Bernadette Bensaude
email: bernadette.bensaude-vincent@univ-paris1.fr

Between the possible and the actual

Keywords: synthetic biology, economy of promises, design, engineering, futures, technoutopia

A number of research projects in synthetic biology funded by public institutions sound like science fiction: for instance DARPA Biodesign project in 2011 aimed at eliminating "the randomness of natural evolutionary advancement, and designing proteins with a 99.5% precision. How should we take such claims? Are they just fantastic visions or hype meant to impress funding agencies and the public or are they symptoms of a specific epistemic attitude?

In considering the visions of the future developed in synthetic biology the paper attempts to evaluate the research agendas of synthetic biology from philosophical perspectives. It will first characterize the ambition to re-engineer life and distinguish the visions of the future underlying the various research agendas assembled under the umbrella synthetic biology. Then addressing the question 'who actually believes in such futures?' the paper contrasts the complicit belief of ethicists and critical activists with the doubts occasionally formulated by synthetic biologists. Doubts however never generate scepticism within the synthetic biology community, which develops an epistemic opportunism. Finally the third section discusses to what extent, and in which sense, the futuristic visions of synthetic biology belong to the genre of techno-utopia. It concludes that imagined futures are integral part of the techno-epistemic culture of synthetic biology and that the tension between the possible and the actual is a criterion of distinction between the various trends that constitute synthetic biology.

Introduction:

DARPA Biodesign project for fiscal year 2011 raised $ 6 million for research aimed to "eliminate the randomness of natural evolutionary advancement primarily by advanced genetic engineering and molecular biology technologies to produce the intended biological effect" and planed to "identify and initiate strategies that would enable a new generation of regenerative cells that could ultimately be programmed to live indefinitely until needed for an injury repair or therapeutic application." The project of making immortal organisms so much challenges common sense views of life that it raised a wave of ironic comments about the mad scientists of the Pentagon. 1 Still such fantasies continue to attract interest and million of dollars from various sources. 2Like nanotechnology synthetic biology is driven by visions of incredible futures. The prospect of design and manufacture of biological organisms triggers all sorts of fantasies: From bacteria-workers to the creation of new forms of life. Just as in nanotechnology, synthetic biology develops an "economy of promises". [START_REF] David | Nano-Hype : The Truth Behind Nanotechnology Buzz[END_REF] However synthetic biology seems to go one step further. While nanotechnology has been advertised with the slogan "shaping the world atom by atom" in the 2000 US National NanoInitiative, synthetic biology opens up the more challenging perspective of designing organisms that will remake the world for us. Reengineered yeasts or bacteria will serve as pharmaceutical plants producing drugs. Synthetic algae will provide renewable fuel for our daily consumption of energy. Synthetic bacteria will decontaminate the soils polluted by chemicals and nuclear waste. It is tempting to dismiss such promises as hype forged by the mass media. However it is not limited to popular press since it is used by scientists and policy makers as well. Scientists have their own way to justify their bold claims about the potentials of synthetic biology by the pressures from funding agencies for research aimed at useful applications. Is the concern with potential impacts of research on economy or medicine the only justification for utopic visions in today synthetic biology? I will argue that the economy of promises developed in synthetic biology is more than a rhetorical device for the purpose of raising funds or public attention. Rather it seems that synthetic biology -like nanotechnology -did unleash imagination and enhance its role in scientific research. However synthetic biology is extremely multi-faceted and not all research programs gathered under this umbrella are promising incredible futures. Therefore in examining to what extent futuristic visions and bold claims characterize the epistemic culture of today synthetic biology, it is important to take into account the epistemic pluralism of synthetic biology. [START_REF] Vincent | forthcoming) Discipline building in synthetic biology[END_REF] The first section of this paper considers the visions of the future outlined in the various research agendas assembled under the umbrella synthetic biology. The second section discussing the credibility of various projects in synthetic biology points to a striking contrast between the complicit belief of the critical activist community and the more sceptical views developed within the community of synthetic biologists. Finally the third section argues that a significant part of the research programs currently conducted under the umbrella synthetic biology belong to the genre of techno-utopia and uses the tension between the possible and the actual as a criterion for distinguishing the various research trends within synthetic biology.

Imagined futures

Speech-acts

The power of the promises of synthetic biology rests on the design of predictably functional organisms that will perform a variety of useful and desirable tasks for us. The main messages conveyed in the popular press as well as in the pages and blogs of scientific periodicals is that life and nature are no longer to be viewed as a given, as the inexorable necessity opposing resistance or constraints to all technological projects. New biological systems can be constructed and "might be harnessed to serve humanity". [3] A strong vision of life as a resource for human design is being shaped through pictures, slogans, and papers. For instance, on the cover page of an issue of Nature dedicated to biodesign one could find a cartoon subtitled "Life is what we make it". [4] In taking up this formula of popular wisdom, a strong message was delivered: life-as-it-is should give way to life-by-design. The message was reinforced on the cover of an issue dedicated to genome sequencing bearing the title "Your life is in your hands". [5] The assertive mode of this cover pages echoes Rob Carlson's title Biology is Technology. [START_REF] Carlson | Biology is Technology: The Promise, Peril, and New Business of Engineering Life[END_REF] Such propositions can be viewed as typical "performative sentences", sentences, which do something in the world rather than describing something about it. [START_REF] Austin | How to do Things with Words: The William James Lectures delivered at Harvard University in[END_REF] What can be the impact of such "speech acts"? They invite researchers to action, whether they be driven by curiosity, by the desire to advance our understanding of natural life or by more practical goals with environmental or pharmaceutical applications in view. The use of this kind of language in scientific journals reveals a remarkable pragmatic turn toward action, which goes far beyond the promises of industrial applications. This proactive attitude is said to bring about powerful research tools for better understanding the conditions and dynamics of life. Constructing organisms rather than simply experimenting on them, is the road for increasing our knowledge of life in the future. Knowing through making, the cognitive strategy developed and routinely used in chemistry is applied to biology. Over the past decade the regular quotations of Richard Feynman's alleged statement that "What I cannot create, I do not understand" became a catchword and marker for creating a community.

