P Gasnier
email: pierre.gasnier@cea.fr

J Willemin

S Boisseau

Goubault De Brugière

G Pillonnet

B Gomez

I Neyret

A cm-scale, low wind velocity and 250°C-compliant airflowdriven harvester for aeronautic applications

This paper reports the design, fabrication, and testing of a centimeter-scale (Ørotor=35mm), 250°C-compliant microturbine for aeronautic applications. Dedicated to lowspeed air flows (≈3 m/s and down to 2 m/s), this device is the first flow-driven harvester withstanding such high temperatures and high vibration levels (10 7 cycles at 20G). Furthermore, the proposed harvester exhibits the highest output power per unit cross sectional area compared to prior art in the cm-scale and low velocity ranges.

Introduction

Among small-scale ambient energy sources (light, thermal gradients, vibrations, strains and shocks), airflow energy harvesting has revealed a great innovation potential and particularly at small-scale. Particularly in environments where light or thermal powers are insufficient, this source of power can be relevant to supply Wireless Sensors Nodes (WSN). Moreover, airflow energy harvesting can be a reliable source of power as it mainly depends on the fluid velocity amplitude contrary to vibration harvesters which are also frequency-dependents. Even if airflow energy harvesters exploiting aeroelastic flutter phenomena [START_REF] Perez | An electret-based aeroelastic flutter energy harvester Smart[END_REF] can be an alternative to lift-based rotors, horizontal axis propellers show better aerodynamic efficiencies (Cp above a few percent) and very low cut-in speeds (vstart below a few m/s). Furthermore, the classical electromagnetic conversion has been chosen here for its interesting power density above a few cubic centimeters. In recent years, many works have been proposed on the miniaturization of classical wind turbines, from 2cm to 6cm rotor diameters [START_REF] Howey | Design and performance of a centimetre-scale shrouded wind turbine for energy harvesting Smart[END_REF][START_REF] Carli | A high-efficiency wind-flow energy harvester using micro turbine[END_REF][START_REF] Flammini | An autonomous sensor with energy harvesting capability for airflow speed measurements[END_REF] but most of them show poor performances at low-speed air flows and/or exhibit large cross-sectional areas. Our work aims to compete with previous electromagnetic-based microturbines whose cross-sectional areas are below 30cm 2 (Ø<6cm) while addressing harsh operational environments.

Our harvester has been designed to supply a wireless sensor system interfacing with aeronautic-grade pressure/strain transducers [START_REF] Grezaud | A robust and versatile, -40C to +180C, 8Sps to 1kSps, multi power source wireless sensor system for aeronautic[END_REF]. Thus, in order to withstand severe aeronautic environment constraints such as high temperature values and vibrations amplitudes, particular materials and specific design rules have been used. Design, fabrication, performances measurements and ageing tests of the device are summarized in the following sections.

Design and fabrication

Our prototype (figure 1) is a horizontal axis wind turbine coupled to a coreless permanent magnet generator which converts mechanical rotations into electricity. Indeed, coreless alternators are particularly suitable when targeting low cut-in speeds as they do not suffer from cogging torque.

The device is composed of several mechanical parts screwed and/or glued together as depicted in figure 1 and figure 2. The coreless alternator (figure 2.a) includes a machined shaft (Aluminum), a 250°C tolerant copper wire wrapped around two casings slotted together and a drilled cylindrical magnet with radial orientation glued to the shaft. The magnet is made of SmCo (Curie temperature: 800°C). Two micro ceramics bearings allow the magnet and the shaft to rotate inside the alternator's casings. A 35mm diameter three-blade propeller (figure 2.b) is screwed to the shaft and converts the kinetic power of the air into rotation. A carter, made of two parts screwed together (figure 2.c) embeds the alternator and protects the propeller from unexpected shocks which may occur during the installation. The propeller, the alternator's casing and the carter have been machined in a high-performance polyimide-based plastic (Vespel®) withstanding a maximum continuous operating temperature of 260°C. A picture of the final assembled prototype is given in figure 2.d.

The propeller has been optimized thanks to an empirical study performed on several propeller designs and based on the Schmitz's theory. Various pitch angles, chord lengths and blade profiles have been tested. This empirical study, which cannot be covered in this paper, led us to choose a flat, 700µm (tip) to 1mm (noze) thick, 3-blade propeller and whose projected area is around 70% of the entire flow section. Each blade keeps a constant pitch angle of 25° and its chord varies linearly with the radius. This design was found to be the best compromise between performance (high power, low cut-in speed), mechanical robustness and ease of manufacturing (machining). It is worth mentioning that the carter surrounding the propeller has a significant impact on the propeller performances, more than the propeller design itself.

Thanks to finite element simulations, the Von Mises stress of all parts (propeller alone and assembled device) at 20G have been kept below 20MPa for all their resonant frequencies in the x, y and z directions. Furthermore, several design iterations led us to thicken the arms' casing and their fillets and to increase the number of arms (from 3 to 4) to reject the resonant frequencies as much as possible above the [0-2kHz] band at 250°C. These choices slightly disadvantage the device performances but they allow its operation in harsh aeronautic environment.

