped them in reaching faster the via-points. Results prove the viability and effectiveness of haptic feedback and of our device for robotic teleoperation applications. We should also consider that humans are used to follow visual navigation cues (e.g., turn-by-turn car navigation systems, road signs), while they are not used to follow haptic navigation stimuli at all. For this reason, we expect more training to significantly improve the performance of the haptic modality in all the considered applications.

In the next future, we will focus on reducing the form factor and weight of our device, with the objective of improving the comfort and wearability of the system. We also plan to study the role of skin elasticity in the actuation of our elastic belt, as well as how the perception of our haptic stimuli changes as the dimensions of the arm change. We also want to study the best way of fixing the device onto the forearm, with the objective of eliciting the most effective sensations. In this paper, we used a second stationary belt to fasten the upper static platform to the proximal side of the arm. In this respect, we are interested in exploring different, and potentially more effective, fasting approaches (e.g., directly clamping the upper structure onto the forearm). Finally, we will extend the evaluation of our device to real-world applications, starting from the teleoperation of a real robotic manipulator and quadrotors fleet, where we will also try to use all the three feedback modalities (skin stretch, pressure, vibrotactile) at the same time.

  (a) CAD design. (b) Device worn on the forearm.

Fig. 1 .

 1 Fig. 1. The proposed wearable device for the arm. It consists of a static platform (A) that accommodates two servomotors (B) and two pulleys (C), a fabric belt (D), and four vibrotactile motors (E). The device is able to provide skin stretch, pressure, and vibrotactile stimuli to the arm.

Fig. 2 .

 2 Fig. 2. Device characterization. Commanding different stretches to the belt, we analyzed the real belt displacement w.r.t. the stretch imparted on the arm skin around the belt. We considered 12 points on the arm and one of the belt.

Fig. 3 .

 3 Fig.3. Skin stretch characterization. It shows the real belt displacement ∆b l,r and skin stretch ∆s resulting from a commanded belt displacement ∆b l,c . To make the graph easier to read, we aggregated the data coming for neighboring points on the skin, showing their mean displacement three by three.

Fig. 4 .

 4 Fig. 4. Pressure characterization. Commanding different displacements ∆bp,c of the belt, we analyzed the force applied by the device on the volar skin of the user's arm.

Fig. 5 .

 5 Fig. 5. Experiment #1. The human operator wears the wearable cutaneous device on his right arm, and a Leap Motion controller tracks the position and the orientation of his hand. The slave side is composed of a simulated 6-DOF Viper S850 robotic arm carrying a gripper, and two objects to grasp.

Fig. 7 .

 7 Fig. 7. Experiment #2. The human operator wears the wearable cutaneous device, and a Leap Motion controller tracks the position of his hand. The slave side is composed of a fleet of ten quadrotors. One is controlled by the operator, and the rest moves according to the connectivity-maintenance strategy of [21].

Fig. 8 .

 8 Fig.8. Experiment #2. The task consisted of following a predetermined trajectory in the simulated space. Completion time, path followed by the robot, and perceived effectiveness (with range from 1 to 9) provided a measure of performance. Mean and 95% confidence interval are plotted.

  , being SE(3) the special Euclidean group and H(t) composed of R H (t), p H (t) , where R H ∈ SO(

This paper was recommended for publication by Editor Yasuyoshi Yokokohji upon evaluation of the Associate Editor and Reviewers' comments. This research has received funding from Rennes Métropole under the "Allocation d'Installation Scientifique (AIS) 2017" programme and from the European Union H2020 RoMaNS project 645582.