Variations and convergences

As many outsiders have emphasized, synthetic biology is extremely multi-faceted and not all research programs gathered under this umbrella are promising incredible futures. [START_REF][END_REF] In fact, the emphasis on design could well be the unique link that ties together the constellation of multidisciplinary groups usually gathered under the umbrella synthetic biology, since they have quite diverse research agendas. To the question "What's in a name" [synthetic biology] the editor of an issue of Nature Biotechnology [START_REF] O'malley | Knowledge-making distinctions in synthetic biology[END_REF] did not get a uniform answer, but a consensus emerged from a dozen of answers around two key notions: engineering and design. There are striking contrasts between the Biobrick program developed around the MIT Registry of standardized bioparts, driven by the ambition of applying the principles of civil engineering to living organisms and research programs aimed at making protocells. While the latter program combines theoretical modelling and experimental construction with the view to disentangle the basic conditions required for the origin of life, the champions of the Biobrick program promise a more exciting future. The living world will become part of the industrial infrastructure and the economy will be restructured around biological manufactures assembling bioparts. For instance, in 2001 Robert Carlson envisioned 2050 in these terms: "In fifty years, you may be reading The Economist on a leaf. The page will not look like a leaf, but it will be grown like a leaf. It will be designed for its function, and it will be alive. The leaf will be the product of intentional biological design and manufacturing." [START_REF] Robert | Biological Technology in 2050[END_REF] Constructing reliable artefacts that get rid of all the messiness and unpredictability of natural systems is the condition for the industrial future of biotechnology. Drew Endy claims to "make routine the engineering of synthetic biological systems that behave as expected". [START_REF] Endy | Foundations for engineering biology[END_REF] Research programs aimed at making minimal genomes share the project of implementing desired functionalities in a chassis. However they differ from the Biobrick program as the emphasis is less on intentional design and more about demonstrating capabilities of challenging nature. Proofs of concept are celebrated as records in sports competitions. For instance in 2010 Craig Venter presented his synthetic bacterium Mycoplasma Mycoïdes as "the first self-replicating species we've had on the planet whose parent is a computer." More ambitious is the branch of synthetic biology often referred to as "unnatural-biology". The synthesis of new types of nucleic acids aims at generating novel forms of life that evolution could have made. It is part of a broader discipline named "universal biology", whose objective is to move beyond the circumstances of the particular origins of life on earth and to identify the general principles of life according to the universal laws of physics and chemistry. It aims at investigating, both the past, the origin of life and the principles that will shape life in the future. However this cognitive goal is intertwined with more practical expectations to design bio-devices and machines for more immediate technological or medical applications. Underlying the project of designing unnatural forms of life is a specific vision of progress as a process of emancipation from an imperfect nature. DNA, RNA and the cell machinery are certainly robust but natural life is not optimal as they are fundamentally contingent. By changing one or two of the four bases of DNA while keeping the acceptor-donor hydrogen bonds in its structure, it is possible to build up artificial genetic systems that can support evolution. For Steve Benner, synthetic biology is a promising pathway to explore alternative forms of life that might exist in other environments. [START_REF] Paul | Hypothesis article: Signatures of a shadow biosphere[END_REF] In this respect, synthetic biology is a matter of interest for NASA: Biology on Earth readily demonstrates that life is an efficient user of resources around it, turning those resources into habitats, materials and forms that perform various functions. Synthetic biology in space represents a new challenge, the challenge of designing organisms to perform reliable functions that an astronaut may one day depend on. If the promise of an engineerable biology on Earth is within reach the initiative aims to develop applications for synthetic biology in space". [13] Benner even claim that synthetic biology overtakes nature and allows us to get rid of its imperfections. In a lecture eloquently entitled "Redesigning DNA: Fixing God's mistakes", Benner argued that: " The structure of DNA is far from perfect because it reflects events that occurred at the origin of life and in subsequent episodes of life where constraints on the structure of biomolecules were quite different from those today, and certainly not conducive to creating a biomolecule that serves the goals of the analytic chemists, the bioengineer, or the synthetic biologists. Fortunately the theory and synthesis are both adequate to make a better genetic material". [START_REF] Benner | Redesign DNA: Fixing God's mistakes[END_REF] In addition, a number of synthetic biologists argue that artificial genetic codes may be needed in case of a catastrophic event that would erase life as it is from the earth. In other terms generating artificial biodiversity thanks to unnatural molecular biology is presented as a plausible response to environmental protection. [START_REF] Marlière | Prométhée, Pandore et Pétri-Vie synthétique et OGM[END_REF] 1.3 Technomimetism Slight differences are thus perceptible in the ambitions of the various trends of research in synthetic biology: improving on nature in the Biobrick program, challenging nature in genome engineering, emancipation from nature in unnatural biology. Nevertheless all discourses and images about synthetic biology converge towards the vision of redesigning life along technological principles, implementing the principles of rational design in natural systems. In stark contrast to the tradition of biomimicry, or bio-inspired chemistry, synthetic biology is promoted as an alternative strategy that could be labelled "technomimetism". Biomimetism is dismissed as a poor amateurish strategy: "If biological engineering were aviation, it would be at the birdman stage: some observation and some understanding, but largely naive mimicry. For the field to really take flight, it needs the machinery of synthetic biology. […] At the turn of the last century, the Wright brothers achieved manned flight not by mimicking natural systems, but by applying the principles of engineering and aerodynamics. Similarly, synthetic biology allows us to dispense with biological mimicry and design life forms uniquely tailored to our needs. In doing so, it will offer not only fundamental insights into questions of life and vitality but also the type of exquisite precision and efficiency in creating complex traits that genetic engineers could previously only dream of". [START_REF] Editorial | Unbottling the genes[END_REF] The synthetic biology community thus gradually built up a grand narrative based on a broad historical picture of technology disqualifying nature. They shape a specific vision of the future, as being in the hands of humans, that Barbara Adam named the future as fortune by contrast to the mythical and religious view of the future as fate. [START_REF] Adam | Future Matters, Action, Knowledge[END_REF] The view of a predestined fate unveiled by prophets has been destroyed in the Enlightenment and replaced by the view of the future as an open and empty space to be colonized and controlled by humans. The future has to be made through action and planning. No longer do we need priests or prophets pretending to prepare people to accept their fate we need scientists and engineers to act upon the future. This shift from "future presents" (predetermined) to "present futures"( as outcome of human choice and action) often implies a devaluation of nature and of life-as-it-is. The present future similarly downplays the current state of the art for the benefit of the imagined future. Many synthetic biologists (Endy and Carlson in particular) like to contrast the clumsy practices of customized genetic engineering, depicted as poor and inefficient tinkering with the clean practices of synthetic biology, based on sound engineering principles such as standardization, abstraction and decoupling. The shift towards abstraction that we observe in the vision of the future is paralleled by a shift toward abstraction in the concept of life, although life is not exactly an empty space to colonize. Rather it is a text that can be deciphered and rewritten. Decoding commands recoding and redesigning. To sum up this section, strong visions of the future as improving nature, challenging nature or emancipating from nature, permeate synthetic biology. In spite of diverse engineering ideals and practices, all synthetic biologists seem to share the conviction that designing or redesigning organisms is making the future. At the frontier of science, they are transforming life into a major resource. The future they have in mind is a future of unrestricted economic growth and expansion of the mastery of humans over nature. In short, they revive the common modernistic views of progress, with science and technology as a driving force.

Issues of credibility

While the faith in progress is not really the dominant view in today Western societies, one may wonder who can share the visions of the future underlying synthetic biology. The extravagant promises of synthetic biology decided policy makers to bring social scientists in the field, to assess the potential societal, legal, economic and ethical impacts of synthetic biology. [START_REF] Henkel | The economics of synthetic biology[END_REF] Their bold claims prompted worries and strong reactions from a number of civil associations and NGOs.