Power performances and ageing tests in temperature

In a dedicated low speed wind tunnel, 50 resistive loads [5Ω to 10 MΩ] were successively connected to the generator at constant wind velocity. The RMS output voltages were measured to compute the AC electrical power (Vrms 2 /Rload). The rotation frequency of the harvester is also measured. It enables to plot the output power vs the Tip Speed Ratio (TSR), shown in figure 4. The TSR corresponds to the ratio between the tangential speed of the blade tip and the actual speed of the wind. Four harvesters have been assembled and characterized at room temperature in a wind tunnel at various wind velocities (2 to 4 m/s) and up to 10 m/s. Their performances in the low speed range are plotted in figure 5. The harvester has a cut-in speed of 2.5m/s and outputs 350µW @ 2 m/s and up to 4.5mW @ 4 m/s at maximum power points (figure 4 and5).

After fabrication, accelerated ageing tests in the x, y and z directions have been carried out in a vibration chamber (figure 3) during 10 7 cycles at 20G / 250°C on each resonant frequencies in the [0-2kHz] band. Ageing tests at 180°C (200 hours) and 250°C (4 hours) have been performed with an airjet actuating the propeller inside the climatic chamber, showing a permanent power decrease of about 8% at 180°C and 34% at 250°C and validating its good operation in harsh environments.

Comparison to the state of art

Figure 6 shows the power density, i.e the maximum electrical power per unit cross sectional area of the harvester as a function of the air speed. It should be clarified that the two lower velocity values (vair = 2 and 2.25 m/s) have been measured after launching the propeller at 2,5 m/s (cut-in speed). Its cut-off speed is around 1,9 m/s. A suggested improvement could be the downsizing of the generator to reduce the influence of the rotor's inertia and the Earth's magnetic field on the cut-in speed of the device.

Regarding the power density, our harvester exhibits the highest performances compared to prior art [START_REF] Howey | Design and performance of a centimetre-scale shrouded wind turbine for energy harvesting Smart[END_REF][START_REF] Carli | A high-efficiency wind-flow energy harvester using micro turbine[END_REF][START_REF] Flammini | An autonomous sensor with energy harvesting capability for airflow speed measurements[END_REF][START_REF] Rancourt | Evaluation of Centimeter-Scale Micro Wind Mills: Aerodynamics and Electromagnetic Power Generation PowerMems[END_REF][START_REF] Zakaria | Experimental investigation and performance modeling of centimeter-scale micro-wind turbine energy harvesters[END_REF] in the cm-scale (Ø<6cm) and low velocity (vair<5 m/s) ranges. This is moreover the first one to be compatible with harsh aeronautic environments, that is to say high temperature (250°C) and high accelerations (20G).

Conclusion

This paper presents a cm-scale (Ø=35mm), low wind velocity (down to 2m/s) and 250°C compliant airflow-driven harvester dedicated to aeronautic applications. It is based on a horizontal axis propeller coupled to a permanent magnet (magnet -air coil) converter. Its compliance to harsh environments was achieved by the use of dedicated materials (Vespel®, SmCo), thermal/vibration analysis thanks to FEM modelling and specific tests in vibration chamber. In this dimension and flow velocity range, our harvester exhibits the highest performances compared to prior art while withstanding high temperature and high acceleration. It also provides a practical solution to harvest power from low-speed air flows in highly constrained environments where batteries cannot operate, as well as more conventional applications like inside buildings equipped with ventilation (HVAC).

Figure 1 .

 1 Figure 1. Cross-sectional view of the proposed harvester and its overall dimensions

Figure 2 .

 2 Figure 2. a) to c) main parts of an unassembled harvester and d) photograph of the final harvester

Figure 3 .

 3 Figure 3. Photograph of the harvester under tests in the vibration chamber (x direction)

Figure 4 .Figure 5 .

 45 Figure 4. Electrical power as a function of the Tip Speed Ratio (Harvester n° 1) for various air speeds at room temperature

Figure 6 .

 6 Figure 6. Performance comparison for air flow harvesters reported in the literature

Acknowledgments

This project was supported by CEASARLab -SAFRAN/CEA joint laboratory and has received funding from the European Union's FP7 CLEANSKY No. 632614 HiTEAS.


Fabien Despinoy

Leon Torres

Jonathan Roberto

; Vitrani

Benoît Marie-Aude Herman

Toward

"Toward remote teleoperation with eye and hand: A first experimental study"

Keywords:

Despite the continuous improvement of master interfaces, distant robot teleoperation remains a challenging task. In many applications, including robotassisted single port laparoscopic surgery and NOTES, the operator has only an indirect vision of the remote environment, provided by a video camera usually mounted on the robot end-effector itself and displayed on a monitor. Driving such an eye-in-hand robot smoothly in real time is known to be difficult. Even a skilled user will likely generate a succession of independent translations and rotations punctuated by frequent stops, in order to keep the target inside the camera field of view. Since one can assume that she/he will look most of the time at the target on the video monitor, our idea is to gather additional information on the remote target with an eye-tracking device, and to use it to facilitate teleoperation. This paper introduces a novel remote teleoperation system that comprises such a device, as well as an 6-axis joystick. ...

Toward Remote Teleoperation with Eye and Hand: A First Experimental Study

Fabien Despinoy, Jonathan Leon-Torres, Marie-Aude Vitrani, and Benoît Herman Abstract-Despite the continuous improvement of master interfaces, distant robot teleoperation remains a challenging task. In many applications, including robot-assisted single port laparoscopic surgery and NOTES, the operator has only an indirect vision of the remote environment, provided by a video camera usually mounted on the robot end-effector itself and displayed on a monitor. Driving such an eye-in-hand robot smoothly in real time is known to be difficult. Even a skilled user will likely generate a succession of independent translations and rotations punctuated by frequent stops, in order to keep the target inside the camera field of view. Since one can assume that she/he will look most of the time at the target on the video monitor, our idea is to gather additional information on the remote target with an eye-tracking device, and to use it to facilitate teleoperation. This paper introduces a novel remote teleoperation system that comprises such a device, as well as an 6-axis joystick. Several eye-hand couplings are compared in a first experimental trial. It is shown that the system is effective and intuitive.