Believers in alert

Ethicists and activists seldom question the credibility of the promises made by the champions of synthetic biology. On the contrary, they often take them seriously as they try to draw the public's attention to their potential impacts on civil society and environment. For instance, a report Extreme Genetic Engineering, published by the Canadian action group on Erosion, Technology and Concentration (ETC Group) in January 2007 spread the view that unnatural organisms were already among us. [START_REF]Extreme Genetic Engineering[END_REF] In their effort to alert the public and decision makers about the risks of bio-terrorism and environmental issues, they made the imagined future more present, more real. In the aftermath of Craig Venter's announcement of the design of a bacterium with synthetic genome, in May 2010, the ETC Group went even further in turning speculative futures into present futures, when they issued a short alarming note entitled Synthia is alive … and breeding. In order to raise legitimate doubts about the potentials of synthetic genomes, they took Venter's claims and promises at face value. For instance, Jim Thomas of commented: "This is the quintessential Pandora's box moment -like the splitting of the atom or the cloning of Dolly the sheep. We will all have to deal with the fall-out from this alarming experiment." [START_REF] Breeding | Panacea or Pandora Box?[END_REF] A similar perverse effect resulted from the ethical discussions about the societal aspects of synthetic biology. The list of issues -biosafety, biosecurity, biohackers, etc -resulting from the accompanying ELSI programs tends to reinforce the credibility of the promises made by synthetic biologists. [START_REF]Synthetic Biology: The Technoscience and its Consequences[END_REF] As pointed out by Alfred Nordmann about the ethics of nanotechnology ethicists endorse and validate an incredible future in their effort to evaluate their impacts. "This discourse violates conditions of intelligibility, squanders the scarce and valuable resource of ethical concern, and misleads by casting remote possibilities or philosophical thought experiments as foresight about likely technical developments." [START_REF] Nordmann | If and then. A critique of speculative ethics[END_REF] Instead of developing a critical assessment of the promises of synthetic biology, ethicists and activists seem to be fascinated by their visions and follow their futuristic talk.

Objections from within

Ironically, it is among the practitioners of synthetic biology that one can find more sceptical attitudes. The most serious doubts about the promised futures are to be found in a number of remarks coming from within the community. They are based on quite different grounds: on theoretical assumptions, on experimental aspects, and on economic considerations. On the theoretical front, Antoine Danchin, a French geneticist working on bacterial genomes questioned the ambition of building synthetic bacteria affording permanent capabilities. His argumentation is based on the informational paradigm and the analogy between cells and computers, which drive many projects of synthetic biology. Looking more closely at this analogy, Danchin assumes that in both cases there is a separation of functions: the hardware is separated from the software in computers and the processes of gene expression separate the genome from the cell machinery. [START_REF] Antoine | Information of the chassis and information of the program in synthetic cells[END_REF] However "computers do not make computers". Whereas the program replicates, the cell reproduces itself (make similar copies). Synthetic biologists tend to focus on the role of the genome and consequently overlook the role of the chassis.

"The present avatar of Synthetic Biology (SB) assumes that we know enough of what life is to allow us to construct life from scratch, or, at least, to modify existing cells and organisms so that they work as cell factories. With this view SB puts together two separate entities, a program (the conceptual extension of the genetic program) and a chassis (the conceptual extension of the living cell). Yet a rapid browsing of publications in the domain indicates that the vast majority of SB-related work deals with the program, not the chassis. Most investigators do as if the recipient cell would be nice enough to accommodate entirely artificial constructs and behave as expected, producing the right products, with the right yield, at the right time. Several hard constraints however make this dream difficult, if not impossible to achieve." [START_REF] Danchin | Scaling up synthetic biology: Do not forget the chassis[END_REF] The genome itself raises a dilemma because in its most entrenched part (paleome) the same genes have a dual function: they secure innovation and they work for the maintenance of the system. They act as information-traps and power stations at the same time. They thus combine evolution and prevention of the degradation of the functional units. Consequently synthetic organisms will either unpredictably evolve and their functional units cannot be guaranteed; or they will be ageing and require a continuous reconstruction of the functional units. The prospect of immortal cells is accordingly dismissed as unrealistic, and the promise of making sustainable microorganisms that would afford permanent functional capacities is also questioned. Whereas Danchin's doubts proceed from broad theoretical assumptions about life, other doubts about the feasibility of some promises emerge from the practice of synthesis. For instance, Steve Benner who is one of the pioneers of synthetic biology (he synthesized DNA in the 1980s) still active in the synthesis of exotic unnatural DNAs, raise doubts about the basic assumptions underlying the Biobrick project. The foundational principles necessary for the achievement of engineering biology have been clearly formulated by Endy in a famous article: standardization (full description & characterization of bioparts); decoupling (divide a task into manageable operations) and abstraction (dealing separately with each level of complexity). [START_REF] Marlière | Prométhée, Pandore et Pétri-Vie synthétique et OGM[END_REF] To Benner who has life-long experience of biochemical synthesis, making standard interchangeable parts is an ideal far from the actual practices of synthesis. Unlike the parts assembled in mechanical engineering the building blocks of synthesis inevitably interact. Their identity and behaviour is to a certain extent determined by their interactions. In thus emphasizing the gap between the engineering ideals of the Biobricks program and the principles at work in actual practice, Benner suggests that the delivery of this ambitious and attractive research program will never be up to the promises. Actually the rare successes of synthetic biology such as the biosynthesis of artemisinin -an anti-malaria drug -were not the result of rational design. In reality they were achieved through more messy and painstaking practices with trials-and-errors and workarounds. As Maureen O'Malley convincingly argues the engineering of metabolic pathways requires a lot of 'kludging' (a term made out the initials of klumsy, lame, ugly, dumb, but good enough) referring to quick and dirty solutions. [START_REF] O'malley | Making knowledge in synthetic biology: Design meets kludge[END_REF] The self-image of synthetic biologists as responsible engineers relying on sound and rational principles is at odds with their actual practice of skilled and astute biochemists. The designing practices of another pioneer of synthetic biology, Michael Elowitz, could also lead to subvert the engineering principles underlying synthetic biology. Elowitz's initial efforts to design genetic circuits "from scratch" were based on the principles of decoupling and abstraction. [START_REF] Elowitz | A Synthetic Oscillatory Network of Transcriptional Regulators[END_REF]. He worked hard to build a circuit that would function as independently as possible from the underlying cellular system turned out to be unrealistic. However he realized that noise and interaction with the host cell were inevitable. [START_REF] Nandagopal | Synthetic Biology: Integrated Gene Circuits[END_REF] The basic assumption of the modularity of biological systems thus seems to be really threatened by his research over the past ten years, The industrial and commercial future of synthetic biology has been questioned by one of its most dedicated champions, Rob Carlson. Carlson casts doubts on the economic potentials of the Registry of Standard Biobricks when he assumes that most biological parts designed by the students in iGEM competition do not work and would be useless for industrial developments because they are not well characterized. [6: 51] His most serious doubts rest on economic arguments. In his view, bio-economy as it is currently envisaged is not sustainable. In particular Carlson undermines the commercial future of Venter's celebrated prowess. He argues that given the amount of money invested just for the first phase of experimental assembly of a synthetic genome the cost of the next stage and the scaling up of synthetic bacteria would exceed all plausible capacities of financial investments. [6:104-105] Thus Carlson unwillingly provides the critics of synthetic biology with powerful arguments, although to him such defects are just an encouragement to go further and reach the critical mass necessary for the take-off of synthetic biology by getting rid of all patents and regulations.

Practitioners believers?

The three types of objections -theoretical, experimental and economic -raised by a number of practitioners of synthetic biology are clearly articulated and publicized. They could be factors of division within the community of synthetic biologists. But doubts are not presented as potential criticisms undermining the future of synthetic biology itself. Remarkably, Elowitz and his team never openly questioned the central assumptions of the Biobrick program. Instead they are developing "a new generation of synthetic circuits that integrate more closely with endogenous cellular processes". [START_REF] Nandagopal | Synthetic Biology: Integrated Gene Circuits[END_REF] It is expected that the second generation of genetic circuits will result in a deeper understanding of regulatory processes and evolution of genetic circuits. From the basic science perspective it is a remarkable gain of knowledge. However from an engineering perspective, such integrated circuits would be at odds with the strict control and predictability promised by the champions of the Biobrick program. [START_REF] Knuuttila | Basic science through engineering? Synthetic modeling and the idea of biology-inspired engineering[END_REF] Elowitz's recent results could thus have been interpreted as a refutation of the engineering program. Instead they are presented as an invitation to move further on. Noise and interactions are supposed to be advantageous to achieve specific functions in genetic circuits, rather than undermining the ambition to improve on nature. Here points a distinctive feature of the community of synthetic biology. Negative results never generate blame or disqualification of colleagues. They are not sceptical scientists. They ignore Robert Merton's famous "organized scepticism", one of the four pillars of the scientist' ethos. They are hard-rock optimists who spring back on all obstacles. Failures or difficulties are never seen as refutations that could threaten the promises. Objections are turned into challenges, negative results into new opportunities. The prohibiting costs of today achievements in synthetic biology are just defects, due to a science still in infancy and it is expected that the costs will exponentially decrease in the next decades. Synthetic biologists are never disappointed, never deeply disturbed in their credo. They are flexible; they are versatile, open to all opportunities. Furthermore, there is no apparent competition between the various branches of synthetic biology. Instead of behaving as a pack of rival ambitious hounds, they run all united behind a flag in the conquest of the future. Far from dividing the community, the promises and imagined futures of plenty seem to act as the cement building a collective out of a cluster of researchers coming from various disciplines and using various tools and languages. The promises need not to be taken as real objectives. They may be just buzzwords generating an atmosphere of activity and excitement around a target. Buzzwords have a dual power: They point to the direction of an attractive and highly desirable object, acting like a magnet; at the same time they conjure up the image of a swarm of bees moving together to found a new colony, thus blurring all potential conflicts, tensions or even frictions in the group. Or to use a biblical metaphor, the imagined futures can be seen as a secular version of the mythical Promised Land. Just as the Promised Land made one people out of twelve tribes, the imagined futures create a horizon of expectations cementing a community of practitioners with different backgrounds and languages. The indefectible optimism cementing the community of synthetic biologists is encouraged by the prevailing knowledge economy. 3 The promises of synthetic biology rely on the assumption that investing in a specific domain of research is good for national competitiveness in a global market. New technologies such as synthetic biology seem a necessary -if not sufficient -condition to deal with pressing economic, societal and environmental issues. So strong is the power of the equation technological innovation=economic growth underlying all this techno-futuristic talk, that technological solutions are expected to all issues. In sum, the question opening this section -'who believes in the incredible futures of synthetic biology'? -may not be the right question. Synthetic biologists do not need to believe they just share the enthusiasm raised by the grand futuristic visions displayed on the front-covers of their favourite journals. As for activists and ethicists they might well question the basic assumptions rather than discussing the promises.

Technoscientific utopia?

The importance of futuristic visions in synthetic biology or nanotechnology raises a broader issue: to what extent utopia permeates the epistemic culture of those technosciences? Instead of comparing the promises of technosciences to a conventional definition of utopia, it seems more relevant to start from the notion of the future and display the variety its modalities. Aristotle characterized the future as contingent, but there are various distinguishable ways of being in-between the impossible and the necessary. The "potential" waiting to be actualized (the seed that will produce the flower) is not unreal in the same manner as the "possible". The possible in turn denotes either a logical possibility (no intrinsic contradiction), or a physical possibility (compatible with natural laws), or a practical possibility (feasible), or a moral possibility (permitted or acceptable). Moreover there is no point of view of outside observer to envision the future. No privileged standpoint allows to objectively assert that the promises of synthetic biology are either unrealistic or realistic. Therefore three different perspectives -historical, epistemological and metaphysical -are displayed without claiming that no other perspectives could be adopted.

Historical perspective

Like many new promising technologies, synthetic biology is advertised as the Nth Industrial Revolution opening up a new era of clean and sustainable development based on the systematic exploitation of biological resources. These claims have an air of déjà vu. As Steven Shapin notes: "Few things are as passé as past futures" [START_REF] Shapin | What else is new?[END_REF]. In the mid-twentieth century a world was promised where beefsteaks made out of oil would alleviate world poverty. Fifty years later the promise has switched to substitutes of oil, fuels made out of feedstock. Futuristic visions are so attractive that they blind the past. Synthetic biologists are blind to innumerable previous attempts at making life in a test-tube, from medieval alchemy to twentieth-century dreams of engineering life. [START_REF] Kirkham | Is biotechnology a new alchemy?[END_REF] Although synthetic biologists benefit from unprecedented technical means, their strategies are not always novel. They re-enact schemes developed by alchemists and synthetic chemists. 4Indeed one cannot conclude from past successes or failures to success or failure in the future. However planning the future without taking into account similar past experiences is not the most reliable route. The futuristic visions of today technoscientists are strikingly amnesic, so detached from the past that they are totally abstract. In particular, synthetic biology is often promoted as a source of technological fixes: biofuels meant to stop the overconsumption of fossile fuels, synthetic bacteria to repair the damages caused by chemical industries and nuclear power stations. But the concern with the damages due to the previous generations of "new technologies" does not invite reflections about the next new generations, i.e. the longterm unintended consequences of all technological innovations. To be sure, the advocates of synthetic biologists include risk assessment in their programs in the name of 'responsible innovation'. They earnestly address issues of biosafety and biosecurity in their annual conferences. Yet they only rely on technological solutions of confinement (physical, biological, evolutionary confinements), which in turn will raise new issues. Whatever the plausibility of the technical solutions for preventing the dissemination of and contamination by synthetic organisms, they never take into account past experiences, never draw lessons from the past. As the historian of technology David Edgerton argues the current "futuristic guru-talk" relies on a superficial and biased view of the history of technology. [START_REF] David | The charge of technology[END_REF] Synthetic biologists only look at the past to select a few conventional standard examples (preferably taken from synthetic chemistry or aviation) detached from their contexts. [START_REF] Vincent | Synthetic biology as a replica of synthetic chemistry? Uses and misuses of history[END_REF] They ignore the complex reality of past technological achievements, how they combined innovations and old familiar techniques. They overlook the path-dependency created by technological trajectories, and their negative impact on attempts at optimization. In the light of past technologies, of synthetic biologists look like a self-fulfilling prophecy, and their capacity to plan and master the future as a utopia.