I. CONTEXT, MOTIVATION, AND OBJECTIVES

When operating an interactive robotic device in real time, both user and robot sensorimotor loops act in synergy to perform a desired task under the user's supervision. Implementation of such a system depends on the chosen interaction between user and robot. With most master-slave systems used for teleoperation and virtual reality, interaction is limited to the exchange of kinematic information (e.g. position and/or velocity) with or without haptic feedback, and the slave arm simply copies the operator's motion. In many applications where robotic arms operate in inaccessible locations (e.g. space, nuclear plants, or during minimally access surgery), the robotic end-effector and its environment are captured by a scene camera and displayed on a monitor in front of the operator. The da Vinci teleoperation system from Intuitive Surgical [START_REF] Ballantyne | The da Vinci telerobotic surgical system: The virtual operative field and telepresence surgery[END_REF] and the DLR MiroSurge [START_REF] Hagn | DLR MiroSurge: A versatile system for research in endoscopic telesurgery[END_REF] are typical examples of such an hand-in-eye configuration in surgery, and are known to be easy to drive with their master arms. The ongoing trend to reduce invasiveness leads to the design of poly-articulated devices (e.g. iSnake form Imperial College [START_REF] Newton | Robot-assisted transvaginal peritoneoscopy using confocal endomicroscopy: A feasibility study in a porcine model[END_REF]) or continuum robots using concentric tubes (e.g. robots from Dupont at Boston University [START_REF] Dupont | Design and control of concentric-tube robots[END_REF] and Webster at Vanderbilt University [START_REF] Rucker | A geometrically exact model for externally loaded concentric-tube continuum robots[END_REF]). When the camera is embedded in such a snake-like system, creating an eye-in-hand configuration as with a flexible endoscope, teleoperation difficulty can increase rapidly. Even a skilled user will not make the most of a 6-axis master interface that could normally induce a smooth and continuous motion. Instead, she/he will likely generate a succession of basic motions in Cartesian or even joint space, punctuated by frequent stops [START_REF] Cotti | Adaptation of voluntary saccades, but not of reactive saccades, transfers to hand pointing movements[END_REF]. The main reason is that the target (e.g. tissue to grasp or to cut) must be kept inside the camera field of view during robot motion. A smoother motion would require to combine translations and rotations of the end-effector, with a ratio between linear and angular velocities that depends on the distance to the target, the latter being usually unknown.

The development of numerous eye-and gaze-tracking devices allows to add a new channel for human-robot interaction. Several systems have been developed for providing assistance to disabled people. In [START_REF] Barea | System for assisted mobility using eye movements based on electrooculography[END_REF], authors present an actuated wheelchair with eye-control, using the eye as a computer mouse. The same idea is used in [START_REF] Latif | Teleoperation through eye gaze (TeleGaze): A multimodal approach[END_REF] to drive a mobile robot remotely. Yet, the gaze is generally not combined with any other interactive information. Furthermore, user needs to perform a specific eye gesture to send an order to the robot, using his/her eyes in an unnatural way. In this paper, we address the following question: Can natural gaze tracking help to teleoperate a robot? Interestingly, Pinpin et al. [START_REF] Pinpin | Gaze interface: Utilizing human predictive gaze movements for controlling a hbs[END_REF] predict the motion of a robot based on user's natural gaze. The main limitation of their setup is that gaze is only reliable for predicting which actions from a predefined list will occur. To overcome this predefined list, we propose to drive a robot using the user's eye movement, without any pattern recognition. Spindler and Chaumette already implemented a visual servoing of the orientation of a robot-mounted camera, using human eye-movement [START_REF] Spindler | Gaze control using human eye movements[END_REF]. This device is however limited to a 2D task. We propose to extend this work to a general 3D task.

The paper introduces a novel teleoperation system that combines joystick and natural gaze inputs in several ways to teleoperate an eye-in-hand robot. Section II presents the robotic platform, and 3 control schemes are described in Section III. A first experimental performance assessment is then reported and discussed in Section IV.

II. REMOTE TELEOPERATION PLATFORM

A proof-of-concept prototype was built around a 6 degrees-of-freedom (DOF) Adept Viper s650 robot with a MotionBlox 60R real-time controller, as depicted in Fig. 1. The teleoperation system comprises a webcam mounted on the robot end-effector to capture the task environment, a head-mounted eye-tracking camera (EYE-TRAC H6 from Applied Science Laboratories, Bedford, MA), a stand to prevent any head motion with respect to a monitor where the webcam images are displayed, and a 6-axis joystick (SpaceNavigator from 3Dconnexion, Waltham, MA).

The platform specificity lies in the original combination of control interfaces. The 6-axis joystick is used to drive the robot in a pose/velocity scheme: Robot linear and angular Cartesian velocities are linked to the joystick position and orientation imposed by the user's hand. An eye-tracker device is also used to record user's pupils movements and to compute gaze direction with respect to the monitor. These two interfaces can then be combined in several ways to control the robot more easily and efficiently.