Epistemological perspective

Imagination strikes as a prominent feature in the epistemology of synthetic biology. In standard scientific methods, imagination is usually strictly confined and duly controlled for being a capacity generating fictions rather than facts, unrealistic scenarios rather than practical solutions. In synthetic biology and recent biotechnologies, imagination takes a more positive and important role, which is by no means in contradiction with the exercise of reason and reasoning. Imagination provides guidelines for action because it bridges the gap between the possible and the real. [START_REF] Hunyadi | The imagination in charge[END_REF] It is precisely a reflection on the space between the possible and the real, which inspires the design of unnatural forms of life. Synthetic biologists working in this field, are engaged in a systematic exploration of the realm of the possible, and only indirectly concerned with understanding life or coming up with profitable innovations. This epistemic attitude is instantiated in Benner and Sismour's attempts at synthesizing novel forms of life, based on new types of nucleic acid or a different genetic code. [START_REF] Benner | Synthetic biology[END_REF] The modification of the standard code is not just meant to design living tools for remediating natural defects or improving on nature. Rather it seems to be driven by a concern for exploring all kinds of potentials and actualizing them. It is the coming into being of alternative forms of life that could have been achieved by biological evolution. For instance, a research program named OUGEPO developed by a French geneticist (Philippe Marlière) and a mathematician (Philippe Bourguignon) systematically explores the horizons that natural selection discarded. The project is driven by the analogy with a surrealist literary game OULIPO designed by Raymond Queneau and Georges Perec. [START_REF] Ougepo | ouvroir de génétique potentielle[END_REF] It rests on the assumption that since current natural life is based on 20 molecules of amino acids while about 300 amino acids are used to make a protein, 20 300 sequences are possible and consequently worth to be explored. While the discrimination between the physically possible and the technically feasible is the landmark of engineering work, the epistemological attitude developed in this project is exclusively concerned with the physically possible. It is aimed at unveiling potentials, regardless of their technological, ecological or ethical consequences. The world becomes a field of potentials that need to be systematically explored because of the opportunities to play with. [START_REF] Nordmann | Enhancing Material Nature[END_REF] Epistemic opportunism requires a mixture of imagination and reasoning but it easily generates techno-utopia as long as it is not concerned with the actualization of opportunities in the real world.

Metaphysical perspective

At the metaphysical level, the utopic dimension of novel forms of life is even more obvious. Unnatural nucleic acids would require unnatural amino acids, a new cell-machinery, and …. An entire world, parallel to the natural world, would have to be created for their actual existence. Indeed the notion of parallel worlds is logically possible. Leibniz's God, acting as a super-computer, was able to display all possible worlds. But knowing that they were not all physically com-possible, God decided to create only the best possible world. Unnatural biology ignores the condition of com-possibility. Although they occasionally claim to make better genetic material, synthetic biologists who craft exotic DNAs do not seem to be concerned with the actualization of the best possible form of life. They are content with the administration of "proofs of principle", demonstrating that other genetic codes, other cell machineries could exist, or have existed. They certainly legitimate novel forms of life as a preventing measure to avoid risks of contamination because alien genetic codes would not interfere or interact with natural ones. They would work independently from nature, in isolation. But what could be the mode of existence of isolated organisms? Isolation, this seems to be the most prominent, and most problematic, feature of all artefacts designed in synthetic biology, not just in the unnatural biology branch. Biobricks are also designed as isolated functional artefacts made of synthetic, well-characterized units. They are supposed to perform well-controlled functions, independently from their environment, whatever the circumstances, overtime. As such they belong to the genre of utopia in a more literal sense. They exist in no-place (u-topia). They are both out-of-place, and out of time. The doubts -mentioned above in section 2.2 -about the possibility of securing permanent functionalities, and the recent evolution of metabolic engineering towards the integration of interactions between the synthetic system and the host cell, reveal the idealistic character of these foundational assumptions. Without entering into a detailed analysis of the ontological status of biosynthetic products, it is clear that these mixtures of nature and artefact are necessarily context-sensitive by their dual nature. [START_REF] Kroes | The dual nature of technical artefacts[END_REF] As technological objects they have to go through a process of "concretization". This term can be understood in two senses. In its common use it refers to the various steps leading from the abstract (theoretical principles, purposes and list of requirements) to the concrete (the finished product). In its more technical definition forged by the philosopher Gilbert Simondon, "concretization" is quite the contrary of the materialization of ideas or intentions. It refers to the process of convergence of functions in a structural unit and gradual inclusion of the environment in the operational scheme of the technical object. [START_REF] Simondon | Du Mode d'existence des objets techniques[END_REF] Interestingly Simondon illustrated this notion with examples taken from the biological world. Biological objects are multifunctional and robust, not only because they are made of smart molecules but also because they are the outcome of evolution and genesis. Time is part of the stuff they are made of. Artefacts, just as natural objects are never permanent substances, they are rather processes. A fortiori, biotechnological objects, which combine genesis and poiesis, cannot exist out of time. What can be expected from organisms deprived of history? The question was raised by Lewis Mumford a few decades before the dawn of bioengineering and synthetic [START_REF] Mumford | The Myth of the Machine[END_REF] He condemned the chemists' ambitions of crafting viruses or bacteria in test-tubes as being i) redundant (why spend billion of dollars, thousands of hours and the elite of scientists for making micro-organism that can be found in abundance in oceans, in the air and everywhere?); ii) retrogressive (why moving three billion years backward and start making life from scratch?). These are certainly common-sense arguments. Nevertheless they are useful to highlight the reconfiguration of time initiated in synthetic biology. While the projects of making life de novo reverses the time arrow, the exploration of all possible forms of life breaks with the linearity of the time arrow and displays all possible forms of life in a space-like anisotropic dimension of time. In the latter configuration there is no future, just an eternal present of play with potentials. Nevertheless, this out-of space, out-of time game may not be without impact on the real world. Even though the probability of interference between alternative genetic codes and the code of natural life is minimal, all biological systems are subject to mutations and natural selection. The novel forms of life will not escape the contingencies of evolution. Since all synthetic organisms will have to take place in the world, it is unrealistic to imagine that they will have a predictable behaviour. If they are capable of evolution they are not under human control.

In conclusion, the incredible promises characterizing the discourses about synthetic biology are not just rhetorical devices for attracting funds and creative young talents. They are integral part of the methods and expectations of the synthetic biology community. Between knowing and making, between understanding and constructing, synthetic biology develops a new vision of nature as a field of potentials to be explored for its affordances. Visions of the future are systematically engaged in dealing with the present for guiding action. However there are striking differences between the various branches of synthetic biology with respect to their balance between the possible and the actual. It seems that metabolic engineering and genomic engineering, whether driven by cognitive goals or by practical goals for making technological applications, put the emphasis on the actual. They seize all opportunities to construct real operational systems that can work as prototypes and shed new light on natural systems on the way. By contrast, the basic principles guiding the Biobrick program so much deny the complexity of living organisms that this movement can be characterized as a utopia, in the sense of using a fiction to change the world. The fiction of isolated modulus working independently can be compared to the "Cartesian novel" (the mechanistic vision of shaped and moved tiny particles making up the world), which deeply influenced the development of modern science. The branch of unnatural biology is more strictly speaking a utopia. In designing out-of-space and out-of-time forms of life, which are not adapted to any real context that we know, it forges an imaginary parallel world. It also captures the anticonformist flavour of all utopias since it intentionally subverts the order of nature. In this respect, it is closer to artistic creation than to traditional scientific investigation. Thus the tension between the possible and the actual provides a criterion for discriminating the various research agendas of synthetic biology.

See for instance Wired on http://www.wired.com/dangerroom/2010/02/pentagon-looks-to-breed-immortalsynthetic-organisms-molecular-kill-switch-included/and DARPAS's response http://techinsider.nextgov.com/2010/02/the_misunderstood_mad_science_of_darpas_biodesign_project.php

[START_REF] Vincent | forthcoming) Discipline building in synthetic biology[END_REF] see for instance the call from Templeton University, http://chronicle.com/article/Intimations-of/133287/).

The knowledge economy initiated by the Organization for Economic Cooperation & Development (OECD) in the 1990s and translated into a roadmap for Europe in the 2000 Lisbon Agenda is based on the pivotal role of innovation in economic development. Increasing knowledge generates capital, increases the Gross National Product more than material resources. Against the background of economic depression in Europe the mission assigned to R&D was to increase economic competitiveness and create new jobs.

For instance, the minimal genome approach seeking to construct new organisms and new functionalities on the basis of the minimum equipment required for sustaining life resembles the strategy used by nineteenth-century chemists to branch chemical functions on the benzene ring for making all sorts of aromatic compounds. It also evokes pre-modern alchemical schemes such as Daniel Sennert's attempts at making gold by reducing metals to their primitive state (reductio in pristimum statum) in order to recover the potentials of matter to make all metals.


Richard Béarée
email: richard.bearee@ensam.eu

Jean-Yves Dieulot
email: jean-yves.dieulot@polytech-lille.fr

Patrice Rabaté
email: patrice.rabate@eads.net

J.-Y Dieulot

An innovative subdivision-ICP registration method for tool-path correction applied to deformed aircraft parts machining

Keywords: Iterative Closest Point, Image registration, Deburring, Aeronautics, Machining robot

A new and fast registration algorithm has been proposed to update the tool-path of a deburring robot, intended to machining composite workpieces under gravity and clamping deformations. A Subdivision Iterative Closest Point algorithm, which considers different parts of the contour with respect to curvature, allows to obtain far better results than classical methods, without complicated assumptions or computations. The procedure has shown to be effective for porthole and nose-cone deburring. Experimental tests conducted on robotic milling workcell demonstrated the efficiency of the registration method.

Introduction

New aircraft designs make a more extensive use of composite materials, which allow to construct large components. However, increasing the size of workpieces requires the use of bigger CNC machines. As an alternative, 6 d.o.f. robots equipped with a spindle have been considered for deburring operations, allowing higher flexibility and the use of coarse clamping devices [START_REF] Tsai | Robotic polishing of precision molds with uniform material removal control[END_REF]. In this case, the workpiece will be roughly positioned, and the trajectory of the robot within space should be registered using the original computer-aided design (CAD) model. However, clamping and gravity forces generate deformations which are not negligible when considering high-dimension composite plates, and should be taken into account in the registration procedure (Fig. 1) As an example, it will be necessary to cut an oval shape into the fuselage to obtain a round porthole once the clamping devices are released. Lengthy re-engineering procedures can be considered which consist of estimating the deformations using finite-element methods and embed those into the CAD/CAM design (see e.g. [START_REF] Saadat | Deformation analysis of large aerospace components during assembly[END_REF]). However, when considering deburring operations, only the contours of the hollow moulded pieces should be processed, which, combined with small deformation hypotheses, allows on-site registration. Typically, a cloud of CAD model points has to be matched with the measurements given by a 3D measurement device (e.g. advanced CMMs, laser trackers, articulated arms with scanner, handscanner), usable in a manufacturing context.

The Iterative Closest Point (ICP) approach is the most popular for modelling a geometric transformation which allows the matching of a cloud of points with a model [START_REF] Besl | A method for registration of 3-D shapes[END_REF]. ICP is an iterative descent procedure which Fig. 1 Deformation of a complex composite part under clamping and gravity forces aims at minimizing the sum of the squared distances between all points in a dataset (a cloud of points representing a real object) and their closest points in a model. The rigid transformation (find a translation coupled to a rotation) which aligns those sets can be solved in a least squares sense with a closed form solution, and has been shown to be performing for nondeformed images [START_REF] Ezra | On the performance of the ICP algorithm[END_REF][START_REF] Liu | Improving ICP with easy implementation for free-form surface matching[END_REF], with applications in workpiece manufacturing, mainly for CAI purposes after machining [START_REF] Boukebbab | Applied iterative closest point algorithm to automated inspection of gear box tooth[END_REF][START_REF] Chuang | A reverse engineering approach to generating interference-free tool paths in threeaxis machining from scanned data of physical models[END_REF][START_REF] Pahk | Precision inspection system for aircraft parts having very thin features based on CAD/CAI integration[END_REF][START_REF] Seltzer | High speed cutting with industrial robots: towards model based compensation of deviations[END_REF]. Pottmann et al. [START_REF] Pottmann | Registration without ICP[END_REF] reveal that the ICP algorithm needs many iterations to converge, but that using local quadratic approximants of the squared distance function allows faster registration. Kim and Kim [START_REF] Kim | A fast ICP algorithm for 3-D human body motion tracking[END_REF] and Jost and Hügli [START_REF] Jost | Fast ICP algorithms for shape registration, pattern recognition[END_REF] propose a hierarchical model point selection and a 2D logarithm data point search which accelerate the ICP convergence, at the price of constructing a K-D tree. Okada and Hebert [START_REF] Okada | Fast 3D tracking of non-rigid objects[END_REF] use a combination of a fast ICP, a two-step ICP, and an estimation of deformation vectors using an accelerated version of the RPM algorithm, splitting the point cloud into deformed and non-deformed parts. In [START_REF] Zhu | Efficient registration for precision inspection of free-form surfaces[END_REF], authors use signal-to-noise ratio to improve ICP for inspection of deformed part. No work has been reported for deburring operations, and very few for robot registration [START_REF] Ravishankar | Automated inspection of aircraft parts using a modified ICP algorithm[END_REF]. Extensions to deformed images can be handled by introducing additional stiffness terms into the model [START_REF] Amberg | Optimal step nonrigid icp algorithms for surface registration[END_REF][START_REF] Kambhamettu | 3D nonrigid motion analysis under small deformations[END_REF][START_REF] Laskov | Curvature-based algorithms for nonrigid motion and correspondence estimation[END_REF][START_REF] Li | Nonrigid motion recovery for 3D surfaces[END_REF], but the main drawback lies in the expertise needed for working these methods out, and the computational burden which is not always compatible with real-time production.

As, in practice, the workpiece curvature and flexure are rather small (e.g. deburring a hole on a curved plate), it is possible to obtain the deformation of the contour using a projection of the trajectory (once registered using ICP); this deformation can be modelled and then compensated, which avoids the introduction of the aforementioned non-rigid terms. However, when considering more complex structures (e.g. deburring the cockpit into the nose-cone barrel), a simple model is impossible to obtain and a region-based (where regions exhibit small curvature) algorithm can be considered. R-ICP approaches determine generally a unique rigid transformation using a regional weighted Least Square criterion [START_REF] Amor | New experiments on ICP-Based 3D face recognition and authentication[END_REF], which can also be found when considering images with multiple views [START_REF] Chen | Object modelling by registration of multiple range images[END_REF].