III. CONTROL SCHEMES WITH EYE AND HAND INPUTS

When performing a task on an object-directly by hand or with any teleoperation means-one usually looks at this object or point of interest (POI): The brush tip while painting, the nail when holding a hammer, or the blood vessel the surgeon wants to clip. This basic assumption lead us to use a gaze-tracking device as a supplement to a joystick or master arm so as to make teleoperation faster, smoother, and easier. Knowing where on the monitor the operator looks thanks to a gaze-tracker, a couple of end-effector motions can be automated, or information about robot-target distance or relative orientation can be deduced, even without any image processing of what is displayed. We designed and implemented 3 control schemes that use in different ways the 2D-coordinates of the focus point on the screen.

The two first possibilities are to maintain the target at the center of the screen, either by correcting automatically the end-effector orientation during a manual translation (autonomous rotation, see Fig. 2), or conversely, by shifting the robot when the user rotates it (autonomous translation).

In these two similar scenarios, the user has to control only 4 DOF manually, the remaining two being handled by the controller thanks to the eye-tracker 2D signal. This signal can be seen as a measure of the on-screen distance between POI and monitor center. The control loop then simply tries to minimize this distance by using the 2 automatic rotational or translational DOF, respectively.

A third possibility is to estimate the distance between endeffector and target, so as to make the robot rotate around the target instead of around its end-effector. This can be achieved iteratively by measuring the user's eye motion induced by a robot displacement, in order to move the center of rotation from the end-effector to the target. In opposition to the first two above, this distance estimation control scheme does not induce any autonomous motion. All translations and rotations are then induced manually but the rotations are more natural. If this distance is underestimated, a manual rotation will induce a shift of the POI in the image in the opposite direction. Conversely, an overestimation of the distance will also translate the POI in the same direction. An iterative algorithm can use this error measured by the gaze-tracker to update the distance estimation.

All these modes, along with a fully manual mode without eye-tracking, were implemented on the robot controller. After initial calibration of the gaze direction with respect to the monitor center, signals form joystick and eye-tracker are acquired, filtered and scaled. They are then used and combined, depending on the scheme, to update the robot pose in real-time. IV. PERFORMANCE ASSESSMENT An experimental study was conducted in order to assess the feasibility and usability of the proposed teleoperation system, and to compare the performance of the proposed control schemes. This section describes the teleoperation task proposed, the metrics used for performance comparison, the experimental protocol and the results.

A. Teleoperation Task

A wide variety of tasks can be performed with a teleoperated robot, depending on the context and the tool held by the robot. However, most tasks can be divided in two phases, that can occur at the same time but are more frequently successive: First, the robot is driven to approach a target, and then an action is performed with the tool. Since our teleoperation system only intends to increase the performance of robot motion, the task is a simple point-topoint displacement combined with an effector reorientation so as to reach a target under a prescribed pose. A specific target (see Fig 3) was designed to ensure that all 6 DOF must be used. It also gives to the operator visual clues on the position and orientation of the end-effector.

B. Metrics

Task duration is the main criterion to compare the schemes. Duration corresponds to the time elapsed between the first operator manual instruction with the joystick and the arrival next to the target. A tolerance zone was programmed to ensure the experiment stop and each linear or angular tolerance was tuned experimentally to define the task difficulty. Moreover, during the experiments, articular robot positions and input signals were also recorded. Graphical analysis of the kinematic data showed that the task could be decomposed into two successive phases: A first approach phase where the error decreases rapidly, followed by an adjustment phase where motions are much shorter and slower. It was then decided to compute the duration of these two phases for each trials, as secondary metrics. Subjects were also asked to estimate, on an integer scale from 0 to 10, the performance, precision and intuitiveness of each mode. Performance indicates speed perception to perform the task. Precision reflects the possibility to drive the robot exactly where the operator wants to. Finally, intuitiveness reveals the operator mental workload to control the system correctly.

C. Experimental Protocol

Six researchers from the CEREM were enrolled in the experimental campaign and repeated the same target-reaching task with the various control schemes, being asked to be as quick and precise as possible. Each subject started the protocol with the fully manual mode so as to learn to use the system. He/she repeated the task until the reach of a plateau on the learning curve fitted and refreshed after each trial. The last 3 trials (on the plateau) were taken into account for statistical analysis. Afterwards, the subject tried the 3 assisted modes in randomized order. For each assisted mode, 8 repetitions were performed. The 4 best out of the 5 last trials were taken for analysis.

D. Statistical Analysis

Data were analyzed using General Linear Model (GLM) with JMP 9.0.3 and SAS 9.3 statistical software. The mixed model contained the following effects: subject and control scheme, and the two-factors interaction. Control scheme was defined as fixed factor for the ANOVA (analysis of variance). Subject and its interaction with the fixed factor were defined as random. The model was solved using REML (Restricted Maximum Likelihood). The Tukey HSD test of multiple comparisons was used to compare modalities of significant factors.

E. Results

Figure 4 depicts the average value and standard deviation of approach, adjustment and total duration for all trials of all subjects, for each control mode. Standard deviation for each mode is similar, ensuring that the ANOVA can model the data properly. One can observe that the autonomous translation mode is the slowest, especially during the adjustment phase. This is confirmed by the statistical analysis, since this mode is significantly different from all others regarding the adjustment duration. For the approach duration, the only significant difference is between the fully manual (slowest) and the autonomous rotation (fastest). Finally, analysis of the overall task duration shows that the autonomous translation (slowest) is significantly different form the autonomous rotation (fastest). Average values of subjective indicators are presented on Fig. 5, after normalization. It appears that the autonomous translation mode is believed to be the less efficient, precise and intuitive mode. In opposition, the distance estimation is perceived as the best mode, followed closely by the autonomous rotation mode.