This paper presents a rapid registration procedure applied to contour deburring into a flexible composite workpiece, which uses an ICP method with contour subdivision, denoted as Subdivision-ICP (S-ICP). Each region is registered, and next, smoothing procedures are provided for connecting adjacent regions. Finally, the inverse transforms will be applied to update the robot trajectory.

2 Registration methods using deformation modelling 2.1 Iterative Closest Point and contour matching ICP allows to register two points clouds in an iterative way, and to estimate the rigid transformation which aligns them; at each step, pairwise points (in a closest point sense) of the two sets are matched, so that the mean distances between the pairs are minimized.

Let two sets of points, X = -→ X i , i = 1 • • • N x , which represents the model set to be registered and Y = -→ Y i , i = 1 • • • N y the data set which represents the actual surface. As the number of points can be different, the sets X and Y generally do not match one-toone. In order to match the two datasets, quaternion algebra is introduced [START_REF] Horn | Closed-form solution of absolute orientation using unit quaternions[END_REF]. The quaternion denoted q = [q 0 , q 1 , q 2 , q 3 ] is used to represent the 3D rotation, T is a translation vector, R is a rotation matrix and is a function which associates an element

-→ X i of the set X with its closest neighbor -→ Y i in the set Y, -→ X i = min -→ X i - -→ Y .
The 3×3 rotation matrix generated by a unit quaternion is given by: R = ⎡ ⎢ ⎣ q 2 0 +q 2 1 -q 2 2 -q 2 3 2 (q 1 q 2 -q 0 q 3 ) 2 (q 1 q 3 +q 0 q 2 ) 2 (q 1 q 2 +q 0 q 3 ) q 2 0 +q 2 2q 2 1 -q 2 3 2 (q 1 q 3 -q 0 q 1 ) 2 (q 1 q 3 -q 0 q 2 ) 2 (q 2 q 3 +q 0 q 1 ) q

2 0 + q 2 3 -q 2 1 -q 2 2 ⎤ ⎥ ⎦ (1)
The ICP algorithm can be split up in four steps which yield the transform F = (R, T) that minimizes the distance between the registered model set X and the actual dataset Y:

a. Initialize X k with X, initialize k = 0 b. For every point -→ X i of X k , search the closest neigh- bor -→ Y in Y. c.
Using the corresponding pairs of points, determine rotation R k and translation T k which minimize the mean quadratic error J k :

J k (R k , T k ) = 1 N X N X i=1 R k -→ X i + T k - -→ X i (2)
Several techniques can be used to solve the problem, in our case quaternion algebra allows to obtain the transformation

F k . d. The new set is updated as X k = R k X k-1 + T k , and step b) is repeated if J k (R k , T k ) ≤ δ, where δ is a predefinite threshold, or if k ≥ k max ,
where k max is the maximum number of iterations. The final transform is noted F = (R, T).

The deburring application allows to reduce the number of points to a neighborhood X = -→ X i of the closed contour C to be processed (e.g. a porthole) lying on a surface S (a composite plate). The theoretical surface (model set) is obtained via a CAD package and is originally represented by a set of triangles obtained via Delaunay triangulation. The actual surface can be obtained by many 3D measurement devices (e.g. a hand-scanner moved by an operator or any automated device) in a neighborhood of the contour to be processed. A first common step to the methods which are considered in this paper consists of selecting points (theoretical and real) that belong to the contour. This can be performed by comparing, for every point, the number of neighbors (using the mean distance between points) and the number of triangles which the point belongs to. When these numbers are different, the point belongs to a contour. When considering the actual workpiece, the contours should be validated by the user to eliminate outliers or wrong contours resulting from imprecise, noisy or missed data points. It is well known that the registration precision increases with the sampling density, until some threshold is reached, for which no further appreciable improvement can be noticed [START_REF] Saadat | Deformation analysis of large aerospace components during assembly[END_REF].

A pragmatic determination of the deformation using projection methods

When considering high-dimension and small curvature workpieces and assuming small deformations, an approximate of the deformation may be obtained using an orthogonal projection of the (coarsely) registered model onto the plane P that approximates the registered model in the Least Square sense. Let X the set registered by the initial ICP algorithm, -→

X i = R -→ X i + T. The orthogonal projection Q of the registered set X onto P is performed as: -→ q i = Q -→ X i : An approximate of the deformation will be -→ d i = -→ q i - -→ X i (3)
If the deformation can be modelled using some function f (.)-e.g. a spline function-, this model can be embedded into the registration procedure. As an example, the deformation of a composite plate can be very roughly approximated, in a neighborhood, by a parabola of equation z = αw 2 , where α is a parameter to be determined. Of course, such an approximation of the deformation is not exact, since a projection on a plane which approximates the registered surface is performed, but it may be well enough when assuming small deformations. In order to debur the final contour C = -→ C i (e.g. of the porthole on the fuselage of the aircraft) the following transform should be applied:

/ -→ γ i = f R -→ C i + T (4)
Note that the normals to the workpiece should be computed again so that machining should be performed in a correct way (e.g. orthogonal to the workpiece). The idea of handling a finer registration procedure after a coarse matching has been worked out in CAI problems such as in [START_REF] Pahk | Precision inspection system for aircraft parts having very thin features based on CAD/CAI integration[END_REF].

Figures 2 and3 present the results of the registration of a porthole in a composite plate; for the sake of simplicity, only 1,500 theoretical points have been Theoretical contour after ICP Theoretical contour after ICP with deformation approximation Real contour Fig. 2 Deburring a porthole in a double curvature plate: theoretical contour registered on the real deformed workpiece Fig. 3 Deburring a porthole in a double curvature plate: normals of the trajectories on the theoretical workpiece (left) and registered trajectories on the deformed workpiece (right) considered, which corresponds to a precision of 5 mm. The maximum deformation in a neighborhood of the contour is 11 mm. The mean square error after registration (ICP and deformation compensation) is 3.3 mm, which corresponds to 65 % of the standard error obtained with a rigid ICP.

The precision of registration can be increased when considering a higher number of points and thus finer scanning measurements (typically, the required precision will lie around 0.1 mm). Performances are, of course, clearly dependent on the accuracy of the deformation model.