F. Discussion

This first experimental session demonstrates the feasibility and efficiency of our teleoperation approach using eye and hand. Although the statistical analysis does not highlight many large differences, results are promising. In particular, it seems that the approach phase can be shortened with a gazetracker, since all 3 assisted modes are faster that the fully manual one. This might be thanks to the fact that, during large motions, it is easier to keep the target close to the center of the monitor with assistance.

However, during the adjustment phase, a fully manual control seems faster. Subjects declared that this was because it was difficult to keep the focus on the center of the target while perform fine adjustments, since several parts of the targets were to be used to check alignment. Therefore, autonomous motions were sometimes felt like a small disturbance during the final adjustment phase. This suggests that the eye-tracking system should be switched off after the initial approach phase.

Additionally, subjective indicators show the same trend than the objective measurements, with the autonomous translation mode that seems less efficient, precise and intuitive. This suggests to abandon this mode and to keep improving the two other assisted schemes.

V. CONCLUSION AND FUTURE WORK

In this paper, we introduced an original system that uses both hand and eyes as master inputs to drive a robot in real time. Three control schemes were proposed that use the gaze information in different ways to reduce the number of DOF that must be driven with a joystick, or to facilitate rotation around the target. A first experimental session demonstrated that teleoperation with eye and hand is feasible and efficient for remote teleoperation with an eye-in-hand configuration.

An improved version of the system is currently being developed, using a 3D tracking system to allow head motion, in order to decrease mental and physical workload. As a consequence, this should probably increase the performance level of the whole system. In addition, the task difficulty should be adjusted, since duration decomposition showed that nearly half of the time was spent for small adjustments that might not be required in real applications. The system and control schemes should also be studied in case of a change of POI during the task, which is not unusual.

Fig. 1 .

 1 Fig. 1. Experimental setup built around the Viper robot.

3 Fig. 2 .

 32 Fig. 2. Autonomous rotation to bring the target back to the center of the monitor after a manual translation.

Fig. 3 .

 3 Fig. 3. Target that requires 6 DOF motions of the end-effector.

Fig. 4 .

 4 Fig. 4. Average value and standard deviation of task duration for all subjects and repetitions.

Fig. 5 .

 5 Fig. 5. Average value of subjective indicators normalized between subjects.

  This work was supported by Fondation Belge Contre le Cancer (Grant No. SCIE 2006/20), and by own funds from UCL, ISIR, and F.R.S.-FNRS.F. Despinoy, J. Leon-Torres, and B. Herman are with the Center for Reserach in Energy and Mechatronics, Université catholique de Louvain, Louvain-la-Neuve, Belgium. J. Leon-Torres is also with Université Pierre et Marie Curie, Paris, France. B. Herman is also with Fonds de la Recherche Scientifique -FNRS benoit.herman@uclouvain.be.F. Despinoy is now with the Laboratoire d'Informatique, de Robotique et de Microélectronique de Montpellier, Université Montpellier 2 -CNRS (UMR5506), Montpellier, France fabien.despinoy@lirmm.fr.M.-A. Vitrani is with UPMC Univ. Paris 06, UMR 7222, ISIR, Paris, France marie-aude.vitrani@upmc.fr.

ACKNOWLEDGEMENT

Authors would like to thank warmly all colleagues from the CEREM who took part in the experiments. Authors are also grateful to Catherine Rasse, from the Institute for Multidisciplinary Research in Quantitative Modelling and Analysis (UCL), for her support and advices regarding experimental methodology and statistical analysis.


Nathalie Glaudert

The The The The

Introduction

The object of this paper is to demonstrate that most of the structural characteristics or features of creole languages-as compared to the languages they originate from-can be accounted for without appealing to external factors or substratum theories. I shall argue that the classical theory of markedness sheds light on the linguistic changes that took place during the process of creolization.

I shall limit my investigation to the field of Indian Ocean creoles-more precisely Reunion and Mauritian Creoles-and to the question of phonemic systems, bearing in mind that the same results could be obtained at morphosyntactic level.

Indian Ocean Creoles

Reunion and Mauritius are two islands located in the southwest of the Indian Ocean, east of Madagascar. Together with Rodrigues, they make up the Mascarene Islands.

Concerning the linguistic situations of the two islands, we can observe that a form of regional French and a creole, namely Reunion Creole, are spoken in Reunion. In Mauritius, although English is the official language, it is mastered only by approximately eight per cent of the population, and the most commonly used languages are Mauritian Creole and French.

The origins of both Reunion and Mauritian Creoles can be traced back to the French colonization of the islands, which took place in the 17 th and 18 th centuries, and thus to the background of a plantation society and of its accompanying system of slavery. Reunion and Mauritius were two uninhabited islands when they were colonized and the servile groups came mainly from Madagascar but also from the East African coast.

Two competing theories of creolization

Studies concerning creolization oppose several theoretical frameworks. I shall mention only two of them, which may be regarded as particularly important. The first is advocated by linguists (such as [START_REF] Holm | Pidgins and Creoles (2 volumes)[END_REF]) who underline the supposed structural specificity of creoles in what I shall call, by convention, a "pidgin-creole theory." Opposed to that, Chaudenson2 argues that the specificity of creoles lies in their genesis-more precisely in the socio-historical context from which they originate-rather than at any structural level proper.

According to advocates of the "pidgin-creole theory" (see above), a creole is supposed to be the nativization of a preliminary pidgin. They describe pidgins as hybrid languages, which are presumably simple and rudimentary, resulting from contacts between linguistically and socially heterogeneous groups. In such a view, when a pidgin becomes the mother tongue of a new generation of speakers, it is transformed and becomes more complex, and it is the output of this restructuring process which is regarded as a creole:

A creole has a jargon or pidgin in its ancestry; it is spoken natively by an entire speech community, often one whose ancestors were displaced geographically . . . . The children of the New World were usually exposed more to this pidgin-and found it more useful-than to their parents' native languages. . . . Although it appears that the children were given highly variable and possibly chaotic and incomplete linguistic input, they were somehow able to organize it into the creole that was their native language. (Holm 1989: 6) In this theory, the interference of a substratum is generally acknowledged: it is hypothesized that, from the pidginization process, the language of the dominated groups plays the role of a substratum, which is consequently supposed to leave traces in the creole. Chaudenson (2003) is highly critical of this view. He claims that the creolization process in the Indian Ocean is not the result of a preliminary pidginization step, but of two "approximate" acquisitions of French in parallel to two successive waves of immigrant populations:

The French-based creoles, originating from colonization dating back to the 17 th and 18 th centuries, resulted, in the socio-historical context of a proslavery plantation system supplied by linguistically heterogeneous immigrant populations, from the appropriation of approximate varieties of a form of French which had already been koineised during the previous phase of colonial settlement. (Chaudenson 2003 : 204, my translation) 3Moreover, Chaudenson makes a fundamental distinction between endogenous and exogenous creoles. In his conception, an endogenous creole has its origins in a colonial settlement where the servile populations are native. Such a context may favour the underlying influence of a substratum, as the colonized groups maintain their original languages. In contrast with an endogenous creole, an exogenous creole originates in a colonial settlement where the servile populations are immigrants. In this case, the hypothesis of a substratum is certainly less valid, as the servile groups are linguistically heterogeneous, and the only common language that they are liable to use is the dominating European language of the colonizers.

The historical contexts in which Reunion and Mauritius were colonized draw the limits of the pidgin-creole theory since, in these very cases, no pidginization is likely to have preceded the creolization process. As a result, [START_REF] Holm | Pidgins and Creoles (2 volumes)[END_REF], for example, far from giving up his theoretical postulates, suggests that Reunionese would in fact be a "semi-creole" resulting from a "partial creolization".

Obviously, Chaudenson's theoretical model proves to be more adequate in accounting for creolization in the Indian Ocean. His thesis provides reliable data concerning the lexicon of Reunion Creole, in which, for instance, the contribution of Malagasy represents no more than 4.3%, that of Indo-Portuguese languages 3.2%, and that of African languages 0.3% [START_REF] Chaudenson | Le lexique du parler créole de la Réunion[END_REF]. These data confirm that Reunion and Mauritian Creoles are not only two French-based creoles but also almost exclusively of French origin. Therefore, we have every reason to think that Reunion and Mauritian Creoles are two exogenous contact languages resulting from a particular socio-historical situation (the contacts between masters and slaves) which permits us to call them "creoles". This particular situation is characterized by the appropriation of 17 th and 18 th centuries' dialectal French by the servile populations.

Phonemic systems

I shall now proceed to compare the phonological systems (i) of standard French, (ii) of the variety of French spoken in Reunion; (iii) of Reunion Creole; and (iv) of Mauritian Creole. In Reunion Creole, two phonological systems co-exist and, following Watbled (2009), I shall call them "variety A" and "variety B" respectively.

Moreover, following a tradition dating back to Jakobson, I propose a feature system based on binary values. The description is thus simplified and the degree of complexity can be more easily assessed. In addition, rules and processes are expressible in a straightforward way. As far as the vowel systems are concerned, the binary features I shall use are:

[±front] and [±back] for tongue position;4 [±high], [±low], [±open] for tongue height;5 [±rounded] for lip gesture.

In the tables below, I present oral vowels only:

i u -high -low e ([e], [ε]) o ([o], [⊃]) -high +low a
Table 1 shows that the French oral vowel system includes eleven phonemes, three of which are front rounded vowels: /y/, /ø/ and /oe/ ([+front; +rounded]).

In table 2, we can observe that in Reunion French and in what I call variety A of Reunion Creole, the vowel system is limited to seven units. Among the front rounded vowels, /y/ and /ø/ have been retained (but [e] Table 3 shows that in variety B of Reunion Creole and in Mauritian Creole, the vowel system has retained only five phonemes, which, incidentally, is close to the fundamental vowel triangle /i/, /u/ and /a/ [START_REF] Jakobson | Rapports internes et externes du langage[END_REF]). It may be observed that the three front rounded French vowels do not appear in this system and that, as in table 2, [e] and [ε], and [o] and [⊃] are allophones of single phonemes /e/ and /o/ respectively.

Concerning consonants, it is only necessary to limit the investigation to the four sibilant or sulcal consonants (see Martinet 1970: 51, for example): two hissing alveolar sibilants /s/ and /z/, and two hushing palatoalveolar sibilants /š/ and /ž/ in standard French.

The binary features I use account for the various places of articulation of consonants:

[±labial]: [+labial] means that the lips are involved in the primary articulation;

[±coronal]: [+coronal] for apical and laminal consonants;

[±domal]: this term is borrowed from Catford (1977: 142).

In Catford's work, the term "domal" refers to "the entire 'domed' part of the roof of the mouth behind the dentalveolar region." In other words, it describes consonants from palatals to uvulars. However, in my analysis, following Watbled's suggestion 6 , and for phonological reasons, the feature [+domal] also includes the lamino-palatoalveolar consonants, which may be regarded as prepalatals in the broad sense of the term, in view of their affinities with the glide [j] for example, or of various assimilatory processes in many languages. Furthermore, I limit the application of the feature [+domal] to nonapical consonants. 6 Personal communication.