Subdivision method

Contour subdivision

In the previous method, the deformation was obtained using a projection on a plane which would approximate the contour of the surface to be deburred, and has proven to be successful for such simple surfaces as a porthole. This method is untractable when considering complex workpieces with complicated deformations such as deburring a cockpit into a nose cone (Fig. 4). In this case, it is proposed to divide the surface into quasi-planar surfaces and the contour into a number of quasi-linear curves. An appropriate subdivision method which addresses the problem of overlapping contours and/or connection between registered contours should be designed. The S-ICP algorithm can be split up in the following steps: a. Detect high curvature contours (with user-defined tuning parameters) and store corresponding points { -→ H 1 }. b. Subdivide low-curvature contours-using subdivision points { -→ H 2 } -, for precision purposes. c. Determine neighboring areas (using user-defined maximum distance to the contour) d. Add stability points to the set H 2 e. Perform rigid ICP, and transform the theoretical contour and the stability points f. Detect the nearest neighbor in the real image for every registered theoretical point g. Perform ICP on every sub-contour piece (note that every sub-contour overlaps its neighbors) h. Perform a smooth weighting procedure for connecting overlapping areas i. Perform the transformation on the theoretical tooltrajectory (using the nearest neighbor on the theoretical contour)

Many methods can be considered to define the curvature of a point belonging to the contour. The radius of curvature was computed as the radius of the osculating circle (which passes through the point and its nearest neighbors belonging to the contour). Now, a set of subdivision points can be designed such as:

H 1 = --→ H 1,i , i = 1 • • • N h , where --→ H 1,i ∈ H/ρ --→ H 1,i > ρ T , ( 5
)
Fig. 4 Deburring a nose-cone (full-barrel) in composite materials where ρ T is a threshold curvature radius. For practical purposes, it is possible to divide long contour pieces into small ones, introducing additional subdivision points:

k < s --→ H 2,i -s ---→ H 2,i-1 < (k + 1) , k ∈ N (6)
where s is the curvilinear abscissa and is a threshold corresponding to the maximum distance between points, k is the number of points between ---→ H 2,i-1 and --→ H 2,i . Such a resulting contour is represented in Fig. 5 for the porthole example. The different segments (a number of 6) are located between the markers. Note that all these segments overlap.

Scaling ICP registration

A first ICP procedure is performed on the contour X as before, yielding a registered contour X . Thus, the S-ICP procedure will only refine the results of the coarse ICP registration, and stability and segment connection problems will be easier to handle since only slight modifications will have to be done. Since the initial contour was divided into a set of segments, a new set of registered segments are obtained using the ICP procedure. Now, an ICP algorithm can be performed for each of these new segments (step g.), i.e. every segment of X is registered with respect to the dataset Y.

Fig. 5 Subdivision for a porthole

To avoid connection problems, an overlapping range (from 20-40%) between segments was allowed; the procedure which handles overlapping part consists of computing a weighted mean of the registered neighboring segments. Let -→ v z , --→ v z+1 be the vertices of zones z, z + 1. Weights were chosen as sinusoidal and the final transformed points -→ P i of the overlapping areas will be obtained as:

H -→ P i = ω z R z -→ P i + T z + ω z+1 R z+1 -→ P i + T z+1 ,
where

ω z = g v z p i , v z+1 p i (7)
e.g.

g v z p i , v z+1 p i = sin πv z+1 p i 2v z p i (8)
Note the existence of stability points → SP (step d.), chosen in the surface S, outside the contour.

-→ SP is the set corresponding to the transformation of -→ SP by the transform F = (R, T) of the primary ICP, for which the nearest neighbors are searched in Y. During the S-ICP procedure, these points are added to every segment of X , and their nearest neighbors to Y. This allows to obtain realistic rotation matrices and can be considered as a security parameter, in particular with respect to missing data in the scanned image. Their influence will be tackled in a further study.

Results

Practical applications of the registration method

First results are shown on a simplified workpiece (deburring a porthole with a double curvature). Next, the method was tested on a nose cone for which the deformation under gravity and clamping forces was obtained using a finite element software (ANSYS™).

Figure 6 shows that efficient registration can be obtained for porthole deburring.

For a complex workpiece (Fig. 7), one notes that the stability points, subdivision points, overlapping segments determination procedure are fully automated, one such tuning parameters being defined by the user, resulting into the subdivision in Fig. 8. Results in Fig. 9 underline the relevance of the S-ICP algorithm, which does not require a computation of the deformation Real workpiece points Theoretical contour after ICP Real contour Theoretical contour after S-ICP Fig. 6 Registration of a porthole with ICP and S-ICP (which is quite complex, as suggested by Figs. 2 and7). The workpiece can be registered quite fast, with the only requirement to obtain a real dataset. This is really one of the main interests of the method, because on-site registration can be performed by a trained operator and avoids further re-engineering. Figure 10 shows results for primary ICP and S-ICP. Note that Theoretical points Real points 1.9 mm when the distance between points is 2 mm for the theoretical contour, i.e. the mean error has been reduced from 57% and the peak error from 45% with respect to rigid ICP. Computing time is below 1 s for the porthole example (around 1,500 points) and is 26 s for the nose-cone (around 10,000 points) on a PC equipped with a Dual core processor 2,16 GHz and 1 G Ram, which is in accordance with a tuning procedure schedule within production context of small series. Note that results in Fig. 10 correspond to a point cloud (object) with measurement noise and missing points. A residual random noise can indeed be noticed when using the S-ICP method, whereas a basic ICP method suffers from matching errors depending from contour curvature (which can be seen when analyzing residuals). Of course, other methods, including nonrigid ICP methods, can lead to similar results, but at the Moreover, the resulting transformation can be applied to the deburring of the workpiece; Fig. 11 shows the updated tool trajectory designed to machine the actual workpiece. The robot trajectory is updated as follows:

a. For every trajectory point, find the nearest neighbor in the set of model points b. Apply the S-ICP transform to the point if the neighboring point belongs to a region with no overlap, otherwise, apply the transforms for both segments and use the overlapping weighted procedure. c. The normals to the tools are updated using the new trajectory points.

Experimental results on a simple test case

In the present work, a robotic milling work cell is setup with a Staubli RX170 industrial manipulator depicted in Fig. 12. A high-speed spindle (30,000 tr/min) is mounted on the robot wrist while the workpiece is fixed on a steel table. A simple test case is presented which consists in deburring a circular contour on a constrained composite material. The programmed toolpath assumes that the unconstrained composite sheet is flat. The real deformation of the sheet is imposed by the supporting piece, and hence is known. Figure 12 shows the experimental setup. The sheet deformation was measured using a 3D scanner device (HandyscanTM) in the machining region (about 3,000 points). After completing the machining operation, the composite sheet is unclamped and the performance criterion is chosen as the absolute abscissa error according to the theoretical contour, noted ε. Figure 13 presents a photo of the finished part and the measured path error is represented in Fig. 14. A better circularity of the machining part after the S-ICP registration method can be clearly observed. The residual error after S-ICP (around 0.2 mm) is in the range of the industrial robot accuracy required for deburring operations. Hence, this simple test case demonstrates the effectiveness of the proposed algorithm.

Conclusion

A new registration algorithm has been applied to update the tool-path for deburring operations on deformed workpieces, which uses techniques derived from the Iterative Closest Point method. After a first coarse ICP, the deformation was modelled in a neighborhood of the contour to be deburred. This technique is only suitable for very simple shapes and is not realistic for registration during production of such complex parts as aircraft nose-cones. Using contour subdivision, a performing method has been found that it has the advantage of being model-free. Experimental validations, using a machining 6-axis robot, have demonstrated the effectiveness of the proposed approach. Hence, trajectory update can be directly derived, allowing on-site machining by robot without accurate clamping or reengineering design.

Fig. 7 Fig. 8 Fig. 9 Fig. 10

 78910 Fig. 7 Nose-cone model and measurements

Fig. 11

 11 Fig. 11 Trajectory update and tool normals after S-ICP

Fig. 13 Fig. 14

 1314 Fig. 13 Finished parts

Acknowledgements

The Authors wish to thank Mr. Xu Z. for programming the algorithm and Mr. Helle X. and Mr. Olabi A. for their contributions to the experimental validations.