The consonantal system of standard French thus includes two hissing sibilants /s/ and /z/, which are [-labial, +coronal, -domal] and two hushing sibilants /š/ and /ž/, which are [-labial; +coronal; +domal]. In Reunion French and in variety A of Reunion Creole, these four sibilants are retained, while variety B of Reunion Creole and Mauritian Creole have lost /š/ and /ž/, which have merged with /s/ and /z/ respectively: /š/ > /s/, /ž/ > /z/.

The theory of markedness

From a more theoretical point of view, I suggest applying the theory of markedness to interpretations of phonemic systems. This theory, originally developed in the field of synchronic phonology by Trubetzkoy, Jakobson and the Prague Linguistic Circle in the 1930s, may also be exploited in studies about language evolution.

One of the main advantages of this theory of markedness is that it allows one to measure the complexity of linguistic structures and units through the comparison of paradigms, which are opposed and contrasted. All those who are familiar with the theory know that in a dyadic opposition, the simpler element is termed "unmarked," and the more complex element, "marked."

In this paper, I also apply the theory of markedness to diachronic facts, insofar as the creolization processes under study imply the transition from French to creoles, which are supposedly structurally simpler than their original sources.

The study of the creolization process will introduce two types of relative simplicity or complexity: an intra-systemic simplicity/complexity resulting from the intrinsic features of phonemes, and an intersystemic simplicity/complexity resulting from the comparison between the phonological systems of French and that of the two creoles. The analyses of intra-and inter-systemic complexities will a priori enable one to describe and predict the phonological changes, which take place in the creolization processes.

Application to Indian Ocean Creoles

The French vowels /y/, /ø/ and /oe/ are marked because they combine the features [+front] and [+rounded]. This is a case of intrasystemic complexity resulting from a combination of features (and not from the actual presence of such or such a feature). Note that [+rounded] vowels are unmarked if they are [+back], as in the case of /u/, for example. Thus, in the creoles under study, the unmarked equivalents of /y/, /ø/ and /oe/ are the vowels having the same respective degrees of aperture, but the feature combination [+front;rounded]; that is, /i/, /e/ and /ε/. Therefore, both in variety B of Reunion Creole and in Mauritian Creole, we can observe the disappearance of marked phonemes in favour of unmarked phonemes.

As regards consonants, the hushing sibilants, /š/ and /ž/ are traditionally regarded as more complex than the hissing sibilants /s/ and /z/, the reason being that the articulations of /š/ and /ž/ combine the features [+coronal; +domal], which causes inherent complexity, while /s/ and /z/ are alveolar and combine the features [+coronal, -domal]: they are thus less complex and articulated with an optimal groove of the tongue [START_REF] Catford | Fundamental Problems in Phonetics[END_REF]. The above-mentioned line of reasoning concerning the vowels may also be applied to the consonants: both variety B of Reunion Creole and Mauritian Creole have lost the marked phonemes /š/ and /ž/, which have merged with the same respective unmarked equivalents: /s/ and /z/ (see above).

In order to account for these observations on the phonological systems of Reunion and Mauritian Creoles, two hypotheses are in competition. On the one hand, the theory that acknowledges the underlying interferences of substrata claims that creoles are mixed languages resulting from a blend of a European language and of one or several non-European languages; on the other hand, advocates of the theory of markedness, which focuses on the disappearance of marked phonemes in favour of their unmarked counterparts, simply argue that creoles result from the optimization of the base language7 . In the case of Reunion Creole and Mauritian Creole, these processes visibly tend towards simplification8 . I argue that the theory of markedness stands a better chance of providing a sound explanation of the creolization processes. The examination of the above-mentioned linguistic systems shows that the marked elements of a language form the weak points of its linguistic system, i.e. a category of elements that are likely to disappear and/or merge with their unmarked equivalents 9 .

In sum, in Creole studies, the application of Ockham's Razor leads us to favour the theory of markedness. This theory demonstrates that internal factors, i.e. the systemic analyses of creoles, are sufficiently revealing to investigate issues concerning their typology and their structure.

"Negative" substratum and convergence

The validity of the theory of markedness has now been confirmed. However, even if markedness explains how creolization functions, and reveals the underlying causes of the processes at work, we still have to account for the "selection" of some linguistic changes to the exclusion of others. Concerning this point, it is possible to acknowledge a convergence between the theory of markedness and the hypothesis of what we may call, following Watbled (2009), a "negative substratum". Let us hypothesize that, in the context of an exogenous creolization, the underlying influence of the non-European language on the European language can be negative, in the sense that, in a creole, the non-European language remains discernable, not through the presence of some of its features, but through the absence of the features which it does not share with the European language. Consequently, the negative substratum induces the reduction of the base language without any transfer of new phonemes:

[In cases of relative complexity,] the initial language causes the disappearance of a type of articulation. We can think that this negative principle is at work in creolization. We must make a distinction between this case and the introduction of unknown articulatory types in the base language: yet, this cannot be noted in the Indian Ocean creoles. (Watbled 2008, my translation) Variety B of Reunion Creole and Mauritian Creole have undergone the same process of decreasing complexity. The phonological systems of these creoles actually correspond to subsets of the French phonological system. Moreover, it is interesting to note that these phonological modifications of French are not specific to the creoles under study: for instance, they are also found in the speech of Italian, Spanish, or Arabic learners of French. In other French creoles, these modifications essentially affect the vowels. For instance, in Haitian or West Indian French-based Creoles, the three front rounded French vowels, /y/, /ø/ and /oe/ have disappeared as well. However, as far as French-based creoles are concerned, the disappearance of /š/ and /ž/ is specific to the Indian Ocean. Variety B of Reunion Creole and Mauritian Creole have lost these two phonemes, not only because of their intrinsic complexity, but also because of their relative complexity, probably resulting from the absence of /š/ and /ž/ in the Malagasy consonantal system, Malagasy having certainly been the main servile language during colonization (see above).

The negative substratum validates and reinforces the hypothesis of internal factors. The convergence between the application of the theory of markedness and the negative substratum actually underscores the general value of the present study. Moreover, the above-mentioned phenomena concerning the creoles follow natural evolution processes, which have also been observed in many other (non-creole) languages.

VIII. Conclusion

As far as creoles or exogenous contact languages are concerned, I have favoured Chaudenson's theory and have taken issue with details of the "pidgin-creole theory" on creolization processes. The details criticized are: (i) the fact that this theory does not take sufficiently into account the actual genesis of creoles, and (ii) that consequently, this theory is automatically liable to resort to external factors in order to account for processes of creolization.

As regards the first critique, I have argued that creoles do not actually present any structural specificity, but only a few recurring tendencies, which are largely due to their initial states. According to Chaudenson's fundamental distinction between endogenous and exogenous creoles, the specificity of creoles is only to be found in the particular socio-historical situations from which they originate. Reunion and Mauritian Creoles are two exogenous creoles and, on that account, substrata are unlikely to have interfered in the construction of their linguistic systems.

As regards the second critique, I have demonstrated that the internal factors, i.e. the analyses of the linguistic systems of creoles and of their base language, combined with markedness principles, are sufficiently revealing to deal with the issues concerning the structural characteristics of creole languages. I hope that I have succeeded in, at least, casting doubt on the validity of theories appealing to external factors or regarding their role as crucial or essential.

In this paper, the analyses of the linguistic systems have been carried out alongside the theory of markedness. I have demonstrated that this theory can account for the linguistic changes that took place during the process of creolization. Markedness can construe and account for the transition from the base language to the creoles without necessarily resorting to external factors. What is more, this theory appeals to universal principles, which could also be applicable in other linguistic evolution processes. It also appears that in the case of Reunion and Mauritius, the transition from the base language to the creoles has tended towards general simplification. The negative substratum I have mentioned may have reinforced the application of markedness principles, favouring the disappearance of the marked phonemes and their merging with their respective unmarked equivalents in the creolization process.

Nathalie Glaudert

  and [ε], [ø] and [oe], and [o] and [⊃] are now allophones of single phonemes /e/, /ø/, and /o/, respectively. The midhigh allophones [e], [ø], [o] and the midlow allophones [ε], [oe], [⊃], appear in free and checked syllables respectively).

Table 1 :

 1 Standard French

				-nasal
			+front
	VOWELS		-back	-front -back	-front +back
		-rounded	+rounded	-rounded
	+high -low -open	i	y		u +rounded
	-high -low -open	e	ø		o +rounded
	-high -low +open	ε	oe		⊃ +rounded
	-high +low +open			a	α -rounded

Table 2 :

 2 Reunion French = variety A of Reunion Creole

			-nasal
		+front
	VOWELS		-back	-front -back	-front +back
			-rounded	+rounded
		-rounded	+rounded
	+high -low	i	y		u
	-high	e	ø		o
	-low	([e], [ε])	([ø], [oe])		([o], [⊃])
	-high +low			a

Table 3 :

 3 Variety B of Reunion Creole = Mauritian Creole

			-nasal
	VOWELS	+front -back	-front -back	-front +back
		-rounded	-rounded	+rounded
	+high
	-low

An earlier version of this paper was presented at the 5th International Contrastive Linguistics Conference in Louvain, Belgium, July 9-12

, 2008.

See Chaudenson : 1974[START_REF]Textes créoles anciens, La Réunion et Ile Maurice: Comparaison et essai d'analyse[END_REF], 2003.

« Les créoles français, nés de la colonisation des XVII ème et XVIII ème siècles, résultent, dans le contexte socio-historique de la plantation esclavagiste alimentée en main-d'oeuvre par des populations immigrées linguistiquement hétérogènes, de l'appropriation non guidée de variétés approximatives d'un français, déjà koïnèisé durant la phase antérieure de société d'habitation ».

Front vowels are [+front; -back], central vowels are [-front; -back] and back vowels are [-front; +back]. The use of two features accounts for three degrees of tongue position.

In the same logic, the use of three features accounts for four degrees of vowel height.

In this view, the question is whether appealing to external (non-French) factors is necessary or not.

But let us note that, in some cases, the interaction of various successive processes may entail linguistic evolutions tending towards complication, even within a theoretical framework including markedness principles.

Interestingly enough, the phonological changes described in the two creoles under study only concern phonemes that are considered as "complex" during French language acquisition by children and by some foreign speakers. In these contexts too, these phonemes tend to merge with their unmarked equivalents.

« [Dans les cas de complexité relative,] la langue première est à l'origine de la disparition d'un type d'articulation. On peut penser que ce principe négatif a été à l'oeuvre lors de la créolisation. Ce cas est à distinguer nettement de l'introduction de types articulatoires inconnus de la langue de base : or on ne constate rien de tel dans les créoles de l'océan Indien ».